452 ZambranoOtero

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11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic
Magnetic Flux Leakage Measurement System
to Detect Flaws in Small Diameter Metallic Wire Ropes
Maria X. ZAMBRANO1, Ane MARTÍNEZ-DE-GUERENU2, Fernando ARIZTI2
1
CEIT
CEIT and Tecnun (University of Navarra)
Manuel de Lardizabal 15, 20018 San Sebastián, Spain
Phone: +34 943 212800, Fax: +34 943 213076;
e-mail: [email protected], [email protected], [email protected].
2
Abstract
This paper describes a magnetic flux leakage measurement system that has a magnetizing circuit fitted to the
geometry of a special wire rope with a reduced diameter (approximately 5 mm in its metallic part) and a plastic
sheath. The ability of the system to detect flaws was assessed by means of artificial flaws, and certain parameters
related to its functioning were analysed and compared with other instruments reported in the literature. The
results show that the system is able to detect flaws on the surface of the special wire rope with very good
sensitivity, resolution, signal-to-noise-ratio and repeatability relative to other instruments; moreover, there was
no considerable effect of the test speed on the measurements. With these features, the measurement system
becomes a useful non-destructive testing tool, which could be used to count the number of broken wires on the
surface of wire ropes with reduced diameter, as well as studying the fatigue to which these kind of wire ropes are
subjected in different applications.
Keywords: Electromagnetic testing (ET), magnetic flux leakage (MFL), wire rope, flaw, non-destructive testing.
1
Introduction
Steel wire ropes are structural elements used in many areas, such as transport systems (e.g.
elevators, ski lifts), the mining industry, construction and other applications. In the area of
transport systems, steel wire ropes that have a special structure with a reduced diameter have
emerged in response to the current trend in elevator design of minimizing the space required
to install an elevator. Therefore, currently it is very common to use direct drive motors, which
provide lower torques, rather than the traditional geared motors. As a result of this trend, the
diameter of the pulley and therefore the diameter of the steel wire rope has been reduced to
compensate for the limitation of torque and also to ensure the elevator operates properly.
The steel wire ropes in an elevator deteriorate with time because they are subjected to various
bending, tensile stresses, in addition to stresses due to the wire rope’s contact with other
elements of the elevator, such as pulleys, which can cause the wire rope to break or fail. For
this reason, monitoring and controlling their integrity is a major concern for manufacturers,
users, owners and safety authorities. A wide variety of instruments described in the literature
for inspecting and detecting flaws in metallic wire ropes [1-5] employ the magnetic flux
leakage (MFL) electromagnetic non-destructive testing method. But there are only a few
instruments, such as the ones developed by Intron Plus, Rotesco, AMC Instruments and
Laboratory Roman Martyna, among others, which employ the same technique to inspect
special wire ropes with a reduced diameter (less than 10 mm) [1-3].
As there are very few instruments available to inspect special wire ropes with a reduced
diameter (approximately 5 mm in its metallic part) and a plastic sheath, the aim of this work is
to show that a MFL instrument with a magnetizing circuit fitted to the geometry of a special
wire rope with a reduced diameter provides very good sensitivity and signal-to-noise ratio for
detecting flaws. The system could be used in the future to count the number of broken wires
1/10
on the surface of special wire ropes and to study the fatigue to which these kind of wire ropes
are subjected in different applications.
The paper is organised as follows: first, the measurement system and the samples used to test
it are described, then the ability to detect flaws is assessed, and finally certain parameters
(such as sensitivity, resolution, signal-to-noise-ratio and repeatability and the effect of the test
speed on the sensitivity) related to its functioning are analysed and compared with other
instruments reported in the literature. The measurement system yields better results than do
other instruments used to inspect wire ropes with larger diameters.
2
Description of the magnetic flux leakage measurement system
This paper describes and analyses a measurement system for inspection of localized faults
(LF) by means of the MFL method. The operating principle of the MFL method for detecting
flaws in metallic wire ropes is illustrated in Figure 1a. In this method, the wire rope is
homogeneously magnetized close to saturation in the longitudinal direction of the steel wire
rope. Broken wires, abrasion or corrosion will generate localized discontinuities in the
magnetic flux lines through the wire rope [6]. By using magneto resistive sensors located near
the surface of the wire rope, changes in the magnetic leakage fields are detected, recorded and
analysed to detect broken wires.
A schematic representation of the measurement system implemented is shown in Figure 1b. It
is composed of a set of powerful permanent magnets, which are supported by cross-shaped
metallic blocks. By using a set of yokes, the permanent magnets are joined to create a closed
magnetic circuit around the wire rope; further details about the geometry and dimensions of
the measurement system can be found in [7].
The measurement system has two groups of magneto resistive sensors, which are used to
measure the magnetic leakage fields near the wire rope surface. Each group is formed by four
sensors located around the perimeter of the wire rope and separated from each other by a 90º
angle. One group is used to measure the longitudinal component of the magnetic field (HL in
Figure 2). The sensors in this group will be identified as longitudinal sensors in what follows.
The other group is used to measure the radial component of the magnetic field (HR in
Figure 2), and they will be identified as radial sensors in what follows.
AA003-02 sensors from the manufacturer NVE are used to measure the radial component of
the magnetic field. They have a linear response in the range of 2 to 14 Oe, and beyond 20 Oe
they saturate. AA005-02 sensors from the same manufacturer are used to measure the
(a)
(b)
Figure 1. (a) Operating principle of the MFL method and (b) sketch of the measurement system
2/10
(a)
Figure 2. Direction of the longitudinal (HL)
and radial (HR) components of the magnetic
leakage fields near the wire rope surface
(b)
Figure 3. (a) Position of the electrode to
machine artificial flaws, and (b) definition of
the dimensions
longitudinal component of the magnetic field, and they have a linear response in the range of
10 to 70 Oe and saturate beyond 100 Oe.
Signals generated by the sensors are amplified and filtered before being acquired. Data
acquisition is carried out by means of a USB-6211 data acquisition card (DAQ), which is
controlled by an application implemented in LabVIEW software. Finally, the signals are
processed with MATLAB software.
3
Samples used to test the measurement system
The rope analysed in this study is a stranded steel wire rope covered with a plastic sheath. The
nominal diameter of the rope (including the cover) is 6.5 mm, and the nominal diameter of the
metallic part is approximately 5 mm. The steel wire rope consists of 6 Seale strands in the
outer layer and an independent wire rope core in the inner layer. Several wire rope samples
with different features were used in this study, and they are described below:
3.1
Sample 1
Sample 1 is a wire rope without a plastic sheath and with 15 artificial flaws, which were
machined via electrical discharge machining (EDM) and with a distance of 170 mm between
consecutive flaws. Figure 3a shows the position of the electrode used to machine a generic
flaw of size “a” x “b” mm, shown in Figure 3b. The red arrow indicates the direction of
penetration, and the red shaded area represents an estimation of the amount of removed
material. Flaws with width values (“a”) of 0.8 and 1 mm and depth values (“b”) between 0.3
and 1 mm were machined.
3.2
Sample 2
Sample 2 is a wire rope without a plastic sheath and with 24 artificial flaws, which were
machined by using wire EDM and with a distance of 170 mm between consecutive flaws.
Flaws with width values (“a”) between 0.25 and 1.5 mm and depth values (“b”) of 0.25 and
0.3 mm were machined.
3.3
Sample 3
Sample 3 is a wire rope with a plastic sheath and 5 artificial flaws, which were machined by
using a cutting tool. These flaws were machined with a distance of 1 m between consecutive
flaws. It was not possible to control the width and the depth of the flaws made by the cutting
tool, but after cutting the number of broken wires were counted and the amount of loss section
that these broken wires represent in each flaw was calculated. These values are listed in
Table 1.
3/10
Table 1. Features of the artificial flaws machined in sample 3
Number of
Percentage of
Flaws
broken wires
loss section (%)
1
17
11.69
2
8
5.51
3
6
4.13
4
8
5.51
5
12
8.27
4
Performance of the measurement system
4.1
Detection of broken wires
In order to verify the ability of the measurement system to detect flaws, tests were carried out
on sample 1 with flaws machined on the surface. Figure 4a and Figure 4b show the amplitude
of the signals of the four longitudinal and the four radial sensors, respectively, obtained in a
section of sample 1 containing 9 flaws. It can be seen that all flaws were detected by a pulse
in the magnetic signal with at least one of the four longitudinal and the four radial sensors.
Large flaws, such as flaw 9 (depth value of 1 mm), were detected by all longitudinal and
radial sensors; while the flaws with the smallest sizes, such as flaw 2 (depth value of
0.3 mm), were only detected by the sensors located closer to them, in this case the ones in the
90º angle position. It is important to point out that the flaws were detected by pulses with
different amplitude values, which depended both on the size of the flaws and how close they
were from each of the sensors.
The results show that there is a clear relation between the size of the flaws and the number of
sensors that are able to detect the flaws, and there is also a relation between the size and the
position of the flaws and the amplitude of the pulses obtained. These results are consistent
with those in [4], which simulated wire ropes of a diameter greater than 10 mm. A flaw in a
wire rope produces amplitude fluctuations in the longitudinal and radial components of the
flux density inside the wire rope, which are reflected in different angular positions around the
0º
0.4
0.2
2
3
4
5
6
7
8
0.5
9
4000
4200
4400
4600
4800
0.5
0
3800
180º
4000
4200
4400
4600
4800
5000
0.2
0
0.4
3
4
5
6
7
8
9
3800
270º
-0.5
5000
4000
4200
4400
4600
4800
5000
Output voltage (V)
Output voltage (V)
3800
90º
0.2
0.4
2
0
0
0.4
0º
1
1
3800
90º
4000
4200
4400
4600
4800
5000
3800
180º
4000
4200
4400
4600
4800
5000
3800
270º
4000
4200
4400
4600
4800
5000
3800
4000
4200
4400
4600
Wire rope length (mm)
4800
5000
0
-0.5
0.5
0
-0.5
0.5
0
0.2
0
3800
4000
4200
4400
4600
Wire rope length (mm)
4800
5000
-0.5
(a)
(b)
Figure 4. Magnetic flux leakage field measured by the (a) four longitudinal sensors and (b) four
radial sensors, obtained in a section of sample 1 containing flaws 1-9
4/10
wire rope. The maximum fluctuation is produced in the angular position where the flaw is
located [4]. As a consequence, it is expected that larger size flaws would be detected by all
sensors, while smaller size flaws would only be detected by the sensor located in front of or
near the flaw.
The quantitative analysis of the performance of the measurement system was carried out
based on the signals resulting from the sum of the four longitudinal sensors, which will be
identified as VLsum, and from the sum of the signals of the four radial sensors, which will be
identified as VRsum (see Figure 5a and Figure 5b). This way the relation between the size and
position of the flaw and the amplitude of the pulses obtained is considered in the analysis, and
the information provided by all sensors, regardless of what sensor is able to detect each flaw,
is taken into account.
4.2
Sensitivity
The sensitivity of the measurement system was determined by establishing a threshold value
in the signals of the longitudinal and radial sensors, as it has been considered in [8].
Figure 6a and Figure 6b show the amplitude of the longitudinal (VLsum) and radial (VRsum)
signals, respectively, when measuring a section of sample 2.
On the one hand, it can be seen that flaw 7 is the smallest flaw that produces a pulse with an
amplitude value higher than the threshold value in the longitudinal signals (Figure 6a). This
flaw has only a broken wire and represents a small section loss of 1.02 %. On the other hand,
it can be seen that in the radial signals (Figure 6b) flaw 16 is the smallest flaw that produces a
pulse with negative amplitude followed by a pulse with positive amplitude, and both of them
have amplitude values that surpass the threshold values. This flaw has only three broken
wires, which represent a small section loss of 2.04 %.
It should be noted that the diameter of the wire rope inspected with the measurement system,
is significantly smaller than the diameter of wire ropes inspected with some instruments
1
VLsum (V)
1
2
3
4
5
6
7
8
9
0.5
0
-0.5
3800
4000
(a)
1.5
1
2
3
4200
4400
Wire rope length (mm)
4
5
6
4600
7
4800
8
5000
9
VRsum (V)
1
0.5
0
-0.5
-1
-1.5
3800
4000
4200
4400
4600
4800
5000
Wire rope length (mm)
(b)
Figure 5. Signals resulting (a) from the sum of the four longitudinal sensors (VLsum), and (b) from
the sum of the four radial sensors (VRsum), in a section of the sample 1 containing the flaws 1 – 9
5/10
0.4
1
0.5
0.2
VRsum (V)
VLsum (V)
0.3
0.1
0
0
-0.5
-0.1
6
-0.2
0
7
200
14
8
9
10
11
-1
400
600
800
1000
1400
15
1600
16
17
18
19
1800
2000
2200
2400
Wire rope length (mm)
Wire rope length (mm)
(b)
(a)
Figure 6. (a) Longitudinal (VLsum) signal and (b) radial (VRsum) signal, in a section of sample 2
containing 6 flaws
reported in the literature. An example of these instruments is the one described in [9, 10],
which is able to inspect wire ropes of a diameter of 38 mm with a sensitivity of a flaw of
1 mm in width and 2 mm in depth, which represents a section loss of 5.26 % of the wire rope
section. Another example is the instrument described in [11], which is able to inspect wire
ropes of 64 mm diameter with a sensitivity of a flaw of a depth of 2 mm, which represents a
section loss of 3.12 % of the wire rope section. Therefore, the present measurement system
has very good sensitivity in both groups of sensors, which allows the detection of flaws as
small as only one broken wire, which represents a low percentage of section loss (1.02 %), in
wire ropes with a reduced diameter (approximately 5 mm in its metallic section).
4.3
Resolution
In order to quantify the resolution of the measurement system as it has been defined in [9, 10],
some tests were carried out with a sample without a plastic sheath and with three flaws of
0.50 mm in width and 0.30 mm in depth. The flaws were machined by using wire EDM, with
a distance of 6 mm between consecutive flaws, which placed each flaw in consecutive
strands. Figure 7a shows an image of the three flaws, and Figure 7b shows the longitudinal
(VLsum) signal obtained for the three flaws, where it is possible to identify three pulses, which
correspond to the three flaws. This shows that the measurement system has a resolution of
6 mm for a wire rope with a diameter of approximately 5 mm in the metallic part.
4.4
Signal to noise ratio (SNR)
The signal-to-noise ratio (SNR) of the measurement system for detecting flaws was
determined by analysing wire rope sections with and without flaws as in [4, 9, 12, 13]. The
VLsum (V)
1
12
0.5
3
0
-0.5
680
700
720
740
760
780
Wire rope length (mm)
(a)
(b)
Figure 7. (a) Image of three flaws machined in a sample without plastic sheath, and
(b) longitudinal (VLsum) signal obtained for the three flaws
6/10
lay-noise or background noise produced by the geometry of the wire rope was measured in a
section of the wire rope without flaws. In the longitudinal (VLsum) and radial (VRsum) signals
the lay-noise — identified as (Vn) and (Vn(p-p)) — varies between ± 50 mV and ± 150 mV,
respectively.
Figure 8a and Figure 8b show, respectively, the longitudinal (VLsum) signal in a section of
sample 2 where there are 11 flaws, and the radial (VRsum) signal in a section of sample 1
where there are 10 flaws. Flaws 6, 7, 8, 9, 11, 14 and 15 are the smallest ones in the section of
the wire rope shown in Figure 8a. Each one has a broken wire in the wire rope surface, which
represents a section loss of 1.02 % (except for flaw 7, which only has a worn wire). These
flaws produce pulses with amplitude values between 160 and 300 mV with a mean value
(VLp) of 210 mV in the longitudinal (VLsum) signal; thus, the SNR of the longitudinal signal
(SNRlong) to detect the smallest flaw in that section is:
SNRlong =
VLp
Vn
=
210 mV
= 4.38 , SNRlong (dB ) = 20 ⋅ log10 (SNRlong ) = 12.82 dB
50 mV
(1)
Flaw 12 is the smallest in the section of the wire rope shown in Figure 8b. It has three broken
wires that represent a section loss of 2.40 % and produce a peak-to-peak value (VR(p-p)) of
820 mV in the radial (VRsum) signal. Therefore, the SNR of the radial signal (SNRrad) to detect
the smallest flaw in that section is:
SNRrad =
VR ( p − p ) 820 mV
=
= 2.73 , SNRrad (dB ) = 20 ⋅ log10 (SNRrad ) = 8.73 dB
Vn ( p − p ) 300 mV
(2)
European Standards [14] establish that the amplitude of pulses obtained by means of LF
instruments must be at least two times superior to the lay-noise; thus, taking into account this
criterion, the measurement system has a proper SNR for detecting flaws. The SNR is in the
same range (between 1 and 5) as the instrument described in [10], but it should be noted that
0.8
VLsum (V)
0.6
0.4
0.2
0
-0.2
0
6
7
200
8
9
400
10
600
11
12
13
800
1000
1200
Wire rope length (mm)
(a)
14
1400
15
1600
16
1800
2000
1.5
VRsum (V)
1
820 mV
0.5
0
-0.5
-1
-1.5
5000
6
7
5200
5400
8
9
5600
12
13
14
15
5800
6000
6200
Wire rope length (mm)
10
11
6400
6600
6800
7000
(b)
Figure 8. (a) Longitudinal (VLsum) signal in a section of sample 2 where the flaws 6 - 16 are located,
and (b) radial (VRsum) signal in a section of sample 1 where the flaws 6 - 15 are located
7/10
the measurement system has better sensitivity, as it is capable of detecting a smaller
percentage of section loss (1.02 % and 2.04 % against 5.26 % in [10]) in a wire rope with a
smaller diameter (approximately 5 mm in the metallic part against 38 mm in [10]).
4.5
Repeatability
To verify the degree of consistency and to assess the reliability of the measurements obtained
with the system, the repeatability of the measurement system was evaluated with sample 3.
This sample was tested six times on the same day, and under the same conditions; the
longitudinal (VLsum) and radial (VRsum) signals obtained in all repetitions were compared and
the accuracy of the measurement was also assessed.
For each flaw in sample 3, the mean value of the amplitude of the pulses produced, the
standard deviation (σ) and the σ in percentage with respect to the mean value of the
measurements were calculated. Table 2 shows the values of the pulses obtained in the
longitudinal (VLsum) and radial (VRsum) signals. According to the size of the flaws analysed
(see Table 1), measurements carried out with the longitudinal and the radial sensors with a
large number of broken wires may vary up to 8 and 6 %, respectively.
4.6
Effect of test speed on the sensitivity of the measurement system
The effect of the test speed on the sensitivity of the measurement system was estimated by
measuring the amplitude of the pulses produced by three flaws from sample 2, tested at
different speeds between 1 – 3 m/s, with a step of 0.5 m/s. Figure 9a and Figure 9b show the
amplitude of the pulses in the longitudinal (VLsum) and radial (VRsum) signals under these
conditions. Table 3 shows the values of the pulses in the longitudinal and radial signals
respectively, along with the mean value, σ (in volts) and σ (in percentage) with respect to the
mean value of the amplitude of the pulses. It can be seen that there is no clear correspondence
between the amplitude of the pulses and the test speed. The percentage of variation in the
amplitude of the pulses due to changes in the test speed is less than 5 % in both the
longitudinal and the radial signals.
The results show that the test speed in the range 1 – 3 m/s does not have a significant effect on
the magnetic flux leakage measurements. This is due to the magnetoresistive sensors used in
the present measurement system, as they provide an absolute measurement of the magnetic
field according to changes in the electrical resistance of the material inside them, no matter
what the test speed is. The behaviour of these kinds of sensors represents an advantage over
instruments with coil sensors, such as the ones described in [3, 9, 10, 15], in which the test
speed has a negative effect in that the signal amplitude is reduced with an increase in the test
Table 2. Mean value and standard deviation data for the pulses obtained in the longitudinal
(VLsum) and radial (VRsum) signals due to the flaws in sample 3
Longitudinal signal
Radial signal
Flaws Mean value
Mean value
σ (V)
σ (%)
σ (V)
σ (%)
(V)
(V)
1
2.15
0.050
2.32
3.03
0.027
0.8
2
1.27
0.035
2.75
1.40
0.050
3.57
3
0.65
0.036
5.35
0.97
0.031
3.19
4
1.03
0.081
7.86
1.01
0.059
5.84
5
1.57
0.097
6.17
2.06
0.112
5.43
8/10
0.8
1.2
VRsum (V)
VLsum (V)
0.7
1.4
Flaw 22
Flaw 23
Flaw 24
0.6
0.5
0.4
0
Flaw 22
Flaw 23
Flaw 24
1
0.8
0.6
1
2
3
Test speed (m/s)
0.4
0
4
1
2
3
4
Test speed (m/s)
(a)
(b)
Figure 9. Amplitude of the pulses in the (a) longitudinal (VLsum) signal, and (b) radial (VRsum)
signal, in a section of sample 2 where flaws 22 – 24 are located
Table 3. Amplitude of the pulses obtained in the longitudinal (VLsum) and radial (VRsum) signals,
in a section of sample 2 containing flaws 22 - 24
Pulse amplitude, long. signal (V)
Positive pulse amplitude, radial signal (V)
Test speed
(m/s)
Flaw 22
Flaw 23
Flaw 24
Flaw 22
Flaw 23
Flaw 24
1.0
0.489
0.640
0.504
0.575
0.804
0.942
1.5
0.499
0.640
0.496
0.627
0.841
1.005
2.0
0.501
0.646
0.510
0.612
0.844
1.017
2.5
0.524
0.643
0.506
0.615
0.843
1.016
3.0
0.522
0.630
0.504
0.597
0.843
1.020
Mean (V)
σ (V)
σ (%)
0.507
0.015
2.98
0.640
0.0058
0.90
0.504
0.0053
1.06
0.605
0.019
3.30
0.835
0.017
2.10
1.000
0.032
3.27
speed. Thus, with the measurement system described here it is possible to inspect wire ropes
at different test speeds without having to compensate for the amplitude of the signals.
5
Conclusions
The results demonstrate that it is possible to detect flaws on the surface of a wire rope with a
very small diameter (approximately 5 mm) by means of the magnetic flux leakage testing
technique by using the measurement system described in this work.
The measurement system´s sensitivity is equivalent to a section loss of 1.02 % and 2.04 % of
the wire rope section in the longitudinal and radial signals, respectively, and has a resolution
of 6 mm. The repeatability of the measurements varies up to 8 and 6 % in the longitudinal and
radial signals. The SNR of the longitudinal and radial signals to detect the smallest flaw is
4.38 and 2.73, respectively; these SNR values are acceptable for detecting flaws according to
European Standards. The test speed in the range 1 - 3 m/s has no effect on the measurements.
Finally, all the features of the measurement system make it a useful non destructive testing
tool, which could be used to count the number of broken wires on the surface of wire ropes
with reduced diameter. Moreover, it could be used to study the fatigue to which these kinds of
wire ropes are subjected in different applications.
9/10
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