Commutation pulse detector in a bi-directional motor

Commutation pulses of a d.c. motor (1) are accurately counted to determine the position of an associated power accessory (11) by using high speed switches in a bridge circuit (S1-S4). When power is disconnected from the motor, the switches recirculate a reverse current resulting from coasting of the motor. Recirculation of this current provides dynamic braking of the motor and also allows commutation pulses to be sensed so that the relatively small coasting movement of the motor and its associated accessory can be precisely determined.