Backlash compensation circuit for use with an incremental encoder

Abstract

A circuit compensates backlash of a shaft on an incremental encoder having a quadrature and index pulse output lines. Quadrature is represented as two signal lines phased apart by 90 degrees and an index pulse occurring once per revolution of the encoder shaft. The logical combination of the quadrature signals produce a four state condition which, through sequential examination of the signal levels, allows discriminating circuitry to determine the rotational direction of the encoder shaft. The circuit allows the definition of clockwise or counterclockwise rotation as being the forward direction. When the circuit discerns the positive forward rotational progression of the two signal lines, the circuit passes such signals unaltered to a pulse responsive system. Upon discernment of an encoder's shaft reverse rotation, the circuit causes the last forward rotational quadrature state to be latched on its output lines. The current condition of the index pulse is latched and the incidence of reverse pulses in a counter circuit is accumulated. Upon detection of a return to a forward rotational direction, the circuit counts down the number of forward pulses sufficient to exhaust the contents of the counter circuit. When the accumulated reverse rotational pulses have been counted down to zero and the state of the encoder's output signal lines match that of the latched condition, the circuit releases the latch and allows passage of subsequent quadrature signals and the index pulse.