Body stiffness in orthogonal directions oppositely affects worm-like robot turning and straight-line locomotion

Earthworms locomote using traveling waves of segment contraction and expansion, which when symmetric result in straight-line locomotion and when biased result in turning. The mechanics of the soft body permit a large range of possible body shapes which both comply with the environment and contribute to directed locomotion. Inspired by earthworms, our new platform Compliant Modular Mesh Worm robot with Steering (CMMWorm-S) has been developed to study this type of locomotion. Compared with our previous robots, CMMWorm-S is capable of an entirely new movement (turning) using two actuated degrees of freedom per segment (a total of 12 motors). The modularity of the mesh, composed of 3D printed rigid pieces and flexible tubes, allows for the interchange of components to vary the stiffness of the robot. On this robotic platform, we show that locomotion efficiency is sensitive to body stiffness. In particular, greater bending stiffness improves turning locomotion, whereas greater circumferential stiffness speeds straight-line locomotion. The data presented demonstrate the contribution of each component towards the longitudinal, circumferential and bending stiffness of mesh-based robots. These analyses can help in the development of design criteria useful for future soft robotic peristaltic devices.