Additional questions about improving pneumatic catapult

[hrench]

Originally Posted by tr6scott

This is a problem, to get a fast reaction time, you need to have cylinders at the bottom of the stroke. So this is a problem if you are not using a latch and pre-pressuring the cylinder prior to launch.


If you go with the latch method, they that volume of air in the cylinder, is basically a low pressure storage tank, which would be beneficial to that system.

I'm on the other side of this--I think there are two ways (at least) to make a pneumatic catapult work and the one my team used is to not retract the cylinders the whole way and instead fill the cylinders 'chamber' to 60 psi while the catapult is latched. If you simply hold down the catapult with your hand while these fill and then release, you'll see how that would work.

The other way is what tr6scott is talking about, it would be to use small diameter cylinders and lots of stroke and large Cv valve, hoping to get enough travel to get speed.

Since you have large diameter cylinders, I think you need to stick with the first plan. You need a latch. The tractor-supply gate latch will work fine, but if you want simplicity and no failure, I recommend you put a small piece of steel on the end of the catapult and hold it down with an electro magnet. The one we use is rated at 180 lbs of pull and uses 5.6 watts.

www.mcmaster.com
In stock


$62.19 Each

5698K116

Also, I think your current catapult arm is too heavy. We use PVC. Really. Just clamp it with u-bolts---don't drill holes in it.

I don't recommend three cylinders--recovery time will take too long using that much air.

[TheHolyHades1]

So, part of the issue we found was that the ball was rolling off the ramp as it was firing. We fixed that by adding a bit of metal to the end, the ball now follows a much more predictable path.

We've ordered high flow solenoids as suggested by team 842 on another of my threads; this should hopefully allow the air to get in a lot faster. The other thing that we did is adjust the angle a bit so that the pistons start at the bottom of their extension and are stopped half way through or so by a rope.

We have, however, come across a new problem. Namely, the cheap hinges that we are using take a heavy beating from the pistons and experience a lot of torque against the point where they're supposed to be hinging; we're looking at steel hinges right now, and we plan on hinging the entire ramp instead of just at 2 points. Hopefully this will allow for better power conversion.

After doing these changes, the ball is able to follow a path that's just barely high enough for it to get over the truss; we hope that the high flow solenoids, which will arrive tomorrow, coupled with a stronger hinge or two, will allow us to fully use the power.

[Christopher149]

I have no particularly good pictures offhand, but here's something:

early prototype video
picture from before first event, I think

We have two cylinders (I'm not sure of bore size) that are 8-10 inches stroke, I believe. The up and down strokes are powered by separate SMC double solenoids like http://team358.org/files/pneumatic/S...alOverride.jpg from kits past, so not particularly high flow. This allows the shot to only vent the topside from atmospheric instead of 60psig. Also, only one downstroke port is connected; the other has no fitting. These push a bar of c-channel whose fulcrum is a bolt. The cylinders point just above the fulcrum, allowing pressure to build before overcoming the weight of ball and bar. The ball is held in place over the bar by the surrounding 80/20 bars. It can make the truss and the high goal from in front of the low goal.

It took a while of playing with dimensions and number of cylinders to get right. There have been several tweaks since these images: a little spacer to elevate the arm at the rear, moving the connection between arm and cylinders a couple holes forward, adding the second valve to not fight pressure on top half of cylinder. Presently, we have 7 accumulators (of at least 3 models, darn space problems) to store air and have an on-board Viair compressor.

Compared to the OP, the shooter is quite close to the ground, so the power is available for a good shooter.

It was finalist at 16-3-0 (11-1-0 through quals) at Traverse City.


Does this help?

Edit: summary of my suggestions: place cylinders more in line with fulcrum, try to use more of stroke, lighten the arm, eliminate flex, keep the ball steady. Use at own risk