Design goal is to have a mobile robot that operates on 3 large casters, essentially 2 to 4 inch diameter steel ball bearings, that are motorized. No other mechanism would touch the surface. The robot should thus be able to move in any XY direction on a flat surface, with steering being achieved by varying the speed and rolling direction of these wheels. The robot has no designated "front" side, so it does not need to (and should not have to) bodily turn, in order to move off in any given direction.
Conventional wheels or tracks are not the preferred approach.
Looking for suggested mechanical layouts of multiple rubber wheels, pressing down onto the steel ball from within the castor housing, to drive the ball in any direction. A single wheel on a stepper, rotated around the vertical axis using a sail-winch servo, is one approach under consideration. Would this be ideal, or are there any serious flaws in this approach?
Alternatively, is there any other suggested method of driving such a steel ball in any arbitrary direction under electronic control?