Valve Flow Control Using Pulse Width Modulation


Problem:
How can a valve which is inherently bi-stable be cycled to control average flow?


Solution:
Pulse width modulation (PWM) can be used to control average flow by varying the valve duty cycle on some (usually) fixed base frequency to match the flow requirement.

By controlling the relative amount of time the valve is in the on state, one can vary the average amount of flow in proportion to the on time. The graph above shows a typical pulse width sequence for a valve that is decreasing normally open port flow over time, as in the graph below.

For example, if a valve can flow 3.0 liters per minute in the full on state, then the valve is cycled so that it is ON for 33% of the time, the valve will flow approximately 33% of the total flow capacity, or about 1 liter per minute. By controlling the pressure and system constraints and characterizing the dynamic performance, a very reliable means of controlling flow can be obtained. With the PWM method, flow can be easily controlled in a range exceeding 10:1, up to a possible 40:1. This ratio of maximum total flow to minimum total flow is sometimes called the "turn-down ratio" of a valve or system. For example, if flow can be controlled between 0.3 and 3.0 liters per minute, then the turn-down ratio is 10:1. More or less resolution can be obtained by altering pressure and frequency, or by changing valve dynamics.

Using pulse width modulation, a bi-stable solenoid valve can be used to very effectively control flow over a range exceeding 10:1.


The Lee Company can help solve your problem. Give us a call at 1 800 LEE PLUG, and let our Sales Engineers show you how.

AN5D 6/97

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