Note: Descriptions are shown in the official language in which they were submitted.
CA 02779226 2012-06-07
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File No. P2031CA00
PULSED EDDY CURRENT SENSOR FOR PRECISION LIFT-OFF
MEASUREMENT
BACKGROUND
(a) Field
[0001] The subject matter disclosed generally relates to non-
destructive
testing (NDT). More particularly, the subject matter relates to NDT using
pulsed
eddy currents (PEC).
(b) Related Prior Art
[0002] Metallic surfaces of engineered systems such as,
without
limitations, aircraft systems, naval systems, nuclear systems, oil and gas
systems, and the like, often require inspection of the surfaces for the
purposes
of, without limitations, precise geometric characterization, corrosion
detection,
crack detection, water ingress, coating delamination, impact damage
assessment, and the like.
[0003] A range of non destructive inspection (NDI)
technologies, including
metrology instruments, are available in the industry for such measurements.
However, the metal surface is frequently hidden by thin coatings such as
paints
applied for corrosion protection, or thick coatings applied for, without
limitations,
insulation, radar and/or sonar absorption, damage protection, aesthetics and
the
like. Thus, thick surface coatings block access to the metallic surfaces for
metrology instruments requiring physical contact, and block the line-of-sight
required for non-contacting instruments.
[0004] Typically, inspection of metal under thick coatings
requires removal
of the coating to expose the metal substrate as current technology cannot
effectively execute a measurement with the coating in place. Such removal and
replacement is costly and discourages or restricts thorough wide-area
inspection.
[0005] In recent years, various new metrology technologies
have matured
to address the need for improved field deployability, simplicity and precision
in
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the process of satisfying the geometric characterization needs of maintenance
planners and engineering analysts. Systems using, without limitations, lasers,
photogrammetric triangulation, and other principles are commercially
available.
Requiring either line-of-sight or physical contact with the measurement point,
use
of these systems to measure points on a surface beneath a thick coating can at
best measure a coordinate on the coating above the actual point of interest.
[0006] Although various technologies are available for inspections when
the metal surface is exposed by removal of the coating, no solution is
commercially available for inspection through thick coatings for the surface
topology measurements at high resolution.
[0007] For example, to provide stealth during operations, the bulk of the
outside surface of submarines is covered with specially designed anechoic
tiles.
These tiles are engineered to absorb sound energy originating from inside the
submarine and to damp reflections of sound generated by external sonar
detection systems. Unfortunately, the presence of anechoic tiles obviously
complicates inspections of the substrate structural hull, such as routine
circularity
checks and evaluation of hull surface condition. Random variations in
thickness
of the tile and applied adhesive epoxy layer preclude any prospect of
extrapolation of the shape of the steel hull from the external shape of the
tiled
submarine to the required precision (i.e., within 1 mm).
[0008] Furthermore, occasional disbond of the tiles from the hull surface
results in tile deformations, which further impact external measurements.
Tiles
may also conceal hull surface conditions arising from corrosion, mechanical
damage, fabrication flaws or weld boundaries. The circularity measurement and
inspection requirements are currently addressed by partial tile removal. The
cost
of anechoic tile removal and replacement, as well as time spent in dry dock
for
submarines undergoing maintenance checks, provides an incentive to apply a
through-tile inspection technique. However, the sound absorbing nature of the
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tiles hinders ultrasonic methods, combined with the tile-thickness challenge,
severely limits the number of available non-destructive evaluation methods
that
can potentially provide accurate through-tile thickness or lift-off distance
measurements.
[0009] Precise
outer hull surface measurements can be achieved with a
number of available spatial measurement technologies, including the Laser
Tracked Ultrasonic Technology (LTUT), laser tracker, laser radar, or
photogrammetry. In order to achieve an exact topology of the steel submarine
hull beneath the tiles, an accurate measurement of the perpendicular distance
from the submarine hull surface to the outer tile surface, i.e., lift-off
distance is
required. Such a dimension is necessary to affect a strategy of mathematical
correction to a
conventionally measured tile-surface or super-surface
coordinates in order to characterize the true metal pressure-hull substrate.
[0010] In
general, signal response in eddy current testing is always
sensitive to lift-off, which is defined as the vertical distance of a probe to
a
conducting surface. Conventional eddy current technology is commonly used for
high-accuracy thickness measurement of paint, cladding of different metals, or
corrosion product thickness, but the range of sensitivity is currently limited
to
several mm. Sensitivity to lift-off decays exponentially, but nominally scales
with
the size of the probe diameter. Therefore, a larger diameter may be used to
compensate for this loss.
[0011]
Inspection through thick ( 10 mm) coatings for the surface
topology measurements at high resolution cannot be done with current state of
the art devices. One system is available to measure simple linear lift-off
distance
(offset) over a flat metal surface through a thick low conducting coating.
There
are a few commercially available systems capable of gross identification of
corrosion patches under thick coatings. Various technologies are available for
inspections when the metal surface is exposed by removal of the coating.
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[0012] There is therefore a need for an improved eddy current sensor for
precision lift-off distance measurements.
SUMMARY
[0013] According to an embodiment, there is provided a pulsed eddy
current sensor for measurement of a lift-off distance over a surface of a
metallic
substrate (a metallic surface), the pulsed eddy current sensor comprising: a
primary excitation coil to which are applied voltage pulses for generating
primary
magnetic fields, wherein an interaction of the primary magnetic fields with
the
metallic substrate produces secondary magnetic fields; and secondary pick-up
probes, each of the secondary pick-up probes located at a different vertical
distance from the metallic surface, the secondary pick-up probes used for
measuring the primary and secondary magnetic fields and for producing a
differential signal that is representative of the lift-off distance.
[0014] According to another embodiment of the sensor, the secondary
pick-up probes are provided within the primary excitation coil.
[0015] According to another embodiment of the sensor, the secondary
pick-up probes are vertically aligned.
[0016] According to another embodiment of the sensor, the primary
excitation coil comprises one or more coaxial excitation coils.
[0017] According to another embodiment of the sensor, the secondary
pick-up probes comprise a pair of secondary pick-up coils.
[0018] According to another embodiment of the sensor, one of the
secondary pick-up coils is wound in a first direction and the other one of the
secondary pick-up coils is wound in a second direction, opposite to the first
direction, thereby subtracting a signal induced in the one of the secondary
pick-
up coils from another signal induced in the other one of the secondary pick-up
coils thereby producing the differential signal.
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[0019] According to another embodiment of the sensor, both secondary
pick-up coils are wound in the same direction, but connected with opposing
leads
thereby subtracting a signal induced in the one of the secondary pick-up coils
from another signal induced in the other one of the secondary pick-up coils
thereby producing the differential signal.
[0020] According to another embodiment of the sensor, the secondary
pick-up probes are connected separately to a differential amplifier that
subtracts
a signal induced in the one of the secondary pick-up probes from another
signal
induced in the other one of the secondary pick-up probes thereby producing the
differential signal.
[0021] According to another embodiment of the sensor, wherein the
secondary pick-up probes are connected together at a fixed distance and the
pulsed eddy current sensor further comprises an adjustment mechanism for
vertically adjusting the position of the secondary pick-up probes within the
primary excitation coil for substantially balancing the secondary pick-up
probes.
[0022] According to another embodiment, the sensor further comprises an
adjustment mechanism for adjusting a distance between the secondary pick-up
probes for substantially balancing the secondary pick-up probes.
[0023] According to another embodiment, the sensor further comprises a
ferrite core provided within each one of the secondary pick-up probes.
[0024] According to another embodiment, there is provided a pulsed eddy
current sensing system for measurement of a lift-off distance over a surface
of a
metallic substrate (a metallic surface), the pulsed eddy current sensing
system
comprising: a pulse generator for generating voltage pulses; a primary
excitation
coil to which are applied voltage pulses for generating primary magnetic
fields,
wherein an interaction of the primary magnetic fields with the metallic
substrate
produces secondary magnetic fields; and secondary pick-up probes, each of the
secondary pick-up probes located at a different vertical distance from the
metallic
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surface, the secondary pick-up probes used for measuring the primary and
secondary magnetic fields and for producing a differential signal that is
representative of the lift-off distance.
[0025] According to another embodiment, the pulsed eddy current sensing
system may further comprise an amplifier and a filter for removing high
frequency
noise components from the differential signal.
[0026] According to another embodiment, the pulsed eddy current sensing
system may further comprise a local degaussing circuit for neutralizing a
residual
magnetization in the metallic surface.
[0027] According to another embodiment, the pulsed eddy current sensing
system may comprise separate amplifiers for each secondary pick-up probe for
substantially balancing the secondary pick-up probes.
[0028] According to another embodiment of the system, the secondary
pick-up probes are provided within the primary excitation coil.
[0029] According to another embodiment of the system, the secondary
pick-up probes are vertically aligned.
[0030] According to another embodiment, there is provided a method for
mapping a metallic surface under a low conductivity coating, a distance from
the
top of the low conductivity coating to the metallic surface defining a lift-
off
distance. The method comprises: performing a series of measurements of the
lift-
off distance (depth) at reference positions covering two dimensions (latitude
and
longitude) over the low conductivity coating; and integrating the series of
measurements with the reference positions to produce a three-dimensional map
of the metallic surface under a low conductivity coating.
[0031] According to another embodiment of the method, the performing of
one of the series of measurements at one of the reference positions comprises:
generating a primary magnetic field centered on a vertical axis normal to the
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metallic surface at the one of the reference positions, wherein an interaction
of
the primary magnetic field with the metallic surface produces a secondary
magnetic field; measuring the primary and secondary magnetic fields at two
distinct vertical positions on the vertical axis; and from the measured
primary and
secondary magnetic fields, calculating a differential signal that is
representative
of the lift-off distance.
[0032] The term "dielectric" is intended to mean an electrical insulator
that
can be polarized by an applied electric field. When a dielectric is placed in
an
electric field, electric charges do not flow through the material as they do
in
a conductor, but only slightly shift from their average equilibrium positions
causing dielectric polarization. Because of dielectric polarization, positive
charges are displaced toward the field and negative charges shift in the
opposite
direction. This creates an internal electric field which reduces the overall
field
within the dielectric itself.
[0033] Features and advantages of the subject matter hereof will become
more apparent in light of the following detailed description of selected
embodiments, as illustrated in the accompanying figures. As will be realized,
the
subject matter disclosed and claimed is capable of modifications in various
respects, all without departing from the scope of the claims. Accordingly, the
drawings and the description are to be regarded as illustrative in nature, and
not
as restrictive and the full scope of the subject matter is set forth in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Further features and advantages of the present disclosure will
become apparent from the following detailed description, taken in combination
with the appended drawings, in which:
[0035] Fig. 1 is a schematic cut-out illustration of a pulsed eddy current
sensor in accordance with an embodiment;
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[0036] Fig. 2 is a perspective view showing a disassembled pulsed eddy
current sensor in accordance with another embodiment;
[0037] Fig. 3 is a perspective view showing the pulsed eddy current sensor
of Fig. 2 assembled;
[0038] Fig. 4A is a graph which illustrates a differential probe response
to
the leading edge of a rectangular pulse from 25mm to 35mm lift-off distance
from
a metallic substrate at 2mm intervals in a laboratory setting in accordance
with
an embodiment;
[0039] Fig. 4B is a picture illustrating a test where a pulsed eddy
current
sensor is positioned on a plurality of low conducting coatings and provides
the
lift-off distance increments over a metallic substrate;
[0040] Fig. 5 is a graph showing lift-off distance measurements (offset)
as
measured from a flat datum over a dented metal plate using a 3" excitation
coil
for comparison with vernier caliper reference datum;
[0041] Fig. 6A is a picture illustrating an upper dotted line with
numbered
demarcations (cm) which shows the low conducting coating path measured by
the pulsed eddy current sensor with underlying metallic substrate profile
shown
as black line in Fig. 6B in accordance with another embodiment (white lines
mark
the low conducting tile boundaries);
[0042] Fig. 6B is a graph which illustrates a metallic surface profile
along
Fig. 6A line interpreted from a pulsed eddy current sensor, compared with a
Laser Tracked Ultrasonic direct measurement of surface profile low conducting
coating was off;
[0043] Fig. 7A is a graph which illustrates a calibration curve for
conversion of voltage response to lift-off distance of Fig. 4A, where the
solid
curve shows a cubic polynomial fit to the data;
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[0044] Fig. 7B is a graph which illustrates line-scans over flat bottom
holes
of different diameters where the holes are 3mm deep and the scans are shifted
vertically for clarity in accordance with another embodiment;
[0045] Fig. 70 is a graph which illustrates lift-off distances from
deformed
plate obtained with a pulsed eddy current sensor and measured directly with a
caliper in accordance with another embodiment;
[0046] Fig. 8A picture with a corresponding schematic drawing that
illustrate hull regions of the steel surface used in a field test in
accordance with
another embodiment;
[0047] Fig. 8B is a graph illustrating relative lift-off distance data
collected
in the first region, where the dotted and solid curves are presents pulsed
eddy
current sensor lift-off distances from the steel surface of Fig. 8A under
tiles, and
where the dashed curve illustrates the smoothed lift-off distance profile of
the
laser-profiler;
[0048] Fig. 80 is a graph of the profile of the steel surface under the
first
scanned region where the light-dotted and solid lines presents profiles
obtained
with and without local demagnetization, respectively and where the dashed line
shows the actual profile of the steel surface where the topology data are
available from 0 to 60 cm in accordance with another embodiment;
[0049] Fig. 9A is a flowchart showing a method for three-dimensional
mapping of a metallic surface under a low conductivity coating according to an
embodiment; and
[0050] Fig. 9B is a flowchart showing the step of performing of one of the
series of measurements at one of the reference positions from the method of
Fig.
9A.
[0051] It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
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DETAILED DESCRIPTION
[0052] In embodiments discussed herein, there are disclosed a pulsed
eddy current sensor for precision lift-off distance measurement and a method
for
precision lift-off distance measurement.
[0053] In comparison with a conventional eddy current inspection
technology, in which the magnetic field is generated by a sinusoidal voltage
applied to a coil, a pulsed eddy current (PEC) sensor, or a transient eddy
current
sensor, uses voltage pulses to excite a coil.
[0054] Referring now to the drawings and more particularly to Figs. 1, 2
and 3, there is shown a pulsed eddy current sensor 10 positioned at a
distance,
called lift-off distance 12, from the metallic surface 16. The pulsed eddy
current
sensor 10 is for measurement of a lift-off distance 12 of a non- or low-
conducting
coating 14 (e.g., an insulator material or a dielectric material) over a
metallic
surface 16 of a metallic substrate 17. The pulsed eddy current sensor 10 is
also
used for detection of defects on the metallic surface 16 through the low
conducting coating 14 as well as the 2D and/or 3D mapping of the metallic
surface 16.
[0055] The pulsed eddy current sensor 10 includes a primary excitation
coil 18 having opposite ends 20, 22. One of the opposite ends 20 is for
interfacing with the low conducting coating 14. The primary excitation coil
18, to
which are applied voltage pulses, is for generating a primary magnetic field
24,
which in turn creates a plurality of pulsed eddy currents in the metal and
magnetization of the metal (if it is ferromagnetic). These eddy currents and
magnetization, if present, in turn produce a secondary magnetic field (i.e.,
an
interaction of the primary magnetic field with the metallic substrate 17
produces
the secondary magnetic field) between metallic surface 16 and the pulsed eddy
current sensor 10. The pulsed eddy current sensor 10 also includes secondary
pick-up probes 28, 30 located within the primary excitation coil 18. The
secondary pick-up probes 28, 30 are used for measuring the primary and
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secondary magnetic fields and for producing a differential signal that is
representative of the lift-off distance.
[0056] According to an embodiment, the secondary pick-up probes 28, 30
are provided within the primary excitation coil 18. In an embodiment, the
secondary pick-up probes 28, 30 are horizontally aligned with the bottom end
of
the primary excitation coil 18. Furthermore, the secondary pick-up probes 28,
30
are vertically, however, it is not a need for the secondary pick-up probes 28,
30
to be within the primary excitation coil 18. In an embodiment, the secondary
pick-up probes 28, 30 are centered within the primary excitation coil 18.
[0057] According to an embodiment, the secondary pick-up probes 28, 30
include a pair of secondary pick-up coils.
[0058] According to another embodiment the secondary pick-up probes
28, 30 include Giant magnetoresistance (GMR) magnetic sensors Anisotropic
magnetoresistance (AMR) sensors and Hall sensors, or a combination thereof.
[0059] Secondary pick-up coil may be wound in a first direction (not
shown) and the other one secondary pick-up coil 30 may be wound in a second
direction (not shown), opposite to the first direction. This allows the
subtraction of
a signal induced in the secondary pick-up coil from another signal induced in
the
other secondary pick-up coil thereby producing the differential signal. Thus,
the
plurality of pulsed eddy currents and magnetization in the case of a
ferromagnetic substrate produce a secondary magnetic field and the secondary
magnetic field is detected by the secondary pick-up coils for providing
measurement of the lift-off distance 12 of the low conducting coating 14 over
the
metallic surface 16.
[0060] According to another embodiment, the pulsed eddy current sensor
may be configured such that both secondary pick-up coils are wound in the
same direction, but connected with opposing leads. This configuration may
allow
subtracting a signal induced in the one of the secondary pick-up coils from
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another signal induced in the other one of the secondary pick-up coils thereby
producing the differential signal.
[0061] According to another embodiment, the pulsed eddy current sensor
may be configured such that the secondary pick-up probes 28, 30 are
connected separately to a differential amplifier that subtracts a signal
induced in
the one of the secondary pick-up probes 28 or 30 from another signal induced
in
the other one of the secondary pick-up probes 28 or 30 thereby producing the
differential signal.
[0062] According to an embodiment, the secondary pick-up probes 28, 30
may be connected together at a fixed distance and the vertical position may be
adjusted within the primary excitation coil 18 for substantially balancing the
secondary pick-up probes. The secondary pick-up probes 28, 30 may be
adjustable relative to the metallic surface 16 using an adjustment mechanism
such as a screw mechanism (e.g., threads 35 at the top of spindle 31,
adjustment
nut 32 and locking nut 33). Indeed, the secondary pick-up probes 28, 30 may be
fixed on a spindle 31 and fixed relative to each other. It may be advantageous
to
minimize the differential signal in air to maximize sensitivity to the
substrate.
[0063] According to an embodiment and still referring to Fig. 1, a
distance
32 between the secondary pick-up probes 28, 30 may be adjustable for
minimizing a differential signal in air, and thereby substantially balancing
the
secondary pick-up probes 28, 30. In this embodiment, the adjustment
mechanism could be a screw type arrangement on the spindle for either one or
both pick-up probes or a sliding and locking mechanism for adjusting the
vertical
height of either one or both pick-up probes.
[0064] According to another embodiment and still referring to Figs. 1, 2
and 3, the pulsed eddy current sensor comprises a ferrite core 34 provided
within
each of the secondary pick-up probes 28, 30 respectively.
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[0065] According to an embodiment, adjustment may be performed away
from any conducting or magnetic objects that may be sensed by the secondary
pick-up probes 28, 30. At the time of measurement, when one end 20 of the
pulsed eddy current sensor 10 is positioned near a conducting or magnetic
material, an imbalance between the secondary pick-up probes 28, 30 develops
resulting in an increase of the differential signal.
[0066] According to another embodiment, voltage pulses comprise a
plurality of rectangular pulses 38 having a leading edge such as shown in Fig.
4A, without limitation. Each one of the plurality of rectangular pulses 38 may
include a pulse width (not shown) and wherein the pulse width is longer than a
transient response of the system which includes the pulsed eddy current sensor
and metallic surface 16. Therefore, the rectangular pulse 38 generates two
separated-in-time pulses of eddy current within the sample (i.e. metallic
substrate
17). The pulsed eddy currents and magnetization for the case of a
ferromagnetic
substrate generate a secondary magnetic field which combined with the primary
field 18 is detected by one or more of at least two secondary pick-up probes
28,
30.
[0067] According to an embodiment, the primary excitation coil 18 may
include one or more coaxial excitation coils.
[0068] Referring now to Figs. 2 and 3, there are shown a disassembled
pulsed eddy current sensor 10 and an assembled pulsed eddy current sensor 10
respectively. Fig. 2 shows the primary excitation coil 18, and the secondary
pick-
up probes 28, 30 for insertion within the primary excitation coil 18. Primary
excitation coils 18 having an internal diameter between 1" 7/8 to 2" 1/2 have
been shown to work. Fig. 3 pulsed eddy current sensor 10 when the secondary
pick-up probes 28, 30 (not visible) are installed within the primary
excitation coil
18.
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[0069] Fig. 4A is a graph which illustrates a differential probe response
to
the leading edge of a rectangular pulse from 25mm to 35mm lift-off distance
from
a metallic substrate 17 at 2mm intervals in a laboratory setting. Fig. 4B
illustrates
a pulsed eddy current sensor 10 positioned on a plurality of low conducting
tiles
15, i.e., on the low conducting coating which provides the lift-off distance
12
increments over the metallic surface 16.
[0070] According to another embodiment, there is provided a pulsed eddy
current sensing system for measurement of a lift-off distance 12 over a
surface
16 of a metallic substrate 17. The pulsed eddy current system may include a
pulse generator (not shown) for generating voltage pulses. The pulsed eddy
current system may also include a primary excitation coil 18 to which are
applied
voltage pulses for generating primary magnetic fields 24, which induce pulsed
eddy currents in the metallic substrate 17, the pulsed eddy currents producing
secondary magnetic fields. The pulsed eddy current system also includes
secondary pick-up probes 28, 30, where each of the secondary pick-up probes
28, 30 are located at a different vertical distance from the metallic surface
16 and
where the secondary pick-up probes 28, 30 are used for measuring the primary
and secondary magnetic fields and for producing a differential signal that is
representative of the lift-off distance 12.
[0071] An amplifier and a filter (not shown) located downstream from the
secondary pick-up probes 28, 30 are used.
[0072] According to an embodiment, the pulsed eddy current system may
further include an amplifier (not shown) and a filter (not shown) to amplify
and
filter the differential signal from the secondary pick-up probes 28, 30
thereby
removing high-frequency noise components from the differential signal. The
pulsed eddy current system may further include a local degaussing circuit (not
shown) for neutralizing a residual magnetization in the metallic surface 16.
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[0073] According to another embodiment, the pulsed eddy current system
described above may include separate amplifiers (not shown) for each secondary
pick-up probe 28 or 30 for substantially balancing the secondary pick-up
probes
28, 30.
[0074] It is to be noted that the pulsed eddy current sensor 10 can serve
as an adjunct to existing metrology systems such as, without limitations,
photogrammetry, laser tracker, articulating metrology arm, and the like, to
allow
3D mapping/reverse engineering of substrate surfaces, i.e. metallic surface,
that
are otherwise inaccessible due to presence of a low conducting coating.
[0075] The design of the pulsed eddy current probe is optimized to obtain
a required precision, sensitivity, and signal-to-noise ratio most notably by
use of
adjustable upper secondary pick-up probe and local degaussing ability for
operation over ferromagnetic substrates, such as submarine hulls, or pressure
vessels and piping used in oil and gas processing or power generation systems.
[0076] The use of the pulsed eddy current sensor 10 to perform lift-off
distance measurements demonstrates lift-off distance 12 measurement accuracy
to better than 0.5 mm at a nominal lift-off distance 12 of 30 mm of low
conducting coating 14. Fig. 4A shows the signal response for lift-off
distances 12
in this range (25, 27, 29, 31, 33 mm). It also demonstrates the capability to
detect
a 3 mm deep and 20 mm diameter feature at this specific lift-off distance 12.
[0077] According to an embodiment, there is provided a method for
mapping a metallic surface 16 under a low conductivity coating 14. The method
includes the step of taking a series of measurements of the lift-off distance
over a
surface of a metallic substrate (a metallic surface) using a pulsed eddy
current
sensor 10. Between measurements, the pulsed eddy current sensor 10 is
displaced over the low conducting coating 14 according to a reference pattern.
The reference pattern includes the 3D coordinates of the position of the
pulsed
eddy current sensor 10 over the low conducting coating 14. The position of the
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pulsed eddy current sensor 10 is normally the center of the bottom of the
secondary pick-up probe 28.
[0078] The reference pattern can be comprised of substantially straight
lines such that a profile of the metallic surface 16 in a plane can be
established.
The reference pattern can also be determined by existing commercial
measurement systems which can give the precise location of a measurement
point of the pulsed eddy current sensor 10 (its "position") at all times.
[0079] The lift-off measurements are performed using the pulsed eddy
current sensor 10 in the manner described earlier.
[0080] Now referring to Fig. 9A, a method 900 for mapping a metallic
surface under a low conductivity coating, wherein a distance from the top of
the
low conductivity coating to the metallic surface defines a lift-off distance.
The
method 900 comprises: performing a series of measurements of the lift-off
distance (depth) at reference positions covering two dimensions (latitude and
longitude) over the low conductivity coating (step 902); and integrating the
series
of measurements with the reference positions to produce a three-dimensional
map of the metallic surface under a low conductivity coating (step 904).
[0081] Now referring to Fig. 9B, the step 902 of performing of one of the
series of measurements at one of the reference positions is detailed. Step 902
comprises generating a primary magnetic field centered on a vertical axis
normal
to the metallic surface at the one of the reference positions, wherein an
interaction of the primary magnetic field with the metallic surface produces a
secondary magnetic field (step 906); measuring the primary and secondary
magnetic fields at two distinct vertical positions on the vertical axis (step
908);
and from the measured primary and secondary magnetic fields, calculating a
differential signal that is representative of the lift-off distance (step
910).
[0082] Now turning to Fig. 5, there is shown the relationship between lift-
off distance measurements (offset) and the distance on a straight line as
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measured from a flat datum over a dented metal plate using a 3" excitation
coil
for comparison with vernier caliper reference datum. This is an example of 2D
mapping of a metallic surface. In order to obtain a 3D map of the metallic
surface
according to an embodiment, a plurality of measurements such as the ones
shown in Fig. 5, taken along a plurality of parallel straight lines, each
providing
probe location, are integrated into a 3D modeling software. The 3D modeling
software will produce the 3D model of the metallic surface. In this particular
case,
location and orientation of probe is required.
[0083] The
pulsed eddy current sensor may be used in different
applications, such as, without limitations, submarine hull inspections
(circularity
measurements, under tile corrosion detection, geometric characterization) and
presents clear cost savings. Other
possible applications include, without
limitations, metallic substrate inspection of polymer-armoured vehicles,
composite reinforced structure, thermal protected structure, radar absorbent
coated airframes and the like.
[0084] Fig. 6A
illustrates an upper dotted line with numbered demarcations
(in cm) which shows the low conducting coating path measured by the pulsed
eddy current sensor 10 with underlying metallic substrate profile shown as a
continuous black line in Fig. 6B. "White" lines on Fig. 6A mark the low
conducting
tile boundaries. Fig. 6B is a graph which illustrates a metallic surface
profile
along Fig. 6A line interpreted from a pulsed eddy current sensor, compared
with
a Laser Tracked Ultrasonic direct measurement of surface profile low
conducting
tiles were off. Fig. 6A and Fig. 6B will be more readily understood by
referring to
the following examples 1 and 2.
[0085] To avoid
the cost of penetration or removal and subsequent repair
of the coating to access the objective metal inspection surface, there is a
need
for a sensor that can measure the vector from the commercial systems closest
possible measurement point on or over the coating, to that point of interest
on the
underlying metallic substrate. Using this so called stand-off or lift-off
distance, a
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mathematical correction can be applied to the coordinate provided by the
commercial system to establish the true coordinate on the hidden substrate.
Such a device effectively extends the "reach" of the commercial system to
access the hidden surface through the coating, while avoiding the costs of
physical penetration and repair.
[0086] The present invention will be more readily understood by referring
to the following examples which are given to illustrate the invention rather
than to
limit its scope.
EXAMPLE 1
Laboratory Tests
[0087] According to a first example and returning to Fig. 4A, there is
shown the differential probe response to the leading edge of the rectangular
pulse at various lift-off distances from a metallic substrate, which is used
to
simulate a submarine hull. The pulsed eddy current is calibrated by stacking,
i.e., 1 mm thick, plastic sheets under the pulsed eddy current and taking
measurements (voltage response to lift-off distance) as each additional
plastic
sheet is inserted. More particularly, Fig. 4A shows differential probe
response to
the leading edge of the rectangular pulse from 25 mm to 35 mm lift-off
distance
from the metallic surface at 2 mm intervals. Instead of the signal from the
leading edge, the signal from the trailing edge of the pulse, which is
essentially
an inverted form of that from the leading edge, can also be used.
Alternatively,
the signal from both the leading edge and the trailing edge can be combined
and
the peak-to-peak signal can be used.
[0088] Fig. 7A shows a calibration curve for conversion of voltage
response to lift-off distance based on the peak-to-peak response. Solid curve
represents a cubic polynomial fit to the data (other functions could be used).
More particularly, Fig. 7A shows how the peak-to-peak signal response varies
with gradually increasing lift-off distance up to 45 mm, covering the range of
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values found on the hull. It is to be noted that Figs. 4A and 7A are not the
same
calibration and Figure 7a is a peak-to-peak measurement of lead and trailing
edge. Leading edges, trailing edges or combination of the two edges could be
used.
[0089]
Laboratory tests of the system exploited by the pulsed eddy current
sensor demonstrate that the signal noise level corresponded to only 0.03 mm at
a lift-off distance of 35 mm. This noise level is essentially independent of
lift-off
distance. However,
the signal increased approximately exponentially with
inverse lift-off distance, so the signal-to-noise ratio is substantially
better at
smaller lift-off distances. The pulsed eddy current sensor's sensitivity is
sufficient
to detect a 1.5 cm diameter and 3 mm deep flat-bottom hole in a steel plate as
shown in Fig. 7B.
[0090]
Referring now to Fig. 7B, there are shown line-scans over flat-
bottom holes of different diameters. The holes are 3 mm deep. The scans are
shifted vertically for clarity. It is to be noted that the pulsed eddy current
also
performs well in mapping surface topology of a deformed steel plate. Fig. 7C
compares pulsed eddy current lift-off distance measurements with lift-off
distance
measurements made with a conventional caliper. As can be seen, the correct
sample profile, to within 1 mm, is obtained.
EXAMPLE 2
Field Tests
[0091]
According to another example, the pulsed eddy current sensor is
tested on a dry docked submarine in an area with a previously characterized
hull
surface, which is now covered with anechoic tiles, a particular low conducting
tile.
Fig. 8A shows the test location, the profiled area and lines along which the
pulsed eddy current sensor and complementary laser profiler measurements
were performed. The first measurements on the hull of the submarine are
conducted along a line parallel to the axis of the sub, near top dead centre
(lines
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1 and 2 in Fig. 8A). Then measurements are performed in the hull
circumferential
direction (line 3 in Fig. 8A).
[0092] A laser profiler is used to measure lift-off distances to the tile
surface from a straight bar. Fig. 8B demonstrates the testing procedure on the
example of line 1. The hull is scanned twice along this line at 1 cm intervals
to
obtain the distance from the top of the tiles to the hull. During the first
scan, i.e.,
the dotted curve, local demagnetization functionality of the pulsed eddy
current
sensor is disabled intentionally. In order to determine the steel surface
profile,
the pulsed eddy current sensor lift-off distance data, i.e., the local
thickness of
the tiling is subtracted from the profile of the tile surface, i.e., the
smoothed and
inverted lift-off distance data from the laser profiler, the dashed curve in
Fig. 8B.
More particularly, Fig. 8B shows relative lift-off distance data collected in
the first
region, where the dotted and solid curves presents pulsed eddy current sensor
lift-off distances from the steel surface under the tiles. On the other hand,
the
dashed curve is the smoothed lift-off distance profile of the laser-profiler,
defined
up to a vertical shift constant.
[0093] The calculated profiles of the steel surface are then compared with
the surface profile data, previously obtained without the tiles, where the
data
overlap. Fig. 80 shows the steel surface profiles calculated from the lift-off
distance data shown in Fig. 8B. More particularly, Figure 80 illustrates the
profile
of steel surface under the first scanned region. The light-dotted and solid
lines
are profiles obtained with and without local demagnetization, respectively.
The
dashed line shows the actual profile of the surface where the topology data
are
available from 0 to 60 cm.
[0094] The vertical shift in the laser-profiler data is cancelled by
shifting
the origin of the plot to the starting point of the scans. The dashed curve in
the
plot represents previously obtained topology data. It is to be noted that the
profile obtained with the local demagnetization, i.e., the solid curve,
matches the
actual profile better than the profile obtained when the demagnetization
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functionality was turned off, i.e., the dotted curve. The effect is most
significant
at the beginning of the scans. The same effect is previously observed in lab
tests. Deviations of profiles, which were obtained without demagnetization,
from
actual profiles, were the largest near the scan starting points, yet remained
close
to 1 mm. These observations suggest that the local demagnetization feature is
necessary for better than 1 mm precision, especially for separated-point
measurements and short scans.
[0095] While
preferred embodiments have been described above and
illustrated in the accompanying drawings, it will be evident to those skilled
in the
art that modifications may be made without departing from this disclosure.
Such
modifications are considered as possible variants comprised in the scope of
the
disclosure.
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