Abstract:
A pulsed eddy current two-dimensional sensor array probe for electrically conducting component inspection includes a drive coil disposed adjacent to a structure under inspection, a pulse generator connected to the drive coil and operable to energize in a pulsed manner the drive coil to transmit transient electromagnetic flux into the structure under inspection, and an array of sensors arranged in a two-dimensional array and substantially surrounded by the drive coil and operable to sense and generate output signals from the transient electromagnetic flux in the structure under inspection.

Description:
BACKGROUND OF INVENTION 
     The present invention generally relates to nondestructive evaluation of metallic structures and, more particularly, is concerned with a pulsed eddy current two-dimensional sensor array probe and system for electrically conducting component inspection. 
     The presence of surface cracks and subsurface flaws in metallic structures, such as aircraft skin structures, have the potential to lead to catastrophic failure. Various inspection methods have been used heretofore for crack and flaw detection with varying degrees of success. 
     One prior art inspection method uses eddy current probes which can give an indication of depth to ascertain crack and flaw severity. The probes requires close contact with the part and have limited resolution and inspection speed. Also, complicated contour following is needed to inspect curvatures of some parts. Lift off between the probes and the part is a major concern. Full coverage with eddy current probes is very time consuming, which leads to spot checking which may miss critical crack and defect areas. 
     Consequently, a need still exists for an innovation which will provides a solution to the aforementioned problem without introducing any new problems in place thereof. 
     SUMMARY OF INVENTION 
     The present invention provides a pulsed eddy current two-dimensional sensor array probe, system and data visualization for electrically conducting component inspection designed to satisfy the aforementioned need. The probe uses a two-dimensional array of magnetic field sensors for inspection of electrically conducting components without requiring mechanical scanning. A coil (solenoid) is used to transmit an electromagnetic pulse of a fixed duration into a component. The induced eddy current penetrates the structure and serves to produce a pattern of electromagnetic transient field above a defect. The two-dimensional array of sensors is located in a plane within the drive coil. Static scanning is employed to collect time evolution response of the magnetic field from all the sensors. This is performed by electronically switching the sensors while the probe is placed in a stationary position on the conducting structure. The response from the sensors is processed to determine the material thickness with a two-dimensional map representing a full field of view. 
     In one embodiment of the present invention, a pulsed eddy current two-dimensional sensor array probe for electrically conducting component inspection is provided which comprises a drive coil disposed adjacent to a structure under inspection, a pulse generator connected to the drive coil and operable to energize in a pulsed manner the drive coil to transmit transient electromagnetic flux into the structure under inspection, and an array of sensors arranged in a two-dimensional array and substantially surrounded by the drive coil and operable to sense and generate output signals from the transient electromagnetic flux in the structure under inspection. 
     In another embodiment of the present invention, a pulsed eddy current two-dimensional sensor array system for electrically conducting component inspection is provided which comprises a pulsed eddy current two-dimensional sensor array probe for transmitting in a pulsed manner a transient electromagnetic flux into a structure under test and for sensing and generating output signals from the transient electromagnetic flux in the structure under inspection, and a data acquisition apparatus for receiving and processing the output signals and outputting the processed signals in the form of a two-dimensional image made up of pixels that correspond to the array of sensors. The two-dimensional image is formed by a given gray level to the amplitude of an informative parameter that is computed for each individual sensor in the two-dimensional array. Each element (pixel) of the image is located according to the spatial position of the sensor in the array. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagrammatic representation of a pulsed eddy current two-dimensional sensor array inspection probe and system of the present invention. 
     FIGS. 2 to  5  are diagrammatic representations of several different mutual locations of the sensors and the drive coil in the array probe 
     FIG. 6 is a graph depicting transient signals from the sensors located within and outside of a drive coil obtained during evaluation of the probe of the present invention while the probe was placed over a flawless region. 
     FIG. 7 is a graph depicting signal differences from the sensors within a drive coil of the probe of the present invention while the probe was placed over a flat bottom hole. 
     FIG. 8 is a set of two-dimensional images obtained with an array of 5×8 sensors while the probe of the present invention passes over a flat bottom hole. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, and particularly to FIG. 1, there is diagrammatically illustrated an eddy current-based nondestructive inspection system, generally designated  10 , of the present invention used for nondestructive evaluation of metallic structures, such as aircraft skin structures, for detection and visualization of surface cracks and subsurface flaws. The inspection system  10  generally includes a pulsed eddy current two-dimensional sensor array probe, generally designated  12 , and a data acquisition apparatus, generally designated  14 . 
     The pulsed eddy current two-dimensional sensor array probe  12  includes a drive coil  16 , an array of sensors  18  arranged in a two-dimensional array, and a square pulse generator  20 . The data acquisition apparatus  14  includes a computer interface  22 , a computer  24 , such as a personal computer, and a computer monitor  26 . The drive coil  16  is a multiple-turn solenoid of generally rectangular configuration surrounding the array of sensors  18 . The rectangular drive coil  16  is used for transmitting a transient electromagnetic flux into a structure under test. 
     The array of sensors  18  forms a rectangular matrix inside the drive coil  16 . The pulse generator  20  is used to excite the rectangular drive coil  16  with a square-shaped short duration pulse of electrical current. The computer interface  22  provides multiplexing of the responses from the bank of sensors  18  and communicates the digitized signals into the computer  24 . Customized software stored in the computer  24  processes the acquired data and displays the results on a monitor  26  of the computer  24  in the form of a two-dimensional image  28  where each pixel represents a result of data processing of the corresponding sensor  18 . Using the computer  24 , a wide variety of digital image processing methods can be used to obtain better representation of a flaw. 
     Sensors  18  can be located inside or outside as well as above or below the drive coil  16 . Particularly, FIGS. 2 to  5  demonstrate four different settings of a drive coil and sensors that were tested for the purpose of the present invention. In FIG. 2 a two-dimensional array of sensors  18  is placed inside of the rectangular drive coil  16 . In FIG. 3, a two-dimensional array of sensors  18  is placed inside the drive coil  16  and two additional one-dimensional arrays of sensors are placed outside of the rectangular drive coil  16 . Two probe configurations with a flat drive coil of pancake or spiral type are shown in FIGS. 4 and 5. FIG. 4 depicts a case where the sensors  18  are placed on top of the drive coil  16 , while FIG. 5 represents a case where the sensors are placed under the drive coil  16 , closer to the surface of an inspected component  30 . 
     Probes  12  of different size and number of sensors  18  in the array can be fabricated depending on the particular application. For example, a drive coil  16  with internal dimensions of 13 mm×66 mm and external dimensions of 26 mm×79 mm having 260 turns of AWG 30 wire has been fabricated and evaluated. Array of 3×8 GMR sensors was placed inside and two arrays of 1×8 GMR sensors were placed outside of the drive coil  16  as in the setting shown in FIG.  3 . The probe  12  was placed over a structure that consists of two layers of 1.6 mm thick aircraft aluminum. A 2 A square pulse with duration of twenty microseconds is used to excite the drive coil  16 . 
     The time evolution signals from the sensors  18  were digitized with a sampling frequency of 2.5 MHz. Typical signals obtained from the GMR sensors for a period of 400 microseconds while the probe  12  was positioned over a flawless region are presented in FIG.  6 . Magnitude of the sensor response rises during the time when the current is on and drops to the initial value after the end of the drive pulse. Curve  34  is a signal obtained from a GMR sensor that is located at the center of the probe  12 , curves  36  and  38  were obtained from the sensors that are located next to the wound wires inside and outside of the coil  16  respectively. Because of variable proximity to the drive coil wires, the significant variations of the responses from the sensors  18  have been observed. A reference signal recording for every sensitive element in the array may be employed to eliminate these variations. 
     A transient electromagnetic signal obtained from the sensor  18  depends on its position inside or outside the drive coil  16  and the geometry of the component directly under this sensor  18 . Nulling of the probe is made on a flawless region of the component to record into the computer memory the transient responses from each sensor  18 . During inspection, the response signal is subtracted from the signal obtained for this sensor  18  during the nulling phase. The signal differences after the end of the exciting pulse that have been obtained from the GMR sensors while the probe  12  was positioned over a flaw are presented in FIG.  7 . The flaw was simulated by a 19 mm in diameter flat bottom hole 0.35 mm deep. The hole is positioned on the top of the second layer of the specimen as shown in FIG.  3 . Curve  40  is a signal difference obtained from a GMR sensor that is located over the center of the flaw. Curve  42  is a signal difference obtained from a GMR sensor that is located over the edge of the flaw. Curve  44  is a signal difference obtained from a GMR sensor that is located over flawless region. The variations between the signal differences from the sensors  18  located in the probe  12  differently in respect to the flaw allow forming an image of the flaw. 
     A two-dimensional image  28  is formed by giving a gray level to the amplitude of an informative parameter that is computed for each individual sensor in the two-dimensional array. Each element (pixel) of the image is located according to the spatial position of the sensor in the array. Using a color map (also called a color palette), a color image is formed as the color values are given to each pixel of the gray scale image from the corresponding look-up tables. 
     A wide variety of algorithms can be used to compute the informative parameter for the sensors. For example, the informative parameter S that can be used to form an image is the mean of the signal difference U during a fixed time interval t 1 −t 2 :                  S        (     i   ,   j     )       =       1   N            ∑     n   =   1     N                     U        (     i   ,   j   ,   n     )             ,           (   1   )                                
     where i and j are the coordinates of the sensors  18  in the two-dimensional array and N is the number of sampled values of the signal difference U(i,j,n) during the time interval t 1 −t 2 . 
     Alternatively, the informative parameter S can be computed by using a sine filter by convolving the signal difference U(i,j,n) with the sin( ) function:                S        (     i   ,   j     )       =       1   N            ∑     n   =   1     N                       U        (     i   ,   j   ,   n     )              sin        (       2        π   ·     (     n   -   1     )         N     )       .                   (   2   )                                
     Since the informative parameter S defined by the equation (2) is not sensitive to the constant level bias of the signal U, this algorithm was found to be effective in presence of an external magnetic field with constant level during the measurement cycle. Generally, most of the known discrete transforms (Fourier, Laplace, wavelet, etc.) applied in the time domain can be used to compute the informative parameter S. 
     FIG. 8 depicts the images (a) (f) that have been obtained from the above described setup by using equation (2) for informative parameter computations. The image (a) was obtained while the probe was placed over flawless region. The images (b) (f) were obtained while the probe passed over the simulated flaw (flat bottom hole  32 ). As can be seen in FIG. 7, a choice of the time interval t 1 −t 2  is pertinent to the flaw detection. The time interval from t 1 =8 μs to t 2 =320 μs beyond the drive pulse was found to be suitable for detecting the simulated flaw when it is located on the top of the second layer of the specimen as shown in FIG.  3 . 
     Customized software stored in the computer  24  controls the data acquisition, processes the acquired data and displays the results on a monitor  26 . Pulse generator  20  produces current pulses through the drive coil  16  with a repetition rate of 100 Hz. The electronic switching the sensor outputs by row and column is made by means of computer interface  22  after completion of data collection cycle for each sensor. In the setup used, based on a personal computer with 200 MHz processor and developed software, the images were updated two times per second. This arrangement provides real-time monitoring of a component under test. The images can be updated faster or slower depending on the used software program and computer hardware. 
     In the above described example, GMR sensors were used as the magnetic field sensors  18 . Alternatively, other magnetic field sensors such as wound coil sensors, Hall elements, flux gate sensors, etc. can also be used. As an alternative to an array of discrete elements fabricated individually, an array of sensors  18  can be performed as one integral module using eddy current array probe (ECAP) technology or integral circuit technology. 
     Productivity and turnaround time are the drivers in today&#39;s economic environment. The array of sense elements, such as Giant Magnetoresistive (GMR) detectors, wound and deposited coils, flux gate sensors or Hall elements, combined with pulsed electromagnetic excitation provides the competitive edge. Also, the two-dimensional representation is much more informative and provides a real-time visual representation of the flaws in the structure under test. 
     It is thought that the present invention and its advantages will be understood from the foregoing description and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely preferred or exemplary embodiment thereof.