## Sensor Manager

In this lecture, we discuss Android's sensors and accessing them via
the
[SensorManager](http://developer.android.com/reference/android/hardware/SensorManager.html). 

Nexus-4 as a number of interesting sensors beyond the camera and the
microphone, specifically:

- Accelerometer
- GPS
- Gyroscope
- Barometer
- Ambient light
- Compass

In this lecture we will focus mainly on the accelerometer. 
The Sensor Manager provides access to the 3-axis accelerometer
sensor. We use the signals (x, y, and z axis) to infer activity in
MyRuns5; specifically, we build an accelerometer pipeline that
extracts "features" from the signals and uses these features to infer
the activity of the user. More on that in MyRuns5 lab.

We first provide a demo of the accelerometer signals and the Sensor
Manager.  We will also show you how to access some of the other
sensors such as light, barometer.

**Note, these notes are incomplete.**

## What this lecture will teach you

- Sensor Manager
- Accessing the X, Z, Y axis of the accelerometer
- Barometer, light and temperature sensors
 
## Demo projects

The demo code used in this lecture include:

* [Android Sensor
  Box](https://play.google.com/store/apps/details?id=imoblife.androidsensorbox&hl=en)
  is a great app that allows you to quickly check out the sensors on
  your phone and interact with them. This is a very cool and geeky
  app. 

* We will use the [shakesensordemo.zip](../code/shakesensordemo.zip) app to
demonstrate how to read the accelerometer. This demo is an extended
version of the code found in the [Sensor Manger
tutorial](http://www.vogella.com/articles/AndroidSensor/article.html).

* We will use the [weatherdemo.zip](../code/weatherdemo.zip) app to
demonstrate how to access the temperature, barometer and light
sensors. Note, that the nexus-4 does not have a temperature sensor!

## Resources

Some excellent references.

* Course book does not seem to have much on sensors.
* Lars Vogel's [SensorManger tutorial](http://www.vogella.com/articles/AndroidSensor/article.html)
* Developers [Sensor Manager](http://developer.android.com/reference/android/hardware/SensorManager.html). 

## ShakeSensor Demo

This simple demo displays the x, y and z axis accelerometer readings
in a continuous fashion. If the phone is shaken the background color
of the text changes and a toast is displayed, as shown in the example below.

![](images/shake.png) 

## Set up Sensor Manager

We first get a sensor service and sensor manager before we can access
the accelerometer data.

## Set the Sensor Data Type of Interest

We set the sensor manager to get accelerometer data --
Sensor.TYPE_ACCELEROMETER. But we could have asked for any sensor data
that the phone produces -- light, proximity, etc. See the list of
[sensor types here](http://developer.android.com/reference/android/hardware/Sensor.html).

# Phone Coordinate-system orientation

The figure below shows the x, y and z axis of the accelerometer sensor
with respect to the phone's orientation.

See [developers notes](http://developer.android.com/reference/android/hardware/SensorEvent.html) on the definition of the coordination and the
SensorEvent object definition which is based to the onSensorChanged()
callback when new data is available.
 
The X axis is horizontal and points to the right, the Y axis is vertical and points up and the Z axis points towards the outside of the front face of the screen. In this system, coordinates behind the screen have negative Z values.

![](images/accel.png) 


~~~{.java}
public class SensorTestActivity extends Activity implements SensorEventListener {
	private SensorManager sensorManager;
	private boolean color = false;
	private View view;
	private long lastUpdate;
	TextView textx, texty, textz;

	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		requestWindowFeature(Window.FEATURE_NO_TITLE);
		getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
				WindowManager.LayoutParams.FLAG_FULLSCREEN);

		setContentView(R.layout.main);
		
		// get textviews
		textx = (TextView) findViewById(R.id.xval);
		texty = (TextView) findViewById(R.id.yval);
		textz = (TextView) findViewById(R.id.zval);

		
		view = findViewById(R.id.textView);
		view.setBackgroundColor(Color.BLUE);

		sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
		sensorManager.registerListener(this,
				sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
				SensorManager.SENSOR_DELAY_NORMAL);
		lastUpdate = System.currentTimeMillis();
	}

	@Override
	public void onSensorChanged (SensorEvent event) {
		if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
			displayAccelerometer(event);
			checkShake(event);
		}

	}
~~~

## Display accelerometer readings

The two helper functions display the accelerometer and check to see if
the phone has been shaken using a simple test -- if so change the
background color of the text.

~~~{.java}
	private void displayAccelerometer(SensorEvent event) {

		// Many sensors return 3 values, one for each axis.
		float x = event.values[0];
		float y = event.values[1];
		float z = event.values[2];

		// display values using TextView
		textx.setText("X axis" + "\t\t" + x);
		texty.setText("Y axis" + "\t\t" + y);
		textz.setText("Z axis" + "\t\t" + z);

	}

	private void checkShake(SensorEvent event) {

		// Movement
		float x = event.values[0];
		float y = event.values[1];
		float z = event.values[2];

		float accelationSquareRoot = (x * x + y * y + z * z)
		     / (SensorManager.GRAVITY_EARTH * SensorManager.GRAVITY_EARTH);
		long actualTime = System.currentTimeMillis();
		if (accelationSquareRoot >= 2) //
		{
			if (actualTime - lastUpdate < 200) {
				return;
			}
			lastUpdate = actualTime;
			Toast.makeText(this, "Don't shake me!", Toast.LENGTH_SHORT).show();
			if (color) {
				view.setBackgroundColor(Color.BLUE);

			} else {
				view.setBackgroundColor(Color.RED);
			}
			color = !color;
		}
	}

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

	}

	@Override
	protected void onResume() {
		super.onResume();
		// register this class as a listener for the orientation and
		// accelerometer sensors
		sensorManager.registerListener(this,
				sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
				SensorManager.SENSOR_DELAY_NORMAL);
	}

	@Override
	protected void onPause() {
		// unregister listener
		super.onPause();
		sensorManager.unregisterListener(this);
	}
}
~~~