A device for detecting moving metal objects. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A device for detecting moving metal objects. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Purpose and principle of operation

The principle of operation of the described device is based on the effect of eddy currents arising in a metal object on the quality factor of the coil that creates a magnetic field.

It can be used in many cases:

to detect unwanted metal objects in the moving mass of any raw material or finished product;
to count metal parts or the number of vehicles passing through a checkpoint, etc.

Schematic diagram

The schematic diagram of the device is shown in fig. 2.26. It consists of four main parts:

generator (VT1, VT2);
its signal processing unit (VT3, DA1);
electronic relay (VT4, VT5, K1);
sensor, which is the coil L1, which forms the oscillatory circuit of the generator with capacitors C1 and C2.

Transistor VT2 performs the functions of a stable current source and dynamic load of transistor VT1. The amplitude of the generated oscillations is stabilized by applying to the gate of this transistor (through the integrating circuit R4C6R5 and the coil L1) the constant component of the generator output voltage rectified by the diodes VD1, VD2.

The signal processing unit contains:

emitter follower transistor VT3;
rectifier, made according to the scheme of doubling the voltage on the diodes VD3, VD4;
integrating circuits R9C11R11C13 and R10C12R12C14 with different time constants and a comparator on the op-amp DA1.

Rice. 2.26. Schematic diagram of a device for detecting moving metal objects (click to enlarge)

In steady state, the output voltage of the comparator is zero, transistors VT4, VT5 are closed, and relay K1 is de-energized. At the moment of power-up, the gate potential of the transistor VT1 is zero, its steepness is maximum, and the generator is self-excited. As the capacitor C6 is charged, the oscillation amplitude gradually decreases and after a few seconds it stabilizes at a certain level. Around the same time, zero voltage is set at the output of the op-amp DA1.

When a metal object approaches coil L1, the losses in the L1C1C2 circuit increase, and the amplitude of the generated oscillations decreases for some time (determined by the parameters of the R4C6R5 circuit elements). As a result, the voltage at the output of the rectifier (VD3, VD4), and with a slight delay - and at the inverting input of the op-amp DA1 decreases. Since the potential of its other input does not have time to change by this time (due to the larger time constant of the R10C12R12C14 circuit), the output voltage of the comparator drops abruptly.

At the same time, transistors VT4, VT5 open, and relay K1 is activated, giving a command to stop the conveyor belt or turning on the alarm. With the ratings of the elements of the integrating circuits indicated on the diagram, the device detects objects moving at a speed of more than 0,5 m/s.

Adjustment

The sensitivity can be adjusted by a tuned resistor R1. Slow voltage fluctuations at the inputs of the integrating circuits (when the speed of movement of metal objects is less than the specified one) does not cause a potential difference at the inputs of the op-amp DA1, so the comparator does not work.

The device does not respond to stationary metal objects in the vicinity of the L1 coil.

Applied Parts

Coil L1 is made in the form of a round frame with a diameter of 320 mm and contains 250 turns of stranded winding wire (litz wire) PEL 35 x 0,05. Coil inductance 42 mH, DC resistance 32 ohms. The frequency of oscillations generated by the device is about 23,5 kHz.

In the device, you can use the K553UD1A OU, transistors of the KT315 (VT3), KT349 (VT4), KT608A (VT5) series, diodes of the D220 (VD1-VD5) and D223 (VD6) series, RES9 relay (passport RS4.524.202).

Author: Tenev L.

See other articles Section metal detectors.

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