Is a magnetometer important?

[CameronMakesRockets]

Hello All!

I am working on my own flight computer and in previous designs I have used the Bosch BNO055 9DOF IMU sensor. Currently this sensor is really difficult to get a hold of, so I am on the hunt for replacements. A lot of the different IMU's that I have seen out there are 6DOF sensors that include a gyroscope and accelerometer. The MNO055 includes an extra magnetometer sensor on board, but how important is it really? Does it really help calculate orientation or is it just another data point?

Any help and explanation on magnetometers is greatly appreciated!

 

[OverTheTop]

Depending on how you do your firmware you might find the magnetometer easier to use to determine maximum tilt. Trying to use gyros and accelerometers to determine verticality is, of course, doable, but the task is non-trivial and fraught with vagaries for anyone inexperienced with that mathematics. Some of our experts in that area are sure to chime in regarding that.

A magnetometer will basically get you a unit vector pointing (x^2 + y^2 +z^2) = 1 in the direction of the magnetic field. Remember that the field is NOT horizontal, so the resulting vector will be non-vertical, and rotate around the vertical axis when the rocket spins during flight. Think of that vector making a cone-shaped path in 3D space. You can check that vector against a limit to determine that the rocket is arcing over.

If you have a look at some of the documentation about Magnetic Apogee Detectors you might gain some insight there.
https://www.rocketreviews.com/scratch-magnetic-apogee-detection-sensor-by-robert-galejs.html
https://en.wikipedia.org/wiki/Magnetic_declination

 

[rocket_troy]

A good 9DOF (or more) will at least provide the option of using the magnetometer to sanity check the gyros or even fuse them together in varying degrees. For example, the MTi-3 - MTi-8 series of IMUs/INSs will provide you various modes of how much weight you want the magnetometer to provide to the fused bearing.
For a typical *unguided* HPR IMU or INS application, the magnetometer isn't generally of central importance. However, for something like a steerable parachute recovery system, it's generally the most important thing.

TP

 

[Adrian A]

I find it's somewhere between difficult and impossible to get the magnetometer calibrated well enough to actually improve the attitude estimate. Today's gyros are easily good enough to have 1-4 degrees of attitude error at the first deployment if you have a fast update rate and do the calculations with quaternions.

 

[jderimig]

A magnetomer is useful if you want to do full 3D inertial navigation in the earth frame. On paper magetometers can be used to compensate gyro drift. But as Adrian has posted in practice the calibration errors are probably an order of magnitude greater than gyro drift errors for the relatively short durations of our rocket flights.

 

[waltr]

I have been flying a 10DOF sensor logger and have not yet been able to use the Magnetometer data.
The Math gets real complex real fast.

I have been able to get good info on what the rocket did using the Accel, Gyro and Baro,

 

[AllDigital]

I also gave up trying to use the magnetometer on the rocket. I just don't find it necessary and they are a huge PITA to work with. The gyro gives me accurate roll/tilt, the accelerometer gives me good vertical velocity integration, the baro is also a good sensor <60k feet, and the GPS gives me absolute coordinates.

That said, I've found that a magnetometer is a must on the ground to have a good tracking solution. I switched all of my ground tracking over to an iPad mini a long time ago, because the internal GPS and magnetometer are just so damn good right out of the box. Have that precise bearing allows me to point an arrow directly at the rocket on the ground and an azimuth angle to the rocket in the air (using the difference in rocket long, lat,alt and base long, lat, alt).