Topics Related. SensorManager & Sensor SensorEvent & SensorEventListener Filtering Sensor Values Example applications

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1 Sensors Lecture 23

2 Context-aware System a system is context-aware if it uses context to provide relevant information and/or services to the user, where relevancy depends on the user s task. adapt operations to the context without explicit user intervention increase usability by taking environmental context into account

3 Context Categories Computing context, such as network connectivity, communication costs, and communication bandwidth, and nearby resources such as printers, displays and workstations. User context, such as the user s profile, location, people nearby, even the current social situation. Physical context, such as lighting, noise levels, traffic conditions, and temperatures. Time context, such as time of a day, week, month, and season of the year

4 Topics Related SensorManager & Sensor SensorEvent & SensorEventListener Filtering Sensor Values Example applications

5 Sensors Hardware devices that measure the physical environment Motion Position Environment

6 Some Example Sensors Motion: 3-Axis Accelerometer Position: 3-Axis Magnetic field Environment: pressure

7 Question 1 Which of the following terms describe a class of Sensors that can exist on an Android device? a) Motion b) Time c) Position d) Environment

8 SensorManager System service that manages sensors Get instance getsystemservice(context.sensor_service) Access a specific sensor: SensorManager.getDefaultSensor(int type)

9 Sensor type constants Accelermeter: Sensor.TYPE_ACCELEROMETER Magnetic field:sensor.type_magnetic_field Pressure: Sensor.TYPE_PRESSURE

10 SensorEventListener When the application wants to receive information from the sensors, need to implement SensorEventListener Interface for SensorEvent callbacks Called when the accuracy of a sensor has changed void onaccuracychanged(sensor sensor, int accuracy)

11 SensorEventListener Called when sensor values have changed void onsensorchanged(sensorevent event)

12 Register for SensorEvents Before the application can receive the sensor information, need to register the sensor event Once is done with the sensor, need to unregister the sensor event (in order to save battery, for example)

13 Register for SensorEvents To register a SensorEventListener for a given sensor public boolean registerlistener( SensorEventListener listener, Sensor sensor, int rate)

14 Unregister for SensorEvents Unregisters a listener for the sensors public void unregisterlistener ( SensorEventListener listener, Sensor sensor)

15 SensorEvent Represent a sensor event Data is sensor-specific Sensor type Time-stamp Accuracy Measurement data: need to know how measurement is interpreted for a specific sensor

16 Sensor Coordinate System When default orientation is portrait & the device is lying flat, face-up on a table, axis run as the following: X: right to left Y: bottom to top Z: down to up

17 Sensor Coordinate System This coordinate system is different from the one used in the Android 2D APIs where the origin is in the top-left corner. Coordinate system does not change when device orientation changes

18 Question 2 If a device is lying flat, face up on a normal table, which sensor coordinate axis runs through the device from the floor up to the sky? a) X b) Y c) Z

19 Demo 1: RawSensorExample App Sensor Raw Accelerometer Displays the raw values read from the device s accelerometer Hand is shaking, not holding the device perfectly, the numbers are changing 1. Starting point: Y has largest value 2. Turn counter-clockwise 90 degree: X has the largest value 3. Another counter-clockwise 90 degree: Y has negative value 4. Another counter-clockwise 90 degree: X has negative

20 MainActivity.java public class MainActivity extends AppCompatActivity implements SensorEventListener { private static final int UPDATE_THRESHOLD = 500; private SensorManager msensormanager; private Sensor maccelerometer; private TextView mxvalueview, myvalueview, mzvalueview; private long protected void oncreate(bundle savedinstancestate) { super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); mxvalueview = (TextView) findviewbyid(r.id.x_value_view); myvalueview = (TextView) findviewbyid(r.id.y_value_view); mzvalueview = (TextView) findviewbyid(r.id.z_value_view);

21 MainActivity.java //Get reference to SensorManager msensormanager = (SensorManager) getsystemservice(sensor_service); //Get reference to Accelerometer if(null == (maccelerometer = msensormanager.getdefaultsensor(sensor.typ E_ACCELEROMETER))) finish();

22 Register //Register protected void onresume(){ super.onresume(); msensormanager.registerlistener(this, maccelerometer, SensorManager.SENSOR_DELAY_UI); } mlastupdate = System.currentTimeMillis();

23 Unregister //Unregister protected void onpause() { super.onpause(); } msensormanager.unregisterlistener(this);

24 Get the values from SensorEvent //Process new public void onsensorchanged(sensorevent event){ if(event.sensor.gettype() == Sensor.TYPE_ACCELEROMETER) { long actualtime = System.currentTimeMillis(); if(actualtime - mlastupdate > UPDATE_THRESHOLD) { mlastupdate = actualtime; float x = event.values[0]; float y = event.values[1]; float z = event.values[2]; } } } mxvalueview.settext("raw X: " + String.valueOf(x)); myvalueview.settext("raw Y: " + String.valueOf(y)); mzvalueview.settext("raw Z: " + String.valueOf(z));

25 Accelerometer values If the device are standing straight up(perfectly), the accelerometer would ideally report: X 0 M/S 2 Y 9.81 M/S 2 Z 0 M/S 2

26 Accelerometer values But these values will vary due to natural movements, non-flat surface, noise, etc.

27 Filtering Accelerometer Values When developing sensor enables apps, the developer usually need to transform the raw sensor values to smooth out Two common transforms Low-pass filter High-pass filter

28 Low-pass filters Deemphasize small transient force changes Emphasize large constant force components Example: when an app need to pay attention to long constant gravity, not the small hand shake A bubble needs to move based on the gravity, not the hand shakes

29 High-pass filters Emphasize transient force changes Deemphasize constant force components Example: when an app need to ignore the long constant force gravity, but should respond to the moves user made Percussion instrument (music instrument)

30 Second Demo RawSensorFilters App Display raw x, y, and Z Display low-pass of x, y and Z try to make Y to be the ideal 9.8 Display high-pass of x, y and Z trying to make x, y and z to be close to 0

31 MainActivity.java public class MainActivity extends AppCompatActivity implements SensorEventListener { //References to SensorManger and acceleromter private SensorManager msensormanager; private Sensor maccelerometer; private static final int UPDATE_THRESHOLD = 500; //Filtering constant private final float malpha = 0.8f; //Arrays for storing filtered values private float[] mgravity = new float[3]; private float[] maccel = new float[3]; private TextView mxvalueview, myvalueview, mzvalueview, mxgravityview, mygravityview, mzgravityview, mxaccelview, myaccelview, mzaccelview; private long mlastupdate;

32 protected void oncreate(bundle savedinstancestate) { super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); mxvalueview = (TextView) findviewbyid(r.id.x_value_view); myvalueview = (TextView) findviewbyid(r.id.y_value_view); mzvalueview = (TextView) findviewbyid(r.id.z_value_view); mxgravityview = (TextView) findviewbyid(r.id.x_lowpass_view); mygravityview = (TextView) findviewbyid(r.id.y_lowpass_view); mzgravityview = (TextView) findviewbyid(r.id.z_lowpass_view); mxaccelview = (TextView) findviewbyid(r.id.x_highpass_view); myaccelview = (TextView) findviewbyid(r.id.y_highpass_view); mzaccelview = (TextView) findviewbyid(r.id.z_highpass_view); //Get reference to SensorManager msensormanager = (SensorManager) getsystemservice(sensor_service); //Get reference to Accelerometer if(null == (maccelerometer = msensormanager.getdefaultsensor(sensor.type_accelerometer))) finish(); } mlastupdate = System.currentTimeMillis();

33 MainActivity.java //Process new public void onsensorchanged(sensorevent event){ if(event.sensor.gettype() == Sensor.TYPE_ACCELEROMETER) { long actualtime = System.currentTimeMillis(); if(actualtime - mlastupdate > UPDATE_THRESHOLD) { mlastupdate = actualtime; float rawx = event.values[0]; float rawy = event.values[1]; float rawz = event.values[2]; //apply low-pass filter mgravity[0] = lowpass(rawx, mgravity[0]); mgravity[1] = lowpass(rawy, mgravity[1]); mgravity[2] = lowpass(rawz, mgravity[2]); } //apply high-pass filter maccel[0] = highpass(rawx, mgravity[0]); maccel[1] = highpass(rawy, mgravity[1]); maccel[2] = highpass(rawz, mgravity[2]);

34 lowpass & highpass //Deemphasize transient forces private float lowpass(float current, float gravity){ return gravity * malpha + current * (1 - malpha); } //Deemphasize constant forces private float highpass(float current, float gravity){ return current - gravity; }

35 Question 3 Low-pass filters are used when you want to emphasize the transient changes in a sensor s readings, while deemphasizing the constant portions of the reading? True False

36 Demo3: MyCompass App

37 MainActivit.java public class MainActivity extends AppCompatActivity implements SensorEventListener protected void oncreate(bundle savedinstancestate) { //Get a reference to the SensorManager msensormanager = (SensorManager) getsystemservice(sensor_service); //Get a reference to the accerometer accelerometer = msensormanager.getdefaultsensor(sensor.type_accelerometer); //Get a reference to the magnetometer magnetometer = msensormanager.getdefaultsensor(sensor.type_magnetic_field); //Exit if both sensors are unavailable if(accelerometer == null magnetometer == null) { finish(); } }

38 public void onsensorchanged(sensorevent event) { //Acquire accelerometer event data if (event.sensor.gettype() == Sensor.TYPE_ACCELEROMETER) { mgravity = new float[3]; System.arraycopy(event.values, 0, mgravity, 0, 3); } //Acquire magnetometer event data if (event.sensor.gettype() == Sensor.TYPE_MAGNETIC_FIELD) { mgeomagnetic = new float[3]; System.arraycopy(event.values, 0, mgeomagnetic, 0, 3); }

39 //If we have readings from both sensors then //use the readings to compute the device's orientation //and then update the display if (mgravity!= null && mgeomagnetic!= null) { float rotationmatrix[] = new float[9]; //Users the accelarometer and magnetometer readings //to compute the device's rotation with respect // a real world coordinate system boolean success = SensorManager.getRotationMatrix(rotationMatrix, null, mgravity, mgeomagnetic); if (success) { float orientationmatrix[] = new float[3]; //returns the device's orientation given //the rotationmatrix SensorManager.getOrientation(rotationMatrix, orientationmatrix); //get the rotation, measured in radians, around the Z-axis //Note: this assumes the devices is held flat and parallel //to the ground float rotationinradians = orientationmatrix[0]; //convert from radians to degrees mrotationindegrees = Math.toDegrees(rotationInRadians); //request redraw mcompassarrow.invalidate(); } } //reset sensor event data arrays mgravity = null; mgeomagnetic = null;

40 // this method draws on the protected void ondraw(canvas canvas) { super.ondraw(canvas); } //save the canvas canvas.save(); CompassArrowView //Rotate this view canvas.rotate((float)-mrotationindegrees, mparentcenterx, mparentcentery); //redraw this view canvas.drawbitmap(mbitmap,mviewlefty,mviewtopx,null); //restore the canvas canvas.restore();

41 Check available sensors

42 Check available sensors

43 Check available sensors SensorManager sm = null; sm = (SensorManager) getsystemservice(context.sensor_service); List<Sensor> allsensors = sm.getsensorlist(sensor.type_all); Log.i(TAG, "allsensors: " + allsensors.size()); //display all the available sensors information for (Sensor sensor : allsensors) { String name = sensor.getname(); String vendor = sensor.getvendor(); int version = sensor.getversion(); Log.i(TAG, "name: " + name + ", vendor: " + vendor + ", version: " + version); }

Sensor & SensorManager SensorEvent & SensorEventListener Filtering sensor values Example applications

Sensor & SensorManager SensorEvent & SensorEventListener Filtering sensor values Example applications Hardware devices that measure the physical environment Motion Position Environment Motion - 3-axis