SYSTEM FOR MEASURING ULTRASONICALLY THE ELASTIC PROPERTIES OF A MOVING PAPER WEB

Abstract: The invention relates to a system for measuring ultrasonically the elastic properties of a moving paper web. A noise type ultrasonic sound wave is generated by an ultrasonic sound generating means (4) in the web at an excitation point. A reference ultrasonic wave reradiated into the air from the excitation point is indicated. Pick-up means (M2; Mic2A, Mic2B; Mic3A, Mic3B) is provided for receiving ultrasonic sound reradiated from the paper web at a predetermined distance from the location of the sound generating means (4). The ultrasonic sound generating means (4) and the reference ultrasonic receiving means (M1,5) are provided on the same side of the paper web. At least one of these two kind of means comprises a number of elements provided symmetrically in relation to the other means.

Description: System for measuring ultrasαnically the elastic properties of a moving paper wab This invention concerns the measurement of the velocity of ultrasound, in-plane, for a moving paper web. The ultrasound velocity in paper is known to be related to various measures of paper strength and stiffness. BACKGROUND OF THE INVENTION The most important values for the papermaker to consider from ultrasound velocity measurements on paper web are: TSO Tensile Stiffness orientation, i.e. the orientation of the elastic properties in-plane of the paper sheet, TSI MD Tensile Stiffness Index in the machine direction of the paper machine, TSI CD TTeennssiillee SSttiiffffnneessss Index in the cross direction of the paper machine. It is possible to determine these quantities and also the anisotropy ratio TSI MD /TSI_ D by performing the ultrasound velocity measurements in the machine direction (MD) , cross direction (CD) and directions between (MD) and (CD) . The tensile stiffness and anisotropy 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 relationship can be expressed by: TSI = v 2 * c where TSI is measured in kNm/g, v is the propagation velocity (km/sek) for the ultrasonic pulse, and c is a diraensionless constant close to 1 depending on Poisson's ratio for the paper. The velocity is easily determined by measuring the propagation time for an ultrasonic pulse between a transmitter and a receiver. These quantities are often measured statically on samples taken from a paper web. However, it is desirable to measure these paper qualities on-line by an on-line meter used as a sensor for the continuous control of a paper manufacturing process. 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 wheels, which contain transmitters and receivers of ultrasonic waves. These wheels are rotated by a moving paper web, what 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 measurement accuracy the wheels must by synchronized which makes the system extremely complicated and unreliable. An arrangement described in U.S. Pat. No. 4,688,423 overcomes this drawback by employing disk type transducers which can be excited continuously and, therefore, synchronization of the wheels is not necessary. However, the arrangements described in the above-mentioned patent specifications need a direct mechanical contact between the ultrasonic transducers 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 particular ultrasonic transducer and the web. Due to the randomly changing force, the amplitudes of received signals fluctuate, thereby making the results of measurements less accurate. The physical contact with the web is not needed if ultrasonic waves are excited and detected optically, as described in U.S.Pat. No. 5,025,665. 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 obtained 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 amplitudes of the ultrasonic waves propagating in-plane of the web are very small. A very strong acoustic noise exists in papermaking machines, which is accompanied by the vibrations of the moving web. In fact this makes the optical detection of the lowest order symmetrical Lamb waves impossible, and only these waves are suitable for measurements of the stiffness and tensile strength of paper. Other prior on-line paper measuring systems 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 inventor of the present invention) , Proceedings of 20th international 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 dry friction, receiving the ultrasonic wave reradiated by the web by two non-contacting ultrasonic receivers and calculating the cross-correlation function between these two received signals. The first receiver was placed opposite to the ultrasonic transmitter and the second at a determined distance from the transmitter along the web. In order to improve the signal/noise ratio, a rotating cylinder was placed underneath the web 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 involved in the active measuring facilities. The disadvantages of the measuring system described in the above-mentioned USSR-patents are that the excitation and reception of the ultrasonic waves are performed from the opposite sides of the web. Also, the signal/noise ratio is not sufficient high enough to permit reliable continous on¬ line measurements in a mill environment. Another problem which is encountered in performing measurements in other directions than the web propagation direction is an even worse signal/noise ratio due to the higher losses of ultrasonic waves in an anisotropic material. The main object of the present invention is to provide an on¬ line measuring system with single side access to the paper web, performing measurements at different directions in-plane of a moving web. Another object of the present invention is to provide an on¬ line measuring system having improved in noise robustness for the system in a paper mill environment. Still another object is to provide an on-line dust- insensitive measuring system. Yet another object is to provide an on-line measuring system providing well-defined and precise measuring results. These objects are achieved with a system having the characterizing features disclosed in the main claim. Further features and further developments of the invention are disclosed in the subclaims. Summary of the invention 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 accomplished by exciting a broad-band noise type Lamb wave in the web by having the source of ultrasonic waves and all the ultrasonic receivers placed on a single side of the web. The broad band noise-like Lamb wave in the paper web is generated by means of dry friction between the moving web and the friction head. The system has no moving parts and all signals are received by non-contacting means. The delay time of the ultrasonic wave is preferably determined as a zero cross of the Hubert transform of the cross-correlation function of the received signals, corresponding to the maximum value of the cross-correlation function. In order to make the system noise robust, i.e. provide a low signal/noise ratio, the receiving of the reradiated ultrasonic waves is performed above the rotating cylinder of a paper making machine at the particular position in respect to the line, where the moving web touches the cylinder for the first time. The pick-up is preferably made by two microphones having such a distance from each other that airborne ultrasound waves from the friction head are reduced and ultrasound waves propagating through the paper web are enhanced. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: FIG. 1 illustrates schematically a measuring system according to the prior art, FIG. 2 illustrates schematically a first embodiment of a measuring system according to the invention, FIG. 3A and 3B illustrates different friction head/microphone combinations, FIGs 4A,4B and 5A,5B and 6A,6B illustrate different embodiments of friction head and microphone units, FIG. 7A illustrates schematically a side view of a second embodiment of a measuring system according to the invention, Figs 7B and 7C illustrate a schematic view from above of two embodiments of the system i FIG 7A having the possibility of measuring the ultrasound velocity in different directions, FIG. 7D illustrates a graph to provide an extrapolated value of the ultrasound velocity in the cross direction, FIGs 8A to 8B are diagrams of signals provided in different operation steps in searching for the delay time of the ultrasonic wave transmitted through the paper web, and FIG. 9 is a flow chart of the processing operation in order to provide the delay time of the ultrasonic wave in-plane of the paper web, The same references are used for the same elements in the Figures. With reference to FIG. 1, a prior art on-line paper measuring system disclosed in the U.S.S.R. Pat. No. 489018 includes a friction head 1 provided on one side of a moving paper web 2 generating a noise-like ultrasonic signal v as a result of dry friction between the head 1 and the web 2. A random signal with a normal law of distribution up to 70 to 90 kHz is excited. The part of this signal v propagating in the paper web 2 as the zero order symmetrical Lamb wave s Q is the interesting one to examine. The excited wave is reradiated partially into the surrounding air and is picked up by a contactless reference microphone Micl provided opposite the head 1 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 1 0 away from, i.e. downstream from, the head 1 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 supported by a rotating cylinder 3 opposite the pick-up microphone Mic2. The signals from the microphones 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 results presented on a display 5' . From a commercial point of view, a measuring system in which all units are located at the same side of a paper web has many advantages. However, in order to implement one side access approach it is necessary to overcome a lot of problems. 1. According to prior art, the reference microphone could not be put at the same distance from a signal source as the pick-up microphone is from a paper 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 usually could not be perpendicular to a propagation direction of the 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 both the reference microphone and the pick-up microphone for the waves propagated along the web on the same side creates a direct wave propagating in air that is much stronger than in the case of a two-side access, because then the paper web is not shielding the airborne ultrasonic wave. It reduces a degree of correlation between the transmitted and recieved signals too. 3. The friction 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 be transported to the microphones, which will reduce noticeably their sensitivity and distort their frequency response, if the same kind of friction heads are used as in prior art. Therefore, a new kind of friction head 4 adapted to a reference microphone Ml is provided according to the invention illustrated schematically in the embodiments of the invention shown in FIGs 2 and 6. The main feature of the combination of the friction head 4 and the reference microphone Ml 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 microphones in each pair 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 friction elements have preferably a nearly pointlike contact with the paper web. FIG 3A shows a first embodiment of the friction head/reference microphone combination in which one element A of the first kind is provided between two elements Bl, B2 of the other kind. FIG 3B shows another embodiment in which one element A' of the first kind is surrounded by four elements Cl, C2 and Dl, D2 of the other kind, each pair C1,C2 and D1,D2 being placed diagonally in relation to the first kind of element A' . From a practical point of view the element A or A' is preferably a microphone element, since then it is not necessary to balance the signals from several elements. 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 embodiment, shown in FIG 3A, is to have a microphone between two pointlike friction elements placed along a line perpendicular to the machine direction, i.e. the moving direction of the paper web. If 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 onto the microphone. Three embodiments of the unit of the friction head and reference microphone having the preferred configuration mentioned above are shown in FIGs 4A, 4B and 5A, 5B and 6A, 6B, respectively. The FIGs 4A, 5A, and 6A show the actual appearance of the units when a noise shield is provided in order to minimize air borne sound waves from reaching the microphone, and the FIGs 4B, 5B, and 6B show the units without the noise shield. Two friction parts, 6, 7 in FIG 4B, 8, 9 in FIG 5B, and 10, 11 in FIG 6B, are placed such that their contact point with the paper web is provided on each side of the ultrasonic reference microphone 13, 14 or 15, respectively, along a line perpendicular to the machine direction of the web. The friction parts are preferably made from a hard alloy material, for instance wolfram carbide. The friction parts are held by a holder 16, 17 or 18, respectively. The distance D between the friction parts is much less than the wavelength of the ultrasonic wave in the web. * Also, the dimensions of the contact area between the friction parts and the paper web are less than the wavelength of the ultrasonic wave in the web. Thus, this kind of friction source acts substantially like a two-point-source. The distance between the contact areas and the plane of the reference microphone is comparable with the wavelength in air for the generated ultrasonic waves (for f Q =40 kHz, λ /2=4.3 mm, where f 0 is the center frequency of the signal spectrum, and λ a is the wavelength of the center frequency in air) . The signals provided in the web by the friction parts are captured by the reference microphone Ml provided as close as possible to the middle point between the friction parts. In order to reduce the waves radiated not by the contact area by the friction parts and transmitted through the air to the reference microphone Ml the noise shield 19, 20 and 21, respectively, is placed around the friction parts and held by their holder and is provided with an opening adapted to hold the reference microphone in place. The noise shield also protects the microphone from at least some of the dust caused by the friction between the parts and the web, and this effect is enhanced if the noise shield has its sides turned towards the microphone somewhat nearer to the web than its outer sides thus directing the dust outwardly. Each of the two parts 6, 7 of the friction head in FIGs 4A and 4B has a triangular form having its base angled to the paper web. This friction head provides the best measuring result but it scratches the paper. Each of the two parts 8, 9 of the friction head in FIGs 5A and 5B are formed as hemispheres, which has a substantially lesser affect on the paper web than the head in FIGs 4A, 4B; however, the measuring result is satisfying but not as good as from the head 6, 7. The friction parts 8, 9 are made of hard alloy and comprise tips, contacting the web, covered by a material absorbing ultrasonic waves, for example, a soft rubber. Each of the two friction parts 11, 12 of the friction head in FIGs 6A and 6B are formed as rounded pins angled to the web and could be seen as a compromise between the other two embodiments of the friction head as regards scratching the paper web and providing well distinguishable signals through the web. Returning to FIG 2, the signal part of the ultrasonic wave v transmitted through the web of interest to be indicated is the s- wave signal, which corresponds to the symmetric zero order Lamb waves propagated in the web 2, i.e. the fastest propagating wave. The s 0 wave reradiated from the web is captured by a pick-up microphone M2 located opposite to the contact line Cl between the rotating cylinder and the web 2 from which the best reradiation into the air of the s 0 wave propagated in the web is provided. The lateral dimension of the pick-up microphone M2, and also of the reference microphone Ml, are at least 10 times less than a wavelength of the ultrasonic wave in the paper web, and the microphones are placed at a distance from the web less than a wavelength of the ultrasonic wave s Q reradiated by the web into air. Noise is also radiated into the air from the contact line c_ where the web 2 first meets the cylinder 3. This noise should preferably be suppressed as far as possible and therefore a noise suppressing shield 25, for example made of rubber, is provided around the microphone M2 shielding it from the noise from the contact line C-, and also from ambient noise. Thus, its outer edge nearest to the contact line C 2 is located downstream of this line. The shield has a small inner diameter so that the microphone M2 is placed close to its internal edge. In order to make an extra shield for the microphone M2 both regarding the airborne noise from the friction head and against the dust from it a number of shields 26 are provided above the paper web between the microphone Ml and the rotating cylinder 3. The signal from the reference microphone Ml is fed to a first input of a processor 27 through an amplifier 28 and a bandpass filter 29. The signal from the pick-up microphone M2 is fed to a second input of the processor 27 through an amplifier 30 and a bandpass filter 31. The processor 27 will make a cross-correlation operation on the signals from the two microphones. Preferably this operation will include a Hubert transform. An example of this kind of operation will be given below in connection with the embodiment shown in FIG 7. The example given there for the somewhat more complicated embodiment could easily be amended to be adapted to the embodiment in FIG 2 by a person skilled in the art. In accordance with the invention measures are taken to enhance the signal/noise ratio of the correlated signals, particularly in a noisy environment. Therefore, in accordance with a second embodiment of the invention, shown in FIG 7A, a double channel measuring receiving microphone device is provided to receive the wave propagated along the web 2. It is, however, to be noted that more than two pick-up microphones can be provided according to the invention. In the second embodiment of the invention at least two pick¬ up ultrasonic microphones Mic2A and Mic2B, being the pick-up elements of the pick-up receivers, are placed a distance 1 from each other, the distance being chosen to be a half- wavelength of the wave in air at the centre frequency of the bandwidth of the ultrasonic wave transmitted through the web. The microphone Mic2A is located opposite the contact line C.. The microphone Mic2B is located on the side of the microphone Mic2A turned away from the friction head 4. Thus the principle of the operation is based on a difference of ultrasound velocities in air (v =343 /sek) and paper (v -=1.5 to 4 km/sek) . The signals at the outputs of the microphones Mic2A and Mic2B are given by: u 2a<*>"*■<*>^a<*> +n md<*> +n 0a<*> where u 2a (t) and ^ 2b (t) are the complete wave signal at the output of the microphones Mic2A and Mic2B, respectively, y s (t) is the s Q wave signal at the output of the microphone Mic2A, y_ is the airborne wave generated by the friction head, n md is the noise propagating along the machine direction at the output of the microphone, n U_α_, (t) and n Λ UJ.D(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 Mic2A and Mic2B for the appropriate waves, is the delay time of the so_ wave between the microphones, and At =1 /v is the delay time of airborne waves between the microphones propagating along the machine direction. Due to extensive differences in the ultrasonic velocities in the web and in air, Δt s «Δt a , Δt B <t 0 , t 0 =l/f Q , where f is the center frequency of the signal spectrum. Therefore, the spectral components with frequencies equal or close to frequency f Q are approximately: y « .(t)«y β (t-Δt <β ) y a (t)*-y a (t-Δt a ) Then, addition of the signals from the two microphones Mic2A and Mic2B gives the following result: u 2 (t)=u 2a (t)+u 2b (t)=(1+k s > * v s (t)+(1"k a ) * y a (t)+(1 ~ The coefficients k , k , k are close to 1, which gives approximately: u 2 (t)=2*y s (t)+ε a y a (t) + ε n n md (t)+[n 0a (t)+n Qb (t) ] where εα_ and e_π are much lower than 1, which indicates that the amplitude of the s Q wave signal is amplified twice and the amplitudes of the wave propagating in air along the machine direction from the friction head is substatially reduced, like the noise propagating in the machine direction. The electronic noises or the noises arriving from directions different from the machine direction are not suppressed and are added as partially correlated or uncorrelated random processes. It is to be noted that the distance l m between the pick-up microphones could be chosen in another way, but then the equations above and the combination of them will be changed. The main feature of the choise of distance is that the term y B (t) is essentially enhanced and the term y a (t) essentially reduced in the combination. Also, as in the embodiment shown in FIG 2, a noise reducing shield 35, for instance of rubber, is placed around the microphones Mic2A and Mic2B in order to reduce the noise from the noisy surroundings. The shield 35, having the same function as the shield 25 in the embodiment 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 cylinder 3, and has its wall placed quite close to the microphones Mic2A and Mic2B. Referring now to the embodiment having the pick-up microphones placed one half-wavelength of the airborne ultrasonic wave apart, in order to estimate the velocity of the s Q wave a cross-correlation should be made on the signals from the reference microphone Ml and the added signals from the two pick-up microphones Mic2A and Mic2B. The signals from the pick-up microphones are amplified in respective amplifiers 36 and 37 and then added in an adder 38. The signal from the adder 38 is fed to the second input of the processor 27 through a bandpass filter 39. The processor 27 is provided with a program for performing an automatic time delay measurement in order to obtain the velocity of the wave in the actual paper web. The delay time is determined from the cross-correlation function. For this purpose two methods are combined, namely, cross-correlation function envelope peak detection for a coarse evaluation and zero-crossing detection of the cross- correlation function Hubert transform for the accurate measurements. Time diagrams illustrating this technique are given in FIGs 8A to 8D. This techique is efficient in the case of relatively narrow-band signals, i.e., when a cross- correlation function has an oscillating character. Therefore, as shown in FIG 8A a cross-correlation function R vV ( T ) between transmitted and received s Q wave signal at the outputs of the receivers Ml, 28, 29, and Mic2A, Mic2B, 36, 37, 38, 39 is provided R χ y(τ)=(l/T)J[x(t)+n 1 (t)]*[2y a (t+τ)+ε[y a (t+τ)+n md (t+τ)]+ n 2 (t+τ)]dtO where T is the signal duration used for calculation, x(t) and y(t+τ) are the signals from input channel Ml, 28, 29, and the output channel Mic2A, Mic2B, 36, 37, 38, 39, respectively, and n. (t) is the noise received by the microphone Ml and n 2 (t+τ) is the added noise received by the micropones Mic2A and Mic2B. A zero-cross of the Hubert transform of the cross- correlation corresponding to the maximum value of the cross- correlated function is made. Then, the envelope, as shown in FIG 8B, of a cross- correlation function Rxy(τ) is obtained by means of the Hubert transform: A χy (τ)=V[R 2 xy (T) + R 2 xy (T)] (see is th function RR xxyy ((TT)) aanndd sshhoowwnn iinn FFIIGG 88DD.. FFIIGG I8C shows the detection of the envelope peak shown in FIG 8B. In the presence of signals propagating through multiple paths, the cross-correlation function has a few peaks, corresponding to different delays. Then the envelope function can be presented as delays in the corresponding paths. Therefore, in a general case not just one but a few peaks will be detected. The proper peak is found by taking into account prior knowledge of the expected time of arrival, and usually is that closest tσ the zero instant. The obtained rough estimate of the delay time τ d is used to i produce a window H(t) in a time domain the width of which oτ is slightly less than half a period of oscillation of the band-limited cross-correlation function Δτ<t Q /2 The window is located symmetrically in respect to the determined delay time τ d . 1, for τ d -(Δτ/2) < t < τ d ,+(Δτ/2) i J- H(t-τ di ) « 0, otherwise The accurate delay time estimation is obtained from the windowed Hubert transform R w (t) of the initial cross- correlation function: The peak value of the envelope function Axy(T) corresponds to the peak value of the cross-correlation function Rx v y v (τ) only in the case of non-dispersive propagation. As it was noted above, the symmetrical s_ wave used for the measurements propagates without noticeable dispersion. On the other hand, the uncertainty in detecting the rough delay time should be less than t Q /2. For a 35 kHz center frequency, rough delay time uncertainties of as much as t Q /2=14 μs can be allowed. Usually this requirement is easily funfilled and no ambiguity occurs. The peak values of the cross-correlation function Rxy(τ) correspond to the zero values of the Hubert transform R,.,. (T) . Hence, the time of signal arrival now can be found using simple zero-crossing technique (FIG. 8D) : R w (t) t =r dl =H(t-? d .) *R xy (r) =r dl =0 It is worthwhile to remember, that by shifting the window function H(t) to the locations of other envelope peaks τ d , i the accurate delay times of signals propagating through different paths may be automatically determined. A flowchart of a program in the processor 27 for automatically deriving the time delay is shown in FIG. 9 and includes shifting of the window ΔT, shown in FIG 8D, in several steps in order to find the searched time delay r, for the paper web 2. The algorithm consists of three main stages: cross- correlation envelope function fitting by 2nd order polynomial; finding the peaks; and finding their classification according to a sharpness. The algorithm starts from the window generation in the time domain. The width of 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 subsequent 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 by a random noise or spurious waves would be ignored. The cross-correlation envelope function fitting is needed for finding the peak and is performed by the least-square method using the 2nd order polynomial. Such a polynomial can have a positive or negative curvature depending on what kind of local extremity - a peak or a minimum has been found. Strictly speaking, 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 reduced by increasing the width of the window. Then the peaks caused by delayed waves are usually sharper than the other, spurious, peaks. Therefore, the peak finding procedure consists of the first order derivative calculation, which enables the determination of the locations of all extremities and the 2nd order derivative calculation, which allows sorting them into maximums and minimums and, consequently, selection of the proper peak (or peaks) according to its (or their) sharpness. The sharpness e is given by Tdi the magnitude of the 2nd derivative of the peak. The delay time estimate τ d - obtained from this peak is used to generate the window H(t) mentioned above. The Hubert transform of the cross-correlation function "~ xy (t) is multiplied by the windowing function H(t) . All these functions are discrete in the time domain. The spacing between two adjacent points is equal to the sampling period ώts. In order to obtain measurement errors less than the signal sampling interval d , the segment of the Hubert transform is fitted using the least-square method by the 5th order polynomial. Then the Equation has five roots, but only the root inside the created window is selected. This root is a fine time delay t,. estimation. The wave velocity v =l n /t,., and the tensile stiffness TSI=c 1 *v 0 2 , where c is a dimensionless constant close to 1 depending on Poisson's ratio for the paper. The flowchart in FIG 8 is believed to be self-explanatory and is therefore not described in further detail. It is necessary to point out that if the peak of the cross- correlation function caused by the s 0 -Lamb wave is the biggest, then the envelope function fitting can be omitted and the rough estimate of the peak delay obtained directly from the measured cross-correlation or envelope function. The other steps in the algorithm remain the same. The method above has been described for measurement of the time delay in the machine direction and this will give the tensile stiffness index TSI ™ ML). in the machine direction of the paper machine. The friction head 4, the microphones Ml, Mic2A and Mic2B are then located in line with the machine direction. However, as mentioned in the introductory part of the specification the tensile stiffness index TSI_ D in the cross direction of the paper machine, and in directions between TSI JQJ and TSI CD , are also needed in order " to calculate the anisotropy ratio and the tensile stiffness orientation. An embodiment for also providing these quantities will now be described with reference to FIGs 7B and 7C, even though the same feature naturally can also be provided for the embodiment shown in FIG 2. As is apparent from FIG 7B, several sets of microphones Mic3A, Mic3B; Mic4A, Mic4B etc are shown located parallel to each other and oblique to the microphone Ml in relation to the machine direction (the respective angular directions α N _ l r α N etc) , such that each microphone Mic3A, Mic4A is situated tangentially in the same location above the rotating cylinder 3 as the microphone Mic2A. The delay time of the symmetrical Lamb wave propagating in that oblique direction, Qfu- i , α N etc, is measured in the same way as described above for the ultrasonic Lamb wave propagation in the machine direction, account being taken 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 cylinder above the web so as to be put in different oblique positions, i.e. scanning along the line Cl. In this case, it is important to place the set of microphones, accurately in precise positions above the web (same distance to the web and along line Cl) in order to have the same measuring conditions for each measured oblique setting (not shown in a separate figure, however the pick-up microphone set will be placed in the same way as shown in FIG 7B) . Another embodiment shown in FIG 7C, has only one pick-up microphone set Mic2A', Mic2B' and moves, as a unit, friction head 4 and reference microphone Ml across the web, for instance along a straight line Fl parallel to the line Cl, as shown, and derives the delay time for the s 0 wave for a chosen amount of settings of the unit 4,Ml having different angular positions in relation to the pick-up microphone set. It is also possible to move the friction-head/microphone set 4, Ml along a curved line F2 (dashed), or to provide the velocity measurement along the machine direction separately and the measurements in the oblique directions along a line F3 (dot/dashed) perpendicular to the line Cl. It should be noted that, even for the embodiments having scanning 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 result. Many different kinds of numerical methods may be used to provide a quite precise guess about the s 0 wave rate in the cross direction of the paper web. One method is to fit the measured s 0 wave rates for the different oblique positions in some kind of periodic function, e.g. the function for an ellipse 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 s 0 wave has been measured in three different directions and these three different values are used for determining constants aO, al and bl. The constants are then inserted in the following formula: f(c-)=a0+al*cos2α+bl*sin2α (1) The estimated velocity is also dependent on formula 2: f(x)=kl*x+k2 (where x=f(α)max/f(α)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 s 0 wave velocity in the cross direction (α=90°) . v(CD)=f(x)*(a0-al) (3) By changing the constants kl and k2 it is possible to get the velocity in any direction from the formula 4: v(α,max/min)=(kl(α)*x+k2(α) )*(aθ+al*cos2α+bl*sin2α) (4) Another advantageous way to derive the velocity of the s 0 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 of the s 0 wave rates in a coordinate system, with the rate in the machine direction along the X-axis and the rate in the cross direction of the web along the Y-axis, in relation to the respecive angular deviation α N _ lf α N etc of each set to the machine direction in the way shown in FIG 7D. A curve is drawn through the different measuring results and extrapolated to cut the Y- axis in order to provide the velocity of the s 0 wave in the web in the cross direction. A small extrapolation error is unavoidable but is minimized 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 technique as shown in FIG 7D can be used also for the embodiments shown in FIG 7C. While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention as apparent from the Claims. In addition, modifications may be made without departing from the essential teachings of the invention. For instance, more than two pick-up microphones could be provided at the rotating cylinder.

Claims: We claim: 1. A system for measuring ultrasonically the elastic properties of a moving paper web, comprising: a. means (4) for generating a noise type ultrasonic sound wave in the web at an excitation point; b. reference means (Ml;5) for receiving contactlessly a reference ultrasonic wave reradiated into the air from the excitation point; c. pick-up means (M2;Mic2A,Mic2B;Mic3A,Mic3B) for receiving ultrasonic sound reradiated from the paper web a predetermined distance from the location of the ultrasonic sound generating means (4) , characterized in that the ultrasonic sound generating means (4) and the reference ultrasonic receiving means (Ml;5) are provided on the same side of the paper web; and that at least one of the two kind of means comprises a number of elements provided symmetrically in relation to the other means. 2. A system according to claim 1, characterized in that said number of elements is even and that the means having said number of elements have the elements arranged pairwise such that the elements in each pair have equal distances to an element of the other means. 3. A system according to claim 1 or 2, characterized in that it is the ultrasonic sound generating means (4) that comprises said number of elements; and that all these elements are provided at the side of a line through an ultrasonic sound receiving element (Ml;5) of the reference means directed in the machine direction of the moving paper web. 4. A system according to any one of the preceding claims, characterized in that the ultrasonic sound generating means (4) comprises at least two hard dry friction providing parts (6,7;8,9;11,12) in contact with the moving paper web, the contact dimension perpendicular to the direction of the moving paper web and the distance between the friction contact parts are much less than a wavelength of the ultrasonic wave (s 0 ) of interest to be indicated generated in the paper web; and that the ultrasonic receiving element (Ml) of the reference ultrasonic receiving means is located in between the dry friction providing parts. 5. A system according to claim 4, characterized in that the friction parts are made of hard alloy material, for instance wolfram carbide, and comprise tips, covered by a material absorbing ultrasonic waves, for example soft rubber, contacting the web. 6. A system according to any one of the preceding claims, characterized in that each friction providing element of the ultrasonic sound generating means on parts turned away from the paper web is covered by a noise shield at least turned towards the reference ultrasonic sound receiving means. 7. A system according to any one of the preceding " claims, characterized in that the ultrasonic pick-up receiving means (M2;Mic2A,Mic2B) is located above and close to a rotating cylinder (3) of the paper-making machine provided under the web and shifted along the direction of movement of the web at a predetermined location (Cl) downstreams from a line (C2) where the moving web the first time touches the cylinder surface for the first time; and in that a first noise shield (25;35) , for example a rubber shield, is located near to the pick-up means shielding the pick-up means from airborne noise, the outer edge of the first shield being located downstream outside a line (C2) where the moving web touches the cylinder for the first time, and the ultrasonic element(s) of the pick-up means is (are) placed close to an internal edge of the shield. 8. A system according to any one of the preceding claims, characterized in a second noise shield (26) is located somewhere between the ultrasonic sound generating means and the pick-up means for shielding the pick-up means from sound and dust generated from the ultrasonic sound generating means. 9. A system according to any one of the preceding claims, characterized in that the lateral dimensions of ultrasonic indicating elements (Ml;14-15;Mic2A,Mic2B) of all the ultrasonic receiving means are at least 10 times less than a wavelength of the ultrasonic wave in the paper web, and all the ultrasonic indicating parts of the receiving means are placed at a distance from the web which is less than a wavelength of the ultrasonic wave reradiated by the web into air. 10. A system according to any one of the preceding claims, characterized in that * the pick-up means comprises at least two adjacent pick¬ up ultrasonic receiving elements (Mic2A,Mic2B) to receive contactlessly the ultrasonic wave generated in the web by ultrasonic sound generating means and reradiated by the web; * processing means (28-31;28-29,36-39) to combine the outputs from the pick-up ultrasonic receiving elements; * computing means (27) for processing the outputs from the reference receiver and from the processing means, and determining the delay time between these outputs. 11. A system according to Claim 10, characterized in that the distance (1 ) between the adjacent pick-up elements (Mic2A,Mic2B) , 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. 12. A system according to any of the preceding Claims, characterized in that the receiving means (Ml,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, propagating in the web and the reradiation of the same wave at the pick-up elements in order to derive the Tensile Stiffness Index in the machine direction. 13. A system according to any of the preceding Claims, characterized in that the ultrasonic pick-up receiving means (M2;Mic2A,Mic2B) can be oriented obliquely in relation to the reference receiving means (Ml) at chosen angle(s) to the machine direction of the moving paper web, in order to obtain the time between generation of the ultrasound wave part of interest propagating in the 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. 14. A system according to claim 13, 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. 15. A system according to any one of the preceding Claims, characterized in that the computing means (27) determines the delay time as a zero-cross of the Hilbert transform of the cross-correlation function between the outputs of the reference receiving means and the pick-up receiving means, corresponding to the maximum value of the cross-correlation function. 16. A system according to claim 18, 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.

Inventors: KAZYS RYMANTAS J, STOLPE T PATRICK, KAZYS, RYMANTAS, J., STOLPE, T., PATRICK
Applicants: KAZYS RYMANTAS J, LORENTZEN & WETTRE AB, STOLPE T PATRICK, AB LORENTZEN & WETTRE, KAZYS, RYMANTAS, J., STOLPE, T., PATRICK
Application year: 1995
Main classification: G01N29
IPC codes: G01N29, G01N33