Central Pattern Generator

A neural circuit or oscillator that produces rhythmic output on its own, with no rhythmic input, used to drive a gait.

The central pattern generator was originally a neuroscience concept: neural circuits in the spinal cord and elsewhere can generate rhythmic output on their own, with no rhythmic input, driving walking, swimming, and breathing. Graham Brown's 1911 experiments were the first to show that the spinal cord could independently generate a stepping pattern, with the lamprey and the cat as classic study subjects. Robotics borrows this idea: a set of coupled oscillators generates a periodic signal for each leg or joint, where amplitude sets stride length, frequency sets stepping speed, and the phase difference between oscillators switches the gait. The Ijspeert team's salamander robot, published in Science in 2007, used a spinal CPG model to switch between swimming and walking. CPGs give stable rhythms with few parameters, but handling complex terrain requires added sensory feedback. The ‘gait phase clock’ commonly used in reinforcement-learning locomotion is similar in spirit, and work such as CPG-RL instead has a neural network modulate the oscillators' parameters.

ExampleEPFL's CPG-RL (2022) gives each quadruped leg its own oscillator, with a reinforcement-learning policy only outputting each oscillator's amplitude and frequency; deployed on a Unitree A1, it could carry an extra 13.75 kg load — 115% of the robot's own weight.

Also called

CPG, CPG Oscillator

Related

Gait · Gait Phase (Phase Clock) · Legged Locomotion · Bio-inspired Robot · RL-based Locomotion Control · Limit Cycle

Sources

Wikipedia: Central pattern generator
Ijspeert A. J. Central pattern generators for locomotion control in animals and robots: a review. Neural Networks, 2008
Bellegarda, Ijspeert. CPG-RL: Learning Central Pattern Generators for Quadruped Locomotion (arXiv:2211.00458)