How they differ from conventional detectors and where they are best used, let's look at examples.
Principle of operation
Any metal detector generates a magnetic field around the transmitter coil. Thanks to this, a magnetic flux also appears at the target under the coil, which is caught by the coil’s receiver. This magnetic flux is then converted into visual information on the screen and into an audio signal.
Conventional ground metal detectors (VLF) generate a constant current in the transmitter coil, and changes in the phase and amplitude of the voltage at the receiver indicate the presence of metal objects. But devices with pulse induction (PI) differ in that they generate a transmitter current that turns on for a while and then turns off abruptly. The coil field generates pulsed eddy currents in the object, which are detected by analyzing the attenuation of the pulse induced in the receiver coil. This cycle repeats continuously, perhaps hundreds of thousands of times per second.
Pros of metal detectors with pulse induction
1. Detection speed does not depend on the material between the metal detector and the target. This means that the search can be carried out through air, water, silt, corals, and various types of soil.
2. The sensors are highly sensitive to all metals and do not react in any way to high levels of soil mineralization, hot stones and salt water.
3. You can search for metal objects and find them at greater depths; this works especially well on mineralized soils.
4. There will be no interference in mineralized soils, salty sand, salt water, and the performance will be higher than VLF detectors.
5. Pulse induction metal detectors have been specially designed to find gold objects, even very small ones (nuggets, chains).
The disadvantages of metal detectors with pulse induction may be not very good discrimination and high price.
Where do pulse induction metal detectors perform best?
The pulse repetition rate (transmitter frequency) of a typical pulse induction metal detector is approximately 100 hertz. Different MD models use frequencies from 22 hertz to several kilohertz. The lower the transmission frequency, the greater the radiated power. At lower frequencies, greater depth and sensitivity for detecting objects made of silver are achieved, but sensitivity to nickel and gold alloys decreases. Such devices have a slow response and therefore require very slow movement of the frame.
Higher frequencies increase sensitivity to nickel and gold alloys, but are less sensitive to silver. The signal may not penetrate as deep into the ground as at lower frequencies, but the coil can be moved more quickly. This allows you to check a larger area over a given period of time, and such devices are also more sensitive to the main beach finds - gold items.
Thus, it is best to use PI metal detectors for beach searching on the coasts of seas and oceans, underwater searching, gold searching, searching in desert and mountainous areas. They are also good at clearing out “knocked-out” areas and during geological exploration.
Top 5 best pulse induction metal detectors:
The decay time of this electrical pulse depends on the magnitude of the electrical resistance of the coil with the wire. A complete absence of resistance, or, on the contrary, a very high value of it, will cause the impulse to fluctuate. It's like throwing a rubber ball onto a very hard surface and having it bounce off multiple times before finally settling down. With sufficient electrical resistance, the pulse decay time is shortened and the reflected pulse is “smoothed out.” This is similar to throwing a rubber ball at a pillow. A pulse induction detector coil is said to be critically damped when the reflected pulse quickly decays to zero without oscillation. Excessive or insufficient suppression will introduce instability and mask signals from highly conductive metals such as gold and reduce detection depth. When a metal object is close to the search coil, it stores some of the pulse energy, which leads to a delay in the process of attenuation of this pulse to zero. The change in the width of the reflected pulse is measured and signals the presence of a metallic object. In order to isolate the signal of such an object, we must measure that part of the pulse where it decreases to zero (tail). At the input of the coil receiver there is a resistor and a limiting diode circuit, which cut the input pulse voltage to 1 volt so as not to overload the circuit input. The signal at the receiver consists of a pulse from the transmitter and a reflected pulse. Typically the receiver gain is 60 decibels. This means that the area where the reflected signal drops to zero can be increased by a factor of 1000.
Gate circuit.
The amplified signal from the receiver enters a circuit that measures the time the voltage drops to zero. The reflected pulse is converted into a train of pulses. When a metal object approaches the coil, the shape of the transmitter pulse will not change, but the reflected pulse will become slightly longer. Increasing the duration of the pulse tail by just a few millionths of a second (microsecond) is enough to determine the presence of metal under the coil. Pulses (strobes) synchronized with the beginning of the transmitter pulse are superimposed on this reflected pulse, and at the output of the electronic circuit a series of strobes is obtained, the number of which is proportional to the length of the “tail” of the pulse. The most sensitive pulse is located as close as possible to the end of the tail, where the voltage is very close to zero. Typically this is a time domain of about 20 microseconds after the transmitter is turned off and the reflected pulse begins. Unfortunately, this is also an area where the operation of a pulsed induction metal detector becomes unstable. For this reason, most models of pulse induction metal detectors continue to produce gate pulses for another 30-40 microseconds after the reflected pulse has completely decayed.
Integrator.
Next, the gated signal must be converted to DC voltage. This is accomplished by an integrator circuit that averages the sequence of pulses and converts them into a corresponding voltage, which increases when the object is close to the frame and decreases when the object moves away. The voltage is further amplified and drives the audio control circuit.
The period of time during which the integrator collects incoming gates is called the integrator time constant (TI). It determines how quickly the metal detector responds to a metal object. A long PVI (on the order of seconds) has the advantage of reducing noise and simplifying detector tuning, but it also requires very slow movement of the search coil, since the object may be missed if it moves quickly. A short PVI (on the order of tenths of a second) reacts faster to the target, which allows you to move the coil faster, but noise immunity and operational stability deteriorate.
DISCRIMINATION (recognition).
Pulse induction metal detectors are not capable of the same degree of discrimination as VLF devices. By measuring the increasing period of time between the end of the transmitter pulse and the point at which the reflected pulse dissipates to zero (delay time), objects composed of certain metals can be filtered out. In terms of this characteristic, aluminum foil comes first, followed by small nickel coins, buttons and gold. Some coins can be identified from a very long pulse tail, however the iron is NOT identified in this way.
Many attempts have been made to create a pulse induction metal detector capable of detecting iron, but all these attempts have had very limited success. Although iron has a long tail, silver and copper have the same characteristics. Such a long delay has a bad effect on determining the depth. The mineral content of the soil will also lengthen the reflected pulse, changing the point at which an object is detected or rejected. If the integrator time constant is adjusted so that the golden ring is not detected in the air, the same ring can “glow” in soil saturated with salts. Thus, salt-saturated soil changes everything about the dwell time and selectivity of a pulsed induction metal detector.
DEVELOPMENT FROM THE GROUND.
Ground offset is very critical for VLF devices, but not for pulse induction metal detectors. On average, the soil does not store any significant amount of energy from the search coil and usually does not produce any signal itself. The soil will not mask the signal from the object and, on the contrary, soil mineralization slightly lengthens the signal in proportion to the increase in the depth of the object. In relation to MDs with pulse induction, the term “automatic ground balance” is often used; they usually do not react to excessive soil mineralization and do not require external adjustment for different soil types. The exception is one of the most unpleasant soil components - magnetite (Fe3O4), or magnetic iron oxide. It overloads the input coils of VLF type detectors, greatly reducing their sensitivity; pulse induction metal detectors will work, but may show false targets if the coil is brought too close to the ground. You can minimize this detrimental effect by lengthening the delay between the end of the transmitter pulse and the start of gating. By adjusting this time constant, you can tune out interference caused by soil mineralization.
AUTOMATIC AND MANUAL SETUP.
Most pulse induction metal detectors have manual settings. This means that the operator must turn the setting until a clicking or itching sound is heard in the headphones. If the soil in the search area varies from and to neutral sand or from dry soil to seawater, then adjustments are necessary. If you don't do this, you may lose detection depth and miss some objects. Manual tuning is very difficult when using a short integrator time constant (TITC). Therefore, many manually tuned instruments have a long PIR and require the search coil to be moved slowly.
There are no problems with using pulse induction MD for underwater searching because it does not move the search coil quickly. When used in the surf, the coil will be either in the water or under the water, and in such conditions, using devices with manual settings can be very disappointing, since you will have to constantly adjust the response threshold. In this case, some operators immediately set the device just below the response threshold. But this can lead to a decrease in detection depth when soil characteristics change.
Automatic adjustment (SAT-self adjusting Threshold) gives a significant advantage when searching in and over salt water or soil with high salt content. It allows you to use the detector at maximum sensitivity without constant adjustment. This improves operating stability, noise immunity and allows the use of higher gain. Pulse induction MDs do not emit strong negative signals like VLF devices. Therefore, they do not go off scale on pits with minerals. It is necessary to continuously move the coil of a metal detector equipped with an auto-tuning system; if you stop the coil, the setting is lost or the device stops responding.
Audio control.
Pulse induction MD audio signaling circuits fall into two categories: variable frequency and variable volume. Variable-frequency circuits based on a voltage-controlled oscillator are good for recording small objects, since changes in frequency are easier to detect by ear than changes in volume, especially at low volume levels, especially for instruments with manual threshold adjustment. However, the sound of a fire siren quickly becomes tiresome, and some people are unable to distinguish high-pitched tones. One good option is mechanical vibration, which was originally used for underwater vehicles. Such a device produces sounds and vibration, which increases to a buzzing sound when an object is detected. The signals from such a mechanical device are easy to recognize and are not drowned out by the air supply system.
Many people prefer a more traditional audio tone that increases in volume rather than frequency. Such sound control systems work well in devices with fast frame movement, those in devices with automatic adjustment, and they sound similar to devices with VLF.
Conclusions on MD with pulse induction.
These are specialized tools. They are of little use for searching for coins in urban environments, since they cannot filter out iron and ferrocontaining debris. They can be used for archaeological searches in rural areas where there is no iron debris in large quantities, searching for gold nuggets and for searching at maximum depth in extreme conditions, such as sea coasts or places where the ground is highly mineralized. Such metal detectors show excellent results in such conditions and are generally comparable to VLF devices, especially in their ability to tune out such soils and “pierce” them to the maximum depth.
Andrey Shchedrin
Moscow
Yuri Kolokolov
Donetsk
The pulse metal detector we bring to your attention is a joint development of Yuri Kolokolov and Andrey Shchedrin. The device is intended for amateur searches for treasures and relics, searches on the beach, etc. After the publication of the first version of the metal detector in, this device was highly appreciated among amateurs who repeated the design. At the same time, useful comments and suggestions were made, which we took into account in the new version of the device.
Currently, the metal detector is mass-produced by the Moscow company MASTER KIT in the form of do-it-yourself kits for radio amateurs under the designation NM8042 (an updated version of the metal detector is currently being produced in the form of a ready-made microprocessor module). The kit contains a printed circuit board, a plastic case and electronic components, including a pre-programmed controller. Perhaps, for many amateurs, purchasing such a kit and its subsequent simple assembly will be a convenient alternative to purchasing an expensive industrial device or completely making a metal detector on their own.
The operating principle of a pulsed or eddy current metal detector is based on the excitation of pulsed eddy currents in a metal object and the measurement of the secondary electromagnetic field that these currents induce. In this case, the exciting signal is supplied to the transmitting coil of the sensor not constantly, but periodically in the form of pulses. In conducting objects, damped eddy currents are induced, which excite a damped electromagnetic field. This field, in turn, induces a damped current in the receiving coil of the sensor. Depending on the conductive properties and size of the object, the signal changes its shape and duration. In Fig. 1. The signal on the receiving coil of a pulse metal detector is shown schematically. Oscillogram 1 – signal in the absence of metal targets, oscillogram 2 – signal when the sensor is near a metal object.
Pulse metal detectors have their advantages and disadvantages. The advantages include low sensitivity to mineralized soil and salt water, the disadvantages are poor selectivity by metal type and relatively high energy consumption.
Fig.1. Signal at the input of a pulse metal detector.
Most practical designs of pulsed metal detectors are built using either a two-coil circuit or a single-coil circuit with an additional power source. In the first case, the device has separate receiving and emitting coils, which complicates the design of the sensor. In the second case, there is only one coil in the sensor, and to amplify the useful signal, an amplifier is used, which is powered by an additional power source. The meaning of this construction is as follows - the self-induction signal has a higher potential than the potential of the power source that is used to supply current to the transmitting coil. Therefore, to amplify such a signal, the amplifier must have its own power source, the potential of which must be higher than the voltage of the signal being amplified. This also complicates the design of the device.
The proposed single-coil design is built according to an original scheme, which is devoid of the above disadvantages.
Specifications
Coin with a diameter of 25 mm - 20 (cm)
- pistol - 40 (cm)
- helmet - 60 (cm)
The block diagram of the metal detector is shown in Fig. 2. The basis of the device is a microcontroller. With its help, time intervals are formed to control all components of the device, as well as indication and general control of the device. Using a powerful switch, energy is pulsedly accumulated in the sensor coil, and then the current is interrupted, after which a self-induction pulse occurs, exciting an electromagnetic field in the target.
Fig.2. Block diagram of a pulse metal detector.
The highlight of the proposed circuit is the use of a differential amplifier in the input stage. It serves to amplify a signal whose voltage is higher than the supply voltage and bind it to a certain potential - + 5 (V). For further amplification, a receiving amplifier with a high gain is used. The first integrator is used to measure the useful signal. During forward integration, the useful signal is accumulated in the form of voltage, and during reverse integration, the result is converted into pulse duration. The second integrator has a large integration constant and serves to balance the amplification path with respect to direct current.
Fig.3. Schematic diagram of a simple pulse metal detector
The proposed design of the device is developed entirely on imported element base. The most common components from leading manufacturers are used. You can try to replace some elements with domestic ones, this will be discussed below. Most of the elements used are not in short supply and can be purchased in large cities in Russia and the CIS through companies that sell electronic components.
Differential amplifier assembled at op amp D1.1. Chip D1 is a quad operational amplifier type TL074. Its distinctive properties are high speed, low consumption, low noise level, high input impedance, and the ability to operate at input voltages close to the supply voltage. These properties determined its use in a differential amplifier in particular and in the circuit in general. The gain of the differential amplifier is about 7 and is determined by the values of resistors R3, R6...R9, R11.
Receiving amplifier D1.2 is a non-inverting amplifier with a gain of 57. During the action of the high-voltage part of the self-induction pulse, this coefficient is reduced to 1 using the analog switch D2.1. This prevents overloading the input amplification path and ensures rapid entry into mode to amplify a weak signal. Transistors VT3 and VT4 are designed to match the levels of control signals supplied from the microcontroller to analog switches.
By using second integrator D1.3 automatically balances the input amplifier circuit for direct current. The integration constant of 240 (ms) is chosen to be large enough so that this feedback does not affect the gain of the rapidly changing desired signal. With the help of this integrator, the output of amplifier D1.2, in the absence of a signal, maintains a level of +5 (V).
Measuring first integrator executed on D1.4. During the integration of the useful signal, key D2.2 opens and, accordingly, key D2.4 closes. A logical inverter is implemented on switch D2.3. After signal integration is completed, key D2.2 closes and key D2.4 opens. Storage capacitor C6 begins to discharge through resistor R21. The discharge time will be proportional to the voltage that has settled on capacitor C6 by the end of integration of the useful signal. This time is measured using microcontroller, which performs analog-to-digital conversion. To measure the discharge time of capacitor C6, an analog comparator and timers are used, which are built into the microcontroller D3.
Button S1 is intended for the initial reset of the microcontroller. Using switch S3, the device display mode is set. Using variable resistor R29, the sensitivity of the metal detector is adjusted.
Using LEDs VD3...VD8 it is possible to produce light indication.
Functioning algorithm
To explain the operating principle of the described pulse metal detector, Fig. 4 shows oscillograms of signals at the most important points of the device.
Fig.4. Oscillograms.
During interval A, key VT1 opens. A sawtooth current begins to flow through the sensor coil - oscillogram 2. When the current reaches a value of about 2 (A), the key closes. At the drain of transistor VT1, a surge of self-induction voltage occurs - oscillogram 1. The magnitude of this surge is more than 300 Volts (!) and is limited by resistors R1, R3. To prevent overload of the amplification path, limiting diodes VD1, VD2 are used. Also for this purpose, during interval A (accumulation of energy in the coil) and interval B (release of self-induction), key D2.1 is opened. This reduces the end-to-end gain of the path from 400 to 7. Oscillogram 3 shows the signal at the output of the amplification path (pin 8 of D1.2). Starting from interval C, switch D2.1 closes and the path gain becomes large. After the completion of the guard interval C, during which the amplification path enters the mode, key D2.2 opens and key D2.4 closes - integration of the useful signal begins - interval D. After this interval, key D2.2 closes and key D2.4 opens – “reverse” integration begins. During this time (intervals E and F), capacitor C6 is completely discharged. Using a built-in analog comparator, the microcontroller measures the value of the interval E, which turns out to be proportional to the level of the input useful signal. The following interval values are set for current firmware versions:
A – 60…200 µs, B – 12 µs, C – 8 µs, D – 50 (µs), A + B + C + D + E + F – 5 (ms) - repetition period.
The microcontroller processes the received digital data and indicates, using LEDs VD3...VD8 and sound emitter Y1, the degree of impact of the target on the sensor. The LED indication is an analogue of a dial indicator - if there is no target, the VD8 LED lights up, then, depending on the level of impact, VD7, VD6, etc. light up sequentially.
Fig.5. Schematic diagram of the second improved version of the microprocessor pulse metal detector
The differences (Fig. 5) from the first version of the device (Fig. 3) are as follows:
1. Added resistor R30. This is done in order to reduce the influence of the internal resistance of various batteries on the device settings. Now you can painlessly change the acid battery for 6-8 pieces of salt batteries. The device settings will not change in this case.
2. Added “accelerating” capacitors C15, C16, C17. Thanks to this, the thermal stability of the circuit has significantly improved. In the old scheme, keys VT2...VT4 were the most vulnerable point in this regard. Plus, continuous automatic zero balancing has been added to the program.
3. Added chain R31, R32, C14. This circuit allows you to continuously monitor the condition of the battery. Using resistor R32, you can now set any threshold for safe (for the battery) discharge of batteries of various types. For example, for 8 pcs NiCd or NiMH AA batteries you will need to set the level to 8 Volts, and for a 12 V acid battery - 11 Volts... When the threshold level is reached, the light and sound indication will be turned on.
This mode is easy to set up. The device is powered from a power supply. The required threshold voltage is set on the power supply, the resistor R32 slider is first placed in the “upper” position according to the diagram, and then, by rotating the rotor of resistor R32, you need to achieve the indication - the VD8 LED will start blinking, the sound source will emit an intermittent signal. The device exits this mode only after a reset.
4. As an alternative display device, you can now use a two-line sixteen-character LCD. This mode is activated when switch S3 is closed. In this case, the LCD signal pins are connected according to the diagram instead of LEDs. You also need to apply +5 V to the LCD module and connect the ground wire. Resistor R33 is mounted directly on the contacts of the LCD module (Fig. 6).
Fig.6. Alternative LCD indicator.
In this case, the name of the metal detector is always displayed in the top line, and in the bottom line, depending on the mode: “Autotuning”, “Low battery”. In search mode, a column of 16 gradations of signal level is drawn in this line. In this case, the sound signal also has 16 gradations of tone.
Types of parts and design
Instead of the operational amplifier D1 TL074N, you can try using the TL084N.
The D2 chip is a quad analog switch of the CD4066 type, which can be replaced with the domestic K561KT3 chip.
The D4 AT90S2313-10PI microcontroller has no direct analogues. The circuit does not provide circuits for its in-circuit programming, so it is advisable to install the controller on a socket so that it can be reprogrammed.
You can try replacing transistor VT1 type IRF740 with IRF840.
Transistors VT2...VT4 type 2N5551 can be replaced with KT503 with any letter index. However, you should pay attention to the fact that they have a different pinout.
LEDs can be of any type; it is advisable to take VD8 in a different color. Diodes VD1, VD2 type 1N4148.
Resistors can be of any type, R1 and R3 should have a power dissipation of 0.5 (W), the rest can be 0.125 or 0.25 (W). It is advisable to select R9 and R11 so that their resistance differs by no more than 5%.
Capacitor C1 is electrolytic, for a voltage of 16V, the rest of the capacitors are ceramic.
Button S1, switches S3, S4, variable resistor R29 can be of any type that fits the dimensions. You can use a piezo emitter or headphones from the player as a sound source.
The design of the device body can be arbitrary. The rod near the sensor (up to 1 meter) and the sensor itself should not have metal parts or fastening elements. It is convenient to use a plastic telescopic fishing rod as the starting material for making a rod.
The sensor contains 27 turns of wire with a diameter of 0.6 - 0.8 mm, wound on a 190 (mm) mandrel. The sensor does not have a screen and must be attached to the rod without the use of massive screws, bolts, etc. (!) A shielded cable cannot be used to connect the sensor and the electronic unit due to its high capacitance. For these purposes, you need to use two insulated wires, for example MGShV type, twisted together.
Setting up the device
ATTENTION! The device contains high, potentially life-threatening voltage - at the VT1 collector and at the sensor. Therefore, when setting up and operating, electrical safety precautions should be observed.
1. Make sure installation is correct.
2. Apply power and make sure that the current consumption does not exceed 100 (mA).
3. Using tuning resistor R7, achieve such balancing of the amplification path so that the oscillogram at pin 7 of D1.4 corresponds to oscillogram 4 in Fig. 4. In this case, it is necessary to ensure that the signal at the end of the interval D remains unchanged, i.e. The oscillogram at this point should be horizontal.
A properly assembled device does not require further adjustment. It is necessary to bring the sensor to a metal object and make sure that the indicators are working. A description of the operation of the controls is given below in the software description.
Software
At the time of writing this article, software versions V1.0-demo, V1.1 for the first version of the device and V2.4-demo, V2.4 for the second version were developed and tested. The demo version of the program is fully functional and differs only in the lack of precise sensitivity adjustment. Full versions are supplied in already flashed microcontrollers included in the MASTER KIT NM8042 kit. HEX file of firmware V1.0-demo and V2.4-demo can be downloaded.
Work on new versions of the software continues, and it is planned to introduce additional modes. New versions, after their comprehensive testing, will be available in MASTER KIT sets.
Working with the device
To begin work, you need to turn on the power of the device, raise the sensor to a level of 60-80 cm from the ground and press the “Reset” button. Within 2 seconds the device will perform auto-tuning. At the end of auto-tuning, the device will emit a characteristic short sound. After this, the sensor must be brought closer to the ground (in a place where there are no metal objects) at a distance of 3-7 cm and the sensitivity adjusted using resistor R29. The knob must be rotated until false responses disappear. After this, you can start searching. When an indication of low battery appears, you must stop searching, turn off the device and replace the power source.
Conclusion
To save time and relieve you from the routine work of finding the necessary components and making printed circuit boards, MASTER KIT offers the NM8042 kit.
Figure 7 shows a drawing of the printed circuit board (for the circuit in Figure 3) and the location of the components on it.
Rice. 7.1. Top view of the printed circuit board.
Fig.7.2. Bottom view of the printed circuit board.
The kit consists of a factory printed circuit board, a firmware controller with program version V 1.1, all necessary components, a plastic case and instructions for assembly and operation. Design simplifications were made deliberately in order to reduce the cost of the kit.
Making a search coil
The coil consists of 27 turns of enameled wire with a cross section of 0.7-0.8 mm, wound in the form of a ring of 180-190 mm. After winding the coil, the turns must be wrapped with insulating tape. To connect the sensor, you need to make a twisted pair from the installation wire. To do this, take two pieces of wire of the required length and twist them together at the rate of one twist per centimeter. On one side this cable is soldered to the coil, on the other to the board. The sensor body and the metal detector rod must not contain metal parts!
Refinement of the body
Before installing the metal detector board into the housing, it is necessary to make holes in it for remote elements.
Figure 8 shows the holes on the front panel for the LEDs, sensitivity control R29, power switch S4 and reset button S1. In Fig. 9 there is a hole on the side surface of the case for the Earphone JACK telephone connector. In Fig. 10 there are holes on the rear panel for the power cable and for the search coil cable.
The appearance of the assembled electronic filling is shown in Fig. eleven.
Fig.8. Holes on the front panel of the case for LEDs.
Fig.9. A hole on the side of the case for a telephone connector.
Fig. 10. Holes on the back panel for the power cable and search coil cable.
Fig. 11. Appearance of the electronics of the microprocessor pulse metal detector from the NM8042 kit.
Information sources
1. Shchedrin A.I. New metal detectors for searching treasures and relics: -M.: “Hot Line-Telecom”, 2003. -173 p.
1.1. Work principles
The terms "transmit-receive" and "reflected signal" in various detector devices 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 (emitted) and received (reflected) signals are continuous, they exist simultaneously and coincide in frequency.
The operating principle of transmit-receive metal detectors is to register a signal reflected (or, as they say, re-emitted) by a metal object (target), see, pp. 225-228. The reflected signal arises due to the influence of the alternating magnetic field of the transmitting (emitting) coil of the metal detector on the target. Thus, a device of this type implies the presence of at least two coils, one of which is transmitting and the other is receiving.
The main fundamental problem that is solved in metal detectors of this type is the choice of the relative 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 direct impact of the transmitting 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 an alternating electromotive force (emf) in the receiving coil.
At first it may seem that in nature there are only two options for the relative arrangement of coils, in which there is no direct transmission of a signal from one coil to another (see Fig. 1, a and b) - coils with perpendicular and crossing axes.
A more thorough study of the problem shows that there can be as many different systems of metal detector sensors as desired. But these are more complex systems with more than two coils, electrically connected accordingly. For example, in Fig. 1, c shows a system of one emitting (in the center) and two receiving coils, connected counter-currently according to the signal induced by the emitting coil. Thus, the signal at the output of the system of receiving coils is ideally equal to zero, since the emf induced in the coils. 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 located in the ground, and bringing the sensor closer to the minimum distance to the surface of the earth is possible only 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 relative arrangement of coplanar coils are shown in Fig. 2, a and b. In the diagram in Fig. 2, and the relative position of the coils is chosen such that the total flux of the magnetic induction vector through the surface limited by the receiving coil is equal to zero. In the diagram of Fig. 2, b one of the coils (receiving) is twisted in the form of a “figure of eight”, so that the total emf induced on the halves of the turns of the receiving coil located in one wing of the “figure of eight” compensates for a similar total emf induced in the other wing of the G8. Various other designs of sensors with coplanar coils are also possible, for example Fig. 2, e.
The receiving coil is located inside the emitting coil. The emf induced in the receiving coil. is compensated by a special transformer device that selects part of the signal from the emitting coil.
The name “beat metal detector” is an echo of the terminology adopted in radio engineering since the days of the first superheterodyne receivers. Beats are a phenomenon that most noticeably manifests itself when two periodic signals with similar frequencies and approximately equal amplitudes are added and consists of a pulsation in the amplitude of the total signal. The ripple frequency is equal to the difference in frequencies of the two added signals. By passing such a pulsating signal through a rectifier (detector), it is possible to isolate the difference frequency signal. Such circuitry has been traditional for a long time, but currently it is no longer used either in radio engineering or in metal detectors. In both cases, amplitude detectors were replaced by synchronous detectors, but the term “on beats” has remained to this day.
The principle of operation of a beat metal detector is very simple and consists in recording the frequency difference from two generators, one of which is stable in frequency, and the other contains a sensor - an inductor in its frequency-setting circuit. The device is adjusted in such a way that, in the absence of metal near the sensor, the frequencies of the two generators coincide or are very close in value. The presence of metal near the sensor leads to a change in its parameters and, as a consequence, to a change in the frequency of the corresponding generator. This change is usually very small, but the change in the frequency difference between the two oscillators is already significant and can be easily recorded.
The frequency difference can be recorded in a variety of ways, from the simplest, when the difference frequency signal is listened to on headphones or through a loudspeaker, to digital methods of frequency measurement. The sensitivity of a metal detector to beats depends, among other things, on the parameters for converting changes in the impedance of the sensor into frequency.
Typically, the conversion consists of obtaining the difference frequency of a stable generator and a generator with a sensor coil in the frequency-setting circuit. Therefore, the higher the frequencies of these generators, the greater the frequency difference will be in response to the appearance of a metal target near the sensor. Registration of small frequency deviations presents a certain difficulty. Thus, by ear you can confidently register a shift in the frequency of the tone signal of at least 10 Hz. Visually, by blinking the LED, you can register a frequency shift of at least 1 Hz. In other ways, it is possible to achieve registration of a smaller frequency difference, however, this registration will require considerable time, which is unacceptable for metal detectors that always operate in real time.
Selectivity for metals at such frequencies, which are very far from optimal, is very weak. In addition, it is almost impossible to determine the phase of the reflected signal from the generator frequency shift. Therefore, the metal detector has no selectivity on beats.
A positive side for practice is the simplicity of the design of the sensor and the electronic part of metal detectors based on beats and on the principle of a frequency meter. Such a device can be very compact. It is convenient to use when something has already been detected by a more sensitive device. If the discovered object is small and located deep enough in the ground, then it can “get lost” and be moved during the excavation. In order not to “look through” the excavation site many times with a bulky, sensitive metal detector, it is advisable to control its progress at the final stage with a compact device of short range, which can be used to more accurately determine the location of the object.
The word “induction” in the name of metal detectors of this type fully reveals the principle of their operation, if you remember the meaning of the word “inductio” (Latin) - guidance. A device of this type contains a sensor of one coil of any convenient shape, excited by an alternating signal. The appearance of a metal object near the sensor causes the appearance of a reflected (re-emitted) signal, which “induces” an additional electrical signal in the coil. All that remains is to highlight this additional signal.
The induction-type metal detector has gained the right to life, mainly due to the main drawback of devices based on the “transmission-reception” principle - the complexity of the sensor design. This complexity leads either to the high cost and complexity of manufacturing the sensor, or to its insufficient mechanical rigidity, which causes false signals to appear when moving and reduces the sensitivity of the device.
If you set yourself the goal of eliminating this drawback from devices based on the “transmission-reception” principle by eliminating its very cause, then you can come to an unusual conclusion - the emitting and receiving coils of the metal detector must be combined into one! In fact, in this case there are no very undesirable movements and bends of one coil relative to the other, since there is only one coil and it is both emitting and receiving. The sensor is also extremely simple. The price for these advantages is the need to isolate the useful reflected signal from the background of a much larger excitation signal of the emitting/receiving coil.
The reflected signal can be isolated by subtracting from the electrical signal present in the sensor coil a signal of the same shape, frequency, phase and amplitude as the signal in the coil in the absence of metal nearby. *How this can be implemented in one of the ways is shown in Fig. 3.
The generator produces an alternating voltage of a sinusoidal shape with a constant amplitude and frequency. The voltage-to-current converter (VCT) converts the generator voltage Ur into current Ig, which is supplied to the oscillatory circuit of the sensor. The oscillatory circuit consists of a capacitor C and a sensor coil L. Its resonant frequency is equal to the frequency of the generator. The PNT conversion coefficient is selected so that the voltage of the oscillatory circuit id is equal to the generator voltage Ur (in the absence of metal near the sensor). Thus, the adder subtracts two signals of the same amplitude, and the output signal - the result of the subtraction - is equal to zero. When metal appears near the sensor, a reflected signal occurs (in other words, the parameters of the sensor coil change), and this leads to a change in the voltage of the oscillating circuit 11d. A non-zero signal appears at the output.
In Fig. Figure 3 shows only the simplest version of one of the diagrams of the input part of metal detectors of the type under consideration. Instead of a PNT in this circuit, it is in principle possible to use a current-setting resistor. Various bridge circuits can be used to turn on the sensor coil, adders with different transmission coefficients for inverting and non-inverting inputs, partial connection of an oscillating circuit, etc.
In the diagram in Fig. 3 an oscillatory circuit is used as a sensor. This is done for simplicity in order to obtain zero phase shift between the Ur and 11d signals (the circuit is tuned to resonance). You can abandon the oscillatory circuit with the need to fine-tune it for resonance and use only the sensor coil as a PNT load. However, the PNT gain for this case must be complex to correct for the 90° phase shift resulting from the inductive nature of the PNT load.
In the types of electronic metal detectors discussed earlier, the reflected signal is separated from the emitted one either geometrically - due to the relative position of the receiving and emitting coils, or using special compensation circuits. Obviously, there may also be a temporary method for separating the emitted and reflected signals. This method is widely used, for example, in pulse echo and radar. During location, the mechanism of delay of the reflected signal is due to the significant time it takes for the signal to propagate to the object and back.
In relation to metal detectors, such a mechanism may be the phenomenon of self-induction in a conductive object. How to use this in practice? After exposure to a magnetic induction pulse, a damped current pulse appears in a conducting object and is maintained for some time (due to the phenomenon of self-induction), causing a time-delayed reflected signal. It carries useful information and should be registered.
Thus, another scheme for constructing a metal detector can be proposed, fundamentally different from those discussed earlier in the method of signal separation. This type of metal detector is called a pulse detector. It consists of a current pulse generator, receiving and emitting coils, which can be combined into one, a switching device and a signal processing unit.
The current pulse generator generates short current pulses in the millisecond range that enter the emitting coil, where they are converted into magnetic induction pulses. Since the emitting coil - the load of the pulse generator - has a pronounced inductive nature, overloads in the form of voltage surges occur at the generator at the pulse fronts. Such bursts can reach tens to hundreds (!) of volts in amplitude, but the use of protective limiters is unacceptable, since it would lead to a delay in the front of the current pulse and magnetic induction and, ultimately, to complicate the separation of the reflected signal.
The receiving and emitting coils can be positioned relative to each other quite arbitrarily, since the direct penetration of the emitted signal into the receiving coil and the effect of the reflected signal on it are separated in time. In principle, one coil can serve as both a receiving and an emitting coil, but in this case it will be much more difficult to decouple the high-voltage output circuits of the current pulse generator from the sensitive input circuits.
The switching device is designed to perform the above-mentioned separation of the emitted and reflected signals. It blocks the input circuits of the device for a certain time, which is determined by the duration of the current pulse in the emitting coil, the discharge time of the coil and the time during which short responses of the device from massive weakly conductive objects such as soil are possible. After this time, the switching device must ensure the transmission of the signal from the receiving coil to the signal processing unit.
The signal processing unit is designed to convert the input electrical signal into a form convenient for human perception. It can be designed based on solutions used in other types of metal detectors. The disadvantages of pulse metal detectors include the difficulty of implementing in practice the discrimination of objects by type of metal, the complexity of the equipment for generating and switching current and voltage pulses of large amplitude, and the high level of radio interference.
Magnetometers are a broad group of devices designed to change the parameters of a magnetic field (for example, the module or components of the magnetic induction vector). The use of magnetometers as metal detectors is based on the phenomenon of local distortion of the Earth's natural magnetic field by ferromagnetic materials, such as iron. Having detected with the help of a magnetometer a deviation from the module or direction of the magnetic induction vector of the Earth's field that is usual for a given area, we can confidently say that there is some magnetic inhomogeneity (anomaly) that can be caused by an iron object.
Compared to the previously discussed metal detectors, magnetometers have a much greater detection range of iron objects. It is very impressive to know that using a magnetometer you can register small shoe nails from a shoe at a distance of 1 m, and a car at a distance of 10 m! Such a large detection range is explained by the following. An analogue of the emitted field of conventional metal detectors for magnetometers is the uniform (on the search scale) magnetic field of the Earth. Therefore, the response of the device to an iron object is inversely proportional not to the sixth, but only to the third power of the distance.
The fundamental disadvantage of magnetometers is the inability to detect objects made of non-ferrous metals with their help. In addition, even if we are only interested in iron, the use of magnetometers for searching is difficult - in nature there is a wide variety of natural magnetic anomalies of various scales (individual minerals, mineral deposits, etc.). However, when searching for sunken tanks and ships, such devices are unrivaled!
It is a well-known fact that with the help of modern radars it is possible to detect an aircraft at a distance of several hundred kilometers. The question arises: does modern electronics really not allow us to create a compact device that allows us to detect objects of interest to us at least at a distance of several meters?9 The answer is a number of publications in which such devices are described.
Typical of them is the use of the achievements of modern microwave microelectronics and computer processing of the received signal. The use of modern high technologies makes it almost impossible to independently manufacture these devices. In addition, their large overall dimensions do not yet allow them to be widely used in field conditions.
The advantages of radars include a fundamentally higher detection range - the reflected signal, in a rough approximation, can be considered to obey the laws of geometric optics and its attenuation is proportional not to the sixth or even the third, but only to the second power of the distance.
Pulse metal detector ( Pulse metal detector or - English) the most sensitive among all detectors, reacts to any metals, does not distinguish ferromagnets from diamagnets. Search features allow the detector to detect gold and gold nuggets in alkaline conditions and extreme ground (or rock) temperatures that are too challenging for VLF/TR devices. It can also detect metal ores found in rocks and clay.
Pulse metal detectors are indispensable when searching in the coastal zone, underwater and in highly mineralized soil. The operation of the devices does not depend on the influence of earth and water. They work equally well underwater and on land. That's why PI technology used in underwater metal detectors. The devices have good results when searching on sandy and wet beaches. The depth of detection of objects in the ground and salt water is greater compared to VLF metal detectors.
Pulse metal detectors perform better than VLF metal detectors near power lines, as well as transmitting antennas of mobile communication systems. Servicing this type of metal detector is quite simple. As a rule, they are equipped with a single sensitivity control, although more advanced models may have other controls.
The devices have high energy consumption and require powerful batteries to operate. Conventional batteries last no more than 12 hours of continuous operation. If alkaline batteries are used, the operating time increases.
Technology Pulse Induction is not universal, and the shortcomings of pulse metal detectors limit their capabilities. Currently, the best metal detectors for all purposes are those using VLF (Very Low Frequency) technology. However, PI technology may be further developed and new detectors with new capabilities may be developed in the future.
These currents are the source of a secondary signal (reflected pulse, response). In the intervals between pulses, the receiver receives a response, which is amplified and processed by the analyzer and then output to the display unit.
The decay time of the reflected pulse is longer than the decay time of the emitted pulse (due to the phenomenon of self-induction). The time difference is a parameter for analysis and recording. The attenuation of eddy currents from soil or water occurs much faster and is not detected by the device. This is why pulse metal detectors work effectively underwater, on mineralized, salty and wet soils.
Related tags: pulse metal detectors, pulse metal detectors, PI technology, Pulse Induction, operating principle of pulse metal detectors, device of pulse metal detectors, how a pulse metal detector works
