Abstract:
In a method for determining geometrical characteristics (d) of an anomaly ( 12 ) which changes the electrical conductivity in the region near the surface of an electrically conducting, in particular a metallic test object ( 10 ), a considerable simplification is achieved in that, in the region of the anomaly ( 12 ) in the test object ( 10 ), eddy currents ( 13, 14 ) of different frequencies are excited, and the magnetic field (B y,0 ), which is produced by the excited eddy currents, is scanned in the vicinity of the anomaly ( 12 ) and the geometric characteristics of the anomaly are exclusively deduced from the distribution of the magnetic field (B y,0 ).

Description:
[0001]    This application claims priority under 35 U.S.C. § 119 to Swiss application no. 01679/07, filed 29 Oct. 2007, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of Endeavor 
         [0003]    The present invention relates to the field of non destructive testing of test objects. It relates to a method for determining geometrical characteristics of an anomaly in a test object and to a measuring apparatus for carrying out the method. 
         [0004]    2. Brief Description of the Related Art 
         [0005]    Applicant is aware that the maximum of the surface plane quadrature component (out-of-phase component) of the crack-related magnetic field anomaly (B y,0 ″(ω)) in the alternating magnetic field of a crack which extends into an electrically conducting test object from the surface of the test object, contains useful information with respect to the geometry of the crack. The advantage of such a process is that the measured magnetic field variable primarily depends on the depth of the crack rather than on the crack volume, as is the case, for example, in a static magnetic field. However, the crack depth cannot be correlated with the measured B y,0 ″ max (ω) without additionally knowing the skin depth and the magnetic permeability of the test object. 
       SUMMARY 
       [0006]    One of numerous aspects the present invention includes methods and apparatus which can be used to determine the geometrical characteristics of an anomaly, in particular the depth of a crack, in an electrically conducting, in particular metallic, test object without having additional knowledge about the test object. 
         [0007]    Another aspect of the present invention relates to excited eddy currents in the region of an anomaly in the test object and the scanning of the magnetic field, which is produced by the excited eddy currents, in the vicinity of the anomaly and the inferring of geometrical characteristics of the anomaly exclusively from the distribution of the magnetic field. 
         [0008]    An exemplary embodiment of the method adhering to principles of the present invention can be characterized in that anomalies, in the form of cracks, are investigated, in that the crack depth is determined as the geometric characteristic of the cracks, in that in a first step the position of the crack on the surface of the test object is determined, and in that in a second step the distribution of the magnetic field transversely with respect to the longitudinal direction of the crack is scanned and evaluated, wherein the eddy currents in the region of the crack are excited by applying an alternating magnetic field, and the quadrature component, which is located in the plane of the surface and transversely with respect to the longitudinal direction of the crack, of the magnetic field is scanned and evaluated. 
         [0009]    The quadrature component of the magnetic field is preferably scanned at different frequencies of the alternating magnetic field and evaluated, wherein in particular the frequency of the alternating magnetic field traverses a prespecified frequency range, from approximately 1 kHz to approximately 1 MHz. 
         [0010]    Another exemplary embodiment of the method adhering to principles of the present invention can be characterized in that, starting from the crack, the first zero crossing of the quadrature component, which is located in the plane of the surface and transversely with respect to the longitudinal direction of the crack, of the magnetic field is determined and the maximum current layer width of the current layers, linked to the crack, of the eddy currents is ascertained from the first zero crossing, and in that the depth of the associated crack is deduced from the ascertained maximum current layer width. 
         [0011]    For measuring the magnetic field, a measuring head operating on the basis of the Hall effect is preferably used, wherein in particular the measuring head comprises a plurality of sensor elements, which are arranged in a linear sensor array, on CMOS (complementary metal-oxide-semiconductor) basis in the form of vertical Hall elements. For measuring the quadrature component, which is located in the plane of the surface and transversely with respect to the longitudinal direction of the crack, of the magnetic field, the linear sensor array is here aligned transversely to the longitudinal direction of the crack. 
         [0012]    The maximum current layer width is preferably determined with an accuracy of 10 μm and the crack depth with a resolution of approximately 50 μm. 
         [0013]    Another exemplary embodiment of the measuring apparatus according to the invention can be characterized in that first means comprises a magnetic coil, in that second means comprises a measuring head arranged inside or below the magnetic coil, in that the magnetic coil and the measuring head are combined in a scanning apparatus, and in that the scanning apparatus can be moved over the surface of the test object, wherein the scanning apparatus can preferably be moved using a robot. 
         [0014]    In particular, the magnetic coil is connected to a frequency generator generating frequencies in the range between approximately 1 kHz and approximately 1 MHz. The measuring head comprises a linear sensor array with sensor elements which operate according to the Hall effect and is connected to a signal processing unit. The sensor elements of the linear sensor array are here preferably configured on CMOS basis in the form of vertical Hall elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be explained in more detail below with reference to exemplary embodiments in conjunction with the drawings, in which: 
           [0016]      FIG. 1  shows, in a schematic representation, the initial situation in the method according to the invention, in which a crack, with a crack depth, extends from the surface into an electrically conducting test object and forms eddy currents with current layers when an alternating magnetic field is applied; 
           [0017]      FIG. 2  shows the calculated curves of the quadrature component of the alternating magnetic field in the direction transverse to the crack for different frequencies of the alternating magnetic field; 
           [0018]      FIG. 2   a  shows the current layer width Δ as a function of the interrogating frequency; 
           [0019]      FIG. 3  shows the calculated relationship between the maximum current layer width determined from  FIG. 2 , with crack depth; 
           [0020]      FIG. 4  shows an exemplary scanning apparatus for ascertaining the maximum current layer width at a crack; 
           [0021]      FIG. 4   a  shows the linear arrangement of VHD sensor elements of the scanning apparatus according to an exemplary embodiment of the invention; and 
           [0022]      FIG. 5  shows a measuring apparatus, equipped with a robot, for determining the crack depth according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0023]      FIG. 1  shows, in a schematic representation, the typical initial situation in the method according to the invention. A test object  10  composed of an electrically conducting material, e.g., a metal, has a surface  11 , whence a crack  12 , with a crack depth d, extends into the test object. The crack  12  further extends in a longitudinal direction (x-direction in the coordinate system in  FIG. 1 ). If an alternating magnetic field is now applied to the region of the test object  10  which contains the crack  12 , eddy currents are induced in the test object  10  near the surface, which in turn produce a magnetic field. Current layers  13 ,  14 , which are orientated by the profile of the crack  12  (the width of the current layers  13 ,  14  is shown in  FIG. 1  in an exaggerated fashion), are formed in the region of the crack  12  which represents an insulating barrier for the eddy currents. The (alternating) current layers  13 ,  14  produce an (alternating) magnetic field, which influences the component B y,0  in the plane of the surface  11 , perpendicular to the longitudinal direction of the crack  12 . 
         [0024]    The current layer width Δ is a function of the crack depth d, the material properties of the test object  10 , and the interrogating frequency F int  of the eddy current. The dependency of current layer width Δ on the interrogating frequency F int  is schematically shown for different materials material m 1  to m 5  in  FIG. 2   a  for three different cracks: crack I to crack III. In all cases, the current layer width Δ is a continuous function of the interrogating frequency F int  of the eddy current in the range 10 3  Hz to 10 7  Hz. These functions have only one maximum. As can be seen from all examples, the maximum current layer width Δ max  is independent of the material properties and is a characteristic value for each crack depth d. In other words, the same maximum current layer width Δ max  can be observed for a given crack depth d. However, the interrogating frequency F int  for which the maximum current layer width Δ max  will be observed depends on the material properties. To find the maximum current layer width Δ max , the interrogating frequency F int  has to varied over a wide frequency range, typically between 10 3  Hz and 10 7  Hz. This can be done as a continuous scan or with a stepwise variation of the interrogating frequency F int . A stepwise variation of the frequency reduces the amount of data to be processed and allows a faster scan. Scanning processes with different step sizes are conceivable. For example, simple constant steps, or steps which are a fraction or multiple of the last interrogating step, can be used. Scanning can be stopped once the maximum current layer width Δ max  is identified. 
         [0025]    In view of the dependency of the quadrature component portion B y,0 ″ (y) on the y coordinate, the profile illustrated in  FIG. 2 , showing different curves for different frequencies of the exciting magnetic field (10 kHz, 30 khz, 100 kHz, 300 kHz, 1 Mhz, and 3 Mhz), can be produced. All of the shown curves (viewed from crack  12  at y=0) having a first zero crossing which is interpreted as width Δ of the current layers  13 ,  14 . When viewed over the frequency range of the excitation frequency, ranging from 1 kHz to 1 MHz, the current layer width Δ passes through a maximum Δ max  dependent exclusively on the crack depth d and not on the electric conductivity and magnetic permeability of the material of which the test object  10  is made. The result is then, for the general case, the relationship presented in  FIG. 3  between crack depth d and the maximum current layer width Δ max , which can be described in an approximate fashion by the proportionality relation Δ max ˜d 0.3 . This relation forms a basis for the methods and apparatus described herein, which determines the crack depth d of a crack  12  by the measurement of the maximum current layer width Δ max  caused by the anomaly. 
         [0026]    As shown in  FIG. 4  and  FIG. 5 , the measuring apparatus  18  for measuring the crack depth includes a scanning apparatus  15  with a magnetic coil  16  having a frequency generator  20 . The frequency generator  20  has a driver circuit  19  and is settable and tuneable to provide alternating current in the frequency range between 1 kHz and 10 MHz. The magnetic coil  16 , which can for example have an external diameter of 4 mm and an internal diameter of 1.5 mm for determining crack depths of 1 mm or less, produces an alternating magnetic field which in turn produces eddy currents in the test object  10 , used to interrogate the test object  10 . Arranged inside or below the magnetic coil  16  is a measuring head  17  which is moved simultaneously with the magnetic coil  16  and which scans the magnetic fields produced by the eddy currents. At the core of the measuring head  17  is a semiconductor chip which is produced as per standard CMOS technology and contains a linear sensor array  26  of VHD sensor elements  25  ( FIG. 4   a ) which are configured in the form of vertical Hall elements (Vertical Hall Devices: VHD). 
         [0027]    This sensor array  26  can be used to measure the local components of the magnetic field in and outside of the plane of the surface  11 . To achieve this the measuring head  17 , and more specifically the sensor array  26 , is connected to a sensor driving unit  21  which, among other things, applies a biasing voltage to the sensor array  26  and preamplifies the VHD signal. A signal processing unit  22 , which is connected downstream of the frequency generator  20 , demodulates and filters the signal. An evaluation unit  23  controls the frequency generator  20 , evaluates the output signal of the signal processing unit  22  and controls a robot  24  (in the example of  FIG. 5 , a robot arm) which guides the scanning apparatus  15  with the magnetic coil  16  and the measuring head  17  over the surface  11  of the test object  10  under investigation. During the measurement, the robot  24  can move and rotate the scanning apparatus  15 . 
         [0028]    Using the measuring apparatus  18  shown in  FIG. 5 , an exemplary method can be carried out as follows: 
         [0029]    1. Crack detection: with quick measurement technology, based on a change in the impedance, that is to say the resistance of the magnetic coil  16  and/or the presence of a peak in the component of the local magnetic field which is not in the surface plane, the presences of a crack is detected. During this process the robot  24  moves at a speed of, for example, 1 cm/s. 
         [0030]    2. Positioning: if a crack is detected, the robot  24  stops. The sensor array  26  is then set in a position at right angles to the longitudinal direction of the crack. 
         [0031]    3. Determining the crack depth: in the set position, the excitation frequency of the magnetic coil is tuned, the zero crossing of B y,0 ″(y) located, and the maximum current layer width Δ max  measured. From this the crack depth d, with the aid of the curve in  FIG. 3 , is determined. 
         [0032]    An important factor in the present method can be the accuracy with which the position of the zero crossing of B y,0 ″ (y) on the y-axis is determined. In order to achieve a resolution of the crack depth d of 50 μm at 1 mm, i.e., a resolution of 5%, Δ max  needs to be measured with an accuracy of 10 Δm. This can be achieved using a sensor array  26  with very closely packed VHD sensor elements  25 , such as those developed at EPFL (Ecole Polytechnique Fédérale de Lausanne), Lausanne, Switzerland. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
         
           
               10  test object (metallic) 
               11  surface (test object) 
               12  crack 
               13 ,  14  current layer 
               15  scanning apparatus 
               16  magnetic coil 
               17  measuring head 
               18  measuring apparatus 
               19  driver circuit 
               20  frequency generator 
               21  sensor driving unit 
               22  signal processing unit 
               23  evaluation unit 
               24  robot 
               25  VHD sensor element 
               26  sensor array 
             B y,0  magnetic field (parallel to the surface, at right angles to the crack) 
             d crack depth 
             Δ current layer width 
             Δ max  maximum current layer width 
             F int  Interrogating frequency 
           
         
       
     
         [0054]    While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.