Post-processing sensor readings from smartphone - gravity calculated from magentometer

Question

I'm trying now to extract in Matlab information from android app that records output of accelerometer, gyrometer and magnetometer. As I need to find linear acceleration, I assumed that I can calculate it from accelerometer taking gravity into account. I thought that it should be possible to find by checking phone orientation based on magnetometer output (Earth magnetic field). But I can't get correct results with any formula. Am I right that it's possible to find gravity vector thanks to magnetometer? If so, how should I interpret sensor readings?

Answer 1

No, it is not possible to find the gravity from the magnetometer.

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What you are looking for is called sensor fusion. I have implemented sensor fusion for the Shimmer platform based on this manuscript, it's basically a tutorial:

Direction Cosine Matrix IMU: Theory

Another resource I highly recommend is Sensor fusion on Android devices.

But why are you doing it yourself? The SensorManager already offers what you want.

Answer 2

You can get linear acceleration by doing a low pass filter on accelerometer values to get gravity and then subtract accelerometer values from gravity. The code below is from http://developer.android.com/reference/android/hardware/SensorEvent.html

public void onSensorChanged(SensorEvent event)
 {
      // alpha is calculated as t / (t + dT)
      // with t, the low-pass filter's time-constant
      // and dT, the event delivery rate

      final float alpha = 0.8;

      gravity[0] = alpha * gravity[0] + (1 - alpha) * event.values[0];
      gravity[1] = alpha * gravity[1] + (1 - alpha) * event.values[1];
      gravity[2] = alpha * gravity[2] + (1 - alpha) * event.values[2];

      linear_acceleration[0] = event.values[0] - gravity[0];
      linear_acceleration[1] = event.values[1] - gravity[1];
      linear_acceleration[2] = event.values[2] - gravity[2];
 }