What is Dither versus PWM?

Traditional solenoid driver electronics rely on linear current, which is the application of a constant voltage across a resistance to produce an output current that is directly proportional to the voltage.

Feedback can be used to achieve an output that exactly matches the control signal. However, this scheme dissipates a lot of power as heat, and it is therefore very ineffective. This heat can cause overheating the solenoid coil to a point where its operation becomes unstable and may even result in either malfunction or no function! Solenoid coils lose their force power when overheated.

Two alternative means to control solenoids on directional valves include Pulse Width Modulation (PWM) and Dither.

Pulse width modulation produces constant current through the coil. A PWM signal itself is not constant. Rather, the signal is on for part of the time, and off for the rest of the time in somewhat of a cyclic fashion. This pulsation rate can vary as compared to the input signal and produces a much more effective means to control proportional control valves.

Static friction, stiction, and hysteresis can cause the control of a hydraulic valve to be erratic and unpredictable. Stiction can prevent the valve spool from moving when given small input changes, and hysteresis can cause the shift to be different for different applications of the same input signal. In order to counteract the effects of stiction and hysteresis, small vibrations (cyclic frequency) around the desired position are created in the spool. This constantly breaks the static friction ensuring that the spool will move even with small input changes, and the effects of hysteresis are averaged out.

Dither is a small ripple frequency that is superimposed over the PWM signal to the solenoid current that causes the desired vibration and thereby increases the linearity of the valve and improves valve response.

Dither and PWM frequencies complement each other for improved spool control and are, in most amplifiers, adjustable independently. This allows the user to customize these signals to each individual application for optimum performance.