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Reducing False Alerts by improving

Vibrating Wire Strain Gauge Data

Quality using Fast Fourier Transform

International Conference on Deep

Excavations 2008

By:

G. H. Tan

Y. K. Poh

N.H. Osborne

C.C. Ng

J.M. Brownjohn

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Content

•Real Time Monitoring & Alert System in

Deep Excavations in Singapore

•Main cause of False Alerts in

Construction sites

•Improving Data Quality without altering

the original raw data using the Fast

Fourier Transform Technique

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Excavation Works in Close Proximity to Buildings have

Real Time Monitoring & Alerts Systems

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What is Real Time Monitoring & Alert System?

Doctor tells the Nurse:

“Please monitor the patient

and call me immediately if

there is any change in his

condition.”

Hospital = Construction Site

Patient = Excavation works

Doctor = Consultant

Nurse = Instrumentation Specialist

Call Immediately = Real Time Monitoring of 10min

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Real Time Monitoring & Alert requires:-

Real Time Sensor Measurement

Real Time Data Transmission

Real Time Data Processing

Real Time Data Analysis

Real Time Alert to the Right Person

If any part of the information flow gets

corrupted or fails, then the entire system

fails !

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Real Time Monitoring & Alert

System gives SMS Alerts

within 10 minutes,hence

earlier reaction time for crisis

management

SMS

Alert

Important data

flow path

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• VWSG is an important role in Real Time Monitoring

Systems

• Benefits of VWSG:

– More cost effective for automated real time system

– Man-less Real Time & Alert Capability

– Automated the entire monitoring process

Vibrating Wire Strain Gauges (VWSG)

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Measures 92 VWSG & 4 Load Cells (92+ 4x4

VWSG + 96 ToC) per Data Logger at every 10

minutes.

It generates (92+16) x 24 x 6 = 15,552 VWSG

data points per day. Hence a 5% data corruption

can cause 777 false SMS alerts per day per

person !

Now False SMS Alerts becomes a major

challenge when Electromagnetic Interference

(EMI) can corrupt the Data Logger Readings

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What causes Electro-Magnetic Interference (EMI)?

In ideal condition, data quality can be

maintained over 500m cable length

In reality, site conditions vary & data

corruption is common

Arc welding, power generators & high current AC wires,

exposed cable joints and ungrounded shielded cable ends

can cause EMI

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0

500

1000

1500

2000

2500

00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00

Time

Lo

ad

(kN

)

SG Welding side

SG Non Welding side

Construction Impact - Welding

Load increase (600kN)

due to welding

Welding

Sudden decrease in load (400kN)

due to EMI effect of welding

SG on the side of

welding of strut

SG on the non welding

side of strut

Welding side

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Impact of Electro-Magnetic Interference (EMI)

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Alert

Trigger

Level

For 10 minutes measurement intervals, a re-confirmation is

only 20 minutes

Level

Is the data really moving

or is it a spike ?

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VWSG

VW Signal

Conditioner

Data

Logger

Electro Magnetic

Interference

sources from Arc

Welding

Equipment,

Pumps, Motors,

Measured Data

is 1000 HzAnalogue

Signals

Use of a Fast Fourier

Transform (FFT)

Spectrum Analyzer to

check the Signal to

Noise Ratio to verify

Quality of Signal

presented to the ADC

The reading is actually ?

= 1050 (Sensor) - 50 (Noise) ?

or

= 600 (Sensor) + 400 (Noise) ?

Frequency Domain Analysis

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EMI Standard

EN50082-2

EN55011 ??

Try asking

your

friendly

Local

supplier

EMI Standard

EN50082-2

EN55011

EMI Standard

EN50082-2

EN55011

70dB 50 dB 90 dB 60 dB 96 dB ∞ dB ∞ dB

Sensor Wiring Conditioner Wiring 16Bit DAQ Digital Wire Computer

In the whole signal flow from Sensor to the Data

Logger, the weakest part is only 50 dB !

Signal to Noise Ratio in the

Data Flow Chain

Log

Scale

Value 1000

but at what

Signal to

Noise Ratio

?

Analog

Cable

standards

in Ideal

Conditions

Analog

Cable

Standards

in Ideal

Conditions

Cable

Carrying

Sensor

Data

In

Digital

Format

Computer

for Data

Processing

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Information taken from the websites

of Three known Brands supplying

VWSG

Company A

Company B

Company C

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One solution to improve the Data

Quality and reduce False alert is:-

•A different fundamental approach to the

VWSG measurement technique

•Discriminate the Wanted Signals from

the Unwanted Signals which Corrupt the

measurements

•Use a proven engineering approach

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Signal Decomposition using Fourier Series (1800s !)

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Field test to compare the data quality

measured using a standard data logger and

the Fast Fourier Transform method (Cooley

and Tukey , 1965)

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Waveform from the output of the VWSG

Time Domain VWSG over 1.5 seconds

Time Domain VWSG when zoomed in

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Comparison Data Quality showing the

Robustness of the FFT Measurement

method unaffected by EMI

Measurement results from

the well established data

logger with 100% data

corruption

Measurement results from

the new FFT method with

less than 0.17% error

True reading

is the Blue

dotted line

True reading

is the Blue

dotted line

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VWSG

VW Signal

Conditioner

Data

Logger

Electro Magnetic

Interference

sources from Arc

Welding

Equipment,

Pumps, Motors,

Measured Data

is 1700 HzAnalogue

Signals

Frequency Domain Analysis

The reading is actually

910 Hz (Sensor) + 790 Hz (Noise) ?

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Accepting Corrupt Data is

like going ahead in a

surgery when the X ray

film has spots and also

blur

What do I

do now ?

Clear Spots & Blur

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Conclusions

+Careful

planning in

Instruments

& Layout

Monitoring

System

installed &

Operate by

Qualified &

Experienced

specialist

+

Proactively

analyzing the

various

influencing

factors when

interpreting

data

=High Quality

Information for

Successful field

measurement &

monitoring

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Thank You