In deep excavation construction sites, real time monitoring on struts is used to manage risks. False alerts have to be avoided as work stoppage delays project schedules. A well known Signal Processing method of FFT is used to reduce data corruption.

1. 1
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

2. 2
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

3. 3
Excavation Works in Close Proximity to Buildings have
Real Time Monitoring & Alerts Systems

4. 4
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

5. 5
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 !

6. 6
Real Time Monitoring & Alert
System gives SMS Alerts
within 10 minutes,hence
earlier reaction time for crisis
management
SMS
Alert
Important data
flow path

7. 7
• 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)

8. 8
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

9. 9
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
Load(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

11. 11
Impact of Electro-Magnetic Interference (EMI)

12. 12
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 ?

13. 13
VWSG
VW Signal
Conditioner
Data
Logger
Electro Magnetic
Interference
sources from Arc
Welding
Equipment,
Pumps, Motors,
Measured Data
is 1000 Hz
Analogue
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

14. 14
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

15. 15
Information taken from the websites
of Three known Brands supplying
VWSG
Company A
Company B
Company C

16. 16
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

17. 17
Signal Decomposition using Fourier Series (1800s !)

18. 18
Field test to compare the data quality
measured using a standard data logger and
the Fast Fourier Transform method (Cooley
and Tukey , 1965)

19. 19
Waveform from the output of the VWSG
Time Domain VWSG over 1.5 seconds
Time Domain VWSG when zoomed in

20. 20
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

21. 21
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) ?

22. 22
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

23. 23
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

24. 24
Thank You