Proportional Valve
A hydraulic valve that adjusts flow steplessly with the size of the control signal; unlike an on/off valve it ramps the load up and stops it where you want.
A proportional valve is a hydraulic valve that can adjust flow steplessly according to the magnitude of the control signal. In an on/off valve the spool is either closed or fully open; in a proportional valve, as the signal grows the opening grows, and with the opening the flow grows too. What it amounts to in the field is this: the load accelerates in a ramped way, decelerates in a ramped way and can be stopped at the point you want.
Difference from an on/off valve
| Criterion | On/off valve | Proportional valve |
|---|---|---|
| Spool position | Closed or fully open | Intermediate positions proportional to the signal |
| Start of the movement | Abrupt | Ramped |
| Way of driving it | Voltage is applied to the coil | Driven by regulating the coil current |
| Placing the load | With intermittent corrections | In a single move, the need to correct falls |
| What the circuit requires | A relay or a switch is enough | A driver card and adjustment are required |
The difference is not only about comfort. On an on/off valve the load swings every time the movement starts and stops; a suspended load sways, hoses are strained and the operator makes extra moves to correct it. Because the proportional valve finishes the same job in a single movement, the shock loading both the machine and the load is reduced.
How it is driven
A proportional coil is operated not with voltage but with current-controlled drive, because the coil resistance changes as it heats up and at constant voltage the current, and therefore the force, drifts. The circuit that does this job is called a proportional amplifier: it takes the control signal, produces the target current and holds the current steady by measuring it. The small-amplitude dither superimposed on the drive current keeps the spool in continuous micro-movement and so prevents it from sticking.
What separates two proportional valves from each other
Two valves said to do the same job are often not in the same class. The difference between them is determined less by the nominal flow in the catalogue than by these headings:
- Deadband: the region at the centre where the flow still does not come out while the signal increases. Its width depends on the type of spool overlap; the detail is in the proportional valve deadband entry.
- Hysteresis: obtaining a different flow at the same current when it is approached from above and from below. Hysteresis determines whether or not you can settle the load in a single move.
- Linearity: whether the same amount of increase produces the same amount of flow change in every region of the signal.
- Pressure compensation: whether the same flow is maintained at the same signal when the load changes.
- Feedback: the closed-loop structure in which the spool position is measured and corrected increases repeatability.
When making a price comparison, deciding which of these headings your job really needs is more decisive than putting two products side by side and looking only at the labels.
Not every job needs a proportional valve
If all that is wanted is for the movement to start and to stop, if the load is light and no positioning precision is sought, an on/off arrangement is both simpler and asks for less adjustment. A proportional valve brings a driver card and adjustment work with it. On an existing machine where the control lever is already driven by hand, converting it to electronic control without removing the lever is often a route with less intervention than changing the valve.
What to do in practice
When choosing a valve, first put in writing how precise the movement has to be; then ask the manufacturer about the deadband, hysteresis and pressure compensation headings one by one. During commissioning do not neglect the deadband compensation and the dither setting: a good valve, driven wrongly, does not behave any better than an on/off valve.
The context in which this term is used: Converting a tractor hydraulic lever to electronic control