The cricket compass for context aware mobile applications
Download
1 / 31

The Cricket Compass for Context-Aware Mobile Applications - PowerPoint PPT Presentation


  • 148 Views
  • Uploaded on

The Cricket Compass for Context-Aware Mobile Applications. Nissanka Priyantha, Allen Miu , Hari Balakrishnan, Seth Teller MIT Laboratory for Computer Science http://nms.lcs.mit.edu/. . Cricket Location System. Original Version [mobicom00] Location information: room, floor, building, etc.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'The Cricket Compass for Context-Aware Mobile Applications' - zarita


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
The cricket compass for context aware mobile applications l.jpg

The Cricket Compass for Context-Aware Mobile Applications

Nissanka Priyantha, Allen Miu,

Hari Balakrishnan, Seth Teller

MIT Laboratory for Computer Science

http://nms.lcs.mit.edu/


Cricket location system l.jpg

Cricket Location System

  • Original Version [mobicom00]

    • Location information: room, floor, building, etc.

  • New extensions – The Cricket Compass

    • Position information

      • (x, y, z) coordinates within a space

    • Orientation information

      • direction at which device faces

Mobile device

(x, y, z)


You are here great now what l.jpg

You

are here

You Are Here… Great, now what?!



Orientation is important l.jpg
Orientation is important!

Orientation is a building block that supports a wide variety of mobile applications


Design goals l.jpg
Design Goals

  • Compact, integrated, self-contained

  • Should not rely on motion to determine heading (as in GPS navigation systems)

  • Robust under a variety of indoor conditions

  • Low infrastructure cost; easy to deploy

  • Enough accuracy for mobile applications

    (5o accuracy)


The cricket compass architecture l.jpg

(x2,y2,z2)

(x1,y1,z1)

(x3,y3,z3)

(x0,y0,z0)

vt3

vt1

vt2

vt0

The Cricket Compass Architecture

Beacons on

ceiling

Y

X

RF + Ultrasonic

Pulse

Z

Cricket listener

with RF and ultrasonic

sensors

Mobile device

( x, y, z)

vt3 to solve for unknown speed of sound


Definition of orientation l.jpg
Definition of Orientation

(x2,y2,z2)

(x1,y1,z1)

(x3,y3,z3)

(x0,y0,z0)

(on horizontal plane)

(on horizontal plane)

B

Beacons on

ceiling

Beacons on

ceiling

Y

X

Z

Orientation relative to B

Mobile device


Approach use differential distance to determine orientation l.jpg

Approach: Use Differential Distance to Determine Orientation

Beacon

Assume: Device rests on horizontal plane

Method: Use multiple ultrasonic sensors;

calculate rotation using measured distances d1, d2, z

sin  = (d2 - d1) / sqrt (1 - z2/d2)

where

d = (d1+d2)/2

d

z

d1

d2

  • Need to measure:

  • a) (d2 - d1)

  • z/d

S1

L

S2


Problem measuring d2 d1 directly requires very high precision l.jpg

Problem: Measuring (d2 – d1) directly requires very high precision!

Beacon

  • Consider a typical situation

    • Let L = 5cm, d = 2m, z = 1m,  = 10º

    • (d2 – d1) = 0.6cm

d

  • Impossible to measure d1, d2 with such precision

    • Comparable with the wavelength of ultrasound (  = 0.87cm)

z

d1

d2

S1

L

S2


Solution differential distance d2 d1 from phase difference l.jpg

t

Solution: Differential Distance (d2-d1) from Phase Difference ()

  • Observation: The differential distance (d2-d1) is reflected as a phase difference between the signals received at two sensors

Estimate phase difference between ultrasonic waveforms to find (d2-d1)!

Beacon

f = 2p (d2 – d1)/l

d1

d2

t

S1

S2


Problem two sensors are inadequate l.jpg
Problem: Two Sensors Are Inadequate

  • Phase difference is periodic  ambiguous solutions

  • We don’t know the sign of the phase difference to differentiate between positive and negative angles

  • Cannot place two sensors less than 0.5 apart

    • Sensors are not tiny enough!!!

    • Placing sensors close together produces inaccurate measurements


Solution use three sensors l.jpg
Solution: Use Three Sensors!

  • Estimate 2 phase differences to find unique solution for (d2-d1)

  • Can do this when L12 and L23 are relatively-prime multiples of l/2

  • Accuracy increases!

Beacon

d3

d1

d2

S3

S2

S1

t

L12 = 3l/2

L23 = 4l/2


Slide14 l.jpg

Cricket Compass v1 Prototype

Ultrasound Sensor Bank

1.25 cm x 4.5 cm

RF module (xmit)

Ultrasonic

transmitter

RF antenna

Sensor Module

Beacon


Angle estimation measurements l.jpg
Angle Estimation Measurements

  • Accurate to 3 for  30, 5 for  40

  • Error increases at larger angles


Cricket compass hardware l.jpg
Cricket Compass Hardware

  • Improves accuracy

  • Disambiguates

     in [ -,  ]

Amplifiers, Wave shaping,

and Selection Circuits

Microcontroller

RS 232

Driver

RF RX


Conclusion l.jpg
Conclusion

The Cricket Compass provides accurate position and orientation information for indoor mobile applications

  • Orientation information is useful

  • Novel techniques for precise position and phase difference estimation to obtain orientation information

  • Prototype implementation with multiple ultrasonic sensors

Orientation accurate to within 3-5 degrees

http://nms.lcs.mit.edu/cricket/


Considerations l.jpg
Considerations

  • Beacon placement

    • At least one beacon within range

    • Avoid degenerate configuration (not in a circle)

  • Ultrasonic reflections

    • Use filtering algorithms to discard bad samples

  • Configuring beacon coordinates

    • Auto-configuration, auto-calibration


Current orientation systems are not adequate for indoor use l.jpg
Current Orientation Systems Are Not Adequate for Indoor Use

  • Magnetic based sensors (magnetic compass, magnetic motion trackers)

    • suffers from ferromagnetic interference commonly found indoors

  • Inertial sensors (accelerometers, gyroscopes)

    • used in sensor fusion to achieve high accuracy

    • require motion to determine heading

    • suffer from cumulative errors

  • Other systems require:

    • Extensive wiring: expensive & hard to deploy

    • Multiple active transmitters worn by the user: obtrusive, inconvenient, not scalable


Point in the direction of the service not at the service l.jpg
Point in the direction of the Service… Not at the Service

  • Orientation information provides a geometric primitive that is general and useful among a variety of “direction-aware” applications, e.g.

    • In-building navigation

    • Point and Shoot User Interfaces

  • Line-of-sight systems are limited

    • awkward to use, not robust

    • do not support navigation

Orientation information is useful for context-aware mobile applications!


Is orientation necessary l.jpg
Is orientation necessary?

  • Direction-aware applications could be implemented using “TV remotes!”

  • But orientation information is useful

    • Application-specific semantics are possible

    • Convenient for navigation applications

    • Eliminates the need for a line of sight to target


System model l.jpg

(x, y, z, )…

(x, y, z, )…

System Model

Cricket

Service

Discovery

Database

Services,

Other users


System model23 l.jpg

printer@(x,y,z, )

printer@(x,y,z, )

System Model

Cricket

(x, y, z, )…

Service

Discovery

Database

Services

pda@(x, y, z, )…


Differential distance from phase difference l.jpg

d1

d2

Differential Distance From Phase Difference

  • Observation: The differential distance (d2-d1) is reflected as a phase difference between the signals received at two sensors

Ultrasound signal first hits sensor S1

Beacon

t

S1

S2


Differential distance from phase difference25 l.jpg

d2

Differential Distance From Phase Difference

  • Observation: The differential distance (d2-d1) is reflected as a phase difference between the signals received at two sensors

The same signal then hits sensor S2

Beacon

d1

t

S1

S2


Where am i active map l.jpg
Where am I?(Active map)



Differential distance from phase difference28 l.jpg
Differential Distance From Phase Difference

  • Observation: The differential distance is reflected as a phase difference between the signals received at two receivers

Estimate phase difference between ultrasonic waveforms to find (d2-d1)!

Beacon

f = 2p vt/ l = 2p (d2 – d1)/l

d1

d2

t

R1

t

R2

t <= L/v, where v is velocity of sound


Ambiguous solutions example l.jpg
Ambiguous Solutions: Example

  • We know: t, t’ <= L/v

  • Let L = 

  • Observed time difference is t

  • Possible time differences are t and t’

Beacon

L/v

t

t

t

t’


Requirements l.jpg
Requirements

  • Navigational information

    • Space

      • address, room number

    • Position

      • coordinate, with respect to a given origin in a space

    • Orientation

      • angle, with respect to a given fixed point in a space

  • Low cost, low power

  • Completely wireless

    • Deployable in existing buildings

  • Scalable

  • Autonomous

    • Mobile device determines its own location


Ambiguous solutions example31 l.jpg

In this case, we can find a unique solution

L/v

t

Ambiguous Solutions: Example

  • We know: t <= L/v

  • Let L = /2

Beacon

t