How to read Android accelerometer values in a stable and accurate way?

Question

In a my project I am trying to control a car using data obtained through the accelerometer of an Android device. (Left, Right, Forward, Reverse). Even though I have managed to read values from the accelerometer the readings are subject to frequent changes even the device is in a stable position. Can someone provide me a better way to do this?

Following is the code that I have used

import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;

public class AccelerometerService {
    private static SensorManager sensorManager;
    private static SensorEventListener sensorEventListener;
    private static boolean started = false;

    private static float[] accelerometer = new float[3];
    private static float[] magneticField = new float[3];

    private static float[] rotationMatrix = new float[9];
    private static float[] inclinationMatrix = new float[9];
    private static float[] attitude = new float[3];

    private final static double RAD2DEG = 180/Math.PI;

    private static int initialAzimuth = 0;
    private static int initialPitch = 0;
    private static int initialRoll = 0;

    private static int[] attitudeInDegrees = new int[3];

    public static void start(final Context applicationContext) {
        if(started) {
            return;
        }

        sensorManager = (SensorManager) applicationContext
                .getSystemService(Context.SENSOR_SERVICE);

        sensorEventListener = new SensorEventListener() {

            @Override
            public void onSensorChanged(SensorEvent event) {

                int type = event.sensor.getType();
                if(type == Sensor.TYPE_MAGNETIC_FIELD) {
                    magneticField = event.values.clone();
                } 
                if(type == Sensor.TYPE_ACCELEROMETER) {
                    accelerometer = event.values.clone();
                }

                SensorManager.getRotationMatrix(rotationMatrix, inclinationMatrix, accelerometer, magneticField);
                SensorManager.getOrientation(rotationMatrix, attitude);

                attitudeInDegrees[0] =  (int) Math.round(attitude[0] * RAD2DEG);    //azimuth
                attitudeInDegrees[1] = (int) Math.round(attitude[1] * RAD2DEG);     //pitch
                attitudeInDegrees[2] = (int) Math.round(attitude[2] * RAD2DEG);     //roll
            }

            @Override
            public void onAccuracyChanged(Sensor sensor, int accuracy) {

            }
        };
        sensorManager.registerListener(sensorEventListener,
                sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
                SensorManager.SENSOR_DELAY_UI);
        sensorManager.registerListener(sensorEventListener,
                sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD),
                SensorManager.SENSOR_DELAY_UI);

        started = true;
    }

    public static boolean getStarted() {
        return started;
    }

    public static void stop() {
        if(started) {
        sensorManager.unregisterListener(sensorEventListener);
        started = false;
        }
    }
}

Answer 1

Old question, but I'm answering here for the sake of others that find this question.

First off, following @andrew-morton link is exactly what I needed, but was in pseudo code, so here is Java for Android!

Second: If you can (you're min API Lvl is 9 or higher), simply use the Sensor.TYPE_GRAVITY. As it is already smoothed out. If you need to support older API Lvls (like I had to), this code should do the trick!

Lastly: This code snippet is somewhat modified from what I am using it for, if you want to use this for yourself, you'll need to create a getter for the Vector3 gravity.

Notes: I've found that a rolling average of 5 (MAX_SAMPLE_SIZE) is quite smooth. 10 is even more smooth, but you start to notice lag.

import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;  
import java.util.ArrayList;
import java.util.List;

/**
 * Created by andy on 6/14/14.
 */
public class AndroidGravityUpdate implements SensorEventListener {
    private SensorManager sensorManager;
    Vector3 gravity;
    List<Float>[] rollingAverage = new List[3];

    private static final int MAX_SAMPLE_SIZE = 5;

    AndroidGravityUpdate( SensorManager sensorManager ) {
        this.gravity = new Vector3();
        this.sensorManager = sensorManager;

        if(sensorManager.getSensorList(Sensor.TYPE_GRAVITY).size() > 0){
            sensorManager.registerListener(
                    this,
                    sensorManager.getDefaultSensor(Sensor.TYPE_GRAVITY),
                    SensorManager.SENSOR_DELAY_GAME
            );
        } else if( sensorManager.getSensorList(Sensor.TYPE_ACCELEROMETER).size() > 0) {
            rollingAverage[0] = new ArrayList<Float>();
            rollingAverage[1] = new ArrayList<Float>();
            rollingAverage[2] = new ArrayList<Float>();

            sensorManager.registerListener(
                    this,
                    sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
                    SensorManager.SENSOR_DELAY_GAME
                );
        }

    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        if (event.sensor.getType()==Sensor.TYPE_GRAVITY){
            gravity.z = event.values[0];
            gravity.x = event.values[1];
            gravity.y = - event.values[2];
        }
        else if ( event.sensor.getType()==Sensor.TYPE_ACCELEROMETER) {
            //For whatever reason, my Samsung only has "Accelerometer"
            // But it is incredibly rough, so attempting to smooth
            // it out with rolling averages.
            rollingAverage[0] = roll(rollingAverage[0], event.values[0]);
            rollingAverage[1] = roll(rollingAverage[1], event.values[1]);
            rollingAverage[2] = roll(rollingAverage[2], -event.values[2]);

            gravity.z = averageList(rollingAverage[0]);
            gravity.x = averageList(rollingAverage[1]);
            gravity.y = averageList(rollingAverage[2]);
        }
    }

    public List<Float> roll(List<Float> list, float newMember){
        if(list.size() == MAX_SAMPLE_SIZE){
            list.remove(0);
        }
        list.add(newMember);
        return list;
    }

    public float averageList(List<Float> tallyUp){

        float total=0;
        for(float item : tallyUp ){
            total+=item;
        }
        total = total/tallyUp.size();

        return total;
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }
}

class Vector3 {
    float x;
    float y;
    float z;
}