Abstract:
A system for mapping a surface defect in an electrically-conducting material by measuring a change in the resonance of the material includes a flexible printed circuit board and a two dimensional array of transducers printed on the flexible circuit board, wherein each element of the array includes two transducer coils in a paired arrangement. A receive circuit connected to the coils is tuned to a resonant frequency, and the transducer coils operate in a send/receive mode. In another feature of the invention, there are means for converting a change in measured resonance to a visual display of the depth and width of the surface defect.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This patent application claims the benefit of provisional patent application Ser. No. 60732959, filed Nov. 3, 2005. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     None.  
       REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISC AND AN INCORPORATION BY REFERENCE OF THE MATERIAL ON THE COMPACT DISC.  
       [0003]     None.  
       BACKGROUND OF THE INVENTION  
       [0004]     (1) Field of the Invention  
         [0005]     The invention relates to devices for measuring the geometry of defects that have altered the surface of an electrically conducting pipe.  
         [0006]     (2) Description of the Related Art  
         [0007]     Pipelines and other structures used in the petrochemical industry are generally made of steel, and are pressurized. Defects such as corrosion, gouges, cracks, or other imperfections or features that remove or alter a portion of the steel can affect the ability of the pipe or structure to operate safely. When a defect or imperfection is discovered it must be assessed using engineering techniques defined in the operating code covering the installation, or by using methods developed and approved by the operator. This assessment requires a calculation using the material properties and design standards of the original construction, and measurements of the defect or imperfection.  
         [0008]     Defect assessment in high pressure pipes is typically performed for corrosion, gouges or metal loss defects, and typically follows one of three industry-accepted methods. The most common is B31G (ASME B31G, Manual for Determining the Remaining Strength of Corroded Pipelines). This assessment technique requires the length and maximum depth of a metal loss or corrosion defect, which, with pipe material information and original design standards, can be used to calculate a safe operating pressure for the defect.  
         [0009]     More complex assessment methods can be used to minimize unnecessary repairs. Using modified material properties and a different shape factor, a modified assessment can be made that is less conservative than the original B31G. This is known as the 0.85 dt method. This technique also requires that the length of the defect or imperfection and the maximum depth of the defect or imperfection be known. These measurements are used to calculate a safe operating pressure for the defect or imperfection.  
         [0010]     The assessment technique with the least variability is the exact or effective area technique. This technique uses the exact cross-sectional area of the defect. The area is determined from an axial depth profile which uses the maximum corrosion depth at each axial measurement spacing. This axial depth profile is projected to a linear representation of the defect, and then the area of metal loss is calculated.  
         [0011]     Each of these methods is defined in detail in the appropriate code. Each method specifies the defect measurements required to make the calculation to determine the effect of the defect or imperfection on the safety of the pipeline or installation.  
         [0012]     Regardless of which assessment method is used, the input data are usually provided by local measurements on the outside of the pipe or structure.  
         [0013]     The simplest case is that of a single isolated corrosion pit or area of metal loss. A scale measures the length of the defect area. A dial extension gage (pit gage) is placed over the pit (assuming the base will span the pit) and the maximum depth read and recorded. Slightly more complicated is the case of several overlapping pits or metal loss areas or a small patch of corrosion. In such cases, the length can still be measured with a scale. An attachment, such as a bridging bar, often spans the entire defect or imperfection, providing a reference surface from which to measure depth. It is not always possible to readily locate the deepest pit or metal loss within the grouping from a visual examination, so several independent depth measurements must be taken. It is also difficult to determine if defects in close proximity interact as defined by the rules outlined in the appropriate codes.  
         [0014]     When corrosion or metal loss is extensive and an exact area assessment is needed, it is essential that the defect be accurately mapped to form a contour plot. In these cases, a rectangular grid is drawn or painted on the pipe or structure surface, including the corroded or metal loss area. Depth measurements are taken at each grid intersection. From this array of measurements, either manual or computer-aided processing is used to construct a contour map. The contour map is then used to assess the defect, and calculate a safe operating pressure. All these manual measurement methods are laborious, time-consuming, and error prone.  
         [0015]     There have been some devices that automate some aspects of inspections, using eddy current arrays, as, for example, in the following patents, which are incorporated herein by this reference: U.S. Pat. Nos. 5,793,206, 5,182,513, and 5,262,722. However, these all have various limitations, as set forth in U.S. Patent Application No. 20040232911.  
         [0016]     U.S. Pat. No. 6,545,467, which is incorporated herein by this reference, states in the abstract, “A flexible eddy current array probe is attached to the contoured exterior surface of the backing piece such that the probe faces the contoured surface of the workpiece to be inspected when the backing piece is disposed adjacent to the workpiece. The backing piece is then expanded volumetrically by inserting at least one shim into a slot in the backing piece to provide sufficient contact pressure between the probe and the workpiece contoured surface to enable the inspection of the workpiece contoured surface to be performed.” However, the method disclosed in this patent is primarily concerned with ensuring coupling between a flexible eddy current array probe and a workpiece to be inspected. This patent does not disclose a two-dimensional eddy current array, does not disclose how to use such an array, does not provide the means to map defects, and does not use the flexible substrate to reestablish the contour of the workpiece.  
         [0017]     Another device that uses eddy current arrays has been disclosed in the following patent applications, which are incorporated herein by this reference: U.S. Patent Application Nos. 20030164700 and 20040232911. These patent applications disclose a device that has essentially identical sensor arrays with sensing elements aligned in proximity to the drive elements, and conductive pathways that promote cancellation of undesired magnetic flux. These references do not disclose how to reestablish the original surface contour of a pipe, nor do they disclose any way to expand the dynamic range of the testing device.  
         [0018]     Another device that uses eddy current arrays has been disclosed in the following patent application, which is incorporated herein by this reference: U.S. Patent Application No. 20060170420, which states in the abstract, “An eddy current testing probe has a flexible substrate adapted to face to a surface of a test article, a plurality of coils which are fixed to the flexible substrate and energized one of which is capable of being changed sequentially, a pressing member for pressing the substrate toward the test article, an elastic member arranged between the substrate and the pressing member, and a movement limiting member for limiting a movement of the pressing member toward the test article.” The device disclosed in this patent is again concerned about pressing an eddy current probe against a workpiece, or “test article”, and the attendant problems with exerting such pressure. The patent does not disclose how to map the dimensions of surface defects.  
         [0019]     In light of the foregoing, a need remains for a flexible two-dimensional eddy current array that 1) provides a means to map defects on a pipe, 2) uses the flexible substrate to reestablish the original surface contour of a pipe, and 3) provides means to expand the dynamic range of the eddy current array.  
       BRIEF SUMMARY OF THE INVENTION  
       [0020]     A system for mapping a surface defect in an electrically-conducting material by measuring a change in the resonance of the material comprises: a flexible printed circuit board; and a two dimensional array of transducers printed on the flexible circuit board, wherein each element of the array comprises two transducer coils in a paired arrangement. A receive circuit connected to the coils is tuned to a resonant frequency, and the transducer coils operate in a send/receive mode. In another feature of the invention, there are means for converting a change in measured resonance to a visual display of the depth and width of the surface defect. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0021]      FIG. 1  is an overview showing the conformable eddy current array of the present invention being used to measure a defective surface area of a pipe.  
         [0022]      FIG. 2  is a schematic diagram of the conformable array of the present invention.  
         [0023]      FIG. 3  is a schematic diagram of the individual coils that comprise the array shown in  FIG. 2 .  
         [0024]      FIG. 4  is a schematic diagram showing the individual coils and the depth of the surface defect of a pipe.  
         [0025]      FIG. 5  is a schematic diagram of the electronics that control the eddy current array. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     In  FIG. 1 , the system  10  of the present invention includes a flexible eddy-current sensor array  12 , and a USB cable  14  that connects the array  12  to a laptop computer  16 . The array  12  receives power from the laptop computer  16  through a USB port. As shown, the system  10  is used by an operator  18  to detect the extent of surface corrosion or defect  20  on an underground pipe  22 .  
         [0027]     Referring now to  FIG. 2 , the array  12  includes a flexible section  30 . Although the flexible section  30  can be of any size and number of transducers, in the preferred embodiment the flexible section  30  contains  512  coils  32  arranged in  256  coil pairs  34 , with connections allowing each coil to sense pit depth over the area beneath it. The coil pairs  34  are arranged in a square grid with 16 rows and 16 columns. The grid measures 6-inches on each side. The number and size of the coil pairs can be altered to make larger or smaller versions of the flexible section  30  or to alter the resolution of the entire system  10 . Each coil pair  34  consists of a send coil  32   a  and a receive coil  32   b , one on top of the other, exactly aligned.  
         [0028]     The flexible section  30  is a printed circuit that contains the coil pairs  34 . The flexible section  30  will conform to the shape of the pipe or structure being assessed, and is stiff enough to reestablish the original contour of the pipe or structure. The flexible section  30  is a printed circuit board, manufactured by the Speedy Circuits Division of PJC Technologies, Inc., 5331 McFadden Avenue, Huntington Beach, Calif. 92649. The section  30  provides the reference surface from which lift-off measurements can be referenced, representing the depth of the defect  20  or feature immediately under a coil pair  34 . The axial and circumferential position of each measurement is acquired from the position of the coil pair  34  within the section  30 .  
         [0029]     Referring now to  FIG. 3 , eddy current lift-off is the method of pit depth or metal loss measurement in the pipe  22  used by the conformable array  12 . If an alternating electrical current flows in a coil  32 , a magnetic field  33  is created about the coil  32  ( FIG. 3A ). If the coil  32  is placed near an electrically conducting material, such as the pipe  22 , the magnetic field  33  penetrates the pipe  22  and causes reaction currents  36  (eddy currents) to flow in the pipe  22  ( FIG. 3B ). The effect of this eddy current  36  is to oppose the force or current that created it, which is manifest as a change in the impedance of the driving coil  32   a . The amount of change depends, among other things, on the distance between the coil and the electrically conducting material  22 . For crack-like defects the crack itself will impede the eddy current, resulting in a change of coupling, and a sensor change that can be measured.  
         [0030]     Referring now to  FIG. 4A , if a single coil  32  of a coil pair  34  is subject to an alternating current, and is placed over an electrically conducting material  22 , the coupling between the active coil  32   a  and the second coil  32   b  of the pair  34  will be altered leading to changes that are manifest as a change in the impedance of the coil  32 . This change in impedance may be detected as a measure of the distance  38  from the coil pair  34  to the conducting surface  22 . Referring now to  FIG. 4B , if the coil pair  34  is placed near the surface of the conducting material  22  with a defect  20  present, the coupling change will be a function of the depth  38  and size of the defect  20 .  
         [0031]     Referring again to  FIG. 2 , in addition to the flexible section  30 , the array  12  includes two rigid sections  40 ,  42  that contain the electronics to address each of the coils  32  individually. This requires thirty-four 16-channel multiplexers. Sixteen 16-channel multiplexers are needed to address the 256 send coils  32   a  with one additional 16 channel multiplexer being required to ensure that only one of the multiplexers is active at any given time. The same holds true for the receive coils  32   b . Other systems on the rigid sections  40 ,  42  generate a sine wave signal, filter the analog portion of the circuits to minimize interference, provide a USB interface, regulate and filter power supplies for both the digital circuits and the analog circuits, and provide oscillators to provide clock and timing for the digital systems.  
         [0032]     Referring now to  FIG. 5 , these systems of the rigid sections  40 ,  42  are shown. The laptop computer  16  connects via the wire  14  to the rigid sections  40 ,  42 . Power distribution chip  50  protects the sections  40 ,  42  and the laptop computer  16  from power spikes that can be caused by capacitive loads.  
         [0033]     USB interface chip  52  is model no. FT245BM, manufactured by FTDI. Chip  52  sends and receives data from and to the laptop  16 . Oscillator  54  is a 6 MHz oscillator to operate the chip  52 . EEPROM  56  is a 1K serial EEPROM that can be programmed from the USB chip  52 . The EEPROM  56  tells the USB chip  52  that it is dealing with a Conformable Array. Data lines  58  are outputs from a microcontroller  60 . This is where the coil data is sent from the microcontroller  60  for transfer to the laptop computer  16 . Connector  61  is used to program the microcontroller  60 .  
         [0034]     The microcontroller  60  is model no. MSP430X14X, manufactured by Texas Instruments. The microcontroller  60  controls all the functions of the array  12 . The microcontroller  60  is used to address multiplexers that select a single coil pair  34  for activation. The microcontroller  60  sends a digital stream to a direct digital synthesizer  62 , telling it to output a 4.2 MHz sine wave. The synthesizer  62  is model no. AD9834, manufactured by Analog Devices. The microcontroller  60  sends an output voltage via a 4-channel multiplexer, model no. ADG704, manufactured by Analog Devices. The multiplexer simply outputs 2.5 volts, which is connected to a lamp in a switch  64  through various resistors that simply increase the current to the lamp causing it to flicker or flash. The switch  64  is a data acquire button. When it is depressed it takes 3.3 volts on one of the resistors to ground, which then takes a pin of the microcontroller  60  to ground, initiating data capture.  
         [0035]     The sine wave from the digital synthesizer  62  is output into a 6 pole Butterworth low pass filter  66 . The filter  66  is made from model no. AD8039, manufactured by Analog Devices. The filter  66  removes signals at a frequency greater than 7 MHz. The output of the filter  66  is directed to a high pass filter that removes signals below 106 kHz, and is then input to a high-speed, high-current buffer amplifier  68  to provide the necessary drive current to the send coil  32   a . The amplifier  68  is model no. BUF634, manufactured by Texas Instruments. A capacitor  70 , 1000 pF, creates a resonant circuit with the drive coil at 4.2 MHz.  
         [0036]     The output of the drive amplifier  68  is routed to the input of a 16-channel multiplexer  80 , model no. ADG706, manufactured by Analog Devices.  
         [0037]     The multiplexer  80  connects to sixteen 16-channel multiplexers,  82 - 112 , which are each model no. ADG726, manufactured by Analog Devices. These multiplexers  82  to  112  are in a dual 16-channel packages.  
         [0038]     Each of the multiplexers  82 - 112  is connected to a row of coils  32 . The lowest 4 bits of the binary address counter cause each of the 16 multiplexers  82 - 112  to address one coil  32  of each row at a time. All sixteen multiplexers  82 - 112  are doing this coil sweep continuously and at the same time. The multiplexer  80  ensures that only one coil  32  is being activated at any given time. The multiplexer  80  couples the output of the driver amp  68  to a single multiplexer  82 - 112  at a time. For example, on initial startup the binary address count is 0000 0000. The lowest 4 bits, 0000 to 1111 address the A 1  to A 16  outputs of the multiplexers  82 - 112  and the top 4 bits, 0000 address the multiplexer  80  to output drive signal to only the first of the multiplexers  82 - 112 . As the count progresses the driver signal remains connected to the same multiplexer until the bottom 4 bits have reached 1111, which will be the sixteenth coil in row  1 . At this time the bottom 4 bits return to 0000 but the top 4 bits change to 1000. This then routes the drive signal to the second multiplexer of the group  82 - 112 , and the process begins again. In this manner all coils are independently activated.  
         [0039]     The output of the drive amplifier  68  is also routed to the input of a 16-channel multiplexer  114 , model no. ADG706, manufactured by Analog Devices.  
         [0040]     The output of the multiplexer  114  is the input to a 1000 pF capacitor  116 , which is used to create a resonant circuit with the inductance of the receive coil at 4.2 MHz. The output of the multiplexer  114  is also the input to a non-inverting amplifier  118 , model no. AD8038, manufactured by Analog Devices. A half-wave rectifier at the output of the amplifier  118  converts the AC 4.2 MHz output signal to a DC level, which is the input to a dual operational amplifier  120 , model no. AD8572, manufactured by Analog Devices. The first stage is simply a non-inverting amplifier with a digitally controlled gain. The gain is set by a digital potentiometer, which takes its gain setting from the microcontroller. The output of stage  1  is also the input to the microcontroller. The output of stage  1  is the input to the second stage of this amplifier. This second stage is a low pass filter with a cutoff frequency of 1.2 kHz. This filter ensures a cleaner signal than may be available at the stage  1  output. This output is input to the microcontroller. In video mode the output of stage  1  input of the microcontroller is the active port. When the acquire data button  64  is depressed it makes the output of stage  2  of the final stage of the receive amplifier the active port. At this time the microcontroller  60  samples each received signal multiple times and averages the response. This average value is then sent to the USB interface chip  52  to be sent to the laptop computer  16 .  
         [0041]     In operation, the laptop computer  16  is used to acquire the data from each coil pair  34 . When these measurements are acquired, they are input to the conformable array analysis software where the signal is compensated according to the response curve previously established by a calibration procedure to produce an accurate map of the defect  20  or imperfection being assessed. This defect map can then be used to assess the defect  20  or imperfection and determine the affect that the defect or imperfection will have on the integrity of the structure being assessed. The software produces a contour map of the defect  20 . The map identifies the location and depth of all features within the scan area, and calculates the maximum safe operating pressure if the structure is a high pressure pipeline  22 . The software allows an operator to calibrate measurements and record other pertinent information for record keeping purposes.  
         [0042]     Prior to device operation each of the 256 transducer coil pairs  34  must be calibrated to develop the individual response curve for that transducer pair  34 . This is done by providing a series of known distances from mild steel (or material similar to the material of the structure being assessed if the structure is made from electrically conducting material other than steel) to the coil pairs (called “lift-off”) and measuring the receiver response. As calibration points are acquired for various lift-off distances they are fit to a polynomial curve. When sufficient calibration points are acquired, the coefficients of the polynomial curve are calculate and stored. Each transducer pair  34  will have a set of four polynomial coefficients which define its response to distance between the coil pair and the conducting surface. When calibration is complete, the calibration coefficients are written to, and stored in, the microprocessor on the array printed circuit card. This calibration data will reside with the array  12  during its life or until such time as a new calibration procedure is executed  
         [0043]     Although this description applies to an array consisting of 256 transducers arranged in 16 rows and 16 columns, designed for a resonant frequency of 4.2 MHz, the size of the array can be changed, and the resonant frequency can be changed.  
         [0044]     When the array  12  is first activated by the laptop computer  16 , the display and analysis software requests calibration information from the microprocessor  60 . The microprocessor  60  sends the stored calibration coefficient data back to the array software through the USB cable  14  where it will be stored and used to process that scan. (The calibration data is stored in the microprocessor  60  using a calibration procedure applied to each assembly prior to use.)  
         [0045]     The laptop computer  16  sends a “New Scan” command to the Conformable Array circuit board through a USB cable. The laptop computer  16  provides power to the electronics on the array printed circuit board through the USB cable  14 . The “New Scan” command initiates the microprocessor  60  on the array  12  which in turn enables thirty-four 16-channel multiplexers. Sixteen 16-channel multiplexers are needed to address the 256 send coils with one additional 16 channel multiplexer being required to ensure that only one of the multiplexers is active at any given time. This ensures that only one coil is active. The array cycles through all 256 coils sequentially. The same holds true for the receive coils.  
         [0046]     The send coils  32   a  and receive coils  32   b  are printed on the printed circuit card, one on top of the other, exactly aligned. The microprocessor enables a Direct Digital Synthesis (DDS) system located on the array. The DDS is programmed to output a sine wave drive signal at a frequency of 4.2 MHz and of known fixed amplitude. The frequency (4.2 MHz) is chosen such that the “send coil”  32   a  resonates with a 1000 pF capacitor in air. The receiver coil  32   b  also resonates with a 1000 pF capacitor in air. This 4.2 MHz signal is amplified and buffered and used to drive each of the 256 sender transducers sequentially in turn. The individual transducers are addressed by the microprocessor  60 . The send and receive transducers of a sensor pair  34  are activated in unison. When the array is in close proximity to electrically conducting material the electromagnetic coupling between the coils  32  will be altered and the impedance of the transducer coils changed. The coils will no longer be resonant with the 1000 pF capacitors, and the amplitude of the coupled signal from the receive coil will change. Operating in the resonance range increases the dynamic range of the system  10 . The magnitude of the change will be proportional to the change in inductance of the coil which is proportional to the distance  38  between the coil  32  and the conducting material  22 . The receiver signal is rectified and filtered to provide a DC voltage proportional to lift-off. In the video scan mode the microprocessor will digitize the DC level and send it to the Conformable Array software for viewing. The software will read the amplitude of the signal and apply the appropriate compensation coefficients previously received from the microprocessor prior to displaying the signal as a color contour map. After compensation, the displayed signal will be proportional to the depth  38  of the defect  20 . In this viewing mode each transducer  34  is scanned once by the microprocessor. When the acquire data command is sent to the microprocessor an additional filter is activated to remove minor ripple from the DC level and each receive transducer  34  is scanned multiple times, and an average value stored in the microprocessor. When all  256  transducers  34  are scanned the full data set is sent to the display software for visual inspection and analysis.  
         [0047]     In operation the pipe  22  or other structure to be investigated is exposed or otherwise made available to the technician using the conformable eddy current array  12 . The pipe  22  or structure is cleaned to remove lose debris and electrically conducting deposits that may exist in the defect  20  or imperfection being assessed. The conformable eddy current array  12  is attached to a laptop computer  16 , and operated from the conformable array software residing on the laptop computer. The conformable array  12  is positioned over the defect  20  to be assessed. A contour map representing the length, width, and depth of the defect  20  is visible to the operator on the laptop computer. The operator presses the acquire data button  64  on the array  12 . The conformable array  12  takes and averages multiple readings from each transducer  34 , and sends the final data to the software. If the structure being evaluated is a pipe  22 , the software will immediately calculate the affect of the defect  20  on the operating properties of the pipe using commonly applied methods. The system  10  will alert the operator to input certain calibration measurements to ensure accuracy of the readings. If the area to be assessed is larger than the size of the active portion of the array  12 , it can be assessed using multiple scans with the array  12  being carefully repositioned between each scan. The software is capable of stitching the multiple scans into a singe contour map of the defect  20 .