Abstract: The equipment for determining the position of a metal pin in a bone comprises a support, a radiating self-induction coil mounted on the support and two receiving self-induction coils mounted on the support. A receiving coil is arranged on either side of the radiating coil but the space between the receiving coils facing the radiating coil remains free. The radiating coil is supplied by an oscillator with a frequency of about 30 kHz. The receiving coils control signaling members according to the phase-shift of signals generated by the receiving coils.

Abstract: Method and apparatus for measuring conductivity of test materials at a measurement temperature and displaying material conductivity at standard temperature by compensating the measured conductivity for conductivity changes due to the measurement-standard temperature differential. This is accomplished by developing a first periodic signal having a period which varies as a direct linear function of material conductivity C.sub.T at the measurement temperature, and a second periodic signal having a period which varies as a function of the product of measured conductivity C.sub.T, the measurement/standard temperature differential .DELTA.T and a conductivity constant .beta. characteristic of non-ferrous metals having a given base metal. Standard conductivity C.sub.S is then determined in accordance with the equation C.sub.S =C.sub.T (1 +.beta.C.sub.T .DELTA.T) as a combined function of the periods of the first and second signals, and is displayed in percent IACS at 20.degree. C.

Abstract: Separation of metallic particles from non-metallic particles of processed solid waste such as garbage to obtain a metallic concentrate, including use of such separation as the mid-step in a process which begins with homogenizing and magnetically treating components of the waste and ends with isolating the metallic concentrate as a high-purity metal fraction, for example, high-purity aluminum.

Abstract: Apparatus is provided for the insertion of a recyclable container into a container receiving station in a manually operated rotatable carrier normally maintained in a non-rotatable condition; and for analysis and initial acceptance or rejection of the container for recycling without mechanical movement apparent to the user. The apparatus includes detecting means adjacent the receiving station to determine the presence of a container comprised of material acceptable for recycling and scanning means for reading code markings on the container. The information read from the code markings on the container may be recorded for subsequent use in determining amounts of deposits to be paid as well as sources of containers. Releasing means are provided for subsequently permitting movement of said rotatable carrier if said container comprises a material acceptable for recycling.

Abstract: A plurality of proximity sensors are disposed in a turbine flow guide to measure flow guide-to-turbine blade tip distance at various points around the flow guide. The sensor signals are detected and the peak-to-peak voltage of the detected signals provides an indication of the desired distance measurement. The signals are also monitored and if ambient operating conditions causes the signals to exceed predetermined positive or negative thresholds, corrective signals are generated so as to bring the detected signal back into an acceptable range.A proximity sensor is also provided to measure axial movement of the rotor to which the blades are attached so that the other sensor signals can be modified as a function of the measured axial position.

Abstract: Eddy current type metal detector for locating and distinguishing between metal objects of different types in the ground or other surroundings. Differentiation is employed to eliminate signals from the ground soil or other material in which the object is located, and objects of a particular type are detected from the phases of signals which they produce.

Abstract: Apparatus for high resolution sensing of the position of a rapidly moving member. The apparatus includes at least two linear variable differential transformers (LVDT's) each having a magnetic core coupled to the movable member. A signal source and a phase shifter provide energizing signals to the primary coil of each LVDT. The energizing signals have the same frequency but are phase shifted relative to each other. The induced secondary voltages of the LVDT's are sampled by synchronous detectors, the outputs of which are combined by an adder into a high resolution output signal. When two LVDT's are utilized, the energizing signals have a relative phase shift of 90.degree. and the effective position sampling rate is doubled. The apparatus can be used to sense the operating frequency of a tunable magnetron.

Abstract: Apparatus is provided for receiving and crushing containers consisting essentially of recyclable material comprising an enclosure having a manually operated rotatable carrier with a first station comprising a pocket in the carrier to receive a container. Means are provided for detecting the presence of a container in the pocket and for detecting the type of material comprising the wall of the container. The apparatus also contains latching means to prevent rotation of the rotatable carrier until a container inserted therein has passed all tests performed in the first station. Means are also provided for measuring the weight of the container and for crushing metallic containers having a predetermined acceptable weight.

Abstract: Apparatus is provided for receiving and crushing containers consisting essentially of recyclable material comprising an enclosure having a manually operated rotatable carrier with one or more stations comprising pockets in the carrier to permit insertion of a container. Means are also provided to detect the presence of a container inserted into the pocket. Shock absorbing means are associated with the rotatable carrier to inhibit damage to the apparatus by rapid or jerking motions during rotation of the carrier to transport an inserted container to one or more subsequent stations for testing and eventual crushing of a container accepted for recycling.

Abstract: Nondestructive method of electromagnetically detecting flaws in a metallic object, comprising: placing an object to be tested for flaws adjacent a sensing head which comprises a portion of a sensor ring made of electrically conductive material and arranged inside an electromagnetic coupling coil excited by a high frequency current; causing the object and the sensing head to move relative to each other so that a flaw in the object causes the impedance of the coil to change; and detecting the change of the impedance.An apparatus for carrying out the method is also provided.

Abstract: Synchronous demodulators demodulate the signal developed in a receive coil coupled to an oscillator driven transmit coil and produce signals that include target pulses when the detector head housing the transmit and receive coils passes over a metallic object (e.g., a target) buried in the ground. The signals are processed to remove ground mineralization information and to determine the "phase angle" of the target, which angle uniquely identifies the detected target. The tangent of the phase angle is determined using either implicit division (sometimes referred to as steepest descent division) or direct division. Regardless of which division technique is used, as the signals are processed, they are continuously filtered to remove ground mineralization information. In addition, the target component of the signals is converted from pulse form into ringing signal form.

Abstract: A method of and apparatus for nondestructive testing of spot welds by producing in a test weld high and low frequency electromagnetic fields to induce eddy currents in the weld zone. The difference between the phase values of the resultant electromagnetic field intensities is indicative of the quality of the weld. The apparatus includes an eddy current transducer, high and low-frequency measuring channels, an indicator, and a controlled commutator for alternate connection of the eddy current transducer to the low-frequency measuring channel or to the high-frequency measuring channel. A memory unit is provided for storing the output signal of the high-frequency measuring channel, and a comparator is provided for comparing the signal stored in the memory unit with the output signal of the low-frequency measuring channel.

Abstract: An inductive displacement transducer is described in which the magnitude of the phase difference between A.C. current and voltage in a coil is dependent upon the axial displacement of a monitored object. The phase difference depends upon the axial position relative to the coil of a short-circuit turn which moves with the monitored object. A magnetic flux concentrator, which may be combined with the short circuit turn, enhances the flux linkage between the short-circuit turn and the coil. In order to sense the height of a vehicle suspension strut, the coil is mounted on a plastics dirt shield, the short-circuit turn is provided by a sleeve plated with an electrically conductive metal and secured to the strut cylinder. Various circuits are described for monitoring the height signals produced by the different suspension struts of a vehicle.

Abstract: Pictorial images of selected volume elements of materials are generated by a contactless, non-destructive, substantially hazard free tomographic technique in which the material is brought within the influence of a relatively low-strength electromagnetic field and subjected to plural preselected frequencies to provide output data which is used to generate an image reflective of the magnetic and/or electric and/or conductance properties of the selected volume elements. This data has a high volume information content which enables accurate and reproducible identification of the material under investigation. The technique has application in the medical diagnostic field such as for example in aiding in the detection of small areas of cancerous tissue present in a larger mass of healthy tissue.

Abstract: A rotational angle detection device having two magnetoresistive sensing elements positioned at a 45.degree. angle with respect to each other senses the rotary position of a permanent magnet. Sine and cosine waves are applied to first and second bridge circuits respectively incorporating the two sensing elements. The resulting bridge circuit outputs are differentially combined to provide an output whose phase relative to either of the input sine or cosine waves is dependent upon the rotational position of the permanent magnet. The phase difference is detected and the rotational position is calculated therefrom.

Abstract: Metal detector apparatus includes a controlled phase responsiveness to allow operation with reduced ground effects while discriminating between different types of metals. A phase relationship is predetermined that will reject unwanted signals from mineralized ground and trash metals and will be responsive to the signals from desired metals. The algebraic relationship between two signals is determined so that only input signals falling within the desired algebraic relationship are used to provide an output signal.

Abstract: An apparatus and method for imaging of structural characteristics in test objects using radiation amenable to coherent signal processing methods. Frequency and phase multiplication of received flaw signals is used to simulate a test wavelength at least one to two orders of magnitude smaller than the actual wavelength. The apparent reduction in wavelength between the illumination and recording radiation performs a frequency translation hologram. The hologram constructed with a high synthetic frequency and flaw phase multiplication is similar to a conventional acoustic hologram construction at the high frequency.

Abstract: Parameters of electrically conductive material at high temperature are measured by generating, by means of a transmitter coil, an electromagnetic alternating field which induces currents in the material and by detecting, by means of a receiver coil, a specific component of the secondary electromagnetic alternating field generated by the induced currents and dependent on the measured parameter, the frequency of the generated electromagnetic alternating field being such that the penetration depth of the field into the material is of the same geometrical order of magnitude as the characteristic geometrical dimension of the system, such as the distance between the receiver coil and the material. The specific component of the secondary electromagnetic alternating field is detected by means of a synchronous detector connected to the receiver coil and controlled by a phase-shifted signal from the transmitter.

Abstract: A vehicle presence detection system capable of detecting multiple entry and exiting comprising an electrical signal generating system comprising a tuned circuit having a magnetic field-producing induction loop for receiving a vehicle in proximity thereto to change the field configuration and corresponding signal resulting therefrom. Signals generated by said loop result from changes in the field of said loop and occur as a result of the entrance or exiting of a vehicle into or away from the proximity of the loop. A first or signal amplifier is used for amplification of the signal received from said loop. A second amplifier responds to positive or negative polarity input from said first amplifier to provide an output in response to a rapidly changing input which will activate a logic gate for sensing the polarity of said second amplifier output signal and producing a gated output signal.

Abstract: A synchronous demodulator subsystem produces a ground balance pulse and a discriminate pulse each time a detector head housing the receive coil and an associated transmit coil passes over a metallic object (e.g., a target) buried in the ground. In one form, a pair of synchronous demodulators produce V.sub.X and V.sub.R signals, each of which includes a pulse each time the detector head passes over a target. The V.sub.X and V.sub.R signals are combined to produce the ground balance and discriminate pulses. Alternatively, a pair of synchronous demodulators are used to directly produce the ground balance and discriminate pulses. The polarity of the discriminate pulse is dependent upon on whether the target is a desirable or an undesirable target. Contrariwise, the polarity of the ground balance pulse is always the same.

Abstract: An eddy current test apparatus and method in which multiple sequential test frequencies are employed. The voltage induced in a sensing coil at said test frequencies are multiplexed and frequency translated to provide a single frequency for processing. The apparatus employs a single channel to process the induced voltages.

Abstract: A method and apparatus for monitoring the state of the surface of a moving body wherein a four-terminal eddy-current coil circuit is connected in a feedback loop of an RC resonator. The eddy-current coil circuit is placed in proximity to the monitored surface of the moving body and connected in the feedback loop of an RC resonator such that a change in the state of the surface of the moving body results in a change in the conductivity thereof, which conductivity change produces a phase displacement changing the output frequency of the RC resonator. During operation, a first data set is obtained from the coil circuit and RC resonator presenting a first runout profile of the body when the monitored state is in an initial state. The first data set is stored in a memory, and a subsequent profile is derived to obtain a second data set indicative of the operational state of the surface of the moving body.

Abstract: A method of and apparatus for determining very accurately the position of a device inside biological tissue comprising a detecting instrument with a probe, which probe generates a small magnetic field which can be disturbed by a magnetically permeable metal in the device inside the tissue when a narrow end of the probe is positioned immediately adjacent to tissue containing the metal, whereby after the metal-carrying device is inserted in the tissue, the end of the probe is used to precisely locate the device by scanning the tissue until the magnetic field is disturbed which causes the instrument to emit a signal.

Abstract: An Eddy Currest Tester for detecting defects in conductive materials employs phase rotation circuitry for eliminating the effects of lift-off signals from the reactive component of the output of an eddy current probe. An operator digitally selects a frequency which is input to the eddy current probe which is adjacent a test material. The signal output from the eddy current probe is amplified and demodulated into reactive and resistive components which are processed by phase rotation circuitry. The phase rotation circuitry is calibrated by applying a lift-off signal thereto while a phase adjust switch is on. When the phase adjust switch is on the reactive signal component is applied to a zero crossing detector which in conjunction with a flip flop controls the counting direction of an counter which addresses a sine-cosine table in a eraseable programable read only memory.

Abstract: An eddy current type of inspection device having input signal coils on cores radially arranged around a center and having outer ends of the cores which rest against a surface of a metal assembly to be tested for defects. The input coils are energized by an AC signal of different phase for each respective coil so that a rotating magnetic field is produced in the assembly being tested. An output sensor coil is mounted at the center of the tester immediately adjacent to such test surface for coupling out a signal induced from the rotating field. A specific embodiment has three coils physically spaced 120.degree. apart, for energizing by a three-phase power source.

Abstract: A filter apparatus (2) is disclosed for filtering out any reoccurring noise (5) present in a defect signal (1) received from a flaw detection device (4) such as an eddy current test probe. Filter circuit means (10) receives the defect signal generated by the flaw detector device and produces an output signal (5) duplicative of any reoccurring noise present therein. A differential amplifier (50) subtracts the output signal (5) of the filter circuit means from the defect signal (1) generated by the flaw detection device and amplifies the difference. A filtered defect signal (9) void of any reoccurring noise is thereby produced.

Abstract: A magneto-inductive materials tester has probes which, when brought into contact with the material, produce an electrical signal corresponding to properties of the material desired to be tested. Apparatus for evaluating this test signal has at least one device for producing a compensation voltage to eliminate residual or drift error voltages that may be superimposed on the test signal. The compensating device includes a counting pulse generator, at least one pulse counter connected to the generator at least temporarily, and a digital/analog converter connected to the pulse counter output to provide the desired compensation voltage.

Abstract: A circuit for automatically correcting a signal given by a differential pick-up. Said circuit comprises as many corrective channels as there are frequencies in the signal. Each of said channels, comprises a band-pass filter centered to the frequency ascribed to the respective channel, an analyzer for the resistive (X) and reactive (Y) portions of the filtered voltage, two stages for the separate treatment of said two portions, each of said stages comprising a threshold comparator, a counter, a digital-analog converter, and a multiplier for multiplying by a reference-signal, either in phase (as regards portion X), or in phase-quadrature (as regards portion Y), with respect to the pick-up energization. An adder is adapted to reconstitute a corrective voltage to be substracted from the input signal.

Abstract: Digital apparatus for controlling members by eddy currents comprising a sensor constituted by a primary winding and a secondary winding, an exciting sinusoidal voltage generator, a correcting circuit for eliminating the residual voltage supply by the secondary winding of the sensor, a circuit for analyzing the corrected measuring voltage incorporating two samplers and a means for displaying the two signals supplied by the two samplers. The exciting voltage generator is of the digital type and comprises an oscillator, a counter, a memory and a digital-analog converter. The collection circuit comprises a number selection means, a comparator, a counter, a memory and a digital-analog converter. The means for generating control pulses for the samplers comprises a number selection means, a comparator and a digital phase shifter.A particular application is in the field of non-destructive testing.

Abstract: Digitally controlled eddy current test apparatus for detecting flaws, their extent and location. The apparatus provides absolute and differential measurements. The apparatus employs multi-frequencies with the signals from the test coils mixed in a predetermined manner to reduce unwanted signals such as may be introduced by wobble (lift-off) and support plates.

Abstract: An induction balance metal detector circuit includes a receiver coil output which is connected to one input of a unique phase detector circuit. The reference transmitter coil signal is connected to a second input of the phase detector. The phase detector comprises circuitry which provides an output equal to the smaller of the two input signals. The output is doubled and integrated, then compared to a current indicative of the integrated reference signal in a differential amplifier. The output of the differential amplifier is passed through a dead band and used to excite a first indicator in response to phase difference between the inputs to the phase detector indicative of the location of ferrous metals. A second indicator is excited in the event the phase difference indicates the presence of non-ferrous metals. A balance and threshold control is established between the transmitter and the receiver coils in order to adjust the phase and amplitude of the received signal.

Abstract: Casing inspection apparatus and method are disclosed for determining defects and other anomalies in ferromagnetic oil well casing. The inspection apparatus utilizes a pair of coaxial transmitter coils for generating an alternating magnetic field in the casing thereby inducing circumferential currents in the casing wall. These circumferential currents and the magnetic field associated with the currents are perturbed by corrosion, holes, and other anomalies both inside and outside the casing wall. Differential receiver coils are urged in proximity with the casing wall to generate a differential receiver signal in response to such perturbations in the magnetic field. The differential receivers are provided with three coils serially connected and having N, 2N and N winding turns respectively. The windings of the coils are disposed in adjacent relationship to one another and wound in a direction opposite to the adjacent winding.

Abstract: A method of non-destructive quality testing of spot welds resides in that in the test weld nugget region there is produced a primary electromagnetic field inducing eddy currents in that region. Thereupon, the primary electromagnetic field is displaced in a direction parallel to the surface of contact of the welded parts. The phase angle between the resultant and primary magnetic fields is measured at the points characterized by the maximum of the primary electromagnetic field intensity lying on the same line parallel to the surface of the test weld and located in the zone disposed between the points located on either side of the test weld zone wherein the weld nugget is disposed, the weld nugget thickness is evaluated from the difference between the maximum phase value of the intensity measured in the test weld zone wherein the weld nugget is absent and the minimum phase value of the intensity measured in the test weld zone wherein the weld nugget is disposed.

Abstract: An electric sensing circuit, typically a metal detector includes a bridge including inductors. The out of balance condition of the bridge is stored on a capacitor and a balancing signal is fed to balance the bridge in dependence upon the stored value of the charge. The stored value of the charge is only updated in response to relatively slow changes in the out of balance condition of the bridge. Relatively rapid changes in the out of balance condition are used to provide an output signal for alarm or measuring purposes, and the arrangement is thereby so arranged that the balancing signal does not attempt to automatically balance out the output signal giving the alarm or measurement.

Abstract: The type of conductor, its property, and if a metal, its type and cross-sectional area can be obtained from measurements made at different frequencies for the amount of unbalance created in a previously balanced stable coil detection system. The true resistive component is accurately measured and thus reflects only the voltage loss attributable to eddy currents caused by introduction of the test sample to the coil system. This voltage divided by corresponding applied frequency gives a curve which peaks at a frequency dependent upon type of conductor. For a metal this peak frequency is proportional to the samples resistivity divided by its cross-sectional area.

Abstract: A surface defect detecting apparatus having at least one sensor block for movement in respect of rotating cylindrically shaped electrically conductive metallic material to be inspected by scanning an outer portion of the surface of the material. The sensor block has spaced guide wheels at front and rear parts thereof in the spiral scanning direction which rotate in response to the rotation of the material; and a coil holder for supporting a plurality of eddy-current flaw detecting coils arranged longitudinally in a row in the direction of movement, intermediate between pairs of guide wheels at the front and rear parts opposite to the outer surface of said material with a predetermined gap therebetween. A turning wheel at each of the front and rear parts of the coil holder is rotatable with respect to the outer surface of the material so as to maintain the predetermined gap.

Abstract: An electromagnetic detector designed to monitor road traffic, responsive to a modification of an electromagnetic field produced by the presence of a metallic mass in the vicinity thereof, comprises a metal base on which are disposed a transmitting coil creating a field and a receiving coil whose axis deviates from the perpendicular to the base. The receiving coil detects a measuring field created by the currents induced in the base by the transmitted field. The processed signal is the resultant of the signal produced by the presence of a metallic mass in the transmitted field and by the signal due to the measuring field.

Abstract: The apparatus comprises a parallel coil and a capacitance, the coil having an air gap in which an unknown specimen is inserted, a source of alternating voltage applied to the coil-capacitance combination, scanning means for automatically varying the value of the capacitance in steps, thus varying the phase of the voltage across the coil, means for digitizing the phase of the voltage corresponding to each capacitance value, a memory in which those digitized phase values are stored, a reference memory containing digitized phase values for known specimens, and means for comparing the digitized phase values of the unknown specimen with those of known specimens. A digital tolerance control circuit is also provided which accommodates deviations between the reference and the known phase values, up to predetermined amounts, and means are provided for displaying the generic name or grade of the stored phase values which match the phase values of the unknown specimen.

Abstract: An apparatus for inspecting for cracks under installed fasteners and the like using a multiple frequency digital eddy current system. A computer with a plurality of digital arrays generate drive signals which are converted to analog and applied to the test coil, a second set of digital arrays generate balance signals, are adjusted in amplitude and phase and drive signals so that the output of the test coil is zero in a test specimen, output signals from a work piece are processed in the computer giving phase-sensitive crack detection.

Abstract: An electromagnetic flaw detection system and method having automatic dynamic error balancing is disclosed.The system includes means for producing an alternating electromagnetic field and for passing a metallic workpiece to be tested along a path through the field. A differential detection coil is disposed in the field near the workpiece path. The coil responds to nonuniformities in the field caused by workpiece flaws, such as voids, to produce flaw indications. Detection circuitry actuates alarm apparatus in response to the flaw indications to further indicate need for corrective action.Automatic balancing circuitry compensates for an error signal, which the differential coil generates due to minute undesired assymmetries in the coil's mechanical structure. The balancing circuitry includes means for generating a reference signal. The reference signal is continually adjusted, in response to characteristics of the error signal, to exactly counterbalance the error signal in phase and amplitude.

Abstract: The magnitude and sign of the Hall angle of the material of a wafer (13) are measured by a combined capacitive and inductive coupling technique which does not require physically contacting the wafer (13). Contacting methods in common use introduce surface damage or contamination which may reduce the yield of microelectronic circuits on semiconductor wafers and normally in addition require special sample geometries. In this technique an rf signal is applied to a pair of concentric planar electrodes (11,12) adjacent to the wafer (13), thus capacitively coupling a radial rf current into the wafer. A magnetic field applied perpendicular to the wafer produces a circular component of rf current because of the Hall effect. This circular rf current produces an axial rf magnetic field which couples to a pickup coil (15). The pickup signal is amplified and detected to produce an output signal related to the sign and magnitude of the Hall angle of the wafer material.

Abstract: DC magnetizing pulses produce unidirectional flux in the object under test, advantageously into the saturation region thereof. The DC pulses have, or are accompanied by, an AC component for producing eddy currents in the object, and variations in the eddy currents are sensed and used to produce corresponding signals. In a preferred embodiment composite DC pulses having a relatively broad initial value followed by shorter DC pulses are intermittently applied to a primary coil wound around the core of a probe adapted to be inserted into a tube in situ. Small null coils provide an output signal. Quadrature detection and gating means yield outputs corresponding to sufficient saturation of an object, and free from transients due to the intermittent bursts of pulses. Flaw detector and comparator type apparatus are described, including bridge configurations in which the AC component is applied to the bridge.

Abstract: A metal detector uses a transmitting search coil inductively coupled to a receiving coil for detecting the presence of metal objects near the surface of the ground within the field of the coils. An oscillator generates a signal transmitted by the transmit coil, and the signals detected by the receive coil are coupled to the signal inputs of two synchronous demodulators. The output of the oscillator also is applied at different phases to the reference signal inputs of the two synchronous demodulators. The outputs of these demodulators then are passed through low pass and bandpass filters having a low cutoff frequency which is higher than the highest frequency components generated in the synchronous demodulators due to ground effects.

Abstract: A metal detector has a balanced coil arrangement fed by an audio frequency oscillator. In-phase and quadrature components arising when the arrangement is unbalanced by proximate conductive objects are detected, the sense of the quadrature component depending on whether the object is ferromagnetic or non-ferromagnetic. The quadrature component varies the frequency of another audio frequency oscillator above and below a mean value and the in-phase component controls the strength of the audio frequency signal fed from the second oscillator to a transducer such as headphones.

Abstract: A method and apparatus for measuring conductivity of nonferrous metals using eddy current test principles in which conductivity is measured as a direct linear function of the period of the inducing signal at a preselected phase angle between the inducing and induced signals and is displayed directly in percentage of the International Annealed Copper Standard.

Abstract: An authentication method for standardized objects, such as in particular monetary articles, in which an object to be authenticated is introduced into the field located between an emitter and a receiver delivering at least an alternative physical quantity, whose quantitative alteration produced by the presence of said object in said field is measured, the result being compared with a reference value pre-stored in a memory and distinctive of the object to authenticate, whereby, within a tolerance approximation, an authentication criterion is deducted, method characterized by the fact that said alteration is measured at different frequencies, that corresponding alterations are successively measured and then simultaneously compared with said respective reference quantities.

Abstract: The invention concerns a measuring instrument for measuring phase shifts in four terminal electrical networks, where the network whose phase shift is to be measured is connected into the feedback circuit of an R-C phase shifter oscillator, and where the generated frequency depends on the phase shift of the four terminal network and of the RC-phase shifter, and where the circuit is so dimensioned that the following oscillation conditions are satisfied:.phi.(.omega.) + .phi. = 0 (or integral multiples of 2 .pi.)(1).vertline.F (.omega.).vertline..multidot..vertline.H.vertline..multidot.A = 1 (2)where .phi. = phase shift of the network whose phase shift is to be measured.phi.(.omega.) = phase shift of the R-C phase shifterF (.omega.

Abstract: An electromagnetic detector sensitive to the intrusion of a metallic mass (e.g., a road vehicle) into its field of radiated energy comprises, disposed on a metallic support, a transmitter coil and two strongly coupled receiver coils. These receiver coils are located at a distance from the transmitter coil and on opposite sides of the axis thereof. The axis of the transmitter coil and the common axis of the receiver coils are roughly perpendicular to each other. The presence of a metallic mass in the field around the detector induces signals in the receiver coils varying in relative phase according to the extent of the modification of the generated electromagnetic field. The output voltages of the receiver coils are combined to form a resultant square wave comparison with a reference square wave produces an output signal depending on the phase difference thereof.

Abstract: Apparatus and method for identifying the presence in a pre-established location of an object having preselected electrical or magnetic properties. By applying two periodically varying electro-magnetic fields of the same frequency to the pre-established location, the presence of the object is determined by unique perturbations of the electro-magnetic fields produced by the object. The applied electro-magnetic field is generally nonuniform in the pre-established location to maximize the interaction between the object and the electro-magnetic field. A pick-up coil is utilized to sense the resulting field perturbations. A magnetometer can be optionally incorporated in a sensing system to inhibit erroneous indications produced by magnetic objects.

Abstract: A digital eddy current apparatus for sensing and analyzing metallurgical characteristics of an electrically conductive material. A square wave is supplied to a resonating circuit which includes a coil used as a sensing element for the conductive material. The resonating circuit produces a sinusoidal wave which is converted into a rectangular pulse which, when the resonating means is at resonance condition if superimposed upon the square wave pulse it would be positioned in the center of one-half of the square wave. The width of the rectangular pulse is a direct function of the amplitude of the sinusoidal wave and phase shift of the sinusoidal waveform is detected by movement of the rectangular pulse from the center position within the square wave. The width and position of the rectangular pulse is a direct function of the metallurgical characteristics of the conductive material.