A proportional valve is a valve that can change its output in response to a change in electrical input. These valves are also known as electro-hydraulic valves and they have some similarities with hydraulic servo valves in that they are electrically controlled. In this valve, the spool is actuated with the help of a solenoid. The main difference between servo and proportional valves is that servo valves are used in closed-loop systems, while proportional valves are used in both open- and closed-loop systems. Proportional valves can be used as directional control valves or pressure control valves, but in most cases these valves are used as directional control valves. The actuation is done with the help of electrical signals. The flow direction and flow rate of the valve is influenced by the control signal.
With proportional valves, the relationship between the output flow and the control signal is not perfectly linear. Despite their non-linear characteristics, proportional valves provide an inexpensive way to control force, velocity and position on machinery that requires high-speed response at high flow rates.
Many proportional valves are modified versions of four-way solenoid valves in which a proportional solenoid valve replaces a conventional solenoid valve. In operation, the electromagnetic force is balanced by a spring force to position the spool proportionally to the input signal. Improved positioning accuracy is achieved by replacing the centering spring with a positioning sensor at the end of the spool. When the spool reaches the specified position, the sensor signal then cancels the solenoid signal. As a result, there is considerable overlap in the zero position of the spool in low performance valves. This overlap can lead to flow dead zones, which can cause errors and instability in the positioning system.
A clear trend has emerged in the valve industry: it is becoming increasingly difficult to distinguish between proportional and servo valves. Traditionally, proportional valves cannot match the performance of servo valves, which are used primarily for open-loop applications. They are mass-produced, while servo systems require meticulous and precise manufacturing, which makes them 10 times more expensive. Proportional valves also have wider clearances, so they are more tolerant of contamination than servo valves. All of this will soon be a thing of the past as proportional valves begin to outperform servo systems.
For example, closed-loop proportional valves that function similarly to servo valves are available in many locations. Using powerful continuous-action solenoids with minimal friction moving parts and fast response electronics, a proportional valve has servo-like performance without the contamination and high pressure drop of a conventional valve. It requires precise manufacturing, but it is still cheaper than servo designs.
Proportional valves control flow, actuator position, speed, torque, and they can synchronize the action of many different cylinders. They are suitable for applications such as pressure systems, molding machines, and traditional servo markets (such as flight simulators and airframe testing). In addition, they are proving ideal in areas where proportional valves are already in use but require performance upgrades through closed-loop systems.
These valves are used in applications that require high resolution, high volume flow and low hysteresis.
For hydraulic systems.
Earthmoving machinery.
Robotics.
Machine tools and paper.
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