In this study, a novel size adjustable robot that could overcome an unstructured environment was introduced. To provide the robot with a volume-modifiable function, negative Poisson’s ratio structure with a unique characteristic about deformation of material was applied to the design of the body frame. The robot could simultaneously adjust its width and length with only one directional control with the help of the negative Poisson’s ratio structure. An omni-directional mobile mechanism was adopted to drive its wheels and allow flexible movement in a narrow space. However, during the procedure to adjust the size of the robot, a slip phenomenon occurred, resulting in an unnecessary movement. To solve this problem, the unnecessary offset was measured through repetitive tests and applied to the robot to compensate the position shift. To verify the performance of the robot, a test bed with a narrow space was fabricated. Extensive experiments were conducted to evaluate environmental recognition and size adjustment function by calculating the width of the narrow space and scaling the robot"s body. Results confirmed that the robot sufficiently achieved the motion objective to move in a narrow space with its size adjustment function.

In this study, slip phenomenon that occurs during trajectory tracking motion of an omni-directional mobile robot based on Mecanum wheels was analyzed. Mecanum wheels which generate the omni-directionality to the mobile robot comprise a centered rim wheel and passive sub-rollers. In forward and backward motion, they function like usual wheels to enable rolling along the ground. However, in sideways motion, they create lateral motion of the mobile robot from the rotational actuation using their peculiar structural configuration, during which slip of the sub-rollers occurs. Unnecessary over-slip of the sub-rollers causes tracking errors of the mobile robot motion. To analyze the properties and reasons for the slip phenomenon, squared and circular trajectory tacking experiments were performed. From the experiments, it was observed that sideways motion generated respectively larger tracking errors than forward and backward motion. The geometric analysis regarding the tracking error generation was discussed using the Mecanum wheel structure. Finally, it was confirmed that suspension mechanism to provide four Mecanum wheels of the mobile robot with even reaction forces on the ground is necessary.