Improved Position Sensorless Piston Stroke Control Method for Linear Oscillatory Machine via an Hybrid Terminal Sliding-Mode Observer

Since the piston stroke of linear oscillation machine (LOM) is very important for the safe and reliable operation of the system, it is necessary to effectively control the piston stroke. Besides, considering that the use of position sensor will reduce the system reliability, so the accurate position sensorless method is needed to obtain the stroke signals. In this article, a hybrid terminal sliding-mode observer is designed by combing a second-order nonsingular terminal sliding mode (SNTSM) and a high-order sliding-mode (HOSM) to estimate the back-electromotive force signals of the LOM. The SNTSM surface is selected to ensure that the system state variables can converge in a finite time and the HOSM control law is designed by introducing a variable exponential reaching law, which can strengthen the stability of the observer and greatly reduce the sliding chattering. Since the piston stroke is calculated by integrating the estimated piston velocity, the pure integral saturation will occur due to the presence of dc components in the measured current and voltage. In order to cope with this problem, one self-adaptive band-pass filter is introduced to replace the pure integrator, which can obtain smooth stroke signals with almost no amplitude and phase offset. Moreover, the current amplitude controller is designed to improve the control accuracy and response speed. Finally, comprehensive simulation and experimental results are provided to verify the effectiveness and the superiority of the proposed method.