Effective Viscous Damping Enables Morphological Computation in Legged Locomotion.

An Mo,Daniel F B Haeufle,Fabio Izzi,Alexander Badri-Spröwitz

doi:10.3389/frobt.2020.00110

Abstract

Muscle models and animal observations suggest that physical damping is beneficial for stabilization. Still, only a few implementations of physical damping exist in compliant robotic legged locomotion. It remains unclear how physical damping can be exploited for locomotion tasks, while its advantages as sensor-free, adaptive force- and negative work-producing actuators are promising. In a simplified numerical leg model, we studied the energy dissipation from viscous and Coulomb damping during vertical drops with ground-level perturbations. A parallel spring- damper is engaged between touch-down and mid-stance, and its damper auto-decouples from mid-stance to takeoff. Our simulations indicate that an adjustable and viscous damper is desired. In hardware we explored effective viscous damping and adjustability, and quantified the dissipated energy. We tested two mechanical, leg-mounted damping mechanisms: a commercial hydraulic damper, and a custom-made pneumatic damper. The pneumatic damper exploits a rolling diaphragm with an adjustable orifice, minimizing Coulomb damping effects while permitting adjustable resistance. Experimental results show that the leg-mounted, hydraulic damper exhibits the most effective viscous damping. Adjusting the orifice setting did not result in substantial changes of dissipated energy per drop, unlike adjusting the damping parameters in the numerical model. Consequently, we also emphasize the importance of characterizing physical dampers during real legged impacts to evaluate their effectiveness for compliant legged locomotion.

Highlights

While less understood, damping likely plays an essential role in animal legged locomotion
By studying the response of two damping strategies in numerical dropdown simulations, we investigate how physical damping can influence the dynamics of the impact phase
We focus on the relation between a ground level perturbation h and the change in energy dissipation – and their dependency on the damper characteristics

Summary

Introduction

While less understood, damping likely plays an essential role in animal legged locomotion. In comparison, has been studied extensively in legged locomotion. Its benefits have been shown both in numerical simulations, e.g., through spring-loaded inverted pendulum (SLIP) models (Mochon and McMahon, 1980; Blickhan et al, 2007), and physical springy leg implementations (Spröwitz et al, 2013; Hutter et al, 2016; Ruppert and Badri-Spröwitz, 2019). What combines both mechanical stiffness and intrinsic, mechanical damping is their sensor- and computational-free action. A spring-loaded leg joint starts building up forces exactly at the moment of impact. Mechanical stiffness, or damping, acts instantaneously, and are not subject to delays from postprocessing sensor data (Grimminger et al, 2020), delays from limited nerve conductive velocities (More and Donelan, 2018), or uncertainties in the estimation of the exact timing of swing-tostance switching (Bledt et al, 2018)

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