Optics: measuring and testing – By monitoring of webs or thread
Reexamination Certificate
2000-03-24
2004-04-06
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By monitoring of webs or thread
C250S559090
Reexamination Certificate
active
06717675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a system and a method for determining the orientation of fibers in fibrous material webs, particularly paper webs.
2. Discussion of Background Information
The article “High-Resolution Fibre Orientation and Basis Weight Measurement” by B. Drouin et al. in the
Journal of Pulp and Paper Science
, Vol. 22, No. 7, Jul. 7, 1996 mentions an instrument with which the fiber orientation in paper is measured by a transmission measurement with a rotating plane of polarization. The instrument is based on filtered black body radiation in the far infrared region (FIR) of the electromagnetic spectrum. Some disadvantages of this conventional device are its high cost and the fact that it works with only one wavelength.
SUMMARY OF THE INVENTION
The invention provides for a system and a method of the type initially described, in which the fiber orientation can be determined more efficiently and with high precision.
The invention utilizes at least one source of electromagnetic radiation disposed on one side of a fibrous material web and at least one sensor for receiving the radiation emitted by the source disposed on the other side. The radiation from the source penetrates the fibrous material from one side and is sensed on the other side. Moreover, at least one optical device, for influencing the propagation of the radiation as a function of its polarization properties, can be positioned in the path of the radiation, e.g., between the source and the sensor.
It has been discovered that the interaction between electromagnetic radiation and fibrous material webs having a homogeneous or at least dominant or prevalent fiber orientation, can be used to obtain information from which the fiber orientation can be determined. In principle, any desired wavelengths of the radiation can be used. However, wavelengths in the realm of visible light and/or infrared radiation are preferred. Moreover, NIR (Near Infrared Radiation) is most preferred. Additionally, other wavelengths may be utilized or required at the same time so that the system can cope with the differing ash content and/or other variations of the paper properties.
The optical system of the invention utilizes radiation whose propagation has been influenced to produce linearly polarized radiation. The invention may use completely unpolarized radiation from various sources, such as a natural light source, partially unpolarized light, or unpolarized radiation. This radiation which is emitted from the source can then be linearly polarized prior to entering and/or interaction with the fibrous material web. In this case, the optical system then serves as a polarizer. Alternatively, the radiation which is emitted from the source can then be linearly polarized after passing through and/or interaction with the fibrous material web. In this case, the optical system is used as an analyzer. Such a design allows the direction of polarization of linearly polarized radiation to be detected after the radiation has passed through the fibrous material web.
The invention takes advantage of the fact that the propagation of linearly polarized radiation is influenced to the extent that the intensity of the linearly polarized radiation, that the optical device permits to pass, is a function of its direction of polarization.
In at least one embodiment of the invention, the optical system includes at least one polarizing filter. With filters of this type, it is possible both to produce linearly polarized radiation and to determine the polarization direction of linearly polarized radiation. Moreover, the use of such a polarizing filter permits the implementation of many different arrangements, all of which are characterized by comparatively simple construction in terms of measurement technology as well as a high degree of measuring precision.
Thus, it is possible to arrange a system having a single optical device in the form of a polarizing filter located between the source and the fibrous material web. The invention provides for the intensity of the linearly polarized radiation, which is produced by the polarizing filter and penetrates the fibrous material web, to be measured by sensor changes as a function of the orientation of the polarizing filter relative to the orientation of the fibers in the fibrous material web. Utilizing this technique, the fiber orientation can be determined in a comparatively simple manner by utilizing repeated measurements of intensity at different orientations of the polarizing filter relative to the fibrous material web. Such a system design can utilize a polarizing filter which is mounted such that it can rotate about an axis running perpendicular to the fibrous material web running direction.
The system functions as follows: It is assumed that a measured light intensity depends on the main fiber orientation in the paper and on the orientation of the polarization filters. The measured light intensity will be highest if the polarization filter and the fibers are oriented in the same direction (at 0° and 180°). The measured light intensity will be lowest in the orthogonal directions (at 90° and 270°). It is further assumed that the intensity distribution in polar coordinates has therefore about an elliptical shape: the largest diameter at 0° (a), and the smallest diameter at 180° (b).
If the difference a−b is high, the fibers are very strongly oriented only in one direction. However, if the difference is small, there is only a very small orientation or an almost equal distribution of the fibers.
Accordingly, if the highest signal is attained, when the orientation of the polarization filter corresponds exactly to machine direction of the paper, the fiber orientation is 0° relative to the machine direction. Otherwise, the fibers are not oriented properly in the machine direction. Acceptable or desired values are between approximately −2° and approximately +2°, while unacceptable or undesired values are larger than approximately 10°.
Thus, an algorithm may be utilized which has two parts: the data of the ellipse, and the relationship or how this data relates to paper properties. Accordingly, in order to calculate the orientation/shape of the ellipse, at least (3) three measurements have to be taken (three signals with polarizing filters in three different orientations). This can be performed using the usual quadratic equations which use the known properties of the ellipses.
In order to relate these data to the paper properties, empirical solutions are required. The signals a and b (and/or algorithmical combinations of these signals like a−b, a+b, and a/b), and the orientation angle of the ellipses are compared to lab tests. Thus, one can use e.g., Least Squares methods like “Partial Least Squares” (or other similar methods) in order to derive formulas to derive paper properties from the characteristics of the ellipse.
On the other hand, breaking load ellipses are something similar. Sample paper strips are typically taken having three different angles relative to the machine direction (i.e., −30°, 30°, and 0°). From these three measurement values, an ellipse is calculated. Accordingly, the ellipse reflects the strength properties of the paper. The strength is greatest in the direction of the main fiber orientation and lowest in a direction orthogonal to it. Thus, the strength properties in different angles serve as an indication of the fiber orientation. Unfortunately, this technique cannot be performed on line as it requires that the paper samples or sections be removed or cut from the web.
Accordingly, the system may utilize several fixed polarizing filters which have defined orientations or polarization directions when in the measurement position instead of one or more movable polarizing filters. This design allows the filters to be exchanged quickly. Moreover, the filters may be arranged in a configuration known as a filter wheel.
According to another variant of the invention, a single polarizing filter is positioned between the fi
Greenblum & Bernstein P.L.C.
Smith Zandra V.
Voith Sulzer Papiertechnik Patent GmbH
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