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What is an IMU

The inertial measurement unit is an electronic device that can measure and report the specific gravity and angular rate of an object to which it is attached. In UAVs, the IMU provides crucial data that helps the drone/plane to be self-aware of its stands, heading, and movement, so it is one of the most basic sensors on a drone.

To understand what is an IMU and how it works, you basically just need to learn the components that make up an IMU. An IMU usually references the combination of accelerometers, gyroscopes, and optional magnetometers.

Accelerometer:

  • Measures the acceleration force applied on the three axes of the sensor

  • Could tell the orientation of the drone. Say if the drone is placed levelly on the ground, then the acceleration it is experiencing is zero, except for the z-axis, which is experiencing gravity. If the 1g acceleration is shifted to the y-axis, then we know the drone rotates 90 degrees by its y-axis

  • The acceleration can be further computed to produce velocity and displacement

  • Can be used to sensor-fusing with the GPS data to provide more accurate position information

Gyroscope:

  • Measures the angular rates to a reference frame

  • The X, Y, and Z axis of the gyroscope are referred to as Pitch, Yaw, and Roll by us

  • Very important if we want to maintain the stand of the drone. The gyroscope can tell the orientation in 3-dimensional space to suggest how we can adjust the output to achieve the position we want

Magnetometer:

  • A compass, that tells the absolute heading of the object

  • This is useful when we want to know where the drone is heading on a map. Without a magnetometer, we can know if the drone is moving, but we can’t tell if it is going north or west.

Barometer:

  • An air pressure sensor that can be used to estimate the altitude of the drone, which is a sensor that is mentioned a lot in flight controller sensors, but it doesn’t really belong to the category of IMU

Drone Dancing (IMU calibration)

If you've ever participated in a flying test, chances are good that you've seen someone, or a couple of people, holding the drone and attempting to get it oriented in all directions. Is this mysterious WARG ritual that can bring us success in the flight test? Yes to some extent, doing a drone dance does help the drone perform better in the flight. The motivation behind the drone dance is actually IMU calibration, or more specifically, the drone dance is the calibration process of the magnetometer.

Calibration process for each sensor:

  • Accelerometers: place the sensor in different directions

image-20240128-145552.png

  • Gyroscope: it gets self-calibrated quickly, and usually doesn’t need to move the sensor around to finish the calibration

  • Magnetometer: the most cumbersome calibration. For a magnetometer to sense the Earth’s magnetic field correctly, it needs to remove the magnetic field effect caused by the magnetic materials from the aircraft body and the electronics on board. The calibration requires the sensor to be mounted on the aircraft and rotates the aircraft to different angles until the sensor figures out the correction parameters.

Further Reading

References:

https://www.vectornav.com/resources/inertial-navigation-primer

https://ardupilot.org/copter/docs/common-accelerometer-calibration.html

https://ardupilot.org/copter/docs/common-compass-calibration-in-mission-planner.html#

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