It can be argued that a proportional valve is any valve that can vary its spool or poppet position infinitely between discrete positions, (i.e. anywhere between fully open and fully closed).
For example, if you apply this definition, a relief valve would be a proportional valve. The poppet can crack to any position; not just fully open or fully closed.
The most commonly understood example is a directional control valve that can exert fine control over the extend and retract speed of a cylinder.
Instead we are going to narrow our scope to the valves which are most usually being referred to when someone says a proportional valve.
The classic solenoid driven, spring returned, variable position, directional control valve!
The standard solenoid operated valve is an on/off, or bang-bang valve. The solenoid is either on, or it’s not. The valve is either in a given operational envelope or it is not.
While the transition between envelopes is not actually instantaneous, it is very, very fast.
The proportional solenoid valve, on the other hand, allows for infinite spool positioning between operational envelopes.
This is achieved by varying the control signal (current) to the valve. Increase the strength of the signal and the valve spool moves in ‘proportion’ to the increase.
As I'm sure you know, hydraulic schematic symbols were never created with animation in mind.
For decades, (centuries maybe, know one knows for certain), humanity had to muddle along with their imaginations alone!
But then we came along and saw the potential of animation for explaining what is going on in the world of hydraulics.
A real motor spins.
So we have made the motor symbol spin.
A check valve opens and closes.
So we have made the symbol do likewise.
Unfortunately, not all hydraulic schematic symbols are so intuitive when they are animated.
Clarifying these symbols can, at times, prove to be a challenge.
Which brings us to the schematic symbol for the proportional valve.
At first glance it looks like a straightforward symbol. Four ports, three positions, some operators, and those proportional bars.
Just shift the valve and we're in business!
But wait! What are we to make of this position?
The hydraulic lines look like they are lined up with the valve ports in a crazy, impossible configuration!
What this represents is the idea that the valve is actually in this envelope, but only partially.
As soon as a proportional valve begins to shift from its neutral position the center envelope is no longer a consideration. Instead focus on the envelope that the valve has started moving toward.
To help with this the arrows in the active envelope become colored.
These colors will be the same as the hydraulic lines that are affecting the valve envelope.
Along with this, the other envelopes which are not currently active become faded.
Drag the green circle to the active envelope.
Now we don't use all of these indicators in all situations in our material.
In fact we often try to keep our schematics looking as close to a traditional paper schematic as we can.
But they are helpful in training the eye to know what to look for.
With these animation tricks, we hope it is a little easier to visualize what is going on with the proportional valve schematic symbol.
Now back to the lesson!
Just as you would not slam a truck into reverse while moving forward at full speed, many pieces of hydraulic equipment can be damaged by sudden changes in flow or pressure.
Since proportional valves can vary their position they can be used to soften the change in forces being directed against an actuator.
For example, a proportional valve can be used to decrease the flow of hydraulic fluid into a cylinder as it approaches the end of stroke.
Another common use for a proportional valve is as a speed control.
Use the Control slider to adjust the motor speed.
Proportional valves also make good piloting valves for controlling other, much larger proportional hydraulic valves.
Proportional valves live in a middle ground between the very basic digital movements of a bang-bang valve and the highly precise, infinite movements of a servo valve.
If a servo valve can serve the same function as a proportional valve, why don't we just stick to servo valves?
Proportional valves have slower responses to change and are overall less precise than servo valves.
On the other hand proportional valves are far less expensive and less susceptible to failure due to contamination.
This means that in situations where very fast valve action or very high accuracy are not required, a proportional valve is the preferred choice.
To truly understand how a proportional solenoid valve works one must first understand a bit of how the solenoid itself works.
A solenoid is an electromagnetic device that uses an electrical signal to generate a magnetic field which, in turn, affects the position of a mechanical armature.
In a conventional bang/bang-type solenoid, the force that the magnetic field is able to exert on the armature rapidly decreases as the air gap between the armature and the back pole piece increases.
Minimum force needed
If the solenoid is provided with an electric signal that is only strong enough to move the armature part way through its stroke, then further increasing the strength of the signal will not generate enough force to move the armature further.
Minimum force needed
On the other hand, a proportional solenoid is designed to be able to generate a consistant force across the armatures stroke.
The section of consistant force is known as the constant stroke or working stroke.
This working stroke is made possible by engineering the pole piece in such a way that the magnetic field does not weaken as quickly while the air gap increases.
When a solenoid is de-energized the armature will just stop and sit where ever it happened to be when the magnetic field collapsed.
In order to return, the solenoid needs something to push it back. This is often a spring.
The spring becomes the primary opposing force to the solenoid.
If the force generated in the solenoid fails to overcome the opposing force of the spring, the solenoid stops.
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