1
GUIDELINES FOR CALIBRATION OF FALLING
WEIGHT DEFLECTOMETERS
RENE CLEMEN, PRODUCTION MANAGER
Fuglesangsallé 16, 6600 Vejen
Denmark
Tel.: +45 76 34 73 73
Fax.: +45 76 34 73 74
E-mail: rec@carlbro.dk
ABSTRACT
The interest and need for systematic pavement maintenance has been growing tremendously over the
last decade. This has increased the demand for specialised equipment for planning and maintenance
in practice and for pavement condition survey equipment.
The FWD is today the most used non-destructive testing (NDT) device for testing of load bearing
capacities of road and airfield pavements.
The FWD equipment has undergone an impressive development and is a true state-of-the-art device
using the latest electronics and computer technology.
In Europe and the USA research work has been done in order to harmonise test methods and
calibration procedures for the FWD. In Europe this work is done by the Centre for Research and
Contract Standardisation in Civil and Traffic Engineering (C.R.O.W.), the Netherlands. A special
group - the Study Committee P7 “Falling Weight Deflection Testing” - has developed preliminary
guidelines for Falling Weight Deflectometer calibration.
The calibration protocols are partially developed by the Study Committee P7 and partially with
amendments from the protocols developed by Strategic Highway Research Program (SHRP),
National Research Council at Washington, DC.
Different levels of the calibration programme have been developed, ranging from simple to difficult,
and from low-priced and convenient to more expensive and comprehensive.
This paper will discuss new equipment, which has been developed to comply with the new calibration
tests performed according to the new protocols at verification centres.
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INTRODUCTION
In Europe there are 3 manufacturers of Falling Weight Deflectometers FWD, namely PHØNIX
Pavement Consultants, KUAB and Dynatest. All have their own FWD calibration programmes.
Normally deflection transducers, load cell and temperature sensors are calibrated.
CBPC has as the only manufacturer chosen to base its calibration programme on CROW (Preliminary
Guidelines for Falling Weight Deflectometer Calibration). First of all this programme was chosen
because it has been developed for FWDs operating with load pulse widths within the range of 20-35
msec. and our opinion is that this procedure gives the best calibration result - and it is an European
standard. CROW operates with the following calibration procedures (protocols A-G):
A.
B.
C.
D.
E.
F.
G.
Relative Calibration Verification of FWD deflection sensors.
FWD short-term repeatability verification
FWD long-term repeatability verification
Reference LVDT calibration procedure (Linear Variable Differential Transducer)
FWD deflection sensor calibration verification
FWD group field calibration procedure
FWD field calibration
When CBPC calibrates FWDs this is done on the basis of Protocols A, B, D and E, which normally
give a good and secure calibration result. (Protocols C, F, G are for more comprehensive analysis
programmes in which CBPC takes part every time the Study Committee P8 and CROW arrange
comparison tests among various FWD brands).
In stead of “just” using Protocol A, B, D and E, CBPC has taken a step further in the endeavour to
obtain the optimum calibration and thus the most precise and correct results.
CBPC Calibration Equipment
CBPC has developed calibration equipment, which works with the newest technology and principles
within this sphere. Furthermore the equipment has a state-of-the-art design, which makes it possible
to bring along the equipment to the customer to be used at site.
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BASIC THEORY
When a geophone (deflection sensor) is calibrated the problem is that the curve is not linear in the
low frequency range from 7-0 Hz.
Geophone frequency response
dB
Hz
50
40
30
20
10
0
-10
-20
-30
-40
-50
Geophone frequency
response
0.1
1
10
100
1000
This problem may be compensated for by adding a fast filter at a natural frequency of 4.5 Hz (- 3 dB),
which is similar but in opposite direction of the geophones own frequency characteristics (see below
figure).
Filter compensation
dB
Hz
50
40
30
20
10
0
-10
-20
-30
-40
-50
Geophone frequency
response
Filter compensation
0.1
1
10
100
1000
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In this way a resultant characteristic is obtained which is linear from approx. 0 Hz to 300 Hz. To
compensate for geophone unlinearity within the low frequency area in this way is not a new method
but a method, which has been used for many years.
Geophone frequency response with filter compensation
Hz
50
30
Geophone frequency
response
Filter compensation
dB
10
-10
Frequency response w ith
filter compensation
-30
-50
0.1
1
10
100
1000
CBPC differs from other FWD manufactures in that CBPC provides for the fact that geophones are
not uniform in their characteristics. The natural frequency has according to the geophone
manufacturer a tolerance of +/- 0.5 Hz. This means that it may vary from 4 to 5 Hz. It is very
important to consider this tolerance as it may give an inaccuracy on the peak value of up to 5%,
which will again result in a large inaccuracy when back calculating.
If the geophone has a natural frequency of 4 Hz and a filter compensation of 4.5 Hz is used, this
means that the frequency characteristic and thus the geophone output will be considerably wrong.
That is up to 5% too high.
Geophone frequency response with wrong filter compensation
dB
HZ
50
40
30
20
10
0
-10
-20
-30
-40
-50
Geophone frequency
response
Filter compensation
Frequency response w ith
filter compensation
0.1
1
10
100
1000
5
If a geophone has a natural frequency of 5 Hz and the filter compensation is 4.5 Hz, this will mean
that the frequency characteristic and thus the output of the geophone will be noticeably wrong. That
is up to 5% too low compared to the reference.
Geophone frequency response with wrong filter compensation
Hz
dB
50
40
30
20
10
Geophone frequency
response
Filter compensation
0
-10
-20
-30
-40
-50
Frequency response w ith
filter compensation
0.1
1
10
100
1000
EQUIPMENT FOR ABSOLUTE CALIBRATION
When CBPC makes an absolute geophone calibration, specially developed calibration equipment is
used that is the newest technique within this area.
The system consists of a flight case, which is easy and handy to transport worldwide and a Notebook
Personal Computer with Windows 95 software.
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The flight case is constructed around a
mini-shaker, which is capable of generating
pulses within the entire geophone
frequency and displacement ranges. A
highly accurate LVDT is mounted on the
mini-shaker, which is used for reference
and for control of the shaker’s fluctuation.
NoteBook Computer
MS Windows 95 sw.
Geophone 1-9
GEO
The control of the computer and the
calibration process is taken care of by a
CPU built into the flight
case.
Shaker
LVDT
CPU
A/D
D/A
AMP
FWD
Card
Converter
Converter
Amplifier
Analog Card
CBPC CALIBRATION PROCEDURE
When CBPC calibrates geophones (displacement transducers), three different calibration parameters
are used:
• Off set
• Cut off frequency
• Slope
The off set and the natural noise, which influences the analog card and the channel, and for which the
system compensates automatically. This means that in practice the value is 0.
Cut off frequency is exactly the frequency where the system shall compensate for the non-liniarity in
the low frequency range.
Slope is the amplification required to make geophone and calibration reference show the same peak
value.
These three software values have the great advantage that they do not change over time and the
current development of the calibration may be followed.
Calibration is done at a given rise time
(frequency) and peak value which may be
changed by the operator as required.
When the system has automatically found
offset, cut off frequency and slope, a test drop
is made with the found calibration data. The
operation may now decide whether the found
values shall be accepted.
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Normally the values will be within the below range:
OFFSET
CUT OFF FREQ.
SLOPE
Min.
- 100
4.0
10
Max.
+ 100
5.2
12
ADC
Hz
gg
The found calibration values may now be tested in various ways to find out if they are acceptable. A
number of test drops may be made to check that “max. deviation ratio at peak” does not exceed a
fixed value. Normally the deviation from the reference is below 0.1%. This test does however not
indicate whether the cut off frequency is correct but only whether the slope is correct. In order to
check the cut off frequency a frequency response test must be made.
Frequency response
After a correct calibration a frequency response should
look like this response.
Under frequency response it is possible to see which
frequency characteristic the geophone has within the
range of 0.4-450 Hz.
Here is the same geophone shown with the same
calibration parameter (offset and slope) but without
compensation filter.
As it clearly appears, the geophone frequency response is
no longer linear.
Here is the same geophone again with the same
calibration parameter. However the cut off frequency has
been changed with -0.5 Hz.
As it appears, the geophone becomes unlinear from
approx. 7 Hz.
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Here is the same geophone again with same calibration
parameter. However cut off frequency has been
changed with +0.5 Hz.
As it appears, the geophone is unlinear from approx. 7
Hz but this time in the opposite side.
Peak value
In the below table it appears how large an influence a correct cut off frequency has on the peak value.
Peak value test at diff. cut off frequencies
Cut off frequency
Hz.
Correct cut off
frequency at 4.75 Hz.
Incorrect cut off
frequency at 4.0 Hz.
Incorrect cut off
frequency at 5.2 Hz.
Drop.
No.
1
2
3
4
5
6
7
8
9
10
REFERENCE
µ m.
497
501
500.6
499.4
500.4
499.1
500
500.2
500.7
500.7
GEOPHONE
µ m.
497.1
500.9
500.2
498.8
500.4
498.7
500.6
500.3
499.8
499.9
DEVIATION
µ m.
-0.1
0.1
0.4
0.6
0
0.4
-0.6
-0.1
0.9
0.8
DEVIATION
%
-0.02
0.02
0.08
0.12
0.00
0.08
-0.12
-0.02
0.18
0.16
0.05
1
2
3
4
5
6
7
8
9
10
499.4
499.6
499.4
498.9
500.7
500.4
500.6
499.4
499.8
499.9
478.3
478.7
478.7
478.3
479
479.3
478.7
478
478.1
478.4
21.1
20.9
20.7
20.6
21.7
21.1
21.9
21.4
21.7
21.5
4.23
4.18
4.14
4.13
4.33
4.22
4.37
4.29
4.34
4.30
4.25
1
2
3
4
5
6
7
8
9
10
500.5
499.2
499.4
500.6
499.2
499.6
501.1
500.3
500.3
498.8
512.6
511.4
511.5
512.4
512.3
512.4
513.6
512.6
512.5
511.6
-12.1
-12.2
-12.1
-11.8
-13.1
-12.8
-12.5
-12.3
-12.2
-12.8
-2.42
-2.44
-2.42
-2.36
-2.62
-2.56
-2.49
-2.46
-2.44
-2.57
-2.48
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Linearity test
After the frequency test the system automatically performs a linearity test in accordance with the
conditions set forth in C.R.O.W. protocol E. The linearity test gives a good picture of the quality of
the calibration as more combinations of rise time and amplitude for the pulse are tested.
Data analysis software
CBPC has also developed an advanced user-friendly software which can be used for data analysis and
manipulation.
For many years it has not been quite clear what time history data could be used for. So far only
peak/max. values have been used for back calculation. However a tool is now available, which makes
it easy and quick to make FFT analyses (Fast Fourier Transformation - frequency analyses) and in
this way see what the frequency content of a drop and the peak value.
Another facility of the tool is low and high pass filter. These filters may among other things be used
for noise filtering of time history signals or for comparison with data from other FWD manufacturers
using filter manipulation in data collection software (see below).
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CBPC finds that original signals should not be filtered before presenting it. This is a possibility when
analysing data. In this way it is always possible to return to the “raw” original signals.
The CBPC analysis software has been fully integrated with the CBPC data collection software and
may automatically be integrated with Microsoft EXCEL spreadsheets.
RECOMMENDATIONS
With more than 20 years experience as a manufacturer of falling weight deflectometers and as a user
of back calculation CBPC has learned how important it is to have data collection equipment (FWD)
that measures as accurately as possible. The data collected by the FWD should not “only” be
approximations, which are e.g. added a lot of filters (60 Hz) in order to have equipment with a
reasonable repeatability performance.
CBPC is of the opinion that the data collected by a FWD shall be as realistic as possible. That means
without deliberate data manipulation. The only filters and compensation that are accepted are those
required by the physics and mathematics in order to present correct data. As a consequence it is vital
to know not only the off set and amplification as calibration parameters but also the cut off frequency
varies from geophone to geophone due to the way geophones are constructed. These values may vary
up to 1 Hz, which is of utmost importance for the output. If the FWD (only the types using velocity
transducers) does not consider this parameter, which is the case for much equipment, CBPC strongly
recommends that precautions are taken (maybe via the FWD manufacturer) to provide for this
inaccuracy of the geophone in future.
REFERENCES
E. BEUVING, CROW Preliminary Guidelines for calibration of falling weight deflectometers,
C.R.O.W. Centre for Research and Contract Standardization in Civil and Traffic Engineering.
SENSOR NEDERLAND BV, SM 6 Miniature digital grade long travel geophone, Input/output inc.
specifications for SM-6 geophones.