Laser-ultrasonic measurement of elastic properties of a thin sheet and of tension applied thereon
First Claim
1. A method for non-contact characterization of a rapidly moving sheet, comprising the steps of:
- illuminating the sheet at a first location with a first pulsed laser beam for generating acoustic waves propagating through the sheet thickness and over its surface, illuminating the sheet at a second location with a second laser beam, on the same side or the opposite side of the sheet, this second location being spaced from the first location by a predetermined distance, said sheet scattering the second beam to produce a scattered beam with a plurality of speckles, collecting the scattered beam, such collected scattered beam having a phase or frequency modulation representative of the acoustic motion at the second location, and demodulating the phase or frequency modulation to provide an electrical signal representative of the acoustic motion at the second location, and analyzing the electrical signal to determine a characteristic or a set of characteristics of the sheet, characterized in that said first laser has a wavelength selected to ensure progressive absorption below the surface of the sheet so as to produce a distributed acoustic source below the surface and an intensity below a threshold value to avoid damage to the sheet, and said demodulating of the phase or frequency modulation is carried out with a speckle-insensitive interferometer.
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Accused Products
Abstract
A method and an apparatus for non-contact and non-invasive characterization of a moving thin sheet and in particular of a paper web on a production line. The method uses a laser for the generation of sonic and ultrasonic waves in the thin sheet and a speckle insensitive interferometric device for the detection of these waves. The generation is performed in conditions to avoid damage impeding further use of the sheet. When the generation and detection spots overlap, the method provides a measurement of the compression modulus. When the generation and detection spots are separated by a known distance and plate waves (Lamb waves) are generated and detected, the method provides a measurement of the in-plane modulus and of the tension applied to the sheet. By detecting waves propagating in various directions, either by rotating the detection sensor head or multiplexing the signals provided by several detection or generation locations, the anisotropy of the in-plane modulus is determined.
39 Citations
43 Claims
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1. A method for non-contact characterization of a rapidly moving sheet, comprising the steps of:
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illuminating the sheet at a first location with a first pulsed laser beam for generating acoustic waves propagating through the sheet thickness and over its surface, illuminating the sheet at a second location with a second laser beam, on the same side or the opposite side of the sheet, this second location being spaced from the first location by a predetermined distance, said sheet scattering the second beam to produce a scattered beam with a plurality of speckles, collecting the scattered beam, such collected scattered beam having a phase or frequency modulation representative of the acoustic motion at the second location, and demodulating the phase or frequency modulation to provide an electrical signal representative of the acoustic motion at the second location, and analyzing the electrical signal to determine a characteristic or a set of characteristics of the sheet, characterized in that said first laser has a wavelength selected to ensure progressive absorption below the surface of the sheet so as to produce a distributed acoustic source below the surface and an intensity below a threshold value to avoid damage to the sheet, and said demodulating of the phase or frequency modulation is carried out with a speckle-insensitive interferometer. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
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20. An apparatus for non-contact and non-invasive characterization of a rapidly moving sheet, comprising:
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a first means for illuminating a first location of the sheet with a first laser beam and for generating acoustic waves propagating through the sheet thickness and over its surface, a second means for illuminating a second location of the sheet with a second laser beam, on the same side or the opposite side, the second location being spaced from the first location by a predetermined distance, a third means for collecting the scattered beam, such collected light having a phase or frequency modulation representative of the acoustic motion at this second location, means for demodulating the phase or frequency modulation so as to provide an electrical signal representative of the acoustic motion at this second location, and analyzing means for analyzing the electrical signal so as to determine a characteristic or a set of characteristics of the rapidly moving sheet, characterized in that said first means has a wavelength selected to ensure progressive absorption below the surface of the sheet so as to produce a distributed acoustic source below the surface and an intensity below a threshold value to avoid damage to the sheetd, and said demodulating means comprises speckle-insensitive interferometer. - View Dependent Claims (21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43)
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Specification