What is the difference in flow characteristic between a proportional hydraulic valve and an on/off solenoid valve?
An on/off valve gives only two flow points; in a proportional valve the spool moves continuously with coil current, so any intermediate flow holds.
An on/off solenoid valve produces only two flow points: 0% with the coil de-energised, and 100% when it is energised and the spool goes to full stroke. In a proportional valve the spool position changes continuously with the coil current, so every intermediate flow between 0% and 100% can be held steadily. In one sentence, the difference is this: the characteristic of one is a step, that of the other a sloped ramp.
Why the characteristic curve comes out differently
An on/off (bang-bang) valve works like a light switch: when the coil is energised the spool overcomes the spring force and goes to full stroke, and whatever flow is in the circuit at that moment goes to the actuator. There is no intermediate position; the speed can only be changed through the pump speed or through a throttle valve added to the circuit.
In a proportional valve, on the other hand, the solenoid produces a force proportional to current, and that force is balanced against the spring force. The spool settles at the intermediate position set by the current; flow passes through the metering area that is opened. The relationship between coil current and flow arises from this balance, while the relation between spool stroke and flow is set by the metering geometry of the valve. At the start of the curve there is also a deadband: until the spool overlap is passed, no flow begins even though the current rises. If the driver electronics do not compensate for this region, the first part of the joystick travel is wasted.
What changes in the field
| Subject | On/off solenoid valve | Proportional valve |
|---|---|---|
| Speed control | A single speed; the spool is either closed or fully open | Stepless across the joystick travel |
| Starting and stopping | Sudden opening, hydraulic shock and load swing | Soft entry and exit with a ramp |
| Positioning | Approaching with a series of short pulses | Approaching directly at a low flow rate |
| Driver side | A relay or a simple switch is enough | A current-controlled driver is required |
Proportional is not needed for every job
Proportional valves were developed to fill the gap between the expensive servo valve and the simple on/off valve: they are not as precise as a servo valve, but they are far more rugged and economical in the field. Against that, if the movement has only two end positions, that is if a door is to be either fully open or fully closed, or a lock is to engage and disengage, a proportional valve means extra cost and extra setting up; it brings no gain. If the load is light and a sudden stop causes no trouble, the on/off solution is enough.
Replacing a manual valve with a proportional one on an existing machine is not always the right route either. It requires draining the line, removing the valve and commissioning the amplifier. Retrofit kits that drive the manual lever from outside need none of these steps; what is more, they do not change the working pressure or the flow of the system.
What to do in practice
First define the movement: is it a function that travels between two end points, or an approach that has to be held at intermediate speeds? If it is the second, proportional is needed; for the first, on/off is enough. If you are moving to proportional, plan from the outset to compensate for the deadband and the hysteresis of the valve on the driver side; if these are not set, the control lever works with a dead start and with a delay on direction change. If you want to keep the existing manual valve, a solution that drives the lever from outside gives most of the graduated behaviour without going into the hydraulics at all.
The full topic: Why should it be commanded by an electronic signal?