CA2018689C - Surface impedance measurement device - Google Patents
Surface impedance measurement deviceInfo
- Publication number
- CA2018689C CA2018689C CA002018689A CA2018689A CA2018689C CA 2018689 C CA2018689 C CA 2018689C CA 002018689 A CA002018689 A CA 002018689A CA 2018689 A CA2018689 A CA 2018689A CA 2018689 C CA2018689 C CA 2018689C
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- Canada
- Prior art keywords
- electrode
- probe
- electrodes
- wobbled
- voltage
- Prior art date
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- 238000002847 impedance measurement Methods 0.000 title abstract description 5
- 239000000463 material Substances 0.000 claims abstract description 94
- 239000000523 sample Substances 0.000 claims abstract description 81
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- 238000000576 coating method Methods 0.000 claims description 5
- 239000011248 coating agent Substances 0.000 claims description 4
- 238000005259 measurement Methods 0.000 description 28
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- 238000000691 measurement method Methods 0.000 description 4
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/04—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant in circuits having distributed constants, e.g. having very long conductors or involving high frequencies
Abstract
A surface impedance measurement device comprises a transmitting electrode producing a magnetic field in the area surrounding a material to be tested and a receiving electrode receiving the magnetic field. An electric voltage wobbled in frequency is applied to the transmitting electrode, and a circuit measures the voltage at the terminals of the second electrode. For the electrodes to be applied on the same side of the material to be tested and of which the surface impedance is to be measured, the electrodes are superposed by the intermediary of a dielectric wedge in a compact probe. The impedance measurements are then independent of the dimensions and geometry of the material.
Description
Il 2018689 S~rface ;m~e~Ance meas.-~ ~n~ device BACKGROUND OF T~ INVENTION
1 - Field of the Invention.
This invention relates to a measurement device called impedancemeter for measuring the surface i --Ance of a materLal.
More particularly, the invention deals with materials that are very good conductors electrically, presented in the form of a thin sheet, such as the conductive metals or c ~nd materials, e.g. with carbon fiber, used in aeronautlc equipment. These materials can, if necessary, be covered with paints or insulation coatings.
1 - Field of the Invention.
This invention relates to a measurement device called impedancemeter for measuring the surface i --Ance of a materLal.
More particularly, the invention deals with materials that are very good conductors electrically, presented in the form of a thin sheet, such as the conductive metals or c ~nd materials, e.g. with carbon fiber, used in aeronautlc equipment. These materials can, if necessary, be covered with paints or insulation coatings.
2 - State of the Prior Art.
It is recalled that the surface i~p~An~e ZS characterizing such materials results from the following definition. The material carries a surface current density JA which creates a potential difference per unit of length represented by the longitudinal electrical field Eto~ The impedance ZS i8 defined by the relatin: Etc Z~J~
In the case of the material being conductive, the impedance ZS is related to the electrical conductivity a and to the thickness d of the material by the relation ZS = 1/(a d) as long as there is no occurrence of skin effect. If we consider a square plate of impedance ZS carrying a current I, a potential difference V appears at the t~rminAlR of the plate such that V=Zs I.
The preceding remark can be turned to good account to measure the surface ~ -'on~e ZS directly. Nevertheless, with a very conductive material, the ~ --Anre of the material is in geries with the im7~Ance of the wires that inject the current I. It i8 then no longer possible to distinguish the contribution of the material and that of the measuring circuit in the measuring of the potential dlfference V, which then becomes impossible. MoLeove., such measurement cannot be carried out if the material is covered with insulating.
To obviate this drawback, it has already been proposed that the impedance ZS be measured without any electrical contact between an impedancemeter and the material.
A known i -~An_ ~er further comprises a first electrode, called transmitting antenna, that creates a given electromagnetic field, and a second electrode, called receiving antenna. A measurement chaln connected to the secondantenna enables the electromagnetic field to be measured in a given frequency band. During a first stage, the transmitting antenna is placed a~ a predetermined ' ,~.
~1 2018689 constant dLstance from the receiving antenna and i8 moved away from materlal likely to perturb the field emitted dLrectly by the first antenna and received by the second antenna. During a second stage, a sample of the material to be tested i8 interposed between the transmitting and receiving antennae. Throughout the two stages, an alternating voltage wobbled ln a predetermined frequency range is applied to the transmitting antenna. The voltage measurements at the tQrm~nAls of the receiving antenna for the two stages are analyzed and interpreted in order to determine the surface ~ nce value ZS of the material sample.
The material usually fills a two-dlmensional opening in a metallic panel of large dimensions. Each of the antennae is comprised of a conductive circular loop attached to an insulating holder plate on both sides of the opening plane.
The transmitting antenna creates a magnetic field that is uniform with the area surrounding the material parallel to the plane of the material. During the first stage, the receiving antenna receives a magnetic field from the other side of the opening at a given distance. During the second stage, the receiving antenna receives a magnetic field that i~ attenuated by the presence of the material in the opening.
In the case of a circular opening of radius a, the ratio of the measured voltages V0 and V between the terminals of the receiving antenna during the first and second stages satisfy the approximate relation :
V/Vo = l/(l+jf/fc) with fc = (3ZS / (8,l0a)) (1+2 n Rc / ZS) whereby ZS is the surface ; -~Anre of the material, a is the radius of the opening, Rc is the possible resistance of the joint connecting the material and the metallic plate, and ~0=4n.10-7 H/m is the permeability of vacuum.
If the electrical contact between the material and the metallic panel in the opening is perfect, i.e. if Rc=O, we obtain fco = 3ZS /8~0a.
In this way, the fixing of the material in the opening provokes in the voltage picked up by the receiving antenna an attenuation or transfer function of the "fir~t-order low-pas~,3 filter" type characterized by a cut-off frequency fco proportional to the surface impedance ZS of the material.
The known ~mr~Anf ~er with two completely separate antennae and the measurement method inherent in this device mainly have the three following drawbacks:
i) The expression of fC shows that if the material-conductive panel contact is not perfect, the frequency fC is shifted with regard to fco.,A~ the value of 2~1868~
Rc cannot be quantified, an assembly must be set up to attempt to minimize the interference resistance Rc. The value of the latter is then neglected in the interpretation of the measurement, without it being possible to evaluate the error made. Furthermore, a sample material covered with paint cannot be measured as the contact is bad.
ii) The frequency fc depends on the radius a of the opening, i.e. on the dimension of the sample material tested. Only materials cut out to a given format so as to fix them in the opening can be tested by this method. It is therefore necessary to cut out a sample of each material to be tested, and a material belonging to a complex structure cannot be tested.
iii) The distance between the transmitting electrode lS and the material must be sufficient to obtain a magnetic field that is uniform with the area surrounding the material. As a result, the distance between the electrodes is relatively long and the voltage measured by the receiving electrode is relatively low.
OBJECTS OF THE INVENTION
The main object of this invention is to obviate the disadvantages of the prior art as commented above.
Another object of this invention is to provide a surface impedance measurement device wherein the two electrode are placed on the same side of the material to be tested.
A further object of this invention is to enable measurements without electrical contact between the surface impedance measurement device and the material, but also measurements that are independent of the dimensions and geometry of the material to be tested.
SUMMARY OF THE INVENTION
According to the présent invention, there is provided a device for measuring the surface impedance of a material to be tested, said device comprising:
a first electrode and a second electrode, said first and second electrodes being superposed by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first lo electrode and crossing said second electrode;
voltage measuring means for measuring second and third wobbled voltages supplied successively by said second electrode, said second wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is moved away from said material to be tested so that said material does not perturb said magnetic field, and said third wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is substantially placed on said material to be tested so that said material perturbs said magnetic field;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
-4a-2ol8689 According to the présent invention, there is also provided a device for measuring the surface impedance of a material to be tested, said device comprising:
first, second and third electrodes, said first electrode being interposed between said second and third electrodes by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second and third electrodes;
voltage measuring means for measuring second and third wobbled voltages respectively supplied by said second and third electrodes, said second and third wobbled voltages being respectively induced in said second and third electrodes responsive to said magnetic field when a side of said probe including said second electrode is substantially placed on said material to be tested so that said material to be tested perturbs said magnetic field in the vicinity of second electrode and does not perturb said magnetic field in the vicinity of said third electrode;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio va-riation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
-4b-The invention requires electrodes with a known geometry, preferably in the form of conductive circular loops.
Preferably, the first and second electrodes are superposed by the intermediary of a dielectric material in a compact probe to be applied near a same side of said material to be tested.
Preferably, the probe comprises a wedge in /
material, and two thin board~ in dielectric material that are re~pectively fixed on two parallel side~ of ~aid wedge and on which are printed the fir~t and ~econd electrodec re~pectively. The wedge and printed board~ a~embly ic coated and protected by an exterior dielectric layer. Then, the two electrodec are maintained parallel at a con~tant di~tance. The ~econd electrode, called receiving electrode, can be applied to the material to be te~ted and again~t which the receiving electrode i~ in~ulated by the thin layer of dielectric coating. The ~econd electrode therefore only pick~ up magnetic field emitted by the firct electrode and perturbed by the material.
Preferably, the probe cornprises a third electrode receiving sai~ magnetic field and ~uperpo~ed above the fir~t electrode oppo~ite the cecond electrode. In thi~ embodiment, the voltage mea~uring means ~imultaneou~ly mea~ures a first voltage between the terminal~ of the ~econd electrode and a ~econd voltage between the terminal~ of the third electrode. 8y comparison with the fir~t embodiment, measurement by means of thic cecond embodiment i~ much faster.
Preferably, this probe with three electrodes furthcl comprises first, ~econd and third thin board~ in dielectric material which re~pectively ~upport the fir~t, ~econd and third electrodes, a firct wedge in dielectric material again~t two parallel ~ide~ of which the fir~t and ~econd board~ are respectively fixed, and a ~econd wedge in dielectric material again~t two parallel sidec of which the firAt and third board~ are fixed.
Preferably, the measurement device also comprises means for calculating the variation of the ratio between said first and ~econd voltage~ ac a function of the frequency when said probe ic moved away from ~aid material to be te~ted and ~ub~equently when ~aid probe i~ placed on a came ~ide of ~aid material to be tested, thereby deducting from it the ~urface impedance of ~aid material to be te~ted by ~ecking out, among ~tored low-pa~-type reference curvec, a reference curve neare~t the variation of ~aid ratio between ~aid voltages.
~RIEF DESCRIPTION OF THE DRAWINGS
Further features and advantage~ of the invention will be apparent from the following particular de~cription of ~everal preferred embodiment~ of this invention a~ illu~trated in the corre~ponding accompanying drawing~ in which :
- Fig.l i~ an exploded view in per~pective of a fir~t probe with two _5_ ~1 2018689 electrodes embodying the inventLon;
- Flg.2 is a block diagram of an i --~n~ Ler embodyLng the invention, operating with the probe in Fig.l;
- Figs.3 and 4 are charts showing electrode voltage ratio variations as a function of frequency, particularly for probes with different interelectrode gaps, when these probes are applied to a same material to be tested;
- Figs.5 and 6 are charts showing variations of said electrode voltage ratio as a function of frequency, obtained with a same probe and two dlfferent materials respectively, and theoretical curves close to the -low-pass filter"
type;
- Fig.7 shows theoretical curves of the "low-pass filter" type for various surface impedance values;
- Fig.8 i8 a chart showing variations of said voltage ratio as a function of frequency, obtained with a same probe and for a same material, but for different distances between the probe and the material;
- Fig.9 is an exploded view in perspective of a second probe comprising three circular electrodes;
- Fig.10 is a block diagram of an i --~n- -ter operating with the probe in Fig.9;
- Fig.ll i8 a schematic vLew in perspective of the impedancemeter in Fig.10, in the form of a portable device; and - Fig.12 is an exploded view in perspective of a probe comprising three rectangular electrodes.
DESCRTPTION OF TH~ Ph~ k~l~ EM~nIM~NTS
A measurement probe SM for imp~nr -ter embodying the invention and shown in Fig.1 comprises an in~ection electrode 1 playing the role of a transmitting antenna, and a measuring electrode 2 playing the role of receiving antenna and placed parallel to the first electrode 1. Each of the electrodes 1, 2 is typically comprised of a circular spire printed on one of the sides of an insulating board 10, 20, or by two concentric circular spires printed on the sides of a two-sided printed board. However, the second electrode may be rectangular or of any other form. The n~iqhhoring t~rmin~l~ ll and 12, 21 and 22 of the electrode 1, 2 are comprised of metallized holes in the board 10, 20, from which printed connection conductors can be extended.
Il 2018689 According to the embodiment illustrated ln Fig.l, the elements 20, 3, lO
and 4 have a same square-shaped sectLon and are maintained in a stack by means of four through screws and nuts 50 and 5l. The elements thus stacked 20, 3, lO
and 4 are then coated wLth a thLn protectLve dLelectrLc layer 52 such as epoxy resln. TypLcally, Lf ~ deslgnates the radlus of the transmLttLng electrode l, the radLus of the receLvLng electrode 2 Ls equal to r/2, and the thLckness z of the LnsulatLng wedge 3 Ls equal to r.
As shown Ln FLg.2, to take surface Lmpedance measurements relatLng to a good electrLcal conductLve materLal to be tested, a thLn plate PM of saLd materLal lO 18 used as a sample. The probe SM 18 placed above the plate Ln such a way that the receLvlng electrode 2 Ls facLng the plate and that the axLs of the electrodes l and 2 is substantially perpendicular to the plate PM. In this way, the receiving electrode 2 and its holder board 20 are at a dLstance h from the plate of material PM which can practically vary from 0, plus the small thickness of the coatLng 52, to several m~ Lers. On the other hand, the Lnterelectrode gap, substantlally equal to the thlckness z of the wedge 3, 1a constant.
Furthermore, the electrodes l and 2 are connected to electronLc cLrcuits of the lmpedancemeter IM proper. The latter Ls organLzed around a central process1ng unlt ~CPU) 6 compr1sLng a p.ep1-o4L- -i mLcroprocessor. The unLt 6 20 is connected by an Lnternal bidLrectLonal bus BU to a frequency generator 7 and -~ a vectorLal analyzer 8, and by a standard Lnterface bus BIS to a measurement result dLsplay devLce 9 comprLsLng e.g. a prLnter.
Outputs of the generator 7 are connected to the t~rm;nAls 11 and 12 of the transmitting electrode l via an ~ -lAn~e matching load 13 which Ls also prLnted on the holder board 10. Generator 7 produces a wobbled alternatLng voltage sLgnal of whLch the frequency f perLodLcally sweQps a frequency range fm1n to fmax selected by the processLng unLt 6. The voltage sLgnal produced by the generator 7 Ls translated by an electrLc current Ln the transmLttLng electrode l whLch then creates an Lnduced current Ln the second electrode 2 and consequently an 30 Lnduced voltage at the t~rm~nAl~ 21 and 22. ThLs Lnduced voltage is equal to the time derLvative of the flux of the magnetic field crossing the receiving electrode 2.
The analyzer 8 has lnputs 81 and 82 respectlvely connected to the output term~ ns~ 12 and 22 of the electrodes 1 and 2 co as to measure the voltage produced by the generator 7 on the electrode 1 and the voltage induced in the electrode 2, at each frequency. These voltages are read in digital form by the unit 6.
The measurement of the surface ~peri~nce Z5 of the materLal to be tested is carried out in two stages.
During a first stage, the probe SM is placed at a distance from all materials ln such a way that the magnetic field directly crossing the receiving electrode 2 is not perturbed by any material. The analyzer 8 measures a voltage Vd(f) directly induced in the electrode 2. The unit 6 memorizes each of the values of Vd(f) as a function of the frequency f.
During a second stage, the probe SM i8 placed above the material PM, as previously described in reference to Fig.2, and is maintained at a constant distance h that does not exceed a few millimeters. The distance h is preferably as small as possible, i.e. almost equal to zero, as will be seen hereinafter.
The analyzer 8 measures a voltage Vm (f) induced in the electrode 2 by the electrode 1. The voltage Vm results from the magnetic flux produced by the electrode 1 and perturbed by the plate of the material PM near the electrode 2.
The unit 6 memorizes each of the values of Vm (f) as a function of the frequency f, and calculates the ratio Stf)=Vm/Vd lower than one, expressed in the form 20 log S(f) with a vLew to representlng its variation8 as a function of the frequency f in the display device 9.
The voltage measured on the electrode 1 enables checking the stability of the signal derived by the generator 7 during first and second stages.
Theoretically, it i9 shown that the voltage ratio S is equal to the sum of two coefficients S(f) = Sl + S2(f), one of which-Sl is independent of the frequency. The coefficients Sl and S2 are expressed as a function of the ratios z/r and h/r:
(~.(Z~ ""~2 S2-(l+(z/r)2) 1(U~/l J (U)~-u(z/r.2~/r) whereby Jl(u) is the 8essel function of order l, fco i8 a characteristic frequency such that fco=ZS / (n4Or)l and ~o=4~.10-7 H/m i8 the permeabillty of vacuum. It appears that the coefficient Sl depends only on the geometric parameters z/r and h/r and is independent of the measurement frequency f, and that the coefficient S2 depends on z/r, h/r, fco and on the frequency f.
The function S(f) is now mainly analyzed as a function of the distance h ll 2~18689 between the probe SM and the material PM ln order to preaent the advantages of the choice h~20 when the impedancemeter is used.
In the case of h=0. the probe SM ig applied directly to the material PM.
The coefficient Sl is then equal to 0 and the voltage ratio S iB reduced to:
S-52[1 +(z/r)2]3'21~ u Jl(u)e~~Z/r)du The ratio S only depends on the geometric parameter z/r, on the characterlstic frequency fco which 1B proportional to the surface impedance Zs of the material, and on the measurement frequency f. The function S can then be calculated digitally.
Flgs.3 and 4 show the varlation of s exprensed in ds as a function of log(f/fcO) for several values of z/r equal to 0.0; 0.1; 0.25; 0.5; 1.0; 10.0;
100.0 and 1000Ø
The ratlo S varies like a low-pass type function of which the form and particularly the slope at high frequencies depend on z/r. In this way, for a geometry of the probe SM determined by constant parameters, the position of the variation curve of S compared with the frer~uency axis is directly related to ZS- During measurement, as will be seen hereinafter, the processing unit 6 ~eeks out one of the low-pass type theoretical curves as close as possible to the variation curve of S co..~onding to the material to be tested and obtained by the measurement proper, and identifies the position of the variation curve of S ao a function of the frequency f in the range [fmin/ fmax] by one or 8everal characteristic points, e.g. at 3 dB, 10, 20 and 30 dB, in order to determine Zs -For z/r varying from 0 to 1 as per Fig.3, the form of the varlation curve of S gradually changeg from a second-order curve at approximately 40 dB/decade to a firgt-order curve at approximately 20 dB/decade, the cut-off frequency which ig ugually chosen at 3 dB varying little. For z/r varying above 1 ag per Fig.4, the function S ig little different to a firgt-order low-pasg function, the cut-off frequency at 3 dB moving towards the lower frequencies.
In this way, though any value of z/r will do to meaoure the gurface ~mrr~r~r~nrt~
ZSt the invention r~ ~~rl~ that the geometric parameter z/r be chosen and consequently the geometry of the probe according to the following two criteria.
The voltage induced between the terminals 21 and 22 of the second electrode 2 ig proportional to the time derivative of the magnetic flux~crossing it and ll 2018689 consequently to the frequency f. To extend measurement to the lower frequencies, there must be sufficient flux and consequently sufficient induced voltage, which implies good coupling between the electrodes 1 and 2 and quite a small interelectrode gap z.
In order to determine the imre~Ance Z5~ the unit 6 locates the position of the variation curve of S measured in relation to at least one given straight line parallel to the frequency axis since, for a given value of S, there is only one value of Z5 and crnAe~l~ntly f fco co-.espollding to a ratLo f/fco. It 18 therefore preferable that the unit 6 identify a "simple" curve. According to the invention, the unit 6 seeks out by successive approximations a low-pass function of the first order nearest the function S(f) obtained on measuring.
For instance, the particular case z/r=1 is suitable; Figs.S and 6 show that the function S(f) obtained after measurement is little different to the function FBP=(l+(f/fC)2)-1/2 whereby fc~1.4 fco = 1-4 ZS t (n4Or) Fig.7 shows different curves FBP(f) for various values of ZS and shows that the smaller the imre~Anre Z5~ the more the measurements are taken in the low frequencies.
In this way, to determine the imre~Anre ZS of a material with a probe SM
such that z=r, the unit 6 identifies the curve S(f) for several predetermined values of S or of f, calculates the standard deviation between the curve S(f) and the various functions FBP(f) corresponding to values f fc, and only retains the function FsP with the smallest standard deviation. The impedance of the material ZS=n~Or fC tl.4 is then deduced from the retained function FBP and consequently from the coL.e~onding characteristic frequency fc.
In the rA~ of h ~ _, i.e. when the probe SM is not applied-to the plate of material PM, the coefficient S1 is no longer zero but is equal to a constant for a given geometry and a predet~rmin~d position of the probe SM, i.e. for constant ztr and htr. The second coefficient S2 is always a low-pass type function tending towards 0 when f tends towards infinity. The ratio S = S1 + S2 ls therefore approximately ec,ual to S2 at low frequency and to S1 at high frequency.
Fig.8 shows the filtering function S as a function of the frequency f for several values of the distance h between the probe SM and the material PM, when the interelectrode gap is constant and equal to z=r. The beginning in low frequency of the curves in Fig.6 is little different to the case h=0 as per Figs.4 and 5. At high frequency, the curves in Fig.8 tend towards respective planes equal to Sl(h/r).
When seeking to determine the surface i ~ nce Zs, h should be as small as possible since major dynamics are sought to identify the form of the curve S(f) obtained durlng measurement.
On the other hand, the value of maximum attenuatLon sl only depends on h for constant z/r and Ls in~lt~p~ nt of the i _ --Ant~e of the material. The curve S1(h) can be used to experimentally determine the distance between the probe and a conductive material, this distance being e.g. equal to the thickness of an insulation coating covering the material.
The measurement method implemented by the impedancemeter enables the surface ',mpe~lAnt~e ZS of a conductive material to be evaluated. However, for a homogeneous material, from the formula ZS = 1/(~Jt d), - if a is known, the thickness of the material d = 1/~Z5a) is deduced, and - if d is known, the conductivity of the material (~= l/(ZS d) is deduced.
In order to set ideas down, e.g. a probe SM for which z2r=5 cm, imposes a characteristic frequency fC ~ 1.4 ZS / (l~lor) ~ 2-107 ZS
Supposing a f.e~ue~ y sweep from O up to a predetermined maximum frequency fmaxt the surface i 3-Ance mea9ured will be le99 than ZS max = fmax /(2-107)~
i.e. for in9tance ZS max = 0-5 n for fmax = 10 MHz, and ZS max = 5 n for fmax 100 MHz.
In this way, the i _ ~-An~ emeter w$th the probe embodying this example measures surface impedances of less than a few ohms.
It should be noted that the higher the impedance measured, the lower the frequency at which the measurement must be carried out, as per Fig.7. As previously stated, the voltage measured via the receiving electrode 2 is proportlonal to the product of the flux crossing it and the frequency. The flux emitted by the first electrode 1 must therefore be increased in order to carry out measurementsat low frequency. The transmitting antenna is no longer comprised of a single spire but of a winding of n spires, and the sensitlvity of the probe is then increased by the integer factor n. In this instance, the board 10 has several concentric spires on one or both sides, or is replaced by a multilayer circuit, eaoh layer comprising at lest one spire. According to another ~ - L which can be implemented alone or combined with the previous solution, an amplifier is provided at output of the frec,uency generator 7 for amplify,ing the current ll 2018689 ln the transmitting electrode 1 and the magnetic flux and subsecuently the sensitivity of the probe SM.
The measurement method described above and Lmplemented by the impedancemeter IM equLpped wLth the probe as per FLg.2 Ls comprised of two successLve stages:
a calLbratLon of the probe at a dLstance from all materLals durLng the fLrst stage, then measurement of the surface i ~ n~e by applyLng the probe SM to the materLal PM durLng the second stage.
To Lncrease measurement speed, the calLbratLon stage is carrLed out sLmultaneously wLth the mea~ur~ - t stage, at each frequency value, by usLng a IC probe SMa wLth a transmLttLng antenna 1 and two receLving antennae 2a and 2R, and a corresponding Lmpedancemeter IMa.
In reference to FLg.9, the probe SMa lLke the probe SM comprises the followLng stacked members, namely a printed board 20a supporting a preferably circular receLvLng electrode 2a, an LnsulatLng wedge 3a, a printed board lOa supporting a circular transmitting electrode la, and an insulating cover 4a.
Fur~h~ -.e, between the board lOa and the cover 4a, the probe SMa comprises a second insulating wedge 3R, and a thLrd prLnted board 20R supporting a reference electrode 2R that L6 preferably cLrcular. The board 20R Ls lnterposed between the cover 4a and the wedge 3R. The wedge 3R Ls applLed agaLnst the board lOa.
All the members of the probe SMa have the same sectLon, e.g. square or dLsk-shaped Ln thLs Lnstance, and are fastened to one another by means of four paLrs of screws 50a and nuts 51a. Then the assembly of stacked members of the probe SMa Ls coated Ln resLn 52a. For Lnstance, the wedge 3R and the board 20R are respectLvely LdentLcal to the wedge 3a and the board 20a, and the radius of the reference electrode Ls equal to r/2.
The thLrd electrode 2R Ln the probe SMa plays the same role as the reception antenna 2 Ln the probe SM durLng the fLrst stage of the measurement method. In fact, when the sLde of the probe SMa comprLsLng the electrode 2a Ls applLed to a plate of materLal PM, the reference electrode 2R Ls sLtuated opposLte the plate PM by comparLson wLth the center board lOa supportLng the transmLttLng electrode la. The dLstance between the electrode 2R and the plate PM Ls then sufficiently large for the plate PM not to perturb the magnetic flux emLtted by the electrode la and generatLng the voltage Lnduced at t~rminAls 21R and 22R of the electrode 2R.
As shown Ln FLgs.10 and 11, the i ~--n~ -ter IMa al80 COmprLB-B a central processLng unLt 6a wLth an Lntegrated microp ocessor, a sLne, wave frequency 20~8689 generator 7a, and controlllng and displaying means 9a.
The vectorial analyzer 8 i8 replaced in the impedancemeter IMa by three parallel assemblies each comprisLng ln serles an lnput ampllfier 711, 712, 71R, an voltage limiting circuit 721, 722, 72R, and a analog-to-digital converter 731, 732 and 73R. The outputs of the generator 7a are connected to the terminals lla and 12a of the transmitting electrode la and to the input t~rm~nAl~ of the ampllfler 711. The t~rmlnAlR 21a and 22a of the recelvlng and measuring antenna 2a and the t~rm{ nAl R 21R and 22R of the recelvlng and reference antenna 2R are respectlvely connected to the lnput t~rmin~l~ of the ampllflers 712 and 71R. In these condltlons, when the unlt 6a orders a freguency sweep ln a glven range fmln to fmax, the converters 731, 732 and 73R slmultaneously transmit Ln digital form a voltage V1~f) applied to the ~rmlnAl~ of the transmitting electrode la by the generator 7a, a voltage V2(f) induced in the electrode 2a depending on the ~mpe~Ance ZS Of the material PM, and a reference voltage VR(f) induced in the electrode 73R. For each frequency f, the unit 6a directly calculates the ratio S(f) = V2/VR which makes it possible to rapidly visualize the curve S(f) with the approaching curve FBFC on the screen 9Sf, and to diAplay the digital value ZS of the material PM in a readout 9Z. Prlor to measurlng, the operator selects a frequency band [fmln/ fmax] and consequently the impedance range ZS
by means of a selector 9SE, and the type of probe by means of a button 9S0.
The i --~nf Ler IMa also comprises a standard Lnterface IN with an RS 232C-type bidirectlonal ~le~ icatlons bus for exchanglng digLtal signals wlth a remote calculator storlng the curves and results obtalned.
The ~r~e~An~ Lers descrlbed above apply to materlals that can be Lep~-esenLed by an lsotroplc surface lmpedance. Measurements are then lnterpreted quantltatlvely.
In the case of a hlghly anlsotroplc materlal, such as a carbon compound wlth parallel flbers, the surface i -~nre l~ comprl-ed of an lmpedance Z8x ln a dlrectlon x and of an ~n,--~n~e 28y ln a dlrectlon y perpendlcular to the dlrectlon x. According to the lnventlon, the clrcular electrodes 1 and 2 Ln the probe SM, or the electrodes la, 2a and 2R ln the probe SMa are replaced by rectangularly looped electrodes l'a, 2'a and 2'R as shown ln Flg.12 for a probe SM'a. In this Flg.12, members slmilar to those shown ln Fig.9 are designated by - the same numeral references but accentuated. Each rectangular electrode is thus comprlsed of at least two longltudlnal conductlve wLres connected at one of theLr ends by a conductive bend and havlng a length egual to at lea~t 10 tlme-ll 2018~89 the very smA11 dLstance between these wires, Ln the region of a few mlllLmeters.
The two or three internal electrodes in the probe are laid out in the same longitudlnal direction x, as shown in Pig.12.
If this rectangular-electrode probe is applied to the anisotropic material in the two perpendicular directions x and y, the ~ nc~ ~Ler displays two different curves Sx(f) and Sy(f) which enable the values of Z8x and Z5y to be deduced for the material under conslderation. One of these two curves with the greatest attenuation corresponds to the case of the common longitudinal direction x of the probe electrodes being parallel to the direction of the fibers, i.e.
1O to the direction in which the conductivity of the material is best.
A probe embodying the invention is also capable of detecting any discontinuity in a material which, by nature, modifies the distribution of the magnetlc fleld that generates the current lnduced ln the recelvlng electrode.
For instance, if the probe is applied to a slot existing between two plates comprised of the same material, the measurement supplied by the impedancemeter varies conslderably and slgnals the presence of the slot. This phenomenon 18 accentuated when one chooses to use a probe wlth a rectangularly-looped receiving electrode placed on the slot.
Other material discontinuities may be detected, such as junctions between 20 two plates of a same material. If the contact between these two plates is electrically perfect, the ~mp~ nre measurement carrled out by placing a probe on a plate ~unction should be identical to that of a uniform plate of this material; if the electrical contact at the ~unction is defective, the measurement is slmilar to that of a slot. A calibration enables the state of the electrical contact to be evaluated.
It is recalled that the surface i~p~An~e ZS characterizing such materials results from the following definition. The material carries a surface current density JA which creates a potential difference per unit of length represented by the longitudinal electrical field Eto~ The impedance ZS i8 defined by the relatin: Etc Z~J~
In the case of the material being conductive, the impedance ZS is related to the electrical conductivity a and to the thickness d of the material by the relation ZS = 1/(a d) as long as there is no occurrence of skin effect. If we consider a square plate of impedance ZS carrying a current I, a potential difference V appears at the t~rminAlR of the plate such that V=Zs I.
The preceding remark can be turned to good account to measure the surface ~ -'on~e ZS directly. Nevertheless, with a very conductive material, the ~ --Anre of the material is in geries with the im7~Ance of the wires that inject the current I. It i8 then no longer possible to distinguish the contribution of the material and that of the measuring circuit in the measuring of the potential dlfference V, which then becomes impossible. MoLeove., such measurement cannot be carried out if the material is covered with insulating.
To obviate this drawback, it has already been proposed that the impedance ZS be measured without any electrical contact between an impedancemeter and the material.
A known i -~An_ ~er further comprises a first electrode, called transmitting antenna, that creates a given electromagnetic field, and a second electrode, called receiving antenna. A measurement chaln connected to the secondantenna enables the electromagnetic field to be measured in a given frequency band. During a first stage, the transmitting antenna is placed a~ a predetermined ' ,~.
~1 2018689 constant dLstance from the receiving antenna and i8 moved away from materlal likely to perturb the field emitted dLrectly by the first antenna and received by the second antenna. During a second stage, a sample of the material to be tested i8 interposed between the transmitting and receiving antennae. Throughout the two stages, an alternating voltage wobbled ln a predetermined frequency range is applied to the transmitting antenna. The voltage measurements at the tQrm~nAls of the receiving antenna for the two stages are analyzed and interpreted in order to determine the surface ~ nce value ZS of the material sample.
The material usually fills a two-dlmensional opening in a metallic panel of large dimensions. Each of the antennae is comprised of a conductive circular loop attached to an insulating holder plate on both sides of the opening plane.
The transmitting antenna creates a magnetic field that is uniform with the area surrounding the material parallel to the plane of the material. During the first stage, the receiving antenna receives a magnetic field from the other side of the opening at a given distance. During the second stage, the receiving antenna receives a magnetic field that i~ attenuated by the presence of the material in the opening.
In the case of a circular opening of radius a, the ratio of the measured voltages V0 and V between the terminals of the receiving antenna during the first and second stages satisfy the approximate relation :
V/Vo = l/(l+jf/fc) with fc = (3ZS / (8,l0a)) (1+2 n Rc / ZS) whereby ZS is the surface ; -~Anre of the material, a is the radius of the opening, Rc is the possible resistance of the joint connecting the material and the metallic plate, and ~0=4n.10-7 H/m is the permeability of vacuum.
If the electrical contact between the material and the metallic panel in the opening is perfect, i.e. if Rc=O, we obtain fco = 3ZS /8~0a.
In this way, the fixing of the material in the opening provokes in the voltage picked up by the receiving antenna an attenuation or transfer function of the "fir~t-order low-pas~,3 filter" type characterized by a cut-off frequency fco proportional to the surface impedance ZS of the material.
The known ~mr~Anf ~er with two completely separate antennae and the measurement method inherent in this device mainly have the three following drawbacks:
i) The expression of fC shows that if the material-conductive panel contact is not perfect, the frequency fC is shifted with regard to fco.,A~ the value of 2~1868~
Rc cannot be quantified, an assembly must be set up to attempt to minimize the interference resistance Rc. The value of the latter is then neglected in the interpretation of the measurement, without it being possible to evaluate the error made. Furthermore, a sample material covered with paint cannot be measured as the contact is bad.
ii) The frequency fc depends on the radius a of the opening, i.e. on the dimension of the sample material tested. Only materials cut out to a given format so as to fix them in the opening can be tested by this method. It is therefore necessary to cut out a sample of each material to be tested, and a material belonging to a complex structure cannot be tested.
iii) The distance between the transmitting electrode lS and the material must be sufficient to obtain a magnetic field that is uniform with the area surrounding the material. As a result, the distance between the electrodes is relatively long and the voltage measured by the receiving electrode is relatively low.
OBJECTS OF THE INVENTION
The main object of this invention is to obviate the disadvantages of the prior art as commented above.
Another object of this invention is to provide a surface impedance measurement device wherein the two electrode are placed on the same side of the material to be tested.
A further object of this invention is to enable measurements without electrical contact between the surface impedance measurement device and the material, but also measurements that are independent of the dimensions and geometry of the material to be tested.
SUMMARY OF THE INVENTION
According to the présent invention, there is provided a device for measuring the surface impedance of a material to be tested, said device comprising:
a first electrode and a second electrode, said first and second electrodes being superposed by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first lo electrode and crossing said second electrode;
voltage measuring means for measuring second and third wobbled voltages supplied successively by said second electrode, said second wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is moved away from said material to be tested so that said material does not perturb said magnetic field, and said third wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is substantially placed on said material to be tested so that said material perturbs said magnetic field;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
-4a-2ol8689 According to the présent invention, there is also provided a device for measuring the surface impedance of a material to be tested, said device comprising:
first, second and third electrodes, said first electrode being interposed between said second and third electrodes by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second and third electrodes;
voltage measuring means for measuring second and third wobbled voltages respectively supplied by said second and third electrodes, said second and third wobbled voltages being respectively induced in said second and third electrodes responsive to said magnetic field when a side of said probe including said second electrode is substantially placed on said material to be tested so that said material to be tested perturbs said magnetic field in the vicinity of second electrode and does not perturb said magnetic field in the vicinity of said third electrode;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio va-riation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
-4b-The invention requires electrodes with a known geometry, preferably in the form of conductive circular loops.
Preferably, the first and second electrodes are superposed by the intermediary of a dielectric material in a compact probe to be applied near a same side of said material to be tested.
Preferably, the probe comprises a wedge in /
material, and two thin board~ in dielectric material that are re~pectively fixed on two parallel side~ of ~aid wedge and on which are printed the fir~t and ~econd electrodec re~pectively. The wedge and printed board~ a~embly ic coated and protected by an exterior dielectric layer. Then, the two electrodec are maintained parallel at a con~tant di~tance. The ~econd electrode, called receiving electrode, can be applied to the material to be te~ted and again~t which the receiving electrode i~ in~ulated by the thin layer of dielectric coating. The ~econd electrode therefore only pick~ up magnetic field emitted by the firct electrode and perturbed by the material.
Preferably, the probe cornprises a third electrode receiving sai~ magnetic field and ~uperpo~ed above the fir~t electrode oppo~ite the cecond electrode. In thi~ embodiment, the voltage mea~uring means ~imultaneou~ly mea~ures a first voltage between the terminal~ of the ~econd electrode and a ~econd voltage between the terminal~ of the third electrode. 8y comparison with the fir~t embodiment, measurement by means of thic cecond embodiment i~ much faster.
Preferably, this probe with three electrodes furthcl comprises first, ~econd and third thin board~ in dielectric material which re~pectively ~upport the fir~t, ~econd and third electrodes, a firct wedge in dielectric material again~t two parallel ~ide~ of which the fir~t and ~econd board~ are respectively fixed, and a ~econd wedge in dielectric material again~t two parallel sidec of which the firAt and third board~ are fixed.
Preferably, the measurement device also comprises means for calculating the variation of the ratio between said first and ~econd voltage~ ac a function of the frequency when said probe ic moved away from ~aid material to be te~ted and ~ub~equently when ~aid probe i~ placed on a came ~ide of ~aid material to be tested, thereby deducting from it the ~urface impedance of ~aid material to be te~ted by ~ecking out, among ~tored low-pa~-type reference curvec, a reference curve neare~t the variation of ~aid ratio between ~aid voltages.
~RIEF DESCRIPTION OF THE DRAWINGS
Further features and advantage~ of the invention will be apparent from the following particular de~cription of ~everal preferred embodiment~ of this invention a~ illu~trated in the corre~ponding accompanying drawing~ in which :
- Fig.l i~ an exploded view in per~pective of a fir~t probe with two _5_ ~1 2018689 electrodes embodying the inventLon;
- Flg.2 is a block diagram of an i --~n~ Ler embodyLng the invention, operating with the probe in Fig.l;
- Figs.3 and 4 are charts showing electrode voltage ratio variations as a function of frequency, particularly for probes with different interelectrode gaps, when these probes are applied to a same material to be tested;
- Figs.5 and 6 are charts showing variations of said electrode voltage ratio as a function of frequency, obtained with a same probe and two dlfferent materials respectively, and theoretical curves close to the -low-pass filter"
type;
- Fig.7 shows theoretical curves of the "low-pass filter" type for various surface impedance values;
- Fig.8 i8 a chart showing variations of said voltage ratio as a function of frequency, obtained with a same probe and for a same material, but for different distances between the probe and the material;
- Fig.9 is an exploded view in perspective of a second probe comprising three circular electrodes;
- Fig.10 is a block diagram of an i --~n- -ter operating with the probe in Fig.9;
- Fig.ll i8 a schematic vLew in perspective of the impedancemeter in Fig.10, in the form of a portable device; and - Fig.12 is an exploded view in perspective of a probe comprising three rectangular electrodes.
DESCRTPTION OF TH~ Ph~ k~l~ EM~nIM~NTS
A measurement probe SM for imp~nr -ter embodying the invention and shown in Fig.1 comprises an in~ection electrode 1 playing the role of a transmitting antenna, and a measuring electrode 2 playing the role of receiving antenna and placed parallel to the first electrode 1. Each of the electrodes 1, 2 is typically comprised of a circular spire printed on one of the sides of an insulating board 10, 20, or by two concentric circular spires printed on the sides of a two-sided printed board. However, the second electrode may be rectangular or of any other form. The n~iqhhoring t~rmin~l~ ll and 12, 21 and 22 of the electrode 1, 2 are comprised of metallized holes in the board 10, 20, from which printed connection conductors can be extended.
Il 2018689 According to the embodiment illustrated ln Fig.l, the elements 20, 3, lO
and 4 have a same square-shaped sectLon and are maintained in a stack by means of four through screws and nuts 50 and 5l. The elements thus stacked 20, 3, lO
and 4 are then coated wLth a thLn protectLve dLelectrLc layer 52 such as epoxy resln. TypLcally, Lf ~ deslgnates the radlus of the transmLttLng electrode l, the radLus of the receLvLng electrode 2 Ls equal to r/2, and the thLckness z of the LnsulatLng wedge 3 Ls equal to r.
As shown Ln FLg.2, to take surface Lmpedance measurements relatLng to a good electrLcal conductLve materLal to be tested, a thLn plate PM of saLd materLal lO 18 used as a sample. The probe SM 18 placed above the plate Ln such a way that the receLvlng electrode 2 Ls facLng the plate and that the axLs of the electrodes l and 2 is substantially perpendicular to the plate PM. In this way, the receiving electrode 2 and its holder board 20 are at a dLstance h from the plate of material PM which can practically vary from 0, plus the small thickness of the coatLng 52, to several m~ Lers. On the other hand, the Lnterelectrode gap, substantlally equal to the thlckness z of the wedge 3, 1a constant.
Furthermore, the electrodes l and 2 are connected to electronLc cLrcuits of the lmpedancemeter IM proper. The latter Ls organLzed around a central process1ng unlt ~CPU) 6 compr1sLng a p.ep1-o4L- -i mLcroprocessor. The unLt 6 20 is connected by an Lnternal bidLrectLonal bus BU to a frequency generator 7 and -~ a vectorLal analyzer 8, and by a standard Lnterface bus BIS to a measurement result dLsplay devLce 9 comprLsLng e.g. a prLnter.
Outputs of the generator 7 are connected to the t~rm;nAls 11 and 12 of the transmitting electrode l via an ~ -lAn~e matching load 13 which Ls also prLnted on the holder board 10. Generator 7 produces a wobbled alternatLng voltage sLgnal of whLch the frequency f perLodLcally sweQps a frequency range fm1n to fmax selected by the processLng unLt 6. The voltage sLgnal produced by the generator 7 Ls translated by an electrLc current Ln the transmLttLng electrode l whLch then creates an Lnduced current Ln the second electrode 2 and consequently an 30 Lnduced voltage at the t~rm~nAl~ 21 and 22. ThLs Lnduced voltage is equal to the time derLvative of the flux of the magnetic field crossing the receiving electrode 2.
The analyzer 8 has lnputs 81 and 82 respectlvely connected to the output term~ ns~ 12 and 22 of the electrodes 1 and 2 co as to measure the voltage produced by the generator 7 on the electrode 1 and the voltage induced in the electrode 2, at each frequency. These voltages are read in digital form by the unit 6.
The measurement of the surface ~peri~nce Z5 of the materLal to be tested is carried out in two stages.
During a first stage, the probe SM is placed at a distance from all materials ln such a way that the magnetic field directly crossing the receiving electrode 2 is not perturbed by any material. The analyzer 8 measures a voltage Vd(f) directly induced in the electrode 2. The unit 6 memorizes each of the values of Vd(f) as a function of the frequency f.
During a second stage, the probe SM i8 placed above the material PM, as previously described in reference to Fig.2, and is maintained at a constant distance h that does not exceed a few millimeters. The distance h is preferably as small as possible, i.e. almost equal to zero, as will be seen hereinafter.
The analyzer 8 measures a voltage Vm (f) induced in the electrode 2 by the electrode 1. The voltage Vm results from the magnetic flux produced by the electrode 1 and perturbed by the plate of the material PM near the electrode 2.
The unit 6 memorizes each of the values of Vm (f) as a function of the frequency f, and calculates the ratio Stf)=Vm/Vd lower than one, expressed in the form 20 log S(f) with a vLew to representlng its variation8 as a function of the frequency f in the display device 9.
The voltage measured on the electrode 1 enables checking the stability of the signal derived by the generator 7 during first and second stages.
Theoretically, it i9 shown that the voltage ratio S is equal to the sum of two coefficients S(f) = Sl + S2(f), one of which-Sl is independent of the frequency. The coefficients Sl and S2 are expressed as a function of the ratios z/r and h/r:
(~.(Z~ ""~2 S2-(l+(z/r)2) 1(U~/l J (U)~-u(z/r.2~/r) whereby Jl(u) is the 8essel function of order l, fco i8 a characteristic frequency such that fco=ZS / (n4Or)l and ~o=4~.10-7 H/m i8 the permeabillty of vacuum. It appears that the coefficient Sl depends only on the geometric parameters z/r and h/r and is independent of the measurement frequency f, and that the coefficient S2 depends on z/r, h/r, fco and on the frequency f.
The function S(f) is now mainly analyzed as a function of the distance h ll 2~18689 between the probe SM and the material PM ln order to preaent the advantages of the choice h~20 when the impedancemeter is used.
In the case of h=0. the probe SM ig applied directly to the material PM.
The coefficient Sl is then equal to 0 and the voltage ratio S iB reduced to:
S-52[1 +(z/r)2]3'21~ u Jl(u)e~~Z/r)du The ratio S only depends on the geometric parameter z/r, on the characterlstic frequency fco which 1B proportional to the surface impedance Zs of the material, and on the measurement frequency f. The function S can then be calculated digitally.
Flgs.3 and 4 show the varlation of s exprensed in ds as a function of log(f/fcO) for several values of z/r equal to 0.0; 0.1; 0.25; 0.5; 1.0; 10.0;
100.0 and 1000Ø
The ratlo S varies like a low-pass type function of which the form and particularly the slope at high frequencies depend on z/r. In this way, for a geometry of the probe SM determined by constant parameters, the position of the variation curve of S compared with the frer~uency axis is directly related to ZS- During measurement, as will be seen hereinafter, the processing unit 6 ~eeks out one of the low-pass type theoretical curves as close as possible to the variation curve of S co..~onding to the material to be tested and obtained by the measurement proper, and identifies the position of the variation curve of S ao a function of the frequency f in the range [fmin/ fmax] by one or 8everal characteristic points, e.g. at 3 dB, 10, 20 and 30 dB, in order to determine Zs -For z/r varying from 0 to 1 as per Fig.3, the form of the varlation curve of S gradually changeg from a second-order curve at approximately 40 dB/decade to a firgt-order curve at approximately 20 dB/decade, the cut-off frequency which ig ugually chosen at 3 dB varying little. For z/r varying above 1 ag per Fig.4, the function S ig little different to a firgt-order low-pasg function, the cut-off frequency at 3 dB moving towards the lower frequencies.
In this way, though any value of z/r will do to meaoure the gurface ~mrr~r~r~nrt~
ZSt the invention r~ ~~rl~ that the geometric parameter z/r be chosen and consequently the geometry of the probe according to the following two criteria.
The voltage induced between the terminals 21 and 22 of the second electrode 2 ig proportional to the time derivative of the magnetic flux~crossing it and ll 2018689 consequently to the frequency f. To extend measurement to the lower frequencies, there must be sufficient flux and consequently sufficient induced voltage, which implies good coupling between the electrodes 1 and 2 and quite a small interelectrode gap z.
In order to determine the imre~Ance Z5~ the unit 6 locates the position of the variation curve of S measured in relation to at least one given straight line parallel to the frequency axis since, for a given value of S, there is only one value of Z5 and crnAe~l~ntly f fco co-.espollding to a ratLo f/fco. It 18 therefore preferable that the unit 6 identify a "simple" curve. According to the invention, the unit 6 seeks out by successive approximations a low-pass function of the first order nearest the function S(f) obtained on measuring.
For instance, the particular case z/r=1 is suitable; Figs.S and 6 show that the function S(f) obtained after measurement is little different to the function FBP=(l+(f/fC)2)-1/2 whereby fc~1.4 fco = 1-4 ZS t (n4Or) Fig.7 shows different curves FBP(f) for various values of ZS and shows that the smaller the imre~Anre Z5~ the more the measurements are taken in the low frequencies.
In this way, to determine the imre~Anre ZS of a material with a probe SM
such that z=r, the unit 6 identifies the curve S(f) for several predetermined values of S or of f, calculates the standard deviation between the curve S(f) and the various functions FBP(f) corresponding to values f fc, and only retains the function FsP with the smallest standard deviation. The impedance of the material ZS=n~Or fC tl.4 is then deduced from the retained function FBP and consequently from the coL.e~onding characteristic frequency fc.
In the rA~ of h ~ _, i.e. when the probe SM is not applied-to the plate of material PM, the coefficient S1 is no longer zero but is equal to a constant for a given geometry and a predet~rmin~d position of the probe SM, i.e. for constant ztr and htr. The second coefficient S2 is always a low-pass type function tending towards 0 when f tends towards infinity. The ratio S = S1 + S2 ls therefore approximately ec,ual to S2 at low frequency and to S1 at high frequency.
Fig.8 shows the filtering function S as a function of the frequency f for several values of the distance h between the probe SM and the material PM, when the interelectrode gap is constant and equal to z=r. The beginning in low frequency of the curves in Fig.6 is little different to the case h=0 as per Figs.4 and 5. At high frequency, the curves in Fig.8 tend towards respective planes equal to Sl(h/r).
When seeking to determine the surface i ~ nce Zs, h should be as small as possible since major dynamics are sought to identify the form of the curve S(f) obtained durlng measurement.
On the other hand, the value of maximum attenuatLon sl only depends on h for constant z/r and Ls in~lt~p~ nt of the i _ --Ant~e of the material. The curve S1(h) can be used to experimentally determine the distance between the probe and a conductive material, this distance being e.g. equal to the thickness of an insulation coating covering the material.
The measurement method implemented by the impedancemeter enables the surface ',mpe~lAnt~e ZS of a conductive material to be evaluated. However, for a homogeneous material, from the formula ZS = 1/(~Jt d), - if a is known, the thickness of the material d = 1/~Z5a) is deduced, and - if d is known, the conductivity of the material (~= l/(ZS d) is deduced.
In order to set ideas down, e.g. a probe SM for which z2r=5 cm, imposes a characteristic frequency fC ~ 1.4 ZS / (l~lor) ~ 2-107 ZS
Supposing a f.e~ue~ y sweep from O up to a predetermined maximum frequency fmaxt the surface i 3-Ance mea9ured will be le99 than ZS max = fmax /(2-107)~
i.e. for in9tance ZS max = 0-5 n for fmax = 10 MHz, and ZS max = 5 n for fmax 100 MHz.
In this way, the i _ ~-An~ emeter w$th the probe embodying this example measures surface impedances of less than a few ohms.
It should be noted that the higher the impedance measured, the lower the frequency at which the measurement must be carried out, as per Fig.7. As previously stated, the voltage measured via the receiving electrode 2 is proportlonal to the product of the flux crossing it and the frequency. The flux emitted by the first electrode 1 must therefore be increased in order to carry out measurementsat low frequency. The transmitting antenna is no longer comprised of a single spire but of a winding of n spires, and the sensitlvity of the probe is then increased by the integer factor n. In this instance, the board 10 has several concentric spires on one or both sides, or is replaced by a multilayer circuit, eaoh layer comprising at lest one spire. According to another ~ - L which can be implemented alone or combined with the previous solution, an amplifier is provided at output of the frec,uency generator 7 for amplify,ing the current ll 2018689 ln the transmitting electrode 1 and the magnetic flux and subsecuently the sensitivity of the probe SM.
The measurement method described above and Lmplemented by the impedancemeter IM equLpped wLth the probe as per FLg.2 Ls comprised of two successLve stages:
a calLbratLon of the probe at a dLstance from all materLals durLng the fLrst stage, then measurement of the surface i ~ n~e by applyLng the probe SM to the materLal PM durLng the second stage.
To Lncrease measurement speed, the calLbratLon stage is carrLed out sLmultaneously wLth the mea~ur~ - t stage, at each frequency value, by usLng a IC probe SMa wLth a transmLttLng antenna 1 and two receLving antennae 2a and 2R, and a corresponding Lmpedancemeter IMa.
In reference to FLg.9, the probe SMa lLke the probe SM comprises the followLng stacked members, namely a printed board 20a supporting a preferably circular receLvLng electrode 2a, an LnsulatLng wedge 3a, a printed board lOa supporting a circular transmitting electrode la, and an insulating cover 4a.
Fur~h~ -.e, between the board lOa and the cover 4a, the probe SMa comprises a second insulating wedge 3R, and a thLrd prLnted board 20R supporting a reference electrode 2R that L6 preferably cLrcular. The board 20R Ls lnterposed between the cover 4a and the wedge 3R. The wedge 3R Ls applLed agaLnst the board lOa.
All the members of the probe SMa have the same sectLon, e.g. square or dLsk-shaped Ln thLs Lnstance, and are fastened to one another by means of four paLrs of screws 50a and nuts 51a. Then the assembly of stacked members of the probe SMa Ls coated Ln resLn 52a. For Lnstance, the wedge 3R and the board 20R are respectLvely LdentLcal to the wedge 3a and the board 20a, and the radius of the reference electrode Ls equal to r/2.
The thLrd electrode 2R Ln the probe SMa plays the same role as the reception antenna 2 Ln the probe SM durLng the fLrst stage of the measurement method. In fact, when the sLde of the probe SMa comprLsLng the electrode 2a Ls applLed to a plate of materLal PM, the reference electrode 2R Ls sLtuated opposLte the plate PM by comparLson wLth the center board lOa supportLng the transmLttLng electrode la. The dLstance between the electrode 2R and the plate PM Ls then sufficiently large for the plate PM not to perturb the magnetic flux emLtted by the electrode la and generatLng the voltage Lnduced at t~rminAls 21R and 22R of the electrode 2R.
As shown Ln FLgs.10 and 11, the i ~--n~ -ter IMa al80 COmprLB-B a central processLng unLt 6a wLth an Lntegrated microp ocessor, a sLne, wave frequency 20~8689 generator 7a, and controlllng and displaying means 9a.
The vectorial analyzer 8 i8 replaced in the impedancemeter IMa by three parallel assemblies each comprisLng ln serles an lnput ampllfier 711, 712, 71R, an voltage limiting circuit 721, 722, 72R, and a analog-to-digital converter 731, 732 and 73R. The outputs of the generator 7a are connected to the terminals lla and 12a of the transmitting electrode la and to the input t~rm~nAl~ of the ampllfler 711. The t~rmlnAlR 21a and 22a of the recelvlng and measuring antenna 2a and the t~rm{ nAl R 21R and 22R of the recelvlng and reference antenna 2R are respectlvely connected to the lnput t~rmin~l~ of the ampllflers 712 and 71R. In these condltlons, when the unlt 6a orders a freguency sweep ln a glven range fmln to fmax, the converters 731, 732 and 73R slmultaneously transmit Ln digital form a voltage V1~f) applied to the ~rmlnAl~ of the transmitting electrode la by the generator 7a, a voltage V2(f) induced in the electrode 2a depending on the ~mpe~Ance ZS Of the material PM, and a reference voltage VR(f) induced in the electrode 73R. For each frequency f, the unit 6a directly calculates the ratio S(f) = V2/VR which makes it possible to rapidly visualize the curve S(f) with the approaching curve FBFC on the screen 9Sf, and to diAplay the digital value ZS of the material PM in a readout 9Z. Prlor to measurlng, the operator selects a frequency band [fmln/ fmax] and consequently the impedance range ZS
by means of a selector 9SE, and the type of probe by means of a button 9S0.
The i --~nf Ler IMa also comprises a standard Lnterface IN with an RS 232C-type bidirectlonal ~le~ icatlons bus for exchanglng digLtal signals wlth a remote calculator storlng the curves and results obtalned.
The ~r~e~An~ Lers descrlbed above apply to materlals that can be Lep~-esenLed by an lsotroplc surface lmpedance. Measurements are then lnterpreted quantltatlvely.
In the case of a hlghly anlsotroplc materlal, such as a carbon compound wlth parallel flbers, the surface i -~nre l~ comprl-ed of an lmpedance Z8x ln a dlrectlon x and of an ~n,--~n~e 28y ln a dlrectlon y perpendlcular to the dlrectlon x. According to the lnventlon, the clrcular electrodes 1 and 2 Ln the probe SM, or the electrodes la, 2a and 2R ln the probe SMa are replaced by rectangularly looped electrodes l'a, 2'a and 2'R as shown ln Flg.12 for a probe SM'a. In this Flg.12, members slmilar to those shown ln Fig.9 are designated by - the same numeral references but accentuated. Each rectangular electrode is thus comprlsed of at least two longltudlnal conductlve wLres connected at one of theLr ends by a conductive bend and havlng a length egual to at lea~t 10 tlme-ll 2018~89 the very smA11 dLstance between these wires, Ln the region of a few mlllLmeters.
The two or three internal electrodes in the probe are laid out in the same longitudlnal direction x, as shown in Pig.12.
If this rectangular-electrode probe is applied to the anisotropic material in the two perpendicular directions x and y, the ~ nc~ ~Ler displays two different curves Sx(f) and Sy(f) which enable the values of Z8x and Z5y to be deduced for the material under conslderation. One of these two curves with the greatest attenuation corresponds to the case of the common longitudinal direction x of the probe electrodes being parallel to the direction of the fibers, i.e.
1O to the direction in which the conductivity of the material is best.
A probe embodying the invention is also capable of detecting any discontinuity in a material which, by nature, modifies the distribution of the magnetlc fleld that generates the current lnduced ln the recelvlng electrode.
For instance, if the probe is applied to a slot existing between two plates comprised of the same material, the measurement supplied by the impedancemeter varies conslderably and slgnals the presence of the slot. This phenomenon 18 accentuated when one chooses to use a probe wlth a rectangularly-looped receiving electrode placed on the slot.
Other material discontinuities may be detected, such as junctions between 20 two plates of a same material. If the contact between these two plates is electrically perfect, the ~mp~ nre measurement carrled out by placing a probe on a plate ~unction should be identical to that of a uniform plate of this material; if the electrical contact at the ~unction is defective, the measurement is slmilar to that of a slot. A calibration enables the state of the electrical contact to be evaluated.
Claims (11)
1. A device for measuring the surface impedance of a material to be tested, said device comprising:
a first electrode and a second electrode, said first and second electrodes being superposed by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second electrode;
voltage measuring means for measuring second and third wobbled voltages supplied successively by said second electrode, said second wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is moved away from said material to be tested so that said material does not perturb said magnetic field, and said third wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is substantially placed on said material to be tested so that said material perturbs said magnetic field;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
a first electrode and a second electrode, said first and second electrodes being superposed by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second electrode;
voltage measuring means for measuring second and third wobbled voltages supplied successively by said second electrode, said second wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is moved away from said material to be tested so that said material does not perturb said magnetic field, and said third wobbled voltage being induced in said second electrode responsive to said magnetic field when said probe is substantially placed on said material to be tested so that said material perturbs said magnetic field;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
2. The device as claimed in claim 1, wherein said probe comprises a spacing member of dielectric material with two parallel sides supporting said first and second electrodes, respectively.
3. The device as claimed in claim 2, wherein said probe comprises an exterior dielectric coating on said second electrode at least so as to apply said probe against said material.
4. The device as claimed in claim 1, wherein said probe comprises a spacing member of dielectric material, and two thin support members of dielectric material that are respectively fixed on two parallel sides of said spacing member and on which are printed said first and second electrodes, respectively.
5. The device as claimed in claim 4, wherein said probe comprises an exterior dielectric coating on said second electrode at least so as to apply said probe against said material.
6. The device as claimed in claim 1, wherein at least one of said first and second electrodes is a circular loop.
7. The device as claimed in claim 1, wherein at least one of said first and second electrodes is a rectangular loop.
8. The device as claimed in claim 1, wherein at least one of said first and second electrodes is comprised of several concentric or superposed spires.
9. A device for measuring the surface impedance of a material to be tested, said device comprising:
first, second and third electrodes, said first electrode being interposed between said second and third electrodes by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second and third electrodes;
voltage measuring means for measuring second and third wobbled voltages respectively supplied by said second and third electrodes, said second and third wobbled voltages being respectively induced in said second and third electrodes responsive to said magnetic field when a side of said probe including said second electrode is substantially placed on said material to be tested so that said material to be tested perturbs said magnetic field in the vicinity of second electrode and does not perturb said magnetic field in the vicinity of said third electrode;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
first, second and third electrodes, said first electrode being interposed between said second and third electrodes by an intermediary of a dielectric material to constitute a compact probe;
means for deriving a first electrical voltage wobbled in frequency to be supplied to said first electrode whereby said first wobbled voltage produces a magnetic field emitted by said first electrode and crossing said second and third electrodes;
voltage measuring means for measuring second and third wobbled voltages respectively supplied by said second and third electrodes, said second and third wobbled voltages being respectively induced in said second and third electrodes responsive to said magnetic field when a side of said probe including said second electrode is substantially placed on said material to be tested so that said material to be tested perturbs said magnetic field in the vicinity of second electrode and does not perturb said magnetic field in the vicinity of said third electrode;
means for calculating the variation of a ratio between said second and third wobbled voltages as a function of the frequency of said first wobbled voltage; and means for comparing said ratio variation with low-pass type functions respectively depending on predetermined surface impedances thereby selecting one of said low-pass type functions nearest said ratio variation to determine the surface impedance of said material to be tested.
10. The device as claimed in claim 9, wherein said second and third electrodes are symmetrical with regard to said first electrode in said probe.
11. The device as claimed in claim 9, wherein said probe comprises:
first, second and third thin support members of dielectric material which respectively support said first, second and third electrodes, a first spacing member of dielectric material against two parallel sides of which said first and second support members are respectively fixed, and a second spacing member of dielectric material against two parallel sides of which said first and third support members are fixed.
first, second and third thin support members of dielectric material which respectively support said first, second and third electrodes, a first spacing member of dielectric material against two parallel sides of which said first and second support members are respectively fixed, and a second spacing member of dielectric material against two parallel sides of which said first and third support members are fixed.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8907726 | 1989-06-12 | ||
| FR8907726A FR2648236B1 (en) | 1989-06-12 | 1989-06-12 | SURFACE IMPEDANCE MEASURING APPARATUS |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2018689A1 CA2018689A1 (en) | 1990-12-12 |
| CA2018689C true CA2018689C (en) | 1994-09-20 |
Family
ID=9382606
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002018689A Expired - Fee Related CA2018689C (en) | 1989-06-12 | 1990-06-11 | Surface impedance measurement device |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US5086274A (en) |
| EP (1) | EP0403344B1 (en) |
| BR (1) | BR9002779A (en) |
| CA (1) | CA2018689C (en) |
| DE (1) | DE69013236T2 (en) |
| FR (1) | FR2648236B1 (en) |
| IL (1) | IL94707A0 (en) |
Families Citing this family (28)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA2043347A1 (en) * | 1990-05-30 | 1991-12-01 | Yukio Kohmura | Method and system for inspection of electroconductive film using eddy current and process and system for production of optical fibres using method and system |
| US5453689A (en) * | 1991-12-06 | 1995-09-26 | Massachusetts Institute Of Technology | Magnetometer having periodic winding structure and material property estimator |
| ATE201787T1 (en) * | 1992-11-25 | 2001-06-15 | Simmonds Precision Products | DATA PROCESSING STRUCTURES AND METHODS |
| US5515041A (en) * | 1993-06-14 | 1996-05-07 | Simmonds Precision Products Inc. | Composite shaft monitoring system |
| US5581248A (en) * | 1993-06-14 | 1996-12-03 | Simmonds Precision Products, Inc. | Embeddable device for contactless interrogation of sensors for smart structures |
| US5602540A (en) * | 1993-06-14 | 1997-02-11 | Simmonds Precision Products Inc. | Fluid gauging apparatus with inductive interrogation |
| US5433115A (en) * | 1993-06-14 | 1995-07-18 | Simmonds Precision Products, Inc. | Contactless interrogation of sensors for smart structures |
| JP3011090U (en) * | 1994-11-11 | 1995-05-16 | 株式会社キョクトー | Spot welding nugget inspection equipment |
| US5698978A (en) * | 1995-05-15 | 1997-12-16 | Northrop Grumman Corporation | System and method for measuring electromagnetic radiation absorption |
| US5793206A (en) * | 1995-08-25 | 1998-08-11 | Jentek Sensors, Inc. | Meandering winding test circuit |
| US5936401A (en) * | 1996-09-19 | 1999-08-10 | The United States Of America As Represented By The Secretary Of The Air Force | Device and process for measuring electrical properties at a plurality of locations on thin film superconductors |
| US6486673B1 (en) | 1997-01-06 | 2002-11-26 | Jentek Sensors, Inc. | Segmented field dielectrometer |
| JP2001508178A (en) * | 1997-01-06 | 2001-06-19 | ジェンテク・センサーズ・インコーポレーテッド | Detecting subsurface objects with magnetometers and deelectrometers |
| US6781387B2 (en) | 1997-01-06 | 2004-08-24 | Jentek Sensors, Inc. | Inspection method using penetrant and dielectrometer |
| US6534974B1 (en) | 1997-02-21 | 2003-03-18 | Pemstar, Inc, | Magnetic head tester with write coil and read coil |
| AU1965297A (en) * | 1997-02-21 | 1998-09-09 | Pemstar, Inc. | A magnetic recording head tester |
| DE69814601T2 (en) | 1997-03-13 | 2004-04-01 | Jentek Sensors, Inc., Watertown | MAGNETOMETRIC DETECTION OF FATIGUE DAMAGE IN AIRCRAFT |
| ATE256865T1 (en) * | 1997-10-29 | 2004-01-15 | Jentek Sensors Inc | ABSOLUTE MEASUREMENT OF PROPERTIES WITH AIR CALIBRATION |
| US6377039B1 (en) | 1997-11-14 | 2002-04-23 | Jentek Sensors, Incorporated | Method for characterizing coating and substrates |
| US6380747B1 (en) | 1998-05-12 | 2002-04-30 | Jentek Sensors, Inc. | Methods for processing, optimization, calibration and display of measured dielectrometry signals using property estimation grids |
| US6346807B1 (en) * | 1999-10-22 | 2002-02-12 | Bently Nevada Corporation | Digital eddy current proximity system: apparatus and method |
| KR100505929B1 (en) * | 2003-03-31 | 2005-08-04 | 삼성광주전자 주식회사 | A compressor and A method for connecting pipeline of compressor |
| WO2007088395A1 (en) | 2006-02-03 | 2007-08-09 | Bae Systems Plc | Improvements relating to damage sensors |
| US8550609B2 (en) * | 2007-11-21 | 2013-10-08 | Xerox Corporation | System and method for measuring a supply of solid ink in a solid ink printer |
| WO2017089591A1 (en) | 2015-11-27 | 2017-06-01 | Airbus Safran Launchers Sas | Wind turbine blade including a plating layer having optimized basis weight |
| US10605777B2 (en) * | 2016-03-16 | 2020-03-31 | Ihi Corporation | Method for inspecting electroconductive composite material and device for inspecting electroconductive composite material |
| JP6494551B2 (en) * | 2016-03-28 | 2019-04-03 | アンリツ株式会社 | Field strength distribution measuring apparatus and field strength distribution measuring method |
| CN109188083A (en) * | 2018-08-28 | 2019-01-11 | 深圳市信维通信股份有限公司 | A kind of RF consistency of LDS antenna and line impedance detection system and method |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2629004A (en) * | 1947-06-10 | 1953-02-17 | Maurice L Greenough | Electrical micrometer |
| US3441840A (en) * | 1967-03-31 | 1969-04-29 | Martin Marietta Corp | Electronic thickness meter having direct readout of coating thickness |
| US3764897A (en) * | 1971-12-28 | 1973-10-09 | Singer Co | Electromagnetic thickness gauging using a transmitting coil shaped to provide a constant field over a range of measuring distances |
| US3848183A (en) * | 1973-04-20 | 1974-11-12 | Magnaflux Corp | Eddy current testing system having concentric coils, one being movable for balancing |
| CH568569A5 (en) * | 1974-02-06 | 1975-10-31 | Bbc Brown Boveri & Cie | |
| US4095180A (en) * | 1975-12-29 | 1978-06-13 | K. J. Law Engineers, Inc. | Method and apparatus for testing conductivity using eddy currents |
| US4042876A (en) * | 1976-04-29 | 1977-08-16 | The United States Of America As Represented By The United States Energy Research And Development Administration | Eddy current gauge for monitoring displacement using printed circuit coil |
| US4383218A (en) * | 1978-12-29 | 1983-05-10 | The Boeing Company | Eddy current flow detection including compensation for system variables such as lift-off |
| US4351031A (en) * | 1980-11-07 | 1982-09-21 | Magnaflux Corporation | Nondestructive testing system having automatic set-up means |
| US4706020A (en) * | 1983-12-12 | 1987-11-10 | General Electric Company | High frequency eddy current probe with planar, spiral-like coil on flexible substrate for detecting flaws in semi-conductive material |
-
1989
- 1989-06-12 FR FR8907726A patent/FR2648236B1/en not_active Expired - Lifetime
-
1990
- 1990-06-08 EP EP90401578A patent/EP0403344B1/en not_active Expired - Lifetime
- 1990-06-08 DE DE69013236T patent/DE69013236T2/en not_active Expired - Fee Related
- 1990-06-11 US US07/535,498 patent/US5086274A/en not_active Expired - Lifetime
- 1990-06-11 CA CA002018689A patent/CA2018689C/en not_active Expired - Fee Related
- 1990-06-12 BR BR909002779A patent/BR9002779A/en not_active Application Discontinuation
- 1990-06-12 IL IL94707A patent/IL94707A0/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| EP0403344B1 (en) | 1994-10-12 |
| CA2018689A1 (en) | 1990-12-12 |
| DE69013236T2 (en) | 1995-05-11 |
| FR2648236A1 (en) | 1990-12-14 |
| DE69013236D1 (en) | 1994-11-17 |
| BR9002779A (en) | 1991-08-20 |
| EP0403344A1 (en) | 1990-12-19 |
| FR2648236B1 (en) | 1991-12-20 |
| US5086274A (en) | 1992-02-04 |
| IL94707A0 (en) | 1991-04-15 |
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