CA2201874A1 - System for measuring ultrasonically the elastic properties of a moving paper web - Google Patents
System for measuring ultrasonically the elastic properties of a moving paper webInfo
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- CA2201874A1 CA2201874A1 CA 2201874 CA2201874A CA2201874A1 CA 2201874 A1 CA2201874 A1 CA 2201874A1 CA 2201874 CA2201874 CA 2201874 CA 2201874 A CA2201874 A CA 2201874A CA 2201874 A1 CA2201874 A1 CA 2201874A1
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- ultrasonic wave
- paper web
- pick
- receiving means
- mic2a
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- Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
Abstract
This invention relates to a system for measuring ultrasonically the elastic properties of a moving paper web. An ultrasonic wave generating means (1; 20), generating a noise type ultrasonic signal, creates the ultrasonic wave in the web at an excitation point. A reference ultrasonic receiving means (Mic1, 7;
21, 7) is directed to receive contactlessly the ultrasonic wave from the excitation point. At least two pickup ultrasonic receiving means (Mic2A, 8, Mic2B, 9) receive contactlessly the ultrasonic wave generated by the ultrasonic wave generating means and reradiated by the paper web. A processing means (8 to 10) combines the outputs from the pickup ultrasonic receiving means. A computing means (13) processes the outputs from the reference receiving means and from the processing means, and determines the delay time between these outputs.
21, 7) is directed to receive contactlessly the ultrasonic wave from the excitation point. At least two pickup ultrasonic receiving means (Mic2A, 8, Mic2B, 9) receive contactlessly the ultrasonic wave generated by the ultrasonic wave generating means and reradiated by the paper web. A processing means (8 to 10) combines the outputs from the pickup ultrasonic receiving means. A computing means (13) processes the outputs from the reference receiving means and from the processing means, and determines the delay time between these outputs.
Description
~2~ ~87~
o96/11395 l ~ S/~
~m for m~lr;n~l~lt~qnn~lly ~ e~ic ~l~Lies of a m3ving p~ web.
This invention conc~rns the measurement of the velocity of ultr~o~ln~, in-plane, for a moving paper web. The ultrasound velocity in paper is known to be related to various measures of paper ~LLenyLh and stiffness.
BACKGROUND OF THE lNv~NllON
The most important values for the papermaker to consider from ultrasound velocity measurements on paper web are:
lO TSO Tensile Stiffness orientation, i.e. the orientation of the elastic ~o~lLies in-plane of the paper sheet, TSIMD Tensile S~; ffn~c Index in the machine direction of the paper machine, 15 TSICD Tensile Stiffness Index in the cross direction of the paper machine.
It is possible to determine these quantities and also the anisoL~u~ ratio TSIMD/TSICD by performing ~he ultrasound ~elccity measurements ;~n the machine direction (MD), cross direction (CD), and directions between (MD) and (CD). The tensile stiffness and anisoL~v~y ratio characterize the paper quality.
The velocity of an ultrasonic pulse propagating in-plane of a paper sheet corresponds with the sheet's elastic properties, i.e. the TSI. TSI can be compared to Young's modulus (or "E-modulus") for other materials. The relation~h;p can be expressed by:
TSI = v2 * c where TSI is measured in kNm/g, ~ is the propagation velocity (km/sek) for the ultrasonic pulse, and c is a dimensionless constant close to l dep~;ng on Poisson's ratio for the paper. The velocity is easily determined by measuring the propagation time for an ultrasonic pulse between a ~ ~ WO96111395 ~ 2 ~ 1 8 7 4 ~ J~
transmitter and a receiver.
These quantities are often measured statically on samples taken from a paper we~. Houevel it is desirable to measure these paper quantities on-line by an on-line meter used as a sensor for the continuous co~ ol of a paper manufacturing ~ i~--S .
Most of the known on-line meter arrangements (U.S. Pat. No.
4,291,577, U.S. Pat. No. 4,688,423, U.S. Pat. No. 4,730,492) employ rotating whe~ which contain transmitters and receivers of ultrasonic waves. These wheels are rotated by a moving paper web, which requires a direct physical contact between the wheels and the web. The ultrasound velocity is usually determined from the delay tIme of an ultrasonic signal between the particular transmitter and receiver.
.
In order to obtain a reasonable mea~ ~t accuracy, the wh~ls must be synchronized which makes the system extremely complicated and unr~ hle~ An arrangement described in U.S.
Pat. No. 4,688,423 oveL~- ?~ this drawback by exploying disk type transducers which can be excited continuously and, therefore, synchronization of the wheels is not necec~ry.
However, the arrangements described in the above-mentioned patent ~r~c; fications need a direct me~h~n;cal contact between the ultrasonic trAnC~nc~rs and the web.
In a papermaking machine the fast moving web vibrates in the direction normal to the web surface, creating a randomly changing force applied to the wheels. The amplitude of excited and received ultrasonic waves depends on the pressure between particul~ ultrasonic transducer and the web. Due to the randomly changing force, the amplitudes of received 5t~n~1c fluctuate, thereby making the results of measurements less accurate.
The physical contact with the web is not ~e~ if ultr~c~n;c waves are excited and detected optically, as described in ~ F ~ 2 0 ~ 8 7 4 ~ .
U.S.Pat. No. ~, 025, 66~. Ultrasonic waves in the paper web are generated by means of a laser. This wave is detected at a determined distance from the excitation point by means of another laser beam, reflected from the web. The velocity of the ultrasonic wave is found from the measured delay time between the excitation instant and the time of the wave arrival.
The disadvantage of this optical system is that the amplitu-des of the ultrasonic waves propagating in-plane of the web are very small. A very strong acoustic noise exists in paper--making ma~h;ne~, which is ~c~m~anied by the vibrations of the moving we~. In ~act this makes the optical detection of the lowest orders symmetrical Lamb waves impossible, and only these waves are suitable for the stiffness and tensile strength measuraments of paper.
A method and device for continuously determ; ni n~ the modulus of elasticity o~ adv~n~;~ flexible material, such as paper web, in a contactless ~ashion is disclosed in WO91/17435. An ultrasonic wave train is transmitted through the air towards the web. Fig. 6 shows an embodiment in which the ultrasonic waves scattered through the air by the material are sensed both at a distance d and at a distance d at the same side of the web, no reference ultrasonic wave receiving means being provided for receiving a reference ultrasonic wave from the transmission point. The measured distance is between the two pick-up ultrasonic wave receiving means, thus not between the position where the ultrasonic wave is generated and the position of the pick-up ultrasonic wave receiving means.
Other prior on-line paper measuring ~y~lls are disclosed in the U.S.S.R. Pat. No. 489018 and U S.S.R. Pat. No. 489036, and described in the publication by Kazys (the same inventor as for the present invention), Proceedings of 20th interna-tional conference of ultrasound, Prague, 1976, p. 192-194.
The ultrasound velocity in a moving paper web was determined by exciting broad band noise-like ultrasonic wave by means of a dry friction, receiving the ultrasonic wave reradiated by A~ND~ ~H~T
. 3~ 2 2 0 1 8 7 4 the web by two non-contacting u!tr~sonic re-eivers and _al-culating cross-correlation function between these tw~ recei-ved signals. The first receiver was placed opposite to the ultrasonic transmitter and the second a determined distance from the transmitter along the web.
In order to improve signal/noise ratio, a rotating cylinder was placed underneath the we~ close to the second ultrasonic receiver. The delay time was determined from the delay of the peak value of the cross-correlation function. The advantage of this measuring system compared to the ones described above was that it had no moving or rotating parts.
The disadvantage of the system described in the above-A~A~?qr.-,,,~,L~---~ WOg6/11395 4 2 2 ~ 9 8 7 4CI/SE;95/011~14 ~ mentioned ~SSR-patents is that the signal/noise ratio is not sufficiently high enough to permit reliable continous on-line measurements in a mill environment. Another disadvantage is that excitation and reception of the ultrasonic waves are performed ~rom the opposite sides of the paper web. Another problem which is ~n~o-lntered in performing measurements in other directions than the machine direction ~MD~ is that an even worse signal/noise ratio is then obtA; n~ due to the higher losses of ultrasonic waves in an anisotropic material.
The main object of the invention is to provide i~vv~d noise robustness for the system in a paper mill environment.
Another object of the invention is on-line measuring system with single side access to the paper web, performing measurements at different directions in-plane of a moving web.
Still another object of the invention is to provide an im~oved signal processing method for reliable determination of the ultr~o~n~ velocity in the paper web.
The main object is achieved with a system having the characterizing features disclosed in the main claim. Further ~eatures and further developments of the invention are disclosed in the subclaims.
SnMMA~Y OF THE lN~ NllON
The present invention solves the problems associated with the prior art and other problems by providing a system for continuous measurements of the velocity of ultrasonic waves in a moving paper web. The foregoing is accomp~i ch~ by exciting a ultrasonic wave, such as broad-band noise type Lamb wave, in the web, receivin~ contactlessly the ultrasonic wave reradiated by the web at least at three different points, and determining the delay time between the received signal, received directly by a first reference receiver microphone placed in the vicinity of the excitation point, and the added other two signals, received by pick-up receiver
o96/11395 l ~ S/~
~m for m~lr;n~l~lt~qnn~lly ~ e~ic ~l~Lies of a m3ving p~ web.
This invention conc~rns the measurement of the velocity of ultr~o~ln~, in-plane, for a moving paper web. The ultrasound velocity in paper is known to be related to various measures of paper ~LLenyLh and stiffness.
BACKGROUND OF THE lNv~NllON
The most important values for the papermaker to consider from ultrasound velocity measurements on paper web are:
lO TSO Tensile Stiffness orientation, i.e. the orientation of the elastic ~o~lLies in-plane of the paper sheet, TSIMD Tensile S~; ffn~c Index in the machine direction of the paper machine, 15 TSICD Tensile Stiffness Index in the cross direction of the paper machine.
It is possible to determine these quantities and also the anisoL~u~ ratio TSIMD/TSICD by performing ~he ultrasound ~elccity measurements ;~n the machine direction (MD), cross direction (CD), and directions between (MD) and (CD). The tensile stiffness and anisoL~v~y ratio characterize the paper quality.
The velocity of an ultrasonic pulse propagating in-plane of a paper sheet corresponds with the sheet's elastic properties, i.e. the TSI. TSI can be compared to Young's modulus (or "E-modulus") for other materials. The relation~h;p can be expressed by:
TSI = v2 * c where TSI is measured in kNm/g, ~ is the propagation velocity (km/sek) for the ultrasonic pulse, and c is a dimensionless constant close to l dep~;ng on Poisson's ratio for the paper. The velocity is easily determined by measuring the propagation time for an ultrasonic pulse between a ~ ~ WO96111395 ~ 2 ~ 1 8 7 4 ~ J~
transmitter and a receiver.
These quantities are often measured statically on samples taken from a paper we~. Houevel it is desirable to measure these paper quantities on-line by an on-line meter used as a sensor for the continuous co~ ol of a paper manufacturing ~ i~--S .
Most of the known on-line meter arrangements (U.S. Pat. No.
4,291,577, U.S. Pat. No. 4,688,423, U.S. Pat. No. 4,730,492) employ rotating whe~ which contain transmitters and receivers of ultrasonic waves. These wheels are rotated by a moving paper web, which requires a direct physical contact between the wheels and the web. The ultrasound velocity is usually determined from the delay tIme of an ultrasonic signal between the particular transmitter and receiver.
.
In order to obtain a reasonable mea~ ~t accuracy, the wh~ls must be synchronized which makes the system extremely complicated and unr~ hle~ An arrangement described in U.S.
Pat. No. 4,688,423 oveL~- ?~ this drawback by exploying disk type transducers which can be excited continuously and, therefore, synchronization of the wheels is not necec~ry.
However, the arrangements described in the above-mentioned patent ~r~c; fications need a direct me~h~n;cal contact between the ultrasonic trAnC~nc~rs and the web.
In a papermaking machine the fast moving web vibrates in the direction normal to the web surface, creating a randomly changing force applied to the wheels. The amplitude of excited and received ultrasonic waves depends on the pressure between particul~ ultrasonic transducer and the web. Due to the randomly changing force, the amplitudes of received 5t~n~1c fluctuate, thereby making the results of measurements less accurate.
The physical contact with the web is not ~e~ if ultr~c~n;c waves are excited and detected optically, as described in ~ F ~ 2 0 ~ 8 7 4 ~ .
U.S.Pat. No. ~, 025, 66~. Ultrasonic waves in the paper web are generated by means of a laser. This wave is detected at a determined distance from the excitation point by means of another laser beam, reflected from the web. The velocity of the ultrasonic wave is found from the measured delay time between the excitation instant and the time of the wave arrival.
The disadvantage of this optical system is that the amplitu-des of the ultrasonic waves propagating in-plane of the web are very small. A very strong acoustic noise exists in paper--making ma~h;ne~, which is ~c~m~anied by the vibrations of the moving we~. In ~act this makes the optical detection of the lowest orders symmetrical Lamb waves impossible, and only these waves are suitable for the stiffness and tensile strength measuraments of paper.
A method and device for continuously determ; ni n~ the modulus of elasticity o~ adv~n~;~ flexible material, such as paper web, in a contactless ~ashion is disclosed in WO91/17435. An ultrasonic wave train is transmitted through the air towards the web. Fig. 6 shows an embodiment in which the ultrasonic waves scattered through the air by the material are sensed both at a distance d and at a distance d at the same side of the web, no reference ultrasonic wave receiving means being provided for receiving a reference ultrasonic wave from the transmission point. The measured distance is between the two pick-up ultrasonic wave receiving means, thus not between the position where the ultrasonic wave is generated and the position of the pick-up ultrasonic wave receiving means.
Other prior on-line paper measuring ~y~lls are disclosed in the U.S.S.R. Pat. No. 489018 and U S.S.R. Pat. No. 489036, and described in the publication by Kazys (the same inventor as for the present invention), Proceedings of 20th interna-tional conference of ultrasound, Prague, 1976, p. 192-194.
The ultrasound velocity in a moving paper web was determined by exciting broad band noise-like ultrasonic wave by means of a dry friction, receiving the ultrasonic wave reradiated by A~ND~ ~H~T
. 3~ 2 2 0 1 8 7 4 the web by two non-contacting u!tr~sonic re-eivers and _al-culating cross-correlation function between these tw~ recei-ved signals. The first receiver was placed opposite to the ultrasonic transmitter and the second a determined distance from the transmitter along the web.
In order to improve signal/noise ratio, a rotating cylinder was placed underneath the we~ close to the second ultrasonic receiver. The delay time was determined from the delay of the peak value of the cross-correlation function. The advantage of this measuring system compared to the ones described above was that it had no moving or rotating parts.
The disadvantage of the system described in the above-A~A~?qr.-,,,~,L~---~ WOg6/11395 4 2 2 ~ 9 8 7 4CI/SE;95/011~14 ~ mentioned ~SSR-patents is that the signal/noise ratio is not sufficiently high enough to permit reliable continous on-line measurements in a mill environment. Another disadvantage is that excitation and reception of the ultrasonic waves are performed ~rom the opposite sides of the paper web. Another problem which is ~n~o-lntered in performing measurements in other directions than the machine direction ~MD~ is that an even worse signal/noise ratio is then obtA; n~ due to the higher losses of ultrasonic waves in an anisotropic material.
The main object of the invention is to provide i~vv~d noise robustness for the system in a paper mill environment.
Another object of the invention is on-line measuring system with single side access to the paper web, performing measurements at different directions in-plane of a moving web.
Still another object of the invention is to provide an im~oved signal processing method for reliable determination of the ultr~o~n~ velocity in the paper web.
The main object is achieved with a system having the characterizing features disclosed in the main claim. Further ~eatures and further developments of the invention are disclosed in the subclaims.
SnMMA~Y OF THE lN~ NllON
The present invention solves the problems associated with the prior art and other problems by providing a system for continuous measurements of the velocity of ultrasonic waves in a moving paper web. The foregoing is accomp~i ch~ by exciting a ultrasonic wave, such as broad-band noise type Lamb wave, in the web, receivin~ contactlessly the ultrasonic wave reradiated by the web at least at three different points, and determining the delay time between the received signal, received directly by a first reference receiver microphone placed in the vicinity of the excitation point, and the added other two signals, received by pick-up receiver
- 2~ ~874 WO96/11395 ~ J~h~ 1144 microphones separated by a half-wavelength in air of the transmitted ultrasonic wave at the centre frequency of the frequency band used for measurements.
The distances between the excitation point and the two pick-up receivers is known. The delay time of the ultrasonic wave is prefera~ly determined as a zero cross of the ~;lh~rt transform of the cross-correlation function of the received signals, corresponding to the maximum value of the cross-correlation function. The source of ultrasonic waves and allthe ultrasonic receivers are preferably located on only one side of the web.
In order to ma~e the system noise ro~ust, i.e. provide a low signal/noise ratio, the receiving of the reradiated ultrasonic waves is performed above a rotating cylinder in a paper -k i ng machine at the particular position in respect to the line, where the moving web touches the cyl;n~r for the first time. ~he broad ~and noise-like Lamb wave in the web is generated by means o~ dry friction contact between the moving web and a friction head. Therefore, the system has no moving parts and all signals are received by nor. ~o~lLacting means.
BRIEF DESCRIPTION OF '1'~1~ DRAWINGS
For a more complete unders~n~;ng of the present invention and for ~urther objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:
FIG. l is a schematic side view of a measuring system according to the prior art, FIG. 2 is a schematic side view of a first emho~ nt of a ~ ~ing system according to the invention, 35 FIGs 3A to 3D are diagrams of signals provided in different operation steps in searching for the delay time of the ultrasonic wave transmitted through the paper web, FI&. 4 is a flow chart of the ~Loce sing operation for 22Q ~74 WO96/11395 - 6 P~ s5lcll44 providing the delay time of the ultrasonic wave in-plane of the paper web, FIG. 5A is a schematic side ~iew of a C~con~ embodiment of a measuring system according to the invention, FIGs 5B and 5C illustrate schematic view from above of two embodiments of the system in FI~. 5A having the possibility of measurtng the ultr~olln~ velocity in different directions, 0 FIG. SD illustrates a graph to provide an extrapolated value of the ultrasound velocity in the cross directiont and FIGs 6A, 6B are perspective views of an emho~;ment of a transmitter/microphone element.
i5 With reference to FI&. l, a prior art on-line paper measuring system disclosed in the U.S.S.R. Pat. No. 489018 includes a friction head l provided on one side of a moving papeT web 2 and generating a noise-like ultrasonic signal VW as a result of dry friction between the head l and the web 2. A random signal with a normal law of distribution up to 70 to so k~z is excited. ~he part o~ this signal VW propagating in the paper web 2 as the zero order symmetrical Lamb wave sO is the interesting one to examine. The excited wave is reradiated partially into the ~u~o~ li nq air and, is picked up by a contactless reference microp~on~ Mic l provided opposite the head l on the other side of the web 2, and by a contactless pick-up microphone Mic2 provided on the same side of the web as the reference microphone Micl but a determined distance away from, i.e. downstream from, the head l along the web in its moving direction, below called "the machine direction".
In order to have an enhanced reradiation of the propagated wave from the web to the air the web 2 is ~o~Led by a rotating cylin~T 3 opposite the pick-up microphone Mic2. The signals from the microphn~c Micl and Mic2 are fed to a processing unit 4', which correlates the two signals in order to derive the propagation time through the web, so that the velocity of the ultrasonic wave in the paper web can be computed and the result presented on a display 5'.
` 22~ ~1874 WO96/11395 ~ PCT/SE951oll~
In accordance with the invention measures are taken to -enhance the signal'noise ratio of the correlated signals, particularly in a :! . . isy en~ironment. Therefore, in accordance with a first embod_ ent of the invention, shown in FIG 2A, a double ~-h~nnel measuring receiving microphone device is provided at the vicinity of teh rotating cylinder 3 to receive the wave propagated along the web, since the lowest signal/noise ratio is obt~; n~ at the input of the microphone Mic2 in the prior art system shown in FIG 1. It is, however, to be noted that more than two pick-up microrho~s can be provided according to the invention.
In accordance with the invention at least two pick-~p ultrasonic microphones Mic2a and Mic2b, being the pick-up elements of the pick-up receivers, are placed a distance lm from each other the distance being rhOS~ to be a half-wavelength of the ultrasonic wa~e in air at the centre freguency of the bandwidth of the ultrasonic wave transmitted through the paper web. The microphone Mic2a is located opposite the contact line C1 between the rotating cyl in~ 3 and the web 2 from which the best radiation into the air of the wave propagated in the web is provided. The microphone Mic2b is located on the side of the microphone Mic2a turned away from the friction head 1. The lateral dimensions of the pick-up microphones Mic2a and Mic2b,~and also of the reference microphone Micl, are at least 10 times less than a wavelength of the ultrasonic wave in the paper we~, and all the microrhon~c are placed at a distance from the web less than a wavelength of the ultrasonic wave sO reradiated by the web into air. Noise is also radiated into the air from the contact line C2 where the web first meets the cyl ;n~r 3, This noise should preferably be suppressed as much as possible. Therefore, a noise ~u~e~sir~ shield 6, for instance made of rubber, is provided a~nd the microphones Mic2a and Mic2b shielding them from th~ noise from the contact line C2 and also from ambient noise. Thus, its outer edge nearest to the contact line C2 is located downstream this line. The microrh~n~c Mic2a and Mic2b are placed close to the internal edge of the shield 6.
.
1 8 ~ 4 WO96/11395 8 PCT/SE95~01144 The signal part of interest of the ultrasonic wave VW
transmitted through the web to be indicated is-the sO wave signal, which corresponds to the symmetric zero order Lamb waves propagated in the web 2, i.e. the fastest propagating 5 wave.
Thus the principle of the operation is based on a difference of ultr~oun~ velocities in air (va=343 m/sek~ and paper (v50-l.5 to 4 km/sek). The signals at the outputs of the microphones Mic2a and MIC2~ are given by:
U2a(t) Ys (t)+ya(t)+nmd(t)~noa(t~
2b s Ys~ ts)+ka*ya(t~~ta)~kn*nmd(t+~t )+n (t) where u2a(t) and u2b(t~ are the complete wave signals at the ~uL~uL of the microphones Mic2a and Mic2b, respectively, ys(t) is the sO wave signal at the ou~uL of the microphone ~5 Mic2a, Ya is the airborne wave generated by the friction head, nmd is the noise propagating along the -~h;n~
direction at the o~uL-of the microphone, nOa{t) and nOb(t) are electronic noise and ambient noise propagating along directions others than the.machine direction, kS, ka~ and kn are the coefficients reflecting the asymmetry of the microphones Nic2a and Mic2b for the a~r~liate waves, AtS=lm/vso is the delay time of the sO wave between the microphones Mic2a and Mic2b, and ~ta=lm/va is the delay time of airborne waves between the microphones Mic2a and Mic2b propagating along the machine direction.
Due to extensive differences in the ultrasonic velocities in the web and in air, ~t <~t . Furthermore, ~t~<t ~ to l/fo~
where fO is the centerSfre~ ency of the signal spectrum.
Therefore, the ~e~Lral components with freguencies equal or close to the freguency fO are approximately:
Ys(t)~Ys(t-~ts) Ya(t)~ Ya(t ~ta) nmd~t)~~nmd~t+~ta) Then, addition of the signals from the two microphones Mic2a and MiC2b gives the following result:
U2(t)=u2a(t)+u2b(t)=(l+k5)*Ys(t)+(l-ka)*Ya(t)+(l-kn)*nmd(t)+n0a(t)+nob(t) 2~ ~8~ -o96/11395 9 P~ 5 The coe~ficien~s k5, ka~ kn are close to l, which gives approximately:
U2(t)=2*YS(t)+~aYa(~)+~nnmd(t)+tnoa(t)+nob(t)]
where ~a and ~n are much lower than l, which indicates that the amplitude of the sO wave signal is amplified twice and the amplitude o~ the wave propagating in air along the machine direction from the friction head is substatially re~llc~, like the noise propagating in the machine direction.
The electronic noises or the noises arriving from directions different from the machine direction are not ~ essed and are added as partially correlated or uncorrelated random processes.
It is to be noted that the distance lm between the pick-up microphones could be chos~n in another way, but then the equations above and the combination o~ them will be changed.
The main feature of the choise of distance is that the term y~(t) is essentially enhanced and the term Ya (t) essentially reduced at the combination.
Referring now to an embodiment having the pick-up microphones half-wavelength of the airborne ultrasonic wave apart, in order to estimate the velocity of the sO wave a cross-correlation should be made on the signals from the reference microphone Micl and the added signals~from the two pick-up microphones Nic2a and Mic2b. The signals are first amplified in respective amplifiers 7, 8, 9. The signals from the amplifiers 8 and 9 are added in an adder lO. The signals from the amplifier 7 and the adder lO are fed to a processor 13 through h~n~rAcs filters ll and 12, respectivaly. The proc~cor 13 is provided with a ~ OYL~ ~ for performing an automatic time delay mea~u~. ~t in order to obtain the ~elocity of the wave in the actual paper web.
The delay time is determined from the cross _~ Lelation function. For this purpose two methods are combined, namely, cross-correlation function envelope pea~ detection for a coarse eYaluation and ze.o ~ossing detection of the cross-correlation function ~il h~rt transform for the accurate ~ 2 ~ ~ 8 7 ~
wo s6rll3ss :LO P~ sroll44 measurements. Time diagrams illustrating this tP~hn; que are given in FIGs 3A to 3D. This t~ch; ~ue is efficient in the case of relatively narrow-~and signals, i.e., when a cross-correlation function has an oscillating character.
Therefore, as shown in FIG 3A, a cross CVL ~elation function Rxy(r) between transmitted and received s~ wave signal at the ~L~Ls of the receivers Micl, 7, 11, and Mic2a, Mic2b, 8, 9, 10, 12 is provi~ed 1~ Rxy(~ /T)l~x(t)+nl(t)]*~2Ya~t+~)+~Ya(t+r)+nmd(t+~)]+
n2(ttr)]dt~
where T is the signal duration used for calculation, x(t) and y(t+~) are the signals from the input channel Nicl, 7, 11, and the ~u~ r~Ann~l Mic2a, Mic2b, 8, 9, 10, 12, respectively, and nl(t) is the noise received by the microphone Micl and n2(t+~) is the added noise received by the mi~.o~olles Mic2a and Mic2b.
A zero-cross of the ~;1h~t transform of the cross-correlation correspon~;ng to the maximum value of the cross-correlated function is made.
Then, the envelope, as shown in FIG 3B, of a cross-correlation function Rxy(~) is obtained by means of the ~ilh~rt transform:
AXy(~ tRxy (~) + R 2 (~)]
-(see FIG 3C), where ~
Rxy (r) =HtRxy(~)3=l RXy(t)/t~*(~~t)] dt is the ~il ~rt transform of a cross-correlation function ~ (~) and shown in FIG 3D. FIG 3C shows the detection of the envelope peak shown in FIG 3B.
In the presence of signals propagating through multiple paths, the cross-correlation function has a few peaks, 35 ~Or ~ JOr~ling to different delays. Then the envelope function can be prese~ted as Axy(~ Ai(~-rdi) where ~dl~ ~d2 - are the de3ays in the corr~pon~;ng paths. Therefore, in a general casen, not just one but a few Z ~ 8 7 4 WO 96/11395 11 1 ~1ID~SI~1144 peaks will be detected. The proper peak is found t~k; ~ into a~c~.L a prior-knowledge about an expected time of the arrival and usually is the peak closest to the zero instant.
The obt~; n~ rough estimate of the delay time ~di iS used to produce a window H(t) in a time ~t -; n the width of which is slightly less than half a period of oscillation of the band-limited cross-correlation function ~<to/2 The window is located symmetrically in respect to the determined de ay time ~d~
l~ for ~d~ /2) < t S di H(t-rdi)=
0, otherwise.
The accurate delay time estimation is ob~ine~ from the windowed ~;lh~t transform RW(t) of the initial cross-correlation function:
RW(t)=H(t-~di~ ~ (t) The peak value of the envelope function Axy(~) c~LLe~onds to the peak value of the cross-correlation function Rxy(~) only in the case of non-~i~p~sive propagation. As it was noticed above, the symmetrical s0 wave used for the measurements propagates without a noticeable ~;cr~rsion.
On the other hand, the ~l~e~Lainty in detecting the rough delay time cho~ be less than to/2. For 35 kHz center frequency, rough delay time uncertainties of as much as to/2=14 ~s can be allowed. Usually this requirement is easy fullfilled and no am~iguity O~uL~.
The peak values of the cross C~L elation function Rxy(~) correspond to the zero values of the ~;l~rt transform ~ (r). Hence, the time of signal arrival now can be found using simple zeLo _Lossing te~hn;que (FIG. 3D~:
~ (t)t ~dl=H(t ~d1)* ~ (~ dl=' It is worthwhile to remember, that by shifting the window function H(t) to the locations of other envelope peaks the accurate delay times of s;gn~le propagating tLl~uyh different paths may be automatically determined.
2~ ~87~
WO9611139S 12 ~ J~ 44 A flowchart of a p~Gyr ~m in the pro~so~ 13 for automatically deriving the time delay is shown in FIG. 4 and includes shifting of the window ~, shown in FIG 3D, in several steps in order to find the searched time delay ~d for the paper web 2.
The algorithm consists of three main stages: cross-correlation envelope function fitting by 2nd order polynomial; f; n~; ng the peaks; and f i n~; ng their A~i fication according to a sharpness.
The algorithm starts from the window generation in the time domain. The width o~ the window is given in terms of sampling points and defines the number of points used in the analysis.
The window is shifted step by step in sl~hC~quent algorithm loops. The size of this step defines the separation between two neighbouring peaks and can be chosen in such a way that minor peaks caused ~y a random noise or spurious waves would ~e ignored.
The cross eoL ~ elation envelope ~unction ~itting is needed for fi~inq the peak and is performed by the least-square method using the 2nd order polynomial. Such a polynomial can have a positive or negative -u~v~Lule ~p~n~;ng on what kind of 2~ local extremity - a peak or a minimum has been found.
Strictly ~re~k; n~, the 2nd order polynomial fitting always finds a local minimum or maximum independently of how they were created - by delayed signals or by random noise fluctuations. The influence of local fluctuations can be re~llc~ by increasing the width of the window. Then the peaks c~l~c~ by delayed waves are usually sharper than the other, spurious, peaks.
Therefore, the peak f;n~;ng proce~n~e consists of the first order derivative calculation, which enables the determination of the locations of all exL ~mities and the 2nd order derivative calculation, which allows sorting them into maximums and minimums and, ronc~quently, selection of the ~20 ~874 W096/11395 13 PCT/SE95/011~
proper pea~ (or peaks) according to its (or their) sharpness.
The sharpness ~ is given by the magnitude of the 2nd derivative of the peak.
The delay time estimate rdi obtA; n~ from this peak is used to generate the window H(t) mentioned a~ove.
The ~;lh~rt transform of the cross-correlation function RXy(t~ is multiplied by the windowing function H(t~. All these functions are discrete in the time ~ -; n . The,spacing between two ad~acent points is equal to the sampling period ~tS. In order to o~tain measur~ent errors less than the signal sampling interval ~tS, the se j -nt of the Hilbert transform is fitted using the least-square method by the 5th order polynom-ial. Then the Equation has five roots, but only the root inside the created window is selected. This root is a fine time delay tdi estimation. The wave velocity vo=lo/tdi, and the tensile stiffness TSI=cl~vo2, where cl is a ~; ^n~ionless constant close to 1 der~n~;ng on Poisson's ratio for the paper. The flowchart in ~IG 4 is believed to be self-explanatory and is therefore not described in further detail.
It is n~C~sc~y to point out that if the peak of the cross-correlation function c~nc~ by the sO-~amb wave is the biggest, then the envelope function fitting can be omitted and the rough estimate of the peak delay obt~; n~ directly from the measured cross-correlation or envelope function. The other steps in the algorithm remain the same.
O
From a commercial point of view, a ~-snring system in which all units are located at the same side of a paper web has many advantage; However, in order to implement the single side access approach it is n~c~c~ry to ove~e a 7 ot of problems.
l. According to prior art (FIG l), the reference microphone could not be put at the same distance from a signal source as the ~on~ ~h~nn~l microphone from a paper 22~ ~874 WO96/11395 14 ~ s~0ll44 web, because both the reference microphone and the signal source had to be located on the same side of the web. For the same reason the reference microphone surface could usually,not be perpendicular to a propagation direction of ~he signal in air, and that caused a significant reduction in a normalized cross-correlation (covariance) function value or a distortion of its shape.
2. The location of the signal source unit and ~oth the reference microphone and the receiving microphone (see prior art in FIG l1 for the waves propagated along the web on the same side creates a direct wave propagating in air that is much ~ ~ Gl.~er than in the case of a two-side ~rC~cs, due to no shielding of airborne waves, h~C~l-ce then the paper web is not shi~l~; ng the airborne ultrasonic waves. It r~-~c~s a degree of correlation between the transmitted and recieved signals too.
The distances between the excitation point and the two pick-up receivers is known. The delay time of the ultrasonic wave is prefera~ly determined as a zero cross of the ~;lh~rt transform of the cross-correlation function of the received signals, corresponding to the maximum value of the cross-correlation function. The source of ultrasonic waves and allthe ultrasonic receivers are preferably located on only one side of the web.
In order to ma~e the system noise ro~ust, i.e. provide a low signal/noise ratio, the receiving of the reradiated ultrasonic waves is performed above a rotating cylinder in a paper -k i ng machine at the particular position in respect to the line, where the moving web touches the cyl;n~r for the first time. ~he broad ~and noise-like Lamb wave in the web is generated by means o~ dry friction contact between the moving web and a friction head. Therefore, the system has no moving parts and all signals are received by nor. ~o~lLacting means.
BRIEF DESCRIPTION OF '1'~1~ DRAWINGS
For a more complete unders~n~;ng of the present invention and for ~urther objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:
FIG. l is a schematic side view of a measuring system according to the prior art, FIG. 2 is a schematic side view of a first emho~ nt of a ~ ~ing system according to the invention, 35 FIGs 3A to 3D are diagrams of signals provided in different operation steps in searching for the delay time of the ultrasonic wave transmitted through the paper web, FI&. 4 is a flow chart of the ~Loce sing operation for 22Q ~74 WO96/11395 - 6 P~ s5lcll44 providing the delay time of the ultrasonic wave in-plane of the paper web, FIG. 5A is a schematic side ~iew of a C~con~ embodiment of a measuring system according to the invention, FIGs 5B and 5C illustrate schematic view from above of two embodiments of the system in FI~. 5A having the possibility of measurtng the ultr~olln~ velocity in different directions, 0 FIG. SD illustrates a graph to provide an extrapolated value of the ultrasound velocity in the cross directiont and FIGs 6A, 6B are perspective views of an emho~;ment of a transmitter/microphone element.
i5 With reference to FI&. l, a prior art on-line paper measuring system disclosed in the U.S.S.R. Pat. No. 489018 includes a friction head l provided on one side of a moving papeT web 2 and generating a noise-like ultrasonic signal VW as a result of dry friction between the head l and the web 2. A random signal with a normal law of distribution up to 70 to so k~z is excited. ~he part o~ this signal VW propagating in the paper web 2 as the zero order symmetrical Lamb wave sO is the interesting one to examine. The excited wave is reradiated partially into the ~u~o~ li nq air and, is picked up by a contactless reference microp~on~ Mic l provided opposite the head l on the other side of the web 2, and by a contactless pick-up microphone Mic2 provided on the same side of the web as the reference microphone Micl but a determined distance away from, i.e. downstream from, the head l along the web in its moving direction, below called "the machine direction".
In order to have an enhanced reradiation of the propagated wave from the web to the air the web 2 is ~o~Led by a rotating cylin~T 3 opposite the pick-up microphone Mic2. The signals from the microphn~c Micl and Mic2 are fed to a processing unit 4', which correlates the two signals in order to derive the propagation time through the web, so that the velocity of the ultrasonic wave in the paper web can be computed and the result presented on a display 5'.
` 22~ ~1874 WO96/11395 ~ PCT/SE951oll~
In accordance with the invention measures are taken to -enhance the signal'noise ratio of the correlated signals, particularly in a :! . . isy en~ironment. Therefore, in accordance with a first embod_ ent of the invention, shown in FIG 2A, a double ~-h~nnel measuring receiving microphone device is provided at the vicinity of teh rotating cylinder 3 to receive the wave propagated along the web, since the lowest signal/noise ratio is obt~; n~ at the input of the microphone Mic2 in the prior art system shown in FIG 1. It is, however, to be noted that more than two pick-up microrho~s can be provided according to the invention.
In accordance with the invention at least two pick-~p ultrasonic microphones Mic2a and Mic2b, being the pick-up elements of the pick-up receivers, are placed a distance lm from each other the distance being rhOS~ to be a half-wavelength of the ultrasonic wa~e in air at the centre freguency of the bandwidth of the ultrasonic wave transmitted through the paper web. The microphone Mic2a is located opposite the contact line C1 between the rotating cyl in~ 3 and the web 2 from which the best radiation into the air of the wave propagated in the web is provided. The microphone Mic2b is located on the side of the microphone Mic2a turned away from the friction head 1. The lateral dimensions of the pick-up microphones Mic2a and Mic2b,~and also of the reference microphone Micl, are at least 10 times less than a wavelength of the ultrasonic wave in the paper we~, and all the microrhon~c are placed at a distance from the web less than a wavelength of the ultrasonic wave sO reradiated by the web into air. Noise is also radiated into the air from the contact line C2 where the web first meets the cyl ;n~r 3, This noise should preferably be suppressed as much as possible. Therefore, a noise ~u~e~sir~ shield 6, for instance made of rubber, is provided a~nd the microphones Mic2a and Mic2b shielding them from th~ noise from the contact line C2 and also from ambient noise. Thus, its outer edge nearest to the contact line C2 is located downstream this line. The microrh~n~c Mic2a and Mic2b are placed close to the internal edge of the shield 6.
.
1 8 ~ 4 WO96/11395 8 PCT/SE95~01144 The signal part of interest of the ultrasonic wave VW
transmitted through the web to be indicated is-the sO wave signal, which corresponds to the symmetric zero order Lamb waves propagated in the web 2, i.e. the fastest propagating 5 wave.
Thus the principle of the operation is based on a difference of ultr~oun~ velocities in air (va=343 m/sek~ and paper (v50-l.5 to 4 km/sek). The signals at the outputs of the microphones Mic2a and MIC2~ are given by:
U2a(t) Ys (t)+ya(t)+nmd(t)~noa(t~
2b s Ys~ ts)+ka*ya(t~~ta)~kn*nmd(t+~t )+n (t) where u2a(t) and u2b(t~ are the complete wave signals at the ~uL~uL of the microphones Mic2a and Mic2b, respectively, ys(t) is the sO wave signal at the ou~uL of the microphone ~5 Mic2a, Ya is the airborne wave generated by the friction head, nmd is the noise propagating along the -~h;n~
direction at the o~uL-of the microphone, nOa{t) and nOb(t) are electronic noise and ambient noise propagating along directions others than the.machine direction, kS, ka~ and kn are the coefficients reflecting the asymmetry of the microphones Nic2a and Mic2b for the a~r~liate waves, AtS=lm/vso is the delay time of the sO wave between the microphones Mic2a and Mic2b, and ~ta=lm/va is the delay time of airborne waves between the microphones Mic2a and Mic2b propagating along the machine direction.
Due to extensive differences in the ultrasonic velocities in the web and in air, ~t <~t . Furthermore, ~t~<t ~ to l/fo~
where fO is the centerSfre~ ency of the signal spectrum.
Therefore, the ~e~Lral components with freguencies equal or close to the freguency fO are approximately:
Ys(t)~Ys(t-~ts) Ya(t)~ Ya(t ~ta) nmd~t)~~nmd~t+~ta) Then, addition of the signals from the two microphones Mic2a and MiC2b gives the following result:
U2(t)=u2a(t)+u2b(t)=(l+k5)*Ys(t)+(l-ka)*Ya(t)+(l-kn)*nmd(t)+n0a(t)+nob(t) 2~ ~8~ -o96/11395 9 P~ 5 The coe~ficien~s k5, ka~ kn are close to l, which gives approximately:
U2(t)=2*YS(t)+~aYa(~)+~nnmd(t)+tnoa(t)+nob(t)]
where ~a and ~n are much lower than l, which indicates that the amplitude of the sO wave signal is amplified twice and the amplitude o~ the wave propagating in air along the machine direction from the friction head is substatially re~llc~, like the noise propagating in the machine direction.
The electronic noises or the noises arriving from directions different from the machine direction are not ~ essed and are added as partially correlated or uncorrelated random processes.
It is to be noted that the distance lm between the pick-up microphones could be chos~n in another way, but then the equations above and the combination o~ them will be changed.
The main feature of the choise of distance is that the term y~(t) is essentially enhanced and the term Ya (t) essentially reduced at the combination.
Referring now to an embodiment having the pick-up microphones half-wavelength of the airborne ultrasonic wave apart, in order to estimate the velocity of the sO wave a cross-correlation should be made on the signals from the reference microphone Micl and the added signals~from the two pick-up microphones Nic2a and Mic2b. The signals are first amplified in respective amplifiers 7, 8, 9. The signals from the amplifiers 8 and 9 are added in an adder lO. The signals from the amplifier 7 and the adder lO are fed to a processor 13 through h~n~rAcs filters ll and 12, respectivaly. The proc~cor 13 is provided with a ~ OYL~ ~ for performing an automatic time delay mea~u~. ~t in order to obtain the ~elocity of the wave in the actual paper web.
The delay time is determined from the cross _~ Lelation function. For this purpose two methods are combined, namely, cross-correlation function envelope pea~ detection for a coarse eYaluation and ze.o ~ossing detection of the cross-correlation function ~il h~rt transform for the accurate ~ 2 ~ ~ 8 7 ~
wo s6rll3ss :LO P~ sroll44 measurements. Time diagrams illustrating this tP~hn; que are given in FIGs 3A to 3D. This t~ch; ~ue is efficient in the case of relatively narrow-~and signals, i.e., when a cross-correlation function has an oscillating character.
Therefore, as shown in FIG 3A, a cross CVL ~elation function Rxy(r) between transmitted and received s~ wave signal at the ~L~Ls of the receivers Micl, 7, 11, and Mic2a, Mic2b, 8, 9, 10, 12 is provi~ed 1~ Rxy(~ /T)l~x(t)+nl(t)]*~2Ya~t+~)+~Ya(t+r)+nmd(t+~)]+
n2(ttr)]dt~
where T is the signal duration used for calculation, x(t) and y(t+~) are the signals from the input channel Nicl, 7, 11, and the ~u~ r~Ann~l Mic2a, Mic2b, 8, 9, 10, 12, respectively, and nl(t) is the noise received by the microphone Micl and n2(t+~) is the added noise received by the mi~.o~olles Mic2a and Mic2b.
A zero-cross of the ~;1h~t transform of the cross-correlation correspon~;ng to the maximum value of the cross-correlated function is made.
Then, the envelope, as shown in FIG 3B, of a cross-correlation function Rxy(~) is obtained by means of the ~ilh~rt transform:
AXy(~ tRxy (~) + R 2 (~)]
-(see FIG 3C), where ~
Rxy (r) =HtRxy(~)3=l RXy(t)/t~*(~~t)] dt is the ~il ~rt transform of a cross-correlation function ~ (~) and shown in FIG 3D. FIG 3C shows the detection of the envelope peak shown in FIG 3B.
In the presence of signals propagating through multiple paths, the cross-correlation function has a few peaks, 35 ~Or ~ JOr~ling to different delays. Then the envelope function can be prese~ted as Axy(~ Ai(~-rdi) where ~dl~ ~d2 - are the de3ays in the corr~pon~;ng paths. Therefore, in a general casen, not just one but a few Z ~ 8 7 4 WO 96/11395 11 1 ~1ID~SI~1144 peaks will be detected. The proper peak is found t~k; ~ into a~c~.L a prior-knowledge about an expected time of the arrival and usually is the peak closest to the zero instant.
The obt~; n~ rough estimate of the delay time ~di iS used to produce a window H(t) in a time ~t -; n the width of which is slightly less than half a period of oscillation of the band-limited cross-correlation function ~<to/2 The window is located symmetrically in respect to the determined de ay time ~d~
l~ for ~d~ /2) < t S di H(t-rdi)=
0, otherwise.
The accurate delay time estimation is ob~ine~ from the windowed ~;lh~t transform RW(t) of the initial cross-correlation function:
RW(t)=H(t-~di~ ~ (t) The peak value of the envelope function Axy(~) c~LLe~onds to the peak value of the cross-correlation function Rxy(~) only in the case of non-~i~p~sive propagation. As it was noticed above, the symmetrical s0 wave used for the measurements propagates without a noticeable ~;cr~rsion.
On the other hand, the ~l~e~Lainty in detecting the rough delay time cho~ be less than to/2. For 35 kHz center frequency, rough delay time uncertainties of as much as to/2=14 ~s can be allowed. Usually this requirement is easy fullfilled and no am~iguity O~uL~.
The peak values of the cross C~L elation function Rxy(~) correspond to the zero values of the ~;l~rt transform ~ (r). Hence, the time of signal arrival now can be found using simple zeLo _Lossing te~hn;que (FIG. 3D~:
~ (t)t ~dl=H(t ~d1)* ~ (~ dl=' It is worthwhile to remember, that by shifting the window function H(t) to the locations of other envelope peaks the accurate delay times of s;gn~le propagating tLl~uyh different paths may be automatically determined.
2~ ~87~
WO9611139S 12 ~ J~ 44 A flowchart of a p~Gyr ~m in the pro~so~ 13 for automatically deriving the time delay is shown in FIG. 4 and includes shifting of the window ~, shown in FIG 3D, in several steps in order to find the searched time delay ~d for the paper web 2.
The algorithm consists of three main stages: cross-correlation envelope function fitting by 2nd order polynomial; f; n~; ng the peaks; and f i n~; ng their A~i fication according to a sharpness.
The algorithm starts from the window generation in the time domain. The width o~ the window is given in terms of sampling points and defines the number of points used in the analysis.
The window is shifted step by step in sl~hC~quent algorithm loops. The size of this step defines the separation between two neighbouring peaks and can be chosen in such a way that minor peaks caused ~y a random noise or spurious waves would ~e ignored.
The cross eoL ~ elation envelope ~unction ~itting is needed for fi~inq the peak and is performed by the least-square method using the 2nd order polynomial. Such a polynomial can have a positive or negative -u~v~Lule ~p~n~;ng on what kind of 2~ local extremity - a peak or a minimum has been found.
Strictly ~re~k; n~, the 2nd order polynomial fitting always finds a local minimum or maximum independently of how they were created - by delayed signals or by random noise fluctuations. The influence of local fluctuations can be re~llc~ by increasing the width of the window. Then the peaks c~l~c~ by delayed waves are usually sharper than the other, spurious, peaks.
Therefore, the peak f;n~;ng proce~n~e consists of the first order derivative calculation, which enables the determination of the locations of all exL ~mities and the 2nd order derivative calculation, which allows sorting them into maximums and minimums and, ronc~quently, selection of the ~20 ~874 W096/11395 13 PCT/SE95/011~
proper pea~ (or peaks) according to its (or their) sharpness.
The sharpness ~ is given by the magnitude of the 2nd derivative of the peak.
The delay time estimate rdi obtA; n~ from this peak is used to generate the window H(t) mentioned a~ove.
The ~;lh~rt transform of the cross-correlation function RXy(t~ is multiplied by the windowing function H(t~. All these functions are discrete in the time ~ -; n . The,spacing between two ad~acent points is equal to the sampling period ~tS. In order to o~tain measur~ent errors less than the signal sampling interval ~tS, the se j -nt of the Hilbert transform is fitted using the least-square method by the 5th order polynom-ial. Then the Equation has five roots, but only the root inside the created window is selected. This root is a fine time delay tdi estimation. The wave velocity vo=lo/tdi, and the tensile stiffness TSI=cl~vo2, where cl is a ~; ^n~ionless constant close to 1 der~n~;ng on Poisson's ratio for the paper. The flowchart in ~IG 4 is believed to be self-explanatory and is therefore not described in further detail.
It is n~C~sc~y to point out that if the peak of the cross-correlation function c~nc~ by the sO-~amb wave is the biggest, then the envelope function fitting can be omitted and the rough estimate of the peak delay obt~; n~ directly from the measured cross-correlation or envelope function. The other steps in the algorithm remain the same.
O
From a commercial point of view, a ~-snring system in which all units are located at the same side of a paper web has many advantage; However, in order to implement the single side access approach it is n~c~c~ry to ove~e a 7 ot of problems.
l. According to prior art (FIG l), the reference microphone could not be put at the same distance from a signal source as the ~on~ ~h~nn~l microphone from a paper 22~ ~874 WO96/11395 14 ~ s~0ll44 web, because both the reference microphone and the signal source had to be located on the same side of the web. For the same reason the reference microphone surface could usually,not be perpendicular to a propagation direction of ~he signal in air, and that caused a significant reduction in a normalized cross-correlation (covariance) function value or a distortion of its shape.
2. The location of the signal source unit and ~oth the reference microphone and the receiving microphone (see prior art in FIG l1 for the waves propagated along the web on the same side creates a direct wave propagating in air that is much ~ ~ Gl.~er than in the case of a two-side ~rC~cs, due to no shielding of airborne waves, h~C~l-ce then the paper web is not shi~l~; ng the airborne ultrasonic waves. It r~-~c~s a degree of correlation between the transmitted and recieved signals too.
3. The ~riction head causes an abrasion of the paper and scrapes off fibres which produces dust. If it is placed on the same side of the web as the microphones this dust will ~e tra~ Led to the microphones, which will reduce not;ceAhly their sensitivity and distort their frequency ~e~ol.ce, if the same kind of friction heads are used as in prior art.
Therefore, a new kind of friction head 20 adapted to a reference microphone 2l,is provided according ~o a further development of the invention illustrated schematically in the second embodiment of the invention shown in FIGs 5A, 5B, and 5C.
The main feature of the combination of the friction head and the reference microphone is that friction and microphone elements are provided symmetrically to each other. This means , that there could be one friction element and an even number of microphone elements provided symmetrically in relation to the friction ,element such that the microphonos in each pair 2 2 ~ 11 8 7 4 WO 96111395 lS P~ ,J'~,1144 have the same distance to the friction element, or there could be one microphone element and an even number of friction elements placed around the microphone element. The ~riction elements have preferably a nearly pointlike contact with the paper web.
.
However, friction elements will cause dust in the environment and measures must be taken to minimize the influence of dust on the microphone(s). Thus, the prefered embo~i nt is to o have a microphone between two pointlike friction elements placed along a line perp~n~ r to the machine direction, i.e. the moving direction of the web. I~ more than two friction elements are provided they must all be provided at the side of a line through an ultrasonic sound receiving element of the reference means directed in the machine direction of the moving paper web in order to prevent dust from coming directly on the microrhon~.
An emho~;ment of the unit of the friction head and reference microphone is shown in FIGs 6A and 6B. ~IG 6A shows the actual appearance of the units when a noise shield is provided, and FIG 6B shows the unit without the noise shield.
The new friction head comprises two friction parts 22 placed along a line perpendicular to the machine direction of the paper web. The friction parts are preferably made from a hard alloy material, for instance wolfram carbide. The friction parts 22 are held by a holder 23.
The reference microphone 21 is located between the two f~iction parts 22 of the friction head 20. The distance D ~
between the friction parts 22 is much less than the wavelength of the ultrasonic wave in the web. Also, the dimensions of the contact area between the friction head 20, and the paper web are less than the wavelength of the ultrasonic wave in the web. Thus, this kind of ultrasonic sound source acts substantially like a two-point-source. The distance between the contact areas and the pla-ne of the reference microphone is comparable with the wavelength in air (for fO=40 kHz, ~a/2=4~3 mm).
, 8 7 ~ -- ~ W096nl3g5 16 P~~ I44 - In order to provide a good correlation between the signals ~Luled by the reference microphone 21 and the microphones Mic2A and Mic2B located at the rotating cyl ;n~r 3 (FIG 5A) it is n~cPcc~ry that the reference microphone 21 is placed as s accurately as possible at the same distance from the contact areas between the friction parts 22 and the web. In order to reduce the waves radiated other than by the contact area by the friction parts and transmitted through the air to the reference microphone, a noise shield 24 lFIG. 6A) is placed ~round the ~riction parts 22 and held by their holder 23 and is provided with an opening adap*ed to hold the reference microphone 21 in place.
Each of the two parts of the friction head 22 in FIGs 6A and 6B are formed as hem;crheres. The friction parts of the head 22 are made of hard alloy and comprise tips, covered by a material absorbing ultrasonic waves, for example, a soft rubber contacting the web.
Referring ~ack to FIG 5A, in order to make an extra shield for the microrho~c Mic2A and Mic2B, ~oth regarding the air~orne noise from the friction head and against the dust from it, a number of shields 35 are provided above the paper web between the microphone 21 and the rotating cyl; n~Pr 3.
Also, as in the embodiment shown in FI& 2, a noise reducing shield 36, for instance made of rubber, is placed around the microrhonPs Mic2A and Mic2B in order to reduce the noise from the noisy ~ v~ l; ngs . The shield 36, having the same function as the shield 6 in the ~ hoAiment shown in FIG 2, has preferably the shape of its lower side adapted to the shape of the paper web when it is transferred over the rotating cyl ;n~e~ 3, as seen from FIG 5A (as well as from FIG
2).
The method above has been described for measurement of the time delay in the machine direction and this will give the tensile stiffness index TSIMD in the machine direction of the paper machine. The friction head 20, the microph~n~c 21, Mic2A and Mic2B are then located in line with the mar-h; n~
2 2 0 ~ 8 7 4 WO9611139S 17 r~-~
direction. However, as mentioned in the i~ uctory part of the specification, the tensi-le stiffness ;n~Y TSICD in the cross direction of the paper machine, and in directions between TSIMD and TSICD, are also needed in order to calculate the anisG~y ratio and the tensile stiffness orientation. An : ~o~i nt for providing also these quantities will now be described with reference to FIGs 5B
and 5C, even though the same feature naturally can also be provided for the : ho~; ~nt shown in FIG 2.
As is apparent from FIG 5B, sev~ al sets of microphones Mic3A, Mic3B; Nic4A, Mic4B etc are shown located parallel to each other and oblique to the microphone 21 in relation to the machine direction (the respective angular directions ~N-lt ~N etc)~ such that each microphone Mic3A~ Mic4A is situated tangentially in the same location above the rotating cyl; n~ 3 as the microphone Mic2A. The delay time of the symmetrical Lamb wave propagating in that o~lique direction, ~N-l ' N etc, is measured in the same way as described above for the ultrasonic Iamb wave propagation in the machine direction t~kin~ ac~ of the somewhat longer propagation path for each set.
Instead of providing an array of receiving pick-up microphone sets only one set need be provided, said set being movable along the cylin_-~ above the web so as to be placed in different oblig~: pos~tions, i.e. sc~nn;~g along the line Cl.
In this instance it is important to place the set of microph~n~c in accurately precise positions above the paper web (same distance to the web and along line Cl) in order to have the same measuring conditio~s for each measured oblique settina fnot shown in a separate figure, however the pick-up microp~ ~ set will be placed in the same way as shown in F-G
5B).
Another emho~; ment, shown in FIG 5C has only one pick-up microphone set Mic2A', Mic2B' and moves, as a unit, friction head 20 and reference microphone 21 across the web, for instance along a straight line Fl parallel to the line Cl, as , - 2~ ~8~
096/ll3g5 18 ~ 44 ` ' shown, and to derive the delay time for the sO wave for a oCo~ amount of settings of the unit 20,21 having different angular positions in relation to the pick-up microphone set.
Tt is also possible to move the friction-head/microphone set 20,21 along a curved line F2 (~h~), or to provide the velocity measurement along the maçhi n~ direction separately and the measurements in the oblique directions along a line F3 (dot/~h~A) perpendicular to the line Cl.
It should be noted that even for the em~o*iments having C~nn; ng elements along a line and one element constantly in the same position, each measuring result is provided having both kinds of elements in the same position in relation to each other during the time it takes to get the measuring 5 result.
Many different kinds of numerical methods may be used to provide a quite precise estimation a~out the s0 wave rate in the cross direction of the paper web. One method is to fit the measured sO wave rates for the different o~lique positions in some kind of periodic ~unction, e.g. the function for an ~ ce or some kind of Fourier serie.
Example in which a trigonometric first order Fourier series is used: , We assume that the ultrasonic velocity of the sO wave has been measured in three different directiohs and these three different values are used ~or determining constants ao, al and bl. The constants are then inserted in the following 3~ formula:
f(~)=aO+al*cos2~+bl*sin2~ (1) The estimated velocity is also ~p~n~nt on formula 2:
f(x)=kl*x+k2 (where x=f~)max/f~)min) ~2) The constants kl and k2 are known. A combination of the functions 1 and 2 will give the following function which determines the s0 wave velocity in the cross direction (~=90) v(CD)=f(x)*(aO-al) (3) By changing the constants kl and k2 it is possi~le to get the - 2~0~87~
~ , velocity in any direction from the formula 4:
v(a,max/min)=(kl(a)*x+k2(a))*(aOIal*cos2a+bl*sin2a) (4) Another advantageable way to derive the velocity of the sO wave in the cross direction from the results from the different settings of the friction-head/reference-microphone and the pick-up microphones in relation to each other is to set the measuring results o~ the sO wave rates in a coordinate system, having the rate in the m~h; n~ direction along the X-axis and 0 the rate in the cross direction of the web along the Y-axis, in relation to the respecive angular deviation a~I, aN etc of each set to ~he m~,h; n~ direction in the way shown in FIG 5D. A
curve is drawn through the different measuring results and extrapolated to cut the Y-axis in order to provide the velocity of the sO wave in the web in the cross direction. A small extrapolation error is unavoidable but is m;n;m;~ed by having a lot of settings of the friction-head/reference-microphone in relation to the pick-up microphones, the more the better.
The same extrapolation t~hnique as shown in FIG 5C can be used also for the embodiments shown in FIG 5D.
While the invention has been described with reference to specific embodiments, it will be understood by those skilled in `5 the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as apparent from the Claims. In addition, modifications may be made without departing from the essential t~ch;ngs of the invention. For instance, more than two pick-up microphones could be provided at the rotating cylinder.
h~ n,~.
ANt~N~ED Sl lEET
Therefore, a new kind of friction head 20 adapted to a reference microphone 2l,is provided according ~o a further development of the invention illustrated schematically in the second embodiment of the invention shown in FIGs 5A, 5B, and 5C.
The main feature of the combination of the friction head and the reference microphone is that friction and microphone elements are provided symmetrically to each other. This means , that there could be one friction element and an even number of microphone elements provided symmetrically in relation to the friction ,element such that the microphonos in each pair 2 2 ~ 11 8 7 4 WO 96111395 lS P~ ,J'~,1144 have the same distance to the friction element, or there could be one microphone element and an even number of friction elements placed around the microphone element. The ~riction elements have preferably a nearly pointlike contact with the paper web.
.
However, friction elements will cause dust in the environment and measures must be taken to minimize the influence of dust on the microphone(s). Thus, the prefered embo~i nt is to o have a microphone between two pointlike friction elements placed along a line perp~n~ r to the machine direction, i.e. the moving direction of the web. I~ more than two friction elements are provided they must all be provided at the side of a line through an ultrasonic sound receiving element of the reference means directed in the machine direction of the moving paper web in order to prevent dust from coming directly on the microrhon~.
An emho~;ment of the unit of the friction head and reference microphone is shown in FIGs 6A and 6B. ~IG 6A shows the actual appearance of the units when a noise shield is provided, and FIG 6B shows the unit without the noise shield.
The new friction head comprises two friction parts 22 placed along a line perpendicular to the machine direction of the paper web. The friction parts are preferably made from a hard alloy material, for instance wolfram carbide. The friction parts 22 are held by a holder 23.
The reference microphone 21 is located between the two f~iction parts 22 of the friction head 20. The distance D ~
between the friction parts 22 is much less than the wavelength of the ultrasonic wave in the web. Also, the dimensions of the contact area between the friction head 20, and the paper web are less than the wavelength of the ultrasonic wave in the web. Thus, this kind of ultrasonic sound source acts substantially like a two-point-source. The distance between the contact areas and the pla-ne of the reference microphone is comparable with the wavelength in air (for fO=40 kHz, ~a/2=4~3 mm).
, 8 7 ~ -- ~ W096nl3g5 16 P~~ I44 - In order to provide a good correlation between the signals ~Luled by the reference microphone 21 and the microphones Mic2A and Mic2B located at the rotating cyl ;n~r 3 (FIG 5A) it is n~cPcc~ry that the reference microphone 21 is placed as s accurately as possible at the same distance from the contact areas between the friction parts 22 and the web. In order to reduce the waves radiated other than by the contact area by the friction parts and transmitted through the air to the reference microphone, a noise shield 24 lFIG. 6A) is placed ~round the ~riction parts 22 and held by their holder 23 and is provided with an opening adap*ed to hold the reference microphone 21 in place.
Each of the two parts of the friction head 22 in FIGs 6A and 6B are formed as hem;crheres. The friction parts of the head 22 are made of hard alloy and comprise tips, covered by a material absorbing ultrasonic waves, for example, a soft rubber contacting the web.
Referring ~ack to FIG 5A, in order to make an extra shield for the microrho~c Mic2A and Mic2B, ~oth regarding the air~orne noise from the friction head and against the dust from it, a number of shields 35 are provided above the paper web between the microphone 21 and the rotating cyl; n~Pr 3.
Also, as in the embodiment shown in FI& 2, a noise reducing shield 36, for instance made of rubber, is placed around the microrhonPs Mic2A and Mic2B in order to reduce the noise from the noisy ~ v~ l; ngs . The shield 36, having the same function as the shield 6 in the ~ hoAiment shown in FIG 2, has preferably the shape of its lower side adapted to the shape of the paper web when it is transferred over the rotating cyl ;n~e~ 3, as seen from FIG 5A (as well as from FIG
2).
The method above has been described for measurement of the time delay in the machine direction and this will give the tensile stiffness index TSIMD in the machine direction of the paper machine. The friction head 20, the microph~n~c 21, Mic2A and Mic2B are then located in line with the mar-h; n~
2 2 0 ~ 8 7 4 WO9611139S 17 r~-~
direction. However, as mentioned in the i~ uctory part of the specification, the tensi-le stiffness ;n~Y TSICD in the cross direction of the paper machine, and in directions between TSIMD and TSICD, are also needed in order to calculate the anisG~y ratio and the tensile stiffness orientation. An : ~o~i nt for providing also these quantities will now be described with reference to FIGs 5B
and 5C, even though the same feature naturally can also be provided for the : ho~; ~nt shown in FIG 2.
As is apparent from FIG 5B, sev~ al sets of microphones Mic3A, Mic3B; Nic4A, Mic4B etc are shown located parallel to each other and oblique to the microphone 21 in relation to the machine direction (the respective angular directions ~N-lt ~N etc)~ such that each microphone Mic3A~ Mic4A is situated tangentially in the same location above the rotating cyl; n~ 3 as the microphone Mic2A. The delay time of the symmetrical Lamb wave propagating in that o~lique direction, ~N-l ' N etc, is measured in the same way as described above for the ultrasonic Iamb wave propagation in the machine direction t~kin~ ac~ of the somewhat longer propagation path for each set.
Instead of providing an array of receiving pick-up microphone sets only one set need be provided, said set being movable along the cylin_-~ above the web so as to be placed in different oblig~: pos~tions, i.e. sc~nn;~g along the line Cl.
In this instance it is important to place the set of microph~n~c in accurately precise positions above the paper web (same distance to the web and along line Cl) in order to have the same measuring conditio~s for each measured oblique settina fnot shown in a separate figure, however the pick-up microp~ ~ set will be placed in the same way as shown in F-G
5B).
Another emho~; ment, shown in FIG 5C has only one pick-up microphone set Mic2A', Mic2B' and moves, as a unit, friction head 20 and reference microphone 21 across the web, for instance along a straight line Fl parallel to the line Cl, as , - 2~ ~8~
096/ll3g5 18 ~ 44 ` ' shown, and to derive the delay time for the sO wave for a oCo~ amount of settings of the unit 20,21 having different angular positions in relation to the pick-up microphone set.
Tt is also possible to move the friction-head/microphone set 20,21 along a curved line F2 (~h~), or to provide the velocity measurement along the maçhi n~ direction separately and the measurements in the oblique directions along a line F3 (dot/~h~A) perpendicular to the line Cl.
It should be noted that even for the em~o*iments having C~nn; ng elements along a line and one element constantly in the same position, each measuring result is provided having both kinds of elements in the same position in relation to each other during the time it takes to get the measuring 5 result.
Many different kinds of numerical methods may be used to provide a quite precise estimation a~out the s0 wave rate in the cross direction of the paper web. One method is to fit the measured sO wave rates for the different o~lique positions in some kind of periodic ~unction, e.g. the function for an ~ ce or some kind of Fourier serie.
Example in which a trigonometric first order Fourier series is used: , We assume that the ultrasonic velocity of the sO wave has been measured in three different directiohs and these three different values are used ~or determining constants ao, al and bl. The constants are then inserted in the following 3~ formula:
f(~)=aO+al*cos2~+bl*sin2~ (1) The estimated velocity is also ~p~n~nt on formula 2:
f(x)=kl*x+k2 (where x=f~)max/f~)min) ~2) The constants kl and k2 are known. A combination of the functions 1 and 2 will give the following function which determines the s0 wave velocity in the cross direction (~=90) v(CD)=f(x)*(aO-al) (3) By changing the constants kl and k2 it is possi~le to get the - 2~0~87~
~ , velocity in any direction from the formula 4:
v(a,max/min)=(kl(a)*x+k2(a))*(aOIal*cos2a+bl*sin2a) (4) Another advantageable way to derive the velocity of the sO wave in the cross direction from the results from the different settings of the friction-head/reference-microphone and the pick-up microphones in relation to each other is to set the measuring results o~ the sO wave rates in a coordinate system, having the rate in the m~h; n~ direction along the X-axis and 0 the rate in the cross direction of the web along the Y-axis, in relation to the respecive angular deviation a~I, aN etc of each set to ~he m~,h; n~ direction in the way shown in FIG 5D. A
curve is drawn through the different measuring results and extrapolated to cut the Y-axis in order to provide the velocity of the sO wave in the web in the cross direction. A small extrapolation error is unavoidable but is m;n;m;~ed by having a lot of settings of the friction-head/reference-microphone in relation to the pick-up microphones, the more the better.
The same extrapolation t~hnique as shown in FIG 5C can be used also for the embodiments shown in FIG 5D.
While the invention has been described with reference to specific embodiments, it will be understood by those skilled in `5 the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as apparent from the Claims. In addition, modifications may be made without departing from the essential t~ch;ngs of the invention. For instance, more than two pick-up microphones could be provided at the rotating cylinder.
h~ n,~.
ANt~N~ED Sl lEET
Claims (13)
1. A system for measuring ultrasonically the elastic properties of a moving paper web comprising:
a. means (1;20) for generating a noise type ultrasonic wave in the paper web at an excitation point;
b. reference ultrasonic wave receiving means (Mic1,7;21,7) for receiving contactlessly the ultrasonic wave reradiated from the paper web into the air from the excitation point;
characterized in c. at least two pick-up ultrasonic wave receiving means (Mic2A,8,Mic2B,9) to receive contactlessly the ultrasonic wave generated by the ultrasonic wave generating means and reradiated by the paper web;
d. processing means (8 to 10) to combine the outputs from the pick-up ultrasonic wave receiving means such that terms (Ys(t)) dependent on the velocity of the ultrasonic wave of interest (s0) propagating in the paper web are enhanced and terms (Ya(t)) dependent on the same ultrasonic wave propagating in air are reduced at the combination;
e. computing means (13) for processing the outputs from the reference ultrasonic wave receiving means and from the processing means, and determining the delay time between these outputs.
a. means (1;20) for generating a noise type ultrasonic wave in the paper web at an excitation point;
b. reference ultrasonic wave receiving means (Mic1,7;21,7) for receiving contactlessly the ultrasonic wave reradiated from the paper web into the air from the excitation point;
characterized in c. at least two pick-up ultrasonic wave receiving means (Mic2A,8,Mic2B,9) to receive contactlessly the ultrasonic wave generated by the ultrasonic wave generating means and reradiated by the paper web;
d. processing means (8 to 10) to combine the outputs from the pick-up ultrasonic wave receiving means such that terms (Ys(t)) dependent on the velocity of the ultrasonic wave of interest (s0) propagating in the paper web are enhanced and terms (Ya(t)) dependent on the same ultrasonic wave propagating in air are reduced at the combination;
e. computing means (13) for processing the outputs from the reference ultrasonic wave receiving means and from the processing means, and determining the delay time between these outputs.
2. A system according to Claim 1, characterized in that the the lateral dimensions of ultrasonic wave indicating elements (Mic1, Mic2A, Mic2B) of all the ultrasonic wave receiving means are at least 10 times less than a wavelength of the ultrasonic wave in the paper web, and all the ultrasonic wave indicating elements of the ultrasonic wave receiving means are placed at a distance from the paper web less than a wavelength of the ultrasonic wave reradiated by the paper web into air.
3. A system according to Claim 2, characterized in that the distance (l m) between the adjacent pick-up ultrasonic wave indicating elements, in a plane parallel to the paper web, is half of the wavelength in air at the centre frequency (f 0) of a band width used for measurements.
4. A system according to Claim 1, characterized in that the ultrasonic wave generating means (1;20) of the ultrasonic wave signal and all the ultrasonic wave receiving means are placed on one side of the paper web.
5. A system according to any one of the preceding Claim, characterized in that the pick-up ultrasonic wave receiving elements (Mic2a,Mic2b;Mic2A,Mic2B) are placed along a straight line, preferably in the machine direction of the moving paper web in order to obtain the time between generation of the ultrasound wave part of interest to monitor and propagating in the paper web and the reradiation of the same ultrasonic wave at the pick-up ultrasonic wave elements in order to derive the Tensile Stiffness Index in the machine direction.
6. A system according to any one of the preceding Claims, characterized in that the unit of pick-up ultrasonic wave receiving elements (Mic2a,Mic2b;Mic2A,Mic2B;Mic3A,Mic3B etc) can be oriented obliquely in relation to the reference receiving element (Mic1) at different angles to the machine direction of the moving paper web in order to obtain the time between generation of the ultrasound wave part of interest to monitor and propagating in the paper web and the reradiation of the same wave at the pick-up elements in order to derive the Tensile Stiffness Index in the oblique direction.
7. A system according to claim 6, characterized in that the results from measurements in several oblique direction are combined to derive the Tensile Stiffness Index in the cross direction of the paper machine.
8. A system according to Claim 1, characterized in that the indicating elements (Mic2a,Mic2b;Mic2A,Mic2B) of the pick-up ultrasonic wave receiving means are located above and close to a rotating cylinder of the paper-making machine provided under the paper web and are positioned along the machine direction of the paper web, which is the direction directed from the ultrasonic wave generating means towards the pick-up ultrasonic wave receiving means, at a predetermined location (C1) downstreams from the line (C2) where the moving paper web the first time touches the cylinder surface.
9. A system according to Claim 1, characterized in that a first shield (35) is placed between the ultrasonic wave generating means and the two pick-up ultrasonic wave receiving means for reducing an amplitude of an airborne ultrasonic wave propagating from the ultrasonic wave generating means (1;20) to the pick-up ultrasonic wave receiving means (Mic2A, Mic2B).
10. A system according to any one of the preceding Claims, characterized in that at least the indicating elements (Mic2A, Mic2B) of the pick-up ultrasonic wave receiving means are placed close to the rotating cylinder (3) and are located inside a second shield (6;36) of airborne ultrasonic waves, for example a rubber cylinder, the edge of which turned from the ultrasonic wave elements is located at a location (C1) downstreams in the machine direction outside the line (C2) where the moving paper web the first time touches the cylinder, and the ultrasonic indicating elements of the pick-up ultrasonic wave receiving means are placed close to an edge of the shield turned towards them.
11. A system according to claim 1, characterized in that the computing means (13) determines the delay time as a zero-cross of the Hilbert transform of the cross-correlation function between the outputs of the first receiving means and the processor, corresponding to the maximum value of the cross-correlation function.
12. A system according to claim 11, characterized in that a Hilbert window is created in the time domain and is shifted until a peak location in time of the cross-correlation function is found and a sharp peak is derived.
13. A system according to Claim 1, characterized in that the ultrasonic wave generating means (20) of the ultrasonic waves comprises dry friction elements in contact with the moving paper web, the contact dimension are much less than a wavelength of the ultrasonic wave (s0) in the paper web of interest to be indicated, and the indicating element (Mic1) of the reference ultrasonic wave receiving means is placed in close vicinity of the two friction contact area elements.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE9403383-4 | 1994-10-06 | ||
| SE9403383A SE504575C2 (en) | 1994-10-06 | 1994-10-06 | Device for ultrasonic measurement of elastic properties of a moving web of paper |
| PCT/SE1995/001144 WO1996011395A1 (en) | 1994-10-06 | 1995-10-05 | System for measuring ultrasonically the elastic properties of a moving paper web |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2201874A1 true CA2201874A1 (en) | 1996-04-18 |
Family
ID=29405475
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2201874 Abandoned CA2201874A1 (en) | 1994-10-06 | 1995-10-05 | System for measuring ultrasonically the elastic properties of a moving paper web |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2201874A1 (en) |
-
1995
- 1995-10-05 CA CA 2201874 patent/CA2201874A1/en not_active Abandoned
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