High Country Tek emc-3L - Manual (Page 100)

switch is on compared to the time the switch is off (FIG. 4). The switch would be always open and no coil
current flows at a 0 % duty cycle. The switch is always closed and maximum current flows at 100 % duty cycle.
COIL
CURRENT
SWITCH
ON
OFF
0 A
FULL
TIME
COIL
SUPPLY
-0.5 V
0 V
VOLTS
FIG. 4
Stiction (static friction or friction when the valve is at rest compared to the lower friction when the valve is
moving) and hysterisis can make controlling valves seem erratic and unpredictable.
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 control input changes enough to free it. The
force required to get the spool to move is generally more than is required to go to the desired spool shift.
Friction of a sliding object causes a reduction in distance moved. Hysterisis can cause the spool shift to be
much different for the same control input depending on whether the control is changing up or down. The friction
of the moving spool is resisting the current's attempt to move it, so the spool shift will be less than desired. The
direction the spool was shifting determines if the spool ended up shifted too far or not far enough.
Dither is a rapid, small movement of the spool about the desired shift point. It is intended to keep the spool
moving to avoid stiction and to average out hysterisis. Dither must be large and slow enough to make the spool
move and small and fast enough not to cause pulsing or resonance in the system due to fluid flow variations.
These requirements can conflict. The goal is to provide just enough dither to fix the problems without creating
new ones.
Dither is caused by coil current changes (“ripples”) at some frequency and amplitude about the commanded
average value. The spool will not follow high frequency ripples as well as low frequency ripples due to inertia.
The amplitude of the ripples determines how far, and if, the spool will move at a given frequency.
Low frequency PWM
Low frequency PWM, typically less than 300 Hz, generates dither as a byproduct of the PWM process (FIG. 6).
This is a violation of the earlier assumption that the changes in current will be fast and small enough not to be
noticed by the spool.
P/N: 021-00163, Rev. A.0 - for V5.2 Tools Page | 100