180
ifm
System Manual
ecomat
mobile
SmartController (CR2500) V05
PWM in the ecomatmobile controller
Hydraulic control in PWM
Figure: The profile of the PWM voltage U and the coil current I at 100 % switch-on time:
The effective coil current I
eff
is also 100 %
9.3.3
What is the dither?
If a proportional hydraulic valve is controlled, its piston does not move right away and at first not
proportional to the coil current. Due to this "slip stick effect" – a kind of "break-away torque" – the valve
needs a slightly higher current at first to generate the power it needs to move the piston from its off
position. The same also happens for each other change in the position of the valve piston. This effect
is reflected in a jerking movement, especially at very low manipulating speeds.
Technology solves this problem by having the valve piston move slightly back and forth (dither). The
piston is continuously vibrating and cannot "stick". Also a small change in position is now performed
without any delay, a "running start" so to speak.
Advantage: The hydraulic cylinder controlled in that way can be moved more sensitively.
Disadvantage: The valve becomes measurably hotter with dither than without because the valve coil is
now working continuously.
That means that the "golden means" has to be found.
When is a dither useful?
When the PWM output provides a pulse frequency that is small enough (standard value: up to 250 Hz)
so that the valve piston continuously moves at a minimum stroke, an additional dither is not required
(
→
next figure):
Figure: Balanced PWM signal; no dither required.
