US 20040244625 A1
A magnetic sensor and magnetic field sensing method for ordnance, comprising locating a giant magnetoresistance detector within the ordnance and detecting magnetic fields with the detector. Also other sensing method and sensors employing such detectors.
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 The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. SOL DAAE30-01-BAA-0500 and Contract No. DAAE30-99-C-1068 awarded by the U.S. Army TACOM-ARDEC, by the terms of Contract Nos. DMI-0060397 and DMI-9704080 awarded by the National Science Foundation, and a contract with the U.S. Navy.
 This application is a divisional application of U.S. patent application Ser. No. 09/969,946, entitled ULTRA SENSITIVE MAGNETIC FIELD SENSORS, filed on Oct. 2, 2001, which claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/301,786, entitled ULTRA SENSITIVE MAGNETIC FIELD SENSORS, filed on Jun. 29, 2001, and which is a continuation-in-part application of U.S. patent application Ser. No. 09/265,991, entitled INTEGRATED MAGNETIC FIELD SENSORS FOR FUZES, to David W. Cutler, et al., filed on Mar. 9, 1999, issued on Oct. 2, 2001, as U.S. Pat. No. 6,295,931, which claimed the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/077,525, entitled SENSITIVE INTEGRATED MAGNETIC FIELD SENSORS FOR FUZES, filed on Mar. 11, 1998; and of U.S. Provisional Patent Application Ser. No. 60/092,717, entitled SENSITIVE INTEGRATED MAGNETIC FIELD SENSORS FOR FUZES, filed on Jul. 14, 1998. The specifications and claims of all these references are incorporated herein by reference.
 The present invention relates to sensors made of giant magnetoresistance materials (GMR) or colossal magnetoresistive materials. The term GMR, as used throughout the specification, is intended to include both giant and colossal magnetoresistance or magnetoresistive materials.
 The present invention is of a magnetic sensor and magnetic field sensing method for ordnance, comprising locating a giant magnetoresistance detector within the ordnance and detecting magnetic fields with the detector. In one embodiment, turns of spinning ordnance are counted, with autonulling being employed, such as injecting a charge into a circuit comprising the giant magnetoresistance detector, the injecting being triggered when a rate of spin of the ordnance exceeds a predetermined rate. In another embodiment, ordnance is armed a pre-determined time after exit of the ordnance from a weapon firing the ordnance, which can involve one or more of the following: determining exit of the ordnance from a weapon firing the ordnance (preferably by detecting concentrated earth's magnetic field lines at an opening of a weapon firing the ordnance), determining exit velocity, determining proximity to metallic targets, and determining direction to metallic targets. In another embodiment, incoming munitions are detected, such as by employing a biasing magnet, employing an oscillation frequency, employing a coil proximate the nose of the ordnance, employing multiple detectors and triangulation means, and/or employing an array of detectors. The detector is preferably fabricated on a printed circuit board, hybrid circuit, or integrated circuit. The sensor can also perform one or more of the following functions: outputting a signal characteristic of impact of the ordnance, employing a map of local magnetic fields to determining position and direction of an object, detecting flying objects by looking for response to an emitted magnetic field, and determining eddy currents output by flying object traversing earth's magnetic field.
 The present invention is also of a magnetic field sensor and sensing method for locating defects in objects in two or three dimensions, comprising providing to a sensor a giant magnetoresistance detector and traversing the sensor over an object. In the preferred embodiment, one or more of the following functions are performed: determining type of defect, determining volume lost to defect, determining depth of defect, imaging the defect, determining dimensions of defect, and producing Lissajous plots of amplitude versus phase for magnetic fields.
 The present invention is further of a magnetic field sensor and sensing method for medical imaging, comprising providing to a sensor a giant magnetoresistance detector and traversing the sensor over a patient. In the preferred embodiment, one or more of the following functions are performed: measuring electromagnetic activity in the patient's heart, measuring electromagnetic activity in the patient's brain, performing biomagnetics analysis, performing nuclear resonance imaging, and imaging skin defects.
 The invention is additionally of a magnetometer and magnetometry method comprising providing a giant magnetoresistance detector and operating the detector. In the preferred embodiment, one or more of the following functions are performed: detecting conductive materials on a surface, detecting conductive materials beneath a surface, detecting conductive materials in a body of liquid, locating naval vessels, detecting electrical currents flowing through printed circuit board traces, detecting electrical currents flowing through integrated circuit components, measuring amplitudes of electrical currents, and detecting breaks in electrical conductors. Operation may be in DC detection mode or employing oscillating magnetic fields.
 Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
 The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
FIG. 1 is an electrical schematic of the preferred spinning round sensor of the invention;
FIG. 2 is a diagram of a circuit board incorporating the sensor of the invention;
FIG. 3 is a graph showing signal pulse upon exit from a mortar tube of a munition incorporating the sensor;
FIG. 4 illustrates the sine wave detected by the sensor as generated by the rotation of a munition, with each cycle being equivalent to one rotation of the munition;
FIG. 5 is an electrical schematic of the barrel exit detection embodiment of the invention, useful with mortars and non-rotating rounds;
FIG. 6 is a perspective view of a data logger PCB incorporatable into a 40 mm round for testing purposes;
FIG. 7 is a front perspective view of a round incorporating data logger and sensor PCBs just before final assembly;
FIG. 8 is a block diagram of a system useful in arming a device according to the invention and collecting data from an on-board data acquisition system;
FIG. 9 illustrates a conditioned signal output by the data logger PCB, each square wave transition corresponding to one revolution of the 40 mm round;
FIG. 10 illustrates use of the sensor of the invention within a counter munition;
FIG. 11 is a perspective view of a handheld NDE unit including graphical modes that can resolve defects to a depth of more than 0.4″ in 6061T6;
FIG. 12 is a top view of a 196-element tunnel junction imaging array;
FIG. 13 is an 11 kHz C-scan of first and second layer rivet heads in an aluminum sheet metal section simulating an aircraft wing;
FIG. 14 shows 100 micron wide cuts in an aluminum sheet metal sample;
FIG. 15 is an APET image of a 10% mass loss corroded region measured through 60 mils of aluminum, made from the side opposite the corrosion;
FIG. 16 illustrates use of the sensor of the invention for guidance of a munition according to the invention to an under-sea mine;
FIG. 17 is a top view of a 14×14 element array of GMR sensors produced on a single substrate, each element being approximately 10×10 microns;
FIG. 18 is output from a two-dimensional GMR sensor array according to the invention showing variations in magnetic field strength over an area of metal; and
FIG. 19 is a block diagram of a system according to the invention used for non-destructive evaluation of a sample.
 Magnetic Field Sensors for Munitions
 The present invention is directed to an advanced, giant magnetoresistive (GMR) based sensor platform for detection of spin rate and muzzle exit, muzzle exit velocity, range, proximity, attitude, and/or guidance of munitions from within the fuze assembly of an advanced munition. The sensor can operate with both rotating rounds such as a 40 mm rotating round or with non-rotating rounds such as a 60 mm mortar. The invention can also operate with finned munitions.
 Extensive shock and thermal testing was employed to verify the robustness of the sensor. The sensor and support electronics were transferred to a production board package in order to provide single board sensors for turns counting or barrel exit sensing. Live-fire tests were successfully completed for a 40 mm rotating round. The tests were successful and showed that it is possible to monitor barrel exit, rotations, velocity, and range all with a single sensor. The results showed that the sensor was able to provide data that can monitor munition flight distance with an error of less than 1% (0.84%). Evaluations of barrel exit detection, tolerance of mechanical shock, and environmental durability of the sensors were also positive using an 60 mm mortar tube. Laboratory simulated live-fire tests for the mortar barrel exit sensor were performed.
 The preferred spinning round circuit design is as follows, including an autonulling capability (see FIG. 1). As soon as the sensor begins to spin at a rate above a few Hz, the circuit injects a small charge into the sensor to compensate for any polling (due to large magnetic field exposure) that has taken place during storage of the sensor or munition. The rate of rotation for activation is high enough that it will not null the sensor if the munition is dropped or rolled during handling. When fired, the rotation due to rifling of the gun barrel makes the sensor spin at a sufficient rate, while the sensor is still in the barrel of the gun, to activate the autonulling. This ensures that the sensor is biased properly to detect barrel exit when it is fired, and begin counting turns when the munition exits the barrel.
 The circuit automatically nulls the GMR sensor during power-up. This compensates for any residual magnetization that may have occurred due to close proximity of the sensor to large magnetic fields.
 The circuits are preferably built on printed circuit boards (PCB), which offer low power operation in a small enough package to meet the demands of both 40 mm rounds and 60 mm mortars. Circuits built this way are much less expensive to design layout, build and test, than are circuits made with hybrid techniques. However, hybrid circuits may be used where a smaller package is needed.
 Shock testing was performed prior to firing the 40 mm test rounds. The sensor and electronics system were unaffected by exposure to shock equivalent to live firing of the round. During live fire tests with 40 mm rounds the sensor and electronics withstood the shock with no failures or any signs of damaging effects.
 The present invention can also measure barrel exit time and barrel exit velocity for mortars. Using techniques and sensors made from giant magnetoresistance materials (GMR), highly durable, low power sensors detect barrel exit in both 60 mm mortars, and 40 mm spinning rounds (FIGS. 2 and 5). Larger diameter mortars (e.g., 80 mm and 120 mm) require that the sensor have the ability to dependably detect the edge of the mortar tube using a passive approach such as measuring the earth's magnetic field concentration at the edge (exit) of the mortar, or using an active mode, for example, a small permanent magnet mounted on the end of the mortar tube at a large distance between the sensor and the edge of the mortar tube. Laboratory tests with larger diameter mortars indicate that the invention can measure both exit time and exit velocity in these munitions.
 The present invention also provides an advanced magnetic safing and arming (S&A) sensor technology based on giant magnetoresistive sensors. Extensive tests, including live fire tests with 40 mm munitions were performed with both turns-counting and barrel-exit sensing configurations for rotating rounds. The present invention includes non-rotating rounds, specifically 60 mm mortars. The sensors survived repeated mechanical shock tests in excess of 100,000 gs and environmental cycling ranging from 150 C. to −180 C.
 Tunnel-junction or spin valve GMR sensors with a resistance change of 7-15% in a 2 Oe magnetic field applied perpendicular to the easy axis of the device have been used. In general, high sensitivity to magnetic field is desirable The sensor sizes used range from 0.4 mm×1 mm to 10 μm×10 μm. The device impedance used ranged from 100 Ω to 10 kΩ but other device impedances can be used depending on the application. In general high impedance (ohms per square) helps reduce the size of the sensor, and this saves power and reduces the size of the sensor. The bandwidth of the sensor used was >1 Ghz, making it capable of detecting extremely rapid changes in magnetic fields. Power consumption of the sensor and support electronics is has been shown to be no more than approximately 1 milliamp at 3 volts for the spinning round sensor. Power requirement will vary depending on the sensors and circuitry needed for each application. The size of the sensor and its support electronics, mounted on a standard printed circuit board, is not an impediment since the entire assembly easily fits inside a 40 mm round or a 60 mm mortar without the need for modifications to the munition. If a smaller sensor system is needed the sensor and circuit can be fabricated as a hybrid integrated circuit, greatly reducing the physical size of the sensor device.
 In one embodiment, the sensor operates by detecting the magnetic field of the earth. In the case of a mortar round, the local field gradient is concentrated by the metal mortar tube. Inside the tube there is little or no magnetic field. At the opening of the mortar tube, the earth's magnetic field lines are concentrated by the edges of the mortar tube, resulting in a relatively strong field. When the mortar is fired, the sensor does not respond until it passes the end of the mortar tube where it detects the concentrated magnetic field. The sensor outputs an electrical pulse that can be used as a trigger to mark barrel exit, and arm the mortar (FIG. 3).
 In the case of a rotating round, the sensor is mounted such that it spins along with the munition. As it spins, its sensitive axis cycles from alignment with the earth's magnetic filed, to nonalignment. This results in the sensor outputting a variable amplitude sine wave corresponding to one revolution of the munition (FIG. 4). Since the rifling dimensions for the gun from which the sensor is fired are known, it is possible to detect barrel exit, speed and range of the munition.
 The sensor of the present invention has a wide range of uses including barrel exit and velocity for mortars. It can also be used for trajectory measurements and proximity to metallic targets for non-rotating rounds. The sensor system has highly desirable characteristics including: extremely small size, low power operation, high shock resistance, wide temperature range, and extremely fast response time. Its operation is based on the earth's magnetic field making it highly reliable and independent of other system such as GPS. The sensor system benefits munitions programs by offering a high performance low cost alternative for arming and safing applications for both non-rotating and rotating munitions.
 Design of the Barrel Exit Sensor and Circuitry for a 120 mm Mortar
 Accurate barrel exit detection is related to the ambient noise level, signal state separation, and sensor bandwidth. The sensor output when in the mortar tube compared to sensor output during barrel exit is sufficient reliable barrel exit detection. If the sensor bandwidth is much smaller than the event bandwidth both event detection and timing accuracy are degraded. Bandwidth, and signal output while inside the mortar tube do not pose problems. The sensor can detect the threshold of the mortar tube where the earth's magnetic field is concentrated by the metal mortar tube.
 Several techniques can be used to increase detection capability and reliability:
 Using the same sensors described above for a 60 mm mortar, determine if there is sufficient sensitivity to detect barrel exit. If not, refine the gain and frequency response of the circuitry to improve performance as needed.
 Use a more sensitive sensor and/or sensor arrays with extremely high sensitivity.
 Use a number of offset sensors each oriented in a different direction to determine if any one of them or any combination of them detect the concentrated field at the mouth of the mortar tube. This array of sensor can be read in a logical mode (e.g., AND, OR) to increase the detection capabilities and reliability of the sensor configuration.
 Use permanent magnet in conjunction with the sensor. For example, a permanent magnet may be placed at the end of the mortar tube where the field would be detected by the sensor(s) as the munition exits the mortar tube.
 The preferred barrel exit circuitry (see FIG. 5) is preferably similar to the electronics developed and tested for mortar exit and spinning rounds. In its simplest form, the circuit has a self-nulling capability so that when it is activated by powering the sensor, the bias point of the sensor is set to the center of its full range output. The circuit will then continuously compare the real-time output of the sensor to a preset voltage that corresponds to the output of the sensor under low magnetic field conditions.
 When the sensor (and mortar) exit the mortar tube, the sensor detects the earth's magnetic field concentrated at the mouth of the launch tube (barrel). When the sensor detects the magnetic field concentration, its output causes the on-board detection circuitry to cross a preset comparison voltage. To accomplish the same goal, the sensor may also detect permanent magnets mounted at the end of the launch tube. This results in a high-speed digital pulse that is used to trigger the fuzing system. If two sensors are used with a known distance between the sensors, then the speed of the munition can be determined at barrel exit. This information can be used for range analysis and compensation due to weather, wind, or other conditions. Additional filtering circuitry may be used so that the sensor amplifier responds only to high frequency pulse characteristic of high-speed barrel exit. This helps eliminate noise and other spurious signals due to set back, permanent magnetic fields in the mortar tube, and accidental dropping of the mortar round.
 The barrel exit sensors of the invention preferably use standard printed circuit board (PCB) techniques utilizing SIOC packaged ICs, and chip capacitors and resistors.
 The barrel exit sensors of the invention were evaluated using both a drop tower and a simulation station. The drop tower consists of a metal tube made of materials and dimensions similar to a 120 mm mortar tube. Alternatively, a modified 120 mortar tube may be used. The test apparatus allowed for mechanical acceleration to achieve high velocities for meaningful testing of the barrel exit sensor.
 The simulation test station was based on using an arbitrary waveform generator, coupled to a discharge-capacitor driven current source to simulate the transient of a much faster barrel exit than that achievable using a drop tower configuration. In principle this simulation station accurately reproduce the magnetic field perturbation of a gun barrel exit at speeds in excess of 5,000 m/s, a velocity actually achieved by some munitions such as the kinetic energy rod. This velocity is well within the bandwidth limitations of the sensor of the present invention.
 60, 80, and 120 mm mortar tubes were tested. Barrel exit detectability was enhanced for the larger mortar tubes by improving the sensor and circuitry specifically for the larger mortar by using flux guides, optimized geometry and signal processing techniques. The sensor was able to detect the edge of the larger mortar tubes during barrel exit under simulated conditions.
 Programs were performed related to GMR sensors for applications in both safing and fuzing, and in the use of GMR sensors and sensor arrays for nondestructive evaluation. In the area of safing and fuzing it was shown that the sensor can be used to detect revolutions in a 40 mm round during live fire testing. The sensor provided information that enabled an on-munition system to determine barrel exit, munition velocity, and range.
 Tests with mortars were conducted. A sensor was made that was able to detect barrel exit in a 60 mm mortar. Live fire tests confirmed the utility of the sensor. In addition to sensor and circuit design, mechanical shock testing, and environmental testing were performed with good results.
 Signal conditioning and readout electronics were also developed. This resulted in tunnel junction GMR sensor arrays for NDE. The sensor elements developed were only 10 microns on a side, yet delivered 7% (R in a 2 Oe field with nominal resistance near 10 k). Signal conditioning and readout electronics for the sensors were also developed.
 Sensors for Fuze Applications
 As described previously, the present invention provides technology to measure barrel exit, spin velocity, and range using GMR sensors detecting the earth's magnetic field. Sensor systems were fabricated for live-fire testing. Tests with the 40 mm spinning round were done. Three (3) data recorder/spin sensor assemblies were fabricated, tested and potted into 40 mm back shells. Each final potted test unit was numbered 1, 2 and 3. Once the operation of the data recorder and spin sensor were individually functionally verified, the two PCBs were interconnected and tested (assembly shown in FIGS. 6 and 7).
 Prior to triggering and recording, the data recorder was powered and armed. This was accomplished through a 7.5 v battery or power supply source, laptop computer and an RS-232 interface. This setup is shown in FIG. 8. The computer communicates to the data recorder through the RS-232 interface and cable. The data recorder microprocessor firmware program is a self-contained program that performs BIT functions, arming, data upload, sampling rate and record time selection. The computer used the Windows program “Hyperterminal” to communicate with the data recorder. This program provides communication with the data recorder for operator parameter entry as well as data upload “receive text file” mode to upload the data from the data recorder to a file on the hard disk. This text was then imported to “Excel” to plot the data to a graph (see FIG. 9).
 Flight Distance Calculations During Field Testing
Predicted Flight Distance==Muzzle Velocity*Flight Time=831*0.25=207.75 ft
Turns Calculated Flight Distance=# of Turns*4 ft/Turn=51.5*4=206.0 ft
% Error=((Predicted Flight Distance−Turns Calculated Flight Distance)/Predicted Flight Distance)*100=((207.75−206)/207.75)*100%=0.84%
 Safing and Arming Sensor—Counter Munitions
 The present invention also relates to advanced magnetic safing and arming (S&A) sensor technology based on giant magnetoresistive (GMR) sensors (see FIG. 10). The counter munition would initially be guided by radar which would also sense the munition. Once the counter munition neared its target the GMR sensor(s) would pinpoint its location and set the needed time delay on the fuze for maximum destructive power to the target. The present invention provides a sensor that can detect incoming munitions such as the KE rod or TOW missile. The sensor provides proximity information that is used by ISP to destroy the incoming munition before it reaches its target (e.g., a tank).
 The sensor is mounted in the nose of the counter munition. It senses the local field strength. When nearing an incoming munition, it detects a change in the magnetic field strength due to permanent magnetic fields generated by ferromagnetic metal in the munition. It also detects the change in the Earth's magnetic field due to the variations in the field lines caused by metal in the munition. Changes in field strength detected by the sensor provides the proximity information needed to detonate the counter munition at the most advantageous time
 One approach to increase sensitivity is to place a permanent magnet near the sensor as a biasing magnet. As the sensor approaches the metallic munition, the bias point changes due to the ferromagnetic metal in the munition.
 An active sensor with an oscillation frequency selected for the type of munition can be used. The sensor is inductively programmed at the time of counter munition launch for the type of incoming munition, based on radar information from the ISP system. A coil wrapped around the nose of the counter munition emanates an electromagnetic frequency selected for the type of munition. The magnetic field properties generated by the coil change as the counter munition nears its target, thus providing the information needed for optimum detonation of the counter munition.
 In all of these cases, multiple sensors can be used for triangulation. This increases angular sensitivity, and provide more detailed information about the incoming munition which can be used to enhance the performance of the counter munition.
 The sensor preferably has the following properties:
 Reponse time: 1 ns
 Bandwidth: >1 GHz
 Signal to noise ratio: depending on application cane be >1000
 Power consumption: −3 V at 2 mA for passive sensor
 Field of view: 3600
 Environmental ruggedness: The sensor and circuit on a PCB have been tested to 100,000 g in live fire tests of rotating rounds; temperature tolerance of sensor has been demonstrated at −1800 C. to +1500 C.
 Size and weight: Current fuze circuit and sensor fit on round PCB 25 mm diameter that weighs 1.8 grams. Could be smaller as a hybrid circuit. Actual sensor is approximately 100×100 microns.
 The sensor technology has a wide range of uses including barrel exit and velocity for mortars, and range for spinning rounds. It also can be used for trajectory measurements and proximity to metallic targets. The sensor system has highly desirable characteristics including: extremely small size, low power operation, high shock resistance, wide temperature range, and extremely fast response time. Its operation is based on the local magnetic fields, making it highly reliable and independent of other system such as GPS. The sensor system benefits munitions programs by offering a high-performance, low-cost alternative for arming and safing applications for both non-rotating and rotating munitions.
 Targeting Sensor
 The present invention also provides a targeting sensor. It uses a single sensor or an array of sensors (see FIG. 17). The sensors provide information regarding the location and/or proximity of the munition to its target. Relative signal strengths and frequency responses are used to identify types of targets for positive identification or IFF (identification friend or foe).
 Medical Imaging Sensor
 This type of sensor or sensor array can also be used in the medical field. For example, it can be used to measure electromagnetic activity in the heart or brain. It can be used as a sensor in the field of biomagnetics. It can also be used in nuclear magnetic resonance imaging. The ability of the sensor to detect magnetic fields in a single plane or line makes it suitable for use in imaging skin defects. It can also be used for imaging low volume point defects. The small signals from small volume defects can be extremely difficult to detect using standard NMR imaging because the signals from the small volume area are overshadowed by those from larger volume areas which contain more hydrogen, and therefore produce larger signals.
 GMR Sensors and Sensor Arrays for Nondestructive Evaluation (NDE)
 The GMR sensors and arrays of the present invention can also be employed for NDE of metallic components (see FIG. 19, sample results in FIG. 18). The GMR detectors were used to detect, scan, and image eddy currents in defective metallic components, such as sheet metal parts or pipe welds. Tunnel junction GMR sensor arrays for NDE were developed. The sensor elements developed were only 10 microns on a side, yet delivered 7% (R in a 2 Oe field with nominal resistance near 10 k). Signal conditioning and readout electronics for the sensors may also be employed. Other types of GMR and CMR sensors can also be used rather than the preferred tunnel junction type of GMR device.
 U.S. Pat. No. 6,150,809, entitled “Giant Magnetorestive Sensors and Sensor Arrays for Detection and Imaging of Anomalies in Conductive Materials” relates to magnetic and electromagnetic nondestructive evaluation. This technology is based on a synergistic combination of a traditional eddy-current coil or permanent magnet and a unique magnetic sensor based on a class of materials known as giant magnetoresistors. The result is a sensor exhibiting substantially higher field sensitivity and spatial resolution when compared with traditional methods. Since these sensors directly sense the magnetic field, rather than changes in the field as is the case for coils, they can be operated with high sensitivity at very low frequencies and even with DC generated fields.
 A handheld NDE instrument was developed. This system was able to detect third layer (0.065″ 2024T3 per layer) cracks without the need for bench-top instrumentation. The present invention is a handheld NDE unit along with a variety of sensors as shown in FIG. 11. This unit provides four different graphical data display modes and is capable of resolving defects to a depth of approximately 0.2″ using the smaller diameter probes and to a depth of approximately 0.4″ using the larger probe.
 Current GMR detectors include several configurations. Most are fabricated using molded urethane. The process begins with a sensor, coil and flux focusing (if applicable) alignment on a metrology station. The aligned assembly, along with the electronics subsystem, sapphire wear surface and electrical connector, is then encased in color-coded urethane. This process provides a rugged and compact package for the sensor and eliminates costly machining.
 The sensors are typically driven with a constant-current AC drive between 50 Hz and 25 kHz. At 1 kHz the ⅜″ diameter sensors can easily resolve EDM cuts less than 0.25″ in length through 0.195″ of 2024T3. A larger, ¾″ version of this sensor has be used to image anomalies to over 0.4″ in 6061T6.
 Two-dimensional imaging arrays of GMR elements have been designed and units have been fabricated. Functional arrays consisting of 196 tunnel-junction sensors (14×14). The individual sensors are only a few square microns in area and are positioned on a grid with a sensor-to-sensor spacing of approximately 210 microns. The active area of the entire 14×14 element array is less than 3 mm on a side (9 mm2). FIG. 12 shows a photo of one of the imaging array prototypes. In this photo the magnetic field generation has been removed. Large (4″×4″) arrays provide for real-time imaging of deeply imbedded defects and corrosion.
 High resolution images were obtained by performing raster, “C-Scans” of various test samples using the APET-CR(tm) sensor configuration. FIG. 13 shows an 11 kHz C-scan of first and second layer rivet heads in an aluminum sheet metal section simulating an aircraft wing. FIG. 14 shows 100 micron wide cuts in an aluminum sheet metal sample. This data demonstrates the extremely high signal-to noise ratio obtainable with the GMR sensor technology of the present invention.
FIG. 15 shows an image of a corrosion test sample provided by the Federal Aviation Administration, Nondestructive Inspection Validation Center (FAA/AANC). The sample is a 0.06″ thick, 2024T3 plate with a nominal 10% uniform mass loss corrosion region on one side. The image corresponds to a scan performed on the side opposite the corroded region. The boundary of the corroded region, as well as the pitting within the corroded region is sharply defined in the image and the signal-to-noise ratio in excess of 100 for these features.
 Detection of Conductive Materials
 The present invention also provides a magnetometer (see FIG. 16) for detecting conductive materials either on the surface or underneath other materials such as earth or water (e.g., mines). This sensor is handheld or operated from a vehicle or aircraft. It operates in a DC detection mode or with oscillating magnetic fields. Permanent magnets or flux concentrators are used to enhance the signal detected by the sensor.
 This type of sensor may be used to locate ships, submarines, and other vessels. It can also be used to detect electrical currents flowing through PCB traces or IC components. It can be used to measure the amplitude of electrical currents. It can also be used to detect breaks in electrical conductors which would result in low or non-existent current flow. As such, it can be used for inspection of many types of circuitry and wiring.
 Impact Sensing
 GMR sensors are magnetorestrictive. During impact such sensors are compressed and will output a signal characteristic of impact.
 Magnetic Field Tracking for Guidance
 GMR sensors according to the invention may also be used for more sophisticated means of guidance of ordnance. A sensor can use a map of local magnetic fields to determine position and direction of a munition or any moving object such as an aircraft or truck. Flying objects may be detected by looking for response to an emitted magnetic field (active sensing) or looking for eddy currents output by aircraft as they traverse the earth's magnetic field (passive sensing).
 Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.