[Gary Dillard]

That sounds good in theory Ken, but in order to do that you have to command the robot to drive with the caster rolling backwards where it is inherently unstable. Casters are self aligning in one direction - the pivot point is off axis from the contact point with the floor. So if you're driving straight and want to back up straight you have to keep compensating for the moment induced in the caster that wants to push you sideways. Try turning the casters the wrong way on a shopping cart and see how far you can get them to roll that way, even with all the feedback available to your senses. I won't debate whether or not it's possible as an engineer (it's certainly possible), but it doesn't make good sense to fight it.