With the development of hydraulic transmission and hydraulic servo systems, the production practice has emerged, which requires that the pressure, flow and direction can be continuously controlled, and the hydraulic system does not need its high control accuracy. Because ordinary hydraulic components can not meet certain servo requirements, and the use of electro-hydraulic servo valves because of the control accuracy requirements are not high and too wasteful, so in recent years, a proportional control valve between ordinary hydraulic components (switch control) and servo valves (continuous control) has been produced.