Welding Apparatus and Control System Therefor

The present invention generally relates to resistance-type welding apparatus, and deals more particularly with a control system for sequentially controlling a plurality of welding operations in one or more welding cycles.

The process of resistance welding involves positioning a workpiece to be welded between a pair of electrodes through which electrical current is delivered from a source thereof. The electrodes are clamped under pressure to squeeze the workpiece therebetween so that electrical current is delivered from one electrode through the workpiece to the other electrode at the point of contact between the electrodes and the workpiece. Heat generated by the resistance encountered by the current passing through the parts of the workpiece to be joined melts the contact faces of the parts, thereby melting the parts to create a weld. In some cases, after the welded joint between the parts has reached a sufficiently high temperature, electrical current flow is terminated and the clamping pressure is maintained for a prescribed time interval to unite the parts. In other cases, such as spot welding, the parts need not remain clamped after current flow is terminated.

One type of resistance welding apparatus is described in U.S. Pat. No. 3,074,009 in which the electrodes receive electrical current from the secondary windings of the transformer. The primary winding of the transformer is coupled in series with a capacitor, an electrical supply source and a pair of ignitrons or similar rectifier tubes. The ignitrons are phase shifted such that they begin to conduct just before the voltage peak in each half cycle. Each time an ignitron begins to conduct, it places a charge on the capacitor approximately equal to this peak. The charge remains on the capacitor until the next half cycle when the other ignitron fires. The capacitor rapidly discharges the stored potential into the primary transformer thereby generating a sharp pulse in the transformer secondary circuit which is delivered to the electrodes. Current is therefore delivered to the electrodes from the secondary transformer in a series of spikes or pulses each equal to approximately twice the value of the instantaneous line voltage. This pulsed current supply provides instantaneous localized heating of the welding surface which cannot be achieved by a lower current of proportionally longer duration because of the dissipation of the interface surface temperature by conduction. Thus, the lower value of power is required in these pulse current systems and undesirable secondary heating effects are minimized.

In order to adapt pulse power type welding apparatus for use in various applications, control systems have been devised which allow selective control of each sequence in a welding cycle, i.e., total clamping time, welding time, holding time after termination of welding current, as well as the magnitude of current delivered to the electrodes. These prior art systems employed mechanical timers and potentiometers arranged in an analog circuit to control the various process variables. Systems of this type were undesirable, however, in that the process parameters could not be selected and controlled with the repeatable precision required in many applications. For example, potentiometers and the like employed to select process parameters could not be precisely reset. Also, deterioration of tubes and other electrical components after a period of use adversely affected the overall accuracy of the system.

Accordingly, it is an important object of the present invention to provide a solid state control system employing a microcomputer for accurately and reliably controlling a resistance type pulse power welding apparatus.

A further object of the present invention is to provide welding apparatus and a control system therefor as described above which is highly flexible in terms of its adaptability to various welding applications.

A further object of the invention is to provide a control system as described above in which process parameters are precisely selected and stored in a memory for use in controlling the welding apparatus.

A still further object of the invention is to provide a control system as described above which includes a sensing circuit for detecting the level of line voltage and inhibiting the operation of the microcomputer when the line voltage falls below a prescribed level thereof.

These and further objects of the invention will be made clear, or will become apparent during the course of the following description of a preferred embodiment of the invention.

A resistance-type welder employs a pulsed power supply and a pair of opposed, spaced apart welding electrodes between which workpiece parts to be welded are clamped. At least one of the electrodes is mounted on an extensible motor member, such as a solenoid actuated hydraulic cylinder. A microcomputer based control system coupled with the power supply and solenoid is employed to allow precise, repeatable selection and control of process parameters such as clamping time, welding time, holding time and the magnitude of current delivered to the electrodes. The control means includes memory means for storing data corresponding to the selected process parameters and a central processor unit for controlling data flow and a pair of circuits which respectively operate the solenoid and power supply. Data corresponding to the process variables is digitally input to the memory by a series of bistable switches and a thumbwheel switch. A plurality of manually actuatable switches coupled with the central processor are controlled by the operator to initiate one or more welding cycles. A sensing circuit is provided which detects the level of line voltage and inhibits the operation of the processor when the line voltage drops below a prescribed level. One of the circuits controlled by the processor comprises a pulse oscillator which operates a solid state switch for controlling the solenoid, and the other circuit controlled by the processor includes a frequency divider and logic gate means for gating a divided frequency signal employed to control the power supply.

In the drawings, which form an integral part of the specification and are to be read in conjunction therewith, and in which like components are designated by identical reference numerals in the various views:

FIG. 1 is a perspective view of resistance welding apparatus and a control system therefor according to the preferred embodiment of the present invention;

FIG. 2 is an elevational view of the control panel associated with the control panel of the present invention;

FIGS. 3A and 3B, taken in combination, form a combined block and schematic diagram of the control system;

FIG. 4 is a detailed schematic diagram of the power supply employed in the control system shown in FIGS. 3A and 3B;

FIGS. 5A-5B, taken together, form a detailed schematic diagram of the control system in FIGS. 3A and 3B;

FIG. 5 is a plan view showing how FIGS. 5A-5E are to be arranged for proper viewing;

FIGS. 6A and 6B, taken together, form a detailed schematic diagram of the data input and output devices associated with the control panel shown in FIG. 2;

FIGS. 7 and 8 form a detailed schematic diagram depicting the interconnection of the control system with the power supply and solenoid; and

FIG. 9 is a detailed schematic diagram of a control module which may be optionally employed in connection with the control system of the present invention.

Referring first to FIG. 1, the present invention is concerned with resistance-type welding apparatus generally indicated by the numeral 100 which comprises a pair of spaced electrodes 104 and 106 of conductive material such as copper, between which parts of a workpiece (not shown) to be welded are clamped. One of the electrodes 106 is stationarily mounted while the other electrode 104 is mounted on the output shaft of a hydraulic cylinder 108. Cylinder 108 is operated by a solenoid actuated valve 110 which is controlled by a later-discussed control system. The control system includes a control panel 102 which allows selective input and read-out of process variables. A manually-actuatable switch 112 is also connected with the control system for initiating one or more welding cycles.

After the desired process variables have been entered into the control system via control panel 102, the workpiece parts are appropriately positioned in the gap between the electrodes 104 and 106. Switch 112 is then actuated, whereupon valve 110 is operated to cause hydraulic cylinder 108 to extend which in turn moves electrode 104 toward electrode 106 thus clamping and squeezing the parts therebetween. In some cases, the electrodes 104 and 106 will be clamped on the parts to be welded for a prescribed time interval prior to the delivery of electrical current through the parts.

For purposes of the present description, the following process variables and their corresponding definitions will be employed:

Squeeze time--the time interval between the initial application of electrode force on the parts and the first application of current. In the event that precompression of the parts is desired, squeeze time is the duration between the initial application of electrode force on the parts and the application of maximum force on the parts.

Weld time--the duration during which welding current is delivered through the electrodes to the parts in making a weld.

Weld cycle--the complete series of events involved in the making of a weld on the parts.

Hold time--in spot projection welding, hold time is the duration during which force is applied at the point of weld after the last impulse of current ceases. In seam, flash and upset welding, hold time is the duration during which force is applied to the parts after current flow ceases.

Off time--the duration during which electrodes are out of contact from the parts. This term is generally employed where repetitive weld cycles are used in making multiple welds on a single pair of parts.

Referring now to FIG. 2, the control panel 102 allows data to be input to the control system and provides readout of process variables as well. A power lamp 114 provides a visual indication when the control system is energized and ready for use. A valve lamp 116 operably associated with the solenoid actuated valve 110, is illuminated when the valve 110 is actuated and remains illuminated for the entire duration of the squeeze, weld and hold times. A weld lamp 118 operatively associated with a later-discussed welding transformer provides an indication of whether the transformer is energized, and the brilliance of this lamp is controlled by the percent current selection switch 130.

A weld/no weld switch 120 is of a toggle variety and provides a means of initiating a control cycle without delivering current to the weld transformer. When switch 120 is set at weld, current is delivered to the welding transformer when the control sequence is initiated. A squeeze time switch 122 is of a push button variety and is self-illuminating when actuated to indicate that data may be entered into the control memory relating to squeeze timing. A weld 1 time switch 124 is also of a self-illuminating, push button type and enables data to be entered into the control memory relating to weld time. The push button, self-illuminating hold time switch 126 allows data to be entered into the control memory relating to hold time. Off time switch 128, is similar to switches 122-126, and allows data to be entered into the control memory relating to off time.

A percent current switch 130 is of a push button, self-illuminating type and permits data to be entered into the control memory relating to the welding phase shift or percent current. A power factor calibrate switch 132 is similar in construction to switch 130 and permits data to be entered into memory relating to a power factor expressed in a percentage.

A two-digit LED type display output 134 provides readout from memory of both numerical data and designations to indicate both operator commands and control conditions during the course of the control sequence. Data may be input into the control system, in accordance with the sequential actuation of switches 122-132 by means of a thumbwheel-type, double decade switch 136. A single/repeat switch 138 is of a two position type and is provided to allow selection of either a single or repetitive mode of operation. With switch 138 in the single position, a later-discussed timer times through its entire cycle after initiation thereof. At the end of the hold time, the timer deactivates the welder whether or not the manually actuatable pilot switch 112 has been released. In the repeat position of switch 138, the timer sequences through all positions and automatically initiates the next sequence; off time allows time for the parts of the workpiece to be realigned before the next sequence begins. The control system continues to sequence the operations automatically as long as the pilot switch 112 remains actuated.

A two position type run program switch 140 allows the selection of either a programming or running mode of operation. With switch 140 in the run position, normal control initiation and sequencing may be initiated. However, memory data may not be entered or altered in the memory while the switch 140 is in the run mode. With the switch 140 in the program position, the control sequence is inhibited and memory data may be modified.

A momentary enter switch 142 is provided to allow entry of data into the control memory; once the thumbwheel switch 136 has been set to the desired position, actuation of the enter switch 142 causes the data set on thumbwheel switch 136 to be entered into the control memory.

In addition to the basic operating functions discussed above, a plurality of optional control modules 144 may also be provided to provide additional operations. For example, a dual weld, dual current module 144A may be provided when a second weld time or weld current is required in a given application. A preheat control module 144B may be employed when an auxiliary weld current is required prior to the main welding current.

An up/down slope control module 144C may be employed when a sloped current effect is required when making a weld; this module provides an auxiliary time immediately before the weld and a second auxiliary time immediately after the weld. A pulsation module 144D may be employed to supply multiple impulses of weld current for a predetermined count, with the cooling period between each current pulse.

A quench and temper control module 144E is provided to provide a post weld auxiliary current for tempering purposes, which is preceded by a cooling period for quenching purposes. Finally, a seam weld module 144F may be employed in welding applications where a continuous or intermittent seam weld is required; this module supplies an auxiliary heating time which may be intermittently or continuously employed.

The control modules 144A-144F are each provided with a self-illuminating type push button switch 146A-146F, as well as switches 148A-148C and 150A-150D to allow data input to the control memory in a manner similar to that described with reference to switches 122-132. A series of two position switches 152A-152F are respectively provided to energize the corresponding modules.

As will become apparent hereinafter, the control system of the present invention readily lends itself adaptable for use with numerous types of control modules employed in various specialized welding applications.

Referring now also to FIGS. 3A and 3B, the control system includes an input section generally indicated within the broken line 290 which comprises a plurality of input switches 112, 154, 156A, 158, 160 and 162. As previously discussed, pilot switch 112 is manually actuated by the user. An emergency stop switch 154 is further provided either on the control panel 102, or elsewhere, to allow emergency deactivation of the welder. Switch 156A comprises relay contacts of a later-discussed relay 156. Second stage weld switches 158 and 162 may also be optionally provided in those applications where the welding sequence is of a two-stage variety. Switches 112, 154, 156A, 158, 160 and 162 are connected through corresponding inverters 174-182 to the inputs of a D-type latch 186. The output of inverter 174 is also coupled via line 198 to the disable input of a later-discussed output latch 190.

The parallel outputs of latch 186 are coupled via an 8-bit data bus 188 to the inputs of output latch 190, option latch 192, display latch 194, thumbwheel latch 196 and to the data input of a microprocessor 212. The parallel data output lines of latches 192-196 are coupled via an 8-bit data bus to a pair of later-discussed main sequence connectors 264 and option connectors 266.

Central processor 212 is of the conventional type and may include a read-only memory in which programmed instructions are stored for controlling the processor, however, in connection with the present invention, it has been found desirable to provide an independent read-only memory (ROM) 224 in which such programmed instructions are stored. The source code listing for one program suitable for controlling processor 212 has been deposited with the U.S. Patent and Trademark Office for public inspection, and is therefore incorporated by reference herein. The central processor 212 is connected to a RAM (random access memory) 222 and ROM 224 via data bus 220 over which data is transmitted and received. Data may be read from RAM 222 or written therein in accordance with the state of write line 226 and read line 228 which are controlled by processor 212. Data may be read from ROM 224 by control signals output from processor 212 on read line 228. ROM 224 is enabled by processor 212 via signals delivered on line 230. Control signals are delivered to and received from the central processor 212 on input/output data buses 188 and 218. Data bus 218 interconnects ROM 224, processor 212 and a later-discussed program decoder circuit 234. Processor 212 is adapted to provide output enabling signals on lines 204, 206, 208 and 210 to the correponding latches 196, 194, 192 and 190.

A power failure detection circuit 172 is operably coupled with a source of power, such as transformer 164, and is operative to deliver output signals on line 202 and 244 to processor 212 and input latch 186, respectively, when the primary supply of power falls below a prescribed level. Power derived from the transformer 164 is rectified by bridge 166 and delivered to a combined power supply regulator and battery 170. Power supply 170 provides a plurality of differing output voltages for use by various portions of the circuit. The battery associated with power supply 170 is operably coupled via line 214 to RAM 222 and is operable to maintain voltage on RAM 222 so as to prevent memory loss during a power failure. A synchronizing circuit 168 operably coupled with the secondary of transformer 164 is coupled via line 216 to the processor and is operative to synchronize the processor 212 to the incoming line of frequency.

One output of latch 190 is delivered via line 242 to a pair of relays 248 and 250 which are also controlled by pilot switch 112. Another output of latch 190 on line 240 operates a solenoid valve control circuit 236. The output of control circuit 236 is coupled in series with the main valve solenoid 110, a voltage source, and relay contacts 156C which are controlled by a relay coil 156.

Three additional outputs of the latch 190, one of same being indicated at 238, are connected to the inputs of a weld fire pulse circuit 246. Weld fire pulse circuit 246 is also controlled by processor 212 via line 232. The output of weld fire pulse circuit 246 on line 272 controls firing of the welding transformer, the primary of the welding transformer being indicated at 260 and the secondary thereof being indicated at 262. More particularly, the output of weld fire pulse circuit 246 actuates an AC switch firing circuit 252 which may be selectively disabled by the previously-mentioned weld/no weld switch 120. The output of firing circuit 252 is delivered through relay contact 254 to an AC switch 256. Switch 256, in turn, is connected in series with a pair of firing capacitors generally indicated at 258, the primary side of transformer 260 and a suitable source of voltage, typically 230 volts.

The main sequence and option connectors 264 and 266, respectively, provide interconnection to the various lamps and switches on the control panel 102.

As shown in FIG. 4, the power supply 170 includes a full wave diode rectifier 166, filter capacitor 334, a pair of voltage regulators 336 and 338 and a battery backup circuit. The battery backup circuit allows the retention of logic memory in RAM 222 concerning timing and current information during those periods when the main control power is shut off. During normal control operation, the battery 340 is charged by voltage regulator 338. Regulator 338 also serves to supply some areas of the control circuit via line 214. Diode 342 blocks the battery voltage from attempting to power up any nonmemory aspect of the logic assembly when the main control power is off. Voltage regulator 336 also supplies voltage to the various portions of the control logic via lines 346.

Attention is now directed to FIGS. 5A-5E which show the schematic details of the control system depicted generally in FIGS. 3A and 3B. As previously noted, the synchronizing circuit 168 is operative to synchronize processor 212 to the incoming line frequency and also allows processor 212 to determine whether the incoming frequency is 50 or 60 hertz. Resistor 304, diodes 306 and 308, and capacitor 310 are operative to delay the incoming signal from the power supply; this incoming signal is shaped into a phase-shifted square wave by NAND gates 312 and 324. A diode bridge 302 along with transistor 314 and associated circuitry produces a positive pulse at the collector of transistor 314 whenever the absolute values of the voltage at the junction of resistor 316 and bridge 302 are less than the prescribed value; this creates a positive pulse occurring every one-half cycle and at the zero crossing point of the incoming line frequency. The combination of these two signals, i.e., the phase-shifted square wave and the half-cycle sync pulses, respectively cause a negative train of cycle sync pulses at the output of NAND gate 324 which bracket the positive and negative transitions of the line voltage. Resistor 318 and capacitor 320 are employed to filter the signals before delivery thereof to inverter 322. The outputs of inverter 322 are positive cycle sync pulses which are processed by NAND gate 326 and inverter 328 before delivery thereof to processor 212.

Each of the input signals to the input operation circuit 290 derived from switches 112, 154, 156A, 160 and 162 are delivered to logic portions of the control circuit via a series of corresponding electro-optical isolation switches 292-300. Switches 292-300 isolate the logic circuit from spurious signals from the input switches to prevent error or malfunction. The output signals from switches 292-300 are inverted by inverters 174-182 and delivered to the input latch 186. Latch 186 may be of the D-type and delivers the switch data to processor 212 on data bus 188 only upon receipt thereat of an enable signal on line 210. Data signals stored in latch 186 are scanned at predetermined intervals by processor 212 in order to determine the change in input status. The data output from latch 186 on bus 188 is employed by the processor 212 to perform all further logic functions. A manually operable two position switch 184 is also coupled with one input of latch 186; in the on position, switch 184 delivers a signal to latch 186 which is interpreted by processor 212 as a command to delay the firing signal 87.degree..

The processor 212 controls all logic functions and contains two independent 8-bit ports 189 and 191 which are operative to send and receive information to control specific operations throughout the circuit. As previously mentioned, processor 212 includes its own read-only memory, however, because of the size of the program chosen in connection with the preferred embodiment, it is necessary to employ an independent ROM 224. All program operation data is stored in ROM 224 and is selectively called up by processor 212 as required. RAM 222 is operative to store all data input via control panel 102. As previously indicated, in the event that the supply voltage drops below a prescribed level, backup power is provided via battery 340.

Processor 212 sends and receives information to RAM 222 and ROM 224 in accordance with a specific routine. Processor 212 first sends out a signal on data bus line 220 which is used to address logic memory. This address word is predetermined by the processor 212 and continuously changes throughout the control operation. The address word is locked into an address latch 225 upon receipt thereto of an address latch enable signal output by the microprocessor on line 227. Once latched, processor 212 clears its data bus 220 in preparation for the next operation.

The address signal latched in address latch 225 is also present on the address lines of RAM 222 and ROM 224. Processor 212 then selectively enables one of three lines. Specifically, an enabling signal delivered on line 229 results in the output of an 8-bit data word from an address location in ROM 224. An enabling signal output on line 228 results in the output of stored information from RAM 222 derived from control panel 102. When data is entered via control panel 102, the write input is enabled, thereby allowing information to be entered into RAM 222 for future use.

Latches 186, 190, 190A (which forms an integral part of latch 190) and 192-196 may each be of the D-type and are selectively enabled by processor 212 to receive information pertaining to external switch signals and to control output devices. As previously mentioned, latch 186 is operative to receive signals from the input operation circuit 290. Similarly, latches 192-196 have their data inputs connected via connectors 268 to the corresponding data outputs of the option modules 144A-144F, display 134 and thumbwheel switch 136.

The power fail and processor reset circuit 172 functions to alert processor 212 that a nonstandard operating condition exists and that appropriate action must be taken. The power fail detect portion of the circuit comprises a diode bridge 348, diode 350, capacitor 352, resistors 354, 356 and 358, and transistor 360. In the event that the incoming line voltage derived from transformer 164 drops below a prescribed level, transistor 360 turns on thereby sending an inhibit signal through inverter 364 to the reset line 202 of processor 212. This inhibit signal is also delivered via line 244 to latch 186 and is employed by processor 212 to cause display 134 to indicate the designation of ES, as will be later discussed. Inhibit signal on line 244 inhibits all outputs of latch 186. In order to protect the sensitive RAM 222 during a power failure or reset condition, transistor 362 turns off thereby delivering an inhibit signal to RAM 222 via line 201; this protects RAM 222 from accidental loss of stored information.

As previously mentioned, data output from processor 212 on bus 218 is delivered to control panel 102 via a programmed decoder circuit 234. Decoder circuit 234 is operative to decode data output by processor 212 and selects an appropriate sequence position with respect to the various connections between the 16-bit data bus line 270 and the data bus connecting control panel 102 with connectors 264 and 266. Decoder circuit 234 comprises a pair of three to eight line decoders each having the inputs connected to lines 218 and eight output lines connected to corresponding switches or lamps on the control panel 102. As employed in the drawings, like interconnections between the output of decoder circuit 234 and the devices associated with control panel 102 are designated by the identical single or double letters. The coded multi-bit address output by process 212 on lines 218 is converted by one of the decoders 271 or 273 to allow only one output position at any given time. Along with the function of enabling the devices associated with control panel 102, decoders 271 and 273 are operable to enable latch circuits which obtain information from the thumbwheel switch 136 and display information on display 134.

The fire pulse circuit 246 functions to generate the necessary weld fire pulses required to control firing of the welder. This circuit generates groups of fire pulses according to information derived from the processor 212. The address latch enable signal output from processor 212 on line 232 is delivered as a train of continuous pulses to the input of flip-flop 368. Flip-flops 368, 374, 382 and 386; AND gates 372, 380 and 384; and, divider 370 function to continually divide down the input signal so as to produce a weld fire pulse signal. Divider 376 whose output is coupled with the clear input of flip-flop 374 is also employed in the division process and functions to provide a specific divide rate depending upon the operating frequency of the system. Once processor 212 determines the operating frequency of the system, it delivers a signal on line 377 which causes divider 376 to select a particular divide rate. In the event that the operating frequency of the system is changed, the processor 212 in combination with divider 376 automatically make the necessary changes to correct the change in frequency. As previously mentioned, a main fire pulse enable signal is output from latch 190 on line 238 and is delivered to one input of AND gate 390, the other input thereto being formed from the output of AND gate 388. Thus, high signals present at both inputs of AND gate 390 result in a prefire pulse being delivered via line 391 to the input of a driver circuit 392 which comprises a Darlington transistor array employed to amplify the fire pulse signal. The amplified firing pulse is output from driver 392 on line 272. The output driver 392 is a standard device and is known in the industry as a ULN 2003. An additional output from output driver 392 on line 275 is employed to latch in a safety relay associated with the main valve solenoid 110.

The solenoid valve control circuit 236 receives an enabling signal from processor 212 on line 240 from latch 190 during the control signal. The signal on line 240 is inverted by inverter 241 and then actuates a fire pulse oscillator comprising NAND gates 396 and 398, resistors 400, 402 and 404, diode 406 and capacitor 408. The output of the oscillator is shaped and amplified by transistors 410 and delivered to the primary side of a pulse transformer 412. The secondary side of transformer 412 is connected to a bi-directional thyristor switch (triac) which is successively gated by the pulses passed from the primary to the secondary of transformer 412. A resistor 416 and capacitor 418 are provided as passive noise filters for protecting switch 414. The output of the solenoid valve control circuit 236 is delivered via line 286 and 288 to the main valve solenoid 110.

Reference is now made to FIGS. 7 and 8 which show the schematic interconnection between the primary side 260 of the welding transformer and the various portions of the circuit shown in FIGS. 5A-5E. As previously indicated, the solenoid valve control circuit 236 is operably coupled via lines 286 and 288 in series with the solenoid valve 110, thyristor switch 414 and relay contact 156C. Lines 286 and 288 are further coupled in series with the secondary of a power supply transformer 415. Upon closure of pilot switch 112, relay coil 248 (FIG. 3) is energized, thereby closing contact 156C and enabling the thyristor switch 414 to deliver enabling signals to valve 110. Valve light 116 is coupled in parallel with valve 110 and therefore is illuminated upon actuation of the valve 110.

Assuming now that the weld/no weld switch 120 is in the weld position, and that the pilot switch 112 has been actuated, fire pulse signals are delivered via line 272 to the AC switch firing circuit 272 which outputs firing signals on lines 274-280. The signal on line 276 is delivered through relay contact 250B (which has been closed by relay coil 250) and thence to one input of ignitron 282, a second input to ignitron 282 being formed by line 274. Similarly, lines 278 and 280 are connected in series with ignitron 284 through relay contact 250A. Ignitrons 282 and 284 are coupled in series with the primary 260 of the welding transformer and the primary of power supply transformer 415.

Turning now to FIGS. 6A and 6B, switches 122-132 are standard push button switches having LED type lamps coupled in circuit therewith. Switches 122-132 are interconnected with the programmed decoder 234 in accordance with the interconnection key shown in the drawings. Display 134 is of a two-digit LED type in which each digit is represented by seven LED segments. Display 134 is coupled via decoder/driver to the programmed decoder circuit 234 via interconnection 270. The thumbwheel switch 136 is coupled through a pair of hex inverters 137 to the display 134 and each of the switches 122-132. Switches 138-142 are of the single pole, double throw type and are likewise interconnected with the programmed decoder circuit 234 via interconnection 270.

Referring now to FIG. 9, which depicts a generalized schematic circuit for each of the optional modules 144A-144F, each module is basically provided with four switches, namely switches 146, 148, 150 and 152. Switches 146-152 are connected through a series of inverters 500 to a series of interconnections 502 adapted to be interconnected with the corresponding connection at 270 of the program decoder circuit 234 (FIGS. 5A-5E). A plurality of jumpers 504 are provided to allow switches 146-152 to be selectively coupled in circuit in accordance with the particular control module function.

It has been previously pointed out that the display 134 on control panel 102 provides a visual indication of the data being entered into the control system memory. However, display 134 further displays alphabetic codes relating to the operating status of the control system in order to assist and inform the user. The display codes and their corresponding definitions employed in connection with the preferred form of the invention are as follows:

C1--indicates a closure is required at the first stage initiation input.

C2--indicates a closure is required at the second state initiation input.

OC--indicates that the control sequence has terminated and all initiation switches must be open before the control can be reinitiated.

ES--indicates that the emergency stop input to the control is open. A closure of the emergency stop switch is required before the control can be reinitiated.

PS--indicates that the pressure switch input to the control is open. A closure of the pressure switch is required for the sequence to continue.

E1--indicates that the sloped bottom current is a greater value than the weld 1 current.

E2--indicates that the sloped bottom current is a greater value than the weld 2 current.

EE--indicates that data previously stored in the logic memory (i.e., timing or phase shift information) has been lost due to a low battery condition.

To initialize the control system for use, the run/program switch 140 is placed in the run position. At this point, the code of either C1 or OC will appear on the display 134. If OC is displayed, all initiation switches must be open before sequencing can take place. If C1 is displayed, a closure at the first stage (pilot switch 112) is required. Assuming that a single stage initiation is desired, upon closing of pilot switch 112, the code C1 disappears from display 134 and the control sequence is initiated. If, however, dual stage initiation is desired, upon closure of pilot switch 112, the code designation of C2 appears on display 134, the main solenoid valve 110 is energized and a control sequence is then delayed until a second stage closure switch (similar to pilot switch 112) is actuated. Once the second stage switch is actuated, the control sequence begins and the C2 designation on display 134 disappears. At the end of the control sequence, display 134 will provide an indication of OC; when the initiation switches (112, etc.) are open, C1 again appears on display 134, in preparation for another welding cycle. Switching of the single/repeat switch 138 to the repeat position causes the control system to sequentially initiate additional welding cycles automatically. In order to reprogram the control system for a new welding application, the run/program switch 140 is switched to the program position and data is entered into memory corresponding to the squeeze time, weld time, hold time, off time, percent current and power factor; this is accomplished by successively depressing the push button switches 122-132, selecting a particular parameter on thumbwheel switch 136, and successively toggling the enter switch 142. After all data has been input to memory, the run/program switch is toggled to the run position.

From the foregoing, it is apparent that the welding apparatus and control system therefor as described above not only provide for the reliable accomplishment of the objects of the invention but do so in a particularly effective and reliable manner. It is recognized, of course, that those skilled in the art may make various modifications or additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.