Abstract:
There is described a continuity sensing system that operates with a resistance-welding machine. This sensing system measures continuity between the welding electrodes after the welding machine has been initiated and prevents application of high electrode force between the electrodes if the continuity measured is below a reference level. The result is a fully passive system that prevents serious high electrode force that would cause serious pinch-point injury to the operator of the welder. The present invention also includes methods of controlling pneumatic systems on resistance-welding machines to apply low force between welding electrodes until continuity between the welding electrodes has been detected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 12/159,076, filed Jun. 24, 2008, which is a National Stage of International Application No. PCT/US2006/049131, filed Dec. 21, 2006, which claims the benefit of U.S. Provisional Application No. 60/755,434, filed Dec. 30, 2005, each of which are incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a monitoring system having voltage pickup wires or contacts connected to the resistance-welding machine&#39;s transformer secondary pads or primary input lines. This connection creates a signal which is conditioned and then compared to a voltage reference signal. The output of this sensor system connects to a permissive input on the welding control and through fail-safe logic therein prevents energizing of solenoid valves or other systems that would apply high force between the welder&#39;s electrodes until metal has been detected between the these electrodes. Methods are also claimed that will reduce welder ram dead weight to an acceptable level prior to application of high electrode force. 
       BACKGROUND 
       [0003]    1. Need for Pinch-Point Protection 
         [0004]    Operators of resistance (spot) welding machines are often exposed to potentially serious injury due to the pinch point area between the welding electrodes. A typical resistance welder operates with electrode forces of 250 lbs to 5,000 lbs and higher. Because the force is concentrated on a very small contact surface of the electrode, the force density is extremely high and can easily cause permanent finger injury or total amputation. 
         [0005]    For example, on a welder with 600 lbs of electrode force and a ¼″ contact surface, the force density at the pinch point is 12,229 lb/in 2 . Because a common application of a resistance welding machine requires the operator to load parts between the electrodes as well as keep their hands on these parts during the welding sequence, the possibility for serious injury is present on a daily basis. This invention eliminates serious pinch-point injury by reducing the force between electrodes to a very low level when the operator&#39;s finger or other body part is between the electrodes. 
         [0006]    2. Prior Art 
         [0007]    A typical prior art arrangement will be described with reference to  FIGS. 1 to 4 . In one form of the prior art arrangement used on direct action welders (“press welders”) and shown in  FIG. 1 , a limit switch  1  is mounted on a fixed portion of the resistance-welding machine frame  2  and wired to the welder&#39;s control circuitry  3 . An adjustable finger  4  is mounted on the moving ram  5  and is adjusted so that the limit switch closes only after the ram has lowered to the point where space between the electrodes  6  is below the desired dimension (typically about ¼″). After the initiation switch has been closed, the welder&#39;s ram  5  falls by gravity, or has some other mechanism to restrict force between the electrodes until the limit switch  1  has closed. The control circuitry  3 , then turns another output on to place full welding force between the electrodes  6 . 
         [0008]    A second form of the prior art arrangement is used on rocker arm type welders and shown in  FIG. 2 . Force at electrodes  6  is derived from the force out of the rear piston multiplied by the mechanical advantage of arms C/B. A limit switch  7  for this prior art device is mounted on a fixed portion of the welder frame  2 , and an adjustable finger  8  is installed on some portion of the welder&#39;s moving cylinder mechanism  9 . This finger  8  is adjusted so that the limit switch  7  closes only after the electrodes  6  have closed to the point where space between them is below the desired dimension (typically about ¼″). The control then turns another output on to place full welding force between the electrodes  6 . 
         [0009]    However in both of the above prior arrangements, setting of the limit switch finger  4  or  8  is done by the operator or setup person, and the accuracy of this setting is fully in the hands of this person&#39;s skill. Furthermore, if the electrodes  6  are moved during normal production or subsequent electrode replacement or adjustment, and the limit switch is not adjusted properly, the safety of the system is compromised. 
         [0010]    A third form of the prior art arrangement, as shown in  FIG. 3 , utilizes a system actuated by a motor  9  to close a limit switch  11  (or limit switch cam). During a setup sequence, an air cylinder is activated to bring the electrodes  6  together. Then the motor  9  moves the switch  11  or cam until the switch is closed by the cam, and then backs it up until a specific dimension (typically about ¼″) has been reached. During each welding sequence, electrodes  6  are moved together under reduced force until the limit switch  11  has been closed. The control then turns another output on to place full welding force between the electrodes  6 . 
         [0011]    This prior art allows automatic setting of the correct spacing between electrodes  6 . However, in this third prior art arrangement, proper setting of the cam is mechanical and subject to mechanical adjustment errors. Additionally, as with the first two forms, if the electrodes  6  spacing is changed during normal production or subsequent electrode replacement or adjustment, and the operator does not remember to reset the finger the safety of the system is compromised. 
         [0012]    A fourth form of the prior art arrangement, as shown in  FIG. 4 , utilizes a mechanically or pneumatically moved sensing arm (sometimes called “ring guard”)  11 . When the welder is energized during each welding cycle, the electrodes  6  do not move forward, but this sensing arm  11 , lowers to touch ring  11  a to the part  12  being welded. Sensing arm  11  is mechanically designed to encompass the area around the electrodes  6 . If the distance traveled is past the set point on a limit switch  13 , the sensing arm  11  will retract and the electrodes  6  will be closed under full welding force. If ring  11  a on the sensing arm  11  does not move this minimum set distance, as would happen when the operator&#39;s finger or other body part is under ring  11  a the sensing arm  11  will retract but the welder control will not cause the electrodes  6  to close. 
         [0013]    However, in this fourth prior art arrangement if the welder operator or setup person does not adjust the sensing arm  11  properly, high force can be applied between the welding electrodes  6  even though the operator&#39;s finger or other body part is between the electrodes. Further, just bending this sensing arm  11  out of the proper sensing zone renders this system totally useless. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention is intended to solve the problems of the prior art pinch point protection for resistance-welding machines. This sensing system measures continuity between the welding electrodes after the welding machine has been initiated and prevents application of high electrode force between the electrodes if the continuity measured is below a reference level. The result is a fully passive system that prevents serious high electrode force that would cause serious pinch-point injury to the operator of the welder. 
       OBJECTS AND ADVANTAGES OF THE INVENTION 
       [0015]    It is the object of the invention to provide a pinch point sensing device of the character described. 
         [0016]    Another object of the present invention to provide a passive sensing system that will only allow high electrode force to be applied between welding electrodes if continuity between the electrodes has been detected, whereby the electrodes will clamp only on any low-resistance material, but will not clamp on a high-resistance material such as an operator&#39;s finger or other body part. 
         [0017]    Another object is to provide internal sensing of continuity between the welding electrodes whereby no operator adjustments can be made in either the continuity sensing system or the internal control logic, and, additionally, since the continuity sensing is independent of electrode position, the sensing system will protect the pinch point area even if spacing between the electrodes is changed. 
         [0018]    Another object is to provide switching from low to high electrode force which permits limiting electrode force prior to continuity detection and still provides pinch point protection. Another object is to provide a method to reduce force between electrodes when the welder ram is closed under the force of gravity. 
         [0019]    The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1-4 , as previously discussed, illustrate prior art systems attempting to provide suitable pinch point protection for welding machines. 
           [0021]      FIG. 5  is a block diagram of a system embodying the present invention using only a continuity sensor where the sensor signal is obtained from the secondary (low voltage) side of the welding transformer according to a preferred embodiment of the present invention, which is the most common type of system. 
           [0022]      FIG. 6  is a block diagram of the novel system using only a continuity sensor where the sensor signal is obtained from the primary (high voltage) side of the welding transformer according to a preferred embodiment of the present invention, which is typically used for welders that have remote welding guns with integral transformers such as transguns. 
           [0023]      FIG. 7  is a flow chart showing the control logic for the block diagram of  FIGS. 5 and 6 . when only a continuity sensor system is used, in which case the continuity sensor must be closed for the system to allow high welding force to be applied 
           [0024]      FIG. 8  is a flow chart showing the control logic for the block diagram of  FIGS. 5 and 6 . when both a continuity sensor and a limit switch system is used, so that both the continuity sensor and the limit switch must be closed for the system to allow high welding force to be applied. 
           [0025]      FIG. 9  is a pneumatic drawing showing a method embodying the present invention for preventing high force between the electrodes on welders with substantial ram weight, showing the welder at rest. 
           [0026]      FIG. 10  is a pneumatic drawing like that of  FIG. 9 , but with the first stage of the ram dropping by gravity. 
           [0027]      FIG. 11  is a pneumatic drawing like that of  FIG. 9 , with bucking pressure being applied to balance some of the ram&#39;s weight. 
           [0028]      FIG. 12  is a pneumatic drawing like that of  FIG. 9  with full welding force being applied if the sensor(s) has been satisfied. 
           [0029]      FIG. 13  is a pneumatic drawing of a typical welder embodying the present invention which is at rest that has a ram without substantial dead weight which will not close under gravity and which typically has a rocker arm welder or a welder using a fixture type package cylinder. 
           [0030]      FIG. 14  is a pneumatic drawing similar to  FIG. 13  with low force being applied for travel of the electrode prior to the sensor(s) being satisfied. 
           [0031]      FIG. 15  is pneumatic drawing similar to  FIG. 13  with full force being applied if the sensor(s) has been satisfied. 
           [0032]      FIG. 16  illustrates the mechanical arrangement of a typical welder embodying the present invention that has a ram without substantial dead weight but that will close under gravity when air is removed from the cylinder. 
           [0033]      FIG. 17  is a pneumatic drawing for a system shown at rest to prevent high force applied between the electrodes, similar to that shown in  FIG. 16 . 
           [0034]      FIG. 18  is a pneumatic drawing of the system shown in  FIG. 17  with the ram falling by gravity (weight of weld ram). 
           [0035]      FIG. 19  is a pneumatic drawing of the system shown in  FIG. 17  illustrating application of full welding force if the sensor(s) has been satisfied. 
       
    
    
     DETAILED DESCRIPTION 
     Preferred Embodiments of the Invention 
       [0036]    A resistance-welding machine according to a preferred embodiments of the invention will be described with reference to  FIGS. 5 through 19 . 
         [0037]      FIG. 5  shows the electronic diagram of a typical resistance-welding machine that has only a continuity sensor  20  as the detection system, with components of the preferred embodiment of the invention. When the welding electrodes  18  are not in contact, a low voltage leakage is developed by a resistor/capacitor element  14 , connected across the SCR contactor  15  to put low voltage on the welding transformer&#39;s primary side  16 . The continuity sensor  20  may also include an isolation transformer  20 A, an operational amplified/integrator  20 B and a RMS converter  20 C, as shown in  FIG. 5 . 
         [0038]    This voltage is transmitted inductively to the welding transformer&#39;s secondary  17  and to the welding electrodes  18 . A pair of wires  19  or contacts are connected across the welding transformers secondary  17 , ideally at the points closest to the transformer&#39;s output keep the sensor wires out of the mechanical area of the welder, and to the input of the continuity sensor  20 . 
         [0039]    During a welding sequence, the welding controller  25  energizes low-pressure solenoid valve,  27 . The welding electrodes  18  are now brought together under low force by the welding controller. If these welding electrodes  18  contact low-resistance metal to be welded, impedance of the welder secondary  17  is lowered. 
         [0040]    Since available current from R/C element  14  is very low, the lowered impedance of the secondary  17  greatly reduces voltage from this R/C element on the transformer primary  16  to inductively reduce voltage on transformer secondary  17 . This lowered voltage at secondary  17  is transmitted through wires  19  to the input of continuity sensor,  20 . The first stage of continuity sensor  20  conditions this input voltage. The continuity sensor  20  may include an isolation transformer  20 A, an operational amplifier/integrator  20 B, and an RMS converter  20 C. The conditioned voltage is fed into comparator  22 . If this conditioned input voltage level goes below the comparator&#39;s reference voltage relay  23  is energized. Normally open contacts  24  on this relay are closed to signal welding controller  25  that continuity has been detected between the electrodes  18 . 
         [0041]    Adjustment of the internal reference voltage will match electrical response of the welder to close and open electrode conditions for different types of welders. At this time, logic in welding controller  25  energizes solenoid valve  26  to put full welding force between welding electrodes  18 . However, if this conditioned reference voltage fails to go below the reference voltage of comparator  22  within a time limit, controller  25  releases low-pressure solenoid valve  27 , and electrodes  18  open without ever reaching full force. At the same time, a diagnostic can be displayed on welding controller  25 . 
         [0042]    Alternately, as shown in  FIG. 6 , a voltage parallel to the welding transformer&#39;s primary  16  is connected by wires  29  to a step-down transformer  28  and then to the input of the continuity sensor with wires  30 . The circuitry of the continuity sensor  20  conditions this input signal to eliminate extraneous voltage and prevent high welding (or line) voltage from damaging the low-level circuitry in the system&#39;s comparator  22 . Similar to  FIG. 5 , the continuity sensor  20  may also include an operational amplified/integrator  20 B and a RMS converter  20 C, as shown in  FIG. 6 . 
         [0043]    During a welding sequence, the welding controller  25 , energizes low-pressure solenoid valve  27 . If welding electrodes  18  contact low-resistance metal  21  being welded, impedence of the welding transformers secondary is greatly lowered. Since the available current from the R/C element  14  is very low, the lowered impedance of the transformer&#39;s secondary reduces voltage from this R/C element on the transformer primary  16 . This lowered and isolated voltage is transmitted through wires  29  through a step-down transformer  28  and to input of the continuity sensor  20 . 
         [0044]    The first stage of the continuity sensor  20 , conditions this input voltage. The conditioned voltage is fed into a comparator  22 . If this conditioned input voltage level goes below the reference voltage of the comparator  22 , relay  23  is energized. Normally open contacts  24  on this relay  23  are closed to signal the welding controller  25  that continuity has been detected between the electrodes  18 . 
         [0045]    Adjustment of the internal reference voltage will match electrical response of the welder to close and open electrode conditions for different types of welders. At this time, logic in the welding controller energizes a high pressure solenoid valve  26  to put full welding force between welding electrodes  18 . However, if this conditioned reference voltage fails to go below the reference voltage of the comparator  22  within a time limit, the controller  25  releases low-pressure solenoid valve  27 , and the electrodes  18  open without ever reaching full force. At the same time, a diagnostic can be displayed on the welding controller  25 . 
         [0046]      FIG. 7  shows the logic sequence according to the preferred embodiment of this invention on a welder using the electronic diagram of  FIGS. 5 and 6 . After the welder&#39;s initiation switch has been closed, the control checks to see if continuity sensor output relay contact  24  that is connected to the control “continuity sensor input” has closed. 
       Sequences of Operation (FIG.  5  and FIG.  6  Embodiment) 
       [0047]    Sequence 1: Continuity sensor input is not closed at start: The control turns on the low-pressure solenoid valve  27  to bring the electrodes closed under low force. The control continues to monitor the continuity sensor input. 
         [0048]    Sequence 1a: If the continuity sensor input is not closed within the selected maximum set detect time, the low-force solenoid valve  27  is turned off to open the electrodes, a display fault is shown on the control, and the system locks out until the initiation switch has been opened. 
         [0049]    Sequence 1b: If the continuity sensor input closes before the selected detect time has elapsed, the high-force solenoid valve  26  is closed to put full force on the electrodes  18 . The control then goes through the weld sequence and finally turns off both solenoid valves  26  and  27  to release the electrodes  18 . 
         [0050]    Sequence 2: Continuity sensor input is closed at the start. Because this indicates a failure of the continuity sensor  20 , a faulty setting of the continuity sensor board, an incorrect setting of the reference voltage, or a short in the detector wires ( 19  in  FIG. 5, 29  in  FIG. 6 ), the controller  25  does not energize the low-force solenoid valve  27  and locks out waiting for the initiation switch to be opened before allowing the next attempted cycle. At the same time, a fault indication is shown on the display. 
         [0051]      FIG. 8  shows a flow chart for an alternate logic sequence that requires the detection of continuity as illustrated above plus closing of a ram-position limit switch before allowing full electrode force. This arrangement is typically used for welding of parts that are not flat at the beginning of the welding sequence, or where a redundant sensor is desired. 
         [0052]    The limit switch, like that shown in  FIG. 1 , is mechanically adjusted so that a cam will close this limit switch if electrodes  6  are closer than a desired dimension (typically about ¼″). The same logic can be used on rocker arm welders using limit switch  7  and adjustable finger  8  in  FIG. 2 ; and a motorized set limit switch  11  like that shown in  FIG. 3  may been used. 
         [0053]    The logic chart shown in  FIG. 8  shows the logic sequence with this second detection element according to the preferred embodiment of this invention. After the welder&#39;s initiation switch has been closed, the control checks to see if both the continuity sensor input and the limit switch contacts have been closed 
       Sequences of Operation (FIG.  8  Embodiment) 
       [0054]    Sequence 1: Neither continuity sensor nor limit switch contact closure is detected at start: The control turns on the low-pressure solenoid valve to bring the electrodes closed under low force. The control continues to monitor both input signals. 
         [0055]    Sequence 1a: If contact closure from both the continuity sensor and the limit switch are not detected within the predetermined selected detect time, the low-force solenoid valve is turned off to open the electrodes, a display fault is shown on the control, and the system locks out until the initiation switch has been opened. 
         [0056]    Sequence 1b: If contact closure from both the continuity detector and the limit switch is detected before the detect time has elapsed, the high-force solenoid valve is energized to put full force on the electrodes. The control then goes through the weld sequence and finally turns off both solenoid valves to release the electrodes. 
         [0057]    Sequence 2: Contact closure from either one or both inputs is detected at the start. Because this indicates a continuity circuit failure or faulty setting of the reference voltage, a short in the detector wires, or an improperly set or permanently closed limit switch, the control does not energize even the low-force solenoid valve and locks out waiting for the initiation switch to be opened before allowing the next attempted cycle. 
       Mechanical Methods for Attaining Low Approach Force. 
       [0058]    Methods of mechanical operation to provide both low and high force between the electrodes are required to utilize the logic of the present invention. Different mechanical arrangements of pneumatic and other components are required to operate different types of resistance welding machines. This section covers the most common welding systems. However the use of the present invention is not limited to this machinery group. 
         [0000]    Welders with Significant Ram Weight: 
         [0059]    Force between electrodes produced by gravity closure (weight of the ram) alone on this type of welder is high enough to cause significant pinch point injury.  FIG. 1  illustrates a press welder that typically utilizes a guided ram containing an electrode holder, and is activated by a cylinder. The cylinder can be operated directly or indirectly, by air, hydraulics, or a hybrid of the two. 
         [0060]    In the preferred embodiment of this invention, a system is installed to partially counterbalance the gravity dead weight of the ram during the initial stages of each cycle and until continuity between the electrodes has been established. 
         [0061]    A preferred arrangement for a pneumatically operated cylinder to operate a press welder is shown in  FIG. 9 . In this figure, the welder is at rest with the electrodes fully opened. Line pressure air from 5-way solenoid valve  39  is reduced by regulator  40  and passes through 3-way solenoid valve  41  through flow control valve  42 , to shuttle valve  43 . Because at this time there is no air pressure on the other input port of this shuttle valve, this air passes through the shuttle valve  43  and then to the bottom of the air cylinder at port  44 . Since there is no air pressure on the top cylinder port  45  air pressure on the underside of the welder cylinder piston  46  keeps the welder cylinder in the retracted position to keep the electrodes fully apart. 
         [0062]    The sequence of events during a typical welding cycle is shown in  FIG. 10 . When the welding control is first initiated (closing of foot or other switch), three way solenoid valve  41  is energized to exhaust air from the bottom of the welder cylinder at port  44  through the shuttle valve  43  and to flow control valve  42 . The airflow is metered by this flow control valve and moves through 3-way solenoid valve  41  to this valve&#39;s exhaust. 
         [0063]    Lowering of air pressure on the underside of piston  46  causes the cylinder piston to drop under the force of gravity. As this happens, downward movement of the welder cylinder piston  46  pushes air out of lower port  44 . Because this airflow is restricted by flow control valve  42  air in the lower portion of this cylinder is partially compressed to impose a backpressure on the underside of piston  46 . This back pressure imposes an upward force on piston  46  to partially reduce the gravity-produced weight of the welder&#39;s ram. 
         [0064]    As shown in  FIG. 11 , when air pressure at the solenoid valve side  47  of shuttle valve  43  drops to a pressure lower than that at regulator side  48  of this shuttle valve, the shuttle valve shifts to block additional exhaust through solenoid valve  41  and imposes air pressure of regulator  49 . 
         [0065]    This air pressure on the underside of piston  46  continues to impose an upward force on this piston to partially balance the gravity-produced weight of the welder&#39;s ram. At this stage of the sequence, force in pounds between the welding electrodes  50  is represented by the formula: 
         [0000]        F=RW −( psi×A )
       Where:   F=force between electrodes in pounds   RW=gravity weight of welder ram in pounds   psi=air pressure of regulator  49  in psi       
 
         [0070]    A=underside area of welder cylinder piston  46  in 2  Force can also be calculated using the appropriate formula for metric measurements. 
         [0071]      FIG. 12  shows the sequence that follows if the input contact is closed from the continuity sensor (or both continuity sensor and limit switch if so installed) to the welding control within the maximum customer-set sensor time. 
         [0072]    At this time, the control energizes solenoid valve  39  while still keeping solenoid valve  41  energized. This puts higher-pressure air, as supplied from regulator  50 , on the top of the welder cylinder at port  45 , and exhausts air from the bottom of the welder cylinder  44  through shuttle  43 , through pressure regulator  49 , and out the exhaust of solenoid  39 . 
         [0073]    When the sequence has been completed, the air on the top of the welding cylinder piston  46  will be at the level of pressure regulator  50 , and the air on the bottom of the welding cylinder piston will be zero. The welding electrodes will now be at full force for welding. 
         [0074]    At this stage of the sequence, force in pounds between the welding electrodes  50  is represented by the formula: 
         [0000]    
       
      
       F=psi×A  
      
       
         
           
             Where: 
             F=force between electrodes in pounds 
             psi=air pressure of regulator  50 , in psi 
             A=top area of welder cylinder piston  46  in 2    
           
         
       
     
         [0079]    After the weld sequence has been completed, the welding control will de-energize both solenoid valves  39 , and  41  to return the welder cylinder piston  46  to the retraced position of  FIG. 9 . If the input is not closed from the continuity sensor (or both continuity sensor and limit switch if so installed) to the welding control within the maximum customer-set sensor time, solenoid valve  39  will not be energized, and solenoid valve  41  will be de-energized to return the welder cylinder piston  46  to the retraced position of  FIG. 9 . 
         [0000]    Welders without Significant Ram Weight, 
       And Will not Close Under Gravity; 
       [0080]    These welder types require a different pneumatic scheme for operation. 
       Rocker Arm: 
       [0081]    The typical mechanical design of one type, a rocker arm welder, is shown in  FIG. 2 . This type of welder typically utilizes a pivoted beam arrangement with an air cylinder on one end of the beam to close the electrodes in the opposite direction on the other end of the beam. 
         [0082]    Unless the beam is very long and made of very heavy material, force between electrodes on this type of rocker arm welder is normally zero or very low when air is exhausted from the welding cylinder. In most cases, the electrodes will not even close when air is removed from the cylinder. 
       Low Ram Weight Press Welders: 
       [0083]    A second type, as shown in  FIG. 1 , is a press welder that contains a ram that does not have significant weight to overcome friction in the welder cylinder and will not close the electrodes by gravity when air is removed from the welder cylinder. 
         [0084]    A preferred arrangement for operating the welding cylinder of these two welder types is shown in  FIG. 13 . In this figure, the welder is at rest, with the electrodes  52  fully open. 
         [0085]    In the preferred embodiment of the present invention, regulated air from regulator  54  goes through 5-way solenoid valve  55  and to port  56  of the welder cylinder. This imposes air pressure on the underside of the welder cylinder piston,  57  to keep the welder cylinder piston in the retracted position and the electrodes,  53  open. On a rocker arm welder, this air cylinder is typically installed inverted from the illustration as shown in  FIG. 2 , but the action within the welder cylinder is identical. 
         [0086]    The sequence of events during a typical welding cycle starts as shown in  FIG. 14 . When the welding control is first initiated (closing of foot or other switch), five-way solenoid valve  55  is energized to exhaust air from the bottom of the welder cylinder  56  out the exhaust port of solenoid valve  55  using flow control valve  57  to meter the airflow and control the closing speed. At the same time, low-pressure air is supplied from pressure regulator  58  through 5-way solenoid valve  55  to port  62  on shuttle valve  60 . Because at this time there is no pressure coming into port  63  of the shuttle valve, this low-pressure regulated air passes through to port  61  on the welder cylinder. This moves the welder piston  57  forward under low force until electrodes  53  touch. Force, in pounds, between the electrodes at this stage of the sequence for a press welder is represented by the formula: 
         [0000]    
       
      
       F=psi×A  
      
       
         
           
             Where: 
             F=force between electrodes in pounds 
             psi=air pressure of regulator  58 , in psi 
             A=top area of welder cylinder piston  57  in 2    
             Force, in pounds, between the electrodes at this stage of the sequence for a rocker arm welder, is represented by the formula 
           
         
       
     
         [0000]        F=C/B ×( psi×A )
       Where:   F=force between electrodes in pounds   psi=air pressure of regulator  58 , in psi   A=top area of welder cylinder piston  57  in 2      B and C=dimensions from  FIG. 2     Force can also be calculated for either press or rocker arm systems using the appropriate formula for metric measurements.       
 
         [0098]      FIG. 15  shows the sequence that follows if the input contact is closed from the continuity sensor (or both continuity sensor and limit switch if so installed) to the welding control within the maximum customer-set sensor time. At this time, the control energizes solenoid valve  59  while still keeping solenoid valve  55  energized. This puts higher-pressure air, as supplied from regulator  64 , into port  63  of shuttle valve  60  to shift the shuttle valve and pu higher pressure air on the top of the welder cylinder at port  61 . 
         [0099]    Force, in pounds, between the electrodes at this stage of the sequence for a press welder is represented by the formula 
         [0000]    
       
      
       F=psi×A  
      
       
         
           
             Where: 
             F=force between electrodes in pounds 
             psi=air pressure of regulator  64 , in psi 
             A=top area of welder cylinder piston,  57 , in 2    
           
         
       
     
         [0104]    Force, in pounds, between the electrodes at this stage of the sequence for a rocker arm welder, is represented by the formula 
         [0000]        F=C/B ×( psi×A )
       Where:   F=force between electrodes in pounds   psi=air pressure of regulator  64 , in psi   A=area of welder cylinder piston  57  in 2      B and C=dimensions from  FIG. 2     Force can also be calculated using the appropriate formula for metric measurements or for welder cylinders that use air-over-oil intensifier systems.       
 
         [0111]    After the weld sequence has been completed, the welding control will deenergize both solenoid valves  55 , and  59  to return the welder cylinder piston  57  to the retraced position of  FIG. 13 . 
         [0112]    If the input is not closed from the continuity sensor (or both continuity sensor and limit switch if so installed) to the welding control within the maximum customer-set sensor time, solenoid valve  59  will not be energized, and solenoid valve  55  will be de-energized to return the welder cylinder piston  57  to the retraced position of  FIG. 13 . 
         [0000]    Welders without Significant Weight
 
But that Close Under Gravity:
 
         [0113]    This type of welder can use a much more simple arrangement to utilize the features of this invention. The mechanical arrangement for a press welder, as shown in  FIG. 16 , has enough ram weight to allow it to close under gravity but is light enough to prevent pinch point damage under gravity closing is. This type of welder typically utilizes a guided ram  65  that contains an electrode holder  66  and is activated by a cylinder  67 . The cylinder can be operated directly or indirectly, by air, hydraulics, or a hybrid of the two. 
         [0114]    A preferred arrangement for a pneumatically operated cylinder is shown in  FIG. 17 . In this figure, the welder is at rest with the electrodes fully opened. Line pressure air is reduced by regulator  69  and passes through 3-way solenoid valve  70  to impose air pressure on the underside of the welder cylinder piston,  71 . This keeps the welder cylinder open. 
         [0115]    The sequence of events during a typical welding cycle starts as shown in  FIG. 18 . When the welding control is first initiated (closing of foot or other switch), three-way solenoid valve  70  is energized to exhaust air from the bottom of the welder cylinder using flow control valve  73  to meter the airflow and control the closing speed until the electrodes touch. 
         [0116]    Force, in pounds, between the electrodes  77  at this stage of the sequence is the dead gravity weight of the welder&#39;s ram  74 . 
         [0117]      FIG. 19  shows the sequence that follows if the input contact is closed from the continuity sensor (or both continuity sensor and limit switch if so installed) to the welding control within the maximum customer-set sensor time. At this time, the control energizes three-way solenoid valve  75  while still keeping solenoid valve  70  energized. This puts air, as supplied from regulator  76 , on the top of the welder cylinder at port  77 . Force, in pounds, between the electrodes at this stage of is represented by the formula: 
         [0000]    
       
      
       F=psi×A  
      
       
         
           
             Where: 
             F=force between electrodes in pounds 
             psi=air pressure of regulator  76 , in psi 
             A=top area of welder cylinder piston in 2    
             Force can also be calculated using the appropriate formula for metric measurements or for welder cylinders that use air-over-oil intensifier systems, 
             Welders that Use Servo Motors or Other Motor Driven Systems: 
           
         
       
     
         [0124]    This type of welder utilizes a motor-driven mechanism to close the electrodes. The system embodying this invention communicates with the servo control circuitry to provide low torque prior to electrode continuity detection. If continuity is not detected prior to the maximum detection time has expired, the low-force signal to the servo control circuitry will be turned off to force the servo system to return the electrode to the fully open position. 
         [0125]    While embodiments of the invention have been shown in considerable detail, it is not intended that the inventions should be limited to the exact construction described and many changes and modifications of the structure and methods can be made without departing from the spirit or scope of the invention.