RU2128818C1 - Device for measurement of conducting coating thickness with direct reading - Google Patents

Abstract

FIELD: nondestructive testing. SUBSTANCE: device has generator, eddy-current resonance converter, digital storage unit, analog-to-digital converter, and indicator. Generator frequency control unit introduced into device compensates for reduction of output signal amplitude of eddy-current converter due to time and temperature variations of resonance circuit quality factor. Device also includes analog storage unit, reference level setting unit, unbalance amplifier, and generator amplitude control unit which allows compensation of supply voltage variation. EFFECT: enhanced control efficiency. 3 dwg

Description

 The invention relates to non-destructive methods of quality control and geometric dimensions by the electromagnetic method and can be used in all sectors of the economy where objects with coatings are used or coatings are controlled.

 A device for measuring the thickness of the coating [1] is known, based on measuring the magnetic flux of eddy currents generated in the coating or substrate using an alternating voltage generator.

 A known method and device for measuring the thickness of a thin metal layer on a conductive substrate [2], based on the measurement of heat (joule) losses due to the appearance of eddy currents when a magnetic circuit supplied with alternating voltage is connected to the measured metal surface.

 A known method and device for measuring the thickness of a thin-film metal coating deposited on a conductive substrate [3], based on measuring the imbalance of the magnetic field of an open core by the magnetic field of eddy currents.

 A common drawback of all the mentioned devices is the large error in measuring the coating thickness due to the instability of the frequency and voltage of the generator, the change in the electrical conductivity of the base and coating materials, the temperature and time instability of the inductive and capacitive elements of the converter.

 A device [4] is known which, in order to increase the accuracy of measuring the thickness of coatings, contains three transducers: one low-frequency and two high-frequency. A low-frequency converter is used to obtain information about the thickness of the coating and the conductive properties of the base. The first high-frequency converter serves to obtain information about the electrically conductive properties of the coating, the second high-frequency converter - about the size of the gap between the converter and the coating. The device allows to exclude the influence on the measurement result of the conductive properties of the base and coating. The disadvantage of this device is a large error due to the temperature and time instability of inductive and capacitive elements of the converters.

 The closest technical solutions to the proposed image is a device for measuring the thickness of the coatings [5], containing a transducer, an alternating voltage generator, a detector and an indicator, wherein the excitation input of the transducer is connected to the output of the generator, the output of the transducer is connected to the input of the detector, and the output of the detector is connected to indicator.

 The disadvantage of this device is the large error in measuring the thickness of the coating due to temporary and temperature changes in the inductance and capacity of the oscillatory air of the converter and the instability of the frequency and amplitude of the signals of the exciting generator.

 The purpose of the present invention is to improve the accuracy of measurement.

 This goal is achieved in that the device for measuring the thickness of the conductive coating, comprising a converter, a generator, a peak amplitude detector, an indicator, is further provided with a frequency control unit, a first memory unit, an unbalance amplifier, an amplitude control unit, a reference level setting unit, an analog-to-digital converter and a second, pre-programmed, memory unit, the drive excitation input being connected to the generator output, the first control input of the generator connecting is connected to the output of the frequency control unit, the input of the converter is connected to the input of the peak amplitude detector, the output of the peak amplitude detector is connected to the input of the first memory unit and the input of the analog-to-digital converter, the output of the first memory unit is connected to the first input of the unbalance amplifier, the second input of which is connected to the output reference level reference unit, the output of the unbalance amplifier is connected to the input of the amplitude control unit, the output of which is connected to the second control input of the generator, the digital output of the ADC is connected It is connected with the address input of the second, preprogrammed, memory block, the output of which is connected to the indicator input.

 In FIG. 1 shows a diagram of a device for measuring the thickness of a coating with a direct reference; in FIG. 2 - frequency dependences of the amplitude of the voltage on the oscillatory circuit for different temperatures; in FIG. 3 is a diagram and timing diagrams of the voltage at the output of the peak amplitude detector.

 A device for monitoring the coating thickness with a direct reference contains a converter 1, a generator 2, a frequency control unit 3, a peak amplitude detector 4, a first memory unit 5, an unbalance amplifier 6, a reference level setting unit 7, an amplitude control unit 8, an analog-to-digital converter 9 , the second, pre-programmed, memory unit 10, indicator 11, and the excitation input of the converter 1 is connected to the output of the generator 2, the first control input of the generator 2 is connected to the output of the frequency control unit 3, output d converter 1 is connected to the input of the peak amplitude detector 4 - with the input of the first memory block 5 and the input of the ADC 9, the output of the first memory block 5 is connected to the first input of the unbalance amplifier 6, the second input of which is connected to the output of the reference unit 7, the output of the unbalance amplifier 6 is connected to the input of the amplitude control unit 8, the output of which is connected to the second control input of the generator 2, the digital output of the ADC 9 is connected to the input of the second, pre-programmed memory unit 10, the data output of which is connected en with indicator input 11.

В известном устройстве [5] частота генератора f_г постоянна и выбирается равной резонансной частоте колебательного контура f₀. Однако, вследствие временных и температурных изменений f_г, индуктивности L и емкости C контура преобразователя происходит взаимное смещение частот f_г и f₀. Это приводит к уменьшению амплитуды переменного напряжения на контуре. А так как о толщине измеряемого покрытия судят по уменьшению добротности колебательного контура преобразователя, то уменьшение амплитуды воспринимается как увеличение толщины покрытия. То есть в результат измерения толщины покрытия вносится погрешность.The device operates as follows:
The generator 2 excites forced oscillations in the oscillatory circuit of the converter 1. The amplitude of the alternating voltage on the circuit is determined by its quality factor and the deviation of the generator frequency f _g from the resonant frequency

In the known device [5], the frequency of the generator f _{g is} constant and is chosen equal to the resonant frequency of the oscillatory circuit f ₀ . However, due to temporary and temperature changes f _g , inductance L and capacitance C of the converter circuit, a mutual shift of frequencies f _g and f ₀ occurs. This leads to a decrease in the amplitude of the alternating voltage on the circuit. And since the thickness of the measured coating is judged by the decrease in the quality factor of the oscillatory circuit of the transducer, the decrease in amplitude is perceived as an increase in the thickness of the coating. That is, an error is introduced into the measurement result of the coating thickness.

1,6 кГц, то есть при отклонении частоты генератора f_г на ± 0,8 кГц амплитуда напряжения на контуре уменьшится на 50% от максимума, имеющего место при f_г = f₀. Для поддержания постоянства амплитуды на контуре с точностью ±0,5% отклонение частоты генератора f_г от резонансной частоты f₀ не должно превышать ± 96 Гц, что составляет ± 0,12%.When the quality factor of the resonant circuit of the transducer Q = 50 and the average resonant frequency f ₀ = 80 kHz, the width of its resonant characteristic is

1.6 kHz, that is, when the oscillator frequency deviates f _g by ± 0.8 kHz, the voltage amplitude on the circuit decreases by 50% from the maximum that occurs when f _g = f ₀ . To maintain a constant amplitude on the circuit with an accuracy of ± 0.5%, the deviation of the oscillator frequency f _g from the resonant frequency f ₀ should not exceed ± 96 Hz, which is ± 0.12%.

где Δf_г - частотное расхождение, обусловленное нестабильностью частоты генератора. Для генераторов без кварцевого резонатора стабильность частоты δf не лучше, чем 1 • 10^-5

Это дает в диапазоне температур Δf = 64 Гц [6].Temporary and temperature changes in the inductance L and capacitance C lead to the same result. Satisfying the condition that the voltage amplitude on the oscillatory circuit is constant with an accuracy of ± 0.5% requires that the deviations L and C do not exceed ± 0.05%. In a wide range of operating temperatures, such requirements cannot be ensured. The use of precision elements allows in the temperature range from -30 to 50 ^o C to ensure the stability of L and C within ± 0.08% and ± 0.4%, respectively. The minimum TKL for coils with cores is ± 10 • 10 ^-6 , the minimum TKE is ± 50 • 10 ^-6 [6]). With these parameters, the average discrepancy f _g and f ₀ in the marked temperature range is

where Δf _g is the frequency discrepancy due to the instability of the frequency of the generator. For oscillators without a quartz resonator, the frequency stability δf is no better than 1 • 10 ^-5

This gives in the temperature range Δf = 64 Hz [6].

0,27% или Δf_L = 211 Гц.Δf _L is the frequency discrepancy due to temperature changes in the inductance L ₀ . In the operating temperature range, δL = ± 0.08%, which gives

0.27% or Δf _L = 211 Hz.

Δf _C is the frequency discrepancy due to temperature changes in capacitance C. In the operating temperature range, δC = ± 0.4%, which gives Δf _C = 504 Hz. The total frequency difference is 550 Hz.

 Such a deviation of the supply and resonance frequencies will lead to a decrease in the amplitude of the voltage across the circuit by 5.3%.

 Therefore, in the operating temperature range, an error of ± 2.6% is introduced into the measurement of the voltage amplitude on the circuit.

где f₀ - резонансная частота колебательного контура в середине рабочего диапазона температур и центральная частота генератора;
ΔL = δL•L_o - максимальное отклонение индуктивности в рабочем диапазоне температур;
ΔC = δC•C_o - максимальное отклонение емкости в рабочем диапазоне температур.In the proposed device, the frequency of the generator forcibly changes from f _min to f _max , and

where f ₀ is the resonant frequency of the oscillatory circuit in the middle of the operating temperature range and the center frequency of the generator;
ΔL = δL • L _o - the maximum deviation of the inductance in the operating temperature range;
ΔC = δC • C _o - the maximum deviation of the capacitance in the operating temperature range.

 In this case, the frequency of the generator will always pass through the resonance of the oscillatory circuit of the converter and the amplitude of the voltage across the circuit will change in accordance with FIG. 2.

In the middle of the operating temperature range, the voltage amplitude on the circuit will have a maximum of U $\genfrac{}{}{0ex}{}{\text{0}}{\text{max}}$ at a generator frequency equal to f ₀ (curve 1).

где r - активное сопротивление катушки индуктивности.With temperature changes of L and C, the resonant frequency of the circuit will change, but due to scanning by the frequency of the generator, the resonance will be detected again, only at a different frequency, for example, f ₁ (curve 2). The amplitude of the voltage on the circuit with a new resonance U $\genfrac{}{}{0ex}{}{\text{1}}{\text{max}}$ will be almost equal to f ₀ . From [6] the quality factor of the circuit

where r is the active resistance of the inductor.

где

относительное изменение добротности из-за флуктуаций индуктивности L;

- относительное изменение добротности Q из-за флуктуаций емкости L.Hence the relative change in the quality factor Q due to changes in L and C can be expressed as

Where

relative change in the quality factor due to fluctuations in the inductance L;

- the relative change in the quality factor Q due to fluctuations in the capacitance L.

With the noted values of δL = 0.08% and δC = 0.4%, we obtain δQ _L = 0.04% and δQ _C = 0.2%, which gives δQ = 0.204%. Hence U $\genfrac{}{}{0ex}{}{\text{1}}{\text{max}}$ will be different from U $\genfrac{}{}{0ex}{}{\text{0}}{\text{max}}$ also by 0.204%.

In the proposed device, the voltage at the output of the peak amplitude detector 4 is measured, equal to U _max in each frequency scanning cycle. Therefore, the proposed device has an order of magnitude smaller error (0.204% compared with 2.6%) associated with temperature and time changes in f _g , L and C, compared with the prototype.

Another source of errors is changes in the output voltage of generator 2, leading to a change in U _max in each scan cycle. To eliminate this error, a circuit consisting of the first memory block 5 with a long memory time (for example, an analog memory device with a large time constant), a reference level setting unit 7, an imbalance amplifier 6, and an amplitude control unit for the generator 8 is used.

 The circuit operates as follows.

The change in the output voltage of the generator 2, the speed of which is determined by the rate of temperature changes in the environment and the rate of change of the temperature of the measuring transducer caused by the change in ambient temperature, creates slow (with a time constant of more than 1 minute) changes in the voltage amplitude on the circuit of the transducer 1 at resonance (U _max ) . As a result, the voltage at the output of the peak amplitude detector 4 will also slowly change. The analog memory unit 5 has a large time constant and only the slow component of the changes in the voltage amplitude on the circuit is allocated at its output. This will lead to the appearance of a voltage difference between the inputs of the unbalance amplifier 6. The value of the reference voltage at the output of the reference level reference unit 7 is selected so that the amplitudes of the alternating voltage taken from the circuit of the converter 1 and supplied to the input of the peak amplitude detector 4 does not exceed the maximum of the working section of the input detector characteristics.

 This difference is amplified by an imbalance amplifier 6 and fed to the amplitude control unit of the generator 8, which will reduce (for example, by reducing the supply voltage of the generator 2) the output voltage of the generator to its original value. Thus, the amplitude of the output voltage of the generator 2 is maintained constant and independent of changes in operating temperature.

The voltage at the output of the peak amplitude detector 4 contains both a fast component (information) and a slow (temperature) component. Peak amplitude detector 4 is a device having different charge and discharge times. In FIG. 3a, one of the possible peak detector circuits is shown. The input AC voltage is supplied to the drive (capacitor C) through the diode VD1. The capacitor C is charged through a resistance consisting of R _q (open) and R _{Vn the} signal source. The signal source (an amplifier that transmits voltage from the converter circuit) has a low internal resistance, the VDI diode in the forward direction also has a low resistance. Therefore, the charge time of the capacitor C is small and during the duration of the input signal, it is charged to the amplitude value. When the voltage amplitude on the circuit begins to decrease (when passing through the resonance), then the voltage difference between U _C and the input signal becomes negative and the VDI diode closes. After this moment, the VDI diode begins to discharge. But the discharge passes through a resistance consisting of in parallel connected: the reverse resistance of the diode VDI, the resistor R, the leakage of the capacitor R _ut and the input resistance of the next stage R _in . All these resistances are large, which provides a constant discharge time of 1 min or more. To prepare the detector for the next measurement cycle, the capacitor C is discharged using the key K, which are closed at the end of the frequency scanning cycle (moment t _{2 of} Fig. 3, b), which does not introduce any errors in the amplitude measurement.

Rapid changes in the voltage at the output of the peak amplitude detector 4, associated with the measurement process and occurring when the measuring transducer is mounted on a controlled object and having a time constant of the order of Ic at a total measurement time of 5 s, are input to an analog-to-digital converter 9. A digital output is generated on its digital output code corresponding to the change in voltage on the circuit at the time of resonance U _max coating.

The functional relationship between U _max and coating thickness is non-linear. Therefore, the direct conclusion to the indicator 11 of the value of U _max requires the translation of the recorded value of U _max in units of coating thickness (for example, microns).

Such a translation operation is performed according to the calibration schedule and introduces an additional error. To eliminate the error of the operation of translating relative readings into units of coating thickness in the proposed device, the digital code U _max from the output of the analog-to-digital converter 9 is fed to the address input of the second (digital) memory block 10. This memory block is preliminarily recorded with a set of a sufficiently large number of reference samples dependence between U _max and coating thickness. Moreover, when programming, the values of the coating thickness are given to the data input, and during operation, the values of U _max are transmitted to the address input by selecting a memory cell in which the coating thickness corresponding to the given U _max is recorded. This value of the coating thickness is indicated by indicator 11.

 Thus, this device allows to reduce the error of measuring the coating thickness due to the excitation of the oscillatory circuit of the converter by a voltage of varying frequency, stabilization of the peak amplitude detector with a large time constant and automatic conversion of the maximum voltage amplitude on the circuit to units of the coating thickness using a pre-programmed memory unit.

Used sources of information
1. A.S. N 1186936, 10.23.85, G 01 B 7/06.

 2. Application of France N 2572175, 04.25.86, G 01 B 7/06.

 3. Application EPO N 0179720, 04/30/86, G 01 B 7/10, 7/02.

 4. A.S. N 932206, 05.30.82, G 01 B 7/06, G 01 N 27/90.

 5. Application EPO N 0178916, 04/23/86, G 01 B 7/06.

 6. G.D. Frumkin. Calculation and design of electronic equipment. - M.: Higher School, 1985.

Claims (1)

 A device for measuring the thickness of a conductive coating with a direct reference, containing a converter, a generator, a peak amplitude detector and an indicator, characterized in that it is additionally equipped with a frequency control unit, a first memory unit, an unbalance amplifier, a reference level setting unit, an amplitude control unit, analog a digital converter and a second, pre-programmed, memory unit, the drive excitation input being connected to the generator output, the first control input d of the generator is connected to the output of the frequency control unit, the output of the converter is connected to the input of the peak amplitude detector, the output of the peak amplitude detector is connected to the input of the first memory block and the input of the analog-to-digital converter, the output of the first memory block is connected to the first input of the unbalance amplifier, the second input of which is connected with the output of the reference level reference unit, the output of the unbalance amplifier is connected to the second control input of the generator, the digital output of the analog-to-digital converter is connected to the address the input of the second, pre-programmed, memory block, the data output of which is connected to the indicator input.

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