What Is an IMU? Inertial Measurement Units Explained.

What Is an Inertial Measurement Unit?

An inertial measurement unit, or IMU, is a sensor module that measures information about a device's motion and rotation.

IMUs commonly combine accelerometers and gyroscopes, and some also include magnetometers.

They are used in drones, mobile robots, humanoid robots, vehicles, and wearable devices.

For example, a humanoid robot may use an IMU mounted in its torso to estimate how its body is rotating while walking.

That information can help its control system maintain balance.

Key Takeaways

• IMUs commonly combine accelerometers and gyroscopes in one sensor module.
• They provide information about acceleration, angular velocity, and, in some systems, estimated orientation.
• IMU-based estimates accumulate errors over time and often need to be combined with other sensor measurements.

How Does an IMU Work?

An IMU combines several measurements of physical motion.

Accelerometers measure specific force, which includes the effect of gravity in the sensor's readings.

Gyroscopes measure angular velocity, or how quickly the device rotates.

Magnetometers, when included, measure magnetic fields and may provide additional heading information.

Consider a drone beginning to tilt to one side.

Its gyroscope detects the rotational movement.

Accelerometer measurements provide additional information about motion and orientation relative to gravity.

The drone's controller uses these measurements to estimate its state and adjust motor output.

The process happens repeatedly as the drone moves.

What Is a Six-Axis IMU?

A common six-axis IMU combines:

• Three accelerometer measurement axes
• Three gyroscope measurement axes

A nine-axis IMU generally adds three magnetometer axes.

The number of axes describes measurement channels rather than the number of physical directions in which the robot can move.

Some IMU modules also contain processors that estimate orientation internally.

IMU vs. GPS

An IMU measures motion-related quantities.

GPS and other GNSS systems estimate geographic position using satellite signals.

An IMU can provide rapid measurements without requiring satellite visibility.

However, calculating position by integrating inertial measurements causes errors to accumulate.

Satellite positioning can help correct longer-term drift outdoors.

Indoors, robots may instead combine IMU measurements with cameras, LiDAR, or wheel odometry.

Why Are IMUs Important for Humanoid Robots?

Walking robots must constantly estimate how their bodies are moving.

A humanoid may begin leaning forward while lifting an object.

The IMU helps the controller detect changes in body rotation and motion.

This information can be combined with joint positions and foot-contact measurements to support balance and movement.

The IMU alone does not provide a complete understanding of the robot's posture or location.

Frequently Asked Questions

Can an IMU measure absolute position?

Not directly. Position can be estimated from inertial measurements, but accumulated errors make additional references important.

Does every IMU include a magnetometer?

No. Many use only accelerometers and gyroscopes.

The Bottom Line

IMUs give robots information about how their bodies are accelerating and rotating. They are essential components of many navigation, balance, and motion-estimation systems.