GB1564562A - Resistance welding - Google Patents

Description

(54) RESISTANCE WELDING (71) We, THE WELDING INSTI TUTE, a British company of Abington Hall, Abington, Cambridge, CB1 6AL do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: In the control of resistance welding processes, one of the methods which has been proposed for determining when to terminate the welding current passing between the welding electrodes depends on the electrical resistance between the electrodes. This resistance falls during a weld, and one method of controlling the quality of the weld has been to monitor the resistance between the electrodes and then to terminate the welding current when this resistance has fallen by a predetermined amount.

In our British No. 1,446,652, we have described and shown a control unit for carrying out such a method in which the peak to which the voltage across the welding electrodes rises is detected and held, the subsequent lower cyclic peaks of the monitored voltage are compared with a voltage which is lower by a predetermined proportion than the first-mentioned peak voltage, and the weld is terminated as soon as the subsequent cyclic peaks fall to the lower voltage level.

Welding machines to which such a control unit may be connected have an internal timer which operates after a predetermined interval. Thus, when the control unit is connected to such a welding machine, the internal timer operates to terminate the weld if the resistance-responsive control unit has not previously terminated it within the predetermined interval as a consequence of a reduction in resistance and therefore a fall in voltage between the electrodes, the predetermined interval being the maximum time expected for a weld of acceptable quality. If the weld is terminated by the machine's internal timer at the end of such an interval, it is desirable to indicate that it is a suspect weld. This means that the suspect weld indication system must be able to distinguish between welds terminated by the resistanceresponsive unit and welds terminated by the internal machine timer; this is made more difficult by the fact that when welding current is cut off by the internal machine timer the voltage between the electrodes falls, i.e.

it brings about the condition for operation of the resistance-responsive unit. To ensure that the suspect weld indicator operates only when the welding current has been cut off by the operation of the internal machine timer in Patent No. 1,446,652 we have shown in the control unit an auxiliary timer, set to operate after the same interval as the internal machine timer, the auxiliary timer energising a warning light at the end of the predetermined interval.

The present invention enables the auxiliary timer to be dispensed with.

A weld-termination control unit according to the present invention, for use with a resistance welding machine including switching means controlling the application of a succession of cycles of alternating current to welding electrodes and having an internal timer which, in the absence of a prior termination of the weld by the control unit, operates the switching means to terminate the weld current after a predetermined number of cycles, comprises: means responsive to the voltage between the electrodes at a first time point in relation to each cycle of the welding current waveform to detect a predetermined fall in the resistance of the material between the electrodes ar,d, upon such detection, to operate the swi-ching means to stop the application of we d current to the electrodes; a suspect weld indicator; and means responsive to the supply current waveform between the switching means and the electrodes at a second time point in relation to each cycle of the waveform, earlier in relation to the cycle than the said first time point, to detect termination of that supply waveform and to actuate the suspect weld indicator if the resistance-responsive means has not yet operated, whereby the suspect weld indicator is operated only when termination of the weld is brought about by the internal machine timer.

The elimination of the auxiliary timer from the control unit simplifies the setting up procedure when changing from one welding job to another. With the arrangement described in British Patent No. 1,446,652 both the welding machine internal timer and the auxiliary timer in the control unit had to be adjusted with virtually every change of job, because weld time has to be adjusted with change of material, thickness and configuration of the components to be welded. In a system embodying the present invention, only the machine timer has to be altered. A further disadvantage of the earlier arrangement was that its reliability depended on the accuracy of the two weld timers being adjusted. If the welding machine timer was set at a shorter time than the auxiliary timer (either inadvertently or through malfunction) the suspect warning light would never be activated even at undersized or no weld conditions. The elimination of the auxiliary timer from the control unit therefore increases the reliability of the system.

In the preferred form, the means responsive to the resistance between the electrodes includes a ramp generator which is reset once in each cycle of the electrode waveform if the peak of that waveform exceeds a reference level, together with a comparator operative when the ramp output exceeds a further reference level, i.e. when the ramp is not reset, to terminate the weld; and the means responsive to the supply waveform between the switching means and the electrodes comprises a further ramp generator and a circuit for resetting this further ramp generator twice in each cycle while the supply waveform continues, together with a comparator responsive to the ramp output exceeding a reference level to operate the suspect weld indicator. Thus, because the first ramp generator is reset every mains cycle and the second ramp generator is reset every mains half cycle the second comparator will operate the suspect weld indicator when the supply waveform is terminated by the machine weld timer before the output of the first ramp generator can reach the reference level at which it would provide a belated weld-termination signal.

In order that the invention may be better understood, an example of apparatus embodying the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically the support circuit of a welding machine 10 to which the control unit is connected; and Figure 2 is a circuit diagram of the control unit.

In Figure 1, the supply voltage is applied to a thyristor or ignitron circuit 12 in series with the primary winding of a transformer 14.

Firing circuits 16 and 18 control the operation of the thyristor or ignitron circuit. The secondary winding of the welding transformer 14 is a single turn connected through copper bars to electrodes 19.

An internal machine timer 24 prevents firing of the thyristor or ignitron control if the resistance-responsive control unit has not terminated the weld within a predetermined number of cycles of the welding current.

Figure 2 is a block diagram of an auxiliary control unit embodying the present invention. The voltage across the electrodes (Figure 1) is taken through a low-pass filter 26 to a mains compensator in the form of a divider circuit 28. The object of the mains compensator is to ensure that the subsequent circuit for measuring the fall in electrode voltage is not misled by a variation in the mains voltage. The compensation is achieved as follows.

The mains voltage is applied through a transformer 20 to a low-pass filter 22 having the same characteristics as the low-pass filter 26. The filtered output is taken to a peak detector 24 which is provided with a discharge path permitting the output voltage to droop between successive cycles. The peak voltage is fed to the divider circuit 28 where it acts as a divisor for the electrode voltage.

The output voltage is the true quotient at the peak of an input cycle. The mainscompensated peak to which the electrode voltage builds at the commencement of a weld is detected by the peak detector 30 whose output is applied across the potentiometer 32. The wiper is set to tap off a predetermined proportion of this peak voltage and to apply it to the comparator 34.

The comparator 34 also receives the actual output of the divider circuit 28 during the course of the weld. At the beginning of the weld, once in each cycle the divider output will rise above the tapped voltage from the potentiometer 32 and the comparator 34 will provide a corresponding pulse. This pulse is applied to a missing-pulse detector 36 to reset a ramp generator 38. The ramp voltage is fed to a comparator 40 in which it is compared with a reference voltage from potentiometer 42, the reference level being above the level reached by the ramp when it is reset at every cycle.

When the resistance of the weld decreases by a predetermined amount, corresponding to the voltage drop selected at the potentiometer 32, the divider output no longer rises above the output of potentiometer 32 and the comparator 34 fails to supply the resetting pulse for the ramp generator. When this happens the ramp output continues to rise until it exceeds the reference level of the comparator 40. The comparator 40 then acts through a circuit 44 and an amplifier 46 to operate a relay 48. The relay 48 opens contacts 50 and 52 (Figure 1) in the thyristor or ignitron firing control circuits and thereby terminates the weld. The circuit 44 is a timer which ensures that the welder cannot operate again for a predetermined number of cycles, for example 30 cycles.

The output of lowpass filter 22 is also applied to a Schmitt trigger circuit 54 of a kind operating on both the positive and the negative half of the incoming waveform and therefore supplying pulses at twice the mains frequency. These pulses are applied as resetting pulses to a ramp generator 56 of a further missing pulse detector 58; the ramp output goes to a comparator 60, the combination acting in the same manner as the ramp generator and comparator 38 and 40 respectively. However, while the ramp generator 38 was reset every 20 milliseconds (on a 50 Hz supply), the ramp generator 56 is reset every 10 milliseconds.

Thus when the weld is terminated by the internal machine timer 24 (Figure 1) there is in effect a "race" condition between the two missing-pulse detectors and the output of ramp generator 56 reaches the reference level of its comparator before the ramp generator 38 attains its corresponding reference level, owing to the fact that the lack of a reset pulse affects the ramp generator 56 before the ramp generator 38 is affected in this way. Consequently before the missingpulse detector 36 has operated, the missingpulse detector 58 actuates a timer circuit 62 (having the same function as the timer circuit 44) and simultaneously gives an audible warning through bleeper 64 and a visible warning through light source 66. The timers 44 and 62 are in fact two halves of a single circuit and are cross-coupled so that they can never be actuated together. Consequently when the timer 44 has been operated by the resistance-responsive unit, the suspect weld indicator cannot be actuated through timer 62 and vice versa.

The output of low-pass filter circuit 26 is applied through a Schmitt trigger circuit 68 to a four-stage shift register 70. The Q output of the third stage operates a switching circuit connected to the peak detector to prevent the detector from being effective in the first two cycles of operation of the unit, thereby avoiding false peaks due to transient conditions. The Q output of the fourth stage enables the timer circuits 44 and 62. When the missing-pulse detector 58 detects the termination of the welding supply, a reset signal is applied to the shift register 70.

The second stage of the shift register 70 also supplies a signal to an amplifier 72 con nected to a light source 74. If after the second cycle there is no light from the source 74, it indicates that the electrodes are connected in the wrong phase. The four stage shift register comprises three lines of information: data, clock and reset. Data is supplied from the Schmitt trigger circuit 54 and the clock is supplied from Schmitt trigger 68. Thus the shift register will only function when the data is high during the leading edge of the clock pulse, and the reset enabled. These conditions ensure the correct operation of the shift register.

It will be seen that when a control unit as described with reference to Figure 2 is connected to a welding machine having an internal timer, the operation of switching off the machine if the resistance responsive control unit has not reacted within the normal period is left to the internal timer in the welding machine; the missing-pulse detector 58 operates to detect when the machine has been switched off by the internal timer and causes the suspect weld indication.

WHAT WE CLAIM IS: 1. A weld-termination control unit, for use with a resistance welding machine which includes switching means controlling the application of a succession of cycles of alternating current to welding electrodes and having an internal timer which, in the absence of a prior termination of the weld by the control unit, operates the switching means to terminate the weld current after a predetermined number of cycles, the control unit comprising: means responsive to the voltage between the electrodes at a first time point in relation to each cycle of the welding current waveform to detect a predetermined fall in the resistance of the material between the electrodes and, upon such detection, to operate the switching means to stop the application of weld current to the electrodes; a suspect weld indicator; and means responsive to the supply current waveform between the switching means and the electrodes at a second time point in relation to each cycle of the waveform, earlier in relation to the cycle than the said first time point, to detect termination of that supply waveform and to actuate the suspect weld indicator if the resistance-responsive means has not yet operated, whereby the suspect weld indicator is operated only when termination of the weld is brought about by the machine timer.

2. A control unit in accordance with claim 1, in which the means responsive to the voltage between the electrodes includes a ramp generator and means operative to reset the ramp generator once in each cycle of the electrode waveform if the peak of that waveform exceeds a reference level, together

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. and the comparator 34 fails to supply the resetting pulse for the ramp generator. When this happens the ramp output continues to rise until it exceeds the reference level of the comparator 40. The comparator 40 then acts through a circuit 44 and an amplifier 46 to operate a relay 48. The relay 48 opens contacts 50 and 52 (Figure 1) in the thyristor or ignitron firing control circuits and thereby terminates the weld. The circuit 44 is a timer which ensures that the welder cannot operate again for a predetermined number of cycles, for example 30 cycles. The output of lowpass filter 22 is also applied to a Schmitt trigger circuit 54 of a kind operating on both the positive and the negative half of the incoming waveform and therefore supplying pulses at twice the mains frequency. These pulses are applied as resetting pulses to a ramp generator 56 of a further missing pulse detector 58; the ramp output goes to a comparator 60, the combination acting in the same manner as the ramp generator and comparator 38 and 40 respectively. However, while the ramp generator 38 was reset every 20 milliseconds (on a 50 Hz supply), the ramp generator 56 is reset every 10 milliseconds. Thus when the weld is terminated by the internal machine timer 24 (Figure 1) there is in effect a "race" condition between the two missing-pulse detectors and the output of ramp generator 56 reaches the reference level of its comparator before the ramp generator 38 attains its corresponding reference level, owing to the fact that the lack of a reset pulse affects the ramp generator 56 before the ramp generator 38 is affected in this way. Consequently before the missingpulse detector 36 has operated, the missingpulse detector 58 actuates a timer circuit 62 (having the same function as the timer circuit 44) and simultaneously gives an audible warning through bleeper 64 and a visible warning through light source 66. The timers 44 and 62 are in fact two halves of a single circuit and are cross-coupled so that they can never be actuated together. Consequently when the timer 44 has been operated by the resistance-responsive unit, the suspect weld indicator cannot be actuated through timer 62 and vice versa. The output of low-pass filter circuit 26 is applied through a Schmitt trigger circuit 68 to a four-stage shift register 70. The Q output of the third stage operates a switching circuit connected to the peak detector to prevent the detector from being effective in the first two cycles of operation of the unit, thereby avoiding false peaks due to transient conditions. The Q output of the fourth stage enables the timer circuits 44 and 62. When the missing-pulse detector 58 detects the termination of the welding supply, a reset signal is applied to the shift register 70. The second stage of the shift register 70 also supplies a signal to an amplifier 72 con nected to a light source 74. If after the second cycle there is no light from the source 74, it indicates that the electrodes are connected in the wrong phase. The four stage shift register comprises three lines of information: data, clock and reset. Data is supplied from the Schmitt trigger circuit 54 and the clock is supplied from Schmitt trigger 68. Thus the shift register will only function when the data is high during the leading edge of the clock pulse, and the reset enabled. These conditions ensure the correct operation of the shift register. It will be seen that when a control unit as described with reference to Figure 2 is connected to a welding machine having an internal timer, the operation of switching off the machine if the resistance responsive control unit has not reacted within the normal period is left to the internal timer in the welding machine; the missing-pulse detector 58 operates to detect when the machine has been switched off by the internal timer and causes the suspect weld indication. WHAT WE CLAIM IS:

1. A weld-termination control unit, for use with a resistance welding machine which includes switching means controlling the application of a succession of cycles of alternating current to welding electrodes and having an internal timer which, in the absence of a prior termination of the weld by the control unit, operates the switching means to terminate the weld current after a predetermined number of cycles, the control unit comprising: means responsive to the voltage between the electrodes at a first time point in relation to each cycle of the welding current waveform to detect a predetermined fall in the resistance of the material between the electrodes and, upon such detection, to operate the switching means to stop the application of weld current to the electrodes; a suspect weld indicator; and means responsive to the supply current waveform between the switching means and the electrodes at a second time point in relation to each cycle of the waveform, earlier in relation to the cycle than the said first time point, to detect termination of that supply waveform and to actuate the suspect weld indicator if the resistance-responsive means has not yet operated, whereby the suspect weld indicator is operated only when termination of the weld is brought about by the machine timer.

2. A control unit in accordance with claim 1, in which the means responsive to the voltage between the electrodes includes a ramp generator and means operative to reset the ramp generator once in each cycle of the electrode waveform if the peak of that waveform exceeds a reference level, together

with a comparator operative when the ramp output exceeds a further reference level, indicative of the absence of resetting of the ramp generator, to terminate the weld; and in which the means responsive to the supply current waveform between the switching means and the electrodes comprises a further ramp generator and a circuit for resetting this further ramp generator twice in each cycle while the supply waveform continues, together with a comparator responsive to the output of the further ramp generator exceeding a reference level, to operate the suspect weld indicator.

3. A control unit in accordance with claim 4, including first and second crosscoupled circuits respectively connected to the outputs of the means for terminating the weld as a consequence of a fall in resistance between the electrodes and the means for operating the suspect weld indicator, the cross-coupling being such as to prevent the operation of the resistance-responsive means after the suspect weld indicator has been actuated and vice versa.

4. A control unit in accordance with claim 2, further comprising a circuit connector to receive the supply waveform and operative on both the positive and negative halves of the incoming waveform to supply resetting pulses to the said second ramp generator.

5. A control unit in accordance with any one of claims 1 to 4, in which the means responsive to the voltage between the electrodes at the first time point comprises a peak detector operative to detect and hold the peak voltage across the welding electrodes and a comparator operative to compare a predetermined portion of the said peak voltage with subsequent cyclic peaks of the voltage across the welding electrodes, the comparator providing a signal change to terminate the weld when the said subsequent cyclic peaks fall to the level of the said predetermined proportion of the firstmentioned peak voltage.

6. A control unit in accordance with claim 5, comprising electrically connected between the electrodes and the said peak detector, a mains compensation circuit operative to divide the electrode voltage by a voltage representative of the level of the mains voltage.

7. A weld-termination control unit, substantially as herein described with reference to the accompanying drawings.