The solenoid valves discussed so far are all on/off type valves, which are either completely open or completely closed. There are many applications that require the valve to be proportional, which means that the valve can be open to any value from 0-100%. Proportional valves are typically used in hydraulic applications where larger loads are controlled. For instance, a proportional hydraulic valve can be used to control the pressure and flow of plastic in an injection molding machine. Larger valves can be used to control the amount of fluid flowing to a hydraulic motor. Hydraulic motors were introduced in industrial applications in the 1960s and 1970s prior to the advent of solid-state circuits for AC and DC motor speed control. These motors were typically over 50 hp and their speed could be controlled easily by adjusting the amount of fluid that flowed past their vanes. This type of application was very popular when electronic components could only control small amounts of voltage and current. The small amount of current could be controlled at the proportional valve, and a large amount of horsepower could be controlled at the hydraulic motor.
A proportional valve is similar to the pneumatic valves discussed earlier in that it has a spool that can allow ports to change as well as open or close from 0-100%. Frequently proportional valves are used in feedback systems so they are called servo valves. Two types of feedback are available with these valves. One type of feedback is external positioning sensors such as encoders and resolvers. A second type of feedback is located directly inside the valve to indicate the position of the spool. Internal feedback is used where the valve response must be so fast that it cannot wait for an external feedback signal to be returned to the valve amplifier and then to the valve. Applications that require faster response include hydraulic robots and velocity control valves on plastic injection molding machines that control the speed that plastic is injected into the mold.
