Abstract: A position detection device for a workpiece to be welded including a spot welding gun having a pair of electrodes adapted to be disposed opposite each other across the workpiece; a robot for holding either one of the spot welding gun and the workpiece in a manner movable relate to each other; a servo motor for allowing the pair of electrodes to approach the workpiece and separate from the workpiece; a physical quantity detection section for detecting a physical quantity correlative to a torque of the servo motor when the servo motor allows one of the pair of electrodes to approach a surface of the workpiece so that the one of the pair of electrodes abuts against the surface of the workpiece; a position detection section for detecting positions of the pair of electrodes; a storage section for storing the physical quantity detected by the physical quantity detection section and a value detected by the position detection section; and a computation section for calculating a contact start time at which the one of

Abstract: A spot welding system including a spot welding gun having a pair of electrodes disposed opposite to each other, and a servo motor for allowing the pair of electrodes to approach each other and separate from each other; a robot for movably holding either a spot welding gun or a workpiece so that a workpiece is disposed between the pair of electrodes of the spot welding gun; a physical quantity detection section for detecting a physical quantity correlative to torque or velocity of the servo motor; a position detection section for detecting positions of the pair of electrodes; a mode switching section for switching, by a switching command, an operation mode of the spot welding system between a spot welding mode for spot-welding the workpiece and a position correction mode for correcting a spot welding position of the workpiece; and a processing section for performing a spot welding process in the spot welding mode and a position correction process in the position correction mode.