A sampling type of metal detector is described in which an oscillator transmits an oscillator signal current through the transmit coil of the detector to produce an electromagnetic field in the region of a search head. A received signal is induced in the receive coil of the detector when a metal object is present within that portion of the electromagnetic field which is external to the search head. A sampling pulse generator produces sampling pulses, at a time related to the output signal of the oscillator, which are transmitted to a sampling circuit for sampling the received signal. In the preferred embodiment, the received signal is sampled at a time controlled by a phase shift adjustment means to produce an output sample voltage such that any indication due to mineral soils or other background conditions is excluded from the sample.

Sampling metal detector

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). The converter (360) produces an analog output signal which is passed to an output driver circuit which produces a target indication signal at a speaker (284). The digital signal processing provided by the microprocessor (374) includes concurrent ground cancellation and discrimination without the need for operator selection of these functions. The digital output produced by the microprocessor (374), upon detection of an object in the electromagnetic field, can be displayed in digital readouts (474-482).

Metal detector having digital signal processing

An inductively coupled oscillator method for inducing eddy currents in a semiconductor PN junction wafer while irradiating said wafer with pulsed light of selected intensity The oscillator loading due to the pulsed light modulated eddy current losses is monitored and displayed on an oscilloscope in the form of a decay time plot of voltage amplitude, the plot being a function of the pulsed light intensity and the recombination rate of light-induced electrons and holes on each side of the junctions The leakage characteristics of the junctions which are desired to be measured are one of the factors determining said rate Leakage characteristic is made the predominant factor by setting the intensity of the pulsed light to a value which produces a nearly straight line decay time plot on the oscilloscope display The slope of the line then is a measure of the leakage characteristic

Contactless LSI junction leakage testing method

A metal detector for detecting and distinguishing between ferrous and non-ferrous objects in an area which may contain a ferrous, mineralized soil background is disclosed. The hysteresis effect is used to distinguish between ferrous and non-ferrous objects by generating groups of transmit current pulses, each group including a polarizing pulse of one polarity followed by a pair of read pulses of the opposite polarity and less magnitude. The transmit pulses are applied to a transmit coil to induce corresponding receive voltage pulses in a receive coil. A pair of receive pulses corresponding to the read transmit pulses are fed through a low pass filter and thereafter sampled to produce a sample output signal corresponding to the difference between the sampled voltage levels of the pair of receive pulses. Control means are provided to adjust the sampling times such that when the metal detector is placed over an area containing a ferrous, mineralized soil background and no metallic objects, the sample output signal will be a constant D.C. voltage. This eliminates the effect of mineralized soil, which otherwise tends to prevent the detection of metallic objects. Thereafter, as the metal detector is moved around and used to search for metallic objects, it will indicate the presence of a ferrous object by a first polarity sample output signal and will indicate the presence of a non-ferrous object by an opposite polarity sample output signal. Demodulator means, such as a balanced synchronous demodulator, is provided to generate detector outputs corresponding to the ferrous or non-ferrous sample output signals and to nullify the effect of relative motion between the metal detector and the objects being detected.

Ferrous/non-ferrous metal detector using sampling

A transmit-receive type buried metal detector is described employing wide gating pulses to transmit a portion of the received signal through a gate means to an averaging means for reducing any noise in the signal The gating pulses are centered on the received signal waveform at a centering position where a background signal component of such received signal is of minimum amplitude As a result, equal positive and negative polarity portions of the background signal component are transmitted through the gate means during each pulse and are averaged to zero in order to eliminate such background signal from the output voltage of the averaging means The background signal component may be produced by mineralized soil or by undesired metal objects located near the metal object sought to be detected As a result, the mineral soil signal component or the background signal component of the undesired metal objects is eliminated from the output voltage of the averaging capacitor This enables detection of the desired metal object in mineral soil or enables such desired metal object to be discriminated from undesired objects A wide gating pulse greater than about 1/3 cycle and preferably equal to approximately 1/2 cycle of the oscillator signal, is used for improved noise rejection including random noise of high frequency and harmonics of 60 hertz power line frequency which are close to harmonics of the oscillator signal

Wide pulse gated metal detector with improved noise rejection

A sine wave oscillator feeds a square wave generator. Pulse generator means produces 50 percent duty cycle pulses having leading and trailing edges coinciding with alternate edges of the square wave and 25 percent duty cycle pulses having leading and trailing edges coinciding with successive edges of the square wave. Gate generating means produces one series of gating pulses coinciding with the leading or trailing edges of the 50 percent duty cycle pulses and a second series coinciding with edges of the 25 percent duty cycle pulses lying halfway between the edges of the 50 percent duty cycle pulses or halfway between successive pulses. The 50 percent duty cycle pulses drive test coil means whose output is supplied to an amplifier tuned to the PRF thereof, and then to a pair of quadrature detectors supplied with respective series of gating pulses.

Pulse eddy current testing apparatus using pulses having a 50% duty cycle and precise quadrature gates

Pulses having a pulse width equal to one-quarter wavelength at a multiple (including 1) of the fundamental frequency of the pulses are applied to the test coil assembly of a pulse eddy current testing apparatus, and the output signals are supplied to an amplifier tuned to said multiple of the fundamental frequency. The output of the amplifier is supplied to a pair of phase-sensitive detectors which are gated at the leading and trailing edges of the applied pulses to produce quadrature signal components. These components are utilized to produce indications of defects or flaws in the object under test. The test coil assembly is advantageously designed to produce an approximately null output in the absence of defects or flaws. The pulses may have a 25 percent duty cycle with the amplifier tuned to the PRF thereof.

Brooks R

Papers

Pulse eddy current testing apparatus using pulses having a 50% duty cycle and precise quadrature gates

Pulse eddy current testing apparatus using pulses having a 25% duty cycle with gating at pulse edges