ARRAYED EDDY CURRENT PROBE SYSTEM

Abstract: Eddy current probes for multi-layered structures include a body of a highly permeability material which is shaped to include a central core and an additional wall. A drive coil is wound around the central core of the body and a plurality of sense coils are located in an array in association with the additional wall. An outer drive coil is wound about the outside of the additional wall. The central drive coil is excited with a first alternating high frequency signal producing eddy currents primarily in the top of the layered structure. These eddy currents are sensed with the sense coil array and analyzed for structural defects in the top surface of the layered structure. Next the central core coil is excited with a second signal of a lower frequency than the first signal. Such signal introduces eddy currents in deeper layers of the structure. Such eddy currents are sensed by the sense coils and the signals so sensed are stored. The outer drive coil is now excited with a low frequency signal and the sense coils operated to detect eddy currents in deeper layers of the structure. The signal from excitation of the outer drive coil is scaled with respect to the signal from the central core coil for separating signal indicative of defects from that indicative of differences in the structure of the underlaying components of the layered structure.

Description: ARRAYED EDDY CURRENT PROBE SYSTEM TECHNICAL FIELD c This invention is directed to devices for detecting cracks, flaws and other b defects in a layered structure. More specifically, the devices utilize eddy cur¬ rents that are generated within the layers of the structure for the detection of the cracks, flaws and other defects in the layers of the structure. BACKGROUND ART 0 Various instruments for flaw detection in structures have been developed that rely upon the generation of eddy currents in the body of the structure being tested. Flaws in the metallic structures are detected by their perturbation of the eddy currents. The eddy currents are generated in the metallic structures 5 by positioning the structures within an alternating magnetic field. Perturbation of the eddy currents because of the localized presence of a defect in the metallic structure creates a resultant change in the magnetic flux associated with these eddy currents. When such change is detected, it is an indication of the presence of defects in the structure. Such use of eddy currents has been practiced for 0 detecting flaws in solid metallic slabs, metallic pipes and in layered metallic structures such as the outside surfaces of aircraft. United States patent 3,437,918 to Arnelo describes an eddy current detec¬ tion system for a slab structure. Further, eddy current detection systems for slabs are described in United States patent 4,534,405 to Hulek, et al., U.S.S.R. 5 patent 1,155,930, Japanese patent 60-147648, and Japanese patent 59-162448. Eddy current detection for solid pipe like structures is disclosed in United States patent 3,694,740 to Bergstrand, et al., Japanese patent 61-14569, British patent 631,987 to Rudorff and United States patent 4,855,677 to Clark, et al. Detection systems for layered structures are found in United States patents 4,219,774 to Rogel, et al., 4,414,508 to Davis, et al., 4,495,466 to Lakin, et al., 4,496,904 to Harrison, 4,445,089 to Harrison, 4,677,379 to Arnaud, et al. and 4,095,181 to Harris et al. Many of the above patents describe systems wherein a single coil is utilized to induce a magnetic field in a test piece. Flaws are detected by noting changes in the impedance of the test coil. In other of the above patents, both a drive coil and a sense coil are utilized. Generally detection of flaws is achieved utilizing voltage variations in the sense coil. The voltage variations are detected utilizing a null bridge. In the above noted Clark patent, United States patent 4,855,677, a plurality of coaxial coils, each driven at a different frequency, is utilized to detect flaws at different depths in pipes. In the above noted Japanese patent 59- 162448, the sense elements, which are independent of the drive coil, are arranged in two sets along two radii which are 180° apart. The sets of sense coils are rotated directly underneath the center of the drive coil. As is described in the noted Lakin patent, United States patent 4,495,466, the skins on wings and the body of aircraft are constructed by fastening several layers together with a large number of fasteners. Each of the fasteners is positioned in a hole that passes through each of the layers. Fatigue cracks about the fastener holes develop in response to stress of the aircraft structures. lii aircraft structures, if these cracks are detected when they are small, the fastener having the crack adjacent thereto can be removed, the hole drilled out to a slightly larger diameter that includes the crack within it bounds and a larger fastener inserted. This thus removes the crack and in doing so, eliminates a structural defect without severely compromising the integrity of the part that initially bore the defect. For aircraft structures (or other structures) the above noted United States patent 4,219,774 to Rogel and United States patent 4,414,508 to Davis require removal of the fastener for inspection of the fastener holes. This is a time consuming and laborious process. Further, aside from the time and expense, in the process of removing the fasteners new flaws can be introduced into the layered structure. In order to sense defects in deeper layers, as for instance a second layer of a two layer system, expedients have been devised for separating signal from the first layer from that of the second layer. Such expedients include multiple fre¬ quency sensing as discussed in the above noted United States patent 4,495,466 to Lakin, or sensor movement as is common in above noted United States patents 4,095,181 to Harris, 4,445,089 and 4,496,904 to Harrison and 4,677,379 to Ar- naud. While the methods practiced in these patents have resulted in improve¬ ments over older methods, flaw detection is still a difficult and time consuming process. Composite materials have been increasingly used in aircraft construction, especially military aircraft. Such composite materials serve to shield detection of flaws in the deeper, underlying metallic structures on which the composite materials are fastened. Additionally, the underlying metallic structures tend to be of complex shape that can distort detection systems such as that of the above noted Arnaud patent 4,677,379. This arises because the Arnaud patent relies upon uniform probe movement along uniformly spaced rivet arrays in essentially flat uniform structures. Indeed, as is noted in this patent, the search coils are stated to be separated from the primary windings at a distance that is equal to one-half of the distance separating consecutive rivets in a succession of rivets. This requires that the pattern of the fasteners be very uniform. Such a uni¬ form pattern may or may not be used in those constructions wherein composite materials are fastened to underlying metallic structures. Quadrature detection is mentioned in both the above noted United States patents 4,677,379 to Arnaud, et al. and 4,496,904 to Harrison. Such a detection technique utilizes both a magnitude and a phase component of a signal for analysis of that signal. As noted above, however, the Arnaud patent requires the probe geometry to be related to the fastener geometry and the Harrison 4,496,904 patent requires the probe to be rotated directly over the center of the fastener for defect sensing. Other techniques that have been utilized for the inspection of aircraft struc¬ tures include radiographic methods. Such radiographic methods, however, miss up to 75% of the cracks because there is a lack of density differences between a part having a small crack and a part not having a crack therein. Further, such detection methods are difficult to implement in many areas of aircraft be¬ cause the geometry of the aircraft structure prevents placing an x-ray film on one side of the structure and a suitable device for generating x-rays on the other. DISCLOSURE OF THE INVENTION It is an object of this invention to provide improved detection systems for detecting defects in layered structures as, for instance, aircraft structures. It is a further object of this invention to provide for detection systems that utilize eddy currents that are generated in a metallic layer of a layered structure. It is a further object of this invention to provide for inspection systems and methods of inspection that can be done in a forthright manner on a complex structure as, for instance, an aircraft at a reasonable inspection rate as, for instance, less than 30 seconds per fastener inspected and in a manner that does not require removal of fasteners from the structure being inspected. It is an additional object of this invention to provide for apparatus and methods of inspecting layered structures wherein a metallic layer is imbedded underneath a composite layer and where the metallic layer may have a com- plex shape that differs from one area under inspection to the next area under inspection. In accordance with these and other objects as will become evident on reading of the remainder of this specification, there is provided an eddy current probe having a body formed of a high permeability material. The body is shaped to include a central core and an additional wall displaced from the central core. The central core has a core drive coil wound thereon. The additional wall has a further drive coil wound thereon. A plurality of independent sense coils are located in a symmetrical array in association with the additional wall. Further in accordance with this invention, there is provided an eddy cur¬ rent probe having a body formed of a high permeability material and where the body is shaped to include a central core, a radially extending wall radiat¬ ing from the central core and a circumferentially extending wall extending from the radial wall concentric with the core. The central core has a core drive coil wound thereon. The circumferential wall includes a rim that is distal from the radial wall. The rim includes a plurality of interstices that divide the rim into a plurality of symmetrical, independent partitions that are symmetrically located about the circumferentially extending wall. The partitions are shaped so as to extend axially essentially parallel with the axis of the central core. A plurality of independent sense coils, equal in number to the number of the partitions, are located on the partitions such that a respective one of the sense coils is wound around a respective one of the partitions and each partition includes an inde¬ pendent sense coil wound around it. Further an outer drive coil is wound about the outside of the circumferentially extending wall. In a preferred embodiment the plurality of interstices and plurality of partitions together form a castellated structure composed of essentially prismatic partitions separated by essentially straight sided crenelated openings. Further, in accordance with this invention there is provided an eddy current probe having a body formed of a high permeability material and where the body is shaped to include a central core, a radially extending wall radiating from the core and a circumferentially extending wall extending from the radial wall concentric with the core. The central core has a core drive coil wound thereon. The circumferentially extending wall includes a rim distal from the radial wall. The rim includes a plurality of interstices that divide the rim into a plurality of symmetrical partitions that are symmetrically located about the circumferentially extending wall with each of the partitions being shaped to include a portion thereof that is positioned in space in a plane that is essentially perpendicular to the axis of the central core. A plurality of sense coils, equal in number to the number of partitions, are located on the partitions such that each one of the respective sense coils is wound around that portion of a respective partition that is positioned in the plane in space that is essentially perpendicular to the axis of the central core. As so positioned each independent partition includes an independent sense coil wound around it. In preferred embodiments the structure further includes an outer drive coil wound around the outside of the circumferentially extending wall. Further in accordance with this invention there is provided an eddy current probe that includes a first body formed of a high permeability material and shaped so as to include a central core, a radially extending wall radiating from the central core and a circumferentially extending wall extending from the radial wall concentrically with the core. The central core includes a core drive coil wound thereon. The circumferentially extending wall includes a lip distal from the radial wall. A further body structurally independent of the first body is symmetrically divisible into a plurality of individual sections. The further body is mounted on the circumferentially extending wall of the first body in association with the lip of that wall. A plurality of independent transducer means equal in number to the number of the sections of the further body are located on the further body such that a respective one of the transducers means is positioned in respect to a respective one of the sections of the further body with each of the independent sections including an independent transducer means operatively associated with it. The transducer means are for sensing eddy currents and are located in at least one symmetrical array with respect to the rim of the circumferentially extending wall. In one preferred embodiment of the invention the transducer means each comprise a sense coil and at least portions of the further body are formed of a high permeability material. Preferably such portions of the further body are formed as ferrite cores that are mounted in the further body with a respective sense coil wound around each of the respective ferrite cores. In a first embodi¬ ment of the invention the sense coils are orientated on the further body such that when the further body is mounted on the first body the axis of each of the sense coils is located essentially parallel with the axis of the central core of the first body. In a further embodiment of the invention the sense coils are orientated on the further body such that when the further body is mounted on the first body the axis of each of the sense coils is positioned in space in a plane that is essentially perpendicular with the axis of the central core of the first body. In a further preferred embodiment an outer drive coil is wound around the outside of the circumferentially extending wall. In even further preferred embodiments the plurality of sense coils can be divided into first and second circular arrays with the first circular array positioned radially from the central core at a first radial distance and the second circular array positioned radially from the central core at a second radial distance. The first and second radial distances are different. In further embodiments of the invention the central core of the first body can be hollow and can include a centering coil that is positioned in the hollow central core. Further, an inner body independent of first body, that has a central boss thereon, can be positioned in the hollow central core with the centering coil wound around the inner body central boss. In even further preferred embodiments of the invention each of the trans¬ ducer means, referred to above, can comprise a Hall effect sensor element, with such Hall effect sensor elements positioned in the respective sections on the fur- ther body such that together the plurality of Hall effect sensor elements are oriented in at least one symmetrical array with respect to the rim of the circum¬ ferentially extending wall. In an even more preferred embodiment, the further body is a monolithic body having said Hall effect sensor elements integrally formed thereon. In even further preferred embodiments of the invention, means can be in¬ cluded for moving the probe or portions thereof with respect to a layered struc¬ ture that is being analyzed for defects. This invention thus further includes an eddy current probe that has a first body formed of a high permeability material with the first body shaped to include a central core, a radial extending wall radiating from the core and a circumferentially extending wall extending from the radial wall concentric with the core. The central core has a core drive coil wound thereon. A further body that is structurally independent of the first body is symmetrically divisible into a plurality of individual sections with the further body movably associated with the circumferential wall of the first body. The further body includes a plurality of independent transducer means with each of the sections of the further body including one of the transducer means positioned therein and with the totally of the transducer means oriented in at least one symmetrical array with respect to the rim of the circumferentially ex¬ tending wall. There is further provided means for moving the first body in a prescribed closed orbit with respect to the further body. In one embodiment of the invention the first body rotates about an axis that is parallel to but radi¬ ally displaced from the axis of the central core of the first body. In a further embodiment the body oscillates about an axis that is parallel to but is radially displaced from the axis of the central core of the body, however, the first body does not rotate about this axis. Further in accordance with this invention there is provided an eddy cur- rent probe that includes a linear body formed of a high permeability material. The linear body can be viewed as having a first solid elongated section, a first plurality of individual partitions and a second plurality of individual partitions. The first solid elongated section has opposing sides that extend along the elon¬ gated dimension of the solid elongated section. The first plurality of individual partitions are located in a first linear array along a first of the sides of the solid elongated section. The second plurality of individual partitions are located in a second linear array along the second of the sides of the solid elongated section. As so located the individual partitions of the first array are each spaced apart from one another, are located essentially perpendicular to the solid elongated section and are essentially mutually parallel to one other. Further the individ¬ ual partitions of the second array are also spaced apart from one another, are each orientated essentially perpendicular to the solid elongated section and are essentially mutually parallel to each other. A drive coil is wound around the solid section between the sides of the solid section. A plurality of individual sense coils, equal in number to the number of the second partitions, are located on the second partitions with a respective one of the sense coils wound around a respective one of the second plurality of partitions such that each of the second plurality of partitions includes an independent sense coil wound around it. In one embodiment of this probe an outer drive coil can be wound around the pe- riphery of the body of the probe. In a further embodiment of this linear probe the body can include a second solid elongated section and a third plurality of individual partitions. The second body section is joined with the first body section such that the second plurality of partitions are located in between the first and second body sections. A second drive coil is wound around the second elongated section between the sides of the second section and the third plurality of partitions are located in a third linear array along the opposite side of the second solid elongated section. Further, in accordance with this invention there is provided a method of in¬ specting a layered structure that includes selecting a probe having a body formed of a high permeability material with the body shaped to include a central core having a core drive coil wound thereon. The body further includes an additional wall displaced from the central core. The additional wall includes a plurality of sense coils located in association with it and a further drive coil wound thereon. The probe is positioned against the layer structure approximately centered over the area to be inspected. The core drive coil is driven with a first input signal of a first alternating frequency to generate eddy currents in said structure. The sense coils are operated to detect initial output signals related to eddy currents generated in the structure. The detected initial output signals are analyzed for anomalies indicative of defects in a top layer of the structure. The core drive coil is driven with a second input signal of a second alternating frequency to gener¬ ate eddy currents in the structure. The second alternating frequency is selected to be of a lower frequency than the first frequency. The sense coils are further operated to detect further output signals related to eddy currents generated in the structure. These further output signals are stored. The further drive coil is driven with the second input signal to generate further eddy currents in the structure. Again the sense coils are operated to detect additional output signals related to eddy currents generated in the structure and these additional output signals are also stored. The additional output signals are scaled to the stored further output signals and the scaled output signals are then compared with the further output signals for anomalies indicative of defects in an underlying layer of the structure. The method of the immediate preceding paragraph can further include cen¬ tering the probe by driving the core drive coil with the first input signal and operating each of the sense coils to detect output signals related to eddy cur¬ rents generated in the structure. These detected output signals are analyzed for anomalies indicative of the probe being off center from the area being inspected. The method can further include storing the initial output signals detected by each of the sense coils and scaling the stored initial output signals to the stored further output signals. The scaled initial output signals are then compared with the further output signals for anomalies indicative of the probe being off-centered from the area being inspected or the probe not being oriented parallel, i.e. tilted or inclined, to the area being inspected. The method can include calibrating the probe by positioning and centering the probe against a layered structure known to be free of defects and driving the core drive coil with the second input signal. Each of the sense coils is then operated to detect calibration signals related to eddy currents generated in the structure. These calibrated signals are stored and compared to the additional output signals to determine a scaling factor. Further in accordance with the objects of this invention there is provided a method of inspecting a layered structure that includes selecting a probe having a body formed of a high permeability material with the body shaped to include a central core having a central axis and a core drive coil wound thereon. The body further includes an additional wall displaced from the central core. The addi¬ tional wall includes a plurality of independent sense coils located in association with the additional wall. The method includes positioning the probe against a layer structure approximately near the area to be inspected and moving the probe though a closed circuit that is centered about an off-centered axis that is parallel to and radially displaced from the central axis of the core. The core drive coil is then driven with a signal of an alternating frequency to generate eddy currents in the structure. The sense coils are operated to detect signals related to eddy currents generated in the structure. The detected signals are then analyzed for anomalies indicative of defects in the structure. Movement of the probe in the orbit can be oscillating movement wherein the probe is moved about the off-centered axis without rotating the probe about the off-centered axis or said movement can be rotational movement wherein the probe is rotated about the off-centered axis. In preferred embodiments, the additional wall is a circumferentially extending wall that is radially displaced from the central core and the sense coils are located in a symmetrical array around this circumferen¬ tially extending wall. Further in accordance with the objects of the invention a further method of inspecting a layered structure includes selecting a probe having a first body formed of a high permeability material with the first body shaped to include a central core having a core axis and a core drive coil wound thereon. The probe further includes a further body having a further body central axis and a plurality of independent sense coils located around the further body central axis with the further body located adjacent to the first body and with the further body central axis located parallel to but radially displaced from the core axis. The probe further includes a means for moving said first body in a prescribed closed orbit with respect to the first body. The method includes positioning the first body of the probe against a layered structure approximately near the area to be inspected and moving the first body of the probe though a closed orbit that is centered about the further body axis. The core drive coil is then driven with a signal of an alternating frequency to generate eddy currents in the structure, and the sense coils are operated to detect signals related to the eddy currents. The detected signals are then analyzed for anomalies indicative of defects in the layers of the structure. Movement of the first body through the closed orbit can be movement by rotating the first body around the further body axis or it can be movement wherein the first body is oscillated about the further body axis while holding the first body fixed with respect to rotation of the first body about the further body axis. Further, in accordance with this invention there is provided a structural defect detection system that includes a body formed of a high permeability ma¬ terial. The detection system further includes a plurality of independent drive coils, each of which is independently located on the body and each of which is capable of inducing eddy currents in a metallic structure in response to an AC signal conducted through the drive coils. A plurality of independent transducer means are located in a symmetrical array in operative association with the body. Each of the transducer means is for sensing eddy currents in a structure and producing an output signal in response to said sensed eddy currents. Trans¬ ducer means are formed independent of the drive coils. The detection system further includes signal generating means for generating at least one selected AC signal of a selected frequency and a drive select means for independently driving one of the drive coils with the selected signal. The detection system further includes a signal processing means operatively associated with the plurality of transducer means for independently processing the output of each of the plu¬ rality of transducer means and for producing an output indicative of differences between the output signals of the transducer means. In preferred embodiments the transducer means comprises sensing coils mounted on the body. In further preferred embodiments the signal generating means generates a drive signal and first and second reference signals with at least the second reference signal dif¬ fering in phase from the drive signal and the first reference signal. The drive select means drives the drive coil with the drive signal, and the signal processing means processes the output signal of each of the transducers in association with both the first and second reference signals and in response thereto generates an output having both a phase and a magnitude component. Further, in accordance with this invention there is provided a method of detecting defects in a structure that includes selecting a probe having a body formed of a high permeability material and having at least one drive coil located thereon that is capable of introducing eddy currents in a metallic structure in response to an AC signal conducted through said drive coil. Further it includes a plurality of independent transducer means located in a symmetrical array in operative association with the body and where each of the transducer means is capable of sensing eddy currents in a metallic structure and in response thereto producing an output signal. The transducer means are independent of the drive coil. The probe is located in association with the structure. An AC drive signal is generated as are first and second AC reference signals. At least the second reference signal differs in phase from the drive signal and the first reference sig¬ nal. The drive coil is driven with the drive signal to induce eddy currents in the structure. The transducer means are interrogated for transducer output signals produced in response to eddy currents in the structure. The output signal of each transducer means is processed in association with both the first and second reference signals and in response thereto a multiple component output signal is generated that has both a phase and a magnitude component. The phase and magnitude components of the multiple component output signal for each of the transducer means is compared to detect defects in the structure. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be further understood when taken in conjunction with the drawings wherein: Figure 1 is a pictorial view of a prior art probe; Figure 2 is a pictorial view of a probe of this invention; Figure 3a is representational view of eddy currents surrounding a cross sec¬ tion of a fastener located in structure and Figure 3b is a similar representational view and in addition includes a crack located in the structure; Figure 4 is a side elevational view of a layered structure having a fastener therein; Figure 5 is a top plan view of the bottom most layer of the structure of Figure 4; Figure 6 is a side elevational view in cross section of a probe of the invention and a layered structure having a fastener therein; Figures 7, 8, 9 and 10 are graphs showing pictorial output displays of signals generated using probes of the inventions; Figure 11 is a pictorial view of a further probe of this invention; Figure 12 is a side elevational view in cross section of the probe of Figure ii; Figures 13, 14 and 15 are pictorial views of portions of probes of the inven¬ tion; Figure 16 is a pictorial view of a portion of a probe of the invention; Figure 17 is an isometric view of a further probe of the invention; Figure 18 is an isometric view of even a further probe of the invention; Figure 19 is a synoptic diagram of a detection system of the invention; Figure 20 is a simplified schematic block diagram illustrating in greater detail portions of the system of Figure 19; Figure 21 is a simplified schematic block diagram illustrating in greater detail further portions of the system of Figure 19; Figures 22, 23, 24 and 25 are graphs showing further pictorial output dis¬ plays of signals generated using probes of the inventions; Figure 26 is block diagram showing the steps of a detection method of the invention; Figure 27a is an elevational view of a further probe of the invention and Figure 27b is a plan view schematically showing certain of the components of the probe of Figure 27a and the paths these components traverse as they move with respect to one another; Figure 28a is an elevational view of an even further probe of the invention and Figure 28b is a plan view schematically showing certain of the components of the probe of Figure 28a and the paths these components traverse as they move with respect to one another; and Figure 29a is an elevational view of an even further probe of the invention and Figure 29b is a plan view schematically showing certain of the components of the probe of Figure 29a and the paths these components traverse as they move with respect to one another. MODES OF CARRYING OUT THE INVENTION Aircraft are typically constructed by fastening a multiple layer skin to un¬ derlying structures utilizing fasteners. This design results in thousands of fasten¬ ers holes located in the hidden underlying structures. Fatigue cracks frequently initiate at such fastener holes. An inspection of the area immediately around each hole is necessary to find such fatigue cracks. Locating fatigue cracks is paramount to flight safety of the aircraft. While ultrasonic methods are useful for detecting fatigue cracks in a top layer of a layered structure, they are useless for underlying layers. Radiographic detection and conventional eddy current detection will only detect cracks after they have grown to a size that is costly and difficult to repair. Indeed it has been shown that radiographic methods will not detect fatigue cracks till they have propagated across, as for instance, an entire spar and have developed a significant width. Fastener removal to detect cracks utilizing probes that go into the fastener holes is a difficult and expensive task and in itself can result in damage to the structure. As a design criteria, detection of cracks that are 2.5 mm (0.1 inch) in an underlying layer of an aircraft structures having a 6.35 mm (0.25 inch) top layer, as for instance a 0.25 inch (6.35) aluminum skin, utilizing either steel, titanium or aluminum fasteners is desirable. Further, such inspection should be straight forward, not require highly technically trained personnel and should be able to be accomplished at a fairly rapid inspection rate. In accordance with this invention such design characteristics can be achieved and an inspection rate of 30 seconds or less per fastener hole has been demon¬ strated. Cracks can be detected when they are of such a small size that they can be removed by drilling out the fastener hole and installing an oversized fastener. Further, crack detection can be effected utilizing ferromagnetic and non-ferromagnetic fasteners in, at least, aluminum over aluminum, titanium over aluminum, titanium over titanium, composite over titanium, and compos¬ ite over aluminum air frame structures. Such detection is effected utilizing eddy currents induced in a top layer, if that top layer is metallic, and the bottom layer of metallic structural components of aircraft frames, wing assemblies, tail assemblies and the like. Further, such inspection has been achieved on structural components that have near-by edges or surfaces and fastener features which in themselves cause disruption of the eddy current and therefore an interfering signal. In one embodiment of this invention, this invention comprises an improve¬ ment to the probe system as described by Lakin in United States patent 4,495,466. As such the entire disclosure of United States patent 4,4945,466 is herein incor¬ porated by reference. As is common with Lakin, in this invention flaws are detected by their perturbation of eddy currents. The perturbation of induced eddy currents cause changes in the associated magnetic flux. The change in the magnetic flux is then detected by a probe. Contrary to United States patent 4,495,466, the probes of this invention utilize multiple drive coils that allow for detection of structural features such as near by edges, surfaces and fastener features that in themselves cause disruption of the eddy currents and therefore produce interfering signals. United States patent 4,495,466 did not include such multiple drive coils and thus the ability to detect interfering structural features such as nearby edges, surfaces and fastener features. Shown in Figure 1 is a probe of United States patent 4,495,466. The probe is identified by the numeral 30. The probe 30 includes a core body 32 having a center post or core 34 and a plurality of key hole shaped openings 36 which define what Lakin refers to as cup core segments 38. A center coil 40 is wound about the center core 34. A plurality of sense coils collectively identified by the numeral 42 are wound between the openings 36 such that each of the cup core segments 38 has a sense coil 42 wound around its base. Figure 2 is illustrative of a probe of this invention. In Figure 2 the probe 44 is formed of a body 46 that includes a central core 48 having a drive coil 50 wound thereon. A plurality of straight sided crenelated openings 52 define a plurality of partitions 54 each of which has an independent sense coil 56 wound thereon. Further, an outer drive coil 58 is wound around the perimeter of the body 46 radially outwardly from the sense coils 56. The crenelated openings 52 form a castellated like structure in the rim 60 of a circumferentially extending wall 62 of the body 46. This leaves the partitions 54 with a prismatic like shape having straight walls and a truncated pie like shape in cross section. The body 46 is formed of a high permeability material, as for instance, a ferrite material. The central core drive coil 50 and the outer drive coil 58 axe completely independent of each other. Further, the sense coils 56 are in¬ dependent of each other as well as being independent of the drive coils 50 and 58. In Figures 3a and 3b, operation of the probe 30 of United States patent 4,495,466 and those features of the probe 46 of this invention that are in common with United States patent 4,495,466 are illustrated. In this Figure, when the central core, as for instance the core 34 of the probe 30 of United States patent 4,495,466, or the core 48 of the probe 44 of this invention, is located over a fastener 64 and an AC current is passed through the coil that surrotmds the central core of the probe, eddy currents, as are illustrated by the lines 66 in Figure 3a, are generated in the metallic structure in which the fastener 64 is located. The magnetic flux generated by the current flowing through the coil that surrounds the probe core induces the eddy currents 66 in the metallic structure. As illustrated in Figure 3b, when a defect, as for instance, a crack 68 is located in the structure adjacent to the fastener 64, the eddy currents as are illustrated by lines 70 of Figure 3b, are perturbed by the presence of the crack 68 and this in turn perturbs the magnetic flux associated with those eddy currents. In both the probe 30 of United States patent 4,495,466 and the probe 44 of this invention, the sense coils of the probe are sensitive to the magnetic flux. Current is induced in the sense coils in response to changes in the magnetic flux. The perturbance of the magnetic flux caused by the crack 68 is sensed by the sense coils as these sense coils measure the spatial distribution of current around the fastener 64. Flaws are detected as perturbations in the current distribution. If fasteners were simply positioned in structures that were essentially in- finitely large with respect to the fastener geometry such that other parts of the structures did not affect the eddy current distribution around a fastener, probes such as the probe 30 of United States patent 4,495,466 would be very effective in sensing defects such as the crack 68. In actuality, however, especially with re¬ spect to aircraft formed of composite structures, the underlying or second layer through which a fastener also passes generally has geometry or special features that interfere with the idealized circular distribution of eddy currents as shown in Figure 3a. As is illustrated in Figure 4, a composite outer skin 72 has been layered on an aluminum spar 74 and held in place with a fastener 76. In Figure 5 the spar 74 is isolated. As can be seen, the spar 74 includes a scalloped side 78 and a straight web side 80. Fastener holes collectively identified by the numeral 82 are positioned closer to the scalloped side 78 than they are to the straight web side 80. By itself, the geometry around the fastener holes 82 is sufficiently different between the scalloped side and the straight web side to perturb the eddy current around the fastener holes 82. Probes such as the probe 30 of Figure 1 of United States patent 4,495,466 are unable to differentiate between cracks around the fastener holes 82 and the complex geometry of the aluminum spar 74. A representational probe 84 of the invention is illustrated in Figure 6. The probe 84 has many things in common with the probe 44 of Figure 2 and in addition thereto it has other features. The probe 84 is located over a fastener 86. In conjunction with a nut 88, the fastener 86 holds a first layer 90 and a second layer 92 of a layered structure together. It can be seen that the second layer 92 is of a different geometry than is the first layer 90. Thus eddy currents to the right side of the fastener as viewed in Figure 6 would be different than those to the left side of the fastener as viewed in Figure 6. The probe 84 includes a central core 94 around which is wound a central core coil 96. A radially extending wall 98 radially extends from the central core 94 outwardly and joins a circumferentially extending wall 100. The wall 100 includes a rim 102 that rests against the top of the first layer 90. The rim 102 is castellated as is described for the probe 44 of Figure 2 to form individual partitions around its circumference. Sense coils 104 and 106 are located around two of these partitions on the rim 102 of the wall 100. Of course, not seen in Figure 6 would be other partitions each having its own sense coil as is illustrated in Figure 2. Radially outward from the sense coils 104 and 106 is an outer drive coil 108. The central core 94 of the probe 84 is hollow and includes a central body 110 located therein. Central body 110 has a boss 112 thereon around which is wound a central body coil 114. When current (an input signal) is passed through the central core coil 96, magnetic flux lines 116 penetrate the first and second layers 90 and 92 and if these layers are metallic, introduce eddy currents therein. For the structure of Figure 4, only a small amount of eddy currents would be generated in the composite layer 92 since it is not metallic. However, significant eddy currents would be generated in the aluminum spar 74 since it is metallic. Assuming for illustrative purposes that both the first and second layers 90 and 92 of Figure 6 are metallic, the magnetic flux lines 116 would generate eddy currents in both of these layers. In turn these eddy currents would be sensed by the sense coils 104 and 106 (and other sense coils which would go around the circumference of the circumferentially extending wall 100) . It is evident from Figure 6 that the magnetic flux in the second layer 92 would be quite different on the right hand side of fastener 86 than it would be on the left hand side of the fastener 86 because of the presence of the edge 118 of the layer 92 in this area. Sense coil 104 might therefore see a different eddy current environment within the second layer 92 than that from sense coil 106 simply because of its spatial position. This difference, however, would not be indicative of a crack in the layer 92, but simply would be indicative of the geometry of the layer 92. Utilizing the probes of this invention the perturbations of the eddy currents in the second layer 92 can be differentiated from those caused by cracks and other defects by independently probing the first and second layers 90 and 92 with a magnetic flux that is generated by activating the outer drive coil 108. The magnetic flux lines that emanate from the circumferentially extending wall 100 in response to driving the outer drive coil 108 are sensitive to the geometry of the layers of the structural material adjacent to these outer drive coils, i.e. the edge 118, but not to the fastener 86 since the fastener 86 is displaced inwardly from the outer drive coils 108. If measurements are made with the sense coils 104 and 106 in response to eddy currents induced by the outer drive coil 108, these measurements will be indicative of the geometry of the structure being scanned in those areas of this structure that are outwardly from the fastener 86. If these measurements are then compared to the measurements made when the central core coil 96 is driven and if the two sets of measurements can be scaled one to the other, it is possible to remove that part of the output signal that is generated by the structural geometry such that signal indicative of cracks and other defects can be differentiated from signal that arises only because of the geometry of the structure being tested. The incorporation of the outer drive coil 108 or other such additional drive coil allows this to be achieved. Figures 7, 8, 9 and 10 show response curves for a probe, as for instance the probe 44 of Figure 2, which has 16 individual sense coils. The vertical axis is indicative of probe response amplitude and the horizontal axis is divided to show the response for each of the individual 16 sense coils. The probe 44 of Figure 2 has been positioned against a structure, as for instance the structure of Figures 4 and 5 that contains a complex geometrical component as its lower component, i.e. the layer 74 having scalloped and webbed sides. For identification purposes, the sense coils of the probe can be numbered 1 thru 16. The probe has been positioned such that sense coil number 5 is immediately adjacent the scalloped side and sense coil number 13 is immediately adjacent the webbed side. The individual responses of the sense coils have been joined to form a rough curve. The curve shown in Figure 7 is that which is obtained when the central or core coil 50 of the probe 44 is driven with an appropriate AC signal of a selected frequency. The response shown in Figure 8 shows the probe in the same location except in this instance the outer drive coil 58 was driven by this same signal. As is evident in comparing Figures 7 and 8, the shape of the curve that is derived by connecting the probe responses is very similar except for a scaling factor. The scale of Figure 8 is much larger than the scale of Figure 7. It is also evident that the shape of this curve is somewhat complex and determining whether or not a crack is present around one of the fastener holes 82 is not immediately evident. Even though they look complex, the response of Figures 7 and 8 are illustrative of the response obtained when no crack or other defect is present around the fastener hole in a layered structure of complex geometry. In Figures 9 and 10, a similar response to those shown in Figures 7 and 8 is measured, except in this case there is a crack adjacent to the fastener hole being tested. Figure 9 is the response curve that is generated when the center coil is activated and Figure 10 is the response curve that is generated when the outer coil is activated. In this instance the shape of the curves look different around the scalloped edge which is generally centered about the coil number 5 compared to that around the webbed side which is centered at about coil number 13. The curves are generally similar near the webbed side, but are different near the scalloped side. From the curve of Figure 9, by itself, one would not know whether or not one is simply looking at a response that has been generated because of geometry or a response that has been generated because of a defect. However, when one compares the curve of Figure 9 to the curve of Figure 10, and utilizes the curve of Figure 10 to remove the response that is only indicative of geometry of the structure being tested, the presence of a crack adjacent a fastener hole can be determined. It is apparent that if the curve of Figure 10 was scaled to that of Figure 9 and subtracted, it would be evident that there is a crack positioned adjacent to sensors 4 or 5. As per this invention, by placing a second drive coil near the outside of the probe body, the second drive coil can be used to generate an eddy current distribution that is sensitive to the boundaries of the structure being tested, but not small flaws near fastener holes being tested. Measurements taken when driving this outer coil are then used to remove the effects of the boundary variations from the measurements taken when driving the central coil. What is left is a signal indicative of flaws around the fastener hole. Referring back to Figure 6, by forming the central core 94 of the probe body as a hollow core, the probe can be utilized to inspect the area around steel fasteners. By utilizing a hollow central core, eddy currents generated in steel fastener heads are limited. This in turn provides for greater sensitivity to flaws around the fastener hole. A further advantage can be achieved by placing a further core body, central body 110, into the hollow central core. Activation of the central core body coil 114 directly over a steel fastener allows for convenient centering of the probe 84 over such a steel fastener. By driving the central body coil 114 an eddy current is generated in the fastener head which is sensed by the sense coil arrays. By moving the probe 84 until the signal sensed by the individual sense coils is approximately equal, convenient centering of the probe 84 over a fastener is achieved. Figures 11 through 16 axe illustrative of a further embodiment of this in¬ vention. In this embodiment a probe 120, seen in Figures 11 and 12, is formed of a body 122 that has a central core 124, a radial wall 126 and a circumfer¬ ential wall 128 essentially as illustrated for the probe 84. A central coil 130 is wound around the central core 124 and an outer coil 132 is wound around the circumferential wall 128. The rim of the circumferential wall 128, however, is not partitioned or castellated as per the prior probes. Instead it terminates in a solid lip 134. A further body 136 is mounted on the lip 134. In Figures 11 and 12 the further body 136 is simply shown as a generic body and the sense elements or sense coils that are associated with it are not illustrated. Preferably the main body 122 of the probe 120 is formed of a high permeability material, as for instance a ferrite material. The body 136, however, need not be formed of such ferrite material and can be formed of a material more easily worked, as for instance, a resin or the like. In Figure 11, for illustrative purposes, a series of phantom lines, collectively identified by the numeral 138 are shown dividing the body 136 into a plurality of segments. These segments are symmetrical with respect to the phantom lines 138 that radiate from the core 124. As is further illustrated in Figures 13 through 16, transducers are located in each of the segments defined by the phantom dividing lines 138. Thus, there is a circumferential symmetry in moving around the body 136 with respect to the central core 124. This, of course, is identical to the symmetry as is illustrated in the probe 44 of Figure 2 and the plurality of partitions 54 and sense coils 56 that are part of the structure of the probe 44. In Figure 13, the body 136 includes a plurality of sense coils collectively identified by the numeral 140. Each of the sense coils 140 may be wound around a ferrite core 142. The ferrite cores 142 are simply small segments of ferrite rod that are easily obtained by segmenting an appropriate ferrite rod into appro¬ priate sections. The sense coils 140, wound around their ferrite cores 142, are embedded in a resin matrix 144 to form one embodiment of the further body 136. In this embodiment the axes of the sense coils 140, when the body 136 is mounted onto the body 122, are parallel to and radially displaced from the axis of the central core 124. The sense coils 140 and their accompanying ferrite cores 142 are circumferentially arranged in a symmetrical array within the resin 144 such that they form a symmetrical array within the body 136 when mounted on the body 122. Each of the sense coils 140 and its central ferrite core 142 would be appropriately located in one of the segments that are defined by the lines 138. Referring now to Figure 14, as with Figure 13, a plurality of sense coils 146, which may have a central ferrite core 148, are located in a resin matrix 150. However, contrary to the embodiment of Figure 13, in the embodiment of Figure 14, the axes of the sense coils 146 and their ferrite cores 148 lie in a plane that is perpendicular to the axis of the central core 124 of the body 122. As with the symmetry of Figure 13, each of the sense coils 146 of Figure 14 would be located in one of the segments that are defined by the lines 138 of Figure 11. Li Figure 15 a further embodiment of the invention is illustrated. In this embodiment the sense coils, when considered with respect to the central core 124 axe spaced apart the same arcuate distance but are positioned at different radial dimensions from the central core 124. In the embodiment of Figure 15, an array of sense coils 152 form an outer circular symmetrical array and a further array of sense coils 154 form an inner circular array. For ease of illustration of the Figure, ferrite cores have not been included in the sense coils 152 and 154 of Figure 15, however, such ferrite cores could easily be incorporated therein. The sense coils will operate with or without ferrite cores. Use of ferrite cores is preferred for the outer circular symmetrical array 152. In positioning the sense coils 152 and 154, alternately a sense coil 152 is placed in one of the segments defined by the lines 138 of Figure 11, and then a sense coil 154 is placed in the next adjacent segment and so on and so forth around the totality of the circumference of the probe 120. In use the array of sense coils 152 would be looked at independently from the array of sense coils 154 since they experience slightly different magnetic flux environments when a probe incorporating them is utilized to scan around a fastener hole in a structure. In Figure 16 a further embodiment of the probe 120 of Figure 11 is illus¬ trated. In this embodiment, the further body 136 is formed as a monolithic body 156 having a plurality of Hall effect sensors 158 formed therein. The Hall effect sensors can be integrally formed within monolithic body 156 utilizing appropri¬ ate IC device construction techniques. When so formed this would allow the thickness of the monolithic body 156 to be very thin since the Hall effect sensors could be formed on just the surface of such a monolithic body. In operation the Hall effect sensors 158 would operate in a manner analogous to the sense coils described for the other embodiments. In Figures 17 and 18 even further probes of the invention are illustrated. In Figure 17 a first linear probe 160 is illustrated. The linear probe 160 has a body that has a first elongated section generally identified by the numeral 162. This section has a first side 164 and a second side 166. Wrapped around the elongated dimension of the elongated section 162 is a drive coil 168. The drive coil is essentially located between the first and second sides 164 and 166. A plurality of individual partitions collectively identified by the numeral 170 extend along the first side 164. Each of the partitions 170 is spaced apart from one another and is oriented essentially perpendicular to the solid elongated section 162. Further the partitions 170 axe essentially mutually parallel with each other. The partitions 170 form a first array of partitions. In a like manner, a plurality of partitions 172 are positioned along side 166. As with the first partitions, the partitions 172 are spaced apart from one another, they are oriented essentially perpendicular to the solid elongated section 162 and they are essentially mutually parallel to each other. A plurality of sense coils collectively identified by the numeral 174 are wound around the partitions 172. Thus, each partitions 172 has a sense coil 174 wound around it. The sense coils 174 are independent of each other and are further independent of the drive coil 168. A further drive coil 176 is wound around the periphery of the probe 160. The probe 160 would be operated in an equivalent manner to the above described circular probe. The probe geometry selected for an inspection, whether circular or linear, would depend upon the symmetry of the region to be inspected. In choosing between circular or linear probes, the selected of probe geometry is such that when an eddy current is generated and distributed in a part to be inspected the eddy current is consistent with the symmetry of the region to be inspected and is also that which would be perturbed by anticipated flaws. In Figure 18 a further probe that is an extension of the probe of Figure 17 is illustrated. In this figure a probe 178 has all the features of the probe 160 except that it includes a second elongated section 180 that is joined to the first elongated section 162 essentially about the second side 166 of the first elongated section as described for Figure 17. A third array of partitions, collectively identified by the numeral 182, extend in a linear array along the outside edge of the second elongated section 180. The sense coils 174 are located in the center array of the probe. A second drive coil 184 is located around the second elongated section 180. Figure 19 shows a synoptic diagram of the electronics associated with the probes of the invention. Illustrated in Figure 19 is a portable personal computer 186 and three boards which are inserted in appropriate peripheral slots of the computer 186. The boards connect on the internal bus of the computer, com- puter bus 188. The first of these boards is a generator and programmable signal source board 190. The second board is a 16 channel quadrature detection board 192 and the third board is a data acquisition board 194. Also connected to this system is a probe illustrated by the probe 196. The probe 196 can be any of the above described probes. In use the probe 196 is appropriately positioned over a fastener to be tested, the operator enters an appropriate command in the computer 186 such that a signal is generated on the board 190 and is used to drive the probe 196. Output signals of the sense coils of the probe axe directed to the board 192 for detection and from there to the board 194 for conversion into digital form for introduction back onto the bus 188 for appropriate read out on the computer 186. The board 190 includes a low frequency source means 198, an amplifier means 200, and a drive means 202. The low frequency source 198 is utilized to generate an appropriate AC signal of a selected frequency. This is amplified by the amplifier 200 and directed via the drive means 202 to an appropriate drive coil in the probe 196. Each of the sense coils are connected to an independent quadrature detec- tion means, collectively identified by the numeral 204 on the board 192. Output from the quadrature detection means 204 is directed to a multiplexer means 206 also on the board 192. First and second detected outputs, as are described below, are fed to a standard analog to digital converter means, collectively iden¬ tified by the numeral 208 on the board 194. Digital signal is then fed back onto the bus 188 for analyzing and display by the computer 186. The board 190 of Figure 19 is shown in greater detail in Figure 20. An in¬ terface means 210 connects to the computer bus 188 of Figure 19. This connects an internal board bus 212 to the computer bus 188. The board 190 further in¬ cludes a programmable frequency source 214. An appropriate signal of a chosen frequency from the programmable frequency source is independently sent on the internal bus 212 to first, second and third pre-setable counters 216, 218 and 220, respectively. A clock signal is further sent to the counters on clock line 222. Associated with the respective counters 216, 218 and 220 are respective memory units for storing an arbitrary wave form, as for instance, a sine wave form. These memory units are identified by numerals 224, 226 and 228. They can be selected as EPROMS, ROMS or other suitable IC devices for storing a preselected suitable wave form. Signal of a preset frequency and wave form is fed from the memory unit 224 to a D to A converter 230 and then to a low pass filter 232 for generation of a first reference output of a known and pre- selected phase. In a like manner, a signal of a known frequency and phase from memory unit 226 is fed through D to A convertor 234 and low pass filter 236 for generation of a second reference signal of known frequency and phase. Generally the second reference signal would be of the same frequency and amplitude as the first except it would be out of phase, as for instance by 90°. For example, the signal from low pass filter 232 would be a sine wave signal while the signal from low pass filter 236 would be a cosine wave signal (i.e. 90° out of phase) . In a like manner signal from the memory unit 228 is fed through D to A converter 238 and low pass filter 240. This signal, however, is passed through power amplifier 242 and then to a drive select relay 244. Together the compo¬ nents 214 through 240 comprise the low frequency source means 198 of Figure 19. The amplifier 242 comprises the amplification means 200 and the drive select relay 244 comprises the drive means 202. Further, located on the board 190 as is seen in Figure 20, is a multiplexer and system control module 246 that controls gain and multiplex selection on the board 192 as is described in detail below. Further the module 246 controls the digital output signal of the digital acquisition board 194. Additionally, located on the board 190 is a DC power source 248 for the board 192. In summary the features of the board 190 are that it generates three chan¬ nels of sine wave output (or other arbitrary wave form) with software control over the phase of all three channels and control of magnitude of one of the chan¬ nels, i.e. the power amplifier 242. Frequency is also under software control and can range from 20 Hz to 20 kHz in appropriate steps. The output from the amplifier 242 can be directed, via the drive select relays 244, to various drive coils of the probes, as for instance the center coil, the outer coil or centering coil as described for the probes above. Selection of output to a drive coil is also under software control via the select relays 244 and input from the computer 186 via the busses 188 and 212. Finally, power supply voltages for the quadrature detection board 192 is provided by the power source 248. The quadrature detection board 192 is connected via appropriate cables to the probe 196. It is a 16 channel board and each particular channel is connected to an individual sense coil in the probe 196. Each channel amplifies the sense coils low frequency output, multiples it by quadrature phase components and low pass filters the output. The output are two DC signals representing quadra¬ ture components of the sense coils output. The output of these channels are multiplexed under software control as follows. Shown in Figure 21 is one of the 16 quadrature detection means 204 as seen in Figure 19. Output from a sense coil is directed to a pre-amp 250 that is under the control of the multiplex and system control unit 246 on the board 190. Output from the pre-amp 250 is fed to a first four quadrant multiplier 252 that also receives the first reference phase output signal from the low pass filter 232 on the board 190. Output from the pre-amp 250 is also fed to a second four quadrant multiplier 254 that additionally receives the output of the low pass filter 236, i.e. the second reference signal that is out of phase with respect to the first reference signal. Output from the four quadrant multiplier 252 is fed through a low pass filter 256 and from there to a first multiplexer 258. Output from the second four quadrant multiplier 254 is fed through a low pass filter 260 and from there to a second multiplexer 262. Together the multiplexers 258 and 262 comprise the multiplexer means 206 on the board 192. Output signal from each of the channels 204 of the sense coils of the probe 196 are identically directed to the multiplexers 258 and 262. The signal acquisition of the multiplexers 258 and 262 is under control of the multiplexer and system control module 246 on board 190 as seen in Figure 20. The first detector output from the multiplexer 258 is input to the A to D convertor 208 on the board 194 and the second detector output from the multiplexer 262 is input to the other A to D convertor 208 on the board 194. After conversion to digital form the signals are then fed to the computer bus 188 for appropriate processing and display on the computer 186. In summary, for the components shown in Figures 19, 20 and 21, a struc- tural defect detecting system is illustrated that includes an appropriate probe, as described above, in conjunction with a signal generating means for generating at least one selected AC signal of a select frequency. A drive selection means independently propagates the signal to one of the drive coils of the probe. A signal processing means associated with the transducer means of the probe in- dependently processes the output signal at each of the plurality of transducer means and produces an output thereof that is indicative of the differences in the output signals of the plurality of transducer means. As illustrated in the probes above, preferably the transducer means com¬ prises sensing coils or Hall effect sensors. Further, as shown on these boards, the signal generator means generates a drive signal and first and second reference signals with at least the second reference signal differing in phase from the drive signal and the first reference signal. The drive signal can also differ from the first reference signal, as for instance, " being 45° out of phase. Thus, the first reference signal would be a zero phase signal, i.e. a sine signal, the drive signal a 45° phase drive signal and the second reference signal a 90° phase drive signal, i.e. a cosine signal. The drive select means drives a selected drive coil with the drive signal and the signal processing means processes the output signal of each transducer in association with both the first and second reference signals and in response thereto generates an output. This output, by utilizing the quadrature detection method of the board 192, has both a phase and a magnitude component. In general, the invention as is described in Figures 19, 20 and 21, utilizes a method for detecting defects in a structure wherein the components of Figures 19, 20 and 21 are utilized in conjunction with a probe as described above. The probe is located in association with an area on a structure to be tested. An AC drive signal and first and second AC reference signals are generated with at least the second reference signal differing in phase from the drive signal and the first reference signal. A drive coil is driven with the drive signal to introduce eddy currents in the structure. The sense coils or other transducer means in the probe axe interrogated individually for output signals that are produced in response to eddy currents in the structure. These output signals are processed in association with both the first and second reference signals and in response thereto a multiple component output signal having both a phase and a magnitude component is generated. Comparison can be made of the phase and magnitude components of the multiple component output signal of each of the transducer means to detect defects in the structure. Referring now to Figures 22 and 23, the usefulness of both a phase and a magnitude component of the output signal of the independent quadrature detectors 204 of Figure 19 is illustrated. As with the graphs of Figures 7, 8, 9 and 10, the output of the individual sensor coils is plotted along the horizontal axis. The vertical axis represents relative magnitude. Both the magnitude and phase axe plotted. First layer signals seen in Figure 22, which are best detected utilizing a high frequency signal, have a phase component 263 of a very low magnitude and an amplitude component 264 of a high relative magnitude. For signal that is indicative of defects in an underlying or buried layer, the amplitude component 265 is of high relative magnitude but contrary to first layer signal their phase component 266, as is seen in Figure 23, is also of a high relative magnitude. Comparison of Figures 22 and 23 shows that the relative magnitude of the phase increases with increasing depth of the defect from the first layer into the second layer. It is thus possible to differentiate the depth of defects around a fastener hole, i.e. whether they are in the first layer or in the second layer by looking at both the phase and the magnitude component of the signals detected in the individual sense coils. As is alluded to in the above referenced United States patent 4,495,466, higher frequencies are best suited for detecting cracks in upper layers whereas it is necessary to utilize lower frequencies for detection of cracks in underlying layers. The programmable frequency source 214 of the board described in Figure 20, allows for selection of appropriate detection frequencies in a useful range of from about 20 Hz to about 20 kHz. Generally the magnitude of the crack response is proportional to the crack size. Additionally the phase, spectral content and frequency dependency of the crack response is related in a systematic manner to crack features. For near surface or first layer cracks the high frequency response will be sharp and also possess a phase response that identifies it as a near feature crack as is illustrated in Figure 22. A second layer crack will only be detected in a low frequency response. It will have a broader, less sharper response. Further, the phase response will also indicate the crack as originating from deeper within the structure as is illustrated in Figure 23. As a general consideration the eddy current distribution generated by the central drive coil will not be distributed evenly around the probe. Adjacent structural edges, probe placement and the like will distort the current distribu¬ tion and will be measured by the probe sense coils. While we do not wish to be bound by theory, it is presently believed that the total probe response can be treated as a linear superposition of these effects. Tilting or lift off of the probe from the surface being checked as well as having the probe off center from the fastener being checked do effect crack response, but only to a small degree. It is presently believed that the totality of the probe response is a summation of the crack response, plus the adjacent structure response, plus the off center response, plus the lift off response, plus response related to the fastener itself. Utilizing the probes and circuitry as described above, those responses that axe not crack responses can be eliminated so as to distinguish the crack re¬ sponse from those due to adjacent structure. As described, the geometry of the structure will mask the eddy current response especially for second layer cracks. Additionally, skin thickness and edge to hole distance influences this. As was described for Figures 7, 8, 9 and 10, with use of the outer drive coil, the sense coil array will sense response that is dependent only on adjacent structure and is insensitive to the fastener hole. This response can then be scaled to the response from the center drive coil which is sensitive to the fastener hole. After scaling, the adjacent structure response is removed so as to differentiate those responses that are directly the result of cracks from those that are structural responses. Stated in different language, the response to the activation of the center drive coil contains structural inferences as well as crack components, whereas that from the outer drive coil contains only structural inferences that can be scaled and used to remove the response due to structure. In a like manner, lift off (probe face perfectly parallel to but raised fro the surface being tested), tilt or tilted lift off (probe face at an angle to the surface being tested), or off center responses can also be removed. Tilt, lift off and off center are associated with surface features. Probe response for these surface features is predominately generated by eddy currents near the surface. By utilizing a multi- frequency approach, a high frequency measurement is made to isolate near surface effects from those emanating deeper in the structure. Further, tilt, lift off and off center responses have characteristic phase responses. By measuring these at a high frequency they can then be scaled and removed from the response at low frequency since a near linear relationship exists for the scaling of high frequency to low frequency responses. This allows removal of tilt, lift off and off center response. The removal of off center response is illustrated in Figures 24 and 25. As with previous graphs, the horizontal axis represents the individual segments while the vertical axis represents the relative magnitude. Line 267 of Figure 24 represents an output without compensation for off centering. When off centering is compensated for the response line is that shown by line 268. If off center compensation was not made it would be hard to detect a cracked response that might be in the line illustrated by line 267. However, after center compensation is made it is evident from line 268 that there is no crack present. This is compared to Figure 25 wherein line 269 represents the response prior to centering compensation while line 270 represents that with centering compensation. Line 270 shows the presence of a crack that is simply not evident in line 269 because of the off centering response included therein. Utilizing quadrature detection and plotting on an "impedance diagram" (in this instance on a pseudo impedance diagram wherein the real component of the response is plotted on the abscissa and the imaginary component of the response is plotted on the ordinate), for a fastener with no defects, generally all the response will be near the origin, however if off centering is present the real component will vary for the individual sensors more than the imaginary component. As opposed to this, with tilt or lift off the imaginary component varies to a greater degree than the real component. These characteristics allow for identification and removal of erroneous response resulting from either lift off or off centering. Fasteners can also produce erroneous response. The fastener head may have somewhat of an oval shape, the head pattern could be different or the fastener hole could be tilted. Responses from such fastener characteristics will generally have phase characteristics of surface features, but will not have spectral responses characteristic of near surface cracks. Changes in countersink angle or over size holes, both of which are symmetrical about the fastener, result in only DC shifts in the probe response and since they are symmetrical, do not affect probe response. Accordingly the following steps, as are illustrated in the flow diagram of Figure 26, are utilized to segregate the various responses and isolate defects, as for instance cracks in fastener holes. At step 272 of Figure 26, calibration of the probe is effected by measuring a fastener hole known to be defect free. The probe is then centered at step 274 utilizing a high frequency signal and a real time display at output 276. As was noted above by utilizing the real component of a quadrature analysis the probe can be moved until an appropriate real time display shows little variation between the individual sensor response. With the probe almost centered at step 278, the center drive is driven with a high frequency signal followed by driving the outer drive also with this high frequency signal. Utilizing the techniques as illustrated for Figures 7 through 10, defects in the first layer can then be seen as a real time output at output 280. At step 282 the center drive is now driven with a low frequency signal and this is compared to the high frequency signal for determination of off centering and lift off compensation. Since tilt, lift off and off centering are predominantly surface effects, they axe detectable in the high frequency signal and can be removed from the low frequency signal using appropriately scaled high frequency signals. Such compensation is effected at step 284. At step 286 the outer drive is driven with a low frequency signal to detect adjacent structure. Such adjacent structure compensation is effected at step 288. Input from the calibration step 272 is then used at step 290 for scaling both the inner and outer drive response from an adjacent hole known to be defect free. Upon such scaling, second layer defects axe evident at output 292 and having identified the defects the probe is then moved to the next hole at step 294. As opposed to utilizing adjacent holes for calibration, the totality of the probe or individual parts thereof can be moved. The current density in eddy currents generated when the center coil is driven increase from a position next to the fastener hole to a maximum that is located underneath the body of the probe but away from the fastener hole and then drops to a minimum external of the body of the probe. Advantage can be taken of the maximum current density at a point which is essentially midway between the center drive coil and the outer drive coil. As is shown in Figure 27a, a probe 296 of the invention is located over a fastener 298 that is in a structure 300. The probe 296 fits within a circular guide 302 that has a larger internal diameter than does the external diameter of the probe 296. The probe 296 can then be oscillated within the guide 302 by moving the probe in a circular motion within the confines of the guide 302. This motion is illustrated in Figure 27b. In Figure 27b the path of the center coil is indicated by line 304 and paths of appropriate representative sense coils by lines 306. The probe is oscillated without rotating it about the center of the probe within the guide 302. The center coil and the sense coils are moved through the orbits as illustrated by the lines 304 and 306 of Figure 27b. The sense coils of the probe thus oscillate, but do not rotate about the center of rotation of the probe itself. In Figure 28a a further method of motion is shown. An axle 308 is mounted to a probe 310 off center from the central core of the probe. The axle 308 is positioned essentially on center with the center of a fastener 312. The probe is then rotated about the axle 308 which reposition the center coil and the sense coils about the center line of rotation that is along the axis of the axle 308. This rotates the probe about this axis. The motion path of the off-center rotation is illustrated in Figure 28b. A further embodiment is shown in Figure 29a. In this embodiment sense coils 314 are located in a body 316 that is independent of a body 318 that contains the central drive coil 320. An axle 322 is connected to the body 318 and rotates the body 318 in a case 324 that is mounted around the body 316. The sense coils remain in position, however, the drive coil is rotated off center from the center of a fastener 326. This is equivalent to the rotation seen in Figure 28b for the totality of the probe, however, only the main body of the probe including the central drive coil are so rotated. The motion path of the off-center rotation is illustrated in Figure 29b. In a like manner to that illustrated in Figure 27b the center body 318 and its drive coil 320 could be oscillated (but fixed with respect to rotation) around the case 324. In both instances the axis of the axle is essentially in the center of the body 316, i.e. in the center of the sense coils 314. This invention may be embodied in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments are therefore to be construed in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes that come within the meaning and range of equivalencies of the claims are therefore are intended to be embraced herein.

Claims: What is Claimed is: 1. An eddy current probe comprising: a body formed of a high permeability material; said body shaped to include a central core and an additional wall displaced from said central core; said central core having a central core drive coil wound thereon; a plurality of sense coils, said sense coils located in an array in association with said additional wall; and a further drive coil wound about said additional wall, said further drive coil independent of said core drive coil. 2. An eddy current probe comprising: a body formed of a high permeability material; said body including a central core, a radially extending wall radiating from the central core and a circumferentially extending wall extending from the radial wall concentrically with the core; said central core having a core drive coil wound thereon; said circumferentially extending wall including a rim distal from said radial wall; said rim including a plurality of interstices that divide said rim into a plural¬ ity of symmetrical independent partitions that are symmetrically located about said circumferentially extending wall, said partitions shaped so as to extend axially essentially parallel with the axis of said central core; a plurality of independent sense coils equal in number to the number of said partitions with a respective one of said sense coils wound around a respective one of said partitions such that each of said independent partitions includes an independent sense coil wound around it; and an outer drive coil wound about the outside of said circumferentially ex- tending wall. 3. An. eddy current probe of claim 2 wherein: said outer drive coil is independent of said core drive coil. 4. An eddy current probe of claim 2 wherein: said plurality of interstices and plurality of partitions together form a castel¬ lated structure composed of essentially prismatic partitions separated by essen- tially straight sided crenelated openings. 5. An eddy current probe comprising: a body formed of a high permeability material; said body including a central core, a radially extending wall radiating from the core and a circumferentially extending wall extending from the radial wall concentrically with the core; said central core having a core drive coil wound thereon; said circumferentially extending wall including a rim distal from said radial wall; said rim including a plurality of interstices that divide said rim into a plural¬ ity of symmetrical partitions that are symmetrically located about said circum¬ ferentially extending wall with each partition being shaped to include a portion thereof that is positioned in space in a plane that is essentially perpendicular to the axis of said central core; and a plurality of sense coils equal in number to the number of said partitions with a respective one of said sense coils wound around that portion of a respec¬ tive one of said partitions that is positioned in said plane such that each of said independent partitions includes an independent sense coil wound around it. 6. An eddy current probe of claim 5 further including: an outer drive coil wound about the outside of said circumferentially ex¬ tending wall. 7. An eddy current probe comprising: a first body formed of a high permeability material; said first body including a central core, a radially extending wall radiating from the core and a circumferentially extending wall extending from the radial wall concentrically with the core; said central core having a core drive coil wound thereon; said circumferentially extending wall including a lip distal from said radial wall; a further body structurally independent of said first body, said further body symmetrically divisible into a plurality of individual sections, said further body mounted on said circumferentially extending wall in association with said lip; and a plurality of independent transducer means equal in number to the number of said sections of said further body, said transducers means for sensing eddy currents, said transducer means located on said further body with a respective one of said transducer means positioned in a respective one of said sections such that each of said independent sections includes an independent transducer means operatively associated with it and together said plurality of transducer means axe oriented in at least one symmetrical array with respect to said rim of said circumferentially extending wall. 8. An eddy current probe of claim 7 wherein: each of said transducer means comprises a sense coil; and said sense coils are located on said further body with a respective one of said sense coils positioned in a respective one of said sections such that each of said independent sections includes an independent sense coil operatively associ¬ ated with it and together said plurality of sense coils are oriented in at least one symmetrical array with respect to said rim of said circumferentially extending wall. 9. An eddy current probe of claim 8 wherein: at least a portion of said further body is formed of high permeability ma- terial. 10. An eddy current probe of claim 9 wherein: each of said sections includes a section core formed of said high permeability material. 11. An eddy current probe of claim 8 further including: a plurality of independent ferrite cores equal in number to the number of said sense coils; and said plurality of ferrite cores mounted in said further body with a respec- tive one of said independent sense coils wound around a respective one of said independent ferrite cores. 12. An eddy current probe of claim 8 wherein: said sense coils are orientated on said further body such that when said further body is mounted on said first body the axis of each of said sense coils is located essentially parallel with the axis of said central core of said first body. 13. An eddy current probe of claim 8 wherein: said sense coils are orientated on said further body such that when said further body is mounted on said first body the axis of each of said sense coils is positioned in space in a plane that is essentially perpendicular with the axis of said central core of said first body. 14. An eddy current probe of claim 7 further including: an outer drive coil wound about the outside of said circumferentially ex¬ tending wall. 15. An eddy current probe of claim 12 further including: an outer drive coil wound about the outside of said circumferentially ex¬ tending wall. 16. An eddy current probe of claim 13 further including: an outer drive coil wound about the outside of said circumferentially ex¬ tending wall. 17. An eddy current probe of claim 8 wherein: said plurality of sense coils are divided into a first circular array and a second circular array; and said first circular array is positioned from said central core at a first radial distance and said second circular array is positioned from said central core at a second radial distance and said second radial distance is different from said first radial distance. 18. An eddy current probe of claim 8 further including: said central core being hollow; an inner core body independent of first body, said inner core body having a central boss, said inner core body sized and shaped to fit into and be retained in said hollow core of said first body; and a centering coil wound around said inner core body central boss. 19. An eddy current probe of claim 7 wherein: each of said transducer means comprises a Hall effect sensor element; and said Hall effect sensor elements located on said further body with a respec¬ tive one of said Hall effect sensor elements positioned in a respective one of said sections such that each of said independent sections includes an indepen¬ dent Hall effect sensor element operatively associated with it and together said plurality of Hall effect sensor elements are oriented in at least one symmetrical array with respect to said rim of said circumferentially extending wall. 20. An eddy current probe of claim 7 further including: means associated with said first body for moving said first body in a closed orbit about a point that is off-center from the center of said central core. 21. An eddy current probe comprising: a body formed of a high permeability material; said body shaped to include a central core, a radially extending wall radiat¬ ing from a central core and a circumfer- entially extending wall extending from the radial wall concentrically with the central core; said central core having a central axis; said central core having a core drive coil wound thereon; a plurality of independent transducer means for sensing eddy currents, said transducer means located in operative association with said circumferentially extending wall in a symmetrical circular array; and means for moving said body in a prescribed closed orbit. 22. An eddy current probe of clai 21 wherein: said means for moving said body in a prescribed closed orbit moves said body in an orbit about an off-center axis, said off-center axis being parallel to and radially displaced from the axis of said central core. 23. An eddy current probe of claim 22 wherein: said body rotates about said off-center axis. 24. An eddy current probe of claim 22 wherein: said body oscillates about said off-center axis but does not rotate about said off-center axis. 25. An eddy current probe comprising: a first body formed of a high permeability material; said first body including a central core, a radially extending wall radiating from the core and a circumferentially extending wall extending from the radial wall concentrically with the core; said central core having a core drive coil wound thereon; a further body structurally independent of said first body, said further body symmetrically divisible into a plurality of individual sections, said further body movably associated with said circumferentially extending wall of said first body; a plurality of independent transducer means equal in number to the number of said sections of said further body, said transducers means for sensing eddy currents, said transducer means located on said further body with a respective one of said transducer means positioned in a respective one of said sections such that each of said independent sections includes an independent transducer means operatively associated with it and together said plurality of transducer means are oriented in at least one symmetrical array with respect to said rim of said circumferentially extending wall; and means for moving said first body in a prescribed closed orbit with respect to said further body. 26. An eddy current probe of claim 25 wherein: said further body includes a central axis, said further body central axis located parallel to and radially displaced from the axis of said central core of said first body; and said means for moving said first body in a prescribed closed orbit with re¬ spect to said further body moves said first body in an orbit centered on said central axis of said further body and off-centered from the axis of said central core of said first body. 27. An eddy current probe of claim 26 wherein: said first body rotates about said central axis of said further body. 28. An eddy current probe of claim 27 wherein: said first body oscillates about said central axis of said further body but does not rotate about said central axis of said further body. 29. An eddy current probe comprising: a first body formed of a high permeability material; said first body shaped to include a radial wall radially extending from the core and a circumferentially extending wall extending from the radial wall con¬ centrically with the core; said central core having a core drive coil wound thereon; said circumferentially extending wall including a lip distal from said radial wall; a further body structurally independent of said first body, said further body including a plurality of Hall effect sensor elements symmetrically located in at least one array about said further body; and said further body mounted on said lip of said circumferentially extending wall. 30. An eddy current probe of claim 29 wherein: said further body is a monolithic body having said Hall effect sensor ele¬ ments integrally formed therein. 31. An eddy current probe comprising: a first body formed of a high permeability material; said body shaped to include a hollow central core, a radially extending wall radiating from the central core and a circumferentially extending wall extending from the radial wall concentrically with the core; said hollow central core having a core drive coil wound thereon; said circumferentially extending wall including a rim distal from said radial wall; said rim including a plurality of interstices that divide said rim into a plural¬ ity of symmetrical independent partitions that are symmetrically located about said circumferentially extending wall, said partitions shaped so as to extend axially essentially parallel with the axis of said central core; a plurality of independent sense coils equal in number to the number of said partitions with a respective one of said sense coils wound around a respective one of said partitions such that each of said independent partitions includes an independent sense coil wound around it; and a centering coil located in said hollow core of said first body. 32. An eddy current probe of claim 31 further including: an inner core body independent of first body, said inner core body sized and shaped to fit into and be retained in said hollow core of said first body; said inner core body having a central boss formed of a high permeability material; and said centering coil being wound around said central boss. 33. An eddy current probe of claim 31 further including: an outer drive coil wound about the outside of said circumferentially ex¬ tending wall. 34. An eddy current probe comprising: a linear body formed of a high permeability material, said body having a first solid elongated section, a first plurality of individual partitions and a second plurality of individual partitions; said solid elongated section having opposing sides that extend along the elongated dimension of said solid elongated section; said first plurality of individual partitions located in a first linear array along a first of said sides of said solid elongated section; said second plurality of individual partitions located in a second linear array along a second of said sides of said solid elongated section; said individual partitions of said first array each being spaced apart from one another and each orientated essentially perpendicular to said solid elongated section and essentially mutually parallel to each other; said individual partitions of said second array each being spaced apart from one another and each orientated essentially perpendicular to said solid elongated section and essentially mutually parallel to each other; a drive coil wound around said solid section between said sides of said solid section; and a plurality of individual sense coils equal in number to the number of said second partitions with a respective one of said sense coils wound around a re¬ spective one of said second plurality of partitions such that each partition in said second plurality of partitions includes an independent sense coil wound around it. 35. An eddy current probe of claim 34 further including: an outer drive coil wound around the periphery of said body. 36. An eddy current probe of claim 34 further including: said body having a second solid elongated section and a third plurality of individual partitions; said second solid elongated section having sides; said second solid elongated section joined along a first of its sides to said first elongated solid section about said second side of said first elongated solid section such that said first and second solid elongated sections are co-planar and parallel with each other and said second linear array is positioned between said first and said second elongated sections; a second drive coil wound around said second solid elongated section be- tween said sides of said second solid elongated section; said individual partitions of said third array each being spaced apart from one another and each orientated essentially perpendicular to said second solid elongated section and essentially mutually parallel to each other; and said third plurality of partitions located in a third linear array along the other of the sides of said second solid elongated section. 37. A method of inspecting a layered structure comprising: selecting a probe having a body formed of a high permeability material with the body shaped to include a central core having a core drive coil wound thereon and an additional wall displaced from said central core and further including a plurality of independent sense coils located in association with said additional wall and a further drive coil wound about said additional wall; positioning said probe against said structure approximately centered over the area to be inspected; driving said core drive coil with a first input signal of a first alternating frequency to generate eddy currents in said structure; operating each of said sense coils to detect initial output signal related to eddy currents generated in said structure; analyzing said detected initial output signal for anomalies indicative of de- fects in a top layer of said structure; driving said core drive coil with a second input signal of a second alternating frequency to generate eddy currents in said structure and wherein said second frequency is of a lower frequency than said first frequency; operating each of said sense coils to detect further output signals related to eddy currents generated in said structure; storing said further output signal detected by each of said sense coils; driving said outer drive coil with said second input signal to generate eddy currents in said structure; operating each of said sense coils to detect additional output signals related to eddy currents generated in said structure; storing said additional output signal detected by each of said sense coils; scaling said stored additional output signal from each of said individual sense coils to said stored further output signal from each of said individual sense coils; and comparing said scaled additional output signal with said further output sig¬ nal for anomalies indicative of defects in an underlying layer of said structure. 38. The method of claim 37 including: centering said probe by driving said core drive coil with said first input signal; operating each of said sense coils to detect output signal related to eddy currents generated in said structure; and analyzing said detected output signal for anomalies indicative of said probe being off center from said area to be inspected. 39. The method of claim 37 further including the steps of: storing said initial output signal detected by each of said sense coils; scaling said stored initial output signal from each of said individual sense coils to said stored further output signal from each of said individual sense coils; and comparing said scaled initial output signal with said further output signal for anomalies indicative of said probe being off-centered from said area to be inspected and said probe not being oriented parallel to said area to be inspected. 40. The method of claim 37 further including the steps of: calibrating said probe by positioning and centering said probe against a layer structure known to be free of defects; driving said core drive coil with said second input signal; operating each of said sense coils to detect calibration signals related to eddy currents generated in said structure; storing said calibration signal detected by each of said sense coils; and comparing said additional output signal to said calibration signal to deter¬ mine a scaling factor. 41. A method of inspecting a layered structure comprising: selecting a probe having a body formed of a high permeability material with the body shaped to include a central core having a core drive coil wound thereon and an additional wall having a plurality of independent sense coils located in association with said additional wall; positioning said probe against a layer structure approximately near the area to be inspected; moving said probe though a closed orbit that is centered about an off- centered axis that is parallel to and radially displaced from a central axis of said core; driving said core drive coil with a signal of an alternating frequency to generate eddy currents in said structure; operating each of said sense coils to detect signal related to eddy currents generated in said structure; and analyzing said detected signal for anomalies indicative of defects in the lay¬ ers of said structure. 42. The method of claim 41 including: moving said probe in said orbit by oscillating said probe about said off- centered axis without rotating said probe about said off-centered axis. 43. The method of claim 41 including: moving said probe in said orbit by rotating said probe about said off- centered axis. 44. A method of inspecting a layered structure comprising: selecting a probe having a first body formed of a high permeability material with the first body shaped to include a central core having a core axis and a core drive coil wound thereon, said probe further including a further body having a further body central axis and a plurality of independent sense coils located around said further body central axis with said further body located adjacent to said first body and with said further body central axis located parallel to but radially displaced from said core axis, said probe further including a means for moving said first body in a prescribed closed orbit with respect to said further body; positioning said further body of said probe against said structure approxi¬ mately near the area to be inspected; moving said first body of said probe though a closed orbit that is centered about said further body axis; driving said core drive coil with an signal of an alternating frequency to generate eddy currents in said structure; operating each of said sense coils to detect signal related to eddy currents generated in said structure; and analyzing said detected signal for anomalies indicative of defects in the lay¬ ers of said structure. 45. The method of claim 44 including: moving said first body through said closed orbit by rotating said first body around said further body axis. 46. The method of claim 44 including: moving said first body through said closed orbit by oscillating said first body about said further body axis while holding said first body fixed with re¬ spect to rotation of said first body about said further body axis. 47. An eddy current structural defect detection system comprising: a body formed of a high permeability material; a plurality of independent drive coils, each of said drive coils being indepen¬ dently located on said body, each of said drive coils being capable of inducing eddy currents in a metallic structure in response to an alternating current signal conducted through each of said drive coils; a plurality of independent transducer means located in a symmetrical array in operative association with said body, each of said transducer means for sens¬ ing eddy currents in said metallic structure and producing an output signal in response to said sensed eddy currents, said each of said transducer means being independent of said drive coils; signal generating means for generating said alternating current drive signal of a selected frequency; drive select means for independently driving a selected one of said drive coils with said alternating current drive signal, said alternating current drive signal being conducted only through said selected one of said drive coils, whereby said eddy currents are induced in said metallic structure only by said selected one of said drive coils; and signal processing means operatively associated with said plurality of trans¬ ducer means for independently and concurrently processing said output signal of each of said plurality of transducer means and for producing a detected out- put signal comprising a multiplexed signal formed of each said independently processed output signal of each said transducer means, each said independently processed output signal having a phase component and a magnitude component, said detected output signal being indicative of differences between said output signals of said plurality of transducer means. 48. An eddy current structural defect detection system as claimed in claim 47 wherein each of said transducer means comprises a sensing coil mounted on said body. 49. An eddy current structural defect detection system as claimed in claim 47 wherein: said signal generating means further comprises means for generating first and second reference signals, said second reference signal differing in phase from said alternating current drive signal and said first reference signal; said drive select means drives said drive coil with said drive signal; and said signal processing means independently and concurrently processes said output signal of each of said transducer means in association with both said first and said second reference signals. 50. A method of detecting defects in a metallic structure comprising: selecting a probe having a body formed of a high permeability material and having at least one drive coil located thereon that is capable of inducing eddy currents in said metallic structure in response to an alternating current drive signal conducted through said drive coil and where said probe further has a plurality of independent transducer means located in a symmetrical array in operative association with the body each of which is capable of sensing eddy currents in said metallic structure and producing an output signal in response to said sensed eddy currents and where said transducer means are independent of said drive coil; locating said probe in association with said structure; generating said drive signal and first and second reference signals, said second reference signal differing in phase from said drive signal and said first reference signal; driving a selected drive coil with said drive signal to induce eddy currents in said structure; interrogating said transducer means for transducer output signals produced in response to eddy currents induced in said structure; processing said output signal of each of said transducer means in associ¬ ation with both said first and said second reference signals and in response thereto generating a multiple component output signal having both a phase and a magnitude component; and comparing said phase and magnitude components of said multiple compo¬ nent output signal of each of said transducer means, whereby defects in said structure are detected. 51. An eddy current structural defect detection system as claimed in claim 47 wherein each of said transducer means comprises a Hall effect sensor. 52. An eddy curren tstructural defect detection system as claimed in claim 49 wherein further said drive signal differs in phase from said first reference signal and said second reference signal.

Inventors: SHEPPARD WILLIAM R, TAM KENT KIN-MAN, SHEPPARD, WILLIAM, R., TAM, KENT, KIN-MAN
Applicants: NORTHROP CORP, NORTHROP CORPORATION
Application year: 1993
Main classification: G01N27
IPC codes: G01N27