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Old 11-28-2006, 05:59 PM
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Trevor Trevor is offline
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Pressure Control via Solenoid Valves.

Pressure Control via Solenoid Valves.

In view of the fact that incorrect information has been deliberately posted, it is as well that the facts are correctly recorded.

The Simple Solenoid Valve.

In the interests of simplicity and economy a simple on/off bistable solenoid valve is often used for the pressure/flow control of fluids. It will be appreciated that this form of valve has limited armature travel. When used in this form the component is sometimes referred to as a pulsoid.

The valve is in effect turned rapidly on and off at an adjustable rate, so that the through flow becomes controlled. The output is the sum of the on time, available from the on time verses off time ratio, sometimes expressed as percentage.

The on/off ratio is established by energising the solenoid operating the valve via an on/off signal and the procedure is known as pulse width modulation. (PWM).
It is usual to hold the frequency of the pulses constant while varying the percentage of on/off time. A frequency of around fifty cycles per second is often used.

It will be appreciated that the down stream flow will be in the form of an undulating stream. In many instances devices in the down stream pressure line will not have there operation affected due to inherent reciprocating weight/mass. In any event smoothing components are often incorporated.

Linear Solenoids.

Where the application is more critical a more complicated approach is necessary. A linear solenoid can be used to drive the valve and this provides the necessary degree of travel to enable intermediate positions to be established and held in order to vary valve opening and thus flow/pressure. But it must be appreciated that continual variables must be continually offset as an ongoing contingency.

The answer to this difficult problem utilised within process control equipment, is to provide a feedback loop, established via a feedback signal, whereby control can be balanced/adjusted even though variables exist. In its simplest form this can be a signal from a down stream sensor, but when this can not be arranged sensing can be achieved directly from the controlling device.

In respect of a linear solenoid the operating current is monitored as a means of securing a feedback signal. It will be appreciated that current will vary as result of the mechanical load, which is directly proportional to the magnetic field and the current through the coil. The circuitry involved naturally becomes complex and the construction of the valve must be taken into account when establishing set points and actual flow.

(Edit Addition) A feed back loop operates by feeding back any change which may have altered a desired set point. e.g. The output of the valve falls. There is a resulting feedback signal via the loop and the valve opening is increased to compensate. If the valve is opened excessively and the pressure is raised above the set point the reverse takes place. The process continues until equilibrium is established.

The process of adjustment as a result of feed back can be continuos and can occur at very high speed, i.e. at many cycles per second depending on the dynamics involved.

Proportional Solenoid Valves.

These purpose designed components, incorporate specific means towards directly providing a feedback signal. Often an auxiliary coil winding is incorporated. The most important feature of note, is extended armature travel so as to provide an increased ratio of movement and therefore degree of control.

Please note that there can be confusion in what should, or should not, be termed a proportional/proportioning solenoid valve, as a linear solenoid valve can perform this function.
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Last edited by Trevor; 11-28-2006 at 06:23 PM.
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