A lift pin assembly for use in a reactor for processing a workpiece includes plural lift pins extending generally parallel with a lift direction, each of the plural lift pins having a top end for supporting a workpiece and a bottom end. A lift table faces the bottom ends of the pins and is translatable in a direction generally parallel with the lift direction. A small force detector senses a force exerted by the lift pins that is sufficiently large to indicate a chucked wafer and sufficiently small to avoid dechucking a wafer. A large force detector senses a force exerted by the lift pins in a range sufficient to de-chuck the wafer.