An Adaptive Sliding Mode Observer with PLL-Based Estimation for Sensorless PMSM Drives

Abstract

Permanent magnet synchronous motors (PMSMs) are widely used in high-performance drive systems because of their high efficiency, high torque density, and rapid dynamic response. Field-oriented control (FOC) is the dominant control strategy for such drives, but its performance depends critically on accurate rotor position and speed information. In sensorless PMSM drives, sliding mode observers (SMOs) are attractive because of their robustness and modest computational demand. However, conventional SMOs still exhibit an inherent trade-off between fast convergence and chattering suppression, while direct extraction of rotor position and speed from the estimated back-electromotive-force (back-EMF) signals remains vulnerable to noise and filtering delay. This paper presents a sensorless PMSM drive based on closed-loop FOC and an improved SMO-based estimation scheme. The PMSM is modeled in the synchronous dq reference frame, whereas the observer is constructed in the stationary reference frame from stator current dynamics and back-EMF reconstruction. To improve estimation performance, an adaptive sliding-mode injection gain is introduced to regulate the observer correction level according to the current estimation error, and a smooth switching law is employed to reduce chattering. In addition, a phase-locked loop (PLL) is incorporated to reconstruct the rotor electrical angle and speed from the estimated back-EMF components, thereby improving estimation smoothness and noise immunity. Simulation results obtained in MATLAB/Simulink under start-up, acceleration, steady-state, and load disturbance conditions demonstrate stable drive operation, accurate speed tracking, and improved estimation behavior compared with the conventional SMO. The results confirm that the proposed observer enhances convergence and reduces steady-state oscillations while preserving the simplicity and practical applicability of SMO-based sensorless control for PMSM drives.