4 Wheels vs 3

Dgswaner

Has anyone had any experience with 0 turn radius robots? I'm constructing a cylindrical robot 1' in Diameter and 3' tall. I really want it to have a 0 radius wheel base. my options are to have 2 wheels centered in the middle and have 2 casters front and back to balance the bot. or not have a 0 radius and use 2 wheels set of center and only one caster. I really don't want make this a balancing robot either, it will add another layer of complexity I don't want. the reason that I'm concerned is the that on a previous robot I used 3 wheels and still lost traction at times. my wood floors varied enough that one tire would slip enough to make the bot go in circles but I did have a weight distrabution problem. I tried making it a 4 wheeled robot but then I had turning issues. more info...
I'm using 5" lawn mower tires. the robot drive train will be from 2 reversible motors geared down to about 20 rpm. I estimate the bot to weigh near 30 lbs. I intend for this bot to only operate on flat indoor surfaces. any info is appricated.

Thanks

Bruce Bates

Were you using rubber tread wheels or plastic wheels on your prior robot?

Regards,

Bruce Bates

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Franklin

You could still get four wheel drive and a 0 turning radius if when you want to turn you turn your wheels purpendicular to the center of the circle and then drive the wheels.

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- Stephen

Dgswaner

My previous robot used plastic wheels with a thin ammount of rubber for the tread. they were Lawn mower wheels purchased from Home Depot. I'm using the same wheels on the new bot.

Post Edited (Dgswaner) : 3/12/2007 5:14:18 AM GMT

slamer

If you put 2wd on the center of your drive platform. Yep it will turn quick. Thats about the quickest turning machine to build using wheels. Those machines can be fun but a bit tricky to drive straight.

·If you put the drive wheels on one end and some sort of rigid castor or fixed wheel that helps to act as a rudder and make the machine travel straighter. This is basically how the Boe bot is set up. It's a good comprimise for stable 2WD machines

·If you use a·self aglining·(think shopping cart) type of castor on this 2WD·drive train the machine tends to have a mind of it's own when changing directions until the castor re-aligns it self with the new direction of travel.

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My Terms for the lower part of this post....

Wheel base is the distance frount to rear of the drive axle centerline on one side of you machine.

Tread width is the distance measured from side to side across your machine to the tire center lines across the treads.

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4WD this is the best type of drivetrain if you want the machine to go straight. To make it turn you have a couple of things to consider. The most popular and effective is 4wd Skid steer. There actually is a trick to getting a 4wd skid steer·drive train·to turn quick. When you lay out your platform or frame you need to think about how much wheel base you will have and how much tread width you will need for stability. If you build a square drive train were the Tread width is equal to the wheel base the robot will turn OK but nothing spectacular.

If you want it to turn fast then you want the tread width to be wider than the wheel base to keep the machine stable.· You want the wheel base to be maybe 1/3 less than the tread width·or so. That will turn very nicely in 4wd skid steer mode. If you want to see a great example of how to build a stable and quick steering 4WD platform Take a look at·a BOB Cat Skid Steer Loader.

The worst way to make a skid steer drive platform is to have the wheel base longer than the tread width. That will make a 4wd skid steer drive platform very difficult to turn. A lot of folks make that mistake and are unhappy with the results. This configuration really eats up battery life as well.

One other thing when building for 4WD you need to consider your·components weight distribution. It's usually better to keep as much of the weight as·evenly distributed between the 4 drive axles as possible for best all around traction and stability. Also there is two basic types of turns in skid steer.

·Slow turn: one side of the drive train is not turning and the other side is driving. The non turning side actually Skids or pivots on the tires contact patch.

Fast turn: (Lets say for a·Left turn.) Right drive wheels are turrning forward. The Left drive wheels are turning in reverse. The wheels are chasing each other around in a circle with a lot less friction than in a slow turn.

Fine control can be achived by controlling the speed of each side of the drive platform. The diameter of the turning circle is·influenced by the distance of the drive axle centerlines·(Wheel base) The shorter this distance the quicker the turn. So to wrap this up, Short and Fat can be a good thing in 4WD Skid steer·drive trains.··If you want to turn quicker while retaining stability.


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Steven Kirk Nelson (slamer)

Team K.I.S.S
Build Safe, Build Mean, Build Strong!

Post Edited (slamer) : 3/12/2007 5:57:32 AM GMT

Bruce Bates

I agree with you that the prior problem was indeed insufficient traction. A wider profile rubber tire will probably help to gain the needed traction. Plastic wheels aren't good for much indoors in my experience.

Regards,

Bruce Bates

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Dgswaner

Thanks for the posts guys. bases on your info tho. it looks like I need to go with a 3 wheels system (one a caster) because the best I could do is make a square 4 WD configuration being that the chassis is 1' diameter. the 0 deg. turning radius isn't mission critical, I've just seen my roomba's get out of some tight areas by turning 180 deg. in one spot. the Most frustrating part of all of this is that I have the book "making robot drive trains" but not that I actually want to "design" one I can't find the book.. thanks for the input guys I'll let you know how it goes!