Device for controlling electrical resistance welding apparatus,

Abstract

In electrical resistance welding apparatus comprising two welding electrodes connected through respective supply cables to an electrical power supply which can generate welding current with selectively-variable effective values, a control device is provided which, in use of the apparatus, measures the resistance between the electrodes clamped on the pieces which are being welded and the resistance between the ends of the supply cables. During welding, the device detects the change in the resistance between the electrodes with time and, on the basis of a linear combination of several parameters of the resistance curve detected, establishes whether the spot weld effected should be considered acceptable or unacceptable. In the latter case, the control device causes the welding operation to be continued with a current having a higher, rms value so as to correct the spot weld made. A statistical analysis is carried out on the spots considered acceptable so as to effect, in dependence on the results of this analysis, gradual increases in the duration of the activation intervals (welding angle) of the electrical power supply so as to compensate for the attrition of the electrodes. A gradual increase in the duration of the activation intervals of the electrical power supply is also effected upon an increase in the resistance detected between the ends of the supply cables in order to compensate for the wear of the cables themselves. Once the possibility of correction has been exhausted, the device indicates to the exterior the need to replace the electrodes and/or the supply cables.