## 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
Moreover, the controller based on \(H_{2}\) and \(H_{\infty}\) is designed for the control system. For the synchronous control strategy of multi mechatronic control systems, since the coupling compensation rules is difficult to be determined, and the amount of online computing is very large, [PERSON] et al. [63] put forward a synchronous control idea with the minimum number of related axes. Moreover, the synchronization control algorithm, which is based on the theory of adjacent coupling error and sliding mode control, is developed. For the system model with large model uncertainties and external disturbances,  
Figure 4: Optimal control scheme with hierarchical structure for electric vehicle. [PERSON] et al. [64] proposed a cross-coupling control (CCC) algorithm, which is based on Lyapunov stability criterion and recursive updating technology. Indeed, the contour performance is obviously enhanced by coordinating the motion of multiple axes. In order to solve the motion synchronization problem for double-cylinder electro-hydraulic elevator system, [PERSON] and [PERSON] [65] presented a nonlinear control algorithm including two-layer coordination controller. The synchronization controller of outer loop motion is designed, which is based on robust control technique on linear multi input and multiple output (MIMO) control system. Furthermore, the inner loop pressure controller for lifting oil cylinder is developed on the basis of disturbance observation for nonlinear single input and single output (SISO) control system. Aiming at the cylinder synchronous control system based on independent metering control system, [PERSON] et al. [66] proposed a coordinated control strategy to realize the synchronous motion control of two cylinders. In case of the slight fluctuation of friction force, an adaptive robust pressure controller is designed to keep the pressure stable in the cylinder cavity, which is beneficial to the accurate modeling of friction force. In addition, an adaptive robust controller is designed to improve motion tracking accuracy of the cylinder, and the model compensation via the on-line assessment with flow rate coefficient is enhanced. For inlet/outlet independently metering control of electro-hydraulic system, [PERSON] et al. [67] proposed a three-level controller to improve the energy utilization efficiency of system. The upper level controller selects the appropriate operating mode by the requirement of load and command speed. The lower level controller not only includes speed controller with the calculation flow rate feedback and pressure controller with back-pressure feedback, but also includes the flow rate controller by variable displacement pump. The middle level achieves the coordinated control between pump-control and valve-control, so that the opening of the inlet valve can be kept as large as possible. Due to the regulation of backpressure control and flow rate control, the system pressure loss is reduced to a minimum, and the energy saving control of the system is realized. The control scheme of independent metering valves-pump system for energy saving is shown in Figure 5.  
In order to realize coordinated control of multi-hydraulic-motor traveling system, the influence of different characteristics amount on every hydraulic motor driving subsystem is considered. [PERSON] et al. [68] put forward the adjacent cross-coupling strategy on the basis of the basic control law with the tracking error, which introduces the tracking errors of the adjacent two axes into the controller of local axis. The structure of multi-axis coordinated motion system based on adjacent cross-coupling strategy is shown in Figure 6.  
Figure 5: Control scheme of independent metering valves-pump system for energy saving. From above-surveyed on the control strategy for electro-hydraulic control system, the most important research aspects are summarized, and listed below. 1.