Prototype Small Wheeled Shooter

[Akash Rastogi]

Originally Posted by falconmaster

Ok now we did 5, 3, 1 ratio

Shooting farther! Still have flex in base, will get more when we stop this
http://www.youtube.com/watch?v=wGp3M...ature=youtu.be

Very nicely done.

[falconmaster]

Originally Posted by AllenGregoryIV

How is that 27 feet being measured. With just our two wheel shooter we were getting 30+ feet with the shooter mounted 2 feet off the ground and shooting parallel to the floor.

Think its due to our base flexing, it will not in the final model. We did shoot from around the Pyramid so it may be 30, 27 was in straight line

Got slow mo of two wheel
http://www.youtube.com/watch?v=cq3feDdl6A0

[Ether]

Originally Posted by falconmaster

Ok now we did 5, 3, 1 ratio

Shooting farther!

That tells me you had a lot of slipping in the 3rd stage. Maybe you still do. Try 2:1 for that second stage.

[falconmaster]

Just measured the latest distance in the hallway, from video last posted of three wheel shooter. 48feet! from 2 1/2 feet above floor

[Bob Steele]

Originally Posted by JVN

If you're clever, with a custom mount plate and a trimmed down shaft, you can mount a MiniCIM to a VersaPlanetary gearbox.

-John

isn't it illegal to trim the shaft of the motor down?
Well not illegal in Texas perhaps but illegal in FIRST?

R

[Ether]

Originally Posted by Bob Steele

isn't it illegal to trim the shaft of the motor down?

4.1.7.2 R33-A

[AllenGregoryIV]

Originally Posted by Bob Steele

isn't it illegal to trim the shaft of the motor down?
Well not illegal in Texas perhaps but illegal in FIRST?

R

R33
The integral mechanical and electrical system of any motor may not be modified. Motors, servos, and electric solenoids used on the ROBOT shall not be modified in any way, except as follows:

The mounting brackets and/or output shaft/interface may be modified to facilitate the physical connection of the motor to the ROBOT and actuated part.
The electrical input leads may be trimmed to length as necessary.
The locking pins on the window motors (P/N: 262100-3030 and 262100-3040) may be removed.
The connector housings on the window motors (P/N: 262100-3030 and 262100-3040) may be modified to facilitate lead connections.
The Integrated Encoder Module (P/N: 276-1321) may be installed on the VEX 2-wire Motor 393 (P/N 276-2177).
The VEX 2-wire Motor 393 (P/N: 276-2177) gears may be changed or replaced per the Supplier instructions.

R33 give us permission to modify the output shaft otherwise tightening a set screw too hard would be illegal.

[JVN]

Originally Posted by Bob Steele

isn't it illegal to trim the shaft of the motor down?
Well not illegal in Texas perhaps but illegal in FIRST?

R

Thanks to Allen & Ether for posting the relevant rule. It is allowable per FRC Ultimate Ascent rules to modify a CIM shaft in this way.

I have contacted the Texas state legislature regarding your question, but have not heard back yet. My buddy is a cop, and he says he's never had to arrest anyone for it, and thinks that if there is a law against it, it is probably one of those ones which isn't strictly enforced. I think we can assume it is okay until we hear otherwise back from the TX lawmakers.

Luckily, Gov Rick Perry is a fan of robotics and FRC -- if there is a problem in TX maybe he can help us resolve it?

-John

[JohnSchneider]

Originally Posted by JVN

Thanks to Allen & Ether for posting the relevant rule. It is allowable per FRC Ultimate Ascent rules to modify a CIM shaft in this way.

I have contacted the Texas state legislature regarding your question, but have not heard back yet. My buddy is a cop, and he says he's never had to arrest anyone for it, and thinks that if there is a law against it, it is probably one of those ones which isn't strictly enforced. I think we can assume it is okay until we hear otherwise back from the TX lawmakers.

Luckily, Gov Rick Perry is a fan of robotics and FRC -- if there is a problem in TX maybe he can help us resolve it?

-John

No we just turn robotics into an underground sport down here. Think of the wonderful and terrifying implications.

[Ian Curtis]

Originally Posted by AllenGregoryIV

R33 give us permission to modify the output shaft otherwise tightening a set screw too hard would be illegal.

There is a special place in the afterlife down under (hint: not Australia) for people that crank down set screws into keyways.

[s_forbes]

Originally Posted by Ether

That tells me you had a lot of slipping in the 3rd stage. Maybe you still do. Try 2:1 for that second stage.

Unfortunately, 2:1 is not an easy off-the-shelf gear ratio to come by. These small wheel shooters are super easy to build with a vex planetary and just about any kit motor. Available reductions are 1:1, 3:1, 4:1, 5:1, 10:1 and any combination of those you would like. You can still adjust the tangential velocity of the wheels by changing wheel size (2", 3", 4", 6" are easy from Banebots) or input motor. Bag motor spins at 14k rpm, AM motor at 16k, and Banebots 550 at around 19k Our best setup at the moment is a 5:1 Bag, 3:1 AM, and 1:1 Bag.

Just looking at the math, it's obvious we have a lot of slipping on the last wheel. By my calculations the tangential velocity of a 3" wheel on a 1:1 Bag motor is roughly 175 ft/sec. If a frisbee had no slippage when contacting a wheel in this type of shooter, I would expect it to leave at about 85 ft/second! This is waaay higher than what we are seeing, so I have no doubt that there is a lot of slipping going on.

Something interesting to note: we put a new frisbee into our two wheeled shooter (3:1 Bag, 1:1 Bag) and analyzed the marks left by the shooter wheels afterwards. It looked to me like the 3:1 wheel had more slippage than the 1:1 wheel, since the 3:1 had a shorter 'burn' mark left on the frisbee and shorter overall contact period.

Also interesting to note: given the frisbee compression and wheel spacing involved, the frisbee contacts two adjacent shooting wheels for a substantial amount of time. I'm not sure exactly how they all interact at this point...


TL;DR: I gave up on calculating this stuff, we're just going to go with quantitative results! Also, I need a better high speed camera.

[Ether]

Originally Posted by s_forbes

Something interesting to note: we put a new frisbee into our two wheeled shooter (3:1 Bag, 1:1 Bag) and analyzed the marks left by the shooter wheels afterwards. It looked to me like the 3:1 wheel had more slippage than the 1:1 wheel, since the 3:1 had a shorter 'burn' mark left on the frisbee and shorter overall contact period.

Trying slowing both motors down, to get faster exit speed. Easy enough to do: run them with motor controllers instead of straight off a battery.

[falconmaster]

Originally Posted by Ether

Trying slowing both motors down, to get faster exit speed. Easy enough to do: run them with motor controllers instead of straight off a battery.

We will try slowing down the last motor and then experiment with the others as well. Thanks!

[Kevin Sevcik]

You can also try speeding up the second wheel by putting in a BB RS-550. Or for prototyping purposes, one of the super-powered FP 801-0673 motors from last year. We prototyped with a FP and were getting some pretty impressive velocity. Mind you, we also have a larger wheel spacing, which means less time with the 'bee contacting two wheels.

[Ether]

Originally Posted by Kevin Sevcik

You can also try speeding up the second wheel...

Better yet:

a) remove wheels 2 and 3, and adjust the speed of wheel 1 until you get the fastest exit speed (call it s1), then

b) keeping wheel 1 at speed s1, add wheel 2 and adjust its speed until you get the fastest exit speed (call it s2), then

c) keeping wheels 1 and 2 at speeds s1 and s2, add wheel 3 and adjust its speed until you get the fastest exit speed.