Detect Movement Accurately using Accelerometer in Android

Question

I am implementing a demo TIMER, with Vibration ( at a particular condition ), When I press start my timer starts running.. and when I stop it using stop button, it simply stops.

Now I have to integrate a functionality, when the person shifts the device (while the Timer is running), it should reset the timer. It is working pretty good, but the accelerometer functionality is not working absolutely accurate. It needs a fast jerk to reset the timer.

Suggest me a good solution for the same.

Here is my code

public class SensorAccelerometer implements SensorEventListener {

    private Context context;
    private SensorManager sensorManager;
    private Sensor accelerometer;
    private TextView timelabel;
    private Handler mHandler;
    Runnable run;

    private float mLastX, mLastY, mLastZ;
    private final float NOISE = (float) 3.0;

    public SensorAccelerometer(Context context) {

    }


    public SensorAccelerometer(Context context,TextView timelabel, Handler mHandler2, Runnable mUpdateTimeTask) {
        // TODO Auto-generated constructor stub

        this.context = context;
        this.timelabel = timelabel;
        this.mHandler = mHandler2;
        this.run = mUpdateTimeTask;

        initialiseSensor();
    }


    public void initialiseSensor(){
        sensorManager = (SensorManager)context.getSystemService(Context.SENSOR_SERVICE);
        accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ALL);
        sensorManager.registerListener(this, accelerometer,SensorManager.SENSOR_DELAY_NORMAL);
    }

    public void unregisterSensor(){
        sensorManager.unregisterListener(this);
        Toast.makeText(context, "Sensor Stopped..", Toast.LENGTH_SHORT).show();
    }


    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }

    public void onSensorChanged(SensorEvent event) {
    float x = event.values[0];
    float y = event.values[1];
    float z = event.values[2];

    mAccelLast=mAccelCurrent;

    mAccelCurrent = FloatMath.sqrt(x*x + y*y + z*z);
    float delta = mAccelCurrent - mAccelLast;
    mAccel = mAccel * 0.9f + delta;

    if(mAccel>0.5){
        TimerActivity.mStartTime = SystemClock.uptimeMillis();
        mHandler.removeCallbacks(run);
        mHandler.postDelayed(run, 100);
    }

}

Timer Activity

public class TimerActivity extends Activity {

    public static long mStartTime = 0L;
    private TextView mTimerLabel;

    private Handler mHandler = new Handler();

    String timerStop1;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        mTimerLabel = (TextView) findViewById(R.id.textTimer);

        Button timerStartButton = (Button) findViewById(R.id.btnTimer);       
        timerStartButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view){

                if(mStartTime == 0L){
                    mStartTime = SystemClock.uptimeMillis();
                    mHandler.removeCallbacks(mUpdateTimeTask);
                    mHandler.postDelayed(mUpdateTimeTask, 100);

                    //activating the sensor and the acclerometer
                    SensorAccelerometer acc = new SensorAccelerometer(view.getContext(), mTimerLabel,mHandler,mUpdateTimeTask);
                }                                   
            }
        }); 

        Button timerStopButton = (Button) findViewById(R.id.btnTimerStop);       
        timerStopButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view){

                mHandler.removeCallbacks(mUpdateTimeTask);
                mTimerLabel.setText(timerStop1);
                mStartTime = 0L;

                SensorAccelerometer scc = new SensorAccelerometer(view.getContext(),mTimerLabel,mHandler,mUpdateTimeTask);
                scc.unregisterSensor();
            }
        }); 

    } 


    private Runnable mUpdateTimeTask = new Runnable(){

        public void run() {

            final long start = mStartTime;
            long millis = SystemClock.uptimeMillis()- start;

            int seconds = (int) (millis / 1000);
            int minutes = seconds / 60;
            seconds = seconds % 60;

            mTimerLabel.setText("" + minutes + ":"
                                  + String.format("%02d", seconds));                    

            timerStop1 = minutes + ":"
                      + String.format("%02d", seconds);

            mHandler.postDelayed(this, 200);            

        }   
    }; 

    protected void onPause() {
        super.onPause();
        SensorAccelerometer scc = new SensorAccelerometer(this,mTimerLabel,mHandler,mUpdateTimeTask);
        scc.unregisterSensor();
    };

} 

Answer 1

To find linear acceleration in a particular direction you can use this code too:

    @Override
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 = 0.8f;

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];
}  

after calculating the linear acceleration you can simply use an 'if' statement to check whether you jerk was powerful or not! [Higher value= more powerful jerk]

Answer 2

I think the next stage in the development of your app is to look at values of acceleration that are produced in a spreadsheet. I use Excel for this, but any tool that can produce graphs will do. So alter onSensorChanged() to something like

public void onSensorChanged(SensorEvent event) {
    float x = event.values[0];
    float y = event.values[1];
    float z = event.values[2];

    float mAccelCurrent = FloatMath.sqrt(x*x + y*y + z*z);
    float mAccel = mAccel * 0.9f + mAccelCurrent * 0.1f;
    Log.d("onSensorChanged",System.currentTimeMillis()+","+mAccelCurrent +","+mAccel);

}

and then you can capture the currentTime, mAccelCurrent and mAccel into the Android logging mechanism. Alternatively, create your own text file, write the values there, and open the file in a tool that can produce graphs. From the graphs, you can then decide what values to use for your trigger.

Answer 3

Two suggestions, and one thought:

1. For consistent behaviour, you should look at the length of the event.values vector, i.e. Math.sqrt(x*x+y*y+z*z), rather than the individual values. Mathematically, it's Math.sqrt(x*x+y*y+z*z) that's independent of your co-ordinate system, i.e. how the device is oriented relative to the ground etc, whereas the individual numbers x, y, z aren't.
2. Your current app looks for changes in acceleration. Instead, I think you should just look for high acceleration, i.e. large values of Math.sqrt(x*x+y*y+z*z).
3. I recently wrote this answer about using accelerometers for a pedometer, and I think you might find it interesting.