An omnidirectional mobile robot that consists of two differential drive wheel modules and two links is proposed. Free-rotating joints are used to connect the links with each other and the wheel modules with both ends of the connected links. A caster is attached to the bottom side of the joint that connects the links. A ball caster is also attached to the bottom side of the steering axis of each wheel module. The robot has four driving wheels, two for each module. The robot utilizes the four degrees of freedom (DOFs) of these wheels to control three DOFs for omnidirectional locomotion and one DOF for the angle between the two links. The robot can change its form by changing this angle; this modifies the stable region of the robot. The robot prevents itself from falling down a slope by extending the stable region downward on the slope. As the robot uses regular rubber tire wheels, it has a large load capacity and high stability on irregular road surfaces. A robot with a depth camera is developed. The movement of the robot toward a goal detected using this camera is controlled. Experimental results show that the robot can move in any direction on a horizontal plane.

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