Proportional Valve Hysteresis
Getting a different flow rate at the same drive current depending on whether you arrived at that value by increasing or decreasing the current. It arises from friction and magnetic remanence; in a valve without dither it can reach the order of ten percent of full scale, while with suitable dither it drops to a few percent.
Hysteresis is like the valve "remembering where it came from." The movement speed being different when you slowly open the lever and close it back to the same point is a direct symptom. It has two basic sources: static friction between the spool and the body, and magnetic remanence in the solenoid's iron circuit.
- Measurement is done by slowly raising the current from zero to peak and back down; the largest difference between the two curves is given as a percentage of full scale.
- Dither largely erases the friction-related portion of hysteresis.
- In closed loop valves with spool position feedback, hysteresis typically drops below the one percent level, but valve cost increases.
The practical consequence is this: in a system with high hysteresis you cannot set the load exactly where you want it in one move, you make two or three corrections. On jobs where load placement must be millimeter-precise, asking for this figure when selecting a valve is more decisive than nominal flow rate.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?