Mitigating Peak Impact Forces by Customizing the Passive Foot Dynamics of Legged Robots

Impact forces are a destructive, yet common occurrence in legged locomotion. Every step produces a collision when the leg’s inertia stops as a result of ground contact. This results in peak forces and high-frequency vibrations that resonate through the system, damage components, and complicate control algorithms. Prior research considers how damping material, such as rubber, mitigates these effects. However, this paper shows the benefits of spring protection where both stiffness and maximum compression are customized to the leg. The spring mitigates the impact force peak by gradually bringing the leg’s inertia to rest. The maximum compression point (i.e., a hard stop) then provides a rigid surface that is ideal for stance. We provide a foot design methodology, validated through simulation and physical testing, that first considers springs in isolation, then in tandem with damping. We show that the coupling of springs and dampers reduces rigid body collisions and foot vibrations in a way that traditional methods—reliant on damping—have yet to achieve.