Controller for spot welding

A process for controlling a welding current for a spot welding operation carried out on a workpiece comprising:

• (a) directing ultraviolet radiation through at least one fiber optic filament onto a thermographic phosphor disposed in a well in thermal contact with a welding tip,
• (b) directing the radiation emitted by the phosphor through at least one fiber optic filament to a first radiant-energy sensor to provide an output signal indicative of a temperature in a low temperature range for the welding tip,
• (c) directing infrared radiation emitted from the welding tip within said well through at least one fiber optic filament to a second radiant-energy sensor to produce an output signal indicative of a temperature in a higher temperature range for the welding tip, comparing the sequential output signals of said first and second sensors during a spot welding operation to a corresponding sequence of standard thermal history values to obtain a sequence of error values and continually adjusting the magnitude of the welding current in response to the sequence of error values to reduce differences between the temperature at the welding site in the workpiece during the course of a spot welding operation and the standard thermal history.

A welding tip for a spot welding machine having a welding controller for controlling a welding current in response to infrared radiation providing a measure of a temperature of a weld, the welding tip comprising:

• (a) a generally cylindrical shank made of an electrically conductive material, an end surface of the shank defining a contact surface for making mechanical and electrical contact with a workpiece to transmit a welding current to the workpiece, the end of the shank opposite to the contact surface being securable to the spot welding machine in an electrically contacting relationship to receive the welding current from the machine, the shank having a passageway through it, the intersection of the passageway with the contact surface defining a thermal-radiation entrance port;
• and
• (b) a window element made of a material which is transparent to infrared radiation and capable of enduring temperatures of up to about 600.degree. C., the window element being fitted in passageway of the shank.