Servo control system operable on digital basis with limit cycle suppressed

A digital servo control system responsive to a command signal indicative of a commanded position of a movable element and a pair of incremental position signals having polarities variable with a phase difference therebetween in response to a predetermined incremental angle of rotation of a motor rotatable with a variable angular velocity to vary a present position of said element, for producing a motor drive signal for controllably driving said motor to position said element at said commanded position, said angular velocity having either of two senses at a time, said phase difference being representative of the sense of said angular velocity, said system comprising position pulse producing means responsive to said incremental position signals for producing position pulses representative of a variation in said current position, wherein the improvement comprises:

• control signal producing means responsive to said command signal and said position pulses for calculating a displacement from said present position to said commanded position and for producing a control signal digitally representative of a reference angular velocity selected for said motor in consideration of said displacement until an instant at which said displacement is reduced in absolute value to a predetermined value, a first positional error determined by said displacement during a prescribed interval of time following said instant, and a second positional error having a value increasing in absolute value with relation to said displacement when a predetermined interval of time lapses after said instant, said prescribed interval being not longer in duration than said predetermined interval, said movable element being positioned substantially at said commanded position within said predetermined interval;
• and
• motor drive signal producing means responsive to said control signal and said position pulses for producing said motor drive signal.