METHOD AND APPARATUS FOR MEASURING PROPERTIES OF PAPER

Abstract: The invention relates to a method and an apparatus for measuring the properties of paper from a moving paper web in a paper machine. An infrared beam (2) is directed to the paper web (4). The infrared beam (2), passed through or reflected from the paper web (4), is split into wavelength components with a spectrograph (5), and signals of the different wavelength components are measured, and the properties of paper are determined on the basis of the signals.

Description: METHOD AND APPARATUS FOR MEASURING PROPERTIES OF PAPER The invention relates to a method of measuring the properties of paper from a moving paper web in a paper machine, in which method measurement is carried out in the infrared region, most preferably in the wavelength range of 1 to 10 μm. The invention further relates to an apparatus for measuring the properties of paper on a moving paper web in a paper machine in the infrared region, most preferably in the wavelength range of 1 to 10 μm. US Patent 4,733,078 discloses an apparatus which directs a radia- tion beam to a target through a rotating filter wheel driven by a motor, the filters in the filter wheel emitting only a given wavelength of light at a time to the target. The light that has passed through the paper is detected at different wavelengths and the desired content of the substance is determined from the target on the basis of signals thus obtained. The problem in measuring a moving target is that the measurements of different wavelength channels are successive, since each filter illuminates a different point in the target. Before the following filter has had time to revolve in front of the light source, the target to be measured has slightly moved to a different point, and consequently the penetration point is not quite the same. In this case the proportional measurement, based on a reference wavelength, does not remove the grey factor variation caused by scattering, resulting in a significant error in measurements of non-homogenous, moving material, such as paper. US Patent 3,641 ,349 discloses an apparatus in which a light beam, having passed through the paper sheet, is split into several parts. Each differ- ent part of the beam is detected by a separate detector, in front of which are disposed narrow-band filters, the signals being measured in parallel. Parallel measurement eliminates measuring noise caused by a non-homogenous moving target, but the number of channels is limited in this solution. The solution becomes cumbersome in applications requiring more than four channels, and its optical efficiency declines as the number of channels increases. Furthermore, because of the filters, resolution and central wavelength tolerances are quite bad, complicating the transfer of calibration from one device to another. The filters used also make it difficult to reach a sufficiently narrow band for the right wavelength, e.g. for measuring kaolin. Still further, lead sulphide detectors are extremely dependent on temperature, a different temperature in detectors disposed at different points causing significant errors in measure- ment. WO 96/42010 discloses an apparatus for measuring the colour and colour-related properties of a moving web by reflectance measurements. This colour measurement utilizes a black and white background behind the web, the reflected light beams being measured by two different sensors. The sensor uses a spectrograph which distributes the light beam to the detectors. The measured signals are transmitted serially from the detectors to a computer for further processing. However, serial processing is not always sufficiently efficient. Moreover, the solution of the publication cannot be utilized in applica- tions in which the properties of paper are to be measured in the infrared region. It is an object of the present invention to provide a method and an apparatus which avoid the above drawbacks. The method of the invention is characterized by directing to the pa- per web an infrared beam modulated by a chopper, splitting the infrared beam which has passed through the paper web or reflected therefrom by a spectrograph into different wavelength components, measuring signals corresponding to the different wavelengths in parallel, detecting the signals using parallel detection synchronized to the chopper, and processing the signal in parallel and determining the properties of the paper on the basis of the wavelength components determined by the spectrograph. The apparatus of the invention is characterized by comprising a radiation source for producing an infrared beam, a chopper for modulating the infrared beam, a spectrograph for splitting the infrared beam reflected from or passed through the paper web into different wavelength components, means synchronized to the chopper for parallel detection and processing of the wavelength components, and means for determining the properties of the paper on the basis of the different wavelength components split by the spectrograph. An essential idea of the invention is to measure the properties of a moving paper web by directing an infrared beam modulated by a chopper to the paper, and splitting the beam which has passed through or reflected from the paper into different wavelengths by means of a spectrograph, measuring signals corresponding to the different wavelengths, detecting the signals using parallel detection synchronized to the chopper, and processing the signal in parallel. It is the idea of a preferred embodiment to split a light beam optically by a spectrograph into different wavelengths and to measure the signals of the different wavelengths from the spectrograph with separate detectors. The idea of another preferred embodiment is that the detectors employed are array detectors. It is an advantage of the invention that because all measuring channels and reference channels are measured exactly simultaneously, the desired statistical accuracy is promptly reached when a non-homogenous rapidly moving sample is measured. Furthermore, detecting and processing the beam in parallel and continuously allows a narrow electric pass band for detector electronics, improving the signal to noise ratio of the detection and making the detection more immune to ambient light interference. Furthermore, measuring time can be utilized to a hundred percent for each channel, which also improves the signal to noise ratio of the detection. A spectrograph provides a high number of channels, good resolution, low central wavelength tolerances and repeatability of the wavelength scale from one device to another. The solution is also applicable to different numbers of wavelengths and to measurements of a continuous spectrum. A high number of wavelengths allows the effect of a change in sample temperature on the measurement to be compensated for, accurate measurements of binders, measurements of different bind- ers, better immunity to interfering components and widening of the basis weight region of the paper to be measured by the use of different wavelength in different applications. In a device with a better wavelength resolution, reference channels can be brought nearer the measuring channel, resulting in e.g. better immunity to baseline bend. In an array detector the detector elements are in the same hermetic casing and of the same production lot, allowing a better immunity to be achieved to changes in ambient temperature than when several separate detectors are used. In the present application, in addition to paper, the term "paper" also refers to paperboard. The invention will be described in greater detail in the attached drawings, in which Figure 1 schematically shows an apparatus of the invention, and Figure 2 is a diagram of the spectrum of some paperboard grades. The figure shows a radiation source 1 producing an infrared beam 2. A blackbody radiator, for example, can be used as the radiation source. The infrared beam 2 is modulated by a chopper 3 to eliminate the effect of back- ground light, for example, in a manner known per se. The chopper 3 can be e.g. a rotating disc or some other chopper solution known per se. The structure of the chopper 3 is obvious to a person skilled in the art, and is therefore not dealt with in any greater detail herein. After the chopper 3, the infrared beam 2 is directed to a paper web 4 moving on a paper machine. The paper web moves in the direction of arrow A. The infrared beam 2, passed through the paper web 4, is led to a spectrograph 5. The attached figure shows a spectrograph structure, what is known as a grating spectrograph, in which a mirror 6 and a grate 7 are used to optically split the infrared beam 2 into different wavelength components. A grating spectrograph with no mirror can be used similarly. The different wavelength components are detected from the spectrograph by separate detectors. The wavelength used varies between 1 and 10 μm and the detection is synchronized to the chopper 3 in a manner known per se. As detectors are pref- erably used array detectors 8, in which the detector elements are in the same hermetic casing and are of the same production lot, whereby different temperatures cannot affect harmfully the detector elements disposed at different points. The detected signals of different wavelengths are led in parallel from the array detector 8 via a pre-amplifier 9 to a computer 10, where the meas- ured data is processed in a manner known per se. Most preferably the detectors detect a signal using lock-in detection. For the sake of clarity, in addition to the spectrograph 5, the attached figure does not show the optics needed for focusing and directing the infrared beam 2. The structure used to lead/control the beam can be e.g. imaging optics, optic fibre or an optic fibre bundle. The apparatus is preferably disposed in a measuring frame 11 , shown with a dashed line, comprising means for traversing the apparatus transversely with respect to the travel direction A of the web. This way the apparatus does not measure the entire width of the web at the same time, but operates on substantially the entire width of the web as a scanning measuring device. The spectrograph 5 easily enables e.g. a 24-channel structure, i.e. the infrared beam 2 can be split into 24 different wavelength components. Said technique allows structures with even more channels. An optically splitting spectrograph 5 and an array detector 8 are relatively simple and inexpen- sive components, making the apparatus of the invention extremely useful. The computer 10 is used to measure the properties of paper or pa- perboard, e.g. basis weight with cellulose as the tracer, dry weight of coating with e.g. kaolin, latex or calcium carbonate as the tracer, moisture and/or like magnitudes. Figure 2 shows the spectra of some paperboard grades. The dash- and-dot line shows the spectrum B of paperboard having room, i.e. 5 percent, moisture. The dashed line shows the spectrum C of wet paperboard, i.e. paperboard having a moisture content of 20 percent. The solid line shows the spectrum D of kaolin-coated paperboard. The horizontal axis shows wavelength λ as nanometres and the vertical axis shows absorbency. The solution of the invention allows the measuring range of Figure 2 to be divided into e.g. 24 parts as is shown in Figure 2. In this case the very narrow kaolin peak at the wavelength range λ 2i can be easily distinguished. At the wavelength range λ 5 the absorbencies of all qualities are substantially identical, making said wavelength an ideal reference measurement wavelength. Water has an ab- sorption peak at the wavelength range λ 10 shown e.g. in Figure 2, whereby the amount of water in the paperboard can be determined by measuring the absorption peak of said wavelength. The kaolin peak, in turn, appears at the wavelength range λ 2 ι, that of latex at the wavelength range λ 2 , and consequently said tracers are measured at the wavelength ranges λ 2 and λ 24 of Figure 2. In other words, the solution of the invention allows simultaneous determination of various paper properties swiftly and accurately, because the measurements at different wavelength ranges can be made simultaneously and because the measurement results are detected and processed in parallel. The drawing and the related specification are only intended to il- lustrate the idea of the invention. The details of the invention may vary within the scope of the claims. In other words, the spectrograph may also be arranged to process the different wavelength signals of the light beam by splitting the incoming light beam in an electric form into different wavelength signals, whereby the electric signals can be led directly to separate processing, without any need for separate detectors. Furthermore, the spectrograph can be placed on the same or opposite sides of the paper web with respect to the radiation source, allowing measurement of a light beam that is either reflected from the paper web or has passed through it. However, the solution of the invention provides extremely good results in measurements of a light beam which has passed the paper web even up to a basis weight of 600 g/m 2 .

Claims: CLAIMS 1. A method of measuring the properties of paper from a moving paper web in a paper machine, in which method measurement is carried out in the infrared region, most preferably in the wavelength range of 1 to 10 μm, characterized by directing to the paper web (4) an infrared beam (2) modulated by a chopper (3), splitting the infrared beam (2) which has passed through the paper web (4) or reflected therefrom by a spectrograph (5) into different wavelength components, measuring signals corresponding to the different wavelengths in parallel, detecting the signals using parallel detection synchronized to the chopper (3), and processing the signal in parallel and determining the properties of the paper on the basis of the wavelength components determined by the spectrograph (5). 2. A method as claimed in claim 1, characterized by splitting the infrared beam (2) optically by the spectrograph (5) into the different wavelength components and detecting the signals of the different wavelength components by detectors. 3. A method as claimed in claim 2, characterized in that array detectors (8) are used as detectors. 4. A method as claimed in claim 2 or 3, characterized by detecting the signal by the detector employing lock-in detection. 5. A method as claimed in any one of claims 2 to 4, character i z e d in that the spectrograph is a grating spectrograph. 6. A method as claimed in any one of the preceding claims, characterized by measuring moisture, basis weight or amount of coating of the paper web (4). 7. A method as claimed in any one of the preceding claims, characterized by measuring the infrared beam which has passed through the paper web (4). 8. An apparatus for measuring the properties of paper from a mov- ing paper web in a paper machine in the infrared region, most preferably in the wavelength range of 1 to 10 μm, characterized by comprising a radiation source (1) for producing an infrared beam (2), a chopper (3) for modulating the infrared beam (2), a spectrograph (5) for splitting the infrared beam (2) reflected from or passed through the paper web (4) into different wavelength components, means synchronized to the chopper (3) for parallel detection and processing of the wavelength components, and means for determining the properties of the paper on the basis of the different wavelength components split by the spectrograph (5). 9. An apparatus as claimed in claim 8, characterized in that the spectrograph (5) comprises at least a grate (7) for optically splitting the wavelength components of the infrared beam (2), and that the apparatus further comprises detectors for detecting the optically split wavelength components. 10. An apparatus as claimed in claim 9, characterized in that array detectors (8) are used as detectors. 11. An apparatus as claimed in claim 9 or 10, characterized in that the detectors employ lock-in detection. 12. An apparatus as claimed in any one of claims 8 to 11 , c h a r - acterized by being adapted to measure moisture, basis weight or amount of coating of the paper web (4). 13. An apparatus as claimed in any one of claims 8 to 12, c h a r - acterized by being adapted to measure the infrared beam (2) which has passed through the paper web (4).

Inventors: HYVAERINEN TIMO, KAENSAEKOSKI MARKKU, MAENTYLAE MARKKU, TENHUNEN JUSSI, HYVAERINEN, TIMO, KAENSAEKOSKI, MARKKU, MAENTYLAE, MARKKU, TENHUNEN, JUSSI
Applicants: HYVAERINEN TIMO, KAENSAEKOSKI MARKKU, MAENTYLAE MARKKU, TENHUNEN JUSSI, VALMET AUTOMATION INC, HYVAERINEN, TIMO, KAENSAEKOSKI, MARKKU, MAENTYLAE, MARKKU, TENHUNEN, JUSSI, VALMET AUTOMATION INC.
Application year: 1998
Main classification: G01N21
IPC codes: G01N21