Pneumatic Ram Question

[Magnechu]

Are we allowed to use our Pneumatic Rams as virtual Pneumatic Resevoir Tanks for the match?

Thanks.

[EricH]

The cylinders, as I read the rules, may not be used as storage tanks. Here is the main rule against it:

<R01> Energy used by FIRST Robotics Competition ROBOTS, (i.e., stored at the start of a MATCH), shall come only from the following sources:
 Electrical energy derived from the onboard 12V and 7.2V batteries
 Compressed air stored in the pneumatic system, stored at a maximum pressure of 120 PSI in no more than four Clippard Instruments tanks. Extraneous lengths of pneumatic tubing shall not be used to increase the storage capacity of the air storage system.
 A change in the altitude of the ROBOT center of gravity.
 Storage achieved by deformation of ROBOT parts. Teams must be very careful when incorporating springs or other items to store energy on their ROBOT by means of part or material deformation. A ROBOT may be rejected at inspection if, in the judgment of the inspector, such items are unsafe.

The following Q&A may also help. http://forums.usfirst.org/showthread...3319#post13319

If you are still unsure, ask the Q&A.

[TubaMorg]

You can (and should), however, charge up your pneumatic system prior to starting the match. This would mean that the pneumatic cylinders would have one side pressurized (retracted or extended). This isn't technically storing energy, though, since the pressurized air in the cylinder can't be used any more (except as exhaust).

[MrForbes]

well....not quite! imagine that you have the cylinder in a mechanism in a way that it is not fully extended or contracted when it is at the end of it's travel (it's travel is limited by the mechanism). The pressurized side can hold quite a bit of energy. Enough to launch the ball, if you have a pair of 2" bore cylinders with at least 8" total stroke, and starting out extended about 1/3 of the way.

at least that's what our calculations show.

But we're going to start with the shooting side of the cylinders unpressurized.

[TubaMorg]

Yes, there are several exceptions to my "simplified" scenario.
It would be interesting to know from the GDC if your example would be allowed though!

[MrForbes]

Yes, it would be! Since we are building our robot so it won't be using any energy stored in the cylinders, we don't feel the need to ask the question.

[TubaMorg]

I dunno....those 2 inch cylinders take a lot of volume. If you were allowed to do it your way you would be one shot further away from depleting your air supply. Unless, of course, a redesign has moved you away from the ginormous cylinders.

[MrForbes]

The cylinders do hold a lot of air, but the system starts out with 120 psi in 4 tanks, and the compressor can run continuously between shots. The big air use is in returning the cylinders to the down position.

I'll see if I can get a student to calculate the air use per shot, as well as the amount of air that the compressor can pump in a lap (estimated time 15 seconds, which is probably optimistic).

[TubaMorg]

We've been doing a lot of calculations ourselves. We aren't going with a shooter/flinger though, so different design considerations. It seems with your configuration you could do a full extend/retract cycle before your working pressure begins to seriously decrease. I'm sure you and a lot of other teams have already figured this out, but it seems like if you are using a cylinder to shoot with, you can't get enough spead on the ram to move the ball very far, no matter how big a cylinder you use. The tubing we have to use limits the mass air flow such that the cylinders can't move very fast. I suppose with the creative use of levers, a speed magnification can be achieved, but with a loss of force. Bah, we're just going to lift and drop.

[MrForbes]

Apparently I did not do a very good job of explaining what we're doing in my first post above....we fill part of the big cylinders up and use just that stored air to shoot the ball. The air is already in the cylinders when we shoot, it does not need to go thru the hose or valve....all we have to do is send air another small cylinder that raises an arm that releases the ball.

[TubaMorg]

Sure I think I understand the configuration. Just make sure what terms you use, though. "Stored air" is only allowed in the 4 storage tanks at 120 psi. Static air in the cylinders is only allowed to be at 60 psi working pressure. So assuming you lock in air at the top and bottom of the cylinder piston, the top would have to be at 60 psi, with the bottom at a somewhat lower pressure (assuming no piston movement). To convert to sudden dynamic motion in the extend direction, you need to vent the air off the top of the piston as fast as possible. Doesn't the vent air need to pass through the tiny plumbing?

The last part of your post I'm not too clear about. Are you saying that you are considering using the air from one cylinder to feed air to a smaller cylinder? An interesting configuration that is legally debateable. You would, indeed, be storing air (energy) in this scenario. Even if it were legal, you are still moving air from one place to another via tiny plumbing which limits the mass flow rate of the system which limits how fast the cylinders will travel.

Of course, emperical evidence trumps all speculation. The cool thing about pneumatics is you can slap it together pretty quick and see if it works!

[MrForbes]

Last part first, no, they are two separate parts of the robot, the smaller second cylinder is mounted to an arm at the top of the robot that lets the ball fly up when it is activated. The big cylinders at the bottom are what propel the ball up.

On the cylinders that propel the ball, they will have up to 60 psi in the 3" long portion between the piston and one end of the cylinders, and atmospheric pressure in the 7" portion between the piston and the other end. When the ball is released, the piston moves with the force of that 60 psi pushing on the piston, but as the piston nears the end of it's travel, the pressure on the other side will build up and slow down the action. Air does not need to enter or leave the cylinders to launch the ball.

This is hard to explain.....

[TubaMorg]

Aha! Now I think I've got it, sort of. There are still some details that aren't clear. If no air leaves or enters the big cylinders, then it is a closed system. How do you know what pressure exists on each side of the cylinder? Also if the bottom is at 60 psi and the top is at atmospheric, then you must have a latching mechanism that keeps it from flinging the ball until released. You must then also have a mechanism that "recocks" the cylinders. I believe the GDC has ruled on this and said it is a no-no. If you had pressure gages installed to the top and bottom and could show that 60 psi would not be exceeded, maybe it would be ok. I admire the creativity you guys have though! Have you considered using gas springs instead? They are way FIRST legal and have lots of sproing.

[AdamHeard]

Originally Posted by squirrel

Last part first, no, they are two separate parts of the robot, the smaller second cylinder is mounted to an arm at the top of the robot that lets the ball fly up when it is activated. The big cylinders at the bottom are what propel the ball up.

On the cylinders that propel the ball, they will have up to 60 psi in the 3" long portion between the piston and one end of the cylinders, and atmospheric pressure in the 7" portion between the piston and the other end. When the ball is released, the piston moves with the force of that 60 psi pushing on the piston, but as the piston nears the end of it's travel, the pressure on the other side will build up and slow down the action. Air does not need to enter or leave the cylinders to launch the ball.

This is hard to explain.....

It sounds like a better solution would be to use COTS gas springs that are exempt from the pneumatics rules.

[s_forbes]

Gas springs were considered, but they have they have the problem of being terribly dangerous. If you have a gas spring catapult latched, then it is very hazardous when it is fired without a ball sitting on top of it (think getting hit by a baseball bat).

The system squirrel described is very safe. The system doesn't have pressure built up unless there is a ball sitting on top of the catapult; the ball acts like a latch of sorts. if you fire without a ball, the catapult just slowly raises due to flow control valves.