Dither Frequency
It is the number of repetitions per second of the oscillation superimposed on the drive current. It is typically selected in the 100-400 Hz range: slow enough for the spool to follow, fast enough that the cylinder and the load cannot filter it out. A correctly chosen frequency can bring hysteresis down from the order of ten percent to a few percent.
Choosing the frequency is a balancing act. At very low frequency the spool oscillation is transmitted all the way to the load; the bucket or boom shakes visibly. At very high frequency the inertia of the spool and the compressibility of the oil absorb the oscillation, dither becomes ineffective, and stiction returns.
- On valves with large, heavy spools the suitable frequency shifts toward the lower end of the range, and on small cartridge valves toward the upper end.
- Amplitude and frequency are set together: as frequency rises, a higher amplitude is usually needed to get the same effect.
- When the hydraulic oil is cold and thick, damping of the oscillation increases; a setting that is acceptable in winter may feel too shaky in summer.
The practical field method is this: increase the amplitude until the cylinder starts to shake visibly, then back off one step. Since the dither setting is fixed by the manufacturer on most mobile valves, if motion is not millimeter precise, oil cleanliness and valve condition should be examined first.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?