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United States Patent m
Pettersson et al.
 ARRANGEMENT FOR MEASURING
MECHANICAL PROPERTIES OF A FOIL
MATERIAL THROUGH USE OF AN
EXCITATION UNIT THAT INCLUDES A
 Inventors: Thorulf Pettersson; Jorma Anttila, both of Taby, Sweden
 Assignee: STFI, Stockholm, Sweden
 Appl. No.: 150,259
 Filed: Not. 10,1993
Related U.S. Application Data
 Continuation of Ser. No. 773,951, Jan. 3, 1992, abandoned.
 Foreign Application Priority Data
Mar. 30, 1990 [SE] Sweden 9001162-8
 Int. CI.* G01L 1/24
 U.S. CI 73/800; 73/159;
 Field of Search 73/159, 788, 849, 800,
73/838; 356/432 T, 429, 430, 431, 32, 373
 References Cited
U.S. PATENT DOCUMENTS
3,605,486 9/1971 Anderholm et al 73/788
4,246,793 1/1981 Fairand et al 73/628
4,622,853 11/1986 Leugers 73/597
4,632,561 12/1986 Rosencwaig et al 356/432 T
[li] Patent Number: 5,361,638  Date of Patent: Nov. 8, 1994
4,952,063 8/1990 Opsal et al 356/432
5,025,665 6/1991 Keyes, IV et al 73/159
Primary Examiner—Hezron E. Williams
Assistant Examiner—Nashmiya N. Ashraf
Attorney, Agent, or Firm—Burns, Doane, Swecker &
An arrangement for measuring mechanical properties of foil material (10), preferably paper. The arrangement includes a material excitation unit (12) and a material sensing detection unit (14). The two units are connected electrically to a common arithmetical unit (18). This unit is intended to register and convert electrical signals deriving from the two first mentioned units (12,14) in a manner to produce final signals which represent the material properties to be measured. The excitation unit (12) includes a laser which, through electromagnetic radiation, is intended to generate in the gaseous atmosphere surrounding the material local transient gas-pressure pulses within variable surface zones which are well-defined geometrically, without the excitation unit coming into contact with the material. These gas pressure pulses cause the material (10) to stretch locally in the boundary regions of the zones, such stretching of the material being necessary to the measuring process. The detection unit (14) senses this stretching of the material, without coming into contact with the material, by detecting transient changes in these material zones.
20 Claims, 2 Drawing Sheets
The present invention relates to an arrangement for measuring mechanical properties of foil material, e.g. paper, such as local strength and basis weight, with the aid of a material excitation unit which includes a laser, and also with the aid of a detection unit which senses 15 excited material. The units are connected electrically to an arithmetical unit which registers and converts electric signals which derive from the two first mentioned units and which represent the starting values of the measuring process, for establishing the values of the 20 final signals representative of the material properties to be measured.
BACKGROUND OF THE INVENTION
In paper manufacturing processes, it is very impor- 25 tant that the mechanical properties of the paper be determined continuously, arid then particularly the strength of the paper in different directions, i.e. the strength anisotropy of the paper. The strength anistropy of paper can be determined when the elastic constants 30 of the paper in different directions are known. It is known that these constants can be determined by subjecting the paper to static forces or to ultrasonic sound.
When the elastic constants are measured with the aid of ultrasonic sound, i.e. mechanical oscillations or vibra- 35 tions of very high frequencies, e.g. frequencies which exceed 20 kHz, there is utilized the fact that the speed at which the sound propagates in different directions in the material is associated with the elastic constants of the material. 40
The aforesaid mechanical properties are preferably measured on-line, i.e. directly on the paper web in the paper manufacturing process, while advancing the web continuously, although said properties can also be determined off-line, i.e. on paper samples in a laboratory. 45 Before it is possible to take on-line measurements, it is necessary to solve a number of complicated technical problems which are associated with the specific properties of the material being measured and with prevailing measuring and manufacturing conditions. In this re- 50 spect, it is necessary to take into account the relatively high speed of the paper web—up to 20 meters per second—and, for instance, the fact that movements are liable to occur in the paper web during its manufacture—web flutter—and that an intensive acoustic noise 55 is generated. Thus, this high noise level coupled with the difficulties associated with exciting the material with ultrasonic waves of sufficiently high energy levels makes on-line measuring of the technical properties of paper very difficult to carry-out with the aid of known 60 technology.
U.S. Pat. No. 4,291,577 describes an arrangement in which a contacting measuring device is used for ultrasound measuring purposes. This measuring device includes a transmitter in the form of a piezoelectric ele- 65 ment which generates mechanical oscillations of frequency 20 kHz. Longitudinal high-frequency waves are generated in the excited paper in this way, through the
contacting piezoelectric element, these waves propagating in different directions in the material in the plane of the material web. A receiver, which also consists of a contacting piezoelectric element, is located at a predetermined distance from the transmitter, thereby enabling the phase velocity of the ultrasonic sound to be calculated, this velocity being related to the modulus of elasticity of the paper. This arrangement, however, is highly sensitive to external influences, for instance the aforementioned noise, uncontrollable variations in the distance travelled by the web, etc. It will also be evident to all those skilled in this art that an arrangement which utilizes movable parts which are in direct contact, such as the arrangement illustrated in the aforesaid U.S. Pat. No. 4,291,577, will also incur other serious drawbacks. For instance, the known arrangement is relatively complicated, due to its construction, and consequently malfunctions are very likely to occur. Furthermore, it is not certain that the transmitter/receiver will remain in physical contact with the paper web at high web speeds and with paper webs of high surface roughness. The web-contacting parts of the arrangement are also liable to damage the paper.
It is also known to apply the principle of contactless registration of the propagation of ultrasonic sound waves for the purpose of measuring the mechanical strength of a stationary or a moving material. One such method is described, for instance, in Swedish Patent Application No. 8017/70 (Publication No. 359 962). The complicated relationships which prevail between the measured parameters and the elastic paper constants when practicing this method, and also the sensitivity of the method to uncontrollable variations in air flows adjacent the material web, make it difficult to apply this method in practice for on-line measuring processes.
It is also known to utilize bending waves, so-called Lamb's waves to indicate the thickness of and the faults in sheet-like or foil materials. Arrangements of this kind are described in U.S. Pat. Nos. 2,536,128 and 3,210,120, for instance. With these arrangements, energy from a radiation source is fed into sheet-like or foil material at a given angle of incidence, with the aid of a coupling liquid, thereby enabling the phase velocity of the bending wave to be measured. A method which is based on the use of a contact liquid is not suitable for use when measuring or determining the properties of, for instance, paper, for obvious reasons.
U.S. Pat. No. 4,180,324 teaches another method of measuring material properties by studying wave movements originating from an excitation location in the form of ultrasonic sound. Although this method can be used, in principle, for measuring the strength of foil material in a punctiform fashion, it can only be used to determine the strength of the material in its z-direction, i.e. a direction transversely through the material at right angles to the surface thereof. Another distinguishing feature when using the novel inventive arrangement, in addition to not studying the propagation of ultrasonic waves, is that the novel construction affords the possibility of measuring material strength locally in different directions in the plane of the material. This is achieved by utilizing the discovered changeable relationship between the generated macroscopic elastic extension or stretch of the material and the strength of the material in different directions, through geometric configuration and orientation of the excitation zones used.
Another known method described in U.S. Pat. No. 4,674,332 can, in principle, be configured for non-con