pic: T-Shirt Cannon Bot Concept WIP

[GeeTwo]

We brought our rebuilt air cannon to Homecoming last night, so I have a few comments, some positive, some questioning:

• 6 pneumatic wheels: We are using AM pneumatic wheels with the long (31") nanotube gearbox. It does not turn very well as far as small radius is concerned. We were able to make a sharp turn, on the boardwalk to get into the shed, but it required about a nine-point turn. For sweeping curves, it drives fine! If you want a highly maneuverable robot with pneumatic wheels, I recommend significantly more than a 1/8" center drop.
• Magazine: we've skipped this level of complexity; we have two muzzle-loaded barrels. Consider whether you're really up to it!
• T-shirt shells: we've developed a method to roll t-shirts which is very similar to rolling socks, and ends up flipping a sleeve around the bundle. It works well for us, but we do still load down the muzzle.
• Variable Angle: we started to design for this, and found it to be of limited utility when we did the math, especially when we decided that we never wanted to throw a t-shirt or football directly at anyone, Sandy Koufax style, but would rather do it mortar-style to reduce the likelihood of bruised faces.
• Air system: 2 gallons is probably sufficient (ours is around 4, but we got the tank surplus). The most important features are the size of the pipe leading out from the accumulator to the valve, and the size of the valve. I recommend a minimum of 3/4", and a preference of 5/4" or larger. We currently have a pipe from the accumulator to the solenoid valve of 1"+, a 3/4" valve, and 3/4" rubber tubing to the barrel, which works great with 50-60 psi and about a 50ms valve-open time.
• Air compressors: we're using a scuba tank. When we considered not doing scuba as a supply, the best solution seemed to be to have a 110V compressor under the stands where we would go back to recharge every 10 or 20 shots, rather than trying to compress air from a 12V FRC battery. With a larger marine battery, perhaps you could make battery-powered recharging of the accumulator feasible. Last night, we used a single battery to drive and activate solenoids; we did not use either of our two spares. Had we been dependent on FRC-legal batteries for air pressure, I suspect we would have had to replace those 3 batteries at halftime.
• 100#, back of a minivan: ours is a bit heavier (130-150#), but it will fit in a minivan or even a small SUV or station wagon once the barrels are removed from the chassis (single 1/4" thumb screw). Our biggest "fitting" problem with our air cannon was fitting it through the doors in the classroom before a rebuild about 1 year ago.
• Feature Additions: no comment, as no experience.
• Release valve: whatever you use, be sure it is rated for air, not just water! Compressed air has several orders of magnitude more energy than the same volume of water at the same pressure, so failures of air systems are much more destructive than water. Air-rated systems are almost always metallic, not plastic. This is because metal "tears" and plastic "shatters". If you don't want to put shrapnel in the operators or bystanders, use only air-rated pipes and valves.
• Bore size: the smallest point on our current air cannon is 3/4". It's been adequate, but if we were buying today, we'd go for 1" or larger all the way from the accumulator tank to the barrel inputs. Added: For the barrel, we're using a 3" bore. We had 2-1/2" previously, and it worked well for shirts, but the foam footballs got stuck. We went to 3" and roll the shirts thicker and shorter.
• We're also using a 3" id barrel. I haven't figured the volume, but we're using a 3/4" nominal diameter solenoid valve, and opening it for about 50 ms, utilizing 30-70psi (most commonly 50 psi). With 40-60 PSI in the system, it puts shirts anywhere in the stands we want to. With 70+ psi, we can toss a t-shirt over the top of our home-side stand and onto ninth street if we fire directly perpendicular to the stands. Foam rubber footballs don't take quite as much pressure x time to get to a given location as do t-shirts.

[MrBasse]

We built one similar a few years back and having shells preloaded is great for speed or for targeting sponsors (if we see on in the crowd at a football game we can load three in a row that contain their shirts).

We run ours off of CO2, and only at 12 psi for normal operation. We use one $14 tank for around 70-80 shirts.

I'll put up a picture soon, the only part we haven't figured out yet is the magazine. I like your solutionwith the rotart loading. We simplified a bit and used a pneumatic ejector and plan to spring load shells so they autoload when the spent shell is ejected.

[cbale2000]

Originally Posted by KohKohPuffs

The valve I'm using for my t-shirt cannon was this:
http://www.ascovalve.com/Application...IsTran sSSL=Y

Uh... that's a really long link, but it should direct you to something I found while browsing CD for info on t-shirt cannons.

Now what kind of air tanks are those? I'm having some issues finding the right one, and those look really great for my design

Honestly, I just sort of created that model based on my ideal dimensions of a 1 gallon tank. The interior volume is 1 gallon, but everything else is guess work.

That said, the tanks that otherguy posted are only 1in longer and a bit smaller diameter, if this is to be built, something like that would probably be used instead.

Originally Posted by otherguy

We used these tanks this past season with success. 1 gal each.

http://www.avsontheweb.com/1-gallon-...-port-fitting/

They have quite a few different sizes. Just search their site for "tank".

Thanks, this seems to be a better fit than anything else I've found for this design. Would you be able to post the dimensions of your tank by chance? The site shows 4” x 15.5” but I'm not sure if that includes the mounting brackets, or if it refers to the tank diameter.

Originally Posted by GeeTwo

We brought our rebuilt air cannon to Homecoming last night, so I have a few comments, some positive, some questioning:

6 pneumatic wheels: We are using AM pneumatic wheels with the long (31") nanotube gearbox. It does not turn very well as far as small radius is concerned. We were able to make a sharp turn, on the boardwalk to get into the shed, but it required about a nine-point turn. For sweeping curves, it drives fine! If you want a highly maneuverable robot with pneumatic wheels, I recommend significantly more than a 1/8" center drop.

Currently the drive is designed for a 1/4" drop center. I picked 1/4" figuring that the standard 1/8" would be inadequate for the squishiness of the wheels, but it was still basically a guess. Should I have a bigger drop then 1/4"? 3/8" perhaps? 1/2"?

Originally Posted by GeeTwo

Magazine: we've skipped this level of complexity; we have two muzzle-loaded barrels. Consider whether you're really up to it!

There would definitely need to be prototyping done first to prove the concept, but based on what I've seen from mechanisms other cannons have used, it seems plausible. The mechanism that seals the shell to the barrel is actually based off of this design posted a few years back. The rotary loader is by far the most complicated part to make but not impossible.

Originally Posted by GeeTwo

T-shirt shells: we've developed a method to roll t-shirts which is very similar to rolling socks, and ends up flipping a sleeve around the bundle. It works well for us, but we do still load down the muzzle.

I saw that method you posted on one of the other T-Shirt Cannon threads recently. It's definitely something we'll look into. We had pretty good luck using rubber bands in our tests with an air cannon a mentor brought in, but not having to bind the shirts at all would be ideal.

Originally Posted by GeeTwo

Variable Angle: we started to design for this, and found it to be of limited utility when we did the math, especially when we decided that we never wanted to throw a t-shirt or football directly at anyone, Sandy Koufax style, but would rather do it mortar-style to reduce the likelihood of bruised faces.

In most cases, the lower angles would not be used for firing the cannon (and in fact, we've discussed disabling the firing mechanism at low angles for safety), but instead for transporting the system. In most situations the cannon would fire mortar-style. The variable angle system itself is not very complex to make either (gas spring + winch) so it wouldn't be a huge loss of time to add it.

Originally Posted by GeeTwo

Air system: 2 gallons is probably sufficient (ours is around 4, but we got the tank surplus). The most important features are the size of the pipe leading out from the accumulator to the valve, and the size of the valve. I recommend a minimum of 3/4", and a preference of 5/4" or larger. We currently have a pipe from the accumulator to the solenoid valve of 1"+, a 3/4" valve, and 3/4" rubber tubing to the barrel, which works great with 50-60 psi and about a 50ms valve-open time.

Bore size: the smallest point on our current air cannon is 3/4". It's been adequate, but if we were buying today, we'd go for 1" or larger all the way from the accumulator tank to the barrel inputs. Added: For the barrel, we're using a 3" bore. We had 2-1/2" previously, and it worked well for shirts, but the foam footballs got stuck. We went to 3" and roll the shirts thicker and shorter.

My target output valve size was about 1" since that seemed to be the easiest to find. I am curious though, what are your thoughts on using multiple smaller valve tanks and combining them using larger size Tee connectors and bushing adapters? Could you, for instance, combine 4 1/4" feeds and get the equivalent to a single 1" feed? I ask because I'm finding it difficult to locate small tanks (<= 0.5 Gallon) to use for an accumulator that also have large output ports (~1").

Also, do you have any suggestions for large (~1") ID pneumatic hose? I'm not sure what type to look for or where to look (the only pneumatics I've ever dealt with have come in the FRC kit of parts).

Originally Posted by GeeTwo

Air compressors: we're using a scuba tank. When we considered not doing scuba as a supply, the best solution seemed to be to have a 110V compressor under the stands where we would go back to recharge every 10 or 20 shots, rather than trying to compress air from a 12V FRC battery. With a larger marine battery, perhaps you could make battery-powered recharging of the accumulator feasible. Last night, we used a single battery to drive and activate solenoids; we did not use either of our two spares. Had we been dependent on FRC-legal batteries for air pressure, I suspect we would have had to replace those 3 batteries at halftime.

I considered Scuba tanks and even Co2, my concern was the repeat cost of use for these (as Scuba tanks have to be refilled and Co2 cartridges replaced). My hope is that by using a separate battery for the air compressors, the battery can be swapped without having to reset the robot itself, allowing for faster change out times. The compressors are also rated for continuous duty, unlike the standard FRC compressors, which I would think make them more efficient. Plus, by using rechargeable batteries, the cost to use is essentially zero (aside from the cost of the items you're shooting of course).

Originally Posted by GeeTwo

Release valve: whatever you use, be sure it is rated for air, not just water!.

I've seen enough threads on this topic on CD to know to avoid non air-rated valves.
That said, I'm curious what people think about using PVC for the barrel. It seems to be quite common, and I would think since the barrel isn't really storing air pressure it wouldn't be a problem. People I've proposed this idea to have mixed feelings about PVC despite it being used in literally every example cannon they've showed me.

Originally Posted by GeeTwo

We're also using a 3" id barrel. I haven't figured the volume, but we're using a 3/4" nominal diameter solenoid valve, and opening it for about 50 ms, utilizing 30-70psi (most commonly 50 psi). With 40-60 PSI in the system, it puts shirts anywhere in the stands we want to. With 70+ psi, we can toss a t-shirt over the top of our home-side stand and onto ninth street if we fire directly perpendicular to the stands. Foam rubber footballs don't take quite as much pressure x time to get to a given location as do t-shirts.

In tests we did with a handheld air cannon, it took about 90psi to get the sort of range performance we were looking for when shooting shirts, but that said, the cannon used a manual lever valve and may have had an inadequate accumulator tank. My intention is to find an electronic pressure regulator so the working pressure of the system can be changed on the fly, allowing for a variety of shots.
Does your cannon (or anyone else reading this) use a 2-stage air system (storage->accumulator), and if so, what is the volume of the accumulator?

[Steven Smith]

Could you, for instance, combine 4 1/4" feeds and get the equivalent to a single 1" feed?

Flow rate is proportional to the area, not diameter. Also, tubing is generally specified via the OD not the ID, so a 1/4" tubing might be closer to ~1/8" ID depending on the wall thickness you have.

Finally, tee fittings have a pretty significant pressure drop (relatively speaking). When you are discharging air in a cannon, your air flow speed is likely going to be quite high, so having a significant number of tees might be an issue. If they were between your high pressure tank and your accumulator, it wouldn't be an issue, but everything that is "firing" when that valve opens needs to be relatively large with minimal pressure drop.

[GeeTwo]

Originally Posted by cbale2000

Currently the drive is designed for a 1/4" drop center. I picked 1/4" figuring that the standard 1/8" would be inadequate for the squishiness of the wheels, but it was still basically a guess. Should I have a bigger drop then 1/4"? 3/8" perhaps? 1/2"?

It's going to depend on the length, weight, and tire pressure/size. If you can manage a design where the drop can be adjusted, at least for the prototype, that would be best. You want the drop to be as little as gives you the maneuverability, because a larger drop means less stability in remaining level. You can partially offset this by putting your CoM clearly ahead or behind the middle axle, so that the robot settles the same way each time you stop.

This is from a bit of thought, not experience: Fill a tire with air and put a quarter of the robot's weight on it, and see how much the axle drops. I would use at least that much center drop, probably a bit more.

Originally Posted by cbale2000

In most cases, the lower angles would not be used for firing the cannon (and in fact, we've discussed disabling the firing mechanism at low angles for safety), but instead for transporting the system. In most situations the cannon would fire mortar-style. The variable angle system itself is not very complex to make either (gas spring + winch) so it wouldn't be a huge loss of time to add it.

Our barrel is hinged at the lower end, but we keep it in place with a length of rod. We can pull a pin and drop the barrel to a level orientation for shipping.

Originally Posted by cbale2000

Originally Posted by cbale2000

My target output valve size was about 1" since that seemed to be the easiest to find. I am curious though, what are your thoughts on using multiple smaller valve tanks and combining them using larger size Tee connectors and bushing adapters? Could you, for instance, combine 4 1/4" feeds and get the equivalent to a single 1" feed? I ask because I'm finding it difficult to locate small tanks (<= 0.5 Gallon) to use for an accumulator that also have large output ports (~1").

It would be less expensive and easier to trouble-shoot to use a single valve per shot. 1" should be adequate; we're getting by with 3/4" at 50-60psi. As Steven noted above, you'd need 16 1/4" i.d. inputs to equal one 1" i.d. input (actually, more than that, as turbulent losses in a small hose are greater than a large one).

Originally Posted by cbale2000

Also, do you have any suggestions for large (~1") ID pneumatic hose? I'm not sure what type to look for or where to look (the only pneumatics I've ever dealt with have come in the FRC kit of parts).

Google is your friend here. You can probably find a local company that deals in these items as well.

Originally Posted by cbale2000

I considered Scuba tanks and even Co2, my concern was the repeat cost of use for these (as Scuba tanks have to be refilled and Co2 cartridges replaced). My hope is that by using a separate battery for the air compressors, the battery can be swapped without having to reset the robot itself, allowing for faster change out times. The compressors are also rated for continuous duty, unlike the standard FRC compressors, which I would think make them more efficient. Plus, by using rechargeable batteries, the cost to use is essentially zero (aside from the cost of the items you're shooting of course).

We got our scuba tank inspected and charged at a dive shop this week; I believe it cost $8.50. We get several hundred shots from a tank. When we don't need inspection, we can sometimes get the local fire house to charge it for free, as we're using it for educational purposes.

Originally Posted by cbale2000

I've seen enough threads on this topic on CD to know to avoid non air-rated valves.
That said, I'm curious what people think about using PVC for the barrel. It seems to be quite common, and I would think since the barrel isn't really storing air pressure it wouldn't be a problem. People I've proposed this idea to have mixed feelings about PVC despite it being used in literally every example cannon they've showed me.

We're using PVC for the final barrel right now, but plan to go to a metal tubing next year.

Originally Posted by cbale2000

In tests we did with a handheld air cannon, it took about 90psi to get the sort of range performance we were looking for when shooting shirts, but that said, the cannon used a manual lever valve and may have had an inadequate accumulator tank. My intention is to find an electronic pressure regulator so the working pressure of the system can be changed on the fly, allowing for a variety of shots.
Does your cannon (or anyone else reading this) use a 2-stage air system (storage->accumulator), and if so, what is the volume of the accumulator?

We have been able to toss shirts over our home stands with 60 psi, and over the press box with 70psi. This is using a 3" barrel and 3/4" valve, and 50ms of valve open time. Each of these numbers will affect how far you can throw an item of a given weight. Having a proper (snug, not tight) fit in the barrel is essential for maximum range as well.

Rather than change the angle, we are planning to adjust range by changing the duration that we keep the valve open. I believe we're currently hard coded at 50ms, but we now have a more capable controller and will be adding variable timing.

[MrBasse]

You can find a picture of our semi-semi auto t-shirt cannon here. Apparently when you upload from an Android phone the thumbnail is upside down, but full size is right.

http://www.chiefdelphi.com/media/img...425da706_l.jpg

We have since ditched the FRC control system in favor of the Cheap and Dirty control system from Andymark. The FRC system was super unreliable and this one works up to about 350 feet away. For those shopping for a RC system you can find the same thing as AndyMarks without their stickers for less than half the price here.