Chief Delphi - pic: 2 Speed Switching Module

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Re: pic: 2 Speed Switching Module

Quote:

Originally Posted by crazyStone (Post 1078723)

Yes I understand there is a limit for the amount of torque that can be transmitted through the wheels. I have had a year of physics and am in AP physics now. The reason I used perhaps, is the fact of weight distribution and the surface area of wheels and material used all play into the how much of that torque can be distributed. Perhaps a configuration can be made that 3.5 ft/sec still transfers torque effectively through the wheels. That is the reason for the prototyping, in addition if we can successfully create the modules with gearing inside of them; it should be much easier task to create modules without switching speeds. This is merely a prototyped design that is being constructed. There is no belief that it will work flawlessly or be a great improvement it’s more of skill building project for preseason. The gear ratios have been made smaller in consideration for what everyone on here has been saying, the low speed has been moved up to nearer 4ft/sec.

I think what Sean was saying in one way or another, is that you don't need to prototype to get a good idea of what to expect in this particular scenario. Prototyping is certainly one way to do it, but the problem with prototypes is that they get expensive quickly. If you go on to study engineering, and eventually become an engineer, you will quickly learn that saving resources (time, money, machines, etc.) is always something to consider.

Going back to your original problem... A 135 lb robot (120 lb + battery, etc) can only exert a given force on the ground through whatever number of contact points it will have. If you have a good estimate on the coefficient of friction between the wheel and the carpet, you can easily estimate what the ideal gearing should be; no prototyping required.

-Brando

Re: pic: 2 Speed Switching Module

Quote:

Originally Posted by Brandon Holley (Post 1078808)

That's exactly what I was saying. The whole purpose of engineering and math and theory is so you don't have to experiment to determine if basic systems are going to work or work well. If you want to throw away money on a less than optimized prototype then no one will stop you but if you take the literally 3 minutes required to run basic calculations to determine your traction limited wheel speed you will save precious time in the long run and end up with a much better performing module.

Re: pic: 2 Speed Switching Module

Quote:

Originally Posted by sdcantrell56 (Post 1078843)

The calculations were done today with some help from two physics majors, so hopefully they were correct.
CIM normal load = 4320 rpm
Torque = 64 in-oz’s
Ff= (c.o.f.) * Fnorm
T= r * Ff
Ratio between high and low gears is approximately 1:2.5
These numbers were calculated using the max weight of a full robot , 150 lbs.
4” Traction Wheel (C.o.f. = 1.25 – 1.5)
Low- 1:30 gear ratio, 6000 in-oz’s torque, 2.5 ft/sec, 144 rpm ( before slipping occurs at 6056 oz-inches)
High- 1:12 gear ratio, 2500 in-oz’s torque, 6.3ft/sec¸ 360 rpm

6” Omni Wheel (C.o.f. =1)
Low- 1:18 gear ratio, 4000 in-oz’s torque, 6.0ft/sec, 230 rpm
High- 1:7.4 gear ratio, 1800 in-oz’s torque, 15.25ft/sec, 580 rpm
(forward/backward direction only)

Slipping occurred at 10.7nM and at 12.7Nm depending on the tread material, so these projected ratios have the torque coming in lower than that value. If the math proves to be grossly off the only true loss is in the traction wheel. So the sprocket ratio from Omni to traction would simply be changed and that should solve any issues with stalling or being too slow and not gaining torque. Any corrections of the math are welcomed, better to fix it now before the pieces are made/ordered.

Re: pic: 2 Speed Switching Module

No need for physics majors.

I've got to refer to this guy. Just type in gear ratios, it spits out torque, speed, and current draw numbers. It'll also indicate where cut offs for traction limiting is when you play around with the numbers.

http://www.chiefdelphi.com/media/papers/2059

You did your numbers with CIM normal load value, this means that the CIM can do this loading all day. Realistically for an FRC application, you should use at minimum the max power numbers (171 oz-in, 2655 rpm, 67.9 amps). While there are other things constraining it (speed controllers, breakers, wiring, efficiencies, etc.), this will give you a better value. When I say FRC application, I mean short pushing matches, two minute matches, etc.

Let me just say, we used 4" x 2" roughtop wheels last year at a 10.75:1 ratio and they were still spinning up the wheels.

Do yourself a favor, use JVN's calculator.

I got to ask, how are you doing your center to center distances on gears?

Edit: Forgot to say, while I wrote about the max power numbers up there, the CIMs are capable of putting out 343.4 oz-in at stall current.

Re: pic: 2 Speed Switching Module

Quote:

Originally Posted by James Tonthat (Post 1078910)

No need for physics majors.

Do yourself a favor, use JVN's calculator.

I got to ask, how are you doing your center to center distances on gears?

Edit: Forgot to say, while I wrote about the max power numbers up there, the CIMs are capable of putting out 343.4 oz-in at stall current.

Thanks for that link. I got all the stats from probably the same data sheet you did for the CIM's, the normal load was picked because it would prove to some more conservative members of our team it could maintain that torque. The gears are standard andymark shifting gears for the most part and in addition their charachteristics were placed with the gear portion of design accelerator on Autodesk Inventor. On top of hand calculations to ensure proper meshing. The modules are being lasercut to ensure the precision.