GB1561930A - Resistance welding - Google Patents

Description

(54) RESISTANCE WELDING (71) We, THE WELDING INSTITUTE, a British company of Abington Hall, Abington, Cambridge, CBl 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 moni tored voltage are compared with a voltage which is lower by a predetermined propor tion than the first-mentioned peak voltage, and the weld is terminated as soon as the subsequent cyclic peaks fall to the lower voltage level.

However, this method will give a false indication of the drop in resistance if the mains voltage falls in the course of the weld.

Consequently, in Patent No. 1,466,652 we have described a circuit in which the mains voltage is applied to a zero crossing detector, the output of which starts a 5 mifli- second timer; the period of 5 milliseconds is chosen for 50 Hz operation because it is one quarter of a cycle at this frequency. In this way the peak of the supply voltage is located in time and a corresponding voltage is held on a capacitor. This peak voltage is applied as a divisor to a devider circuit receiving as its other input the electrode voltage. The resulting quotient voltage is compensated for mains variations.

A disadvantage of this method is that if the control circuit is to be used on a mains supply with a different frequency, the timer must be changed to detect the peak correctly.

According to the present invention, a con- troi circuit for controlling the duration of a resistance weld comprises a pair of welding electrodes for connection to an AC supply to receive welding current therefrom, a first peak detector circuit connected to receive the supply voltage, or a voltage proportional thereto, and of the kind in which a capacitor is charged to the peak voltage and has a discharge path permitting a flow of current from the charged capacitor in the period up to the arrival of the next of the cyclic peaks of voltage, a divider circuit for dividing a voltage obtained from the welding electrodes by the said peak supply voltage to obtain a compensated electrode voltage, a second peak detector circuit for detecting and hoIding the peak value to which the compensated electrode voltage rises, a voltage proportioning circuit set to derive a voltage at least 2% but not more than 20% below the compensated electrode peak voltage, a comparator for comparing the compensated electrode voltage with the said derived voltage, and means for terminating the weld current through the welding electrodes whenever the subsequent lower cyclic peaks of the compensated electrode voltage fall to the level of the said derived voltage.

With a first peak detection circuit of the kind described, the control circuit is not confined to a single mains frequency. 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 the supply circuit for the welding electrodes; Figure 2 is the control circuit which is connected to the welding electrodes; and Figure 3 shows the peak detector circuit used for the mains compensation.

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 ignition 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 main votlage 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 mains-compensated 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 missingpulse- detetcor 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 voltae - dron selected at the potentiometer 32, the divider- output no longer rises above the output of potentiometer 32 atnd 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 ignition 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 rest 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 output of the third stage operates a switching circuit connetced to the peak detector to prevent the detector from being effectiye 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 connected 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.

In Figure 3 there is shown a more detailed circuit diagram of the peak detector in the mains compensation circuit. The output of the lowpass filter 32 is applied through resistor R20 to the noninverting input of an integrated circuit amplifier IC9, the output of which is connected back through a diode D9 to its inverting input. The amplifier is of the 741 type. The signal which has passed through diode D9 charges a capacitor C10 of 0.15 microfarads in parallel with a resistor R21 of 820 kilohms. The capacitor voltage is applied through resistor R22 to the noninverting input of a further integrating circuit amplifier IC10, provided to boost the output of the IC9. The output of circuit ICIO is connected through a diode D10 to a junction 76 from which a feedback loop extends to its inverting input; the output of circuit IC10 also acts a divisor in the divisor circuit 28, as described above. The circuit 28 may be of the standard 4291J type.

WHAT WE CLAIM IS:- 1. A control circuit for controlling the duration of a resistance weld, comprising a pair of welding electrodes for connection to an AC supply to receive welding current therefrom, a first peak detector circuit connected to receive the supply voltage, or a voltage proportional thereto, and of the kind in which the capacitor is charged to the peak voltage and has a discharge path permitting a flow of current from the charged capacitor in the period up to the arrival of the next of the cycle peaks of voltage, a first divider circuit for dividing a votlage obtained from the welding electrodes by the said peak supply voltage to obtain a compensated electrode voltage, a second peak detector circuit for detecting and holding the peak value value to to which the compensated electrode voltage rises, a voltage proportioning circuit set to derive a voltage at least 2% but not more than 20% below the compensated electrode peak voltage, a comparator for comparing the compensated electrode voltage with the said derived voltage, and means for terminating the weld current through the welding electrodes whenever the subsequent lower cyclic peaks of the compensated electrode voltage fall to the level of the said derived voltage.

2. A control circuit according to claim 1 and further comprising a ramp generator connected to the output of the comparator the ramp generator being reset to zero in response ot an output from the comparator whenever the subsequent lower peaks of the compensated electrode voltage exceed the level of the said derived voltage.

3. A control circuit according to Claim 1 or 2 further comprising an active low pass filter connected between the welding electrodes and the second peak detector circuit.

4. A control circuit for controlling the duration of a resistance weld, substantially as herein described with reference to the accompanying drawings.

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. also supplies a signal to an amplifier 72 connected 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. In Figure 3 there is shown a more detailed circuit diagram of the peak detector in the mains compensation circuit. The output of the lowpass filter 32 is applied through resistor R20 to the noninverting input of an integrated circuit amplifier IC9, the output of which is connected back through a diode D9 to its inverting input. The amplifier is of the 741 type. The signal which has passed through diode D9 charges a capacitor C10 of 0.15 microfarads in parallel with a resistor R21 of 820 kilohms. The capacitor voltage is applied through resistor R22 to the noninverting input of a further integrating circuit amplifier IC10, provided to boost the output of the IC9. The output of circuit ICIO is connected through a diode D10 to a junction 76 from which a feedback loop extends to its inverting input; the output of circuit IC10 also acts a divisor in the divisor circuit 28, as described above. The circuit 28 may be of the standard 4291J type. WHAT WE CLAIM IS:-

1. A control circuit for controlling the duration of a resistance weld, comprising a pair of welding electrodes for connection to an AC supply to receive welding current therefrom, a first peak detector circuit connected to receive the supply voltage, or a voltage proportional thereto, and of the kind in which the capacitor is charged to the peak voltage and has a discharge path permitting a flow of current from the charged capacitor in the period up to the arrival of the next of the cycle peaks of voltage, a first divider circuit for dividing a votlage obtained from the welding electrodes by the said peak supply voltage to obtain a compensated electrode voltage, a second peak detector circuit for detecting and holding the peak value value to to which the compensated electrode voltage rises, a voltage proportioning circuit set to derive a voltage at least 2% but not more than 20% below the compensated electrode peak voltage, a comparator for comparing the compensated electrode voltage with the said derived voltage, and means for terminating the weld current through the welding electrodes whenever the subsequent lower cyclic peaks of the compensated electrode voltage fall to the level of the said derived voltage.

2. A control circuit according to claim 1 and further comprising a ramp generator connected to the output of the comparator the ramp generator being reset to zero in response ot an output from the comparator whenever the subsequent lower peaks of the compensated electrode voltage exceed the level of the said derived voltage.

3. A control circuit according to Claim 1 or 2 further comprising an active low pass filter connected between the welding electrodes and the second peak detector circuit.

4. A control circuit for controlling the duration of a resistance weld, substantially as herein described with reference to the accompanying drawings.