[theCADguy]

Omni wheels have a set of rollers perpendicular to the front of the wheel. This allows them to have sideways and forward movement, that doesn't require turning. You can put the wheels in an orientation like this:
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The wheel(s) in the center can drive the robot sideways, because the rollers on the side wheel allow perpendicular movement. Considering that your center of mass is directly over the center wheels, this style of drive (referred to as slide or H - drive) is capable of excellent omni directional maneuvering.