A solenoid actuator is one of a host of different ways to operate compressed air valves. Other valve actuators include manual levers, push buttons, palm buttons, roller cams and whisker switches, to name just a few. Here’s a page that focuses on the solenoid actuator, just the information you need when specifying an air valve for an application.

When I write about the various methods to actuate an air valve I use the term actuator to refer to the electrical or mechanical device that shifts the internal workings of an air valve. I use the term operator to refer to the person that moves the lever or pushes the button to actuate the valve. Here is some background information on air valves on this site if you are interested.

Two Kinds of Solenoid Valve Actuators

There are two kinds of solenoid actuators for air valves. The Direct Acting Solenoid actuator is discussed first.

The other is the Air Pilot Solenoid actuator which is covered at the bottom of this page.


Direct Acting Solenoid Valve Actuator

Solenoid Definition

Here is a quick definition of a solenoid. “In compressed air valves, a solenoid is a device that, when energized with electricity, creates a magnetic field that generates motion.”

Motion in what? The item the solenoid magnetic field moves is a tiny valve to control air to an actuator on an air valve.

More information follows as we first talk about what a direct acting solenoid is.

Direct Acting Solenoid Actuator

On the valve in the image is a coil, containing item 3, which is explained below. This coil operates the valve via creating magnetic force.

solenoid actuator - 2/2 Air Valve Schematic

In the drawing above, item #1 is the coil, and inside the black exterior are the coil windings are depicted by the gold color.

Item #2 depicts the compressed air supply to this small 2/2 normally closed valve. The 2/2 refers to a two position, two ported valve.

Note how the compressed air enters the valve through the supply port,and is blocked from flowing through the valve by a poppet (purple color). The poppet is sitting on an “O” ring or some sort of rubbery surface, thus sealing the incoming air stream from flowing through the valve and out the other port.

The coil on this valve drawing is de-energized, meaning that electricity is not flowing to it, and the internal valve actuator, the spring (located on the right in the drawing) is in control at this time, shutting off the air flow.

The solenoid actuator, when it is energized with electricity, creates a magnetic field.

That electrical field will move the pole piece (item #3 – brown color in the image above) inside the coil housing.  The magnetic field will move the pole piece to the right.

Since the pole piece is attached to the poppet inside the valve, when the coil is energized and the pole piece is moved by the magnetic field, the poppet will move as well.

Item #4 (image above) depicts the outer valve body. The arrow is pointing towards the internal valve actuator, the coil spring.

Operation of this 2/2 air valve

solenoid actuator - 2/2 Air Valve Schematic

When the solenoid is energized the coil creates a magnetic field pulling the pole piece to the right as shown in the drawing just above.

The poppet, connected directly to the pole piece, also shifts (#3), moving off one seat and onto another, now allowing the compressed air to flow through the valve.

Item #2 points toward the inner valve actuator, the coil spring. It has now been compressed by the force generated by the coil and the movement of the pole piece and the poppet to the right. As long as the coil has power too it, this spring will stay compressed, the air path will be open, and air will flow through the valve.

When this direct acting solenoid valve is de-energized, the return spring shifts the poppet and the pole piece the other way inside the valve, and the compressed air is, once again, blocked.

Solenoid Actuators Have Limited Capacity

A solenoid actuator is a pretty neat device. Maybe it makes you wonder, if all you need is a solenoid and electricity to make things move, why compressed air circuit designers even use air actuators at all if a solenoid can move things with just electricity. Why not just use solenoids to do work instead of air cylinders?

While a solenoid coil can generate sufficient force to move a small, very light object over a very short short distance, the movement distance and the weight of the item being moved is limited by the relatively weak force generated by an electric coil.

It would take a huge electric coil to move any meaningfully sized device over a distance greater than about 1/10th to 1/8th of an inch. In order to have a coil actuator that would do larger physical work, like moving some heavy tooling 18″ for example, coil would be massive.

The coil size and the cost of the coil to move that large object that distance can be replaced by a relatively small air cylinder using the power of compressed air to easily do so.

Smaller air valves are OK for direct acting solenoids, larger valves, not so much!

If it is true, and it is, that compressed air valves that are actuated by a direct acting solenoid must be small, limited in size because solenoids cannot generate large force over a long distance, what can we use to accomplish shifting larger air valves electrically?

We use a small air valve, with a small solenoid, and use the air through that small solenoid to provide the shifting forces needed for a larger air valve.

A small valve ensures that the poppet or spool that controls the air flow inside that valve is even smaller yet.

Being a small valve attached to a larger valve, it is light weight, with even smaller internal components. Small valves poppets or spools only have to move a limited distance to open or close the flow path inside themselves, meaning a quite high force-to-shift ratio for reliable operation of the mini-solenoid valve opeators.

We understand that direct Acting Solenoid Actuators have limits!

Direct acting solenoid valves are relatively inexpensive to manufacture which is why air valve engineers are always trying to maximize flow through the small, direct acting valve.

They keep trying to increase the air flow capacity throughput without enlarging the body of the valve. If the valve body is small, the coil only has to move the pole piece a very short distance and can generate maximum force over that short distance for greater reliability.

Why Direct Acting Solenoid valves stick

Water and crud that comes down the air line can deposit crud inside the valve body.

Compressed air is pretty much filthy. It has water. It has dust. It has pipe scale. Lots of “stuff” to get inside the air valve.

Then, between operations, the valve sits idle. Shut down for an hour or two, overnight, or for days.

What happens is that the gunk deposited inside the valve from the air flow dries out and essentially glues the moving parts of the valve together. When the valve is signaled to fire again, the force generated by the small coil cannot overcome the stickiness inside the valve body and bingo, you have a stuck valve.

Anybody have a hammer to smack the valve with to unstuck it?

There are limitation and benefits of Direct Acting Valves

Some positives include the fact that direct acting solenoid valves are the first choice when low pressure (under 25 PSI) air pressure is being controlled, or the valve is controlling a vacuum. Since there is limited, or no, air pressure available to help shift a valve poppet or spool in the main valve, either the valves controlling low pressure or vacuum are small enough to be operated with a direct acting solenoid valve, or they must be operated via other means.

Direct acting solenoid valves can be very, very small, with extremely low electrical demands. This is ideal for circuits that need the solenoid valves powered right from a PLC (Programmable Logic Controller) or, perhaps with the min-valves installed right onto a circuit board.

Direct acting solenoid valves operate very quickly. I recall one time I was giving a demonstration to a group of maintenance engineers on the relative merits of a particular brand of air valve. I used an oscilloscope that could generate a wave on the screen to demonstrate that the little direct acting valve I was demonstrating was cycling at over 200 times per second.

That is over 200 hertz! Opening, allowing air to flow, and closing, shutting off the compressed air, more than 200 times every second. That sort of response time is limited to valves operated with the lightning like response of electricity.

Some negatives to directing acting solenoid valves include the fact that direct acting solenoid valves have, by nature of how they operate, to be quite small. That means flow paths are small too with relatively limited air flow.

If the compressed air supply is full of water, water vapor, compressor oils, rust and scale from black pipe, then small direct acting solenoid valves may tend stick more often than their larger, more powerful, cousins.

If the application calls out for a physically large valve with higher compressed air flow, and the large air valve cannot be operated with a direct acting solenoid coil, what happens when you need to use a solenoid actuator to operate a bigger valve?

That brings us to another design of air valve. It is the…


Solenoid Pilot Air Valve

A Solenoid Pilot air valve uses compressed air to shift the spool or poppet of a large air valve.

They use a small, relatively (for its size) powerful and reliable small direct acting solenoid valve to solenoid pilot-operate a larger valve?

In the drawing just above I show a small, direct acting solenoid valve (item #2)  installed on the end of a much larger spool valve. The small valve provides the energy source to shift the spool in the big valve.

Item #3 shows the supply line of compressed air to that small, direct acting solenoid valve. The air supply to the small air valve is siphoned internally from the compressed air supply to the main valve. The small direct acting pilot valve is drawn as de-energized. As a result, the main, larger air valve is in it’s resting position. The spool is to the left, and the internal main valve actuator, the spring, is extended.

For simplicity, I do not show the flow paths in the main valve, or those in the small direct acting valve.

Inside the larger air valve will be a spool or a poppet, depicted as a black rectangle is the drawing above. The end of that spool or poppet will be as big as the valve body will allow since when we are talking about power to shift a main, big valve, Force = Pressure x Area.

With compressed air force measured in PSI.

Compressed Air Valve

In the drawing 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 valve spool, the black rectangle in the drawing.

This pilot air is pushing on the end of the spool . This process is similar to what happens to the piston inside an air cylinder. Since the air is trapped at the end of the spool, it builds pressure and almost instantly exerts force on the end of the spool. When that force becomes great enough, the spool will be shifted over against the internal spring at the other end of the spool, and the flow paths through the main valve will shift. This all happens in a fraction of a second.

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.

Solenoid Pilot is A Miniature Air Cylinder

What we have here is essentially a miniature air cylinder inside the main valve body.

If the end of the poppet or spool inside the larger valve body has a surface area of 1/2 a square inch for example, and the air pressure flowing through the lines is 100 PSI, the compressed air pilot signal will be able to generate 50 lbs. of pushing force on the end of that poppet or spool. This is a huge force available to shift a valve poppet or spool.

Solenoid Pilot Operation

The solenoid air piloted valve will typically operate as follows.

  • the solenoid gets an electrical signal
  • electricity will magnetize the coil inside the solenoid
  • the magnetized 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
  • and compressed air flow into and will be used to shift the bigger valve spool
  • when the air is exhausted, typically an internal spring will shift the poppet or spool back

Very neat, and very effective!

Benefits Of Solenoid Piloted Air Valves

A small amount of compressed air bled off the main supply line of the large power valve can shift massive valves using solenoids with low electrical demand (and low wattage) and use very small sized coils.

The larger the spool inside the main valve, the larger the spool or poppet end-surface area can be, and the greater the shifting force in that valve when air flows into the spool / poppet area from the solenoid pilot valve.

The greater the shifting force available from the solenoid pilot, the larger the spool or poppet return spring can be, giving designers the ability to select higher-flowing air valves, with the expectation of a high level of reliability of air valve operation.

Issues With Solenoid Pilot Valves

The solenoid pilot operated air valve will typically have a minimum operating pressure. How does a solenoid pilot operated valve control vacuum then, since there is no air supply.

To use a solenoid pilot actuator on a valve to control vacuum, an air supply must be plumbed to the solenoid actuator.

Typically, solenoid air piloted valves have a small port on the direct acting solenoid valve so that higher compressed air pressure can be brought to the valve, to provide the shifting force, while the larger valve is controlling the flow of low pressure compressed air or vacuum.


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