Title of Invention

A METAL DETECTOR

Abstract

ABSTRACT A metal detector comprising an LC oscillator circuit; sensing means for sensing the variation in the amplitude of the output waveform of the said oscillator circuit whenever the inductor coil thereof is brought near a metallic object; activating means coupled to the sensing means and also to indication means; whereby whenever the sensing means senses a variation in the amplitude of the output wave form of the oscillator circuit the activating means are triggered to activate the indication means.

Full Text

This invention relates to a metal detector. Metal detectors are electronic devices used for detecting the presence of metals. They are used for security checks detecting metallic objects, such as, concealed wiring, in industrial applications, detecting mines in military operations and for other purposes which include internment e.g. in games. In the case of security checks, the important types used are hand held metal detectors and door frame metal detectors. Practical circuits of metal detectors usually comprise three categories described below: BFO (Beat Frequency Oscillator) In this system, a signal with a variable frequency is mixed with another signal having a fixed frequency. The difference (beat) frequency produced by the mixer is in the audible range. If the search head comes close to a metal object, the oscillator producing the variable frequency is de-tuned, resulting in a different tone which the user hears from a loudspeaker. TR / IB (Transmit - Receive / Induction Balance) As indicated by name, method utilizes a transmitter as wee as a receiver. If the metal object comes within the scan range of the search head, the degree of magnetic coupling between the inductors is modified. This change in turn produces a change in the oscillator output signal level. PI (Pulse Induction) In this system, pulses are generated and transmitted. The intensity and shape of the received pulses provide an indication about the presence of metal objects within the scan lance. This invention proposes a different category of metal detector. When a metal object comes close to the inductor (search head) of an LC oscillator, it produces a variation in the amputee of the oscillation, which is effectively employed for any kind of apphcations of metal detection. The advantages of this invention over tike known metal detectors are: 1. Simpler Circuit: In case of the existing detectors they need two oscillators or a transmitter receiver pair. But in the present case a single oscillator suffices and hence the circuit becomes simpler and more compact. 2. Greater Range: The range of any metal detector depends upon the type, size and shape of the coil used. When the metal comes near the coil it wig increase its inductance as the relative permeabiHty of the medium increases. (Here metal acts as the medium). The increased value in the inductance will increase the impedance of the inductor and capacitor of the oscillator . This win decrease the base drive of the transistor of the oscillator circuit and hence the amplitude of the output voltage will decrease. The ac Voltage across the inductor will produce a varying magnetic field. When metal comes in this field it will induce an Since these two effects act cumulatively, the variation produced when metal comes close to the search coil is more in the circuit according to this invention and hence the range is more for a particular size of the co. 3. Small Size: For a particular range, the size of the coil is smaller in the case of the detector proposed herein. The simpler circuit also reduces the size of the detector as a whole. 4. Low Cost'. In view of the simpler circuit and smaller size of the coil, the cost of the detector will be less. Various other features of this invention will be apparent from the following further description thereof The metal detector, according to this invention, comprises an LC oscillator circuit; sensing means for sensing the variation in the ampHtude of the output waveform of the said oscillator circuit whenever the inductor coil thereof is brought near a metallic object; activating means coupled to the sensing means and also to indication means, whereby whenever the sensing means senses a variation in the ampHtude of the output wave form of the oscillator circuit, the activating means are triggered to activate the indication means. One of various possible forms of the sensing and activating means comprise a rectifier circuit for rectifying the output of the oscillator circuit; a filter circuit for smoothening the dc pulse output of the rectifier circuit; a buffer and amplifier circuit for serving as a buffer and for ampHfying the output of the filter circuit; a comparator circuit for comparing the value of the said amplified output with a preset voltage value. The indication means receive the output of the comparator circuit, said means, however, being inactive when the comparator circuit output is zero and being activated by the comparator circuit whenever the inductor coil of the said oscillator circuit is brought near a metallic object to result in. a decrease in the ampHtude of the oscillator output and a corresponding increase in the output value of the comparator circuit. This invention will now be described with reference to the accompanying drawings which illustrate, by way of example, the circuit of one of various possible embodiments of the metal detector proposed herein Fig. 1 illustration a block circuit diagram and Fig.2 illustrating the circuitry in greater detail of there said embodiment. The oscillator circuit O comprises the inductor coil L and the capacitors CI and C2. As amitotic object is brought close to the inductor coil L there is near maximum variation in the oscillator output. Correspondingly, the distortion in the sinusoidal oscillation is near minimum, when the loop gain is near unity. While this can be controlled by varying the collector resistor R2, the current through the inductor L will be affected. To avoid this, the collector resistance R2 is kept constant; however, a resistance RS incorporated in the feed back network of the oscillator can be varied to directly control the loop gain. A rectifier circuit B receives the output of the escalator circuit O. The circuit B rectifies the sinusoidal output of the oscillator to dc pulses and passes on the same to the filter circuit F which smoothens the same. The ac voltage at the oscillator output is thus rectified and filtered to produce a dc voltage. Hence the variation in the amplitude of the ac oscillation will become a perfect dc vermilion. To the output of the filter circuit F is connected a buffer and campfire circuit G A. This circuit reduces the loading effect of the successive components. Amplification of the voltage output of the filter circuit also takes place at this stage. Next is a comparator E for receiving the amplified output and for comparing the value of the said output with a preset voltage value. Indication means such as the LED T and buzzer Y receive the output of the comparator E. These means are activated when the inductor coil L, housed in the search head, is brought near a metallic object ,- since such a situation results in a decrease in the amputee of the oscillator output and a corresponding increase in the output value of the comparator E. When no metal is present the output of the comparator is zero and the indication means are inoperative. The comparator circuit, preferably, incorporates an auto-tuning circuit V. The auto-tuning circuit sets the input to the comparator to zero in the event of a continued presence of a metal object or metal objects e.g. the hinges of door frame so that the indication means are not continuously activated. Such means thus become operative only when there is a supervening influence of an additional metal object being brought within the filed of the inductor coil. In the circuit of the embodiment illustrated the supply is obtained from a single 9 volt dc cell or battery. The oscillator used is a culprits oscillator of frequency 12 KHz. When the metal comes close to the inductor, the amphtude of the ac voltage at the output of the oscillator with decrease. It may be noted that any LC oscillator, designed for any frequency, can be used in the circuit e.g. Hartley, Culprits, Clapp oscillators, among others. The oscillators used can also be of the series feed back type or the shunt feed back type. The terms and expressions in this specification are of description and not of limitation there being no intention in the use of such terms and expressions of excluding any equivalents of the features illustrated and described, but it is understood that various other embodiments of the metal detector proposed herein are possible without departing from the scope and ambit of this invention. Wc Claim: 1. A metal detector comprising an LC oscillator circuit sensing means for sensing the variation in the amphtude of the output waveform of the said oscillator circuit whenever the inductor coil thereof is brought near a metallic object; activating means coupled to the sensing means and also to indication means, whereby whenever the sensing means senses a variation in the amplitude of the output wave form of the oscillator circuit, the activating means are triggered to activate the indication means. 2. A metal detector as claimed in Claim 1 wherein the sensing and activating means comprise a rectifier circuit for rectifying the output of the oscillator circuit; a filter circuit for smoothening the dc pulse output of the rectifier circuit; a buffer and campfire circuit for serving as a buffer and for ampHfying the output of the filter circuit; a comparator circuit for comparing the value of the said amplified output with a preset voltage value, the indication means receiving the output of the comparator circuit and being inactive when the comparator circuit output is zero and being activated by the comparator circuit whenever the inductor coil of the said oscillator circuit is brought near a meteoric object to result in a decrease in the amphtude of the oscillator output and a corresponding increase in the output value of the comparator circuit. 3. A metal detector as claimed in Claim 1 or Claim 2 wherein the oscillator circuit incorporates a series resisor in the feed back network. 4. A metal detector as claimed in Claim 2 or Claim 3 wherein the comparator circuit incorporates an auto-tunning circuit. 5. A metal detector as claimed in any one of the preceding Claims wherein the indication means comprise a buzzer and LED. 6. A metal detector substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

Documents:

2223-mas-1996 abstract.pdf

2223-mas-1996 claims duplicate.pdf

2223-mas-1996 claims.pdf

2223-mas-1996 correspondence others.pdf

2223-mas-1996 correspondence po.pdf

2223-mas-1996 description (complete) duplicate.pdf

2223-mas-1996 description (complete).pdf

2223-mas-1996 drawing.pdf

2223-mas-1996 form-1.pdf

2223-mas-1996 form-26.pdf