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An adaptive sliding mode speed and position observer for sensorless control of brushless DC motor (BLDCM) is proposed in this paper. According the mathematical model of BLDCM, the sliding surface was defined based on the errors between actual and estimated currents. Equivalent control and model reference adaptive control (MRAC) is used to obtain position and speed signals of the rotor. The Lyapunov theory is introduced to prove the convergence of this system. A sensorless control system for BLDCM based on the sliding mode observer is implemented. The stability of this observer which shows good robustness is not influenced by both load disturbance and measurement noise. The result of simulation shows that the proposed method can correctly estimate the speed and position of the rotor, and the system has good dynamic and static performances.