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Frequently Asked Questions
How does a proportional valve differ from a solenoid valve?
It holds an intermediate position in response to a variable signal, rather than switching between two fixed states.
A conventional solenoid valve is a switch. Energise the coil and the element moves fully to one position; de-energise and it returns. There is no in-between - and if it lingers in between, something is wrong.
A proportional valve is designed to sit in between. Its actuator produces a force proportional to the input signal, balanced against a spring or against internal feedback, so the element takes a position corresponding to the command. A signal at half scale produces roughly half the output.
The output that is being controlled is either pressure - a proportional regulator - or flow and direction, in a proportional directional valve.
So the control system's relationship with the pneumatics changes from commanding events to commanding values, which is what allows force and speed to be controlled rather than merely switched on.
What can be achieved that on/off valves cannot?
Setting and varying force, tension and speed under control, rather than fixing them mechanically.
With on/off valves, a cylinder's force is whatever the supply pressure and the bore give, adjusted by a manual regulator and then left. Its speed is set by a flow restrictor, turned by hand at commissioning.
With proportional control, force becomes a variable the control system sets - so one station can clamp a delicate part gently and a robust one firmly, changing between products without any mechanical adjustment.
Web tension control becomes possible: as a reel's diameter changes through a run, the required torque changes continuously, and a proportional regulator adjusts a brake or a dancer accordingly.
Test rigs can apply programmed pressure profiles. Leak test benches can hold a precise pressure. Assembly presses can apply a controlled force and detect a fault by the force required.
All of it is closed-loop control rather than a mechanical setting.
What signal do they accept?
Standard analogue signals, with digital and network control increasingly common.
Most proportional valves accept a voltage or current input over a standard industrial range, generated by an analogue output card on the controller. The valve maps that range onto its output span, so the low end of the signal corresponds to minimum output and the top end to maximum.
Many also offer fieldbus or IO-Link control, which removes the analogue card and allows the setpoint to be sent as a number - along with diagnostic information coming back, and often the ability to change parameters such as ramp rates from the controller.
Some accept a pneumatic pilot signal instead, for use where electrical control is unavailable.
When specifying, confirm the signal type matches the controller's available outputs, and check whether the valve needs a separate supply voltage in addition to the signal - most do, and it is a common commissioning oversight.
How accurate and repeatable are they?
Good, and considerably better on closed-loop valves than on open-loop ones - which is the distinction worth checking.
An open-loop proportional valve positions its element according to the input signal, and the resulting output depends on that position plus whatever else affects it: supply pressure variation, temperature, friction and hysteresis in the mechanism.
A closed-loop valve measures its own output with an internal sensor and adjusts until it matches the command. That corrects for all of those disturbances, so the output tracks the setpoint far more closely and repeats better.
Manufacturers publish linearity, hysteresis and repeatability figures, and they differ substantially between the two types.
For a simple force adjustment, open-loop may be adequate and is cheaper. For tension control, test work or anything where the value matters rather than just being adjustable, closed-loop is the appropriate choice - and the specification should say which.
What air quality do they require?
Clean and dry, to a better standard than ordinary valves - because the internals are small and must move precisely rather than decisively.
A conventional solenoid valve drives its element hard from one end to the other, and a certain amount of contamination is simply pushed aside. A proportional valve holds a small element at a precise intermediate position against a fine balance of forces, and any particulate or moisture disturbs that.
The symptoms are drift, hysteresis, sticking and eventually failure to respond smoothly - all of which present as a control problem rather than an obvious valve fault.
So manufacturers specify a filtration standard, usually finer than the general plant requirement, and it should be provided locally to the valve rather than relying on the main air treatment.
Oil is a particular problem on piezo-actuated valves. Check whether lubricated air is acceptable - for many proportional valves it is not, and running them on lubricated air will ruin them.
Where are proportional valves typically used?
Force control, tension control, test and measurement, and anywhere a pneumatic value must be varied under control.
Assembly and press applications use them to apply a controlled force - and to detect faults, since the force required to complete an operation reveals a missing or wrong component.
Web handling uses them for tension control on unwind and rewind, adjusting a brake or dancer continuously as reel diameter changes.
Test benches, leak testing and burst testing use them to apply and hold precise pressures and to run programmed profiles.
Other applications include controlling clamping force on delicate product, balancing and load compensation, paint and coating pressure control, and the pressure setpoint on any process that must vary.
What they are not is a general replacement for on/off valves. Most pneumatic motions want to go from one end to the other as quickly as possible, and a switch does that better and far more cheaply.
What happens on loss of signal or power?
It depends on the valve, and it must be established rather than assumed - because a proportional valve's fail state is less obvious than a spring-return solenoid's.
Some proportional valves fail to zero output, venting the downstream pressure. Some hold their last position. Some fail to a defined value.
Which behaviour applies depends on the valve's construction and sometimes on how it is configured, and the consequence for the machine differs completely: a clamp that vents releases the workpiece, while one that holds keeps it gripped.
That has to be decided from the risk assessment, exactly as the choice between spring-return and double-solenoid valves does.
Where the fail state matters for safety, a proportional valve should not be relied on alone - a separate on/off safety valve or dump valve, of appropriate rating, is normally required to bring the system to a safe state independently of the proportional control.