A novel sliding mode observer (SMO) with adaptive feedback gain is presented for sensorless vector control of permanent magnet synchronous machines (PMSM). The rotor position angle is estimated based on the equivalent control in the sliding mode observer. By selecting adaptive feedback gain, the equivalent control has the same constant magnitude as rotor flux over full speed range. Compared to other back-EMF-based sliding mode observers, the proposed one features: 1) robust estimation performance due to the adaptation of switching gain to variable operating speed; 2) easy design of the switching gain in SMO; and 3) the reduced estimation error of rotor position at low speeds when the induced back-EMF is too small to be accurately estimated. Simulation and experimental results are provided to verify its feasibility and effectiveness.
