Tank Stearing - 4 wheel drive
 

Our team has tank stearing with a 4 motor drive system.
Most of our weight is over 2 of the wheels. We found out that the
robot turns better when the weight is towards the front, therefore we are sort of stuck with running our robot with the weight towards the front. Question, If we would have kept the weight in the center of the robot, would the robot turn just as good in both frontwards and backwords configuration.

Re: Tank Stearing - 4 wheel drive
 

With the balance point midway between the front and rear wheels, the machine should behave the same way going forward and backward if all four wheels are alike. With the center of gravity near the center of the robot, you would need more power to turn, though. When turning a tank drive robot with wheels, you are sliding wheels sideways when you turn. By having a forward or rearward weight bias, you need less power to turn and can use "taller" gearing without tripping breakers and/or smoking motors.

turning
 

Although I'm not sure just where the center of balance is on our 4-wheel drive 'bot, we put "slipperier' poly wheels in front to assist turning, and pneumatic rubber wheels in back. This may have helped shift the pivot point to approximately 1/3 of the way between the rear wheels and the front. We wanted to avoid drivetrain stress and assist turning but gain a bit from having the extra traction on the ramp. So far, it works very well.

tank steering with 4 powered wheels can be tricky

in 2001 our team had 4 wheelchair wheels for our drive.
our center of gravity was around the middle of the bot, and our wheels were sticky...so we ended up jumping and hopping around when our bot turned.

not good. so in 2002, we designed (patrick) some wicked cool omniwheels, that would be powered and help with manueverability. it was awesome we could spin like nothing else, and we were able to push like one crazy mofo.

We are currently using a four wheel drive with four pneumatic rubber slicks. They get great traction. They are so good on carpet that the robot would jump. They had no problem on the ramp. To fix the jumping we added 2 casters on the back which are raised up and down with a pneumatic actuator. When we need turning on the carpet we push a button and they are down. They don't lift the back tire off the ground but push down enough to reduce the frictional force. Now it doesn't dance, unless we want it to, and we have more than enough traction for hardcore pushing. Yay!

Two words:

Tank treads :)

Two words:

Can't turn

Many Words:
Can turn if it is done correctly!

If your gearbox is geared low enough to over come the "side loads" brought on by the treads, you can certainly turn. It is just one more thing that needs to be engineered correctly.

When done well, a tank tread system can be an amazing thing.

Also, there are other ways to get away from side loads on treads.
Just check this out:

The tank treads don't have to be touching on all surfaces. Our weight is centered about 1/3 of the way from the front of our robot, and our track pivots on approximately 7 inches of track, with the rest about 1/32 of an inch above that.

If you want to turn on a tank track, you just have to put in a pivot point. Look at this center wheel we move it up or down to give us the max traction and still turning on a dime.

http://www.valleytech.k12.ma.us/Robo...otAutoMode.JPG

Interesting approach, lifting the treads off the floor, makes nice, tight turns, I bet. Oh, BTW, you are missing hardware at #4 just in case you didn't catch it before.
Can't wait to see it in action. Good Luck!

Al,
It's not ours!
This drive was made by team 1064. (yay rookies!)
I just lifted the picture from firstrobotics.net to make a point.

I'm sure they appreciate your compliments though!

Hmmmm...Well, if that's the case, we'd better tell our robot before he gets dizzy. :D

The editor took some liberties (embellishment)-- we only go 10 fps. and can only stack 7 high :(
we didn't get the chance to video stacking although that came easy enough--though not as fast as we would have liked.

http://www.fleef.net/robo/robo2rm.rm