Catapult issues with boulders

[wilsonmw04]

Greetings,
We have had great success throwing boulders from outworks with year with a pneumatic catapult. I say, "had" because we ran into a problem during Elims at DCHP. They changed the boulders. The new balls are stiffer than the older boulders. This difference changed the trajectory nearly a foot lower than what we were doing in Quals. I am shocked that the condition on the balls would affect the shot this much.

We took some slow-mo video of an old boulder(on the right) and a new one (on the left).
https://www.youtube.com/watch?v=dKPg...ature=youtu.be

We could use some different eyes on this problem. Your suggestions and comments would be appreciated.

[Ernst]

Is the bucket that the ball rests in just a bucket, or is it some kind of pneumatic suction cup?

I would consider either replacing it with a different shaped bucket that has longer guides to set the ball's trajectory, or just adding additional guides after the bucket ends. In the video you showed, they would go on the left side and curve up to better cradle the ball while firing. We use a catapult on our robot this year and have had very good results with a curved bucket. Instead of just pushing the ball out, the ball rolls off. This would definitely change your hard stop and release point, but I think it could take out some of the variability from the balls deflecting differently.

[JesseK]

Quick observation - it looks like the one on the right sits higher in the catapult 'cup' or 'tray' than the other one. More of the ball is exposed rather than supported.

[Brandon Holley]

The left video (by my naked eye) appears to be sitting in the catapult cradle just a smidge longer, which to me would point to it shooting a tad lower against the tower.

Is this what you are seeing?

-Brando

[Darkseer54]

Originally Posted by Brandon Holley

The left video (by my naked eye) appears to be sitting in the catapult cradle just a smidge longer, which to me would point to it shooting a tad lower against the tower.

Is this what you are seeing?

-Brando

Yes, the left video is the new ball, while the right one is the old ball.

[chapman1]

Since the mass of the ball wouldn't change with age, the difference must come from the "squishiness". More contact between ball and cradle would help reduce the difference in "squish"

After watching your video, I'd say the critical place to add support is at the back of your ball cradle. We experienced this in 2014. As the catapult accelerates, centrifugal force wants to move the ball away from the fulcrum and up over the back lip of the cradle. Balls of differing "squishiness" will behave differently going over the lip, and if the last point of contact is the lip rather than the full cradle, ball differences will be magnified. If you can add support to limit that roll away from the fulcrum, I bet your launch differences will go away.

If your cradle cannot be made larger, try making it more solid and tipping it toward the fulcrum to reduce "roll away".

[pandamonium]

We are another catapult team and we experienced a similar issue with our shot suddenly being different. Our catapult is not pneumatic and we didn't figure this out until after but changing the weight of the cradle can be adjusted to tune shots.

[tr6scott]

We are shooting with wheels this year, and I feel your pain. We have had issues with jamming competition balls all year. What really toasts my goats, is we ordered comp balls from the manufacture the day of kickoff. The called a week later and said they needed to substitute a "like" ball to willful the order, which we accepted, and they came in orange. We played week 1, and had shooter issues with jamming, we ended up burning up 4 775pros, so we redesign was in order. We had better success at week 4 until eliminations when the introduced all new balls. Again jams. I understand that game piece variability is part of the engineering challenge, but we have not even been able to buy an actual game piece since week two so that we could test the redesigned shooter.

To that I call foul.

Off soapbox.

To me, the issue with the new balls is the stickyness of the coating, not squishyness of the ball. So it appears that you use a vacuum generator to hold the ball, the funnel and the hose. I do not know your sequence, but I would look at making the vacuum a little less effective with a new ball. maybe some slits in the funnel? Maybe releasing vacumm prior to shooting, then move arm? Maybe move the vacuum funnel towards the bottom of the arm.

We shot with a pneumatic catapult in 2014, and the angle is all about release point. It seems the old ball is moving from the pocket before the arm hits the hard stop, where the new ball is still sitting in the pocket a little longer. You may be able to choke up on the shooter a bit and force the release point to happen with the hard stop of the arm, and gain some consistency. Not sure how that impacts making the goal.

[wilsonmw04]

Originally Posted by tr6scott

To me, the issue with the new balls is the stickyness of the coating, not squishyness of the ball. So it appears that you use a vacuum generator to hold the ball, the funnel and the hose. I do not know your sequence, but I would look at making the vacuum a little less effective with a new ball. maybe some slits in the funnel? Maybe releasing vacumm prior to shooting, then move arm? Maybe move the vacuum funnel towards the bottom of the arm.

We have nothing that fancy. The cup is simply a drip pan from a stove.

[wilsonmw04]

would it make sense to increase air flow to the cylinders? would two solenoids per cylinder work (or even legal)?

[notmattlythgoe]

Originally Posted by wilsonmw04

would it make sense to increase air flow to the cylinders? would two solenoids per cylinder work (or even legal)?

Are you using high flow solenoids? We used them in 2014 on our pneumatic catapult. To increase flow even further we allowed the return port on the cylinders to vent to atmosphere.

[ollien]

Originally Posted by wilsonmw04

would it make sense to increase air flow to the cylinders? would two solenoids per cylinder work (or even legal)?

It's not legal. See R90. That being said, what's the flow rate of your solenoid?

[Brandon Holley]

Originally Posted by Darkseer54

Yes, the left video is the new ball, while the right one is the old ball.

Catapults are finicky mechanisms. We went through a very intense and crazy design process with our 2014 catapult. When we started considering using one for this year's game (for consistency), we very quickly implemented two design rules based on our 2014 learnings.

#1. Pneumatic powered, no questions asked. This rule was easily accepted by having a nice small and lightweight game piece.

#2. Roll off the tips. This was a huge part of our 2014 design and something we jumped on right away for our 2016 version. By giving the ball a predictable spin we found it helped keep it on a more predictable trajectory.


When it came to diagnostics, slowmo video like you posted is mission critical. It's actually very encouraging to find a 'smoking gun' in the slowmo you provided. It gives you something to react to in your design aside from just adjusting things until it works right again.

This is my personal critique of what I see: Your catapult is stuck somewhere between a 'stay in the cradle' and a 'roll off the tips' design. It stays in the cradle for a while, but eventually does roll off, and the mechanism that is keeping it in the cradle for as long as it does appears to be the 'squishiness' factor of the ball.

If the boulders you are seeing on the field are consistently stiffer than what you originally tuned for, the solution is obvious - just retune your catapult to that ball. If you are seeing variation in the balls that are on the field between stiff and less stiff, then the real fix may be to adjust your cradle to either hold that ball in the cradle through the stroke - or to completely adapt 'rolling off the tips'.

Just my $0.02.

-Brando

[Kevin Ainsworth]

We did a catapult in 2014 and this year.

There are two reasons we found that the balls behave differently.
Your issue looks to be #2, but #1 might be contributing also.
Read below for more detailed information.
The new ball doesn't move as far up the cup as the old ball does.
Looks like your old ball slides up further, causing the ball to be further from the catapult fulcrum, causing more height/speed/etc. Sounds like the new balls are shooting lower, which is to be expected. They are not sliding up as far and shot from a slower/lower position on the catapult arm. We have a soft noodle behind the ball, mounted horizontally, and we do not allow the ball to move in relation to the catapult arm during the firing. This results in more consistent shot trajectory. A ball resting in the catapult where a huge chunk is missing can still cause issues, but not much you can do about that.
The higher spring back to the older balls also can add launch force and would add to the issues listed above, but to a lesser amount. The large surface area you have should reduce this effect to a minimal contribution, one benefit to the large cup method of containing the ball.

#1 The ball is "squished" when the catapult is fired and when the catapult comes to a dead stop the ball rebounds (springs) off the catapult. The different balls have different spring rates, for different reasons. Balls with large chunks missing tend to rebound back to their original shape faster since the foam can fill back up with air faster due to the large opening in the skin. Whereas the new balls expand back slower due to the single small vent in the skin requiring time for the air to reenter the ball. The amount of foam added also differs so some are denser than others. This requires you to slow the catapult arm down at the end of its travel to allow the different balls to rebound (return to their original shape) slightly before leaving the catapult. We found that allowing the surgical tubing to have minimal preload when the catapult arm was at firing position, allowed the catapult arm to slow down slightly before the limit straps stopped the catapult arm. This allowed the catapult to decelerate slightly and therefore allowed the ball to start returning to its normal shape before leaving the catapult.

Another cool catapult was 469's catapult from 2014. We were planning on using this type of device until we prototyped and found it was not necessary for the low forces required this year. 469 used a technique for reducing the stopping force distributed into the robot when the catapult reached the end of its travel. We were planning on using it to slow the initial arm speed so the ball moved a minimal amount with our long travel, tall catapult. 469 had an intermediate arm which they pulled on to load the catapult, it was tied to the main catapult arm with surgical tubing that acted as a damper. This allowed the main catapult arm to start off slower due to the rubberband effect. The intermediate arm pulled the catapult arm, and ball, via the surgical tubing. The intermediate arm also kept pulling after the catapult arm hit its stop reducing the force the catapult arm was exposed to and lessening the movement of the robot when the ball was released. Check it out, pretty cool idea. I would love to hear 469's explanation for their 2014 catapult, but that's our take on it.

#2 Depending on your "cup", "flinger", "cradle", etc. the way the ball leaves the catapult is also important. In 2014 we used manually adjustable flinger ramps that allowed us to vary the angle of release slightly to dial in our shot. The large balls would roll up the flinger fingers and the angle of release allowed us to adjust the height/release angle of the shot. That worked in a year when the balls "slide" along the flinger consistently. This year we found retaining the ball for the entire shot gave us more consistency. This years balls are tacky when new and more slippery when worn. So allowing the balls to slide or grab onto elements of the catapult can cause them to move up or down on the catapult arm (cup for you) and mess with the shot trajectory.

Hope this helps.

[JesseK]

Originally Posted by Brandon Holley

If the boulders you are seeing on the field are consistently stiffer than what you originally tuned for, the solution is obvious - just retune your catapult to that ball. If you are seeing variation in the balls that are on the field between stiff and less stiff, then the real fix may be to adjust your cradle to either hold that ball in the cradle through the stroke - or to completely adapt 'rolling off the tips'.

The boulders would start out stiff, and then they would get run over by a robot. A few boulders were completely flattened by robots that came over the CDF. In many matches there was a broad mix of new/used boulders, particularly on Saturday morning.