Quad- 1/2 track drive system

[ajlapp]

you need to really think about your objective and how you want to accomplish it rather than get wrapped up with wheels or treads. i've built both....treads are definately harder to build, and not always the answer. i've taken treaded robots against The Chief with two 8" wheels and lost horribly. it's all a matter of weight distribution, time, and desires. i know it's cryptic, but there are die-hard tread people, and there are cavemen(wheels), there is no clear cut winner. by the way, we definately don't have a standard drivetrain either...it should be a FIRST first!

Anthony Lapp
team 221 ----> now 857

[Chris Hibner]

Apparently team 401 and 857 are doing a totally new drivetrain concept. I want to take a guess at it:

First, we know what it isn't:

Typical drivetrains:

- wheels
- treads

Not-so-typical motive device:

- walking

However, I believe either 401 or 857 are considering something a bit different. I think it will be: A SCREW DRIVE SYSTEM!!!!!

Instead of wheels or treads, how about making two auger screws that are about 33 inches long (one for each side of the robot), and just spin the screws like crazy? You get a a lot of traction and it is a VERY simple design (except for making the screws, but that might not even be that difficult). I'm kind of surprised no one has tried this in the past (or did someone try it and I just missed it?).

Anyway, is that the secret system? Are you planning on using screws to move the robot? I guess you don't have to answer if you don't want to.

-Chris

[kevinw]

Why not coat the bottom of your base with some extremely low coefficient of friction material (available from SPI, of course), and use the motors to turn fans and propel you? Of course, you'd need to drop part of your chassis down to prevent people from pushing you around when you didn't want to move...

[LittleDave]

The formula for the force of Friction is F = u N NOTICE THAT THERE IS ABSOLUTLY NO CONCIDERATION FOR SURFACE AREA!! The thing thats going to make the biggest difference in traction is the material you are using and the coefecent of friction it provides with the carpet. Timng belts and other track materials are made of hard rubber and that means they have a low coeffecent of friction! I suggest that some of you re-think your driving mechs.



-Im sorry for my * spelling errors. * means i couldent spell terrible or atroshus

[Matt Leese]

Originally posted by LittleDave
The formula for the force of Friction is F = u N NOTICE THAT THERE IS ABSOLUTLY NO CONCIDERATION FOR SURFACE AREA!! The thing thats going to make the biggest difference in traction is the material you are using and the coefecent of friction it provides with the carpet. Timng belts and other track materials are made of hard rubber and that means they have a low coeffecent of friction! I suggest that some of you re-think your driving mechs.



-Im sorry for my * spelling errors. * means i couldent spell terrible or atroshus

While surface area doesn't matter, weight distribution does. The larger area in contact with the ground means that more of the weight will be positioned over the drive wheels (or treads,etc....therefore increasing N). Treads (as in with teeth) work a bit differently because they don't work on the properties of friction. They instead apply a contact force to the carpet parallel to the carpet (ie, they dig in and grip the carpet). In this case, larger contact area does help. Also, hard rubber has a satisfactory coefficient of friction (meaning not particularly high nor low).

Matt

[ajlapp]

we do have wheels.......but the drivetrain is untraditional, think the first time you saw the chief. walking would be cool though, and i seen screws used before, but speed is good for us this year!

[Ben Mitchell]

I've seen robots with treads perform fantastically (team 365 is a great example)

My whole purpose is to combine the high points of treads and wheels, while minimizing the negative qualities of both.

Treads would make a robot much harder to push or move, simply becuase of the friction spread over the surface area.

Treads also bear the weight of the robot over a larger area, making less stress on the drive.

As for 2 auger screws, think about it: they'd only be able to go forwards and backwards. They'd need 4, and thats the same as wheels ( ). duhh.

Getting back to the subject of treads versus wheels, I think that is you want a muscle robot, you'll probably want treads. I compare such robots to bulldozers: they may be slower, but they don't stop!

The advantage of wheels is that they are much faster, but they are also more vulnerable to being shoved across the room, are prone to lack of traction, and bear the weight of the robot on 4 smaller surfaces.

Having 4 wheel drive conpensates for some problems, but |||nothing||| compares with some nice solid treads.

Anyone beg to differ? Also, how have teasm improved wheel traction in the past.

And how about them pnuematic wheels (the ones w/ air inside them) Would'nt robots bounce around with them on??

Hmmm..

--Ben Mitchell

[Chris Hibner]

Originally posted by LittleDave
The formula for the force of Friction is F = u N NOTICE THAT THERE IS ABSOLUTLY NO CONCIDERATION FOR SURFACE AREA!! The thing thats going to make the biggest difference in traction is the material you are using and the coefecent of friction it provides with the carpet. Timng belts and other track materials are made of hard rubber and that means they have a low coeffecent of friction! I suggest that some of you re-think your driving mechs.

You are not quite correct in your assumptions.

Actually, the friction formula does not account for surface area, so you are right in that regard. However, what people are forgetting is that we are not dealing with pure friction here.

The carpet acts more like snow or sand rather than a smooth floor surface. When travelling on snow or sand, most of the pushing force of a good wheel design does not come from friction - it comes from a normal (normal meaning orthogonal) pushing force. That is why there are treads on snow tires (or paddle ribs on dune buggy sand tires). The treads or ribs sink into the snow or sand and push off of the snow or sand much like you pushing off a wall using your arms.

A simliar idea is the track design on a snowmobile. There are paddle ribs that protrude from the track (now most snowmobiles come with cleats for ice, but if you've seen an older snowmobile the track was made of formed rubber). Once again, the ribs sink into the snow to push off of the snow. The friction between the rubber and the snow accounts for a very small fraction of the total motive force. The snow can only hold so much stress before breaking away, so more track area means more paddle area, which means less stress on on the snow, which means more traction.

The carpet acts quite a lot like the snow or sand since it is quite deformable. Good tank treads act like the paddle ribs on the snowmobile track. The teeth of the treads sink into the carpet between the fibers and push off of the carpet with a normal force. Friction plays a much smaller role in this design.

If you want proof of this, look at either our wheels of the past few years or the treads of the TechnoKats from last year. The coefficient of friction between the carpet and lexan (our wheels) or smooth metal (the TechnoKats treads) is quite low compared to any rubber. However, we both got traction that was equal to or superior to rubber because we dug into the carpet and used "normal force" rather than friction.

Just like the snow, if you put too much stress on the carpet fibers, they will deflect enough to let go of your tread and you will begin to slip. Just like the snowmobile track, in this design the more cleats you can dig into the carpet, the less stress you put on the carpet, which means you are less likely to have the carpet deflect enough to cause slippage, which means the more traction you'll get.

[Chris Hibner]

Originally posted by Ben Mitchell

As for 2 auger screws, think about it: they'd only be able to go forwards and backwards. They'd need 4, and thats the same as wheels ( ). duhh.

You can turn using the auger screws. Turn one forward and the other in reverse and you turn just like the tank steer system with treads. Should I say "duhh" now?

[AdamT]

I'll spill the beans in a week or two, but here's another hint. It may not be WAY out there. There is a heavy machine that uses them already. Yeah, that's the ticket.