The limits of running speed: Experiments with a legged robot

The top running speed of a legged system depends on the design of its mechanism and on the algorithms that control its motion. The important parts of the mechanism are the legs and the hips. To run fast, the legs should be long, strong, and stiff, and the hips should be able to rotate quickly through a large range of motion. The control algorithrns must coordinate the actions of the legs and hips to regulate the momentum of the system in the horizontal, vertical, and rotational directions. Rotation of the body is improved by control algorithms that move the legs symmetrically, compensate for the properties of the hip actuators, and anticipate the collisions of the feet with the ground.A dependency tree expresses the qualitative dependence of a running system's speed on characteristics of its motion and mechanism. Quantifying and combining the relationships in the tree produces an equation that expresses the speed of a running system as a function of its parameters and its running motion. The equation indicates that a fast running machine should have long legs and fast actuators, and that it should be lightweight and have stiff leg springs.Experiments with a planar two-legged robot provid insight into how the design and control of a legged system affect its top running speed. Experiments with a planar biped running machine show that it runs faster with long legs than with short legs, and faster with stiff legs than with soft legs. The experiments make it clear that there are speed dependent disturbances to body attitude, and that fast running requires that the control system reject or correct those disturbances. Two control techniques reduced disturbances to body attitude: moving the legs symmetrically, and compensating for velocity dependent hip actuator force. The biped has run at a steady state speed of 5.9 m/s (13.1 mph) for a distance of over 50 m.