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Personal computers and high quality color printers andcopiers have made detection of counterfeit currency anincreasing worldwide problem. 9 Magnetic fields areproduced by magnetized small particles of iron oxidecommonly used in black ink. These small fields canproduce signatures when read by magnetic sensors that canbe used to identify the denominations of currency presentedto vending machines and bank sorting machines. Thesignature of additional magnetic information encoded intomany counties’ currencies can be used to distinguish validcurrency from copies. At present magnetic signatures areread by one, or at the most a few, magnetic sensors as a billpasses a detection head. Optical detectors are used toobtain a more complete image of the bill. High-resolutionGMR sensor arrays could obviate the need for opticalimaging by providing a magnetic image of the bill.The magnetic imaging of currency is difficult becausethe low fields produced by the magnetic ink. Themaximum field measured immediately above U. S.currency is less than 100 mOe or 8 A/m. Inductive readheads designed similarly to tape recorder heads need to bein direct contact to yield an adequate signal from U. S.currency. Similarly, present commercially availablemagnetoresistive currency detection heads must be in directcontact with the passing currency. To avoid jamming inhigh-speed transport mechanisms it is desirable to be ableto read the bill from a few mm away. To achieve this goal,sensitive, low-field sensors such as GMR sensors areutilized with amplification and filtering. The small size ofGMR sensors offers the possibility of making closelyspaced arrays of sensors to image a bill rather than justobtaining an image of one trace along the length of the billor across the bill. Figure 12 shows a magnetic trace acrossthe right half of a new U.S. $20 bill. The upright sectionsof the letters in the word “TWENTY” cause the multiplepeaks in the center. The border surrounding the portraitcauses the large peak on the right and the framesurrounding the bill, the peak on the left. The GMR sensoroutput is amplified by an instrumentation amp and anoperational amp with a combined gain of several thousand.A low-frequency, high-pass filter and a high-frequency,low-pass filter limit noise and eliminate dc bias problems.Credit card fraud is an increasing problem in manyparts of the world. Skimmers use small computer devicesto obtain the information from credit cards at ATMterminals and use that information to create bogus “clone”cards. These cards are then used at other ATM terminals towithdraw money. Methods of increasing the security ofmagnetic stripe credit cards include using higher resolutionreaders to read unique fingerprints in each card in therandom arrangement of the magnetic particles or in randommagnetic particles added to the plastic itself. 10 Highresolutionmagnetic sensors and magnetic sensor arrays arethe basis for these security methods.Output (V)43210-1-20 0.1 0.2 0.3 0.4 0.5Time (s)Figure 12. Amplified output from a GMR magnetic sensorwhen passed over the center of the right half of a newstyleU.S. twenty-dollar bill.The same method of using magnetic “noise” toincreasing security of magnetic stripe credit cards can beapplied to verifying the authenticity of personal identitycards and documents – vital to the increased security needsfor Homeland Security.Nondestructive Evaluation -- Solid-state magneticsensors based on GMR and SDT effects have high,frequency-independent sensitivity that extends to the lowfrequencies necessary for deep, eddy-current penetration.The use of these sensors has been demonstrated in NDEdetection and magnetic imaging of surface cracks andfeatures, deep cracks, and cracks initiating from edges of11 12 13 14holes. The small size and low power consumptionof these solid-state magnetic sensors allow them to be usedin arrays of multiple sensors on a single chip facilitatingrapid scanning of an area for defects in a single pass ratherthan by single-point, raster scanning.The main components of an eddy current probe forNDE comprise a pancake-type coil and a GMR or SDTsensor. During measurement the sensing axis of the GMRor SDT probe is maintained to be coplanar with the surfaceof the specimen. The excitation field on the coil axis, beingperpendicular to the sensing axis of the GMR or SDT films,has no effect on the sensor. In this way, the detected field,which is the result of the perturbation of the eddy currentflow paths due to the crack, is separated from the excitationfield. Due to the circular symmetry of the field producedby the coil, corresponding eddy currents induced in thesurface of a defect free specimen are also circular. In thiscase, the tangential component of the field created by theeddy currents is zero at the location of the sensor. In thepresence of defects, the probe provides an absolute measureof the perturbed eddy currents.6
The size of the coil is related to the resolutionnecessary to detect the defects. For large defects and fordeep defects, large coils surrounding the sensor arerequired. To resolve small defects, small coils located closeto the specimen are necessary. It is possible to incorporatethe excitation coils directly on GMR or SDT sensors.Eddy currents shield the interior of the conductingmaterial with the skin depth related to the conductivity andthe frequency. Therefore, by changing the frequency,differing depths of the material can be probed. GMR orSDT sensors with their wide frequency response from dcinto the multi-megahertz range are well suited to thisapplication. The small size of a GMR sensing elementincreases the resolution of defect location if the detector israster scanned over the surface.A prototype of an SDT based eddy current probe hasbeen built and successfully tested for detecting cracks ofcalibrated width and depth. Figure 13 shows the output ofsuch a probe. The asymmetry in the magnetic field isdetected on either side of the crack when the sensitive axisis perpendicular to the crack. In the left figure, theasymmetry is detected at the ends of the crack when thesensitive axis is parallel to the crack. It has beendemonstrated that the unidirectional sensitivity of GMRsensors enable the detection of cracks at, and perpendicularto, the edge of a specimen. This discrimination is possiblebecause the sensitive axis of the GMR sensor can be rotatedto be parallel to the edge; consequently, the signal is dueonly to the crack. With inductive probes, the edge willproduce a large signal that can mask the signal produced bya crack. This capability represents a very simple solution toa difficult problem encountered in the aircraft industry --detecting cracks that initiate at the edge of turbine disks ornear the rivets.output(V)x (mm)y (mm)output(V)x (mm)y (mm)Figure 13. The output of an SDT eddy-current probe with20 kHz excitation scanned over a 15 mm by 2 mm deepcrack. a) sensitive axis parallel to the crack. b)sensitive axis perpendicular to the crack. (Resultscourtesy of Albany Instruments.)SDT sensors are in attractive for NDE low frequencyapplications, such as for the detection of deeply buriedflaws. In contrast, inductive probes have poor sensitivity atlow frequencies because they are sensitive to the timederivative of the magnetic field rather than to themagnitude of the magnetic field created by the flaw. Fordetecting deep cracks, it is necessary to use large diameterexcitation coils in order to increase the penetration depth ofthe eddy currents into the material under test. SDT eddycurrent probes were tested for detecting edge cracks at thebottom of both a single plate and a stack of thick aluminumplates. A short edge crack of 3 mm length and 3 mm heightwas detected at a depth of 18 mm below surface, and acrack of 15 mm length and 3 mm height was detected at 23mm at the bottom of a two-layer aluminum structure.Figure 13 shows the results of a single scan along the edgeand across the buried crack in both magnitude and phaserepresentations.Output (V) (mV)80706050400 20 40 60Displacement (mm)Phase (deg)16012080400 20 40 60Displacement (mm)Figure 14. The signature of a 15 mm long edge crack 9 mmbelow the surface in both magnitude and phase of theoutput of an SDT eddy-current probe. Excitingfrequency was 200 Hz. (Results courtesy of AlbanyInstruments.)Aging aircraft must be inspected for defects thataccumulate in the skin including cracks around rivet holesand corrosion between layers in multi-layer airframe skins.Eddy-current probes are of primary use for thisnondestructive evaluation. For deep flaws, the lowfrequency response of GMR and SDT sensors make themideal for this application. A demonstration of thecapabilities of GMR-based eddy-current probes was carriedout at Albany Instruments. For this experiment, pinholes ofsmall diameters (1.0 and 0.75 mm) of different depth weredetected on the backside of an aluminum plate of thickness1.6 mm. Two rows of holes were machined in the bottomsurface of the plate to simulate corrosion-type defects. Thefirst row consisted of four holes of 1 mm (0.04 inch)diameter, the second one of four holes of 0.75 mm diameter(0.03 inch). The depths of the holes in each row were 1mm, 0.75 mm, 0.5 mm and 0.25 mm. The space betweenholes in each row was 15 mm.The goal of the experiment was to detect these defectsfrom the opposite side of the plate. To obtain thepenetration depth and the resolution required for this typeof defects, an excitation coil of mean diameter of about 2mm was chosen. The optimum detection of defects wasobtained at the frequency of 8 kHz. A schematic diagramof the probe above the specimen during the experiment isrepresented in Figure 15.7
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very dense magnetic sensor arrays for precision ... - NVE Corporation

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