User guide

P/N: 021-00154, Rev. A.6 - updated for V4.7 Tools Page | 155
Appendix E Pulse Width Modulation (PWM) and Dither
HCT’s DVC products provide selectable PWM and Dither capabilities for each proportional valve in your system.
Our default settings will generally suffice for most applications but a user can specify different values if desired.
The DVC product’s control circuits and internal BIOS automatically handle the application of the PWM and
Dither control signals.
Current flowing through a valve’s coil creates a magnetic field. This field provides the force to move the valve’s
spool and thereby adjust the fluid flow in the valve. The voltage across the coil divided by the coil resistance is
equal to the coil current. This current is supplied by an external power supply, which generally is a battery. The
total circuit is made up of all the components from the power supply’s positive terminal to the power supply’s
negative terminal. The circuit’s accumulative resistance due to the connecting wire lengths, the coil, and
switches determine the actual current. Very easy so far, but proportional valves are only useful if the current
can be changed.
A potentiometer can be used to vary the resistance in series with the coil to set the coil current to the desired
value. Unfortunately this simple technique is very inefficient and not practical for higher currents. Adding
resistance to control the current prevents the use of the electronics that provide current regulation, dither (static
friction compensation), short circuit protection, current ramping, and dead band elimination. Pulse width
modulation (PWM) is an efficient technique for driving current through a valve’s coil that allows these features to
exist. PWM does not waste any significant power or generate unnecessary heat.
How PWM works
The coil current value is set by turning a low resistance switch on and off at the PWM frequency (FIG. 1).
SUPPLY
-
+
COIL
VALVE
POWER
DRIVER
VALVE
+POWER +COIL
-COIL
PWR COM
SWITCH
PWM
DIODE
BLOCK DIAGRAM
FIG. 1