GB2139361A

GB2139361A - Monitoring weld voltages

Info

Publication number: GB2139361A
Application number: GB08334660A
Authority: GB
Inventors: Keith Buxton
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1983-12-30
Filing date: 1983-12-30
Publication date: 1984-11-07
Also published as: GB8334660D0; GB2139361B

Abstract

A monitoring circuit for an a.c. resistance welding apparatus for can bodies is arranged to produce an output signal which is representative of the integral with respect to time of the resistive or in-phase component of the welding voltage taken over the half-cycles of welding current. The output signal is representative of the energy expended during welding assuming constant current, and may be used for indication or control. The integral is derived by an integrator arranged repetitively to integrate the monitored weld voltage over the half-cycles of welding current. <IMAGE>

Description

SPECIFICATION Monitoring resistive component of weld voltage This invention relates to electric welding, in particular (but not exclusively) the a.c. resistance welding of the longitudinal seams of tubular bodies for three-piece cans.

The effective or in-phase voltage across the electrodes of a can seam a.c. resistance welding apparatus is accompanied by a reactive voltage which is associated with the a.c. supply circuit to the electrodes. In addition, any attempt to monitor the welding voltage will almost unavoidably be attended by a further reactive voltage induced in the pick-up connections of the monitoring circuit itself. The welding voltage as monitored will therefore be formed of the effective or in-phase component in relation to which the monitoring is required, and a reactive component which may be substantially larger than the resistive component so as to mask the resistive component in the monitored welding voltage.

The present invention seeks to provide a monitoring circuit for the weld voltage of an electric welding apparatus, which is capable of deriving from the weld voltage a signal representative of the integral with respect to time of only the resistive or in-phase component of the weld voltage.

Accordingly, a monitoring circuit for monitoring the resistive or in-phase component of the weld voltage of an a.c. resistance welding apparatus comprises an integrator arranged for integrating the monitored weld voltage with respect to time and for producing an output signal representative of the integral, the integral comprising only the resistive or inphase component of the weld voltage.

In order that the invention may be more fully understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 diagrammatically shows a monitoring circuit in accordance with the invention, in relation to the wheel electrodes and supply busbars of a can seam resistance welding apparatus; and Figs. 2 and 3 show various waveforms of the monitoring circuit, in their phase relation to the welding current.

Referring firstly to Fig. 1, a monitoring circuit is shown for deriving a signal representative of the time integral of the resistive or inphase component VR of the voltage across the wheel electrodes 10 of a can body seam welding apparatus such as is sold in U.K. by Soudronic AG under the type designations ABM and FBB. The wheels are shown when the apparatus is in operation, that is, in relation to a pair of overlapped marginal edges 11 destined to form the side seam of a tinplate body of a three-piece can. The body is tubular and open-ended, the lower wheel 10 moving along its inside as the seam is progressively formed along its length.

The seam is formed by reistance welding using an a.c. electrical supply. Accordingly, and in known manner, it is formed of a succession of regularly spaced but overlapping weld "nuggets" each created during a respective half-cycle of the a.c. waveform.

Although not apparent from the drawing, the supply circuit is conventional. It has a rotary frequency converter fed from a standard 3-phase a.c. supply and having a singlephase output winding with an earthed centretapping. A welding transformer connected across this winding supplies the wheels 10 through the upper and lower busbars 13,14 shown in the drawing. A pair of inverseparallel connected thyristors is connected into circuit between the rotary converter and the welding transformer, and controllable to provide phase angle control of the welding current and so regulate the electrical energy used per nugget.

The monitoring circuit is connected for receiving the weld voltage Vw on leads 15, and has a differential amplifier 41 with the leads 15 connected across its input terminals. The weld voltage Vw as it appears on the leads 15 is formed of the resistive or in-phase component VR with the addition of a substantially larger inductive component VL due to the distributed circuit inductance between the monitoring points. In addition, a voltage VE will be unavoidably induced on the leads 15 themselves; this voltage may, in relative terms, be substantial.

The voltage appearing at the input to the amplifier 41, accordingly equal to VR + VL + VE (or VW 5 + VE), is denoted in Fig. 1 by the reference numeral V'w. It is hereinafter referred to as the "monitored weld voltage".

The monitoring circuit shown in Fig. 1 relies for its operation upon the fact that, since both of the reactive voltage components of V'w (i.e. VL and VE) are proportional to the rate of change of magnetic flux, when V'w is integrated they will produce an output proportional to the magnetic flux and so will disappear when the integration is taken over one complete half-cycle, i.e. between zero flux conditions. Therefore, by integrating V'w with respect to time over each pulse of the weld current, a voltage signal can be obtained which is substantially free of any reactive component and is therefore representative only of the resistive or in-phase compnent VR.

Moreover, the signal is already in an integral form (JVR dt) and so is suitable for can rejection and/or any other indication or control function which requires a measure of the electrical energy expended per nugget.

A bidirectional integrator 42 is connected to the output of the amplifier 41 for generating the signal representative of J-VR dt on a line 43. For a constant value of weld current Iw this integral is representative of electrical energy expended in the welding operation. The integrator is reset by a resetting circuit 44 prior to each half-cycle of the current IW, so that as a seam is being welded there appears on the line 43 a train of signals indicative of the electrical energies used for the individual nuggets of the seam. These signals are passed to a circuit 29 which, in known manner, samples their final absolute values and holds the samples on a capacitor 9 until updated by the next signal to arrive.

Statistically it is known that for satisfactory nugget formation the energy used should lie within predetermined upper and lower limits.

Cans which have one or more nugget energy values lying outside this range are automatically rejected by can rejection apparatus 25 responsive to the output signal from the sample and hold circuit 29. The apearatus 25 may have provision for an operator to alter the limits after visual (or other) inspection of the welded can bodies; accordingly, it has inputs 26,27 at which the upper and lower limits (in suitable form) can respectively be entered.

The signal on the line 43 may be used for indication and/or control functions other than the rejection of defective can bodies as described.

Figs. 2 and 3 show two successive halfcycles of various voltage waveforms illustrating the operation of the monitoring circuit. For the purposes of comparison the-weld current 1w is shown in dashed line in Fig. 2 when superimposed upon the monitored weld voltage V'w which is seen to be largely reactive.

The welding current falls naturally to zero at the positions indicated Iwo on the waveform, the following rise being delayed artificially, to the positions denoted IwB, by the phase angle control previously mentioned.

Fig. 3 shows the resistive or in-phase component VR of the monitored weld voltage V'w.

Because of leakage inductance in the welding transformer a positive voltage exists during the time that the phase angle control is effective, i.e. between the positions Iwo and lwB on the waveform. This voltage is ignored by the integrator 42 in its determination of SV, dt.

This invention is not to be considered as limited to the welding of seams on tubular can bodies using wheel electrodes, but may have application to other welding apparatus for other applications. Moreover, many variations and modifications of the monitoring circuits are possible within the scope of the appended claims.

Claims

1. A monitoring circuit for monitoring the resistive or in-phase component of the weld voltage of an a.c. resistance welding apparatus, which comprises an integrator arranged for integrating the monitored weld voltage with respect to time and for producing an output signal representative of the integral, the integral comprising substantially only the resistive or in-phase component of the weld voltage.

2. A circuit according to claim 1, which includes means responsive to the output signal for producing an indication signal whenever the output signal falls outside predetermined upper and lower limits.

3. A circuit according to claim 2, wherein the said means is arranged to control movement of the welded article in response to the indication signal.

4. An a.c. resistance welding apparatus for the longitudinal seams of tubular bodies for cans, which includes first and second electrodes for contacting opposed parts of a said tubular body to be welded together, supply connections connected to the electrodes for supplying the same with a.c. electrical current, a monitoring circuit for monitoring the weld voltage across said electrodes and for producing an output signal representative of the electrical energy expended during each half-cycle of the welding current, said monitoring circuit comprising an integrator arranged for integrating the monitored weld voltage with respect to time and for producing an output signal representative of the integral, the integral comprising substantially only the resistive or in-phase component of the weld voltage.

5. An apparatus according to claim 4, which includes means responsive to the output signal for segregating and said tubular bodies having a said output signal outside predetermined upper and lower limits from said tubular bodies of which the output signals are within said limits.

5. An apparatus according to claim 4, which includes means responsive to the output signal for segregating any said tubular bodies having a said output signal outside predetermined upper and lower limits from said tubular bodies of which the output signals are within said limits.

6. A monitoring circuit for monitoring the weld voltage of an a.c. resistance welding apparatus, substantially as hereinbefore described with reference to the accompanying drawings.

1. A monitoring circuit for monitoring the resistive or in-phase component of the weld voltage of an a.c. resistance welding apparatus, which comprises means to monitor the weld voltage and produce a monitored weld voltage signal therefrom, an integrator arranged for integrating the monitored weld voltage signal with respect to time and for producing an output signal representative of the integral, the integral comprising substantially only the resistive or in-phase component of the monitored weld voltage signal, and the output signal being therefore representative of substantially only the resistive or in-phase component of the weld voltage.

3. A circuit according to claim 2, wherein the said means responsive to the output signal is arranged to control movement of the welded article in response to the indication signal.

4. An a.c. resistance welding apparatus for the longitudinal seams of tubular bodies for cans, which includes first and second electrodes for contacting opposed parts of a said tubular body to be welded together, supply connections connected to the electrodes for supplying the same with a.c. electrical current, a monitoring circuit for monitoring the weld voltage across said electrodes and for producing an output signal representative of the electrical energy expended during each half-cycle of the welding current, said monitoring circuit comprising means to monitor the weld voltage and producing a monitored weld voltage signal therefrom, an integrator arranged for integrating the monitored weld voltage signal with respect to time and for producing an output signal representative of the integral, the integral comprising substantially only the resistive or in-phase component of the monitored weld voltage signal, and the output signal being therefore representative of substantially only the resistive or in-phase component of the weld voltage.