Single-coil metal detector of induction type, theory. Scheme, description

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Single-coil metal detector of induction type, theory. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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The word "induction" in the name of metal detectors of this type fully reveals the principle of their work, if we remember the meaning of the word "inductio" (lat.) - guidance. The device of this type has as part of the sensor 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-radiated) signal, which "induces" an additional electrical signal in the coil. It remains only to select this additional signal.

The induction-type metal detector gained the right to life, mainly due to the main drawback of devices based on the "transmission-reception" principle - the complexity of the design of the sensors. This complexity leads either to the high cost and complexity of manufacturing the sensor, or to its insufficient mechanical rigidity, which causes the appearance of false signals during movement and reduces the sensitivity of the device.

If we set ourselves the goal of eliminating this shortcoming in devices based on the "transmission-reception" principle by eliminating its very cause, then we can come to an unusual conclusion - the emitting and receiving coils of a metal detector must be combined into one! In fact, there are no very undesirable movements and bends of one coil relative to the other in this case, since there is only one coil and it is both emitting and receiving at the same time. There is also the extreme simplicity of the sensor. The trade-off for these advantages is the need to isolate the useful return signal from the much larger transmitter/receiver coil drive signal.

The reflected signal can be distinguished 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 done in one of the ways is shown in Fig. 3.

Single-coil metal detector of induction type, theory
Rice. 3. Structural diagram of the input node of the induction metal detector

The generator generates a sinusoidal alternating voltage with a constant amplitude and frequency. The "voltage-current" converter (PNT) converts the generator voltage Ug into current Ig, which is set in the oscillatory circuit of the sensor. The oscillatory circuit consists

from capacitor C and coil L of the sensor. Its resonant frequency is equal to the frequency of the generator. The PNT conversion coefficient is chosen so that the voltage of the oscillatory circuit id is equal to the voltage of the generator Ug (in the absence of metal near the sensor). Thus, two signals of the same amplitude are subtracted at the adder, 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 oscillatory circuit Ud. The output is a non-zero signal.

On fig. 3 shows only the simplest version of one of the schemes of the input part of the metal detectors of the type under consideration. Instead of PNT in this circuit, in principle, it is 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 inclusion of an oscillatory 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 a zero phase shift between the signals Ug and Ud (the circuit is tuned to resonance). It is possible to abandon the oscillatory circuit with the need to fine-tune it to resonance and use only the sensor coil as a PNT load. However, the POT gain for this case must be complex to correct for the 90° phase shift resulting from the inductive nature of the POT load.

Author: Shchedrin A.I.

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