Improbability Roller: A Hybrid Mobile Robot with Variable Diameter Wheels

Abstract

This thesis presents the design, development, and experimental study of the Improbability Roller, a hybrid mobile robot that utilizes variable-diameter wheels to adapt to various terrains while maintaining a purely wheeled mode of operation. Wheeled robots typically struggle on uneven ground, whereas legged systems introduce mechanical and energy overhead. This work investigates wheel size modulation as a way to adjust geometry during motion without changing the locomotion mode. The first version used a cable-driven, passive spring-assisted mechanism that allowed continuous size adjustment with three actuators. Experiments in trajectory tracking, slope climbing, and cost of transport showed stable behavior across different terrains and lower energy use with larger diameters at higher speeds. Limitations in structural layout and maneuverability motivated a second iteration. The second version introduced a rhombus-folding wheel mechanism with a 1.87 size-change ratio, the highest reported among comparable systems, along with lighter composite components and two steering modes. Size-disparity steering provided better stability at higher speeds, while smaller diameters reduced vibration for indoor and sensor-sensitive tasks. Overall, the study demonstrates that adjusting wheel geometry without altering morphology can enhance terrain adaptability while maintaining simplicity in actuation and control, providing a practical approach for cluttered, deformable, and space-constrained environments.