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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 sensors

Physical components built into a handset
They 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 List

Sensor	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 Kernel

GPS

Driver

Accelerometer Driver

Gyroscope

Driver

Thermometer

Driver

Other Drivers

Hardware

GPS

Accelerometer

Gyroscope

Thermometer

….

Android

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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)

Disadvantages

GPS does not work indoors
GPS quickly kills your battery
Building 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 values

Pitch 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 devices

Most 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 automatically

When 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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Thank You

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Exercise

Q1. What is sensor?

Q2. Explain category 1 sensors.

Q3. Explain category 2 sensors.

Q4. Explain accelerometer sensor.

Q5. Explain gravity sensor.

Q6. Explain gyroscope.

Q7. Explain proximity sensor.

Q8. Accelerometer is a

Software sensor
Hardware sensor
Framework
Document

Q9. Orientation sensor is a

Software sensor
Hardware sensor
Framework
Document

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Q10. Most of the sensors use the same

Software
Hardware
Coordinate system
Electronics

Q11. Acceleration is measured on

Azimuth
Pitch
Y axis
X axis

Q12. Measures the rate of rotation (angular speed) around an axis

Gyroscope
Accelerometer
Gravity meter
Thermometer

Q13. It is usually used by mobile device to determine how far a person’s head is from the face of a handset

Gyroscope
Accelerometer
Gravity meter
Proximity sensor