A device for detecting moving metal objects. Scheme, description

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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 / Radio amateur designer

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

The schematic diagram of the device is shown in the figure. It consists of a generator (VT1, VT2), a node for processing its signal (VT3, DA1) and an electronic relay (VT4, VT5, K1). Coil L1 serves as the sensor. forming with capacitors C1 and C2 the oscillatory circuit of the generator. 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.

(click to enlarge)

The signal processing unit contains an emitter follower on a VT3 transistor, a rectifier. made according to the scheme of doubling the voltage on diodes VD3, VD4, integrating targets R9C11R11C13 and R10C12R12C14 with different time constants and a comparator on the op-amp DA1. In steady state, the output voltage of the comparator is 0, 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 equal to 0, 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 the 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, and since the potential of its other input does not have time to change by this time (due to the greater time constant of the R10C12R12C14 circuit), the output voltage comparator jumps down. At the same time, transistors VT4, VT5 open, and relay K1 is activated, giving a command to stop the conveyor belt or turning on an 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. The sensitivity is adjusted by the trimmer resistor R1. Slow voltage fluctuations at the inputs of the integrating circuits (the speed of movement of metal objects is less than the specified one) do 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.

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 35X0,05. 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.; Publication: N. Bolshakov, rf.atnn.ru

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