Bang-bang servo system

Claim 1

Servo control apparatus for a drive motor operable to drive a paper tractor, including: a counter settable to register an initial digital count corresponding to a required paper tractor advance expressed as increments of movement distance; means for progressively decrementing the count registered by the counter in response to accomplishment of increments of actual movement; a motor drive current supply arranged to provide acceleration and deceleration drive current to the motor including a network of parallel current paths respectively arranged to provide current having a mutual binary power relationship and of opposite polarities, the polarities corresponding to acceleration and deceleration drive conditions respectively; and control means including means arranged to respond to the count registered during the movement to produce control signals; a comparator responsive to the initial digital count and to the previously decremented registered count to produce, if the required advance is less than a predetermined magnitude, a first changeover signal when the registered count indicates that half the required advance has been completed; and a gating control arrangement effective, if the required advance is equal to said predetermined magnitude and if the required advance is greater than said predetermined magnitude, to disable the comparator and to produce a second changeover signal when the registered count indicates that a predetermined distance before completion of the advance has been reached, said control signals being operative to select the current paths in the network to be energized singly and in combination to provide a drive current of required magnitude and said first and second changeover signals being operative to alter the polarities of the drive current from an acceleration drive condition to a deceleration drive condition.