DISCRIMINATOR FOR MATERIAL SURFACE

PURPOSE:To automatically discriminate whether a plate material surface is plated or not by sandwiching a plate material to be welded between two couples of electrodes and supplying constant currents to one couple of electrodes, and then detecting a voltage drop across the other couple of electrodes. CONSTITUTION:Steel materials A and B are stacked between electrodes 11a and 11b, and electrodes 5a and 5b, and then the electrode 5a has its top end pressed against the reverse surface of the steel plate A by the repulse force of a spring 6. When a guide 4b is pressed, the electrode 5b is pressed against the steel plate B by the repulse force of a spring 10 and the electrodes and 11a and 11b are also brought into contact with the steel plates A and B respectively. Then, a constant current flows from a constant current source 12 to the steel plates A and B through the electrodes 5a and 5b to cause a voltage drop across the welding electrodes 11a and 11b corresponding to the contact resistance of the faying surface between both the steel plates A and B, and the drop is detected by a voltage detector 13. The voltage is low when plated steel plates are contacted and high when nonplated surfaces are contacted together.