Magnetometer calibration

BenjaminKiefer · Aug 29, 2023

Hi!

I'm relying on the magnetometer for my application. However, its current outputs are not useful, e.g.

Just restarting the cam gives magnetometer values of +-10 degrees difference
When driving around and rotating, the values drift
I'm comparing to course over ground and it almost never aligns (when going straight)

I'd want to calibrate the magnetometer using tutorials as such:

(8) How to Calibrate a Magnetometer | Digi-Key Electronics - YouTube

However, I'm unsure where I could add these values such that the IMU can use that for its own computation. E.g., I want the rotation vector from the IMU which is computed on the IMU I believe. Is there a way to do that?

Thanks a lot!

erik · Aug 31, 2023

Hi BenjaminKiefer ,
There are 3 different magnetometer sensors available which you can use, mentioned below. If you have additional user-calibration you'd want to apply, you'd need to apply that either on host computer, or inside Script node, as IMU doesn't allow custom user-calibration.
Thanks, Erik

 /**
     * Section 2.1.3
     *
     * Magnetic field measurement without any postprocessing, straight from the sensor.
     * Units are [uTesla]
     */
    MAGNETOMETER_RAW = 0x16,
    /**
     * Section 2.1.3
     *
     * The fully calibrated magnetic field measurement.
     * Units are [uTesla]
     */
    MAGNETOMETER_CALIBRATED = 0x03,
    /**
     * Section 2.1.3
     *
     * The magnetic field measurement without hard-iron offset applied.
     * Units are [uTesla]
     */
    MAGNETOMETER_UNCALIBRATED = 0x0f,

BenjaminKiefer · Aug 31, 2023

Thanks for your reply!

Essentially, I need a precise rotation vector with absolute heading (referenced to magnetic north).

However, the measurements I get often times are completely wrong and always noisy. I wondered whether I can calibrate the magnetometer to account for hard and soft iron magnetic compensation.

So, am I correct in the conclusion that I'd have to take either of these three variables and compute magnetic north separately (with calibration parameters included)?

Thanks!

erik · Aug 31, 2023

Hi BenjaminKiefer ,
To get accurate compass functionality out of 9axis IMU you'd need to calibrate the device. I have found some python library that might help: https://github.com/michaelwro/mag-cal-example. You'd then apply calibration values on host after getting results from the camera.
Gpt4 also provides good answer on approaches.

BenjaminKiefer · Aug 31, 2023

Thanks for the resources.

I've also looked at some resources that describe how to calibrate the sensor based on the raw microtesla measurements. However, I also read that many of the easier approaches might not really reliably work and instead some more advanced methods, such as Kalman filter smoothing (combined with accelerometer/gyrometer), need to be implemented.

Hence, I was hoping for a calibration procedure that already changes parameters inside the IMU, so that I can still rely on the (stabilized) rotation vector I can get from it.

Anyways, thank you!