Monopedal robot branch-to-branch leaping and landing inspired by squirrel balance control

Abstract

Locomotors traversing arboreal environments must often leap across large gaps to land on small-diameter supports. Balancing these dynamic landings is challenging because of high incident momentum, restricted foothold options, and reduced capacity to produce reaction torques on narrow supports. We hypothesized that leg length control to enhance branch reaction control authority would markedly expand the range of successful landing conditions, drawing on the same powerful leg actuation required for leaping. Exploring this balance strategy, the monopedal robot Salto-1P demonstrates branch-to-branch leaps, including some upright balanced landings, despite negligible grasping torque. We also compared this landing strategy with the landings of squirrels, which similarly lack the grip strength found in other arboreal species. We demonstrate that greater radial force control reduces the inertial body torque and/or grasping torque at the support required to balance a given landing. Adding simple radial force balance control strategies to conventional balance controllers greatly expands potential landing conditions, increasing the range of initial angular momentum that can be balanced by 230 and 470% across ranges of landing angles for low-order models of the robot and squirrel, respectively.