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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Differential magnetometer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The differential magnetometer brought to your attention can be very useful for searching for large iron objects. It is practically impossible to search for treasures with such a device, but it is indispensable when searching for shallowly sunken tanks, ships and other types of military equipment. The principle of operation of a differential magnetometer is very simple. Any ferromagnetic object distorts the Earth's natural magnetic field. These items include everything made of iron, cast iron and steel. The intrinsic magnetization of objects, which often takes place, can also significantly affect the distortion of the magnetic field. Having fixed the deviation of the magnetic field strength from the background value, we can conclude that there is an object made of ferromagnetic material near the measuring device. The distortion of the Earth's magnetic field far from the target is small, and it is estimated from the difference in signals from two sensors separated by some distance. Therefore, the device is called differential. Each sensor measures a signal proportional to the strength of the magnetic field. Ferromagnetic sensors and sensors based on the magnetonic precession of protons are most widely used. The device under consideration uses sensors of the first type. The basis of a ferromagnetic sensor (also called a fluxgate) is a coil with a core of ferromagnetic material. A typical magnetization curve for such a material is well known from a school physics course and, taking into account the influence of the Earth's magnetic field, has the following form, shown in Fig. 29.
The coil is excited by an alternating sinusoidal signal of the carrier frequency. As can be seen from fig. 29, the displacement of the magnetization curve of the ferromagnetic core of the coil by the external magnetic field of the Earth leads to the fact that the field induction and the associated voltage on the coil begin to distort in an asymmetric manner. In other words, the voltage of the sensor with a sinusoidal current of the carrier frequency will differ from the sinusoid by more "flattened" tops of the half-waves. And these distortions will be asymmetrical. In the language of spectral analysis, this means the appearance in the spectrum of the output voltage of the coil of even harmonics, the amplitude of which is proportional to the strength of the bias magnetic field (Earth's field). It is these even harmonics that must be "caught".
Before mentioning the synchronous detector that naturally suggests itself for this purpose, operating with a reference signal of a double carrier frequency, let us consider the design of a complicated version of a ferromagnetic sensor. It consists of two cores and three coils (Fig. 30). At its core, it is a differential sensor. However, for simplicity, we will not call it differential further in the text, since the magnetometer itself is already differential :). The design consists of two identical ferromagnetic cores with identical coils placed in parallel next to each other. In relation to the excitatory electrical signal of the reference frequency, they are included in the opposite direction. The third coil is a winding wound on top of the first two core coils stacked together. In the absence of an external biasing magnetic field, the electrical signals of the first and second windings are symmetrical and ideally act so that there is no output signal in the third winding, since the magnetic fluxes through it are completely canceled. In the presence of an external biasing magnetic field, the picture changes. Either one or the other core at the peak of the corresponding half-wave "flies" into saturation deeper than usual due to the additional influence of the Earth's magnetic field. As a result, a double frequency mismatch signal appears at the output of the third winding. The fundamental harmonic signals are ideally fully compensated there. The convenience of the considered sensor lies in the fact that its coils can be included to increase the sensitivity in oscillatory circuits. The first and second - into the oscillatory circuit (or circuit), tuned to the carrier frequency. The third - into an oscillatory circuit tuned to the second harmonic. The described sensor has a pronounced radiation pattern. Its output signal is maximum when the longitudinal axis of the sensor is located along the lines of force of the external constant magnetic field. When the longitudinal axis is perpendicular to the lines of force, the output signal is zero. The sensor of the considered type, especially in combination with a synchronous detector, can successfully work as an electronic compass. Its output signal after rectification is proportional to the projection of the Earth's magnetic field strength vector onto the axis of the sensor. Synchronous detection also makes it possible to find out the sign of this projection. But even without a sign - by orienting the sensor at the signal minimum, we get the direction to the west or east. Orienting to the maximum - we get the direction of the magnetic field line of the Earth's field. In middle latitudes (for example, in Moscow), it goes obliquely and "sticks" into the ground in the direction to the north. According to the angle of magnetic declination, one can approximately estimate the geographical latitude of the area. Differential ferromagnetic magnetometers have their advantages and disadvantages. The advantages include the simplicity of the device, it is no more complicated than a direct amplification radio receiver. The disadvantages include the complexity of manufacturing sensors - in addition to accuracy, an absolutely exact match of the number of turns of the corresponding windings is required. An error of one or two turns can greatly reduce the possible sensitivity. Another drawback is the "compass" of the device, i.e., the impossibility of complete compensation of the Earth's field by subtracting signals from two spaced sensors. In practice, this leads to false signals when the sensor is rotated around an axis perpendicular to the longitudinal one. Practical Design The practical design of a differential ferromagnetic magnetometer was implemented and tested in a prototype version without a special electronic part for sound indication, using only a microammeter with zero in the middle of the scale. The sound indication scheme can be taken from the description of the metal detector according to the "transmission-reception" principle. The device has the following parameters. Main Specifications
Depth of detection:
Structural scheme The block diagram is shown in fig. 31. A quartz-stabilized master oscillator provides a clock frequency for the signal conditioner.
At one of its outputs, there is a square wave of the first harmonic, which enters the power amplifier, which excites the radiating coils of sensors 1 and 2. The other output forms a square wave of the reference double clock frequency with a shift of 90 ° for the synchronous detector. The difference signal from the output (third) windings of the sensors is amplified in the receiving amplifier and rectified by a synchronous detector. A rectified constant signal can be registered with a microammeter or sound indication devices described in previous chapters. Schematic diagram The schematic diagram of a differential ferromagnetic magnetometer is shown in fig. 32 - part 1: master oscillator, signal conditioner, power amplifier and radiating coils, fig. 33 - part 2: receiving coils, receiving amplifier, synchronous detector, indicator and power supply.
The master oscillator is assembled on inverters D1.1-D1.3. The oscillator frequency is stabilized by a quartz or piezoceramic resonator Q with a resonant frequency of 215 Hz = 32 kHz ("clock quartz"). The R1C1 circuit prevents the excitation of the generator at higher harmonics. Through the resistor R2, the OOS circuit is closed, through the resonator Q, the POS circuit is closed. The generator is characterized by simplicity, low current consumption, reliable operation at a supply voltage of 3 ... 15 V, does not contain tuned elements and overly high-resistance resistors. The output frequency of the generator is about 32 kHz. Signal conditioner (Fig. 32) The signal conditioner is assembled on a binary counter D2 and a D-trigger D3.1. The type of binary counter is not fundamental, its main task is to divide the clock frequency by 2, by 4 and by 8, thus obtaining meanders with frequencies of 16, 8 and 4 kHz, respectively. The carrier frequency for excitation of the radiating coils is 4 kHz. Signals with frequencies of 16 and 8 kHz, acting on the D-flip-flop D3.1, form at its output a meander doubled with respect to the carrier frequency of 8 kHz, shifted by 90 ° relative to the output signal of the 8 kHz binary counter. Such a shift is necessary for the normal operation of the synchronous detector, since the same shift has a useful double frequency mismatch signal at the sensor output. The second half of the microcircuit of two D-flip-flops - D3.2 is not used in the circuit, but its unused inputs must be connected to either logic 1 or logic 0 for normal operation, which is shown in the diagram. Amplifier (Fig. 32) The power amplifier does not look like this and is just powerful inverters D1.4 and D1.5, which in antiphase swing an oscillatory circuit consisting of series-parallel connected radiating coils of the sensor and capacitor C2. An asterisk near the value of the capacitor means that its value is indicated approximately and that it must be selected during commissioning. An unused inverter D1.6, in order not to leave its input unconnected, inverts the signal D1.5, but practically runs "idle". Resistors R3 and R4 limit the output current of the inverters to an acceptable level and, together with the oscillatory circuit, form a high-quality band-pass filter, due to which the shape of the voltage and current in the radiating coils of the sensor almost coincides with a sinusoidal one. Receiving amplifier (Fig. 33) The receiving amplifier amplifies the difference signal coming from the receiving coils of the sensor, which together with the capacitor C3 form an oscillatory circuit tuned to twice the frequency of 8 kHz. Thanks to the tuning resistor R5, the signals of the receiving coils are subtracted with some weighting coefficients, which can be changed by moving the slider of the resistor R5. This achieves compensation for nonidentical parameters of the receiving windings of the sensor and minimization of its "compass". The receiving amplifier is two-stage. It is assembled on the D4.2 and D6.1 op amps with a parallel voltage OS. Capacitor C4 reduces the gain at higher frequencies, thereby preventing overload of the amplifying path with high-frequency pickups from power networks and other sources. Op-amp correction circuits are standard. Synchronous detector (Fig. 33) The synchronous detector is made on the OS D6.2 according to a typical scheme. The D5 CMOS 8 by 1 multiplexer-demultiplexer chip is used as analog keys (Fig. 32). Its digital address signal is moved only in the least significant bit, providing alternate switching of points K1 and K2 to a common bus. The rectified signal is filtered by capacitor C8 and amplified by op-amp D6.2 with simultaneous additional attenuation of unfiltered RF components by circuits R14C11 and R13C9. The op amp correction circuit is standard for the type used.
Indicator (Fig. 33) The indicator is a microammeter with zero in the middle of the scale. In the indicator part, the circuitry of other types of metal detectors described earlier can be successfully used. In particular, as an indicator, you can use the design of a metal detector based on the principle of an electronic frequency meter. In this case, its LC oscillator is replaced by an RC oscillator, and the measured output voltage is fed through a resistive divider to the frequency setting circuit of the timer. You can read more about this on the website of Yuri Kolokolov. Chip D7 stabilizes the unipolar supply voltage. The D4.1 op amp creates an artificial middle feed point, allowing the use of conventional bipolar op amp circuitry. Ceramic blocking capacitors C18-C21 are mounted in close proximity to the housings of digital circuits D1, D2, D3, D5. Part types and design The types of microcircuits used are indicated in Table. 6. Table 6. Types of microcircuits used
Instead of K561 series microcircuits, it is possible to use K1561 series microcircuits. You can try to use some chips of the K176 series or foreign analogues of the 40XX and 40XXX series. Dual operational amplifiers (op-amps) of the K157 series can be replaced by any general-purpose op-amps of similar parameters (with corresponding changes in the pinout and correction circuits). There are no special requirements for the resistors used in the differential magnetometer circuit. They only need to be robust and miniature in design and easy to install. The power dissipation rating is 0,125 ... 0,25 W. Potentiometers R5, R16 are desirable multi-turn for the convenience of fine-tuning the device. The R5 potentiometer handle must be made of plastic and must be of sufficient length so that the touch of the operator's hand during adjustment does not cause changes in the indicator readings due to pickups. Capacitor C16 - electrolytic of any small-sized type. The capacitors of the oscillatory circuits C2* and C3* consist of several (5-10 pcs.) Capacitors connected in parallel. Tuning the circuit into resonance is carried out by selecting the number of capacitors and their rating. The recommended type of capacitors is K10-43, K71-7 or foreign thermostable analogues. You can try to use conventional ceramic or metal film capacitors, however, with temperature fluctuations, you will have to adjust the device more often. Microammeter - any type for a current of 100 μA with zero in the middle of the scale. Small-sized microammeters, for example, type M4247, are convenient. You can use almost any microammeter, and even a milliammeter - with any scale limit. To do this, you must adjust the values of the resistors R15-R17 accordingly. Quartz resonator Q - any small-sized watch quartz (similar ones are also used in portable electronic games). Switch S1 - any type, compact. The sensor coils are made on round ferrite cores with a diameter of 8 mm (used in magnetic antennas of MW and LW radio receivers) and a length of about 10 cm. Each winding consists of 200 turns of copper winding wire with a diameter of 0,31 mm, evenly and tightly wound in two layers in double lacquer-silk insulation. A layer of screen foil is attached over all windings. The edges of the screen are isolated from each other to prevent the formation of a short-circuited coil. The screen output is carried out with a tinned copper single-core wire. In the case of an aluminum foil screen, this lead is superimposed on the screen for its entire length and is tightly wound with electrical tape. In the case of a shield made of copper or brass foil, the lead is soldered. The ends of the ferrite cores are fixed in fluoroplastic centering discs, thanks to which each of the two halves of the sensor is held inside a plastic tube made of textolite, which serves as a housing, as is schematically shown in Fig. 34.
The length of the pipe is about 60 cm. Each of the halves of the sensor is located at the end of the pipe and is additionally fixed with silicone sealant, which fills the space around the windings and their cores. Filling is carried out through special holes in the body-pipe. Together with fluoroplastic washers, such a sealant gives the fastening of fragile ferrite rods the necessary elasticity, which prevents them from cracking during accidental impacts. Setting up the device 1. Make sure the installation is correct. 2. Check the current consumption, which should not exceed 100 mA. 3. Check the correct operation of the master oscillator and other elements of the formation of pulse signals. 4. Adjust the oscillatory circuit of the sensor. Radiating - at a frequency of 4 kHz, receiving - at 8 kHz. 5. Make sure that the amplifying path and the synchronous detector work correctly. Working with the device The procedure for setting up and working with the device is as follows. We go to the place of search, turn on the device and begin to rotate the antenna-sensor. Best in a vertical plane passing through the north-south direction. If the sensor of the device is on the rod, then you can not rotate it, but swing it as far as the rod allows. The indicator needle will deflect (compass effect). Using a variable resistor R5, we are trying to minimize the amplitude of these deviations. In this case, the middle point of the microammeter readings will "move out" and it will also need to be adjusted with another variable resistor R16, which is designed to set zero. When the "compass" effect becomes minimal, the device is considered balanced. For small objects, the method of searching with a differential magnetometer does not differ from the method of working with a conventional metal detector. Near the object, the arrow can deviate in any direction. For large objects, the indicator arrow will deviate in different directions over a large area. Author: Shchedrin A.I.
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