Abstract:
There is provided a part holding head assembly of a part mounting device including: a rotary head; a spindle configured to rotate in a first direction T with respect to a central axis of the spindle and configured to move in a second direction Z substantially parallel with an extending direction of the central axis of the spindle; a nozzle provided at a first end of the spindle; a raising part configured to move the spindle to in the second direction Z; and a contact detecting sensor configured to sense the nozzle contacting a part or the part held by the nozzle contacting a substrate and configured to generate a contact sensing signal according to the sensing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2014-205016 filed on Oct. 3, 2014 and Japanese Patent Application No. 2014-205017 filed on Oct. 3, 2014 in the Japanese Patent Office the disclosures of which are incorporated herein in their entireties by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Apparatuses consistent with exemplary embodiments relate to a part holding head assembly for a chip mounting device, and more particularly, to a part holding head assembly of a part mounting device mounting an electronic part, the part holding head assembly including a nozzle configured to hold the electronic part such as an integrated circuit (IC) chip or the like, on a substrate. 
         [0004]    2. Description of the Related Art 
         [0005]    In the related art, a part mounting device moves a part holding head assembly above a part supply unit and enables a nozzle of the part holding head assembly to perform descending and ascending operations at that location. Thereafter, the part mounting device adsorbs a part at a lower end of the nozzle in a vacuum to pick up the part, moves the part holding head assembly above a substrate, and enables the nozzle to re-perform the descent or ascent operation so as to mount the part in a preset coordinate position of the substrate. 
         [0006]    During the operation described above, if the descending and ascending operations of the nozzle are performed to pick up the part into the nozzle, and an ascent stroke of the nozzle is too strong, the lower end of the nozzle may press on an upper surface of the part with an extensive force and thus may break the part. If the descent stroke is too weak, the nozzle may fail to contact the upper surface of the part and thus may fail to pick up the part. Further, if the descent stroke of the nozzle is too strong, the part adsorbed at the lower end of the nozzle may be pressed with an extensive force and may be broken on the substrate. Meanwhile, if the descent stroke is too weak, the part held in the nozzle fails to contact the upper surface of the substrate and thus fails to mount the part. Therefore, the descent stroke of the nozzle should be accurately controlled. 
         [0007]    As a method of accurately controlling an ascent/descent stroke of a nozzle, Japanese Patent No. 3543044 discloses a method of using a sensing unit (a contact detecting sensor) for sensing a contact of a nozzle. 
         [0008]    As another method of accurately controlling a descent stroke of a nozzle, Japanese Patent Application No. 2013-212220 discloses a method of using a reflective light sensor (an optical fiber sensor) as a contact detecting sensor. The optical fiber sensor includes a light-emitting unit which emits light toward an outside reflective surface of the nozzle, a light-receiving unit that receives reflected light reflected from the outer reflective surface, and a sensor unit that continuously measures a received amount of reflected light. When the received amount decreases to be lower than or equal to a threshold value, the optical fiber sensor determines that the nozzle contacts a part to generate a contact sensing signal. 
         [0009]    In other words, light emitted from the light-emitting unit of the optical fiber sensor is focused on the reflective surface before the nozzle contacts the part. Therefore, if upper and lower positions of the nozzle are changed due to a contact of the nozzle with the part, an amount of reflected light reflected from the reflective surface decreases, and an amount of light received by the light-receiving unit of the optical fiber sensor decreases. In the related art, an amount of received light by the light receiving unit is compared with a predetermined threshold value, and when the amount of received light by the light receiving unit decreases to be lower than or equal to the threshold value, the optical fiber sensor determines that the nozzle contacts the part and thus stops descending of the nozzle based on the determination. 
         [0010]    In the related art, the threshold value is set to a fixed value based on a pre-experimental result using an optical fiber sensor. Therefore, each type of different nozzles requires the pre-experimental results on the threshold value A. Also, as a nozzle is used, foreign material may become stuck onto a reflective surface of the nozzle. Therefore, before the same nozzle contacts a part, an amount of received light may be changed as time elapses. If an amount of received light is changed according to a nozzle or a time elapse before a nozzle contacts a part as described above, and the threshold value A is set to a fixed value, the following problems occur. 
         [0011]    If, for example, an amount of received light, which is measured before a nozzle contacts a part, is lower than the threshold value A, an optical fiber sensor erroneously determines that the nozzle contacts the part when the nozzle actually does not contact the part. In this case, before the nozzle actually contacts the part, a picking-up (adsorbing) operation or a mounting operation of the part starts, and thus a picking-up error or a mounting error occurs. Also, the nozzle may be in a state where a contact of the nozzle may not be sensed. 
         [0012]    If, for example, an amount of received light is significantly higher than the threshold value before a nozzle contacts a part, the amount of received light does not each a value lower than or equal to the threshold value even after the nozzle contacts the part. Therefore, although the amount of received light decreases, the amount of received light takes a long time to reach the threshold value A. As a result, a long time is taken from when the nozzle contacts the part to when the nozzle stops, and thus the nozzle excessively presses on the part, and the part may be damaged. In other words, the pressing of the nozzle onto the part may not be accurately controlled. 
         [0013]    A contact sensing function performed by an existing contact detecting sensor (or an optical fiber sensor) is not sufficient for an adaptability to a change in an amount of received light before the nozzle contacts the part due to a difference in a shapes or status of the nozzle or the like. In other words, a robust performance of contact sensing function is still desired. 
         [0014]    Also, in the related art, an outside reflective surface of a nozzle may become dirty or a reflective plate may be installed to form a reflective surface in the nozzle. In this case, a sufficient amount of received light may not be acquired from before the nozzle contacts the part. Therefore, a contact of the nozzle may not be sensed or although the contact of the nozzle is sensed, an accuracy of the contact of the nozzle is lowered. 
         [0015]    Therefore, an optical abnormality of a nozzle, cleanness of a reflective surface, uninstalling of the reflective surface, or the like, needs to be accurately sensed in real time to accurately sense a contact of the nozzle through a contact detecting sensor, such as an optical fiber sensor that is a reflective optical fiber sensor, or the like. 
       SUMMARY 
       [0016]    One or more exemplary embodiments provide a part holding head of a part mounting device including a contact detecting sensor sensing a contact of a nozzle to improve a robust performance of a contact sensing function of the contact detecting sensor. 
         [0017]    One or more exemplary embodiments provide a part holding head of a surface mounting device including a contact detecting sensor sensing a contact of a nozzle to accurately sense an optical abnormality of a part holding unit interfering with contact sensing of the contact detecting sensor online. 
         [0018]    Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments. 
         [0019]    According to an aspect of an exemplary embodiments, there is provided a part holding head assembly of a part mounting device including a holding head, a spindle configured to rotate in direction T with respect to a central axis of the holding head and ascend in direction Z following a longitudinal direction of the central axis of the holding head, a nozzle configured to be installed at a lower end of the spindle through an elastic body, a raising part configured to enable the spindle to ascend in the direction Z, and a contact detecting sensor configured to operate with the raising part to enable the raising part to ascend in the direction Z and sense that the nozzle contacts a part or the part held by the nozzle contacts a substrate so as to generate a contact sensing signal. The contact detecting sensor may include a light-emitting unit configured to emit light toward an outer reflective surface of the nozzle, a light-receiving unit configured to receive reflected light reflected from the outer reflective surface, and a sensor unit configured to continuously measure a received amount of the reflected light and generate the contact sensing signal when the received amount of the reflected light decreases to be lower than or equal to a threshold value, wherein the contact detecting sensor sets the threshold value based on the received amount of the reflected light after ending a T-direction rotation operation of the spindle before the nozzle to be sensed contacts the part. 
         [0020]    Whenever performing an operation of enabling the nozzle to contact the part, the contact detecting sensor may set the threshold value based on the received amount of the reflected light after ending the T-direction rotation operation of the spindle before the nozzle contacts the part. 
         [0021]    A plurality of spindles may be disposed along a circumferential direction of a rotary head configured to be installed so as to rotate in direction R with respect to the central axis of the holding head. The contact detecting sensor may set the threshold value based on the received amount of the reflected light after ending the T-direction and R-direction rotation operations of the spindle before the nozzle to be sensed contacts the part. 
         [0022]    Whenever an operation of enabling the nozzle to contact the part, the contact sensing sensor may set the threshold value based on the received amount of the reflected light after ending the T-direction and R-direction rotation operations of the spindle before the nozzle contacts the part. 
         [0023]    In descriptions of exemplary embodiments, “a contact of a nozzle” is a concept that includes all of an operation of enable a lower end of the nozzle to contact an upper surface of a part and an operation of enable the part held at the lower end of the nozzle to contact an upper surface of a substrate. Therefore, even if only the operation of enabling the nozzle to contact the part is described in relation to “the contact of the nozzle”, it is understood that “the contact of the nozzle” is equally applied to the operation of enabling the part held at the nozzle to contact the substrate. Also, “the contact” of the nozzle may be used as the term “landing”. 
         [0024]    According to an aspect of an exemplary embodiments, there is provided a part holding head assembly of a part mounting device including: a rotary head; a spindle configured to rotate in a first direction T with respect to a central axis of the spindle and configured to move in a second direction Z substantially parallel with an extending direction of the central axis of the spindle; a nozzle provided at a first end of the spindle; a raising part configured to move the spindle to in the second direction Z; and a contact detecting sensor configured to sense the nozzle contacting a part or the part held by the nozzle contacting a substrate and configured to generate a contact sensing signal according to the sensing, wherein the contact detecting sensor may include: a light-emitting unit configured to emit light toward an outer reflective surface of the nozzle; a light-receiving unit configured to receive reflected light reflected from the outer reflective surface; and a sensor unit configured to measure a received amount of the reflected light and to generate the contact sensing signal in response to the received amount of the reflected light decreases to be lower than or equal to a threshold value, and wherein the contact detecting sensor is configured to set the threshold value based on an amount of the reflected light received by the light receiving unit after completion of a T-direction rotation operation of the spindle before the nozzle contacts the part. 
         [0025]    The contact detecting sensor may be configured to set the threshold value based on the received amount of the reflected light after the completion of the T-direction rotation operation of the spindle before the nozzle contacts the part at each time the raising part moves the nozzle to contact the part or move the part held by the nozzle to contact the substrate. 
         [0026]    The contact detecting sensor may be configured to generate an abnormality signal in response to the received amount of the reflected light exceeding a preset range and configured to set the threshold value in response to the received amount of the reflected light being in the preset range, the received amount of the reflected light being measured after the completion of the T-direction rotation operation of the spindle. 
         [0027]    The spindle may include a plurality of spindles, the plurality of spindles provided along a circumferential direction of the rotary head and configured to rotate in a third direction R with respect to a central axis of the rotary head, and the contact detecting sensor may be configured to set the threshold value based on the received amount of the reflected light after completion of the T-direction rotation and R-direction rotation operations of the spindle before the nozzle contacts the part. 
         [0028]    The contact sensing sensor may be configured to set the threshold value based on the received amount of the reflected light after the completion of the T-direction rotation and R-direction rotation operations of the spindle before the nozzle contacts the part at each time the raising part moves the nozzle to contact the part or move the part held by the nozzle to contact the substrate. 
         [0029]    The contact detecting sensor may be configured to generate an abnormality signal in response to the received amount of the reflected light exceeding a preset range and configured to set the threshold value in response to the received amount of the reflected light being in the preset range, the received amount of the reflected light being measured after the completion of the T-direction rotation and R-direction rotation operations of the spindle before the nozzle contacts the part. 
         [0030]    The part holding head assembly may further include: a raising controller configured to control a movement of the raising part so as to control a corresponding movement of the nozzle, wherein the raising controller may be configured to control the movement of the nozzle with a movement profile heading toward a preset target contact position, and in response to the raising controller receiving the abnormality signal, the raising controller may be configured to control the nozzle to move in a direction away from the part when the nozzle reaches the target contact position regardless of the contact sensing signal being received. 
         [0031]    The contact detecting sensor may be configured to set the threshold value based on the received amount of the reflected light measured at a measured position along a circumferential direction of the outer reflective surface, the measured position corresponding to a final position along the circumferential direction of the outer reflective surface sensed by the contact detecting sensor when the nozzle contacts the part. 
         [0032]    The contact detecting sensor may be fixedly attached to the raising part. 
         [0033]    The contact detecting sensor may be configured to emit the light toward the outer reflective surface of the nozzle at an intersecting direction of the second direction. 
         [0034]    An elastic body may be provided between the nozzle and the spindle. 
         [0035]    The contact detecting sensor may be configured to determine whether the nozzle contacts the part based on a comparison between the threshold value and an amount of the reflected light received by the light-receiving unit at a final position along a circumferential direction of the outer reflective surface sensed by the contact detecting sensor when the nozzle contacts the part. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The above and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
           [0037]      FIG. 1  is a perspective view of a structure of a part holding head assembly according to an exemplary embodiment; 
           [0038]      FIG. 2  illustrates a device that enables a spindle (or a nozzle) to ascend and/or descend in a vertical direction in the part holding head assembly of  FIG. 1 ; 
           [0039]      FIG. 3  illustrates an element disposed around a raising part that enables the spindle (or the nozzle) of  FIG. 2  to ascend and/or descend in a vertical direction; 
           [0040]      FIG. 4A  illustrates a rotary head including a plurality of spindles that is in an initial position; 
           [0041]      FIG. 4B  illustrates a selected spindle of the plurality of spindles descending in the vertical direction; 
           [0042]      FIG. 5  is an enlarged perspective view of a cross-section of a nozzle installed at a lower end of each of a plurality of spindles; 
           [0043]      FIG. 6  is a graph schematically illustrating changes in an amount of received light of an optical fiber sensor when a nozzle contacts a part; and 
           [0044]      FIG. 7  illustrates a method of setting a threshold value in the part holding head assembly of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain aspects of the present description. 
         [0046]      FIG. 1  is a perspective view of a structure of a part holding head assembly  10  according to an exemplary embodiment. 
         [0047]    In the part holding head assembly  10  of a part mounting device according to the exemplary embodiment, a rotary head  30  is a rotary head type part holding head and is installed in a head body  20 , which is fixedly disposed, to rotate in a first rotational direction R with respect to a central axis of the head body  20 . A plurality of spindles  31  are disposed in the rotary head  30  at equal intervals along a circumferential direction thereof, and nozzles  32  are respectively installed at lower ends of the plurality of spindles  31  to adsorb a part  60 . 
         [0048]    The rotary head  30  rotates in the first rotational direction R due to driving of an R servo motor  21  installed in the head body  20 . The spindles  31  rotate in a second rotational direction T around an axis of a T servo motor  22  due to driving of the T servo motor  22 . 
         [0049]    Also, a Z servo motor  23  is disposed in the head body  20  to move a selected spindle  31   a  of the plurality of spindles  31 , which is installed in a particular position, in a vertical direction Z (i.e., a gravitational direction) corresponding to a longitudinal direction of the selected spindle  31   a . A device that rotates the rotary head  30  in the first rotational direction R due to driving of the R servo motor  21  and a device that rotates each of the plurality of spindles  31  in the direction T due to driving of the T servo motor  22  are well known, and thus descriptions of such devices are omitted. A device that moves the selected spindle  31   a  to descend/ascend due to driving of the Z servo motor  23  will now be described. 
         [0050]      FIG. 2  illustrates a device that moves a selected spindle  31   a  having a nozzle  32  to ascend/descend in the vertical direction Z in the part holding head assembly  10  of  FIG. 1 . 
         [0051]      FIG. 2  illustrates a device that moves the selected spindle  31   a  to ascend/descend in the vertical direction Z in the part holding head assembly  10  of  FIG. 1 . A motor shaft of the Z servo motor  23  disposed in the head body  20  is connected to a screw shaft  24   a  of a ball screw device  24 , and a nut  24   b  is installed at the screw shaft  24   a . A raising part  25  is connected to the nut  24   b . Therefore, the nut  24   b  and the raising part  25  move together in the vertical direction Z in response to driving of the Z servo motor  23 . 
         [0052]    The raising part  25  is provided on a side of the head body  20 . Before the selected spindle  31   a  is moved in the vertical direction Z, the rotary head  30  including the plurality of spindles  31  rotates relatively with respect to the raising part  25  to select a selected spindle  31   a  of the plurality of the spindles disposed in a particular position and move the raising part  25  in the vertical direction Z so as to move the selected spindle  31   a  in the vertical direction Z. 
         [0053]      FIG. 3  illustrates an element disposed around a raising part  25  that enables the spindle  31  (or the nozzle) of  FIG. 2  to move in the vertical direction Z. 
         [0054]    In the exemplary embodiment, as shown in  FIG. 3 , the rotary head  30  is rotated in the first rotational direction R to move the plurality of spindles  31  with respect to the raising part  25  and enables a selected spindle  31   a  of the plurality of spindles  31 , which is positioned directly below the raising part  25 , to descend/ascend in the vertical direction Z. However, the selected spindle  31   a  of the plurality of spindles  31  disposed in the particular position along the circumferential direction of the rotary head  30  to ascend/descend is not particularly limited thereto. For example, the part holding head assembly  10  of a part mounting device may move a raising part  25  to select a spindle  31   a  to be descended/ascended. Also, the particular position may be two or more depending on the design of the part holding head assembly  10  including the raising part  25 . 
         [0055]    Referring back to  FIG. 2 , an optical fiber sensor  40 , which is an example of a contact detecting sensor, is connected to the nut  24   b , to which the raising part  25  is connected, through a connection bar  26  and a spline nut  28  installed at a spline shaft  27  fixedly provided in the head body  20 . In other words, the optical fiber sensor  40  is fixedly attached to the raising part  25  via the spline nut  28 , the spline shaft  27  and connection bar  26 . Therefore, if the raising part  25  moves in the vertical direction Z due to driving of the Z servo motor  23 , the optical fiber sensor  40  moves along together with the raising part  25  in the vertical direction Z. Here, “the moving of the optical fiber sensor  40  together with the raising part  25 ” means that the optical fiber sensor  40  and the raising part  25  are mechanically formed into one body to move together or to be driven by different driving units but the optical fiber sensor  40  and the raising part  25  move at the same speed as illustrated in  FIGS. 4A and 4B . 
         [0056]      FIGS. 4A and 4B  illustrate the selected spindle  31   a  with the nozzle  32  attached thereto that is moved (e.g., the selected spindle  31   a  descends) by the raising part  25  of  FIG. 2 .  FIG. 4A  illustrates the spindle  31   a  that is in an initial position.  FIG. 4B  illustrates the selected spindle  31   a  of the plurality of spindles  31  that descends by the raising part  25  of  FIG. 2 . Also, the selected spindle  31   a  is constantly biased to the initial position as shown in  FIG. 4A  by an elastic body  33  (refer to  FIG. 2 ) formed of two coil springs. 
         [0057]      FIG. 5  is an enlarged perspective view of a cross-section of a nozzle  32  installed at a lower end of each of a plurality of spindles  31 .  FIG. 6  is a graph schematically illustrating changes in an amount of received light of an optical fiber sensor  40  when the nozzle  32  contacts a part  60  ( FIG. 7 ). 
         [0058]    A light-emitting unit  41  and a light-receiving unit  42  are assembled together with an optical fiber or a lens  40   a  on the same shaft line to form the optical fiber sensor  40 , and the optical fiber sensor  40  is well known. In the exemplary embodiment, as shown in  FIG. 2 , the optical fiber sensor  40  is disposed above the nozzle  32  to be slanted or inclined with respect to the vertical direction Z, wherein the nozzle  32  is installed at the lower end of each of the plurality of spindles  31 . In the exemplary embodiment, a coil spring  34  (or an elastic member) is provided between the nozzle  32  and each of the plurality of spindles  31 . The light-emitting unit  41  of the optical fiber sensor  40  emits light P toward a reflective surface  32   a  of the nozzle  32 . The emitted light P from the light-emitting unit  41  is reflected by the reflective surface  32   a  of the nozzle  32  and is received by the light-receiving unit  42  of the optical fiber sensor  40 . 
         [0059]    Here, as described above, the nozzle  32  is installed at the lower end of each of the plurality of spindles  31  through the coil spring  34 . Here, if the nozzle  32  installed at the lower end of the spindle  31  contacts the spindle  31  due to the vertical movement of the selected spindle  31   a , the coil spring  34  is compressed and the position of the nozzle  32  is changed with respect to the spindle  31 . In detail, the nozzle  32  relatively moves toward the lower end of the spindle  31 . 
         [0060]    “The contact of the nozzle  32 ” means that a force is applied at a lower part of the nozzle  32 . This case includes a case where the nozzle  32  moves downwards in a process of picking up a part, and then the lower part of the nozzle  32  contacts an upper surface of the part and a case where the part adsorbed by the lower part of the nozzle  32  contacts an upper surface of a substrate in a process of mounting the part. 
         [0061]    The light P emitted from the light-emitting unit  41  of the optical fiber sensor  40  is focused by a lens  40   a  of  FIG. 2  on the reflective surface  32   a  before the nozzle  32  moves toward the spindle  31  and contacts the spindle  31 . Therefore, if the nozzle  32  moves toward the spindle  31  and contacts the spindle  31 , and thus the position of the nozzle  32  is changed with respect to the spindle  31 , an amount of reflected light reflected from the reflective surface  32   a  may change (i.e., decreases), and an amount of received light received by the light-receiving unit  42  of the optical fiber sensor  40  may also change (i.e., decreases) as illustrated in  FIG. 6 . In the exemplary embodiment, the decrease in the amount of received light is sensed by a sensor unit  43  of the optical fiber sensor  40 . 
         [0062]    Also, the sensor unit  43  continuously measures the amount of received light, and when the amount of received light is determined to be lower than or equal to a threshold value A as illustrated in  FIG. 6 , the sensor unit  43  determines that the nozzle  32  contacts the spindle  31  and generates a contact sensing signal. 
         [0063]      FIG. 7  illustrates a method of setting a threshold value A in the part holding head assembly  10  of  FIG. 1 . 
         [0064]    A method of setting the threshold value A in a part holding head assembly  10  according to an exemplary embodiment will now be described with reference to  FIG. 7 . Also,  FIG. 7  illustrates a process of mounting a part  60  on a substrate  70 , but the method of setting the threshold value A is the same for a process of picking up a part  60 . 
         [0065]    As shown in  FIG. 7 , a Z-direction (i.e., a vertical direction) descending operation (or Z-axis descending) of the nozzle  32  starts at time A. In an initial step of the Z-axis descending operation, the nozzle  32  simultaneously performs a T-direction rotation operation (or a T-axis rotation) and an R-direction rotation operation (or an R-axis rotation) described in  FIG. 3  to descend. 
         [0066]    The sensor unit  43  of the optical fiber sensor  40  automatically sets the threshold value A based on an amount of received light actually measured immediately after the T-axis rotation and the R-axis rotation are completed (at time B of  FIG. 7 ) before the nozzle  32  to be sensed contacts the part and determines whether the nozzle  32  contacts the part, based on the threshold value A. In an example of  FIG. 7 , the amount of received light is lower than or equal to the threshold value A at time C, and thus the sensor unit  43  determines that the nozzle  32  contacts the part at the time C. The setting of the threshold value A is repeated in each operation of mounting (picking up) a part by the nozzle  32 , and the threshold value A is updated on each case (e.g., at each descending operation). 
         [0067]    If the amount of received light exceeds an allowable range at the time B, the optical fiber sensor  40  determines that an optical abnormality occurs in the nozzle  32  and generates an abnormality signal. If a controller  50  (a raising controlling unit of  FIG. 2 ) that controls the Z servo motor  23  receives the abnormality signal, the controller  50  controls the Z servo motor  23  so as to move the selected spindle  31   a  having the nozzle  32  in an ascending direction if the nozzle  32  reaches a target contact position regardless of whether to receive the contact sensing signal. 
         [0068]    Here, the target contact position is set based on device configuration data of a surface mounting device, data about heights of a substrate and a part, or the like and is pre-given with respect to the controller  50 . The controller  50  controls the Z servo motor  23  to control the descent of the nozzle  32  with a descent profile (e.g., a Z-axis speed profile as shown in  FIG. 7 ) heading toward the target contact position. 
         [0069]    In the exemplary embodiment, the controller  50  (or the Z servo motor  23 ) uses the reception of the contact sensing signal as a trigger as shown in  FIG. 7  and stops descending of the nozzle  32  in consideration of a necessary amount of pressing force on the part  60 . However, if the controller  50  receives the abnormality signal indicating the optical abnormality of the optical fiber sensor  40 , an accurate contact sensing signal may not be acquired. Therefore, the controller  50  (or the Z servo motor  23 ) moves the selected spindle  31   a  having the nozzle  32  in the ascending direction if the nozzle  32  reaches the target contact position regardless of whether to receive the contact sensing signal. Therefore, the nozzle  32  may be prevented from descending excessively. 
         [0070]    A control program of the part holding head assembly  10  may control a timing of determining whether the nozzle  32  has the optical abnormality and a timing of ending the T-axis rotation and the R-axis rotation for setting the threshold value A. In practice, for example, determining the time B of  FIG. 7  (timing of time B) may be determined based on an encoder value of the Z servo motor  23  that enables the nozzle  32  to ascend. 
         [0071]    The controller  50  may determine whether the optical abnormality occurs in the nozzle  32 , based on an amount of received light actually measured after ending the T-axis and R-axis rotations, to accurately sense the optical abnormality of the nozzle  32 . In other words, if foreign material is stuck onto a part of a T-direction circumference of the reflective surface  32   a  of the nozzle  32 , the foreign material affects the amount of received light according to a T-direction position of the nozzle  32 . Therefore, the controller  50  may measure an amount of received light before the nozzle  32  contacts the part  60 , in the same position as a final position of the nozzle  32  taken when the nozzle  32  contacts the part, to accurately sense the optical abnormality of the nozzle  32 . 
         [0072]    The controller  50  may set the threshold value A based on the amount of received light actually measured after ending the T-axis and R-axis rotations to set a best threshold value A in each nozzle  32  that is an object to be sensed. In other words, if the T-axis and R-axis rotations are completed, a position of a rotation direction of the nozzle  32  becomes equal to the final position taken when the nozzle  32  contacts the part. The threshold value A is set based on the amount of received light measured before the nozzle  32  contacts the part when the position of the nozzle  32  is equal to the final position. Therefore, the threshold value A is best as a determination reference value for sensing contact using a decrease in an amount of received light. Also, as described above, the controller  50  determines whether the nozzle  32  contacts the part, by using the threshold value A set in each nozzle  32 . Therefore, although an amount of received light measured before the nozzle  32  contacts the part is changed according to a difference in shapes or status of the nozzle  32  or the like, the threshold value A is set in real time so as to improve a robust performance of a contact sensing function of the optical fiber sensor  40 . 
         [0073]    Although the amount of received light measured before the nozzle  32  contacts the part decreases due to the foreign material sticking on the nozzle  32  or the like as time elapses, an accurate threshold value A is set according to an amount of received light measured whenever the nozzle  32  is used. That is, threshold value A is set in real time during the operation. Therefore, the robust performance may be improved, and an available period of the nozzle  32  may be extended. If the threshold value A is a fixed value, and the amount of received light measured before the nozzle  32  contacts the part decreases, the above-described problems occur. Therefore, the nozzle  32  may be replaced with a new one in an initial stage. In the above-described structure, the part mounting device including the part holding head assembly  10  adsorbs and picks up a part from a part supply unit through the nozzle  32  installed at the lower end of the spindle  31  to transfer the part onto a print board and mount the part on a preset position of the print board. When adsorbing and mounting the part, the sensor unit  43  of the optical fiber sensor  40  determines whether the nozzle  32  contacts the part, based on the threshold value A set in each nozzle  32  according to the above-described method. If the contact of the nozzle  32  is sensed, the sensor unit  43  generates a contact sensing signal. The contact sensing signal is transmitted to the controller  50  (or a controlling unit) of  FIG. 2 . If the controller  50  receives the contact sensing signal, the controller  50  stops the Z servo motor  23  that enables the raising part  25  to descend. As a result, a descent stroke of the nozzle  32  is accurately controlled so as to enable the nozzle  32  to accurately contact the part. 
         [0074]    In the above-described exemplary embodiment, the sensor unit  43  of the optical fiber sensor  40  is installed separately from the controller  50 , but a function of the sensor unit  43  may be assembled with the controller  50 . Also, in addition to the optical fiber sensor  40 , a reflective optical sensor may be used as the contact detecting sensor. In addition, the exemplary embodiment may be applied a rotary head type and a part holding head, i.e., a part holding head that does not accompany an R-direction rotation operation. In this case, the sensing of the optical abnormality of the nozzle  32  and the setting of the threshold value A are performed after ending the T-axis rotation. 
         [0075]    In a part holding head assembly of a part mounting device according to exemplary embodiments as described above, a threshold value of an amount of received light that is a standard of contact sensing is set based on an amount of received light actually measured before a nozzle to be sensed contacts a part. Also, a measurement of an amount of received light used for setting the threshold value is performed after ending T-direction and R-direction rotation operations. In other words, the setting of the threshold value is performed before the nozzle contacts the part in the same position (or a rotation direction position) as a final position taken when the nozzle contacts the part. 
         [0076]    Therefore, a robust performance of a contact sensing function of a contact detecting sensor may be improved. Also, a descent stroke of the nozzle holding the part may be further accurately controlled. 
         [0077]    While exemplary embodiments have been particularly shown and described above, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventive concept as defined by the following claims.