A Sensorless Control Method for Permanent Magnet Synchronous Machine Based on Dual Position Observers at Low Speed

For low-speed sensorless control of permanent magnet synchronous machine (PMSM), high frequency (HF) pulsating voltage injection method has been widely adopted to obtain the rotor position. However, the HF noise signal caused by HF signal injection imposes a negative impact on system bandwidth and dynamic performance. To solve this problem, some methods are proposed by designing novel observers and filters, but there is still an unavoidable trade-off between low noise and fast response. Therefore, new solution is necessary to design different position observers to decouple the low noise steady state and fast response transient state. To this end, the closed-loop bandwidths, filter characteristics and operation models for different observers should be analyzed, and a solution to combine different observers is required. Firstly, a conventional HF pulsating voltage injection method is presented to obtain a phase-locked loop (PLL) position observer and its parameter design method. Secondly, a second-order sliding mode observer (SMO) with variable gains is used for another position observer to realize low noise or low delay operations by changing its gains. Thirdly, the control structure and parameter design of the speed loop are derived through the similar structure with PLL, and its dynamic performance and filter characteristic are close to first-order low pass filter by comparing their Bode plots, which means it is difficult to achieve the low noise and low delay operations together. Finally, to decouple the low noise and low delay operations, the sensorless control system with a PLL position observer and a SMO is built which can switch low and high bandwidth operation models utilizing variable parameters corresponding to low noise and low delay operations respectively. In the sensorless control system, HF signal is adopted to implement signal injection based on a PLL position observer. An independent position observation system with a high closed-loop bandwidth is constructed to guarantee that the position estimation error converges to zero fast. Based on fundamental signal processing, a field-oriented control (FOC) system is built which obtains the rotor position and speed from the SMO. The SMO can get the rotor position with low delay from the PLL and obtain new position and speed estimations, then provide estimations with low noise or low delay for the FOC system corresponding to steady state and transient state respectively. According to the differences of response speed between the PLL and SMO, the conditions are presented to determine the system states and switch the controller bandwidths and a strategy is proposed using the integrators of the speed loop and SMO to smooth the bandwidth switching process. The experimental results describe the performances of the sensorless control system with dual position observers at different operations. When the PMSM starts up from 0 to 100r/min, comparing with conventional method, the low bandwidth model has lower speed and current fluctuations but longer position error convergence time. While the maximum speed fluctuation of high bandwidth model is close to 40r/min but the average speed is still 100r/min and the system can keep steady state. When the PMSM starts up with a rated load, the low bandwidth model and conventional method need 5s to reach the reference value but the high bandwidth model only need 0.02s, which means that the high bandwidth model has better dynamic performance. When the load or reference speed has a sudden change, the system can switch to high bandwidth model to regulate speed fast then return to low bandwidth model smoothly based on the bandwidth switching strategy. The theoretical analysis and experimental results show that the HF and fundamental signal processing can be decoupled utilizing dual position observers and the sensorless control system can be divided into independent position observation and FOC systems. Based on variable bandwidth and smooth switching strategy, the system can realize low noise and fluctuation at steady state and fast response and regulating at transient state. © 2023 Chinese Machine Press. All rights reserved.