handling inversion

[belm0]

The imu get_acc_angles() function output is incorrect when the IMU is inverted (phi and theta converge back to 0).

It seems to be due to loss of sign in the 2nd argument to the atan2 calls. I'd like to figure out how to correct it.

https://github.com/pms67/Attitude-Estimation/blob/44a4970d33268dc9ac06fc548e2935200238ffd3/imu.py#L48-L52

Thank you for the excellent article and sample code!

[belm0]

This seems to work (from https://stackoverflow.com/questions/3755059/3d-accelerometer-calculate-the-orientation):

phi = math.atan2(ay, az)

However from https://www.nxp.com/files-static/sensors/doc/app_note/AN4248.pdf, there also seems to be a different way to compute theta:

phi = math.atan2(ay, az)
theta = math.atan2(-ax, ay * math.sin(phi) + az * math.cos(phi))