  Animals run robustly in diverse terrain. This locomotion robustness is
puzzling because axon conduction velocity is limited to a few ten meters per
second. If reflex loops deliver sensory information with significant delays,
one would expect a destabilizing effect on sensorimotor control. Hence, an
alternative explanation describes a hierarchical structure of low-level
adaptive mechanics and high-level sensorimotor control to help mitigate the
effects of transmission delays. Motivated by the concept of an adaptive
mechanism triggering an immediate response, we developed a tunable physical
damper system. Our mechanism combines a tendon with adjustable slackness
connected to a physical damper. The slack damper allows adjustment of damping
force, onset timing, effective stroke, and energy dissipation. We characterize
the slack damper mechanism mounted to a legged robot controlled in open-loop
mode. The robot hops vertically and planar over varying terrains and
perturbations. During forward hopping, slack-based damping improves faster
perturbation recovery (up to 170%) at higher energetic cost (27%). The tunable
slack mechanism auto-engages the damper during perturbations, leading to a
perturbation-trigger damping, improving robustness at minimum energetic cost.
With the results from the slack damper mechanism, we propose a new functional
interpretation of animals' redundant muscle tendons as tunable dampers.
