Most Powerful FIRST Team created T-Shirt Cannon

[JamesCH95]

I also seem to recall that in a particular experiment on Myth Busters (shooting a bird out of an air cannon?) they they were getting essentially the same results if they used around 100psi or around 20psi. I can't recall the exact details... Maybe a t-shirt cannon can operate at a much lower pressure, reducing the potential danger.

One could make the barrel wider to extract the same PV work from the lower-pressure air, but heaven forbid we actually do some engineering math

As an aside: I'd be amazed if any of the people on the show actually have a "pressure vessel" welding certification from ASME or AWS.

[EricH]

Originally Posted by JamesCH95

As an aside: I'd be amazed if any of the people on the show actually have a "pressure vessel" welding certification from ASME or AWS.

I would suspect that they mainly use stock tanks, or get someone else who does have it to build the custom ones.

Or, maybe someone does have one. I'd actually not be that surprised, with the range of experience they have available...

[davidthefat]

Now, that gives me an idea... Make a tshirt launcher on the existing robot, are 2 servos strong enough to support the launcher (obviously not the WHOLE launcher) I can just use the existing compressor and stuff just mount a barrel to the servos

[JamesCH95]

If you're talking about the hobby-type servos that come in the KOP then do the math:

Typical holding torque of about 2.8lb-inches

So it will hold 2.8lbs 1 inch from its center of rotation, 1lb 2.8 inches from its center of rotation, etc. You won't have much of a cannon...

[apalrd]

Of course, you could always make a servo with a big motor and a pot. A window motor would probably work, unless it was really really heavy, then you could go to a FP or gear it down a ton.

[davidthefat]

http://www.truetex.com/servomod.htm

that? I might ask one of the electrical people to do it, my mentor will NEVER trust me to mess around with motors, only as a programmer

[apalrd]

Why not just have feedback through whatever processor is already on the robot?

Mount the pot and motor (or a sprocket chained to the motor) on the shaft of the cannon. If there is no place for the pot it can be on a sprocket on the motor chain as well. If the shaft is hollow, you can put in a plastic (HDPE, Delrin, etc.) plug with a set-screw for the pot, and mount the pot with a piece of folded sheet metal.

Then, wire the pot to an analog input, and the motor, through a speed controller, to a PWM output.

You can then have fun writing and tuning PID code. If it dosen't have a whole lot of mass to it then P would work fine, if it can't get to the setpoint without overshooting you can add a little bit of I to help it along, and if it has a giant amount of mass (e.g. a big steel pipe) you can add D to help with momentum.

[JamesCH95]

I don't think that any of the KOP motors are rated for continuous stall (the exact term is escaping me right now) the way a servo motor is generally used. To use a FP or CIM you would really want to gear it down through a worm gear, like the window motor, to prevent back-drive. Alternatively you could use a Dewalt or other drill transmission assuming that it uses an anti-back-drive mechanism.

[billbo911]

Originally Posted by JamesCH95

I don't think that any of the KOP motors are rated for continuous stall (the exact term is escaping me right now) the way a servo motor is generally used. To use a FP or CIM you would really want to gear it down through a worm gear, like the window motor, to prevent back-drive. Alternatively you could use a Dewalt or other drill transmission assuming that it uses an anti-back-drive mechanism.

Continuously running any motor in a stalled state is never a good idea. Even a hobby servo doesn't like it. Position feedback keeps the servo in a set position. Once the desired position is achieved, motor current is removed.

Your best bet is to keep the mechanism you are positioning in balance. That way, the motor is primarily used to move the mechanism, not hold it.

[JohnFogarty]

how about what we did, Lazy Susan+Window Motor

[JamesCH95]

Originally Posted by billbo911

Continuously running any motor in a stalled state is never a good idea. Even a hobby servo doesn't like it. Position feedback keeps the servo in a set position. Once the desired position is achieved, motor current is removed.

Your best bet is to keep the mechanism you are positioning in balance. That way, the motor is primarily used to move the mechanism, not hold it.

After doing a little more research I found that some motors have a published "Maximum Continuous Stall Torque" rating, usually a small fraction of their maximum rated torque. It would be safe to operate the motor under these conditions, though a non-KOP motor designed for this purpose would be capable of a higher sustained torque.

On GOOGLE Books one can find Handbook of Electric Motors By Hamid A. Toliyat, G. B. Kliman which has many of its pages readable and searchable for more information. It has block diagrams and equations one could use to start making a PID loop to control a DC motor.

I heartily agree with your second point, mounting the servo so it is close to the CG of what it's manipulating will undoubtedly reduce the load on whatever motor or servo is used.

[David Brinza]

Prior post reported.

[martin417]

Just a note about safety. In industry, there are several factors that are considered when characterizing the "safety" of a device or design. When considering a dangerous event that might occur, the two most important are:

1. Likelihood of the event occurring
2. Cost of the event occurring.

If either one of these is high, that device or design is considered unsafe, even if the other is low. The reason PVC is not rated for pressurized gas is the cost of failure, not the likelihood of failure. When gas is pressurized in a reservoir, energy is stored (not so much the case with non-compressible fluids like water or oil). If the reservoir fails, that energy is released. If the material is brittle plastic, that energy release is almost instantaneous, and is accompanied by flying shrapnel.

Metal reservoirs can also fail catastrophically. If you look at pressure vessels like SCUBA tanks, you will see dates stamped on the side. Each SCUBA tank must have a visual internal inspection every year (to look for corrosion or visible flaws), and a hydrostatic test every three years (the tank is filled with water and pressurized to measure the growth of the tank. Water is used instead of air so that if the tank fails, little or no energy will be released). With higher pressures and higher volumes come more energy and more danger. Here is the result of a SCUBA tank failure.



So, PVC pipe is not more likely to fail when pressurized with gas instead of water, but the cost of that failure, in terms of safety, is much higher. Please consider the cost of failure of any design you wish to build, as well as the likelihood of that failure.

[will_1359]

lol i'm sure the boys doing the t-shirt cannon for team 1359 will take this as a challenge.

right now our most powerful launch we've had was using a 3 foot x 4in(i don't feel like getting the cubic inches) air reservoir(yes it was PvC, but it was the highest rated pressurized PvP we could find, what we're worried about blowing are the seals holding everything together), pumped to 125psi. It may have actually traveled further then a football field lol, might have to try it again.

only downside is that we havn't set up the robot to fire it yet, it's all hand operated.

[JamesCH95]

Doing the *quick* numbers shows that it holds on the order of 24,000joules of potential energy. That would be roughly equivalent to a 9lb weight going 170mph, think of a sledge hammer swinging many times faster than you could possibly hope to swing it, that's the sort of energy you're playing with here. Do a little Energy=Mass*(speed)^2 math and figure out what would happen if one of your end-caps blew off. Bad news.