Abstract:
A Barkhausen probe comprising a magnetizing field generator, a magnetoresistive magnetic field sensor, a direct current power supply for biasing the magnetoresistive magnetic field sensor and signal conditioning electronics is described. The Barkhausen probe being capable of generating and sensing the Barkhausen noise emanating from the surface of a cyclic magnetized specimen. The conditioned Barkhausen noise output of the signal conditioning electronics being usable as input to various analysis systems where textural analysis of the material can be performed.

Description:
CROSS REFERENCE OF RELATED APPLICATIONS  
       [0001]    Pursuant to 35 U.S.C. Section 119, the benefit of priority from Provisional Application Number 60/227,265 with filing date Aug. 24, 2000 is claimed for this Non-Provisional Application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field of the Invention  
           [0003]    The present invention relates generally to a probe for detecting the Barkhausen signals from the sample when it is exposed to a varying external magnetic field.  
           [0004]    2. Background and Objects of the Invention  
           [0005]    The Barkhausen noise method is based on the concept of ferromagnetic domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization.  
           [0006]    Walls within which the direction of the magnetization usually turns 180 or 90 degrees separate domains from one another. When a magnetic field or mechanical stress is applied to a ferromagnetic material changes take place in its domains structure by abrupt movement of domain walls or rotation of domain magnetization vectors. If a coil of conducting wire is placed near a sample while the domain wall moves, the resulting change in the magnetization will induce an electrical pulse in the coil. These electrical pulses, which are stochastic in nature, make up what is called barkhausen noise. The characteristics of the barkhausen noise give rise to a power spectrum that extends up to frequencies as high as several megahertz. The power spectrum has information at virtually all frequencies. For example, the frequency content of the Barkhausen signals will start at the magnetizing frequency and extend up to several megahertz. The magnetic signals are exponentially damped as a function of distance it has to travel from within the material to the surface where it undergoes damping due to counter magnetic fields set up within the material by transient eddy currents as the magnetic field pulses travels to the surface of a test specimen. The amount of damping determines the depth from which information can be obtained. The main factors affecting this depth are the frequency range of the signal analysis and the conductivity and permeability of the test material. The conductivity and permeability of the test material are independent of the size and type of Barkhausen noise detection probe. The selected frequency analysis range is a function of the type, size and geometry of the Barkhausen detector. It is an object of this invention to provide a Barkhausen sensor that has high sensitivity at very low frequencies. It is well known that an increase in the excitation frequency decreases the depth of penetration of the excitation magnetic field and an increase in the detection frequency allows the detection of Barkhausen signals primarily from the surface region of the material.  
           [0007]    Sakamoto et al. in U.S. Pat. No. 6,073,493 described a Barkhausen apparatus having a U-shaped magnetic excitation head and a magnetic detection head consisting of a wire wound air-core coil. The U-shaped excitation head is made of soft magnetic material and an excitation coil. The apparatus of Sakamoto is designed to diagnose the fatigue life structural steel work using the root-mean-square (RMS) voltage or voltage amplitude value of the Barkhausen noise.  
           [0008]    Kohn et al., U.S. Pat. No. 5,619,135 described a steel hardness measuring system that comprises an energizing yoke, a hall probe, a Barkhausen signal sensor and a signal-processing unit. The coil wound energizing yoke is used to provide the magnetic flux to generate the Barkhausen noise within the sample. The Hall probe was provided to measure the tangential magnetic field intensity H at the surface of the sample to provide for calibrations of the device of the U.S. Pat. No. 5,619,135. The Barkhausen signal sensor in this case was of one of a variety of known coil type sensors designed to sense a changing magnetic flux. The invention of this provisional application has only to do with the probe for creating and detecting the BL noise.  
           [0009]    Perry in U.S. Pat. No. 5,166,613 invented a system for identifying and measuring stress at specific locations within a ferromagnetic material by the production and detection of Barkhausen noise during magnetization of the material. The apparatus of Perry consist of a signal generator for generating a combined cyclic and spatially varying magnetic field signals using two electromagnets located on opposing faces of a sample He teaches the use of acoustical Barkhausen noise sensors located near the two electromagnets coupled to computing and analysis capability for analyzing the acoustic Barkhausen noise. The acoustic sensors, which must be attached to the sample, are design to operate in the frequency range from 75 to 450 kHz. These sensors provided for greater penetration into the thickness of a test sample than that normally associated with wound coils. The aim of this invention is to provide a probe that will provide for the sensing of Barkhausen noise from depths not possible with the typical wire wound coils. A still further aim is to provide a sensor that will not have to be physically attached to the surface of the test specimen.  
           [0010]    Typically the sensor used to detect Barkhausen noise may comprise any of a variety of known coil sensor types. Examples of prior art Barkhausen signal sensors include encircling coils, surface pancake coils, ferrite core surface coils, etc. All detection coils are based upon Faradays law, which states that the total emf induced in a closed circuit is equal to the time rate of decrease of the total magnetic flux linking the circuit. That is, the induced emf is a function of the area of the coil and the rate of change of the magnetic flux at the coil. This suggests that a wire wound coil has reduced sensitivity at low frequencies. In addition to the aims of this invention discussed above. It is an object of this invention to provide a Barkhausen noise generator and detector with increased sensitivity at lower frequencies. It is a further object to provide a Barkhausen generating and sensing probe that can sample a ferromagnetic material at depths not available with coil Barkhausen signal detectors. Moreover it is an additional object of this invention to provide a Barkhausen probe that can be used with commonly available signal processing systems to measure residual stress, detect faults in structures and other material properties. . Other objects of the probe of this invention will be come obvious during the course of the description of the probe.  
         OTHER REFERENCES  
         [0011]    Pasley, R., Barkhausen Effect—An Indication of Stress, Material Evaluation, Vol. 28, No 7, July 1970, pp. 157-161.  
           [0012]    Dhar, A., Jagadish, C., and Atherton, D. L., Using the Barkhausen Effect to Determine the Easy Axis of Magnetization in Steels, Material Evaluation, October 1992, pp. 1139-1141.  
           [0013]    Francino, P., and Tiitto, K., Evaluation of Surface and Subsurface Stresses with Barkhausen Noise: A Numerical Approach, Practical Applications of Residual Stress Technology, Proceedings of the Third International Conference, Indianapolis, Ind. May 15-17, 1991.  
         SUMMARY OF THE INVENTION  
         [0014]    According to the principles of the first embodiment of the present invention, a Barkhausen probe comprising a magnetizing field generator, a magnetoresistive magnetic field sensor, a direct current power supply for biasing the magnetoresistive magnetic field sensor and signal conditioning electronics. The Barkhausen probe being capable of generating and sensing the Barkhausen noise emanating from the surface of a cyclic magnetized specimen. The conditioned Barkhausen noise output of the signal conditioning electronics being usable as input to various analysis systems. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0015]    [0015]FIG. 1 is sketch showing a schematic representation of a side view of the Barkhausen probe of the first embodiment of this invention with its sensitive axis essentially perpendicular to the surface of the test specimen.  
         [0016]    [0016]FIG. 2 is a sketch showing a schematic representation of the Barkhausen noise detector and signal conditioning electronics of the probe of the first embodiment of the invention.  
         [0017]    [0017]FIG. 3 is sketch showing˜ a schematic representation of the probe of second embodiment of this invention  
         [0018]    [0018]FIG. 4 is a schematic representation of side view of the probe of the first embodiment of this invention with the sensitive axis of the Barkhausen probe essentially parallel to the surface of the test specimen.  
         [0019]    [0019]FIG. 5 is a bottom view of a multiplicity of Barkhausen probes arrayed in an integral housing.  
         [0020]    [0020]FIG. 6 is a Cross sectional side View of one of the multiplicity of Barkhausen probes of FIG. 5.  
         [0021]    [0021]FIG. 7 is a schematic representation of the probe of this invention and ancillary equipment employed in the use of the probe. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Several preferred embodiments of the present invention will be described below with reference to the accompanying figures.  
         [0023]    First embodiment  
         [0024]    The first embodiment will now be described. The first embodiment of the device of this invention is schematically represented in FIG. 1. As shown in FIG. 1, the Barkhausen probe is indicated generally by the numeral  2 . The Barkhausen probe  2  comprises magnetic excitation head  4  and magnetoresistive material based Barkhausen noise pickup sensor  6  with its sensitive axis  7  having a direction that is substantially perpendicular to the surface of test specimen  3 . The magnetic excitation head  4  (hereafter referred to as electromagnet  4 ) has C or U-shaped core  8  made of soft or easily magnetized material.  
         [0025]    The soft magnetizeable core will be hereafter referred to as a U-shaped core  8 . The U-shaped core  8  of electromagnet  4  may consist of laminated transformer steel such as grain oriented silicon steel or any material that has good magnetic permeability and sufficiently high magnetic saturation. The electromagnet  4  is formed with electrically insulated wire (e.g. enamel transformer wire)  10  coiled around the U-shaped core  8 . Materials such as mu-metal, or an amorphous magnetic material will also make sufficiently adequate core materials.  
         [0026]    Pickup sensor  6 , which is a commercially available solid state device, comprises four magnetoresistive elements  13   a ,  13   b ,  13   c  and  13   d  configured in a solid state Wheatstone bridge configuration (WBC)  14  as shown schematically in FIG. 2. Magnetoresistive elements  13   a  and  13   c  are designed to respond to the Barkhausen generated noise while elements  13   b  and  13   d  are magnetically shielded from stray and test specimen  3  originated magnetic field Barkhausen noise. The Wheatstone bridge is hereafter referenced to by numeral  14 . Nodes  15   a  and  b  of the Wheatstone bridge  14  are connected to the positive and ground terminals, respectively, of an ordinary 5 volt power supply  12  of FIGS. 1 and 2. The output voltage of the solid-state bridge circuit WBC  14  is sensed at the two opposing nodes  15   d  and  15   c , which are connected to channel  18   a  of signal conditioning electronics  18  as shown schematically in FIG. 2. The pickup sensor  6  (see FIGS. 1,2 and  3 ) is, as mentioned before, is a solid-state representation of WBC of FIG. 2. The elements of the Wheatstone bridge  14  are not very sensitive to magnetic fields whose directions are not parallel to the vertical axis of the solid-state bridge circuit. Even though the WBC  14  offers certain advantages for detecting fluctuating signals, single element giant magnetoresistive sensor could be used for detecting Barkhausen noise signals. Further, the intensity of the magnetic field proximate the surface of test sample  3  can be monitored by recourse to electrical connections between nodes  15   d  and  15   b  and channel  18   b . Channel  18   b  is one channels defined by the dual signal conditioning electronics  18  shown in FIG. 2. AS shown in FIG. 2, sensor  6 , makes it possible to simultaneously monitor the Barkhausen noise being generated and the intensity of the applied magnetic field. The sensitive axis  7  of sensor  6  is positioned so as to be substantially perpendicular to the surface of test specimen  3  and is collinear with the central vertical axis of cylindrical sleeve  20 . The cylindrical sleeve  20  is used to ensure the position of sensor  6  in regards to its symmetrical location at the first terminal of tube  40 . The cylindrical sleeve  20  of FIG. 1 is made of dielectric material. It could however been made of mu-metal or other magnetic field shielding materials. When the cylindrical sleeve  20  is made of mu-metal, it forms a barrier against magnetic fields and hence substantially reduces the strength of fluctuating and non-fluctuating magnetic fields at the location of sensor  6  whose directions are not substantially perpendicular to the surface of the specimen to which the Barkhausen probe is applied in non-destructive testing. The WBC  14  or sensor  6  is inserted into sleeve  20  in a non-touching position with the inner surface of cylindrical sleeve  20 . The air filled volume of probe  2  defined its first terminal end  41  and disk  5  is filled with potting compound  22 . The magnetic excitation head  4  and the pickup sensor  6 , which can be made very small because the solid-state nature of the pickup sensor  6 , are potted in dielectric potting compound  22 . The active end of the probe could be made even smaller using hybrid circuitry where the excitation head  4 , pickup sensor  6  and signal conditioning electronics  18  are packaged using hybrid and integrated circuitry technology(see FIGS. 5 and 6). In FIG. 5, a multiplicity of magnetic field excitation heads  4 , cylindrical sleeves  20  and Barkhausen noise sensors  6  are potted in potting compound  22 . The aggregate is housed in plastic housing unit  38  thereby forming an array of sensors for use in non-destructive material testing. The signal conditioning electronics  18  (not shown in FIG. 5) is shown schematically in the cross sectional view of FIG. 6. In FIG. 6, the outputs from the array of Barkhausen probes are fed from the signal conditioning electronics to a multi input analysis unit (not shown) via multiple pair electrical leads  17 .  
         [0027]    The single probes of this invention had an overall diameter of approximately 12.5 mm at its active end. However, the die of pickup sensor  6  can be made with volumes less than 0.5 mm×0.5 mm×0.75 mm. When made using hybrid and integrate circuitry technology probes smaller than  4  mm will be possible making the use of the Barkhausen noise technique applicable in measurement applications that are not realizable with the existing Barkhausen noise detection technology that is based on the use of wire wound coils as pickup sensors. Further this makes possible good spatial resolution since the sensed Barkhausen noise will be averaged over a much smaller volume due to the size of the active end of the probe. The probe will be usable on components with small radiuses of curvature as is found in many components of high performance engines.  
         [0028]    While the magnetic field sensor of the Barkhausen probe  2  is made of magnetoresistive elements in a Wheatstone bridge configuration. It could, however, been made of anisotropic magnetoresistive or Giant magnetoresistive materials in Wheatstone bridge configuration.  
         [0029]    The signal conditioning circuitry  18 , which is composed of dual electronic channels  18   a  and  18   b , shown in box representation in FIG. 1 and schematically in FIGS.  2  comprises commercially available ordinary instrumentation amplifier  19   a  and  19   b  in a dual integrated circuitry package, low noise high gain bandwidth amplifiers  21   a  and  21   b , and bandpass filters  23  and lowpass filter  25  which are electrically connected first to pickup sensor  6  via electrical leads  17  and secondly to connector  45 . A pair of the electrical leads  17  are coupled to nodes  15   d  and  15   c  WBC  14  and to the differential input of instrumentation amplifier  19   a  of channel  18   a . A second pair is connected to nodes  15   d  and  15   b  and to the single input of instrumentation amplifier  19   b . The bandpass filter  23  and the lowpass filter  25  were selected from commercially available units that like the amplifiers avoided the use of components with ferromagnetic constituent parts. The active integrated circuit components maybe powered by dual or single sided power supplies such as the 5 Vdc supply  12 . The bipolar solid-state power supply  28  of FIGS. 1 and 3, which is electrically coupled to electromagnet  4  as by electrical leads  29 , is also electrically connected to connector  44 . The first end of Barkhausen probe  2  is housed in cylindrical tube  40  and the handheld portion in closed end cylindrical tube  42 .  
         [0030]    [0030]FIG. 4 shows a side view of the device of the first embodiment of the invention shown in FIG. 1 wherein the pickup sensor  6  is deployed with the direction of its sensitive axis  7  normal or orthogonal to the inner surface of tube  40 .  
         [0031]    The probe of this patent application is designed to be operated between at frequencies of a few Hz and several hundreds Hz. For example, during the course of the development of this invention the probe was operated between 5 Hz and 5000 Hz. The lower the frequency the greater the penetration depth. Correspondingly, an increase in excitation frequency decreases the depth of the excitation magnetic field. The output pulse rate of the Barkhausen noise pulses are of the order of thousands per second as the domain flips from one direction of easy magnetization to another. The theoretical energy content of these pulses is almost uniform from essentially zero Hz up to several Mega Hz. However, when the Barkhausen noise is Fourier transformed to obtain a frequency spectrum, the signal intensity generally decreases with an increase in frequency. The detection efficiency is then related to the bandwidth of the Barkhausen signal used in the analysis. The giant magnetoresistive elements  13   a - 13   d  are magnetic field dependent resistive elements usable, as mentioned previously, singly or in WBC configuration.  
         [0032]    Second Embodiment  
         [0033]    The probe of the second embodiment will now be described. The second embodiment of the device of this invention is schematically represented in FIG. 3. As shown in FIG. 3, the Barkhausen probe of the second embodiment is indicate generally by the numeral  30 .  
         [0034]    The Barkhausen probe  30  comprises magnetic excitation head  31  and magnetoresistive magnetic field sensor  32 . The magnetic excitation head  31  (hereafter referred to as electromagnet  31 ) has U-shaped core  34  made of soft or easily magnetized material. The U-shaped core  34  of electromagnet  31  may consist of laminated transformer steel such as grain-oriented silicon steel or any material that has good magnetic permeability and sufficiently high magnetic saturation. The electromagnet  31  then being formed with electrically insulated wire (e.g. enamel transformer wire)  10  coiled around the U-shaped core  34 . Juxtaposed next to magnetoresistive magnetic field sensor  32  with its sensitive axis in a plane perpendicular to the plane containing the vertical axis of the magnetoresistive magnetic field sensor  32  is solid-state magnetic field sensor  36 , which is used to monitor changes in the intensity of the applied magnetic field proximate the surface a test specimen  3 ′. The giant magnetoresistive magnetic field sensor  32  performs the same function as the pickup sensor  6  of the first embodiment of this invention. The bias voltage needed to power the Wheatstone bridge  36  will be the same as that that is used to bias magnetoresistive magnetic field sensor  32 . The cylindrical sleeve  33  is a magnetic field shield that is recessed into the inner volume of tube  40  proximate its first terminal end  41 ′ being displaced linearly from the first terminal end  41 ′ to accommodate the first placement of the magnetic field sensor  32 . While the cylindrical sleeve  33  is a conductive magnetic field shield, it could just as well been made of dielectric material. The Barkhausen noise detector and the applied magnetic field detector are potted in dielectric compound  38  which is housed in plastic cylindrical tube  40 ′ which in turn mechanically connected to hand held handle  42 ′ made of plastic material. The handle  42 ′ contains multi-pin connectors  44 ′ and  45 ′. Connector  44 ′ is used to input the control signal to bipolar power supply  28 ′. Connector  45 ′ connects the output of both bridge circuits to the analysis units, which may be a basic true RMS digital meter or a data acquisition and digital analysis system. It also provides the control signal for 5 volt power supply  12 ′. The oscillatory drive signal used to power the electromagnet  31  is conveyed thereto by electrical leads  29 ′.  
       A MODE OF OPERATION OF THE APPARATUS OF THIS INVENTION  
       [0035]    The operation of the probe of the first embodiment of this invention will now be briefly described. It should be obvious to any one of ordinary skill in the art that the probe may be employed in various measurements where the Barkhausen phenomenon is exploited. During the development of the probe, the electromagnet  4  was driven by waveform generator  48  (see FIG. 7) coupled to commercial flat pack bipolar power supply  28 . The electromagnet  4 , which is an integral part of the active end of Barkhausen probe  2 , which in the case of the first embodiment also contains magnetoresistive magnetic field sensor  6 , was place in contact test sample  3 . When in contact with test sample  3 , the output of the waveform generator  48 , which was connected to bipolar power supply  28 , was varied manually causing a variable magnetic field to be applied to the steel sample thereby sweeping it through a hysteresis loop. The control of the variable magnetic field could have been applied either manually, automatically, as by digital controller/analysis unit  50  of FIG. 7, which is connected to waveform generator  48  with control cable  52 . The waveform generator  48  is connected to bipolar power supply  28  via power cable  49  which transmits the waveform and bias voltages to bipolar power supply  28 . The Barkhausen signal generated by the variable magnetic field was then detected by the giant magnetoresistive magnetic field sensor  6  whose output pulses were amplified by instrumentation amplifier  19   a  and fed to low noise high gain bandwidth amplifier  21   a  whose output was fed to bandpass filter  23 . The output of the bandpass filter  23  was fed to ordinary digital controller/ analysis unit  50  of FIG. 7 via cable  51  where it is stored offline analyses. The ordinary digital controller/analysis unit  50  being equipped with analog-to-digital and digital-to-analog electronic capability. The method and choice of analysis of the output voltage signal depending on the objective of the measurement is independent of the manner in which the signals are derived. For example if one is interested in determining the easy axis of magnetization in the steel sample a simple true RMS digital voltmeter may be used. The complexity of the analysis unit may vary considerably depending on the goal a measurement program in which the probe is used.  
         [0036]    The Barkhausen probe of the first and second embodiments of this invention is here described as a cylindrical handheld probe for use portable and non-portable analysis equipment in the nondestructive testing of ferromagnetic specimens. It could, however, have one of many different three-dimensional shapes with an array of Barkhausen probes with onboard or remote analysis and readout components.  
         [0037]    Other variations will be readily apparent to those of ordinary skill in the art. The foregoing is not intended to be an exhaustive list of modifications but rather is given by way of example. It is understood that it is in no way limited to the above embodiments, but is capable of numerous modifications within the scope of the following claims.