Sliding Mode Observation Method for Position Based on SOGI-FLL in Stirling Linear Generator Systems

Aiming at the problem that Stirling linear generator systems lack reliable displacement sensors for realtime piston position acquisition due to the high-pressure enclosed chamber and compact internal structure, this paper investigates a sensorless detection method for the moving-magnet piston. The motor's back-EMF is observed using a sliding mode observer based on the motor's output voltage and current. A SecondOrder Generalized Integrator (SOGI) is employed to filter the observed results. Furthermore, to avoid frequency fluctuations during system operation, a Frequency-Locked Loop (FLL) is incorporated to enhance the robustness of the filtering effect and frequency adaptability. A Phase-Locked Loop (PLL) is used to observe the amplitude and phase of the back-EMF. The piston's amplitude and phase results are derived based on the relationship between the back-EMF and the position characteristics of the moving-magnet piston. Experimental results demonstrate that this method can stably track the motion amplitude, with an amplitude detection error of less than 5% and a phase detection error of less than 0.15 rad, proving its suitability for system control operation.