Stiction & Hysteresis
Stiction describes the valve's spool sticking when small changes are attempted. Friction of a sliding object is less than when it is stationary. Stiction can keep the spool from moving for small control input changes, and then the spool moves too far when the current changes enough to free it. The result of stiction is that the force required to get the spool to move is more than is required to go to the desired spool shift.
Hysteresis describes the spool's final position in response to a given
current depending on which direction it was moving. Friction of a sliding
object causes a reduction in distance moved. Residual magnetism in the spool
or pole piece will also cause hysteresis by resisting changes to the
magnetic field attempting to shift the spool. Hysteresis can cause the spool
shift to be much different for the same current depending on whether the
current is increasing or decreasing. Hysteresis forces resist the current's
attempt to move it, so the spool shift will be less than desired. The result
of hysteresis is that the direction the spool was shifting determines if the
spool ended up shifted too far or not far enough. Stiction and hysteresis
can make controlling the proportional valve seem erratic and unpredictable.