Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-08-06
2003-04-08
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S655000, C073S657000, C073S159000
Reexamination Certificate
active
06543288
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to the determination of the elastic properties of a thin sheet and to the measurement by laser-ultrasonics of the tension applied to a thin sheet. This invention is particularly useful for paper making and printing.
DESCRIPTION OF PRIOR ART
There is currently a great interest in industry to measure the elastic properties of thin sheets, either on the manufacturing line or off-line and to evaluate the tension applied to these thin sheets during processing. These measurements are particularly important for papermaking and printing as well as for aluminum foil rolling and plastic sheet fabrication. Regarding papermaking and printing, adequate tension applied to the web is fundamental for avoiding breaks and insuring optimum runnability. The in-plane elastic modulus of a paper web and its variation with direction are also important parameters to measure for evaluating the paper web quality. They are usually measured off-line with an apparatus that measures a parameter related to the in-plane modulus and called the tensile stiffness index (TSI). More precisely the TSI in a given direction is defined as the square of the acoustic velocity in this direction. The variation of the TSI with direction and the direction of its maximum value, called the Tensile Stiffness Orientation parameter (TSO), are also evaluated with this off-line apparatus. The paper web compressibility is also of interest, especially for printing, and can be estimated by through-thickness elastic modulus measurements.
Regarding the measurement of the tension applied on a thin sheet, the conventional method is to measure the load the sheet causes on a roller. Consequently, only the mean value over the sheet width is accessible and local tension peaks cannot be measured. To evaluate the variation of tension across a sheet, particularly across a paper web, sensors based on an applied force, sound waves or applied pressure have been designed. These sensors do not usually meet all the requirements of industry to measure the tension on a paper web running at high speed, as reviewed by H. Linna and P. Moilanen in
Comparison of methods for measuring web tension, Tappi Journal
, vol. 71, p. 134-138 (1988). They have prohibitive characteristics such as being slow, invasive, and sensitive to temperature changes. Although piezoelectric transducers have been developed for on-machine measurements, they have the drawback to be based on a contact approach. A non-contact method to measure the tension applied to a paper web was proposed by M. Luukkala and T. Marttinen in U.S. Pat. No. 4, 833, 928 entitled Method and apparatus for noncontacting tension measurement in a flat foil and especially in a paper web. The method is based on the generation of a sound wave by a piezoelectric transducer or a microphone and the detection of this sound wave by a laser-based triangulation technique. Although an improvement over other methods, the method proposed by M. Luukkala et al. suffers from the low sensitivity of laser-based triangulation techniques to acoustic motion.
Regarding the measurement of the elastic properties, various techniques for measuring the elastic properties of a static or moving paper web were recently reviewed in a publication by P. H. Brodeur, M. A. Johnson, Y. H. Berthelot, J. P. Gerhardstein, entitled
Noncontact laser generation and detection of Lamb waves in paper
, published in the Journal of Pulp and Paper Science, 23, J238, (1997). We summarize hereinbelow the main elements of this review. Contact methods based on a mechanical sensor for force measurement, ultrasonic transducers for Lamb wave generation and detection and friction-induced noise generator and microphone detection have been developed. As mentioned before, contact methods are not suitable for on-line measurement on a paper web moving at high speed. Two non-contact approaches have been investigated. The first method is based on air-coupled transducers to generate and detect ultrasonic and sonic waves propagating across the web. The reliability of this method is limited by the effect of ultrasonic absorption in air, sensitivity to moisture and air turbulence. The second method relies on lasers to generate and detect ultrasonic and sonic Lamb waves propagating across the web. Laser measurements of the ultrasonic and sonic surface motion of the paper web can be performed by laser-based triangulation methods and by interferometric methods, the later being much more sensitive. These laser methods have been described in various patents and publications and are discussed below.
The generation of acoustic waves has been reported to be performed by a Nd:YAG laser operated at its fundamental wavelength in the near infrared or at its second harmonic in the visible in U.S. Pat. No. 4,622,853 by M. A. Leugers entitled Laser induced acoustic generation for sonic modulus and in U.S. Pat. No. 5,814,730 by P. H. Brodeur, Y. H. Berthelot, J. P. Gerhardstein, M. A. Johnson entitled Material characteristic testing method apparatus using interferometry to detect ultrasonic signals in a web and also in the publication of B. Pouet, E. Lafond, B. Pufahl, D. Bacher, P. Brodeur, M. B. Klein entitled On-machine characterization of moving paper using a photo-electromotive force laser ultrasonic method published in Proceeding of SPIE, vol. 3589, p. 160, (1999). The use of an ultraviolet nitrogen laser was reported in U.S. Pat. No. 4,674,332 by S. A. Pace, S. S. Salama entitled Laser induced acoustic generation for sonic modulus. These two lasers, Nd-YAG and N
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produce strongly fluctuating measurements from laser shot to laser shot and tend to induce damage on the paper web, as recognized by Y. H. Berthelot and M. A. Johnson in Laser ultrasonics in copy paper, Optical Engineering, vol. 36, pp. 408-416, (1997). In U.S. Pat. No. 5,025,665 entitled Non-contacting on-line paper strength measuring system, M. A. Keyes IV, W. L. Thompson considered the use of an infrared CO
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laser (around a wavelength of 10 &mgr;m) with very long pulses, but this disclosure failed to indicate and discuss why this wavelength could be more appropriate for damage free generation of ultrasound in paper.
Regarding the detection of laser generated ultrasound, several methods were proposed in the patents and publications mentioned above. Detection can be performed by using a contact piezoelectric transducer or a microphone as proposed by M. A. Leugers in U.S. Pat. No. 4,622,853 and S. A. Pace, S. S. Salama in U.S. Pat. No. 4,674,332, or by using non-contact laser-based methods. As already mentioned, contact methods are not suitable for on-line measurements on a moving paper web. The non-contact methods proposed are either based on laser triangulation (M. A. Keyes IV, W.L. Thompson in U.S. Pat. No. 5,025,665), speckle sensitive interferometric heterodyne detection of the phase modulation produced by the ultrasonic motion of the paper web (P. H. Brodeur, Y. H. Berthelot, J. P. Gerhardstein, M. A. Johnson in U.S. Pat. No. 5,814,730) or a speckle insensitive photo-electromotive force based demodulator (B. Pouet et al, publication mentioned above). The heterodyne interferometric method used is sensitive to the speckle structure of the collected light, which results in a strongly fluctuating light level collected from the surface and a very large increase of the intensity noise seen by the optical detector. The speckle insensitive photo-electromotive force can be a basis of a laser-ultrasonic system for paper characterization, although the data reported with this device appears very noisy when the sheet is in rapid motion. Data is also only reported for the cross-machine direction. In the machine direction the Doppler effect produced by the sheet motion will decrease even more the signal-to-noise ratio.
In conclusion, all the previously described methods and techniques to either measure the tension or the mechanical properties of a sheet in motion (in particular a paper web) have drawbacks which limit their practical use. We have realized t
Blouin Alain
Monchalin Jean-Pierre
Reid Benoit
(Marks & Clerk)
Larkin Daniel S.
Miller Rose M.
National Research Council of Canada
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