The cost of a proportional valve can be as much as 50 percent of the cost of a servo valve of the same size. However, servo valves continue to be used where precise positioning or speed control is required in aircraft, aerospace and turbine generator applications.
Proportional valves can be used as flow and pressure controls, but the most common use is as a directional valve. While the design of proportional directional valves may vary from one manufacturer to another, they all essentially perform the same function: controlling the direction and speed of a hydraulic cylinder or motor.
By using feedback devices like linear-displacement transducers or rotary encoders, the position of the actuator can be precisely controlled.