According to the purpose, it is divided into one-way valve and reversing valve. One-way valve: Only allow the fluid to be connected in one direction in the pipeline, and the reverse is cut off. Reversing valve: Change the on and off relationship between different pipelines. According to the number of working positions of the spool in the valve body, it can be divided into two positions, three positions, etc.; according to the number of controlled channels, it can be divided into two-way, three-way, four-way, five-way, etc.; according to the driving mode of the spool, it can be divided into manual, motorized and electric , hydraulic etc. Figure 2 shows the working principle of the three-position four-way reversing valve. P is the oil supply port, O is the oil return port, and A and B are the output ports leading to the actuator. When the spool is in the neutral position, all the oil ports are cut off and the actuator does not move; when the spool is moved to the right position, P and A are connected, and B and O are connected; when the spool is moved to the left position, P and B are connected Pass, A and O pass. In this way, the actuator can move forward and reverse.

In the late 1960s, an electro-hydraulic proportional control valve was developed on the basis of the above-mentioned hydraulic control valves. Its output (pressure, flow) can change continuously with the input electrical signal. Electro-hydraulic proportional control valve is divided into electro-hydraulic proportional pressure control valve, electro-hydraulic proportional flow control valve and electro-hydraulic proportional directional control valve according to different functions.