Design of an improved adaptive sliding mode observer for charge/discharge control in flywheel energy storage systems

Aiming to address severe sliding mode chattering in traditional sliding mode observer (SMO), high-frequency harmonics in the back electromotive force (EMF), and low rotor position estimation accuracy, this paper proposes an improved adaptive SMO. And considering the characteristics of the flywheel energy storage system-such as high flywheel operating speeds, a wide range of speed variations, and frequent switching of control strategies-the sliding mode surface and reaching law are redesigned. Additionally, a charge and discharge control strategy tailored for the flywheel energy storage system is developed. First, a continuous sigmoid function is established as the sliding mode switching function to accelerate system convergence and mitigate chattering. An integral sliding mode surface and a power reaching law are incorporated into the sliding mode surface function, which not only suppresses chattering but also improves the system's dynamic response speed. Second, adaptive control of the back EMF is introduced to filter out high-frequency harmonic signals in the estimated extended back EMF, thus avoiding the chattering and phase delay caused by a low-pass filter. An improved phase-locked loop (PLL), which filters out the effects of motor rotation, is introduced to correct the impact of rotor speed variations, further enhancing the accuracy of rotor speed and position estimation. Finally, a simulation model is built on the MATLAB/Simulink platform to validate the practicality and effectiveness of the proposed control strategy.