Nonlinear Control With High-Gain Extended State Observer for Position Tracking of Electro-Hydraulic Systems

In this article, a method for nonlinear control is proposed using an extended state observer for position tracking of an electro-hydraulic system (EHS) with only position feedback. The proposed method consists of a high-gain extended state observer (HGESO) and a nonlinear controller. The EHS model is transformed into the normal form to lump a system function and an external disturbance into a disturbance. The HGESO observer is accordingly designed to estimate the full state and disturbance. The nonlinear controller is then developed using backstepping to suppress the position tracking error using the input-to-state stability property. The key innovation of the proposed method is the design of the nonlinear gain to suppress the position tracking error according to the disturbance estimation error. Thus, the use of the high observer gain to accurately estimate the disturbance can be avoided. The stability of the closed loop including the EHS, controller, and observer is then mathematically demonstrated without the need for an approximation of the "sgn" function in the EHS dynamics. The proposed method is verified using simulated and experimental testing.