Abstract:
Method and apparatus for detecting unacceptable misalignment of welding electrodes. An electrode-alignment station is positioned adjacent a welding station. A robot controls the movement of the electrodes, and occasionally moves the electrodes out of the welding station and into an electrode-alignment station. At the electrode-alignment station, the electrodes are closed on a fixture that transfers electrode-misalignment into movement of a plate that triggers an alarm if the misalignment is greater than a prescribed amount.

Description:
TECHNICAL FIELD 
     This invention relates to method and apparatus for checking opposing welding electrodes for misalignment. The invention is particularly useful in determining misalignment of electrical resistance welding electrodes. 
     BACKGROUND OF THE INVENTION 
     Electrical resistance welding (e.g. spot welding, projection welding, etc.) is known, and involves squeezing/pinching the components of a work-piece to be welded between a pair of opposed electrodes, and passing sufficient electrical current through the electrodes and components to melt and fuse the components together. 
     It is also known to sequentially pass a series of work-pieces to be welded through a welding station, and to use a robot to move the welding electrodes into place in the welding station to make the weld. Thereafter, the robot moves the electrodes out of the way as another work-piece moves into the welding station. It is also known to periodically move the electrodes into an electrode dressing station to machine or otherwise shape the electrode tips to a prescribed configuration. 
     It is essential for weld quality that the welding electrodes be aligned with each other, and with the work-piece, within suitable tolerances. Unfortunately, such tolerances are sometimes exceeded in the course of welding numerous work-pieces one after the other, or between production runs, incident to the welding equipment being damaged through rough handling, violent contact with fixtures, or incorrect presentation to the work-piece which can result in the electrodes being forced out of alignment with each other and with the work-piece. 
     Heretofore, alignment of the welding electrodes has been determined either by occasional visual inspection of the electrodes by equipment operators or maintenance persons, or by the observance of poor welds on the work-piece downstream of the welding station. The present invention provides a method and apparatus for the automatic mechanized inspection of electrode alignment, and signaling of the welding operator when untoward misalignment exceeds a prescribed amount. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a welding method and apparatus involving a welding station having pair of aligned, electrical resistance welding electrodes having an inter-electrode gap in which the welding takes place. A series of work-pieces to be welded are passed through the gap, and therein pinched between the electrodes while sufficient electrical current is passed through the electrodes and work-pieces to fuse the work-pieces together. Occasionally throughout a welding run, the electrodes are moved out of the welding station (e.g. by a robot) and into an adjacent, electrode-alignment checking station to check the alignment of the electrodes with each other and with the work-piece. The electrode-alignment station has an inspection fixture having a reference target against which alignment, or misalignment, of the electrodes is determined. The electrodes are positioned in the electrode-alignment station so as to locate the electrodes opposite the target. The target, in turn, is positioned in the alignment station such that when the electrodes are registered with the target, they will be aligned (1) with each other, and (2) with the weld site on the work-piece when the robot moves the electrodes back into the welding station. In the electrode-alignment station, the electrodes are brought together to engage the inspection fixture which triggers an alarm if the electrodes are misaligned from the target (i.e. “off-target”) by more than a prescribed amount. Preferably, the inspection fixture comprises an electrode-misalignment detector comprising a moveable plate (preferably a disk) having the alignment target at its center, and a peripheral portion, outboard the target, which moves to trigger the alarm when sufficiently misaligned electrodes are closed on the plate. According to one embodiment, the closed electrodes cause the plate to move laterally. According to another embodiment, the closed electrodes cause the plate to slant from its “at rest” position. 
     The present invention also contemplates an inspection fixture for use with the aforesaid welding method that is adapted to detect unacceptable misalignment of opposing welding electrodes. The inspection fixture comprises a housing and a plate mounted for movement in the housing when it is engaged by misaligned electrodes. The plate has (1) a reference target at its center against which the electrodes are registered at the beginning of a welding sequence (i.e. the electrodes are aligned along a target-line that passes through the target perpendicular to the plate), and (2) a peripheral portion that is outboard the target and is displaceable when one or both of the electrodes forcibly engage the plate off-target. The housing carries at least one (preferably more) displacement-detector(s) adjacent the peripheral portion of the moveable plate to detect such displacement of the peripheral portion as is above a prescribed limit indicative of unacceptable electrode misalignment. An alarm, responsive to the displacement-detector, signals the need for realignment of the electrodes when unacceptable displacement of the peripheral portion occurs. 
     According to one embodiment, the moveable plate has a conical depression formed in one of its faces and surrounding the target. The conical depression is defined by a wall that slopes inwardly toward the target to provide a camming surface against which a misaligned electrode reacts to laterally displace the plate when the electrodes are forcibly closed on the plate. 
     According to a variation of this embodiment, the inspection fixture comprises a pair of abutting such plates that slide relative to each other along an interfacial plane that is perpendicular to the target-line. Both plates have a conical depression surrounding the target each for receiving one of the electrodes. The plates slide relative to each other when forcibly engaged by off-target electrodes. 
     The displacement-detector may comprise any of a variety of sensors, but will preferably comprise either a pressure transducer, or an electrical contact engageable by an electrical contact on the plate to trigger the alarm. According to one embodiment of an electrical-contact-type displacement-detector, the detector will preferably comprise a contact ring that is centric with the target-line and acts, as a switch, with a contact ring on the peripheral portion of the plate 
     According to still another embodiment of the invention an inspection fixture is provided wherein: (1) the moveable plate has first and second opposing faces; (2) the fixture includes at least one first spring extending between the housing and at least the first face for elastically separating the peripheral portion of the plate from a first displacement-detector; and (3) the plate is adapted to slant sufficiently from its rest position toward the first displacement-detector to engage the peripheral portion with the displacement-detector and thereby trigger the alarm when the electrodes are unacceptably misaligned. A preferred variation of this embodiment includes at least one second spring extending between the housing and the second face of the plate for elastically separating the peripheral portion of the plate from a second displacement-detector. The plate is adapted to slant sufficiently from its at rest position toward the second displacement-detector to engage the peripheral portion with the second displacement-detector and thereby trigger the alarm. Preferably, adjusting screws are provided at the ends of each spring for varying the spring&#39;s compression, and thereby varying the amount of misalignment tolerable by the fixture before triggering the alarm. The fixture will preferably includes a damper between the plate and the housing for cushioning the impact of the electrical contacts on each other when the switch is closed by misaligned electrodes. 
     The invention will be better understood when considered in the light of the following detailed description of certain specific embodiments thereof which is given hereafter in conjunction with the following drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  depict aligned and misaligned electrodes respectively; 
         FIG. 2  depicts, in simplified fashion, a welding station and adjacent electrode-alignment station; 
         FIG. 3  is a top view of  FIG. 2 ; 
         FIG. 4  is a side, sectional view in the direction of  4 — 4  of  FIG. 5 ; 
         FIG. 5  is a plan view of  FIG. 4 ; 
         FIG. 6  is a view similar to  FIG. 4  but depicting, in exaggerated fashion, misaligned electrodes and compressed pressure transducer; 
         FIG. 7  is a plan view of another embodiment of the invention; 
         FIG. 8  is a side, sectional view in the direction of  8 — 8  of  FIG. 7 ; 
         FIG. 9  is a side, sectional view of still another embodiment of the invention which is a variation of the embodiment shown in  FIG. 8 ; 
         FIG. 10  is a side, sectional view of still another embodiment of the present invention; and 
         FIG. 11  depicts a side, sectional view of still another embodiment of the present invention similar to that shown in FIG.  10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1A  depicts a pair of opposing electrical resistance welding electrodes  2 ,  4  aligned one with the other.  FIG. 1B  depicts the same electrodes  2  and  4 , but misaligned. 
       FIGS. 2 and 3  depict, a segment of a production line where welding occurs. The station depicted in the drawings has only a single pair of opposed welding electrodes (i.e. a single welding gun). In practice however, such stations often have multiple such welding guns, and such multi-gun welding stations are within the scope of the present invention. More particularly,  FIGS. 2 and 3  show a welding station  6 , an adjacent electrode-alignment station  8  and a robot  10  for moving the welding electrodes in and out of the welding and electrode-alignment stations. In operation, a work-piece  12  moves into the welding station  6  with flanges  14  and  16  to be welded positioned in a gap  13  between the welding electrodes  18  and  20  which are carried by an arm  22  of the robot  10 . In the welding station  6 , the flanges  14  and  16  are squeezed between the electrodes  18  and  20  and welding current passed therethrough. After the weld is made, the robot  10  rotates the arm  22  (and hence the electrodes  18  and  20 ) out of the welding station  6  to allow removal of the work-piece  12 . Thereafter, a new work-piece  12  moves into the welding station  6 , and the robot  10  rotates the arm  22  back into the welding station  6  for welding the new work-piece  12 . This process is repeated over and over many times in the course of a production run. From time to time, it is desirable to check the alignment of the electrodes  18  and  20  with respect to each other, and with respect to their location in the welding station  6  relative to the work-piece  12 . To this end, the robot  10  is programmed to rotate the electrodes  18  and  20  into the electrode-alignment station  8  to check the alignment of the electrodes  18  and  20  with respect to each other and to the welding station  6 . In this regard, the electrode-alignment station  8  includes an inspection fixture  24  rigidly mounted in a location selected to preferably mirror the location of the flanges  14  and  16  in the welding station  6  such that when the electrodes  18  and  20  are aligned with the fixture  24  they will likewise be properly aligned with respect to the flanges  14  and  16  of the work-piece  12  in the welding station  6 . The location of the fixture  24  may be other than a mirror position, and the robot programmed to coordinate electrode registration between the alignment station  8  and the welding station  6 . This latter alternative is particularly applicable to multi-gun welding equipment wherein the robot may be programmed to present all of the guns to a single inspection fixture or to multiple inspection fixtures positioned to coordinate electrode registration between the alignment station and the welding station. 
     In the electrode-alignment station  8 , the electrodes  18  and  20  are closed to pinch the inspection fixture  24  therebetween. The inspection fixture  24 , in turn, is designed (1) to determine if the electrodes are sufficiently out of alignment with each other, or the work-piece, as to produce poor welds, and (2) to trigger an alarm (e.g. audible or visual) to alert the welding operator that alignment of the electrodes  18  and  20  is needed. Specific examples of such inspection fixture  24  will be discussed hereinafter. For the moment however, suffice to say the fixture  24  includes a reference target at its center against which the electrodes are registered at the beginning of a welding sequence (e.g. start of a production run). After the electrodes have been properly registered with the target, they are returned to the welding station  6  for sequentially welding a plurality of work-pieces. Thereafter the electrodes are occasionally returned to the electrode-alignment station  8  to check and see if the electrodes&#39; alignment remains within acceptable tolerances. 
       FIGS. 4-6  depict one embodiment of an inspection fixture in accordance with the present invention.  FIGS. 4-6  depict an inspection fixture  24  having a housing  26  encasing a pair of relatively slidable disk-shaped plates  28  and  30 . The plates  28  and  30  each have a conical depression  32  and  34  respectively in their exposed faces for receiving the electrodes  40  and  42 . The pieces  36  and  38  of the conical depressions  32  and  34  serve as targets for aligning the electrodes  40  and  42  along a target line  44 . The plates  28  and  30  are slidable relative to each other along an interfacial slide plane  40  and comprise a peripheral portion  42  and  44 , respectively, lying outboard the targets  36 ,  38 . One or more displacement-detectors  46  surround(s) the peripheral portions  42  and  44  of the plates  28  and  30  and serve(s) to determine if the plates  28  and  30  move laterally more than a prescribed amount when pinched between the electrodes  40  and  42 . In the embodiment shown in  FIGS. 4-6 , the displacement-detector comprises an annular load cell (i.e. pressure transducer) that deforms minutely (shown exaggerated in  FIG. 6 ) when pressurized by the sliding of the plates  28  and  30  relative to each other. The conical depressions  32  and  34  in the plates  28  and  30  are defined by walls that slope inwardly toward the targets  36  and  38 , and provide a camming surface against which the electrodes  40  and  42  react when they are misaligned. In this regard when the targets and the electrodes are aligned with the target line  44  (see FIG.  4 ), no lateral movement of the plates  28  or  30  occurs. However and as best depicted in  FIG. 6 , when the electrodes  40  and  42  are misaligned, they will miss one or both of the targets  36 ,  38  and engage the sidewall of the depression. When pinching force is applied to the electrodes  40  and  42 , camming action forces the plates  28  and  30  to slide relative to each other as illustrated in FIG.  6 . This camming/sliding action is best achieved by using plates  28  and  30  that comprise a highly lubricious material such as polytetrafluoroethylene, or other similar fluorocarbons, or the like. 
       FIGS. 7 and 8  depict another embodiment of the present invention. More specifically,  FIGS. 7 and 8  depict an inspection fixture comprising a housing  48  formed from two nesting sections  50  and  52 , bolted together as shown. The housing  48  encases a movable plate  54  (shown in its “at rest” position) having first and second faces  56  and  58 , each having a conical depression  60  and  62  respectively therein. The plate  54  comprises an electrically conductive material (e.g. metal), and carries an electrical contact in the form of a ring  64  on the peripheral portion  70  of the plate  54 . Insulation  66  electrically insulates the plate  54  from the housing  48 . Springs  68  elastically separate the peripheral portion  70  of the plate  54  from the housing  48 . Insulation  69  electrically insulates the springs  68  from the plate  54 . Adjusting screws  72  allow for adjustment of the compression on the springs  68  for adjusting/setting the inspection fixture to the prescribed amount of tolerable misalignment. An alarm  74  is electrically connected to the housing  48  and the plate  54  by means of a circuit  76  that includes a battery or other source of electricity  78 . The plate  54  includes targets  80  and  82  for registering the electrodes. When the electrodes are “on target”, no movement of the plate  54  occurs. However when the electrodes are “off target”, the plate  54  slants (i.e. tilts from its at rest position) so as to move the contact ring  64  closer to the housing  48 . If the extent of electrode misalignment is severe enough, the contact ring  64  will engage the housing  48  and close the circuit  76 , thereby causing the alarm  74  to go off to alert the operator that electrode alignment is required. 
       FIG. 9  depicts another embodiment of the invention similar to that of FIG.  8 .  FIG. 9 , however, employs a non-conductive plate  84  that carries a plurality of electrical contacts  86  on its peripheral portion  88 . The contacts  86  carried by the plate  84 , will engage contacts  90  on the housing  96 , and close an alarm circuit  92 , when the plate  84  slants enough to indicate that the electrodes are unacceptably misaligned. An annular, elastomeric bumper/damper  94  is carried by the housing  96  to dampen the impact of the contacts  86  on the contacts  90 . 
       FIG. 10  depicts still another embodiment of the present invention and depicts a housing  98  encasing a floating plate  100  suspended between springs  106  and  108 . The floating plate  100  has a first face  102  and a second face  104 . A plurality of first springs  106  extend between the housing  98  and the first face  102  and a plurality of second springs  108  extend between the second face  104  and the housing  98 . A plurality of electrical contacts  110  and  112  are embedded in the peripheral portion  114  of the plate  100 . The center of the plate  100  serves as the alignment target for the electrodes and may be scored or etched with a suitable marking (e.g. an X). Annular elastomeric dampers  116  and  118  are provided to dampen the impact of contacts  110  and  112  on ring contacts  120  and  122  carried by the housing  98 . Sufficiently misaligned electrodes will cause the plate  100  to slant sufficiently to cause a contact  112  to engage the ring contact  122 , and a contact  110  to engage the ring contact  120  thereby completing the alarm circuit  124  and triggering the alarm  126 . In this embodiment, the plate  100  is made from a conductive material (e.g. metal). Insulation  128  isolates the ring contact  122  from the housing  98  while insulation  130  insulates the ring contact  120  from the housing  98 . Similarly, insulation  132  insulates the springs  106  from the housing  98 , and insulation  134  insulates the springs  108  from the housing  98 . Adjusting screws  136  adjust the compression on the springs  106 , 108  for setting the fixture to the desired tolerance. 
     The embodiment shown in  FIG. 11  is similar to that shown in  FIG. 10  with the principal exception being that the floating plate  136  comprises an insulating material and carries a ring contact  138  on its peripheral portion  140 . Canting of the plate  136  incident to the force supplied by sufficiently misaligned electrodes, causes the ring contact  138  to engage the ring contacts  140  and  142  carried by the housing  144  to thereby close the warning circuit  146  and trigger the alarm  148 . 
     While the invention has been described in terms of certain specific embodiments thereof, it is not intended to be limited thereto, but rather only to the extent set forth hereafter in the claims which follow.