Passive Dynamic Walking

A style of biped walking with no motors, powered purely by gravity and the legs' natural swing down a gentle slope.

Passive dynamic walking was introduced by Tad McGeer in the late 1980s, with the representative paper 'Passive Dynamic Walking' published in IJRR in 1990: a two-legged mechanism with no motors and no controller, placed on a shallow slope, can walk with a stable periodic gait powered by gravity alone, with each leg swinging naturally like a pendulum. The energy lost with each step is exactly repaid by the downhill slope, and the gait converges to a limit cycle (a stable periodic motion). It demonstrates that much of walking's stability comes from the mechanism's own dynamics, without needing every joint to be precisely controlled. In 2005, Collins and colleagues showed in Science a robot built on this principle, with only a small amount of added actuation, that could walk on level ground with a cost of transport of about 0.2 — close to a human's — versus about 3.2 for ASIMO. It's a classic example for understanding low-energy gaits, limit-cycle stability, and morphological computation.

ExampleA McGeer-style passive walker: two legs with knees, no motors at all, given a light push on a very shallow slope, will walk itself step by step down to the bottom.

Also called

Passive Walking, Passive Dynamic Walker

Related

Bipedal Locomotion · Limit Cycle · Cost of Transport · Morphological Computation · Inverted Pendulum Model (IPM) · Honda ASIMO

Sources

Wikipedia: Passive dynamics（含 McGeer 1990 与 Collins et al. 2005 Science 引用） (Chinese)