Measuring and testing – Vibration – By mechanical waves
Patent
1997-03-26
1998-09-15
Williams, Hezron E.
Measuring and testing
Vibration
By mechanical waves
73159, 73644, 364550, G01N 2918, G01N 2924
Patent
active
058081992
DESCRIPTION:
BRIEF SUMMARY
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: properties in-plane of the paper sheet, machine, machine.
It is possible to determine these quantities and also the anisotropy ratio TSI.sub.MD /TSI.sub.CD 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 "Emodulus") for other materials. The relationship can be expressed by: the ultrasonic pulse, and c is a dimensionless 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, that 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.
A method and device for continuously determining the modulus of elasticity of advancing flexible material, such as paper web, in a contactless fashion is disclosed in WO91/17435. An ultrasonic wave train is transmitted through the air towards the web. The ultrasonic
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Kazys Rymantas J.
Stolpe T. Patrick
AB Lorentzen & Wettre
Miller Rose M.
Williams Hezron E.
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