4-wheel drive trouble

[Guilherme]

We're considering solutions, the faster solution to tell the truth, 'cos we have to deliver our robot thuesday (because of the carnival holidays)

Thanks all.

[Al Skierkiewicz]

Originally Posted by Joe

We're considering solutions, the faster solution to tell the truth, 'cos we have to deliver our robot thuesday (because of the carnival holidays)

Thanks all.

Joe,
A technique we used last year was to have a foot descend pnuematically when we wanted to turn. The foot had a plastic or delrin surface that had little friction with the carpet. This would lift one set of wheels off the ground and let the other two do the driving. The side friction is not only killing your turns but your current skyrockets during the manuever as well. That shortens your running time and drags down everything else on the robot that runs off the battery. In the past (pre-backup battery) this would have reset the robot controller.

[T967]

Reduce the traction of either the front or rear tires. Or get a stronger transmission. As rookies we just went with two wheel/castors and didn't mess with 4 wheel. Put more weight on one end maybe? ALOT MORE. Shrink your distance between your front and rear wheels. Gear down Front/rear slightly less than the Rear/Front. Flip the Gears in the Tranny so you are geared a lot lower. (kit tranny)

[Matt Adams]

If you're using the kit drive system from FIRST... it sounds like your solution might be easier than you think: Is your robot in high gear?

I just have to ask.

If it's not in low... you'll be hurting to turn with those ratios.

Good luck.

Matt

[jskene]

With 4-wheel, tank-drive, as you are using, your robot must be wider than it is long, or you will have trouble turning. Make sure your robot is 36" wide and 30" long, not 30" wide and 36" long. This will make a huge difference in the ability of your robot to turn.

If you cannot do this, then ensure that the lateral friction of your wheels is less than the longitudinal friction. That is, make sure that it is easier to push your wheels sideways on the carpet than it is to push them forward or backward. You may be able to do this by modifying the tread pattern on the tire, as mentioned in an earlier post.

Originally Posted by jskene

With 4-wheel, tank-drive, as you are using, your robot must be wider than it is long, or you will have trouble turning. Make sure your robot is 36" wide and 30" long, not 30" wide and 36" long. This will make a huge difference in the ability of your robot to turn.

Actually, its the distances between wheels that matters (technically, it's the distance between the contact points of the wheels that matters). Measure the distance between two wheels one side of the robot and call that y and measure the distance between two wheels on opposite sides of the robot, but both in the front (or back), and call that distance x. If y > x, your turning efficiency is drastically reduced. If x > y, you should be good.

To understand why, let's look at some physics. Let's say your pivot point is the center location between all 4 wheels. Then, the distance to each wheel is called r. To find the torque used to turn the wheel you take the cross product between Force and Distance (both vectors). Look at the atttached drawing. The force that the ground exerts on the wheel (friction) is always parallel to the wheel. Draw a line from the wheel's contact point to the center. When the angle between force and this line is brought closer to 90 degrees, you can turn better. (A cross product between vectors A and B is equal to A*B*sin(d) where d is the angle between the two vectors. So when d is closest to 90 degrees, sin (d) is closest to 1, so the cross product has its largest value.)

Moral: Move the left set of wheels farther from the right set of wheels. And make the wheels on the left closer to each other, and the wheels on the right closer to each other. Or don't use 4-wheel drive. Our team made this "mistake" the first two years - we could hardly turn or maneuver. This year we went with track drives and they turn on a dime...

[Elgin Clock]

Put in a freshly charged battery and then find out the following:

What is your OI reading for voltage when you are tring to turn?? You may be spiking out the system with using only two drill motors for 4 wheel drive?

If it is reading anything less than 6 or 7 Volts then I suggest you either redesign your tranny or for goodness sakes, make some wheel adjustments like stated above!!

[Arefin Bari]

Originally Posted by Al Skierkiewicz

Joe,
A technique we used last year was to have a foot descend pnuematically when we wanted to turn. The foot had a plastic or delrin surface that had little friction with the carpet. This would lift one set of wheels off the ground and let the other two do the driving. The side friction is not only killing your turns but your current skyrockets during the manuever as well. That shortens your running time and drags down everything else on the robot that runs off the battery. In the past (pre-backup battery) this would have reset the robot controller.

i agree.... we were having the same problem last year and probably this year too... that really helped us out...

another thing, think of different places that you can place pneumatics in of the robot which might help you to get up to the platform...

[Tom Bottiglieri]

Originally Posted by Joe

we have to deliver our robot thuesday (because of the carnival holidays)

Is that tuesday or thursday?

[velocipenguin]

Originally Posted by ToMMan b182

Is that tuesday or thursday?

It is both, yet neither.

[Geoff]

Originally Posted by velocipenguin

It is both, yet neither.

Stop stealing my lines.