Patent Publication Number: US-6336548-B1

Title: Apparatus for positioning electronic component holder head and apparatus for transferring electronic component

Description:
This application is a Divisional application of Ser. No. 08/907,882 filed on Aug. 11, 1997 and since patented as U.S. Pat. No. 6,168,009 which in turn is a Continuation-in-Part of application Ser. No. 08/769,700 filed Dec. 18, 1996 and since patented as U.S. Pat. No. 5,926,950. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for positioning an electronic component holder head and an apparatus for transferring an electronic component (“EC”). 
     2. Related Art Statement 
     Japanese Patent Application laid open for inspection under Publication No. 2-69999 discloses an EC holder head which is employed by an EC mounting system for mounting one or more ECs on an object such as a printed circuit board (“PCB”). The prior EC mounting system includes a rotatable table which is intermittently rotatable about a vertical axis line, and a plurality of head supporting devices which are supported by the rotatable table such that the head supporting devices are equiangularly spaced from each other about the vertical axis line of- the rotatable table and such that each of the head supporting devices is movable in a direction parallel to the vertical axis line. Each of the head supporting devices supports an EC holder head such that the EC holder head extends parallel to the vertical axis line of the rotatable table and such that the EC holder head is rotatable about an axis line thereof. As the rotatable table is intermittently rotated, each of the head supporting devices is sequentially stopped at each of the same number of stop stations as that of the supporting devices. At an EC sucking station as one of the stop stations, each of the head supporting devices is moved up and down by an elevating and lowering device, so that the EC holder head supported by said each head supporting device sucks up an EC from an EC supplying device. Meanwhile, at an image taking station as one of the stop stations, an image of the EC held by the EC holder head is taken by an image taking device, so that an angular position error of the EC about an axis line of the EC holder head is calculated based on the image data provided by the image taking device and the calculated angular position error of the EC is corrected by rotating the EC holder head about the axis line thereof. After the angular position error of the EC is corrected, the head supporting device is moved to an EC mounting station as one of the stop stations, where the head supporting device is lowered by another elevating and lowering device, so that the EC holder head supported thereby mounts the EC on a PCB. 
     However, the above-identified prior EC mounting system can stop each of the head supporting devices at only one circumferential stop position in each of the plurality of stop stations, and cannot change or move the only one circumferential stop position in the circumferential direction of the rotatable table. Likewise, the prior EC mounting system can position each of the EC holder heads at only one position relative to the corresponding head supporting device in the direction parallel to the vertical axis line of the rotatable table, and cannot change or move the only one relative position in the direction parallel to the vertical axis line. In addition, the prior EC mounting system can move each of the EC holder heads along only one movement path and cannot change or move at least a portion of the movement path to another path whose position in a direction normal to that portion of the movement path is different from that of the portion of the movement path. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus wherein at least one of (a) a circumferential or rotating-direction stop position of a rotary member which carries an EC holder head and is rotatable about an axis line while describing a circular locus and (b) a position of the EC holder head relative to the rotary member in a direction parallel to the axis line is selectively changeable. 
     It is another object of the present invention to provide an apparatus which includes a plurality of rotary members which are rotatable about a common axis line independently of each other and each of which carries an EC holder head and wherein at least one of (a) a rotating-direction position of at least one of the rotary members and (b) a position of the EC holder head carried by at least one of the rotary members relative to said one rotary member in a direction parallel to the common axis line is selectively changeable. 
     It is another object of the present invention to provide an apparatus which includes a drive cam and at least one cam follower which cooperate with each other to rotate a rotary member and wherein a rotating-direction stop position of the rotary member is easily and selectively changeable. 
     It is another object of the present invention to provide an apparatus wherein a stationary cam and at least one cam follower are employed for easily and selectively changing a position of an EC holder head relative to an rotary member in a direction parallel to an axis line about which the rotary member is rotatable. 
     It is another object of the present invention to provide an apparatus which includes an EC holder head which is movable along a circular movement path or an EC holder head which is movable along a straight movement path and wherein at least one of (a) an on-path stop position on the circular or straight movement path and (b) an intersecting-direction stop position spaced from the circular or straight movement path in a direction intersecting the movement path is selectively changeable. 
     It is another object of the present invention to provide an EC transferring apparatus which can select, as at least a portion of a movement path along which an EC holder head is movable, one of a plurality of selectable paths which have different positions, respectively, in a direction intersecting that portion of the movement path. 
     The present invention provides an EC holder head positioning apparatus, an EC transferring apparatus, and an EC mounting system which have one or more of the technical features which are described below in respective paragraphs given parenthesized sequential numbers (1) to (25). Any technical feature which includes another technical feature shall do so by referring, at the beginning, to the number given to that technical feature. Thus, two or more of the following technical features may be combined, if appropriate. Each technical feature may be accompanied by a supplemental explanation, as needed. 
     (1) According to a first feature of the present invention, there is provided an apparatus for positioning each of a plurality of electronic component holder heads each for holding an electronic component, at a selected one of a plurality of positions of at least one stop station, comprising a plurality of rotary members each of which is rotatable about a common axis line and is stoppable at the at least one stop station, the electronic component holder heads being carried by the rotary members, respectively, such that each of the holder heads is movable relative to a corresponding one of the rotary members in a direction parallel to the common axis line; and at least one of (a) a circumferential position selecting device which selects, for the stop station, one of a plurality of circumferential positions which are spaced from each other in a direction of rotation of the rotary members about the common axis line, so that at least one of the rotary members is stopped at the selected circumferential position and accordingly the electronic component holder head carried by the one rotary member is positioned at the selected one position, and (b) an axial position selecting device which selects, for the stop station, one of a plurality of axial positions which are spaced from each other in the direction parallel to the common axis line, so that at least one of the holder heads is positioned at the selected axial position as the selected one position. The present positioning apparatus can select one of the plurality of circumferential positions and/or one of the plurality of axial positions. Accordingly, in the case where the present positioning apparatus is employed by, e.g., an EC mounting system having an EC sucking station and an EC mounting station, the present apparatus can select, as an EC sucking position at the EC sucking station or as an EC mounting position at the EC mounting station, one of the plurality of circumferential positions so that at least one of the rotary members, i.e., the EC holder head carried on said one rotary member is stopped at the selected one circumferential position as the EC sucking position or as the EC mounting position. In addition, depending upon a dimension of an EC as measured in the direction parallel to the common axis line, the present apparatus can select, as a stroke of movement of the EC holder head for sucking the EC or mounting the EC, one of the plurality of axial positions spaced from each other in the direction parallel to the common axis line, so that the EC holder head is moved to, and positioned at, the selected one axial position corresponding to the stroke of movement of the EC holder head. Thus, the present apparatus contributes to improving the EC mounting efficiency of the EC mounting system. 
     (2) According to a second feature of the present invention which includes the first feature (1), the positioning apparatus further comprises a rotary member supporting device which supports the rotary members such that the rotary members are rotatable about the common axis line independently of each other; and a rotary member rotating device which rotates the rotary members supported by the rotary member supporting device, independently of each other. Since the rotary members are rotatable independently of each other, the present positioning apparatus can rotate one of the rotary members while having another rotary member stopped at a stop station. Therefore, as will be described on a preferred embodiment of the present invention, the present apparatus can shorten a transferring pitch at which each of the rotary members is transferred from one stop station to the next stop station. Since this technical feature is combined with the above-indicated feature that the present apparatus can select one of the plurality of circumferential positions and/or one of the plurality of axial positions, the present apparatus can enjoy a high EC transferring efficiency of the EC holder heads. 
     (3) According to a third feature of the present invention which includes the second feature (2), the rotary member rotating device comprises a plurality of cam followers each of which is connected to a corresponding one of the rotary members; and at least one drive cam which has a cam groove engageable with each of the cam followers and which is rotatable about an axis line to move the each cam follower and thereby rotate a corresponding one of the rotary members about the common axis line, at least a portion of the cam groove having a width which permits the each cam follower to be moved in a direction of the width, and the circumferential position selecting device comprises a plurality of circumferential-direction pressing devices each of which selectively presses a corresponding one of the cam followers against each of a pair of opposed side surfaces of the cam groove. In the case where the cam groove of the drive cam includes an inclined portion which is inclined relative to the axis line of the cam, and a perpendicular portion extending in a direction perpendicular to the axis line, each of the rotary members is rotated when the drive cam is rotated with the cam follower connected to said each rotary member being engaged with the inclined portion of the cam groove, and is stopped when the drive cam is rotated with the cam follower being engaged with the perpendicular portion of the cam groove. Therefore, two stop positions which are spaced from each other in the direction of rotation of the rotary members can be defined by employing a cam groove having a width which permits each cam follower to be moved in a direction of the width. That is, one of the two stop positions corresponds to the state in which each cam follower is pressed against one of the two opposed side surfaces of the cam groove, and the other stop position corresponds to the state in which each cam follower is pressed against the other side surface of the cam groove. Thus, each rotary plate can be selectively positioned at one of the two stop positions by pressing, using a rotating-direction or circumferential-direction pressing device, the cam follower connected to each rotary plate against a corresponding one of the two opposed side surfaces of the cam groove. Since in the present positioning apparatus the plurality of circumferential positions are defined by providing a drive cam employed for rotating each rotary member, with a cam groove having a width which permits each cam follower to be moved in a direction of the width, the present apparatus can select one of the plurality of circumferential positions in a simple and low-cost manner. 
     (4) According to a fourth feature of the present invention which includes the second feature (2), the rotary member rotating device comprises a plurality of pairs of cam followers each pair of which are connected to a corresponding one of the rotary members, respectively; and at least one drive cam which has a cam rib engageable with each pair of cam followers out of the pairs of cam followers and which is rotatable about an axis line to move the each pair of cam followers and thereby rotate a corresponding one of the rotary members about the common axis line, at least a portion of the cam rib of the drive cam having a thickness which permits the each pair of cam followers to be moved in a direction of the thickness, and the circumferential-position selecting device comprises a plurality of circumferential-direction pressing devices each of which selectively presses each of the two cam followers of a corresponding one pair of the pairs of cam followers against a corresponding one of a pair of opposite side surfaces of the cam rib. In the case where the cam rib of the drive cam includes an inclined portion which is inclined relative to the axis line of the cam, and a perpendicular portion extending in a direction perpendicular to the axis line, each of the rotary members is rotated when the drive cam is rotated with the pair of cam followers connected to said each rotary member being engaged with the inclined portion of the cam rib, and is stopped when the drive cam is rotated with the pair of cam followers being engaged with the perpendicular portion of the cam rib. Therefore, two stop positions which are spaced from each other in the direction of rotation of the rotary members can be defined by employing a cam rib having a width which permits each pair of cam followers to be moved in a direction of the width. That is, one of the two stop positions corresponds to the state in which one of the two cam followers of each pair is pressed against one of two opposite side surfaces of the cam rib, and the other stop position corresponds to the state in which the other cam follower of each pair is pressed against the other side surface of the cam rib. Thus, each rotary plate can be selectively positioned at one of the two stop positions by pressing, using a circumferential-direction pressing device, one of the two cam followers connected to each rotary plate against a corresponding one of the two opposite side surfaces of the cam rib. 
     (5) According to a fifth feature of the present invention which includes any one of the first to fourth features (1) to (4), the positioning apparatus further comprises a stationary cam which has a cam groove formed along a cylindrical surface having a center line on the common axis line; and a plurality of cam followers each of which is connected to a corresponding one of the electronic component holder heads and is engageable with the cam groove of the stationary cam, at least a portion of the cam groove of the stationary cam having a width which permits the each cam follower to be moved in a direction of the width, and the axial position selecting device comprises a plurality of axial-direction pressing devices each of which selectively presses a corresponding one of the cam followers against each of a pair of opposed side surfaces of the cam groove. In the present positioning apparatus, each of the cam followers is pressed against one of the pair of opposed side surfaces of the cam groove and, when each of the rotary members is rotated, the cam follower connected to the EC holder head carried by said each rotary member is moved along said one side surface of the cam groove. The pair of opposed side surfaces of the cam groove are spaced from each other in the direction parallel to the common axis line, and the portion of the cam groove which has a width which permits each cam follower to be moved in a direction of the width, defines two axial positions which are spaced from each other in the direction parallel to the common axis line. Therefore, in the case where a portion of the cam groove which corresponds to a stop station has a width which permits each cam follower to be moved in a direction of the width, the present positioning apparatus can select one of the two axial positions at the stop station. 
     (6) According to a fifth feature of the present invention which includes the fifth feature (5), the cam groove of the stationary cam includes a portion whose position in the direction parallel to the common axis line continuously changes. In the present positioning apparatus, when each of the cam followers is moved along the above-indicated portion of the cam groove, the EC holder head to which said each cam follower is connected is moved in the direction of rotation of the rotary members while being moved in the direction parallel to the common axis line (hereinafter, referred to as the “axial direction”). Thus, the stop positions at which the EC holder head is stopped in the axial direction at one of a plurality of stop stations may differ from those at another stop station. Each of the axial-direction pressing devices can press a corresponding one of the cam followers against a selected one of the opposed side surfaces of the cam groove, thereby selecting a corresponding one of the stop positions at each of the stop stations. For example, in the case where the present positioning apparatus is employed by an EC mounting system which includes an EC supplying device and an EC mounting device and wherein the EC supplying and mounting devices are provided at different height positions, respectively, the height difference between the two devices can be compensated for by moving each of the EC holder heads in the direction parallel to the common axis line which extends vertically. In the case where the present positioning apparatus is employed by an EC mounting system wherein an EC supplying device and an EC mounting device are provided at the same height position or level, or wherein an EC supplying device and an EC mounting device can be moved up and down by respective elevating and lowering devices, respectively, the cam groove of the stationary cam need not include any portion whose position in the axial direction continuously changes, because there is no need to move each EC holder head in the axial direction. In the latter case, the stationary cam may have the cam groove whose position in the axial direction does not change at all. This is also the case with the EC holder head positioning apparatus wherein the stationary cam has a cam rib or ridge in place of the cam groove. 
     (7) According to a seventh feature of the present invention which includes any one of the first to fourth features (1) to (4), the positioning apparatus further comprising a stationary cylindrical cam which has a cam rib formed along a cylindrical surface having a center line on the common axis line; and a plurality of pairs of cam followers each pair of which are connected to a corresponding one of the electronic component holder heads and is engageable with the cam rib of the stationary cam, at least a portion of the cam rib of the stationary cam having a thickness which permits the each pair of cam followers to be moved in a direction of the thickness, and wherein the axial position selecting device comprises a plurality of axial-direction pressing devices each of which selectively presses each of the two cam followers of a corresponding one pair of the pairs of cam followers against a corresponding one of a pair of opposite side surfaces of the cam rib. The “thick” portion of the cam rib that is other than the “thin” portion thereof which has the thickness which permits each pair of cam followers to be moved in the direction of the thickness, may have a thickness which is slightly greater than the distance between the two cam followers in the direction of the thickness of the “thin” portion, so that the two cam followers may engage the opposite side surfaces of the cam rib, respectively. In this case, each of the EC holder heads can be moved along the cam rib without any rattling in the axial direction. Although only one of the two cam followers is pressed against a corresponding one of the two side surfaces of the cam rib by the axial-direction pressing device, the two cam followers can be moved along the cam rib while sandwiching the rib without any spaces provided therebetween. The thickness of the “thin” portion of the cam rib is smaller than the distance between the two cam followers in the direction parallel to the direction of the thickness of the “thin” portion, i.e., the axial direction. Along only the “thin” portion of the cam rib, the two cam followers are moved while only one of the two followers is pressed against, and engaged with, a corresponding one of the two side surfaces of the rib by the axial-direction pressing device. Thus, depending upon which one of the two cam followers is pressed against the corresponding one of the two side surfaces of the cam rib, the present positioning apparatus can select, for each EC holder head, one of two axial positions which are distant from each other by a distance equal to the difference between the thickness of the “thick” portion of the cam rib and the thickness of the “thin” portion of the same. 
     (8) According to an eighth feature of the present invention which includes the seventh feature (7), the cam rib of the stationary cam includes a portion whose position in the direction parallel to the common axis line continuously changes. Like the positioning apparatus in accordance with the above-indicated sixth feature (6), the present positioning apparatus can select, for each of a plurality of stop stations, one of a plurality of stop positions at which each of the EC holder heads is stopped in the axial direction. 
     (9) According to a ninth feature of the present invention which includes any one of the first and fifth to eight features (1), (5) to (8), the rotary members are supported on a common, intermittently rotatable table such that each of the rotary members is selectively movable to a plurality of positions thereof located on a circle having a center on the common axis line, and wherein the circumferential position selecting device comprises a plurality of rotary member moving devices each of which selectively moves a corresponding one of the rotary members to one of the plurality of positions thereof. 
     (10) According to a tenth feature of the present invention which includes any one of the third to eight features (3) to (8), the at least one drive cam comprises at least one concave globoidal cam having an outer circumferential surface which is defined by a locus which is described by a circular arc having a center on the common axis line when the circular arc is rotated about a circular-arc axis line which is perpendicular to the common axis line and which is positioned relative to the circular arc and the common axis line such that the circular arc is interposed between the circular-arc axis line and the common axis line. The outer circumferential surface of the concave globoidal cam contains the circular arc having the center on the common axis line. Therefore, when the globoidal cam which has an appropriate cam surface including a cam groove or a cam rib is rotated about an axis line thereof, the cam follower or followers which is or are engaged with the cam groove or rib is or are moved by the rotation of the cam, so the rotary member to which the cam follower or followers is or are connected is rotated about the common axis line over an angle corresponding to the length of the cam in the direction of rotation of the rotary members. 
     (11) According to an eleventh feature of the present invention which includes any one of the third to tenth features (3) to (10), at least one of the circumferential-direction pressing devices comprises a fluid-pressure-operated cylinder device. In this case, the circumferential-direction pressing devices can be produced at low cost. The fluid-pressure-operated cylinder device may be a double-acting air cylinder device. 
     (12) According to a twelfth feature of the present invention which includes any one of the fifth to eleventh features (5) to (11), at least one of the axial-direction pressing devices comprises a fluid-pressure-operated cylinder device. The fluid-pressure-operated cylinder device may be a double-acting air cylinder device. 
     (13) According to a thirteenth feature of the present invention which includes any one of the fifth to twelfth features (5) to (12), the stationary cam includes a movable portion which is movable in the direction parallel to the common axis line, and wherein the apparatus further comprises a movable portion moving device which moves the movable portion of the stationary cam and thereby moves at least one of the electronic component holder heads in the direction parallel to the common axis line. The movable portion of the stationary cam has a cam groove or a cam rib which provides part of the cam groove or rib of the stationary cam. As each of the rotary members is rotated, the cam follower or followers engages or engage the cam groove or rib of the movable portion. In this state, if the movable portion is moved by the movable portion moving device, the EC holder head carried on the rotary member is moved in the axial direction. 
     (14) According to a fourteenth feature of the present invention which includes any one of the first to thirteenth features (1) to (13), each of the electronic component holder heads comprises a sucking pipe which sucks and holds, by vacuum, the electronic component. 
     (15) According to a fifteenth feature of the present invention, there is provided an apparatus for selectively positioning at least one electronic component holder head for holding an electronic component, at a selected one of a plurality of positions of at least one station, comprising a holder head moving and stopping device which moves the electronic component holder head along a predetermined movement path and stops the holder head at the at least one stop station on the movement path; and at least one of (a) an on-path stop position selecting device which selects, for the stop station, one of a plurality of on-path stop positions on the movement path, so that the electronic component holder head is stopped, and accordingly positioned, at the selected on-path stop position as the selected one position, and (b) an intersecting-direction stop position selecting device which selects, for the stop station, one of a plurality of intersecting-direction stop positions which are spaced from each other in a direction intersecting the movement path, so that the electronic component holder head is stopped, and accordingly positioned, at the selected intersecting-direction stop position as the selected one position. The movement path of the EC holder head is not limited to the circular path employed in the EC holder head positioning apparatus in accordance with the first feature (1), but may be a curve other than the circular arc, or a straight line, or a composite of one or more curves and one or more straight lines. For example, the EC holder head may be mounted on a movable member which is movable along a straight movement path and is stoppable at each of a plurality of stop stations on the movement path, and a plurality of stop positions may be provided for at least one of the stop stations. In this case, the present positioning apparatus can select one of the stop positions so that the movable member or the EC holder head may be stopped at the selected stop position. A head moving device which moves the EC holder head may be provided on the movable member. In this case, the positioning apparatus can select one of a plurality of stop positions, by operating the moving device to move the head holder to a selected one of a plurality of positions on the movable member. The movable member may support another or second movable member which carries the EC holder head, such that the second movable member is movable along a straight line perpendicular to the straight movement path on a common plane. In this case, the EC holder head can be moved to any desired position on the common plane by the combination of respective movements of the two movable members. The positioning apparatus can select one of a plurality of stop positions at which the EC holder head is stopped, by moving one of the two movable members to a selected one of a plurality of stop positions therefor and moving the other movable member to a selected one of a plurality of stop positions therefor. An EC holder head moving device may be mounted on the second movable member so as to move the EC holder head to a select one of a plurality of stop positions. In the case where the present positioning apparatus is employed by an EC mounting system including an EC supplying device which utilizes a palette for supplying ECs, the EC holder head and the palette may be moved in respective directions perpendicular to each other in a common plane, so that the EC holder head may pick up an EC accommodated in each of a number of EC accommodating pockets of the palette. Alternatively, each of the head and the palette may be moved in both the two directions, so that the head may pick up the ECs from the palette. Moreover, one of the head and the palette may be moved in only one direction and the other may be moved in both the two directions, so that the head may pick up the ECs from the palette. In either case, the positioning apparatus can select one of the stop positions at each of which the EC holder head is stoppable, thereby selecting one of a plurality of EC pick-up positions at each of which the EC holder head can pick up an EC from the palette. This arrangement contributes to reducing the cumulative distances of movement of the palette. The intersecting direction in which the stop positions are spaced from each other may be any direction intersecting the movement path of the EC holder head, for example, a vertical direction. In the case where the EC holder head is supported on a movable member which is movable along the movement path, such that the holder head is movable on the movable member in a direction perpendicular to the movement path, the intersecting direction may be a direction which is perpendicular to both the movement path and the direction in which the EC holder head is movable on the movable member, or a direction which is perpendicular to the movement path and is parallel to the direction of movement of the EC holder head on the movable member. The present EC holder head positioning apparatus can select, for a stop station, one of a plurality of stop positions at each of which the EC holder head is stoppable when the head is moved along the movement path which may contain a straight line and/or a curve. Therefore, irrespective of the kind of the EC mounting apparatus which employs the present positioning apparatus, the positioning apparatus contributes to improving the EC mounting efficiency of the mounting apparatus. 
     (16) According to a sixteenth feature of the present invention which includes the fifteenth feature (15), the positioning apparatus further comprising a movable member which supports the electronic component holder head such that the holder head is movable in a direction intersecting the movement path and which is movable along the movement path; a stationary cam having a cam groove extending along the movement path; a cam follower which is connected to the movable member such that the cam follower is movable with the holder head and which is engageable with the cam groove of the stationary cam, at least a portion of the cam groove having a width which permits the cam follower to be moved in a direction of the width, and wherein the intersecting-direction stop-position selecting device comprises an intersecting-direction pressing device which selectively presses the cam follower against each of a pair of opposed side surfaces of the cam groove. The intersecting-direction pressing device may comprise a fluid-pressure-actuated cylinder device, such as a double-acting air cylinder device. 
     (17) According to a seventeenth feature of the present invention which includes the fifteenth feature (15), the positioning apparatus further comprises a movable member which supports the electronic component holder head such that the holder head is movable in a direction intersecting the movement path and which is movable along the movement path; a stationary cam having a cam rib extending along the movement path; a pair of cam followers which are connected to the movable member such that the pair of cam followers are movable with the holder head and which are engageable with the cam rib of the stationary cam, at least a portion of the cam rib having a thickness which permits the pair of cam followers to be moved in a direction of the thickness, and wherein the intersecting-direction stop-position selecting device comprises an intersecting-direction pressing device which selectively presses each of the two cam followers of the pair against a corresponding one of a pair of opposite side surfaces of the cam rib. The intersecting-direction pressing device may comprise a fluid-pressure-actuated cylinder device, such as a double-acting air cylinder device. 
     (18) According to an eighteenth feature of the present invention which includes any one of the fifteenth to seventeenth features (15) to (17), the electronic component holder head comprises a sucking pipe which sucks and holds, by vacuum, the electronic component. 
     (19) According to a nineteenth feature of the present invention, there is provided an apparatus for transferring an electronic component, comprising at least one electronic component holder head which holds an electronic component; a holder head moving device which moves the electronic component holder head along a predetermined movement path; and a path selecting device which selects, as at least a portion of the movement path, one of a plurality of selectable paths which have different positions, respectively, in a direction intersecting the portion of the movement path, so that the electronic component holder head is moved along the movement path including the selected one selectable path. In the case where a stop station is provided on the above-indicated portion of the movement path, the present EC transferring apparatus can select, for the stop station, one of a plurality of stop positions at each of which the EC holder head is stoppable. However, this feature is not essential to the present EC transferring apparatus. For example, the EC holder head is not essentially required to be stopped at an image taking station where an image of an EC held by the holder head is taken for confirming the kind of the EC and/or detecting the possible position errors of the EC held by the holder head. That is, it is possible to take an image of the EC being moved. However, if the kind of one EC held by the EC holder head is different from that of another EC held by the holder head, the distance between an image taking device and the surface of each EC whose image is to be taken by the image taking device at the image taking station may not coincide with the focal length of the image taking device. In this case, the above distance may be so changed as to coincide with the focal length, by changing at least a portion of the movement path along which the EC holder head is moved. The present EC transferring apparatus may be advantageously used in this way, as well. The movement path may be a circular arc, a straight line, a curve other than the circular arc, or a composite of those lines. The intersecting direction may be a direction perpendicular to the above portion of the movement path, such as a vertical direction. 
     (20) According to a twentieth feature of the present invention which includes the nineteenth feature (19), the transferring apparatus further comprises an image taking device which is provided in association with the portion of the movement path and which takes an image of the electronic component held by the electronic component holder head. 
     (21) According to a twenty-first feature of the present invention which includes the twentieth feature (20), the image taking device comprises a line sensor which takes, as the image of the electronic component, a set of respective images of contiguous linear portions of the component as the component is moved. The line sensor comprises a number of image pick-up elements which are arranged in a straight array extending in a direction intersecting the direction in which an EC is moved, and takes an image of a linear portion of the EC at regular intervals of time as the EC is moved. Thus, the line sensor takes, as the image of the EC, a set of respective images of the contiguous linear portions of the EC as the EC is moved. When the EC finishes passing by the line sensor, the line sensor finishes taking the entire image of the EC. Thus, a two-dimensional image of the EC is obtained by the combination of the iterative linear-image taking of the line sensor and the movement of the EC held by the EC holder head. 
     (22) According to a twenty-second feature of the present invention which includes the twentieth feature (20), the image taking device comprises a high-speed camera which emits, when the electric component held by the electronic component holder head passes, a single flashlight toward the component for so short a time that an amount of movement of the component during the time is negligible, and which takes the image of the component exposed to the single flashlight. For example, in the case where the high-speed camera has a stroboscope, the stroboscope emits a strong flashlight toward an EC which is passing by the camera, so that the camera takes an image of the EC. Although the EC is moving, the camera can take an image of the EC, as if the EC were stopped, by employing a very high shutter speed or a very short light-emitting time. 
     (23) According to a twenty-third feature of the present invention, there is provided a system for mounting an electronic component on an object, comprising an electronic component holder head positioning apparatus according to the first feature (1); an electronic component supply device which is provided at a first one of a plurality of stop stations comprising the at least one stop station and which supplies an electronic component to each of the electronic component holder heads so that the each holder head holds the electronic component; and an object supporting and positioning device which is provided at a second one of the stop stations and which supports and positions an object on which the electronic component held by the each holder head is to be mounted. 
     (24) According to a twenty-fourth feature of the present invention, the mounting system further comprises an image taking device which is provided at a third one of the stop stations and which takes an image of the electronic component held by the each holder head. 
     (25) According to a twenty-fifth feature of the present invention, there is a system for mounting an electronic component on an object, comprising an electronic component holder head positioning apparatus according to the feature (15); an electronic component supply device which is provided at a first one of a plurality of stop stations comprising the at least one stop station and which supplies an electronic component to each of the electronic component holder heads so that the each holder head holds the electronic component; and an object supporting and positioning device which is provided at a second one of the stop stations and which supports and positions an object on which the electronic component held by the each holder head is to be mounted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and optional objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
     FIG. 1 is a plan view schematically showing an electronic component mounting system according to one embodiment of the present invention, including an electronic component transferring and mounting apparatus equipped with one embodiment of an electronic component transferring device of the invention; 
     FIG. 2 is a front elevational view in cross section of the electronic component transferring and mounting apparatus of FIG. 1; 
     FIG. 3 is a plan view of the present electronic component transferring and mounting apparatus; 
     FIG. 4 is a front elevational view in cross section showing rotary plates supported by a stationary shaft in the present transferring and mounting apparatus; 
     FIG. 5 is a view indicating three positions at which component holder heads are stopped in the transferring and mounting apparatus; 
     FIG. 6 is a front elevational view in cross section showing one of the component holder heads in the transferring and mounting apparatus; 
     FIG. 7 is a front elevational view in cross section of the component holder head of FIG. 6 placed in an operating state in which a sleeve and an inner shaft are disconnected from each other; 
     FIG. 8 is a bottom plan view showing a head elevating and lowering device and a stationary cylindrical cam in the transferring and mounting apparatus; 
     FIG. 9 is a block diagram illustrating a part of a control device for controlling the present electronic component mounting system, which part relates to the present invention; 
     FIG. 10 is a timing chart indicating a relationship between the rotating angle of the twelve rotary plates and the time in the transferring and mounting apparatus; 
     FIGS. 11A-1,  11 A- 2  and  11 A- 3  are graphs showing changes in the position, rotating speed and acceleration or deceleration of the rotary plate in the present transferring and mounting apparatus, in the vicinity of a stop position of the rotary plate; 
     FIGS. 11B-1,  11 B- 2  and  11 B- 3  are graphs corresponding to those of FIGS. 11A, in a conventional transferring and mounting apparatus; 
     FIG. 12 is a graph for explaining reduction in the time required for one pitch of rotary movement of the rotary plate to the stop position in the present transferring and mounting apparatus; 
     FIG. 13 is a front elevational view in cross section of an electronic component transferring and mounting apparatus in an electronic component mounting system constructed according to another embodiment of this invention; 
     FIG. 14 is a plan view partly in cross section showing the transferring and mounting apparatus of FIG. 13, taken at a vertical position in which cam follower rollers of rotary plates engage concave globoidal cams; 
     FIG. 15 is a view schematically showing a cam groove in one of the concave globoidal cams of FIG. 14, and the corresponding cam follower roller which is selectively brought into engagement with two side surfaces of a non-lead portion of the cam groove; 
     FIG. 16 is a view showing the cam groove of FIG. 15 in a developed state; 
     FIG. 17 is a view showing a cam groove in a stationary cylindrical cam used in the electronic component transferring and mounting apparatus of FIG. 13; 
     FIG. 18 is a plan view showing a portion of the apparatus of FIG. 13 in which there are provided stop position changing air cylinders, stop position changing valves, height position changing valves, first and second valve switching devices for the stop position changing valves, and valve switching devices for the height position changing valves; 
     FIG. 19 is a front elevational view showing the stop position changing valve, height position changing valve, first stop position changing valve switching device, and their vicinities in the apparatus of FIG. 13; 
     FIG. 20 is a front elevational view in cross section showing the stop position changing valve, height position changing valve, height position changing switching device and their vicinities in the present apparatus; 
     FIG. 21 is a block diagram showing a part of a control device for controlling the electronic component mounting system including the transferring and mounting apparatus of FIG. 13, which part relates to the present invention; 
     FIGS. 22A-22C are views for explaining a change in the component sucking position where a cartridge support is fed in a forward direction, in the apparatus of FIG. 13; 
     FIGS. 23A-23D are views for explaining a change in the component sucking position where the cartridge support is fed in a reverse direction, in the apparatus of FIG. 13; 
     FIG. 24 is a view for explaining changes in the height position of the component holder head and the vertical head stroke in the apparatus of FIG. 13; 
     FIG. 25 is a front elevational view in cross section showing a transferring and mounting apparatus in an electronic component mounting system constructed according to a further embodiment of this invention; 
     FIG. 26 is a plan view showing the transferring and mounting apparatus of FIG. 25; 
     FIG. 27 is a front elevational view in cross section showing one of concave globoidal cams in the transferring and mounting apparatus of FIG. 25; 
     FIG. 28 is a view showing cam grooves of the concave globoidal cam of FIG. 27, in a developed state; 
     FIG. 29 is a cross sectional view taken along line  29 — 29  of FIG. 28, showing the cam groove of the concave globoidal cam; 
     FIG. 30 is a view for explaining an offset of center lines of wide and narrow portions of the cam groove of the concave globoidal cam of FIG. 27; 
     FIG. 31 is a timing chart indicating a relationship between the rotating angle of fifteen rotary plates and the time in the apparatus of FIG. 25; 
     FIGS. 32A-32D are graphs showing changes in the torque of the concave globoidal cam at the stop position and its vicinity of the rotary plate in the apparatus of FIG. 25; 
     FIG. 33 is a plan view schematically showing means for preventing releasing of a cam follower in an electronic component transferring apparatus according to a still further embodiment of the invention; 
     FIG. 34 is a plan view schematically showing means for preventing the releasing of the cam follower in an electronic component transferring apparatus according to a yet further embodiment of this invention; 
     FIG. 35 is a view schematically showing an example of cam follower releasing preventing means provided in an apparatus in which a concave globoidal cam is used in combination with a cylindrical cam; 
     FIG. 36 is a timing chart indicating a relationship between the rotating angle of the twelve rotary plates and the time in the transferring and mounting apparatus of FIG. 13; 
     FIG. 37 is a view for explaining changes in the height position of a component holder head of an electronic component transferring apparatus according to another embodiment of this invention; 
     FIG. 38 is a view for explaining changes in the height position of a component holder head of an electronic component transferring apparatus according to yet another embodiment of this invention; 
     FIG. 39 is a view for explaining changes in the height position of a component holder head of an electronic component transferring apparatus according to yet another embodiment of this invention; 
     FIG. 40 is a view for explaining the state in which an elevator plate which carries a component holder head is engaged with a cam rib of a stationary cam which is employed by a component holder head positioning device as part of an electronic component transferring and mounting apparatus according to yet another embodiment of this invention; 
     FIG. 41 is a view of an essential portion of an electronic component mounting system which includes an intermittently rotatable table and a plurality of component holder heads which are supported by the table and each of which sequentially sucks up an EC and mounts the EC as the table is intermittently rotated, wherein the mounting system additionally includes a circumferential-position selecting device or an on-path stop position selecting device as part of a component holder head positioning apparatus; 
     FIG. 42 is a view of an essential portion of an electronic component mounting system which includes a movable member which is movable along a straight movement path, and a component holder head which is supported by the movable member such that the holder head is movable with the movable member so as to suck up an EC and mounts the EC, wherein the mounting system additionally includes an intersecting-direction stop position selecting device as part of a component holder head positioning apparatus, or a path selecting device as part of an electronic component transferring apparatus; 
     FIG. 43 is a perspective view of the head elevating and lowering device and the stationary cylindrical cam of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1-12, there will be described an electronic component mounting system including an electronic component transferring and mounting apparatus  12  which is equipped with an electronic component transferring device constructed according to one embodiment of the present invention. 
     In FIG. 1, reference numeral  10  denotes a base on which are mounted the electronic component transferring and mounting apparatus  12 , an electronic component supply device  14 , and a workpiece supporting and positioning device in the form of a board supporting and positioning device  16 . The electronic component supply device  14  includes a cartridge support block  20  which carries a multiplicity of component supply cartridges  22  (hereinafter referred to as “cartridges  22 ) such that the component supply portions of the cartridges  22  are arranged along a straight line. 
     The cartridge support block  20  is provided with a nut (not shown) engaging a feed screw  24 . When the feed screw  24  is rotated by a drive source in the form of a cartridge feed servomotor  26 , the cartridge support block  20  is fed in an X-axis direction while it is guided by guide members in the form of a pair of straight guide rails  28 . Thus, one of the cartridges  22  which are arranged in the X-axis direction is brought to a predetermined component supply position. The nut, feed screw  24  and cartridge feed servomotor  26  constitute a major portion of a cartridge feeding device  30  for feeding the cartridges  22  in the X-axis direction. 
     The board supporting and positioning device  16  is disposed at a level lower than that of the cartridges  22  of the electronic component supply device  14 . This device  16  includes an X-axis table  34  movable in the X-axis direction, and a Y-axis table  36  which is mounted on the X-axis table  34  and which is movable in a Y-axis direction which is perpendicular to the X-axis direction in a horizontal plane. The Y-axis table  36  has a board holding device (not shown) mounted thereon to position a workpiece in the form of a printed-circuit board  38 . The X-axis table  34  is moved in the X-axis direction while it is guided by guide members in the form of straight guide rails  44  when a feed screw  40  is rotated by a drive source in the form of an X-axis drive servomotor  42 . The Y-axis table  36  is moved in the Y-axis direction while it is guided by guide members in the form of straight guide rails  50  when a feed screw  46  is rotated by a drive source in the form of a Y-axis drive servomotor  48 . With the X-axis table  34  and the Y-axis table  36  being moved in the horizontal plane, the printed-circuit board  38  is positioned such that a multiplicity of component mounting portions of the board  38  are sequentially aligned with a predetermined component mounting position. The X-axis table  34  extends below the electronic component transferring and mounting apparatus  12 , and a portion of the table  34  is located below the cartridges  22 . The printed-circuit board  38  is loaded and unloaded onto and from the board holding device of the board supporting and positioning apparatus  16 , in the X-axis direction. 
     The electronic component transferring and mounting apparatus  12  is disposed above the electronic component supply device  14  and the board supporting and positioning device  16 . The apparatus  12  is adapted to receive the electronic components from the electronic component supply device  14  and transfer the electronic components onto the printed-circuit board  38 , namely, mount the electronic components on the board  38 . Thus, the apparatus  12  serves not only as an electronic component transferring device, but also as an electronic component mounting device. 
     Referring to FIG. 2, the electronic component transferring and mounting apparatus  12  has a main body including a frame  60 , which is supported by support members (not shown) fixed to the base  10  indicated above. That is, the frame  60  is disposed above the base  10 . The frame  60  has a mounting structure  62  in the form of a box having a U-shape in cross section, and a support plate  64  which extend between and are fixed to a pair of side walls of the mounting structure  62 . 
     A support shaft in the form of a stationary shaft  66  having a circular cross sectional shape is fixed at its upper end by the support plate  64  of the frame  60 , and extends downwards through an opening  69  formed through a bottom wall  67  of the mounting structure  62 . The lower end portion of the stationary shaft  66  is located outside and below the mounting structure  62 , and is fixed at its lower end by a support plate  68  secured to the base  10 . As shown in FIGS. 2 and 3, twelve rotary members in the form of rotary plates  70  are fixed to respective pairs of bearings  72 , such that the rotary plates  70  are rotatable about an axis of the stationary shaft  66 . This axis of the stationary shaft  66  serves as a common axis about which the rotary plates  70  are rotatable. 
     The bearings  72  consists of two arrays  74  of bearings which are fixedly disposed on the stationary shaft  66  such that the two arrays  74  are spaced apart from each other in the axial direction of the shaft  66 . Each of these two arrays  74  consists of twelve bearings  74  which are arranged in the axial direction, namely, superposed on each other in a stack. The twelve rotary plates  70  are associated with respective pairs of support arms  76  fixed thereto. Each pair of support arms  76  are fixed to and supported by the corresponding pair of bearings  72  which belong to the two arrays  74 , respectively. All the pairs of bearings  72  corresponding to the respective pairs of support arms  76  of the twelve rotary members  70  have the same distance therebetween in the axial direction of the stationary shaft  66 . 
     As shown in FIG. 4, each of the support arms  76  consists of an annular fitting  78 , and a radial arm  80  which extends from a portion of the circumference of the annular fitting  78  in a radial outward direction of the annular fitting  78 . Each pair of support arms  76  has a connecting portion  82  which connects the two radial arms  80 . The connecting portion  82  of each pair of support arms  76  and the end portions of the radial arms  80  connected to the connecting portion  82  are fixed to the corresponding rotary plate  70 , while the annular fittings  78  are fitted on respective outer casings  84  of the corresponding pair of bearings  72  and are fixed to the outer casings  84  by a plurality of bolts. The twelve pairs of support arms  76  have different axial positions at which the arms  76  are fixed to the respective rotary plates  70 . Accordingly, the support arms  76  do not interfere with each other, even though the rotary plates  70  have the same position with respect to the stationary shaft  66  in the axial direction of the shaft  66 . In this arrangement, the twelve rotary plates  70  are rotatable about the common axis, namely, about the axis of the stationary shaft  66 , such that all the rotary plates  70  maintain a predetermined position or height in the axial direction of the stationary shaft  66 . The pair of radial arms  80  and the connecting portion  82  are provided with a rib  86 , which has a thinned fixed end portion on the side of the stationary shaft  66 , as indicated by broken lines in FIG.  3 . The thickness of the thinned fixed end portion of the rib  86  decreases in a radial direction toward the stationary shaft  66 , so that the fixed end portions of the adjacent ribs  86  and the adjacent radial arms  80  do not interfere with each other. Each rotary plate  70  extends downwards through the opening  69  formed through the bottom wall  67  of the frame  60 , and the lower end portion of the rotary plate  70  is located below the bottom wall  67 . 
     As shown in FIG. 2, a cam follower in the form of a cam follower roller  88  is attached to a radial end of each rotary plate  70  remote from the stationary shaft  66 , such that the roller  88  is rotatable about a horizontal axis extending in a radial direction of the stationary shaft  66 . As shown in FIG. 3, the rollers  88  of the twelve rotary plates  70  are held in rolling engagement in cam grooves  92   a,    92   b,    92   c,    92   d  formed in four concave globoidal cams  90   a,    90   b,    90   c,    90   d  which are rotatably mounted on the frame  60 . In the interest of brevity and simplification, the twelve rotary plates  70  are shown in FIG. 3 as positioned relative to each other such that the rotary plates  70  are evenly or equi-angularly spaced apart from each other about the stationary shaft  66 . 
     The four concave globoidal cams  90   a,    90   b,    90   c,    90   d  have respective outer circumferential surfaces  93   a,    93   b,    93   c,    93   d.  The outer circumferential surface  93  is defined by a locus which is to be described by a circular arc having a center at the axis of the stationary shaft  66  when the circular arc is rotated about an axis which is located such that the circular arc is interposed between this axis and the axis of the stationary shaft  66  and which is perpendicular to the axis of the stationary shaft  66 . The axis about which the circular arc is rotated to describe the above-indicated locus defining the circumferential surface  93  is an axis of a rotary shaft  94  of the concave globoidal cam  90 , which will be described. The four concave globoidal cams  90   a,    90   b,    90   c,    90   d  are disposed symmetrically with respect to the axis of the stationary shaft  66 , such that lines of intersection of the outer circumferential surfaces  93   a,    93   b,    93   c,    93   d  of the cams  90   a,    90   b,    90   c,    90   d  with a plane (horizontal plane) including the axes of the cams  90  and perpendicular to the axis of the stationary shaft  66  cooperate to define a substantially continuous circle which has a center at the axis of the stationary shaft  66 . The cam grooves  92   a,    92   b,    92   c,    92   d  formed in the outer circumferential surfaces  93   a,    93   b,    93   c,    93   d  are substantially connected to each other. 
     The concave globoidal cams  90   a,    90   b,    90   c,    90   d  are fixedly mounted on respective rotary shafts  94   a,    94   b,    94   c,    94   d,  as shown in FIG.  3 . The rotary shafts  94  are rotatably supported by respective pairs of brackets  96   a,    96   b,    96   c,    96   d  fixed to the mounting structure  62  of the frame  60 . The four concave globoidal cams  90   a - 90   d  have respective pairs of bevel gears  98   a,    100   a,    98   b,    100   b,    98   c,    100   c,    98   d,    10   d.  The bevel gears  98 ,  100  of each globoidal cam  90  are formed integrally and coaxially with the cam  90 , at the axially opposite ends. The bevel gears  98 ,  100  of the adjacent concave globoidal cams  90   a - 90   d  are held in meshing engagement with each other. 
     The rotary shaft  94   a  to which the concave globoidal cam  90   a  is attached has a larger axial length than the other rotary shafts  94   b - 94   d,  and is rotatably supported also by another bracket  104  fixed to the frame  60 , as shown in FIG.  3 . At the free end of the rotary shaft  94   a,  there is fixed a timing pulley  106 , which is connected by a timing belt  112  to a timing pulley  110  fixed to an output shaft of a main drive source in the form of an electrically operated main drive servomotor  108 . When the rotary shaft  94   a  is rotated by the main drive servomotor  108 , the four concave globoidal cams  90  are contemporaneously rotated in synchronization with each other, with the bevel gears  98   a - 98   d  meshing with each other, so that the twelve rotary plates  70  are rotated about the stationary shaft  66  or held stationary, as described below. 
     In the present electronic component transferring and mounting apparatus  12 , the rotary plates  70  are stopped at a component sucking station, an image taking station and a component mounting station. In these stations, there are set a component sucking position, an image taking position and a component mounting position, respectively, as shown in FIG.  5 . At the component sucking position, the apparatus  12  receives the electronic components from the electronic component supply device  14 , which is located on the base  60 , at a position in the vicinity of the component sucking position. A CCD (charged-coupled device) camera  114  (FIG. 9) is located on the base  10 , at a position corresponding to the the image taking position, while the board supporting and positioning device  16  is located on the base  10 , at a position corresponding to the component mounting position. 
     The concave globoidal cam  90   a  is located at a position corresponding to the component mounting position, and the concave globoidal cam  90   c  is located at a position corresponding to the component sucking position, while the concave globoidal cam  90   d  is located at a position corresponding to the image taking position. The cam grooves  92   a,    92   c,    92   d  of the cams  90   a,    90   c,    90   d  are formed or shaped so that the rotary plates  70  (cam follower rollers  88 ) are held stopped at the component sucking and mounting positions and the image taking position, and are decelerated and accelerated during movements thereof toward and from those three positions, and such that the rotary plates  70  are rotated at a predetermined constant angular velocity when the corresponding cam follower rollers  88  are moving into and from the cam grooves  92   a,    92   c,    92   d.  Each of the cam grooves  92   a,    92   c,    92   d  of the concave globoidal cams  90   a,    90   c,    90   d  has an inclined portion having a lead angle with respect to a plane perpendicular to the axis of the rotary shaft  94   a,    94   c,    94   d,  and a non-lead portion perpendicular to that axis. The inclined portion includes a curved section and a straight section. The cam groove  92   b  of the concave globoidal cam  90   b  has only a straight inclined portion having a lead angle with respect to a plane perpendicular to the axis of the rotary shaft  94   b.  This lead angle is determined to permit the rotary plates  70  to be rotated at the above-indicated constant angular velocity. 
     It is noted that a component container (not shown) is provided in a component discarding area between the component mounting and sucking stations of the apparatus  12 . Described more specifically, the component container is provided along a circular arc path of component holder heads  120  carried by the rotary plates  70 , and are located below the component holder heads  120 , so that the component container accommodates electronic components which are discarded from the component holder heads  120  during rotation of the rotary plates  70  from the component mounting station to the component sucking station. Namely, the electronic components which have been sucked up by the component holder heads  120  are not mounted on the printed-circuit board  38  and discarded into the component container, if the components have not been adequately positioned with respect to sucking nozzles of the heads  120 , that is, if the components held by the heads  120  are dislocated to such an extent that the position of the components cannot be corrected. The components are also discarded if the kinds of the components which have been held by the heads  120  are different from those of the components that should be mounted. 
     Each of the twelve rotary plates  70  carries a component holder head  120 , as shown in FIG.  2 . Each rotary plate  70  has guide members in the form of a pair of guide blocks  122  fixed thereto such that the guide blocks  122  are spaced apart from each other in the vertical direction. A movable member in the form of a vertical slide  124  engages the guide blocks  122  such that the vertical slide  124  is vertically slidably movable. To an upper part of the vertical slide  124 , there is attached a cam follower in the form of a cam follower roller  126  such that the roller  126  is rotatable about an axis extending in a radial direction of the stationary shaft  66 . To a lower part of the vertical slide  124 , there is attached the component holder head  120 . 
     A stationary cylindrical cam  128  is fixed to the underside (lower surface) of the bottom wall  67  of the frame  60 , in coaxial relationship with the stationary shaft  66 . The cam follower roller  126  indicated above is held in rolling engagement with a cam groove  130  formed in the inner circumferential surface of the cylindrical cam  128 . The cam groove  130  has a height varying portion whose height (in the axial direction of the shaft  66 ) gradually varies in the circumferential direction of the cam  128 , and a level portion whose height is held constant in the circumferential direction. The cam groove  130  is formed so that each component holder head.  120  is placed in its upper end position when the rotary plate  70  is located at the component sucking position, and is placed in its lower end position when the rotary plate  70  is located at the component mounting position, and so that the component holder head  120  is moved in a horizontal plane when the rotary plate  70  is rotated around each of the component sucking and mounting positions and the image taking position. In the present arrangement, the vertical slide  124  is moved up and down to move the component holder head  120  in the vertical direction when the cam follower roller  126  is moved in rolling contact with the height varying portion of the cam groove  130 , with the rotary plate  70  being rotated with the cam follower roller  126 . 
     As shown in FIG. 6, a bracket  136  is fixed to the lower end portion of the vertical slide  124 , and a sleeve or hollow shaft  138  is supported by the bracket  136  via bearings  140 ,  142  such that the sleeve  138  is rotatable relative to the bracket  136  and is axially immovable relative to the bracket  136 . An inner shaft  144  is fitted in the sleeve  138  such that the inner shaft  144  is rotatable and is axially immovable relative to the sleeve  138 . A holder portion  146  which is generally U-shaped in cross section is fixed to the lower end of the sleeve  138  located outside the bracket  136 . The holder portion  146  has a pair of side walls  148 ,  150 , and a support shaft  152  which is fixed at its opposite ends to the side walls  148 ,  150 . The support shaft  152  carries a cylindrical nozzle holder  154  rotatably mounted thereon. 
     The nozzle holder  154  has six nozzle engaging radial holes  156  which are equi-angularly spaced from each other in the circumferential direction of the support shaft  152 . The radial holes  156  accommodate respective cylindrical suction nozzles  158  such that the nozzles  158  are axially movable in the radial holes  156  and are not rotatable relative to the nozzle holder  154 . The suction nozzles  158  are biased in a radially outward direction of the nozzle holder  154  by biasing means in the form of elastic members in the form of respective compression coil springs  160  accommodated in the radial holes  156 . Rotation of each suction nozzle  158  in the radial hole  156  and axial movement of the suction nozzle  158  out of the radial hole  156  are prevented by engagement of a pin (not shown) provided on the suction nozzles  158  with the opposite ends of a corresponding groove (not shown) formed in the nozzle holder  154 . The six suction nozzles  158  are provided for sucking up electronic components  164  of respective different sizes. The six suction nozzles  158  have respective suction tubes  162  having respective different diameters. Each of the suction nozzles  158  has a reflector plate  163 . The diameter of the reflector plates  163  of the suction nozzles  158  whose suction tubes  162  have a comparatively small diameter is smaller than that of the reflector plates  163  of the suction nozzles  158  whose suction tubes  162  have a comparatively large diameter. The suction tubes  162  of the six suction nozzles  158  have the same length, so that the free ends of the suction tubes  162  lie on a circle having a center on the axis of rotation of the cylindrical nozzle holder  154 . 
     The six suction nozzles  158  are selectively placed in an operating position by rotation of the nozzle holder  154 , as described below. When a selected one of the suction nozzles  158  is in the operating position, its axis extends in the vertical direction, and the free end of its suction tube  162  is located right below the axis of the support shaft  152 . The axis of the suction nozzle  158  placed in the operating position is aligned with the axis of the sleeve  138 . 
     The suction nozzles  158  are adapted to hold the electronic component  164  by air suction. As shown in FIG. 6, passages  168 ,  170 ,  172 ,  174 ,  176 ,  177  are formed through the nozzle holder  154 , support shaft  152 , side wall  150 , inner shaft  144 , sleeve  138  and bracket  136 , respectively. A switching device  178  is attached to the bracket  136 , so that the suction nozzle  158  placed in the operating position is communicated selectively with a vacuum source (not shown) or the atmosphere by an operation of the switching device  178 . The vacuum source is connected through a conduit (not shown) to a passage (not shown) formed through the stationary shaft  66 , and this passage is connected through a rotary valve (not shown) to the twelve switching devices  178  through respective hoses. The rotary valve is disposed at a position of the stationary shaft  66 , which is lower than the bearing arrays  74 . A rotary motion of a valve drive servomotor (not shown) disposed on the support plate  68  is transmitted to the rotary valve through timing pulley and belt, so that the rotary valve is held operated at the same angular velocity as that of the constant-velocity movement of the rotary plate  70 , whereby the switching device  178  is held connected to the vacuum source. When the rotary plate  70  is stopped, the switching device  178  and the rotary valve are rotated by a small angle relative to each other. This relative rotation is permitted by the elastic deformation of the hose. 
     Each switching device  178  has a solenoid-operated switch valve operated for selective communication of the suction nozzle  158  with the vacuum source or the atmosphere, so that the electronic component  164  is sucked up by the suction tube  158  or released therefrom. The passage  170  formed through the support shaft  152  is connected to the passage  168  which communicates with the suction nozzle  158  placed in the operating position. 
     A gear  180  is rotatably mounted on the support shaft  152 , and is fixed to the nozzle holder  154  by a connecting member in the form of a pin  182  so that the nozzle holder  154  is rotated with the gear  180 . The gear  180  meshes with a gear  186  fixed to a support shaft  184  which is rotatably attached to the side wall  148  of the holder portion  146 . The gears  180 ,  186  have the same diameter. The gear  186  has an integrally formed bevel gear  188 , while the inner shaft  144  has a bevel gear  190  integrally formed at its lower end. The bevel gears  188 ,  190  mesh with each other, and have the same diameter. The upper end portion of the inner shaft  144  projects upwards from the sleeve  138 , and is connected through a connecting member  196  to an output shaft  194  of a nozzle rotating/selecting servomotor  192 . The inner shaft  144  is axially movable and is not rotatable relative to the connecting member  196 . The nozzle rotating/selecting servomotor  192  is bidirectionally operated at a suitable speed, and an angle of rotation of the output shaft  194  is detected by an encoder (not shown). 
     The supplying of electric energy from a power source to the solenoid-operated valves as the switching devices  178  and the nozzle rotating/selecting servomotors  192  may be achieved by using a common slip ring. However, in the present embodiment, the electric energy is supplied by a no-contact electricity supplying device disclosed in, e.g., U.S. Pat. No. 5,588,195. The no-contact electricity supplying device includes twelve electricity suppliers and twelve electricity receivers for the twelve switching devices  178  and the twelve servomotors  192 , respectively. The electricity suppliers are provided on the stationary shaft  66 , and include respective supply-side coils each connected to the power source. The electricity receivers include respective receiver-side coils which are opposed to the supply-side coils with small clearances therebetween, and are rotated at a constant speed together with the above-indicated rotary valve (not shown) employed for the supplying of vacuum, by the above-indicated valve drive servomotor (not shown). The receiver-side coils are connected via conductive lines to the solenoid-operated valves  178  and the nozzle rotating/selecting servomotors  192 , so as to receive the electric energy supplied from the electricity suppliers. 
     The connecting member  196  takes the form of a sleeve which engages the output shaft  194  and is connected to the output shaft  194  through a connecting member in the form of a pin  198  such that the sleeve  196  is not rotatable and axially immovable relative to the output shaft  194 . The connecting member  196  has two cutouts  206  formed on diametrically opposite portions. These cutouts  206  extend in the axial direction of the connecting member  196  and are open in the lower end face of the connecting member  196 . The upper end portion of the inner shaft  144  is fitted in the connecting member  196  such that the inner shaft  144  is axially movable relative to the connecting member  196 . The inner shaft  144  has a pin  208  extending diametrically through its upper end portion such that the opposite end portions of the pin  208  project outwardly of the inner shaft  144 . These opposite end portions of the pin  208  engage the respective cutouts  206  such that the opposite end portions of the pin  206  are movable in the cutouts  208  in the axial direction of the inner shaft  144 . With the pin  206  and the cutouts  206  engaging each other, a rotary motion of the nozzle rotating and selecting servomotor  192  is transmitted to the inner shaft  144  through the connecting member  196 . Thus, the pin  208  and the cutouts  206  serve as a first  5  engaging member and a second engaging member, respectively, which engage each other and cooperate to permit the connecting member  196  and the inner shaft  144  to be connected to each other such that the connecting member  196  and inner shaft  144  are axially movable relative to each other and are not rotatable relative to each other. 
     A drive member  202  in the form of a ring is fitted on the inner shaft  144  such that the drive member  202  is axially movable relative to the inner shaft  144 . The drive member  202  includes a portion which is concentrically and fixedly fitted in the lower end of the connecting member  196  so as to close the lower open ends of the cutouts  206 . The drive member  202  has a toothed peripheral portion  200  formed by six grooves, which are formed through the thickness of the drive member  202  such that the grooves are equi-angularly arranged in the circumferential direction of the drive member  202  and are open in the radially outward direction. 
     The vertical slide  124  carries a motor support in the form of an elevator  210  on which the nozzle rotating/selecting servomotor  192  is mounted. The elevator  210  is generally U-shaped in cross section, and includes two opposed side wall  212 ,  216  which partly defines the U-shape. The elevator  210  has a guide member in the form of a guide block  214  fixed to the side wall  212 , and is vertically slidable on the vertical slide  124  through the guide block  214 . To an outer surface of the other side wall  216 , there is attached a cam follower in the form of a cam follower roller  218 , which is rotatable about an axis extending in a radial direction of the stationary shaft  66 . The side wall  216  has a cylindrical engaging boss  220  formed on its lower end face. The engaging boss  220  has a relatively small diameter. 
     The elevator  210  is biased upwards by biasing means in the form of an elastic member in the form of a compression coil spring  224  interposed between the drive member  202  and the upper end of the sleeve  138  which is located above the bracket  136 . That is, the elevator  210  is biased by the spring  224  in a vertical direction away from the sleeve  138 . An upward movement of the elevator  210  under the biasing action of the compression coil spring  224  is limited by abutting contact of the drive member  202  with the pin  208  fixed to the inner sleeve  144 . In other words, the upper stroke end position of the elevator  210  is established by this abutting contact of the drive member  202  with the pin  208 . A driven member  232  is fixed to the upper end portion of the sleeve  138 . The driven member  232  includes an annular portion  234  fixedly fitted on the sleeve  138 . The annular portion  234  has a radial extension which extends radially outwardly in a direction away from the vertical slide  124  and which has an engaging cutout  238 . 
     The driven member  232  further includes an arm portion  240  extending upwards from the annular portion  234 . The arm portion  240  has teeth  242  formed on an inner surface thereof facing the inner shaft  144 . The teeth  242  provides part of an internally gear, which cooperates with the toothed peripheral portion  200  of the drive member  202  to constitute a claw clutch. When the elevator  210  is placed in its upper stroke end position under the upward biasing action of the compression coil spring  224 , the toothed peripheral portion  200  of the drive member  202  is held in meshing engagement with the teeth  242  of the arm portion  240  of the driven member  232 , such that the toothed peripheral portion  200  and the teeth  242  are movable relative to each other in the axial direction of the sleeve  138  and are not rotatable relative to each other, whereby the sleeve  138  and the inner shaft  144  are not rotatable relative to each other. In this condition, the engaging boss  220  provided on the elevator  210  is aligned with the engaging cutout  238  formed through the driven member  232 , in the circumferential direction of the sleeve  138 , and the engaging boss  220  is engageable with the cutout  238  when the elevator  210  is moved down. This position in which the boss  220  is engageable with the cutout  238  will be referred to as an “original circumferential position” of the sleeve  138  and the component holder head  120 . In this original circumferential position, the horizontal axis of the support shaft  152  rotatably supporting the nozzle holder  154  extends in a radial direction of the stationary shaft  66  (axis of rotation of the rotary plate  70 ), which is parallel to the plane of the rotary plate  70 . 
     A stationary nozzle selecting cam  248  is located in an area which is between the component discarding area and the component sucking station and in which the rotary plates  70  are rotated at the predetermined constant velocity while the component holder heads  120  are moved in a horizontal plane maintaining a constant height. The nozzle selecting cam  248  has a cam surface  250  which is elongated in the rotating direction of the rotary plates  70 , along a circular arc having a center at the axis of the stationary shaft  66 . The cam surface  250  consists of a downwardly inclined region, a level region, and an upwardly inclined region, which are arranged in the rotating direction of the rotary plates  70 . The downwardly inclined region is inclined downwards in the rotating direction of the rotary plates  70 , namely, in the direction from the upwardly inclined region toward the level region. The level region extends from the downstream or lowest end of the downwardly inclined region, at the same level or height as the lowest position at the downward end of the downwardly inclined region. The upwardly inclined region extends from the downstream end of the level region remote from the downwardly inclined region, and is inclined upwards in the rotating direction of the rotary plates  70 , namely, in the direction from the level region toward the downwardly inclined region. As the cam follower roller  218  is moved in rolling contact with the downwardly inclined region of the cam surface  250 , the elevator  210  is moved downwards. The elevator  210  is held level while the cam follower roller  218  is moved in rolling contact with the level region. As the roller  218  is moved in rolling contact with the upwardly inclined region, the elevator  210  is permitted to be moved upwards. The height of the nozzle selecting cam  248  is determined so that the downwardly inclined region of the cam surface  250  is engageable with the cam follower roller  218  when the elevator  210  is placed in its upper stoke end position. The initial portion of the downwardly inclined region is shaped to have a part-cylindrical surface, for facilitating the engagement with the roller  218 . The elevator  210  is placed in its lower stroke end position while the roller  218  is in rolling contact with the level region of the cam surface  250  of the cam  248 . 
     While a certain rotary plate  70  is moved from the image taking position toward the component mounting position, the nozzle rotating/selecting servomotor  192  is actuated in a controlled manner to correct a positioning error of the electronic component  164  held by the component holder head  120 , more specifically, to remove a deviation of the angular position of the electronic component  164  about the axis of the suction tube  162 . While the rotary plate  70  is located between the image taking position and the component mounting position, the sleeve  138  and the inner shaft  144  are connected to each other through engagement of the teeth  242  of the driven member  232  with the toothed peripheral portion  200  of the drive member  200 , as shown in FIG. 6, so that a rotary motion of the nozzle rotating/selecting servomotor  192  is transmitted to the sleeve  138  through the connecting member  196 , drive member  202  and driven member  232 . A rotary motion of the sleeve  138  causes a rotary motion of the suction nozzle  158  placed in the operating position, about its axis, whereby the electronic component  164  held by suction on the end face of the suction tube  162  is rotated about the axis of the suction tube  162 , by an amount suitable to remove the angular positioning error of the component  164 . This amount of rotation of the component  164  is calculated on the basis of angular positioning data obtained from an output of the CCD camera  144  at the image taking position. 
     It is noted that the inner shaft  144  is rotated with the sleeve  138 , whereby the meshing position of the two bevel gears  188 ,  190  remains unchanged, so that the bevel gear  188  remains stationary, and the nozzle holder  154  is prevented from being rotated about the axis of the support shaft  152 , to thereby prevent changing the suction nozzle  158  placed in the operating position. 
     As a result of rotation of the sleeve  138 , the engaging cutout  238  formed in the driven member  232  is offset from the engaging boss  220  provided on the elevator  210 , in the rotating direction of the sleeve  138 . After the electronic component  164  is mounted on the printed-circuit board  38  and while the rotary plate  70  is rotated toward the component discarding area, the sleeve  138  is rotated in the direction opposite to the direction of the rotation effected to remove the angular positioning error of the component  164 , by the same amount as in the removal of the angular positioning error, so that the engaging cutout  238  is brought into alignment with the engaging boss  220  in the circumferential or rotating direction of the sleeve  138 . 
     While the rotary plate  70  is rotated from the component discharging area toward the component sucking station, with the component holder head  120  being moved at the constant velocity while maintaining the same height, the cam follower roller  218  is brought into engagement with the cam surface  250  of the nozzle rotating/selecting cam  248 . Described in detail, the roller  218  initially contacts the downwardly inclined region of the cam surface  250 , pushing down the elevator  210  against the biasing force of the compression coil spring  224 . As a result, the teeth  242  of the driven member  232  is disengaged from the toothed peripheral portion  200  of the drive member  202 , and the sleeve  138  is disengaged from the inner shaft  144 , as shown in FIG.  7 . Before the driven member  232  and the drive member  202  are completely disengaged from each other, the engaging boss  220  on the elevator  210  is moved into the engaging cutout  238  formed in the driven member  232 , whereby the sleeve  138  is prevented from rotating and is held in the original circumferential position. The sleeve  138  is returned to the original circumferential position before the cam follower roller  218  is brought into contact with the cam surface  250 , that is, before an operation to select the suction nozzle  158  is initiated. Consequently, the engaging boss  220  can be moved into the engaging cutout  238  to prevent rotation of the sleeve  138  when the elevator  210  is moved down by the cam follower roller  218 . While the rotary plate  70  is rotated after the driven member  232  and the drive member  202  are disengaged from each other, the roller  218  is held in contact with the level region of the cam surface  250 , so that the sleeve  138  and the inner shaft  144  are held disengaged from each other with the driven and drive members  232 ,  202  being held disengaged from each other. 
     When it is desired to change the suction nozzle  158  used to hold the electronic component  146 , the nozzle rotating/selecting servomotor  192  is actuated in the condition of FIG.  7 . As a result, only the inner shaft  144  is rotated, and a rotary motion of the inner shaft  144  is transmitted to the nozzle holder  154  through the bevel gears  188 ,  190  and the gears  180 ,  182 , so that the nozzle holder  154  is rotated about the axis of the support shaft  152  to place a desired one of the six suction nozzles  158  in the operating position. The suction nozzle  158  currently placed in the operating position can be determined by the angle of and direction of rotation of the inner shaft  144 , and the kind of the suction nozzle  158  that should be used for holding the next electronic component  164  can be determined according to a component mounting program which is formulated to mount various electronic components  164  in a predetermined order on the printed-circuit board  38 . The angle and direction of rotation of the servomotor  192  can be determined based on the determined kind of suction nozzle  158  currently placed in the operating position and the determined kind of the electronic component  164  to be mounted next. Thus, the appropriate suction nozzle  158  is brought into the operating position by an operation of the servomotor  192 . The direction of rotation of the servomotor  192  is determined so as to reduce the angle of rotation of the nozzle holder  154  required to select the desired suction nozzle  158 . 
     Since the bevel gears  189 ,  186  have the same diameter while the gears  180 ,  182  have the same diameter, the nozzle holder  154  is rotated by the same angle as the inner shaft  144 . The teeth of the bevel gears  188 ,  190  and gears  180 ,  186  are accurately shaped to minimize an amount of gear backlash, so that the desired suction nozzle  158  can be positioned at the operating position with high accuracy by rotation of the servomotor  192  by the determined angle. 
     When only the inner shaft  144  is rotated, the sleeve  138  is prevented from rotating by engagement of the engaging boss and cutout  220 ,  238 , so that the sleeve  138  is not rotated through friction between the sleeve  138  and the inner shaft  144 , that is, so that the nozzle holder  154  is rotated about the horizontal axis which extends in the radial direction of the stationary shaft  66  and which is parallel to the plane of the rotary plate  70 . 
     During an operation to select the desired suction nozzle  158 , the elevator  210  is held in its lower stroke end position with the cam follower roller  218  held in rolling contact with the level region of the cam surface  250  of the cam  248 , so that the inner shaft  144  and the sleeve  138  are held disengaged from each other. After the desired suction nozzle  158  has been selected, the roller  218  is brought into contact with the upwardly inclined region of the cam surface  250 . As the rotary plate  70  is further rotated, the elevator  210  is gradually elevated by the compression coil spring  224 . As a result, the teeth  242  of the driven member  232  are brought into engagement with the toothed peripheral portion  200  of the drive member  202 , and the engaging boss  220  is disengaged from the engaging cutout  238 , whereby the sleeve  138  and the inner shaft  144  are connected to each other for simultaneous rotation, and the sleeve  138  is disengaged from the elevator  210 . The teeth  242  are brought into engagement with the toothed peripheral portion  202  before the boss  220  is disengaged from the cutout  2238 . Since the sleeve  138  is prevented from rotating during rotation of the inner shaft  144  to select the suction nozzle  158 , the teeth  242  can engage the toothed peripheral portion  200  of the drive member  202  which has been rotated during the operation to select the suction nozzle  158 . Since the bevel gears  188 ,  190  have the same diameter and the gears  180 ,  186  have the same diameter, the nozzle holder  154  is rotated by the same angle as the inner shaft  144  during a nozzle selecting operation to select the suction nozzle  158 . The toothed peripheral portion  200  consists of six grooves. The angular phase of the toothed peripheral portion  200  in which the teeth  200  engage the portion  200  after the nozzle selecting operation is offset from that before the nozzle selecting operation, by the angle of rotation of the nozzle holder  154  during this operation. 
     The selection of the desired suction nozzle  158  as described above is effected before the electronic component  164  is sucked up by the component holder head  120 . When the rotary plate  70  reaches the component sucking position, the desired suction nozzle  158  for holding the next component  164  has been placed in the operating position. At this time, the head  120  is placed in the original circumferential position. When the angular position of the electronic component  164  as held by the suction nozzle  158  is corrected, the head  120  is rotated from the original circumferential position in the forward or reverse direction. 
     As described above, the nozzle rotating/selecting servomotor  192  provided on the component holder head  120  is used as the drive source for selecting the desired suction nozzle  158  and for removing an angular positioning error of the component  164  held by the head  120 . This arrangement makes it possible to effect the above operations during rotation of the component holder head  120  about the stationary shaft  66 , and allows for a relatively long time in which these operations should be completed. Accordingly, the present arrangement is effective to reduce the angular velocity upon rotation of the suction nozzle  158  about its axis, and prevent or minimize the vibration which would take place during the operations to select the suction nozzle  158  and correct the angular position of the component  164 . The present arrangement is also effective to increase the maximum angle of rotation of the nozzle holder  154 . Further, since the servomotor  192  is used not only as the drive source for selecting the suction nozzle  158  but also as the drive source for correcting the angular position of the component  164 , these operations can both be performed during movement of the head  120  about the stationary shaft  66 , whereby the electronic component transferring and mounting apparatus  12  can be simplified in construction, and manufactured with a reduced weight and at a reduced cost. 
     At two positions corresponding to the component sucking and mounting positions of the frame  60 , there are disposed two head elevating and lowering devices  260  as shown in FIGS. 2 and 8. More precisely, these figures show the head elevating and lowering device  260  disposed at the position corresponding to the component mounting position. FIG. 8 is a bottom plan view of the device  260  as seen from under the frame  60 . Since the two head elevating and lower devices  260  have the same construction, the device  260  at the position corresponding to the the component mounting position of the frame  60  will be explained. 
     The stationary cylindrical cam  128  has an engaging groove  262  formed at a circumferential position thereof corresponding to the component mounting position. This groove  262  is open in the inner circumferential surface and the upper and lower surfaces of the cylindrical cam  128 , as shown in FIGS. 2 and 8. The cam  128  further has a radial opening  264 , which is formed through an upper part of a radially outer portion of the cam  128  in the radial direction, for communication with the engaging groove  262 . A vertically movable member  266  is vertically movably received in the groove  262 . The cam  128  further has a guide member in the form of a straight guide rail  268  formed on the bottom wall of the groove  262  in the vertical direction (in the axial direction of the cam  128 ). On the other hand, a pair of guide blocks  270  are fixed to the vertically movable member  266 . These guide blocks  270  slidably engage the guide rail  268 . 
     A lower portion of the vertically movable member  266  has a circumferential dimension (as measured in the circumferential direction of the cylindrical cam  128 ), which is determined to provide a small clearance between the lower portion and the inner surfaces of the groove  262 , for permitting the vertically movable member  266  to move vertically in the groove  262 . A groove  272  is formed through this lower portion of the vertically movable member  266 , such that the groove  272  is open in the inner surface of the member  266  corresponding to the inner circumferential surface of the cylindrical cam  128 , and extends in a horizontal plane in a direction parallel to a tangent line at a circumferential point of the cam groove  130  corresponding to the component mounting position. 
     The vertically movable member  266  has an upper portion whose circumferential dimension is smaller than that of the lower portion described above. This upper portion is pivotably connected at an upper part thereof to a forked end portion of a pivotal member in the form of a lever  276  through a shaft member in the form of a pin  277 . The lever  276  extends through the radial opening  264  and projects radially outwardly from the outer circumferential surface of the cam  128 . The lever  276  is supported at an intermediate portion thereof by a support portion  280  of a support member  278  through a bearing  282  such that the support member  278  is moved relative to the lever  276  in the longitudinal direction of the lever  276 . The bearing  282  has two support pins  284  formed on two side surfaces thereof parallel to the longitudinal direction of the lever  276 , such that the support pins  284  extend perpendicularly to the longitudinal direction of the lever  276 , as shown in FIG.  8 . These support pins  284  function as a shaft portion which rotatably engages the support portion  280  of the support member  278 , so that the lever  276  is pivotable together with the bearing  282 , about the axis of the support pins  284 . 
     As shown in FIG. 8, the support member  282  has an arm portion  288  extending horizontally from the support portion  280  into the interior of the frame  60  through an elongate hole  286  formed through the frame  60 . The arm portion  288  carries a nut  292  fixed at its free end within the frame  60 . The nut  292  engages a feed screw  294 , which is rotated by a drive source in the form of a head stroke adjusting servomotor  296  for moving the support member  270  in the longitudinal direction of the lever  276 . A movement of the support member  278  in the longitudinal direction of the lever  276  causes a longitudinal movement of the bearing  282  and the support pins  284  relative to the lever  276 , whereby the pivoting axis of the lever  276  is changed in the longitudinal direction of the lever  276 , to thereby change the vertical stroke of the vertically movable member  266 . Thus, the feed screw  294  and the servomotor  296  constitute a major portion of a device for changing the vertical stroke of the vertically movable member  266 . A guide block  298  is fixed to the support portion  280 . This guide block  298  is in sliding contact with a guide rail  299  fixed to the frame  60 , so that the support member  278  is guided by the guide block  298  and the guide rail  299 . 
     As shown in FIG. 2, a generally L-shaped lever  300  is rotatably supported by a shaft  302  fixed to the frame  60 . The lever  300  has an arm  303  whose free end is pivotally connected to an upper end portion of a connecting member in the form of a rod  304  through a shaft member in the form of a pin  306 . The upper end portion of the rod  304  is movable relative to the pin  306  in the axial direction of the pin  306 . The rod  304  extends through an elongate hole  305  formed through the mounting structure  62 , and the lower end of the rod  304  is pivotally connected through a pin  308  to an end portion of the lever  276  remote from the end portion connected to the vertically movable member  266 . 
     The generally L-shaped lever  300  has another arm  312  which carries a cam follower roller  314  pivotably attached at its free end portion. The lever  300  is biased by biasing means in the form of an elastic member in the form of a tension coil spring  316  so that the cam follower roller  314  is held in rolling contact with a cam surface  320  of a head elevating and lowering cam  318  which is rotatably attached to the frame  60 . 
     The head elevating and lowering cam  318  is supported by a support shaft  322  which is rotatably supported by the frame  60 . As shown in FIG. 3, a timing pulley  324  is fixedly mounted on the support shaft  322 , and the timing pulley  324  is connected through the timing belt  112  to the timing pulley  110  fixed to the output shaft of the main drive servomotor  108 . Thus, the head elevating and lowering cam  318  is driven by the same servomotor  108  as used for driving the concave globoidal cams  90   a - 90   d.  The diameter of the timing pulley  324  is determined so that the cam  318  is rotated through 360° for a component mounting time interval equal to a time interval at which the rotary plates  70  are successively stopped at the component mounting position. 
     While FIG. 2 shows that the head elevating and lowering cam  318  has a cam surface  320  which has a circular shape, the cam  318  actually has a heart-like shape in cross section, so that a rotary motion of the cam  318  causes the lever  300  to be pivoted to vertically move the rod  304  for thereby pivoting the lever  276  so as to vertically move the vertically movable member  266 . Since the rod  304  and the vertically movable member  266  are connected to the opposite ends of the lever  276  on the opposite sides of the support pins  284 , the member  266  is moved down while the rod  304  is moved up, and vice versa. The lowermost and uppermost positions of the rod  304  are determined by the profile of the cam surface  320 . Described more specifically, the lowermost position of the rod  304  is adjusted by adjusting the axial position of the rod  304  so that the groove  272  formed through the vertically movable member  266  is aligned or contiguous with the cam groove  130  in the cylindrical cam  128  in the vertical direction, so as to form a horizontal groove, and so that the lever  276  has a horizontal attitude parallel to the feed screw  294  and the guide rail  299  when the rod  304  is placed in the lowermost position. The uppermost position of the vertically movable member  266  is determined by the lowermost position of the rod  304  determined as described above. In the uppermost position of the vertically movable member  266 , this member  266  cooperates with the adjacent portions of the cam groove  130  to hold the component holder head  120  at the uppermost position for a predetermined time. To mount the electronic component  164  on the printed-circuit board  38  when the rotary plate  70  is located at the component mounting position of the cylindrical cam  128 , the vertically movable member  266  disposed at the position corresponding to the component mounting position is first lowered from the uppermost position to the lowermost position and then elevated back to the uppermost position. The member  266  is held at the uppermost position at the positions other than the component mounting position. 
     As is apparent from the above description, the lever  276  is parallel to the guide rails  299  for guiding the support member  278  when the vertically movable member  266  is placed in the uppermost position with the rod  304  moved to the lowermost position. In this condition, the support member  278  is moved as needed, in the direction parallel to the guide rails  299 , to change the position of the pivoting axis of the lever  276 . Thus, the operating stroke of the vertically movable member  266  can be changed by changing the lowermost position of the member  266 , without changing the uppermost position of the member  266 . 
     The electronic component transferring and mounting apparatus  12  is controlled by a control device  330  illustrated in the block diagram of FIG.  9 . The control device  330  is constituted principally by a computer  340  incorporating a central processing unit (CPU)  332 , a read-only memory (ROM)  334 , a random-access memory (RAM)  336 , and a bus  338  connecting these elements  332 ,  334 ,  336 . To the bus  338 , there is connected an input interface  342  which receives an output of the CCD camera  114 . Also connected to the bus  338  is an output interface  344  which is connected to the cartridge feed servomotor  26 , X-axis drive servomotor  42 , Y-axis drive servomotor  48 , main drive servomotor  108 , nozzle rotating/selecting servomotors  192  and head stroke adjusting servomotor  296 , through respective driver circuits  346 ,  348 ,  350 ,  352 ,  354  and  356 . The ROM  334  stores various control programs such as those for sucking up the electronic component  164 , taking an image of the component  164  and mounting the component  164  on the printed-circuit board  38 . 
     When the electronic components  164  are mounted on the printed-circuit board  38  in the electronic component mounting system including the transferring and mounting apparatus  12 , the four concave globoidal cams  90   a - 90   d  are concurrently rotated by the main drive servomotor  108  to rotate and stop the twelve rotary plates  70  such that the acceleration, deceleration, constant-velocity movement and stopping of the rotary plates  70  are effected independently of each other. The rotary plates  70  are stopped at the component sucking position, image taking position and component mounting position, so that the electronic components  164  are sucked up by the heads  120  at the component sucking position, subjected to an image taking operation at the image taking position, and mounted on the board  38  by the heads  120  at the component mounting position. 
     The operation for the component holder head  120  to hold the electronic component  164  will be first explained. 
     When a given rotary plate  70  is rotated about the stationary shaft  66  toward the component sucking position, the cam follower roller  126  in rolling contact with the cam groove  130  is moved into the groove  272  formed through the vertically movable member  266  received in the groove  262  formed in the cylindrical cam  128 . The roller  126  is moved from the cam groove  130  into the groove  272  before the rotary motion of the rotary plate  70  to the component sucking position is completed. After the cam follower roller  126  has been moved into the groove  272  and before the rotary plate  70  is stopped at the component sucking position, a downward movement of the vertically movable member  266  from the uppermost position is initiated, whereby the roller  126  is lowered as the member  266  is lowered. As a result of the downward movement of the roller  126 , the vertical slide  124  is lowered, and the component holder head  120  is accordingly lowered. Thus, a movement of the head  120  about the stationary shaft  66  and a downward movement of the head  120  are effected contemporaneously. 
     As a result of the downward movement of the head  120 , the suction tube  162  of the suction nozzle  158  is brought into contact with the upper surface of the electronic component  164 , and the switching device  178  is switched to a position in which the electronic component  164  is attracted under suction to the lower end of the suction tube  162 . The rotary plate  70  has been stopped at the component mounting position when the suction tube  162  is brought into contact with the electronic component  164 , so that the component  164  can be attracted to the suction tube  162  with high reliability. After the component  164  is sucked up by the head  120 , the vertically movable member  266  is moved up, and the cam follower roller  126  is accordingly moved up, whereby the vertical slide  124  is elevated to elevate the head  120 , so that the component  164  sucked up by the suction nozzle  158  is picked up from the cartridge  22 . 
     After the component  164  is picked up from the cartridge  22  by the head  120 , the rotation of the rotary plate  70  about the stationary shaft  66  is resumed before the vertically movable member  266  has reached its uppermost position and before the groove  272  has been aligned with the cam groove  130 . The roller  126  is elevated in rolling contact with the groove  272  in the member  266 , and is moved from the groove  272  into the cam groove  130  of the cylindrical cam  128  immediately after the member  266  has reached the uppermost position. Thus, the head  120  is simultaneously elevated and rotated after the component  164  is sucked up from the cartridge  22 . 
     Upon sucking of the electronic component  164 , the vertical stroke of the head  120  is changed as needed if necessary, depending upon the height dimension or level of the upper surface of the electronic component  164 . The upper surfaces of the electronic components  164  may have different levels depending upon the height dimensions of the components  164 , where a component accommodating portion of a component holder tape in the component supply cartridge  22  is supported by a body of the cartridge  22  which is located below the component accommodating portion. 
     The component holder tape has a multiplicity of component accommodating recesses in which the components  164  are accommodated. These recesses are closed by an upper covering tape to prevent removal of the components  164  from the recesses. The thickness of the component holder tape increases with an increase in the height dimension of the components  164 . However, the height of the tape guiding surface of the body of the cartridge  22  is constant, so that the level of the upper surface of the component  164  is raised as the height dimension of the component  164  increases. Consequently, the vertical stroke of the component holder head  120  should be reduced with an increase in the height dimension of the electronic component  164  to be sucked up by the head  120 . This adjustment of the vertical operating stroke of the head  120  is effected before the component  164  is sucked up by the suction nozzle  158 . To this end, the head stroke adjusting servomotor  296  is actuated to move the support member  278  in the longitudinal direction of the lever  276 , for thereby changing the position of the pivoting axis of the lever  27 . 6 . For reliable attraction of the component  164  to the suction tube  162  of the suction nozzle  158 , the vertical stroke of the head  120  is accurately determined on the basis of not only the distance between the lower end face of the suction tube  162  of the head  120  and the upper surface of the component  164 , but also a height error of the component  164  due to a positioning error associated with the electronic component supply device  12 . An excessive downward movement of the suction nozzle  158  may be accommodated or absorbed by compression of the coil spring  160  biasing the suction nozzle  158 . 
     After the electronic component  164  has been held by the head  120 , the rotary plate  70  is rotated to the image taking position, at which the rotary plate  70  is stopped so that the component  164  sucked up by the suction nozzle  158  is held stationary. In this condition, an image of the component  164  is taken by the CCD camera  114 . On the basis of an output of the CCD camera  114 , an angular position error, and X-axis and Y-axis position errors of the component  164  are calculated. The angular position of the component  164  is adjusted to remove the angular position error before the rotary plate  70  has reached the component mounting position. To this end, the nozzle rotating/selecting servomotor  192  is actuated to rotate the sleeve  138  and the inner shaft  144  so that the suction nozzle  158  placed in the operating position is rotated about its axis to rotate the component  164  by an angle suitable to remove the angular position error. 
     Before an operation to mount the electronic component  164  on the printed-circuit board  38 , an image of a fiducial mark provided on the board  38  is taken to calculate X-axis and Y-axis positioning errors of the board  38  for each of the component mounting locations. When the electronic components  164  are mounted on the printed-circuit board  38 , the board  38  is moved in the X-axis and Y-axis directions, so that the locations at which the electronic components  164  are mounted are right under the component holder head  120  located at the component mounting position. On the basis of the calculated positioning errors of the board  38  and the X-axis and Y-axis position errors of the component  164 , the distances of movement of the board  38  in the X-axis and Y-axis directions are adjusted to mount the component  164  at the nominal X-axis and Y-axis positions on the board  38 . 
     When the rotary plate  70  is rotated toward the component mounting position, the cam follower roller  126  is moved from the cam groove  130  of the cylindrical cam  128  into the groove  272  in the vertically movable member  266  at the component mounting position. After the roller  126  has entered the groove  272  and before the rotary plate  70  has reached the component mounting position, a downward movement of the vertically movable member  266  is initiated to lower the component holder head  120  while the head  120  is rotated toward the component mounting position. The rotary plate  70  has been stopped at the component mounting position before the component  164  is mounted on the printed-circuit board  38 . Namely, the head  120  is further lowered to mount the component  164  at the predetermined point on the board  38  after the rotary plate  70  has reached the component mounting position. 
     The vertical operating stroke of the head  120  is changed depending upon the height dimension of the component  164  when the component  164  is mounted on the board  38 . The stroke of the head  120  is reduced as the height dimension of the component  164  increases. Precisely, the stroke of the head  120  should accommodate positioning errors such as dimensional and positioning errors in the manufacture of the board supporting and positioning device  16 , for example, so that the electronic components  164  can be accurately mounted on the board  38 . After the component  164  has been placed at the predetermined point on the board  38 , the switching device  178  is switched to a position to stop the application of a vacuum pressure to the suction nozzle  158 , whereby the component  164  is released from the suction nozzle  158 . After the component  164  is mounted on the board  38 , the vertically movable member  266  is elevated to elevate the head  120 . In this case, too, the rotation of the rotary plate  70  is resumed before the member  266  has reached the uppermost position, so that the roller  127  is elevated while it is moved in rolling contact with the groove  272 . Immediately after the vertically movable member  266  has reached the uppermost position, the roller  126  is moved into the cam groove  120 , and the rotary plate  70  is rotated toward the component sucking position. 
     While the rotary plate  70  is rotated from the component mounting position toward the component sucking position, the sleeve  138  is rotated in the direction opposite to the direction in which the sleeve  138  was rotated to correct the angular position of the component  164  before mounting thereof on the board  38 . The amount of rotation of the sleeve  138  at this time is the same as that for correcting the angular position of the component  164 , so that the sleeve  138  and the head  120  are returned to their original circumferential position. 
     In the event of some error at the component sucking position, the vertical stroke of the vertically movable member  266  at the component mounting position is reduced to a smallest value to avoid drawbacks described below. Such component sucking error may be a failure of the suction nozzle  158  to pick up the component  164 , an erroneous operation of picking up of the wrong component  164 , or an excessively large error of the angular position of the sucked component  164  that cannot be removed. In such event, the vertical stroke of the head  120  is reduced to a smallest value to prevent abutting contact of the suction nozzle  162  of the suction nozzle  158  (without the component  164  being attracted thereto) or the wrong component  164  with the printed-circuit board  38  when the head  120  is lowered to the lowermost position at the component mounting position. In this case, the switching device  178  is not switched at the component mounting position, and the vacuum pressure is kept applied to the suction nozzle  158 , so that the component  164  is carried by the head  120  to the component discarding area. In the component discarding area, the switching device  178  is actuated to stop the application of the vacuum pressure to the suction nozzle  158 , for discarding the component  164  into the component container. Where the suction nozzle  158  fails to pick up the electronic component  164 , this failure may be detected on the basis of the image taken at the image taking position. In this case, the switching device  178  is actuated to stop the application of the vacuum pressure to the suction nozzle  158 . 
     When the rotary plate  70  is further rotated, the cam follower roller  218  provided on the elevator  210  is brought into engagement with the cam surface  250  of the nozzle selecting cam  248 , so that the sleeve  138  is disconnected from the inner shaft  144 . If the kind of the suction nozzle  158  is changed, the nozzle rotating/selecting servomotor  192  is operated to rotate the nozzle holder  154  so that the suction nozzle  158  to be used next is brought into the operating position. 
     As described above, there are three stop positions at which the rotary plate  70  is stopped. Between the component sucking position and the image taking position, and between the image taking position and the component mounting position, the rotary plate  70  is rotated through 90°, with acceleration, constant-velocity movement and deceleration. Between the component mounting and sucking positions, the rotary plate  70  is rotated through 180° at a constant velocity. The timing chart of FIG. 10 shows a relationship between the time and the angle of rotation of each of the twelve rotary plates  70 . In FIG. 10, “T” represents the time required for each rotary plate  70  to be rotated through 360°, and this time is taken along the abscissa, with a graduation unit being equal to T/12 in view of the twelve rotary plates  70 . On the other hand, the angle of rotation of the rotary plate  70  is taken along the ordinate, with a graduation unit being equal to an angular spacing pitch of the twelve rotary plates  70  or heads  120 . The time-angle relationship of each rotary plate  70  is expressed by a line having a straight portion, an upwardly curved or convex portion and a downwardly curved or concave portion. The straight portion indicates the constant-velocity movement of the rotary plate  70 . The convex portion indicates the deceleration of the rotary plate  70 , while the concave portion indicates the acceleration of the rotary plate  70 . It will be understood from the timing chart of FIG. 10 that the individual rotary plates  70  are rotated independently of each other so that the nine rotary plates  70  can be rotated about the stationary shaft  66  while the three rotary plates  70  are held stopped at the component sucking, image taking and component mounting positions, for the corresponding heads  120  to pick up and mount the components  164 , and for the CCD camera  114  to take an image of the component  164 . 
     The present electronic component transferring and mounting apparatus  12  having the twelve component holder heads  12  has only three stop positions, so that the required acceleration and deceleration values of the heads  120  can be reduced, as compared with those in the conventional component transferring device in which a plurality of component holder heads are carried by an intermittently rotated rotary table. Suppose the conventional rotary table carries twelve heads arranged along a circle whose diameter is the same as that of the circle along which the heads  120  are rotated, and suppose the time required for one full rotation of the conventional rotary table is the same as the time T in the present apparatus  12 , the required acceleration and deceleration values of the heads  120  in the present apparatus  12  are smaller than those in the conventional apparatus using the conventional rotary table. The reason for this will be described below. 
     Each of FIGS. 11A-1 through  11 A- 3  shows four adjacent blocks of FIG. 10 according to the present embodiment, wherein a stop position of the component holder head  120  is indicated in the vicinity of an intersection of two mutually perpendicular partition lines of the blocks. Each of FIGS. 11B-1 through  11 B- 3  shows the corresponding four adjacent blocks according to the conventional apparatus. Two-dot chain lines (inclined straight lines) in FIGS. 11A-1 and  11 B- 1  indicate a continuous movement of the component holder head  120  as indicated in the other four adjacent blocks of FIG. 10 which do not include a stop position. When the head  120  in the present apparatus  12  is stopped at the stop position at the end of a rotary movement thereof corresponding to the angular spacing pitch of the rotary plates  70  or heads  120 , the head  120  is initially moved at a constant velocity which is represented by the gradient or angle of inclination of the inclined straight two-dot chain line in FIG. 11A-1. Then, the head  120  is accelerated as indicated by an initial portion of a solid line in FIG. 11A-1 which initial portion is located above the two-dot chain line. By this acceleration, the amount of angular displacement of the head  120  is larger than that by the constant-velocity movement represented by the two-dot chain lines. Finally, the head  120  is decelerated and eventually stopped at the stop position. Thus, the acceleration is effected to compensate for a sum of a time corresponding to a difference between the average velocity during the deceleration period and the constant velocity represented by the two-dot chain lines, and a time during which the head  120  is held stopped as represented by a horizontal portion of the solid lines. When a rotary motion of the head  120  is resumed after stopping at the stop position, the sequence of events (acceleration, constant-velocity movement and deceleration) is reversed with respect to the sequence where the head  120  is stopped. 
     In the conventional apparatus in which the component holder heads are carried by the intermittently rotated rotary table, each head should be stopped at all of the stop positions. During a movement of the head corresponding to the angular spacing pitch, therefore, the head is initially accelerated from the zero velocity, and is then decelerated to the zero velocity to be stopped at each stop position, as indicated by a solid line in the lower left block in FIG. 11B-1. To complete the movement of the head corresponding to the angular spacing pitch within the same time as in the present apparatus  12 , the maximum velocity of the head in the conventional apparatus as indicated in FIG. 11B-2 should be higher than that of the head  120  in the present apparatus  12  as indicated in FIG. 11A-2. Accordingly, the acceleration and deceleration values of the head in the conventional apparatus as indicated in FIG. 11B-3 should be higher than those of the head  120  as indicated in FIG. 11A-3. Thus, the required condition of movement of the head  120  in the present electronic component transferring and mounting apparatus  12  is more moderate than that in the conventional apparatus. Accordingly, the present apparatus  12  is required to have a comparatively low degree of structural rigidity, and the required capacity of the main drive servomotor  1 . 08  for rotating the rotary plates  70  can be significantly reduced, provided the component transferring efficiency of the present apparatus  12  is the same as that of the conventional apparatus. Therefore, the cost of manufacture of the present apparatus  12  can be reduced, or the accuracy of positioning of the heads  120  at the stop positions can be improved. 
     Conversely, the operating efficiency of the present apparatus  12  can be improved by reducing the time of movements of the heads  120  corresponding to the angular spacing pitch of the heads  120  or rotary plates  70 , namely, by reducing the time interval at which the successive rotary plates  70  sequentially reach a given stop position. 
     In the conventional apparatus in which the rotary table carrying the heads is intermittently rotated stopping at each stop position, each head should be accelerated from zero and decelerated to zero each time the head is moved between the adjacent stop positions at which the head is held stopped for a given time, as described above. Suppose the time required for the movement corresponding to the angular spacing pitch is 60 ms and the stop time is 20 ms, the time of the movement by acceleration and deceleration is 40 ms. Therefore, the average velocity of the head should be increased as compared with that in the present apparatus  12 . 
     In the present apparatus  12 , the stop positions are provided at every three or six angular spacing positions which are evenly spaced from each other about the stationary shaft  66 . Each head  120  is moved at a constant velocity into a given angular spacing region between the non-stop position and the stop position, and is decelerated to be stopped at the stop position. Therefore, a comparatively long time of 50 ms is allowed for the actual movement. Accordingly, the overall time corresponding to the angular spacing pitch can be reduced by a time which is substantially equal to a sum of a half (10 ms) of the stop time (20 ms) and a difference, a, between the time required for the accelerating movement by an angle r and the time required for the constant-velocity movement by the same angle, as indicated in FIG.  12 . Consequently, the required time of movement of the head  120  corresponding to the angular spacing pitch can be reduced by a time corresponding to the above-indicated sum, and the component mounting efficiency can be accordingly increased. For instance, the conventionally required time of 90 ms corresponding to the angular spacing pitch can be reduced to as short as 60 ms in the present electronic component transferring and mounting apparatus  12 . 
     The present apparatus  12  is further advantageous in that the twelve component holder heads  120  are held by the respective rotary plates  70 , and are rotated independently of each other, whereby the mass that should be accelerated and decelerated can be reduced, making it possible to prevent an increase in the vibration and noise even if the acceleration and deceleration values of the mass (head  120 ) are increased. This results in a further decrease in the required time of movement of the head  120  corresponding to the angular spacing pitch. 
     In the conventional apparatus in which the rotary table carrying a plurality of component holder heads is intermittently rotated, the rotary table is generally provided with a plurality of cam follower rollers corresponding to the heads, and these rollers are sequentially brought into engagement with a roller gear cam, so that the rotary table is intermittently rotated. This arrangement requires a comparatively high degree of accuracy in the spacing pitch of the cam follower rollers, to prevent the rollers from exerting an excessive force on the surface of the cam, which may reduce the life expectancy of the rollers and cause vibration and noise. In the present apparatus  12 , on the other hand, the individual rotary plates  70  has respective cam follower rollers  88  which engage the cam grooves  92  of the concave globoidal cams  90  for rotating the rotary plates  70 . In this arrangement, the twelve cam follower rollers  88  engage the concave globoidal cams  90 , independently of each other and without influencing each other, excessive forces will not act on the rollers  88  engaging the cams  90 , whereby the life expectancy of the rollers is improved while the vibration and noise are reduced. 
     Referring next to FIGS. 13-24, there is shown an electronic component mounting system equipped with an electronic transferring and mounting apparatus according to another embodiment of this invention. In the present system, the electronic component transferring and mounting apparatus generally indicated at  380  in FIG. 13 uses two concave globoidal cams and a constant-velocity rotor for rotating twelve rotary members such that each rotary member is stopped at a component sucking position and a component mounting position, but is not stopped at an image taking position. The present apparatus is adapted such that two positions are available for each of the component sucking and mounting positions, and such that the height position of the component holder head  120  upon a downward movement thereof at the component sucking and mounting positions and the height position of the electronic component at the image taking position can be changed in two steps. In the other aspects of the apparatus according to the present second embodiment, the present apparatus  380  is identical with the apparatus  12  of the first embodiment. The same reference numerals as used in the first embodiment will be used to identify the functionally corresponding elements, and redundant description of these elements of the present apparatus will not be provided. 
     The electronic component transferring and mounting apparatus  380  has a frame  382  including a support plate  384  to which an engaging cam  386  is fixed, as shown in FIG. 13. A support shaft in the form of a main shaft  388  is rotatably supported at its upper end portion by the engaging cam  386 . The main shaft  388  extends downwards through an opening  393  formed through a bottom wall  391  of a mounting structure  389  of the frame  382 , such that the lower end of the main shaft  388  is located outside and below the frame  382 . The main shaft  388  is rotatably supported at its lower end portion by a support plate  390  secured to the base  10 . Thus, the main shaft  388  is supported rotatably about a vertically extending axis thereof. On this main shaft  388 , there are rotatably supported a plurality of rotary members in the form of twelve rotary plates  392  through respective twelve pairs of bearings  394 . The rotary plates  392  are rotatable about a common axis, which is the axis of the main shaft  388 . 
     The bearings  394  are identical with the bearings  72  in the first embodiment, and are arranged in two arrays  396  which are spaced apart from each other in the axial direction of the main shaft  388 . Each array  396  consists of twelve bearings  394 . The vacuum source is connected through a conduit (not shown) to a passage (not shown) formed through the main shaft  388 , and this passage is connected to the switching device  178 . Since the main shaft  388  is rotated, the above-indicated conduit and the passage in the main shaft  388  are connected by a valve (not shown) which permits communication between the conduit and the passage even while the main shaft  388  is rotated. The passage in the main shaft  388  is connected to the twelve switching devices  178  through respective hoses. A small angle of relative rotation of the switching device  178  and the rotary plate  392  is permitted by elastic deformation of the hose. 
     The upper end portion of the main shaft  388  projects upwards from the engaging cam  386 , and has a timing pulley  400  fixed thereto. The timing pulley  40  is connected through a timing belt  406  (FIG. 14) to a timing pulley  404  which is fixed to an output shaft of a drive source in the form of a main drive servomotor  402  (FIG.  21 ). The timing pulley  40  is rotated at a predetermined constant speed by the servomotor  402 . 
     Two concave globoidal cams  410   a,    410   b  are disposed at respective positions of the frame  382  corresponding to the component sucking and mounting positions. As shown in FIGS. 13 and 14, these concave globoidal cams  410   a,    410   b  are disposed symmetrically with each other with respect to the axis of the main shaft  388  such that axes of the cams  410  lie in a horizontal plane and are parallel to each other. The cams  410   a,    410   b  are fixedly mounted on respective rotary shafts  412   a,    412   b,  which are rotatably supported by respective pairs of brackets  413   a,    413   b.  As shown in FIG. 14, each rotary shaft  412   a,    412   b  has a bevel gear  414   a,    414   b  fixed thereto at one end thereof. The bevel gears  414   a,    414   b  mesh with respective bevel gears  416   a,    416   b  (FIG. 13) which are rotatable about vertical axes. These bevel gears  414   a,    414   b,    416   a,    416   b  have the same diameter. 
     As shown in FIG. 13, brackets  420   a,    420   b  are fixed to the mounting structure  389  of the frame  382 , and rotary shafts  422   a,    422   b  are supported by the support plate  384  of the frame  382  and the brackets  420   a,    420   b  such that the rotary shafts  422  are rotatable about vertical axes. The bevel gears  416   a,    416   b  are fixedly mounted on the rotary shafts  422   a,    422   b,  respectively. The upper end portions of the rotary shafts  422   a,    422   b  project upwards from the support plate  384 , and have respective timing pulleys  424   a,    424   b  fixed thereto. These timing pulleys  424  are held in rolling contact with the timing belt  406 , which is provided for transmitting a rotary motion of the main drive servomotor  402  to the main shaft  388  as described above. The timing belt  406  is also engaged with an idler timing pulley  426 . In the present arrangement, the rotary shafts  422   a,    422   b  are rotated by the main drive servomotor  402 , so that the bevel gears  416   a,    416   b,    414   a,    414   b  are rotated to rotate the concave globoidal cams  410   a,    410   b  simultaneously in synchronization with each other. 
     The concave globoidal cams  410   a,    410   b  have respective cam grooves  430   a,    430   b,  which are formed to rotate the rotary plates  392  at the velocities described below. The cam grooves  430   a,    430   b  are engageable with cam follower rollers  436  fixed to the respective rotary plates  392 . Immediately after these rollers  436  have entered the cam grooves  430   a,    430   b,  the rotary plates  392  are rotated at the same velocity as the main shaft  388 . Then, the rotary plates  392  are accelerated for an increased rotary movement distance per unit time, and are then decelerated to be stopped at the component sucking or mounting position. During an operation to pick up or mount the electronic component at the sucking or mounting position, the appropriate rotary plate  392  is held stopped. After the operation, the rotary plate  392  is accelerated, and is then decelerated so that the velocity of the rotary plate  392  upon movement of the roller  436  away from the cam groove  430   a,    430   b  is made equal to the velocity of the main shaft  388  and so that the angular interval of this rotary plate  392  with respect to the adjacent rotary plate  392  located downstream in the rotating direction is 30°. 
     The cam groove  430   a  of the concave globoidal cam  410   a  is shown in FIGS. 15 and 16, by way of example. The cam groove  430   a  has an inclined portion  432  having a lead angle with respect to a plane perpendicular to the axis of rotation of the concave globoidal cam  410   a,  and a non-lead portion  434  which does not have such a lead angle and which is perpendicular to the axis of rotation. The inclined portion  432  has a comparatively small axial width as measured in the axial direction of the cam  410   a,  so that the cam follower roller  436  engages this narrow inclined portion  432  with a small amount of clearance to the side surfaces of the narrow inclined portion  432  in the diametric direction of the roller  436 . The non-lead portion  434  has a larger axial width than the inclined narrow portion  432 , so that this wide non-lead portion  434  permits the roller  436  to move in the axial direction. The cam groove  430 a also has an intermediate width varying portion  438  between the narrow inclined portion  432  and the wide non-lead portion  434 . The axial width of the width varying portion  438  gradually increases in the direction from the narrow inclined portion  432  toward the wide non-lead portion  434 . 
     When the cam follower cam  436  is in engagement with the wide non-lead portion  434 , the rotary plate  392  is held stopped. Since the axial width of the non-lead portion  434  is larger than the diameter of the roller  436 , two positions, namely, first and second positions, are available as the actual stop position of the rotary plate  392 . In the first position, the roller  436  is in contact with an upstream side surface  440  of the non-lead portion  434 , as indicated in solid line in FIG.  15 . In the second position, the roller  436  is in contact with a downstream side surface  442  of the non-lead portion  434 . The side surfaces  440 ,  442  are located on the upstream and downstream sides, respectively, as viewed in the rotating direction of the rotary plate  392 . One of the first and second positions is selected as the actual stop position (component sucking or mounting position). The concave globoidal cams  410   a,    410   b  are disposed at the positions of the frame  382  corresponding to the component sucking and mounting positions, which are spaced apart from each other by 180°. The axial dimension of the non-lead portion  434  is determined so that the distance between the first and second positions in the X-axis direction is equal to 8 mm. The first and second positions selectively established as the component sucking position will be referred to as first and second sucking positions, while the first and second positions selectively established as the component mounting position will be referred to as first and second mounting positions. 
     The cam grooves  430   a,    430   b  of the concave globoidal cams  410   a,    410   b  are formed such that the rotary plate  392  is rotated at the same velocity as the main shaft  388  after the cam follower roller  436  is disengaged from the cams  410   a,    410   b,  and the angular interval of this rotary plate  392  with respect to the next downstream rotary plate  392  is 30° when the roller  436  is disengaged from the cams  410 , irrespective of whether the rotary plate  392  is stopped at the first or second component sucking or mounting position. 
     In the present electronic component transferring and mounting apparatus  380 , the two head elevating and lowering devices  260  described above with respect to the first embodiment are disposed at the positions of the frame  382  corresponding to the component sucking and mounting positions. In FIGS. 13 and 14, only the device  260  corresponding to the component mounting position is shown. In the present apparatus  380 , however, a hypoid gear (not show) is fixedly and concentrically mounted on the support shaft  322  of the head elevating and lowering cam  318  of the head elevating and lowering device  260 . This hypoid gear meshes with a hypoid gear  446  (FIG. 13) rotatably attached to the frame  382 . These hypoid gears have the same diameter. The hypoid gear  446  is fixedly mounted on a rotary shaft  450  which is supported by the support plate  384  of the frame  382  and a bracket  448  fixed to the mounting structure  389  of the frame  382 . The rotary shaft  450 , which is rotatable about a vertical axis thereof, has a timing pulley  452  fixed to its upper end portion projecting upwards from the support plate  384 . The timing pulley  452  is held in rolling contact with the above-indicated timing belt  406 , so that a rotary motion of the main drive servomotor  402  is transmitted to the head elevating and lowering cam  318  through the hypoid gear  446 , whereby the cam  318  is rotated. 
     Thus, the drive source in the form of the main drive servomotor  402  is commonly used for the main shaft  388 , two concave globoidal cams  410   a,    410   b  and head elevating and lowering cam  318 . The diameter of the timing pulley  452  for rotating the cam  318  is determined so that the timing pulley  452  and the cam  318  are rotated through 360° during rotation of the main shaft  388  through 30°. The diameter of the timing pulleys  410   a,    410   b  is determined so that these pulleys  410  are rotated through 180° during rotation of the main shaft  388  through 30°. Thus, the cam  318  is rotated one full turn within a time corresponding to the angular spacing pitch of the rotary plates  392 , namely, each time the rotary plates  392  sequentially reach the component mounting position. 
     As shown in FIGS. 13 and 14, a constant-velocity rotary disc  460  is fixedly and concentrically mounted on a portion of the main shaft  388  between the engaging cam  386  and the upper array  396  of the bearings  394  to which the rotary plates  392  are fixed. The main shaft  388  and the constant-velocity rotary disc  460  cooperate to constitute a constant-velocity rotor. The constant-velocity rotary disc  460  is rotated at a predetermined constant velocity together with the main shaft  388 . The rotary disc  460  carries engaging members in the form of twelve engaging pins  468  fixed thereto. These pins  468 , which constitute an engaging device  464 , are equi-angularly spaced from each other about the axis of the rotary disc  460 . 
     The constant-velocity rotary discs  460  further carries guide members in the form of twelve guide pins  466 . These guide pins  466  engage the rotary disc  460  such that the pins  466  are movable relative to the rotary disc  460  in the axial direction of the rotary disc  460 . The engaging pins  468  are located radially outwardly of the guide pins  466 , and engage the constant-velocity rotary disc  460  such that the pins  468  are movable relative to the rotary disc  460  in the axial direction of the disc  460 . The lower portion of each engaging pin  468  projects downwards from the rotary disc  460 , and has a tapered lower end  470  whose diameter decreases in the downward direction. The guide pin  466  and the corresponding engaging pin  468  are connected to each other at their upper ends by a connecting plate  474 , so that the guide pins  466  guides the corresponding engaging pin  468  in the axial direction, and prevents rotation of the engaging pins  468 . 
     To the upper surface of the connecting plate  474 , there is attached a cam follower roller  478  such that the roller  478  is rotatable about an axis which extends in a radial direction of the main shaft  388 . The roller  478  is held in engagement with a cam groove  480  formed in the engaging cam  386  described above. The cam  386  has a cylindrical outer circumferential surface coaxial with the main shaft  388 , and the cam groove  480  is formed and open in the outer circumferential surface of the cam  386 . The cam groove  480  has a height-varying portion and a non-lead portion. The height-varying portion is a portion whose height position in the axial direction of the main shaft  388  gradually varies, and the non-lead portion is a portion whose height position is unchanged. When the constant-velocity rotary disc  460  is rotated with the main shaft  388 , the cam follower roller  478  is rotated with the rotary disc  460  about the axis of the main shaft  388 , and at the same time is elevated and lowered or held at a predetermined level. As a result, the engaging pins  468  are similarly rotated and vertically moved. 
     The twelve rotary plates  392  have respective engaging recesses  486  open in their upper end faces. Each of these engaging recesses  486  has a tapered surface whose taper angle corresponds to the taper angle of the tapered end of the corresponding engaging pin  468 . The engaging pins  468  are engaged with and disengaged from the respective recesses  486  when the pins  468  are moved up and down. With the engaging pin  468  engaging the recess  486  of a given rotary plate  392 , this rotary plate  392  is rotated at a predetermined velocity by the constant-velocity rotary disc  460 . 
     The cam groove  480  of the cam  386  is shaped so that the following events will take place for a given rotary plate  392  when the main shaft  388  is rotated. 
     The engaging pin  468  corresponding to the rotary plate  392  in question is disengaged or released from the engaging recess  486  (namely, the pin  468  is moved upwards), after the cam follower roller  436  has been moved into the cam groove  430   a,    430   b  and while the rotary plate  392  is rotated by the concave globoidal cam  410   a,    410   b  at the same constant velocity as the constant-velocity rotary disc  460 . Then, the engaging pin  468  is held released from the recess  486  until the pin  468  is again brought into engagement with the recess  468 , so that the pin  468  does not disturb a rotary motion of the rotary plate  392  by the concave globoidal cam  410   a,    410   b.  The engaging pin  468  is engaged with the recess  486  (namely, the pin  468  is moved downwards), before the roller  436  has been moved out of the cam groove  430   a,    430   b  and while the rotary plate  392  is rotated at the same velocity as the constant-velocity rotary disc  460 . The pin  468  is held engaged with the recess  486  to cause the rotary plate  392  to be rotated by the constant-velocity rotary plate  460  until the roller  346  is again brought into engagement with the cam groove  430   a,    430   b  to cause the rotary plate  392  to be rotated by the concave globoidal cam  410   a,    410   b.  Since the engaging pin  468  has the tapered end  470 , the pin  468  can be smoothly brought into engagement with the engaging recess  486 . 
     The rotary plate  392  is rotated by the concave globoidal cams  410   a,    410   b,  in the vicinities of the component sucking and mounting positions, and is rotated at the predetermined constant velocity by the constant-velocity rotary disc  460 , between the component sucking and mounting positions. The rotary plate  392  can be stopped by the concave globoidal cams  410   a,    410   b,  at the component sucking and mounting positions with high positioning accuracy. The image taking position at which the image of the electronic component is taken is located between the component sucking and mounting positions, and a high-speed camera  562  with a stroboscope (FIG. 21) is provided at a position of the base  10  corresponding to the vicinity of the image taking position. The rotary plate  392  is rotated at the constant velocity by the constant-velocity rotary disc  460 , between the component sucking and mounting positions, without stopping at the image taking position. The image of the electronic component  164  is taken by the high-speed camera  562  when the component  164  passes a position right above the camera  562 . 
     Between the twelve rotary plates  392  and the constant-velocity rotary disc  460 , there are provided twelve stop position changing air cylinders  490  for changing the stop position of the rotary plate  392 , as shown in FIGS. 13 and 18. Each air cylinder  490  has a cylinder housing  492  which is connected by a pin to a bracket  494  fixed to the constant-velocity rotary disc  460 , such that the cylinder housing  492  is rotatable about an axis which extends in the axial direction of the rotary disc  460 . The air cylinder  490  further has a piston rod  498  which is connected by a pin to the rotary plate  392  such that the piston rod  498  is rotatable about an axis which also extends in the axial direction of the rotary disc  460 . 
     The twelve stop position changing air cylinders  490  are connected to the constant-velocity rotary disc  460  such that the connections of the adjacent two air cylinders  490  to the rotary disc  460  are located at different radial positions (i.e., at relatively outer and inner positions) of the rotary disc  460 , in order to prevent an interference of the adjacent air cylinders  490 . To this end, the every two rotary plates  392  have respective radial extensions  500  radially outwardly extending from the upper ends thereof, as shown in FIGS. 13 and 18. The radial extensions  500  have respective cutouts  501  formed in their upper surfaces, as shown in FIGS. 13 and 18, to avoid interferences of those radial extensions  500  with the air cylinders  490  connected to the rotary plates  392  not provided with the radial extensions  500 . The bracket  494  for supporting the air cylinder  490  connected to the rotary plate  392  not provided with the radial extension  500  is located above or horizontally spaced apart from the cylinder housing  492  of the air cylinder  490  connected to the adjacent rotary plate  392 , so that the bracket  494  does not interfere with that cylinder housing  492 . 
     Each stop position changing air cylinder  490  is a double-acting cylinder having two air chambers. By controlling flows of compressed air into and from these two air chambers, the corresponding rotary plate  392  is pivoted by the air cylinder  490  about the axis of the main shaft  388 , so that the cam follower roller  436  is selectively moved to the first and second sucking or mounting positions in which the roller  436  is forced against the upstream and downstream side surfaces  440 ,  442  of the cam groove  430   a,    430   b,  respectively, as indicated by solid and two-dot chain lines in FIG.  15 . 
     In the present electronic component transferring and mounting apparatus  380 , too, a stationary cylindrical cam  504  is fixed to the underside of the frame  382 , and cam follower rollers  502  are held in rolling engagement with a cam groove  506  formed in the cylindrical cam  504 . Like the rollers  126  provided in the apparatus  12  according to the first embodiment, each of the rollers  502  is fixed to the vertical slide  124  which is vertically movably supported by the rotary plate  392 . As described above with respect to the apparatus  12 , the component holder head  120  is attached to the vertical slide  124 . In the present apparatus  380 , the circumferential dimension (as measured in the rotating direction of the rotary plate  392 ) of the vertically movable member  266  of the head elevating and lower device  260  disposed at the position corresponding to the component sucking position is determined so that the cam follower roller  502  is located in the groove  272  when the rotary plate  392  is stopped at the first or second sucking position, namely, irrespective of whether the rotary plate  392  is located at the first or second sucking position. The circumferential dimension of the vertically movable member  266  of the head elevating and lowering device  260  disposed at the position corresponding to the component mounting position is similarly determined. 
     As schematically illustrated in FIG. 17, the cam groove  506  has a first wide portion  508 , a second wide portion  509  and a third wide portion  511 , which have respective constant heights or whose levels do not vary in the circumferential direction of the cylindrical cam  504 . The first wide portion  508  corresponds to the component sucking position and its vicinity. The second wide portion  509  corresponds to the component mounting position and its vicinity. The third wide portion  511  corresponds to the image taking position and its vicinity. The width or vertical dimension of the first wide portion  508  as measured in the axial direction of the main shaft  388  is equal to a diameter of the cam follower roller  502  plus 6 mm, so that the roller  502  is permitted to be moved in the first wide portion  508  in the vertical direction. The width or vertical dimension of the second wide portion  509  is equal to the diameter of the roller  502  plus 4 mm, while the width or vertical dimension of the third wide portion  511  is equal to the diameter of the roller  502  plus 3 mm. These second and third wide portions  509 ,  511  also permit the roller  502  to move therein in the vertical direction. The cam groove  506  further has a first narrow portion  510  which has a constant height and which is located upstream of the component sucking position as viewed in the rotating direction of the rotary plates  392 . The width or vertical dimension of this first narrow portion  510  is determined such that the roller  502  is in rolling engagement with the first narrow portion  510  with substantially no clearance therebetween in the vertical direction. The cam groove  506  also has a second narrow portion  514  whose height or level varies in the circumferential direction. The cam groove  506  has width varying portions  512  which connect the portions having different widths. The width of these varying portions  512  gradually varies to smoothly connect the adjacent portions of the groove  506 . 
     The width or vertical dimension of the first wide portion  508  is equal to the vertical dimension (as measured in the axial direction of the main shaft  388 ) of the groove  272  formed in the vertically movable member  266  of the head elevating and lowering device  260  disposed at the position corresponding to the component sucking position. The width or vertical dimension of the second wide portion  509  is equal to the vertical dimension of the groove  262  of the vertically movable member  266  of the head elevating and lowering device  260  disposed at the position corresponding to the component mounting position. A significance of the difference between the vertical dimensions of the first and second wide portions  508 ,  509  will be described. 
     As shown in FIG. 13, each of the twelve rotary plates  392  is provided with a height position changing air cylinder  520  attached thereto so as to extend in the vertical direction. The air cylinder  520  has a piston rod  522  which has an engaging member  524  fixed thereto. The engaging member  524  is fixed to the vertical slide  124  which carries the component holder head  120 . The height position changing air cylinder  520  is a double-acting cylinder whose piston rod  522  is adapted to vertically move the vertical slide  124  and the cam follower roller  502  fixed thereto, so that the roller  502  can be selectively brought into pressing contact with an upper side surface  526  and a lower side surface  528  (FIG.  17 ). When the vertical slide  124  is vertically moved by the stationary cylindrical cam  504  and the cam follower roller  502 , the direction of movement of the vertical slide  124  may be reversed with respect to the direction in which the cam follower roller  502  is biased by the height position changing air cylinder  520 . In this case, the vertical movement of the vertical slide  124  may be permitted by compression of the air in one of the two air chambers of the air cylinder  520  which is connected to the air source. However, it is desirable that the vertical movement of the vertical slide  124  is permitted by permitting a discharge flow of the air from the air chamber connected to the air source. This latter arrangement prevents a variation in the air pressure in that air chamber, making it possible to force the roller  502  onto the side surface of the cam groove  506  with a constant force. 
     When the cam follower roller  502  is in pressing contact with the upper side surface  526 , the component holder head  120  has a larger height at the first or second sucking position or first or second mounting position, than when the roller  502  is in pressing contact with the lower side surface  528 . Thus, the height position changing air cylinder  520  permits the height position of the head  120  (in the axial direction of the main shaft  388 ) to be changed in two steps when the head  120  is stopped at the component sucking and mounting positions. Further, the air cylinder  520  permits the height position of the head  120  to be changed in two steps when the head  120  passes the image taking position. Thus, there are available two height positions of the head  120  at each of the first and second sucking positions (in the component sucking station), first and second mounting positions (in the component mounting station), and image taking position (in the image taking station). 
     Flows of compressed air into and from the stop position changing air cylinders  490  and the height position changing air cylinders  520  are controlled by twelve stop position changing valves  530  and twelve height position changing valves  532 , as shown in FIGS. 13 and 19. These valves  530 ,  532  are attached to the constant-velocity rotary disc  460  such that the twelve pairs of these valves  530 ,  532  corresponding to the twelve rotary plates  392  are equi-angularly spaced from each other in the circumferential direction of the rotary disc  460 . Each of these valves  530 ,  532  includes a switching member in the form of a spool which is moved between two operating positions for mechanically controlling the supply and discharge flows of the compressed air into the appropriate cylinder  490 ,  520 . The stop position changing valve  530  and the height position changing valve  530  of each pair are spaced apart from each other in the radial direction of the rotary disc  460 , as shown in FIG.  19 . The stop position changing valves  530  are controlled by first and second valve switching devices  534 ,  536 , while the height position changing valves  532  are controlled by a third valve switching valve  538 , as shown in FIGS. 18-20. 
     Referring to FIG. 18, there are shown twelve angular positions which are equi-angularly spaced from each other at an angular interval of 30° in the clockwise direction about the main shaft  388 . Suppose the stop and height changing valves  530 ,  532  corresponding to the rotary plate  392  located at the component sucking position are located at the first angular position, the valves  530 ,  532  corresponding to the rotary plate  392  located at the component mounting position are located at the seventh angular position, and the valves  530 ,  532  corresponding to the rotary plate  392  at the image taking position for taking an image of the corresponding electronic component are located at the fourth angular position. 
     As shown in FIG. 18, the first and second valve switching devices  534 ,  536  for the stop position changing valves  530  are located at the fifth and eleventh angular positions, respectively, while the third valve switching device  538  for the height position changing valves  532  is located at the tenth angular position. 
     The three valve switching devices  534 ,  536 ,  538  have the same construction. The first valve switching device  534  for the stop position changing valves  530  will be described by way of example, by reference to FIG.  19 . The first valve switching device  534  has two actuating members in the form of engaging rollers  540 ,  542  disposed such that these rollers  540 ,  542  are located above and below the spool of each stop position changing valve  530  when the valve  530  is located at the fifth angular position (FIG.  18 ). The first valve switching device  534  further has a drive device in the form of two valve switching air cylinders  544 ,  546  fixed to the frame  382 . The air cylinders  544 ,  546  have respective piston rods  548 ,  550 . The rollers  540 ,  542  are fixed to the free ends of the piston rods  548 ,  550  such that the rollers  540 ,  542  are rotatable about horizontal axes extending in the radial direction of the main shaft  388 . The air cylinders  544 ,  546  are single-acting cylinders whose pistons rods  548 ,  550  are spring-biased so that the rollers  540 ,  542  are normally held in their positions in which the rollers  540 ,  542  are spaced apart from the end faces of the spool of the stop position changing valve  530 . Like the first valve switching device  534 , the second valve switching device  536  located at the eleventh angular position is disposed at a position aligned with the stop position changing valves  530  in the radial direction of the main shaft  388 , and the third valve switching device  538  located at the tenth angular position is disposed at a position aligned with the height position changing valves  532  in the radial direction of the main shaft  388 . 
     The compressed air is supplied from the air source through a conduit (not shown) to a passage formed through the main shaft  388 . This passage is connected to the stop position changing valves  530  and the height position changing valves  532  through the constant-velocity rotary disc  460 . Each stop position changing valve  530  is connected to the stop position changing air cylinder  490  through a hose (not shown), while each height position changing valve  532  is connected to the height position changing air cylinder  520  through a hose (not shown). Since the main shaft  388  is rotated, the above-indicated conduit and passage in the main shaft  388  are connected to each other by a rotary joint. 
     The present electronic component mounting system including the electronic component transferring and mounting apparatus  380  is controlled by a control device  560  illustrated in the block diagram of FIG.  21 . This control device  560  is principally constituted by the computer  340  as used in the control device  330  in the first embodiment. The high-speed camera  562  is connected to the input interface  342  of the computer  340 . To the output interface  344  of the computer  340 , there are connected the driver circuit  352  for the main drive servomotor  402 , and driver circuits  564  for controlling the valve switching air cylinders  544 ,  546  of each of the first, second and third valve switching devices  534 ,  536 ,  538 . The ROM  334  of the computer  340  stores various control programs including programs for controlling operations for the component holder heads  120  to hold the electronic components and mount them on the printed-circuit board  38 , programs for selecting the first or second sucking position and the first or second mounting position, and programs for selecting the height positions of the heads  120  at the component sucking and mounting position and image taking position. 
     When the present electronic component mounting system is operated to mount the electronic components on the printed-circuit board  38 , the main drive servomotor  402  is activated to rotate the constant-velocity rotary disc  460  and the concave globoidal cams  410   a,    410   b,  so that the twelve rotary plates  392  are rotated independently of each other, namely, accelerated, decelerated, moved at a predetermined constant velocity or stopped, depending upon their positions in their circular path. Each rotary plate  392  is rotated by the constant-velocity rotary disc  460  at the constant velocity while the engaging pin  468  of the corresponding engaging device  464  is held in engagement with the engaging recess  486  of the rotary plate  392  in question. The rotary plate  392  is rotated by the concave globoidal cams  410   a,    410   b  while the corresponding cam follower roller  436  is held in engagement with the cam grooves  434   a,    434   b  of the cams  410   a,    410   b  and while the roller  436  is located at positions other than the component sucking and mounting positions. The rotary plate  392  is stopped at the component sucking and mounting positions so that the electronic component  164  is picked up by the corresponding component holder head  120  and mounted on the printed-circuit board  38 . 
     When the rotary plate  392  is passing the image taking station, the rotary plate  392  is rotated by the constant-velocity rotary disc  460  at the constant velocity with the pin  468  in engagement with the recess  486 . The image of the electronic component  164  held by the head  120  is taken by the high-speed camera  562  when the rotary plate  392  passes the image taking position during its rotary movement at the constant velocity. Since the high-speed camera  562  is provided with a stroboscope, the image of the electronic component  164  can be taken with a sufficiently high quality without having to stop the electronic component  164 . The X-axis and Y-axis and angular position errors of the electronic component  164  are calculated on the basis of the output of the high-speed camera  562 , and the angular position of the electronic component  164  is adjusted to remove the angular positioning error by rotating the suction nozzle  158  about its axis while the head  120  is moved from the image taking position to the component mounting position. 
     As described above, there are two stop positions at which the rotary plate  70  is stopped. Around each of the component sucking position and the component mounting position, the rotary plate  70  is rotated with acceleration, at a constant velocity, and with deceleration. At the remaining portion of the circular movement path, the rotary plate  70  is rotated at a constant velocity. The timing chart of FIG. 36 shows a relationship between the time and the angle of rotation of each of the twelve rotary plates  70 , in the same manner as that employed in the timing chart of FIG.  10 . Although in the present embodiment there are provided two selectable component sucking positions and two selectable component mounting positions, the timing chart of FIG. 36 is, for easier understanding purposes only, based on the assumption that the respective non-lead portions  434  of the cam grooves  430   a,    430   b  of the concave globoidal cams  410   a,    410   b  have a width which does not permit the roller  436  to be moved in the direction of the width and accordingly there is provided a single component sucking position and a component mounting position. This assumption applies to FIGS. 11A-1 to  11 A- 3  and  11 B- 1  to  11 B- 3  which will be referred to, as needed, in the following description. It will be understood from the timing chart of FIG. 36 that the individual rotary plates  70  are rotated independently of each other so that the ten rotary plates  70  can be rotated about the stationary shaft  66  while the two rotary plates  70  are held stopped at the component sucking and component mounting positions, for the corresponding heads  120  to pick up and mount the components  164 . The present electronic component transferring and mounting apparatus  380  having the twelve component holder heads  120  has only two stop positions, so that the required acceleration and deceleration values of the heads  120  can be reduced, as compared with those in the conventional component transferring device in which a plurality of component holder heads are carried by an intermittently rotated rotary table. Suppose the conventional rotary table carries twelve heads arranged along a circle whose diameter is the same as that of the circle along which the heads  120  are rotated, and suppose the time required for one full rotation of the conventional rotary table is the same as the time T in the present apparatus  380 , the required acceleration and deceleration values of the heads  120  in the present apparatus  380  are smaller than those in the conventional apparatus using the conventional rotary table, for the reason described above with respect to the timing chart of FIG.  10 . 
     Meanwhile, in the present embodiment, there are provided two selectable component sucking positions and two selectable component mounting positions. When the rotary plate  70  is stopped at the upstream one of the two selectable component sucking or mounting positions in the rotating direction thereof, the component holder head  120  is rotated at a slope smaller than that of a curve indicated at solid line in FIG. 11A-1, that is, rotated along a curve slightly below the solid-line curve, before the head  120  is stopped at a position slightly below the stop position indicated by the solid-line curve (i.e., position with zero or no dimensionless displacement). Thus, the component holder head  120  can be moved more slowly to the upstreamside stop position. After the rotation of the rotary plate  70  is resumed, the holder head  120  is rotated at a slope greater than that of the solid-line curve shown in FIG. 11A-1, that is, rotated along a curve slightly above the solid-line curve, so as to compensate for the distance between the upstream-side and downstream-side stop positions in the rotating direction. Thus, the holder head  120  is placed in the state in which the head  120  is rotated at a constant velocity, at the same timing as that at which the holder head  120  is rotated in the first embodiment wherein the globoidal cams  90  each having no no-lead portion  434  are employed. Therefore, the component holder head  120  is moved at velocities slightly smaller than those of a curve indicated at solid line in FIG. 11A-2, and at accelerations or decelerations slightly smaller than those of a curve indicated at solid line in FIG. 11A-3, before the head  120  is stopped. On the other hand, after the rotation of the rotary plate  70  is resumed, the holder head  120  is rotated at velocities slightly greater than those of the solid-line curve shown in FIG. 11A-2, and at accelerations or decelerations slightly greater than those of the solid-line curve shown in FIG. 11A-3. 
     When the rotary plate  70  is stopped at the downstream one of the two selectable component sucking or mounting positions in the rotating direction thereof, the component holder head  120  is rotated, conversely, at a slope greater than that of the solid-line curve shown in FIG. 11A-1, that is, rotated along a curve slightly below the solid-line curve, before the head  120  is stopped at a position slightly above the stop position indicated by the solid-line curve (i.e., position with zero or no dimensionless displacement). In order to move additionally over the distance between the upstream-side and downstream-side stop positions in the rotating direction, the component holder head  120  is rotated more quickly to the downstream-side stop position. After the rotation of the rotary plate  70  is resumed, the holder head  120  is rotated at a slope smaller than that of the solid-line curve shown in FIG. 11A-1, that is, rotated along a curve slightly below the solid-line curve, because the head  120  was stopped at the downstream-side stop position. The holder head  120  is placed in a constant-velocity state, at the same timing as that at which the holder head  120  is rotated in the first embodiment. Therefore, the component holder head  120  is moved at velocities slightly greater than those of the solid-line curve shown in FIG. 11A-2, and at accelerations or decelerations slightly greater than those of the solid-state curve shown in FIG. 11A-3, before the head  120  is stopped. On the other hand, after the rotation of the rotary plate  70  is resumed, the holder head  120  is rotated at velocities slightly smaller than those of the solid-line curve shown in FIG. 11A-2, and at accelerations or decelerations slightly smaller than those of the solid-line curve shown in FIG. 11A-3. 
     In the present embodiment wherein there are provided two selectable component sucking positions and two selectable component mounting positions for the component holder heads  120 , each of the heads  120  is rotated at different accelerations and decelerations from those in the case where there are provided a single component sucking position and a single component mounting position. However, those differences are small. On the other hand, like the first apparatus  12 , the present apparatus  380  enjoys the advantage that the component holder heads  120  are moved at smaller accelerations and decelerations than those at which the holder heads of the conventional apparatus are moved. Thus, each holder head  120  can reach each of the two stop positions in a shorter time. In addition, since each rotary plate  70  is rotated by the concave globoidal cams  410   a,    410   b  around the component sucking and mounting positions, respectively, it can be stopped with accuracy at each of the sucking and mounting positions. 
     As the two rotary plates  392  which are 180° spaced from each other about the axis of the main shaft  388  are rotated toward the component sucking and mounting positions, the corresponding cam follower rollers  502  are moved from the cam groove  506  of the stationary cylindrical cam  504  into the grooves  272  of the vertically movable members  266  of the two head elevating and lowering devices  260  disposed at the positions corresponding to the sucking and mounting positions. With the devices  260  operated to elevate the vertically movable members  266 , the corresponding two component holder heads  120  are vertically moved so that the electronic component  164  is picked up by one of these two heads  120  while the electronic component  164  held by the other head  120  is transferred to the printed-circuit board  38 , as described above with respect to the first embodiment. In portions of these component pickup and mounting operations, the rotation and vertical movement of the heads  120  are contemporaneously effected, as also described above. In the present electronic component transferring and mounting apparatus  380 , however, one of the first and second sucking positions is selected as the component sucking position, and one of the first and second mounting positions is selected as the component mounting position, so that the height positions of the heads  120  at the component sucking and mounting positions and the image taking position are changed according to the selected sucking and mounting positions. 
     An operation to pick up the electronic component  164  will be first described. Before the apparatus  380  is started, the multiplicity of cartridges  22  are arranged on the cartridge support block  20 , in the order in which the electronic components are mounted on the printed-circuit board  38 . 
     Usually, the successive component holder heads  120  successively receive a plurality of electronic components from one of the cartridges  22 , and the following successive component holder heads  120  receive a plurality of electronic components from the next cartridges  22 . Thus, the successive heads  120  receive the electronic components from the successive cartridges  22  as the cartridge support block  20  is intermittently fed to sequentially bring the successive cartridges  22  at the predetermined component supply position at which the predetermined number of electronic components is supplied from this cartridge  22  to the corresponding number of successive heads  120 . When the predetermined number of electronic components have been supplied from one cartridge  22 , the cartridge support block  20  is fed by a predetermined distance. At this time, the component sucking position is changed from one of the first and second sucking positions to the other position, for the reason explained below. 
     For example, the cartridges  22  are arranged on the cartridge support block  20  at a spacing pitch of 16 mm, and the cartridge support block  20  is intermittently fed at a feed pitch of 8 mm, as indicated in FIG.  22 A. When the cartridge  22  placed in the predetermined component supply position is changed from one cartridge to another, the cartridge support block  20  must be fed by a distance equal to the feed pitch multiplied by two. 
     If the operation to pick up the electronic component at the component sucking position is initiated only after the new cartridge  22  has been moved to the component supply position by two intermittent feeding movements of the cartridge support block  20  by a distance of 16 mm equal to the spacing pitch of the cartridges  22 , the required operation time corresponding to the angular spacing pitch of the heads  12  is inevitably increased with a result of lowering of the component mounting efficiency of the apparatus  380 . 
     If the electronic components are supplied from the cartridges  22  while the cartridge support block  20  is intermittently fed in a forward direction indicated by arrow P in FIG. 22 (which is opposite to the direction of rotation of the rotary plates  392  indicated by arrow Q), the rotary plates  392  are normally stopped at the first sucking position of FIG.  22 A. In this first sucking position, the cam follower roller  436  is in pressing contact with the upstream side surface  44  of the non-lead portion  434  of the cam groove  430   a  of the concave globoidal cam  410   a.  The position in the electronic component supply device  14  right below the head  120  stopped at the first sucking position will be referred to as “first component supply position”. 
     After the last one of the predetermined number of electronic components is picked up from the cartridge  22 C by the head  120 A, as indicated in FIG. 22A, the cartridge support block  20  is fed in the forward direction P by a distance of 8 mm, and the rotary plate  392  carrying the next head  120 B for picking up the first one of the predetermined number of electronic components from the next cartridge  22 B is stopped at the second sucking position as indicated in FIG.  22 B. As described above, the first and second sucking positions are spaced apart from each other by a distance of 8 mm in the X-axis direction, that is, in the feeding direction of the cartridge support block  20 . Consequently, the cartridge  22 B and the rotary plate  392 B are moved relative to each other by a distance of 16 mm. Therefore, the head  120 B does not have to wait for the completion of the 16 mm movement of the cartridge  22 B before the head  120 B starts picking up the first one of the predetermined number of electronic components from the cartridge  22 B. The position of the electronic component supply device  14  right below the head  120  stopped at the second sucking position will be referred to as “second component supply position”. 
     When the rotary plate  392  is stopped at the second sucking position, the cam follower roller  436  is moved by the stop position changing air cylinder  490  to be forced against the downstream side surface  442  of the cam groove  430   a,  as indicated by two-dot chain line in FIG.  15 . To this end, the stop position changing valve  530  corresponding to the rotary plate  392  to be stopped at the second sucking position is switched by the second valve switching device  536  to stop the rotary plate  392  at the second sucking position, when the above-indicated stop position changing valve  530  has reached the eleventh angular position shown in FIG.  18 . However, this switching operation of the valve  530  by the switching device  536  is effected only where the valve  530  has been held in the position corresponding to the first sucking position before the corresponding rotary plate  392  has reached the eleventh angular position. 
     The concave globoidal cams  410   a,    410   b  and the main shaft  388  are driven by the main drive servomotor  402  such that the angles of rotation of the cams  410  and main shaft  388  are proportional to the angle of rotation of the servomotor  402 . Accordingly, the positions of the twelve rotary plates  392  about the main shaft  388  can be determined on the basis of the amount of rotation of the servomotor  402 . Therefore, the rotary plate  392  which has reached the eleventh angular position can be identified. Further, the electronic component that is to be picked up by the head  120  carried by that rotary plate  392 , and the first or second sucking position that should be selected for that rotary plate  392  can be determined from the component sucking and mounting control programs. Accordingly, the stop position changing valves  530  of the rotary plates  392  can be suitably controlled when the rotary plates  392  have reached the eleventh angular position, so that the rotary plates  392  can be suitably stopped at the first or second sucking position depending upon the electronic component to be picked up from one of the cartridges  22 . 
     The air chamber of each valve switching single-acting air cylinder  544 ,  546  is kept open to the atmosphere except when the stop position changing valve  530  is switched. Therefore, the piston rods  548 ,  550  of these air cylinders  544 ,  546  are normally kept by the biasing springs in the positions to hold the rollers  540 ,  542  in their non-operating positions in which the spool of the valve  30  is not operated even when the valve  30  has reached the eleventh angular position. When the valve  530  is switched, one of the two valve switching air cylinders  544 ,  546  is actuated to move the corresponding one of the rollers  540 ,  542  to the operating position, whereby the spool of the valve  530  is moved for actuating the air cylinder  490 . The spool of the stop position changing valve  530  moved with the constant-velocity rotary disc  460  is brought into contact with one of the two rollers  540 ,  542  which is now placed in the operating position, so that the spool is pushed by contact with the circumferential surface of that roller  540 ,  542 . Thus, the valve  530  is switched to actuate the air cylinder  490  for biasing the rotary plate  392  so as to cause the cam follower roller  436  to be forced against the upstream side surface  440  or downstream side surface  442  of the cam groove  430   a  of the concave globoidal cam  410   a,  whereby the rotary plate  392  is stopped at the first or second sucking position. The roller  540 ,  542  which has been moved to the operating position is returned to the non-operating position if the component sucking position is not changed for the next rotary plate  392  which reaches the eleventh angular position. If the component sucking position is changed for the next rotary plate  392 , the roller  540 ,  542  is held in the operating position. 
     The valve switching air cylinders  544 ,  546  of the second valve switching device  536  at the eleventh angular position are provided to switch the stop position changing valves  530  while the rotary plates  392  are rotated about the main shaft  388 . Since the air cylinders  544 ,  546  are provided with the rollers  540 ,  542  for contact with the spools of the valves  530 , the valves  530  can be smoothly switched without disturbing the rotary movement of the rotary plates  392 . The same advantage is offered by the valve switching air cylinders  544 ,  546  of the first valve switching device  534  (for switching the stop position changing valves  530  at the fifth angular position) and the third valve switching device  538  (for switching the height position changing valves  532  at the tenth angular position). Since the rollers  540 ,  542  are normally held in their non-operating positions, these rollers do not move into the path of the spools and operate the valves  530 , except when valves  530  should be switched. In other words, neither the roller  540  nor the roller  542  lies on a movement path of the spools of the valves  530  when the air chambers of the air cylinders  544 ,  546  are held open to the atmosphere. 
     The switching of the stop position changing valve  530  is initiated at the eleventh angular position, which is spaced from the component sucking position in the direction opposite to the rotating direction of the rotary plates  392 , by an angular distance equal to the angular spacing pitch multiplied by two. Immediately after the valve  530  has been switched to the position corresponding to the second sucking position, for example, the corresponding rotary plate  392  is rotated with the constant-velocity rotary disc  460  with the engaging pin  468  still kept in the engaging recess  486 . In this condition, the rotary plate  392  is not pivoted by the stop position changing air cylinder  490 . After the pin  468  is disengaged from the recess  486 , the cam follower roller  436  is moved into the cam groove  430   a  of the concave globoidal cam  410   a  and is moved in pressing contact with the downstream side surface  442  of the cam groove  430   a  under the biasing force of the air cylinder  490 , and the rotary plate  392  is stopped at the second sucking position. Although the width of the non-lead portion  434  of the cam groove  430   a  is larger than the diameter of the roller  436 , the roller  436  will not be oscillated within the width of the non-lead portion  434 , since the roller  436  is forced against the downstream side surface  442  by the air cylinder  490  through the rotary plate  392 . Accordingly, the rotary plate  392  can be stopped at the second sucking position with high positioning accuracy, permitting the component holder head  120  to pick up the electronic component  164  with high reliability. This advantage is also enjoyed when the rotary plate  392  is stopped at the second sucking position, and at the first and second mounting positions, permitting the electronic component  164  to be mounted on the printed-circuit board  38  with high positioning accuracy. 
     After the first one of the predetermined number of the electronic components is picked up from the cartridge  22 B by the head  120 B, the cartridge support block  20  is further fed by a distance of 8 mm in the forward direction from the position of FIG. 22B, so that the head  120 C can pick up the second one of the predetermined number of the electronic components from the cartridge  22 B, at the first sucking position, as indicated in FIG.  22 C. 
     The electronic component supply device  14  is adapted to change the direction of feeding of the cartridge support block  20  when all of the predetermined number of the electronic components have been picked up from the most upstream cartridge  22 A as seen in the forward feeding direction P as indicated in FIG. 23A, namely, when all of the electronic components to be mounted on the printed-circuit board  38  have been picked up from the cartridges  22 . That is, the cartridge support block  20  is intermittently moved in a reverse direction as indicated by arrow R in FIG. 23B, namely, in the rotating direction Q of the rotary plates  392 , so that the electronic components are successively picked up by the heads  120 , to be mounted on the next printed-circuit board  38  in the order which is reversed to that in the case where the cartridge support block  20  is intermittently fed in the forward direction P. 
     In this case where the cartridge support block  20  is fed in the reverse direction R, the electronic components are first supplied from the cartridge  22 A, and the heads  120  are normally stopped at the second sucking position. Initially, the block  20  is fed in the reverse direction R by a distance of 8 mm, and the first one of the predetermined number of electronic components is picked up from the cartridge  22 A by the appropriate head  120 , for example, by the head  120 A, when the rotary plate  392  carries the head  120 A is stopped at the second sucking position, as shown in FIG.  23 B. 
     When all of the predetermined number of the electronic components have been picked up from the cartridge  22 A, the cartridge support block  20  is fed by a distance of 8 nm in the reverse direction R, and the rotary plate  392  (e.g., plate  392 E) carrying the head  120  (e.g.,  120 E) for picking up the first electronic component from the cartridge  22 B is stopped at the first sucking position, so that the electronic component is picked up by the head  120 E at the first sucking position. In this case, therefore, the cartridge  22 B is moved relative to the head  120 E by a distance of 16 mm. The stop position changing valve  530  corresponding to the rotary plate  392 E is switched by the valve switching device  536  to stop the rotary plate  120 E at the first sucking position, when the above-indicated valve  530  has reached the eleventh angular position. This switching operation of the valve  530  by the valve switching device  536  is effected only where the valve  530  has been held in the position corresponding to the second sucking position before the valve  530  has reached the eleventh angular position. After the first one of the predetermined number of the electronic components has been picked up from the cartridge  22 B by the head  120 E, the block  20  is further fed by a distance of 8 mm in the reverse direction R, as indicated in FIG. 23D, so that the head  120 F for picking up the second electronic component from the cartridge  22 B may pick up this electronic component at the second sucking position. 
     Thus, the stop position of the rotary plates  392  at the component sucking position is suitably changed from the first sucking position to the second sucking position or vice versa, when the cartridge  22  from which the electronic components are picked up by the head  120  is changed from one to another. This arrangement is effective to reduce the required distance of feeding movement of the cartridges  22 , permitting a decrease in the time required for the suction nozzles  158  to reach the component sucking position, without increasing the acceleration and deceleration values of the cartridge support block  20  upon changing of the cartridges  22 , whereby the components  164  can be picked up and mounted with improved efficiency. Further, since the cartridges  22  can be fed with reduced vibration, the electronic components  164  contained in the component holder tapes are prevented from taking a vertical posture rather than a horizontal posture in which the components are normally picked up. In addition, the present arrangement is effective to reduce the required output of the cartridge feed servomotor  26 , and the required strength of the floor or support structure on which the electronic component mounting system is installed. 
     After the electronic component is picked up by the head  120  at the component sucking position, the head  120  is rotated toward the component mounting position through the image taking position. When the electronic component passes the image taking position during movement of the rotary plate  392  with the constant-velocity rotary disc  460  at a predetermined constant velocity, an image of the electronic component is taken by the high-speed camera  562  with a stroboscope. Thus, the image of the electronic component is taken without having to stop the electronic component at the image taking position. The image taking position in the horizontal direction is fixed irrespective of the selection of the first or second sucking position, since the image is taken while the rotary plate  392  is rotated by the constant-velocity rotary disc  460  rather than the concave globoidal cam  410 . 
     As described above, the first and second mounting positions are selectively available as the component mounting position at which each rotary plate  392  is stopped for mounting the electronic component  164  on the printed-circuit board  38 . After one electronic component  164  is mounted, one of the first and second mounting positions is selected so that the selected mounting position reduces the required distance of movement of the printed-circuit board  38  in the X-axis direction for positioning the board  38  such that the location at which the next electronic component  164  is mounted is right below the component holder  120  carrying the next electronic component  164 . Based on the selected first or second mounting position, the distance of movement of the board  38  is determined. This arrangement makes it possible to reduce the required distance of movement of the printed-circuit board  38  in the X-axis direction, and shorten the required time of movement of the suction nozzle  158  corresponding to the angular spacing pitch of the heads  120 , without increasing the acceleration and deceleration values of the board  38 . Since the required acceleration and deceleration values of the board  38  can be reduced, otherwise possible dislocation of the electronic components  164  mounted on the board  38  can be avoided, and the required outputs of the X-axis and Y-axis drive servomotors  42 ,  48  can be reduced. 
     The kind of the electronic component  164  held by the head  120  of each rotary plate  392 , the location at which the electronic component is to be mounted, and the first or second sucking position selected for the rotary head  392  are known from the component mounting program. If the first or second mounting position that should be selected for mounting the electronic component  164  at the above-indicated location is the same as the selected first or second sucking position, the stop position changing valve  530  corresponding to the rotary plate  392  in question is not switched by the first valve switching device  534 . If the selected first or second mounting position is different from the selected first or second sucking position, the valve  530  is switched by the first valve switching device  534  when the valve  530  passes the fifth angular position of FIG. 18, so that the rotary plate  392  is stopped at the selected first or second mounting position. 
     If the first or second mounting position selected for a given rotary plate  392  is different from the first or second sucking position that should be selected for the same rotary plate  392  to pick up the next electronic component  164 , the stop position changing valve  530  is switched by the second valve switching device  536  when the valve  530  passes the eleventh angular position of FIG. 18, so that the rotary plate  392  can be stopped at the selected first or second sucking position. 
     There will next be described the manner of changing the height position of the component mounting heads  120  at the component sucking and mounting positions and the image taking position. 
     The present electronic component transferring and mounting apparatus  380  is provided with one height changing valve switching device in the form of the third valve switching device  538  which is disposed at the tenth angular position of FIG. 18 as, described above. In other words, the height position of each head  120  can be changed at one angular position during one full rotation of the rotary plate  392  about the main shaft  388 . Therefore, if the height position is changed to the higher level at the component mounting position, for example, this higher height position or level is maintained at the component sucking and image taking positions, or vice versa. 
     The height of the head  120  at the component mounting position is determined or selected in the following manner: 
     When the electronic component  164  is mounted on the printed-circuit board  38 , the head  120  carrying the electronic component  164  is moved with the vertical slide  124  with an initial downward movement and the following horizontal movement in the vicinity of the component mounting position, with the cam follower roller  502  being guided by the cam groove  506  of the stationary cylindrical cam  504 , as indicated in FIG.  17 . The height position of the head  120  during the horizontal movement should be determined so as to prevent an interference of the electronic component  164  held by the head  120  with the electronic components  164  already mounded on the printed-circuit board  38 . The possibility of this interference decreases with a decrease in the height of the electronic component  164  held by the head  120 , and therefore the height position of the head  120  can be reduced as the height of the electronic component  164  carried by the head decreases. If the height position of the head  120  at the component mounting position is fixed at a relatively low level corresponding to an electronic component  164  having a relatively small height, another electronic component  164  having a relatively large height may interfere with the already mounted electronic components  164 . If the height position of the head  120  is fixed at a relatively high level corresponding to an electronic component having a relatively large height, the vertical stroke of the head  120  required upon mounting of an electronic component having a relatively small height is undesirably increased. Therefore, the height position of the head  120  at the component mounting position is changed in two steps (high and low height positions) depending upon the height of the electronic component to be mounted. 
     The present electronic component mounting system is adapted to deal with the electronic components  164  whose heights range from 0 mm to 6 mm. The electronic components  164  whose heights are 2 mm or smaller will be referred to as “thin components”, while the electronic components  164  whose heights range from 2 mm to 6 mm will be referred to as “thick components”. Where a thick component  164  having the largest height of 6 mm has been already mounted on the board  38  and another thick component having the largest height of 6 mm is to be mounted on the board  38 , the distance between the end face of the suction tube  162  of the suction nozzle  158  and the upper surface of the board  38  should be at least 14 mm if an interference of the thick components is avoided by providing a clearance of at least 2 mm between the upper surface of the thick component already mounted on the board  38  and the lower surface of the thick component head by the head  120 . 
     Where a thin component  164  having the largest height of 2 mm and a highest possibility of interference with the already mounted thick component having the largest thickness of 6 mm is to be mounted on the board  38 , the distance between the end face of the suction tube  162  and the upper surface of the board  38  should be at least 10 mm if the interference is avoided by providing the same clearance of 2 mm between the thin and thick components. Hence, the required height difference of the head  120  in the above two cases is 4 mm. 
     In the light of the above analysis, the width or vertical dimension of the second wide portion  509  of the cam groove  506  is determined to be equal to a sum of the diameter of the cam follower roller  502  and 4 mm, so that the head  120  may be stopped at different heights at the component mounting position, namely, so that the distance between the suction tube  162  and the upper surface of the board  38  is 14 mm upon mounting of the thick components, and is 10 mm upon mounting of the thin components. 
     It is noted that the end faces or suction surfaces of the suction tubes  162  of all suction nozzles are located on a circle having a center at the axis of the support shaft  152 , and that the suction nozzles  158  for different kinds of the electronic components have the same vertical operating stroke for sucking and mounting operations. 
     At the component sucking position, the difference between the high and low height positions of the heads  120  is 6 mm. As previously explained, the cartridges  22  of the electronic component supply device  14  of the present mounting system are adapted such that the electronic components  164  are accommodated in component holder tapes whose component accommodating recesses are covered by a top covering tape. Each component holder tape has opposite support portions which define the width of the tape and which extend in the longitudinal direction of the tape. The component holder tape further has a component accommodating portion which is supported by the support portions and which has a multiplicity of component accommodating recesses whose bottom walls take the form of bosses extending downwards between the support portions. The electronic components  164  are accommodated in the respective recesses. This component holder tape is referred to as “embossed type” holder tape. The support portions of the component holder tape are supported by the body of the cartridge  22 . The component accommodating recesses have different depths depending upon the heights of the electronic components  164  accommodated therein. However, the support portion of the cartridge body supporting the support portions of the component holder tape has a constant height over the entire length of the tape, and the upper open ends of all the recesses have the same height, namely, the upper surfaces of the electronic components  164  accommodated in the recesses have the same height. 
     When the electronic component  164  is picked up by the head  120 , the suction tube  162  is first lowered into abutting contact with the upper surface of the electronic component  164 , and is then elevated with the electronic component held thereon under suction, so that the entirety of the electronic component is moved up out of the component accommodating recess. The level to which the electronic component is elevated by the head  120  at the component sucking position is determined so as to avoid an interference of the electronic component with any members surrounding the cartridge  22 , such as a cover member covering the component holder tape, which interference may occur when the rotary plate  392  is rotated toward the image taking position. The above-indicated level differs depending upon the specific cartridge  22  from which the electronic component  164  is picked up. The above level from the level of the upper surface of the component holder tape is 2 mm for the cartridges  22  accommodating the thin components (whose height is 0-2 mm), and is 4 mm for the cartridges  22  accommodating the thick components (whose height is 2-6 mm). When the thin component is picked up, therefore, the distance between the suction tube  162  and the upper surface of the component holder tape is a sum of the above-indicated 2 mm and the largest height of 2 mm of the thin components, that is, 4 mm. When the thick component is picked up, the above distance is a sum of the above-indicated 4 mm and the largest height of 6 mm of the thick components, that is, 10 mm. Thus, the difference between these distances is 6 mm. Accordingly, the width or vertical dimension of the first wide portion  508  of the cam groove  506  of the stationary cylindrical cam  504  is made equal to a sum of the diameter of the roller  502  and 6 mm, so that the head  120  may be stopped at different heights at the component sucking position, namely, so that the distance between the end face of the suction tube  162  and the upper surface of the component holder tape is 10 mm upon sucking of the thick components, and is 4 mm upon sucking of the thin components. 
     If the difference of 6 mm of the high and low height positions of the head  120  selected depending upon the height of the electronic component to be picked up is maintained at the image taking position, the distance between the lower surface of the electronic component and the high-speed camera  562  may vary by a maximum of 6 mm, and the image of the electronic component taken by the camera tends to be deteriorated. To avoid this drawback, the width or vertical dimension of the third wide portion  511  of the cam groove  506  of the cam  504  is determined to be equal to a sum of the diameter of the roller  502  and 3 mm, namely, to be smaller than that of the first wide portion  508  by 3 mm, so as to reduce the amount of variation of the distance between the electronic component and the high-speed camera  562  for thereby minimizing the deterioration of the image taken by the camera  562 . 
     The selection of the height position of each head  120  is effected between the component mounting and sucking positions, more specifically, when the height position changing valve  532  corresponding to the head  120  whose height should be changed passes the tenth angular position of FIG.  18 . The movement to the tenth angular position of the valve  532  of the head  120  whose height should be changed can be detected on the basis of the amount of operation of the main drive servomotor  402 . Further, the current high or low height position of the head  120  with respect to the rotary plate  392 , and the high or low height position of that head  120  that should be selected at the component sucking and mounting positions, can be determined according to the component sucking and mounting programs. Based on these information, the height position changing valve  532  of each rotary plate  392  is switched by the third valve switching device  538  at the tenth angular position to activate the corresponding air cylinder  90  to change the height position of the corresponding head  120 , if the currently selected height position is different from the height position that should be selected for the next electronic component. If the thin component having a height up to 2 mm is to be mounted, the high height position is selected. If the thick component having a height of 2 mm-6 mm is to be mounted, the low height position is selected. The rollers  540 ,  542  of the third valve switching device  536  are normally placed in their non-operating positions, and one of these rollers  540 ,  542  is moved to the operating position to switch the height changing valve  532 , as described above with respect to the first and second valve switching devices  534 ,  536 . 
     The switching of the height position changing valves  532  to select or change the height position of the heads  120  is effected while the cam follower roller  502  is moved through the horizontal first narrow portion  510  of the cam groove  506 , which is located upstream of the first wide portion  508  as viewed in the rotating direction of the rotary plate  392 , as indicated in FIG.  17 . In this arrangement, the roller  502  is prevented from being moved in the first narrow portion  510  in the vertical direction even when the direction in which the roller  502  is biased by the air cylinder  520  is changed upon switching of the valve  532 , whereby the cam follower roller  502  is smoothly moved through the first narrow portion  510  upon activation of the air cylinder  520  by the height position changing valve  532 . When the roller  502  is biased upwards against the upper side surface  526 , the head  120  is located at its high height position at the component sucking, image taking and component mounting positions. When the roller  502  is biased downwards against the lower side surface  528 , the head  120  is located at its low height position at the component sucking, image taking and component mounting positions. 
     Where the thick electronic component  164  is to be mounted, the head  120  at the component sucking position is placed in the high height position in which the distance between the suction tube  162  and the upper surface of the component holder tape is 10 mm, so that the stroke of the vertical movement of the head  120  by the elevating and lowering device  260  at the component sucking position is equal to 10 mm plus an expected possible vertical positioning error of the apparatus  380  due to some manufacturing error of associated elements or devices such as those of the electronic component supply device  14 . Where the thin electronic component  164  is to be mounted, the head  120  at the component sucking position is placed in the low height position in which the above-indicated distance is 4 mm, so that the vertical stroke of the head  120  is equal to 4 mm plus the expected possible vertical positioning error of the apparatus  380 . To obtain these vertical strokes of the head  120 , the position of the pivoting axis of the lever  276  of the head elevating and lowering device  260  is changed by the head stroke adjusting servomotor  296 , to change the vertical stroke of the vertically movable member  266 . 
     Where the head  120  located at the component sucking position is placed in its low height position for picking up the thin component  164 , the cam follower roller  502  is moved in the cam groove  506  while it is forced downwards against the lower side surface  528 . In this case, the head  120  is raised by 3 mm when the roller  502  is moved from the first wide portion  508  into the third wide portion  511 , as is apparent from FIG.  17 . That is, the height position of the head  120  when the roller  502  is located in the third wide portion  511  or when the head  120  passes the image taking position or the high-speed camera  562  is 3 mm higher than that when the roller  502  is located in the first wide portion  508  or when the head  120  is located at the component sucking position. Where the head  120  located at the component sucking position is placed in its high height position for picking up the thick component  164 , the roller  502  is moved in the cam groove  506  while it is forced upwards against the upper side surface  528 . In this case, the head  120  located at the image taking position has the same height as that at the component sucking position. Thus, the height difference of the head  120  between the high and low height positions at the image taking position is reduced to 3 mm from the height difference of 6 mm at the component sucking position, so that the tendency of deterioration of the image quality obtained by the high-speed camera  562  is reduced. 
     The cam follower roller  502  is then moved into the second wide portion  509  through the second narrow portion  514 , and the head  120  reaches the component mounting position. Where the high height position of the head  120  is selected for the component mounting position, the roller  502  is moved in rolling contact with the upper side surface  526 , so that the head  120  is stopped at the component mounting position such that the lower end face of the suction tube  162  is spaced 14 mm apart from the upper surface of the printed-circuit board  38 , as shown at right in FIG.  24 . Where the low height position of the head  120  is selected, the head  120  is stopped at the component mounting position such that the lower end face of the suction tube  162  is spaced 10 mm apart from the upper surface of the board  38 , as shown at left in FIG.  24 . Thus, there exists a height difference of 4 mm in these two cases. When the thick component  164  is mounted on the board  38 , the vertical stroke of the head  120  is equal to S 1  plus an expected possible vertical positioning error of the apparatus  380  due to some manufacturing error of associated elements or devices such as those of the device for positioning the board  38 . The value S 1  is the distance of 14 mm between the lower end face of the suction tube  162  and the upper surface of the board  38 , minus the height of the component  164 . When the thin component  164  is mounted on the board  38 , the vertical stroke of the head  120  is equal to S 2  plus the vertical positioning error. The value S 2  is the distance of 10 mm minus the height of the component  164 . To obtain these vertical strokes S 1 , S 2 , the position of the pivoting axis of the lever  276  of the head elevating and lowering device  260  is adjusted to change the vertical stroke of the vertically movable member  266 . 
     As described above, the height position of the head  120  at the component mounting position is increased for the thick component  164 , and is decreased for the thin component  164 . The high and low height positions for the thick and thin components are determined so as to avoid an interference of the electronic component held by the head  120 , with the electronic components already mounted on the printed-circuit board  38 . Further, the present arrangement is effective to reduce the required vertical stroke of the head  120  when the thin components are mounted. 
     It will be understood from the above explanation that the rotary plates  70  provide rotary members; the stop position changing air cylinders  490  provide circumferential-direction or rotating-direction pressing devices as part of a circumferential position selecting device; the circumferential-direction pressing devices cooperate with the cam-follower rollers  436  and the non-lead portions  434  of the cam grooves  430   a,    430   b  of the concave globoidal cams  410   a,    410   b  to provide the circumferential position selecting device; and the air cylinders  490 , the rollers  436 , and the non-lead portions  434  cooperate with one another to provide an on-path stop position selecting device as well. In addition, the height position changing air cylinders  520  provide axial-direction pressing devices as part of an axial position selecting device; the axial-direction pressing devices cooperate with the cam-follower rollers  502  and the first to third wide portions  508 ,  509 ,  511  of the cam groove  506  of the stationary cylindrical cam  504  to provide the axial position selecting device; the air cylinders  520 , the rollers  502 , and the wide portions  508 ,  509 ,  511  cooperate with one another to provide an intersecting-direction stop position selecting device or a path selecting device as well. Moreover, the frame  60 , the stationary shaft  66 , and the groups of bearings  74  cooperate with one another to provide a rotary member supporting device; and the concave globoidal cams  410   a,    410   b  as drive cams cooperate with the rollers  436  as cam followers to provide a rotary member rotating device. The cams  410   a,    410   b  and the rollers  436  cooperate with the constant-velocity rotary disc  460  and the engaging devices  464  to provide EC holder head moving and stopping devices. 
     Referring next to FIGS. 25-31 and  32 A- 32 D, there will be described an electronic component mounting system equipped with an electronic component transferring and mounting apparatus  600  constructed according to a third embodiment of the present invention. 
     The electronic component transferring and mounting apparatus  600  includes four concave globoidal cams  602   a - 602   d  similar to the concave globoidal cams  90   a - 90   d  provided in the transferring and mounting apparatus  12  of the first embodiment. In the present apparatus  600 , however, fifteen rotary plates  604  are rotated by the four concave globoidal cams  602   a - 602   d.    
     As shown in FIG. 25, a base plate  610  is fixed in a horizontal posture, to top surfaces of a plurality of columns  608  extending upright from a base (not shown). Upper and lower support members  612 ,  614  are fixed to the upper and lower surfaces of the base plate  610 . The upper and lower support members  612 ,  614  cooperate with the columns  608  and the base plate  610  to constitute a frame  616  of the present electronic component transferring and mounting apparatus  600 . The upper and lower support members  612 ,  614  support a support shaft in the form of a stationary shaft  618  at their upper and lower ends. The stationary shaft  618  extends in the vertical direction through an opening  620  formed through the base plate  610 . On two axial portions of the stationary shaft  618  which are spaced apart from each other in the vertical direction, there are disposed two arrays  624  of bearings  622 . Each array  624  consists of fifteen bearings  624 . Each of the fifteen rotary plates  604  is supported through a pair of support arms  626  by the corresponding pair of bearings  622  one of which belongs to the upper array  624  and the other of which belongs to the lower array  624 . Thus, the fifteen rotary plates  604  are supported by the bearings  622  rotatably about a common axis, which is the axis of the stationary shaft  618 . In FIG. 26, the fifteen rotary plates  604  are shown as being equi-angularly spaced from each other about the stationary shaft  618 . 
     The fifteen rotary plates  604  extend through the opening  620  formed through the base plate  610  such that the upper portion of each rotary plate  604  projects upwards from the base plate  610 . Each rotary plate  604  has a bracket  630  fixed to its upper end, and carries a large-diameter roller  632  and a small-diameter roller  634  attached to its upper end through the bracket  630  such that the rollers  632 ,  634  are rotatable about a horizontal axis which extends in a radial direction of the stationary shaft  318 . These rollers  632 ,  634  cooperate to constitute a cam follower  636 . A support shaft  638  is fixed to the bracket  630 . The large-diameter roller  632  is rotatably mounted through a bearing (not shown) on a fixed end portion of the support shaft  638  which is adjacent to the bracket  630 , while the small-diameter roller  634  having a smaller diameter than that of the large-diameter roller  632  is rotatably mounted through another bearing on a free end portion of the support shaft  638  which is remote from the bracket  630 . 
     As shown in FIG. 26, the four concave globoidal cams  602   a - 602   d  are disposed symmetrically with respect to the axis of the stationary shaft  618  such that lines of intersection of the outer circumferential surfaces of the concave globoidal cams  602  with a plane including axes of all of the concave globoidal cams  602  cooperate to define a substantially continuous circle which has a center at the axis of the stationary shaft  618 . The concave globoidal cams  602   a - 602   d  are coaxially mounted on respective rotary shafts  644   a - 644   d,  as shown in FIGS. 25-27, such that the cams  602  are rotated with the rotary shafts  644  and are not axially immovable relative to the rotary shafts  644 . The rotary shaft  644   b  of the cam  602   b  is not shown in these figures. The four rotary shafts  644   a - 644   d  are rotatably supported by respective pairs of brackets  646   a - 646   d  through respective bearings. 
     The concave globoidal cam  602   d  will be described in detail by reference to FIG. 27, by way of example. The rotary shaft  644   d  is inserted through a bore  652  formed through the concave globoidal cam  602   d,  and has an externally threaded portion  654  at one end thereof, a nut  656  screwed on the externally threaded portion  654 , and a flange  658  formed at the other end portion. The nut  656  and the flange  658  cooperate to hold the concave globoidal cam  602   d  at the opposite ends of the bore  652 , so that the concave globoidal cam  602   d  and the rotary shaft  644   d  are not axially movable relative to each other. A rotary motion of the rotary shaft  618  is transmitted to the concave globoidal cam  602   d  through a key  660 . 
     The rotary shaft  644   d  is rotatably supported at its opposite ends by the brackets  646   d  through respective bearings  662 ,  664 . The rotary shaft  644   d  is provided with two bevel gears  666 ,  668  fixed to its opposite end portions extending from the bearings  662 ,  664 . Portions of the brackets  646   d  and bearings  662 ,  664  are accommodated in tapered recesses  670 ,  672  formed in the opposite end portions of the concave globoidal cam  602   d.  For the bevel gears  666 ,  668  of the rotary shaft  644   d  to mesh with the adjacent bevel gears of the rotary shafts  644   a  and  644   c,  the bevel gears  666 ,  668  are required to be located outwardly of the bearings  662 ,  664  as viewed in the axial direction of the rotary shaft  644   d.  If the bearings  662 ,  664  were located axially outwardly of the bevel gear  666 ,  668 , it would be difficult to have the bevel gears  666 ,  668  mesh the adjacent bevel gears while avoiding an interference between the bearings  662 ,  664  and the bearings of the rotary shafts  644   a,    644   c.    
     Like the concave globoidal cam  602   d,  the rotary shaft  644   a  of the concave globoidal cam  602   a  has bevel gears  676 ,  678 . The bevel gear  678  on the side of the cam  206   d  meshes with the bevel gear  666 . Unlike the rotary shafts  644   a,    644   d,  each of the rotary shafts  644   b,    644   c  of the cams  602   b,    602   c  has a bevel gear  680 ,  682  at only one of its ends. The bevel gear  680  of the rotary shaft  644   b  meshes with the bevel gear  676  of the rotary shaft  644   a,  while the bevel gear  682  of the rotary shaft  644   c  meshes with the bevel gear  668  of the rotary shaft  644   d.    
     The rotary shaft  644   a  of the concave globoidal cam  602   a  has an extension which extends further from the bevel gear  678  and at which the rotary shaft  644   a  is rotatably supported by a bracket  690  through a bearing  692 . The bracket  690  is fixed to the base plate  610 . This extension of the rotary shaft  644   a  has a timing pulley  694  fixed thereto. A rotary motion of a drive source in the form of a main drive servomotor  696  is transmitted to the timing pulley  694  through a timing pulley  698  and a timing belt  700 , so that the concave globoidal cam  602   a  is rotated, whereby the four concave globoidal cams  602   a - 602   d  are contemporaneously rotated in synchronization with each other through meshing engagement of the bevel gears  666 ,  668 ,  676 ,  678 ,  680 ,  682  with each other. Thus, the rotary shaft  644   a  functions as an input shaft which receives the rotary motion of the servomotor  696  to rotate the four concave globoidal cams  602   a - 602   d.    
     The concave globoidal cams  602   a - 602   d  have respective cam grooves  710   a - 710   d.  Described more specifically, each of the cams  602  has two identically shaped cam grooves  710  formed in the outer circumferential surface. The cam grooves  710   a  of the cam  602   a  are illustrated in FIG. 28, by way of example. The two cam grooves  710   a  are formed with an angular phase difference of 180°, and receive cam followers  636  of the two adjacent rotary plates  604 . 
     The concave globoidal cam  602   a  is disposed at a position of the base plate  610  corresponding to the image taking position. Each cam groove  710   a  has an inclined portion  714  having a lead angle with respect to a plane perpendicular to the axis of the rotary shaft  744   a,  and a non-lead portion  716  which does not have such a lead angle and which is perpendicular to the axis of the rotary shaft  644   a.  When the rotary plate  604  is stopped, the inclined portion  714  functions to initially accelerate the rotary plate  604  (which has been rotated at a constant angular velocity) for rotating it over a relatively large angular distance per unit time, and then decelerating the rotary plate  604  to be stopped at the stop position (i.e., image taking position). When the rotary motion of the rotary plate  604  is resumed after a predetermined time of stopping at the stop position, the inclined portion  714  functions to initially accelerate the rotary plate  604 , and then decelerating it down to the constant angular velocity. The function of the inclined portion  714  is the same as that of the inclined portion of the concave globoidal cams  62 ,  410  of the first and second embodiments. The cam grooves  710   d,    710   b  of the cams  602   d,    602   b  disposed at respective positions of the base plate  610  corresponding to the component sucking and mounting positions have the same inclined and non-lead portions as those of the cam grooves  710   a.  The cam grooves  710   c  of the cam  602   c  have only an inclined portion, so that the rotary plate  604  rotated by this cam  602   c  is not stopped, and is rotated at the constant angular velocity. 
     The cam grooves  710   a,    710   b,    710   c  of the concave globoidal cams  602   a,    602   b,    602   c  are formed such that the appropriate three rotary plates  604  reach the image taking. and component mounting and sucking positions at respective different times, more specifically, such that the points of time at which the three rotary plates  604  reach those three stop positions differ from each other by a time length substantially equal to one third of the time interval at which the fifteen rotary plates  604  reach each of those three stop positions. This time interval is the required time of rotary movement of each rotary plate corresponding to the angular spacing pitch of the rotary plates  604 . In the present embodiment in which the three stop positions are provided, the above-indicated time difference of the points of time at which the three rotary plates  604  reach the three stop positions is determined to be equal to one third of the above-indicated time interval. The inclined portions  714  of the cam grooves  710   a,    710   b,    170 c have the same angle of inclination at their curved and straight sections, so that the three cams  602   a,    602   b,    602   c  cause the rotary plates  604  to have the same velocity for the constant velocity movement, and the same acceleration and the deceleration. However, the straight sections of the inclined portions  714  of the cam grooves grooves  610   a,    710   b,    710   b  have different lengths, so that the angular distance between the component sucking position and the image taking position is different from the distance between the image taking position and the component mounting position. This arrangement causes the appropriate three rotary plates  604  to reach the image taking and component sucking and mounting positions at times different from each other by a time substantially equal to one third of the time interval corresponding to the angular spacing pitch of the rotary plates  604 . The cam groove  710   c  of the cam  602   c  is formed such that the angle of inclination of the inclined portion  714  is the same as that of the straight sections of the inclined portions  714  of the cam grooves  710   a,    710   b,    710   d,  so that the rotary plates  604  are rotated by the cam  602   c  at the same velocity as that when the rotary plates are rotated by the cams  602   a,    602   b,    602   d.    
     Referring to FIG. 29, there is shown the cam groove  710   a  by way of example. Each of the cam grooves  710   a - 710   d  is a two-step groove having a wide portion  720  having a relatively large width, and a narrow portion  722  which is formed in the bottom surface of the wide portion  720  and which has a smaller width than the wide portion  720 . The large-diameter roller  632  and the small-diameter roller  634  are held in rolling engagement with the wide portion  710  and the narrow portion  722 , respectively. 
     As shown in FIG. 30, the width of the wide portion  720  is equal to a diameter  2 R of the large-diameter roller  632  plus δ 1 , while the width of the narrow portion  722  is equal to a diameter  2 r of the small-diameter roller  634  plus δ 1 . The wide and narrow portions  720 ,  722  have center lines which are offset from each other by a small distance. The wide and narrow portions  720 ,  722  are formed such that the center lines of the wide and narrow portions  720 ,  722  are offset from each other by a distance of δ 1 +δ 2 . That is, the center lines of the wide and narrow portions  720 ,  722  are shifted or offset in the opposite directions perpendicular to the direction of length of the cam groove  710 , by a distance of (δ 1 /2+δ 2 /2), from the state of FIG. 30 in which one-dot chain line indicates the path of movement of the axes of rotation of the rollers  632 ,  634  and in which there exist clearances of δ 1 /2 between the rollers  632 ,  634  and the opposite side surfaces of the wide and narrow portions  720 ,  722  of the cam groove  710 . The value δ 2  indicates an expected total amount of elastic deformation of the rollers  632 ,  634 , support shaft  638  and side surfaces of the cam grooves  710   a.  Therefore, the offset distance of the center lines of the wide and narrow portions  720 ,  722  is equal to (δ 1 +δ 2 ). The large-diameter roller  632  is held in pressing rolling contact with one of the opposite side surfaces of the wide portion  720 , while the small-diameter roller  634  is held in pressing rolling contact with the side surface of the narrow portion  722  which is opposite to the above-indicated one side surface of the wide portion  720 . 
     The end sections of the cam groove  710   a  of the cam  602   a  constitute a part of the inclined portion  714 , and are inclined with respect to a plane perpendicular to the axis of rotation of the cam  602   a.  One of the opposite side wall portions of these end sections which define the width of the cam groove  710   a  has a comparatively small wall thickness and a comparatively low strength. This fact is true for both of the wide and narrow portions  720 ,  722  of the cam groove  710   a.  The side wall portion having the small strength, particularly, of the narrow portion  722 , is removed by cutting, as indicated by solid black areas in FIG. 28, since these end sections have a small width and a low strength and may deform due to a load applied by a cutting tool during cutting of the groove  710  or a load applied from the large-diameter and small-diameter rollers  632 ,  634  during movements thereof in the wide and narrow portions  720 ,  722 . The above-indicated side wall portion of the end sections of the cam groove  710   a,  if left on the cam  602   a,  may disturb the rotation of the cams  602 . The removed side wall portions of the wide and narrow portions  720 ,  722  are spaced apart from each other in the circumferential direction of the cam  602   a.    
     As shown in FIG. 25, the concave globoidal cams  602   a - 602   d  are covered by covers  730 ,  732  fixed to the base plate  610 . The cover  730  covers an inner part of the assembly of the cams  602 , while the cover  732  covers inner, outer and upper parts of the assembly of the cams  602 . 
     As in the first and second embodiments, each of the fifteen rotary plates  604  has a vertical slide  740  vertically movably attached thereto. The vertical slide  740  carries a component holder head  742 , as schematically shown in FIG. 25 in which the suction nozzle and other elements are not shown. The selection of the suction nozzles and the operation to remove the angular positioning error of the electronic component are effected by using the same servomotor as a drive source, as in the previous embodiments. 
     When the rotary plate  604  is rotated, two cam follower rollers  744  fixed to the vertical slide  740  are moved in rolling engagement with a cam groove  748  formed in a stationary cylindrical cam  746 , whereby the vertical slide  740  is vertically moved. The two rollers  744  are rotatably fixed on the vertical slide  740  such that the two rollers  744  are spaced apart from each other in the axial direction of the stationary shaft  618 . The upper and lower rollers  744  are held in pressing rolling contact at its circumferential surface with the upper and lower side surfaces of the cam groove  748 , with a small gap between the upper and lower rollers  744 . This arrangement assures smooth vertical movements of the vertical slide  740  without vibration as the vertical slide  740  is rotated relative to the stationary cylindrical cam  746 . As in the first embodiment of FIGS. 1-12, head elevating and lowering devices similar to the device  260  are provided at positions corresponding to the component sucking and mounting positions, and a device for taking an image of the electronic component carried by the head  742  is provided at a position corresponding to the image taking position. 
     When the electronic component is mounted by the present electronic component transferring and mounting apparatus  600 , the main drive servomotor  696  is started to rotate the four concave globoidal cams  602   a - 602   d  simultaneously in synchronization with each other. As a result, the fifteen rotary plates  604  are rotated, and stopped at the component sucking position to pick up the electronic component, at the image taking position to obtain the image of the electronic component, and at the component mounting position to mount the electronic component on the printed-circuit board  38 . 
     As indicated in the time chart of FIG. 31, the points of time at which the appropriate three rotary plates  604  stop at the component sucking, image taking and component mounting positions differ from each other by one third of the required time of rotary movement of each rotary plate  604  corresponding to the angular spacing pitch of the rotary plates  604 . This arrangement is effective to smooth the load torque of the main drive servomotor  696 , or reduce a variation in the load torque. In the present third embodiment in which the fifteen rotary plates  604  are used, the angular spacing pitch is equal to T/15, where T represents the required time of one full rotation of the rotary plate  604  about the axis of the main shaft  316 . In the time chart, the time is taken along the abscissa, while the angle of rotation about the axis of the stationary shaft  618  is taken along the ordinate. In the present embodiment, thirteen stations are evenly spaced from each other about the axis of the stationary shaft  618 . The thirteen stations include three stations which correspond to the component sucking and mounting positions and the image taking position. The rotary plates  604  are not stopped at the other ten stations. 
     FIG. 32A indicates a variation in the load torque of a given concave globoidal cam  710  when the corresponding rotary plate  604  comes to a stop and resumes a rotary motion. The cam  710  is rotated a half turn (through 180°) during a rotary movement corresponding to the angular spacing pitch of the rotary plates  604 . During the half turn of the cam  710 , the corresponding rotary plate  604  is accelerated and decelerated and is held stopped at the stop position for a half of the predetermined stop time. During the next half turn, the rotary plate  604  is held stopped for the remainder of the stop time, and is accelerated and decelerated to move at a predetermined constant velocity. FIG. 32A shows the load torque of the cam  710  during one full turn thereof corresponding to the rotary motions of the two successive rotary plates  604  by the cam  710 , that is, corresponding to the angular spacing pitch of the rotary plates  604  multiplied by two. The displacement, speed and acceleration of the rotary plates  604  during this time period change as indicated in FIGS. 11A-1,  11 A- 2  and  11 A- 3 , respectively. 
     Prior to stopping of the rotary plate  604 , the acceleration of the rotary plate  604  from the predetermined constant velocity causes a positive load torque to act on the concave globoidal cam  602  since the rotary plate  604  tends to rotate at the constant velocity. The subsequent deceleration of the rotary plate  604  to be stopped at the stop position causes a negative load torque to act on the cam  602  since the rotary plate  604  tends to rotate at the constant velocity. Following the stopping of the rotary plate  604 , too, the acceleration and deceleration cause a positive and a negative torque to act on the cam, respectively. 
     Since the two cam grooves  710  are formed in each of the cams  602   a - 602   d,  one cam follower roller  636  comes into engagement with one of the two cam grooves  710  each time the cam  602  is rotated by a half turn. Namely, the cam follower rollers  636  of the two successive rotary plates  604  alternately come into engagement with one and the other of the two cam grooves  710  one after another, during one full turn of the cam  602 . As shown in FIG. 32B, therefore, the phases of two torque variation curves of the same pattern of the cam  602  corresponding to the two successive rotary plates  604  are offset from each other by 180°. In FIG. 32C, two broken lines indicate these two torque variation curves of the 180°-offset phases, while a solid line indicates a variation in the total torque of the cam  602 , which is a sum of the load torque values represented by the load torque variation curves indicated by the broken lines. As indicated in FIG. 32A, the negative and positive load torque values of the cam  602  before and after the stopping of the rotary plate  604  symmetrically change with respect to the abscissa. Accordingly, the torque values of the cam  602  which correspond to the successive rotary plates  604  and which are represented by the broken lines are partially offset by each other, as indicated by the solid line in FIG. 32C, since the rotary motions of the two successive rotary plates  604  have a phase difference of 180°. 
     As described above, the points of time at which the rotary plates  604  stop at the three stop positions during simultaneous rotations of the four cams  602   a - 602   d  by the common main drive servomotor  686  differ from each other by a time substantially equal to one third of the required time of movement of each rotary plate  604  corresponding to the angular spacing pitch of the rotary plates  604 . Therefore, the load torque variation curves of the three cams  602   a,    602   b,    602   d  are offset from each other by the time substantially equal to one third of the movement time corresponding to the angular spacing pitch, as indicated in FIG. 32D, whereby the positive and negative load torque values of the three cams  602   a,    602   b,    602   d  are almost completely offset by each other, so that the variation in the load torque of the main drive servomotor  696  is effectively reduced. 
     Since the center lines of the wide and narrow portions  720 ,  722  of each cam groove  710   a - 710   d  are offset from each other, the large-diameter and small-diameter rollers  632 ,  634  are held in pressing contact with the opposite side surfaces of the wide and narrow portions  720 ,  722 , as described above. Accordingly, the rollers  632 ,  634  can smoothly roll in the wide and narrow portions  720 ,  722  of the cam groove  710 , without causing vibration or noise of the rotary cam  604 . 
     The present apparatus  600  also permits smooth transition of the cam followers  636  from the cam groove of one of the cams  602  to the cam groove of the adjacent cam  602 , without vibration or shock. The opposite end sections of each cam groove  710  are part of the inclined portion  714 , which is inclined with respect to the axis of the cam  602 . A straight line connecting the points of contact of the large-diameter and small-diameter rollers  632 ,  634  with the side surfaces of the wide and narrow portions  620 ,  722  of the cam groove  710  is inclined with respect to a straight line connecting the open ends of the wide and narrow portions  720 ,  722  which are open in the end faces of the cam  602 . 
     Accordingly, the large-diameter and small-diameter rollers  632 ,  634  reach the ends of the wide and narrow portions  720 ,  722  at different times. Namely, when the cam follower  636  is moved from the cam groove of the two adjacent cams  602  into the cam groove of the other cam  602 , the large-diameter roller  632  first reaches the joint or interface between the cam grooves of the adjacent cams  602 . Since there exist some gaps between the adjacent cam grooves, in particular, due to cutting of the end sections of the wide portion  720 , the large-diameter roller  632  located at the joint is placed in a released state in which the roller  634  is not interposed between the opposite side surfaces of the wide portion  720 . At this time, that is, when the large-diameter roller  632  is located at the joint of the two cam grooves  710 , the small-diameter roller  634  has not reached the joint, and is still interposed between the opposite side surfaces of the narrow portion  722 . Since the small-diameter roller  634  is thus supported by the narrow portion  722 , the large-diameter roller  632  can smoothly transit from from one of the two adjacent cam grooves  710  into the other cam groove, without vibration. When the small-diameter roller  634  reaches the joint of the two cam grooves  710 , the large-diameter roller  632  has already been interposed between the opposite side surfaces of the wide portion  720  of the above-indicated other cam groove  710 , whereby the small-diameter roller  634  can smoothly transit into this other cam groove, without vibration. 
     It is noted, in particular, that the cam grooves  710   a - 710   d  are formed such that the rotary plates  604  are moved at the predetermined constant velocity while the cam followers  636  are located in the opposite end portions of the cams  602   a - 602   d.  This arrangement assures reduced vibration and shock of the cam followers  636  upon transition thereof between the adjacent cams  602 , than in the case where the rotary plates  604  are accelerated or decelerated while the cam followers  636  are located in the opposite end portions of the cams. 
     As described above, the large-diameter roller  632  and the small-diameter roller  634  are placed in the released state in which the rollers  632 ,  634  are not interposed between the opposite side surfaces of the wide and narrow portions  720 ,  722  of the cam grooves  710  of the adjacent cams  602  when the rollers  632 ,  634  are moved from one of the cam grooves to the other. Since one of the rollers  632 ,  634  is interposed between the opposite side surfaces of one of the adjacent cam grooves, the cam follower  636  can be smoothly moved between the adjacent cam grooves, without shock or vibration. 
     While each of the cam followers  636  engaging the cam grooves  710   a - 710   d  of the concave globoidal cams  602   a - 602   d  used in the third embodiment of this invention consists of the large-diameter roller  632  and the small-diameter roller  634 , the cam follower  636  may be a tapered roller  770  whose diameter continuously decreases in the direction toward the bottom of a cam groove  780  of a concave globoidal cam  778 . The tapered roller  770  is rotatably supported through a bearing (not shown) by a support shaft  776  fixed to a rotary plate  774 . In this case, the cam groove  780  is a generally V-shaped groove having a trapezoid shape in transverse cross section. 
     When the tapered roller  770  is moved from one of the cam grooves  780  of the adjacent cams  778  into the other cam grooves, the portion of the tapered roller  770  having a relatively large diameter first reaches the joint between the two cam grooves, but at this time the portion of the tapered roller  770  having a relatively small diameter does not reach the joint and is still interposed between the opposite side surfaces of the above-indicated one cam groove  780 , so that the large-diameter portion of the tapered roller  770  can be smoothly moved through the joint. When the small-diameter portion of the tapered roller  770  reaches the joint, the large-diameter portion has been already interposed between the opposite side surfaces of the above-indicated other cam groove  780 , the small-diameter portion can be smoothly moved into this other cam groove. 
     The cam follower may be a cylindrical roller having a constant diameter, as indicated at  790  in FIG.  34 . In this case, the radially outer portions of the opposite end faces of adjacent concave globoidal cams  792 ,  794  have tapered surfaces  796 ,  798  which are parallel and adjacent to each other in a plane including the axes of rotation of the cams  792 ,  794 . The generators of the tapered surfaces  796 ,  798  in the above-indicated plane are inclined in this plane, at selected points on the generators, with respect to a normal line perpendicular to the common axis about which the rotary plates are rotated. The roller  790  has an axis of rotation which is perpendicular to the above-indicated common axis and parallel to the above-indicated normal line. When the tapered roller  790  is moved through the joint of the adjacent cam grooves, the circumferential surface portions of different axial portions of the roller  790  reach the joint at different times. When a given portion of the roller  790  reaches the joint, the other portion is still interposed between the opposite side surfaces of one of the adjacent cam grooves, so that the roller  790  is not placed in a released state at the joint of the two cam grooves, whereby the roller  790  can be smoothly moved into the other cam groove. 
     Movable members may be moved by a combination of a concave globoidal cam  810  and a cylindrical cam  812 , as shown in FIG.  35 . In this case, too, suitable means may be provided for preventing a cam follower from being placed in the released state at the joint of adjacent cam grooves. The movable members are rotated by rotation of the concave globoidal cam  810 , about an axis which is perpendicular to a plane including axes of rotation P, Q of the cams  810  and  812  and which passes a center of a circular arc which is defined by a line of intersection between the above-indicated plane and the circumferential surface of the globoidal cam  810 . Then, the movable members are linearly moved by rotation of the cylindrical cam  812  in a direction parallel to the axis of rotation Q of the cylindrical cam  812 . In this arrangement, such means as provided in the illustrated embodiments may be used for preventing the releasing of the cam follower at the joint of the adjacent cam grooves. For instance, the means may include the provision of two-step cam grooves having a wide portion and a narrow portion, and the provision of cam followers each including a large-diameter portion and a small-diameter portion. The combination of cams need not be a combination of a single concave globoidal cam and a single cylindrical cam. Namely, the combination may consist of at least one concave globoidal cam and at least one cylindrical cam, which cooperate to move movable members along a desired path such as a path like a track defined by two opposite parallel straight lines and two circular arcs connecting the ends of the parallel straight lines. 
     In the second embodiment shown in FIGS. 13 to  24 , the two selectable component mounting positions where the component holder heads  120  can mount electronic components (“ECs”) on a printed circuit board (“PCB”)  38  is determined such that the holder heads  120  can freely mount the ECs on the PCB  38 , without being interfered with by the ECs which have already been mounted on the PCB  38 , irrespective of whether the ECs mounted on the PCB  38  may be large or small. However, as shown in FIG. 37, the two selectable component mounting positions may be determined such that at the higher one of the two mounting positions the holder heads  120  can prevent interference with large ECs  900  mounted on the PCB  38  and at the lower mounting position the holder heads  120  to can prevent interference with small ECs  902  mounted on the PCB  38 . For example, in the case where the PCB  38  has a large-EC mounting area for mounting a group of large ECs  900  and a small-EC mounting area for mounting a group of small ECs  902 , the two component mounting positions each of which corresponds to the lower ends of respective component suction nozzles  904  of the holder heads  120  may be determined at different height positions, P 1  and P 2 , respectively. 
     The “large” ECs  900  may comprise ECs  900  in different larger sizes. In this case, a height, h 1 , shown in FIG. 37 indicates the height of the largest-size EC  900 . The height position P 1  of the higher mounting position (i.e., the stroke of vertical movement of each suction nozzle  904 ) may be adjusted depending upon the height h 1 . Similarly, the “small” ECs  902  may comprise ECs  902  in different smaller sizes. In this case, a height, h 2 , shown in FIG. 37 indicates the height of the largest EC  902  of the small-size ECs  902 . The height position P 2  of the lower mounting position may be adjusted depending upon the height h 2 . It is preferred to provide a clearance, L 1 , between the ECs  900 ,  902  mounted on the PCB  38  and the EC  900 ,  902  held by the suction nozzle  904 . 
     The size-different ECs which belong to each of the above-indicated large-size and small-size EC groups can be mounted by using different sorts of suction nozzles  904  having different axial lengths. Since the respective distances between the axis line of the support shaft  152  and the respective free end faces (i.e., suction openings) of the different suction nozzles  904  differ from each other, each of the size-different ECs can be mounted by using a corresponding one of the suction nozzles  904  which has an appropriate distance between the axis line of the shaft  152  and the free end face of the one nozzle  904 . 
     In addition, as shown in FIG. 38, the height position of the component suction nozzles  904  may be determined at a lower position, P 3 , for mounting the large ECs  900 , and may be determined at a higher position, P 4 , for mounting the small ECs  902 . For example, in the case where a group of large ECs  900  are mounted before a group of small ECs  902  such that the path along which the large ECs  900  are sequentially mounted on the PCB  38  does not intersect itself, there is no possibility that the EC held by each suction nozzle  904  be interfered with by the ECs mounted on the PCB  38 . Therefore, the height position of the component suction nozzles  904  may be determined at the lower position P 3 , for mounting the large ECs  900 . When the small ECs  902  are mounted after the large ECs  900 , the component suction nozzles  904  are positioned at the higher position P 4 , for preventing interference with the large ECs mounted on the PCB  38 . In this case, the mounting apparatus  380  can freely mount the small ECs  902  on the PCB  38 . 
     In the case where the path along which the large and/or small ECs  900 ,  902  are mounted on the PCB  38  is so determined not to intersect itself, the height position of the component suction nozzles  904  may be determined, as shown in FIG. 39, at a higher position, P 5 , equal to the sum of the above-defined height h 1  and clearance L 1 , for mounting the large ECs  900 , and may be determined at a lower position, P 6 , equal to the sum of the above-defined height h 2  and clearance L 1 , for mounting the small ECs  902 . 
     In the case where the component holder heads  120  are stopped at the higher one of the two selectable component sucking positions and at the lower one of the two selectable component mounting positions, or vice versa, the height position of each holder head  120  may be changed while the holder head  120  is moved from the sucking position to the mounting position. For example, the cam groove  506  of the stationary cylindrical cam  504  may include a horizontal narrow (not wide) portion between the image taking position and the component mounting position, and a height-position-changing-valve switching device may be provided in association with the narrow portion for pressing the cam-follower roller  502  connected to each head holder  120 , against the other side surface of the wide portion of the cam groove  506 . This is also true for the case where the holder heads  120  are stopped at the higher one of the two selectable component sucking or mounting positions and at the lower one of the two selectable image taking positions, or vice versa. 
     In the second embodiment shown in FIGS. 13 to  24 , the component holder heads  120  are moved up and down by the stationary cylindrical cam  504  having the cam groove  506 , and the height position of the holder heads  120  is changed in the two steps by selectively pressing the rollers  502  against each of the upper and lower side surfaces  526 ,  528  of the cam groove  506 . However, as shown in FIG. 40, the apparatus  380  may employ, in place of the cam  504 , a stationary cylindrical cam  910  having a cam ridge or rib  912  with which a pair of rollers  916  as cam followers which are provided on an elevator plate  914  of each rotary plate  70  are engageable. 
     The cam rib  912  includes a thick portion  918 , and three thin portions  920  (only one  920  is indicated at broken line in FIG.  40 ). The thickness or dimension of the thick portion  918  in a direction (hereinafter, referred to as the “axial direction”) parallel to the axis line about which the rotary plates  70  supporting the component holder heads  120  are rotatable, is slightly greater than the distance between the paired rollers  916 . The three thin portions  920  correspond to the component sucking, image taking, and component mounting positions, respectively, and the thickness of each thin portion  920  in the axial direction is smaller than the distance between the paired rollers  916 , so that the rollers  916  are permitted to be moved in the direction of the thickness of the think portion  920 . In addition, the cam rib  912  includes at least one portion (not shown) whose position in the axial direction continuously changes, so that the two selectable component sucking positions are higher than the two selectable component mounting positions. The elevator plate  914  is moved up and down by one or more height position changing air cylinders (not shown) provided between the elevator plate  914  and the corresponding rotary plate  70 , so that a selected one of the two rollers  616  is pressed against a corresponding one of upper and lower side surfaces  922 ,  924  of the cam rib  912 . 
     In addition, each of the concave globoidal cams  410   a,    410   b  may have a cam rib including a thick and a thin portion and each of the rotary plates  70  may have a pair of rollers as cam followers which are engageable with the cam rib. In this case, the thin portion has a thickness which permits the pair of rollers to be moved in the direction of rotation of the rotary plates  70  about the stationary shaft  66 . 
     The principle of the present invention applied to the apparatus  380  shown in FIGS. 13-24 may be applied, as shown in FIG. 41, to an EC mounting system including a table  930  which is intermittently rotatable about an axis line and a plurality of EC holder heads  932  which are supported by the rotatable table  930  such that the holder heads  932  are equiangularly spaced from each other in a rotating direction of the table  930 . That is, the stop position of the holder heads  932  at a stop station is changed selectively in two steps in the rotating direction. The table  930  has the same number of part-annular guide grooves  934  as that of the holder heads  932 , such that the guide grooves  934  have a common center at the stationary shaft  66  and are equiangularly spaced from one another. Each holder head  932  is supported by a rotary member  936  to which a pair of guide rollers  938  are fixed such that each of the guide rollers  938  is rotatable about an axis line parallel to the axis line about which the table  930  is rotatable. The guide rollers  938  are rotatably fit in the corresponding guide groove  934 . Thus, the rotary member  936  is guided by the guide groove  934  via the rollers  938 . A piston rod  942  of a rotating-direction position changing air cylinder device  940  which is rotatably attached to the rotatable table  930  is rotatably connected to the rotary member  936 . When the piston rod  942  is projected from, and retracted into, a housing of the cylinder device  940 , the rotary member  936  is moved along a circular arc having a center at the rotation axis line of the table  930 , so that the stop position of the holder head  932  is changed between a first position corresponding to one of opposite ends of the guide groove  934  with which one of the guide rollers  936  is engaged, and a second position corresponding to the other end of the guide groove  934  with which the other roller  936  is engaged. 
     In addition, the principle of the present invention applied to the apparatus  380  shown in FIGS. 13-24 may be applied, as shown in FIG. 42, to an EC mounting system wherein a component holder head (not shown) is moved along a straight movement path for sucking and mounting an EC. That is, the holder head can take a selected one of two positions which are spaced from each other in a direction perpendicular to the straight movement path, e.g., a vertical direction. A nut  954  is fixed to a movable member  950  on which the component holder head is mounted, and is screwed on a ball screw  956  which is coupled to a servomotor (not shown). When the screw  956  is driven or rotated by the servomotor, the movable member  950  is linearly moved by being guided by a pair of guide rails  958 . 
     A guide rail  962  is provided on a vertical side surface  960  of the movable member  952 , and an elevator member  964  is fit on the guide rail  962  such that the elevator member  964  is movable up and down. The component holder head (not shown) is mounted on the elevator member  964  such that the holder head is movable up and down and is rotatable about its axis line, and is moved up and down by a holder-head elevating and lowering device (not shown) and rotated by a holder-head rotating device (not shown). The elevator member  964  has an arm which extends from one side of the movable member  950  to the opposite side thereof over the upper surface thereof. A roller  966  as a cam follower is attached to a free end of the arm of the elevator member  964 , such that the roller  966  is rotatable about a horizontal axis line perpendicular to the direction of movement of the movable member  950 . The roller  966  is rotatably fit in a cam groove  970  of a stationary cam  968 . The stationary cam  968  extends along the movement path of the movable member  950 , and the cam groove  970  includes at least one narrow portion  972 , and three wide portions  974  (only one  974  is indicated at broken line) corresponding to an EC sucking position, an image taking position, and an EC mounting position, respectively. The narrow portion  972  has a small width in which the roller  966  is fit with only small clearances left therebetween. Each wide portion  974  has a great width which permits the roller  966  to be moved in the direction of the width. The cam groove  970  additionally includes, between the narrow and wide portions  972 ,  974 , a portion whose width continuously changes. 
     A piston rod  980  of a normal-direction stop position changing air cylinder device  978  provided on the movable member  950  is connected to the horizontal arm of the elevator member  964 . When the piston rod  980  is extended from, and retracted into, a housing of the cylinder device  978 , the cam-follower roller  966  is pressed against a selected one of an upper side surface  982  and a lower side surface  984  of the cam groove  970 , so that the elevator member  964  is moved up and down. In this way, the height position of each of the component holder heads  120  can be changed in two steps at the component sucking position, the image taking position, and the component mounting position. 
     The cam groove  970  may include at least one portion whose height position continuously changes, so that the height position of each holder head  120  at one or two of the component sucking and mounting positions and the image taking position may differ from those or that of the same  120  at the other positions or position. 
     While the presently preferred embodiments of the present invention have been described above in detail, it is to be understood that the invention is not limited to the details of the illustrated embodiments, but may be otherwise embodied. 
     In the illustrated embodiments, the component holder head  120 ,  742  provided on the vertical slide  124 ,  740 ,  914  are vertically moved together with the vertical slide  124 ,  740 ,  914 , by the vertically movable member  266  provided in the stationary cylindrical cam  128 ,  504 ,  746 , at each of the component sucking and mounting positions (i.e., stop positions). However, members for vertically moving the heads  120 ,  740  need not be part of the stationary cam. For instance, the component holder head may be fixed to the vertical slide while the vertical slide is vertically moved by vertically moved by the stationary cam, and may be vertically moved relative to the vertical slide at each of the component sucking and mounting positions where the locking of the holder head to the vertical slide is released. Even in the latter case, the apparatus  380  as the second embodiment shown in FIGS. 13-24 may maintain the technical feature that the height position of the heads  120  at each of the component sucking, image taking and component mounting positions is selectable from two or more height positions. 
     Like the apparatus  380  of FIGS. 13-24, the apparatus  12  of FIGS. 1-12 and the apparatus  600  of FIGS. 25-32 may be adapted such that each of the component sucking and mounting positions is selectable from two or more circumferential positions. In addition, or alternatively, the apparatus  12 ,  600  may be adapted such that the height position of the heads  120 ,  742  at each of the component sucking, image taking and component mounting positions is selectable from two or more height positions. 
     Further, the apparatus  12  of FIGS. 1-12 and the apparatus  600  of FIGS. 25-32 may be adapted such that the images of the electronic components  164  held by the suction nozzles  158  under suction are obtained without stopping the rotary movements of the rotary plates  70 ,  604 , as in the apparatus  380  of FIGS. 13-24. In this case, a high-speed camera with a stroboscope or a device including for example a line sensor capable of obtaining images of the electronic components  164  in motion is provided at the image taking position, and the cam groove  92   d,    710   a  of the concave globoidal cams  90   d,    602   a  disposed at the image taking position are formed so that the rotary plates  70 ,  604  are rotated continuously, without stopping at the image taking position. The height position of the head  120  while the head  120  passes the image taking position may be changed in at least two steps, or may not be changed (may be held constant). 
     In the second embodiment of FIGS. 13-24, the stop position (i.e., circumferential position) changing air cylinders  490  and the height position changing air cylinders  520  are controlled by mechanically switching the stop position (i.e., circumferential position) changing valves  530  and the height position changing valves  532 . However, these valves  530 ,  532  may be replaced by solenoid-operated valves, which may be switched on the basis of output signals of sensors which detect the passing of each rotary plate  392  through predetermined positions. Alternatively, the position of each rotary plate  392  may be determined on the basis of the amount of rotation of the main drive servomotor  402  or the angular position of the main shaft  388 , and the solenoid-operated valves may be switched when the determined position of the rotary plate  392  coincides with the predetermined positions, more precisely, at the positions where the stop positions (i.e., circumferential positions) at the component sucking and component mounting positions should be changed, and at the positions where the height positions at the component sucking, image taking, and component mounting positions should be changed. 
     Further, the air cylinders  544 ,  546  used for the first, second and third valve switching devices  534 ,  536 ,  538  may be replaced by solenoids adapted to move the rollers  540 ,  542  for switching the stop position (i.e., circumferential position) changing valves  530  and the height position changing valves  532 . 
     In the second embodiment of FIGS. 13-24, the high height positions of the heads  120  are selected at the component sucking, image taking and component mounting positions, when the electronic component  164  has a relatively large height, and vice versa. However, the high and low height positions of the heads  120  may be selected when the electronic components  164  have relatively small and large heights, respectively. In this case, for example, all of the relatively thick electronic components are first mounted on the printed-circuit board in a predetermined order, from one end of the board in the X-axis direction, and then the relatively thin electronic components are mounted at random. The mounting of the relatively thick electronic components does not cause any interference between the components to be mounted and the components already mounted on the board, since the relatively thick components are all mounted first in the predetermined order in one direction. Therefore, the height position of the heads  120  can be reduced during mounting of the relatively thick components. During the mounting of the relatively thin components, on the other hand, the height position of the heads  120  is made high enough to avoid an interference between the components to be mounted and the already mounted components. The height position of the heads  120  may be made comparatively low for all the electronic components, if the components are all mounted in the predetermined order from one end of the printed-circuit board in the predetermined one direction. 
     In the second embodiment of FIGS. 13-24, only one switching device  538  for switching the height position changing valves  532  is provided at the tenth angular position of FIG. 18 corresponding to the first narrow portion  510  between the component mounting and sucking positions. In this arrangement, if the high height position of the heads  120  is selected for the component sucking position, for example, the high height position is also selected for the image taking and component mounting positions. However, another switching device for switching the height position changing valves  532  may be provided at a suitable position, for instance between the component sucking and image taking positions. In this case, the cam groove  506  of the stationary cylindrical cam  504  has a third narrow portion between the first and third wide portions  508 ,  509 . According to this arrangement, the low height position of the heads  120  may be selected at the image taking and component mounting positions even if the high height position is selected at the component sucking position. 
     The illustrated embodiments are adapted such that the twelve or fifteen rotary plates  70 ,  392 ,  604  are stopped at three or two stop positions. However, the number of the rotary members and the number of the stop positions (and the kinds of the stop positions) may be suitably determined. 
     In the illustrated embodiments, the rotary members in the form of the rotary plates are equi-angularly spaced from each other at a predetermined angular interval about the axis of the stationary shaft  66  or main shaft  388 ,  618 . However, it is possible that some of the rotary members are spaced from each other at a first angular interval, while the other rotary members are spaced from each other at a second angular interval different from the first angular interval. 
     The rotary plates  70 ,  392 ,  604  have a relatively smaller thickness at their inner end portions as viewed in radial directions of the stationary or main shaft  66 ,  388 ,  618 , in order to avoid an interference between the adjacent rotary plates. However, the interference may be avoided by increasing the radial distance between the inner ends of the rotary plates and the axis of the stationary or main shaft. 
     The illustrated electronic component transferring and mounting apparatus and electronic component mounting system are adapted to transfer and mount various electronic components whose heights are not larger than 6 mm, the principle of the present invention is equally applicable to such apparatus and system capable of transferring and mounting electronic components whose heights are greater than 6 mm. 
     In the embodiments shown in FIGS. 13-24 and  36  to  42 , the various electronic components are grouped into the two groups, i.e., the small components whose heights are not greater than 2 mm and the large components whose heights are greater than 2 mm and not greater than 6 mm. However, the criterion of 2 mm may be replaced by 3 mm or any other appropriate height. The component holder head positioning apparatus in accordance with the present invention is applicable to various component transferring and mounting apparatuses or various component mounting systems. 
     The component holder heads  120 ,  742  in the illustrated embodiments may be provided with a positioning device for locking the nozzle holder  154  after one of the six suction nozzles  158  is placed in its operating position. In this case, the positioning device may include a positioning pin engageable with a selected one of six positioning holes corresponding to the six suction nozzles  158 . The positioning holes may be tapered holes which are formed in one of the nozzle holder  154  and the stationary support shaft  152  rotatably supporting the nozzle holder  154 , such that the tapered holes are equi-angularly spaced from each other in the circumferential direction of the nozzle holder  154  or support shaft  152 . In this instance, the positioning pin is provided on the other of the nozzle holder  154  and the stationary support shaft  152 , and is biased by suitable biasing means toward a position for engagement with the selected positioning hole. When one of the six suction nozzles  158  is selected, the positioning pin is disengaged from the positioning hole against the biasing force of the biasing means, to permit the rotation of the nozzle holder  154  relative to the stationary support shaft  152 . A plurality of positioning pins may be provided so that a selected one of these pins is engageable with a single positioning hole. 
     In the third embodiment of FIGS. 25-32, the center lines of the wide and narrow portions  720 ,  722  of the cam groove  710  are offset from each other. This offset arrangement is not essential. Namely, the center lines of these wide and narrow portions  720 ,  722  may be aligned with each other. 
     Where the rotary plates are rotated by only the concave globoidal cams as in the first and third embodiments of FIGS. 1-12 and FIGS. 25-32, the number of the concave globoidal cams is not limited to four. That is, three or smaller or five or more concave globoidal cams may be used. 
     In the illustrated embodiments, the application of the vacuum pressure to the suction nozzles  158  is electrically controlled by switching devices including solenoid-operated directional control valves. However, the application of the vacuum pressure may be mechanically controlled. For instance, each of the members which are movable with the component suction nozzles, such as the rotary members (rotary plates), is provided with a component-suction-nozzle-related switching device having a switching member which is movable between a vacuum supply position in which the switching device permits vacuum to be supplied to the suction nozzle and a vacuum cut position in which the switching device does not. Additionally, a component-sucking-position-related switching device which moves the switching member from the vacuum cut position to the vacuum supply position and thereby switches the component-suction-nozzle-related switching device is provided at the component sucking position, and a component-mounting-position-related switching device which moves, after the mounting of the electronic component on the printed circuit board, the switching member from the vacuum supply position to the vacuum cut position and thereby switches the component-suction-nozzle-related switching device is provided at the component mounting position. 
     In the second embodiment of FIGS. 24-32, switching devices may be provided on the constant-velocity rotary disc  460 . The switching member of each switching device is moved to the vacuum supply position by a suitable actuator device which is disposed at a position corresponding to the component sucking position. The actuator device is activated to move the switching member to the vacuum supply position, in synchronization with the downward movement of the component holder head by the head elevating and lowering device  260 . Another actuator device is disposed at a position corresponding to the component mounting position. 
     This actuator device is actuated to move the switching member to the vacuum cut position, in synchronization with the downward movement of the head by the head elevating and lower device  260 . The switching member is arranged to be held in the vacuum supply or cut position once this position is established. 
     Where the application of the vacuum pressure to the suction nozzles is mechanically controlled, too, the vertical stroke of the vertically movable member of each head elevating and lowering device  260  is reduced to the minimum value in the event of a failure of the suction nozzles to normally pick up the electronic component. This arrangement prevents a contact of the electronic component or suction tube with the printed-circuit board, and a movement of the switching member of the switching device to the vacuum cut position by the actuator device disposed at the position corresponding to the component mounting position, whereby the electronic component erroneously picked up by the suction nozzle is not mounted on the printed-circuit board. A further actuator device is provided at a position corresponding to the component discarding area, so that the switching member of the switching device is moved to the vacuum cut position to release the electronic component from the suction nozzle when the switching device has reached the position of the actuator device in the component discarding area. The switching member of the switching device is also moved to the vacuum cut position when the component holder head fails to pick up the electronic component. 
     The illustrated embodiments are adapted such that the downward movement of the component holder head is initiated before the corresponding rotary plate is stopped at the component sucking and mounting positions, and such that the rotary movement of the rotary plate is initiated before the head reaches its upper stroke end after the sucking or mounting of the electronic component. Thus, the rotary movement and the vertical movement of the head take place contemporaneously, whereby the required time of movement of each head corresponding to the angular spacing pitch of the heads can be reduced, with a result of improving the component mounting efficiency. However, the contemporaneous rotary and vertical movements of the head is not essential. Namely, the head may be moved down only after the rotary plate has been brought to the component sucking and mounting positions, and the rotary plate may be rotated only after the head has been moved to its upper stroke end. 
     In the third embodiment of FIGS. 25-32, each rotary plate is stopped at three stop positions, and the points of time at which the three rotary plates corresponding to the three stop positions are stopped at these three stop positions differ from each other by a time length equal to one third of the required time of movement of the rotary plates corresponding to the angular spacing pitch of the rotary plates, so that the positive and negative values of the load torque of the three concave globoidal cams are at least partially offset by each other. In the third embodiment in which each rotary plate is stopped at the three position, the above-indicated time length (time difference of the above-indicated points of time) is determined to be equal to one third of the required time of rotary movement of each rotary plate corresponding to the angular spacing pitch of the rotary plates. However, the above-indicated time length (time difference) may be equal to one fourth of the required time of rotary movement corresponding to the angular spacing pitch of the rotary plates, if each rotary plate is stopped at four stop position. Namely, the time difference decreases with an increase in the number of the stop positions. As the time difference decreases, the effect of offset of the positive and negative load torque values of the concave globoidal cams or main drive servomotor is increased. 
     In the first embodiment of FIGS. 1-12, the rotary plates  70  are supported by the stationary shaft  66  which is not rotated. However, the stationary shaft  66  may be replaced by a rotary shaft which is rotated with the rotary plates  70 . In this case, the rotary shaft is rotated by the main drive servomotor provided for rotating the concave globoidal cams. The rotary shaft has a vacuum passage which is connected to the vacuum source through a conduit and a rotary joint, and is connected through hoses to vacuum switching devices corresponding to the rotary plates. 
     In the first embodiment, the rotary valve provided on the stationary shaft  66  for connecting the vacuum passage in the stationary shaft  66  to the switching devices  178  is driven by an exclusive drive source. However, the rotary valve may be rotated with one of the rotary plates. 
     In the illustrated embodiments, the vacuum switching devices for applying the vacuum pressure to the suction nozzles of the component holder heads are rotated with the component holder heads. However, the switching devices may be provided on a stationary member. The switching devices are connected to the suction nozzles of the respective component holder heads through suitable passages, rotary valve and hoses, as described above. 
     The component holder positioning apparatus embodied as the apparatus  380  shown in FIGS. 13-24,  36  or the apparatuses shown in FIGS. 37-40 may be employed by an electronic component mounting system including an electronic component transferring apparatus as disclosed in the Japanese patent publications JP-A-6-77693 and JP-A-6-45787. In the electronic component transferring apparatus of this system, a plurality of component holder heads are disposed at a predetermined angular interval on a rotary table which is continuously rotated at a constant velocity, and the heads that should be stopped at the predetermined stop positions such as the component sucking and mounting positions are rotated about the axis of the rotary table at the same velocity in the direction opposite to the direction of rotation of the rotary table, so that those heads are virtually stopped at the stop positions. 
     In the embodiments shown in FIGS. 13-24,  36 - 39 ,  41 , and  42 , the cam groove of the stationary cam  504 ,  968  or the drive (concave globoidal) cam  410  may be adapted to engage a pair of rollers as cam followers which are connected to each of the component holder heads  120 , like the rollers  744  employed in the third embodiment shown in FIG.  13 . The two rollers are provided at different positions, respectively, in the direction of width of the cam groove. The cam groove includes at least one narrow portion whose width is slightly shorter than the distance between the two rollers plus the respective diameters of the two rollers, so that the two rollers are fit in the narrow portion without any spaces left therebetween. The cam groove additionally includes at least one wide portion whose width is greater than that of the narrow portion and which permits the two rollers to be moved in the direction of the width. In this case, when the two rollers are moved through the narrow portion of the cam groove, the component holder head is effectively prevented from being out of position in the direction of width of the cam groove. 
     In the embodiments shown in FIGS. 13-24,  36 - 39 ,  41 , and  42 , the cam groove of the stationary cam  504 ,  968  includes both the narrow and wide portions. However, it is not essential that the cam groove include at least one narrow portion. That is, the cam groove may consists of only a single wide portion or a plurality of wide portions which permits or permits one or two cam followers to be moved in the direction of width of the groove. This is also true in the case where the stationary cam has, in place of a cam groove, a cam ridge or rib like the cam rib  912  shown in FIG.  40 . 
     In the embodiments shown in FIGS. 13-24 and  36 - 42 , the two selectable stop positions (i.e. circumferential positions) are provided at each of the component sucking and mounting positions (i.e., stop stations), and the two selectable height positions (i.e. axial positions spaced from each other in the axial direction of the stationary shaft  388  perpendicular to the circular movement path of the rotary members  392 ) are provided at each of the component sucking, image taking, and component mounting positions. However, three or more selectable circumferential positions may be provided at at least one of the stop stations, and three or more selectable axial positions may be provided at at least one station which may comprise one or more of the component sucking, image taking, and component mounting positions. 
     In the first embodiment shown in FIGS. 1-12 or the third embodiment shown in FIGS. 25-42, the rotary plates are not stopped at the image taking position. However, the apparatus  12 ,  600  may be modified such that the rotary plates are not stopped at the image taking position. That is, an image of an electronic component is taken while the component is moved through the image taking position. In either case, the height position of the component holder heads may be changed in two or more steps at the image taking position. 
     In the illustrated embodiments, each of the component suction nozzles  158  is provided with the reflector to plate  163 . However, the reflector plates  163  may be replaced by luminous plates. 
     The component suction nozzles  158  may be replaced by chucks each of which includes two pairs of grasping hands one pair of which are opened and closed in a first direction and the other pair of which are opened and closed in a second direction perpendicular to the first direction. 
     It is further noted that the present invention is not limited to an electronic component holder head positioning apparatus or an electronic component transferring apparatus which is employed by an electronic component mounting system adapted to transfer electronic components and mount them on an object such as a printed-circuit board, and that the principle of the present invention is also applicable to an electronic component holder head positioning apparatus or an electronic component transferring apparatus adapted to simply transfer electronic components from a given device to another device. 
     It is also noted that the present invention may be embodied with various other combinations of the elements employed in the illustrated embodiments. 
     It is to be understood that the present invention may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, in the light of the foregoing teachings, without departing from the scope of the invention defined in the following claims.