What is dither and why does it reduce hysteresis in a proportional valve?
Dither is a small-amplitude, high-frequency oscillation deliberately superimposed on the proportional solenoid current. It is typically applied in the 100-400 Hz range and keeps the spool in constant micro-motion, breaking static friction (stiction). This can bring valve hysteresis down from around 10% to below 3%.
The spool slides on a thin oil film inside the body bore. When it is stationary, static friction is greater than dynamic friction; a small current change is not enough to move the spool. The result is hysteresis: when you approach the same current value from below and from above, you get two different flow rates.
Dither solves this problem by never letting the spool fully stop. The constant small oscillation ensures the system is subject to dynamic rather than static friction; so the spool responds the moment the command changes.
The tuning balance matters:
- Frequency too high: The spool mass and the oil film damp the oscillation, and dither becomes ineffective.
- Frequency too low / amplitude too large: The vibration transfers to the actuator, producing visible juddering and noise at the cylinder.
Do not confuse dither with PWM frequency: PWM is there to produce current and is user-adjustable in most controllers; dither, on the other hand, is specific to the mechanics of the valve and is generally fixed by the manufacturer.
Full coverage of this topic: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?