Robotics
Inertial measurement unit
Definition
An inertial measurement unit is a sensor assembly that typically combines accelerometers and gyroscopes to measure specific force and angular velocity. Some devices also provide magnetometer readings or estimated orientation.
Also known as: IMU
Updated
Measurements expressed in sensor axes
Gyroscopes measure rotational rate. Accelerometers measure specific force, which differs from a gravity-free acceleration estimate. ROS REP 145, a draft convention for IMU drivers, explains that a stationary supported sensor reports a gravity-related accelerometer reading.
For a humanoid, an IMU mounted in the torso can contribute rapid motion measurements to a body state estimator. The sensor's mounting orientation determines how its axes relate to the robot.
Orientation can be an estimate
A device may calculate orientation by fusing several measurements internally. It may instead output only raw acceleration and angular velocity. The ROS Imu message specification explicitly allows an orientation estimate to be unavailable and provides covariance fields for the reported measurements.
A sensor is not a complete positioning system
An IMU does not directly measure absolute position. Coordinate conventions and uncertainty must be understood before combining its output with other sensors. In visual-inertial odometry, camera observations provide additional constraints while the estimator accounts for inertial bias and motion.
Sources
Related terms
Sensor fusion
Sensor fusion combines information from multiple sensors or estimation sources to produce a shared estimate. The combination must account for coordinate frames, timing, uncertainty, and dependence between inputs.
Visual-inertial odometry
Visual-inertial odometry estimates a moving system's motion by combining camera observations with inertial measurements. It typically estimates position, orientation, velocity, and sensor biases over time.
State estimation
State estimation infers quantities describing a robot or its environment from measurements and a model. A robot state may include position, orientation, velocity, and other variables that are not all directly measured.