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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.

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the cricket compass for context aware mobile applications

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

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)

orientation is important
Orientation is important!

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

design goals
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

(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
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

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

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

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
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
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

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
Angle Estimation Measurements
  • Accurate to 3 for  30, 5 for  40
  • Error increases at larger angles
cricket compass hardware
Cricket Compass Hardware
  • Improves accuracy
  • Disambiguates

 in [ -,  ]

Amplifiers, Wave shaping,

and Selection Circuits

Microcontroller

RS 232

Driver

RF RX

conclusion
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
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
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
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
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

(x, y, z, )…

(x, y, z, )…

System Model

Cricket

Service

Discovery

Database

Services,

Other users

system model23

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

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

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

differential distance from phase difference28
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
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
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