The swivel castor (or caster) is a wheel that can freely rotate with the minimum of possible resistance in any direction, in order to give stability to the vehicle.

A swivel caster includes a small amount of offset distance between the center axis of the vertical shaft and the center axis of the caster wheel. When the caster is moved and the wheel is not facing the correct direction, the offset will cause the wheel assembly to rotate around the axis of the vertical shaft to follow behind the direction of movement, avoiding either preventing motion or dragging across the ground. When in motion along a straight line, a swivel caster will tend to automatically align to, and rotate parallel to the direction of travel.

Another safety sensor is mounted on the castor wheel support. If the castor is not present or not in contact with the floor the robot will not move.

A little screen is attached to the wheel support. When it's pushed up it blocks the infrared beam of the fork IR barrier enabling the robot.