Android Sensors

Presentación utilizada en XI Jornadas SLCENT de Actualización Informática y Electrónica. 19 de Noviembre de 2014, Almería.

http://hacklabalmeria.net/evento/20141119/

Esta charla trata de ofrecer una visión general sobre los diferentes tipos de sensores que podemos encontrar en los dispositivos Android y cómo podemos trabajar con ellos.

Transcript

1. Android Sensors
   XI Jornadas SLCENT de Actualización
   Informática y Electrónica
   

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2. About me
   José Juan Sánchez Hernández
   !
   Android Developer (In my spare time :)
   !
   Member and collaborator of:
   - Android Almería Developer Group
   - HackLab Almería
   @josejuansanchez
   2
   

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3. ¿What is a sensor?
   3
   

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4. Microelectromechanical sensors
   (MEMS)
   MEMS are sensors that have been made on a tiny
   scale, usually on silicon chips using techniques
   borrowed from computer-chip manufacturing.
   4
   

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5. What are Android Sensors?
   Android sensors give applications access to a
   mobile device's underlying physical sensors:
   !
   accelerometers, gyroscopes, magnetometers,
   barometer, humidity, pressure, light, proximity and
   heart rate sensors.
   !
   Camera, microphone and touch screen are currently
   not in the list of physical devices providing data
   through “Android sensors.” They have their own
   reporting mechanism.
   5
   

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6. Sensor stack
   Layers of the Android sensor stack and their respective owners
   6
   

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7. SDK
   Applications access sensors through the
   Sensors SDK (Software Development Kit)
   API.
   !
   The SDK contains functions to list available
   sensors and to register to a sensor.
   7
   

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8. Framework
   The framework is in charge of linking the
   several applications to the HAL. The HAL
   itself is single-client.
   !
   Without this multiplexing happening at the
   framework level, only a single application
   could access each sensor at any given time.
   8
   

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9. HAL (Hardware Abstraction Layer)
   The Sensors Hardware Abstraction Layer
   (HAL) API is the interface between the
   hardware drivers and the Android framework.
   !
   It consists of one HAL interface sensors.h
   and one HAL implementation we refer to as
   sensors.cpp.
   9
   

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10. HAL / sensors.h
    https://android.googlesource.com/platform/hardware/libhardware/+/master/include/hardware/sensors.h
    10
    Let’s see the code!
    

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11. Kernel driver
    The sensor drivers interact with the
    physical devices. In some cases, the HAL
    implementation (sensors.cpp) and the
    drivers are the same software entity.
    !
    HAL implementation and kernel drivers are
    the responsibility of the hardware
    manufacturers, and Android does not
    provide preferred approaches to write them.
    !
    11
    

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12. Sensor hub
    The sensor stack of a device can optionally
    include a sensor hub, useful to perform
    some low-level computation at low power
    while the SoC (System on Chip) can be in a
    suspend mode.
    !
    It is sometimes a separate chip, and
    sometimes included on the same chip as
    the SoC.
    !
    Important characteristics of the sensor hub
    is that it should contain sufficient memory
    for batching and consume very little
    power to enable implementation of the low
    power Android sensors.
    12
    

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13. 13
    Source: http://www.lapis-semi.com/en/semicon/miconlp/sh-mcu.html
    

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14. Sensors
    Those are the physical MEMs chips
    making the measurements. In many cases,
    several physical sensors are present on the
    same chip.
    !
    For example, some chips include an
    accelerometer, a gyroscope and a
    magnetometer. Such chips are often called
    9-axis chips, as each sensor provides data
    over 3 axes.
    !
    14
    

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15. What is Batching?
    “Batching” refers to storing sensor events in a
    hardware FIFO before reporting them through the
    HAL instead of reporting them immediately.
    !
    Batching can enable significant power savings by
    preventing the SoC from waking up to receive each
    event. Instead, the events can be grouped and
    processed together.
    !
    Android 4.4 (KitKat) introduced platform support
    for hardware sensor batching.
    15
    

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16. Types of Sensors
    The Android platform supports three broad categories of sensors:
    !
    Motion sensors: These sensors measure acceleration forces and
    rotational forces along three axes. This category includes
    accelerometers, gravity sensors, gyroscopes and rotational vector
    sensors.
    !
    Environmental sensors: These sensors measure various
    environmental parameters, such as ambient air temperature and
    pressure, illumination, and humidity. This category includes
    barometers, photometers and thermometers.
    !
    Position sensors: These sensors measure the physical position of a
    device. This category includes orientation sensors and
    magnetometers.
    16
    classification 1
    

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17. Raw and Composite Sensors
    Raw sensors (hardware-based): give raw data from a sensor, and
    one raw sensor corresponds to one actual physical component
    inside the Android device.
    !
    !
    Composite sensors (software-based): provide an abstraction layer
    between application code and low-level device components by
    either combining the raw data of multiple raw sensors, or by
    modifying the raw sensor data to make it easier to consume.
    17
    classification 2
    

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18. 18
    Sensor Type Description Common Uses
    TYPE_ACCELEROMETER Hardware Measures the acceleration force in m/s
    is applied to a device on all three physical
    axes (x, y, and z), including the force of
    gravity.
    Motion detection
    (shake, tilt, etc.).
    TYPE_AMBIENT_TEMPERATURE Hardware Measures the ambient room temperature in
    degrees Celsius (°C). See note below.
    Monitoring air
    temperatures.
    TYPE_GRAVITY Software or
    Hardware
    Measures the force of gravity in m/s
    applied to a device on all three physical axes
    (x, y, z).
    Motion detection
    (shake, tilt, etc.).
    TYPE_GYROSCOPE Hardware Measures a device's rate of rotation in rad/s
    around each of the three physical axes (x, y,
    and z).
    Rotation
    detection (spin,
    turn, etc.).
    TYPE_LIGHT Hardware Measures the ambient light level
    (illumination) in lx.
    Controlling
    screen
    brightness.
    TYPE_LINEAR_ACCELERATION Software or
    Hardware
    Measures the acceleration force in m/s
    is applied to a device on all three physical
    axes (x, y, and z), excluding the force of
    gravity.
    Monitoring
    acceleration
    along a single
    axis.
    Sensor types supported by the Android platform
    Source: http://developer.android.com/guide/topics/sensors/sensors_overview.html
    

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19. 19
    TYPE_MAGNETIC_FIELD Hardware Measures the ambient geomagnetic field for
    all three physical axes (x, y, z) in μT.
    Creating a
    compass.
    TYPE_ORIENTATION Software Measures degrees of rotation that a device
    makes around all three physical axes (x, y, z).
    As of API level 3 you can obtain the
    inclination matrix and rotation matrix for a
    device by using the gravity sensor and the
    geomagnetic field sensor in conjunction
    with the getRotationMatrix() method.
    Determining
    device position.
    TYPE_PRESSURE Hardware Measures the ambient air pressure in hPa or
    mbar.
    Monitoring air
    pressure
    changes.
    TYPE_PROXIMITY Hardware Measures the proximity of an object in cm
    relative to the view screen of a device. This
    sensor is typically used to determine
    whether a handset is being held up to a
    person's ear.
    Phone position
    during a call.
    TYPE_RELATIVE_HUMIDITY Hardware Measures the relative ambient humidity in
    percent (%).
    Monitoring
    dewpoint,
    absolute, and
    relative
    Sensor Type Description Common Uses
    

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20. 20
    TYPE_ROTATION_VECTOR Software
    or
    Hardware
    Measures the orientation of a device
    by providing the three elements of the
    device's rotation vector.
    Motion
    detection and
    rotation
    detection.
    TYPE_TEMPERATURE Hardware Measures the temperature of the
    device in degrees Celsius (°C). This
    sensor implementation varies across
    devices and this sensor was replaced
    with the
    TYPE_AMBIENT_TEMPERATURE
    sensor in API Level 14
    Monitoring
    temperatures.
    Sensor Type Description Common Uses
    

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21. 21
    Sensor Android 4.0
    (API Level 14)
    Android 2.3
    (API Level 9)
    Android 2.2
    (API Level 8)
    Android 1.5
    (API Level 3)
    TYPE_ACCELEROMETER Yes Yes Yes Yes
    TYPE_AMBIENT_TEMPERATURE Yes n/a n/a n/a
    TYPE_GRAVITY Yes Yes n/a n/a
    TYPE_GYROSCOPE Yes Yes n/a n/a
    TYPE_LIGHT Yes Yes Yes Yes
    TYPE_LINEAR_ACCELERATION Yes Yes n/a n/a
    TYPE_MAGNETIC_FIELD Yes Yes Yes Yes
    TYPE_ORIENTATION Yes Yes Yes Yes
    TYPE_PRESSURE Yes Yes n/a n/a
    TYPE_PROXIMITY Yes Yes Yes Yes
    TYPE_RELATIVE_HUMIDITY Yes n/a n/a n/a
    TYPE_ROTATION_VECTOR Yes Yes n/a n/a
    TYPE_TEMPERATURE Yes Yes Yes Yes
    1 This sensor type was added in Android 1.5 (API Level 3), but it was not available for use until Android 2.3 (API Level 9).
    2 This sensor is available, but it has been deprecated.
    Sensor availability by platform
    

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22. Android 4.1.x (Jellybean): New sensor types allow apps to better
    manage sensor readings. A game rotation vector lets game
    developers sense the device’s rotation without having to worry about
    magnetic interference. Uncalibrated gyroscope and uncalibrated
    magnetometer sensors report raw measurements as well as
    estimated biases to apps .
    !
    Android 4.4 (KitKat): Introduces platform support for hardware
    sensor batching, a new optimization that can dramatically reduce
    power consumed by ongoing sensor activities. Adds platform
    support for two new composite sensors: step detector and step
    counter.
    !
    Android 5.0 (Lollipop): A new tilt detector and a heart rate sensor
    reports the heart rate of the person touching the device. New
    interaction composite sensors are now available to detect special
    interactions such as a wake up gesture, a pick up gesture, and a
    glance gesture.
    22
    

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23. Sensors SDK API
    SensorManager!
    Is the Android system service that gives an app access to hardware
    sensors.
    !
    Sensor!
    Is the Android representation of a hardware sensor on a device.
    !
    SensorEventListener!
    Is an interface that provides the callbacks to alert an app to sensor-
    related events.
    !
    SensorEvent!
    Is the data structure that contains the information that is passed to an
    app when a hardware sensor has information to report.
    23
    

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24. SensorManager
    SensorManager lets you access the device's sensors. Get an
    instance of this class by calling getSystemService() with the
    argument SENSOR_SERVICE.
    24
    !
    private SensorManager mSensorManager;!
    ...!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    

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25. SensorManager
    SensorManager provides two methods to access Sensor objects:
    !
    • getSensorList(): returns all the sensors.
    • getDefaultSensor(): returns the default sensor for the
    specified type.
    !
    Example 1:
    25
    !
    List deviceSensors = mSensorManager.getSensorList(Sensor.TYPE_ALL);!
    !
    List deviceSensors = mSensorManager.getSensorList(Sensor.TYPE_ACCELEROMETER);!
    Example 2:
    

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26. SensorManager
    Example 3:
    26
    !
    private SensorManager mSensorManager;!
    ...!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    !
    if (mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD) != null){!
    // Success! There's a magnetometer.!
    }!
    else {!
    // Failure! No magnetometer.!
    }!
    

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27. Sensor
    Sensor represents a hardware sensor on a device.
    !
    This class provides information about the sensor, such as:
    !
    • Maximum range
    • Minimum delay
    • Name
    • Power
    • Resolution
    • Type
    • Vendor
    • Version
    !
    Source code: Sensor.java
    27
    

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28. SensorEventListener
    SensorEventListener is used for receiving notifications from the
    SensorManager when sensor values have changed. In this class
    there are two public methods:
    !
    • onAccuracyChanged(Sensor sensor, int accuracy)
    Called when the accuracy of a sensor has changed.
    !
    • onSensorChanged(SensorEvent event)
    Called when sensor values have changed.
    28
    

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29. 29
    !
    public class SensorActivity extends Activity implements SensorEventListener {!
    private SensorManager mSensorManager;!
    private Sensor mLight;!
    !
    @Override!
    public final void onCreate(Bundle savedInstanceState) {!
    super.onCreate(savedInstanceState);!
    setContentView(R.layout.main);!
    !
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    mLight = mSensorManager.getDefaultSensor(Sensor.TYPE_LIGHT);!
    }!
    !
    @Override!
    public final void onAccuracyChanged(Sensor sensor, int accuracy) {!
    // Do something here if sensor accuracy changes.!
    }!
    !
    @Override!
    public final void onSensorChanged(SensorEvent event) {!
    // The light sensor returns a single value.!
    // Many sensors return 3 values, one for each axis.!
    float lux = event.values[0];!
    // Do something with this sensor value.!
    }!
    Example:
    

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30. 30
    !
    @Override!
    protected void onResume() {!
    super.onResume();!
    mSensorManager.registerListener(this, mLight, SensorManager.SENSOR_DELAY_NORMAL);!
    }!
    !
    @Override!
    protected void onPause() {!
    super.onPause();!
    mSensorManager.unregisterListener(this);!
    }!
    }!
    Sensor Rate
    

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31. Sensor Rates
    When you register a listener, you specify the delay or measurement
    rate for the listener. The predefined rates are:
    !
    • SENSOR_DELAY_FASTEST: get sensor data as fast as possible.
    (0 microsecond delay)
    !
    • SENSOR_DELAY_GAME: rate suitable for games.
    (20.000 microseconds delay = 0.02 seconds)
    !
    • SENSOR_DELAY_UI: rate suitable for the user interface functions.
    (60.000 microseconds delay = 0.06 seconds)
    !
    • SENSOR_DELAY_NORMAL: rate suitable for screen orientation
    changes. (The default value).
    (200.000 microseconds delay = 0.2 seconds)
    31
    

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33. SensorEvent
    SensorEvent is the data structure that contains the information that
    is passed to an app when a hardware sensor has information to
    report. The data members of the SensorEvent are:
    !
    • accuracy: The accuracy of the event. Can have the following
    values:
    SensorManager.SENSOR_STATUS_ACCURACY_HIGH
    SensorManager.SENSOR_STATUS_ACCURACY_MEDIUM
    SensorManager.SENSOR_STATUS_ACCURACY_LOW
    SensorManager.SENSOR_STATUS_UNRELIABLE
    • sensor: An instance of the Sensor class that generated the
    SensorEvent.
    • timestamp: The time in milliseconds when the SensorEvent
    occurred.
    • values: An array of values that represent sensor data.
    33
    

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34. Ensuring that a given sensor is
    present on a device
    Detect sensors at runtime and enable or disable application features
    as appropriate.
    34
    !
    private SensorManager mSensorManager;!
    ...!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    if (mSensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE) != null){!
    // Success! There's a pressure sensor.!
    }!
    else {!
    // Failure! No pressure sensor.!
    }!
    

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35. Ensuring that a given sensor is
    present on a device
    Use Google Play filters to target devices with specific sensor
    configurations.
    35
    !
    android:required="true" />!
    

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36. Sensor Coordinate System
    In general, the sensor framework uses a standard 3-axis coordinate
    system to express data values.
    36
    The coordinate-system is defined relative to the screen of the phone in its
    default orientation. The axes are not swapped when the device's screen
    orientation changes.
    

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37. Best Practices for Accessing and Using Sensors
    1) Unregister sensor listeners!
    Always make sure to disable sensors you don't need, especially
    when your activity is paused. Failing to do so can drain the battery
    in just a few hours. Note that the system will not disable sensors
    automatically when the screen turns off.
    37
    !
    private SensorManager mSensorManager;!
    ...!
    @Override!
    protected void onPause() {!
    super.onPause();!
    mSensorManager.unregisterListener(this);!
    }!
    

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38. Best Practices for Accessing and Using Sensors
    !
    2) Don’t block the onSensorChanged() method!
    !
    Sensor data can change at a high rate, which means the system may
    call the onSensorChanged(SensorEvent) method quite often. As
    a best practice, you should do as little as possible within the
    onSensorChanged(SensorEvent) method so you don't block it.
    !
    3) Avoid using deprecated methods or sensor types!
    !
    Several methods and constants have been deprecated. In particular,
    the TYPE_ORIENTATION sensor type has been deprecated.
    38
    

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39. Best Practices for Accessing and Using Sensors
    4) Verify sensors before you use them!
    !
    Always verify that a sensor exists on a device before you attempt to
    acquire data from it. Don't assume that a sensor exists simply
    because it's a frequently-used sensor.
    !
    5) Choose sensor delays carefully!
    !
    Sensors can provide data at very high rates. Allowing the system to
    send extra data that you don't need wastes system resources and
    uses battery power.
    39
    

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40. How to use sensors…
    40
    

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41. 41
    TYPE_PROXIMITY SensorEvent.values[0] Distance
    from object.
    cm
    Using the Proximity Sensor
    Sensor Sensor event data Description Units of
    measure
    The proximity sensor lets you determine how far away an object is
    from a device.
    !
    The following code shows you how to get an instance of the default
    acceleration sensor:
    

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42. Using the Proximity Sensor
    42
    Most proximity sensors return the absolute distance, in cm, but
    some return only near and far values.
    !
    private SensorManager mSensorManager;!
    private Sensor mSensor;!
    ...!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_PROXIMITY);!
    ...!
    !
    @Override!
    public final void onAccuracyChanged(Sensor sensor, int accuracy) {!
    // Do something here if sensor accuracy changes.!
    }!
    !
    @Override!
    public final void onSensorChanged(SensorEvent event) {!
    float cm = event.values[0];!
    // Do something with this sensor data.!
    }!
    Most proximity sensors return the absolute distance, in cm, but
    some return only near and far values.
    

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43. Using the Light, Pressure, and
    Temperature Sensors
    43
    The raw data you acquire from the light, pressure, and temperature
    sensors usually requires no calibration, filtering, or modification,
    which makes them some of the easiest sensors to use.
    Sensor Sensor event data Units of
    measure
    Data description
    TYPE_AMBIENT_TEMPERATURE event.values[0] °C Ambient air
    temperature.
    TYPE_LIGHT event.values[0] lx Illuminance.
    TYPE_PRESSURE event.values[0] hPa or mbar Ambient air pressure.
    TYPE_RELATIVE_HUMIDITY event.values[0] % Ambient relative
    humidity.
    TYPE_TEMPERATURE event.values[0] °C Device temperature.
    1 Implementations vary from device to device. This sensor was deprecated in Android 4.0 (API Level 14).
    

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44. 44
    !
    public class SensorActivity extends Activity implements SensorEventListener {!
    private SensorManager mSensorManager;!
    private Sensor mPressure;!
    !
    @Override!
    public final void onCreate(Bundle savedInstanceState) {!
    super.onCreate(savedInstanceState);!
    setContentView(R.layout.main);!
    !
    // Get an instance of the sensor service, and use that to get an instance of!
    // a particular sensor.!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    mPressure = mSensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE);!
    }!
    !
    @Override!
    public final void onAccuracyChanged(Sensor sensor, int accuracy) {!
    // Do something here if sensor accuracy changes.!
    }!
    !
    @Override!
    public final void onSensorChanged(SensorEvent event) {!
    float millibars_of_pressure = event.values[0];!
    // Do something with this sensor data.!
    }!
    

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45. Using Accelerometer
    45
    Sensor Sensor event data Description Units of
    measure
    TYPE_ACCELEROMETER SensorEvent.values[0] Acceleration force
    along the x axis
    (including gravity).
    m/s
    SensorEvent.values[1] Acceleration force
    along the y axis
    (including gravity).
    SensorEvent.values[2] Acceleration force
    along the z axis
    (including gravity).
    

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46. Using Accelerometer
    46
    An acceleration sensor measures the acceleration applied to the
    device, including the force of gravity.
    !
    The following code shows you how to get an instance of the default
    acceleration sensor:
    !
    private SensorManager mSensorManager;!
    private Sensor mSensor;!
    ...!
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);!
    mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);!
    

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47. Using Accelerometer
    47
    The norm of should be close to 0 when in free fall.
    

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48. Using Accelerometer
    48
    To measure the real acceleration of the device, the contribution of
    the force of gravity must be removed from the accelerometer
    data.
    !
    This can be achieved by applying a high-pass filter. Conversely, a
    low-pass filter can be used to isolate the force of gravity.
    !
    !
    The following example shows how you can do this:
    

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49. 49
    !
    public void onSensorChanged(SensorEvent event){!
    // In this example, alpha is calculated as t / (t + dT),!
    // where t is the low-pass filter's time-constant and!
    // dT is the event delivery rate.!
    !
    final float alpha = 0.8;!
    !
    // Isolate the force of gravity with the low-pass filter.!
    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];!
    !
    // Remove the gravity contribution with the high-pass filter.!
    linear_acceleration[0] = event.values[0] - gravity[0];!
    linear_acceleration[1] = event.values[1] - gravity[1];!
    linear_acceleration[2] = event.values[2] - gravity[2];!
    }!
    

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50. Gyroscope
    50
    http://crypto.stanford.edu/gyrophone/
    News!
    

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51. 51
    Good Book! :)
    

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52. Android Sensors
    XI Jornadas SLCENT de Actualización
    Informática y Electrónica
    

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