Mechanical Design and Kinematics of a Multimodal Two-Wheeled Robot

A two-wheeled vehicle has a compact structure and high mobility in crowded and complex environments, which has been widely used in urban logistics. The bicycle and self-balancing vehicle are the two main modes of the two-wheeled vehicle, and their combination allows for good balance-control stability at both high and low speeds. Four control inputs by two steerable driving wheels are required to implement transformations between the two modes due to the difference in their configuration spaces. However, the control inputs are redundant for planar motions, which results in an over-constraint of the vehicle. In this work, a two-wheeled robot with an additional structural degree of freedom (DOF) is designed to balance inputs and DOFs to avoid over-constraint. A transition mode based on oblique vehicle motions is used to bridge the transformation of the bicycle and self-balancing vehicle modes. A general kinematic model is developed for the two-wheeled robot’s planar motions, and the three modes’ kinematics are special cases with particular servo constraints. Structural DOF control laws are developed and experimentally validated on a prototype robot. Smooth transformations of the multimodal motions are also validated by using the prototype.