Placing a Tank after a solenoid valve

[Richard.Varone]

We actually did this in 2008 and it was legal, but it's possible the rules have changed so this sounds like a good Q&A to me.

[Peter Matteson]

Mechanically locking a cylinder and keeping the valve open won't do anything.

Once you have 60 psi air in the area of the cylinder it can fill it will stop flowing. Adding low pressure volume does nothing but make your compressor work harder for no reason. The reason you have high pressure storage is the change in volume of the air as it expands to a lower pressure acts as energy storage. Without the expansion just oure volume does nothing.

Please rethink your plan and consult a mentor that has some experience in fluid dynamics.

[Mr V]

Originally Posted by gpetilli

I too am not an expert, but I believe the standard FRC solenoids use a supply "pilot" which means that if the input air pressure goes away (think manual dump) then the solenoid opens allowing the output air to back through the output to the input. This works only if there are no check values in the system and does not depend on any electrical signals. We have had issues in the past that if the working pressure is too low, there is not enough energy to "engage the pilot" and the solenoid does not activate. If I am correct, then this would allow the low pressure storage tank to vent when the manual release is opened.

Yes the common solenoids are air piloted but that is not relevant to their operation. They just use air pressure to move the shuttle, once the shuttle is moved it stays there and w/o enough air pressure they can't move the shuttle.

Since the shuttle stays in the same position the pressure on the working side of the valve can vent back through the valve. So yes tanks on the working side of a commonly used FRC solenoid will vent assuming as you noted that there aren't any check valves added into the system.

[pmangels17]

Originally Posted by Peter Matteson

Mechanically locking a cylinder and keeping the valve open won't do anything.

Once you have 60 psi air in the area of the cylinder it can fill it will stop flowing. Adding low pressure volume does nothing but make your compressor work harder for no reason. The reason you have high pressure storage is the change in volume of the air as it expands to a lower pressure acts as energy storage. Without the expansion just oure volume does nothing.

Please rethink your plan and consult a mentor that has some experience in fluid dynamics.

In 2008 we used a lock and air tanks on the cylinder because the flow rate through the solenoid wasn't fast enough to accelerate the ball quickly. The extra tanks were mounted using brass fitting to allow a larger volume of air to be stored, so that when the piston is released, the air can flow faster into the cylinder, thus moving the piston faster. This gave us more power and a quicker shot. Adding a tank between the valve and cylinder facilitates a more powerful shot.

[FrankJ]

Think of this way. You use cylinder that has more stroke than you need. The start position is mid way in the cylinder. You effectively use part of the cylinder as the accumulator. And you don't have that pesky 1/4 tube slowing things down. If the vent valve can dump all the pressure, you should be legal. If it doesn't then the design is not legal regardless of the legality of the individual parts.

This is safer than using a big spring for your stored energy since you can relieve the pressure, getting rid of the stored energy. The catapult is ultimately going to have the stored energy to throw the ball at the required speed regardless of the method used. So the wise thing to do is thing about how to safely handle that energy.

[Richard.Varone]

For those interested this was actually a prototype from 2008 which was the first thing we tested after kickoff. This is an 8" with a 1 1/2" bore and it shoots the ball about 7ft ish into the air.

If anyone would like more info on it let me know and maybe we can post a vid or something along those lines.


EDIT: DISCLAIMER - This was legal in 2008, we haven't looked into it's legality in 2014.

[Peter Matteson]

Originally Posted by pmangels17

In 2008 we used a lock and air tanks on the cylinder because the flow rate through the solenoid wasn't fast enough to accelerate the ball quickly. The extra tanks were mounted using brass fitting to allow a larger volume of air to be stored, so that when the piston is released, the air can flow faster into the cylinder, thus moving the piston faster. This gave us more power and a quicker shot. Adding a tank between the valve and cylinder facilitates a more powerful shot.

The flow restriction is still the valve. Your are not getting full working pressure but something signifigantly less that weakens quickly as the air expands. Again going back to my point you are get the force you think you are when you do this, unless you have calculated out the pressure when the volume you trap fully expands.

[Al Skierkiewicz]

I am looking at this more from a safe design that plans for failures view. If the system dump valve is downstream from a regulator(s) that potentially could plug in a failure, then the valve couldn't release all system pressure. I believe that is what the original rule was addressing. With working regulators, system pressure would be released.

[FrankJ]

Q/A Q62 addresses this.

Q62 Q. Is it legal to have air storage tanks (accumulators): A) On the "working" pressure side of the regulator? and/or B) Between a solenoid valve and the pneumatic cylinder(s)? I can find nothing in the rules prohibiting either of these cases.

A. There are no Rules explicitly prohibiting either scenario.

[ToddF]

Originally Posted by Peter Matteson

The flow restriction is still the valve. Your are not getting full working pressure but something signifigantly less that weakens quickly as the air expands. Again going back to my point you are get the force you think you are when you do this, unless you have calculated out the pressure when the volume you trap fully expands.

Valve flow only matters if the reservoir volume is small compared to the cylinder displacement volume. If I have a 5 gallon low pressure reservoir, and a tiny cylinder with only 1cc of stroke volume, flow through the valve doesn't matter at all during cylinder extension (assuming a mechanical trigger restraining a pre-charged cylinder from actuating). At the other extreme, a 6 inch diameter cylinder with a 12 inch stroke having a reservoir composed only of the hose feeding it, the valve flow matters a great deal.

The trick is selecting an appropriate reservoir volume that balances the need for a fast stroke with the time needed to recharge the reservoir after venting for retraction.

[Ed Sparks]

Originally Posted by Richard.Varone

For those interested this was actually a prototype from 2008 which was the first thing we tested after kickoff. This is an 8" with a 1 1/2" bore and it shoots the ball about 7ft ish into the air.

If anyone would like more info on it let me know and maybe we can post a vid or something along those lines.


EDIT: DISCLAIMER - This was legal in 2008, we haven't looked into it's legality in 2014.

I see a potential problem with this device depending on how the system is pneumatically controlled. The cylinder may be considered a second compressor if it restores the air used to push the plunger back into the storage devices when the plunger is re-cocked. In this case, I'm assuming something mechanical is re-cocking the cylinder. A solenoid valve would have exhausted the air to atmosphere in both the forward stroke and reverse stroke.

[Richard.Varone]

Originally Posted by Ed Sparks

I see a potential problem with this device depending on how the system is pneumatically controlled. The cylinder may be considered a second compressor if it restores the air used to push the plunger back into the storage devices when the plunger is re-cocked. In this case, I'm assuming something mechanical is re-cocking the cylinder. A solenoid valve would have exhausted the air to atmosphere in both the forward stroke and reverse stroke.

The cylinder retracted with gravity/a small spring, the valve was open in this condition so the pushed air never re-entered the system

EDIT: Also we've found that in a system like this it's vital to not have any fittings on the other end of cylinder

[Al Skierkiewicz]

In rereading this thread and thinking always about safe operation, the precharged and locked cylinder makes me wonder what happens when the vent plug valve is opened. (I do a lot of "what if" analysis. Sorry, but that is what I do) So these are my questions as I consider operation...
1. Is there ever a chance that the locking cylinder could release the piston as system pressure is vented?
2. Is there a mechanical lock that prevents uncontrolled movement when the robot is powered down?
3. Is the robot carried off the field with the cylinder under pressure and what happens if the robot is accidentally dropped or shocked?
4. When the cylinder moves, is the movement exposed to the outside of the robot?
5. Knowing that the PVC storage tanks have failed in the past (from external forces), are they being used as intended with rigid fittings to a moving cylinder?

Did I miss anything?

[Richard.Varone]

Not sure if this was directed towards me or just in general but:

Originally Posted by Al Skierkiewicz

1. Is there ever a chance that the locking cylinder could release the piston as system pressure is vented?

This was a while ago so my memory might be a bit shaky, but I believe that the trigger was designed such that when the cylinder was pressurized it could only release by powering the lock in the other direction.

Originally Posted by Al Skierkiewicz

2. Is there a mechanical lock that prevents uncontrolled movement when the robot is powered down?

We had a pin that we would install and remove on the field before and after the match

Originally Posted by Al Skierkiewicz

3. Is the robot carried off the field with the cylinder under pressure and what happens if the robot is accidentally dropped or shocked?

Regardless of the year/robot we always open the main release valve before taking the robot off the field.

Originally Posted by Al Skierkiewicz

4. When the cylinder moves, is the movement exposed to the outside of the robot?

Ours was positioned towards the back of the robot and was always within the frame.

Originally Posted by Al Skierkiewicz

5. Knowing that the PVC storage tanks have failed in the past (from external forces), are they being used as intended with rigid fittings to a moving cylinder?

We use and I'd also recommend that anyone doing this setup use metal storage tanks and brass fittings, along with making sure that it is securely mounted so that the robot is taking the recoil and not the pneumatic system.

This setup had potential to be pretty dangerous and extreme caution should be taken into it's design and execution. While it was able to be dry fired we never recommended it, hitting the ball acted as a dampener taking a lot of the stress of the shot. Another point in the safety of a device like this is to attach a rope from the robot to the end of what ever is on the piston.

Now in terms of safety, this years ball is half the size, and with our testing we were able to use a cylinder half the size which makes for a safer implementation

[ToddF]

Originally Posted by Ed Sparks

I see a potential problem with this device depending on how the system is pneumatically controlled. The cylinder may be considered a second compressor if it restores the air used to push the plunger back into the storage devices when the plunger is re-cocked. In this case, I'm assuming something mechanical is re-cocking the cylinder. A solenoid valve would have exhausted the air to atmosphere in both the forward stroke and reverse stroke.

Teams implementing a system such as this need to ensure that when the cylinder is "recocked", the pressure side of the piston is vented to atmosphere. It is possible to use a winch type system to recock the mechanical latch. If the solenoid valve were in the closed position, the recocking action could raise the pressure between the cylinder and the valve over the allowable low pressure limit (or, depending on the geometry, over the high pressure limit). This could possibly cause a failure in whatever is the "weakest link" in the low pressure system.

If we go this route, we won't use a recocking mechanism that is capable of overpressurizing the low pressure side of the system. If your team does so, I would highly suggest installing a pressure relief valve set at 60psi between your solenoid and the catapult cylinder. This will help keep you legal and safe.