Trajectory Design Trade-Offs in a 1-DoF Transformable Wheel for Obstacle Climbing

Abstract

As mobile service robots expand into human living environments, their ability to negotiate structured obstacles—such as thresholds, curbs, and stairs—has become increasingly important. Transformable wheels offer a compelling alternative to high-DoF locomotion by preserving the efficiency and maneuverability of conventional wheels on flat ground while selectively reconfiguring their geometry only when obstacle negotiation is required. Among such systems, the 1-DoF RPRP transformable wheel achieves step climbing with minimal actuation by mechanically coupling radial transformation and spoke tilting through an internal linkage. This reduced-actuation architecture, however, also creates a distinctive design challenge: because the mechanism lacks kinematic redundancy, a very small number of trajectory parameters exert a disproportionately large influence on climbing behavior. As a result, performance is governed less by control flexibility and more by how transformation timing and posture are coordinated throughout the climbing cycle. Despite this, prior studies on transformable wheels have largely focused on mechanism design and kinematic feasibility, leaving insufficient understanding of how trajectory design shapes the trade-offs among motion smoothness, actuator load, and power demand [1]–[3]. To address this gap, this study presents a trajectory-level design-space exploration framework for a 1-DoF transformable wheel, in which the obstacle-climbing motion is parameterized us