Compliant cable-driven limbless robot for complex terrain navigation

Limbless robots have been developed to study the mechanisms underlying the capabilities of their biological counterparts, but still, fail to match their performance. We posit that the bilateral actuation mechanism of muscles and passive body mechanics in animals facilitates effective limbless locomotion; such mechanisms have rarely been applied to limbless robots. Building on previous work (Schiebel, et al., 2020), we developed a limbless robot featuring a bilateral actuation strategy (L=86cm, W=7cm, 7 joints). Servo motor pairs in each joint wind/unwind Kevlar strings ("muscles") on either side of the body, bending each body segment. The robot also features an elastic mesh skin for smooth robot-environment contact and detachable modular wheels for variable drag anisotropy. The cable-driven mechanism allows variation of body stiffness, and we study how such mechanics facilitate terrain traversal in model laboratory complex terrains (i.e., lattices, channels, granular media). Results show that the appropriate body passivity reduces the mechanical cost of transport by 50% compared with a fully rigid body. Further, results show that using body passivity improves the open-loop traversal rate of the robot in dense lattice environments (20cm spacing, 15cm post diameter), allowing the robot to navigate a complex terrain without the need for sensing and electronic feedback control. Moreover, we designed vertically bending modules with universal joints, extending the robot’s capabilities to 3D spaces.