Metal detector with the principle of receiving transmission. Metal detector transmit-receive. What determines the depth of the search for goals

Transmission-reception metal detector - Theory

The terms "transmit-receive" and "reflected signal" in various search instruments are usually associated with methods such as pulse echo and radar, which is a source of confusion when it comes to metal detectors.

Unlike various types of locators, in metal detectors of this type, both the transmitted signal (radiated) and the received signal (reflected) are continuous, they exist simultaneously and coincide in frequency.

Operating principle

The principle of operation of metal detectors of the “transmission-reception” type is to register a signal reflected (or, as they say, re-emitted) by a metal object (target), see, pp. 225-228. The reflected signal occurs due to the impact on the target of an alternating magnetic field of the transmitting (radiating) coil of the metal detector. Thus, a device of this type implies the presence of at least two coils, one of which is a transmitter and the other is a receiver.

The main fundamental problem that is solved in metal detectors of this type is the choice of the mutual arrangement of the coils, in which the magnetic field of the emitting coil, in the absence of foreign metal objects, induces a zero signal in the receiving coil (or in the system of receiving coils). Thus, it is necessary to prevent the direct impact of the emitting coil on the receiving coil. The appearance of a metal target near the coils will lead to the appearance of a signal in the form of a variable emf. in the take-up coil.

Sensor circuits

At first it may seem that in nature there are only two options for the mutual arrangement of coils, in which there is no direct signal transmission from one coil to another (see Fig. 1a and 16) - coils with perpendicular and with crossing axes.

Rice. 1. Options for the mutual arrangement of the coils of the metal detector sensor according to the "transmission-reception" principle.

A more thorough study of the problem shows that there can be an arbitrarily large number of such different metal detector sensor systems, however, they will contain more complex systems with more than two coils, electrically connected accordingly. For example, Fig. 1c shows a system of one emitting (in the center) and two receiving coils connected in opposite directions according to the signal induced by the emitting coil. Thus, the signal at the output of the receiving coil system is ideally equal to zero, since the emfs induced in the coils are mutually compensated.

Of particular interest are sensor systems with coplanar coils (i.e. located in the same plane). This is explained by the fact that metal detectors are usually used to search for objects in the ground, and it is only possible to bring the sensor closer to the ground surface if its coils are coplanar. In addition, such sensors are usually compact and fit well into protective housings such as "pancake" or "flying saucer".

The main options for the mutual arrangement of coplanar coils are shown in Figs. 2a and 26. In the diagram in Fig. 2a, the mutual arrangement of the coils is chosen so that the total flux of the magnetic induction vector through the surface bounded by the receiving coil is equal to zero. In the scheme of Fig. 26, one of the coils (receiving) is twisted in the form of a "figure eight", so that the total emf, induced on the halves of the turns of the receiving coil, located in one wing of the "figure eight", compensates for a similar total emf. s., induced in the other wing of the G8.

Rice. 2. Coplanar options for the mutual arrangement of the metal detector coils according to the "transmission-reception" principle.

Other various designs of sensors with coplanar coils are possible, for example, Fig. 2c. The receiving coil is located inside the emitting coil. Emf induced in the receiving coil is compensated by a special transformer device that selects part of the signal from the radiating coil.

Practical Considerations

Sensitivity metal detector depends primarily on its sensor. For the considered variants of sensors, the sensitivity is determined by formulas (1.20) and (1.33). With the orientation of the sensor to the object optimal for each case in terms of the roll angle y, it is determined by the same coefficient K 4 and the functions of the normalized coordinates F(X,Y) and G(X,Y). For comparison, in the square X O[-4.4], Y O[-4.4], the modules of these functions are shown as an axonometric set of sections on a logarithmic scale in Fig.12 and Fig.13.

The first thing that catches your eye is the pronounced maxima near the locations of the sensor coils (0,+1) and (0,-1). The maxima of the functions F(X,Y) and G(X,Y) are of no practical interest and, for the convenience of comparing the functions, they are cut off at the level of 0(dB). It is also seen from the figures and from the analysis of the functions F(X,Y) and G(X,Y) that in the specified square the modulus of the function F almost everywhere slightly exceeds the modulus of the function G, except for the most remote points at the corners of the square and except for a narrow area near X=0, where the function F has a "ravine".

The asymptotic behavior of these functions far from the origin can be illustrated for Y=0. It turns out that the modulus of the function F decreases with distance proportionally to x^(-7), and the modulus of the function G decreases proportionally to x^(-6). Unfortunately, the advantage of the G function in terms of sensitivity is manifested only at large distances exceeding the practical range of the metal detector. The same values of modules F and G are obtained at X>>4.25.

Rice. 12. Graph of the function F(X,Y).

Fig.13. Graph of the function G(X,Y).

The “ravine” of the function F is of great practical importance. First, it indicates that the sensor of the system of coils with perpendicular axes has a minimum (theoretically zero) sensitivity to metal objects located on its longitudinal axis. Naturally, many elements of the design of the sensor itself also belong to these items. Consequently, the useless signal reflected from them will be much smaller than that of a crossed coil system sensor. The latter is very important, given that the reflected signal from the metal elements of the sensor itself can exceed the useful signal by several orders of magnitude (due to the proximity of these elements to the sensor coils). It's not that the useless signal from the metal elements of the sensor design is difficult to compensate. The main difficulty lies in the slightest changes in these signals, which are usually caused by thermal and especially mechanical deformations of these elements. These slightest changes may already be comparable to the useful signal, which will lead to incorrect readings or false alarms of the device. Secondly, if a small object has already been detected with the help of a metal detector of a system of coils with perpendicular axes, then the direction of its exact location can be easily “beared” by the zero value of the metal detector signal with the exact orientation of its longitudinal axis to the object (for any roll orientation) . Considering that the area of "capture" of the sensor during the search can be several square meters, the last quality of the system

topics of coils with perpendicular axes are quite useful in practice (less useless excavations).

The next feature of the graphs of the functions F(X,Y) and G(X,Y) is the presence of an annular “crater” of zero sensitivity passing through the centers of the coils (a circle of unit radius centered at the point (0,0)). In practice, this feature allows you to determine the distance to small objects. If it is found that at some finite distance the reflected signal is zero (with optimal roll orientation), then the distance to the object is half the base of the device, that is, the value L / 2.

It should also be noted that the directional patterns in terms of roll angle y for metal detector sensors with different mutual arrangement of coils also differ. Figure 14b shows the radiation pattern of the device with perpendicular axes near the coils, and in Figure 14a - with crossed ones. Obviously, the second diagram is more preferable, as it has fewer roll deadbands and fewer lobes.

In order to evaluate the dependence of the voltage induced in the receiving coil on the parameters of the metal detector and the object, it is necessary to analyze the expression (1.19) for the coefficient K 4. The voltage induced in the receiving coil is proportional to (L / 2) ^ 6. The arguments of the functions F and G are also normalized to the value L/2, the decrease of which occurs with the 6th - 7th power of the distance. Therefore, as a first approximation, other things being equal, the sensitivity of a metal detector does not depend on its base.

Roll patterns of sensors of coil systems:
- with crossed axles (a)
- with perpendicular axes (b).

In order to analyze selectivity metal detector, that is, its ability to distinguish between objects made of various metals or alloys, it is necessary to refer to the expression (1.23). The metal detector can distinguish objects by the phase of the reflected signal. In order for the resolution of the device by type of me

tall was maximum, it is necessary to select the frequency of the signal of the radiating coil accordingly, so that the phase of the signal reflected from the objects is about 45 °. This is the middle of the range of possible changes in the phase of the first term of expression (1.23), and there the steepness of the phase-frequency characteristic is maximum. We consider the second term of expression (1.23) to be zero, since when searching, we are primarily interested in selectivity for non-ferrous metals - non-ferromagnets. Naturally, the optimal choice of signal frequency implies knowledge of the typical size of the intended objects. In almost all foreign industrial metal detectors, the size of a coin is used as such a size. The optimal frequency is:

With a typical coin diameter of 25(mm), its volume is about 10^(-6) (m^3), which, according to formula (1.25), corresponds to an equivalent radius of about 0.6(cm). From here we obtain the optimal frequency value of about 1 (kHz) with a conductivity of the coin material of 20 (n0m × m). In industrial devices, the frequency is usually an order of magnitude higher (for technological reasons).

conclusions

1. According to the author, a system of coils with perpendicular axes is preferable for searching for treasures and relics than a system of coils with crossed axes. Ceteris paribus, the first system has a slightly higher sensitivity. In addition, with its help it is much easier to determine (“bearing”) the exact direction in which the detected object should be searched.

2. The considered systems of coils have an important property that makes it possible to estimate the distance to small objects by zeroing the reflected signal at a distance to the object equal to half the base.

3. Other things being equal (dimensions and number of turns of the coils, sensitivity of the receiving path, current value and its frequency in the emitting coil), the sensitivity of the metal detector according to the “transmission-reception” principle practically does not depend on its base, that is, on the distance between the coils.

A device that allows you to search for metal objects located in a neutral environment, for example, the ground, due to their conductivity is called a metal detector (metal detector). This device allows you to find metal objects in various environments, including in the human body.

Largely due to the development of microelectronics, metal detectors, which are produced by many enterprises around the world, have high reliability and small overall weight characteristics.

Not so long ago, such devices could most often be seen with sappers, but now they are used by rescuers, treasure hunters, public utilities workers when searching for pipes, cables, etc. Moreover, many "treasure hunters" use metal detectors that they assemble with their own hands .

The design and principle of operation of the device

Metal detectors on the market operate on different principles. Many believe that they use the principle of pulsed echo or radar. Their difference from locators lies in the fact that the transmitted and received signals operate constantly and simultaneously, in addition, they operate at the same frequencies.

Devices operating on the principle of "reception-transmission" register the signal reflected (re-radiated) from a metal object. This signal appears due to the impact on a metal object of an alternating magnetic field, which is generated by the metal detector coils. That is, the design of devices of this type provides for the presence of two coils, the first is transmitting, the second is receiving.

Devices of this class have the following advantages:

simplicity of design;
great ability to detect metallic materials.

At the same time, metal detectors of this class have certain disadvantages:

metal detectors can be sensitive to the composition of the soil in which they search for metal objects.
technological difficulties in the production of the product.

In other words, devices of this type must be configured by hand before operation.

Other devices are sometimes referred to as a beat detector. This name comes from the distant past, more precisely from the time when superheterodyne receivers were widely used. Beating is a phenomenon that becomes noticeable when two signals with close frequencies and equal amplitudes are summed. The beating consists in pulsing the amplitude of the summed signal.

The pulse frequency of the signal is equal to the difference in the frequencies of the summed signals. By passing such a signal through a rectifier, it is also called a detector, the so-called difference frequency is isolated.

Such a scheme was used for a long time, but today, it is not used. They were replaced by synchronous detectors, but the term remained in use.

The beat metal detector works using the following principle - it registers the frequency difference from two generator coils. One frequency is stable, the second contains an inductor.

The device is set up by hand so that the generated frequencies match or at least are close. As soon as metal enters the coverage area, the set parameters change and the frequency changes. The frequency difference can be recorded in many ways, ranging from headphones to digital methods.

Devices of this class are characterized by a simple sensor design, low sensitivity to the mineral composition of the soil.

But besides this, during their operation it is necessary to take into account the fact that they have high energy consumption.

Typical design

The structure of the metal detector includes the following components:

The coil is a box-type design, it houses the receiver and transmitter of the signal. Most often, the coil has an elliptical shape and polymers are used for its manufacture. A wire is connected to it, connecting it to the control unit. This wire transmits the signal from the receiver to the control unit. The transmitter generates a signal when metal is detected, which is transmitted to the receiver. The coil is installed on the lower rod.
The metal part on which the coil is fixed and its angle of inclination is adjusted is called the lower rod. Thanks to this solution, a more thorough examination of the surface occurs. There are models in which the lower part can adjust the height of the metal detector and provides a telescopic connection with the rod, which is called the middle one.
The middle shaft is the node located between the lower and upper shafts. Fixing devices are fixed on it, allowing you to adjust the size of the device. on the market you can find models that consist of two rods.
The top bar is usually curved. It resembles the letter S. This form is considered optimal for fixing it on the hand. An armrest, a control unit and a handle are installed on it. The armrest and handle are made of polymeric materials.
The metal detector control unit is required to process the data received from the coil. After the signal is converted, it is sent to headphones or other means of indication. In addition, the control unit is designed to adjust the operating mode of the device. The wire from the coil is connected using a quick-release device.

All devices included in the metal detector are waterproof.

This is the relative simplicity of the design and allows you to make metal detectors with your own hands.

Varieties of metal detectors

The market offers a wide range of metal detectors used in many areas. Below is a list that shows some of the varieties of these devices:

Most modern metal detectors can find metal objects at a depth of up to 2.5 m, special deep products can detect a product at a depth of up to 6 meters.

Operating frequency

The second parameter is the frequency of operation. The thing is that low frequencies allow the metal detector to see to a fairly large depth, but they are not able to see small details. High frequencies allow you to notice small objects, but do not allow viewing the ground to a great depth.

The simplest (budget) models operate at one frequency, models that are classified as average price levels use 2 or more frequencies in operation. There are models that use 28 frequencies when searching.

Modern metal detectors are equipped with such a function as metal discrimination. It allows you to distinguish the type of material located at depth. In this case, when ferrous metal is detected, one sound will sound in the searcher's headphones, and another when non-ferrous metal is detected.

Such devices are referred to as pulse-balanced. They use frequencies from 8 to 15 kHz in their work. Batteries of 9 - 12 V are used as a source.

Devices of this class are able to detect a gold object at a depth of several tens of centimeters, and ferrous metal products at a depth of about 1 meter or more.

But, of course, these parameters depend on the device model.

How to assemble a homemade metal detector with your own hands

There are many models of devices on the market for searching for metal in the ground, walls, etc. Despite its external complexity, making a metal detector with your own hands is not so difficult and almost anyone can do it. As noted above, any metal detector consists of the following key components - a coil, a decoder and a power supply signaling device.

To assemble such a metal detector with your own hands, you need the following set of elements:

controller;
resonator;
capacitors of various types, including film ones;
resistors;
sound emitter;
Voltage regulator.

The simplest do-it-yourself metal detector

The metal detector circuit is not complicated, and you can find it either in the vastness of the global network, or in specialized literature. Above is a list of radio elements that are useful for assembling a metal detector with your own hands at home. A simple metal detector can be assembled with your own hands using a soldering iron or another available method. The main thing at the same time, the parts should not touch the body of the device. To ensure the operation of the assembled metal detector, power supplies of 9-12 volts are used.

To wind the coil, a wire with a cross-sectional diameter of 0.3 mm is used, of course, this will depend on the selected circuit. By the way, the wound coil must be protected from the effects of extraneous radiation. To do this, it is screened with your own hands using ordinary food foil.

To flash the controller, special programs are used, which can also be found on the Internet.

Metal detector without chips

If a novice "treasure hunter" has no desire to get involved with microcircuits, there are schemes without them.

There are simpler circuits based on the use of traditional transistors. Such a device can find metal at a depth of several tens of centimeters.

Deep metal detectors are used to search for metals at great depths. But it is worth noting that they are not cheap and therefore it is quite possible to assemble it with your own hands. But before you start making it, you need to understand how a typical circuit works.

The scheme of a deep metal detector is not the simplest and there are several options for its execution. Before assembling it, it is necessary to prepare the following set of parts and elements:

capacitors of various types - film, ceramic, etc .;
resistors of different ratings;
semiconductors - transistors and diodes.

Nominal parameters, quantity depend on the selected circuit diagram of the device. To assemble the above elements, you will need a soldering iron, a set of tools (screwdriver, pliers, wire cutters, etc.), material for making the board.

The process of assembling a deep metal detector is approximately as follows. First, a control unit is assembled, the basis of which is a printed circuit board. It is made from textolite. Then the assembly scheme is transferred directly to the surface of the finished board. After the drawing has been transferred, the board must be etched. To do this, use a solution that includes hydrogen peroxide, salt, electrolyte.

After the board is etched, holes must be made in it to install the circuit components. After the board has been tinned. The most important step is coming. Do-it-yourself installation and soldering of parts on a prepared board.

To wind the coil with your own hands, use a wire of the PEV brand with a diameter of 0.5 mm. The number of turns and the diameter of the coil depend on the chosen scheme of the deep metal detector.

A little about smartphones

There is an opinion that it is quite possible to make a metal detector from a smartphone. This is not true! Yes, there are applications that install under the Android OS.

But in fact, after installing such an application, he will really be able to find metal objects, but only pre-magnetized ones. He will not be able to search and, moreover, discriminate against metals.

The principle of operation of metal detectors of this type is based on the impact on the object under study of an alternating magnetic field of the transmitting coil and the registration of the signal that appears as a result of the induction of eddy currents in the target. Thus, they belong to the location type devices and must have at least 2 coils - transmitting and receiving.

Both the transmitted and received signals are continuous and coincide in frequency.

The fundamental point for metal detectors of this type is the choice of the location of the coils. They must be located so that in the absence of foreign metal objects, the magnetic field of the emitting coil induces a zero signal in the receiving coil.

Coils that create radiation or receive a signal are made in the form of a certain structure called a search frame. The parallel arrangement of the coils is called coplanar.

Usually, in metal detectors of this type, the search frame is formed by 2 coils located in the same plane and balanced so that when a signal is applied to the previous coil, the receiving coil has a minimum signal. The operating frequency of radiation is from one to several tens of kHz.

Beat metal detectors

A beat is a phenomenon that occurs when two periodic signals with similar frequencies and amplitudes are multiplied. The resulting signal will ripple with a frequency equal to the frequency difference. If a low frequency signal is applied to the speaker, then we will hear a characteristic "gurgling" sound.

The metal detector contains two generators: reference and measuring. The first has a stable frequency, and the second can change frequency when approaching a metal object. Its sensitive element is an inductor made in the form of a search frame.

The signals from the generators are fed to the detector, at the output of which an alternating voltage is emitted with a frequency equal to the difference between the frequencies of the reference and measuring generators. Further, this signal increases in amplitude and enters the light sound indicators.

The presence of metal near the measuring frame leads to a change in the parameters of the surrounding magnetic field and to a change in the frequency of the corresponding generator. A frequency difference arises, which is extracted and used to form a signal.

The greater the mass of metal and the closer the metal object, the more the generator frequencies differ and the higher the frequency of the generator output voltage.

How some modification of metal detectors on beats can be considered metal detectors - frequency meters . They have only a measuring generator. When the measuring frame of the metal detector approaches a metal object, the frequency of the generator changes. Then the length of the period in the absence of metal is subtracted from it.

Single-coil metal detectors of induction type

This metal detector has one coil, which is both emitting and receiving at the same time.

An electromagnetic field is created around the coil, which, having reached a metal object, creates eddy currents in it, which cause a change in the magnetic induction of the field around the coil.

The currents that have arisen in the object change the magnitude of the magnetic induction of the electromagnetic field around the coil. The compensating device maintains a constant current through the coil. Therefore, when the inductance changes, the indicator will work.

Pulse metal detectors

A pulse metal detector consists of a current pulse generator, receiving and emitting coils, a switching device and a signal processing unit. According to the principle of operation - a metal detector of a location type.

With the help of a switching unit, the current generator periodically generates short current pulses that enter the radiating coil, which creates pulses of electromagnetic radiation. When this radiation is exposed to a metal object, a damped current pulse arises in the latter and persists for some time. This current creates radiation from a metal object, which induces a current in the coil of the measuring frame. By the magnitude of the induced signal, one can judge the presence or absence of conductive objects near the measuring frame.

The main problem of metal detectors of this type is to separate weak secondary radiation from much more powerful radiation.

Most pulse-type metal detectors have a low repetition rate of the current pulse applied to the emitting coil.

Magnetometers

For magnetically sensitive metal detectors, sensitivity is usually denoted by the value of the magnetic field induction, which the device is able to register. Sensitivity is usually measured in nanoteslas.

In addition to sensitivity, a resolution is used to determine the qualities of a magnetometer, which determines the minimum difference in induction.

Devices whose operation principle is based on the use of the nonlinear properties of ferromagnetic materials have become widespread.

Sensing elements that implement this principle are called fluxgates .

A typical magnetometer design includes a rod with a battery pack and an electronic unit placed on it, as well as a fluxgate transducer on an axis perpendicular to the rod.

Before use, the device is pre-calibrated to compensate for the effect of the Earth's field in the absence of ferromagnetic test objects.

There are magnetometers that work on other physical principles. So, known quantum devices based on the effect of nuclear magnetic resonance and the Zeeman effect, with optical pumping. They have great sensitivity.

Hand held metal detectors

They are not large and heavy. During the search, they are manually moved along the control object.

The ability of an object to perceive metal objects is determined by its sensitivity. Hand-held metal detectors can detect an object the size of a small coin from a distance of 5-10 to several tens of centimeters.

Sensitivity depends on the orientation of the metal detector frame relative to the test object. It is recommended to draw a search frame along the control object several times at different angles.

Examples of handheld metal detectors:

selective metal detector AKA 7215 :

The tone of the alarm signal depends on the type of metal detected

It has a potentiometer for smooth sensitivity adjustment, as well as a switch - ferrous and non-ferrous metals

Continuous operation time from a fresh 9V battery - at least 40 hours

Weight 280 g.

GARRETT Handheld Metal Detector:

The presence of a switch to reduce sensitivity

Automatic control of the degree of discharge of the battery

Alarm indication - sound and LED

Impact-resistant housing

Headphone/battery jack

Satisfies hygiene certificates

Time of continuous work - up to 80 hours

The developments of recent years are characterized by an increase in the "electronic complexity" of devices. They are equipped with microprocessors, displays, etc. All this allows you to expand the functionality of the devices.

The displays show information about the detected object and its conductivity.

Metal detectors are often needed, for example, when searching for lost metal objects or pipes, cables, tanks buried underground. Metal detectors are also associated with treasure hunters and miners 🙂

Types of metal detectors

The most complex and sensitive, but also the most expensive, are built on the principle transmission / reception of a radio signal. The complexity and high cost lies not only in the abundance of electronic components of the circuit, but also in the need for qualified circuit tuning.

There are several more types according to different principles: induction, frequency meters, pulse, generation attenuation, beat method, pulse induction, resonance disruption ...

The meaning of all metal detectors in one: change in the frequency of the generator when a metal object enters the coil field. This change in frequency, as a rule, is very insignificant, and the second essence of this or that scheme is to catch this slightest change and convert it into something.

A diagram of a simple metal detector is presented below.

Having made such a metal detector compact and taking it with you on a trip to the sea, it will help you when searching for a gold jewelry lost by you or relatives on the beach. But what is closer to you is the search for hidden wiring in the wall or some kind of carnation. We will consider such a simple and proven metal detector circuit for such purposes here in order to assemble it with our own hands.

Scheme of a simple metal detector on transistors

The scheme of this simple metal detector, which can be repeated by an amateur without much experience.

Metal detector characteristics:

Detection of a coin - 10-15 cm (with a good adjustment, some grab it, which is up to 50 cm!);
Steel scissors - 20-25 cm;
Large items - 1-1.5 meters.

The circuit consists of two high-frequency generators, each on one transistor (VT1 and VT2). The frequency of the left oscillator (VT1) changes when metal enters the L1 field, while the frequency of the right oscillator (VT2) remains unchanged. The ratings of the elements of both generators are selected so that the frequencies of the generators differ only slightly. The generators operate at a radio frequency (more than 100 kHz), and such a sound is not heard either by our ear or reproduced by a speaker. But their small difference, for example, 160 kHz and 161 kHz is equal to 1 kHz - these are oscillations already audible to the ear. And both generator coils (L1, L2) are inductively coupled (located nearby), so both signals from the generators with a difference of 1 kHz are combined and we hear the so-calledamplitude beats frequency 1 kHz.