pic: Shooter damaged a Boulder
 

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The ghosts of Rebound Rumble have returned...

Re: pic: Shooter damaged a Boulder
 

What tread were you using on the plaction wheels? I'd avoid roughtop and go for a smoother tread.

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Who'd have thought having a fast moving rough piece of plastic pressed up against a thin plastic coating on a foam ball would do damage?!? :p

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After playing with the ball for a few minutes, I have very little interest in trying to shoot it using a spinning wheel. It's not that type of ball.

However, building a medieval siege engine (catapult) looks like a neat plan!

Re: pic: Shooter damaged a Boulder
 

IMO, the million dollar question: does the puncture in the outer layer change its squishiness and/or how quickly it returns? It seems like the squishiness is at least partly driven by forcing air in and out through the one little hole in the jacket/coating... I'd think more puncture holes could dramatically change that (and thus how it shoots/flies... particularly in a flywheel/compression shooter).

That's what I've been thinking since I first saw their differences from the Basketballs & Poof Balls... without having tried shooting Boulders (worn and unworn) with both catapults and flywheels (and impact launchers, I suppose) it's obviously all just educated guess-work.

Re: pic: Shooter damaged a Boulder
 

I think that there are good qualities to both methods, but I think I would still rather have a flywheel shooter because it's more accurate. I may be wrong, though. Also, I'm sure that there will be replacement balls available between matches.

Re: pic: Shooter damaged a Boulder
 

Look at Team 16's catapult from 2012.

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One thing about using a shooter is that it kind of tears up balls, which is the subject of this thread...and the balls cost a lot. I don't think our team can afford to test and tune a shooter this year!

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DITTO

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May I ask: were you commanding full voltage to those miniCIMs, or some fraction thereof?

Re: pic: Shooter damaged a Boulder
 

We've run a ball through a shooter prototype around 10-15 times with no damage at a few different compression setting. I strongly suggest using a wheel with a smooth surface (Colson, Stealth, etc) if working on a flywheel type shooter.

Re: pic: Shooter damaged a Boulder
 

Are you gently pushing the ball towards the spinning wheels and allowing them to grab the ball or are you vigorously pushing the ball into the space between the spinning wheels?

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The GreenHorns' wheeled shooter is at the point where we aren't tearing up the ball anymore (initial tests were brutal on that ball!) Thanks to Andymark for the donated game piece!!! Now that it's tuned we're throwing about 15-20 feet and it's very accurate. Our current game piece is beat to heck and the flight path hasn't changed since all the little nicks and cuts developed. We have a new ball and an old ball and they fly the same.

Don't be afraid of wheeled shooters this year. The 1114 style articulating arm is going to be gold this year (it is almost every year...).

Re: pic: Shooter damaged a Boulder
 

Thanks for the report!

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DITTO

Re: pic: Shooter damaged a Boulder
 

Our team has found that the signature Vex "Colson Performa" (The smooth grey ones) wheels dont tear up the ball without sacrificing initial performance

Re: pic: Shooter damaged a Boulder
 

The boulders do compress and take some time to return to full size. Any thoughts on compressing in robot to make a smaller "object" to get into the tower opening?

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I don't think you'll get much out of compressing the ball. Shooting a ball has been done year after year. I think teams has mastered the techniques of catapult and ball pitcher methods.

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Fingers cross they don't introduce brand new balls with different properties for the championship eliminations again...

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I don't think this is something I would design my robot around. We're compressing the ball about 3" and it doesn't noticeably change the shape of the ball after it's out of the shooter. If you compress it any more you'll have to worry about things like stalling your shooting motors. Then you can try putting bigger wheels on the gain some linear speed while gearing down the motors. By then you'll notice the shooter is clunky and doesn't fit into tight places where you want it. Then you'll realize wow, this change is causing more trouble than it's worth. Smaller wheels, fine tuned compression, low gear ratios, and high rpm's. Those are the constraints I'd be designing my shooter around.

Re: pic: Shooter damaged a Boulder
 

I have managed to find to different places where you can get the balls from. Here are the links.
http://www.andymark.com/FRC16-p/am-3276.htm
http://www.gophersport.com/pe/premiu...oam-dodgeballs

Hope this helps.

Re: pic: Shooter damaged a Boulder
 

I have a feeling that this is what the GDC wanted you to feel. It was no coincidence that from the material down to the name of the game piece it automatically scream catapult.

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Well...
BALLISTA FOR THE WIN

Maybe.

Anyways, had to stop a rookie from picking at the skin of our boulder... It seems heavier than it looks.

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Thanks for the suggestions! We were prototyping on the second day and grabbed whatever 6'' wheels we had on hand to throw something together:D . Now that we've seen what plaction wheels can do to our ball, I think we'd be using Vexpro colsons or one of Andymark's colored stealth wheels, depending on how they perform in a later prototype.
We were running the motors as hard as we could. I think we can safely assume that the motors were getting full voltage.

I wasn't the one putting it in, from from my perspective, we were not vigorously pushing the ball in, nor were we especially gentle. We just put it forward and watched it shoot.
Is that a significant factor in damaging/shooting the Boulder? After all, the robot would be pushing the ball into the shooter with some amount of force anyhow.
Thanks for the advice! We were going to use smaller wheels, but we had a scrap piece of 2x1 handy to fit a 6'' wheel shooter. Can you describe your "fine tuned compression"? I've always assumed that moving the 2 flywheels on the same shaft closer together would be enough to vary compression. Is that true?
What is the 1114 style articulating arm? A quick google search yields this thread.:confused:

Re: pic: Shooter damaged a Boulder
 

http://www.chiefdelphi.com/forums/sh...14&postcount=7

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Fine tuned compression: The point where the wheels don't slip on the ball and it doesn't leave black streaks, but it isn't over compressed. It isn't an exact science (for Ri3D at least...), but we found a compression of 3 inches gave us the results we were looking for. I can't speak to moving the flywheels on the same shaft closer together and how that affects the compression. Making an easily adjustable prototype is the best way to determine this :D

Simbot SS, 1114's robot in 2008 inspired their 2014 Robot.

Re: pic: Shooter damaged a Boulder
 

I'm more impressed that you actually have a shooter. Or maybe my team is just a bit slow off the mark.

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A Ballista is a type of catapult, along with the Onager and Trebuchet are the 3 major types of catapult.


The More You Know!
/wavehandfromlefttoright

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Just want to comment on this, we also found that 3 inches of compression is ideal. This ball can really fly.

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Has FIRST been able to offer a good reason WHY basic foam balls are priced so highly?

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- See more at: http://www.firstinspires.org/robotic....4XOPPEDm.dpuf

Not so basic. They had a lot of criteria to hold.

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It may be that it takes longer for the wheels to grab the ball and shoot it when the ball is pushed forward gently allowing the wheels more time to abrade through the outer coating of the ball. This is just a hypothesis and you would have to specifically test for the difference.

Re: pic: Shooter damaged a Boulder
 

Some new testing with our wheeled shooter for the GreenHorns showed some great results. We started playing basketball with one of our boulders around 3 AM (used the tower for our hoop... hopefully robots will be more accurate than me...)while waiting for the programmers and it got pretty beat up. When they were finally finished we set our robot up in an orientation that it was making 100% of its shots with that beat up ball. Next we introduced an untouched ball to the shooter and it was still successful.

The tests tell me that wheeled shooters aren't terribly affected by wear on the boulder. For reference we were shooting from about 7-10 feet away from the tower.

Re: pic: Shooter damaged a Boulder
 

We have already tried both a wheeled shooter and catapult prototype design with the option to change certain variables.
Compression on a wheeled shooter seemed optimal at 3" compression. We tested 2" also with poor results. We also used direct drive CIMs on an older AM wheel similar in texture to the stealth wheel.
We didnt even bother trying the plaction wheels because our goal isnt to create a shredder.:p

Shooters and catapults have already been built by many teams and there are a ton of old video clips from previous seasons.
IMO, building either is the trivial part.

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Hm. It just seems like a foam ball, even built to specifications, shouldn't be quite that pricey, although as always I appreciate the thought from FIRST that went into the game piece even when it seems basic!

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I will testify that the catapult works, I will also testify that it creates one hell of an accuracy problem.

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Please tell us more about the design of your catapult that has an accuracy problem.

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I wouldn't expect that to work either, but it's worked for my team so far. I have a video here:

https://www.instagram.com/p/BAdNlxTj...y=team293spike

Re: pic: Shooter damaged a Boulder
 

I suggest you take a look at the end of the catapult. Do some slow motion video on it - even the slow motion that a Samsung phone can shoot. We did quite a bit of testing in 2014 and found the biggest contributor to inaccuracy from a catapult is when the ball 'rolls' off the ends of the catapult as it is shooting. As a result, we put wedges on the end of the catapult to keep the ball on until the end of motion.

In addition, keep close mind of how you are powering the catapult. If you are using motors, it will be inaccurate unless you are accounting for battery voltages, motor speeds, etc. If you are using some type of spring, get a fish scale so you can measure the spring tension and make sure it is consistent. Also insure you are shooting from a hard stop to another hard stop on the motion so that the catapult follows a proscribed motion each time.

People with wheels need to worry about slip, moment of inertia, and wheel velocity and how the ball is introduced to the shooter. Catapults just offer different challenges.

Re: pic: Shooter damaged a Boulder
 

We just received two Rhinoskin balls.
Both measured the same diameter as our KOP ball +about 1/16th".


In addition to measuring their diameter, we compared their compress-ability to both a new KOP ball and the well used one in the image.

We placed a 13.875 LB battery on the ball and measured it's compressed height.

Used KOP boulder = 8.25"
New RhinoSkin = 8.5"
New KOP boulder = 8.675"

So, all in all, they appear to be a good match. That said. actually shooting them will be the real test!

Re: pic: Shooter damaged a Boulder
 

We put a small tear in the skin not by the spinning wheels but by hitting a corner inside the tower. We have since put a stop (piece of lexan) in the tower to stop the boulder and roll it back.

We are using 4" plaction wheels spinning at 2950RPM and have only minor scuffing on the skin. We are compressing the boulder 2" and are able to hit the high goal consistently (19 for 20) sitting on the outer works.

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How are you controlling the 2950RPM?

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We aren't really controlling it. Using a Banebot RS775 with a 3:1 transmission. Running at full power our tach shows a constant 2950 +- 15RPM. No encoders installed. The shooter is a prototype but it shows a lot of promise.

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The Banebots M7-RS775-18 motor has a 12v spec free speed of 13,000 rpm.

But you say the motor is spinning at only 2950*3 = 8850 rpm with a fully charged battery.

8850 rpm is only 68% of free speed. Of course the motor speed will be less than its free speed, but 68% is quite a bit less. Some rough calculations indicate the motor would be drawing in the neighborhood of 30 amps. Can you measure the amps the motor is drawing to confirm this?

Try reducing the motor voltage by half a volt and see what happens to your RPM and whether you still hit the goal.

Re: pic: Shooter damaged a Boulder
 

We are running 4" stealth wheels that are connected 1:1 onto mini cims. The only time we damaged the ball was when we were testing, and somebody forgot to reverse the left motor. This made both motors spin the same direction. It made two small gashes in the ball but it's still usable. We have another ball we bought, but we don't want to use it until we get the final version of our shooter made. Has anybody noticed a difference in performance in a wheeled shooter from a new ball compared to an old vall? And does anybody know how often they will switch balls out in the game?

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What speed are you running it at?

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To be honest, I don't really know. We have all of our electrical on a practice robot so we just plugged the motors into an old power distributor, with no motor controllers. It shoots more than far enough, just need the correct angle.

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The correct angle when you are running it straight off a fully-charged battery may not be the correct angle toward the end of an actual match when your battery is getting tired.

That's why it's important to test your prototype to see how much the correct angle is affected by voltage. If the effect is enough to be a concern, you will want to design your shooter to operate at a lower voltage (say 9 volts) and then used a closed-loop controller (PID, bang-bang, Take-Back-Half, etc) to hold the wheel speed constant as the battery gets weaker.

Re: pic: Shooter damaged a Boulder
 

We were planning on getting a gear box to run at 2:1 so we could decrease the speed of the motor but still shoot the same. The mini cims have no issues with shooting the ball through a 7" opening, the motors don't really even make a noise as the ball goes through. We just need to figure if we can afford to run all the motors we want on our robot (11 total).

Re: pic: Shooter damaged a Boulder
 

2:1 would reduce your wheel speed, not your motor speed.

Did you mean to say 1:2 ?

If so, you might want to instead consider increasing your wheel diameter by 50% and reducing your motor speed appropriately.

Re: pic: Shooter damaged a Boulder
 

We made a second iteration of our shooter that uses 4'' colsons and no longer rips up boulders. We're running that shooter off 2 775pros. We've noticed a significant and problematic difference between a fairly new boulder and the boulder pictured here, which is now even more torn up than before. I think that this is caused by the torn up boulder losing air faster and not "pushing back" as much on the shooter wheels. I guess catapults have the edge on this problem, although they might have their own problems.

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Yes, that is what I meant. So every 1 rotation of the motor, the wheel rotates twice. But it appears you understood what I was trying to say atleast. Thank you for correcting me.

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You might want to ask around for opinions from other teams who have tried that, before investing too much time in it. Just a thought.

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I also had made a prototype that resulted in this. I used a 4 inch hi-grip wheel. Only shot it about 10 times and it is covered in bruises. We named the ball Ricardo, so out little Ricardo was in a little bit of pain.::ouch::

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What kind of compression, gear ratio, etc. did you have for your shooter?