Torque Reduction of 1-DOF Transformable Wheel With Passive Springs

Transformable wheels provide an effective solution for mobile robots to overcome structured obstacles by altering their shape. However, they often encounter a peak torque during the critical climbing phase, particularly at contact-point transitions. This significantly increases actuator requirements and overall system costs (e.g., motor size and price), ultimately limiting the practicality of transformable wheels in mobile service robots. To address this issue, we integrated a passive spring into the previously developed one-degree-of-freedom (1-DOF) RPRP transformable wheel (STEP II), aiming to reduce peak torque and redistribute internal forces during obstacle climbing. A quasi-static model was first developed to analyze torque behavior and simulate a 6 kg wheel climbing a 100 mm step. Simulations identified an optimal spring constant of k = 0.9 N/mm, achieving approximately 12% peak torque reduction compared to the no-spring baseline. Additional simulations indicated that this approach remains effective for various obstacle dimensions. An experimental prototype validated this result and further evaluated five spring configurations (k = 0 to 3.0 N/mm) across four obstacle heights (50–140 mm). The results confirmed that moderate stiffness values (k = 1.8–2.4 N/mm) yield the greatest benefit, with peak torque reductions of up to 35% depending on the obstacle height. These results demonstrate that passive spring integration effectively mitigate the high-torque issue inherent in transformable wheels, paving the way for enhanced energy efficiency and cost-effectiveness, and potentially enabling smaller actuators and more practical service robot deployments.