Motor and caster wheel placement questions

[jocatch]

My old robot uses 2 motors and a caster wheel in a triangular design which works well for going over bumps, etc. The problem is when it backs up and then goes forward, it is never straight. The caster wheel swevels and the robot never goes back and covers the same ground. (It can't make perpendicular turns well).

I have seen alot of plans which call for the 2 motors to be placed in the center of the base and caster wheels on each end. However when I tried this design, the base would get hung up going over bumps (the drive wheels would lose contact with the floor and stop).

What are the common tricks in using either design so that the robot doesn't get hung up going of things like the metal floor strips seperating carpet in different rooms and so it tracks back and forth accurately? THanks for any input.

Joe

[Webbot]

Ball casters (also known as Ball Transfer Units) http://www.pololu.com/catalog/product/66 are quite good as they can rotate freely in any direction. But the diameter of the ball will dictate the size of bump it can go over.

A cheap home made solution is to use some stiff wire to make a 'skid' (ie bend the wire into a shallow 'V' shape where the bottom of the V is in contact with the ground).

Another solution is 'omni wheels' http://en.wikipedia.org/wiki/Omni_wheel  http://www.societyofrobots.com/robot_omni_wheel.shtml

As you've discovered 3 legs are better than 4 - as it makes sue that all legs are on the floor. You can put the drive wheels in the center so that the robot can spin on the spot and some sort of caster at either the front or the back. You will then need to place the battery pack so that the robot doesn't tip over when braking.