JPH07280503A - Device for measuring surface film thickness - Google Patents

Abstract

PURPOSE:To measure surface film thickness on-line by forming an electrode into a rotatable cylindrical type and bringing it into contact with the surface film of a metal belt during running. CONSTITUTION:An collector ring 2 is mounted on each electrode 1 formed into a cylindrical type and a slider 3 which comes into contact with the collector ring 2 is connected to a measurement terminal 6. When the electrode 1 is brought into contact with the surface film 5 of a running metal belt 4, it rotates, and even during running of the metal belt 4, the geometrical position relation of them is always kept constant. The electrode 1 and the metal belt 4 constitute an electrostatic capacity C to sandwich a film 5, since the capacity C is determined by the thickness (t) of the film 5 and permittivity (specific permittivity) epsilon, the permittivity epsilon corresponds to the thickness of the film 5 by a ratio of 1 to 1 if the permittivity epsilon is constant. Accordingly, if the electrostatic capacity C is measured, the thickness (t) of the film 5 can be measured. Guide rolls are provided before and behind the electrode 1 so that the electrode 1 and the film 5 may be always Kept line contact or if the metal belt 4 is disposed to be sandwiched by the electrode 1 and a control roll, it is effective.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the thickness of an insulative surface coating applied to the surface of a running metal strip or the like.

[0002]

2. Description of the Related Art Various techniques have been proposed as a technique for measuring the thickness of an insulating surface coating applied to the surface of a conductive material such as a metal band. Among them, the method to which the capacitance measuring technique is applied is widely used because it is simple. In this method, a flat plate-shaped electrode is brought into contact with the surface coating of a subject, the body of the subject (metal strip, etc.) is regarded as the other electrode, and the capacitance between these two electrodes is measured. .

FIG. 3 is a diagram showing the structure of a conventional measuring apparatus. In the figure, 1 is an electrode, 4 is a metal band, 5 is a surface film, 6
Indicates the capacitance measuring terminals, respectively. Here, the metal strip 4 (and the surface coating 5 of the metal strip) that is the subject is sampled sufficiently larger than the flat electrode 1. If the contact area of the flat plate-shaped electrode 1 is S, the relative permittivity of the film 5 is ε, the permittivity of vacuum (air) is ε ₀ , and the measured capacitance is C, then the film 5
The thickness t is expressed as t = ε · ε ₀ · S / C. If the relative permittivity ε of the film 5 is known, the thickness t of the film 5 can be calculated from this equation.

[0004]

The prior art is an inexpensive and relatively highly accurate method, but it is not applicable to the measurement of the surface coating of a running metal strip, such as in a metal strip production line. Have difficulty.

First, in order to bring the electrode into contact with the surface of the traveling metal strip, it is necessary to make the electrode completely follow the traveling of the metal strip, which complicates the mechanism and driving device. Even if a follow-up device is created, if there is a slight gap between the surface of the metal strip and the electrode, the capacitance will decrease and the surface coating thickness will be excessively measured.

Next, it may be considered to face the running metal strip with a small gap, but it is difficult to keep the gap constant. This is because, generally, in a production line in which a metal strip runs, vibration of the metal strip called fluttering or line vibration is unavoidable. Also,
This line vibration depends on the tension of the metal strip and the traveling speed.
In the end, it is impractical because the amplitude can increase and the measurement can damage the detector as well as the detector itself.

The present invention solves these problems that would be expected when applying the prior art to running metal strips, and provides an on-line applicable surface coating measurement device.

[0008]

According to a first aspect of the present invention, an electrostatic capacitance type surface film thickness measurement is carried out in which an electrode is brought into contact with the surface film of a metal strip to measure the electrostatic capacitance and measure the thickness of the surface film. In the apparatus, the structure of at least one electrode is a rotatable cylindrical type, which is a surface film thickness measuring apparatus. The invention of claim 2 is the surface film thickness measuring device according to the invention of claim 1, which is provided with two electrodes.

[0009]

According to the present invention, when the cylindrical electrode is brought into contact with the surface coating of the metal strip, the electrode rotates, and the geometrical positional relationship between the electrode and the metal strip is always kept constant while the metal strip is running. Be done. The electrode and the metal strip form a capacitance that sandwiches the surface coating, and its value is determined by the thickness of the surface coating and the dielectric constant (relative permittivity). This capacitance has a one-to-one correspondence with the thickness of the surface coating if the dielectric constant of the surface coating is constant.
Therefore, the thickness of the surface film can be measured by measuring this capacitance.

According to the second aspect of the present invention, since the two cylindrical electrodes are provided, the capacitance can be measured without electrically connecting the metal portion of the main body of the running metal strip. In the present invention, the two capacitances formed by the metal strip and these two electrodes are connected in series by the metal strip in terms of an electric circuit. These two capacitances have a one-to-one correspondence with the thickness of the surface film if the dielectric constant of the surface film is constant.
The combined capacitance of these connected in series also corresponds to the thickness of the surface coating on a one-to-one basis.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing one embodiment of the measuring apparatus of the invention. In the figure, 1 is an electrode, 2 is a current collector, 3 is a slider, and 4
Is a metal strip, 5 is a surface film, 6 is a measuring terminal, 10 is a capacitance measuring device, 11 is an AC power supply, 12 and 13 are first and second
, Multipliers 14, 15 are first and second integrators, 16 is a divider, 17 is a reference impedance, and 18 is a phase shifter.

A current collector 2 is attached to each of the electrodes 1, and the current collector 2 is in contact with the slider 3. The slider 3 is connected to the measurement terminal 6, and thus the electrode 1 and the measurement terminal 6 are electrically connected. With the electrode 1 in contact with the surface coating 5 of the metal strip 4, the capacitance of the measuring terminal 6 is measured by the capacitance measuring device 10.

In order to measure the thickness t of the surface film from the capacitance C thus obtained, it is preferable to measure the relation between the two in advance, but it may be theoretically handled. Figure 2
a and b are schematic views for considering an electrostatic field composed of an electrode and a metal band. First, FIG. 2a shows the geometry of the metal strip and the respective electrodes. Focusing on the electric field around one of the electrodes, the metal strips are conductors, so the potentials are equal. Therefore, the interface between the metal strip 4 and the surface coating 5 becomes an equipotential surface. Similarly for other electrodes, the interface between the metal strip 4 and the surface coating 5 becomes an equipotential surface.

FIG. 2b is a back-to-back layout of the equipotential surfaces for these two electrodes. In the figure,
Reference numeral 45 indicates an equipotential surface. After all, as shown in this figure, with respect to the arrangement in which the dielectric with a thickness of 2 t is sandwiched between two electrodes,
It is enough to consider the electric field.

Assuming that the thickness of the surface film is t and its relative permittivity is ε, a dielectric having a thickness of 2t and a relative permittivity of ε is equivalent to a vacuum (air) having a capacitance of 2t / ε. Is. Therefore, the diameter D and the width L, which are arranged with the gap 2t / ε, are set to 2
It suffices to find the capacitance between two cylindrical conductors.

The width (L) of the electrode is the diameter (D)
If the electric field is sufficiently larger and the electric field can be regarded as a two-dimensional electric field, that is, for a cylinder with an infinite length, the capacitance C between the two cylinders can be theoretically obtained and expressed as X = t / (ε · D). , C = π · ε ₀ · L / ln [1 + X + (2X + X ² ) ^1/2 ]. Therefore, t can be calculated by obtaining X from the electrostatic capacitance C.

Also, the thickness t of the surface coating is usually several tens of μm.
For m, the diameter D of the electrode is several tens of mm or more, and t <<
Since it is D, the approximation may be further advanced to C = π · ε · ε ₀ · L · D / t.

It should be noted that the two electrodes 1 and 1 must be surely in contact with the surface coating 5 of the metal strip. for that purpose,
The surface coating of the metal strip may be brought into contact with the electrodes using a guide roll or the like. At that time, when the metal strip comes to be wound around the electrode, the surface coating of the metal strip comes into surface contact with the electrode, and the electrostatic capacitance is changed, so that the surface coating thickness cannot be measured by the electrostatic capacitance. Further, as a problem other than the measurement accuracy, there is a risk that a large force is applied to the electrode and the device is damaged.

Therefore, it is necessary to adjust the electrodes so that they always maintain line contact with the surface coating of the metal strip. for that purpose,
Various arrangements are effective, such as providing guide rolls in front of and behind the electrodes, and placing a metal strip between the electrodes and a holding roll. The electrode used in the present invention may be a conductor,
It may be hollow or solid as long as the portion of the metal strip that comes into contact with the surface coating is cylindrical.

[0020] Incidentally, the capacitance of the measurement terminal 6 thus obtained, since precisely the value of C + C ₀ plus the capacitance C ₀ between the two electrodes, in fact the C ₀ value The value obtained by subtracting may be used as the capacitance C of the measurement terminal 6.

Here, C ₀ can be calculated as the electrostatic capacitance between the two cylindrical conductors, but it is also the electrostatic capacitance of the measuring terminal 6 when there is no subject. Therefore, it is most accurate to measure the electrostatic capacitance of the measurement terminal 6 when there is no object, and use it as the device constant of the measuring device itself. However, if the distance between the electrodes is much larger than the film thickness,
The capacitance C ₀ may be ignored with respect to the capacitance C of the film.

Although the embodiment of FIG. 1 uses two electrodes, the capacitance between one electrode and the metal strip may be measured. In that case, one electrode and the metal part main body 4 of the metal strip are electrically connected to the measurement terminal 6 of FIG.
Here, if the surface coating of the metal strip is one-sided, the metal strip is grounded by a transport roll or the like, so the ground potential may be used as the potential of the metal strip. In addition, it is not necessary to directly connect it, as long as it is electrically connected. For example, it is possible to use capacitive coupling between a metal roll and a roll that comes into contact with the surface coating of the metal strip over a wide area such as a deflector roll. .

There are various measuring devices for the capacitance measuring device, but here, a simple circuit having a good responsiveness to the fluctuation of the surface film thickness was used. In FIG. 1, the measurement terminal 6 is connected to the AC power supply 11 via the reference impedance 17, and supplies a current to the capacitance between the measurement terminals.

A voltage equal to the product (IZ, IZr) of the respective AC resistance values (Z and Zr) and the current (I) is generated at the measuring terminal and the reference impedance. By the first and second multipliers 12, 13,
The product of these two voltages (I ² · Z · Zr) and the square of either one (I ² · Z ² or I ² · Zr ² ) are obtained as outputs.

Here, the outputs of the multipliers 12 and 13 (I ² ·
If the ratio of Z · Zr, I ² · Z ² or I ² · Zr ² ) is taken, the square of the current (I ² ) is reduced and the ratio of the two AC resistance values (Z, Zr) is obtained. .

The phase shifter 18 and the first and second integrators 14 and 15 determine the amplitude of the AC signal. Integrator 1
In 4, 15, integral with one of the square of the effective current value the product of the integration and two AC resistance value of the product of ^{(I 2) (Z · Zr} ), and the square of the effective current value (I ²⁾ The product of one of the squares (Z ² or Zr ² ) is obtained as an integrated value.

The divider 16 outputs the ratio of these two integral values, but the integral of the square of the effective current value (I ² ) is reduced, and as a result, the product of the two alternating resistance values is multiplied. And the squared ratio of either one (Z · Zr / Z ² or Z · Z
r / Zr ² ) remains. This is further reduced, and in the end, the ratio output by the divider is the ratio of AC resistance values (Zr / Z or Z
/ Zr). Since the AC resistance (Zr) of the reference impedance 17 is known, the AC resistance (Z) of the measurement terminal 6 can be easily understood from this ratio, and if the AC resistance is converted into capacitance, the thickness of the surface film can be calculated. Can be measured.

In FIG. 1, the reference impedance 17
A case in which a resistor is used for will be described. The current at the measuring terminal 6 is I, its frequency is ω, the resistance value of the resistor is R, and the capacitance of the measuring terminal 6 is C. First, the voltage at the measurement terminal 6 becomes I / jωC, which is phase-shifted by 90 ° by the phase shifter 18 and becomes I / ωC. Next, the voltage of the reference impedance (resistor) 17 becomes I · R.

Since these voltages are input to the first and second multipliers, the outputs of the multipliers are I ² · R / ω, respectively.
C and I ² · R ² . From this, the ratio of the output after integration is 1 / ωC · R, and the value of R is known, so the electrostatic capacitance C of the surface film is obtained.

A capacitor may be used as the reference impedance 17. In that case, the phase shift amount of the phase shifter 18 may be 0 °, or the phase shifter itself may be omitted. The reference impedance (capacitor) capacity is C
If r, the voltage is I / jωCr.

This voltage and the voltage I / jωC of the measuring terminal 6
Becomes the input of the first and second multipliers, so the output of the multiplier
Each force is the phase (90 ° deviation from the current I)
I ²/ (Ω²C / Cr), I²/ (Ω²Cr²)When
Become. From this, the ratio of the output after integration becomes Cr / C and C
Since r is known, the capacitance C of the surface film can be obtained.

[0032]

According to the present invention, since the rotatable cylindrical electrode is used, it is possible to measure the surface coating thickness online by bringing it into contact with the surface coating of the running metal strip.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a measuring apparatus of the invention.

FIG. 2 is a schematic diagram for considering an electrostatic field composed of an electrode and a metal band.

FIG. 3 is a diagram showing a configuration of a conventional measuring device.

[Explanation of symbols]

 1 electrode 4 metal band 5 surface film

Claims (2)

[Claims] 1. A capacitance type surface coating thickness measuring apparatus for measuring the capacitance by contacting an electrode with a surface coating of a metal band to measure the capacitance, wherein at least one electrode structure is rotated. A surface coating thickness measuring device characterized by being capable of being a cylindrical type. 2. The surface film thickness measuring device according to claim 1, wherein the two electrodes are provided.