How does proportional solenoid coil current determine hydraulic flow rate?
Magnetic force follows coil current; balanced by the spring the spool settles and flow passes through the metering area opened. Control is by current.
In a proportional solenoid the magnetic force produced is proportional to the coil current. When this force is balanced against the spring force opposing it, the spool settles at a definite position and flow passes through the metering area that is opened. In other words, the quantity being controlled is not voltage but current; and flow is a direct function of the spool stroke.
How the chain works
The order is this: current → magnetic force → spool position → metering area → flow rate. Each link has some non-linear behaviour, which is why the current-flow curve is not a perfectly straight line. That is also what separates a proportional coil from an on/off solenoid: an on/off coil either pulls in or does not, while a proportional coil is designed to produce a steady force at intermediate positions.
The proportionality the operator feels at the joystick is the overall behaviour of this chain. If one of the links has backlash, friction or saturation, moving the joystick more slowly will not cure it; you have to find which link is spoiling it. That is why, when hunting a fault, going through the chain link by link from one end to the other gives a result faster than simply replacing the valve.
Why current is controlled rather than voltage
The reason is temperature. As the coil heats up, copper resistance rises; if you apply a constant voltage the current drops and the flow slowly falls at the same joystick position. If a machine runs correctly in the morning and slows down in the middle of the shift, this is the first place to look. A current-regulated proportional amplifier removes this drift: it measures the actual current, compares it with the target and continuously corrects the PWM duty cycle. So even if the coil heats up, the spool stays at the same position.
The three regions of the curve
| Region | What happens | What is done |
|---|---|---|
| Lower end (deadband) | Below the minimum current the spool stays within the overlap region and no flow passes at all | Minimum current setting in the controller |
| Middle region | Current and flow change together in a usable way | The joystick curve works here |
| Upper end (saturation) | The spool runs to full stroke; increasing the current does not increase the flow | Limit the maximum current so the coil does not heat up for nothing |
The gap at the lower end is closed with deadband compensation: as soon as the joystick leaves zero the controller takes the current straight to the minimum threshold and does not make the operator travel the empty distance in between. The hysteresis that comes from friction — the spool settling at a different position for the same current depending on the direction it came from — is reduced with a small-amplitude dither current.
Current alone does not determine flow
The flow that passes also depends on the pressure differential across the valve. At the same current, if the load rises and the pressure differential changes, the flow changes; if a bucket moves fast when empty and slowly when loaded, this is usually the reason. That is why pressure compensated proportional valves are used in precision applications: the compensator holds the pressure differential across the valve constant, so the flow becomes independent of the load.
The coil itself is a limit too. A coil left at a high current continuously heats up, its resistance rises and it narrows the driver's regulation margin; if the supply voltage is low, beyond a certain point the driver cannot reach the target current at all. Choosing the driver to suit the coil data of the valve is therefore not a detail that only matters on paper.
Verify in order in the field
First confirm that the driver is current-regulated; if it is not, you cannot correct the drift that comes with heating by adjustment. Then set the minimum and the maximum current according to the valve's own catalogue and confirm that dither is switched on. If these three items are in place, the joystick response does not drift as the oil warms up. Any remaining irregularity is most likely in the hydraulics; measure the pressure differential before you change the valve.
If you are making a budget comparison, assess the valve not on its own but together with the amplifier that will drive it. A driver without current regulation spoils the behaviour of a good valve as well; in that case the precision you paid for does not come back in the field.
The full topic: Why should it be commanded by an electronic signal?