Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR SCANNING A TEST OBJECT
TECHNICAL FIELD
[0001] The present disclosure relates to the field of non-
destructive
testing. More specifically, the present disclosure relates to a method and an
apparatus for scanning a test object.
BACKGROUND
[0002] Mechanical scanning for ultrasonic inspection of regularly
shaped objects, such as for example flat or slightly curved objects, is well
documented and widely used. An object is typically mapped by software using
a grid, for example a rectangular grid, in which a length and a width of each
rectangular pixel of the grid is identical. The grid is typically aligned with
main
axes of a scanner, designated axes x and y, and a scanning motion is obtained
by moving an ultrasonic probe along one axis or along a combination of axes
(for instance x) and repeated after indexing along a perpendicular direction
by
moving one or a set of axes (for instance y), while maintaining a distance
between the probe and the structure surface. Some systems use a plurality of
such probes. An example of a method and apparatus for scanning an object is
described in Patent Application No. CA 2,820,732, entitled "Method and
Apparatus for Scanning an Object", to Grimard etal., filed on June 27, 2013.
[0003] Trigger signals are generated to initiate ultrasonic pulse
generation and data acquisition when the probe reaches positions set by the
grid. Encoder signals of the main scanning axis (for example x) are monitored
by an encoder counter that generates the trigger signals at required probe
positions. The probe is indexed in a perpendicular direction by a distance
dictated by dimensions of the pixels on the grid at the end of the scanning
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motion. The scanning motion is then repeated for a new index position.
[0004] The ultrasonic signal acquired at each position of the grid is
amplified and filtered by a receiving apparatus using fixed filter and
amplification settings. A selected time interval of this outputted signal is
converted to digital data using an analog to digital converter apparatus. This
digital data is recorded on a computer and the amplitude of the signal within
a
selected time interval (for instance the maximum absolute amplitude) or the
time of occurrence of an echo within a selected time interval is associated to
each position of the grid. An image named the C-scan is produced by
associating colors of a chart to the values associated with each pixel of the
grid.
[0005] Flaws are typically detected by analyzing the maximum
amplitude of an ultrasonic signal within a selected time interval containing
an
ultrasonic echo that interacted with the test object. In the specific case of
through transmission inspection, the echo that travelled from the emitting
probe to the receiving probe and through the thickness of the tested object is
monitored. Flaws are identified by comparison of the amplitude of one or a
group of grid pixels with the amplitude of the surrounding pixels. An increase
or
decrease of signal amplitude exceeding a level obtained by calibration of the
scanning apparatus on calibrated flaws, for example flat bottom holes, reveals
the presence of a flaw.
[0006] The amplitude of an ultrasonic echo is a function of the
presence of flaws, but also of variables associated with the object geometry
and composition. Examples of variables that affect the amplitude of an
ultrasonic echo interacting with an object are attenuation, diffraction,
scattering, as well as changes of mechanical impedance. These variables can
have various causes such as, for example, variations of thickness, surface
curvature, material composition or the anisotropy of the material.
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[0007] The amplifier gain of the receiver apparatus is set at a fixed
value that allows monitoring the echo and the variations of amplitude of the
echo in the presence of a flaw with an acceptable signal-to-noise ratio based
on calibration.
[0008] For the example of a signal acquired by transmission of
ultrasounds through a material, the amplifier gain must be set at a high
enough
level to monitor the expected variations of amplitude in the presence of a
flaw,
but low enough to avoid saturation of the echo by the receiver output limits
or
the acquisition input limits. In addition, the ratio of the signal amplitude
recorded on a flawless area to the electronic noise level (i.e. the signal to
noise
ratio) must be higher than the expected signal amplitude loss due to a flaw.
[0009] Identification of flaws in objects of arbitrary shape, thickness
and material composition is complicated by variations of echo amplitude
naturally occurring within the tested material. If a sound material presents
large
differences in attenuation, the optimal receiver amplifier gain changes as a
function of the attenuation. If the variation of attenuation between two areas
of
a sound object is too high, it can be impossible to set a unique receiver
amplifier gain value that allows evaluating the condition of all areas of the
object with the proper signal-to-noise ratio.
[0010] If the inspection of an object can be achieved with a fixed
amplifier gain, data analysis must be performed with an a priori knowledge of
the natural echo amplitude variations of the tested object in order to
properly
separate signal losses caused by flaws from natural signal amplitude
variations.
[0011] Sizing of flaws is also complicated by variations of echo
amplitude that are not related to flaw. Typical image flaw sizing methods such
as -6 dB drop sizing or other C-scan based sizing that make use of echo
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amplitude require a constant amplitude reference to be reliable.
[0012] If the inspection of an object cannot be achieved with a fixed
amplifier gain, current practice is to inspect different areas of the object
individually at their optimal receiver gain or, when the geometry of the
object
allows it, to pause the scanning process and manually change the amplifier
gain when the probe enters an area that required a different amplifier gain.
In
the case of an arbitrary object, both alternatives are impractical.
[0013] Therefore, there is a need for techniques that enable efficient
scanning of test objects having complex geometries.
SUMMARY
[0014] According to the present disclosure, there is provided a
method of scanning a test object. A reference object is scanned to build a
gain
correction map including a plurality of gain values for a corresponding
plurality
of scanning points on a surface of the reference object. The test object is
also
scanned to obtain a plurality of measurements for a corresponding plurality of
scanning points on a surface of the test object. Amplitudes of the plurality
of
measurements obtained for the plurality of scanning points on the surface of
the test object are normalized using the gain values of the gain correction
map.
[0015] According to the present disclosure, there is also provided an
apparatus for scanning a test object. The apparatus comprises a mechanical
scanner, a probe and a controller. The probe is mounted on the mechanical
scanner. The controller is operably connected to the mechanical scanner and
to the probe. The controller controls a scanning motion of the probe to obtain
a
plurality of measurements for a corresponding plurality of scanning points on
a
surface of a reference object, builds a gain correction map including a
plurality
of gain values calculated as a function of the plurality of scanning points on
the
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surface of a reference object, controls a scanning motion of the probe to
obtain
a plurality of measurements for a corresponding plurality of scanning points
on
a surface of the test object, and normalizes amplitudes of the plurality of
measurements obtained for the plurality of scanning points on the surface of
the test object using the gain values of the gain correction map.
[0016] The foregoing and other features will become more apparent
upon reading of the following non-restrictive description of illustrative
embodiments thereof, given by way of example only with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Embodiments of the disclosure will be described by way of
example only with reference to the accompanying drawings, in which:
[0018] Figure 1 is a flowchart showing operations of a method for
scanning a test object;
[0019] Figure 2 is a perspective view of an apparatus for scanning a
test object according to an embodiment;
[0020] Figure 3 is a detailed view of an example of emitting and
receiving ultrasonic probes operating in transmission mode;
[0021] Figure 4 is a block diagram of the scanning apparatus of
Figure 2 showing an information flow within the apparatus
when building a gain correction map; and
[0022] Figure 5 is a block diagram of the scanning apparatus of
Figure 2 showing an information flow within the apparatus
when scanning a test object.
[0023] Like numerals represent like features on the various drawings.
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DETAILED DESCRIPTION
[0024] Various aspects of the present disclosure generally address
one or more of the problems of scanning test objects having complex
geometries.
[0025] The following terminology is used throughout the present
disclosure:
[0026] Test object: any object for which it is desired to obtain a
physical analysis.
[0027] Reference object: an object similar in geometry to the test
object; as an example, the test object and the reference object
may be similar items, the test object having potential flaws due
to wear and tear, the reference object being new.
[0028] Scanning point: one of a plurality of points on the surface of an
object where a scanning measurement is obtained.
[0029] Gain value: a value representative of a level of attenuation of a
scanning signal at a given scanning point of an object.
[0030] Gain correction map: a list of gain values for a number of
scanning points of the reference object.
[0031] Normalization: adjustment of a measured value using a
corresponding gain value.
[0032] Pulse: a brief signal emitted by a probe.
[0033] Pulse-echo mode: signal acquisition mode of a probe in which
a pulse is emitted by the probe and a reflected signal is
acquired by the same probe.
[0034] Transmission mode: signal acquisition mode in which a pulse
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is emitted by an emitting probe and acquired by a receiving
probe after transmission through an object.
[0335] Probe: a physical device capable of sending and/or receiving a
signal towards an object being scanned.
[0036] Mechanical scanner: a mechanical device capable of moving
under control of a controller while supporting a probe.
[0037] Controller: a processor, a computer, a combination of
processors and/or computers, possibly including a memory, an
interface, and similar components, the controller may be hard-
wired for carrying a function or may comprise programmable
code for carrying a function.
[0038] Non-transient memory: a memory device that maintains stored
information in the absence of electrical power.
[0039] Command: a control signal sent from a first component to a
second component for initiating an action of the second
component.
[0*40] Non-destructive testing: a material evaluation technique that
does not cause damage to an object under test.
[0041] C-scan image: an image constructed based on information
from signals resulting from scanning of an object, the signals
being obtained at a plurality of scanning points on the surface
of the object.
[0042] In an embodiment, the present disclosure presents a method
of scanning a test object. In another embodiment, the present disclosure
presents an apparatus for scanning a test object. The disclosed method and
apparatus can be used for various applications, including without limitation
for
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non-destructive testing purposes.
[0043] Referring now to the drawings, Figure 1 is a flowchart showing
operations of a method for scanning a test object. In Figure 1, a sequence 100
comprises a plurality of operations that may be executed in variable order,
some of the operations possibly being executed concurrently, some of the
operations being optional. Operation 110 comprises scanning a reference
object, similar in geometry to the test object, to build a gain correction map
including a plurality of gain values for a corresponding plurality of scanning
points on a surface of the reference object. The scanning points on the
surface
of the reference object may be defined on a rectangular grid. The gain values
for the plurality of scanning points on the surface of the reference object
may
be stored in a non-transient memory. In order to minimize possible noise
effects or localized effects that may not be reproducible amongst similar
objects, the gain values may be filtered or smoothed prior to their use or
storage.
[0044] The test object is also scanned at operation 120 to obtain a
plurality of measurements for a corresponding plurality of scanning points on
a
surface of the test object. There is at least some correspondence between the
scanning points on respective surfaces of the reference object and of the test
object. In a first variant of the method, scanning paths on the surfaces of
both
objects may be identical and there may be a one to one correspondence
between scanning points on surfaces of the reference object and of the test
object. In a second variant of the method, the scanning points may follow
similar scanning paths on the surfaces of both objects, possibly following
similar rectangular grids or any other similar grids. The gain values of the
gain
correction map may be interpolated if a number of scanning points on the
surface of the reference object is not equal to a number of scanning points on
the surface of the test object.
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[0045] Scanning the reference object and the test object may be
executed using ultrasonic pulses applied at each scanning point of the
surfaces of the reference object and of the test object. The scanning may
involve detecting amplitudes of signals resulting from pulse-echo or
transmission of the ultrasonic pulses on each of the scanning points on the
surfaces of the reference object and of the test object. Amplitudes of the
plurality of measurements obtained for the plurality of scanning points on the
surface of the test object are normalized at operation 130, using the gain
values of the gain correction map. An image of the test object may be
constructed at operation 140, using the normalized amplitudes of the plurality
of measurements obtained for the plurality of scanning points on the surface
of
the test object.
[0046] In the same or other variants of the method, when detecting
the amplitudes of the signals resulting from pulse-echo or transmission of the
ultrasonic pulses on each of the scanning points on the surfaces of the
reference object and of the test object, a maximum of each amplitude signal
over a predetermined time interval is determined. A reference amplitude for
the
reference object may be determined by selecting a maximum value among the
amplitudes of the signals resulting from pulse-echo or transmission of the
ultrasonic pulses on each of the scanning points on the surface of the
reference object. The gain values for the plurality of scanning points on the
surface of the reference object may then be set as a function of corresponding
amplitudes of the signals resulting from pulse-echo or transmission of the
ultrasonic pulses on each of the scanning points on the surface of the
reference object in relation to the reference amplitude, for example by
calculation ratios of corresponding amplitudes of the signals resulting from
pulse-echo or transmission of the ultrasonic pulses on each of the scanning
points on the surface of the reference object and of the reference amplitude.
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[0047] To normalize the amplitudes of the signals resulting from
pulse-echo or transmission of the ultrasonic pulses on each of the scanning
points on the surface of the test object, they may be amplified using gain
values calculated as a function of the amplitudes of the signals resulting
from
pulse-echo or transmission of the ultrasonic pulses on each of the scanning
points on the surface of the reference object.
[0048] When the reference object is symmetrical along an axis,
execution time of the sequence 100 may be reduced by scanning the
reference object over a part of its surface on one side of the axis. Gain
values
determined for a corresponding plurality of scanning points on the one side of
the axis are then replicated on corresponding unscanned points on another
side of the axis.
[0049] Figure 2 is a perspective view of an apparatus for scanning a
test object according to an embodiment. An apparatus 200 as illustrated is one
of many possible realizations of a scanning apparatus capable of executing the
operations of the above described sequence 100. Some of the shown
components of the apparatus 200 are optional and may not be present in some
embodiments.
[0050] The apparatus 200 comprises a mechanical scanner 210, a
probe 220 mounted on the mechanical scanner 210, and a controller 230. As
illustrated, the probe 220 is held within a basin 240 that may be filled with
a
couplant material, for example water, air, or another fluid, the couplant
material
forming a controlled environment for transmission of probe signals. The
mechanical scanner 210 as shown is capable of moving the probe 220 along
three (3) axes by raising or lowering a vertical member 212, moving the
vertical
member 212 laterally along a length of an horizontal beam 214, and by moving
the horizontal beam 214 longitudinally along a length of the basin 240. The
vertical member 212 and the horizontal beam 214 are moved by a motion
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controller 216, which itself receives commands from the controller 230. The
apparatus 200 can thus scan an object over a three-dimensional (3D) space.
The mechanical scanner 210 and the probe 220 are connected by wires (not
shown) to the controller 230; they may be connected wirelessly to the
controller 230 in other variants. The controller 230 includes a screen 232 and
a
keyboard 234 allowing an operator to control the apparatus 200 and its
components.
[0051] The controller 230 controls a scanning motion of the probe 220
to obtain a plurality of measurements for a corresponding plurality of
scanning
points on a surface of the reference object. It builds a gain correction map
including a plurality of gain values calculated as a function of the plurality
of
scanning points on the surface of a reference object. The controller 230 also
controls a scanning motion of the probe 220 to obtain a plurality of
measurements for a corresponding plurality of scanning points on a surface of
the test object. Finally, the controller 230 normalizes amplitudes of the
plurality
of measurements obtained for the plurality of scanning points on the surface
of
the test object using the gain values of the gain correction map.
[0052] The apparatus 202 may comprise a memory (not specifically
shown) in which the controller 230 stores and reads the correction gain map.
An image generator constructs an image, for example a C-scan image, of the
test object using the normalized amplitudes of the plurality of measurements
obtained for the plurality of scanning points on the surface of the test
object. A
visual display such as the screen 232 may show the image. Alternatively, the
image may be printed on a printer (not shown) or an output port (not shown) of
the controller 230 may transmit data representing the image.
[0053] Figure 3 is a detailed view of an example of emitting and
receiving ultrasonic probes operating in transmission mode. The probe 220 of
Figure 2 includes an emitting probe 222 and a receiving probe 224, both of
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which operate in ultrasonic mode in the non-limiting example of Figure 3. The
vertical member 222 of Figure 2 doubles at its lower end into members 212a
and 212b for respectively mounting the emitting probe 222 and the receiving
probe 224. An object 250, which may be a reference object or a test object, is
held between the emitting probe 222 and the receiving probe 224. While
Figure 3 shows an example of emitting and receiving ultrasonic probes
operating in transmission mode, other types of probes such as air coupled
probes, eddy current probes, laser probes, hybrid probes, and the like,
operating in transmission mode or in pulse-echo mode, are also contemplated.
Also, while Figure 3 shows one (1) pair of emitting and receiving probes 222
and 224, use of multi-element probes and use of phased arrays or probes,
operating in pulse-echo mode or in transmission mode, are also contemplated.
[0054] Figure 4 is a block diagram of the scanning apparatus of
Figure 2 showing an information flow within the apparatus when building a gain
correction map. Figure 5 is a block diagram of the scanning apparatus of
Figure 2 showing an information flow within the apparatus when scanning a
test object. Figures 4 and 5 depict the same apparatus 200 and only differ in
the manner in which they show their information flows. For illustration
purposes, Figures 4 and 5 are described in the context of the apparatus 200
having a 3D mechanical scanner 210.
[0055] Referring at once to Figures 4 and 5, the apparatus 200
comprises the mechanical scanner 210 having the motion controller 216 and
one or more probes 220. A control software module 300, a data acquisition unit
330 and an encoder counter/trigger generator 360 are interconnected to the
mechanical scanner 210 and to the probes 220. The control software module
300, the data acquisition unit 330 and the encoder monitoring module 360 may
in whole or in part be integrated in the controller 230 of Figure 2.
[0056] Features of the apparatus 200 common to both Figures 4 and
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will first be described. Based on a predefined scanning path 302, for example
a rectangular grid, defined according to the geometries of the reference
object
and test object, the control software module 300 includes a probe position
information calculator 304 that calculates axis encoder positions 304 (i.e.
three
(3) positions over 3D for each scanning point) for each subsequent scanning
point on the scanning path 302. The control software module 300 provides
each subsequent axis encoder positions 304 to the encoder counter/trigger
generator 350 and to the motion controller 216.
[0057] The mechanical scanner 210 includes three (3) motors and
axis encoders 211 that detect movements 226 of the probe 220 over 3D and
provide probe position information 215 to axis controllers 218 of the motion
controller 216. The axis controllers 218 use the axis encoder positions 304
and
the probe position information 215 to provide positioning commands 217 to the
mechanical scanner 210 to move the probes 220 while minimizing differences
between the desired axis encoder positions 304 and the actual probe position
information 215.
[0058] The mechanical scanner 210 also provides the probe position
information 215 to an axis input port 362 of the encoder counter/trigger
generator 360. An encoder monitoring process 364 analyses the desired axis
encoder positions 304 and the actual probe position information 215 and
causes a 3D trigger module 366 to generate pulse commands 368 when the
probes 220 are at the desired position.
[0059] In the data acquisition unit 330, a pulser 332 receives the
pulse commands 368 and forwards them to the probes 220. The pulse
commands 368 are also applied to an analog to digital converter 334. A signal
amplification and filtering module 336 (that at the time does not use any gain
signal) receives response signals 338 from the probes 220. Amplified signals
340 are also fed to the analog to digital converter 334 digitized versions 342
of
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the pulse commands 368 and of the response signals 338 are sent to the
control software module 300.
[0060] When building the gain correction map, the reference object is
being scanned by the apparatus 200. As shown on Figure 4, the digitized
versions 342 of the pulse commands 368 and of the response signals 338 are
gated 306 to extract peak values of the response signals 338. An image
generator 308 produces a C-scan image of the reference object. A calculator
310 detects a maximum amplitude 312 among the peaks of the C-scan image.
A normalizer 314 evaluates gains required to normalize the C-scan image,
optionally computing the gain values as a function of the maximum amplitude
312, for example as a ratio of each peak value over the maximum amplitude
312. A selector 316 choses a gain map data type, in which gain values are
determined in terms of image or line values. The selector 316 optionally
filters
and/or smoothes the gain values. The resulting gain values are stored in a
memory (not specifically shown) as a gain correction map 320. The gain
correction map 320 contains gain values (i.e. amplification values) for each
scanning position of the scanning path 302.
[0061] When the test object is being scanned by the apparatus 200,
as shown on Figure 5, the gain correction map 320 for a reference object
having sharing a similar geometry with the test object is loaded. Gain values
from the gain correction map 320 are used both in the control software 300
and in the data acquisition unit 330.
[0062] In the data acquisition unit 330, the gain correction map 320
provides gain values 344 for each scanning point. The pulse commands 368
are used as triggers for reading the gain value 344 relevant to a given point
being scanned. The gain value 344 is used to generate a gain command 346
that is applied to the signal amplification and filtering module 336. As such,
the
response signal 338 is normalized at least in part using the gain command 346
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before being submitted to the analog to digital converter 334. The digitized
versions 342 of the pulse commands 368 and of the amplified response
signals 338 are fed to a software amplification module 322. The software
amplification module 322 applies to the digitized signal 342 a gain command
323 calculated based on a complementary software gain 324 obtained by
selecting a gain value for the appropriate scanning point from the gain
correction map 320, producing further amplified and normalized outputs.
Outputs of the software amplification module 322 are gated 326 to extract peak
values of these outputs. An image generator 328 collects the outputs of the
gate 326 to form a normalized C-scan image of the test object.
[0063] Various embodiments of the method and apparatus for
scanning a test object, as disclosed herein, may be envisioned. One such
embodiment may comprise a scan gain mapping method dedicated to
ultrasonic inspection of complex materials such as aerospace composite parts
of arbitrary shape, thickness and material composition.
[0064] The method provides a normalized ultrasonic inspection output
of a test sample of arbitrary geometry and/or having varying material
composition. The amplitude of an ultrasonic echo signal is a function of the
presence of flaws, but also of variables associated with the object geometry
and composition. Examples of variables of an object that affect the amplitude
of an ultrasonic echo are attenuation, diffraction, scattering, reflection
coefficient, transmission coefficient. These variables can have various causes
such as, for example, variations of thickness, variations of surface
curvature,
variation of material composition or the anisotropy of the material.
[0065] An inspection is performed by moving one or multiple probes
along the surface of the test object on a rectangular grid pattern within a
given
orthogonal space (Euclidian, cylindrical, polar or generalized parametric
space
for complex surfaces). Ultrasonic waves are emitted, received, amplified
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filtered and stored each time the probe reaches the positions set by the grid.
A
C-scan image is typically built by monitoring the amplitude of an echo within
a
given time frame (gate). It could also represent the time-of-flight of an
echo, its
equivalent thickness inside a material or any other amplitude/time feature
observable on the ultrasonic signal.
[0066] Conventional practice has been to set the amplifier gain of the
receiver at a value that allows seeing the echo of a flawless area below the
saturation limit of the ultrasonic apparatus (receiver and acquisition card)
and
with an amplitude that is high enough to detect variations of echo amplitude
at
a level established using a reference sample.
[0067] In the specific case of through transmission inspection
(ultrasonic waves propagating from an emitting probe, through the thickness of
the object and to a receiving probe on the other side of the object), a flaw
is
identified by an attenuation of the transmitted signal, i.e. a drop of its
amplitude
relative to an adjacent flawless area. This is typically detected by the use
of an
amplitude threshold gate which level is set as a given drop referred to the
amplitude of a flawless signal. A minimum signal-to-noise ratio is therefore
required in order to identify a signal drop corresponding to a flaw.
Considering
the example of a signal acquired by transmission of ultrasounds through a
material, the signal amplification (receiver gain) is set at a level that
limits the
signal amplitude within the range of the acquisition instrument, while being
high enough to monitor variations of amplitude caused by the presence of a
flaw. If a single amplification level is used for all areas of the object
(common
practice), the detection of flaws can be compromised; in areas where natural
signal loss is high, it becomes impossible to identify a flaw if the
electronic
noise level is higher than the amplitude that is expected in the presence of a
flaw. Furthermore, if the echo amplitude of the flawless areas vary too much
during the inspection of the object, it becomes impossible to set a unique
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amplitude detection threshold gate since the reference from which the
threshold is established changes as a function of probes position. Thus flaws
can be missed.
[0068] The disclosed method normalizes the ultrasonic response of
the object in order to minimize or remove the amplitude variations that are
not
related to flaws, i.e. solely due to material properties and geometry, as well
as
reproducible mechanical imprecisions of the scanning apparatus. It makes use
of the positional reproducibility of an automated scanner combined with a
reference, flawless object to create a map of the differences in ultrasonic
signal
amplitude caused by attenuation along the test object. This map is then used
in conjunction with the gain control of the amplifier of the ultrasonic
receiver to
normalize the ultrasonic response when scanning a comparable object. With
such a normalized response, flaws can be identified more easily by threshold
gating.
[0069] Objects with arbitrary geometries are inspected by a computer
controlled motion control system adjusting the position and orientation of an
ultrasonic test probe (or a plurality of probes) so that it follows the
surface of
the arbitrary object at a constant distance. The orientation of the probe is
adjusted to compensate for surface curvature and refraction effects. At each
sampling point on the test specimen, a trigger signal is sent to a computer
controlled pulser-receiver which sends an ultrasonic pulse and receives and
amplifies the transmitted and/or reflected echoes. A waveform digitizer is
then
used to transfer the amplified signal to the computer. The echo amplitudes of
the sampling points are organized into the C-scan image.
[0070] Variations in local thickness and material composition of
arbitrary objects produce variations in the ultrasonic echo amplitude acquired
at different locations on the object. Furthermore, mechanical imprecisions
also
contribute to variations in the echo amplitudes over the part. Without
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normalization, the resulting variation in the C-scan image amplitudes would
decrease the reliability of flaw detection and sizing.
[0071] Normalization through scan gain mapping corrects such
variations, effectively generating a uniform ultrasonic echo amplitude on all
the
sample points. At a first operation of the process, a reference sample
containing no flaws is inspected. The resulting amplitude C-scan image is
processed to determine, for each point, the gain adjustment required to obtain
a uniform amplitude response over the entire C-scan image. This gain
adjustment matrix is called the scan gain map. For bidirectional scanning
applications, two different gain matrices are produced, one for the positive
direction scan lines and the other for the negative direction scan lines,
which
are typically different due to mechanical backlash. Filtering or B-Spline
surface
fitting can be applied to the gain adjustment matrices in order to eliminate
the
effects of structural noise that are not reproducible from one sample to
another.
[0072] Once the gain matrices are obtained from the reference
sample, tests samples having the same geometry can be inspected. Before
performing the scan, a gain control sequence is constructed from the gain
matrices and sent to the computer controlled pulsar-receiver unit. Using this
gain control sequence and the sampling point trigger signal, the pulser-
receiver
unit automatically changes the signal gain for each sampling point on the test
sample. The resulting C-scan, in the absence of flaws, will have a constant
amplitude throughout the whole grid, thus allowing for more reliable flaw
detection and sizing.
[0073] This method is particularly well suited for through-transmission
scanning, where loss of amplitude is typically monitored to detect the
presence
of flaws. It is common for samples of complex shapes to present thickness
variations that result in natural signal amplitude loss in thick areas that is
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comparable to a signal amplitude loss associated to flaws in thinner sections.
Using scan gain mapping normalizes the amplitude of the transmitted echoes,
compensating for natural sources of amplitude variations. This results in
ultrasonic signals which amplitude losses are solely due to the presence of
flaws. This improves the capabilities of performing measurements such as
sizing and signal-to-noise ratio of flaws.
[0074] The process is separated in two operations: 1) Construction of
a gain correction map for the object using a reference object; 2) Application
of
the gain correction map during an inspection to normalize the response of the
object to the response of the reference object.
[0075] Here, scanning and indexing lines represent consecutive grid
pixels along the axes of the width and length of the rectangular grid
arrangement. The scanning line is aligned along the physical or virtual axis
that
moves continuously along the object, while the indexing line is aligned with
the
other physical or virtual axis that indexes the probe from one scan line to
the
other during the scan.
[0076] A mechanical scanner comprising a combination of linear
and/or angular axes of motion is used to move one or multiple probe(s) along
the surface of an object. Each axis is moved by a motor and the position of
the
axis is given by a positional encoder device. A motion controller is used in
combination with amplifier drives to control the motors.
[0077] The surface of the object is mapped using a rectangular grid
where each element of the grid represents a position where ultrasonic waves
are generated and received by the ultrasonic probe, and where signal
acquisition is done.
[0078] For each grid position, the control software determines the
optimal axis encoder to be monitored in order to know when the scanner
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reaches the grid position. A trigger pulse signal is generated when the axis
is
at the grid position. The trigger signal is received by the ultrasonic pulser
to
generate an electrical impulse that is sent to the emitting probe(s). The
trigger
signal is also received by the analog-to-digital converter to synchronize the
waveform acquisition with the emitted electrical impulse.
[0079] The ultrasonic probe converts the electrical impulse into an
ultrasonic wave. The ultrasonic wave interacts with the object, and then
travels
to the receiving probe which converts it to an electrical signal. The
receiving
probe can be the same probe as the one that generated the ultrasonic signal
or a different probe.
[0080] The electrical signal generated by the receiving probe(s) inputs
the receiver and is filtered and amplified by the receiver. The amplification
is
set to avoid any saturation of the signal after amplification.
[0081] The amplified analog signal is then converted to a digital signal
by an analog-to-digital converter (digitizer).
[0082] The digitized signal is transferred to the computer and a gate
is used to extract an amplitude from the signal within a time interval set by
the
gate. This amplitude value is typically but not restricted to the peak
amplitude
of the signal within the gate. Alternatively, the gate could be applied on the
analog signal and the resulting amplitude data could be digitized and
transferred to the computer.
[0083] The amplitude value obtained by the gate is then stored and
associated to the current point of the grid. A C-scan image consisting of all
the
amplitude values placed within a rectangular grid corresponding to the
scanned grid is created.
[0084] The maximum amplitude of the C-scan image is found and
used as the reference amplitude. The amplification required to increase the
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21
amplitude of all other positions of the grid to match the maximum amplitude of
the C-scan is calculated.
[0085] A new data set utilizing the same grid is obtained, where each
item of the grid contains the amplification calculated in the previous
operation.
The gain map is the result of this operation.
[0086] Alternatively, if the object is symmetrical along one axis of the
scanned grid, for instance the scanning axis, the gain map can be obtained by
selecting a representative row of pixels of the grid along the other axis,
i.e. the
indexing axis, and by filling the gain map grid by reproducing this row of
pixels
along the scanning axis.
[0087] Filtering and smoothing algorithms can also be applied on the
gain map to minimize the effects of noise or localized effects that may not be
reproducible amongst identical objects.
[0088] A mechanical scanner comprising a combination of linear
and/or angular axes of motion is used to move one or multiple probes along
the surface of an object. Each axis is moved by a motor and the position of
the
axis is given by a positional encoder device. A motion controller is used in
combination with amplifier drives to control the motors.
[0089] The surface of the object is mapped using a rectangular grid
where each element of the grid represents a position where ultrasonic waves
are generated and received by the ultrasonic probes.
[0090] A gain map corresponding to the object and the grid is recalled
from the computer memory. If the number of elements of the grid is not the
same as the number of elements of the gain map, the content of the latter is
sampled or interpolated to match the grid size. For each element of the grid,
the maximum control gain value that is below the required gain defined in the
gain map is stored in the internal memory of the ultrasonic unit. Since the
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22
ultrasonic unit gain can only be controlled by discrete values, the difference
between the gain map value and the control gain calculated for each element
of the grid is stored as a complementary software gain data on the computer.
[0091] For each grid position of the scanning line, the control software
determines the optimal axis encoder to be monitored in order to know when
the scanner reaches the grid position. A trigger pulse signal is generated
when
the axis is at the grid position. The trigger signal is received by the
ultrasonic
pulser to generate an electrical impulse that is sent to the emitting
probe(s).
The trigger signal is also received by the analog-to-digital converter to
synchronize the waveform acquisition with the emitted electrical impulse. The
ultrasonic unit also uses the trigger signal to select the gain control
command
associated with the current element of the grid.
[0092] The ultrasonic probe(s) converts the electrical impulse into an
ultrasonic wave. The ultrasonic wave interacts with the object and is
converted
to an electrical signal by the receiving probe(s). The receiving probe(s) can
be
the same probe(s) as the one that generated the ultrasonic signal or a
different
probe (s) .
[0093] The electrical signal generated by the receiving probe(s) signal
inputs the receiver and is filtered and amplified by the receiver at the
amplification level corresponding to the gain command selected at the
reception of the trigger signal.
[0094] The amplified analog signal is then converted to a digital signal
by an analog-to-digital converter (digitizer).
[0095] The digitized signal is transferred to the computer. The
complementary software gain value of the current grid position is then applied
on the digital signal by mean of software amplification.
[0096] A gate is used to extract an amplitude from the signal within a
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23
time interval set by the gate.
[0097] This amplitude value is typically but not restricted to the peak
amplitude of the signal within the gate. The amplitude value obtained by the
gate is then stored and associated to the current point of the grid. A C-scan
image consisting of all the amplitude values corresponding to the rectangular
grid and having the same dimensions as the grid is created.
[0098] Those of ordinary skill in the art will realize that the
description
of the method and apparatus for scanning a test object are illustrative only
and
are not intended to be in any way limiting. Other embodiments will readily
suggest themselves to such persons with ordinary skill in the art having the
benefit of the present disclosure. Furthermore, the disclosed method and
apparatus may be customized to offer valuable solutions to existing needs and
problems of scanning test objects having complex geometries.
[0099] In the interest of clarity, not all of the routine features of the
implementations of the method and apparatus are shown and described. It will,
of course, be appreciated that in the development of any such actual
implementation of the method and apparatus, numerous implementation-
specific decisions may need to be made in order to achieve the developer's
specific goals, such as compliance with application-, system-, and business-
related constraints, and that these specific goals will vary from one
implementation to another and from one developer to another. Moreover, it will
be appreciated that a development effort might be complex and time-
consuming, but would nevertheless be a routine undertaking of engineering for
those of ordinary skill in the field of non-destructive testing having the
benefit of
the present disclosure.
[00100] In accordance with the present disclosure, the components,
process operations, and/or data structures described herein may be
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implemented using various types of operating systems, computing platforms,
network devices, computer programs, and/or general purpose machines. In
addition, those of ordinary skill in the art will recognize that devices of a
less
general purpose nature, such as hardwired devices, field programmable gate
arrays (FPGAs), application specific integrated circuits (ASICs), or the like,
may also be used. Where a method comprising a series of operations is
implemented by a computer or a machine and those operations may be stored
as a series of instructions readable by the machine, they may be stored on a
tangible medium.
[00101] Systems and modules described herein may comprise
software, firmware, hardware, or any combination(s) of software, firmware, or
hardware suitable for the purposes described herein. Software and other
modules may reside on servers, workstations, personal computers,
computerized tablets, personal digital assistants (PDA), and other devices
suitable for the purposes described herein. Software and other modules may
be accessible via local memory, via a network, via a browser or other
application or via other means suitable for the purposes described herein.
Data
structures described herein may comprise computer files, variables,
programming arrays, programming structures, or any electronic information
storage schemes or methods, or any combinations thereof, suitable for the
purposes described herein.
[00102] Although the present disclosure has been described
hereinabove by way of non-restrictive, illustrative embodiments thereof, these
embodiments may be modified at will within the scope of the appended claims
without departing from the spirit and nature of the present disclosure.
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