Passive wheels on legged robots: a survey

In recent years, highly dynamic robots have seen a significant rise in both consumer and research applications. Much of the focus on legged robots stems from their ability to adapt to a world built for humans, but energy inefficiency and battery capacity remain significant challenges to widespread adoption. Wheel-legged hybrid locomotion is an increasingly popular solution, as robots combine the energy efficiency and speed of wheels with the versatility and adaptability of legs. Most of the literature focuses on the actuated or active-wheel variant because it is simpler to control. However, recent advances in control strategies and computing power have made the advantages of unactuated or passive wheel-legged robots, such as their mechanical simplicity, low weight, and high energy efficiency, accessible. In this survey, we provide a comprehensive analysis of the unactuated wheel-legged hybrid robot literature, together with similar systems, to review the alternative implementations and design techniques in the context of the inherent challenges of controlling a legged robot that has passive skates on its feet. We aim to outline the critical factors that determine the system's viability and propose some directions for future research.