Patent GB-1434349-A (GB)
Title: Monitoring weld quality in electric resistance welding
IPC: B23K11/25
Filed: 1972-11-21
Abstract: 1434349 Welding by pressure RUBERY OWEN & CO Ltd and WELDING LIAISON Ltd 21 Feb 1974 [21 Nov 1972] 53798/72 Heading B3R In monitoring the quality of a weld made by electric resistance welding, the dynamic resistance of the weld (i.e. the quotient, instantaneous voltage drop across the welding electrodes / instantaneous welding current) is measured at predetermined successive instants during welding and the measured values are compared with corresponding values known to be characteristic of a good quality weld. An apparatus for spot welding comprises a pair of electrodes 1 which are fed a predetermined number of weld pulses at regular intervals during a welding period, each pulse being formed by a predetermined number of rectified cycles of three phrase mains supply. During each weld pulse a voltage signal produced by a variable gain amplifier 3, having input connections from the electrodes 1, is fed to a divider circuit 2. At the same time a welding current signal produced by a toroidal coil 4 about the lower electrode is fed to a variable gain amplifier 5 which is connected to a further amplifier 6 which produces a smoothed output signal and shifts the phase of the current signal so that it is in phase with the welding voltage signal and this smoothed phase shifted signal is also fed to the divider 2. The divider 2 divides the voltage signal by the current signal to produce a signal proportional to the resistance of the weld during the weld pulse, the weld pulse being sufficiently short for the resistance to remain substantially unchanged during that time. A succession of signals proportional to the instantaneous resistance of the weld are produced during a welding period and as each is produced it is compared with a corresponding one of a succession of reference values of resistance known to correspond to a good quality weld, these reference values being stored in a memory circuit 7 and fed successively to a comparator 10. Each signal proportional to the weld resistance is fed from the divider 2 to the comparator 10 via a low pass filter 8 and the comparator 10 compares each derived signal with each reference signal, a synchronizing pulse generator 9 in synchronism with the divider 2 ensuring that the second derived signal is compared with a second reference signal, &c. There are typically eight weld pulses and thus eight comparisons. An analogue switch circuit 11 driven by the generator 9 gates the reference signals. The comparator 10 feeds an error signal to a discriminator circuit 12 which produces a pass signal or a fail signal and these signals are fed to a device 13 which issues a warning so that an operator can take appropriate action or automatically rejects the failed weld. The rate of rise of resistance may also be measured. The apparatus may be used in the production of spot welded wheels for road vehicles, the rim being spot welded to the nave pressing at a number of points and wheels of doubtful quality can be automatically rejected by the application of a paint mark or by delivery to a reject shoot or both and an alarm can be signalled. The welding apparatus used may have multiple heads and successive sets of welds are made by rotating the wheels between welds. The resistance welding may be seam or flash-butt.