[archiver]

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.