Gait Phase (Phase Clock)

A number that cycles from 0 to 1, indicating where the robot currently is in its gait cycle.

Legged walking is periodic motion, with each leg alternating between stance phase (on the ground, bearing weight) and swing phase (lifted, swinging forward). Gait phase φ normalizes one cycle to [0,1): φ = (t mod T)/T, where t is time and T the gait period. Giving each leg its own phase offset defines a gait — for example, left and right legs offset by 0.5 gives alternating stepping in a biped, and diagonal legs sharing the same phase gives a quadruped trot. In model-based control it's produced by the gait scheduler and determines when contact switches; in reinforcement learning, Siekmann et al. (ICRA 2021) used it to build a periodic reward — penalizing foot velocity during stance and foot force during swing — learning walking, running, and hopping gaits on the Cassie biped with no reference motion at all.

Exampleunitree_rl_gym's G1 walking environment uses a 0.8-second period with a 0.5 phase offset between legs; a leg with phase under 0.55 is treated as expected to be in stance, and the policy is rewarded when the actual contact state matches.

Also called

Gait Clock

Related

Gait · Stance Phase / Swing Phase · Gait Cycle and Duty Factor · Gait Planning · Trot Gait · Gait Phase Variable / Clock Input (sin/cos Phase Encoding)

Sources

Siekmann et al., Sim-to-Real Learning of All Common Bipedal Gaits via Periodic Reward Composition (arXiv 2011.01387, ICRA 2021)
unitree_rl_gym: legged_gym/envs/g1/g1_env.py