What is the difference in flow characteristic between a proportional hydraulic valve and an on/off solenoid valve?
An on/off solenoid valve produces only two flow points: 0% with the coil de-energized and 100% when energized and the spool goes to full stroke. In a proportional valve the spool position varies continuously with coil current, so any intermediate flow between 0% and 100% can be held stably. The result is graduated speed and soft stops on the same cylinder.
An on/off (bang-bang) valve works like a light switch: when the coil is energized 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; speed can only be changed via pump speed or a throttle valve.
In a proportional valve, 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 opened. That is why the characteristic curve is a sloped ramp, not a step.
The differences in practice:
- Speed control: on/off gives a single speed; proportional allows continuous speed adjustment across the joystick travel.
- Shock load: the sudden opening of an on/off valve produces hydraulic shock and load swing; a proportional valve suppresses this with a ramp.
- Positioning: millimetric approach with on/off can only be attempted with a series of short pulses; with proportional it is done directly at a low flow rate.
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.
Full coverage of this topic: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?