Dither Current (Dither)
It is a small amplitude oscillation deliberately superimposed on the drive current of a proportional valve solenoid. By keeping the spool in constant microscopic motion it breaks static friction (stiction); its amplitude is typically a few percent of nominal current, and when set correctly it noticeably lowers hysteresis.
A stationary spool tends to stick inside the body because of the oil film and contamination. When the command increases by a small amount the solenoid force cannot overcome this stiction and the spool does not move at all; then, once the threshold is passed, it jumps all at once. Operators describe this as "the lever does not move by millimeters, it skips." Dither keeps the spool in a very small vibration that never fully stops, reducing friction from static to kinetic.
- If the amplitude is too small, stiction is not broken and the benefit is lost.
- If the amplitude is too large, cylinder motion visibly shakes and wear in the valve body increases.
- Amplitude is usually fixed by the manufacturer according to valve type; it is not exposed for user adjustment on every card.
Dither and PWM are not the same thing: PWM exists to switch power efficiently, while dither is a disturbance deliberately added to solve a mechanical problem. On some drivers the ripple of the PWM carrier itself takes over the dither function.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?