This design is cool. I don’t want to be a party pooper, but I have a few questions.
First, what do you mean by 2 CIMs and 4 motors? I only see 4 fisher-price type motors in the pictures, no CIMs.
Second, why in the name of Kamen would you smooth your wheels? the fact that you’re only getting 16 feet from a 4 motor shooter means you’re losing a lot of energy between the energy and the ball. The usual idea is to make the wheels high-traction so that less energy is lost to the wheels sliding along the surface of the ball.
Oh - thanks for spotting my goof. Thanks for commenting
I mean to say (CIMSim), the Andy Mark, Product but only managed to get the CIM written down. We left all 4 CIM’s in our drivetrain.
Good question on the wheels. I had read about this year’s knobbed FRC wheels tearing up balls so I took the wheels we have which were inverse crowned and had them smoothed flat thinking that the judges couldn’t complain about that at the competition. That leaves spacing of the wheels and compression of the ball being the major factor in tearing apart balls. Reading posts on CD I realized that 1 to 2 inches seems to be popular so I chose 2.5 inches at the major diameter of the ball. so the compression is 3 + 5.5 + 3 at the very middle, but the wheels contact further out where the dimension is more like 3+ 6 + 3 ( 1 inch on both sides compression).
BTW, the ball is nominally marked up so far with a very small number of test firings.
I talked to someone today and they said running the top rollers and bottom rollers at 10% speed difference will provide sufficient ball spin. I am eager to test that concept when I get to the high school again.
The 16 feet horizontal shooting was when the ball shooter was mounted onto the desktop as shown. When we tipped the device to 45 degrees we obviously shoot further but hit the wall where I was unable to measure true distance.
I think shooting at far distances may be difficult due to the ball being very bouncy. We are planning on dialing down the speed to “toss” the ball into the net. Experimentation is of course required.
I see. Also, what do you mean by the balls are “bouncy?” I’m not sure I understand what you’re saying. Are you talking about the deflection inwards made by the wheels? In addition, will you need to change the angle of the shooter to give you different angles/range?
I had a chance to visit the HOT (67) teams field and bounce a game basketball off their backboards. I concluded very quickly that a hard thrown basketball without some kind of ball spin would have the ball “bounce back” hence my term bouncy used above. I am sure teams that constructed practice game fields made a similiar conclusion early in the season.
There are other discussion threads related to ball spin that you may want to read if you need to read up on ball “back spin”. We intend to have the upper set of wheels spin at a different rate to induce back spin.
I am wondering what other team’s ball spin speed ratio has been experimentally determined to provide “adequate” back spin.
We’re pretty darn lazy, and it seems to take us forever to make just a single moving part for the robot. So we just used one motor, one wheel, one gear, and let the ball get a lot of back spin, which conveniently makes it like to go into the basket when shot at the backboard.
So to answer the percent spin difference question…we “spin” the upper “wheels” at zero speed (it’s just a couple pieces of metal, not wheels). and spin the lower wheel at about 3500-4000 rpm.
Have you guys been able to test with a polycarbonate backboard like the actual field will have? We haven’t shot at wooden backboard so I not sure how it compares to our polycarbonate one. From the testing we’ve conducted it looks to have a different bounce characteristic than shown in your video