Thread: tank or go-cart style steering?
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Unread 23-06-2002, 22:42
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#0047 (ChiefDelphi)
  
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Re: A warning about tank steer (how to do 4WD effectively)
Posted by Chris Hibner at 1/17/2001 8:04 AM EST


Coach on team #308, Walled Lake Monster, from Walled Lake Schools and TRW Automotive Electronics.


In Reply to: Re: A warning about tank steer
Posted by CArobot on 1/16/2001 6:25 PM EST:



We are a team that uses 4WD and swears by it since it is powerfual and the robot is extremely easy to drive if done properly.

As Joe mentioned below, in a 4WD drive system, the wheels must scrub sideways as the robot turns. If you sum the moments about the robot as it turns, you will see that you need one of two things in order to turn effectively:

1) Your track must be larger than your wheelbase (considerably larger to be effective).

or

2) Your coefficient of friction between your wheels and the ground must be greater in the fore/aft axis than in the side-to-side axis. This is why we make custom wheels for our robot. We use metal or plastic and keep the edges of the wheel teeth "sharp" while we round the sides of the wheels. Then, voila! - you get a significant difference in the coefficient of friction from fore/aft to side-to-side. Of course, this only works if you are on carpet (or another surface that will deform under a wheel) and your wheel teeth are a good thickness. The wheels are esentially like skis - sharp in one direction and rounded and slippery in the other direction.

The residual friction in the 4WD system makes the robot extremely easy to drive since when you release the controls, the robot stops turning very quickly. Thus, the drivers don't really need to worry about compensating for overshoot in the turns.

I hope this helps anyone considering a 4WD tank steer system.

-Chris


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