Enamel thickness control on the body. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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In "Radio", 2002, No. 2 in the article by A. Belsky "Evaluation of paintwork thickness"A simple device was described that can detect places with an uneven thickness of the protective layer of enamel on a car body. Below we provide a description of another coating thickness gauge that has higher performance.

Over the past few years, Russia has seen an increased demand for foreign-made used cars. It's no secret that many of these cars have bodies that have undergone refurbishment after various traffic accidents.

An almost flawless layer of enamel often hides heavily dented places underneath, and even the fact of welding a two-piece body. They sell these cars at "unbeaten" prices. A coating thickness gauge can help identify such defects.

The device described by A. Belsky in "Radio", 2002, No. 2, p. 57 can measure the thickness of enamel applied to a ferromagnetic base only. The sensor is designed to work with flat surfaces - even with a slight curvature, the measurement error increases sharply. In addition, 220 V AC is required to power the instrument.

The coating thickness meter described below is a capacitive probe connected to a low capacitance meter (which can be any handheld multimeter with "Cx" function). The readings of the meter do not depend on the metal on which the controlled coating is applied (steel, galvanized steel, aluminum alloy, etc.), nor on its thickness.

The sensor consists of two plates of two capacitors connected in series, the third common plate of which is a metal surface with the coating under study (Fig. 1, a and b). If we assume that the area of ​​the plates is strictly the same and the coating is uniform in thickness and dielectric constant, the capacitance of the capacitors is C1=C2=C, and the capacitance of the sensor is Cd = 0,5C + Cp, where Cp is the parasitic capacitance between the working plates and the conductors connecting the sensor to capacitance meter.

It is easy to see that, firstly, the resolution of the sensor is greater, the smaller the parasitic capacitance C with respect to C, and, secondly, the sensor capacitance and coating thickness are inversely related.

The capacitance meter (I use the MASTECH M890G) is powered by a built-in battery. No power sensor required.

The design of the sensor is not very critical. One of its variants is shown in Fig. 2. A standard mains plug is used as a housing, in which the contact pins are removed (the pin holder is retained) and the working edge is aligned with a flat file.

A disk with a diameter of 37 mm, cut out of fiberglass with a thickness of 1,5 ... 2 mm, is attached to the edge with Moment glue. Previously, two holes are drilled in the disk for the leads of the plates. A disk of the same diameter is cut out of soft sheet microporous rubber with a thickness of 3 ... 4 mm with smooth surfaces and the same holes are cut in it. The rubber disc is glued to the fiberglass disc so that the holes match.

The plates are cut out of copper or brass foil 0,05 ... 0,1 mm thick with an allowance along the outer contour. To the plates, opposite the holes, solder MGTF or MGSHS wires with a cross section of 300 ... 0,07 mm0,25 along a 2 mm long lead. The output at the place of soldering must be perpendicular to the lining. The conclusions are passed through the holes and the plates are glued to the rubber disk with the same glue, as shown in the drawing. The protruding excess foil is cut off with scissors, aligned along the edges and deburred from the side of the working surface with fine-grained sandpaper.

To ensure the minimum own capacitance of the connecting cable, it is better to make it yourself. To do this, each sensor output is put on a segment of a polyethylene tube - the internal insulation of a television cable. If the hole in the tube is so small that it is not possible to thread the leads into it, they must be replaced with pieces of winding wire with a diameter of 0,1 ... 0,12 mm. The ends of the cable must be fixed in the sensor housing and on the connector block.

The block is cut out of foil fiberglass with a thickness of 1 mm. The foil must be removed, leaving it only on the contact pins.

With careful execution of the sensor, its total capacitance does not exceed 10 pF. For comparison: the measured capacitance when determining a coating thickness of about 0,05 mm is about 100 pF.

Now it remains only to compile the calibration table in the same order as described in the mentioned article by A. Velsky. Sheets of paper of known thickness are used as the "gauge". Despite the fact that the calibration dependence is non-linear, in practice this is not so important, since the main purpose of the device is to search for areas with a coating of uneven thickness.

Briefly about working with the device. Before measurement, the surface to be examined must be cleaned of dust and dried. The sensor is pressed tightly against the surface in a place with minimal curvature, and the maximum reading of the device is achieved. The measurements are repeated at several characteristic points. If the object of study is a car, it is advisable to first take a similar table of instrument readings from a new car of this model in a car dealership.

In conclusion, I note that many will find it more convenient to work with an analog capacitance meter with a microammeter at the output and battery power. Descriptions of such devices can be found in the magazine "Radio".

Author: I. Chekhovsky, Shchelkovo, Moscow Region

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