Analysis of locomotion in a soft inchworm robot driven by friction asymmetry and phase-dependent motion

Efficient and controllable crawling remains a key challenge in soft robotics, where adaptability to diverse surfaces underpins practical deployment. This paper presents a unified theoretical and experimental framework for bidirectional locomotion in inchworm-inspired soft robots, emphasizing the interplay between friction asymmetry, bending configuration, and actuation phase dependency. A three-segment bending model is introduced to predict robot kinematics and contact force evolution throughout the motion cycle. Integration of Lagrangian dynamics with Finite Element Analysis (FEA) enables systematic quantification of force redistribution and transitions between energy-dominated and friction-dominated regimes. Experiments across multiple surfaces confirm that forward motion exhibits high model agreement (± 6.4 mm) due to elevated energy states and predictable tail anchoring, whereas backward locomotion occurs in a lower-energy, friction-dominated regime with partial force redistribution between head and tail, resulting in reduced net displacement. These results highlight the importance of bending behavior and energy state control for locomotion efficiency and feasibility. The insights provide a foundation for future soft robot design and control strategies aimed at reliable, bidirectional crawling.