Abstract:
An assembly system, such as welding system, includes an assembly station that is configured to assemble a component to a part in a loaded position. A drag conveyor includes a movable conveyor member that supports multiple locating elements configured to cooperate with a locating feature on a part. A friction bar is arranged along the locating elements, and the locating elements extend beyond the friction bar. The friction bar is configured to support the part and reorient the part from an unoriented position to an oriented position while the movable conveyor member drags the part along the friction bar. A robot is configured to transfer a part from the oriented position on the conveyor to the loaded position at the assembly station, such as a welding unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/020,064, which was filed on Jul. 2, 2014 and is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to drag conveyor that is used, for example, with a welding system and welder used to secure fasteners to stampings. 
         [0003]    The production of complex assemblies such as vehicles, appliances, consumer goods, and furniture often requires the fabrication of welded subassemblies, such as sheet metal workpieces with one or more welded fasteners. In the production of an assembly as complex as a vehicle, there can be a considerable quantity and variety of such sheet metal workpieces that are produced in a stamping operation for later welding in batches to produce the required welded subassemblies. A machine operator must remove a randomly oriented and sometimes interlocked sheet metal workpiece from the storage bin before loading it into the welding machine. A second component is then added manually or by automation in preparation for welding. In the case of resistance welding, two electrodes close upon the workpiece and second component before applying force and a high current necessary for completing the resistance weld. The finished subassembly is then removed from the machine in preparation for repeating the welding operation. 
         [0004]    As automotive component production costs continue to be pressured lower, labor productivity is under increased scrutiny. The common direction in the industry is to replace labor with significant automation. To be competitive, it is important to maximize the productivity of both the equipment and operator. Much effort is being directed to address this challenge by building equipment with elements such as one or more fixtures to orient the workpiece or complex vision systems that permit a robot to accurately grasp and position the workpiece. Such systems can be challenging to set-up and may require specialized and costly skills to configure, troubleshoot and maintain. The problem this approach has created is equipment that is too hard for many customers (or specific plants) to set-up, operate and maintain. Some cannot access skilled labor, or they do not have enough complex equipment to justify investment in skilled labor and tools. Excessive sophistication may limit the operational reliably in the production environment. The complexity also increases the capital cost of the equipment and the inventory of spare parts to keep it in operation. 
         [0005]    It is desirable to use equipment that is simple to configure, operate, troubleshoot, and maintain. It is also desirable to minimize the equipment changeover time while also minimizing its complexity. 
       SUMMARY 
       [0006]    In one exemplary embodiment, an assembly system includes an assembly station that is configured to assemble a component to a part in a loaded position. A drag conveyor includes a movable conveyor member that supports multiple locating elements that are configured to cooperate with a locating feature on a part. A friction bar is arranged along the locating elements. The locating elements extend beyond the friction bar. The friction bar is configured to support the part and reorient the part from an unoriented position to an oriented position. A robot is configured to transfer a part from the oriented position on the conveyor to the loaded position at the assembly station. 
         [0007]    In a further embodiment of the above, the conveyor member is a continuous chain that supports the multiple locating elements. 
         [0008]    In a further embodiment of any of the above, the conveyor includes multiple rows of conveyor members with locating elements. 
         [0009]    In a further embodiment of any of the above, the locating elements are transfer pins. 
         [0010]    In a further embodiment of any of the above, the conveyor includes a sprocket coupled to the chain. A support bar is arranged beneath the chain and adjacent to the sprocket. The support bar is configured to position the locating element as the locating element approaches an end of the conveyor. 
         [0011]    In a further embodiment of any of the above, the conveyor has an adjustment assembly connected to the friction bar and is configured to move the friction bar relative to the locating elements. 
         [0012]    In a further embodiment of any of the above, the adjustment assembly includes a linkage interconnecting a cylinder to a frame that supports the friction bar. The frame is configured to move vertically in response to the cylinder pivoting the linkage. 
         [0013]    In a further embodiment of any of the above, the conveyor includes a guide arranged on the friction bar adjacent to the locating element. The guide is configured to reorient the part from the unoriented position to the oriented position. 
         [0014]    In a further embodiment of any of the above, the guide has a ramp that is configured to cooperate with a front of the part to lift the front off of the friction bar. 
         [0015]    In a further embodiment of any of the above, there are multiple conveyors. Each conveyor is configured to carry a different part. 
         [0016]    In another exemplary embodiment, a method of positioning a part with a conveyor that includes the step of manually loading a part onto a conveyor locating element in an unoriented position. The part is dragged along a conveyor friction bar to an oriented position. The part is picked up robotically from the oriented position. 
         [0017]    In a further embodiment of any of the above, the multiple locating elements are arranged at a regular spacing. The manually loading step includes manually loading parts onto the locating elements at an irregular spacing. 
         [0018]    In a further embodiment of any of the above, the locating element is a transfer pin. The manually loading step includes placing the transfer pin through a hole in the part. 
         [0019]    In a further embodiment of any of the above, the dragging step includes pivoting the part about the transfer pin from the unoriented position to the oriented position. 
         [0020]    In a further embodiment of any of the above, the manually loading step includes manually loading parts onto multiple rows of conveyor locating elements. 
         [0021]    In a further embodiment of any of the above, the manually loading step includes supporting the part on the conveyor friction bar and comprising the step of adjusting a height of the friction bar relative to the conveyor locating element prior to the manually loading step. 
         [0022]    In a further embodiment of any of the above, the dragging step includes stabilizing the locating element near an end of the conveyor prior to performing the part picking up step. 
         [0023]    In a further embodiment of any of the above, the method includes the step of laterally adjusting a position of the part from a first oriented position to a second oriented position near an end of the conveyor prior to performing the part picking up step. 
         [0024]    In another exemplary embodiment, an assembly system conveyor includes a continuous conveyor member that each include multiple transfer pins that are configured to cooperate with a hole in a part. Friction bars are arranged along lateral sides of a row of the transfer pins. The transfer pins extend beyond the friction bars. The friction bars are configured to support the part and reorient the part from an unoriented position to an oriented position. 
         [0025]    In a further embodiment of any of the above, a support bar is arranged beneath the continuous conveyor member. The support bar is configured to position the transfer pin as the transfer pin approaches an end of the conveyor. 
         [0026]    In a further embodiment of any of the above, an adjustment assembly is connected to the friction bar and is configured to move the friction bar vertically relative to the transfer pin. 
         [0027]    In a further embodiment of any of the above, the conveyor includes a guide arranged on the friction bar adjacent to the locating element. The guide is configured to reorient the part from the unoriented position to the oriented position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0029]      FIG. 1  is a highly schematic view of a welding cell embodiment; 
           [0030]      FIG. 2  is a simplified perspective view of a welding system embodiment without guarding or material handling; 
           [0031]      FIG. 3  is a perspective view of a part conveyor with random workpiece placement with a fixed table; 
           [0032]      FIG. 4A  is a perspective view of another part conveyor; 
           [0033]      FIG. 4B  is a side view of the part conveyor shown in  FIG. 4A  with a side cover removed for clarity; 
           [0034]      FIG. 4C  is a side view of the part conveyor shown in  FIG. 4B  with conveyor chains removed for clarity; 
           [0035]      FIG. 4D  is an enlarged partial side view of the part conveyor shown in  FIG. 4C  with the conveyor chain, friction bar and transfer pins; 
           [0036]      FIG. 5  is a perspective view of a portion of the part conveyor shown in  FIG. 4D  illustrating the transfer pins; 
           [0037]      FIG. 6A  is a perspective view of a front portion of the part conveyor shown in  FIG. 4A  with a first set of guides installed; 
           [0038]      FIG. 6B  is a perspective view of a front portion of the part conveyor shown in  FIG. 4A  with a second set of guides installed; 
           [0039]      FIG. 7  is a simplified perspective view of a welding machine; 
           [0040]      FIG. 8  is an enlarged perspective view of the resistance welding gun incorporated in the welding machine; 
           [0041]      FIG. 9A  is another enlarged perspective view depicting the welding gun with a fastener feeder in a feeder retracted position, a pin in a pin retracted position, and an electrode in an electrode advanced position; 
           [0042]      FIG. 9B  is an enlarged perspective view depicting the feeder in a feeder advanced position and the electrode in the electrode advanced position; 
           [0043]      FIG. 9C  is an enlarged perspective view depicting the pin in a pin advanced position and the fastener feeder moving from the feeder advanced position to the feeder retracted position; and 
           [0044]      FIG. 9D  is an enlarged perspective view depicting the pin in the pin advanced position and the fastener feeder in the feeder retracted position. 
       
    
    
       [0045]    The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
       DETAILED DESCRIPTION 
       [0046]    The disclosed system provides low-cost automation to weld fasteners, or secure other parts such as pins, clips, or brackets to sheet metal stampings at high speeds. The same system can be applied to other assembly processes such as rivets, self-piercing fasteners, mechanical fasteners to a variety of workpieces including those made of plastic. Thus, although the disclosed system is discussed primarily in terms of a welding system and method, it should be understood that other types of securing systems are contemplated. 
         [0047]    A welding system  10  is schematically illustrated in  FIG. 1 . First and second conveyors  12   a,    12   b  feed different parts, such as stampings, into a robot  14  within a welding station  15 . The robot  14  transfers the parts from the conveyors  12   a,    12   b  to welding machines  16   a,    16   b.  One welding machine  16   a  secures studs supplied by a vibratory feeder bowl  17   a,  and the other welding machine  16   b  secures nuts supplied by a vibratory feeder bowl  17   b.  The arrangement shown in  FIG. 1  is exemplary only and may be configured differently than illustrated. 
         [0048]      FIG. 2  shows one example welding system  10  for high-rate production in more detail that includes a drag conveyor  12 , high-speed robot  14 , welding machine  16 , unload chute  18 , and control system  20 . The drag conveyor  12  provides a number of equally spaced load stations, each having a transfer pins  110  for engaging a workpiece to which a component such as a fastener will be welded. The drag conveyor  12  advances workpieces loaded by an operator onto transfer pins  110  at one end towards an unload end (shown in more detail in  FIG. 3 ). The high-speed robot  14  is positioned in close proximity to the unload end of the drag conveyor  12  where it will grasp a workpiece and the welding machine  16  to which it will present the workpiece for welding. A control system  20  is interconnected to each of the elements of the welding system  10  to provide functional control and coordination of the motions of the elements to minimize the operating cycle time. In one example, the welding system  10  may be configured to weld than thirty or more different assemblies. The control system  20  maintains data for each assembly needed to manage the position and number of fasteners in each assembly, instructions for when the robot can release the assembly, error recovery protocols, weld signatures, and other information for automating the welding of the assembly. 
         [0049]      FIG. 3  is a perspective view of the drag conveyor  12  with the addition of a number of workpieces W. The workpieces W are engaged on number of evenly spaced transfer pins  110  are affixed to conveyor chains  112 . Three such conveyor chains  112  are supported and driven by a sprocket assembly at each end of the drag conveyor  12 , which has a fixed table in this example. The conveyor chains  112  are aligned with spaces provided between adjacent friction bars  116  to permit the travel of the transfer pins  110  and thereby the workpieces W. 
         [0050]    The workpiece W shown in  FIG. 3  is used to illustrate the principles of operation and is not representative of the range and size of permissible workpieces W. While not defined in the hardware, a number of zones are illustrated in  FIG. 3  to explain the operation of the drag conveyor  12 . An operator loading zone A, is within easy reach of an operator who will manually sort, orient, and place a workpiece W so the selected attribute of the workpiece W, such as a specific hole, engages a transfer pin  110 . Within zone A, the orientation of the workpiece W is not required to be precise and the number of occupied transfer pins  110 , as well as the sequence in which they are loaded is not important. The sprocket assembly  114  drives the conveyor chains  112  to advance the transfer pins  110  towards the unloading zone C. Along the length of the drag conveyor there is a workpiece orientation zone B in which the friction of the workpiece W acting against the friction bar  116  urges the workpiece W into a generally consistent orientation. Also illustrated is an empty zone B 1  which corresponds to a time when the operator was unable to load a workpiece W. This could be for any number of reasons such as when convenient access to workpieces W is not possible. This could occur when there is an interruption of the supply of workpieces, such as might occur when the container holding them is empty and requires replacement. In the oriented part zone B 2  the workpieces are progressively oriented such that when leaving this zone and entering the unloading zone C, they are in a position identifiable by the high-speed robot  14 . 
         [0051]    In the unloading zone C, a detection method would be employed so the control system  20  can communicate the workpiece W location to the high-speed robot  14 . The detection method could be an inductive proximity switch, photo switch, laser, or imaging system. The detection method could detect and verify the workpiece W in the unloading zone C, or it could be incorporated into the high-speed robot  14  tooling that is used to capture the workpiece W for loading into the welding machine. The drag conveyor  12  would be advanced to position at least one workpiece W in the unloading zone C within reach of the robot. The high-speed robot  14  could wait until a workpiece W has reached a fixed unloading position, or it could capture the workpiece while it is moving within the unloading zone C if the drag conveyor  12  and high-speed robot  14  are operated in coordinated motion. 
         [0052]    The detection method employed to detect and verify the workpiece W is within the unloading zone C can also be used to verify an attribute of the workpiece W so that a misaligned or incorrect workpiece W can be discharged from the drag conveyor  12  simply by moving it past the point at which the workpiece W remains engaged with the friction bars  116  and transfer pin  110 . 
         [0053]    The conveyor frame  118  shown in  FIG. 3  is illustrative only. It can be a free-standing unit as shown or integrated with the frame supporting other components of the welding system  10 . The number and length of conveyor chains  112  is dependent on the number and spacing of transfer pins  110  required to support the welding system  10  operating requirements. The transfer pins  110  may not have a circular cross section and may be of any length required to engage the workpiece W while it is resting on a surface. The length and definition of the illustrated drag conveyor  12  zones is a function of the welding system  10  operating requirements and other factors such as machine guarding, and the duration of friction application required to ensure the workpieces W are consistently oriented. The friction bar  116  would commonly be made of sheet or plate steel with a width, length, and thickness suited to the size of the workpiece W. Instead of changing the length of transfer pins  110  to suit different workpieces W, provision can be made to change the spacing between the friction bar  116  and conveyor chain  112 . 
         [0054]    The drag conveyor  12  shown in  FIG. 3  is an example configuration. The transfer path does not need to be linear as illustrated. The transfer path could be circular as a rotary table, a serpentine shape, or have transitions to different shapes along the length of travel. The transfer path also does not need to be in one plane or in a plane parallel to the floor. The conveyor chain  112  can be driven by any number of means such as the high-speed robot  14  controller to provide for coordinated motion, a speed-controlled motor, or a ratcheting drive connected to a pneumatic cylinder. The friction bars  116  can be made of any number of metal, other material such as self-healing polymer, or a combination of materials. The friction bars  116  may also be supplemented with risers or guides if, for example, they are beneficial to speed up workpiece W orientation, reduce the chance the workpiece W will lock on the transfer pin  110 , or prevent motion that would cause interference between adjacent workpieces W. The space between friction bars could be occupied with conveyor chain  112  link, accessory, or cover that prevents the workpiece W from engaging with the gap between adjacent friction bars  116  if the such engagement would prevent the workpiece W from moving to the desired alignment. 
         [0055]    The welding system  10  is configured to accept a workpiece W such as a sheet metal stamping and weld a component part to it, such as a fastener F (e.g.,  FIGS. 9A-9D ), in the shortest time possible. The individual stations of a drag conveyor  12  include a transfer pin  110  which engages in a hole in the workpiece to pull the workpiece W from the loading point to an unloading point. The geometry and mass of the workpiece W will determine its attitude when resting on the drag conveyor  12  and engaged with the transfer pin  110 . Friction between the bed of the drag conveyor  12  and the workpiece W is sufficient to urge all of the workpieces into a sufficiently consistent orientation as they are moved towards the unload position. 
         [0056]    The number and spacing of the transfer pins  110  will determine the length and width of the drag conveyor  12 . The diameter of the transfer pins  110  is selected to engage the hole in the range of workpieces to be welded, have sufficient strength to minimize bending, and to provide some clearance between the hole and workpiece W so the high-speed robot  14  can reliably remove the workpiece W. The length of the transfer pin  110  is based on the height above the drag conveyor  12  where the workpiece engages the transfer pin  110 . The height of the drag conveyor  12  depends on whether it is to be manually loaded at a convenient height for an operator, or by automation directly from the stamping equipment used in production of the workpiece. 
         [0057]    The drag conveyor  12  has three rows of twenty four stations. The number of rows and stations depends on a number of factors such as permissible floorspace, workpiece loading time, welding sequence time, buffer inventory requirement, and interaction between workpieces in adjacent stations. 
         [0058]    The high-speed robot  14  is programmed to properly grasp the workpiece W from a station at the unload end of the drag conveyor  12  and transition it to an attitude and position necessary to engage with the welding machine  16 . Since the tooling uses a simple transfer pin  110 , there is no requirement beyond programming, for tooling or tool set-up to accommodate different workpieces. 
         [0059]    Various features of the drag conveyor  12  are shown in more detail in  FIGS. 4A-6 . The drag conveyor  12  includes side covers  120  and front cover  122 , as shown in  FIG. 4A , that enclose the drive elements, such as the chains  112  and sprocket  114 . The covers  120 ,  122  are removed for clarity in  FIGS. 4B-5 . 
         [0060]    Referring to  FIGS. 4A-4B , a barrier system  124  may be provided to prevent the machine operator from getting too close to the robot  14  during operation. Barrier system  124  may incorporate a means for sensing a condition, which requires an immediate halt to the progression of the transfer pins  110 . An example is a sensor monitoring the position of a hinged barrier panel, to ensure the barrier panel is not dislocated from its normal position by the operator or something improperly placed on the drag conveyor  12 . A photoelectric light curtain may be an alternative to the hinged breakaway barrier panel for assembly processes that do not have requirements for containment of metal expulsion and ultraviolet radiation. 
         [0061]    Referring to  FIGS. 4C-4D , an adjustment assembly  126  is supported on the conveyor frame  118  and is used to provide a desired amount of pin exposure for each workpiece W. The adjustment assembly  126  raises and lowers the friction bars  116 , which are secured to mounting blocks  130  on a table  128 , to effectively lengthen or shorten the transfer pins  110  by changing the amount the transfer pins  110  are exposed relative to the friction bars  116 . A linkage  132  includes arms  136  that are pivotally supported by the conveyor frame  118 . The arms  136  are interconnected to a bar  134  at one end, and the table  128  is pivotally connect to the arms  136  at an opposite end. Actuator cylinder  138  is connected to the bar  134  and is controlled to rotate the arms  136 , thereby moving the table  128  and supported friction bars  116  up or down. 
         [0062]    Support bars  140  are arranged beneath the upper portion of the chain  112 , in particular, near the sprocket assembly  114 . The support bars  140  prevent the chain  112  from sagging or vibrating, which more precisely locates the workpiece W in preparation for the robot  14  picking up the part. 
         [0063]    Referring to  FIG. 5 , the conveyor chains  112  include links having tabs  142  to which pin blocks  144  are secured. The transfer pins  110  are mounted to the pin blocks  144 , for example, by a threaded interface. Different sized and shaped transfer pins  110  can be mounted to the pin blocks  144  when the welding system  10  is reconfigured to weld different workpieces. 
         [0064]    Typically the friction between the friction bars  116  and the workpiece W is sufficient to locate the workpiece accurately enough for the robot  14 . Guides  146  may be used if more precise alignment of the workpiece W is desired, as shown in  FIGS. 6A-6B . The guides  146  may be provided as a set for a given workpiece W. The guides  146  include handles  148  for manipulation by the operator so the guides  146  may be easily positioned on the friction bars  116  and secured thereto by fasteners  150 . Adjacent edges  152 ,  154  of adjacent guides  146  interacted with the sides of the workpiece W to ensure a desired orientation of the workpiece W as it nears the end of the drag conveyor  12  for pick up by the robot  14 . The example guides  158  shown in  FIG. 6B  include ramps  156  that lift the front end of the workpiece W so the workpiece is more easily grasped by the robot  14  using its grippers. The robot  14  may use a magnet instead of a gripper, if desired. 
         [0065]    The high-speed robot  14  and the welding machine  16  are coordinated by a control system  20  to engage the workpiece W with the welding machine  16  as quickly as possible so the welding process can be completed and the completed assembly discharged from the welding system  10 . Referring to  FIGS. 7 and 8 , the welding machine  16  includes provisions, such as the float assembly for accommodating some misalignment of the workpiece due to variation of the attitude of the workpiece on the drag conveyor  12  and positioning error of the high-speed robot  14  plus any positioning error due to variations in the workpiece W, or movement of the workpiece W in the gripping device of the high-speed robot  14  during the workpiece transfer. 
         [0066]    In further detail, with continuing reference to  FIG. 2 , the size and configuration of the welding system are suited to the range of assemblies to be welded. The welding machine  16  capability, including its physical size, welding current or power capability, and electrode force range are determined by the workpiece characteristics and the requirements of the welding process. The configuration and height of the unload chute  18  is based on the type of container in which the completed weldment is discharging. 
         [0067]    The welding machine  16  shown in  FIGS. 7 and 8  is a resistance welding machine configured for welding projection weld nuts. It could also be configured to weld projection weld studs, to weld by other means such as laser welding, or to perform other processes such as assembly. The versatility of the welding system  10  can be expanded by incorporating multiple welding machines  16  that weld different sized fasteners F or different types of operations. These either can be located within easy reach of the high-speed robot, manually repositioned, or supplied with automation to index one or more welding machines  16  to the active position within the reach of the high-speed robot  14 . 
         [0068]    The welding machine  16  as shown in  FIG. 7  includes a welding machine frame  30  supporting a resistance welding gun  32  and a feeder  34  for delivering a fastener F to be welded to a workpiece W. The welding machine frame  30  maintains the alignment and orientation of the components and includes provisions for leveling to ensure the desired function. The resistance welding gun  32  supports a resistance welding transformer  36  to produce the required welding current and a welding actuator  38  for achieving the required coordinated motion of the welding electrodes and the necessary electrode force for welding. The resistance welding transformer can be of any type, such as alternating current or inverter. The resistance welding gun is oriented to exploit gravity to maintain engagement with the fastener F delivered by a feeder  34  as it is moved to the welding position. The feeder  34  dispenses one fastener F to be welded to the workpiece at a time onto the movable welding electrode  44 . 
         [0069]    The feeder  34  shown is for feeding projection weld nuts. The design of the feeder  34  will be based on the requirements of the fastener F to be welded and may for example be a stud, pin, or bracket; or other process such as feeding of a rivet, screw, clinch nut, mechanical clip, or other mechanical fastener. Thus, the welding machine  16  can employ a process other than resistance welding or a process that does not involve such mechanical fastening involving riveting, self-piercing fastener, bolting, or the like. 
         [0070]    The welding machine frame  30  is rigid enough to ensure consistency of the resistance welding gun  32  position when the mass of the gun is shifting and when it is subjected to external forces. When a welding cycle is requested, the welding actuator  32  extends to a position that is aligned with the feeder  34  when it advances to deliver a component to be welded. When the feeder  34  retracts to its home or feeder retracted position, the welding actuator  32  extends fully to close the electrodes and press the fastener F to be welded against the workpiece W positioned by the high-speed welding robot  14 . During the welding sequence, welding current delivered by the resistance welding transformer  36  creates the heat for welding. At the completion of the weld, the output of the welding actuator  38  is retracted to return the moveable welding electrode  44  to its home or electrode retracted position. 
         [0071]    The welding machine frame  30  of  FIG. 7  holds the resistance welding gun  32  at the required working height determined in large part by the required height of the unload chute  18  or by clearance necessary to ensure there is no interference with the operation of the resistance welding gun  32 . The resistance welding gun  32  is sized to accommodate the physical size of the workpiece W or workpieces, and the required electrode force and welding current. The stroke of the welding actuator  38  is determined by the required resistance welding gun  32  opening and that in turn determines the length of the welding actuator  38 . 
         [0072]    The configuration of the welding machine frame  30  illustrated in  FIG. 7  is a generic design for illustration purposes. The welding machine frame  30  can be a standardized version or a custom design created for a specific application. The welding machine frame  30  can stand alone, it can be mounted to a frame that supports the other components of the welding system  10 , or it can be incorporated into a larger piece of equipment providing multiple functions. The C-type resistance welding gun  32  shown employs a welding actuator  38  that moves the electrode in a linear motion towards an opposing stationary welding electrode  48  ( FIG. 8 ). Linear motion is desirable for projection welding because it is most effective for providing consistent force on each projection and to follow the projection collapse during the weld. 
         [0073]    The resistance welding gun  32  can be of any design, construction or material that achieves the requirements for the particular project scope. The welding actuator illustrated is an electric servo type but it could also be a pneumatic cylinder with an intermediate stroke position, such as a retract cylinder or other similar device. 
         [0074]      FIG. 8  shows elements of the welding machine  16  in more detail. The resistance welding gun  32  includes two spaced apart welding gun side frames  40  and  42  that provide the main structure to hold the components of the gun, contain the welding force, and provide a means for mounting within the welding machine  16 . A moveable welding electrode  44 , containing a fastener rough locating pin  46 , is connected to the output shaft of the welding actuator  38 . A stationary welding electrode  48  containing a fastener locating pin  50 , opposes the moveable welding electrode  44 . The fastener locating pin  50  is actuated by a locating pin cylinder  52  and its position is sensed by a locating pin position sensor  53 . 
         [0075]    Examples relating to suitable pin position sensing can be found in U.S. Pat. No. 6,576,859, entitled “Resistance Welding Fastener Electrode,” issued Jun. 10, 2003 and PCT International Application No. PCT/CA2014/050896, entitled “Welder with Indirect Sensing of Weld Fastener Position”, filed Sep. 18, 2014, which are incorporated by reference herein in their entirety. 
         [0076]    The sensor(s) can be used to track the electrode and welding pin movement to monitor situations that might generate an error leading to a defective part or a fault in the welding system  10 . For example, if the pin locating the fastener is depressed when the welding gun closes on the workpiece presented by the robot, the workpiece may not have a clearance hole, or the workpiece may have slipped in the robot gripper too far for the floating action of the welding unit to accommodate (discussed in more detail below). The control system  20  in this case can instruct the robot  14  to deliver the workpiece to a containment area and pick up a new workpiece from the drag conveyor  12 . In another example, if the electrode is not closed to the expected height, there may be an improper workpiece, fastener, or perhaps two fasteners. In this scenario, the system may first try to eject the fastener and reload. If the same error occurs, the workpiece is replaced as above. 
         [0077]    The stationary welding electrode  48  and fastener locating pin  50  are specifically designed to suit the thickness of the workpiece W plus the clearance hole and fastener F locating diameter. The fastener locating pin  50  needs to be designed with gentle and smoothly transitioned curves to help urge the resistance welding gun  32  into the correct position. Excessive roughness, sharp angles, or steps on the fastener locating pin  50  may cause the pin to hang up on the workpiece, thereby inhibiting the locating pin cylinder  52  from advancing the fastener locating pin  50  to its fully extended position. Significant deviation of the workpiece W hole location can be accommodated by this arrangement. The maximum deviation would be in the range of 40% of the fastener thread being gaged. Our demonstration system is somewhat higher—providing a 5 mm window of compensation for a fastener having an 8 mm thread. 
         [0078]    The stroke and force capability of the locating pin cylinder  52  needs to be sufficient to overcome the force applied to the fastener rough locating pin  46  in the movable welding electrode  44 . 
         [0079]    The resistance welding gun  32  is connected to the machine frame  30  by one or more float assemblies  59  which permit motion within a plane. For simplicity, the description of one mechanism will be described although  FIG. 4  shows there is one such mechanism located on opposing sides of the resistance welding gun  32 . The planar alignment of bracket  54  on frame with the bracket  56  on welding unit is provided by link  58  to frame and link  60  to welding unit. Pins and bearings are provided at the attachment points between the components of this mechanism to enable welding gun  32  to float freely relative to the frame  30 . A homing assembly  61  includes a homing actuator  66  that engages guide pins  62  and  64  to urge the resistance welding gun to a fixed home position that is the nominal position of the fastener locating pin  50  where the high-speed robot  14  has been programmed to position the clearance hole for the fastener locating pin  50 . 
         [0080]    The resistance welding gun  32  of  FIG. 8  provides clearance between the welding gun side frames  40  and  42  to accommodate the feeder  34 . Like the welding gun side frames  40  and  42 , the feeder  34  represents a potential point of interference between the resistance welding gun  32  and the workpiece or high-speed welding robot gripper so it is desirable to keep it out of the way and this is accomplished by putting it in the throat of the resistance welding gun  32 . The feeder  34  is commonly provided with components, such as fasteners, from an automatic feeding system by way of a tube, track, or carrier (e.g., vibratory feeder bowls  17   a,    17   b  shown in  FIG. 1 ). The feed path required to accommodate this apparatus can be quite long and it can be provided for and accommodated within the welding machine frame  30 . 
         [0081]    To load the component such as a fastener, the rod of the welding actuator  38  is advanced (with the feeder  34  in the feeder retracted position) to raise the movable welding electrode  44  to the electrode advanced position ( FIG. 9A ) necessary to interact with the feeder  34 . The feeder  34  advances in a linear motion to a feeder advanced position ( FIG. 9B ) where the fastener is aligned with the fastener rough locating pin  46 . At this time, the fastener rough locating pin  46  is advanced to its fully extended pin advanced position ( FIG. 9C ) to capture the fastener. Then the feeder  34  is withdrawn to its home feeder retracted position ( FIG. 9D ), the action of which causes the feeder  34  to release the fastener on the fastener rough locating pin  46 . With the feeder  34  clear of the moveable welding electrode  44 , the welding actuator  38  is free to advance the moveable welding electrode  44 , as soon as the high-speed welding robot  14  confirms it has placed a workpiece over the fastener locating pin  50  in the stationary welding electrode  48 . 
         [0082]    An example operating sequence of each of the principle welding system  10  components is as follows. The equipment operator or automation will load workpieces on the drag conveyor  12  whenever there is an available station within reach. The drag conveyor  12  will advance workpieces W towards the unloading zone C whenever there are no workpieces W properly oriented for pick-up within the unloading zone C. The high-speed robot  14  will move to position to engage a workpiece W when it is free to begin the transfer sequence and a workpiece W has been detected in an orientation conducive to engagement. When the resistance welding gun  32  is opened sufficiently to accept the high-speed robot  14  to load a workpiece W, the high-speed robot  14  will move the workpiece W to a position in alignment with the stationary welding electrode  48 . Independently, or simultaneously with the operation of the high-speed robot  14 , when the moveable welding electrode  44  has been moved by the welding actuator  38  to the position to receive the fastener F from the feeder  34 , the feeder  34  will advance to the location in which the fastener F is aligned with movable welding electrode  44 . The fastener rough locating pin  50  is then advanced to engage with the fastener F. The feeder  34  is then retracted to its rest position. When the feeder  34  has retracted, the fastener F will be raised towards the workpiece W by the movable welding electrode  44  on which it is resting. When the welding actuator  38  reaches the travel distance at which the workpiece W and fastener F should be in contact between the moveable welding electrode  44  and stationary welding electrode  48 , homing guide pins  62  and  64  will be released from the homing guides  68  and  70 . The fastener locating pin  50  will then advance to monitor and verify the part position as well as to bring the parts to be welded, if present, into alignment by urging the resistance welding gun  32  to move. The welding machine  16  will perform the welding operation. When the workpiece W does not require an additional fastener F and can be supported by the welding electrodes, the high-speed robot  14  will disengage from the workpiece W and move to retrieve another workpiece W from the drag conveyor  14 . Otherwise, it will continue to hold and support the workpiece W until the moveable electrode  44  has retracted sufficiently to either permit the high-speed robot  14  to index the subassembly to the next welding position, or to release the completed assembly so it can exit the welding system  10 . The welding actuator  38  will return the moveable welding electrode  44  to its fully lowered position to begin the fastener feeding and welding cycle over again. 
         [0083]    If there is a requirement to weld additional fasteners the feeder  34  operating sequence and welding sequence can be repeated as soon as the moveable welding electrode  44  has returned to the feeder  34  cycling position. If the welding is complete, the high-speed robot  14  can move the assembly to the position where it can be released. 
         [0084]    The disclosed welding system welds fasteners at high speeds to maximize labor productivity while reducing complexity to minimize downtime and capital cost. Both the equipment and operator productivity are improved by freeing the operator from having to load components synchronously with the welding machine cycle. When the operator can grab a number of workpieces from the storage bin and load them into the equipment at a higher rate than the equipment cycle time, the time required for the operator to get more workpieces from the bin can be buried in the system cycle time so the welding process can proceed at the maximum production rate of the welding machine. 
         [0085]    Maximizing the production rate allows the capital cost of the welding machine or assembly processes to be amortized over more assemblies. In addition to the labor savings that comes from increasing the rate of production welding, increased production volume provides an opportunity to amortize the cost of any supplemental error avoidance systems over more assemblies. 
         [0086]    The system provides sufficient speed, versatility, and reliability to be located in the stamping bay where the stampings are produced or where injection molding occurs to eliminate storage of inventory and extra material handling. 
         [0087]    It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom. Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention. 
         [0088]    Although the different examples have specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
         [0089]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.