0 2 4 6 8 10 12 14 16 18 2010

-2

100

102

104

Frequency (Hz)

abs(

FF

T(.

))

Frequency plot, vertical sensors at L1-L5

V2V4V13V7V9

0 1 2 3 4 5 6 7 8 9 10-8

-6

-4

-2

0

2

4

6

8

Time (sec)

Acc

eler

atio

n (

mg)

Time plot, vertical sensors at L1-L5

V2V4V13V7V9

Wireless Sensor Networks for Structural Health Monitoring

Overview• Ambient vibrations of the structure are monitored and used to determine the health status of the structure.• With a Wireless Sensor Network, low cost monitoring is possible without interfering with the operation of the structure.

Accelerometer Board

Challenges• High Fidelity Data• High Frequency Sampling with Low Jitter• Time Synchronized Sampling [FTSP]• Large-scale Multi-hop Network [Mint]• Reliable Command Dissemination [Broadcast]• Reliable Data Collection [Straw]

Silicon Designs 1221LADXL 202E

ADXL202E

Silicon Designs221L

Range -2G ~ 2G -0.1G ~ 0.1G

Systemnoise floor

200(μG/√Hz) 30(μG/√Hz)

Price $10 $150

• Two measurement axis each with two accelerometers• Thermometer, 16bit ADC, Low-pass filter• On-board Digital Signal processing• Calibration for manufacturing variation and temperature

-0.99

0.19-0.73

1.000.74

First Vertical Mode of Vibration

Estimated results match with a FE model of the bridge (SAP)

Deployment at the Footbridge

Berkeley SF Bay

mid-spanquarter-span

59

Base Station

260ft

16ft27 1

1310 38 4

121114

Sukun Kim*, Shamim Pakzad+, David Culler*, James Demmel*, Gregory Fenves+, Steve Glaser+, Martin Turon#

* EECS, UC Berkeley +CEE, UC Berkeley #Crossbow

Software Architecture

Best-effort Single-hop Communication

Broadcast MintRouteFTSP

Low-level FLASH

BufferedLogStraw

Sentri (Application Layer)

• When sampling, only necessary components are turned on to reduce jitter• Straw provides reliable data collection

• Selective-NACK is used – complexity is drawn from the sender (mote) to the receiver (PC)• Rate-based control• Pipelining increases channel utilization

Deployment at theGolden Gate Bridge

1st mode 2nd mode 3rd mode

Vertical Frequency (Hz) 1.35 1.79 11.47

Damping Ratio 0.055 0.02 0.043

Horizontal Frequency (Hz) 2.37 7.87 11.91

Damping Ratio 0.26 0.16 0.123

• Nodes on the main span and the south tower• Distance between nodes on the west span is either 100ft or 50ft• Exposed to strong and salty wind and fog

8 nodes

56 nodes

1125 ft 4200 ft

500 ft

246 ft

SF(south)

Sausalito(north)

Node, Battery, Antenna Rusting of C-clamp Base station in Tower

0 100 200 300 400 500 600-20

0

20

Time (sec)

Acc

el (

mg)

Time and Frequency plots, Vertical sensors, s284n62

45 50 55 60 65-5

0

5

Time (sec)

Acc

el (

mg)

0 5 10 15 20 25 30 35 40 45 500

1

2

frequency (HZ)

PS

D (

mg/

Hz)

0 0.5 1 1.5 2 2.50

1

2

frequency (HZ)

PS

D (

mg/

Hz)

0 100 200 300 400 500 600-50

0

50

Time (sec)

Acc

el (

mg)

Time and Frequency plots, Vertical sensors, s284n45

45 50 55 60 65-10

0

10

Time (sec)

Acc

el (

mg)

0 5 10 15 20 25 30 35 40 45 500

2

4

frequency (HZ)

PS

D (

mg/

Hz)

0 0.5 1 1.5 2 2.50

2

4

frequency (HZ)

PS

D (

mg/

Hz)

(a) Vertical, Quarter spanNorth of the South Tower

(b) Vertical, Quarter spanSouth of the North Tower

Bandwidth versus Hop Count

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50Hop Count

Bandw

idth

(B

/s)

Vibration Data from the Footbridge