In this paper, a robust sensorless speed observer- controller scheme for a permanent magnet synchronous motor (PMSM) is proposed. To estimate both position and speed, without any mechanical sensors and only from the electrical measurements, a back-EMF-based sliding mode is designed. The convergence of this observer is proved. Then a high order sliding mode controller is developed. By using this technique, the drawback of the sliding mode technique called "chattering phenomenon" is strongly attenuated. After that, the convergence of the overall system is analyzed. Moreover, the robustness of this method with respect to the parameters uncertainties is verified by significant simulation results in the framework of an industrial benchmark.