Re: tank drive in this years game
 

Would anyone be able to show me a link to a CRAB drive? I have an idea what it is but would like to see me about it.

Re: tank drive in this years game
 

There are several ways to do a crab drive, but this will give you an idea...

http://www.chiefdelphi.com/media/photos/28618

Re: tank drive in this years game
 

Here's a fantastic white paper describing many types of drives.

Re: tank drive in this years game
 

Here is why I do not like the tank drive. The field still has carpet along the edge. If you get two wheels (based on a 4 wheel system) of one side of the robot on the carpet, the other side of the robot will not have enough traction to turn. So your robot will be stuck on that edge of the field for the entire map.

This is why I am pushing for a hybrid between a swerve and "monster truck" style steering. Wheels are powered individually and move to steer, so if you get caught on that carpet, you can just turn off it.

Re: tank drive in this years game
 

Thanks Taylor and Taz!!

Re: tank drive in this years game
 

Is their a limit to the number of wheels you can use?

(I am thinking inline skates) Not sure on the physics since friction is a product of weight and area, putting more wheels on increases area but lowers the force on each wheel, so I am not sure if overall traction, ability to accelerate improves. Tank style steering is also called 'skid' steering for a reason. Putting more wheels on will make turning harder, so aiming the robot to shoot is harder (Unless your shooter is independent of robot position). Most of the successful 6 wheel drive systems really cheat a bit to let them turn, typically the center wheel is a bit lower then front and back, so the robot rocks a bit when turning - and typically only 4 wheels are providing traction at any one time.

This year's game is going to be a gas - I think rookies are going to have a good chance to do well. I don't think we'll see a lot of use of G14. Just gonna be to hard to score. I hope you teams are gonna recruit good human players. I'd start bringing donuts to the basketball team now. A good three point shooter could spell the difference. If your school does hockey, I'd recruit my driver from there too!

Re: tank drive in this years game
 

I've seen the difference between the coefficient of friction in line with the wheels or sideways to the wheels mentioned in a couple of places. I can't think of a reason for this. Was physics class so long ago that I have forgotten some fundamental concept?

Ivan

Re: tank drive in this years game
 

Experiment trumps theory on this one. We took last year's frame, loaded it up with ~140 lbs, and put it half on FRP and half on carpet... While towing a hand truck (casters on the front, high-mu in-line rubber wheels on the back), and turned with no problem. The wheels on the carpet spun more than the wheels on the FRP, and the bot just pivoted right off, easy as pie. Pie on ice. Or the moon. But pie.

...and Ivan, the tests we've done show no difference in inline vs. lateral coefficient of friction. I don't know where the official numbers came from, but they don't jive with the tests done by several other teams, either.

Patrick

Re: tank drive in this years game
 

Thanks Patrick, that makes more sense. Thanks also for saving me the time of doing the test myself.

Ivan

Re: tank drive in this years game
 

Hysteresis is usually a ferromagnetic term, but in this case it's basically the tendency to over-steer. (official definition: the lag in response exhibited by a body in reacting to changes in the forces...affecting it.)
So, in it's simplest terms--which is about all I can actually explain, though I'll try more if you'd like--the "easier" it is to steer, particularly the less you have to skid, the less hysteresis you'll have. I feel like I should explain its relation to static and kinetic frictional coefficients, but I'm not sure that I can do it understandably.

There is no limit to the number of wheels (except for your maximum expenditure and the number that can fit in the 28"x38"60" box).
In a perfect world (i.e. Coulomb's), surface area has zero effect on friction. Ff=mu*Fn, and we can all go home now. In reality, surfaces aren't perfect, and their mu's can vary across the surface. At first I didn't think this was a big deal in this game, but from what people say about floor wear-and-tear, it might be a minor advantage. I still seriously doubt it could play a major part.
Can you explain your more wheels=harder steering conclusion? My train of thought (and experimentation) was to consider the difference angular distance for wheels closer and farther from the center of rotation. Without sacrificing wheel base (stability), more wheels means wheels closer to the com, which means less skidding. Our prototypes have supported this; 6wd (even without dropped center) does turn better, chiefly because the center wheels do not have to skid as much. Someone on our team put together a beautiful analysis of this, I'll ask him to post it.

Are your wheels worn in? We did a loaded incline test with the fresh wheels and found inline as .09 and transverse as .13 (static). Not quite FIRST's numbers, but I can't blame them for giving us the bottom of the range. Does this change as the wheels wear?
To answer Ivan's question, in theory the difference between transverse and inline coefficients is based on the structure/pattern of the material. Almost all materials have these differences. However, if the difference disappears with wear, I might be inclined to blame it on that nice seam running down the wheel. If not, it's surprising given that there are no observable significant grains on the wheels, but certainly not physics-defying.

Re: tank drive in this years game
 

I was thinking along the lines of how Ice skates work. Also how the keel in a sailboat tends to keep the boat going straight. In skating you turn to the side in order to slow more quickly, but I am not sure this would be a good analogy. I think it works in skating because the skate actually starts breaking the ice and making snow cones, as opposed to straight skating where the pressure of the skate melts the ice and forms a frictionless layer of water.

This is all academic - teams will experiment and find what works best for them. From what I see 4wd can turn and accelerate and clever speed control programming along with driver practice will pay off. Be aware the trailer is really part of the robot this year - it will sort of act like a big sea anchor. It probably will make it easier to go straight but harder to turn sharply.

Has anyone found a cheap substitute for regolith? Would something like freshly acrylic waxed linoleum be close? Looks like I may be mentoring some rookies this year so any suggestions to cut costs would be appreciated.

Re: tank drive in this years game
 

Not tremendously, no -- but they're not perfectly fresh, either.

Patrick

Re: tank drive in this years game
 

Ours have become a bit dirty, but not worn either.

Re: tank drive in this years game
 

Not sure what makes you think more wheels means more traction. In general it doesn't.

Siri: Thanks for the explanation, to mirror it back and ensure my understanding, the lag between steering input and robot reaction could be called Hysteresis.

Don

Re: tank drive in this years game
 

We built a simple "wide" test bot today with two caster wheels made from 2009 "slick wheels" on the front and 2 powered "slick wheels" on the back. We weighed the test bot to 140 lbs and attached a 35 lb trailer on the back. I was shocked at how well the thing actually did. I was expecting poor performance but it wasn't too bad. We have something more complicated in our build plan. If that doesn't pan out, We may go back to the caster bot.