Robophysical Modeling of Bilaterally Activated and Soft Limbless Locomotors

Abstract

Animals like snakes use traveling waves of body bends to move in multi-component terrestrial terrain. Previously we studied [Schiebel et al., PNAS, 2019] a desert specialist, Chionactis occipitalis, traversing sparse rigid obstacles and discovered that passive body buckling, facilitated by unilateral muscle activation, allowed obstacle negotiation without additional control input. Most snake robots have one motor per joint whose positions are precisely controlled. In contrast, we introduce a robophysical model designed to capture muscle morphology and activation patterns in snakes; pairs of muscles, one on each side of the spine, create body bends by unilaterally contracting. The robot snake has 8 joints and 16 motors. The joint angle is set by activating the motor on one side, spooling a cable around a pulley to pull the joint that direction. Inspired by snake muscle activation patterns [Jayne, J. Morph., 1988], we programmed the motors to be unilaterally active and propagate a sine wave down the body. When a motor is inactive, it is unspooled so that its wire cannot generate tension. Pairs of motors can thus resist forces which attempt to lengthen active wires but not those pushing them shorter, resulting in a kinematically soft robot that can be passively deformed by the surroundings. The robot can move on hard ground when drag anisotropy is large, achieved via wheels attached to the bottom of each segment, passively re-orient to track a wall upon a head-on collision, and traverse a multi-post array with open loop control facilitated by buckling and emergent reversal behaviors. In summary, we present a new approach to design limbless robots, offloading the control into the mechanics of the robot, a successful strategy in legged robots [Saranli et al., IJRR, 2001].

Supported by NSF PoLS PHY-1205878, PHY-1150760, and CMMI-1361778. ARO W911NF-11-1-0514, U.S. DoD, NDSEG 32 CFR 168a (P.E.S.), and the NSF Simons Southeast Center for Mathematics and Biology (SCMB).

P. E. Schiebel and M. C. Maisonneuve—These authors contributed equally to the work.