Additional questions about improving pneumatic catapult

[Wren Hensgen]

There are several things you can do to help speed up your shot. The most dramatic way that we have discovered is to latch the cylinders. We use a simple gate latch to hold the end of our catapult down. While it's held down, we open the valves, allowing air to flow into the bottom of the cylinders. When you release the latch, there's already considerable pressure built up behind the pistons. This method also allows easy varying of the shot by choosing different times (in milliseconds) to latch before each shot. If you have any questions on this method, please ask.

[dougwilliams]

We also implemented a dual cylinder pneumatic launcher - similar to what you are trying to achieve. There are a few things I would do to yours to maximize your launch:

1- At the back of your launcher arm you need small "armlets" (we called them) that will cradle and hold the ball. This will be the last point of contact as the ball is going through the arc of the launcher and will (drastically) effect the launch trajectory. Your launch appears to be going more up, and needs more forward. The angle and length makes a big difference - even a 1/4 inch changed distances and heights for us.

I had a proto launch video that I cant find , but I will link these two of our completed robot:

http://www.wicz.com/news/video.asp?v...Eflv&zone=News

http://www.wbng.com/news/local/Tiger...249661331.html

You can see the geometry better than I can describe.

2 - You are giving up a good amount of energy by only using half of the stroke on those pistons. You can pull them backward or raise them up and use up more stroke. I don't think you need a third. Based on that, you need to be mindful of cam-locking the throwing arm.

3 - I agree with the gate latch and pre-pressurize the cylinders in a retracted position and then just release the latch. (It worked for us).


Looking at your construction - You will need to heavily reinforce it. I know it's a prototype- even what we did in our competition robot was put under immense stress and pulled itself apart and needed reinforcement during the regionals.

Airflow was a bit of an issue as well and we minimized all runs of tubing and extra T's and such in the system.

[TheHolyHades1]

Originally Posted by dougwilliams

We also implemented a dual cylinder pneumatic launcher - similar to what you are trying to achieve. There are a few things I would do to yours to maximize your launch:

1- At the back of your launcher arm you need small "armlets" (we called them) that will cradle and hold the ball. This will be the last point of contact as the ball is going through the arc of the launcher and will (drastically) effect the launch trajectory. Your launch appears to be going more up, and needs more forward. The angle and length makes a big difference - even a 1/4 inch changed distances and heights for us.

That's certainly one thing we were considering. However, the problem is that if an opposing team's ball was to get into our system then we wouldn't be able to release it without shooting it, which would be a penalty on us. How were you able to avoid this problem?

Originally Posted by dougwilliams

I had a proto launch video that I cant find , but I will link these two of our completed robot:

http://www.wicz.com/news/video.asp?v...Eflv&zone=News

http://www.wbng.com/news/local/Tiger...249661331.html

You can see the geometry better than I can describe.

2 - You are giving up a good amount of energy by only using half of the stroke on those pistons. You can pull them backward or raise them up and use up more stroke. I don't think you need a third. Based on that, you need to be mindful of cam-locking the throwing arm.

Interesting design, congrats on going to nationals and on making the news. The problem that I'd think we'd have with implementing something like that is that you've got the pistons pointed horizontally (roughly), instead of roughly vertically as we do. I'm not entirely sure how we could stick to this and still be able to have the pistons start at their low point and extend fully, though I'm certainly open to any suggestions for anything that may have slipped my mind. We considered mounting it similar to the way you do, ie. horizontally, but the way the robot is built right now I'm not exactly sure where we could go about doing that; again, if you have any suggestions I would love to hear them.

Originally Posted by dougwilliams

3 - I agree with the gate latch and pre-pressurize the cylinders in a retracted position and then just release the latch. (It worked for us).

Could you perhaps post a bit more information about your gate latch system, and what you're using to latch/unlatch it?

Originally Posted by dougwilliams

Looking at your construction - You will need to heavily reinforce it. I know it's a prototype- even what we did in our competition robot was put under immense stress and pulled itself apart and needed reinforcement during the regionals.

Gotcha, thanks. If this ends up getting mounted to the robot, we'll still have approximately 10 pounds to work with, so nearly all of that can go to reinforcements and it almost certainly will; I figure a bit of energy is being lost in vibrations (as seen on the testing rig), so some reinforcement should certainly help.

Originally Posted by dougwilliams

Airflow was a bit of an issue as well and we minimized all runs of tubing and extra T's and such in the system.

We will make sure we do that, thanks. Did you find that the solenoids were limiting you after you did this optimization?

[dougwilliams]

Originally Posted by TheHolyHades1

That's certainly one thing we were considering. However, the problem is that if an opposing team's ball was to get into our system then we wouldn't be able to release it without shooting it, which would be a penalty on us. How were you able to avoid this problem?

With our entire robot, our intake arms could raise or lower and allow the ball to be captured, or not captured. Our easy floor pass was raising our arms all the way a short burst backwards driving and the forward and the ball would fall on the floor where it was. In competition we wound up playing forward and generally shooting the ball, or playing defense and shooting over truss and didn't need the pass often. In my own opinion, the chance of an accidental "catch" was so slight we didn't worry about it. We would have to actively try and pick up an opposition ball.

Originally Posted by TheHolyHades1

Interesting design, congrats on going to nationals and on making the news. ?

Ha - we aren't - you can chalk that up to news reporter misunderstanding. We are in NY and went to the Troy, NY and Richmond, VA regional competitions. We did make it to semifinals and got to compete against the winning alliance though, and that was great fun. We live in a small area though, and were trying to drum up some interest in the program. I posted that video because it was the final configuration, and had some decent shots of the geometry.

Originally Posted by TheHolyHades1

The problem that I'd think we'd have with implementing something like that is that you've got the pistons pointed horizontally (roughly), instead of roughly vertically as we do.

It's all a trade off and I think either could work. We are more horizontal, and in fact I was trying to have the whole arm go beyond the pivot point as azcalg mentioned, effectively cam-locking it so we wouldn't need any latch. In the end, we were concerned that we would accidentally fire without a real latch if we got bumped too hard. with your specific robot and mounting points, you probably need to adjust some geometry until you find something that works.

Originally Posted by TheHolyHades1

Could you perhaps post a bit more information about your gate latch system, and what you're using to latch/unlatch it?

http://www.tractorsupply.com/en/stor...ate-latch-zinc

Fairly standard style gate latch. We actually used a small pneumatic piston to retract the gate latch. 1.5 in stroke or so, and 1/2 diameter? It was tough to get it working right though. We ended up angling the latch mount so the whole latch was mounted on an incline so the opening followed the arc of the launch arm. We also mounted a u-bolt to a back cross vrace across teh launcher arm as the piece that was latched.

DONT DO THAT.

Because it's curved it had a tendency to let the sides of the launch arms rock back and forth awkwardly. Instead - use a straight bar stock somehow- or we assume that would have been better. I can try and get a picture tomorrow to show what I mean.

Originally Posted by TheHolyHades1

We will make sure we do that, thanks. Did you find that the solenoids were limiting you after you did this optimization?

The force and sudden stop on this was able to rip through a number of plastic bearings. Don'd underestimate how much reinforcement you need. We were fine for a practice and had to strengthen it at competition because it was slowly dismantling itself.

Also - be safe. Don't stick your head over it. Our was certainly strong enough to seriously injure someone. We used a nylon safety strap around the whole launcher for carrying on and off field.

Yes, it appears solenoids were limiting us. We wound up using a storage tank on the typical low side behind the solenoid, and another on the front side of the solenoid on the launch tubing. That was plumbed straight in to the bottom of the piston and we did did not use any exhaust side fittings on one of the pistons. This was also pre-charged, and allowed a quicker transfer of air in to the piston. Our launcher would stay upright, so to retract we only plumbed one of the retract side cylinder ports to the solenoid.

We have a few post season ideas to play with and see if some other arrangement would have made for quicker cycling times on the pneumatics.

[azcalg]

I know I told you guys to move the pistons closer to the pivot point but have you tried it with them not that close? You really should be using the full extension of those pistons, we have the same cylinders as you and initially used the same solenoids so it's definitely possible. You may also want to decrease the angle between the pistons and the catapult if you want to avoid using a latch. I'll try to get some pictures of our shooter for you. Our shooter's resting angle is only several degrees below the horizontal and is ~60 degrees above the horizontal when fully extended and because it's nearly horizontal when it's resting the ball sits neatly on top and doesn't have to be held. We dominated with our shooter during qualifying matches finishing in 4th so it's a proven design, just requires some tweaking. Also you should avoid dry shooting like you did in that video like the plague. Good luck guys.

[MrForbes]

We have our pistons extended about 1/3 of their 8" stroke. We don't use a latch on ours...instead, we rely on the geometry of the system to keep the ball from launching until the cylinders build air pressure, and then whammo, they move and launch the ball. The cylinders start out almost parallel to the arms.

I think it works ok, we were finalists...but if you want to build a really good one, find team 842's web page, where they show how they built theirs. It's different, and better.

https://sites.google.com/site/falcon...ots/2014-dream