Abstract: This positioning servo-system consists of a 16 bits microcomputer, PWM, and optocoders. In order to accomplish the positionting control fast and precisely, a Bang-bang control scheme combined with a non-linear velocity feedback is proposed. The Bang-bang is responsible for the large deviation, while the velocity feedback for the small deviation. The switching between these two states is determined by the microcomputer. The principles of non-linear feedback control and its applications to the saturated systems and high order systems are described, and the optimality of the above control principles is discussed. The discrimination criterion of the segment transformation is deduced. The ,results of simulations and real time control systems are illustrated.