PJRC Prop shield (+ Teensy 3.5): how to get reliable heading values?

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XFer

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Hi,

after having tried a few different IMU boards (including Adafruit's NXP and Bosch BNO055) with bad results, I now got a PJRC Prop Shield, just to start fresh on a level ground.

I'm using Arduino 1.8.8 and Teensyduino 1.45.
I hooked it to a Teensy 3.5, loaded the "Calibration" sketch, calibrated the sensors with the "MotionCal" GUI and got a nice centered sphere (all errors < 2%).

Sent the calibration data to the Teensy EEPROM, closed the GUI and loaded the sample "Orientation" sketch (which uses the "NXPSensorFusion" filter).

By looking at the orientation angles in the Serial monitor, it looks like Pitch and Roll are more or less stable and "reasonable", but heading shows totally unreliable values (I have a traditional compass too).

In particular:

Let's say heading "initial value" stabilizes (after at least a minute! Too much time, by the way..?) at about 190 (by the way: my compass says 225).
If I turn the board by 90 degrees, I should read 100; instead, after the usual minute or two, it settles at 125.
If I turn the board to back to the initial position, it gives 180 (instead of the original 190).
Now, if I turn the board by 180 degrees, I read 40 instead of 0!

I did the calibration in the very same position and conditions (and just a few minutes before), so I would exclude calibration issues.

So, my questions (please note I'm not an IMU expert):

- How to fix heading reliability? Not talking about a couple degrees: it gives totally unreliable values, off by tens of degrees (unusable on field)
- Why, when turning the board (keeping it flat), the heading value does not follow the rotation? My compass, in the same conditions (couple inches away), follows the rotation appropriately
- Why heading takes so much to settle to a (more or less) stable value? I can't think one has to wait a minute or two to get a heading value...?

Maybe I'm doing it wrong? I followed this page:
https://www.pjrc.com/store/prop_shield.html

but with a Teensy 3.5 in place of the 3.2.

Many thanks

Fernando
 
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