Pulse Width Modulation (PWM)
This is a method of driving current through a valve's coil or motor's windings, typically used in current or process regulation.
Current flowing through the valve coil creates a magnetic field which
provides the force to shift the spool. The voltage across the coil divided
by the coil resistance is equal to the coil current. Very easy so far, but
proportional valves are only useful if the current can be changed. A
rheostat can add more resistance in series with the coil to set the current
to the desired value. Unfortunately, this simple technique is very
inefficient and not practical for higher currents. The heat generated by
adding resistance to control current prevents the use of the electronics
that provide other useful features.
Pulse width modulation (PWM) is an efficient technique for driving current
through a valve coil that allows these features to exist at higher coil
currents. An over simplified explanation is that the coil is turned on and
off so quickly that the spool can not follow the current command to turn on
or off. The spool tries to follow the current being turned on and off by
averaging the on time vs. the off time. The PWM% is the on time divided by
the on time plus the off time. The PWM% is proportional to the spool shift.
The repetition frequency of turning on and off is constant. The PWM driver
is fully on (very little resistance and heat) or fully off (little heat),
thus the process is quite efficient. Coil inductance usually plays a large
part in averaging the on and off to a steadier current through the coil.
Note that turning on and off large currents will cause noise in the system,
so be careful to keep sensitive signals away from wires with PWM on them,
BOTH leads of the coil and BOTH leads of the power supply to the unit.