# IMU orientation using AHRS filter

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Adam Levschuk on 22 Aug 2020
Commented: Adam Levschuk on 24 Aug 2020
Hello, I am having IMU orientation troubles I am using the AHRS Filter to output Angular Velocity and Quaternions relative to the NED reference frame.
To do this i use the code:
[quaternions, angularVelocity] = ahrsfilter(acc,gyr,mag);
Where acc, gyr, and mag are accelerometer, gyroscope, and magnetometer readings, respectively. Each acc, gyr, and mag are 3xN.
I want to then rotate the sensor signals so they are aligend the the NED refernce frame. I am trying to do this with the following code
for i = length(1:end(acc))
acc(i,:) = rotateframe(quaternions(i),acc(i,:))
gyr(i,:) = rotateframe(quaternions(i),gyr(i,:))
end
after i rotate the accelerometer and gyroscope readings, the rotated signals are not correct. However, the angularVelocity output from the ahrsfilter is correct. Does anyone have any ideas why this may be?
Thank you

James Tursa on 23 Aug 2020
I haven't used either of those functions, but from reading the doc maybe you need to use the conjugate of the quaternions in your rotateframe call. I.e., if the sensor signals are in the BODY frame and the quaternions are NED->BODY, then maybe you need the conjugate of the quaternions to do the BODY->NED conversion.

Adam Levschuk on 24 Aug 2020
yes the original data and the angular velocity from the AHRS filtrer match up
James Tursa on 24 Aug 2020
So then I don't understand why you are comparing a rotated gyr signal against the angularVelocity filter result. The original gyr signal is in BODY frame and that already matches the angularVelocity result in BODY frame, so why the rotated gyr data comparison? I am still not following.
Adam Levschuk on 24 Aug 2020
Ok, Perhaps i am not uindertsanding my own problem. Im am going to work on this myslef more adn hopefully ill figure it out. I may post abck here in the future if i cna better explain it.