accelerometer - Android TYPE_LINEAR_ACCELERATION sensor - what does it show?

Question

I am developing application for car acceleration tracking. I used standard accelerometer, calibrating it in specific position beforehand.

Then, assuming phone's orientation is not changing, I logged the accelerometer data for a specified time and calculated move parameters, one of which is the car's speed at the end of the test.

It works rather well, on a straight, horizontal road: error of a few percent.

But then I found out, that in API-level 10 there is a virtual sensor called TYPE_LINEAR_ACCELERATION and, as far as I understand, it must do what I need: filter gravity, orientation changes - so I may use it and get pure linear acceleration of mobile device.

BUT in real life..

I made a simple application, that does a little test:

//public class Accelerometer implements SensorEventListener { ...
public void onSensorChanged(SensorEvent se) 
{
    if(!active)
        return;

    lastX = se.values[SensorManager.DATA_X];
    lastY = se.values[SensorManager.DATA_Y];
    lastZ = se.values[SensorManager.DATA_Z];
    long now = System.currentTimeMillis();
    interval = now - lastEvetn;
    lastEvetn = now;
    out.write(Float.toString(lastX) + ";" + 
                    Float.toString(lastY) + ";" + 
                    Float.toString(lastZ) + ";" + 
                    Long.toString(interval) + "
");
}

I bind a listener with the following parameters:

  mSensorManager.registerListener(linAcc,
                mSensorManager.getDefaultSensor(Sensor.TYPE_LINEAR_ACCELERATION),
                SensorManager.SENSOR_DELAY_GAME);

It works OK, but when I analyzed data dump, calculating speed like V = V0 + AT, where V0 = 0 at first, then - speed of interval before this, A = acceleration (SQRT (x*x+y*y+z*z)) (t = time of interval), eventually I get a very low speed - three times less than real speed.

Changing Sensor type to TYPE_ACCELEROMETER, calibrating and using same formula to calculate speed - I get good results, much closer to reality.

So, the question is:

What does Sensor.TYPE_LINEAR_ACCELERATION really show?

Where am I wrong, or is something wrong with Sensor.TYPE_LINEAR_ACCELERATION implementation?

I used Samsung Nexus S phone.

Answer 1

Very interesting question!!!!

I'm developing somethig similar to your application. What i found about TYPE_LINEAR_ACCELERATION isn't happy for me.

1) TYPE_LINEAR_ACCELERATION, TYPE_GRAVITY, ecc are implemented only for Android 2.3 (and up) So i have Android 2.2 and i can't test them.

2) TYPE_LINEAR_ACCELERATION isn't so accurate as it would be, because there are some simple problem when substract the gravity. In fact is a "sensor fusion" that uses accelerometer and orientation to know where is directed the gravity and then subs. it.

Here i found a very usefull answer that explain it:

https://groups.google.com/forum/#!topic/android-developers/GOm9yhTFZaM

TYPE_ACCELEROMETER uses the accelerometer and only the accelerometer. It returns raw accelerometer events, with minimal or no processing at all.

TYPE_GYROSCOPE (if present) uses the gyroscope and only the gyroscope. Like above, it returns raw events (angular speed un rad/s) with no processing at all (no offset / scale compensation).

TYPE_ORIENTATION is deprecated. It returns the orientation as yaw/ pitch/roll in degres. It's not very well defined and can only be relied upon when the device has no "roll". This sensor uses a combination of the accelerometer and the magnetometer. Marginally better results can be obtained using SensorManager's helpers. This sensor is heavily "processed".

TYPE_LINEAR_ACCELERATION, TYPE_GRAVITY, TYPE_ROTATION_VECTOR are "fused" sensors which return respectively the linear acceleration, gravity and rotation vector (a quaternion). It is not defined how these are implemented. On some devices they are implemented in h/w, on some devices they use the accelerometer + the magnetometer, on some other devices they use the gyro.

On Nexus S and Xoom, the gyroscope is currently NOT used. They behave as if there was no gyro available, like on Nexus One or Droid. We are planing to improve this situation in a future release.

Currently, the only way to take advantage of the gyro is to use TYPE_GYROSCOPE and integrate the output by hand.

I hope this helps,

Mathias

Anyway, in various place on the web i found no best word about phone sensor and their potential, due to the fact that aren't accurate...

Some more precision can be reached using Kalman filter, but i have no idea how...