Thickness gauge for insulating coatings. Encyclopedia of radio electronics and electrical engineering

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The principle of operation is based on measuring the quality factor and inductance of the coil when it approaches a conductive metal surface.

This simple device can measure the thickness of insulating coatings on metals and determine the type of substrate metal (colored or black) without destroying the coating. With it, you can, for example, find putty under a layer of paint on a car body and at the same time check whether the body metal is galvanized. The measurement limit is 0,5-8 mm for steel and cast iron, 0,3-5 mm for non-ferrous metals.

The measuring coil L1 is part of the oscillatory circuit (L1, C1, C2, C3) of the generator on the elements DD1.1, DD1.2 with a generation frequency of about 350 kHz. A feature of the generator circuit is its ability to operate stably with a significant change in the voltage amplitude on the circuit, this is achieved due to the high gain in the feedback loop. Since the "pump" power (a rectangle with a CMOS level coming from the output of the DD1.2 element through R3 to C2) does not depend on the voltage at the inputs of DD1.1, the amplitude of the oscillations in the circuit decreases with increasing losses in L1 and vice versa.

When the sensor approaches the metal, the alternating magnetic field of the coil induces eddy currents on its surface, causing an increase in losses (decrease in the quality factor) and changing the inductance. This, in turn, affects the amplitude and frequency of oscillations. The sinusoidal signal is taken through R2, amplified by VT1, rectified by diodes VD3, VD4 and fed to the PA1 measuring device, which determines the coating thickness. Resistor R2 sets the arrow of the device to the final division of the scale before starting the measurement. Since non-ferrous metals have better conductivity and worsen the quality factor to a lesser extent, but significantly reduce the inductance (the generator frequency increases by 10-15%), a non-ferrous metal detector is introduced into the device (threshold frequency detector on elements DD1.3, DD1.4 and transistor VT1 ).

The detector works as follows: A rectangular signal with a generation frequency is taken from pin 11 of the microcircuit, goes to input 6 of the DD1.3 element directly and to input 5 through a phase-shifting circuit and an inverting amplifier-shaper DD1.4. If the tuning frequency of the phase-shifting circuit coincides with the generation frequency (there is no phase shift in the circuit R4, L2, C6), there is an anti-phase voltage at pins 5 and 6 and, accordingly, logical 0 at pin 4. When the frequency of the measuring generator increases (lowers), the circuit begins to shift the phase of the signal . At the input 5 DD1.3 pulses come with a phase lag. A log appears at pin 4 MC. 1 at the moments of coincidence log. 0 at the inputs DD1.3. From the pulses, a constant component is selected by the chain R7, C10, and when the opening voltage of VT2 and VD2 is reached, the VD1 LED will light up. The device is powered by a Krona battery (6F22). The consumed current does not exceed 5 mA. Buttons SB1 and SB2 of the type MP10 or MP-1, the first - turning on the device, the second - monitoring the battery voltage (switches the device to the battery circuit through R11 and diodes VD3, VD4 of the rectifier). Button pushers are cut from thick rubber.

Measuring coil L1 contains - 100 turns of PEV 0,1. It is wound in a half of the SB-12 core made of carbonyl iron, filled with epoxy resin and glued flush into the front wall of the case with the open part outward. Coil L2 is also wound, only the core is assembled and installed on the board. Loop capacitors C1 and C6 for better thermal stability were used of the same type with a small TKE. R2 type SP4-1. Measuring device M4247 (total deflection current 100 μA, frame resistance 2,9 kOhm) is glued into the cutout of the front wall (mounting lugs are sawn off). Rectifier diodes VD3, VD4 are necessarily germanium, and VD5 and VD2 are silicon. The rest of the details are unremarkable. The side walls of the case (dimensions 160x54x26) are made of facing three-layer plastic with a thickness of 3mm, the front and rear walls are made of textolite (8mm). The printed circuit board is installed on 4 racks 4-5 mm high.

Set R2 to the minimum gain position and select R3 to set the instrument pointer to the middle of the scale. Then, using R2, set the arrow to the final division and bring a flat plate of steel or cast iron close to the sensor, select R8 to set the arrow of the device to 0. Select C6 roughly and L2 core accurately, start ignition of VD1 when the sensor approaches the aluminum or copper plates on 4-6 mm (it should be noted that when the sensor comes into contact with non-ferrous metals, the device will show 20-30 μA). For accurate measurements, the device must be calibrated by placing insulation plates of known thickness between the sensor and the metal. The results can be entered in a table or graph and glued to the top cover of the housing (for ferrous and non-ferrous metals, the graduation is different). If frequent measurements of the same products are required, the measurement accuracy can be improved.

To do this, you need to make a metal ruler from the same metal as the product being measured, apply an insulation layer on it in any way with a smooth change in thickness and put down divisions in accordance with the current layer thickness. The meter is first applied to the surface to be measured, then the arrow is set to the maximum possible division of the scale by resistor R2, after which the device is transferred to the manufactured strip and moves along until the readings match. The thickness is read by divisions on the ruler. With this method of measurement, instrument errors do not affect the measurement accuracy.

As practice has shown, the accuracy of measurements is little affected by the humidity of the coating and the thickness of the substrate metal, but when working with non-ferrous metals, the error is introduced by the cleanliness of the surface treatment. It should be taken into account that the device reacts only to the surface layer of metal and if the substrate, for example, galvanized steel, then the LED will show non-ferrous metal and the measurement will be carried out, respectively, according to the scale of non-ferrous metal. Also, the device can determine ferrite materials, while the LED will light up (by lowering the frequency) and the quality factor of the coil will increase (the arrow of the device will go up).

Literature

1. V. Naryzhny. Polymer film thickness gauge. Radio, No. 3, 2004 p. 47.

Author: S.Bartsov

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