Hydraulic transmission technology has the advantages of high power-mass ratio and convenient speed regulation. It has been widely used in heavy metallurgical equipment, construction machinery, aerospace and other fields. The traditional hydraulic control system (such as system controlled by three-position four-way valve) is limited by the fixed structure of the spool valve. Its valve throttles both in the meter-in and meter-out oil circuit, so there is redundant throttling loss in the system, and there is still room for improving the overall efficiency of the system. At the same time, there is a direct coupling relationship between the pressure and flow parameters in the two chambers of the actuator, and the degree of freedom that the system parameters can be adjusted is relatively small. In order to solve both problems, Professor Palmberg of LinköPIng University in Sweden first proposed the concept of separate meter-in and separate meter-out system in 1990 [1], that is, two or more valves are used to control the inlet and outlet of actuator respectively. At present, the research in this field mainly focuses on three aspects: system principle design [2] [3], selection of working mode of actuator [4] [5], and speed and pressure control strategy [6] [7]. At present, many scholars have studied a calculated flow feedback method (virtual dynamic flow test) [8] [9] [10]. This method owns the advantages of high response and accuracy control, but it also has shortcomings of high system design cost and less practical application.

Energy-saving has become one of the research focuses on hydraulic technology [11]. In order to promote and apply the technology of independent metering control in practical projects, from the perspective of reducing system design cost, combining the traditional principle of load-sensing system, this paper proposes a new independent metering control system principle. In the early stage, control components (integrated valve, which includes main valve and pressure compensation valve) have been calculated and simulated. In the later stage, relevant simulation studies on the speed control, energy-saving and anti-flow saturation characteristics of the system have been carried out. The above work can provide a reference for the development of this system.

The hydraulic principle of the new independent metering control system presented in this paper is shown in Figure 1. Before introducing the principle of this system, the pressure compensation in hydraulic technology is explained as follows. The hydraulic system in construction machinery is often driven by a single pump with multiple actuators. The loads of actuator are often time-variant and

different. Therefore, in order to maintain the speed of actuator without the influence of load changes, typical mechanical-hydraulic load-sensing system usually takes compensatory measure for pressure difference of the directional valve, such as the PVG series multi-way valve produced by Danfoss and M4 series multi-way valve produced by Bosch Rexroth. However, the load-sensing system based on above compensation principle has the disadvantages of slow control response and unstable speed regulation in terms of dynamic characteristics [12]. With the rapid development of electronic transducer technology, in the 1990s, western universities such as RWTH Aachen University began to study the electronic-hydraulic load-sensing system using electrical signals to replace mechanical and hydraulic signals earlier [13]. The pressure signal of the system is detected by pressure sensors, and the systems start to use the electronic-hydraulic proportional pump. Relevant research results show that the electronic-hydraulic load-sensing system is superior to traditional mechanical-hydraulic load-sensing system in many aspects [14] [15]. Combining the electronic-hydraulic load-sensing technology and independent metering control technology, the innovative system design is carried out and its principle is as follows. Giving the control signals of two three-position five-way proportional directional valves (dv), the two valves can be in three different working states. By switching the working position of the two directional valves, different control functions can be realized. For example, when both directional valves are in the right position, the inlet and return oil circuits of the actuator are in the throttling state, which implements the independent control of the actuator. In addition, when the left directional valve works in the right position and the right directional valve works in the left position, the hydraulic cylinder can realize differential control. The pressure compensation valve (pcv) senses the maximum load pressure of all actuators and ensures that the pressure drop of the inlet valve in each valve group unit is similar. The opening of the pressure compensation valve is determined by the difference between the maximum load pressure in the system and the load pressure of the corresponding actuator. The pressure sensors are installed at the outlet of the pump and the pressure feedback pipe to compare the actual pressure difference with given pressure margin. The displacement controller outputs corresponding electrical signal to control the electronic-hydraulic variable displacement pump to output the flow and pressure corresponding to the system requirements.

The cross-sectional view of the symmetrical plane of the three-position five-way electric proportional direction valve is shown in Figure 2.

The maximum flow rate of the directional valve is set at 60 L/min. The size values of the directional valve are calculated by using the traditional design methods of hydraulic valve as shown in Table 1.

The orifice of directional valve adopts U-shaped throttle groove. The rectangular coordinate system shown in Figure 3 is established. The flow area of U-shaped

throttle groove is replaced by two main throttle surfaces in series. The calculation formula is as follows: (1)-(3). The specific parameters are substituted as follows: groove diameter r = 1.5 mm, groove depth h = 2.2 mm, groove length l = 3.5 mm, groove number n = 4, flow coefficient C de = C d1 = 0. 61 , and C d = 0. 72 . By using the software of MATLAB/Simulink, the relationship curve between the flow area and the opening of the valve is obtained as shown in Figure 4.

A 1 = { n ∫ 0 x 1 R ⋅ 2 arcsin z R d x 0 < x 1 ≤ r n ( ∫ 0 r R ⋅ 2 arcsin z R d x + ( x 1 − r ) ⋅ R ⋅ 2 arcsin r R ) r < x 1 ≤ l (1)

A 2 = { n [ y z + R 2 arcsin z R − 2 z ( R − h ) ] 0 < x 1 ≤ r n [ r R 2 − r 2 + R 2 arcsin r R − 2 r ( R − h ) ] r < x 1 ≤ l (2)

A e = C d1 C d2 A 1 A 2 C de C d1 2 A 1 2 + C d2 2 A 2 2 (3)

The cross-sectional view of the symmetrical plane of the pressure compensation valve is shown in Figure 5.

The size values of the pressure compensation valve are shown in Table 2.

In this paper, a new type of independent metering control system is proposed. The simulation results show that the system can achieve better speed control effect of the actuator and has good energy-saving performance. Compared with the existing system structure, the system proposed in this paper can also achieve anti-flow saturation function under multi-actuator working condition, which provides a new way to reduce the design cost of independent metering control system. Of course, this paper only simulates the proposed system characteristics, and subsequently we need to build up a test platform to verify the accuracy of the simulation results.

This work is supported by the National Nature Science Foundation of China (Grant No. 51805228).

The authors declare no conflicts of interest regarding the publication of this paper.

Zhang, G.T., Xu, J., Yang, J. and Liu, K.L. (2019) Simulation Analysis of Characteristics for Independent Metering Control System Based on Downstream Compensation. World Journal of Engineering and Technology, 7, 536-547. https://doi.org/10.4236/wjet.2019.73038