pic: Shooter damaged a Boulder

[abigailthefox]

Originally Posted by MrForbes

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!

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

[Jon K.]

Originally Posted by abigailthefox

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

Originally Posted by FRC Blog

Boulders The game pieces for FIRST STRONGHOLD are more expensive than we would have liked. We couldn’t find less expensive alternatives that had all the characteristics we were looking for, like being foam (so we didn’t need to worry about inflation levels or popped pieces), having some level of durability (though spinning shooters will put them to the test), being the right size, and, very importantly, being low bounce (to minimize the chance of boulders bouncing somewhere you really don’t want them to go). Hopefully, you won’t need as many of them as you did for a game like Rebound Rumble, in which you could carry several game pieces without penalty.

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

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

[philso]

Originally Posted by Chak

... 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...

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.

[Ginger Power]

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.

[waialua359]

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.

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.

[abigailthefox]

Originally Posted by Jon K.

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

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

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!

[electronicsdude]

Originally Posted by MrForbes

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!

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

[Ether]

Originally Posted by electronicsdude

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

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

[GMeyer]

Originally Posted by MrForbes

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!

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

[Tom Line]

Originally Posted by electronicsdude

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

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.

[billbo911]

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!

[bobl]

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.

[Ether]

Originally Posted by bobl

We are using 4" plaction wheels spinning at 2950RPM

How are you controlling the 2950RPM?

[bobl]

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.

[Ether]

Originally Posted by bobl

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.

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?

Originally Posted by bobl

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.

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