Machine Design

Proportional Valves

The performance of proportional valves falls in the wide spectrum between on/off solenoid valves and electrohydraulic servovalves. The valves are termed proportional because output flow is not exactly linear in relation to input current. Despite their nonlinear response, the valves are an inexpensive way to control position, velocity, or force on equipment requiring high-speed response at high flow rates.

Many proportional valves are modified versions of four-way, on/off solenoid valves in which proportional solenoids replace conventional solenoids. In operation, solenoid force is balanced by spring force to position the spool in proportion to the input signal. Positioning accuracy can be improved by removing the centering springs and adding a positioning sensor to the end of the spool. The sensor signal then cancels the solenoid signal when the spool reaches the specified position.

Proportional valve parts are built to be interchangeable; thus, the spool in low-performance valves can have considerable overlap in the null position. This overlap causes flow deadband, which is not critical for flow-control systems but can cause errors and instability in positioning systems.

However, a definite trend in the valve industry is the increasing difficulty in differentiating between servo and proportional valves. Historically, proportional valves could not match servovalve performance and were primarily used in open-loop applications. They were mass produced, while servovalves required meticulous manufacturing and fit-up, making them up to ten times more expensive. Proportional valves also had wider clearances, making them more forgiving and more tolerant of contamination. However, such definitions no longer hold in many cases.

For example, closed-loop proportional valves are available that function much like servovalves. By using high-force, continuous-action solenoids, minimum-friction mechanical moving parts, and rapid-response electronics, the valves offer servolike performance without drawbacks like contamination sensitivity and high pressure drop. A key feature in the valve is a spool and sleeve assembly with no overlap in midposition. While this requires precise manufacturing, it is less costly than other servo designs.

The valves control flow or pressure, or actuator position, velocity, force, or torque, and can synchronize the action of a number of cylinders. They are suited for applications such as press systems and molding machines, for traditional servo markets like flight simulators and airframe testing, and for those areas currently using proportional systems that need to further upgrade performance with a closed-loop system.

Some manufacturers are producing proportional valves that are essentially servovalves made to mass-production specifications, with much greater tolerance allowances and looser fits than in their standard servo line. However, adding electronic feedback results in performance almost as good as that of a servovalve. In many cases, this gives performance perfectly suited to an application at a lower cost.

TAGS: Hydraulics
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