Note: Descriptions are shown in the official language in which they were submitted.
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DETECTION OF NEAR SURFACE DEFECT'S BY REVERSED THERMOGRAPHY
Field of the invention
The present invention relates to a method for the detection of near surface
defects by
reversed thermography.
The invention also relates to an apparatus for carrying this method which is
particularly
useful for the inspection of thermal sprayed engine block cylinder wall
linings.
Background
Thermal spraying is a generic term that encompasses an array of processes used
to
apply coatings of various high melting point materials (metals, ceramics,
etc.) as a spray of
droplets heated and propelled by some form of flame or plasma. The coating is
built as
"splatted" droplets solidify and accumulate on the substrate's surface.
Thermal spraying is
extensively used in the aerospace industry and, more recently, has moved into
the automobile
industry, in particular, for the manufacturing of engine blocks.
Presently, engine blocks are made of cast aluminum with steel sleeves lining
the inner
surface of the cylinders. The sleeves, a few millimeters th ick, are inserted
in the mold prior
to the casting of the aluminum. For several years now, auto makers, sometimes
in collaboration
with thermal spraying equipment manufacturers, have been developing coating
processes that
would replace the steel inserts. Significant savings in manufacturing costs as
well as weight
reduction and performance enhancement has fuelled this general trend
throughout the industry.
Because it is very thin (.005" to .008"), the wear resistant coating
(generally steel)
performs better because heat is extracted more. efficiently than through the
much thicker
sleeve. However, bonding of the coating to the aluminum substrate is critical
and bonding
defects are generally considered unacceptable. Since an in-service breakdown
of the coating
leads to failure of the engine, the quality and reliability requirements for
these processes are
extremely high and it is uncertain if they can be met without some form of
inspection method.
There is presently no method of detecting; these defects that could reasonably
be used
in a production environment. The present Applicant, Tecnar Automation Ltee,
became aware
of this situation through its contacts with a major car manufacturer on a
related matter and the
decision was made to pursue this opportunity and develop a suitable method.
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l~ns~ection by thermography
The basic principle of thexmographic inspection is to establish heat flow
conditions
through the inspected component and detect disx~zptions in the heat flow
pattern caused by
potential defects. Some form of infrared (IR) detection device (IR camera for
example)
measures the surface temperature of the component. Defects perpendicular to
the direction of
the heat flow will cause the most disruption and therefore will be the easiest
to detect.
Since bonding defects in coatings are obviously parallel to the component
surface, one
must establish heat flow conditions perpendicular to the component surface in
order to
maximize contrasts caused by these defects. A typical setup will use some form
of heat source
(generally a laser) to uniformly heat an area of the compoxient surface. If
there is no defect, the
temperature of the area is uniform. Any defect that restricts the flow of heat
from the surface
toward the inside of the component will appear ;is a hot spot within the
heated area. Figure
identified as "prior art" illustrates such a setup.
Of course, the mechanism used to heat the surface must not itself generate IR
radiation
that would reflect on the inspected area and blind the IR detector. . A laser
is the most
convenient heat source from that perspective because its precise color can
easily be filtered out
of the IR detection device. However, the cost per watt of power can be
prohibitive.
Furthermore, safety and limited mean time between failure are also major
concerns. Okher heat
sources have been used such as joule heating with an electrical current or
even a flow of hot air.
Summar of the invention
The present invention is based on the discovery that, since engine blocks come
out of
the coating station at a temperature of aboul: 90 ° C, one could
actually use reversed
thermography to inspect the coating on cylinder walls.
More specifically, the invention provides a method for the detection of near
surface
defects in components, including especially but not exclusively engine block
cylinder walls,
comprising the steps of:
(a) applying a flow of cold air to an area to be inspected, thereby
establishing a
heat flow from the inside-out, and
(b) subjecting the cooled area to an inspection by reversed thexrnography in
order
to locate any cold spots which would be indicative of defects.
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As it can be understood, the amount of heat extracted by the cold air is a
function of
the difference in temperature between the air and the surface. In steady-state
conditions, that
temperature reaches an equilibrium where the cooling rate equals the amount of
heat
continuously flowing from the hot mass underneath the cooler surface. Where
there is a
bonding defect, the equilibrium temperature will be lower because heat flow
towards the
surface is disrupted. Therefore, defects will appear as cold spots, in
reversed contrast with
traditional thermography.
The invention also provides an apparah~s for carrying out the above method.
This
application comprises:
(a) means for generating a flow of a cold gas;
(b) means for applying the flow of cold gas to a surface area of the component
to
be inspected, thereby establishing a heat flow from said surface area through
said component;
and
(c) means for inspecting the cooled area by reversed thermography in order to
locate any cold spots that would be indicative of defects.
Brief description of the drawi~s
Figure 1 identified as "prior art" is a schematic representation of the
detection of
bonding defects by laser thermography;
Figure 2 is a schematic representation of tlne detection of bonding defects by
the method
according to the invention, using air cooled reversed thermography;
Figure 3 is an IR camera image of a cylinder wall surface with defects while
cooled
with compressed air;
Figure 4 is a curve giving the temperaW re profile along a line running across
two
defects;
Figure 5 is a schematic representation of an apparatus according to the
invention for
use to inspect engine block cylinders.
Figure 6 is a side elevational view of an apparatus according to a preferred
embodiment
of the invention, hereinafter called "cylinder inspection system";
Figure 7 is a schematic perspective view of the head assembly of the system
shown in
Figure 6;
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Figure 8 is a top perspective view of parl:ial cross-section of one of the air
injectors
used in a system shown in Figure 6;
Figure 9 is a top view of a cylinder subj ected to inspection, showing the
respective
location of the air injectors and mirror of the system shown in Figure 6;
Figure 10 is a block diagramm of the conlTO1 device of the system shown in
Figure 6;
and
Figure 11 is a typical screen display of a cylinder with defects inspected
with the
system shown in Figure 6.
Detailed description of the invention
As aforesaid, the way bonding defects can be detected by the method according
to the
invention is schematically illustrated in Figure. 2. As already indicated
hereinabove, this
method is not restricted to the detection of defects in engine block cylinder
walls. As a matter
of fact, it can detect any defect whose size, shape and orientation
significantly disrupts the heat
flowing perpendicularly to the surface of a component.
If such is needed, cold components can be heated prior to inspection. All that
is
required is that the component be massive enough to act as a "virtually
infinite" heat sink (or,
in this case, "cold" sink) for as long as it takes to establish steady state
heat flow conditions
through the inspected layer:In other words, the component has to be much
thicker than the
inspected layer.
In carrying out the method according to the invention, it is assumed that heat
flows in
only one direction (inside-out). Of course, heat also flows sideways and can
go around a defect,
blurring the contrast at the edge of deeper ones. Therefore, the resolution of
the method is
limited by the depth of potential defects, i.e. the thickness of the inspected
layer.
Another factor that contributes to the efficiency of the method according to
the
invention is the thermal conductivity of the material. If it is high, a
physical discontinuity will
cause a much greater disruption in the heat flow than if the material is
itself a good thermal
insulator. Therefore, contrasts in thermal barrier coatings will be lower than
in the case of
cylinder walls where the steel coating is a fairly good heat conductor and
aluminum is an
excellent one.
Figure 3 shows images of artificial bonding defects in a test cylinder wall.
The
aluminum cylinder was 3" in diameter and 5" long. Wall thickness was 3/8" with
a .010" thick
CA 02267586 1999-04-13
steel coating on the inside. For convenience, the ~oylinder was longitudinally
cut in two.
An IR camera was aimed toward the inside surface of one of the half cylinders
which
was then heated with a heat gun (a kind of "industrial" hair dryer) to about
60 ° C. The picture
from the IR camera showed a uniformly warm surface.
5 Compressed, room temperature air was thc;n blown from a nozzle directly onto
an area
at the center of the camera's field of view where dc;fects were known to be
present. As can be
seen in Figure 3, the cone shaped area struck by the compressed air appears as
shaded in the IR
image because it is understandably cooler. Near the center, two defects are
clearly visible as
cold spots within the continuously cooled area. '.Che shape and location of
these spots are in
perfect agreement with the defect map given by tJhe test cylinder maker.
Figure 4 gives the temperature profile along the faint line marker visible in
Figure 3.
As can be seen, the defects are 4 to 6°C cooler than the surrounding
area. Considering that the
temperature difference between the sample and the compressed air is about 50
° C, 4 to 6 ° C is
a very significant contrast.
Inspection of cylinders
In the simple experiment described above, the geometrical complexity of
inspecting
the inside of a 360° cylindrical surface was circumvented by cutting it
in half, thus allowing
direct, perpendicular viewing of the interior. For real engine blocks, an
optical arrangement has
to be designed to allow viewing from the top, such as illustrated by way of
example only in
Figure 5.
As can be seen, a mirror 1 allows an IR camera 3 located on top of the block
to inspect
an area 9 inside a cylinder 7. Compressed air should of course be blown via a
nozzle 5 onto the
same area. The minor 1 and air nozzle 5 are located inside a cylindrical
housing that shrouds
the remainder of the cylinder's inner surface 7 to prevent IR radiation from
that surface to blind
the IR camera. The whole assembly pivots 360° ('see the arrow) to scan
a complete cylindrical
section of the surface. The head is moved up and the process is repeated until
the whole
cylinder has been covered.
As can be easily understood, the whole process can be automated. A computer
can
easily collate the various scan results to display an unfolded, continuous
image of the whole
cylinder which can be processed by image analysis software to signal an
operator if the area and
density (i.e. intensity of the contrast) of defects exceeds preset thresholds.
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zam
A complete cylinder inspection system was developed based on the arrangement
illustrated in Figure 5. As it can be understood, thc; actual design of the
inspection head of the
system that was developed is much more comple:~c than the one illustrated in
Figure S.
As shown in Figure 6, this inspection system comprised an inspection head 21
and a
camera 3 mounted on a turntable 23 to provide roW tion for the scans. A
vertical slide 25 moved
the turntable/camera/head assembly from a bottom scan position to a top scan
position within
the cylinder, as well as to a completely retracted position out of the engine
block being
inspected. The centerline of the engine block cylinder is identified by letter
"A" . The assembly
also comprised an assembly line conveyor 27 to move the engine block when the
head 21 and
camera 3 were in retracted position to align the nc;xt cylinder for inspection
or to bring a new
block for inspection. Of course, movements of the conveyor were coordinated
with the
inspection sequence.
As shown in Figure 7, the inspection head 21 comprised a top plate 29, a
bottom plate
31 and two side plates (not shown) in the back for stiffening the assembly and
hiding the rest
of the cylinder's inner surface from the camera.. The assembly 21 also
comprises two air
injectors 11, 11' fixed to the bottom plate 31 on which the mirror 1 was also
mounted. The top
plate 29 had a view-port 33 through which the camera had access to the mirror
1. The top plate
was thick enough to house the air ducts that feed the injectors. All elements
of the head except
the minor surface were coated with IR absorbing black paint.
The air injectors 11, 11' were designed to focus the flow onto a narrow
vertical strip
along the full length of the cylinder portion viewed through the mirror. The
air injectors were
also designed to minimize obstruction of the camera lens aperture and allow
for a maximum
of IR radiation to be collected.
As is better shown in Figure 8, each air injiector 11, 11' comprised an
aluminium body
13 closed by a steel cover 15 with a gasket 17 in between. The air injector 11
defined a square
cavity 19 which series as a manifold into which compressed air is brought from
the top. The
compressed air escaped through a series of 50, 10 mm long, 1 mni2 small
nozzles 21. Because
of its aspect ratio, each nozzle 21 maximized air velocity in the longitudinal
direction, thus
producing an air jet that remains fairly well collimated up to about 10 mm
away. Disposing 50
of those along a line therefore produced an air jet i.n the shape of a blade.
This principle is well
known to manufacturers of air nozzles for various industrial applications such
as cleaning,
stripping debris off a conveyor belt, etc.
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In the system that was developed, two such air injectors 11, 11' were used. As
is better
shown in Figure 9, the two air injectors 11, 11' lhereinafter called
"clockwise injector" and
"counter-clockwise injector", respectively, were positioned within the
cylinder 7 to be
inspected so as to inject air at an angle of 45° toward a same spot.
The mirror 1 was positioned
between the injectors. Preliminary tests have shown that up to that angle of
45°, the cooling
effect of the air jet is almost as good as when it was aimed straight at the
surface (0°). This
allows the least amount of obstruction of the camc;ra leans aperture, as
previously mentioned.
For the same reason, the body 13 of each injector 11, 11' containing the
manifold was located
on the exterior side of the injection holes.
The reason why two air injectors 11, 11' were used, is as follows. A complete
inspection sequence requires two scans, one for thc; bottomp art of the
cylinder wall and one for
the top part. For the second scan, the head rotates in the reverse direction
from the first one.
This reduces twisting of all the cables connecting; to the inspection head
without the need for
a "cable untwisting" reverse rotation between the two scans. Since better
contrasts were
obtained when the inspection head rotated in the direction of the flow, it was
found that two
injectors were actually required, one for the clockwise scan and one for the
counter-clockwise
one.
As aforesaid, Figure 7 also shows the 4:i°mirror 1. As is illustrated,
the mirror was
wider at the top where it was the most distant from the inspected area than at
the bottom, where
it was very close to it. Again, this was made to exploit the full aperture of
the camera lens, i.e.
every part of the inspected area "accesses" the whole aperture of the lens.
Figure 10 is a block diagramm of the central device of the inspection system
that was
developed and tested. As is shown, the developed system was controlled by a PC
type computer
which captured the camera's video signal throu~;h a frame-grabber card (also
called "video"
card) located in one of the computer's expansion slots and which also drove
the vertical and
rotational axis stepper motors through a stepper motor controller connected to
the computer's
printer port.
Figure 11 illustrates a typical screen display that was obtained after
inspection of a
cylinder with the system shown in Figures 6 to 10. The complete thermal image
of the
cylinder's inner surface was displayed. Darker areas are regions where the
cooling effect of the
air jet was most effective in reducing surface temperature, indicating
inferior heat transfer
properties that may be caused by delamination or other forms of sub-surface
defects.
The top and bottom scans produced the top and bottom part of the image which
were
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seamlessly morphed together to display the whole cylinder at once. Each half
image was
obtained by juxtaposing data from a single vertical line captured about 600
times during the
360° rotation of the head. Within the field of view of the camera, this
line was chosen as the
centerline of the area cooled by the air injectors. Since the frame grabber
card acquired 60
images/s from which 60 lines/s were extracted, each scan took approximately 10
seconds, and
therefore a complete inspection, a little over 20 seconds.
The software saved all cylinder images as it automatically sequenced through
inspection of the whole engine block. Various standard image analysis criteria
such as severity,
size or density of defects could be adjusted to provide a simple "passed" or
"failed" signal to
the operator.
"Failed" blocks were diverted out of the .assembly line for closer examination
by QC
personal later on who could recall individual cylinder images form these
blocks and make the
final decision to scrap or process them.
Of course, modifications could easily be made to the method and apparatus
disclosed
hereinabove and illustrated in the accompanying drawings without departing
from the scope
of the present invention.
