What is a hydraulic proportional valve and when should you use one?

If you've ever seen a hydraulic system struggle to control the actuator moving in a jerky manner, pressure spikes, or poor control of speed, the cause is usually the same: the valve that controls the flow of fluid is open or closed completely, and there is nothing else between. This kind of binary operation is fine for simple applications that require on/off; however, many modern hydraulic systems require some sort of more accuracy. This is where proportional valves step into the picture.

This article will explain how hydraulic proportional valves are different from traditional directional control valves and, importantly, when deciding if one is a good idea for your particular application.

The concept at the heart of it is ratio vs. on/off.

A typical solenoid-operated directional valve operates as the light switch. When the solenoid turns on the spool, it shifts to a single position. The flow is directed in a single direction at the speed the pressure of the system and the orifice's size permit. There is no middle ground.

A hydraulic proportional valve operates as a dimmer. The solenoid is able to receive an electrical signal that is variable—typically a voltage of 4-20 mA or 0-10 V DC input from a controller—and the spool is moved in proportion to the signal. The greater the movement of the spool, the greater the flow path is opened and the greater the rate of flow or output pressure. This means that the valve can be maintained at any point in between completely closed and completely open, allowing engineers to have continuous reliable control of actuator force, speed, and direction.

The electrical input is provided by the proportional amplifier card or integrated driver that also performs functions such as the control of ramps, dither, and even signal conditioning. The majority of modern proportional valves have the linear displacement transducer (LVDT) or a similar sensor that feeds the spool's position into the amp, thereby increasing the control's accuracy significantly.

What proportional valves can be constructed?

Proportional valves appear similar to conventional D03 or D05 directionally oriented valves from the outside; however, the insides are different. They are made of solenoid coils that have been wound to provide a proportional response, not a snap-action switch, and the spool is machined to tighter tolerances to ensure consistent positioning across its entire travel range.

The majority of manufacturers provide proportional valves that come in two basic types:

Proportional directional control valves (PDCVs) regulate both the direction and the velocity of the hydraulic fluid concurrently. One valve can replace what would require an additional directional valve and an additional flow valve, reducing the circuit and minimizing potential leaks.

Proportional pressure relief valves permit the pressure setting point for the system to be dynamically adjusted in real-time without having to manually turn the knob or switch the cartridge. This is useful in cases where the clamping force and press tonnage or back pressure must be adjusted in various cycles of work.

The proportional valves control the flow rate independent of pressure fluctuations and often incorporate pressure compensation to ensure the same flow rate when load conditions change.

High-end proportional valves that incorporate electronics are often referred to as servo proportional valves. They are situated between a conventional proportional valve and the servo valve. They provide quicker response times and lower hysteresis levels than the basic proportional valves, but without the demands for strict filtration that come with the one-way valve.

When is the best time to utilize an equal value?

This is a question: Which is the most important factor in the design of systems? Proportional valves can be more expensive than solenoid valves that are conventional—often significantly so—and make electronic components more complex in the system. They're not the best option for every situation. Here's how they are rewarded:

Controlling the speed of a variable is necessary. If your hydraulic cylinder or motor requires it to gradually accelerate and hold a particular interval of speed or slow down quickly before completing the stroke A proportional valve can be the best device. Controlling forklift masts and carriage movement by injection molding the crane's boom's lowering are all classic examples of the smooth operation of variable speed directly impacting the quality of the cycle and the experience of the operator.

The system needs to be able to react rapidly to changes in load conditions. For instance, in applications such as active material testing and adaptive clamping fixtures as well as hydraulic robotic joints, the load is constantly changing, and the valve has to react to feedback in real-time. A proportional valve that is paired with an electronic pressure transducer or sensor can be controlled with a closed loop, which adjusts automatically.

It is essential to get rid of the water hammer and pressure shock. A rapid switch of standard valves can cause pressure spikes that cause damage to seals, crack fittings, and decrease the lifespan of pumps. Proportional valves that have ramp control in the amplifier card allow for gradual movement of the spool, which results in the flow gradually changing and dramatically reduced pressure transients within the system.

The application needs the use of a remote or programmable adjustment for pressure. Presses for hydraulics, testing equipment, and industrial forming equipment typically require different pressure profiles for various materials or parts. By using a proportional pressure relief valve, it is possible for the PLC or controller to move the relief setting point up or down during cycles without the need for manual intervention.

replacing multiple valves using one more efficient component. In certain circuits there is a single proportional control valve that could replace the combination of a directionally oriented valve, a metering flow control, as well as a meter-out flow control. A smaller number of components means fewer connections and less heat generated due to throttling losses, as well as simpler troubleshooting.

If proportional valves aren't the best option

It's important to be aware of what you don't need.

If your job is strictly switched on or off—such as a clamp that fully opens and then holds the clamp—a conventional solenoid valve will be simpler, cost-effective, more affordable, and sufficient. Proportional valves require an amplifier that has its own wiring as well as tuning and diagnostics. It's a waste of time and money for devices that don't utilize the proportional function.

They are more susceptible to contamination of fluids than traditional directional valves; however, they are less sensitive than servo-type valves. The majority of proportional valves need clean hydraulic fluid to be ISO 16/14/11 or greater. If the system has poor filters or operates in areas that are heavily contaminated and do not have sufficient maintenance discipline, the tighter spool clearances in a proportional valve can become an issue.

Then, think about your control system. Proportional valves require an input signal such as an output card from a PLC or a dedicated amplifier or the variable voltage signal of an appropriate controller. If the necessary infrastructure doesn't exist and the purpose of the application isn't enough to justify the need for it, then the economics aren't always working out.

The most important specifications to assess

When choosing an appropriate proportional valve, you must look beyond flow ratings and prices. Hysteresis—the variation in output between an increasing and decreasing signal directly impacts the repeatability of positioning. Lower hysteresis values are more reliable, especially in closed-loop applications. Response time (the time it takes for the spool's spool to go between 0 percent and 100 percent signal) is important in fast-cycle applications. Be sure to check the flow rating at specific pressures because the curves of manufacturers are usually based on a 35-bar (500 PSI) pressure differential. The actual operating conditions could change the flow numbers significantly.

Make sure to confirm that the electrical interface is compatible early. Valve coils designed for 24VDC, 12VDC, or mA inputs all need different amplifier configurations. Mixing them can be a common error in commissioning.

Proportional valves address a particular issue: the requirement for constant electronic control of variable flow of hydraulics in applications where switching off and on isn't enough. When the system really needs smooth movement, dynamic load responses or programmable flow and pressure management, these valves can reduce the number of components, enhance system performance, and allow capabilities that aren't possible using conventional valves. If your application doesn't require these features, make the circuit as simple as possible and make sure you have enough money for a more important thing. The ability to match the valve technology to the requirements of the system and not define the valve's position up or down is what distinguishes quality engineering of a hydraulic circuit from costly and speculative work.