Hopping with Elastic Restitution Is More Difficult Than It Seems

Bouncing rubber balls and hopping on pogo sticks might suggest that building a robot that hops with a significant fraction of elastic energy restitution would be straightforward. Using spring steel legs with 99% restitution individually, we built a three-legged hopper. The hopper achieved a minute 11.2% restitution when using the springs in series with the actuators. We present five generations of three-legged hoppers: four have actuators in series with springs. Three different types of ground contact were tested. Our best design so far uses actuators in parallel with the springs, and achieves 49+/-5% restitution. This work will be of use to those interested in using elastic energy storage in legged locomotion.