Position tracking and flatness controller with disturbance observer in hydraulic servo systems

A position tracking and flatness controller with disturbance observer (DOB) is proposed in hydraulic servo systems (HSSs). In this paper, we assume that the disturbance is a biased sinusoidal signal with unknown frequency. DOB is designed in the form of 2^nd high pass filter to estimate the disturbance. The nonlinear controller is designed to compensate the estimation error of the disturbance and to track the desired position and load pressure as a near IO linearizing inner-loop load pressure controller and a feedback plus feedforward outer-loop position controller. The desired load pressure is designed using differential flatness property of the HSS mechanical subsystem. The performance of the proposed method is validated via simulations.