Abstract:
An apparatus for nondestructive detecting of cracks in lapped electrically conductive upper and lower plates characterized by a probe having a square shape drive coil and a magnetoresistor sensor aligned with the longitudinal axis of the drive coil. The drive coil is intended to extend across the lap joint above the plates with the sensor mounted between the drive coil and plates. A signal generator applies periodic unipolar pulses to the drive coil.

Description:
RELATED APPLICATION  
       [0001]     This application is a continuation of PCT/US2006/24324 filed on 23 Jun. 2006 which claims priority based on U.S. provisional application 60/694,570 filed on Jun. 28, 2005. This application claims the benefit of both aforecited applications. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to nondestructive evaluation (NDE) equipment and more particularly to a giant magnetoresistive (GMR) sensor based apparatus configured to detect cracks in electrically conductive material, particularly cracks near lap joints of an aircraft fuselage.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Pat. No. 6,888,346 describes a probe for detecting deep flaws in thick multilayer conductive materials. The probe uses an excitation coil to induce eddy currents in conductive material oriented perpendicular to the coil&#39;s longitudinal axis. A giant magnetoresistive (GMR) sensor, surrounded by the excitation coil, is used to detect generated fields. Between the excitation coil and the GMR sensor is a highly permeable flux focusing lens which magnetically separates the GMR sensor and excitation coil and produces high flux density at the outer edge of the GMR sensor. The use of feedback inside the flux focusing lens enables cancellation of the leakage fields at the GMR sensor location and biasing of the GMR sensor to a high magnetic field sensitivity.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is directed to an enhanced NDE probe apparatus which includes a drive coil for producing a primary magnetic field to induce eddy currents in adjacent conductive material (e.g., a metal aircraft fuselage) and a GMR sensor for detecting nonuniformities in a generated secondary magnetic field which nonuniforminities are indicative of discontinuities, or “cracks” in the conductive material.  
         [0005]     In accordance with the present invention, the probe uses a square shape drive coil (i.e., having a substantially square cross section perpendicular to the coil&#39;s longitudinal axis) to maximize the interaction zone with a crack in the conductive material.  
         [0006]     In accordance with a preferred embodiment, to enhance the probe&#39;s sensitivity to cracks in conductive plates adjacent to a lap joint formed by a bottom conductive plate lapped by a top conductive plate, the GMR sensor is mounted so that its axis of sensitivity is located immediately adjacent and parallel to the skin of the bottom plate. To further enhance sensitivity, the square shape drive coil is preferably constructed of minimal height, i.e., pancake fashion, and longitudinally spaced from the sensor to allow the drive coil to extend across the lap joint above the skin of the top plate.  
         [0007]     In accordance with a further feature of the preferred embodiment, bias means are provided to produce a bias magnetic field to keep the sensor operating in the linear region of the sensor&#39;s response curve. The bias field is oriented perpendicular to the sensor axis of sensitivity to avoid interacting with the eddy current producing secondary magnetic field.  
         [0008]     In accordance with a still further feature of a preferred embodiment, the drive coil is excited by periodic unipolar pulses (e.g., half sine wave, saw tooth pulse, square pulse) to vary the magnitude, but not the direction, of the eddy current producing primary magnetic field. As a consequence, the GMR sensor can operate unidirectionally and provide a D.C. output signal thereby minimizing the downstream signal processing requirements because unwanted A.C. components can be readily filtered. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]      FIG. 1  schematically illustrates the use of a square drive coil in accordance with the present invention for generating eddy currents in a conductive plate to produce a secondary magnetic field whose characteristics identify cracks in the plate;  
         [0010]      FIG. 2  is a block diagram of a preferred GMR sensor based eddy current crack detector system consistent with  FIG. 1 ;  
         [0011]      FIG. 3  is a top plan view of a preferred probe in accordance with the present invention;  
         [0012]      FIG. 4  is a side view of the probe of  FIG. 3 ;  
         [0013]      FIG. 5  is a top plan view showing the probe of  FIG. 3  being used to detect cracks in a bottom plate of a lap joint;  
         [0014]      FIG. 6  is a side view of the probe and lap joint as represented in  FIG. 5 ;  
         [0015]      FIG. 7  diagrammatically illustrates the effective interaction zone produced by a square drive coil in accordance with the present invention;  
         [0016]      FIG. 8  illustrates a typical interaction zone of a conventional circular drive coil;  
         [0017]      FIG. 9  is an enlarged schematic view of a preferred probe in accordance with the invention showing the physical relationship between the drive coil and the GMR sensor;  
         [0018]      FIG. 10  is a diagrammatic view of an exemplary prior art probe showing the relationship between a drive coil and a GMR sensor;  
         [0019]      FIG. 11  diagrammatically illustrates the utilization of a conductive trace on a circuit board supporting the GMR sensor for producing a bias magnetic field; and  
         [0020]      FIG. 12  depicts an exemplary GMR sensor response curve. 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 1  schematically illustrates the basic operation of an eddy current system  10  in accordance with the present invention for detecting cracks (which term should be understood to mean any type of flaw or discontinuity) in conductive material  12 , typically a metal plate  14  of an aircraft fuselage. The system  10  includes a square shape drive coil  16  which is excited by periodic unipolar pulses supplied by D.C. pulse source  18 . In use, the coil  16  is positioned above plate  14  and oriented with its longitudinal axis extending substantially perpendicular to the plate. Excitation of the coil  16  by source  18  generates a primary magnetic field  20  which in turn induces eddy currents  22  in the plate  14 . The eddy current flow in the plate generates a secondary magnetic field  24 . If there are no cracks in the plate, the secondary magnetic field will be substantially uniform across the entire plate area. However, if the eddy current flow is disturbed by a crack, then the secondary magnetic field will exhibit nonuniformities across the plate area thereby forming tangential vector components near the crack. Such nonuniformities can be detected by a sensor located near the plate  14 .  
         [0022]      FIG. 2-4  illustrate a preferred system  30  in accordance with the invention depicted as including a probe  32  and support electronics  34 . The probe  32  is comprised of a housing  36  formed by a top wall  38  and a bottom wall  40  ( FIG. 4 ). A substantially planar drive coil  42  is mounted in the housing preferably adjacent to the underside of the top wall  38  with the longitudinal axis of the drive coil oriented essentially perpendicular to wall  38 . The drive coil  42  is configured with a square cross section, or profile, ( FIGS. 2, 3 ) to maximize the zone of interaction with cracks  44  in a conductive plate to be evaluated. The drive coil  42  is preferably pancake shaped meaning that its turns are densely packed and that its axial dimension is minimized.  
         [0023]      FIGS. 3 and 4  show the probe  32  with a substantially planar GMR sensor  50  supported in the housing  36  on the housing bottom wall  40  which can comprise a standard circuit board. The sensor  50  is preferably aligned with the longitudinal axis of the drive coil  42  and is oriented substantially parallel to and spaced from the drive coil. Particularly note the physical relationship between the drive coil  42  and the GMR sensor  50  as shown in  FIG. 4 . That is, the square planar profile of the drive coil  42  is larger than that of sensor  50  so that the front edge  52  of the drive coil extends beyond the front edge  54  of sensor  50 . This physical relationship facilitates detecting cracks adjacent to lap joints as will be further discussed in connection with  FIGS. 5 and 6 .  
         [0024]     With reference to  FIG. 2 , it should be noted that the support electronics  34  includes a D.C., or unipolar, signal source  56 , preferably a half sine wave generator, and signal amplifier  58  for supplying signal energy to excite drive coil  42 . The support electronics  34  also includes a D.C. power supply  60  for powering the GMR sensor  50  as well as a bias winding to be discussed in connection with  FIG. 11 . Further, a signal conditioning circuit  62  is provided for responding to the output of sensor  50  to control circuit  64  which drives a bank of LED indicators  66  to indicate the presence and magnitude of a detected crack.  
         [0025]     The GMR sensor  50  can be of conventional design defining a preferred axis of sensitivity  68  which is oriented perpendicular to the sensor front edge  54  ( FIG. 4 ). The sensor  50  and drive coil  42  are arranged in such a way that a tangential vector component of the secondary magnetic field  24  extends parallel to the axis of sensitivity  68 . The axis of sensitivity  68  extends essentially perpendicular to the length of a typical crack  44  in conductive material under inspection. Consequently, the sensor  50  is insensitive to both the primary magnetic field  20  ( FIG. 1 ) generated by the drive coil  42  ( FIG. 2 ) and the resulting secondary magnetic field  24  except when cracks exist in the material  12  under inspection. The level of the output signal from the sensor  50  can be correlated to the depth and width of a crack  44  to enable the LED drive circuit  64  to control multiple LEDs  66  which are preferably color coded to indicate the existence and quality of a crack. The circuit  64  preferably includes means for adjustably setting a threshold corresponding to the minimum crack depth to be detected.  
         [0026]      FIGS. 5 and 6  illustrate the utilization of the probe  32  for detecting cracks  44  adjacent to a lap joint  70  (comprised of a top plate  72  and a bottom plate  74  held together by e.g., fasteners, rivets  76 ) which are characteristically formed in a typical aircraft fuselage. Note in  FIGS. 5 and 6  that the sensor front edge  54  is held against the edge  78  of the top plate  72  as drive coil front edge  52  is moved along edge  78  (represented by scan arrow  79 ). Also note that the sensor  50  is positioned immediately adjacent to the skin of the bottom plate  74  whereas the substantially planar drive coil  42  is positioned to bridge both the top plate  72  and bottom plate  74 . This arrangement of the square drive coil  42  and GMR sensor  50  facilitates the detection of hidden cracks adjacent the lap joint  70  of an aircraft fuselage within the foot print of the drive coil  42 .  
         [0027]      FIG. 7  schematically depicts the enlarged zone of interaction with typical plate cracks  44  ( FIG. 5 ) achieved by using the square drive coil  42  in accordance with the invention as contrasted with the smaller interaction zone afforded by the use of a more conventional circular drive coil  77  depicted in  FIG. 8 . (Note: The circled dots represent magnetic lines of force going into the plane of the paper, while the circled Xs represent magnetic lines of force coming out of the plane of the paper.)  
         [0028]      FIG. 9  schematically depicts the physical relationship between the drive coil  42  and sensor  50  which allows the sensor to touch the skin of lower plate  74  for maximum sensitivity and allows the coil  42  to bridge the lap joint  70  for maximum coverage. This arrangement in accordance with the invention ( FIG. 9 ) is readily distinguishable from the more conventional arrangement depicted in  FIG. 10 .  
         [0029]      FIG. 11  shows the inclusion of a bias winding  80  which preferably comprises a conductive trace  82  formed on the bottom wall circuit board  40  under the sensor  50 . The bias winding  80  is energized from power supply  60 .  
         [0030]      FIG. 12  shows a typical GMR sensor response curve  83 . By application of an appropriate voltage across bias winding  80 , the sensor  50  can be operated in a linear zone of its response curve  83  for optimum performance. The bias signal is preferably generated with DC voltage (0-5 Volts with maximum 1 AMP current) applied across the trace  82  printed on the circuit board  40 . Since the trace  82  is under the GMR sensor  50  and applies a bias magnetic field perpendicular to the axis of sensitivity  68 , the bias field does not interact with the secondary field crack signal but it does function to keep the background magnetic field strength above the ambient field, i.e. field attributable to the earth&#39;s magnetic field and/or fields generated by adjacent electronic equipment. In order to maximize the effect of the bias field on the GMR sensor  50 , a magnetic shield  84  ( FIG. 2 ) is preferably provided on top of the drive coil  42 . When the probe  32  is placed on an aircraft skin for inspection, the skin shields any unwanted field coming from under the probe and any unwanted field coming from above the probe is shielded by shield  84 . In this way, the bias field is effective to keep the sensor in the linear regions of the GMR signal response curve  83 . If the bias is not correctly set (either lower section or top section of the curve), then the response to the crack signal can depart from maximum sensitivity.  
         [0031]     It has previously been mentioned that the square drive coil  42  is preferably excited by periodic unipolar pulses. Although it is preferable to use a half sine wave generator (e.g.  56  in  FIG. 2 ), alternatively, the unipolar pulses can be square shaped, saw tooth shaped, etc. The parameters of the excitation signal, e.g., repetition rate, pulse width, pulse amplitude can be adjusted to optimize each particular system. Inasmuch as unipolar pulses are used to create the primary magnetic field, the sensor  50  will have a unidirectional response, i.e., provide a D.C. output voltage whose level is proportional to the magnitude of the detected secondary magnetic field tangential vector components. Accordingly, the signal conditioning circuit  62  ( FIG. 2 ) can be readily inexpensively implemented to filter out all unwanted A.C. components including intrinsic noise coming from the GMR sensor itself.  
         [0032]     The foregoing describes a preferred crack finder in accordance with the invention particularly suited for detecting cracks in conductive plates adjacent to a lap joint. It is recognized that variations and modifications of the preferred embodiment will occur to those skilled in the art which fall within the spirit of the invention and the intended scope of the appended claims.