Acceleration from device's coordinate system into absolute coordinate system

android vector accelerometer coordinate-systems coordinate-transformation
alex picture alex · Jul 20, 2012 · Viewed 21.6k times · Source

From my Android device I can read an array of linear acceleration values (in the device's coordinate system) and an array of absolute orientation values (in Earth's coordinate system). What I need is to obtain the linear acceleration values in the latter coord. system.

How can I convert them?

EDIT after Ali's reply in comment:

All right, so if I understand correctly, when I measure the linear acceleration, the position of the phone completely does not matter, because the readings are given in Earth's coordinate system. right?

But I just did a test where I put the phone in different positions and got acceleration in different axes. There are 3 pairs of pictures - the first ones show how I put the device (sorry for my Paint "master skill") and the second ones show readings from data provided by the linear acc. sensor:

  1. device put on left side

first position first readings

  1. device lying on back

second position second readings

  1. device standing

enter image description here enter image description here

And now - why in the third case the acceleration occurs along the Z axis (not Y) since the device position doesn't matter?

Answer

alex picture alex · Jul 23, 2012

I finally managed to solve it! So to get acceleration vector in Earth's coordinate system you need to:

  1. get rotation matrix (float[16] so it could be used later by android.opengl.Matrix class) from SensorManager.getRotationMatrix() (using SENSOR.TYPE_GRAVITY and SENSOR.TYPE_MAGNETIC_FIELD sensors values as parameters),
  2. use android.opengl.Matrix.invertM() on the rotation matrix to invert it (not transpose!),
  3. use Sensor.TYPE_LINEAR_ACCELERATION sensor to get linear acceleration vector (in device's coord. sys.),
  4. use android.opengl.Matrix.multiplyMV() to multiply the rotation matrix by linear acceleration vector.

And there you have it! I hope I will save some precious time for others.

Thanks for Edward Falk and Ali for hints!!

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