RU2456589C1 - Method for eddy current-measurement of thickness of metal coatings - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: for probing, a harmonic signal at a selected frequency is transmitted to an exciting coil placed in the middle of a ferrite core; signals are picked up from identical measurement and compensation coils placed on opposite ends of the ferrite core; their difference signal is found and its phase φ is measured; when probing the measuring object, the end of the ferrite rod with the measurement coil is pressed to said object; according to the invention, the optimum frequency of the exciting coil is selected based on the relationship: f_opt=(0.3…0.7)/(πµ₀σ_n T² _nax), where µ₀ is the magnetic constant; σ_n is conductivity of the coating; - maximum thickness of the coating, before the beginning of measurement, the measurement and compensation coils are balanced by achieving a zero difference signal in the absence of the measuring object; a spacer made from non-ferromagnetic metal whose electroconductivity is close to that of the coating material is placed between a ferrite core and the measuring object; the thickness of the spacer is such that with tentatively known parameters of the analysed object and the ferrite core with windings, maximum calculated sensitivity of amplitude-phase characteristics of the difference signal from the thickness of the coating of the object is provided; during probing, the amplitude A of the difference signal is further measured; the measuring device is graduated, for which measuring objects are probed, having characteristics close to the analysed object, with several known coating thicknesses T_n and for several known values of gaps h between the ferrite core with the spacer and the surface of the measuring objects; the relationship between the amplitude and phase of difference signals and the thickness of the coating and gaps A, φ(T_n h) is stored; the analysed object is probed; the thickness of the coating T_n and the gap h between the ferrite core with the spacer and the surface of the coating of the object is calculated based on measured values of amplitude A and phase φ of the difference signal using the relationship A, φ (T_n h).

EFFECT: possibility of measuring thickness of thin non-ferromagnetic metal coatings deposited on non-ferromagnetic bases, including in conditions when electroconductivity of the coating is lower than that of the base, with possibility of measuring objects with rough surfaces and curvilinear surfaces, having an additional non-electroconductive coating.

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Description

The invention relates to the field of non-destructive testing by the method of eddy currents and can be used to measure the thickness of thin non-ferromagnetic coatings of bismuth, lead, zinc, cobalt, cadmium and their alloys having lower electrical conductivity than non-ferromagnetic bases of copper, brass, bronze, silver, etc. .P. In addition, the method can be used to measure coating thicknesses of products with a rough and (or) curved surface, as well as having a non-conductive coating in the form of varnish, paint, etc.

The listed materials and alloys are widely used in the electrical industry. Moreover, the coating thicknesses T _p are usually small - from 0.5 to 21 microns [1]. The electrical conductivity of the coatings σ _p usually lies in the range (6-9.5) MSm / m, and the conductivity of the bases σ _о can be (16-60) MSm / m. Thus, the relative conductivity is less than unity, σ _p / σ _o = (0.11-0.55). The bases often turn out to be small-sized products with a measuring zone of diameter 2 ... 5 mm, with non-planar forms, for example cylindrical, with a diameter D (2-10 mm), tubular with a minimum wall thickness of up to 0.3 mm, including a rough surface R _z = 5-15 microns. In addition, additional non-conductive coatings - varnishes, paints, etc., are sometimes applied to finished products. Thus, the features of the problem of measuring the thickness of the coating under the considered conditions are that:

- the electrical conductivity of the coating is close and less than the electrical conductivity of the base, which greatly complicates the measurement;

- the curvature of the products, their roughness and additional coatings significantly affect the contact of the meter with the measurement object and, accordingly, the measurement accuracy.

These features make high demands on the thoroughness of the choice of methods and means of measurement.

A known method of measuring the properties of metal coatings [2], which consists in the fact that the inductor in the coating material with different gaps h between the meter and the measurement object excites a pulsed current with a frequency that provides the maximum degree of dependence on it of controlled thermophysical parameters, measure the temperature of the coating material, according to which the thermal properties of the coating are judged, and the informative parameters of the mother are judged by the output electrical signals of the vortex-thermal converter la coating.

The disadvantages of this method are the low accuracy and speed of measurements associated with the complexity of the quick and accurate measurement of temperature indicators of the material. The method for determining the informative parameters of the coating material, for example thickness, is not considered in the patent [2].

Known instruments for measuring thicknesses, including electrically conductive non-ferromagnetic coatings on electrically conductive non-ferromagnetic substrates, in particular PosiTector 6000 NAS2 Standard [3], Pocket-Surfix [4] and others. Such devices are designed to measure the thickness of non-ferromagnetic coatings, provided that the conductivity of the coating is higher than the conductivity of the base: σ _p / σ _o = 6-30, for example, copper on stainless steel, silver on brass, etc.

Thus, the disadvantage of these devices is the limited scope in terms of the ratio of the characteristics of the materials of the bases and coatings.

Closest to the claimed is a method of eddy current measurement of the thickness of the coatings [5], which consists in the fact that for probing a harmonic signal of the selected frequency is applied to the excitation coil installed in the middle of the ferrite core, signals are taken from the same measuring and compensation windings installed at opposite ends of the ferrite core, find their difference signal and measure its phase φ, when probing the measurement object, press the end of the ferrite rod with the measurement skein.

It is known [6] that the possibilities of using eddy current methods using phase measurements of a difference signal are significantly limited due to the low sensitivity in the coating region up to 20 μm thick. Especially for the case of σ _p / σ _o = (0.11-0.55).

The objective of the proposed method is to measure the thickness of thin non-ferromagnetic metal coatings deposited on non-ferromagnetic substrates, including under conditions when the conductivity of the coating is less than the conductivity of the base, with the possibility of measuring objects with rough coatings and (or) curved surfaces, as well as having an additional non-conductive coating.

To solve the problem in the method of eddy current measurement of the thickness of metal coatings, which consists in the fact that for probing a harmonic signal of the selected frequency is applied to the field coil installed in the middle of the ferrite core, signals are taken from the same measuring and compensation windings installed on opposite ends of the ferrite core, find their difference signal and measure its phase φ, when probing the measurement object, press the end of the ferrite rod to it, measure noy winding selected optimal frequency f _opt excitation winding from the relation:

f _opt ≈ (0.3 ... 0.7) / (π µ ₀ σ _p T ² _{p max} ),

where µ ₀ is the magnetic constant; σ _p - electrical conductivity of the coating; T _{p max} - maximum coating thickness,

Before starting the measurements, the measuring and compensation windings are balanced, achieving a zero difference signal in the absence of the measurement object, a gasket made of a non-ferromagnetic metal close in electrical conductivity to the coating material is installed between the ferrite core and the measurement object, the thickness of the gasket is selected so that for approximately known parameters of the studied object and a ferrite rod with windings provide the maximum design sensitivity of the amplitude-phase characteristics of the difference of the signal from the coating thickness of the object, when probing, the amplitude A of the difference signal is additionally measured, the meter is calibrated, for which measuring objects with characteristics close to the object under investigation are probed with several known coating thicknesses T _p and with several known values of the gaps h between the ferrite core with the gasket and the surface of the measured objects, keep the dependence of the amplitudes and phases of the difference signals on the thickness of the coating and the gaps A, φ (T _p , h), probe the object under study, calculate the thickness p coverage T _p and the gap h between the ferrite core with the gasket and the coating surface of the object according to the measured values of the amplitude and phase of the difference signal of the object under study, using the dependence A, φ (T _p , h).

The inventive method is illustrated by the following graphic materials:

Figure 1 - eddy current meter, where

1 - field winding;

2 - ferrite core;

3 - measuring winding;

4 - compensation winding;

5 - measurement object;

6 - coating;

7 - base;

8 - gasket;

9 - balancing scheme;

10 - balancing input;

11 - eddy current transducer;

12 is a detection unit.

Figure 2 - dependence of the complex differential signal from the thickness of the coating.

Figure 3 - dependence of the complex difference signal on the thickness of the coating and the height of the meter above the surface of the measurement object.

Figure 4 - meter above the rough surface of the measurement object.

5 is a meter above the rounded surface of the measurement object.

6 is a structural diagram of a device for measuring the thickness of the coating, where:

13 - microcontroller;

14 - control and indication device.

. Корректное использование этих сигналов позволяет определить искомые характеристики Т_п и h. Для решения этой задачи необходимо выбрать оптимальные параметры вихретокового преобразователя.For measuring the thickness of the coatings, eddy current transducers are usually used, Fig. 1. The principle of their action is that when sensing, a harmonic signal u _{at a} selected frequency f is supplied to the field winding 1. The field arising in the ferrite core 2 causes the appearance of the signals u _and and u _k in the same measuring 3 and compensation 4 windings, respectively. These signals are fed to the balancing circuit 9, where they are subtracted. In the absence of the object of study, the signals are equal to u _and = u _k = u ₀ , and the difference signal of the measuring 3 and compensation 4 windings (Δu) = (u _and -u _k ) at the output of the balancing circuit 9 is zero. If the end of the ferrite core 2, on which the measuring winding 3 is located, is pressed against the object of study 5, then the signal u _and changes the amplitude A and phase φ depending on the coating thickness T _p and the height (gap) h of the eddy current transducer over the measurement object, the conductivity of materials coating σ _p and base σ _about and other parameters. The difference signal of the measuring 3 and compensation 4 windings Δu is supplied to the detection unit 12. The signal u _to from the compensation winding 4 is used as a reference. At the output of the detection unit, signals are generated proportional to the amplitude modulus A (T _p , h) and phase φ (T _p , h) of the difference signal. These signals, Figure 2, can be depicted in a complex plane

. The correct use of these signals allows you to determine the desired characteristics of T _p and h. To solve this problem, it is necessary to choose the optimal parameters of the eddy current transducer.

, где R - эквивалентный радиус обмотки возбуждения 1; σ_и - интегральная электропроводность объекта измерений; µ₀ - магнитная постоянная. Для измерения малоразмерных объектов следует выбирать минимальные из технически реализуемых размеры ферритового стержня. Если ферритовый стержень 2 имеет диаметр 1 мм, то эквивалентный радиус обмотки возбуждения 1 может составлять R=0,5…0,7 мм. Вариация Т_п будет приводить к изменению интегрального значения электропроводности σ_и в зоне измерения и, соответственно, параметра β. Для обеспечения приемлемой чувствительности величину β(σ_п) следует выбирать на уровне порядка 5…30 [7].It is convenient to use the generalized parameter to calculate the parameters of the measuring transducer

where R is the equivalent radius of the field winding 1; σ _and - integrated electrical conductivity of the measurement object; µ ₀ is the magnetic constant. To measure small objects, you should choose the minimum of the technically feasible dimensions of the ferrite core. If the ferrite rod 2 has a diameter of 1 mm, then the equivalent radius of the field winding 1 can be R = 0.5 ... 0.7 mm. Variation T _n will lead to a change in the integral values of electrical conductivity σ _and a measurement zone and, accordingly, the parameter β. To ensure acceptable sensitivity, the value of β (σ _p ) should be chosen at the level of the order of 5 ... 30 [7].

, где σ - электропроводность материала (покрытия или основания).In the General case, the penetration depth of the eddy currents 8, which determines the maximum measured coating thickness T _{p max} and the minimum base thickness T _omin , is equal to:

where σ is the electrical conductivity of the material (coating or base).

In accordance with [8], the measuring range is T _{p max} = (0.6 ... 0.8) δ, and the minimum allowable base thickness _Tmin = 2.5δ. From the above relations it follows that the optimal value of the frequency of the Converter is equal to:

To ensure optimal β, the value of the equivalent radius R of the field winding for the eddy current phase transformer must be selected from the condition:

Consider in more detail the inventive method for estimating the coating thickness of the measurement object based on the amplitude A (T _p , h) and phase φ (T _p , h) of the difference signal.

может быть рассчитан теоретически по формулам [9]:The vector of the complex relative differential voltage

can be calculated theoretically by the formulas [9]:

at

- модуль напряжения на выходе измерительной обмотки при отсутствии объекта измерений, R_В - радиус обмотки возбуждения; R_И -радиус измерительной обмотки; z_и, z_в - расстояние от измерительной обмотки и обмотки возбуждения до поверхности изделия соответственно; J_i(λR_i) - функция Бесселя первого порядка; λ - параметр преобразования; М - коэффициент начальной взаимоиндукции между обмотками; ω=2πf -круговая частота тока возбуждения; U_К - напряжение на выходе компенсационной обмотки. При расчетах можно принять радиусы обмоток равными радиусу ферритового сердечника [10].Where

- voltage module at the output of the measuring winding in the absence of the measurement object, R _B - radius of the field winding; R _{And the} radius of the measuring winding; z _and , z _in - the distance from the measuring winding and the field winding to the surface of the product, respectively; J _i (λR _i ) is the first-order Bessel function; λ is the transformation parameter; M is the coefficient of initial mutual induction between the windings; ω = 2πf is the circular frequency of the excitation current; U _K - voltage at the output of the compensation winding. In the calculations, we can take the radii of the windings equal to the radius of the ferrite core [10].

в виде вещественной RE(U*_P) и мнимой IM(U*_P) составляющих для оловянно-свинцового покрытия медного основания, при h=0. В точке а Т_п=0, а в точке е Т_п=∞. На участке а-b в диапазоне Т_п=(0…15) мкм фаза φ практически не изменяется, а линия a-b-c-d-e совпадает с линией Н, которая характеризует изменение комплексного относительного разностного напряжения

при увеличении высоты h между ферритовым стержнем и измеряемым объектом, т.е. покрытия такой толщины с точки зрения вихретоковых измерений воспринимаются как зазор. Таким образом, участок а-b оказывается малоинформативным и не пригоден для измерения толщины покрытия Т_п. На участке b-с указанная чувствительность возрастает, но не достаточна для точного измерения толщины покрытия Т_п. Наконец после точки с при реализуемых на практике точностях измерения фазы φ и амплитуды А появляется возможность измерять Т_п=(0…15) мкм с абсолютной погрешностью на уровне ±(1…1,5) мкм. Для обеспечения возможности измерения толщины тонких покрытий (в диапазоне толщин 0…20 мкм) необходимо перенести начало измерений в точку с, например, путем установки, Фиг.1, прокладки 8 из материала, соответствующего или близкого по электропроводности материалу покрытия 6 (например, сплава олово-свинец), толщиной порядка Т_пр=(15-60) мкм, между ферритовым стержнем 2 и объектом измерения 5 (покрытием 6). На Фиг.2 показано, что при установке прокладки толщиной 55 мкм в точке с Т_п=0 и годограф

при изменении Т_п от нуля до Т_пmax будет располагаться между точками с-е.Figure 2 on the complex plane presents calculated in accordance with (2) the dependence (line a-bcde) of the complex relative differential voltage from the coating thickness -

in the form of real RE (U * _P ) and imaginary IM (U * _P ) components for the tin-lead coating of the copper base, at h = 0. At the point a Т Т _п = 0, and at the point е Т Т _п = ∞. On the plot a-b in the range T _p = (0 ... 15) μm, the phase φ practically does not change, and the line abcde coincides with the line H, which characterizes the change in the complex relative difference voltage

with increasing height h between the ferrite core and the measured object, i.e. coatings of this thickness from the point of view of eddy current measurements are perceived as a gap. Thus, section a-b is not very informative and is not suitable for measuring the coating thickness T _p. In section b-c, the indicated sensitivity increases, but is not sufficient for accurate measurement of the coating thickness T _p. Finally, after point c with practical phase measurement accuracy φ and amplitude A, it becomes possible to measure T _p = (0 ... 15) μm with an absolute error of ± (1 ... 1.5) μm. In order to be able to measure the thickness of thin coatings (in the thickness range 0 ... 20 μm), it is necessary to transfer the beginning of measurements to point c, for example, by installing, Fig. 1, gaskets 8 from a material corresponding to or similar in electrical conductivity to the coating material 6 (for example, tin-lead), a thickness of the order of T _ol = (15-60) microns, between the ferrite rod 2 and the measurement object 5 (coating 6). Figure 2 shows that when installing gaskets with a thickness of 55 μm at a point with T _p = 0 and hodograph

when T _p changes from zero to T _{p max} will be located between points _cf.

при разных значениях h. Эти зависимости могут быть рассчитаны теоретически, но большей точности измерений можно добиться за счет градуировки реальных приборов на образцовых объектах.Figure 3 presents a family of dependencies

at different values of h. These dependences can be calculated theoretically, but more accurate measurements can be achieved by calibrating real instruments on exemplary objects.

Significant differences of the proposed method are as follows.

The optimal f _opt frequency of the field winding is selected from relation (1). This choice of frequency allows you to select the best conditions for measuring the desired parameters of the measurement object.

In the prototype, the frequency of the field winding is selected by the operator based on the parameters of the material of the object of study, the requirements for resolution, etc., however, specific recommendations for choosing the frequency are not considered.

Before starting measurements, the measuring and compensation windings are balanced, achieving a zero difference signal in the absence of the measurement object, which eliminates the influence of various (parametric, temperature, etc.) sources of instability.

In the prototype, balancing is not performed.

A non-ferromagnetic metal gasket is installed that is close in electrical conductivity to the coating material between the ferrite core and the measurement object, and the gasket thickness is selected so that, with approximately known parameters of the test object and the ferrite core with windings, the maximum calculated sensitivity of the amplitude-phase characteristics (5) of the difference signal from the thickness of the coating of the object (Figure 2). The indicated actions make it possible to achieve a satisfactory sensitivity of the meter under difficult conditions of measuring materials similar in properties.

In the prototype, the sensitivity of the meter is not considered.

When probing, the amplitude A of the difference signal is additionally measured, which allows the use of the amplitude-phase characteristic of the difference signal, i.e. more complete information about the properties of the measurement object.

In the prototype, only the phase of the difference signal is measured.

Calibration of the meter by sensing measuring objects having characteristics close to the object under study, with several known thicknesses T _{p of the} coating and with several known values of the gaps h between the ferrite core with the gasket and the surface of the measuring objects and maintaining the dependence of the amplitudes and phases of the difference signals on the coating thicknesses and the gaps A, φ (T _p , h) allows you to get the dependence shown in figure 2, and due to this, to carry out measurements taking into account specific materials and measurement conditions.

In the prototype, graduation is not provided.

The thickness of the coating and the gap between the ferrite core with the gasket and the surface of the coating of the object are determined from the measured values of the amplitude and phase of the test object using the indicated dependence A, φ (T _p , h). The coating thickness is the desired parameter of the measurement object for all such devices, however, in this case, it becomes possible to measure thin coatings that are not able to measure other methods and devices known to the authors. The ability to measure the gap between the ferrite core with the gasket and the coating surface of the object allows you to evaluate the thickness of the dielectric coating, such as varnish, paint, etc., applied to the measurement object, to clarify the thickness of the coating in the presence of roughness and (or) curvature of the test object.

The prototype determines only the thickness, and not thin coatings.

Consider the possibility of implementing the proposed method.

.For sounding, a harmonic signal u _{in the} selected frequency f _{opt is} supplied to the field winding 1 installed in the middle of the ferrite core 2, signals are taken from the same compensation windings 3 - u _to and measuring 4 - u _and installed on opposite ends of the ferrite core 2. In practice to simplify the processing, instead of u _and easier to remove the residual signal Δu = u -u _{and to} a balancing circuit 9. Find the phase φ and amplitude a difference signal

.

In the manufacture of the meter balance measuring 4 and compensation 3 windings, achieving a zero difference signal at the output of the balancing circuit 9 in the absence of the measurement object 5.

и определяют необходимую толщину прокладки 8 так, чтобы обеспечить максимальную расчетную чувствительность амплитудно-фазовых характеристик разностного сигнала

от толщины покрытия Т_п объекта измерения 7. Устанавливают прокладку 8 из неферромагнитного металла, близкого по электропроводности к материалу покрытия 6, между ферритовым стержнем 2 и объектом измерений 5.According to the known characteristics of the materials of the base 5 and coating 6 of the measured object 7 according to the formulas (2) calculate the dependence

and determine the required thickness of the strip 8 so as to ensure maximum design sensitivity of the amplitude-phase characteristics of the difference signal

from the thickness of the coating T _p of the measurement object 7. Install a gasket 8 of non-ferromagnetic metal, close in electrical conductivity to the coating material 6, between the ferrite core 2 and the measurement object 5.

The meter is calibrated, for which measuring objects with characteristics close to the object under investigation are probed, with several known coating thicknesses T _p and with several known values of the gaps h between the ferrite core 2 with gasket 8 and the surface of the measuring objects 5. As a result, a calibration dependence A is obtained , φ (Т _п , h), which is preserved. This dependence allows us to solve the inverse problem: by the known values of A and φ find T _p and h.

, по которым с использованием сохраненной зависимости А,φ(Т_п, h) определяют толщину покрытия Т_п и зазор h между ферритовым стержнем с прокладкой и поверхностью покрытия объекта.Probe the test object 5, measure the amplitude A and phase φ of the difference signal

according to which, using the stored dependence A, φ (T _p , h), the coating thickness T _p and the gap h between the ferrite core with the gasket and the coating surface of the object are determined.

Thus, knowing the value of the gap h allows you to determine the thickness of the coating T _p , including for objects with an additional non-conductive coating (varnish, paint, etc.), having a roughness of the metal coating R _z (Figure 5) and curved surfaces ( radius r - Fig.6), since all these factors can be considered as an equivalent gap h _e [11].

Consider a device that implements the inventive method, Fig.1, 6.

The eddy current transducer 11 contains a ferrite rod 2, on which the field windings 1, measuring 3 and compensation 4 are wound. A gasket is installed at the end of the rod 2. For sounding, the rod 2 with the gasket 8 is pressed against the measured object 5, consisting of a base 7 with a coating 6. Note that in FIGS. 1, 6, for clarity, the scale is not respected, since with a rod thickness of several millimeters, the thickness of the gasket 8 and the coating 6 can be tens of micrometers. The balancing circuit 9 is made in the form of a digital-controlled resistor from input 10 and allows you to set the zero difference signal of measuring 3 and compensation 4 windings. The detection unit 12 contains amplitude and phase detectors, which allow determining the amplitude and phase of the difference signal of the measuring and compensation windings, which depend on the coating thickness and the height of the eddy current transducer above the surface of the object, i.e. A, φ (T _p , h). The microcontroller 14 contains a digital-to-analog converter for generating a U _in signal for the field coil and analog-to-digital converters for receiving signals proportional to the amplitude and phase A, φ (T _p , h) of the difference signal. The microcontroller is equipped with a control and display panel 14, from which the operating modes of the meter are set and the measurement results are displayed.

At manufacturing, the meter is balanced. To do this, probe the space without an object and, changing through the balancing input 10 the code of the balancing circuit 9 from the microcontroller 13, achieve zero difference signals at the output of the detection unit 12.

Before starting the measurements, it is necessary to have a priori information about the electrical conductivities of the base σ _о and the coating σ _p , their expected thicknesses T _{p max} , T _min .

The optimal excitation frequency f _{opt is} calculated using relation (1).

Using the relations (2), the theoretical line abcde is constructed, Fig. 2, and the required thickness of the gasket T _{pr is} determined. Install the gasket 6 at the end of the core 2 of the eddy current transducer 11.

When probing the microcontroller 13 generates a square wave with a frequency f _opt which, after smoothing in a lowpass filter (not shown in Figure 6) as a sinusoidal signal u _in applied to the excitation winding 1. Read the signals u and Δu _to a balancing circuit 9 and the compensation 4 windings respectively. Based on the received signals, the detection unit generates signals proportional to the amplitude and phase of the difference signal A, φ (T _p , h).

Installation of the appropriate gasket and calibration of the meter can be carried out during the manufacturing process of the device, if its purpose and application conditions are a priori known, or immediately before measurements. The calibration of the device represents the procedure for setting the sensitivity using a set of measures of the thickness of the coating T _p on the basis. Gap calibration is carried out, for example, by gluing a dielectric film of known thickness onto thickness measures. During calibration, soundings are performed, the amplitudes A and phases φ of the difference signals are determined for known coating thicknesses and T _p and gaps h. The results A, φ (T _p , h) are stored in the permanent memory of the microcontroller 13. Subsequently, this permanent memory is used as a functional converter for solving the inverse problem: T _p , h (A, φ).

The object under investigation is probed, the amplitude and phase of the difference signal are measured, the coating thickness T _p and the gap h between the ferrite core with the gasket and the object coating surface are determined using the described functional transducer.

For the studied objects with rough coatings and (or) curved surfaces, the measured gap value h characterizes with the degree of roughness R _z and (or) curvature r.

Thus, the inventive method is technically feasible and allows you to measure the thickness of the coatings under these conditions.

Devices developed in accordance with the above allow measuring the thickness of non-ferromagnetic metal coatings with conductivity from 6 to 9.5 MS / m on non-ferrous non-ferrous metal substrates with conductivity from 16 to 60 MS / m. The minimum diameter of the bases d _min = 2 mm, the maximum roughness R _zmin = 20 microns. The main permissible error in measuring the coating thickness ΔТ _{p is} not more than ± (1 ... 2) microns.

Information sources

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10. Dorofeev A.L., Nikitin A.I., Rubin A.L. Induction thickness gauge. - M .: "Energy", 1978. - 184 p.

11. Syasko V.A., Ivkin A.E. Eddy current thickness measurement of non-ferromagnetic metal coatings on non-ferrous metal products // World of Measurements, 2010, No. 6, p. 18-23.

Claims (1)

The method of eddy current measurement of the thickness of thin non-ferromagnetic metal coatings, which consists in the fact that for probing a harmonic signal of the selected frequency is applied to the excitation coil installed in the middle of the ferrite core, signals are taken from the same measuring and compensation windings installed at opposite ends of the ferrite core, their difference is found the signal and measure its phase φ, when probing the measurement object, press the end of the ferrite rod with the measuring coil to it Coy, characterized in that the selected optimal frequency f _opt excitation winding from the relation f _opt = (0,3 ... 0,7) / ( π μ ₀ σ _n ² T _pmax), where μ ₀ - magnetic constant; σ _p - electrical conductivity of the coating; T _pmax is the maximum coating thickness, balance the measuring and compensation windings before starting measurements, achieving a zero difference signal in the absence of the measurement object, install a non-ferromagnetic metal gasket that is close in electrical conductivity to the coating material between the ferrite core and the measurement object, and select the gasket thickness so that with approximately known parameters of the studied object and the ferrite core with windings, ensure the maximum calculated sensitivity amplitude the bottom-phase characteristics of the difference signal from the coating thickness of the object, when probing, the amplitude A of the difference signal is additionally measured, the meter is calibrated, for which measuring objects with characteristics close to the object under study, with several known coating thicknesses T _p and with several known gap values are probed h between the ferrite core with the gasket and the surface of the measured objects retain the dependence of the amplitudes and phases of the difference signals on the thickness of the coating and the gaps A, φ (T _p , h), probe investigated object, calculate the coating thickness T _p , the gap h between the ferrite core with the gasket and the surface of the coating of the object according to the measured values of the amplitude A and phase φ of the difference signal, using the dependence A, φ (T _p , h).

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