Abstract:
A wiring unit is provided for robots having a rotary joint. This unit includes a wiring case, a slide aid contained in the wiring case, and a flexile printed wiring board for electric wiring. The wiring case includes a cylindrical housing and a reel with a cylindrical core member. The reel is relatively rotatable to the housing. The reel and housing are linked with robot&#39;s two members requiring the rotary joint. In the slide aid, an annular rotary plate surrounds the core member and is rotatable about the core member. Plural rollers are provided on the rotary plate and self-rotatable. The wiring board has electrical wires and is connected to the core member and housing and is wound and unwound in radially inside and outside spaces of the rollers via the vacant space by moving on and along the rollers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims the benefit of priority from earlier Japanese Patent Applications No. 2009-064482 filed Mar. 17, 2009 and No. 2010-018580 filed Jan. 29, 2010, the description of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field of the Invention 
         [0003]    The present invention relates to a rotary joint wiring unit for robots, which is used for providing electric wiring between two members joined by a rotary joint, and in particular, to a rotary joint wiring unit for industrial robots, which uses a flexible printed wiring board for electric connections through the rotary joint. 
         [0004]    2. Related Art 
         [0005]    Industrial robots are generally provided with a base and a plurality of arms sequentially joined via rotary joints. The end arm of the joined arms has a tip end that is a wrist part on which an end effector (or an actuator), such as a hand, is usually mounted. 
         [0006]    A cable is wired in such an industrial robot to supply electric power to motors which serve as drive sources for the arms, the end effector, and the like, and to transmit/receive control signals between the motors and a robot controller. Wiring methods that can be used to wire such a cable include an internal wiring method in which a cable is wired inside the base and the arms, and an external wiring method in which a cable is wired along an outer surface of the robot. 
         [0007]    In using either of the above two wiring methods, the wiring structure employed is required to have rotary joints that will not block relative rotation between two members (e.g., base and arm, arm and arm, arm and wrist part, etc.). 
         [0008]    Patent documents JP-A-H06-143186 and JP-A-H10-034588 each disclose a wiring structure of a rotary joint using the internal wiring method. In the wiring structure, a cable is wired through the center of rotation of a rotary joint to absorb the relative rotation between two members with the twisting of the cable. 
         [0009]    Another patent document JP-A-H01-153290 discloses a wiring structure of a rotary joint using the external wiring method. This wiring structure uses a flexible sheet in which a wiring pattern is formed. The flexible sheet is fixed to two relatively rotating members, bridging therebetween, in the state of being loosened into an arc form. Thus, the relative rotation between the two members is ensured to be absorbed by the arc loosened portion of the flexible sheet. 
         [0010]    Although not related to robotic rotary joints, a patent document JP-A-2003-324835 discloses a spiral cable unit that electrically connects the steering wheel of the vehicle with the vehicle body. The spiral cable unit is provided with a casing having an outer cylinder and an inner cylinder which is disposed inside the outer cylinder so as to be rotatable relative to the outer cylinder. The casing accommodates in its interior a band of long flexible flat cable, being wound about the outer periphery of the inner cylinder. One end of the flexible flat cable is connected to the inner cylinder and the other end thereof is connected to the outer cylinder. 
         [0011]    With the structure disclosed in JP-A-H06-143186 and JP-A-H10-034588, the relative rotation between the two members is absorbed by the twisting of the cable. Therefore, if the portion corresponding to the center of rotation of a rotary joint for wiring the cable has a small length, the rate of twisting per unit length will be high. The high rate of twisting is likely to hasten damage or disconnection of the cable. Therefore, the portion corresponding to the center of rotation of the rotary joint is required to be elongated. This means that the space required for wiring the cable has to be increased in the direction of the rotation axis of the rotary joint. The increase of such a space will increase the size of the rotary joint, which will be a disadvantage in a small-size robot, in particular, in which a number of parts are closely arranged. 
         [0012]    In the structure disclosed in JP-A-H01-153290, the flexible sheet is flexed and extended with the relative rotation between the two members. However, since the force is concentrated in certain areas, fatigue and breakage may be accelerated especially at areas of maximum flexure. 
         [0013]    The spiral cable unit disclosed in JP-A-2003-324835 has a structure in which the rotation of a vehicle steering wheel is absorbed by allowing the flexible flat cable to be wound about and unwound from the inner cylinder. Therefore, comparing with the structure using the twisting of a cable, the thickness can be reduced in the direction of the rotation axis of the unit. In addition, the life of the flexible flat cable (hereinafter referred to as “flexible printed wiring board”) will be lengthened because no local flexing and extending motion is imparted. 
         [0014]    However, since the steering wheel of a vehicle is manipulated by a person, the speed of rotation is relatively slow and the wheel rotation is usually only maintained for a short time. On the other hand, when robotic arms are concerned, the speed of rotation of the arms is much higher than the speed of rotation of a steering wheel. Moreover, industrial robots are operated for a long time, which may continue as long as twenty-four hours. Thus, the spiral cable unit disclosed in JP-A-2003-324835, while suitable for absorbing the rotation of a steering wheel of a vehicle, is not configured assuming high-speed and long-duration rotation. Accordingly, it is expected that use of the flexible printed wiring board in a robotic rotary joint may problematically hasten wearing or the like. Thus, the flexible printed wiring board cannot be applied, as it is, to a robotic rotary joint. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention has been made in light of the problem mentioned above, and has as its object to provide a rotary joint wiring unit for robots, which wiring unit employs a structure for preventing early wearing to thereby lengthen the life of the wiring unit in the case where the wiring unit is configured using a flexible printed wiring board. 
         [0016]    In order to achieve the object, the present invention provides a wiring unit for robots having a rotary joint with which two members are mutually rotatably coupled, an electronic wiring passing the two members and the rotary joint. The wiring unit includes a wiring case, a slide aid, and a flexile printed wiring board. Of these components, the wiring case includes i) a cylindrical housing whose both ends are opened in an axial direction of the housing and ii) a reel i) having a cylindrical core member located in an inner space of the housing and the reel comprising two lid members mutually integrated with the core member and located on both axial end sides of the inner space of the housing, and ii) being relatively rotatable to the housing, the housing and the reel respectively being linked with the two members of the robot. The slide aid includes an annular rotary plate i) having both ends which produce a vacant space therebetween, ii) being placed to surround the core member in a radial direction of the housing, and iii) being rotatable about the core member, and a plurality of rollers provided on the rotary plate to be located around the core member and rotatable about an axis thereof. The flexile printed wiring board ( 14 ) is belt-shaped. In this flexile printed wiring board, a plurality of electrically conductive wires for the electrical wiring are contained with electrical insulation therebetween, and which has i) two length-width end portions one of which is linked with the core member and the other of which is linked with the housing and ii) a rest portion other than the two length-width end portions, which is located within the inner space of the housing, is wound and unwound in inside and outside spaces of a set of the rollers in the radial direction via the vacant space such that that the rest portion is moved on and along the rollers and is wound and unwound oppositely in directions between the inside and the outside spaces of the set of the rollers in the radial direction. 
         [0017]    With this configuration, when the first and the second member relatively rotate, the flexible printed wiring board moves such that it comes into contact with the rollers of the slide aid so as to be wound about and unwound from the core member and the slide aid, while allowing the rollers to rotate. Therefore, the flexible printed wiring board can move duly following the high-speed relative rotation of the first and second members of the robot. Moreover, the flexible printed wiring board will not be entangled or worn out excessively quickly, will satisfactorily endure periods of operation of the robot. 
         [0018]    For example, each of the rollers has an outer diameter and the rotary plate has a width in the radial direction and the outer diameter is larger than the width. Thus, the flexible printed wiring board can be prevented from being worn out by being in contact with the rotary plate. 
         [0019]    It is preferred that, in the foregoing configurations, the flexible printed wiring board is folded back into the vacant space produced by the rotary plate so that the flexible printed wiring board has an arc folded-back portion which changes a winding direction thereof in the vacant space, and a roller among the plurality of rollers is provided at one end of both ends of the rotary plate, the arc folded-back portion being wound around the one end of both ends of the rotary plate, and the unit further includes a protective pole provided at the other end of both ends of the rotary plate, wherein the protective pole has an arc concave surface which faces and accepts the arc folded-back portion for protecting the arc folded-back portion. 
         [0020]    Thus, when the first and second members of the robot relatively rotate in the direction of allowing the portion of the flexible printed wiring board outside the slide aid to be wound about the slide aid, the flexible printed wiring board in the arc fold-back portion comes into contact with the roller concerned to pull the slide aid. As a result, the slide aid will rotate about the core member. 
         [0021]    On the other hand, when the first and second members of the robot relatively rotate in the direction of allowing the portion of the flexible printed wiring board outside the slide aid to be unwound from the slide aid, the flexible printed wiring board in the arc fold-back portion comes into contact with the arc concave surface of the protective pole to push the protective pole. As a result, the slide aid will efficiently receive the pressing force of the flexible printed wiring board and smoothly rotate about the core member. 
         [0022]    The protective pole may be biased outward in the axial direction on the rotary plate. Thus, the flexible printed wiring board can be prevented from being pulled into the gap between the protective pole and the core member. 
         [0023]    It is preferred that the wiring unit further includes a projection fixedly located between two mutually adjacent rollers among the plurality of rollers, wherein the two rollers are located from one end of both ends of the rotary plate and composed of a first roller and a second roller in this order from the one end, and the flexible printed wiring board has an arc folded-back portion which changes a winding direction thereof around the two rollers. In this configuration, the arc folded-back portion of the flexible film board is located between the projection and the second roller and the flexible printed wiring board layers are located around the first roller via the vacant space produced between both ends of the rotary plate. Therefore, the flexible film board is first wound around the second roller before the flexible printed wiring board layers are wound around the first roller in response to a relative rotation between the reel and the housing to have the flexible printed wiring board wound around the rollers. In contrast, the flexible film board is first touched to the projection before the arc folded-back portion of the flexible printed wiring board is made to touch a roller provided on the other end of the rotary plate by being unwound, in response to a relative rotation between the reel and the housing to have the flexible printed wiring board unwound around the rollers. 
         [0024]    With this configuration, the flexible printed wiring board will no longer be wound about the rollers of the slide aid or be in contact with the end of the slide aid. Therefore, the flexible printed wiring board can be prevented from being worn out. At the same time, with the above configuration, sliding properties similar to the ones set forth above can also be realized. 
         [0025]    Preferably, the protrusion has a side to which the arc folded-back portion of the flexible film board is made to touch and the side of the protrusion has an arc concave surface to accept the arc folded-back portion. Thus, when the flexible printed wiring board in the arc fold-back portion comes into contact with the arc concave surface of the drive projection to push the drive projection, the slide aid will efficiently receive the pressing force of the driving film sheet and smoothly rotate about the core member. 
         [0026]    As another preferred example, the plurality of rollers include a roller provided on one end of both ends of the rotary plate, the flexible printed wiring board being folded back from the radial inside of the set of rollers to the radial outside of the set of rollers around the roller provided on the one end. In this case, the wiring unit further includes two protective rollers provided on the other end of both ends of the rotary plate, the two protective rollers aligning in a width direction of the rotary plate, each of the protective rollers being cylindrical and having a radius smaller than the radius of each of the rollers. 
         [0027]    Thus, when the flexible printed wiring board is unwound, the flexible printed wiring board in the arc fold-back portion is ensured to contact and push the two protective rollers. Therefore, the wearing of the flexible printed wiring board can be further reduced. 
         [0028]    Still preferably, the two protective rollers consist of a first protective roller located outward in the radial direction and a second protective roller located inward in the radial direction and the two protective rollers are located such that the first protective roller first touches the arc folded-back portion and the second protective roller then touches the arc folded-back portion. 
         [0029]    With this configuration, the flexible printed wiring board in the arc fold-back portion, when it first comes into contact with the outer protective roller, will be driftingly drawn to the inner peripheral side. Therefore, the flexible printed wiring board in the arc fold-back portion is unlikely to be pulled into the gap between the outer protective roller and the housing. Thus, the fold-back portion can be prevented from being pulled into the above gap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In the accompanying drawings: 
           [0031]      FIG. 1  is a perspective view illustrating an industrial robot to which embodiments of the present are applied; 
           [0032]      FIG. 2  is a horizontal cross-sectional view illustrating a rotary joint wiring unit together with a rotary joint structure, according to a first embodiment of the present invention; 
           [0033]      FIG. 3  is a side view illustrating a mounting frame with an outer cover detached, as viewed along an A-A line of  FIG. 2 ; 
           [0034]      FIG. 4  is an exploded perspective view of the rotary joint wiring unit; 
           [0035]      FIGS. 5A and 5B  are enlarged cross-sectional views each illustrating a flexible printed wiring board (FPC board) used in the rotary joint wiring unit; 
           [0036]      FIGS. 6A and 6B  are cross-sectional views each illustrating a plurality of the FPC boards in the state of being laid one over the other; 
           [0037]      FIG. 7  is a partial perspective view of the plurality of FPC boards in the state of being laid one over the other; 
           [0038]      FIG. 8  is a horizontal cross-sectional view illustrating a body of the rotary joint wiring unit; 
           [0039]      FIG. 9  is a vertical cross-sectional view illustrating the body of the rotary joint wiring unit; 
           [0040]      FIG. 10  is a vertical cross-sectional view illustrating the body of the rotary joint wiring unit in a state different from the one illustrated in  FIG. 9 ; 
           [0041]      FIG. 11  is an enlarged cross-sectional view illustrating the function of a protective pole; 
           [0042]      FIG. 12  is a vertical cross-sectional view illustrating a body of a rotary joint wiring unit, according to a second embodiment of the present invention; 
           [0043]      FIG. 13  is a vertical cross-sectional view illustrating a body of a rotary joint wiring unit, according to a third embodiment of the present invention; 
           [0044]      FIGS. 14A to 14C  are vertical cross-sectional views each illustrating the movement of the rotary joint wiring unit according to the third embodiment; and 
           [0045]      FIGS. 15A to 15C  are vertical cross-sectional views each illustrating the movement of a rotary joint wiring unit, according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    With reference to the accompanying drawings, hereinafter will be described some embodiments of the present invention. 
       First Embodiment 
       [0047]    Referring to  FIGS. 1 to 11 , hereinafter will be described a first embodiment of the present invention. 
         [0048]      FIG. 1  illustrates an industrial robot  1  in which a rotary joint according to the present invention is put into practice. The industrial robot  1  is configured, for example, as a vertically articulated six-axis robot. The industrial robot  1  includes a base  2 , shoulder  3 , lower arm  4 , upper-rear arm  5 , upper-front arm  6 , wrist  7  and flange  8 . The base  2  is set up on a floor. The shoulder  3  is supported by the base  2  so as to be pivotably movable in the horizontal direction. The lower arm  4  is supported by the shoulder  3  so as to be pivotally movable in the vertical direction. The upper-rear arm  5  is supported by the lower arm  4  so as to be pivotally movable in the vertical direction. The upper-front arm  6  is supported by the upper-rear arm  5  so as to be twistingly rotatable. The wrist  7  is supported by the upper-front arm  6  so as to be pivotally movable in the vertical direction. The flange  8  is supported by the wrist  7  so as to be twistingly rotatable. The flange  8 , or an arm end, is configured to be attached with an end effector (i.e., an actuator; not shown), such as a hand. 
         [0049]    Besides a hand, a camera of a visual checking device may be attached as an end effector. The visual checking device refers to a device that picks up an image of a desired checking point of a piece of work, transmits the image to a robot controller and displays the image received by the robot controller. Good or bad assembly, processing and the like are checked based on the image visually indicated on the display. 
         [0050]    Thus, rotary joint structures are provided, in which respective rotatable supports are achieved, i.e. rotatable support of the shoulder  3  by the base  2 , rotatable support of the lower arm  4  by the shoulder  3 , rotatable support of the upper-rear arm  5  by the lower arm  4 , rotatable support of the upper-front arm  6  by the upper-rear arm  5 , rotatable support of the wrist  7  by the upper-front arm  6 , and rotatable support of the flange  8  by the wrist  7 . 
         [0051]    Of these rotary joint structures,  FIG. 2  shows the rotary joint structure of the wrist  7  for the upper-front arm  6 . As shown in  FIG. 2 , the upper-front arm  6  is configured to have an arm base frame  9  which is covered with a plurality of outer covers  10 . The arm base frame  9  of the upper-front arm  6  has a tip end portion in which a shaft hole  9   a  is formed. Meanwhile, the wrist  7  is provided with a projected cylindrical shaft portion  7   a . The shaft portion  7   a  is fitted to the shaft hole  9   a  of the arm base frame  9  of the upper-front arm  6  and supported by a cross roller bearing  11  to enable relative rotation. In such a rotary joint structure, the wrist  7  is supported by the upper-front arm  6  so as to be pivotally movable in the vertical direction. The similar structure is applied to the rotary joints of other robotic elements, such as the shoulder  3 , lower arm  4 , upper-rear arm  5  and flange  8 . 
         [0052]    The shoulder  3 , lower-arm  4 , upper-rear arm  5 , upper-front arm  6 , wrist  7  and flange  8 , which are pivotally movable or twistingly rotatable robotic elements, each have an actuator, e.g., servomotor (not shown), as a drive source. An electric cable (not shown) is wired inside the robot, extending from the base  2  to the wrist  7 , the tip end, for the purposes of supplying power to the servomotors, or transmitting control signals from the robot controller to the drive circuits of the respective servomotors, or transmitting rotation sensing signals from the rotary encoders of the respective servomotors to the robot controller. 
         [0053]    In the case where the end effector attached to the flange  8  is a hand, an electric cable is wired in the robot for the purposes of supplying power to the servomotor, i.e. actuator, of the hand, or transmitting/receiving control signals or rotation sensing signals between the servomotor of the hand and the robot controller. In the case where the end effector is the camera of a visual checking device, an electric cable is wired in the robot for the purposes of supplying power to the camera, or transmitting pickup image signals of the camera to the robot controller. 
         [0054]    In the wiring that is the cable wired in the robot, a rotary joint wiring unit  12  shown in  FIGS. 2 to 4  is used for the wiring through a rotary joint. The rotary joint wiring unit  12  shown in  FIGS. 2 to 4  is used for the rotary joint of the wrist  7 . Wiring units used for other rotary joints have the similar configuration. The rotary joint wiring unit  12  is configured with a wiring case  13  and a band of long flexible printed wiring board assembly  14  accommodated in the wiring case  13 . 
         [0055]    In the following description, the flexible printed wiring board assembly  14  is referred to as “FPC board assembly  14 ”. 
         [0056]    The wiring case  13  of the rotary joint wiring unit  12  includes a cylindrical housing  15 , a first disc  17  and a second disc  18 . The cylindrical housing  15  is shaped into an approximately cylindrical appearance, so that, using this housing  15 , an axial direction, a radial direction and a circumference direction can be defined as shown in  FIGS. 2-4 . This cylindrical housing  15 , which is made of transparent plastic, has a cylindrical bore (i.e., inner space), and both axial ends are open. 
         [0057]    The first disc  17  is made of transparent plastic and is integrally provided with an approximately cylindrical core member  16  at its center portion. The second disc  18  is also made of transparent plastic. 
         [0058]    The first and second discs  17  and  18  are used for closing both of the open ends of the housing  15  in the axial direction. The first disc  17  has a circular fitting projection  17   a  to be fitted to the inner side of the housing  15 . The second disc  18  has an annular fitting rib  18   a  to be fitted to the outer side of the housing  15 . The second disc  18  is fixed to the core member  16  of the first disc  17  by a screw  19  shown in  FIG. 8 , for integration with the first disc  17 . The integrated first and second discs  17  and  18  constitute a reel  20  having the core member  16  at the center. 
         [0059]    In constituting the reel  20  by integrating the first and second discs  17  and  18 , the circular fitting projection  17   a  is fitted to the inner side of the housing  15 , while the annular fitting rib  18   a  is fitted to the outer side of the housing  15 . Thus, the first and second discs  17  and  18  are arranged sandwiching the housing  15 . 
         [0060]    In this case, very small gaps are ensured to be formed between the housing  15  and the first disc  17  and between the housing  15  and the second disc  18 . The housing  15  and the reel  20  are assembled in this way, enabling relative rotation therebetween, to constitute the wiring case  13 . 
         [0061]    The core member  16  located at the center portion of an imaginary plane perpendicular to the axial direction of the housing  15  is provided with a holding groove  16   a  into which a stopper  21  having an insertion groove  21   a  is fitted. On the other hand, the housing  15  is provided with a slit  15   a  that draws an arc from the inner peripheral surface of the housing  15  toward the outer peripheral surface thereof. The housing  15  is also provided with a deep groove  15   b  that unidirectionally extends from the slit  15   a  to the opposite side. At the tip end portion of the deep groove  15   b , the outer sidewall portion of the groove is left open to the outside at the outer peripheral surface of the housing  15 , while the bottom face portion of the groove is also left open to the outside at the end surface of the housing  15 . 
         [0062]    The first disc  17  has an inner surface (the surface that constitutes an inner surface of the wiring case  13 ) in which two annular ribs  17   b  and  17   c  with different diameters are formed so as to be coaxial with the core member  16 . An annular groove sandwiched between the two annular ribs  17   b  and  17   c  serves as a guide groove  22 . The inner surface of the first disc  17  is also provided with a plurality of radial ribs  17   d  extending radially from the core member  16 . The radial ribs  17   d  are not formed inside the guide groove  22 . 
         [0063]    The wiring case  13  is provided in the interior thereof with a slide aid  23 . The slide aid  23  has, as a base, an annularly shaped rotary plate  24  made of plastic. The rotary plate  24  has an incomplete annular shape and thus has ends E 3  and E 4  as shown in  FIG. 9 , for example, forming an opening therebetween. One of two opposing surfaces of the rotary plate  24  is provided with a projection  24   a . The other surface of the rotary plate  24  is provided with a row of a plurality of upright support shafts  24   b  along the arc of the rotary plate  24 . The rotary plate  24  is also provided with rollers  25  which are rotatably supported by the respective support shafts  24   b . Each roller  25  is ensured to have an outer diameter larger than the width of the rotary plate  24 , so that each roller  25  sticks out of the inner and outer peripheral edges of the rotary plate  24 . 
         [0064]    In the vicinity of the end E 4  on the surface of the rotary plate  24  where the support shafts  24   b  are uprightly provided, a protective pole  26  is uprightly provided. The protective pole  26  has an arc concave surface  26   a  that faces the end E 3 . The protective pole  26  has a width smaller than the width of the rotary plate  24 . For example, the width of the protective pole  26  is set to approximately half of that of the rotary plate  24 . The protective pole  26  is provided at a position deviated to the side of the outer peripheral edge of the rotary plate  24 . At the end E 3  of the rotor  24 , which end is opposite to the end E 4  where the protective pole  26  is uprightly provided, a roller  25  is positioned. The roller  25  concerned sticks out more outer side than at the end E 4  of the rotary plate  24 . 
         [0065]    Thus, the slide aid  23  is configured by arranging the plurality of rollers  25  on the rotary plate  24 . As shown in the enlarged view of  FIG. 8 , the slide aid  23  is accommodated in the wiring case  13  so that the projection  24   a  of the rotary plate  24  is slidably fitted to the guide groove  22  of the first disc  17 . By allowing the projection  24   a  of the rotary plate  24  to slidably fit to the guide groove  22 , the slide aid  23  is ensured to be rotatably movable about the core member  16  and centered thereon. 
         [0066]    As also shown in  FIG. 7 , the FPC board assembly  14  is provided with a plurality of power and signal FPC boards (sheets)  14 - 1  and  14 - 2  (described later) laid one over the other and accommodated in the wiring case  13 . 
         [0067]    As shown in  FIGS. 5A and 5B , these FPC boards  14 - 1  and  14 - 2  are each basically configured with adhering a plastic film, such as a polyimide film  28 , in which a plurality of electrically conductive lines  27  are formed, to a plastic film, such as a polyimide film  29 , as an insulation coating, using an adhesive  30 . The conductive lines  27  are used as power lines or signal lines. Thus, two types of FPC boards  14  are prepared. One type is the power FPC board  14 - 1  in which the conductive lines  27  are used as power lines  27 - 1  as shown in  FIG. 5A . 
         [0068]    The other type is the signal FPC board  14 - 2  in which the conductive lines  27  are used as signal lines  27 - 2  as shown in  FIG. 5B . Each power line  27 - 1  and each signal line  27 - 2  have the same thickness, however, and the width of the latter is smaller than that of the former. 
         [0069]    In the present embodiment, the power FPC board  14 - 1  plays a role of supplying power to the servomotors that are actuators (drive sources) of the wrist  7 , flange  8  and the like, and of supplying power to the end effector. The signal FPC board  14 - 2  plays a role of transmitting/receiving signals between the servomotors and the robot controller, and of transmitting/receiving signals between the end effector and the robot controller. 
         [0070]    In effect, the FPC board assembly  14  is provided with a predetermined number of the power FPC boards  14 - 1  and a predetermined number of the signal FPC boards  14 - 2 . In laying the plurality of FPC circuit boards  14 - 1  and  14 - 2  one over the other, it is ensured that the signal FPC boards  14 - 2  are sandwiched between the power FPC boards  14 - 1 . These FPC boards may be laid in various ways. 
         [0071]    For example, as shown in  FIG. 6A , the signal FPC board  14 - 2  may each be sandwiched by two single power FPC boards  14 - 1 . Alternatively, as shown in  FIGS. 6B and 7 , a plurality of the single FPC boards  14 - 2  may be collectively sandwiched by the power FPC boards  14 - 1 . As shown in  FIG. 4 , the power FPC board  14 - 1  and the signal FPC board  14 - 2  have a different length. Specifically, the signal FPC board  14 - 2  is made longer than the power FPC board  14 - 1 . These two types of FPC boards  14 - 1  and  14 - 2  of different length are laid one over the other with the end portions on one side (hereinafter collectively referred to as “end E 1 ”) being aligned. Accordingly, the end portions of the long signal FPC boards  14 - 2  on the other side (hereinafter collectively referred to as “end E 2 - 2 ”) are allowed to further extend from the end portions of the short power FPC boards  14 - 1  on the other side (hereinafter collectively referred to as “end E 2 - 1 ”). At the one end E 1 , the justified end portions of the two types of FPC boards  14 - 1  and  14 - 2  are adhered to each other, while at the other ends E 1 - 1  and E 2 - 2 , the justified end portions of the two types of FPC boards  14 - 1  and  14 - 2  are adhered to each other, (though not shown in  FIG. 4 ). However, it should be appreciated that these FPC boards  14 - 1  and  14 - 2  may not necessarily be adhered to each other. 
         [0072]    The ends E 1 , E 2 - 1  and E 2 - 2  of the FPC board assembly  14  are connected to the core member  16  and the housing  15 . Specifically, the justified end portions of the FPC boards  14 - 1  and  14 - 2  (i.e. the end E 1  of the of FPC board assembly  14 ) are inserted into and held by the insertion groove  21   a  of the stopper  21  which is fitted to the core member  16 . Meanwhile, the portion of the FPC boards extending from near the end portions of the short power FPC boards  14 - 1  to the end portions of the long signal FPC boards  14 - 2  (i.e. the portion of the FPC board assembly  14  extending from near the end E 2 - 1  to the end E 2 - 2 ) is entirely inserted into the slit  15   a  and the deep groove  15   b  that continues from the slit  15   a . Thus, this portion of the FPC boards closes the opening of the slit  15   a  on the outer peripheral side and the opening of the deep groove  15   b  at the tip end on the outer peripheral side, being held by pressers  31  and  32  that are fixed to the housing  15 . 
         [0073]    As also shown in  FIG. 9 , the FPC board assembly  14 , with the ends E 1 , E 2 - 1  and E 2 - 2  being connected to the core member  16  and the housing  15 , is wired through the opening between the ends E 3  and E 4  of the slide aid  23  (rotary plate  24 ) so that an intermediate portion of the FPC board assembly  14  is located at the opening. In the wiring, a portion of the FPC board assembly  14  located inside the slide aid  23  is wound about the core member  16  by a predetermined number of times, while a portion of the FPC board assembly  14  located outside the slide aid  23  is wound about the slide aid  23  by a predetermined number of times in a direction reverse of the winding direction of the FPC board assembly  14  inside the slide aid  23 . 
         [0074]    Thus, the FPC board assembly  14  is wound in opposite directions on the inner and outer sides of the slide aid  23 . Accordingly, the FPC board assembly  14  is folded back forming a U-shape at the opening between the ends E 3  and E 4  of the slide aid  23  (rotary plate  24 ). Inside the U-shaped folded back portion T (hereinafter referred to as “fold-back portion T”), the roller  25 , as mentioned above, is provided at the end E 3  of the rotary plate  24 . 
         [0075]    Although five FPC boards  14 - 1  and  14 - 2  are drawn in  FIGS. 6A ,  6 B and  7 , only three of these components are drawn in  FIGS. 4 ,  9  and  10  for the sake of brevity. 
         [0076]    As shown in  FIG. 4 , the ends E 1 , E 2 - 1  and E 2 - 2  of the FPC board assembly  14  are formed into extensions  14   a  and  14   b . Each of the extensions  14   a  and  14   b  extends perpendicular to the FPC board assembly  14 . The extension  14   a  formed at the end E 1  of the FPC board assembly  14  and connected to the core member  16  has a tip end to which a connecting terminal  33  is attached for establishing connection with the conductive lines  27  of the FPC board assembly  14 . The extensions  14   b  formed at the ends E 2 - 1  and E 2 - 2  of the FPC board assembly  14  and connected to the housing  15  have respective tip ends to which connecting terminals  34  and  35  are attached, respectively. 
         [0077]    The extension  14   a  formed at the end E 1  of the FPC board assembly  14  and connected to the core member  16  is wired through a slit  18   b  linearly formed in the second disc  18  so as to extend from the outer periphery of the second disc to the center portion thereof, and drawn out of the wiring case  13 . The extension  14   b  formed at the end E 2 - 1  of the FPC board assembly  14  is wired through a portion of the slit  15   a , which portion is not closed by the first disc  17 , and drawn out of the wiring case  13 . The extension  14   b  formed at the end E 2 - 2  of the FPC board assembly  14  is drawn out of the wiring case  13  from the opening at the bottom surface portion (on the side of the first disc  17 ) of the tip end portion of the deep groove  15   b.    
         [0078]    The FPC boards  14 - 1  and  14 - 2 , as far as they are located inside the wiring case  13 , are left unadhered to each other so as to be slidably movable. As also shown in  FIG. 9 , the length of the boards  14 - 1  and  14 - 2  in the unadhered portion is set so that the FPC board  14 - 1  or  14 - 2 , as it is located more outer side in the U-shaped fold-back portion T at the opening of the slide aid  23 , will have a larger length. Thus, the FPC boards  14 - 1  and  14 - 2  located in the fold-back portion T will be separated from each other with a space being formed therebetween. 
         [0079]    The rotary joint wiring unit  12  is configured as described above. Hereinafter will be described an example of a procedure for assembling the rotary joint wiring unit  12 . 
         [0080]    First, the plurality of FPC boards  14 - 1  and  14 - 2 , which are for the FPC board assembly  14 , are laid one over the other so that the end portions are adjusted on one side (end E 1 ). Then, the adjusted end portions at the end E 1  are adhered to each other. Also, the end portions of the short FPC boards  14 - 1  at the end E 2 - 1  are adhered to each other, while the end portions of the long FPC boards  14 - 2  at the end E 2 - 2  are also adhered to each other. Subsequently, the connecting terminals  33 ,  34  and  35  are connected to the tips of the extensions  14   a  and  14   b  provided at the ends E 1 , E 2 - 1  and E 2 - 2 , respectively. 
         [0081]    Meanwhile, the first disc  17  is placed on a workbench and then the housing  15  is placed on the first disc  17  so as to be fitted to the circular fitting projection  17   a . Subsequently, the slide aid  23  is accommodated in the housing  15  such that the projection  24   a  of the rotary plate  24  of the slide aid  23  is fitted to the guide groove  22  of the first disc  17 . 
         [0082]    After that, the end E 1  of the PCT board assembly  14  is inserted into the insertion groove  21   a  of the stopper  21 , followed by fitting the stopper  21  to the holding groove  16   a  of the core member  16 . Then, the FPC board assembly  14  inside the slide aid  23  is loosely wound about the core member  16  by a predetermined number of times, wired through the opening between the ends E 3  and E 4  of the slide aid  23 , and drawn out of the slide aid  23 . Then, the FPC board assembly  14  is folded back and wound about the slide aid  23  by a predetermined number of times in a direction reverse of the winding direction of the FPC board assembly  14  inside the slide aid  23 . 
         [0083]    Then, the extension  14   b  at the end E 2 - 1  of the FPC board assembly  14  is led into the slit  15   a  of the housing  15  from the outer peripheral side. Meanwhile, the extension  14   b  at the end E 2 - 2  of the FPC board assembly  14  is led into the inside of the tip end portion of the deep groove  15   b  from the outer peripheral side opening of the housing  15 . Then, the portion extending from the end E 2 - 1  to the end E 2 - 2  is inserted from above into the slit  15   a  and the deep groove  15   b . Thus, the extensions  14   b  at the ends E 2 - 1  and E 2 - 2  are brought into the state of being drawn out to the side of the first disc  17  from the tip end to portions of the slit  15   a  and the deep groove  15   b.    
         [0084]    Then, the second disc  18  is arranged on the housing  15  such that the extension  14   a  drawn out upward from the core member  16  is inserted into the slit  18   b  from the outer peripheral side toward the center side to thereby fit the annular fitting rib  18   a  to the outer periphery of the housing  15 . Finally, the second disc  18  is fixed to the core member  16  by the screw  19 . Thus, the rotary joint wiring unit  12  has been assembled. 
         [0085]    As shown in  FIG. 2 , after attaching the housing  15  to a metal holding cylinder  36 , the wiring case  13  accommodating the FPC board assembly  14  is arranged at a rotary joint. In arranging the wiring case  13 , the center of the relative rotation between the housing  15  and the reel  20  is ensured to align with the center of the relative rotation between the wrist  7  and the upper-front arm  6 . The holding cylinder  36  is provided with a flange  36   a  which is fixed to the shaft portion  7   a  of the wrist  7 , for example, by a screw  37 . Thus, the housing  15  is fixed to the wrist  7  (one mating member for the relative rotation of the robot) via the holding cylinder  36 . 
         [0086]    The wiring case  13 , with the housing  15  being fixed to the shaft portion  7   a  of the wrist  7 , is accommodated in a cylindrical accommodation portion  39   a  of a mounting frame  39  which is fixed to the arm base frame  9  of the upper-front arm  6  by a screw  38 . The cylindrical accommodation portion  39   a  has ends both of which are open, with a T-shaped connecting frame  39   b  being formed at one of the ends. The connecting frame  39   b  is fixed to the core member  16  by a screw  40  via the second disc  18 . 
         [0087]    Thus, the reel  20  is fixed to the upper-front arm  6  (the other mating member for the relative rotation of the robot) via the connecting frame  39   b . The connecting frame  39   b  is provided with a slit  39   c  for passing the extension  14   a  of the FPC board assembly  14  drawn out of the slit  18   b  of the second disc  18 . The outer covers  10  mentioned above are fixed to the arm base frame  9  and the mounting frame  39  by a plurality of screws  41  to cover the mounting frame  39 . 
         [0088]    The connecting terminal  33  of the extension  14   a  at the end E 1  of the FPC board assembly  14  drawn out of the second disc  18  of the wiring case  13  is fixed to a predetermined position in the upper-front arm  6 . The connecting terminal  33  is connected, via a terminal to a cable, not shown, wired in the upper-front arm  6 . The two connecting terminals  34  and  35  of the extensions  14   b  at the ends E 2 - 1  and E 2 - 2 , respectively, of the FPC board assembly  14  drawn out to the side of the first disc  17  from the housing  15  of the wiring case  13  are fixed to predetermined positions in the wrist  7 . The connecting terminals  34  and  35  are connected, via respective terminals, to cables, not shown, wired in the wrist  7 . 
         [0089]    In this way, the cable wired in the upper-front arm  6  and the cables wired in the wrist  7  are connected via the rotary joint wiring unit  12 . 
         [0090]    Hereinafter, the rotary operations obtained in the above configuration will be described. First, there is no relative rotation between the housing  15  and the reel  20  in the state where the wrist  7  is pausing without rotating relative to the upper-front arm  6 . In this state, the FPC board assembly  14  is urged to restore to the state before being accommodated in the wiring case  13 , i.e. to the unwound straight state. Therefore, as shown in  FIG. 10 , accompanying the restoration, the portion of the FPC board assembly  14  inside the slide aid  23  comes into contact with the slide aid  23  and the portion of the FPC board assembly  14  outside the slide aid  23  comes into contact with the inner peripheral surface of the housing  15 . Also, accompanying the restoration again to the unwound straight state, the portion of the FPC board assembly  14  in the fold-back portion T inside the slide aid  23  comes into contact with the core member  16 , leaving the slide aid  23 . 
         [0091]    From this state, the wrist  7  rotates relative to the upper-front arm  6 . Accompanying this relative rotation, the housing  15  rotates relative to the reel  20  in the direction indicated by an arrow L in  FIG. 9 . In this case, the direction of the relative rotation of the housing  15  coincides with the direction towards the ends E 2 - 1  and E 2 - 2  (the side connected to the housing  15 ) from the FPC board assembly  14  in the fold-back portion T outside the slide aid  23 . Therefore, the portion of the FPC board assembly  14  outside the slide aid  23  will be pulled in the direction of the arrow L. Then, the portion of the FPC board assembly  14  outside the slide aid  23  will be apart from the inner surface of the housing  15  and wound about the slide aid  23 . At the same time, the fold-back portion T will pull the slide aid  23  to rotate the slide aid  23  in the direction of the arrow L. 
         [0092]    In this case, the slide aid  23  rotates with the number of rotations approximately half of that of the housing  15 . Accordingly, the portion of the FPC board assembly  14  inside the slide aid  23  will be unwound from the core member  16  and will be gradually led out of the slide aid  23 , and thus will be gradually apart from the rollers  25 . As a result, the portion of the FPC board assembly  14  outside the slide aid  23  will rotate in the direction of the arrow L in integration with the housing  15 , with the portion of the FPC board assembly  14  led out from the inside of the slide aid  23  being permitted to be wound about the slide aid  23 , i.e. being in contact with the rollers  25  to rotate the rollers  25 . 
         [0093]    Thus, when the housing  15  relatively rotates in the direction of the arrow L, the FPC board assembly  14  moves such that, inside the slide aid  23 , it is unwound from the core member  16 , and outside the slide aid  23 , it is wound about the slide aid  23 . 
         [0094]    On the other hand, accompanying the relative rotation of the wrist  7  with respect to the upper-front arm  6 , the housing  15  rotates relative to the reel  20  in the direction indicated by an arrow M in  FIG. 10 . In this case, the direction of the relative rotation of the housing  15  is opposite to the direction from the FPC board assembly  14  in the fold-back portion T outside the slide aid  23  towards the ends E 2 - 1  and E 2 - 2  of the FPC board assembly  14 , which are connected to the housing  15 . Therefore, the portion of the FPC board assembly  14  outside the slide aid  23  will be pushed by the housing  15 . 
         [0095]    Thus, the portion of the FPC board unit  15  outside the slide aid  23  will be loosened and be apart from the slide aid  23  to thereby come into contact with the inner surface of the housing  15  and thus to rotate in integration with the housing  15  in the direction of the arrow M. With this rotation, the portion of the FPC board assembly  14  outside the slide aid  23  will be unwound from the slide aid  23  in pause, and at the same time, the fold-back portion T will be apart from the roller  25  at the end E 3  of the slide aid  23  and come into contact with the protective pole  26  at the end E 4  thereof. In this case, since a side face of the protective pole  26  is formed into the arc concave surface  26   a  for the fold-back portion T to come into contact therewith, the fold-back portion T will not come off from the protective pole  26 , whereby the protective pole  26  can be efficiently pushed. 
         [0096]    When the protective pole  26  receives a pressing force from the fold-back portion T, the slide aid  23  will rotate in the direction of the arrow M. In this case, the number of rotations of the slide aid  23  will be approximately half of that of the housing  15 . Therefore, the portion of the FPC board  14  outside the slide aid  23  will be kept being unwound from the slide aid  23 , so that the unwound portion will be pushed into the slide aid  23 . Thus, inside the slide aid  23 , the portion of the FCP board  14  pushed there from outside will come into contact with the rollers  25  of the slide aid  23  and will be wound about the core member  16 , while rotating the rollers  25 . 
         [0097]    In this way, when the housing  15  relatively rotates in the direction of the arrow M, the FPC board assembly  14  moves such that, inside the slide aid  23 , it is wound about the core member, and outside the slide aid  23 , it is unwound from the slide aid  23 . 
         [0098]    As described above, according to the rotary joint wiring unit  12  of the present embodiment, the portion between the end E 1  and the ends E 2 - 1  and E 2 - 2  of the FPC board assembly  14  is flexibly bent so as to be wound about and unwound from the core member  16  and the slide aid  23 , while the end E 1  and the ends E 2 - 1  and E 2 - 2  are connected to the core member  16  and the housing  15 , respectively. Thus, the relative rotation can be absorbed from between the housing  15  and the reel  20 , and therefore between the wrist  7  and the upper-front arm  6 . Therefore, the axial dimension of the wiring case  13  may be comparatively as small as a thickness that may only allow accommodation of the FPC board assembly  14 . At the same time, the dimension in the radial direction may also be comparatively as small as a radial dimension that may only allow the FPC board assembly  14  to be wound about and unwound from the core member  16  and the slide aid  23 . Therefore, the wiring case  13  can be disposed in a small space. In other words, the wiring case  13  can be used for a small robot having less extra space without making the robot larger, or even when the robot is made larger, the degree of enlargement may be very small. 
         [0099]    Particularly, when a camera is selected as an end effector in a conventional configuration, the cable used for transmitting the pickup image signals of the camera has been vulnerable to twisting due to the use of the cable in which a core line is enclosed by a mesh conductor. Accordingly, as disclosed in the patent documents JP-A-H06-143186 and JP-A-H10-034588 mentioned above, it has been difficult to employ a configuration in which a sealed cable is disposed at the center portion of rotation of a rotary joint of a robot to absorb the relative rotation between two members of the robot by twisting the sealed cable. Therefore, use of a sealed cable has necessitated the use of the external wiring method. The use of the external wiring method has further necessitated provision of an extra space for the externally wired sealed cable. Thus, in performing a certain motion, robots using such a sealed cable have created a problem of limiting (narrowing) the motion area compared to those robots which are not provided with such an external cable and have an area of the same size. 
         [0100]    In this regard, according to the present embodiment, relative rotation between two members of a robot is configured to be absorbed by flexibly bending the FPC board assembly  14 . Specifically, the present embodiment uses an FPC board unit having a sealed structure instead of a coaxial cable structure. Thus, the cable for transmitting the image pickup signals can also be incorporated into the FPC board assembly  14  of the rotary joint wiring unit  12 , realizing use of the internal wiring method. 
         [0101]    Moreover, the slide aid  23  is arranged between the housing  15  and the core member  16  so as to be rotatably movable. Therefore, when the FPC board assembly  14  receives a pressing force with the relative rotation of the housing  15 , the PCT board assembly  14  is supported by the housing  15  and the slide aid  23 , or by the core member  16  and the slide aid  23  to prevent the FPC board assembly  14  from being buckled. Thus, the FPC board assembly  14  can be effectively prevented from being entangled within the wiring case  13 . 
         [0102]    In addition, when the housing  15  rotates relative to the reel  20 , the FPC board assembly  14  moves so as to be wound about and unwound from the slide aid  23 , while being in contact with the slide aid  23 . Since the contact with the slide aid  23  is actually the contact with the rollers  25 , the contact is actually a rolling contact that will decrease friction and wearing. Thus, the FPC board assembly  14  will be smoothly wound about and unwound from the slide aid  23 , while allowing the slide aid  23  to rotate smoothly. 
         [0103]    In the winding and unwinding motion of the FPC board assembly  14 , the FPC board assembly  14  will not be buckled or entangled, owing to the presence of the slide aid  23 . In addition, the FPC board assembly  14  achieves a rolling contact with the slide aid  23  via the rollers  25  to reduce friction and wearing. As a result, the FPC board assembly  14  can be wound about and unwound from the slide aid  23 , while allowing the slide aid  23  to rotate smoothly. Therefore, the industrial robot  1  can be continuously operated for a long time, with the wrist  7  pivotally rotating at high speed with respect to the upper-front arm  6 . In realizing such a long-time use, the FPC board assembly  14  is prevented, as much as possible, from being problematically entangled or quickly worn out. 
         [0104]    Additionally, in the present embodiment, since the width of the rotary plate  24  is made smaller than the diameter of each roller  25 , there is no potential for the FPC board assembly  14  to be in contact with the rotary plate  24 . Also, of the ends E 3  and E 4  of the rotary plate  24 , the end E 3 , i.e. the end located inside the fold-back portion T, is provided with a roller  25 . Therefore, when the FPC board assembly  14  in the fold-back portion T pulls the slide aid  23  for rotation, the fold-back portion T comes into contact with the roller  25  concerned. Thus, the FPC board assembly  14  is configured to be wound about and unwound from the slide aid  25 , while being in contact with the rollers  25 . Therefore, low-frictional (low-wear) contact of the FPC board assembly  14  with the slide aid  23  can be more reliably achieved. 
         [0105]    In the present embodiment, the protective pole  26  is provided at a position deviated to the side of the outer periphery of the rotary plate  24 . Thus, such a concern can be prevented as much as possible that the gap between the core member  16  and the protective pole  26  becomes large to have the fold-back portion T pulled into the gap. 
         [0106]    Specifically, when the housing  15  is rotated in the direction of the arrow M as shown in  FIG. 10  and then reversely rotated in the direction of the arrow Las shown in  FIG. 9 , the slide aid  23  may also be rotated in the direction of the arrow L to allow the protective pole  26  to push the FPC board assembly  14  in the fold-back portion T. In this case, the FPC board assembly  14  may be softened due to the increase of the temperature as a result of long-time operation, or grease may have been coated on the core member  16  and the FPC board assembly  14  to reduce the frictional coefficient. Under these conditions, the FPC board assembly  14  may be in intimate contact with the core member  16  or with the FPC board assembly  14  wound about the core member  16 . As a result, the fold-back portion T may be curved to enter the gap between the core member  16  and the protective pole  26  as indicated by the dash-dot-dot line in  FIG. 11 . 
         [0107]    In this case, if the protective pole  26  has not been set up being deviated to the outer peripheral edge of the rotary plate  24  but has been set up covering the entire width of the rotary plate  24 , the gap between the core member  16  and the protective pole  26  will be narrow. As a result, the portion “t” of the fold-back portion T curved to enter the narrow gap (hereinafter revered to as “curved protrusion”) between the core member  16  and the protective pole  26  is undesirably pulled into the narrow gap. 
         [0108]    However, with the present embodiment, since the gap between the core member  16  and the protective pole  26  is ensured to be wide, the curved protrusion “t” can be prevented from being pulled into the gap between the core member  16  and the protective pole  26 . 
         [0109]    In the present embodiment, since the FPC board assembly  14  is made of plastic (e.g., polyimide resin) and thus has low frictional properties, the progress of wearing in the front and rear of the FPC board assembly  14  is very slow, enabling it to endure use over a long period of time. It is true that, when the housing  15  rotates relative to the reel  20 , the plurality of FPC boards  14 - 1  and  14 - 2  laid one over the other is slightly in friction. In the actual use, however, the low frictional properties coupled with the application of oil, will contribute to the very slow progress of wearing. 
         [0110]    Moreover, the FPC board assembly  14  is configured such that the signal FPC boards  14 - 2  having the narrow-width signal lines  27 - 2  are sandwiched between the power FPC boards  14 - 1  having the wide-width power lines  27 - 1 . Therefore, even when the FPC board assembly  14  is damaged being in contact with the rollers  25 , housing  15 , core member  16 , and the like, the narrow-width signal lines  27 - 2  can be prevented from being disconnected at an earlier stage. Accordingly, under the conditions where the wrist  7  is in high-speed relative rotation or operated for a long time, the FPC board unit can be prevented from being quickly damaged to thereby achieve a long life. 
         [0111]    In the present embodiment, the radial ribs  17   d  are formed in the inner surface of the first disc  17 . Therefore, the winding and unwinding motions of the FPC board assembly  14  are carried out with the FPC board assembly  14  being slidably in contact with the radial ribs  17   d . Accordingly, compared to the case where the FPC board assembly  14  is in contact with the inner surface of the first disc  17 , the wearing of the FPC board assembly  14  can be reduced. 
         [0112]    In fact, the second disc  18  is not provided with such radial ribs in its inner surface. As described above, the FPC board assembly  14  located inside the wiring case  13  has the ends E 2 - 1  and E 2 - 2  at the respective tips of which the extensions  14   b  are formed for connection with the housing  15 . It is considered that the reason why the second disc  18  is not provided with radial ribs is because the FPC board assembly  14  is pulled to the side of the first disc  17  via the extensions  14   b  and thus tends to be in contact with the inner surface of the first disc  17  while being in less contact with the inner surface of the second disc  18 . As a matter of course, the second disc  18  may be provided with radial ribs at its inner surface. 
         [0113]    In the present embodiment, the wiring case  13  is provided with the housing  15 , first disc  17  and second disc  18 , which are made of transparent plastic. Therefore, if only the outer covers  10  are detached in conducting periodic checks, the conditions of the FPC board assembly  14  inside the wiring cover  13  can be visually checked. Thus, the periodic checks such as on the entangling and wearing conditions or the occurrence of disconnection of the FPC board assembly  14 , can be readily conducted without the necessity of detaching the rotary joint wiring unit  12  from the rotary joint. 
         [0114]    The present invention is not limited to the first embodiment described above and illustrated in the drawings. The present invention may be extended or modified as set forth below. 
         [0115]    The plastic films of the FPC board assembly  14  are not limited to polyimide films, but may be PET (polyethylene terephthalate) films or other types of resin materials, such as polyamide resin. The length of the power FPC boards  14 - 1  may not be necessarily be differentiated from that of the signal FPC boards  14 - 2 . The guide groove  22  of the slide aid  23  may be formed on the side of the second disc  18 . In addition, the protective pole  26  may be formed to extend over the entire width of the rotary plate  24 . 
       Second Embodiment 
       [0116]    Referring now to  FIG. 12 , hereinafter will be described a second embodiment of the present embodiment. In the second and the subsequent embodiments, the components identical with or similar to those in the first embodiment are given the same reference numerals for the sake of omitting explanation. 
         [0117]      FIG. 12  is a vertical cross-sectional view illustrating a body of a rotary joint wiring unit, according to the second embodiment. As shown in  FIG. 12 , similar to the first embodiment, the FPC board assembly  14  is provided with an arc fold-back portion from where the winding direction of the FPC board assembly  14  is reversed. In the present embodiment, an inner protective board  51  is further laid over the plurality of FPC boards  14 - 1  and  14 - 2  that are laid one over the other, so that the inner protective board  51  is located innermost in the arc fold-back portion T. Also, an outer protective board  52  is further laid over the plurality of FPC boards  14 - 1  and  14 - 2 , so that the outer protective board  52  is located outermost in the arc fold-back portion T. 
         [0118]    The inner protective board  51  is made up of a plastic film, such as a polyimide film or a polyester film, with its thickness being set to substantially the same as that of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . The inner protective board  51  has a tension (i.e. rigidity) substantially of the same level as that of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . 
         [0119]    The outer protective board  52  is also made up of a plastic film, such as a polyimide film or a polyester film, with its thickness being made thicker than that of the inner protective board  51 . The outer protective board  52  has a tension appropriately stronger than the tension of the inner protective board  51 . 
         [0120]    When the housing  15  rotates relative to the reel  20  in the direction shown in an arrow L in  FIG. 12  in the configuration described above, the portion of the FPC board assembly  14  outside the slide aid  23  will be pulled in the direction of the arrow L. Thus, the FPC board assembly  14  outside the slide aid  23  will be apart from the inner surface of the housing  15  and wound about the slide aid  23 . Meanwhile, inside the fold-back portion T, the inner protective board  51  of the FPC board assembly  14  will pull and rotate the slide aid  23  in the direction of the arrow L. 
         [0121]    With this configuration, the inner protective board  51  is wound about the slide aid  23  to pull the slide aid  23 . Accordingly, the FPC board assembly  14  will not be directly in contact with the slide aid  23 , whereby wearing or the like of the FPC board assembly  14  can be prevented. 
         [0122]    On the other hand, when the housing  15  rotates relative to the reel  20  in the direction opposite to the direction of the arrow L in  FIG. 12 , i.e. in a direction indicated by an arrow M, the portion of the FPC board assembly  14  outside the slide aid  23  will be pushed by the housing  15 . Thus, the portion of the FPC board assembly  14  outside the slide aid  23  will be loosened, be part from the slide aid  23 , and be in contact with the inner surface of the housing  15  for rotation in the direction of the arrow M in integration with the housing  15 . With this rotation, the portion of the FPC board assembly  14  outside the slide aid  23  will be unwound from the slide aid  23  in pause. 
         [0123]    At the same time, in the fold-back portion T, the outer protective board  52  of the FPC board assembly  14  will be apart from the roller  25  at the end E 3  of the slide aid  23  to contact and push the protective pole  26  at the end E 4 . Then, when the protective pole  26  receives a pressing force from the fold-back portion T, the slide aid  23  will rotate in the direction of the arrow M. In this case, the FPC board assembly  14  moves such that, inside the slide aid  23 , it is wound about the core member  16 , and outside the slide aid  23 , it is unwound from the slide aide  23 . 
         [0124]    With the configuration described above, the outer protective board  52  is configured to contact and press the protective pole  26  of the slide aid  23 . Therefore, the FPC board assembly  14  will no longer be directly in contact with the protective pole  26 , whereby wearing or the like of the FPC board assembly  14  can be prevented. 
         [0125]    If the rigidity of the outer protective board  52  is low, the outer protective board  52  in the fold-back portion T, after being in contact with the protective pole  26 , will curves further (i.e. cannot keep the curved shape as it is, which shape has been achieved at the time of contacting the protective pole  26 ). Thus, the curved portion is in danger of being pulled into the gap between the core member  16  and the rollers  25  (or the gap between the rollers  25  and the housing  15 ). 
         [0126]    In this regard, in the present embodiment, the rigidity of the outer protective board  52 , per se, is ensured to be well strengthened, so that the outer protective board  52  has no excessive curved portion and is prevented from being pulled into the above gap. 
         [0127]    The other parts of the body of the rotary joint wiring unit of the second embodiment are configured in the same manner as in the first embodiment. Accordingly, in the second embodiment as well, substantially the same advantages as in the first embodiment can be obtained. 
         [0128]    In the second embodiment described above, the inner and outer protective boards  51  and  52  have been made up of plastic films. Alternatively, these protective boards may each be made up, for example, of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . In such a configuration, however, it is desirable that no current or signal is passed through the conductive lines of the power FPC board  14 - 1  or the signal FPC board  14 - 2  serving as the inner or outer protective board  51  or  52 . 
       Third Embodiment 
       [0129]    Referring to  FIG. 13  and  FIGS. 14A to 14C , hereinafter will be described a third embodiment. 
         [0130]      FIG. 13  is a vertical cross-sectional view illustrating a body of a rotary joint wiring unit, according to the third embodiment. As shown in  FIG. 13 , a substantially rectangular drive projection  53  is set up at the portion between two rollers  25  at the end E 3  of the rotary plate  24  of the slide aid  23 . The lower surface, as viewed in  FIG. 13 , of the drive projection  53  is shaped into an arc concave surface  53   a.    
         [0131]    In the third embodiment, the end E 4  of the rotary plate  24  is not provided with the protective pole  26 , but the rotary plate  24  is shortened by the length corresponding to the length required for providing the protective pole  26 . Thus, the rotary plate  24  is ensured to have a larger length between the ends E 3  and E 4 . 
         [0132]    In the third embodiment, as shown in  FIG. 13 , a flexible driving film sheet  54  is further laid over the plurality of FPC boards  14 - 1  and  14 - 2  that are laid one over the other, so that the driving film sheet  54  will be located innermost in the arc fold-back portion T. The driving film sheet  54  is made up of a plastic film, such as a polyimide film or a polyester film, with its thickness being substantially the same as that of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . The driving film sheet  54  has a tension (i.e. rigidity) substantially of the same level as that of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . The thickness of the driving film sheet  54  may be made larger than that of the power FPC board  14 - 1  or the signal FPC board  14 - 2  to provide a configuration having an appropriately strong tension. 
         [0133]    As shown in  FIG. 13 , the FPC board assembly  14 , when it is accommodated in the wiring case  13 , is configured to be located in the same manner as in the first embodiment, except a portion of the driving film sheet  54 . Specifically, a portion of the driving film sheet  54 , i.e. the portion of a fold-back portion U shown in  FIG. 13 , is accommodated in the wiring case  13  so as to be positioned between the second roller  25  from end E 3  of the slide aid  23  and the drive projection  53 . 
         [0134]    In this case, the length of the driving film sheet  54  is set to be shorter than that of the FPC board assembly  14 , so that the FPC board assembly  14  and the driving film sheet  54  can be accommodated in the wiring case  13  as described above. 
         [0135]    Referring to  FIGS. 14A to 14C , the operations of the rotary joint wiring unit according to the present embodiment will now described.  FIGS. 14A to 14C  are vertical cross-sectional views illustrating the operation of the rotary joint wiring unit of the present embodiment. 
         [0136]    First, referring to  FIG. 14A , the operation of the rotary joint wiring unit of the present embodiment is described in the case where, first, the housing  15  rotates relative to the reel  20  in the direction indicated by an arrow M in  FIG. 14A , with the driving film sheet  54  being wound about the roller  25  at the end E 3  of the slide aid  23 . With the rotation of the housing  15  in the direction of the arrow M, the portion of the FPC board assembly  14  outside the slide aid  23  will be pushed by the housing  15 . Thus, the portion of the FPC board  14  outside the slide aid  23  will be loosened, be apart from the slide aid  23 , and be in contact with the inner surface of the housing  15  for rotation in the direction of the arrow M in integration with the housing  15 . 
         [0137]    With this rotation, the portion of the FPC board  14  outside the slide aid  23  will be unwound from the slide aid  23  which is in pause. Meanwhile, the driving film sheet  54  disposed inner side of the FPC board assembly  14  and positioned in the fold-back portion U is permitted to be apart from the second roller  25  from the end E 3  of the slide aid  23  (see  FIG. 14B ). After that, with the further rotation of the housing  15  relative to the reel  20  in the direction of the arrow M, the drive film  54  in the fold-back portion U will contact and push the arc concave surface  53   a  of the drive projection  53  of the slide aid  23 . Then, when the drive projection  53  receives a pressing force from the driving film sheet  54  in the fold-back portion U, the slide aid  23  will rotate in the direction of the arrow M. In this case, the FPC board assembly  14  moves such that, inside the slide aid  23 , it is wound about the core member  16 , and outside the slide aid  23 , it is unwound from the slide aid  23 . 
         [0138]    With the configuration described above, the driving film sheet  54  is configured to contact and push the drive projection  53  of the slide aid  23 . Therefore, the FPC board assembly  14  will no longer be directly in contact with the rollers  25  of the slide aid  23 , whereby the wearing or the like of the FPC board assembly  14  can be prevented. 
         [0139]    It is likely that the driving film sheet  54  will eventually become worn because the driving film sheet  54  is configured to contact and push the drive projection  53 . However, since the driving film sheet  54  is dedicated to driving the rotary operations and does not carry power or signals, the possible wearing of the driving film sheet  54  will not cause serious problems. Also, the driving film sheet  54 , which is not required to carry signals or power, is not required to have insulation properties. Thus, the material or the thickness of the driving film sheet  54  can be determined as desired. Accordingly, the driving film sheet  54  may have a large thickness, or may be made of a less expensive material having high resistance to wearing. Thus, a board having high resistance to wearing can be used as the driving film sheet  54 . 
         [0140]    If the tension of the driving film sheet  54  is weak (or if the speed of the relative rotation between the housing  15  and the reel  20  is high), the driving film sheet  54  in the fold-back portion U, after being in contact with the drive projection  53 , may be inflated outward in the fold-back direction (i.e. cannot keep the curved shape as it is, which shape has been achieved at the time of contacting the drive projection  53 ). Hence, the curved portion may be undesirably pulled into the gap between the rollers  25  and the housing  15  (or the gap between the core member  16  and the rollers  25 ). 
         [0141]    In this regard, in the present embodiment, the tension (rigidity) of the driving film sheet  54  is ensured to be well strengthened (e.g., the tension) is set to a level equal to or stronger than that of the FPC board assembly  14 ), so that the curved portion will not be formed in the driving film sheet  54 , which curved portion may otherwise be pulled is into the above gap. In this case, the rigidity the driving film sheet  54  depends on the material or the thickness of the driving film sheet  54  (i.e. the larger the thickness is, the stronger the tension will be). Accordingly, the material and the thickness of the driving film sheet  54  may only have to be adjusted to obtain required intensity of tension. 
         [0142]    Referring to  FIG. 14C , the operation is described in the case where the housing  15  rotates relative to the reel  20  in the direction opposite to the direction indicated by the arrow M, i.e. in the direction indicated by an arrow L, with the driving film sheet  54  being in contact with the drive projection  53  of the slide aid  23  (see  FIG. 14C ). With the rotation of the housing  15  in the direction of the arrow L, the portion of the FPC board assembly  14  outside the slide aid  23  will be pulled in the direction of the arrow L, and thus the driving film sheet  54  located inner side of the FPC board assembly  14  and positioned in the fold-back portion U will be apart from the arc concave surface  53   a  of the drive projection  53  (see  FIG. 14B ). 
         [0143]    After that, with the further rotation of the housing  15  relative to the reel  20  in the direction of the arrow L, the driving film sheet  54  in the fold-back portion U will be wound about the second roller  25  from the end E 3  of the slide aid  23 , and will pull the roller  25  concerned as well as the slide aid  23  to rotate the slide aid  23  in the direction of the arrow L (see  FIG. 14A ). 
         [0144]    With this configuration, the driving film sheet  54  is configured to be wound about the second certain roller  25  of the slide aid  23  to pull the second roller  25 , i.e. the slide aide  23 . Thus, the FPC board assembly  14  will no longer be directly in contact with the rollers  25 , whereby the wearing or the like of the FPC board assembly  14  can be prevented. 
         [0145]    It is likely that the driving film sheet  54  becomes worn out because the driving film sheet  54  is configured to be wound about the second roller  25 . However, since the driving film sheet  54  is dedicated to driving the rotary operations and does not carry power or signals, the possible wearing of the driving film sheet  54  will not cause serious problem. Also, the driving film sheet  54 , which is not required to carry signals or power, is not required to have insulation properties. Thus, the material or the thickness of the driving film sheet  54  can be determined as desired. Accordingly, the driving film sheet  54  may have a large thickness, or may be made of a less expensive material having high resistance to wearing. Thus, a board having high resistance to wearing can be used as the driving film sheet  54 . 
         [0146]    In the third embodiment, the end E 4  of the rotary plate  24  is not provided with the protective pole  26  to decrease the length of the end E 4  by the length corresponding to the length required for providing the protective pole  26 , but to increase the distance between the ends E 3  and E 4  of the rotary plate  24 . Thus, the number of the FPC boards  14 - 1  and  14 - 2  may be increased. 
         [0147]    In the third embodiment, the configuration other than the configuration described above is the same as the configuration in the first embodiment. Accordingly, in the third embodiment as well, substantially the same advantages as in the first embodiment can be obtained. 
         [0148]    In the third embodiment, the driving film sheet  54  has been made up of a plastic film. Alternatively, the driving film sheet  54  may be made up, for example, of the power FPC board  14 - 1  or the signal FPC board  14 - 2 . In such a configuration, however, it is desirable that no current or signal is passed through the conductive lines of the power FPC board  14 - 1  or the signal FPC board  14 - 2  as the driving film sheet  54 . 
       Fourth Embodiment 
       [0149]    Referring to  FIGS. 15A to 15C , a fourth embodiment is described. 
         [0150]      FIGS. 15A to 15C  are vertical cross-sectional views illustrating the operation of a rotary joint wiring unit according to the fourth embodiment. 
         [0151]    In the fourth embodiment, as shown in  FIG. 15A , the end E 4  of the rotary plate  24  of the slide aid  23  is rotatably provided with two small-diameter protective rollers  55  and  56 , instead of the protective pole  26 , via respective shafts  24   c  and  24   c . The two protective rollers  55  and  56  are arranged along the widthwise direction of the rotary plate  24 . 
         [0152]    With this configuration, as shown in  FIG. 15A , when the housing  15  rotates relative to the reel  20  in the direction indicated by an arrow M, the portion of the FPC board assembly  14  outside the slide aid  23  will be pushed by the housing  15 . Thus, the portion of the FPC board assembly  14  outside the slide aid  23  will be loosened, be apart from the slide aid  23 , and be in contact with the inner surface of the housing  15  for rotation in the direction of the arrow M in integration with the housing  15 . 
         [0153]    With this rotation, the portion of the FPC board assembly  14  outside the slide aid  23  will be unwound from the slide aid  23  in pause. 
         [0154]    Meanwhile, the FPC board assembly  14  in the fold-back portion T will be apart from the roller  25  at the end E 3  of the rotary plate  24  of the slide aid  23 , and will be in contact, first, with the outer protective roller  55  of the two protective rollers  55  and  56  at the end E 4  so as to be driftingly drawn to the lower side (inner peripheral side) as viewed in  FIG. 15B . 
         [0155]    Thus, the FPC board assembly  14  in the fold-back portion T will be unlikely to be pulled into the gap between the protective roller  55  and the housing  15 . Also, when the FPC board assembly  14  in the fold-back portion T contacts and pushes the protective roller  55 , the protective roller  55  will rotate in the direction indicated by an arrow N 1  in  FIG. 15B . Thus, the sliding resistance will be lessened between the FPC board assembly  14  in the fold-back portion T and the protective roller  55 . 
         [0156]    Subsequently, as shown in  FIG. 15C , with the further relative rotation of the housing  15  in the direction of the arrow M, the FPC board assembly  14  in the fold-back portion T will also contact and push the inner protective roller  56  of the two protective rollers  55  and  56 . In this case, the protective roller  56  will rotate in the direction indicated by an arrow N 2  in  FIG. 15C . Thus, the sliding resistance will be lessened between the FPC board assembly  14  in the fold-back portion T and the protective roller  56 . 
         [0157]    The two protective rollers  55  and  56  are ensured to be arranged such that the FPC board assembly  14  in the fold-back portion T, when comes into contact with the two rollers  55  and  56 , may first contact the outer roller  55  and then, a little later, contact the inner roller  56 . Thus, the two protective rollers  55  and  56  will receive a pressing force from the fold-back portion T to rotate the slide aid  23  in the direction of the arrow M. 
         [0158]    With the configuration described above, the FPC board assembly  14  in the fold-back portion T contacts and pushes the two protective rollers  55  and  56  of the slide aid  23 . Accordingly, compared to the configuration in which the FPC board assembly  14  comes into contact with the protective pole  26  of the slide aid  23 , the sliding resistance between the FPC board assembly  14  and the protective rollers  55  and  56  is reduced (i.e. the friction is reduced), by an amount corresponding to one free rotation of the protective rollers  55  and  56 . Therefore, the wearing or the like of the FPC board assembly  14  can be further prevented. 
         [0159]    In particular, in the fourth embodiment, the FPC board assembly  14  in the fold-back portion T, when comes into contact with the two protective rollers  55  and  56 , is configured to contact the outer roller  55  first and then, a little later, contact the inner roller  56 . With this configuration, the FPC board assembly  14  in the fold-back portion T, when it first contacts the outer roller  55 , is driftingly drawn to the lower side as viewed in  FIG. 15B . Thus, the FPC board assembly  14  in the fold-back portion T will be unlikely to be pulled into the gap between the protective roller  55  and the housing  15 . As a result, the fold-back portion T can be prevented from being pulled into the above gap. 
         [0160]    In the fourth embodiment, the configuration other than the configuration described above is the same as the configuration in the first embodiment. Accordingly, in the fourth embodiment as well, substantially the same advantages as in the first embodiment can be obtained. 
         [0161]    The present invention may be embodied in several other forms without departing from the spirit thereof. The embodiments and modifications described so far are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them. All changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.