Mobile Handset Sensors

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Outline

Overview GPS Other Sensors

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Sensor

A sensor is a converter that measures a physical quantity and converts it into a signal which can be read by an instrument

Visual Sensor Ultrasound Sensor Infrared Sensor

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Mobile Handset Sensor

Modern mobile handset not only serves as the key computing and communication device, but it also comes with a rich set of embedded sensors

Enabling new applications across a wide variety of domains, such as transportation, social networks, environmental monitoring, healthcare, etc.

Giving rise to new research areas such as mobile sensing, mobile data mining, etc.

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Sensor Categories (1)

Hardware-based sensorsPhysical components built into a handsetThey derive their data by directly measuring

specific environmental properties Software-based sensors

Not physical devices, although they mimic hardware-based sensors

They derive their data from one or more hardware-based sensors

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Sensor Categories (2)

Motion sensors Measure acceleration forces and rotational forces along

three axes, e.g., accelerometer, gyroscope, etc. Position sensors

Measure the physical position of a device, e.g., GPS, proximity sensor, etc.

Environmental sensors Measure various environmental parameters, e.g., light

sensor, thermometer, etc.

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Sensor ListSensor Function Type Software-based or

Hardware-based

Accelerometer Motion Sensor Hardware-based

Gyroscope Motion Sensor Hardware-based

Gravity Motion Sensor Software-based

Rotation Vector Motion Sensor Software-based

Magnetic Field Position Sensor Hardware-based

Proximity Position Sensor Hardware-based

GPS Position Sensor Hardware-based

Orientation Position Sensor Software-based

Light Environmental Sensor Hardware-based

Thermometer Environmental Sensor Hardware-based

Barometer Environmental Sensor Hardware-based

Humidity Environmental Sensor Hardware-based

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Sensors in Android OS

Applications

Application Framework

Libraries

Linux KernelGPS

DriverAccelerometer

DriverGyroscope

DriverThermometer

DriverOther

Drivers

Hardware

GPS Accelerometer Gyroscope Thermometer….

Android OS

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Outline

Overview GPS Other Sensors

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GPS (1)

Need connect to 3 satellites for 2D positioning, 4 satellites for 3D positioning

More visible satellites increase precision Typical precision 20-50m; maximum

precision: 10m

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GPS (2)

Location service using GPS in Android consists of five architectural components

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GPS (3)

GPS chip: Radio frequency receiver that directly communicates with GPS satellites

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GPS (4)

GPS Driver Communicates with the

GPS chip Provides low level APIs to

high level software

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GPS (5)

GPS Engine The heart of the system It uses configuration parameters

to configure GPS It instructs the GPS driver to

detect multiple GPS satellites Timing information can be got

from NTP servers via internet (fast) or directly downloaded from Satellites (slow)

The GPS chip locks onto satellites using the timing information

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GPS (6)

Android Location Service It consists of Android

framework classes like location manager that provide services to applications

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GPS (7)

User Application Location-based applications

like Google maps, Navigation, etc.

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GPS (8)

DisadvantagesGPS does not work indoorsGPS quickly kills your batteryBuilding reflect and occlude satellite signals

(reducing precision of positioning in urban environments)

Alternative positioning: GPS, cell towers, Wi-Fi or combination of them

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GPS (9)

An example to obtain the locations (not all application do this way)

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Outline

Overview GPS Other Sensors

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Sensor Coordinate System

Most of the sensors use the same coordinate system When a device’s screen is facing the user

The X axis is horizontal and points to the right The Y axis is vertical and points up The Z axis pints toward outside of the screen face

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Accelerometer (1)

Measure proper acceleration (acceleration it experiences relative to freefall)

Units: g

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Accelerometer (2)

Acceleration is measured on 3 axes Note that the force of gravity is always included in

the measured acceleration When the device is sitting on the table stationary, the

accelerometer reads a magnitude of 1g When the device is in free fall, the accelerometer reads a

magnitude of 0g To measure the real acceleration of the device, the

contribution of the force of gravity must be removed from the reading, for example, by calibration

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Accelerometer (3)

When the device is lying flat gives +1g (gravitational force) reading on Z axis

Stationary device, after 45 degree rotation Same magnitude, but rotated

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Gravity Sensor

Gravity sensor is not a separate hardware It is a virtual sensor based on the

accelerometer It is the result when real acceleration

component is removed from the reading

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Gyroscope (1)

Measures the rate of rotation (angular speed) around an axis

Speed is expressed in rad/s on 3 axis When the device is not rotating, the sensor

values will be zeros

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Gyroscope (2)

It gives us 3 valuesPitch value (rotation around X axis)Roll value (rotation around Y axis)Yaw value (rotation around Z axis)

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Gyroscope (3)

Unfortunately, gyroscope is error prone over time.

As time goes, gyroscope introduces drift in result

By sensor fusion (combining accelerometer and gyroscope), results can be corrected and path of movement of device can be obtained correctly

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Magnetic Field Sensor

Measures direction and strength of earth’s magnetic field

Strength is expressed in tesla: T

Typical application: compass

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Proximity Sensor (1)

A proximity sensor can detect the presence of nearby objects without physical contact

It often emits an electromagnetic field (e.g., infrared) and looks for changes in the field or return signal

It is usually used by mobile device

to determine how far a person’s

head is from the face of a handset E.g., a user is making a phone call

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Proximity Sensor (2)

The measured results could be different based on different devicesMost proximity sensors return the absolute

distance in centimeters (cm)Some return only a flag that represents near or far Some return either 0.0 or the maximum value only

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Light Sensor (1)

It gives a reading of the light level detected by the light sensor of the device

Located at front of mobile device near to front facing camera

The units are in SI lux units

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Light Sensor (2)

The device uses the data to adjust the display’s brightness automaticallyWhen ambient light is plentiful, the screen’s

brightness is pumped up and when it is dark, the display is dimmed down

High-end Samsung galaxy phones use an advanced light sensor that can measure white, red, green, and blue light independently to fine tune image representation

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Thermometer

The thermometer here measures ambient temperature outside of the device

In fact, there’s thermometer in almost every mobile device and some handsets might have more than one of them. However, they are used to monitor the temperature inside the device and its battery to detect overheating

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Pressure Sensor

Some higher-end mobile devices have a built-in pressure sensor (barometer) which can measure atmospheric pressure

The data is used to determine how high the device is above sea level, which in turn can help improve GPS accuracy

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References

http://developer.android.com/guide/topics/sensors/sensors_overview.html http://www.cs.odu.edu/~cs495/materials/Lec-05_Android-Sensors.pdf http://mobilemining.clusterhack.net/pdf/2-tech_6up.pdf https://

www.uni-weimar.de/medien/wiki/images/Zeitmaschinen-smartphonesensors.pdf

http://www.slideshare.net/datta_jini/android-sensors-18449038 http://www.slideshare.net/paller/motion-recognition-with-android-devices http://

www.phonearena.com/news/Did-you-know-how-many-different-kinds-of-sensors-go-inside-a-smartphone_id57885

http://www.cs.dartmouth.edu/~campbell/papers/survey.pdf