DEVICE FOR INSPECTING THE SURFACE OF AN ELECTRICALLY
CONDUCTIVE PART1
Technical context
The invention lies in the field of techniques for
inspecting mechanical parts, in particular engine parts,
for example parts of turbine engines, and in particular
parts that include holes.
Numerous engine parts are critical, and if they
10 rupture they are likely to have severe repercussions on
the entire system, e.g. an aircraft.
The present invention relates particularly, but not
exclusively, to parts in Ylhich a hole has been made.
Such holes, Ylhich may be of various shapes, and in
15 particular of elongate shapes Ylith a middle axis, might
include flaYis, i.e. surface roughnesses that may act as
starting points for rupture of the part.
Such fla\~S may appear Ylhile the hole is being
machined, e:·g. by using electrical discharge machining
20 (EDM), or else Ylhile the part is in use, e.g. in an
engine.
In numerous situations, it is necessary to inspect
parts to ensure that there is no risk caused by the
presence of such fla\ols. Such inspections may be
25 performed on parts that are ne\ol or on parts that have
already been used.
One kno\oln inspection technique is the eddy current
method, consisting in verifying that the material is
continuous by measuring currents that have been induced
30 by a magnetic field.
Such a method is performed using a sophisticated
measurement device generally having a plurality of eddy
current probes or points. Such devices are knmm that
are mounted on articulated metal arms Ylith springs or
t Translation of the title as established ex officio.
2
brushes, which are fragile, thereby constituting a first
problem.
Furthermore, the construction of such probe systems
generally needs to be revised whenever attention is given
5 to a hole of a shape that is different from the shape of
the hole for 1vhich the system was initially designed, in
order to take account of the curvature of the middle axis
of the hole. Thus, the positioning and the length of the
arms or the brushes need to be revised, 1-1hich is
10 complicated and expensive. Furthermore, it is very
difficult to position such devices 1-1hile using an
automatic protocol, Hith a robot or Hith complex
positioning tooling.
Finally, it is often necessary to change the probe
15 in order to finish off observations both on a given hole
and also on its opening or its end. Specifically,
guiding a system of probes at the opening or at the end
of a hole is particularly aHkl-lard, and it is preferred to
use speci·fic ·probes for those difficult zones.
20 Finally, existing systems are complex to implement,
25
30
and excessively expensive because of the multiplicity of
devices that need to be designed for holes that are
different or even for only a single type of hole.
The invention seeks to resolve those difficulties.
Definition of the invention and the associated advantages
To solve these difficulties, there is proposed a
device for inspecting a surface of an electrically
conductive part, in particular the inside surface of a
hole, the device having a plurality of eddy current
probes arranged on a convex surface of the device
together 1-1ith application means for applying the probes
against the surface to be inspected into 1-1hich the.device
is inserted, the device being characterized in that the
35 probes are fastened on flexible strips extending beside
one another in a longitudinal direction of the device,
said application means comprising a deformable material
3
that, on being compressed along said longitudinal
direction, gives rise to expansion transversely to the
longitudinal direction, said expansion deforming said
strips so as to apply the probes against the surface.
5 By means of this device, a tool is made available
that is much simpler to use, that is capable of adapting
to numerous situations, and that is robust and
inexpensive to fabricate and to use. The number of eddy
current probes arranged on the tool may be large, insofar
10 as there is no need to use a respective metal arm with a
spring for each of them.
15
The probe may be handled by a robot, and measurement
is entirely satisfactory, because of the way the probe
adapts to the surfaces against which it is applied.
The surfaces in question are constituted in
particular by the inside surfaces of holes, however the
device can be used for inspecting other surfaces. In
general manner, with this device there is no need to
develop numerous probes for one or more particular
20 applications, since the device can be adapted to most
configurations. In particular, when inspecting the
surfaces of holes, the device generally makes it possible
to inspect not only the main surface of the hole, but
also its end wall and its opening, thereby providing an
25 improved field of observation. Finally, the time
required for inspection is shortened.
Depending on the embodiment, the flexible strips are
flexible printed circuits, which are advantageous since
they are in expensive, or flexible metal strips. The
30 deformable material may be silicone or any other
deformable material.
In an advantageous embodiment, the deformable
material is placed longitudinally between two compression
parts, and the longitudinal compression of the deformable
35 material is obtained by using a cable that is fastened to
one of the two parts and that is slidable relative to the
second part. In certain embodiments, it is proposed that
4
the compression in the longitudinal direction is limited
by an abutment.
Advantageously, said convex surface is a circular or
elliptical circumference of the device. This makes it
5 possible to inspect holes for which the circumference of
the device is adapted.
In such a configuration, the device includes at
least one additional set of flexible strips on a
circumference of the device, the strips being compressed
10 outwards from the device in order to guide the probe·
perpendicularly to the \vall of the hole or to protect the
eddy current probes on the device entering or leaving the
hole. The device thus protects the eddy current probes
so as to avoid them being damaged when the device is
15 being inserted into or extracted from the hole that is to
be inspected.
The device may include two such additional sets of
flexible strips, referred to as "guides", one upstream
from said plurality of eddy current probes and the other
20 downstream from said plurality of eddy current probes.
25
The flexible strips of the guide assembly preferably
exert pressure on the walls that is greater than the
pressure exerted by the flexible strips carrying the
probes when they are applied against the walls.
The invention also provides a method of fabricating
a device as described above, comprising forming slits in
a flexible plate, so as to form flexible strips that are
held together at their ends, so as to carry at least one
probe per strip, and a step of molding the deformable
30 material against the face of the flexible plate that is
opposite from the eddy current probes. This fabrication
method is particularly simple and practical to perform,
and constitutes one of the contributions of the present
invention.
35 The invention is described below with reference to
the figures.
5
List of figures
Figure 1 shows a part including a hole that is to be
inspected.
Figure 2 shows an inspection device that has been
5 developed to inspect the hole in the part of Figure 1, in
compliance with the principles of the invention.
Figure 3 sh01vs a step in fabricating such an
inspection device.
Figure 4 sh01vs a subsequent fabrication step.
10 Figure 5 shows a central assembly of the Figure 2
inspection device, obtained after performing the steps
shown in Figures 3 and 4.
Figure 6 shows the same assembly in operation.
15 Detailed description
With reference to Figure 1, there can be seen a part
for inspection, specifically a disk 10, having an axis 11
on the right-hand side of the figure, and including an
e1ongate·hole 12 of elliptical section and of curvilinear
20 axis. Until now, it has been necessary to use a
plurality of eddy current probe tools for inspecting such
a hole. Unfortunately, such tools are fragile and
expensive, thereby making the operation difficult.
Furthermore, the hole needs to be inspected in
25 particularly careful manner since the part in question is
extremely critical.
It should be observed at this point that the hole 12
has two openings, since both of its ends open out into
faces of the part, however there are numerous situations
30 in which it is likewise necessary to inspect holes having
only one opening.
Figure 2 shows the inspection device 100 designed
for searching for flaws in the hole 12. The device is of
elongate shape, and it comprises a flexible drive sheath
35 110 having a head 120 fastened to one of its ends, which
head is made up of a plurality of elements. More
precisely,_ the head comprises, starting from the sheath
6
110 and going towards the free end of the device: a first
assembly that is flexible in compression, referred to as
a ''guide'' 121; a flexible connection 122; an adjustable
assembly that is flexible in compression, constituting
5 the functional core 123 of the inspection device; a
second flexible connection 124; and a second assembly
that is flexible in compression and referred to as a
"guide" 125. A tip 126 in the form of a cone terminates
the device. It should be understood that even though the
10 guides 121 and 125 are advantageous, they are not
absolutely essential for performing the invention.
Concerning the functional core, the concept
governing the operati-on of the functional core is novel
and, compared with prior devices, it makes it possible to
15 increase the number of eddy current probes that can be
applied against the surface for inspection, while also
reducing the associated inspection time.
Figure 3 shows a.step in a process of fabricating an
inspecti-on-devi-ce of the kind shown in Figure 2. A
20 flexible printed circuit plate 300 of rectangular shape
is formed w_ith through slits 310, 311, 312, that are
mutually parallel, and in this example parallel to the
long dimension of the plate. By way of. example, each
slit extends lengthwise over the central two-thirds of
25 the plate 300. The plates are spaced apart from one
another equidistantly so as to define strips 320, 312,
322, ... , that are held to one another by the non-slit
material outside the central t\vo-thirds of the plate 300.
Between two adjacent slits, at equal distances from each
30 of them, and halfway along them, eddy current probes 320,
331, 332 are put into place either before or after the
slits are made. Each probe is provided with a respective
electrical connection 340, 341, 342,
Figure 4 shows the subsequent step of fabricating
35 the inspection device. The plate 300 is rolled into a
cylinder 400 around an axis that is parallel to the
slits, so that the probes face outwards, and so that
7
deformable material 490 can be injected or cast into the
inside of the cylinder through one of its two openings in
order to match its shape by molding.
Prior to injecting or casting the deformable
5 material 490, the electrical connections 340, 341, 342,
of the probes are arranged so as to exit the cylinder
via its second opening. A pin 410 is placed in the
center of the cylinder so as to reserve a cylindrical
bore in the middle of the injected or cast material. An
10 end wall 420 also serves to prevent the deformable
material 490 escaping from the cylinder through the
second opening. Finally, a banding tool 430 (or outer
molding tool) surrounds the strips to prevent them from
splaying axially while the deformable material 490 is
15 being deposited. The tool preferably has the same shape
as the hole to be inspected for which the probe is being
fabricated, and it preferably has dimensions that are
slightly smaller than those of the hole.
It .. shou'lx:l .. be understood that the cylinder shown in
20 the figures is on a circular base, but that it is also
possible to use a cylinder on a base_ that is elliptical
or indeed of some other shape.
As mentioned at the end of this document, the plate
300 is not necessarily shaped to take the form of a
25 closed cylinder, although the example shown relates to
such a closed cylindrical shape, and in any event it
becomes deformed to take on the shape of a convex
cylinder, within 1vhich the molding is performed.
In a particular embodiment, the assembly of Figure 4
30 is made by placing the plate of Figure 3 on two supports
(not shmvn) that define the upstream and do~mstream
openings of the structure. A sheath or adhesive tape
serves to hold the printed circuit on the support and to
provide the necessary sealing at these openings. In
35 certain embodiments, the pin 410 is used to align the t1-10
supports. In this embodiment, once the cylinder has been
formed, it is inserted in two molding preforms, one being
a central cylindrical
and the other being an
the banding tool 430).
8
axis (corresponding to the pin 410)
outer enclosure (corresponding to
Both molding preforms are coated
in unmolding substances, such as Teflon (registered
5 trademark), v1hich may be obtained by spraying or by being
deposited in sheet form. Teflon sheets also make it
possible to leave clearance between the outer enclosure
and the printed circuit. A stopper (corresponding to the
end wall 420), e.g. a silicone stopper, is put into place
10 to obstruct one of the openings of the cylinder, through
which the electrical connection wires for the probes are
nevertheless extracted.
The deformable substance 490 is injected or cast in
such a manner as to fill the volume of the cylinder 400,
15 in part or in full.
Once the deformable material 490 has been injected
or cast and once it has finished setting, the banding 430
and the pin 410 are removed. The banding has enabled the
deformabl>e"'Substance 490 to take the shape of the
20 cylinder 400 without deforming it, and the pin 410 has
served to create a central cylindrical bore from one end
of the device to the other, inside the deformable
substance 490.
Figure 5 shows the device at a later step in its
25 fabrication. A cable 510 has been inserted in the bore
created that the center of the device and it is fastened
via one end to a presser part 520 that is placed in one
of the openings of the cylinder 400 so as to be in a
position to press against the deformable substance 490
30 molded inside the cylinder 400. Where necessary, the
presser part 520 is fastened to the cylinder 400, e.g.
with an adhesive tape or with a sheath placed on the end
of the cylinder. The cable 510 leaves the cylinder 400
via the other opening, either through or beside a
35 retaining part 530 that also bears against the deformable
substance 490 and that, \~here necessary, is fastened to
9
the cylinder 400, e.g. using adhesive tape or a sheath
placed on the end of the cylinder.
By way of example, the presser and retaining parts
520 and 530 may be the support used for holding the plate
5 300 in the form of the cylinder 400 prior to molding.
Other solutions are possible, it being possible for these
two parts to be inserted into the structure at various
potential moments. The end wall 420 as shown in Figure 4
is retained; however in certain embodiments, it could be
10 removed, where necessary, once the deformable substance
490 has been molded.
The cable 510 may in particular be crimped onto the
presser part 520. Furthermore, the cable 510 may have a
stroke that is limited by an abutment formed by a pair
15 constituted by a cable tube (not shown) and an abutment
washer (not shown). In such an embodiment, the cable 510
is inserted in a tube that begins level with the presser
part 520 and that terminates, when the cable is not being
20
subjected--to traction, at a given
outlet of the retaining part 530.
distance from the
The cable 510 and the
tube are secured to each other for movement in
translation relative to the deformable substance 490 and
the retaining part 530. The abutment washer surrounds
the cable at its exit from the retaining part 530,
25 allowing it to move, but preventing movement ()f the tube.
Thus, when traction is applied to the cable 510, its
stroke is blocked by the tube meeting the washer.
As shown in Figure 6, beside the retaining part 530,
the cable 510 may be subjected to traction that acts by
30 pulling on the presser part 520. Traction on the cable
510 then leads to longitudinal pressure (in the direction
L) between the presser part 520 against the deformable
substance 490 molded in the cylinder 400. The function
of the retaining part 430 is to retain the deformable
35 material, which is then being compressed longitudinally.
This has the consequence of the deformable material
bearing against the walls of the cylinder, enabling the
10
strips 320, 321, 322, ... to expand radially, the strips
becoming curved and splaying apart from one another.
Preferably, since the retaining and presser parts
530 and 520 are arranged symmetrically relative to the
5 eddy current probes 330, 331, 332, ... , and relative to
the mass of deformable substance 490 molded in the
connect, the maximum expansion of the deformable
substance takes place along the circumference of the
cylinder that carries the probes 330, 331, 332, ... , so
10 that, when the device is in use, the probes are pressed
against the wall of the hole that is to be inspected and
into vlhich the device has been inserted. The length of
the stroke of the cable determines the maximum expansion
that can be given, in use, to the circumference of the
15 cylinder, and thus the pressure with which the eddy
current probes 330, 331, 332, ... , are applied. It is
proposed to define this length by performing tests on the
probe, as a function of the type of hole that is to be
inspected.
20 It should be understood that other longitudinal
compression systems could be used, for the purpose of
causing the deformable substance 490 to expand in a plane
perpendicular to the direction L.
Compression is controlled from a control station
25 (not shown), by a human operator or by a robot.
Returning to the structure shown in Figure 2, the
guides 121 and 125 may be made in the same manner as the
functional core 123, using the same principles as those
shmm in Figures 4 to 6, while naturally omitting the
30 eddy current probes and their electrical connections.
However for the guides 121 and 125, the compression of
the flexible printed circuit may be defined in final
manner on assembly, by securing the second end of the
cable 510 or by setting the compression 1-1ith means other
35 than a cable. The compression could also be adjusted
when,making the hole that is to be inspected, in order to
take account of the exact dimensions of the hole.
11
This is different from that 1·1hich is done with the
functional core 123, where the cable 510 enables the
degree of compression to be controlled from a control
station, after the device has been inserted inside the
5 hole for inspection. On this topic, it should be
understood that the cable 510 of the functional core 123
and the electrical connections 340, 341, 342, ... are
taken to a control station inside the flexible drive
sheath 110.
10 The invention is not limited to the embodiments
described, but extends to any variant coming Hithin the
ambit of the scope of the claims.
In particular, it is not necessary to place the eddy
current probes around a closed circumference of the
15 device, in particular in the event Hhere the surface that
is to be inspected does not constitute a closed section
of a hole.
It is thus possible to use the principles of the
invention·Hh'i'!:e· shaping the plate 300 into a cylinder
20 having a base that is a semicircle or half an ellipse,
thereby constituting a surface that is convex, Hith its
shape peing closed by an extra part, loJhich may for
example be plane, and Hhich is preferably not deformable.
The deformable substance (silicone) is then molded in a
25 cavity formed by the plate 300 and the extra part. Thus,
the effect of deforming the strips takes place only over
a semicircle or ~alf an ellipse.
Independently of the above comments, it should also
be understood that in order to implement the invention it
30 is not essential for the probes to be equidistant, nor
even for them to be uniformly distributed over the convex
surface, nor is it essential for all of the strips to be
of the same loJidth.
12

CLAIMS
1. A device (100; 500) for inspecting a surface of an
electrically conductive part, the device having a
plurality of eddy current probes (330, 331, 332, ... )
5 arranged on a convex surface of the device together with
application means for applying the probes against the
surface to be inspected into which the device is
inserted, the device being characterized in that the
probes (330, 331, 332, ... ) are fastened on flexible
10 strips (320, 321, 322, ... ) extending beside one another
in a longitudinal direction (L) of the device, said
application means comprising a deformable material (490)
that, on being compressed along said longitudinal
direction (L), gives rise to expansion transversely to
15 the longitudinal direction, said expansion deforming said
strips (320, 321, 322, ... ) so as to apply the probes
(330, 331, 332, ... ) against the surface.
2. A devi·ce according to claim 1, wherein the flexible
20 strips (320, 321, 322, ... ) are flexible printed
circuits.
25
3. A device according to claim 1, wherein the flexible
strips (320, 321, 322, ... ) are flexible metal strips.
4. A device according to any one of claims 1 to 3,
wherein the deformable material (490) is silicone.
5. A device according to any one of claims 1 to 4,
30 wherein the deformable material (490) is placed
longitudinally bet1veen two compression parts ( 520, 530) ,
and the longitudinal compression of the deformable
material (490) is obtained by using a cable (510) that is
fastened to one of the two parts (520) and that is
35 slidable relative to .the second part (530).
13
6. A device according to any one of claims 1 to 5,
wherein the compression in the longitudinal direction is
limited by an abutment.
5 7. A device according to any one of claims 1 to 6,
wherein said convex surface is a circumference of the
device.
8. A device according to claim 7, 1·1herein the device
10 further includes at least one additional set of flexible
strips (121, 125) on a circumference of the device, the
strips being compressed outwards from the device in order
to guide the probes perpendicularly to the wall of the
hole or to protect the eddy current probes on the device
15 entering or leaving the hole.
9. A device according to claim 8, wherein it further
includes two such additional sets of flexible strips
(121, 125)-, ··one upstream £rom .said plurality of eddy
20 current probes (330, 331, 332, ... ) and the other
downstream from said plurality of eddy current probes
(330, 331, 332, ... ).
10. A device according to claim 8 or claim 9, wherein the
25 flexible strips o£ the guide assembly (121, 125) exert
pressure on the walls that is greater than the pressure
exerted by the flexible strips (320, 321, 322, ... )
carrying the probes (330, 331, 332, ... )when they are
applied against the walls.
30
11. A method of fabricating a device according to any one
of claims 1 to 10, comprising forming slits (310, 311,
312, ... ) in a flexible plate, so as to form flexible
strips (320, 321, 322, ... ) that are held together at
35 their ends, so as to carry at least one probe per strip,
and a step of molding the deformable material (490)
14
against the face of the flexible plate that is opposite
from the eddy current probes.