First, the prior art of this idea is every sort of two wheeled
self balancing gadget which uses a left and a right wheel.
Segways, hoverboards, and various clones.
That sort of balancing doesn't specifically need two wheels
-- you could use just one wide cylindrical wheel, but you'd
lose
the ability to steer.
This idea is to balance and steer in the same was as these
gadgets, and *add* the ability to move sideways, without
turning.
We accomplish this by using mecanum wheels for the left
and right wheel, and an omni wheel for the middle wheel.
To balance in place without spinning or moving sideways, all three wheels are equally spun forwards or backwards,
just as in a two wheeled balancing device.
To spin, without sliding sideways, the mecanum wheels are
spun in opposite directions, while the middle omni wheel is
kept still.
It move sideways, without spinning, the mecanum are spun
in the same direction as one another, and the middle omni
wheel is kept still.
Obviously, combinations of various motion are also
possible.