pic: T-Shirt Cannon Bot Concept WIP

[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.