A proportional valve, also referred to as an electro-proportional valve produces an output (direction, pressure, or flow) proportional to an electronic control input. They are often used in applications that require accurate position, velocity, or force and are well suited for both mobile and industrial applications. Proportional valves are beneficial for circuits that need to vary either flow or pressure to reduce lunge and shock.
In earlier days, there were only two types of electrically operated valves – solenoid valves and servo valves. Solenoid valves are also referred to as “bang-bang” solenoids. They are either actuated or unactuated or ON-OFF. The spool in a standard 2-position, solenoid-operated valve shifts all the way to its new position at high speed. Since they have no intermediate position, they facilitate very little control. This rapid, full shift can cause an actuator to jump or lunge on startup and produce excessive shock when stopping which may cause machine damage or adversely affect piping, causing leakage. Pressure spikes and noise are also common.
In contrast, servo valves are continuously controlled high-frequency response devices that receive commands through their electronic control systems to provide a high degree of control over position, velocity, and acceleration. They can accept and accurately respond to command signals at frequencies exceeding 100 Hz. Their continuous feedback from electronic transducers ensure high accuracy.
There are three main types of valve controllers.
- Electric controls
Proportional valves are positioned between the two extremes of solenoid and servo valves. They are similar to low-cost, low-performance servo valves and are operated by proportional solenoids whereas servo valves are operated by servo motors. Also, proportional control valves only work with electronic controllers which may be located "on board" (built into the valve) or may be located external to the valve such as:
- in a control cabinet
- mounted to a machine frame
- mounted to the falve by various fixing devices
- integrated into wired or wireless machine controls
Simple proportional valves can be purchased for not much more than a lever valve, less than $100. A combination of proportional valve and controller can be had for a few hundred dollars, which although more than a lever valve, still provides variable flow control. Worth noting however, inexpensive elector-proportional valve and controller systems are not made for high performance systems. Their coil strength is often lower, have higher hysteresis, and have a lower frequency response. They are also susceptible to performance variances due to changes in inlet pressure and actual pressure drop. However, they serve their purpose based on performance, control, and cost.
Directional control valves are the most common type of proportional control valve. They are available in a variety of accuracy and feedback options and are more tolerant of contamination. They also cost less than servo valves. When a circuit does not require extreme accuracy or flow repeatability, the cost-savings, plus a less-expensive filtration requirement make this valve a good choice.
Other considerations – Proportional valves often work best with a pressure-compensated pump in a closed-center circuit. An accumulator in the circuit enhances cycle response time and protects the pump from pressure spikes. Systems that use proportional valves usually require a heat exchanger because energy waste is higher with this type of circuit.Higher end valves have benefits like on-board electronics or spool position feedback.
In conclusion, depending on the application, they may be considered money well spent.