Pulse Width Modulation (PWM) is a DC supply voltage that is switched on and off at a given frequency for a modulated period of time (duty cycle). The duty cycle is the “on” time of the voltage and is expressed as a percentage of the time period. At 50% duty cycle the voltage is “on” for 50% of the time period and “off” for the remaining 50%. Therefore the time averaged voltage is only 50% of the maximum supply voltage and the current to the solenoid is only 50% of maximum current as well. It is this time averaging that allows PWM signals to be used for proportionally controlling solenoids.
PWM Solenoid Theory
Figure 1
Typical uses of a PWM controlled solenoid are:
- Proportional Hydraulic or Pneumatic Solenoid Valves (Digital Control)
- Proportional Current Control (Position Control)
Proportional Hydraulic or Pneumatic Solenoid Valves (Digital Control)
The principal behind a proportional solenoid valve is to proportionally control the flow (or pressure) from zero to a maximum flow as a function of the duty cycle. At zero duty cycle the solenoid does not move, the valve is not opened and therefore the flow is zero. At 50% duty cycle the solenoid moves through full stroke and opens the valve to full flow. Since the valve is only allowing full flow for 50% of the time, the time averaged flow in theory will be 50% of maximum flow. This type of control is called “digital” because the valve is fully open or fully closed, “on” or “off”.
Figure 2
The PWM frequency of a “digital” proportional solenoid valve is typically in the range of 25 to 200 Hz. These lower frequencies allow the solenoid to respond (full stroke) over the duty cycle range of control.
Proportional Current Control (Position Control)
With “digital” control the solenoid moves full stroke and is held at this position for a duration of time. With “position control” the solenoid moves from zero to full stroke as a function of the time averaged current applied. At 50% duty cycle the solenoid is supplied 50% of the current and in theory will move 50% of full stroke.
Current controlled solenoids can be used for linear position control and also in hydraulic and pneumatic solenoid valves.
The PWM frequency of a proportional current control solenoid valve is typically in the range of 200 to 1000 Hz. These higher frequencies produce the time averaged current but do not allow the solenoid to fully respond as in digital control.
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