SU1425431A1 - Eddy-current thickness gauge - Google Patents

Description

4ib Yu

O1 41

WITH

The invention relates to the field of instrumentation technology and can be used in all sectors of the national economy where it is necessary to measure crowds of different dielectric coatings deposited on non-magnetic electrically conductive bases.

The purpose of the invention is to improve the measurement accuracy by eliminating the temperature drift of the gain of the logarithmic amplifier.

The drawing shows the block diagram of the proposed thickness gauge.

The eddy current thickness gauge contains serially connected autogenerator 1, eddy current transducer 2 (VTP), amplifier 3 amplitude detector 4, logarithmic amplifier 5, voltage divider 6 and indicator 7, successively connected to source 8 of the reference voltage, adder 9, the second input -; Go is connected to the output of the logarithmic amplifier 5, and the output is connected to the second input of the divider of 6 voltages.

Eddy current thickness gauge works as follows.

The auto-oscillator 1 feeds a sinusoidal current converter 2, the signal from the measuring winding of which, which appears during the interaction of the converter with the object, is amplified by amplifier 3 and detected by an amplitude detector 4, after which it is logarithmized by a logarithmic amplifier 5. The output signal is last proportional to In A, is supplied to the first input of the voltage divider 6 and simultaneously to the second input of the adder 9. The output signal of the adder 9, proportional to the value

Vo

+ InA,

where V (is the output voltage of the source 8 of the reference voltage; A is the normalized amplitude

voltage applied to the second divider 6 voltage, the output signal of which is proportional to the value of V ci

l, inA

- enters the Von + 1PA indicator

7, the scale of which is calibrated in units of thickness measurement h.

The voltage applied to the invoice transducer can be expressed in the following way:

., .., X-W + jftZ -2L J r Trffef

BUT

where 1 is constant;

f

i R “;

where D is the diameter of the contour of the maximum density of eddy currents D 2 (Rff + Kh), while K is constant; - - circular frequency; electrical conductivity

G msm I

in our case 1. Then (Vj А rxe-2-Л.

However, the true dependence A (h) is described by the expression approximately, which is also a consequence of the fact that the diameter of the contour of the maximum density of eddy currents depends on the gap.

Such dependence will be taken into account if the parameter D is represented as

 I

L - |, g and- „||.

1 + kjh

where K and K are constant, and

Kg 0.13. Then from the expression for A taking into account

And write down.,.

. y And what about

where Kj is constant.

 Solving this equation for h is

h l + CilnA

where h is the coating thickness (gap);

C, C - constant. In practice, in the Idou, simple implementation and high accuracy of generating the logarithmic signal conversion function (FPS), logarithmic amplifiers are obtained with the inclusion of a differential transistor pair in the operational amplifier feedback.

31425А31

The function of the FPS converter of the input signals of such an amplifier has the form

V N fr-t-ic PTlnA N + lnA

1PA

t f,

- constant;

temperature potential; f (t).

about

The output voltage of the source of the reference voltage is also avis, with Vo low-frequency r, where N is a constant coefficient.

Based on the above, one of the main advantages of the proposed LRF is that its structure allows to effectively exclude the multiplicative temperature error of the entire measuring path.

Indeed, by taking into account the FPS of logarithmic amplifier 5 and the dependence of the reference voltage, on, the output voltage of the divider 6 voltage (V) can be expressed as follows:

t

ten

20

where Kj and K. are permanent.

From this expression it follows that the indications of the indicator 7 do not depend on the temperature instability of the transfer coefficient of the logarithmic amplifier 5 and are proportional to the measured thickness h of the coating.

. -Exclude the temperature drift of the gain of the logarithmic amplifier in the best way

15, it is possible, if instead of the diodes included in the source of the reference voltage 8, to use transistors in a diode connection, made in the same integrated case with a differential transistor pair, which enters the logarithmic amplifier.

Due to the independence of the testimony of such a thickness gauge from changes in ambient temperature, it can

25 to be used in a wide variety of temperature conditions.

Claims (1)

VICHRET THICKNESS THERMOMETER containing a serially connected oscillator, eddy current transducer, amplifier, amplitude detector and a logarithmic amplifier, a series-connected reference voltage source and adder, the second input of which is connected to the output of a logarithmic amplifier, and an indicator, characterized in that, in order to increase the measurement accuracy, it is equipped with a voltage divider, the first input of which is connected to the output of the logarithmic amplifier, the second input to the output of the adder, and the output is connected ikator, the logarithmic amplifier is made in the form of an operational amplifier with a transistor pair in feedback, the reference voltage source is made in the form of a diode-resistive bridge with an adjustable resistor in the diagonal, in which diodes are used transistors and the bridge diodes and differential transistor pair The logarithmic amplifier is housed in a single integrated housing. ' SU „1425431