Proportional Solenoid
A solenoid type that produces a push force proportional to the current through it, so it can hold the valve spool not only at the end positions but also at intermediate positions. An on/off solenoid knows two positions; on a proportional solenoid a drive current typically in the 0–1.5 A range sets the flow rate steplessly.
A classic solenoid is a magnet whose force changes rapidly over the pull distance; that is why it either pulls fully or not at all. In a proportional solenoid the magnetic circuit is specially shaped so that over a certain stroke range the force stays largely independent of position and depends only on current. This push, balanced against the counterforce of the spool spring, produces a different spool position for every value of current.
- Force is proportional to current, not voltage. That is why the driver card regulates current, not voltage.
- Usable stroke is typically on the order of 1–3 mm; over this short travel the spool carries the flow rate from zero to full opening.
- As solenoid temperature rises so does resistance; without current-controlled drive, flow rate drifts over the day at the same command.
AXI's HCT400 and HCT402 retrofit kits act not on the solenoid but on the vehicle's own manual lever. Even so, this distinction matters when troubleshooting: if lever motion is correct but the machine does not respond, the problem is not in the kit but on the vehicle's own valve and solenoid side.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?