Context Sensing

Context Sensing

金仲達教授清華大學資訊系統與應用研究所

九十三學年度第一學期

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Outline Location sensing RFID

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Location Sensing“Location Systems for Ubiquitous Computing,

” Jeffrey Hightower, Gaetano Borriello, University of Washington, IEEE Computer Magazine, August 2001

Systems and technologies that automatically locate people, equipment, and other tangibles.

Develop a taxonomy to help developers of location-aware applications better evaluate their options when choosing a location-sensing system

Outline: Location Sensing Introduction Location Systems

Techniques Properties

Survey of Location Systems Research Directions Summary

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Techniques Triangulation

Use the geometric properties of triangles to compute object locations.

Proximity Measure nearness to a known set of points. The object’s presence is sensed using a

physical phenomenon with limited range. Scene Analysis

Use features of a scene observed from a particular vantage point to draw conclusions about the location of the observer or of objects in the scene.

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Techniques – Triangulation Lateration: Compute the position of an object

by measuring its distance from multiple reference positions Direct

Physical action or movement. Time-of-Flight

Measure the time it takes to travel between the object and point P at a known velocity.

Attenuation Given a function correlating attenuation and distanc

e for a type of emission and the original strength of the emission

Techniques – Triangulation

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Techniques – Angulation Use primarily angle or bearing measurements 2D angulation requires two angle measureme

nts and one length measurement such as the distance between the reference points.

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Techniques – Scene Analysis Static

Observed features are looked up in a predefined dataset that maps them to object locations.

Differential Tracks the difference between successive

scenes to estimate location.

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Techniques – Scene Analysis Advantages

The location of objects can be inferred using passive observation and features that do not correspond to geometric angles or distances.

Disadvantages The observer needs to have access to the

features of the environment against which it will compare its observed scenes.

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Techniques – Proximity Detecting physical contact

Detect physical contact with an object Include pressure sensors, touch sensors,…

Monitoring wireless cellular access points Monitoring when a mobile object device is in

range of one or more access points in a wireless cellular network

Observing automatic ID systems If the device scanning the label, interrogating

the tag, or monitoring the transaction has a know location, the location of the mobile object can be inferred.




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Properties Physical position and Symbolic position Absolute versus Relative Localized location computation Accuracy and Precision Scale Recognition Cost Limitations

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Properties – Physical and Symbolic Positions Physical

A building situated at 47°39’17”N by 122°18’23”W, at a 20.5-meter elevation

E.g., GPS Symbolic

Encompass abstract ideas of where something is

E.g., in the kitchen, near to a mailbox, a train approach Denver, …

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Properties – Physical and Symbolic Positions Physical position:

Can be augmented to provide symbolic location with additional information, infrastructure, or both, e.g., linking train positions to reservation and ticketing database can help locate a passenger on a train

Can determine a range of symbolic information, e.g., use a GPS to find the closest printer, or link GPS with calendar to provide current activity of a person

Purely symbolic location systems typically provide coarse-grained physical positions Increase accuracy may need multiple readings or

sensors

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Properties – Absolute versus Relative Absolute

Use a shared reference grid of all located objects All GPS receivers use latitude, longitude, and

altitude; Two GPS receivers placed at the same position will report same position readings, and refers to the same place regardless of GPS receiver

Relative Each object can have its own frame of reference For example, a mountain rescue team searching

for avalanche victims can use handheld computers to locate victims’ avalanche transceivers. Each rescuer’s device reports the victims’ position relative to itself.

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Properties – Absolute versus Relative An absolute location can be transformed

into a relative location Relative to a second reference point, but may

not always available In reverse, we can use triangulation to

determine an absolute position from multiple relative reference points.

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Properties – Localized Location Computation Ensures privacy by mandating that no other

entity may know where the located object is unless it specifically publicizes that information

Some systems require the located object to periodically broadcast, respond with, or otherwise emit telemetry to allow the external infrastructure to located it Infrastructure can find objects in its purview

without directly involving the objects in the computation

Reduce computational and power demands on the objects being located => lower costs and smaller form factors

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Properties – Accuracy and Precision Some GPS receiver can locate position to

within 10 m for 95% of measurements The distances denote the accuracy, or grain

size, of the position information GPS can provide

The percentages denote precision, or how often we can expect to get that accuracy

Sensor Fusion seeks to improve accuracy and precision by integrating many location or position systems to form hierarchical and overlapping levels of resolution, e.g., Robot

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Properties – Scale Scale: locating objects worldwide, within a

metropolitan area, throughout a campus, in a particular building, or within a single room

Assessing: coverage area per unit of infrastructure and # objects the system can locate per unit of infrastructure per time interval Systems can often expand to a larger scale by

increasing the infrastructure, e.g., a tag system that locates objects in a single building can operate on a campus by outfitting all campus buildings and outdoor areas with the necessary sensor infrastructure

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Properties – Scale Hindrances to scalability in a location

system include not only the infrastructure cost but also middleware complexity

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Properties – Recognition For applications that need to recognize or

classify located objects to take a specific action based on their location, an automatic identification mechanism is needed. For example, a modern airport baggage handling

system needs to automatically route outbound and inbound luggage to the correct flight or claim carousel

Systems with recognition capability may recognize only some feature types For example, cameras and vision systems can

easily distinguish the color or shape of an object but cannot automatically recognize individual people

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Properties – Recognition A general technique for providing

recognition capability assigns names or globally unique IDs (GUID) to objects the system locates It can also combine the GUID with other

contextual information so it can interpret the same object differently under varying circumstances.

For example, a person can retrieve the descriptions of objects in a museum in a specified language.

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Properties – Cost Time Cost

Installation process’s length and the system’s administration needs

Space Cost Amount of installed infrastructure and the

hardware’s size and form factor Capital Cost

Price per mobile unit or infrastructure element and the salaries of support personnel

GPS receivers need an antenna of sufficient size for adequate satellite reception and may need a second antenna to receive the land-based differential signal.

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Properties – Limitations Some system will not function in certain

environments GPS: receivers usually cannot detect satellites’

transmissions indoors Possible solution: uses a system of GPS

repeaters mounted at the edges of buildings to rebroadcast the signals inside

In general, we assess functional limitations by considering the characteristics of the underlying technologies that implement the location system.




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Survey of Location Systems Global Positioning System (GPS) Active Badge

Developed at Olivetti Research Lab., now AT&T Cambridge

Active Bat Developed by AT&T

MotionStar magnetic tracker Developed by Ascension

SpotON Developed by Washington

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Global Positioning System (GPS) GPS satellites (24+3):

Have no knowledge about who uses the signals they transmit

Are precisely synchronized with each other and transmit their local time in the signal allowing receivers to compute the difference in time-of-flight

Receivers: Allow receivers to compute their location to

within 1 to 5 meters Receivers can compute 3-D position using 4

satellites

Global Positioning System (GPS)

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Active Badge A cellular proximity system using diffuse

infrared Usage:

Person wears a small infrared badge Badge emits a unique id every 10 sec or on demand Central server collects data from fixed infrared

sensors around the building, aggregates it, and provides an API for using the data

Provide absolute location information Location is symbolic, representing, for

example, the room in which the badge is located

Active Badge

Network sensor (contains two 87C751 microprocessor

Four generations of the Active Badge

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Active Badge: Limitations As with any diffuse infrared system, Active

Badges have difficulty in locations with fluorescent lighting or direct sunlight because of the spurious infrared emissions that these light sources generate

Diffuse infrared has an effective range of several meters, which limits cell sizes to small- or medium-sized rooms

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Active Bat Uses ultrasound time-of-flight lateration

Provide more accurate positioning than Active Badge Usage:

Users and objects carry Active Bat tags Each Bat has a GUID for addressing and recognition In response to a request the controller sends via sho

rt-range radio, a Bat emits an ultrasonic pulse to a grid of ceiling-mounted receivers

Each ceiling sensor measures time interval from reset to ultrasonic pulse arrival and computes the distance

Local controller then forwards the distance measurements to a central controller

Active Bat

Bat (7.5cm*3.5cm*1.5cm)Power: 3.6 VLifetime: around 15 monthsUnique 48-bit codeBi-directional 433MHz radioTwo buttons, two LEDs, a speaker,and a voltage monitor

Placed in a square grid, 1.2m apartConnected by a high-speed serial networkThe serial network is terminated by aDSP calculation board

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Active Bat: Limitations The system can locate Bats to within 9 cm

of their true position for 95 percent of the measurements, and work to improve the accuracy even further is in progress

Require a large fixed-sensor infrastructure throughout the ceiling

Rather sensitive to the precise placement of these sensors.

Disadvantages: scalability, ease of deployment, and cost

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MotionStar Magnetic Tracker Use scene analysis, lateration and electronma

gnetic sensing Tracking systems uses DC magnetic sensors to

overcome blocking and post processing delays .

System computes the position and orientation of the receiving antennas by measuring the response in three orthogonal axes to the transmitted field pulse, combined with the constant effect of the earth’s magnetic field

MotionStar Magnetic Tracker

MotionStar Controller MotionStar Wireless(Magnetic pulse transmitting antennas receiving antennas and Controller)

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MotionStar Magnetic Tracker Sense precise physical positions relative to

the magnetic transmitting antenna Advantages

Very high precision and accuracy Disadvantages

Steep implementation costs and the need to tether the tracked object to a control unit

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SpotON Implement ad hoc lateration with low-cost tag

s Ad-hoc location sensing is a fusion of concepts

from object location tracking and the theories of ad-hoc networking

SpotON tags use radio signal strength information (RSSI) as a distance estimator to perform ad-hoc lateration.

SpotON




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Sensor Fusion The use of multiple technologies or

location systems simultaneously to form hierarchical and overlapping levels of sensing Provide aggregate properties unavailable when

using location systems individually For example, integrating several systems with

different error distributions may increase accuracy and precision beyond what is possible using an individual system

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Ad hoc Location Sensing Locating objects without drawing on the

infrastructure or central control In a purely ad hoc location-sensing system,

all of the entities become mobile objects with the same sensors and capabilities To estimate their locations, objects cooperate

with other nearby objects by sharing sensor data to factor out overall measurement error

Cluster-based approach: Objects in the cluster are located relative to one another or absolutely if some objects in the cluster occupy known locations

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Location Sensing System Accuracy Comparing the accuracy and precision of

different location sensing systems Need quantitative evaluations

Should include error distribution, summary of accuracy and precision and any relevant dependencies, e.g. density of infrastructural elements

Accurately described error distribution can be used as partial input for simulating a system=> use of simulation for evaluation




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Location Sensing: Summary Location sensing is a mature enough field

to define a space within a taxonomy that is generally populated by existing systems

Future work should generally focus on Lowering cost Reducing the amount of infrastructure Improving scalability Creating systems that are more flexible within

the taxonomy

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Outline Location sensing RFID

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Outline: RFID System Components

Transponders/Tags Reader/Interrogator RF Transponder Programmers

RFID System Categories Areas of Application for RFID

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What is RFID? Radio Frequency Identification Basic components:

An antenna or coil A transceiver (with decoder) or reader A transponder (RF Tag), electronically

programmed with unique information Data are carried in

transponders to provideidentification

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System Components

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Transponder

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Active Transponders Powered by an internal battery and are

typically read/write devices Principle of operation: propagation

coupling based upon propagating electromagnetic

waves Tags have

Microprocessor Memory (up to 1MB) Metal coil (antenna) Separate power source

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Passive Transponders Operate without an internal battery

source, deriving the power to operate from the field generated by the reader

Principle of operations: inductive coupling based upon close proximity electromagnetic

Tags have Microprocessor Small memory (32-128 bits) Metal coil (antenna) Separate power source

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Comparing Passive and Active Tags Active

greater communication range

better noise immunity higher data

transmissions rates usually capable of

operating over a temperature range of -50° C to +70° C

Greater size Greater cost Limited operation life

(10 years)

Passive lighter, smaller less expensive unlimited operation life small read range require a higher-

powered reader sensitivity and

orientation performance may also be constrained by the limitation on available power

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Data Carrying Options Tags may be used to carry:

Identifiers:a numeric or alphanumeric string is stored for identification purposes or as an access key to data stored elsewhere in a computer or information management system

Portable data files:information can be organized, for communication or as a means of initiating actions without recourse to, or in combination with, data stored elsewhere

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Data Capacity Single bit

For surveillance, e.g., retail electronic article surveillance (EAS)

May also be used for counting Up to 128 bits

Can hold serial no. or id with parity check May be manufacturer or user programmable

Up to 512 bits Mostly user programmable Can hold id, package content, process

instructions Around 64 kilobits

As carriers for portable data files

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Transponder Memory ROM

Security data OS instructions: in conjunction with the

processor or processing logic deals with the internal "house-keeping" functions such as response delay timing, data flow control and power supply switching.

RAM Used to facilitate temporary data storage

during transponder interrogation and response.

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Transponder Memory Non-volatile programmable memory

Electrically erasable programmable read only memory (EEPROM) being typical

Store transponder data and ensure that the data is retained when the device is in its quiescent or power-saving "sleep" state

Data buffers are further components of memory Used to temporarily hold incoming data

following demodulation and outgoing data for modulation and interface with the transponder antenna

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Data Read Rate

Long read rangeExpensive High reading speedLine of sight required

High850-950 MHz2.4-5.8 GHz

Short to medium read rangePotentially inexpensiveMedium reading speed

Intermediate10-15 MHz

Short to medium read rangeInexpensiveLow reading speed

Low100-500 kHz

Characteristics Frequency Band

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Communication Range The range is determined by:

The power available at the reader/interrogator to communicate with the tag(s)

The power available within the tag to respond The antenna design will determine the shape of the

field or propagation wave delivered. The environmental conditions and structures

noise ratio obstructions or absorption mechanisms moisture

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Data Programming Options read-only write once read many (WORM)

User-programmable (at beginning) read/write

User-programmable Allowing the user to change data stored in a

tag

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Physical Form Animal tracking tags inserted beneath the

skin, can be as small as a pencil lead in diameter and ten millimeters in length

Tags can be screw-shaped to identify trees or wooden items

Credit-card shaped for use in access applications

Plastic or printed for placing on packages

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Reader/Interrogator/Transceiver Transmitter and receiver Larger antenna Larger coil (energizing the tag) Draws power from external power supply

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Reader Command response protocol (hands down

polling) Once the signal from a transponder has been

correctly received and decoded, algorithms may be applied to decide whether the signal is a repeat transmission, and may then instruct the transponder to cease transmitting

Used to circumvent the problem of reading multiple tags in a short period of time

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Reader Hands up polling

The interrogator looks for tags with specific identities, and interrogates them in turn

This is contention management, and a variety of techniques have been developed to improve the process of batch reading

A further approach may use multiple readers, multiplexed into one interrogator, but with attendant increases in costs

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RF Transponder Programmers Programming is generally carried out off-

line, e.g., at the beginning of a batch production run

For some systems re-programming may be carried out on-line, particularly if it is being used as an interactive portable data file within a production environment

By combining the functions of a reader/interrogator and a programmer, data may be appended or altered in the transponder as required

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RFID System Categories EAS (Electronic Article Surveillance) systems

Typically a one bit system to sense the presence/absence of an item, usually in retail stores

Portable data capture system Portable data terminals with integral RFID reader Capture data which is then either transmitted

directly to a host information management system via a radio frequency data communication (RFDC) link or held for delivery by line-linkage to the host on a batch processing basis

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RFID System Categories Networked systems

Fixed position readers deployed within a given site and connected directly to a networked information management system

The transponders are positioned on moving or moveable items, or people

Positioning systems Readers are positioned on the vehicles and

linked to an on-board computer and RFDC link to the host information management system

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Typical Areas of Application Transportation and logistics Manufacturing and processing Security Miscellaneous

Animal tagging Waste management Time and attendance Postal tracking Airline baggage reconciliation Road toll management

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New Areas of Application Electronic article surveillance Vehicle anti-theft systems Electronic monitoring of offenders at home Time and attendance

To replace conventional "slot card" time keeping systems.

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Example Application 1 Consider a book consisting of a collection of pr

inted pages When a computational device detects the tag, an as

sociated virtual document is displayed You can always read the latest electrical version

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Example Application 2 Augmenting business cards

A tag is placed on the back of a regular business card

When the card is brought close to the computer, the corresponding homepage is displayed

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Example Application 3 Extending document functionality: services

We can link to the corresponding Amazon.com web page to order a copy of the book

We can additionally link in theauthor’s homepage, reviews of thebook, or other correspondencerelated to the book

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Example Application 4 Augmenting “non-document“ objects: wrist

watch A tag is embedded in a wristwatch When the watch is close to the

computer, the calendar applicationfor the particular user is shown forthe current day, at the current time

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References http://www.aimglobal.org http://www.TLCdelivers.com www.ems-rfid.com

Documents

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