Body Frame

A coordinate frame fixed to a robot's own body, moving and rotating along with it.

The body frame — often written {b} in textbooks — is a coordinate frame attached to a rigid body or a robot's torso, with its origin and three axes translating and rotating right along with the body; its counterpart is the fixed world frame. Describing an object's position and orientation is, at bottom, just giving the body frame's pose relative to the world frame; quantities like velocity and force can likewise be expressed in either the world frame or the body frame. Reinforcement-learning policies for legged robots usually put observations in the body frame: in Isaac Lab, base_lin_vel, base_ang_vel, and projected_gravity (the direction of gravity projected into the body frame, used to sense body tilt) are all defined relative to the robot's root frame, so the policy never has to care which way the robot happens to be facing in the world.

ExampleCommanding a quadruped robot to “walk forward at 0.5 m/s” gives that velocity in the body frame, where x means forward relative to the body's own facing direction — whether the robot is actually facing east or north in the world, the same command always means “walk toward your own front.”

Also called

Robot Body Frame

Related

World Frame · Base Frame · Coordinate Transformation · Projected Gravity · Pose · Velocity Command Tracking

Sources

Modern Robotics（Lynch & Park, 2017 预印本）第 3 章 Rigid-Body Motions (Chinese)
isaaclab.envs.mdp API（base_lin_vel / projected_gravity 等观测） (Chinese)