My team has been trying to use a pneumatic launching mechanism and we hit a brick wall today when we realized that it didn’t shoot high enough. At the practice field we were at, another team’s mentor mentioned that we should use a higher bore size than we are using. We presently use a 7" stroke and 3/4" bore. I know that increasing the bore size would increase the force, given F = PA but wouldn’t it also decrease the speed of the pistons, given Av = k, thus lowering the initial speed of the boulder and decreasing the effectiveness of our catapult? Am I missing something?
One trick you can use is employ a lock on the mechanism so you can pre charge your cylinder/s then release the lock. We did this on our 2014 bot.
Could you clarify what you mean? If I’m understanding correctly, the lock would hold the piston in place while it was actuating, but how would this increase the effectiveness?
you can also add another cylinder, or a longer cylinder (and move the mounting points). You’re correct that a larger bore cylinder might move too slowly…but then again, it might not…we had a couple of 1.5" bore cylinders on the pneumatic catapult that we used to launch the huge balls in 2008 and 2014.
but we also did a trick with pre charging them, we didn’t latch them…instead, we set up the geometry so the piston had to develop quite a bit of force, before it had enough umph to launch the ball. The trick is to have the cylinder mounted so one end is just above the line from it’s pivot, to the catapult’s pivot.
It’s easier to provide input if you have a picture of your current setup. Are you using just one piston? How far away from the goal are you trying to shoot from?
You can make larger bore cylinders move very fast with the right solenoids and layout of your pneumatic components. We currently have two 6" stroke 1.25" diameter cylinders that launch the ball very hard from near the outer works.
This seems quite similar to what we’re trying to do…I’ll get you a picture tomorrow to see if there’s a way we can change our design to do what you’re mentioning. I can provide a rough sketch, but it would be just that: a rough sketch.
I can get you a picture tomorrow once I’m in the lab. As it stands, we’re using two pistons. We’re trying to shoot from the outer works but I wouldn’t be opposed to shooting from the alignment line. How would ‘the right’ solenoids play a part here? We’re presently using these solenoids.
To move a pneumatic cylinder quickly, you need to get rid of as many things that slow down the flow of air as possible. Pneumatic tubing, the primary regulator, and the solenoids all restrict air flow to some degree. If you can optimize each of those to flow more air, then you can make you’re catapult cylinders fill faster and deliver more energy to the ball.
I’ve posted about this before (see this post), but the general idea is to minimize the obstructions that the air has to go through on its way from the storage tank to the cylinder. Use short lengths of tubing, get high flow solenoids, and put an air tank right before the solenoid so that it doesn’t have to go through the main regulator.
The solenoid you’re using has a Cv (basically, how much volume can flow through it in a minute) of 0.22. McMaster has a solenoid that we like with a posted flow constant of 0.75. The solenoid might not be what’s restricting you, though. It might be the tubing and the regulator that’s taking away most of your flow.
We did a few tests with prototypes using our maxed out flow setup and found that we were just reaching the high goal from the outerworks with two 1.0625" bore 6" stroke cylinders. Smaller volume cylinders weren’t quite making it. I suspect that you might need more volume in your cylinders than you’re currently using to put enough energy into the ball.
When we were testing a piston launcher we had the same issue and to fix it we only plugged one side into the solenoid and left the other side of the piston open.
Edit: We retracted it with rubber bands
I’m not sure about the rules on having only one side connected so I would double check that.
Also we were using the higher flow rate solenoids mentioned above. And I think we were shooting from 3 feet off the ground and at the outerworks.
You want the arm down position to have the direction of extension be pointed at the axle that the arm rotates about. You can then pre-charge the cylinder with out it moving. Then use a cam to break the linkage out of a straight line and it will extend rapidly gaining speed as the leverage on the arm increases.
It would be best if the piston wasn’t all the way in when the arm is down. Hopefully geometry of the existing robot will favor you.
I’ll try moving an air tank tomorrow and messing with some geometry. If that doesn’t work, we’ll order some larger pistons. Could you link me to the solenoid you’re referring to?
Sorry, what do you mean by a cam?
Something like this
I’ve tended to think that something shape like this gives the most amount of force to start the system moving.
Team 1108 has been doing pneumatic catapults for years with some winning designs.
We design so the cylinder isn’t fully retracted so there is a ‘chamber’ in the back half. Something like 4-inches for a 12 inch stroke. We fill the cylinder with any-old slow valve while we’re holding it down with an electro magnet holding a puck of steel on the catapult. Let go of the magnet and it flys!
Here’s the magnet we’re using this year…5.6 watts/ http://www.mcmaster.com/#5698k116/=118b3ox
So I would use the cam to push the piston arm out slightly?
Wow this is neat but I’m not sure how well we’d be able to mount it given our current setup. I will be sure to look into this though. Thank you! Do you have any pictures of how you’ve mounted this on your bot?
I don’t have any straight-on pic available right now, but I think you can see it in this one okay. The cylinders are on the top and the magnet is at the left, under the hoop of the gray PVC catapult. And yes, just PVC pipe has worked great for us. Least expensive material in the world, but the flex it has actually makes longer shots than an aluminum catapult.
I must be missing something here. Under that gray PVC loop I see a bunch of air tanks but I don’t see a magnet anywhere, nor do I see any pistons that go into the PVC. Could you point me to where I should look at this? Sorry! I hate to bother you with this.
I’ll see how I can utilize this tomorrow… Hopefully I’m able to fit something in there. Thanks!