Abstract:
A pogo welding apparatus especially for spot welding metal sheets at selected locations spaced away from their edges wherein the sheets are held in a temporary assembly with the selected locations engaged and separate programmable positioners move a primary (pogo) electrode and a backup electrode sequentially against opposite sides of the selected locations to spot weld the sheets at the selected locations in sequence. Various alternative programmable positioners, including robots of varying types, may be used to carry the electrodes. A multi head back-up electrode is adapted for efficient use of the apparatus and method in difficult to reach workpiece locations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in part of U.S. patent application Ser. No. 09/778,621 now U.S. Patent 6,429,397, filed Feb. 8, 2001 and assigned to the assignee of the present invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to spot welding apparatus, particularly to so-called pogo welding with a single electrode welding gun. 
     BACKGROUND OF THE INVENTION 
     It is known in the art relating to spot welding to connect the central portions of large sheet assemblies by so-called pogo welding. In this method, a welding gun having a single electrode is moved sequentially to a series of welding locations at each of which spot welding is performed by transmitting current from the welding gun electrode through the sheets to be joined to backup electrodes located below the specified welding locations. 
     FIG. 1 shows the concept of a prior pogo welding apparatus  10 , which includes a programmable robot  12  having a linearably movable base  13  supporting a movable jointed arm  14  carrying a welding gun  16  with a single pogo electrode  18 . The electrode is connected electrically with a transformer  20 , also connected with an electrically conductive stand  22 . The stand supports a plurality of copper backup electrodes  24 , each of which engages one of selected welding locations  25  on the lower side  26  of a temporary or loose assembly of two metal sheets, supported on the stand to be spot welded into a permanent assembly  28 . 
     In operation, the pogo electrode  18  is moved sequentially by the robot  12  to each of the selected locations  25  on the opposite upper side  30  of the assembly  28 . There, the electrode engages the upper side  30  opposite from each of the backup electrodes  24  in sequence. At each location  25 , the welding gun  16  is energized to conduct welding current through the assembly  28  to the opposite backup electrode  24 , spot welding the metal sheets together. 
     In this prior arrangement, a part of the welding current travels through the metal sheets to the neighboring backup electrodes  24  while the remainder passes through the assembly between the opposing electrodes to perform the spot welding process. Significant current shunting may lead to discrepant welds. The prior process requires a different table or tool tray specifically designed to engage the selected locations for each differing component design that is to be spot welded. Thus, a new or modified component requires provision of a modified welding fixture with the backup electrodes positioned in the changed locations. 
     SUMMARY OF THE INVENTION 
     The present invention replaces the conductive stand or tray with a suitable support, where needed, and a programmable positioner carrying a single backup electrode. The support holds the metal sheets in temporary assembly during the spot welding procedure. The positioner carries the single backup electrode on a movable head, which is universally movable within a desired envelope. The positioner is programmed to move the backup electrode sequentially to the various selected locations to be spot welded so that the backup electrode is positioned against one side of the sheet assembly at selected locations when the robot carried pogo electrode is moved against the opposite side of the assembly at the respective selected locations. Various types of positioners may be utilized which may be suitable for the particular components to be welded. 
     Unlike, the prior table or tray with multiple prepositioned backup electrodes arranged for use with a single component design, the positioner of the invention may be used with differing component designs as may be desired by merely reprogramming the controller for the positioner instead of requiring a new or modified electrode tray or table. The invention further eliminates shunting of welding currents away from the active welding location, since only two opposing electrodes are provided for conducting current through each of the selected welding locations. Thus, weld quality becomes more consistent and the fixture costs resulting from design changes are substantially reduced. 
     A modification of the invention includes a multi head back-up electrode usable in the inventive apparatus and method to facilitate contact with interior back-up locations that would otherwise be more difficult or less efficient to reach. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic side view of a prior art pogo welding apparatus; 
     FIG. 2 is a view similar to FIG. 1 showing an improved apparatus according to the invention; 
     FIG. 3 is a similar view showing application of an alternative apparatus; 
     FIG. 4 is a view showing multiple positioners for increasing assembly output; 
     FIGS. 5-8 are schematic side views of various planar positioners usable in the apparatus of FIG. 3; 
     FIG. 9 is a view similar to FIG. 2 showing an alternative programmable positioner. 
     FIG. 10 is a front view of a multi head backup electrode according to the present invention; 
     FIG. 11 is a side view of the electrode of FIG. 10; and 
     FIG. 12 is a front view of apparatus including the electrode of FIGS. 10 and 11 and illustrating the method and apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2 of the drawings in detail, numeral  32  generally indicates a programmable pogo welding apparatus according to the invention. Apparatus  32  includes a first positioner in the form of a robot  12  as in the prior art embodiment. If appropriate, any other suitable form of programmable positioner may be substituted for the robot  12  within the scope of the invention. The robot  12  includes a base  13 , a jointed arm  14  carrying a welding gun  16  with a pogo electrode  18  connected with an electrical transformer  20  carried on the base  13  as before. A workpiece in the form of an assembly  28  of two metal sheets has lower and upper sides  26 ,  30  as in the prior art embodiment. 
     Apparatus  32  differs in that the workpiece assembly  28  is carried by a suitable holding fixture or support  34  for supporting the assembly in position to be spot welded. A programmable manipulator or positioner  36  is located beneath the assembly and includes a base  38  that is linearly movable along a rail  40  extending about the length of the assembly  28 . The base  38  carries a positionable head  42  adjustably supported by a plurality of control arms  44 . The head carries a copper backup electrode  46 , which is electrically connected through the positioner  36  to the transformer  20 . The control arms  44  and the base  38  are adjustable by programmable controls, not shown, to adjust the position of the base along the rail  40  and the attitude and position of the head  42  relative to the base  38 . 
     In operation, the backup electrode  46  is thus movable along the length of the assembly  28  by linear movement of the base  38  and is adjustable laterally and by tilting if needed to reach selected welding locations  25  on the back, or lower side  26 , of the workpiece assembly  28 . Spot welding is accomplished by the programmed robot  12  moving the pogo electrode  18  along the upper side  30  of assembly  28  sequentially from one to the next of the selected locations  25  at which spot welding is to be performed. Concurrently, the positioner  36  moves the backup electrode  46  along the lower side  26  sequentially to the corresponding selected locations  25  of the assembly  28 . Each spot weld is then completed one at a time when both electrodes are then in place on opposite sides of a selected location  25  and the process is repeated at each location until the welding sequence is complete. The programmed robot  12  and positioner  36  are then moved to their loading positions, the finished assembly  28  is removed from the holding fixture and a new unfinished assembly is loaded in the holding fixture to be welded. 
     FIG. 3 illustrates an application of the apparatus  32  of FIG. 2 to spot welding of a tunnel undershield  48  to a tunneled floor pan  50  of a vehicle to form a floor pan assembly  52 . The components of the assembly  52  are again carried in a suitable holding fixture or support  54 . The robot  12  and the positioner  36  are then controlled to concurrently move the pogo electrode  18  and backup electrode  46  to opposite sides of the assembly  52  at selected welding locations  56  where spot welding is performed along opposite sides of the tunnel  58 . In this application, the positioner head  42  moves both linearly along the rails  40  with the positioner  36  and also moves laterally, as well as tilting, to place the backup electrode  46  in direct opposition to the pogo electrode  18  at each of the selected locations  56 . 
     FIG. 4 shows a variation of the embodiment of FIG. 3 wherein a pair of positioners  36  are spaced linearly along the rails  40 . Each of the positioners carries a backup electrode  46  on a head  42  adjustably mounted on a base  38  as in FIG.  3 . The spaced positioners  36  are located so that two robots  12  may be operated concurrently with the positioners  36  to weld a floor pan and undershield assembly similar to assembly  52  in a shortened period of time. 
     Because the tunnel assembly  52  of FIG. 3 is aligned longitudinally along the central axis (or X axis) of a vehicle, the rails  40  will be aligned parallel with the central axis and the mechanism carried by the base  38  may be limited to two dimensional operation with three degrees of freedom in the Y-Z plane normal to the X-axis. FIGS. 5-8, wherein like numerals indicate like parts, illustrate four alternative mechanisms which could be used to provide the required motions for the positioners  36  of FIGS. 3 and 4. These embodiments are all based on a 2-loop, 8-bar topology with 3 actuated joints. 
     FIG. 5 shows a mechanism  58  having a base  60  pivotably connected by 2 telescoping bars  62 ,  64  to an upper portion of a head  66  carrying a backup electrode  46 . A third bar  68  of fixed length is pivotable on the base  60  and engages a slide  70  in the lower portion of the head  66 . By extending and/or retracting the bars  62 ,  64 , the head  66  may be raised or lowered and moved laterally from side to side. Rotation of the third bar  68  around its lower pivot  72  tilts the head  66  about the upper pivot  74  and adjusts the attitude of the electrode  46  to a position normal to the portion of the assembly to be welded at each location. 
     FIG. 6 shows a similar mechanism  76  with a base  60 , telescoping bars  62 ,  64 , head  66  and backup electrode  46  as in FIG. 5. A third telescoping bar  78  is pivotally connected between the base  60  and the lower end of the head  66 . Bars  62 ,  64  move the head as in FIG.  5 . Extension and retraction of the third bar  78  tilts the head about its upper pivot  74 . to adjust the attitude of electrode  46 . 
     FIG. 7 shows a mechanism  80  with a base  60  mounting three pivotable bars  82 ,  84 ,  86 , that rotate about lower pivots at the base  60 . Bars  82 ,  84  connect with links  88 ,  90 , respectively, which, in turn, connect to an upper pivot  74  of the head  66 . Bar  86  connects with link  92  that also connects with a lower pivot  94  of the head. Rotating bars  82 ,  84  about their lower pivots, raises or lowers the head  66  or moves it laterally as desired. Rotating bar  86  tilts the head  66  about its upper pivot  74  to adjust the attitude of the electrode  46  as desired. 
     FIG. 8 shows a mechanism  96 , which is a variation of the embodiment of FIG. 7, wherein the head  66  is generally horizontal and the electrode  46  is mounted on an upper side of the head near its pivot  74 . Rotating bars  82 ,  84 ,  86  move and tilt the head  66  and electrode  46  as in FIG. 7, so that only the orientation of the head  66  is changed. 
     FIG. 9 shows an alternative pogo welding apparatus  100  similar to that of FIG. 2 except for the use of a different form of positioner  102 . The positioner  102  includes a linearly movable base  104  supporting a column  106  that is both vertically extendable and rotatable about a vertical axis, not shown. A lateral arm  108  carried by the column supports a vertical head  110  that is rotatable and tiltable to adjust a backup electrode  46  mounted thereon. Positioner  102  is operated by internal motors and mechanisms controlled by a programmable control, not shown, to move the electrode  46  sequentially to engage the lower side  26  of the workpiece assembly  28  at various selected positions  25 . The robot moves the pogo electrode  18  against the upper side  30  of the assembly at the same locations and the welding procedure is completed sequentially at the selected locations. 
     The positioner  102  may be considered as a specialized form of robot that is used to support and move the backup electrode  46  in completing the pogo spot welding procedure. The inventors further contemplate the alternative use of other forms of robots or positioner mechanisms that are programmable to follow the positioning motions of the robot  12 , or other positioner, in sequentially placing the electrodes  18 ,  46  on opposite sides of the workpiece assembly  28  to complete the welding operations at each of the selected locations. 
     Referring now to FIGS. 10 and 11 of the drawings, there is shown an alternative form of back-up electrode generally indicated by numeral  112 . Electrode  112  includes a base  114  adapted to be supported by a suitable positioner, not shown. Base  114  carries an electrical insulating collar  116  in which is supported a copper back-up member  118 . The back-up member has the form of a cylindrical conductive member extending upward from the collar  116  and downward through the base  114  for connection with an electrical cable, not shown. Member  118  has angled sides at an upper end  120  on which are carried three contact heads  121 ,  122 ,  123  positioned in a common plane through a central axis  124  of the electrode  112 . 
     The contact heads are angled outward at suitable angles  126 , such as 60 degrees, to position the heads for contacting varying points along the interior of a tunnel or channel in a panel to be spot welded to another panel, for example in the manner shown in FIG. 3 of the drawings. It should be understood that the arrangement of the contact heads at the end of, or otherwise, on the electrode may be varied as desired to accommodate use of the electrode as a back up for pogo welding of particularly shaped workpieces or for use in other applications. 
     FIG. 12 shows a pogo welding application  128  for welding an overlapping seam of a front floor panel  130  to a floor pan  132  of an automotive vehicle. The tunnel configuration is relatively deep and narrow so that use of a single contact electrode to reach all the back-up locations in the tunnel could be difficult. The problem is solved by employing the triple head electrode  112  of FIGS. 10 and 11. Electrode  112  may be mounted on a multi axis positioner  134 , which allows tilting and positioning of the electrode against sequential back-up locations on the interior of the tunnel slong the overlapping seam. The axially located end head  122  is particularly suited to engaging locations on the upper part  136  of the tunnel while the side angled heads  121 ,  123  are suited for engaging locations along the tunnel sides  138 . 
     Numeral  140  indicates a robot positioner similar to robot  12  of FIG.  3 . Robot  140  is programmed for applying an electrode against sequential welding locations on the upper surface of the tunnel of floor pan  132  to carry out the welding process with the back-up positioner  134 . 
     For other applications, a multi head backup electrode in accordance with the invention may have any number of contact heads arranged as desired to conveniently contact the selected locations on the surfaces of a particular workpiece. The heads may be configured differently if needed to carry out the intended process, but the use of similarly configured heads as shown in FIGS. 10 and 11 is preferred where appropriate. For a dual head electrode the contact heads would preferably be disposed at angles of 45 degrees or more although narrower angles could be used where called for by the application. The triple head electrode of FIGS. 10 and 11 has the contact heads angled 60 degrees apart, however they could be set at 30 degrees or greater as the application directs. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.