IMUs Explained: How Robots Sense Their Own Orientation

The small chip telling a robot which way is up, hundreds of times per second.

An inertial measurement unit (IMU) combines an accelerometer, which senses linear acceleration (including gravity), and a gyroscope, which senses rotational velocity, usually across three axes each. Many also add a magnetometer for absolute heading, though that's more common in drones and vehicles than legged robots.

Why balance depends on it

A humanoid's balance controller needs to know its torso's orientation and rate of tilt continuously to keep its center of mass over its base of support. The IMU is usually the single most safety-critical sensor on the robot for exactly this reason — lose good IMU data and a walking robot falls almost immediately.

Drift and fusion

Gyroscopes drift slowly over time due to tiny integration errors, and accelerometers are noisy at high frequency but don't drift. Sensor fusion algorithms, most commonly a complementary or Kalman filter, blend the two to get an orientation estimate that's both stable long-term and responsive short-term.