## 2 Inlet/Outlet Independent Regulation and Coordinated Control
### Control Strategy for Electro-Hydraulic Control System
#### 2.2.2 Hierarchical Coordination Control Strategy of Electro-Hydraulic Control System  
With the increase of control performance requirement and complexity of control system, the decoupling control for multivariable and complex systems is no longer able to achieve the design requirements. However, for many electro-hydraulic control systems, it is not required to decouple the variables, but to maintain a certain coordination of multiple variables in control process. Therefore, the hierarchical and coordinated control of electro-hydraulic control system becomes an important research field. Multivariable decoupling control is widely used in multivariable and complex electro-hydraulic control system, and the working principle is that the coupling between controllers is used to remove the coupling effect of controlled plant. Moreover, the multivariable control system with coupling effect is decomposed into several single variable control subsystems to regulate separately. In 1960, the first international federation of automatic control and Chinese scholar [PERSON] first proposed the multivariable coordinated control theory. It is not only different from the non-interactional control put forward by American scholars, but also different from the theory of self-regulation by Soviet scholars. At present, the idea of hierarchical coordination control has been widely used in different professional fields. [PERSON] et al. [57] developed a hierarchical optimal controller, which is simultaneous to control both the machining force and the axis position. The whole milling process is divided into two stages, and the two level targets are adjusted in the feasible range by weight. However, during the transient process, a series of simulations show that the control method can only make one target performance better. In order to enhance the vehicle dynamics stability and handling performance for electric vehicle, an optimal two-layer control scheme with hierarchical structure is proposed in the reference [58]. In fact, the upper controller based on sliding-mode control is in charge of motion regulation, where the desired longitudinal and lateral forces and yaw moment is calculated for keeping vehicle dynamics stability. The lower layer assigns the driving/braking torques to each in-wheel motor with an optimization algorithm. Moreover, considering motor power capability and tire workload, a cost function with adjustable weight coefficients is designed. The optimal control scheme with hierarchical structure for electric vehicle is shown in Figure 4.  
[PERSON] et al. [59] developed a hierarchical control architecture based on the Takagi-Sugeno (T-S) fuzzy model. Even if there are serious behavior changes of boiler steam turbines, the hierarchical control can achieve the optimal control. [PERSON] [60; 61] developed the controller for an independent metering control system with five cone valves, and the control characteristics is studied deeply. The two-layer controller included the nonlinear robust adaptive controller and PID are designed, which achieves trajectory tracking control and energy saving control. Moreover, the control system is composed of five cone valves, where four valves are used for independently regulating the flow rate and pressure in two chambers of actuator, and one valve is directly connected with the two cylinders for energy regeneration. In addition, the two-layer controller is used for mode selection and flow-pressure control with proportional valve, which includes the upper level controller with task switching and the lower layer controller with flow-pressure control. Furthermore, both the pressure controller and flow rate controller adopt robust adaptive control based on the nonlinear system model. Aiming at power distribution and coordination problem for the earth moving vehicle, [PERSON] et al. [62] developed a generalized model for the multi-input and multi-output electro-hydraulic transmission system, which adopts the method with linearization of nominal work points.