CA1268816A

CA1268816A - Measuring device for the "contactless" measurement of large thicknesses, for metal materials above the curie temperature

Info

Publication number: CA1268816A
Application number: CA000456428A
Authority: CA
Inventors: Eugenio Caruso; Antonio Gray
Original assignee: Dalmine SpA
Current assignee: SMS Group SpA
Priority date: 1983-06-15
Filing date: 1984-06-13
Publication date: 1990-05-08
Anticipated expiration: 2007-05-08
Also published as: IT1194275B; IT8321637A0; EP0129282A1; ATE37088T1; JPS6013203A; EP0129282B1; DE3473921D1; IT8321637A1; US4771238A

Abstract

Abstract of the Disclosure.

The sensor is constituted by a magnetic core with pole pieces and by an electric coil wound on the body of said core. With said coil there are associated a sinusoidal current generator and impedance measurement means. A processor determines the thickness from the measured value of said impedance.

Description

~2~ 6 Thls Inventlon relates to a measurlng devlce for the "contactless" measurement of large t~icknesses for use on conduc-tlve walls at a temperature exceedlng the Curle temperature. A
typlcal applIcatlon of the measurlng devlce Is to measure the thlckness (up to about 10 cm) of plates and tubes leav1ng hot rolllng mllls.

Measurements of thls ~Ind are currently made by the followlng methods: a) ~ -ray or X-ray rad10graphy; b) echography uslng ultrasonlc electromagnetlc transducers.

These systems have varlous drawbacks, whlch can be sum-marlzed as follows: a) dlmenslons, cost, psychologlcal problems and the need for adeciuate radlatlon shleldlng In the case of ~ -ray systems. Dlmenslons, cost, low accuracy and the need foradequate shleldlng In the case of X-ray systems; b) the complex-lty of ultrasonlc electromagnetlc transducer systems, the fact that the measurlng sensltlvlty Is hlghly dependent on the sensor-wall dlstance, and the need to keep the sensor very close to the wall (In practlce not more than 1 mm dls-tant).

The present Inventlon provldes a measurlng devlce for the "contactless" thlcicness measurement of conductlve wall above the Cur/e temperature.

2~
Accordlng to the present Invention there Is provided a measurlng devlce for the contactless measurement of the thickness of metalllc materlal at a temperature above the Curle tempera-ture, comprlslng a magnetlc core wlth pole pleces f aclng a sur-face of the metalllc materlal, coll means wound on a body of saldcore, generator means for supplylng an alternatlng current to sald coll means, means ~or measurlng the Impedance of sald coll means, sensor means for determlnlng the distance between sald coll means and sald surface and processlng means for processlng ~5 the value of sald Impedance and sald dlstance In order to obtaln the value of thlckness of the metalllc material Independently 12tiiE3E~

from a variatlon of said dlstance. Sultably, sald coll means comprlse sald dlstance sensor means. i~eslrably, sald dlstance sensor means comprlse means for generatlng at least one dlfferent hlgher frequency than that used For the means for measurlng the Impedance of sald coll means. Alternatlv01y, sald dlstance sen-sor means are Independent from sald coll means. Sultably, sald core Is made oi ferromagnetlc materlal.

Accordlng to the Inventlon the measur ! ng devlce com-prlses a sensor constltuted by a core wlth pole pleces whlch canbe made to approach the wall to be examlned and by an electrlc coll wound on the body of sald core, means for supplylng an alternatlng current to sald coll, means for measurlng the Impe-dance of sald coll, and processlng means for obtainlng the thlck-ness of the wall under measurement from the measured value ofsald Impedance.

Large thlcknesses can be measured If the metal alloy of the wall to be measured Is above the Curle temperature. Thls ZO physlcal condltlon means that the materlal Is no longer ferromag-netlc (nor contalns ferromagnetlc phases or preclpltates), but Instead us In a dla or paramagnetlc state. The absence of ferro-magnetlsm enables the llnes of force of the magnetlc fleld to penetrate Into the materlal. The hlgh temperature means that the 2~ metal alloy has a suffIclently hlgh electrlcal reslstlvlty to llmlt the dlssipatlon of the magnetlc fleld energy by the eddy current effect, and thus ensure good penetratlon of the alternat-lng magnetlc fleld.

In one embodiment of the present Inventlon sald coli means are constltuted by a slngle exclter and measurement coll.

In another embodlment of the present inventlon sald coll means comprise more than one coll. Sultably, sald coll means are constltuted by an exclter coll and two secondary mea-surement colls formlng two branches of a Wheatstone brldge.

"` ~L2~ 6 Deslrably, said coll means comprlse at least two colls In -the form of an autotransformer. Preferably, two measurement colls are obtalned from an exclter coll by means of an autotransformer tap.

The core sultably has a C, E,y , I or E-rotatlon shape.
Sultably, when the core has a y shape sald core has a "E" shape.

In another embodlment of the present Inventlon sald generator means comprlse a slnusoldal generator -For provldlng the slnusoldal feed current for the coll means and for provldlng a reference voltage, and sald Impedance measurement means comprlse two synchronous detectors for rectlfylng the voltage developed across the coll means In accordance respectlvely wlth sald refer-ence voltage and wlth a voltage the phase of whlch Is shlftedthrough 90 relatlve to sald reference voltage, and means for feedlng t.he outputs of sald detectors to sald processlng means.

In a further embodlment of the present Inventlon sald generator means comprlse a double slnusoldal generator for pro-vldlng the slnusoldal feed current for the coll means and for provldlng an Identlcal feed current for an Inductor havlng a coeffIclent equal to that of the coll means when the metalllc materlal Is absent, and sald Impedance measurement means com-prlses a dlfferentlal ampllfler for obtalnlng the dlfferencebetween the two resultant voltages and a phase comparator for maklng a phase comparlson between sald dlfference and a reference voltage provlded by sald generator.

In a stlll further embodlment of the present Inventlon sald generator means comprlse a slnusoidal generator for provld-lng a slnusoldal feed current for the co11 means, and sald Impedance measurement means comprlse a detector for rectlfylng the voltage developed across the coll means, the output of sald detector belng fed to sald processlng means. Sultably, sald gen-erator means comprlse means for varylng the frequency of the - 2a -alternatlng current as a function of the thlckness of -the metal-llc materlal.

In another embodIment of the present Inventlon sald measurlng means comprlse means for calculatlng the real part of the Impedance of the coll means. Alternatlvely, sald me~surlng means comprlse means for calculatlng the imaglnary part oF the Impedance of the coll means. Sultably, sald processlng means comprlse means for calculatlng two of the three parameters, modu-lus, real part, Imaglnary part, of the Impedance of the collmeans. Deslrably, means are provlded for coollng the components of the measurlng devlce. Sultably, means are provlded for pro-tectlng the components of the measurlng devlce from the flames and fumes whlch accompany the materlal.
1~
The devlce of the present Inventlon Is partlcularly useful for measurlng the thlckness of a tube or a ferrous plate or determlnlng the eccentrlclty of a tui~e through the measurlng of the Impedance of coll means dlsposed In opposlte posltlon wlth respect to the tube, de-terminlng the transverse proflle oF a fer-rous plate, determlnlng the planarlty of a ferrous plate, deter-mlnlng the dlameter of a ferrous rod, and measurlng the thlckness of metalllc materlal movlng wlth respect to the measurlng devlce.

The present Inventlon wlll be further Illustrated wlth reference, by way of example, to the accompanylng drawlngs, In whlch:-F Igure 1 shows the baslc scheme of a sensor accordlng to the Inventlon;

F /gure 2 shows the equlvalent magnetic clrcuit of sald sensor, In comblna~lon wlth the wall subJected to thlckness mea-surement;

3~

- 2b -Flgures 3, 4 and 5 are dlagrams of three posslble exam-ples of clrcult arrangements sultable for feedlng the coll, mea-surlng Its Impedance and processlng the measurement;

Flgure 6 shows the baslc scheme of a dlfferent type oF
sensor for a measurlng devlce accordlng to the Inventlon; and 1 ~

3~

-- 2c --~8~6 Figure 7 shows a different method for mounting the coils in a sensor for ~ measuring device according to the lnvention.
-- c~
Figure 1 shows a sensor S constituted by a ferromagnetic~ N
~of C ~hape), the pole pieces of which face the conductive wall P to be measured and are disposed fl short distance therefrom.
On the central portion of the ferromagnetic core N there is wound a coil B, which iB fed with sinusoidal current I and hQs a measurable impedance Z.
This described assembly corresponds to the equivalent magnetic circuit of Figure 2, which comprises a branch 1 equivalent to the ferromagnetic core N, a branch 2 equivalent to the passage through air of the magnetic flux between the core N and wall P, and a branch 3 equivalent to the passage in air downstream of the wall P;
ln the equivalent circuit model, the wall P is assumed thin relative to the a{r thicknesses and is thus negligible. On ehe other hand5 the material of P is a~ a temperature hlgher than the Curie eempera-ture, and the magnetic permeability of the air and of thP ma~erial are approximately equal ~ - 1)9 so that the presence of ~he wall does not alter ehe distribution of the magnetic field lines of flux.
The three said branches are characterised by different values of magnetic reluctance, ie:
- in the branch 1 there is a reluctance Rl, whlch i9 ~egligible compared with the other two because of the high premeabillty of the ferromagnetic material ~hich constitutes the core N;
- in the branch 2 there i8 a reluctance R2~ which i9 determined by the air upstream of the wall P;

. . .

~Z6131~6 .

- in the branch 3 there is a reluctance R3, which ~s determined - by the air downstream of the wall P and equivalent to the wall P.
The turns of the coil B, which are n in number, are llnked with the branch 1, whereas one turn p is to be considered linked with the third branch, to represen~ the conduc~ive wall P, which has a resistance r proportional to the resistivity of its constituent material.
In a situation of this kind, feeding the coil 8 with a sinusoidal current I of angular frequency ~ gives rise ~o a magnetomotive force f.nl.m. - n I
to which there corresponds in the branch 1 a magnetic flux 01 = ~2 + 03 where P2 and 03 represent the contributions provided by the fluxes present in the branches 2 and 3 respectively.
~hereas 02 can be calculsted by considering only the reluctance R2, ie P2 - n I/R2 the flux 0~ also depends on the curren~ Ip induced in ~he conduc~i~e wall P ~and thus in the turn p with resistance r), ie O ~ (n I ~ I )/R3 The current I itself depends on the deriva~ive of p3 with respect to ti~e, and on the resistance r offered by the wall P. Using vectorial notation:
Ip 8 ~p3/r from which, using the previous equatlon:

~26~ 6 03 = n I/R3 ~ /rR3 The voltage V applied across the coil B can be calculated thus:
i Pl j~n(nI/R2 + nI/R3 1 + ~/rR3 It follows that the impedance Z of the coil B is given by the equation:
2 V ~ jWL(l - ~2r2) ~ ~w2~ (1) 1 + ~2~ 2 1 t~ ~2~2 in which the following parameters have been introduced:

R2 ; L ~ n2(1 ~ 1 ); T =
2 3 R~ R rR3 where ~ depends on the ratio of the reluctance of the magnetic paths upstream of the wall P to the reluctance external to the : 15 core and thus depends on the distance bet~een the sensor and the wall, L is equal to the inductance of the coil B in the absence of the conductive wall, and T is a constant proportional to ~he inverse of the resistance presented by said wall, and thus proporti-onal to ~he wall thickness.

The above relationship (1) makes it possible to ob~ain the required thickness of the wall P from -the measurement of the impedance Z of the coil B, by introducing the sensor-wall distance parameter as described hereinafter.

~or this purpose it is possible ~o operate in one of the following ways:
~) By obtaining an indepèndent measure of the sensor-wall dis~a~ce, -:L~6~

for example by means of capacitive, opt$oal~ microwave or other ~ensors, the thickness of the wall P can be determined by measuring one of the following constituent parameters of the impedance Z of the coil B:
a) the modulus of the eoil impedsnce Z = ~L ¦1 + ~2~[l ~ ~2~ l)2 + 11 (2) ~I
+ ~2~
b) the real part oE the coil impedance Zr = ~ ~2r .L (3) ~ Z'f2 c) the coefficient of the imaginary part of the coil impedance Zi = ( ~ 321 2 ) 67L (4~
2~2 -The parameter K = ~ is an index of the magnet$c field penetration through the wall P. The smaller the value of K the greater the transparency. In this respect, for penetration to be good, the impedance ~f ~he third branch ~/R3 must be less than the resistance r of the equivalent turn:
- u < r and thus R ~ ~r < 1 The relationship is satisfied the smaller the ~alue of ~. As g = l/rR3~ it is necessary for r to be large, this condition being ~atisfied when the material is at high temperature; as will be the 2S case $n ~hickness measurement at the exit ~f hot rolllng mills.

By introd~cing a fir~g order approxlmat$onl the real part of the ~ ~268t3~6 impedance becomes (where K l) Z _ ~ ~2lL (5) with a very strong dependence (l/r) on the presence of the wall P
From each of the expressions 2, 3, 4, 5, the values of L and ~ being known and having found the values of R2 and R3, it is possible, from a knowledge of the sensor-wall distsnce and using theoretical calculations and tables of theoretical or experimental orlgin, to obtain the value of r and thus obtain the thickness of the wall P, 8S the a~erage resis~ivity of this latter is known. This latter passage can also be effected either on the basis of theoretical calculations or, more conveniently, from experimental results.
B) Si~ultaneous measurements are made of the impedance of the coil B, these measure~en~s provlding its real part and imaginary part, as indicated heretofore.
These measurements can be processed in ~he following ways:
a) by comparing them wi~h experimental tables which provide the corresponding values of thickness and sensor-wall dis~ance;
b) by calculating the expression:
(5iL. _ Z )2 ~S2$:2 Zr I (wL ~ Zi) 2 1 ~ W2~2 ~hich is dependent on l/r and independent of the ra~lo R2/~R2 ~ R3) and hence of the sensor-wall dis~ance.
C) The voltage signal j~LI, present acros~ the coil B when ~he conductive wall P is absent, is subtracted fro~ the voltage measured when the wall is present, and the phase difference ~
between the resultant signal and the current fed to the coil is - ~L26~816 measured.

This phase difference is given by Zi ) = arcsin - -~ (w~ ~ Zi) ~ Zr from which 2~2 I = sln \11 + ~2r2 from which it is possible to deduce the required value of the thickness of the wall P once the sensor~wall distance is known.
D) Several coil impedance measurements are made at differen~
frequencies. In this respect, the magnetic fields at lower frequencies are more sensitive to the thickness, whereas those at hlgher frequencies are more sensitive to ~he sensor-wall dis~ance as a consequence of the skin effect, which limits magnetic flu~
penetration into the conductors at high frequency. The thicknes6 measurement and sensor-wall distance can be obtained by sui~able - mathematical manipulation or by using tables.
: To employ the methods explained in the preceding paragraphs A~ B

~nd D, the circuit arrangement illustrated in Figure 3 can be used, ~hlch is shown and described here by way of non~ iting e~ample.
I~ ~aid figure, it can be seen that a generator G generating sinu-soidal current of angular frequency ~ feeds ~he current I to the sensor coil B7 and a reference voltage Vrif, which a ph~se shifter : SF shifts through 90 so that it becomes V'rif. The voltage V
across the coit B, which indicates the impedance of ~his latter 9 i8 rectified by ~o sy~chronous detectors RSl and RS2 which are piioted by the reference voltages V if and V'rif respec~ively, and , ~Z~;~38~6 g the relative outputs are fed to a processor EL which, in accordance with the proposed methods, is able to determine the value of the thickness of the wall P. If measurements at different frequencies are requ~red, the processor EL can control the frequency of the sinusoidal generator G by means of a frequency control signal CF.
In the case of ~he method described in ehe preceding paragraph C
lt is advisable to use the circuit arrangement of Figure 4, in whLch a double sinusoidal generator GD feeds two identical currents I
to the sensor coil B and to an inductor L ha~ing a coefficient equal to that of the coil when the wall is absent. The difference between the two resultant voltage signals, which is obtained by means of a differen~ial amplifier AD, $s then compared, with regard to ~he i phase~ with a reference signal V i~ provided by the generator GD.
This phase comparison is made by a phase comparator and processor CE, by means of which the thickness of the wall P can be obtained.
When employing method A, and wishlng to use only the modulus value of the impedance Z it is possible to use the more simple circui~
arrangement shown in Figure 5. A generator G generating sinusoidal current with angular frequency ~ feeds current I to the sensor coil B. The voltage V across the coil B, which is indicative of the $mpedance of this latter, is rectlfied by RS and fed to a processor EL able to determine the thickness of the wall P~
Instead of being constituted by a single coil which operates both as an exciter element and as an inductance, the impedance ~aria~ions cf which are measured, the sensor can be constructed differently in order to improve the signal/noise ratio and stability, and ~o 126~38i6 simplify the electrical circuits.
Figure 6 shows the basic sche~e of said sensor. An exciter coil B and two measurement coils Bl and B2 ~onnected inta a Wheatstone bridge are wound on a core. A potentiometer p enable~l the bridge to be zeroed when the input to be measured is absent.
Figure 7 shows a different coil assembly scheme which has been tested experimentally and gives optimum pe~formance. The core is

4 -shaped, the exciter coil B being mounted on the vertical central branch and the two bridge coils Bl and B2 being ~ounted on the ad~acent horizontal branches of the 4 .
In all cases, whatever the type of sensor used, the sensor should be cooled and protected from flames and fumes which may accompany the material to be measured. Although the description has referred heretofore to the measurement of a flat wall, it is possible to use analogous methods to measure the thickness of a tube,whether the tube is at rest or moving.
With two opposlng sensors fed by the same sinusoidal generator, and employing means for measuring the variations in the impedance differences between the two coils of the opposing sensors, it is also possible to measure the eccentrlcity of said tube.
By means of the thickness measurement of more than two sensors it is possible to deduce tube eccentricity.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A measuring device for the contactless measurement of the thickness of metallic material at a temperature above the Curie temperature, comprising a magnetic core with pole pieces facing a surface of the metallic material, coil means wound on a body of said core, generator means for supplying an alternating current to said coil means, means for measuring the impedance of said coil means, sensor means for determining the distance between said coil means and said surface and processing means for processing the value of said impedance and said distance in order to obtain the value of thickness of the metallic material independently from a variation of said distance.

2. Measuring device according to claim 1, wherein said coil means comprise said distance sensor means.

3. Measuring device according to claim 2, wherein said distance sensor means comprise means for generating at least one different higher frequency than that used for the means for mea-suring the impedance of said coil means.

4. Measuring device according to claim 1, wherein said distance sensor means are independent from said coil means.

5. Measuring device according to claim 1, wherein said core is made of ferromagnetic material.

6. Measuring device according to claim 1, wherein said core has a "C" shape.

7. Measuring device according to claim 1, wherein said core has an "E" shape.

8. Measuring device according to claim 1, wherein said core has a "y" shape.

9. Measuring device according to claim 1, wherein said core has an "I" shape.

10. Measuring device according to claim 1, wherein said core has an "E rotation" shape.

11. Measuring device according to claim 1, wherein said coil means are constituted by a single exciter and measure-ment coil.

12. Measuring device according to claim 1, wherein said coil means comprise more than one coil.

13. Measuring device according to claim 12, wherein said coil means are constituted by an exciter coil and two sec-ondary measurement coils forming two branches of a Wheatstone bridge.

14. Measuring device according to claim 12, wherein said coil means comprise at least two coils in the form of an autotransformer.

15. Measuring device according to claim 14, wherein two measurement coils are obtained from an exciter coil by means of an autotransformer tap.

16. Measuring device according to claim 8, wherein an exciter coil is wound on the central branch and two bridge coil are wound on the two lateral, consecutive branches of the "y"
shaped core.

17. Measuring device according to claim 1, wherein said coil means comprises an inductive portion of an oscillating circuit whose self frequency variation is measured to deduce the thickness of the metallic material.

18. Measuring device according to claim 1, wherein said generator means comprise a sinusoidal generator for provid-ing the sinusoidal feed current for the coil means and for pro-viding a reference voltage, and said impedance measurement means comprise two synchronous detectors for rectifying the voltage developed across the coil means in accordance respectively with said reference voltage and with a voltage the phase of which is shifted through 90° relative to said reference voltage, and means for feeding the outputs of said detectors to said processing means.

19. Measuring device according to claim 1, wherein said generator means comprise a double sinusoidal generator for providing the sinusoidal feed current for the coil means and for providing an identical feed current for an inductor having a coefficient equal to that of the coil means when the metallic material is absent, and said impedance measurement means com-prises a differential amplifier for obtaining the difference between the two resultant voltages and a phase comparator for making a phase comparison between said difference and a reference voltage provided by said generator.

20. Measuring device according to claim 1, wherein said generator means comprise a sinusoidal generator for provid-ing a sinusoidal feed current for the coil means, and said Impedance measurement means comprise a detector for rectifying the voltage developed across the coil means, the output of said detector being fed to said processing means.

21. Measuring device according to claim 1, 18 or 19, wherein said generator means comprise means for varying the fre-quency of the alternating current as a function of the thickness of the metallic material.

22. Measuring device according to claim 1, wherein said impedance measuring means comprise means for calculating the modulus of the impedance of the coil means.

23. Measuring device according to claim 1, wherein said measuring means comprise means for calculating the real part of the impedance of the coil means.

24. Measuring device according to claim 1, wherein said measuring means comprise means for calculating the imaginary part of the impedance of the coil means.

25. Measuring device according to claim 1, wherein said processing means comprise means for calculating two of the three parameters, modulus, real part, imaginary part, of the impedance of the coil means.

26. Measuring device according to claim 1, wherein means are provided for cooling the components of the measuring device.

27. Measuring device according to claim 1 or 26, wherein means are provided for protecting the components of the measuring device from the flames and fumes which accompany the material.

28. A method of contactless measurement of the thick-ness of a metallic material at a temperature above the Curie tem-perature comprising positing the device as claimed in claim 1, such that the pole pieces are adjacent to the surface of the metallic material, measuring the impedance of the coil means, determining the distance between the coil means and such surface by such sensor means and processing the value of such impedance and said distance obtaining the value of thickness of the metal-lic material independently from the variation of said distance.

29. A method according to claim 28, comprising deter-mining the eccentricity of a tube through the measuring of the impedance of coil means disposed in opposite position with respect to the tube.

30. A method according to claim 28, comprising deter-mining the transverse profile of a ferrous plate.

31. A method according to claim 28, comprising deter-mining the planarity of a ferrous plate.

32. A method according to claim 28, comprising measur-ing the thickness of metallic material moving with respect to the measuring device.