## 2 Inlet/Outlet Independent Regulation and Coordinated Control
### Flow-Rate Regulation Mechanism of Inlet/Outlet Independent Metering  
Inlet/outlet independent metering system usually uses two electro-hydraulic control valves to determine the motion state of an actuator, which opens the mechanical connection between the inlet and outlet in ordinary electro-hydraulic control valve. The key question is how to coordinate the opening displacement of two spools in control valves, so that the coordinate control of pressure and flow rate can be realized. Because two throttle valves can be respectively responsible for flow rate control in inlet lines and back-pressure control in outlet lines, to reduce the throttling loss in inlet line at least is possible. In 1987, Professor [PERSON] [8] presented the conception of separate control of actuator ports with cartridge valve control theory, the control principle is shown in Figure 1. Through the input liquid resistance and output liquid resistance, pressure and flow rate of hydraulic cylinder chambers are independently controlled. Liquid resistances are realized in four cartridge valves. With the development of electronic technology, [PERSON] applied electronic feedback control technology to hydraulic control system, further optimized the performance of the control system. The feedback signals, which are in the form of pressure feedback, flow rate feedback, speed feedback and acceleration feedback, realize the system feedback control function. Figure 1: Independent metering control system based on poppet valve. [PERSON] [9] formally proposed the concept of inlet/outlet independent metering control. Four electro-hydraulic proportional poppet valves are used to realize the control of hydraulic actuators, and a constant pressure pump is selected for oil supply. In addition, the value and direction of the load are obtained by the measured pressures from two pressure sensors in two chambers, then different control strategies can be selected for optimal control according to the load and speed characteristics. [PERSON] et al. [10] put forward the controllability and possibility of energy recovery under negative load, and the scheme adopts the inlet/outlet oil decoupling control through four-valve independent regulation. Moreover, the mathematical analysis and demonstration is derived. Compared to the traditional valve control system, the saving energy effect of the independent metering mechanism with four-valve can is better by controlling the back pressure. The electro-hydraulic system mechanism of four-valve independent regulation is shown in Figure 2.  
[PERSON] et al. [11] further studied this kind of electro-hydraulic system controlled by four valves. Since the four valves can be independently controlled, the configuration scheme can be operated with a variety of double valves in independent control modes. In addition, the regulation modes of five different working conditions are summarized, which includes power extension, high side regeneration extension, low side regeneration retraction, power retraction, and low side regeneration extension. [PERSON] et al. [12] studied the physical decoupling control of double chamber pressure for independent metering control system, where the two chambers of hydraulic cylinder respectively use an electro-hydraulic proportional valve for flow-rate or pressure control. In this control system, an auxiliary controller and two pressure controllers are designed. The pressure decoupling control of two chambers is achieved with two pressure controllers, and the speed control of actuator is realized by auxiliary controller. [PERSON] et al. [13] proposed a new control method of automatic calibration state trajectory, and applied it to the flow rate control in independent metering system. The system uses five electric hydraulic lifting valves to form the independent metering control structure. The proposed control method is used to adjust four of these valves, with which the flow rate control of hydraulic actuator's inlet/outlet oil is realized.