Abstract:
A fully programmable servo-pneumatic modular weld gun having closed-loop position and pressure control generally includes a weld gun base module, a pneumatic cylinder with a movable piston having a first welding electrode, a J-shaped gun arm having a first end attached to the base module and a second end having a second welding electrode and a controller for providing closed-loop positioning and pressure control of the first welding electrode. The pneumatic cylinder includes an internal positioning encoder for detecting piston position and transmitting piston position information to the controller to control movement of the piston through a proportional flow control valve and a proportional pressure regulator. In a method for controlling the position of a spot weld gun weld tip, one of a plurality of binary bit position sequences corresponding to a different weld tip position are inputted to a controller that compares the inputted sequence to the actual position of the weld tip as detected by an internal positioning encoder. Based on the comparison, the controller activates a control valve of a pneumatic cylinder to extend or retract the weld tip. By manipulating the bit sequences and depending on the input address of the controller, other welding parameters such as weld pressure, equalizer pressure and tip dampening can be controlled.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to spot welding guns and more particularly to a servo-pneumatic modular weld gun system having freely programmable closed loop position and pressure control. The present invention is well suited to rapid assembly line welding applications, particularly in the automotive industry.  
         BACKGROUND OF THE INVENTION  
         [0002]    Resistance or spot welding joins middle sheets or structures through heat produced by resistance to the flow of electrical current. The metal structures to be welded are typically clamped together with rod shaped electrodes applying pressure on opposite sides. An electric current passing between the electrodes meets resistance when it flows across the metal structures producing heat that melts the metal and welds them together.  
           [0003]    Conventional spot welding guns utilize one or more pneumatic cylinders to extend and retract a moveable electrode and to apply the pinching force to the materials to be welded. The pneumatic cylinder moves the electrode weld tip to one of at least three known positions. These hard stop positions include fully retracted, fully extended and a mid-point position. After welding with the weld tip in its fully extended position, the piston rod returns to a mid-point position rather than its fully retracted position. This reduces cycle time between welds. To accomplish this design parameter, normally a three position pneumatic cylinder having a fixed mid-point position is used in the spot weld gun.  
           [0004]    Typically, the specific weld application dictates the design of the weld gun configuration. In other words, weld guns are usually custom made to fit the particular welding application. Several considerations must be addressed in custom designing a weld gun. Design parameters include the gap between the electrode weld tips, the stroke of the movable electrode, the hard positions of the electrode weld tip and the cycle time. Thus, depending upon the weld application, a custom weld gun is designed around a selected three position pneumatic cylinder. Accordingly, in an assembly line environment there may be several different weld gun configurations in operation at any one time. The obvious disadvantage of custom designed weld guns is the total lack of standardization and interchangeability.  
           [0005]    Additionally, when a weld parameter changes in a conventional weld gun system, usually the weld gun must be disassembled and redesigned for the new welding parameter. For example, if it is desired to change the weld tip mid-point position, a new pneumatic cylinder must be substituted for the existing cylinder. During this retrofit, the weld gun must be taken out of the assembly line resulting in production down time. Attempts to avoid this disadvantage include providing weld guns having multiple pneumatic cylinders. However, the disadvantage of multiple cylinder weld guns is the increase in weight and complexity.  
           [0006]    Accordingly, it would be desirable to provide a weld gun configuration having a standard design that can be readily interchanged with other weld guns. It is also desirable to provide a weld gun that is freely programmable to provide for any desired weld tip positioning and/or pressure.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a fully programmable servo-pneumatic modular spot weld gun having closed-loop position and pressure control and a method for controlling the position of a weld gun welding electrode. The weld gun generally includes a weld gun base module, a pneumatic cylinder with a movable piston having a first welding electrode, a J-shaped gun arm having a first end attached to the base module and a second end having a second welding electrode and a controller for providing closed-loop positioning and pressure control of the first welding electrode.  
           [0008]    The pneumatic cylinder includes an internal positioning encoder for detecting piston position and transmitting piston position information to the controller to control movement of the piston through a proportional flow control valve and a proportional pressure regulator. Preferably, the internal positioning encoder is a linear potentiometer fitted within the piston rod of the pneumatic cylinder to accurately determine the extension and retraction of the piston rod.  
           [0009]    The closed-loop positioning control allows for use of a standard pneumatic cylinder having a known stroke and cylinder bore. Thus, the weld gun is adapted for releasable attachment of the pneumatic cylinder and the gun arm, whereby the pneumatic cylinder and the gun arm may be interchanged with other pneumatic cylinders and gun arms. Additionally, the weld gun base module components are made from high strength machined aluminum to reduce weight.  
           [0010]    In a method for controlling the position of a spot weld gun weld tip, one of a plurality of binary bit position sequences corresponding to a different weld tip position are inputted to a controller that compares the inputted sequence to the actual position of the weld tip as detected by an internal positioning encoder. Based on the comparison, the controller activates a control valve of a pneumatic cylinder to extend or retract the weld tip. Preferably the bit sequence comprises four binary bits representing sixteen possible weld tip locations.  
           [0011]    By manipulating the bit sequences and depending on the input address of the controller, other welding parameters such as weld pressure, equalizer pressure and tip dampening can be controlled. For example, a method of controlling weld pressure at the weld tip includes the steps of inputting one of a plurality of binary bit pressure sequences to a controller wherein each pressure sequence corresponds to a different pressure applied at the weld tip. The pressure sequence is compared with the actual weld tip pressure detected by a pressure regulator and, based on the comparison, the controller activates the control valve of the pneumatic cylinder to increase or decrease pressure applied at the weld tip. Similarly multiple weld tip positions may be programmed into the controller to provide a tip dampening feature to the weld gun.  
           [0012]    For a better understanding of the present invention, reference is made to the following detailed description to be read in conjunction with the accompanying drawings and its scope will be defined in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a top perspective view of the servo-pneumatic weld gun formed in accordance with the present invention.  
         [0014]    [0014]FIG. 2 is a cross-sectional view of the pneumatic cylinder taken along the line  2 - 2  of FIG. 1.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring to FIG. 1, a servo-pneumatic modular weld gun  10  formed in accordance with the present invention is shown. The weld gun  10  generally includes a weld gun base module  12 , a J-shaped gun arm  14 , a pneumatic cylinder  16  and a sub-controller  88 .  
         [0016]    The weld gun base module  12  is constructed to act as a platform for a multitude of designs. In other words, the base module  12  remains the same regardless of the type of pneumatic cylinder or gun arm used. By interchanging stationary gun arms, a wide variety of weld conditions can be satisfied, while keeping the total number of weld gun components to a minimum. Reduction in the number of weld gun components benefits the end customer by reducing both spare weld gun requirements and spare part inventories. Maintenance costs and training are also minimized due to the base module commonality.  
         [0017]    The base module  12  comprises a base  18 , a weld transformer  19 , an equalizer assembly  20 , a weld gun base flange  21  and mounting brackets  22 . The base  18 , the base flange  21  and the mounting brackets  22  are machined from high strength, aircraft quality aluminum, thereby reducing the overall weight of the base module  12  but still providing the required strength. The mounting brackets  22  are provided on two sides and the bottom of the base  18  to enable attachment of the weld gun  10  to a robotic arm or other assembly line welding machine in any one of three configurations. All parts are machined from billet material reducing weight and eliminating the need for casting patterns and thereby reducing manufacturing time.  
         [0018]    The transformer  19 , the equalizer assembly  20  and the mounting brackets  22  are connected to the base  18  by bolts. The weld transformer  19  includes electrical connectors (not shown) for connection to an external power source and provides the required current to the weld gun. The open design of the base module  12  allows for a variety of weld transformers including U.S. standard AC and DC styles as well as ISO standard styles.  
         [0019]    The equalizer assembly  20  is a conventional pneumatic cylinder having a piston rod  23  extending outwardly therefrom. Secured to the end of the piston rod  23  is the weld gun base flange  21 . Thus, the base flange  21  moves together with the piston rod  23  of the equalizer assembly  20 . The equalizer assembly  20  includes a proportional control valve  24  and a pressure regulator  25  for controlling and monitoring air pressure supplied to the pneumatic cylinder for extending and retracting the equalizer piston rod  23 . Operation of the equalizer assembly  20  will be discussed in further detail below.  
         [0020]    Also connected to the base module  12  are a transformer bus bar  40 , a movable bus bar  42 , electrode adapters  44 , a stationary shunt  46  and a movable shunt  48  for providing electrical connections from the weld transformer  19 . The movable bus bar  42  and the movable shunt  48  are connected to a movable shank portion  28  of the pneumatic cylinder  16  by an arm  50  which includes a guide shaft  52  that is journaled into a bearing  54  fixed in the base flange  21 . The guide shaft  52  and the bearing  54  help to ensure proper alignment of the movable shank portion  28  of the cylinder piston during travel.  
         [0021]    Not shown on the base module  12  are conventional water line connections and associated internal water passages and manifolds for cooling the components of the weld gun. Any suitable cooling system design may be utilized with the weld gun of the present invention. Such systems are well known in the art and need not be discussed in any further detail.  
         [0022]    The base flange  21  further includes a gun arm receiving cavity  32  for attachment of the gun arm  14 . The gun arm  14  is also made from a machined high strength aircraft quality aluminum to reduce weight. The gun arm  14  is generally J-shaped, having one end secured to the base flange  21  and a stationary shank  34  with a weld tip  36  secured at an opposite end thereof. The gun arm  14  is secured to the base flange  21  by bolts  33  such that the stationary shank portion  34  of the gun arm is axially aligned with the movable shank portion  28  of the cylinder piston and a gap is formed between the opposing weld tips of the shanks. Thus, when the equalizer assembly  20  is activated, the stationary shank portion  34  of the gun arm  14  moves together with the equalizer piston rod  23  toward or away from the movable shank portion  28  of the cylinder piston. By interchanging different gun arms, the gap between the weld tips can be adjusted. Again, the gun arm  14  and the pneumatic cylinder  16  are releasably connected to the base flange  21  to provide for full interchangeability.  
         [0023]    Connected to the base flange  21  by bolts  26  is the pneumatic cylinder  16 . The base flange  21  is formed with a bore  27  through which the cylinder piston rod extends. Connected to the cylinder piston rod is a movable shank portion  28  having a weld tip  30  at its external end.  
         [0024]    An important benefit of the present invention is the provision for the use of a standard ISO 6431 multi-position pneumatic cylinder. A suitable pneumatic cylinder for use with the weld gun of the present invention is Model No. DNC-125-200 manufactured by Festo Corporation of Hauppauge, N.Y. As discussed above, conventional weld guns are usually custom made using a 3-position cylinder. When a new application is required, the weld gun must be disassembled and reconfigured for the new application. Thus, there is virtually no interchangeability with existing weld guns. By standardizing the pneumatic cylinder, the end user can have several identical weld guns on hand that can be easily interchanged. This dramatically reduces down time when tooling applications change. As discussed in further detail below, standardization is achieved by incorporating an internal positioning encoder  78  into a standard cylinder and providing a sub-controller  88  to provide continuous closed-loop positioning control.  
         [0025]    Continuous closed-loop positioning control means that the actual position of the weld tip  30  is continuously monitored by the internal positioning encoder  78  and this information is sent to the sub-controller  88  where it is compared with input information corresponding to a desired weld tip position. The position of the weld tip  30  is then automatically adjusted by the sub-controller  88  based on the comparison of the actual position and the input information.  
         [0026]    [0026]FIG. 2 illustrates a cross-section of the pneumatic cylinder  16  taken along the line  2 - 2  in FIG. 1. The pneumatic cylinder  14  generally includes a cylindrical housing  56 , bearing caps  58  and  60 , a piston  62  and a piston rod  64 . The bearing caps  58  and  60  are fixed to the ends of the cylindrical housing  56  to thereby form a piston chamber  66  therein. The bearing caps  58  and  60  include ports  67  for connection to a compressed air source through a proportional control valve  68  and a proportional pressure regulator  69  shown in FIG. 1. The proportional pressure regulator  69  monitors and controls compressed air going to the proportional control valve  68  through pneumatic lines  71  as will be discussed in further detail below. The front bearing cap  58  further includes a central bore  70  through which the piston rod  64  may travel. The piston  62  is fixed to the piston rod  64  forming an air-tight seal against the interior of the cylinder housing  56  such that when compressed air is applied to one of the ports  67  of one of the bearing caps, the piston moves in a direction away from the applied pressure, thereby moving the piston rod  64  in an axial direction. The standard bore for the internal chamber  66  is 125 mm bore and the stroke of the piston rod  64  is 185 mm.  
         [0027]    Fixed within the central bore  70  of the front bearing cap  58  is a metallic scraper ring  72  which is held in place between a snap ring  73  and a bearing  74 . The scraper ring  72  has an internal diameter sized so that the scraper ring is in intimate contact with the circumferential periphery of the piston rod  64 . Thus, as the piston rod  64  moves, the scraper ring removes any weld slag that may accumulate on the piston rod thereby preventing such debris from entering the internal chamber  66  of the pneumatic cylinder  14 .  
         [0028]    The pneumatic cylinder  14  is retrofitted with a hardened C 45  chrome plated piston rod  64  that has been gun drilled along its centerline to form an axial bore  76  therein. The axial bore  76  is sized to receive the internal positioning encoder  78 . The piston rod  64  further includes a lock ring  80  fixed to its outer end to facilitate attachment of the movable arm  50 , movable shank portion  28  and weld tip  30  thereto.  
         [0029]    The internal positioning encoder  78  is preferably a linear potentiometer fixed at one end to the rear bearing cap  60  with stainless steel nylon lock nuts  81 . The linear potentiometer  78  further includes a probe  82  fixed to the outer end of the piston rod  64 . A suitable linear potentiometer for use with the weld gun of the present invention is Part No. 04LF8258 manufactured by Betatronix Inc. of Hauppauge, N.Y. FIG. 2 shows one method of attaching the piston probe  82  to the piston rod  64 . A screw  83  having an internal axial thread  84  is threaded into the end face of the outer end of the piston rod  64 . The probe  82  is then threaded into the internal thread of the screw thereby fixing the probe to the piston rod. The potentiometer  78  has a working stroke of 10 inches (254 mm) of which only 7.28 inches (185 mm) will be used. When the piston rod  64  is fully retracted, the potentiometer has a dead band of 1.35 inches (34 mm), same as when it is fully extended. The linear potentiometer  78  operates in a known manner whereby the extension of the probe  82  can be accurately measured. Consequently, the position and travel of the piston rod  64  can be accurately monitored.  
         [0030]    An electrical enclosure box  85  is connected to the rear bearing cap  60  to house the potentiometer wiring. The electrical enclosure box  85  further includes electrical connectors  86  to facilitate electrical connection between the linear potentiometer  78  and a sub-controller  88 . The sub-controller  88  is the interface between the weld gun  10  and a master controller (not shown). The sub-controller  88  is essentially a micro-processor that receives signals from the encoder  78 , the proportional pressure regulator  69  and the equalizer pressure regulator  25  and compares these signals with signals received from the master controller. The proportional control valve  68 , the proportional pressure regulator  69 , the equalizer control valve  24  and the equalizer pressure regulator  25  are all electrically connected to the sub-controller  88  through lines  89 .  
         [0031]    The weld gun  10  generally operates in the following manner. With the weld tips  30  and  36  in a full open position, a robotic arm positions the weld gun so that the metal workpiece is between the weld tips. The equalizer assembly  20  is activated to retract the equalizer piston rod  23  and move the weld tip  36  toward the workpiece until it makes contact. This supports the workpiece during welding and prevents damage to the pneumatic cylinder  16 . Once the workpiece is supported, the pneumatic cylinder  16  is activated to move the weld tip  30  toward the workpiece. Once the weld tip  30  contacts the workpiece, the transformer  19  provides a desired current between the weld tips  30  and  36 . The current passes through the metallic workpiece melting the metal and producing a weld. When the weld is complete, the pneumatic cylinder  16  retracts the weld tip  30  to a mid-point position and the workpiece and/or gun is moved to a new location and the cycle repeats.  
         [0032]    As mentioned above, by retracting the weld tip  30  to a mid-point position rather than a fully retracted position, cycle time between welds is greatly reduced. By incorporating an internal positioning encoder  78  and a sub-controller  88 , the present invention allows for programming of any desired mid-point position through the master controller. Additionally, other features such as tip wear compensation, tip dampening and accurate calibration are achieved as discussed in further detail below.  
         [0033]    The master controller coordinates all motion, pressure control and sequencing for the weld gun robotic arm based on its own internal program and on feedback from the sub-controller  88 . This is accomplished by the transmission of bit sequences between the master controller and the sub-controller  88 . The master controller and the sub-controller communicate with each other through a standard communication protocol known as DeviceNet. The following table is a bit map showing the inputs of the master controller from the sub-controller and the outputs of the master controller to the sub-controller.  
                             TABLE 1                       Bit Map                                    Inputs               Nxx:i01 -   System On/System Ok           Nxx:i02 -   Position Achieved           Nxx:i03 -   Positioned for weld (stack-up is ok)           Nxx:i04 -   Position Over Travel (pulled cap)           Nxx:i05 -   Equalizer Pressure Achieved           Nxx:i06 -   Weld Pressure Achieved           Nxx:i07 -   System Calibrated           Outputs           Nxx:o01 -   Gun tip close strobe           Nxx:o02 -   Calibrate weld tip           Nxx:o03 -   Gun forward/backup strobe           Nxx:o04 -   Forward/backup position Bit 1           Nxx:o05 -   Forward/backup position Bit 2           Nxx:o06 -   Forward/backup position Bit 3           Nxx:o07 -   Forward/backup position Bit 4           Nxx:o08 -   Tip close and weld position Bit 1           Nxx:o09 -   Tip close and weld position Bit 2           Nxx:o10 -   Tip close and weld position Bit 3           Nxx:o11 -   Tip close and weld position Bit 4           Nxx:o12 -   Start           Nxx:o13 -   Weld pressure Bit 1           Nxx:o14 -   Weld pressure Bit 2           Nxx:o15 -   Weld pressure Bit 3           Nxx:o16 -   Weld pressure Bit 4           Nxx:o17 -   Jog Tip (+)           Nxx:o18 -   Jog Tip (−)           Nxx:o19 -   Equalizer pressure Bit 1           Nxx:o20 -   Equalizer pressure Bit 2           Nxx:o21 -   Equalizer pressure Bit 3           Nxx:o22 -   Equalizer pressure Bit 4                      
 
         [0034]    The master controller controls the weld gun by manipulating a sequence of binary bits. Bits  1 - 4  are manipulated to establish various operating parameters depending on the output address of the master controller. Bit  5  is a strobe bit carrying the execute command.  
         [0035]    An “on” bit from master controller output Nxx:o12 turns the weld gun system on. Master controller outputs Nxx:o04-07 control the mid-point position of the pneumatic cylinder piston rod 64 based on feedback from the internal encoder 78. The piston rod 64 has a possible stroke of 150 mm for positioning purposes. The forward/backup position addresses, Nxx:o04-07 represent 0 mm -150 mm in 10 millimeter increments according to the following bit sequence shown in Table 2.  
                                     TABLE 2                           Forward/Retract Position Table            Bit 1   Bit 2   Bit 3   Bit 4           Nxx:o04   Nxx:o05   Nxx:o06   Nxx:o07   Stroke Position               0   0   0   0    0 mm closed       0   0   0   1    10 mm       0   0   1   0    20 mm       0   0   1   1    30 mm       0   1   0   0    40 mm       0   1   0   1    50 mm       0   1   1   0    60 mm       0   1   1   1    70 mm       1   0   0   0    80 mm       1   0   0   1    90 mm       1   0   1   0   100 mm       1   0   1   1   110 mm       1   1   0   0   120 mm       1   1   0   1   130 mm       1   1   1   0   140 mm       1   1   1   1   150 mm                  
 
         [0036]    Output Nxx:o03 “on” will position the piston rod 64 to a location as determined by the bit sequence of Nxx:o04-07 and relative to the last operation of the Again, calibrate output Nxx:o02. positioning in, this cannot be achieved without positive closed-loop feedback from the internal positioning encoder 78 sending signals back to the subcontroller 88 for comparison with the outputs from the master controller.  
         [0037]    Master controller outputs Nxx:o08-11 also utilize a four bit sequence to close the weld gun to a desired gap between weld tips 30 and 36. This bit sequence allows for automatic calibration and for a tip dampening feature. By manipulating this bit sequence the weld tips can be positioned to an initial gap prior to welding at a “tip closed” position, according to the following bit sequence shown in Table 3.  
                                     TABLE 3                           Tip Close and Weld Position Table            Bit 1   Bit 2   Bit 3   Bit 4           Nxx:o08   Nxx:o09   Nxx:o10   Nxx:011   Stroke Position               0   0   0   0   0 mm       0   0   0   1   1 mm       0   0   1   1   2 mm       0   0   1   1   3 mm       0   1   0   0   4 mm       0   1   0   1   5 mm       0   1   1   0   6 mm       0   1   1   1   7 mm       1   0   0   0   8 mm       1   0   0   1   9 mm       1   0   1   0   10 mm        1   0   1   1   11 mm        1   1   0   0   12 mm        1   1   0   1   13 mm        1   1   1   0   14 mm        1   1   1   1   15 mm                   
 
         [0038]    The stroke positions listed in Table 3 are relative to the location of the “tip closed” position and must be calibrated prior to operation. The weld tip has a relative stroke of 15 mm for welding purposes wherein the tip closed position bits Nxx:o08-11 represent 0 mm -15 mm in 1 mm increments. Output Nxx:o01 “on” will position the weld tip to a location that is determined by the bit sequence of Nxx:o08-11 and relative to the last operation of the calibrate bit Nxx:o02.  
         [0039]    A third four bit sequence four bit sequence corresponding to outputs Nxx:o13-16 is used to manipulate welding pressure applied is used to manipulate welding pressure applied at the weld tips  30  and  36  and a fourth four bit sequence corresponding to outputs Nxx:o19-22 is used to manipulate equalizer pressure. The weld tip pressure has a range of 90 psi with a 6 psi per bit resolution that is set by sequencing the bits corresponding to outputs Nxx:o13-16. The equalizer assembly pressure also has a range of 90 psi with 6 psi per bit resolution that is set by sequencing the bits from master controller outputs Nxx:o19-22. Table 4 shows the bit sequence for setting both weld tip pressure and equalizer pressure.  
                                     TABLE 4                           Weld and Equalizer Pressure Value Table            Bit 1   Bit 2   Bit 3   Bit 4   Pressure       Nxx:o13   Nxx:o14   Nxx:o15   Nxx:o16   Weld       Nxx:o19   Nxx:o20   Nxx:o21   Nxx:o22   Equalizer               0   0   0   0    0 psi       0   0   0   1    6 psi       0   0   1   0   12 psi       0   0   1   1   18 psi       0   1   0   0   24 psi       0   1   0   1   30 psi       0   1   1   0   36 psi       0   1   1   1   42 psi       1   0   0   0   48 psi       1   0   0   1   54 psi       1   0   1   0   60 psi       1   0   1   1   66 psi       1   1   0   0   72 psi       1   1   0   1   78 psi       1   1   1   0   84 psi       1   1   1   1   90 psi                  
 
         [0040]    Master controller inputs Nxx:i05 and Nxx:i06 “on” are used to indicate the pressure output of the proportional pressure regulator  69  or the equalizer pressure regulator  25  is within a +/−2% accuracy of the intended pressure determined by the bit sequence of Nxx:o13-16 or Nxx:o19-22. The weld pressure command will only operate when the tip close and weld position is executed. The master controller has full control of equalizer pressure and must alter pressure values as necessary by gun orientation.  
         [0041]    Master controller outputs Nxx:o17 and Nxx:o18 are used to manually operate the weld gun. Output Nxx:o17 will jog the cylinder to a tip closed position and output Nxx:o18 will jog the cylinder to a tip open position. These features allow for manual extension and retraction from the master controller.  
         [0042]    Master controller inputs Nxx:i01-07 receives signals from the subcontroller  88  indicating various operating parameters of the weld gun. Input Nxx:i01 “on” indicates to the master controller that the system is on and no fault condition exists. Input Nxx:i02 “on” indicates that a forward/retract position has been achieved. Input Nxx:i03 “on” indicates that the weld stack is within permissible tolerance while “off” indicates that the stack is too thick or something is interfering with the tip travel. Input Nxx:i04 “on” indicates to the master controller the position over travel or a “pulled cap” condition. Input Nxx:i05 “on” indicates the commanded weld pressure is achieved as selected by outputs Nxx:o13-16. Input Nxx:o16 “on” indicates that the commanded equalizer pressure is achieved as selected by outputs Nxx:o19-22. Finally, input Nxx:i07 “on” indicates that the system is in calibration mode whereas “off” indicates calibration is complete and normal operation may resume.  
         [0043]    After initialization and calibration is complete, operation of the weld gun system according to the present invention may be illustrated by the following steps:  
         [0044]    1. Set bits Nxx:o04-Nxx:o07 for an intended position.  
         [0045]    2. Hold bit Nx:o03 high to extend to the position.  
         [0046]    3. Set bits Nxx:o13-Nxx:o16 for the desired pressure.  
         [0047]    4. Set bits Nxx:o08- Nxx:o011 for the desired metal thickness.  
         [0048]    5. Hold bit Nxx:o01 high to execute weld.  
         [0049]    6. Test for bit Nxx:i03 for “positioned to weld”.  
         [0050]    7. Test for bit Nxx:i06 for weld pressure achieved.  
         [0051]    8. Test for bit Nxx:i04, pulled cap, is not on.  
         [0052]    9. Execute weld with weld controlled.  
         [0053]    10. Drop bit Nxx:o01 to back up to location as sequenced by bits Nxx:o04- Nxx:o07 and set by Nxx:o03.  
         [0054]    11. Drop bit Nxx:o03.  
         [0055]    12. Enter new backup location steps 1 through 2.  
         [0056]    13. Set new weld pressure Step 3.  
         [0057]    14. Sent new thickness Step 4.