## 2 Inlet/Outlet Independent Regulation and Coordinated Control
### Control Strategy for Electro-Hydraulic Control System
#### 2.2.1 Nonlinear Control Algorithm of Electro-Hydraulic Control System  
There are many nonlinear characteristics in the electro-hydraulic control system, such as the pressure and flow rate nonlinearity in servo valve, the structure nonlinearity of differential equation, the friction nonlinearity of actuators, and so on. And these nonlinear factors and questions in the most electro-hydraulic servo systems are inevitable to solve. Therefore, more advanced nonlinear control methods need be studied for the electro-hydraulic control system with nonlinear factors. Addressing the uncertainties in nonlinear control systems, [PERSON] and [PERSON] et al. developed various nonlinear control methods, such as adaptive control and variable structure sliding mode control. In the 1990s, [PERSON] et al. proposed the idea of backstepping control for the mismatched and uncertainty problem in electro-hydraulic control, where the core is how to overcome all kinds of uncertainties in nonlinear models. [PERSON] and [PERSON] et al. [32] established the nonlinear models for the electro-hydraulic actuator (EHA), and the robust controller of EHA based on the quantitative feedback theory (QFT) is designed. Meanwhile, the LuGre friction model is introduced to EHA model, the adaptive observer is used to estimate the actuating cylinder acceleration and the friction state, which cannot be observed directly. Moreover, the adaptive control law is designed to compensate the parameters perturbation and load disturbance of the system. For the nonlinear model of EHA, [PERSON] and [PERSON] [33; 34] designed the backstepping controller, which meets the stability of Lyapunov, obtaining a good control effect. [PERSON] [35; 36] applied the existing nonlinear control strategy to inlet/outlet independent metering control system, and the method with linear quadratic Gaussian (LQG) optimal control is proposed. In addition, the speed and pressure controller of the independent metering control system is also designed, which is applied to electro-hydraulic system of the crane. In order to test the actuator controlled by direct drive valve (DDV), [PERSON] et al. [37; 38] developed the dynamic load simulator, and the robust controller for loading system is designed, which is based on the quantitative feedback control technology (QFT). For electro-hydraulic loading system, [PERSON] et al. [39; 40] proposed a fuzzy proportional-integral-derivative (PID) intelligent control algorithm, which combines with the fuzzy control algorithm, grey scale prediction, and PID control. In order to solve the issue that fluid bulk elastic modulus is difficult to quantify, [PERSON] [41] developed a control design process, where additional sensors are not needed. Moreover, the design process makes the variables in bulk modulus have robustness. [PERSON] et al. [42] presented the derivation of nonlinear tracking control law for a hydraulic servo system. An analysis of the nonlinear system equations is used in the derivation of a Lyapunov function that provides for exponentially stable force trajectory tracking. Furthermore, this control law is then extended to provide position tracking. Addressing the nonlinear systems with unknown input dead-zones, [PERSON] et al. [43] proposed the direct/indirect adaptive robust control combining with the desired compensation strategy to synthesize practical high performance motion controllers for precision electrical drive systems[PERSON] Even if the overall system is subjected to parametric uncertainties, time varying disturbances, and other uncertain nonlinearities, certain guaranteed robust transient performance and steady-state tracking accuracy can be achieved. For multi-degree of freedom system, [PERSON] et al. [44] expanded the diagonalization method based on the modal space disturbance observers, which suppresses the interferences between the position control and force control of the system[PERSON] As a result, the bilateral control is realized, and that performance is better than the coordinated control.