## 2 Inlet/Outlet Independent Regulation and Coordinated Control
### Flow-Rate Regulation Mechanism of Inlet/Outlet Independent Metering
Furthermore, an adaptive robust control method based on correction of quiescent operation point is proposed. Aiming at the pump/valves control strategy for an electro-hydraulic flow rate matching system, [PERSON] et al. [25] analyzed the requirements with speed control system under overrunning and impedance conditions, and an improved method of dynamic performance based on compensation with proportional valve is proposed. The proportional valve controller, which considers the compensation of dynamic and static performance at the same time, is developed to improve the performance of speed control system. A typical electro-hydraulic flow rate matching system mechanism of pump/valves control is shown in Figure 3.  
[PERSON] [26] designed a flow-rate and pressure coupling controller to control the piston speed and pressure in each chamber, and a double closed loop control in independent metering control system is used to monitor and modify the pressure and flow rate in real time. Moreover, four types of threads are designed to arrange the task. In reference [27], a programmable valve with inlet/outlet independent metering control is applied to replace the multi-way directional valve in construction machinery, which integrates the advantages of digital signal and inlet/outlet independent metering technology. Due to the increase of control freedom of electro-hydraulic system in construction machinery, the controllability of the control system is improved, and the energy consumption of system cut down by reducing the back pressure of hydraulic actuator. [PERSON] [28] put forward a novel flow rate amplifier valve with digital pilot, where the digital valve is the pilot stage and the Valvistor valve is the main stage. Furthermore, applying it to independent metering control system, not only realizes the requirements of control performance and energy saving of valve control system, but also lays the foundation for realizing modern intelligent control of electro-hydraulic system. [PERSON] et al. [29] used a flow-rate feedback calculation control and pressure calculation control for independent metering system under negative load condition to improve the speed stability. [PERSON] [30] presented a direct proportional flow rate control by pressure compensation with load control valve. In this valve, the flow rate through load control valve is proportional to the pilot pressure in the control stroke, and load pressure compensation function means that when the load pressure is too high, the flow rate of load control valve can be limited to the vicinity of the maximum rated flow. Addressing the control for the speed system of pump controlled parallel variable displacement motor with the flow rate adaptive distribution and multiplying nonlinear link characteristics, [PERSON] et al. [31] proposed a compound control architecture, which realizes pressure control by variable displacement pump and speed control of drive shafts by variable displacement motors. The second order linearization models of pressure control system and speed regulating system are derived, and the expected pressure planning method is proposed to realize the dynamic pressure regulation of the system. In addition, by introducing a disturbance observer to the speed controller of variable displacement motor, the nonlinear disturbance and unknown load disturbance are suppressed, which improves the steady state accuracy with fluctuating pressure. From above-surveyed on flow rate regulation mechanism for inlet/outlet independent metering control system, the most important research aspects are summarized, and listed below. 1. The inlet/outlet independent metering control has become an important research direction for electro-hydraulic power mechanism. It not only improves the accuracy and stability of the motion control system, but also possibly realizes the energy saving and regeneration. While maintaining  
Figure 3: Electro-hydraulic flow rate matching system mechanism of pump/valves control.