Apparatus and method of phase control for resistance welding and other

Abstract

A computer-controlled welding apparatus includes a phase-controlled resistance welding circuit (A) for selectively conducting pulses of a welding current to a workpiece (B) and a control circuit (C) for controlling the conduction of the welding current. The control circuit includes a line clock generator (30) for determining the beginning of each voltage half line cycle and a conduction sensor (34) for determining whether or not a contactor (14) is conducting current to the workpiece. A microcomputer (40) is preprogrammed with a plurality of power factor correction curves, each of which represents weld current vs. firing angle for a fixed hangover angle. The computer is programmed to select one of the power factor correction curves. If the preceding half line cycle was conductive, the program selects the curve which corresponds to the hangover angle of the preceding half line cycle. If the present half cycle is the first conductive half cycle in a weld, the program selects the curve corresponding to a hangover angle of about 87.degree.. During pulsation welding, the curve is selected based upon the hangover angle of the most recent conductive half cycle and the number of nonconductive half cycles therebetween. During each half cycle, the present angle (duration since the beginning of the half cycle) periodically addresses the curve and a corresponding weld current is retrieved. The retrieved weld current represents the weld current which would be produced if the container were rendered conductive at the present time, i.e., the present weld current. A comparator (86) compares a preselected weld current for the present half cycle with the present weld current. When the present weld current becomes coincident with the preselected weld current, the contactor is rendered conductive.