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!
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