Cassie is Agility Robotics’ bipedal research platform, spun out of Oregon State University’s Dynamic Robotics Laboratory — a spring-legged biped whose distinctive knee-forward, bird-like stance and passive elastic leg design have made it one of the most widely used robots in academic legged-locomotion research, from classical control through modern reinforcement learning.
The mechanism
Each leg carries five actuated joints — hip abduction, hip rotation, hip flexion, knee,
and toe — for ten actuated degrees of freedom in total. The signature part of Cassie’s
design is what happens between the knee and the foot: a passive four-bar linkage (an
achilles rod and a spring-loaded shin/tarsus connection) provides compliant, spring-like
ground contact without a dedicated actuator. URDF can’t express a true closed loop, so this
description encodes that linkage as two extra open-chain joints per leg
(knee_to_shin, ankle_joint) — kinematically dependent on the knee joint, not
independently driven, per the repository’s own dependent_joints.yaml.
About this file — read before importing
This links directly to cassie_description, a third-party academic repository from the University of Michigan’s Biped Lab — Agility Robotics has never published a URDF for Cassie itself. See this entry’s license notice above: this repository carries no license of any kind, so review that before building on this design. ARMOR links directly to the repository rather than re-hosting anything, so what you get is exactly what’s published there, unmodified.