Abstract: A detection apparatus for an eddy current in a heat generating tube using a permeability measurement method, and a method using the apparatus are provided. In the detection apparatus, a bobbin type probe acquires detection information with respect to a magnetic flux change by a magnetic phase occurring in the heat generating tube using a coil which is wound in an axis direction of at least one yoke located in a perpendicular direction with a bobbin of the bobbin type probe, and a material having a corresponding magnetic phase and a circumferential defect, which is difficult to be detected by the bobbin type probe, are detected based on the detection information.

Abstract: The invention concerns a method for producing an assembly of at least one transmission coil (B1) and one reception coil (B2) for eddy current testing, the reception coil receiving in the absence of fault a complex amplitude signal VR, subject to a variation ?VR in the presence of a characteristic fault to be detected. The method consists in selecting the distance ?ER between the axes of the transmission coil and the reception coil so as to maximize the ratio I?VR/VRI.

Abstract: An apparatus and method are disclosed for detecting flaws in electrically conductive materials by observing properties of the back-EMF of the eddy current field generated by driving magnetic flux through the object to be examined. The input signal may include sweeps at several frequencies, and may do so at one time under the principle of wave superposition. The sectorial observations of eddy currents summations may be compared to a known datum for a defect free material, the presence of anomalies in eddy field back EMF divergence tending to provide an indication of an irregularity in the underlying eddy field, and hence in the underlying material itself. The portable unit may have a number of different configurations depending on the nature of the object to be examined, be it a flat or large radius plate, a flange, a rail, or some other structural element.

Abstract: An automated, non-destructive anhysteretic magnetization characterization method for studying and modeling soft magnetic materials. This measurement method employs a “reading-waveform” that allows multiple points of reference to be established in tracing out the B waveform. In using the reference values from this waveform, the components of B that cannot be measured directly may be calculated with precision. In turn, the initial magnitude of the B waveform is identified as the unknown component of the anhysteretic state. The processes can be repeated for different values of static fields as well as a function of temperature to produce a family of anhysteretic magnetization curves. The core characterization was performed without physically altering the core, so that the true anhysteretic magnetization curve, and the true B-H loop under applied bias H, is measured.

Abstract: A device for nondestructive and noncontact detection of faults in a test piece, with a stationary measurement device (16) for taking an eddy current or a magnetic stray flux measurements on a test piece (10) continuously advanced relative to the measurement device; and a device (12) for positioning the test piece with respect to the measurement device in a plane perpendicular to the direction of movement of the test piece. The device also has a monitoring unit (20) with at least three distance sensors (22) arranged distributed around the test piece in the peripheral direction for contactless detection of the distance between the surface of the test piece facing the respective sensor and the respective sensor, as well as a unit for evaluating the signals from the sensors.

Abstract: An eddy current probe testing apparatus structured to operate concurrently in a driver pick-up mode and said impedance mode is provided. The eddy current probe has two coils. The eddy current probe testing apparatus also includes a signal producing device, an output device, and a switch assembly. The switch assembly is structured to switch how an input signal from the signal producing device is provided to the two coils. Thus, an inspection may be performed in two modes concurrently.

Abstract: An inspection system for inspecting a part is provided. The inspection system includes a multi-dimensional array of eddy current sensors that conforms to a contour of a three dimensional shape of the part. The inspection system also includes a controller coupled to the multi-dimensional array, wherein the controller is configured to electronically scan the part via an electrical connection of the eddy current sensors to an eddy current instrument. The inspection system further includes a processor coupled to the eddy current instrument, wherein the processor is configured to analyze output from the eddy current instrument and the controller to accomplish inspection of the part.

Abstract: An eddy current probe assembly suitable for inspecting a test object with longitudinal shape, being passed through the assembly in the object's axial direction during an inspection session, the probe assembly comprising multiple probe modules being disposed in a radial plane and with the modules partially overlaying on each other forming an iris structure encircling an inspection zone, wherein a movement in unison of each of the probe modules closer to or further away from the center of the inspection zone makes the inspection zone enlarged or contracted. Spring tension is applied on each of the probe modules so that constant life-off in maintained between the probe modules and the test surface. Array of eddy current elements for each probe module and multiple layers of probe modules can be employed to achieve complete coverage of the test surface. The radial cross-sectional shapes of the test objects can be of round or polygonal.

Abstract: An arrangement and a method for influencing and/or detecting magnetic particles in a region of action includes a selector for generating a magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone having a low magnetic field strength and a second sub-zone having a higher magnetic field strength are formed in the region of action. A driver is provided for changing the position in space of the two sub-zones in the region of action by a magnetic drive field so that the magnetization of the magnetic particles changes locally. A receiver is also provided for acquiring signals, which signals depend on the magnetization in the region of action, which magnetization is influenced by the change in the position in space of the first and second sub-zone. Further, a coil arrangement is provided including a surrounding coil and a magnetic field generator such that the surrounding coil almost completely surrounds the magnetic field generator.

Abstract: An in-line pipe inspection tool has one or more inspection platforms (28, 30) which are connected to an elongate wheeled trolley by link arms (20 to 26). The trolley unit (10, 12) has drive means for driving the point of connection of the first link arm (20, 22) to the trolley (10) relative to the point of connection of the second link arm (24, 26) to the trolley (12), thereby to move the inspection platforms (28, 30) in a direction perpendicular to the direction of elongation of the trolley (10, 12). Thus the inspection platforms (28, 30) are movable which is relative to the trolley (10, 12) to permit the tool to be adapted to pipelines of different diameters. Moreover, the platforms (28, 30) preferably have permanent magnets which contain a rotatable magnet. The rotatable magnet permits the net magnetic field generated by the platform to be varied.

Abstract: The method for detecting at least one structural defect in a spherical particle (33) comprises at least the following steps of passing the particle (33) into at least one induction coil (15); exciting the induction coil (15) in order to induce Foucault currents in the particle (33); acquiring an output signal at the terminals of the induction coil (15); and analyzing the signal in order to establish whether or not the particle (33) comprises a structural defect. A plurality of output signals are acquired by passing the particle (33) successively into one or more induction coils (15) with different positions of the particle (33), the or each induction coil (15) being excited at least each time the particle (33) passes in order to induce Foucault currents in the particle (33).

Abstract: Storage of information, such as baseline information and structure ID, in a memory that is mounted on the structure, rather than inside the diagnosis hardware. This allows for faster and more convenient information retrieval. In particular, this approach allows for a more modular system in which different diagnosis hardware or other analyzers can be simply plugged into a structure's sensor network, whereupon they can quickly download any desired structure-specific information (e.g., baseline information, structure ID, and other useful information) from the on-structure memory.

Abstract: An electromagnetic system for biosensors including two independent electromagnetic units separated in the region of pole shoes of the electromagnetic units positioned under a gap, a cartridge positioned in the gap providing a sample volume and a biosensor having a sensor surface located at one or more inner surfaces of the cartridge proximate to the pole shoes.

Abstract: Described are methods for monitoring of stresses and other material properties. These methods use measurements of effective electrical properties, such as magnetic permeability and electrical conductivity, to infer the state of the test material, such as the stress, temperature, or overload condition. The sensors, which can be single element sensors or sensor arrays, can be used to periodically inspect selected locations, mounted to the test material, or scanned over the test material to generate two-dimensional images of the material properties. Magnetic field or eddy current based inductive and giant magnetoresistive sensors may be used on magnetizable and/or conducting materials, while capacitive sensors can be used for dielectric materials. Methods are also described for the use of state-sensitive layers to determine the state of materials of interest. These methods allow the weight of articles, such as aircraft, to be determined.

Abstract: Disclosed is an eddy current flaw detection probe that is capable of pressing itself against an inspection target whose curvature varies. A flaw sensor is configured by fastening a plurality of coils to a flexible substrate that faces the surface of the inspection target. A first elastic body is positioned opposite the inspection target for the flaw sensor, is obtained by stacking two or more elastic plates, and has an elastic coefficient that varies in a longitudinal direction. A second elastic body is a porous body positioned between the flexible substrate and the first elastic body. A pressure section is employed to press the first elastic body toward the inspection target.

Abstract: Methods and apparatuses for detecting fastener loosening. Sensors query a structure at a baseline value of an environment variable, such as temperature, and this baseline signal is stored for later use. Subsequently, users can query the structure remotely and at any time, and the signals from these queries are compared to the stored baseline signal. In some embodiments, an index is calculated, and the system determines that one or more fasteners have come loose if the calculated index exceeds a predetermined threshold value. It is desirable to select a time window within which the query signal is most sensitive to fastener loosening but least sensitive to variations in the environment variable. Accordingly, embodiments of the invention include methods and apparatuses for determining an optimal time window for use in calculating the above described index.

Abstract: Nondestructive examination is performed on a composite aircraft component including a composite body and a conductive medium. The conductive medium is substantially more conductive than the composite body. The nondestructive examination includes applying an electromagnetic field that penetrates the composite body and heats the conductive medium, and creating a thermal image of the conductive medium to reveal conductivity information about the conductive medium.

Abstract: A system for detecting defects in a combustion duct of a combustion system of a combustion turbine engine while the combustion turbine engine operates, wherein the combustion duct comprises a hot side, which is exposed to combustion gases and, opposing the hot side, a cold side. The system may include: an indicator coating disposed on the cold side of the combustion duct; and a proximity sensor positioned in proximity to the combustion duct and configured to detect a distance between the position of the proximity sensor and the cold side of the combustion duct.

Abstract: The system provides first and second sensor assemblies and processor that work in cooperation to detect flaws in welded tubulars. The first sensor detects and provides an indication of the weld line to the processor, which analyzes the indication and determines the approximate location of the weld line along the tubular. The processor then transmits a signal to the second sensor assembly, which is preferably mounted on an automatic positioning apparatus. In response to the signal, the automatic positioning apparatus adjusts and readjusts the position of the second sensor assembly into proximity with the weld line to search for flaws in the tubular.

Abstract: An eddy current probe testing apparatus structured to operate concurrently in a driver pick-up mode and said impedance mode is provided. The eddy current probe has two coils. The eddy current probe testing apparatus also includes a signal producing device, an output device, and a switch assembly. The switch assembly is structured to switch how an input signal from the signal producing device is provided to the two coils.

Abstract: An eddy current probe uses magnetic gap. The probe has a small size; and coil number in the probe is reduced. Hence, the probe can move easily inside and outside a tube and detect an end of the tube as close as possible.

Abstract: An inspection device that is capable of inspecting all heat-transfer-tube sealing portions in a steam generator and that is also capable of analyzing a defect shape is provided. An inspection device that employs the eddy-current flaw detection method to inspect the presence/absence of a defect in a welded portion (103) between a tube (101) and a tube plate (102) is provided with a main unit (41) that has a circular-column portion (41A), which is inserted into the tube (101), and a flange portion (41B), which is pressed against the tube plate (102), and that is rotatable with respect to the welded portion (103); a probe (42) that is disposed inside the main unit (41), that can be moved close to and away from the welded portion (103), and that detects a defect in the welded portion (103); and a pressing portion (44) that presses the probe (42) toward the welded portion.

Abstract: A system includes at least one strip of ferromagnetic material and a plurality of pulsing/receiving coil circuits. The at least one strip of ferromagnetic material is induced with a bias magnetic field and is coupled to a surface of a structure under test. The plurality of pulsing/receiving coil circuits are aligned with a surface of the at least one strip of the ferromagnetic material. The plurality of pulsing/receiving coil circuits are individually controllable by a number of channels to excite guided waves in the structure under test using at least one of active phased-array focusing or synthetic phased-array focusing of the guided waves.

Abstract: A method for integrating a measurement device for use in measuring a machine component includes providing a coordinate measuring machine (CMM) and combining eddy current (EC) capabilities and CMM capabilities to form an inspection probe. The method further includes installing the inspection probe on the CMM so that the inspection probe measures external boundaries of the machine component with the CMM capabilities and substantially simultaneously measures at least one of internal boundaries, internal defects, surface defects, and material properties of the machine component with the EC capabilities, which are directly linked to actual component dimensional information provided by CMM. The inspection data can be simultaneously linked to and/or displayed with a CAD model to enable a direct comparison between the inspection data and the nominal requirements specified on the CAD model.

Abstract: An eddy current system and method enables detection of sub-surface damage in a cylindrical object. The invention incorporates a dual frequency, orthogonally wound eddy current probe mounted on a stepper motor-controlled scanning system. The system is designed to inspect for outer surface damage from the interior of the cylindrical object.

Abstract: A wire rope flaw detector comprises a back yoke and excitation permanent magnets, which form a main magnetic path in a predetermined section of a wire rope in the axial direction; a magnetic path member arranged in the predetermined section to be magnetically insulated from the back yoke and the permanent magnets and making the leakage flux generated from a damaged part of wire rope detour to the outside of the wire rope; and a detection coil wound around the magnetic path member for detecting leakage flux. The amount of leakage flux can be increased by providing the magnetic path member and since the windable area of the detection coil is increased, the number of turns of detection coil can be increased.

Abstract: This eddy current imaging method includes the steps of: positioning (72), in the vicinity of a large inspection region, elements for the measurement of a surface magnetic field, generating (74, 92) a global exciting magnetic field over the observation inspection region, measuring (76, 84) a resultant magnetic field at the surface, in the form of images, processing (90) the images. The generating step (74, 82) involves generating a set of at least two exciting magnetic field waveforms; the measuring step (76, 86) involves measuring a set of configurations of the resultant magnetic field in the form of images; the step (90) of treating the images by combining them allows defects to be detected and the position and the nature thereof to be determined.

Abstract: A sensor measures the interaction of an applied magnetic field to an iron sample surface to determine whether the sample surface has been graphitized. The sensor measures the magnetic force resulting from the interaction or the magnetic flux density to determine the content of magnetic material in localized regions of the sample surface. The sensor includes a cantilever beam with a strain gauge for measuring magnetic force. Alternatively, the sensor includes a magnetic flux density sensor to measure magnetic flux density.

Abstract: The invention relates to a non-destructive inspection method using eddy currents for detecting flaws in a metal structure (3) by means of an array (5) of coils attached to a surface (31) of said structure (3) comprising activation of the coils, measurement of the electrical signals representative of the eddy currents, and evaluation over time of a variation in the electrical signal of each of the coils (511-536) by taking as a reference an edge effect corresponding to a specific electrical signal emanating from at least one coil installed at the edge of the surface, the level of said variation being indicative of the presence of the flaws in the structure.

Abstract: An inspection system for detecting flaws on a moving metal (e.g., steel) bar coordinates the operation of an eddy current testing (ECT)-based flaw detection apparatus and an imaging-based flaw detection apparatus. The ECT-based flaw detection apparatus and the imaging-based flaw detection apparatus are disposed along a movement path in a predetermined relationship with each other, for example, as a predetermined fixed offset distance therebetween. A synchronizing mechanism synchronizes the output data streams from the two flaw detection apparatuses based on the predetermined relationship, so as to align the data streams as function of the axial position on the metal bar. A processing unit is configured to process the synchronized data streams for the detection of flaws, which are then also synchronized (axial position).

Abstract: An enhanced wireless eddy current probe is disclosed which has means to wirelessly couple to a non-destructive inspection (NDI) system situated some distance away from an inspection point on a material under inspection. The disclosed enhanced wireless eddy current probe provides means for executing advanced functions necessary for a complex eddy current inspection operation. These functions include, but are not limited to, storing, loading, and executing a predetermined firing sequence on an array of coil elements, probe balancing, probe calibration, and providing bibliographic information specific to said probe to a wirelessly coupled NDI system.

Abstract: An apparatus, system, and method are disclosed for scanning metallic surfaces. The apparatus, in one embodiment, comprises a shaft and a rotating member. The rotating member may comprise a sensing end and a featuring engaging element. The shaft further comprises a first coupling element and the rotating member further comprises a second coupling element. The first coupling element may be coupleable with the second coupling element such that the rotating member is pivotable at any three-dimensional angle with respect to the shaft to orient the sensing surface parallel to the scanned surface.

Abstract: A magnetic induction tomography system and method for studying the electromagnetic properties of an object includes generator coils adapted for generating a primary magnetic field, and sensor coils adapted for sensing a secondary magnetic field. Actuators provide relative movement between the generator coils and/or the sensor coils on the one hand and the object to be studied on the other hand.

Abstract: An apparatus is provided for sensing anomalies in a long electrically conductive object to be inspected. The object may be a pipeline, or other hollow tube. The apparatus may have a magnetic field generator, and an array of sensors spaced about the field generator. As relative motion in the longitudinal direction occurs between the apparatus and the object to be inspected, the moving magnetic field, or flux, passed from the field generator into the object to be inspected may tend to cause eddy currents to flow in the object. The sensors may be spaced both axially and circumferentially to permit variation in magnetic flux, or eddy current divergence, to be sensed as a function of either or both of axial position relative to the wave front of the magnetic field (or, effectively equivalently any other known datum such as the radial plane of the midpoint of the field generator), and circumferential position about the periphery of the apparatus as measured from an angular datum.

Abstract: Disclosed is an improved orthogonal eddy current probe with at least three coils, each of the coils is wound across the two facing sides of an at least six-sided right polygonal prism, such as a hexagonal core. At each time interval, two of the three coils are used as driver coils, being charged simultaneously with electric current driven in coherent directions to induce a combined eddy current and one of the coils is used as a receiver coil to sense the eddy current, with the combined eddy current to be orthogonal to the receiver coil. Each coil alternates to be one of the driver coils or the receiver coil at a predetermined switching sequence and a predetermined switching frequency during consecutive time intervals. The eddy current probe as disclosed provides the advantages of inspecting a test surface for flaws of any flaw orientation with one pass of scan, providing sufficient sensitivity and desirable noise cancellation in all directions.

Abstract: An economical, flexible, magnetostrictive sensor (MsS) probe assembly for use on longitudinal cylindrical structures, for guided-wave, volumetric inspection of the structures is described. The paired flexible plate MsS probes each include a flexible strip of magnetostrictive material that is positioned and/or adhered to the base of a generally flat, flexible, conductor coil assembly, preferably with an elastomeric adhesive. The conductor coil assembly has a core composed of a thin flexible layer of metal and a thin bendable permanent magnet circuit. The flexible core is surrounded by a flat flexible cable (FFC) that is folded and looped over the layers of the core. The exposed conductors at the ends of the FFC are shifted from each other by one conductor spacing and joined together so that the parallel conductors in the FFC form a flat, flexible, continuous coil.

Abstract: An insertion type eddy current flaw detection probe that is capable of more accurately detecting flaws in magnetic tubes is provided. A method for inspecting magnetic tubes for flaws with high accuracy is also provided. A eddy current flaw detection probe comprises a cylindrical yoke (1), a plurality of detection coils (5) disposed around the central portion of the cylindrical yoke in the direction of a cylindrical axis thereof, first and second inner excitation coils (6) disposed on both sides of the plurality of detection coils in the direction of the cylindrical axis, and first and second permanent magnets (3, 4) disposed around the yoke on both sides of the first and second excitation coils in the direction of the cylindrical axis so that the direction of magnetizations thereof are parallel to the radial direction of the yoke and magnetic poles on the cylindrical yoke side thereof are different from each other.

Abstract: An eddy current sensor includes: an excitation coil that applies a predetermined alternating-current excitation signal to a measurement target component; and a detection coil that detects a detection signal, generated by eddy current, from the measurement target component to which the alternating-current excitation signal is applied. The excitation coil has a plurality of solenoid coils. The detection coil is arranged at least on one of two sides of the excitation coil in a direction of a central axis of each solenoid coil. Then, the plurality of solenoid coils are arranged in parallel with one another so that winding directions of the adjacent solenoid coils are opposite to each other.

Abstract: A device for non destructive evaluation of defects in a metallic object (2) by eddy currents, comprises a field emitter (3) for emitting an alternating electromagnetic field at a first frequency fi in the neighborhood of the metallic object (2), and a magnetoresistive sensor (1) for detecting a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in the metallic object (2).

Abstract: It is possible to accurately and stably inspect a scratch in the vicinity of a surface of a cell external case formed by a cylindrical copper plate by using a simple device. The cell external case (1) is rotated around the center axis of the cylindrical shape by a rotation device (2) and a magnetic flux is applied to the rotating cell external case from both ends of a frame (4). When a scratch is present on the cell external case (1), leak magnetic flux is generated which is detected by a magnetic sensor (6) arranged in the vicinity of the cell external case (1) and displayed on a display device (12) via a signal processing device (11). Since the cylindrical cell external case (1) rotates around its center axis, the distance between its surface and the magnetic sensor (6) is not changed and a scratch can be detected stably.

Abstract: An ultrasonic probe that uses the magnetostrictive effect to generate and detect a surface-coupled guided wave for the purpose of inspecting a thick-walled structure for surface defects. A transmitter probe and a receiver probe are especially designed to generate and detect short wavelengths that will couple to only one surface of the plate.

Abstract: Disclosed is a method and an NDT/NDI probe deploying a slit or a flexible joint of probe bending region, preferably between two rows of probe elements to allow free bending between rows of probe elements and along the direction of the rows of elements and to allow two adjacent rows of elements to bend individually along its own natural bending lines perpendicular to the direction of the rows of elements. Also disclosed is the use of protective flexible pads to cover the probe elements and other probe components.

Abstract: A method for inspecting a component having a surface profile that includes a local minima and a local maxima. The method includes positioning an eddy current probe proximate to a surface of the component to generate a first position indication, positioning the eddy current probe proximate to the surface of the component to generate a second position indication that is different than the first position indication, and interpolating between the first and second position indications to determine a profile of a portion of the surface of the component.

Abstract: A wireless, integrated, mountable, portable, battery-operated, non-contact eddy current sensor that provides similar accuracy to 1970's laboratory scale equipment (e.g., a Hewlett-Packard GP4194A Impedance Analyzer) at a fraction of the size and cost.

Abstract: An example inspection probe device includes a sensor assembly configured to induce an eddy current in a component. A probe body houses at least a portion of the sensor assembly such that the portion of the sensor assembly is spaced from a target surface of the component when the probe body is in contact with the component. A controller is used to calculate the location of the target surface relative to the probe body using an eddy current parameter sensed by the sensor assembly.

Abstract: Reference standards or articles having prescribed levels of damage are fabricated by monitoring an electrical property of the article material, mechanically loading the article, and removing the load when a change in electrical properties indicates a prescribed level of damage. The electrical property is measured with an electromagnetic sensor, such as a flexible eddy current sensor, attached to a material surface, which may be between layers of the article material. The damage may be in the form of a fatigue crack or a change in the mechanical stress underneath the sensor. The shape of the article material may be adjusted to concentrate the stress so that the damage initiates under the sensor. Examples adjustments to the article shape include the use of dogbone geometries with thin center sections, reinforcement ribs on the edges of the article, and radius cut-outs in the vicinity of the thin section.

Abstract: An inspection system for detecting a flaw in a part is provided. The inspection system includes a generally C-shaped core having an opening for receiving the part. The system also includes a driver coil wrapped around the core for creating a magnetic field in the opening. The system further includes at least one single element or multiple element eddy current sensor disposed in the opening.

Abstract: A first coil configuration (106) for use with a radio frequency metal detector includes a parallel wound oscillator coil (42) having two planar loops (43, 44). An oscillator excitation voltage (46) is applied simultaneously across both planar loops 43. The amount of induced voltage in two adjacent receiving loops (48, 49) is increased by closely spacing the oscillator coil loop (44) to one receiving loop (49) and by closely spacing the oscillator coil loop (43) to the other receiving loop (48). A second coil configuration (107) is a series aiding oscillator coil (55) having two planar loops (108, 54). The series aiding coil arrangement increases the oscillator current by decreasing the inductance of the oscillator coil (55) when a conductive contaminant (18) crosses the plane of the oscillator coil. The receiving or input coil (60) is formed of two separate loops (56, 57) wound in serial opposition to each other.

Abstract: Methods for reclaiming or reconditioning used tubulars are disclosed. One method includes performing non-visual, non-destructive inspection of used tubulars at a site of a reconditioning or reclaiming plant prior to the used tubulars being cleaned in preparation for reclaiming or reconditioning, followed by sorting the used tubulars based principally on the inspection. The non-visual, non-destructive inspection may be selected from magnetic flux leakage inspection, ultrasonic inspection, Eddy current inspection, acoustic emission inspection, and combinations thereof.

Abstract: A method for detecting a position of a cylinder rod includes depositing a plurality of welds substantially along the length of a steel rod and depositing a corrosion resistant material onto the steel rod by laser cladding. The cylinder rod is then placed proximate to a sensor assembly. One of the cylinder rod and the sensor assembly are moved relative to the other. The sensor assembly detects a change in properties between the steel rod and the welds and generates a corresponding signal. A change in position of the cylinder rod or damage to the cylinder rod can be detected by analyzing the number and strength of the signals.