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#12
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6WD -- Highly recommended...
Posted by Joe Johnson at 1/16/2001 9:33 PM EST
Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems. In Reply to: Re: A warning about tank steer Posted by CArobot on 1/16/2001 6:25 PM EST: Turning is a big reason to go with 6 wheel drive. If you are trying to turn with 4WD or with a tank drive, you are going to have to scrub your drive mechanism across the floor. If your front and back wheels are sharing the load about equally, then you have a separation of about a robot length between the scrub forces on the front and back wheels. This puts a torque on your machine that the drive system has to overcome. I have build a number of robots that can barely turn at all without a lot of heat being generated by the motors. By going with 6WD, you effectly half the seperation between the scrubbing forces. More than this, if you look at the velocity vectors, you will see that in the case of a square robot turning about its center with it weight in the middle of the robot, the tires have to move at a 45 degree angle to the direction of rolling. Under the same situation for a 6WD robot with the center wheels lowered so that they take more of the load, there is almost no loss due to scubbing of the wheels because the wheels that scrub are not loaded to any significant amount. Finally, considering the worst case of the CG half way between two sets of wheels and a machine trying to spin about its CG. Again, look at the velocity vectors. Because distance between the drive wheels in the fore/aft direction is about half the distance between the wheels in the side to side direction, the velocity vectors make a much smaller angle with the direction of rolling of the wheels. This means less scrubbing. BOTTOM LINE: This is a great year to consider 6 wheel drive with the middle wheels lowered a bit so the machine rock just about a 1/4 inch. Try it, you'll like it. Joe J. |
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