Abstract:
A device for non destructive evaluation of defects in a metallic object ( 2 ) by eddy currents, comprises a field emitter ( 3 ) for emitting an alternating electromagnetic field at a first frequency fi in the neighborhood of the metallic object ( 2 ), and a magnetoresistive sensor ( 1 ) for detecting a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in the metallic object ( 2 ). The device further comprises: a driving circuit ( 230 ) for driving the magnetoresistive sensor ( 1 ) by a current at a second frequency fc which is different from the first frequency fi, so that the magnetoresistive sensor ( 1 ) acts as an in situ modulator; a detector for detecting a response signal between the terminals of the magnetoresistive sensor ( 1 ); a filter for filtering the response signal detected by the magnetoresistive sensor ( 1 ) to keep either the frequency sum (fi+fc) of the first and second frequencies or the frequency difference (fi−fc) of the first and second frequencies, and a processor for processing the filtered response signal and extract eddy current information on defects in the metallic object ( 2 ).

Description:
This application is a §371 national phase filing of PCT/EP2006/002599 filed Feb. 24, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a device for non destructive evaluation of defects in a metallic object by eddy currents. 
     The invention more specifically relates to a method for non destructive evaluation of defects in a metallic object by eddy currents, the method comprising the steps of emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object and detecting through at least one magnetoresistive sensor a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object. 
     2. Description of the Related Art 
     The technique of detection of defects in metallic objects by observation of the deformation of eddy currents lines has been well known and widely used for a long time. 
     The principle is to use an electromagnetic field emitter in the neighbourhood of the metallic object for creating eddy currents in this object. These eddy currents retransmit a radiofrequency field which can be detected by a receiver. In case of a presence of a defect, the current lines are perturbed and the electromagnetic field which is reemitted is modified in its amplitude and its phase distribution but its frequency remains unchanged. 
     Classical approaches use inductive coils as emitter of a radiofrequency field and as receiver of the re-emitted radiofrequency field. 
     More recently, the use of other kinds of sensors, like magneto-resistive sensors, has been proposed for the receiver part. Magneto-resistive elements (MREs) based on the so called “giant magnetoresistive effect” (GMR effect) have been demonstrated, for example in spin valves consisting of two adjacent magnetic layers, whose resistance R varies as a function of the applied magnetic field. In these GMR devices, variations of resistance ΔR max  have been observed with ΔR max /R-values as large as 15%. 
     Some patents and publications show magneto-resistive sensors applied for eddy current (EC) testing. 
     For instance, in the document US2005/0140355 A1, a specific embodiment of the excitation coil on a printed circuit board (PCB) is used and a magnetoresistive (MR) sensor is located on the opposite side of the PCB to detect the magnetic field related to the eddy currents modified by a defect. 
     In the document US2005/0062470 A1, an eddy current probe having a non planar form allows the displacement of the probe in a close proximity to the object&#39;s surface avoiding an adsorption phenomenon due to static friction. 
     In these documents, the designs of the excitation loop and the sensor are addressed to improve the detection. 
     In the document U.S. Pat. No. 6,693,425 B2, the excitation current in the coil varies to obtain a varying field penetrating in the component under test. This invention allows scanning defects of varying depths. 
     In all these cases, even if the sensitivity is increased, the sensor which might be either a coil or a magnetoresistive element, detects the signal of the eddy current at a frequency equal to the excitation coil&#39;s frequency, and is therefore limited in term of signal-to-noise ratio. 
     SUMMARY OF THE INVENTION 
     The present invention aims at overcoming the shortcomings and drawbacks of the methods and devices of the prior art. 
     More specifically a main object of the present invention is to increase the signal-to-noise ratio and to get rid of the main fluctuations of sensors due for example to the temperature drift or to ageing. 
     These aims are achieved due to a method for non destructive evaluation of defects in a metallic object by eddy currents, the method comprising the steps of emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object and detecting through at least one magnetoresistive sensor a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object, 
     characterized in that it comprises the further steps of driving said at least one magnetoresistive sensor by a current at a second frequency fc which is different from said first frequency fi, so that said at least one magnetoresistive sensor acts as an in situ modulator and filtering the response signal detected by said at least one magnetoresistive sensor to keep either the frequency sum of said first and second frequencies or the frequency difference of said first and second frequencies before processing said response signal to extract eddy current information on defects in said metallic object. 
     The signal due to the eddy current is unambiguously different from the excitation signal and can therefore be highly amplified, thus increasing the signal-to-noise ratio. 
     A main advantage of the method according to the invention lies in the fact that it is possible to get rid of the main fluctuations of the magnetoresistive sensor due for example to the temperature drift or to the ageing. Furthermore, a lot of noise due to the electromagnetic interference (EMI) at the field emitter frequency may be avoided. This interference is due to the field induced by the emitter and preferentially appears for frequencies greater than 100 kHz. 
     According to a preferred embodiment, the filtering step comprises filtering the response signal detected by said at least one magnetoresistive sensor to keep the frequency difference of said first and second frequencies before processing said response signal to extract eddy current information on defects in said metallic object. 
     In such an embodiment, the alternating electromagnetic field is preferably emitted at a single first frequency f 1  which is higher than 100 kHz. 
     According to a specific embodiment, the method comprises the steps of amplifying a voltage between first and second terminals of said magnetoresistive sensor, to obtain an amplified voltage and sending said amplified voltage to a signal input of a mixing and filtering system and mixing a frequency reference signal at said second frequency fc with a frequency reference signal at said first frequency f 1  in a multiplier to obtain a product reference signal f 1 −fc which is applied to a reference input of the same mixing and filtering system whose output is processed as an ordinary output signal of an eddy current testing method. 
     This at least one magnetoresistive sensor has a sensing axis which may be placed either orthogonally or parallely to the emitted alternating electromagnetic field. 
     According to a particular embodiment, the response signal is detected through an array of sensors which are used as in-situ demodulators and are able to detect the different components of the return electromagnetic field which are due to the modification of the eddy currents by a defect. 
     According to a specific embodiment, the at least one alternating electromagnetic field is emitted in the neighbourhood of the metallic object at a set of different first frequencies which are all different from said second frequency fc. 
     In such a case, advantageously, the filtering step comprises filtering the response signal detected by said at least one magnetoresistive sensor to keep the frequency differences of said first and second frequencies before processing said response signal as a simple demodulated signal giving the useful signal created by the modification of the eddy currents by a defect. 
     According to another specific embodiment of the method according to the invention, the at least one magnetoresistive sensor has a non linear behaviour and the response signal detected by said at least one magnetoresistive sensor is filtered to keep either the frequency sum of said first frequency and n times the second frequency or the frequency difference of said first frequency and n times the second frequency, where n is an integer, before processing said response signal to extract eddy current information or defects in said metallic object. 
     The invention further relates to a device for non destructive evaluation of defects in a metallic object by eddy currents, comprising at least one field emitter for emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object, and at least one magnetoresistive sensor for detecting a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object, characterized in that it further comprises:
         driving means for driving said at least one magnetoresistive sensor by a current at a second frequency fc which is different from said first frequency fi, so that said at least one magnetoresistive sensor acts as an in situ modulator,   detecting means for detecting between the terminals of the magnetoresistive sensor a response signal,   filtering means for filtering the response signal detected by said at least one magnetoresistive sensor to keep either the frequency sum of said first and second frequencies or the frequency difference of said first and second frequencies, and   processing means for processing said filtered response signal and extract eddy current information on defects in said metallic object.       

     According to a particular embodiment, the detecting means comprises amplification means for detecting reference signals at said at least one first frequency fi and at said second frequency fc, multiplying means for mixing said at least one first frequency fi and said second frequency fc and at least a lock-in amplifier for detecting the frequency sum of said first and second frequencies or the frequency difference of said first and second frequencies. 
     The magnetoresistive sensor may comprise multiple contact points for voltage measurements or an array of sensors. 
     The magnetoresistive sensor may be a Hall effect sensor or else an anisotropic magnetoresistive sensor (AMR), a giant magnetoresistive sensor (GMR), a giant magnetoimpedance sensor (GMI) or a tunnel magnetoresistive sensor (TMR). 
     The magnetoresistive sensor may be constituted by any element having a resistance presenting a variation as a function of an applied external field. 
     The magnetoresistive sensor may be built on different kinds of substrates, e.g. a very thin silicon substrate, a bevelled substrate or a flexible substrate. 
     According to a specific embodiment, the magnetoresistive sensor has a yoke shape, the length of the yoke and the length of the lateral arms of the yoke each are at least three times the width of the yoke, and the width of the yoke is comprised between 2 μm and 12 μm. 
     Advantageously, the at least one field emitter is a planar coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will appear more readily from the following description of several embodiments of the present invention, given as examples, with reference to the enclosed drawings, on which: 
         FIG. 1  is a block diagram illustrating the main components of a device according to the present invention, 
         FIGS. 2 to 6  diagrammatically show different embodiments of a device for implementing the present invention, 
         FIG. 7  is a schematic cross-section of the main components used for a non destructive control by eddy currents according to a first specific embodiment, 
         FIG. 8  is a schematic cross-section of the main components used for a non destructive control by eddy currents according to a second specific embodiment, with a sensor placed in the plane of a field emitter, 
         FIG. 9  is a schematic drawing of a specific emitter comprising two coils which can be used in a device according to the invention, 
         FIG. 10  shows a specific configuration with the relative positions of a field emitter and a sensor which may be implemented according to the invention, 
         FIG. 11  is a schematic cross-section of a GMR or TMR stack in a spin valve configuration, 
         FIG. 12  shows an example of yoke-shape GMR sensor which may be used in a device according to the invention, 
         FIG. 13  shows another example of GMR sensor which comprises multiple channel output, 
         FIG. 14  is a cross-section of a specific measurement device according to the invention, where a sensor is deposited on a thin silicon substrate, 
         FIG. 15  is a cross-section of a specific measurement device according to the invention, where a sensor is deposited on a bevelled glass substrate, 
         FIG. 16  is a cross-section of a specific measurement device according to the invention, where a sensor is deposited on a flexible substrate, and 
         FIG. 17  is a diagram showing the responses obtained on a GMR sensor when scanning three defects in an inconel plate. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention essentially addresses a method which improves the signal-to-noise ratio of the detection of defects through measurement of eddy currents, by using a magnetoresistive sensor as an in situ demodulator. 
     The principle of the measurement method will be explained hereafter with reference to  FIG. 1 . 
     An emitter  3 , which can be a coil for instance, is fed through an amplifier  220  with an alternating current (AC) or radiofrequency (RF) field at a frequency f 1 . The emitter  3 , which is located in close proximity to a metallic object  2  to be inspected, sends in turn the AC or RF field at the frequency f 1  in the tested object  2 . The object to be inspected  2  re-emits a signal at the frequency f 1 . 
     According to the invention a magnetoresistive (MR) sensor  1  is located in the vicinity of the object to be inspected  2  and is fed through an amplifier  230  with a radiofrequency current at a frequency fc which is different from the working frequency f 1 . 
     The MR sensor  1  is thus used as an in situ mixer. 
     The voltage V measured at the active points of the MR element is then given by:
 
V =RI =( R   0   +R   1   H  cos(2π f   1   t+φ   H )+ . . . )* I   0  cos(2π f   c   t )  (1)
 
which can be developed as:
 
V =RI=R   0   I   0  cos(2π f   c   t )+( R   1   H I   0  cos(2π( f   1   −f   c ) t+φ   H )+ R   1   I   0   H  cos(2π( f   1   +f   c ) t+φ   H )+ R   n    H   n    I   0  cos n (2π( f   1   −f   c ) t+φ   H )+ R   n    H   n    I   0  cos n (2π( f   1   +f   c ) t+φ   H )
 
     This development shows that the two first terms depending on the sum and difference of the frequencies are exactly proportional to H. H is the total field experienced by the sensor, i.e. the vectorial sum of the emitting field by the coil  3  and the field reemitted by the eddy current. The higher order terms (n≧2) depend on frequencies different from the sum and the difference and may be rejected by adequate filtering. Usually a sensor is sensitive to only one direction of field and then the value of H is the projection of the total field along that axis. 
     The main advantages of that approach are first to get rid of the fluctuations of the resistance R 0  due for example to the temperature or the ageing. Finally, non linearity of the MR sensors does not affect the result. The non linearity of a sensor gives the appearance of terms R n  which have a frequency of f 1 −nf c  and f 1 +nf c  different from the measured frequency and which can then easily be eliminated if the frequencies are chosen different enough. 
     Secondly, this principle of measurement avoids a lot of noise effects due to the electromagnetic interference (EMI) at the field emitter frequency. This interference is due to field induced by the emitter  3  and preferentially appears for frequencies greater than 100 kHz. EMI can be induced by the coupling between the field emitter  3  and the parasitic connection loops or the coupling between the field emitter  3  and unshielded tracks. At frequency f 1  comparable or larger than 10 MHz, it is indeed quite impossible to avoid disturbing signals due to the connection loops. 
     The principle of the method according to the present invention will be more clearly described with reference to the examples of  FIGS. 2 to 6 . 
     An emitter coil  3  is fed by a waveform generator  22  at the frequency f 1  ( FIGS. 2 to 4 ). 
     A magnetic field is produced by the coil  3  in the vicinity of a metallic piece  2 . This magnetic field is modified by the presence of a defect  21  in the metallic piece  2  to be controlled. 
     A magnetoresistive sensor  1 , which is powered by a generator  23  at the frequency fc, measures the magnetic field which has been modified by the presence of the defect  21 . 
       FIG. 3  illustrates an embodiment of the acquisition system. The voltage across the sensor  1  is measured by a differential amplifier  25 . The frequency of the signal outputted by the differential amplifier  25  can be |f 1 −fc| or f 1 +fc. 
     The signals outputted by the waveform generators  22  and  23  are mixed in a mixer  26  to form a reference signal. A filter  27  is connected at the output of the mixer  26 . The filter  27  is a low pass filter if a reference signal is selected to be at the frequency |f 1 −fc| and is a bandpass filter if a reference signal is selected to be at the frequency f 1 +fc. 
     A mixer  28  and a filter  29  constitute a lock-in amplifier for receiving on the one hand the output of the differential amplifier  25  and on the other hand the reference signal outputted by the filter  27 . 
     The filter  29  is a low pass filter. 
       FIG. 4  shows an example of acquisition system when the magnetoresistive sensor  1  comprises an array of several magnetic sensors  1   a ,  1   b ,  1   c  which are connected in series. In such a case, several differential amplifiers  24   a ,  24   b ,  24   c  are used to measure the voltages of each of the individual sensors  1   a ,  1   b ,  1   c  respectively. A multiplexer  210  selects one of the voltages outputted by the differential amplifiers  24   a ,  24   b ,  24   c . This selected voltage is then applied to a mixer  28  of a common lock-in amplifier  28 ,  29  in a manner similar to the embodiment of  FIG. 3 . 
     Since the same sensing current is used for all different individual magnetic sensors  1   a ,  1   b ,  1   c , the overall system may remain simple and only one lock in amplifier  28 ,  29  is necessary. 
     The present invention may be in principle implemented with working frequencies varying from DC to GHz frequencies. However, in practice the working frequency fi should be chosen as a function of the type of the investigated defects. 
     For example, defects of the order of 100 μm should be preferably detected with frequencies fi ranging from 1 MHz to 20 MHz. The signal can be detected at a frequency fc which is substantially different from the working frequency fi. For example, a working frequency fi might be 5 MHz whereas the corresponding frequency fc is 5.1 MHz. 
     The signal can thus be detected at low frequency (e.g. under 100 kHz) using the difference fi−fc to allow an easy amplification. 
     However, the signal can also be detected at high frequency, using the sum fi+fc (e.g. 10.1 MHz) to allow an easier filtering and noise suppression. 
     The detection of defects which are larger than 100 μm requires working frequencies fi preferably lower than 1 Mhz. 
     The invention may be implemented with a single working frequency f 1  as disclosed with reference to  FIGS. 1 to 4 . 
     However alternatively a set of several frequencies fi can be sent by the field emitter  3 . A frequency fc which is different from the working frequencies fi is applied to the magnetoresistive sensor  1  and then a set of frequencies |fi−fc| are detected at the output of the sensor  1 . 
       FIG. 5  illustrate a possible embodiment using a set of two working frequencies f 1  and f 2  which are produced by waveform generators  22   a ,  22   b  respectively and are applied to an adder  290  whose output is connected to an emitter coil  3 . The magnetoresistive sensor  1  is fed at a frequency fc by a waveform generator  23  in the same manner as in the embodiment of  FIG. 3 . The voltage across the sensor  1  is measured by a differential amplifier  25 . 
     The sum of frequencies f 1  and f 2  outputted by the adder  290  and the frequency fc are mixed in a mixer  26  which outputs a reference signal which is applied to a first input of a mixer  28  of a lock-in amplifier  28 ,  29 . 
     The output of the differential amplifier  25  is applied to a second input of the mixer  28  which is associated with a low pass filter  29  for measuring the useful signals |f 1 −fc| and |f 2 −fc| which will then permit to detect the defects  21  of the metallic piece  2  simultaneously at different working frequencies. 
       FIG. 6  shows an example of another possible embodiment using two different working frequencies f 1 , f 2 . 
     A set of working frequencies f 1 , f 2  generated by waveform generators  22   a ,  22   b  respectively are applied to the inputs of an adder  290 . The output of the adder  290  is connected to an emitter coil  3  and to a first input (reference input) of a mixer  26 . 
     A frequency fc generated by a waveform generator  23  is applied to a second input of the mixer  26 . 
     The output of the mixer  26  is applied to a filter  27  which is connected to a magnetoresistive sensor  1 . 
     The filter  27  is a low pass filter for outputting signals f 1 −fc and f 2 −fc or a bandpass filter for outputting signals f 1 +fc and f 2 +fc. 
     The output voltage of the sensor  1  may be analyzed with a lock in amplifier or a bandpass filter at the frequency fc in a manner similar to the embodiment of  FIG. 5 . The signals which are provided by the magnetic field emitted by the coil  3  at several frequencies f 1 , f 2  are demodulated at the same frequency fc which is different from f 1  and f 2 . 
     It might also be possible to use a magnetoresistive sensor  1  having a non linear behaviour, such as a sensor including a TMR junction. In such a case, the equation giving the voltage V measured at the active points of the magnetoresistive element comprises terms R n  (e.g. R 2 ) which have a frequency of fi−nf c  and fi+nf c  where n is an integer such as 2. These frequencies fi−nf c  and fi+fn c  can be filtered and the detection may thus be done at |fi−nf c | or fi+nf c . 
       FIG. 7  shows a typical configuration of a device for a non destructive control by eddy currents. 
     The field emitter  1  is placed in the vicinity of the surface of an element  2  to be explored, at a distance d 2  thereof. The sensor  3  is placed near the field emitter  1 , at a small distance d 1  thereof. The field emitter  1  is thus located in the gap between the sensor  3  and the metallic object  2 . 
     Preferably, the distance d 1  between the field emitter  1  and the sensor  3  is well determined and is fixed. The system comprising the field emitter  1  and the sensor  3  may be designed to move along the main surface of the object  2 . During this scanning operation the distance d 2  between the object  2  and the emitter  1  secured to the sensor  3  may slightly vary. 
       FIG. 8  shows an alternative configuration of a device according to the invention. The sensor  3  is placed in the vicinity of the field emitter  1  substantially in the same plane at a distance d 2  from the main surface of the object  2  to be examined. 
     The distance d 2  is all the more critical as the size of the searched flaw is small. 
     To better characterizing the shape of a defect it is possible to measure different components of the magnetic field created by the eddy currents by using a plurality of magnetoresistive sensors  3  having different sensitive axes. 
     In particular at least one magnetoresistive sensor may have a sensing axis which is placed orthogonally to the emitted alternating electromagnetic field and at least one magnetoresistive sensor may have a sensing axis which is placed parallely to the emitted alternating electromagnetic field. 
     Different kinds of field emitters  3  may be used. For example, the field emitters  3  may comprise vertical coils, horizontal coils or planar exciting coils as described in document US2005/0140355 A1. The shape of the field emitter  3  depends on the characteristics of the defect which is to be detected and on the shape of the piece under test. The field emitter  3  may create a field which is perpendicular to the surface of the object under test or along the plane of the surface of the object under test. 
       FIG. 9  shows an example of a current sheet having a plane of symmetry and which may be used to create an homogeneous field over the central part of the emitter  3 . 
     The position of the sensor  1  is preferably chosen to cancel the direct coupling between the sensor  1  and the emitter coil  3 . 
       FIG. 10  shows a possible embodiment where the sensor  1  is sensitive to the field along the x axis whereas the field emitter  3  creates a field along the y axis. The field which is produced by the emitter coil  3  being oriented along the y axis, there is no direct coupling between the sensor  1  and the emitter coil  3 . 
     Alternative configurations for the field emitter  3  and the sensor  1  may be used. The configuration should be optimized as a function of the characteristics of the searched defect and of the shape of the object under test. 
     The MR sensor is characterized in that its resistance varies as a function of the applied field. Within various kinds of MR sensors, one may mention the Hall sensor, Anisotropic Magnetoresistance (AMR), Giant Magnetoresistance (GMR) and Tunnel Magnetoresistance (TMR) sensors. The sensor fabrication is well known. For all AMR sensors, a single layer of soft magnetic material such as permalloy can be used with a MR variation of about 2%. 
     For GMR sensors, a spin valve configuration ( FIG. 10 ) is preferable to a multiple layer GMR like (Co/Cu) n  or (NiFe/Ag) n  in order to avoid static bias field. 
     A spin valve configuration as shown on  FIG. 11  consists of several layers deposited on a substrate  61  which may be typically made of silicon or glass. The spin valve comprises a hard magnetic layer  64 , made of an antiferromagnetic layer coupled to a ferromagnetic layer, and a free magnetic layer  62  made of one or several ferromagnetic layers. These two stacks are separated by a thin metallic layer  63  (typically a copper layer 1-2 nm thick). Simple spin valves or spin valves with an artificial antiferromagnetic layer can be used. Typical MR variation on these systems are 10%. When replacing the metallic layer by an insulating layer, one obtains a TMR system. The MR ratio can reach 350% in these systems. 
     The shape of the MR sensor is essential for the signal-to-noise performance. 
     In case of a Hall sensor, a square or rectangular shape is well adapted for the measurement. The Hall sensor is placed so that it is sensitive to the field reemitted perpendicularly to the surface. AMR, GMR or TMR sensors are usually sensitive to in-plane fields and then will be in general used to detect the in-plane fields reemitted by the defect. 
     For an AMR, GMR or TMR sensor, a preferred shape is a yoke shape sensor  70  presenting a reasonable flux closure as shown on  FIG. 12 . 
     The yoke dimension should preferably be bounded by several constraints:
         the length (L) of the main part  71  of the yoke can be as long as necessary to cover the measuring zone. It can be comprised between 10 μm to several centimeters. For noise reasons, its length should be at least 3 times the width (w) of this main part  71  of the yoke;   the length (l) of the arms  72 ,  73  of the yoke should be at least 3 times the width w;   the width (w) of the yoke should be preferably comprised between 2 μm and 12 μm. The sensitivity is decreasing with the width w and the low frequency noise is increasing with the width w. A width of 10 to 12 μm is optimal. Larger widths are possible, depending on the material, but could induce less stable devices, due to magnetic domain formation. In case of TMR sensors, the free layer only needs to have a yoke shape.       

     The arms  72  and  73  of the yoke are linked to additional arms  74 ,  75  which are parallel to the main part  71  but have a length L arm  which is comprised between about ¼ and ⅓ of the overall length L. 
     In all cases, a resistance between two points of measurement comprised between 50 Ohms and several kOhms can be used. In terms of signal-to-noise ratio, it is better to use a higher resistive value as the signal will be proportional to that resistance and the noise is proportional to the square root of the resistance. A four point measurement should be preferred for the improvement of signal-to-noise ratio. 
     The use of a bridge configuration is not necessary with the proposed method as the offset R 0  is automatically suppressed by the in situ demodulation. Furthermore, a bridge configuration with only one sensitive element would add excess noise with a factor of √2. 
     If a rather large surface should be scanned, an array of sensors or a sensor with multiple contacts can be used. In case of an array of sensors, the same sensing current can be used for all of these. 
       FIG. 13  gives an example of a single sensor  170  with multiple contact points V 1  to V 5 . The resolution of measurement is given by the distance d between two adjacent points. 
     In the case of a single sensor, the individual element is defined by the distance between two adjacent voltage contacts. In order to insure the relative independence of each element, it is then necessary, in the case of an AMR, a GMR or a TMR sensor to have a distance d between each voltage contact larger than the width w of the sensor. 
     The sensor  170  of  FIG. 13  comprises a main part  171  and two current feeding contacts  172 ,  173 . Arms  174 ,  175  close the yoke shape in a way similar to the embodiment of  FIG. 12  with arms  74 ,  75 . 
     MR sensors are usually deposited on silicon or on ceramics for pick up heads. For Non Destructive Evaluation (NDE) applications, silicon can be used for frequencies lower than 10 MHz due to capacitance effects which short circuit the sensor. At higher frequencies glass or insulating ceramics substrates are necessary. An alternative is to use flexible insulating substrate like the material known under the trademark Kapton. 
     The mechanical mounting of the sensor system is particularly important. 
     In order to achieve a high resolution of the detection of defects and to be able to use the small size of the MR sensor, it is necessary to optimize the design of the detecting system. In particular, the distance d 2  (see  FIGS. 7 and 8 ) is all the more critical as the size of the searched flaw is small. The problem is then to be able to electrically contact the sensor as it moves over the surface. 
     Three preferred mechanical mountings will be described with reference to  FIGS. 14 to 16 . These mountings depend on the substrate chosen for the MR sensor and are intended to enhance the signal-to-noise ratio of the probe. These special mechanical mountings of the MR sensor system thus allow an optimized use of the in situ modulation. 
       FIG. 14  shows an embodiment using a silicon substrate  85  of reduced thickness. 
     The sensor  1  is deposited on a silicon wafer  85  which can be as thin as possible. 
     The emitter  3  comprising coils for RF field emission is supported on one side of a printed circuit board (PCB)  81  which supports on its other side tracks  82 ,  83  for electrical contacts. The reference  82  designates deported electrical contacts for the sensors whereas reference  83  designates electrical contacts on the sensors themselves which are bonded to the PCB  81 . Reference  86  designates electrical links between tracks  82  and the MR sensors  1 . These electrical links may be constituted by a low fusion temperature metal such as indium which permits to solder the Si wafer  85  on the PCB  81 . 
     The sensor is thus protected by the substrate  85  and the distance from the surface of the object  2  is given by the thickness of the substrate  85 , which can be reduced down to 20 μm, when the substrate  85  is placed on the object  2  to slide along the surface to be tested. 
       FIG. 15  shows an alternative embodiment which uses a bevelled substrate  94 . 
     The bevelled substrate  94  is glued on a first surface of a PCB  81  which also supports the emitter  3  (coils for RF field emission) on a second surface in a manner similar to the embodiment of  FIG. 14 . 
     The bevelled substrate  94  may be constituted by a ceramic or glass substrate. Reference numeral  82  designates deported electrical contacts for the sensors  1 . 
     The continuity of the contacts can be maintained through the angles allowing a very small distance between the sensor  1  and the scanned object  2 . 
     The sensor  1  can be protected either by a protective insulating layer, such as Si 3 N 4  or SiO 2  or a thin Kapton™ film, allowing the protected sensor to be directly in contact with the surface of the object  2 . The bevelled shape may be created by mechanical or chemical methods. 
       FIG. 16  shows another possible embodiment using a flexible substrate  103 . 
     The substrate  103  may be constituted for example by a polyimide film known under the trademark “Kapton” of the company DUPONT. The sensor  1  and its contacts  82  are deposited on one side of the foil  103 . The emitter  3  is deposited on the other side of the foil  103 . The foil  103  is then bent to allow the tracks  82  to be in contact with a PCB system  101 . 
     The results obtained with a specific embodiment used for scanning a plate having three defects of about 100 μm are given herebelow with respect to  FIG. 17 . 
     The sensing device comprises a field emitter including a double planar coil and a magnetoresistive sensor having multiple measurement points disposed on glass with bevelled edges. 
     The frequency f 1  of the field emitter is 5 MHz and the frequency fc of the current with crosses the sensor is 5.1 MHz. The voltage V measured at the active points of the MR element is then at the frequencies f 1 −fc=100 kHz or f 1 +fc=10.1 MHz. 
     If a low pass filter is used, the useful signal may be measured at the frequency of f 1 −fc=100 kHz and it is thus possible to get rid of the disturbing signal which occurs at the frequencies f 1  and fc. 
     The output signal shown on  FIG. 17  corresponds to the detection of three defects having the following sizes:
     1 st  defect: 200×100×200 μm 3      2 nd  defect: 100×100×200 μm 3      3 rd  defect: 100×100×100 μm 3 .