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					     Recent Developments of Eddy Current
                   Probes
Benoît MARCHANDa, Chiara ZORNIa , Jean-Marc DECITREa and, Olivier CASULAa
               CEA, LIST, F-91191 Gif-sur-Yvette, France



          Abstract. Eddy Currents (EC) technique is a powerful means of detection of
          defects in conductive components. The spectrum of industrial needs is large and EC
          probes have to be optimized to deal with different issues. Thanks to the CIVA-
          simulation platform, the CEA LIST develops high resolution arrays and high
          sensitive flexible probes to bring adapted solutions. On a first part, an original
          scheme, composed with two one-millimeter coils etched on a kapton film is
          presented. Experimental results coming from a flexible 32 elements of this sensor
          are shown and demonstrate the high performances of this technology in terms of
          sensitivity and spatial resolution. The second part of this paper concerns probes
          based on magnetic sensors. Two large arrays using respectively 22 Giant-Magneto
          Resistance sensors and 96 Anisotropic Magneto-Resistance sensors have been
          developed for the detection of very small surface flaws. Their high integration leads
          to a spatial resolution as small as 100µm. Experimental tests are performed and
          demonstrate their efficiency. Eventually, flexible magnetic sensor probes recently
          developed in the labs are presented and their performances in the detection of deep
          defects are valued.

          Keywords. Eddy Currents, multi-elements, flexible, micro-coils, magnetic sensors.




Introduction
Eddy Currents (EC) technique is a powerful mean of defects detection in conductive
components. The spectrum of industrial needs is large and EC probes have to be
optimized to deal with different issue. The first one is the detection of very small
surface flaws. This objective required high sensitivity and high spatial resolution for
fast and accurate inspection. The second one is the detection of deep defects that need
high sensitivity at low frequency. Planar or complex parts are concerned by both issues
and that is the reason why flexible probes have to be developed.
      Thanks to the CIVA-simulation platform, the CEA LIST develops high resolution
arrays and high sensitive flexible probes to bring solutions. In this paper we present
recent developments of such probes achieved in the laboratory.


1. Flexible Array Probes Based on Micro-Coils
Classical winding coils probes present limits coming from their technology itself:
They are hardly able to be integrated in arrays with small pitch, meaning high spatial
resolution. Too rigid, they can hardly be used in flexible probes for the inspection of
complex parts. To overcome these limits, the CEA-LIST has developed a new kind of
sensor that allows flexibility, high resolution and sensitivity at the same time. This
sensor consists in two micro-coils, one as emitter and the other one as receiver, etched
on both sides of a kapton film. Simulations, performed with CIVA software has led to a
special and new pattern, presenting an overlapping of the coils. As the efficiency of this
original sensor has been demonstrated by several experimental testing, large arrays
have been developed. Thanks to the small size of the micro-coils and using an
innovative staggered pattern, the measurement pitch has been reduced to only 350µm.
Furthermore, the kapton film is flexible and can easily be put on a silicone roll, whose
shape fits with the part that has to be inspected. Another consequence of this flexibility
is the reduction of the lift-off noise, since these sensors remain in contact with the
inspected surface [1]. Figure 1 is a photo of the 32 elements (meaning 64 micro-coils)
array probe, presenting a matrix setting (4 rows and 8 lines) that allows the scanning of
a 11mm width strip. Amplifiers have been put close to the sensor to improve the signal
to noise ratio (SNR).




                    Figure 1. Flexible micro-coils array probe with 32 elements.



     This probe is driven by the Eddy-Current device Multi-X, developed by M2M
Company. It allows the driving of 32 independent and multi-frequencies emitting
channels and 64 independent digital demodulators as receivers. Figure 2 is an
experimental testing of the probe. The objective is the detection of a 4mm long surface
notch, located in Inconel elbow mock-up. The 32 overlapped Lissajous curves can be
seen in the impedance plane, and on the right size the defect itself appears. On the top
part, the CSCAN corresponds to data coming from a time encoder trigger. Since the
pattern of the sensor is 4 staggered rows, the signature of the defect is a dot line.
Thanks to mechanical encoders, a real-time shift is carried out to give a continuous
CSCAN of the defect, as shown on the bottom part of the figure.
               Figure 2. Experimental testing: detection of a 4mm long surface notch



2. Probes Based on Magnetic Sensors
Magnetic sensors present a very interesting trade-off between size and sensitivity.
Since their sensitive area is only few micrometers, large high spatial resolution arrays
can be achieved [2]. Furthermore, their large frequency bandwidth allows to using them
for the detection of small surface breaking flaws, when high frequency is required, or
else, for the detection of deeply buried defects, at low frequency [3].

2.1. Large Magnetic Sensors Arrays

Two large magnetic sensors arrays have been developed by the CEA LIST. The first is
based on Giant Magneto-Resistance (GMR) sensors. It’s a linear array, composed with
22 GMR, which sensitive area is a rectangle of 70x8µm2. The 100µm only pitch gives
to this probe a very high spatial resolution, which makes the inspection fast and
efficient. The emitter is a current foil, put above the array.

     Figure 3 presents experimental results that show the efficiency of the probe in the
detection of small surface breaking flaws. An inconel mock-up in which three defects
of different length (200x100x200 µm3, 100x100x200 µm3 and 100x100x100 µm3) has
been used.
     The second magnetic sensor array has been developed in the framework of the
IMAGINE project, supported by the French National Research Agency. It consists of a
linear array composed with 96 Anisotropic Magneto-Resistance sensors. The challenge
of this probe was the integration of so many sensors, with a pitch as small as 100µm to
give to the probe its high spatial resolution. Therefore, almost a 10mm width strip is
inspected which reduces the scanning time but not the efficiency of the probe
considering the small space between two consecutive sensors. The emitter is a large
current foil. Besides, to improve the SNR, a special PCB containing amplifiers has
been added next to the sensors. Figure 4 is a photo of the probe and a zoom of its
sensitive area. As the objective is the detection of small surface defects, the magnetic
sensor array has been put as close as possible of the inspected surface.
     Voie : test                                                                                                         Interpolation : Non
     Composante : Amplitude                                                                                                 Extraction : Oui
                                                                                                                                                Voie : test                                                                                                       Interpolation : Non    Voie : test                                                                                            Interpolation : Non
     Equilibrage : aucun                                                                                              Axe Orthonormé : Oui
                                                                                                                                                Composante : Amplitude                                                                                               Extraction : Oui    Composante : Amplitude                                                                                    Extraction : Oui
     Normalisation : 1.000V 0.000deg                                                                         Echelle de Couleur Fixée : Non
                                                                                                                                                Equilibrage : aucun                                                                                            Axe Orthonormé : Oui      Equilibrage : aucun                                                                                 Axe Orthonormé : Oui
                                                                     test                                                                       Normalisation : 1.000V 0.000deg                                                                       Echelle de Couleur Fixée : Non     Normalisation : 1.000V 0.000deg                                                            Echelle de Couleur Fixée : Non
                                                                                                                                       2.4306
                                                                                                                                                                                                              test                                                                                                                               test
                               4
                                                                                                                                                                                                                                                                                1.1358                                                                                                                       0.56645
                                                                                                                                                                          3.4
                              3.8                                                                                                                                                                                                                                                                                                                                                                         0.55
                                                                                                                                                                                                                                                                             1.1
                                                                                                                                                                                                                                                                                                                 2.6
                                                                                                                                                                          3.2
                              3.6                                                                                                     2                                                                                                                                        1                                                                                                                           0.5

                                                                                                                                                                           3                                                                                                                                     2.4
                              3.4                                                                                                                                                                                                                                            0.9                                                                                                                          0.45

                                                                                                                                                                          2.8                                                                                                0.8                                                                                                                           0.4
                              3.2                                                                                                   1.5
                                                                                                                                                                                                                                                                                                                 2.2
                                                                                                                                                                          2.6                                                                                                0.7                                                                                                                          0.35
                         mm




                               3




                                                                                                                                                                     mm




                                                                                                                                                                                                                                                                                                            mm
                                                                                                                                                                                                                                                                             0.6                                  2                                                                                        0.3
                                                                                                                                                                          2.4
                              2.8                                                                                                     1
                                                                                                                                                                                                                                                                             0.5                                                                                                                          0.25
                                                                                                                                                                          2.2
                              2.6                                                                                                                                                                                                                                                                                1.8
                                                                                                                                                                                                                                                                             0.4                                                                                                                           0.2
                                                                                                                                                                           2
                              2.4                                                                                                   0.5                                                                                                                                      0.3                                                                                                                          0.15
                                                                                                                                                                                                                                                                                                                 1.6
                                                                                                                                                                          1.8
                              2.2                                                                                                                                                                                                                                            0.2                                                                                                                           0.1

                                                                                                                                                                          1.6                                                                                                0.1                                 1.4                                                                                      0.05
                               2                                                                                                      0
                                    25   25.2   25.4   25.6   25.8    26    26.2   26.4   26.6   26.8   27
                                                                                                                                    0.0074744
                                                                     mm                                                                                                           45.2   45.4   45.6   45.8   46     46.2   46.4   46.6   46.8   47                                                                    65.4   65.6   65.8   66          66.2   66.4   66.6   66.8
                                                                                                                                                                                                                                                                             0.0092172                                                                                                                     0.0089227
                                                                                                                                                                                                              mm                                                                                                                                 mm



           200x100x200                                                                                                                                100x100x200                                                                                                                                                 100x100x100
              Défaut n°1                                                                                                                                 Défaut n°2                                                                                                                                                  Défaut n°3
       µm3 200x100x200 µm3                                                                                                                         µm3100x100x200 µm3                                                                                                                                             µm3
                                                                                                                                                                                                                                                                                                                  100x100x100 µm3
                                                                             Figure 3. Detection of three small flaws in an inconel mock-up.




                                                                                                                                                Figure 4. 96 AMRs sensor probe.



    Both magnetic sensor arrays can be driven by the Multi-X Eddy Current device.

2.2. Flexible Magnetic Sensor Probes
     A common issue in ECT is the detection of defect buried in planar or in complex
geometry parts. For this particular need, the CEA LIST has developed three flexible
probes based on magnetic sensors [4]: Giant-Magnetic Resistance (GMR), Giant-
Magnetic Impedance (GMI) and µfluxgate. The GMR one is shown on Figure 5. Their
design is the following: two coils can be used as emitter, the first one is 15mm from the
sensor and the other one is 20mm distant. This configuration allows to improving the
inspection depending on the deepness of the defect to look for. Coils and magnetic
sensor are embedded in silicone that gives flexibility. Parts with a curvature radius up
to 30mm can be inspected. The sensitivity axis of the magnetic sensor is perpendicular
to the surface.
     CSCANs presented in Figure 6 are experimental data obtained using the probe in
which the sensor is a GMR, 15 mm distant from the winding coil emitter. The
inspected is a multilayer Inconel 600 mock-up, containing a 10x0.1x0.78 mm3 flaw.
This mock-up allows to studying the detection efficiency of the probe, regarding the
deepness of the defect. For each configuration the frequency is first valuated by using
the skin depth relationship given by:
                                              f 
                                                              1
                                                                  2



and then experimentally optimized.


                                          magnetic sensor :
                                          GMI, μFluxgate
                                          or GMR

  flexible

                                          emitter :
                                          Winding coils




                Figure 5. Flexible GMR probe for detection of deeply buried defects.




             Figure 6. Detection of a 10 mm long defect vs ligament – experimental data.



    Signal-to-noise ratios have been computed and reveal an almost linearly decrease
with respect to the ligament of the defect, considering the ligament slot [0 – 4] mm, as
shown on Figure 7
                                                    Experimental results




               Figure 7. SNR vs ligament - GMR sensor probe - Experimental results.


3. Conclusions

The innovative pattern, based on staggered rows and presenting an overlapping of the
micro-coils, has been designed using CIVA software. It shows great interests in NDT
probe development. Several flexible array probes based on this sensor technology have
already been achieved. As show in this paper, their high spatial resolution and lift-off
noise reduction, coming from their flexibility, allow them to detect small surface
breaking flaws with good SNR. As the silicone roll can be changed and adapted to the
inspected area, they are able to inspect a large spectrum of complex parts.
     The advantages of magnetic sensors in terms of size and frequency bandwidth
sensitivity have been used to develop high performances probes for special needs: on
one hand, large arrays for the fast and accurate surface inspection, on the other hand
flexible probe for the inspection of deep buried defects in complex parts.
     Experimental results have been shown to demonstrate the good performances of
the advanced NDT probes.


References
[1] Gilles-Pascaud C, Vacher F, Decitre JM, Cattiaux G, EC Array Probe Development
For Complex Geometries, 5th ICNDE, San Diego, July 2006.
[2] T.Dogaru and S. T. Smith, Giant Magnetoresistance-Based Eddy-Current Sensor,
IEEE Transactions on Magnetics, Vol. 37, No. 4, pp.2790-2793, 2001.
[3] Marchand B, Vacher F, Decitre JM, Gilles-Pascaud C, Fermon C, High Resolution
Eddy Current Probes For Non Destructive Testing, QNDE 2007, Golden, July 2007.
[4] Decitre JM, Casula O, Non Destructive Testing with GMR Magnetic Sensor Arrays,
ECNDT, Berlin, 27-29 September 2006.

				
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posted:8/14/2011
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