Inertial Measurement Unit

A sensor combining an accelerometer and gyroscope that measures an object's acceleration and rotation rate.

An inertial measurement unit (IMU) combines an accelerometer and a gyroscope, which measure linear acceleration along three axes (at rest, this reads gravity, which is how the IMU can tell which way is down) and angular velocity around three axes — together called six-axis; adding a three-axis magnetometer makes it nine-axis. IMUs are found in phones, drones, and VR headsets. For a robot, an IMU provides the body's orientation (especially which way gravity points) and its rotation rate, which is basic input for balance in legged and humanoid robots. Its readings carry bias and noise, so integrating them to get angle or position accumulates drift over time, which is why IMUs are usually fused with a camera, lidar, or leg odometry. Colloquially, some people call an IMU an ‘inertial nav’ system, but strictly speaking an inertial navigation system is the larger system that further integrates the IMU's readings to solve for position and velocity.

ExampleIn Unitree's open-source unitree_rl_gym, the first 6 dimensions of the G1 walking policy's observation are the body angular velocity measured by the IMU's gyroscope, plus the projected gravity direction computed from the IMU's attitude quaternion.

Also called

IMU, Six-Axis IMU, Nine-Axis IMU, Inertial Sensor

Related

Sensor Drift · Projected Gravity · State Estimation · Visual-Inertial Odometry · Proprioception · Camera-IMU Calibration

Sources

Wikipedia: Inertial measurement unit
unitree_rl_gym deploy_real.py（IMU 角速度与投影重力作为观测） (Chinese)