# Variable Pnuematics

Hi
From my understanding the double acting pistons are either in or out, but I think that it is possible to make them variable, has anyone tried this?

Mark

I’m not going to go so far as to say it is impossible, but I will call it impractical.

The problem is that if you balance the pressure on both ports of the cylinder to stop the piston away from either end, then there will be no net pressure difference holding the piston in place.

That means an external load (say, the weight of an arm) will work to compress the air on one port, moving the piston. The piston will move until the difference in pressures equals the external force moving the piston.

Hydraulics are different since hydraulic fluid is more-or-less incompressible, but pneumatic cylinders really are an all-or-nothing proposition (or, you are using them as air-shocks )

This doesn’t even address the fact that such a pressure balancing act would be extremely challenging to achieve with competition-legal parts.

Cheers,

• Dean

You could use a pressure regulator to adjust the force on the piston and therefore the distance it is extended if you use something where the force increases as it moves further out. You would also want the system pressure to be constant.

My team has talked about this before… aren’t doubling acting pistons still controlled by one solenoid and one digital out? If so, then when you get the piston in the position you want, and then you flip the output, it’ll instantly push the piston back the other way.

Team 599D uses a pneumatic lift that raises to variable heights…

Do they use a crank-shaft concept to raise to varible heights?

I have no clue what they use. I did no get a very good look at it but all I know is that it is very fast.

On the nose. Simply put its annoying.

There are ways. Consistent and inconsistent. You can do air line set ups like Dontworryaboutit is talking about. Which loses consistency as you use the system.

Or there is a mechanical system. Say you attach a string to the end of your arm attach it to a pulley. From there have a few motors that drive the pulley in, this would allow you to compress the air in the firing end of the cylinder and therefore adjust the angle or distance everything has gone.

This does have risks though Piston are rated for a certain PSI get withing +/- of the safe operating pressure and you might have some problems.

But thats just one way of doing it.

• Andrew

If you run your pneumatic system at, say, 60psi, then it should be pretty consistent over the match, provided you have enough tanks to keep the psi above 60.

With Kit 2 pneumatics, you actually have four “positions”: extended, retracted, neither, and both. Good luck finding a way to utilize all these positions, as the difference between both and neither is not that much (less than 1 lb I think, but enough to engage a ratchet). You have to use two Kit 1 solenoids to do this, but it works.

If you have several cylinders, you could make a variable arm that has different segments, each corresponding to a different height increase. 20" and 11" seems like they would work well, so you can be on the ground, at 11", at 20", or at 31". Two scissor lifts stacked up comes to mind as the best way to do this. Then you have two sets controlled by two solenoids.

You dont have to control it by one solenoid… you can add a second, so if you have one solenoid to each end of the piston then you should be able to put air into the front then when it gets to a certain point put air in the other end and that should keep it at that point. That is how I see it working but im not sure if it will actually work

Oh, it will probably work a bit, but it I’ll be impressed if you get speed/power as well as variability. I think the two necessarily trade off each other.

Even if you had a perfectly controllable pressure source for each port of the cylinder, you still have to react to deflections caused by variable loads.

A double-acting cylinder will only stay away from the ends if the pressure at the two ports is at equilibrium. In that state, the pneumatic cylinder has its weakest holding force, which is governed by the volume and compressibility of the fluid (air) on either port. So to pull this off, you need to work with high pressures, and low volumes, or use an incompressible fluid.

It still seems like more trouble than it is worth to me.

If you have a few specific heights that you care about, you could use servos to engage stops along the path of travel. Then the arm could push up with full force & speed until it hits the stop that is engaged. Pneumatics for the speed and power, mechanical stops for precision.

Cheers,

• Dean

The problem with pressurizing both ends and hoping the piston stays at the same spot is exactly what Quazar described: there is effectively nothing holding the piston where it is.

If the flow control valves can be turned all the way off (so no flow passes through them), you could use those to extend the piston a certain amount and then shut off air.
There will still be issues with holding the piston at the same spot under variable loads, but you can compensate more easily by letting more air in/letting air out then adjusting both pressures and flows of a Kit 2 cylinder.

To control the pressure regulators and flow control valves that I suggested, you would need a motor or a servo.