Abstract: Defects such as corrosion located on a covered metal component are detected through Eddy current inspection using stationary magnetic fields and scanning sensors arrays. A magnetizing yoke is held fixed to the cover of the component and a magnetic flux sensor is used to scan the area between the legs of the yoke over the cover of the component. Also, a fixed array of sensors can be provided between the legs of the yoke for automatic multiplexing of the sensors and sampling in sequence of signal responses picked up by sensors in the sensor array. Additionally, a stationary magnetic field arrangement is provided by a plurality of probes aligned adjacent and parallel with each other. A fixed array of magnetic flux sensors are arranged between the legs of the aligned probes for scanning for defects. This arrangement of fixed sesnsors can be used for underwater use and sealed by a sealing compound.

Abstract: A detection and processing circuitry of a magnetostriction type torque sensor is disclosed. An alternating current voltage signal Vo is generated and output from an oscillator. A shift circuit shifts a phase of the AC voltage waveform of the oscillator by 1/4 period and outputs the phase shifted voltage to a detector. The detector modulates the inverted voltage signal Va on the basis of the phase shifted voltage signal and an integrator integrates the detector output voltage in a time range from (.alpha.-.pi./2) to (.alpha.-3.pi./2). Thus, the output voltage signal Vd from the integrator indicates a resistance value of an imaginary part, i.e., inductance of the impedance of the detection coil of a magnetostrictive shaft of the torque sensor which is not dependent on the change in a temperature.

Abstract: A circuit for detecting and processing a signal from a sensing device such as a torque sensor is disclosed in which a first peak hold circuit detects a first peak value of an output AC signal of an oscillator and a second peak hold circuit detects the peak value of an output signal of a differential amplifier. The differential amplifier is connected across an output end of a bridge circuit for detecting a change in the output voltage from the torque sensor. A sign of a variable in the torque sensor is determined according to the first and second hold values derived from the first and second hold circuits. The magnitude of the variable of the torque sensor is detected according to the magnitude of the second peak value of the second hold circuit.

Abstract: An eddy-current apparatus for measuring the conductivity of a conductive material and for reducing the influence of lift-off on conductivity measurements is provided. The apparatus includes a probe for inducing an eddy-current in a conductive material and a digital LCR meter for measuring the impedance of the probe when it is placed near the conductive material. A digital processor uses calibration impedance data obtained from a series of reference materials and an impedance measurement for a test material to produce a conductivity value independent of lift-off between the probe and the test material.

Abstract: A method and apparatus for nondestructively detecting a flaw such as a hairline crack or defect within a conductive object to be inspected. Surface flaws can be distinguished from deep flaws. The pulse duration of the pulse voltage induced in the secondary coil of a sensor is measured at two separate time instances. At one time instance, the pulse duration is affected by variations in the distance between the sensor and the object but is not affected by the flaw. At the other time instance the pulse duration is affected by said variations and also by the flaw. The effect of said variations is cancelled, using these two measured pulse durations. Only a signal correctly indicating the flaw is extracted to detect the presence of the flaw.

Abstract: The present invention relates to a flaw detector for metal material for maintaining and managing pipelines using a remote field eddy current sensor. An AC signal having a constant level and shifted by predetermined phase angle is added to signals received in receiving coils of the remote field eddy current sensor to obtain accurate phase detection in a normal, nonflawed portion of the pipeline to be tested (e.g. uniform thickness, no holes, etc.). By preventing generation of abnormal flaw data, precise diagnosis of the pipeline can be performed. An AGC circuit may be used to maintain at a constant level of received signal. Two sets of spaced apart receiving coils are spaced from an exciting coil a distance greater than a predetermined distance (about two time the pipeline diameter). The set of receiving coils closest to the exciting coil has a greater number of coil turns than the other set.

Abstract: A device to detect the presence of particles, principally metallic particles, in fluid lines. The fluid to be sampled flows through a sensor coil which is flanked upstream and downstream by two field coils. The field coils are driven by a high frequency signal derived from a crystal oscillator and the upstream and downstream sections are wound in opposite directions such that the magnetic field from each section is cancelled out in the plane of the sense coil. When a ferromagnetic or conductive particle passes through the winding structure, the coupling in the sense coil is disturbed as the particle first interacts with the field in the upstream section of the field coil and later with the field in the downstream section of the field coil as it passes through the Sensor Coil Assembly (SCA). A characteristic signature is generated in the sense coil which results from the vector summation of these transient imbalance signals with the steady-state signal from the opposed field coils.

Abstract: An eddy current imaging system includes an eddy current sensing coil positioned proximate a workpiece for scanning the workpiece to produce a signal indicative of the integrity of the workpiece for regions in an x,y array of data. Such a signal may be a complex impedance signal representative of the amplitude and/or the phase of the signal. Two data processing techniques are disclosed for enhancing the display of a flay in the workpiece. The first includes the step of calculating a spatial derivative of the amplitude and/or phase indicative signal from the coil and providing an image of the spatial derivative to produce a C scan of the spatial derivative. The second includes the steps of determining the regions of maximum ascent and/or descent of the signal for regions in the scan and calculating an impedance plane trajectory from the data array of path points for displaying a reconstructed optimum signal independent of path pattern.

Abstract: Monitoring a series of products of the kind in which the presence of foreign bodies in the product being monitored is detected by the characteristic magnetic field phase difference each material exhibits. The signal generated by a satisfactory product having no foreign body in it is used as the datum for judging whether the next subsequent product is free from a foreign body or not.

Abstract: An improved AC susceptometer and methodology for its use which is particularly suitable for the characterization of the properties of superconducting materials. Added to the circuitry of a conventional AC susceptometer is frequency domain analytical equipment for measuring the induced magnetic response. The addition of frequency domain measuring equipment permits the determination of the harmonic components of the induced magnetic response. The measurement of the harmonic components of the response also provides novel methodology for studying the phenomena of flux penetration, flux pinning and movement and permits the measurement of parameters such as lower critical field, critical temperatures, and the irreversibility line.

Abstract: An improved AC susceptometer and methodology for its use which is particularly suitable for the characterization of the properties of superconducting materials. Added to the circuitry of a conventional AC susceptometer is frequency domain analytical equipment for measuring the induced magnetic response. The addition of frequency domain measuring equipment permits the determination of the harmonic components of the induced magnetic response. The measurement of the harmonic components of the response also provides novel methodology for studying the phenomena of flux penetration, flux pinning and movement and permits the measurement of parameters such as lower critical field, critical temperatures, and the irreversibility line.

Abstract: A device is disclosed for use with heterodyne metal detectors to eliminate the indication of the detected presence of certain specific metals to the operator. A specially tuned circuit rotated adjacent the detector interacts with the circuitry of the detector to eliminate the audio and visual response of the detector to certain predetermined undesired metal objects. The device includes two coils, one of low impedance and high resistance and the other of high impedance and low resistance.

Abstract: A magnetic core, for use in fluxgate magnetometers or gradiometers, for example, comprises a pair of coaxial coils constituted by strips of permeable magnetic material oppositely wound on a non-magnetic cylindrical tube. At each of their opposite ends, the coils terminate in planes perpendicular to the length of the tube. In the manufacture of a core, the strips at each end of the coils may be joined to each other to form a lap extending along the length of the tube. In one embodiment, strip portions extending beyond the respective planes, including a portion of each lap, are cut off and removed. In another embodiment, the entire lap at each end is removed after the strips at each end are welded to each other. By virtue of the core construction, the signal-to-noise ratio of saturable core measuring devices using the cores is substantially improved.

Abstract: An apparatus for rotating a spherical object, which includes suspension means for supporting a spherical object in a fixed position relative to its center. The suspension means holds the sphere by simultaneously contacting its surface with contacting elements at at least four points spatially independent of each other on the surface. Each of the contact elements is of regular shapes, such as, spheres, cylinders, cones or other shapes having symmetry about an axis of rotation. At least one of the contact elements is rotationally driven by a rotating drive means. The rotating drive means is rotated at a fixed or time-varying speed to impart controlled movement to the sphere about an axis through its center. The speeds or time-varying functions thereof cause the resultant controlled rotational motion of the sphere by manipulating an axis of rotation through its center.

Abstract: The present invention relates to a flaw detector for metal material for maintaining and managing pipelines using a remote field eddy current sensor. An AC signal having a constant level and shifted by predetermined phase angle is added to signals received in receiving coils of the remote field eddy current sensor to obtain accurate phase detection in a normal, nonflawed portion of the pipeline to be tested (e.g. uniform thickness, no holes, etc.). By preventing generation of abnormal flaw data, precise diagnosis of the pipeline can be performed. An AGC circuit may be used to maintain at a constant level of received signal. Two sets of spaced apart receiving coils are spaced from an exciting coil a distance greater than a predetermined distance (about two times the pipeline diameter). The set of receiving coils closest to the exciting coil has a greater number of coil turns than the other set.

Abstract: Method and apparatus for determining the nature or property of a non-metallic material in which the material is passed through a time varying magnetic field. The signals which are induced in the sensing coil for the magnetic field are processed for measurement. Although the material is non-metallic, it may have sufficient conductivity at the frequencies employed to cause the flow of eddy-currents to generate a measurable signal. The method of the invention also provides for measurement of permittivity or diamagnetic properties of the material which may also give rise to small measurable signals.

Abstract: A non-destructive eddy current test device using additive flux/substractive flux switching includes a generator supplying a sinusoidal signal, two power amplifiers of equal and opposite gain, a changeover relay, two impedances and two coils forming a Wheatstone bridge, an amplifier controlled by a binary signal to add or subtract and two synchronous detectors for supplying signals representing the real part and the imaginary part of the difference between the complex impedances of the coils. A control device supplies to the changeover relay and to the amplifier a control signal for obtaining additive or subtractive fluxes in the two coils and for respectively subtracting or adding the two output voltages of the Wheatstone bridge. The device is applicable to the non-destructive testing of metal parts.

Abstract: The invention discloses an inspection system for detecting near surface flaws or defects in conductors using nondestructive eddy current testing suitable for industrial application. The system provides a method and apparatus for acquiring real time, synchronized, discrete eddy current measurement signals from a plurality of sufficiently disposed, spatially correlated eddy current probes then processing and formatting said measurement signals automatically over parallel data channels to accommodate digital processing techniques in order to produce on eddy current image. Utilizing digital image processing provides a capability for improving flaw detection limits while simultaneously enhancing image resolution.

Abstract: A highly sensitive magnetic field detecting system includes a DC superconducting quantum interference device (DC SQUID) for detecting a magnetic field to produce a signal voltage, and a flux locked loop (F.L.L.) circuit device for driving the DC SQUID. A phase detecting circuit forming part of the F.L.L. circuit device is composed of two phase detecting circuits having different phases to be detected. By adding the individual output signals of the two phase detecting circuits, the outputs can be unvaried at a phase difference equal to that of the detected phases of the two detecting circuits. The added signal is also unvaried with respect to the phase variations of the detected signal so that the metering procedures can be simplified without any necessity for phase adjustment. Further, the output signals obtained can be stable relative to the phase drifts of the signal to be transmitted from the DC SQUID to improve the metering precision.

Abstract: An embodiment comprises a generator constituted by a quartz crystal oscillator and programmable frequency dividers; a power amplifier providing a periodic excitation signal, of squarewave form; two coils electromagnetically coupled to a metal part to be tested; and measuring means comprising two synchronous detectors for measuring the real part and the imaginary part of the difference between the complex impedances of the coils. The frequency stability of the generator effects an improvement in the accuracy of the impedance measurements, when these measurements are made at different frequencies. For application to non-destructive testing of metal parts by eddy currents, at different frequencies.

Abstract: An eddy current flaw detection apparatus, having: a pair of detecting coils disposed in a separated manner along a transit path of a conductor and coaxial with the conductor, magnetic fields generated by said detecting coils being in anti-phase; an alternating current bridge circuit, two sides of a bridge of which are comprising the detecting coils, and which, when a relative change occurs in inductances of the detecting coils and equilibrium of the bridge is thus disturbed, outputs an abnormality detection signal; a pair of resonance coils, which are disposed between the detecting coils, and coaxial with the detecting coils, and magnetic fields of which are in phase, and which are so set that in a state in which the bridge is in equilibrium, induction currents generated by said resonance coils cancel each other out; and a capacity circuit for the purpose of bringing oscillation frequencies of the detecting coils into resonance, which is connected to the resonance coils.

Abstract: Apparatus for detecting surface flaws in cylindrical articles having central longitudinal axes, including a guide assembly for guiding the cylindrical articles along a displacement path having a central longitudinal axis extending coaxially of the central longitudinal axes of the articles and a detection coil assembly positioned in annular relationship with the displacement path for producing a signal representative of the surface geometry of a cylindrical article which traverses the displacement path.

Abstract: A method and system for determining the depth of an electrically conductive body in a medium having a known conductivity and a known permeability. The method comprises the steps of generating a primary magnetic field having a time-varying field strength, and transmitting that primary magnetic field into the medium. The primary magnetic field induces an electric current in the conductive body, and this electric current generates a secondary magnetic field that also has a time-varying field strength and that propagates outward from the conductive body and through the medium. The method of this invention further includes the steps of sensing the strength of that secondary magnetic field, comparing the primary and secondary magnetic fields to measure the phase difference therebetween, and then calculating the depth of the conductive body in the medium from the detected phase difference.

Abstract: Permeameter suited for measuring permeability of a small, non-magnetic (i.e., the permeability value is close to 1) object. The probe of the permeameter is of the differential transformer type with an exciting coil and two detecting coils placed at both ends of the exciting coil. The difference in the outputs of the detecting coils is analyzed and the effect of the eddy current is effectively eliminated, whereby the signal representing the permeability of the object is extracted from the difference signal by the phase shift analysis. By reducing the distance between the two detecting coils, the influence of the disturbance of the eddy current at the boundary of the object sample is minimized and the permeameter can measure a small object sample without sacrificing accuracy and sensitivity.

Abstract: A variable transformer is disclosed. The transducer includes a uniform pitch winding wound on a hollow bobbin with several continuously variable concentric coils wound over it with layers of insulation in between. The variable coils are wound in accordance with true trigonometric functions by adjusting the pitch between adjacent turns and reversing the direction of winding at each maximum pitch point corresponding to the zero crossover. The variable pitch coils are physically wound in quadrature with respect to a reference point on the bobbin. The transducer is finished by placing a magnetic shield over the bobbin which may include magnetic end washers. Lead wire connections are made and the entire unit is placed in a suitable housing. A moveable core, made from a highly permeable material, is placed inside the unit for slidably engaging the bobbin in its hollow.

Abstract: A diagnostic filter device for monitoring electrically conductive and semiconductive particles entrained by a fluid, composed of:a filter element (4) disposed to receive the fluid and to permit passage of the fluid while retaining particles entrained in the fluid;a component (8) disposed for producing an alternating electromagnetic field which extends across at least one region of the filter element (4); andan eddy current detecting instrument (12) connected to the electromagnetic field producing component (8) for detecting the influence on the field of particles in the filter element region.

Abstract: A tool dullness indicator and method comprising the measurement of eddy currents at a known, controlled frequency directly in the tool by measuring the phase angle shift in a coil when a sample is put into or next to the coil. The phase angle change caused by presence of a workpiece is measured when the tool is sharp to provide a reference reading taking into account the surface hardness and other bulk properties of the workpiece, and arriving at a parameter that is dependent on eddy current changes, such as phase angle shift or other measurable changes related to eddy currents in the part, and then analyzing the workpiece, either while working, or at a set programmed check time to determine when the tool is dull as a function of the change in eddy current characteristics caused by work hardening.

Abstract: An all digital eddy current measurement system is described and illustrated in which an eddy current probe is driven by a driving signal and voltage signals representing the current through and voltage across the probe coil are used to calculate the magnitude and phase angle of a complex probe impedance. Digitization of the voltage signals is controlled by a control logic system which is run separately from but initiated by a microprocessor, the latter of which functions to analyze the acquired data and calculate impedance magnitude and phase angle values therefrom.

Abstract: A foreign matter detector includes an oscillator for generating an oscillating signal, a phase shifter connected to the oscillator and adapted to provide a phase shifted oscillating signal, and an electromagnetic transducer. The transducer includes an excitation coil connected to the oscillator and two interconnected detection coils magnetically coupled to the excitation coil and adapted to produce a differential signal therebetween when an object is passed through the transducer. The detector also includes a first detector adapted to produce a first detected analog signal corresponding to the component of the differential signal in phase with the oscillating signal and a second detector adapted to produce a second detected analog signal corresponding to the component of the differential signal in phase with the phase shifted oscillating signal.

Abstract: The device comprises on the one hand an elongate scale (7) consisting of a succession of magnetic north poles and south poles, and on the other hand a measuring head (9) with at least a first and a second magnetic field sensor (21, 23) which are subjected to the effect of a magnetic field which alternates as a function of time due to the alternating passage of north poles and south poles during a displacement (15) of the scale in its longitudinal direction with respect to the measuring head. The two magnetic field sensors (21, 23) are arranged so that they are always situated opposite one and the same point on the scale (7), the first sensor (21) being sensitive to variations in the magnetic field component extending parallel to the longitudinal direction of the scale (7), the second sensor (23) being sensitive to variations in the component extending parallel to the connecting line (19) between the scale (7) and the measuring head (9).

Abstract: A foreign matter detector includes an oscillator for generating an oscillating signal, a phase shifter connected to the oscillator and adapted to provide a phase shifted oscillating signal, and an electromagnetic transducer. The transducer includes an excitation coil connected to the oscillator and two interconnected detection coils magnetically coupled to the excitation coil and adapted to produce a differential signal therebetween when an object is passed through the transducer. The detector also includes a first detector adapted to produce a first detected analog signal corresponding to the component of the differential signal in phase with the oscillating signal and a second detector adapted to produce a second detected analog signal corresponding to the component of the differential signal in phase with the phase shifted oscillating signal.

Abstract: Several phenomena which occur commonly in graphite composites, as well as other materials, and which affect the impedance of an eddy-current probe have been characterized on the complex impedance plane. An eddy-current method is disclosed by which a target material is stimulated at several frequencies, the impedance of the probe is measured and compared to the impedance of reference material, and the condition of the target material is determined. The method described is capable of finding the signatures of a number of conditions commonly found by eddy-currents, and can be programmed to find others, such as conductive and non-conductive plating thicknesses, and inclusions of material with relative ferrous materials. Because the method determines faults on the normalized impedance plane, it is capable of being used on other conductive materials in addition to graphite composites.

Abstract: An apparatus for measuring the thickness of a coating on a substrate has a bridge circuit including four coils. One coil is placed near the coated substrate and another placed near an uncoated substrate of the same material. An oscillator is connected to the bridge circuit and frequency sweeps, e.g. from 10 KHz to 10 MHz. Phase differences between the voltages induced in the coils are detected to determine conductivity changes with frequency. A method for measuring coating thickness comprises generating variable frequency eddy current in coated and uncoated substrates of the same material and comparing the generated eddy current.

Abstract: A two wire transducer and associated circuit processes an output voltage of the transducer to produce a first signal proportional to the position of a non-contacting movable core and to produce a second signal proportional to the temperature of operation of the transducer. The associated circuit includes an excitation source or current driver which provides a fixed frequency sinusoidal constant current across the transducer coil; and first and second demodulator circuits respectively extract position and temperature signals from the voltage developed across the coil of the transducer device. Thus, both temperature and position information are provided by a single device over a single pair of leads.

Abstract: A phase-difference eddy current imaging system has a bridge circuit including a pair of coils with one or both coils disposed proximate an object that may contain one or more flaws. An oscillator is coupled to the bridge circuit and also to a pair of phase detectors that are coupled to the coils. A differential amplifier is coupled to the phase detectors for providing a comparison signal. A method for eddy current imaging using phase difference detection comprises providing relative movement between an object and at least one of a pair of coils, measuring the phase of the effective reactance in each of the coils, comparing the measured values, and formatting the compared values as a function of spatial position.

Abstract: An eddy current sensor having a coil set with a reference coil and a measng coil, and a sine generator whose output signal is applied to the reference coil and the measuring coil. The output signals of these coils are subtracted from one another by a first subtracter, whose ouput signal is applied to a first modulator and to a second demodulator through a 90 degree phase shifter. The output signal of the sine generator is provided to the first and second demodulators as an additional signal via a phase shifter having an adjustable phase angle. A phase angle is selected such that the interference signals have a specified direction in the formed signal plane. The output signal from the first demodulator is applied through an amplifier having an adjustable amplifying factor (a) to an input of a second subtracter. Similarly, the output signal of the second demodulator is provided through an amplifier having an adjustable amplifying factor (b) to the second subtracter.

Abstract: In order to detect interlamination shorts (8 ) in the case of laminated core stacks, the core stack (1) is magnetized with an auxiliary winding. The iron surface (15) is scanned with a measuring coil arrangement (5) comprising two electrically separated, mechanically interlinked coils (6, 7). The output signals from the coils are compared with each other in a downstream measuring instrument (9); in the case of an interlamination short, the fault current through the short-circuited laminations will induce in the coils different, phase-shifted voltages which make it possible to infer the position and size of the interlamination shorts.

Abstract: A pulsed eddy current testing system for detecting defects in a part. A probe carries an exciting pulse in one coil and receives a measuring signal in a second coil. The measuring signal is sampled at two instants defined by short and long time lags. The two amplitudes obtained are plotted in a representative plane having two reference axes. The point having the signals as coordinates describes a figure eight, when the part has a defect. The size and orientation of the figure gives information on the defect in the part.

Abstract: In a system for detecting stray metal in an article, the article passes through an alternating magnetic field generated from a coil. The magnetic field is detected by coils which generate induced voltages and the difference between the induced voltages is applied to a tuning and phase-shift circuit. An output signal having a phase, generated from the tuning and phase-shift circuit, is detected by a detector and the peak level of the detected output signal is held in a peak hold and comparing circuit. A plurality of the peak levels are stored in a memory circuit and are processed by a CPU to determine the preferred phase of the output signal. The tuning and phase-shift circuit is so switched as to apply the preferred phase to the output signal.

Abstract: A device for accurately detecting and monitoring the position of an object inside biological tissue, particularly an endotracheal tube inside a patient's throat, comprising a circuit which generates an electromagnetic field of limited width and depth, and which is disturbed by the presence of material having a high magnetic permeability such as a metal band on the distal end of such an endotracheal tube, but which is otherwise stable and not affected by temperature or other such factors. Included in the invention is a probe for generating the electromagnetic field, which probe is insensitive to temperature changes.

Abstract: An apparatus for measuring the magnetic content of non-metallic samples includes operating and reference sine wave oscillators of identical frequency, a mutual synchronization loop for said oscillators, and a phase meter connected to the oscillator outputs for measuring the phase shift between their oscillations, when a test sample is placed inside the inductance coil of the operating oscillator. The mutual synchronization loop includes a switchable resistive attenuator for varying the resistance of the mutual synchronization loop according to the magnetic content of the test sample, such that the phase shift between the oscillations of the oscillators, with any value of content within the specified measuring range, is not in excess of 30 deg.

Abstract: This invention relates to a method for nondestructive inspection (NDI) of mposite materials comprising conductive fibers. The depth of broken fiber damage in nonmetal matrix composites such as graphite/epoxy and carbon/carbon materials may be ascertained and eddy current inspection is extended to include thick metal sections by this method. Inspection is possible with access limited to a single surface.

Abstract: This invention relates to a method and device for measuring the resistivity f materials. In particular, the methods measure resistivity in carbon fiber reinforced composites as well as any materials with a relative magnetic permeability of 1. The measurement may be made with access restricted to a single side of the material under test.

Abstract: An eddy current flaw detecting apparatus comprises a pair of detection coils each connected in adjoining arms of flaw detecting bridge circuit, the planes containing the pair of coils are arranged in parallel and inclined to the axis of a fuel clad pipe being examined at an inclination angle between 75.degree. and 85.degree. and an arrangement for rotating the fuel clad pipe relative to the pair of detection coils, when a flaw is detected while moving the fuel clad pipe relative to the pair of detection coils, to obtain a characteristic curve between a flaw signal output and turning angle in order to determine direction of the flaw, such as circumferential and axial, in comparison with characteristic curves between a flaw signal output and turning angle obtained in advance with respect to standard flaw testing pieces.

Abstract: A displacement transducer has a movable slug which varies the mutual inductance of two coil sections. The coil sections are connected in a network which causes a variation of phase shift in a signal from an oscillator on movement of the slug. A phase detector detects this phase shift. The use of a variable phase design overcomes the need for high gain linear amplification and the problem of noise in conventional variable amplitude linear variable displacement transducers. The variable phase shift network obviates the need for balanced amplitude quadrature input signals of one known variable phase transducer, while giving a linear phase shift wihtout the need for a specially shaped slug as in another known variable phase transducer.

Abstract: A walk-through metal detector comprising an array module supporting transmit and receive coil arrays. Surrounding the array module, yet not touching it, is an outer shell protecting the array module from contact. A passageway tube passes through the opening in the array module, and is supported by the outer shell, further isolating the array module from mechanical vibration. The array module has a relatively high resonant frequency; thus, vibration induced signals are higher in frequency than actual target signals and are disregarded by the detection circuitry. The metal detector also includes a bypass chute, mechanically isolated from the array module, in which metallic objects may be placed and transported to the exit of the passageway without setting any alarms. The array module supports a balanced transmit and receive coil array for insensitivity to external electromagnetic interference.

Abstract: A method of monitoring a heating cycle of an induction heating system wherein an inductor encircles a metal workpiece and an alternating current is applied through the inductor from a power supply during the heating cycle. This method comprising the steps of generating an analog signal representative of the voltage across the inductor, as the voltage varies during the heating cycle by changes in the electromagnetic characteristics of the workpiece as the workpiece is being heated; digitizing the voltage representative analog signal; creating a trace of the digitized voltage representative analog signal, with the trace being indicative of the electromagnetic characteristic of the workpiece as sensed by the inductor voltage during the heating cycle; and, comparing the created trace with a preselected pattern.

Abstract: A well logging system and method of the present invention determines the dielectric constant and/or conductivity of earth formations some of which have fluid invasion. The system and method of the present invention includes the transmission of electromagnetic energy into the earth formation from a first location in a borehole in the formations at a frequency which enables the electromagnetic energy to propagate throughout the surrounding earth formations. Electromagnetic energy is received by receivers at three locations in the borehole which are spaced longitudinally from the location of the transmission of the electromagnetic energy. The receivers provide signals representative of the total electromagnetic fields at those locations. The dielectric constant and/or resistivity of the earth formations are determined in accordance with portions of the signals from the three receivers corresponding to secondary electromagnetic fields at the three receiving locations.

Abstract: An apparatus is described for detecting metal objects in an interrogated environment. An alternating magnetic signal is transmitted, and received by a receiving coil. The signal is amplified, and is then synchronously demodulated based on a transmitted frequency. The synchronously demodulated signals are low-pass filtered, and then changes in the amplitude of these low-pass filtered signals are detected. The outputs are cross-correlated and interrograted, and are used to control a controlled phase shifter. The output of the controlled phase shifter is used as a reference input to a first one of the synchronous demodulators, and a shifted transmitted signal is used as a reference to the second synchronous demodulator.

Abstract: For enabling a resolver to have absolute position sensing over a wide displacement range while still obtaining the resolution, accuracy and precision obtainable by operation in an incremental mode, the resolver is provided with a number of terminals for providing offset pitch phase indications as a function of position. A microcomputer is programmed to execute a decoding procedure for reading the offset pitch phase indications and from them computing the absolute position. This method is applicable to linear as well as rotary position sensing. By using multiplexing, digital signal processing and large-scale circuit integration for interfacing the resolver to the microcomputer, the method can achieve absolute position sensing with high reliability and low cost. The offset pitch phase indications are readily provided by inductive coupling between a multiplicity of windings, including a set of offset pitch windings connected to the terminals.