Has 6 wheeled designs surpased tank treads

[Paul Copioli]

jimFortytwo,

Was that aimed at me? If so, then I am ready to put my money where my mouth is. Are you?

-Paul

[Tom Bottiglieri]

I would trust what the UFH Engineer has to say about this.

Thats just me though.

[jimfortytwo]

Originally Posted by Paul Copioli

jimFortytwo,

Was that aimed at me? If so, then I am ready to put my money where my mouth is. Are you?

-Paul

No, I wasn't trying to single you out. Though I imagine you have not yet, in fact, read the white paper in question, and have not done the experiments yourself. Or are you holding out on us?

I suggest reading the whitepaper. Following your challenge, I reread it, and the results still somewhat surprise me. I won't bother summarizing, as it would spoil the surprise.

the paper:
http://www.chiefdelphi.com/forums/sh...threadid=15719

I guess the other part of "putting my money where my mouth is" should be to mention the traction tests I did for my team last year. We were looking at 8" hard rubber tires with an aggressive crosswise tread pattern. I did tests for kinetic friction with 30 pounds and 20 pounds of pressure on the tire. I was working with the risky approximation that in a 4-wheel drive robot each tire sees 30 pounds (30*4 =120), and in a 6-wheel design tires see 20 pounds (20*6 = 120). I can't remember the exact numbers, but my recollection is that I extrapolated a 10-20% benefit in the case of distributed load. Obviously a better procedure is needed to give any meaningful results, however.

I remain convinced that the traction advantage of treads over wheels is so highly dependent on the carpet and material that you can not generalize it. If I can put together materials to do a proper investigation of the subject, I will. Unfortunately we're on winter break, so that might have to wait until January.

[MarkVH]

We have been useing treads since be for I joined the RoboDawgs. We do not have problems with dying batteries. We've only had problems going straight.

[sanddrag]

Originally Posted by MarkVH

We have been useing treads since be for I joined the RoboDawgs. We do not have problems with dying batteries. We've only had problems going straight.

That's actually somewhat surprising. It seems to me that a tread system would "naturally tend" to go straighter than a wheeled system since it is harder to turn. But perhaps this was just because of the bias in your drill motors. Maybe you could elaborate a little.

[Adam Y.]

It seems to me that a tread system would "naturally tend" to go straighter than a wheeled system since it is harder to turn. But perhaps this was just because of the bias in your drill motors. Maybe you could elaborate a little.

Hmmmm.... Did you attach the motors backwards. I don't have a drill motor handy but Im almost certain that there is a positive and negative lead on the motors. Attaching one motor one way and the other one the other way will result in such a situation since the motors go faster in one direction than another.

[Paul Copioli]

Jimfortytwo,

The basic flaw in the distributed load assumption is that in a six wheel drive configuration that lowers the middle wheel (and in treads that have a lowered middle in order to pivot), the center wheel sees most of the weight load. This is true if the CG is close to the center of the robot. In general, the wheel closest to the CG will take most of the load from weight. In many of the six wheel designs you see today, the outboard wheels (all four of them) are simply powered casters. If balance wasn't such a big deal, then the two wheel drive with the wheels directly under the CG would be the most maneuverable and most efficient in transferring load to useful pushing force.

I read that white paper, but the results were not as convincing as the discussion leads us to believe.

Yes. I have done numerous tests and I guess I am holding out on everyone. Not because I want to, but I have not had the time to reduce all the data and put it into a nice format. The results I saw convinced me enough to proceed. I promise I will get my results out soon.

The thing I want the readers of this forum to take from this is that the selection of wheel (tread) material is most important and the differences (if any for our game) between area and friction are negligible and definitely not noticeable on FIRST carpet.

-Paul

[jimfortytwo]

Paul

Yes, weight distribution is the problem with the distributed load assumption. Of course in our 6-wheel drive robot last year we didn't drop the center wheels, so it wasn't quite as egregious an omission.

Presently my hypothesis is that there is a crush load (pressure) that distinguishes wheel surfaces that just ride along top of the carpet and those that mash down into the carpet. Before reaching the crush zone, I would imagine that our basic AP physics friction rules apply, and above the crush zone I would imagine they apply too. I'm not convinced the transition zone between one and the other will be linear like the rest, however. Thats just my present thinking, however.

One problem with the white paper is that the scale of it (smallest test patch 150 square inches) makes it hard to apply with any certainty to wheels.

The main thing I'm suggesting is that tread material choice is most important, and that you can't assume either a positive or inverse relationship between surface area and friction without empirically testing your materials. (sounds roughly familiar)

[Stephen Kowski]

just to throw something interesting into the mix....we did a 6 wheel drive bot last year, and those beadlok wheels from skyway are much heavier than one might think. So if you do chose to go 6 wheel i recommend a small wheel or you are going to be fighting to get that weight back.

[Andrew Schuetze]

It is interesting how this thread has revived itself and has also digressed to the much debated coefficient of friction between a track and a wheel.
If I can sumerize the last few posts to clarify my thinking. The physics of tracks verses wheels as applied to FIRST class robots and the carpet does not present an appretiable difference between the two.

If one is looking to gain traction, one needs to focus more on the tread material of which ever system they employ.

Track systems are more expensive, require a bit more enigneering expertise, draw more battery power but still don't kill the system in the short time frame of a round. One needs to have spare tracks and design the system to keep the track on during turns. They do provide greater obstacle drivability.

A six wheel system will drive straighter and draw less power. One can gain back some manuverability by lowering the center wheels 1/4 inch or less.

If we can put those issues to bed, I am still interested to get opinions or tested data for the following:

Do six wheel designs exist that can approach the obstacle drivability of a track system?

[greencactus3]

Originally Posted by Andrew Schuetze

A six wheel system will drive straighter and draw less power. One can gain back some manuverability by lowering the center wheels 1/4 inch or less.

again, this matters on tread material and width. more width means more scrub which means it tracks straighter. and in the designs with center idlers or lowered center wheels, so you cannot necessarily say a 6wd will track straighter. espescially if it the frame gets tweaked a bit.

Do six wheel designs exist that can approach the obstacle drivability of a track system?

the mars rovers? in a independent suspension view i think 6wd will have better obstacle drivability. but with zero suspension i think treads may have the advantage of being able to angle their treads at the front as in to climb over higher objects. but if the wheel for 6wd and pulley for tread were the same, id say it matters on what obstacles you want to clear. as in sand tread will distribute weight more but in rockcrawling and w/e you probably want a bit more stuff to actually "claw" at the surface. meh. i dunno

[Al Skierkiewicz]

Originally Posted by Andrew Schuetze

Track systems are more expensive, require a bit more enigneering expertise, draw more battery power but still don't kill the system in the short time frame of a round. One needs to have spare tracks and design the system to keep the track on during turns. They do provide greater obstacle drivability.
A six wheel system will drive straighter and draw less power. One can gain back some manuverability by lowering the center wheels 1/4 inch or less.

Andrew,
I need to weigh in here from an electrical standpoint and add some other observations.
Drive systems that cannot steer, eat batteries when turning on carpet. The side loading is incredible and many teams have reported to me that they can barely last one match on a fully charged battery. (I have observed low battery conditions on several robots at the end of matches.) The data we collected with current monitoring suggests that most multi-wheel and tread drive systems regularly run near stall currents on all drive motors during turns. Slow, tight turns are obviously worse than higher speeds. The degree of battery drain depends on driver strategy and operation. A driver who only drives forward and back with little steering should not experience any greater battery drain than other designs. Due to the additional friction in the drive components, (pulleys, drive trucks, etc.) a treaded design generally runs higher electrical demand. Tighten up the belt to prevent it slipping off and those frictions increase. Only low friction bearings can help on these problems. When asked, I recommend that a robot should be able to run two to three matches on a single battery as a rule of thumb. This margin insures enough electrical power be available for a single match.
Although the discussion on treads and carpet friction is very interesting, I think there are too many variables (assumptions) for this to be a general discussion. Designs that have no way of transferring loads to the tread except at the wheels have to be analyzed differently than those that spread the load across the full length of tread in contact with the carpet. Even those designs that use center trucks that are lower than the either the front or back pulleys require a different analysis. Turning friction varies with design as well. Often in turns, belts tend to go "up on edge" so that the full width is no longer in contact with the carpet.
I am not saying that tank designs are bad but there obvious problems and tradeoffs. Teams that have successful tank designs do exist but further investigation into their design criteria is warranted.

[#1Transgirl1140]

My team has been around for 3 years this year and each year that we've been competing we've used treads. We've also used the same transmissions as well and hopefully we can use them again.

[Billfred]

Originally Posted by Al Skierkiewicz

Andrew,
I need to weigh in here from an electrical standpoint and add some other observations.
Drive systems that cannot steer, eat batteries when turning on carpet. The side loading is incredible and many teams have reported to me that they can barely last one match on a fully charged battery. (I have observed low battery conditions on several robots at the end of matches.) The data we collected with current monitoring suggests that most multi-wheel and tread drive systems regularly run near stall currents on all drive motors during turns. Slow, tight turns are obviously worse than higher speeds. The degree of battery drain depends on driver strategy and operation. A driver who only drives forward and back with little steering should not experience any greater battery drain than other designs. Due to the additional friction in the drive components, (pulleys, drive trucks, etc.) a treaded design generally runs higher electrical demand. Tighten up the belt to prevent it slipping off and those frictions increase. Only low friction bearings can help on these problems. When asked, I recommend that a robot should be able to run two to three matches on a single battery as a rule of thumb. This margin insures enough electrical power be available for a single match.
Although the discussion on treads and carpet friction is very interesting, I think there are too many variables (assumptions) for this to be a general discussion. Designs that have no way of transferring loads to the tread except at the wheels have to be analyzed differently than those that spread the load across the full length of tread in contact with the carpet. Even those designs that use center trucks that are lower than the either the front or back pulleys require a different analysis. Turning friction varies with design as well. Often in turns, belts tend to go "up on edge" so that the full width is no longer in contact with the carpet.
I am not saying that tank designs are bad but there obvious problems and tradeoffs. Teams that have successful tank designs do exist but further investigation into their design criteria is warranted.

Al,

I remember back in my BattleBots phase that one builder suggested running your robot against a curb for three minutes (in their case) to check and make sure you absolutely have enough power to last a shoving match. Would such a method work for testing FIRST robots as well?

[Al Skierkiewicz]

Bill,
That seems like a big overkill since our robots won't sit there shoving against another player for the full two minutes. You are not only stressing the battery but the drive motors and controllers as well. I can tell you that there is a huge list of teams that have asked this question, admitting that as little as 1:30 of heavy driving killed the battery. Driving in practice should give you a fair indication of battery use. Teams when pushed to the limit will try to devise faster methods of battery charging (to keep up with demand) which starts that downward spiral of early battery failure. The KOP battery can be safely charged at 6 amps maximum, which would require at least 3 hours to fully charge the battery if it was fully discharged. Remember that the backup battery only hides the real problem. A dead primary battery will keep you from driving whether the RC is functioning or not.