Abstract: A wire loop reference transmitter is coupled with the excitation winding and the receiver winding of metal detection apparatus and which can be alternately opened and closed through a switch. In another version the separation of the reference phase transmitter reference signal from other signals is accomplished by periodically turning the reference signal off and on at a predetermined switching frequency. According to a further version this separation is achieved by a phase control demodulator controlled by a switched-on reference signal of a first controllable phase and by a switched-off reference signal with a control signal to obtain a 180 degree second control phase. Still further, control signals of a phase controlled rectifier can be at 90 degrees to the reference signal of the reference phase transmitter.

Abstract: In eddy current testing of coated metal objects for corrosion, test values are arranged into fields corresponding to metal object and coating characteristics permitting direct readout of corrosion existence underneath coating without requiring coating removal.

Abstract: A turbine blade shroud clearance monitoring system is comprised of a plurality of sensors for measuring the clearance between the stationary portion of the turbine and the shroud segments in each turbine blade row. The measurements are averaged by a microprocessor to produce an average clearance value for each shroud segment. The microprocessor then analyzes the average clearance values for each shroud segment to determine when the clearance between a shroud segment and the stationary portion of the turbine is approaching a critical value. Output responsive to the foregoing analysis is then generated.

Abstract: A metal detector for use in locating objects such as coins includes analog and digital circuits, further combined with a microprocessor and stored program routines, to provide enhanced ground exclusion balance, target discrimination, and ease of use. Both discrimination and ground exclusion balance are under microprocessor control. A ground exclusion balancing routine is automated and can automatically track any changes in ground mineralization, changing the ground balance setting whenever required. A plurality of memory registers, each corresponding to a different range of phase angles, can be coded by the user to indicate whether a target producing a signal in that phase angel range is a desirable object. When a target response is detected, its characteristic phase angle is calculated. The memory register corresponding to the calculated phase angle is then checked. If the register contains an ACCEPT code, the metal detector signals the user that an accepted target has been found.

Abstract: A defect-sensing device for a test object, e.g. for detecting cracks in metal bodies with the aid of the eddy-current technique, has improved phase-selective properties by adding at least one product and/or quotient former to the signal processing equipment. The output signal from an ordinary phase-controlled rectifier can be substantially described via sin .alpha. and/or cos.alpha. functions in which indicates the phase position of the control signal in relation to the input signal. By multiplying sin.alpha. by cos .alpha. sin 2.alpha. cos 2.alpha. functions are obtained which in turn can be multiplied to obtain the function sin 4.alpha. corresponding to narrow sensitivity lobes in the transducer's impedance plane, thus permitting effective separation of the various types of vector signal resulting from detectable defects.

Abstract: A well logging apparatus is disclosed which employs slot antennas having a plurality of conductive probes traversing the slot. Also disclosed are a logging apparatus having electrically isolated antenna probes and a logging apparatus with an improved means for generating and applying a reference signal used in phase detection.

Abstract: Various forms of hydraulic cylinders having axial bores formed in their piston rods, and probes projecting axially from the closed end of the cylinder body with two spaced coils thereon are disclosed. The coils are connected into external circuitry which responds to change in inductance of either coil whereby to generate a reference signal upon the occurrence of preselected changing conditions of inductance of either coil caused by movement of that coil into or out of mutual induction with a shorted turn arrangement formed by or carried by the piston. The coils may be the drive coils of a displacement transducer arranged to indicate either end of the stroke of the piston. Alternatively either a supplementary shorted turn or an additional drive coil may be provided to generate an intermediate stroke reference signal. The reference signals may be used in operation of a system for effecting automatic continuous reciprocation of the piston.

Abstract: A method and apparatus for cancelling cyclically recurring noise induced in a magnetic metal detector by movement of ferrous machine parts in the field of the detector provides for periodically sampling the output of the metal detector, delaying each sample for the duration of one cycle of the cyclically recurring noise, and subtracting the samples taken in one cycle from the output of the metal detector during the following cycle. The delay is accomplished by a delay line or shift register, or by storing the samples under microprocessor control. If the noise has a first component occuring at a first rate and a second component occuring at a second rate cancellation of both components may be accomplished.

Abstract: The invention relates to a method and apparatus to detect the superconductive state by measuring the change in the resistance of a sample. A dc magnetic field is imposed on the sample while the temperature is swept. The strength of the magnetic field is held below the critical value and is frequency modulated by the application of an ac field in such a way that total magnetic field is always positive. The resistance of the sample is measured incrementally as the temperature is swept and is phase detected at the modulation frequency. According to this embodiment, only magnetic field dependent changes demonstrating a precipitous drop in resistance identify a composition as superconductive.

Abstract: A system for measuring the hardness of cartridge cases employs an eddy current probe for inducing and sensing eddy currents in each cartridge case. A first component of the sensed signal is utilized in a closed loop system for accurately positioning the probe relative to the cartridge case both in the lift off direction and in the tangential direction, and a second component of the sensed signal is employed as a measure of the hardness. The positioning and measurement are carried out under closed loop microprocessor control facilitating hardness testing on a production line basis.

Abstract: A shaft monitoring system for sensing axial movement of a shaft, especially in a gas turbine engine, comprises a toothed wheel attached to and rotatable with the shaft, and a pair of inductive sensors disposed radially of the toothed wheel for sensing the passage of the teeth as the shaft rotates. In one embodiment, the toothed wheel has two axially-spaced sets of teeth, one set straight cut and the other set diagonal cut, so that the phase difference between the respective outputs of the sensors varies with axial movement of the shaft. In another embodiment, the sensors are positioned at opposite axial ends of the toothed wheel, so that axial movement of the shaft causes one sensor output to increase in amplitude while the other decreases.

Abstract: A signal processing method in an electromagnetic induction test. A test object is subjected to an electromagnetic induction by use of N (N.gtoreq.1) kinds of test frequencies to obtain M (M.ltoreq.2N) kinds of phase detection outputs corresponding to respective states of the test object. The phase detection outputs are then sampled at a plurality of measuring points so that there exists n (n<M) kinds of disturbance factors of which a portion contribute to the electromagnetic induction test. The phase detection outputs have a variance greater than a variance of a target parameter of the test object and contribute to the electromagnetic induction test. A first coordinate axis Z, is determined in a direction in which a variance of a distribution of the group of the measuring points comes to be a maximum in an M-dimensional space formed with the coordinate axes associated with the M kinds of the phase detection outputs. The remaining coordinate axes are likewise determined.

Abstract: Method and apparatus for identifying and classifying steels in which a steel specimen is presented to a coil which receives an excitation current to produce an electrical output dependent on an induction value of the specimen. An excitation current is applied to the coil at each of a plurality of excitation frequencies, and a respective value of the specimen at each of those frequencies is determined. The value of a discriminant function is derived for each of a plurality of possible steel grades, with the function being of the form a.alpha.+b.beta.+c.gamma.. . . where .alpha.,.beta.,.gamma.. . . are attributes of the specimen comprising the induction values and a, b, c . . . are predetermined coefficients reflecting the degree to which the respective attributes are effective to discriminate that possible steel grade from the others.

Abstract: Method and apparatus for non-destructive inspection of flaws in metal materials include exciting coils centered on a longitudinal axis of a cylindrical or columnar metallic material to be examined being arranged to surround the examinee material. A multiphase alternating current of such a relatively low frequency as to cause the exciting coils to generate a rotating magnetic field which rotates about the longitudinal axis of the examinee material is superimposed with a high frequency alternating current having an inspection frequency selected on the basis of the necessary sensitivity for detection of surface flaws of the examinee material. The composite multiphase current is applied to the exciting coils so that an electromagnetic effect induced in the surface of the examinee material by the high frequency alternating current is rotated for shifting circumferentially about the examinee material along with the rotating magnetic field.

Abstract: An inductive displacement sensor includes an oscillator circuit that is characterized by primary and secondary pairs of coils disposed on opposing surfaces of adjacent optical segments. Each coil in both coil pairs is alternately positioned along a respective axes perpendicular to a segment displacement axis. The secondary coils are respectively tuned to frequencies above and below the oscillator circuit resonant frequency. Any motion between segments along the displacement axis induces a detectable change in phase between the current and voltage of the oscillator signal.

Abstract: A coil assembly including primary coils disposed at predetermined locations axially spaced from one another and secondary coils provided in association with these primary coils is fixed at one end of a fluid powered cyliner. A piston rod is slidably disposed through a space in the primary and secondary coils. Rings made of magnetic substance are provided on the piston rod with a predetermined axial interval therebetween. Each of the primary coils is excited with one of reference AC signals which are out of phase with one another and an output signal which is electrically phase-shifted in accordance with the position of the piston rod is obtained as a sum of output signals from the secondary coils. Piston rod position data is obtained by measuring the amount of the phase-shifting and is utilized for positioning control of the cylinder.

Abstract: Typical defects are detected on a product moving relative to detecting means. An alternating magnetic field is generated in order to provoke the circulation of eddy currents on the surface of the product. A signal representative of the variations of the eddy currents is collected by means of a differential sensor and is demodulated by projection along an angle of adjustable phase in order to detect the disturbances introduced by the defects to be detected. This angle of phase is determined so that the projection of the collected signal has a minimum mean value, in order to eliminate disturbances of the collected signal due to repetitive surface irregularities. The component of the collected signal obtained by the projection is converted into digital form, and the digital signal thus obtained is filtered using a digital filter whose characteristics are predetermined as a function of a type of defect to be detected.

Abstract: A magnetic field is produced by way of an emitter placed close to the surface of a product to be controlled and a receiver separate from the emitter is placed, with respect to said emitter, on the one hand, in such a way as to be aligned with the emitter in a direction corresponding to the general orientation of the defects to be detected on the surface of the product, and on the other hand, so as to avoid any substantial direct influence from the eddy currents generated by the emitter when the product surface is defect-free, so that a significant signal is only picked up by the receiver in the case of a deviation of said eddy currents in its direction due to a discontinuity in the surface of the product. The invention finds an application in the detection of surface defects in semi-finished metallurgical products and in particular continuously cast steel products.

Abstract: A method and apparatus for determining the extent of axially extending defects in ferromagnetic elements, such as tubing comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool detects axial defects, such as sucker rod wear during removal of the tubing from the well. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing and the magnitude of the induced fields and the changes are measured to quantify defects in the tubing.

Abstract: A portable transmit/receive induction balance type metal detector having an automatic ground exclusion balance (GEB) feature to facilitate the screening out of the effects of mineralized ground. The circuitry provides a static GEB in a single try and then, if selected by the user, continual automatic GEB on a dynamic basis as the system is used. The circuit includes a pair of quadrature phase detectors, one of whose outputs is approximately in phase with the ground component of the loop return signal. That output is modified, during an air balance of the detector, to remove the residual offsets in the circuit. This modified output in turn is summed with the other phase detector output to produce a phase detection signal that can be adjusted by automatic means to be precisely in quadrature with the ground component of the loop return signal. A controller is provided, with selectable user inputs, to initiate the static and dynamic GEB operations.

Abstract: The sensor is constituted by a magnetic core with pole pieces and by an electric coil wound on the body of said core. With said coil there are associated a sinusoidal current generator and impedance measurement means. A processor determines the thickness from the measured value of said impedance.

Abstract: A probe (10) inductively couples radio-frequency electromagnetic energy into a test piece that is made of a multi-ply graphite-epoxy composite material whose fiber volume, ply count and/or thickness are to be measured. The probe (10) and the test piece form a system characterized by an electrical impedance vector, whose magnitude and phase are determined by the composition of the test piece. A frequency synthesizer (16) drives a compensating circuit (15), which applies the output of the frequency synthesizer (16) to the probe (10). The waveform and frequency of the frequency synthesizer (16) are selected to optimize the amount of energy coupled from the probe (10) into the test piece.

Abstract: A method and apparatus for measuring the thickness of thin metallic layers deposited on a conductive support, wherein the thickness is determined from the measurement of the losses corresponding to the Joule effect, due to the eddy currents which appear when a magnetic circuit excited by an alternating voltage is brought close to the metallic surface.

Abstract: Apparatus for measuring the electrical conductivity of a subject comprises a coil for producing a substantially uniform magnetic field within a predetermined volume and a transport assembly for causing relative movement between the subject and the predetermined volume so as to move at least portions of the subject into and out of the volume. A measuring circuit is coupled with the coil for measuring the electrical conductivity thereacross and a control circuit causes the measuring circuit to measure the electrical conductivity across the coil a plurality of times during movement of the subject relative to the volume so as to obtain a plurality of measurements of the electrical conductivity across the coil with different portions of the subject within the volume.

Abstract: A digital induction resistivity logging tool is disclosed for digitally measuring both the in-phase (R) and quadrature (X) phase components in a receiver signal generated in response to transmitter induced currents flowing in the earth's sub-surface formations. The digital indication tool includes a digital sinewave generator for generating a highly phase stable, low distortion transmitter signal, the frequency of which is selectable from among a plurality of transmitter frequencies. One phase sensitive detector is used alternately to detect both the R and the X phase component signals. Automatic phase compensation is included to periodically compensate for both static and dynamic temperature dependent phase shift errors due to circuits of the tool involved in the component measurements. A floating point analog-to-digital converter capable of handling the wide dynamic range in the detected phase component signals is provided to convert the phase detector output into digital signals.

Abstract: In a metal detector, such as may be used in a bakery, an adjustable phase shifter is provided to adjust the detectors for different products. The adjustable phase shifter is controlled electronically from the output signal so that the arrangement is self-adjusting.

Abstract: Selective non-magentic detection of non-ferrous metallic particles in a mixture of same with ferrous metallic particles and non-metallic particles derived from homogenized and magnetically treated municipal or like waste by a plurality of electronic detectors and separation of a non-ferrous metallic concentrate from said mixture.

Abstract: A method and apparatus for determining the extent of defects in ferromagnetic tubular elements comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool measures tubing average wall thickness, local defects, such as corrosion pitting, and axial defects, such as sucker rod wear during removal of the tubing from the well. Tubing velocity is also measured, and couplings between tubing sections are detected and counted, in order to specify the axial location of defects on each tube, and also provide a profile of the condition of the overall string. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing and the magnitude of the induced fields and the changes are measured to quantify defects in the tubing.

Abstract: A metal detector circuit includes a transmit coil (12) and a receive coil (56) arranged in a balanced induction configuration in an electromagnetic field. The receive signal from the receive coil (56) is input to electronic switches (146, 148) which receive quadrature reference inputs from a phase shift circuit (112). The phase demodulated outputs of the switches (146, 148) are passed through amplifiers and input to an analog-to-digital converter (324) to produce digital signal samples which are transmitted through a bus (326). The bus (326) is connected to random access memory (414, 416) and a read only memory (418) which includes a stored signal processing program. A microprocessor (374) is connected to the bus (376) for receiving the digital signal samples and the stored program from memory (418). The microprocessor (374) executes the stored signal processing program to produce a digital output signal which is transmitted through the bus (326) to a digital-to-analog converter (360).

Abstract: A metal detector circuit of the balanced induction type is utilized to detect hidden metal objects. A receive signal is provided with two fixed concurrent phase shifts relative to the transmit signals. Each of the phase shifted receive signals is synchronously demodulated to produce first and second vector signals. The vector signals are combined in a summation circuit to produce a third vector signal. The first and third vector signals are then selectively combined by an operator to produce a fourth vector signal having a desired vector range between minimum and maximum vector angles. The metal detector circuit of the present invention provides a response for objects having a phase response signal outside the selected phase range. The first and fourth vector signals are each differentiated twice then level detected. The outputs from the level detection process are logic signals which are input to an AND gate.

Abstract: A noncontacting signal and energy transmission device includes a stationary microstation and a moveable microunit, wherein data is transmitted between the microstation and the microunit simultaneously by phase shifted signals and by synchronously switched signals, which also supply power to the microunit.

Abstract: The target signal produced by the receive coil of an induction-balance metal detector is demodulated and filtered in matched V.sub.X and V.sub.R demodulator and filters (11, 13) driven by quadrature versions (TA/TB) of the signal that drives the transmit coil of the detector. The resultant phase quadrature signals--V.sub.X and V.sub.R --are applied to a ramp computer (15) that internally produces a ramp signal whose slope is a function of the magnitude of the sum of the V.sub.X and V.sub.R signals. The ramp computer (15) inludes a comparator that combines the ramp signal with a fixed signal whose magnitude is a function of the magnitude of the V.sub.X signal. The result is a pulse whose width is a function of the phase angle and, thus, the identity of the target that created the V.sub.X and V.sub.R signals. The pulse is integrated (17) and the results stored at a TI voltage whose magnitude is functionally related to the identity of the target.

Abstract: An eddy current testing system is disclosed in which a pair of thin one-layer windings are disposed against a pair of thin plates of high permeability magnetic material, about aligned openings in the plates through which a wire is passed, pits and inclusions in the wire being detected by comparing signals corresponding to the electrical impedances on the two windings. The plates of high permeability magnetic material operate to concentrate magnetic flux in small regions of the wire, to obtain a high resolution.

Abstract: Metal detection apparatus includes an oscillator coil 101 driven by an oscillator 101, which induces in detector coils 103,104 equal and opposite E.M.F.s such that a nominally zero output is produced at terminals D.E. When metal is detected the E.M.F.s in coils 103,4 move out balance to produce a voltage at D.E., which is detected by circuit IC11 and fed to output circuits 111. Drifts in the nominally zero output at D.E. are corrected by combining quadraphase balancing signals on lines 109,110 with the detector coil output by means of amplifier IC10. The combined signal from IC10 is fed to phase sensitive detectors IC1,2 which respectively detect the quadraphase components of the combined signal from IC10. The outputs of IC1,2 are stored by capacitor circuits 112,113 and used to adjust variable capacity diode bridges 106,7 which adjust the amplitude of the balancing signals to maintain a nominally zero output from IC10 when metal is not being detected.

Abstract: A metal cable corrosion monitor comprises a sensing head (14) located about a multi strand cable (1). The sensing head includes a field windings (3) for applying a magnetic field coaxially to a portion of the cable to be monitored. A pick-up coil (2) encircles the portion and detects magnetic flux parallel with the cable and linking with the area of the pick-up coil. The signals from the pick-up coil are processed by a high gain amplifier (8) and phase sensitive detectors (7) and (6), the former phase sensitive detector producing an output in phase with the field winding driving frequency and the latter phase sensitive detector producing a quadruture output relative to the field winding driving frequency. By observation of the amplitude and the phase output parameters it is possible to detect corrosion of a single or multi strand cable. A particular format of field winding is disclosed whereby the effects of induced inter strand currents can be reduced.

Abstract: The invention relates to a multiple coil eddy current probe equipped with a coil balancing device. This probe comprises several coils forming an eddy current probe supplying in each case one signal. Each coil is regulated to overvoltage by a regulating unit, which acts on the frequency of the high frequency oscillator supplying each coil. Application is to the automatic positioning of a welding torch and to the inspection of the weld.

Abstract: An accuracy control subsystem for an object examination (OE) system is disclosed. The OE system is of typical frequency-based type and includes a drive unit for generating an excitation signal of preselected frequency, a transducer for receiving the excitation signal and for applying it to the object and for generating an output signal indicative of interaction of the excitation signal with the object and a detector for receiving the transducer output signal and for generating first and second output signals respectively indicative of components of the transducer output signal at first and second different phases thereof, typically zero degrees and ninety degrees, i.e., in-phase and quadrature, respectively.

Abstract: A method and apparatus for producing a display of eddy current test data from signals produced by an eddy current detector which is supplied with an alternating current signal composed of alternating currents at several different frequencies, the detector signals including a respective signal associated with each current frequency and varying in amplitude and in phase relative to its associated alternating current as the detector is displaced relative to a test body.

Abstract: A metal body is inspected for defects by relatively longitudinally displacing it and a plurality of pairs of coils connected in respective bridge circuits that are excited with alternating current. The coils are also excited with alternating current so that eddy-current variations caused by defects in the body become impedance variations of the coils which in turn become imbalance signals in the respective bridges. The imbalance signals are demodulated to form respective imbalance voltages which are periodically sampled and the levels of the sampled demodulated voltages are stored. The sampled demodulated voltages of each bridge are averaged to produce respective base lines which are compared with the respective stored levels of each sampled voltage for generation of respective error signals corresponding to the differences. These error signals are compared with a first relatively low threshold corresponding to the smallest defect to be detected and with a second relatively high threshold.

Abstract: A method and apparatus for determining the extent of defects in ferromagnetic tubular elements comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool measures tubing average wall thickness, local defects, and axial defects, such as sucker rod wear during removal of the tubing from the well. Tubing velocity is also measured. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing. Signals corresponding to changes in the induced fields are divided into channels for complete circumferential coverage. Phase shift is then determined by a zero crossing technique to quantify axial defects, such as sucker rod wear.

Abstract: An eddy current defect-detecting system for a metal tube comprises a detector including test coils movable axially through the metal tube for detecting as an impedance variation a variation in an eddy current produced due to a defect present in the metal tube, an oscillator for exciting the detector at a high frequency, a phase adjuster responsive to an oscillated output from the oscillator for adjusting the phase of the oscillated output to produce a first control signal, a phase shifter responsive to the first control signal from the phase adjuster for issuing a second control signal which is 90.degree.

Abstract: Metal detector for locating and classifying metal objects of different types in the ground or other surroundings. Automatic compensation is provided for signals produced by magnetic materials in the soil and by misalignment of the coils in the sensing head.

Abstract: This invention relates to a method and apparatus for compensating the output signal of a reflection type eddy current probe so as to minimize the change in the phase shift of the probe output signal that otherwise would occur as a result of a change in the spacing of the probe from the surface of the metal sample being inspected or investigated.

Abstract: A monitoring device for monitoring an interior conduit wall includes a plurality of sliding members each resiliently urged by equal amounts away from the device body toward the conduit wall, so that the device can be used in conduits of different diameters while always remaining substantially centered within the conduit.

Abstract: An electromagnetic detector device embodying a linear voltage differential transformer. A pair of coils of the differential transformer serve as pick-up coils connected in a detector circuit which is phase sensitive and capable of discriminating between signals representing a target metallic material, such as carbon steel particles trapped in a filter element, and signals induced by other metallic material, as well as stray signals including electrical noise. Input signals representing a target metallic material are fed to an operational amplifier circuit including a first non-inverting amplifier and a second inverting amplifier which produce a non-inverted signal and an inverted signal which are fed to an analog switch. The switch is operated as a chopper by a driver signal having a predetermined phase and the same frequency relative to the signal driving an input coil to the transformer. The phase of the driver signal is adjustable to provide phase discrimination.

Abstract: A magnetic proximity sensor for detecting the proximity of a laterally mog magnetized target moving along a translation axis which is parallel to a line joining the targets magnetic poles, including: two ring-core flux-gate magnetometers disposed adjacent to each other with their axes of sensitivity in parallel, with both axes perpendicular to the magnetized target translation axis. The energizing coils of the flux-gate magnetometers are driven by an ac signal at frequency f.sub.o. The signal on the detecting coils in each flux-gate magnetometer is applied to a phase comparator via a bandpass filter tuned to the second harmonic of f.sub.o. When the signals applied to the phase comparator are of opposite phase, then the center plane equidistant from the target's magnetic poles is between the two axes of sensitivity of the magnetometers. When the signals are of the same phase, then the target's center plane is laterally outside of the these two axes.

Abstract: A uniform eddy current is produced in a part and the field is probed in several orthogonal positions by two coils to yield, for each position and for each coil, a signal that manifests the phase difference between the field and the signal producing the current and a signal that manifests the relative strength of the field.

Abstract: A method and an instrument for testing materials using the eddy current principle, at different frequencies depending on the output quantity of a coil system, compensation values are generated being typical of interfering factors. These compensation values are subsequently superposed on the output values in order to remove the influence of the interfering factors on the measuring. In this manner with a favorable setting of the working point of an amplifier charged with the output values, an overmodulation is avoided and the dynamic range of the amplifier optimally utilized. Therefore good measuring results can be obtained even if the percentage of the interfering signals as compared to the signal controlling the detecting of defects, is relatively high.

Abstract: A device for detecting metallic objects in a material flow, comprising as sensing member a scanning coil consisting of a primary coil and, for example, two oppositely connected secondary windings arranged symmetrically around the primary winding so that the voltage induced in the secondary winding becomes zero as long as no conducting objects appear in the scanning area. To eliminate irrelevant signals from conducting objects in the vicinity of the scanning coil but not in the material flow or from weakly conducting objects in the material flow, the primary winding is fed with at least two alternating currents having different frequencies. The corresponding secondary voltages are rectified in phase-controlled rectifiers, and in a calculating circuit the difference is derived between the output of one rectifier voltage and the output of another rectifier multiplied by a constant factor which includes the relation between the frequencies of the corresponding alternating voltages.

Abstract: Metal detection apparatus includes an oscillator coil 101 driven by an oscillator 101, which induces in detector coils 103,104 equal and opposite E.M.F.s such that a nominally zero output is produced at terminals D.E. When metal is detected the E.M.F.s in coils 103,4 move out balance to produce a voltage at D.E., which is detected by circuit IC11 and fed to output circuits 111. Drifts in the nominally zero output at D.E. are corrected by combining quadraphase balancing signals on lines 109,110 with the detector coil output by means of amplifier IC10. The combined signal from IC10 is fed to phase sensitive detectors IC1,2 which respectively detect the quadraphase components of the combined signal from IC10. The outputs of IC1,2 are stored by capacitor circuits 112,113 and used to adjust variable capacity diode bridges 106,7 which adjust the amplitude of the balancing signals to maintain a nominally zero output from IC10 when metal is not being detected.