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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector on the principle of Transmission-Reception. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The proposed metal detector is designed for "long-range" search for relatively large objects. It is assembled according to the simplest scheme without a discriminator by metal types. The device is easy to manufacture. The detection depth is:
Structural scheme The block diagram is shown in fig. 4. It consists of several functional blocks.
The generator is a source of rectangular pulses, from which a signal is subsequently formed that arrives at the radiating coil. The same signal is used to generate a sound indication signal. The oscillator signal is divided by frequency by 4 using a ring counter on flip-flops. According to the ring scheme, the counter is designed so that two signals can be generated at its outputs, shifted relative to each other in phase by 90 °. A rectangular signal (meander) is fed from the first output of the ring counter to the input of the power amplifier, the load of which is an oscillatory circuit with a radiating coil. By its type, the power amplifier is a voltage-to-current converter, which helps prevent overloading of the output stage when the polarity of the input rectangular signal of the power amplifier is reversed. The receiving voltage amplifier amplifies the signal coming from the receiving coil. In addition to the useful signal, a spurious signal also penetrates into the receiving coil, due to the non-ideal design of the metal detector coil system, ground conductivity and other reasons. To eliminate it, a compensation scheme is designed. The meaning of its operation is that some part of the signal from the output oscillatory circuit is mixed into the signal of the receiving amplifier in such a way as to minimize (ideally, bring to zero) the output signal of the synchronous detector in the absence of metal objects near the sensor. The adjustment of the compensation circuit is carried out using the adjusting potentiometer. The synchronous detector converts the useful alternating signal coming from the output of the receiving amplifier into a constant signal. An important feature of a synchronous detector is the possibility of separating the useful signal against the background of noise and interference, which significantly exceed the amplitude of the useful signal. The reference signal of the synchronous detector is taken from the second output of the ring counter, the signal of which has a phase shift relative to the first output by 90°. The dynamic range of changes in the useful signal both at the output of the receiving coil and at the output of the synchronous detector is very wide. In order for the indication device - a pointer device or a sound indicator to equally well register both very weak signals and very (for example, 100 times) stronger signals, it is necessary to have a device that compresses the dynamic range as part of the device. Such a device is a nonlinear amplifier, the amplitude characteristic of which approaches the logarithmic one. A pointer measuring device is connected to the output of the nonlinear amplifier. The formation of an indication sound signal begins with a minimum limiter, i.e. block having a dead zone for small signals. This means that the sound indication is turned on only for signals that exceed a certain threshold in amplitude. Thus, weak signals associated mainly with the movement of the device and its mechanical deformations do not irritate the ear. The sound indication reference signal shaper generates bursts of rectangular pulses with a frequency of 2 kHz and a burst repetition rate of 8 Hz. With the help of a balanced modulator, this reference signal is multiplied by the output signal of the limiter to a minimum, thus forming a signal of the desired shape and amplitude. The piezo-emitter amplifier increases the amplitude of the signal that is fed to the acoustic transducer - the piezo-emitter. Schematic diagram A schematic diagram of a metal detector developed by the author on the principle of "transmission-reception" is shown in fig. 5 - input block and in fig. 6 - indication block. The division into blocks is conditional and does not reflect the design features.
Generator The generator is assembled on logical elements 2I-NOT D1.1-D1.4. The generator 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 generator from being excited at higher harmonics. The OOS circuit is closed through the resistor R2, and the POS circuit is closed through the Q resonator. The generator is simple, low current consumption from the power source, operates reliably at a supply voltage of 3 ... 15 V, does not contain trimmers and excessively high-resistance resistors.The output frequency of the generator is about 32 kHz. ring counter The ring counter has two functions. First, it divides the oscillator frequency by 4, up to a frequency of 8 kHz. Secondly, it generates two signals shifted relative to each other by 90° in phase. One signal is used to excite an oscillatory circuit with a radiating coil, the other is used as a reference signal of a synchronous detector. The ring counter consists of two D-flip-flops D2.1 and D2.2, closed in a ring with signal inversion around the ring. The clock signal is common to both flip-flops. Any output signal of the first trigger D2.1 has a phase shift of plus or minus a quarter period (ie 90°) relative to any output signal of the second trigger D2.2. Amplifier The power amplifier is assembled on an operational amplifier (op-amp) D3.1. An oscillatory circuit with a radiating coil is formed by elements L1C2. The parameters of the inductor are given in table. 2. Brand of winding wire - PELSHO 0,44. Table 2. Parameters of the sensor inductors
The output oscillatory circuit is included in the OS circuit of the amplifier only by 25%, due to the tapping of the radiating coil L50 from the 1th turn. This allows you to increase the amplitude of the current in the coil with an acceptable value of the capacitance of the precision capacitor C2. The value of the alternating current in the coil is set by resistor R3. This resistor should have a minimum value, but such that the power amplifier op-amp does not fall into the mode of limiting the output signal by current (no more than 40 mA) or, which is most likely with the recommended parameters of the inductor L1, by voltage (no more than ±3,5 .4,5 V at battery voltage ±3.1 V). In order to make sure that there is no limit mode, it is enough to check the waveform at the output of the op amp D3.1 with an oscilloscope. During normal operation of the amplifier, the output should have a signal approaching a sinusoid in shape. The peaks of the sine waves should have a smooth shape and should not be cut off. The correction circuit of the op-amp D3 consists of a correction capacitor C33 with a capacity of XNUMX pF. Receiving amplifier The receiving amplifier is two-stage. The first stage is made on the D5.1 op-amp. It has a high input impedance due to the series voltage feedback. This eliminates the loss of the useful signal due to shunting the L2C5 oscillatory circuit with the input impedance of the amplifier. The voltage gain of the first stage is: Ku \u9d (R8 / R1) + 34 \u5.1d 6. The correction circuit of the op-amp D33 consists of a correction capacitor CXNUMX with a capacity of XNUMX pF. The second stage of the receiving amplifier is made on the D5.2 op-amp with parallel voltage feedback. The input impedance of the second stage: Rin = R10 = 10 kOhm - not as critical as the first, due to the low resistance of its signal source. The isolation capacitor C7 not only prevents the accumulation of a static error in the stages of the amplifier, but also corrects its phase response. The capacitance of the capacitor is chosen so that the phase advance created by the C7R10 circuit at an operating frequency of 8 kHz compensates for the phase lag caused by the finite speed of the op-amp D5.1 and D5.2. The second stage of the receiving amplifier, thanks to its circuit, makes it easy to sum (mix) the signal from the compensation circuit through the resistor R11. The gain of the second stage in terms of the voltage of the useful signal is: Ku = - R12 / R10 = -33, and in terms of the voltage of the compensating signal: Kuk = - R12 / R11 = - 4. The correction circuit of the OA D5.2 consists of a correction capacitor C8 with a capacity of 33 pF . Stabilization scheme The compensation circuit is made on the OA D3.2 and is an inverter with Ku = - R7 / R5 = -1. The adjusting potentiometer R6 is connected between the input and output of this inverter and allows you to remove the signal in the range [-1, +1] from the output voltage of the op-amp D3.1. The output signal of the compensation circuit from the engine of the adjusting potentiometer R6 is fed to the compensating input of the second stage of the receiving amplifier (to the resistor R11). By adjusting the potentiometer R6, a zero value is achieved at the output of the synchronous detector, which approximately corresponds to the compensation of an unwanted signal that has entered the receiving coil. The correction circuit of the OU D3.2 consists of a correction capacitor C4 with a capacity of 33 pF. Synchronous detector The synchronous detector consists of a balanced modulator, an integrating circuit and a constant signal amplifier (CCA). The balanced modulator is implemented on the basis of a multifunctional switch D4, made according to integrated technology with complementary field-effect transistors, both as discrete control valves and as analog switches. The switch works as an analog switch. With a frequency of 8 kHz, it alternately closes the outputs of the "triangle" of the integrating circuit, consisting of resistors R13 and R14 and capacitor C10, to a common bus. The reference frequency signal is fed to the balanced modulator from one of the ring counter outputs. The signal to the input of the "triangle" of the integrating circuit is fed through the decoupling capacitor C9 from the output of the receiving amplifier. Time constant of the integrating circuit t = R13*C10 = R14*C10. On the one hand, it should be as large as possible in order to attenuate the influence of noise and interference as much as possible. On the other hand, it should not exceed a certain limit, when the inertia of the integrating circuit prevents tracking of fast changes in the amplitude of the useful signal. The highest rate of change in the amplitude of the useful signal can be characterized by a certain minimum time during which this change can occur (from a steady value to the maximum deviation) when the metal detector sensor moves relative to a metal object. Obviously, the maximum rate of change in the amplitude of the useful signal will be observed at the maximum speed of the sensor. It can be up to 5 m/s for the "pendulum" movement of the sensor on the rod. The useful signal amplitude change time can be estimated as the ratio of the sensor base to the movement speed. By setting the minimum value of the sensor base equal to 0,2 m, we obtain the minimum time for changing the useful signal amplitude of 40 ms. This is several times greater than the time constant of the integrating circuit for the selected values of resistors R13, R14 and capacitor C10. Consequently, the inertia of the integrating circuit will not distort the dynamics of even the fastest of all possible changes in the amplitude of the useful signal from the metal detector sensor. The output signal of the integrating circuit is taken from the capacitor SU. Since the latter has both plates under "floating potentials", the UPS is a differential amplifier made on the D6 op-amp. In addition to amplifying the constant signal, the OPA performs the function of a low-pass filter (LPF), which additionally attenuates unwanted high-frequency components at the output of the synchronous detector, mainly associated with the imperfection of the balanced modulator. The low-pass filter is implemented thanks to the capacitors C11, C13. In contrast to the other components of the metal detector, the op amp of the UPS should approach precision op amps in terms of its parameters. First of all, this refers to the value of the input current, the value of the bias voltage and the value of the temperature drift of the bias voltage. A good option, combining good parameters and relative accessibility, is an OU of the K140UD14 (or KR140UD1408) type. The correction circuit of the op-amp D6 consists of a correction capacitor C12 with a capacity of 33 pF. Nonlinear amplifier The non-linear amplifier is based on the D7.1 op-amp with a non-linear voltage feedback. Nonlinear OOS is implemented by a two-terminal device consisting of diodes VD1-VD8 and resistors R20-R24. The amplitude characteristic of a non-linear amplifier approaches the logarithmic one. It is a piecewise linear, with four break points for each polarity, approximation of the logarithmic dependence. Due to the smooth shape of the current-voltage characteristics of the diodes, the amplitude characteristic of the nonlinear amplifier is smoothed at the break points. The low-signal voltage gain of the nonlinear amplifier is: Kuk = - (R23+R24)/R19 = -100. As the amplitude of the input signal increases, the gain decreases. The differential gain for a large signal is: dUout/dUin = - R24/R19 = = -1. A pointer measuring device is connected to the output of the nonlinear amplifier - a microammeter with an additional resistor R25 connected in series. Since the voltage at the output of a synchronous detector can have any polarity (depending on the phase shift between its reference and input signals), a microammeter with zero in the middle of the scale is used. Thus, the pointer device has an indication range of -100 ... 0 ... +100 μA. The correction circuit of the op-amp D7.1 consists of a correction capacitor C18 with a capacity of 33 pF. Minimum limiter The minimum limiter is implemented on the D7.2 op-amp with a non-linear parallel voltage feedback. The non-linearity is enclosed in the input two-terminal network and consists of two anti-parallel connected diodes VD9, VD10 and resistor R26.
The formation of an indication sound signal from the output signal of a non-linear amplifier begins with one more adjustment of the amplitude characteristic of the amplifying path. In this case, a dead zone is formed in the region of small signals. This means that the sound indication is turned on only for signals that exceed a certain threshold. This threshold is determined direct voltage diodes VD9, VD10 and is about 0,5 V. Thus, weak signals associated mainly with the movement of the device and its mechanical deformations are cut off and do not irritate the ear. The small-signal limiter gain is at a minimum of zero. The differential voltage gain for a large signal is: dUout / dUin = - R27 / R26 = -1. The correction circuit of the op-amp D7.2 consists of a correction capacitor C19 with a capacity of 33 pF. Balance modulator The sound indication signal is formed as follows. A constant or slowly changing signal at the output of the limiter is multiplied to a minimum by the reference signal of the audible indication. The reference signal sets the shape for the audio signal, and the output signal of the minimum limiter sets the amplitude. The multiplication of two signals is carried out using a balanced modulator. It is implemented on a D11 multifunctional switch operating as an analog key, and a D8.1 op amp. The transfer coefficient of the device is +1 when the key is open and -1 when the key is closed. The correction circuit of the op-amp D8.1 consists of a correction capacitor C20 with a capacity of 33 pF. Reference Signal Conditioner The reference signal shaper is implemented on a binary counter D9 and a counter-decoder D10. Counter D9 divides the 8 kHz frequency from the ring counter output to 2 kHz and 32 Hz. A signal with a frequency of 2 kHz is supplied to the least significant bit of the AO address of the D11 multifunctional switch, thus setting the tone signal with the most sensitive frequency for the human ear. This signal will affect the analog key of the balanced modulator only if there is a logical 1 on the high-order bit of the address A11 of the multifunction switch D1. At a logical zero on A1, the analog key of the balanced modulator is always open. The sound indication signal is generated intermittently so that hearing is less tired. To do this, a counter-decoder D10 is used, which is controlled by a clock frequency of 32 Hz from the output of a binary counter D9 and generates at its output a rectangular signal with a frequency of 8 Hz and a ratio of the duration of a logical unit and a logical zero equal to 1/3. The output signal of the counter-decoder D10 is supplied to the high-order bit of the address A1 of the multifunctional switch D11, periodically interrupting the formation of a tone message in the balanced modulator. Piezo Buzzer Amplifier The piezoelectric amplifier is implemented on the D8.2 op-amp. It is an inverter with a voltage gain Ki = - 1. The amplifier load - a piezoelectric radiator - is connected in a bridge circuit between the outputs of the op-amp D8.1 and D8.2. This allows you to double the amplitude of the output voltage at the load. Switch S is designed to turn off the sound indication (for example, when setting up). The correction circuit of the OU D8.2 consists of a correction capacitor C21 with a capacity of 33 pF. Part types and design The types of microcircuits used are given in Table. 3. Instead of K561 series microcircuits, it is possible to use K1561 series microcircuits. You can try to apply some chips of the K176 series and foreign analogues. Table 3. Types of microcircuits used
Dual operational amplifiers (op-amps) of the K157 series can be replaced by any single general-purpose op-amps of similar parameters (with corresponding changes in the pinout and correction circuits), although the use of dual op-amps is more convenient (the mounting density increases). The operational amplifier of the synchronous detector D6, as already mentioned above, should approach precision op amps in terms of its parameters. In addition to the type indicated in the table, K140UD14, 140UD14 are suitable. It is possible to use OU K140UD12, 140UD12, KR140UD1208 in the corresponding switching circuit. There are no special requirements for the resistors used in the metal detector circuit. They just need to be sturdy and easy to install. The power dissipation rating is 0,125 ... 0,25 W. Compensation potentiometer R6 is desirable multi-turn type SP5-44 or with vernier adjustment type SP5-35. You can get by with conventional potentiometers of any type. In this case, it is advisable to use two of them. One - for rough adjustment, with a nominal value of 10 kOhm, included in accordance with the diagram. The other is for fine tuning, connected according to the rheostat circuit into the gap of one of the extreme conclusions of the first potentiometer, with a nominal value of 0,5 ... 1 kOhm. Capacitors C15, C17 - electrolytic. Recommended types - K50-29, K50-35, K53-1, K53-4 and other small ones. The remaining capacitors, with the exception of the capacitors of the oscillatory circuits of the receiving and emitting coils, are ceramic type K10-7 (up to 68 nF) and metal-film type K73-17 (values above 68 nF). Circuit capacitors - C2 and C5 - are special. They are subject to high demands on accuracy and thermal stability. Each capacitor consists of several (5 ... 10 pcs.) Capacitors connected in parallel. Tuning the circuits into resonance is carried out by selecting the number of capacitors and their rating. The recommended type of capacitors is K10-43. Their thermal stability group is MPO (i.e., approximately zero TKE). It is possible to use precision capacitors and other types, such as K71-7. In the end, you can try using old-fashioned thermostable mica capacitors with silver plates such as KSO or polystyrene capacitors. Diodes VD1-VD10 type KD521, KD522 or similar low-power silicon. Microammeter - any type, designed for a current of 100 μA with zero in the middle of the scale. Small-sized microammeters, for example, type M4247, are convenient. Quartz resonator Q - any small-sized watch quartz (similar quartz resonators are used in handheld electronic games). Power switch - any type of small-sized. Batteries - type 3R12 (according to the international designation) and "square" (according to ours). Piezo emitter Y1 - can be type ЗП1-ЗП18. Good results are obtained when using piezo emitters of imported telephones (they go in huge quantities "to waste" in the manufacture of telephones with caller ID). Device design can be quite arbitrary. When developing it, it is desirable to take into account the recommendations outlined below, as well as in the paragraphs on sensors and housing design. The appearance of the device is shown in fig. 7.
By its type, the sensor of the proposed metal detector refers to sensors with perpendicular axes. The sensor coils are glued from fiberglass with epoxy glue. The windings of the coils together with the fittings of their electric screens are filled with the same glue. The metal detector rod is made of an aluminum alloy pipe (AMGZM, AMG6M or D16T) with a diameter of 48 mm and a wall thickness of 2...3 mm. The coils are glued to the rod with epoxy glue: coaxial (radiating) - with the help of a transitional reinforcing sleeve; perpendicular to the axis of the rod (receiving) - using a suitable form of adapter. These auxiliary parts are also made of fiberglass. The housing of the electronic unit is made of foil fiberglass by soldering. The connections of the sensor coils with the electronic unit are made with a shielded wire with external insulation and are laid inside the rod. The shields of this wire are connected only to the common wire bus on the electronics board of the device, where the shield of the housing in the form of a foil and a rod are also connected. Outside the device is painted with nitro enamel. The printed circuit board of the electronic part of the metal detector can be made by any of the traditional methods; it is also convenient to use ready-made breadboard printed circuit boards for the DIP package of microcircuits (2,5 mm pitch). Setting up the device It is recommended to set up the device in the following sequence. 1. Check the correct installation according to the circuit diagram. Make sure that there are no short circuits between adjacent PCB conductors, adjacent microcircuit legs, etc. 2. Connect batteries or a bipolar power supply, strictly observing the polarity. Turn on the device and measure the consumed current. It should be about 20 mA on each power rail. A sharp deviation of the measured values from the indicated value indicates incorrect installation or malfunction of the microcircuits. 3. Make sure that there is a pure meander at the output of the generator with a frequency of about 32 kHz. 4. Make sure that there is a meander with a frequency of about 2 kHz at the outputs of triggers D8. 5. By selecting capacitor 02, set the output circuit L1C2 to resonance. In the simplest case - by the maximum amplitude of the voltage across it (about 10 V), and more precisely - by the zero phase shift of the circuit voltage relative to the meander at the output 12 of the trigger D2. 6. Make sure that the receiving amplifier is working. Set its input oscillatory circuit L2C5 to resonance. As an input signal, a parasitic signal penetrating from the radiating coil is quite sufficient. Tuning into resonance, as for the output circuit, is carried out by soldering or removing the required number of capacitors of suitable ratings. 7. Make sure that the parasitic signal can be compensated with the potentiometer R6. To do this, first, the output of the op-amp D5.2 is controlled by an oscilloscope. When the axis of the potentiometer R6 is rotated, the amplitude of the signal with a frequency of 8 kHz at the output of the op amp D5.2 should change and in one of the middle positions of the R6 slider this amplitude will be minimal. Next, you should check the output of the synchronous detector - the output of the op-amp D6. When the axis of the potentiometer R6 is rotated, the level of the constant signal at the output of the op-amp D6 must change from the maximum value of +3,5 V to the minimum value of -3,5 V or vice versa. This transition is quite sharp and in order to "catch" it, it is just convenient to use the fine tuning mentioned above. The setting consists in setting, using the potentiometer R6, the voltage at the output of the op-amp D6, equal to zero. Attention! Adjustment with potentiometer R6 must be carried out in the absence of large metal objects near the coils of the metal detector sensor, including measuring instruments! Otherwise, when these objects are moved or when the sensor is moved relative to them, the device will be upset, and if there are large metal objects near the sensor, it will not be possible to set the output voltage of the synchronous detector to zero. See also the paragraph on possible modifications for compensation. 8. Verify the operation of the non-linear amplifier. The easiest way is visually. The microammeter must respond to the tuning process made by the potentiometer R6. At a certain position of the R6 slider, the microammeter needle should be set to zero. The farther the arrow of the microammeter is from zero, the weaker the microammeter should react to the rotation of the R6 engine. It may turn out that an unfavorable electromagnetic environment will make it difficult to adjust the device. In this case, the microammeter needle will make chaotic or periodic oscillations when the potentiometer R6 slider approaches the position in which the signal compensation should take place. The described undesirable phenomenon is explained by the interference of the higher harmonics of the 50 Hz network on the receiving coil. At a considerable distance from the wires with electricity, the arrows should not fluctuate during tuning. 9. Make sure that the nodes that generate the sound signal are working. Pay attention to the presence of a small dead zone on the sound signal near zero on the scale of the microammeter. If there are malfunctions and deviations in the behavior of individual components of the metal detector circuit, you should act according to the generally accepted method:
Possible modifications The scheme of the device is quite simple and therefore we can only talk about further improvements. These include: 1. Adding an additional compensation potentiometer R6 *, connected in parallel with R6 at the extreme conclusions. The engine of this potentiometer is connected through a capacitor with a capacity of 510 pF (it is necessary to clarify experimentally) to the inverting input 5 of the D5.2 op-amp. In this configuration, there will be two degrees of freedom when compensating for a spurious signal (by sine and cosine), which can help tune the device when operating with significant temperature differences in the sensor, with high soil mineralization, etc. 2. Addition of an additional visual indication channel containing a synchronous detector, a nonlinear amplifier and a microammeter. The reference signal of the synchronous detector of the additional channel is taken with a shift of a quarter of the period relative to the reference signal of the main channel (from any output of another ring counter trigger). Having some experience in searching, one can learn to evaluate the nature of the detected object, i.e. according to the readings of two pointer instruments. work no worse than an electronic discriminator. 3. Addition of protective diodes connected in reverse polarity in parallel with power supplies. In case of an error in the polarity of the batteries, in this case it is guaranteed that the metal detector circuit will not suffer (although if you do not react in time, the incorrectly connected battery will be completely discharged). It is not recommended to turn on the diodes in series with the power buses, since in this case 0,3 ... 0,6 V of the precious voltage of the power sources will be wasted on them. Type of protective diodes - KD243, KD247, KD226, etc. Author: Shchedrin A.I.
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