Re: Making your robot drive easier
 

we did six wheel direct drive, with lowered center wheel.

here it is without any end-efffects:

http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=6516&direction =DESC&sort=date&perrow=4&trows=3&quiet=Verbose

Re: Making your robot drive easier
 

Roger, where those red plates powdercoated? or just painted? where did you get those wheels btw?

Re: Making your robot drive easier
 

plates are powdercoated, and the wheels are 2" x 8.5" skyway bead-lok modified using a dremel and a belt sander.

Re: Making your robot drive easier
 

You could aso check out team 25. They have had a great 6 wheel drive system for the last two years.

Re: Making your robot drive easier
 

Well, while we're on the subject of Home Depot cart type drive systems (you know, 6 wheels with center ones lower), 980 and 599 had them too.

Re: Making your robot drive easier
 

Question about "Home Depot Lumber cart Drive-train"
Is the lowering of the center wheels as significant as is with the lumber carts? They have a definate see-saw effect and are truely always driving on just four wheels. Center set plus either set on the corners.

Or is it just enough to lower the contact patch on the corner wheels so that the frictional forces are reduced such that it can turn and not dance around corners?

A post early on indicated approximately 5 thousandths of an inch lower. Is that consistant with other six wheel 'bots? How much did you lower the center wheels?

Thanks a bunch for the posts and pics :]

APS

Re: Making your robot drive easier
 

All you need is 1/4 of an inch maybe less, This allowed us to actually TURN with 12.5" wheels (allot of bots couldn't turn that didn't have a sixth wheel and still had 12 inch wheels) , this way if we decided not to hang we could stilll go around the field and stuff.
PID is pretty cool we haven't tried it but it is way better because then you control the amount of acceleration during turning not simply the voltage ... this works esp good on arms where as you go straight up the lever arm is significantly lower then at 90 degs so less voltage is req'd for the same amount of acceleration...the computer compensates...so the driver doesn't have to jerk the stick back and forth

Re: Making your robot drive easier
 

I don't see how the 6 wheel drive would be optimal. If the center two wheels are lower, then only 4 wheels are touching at one time. If this is true, then you actually have power going to the two wheels that aren't on the ground simply being thrown out the window. Why not have a 4 wheel drive system with the wheels very close together. Am I missing something here?

Re: Making your robot drive easier
 

1. The energy isn't being thrown out the window, because usually the efficiency of a wheel not on the ground is very high. Very little energy is being "lost". There isn't any real voodoo involved, the torque from the gearbox is just transmitted to the wheels under load (the ones touching the ground), and is mostly being turned into pushing force. The inefficiencies of this design aren't any greater or less than a normal gearbox. It is just as easy to make a high efficiency 6WD base, as a high efficiency 4WD base (which is still a tricky thing to accomplish).
Striving for high efficiency is something we've spent a lot of time looking at. Check out any drive team 65 has built. Those pesky Huskies sure know what they're doing. Being able to take the power from the motor, and put most of it to the floor is a huge advantage. ;)




2. Putting 4 wheels close together would work. Except, it is highly unstable, and the robot is vulnerable to flipping (especially if it has a high CG). Think about it. When the robot comes to an abrupt stop, the CG of the robot tries to rotate around the furthest forward point touching the ground. The further forward this point, the less likely the robot is to flip over forwards.


6WD, with the middle wheel dropped allows for excellent turning, while still utilizing high traction wheels for ALL contact with the ground (which maximizes pushing force). It also allows for a looong wheelbase, which as mentioned above increases robot stability.


Problems with this design?
Well, another set of wheels is more weight. The means of driving these wheels is... more weight and more complexity in the drive. Also if the distance the middle wheel is dropped is "too much" then the robot will rock back and forth during driving and "waddle" across the field. 6WD can ve VERY VERY elegant when done correctly, but there are of course pitfalls to overcome. 60/254 overcame them very, very nicely. ;)


I'm a fan,
John

Re: Making your robot drive easier
 

38 used a 6 wheel drive system for the first time this year. I think we used 6" pneumatic skyway wheels, the middle pair about 3/16" lower than the other two. Last year we used 8" pneumatic tires in a 4 wheel drive setup, and danced and hobbled more than we turned... too much lateral friction.

I think one advantage of a 6 wheel system is to have the weight of the robot pivot about an axis in the middle, rather than distribute it to all 4 wheels in contact with the ground at the extremes of the chasis. It seems to help quite a bit with traction, while keeping a low enough cof to turn without dancing. It's sort of cool watching it turn- if you drive one side and leave the other idling, the robot will pivot perfectly about the undriven side's center wheel. This compared to our 4 wheel system, where if one side is drive, the robot will coast in one direction, rather than pivot.

I have to say though, I love 25's entire drive- no chain! Gears the whole way through, to all 6 wheels!

Re: Making your robot drive easier
 

6 wheel drive is cool and all, but...

Why use 6 wheels when you could get away with 2?

2 wheel center drive with skids at 4 corners makes for an extreamly quick and easy to drive bot. Plus the simplicity and light weight, and you've got one honey of a drive train done and running in a week, and plenty of time and weight left over for doohickeys.

If you want to do a 2 wheel drive you pretty much throw out any form of climbing abilty. But, if you're going after balls or want speed, manuverbilty, simplicity and rock solid stabilty (two wheelers almost never get tipped over) all in one, 2 wheel drives are the way to go.

Get the right balence, and its like driving a spunky sports car vs a truck with locked axles.

-Andy A.

Re: Making your robot drive easier
 

Pushing Force!

If you are using 2WD with 4 skids on the corners, you have at most 6 points of contact with the ground, and for most operation at least 4 points of contact. (Sometimes it balances on just 2 wheels, but not in the cases I'll be describing).

This means, there is weight resting on non-powered wheels (casters, skids, whatever). Why is this bad?

Simply:
Pushing Force = (Normal Force of Robot resting on Drive Wheels) * (Wheel coefficient of Friction)

By putting weight on "Dead wheels" you are greatly limiting your pushing force. If you have 4 points of contact, but only 2 of them are powered (i.e. 2 wheels, 2 casters) you are only resting 1/2 your robot weight on the drivewheels, and only utilizing HALF of your potential pushing force. (Yes, this is a simplification assuming weight is evenly distributed over all wheels, but it's still a valid point).

So, here is the blunt (however opinionated ;)) truth:
If you use 2WD, you better not plan on winning any pushing contests.
Physics is against you, and it's not just a polite suggestion -- It's the law!

John

Re: Making your robot drive easier
 

Who said you have to push anything? It's all about driving simplicity and ease here. When it comes down the straight out driving ease and manuverbilty (which is a big component of ease), 2 center wheels are just a dream. Coupled with a PID loop, and getting the bot into tight spaces and making real precise movements is a breeze.

I've driven both types, and if the game calls for climbing or pushing absoulty go for 6 wheels. But, if you want to go after balls or otherwise be quick, 2 wheels are much easier to drive at high speed then any other setup I've seen.

And mark my words, one day FIRST will put out a game that encourages speed and manuverbilty over bulldozing.

Re: Making your robot drive easier
 

2WD with four skid plates will be more maneuverable than most 6WD systems, this much is true. But, when 6WD is implemented very carefully, it will only be slightly behind a 2WD setup, the loss of agility shouldn't be overwhelming. On the other hand the difference in pushing power between 6WD and 2WD is rather substantial.

I'd be willing to give up a small amount of turning ability for a large amount of pushing power any day. Give a little, get a lot. The decision seems simple.

Also, it's hard for me to envision a game where pushing power isn't rewarded in any sort of way. Then again, I'm sure I said this before the 2001 season...:o

Re: Making your robot drive easier
 

When that day comes, the teams with 6WD will have both. :)

It's all about tradeoffs.

6WD is more complex, but has many MANY more benefits.
Check out the 254/60 bot.

John