1. Field of the Invention
This invention relates to an apparatus for controlling resistance welding. This invention also relates to a method of controlling resistance welding.
2. Description of the Prior Art
U.S. Pat. No. 4,634,828 discloses a control method related to resistance welding in which welding conditions such as a welding current and a welding time are controlled in response to a change in material of weldments. The method in U.S. Pat. No. 4,634,828 has the step of detecting the local lowest value of resistance between electrode tips which appears in an initial welding period, the step of comparing the detected lowest value with a plurality of reference resistance values to determine the material of a weldment under process, and the step of changing the welding conditions (such as the welding current and the welding time) depending on the determined material. The reference resistance values are preset correspondingly to materials of weldments of different sorts, and are divided into plural classes.
Japanese published unexamined patent application 5-337655 discloses a method and a device for controlling a welding current in a resistance welding apparatus. The device in Japanese application 5-337655 includes a CPU which outputs a command value of the welding current of a square waveform according to initial data stored in a storage circuit. A secondary side current detector informs the CPU of the actual welding current energized between electrode tips. The CPU commands an arithmetic circuit to calculate a limiting current value from the actual welding current. The limiting current value relates to the generation of expulsion and surface flash. A specified energizing pattern is selected based on the limiting current value from among plural energizing patterns of a stepped waveform. The command value of the welding current is outputted in accordance with the selected energizing pattern. The presence or the absence of expulsion and surface flash is detected, and one of the energizing patterns is reselected based on the detection result.
Japanese published unexamined patent application 5-337657 discloses a resistance welding controller including a detection segment for detecting at least one physical quantity on a part to be welded, and an arithmetic segment for carrying out numerical analysis of the energizing diameter and the nugget diameter in response to the detected physical quantity. The resistance welding controller in Japanese application 5-337657 also includes a comparison segment for comparing the physical quantity with a model waveform or comparing a nugget-diameter model waveform with the nugget diameter estimated by the arithmetic segment, and for outputting the comparison result as a control signal. At least one of a welding current, a welding time, and a welding force is changed in response to the control signal outputted by the comparison segment and the energizing diameter outputted by the arithmetic segment.
Japanese published unexamined patent application 3-8585 discloses feedback control in resistance welding. According to the feedback control in Japanese application 3-8585, a resistance value is calculated from a welding current and a welding voltage applied between electrodes, and the resistance value is compared with a model resistance value. The welding current is controlled in response to the comparison result to stabilize welding conditions.