GB2127564A

GB2127564A - Measuring weld resistance/voltage drop

Info

Publication number: GB2127564A
Application number: GB08226784A
Authority: GB
Inventors: Robert Edwin Arthur Denning
Original assignee: ARO Machinery Co Ltd
Current assignee: ARO Machinery Co Ltd
Priority date: 1982-09-20
Filing date: 1982-09-20
Publication date: 1984-04-11
Also published as: GB2127564B

Abstract

In electric spot welding wherein sensor leads are attached to the main welding current conductors to detect the weld nugget voltage drop for welding control purposes, inductive coupling between the main welding current conductors and the sensor leads is avoided by attaching the sensor leads at positions remote from the welding electrodes. The sensor lead signal waveform (10) is shifted vertically by summing with it a d.c. voltage, so that the waveform becomes asymmetrical (22) about its base or zero voltage level, and the signal required for welding control is derived by detecting the peaks of only the half cycles of the waveform that are thereby reduced in amplitude. <IMAGE>

Description

SPECIFICATION Improvements in or relating to Spot Welding This invention relates to electric spot welding.

In the control of spot welding it is known to monitor the variation in resistance of the weld nugget by means of sensor leads connected to the welding electrodes, or to the welding current conductors close to the electrodes. A problem which arises is that if the main welding current conductors and the sensor leads are led back together along the arm of the welding machine that carries the electrodes inductive coupling takes place which can seriously interfere with the sensor signal. The problem is particularly severe if large components such as doors, aircraft wing panels and so forth are to be welded necessitating a long welding arm with a deep throat to receive the work.

The inductive coupling is reduced if the sensor leads are connected to the main current conductors farther back along the welding arm.

However, this results in a larger voltage across the sensor leads and a corresponding reduction in the ratio to this voltage of the weld nugget voltage drop, making it increasingly difficult to measure the weld nugget voltage drop as the sensor leads are moved back.

The inductive coupling can be avoided if the sensor leads do not run back along the welding arm but are led away from the electrodes in some other direction. But if this is done there is a risk of the work to be welded being fouled by the sensor leads. Indeed, all leads connected to the welding electrodes, or to the welding current conductors close to the electrodes, are vulnerable to a greater or lesser degree.

In out patent Application No. 2,081,459, a solution to the problem is proposed that involves fitting an inductive coil into the sensor leads. This solution works extremely well on certain types of welding machine, notably pedestal welders, but it is not so successful on mobile welding guns. It is therefore an object of the present invention to provide an improved solution.

According to the invention, the sensor leads are attached at positions remote from the electrodes, and in the processing of the sensor lead signal waveform the signal required for welding control purposes is obtained from the half cycles of one polarity, and the whole waveform is shifted vertically relatively to the base or zero voltage level so that the waveform becomes asymmetrical about said level and the major part of the undesirable amplitude increase resulting from connection of the sensor leads at positions remote from the welding electrodes occurs in the half cycles of the opposite polarity.

Preferably, the useful control signal is obtained by detecting the peaks of only the positive half cycles, and a negative d.c. voltage is mixed with the signal to offset the complete waveform vertically in the negative direction.

The nature of the invention will now be described in more detail by way of example with reference to the accompanying drawings.

Figs. 1 to 6 show welding electrodes 1 1, 12 carried on welding arms 13, 14, with a pair of sensor leads 15, 16 attached at various positions to the arms/electrodes. The sensor leads feed the signal waveform picked off from the welding arms/electrodes to a control circuit that monitors the voltage changes occurring during weld nugget formation, the purposes being to detect the peak voltage and to terminate the welding cycle when the voltage has dropped to a level a predetermined amount below the peak voltage which level represents the required weld spot size and is chosen to give weld termination at a point early enough both to avoid deleterious weld splash and to achieve a high rate of spot weld production.

However, it will be observed that if the sensor leads are attached to the electrodes as in Fig. 1, which gives the best signal for the above purpose, there is a considerable likelihood that they will foul the work being welded, especially if it is of large size, and they are vulnerable to being damaged or torn off. The leads can be protected from mechanical damage by being laid along the welding arms, but then the already discussed problem of inductive coupling occurs, to which the technique described in our patent application No.

2081459 is only a partial solution. Clearly, it would be preferable if the sensor leads could be attached at positions of safety remote from the welding electrodes, but if that is done the amplitude of the voltage waveform across the points of attachment of the two leads becomes so large that it is difficult or impossible to achieve reliable detection of the voltage drop across the weldment which is then a much smaller percentage of the total signal. Thus, in the instances shown in Figures 1 to 6 in which 1 mm sheets are being spot welded, acceptable signals can be obtained when one or both sensor leads are attached at or near the electrodes as in Figures 1, 5 and 6, but as both attachment points are moved together along the arms 13, 14 away from the electrodes (Figures 2 to 4) a position is reached (Figure 3) at which the sensor signal ceases to be satisfactory.Automatic adaptive weld cycle termination dependent on the condition of the weld then has to be abandoned and each weld cycle period is extended to the "back stop" time of the welding control.

Hitherto, the sensor signal has been fed via filters and a buffer amplifier to a peak detector and divider circuit, the output of which has been applied directly to a comparator for comparison with the preset "spot size" voltage. The purpose of the divider is to eliminate supply voltage fluctuations by causing them to appear in both the dividend and the divisor of the quotient.

Referring now to Figure 7, the signal waveform 10 from the sensor leads is applied via filters 17 and buffer amplifier 18 to both the divider 20 and a positive peak detector 1 9. The outputs of the peak detector 19 and divider 20 are combined and fed to following circuitry via a buffer amplifier 21. In order to overcome the problem previously discussed, a d.c. voltage is summed with the signal so as to reduce the amplitude of the peaks of one polarity that are detected by the peak detector 19. The effect is represented by the waveform 22; this shows the whole waveform offset vertically with respect to the base or zero volts line 23, so that the amplitude of the positive half cycles is reduced and that of the negative half-cycles is increased.Consequently, the voltage drop through the weldment is a much larger percentage of the half-cycle amplitude detected than it would be if the waveform remained symmetrical about the base line.

Figure 8 is a circuit diagram of the system. The positive peak detector 24 is followed by a unity gain amplifier 25 acting as an inverter, the output of which is applied to a voltage divider 26 and thence to an amplifier 27 which has its signal input terminal coupled to the zero volts line by a capacitor 28. The voltage representing the positive peaks now appears at the input of the amplifier 28 with a smaller amplitude.

Now merely can the sensor leads now be attached to the welding arms at positions remote from the electrodes, but it is even possible for thein to be attached to the castings carrying the arms of the jumpers for the current supply conductors. This not only ensures that there are no sensor leads in vulnerable positions near the electrodes but it also permits the welding arms to be changed without the need to re-fix the sensor leads. Control setting changes and adjustments to the electronic circuits can also be made without interference with the sensor leads.

Whereas in the particular example described, the invention is implemented by an analogue technique, it will be understood that the same result can be achieved by digital techniques, if preferred.

Claims

1. In electric spot welding, a method of avoiding inductive coupling between the main welding current conductors and sensor leads attached thereto to detect the weld nugget voltage drop for welding control purposes, wherein the sensor leads are attached at positions remote from the welding electrodes, in the processing of the sensor lead signal waveform the signal required for welding control purposes is obtained from the half cycles of one polarity, and the whole waveform is shifted vertically relatively to the base or zero voltage level so that the waveform becomes asymmetrical about said level and the major part of the undesirable amplitude increase resulting from connection of the sensor leads at positions remote from the welding electrodes occurs in the half cycles of the opposite polarity.

2. A method according to claim 1, wherein the welding controlsignal is obtained by detecting the peaks only of the positive half-cycles of the sensor lead signal, and a negative d.c. voltage is mixed with the sensor lead signal to offset the complete waveform in the negative direction.

3. Apparatus for performing the method of claim 1, comprising a divider and a peak detector both receiving the sensor lead signal, a buffer amplifier on the input of which the outputs of the divider and the peak detector are combined, and means for summing a d.c. voltage with the buffer amplifier signal.

4. Apparatus according to claim 3, wherein the divider and peak detector receive the sensor lead signal via filters and a buffer amplifier.

5. The method of avoiding inductive coupling between the main welding current conductors of an electric spot welder and sensor leads attached thereto, substantially as described herein with reference to Figure 7 of the accompanying drawings.