Optimum Ball Mechanics

[Jecht22]

I was just wondering if people would be able to help me with a few things before I go into prototyping our shooter.
Although our design is very common, here it is
4 wheels (2 on top and 2 on bottom)
2 CIMs
My 2 questions:

1. what is the optimum gear ratio for shooting the balls (Direct drive using CIM's or gearing them up to go faster)
2. What is the optimum angle to set the shooter at (fundamentally I see a 45 degree angle as the perfect angle to maximize distance but I wanted another opinion)

[Matt H.]

Originally Posted by Jecht22

My 2 questions:

1. what is the optimum gear ratio for shooting the balls (Direct drive using CIM's or gearing them up to go faster)
2. What is the optimum angle to set the shooter at (fundamentally I see a 45 degree angle as the perfect angle to maximize distance but I wanted another opinion)

First welcome to the forums. Your question is hard to answer because optimum is a very fuzzy word. Do you want to maximize the distance the ball travels? Do you want to minimize the error in your shots? Your answers to questions like those will determine the optimal design.

For your first question:
The speed of the balls when they exit the shooter will be roughly proportional the the RPM of the wheels. For a physical model of the system see this post (also by me): http://www.chiefdelphi.com/forums/sh...ad.php?t=99797

The accuracy of your shots will depend on your system's control of the wheel RPM and the amount of symmetry you can build into your system.

It's often a good idea to gear your motors for peak efficiency, which in the case of the CIM means 4614 RPM at 45 oz-in of torque.

For your second question:
45 degrees gives the maximum distance in simple ballistics. There are many reasons to choose a different angle. For example depending on where your shooter is located on the robot, a 45 angle allows your shot to be blocked.

[EricH]

Originally Posted by Matt H.

For your second question:
45 degrees gives the maximum distance in simple ballistics. There are many reasons to choose a different angle. For example depending on where your shooter is located on the robot, a 45 angle allows your shot to be blocked.

IIRC, a number of teams found something more in the 30s was more effective in 2006, once spin effects were factored in, and given that the target sloped into the field. But if you want a high arc for a more vertical shot to the hoops this year, you might want something in the 60s.

Build a mockup of your shooter (once you've answered your first question) and try it at different exit angles in an open area with a hoop under the landing path. The answers you get might surprise you.

[Tom I]

Originally Posted by Matt H.

It's often a good idea to gear your motors for peak efficiency, which in the case of the CIM means 4614 RPM at 45 oz-in of torque.

This may be me being stupid and missing something, but how exactly do you go about gearing motors for peak efficiency? I get that at 4614 RPM with 45 oz-in of torque you'll get the maximum power output from a CIM, but say I'm gearing up a thrower, how would I actually go about this? Do I have to account for the resistance of throwing the ball as the torque and calculate out what that will put on the motor?

[Jecht22]

Originally Posted by Tom I

This may be me being stupid and missing something, but how exactly do you go about gearing motors for peak efficiency? I get that at 4614 RPM with 45 oz-in of torque you'll get the maximum power output from a CIM, but say I'm gearing up a thrower, how would I actually go about this? Do I have to account for the resistance of throwing the ball as the torque and calculate out what that will put on the motor?

This was the exact question I was about to ask.

Also I had wanted to maximize accuracy but also have the option to shoot at range. I know that the further away from the hoop, the less accurate the shot but is it physically possible to run a ball shooting system like this one, and get the balls from one corner of the field to hit the top backboard on the opposite side. This would be useful not just for attempting to score but to feed the balls back to your side of the court.

[Ether]

Originally Posted by Tom I

I get that at 4614 RPM with 45 oz-in of torque you'll get the maximum power output from a CIM

No, you won't. Maximum power is at 2655 rpm and 172 oz-in.

Maximum efficiency is at 4648 rpm and 43 oz-in.

[engunneer]

Originally Posted by Matt H.

For your second question:
45 degrees gives the maximum distance in simple ballistics. There are many reasons to choose a different angle. For example depending on where your shooter is located on the robot, a 45 angle allows your shot to be blocked.

More for your second question. Given the effects of drag, 40 degrees will actually give you more range than 45.

I have a spreadsheet that shows many options for the drag-less case, and someone else has a nice one that includes drag, but doesn't generate the other data.

Estimate your initial velocity, and start at short rage. be sure to put in the height of your shooter off the ground (for mine anyway). You might be surprised at what angles come out.

[roystur44]

We use 1 direct drive CIM running at 9 volts and we can swish 30 foot shots.

The kids found out that ball compression is key. Make sure you test with the same kind of balls. We found the trajectory varies between the balls we bought and the balls that came in the KOP.

135 deg is a good starting point. Build a hood so you can vary the release point.

Being able to control your motor speed accurately is also key. The kids built a test box that can control a Jag with a pot to control the motor speed less the CRIO. They wired up a multimeter to monitor the voltage sent to the motor to vary the motor speed.