Viscoelastic legs for open-loop control of gram-scale robots

Gram-scale insects, such as cockroaches, take advantage of the mechanical properties of the musculoskeletal system to enable rapid and robust running. Engineering gram-scale robots, much like their biological counterparts, comes with inherent constraints on resources due to their small sizes. Resource-constrained robots are generally limited in their computational complexity, making controlled, high-speed locomotion a challenge, especially in unstructured environments. In this paper we show that embedding control into the leg mechanics of robots, similarly to cockroaches, results in predictable dynamics from an open-loop control strategy that can be modified through material choice. Tuning the mechanical properties of gram-scale robot legs promotes high-speed, stable running, reducing the need for active control. We utilize a torque-driven damped spring-loaded inverted pendulum model to explore the behavior and the design space of a spring-damper leg at this scale. The resulting design maps show the trade-offs in performance goals, such as speed and efficiency, with stability, as well as the sensitivity in performance to the leg properties and the control input. Finally, we demonstrate experimental results with magnetically actuated quadrupedal gram-scale robots, incorporating viscoelastic legs and demonstrating speeds up to 11.7 body lengths per second.