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I am developing application for car acceleration tracking. I used
standard accelerometer, calibrating it in specific position before
start, than - considering phone's orientation is not changing -
logging accelerometer data for specified time and calculating move
parameters, one of them - car speed at the end of test. It works
almost good, on strait horizontal road - few percent mistake.
But then I found out, that in apl-level 10 there is virtual sensor
TYPE_LINEAR_ACCELERATION, and, as far as i understand, it must do what
i need: filter gravity, orientation changing - so i may use it and get
pure linear acceleration of mobile device. BUT in real life.. I made
simply application, that does 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) + "\n");
}
I bind listener with following parameters:
mSensorManager.registerListener(linAcc,
mSensorManager.getDefaultSensor(Sensor.TYPE_LINEAR_ACCELERATION),
SensorManager.SENSOR_DELAY_GAME);
It works OK, but when i analyze 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, at the
end i get very little 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 close to
reality. So, the question is: what Sensor.TYPE_LINEAR_ACCELERATION
really shows? Where am I wrong, or something is wrong with
Sensor.TYPE_LINEAR_ACCELERATION realization? I use Samsung Nexus S
phone
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