Abstract: A shelf height selector system for a lift truck provides signals to an operator as he raises or lowers a load carriage relative to a storage shelf level selected by a keyboard in order to perform a store or retrieve operation, and automatically stops carriage movement with load forks at the proper elevation. Pulses from an incremental shaft encoder operated by carriage movement are counted to provide an instantaneous elevation signal upon which indications to the operator and control functions depend. A switch on the truck mast is operated by carriage movement to repeatedly recalibrate a register containing the instantaneous elevation signal, obviating any errors should noise affect tallying of encoder counts. An optical sensor carried on the load carriage to sense reflective markers adjacent some shelves also recalibrates the register, obviating positioning errors due to mast deflection and tire wear.

Abstract: Two-phase logic signals from a reversible shaft encoder or like device provide count occurrence pulses and a count direction logic signal to control incrementing and decrementing of a counting means. One count occurrence pulse is omitted each time the count direction changes to avoid errors which otherwise might result. Up counts and down counts occur at the same shaft encoder positions, and the system has high noise immunity.