Quick Question

[Kersten]

Is it possible, when using a piston, to retain the air instead of lose it so it will work faster instead of having to build up the energy over and over again?

[DonRotolo]

Not really, because it is the change of pressure from ~60 PSI to zero that does the work. You are storing mechanical energy as air pressure, and to use it you have to, er, use it (release the pressure).

If you keep 'all' the pressure, the piston in the cylinder won't move (what's going to push it?)

If you keep 'some'of the pressure, it's the same as turning down the regulator to a lower pressure.

You CAN conserve air by using lower pressure - the lowest pressure that gets the job done will use the least air. Additionally, use the smallest volume (cylinder diameter, stroke length) since that saves air, too.

Don

[Kersten]

would that mean then that it would take just as long if i did what you said as if i let it keep building pressure?

[MrForbes]

Another trick is to have gravity or a weak spring (think surgical tubing) return the cylinder, and only connect the pneumatic valve to one end of it, and plug the other output from the valve. This is how we made our fully pneumatic launcher actually be able to work, since we didn't calculate ahead of time how much air it would use.

[dtengineering]

Originally Posted by Kersten

Is it possible, when using a piston, to retain the air instead of lose it so it will work faster instead of having to build up the energy over and over again?

Possible and FRC legal are two different things.

The short answer is yes, it is possible to retain some of the energy in the compressed air by venting it to a second-stage low pressure side of the pneumatics. I'm simplifying a bit here, but yes, it is possible to build a system that is more efficient than simply venting all "used" air to the atmosphere.

FRC has a few rules that make this challenging, one stating that all compressed air must come from the compressor, and another one stating that only the Clippard tanks may be used for storing compressed air.

So consider the situation where you have a cylinder pressurized to 60psi. Hook it up as normal, but then run the vent from the solenoid valve to another solenoid valve. This second solenoid valve can output to either a regulator set to 30psi feeding a storage tank on a low pressure side of the system, or to atmosphere.

When you change the direction of the first solenoid valve (to move the piston) the 60psi air in the extended part of the cylinder is directed through the second solenoid valve to the 30psi side of the system. When the pressure in the cylinder has dropped a bit, and some the energy in the compressed air has been recovered, then the second solenoid valve shifts and vents to atmosphere, allowing the piston to complete its stroke as normal. This might be legal, so long as it is not determined that the cylinder is acting as a storage tank, and so long as the piston on the cylinder is not 'pushed' in by an external force (thus compressing the air using something other than the compressor).

While I don't see a rule specifically prohibiting this, other than listed above, I would definitely get a Q&A ruling on it before planning to builld it into a robot.

But it IS possible to recover SOME of the energy stored in the compressed air.

Jason

P.S. On the "possible" vs. FRC legal... it is possible to set the working system pressure above 60psi, but not FRC legal. Now that our season is over, however, we have discovered that an extra 10psi or so in the launcher is... impressive.

[Kersten]

I see, Thank you all, that was all very helpful!

[GaryVoshol]

Jason, I believe you've just described the process behind a double-expansion steam engine. There are triple and quadruple steam engines still in existance today. Each cylinder in sequence is larger, being driven by lower pressure, so that the steam from the boiler is used 2 or 3 or 4 times, making the engine more efficient.

Just a sidebar, nothing to do with the original pneumatic question.

[dtengineering]

Originally Posted by GaryVoshol

Jason, I believe you've just described the process behind a double-expansion steam engine. There are triple and quadruple steam engines still in existance today. Each cylinder in sequence is larger, being driven by lower pressure, so that the steam from the boiler is used 2 or 3 or 4 times, making the engine more efficient.

Just a sidebar, nothing to do with the original pneumatic question.

Actually it is quite relevant to the original question and an excellent example of recovering energy from a "used" gas. I seem to recall something from third-year thermodynamics about maximizing energy recovery from a compressed gas, and that if you could expand the gas infinitely slowly that you could recover 100% of the energy. (Those with more recent memories of thermo, please feel free to correct me... it was almost 20 years ago...) So the more stages that ocurr in the expansion process the more efficient the energy recovery from the compressed gas.

In this case it would also be interesting to use a constant pressure, rather than constant volume, compressor. The low pressure side of the system could be fed back into the compressor so that the air would only need to be compressed from 30 psi to max, rather than from atmospheric to max.

Jason

[Rick TYler]

Originally Posted by dtengineering

So consider the situation where you have a cylinder pressurized to 60psi. Hook it up as normal, but then run the vent from the solenoid valve to another solenoid valve. This second solenoid valve can output to either a regulator set to 30psi feeding a storage tank on a low pressure side of the system, or to atmosphere.

Or you could just vent the exhaust side of your solenoid through a small-diameter aperture which blows on a little windmill attached to your driveshaft...

[Kersten]

hm...thanks again for all of your input!

[Eugene Fang]

Originally Posted by Rick TYler

Or you could just vent the exhaust side of your solenoid through a small-diameter aperture which blows on a little windmill attached to your driveshaft...

yay! .00000000000000001 inch pounds of torque!