## 2 Inlet/Outlet Independent Regulation and Coordinated Control
### Flow-Rate Regulation Mechanism of Inlet/Outlet Independent Metering
Moreover, supply pressure is controlled by the fifth valve with open-loop operation. In view of the fact that the excavator system based on inlet/outlet independent metering control is a mechanical-hydraulic coupling system with many unknown parameters, [PERSON] et al. [14] proposed the multi-objective optimization method based on nonlinear programming by quadratic  
Figure 2: Electro-hydraulic system mechanism of four-valve independent regulation. Lagrangian (NLPQL). The optimal value of characteristic parameters, which are difficult to estimate, are found by the method, and the speed and accuracy of parameter identification are improved. [PERSON] and [PERSON] [15; 16] designed an independent metering control system consisted of five independent proportional cartridge valves, which can be programmed to achieve flexible control of the system. Changing the control logics of the valves can achieve different median performance, and the system can realize system energy saving and regeneration without changing the hardware structure by proper programming. For inlet/outlet independent metering system, Professor [PERSON] [17; 18] carried out the study on the control strategy and the influence of control performance by valve structural parameters. The electronic control handle transmits the speed signal instruction to system controller, and the system controller sends control signals to control the proportional directional valve on both sides of the actuator, then the speed control and pressure control of actuator are realized. Meanwhile, the performance of system energy saving is enhanced. In addition, because the throttle loss of check valve is much smaller than balance valve, the efficiency of system is improved by changing components. In the middle of 1990s, [PERSON] et al. [19; 20] carried out a full study on the impact problems during the acceleration and deceleration, the starting and brake of the large inertia load in the hydraulic excavator. A pressure difference sensing technology is put forward to reduce the pressure impact, which is produced during the acceleration and deceleration of actuators. The control method of calculation flow rate feedback is adopted to control the flow rate of throttle valve, so that the speed smooth control is achieved for large inertia load. The core idea of control is to use a piecewise control method while the large inertia load is in the process of acceleration, deceleration and braking, and the method is based on actuator speed observation and parameters online estimation. Moreover, it takes account of its control accuracy of steady state as well as fast stability of dynamic process. In references [21; 22], the speed control characteristic of independent metering control system with multiple actuators is studied when actuators run at the same time. The synchronization model of the dual-actuator for independent metering system is established. Moreover, the controller is developed, which compares the speed control characteristics under different working conditions. Speed control method for piecewise adjustment according to the delay time of the balance valve is proposed, which improves the stability of speed control in balance valve circuit. For independent metering control system, [PERSON] et al. [23] proposed a control scheme for differential hydraulic cylinder, and this method can reduce power loss of electric motor by about 20%, which improves the efficiency of pump-controlled differential cylinder system. In addition, an inlet/outlet independent metering control system based on flow rate matching with pump and valve was put forward. The system ensures that the output flow rate of power source always matches the required flow rate by all the actuators, and it not only achieves the implementation of the individual executor, but also reduces throttle loss of the system to a minimum. [PERSON] [24] studied the control technology of inlet/outlet independent metering, which uses a double servo valve to control the cylinder system.