Solenoid Pilot
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These are devices that are used to operate larger compressed air valves.
Earlier, we talked about Direct Acting Solenoid Actuators. In those pages we discussed how it's impractical to use a direct acting solenoid to actuate larger compressed air valves due to the relative low valve-mechanism shifting force of a direct acting coil. With a small distance (10th of an inch or so) a direct acting coil generates sufficient force.
If longer distances are required, as in the internal air-control mechanisms of larger air valves, the direct acting coil hasn't enough power.
How do we use an electrical coil to shift the internal mechanism of larger valves?
That's where the solenoid pilot valve actuator comes in.
We can accept that for small valves with low shifting distances the direct acting solenoid concept will work, and it does. With reasonable quality compressed air, direct acting solenoids (and the small valves they actuate) can be relied on for millions of cycles with little or no problem. They are the right product for the right job.
What is it that the small, direct acting solenoid valve controls? Compressed air, of course.
And why do we use compressed air to do work? As compressed air is able to exert significant force, more than just a solenoid coil can.
What do we need to shift the internal workings of larger air valves? Enough force from the valve actuator to ensure that the valve poppet or spool will actually shift and control the flow of the compressed air when it's asked to do so.
Then, why not use a small, relatively (for it's size) powerful and reliable small direct acting solenoid valve to solenoid “pilot operate” a larger valve?
And of course, the industry has.
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In the drawing above, we show a small, direct acting solenoid valve, installed on the end of a much larger spool valve, item #2.
Item #3 shows the supply line of compressed air to that small, direct acting solenoid valve. Note that it's taken internally from the main valve's compressed air supply (item #2).
In this drawing, the small direct acting valve is de-energized, and as a result, the main big valve is at rest. The spool is to the left, and the internal main valve actuator, the spring, is extended.
I chose not to try to show the flow paths in the main valve, or those in the small direct acting valve for simplicity.
Inside the larger air valve will be a spool or a poppet. The end of that spool or poppet will be as big as the valve body will allow or as large as the design engineer decided to make it.
Remember; "Force=Pressure x Area".
Compressed air force is measured in PSI.
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In the drawing just above, item #2, the direct acting solenoid pilot valve, has now been energized.
The pilot air (item #3) which had been flowing to the direct acting solenoid pilot valve is now flowing through it (item #1) and filling the area in front of the bigger valve spool.
The pilot air is pushing on the end of the spool (picture a piston in an air cylinder), and since the air is trapped there, it exerts force on the end of the spool. If that force is great enough, the spool will be shifted over against the internal spring, and the flow paths through the main valve will change.
Item #4 shows that in this drawing the air is now flowing out the other cylinder port to the application, and the other port is now flowing to exhaust.
Figured it out yet? What we have here is a miniature air cylinder inside the main valve.
If the end of the poppet or spool inside the big valve has a surface area of ˝ a square inch for example, and the air pressure flowing through the lines is 100 PSI, you can see that the compressed air pilot signal will be able to generate 50 lbs. of force on the end of that poppet or spool. This is huge force to shift a valve.
In a "nutshell" then, the design of the solenoid air piloted valve will be such that when the solenoid gets a signal, it will magnetize the coil, the coil will cause a pole piece (attached to the internal air control mechanism of the small valve) to shift, air will flow through the internal air paths of the small direct acting solenoid valve that is installed on the bigger valve, and compressed air will be used to shift the bigger valve spool.
Neat! And very effective.
Benefits
A small amount of compressed air bled off the main supply line of the large power valve can shift massive valves containing solenoids with low electrical demand (low wattage) and small footprint coils.The larger the spool inside the main valve, the larger the "piston" surface area can be, and the greater the shifting force in that valve.
The greater the shifting force available from the solenoid pilot, the larger the spool or poppet return spring can be, giving this style of valve huge spool and poppet shifting forces in both directions.
Problems
This type of valve will usually have a MOP (Minimum Operating Pressure) rated in the 20-25 PSI range.Therefore, unless you can bring a supply line with higher pressure to the direct acting solenoid pilot, this type of valve will not be able to control low pressure or vacuum.
Most solenoid air piloted valves have a small port on the direct acting solenoid so that higher compressed air pressure can be brought to the valve, so that it will control low pressure and vacuum.
Here's more info on compressors in general solenoid pilot.
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