Electro-hydraulic servo systems are widely used in industrial applications. The load greatly affects the dynamic response of the separating-metering electro-hydraulic system. The current researches mainly aim at the system tracking performance for the hydraulic servo system, but the researches on the damping characteristics are relatively less. For energy-saving reasons, the metering-out chamber is often maintained near a lower pressure. The system will oscillate when the load drastically changes. The active damping strategy is proposed in this work in order to increase the damping and suppress the oscillation in separating-metering electro-hydraulic system. The effectiveness of the active damping strategy is proven by mathematical derivation. Furthermore, the nonlinear mathematical model of the separating-metering electro-hydraulic system is built, and a robust backstepping controller that combines the tracking differentiator and nonlinear disturbance observer is designed. The experimental results indicate that the system oscillation is suppressed and the proposed controller has good tracking accuracy.

Featured Application: To improve energy efficiency while also ensuring position tracking accuracy, this paper gives the design and the analysis of a separate metering electro-hydraulic servo system with varying supply pressure for a rolling shear. This work is also suitable for similar engineering applications.Abstract: The traditional valve-controlled hydraulic servo system has large throttling losses and undergoes serious heat problems when used in electro-hydraulic servo systems (EHSSs) for a rolling shear. In order to improve the energy efficiency of the EHSS for the rolling shear while also ensuring the position tracking accuracy, the separate metering electro-hydraulic servo system with varying supply pressure (VSP-SMEHSS) is proposed in this work. The inlet valve controls the position of a hydraulic cylinder, while the outlet valve controls the back pressure of the hydraulic cylinder. However, due to the disturbance caused by the varying supply pressure, the proportional-integral-derivative (PID) controller or active disturbance rejection controller (ADRC) cannot meet the requirements of accuracy. In order to solve this problem, based on a nonlinear disturbance observer (NDO) and a tracking differentiator (TD), a dynamic surface control (DSC) is proposed in this work. Firstly, the stability of the controller is validated using the Lyapunov method. Then, experiments are conducted to verify the proposed control strategy. As a result, the hydraulic cylinder can accurately track the reference displacement signal and effectively reduce the pressure drop at the valve's orifice, due to which the hydraulic system achieves significant energy-savings. Compared with that of the EHSS, the energy consumption of the VSP-SMEHSS is reduced by 44.6%.Appl. Sci. 2020, 10, 435 2 of 18 undergoes significant throttling losses [1][2][3][4], which generates a lot of heat and affects the reliability of the system. High temperature during the process affects the control of the system. Therefore, improving the energy efficiency of the electro-hydraulic servo system (EHSS) while guaranteeing the tracking accuracy is a topic worth research attention.In order to reduce throttling losses of a valve-controlled hydraulic servo system, one solution is to use a closed pump-controlled system [5][6][7][8], which includes a fixed-displacement pump system driven by a servo motor and a constant-speed variable-displacement pump system [9,10]. The flow rate of the pump is varied to control the speed of an actuator. Due to this reason, theoretically speaking, there are no throttling losses. Due to obvious energy-saving effects, this technology has been applied to hydraulic systems of plastic injection molding machines [11]. However, this scheme has a poor dynamic response, and it is difficult to meet the high-response requirement of a rolling shear EHSS.Load-sensing technology is another solution to reduce throttling losses [12,13], and it is widely used in hydraulic systems of excavators [14] and steering systems of hydraulic power [15]. In the lo...