This invention relates generally to workpiece processing, and more particularly to the adjustment of clamps used to hold a workpiece in position during welding. 
     BACKGROUND OF THE INVENTION 
     The door of an automotive vehicle typically consists of an inner panel and an outer panel, both made of a resilient metal such as steel. The panels are preliminarily held together by overlapping flanges along the edges and then welded together at a welding station where the door is retained in a fixed position by several clamps. 
     It is essential that the door be held correctly without bending or flexing the panels, so that the geometry of the door is right for welding. Otherwise, when released after welding, the door may spring-back to a slightly warped condition. This may prevent the door, when installed in a vehicle, from sealing properly against the frame, producing wind noise, water leaks and an unsightly appearance. To eliminate the problem, the clamps used to hold the door during welding, or at least some of the clamps, must be adjusted. In the past, it has been necessary to make the adjustment manually by shimming the clamps. The operator made this adjustment based on his experience and judgment. This was very time-consuming and subject to error. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, each clamp is mounted on a slide and can be adjusted back and forth along the slide by a motor. In practice, a workpiece such as a vehicle door is placed at the weld station on support pads and gripped by the clamps. After welding, the door is moved to a checking station where critical points on the door are measured. These measurements are preferably taken by laser optic cameras. 
     The measurement data is transmitted to a central processing unit (CPU) at an optical station where it is stored in a computer. The measurement data is then compared with ideal or “nominal” critical point locations to determine the error if any, that is, the departure of the measurement data from the ideal. If the departure or error is outside the maximum tolerance, the CPU will interpolate the data with an algorithm and determine the amount of clamp adjustment needed. 
     In an automatic system, the adjustment data is transmitted from the CPU to a servo-motor controller to actuate the motors to make the necessary adjustment in the position of one or more of the clamps. In a manual system, the CPU will merely record the error which an operator will interpret and then manually input the adjustment to the servo-motor controller. The adjustment can be made rapidly and with a high degree of accuracy. The adjustment causes one or more clamps to bend the workpiece into a profile or shape which corresponds to a preferred or ideal shape. The workpiece is then welded while held in this ideal profile. 
     One object of the invention is to provide method and apparatus for quickly and accurately adjusting the positions of clamps used to hold a workpiece during processing, having the foregoing features and capabilities. 
    
    
     Other objects, features and advantages will become more apparent as the following description proceeds, especially when considered with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing a door supported at a welding station by support pads and clamping fixtures, in accordance with the invention. 
     FIG. 2 is a plan view of the door after it has been welded and transferred to a checking station for measurements by laser optic cameras. 
     FIG. 3 is an elevational view showing the door supported at the welding station. 
     FIG. 4 is an enlarged, fragmentary plan view of one of the clamping fixtures. 
     FIG. 5 is a view taken on the line  5 — 5  in FIG. 4 showing the clamp jaw closed in solid lines and open in broken lines. 
     FIG. 6A is a sectional view of the door after welding. 
     FIG. 6B is a view similar to FIG. 6A but shows a slight outward spring-back of the panels of the door from the ideal condition shown in broken lines. 
     FIG. 6C is a view similar to FIG. 6A but shows a slight inward spring-back from the ideal shown in broken lines. 
     FIG. 7 illustrates hardware for controlling the clamps in an automatic system. 
     FIG. 8 illustrates the hardware for controlling the clamps in a manual system. 
     FIG. 9 is a flow diagram. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now more particularly to the drawings, there is shown a table (FIG. 3)  20  at a welding station  22  for holding a workpiece  24  for welding. The workpiece in this instance is an automotive vehicle door having an inner panel  26  and an outer panel  28 . One of the panels has an integral flange  30  (FIG.  6 A) along one or more edges which is bent over the corresponding edge or edges of the other panel to interconnect the two panels and hold them in a predetermined position relative to one another. The door panels are made of a flexible, resilient material, such, for example, as steel. 
     The door  24  in this condition is supported at the welding station by spaced apart support pads  32  (FIG. 3) and by spaced apart clamping fixtures  34 . The clamping fixtures  34  abut and grip the edge of the door panel along the header portion thereof. 
     Each clamping fixture  34  includes a clamp  36  having a clamp body  38  provided with a fixed jaw  40  and a pivoted jaw  42 . The pivoted jaw  42  is connected to the clamp body  38  by a pivot pin  44  for swinging movement between open and closed positions. When the pivoted jaw  42  is in closed position, the header of the door is clamped between the pivoted jaw  42  and the fixed jaw  40  (FIGS.  3  and  5 ). When the pivoted jaw  42  is swung away from the fixed jaw  40 , the door header is released. The pivoted jaw is swung between open and closed positions by a power cylinder  50  mounted on the clamp body. 
     The clamp body  38  of each clamp fixture  34  has a frame  51  mounted on a slide  52  for reciprocable sliding movement. A motor drive unit  54  is provided for reciprocating the clamp body. The motor drive unit  54  includes a reversible servo-motor  56  secured to the base of the slide  52  and a connection such as a ball screw drive or helical screw drive  58  from the motor  56  to the clamp body  38 . By mounting the screw drive  58  at an inclined angle to the horizontal, gravity tends to pull the screw drive components together so as to eliminate backlash in the drive, and thereby improve drive accuracy. 
     Operation of the motor  56  effects movement of the clamp  36  in one direction or the other along the slide  52  depending on the direction of operation of the motor. In this manner, the clamp  36  of each clamping mixture may be adjusted to a predetermined position for clamping the header of the door during welding. The midpoint of reciprocating travel of the clamp  36  may be set at a “nominal” location corresponding to a desired nominal or critical point on the finished welded door. In this manner, the clamp may provide maximum adjustment on opposite sides of the nominal or critical point. All of the motors  56  for the clamping fixtures  34  are operated by a single servo-motor controller  57  (FIGS. 7,  8 ). Although stepper motors can be used in place of servo motors, servo motors are preferred based on their greater accuracy and control. 
     The panels  26  and  28  of the door, while the door is supported by the pads  32  and the clamps  36 , are welded together at spaced points by welding devices, two of which are indicated at  60  in FIG.  1 . If the door panels are properly and correctly supported during welding, there will be no appreciable spring-back of the door panels and the door will have the desired contour, configuration and orientation as shown in FIG.  6 A. However, if the door after welding springs-back and assumes a slightly warped position such as shown in FIG. 6B or  6 C, where the desired position is shown in broken lines, this indicates the need for an adjustment in the position of one or more of the clamps  36  so that subsequent doors will not be incorrectly held during welding. FIG. 6A shows the flush relationship of the edges  61  and  63  near a weld  65  of the door which was held correctly by the clamps. FIGS. 6B and 6C show the shift in these edges when the door is not held correctly. 
     To measure the contour and configuration of the door after welding and thereby determine if a clamp adjustment is required, the door is transferred to a checking station  62  (FIG. 2) by a transfer mechanism  64 . The welded and transferred door is laid on support pads  66  of a table at the checking station and measured at certain critical points which are the same points where the door is gripped at the welding station by at least several and preferably three of the clamps  36 . Measurement of the critical points is taken by laser optic cameras  70 . 
     The measured data as thus generated by the laser optic cameras is transmitted to a central processing unit (CPU) at an optical station where this data is stored in a computer  74  for comparison by a comparator  75  with certain ideal or “nominal” critical point locations that have been entered. These nominal points correspond to a preferred contour of the door. The hardware for the storing of measurement data and comparison with ideal critical point locations and operation of the servo-motor controller  57  is shown schematically in FIGS. 7 and 8. 
     In use, the door  24  constructed from the assembled and joined door panels  26  and  28 , is placed on the table at the welding station, resting on the support pads  32  and gripped by the clamps  34 . The setting of the clamps determines the geometry of the door for welding and this setting occurs prior to closing the clamps on the door header. After welding, the clamps are opened to release the door and the door is transferred to the checking station  62  where it rests on the support pads  66 . At the checking station, the laser optic cameras  70  take measurements of the specific critical points on the door header which were gripped by the clamps at the welding station. This measurement data is transmitted to the central processing unit (CPU) at the optical station. Such data is stored and compiled in the computer  74  for comparison with the ideal or nominal critical point locations. The comparison is made in the comparator  75 . If the comparison is not within tolerance, then the computer will interpolate the data with an algorithm and produce an adjustment data signal indicative of the amount of adjustment needed for the individual clamps to corrects for the error. The adjustment data signal is automatically transmitted to the servo-motor controller  57  to actuate the motors  56  and make the necessary adjustment in the positions of one or more of the clamps (FIGS.  7  and  9 ). If accomplished manually, the error data is transmitted to a panel  80  from which an operator may read the error and then operate the servo-motor controller  57  via a manual CRT touch screen actuator  79  to actuate the motors  56  to adjust the individual clamps as needed (FIGS.  8  and  9 ).
A workpiece is held at a welding station by several clamps during welding. After welding, the workpiece is transferred to a checking station where a determination is made of the actual location of critical points on the workpiece. The actual critical point locations are compared with ideal critical point locations to determine if there is any error in the manner in which the workpiece was held by the clamps. If an error is determined, one or more of the clamps are adjusted to compensate for the error when holding a subsequent workpiece for welding. Apparatus is also provided for carrying out the method of this invention.