A modified instantaneous speed observer design for brushless AC motors

Low speed control of servo systems is required in some applications. Motor speed is generally acquired from the so-called M method, which averages the number of pulses from the coupled encoder by a fixed time period. As a result, a long detection delay time, due to speeds lower than the minimum measurable value determined by the sampling frequency and the resolution of the encoder, may degrade the performance or even make system unstable. Instantaneous speed observers and/or disturbance torque observers provide merits of simple structure and easy implementation. However, this method is obsessed by the sensitivity to the instantaneous speed estimation due to the parameter variations. In this paper, a simply modified instantaneous speed observer is proposed to overcome the sensitivity problem. The experimental results without and with load will verify the effectiveness of the proposed algorithm such that the controllable motor speed is much lower than the minimum measurable value.