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.