Tuesday, 1 June 2010

Android: Using Accelerometer to Calculate Total Force

I'm in the process of building an app that calculates how far you could throw something, based on the speed you swing your phone. This involves tracking your phone's movement as you swing it, and calculating the total maximum force involved. I couldn't find any examples on how to do this, so I've mocked up a bit of a tutorial below.

How do we get the accelerometer values?

private SensorManager mgr=null;

private SensorEventListener listener=new SensorEventListener() {
        public void onSensorChanged(SensorEvent e) {
            if (e.sensor.getType()==Sensor.TYPE_ACCELEROMETER) {
                //Total acceleration will be sqrt(x^2+y^2+z^2)
                double netForce=e.values[0]*e.values[0];    //X axis               
                netForce+=e.values[1]*e.values[1];    //Y axis
                netForce+=(e.values[2])*(e.values[2]);    //Z axis (upwards)
                netForce = Math.sqrt(netForce) - SensorManager.GRAVITY_EARTH;    //Take the square root, minus gravity
                Log.d("ForceCalculator", "Net force:"+netForce+"");
        public void onAccuracyChanged(Sensor sensor, int accuracy) {
            // unused

This will create and register an event listener for the accelerometer. This listener then calculates the net force.

This calculation is pretty self-explanatory. It involves three components - X axis, Y axis, and Z axis. To calculate the magnitude of these vector components, we simply take the square root of their combined squares:
netForce = sqrt( x^2 + y^2 + z^2 )
The only point to note is that we then subtract the acceleration due to gravity (approx. 9.8m/s^2). Otherwise we could have a resting acceleration of +9.8.

Full code will be coming later, when I've got some more time to play around with it. For now, this should show you how to get net force using the accelerometer. Still to come is code to calculate maximum force over a specified period of time, with threshold starting and finishing velocities.


krisonearth said...

I have a question regarding net force.
What force are you trying to measure using this code? Is it really force or is it acceleration ?


David said...

You're right Krushnaal - this is actually measuring the net acceleration. I've mistakenly been using the word "force" interchangeably. Since the phone is a fixed mass however, a change in acceleration will always be due to a proportional force being applied, so it's still relevant.


krisonearth said...

Is there a way to determine the force applied on the phone by swinging it in a particular direction?


David said...

If you really want to find the force, you'll need to find (or estimate) the mass of the phone first. From standard physics, Force=Mass*Acceleration - we can find the acceleration using the code above, so if you have the mass of the phone you can apply this simple calculation to determine the force applied.

Of course, depending on what you're using this value for you may simply want to use the acceleration value instead.


P.S. For more info on the calculations involved, check out this site.

Anonymous said...

how to find angular momentum in s=android of swinging balls?

Kieran said...

I think that if you are adjusting for gravity you would have to know the orientation of the device wrt/ gravity and subract each component before computing the absolute value.
Depending on how the gravity is oriented along the accelerometer you will get different values of total acceleration.