FI97571C - Paper web moisture sensor and method for adjusting web moisture - Google Patents

Description

97571

Paper web moisture sensor and method for adjusting web moisture 5 This application is divided by application 880728.

The present invention relates to a moisture sensor for a paper web and to a method for controlling the moisture of a paper sheet by measuring the transmittance of infrared rays of different wavelengths applied to the sheet.

In commercial papermaking, paper is produced as a continuously moving sheet called a "paper web." Because the paper web is made from a liquid suspension containing wood pulp fibers, cotton fibers, and various chemicals, the paper web initially contains a significant amount of moisture. Most of this moisture is removed during the papermaking process. However, for many reasons, it is desirable that at least some 20 moisture remain in the paper web. For example, if the paper web is too dry, the edges of the paper tend to curl.

The paper web is normally dried by passing it through hot drying drums. However, this technique-dried paper web dries unevenly. Such uneven drying results in uneven paper quality. Various devices have therefore been developed for the controlled wetting or drying of certain parts of a paper web after the paper web has passed through dryers in order to produce a paper web with a uniform moisture content. Clearly, the paper mill operator or paper mill process control computer must be familiar with the moisture profile of the paper web before such wetting and drying devices can be used effectively. For this reason, 35 paper web moisture sensors have been developed that swipe back and forth in the width direction of the paper web, determining the moisture of the web at various points.

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Water absorbs infrared radiation. Certain types of humidity sensors take advantage of this phenomenon by directing an infrared beam toward the paper web and measuring the intensity of the infrared beam after passing through the paper web. The more moisture there is in the paper web, the greater the absorption of infrared radiation.

Certain infrared humidity sensors utilize infrared detectors with an infrared-10 bandpass filter in front of each detector. The bandpass of each filter is selected so that each detector receives energy only for a small area of the infrared spectrum. One filter is selected so that it transmits infrared radiation in an area where the water absorption of the paper web is high. The detector associated with this filter-15 is therefore mainly sensitive to water in the paper web. The first detector receives more infrared radiation when the paper web is dry, and less infrared radiation when the paper web is wet.

The bandpass filter associated with the second detector is selected to transmit infrared radiation in an area of the infrared spectrum where there is relatively little absorption by moisture. Most of the infrared absorption in this second region of the infrared spectrum is due to the fibers of the paper web, not the moisture of the paper web. It follows that as the fiber weight per unit area (i.e., "pin weight") increases, this second detector receives less infrared radiation. The output of this second detector can be used to compensate for the moisture-30 measurements of the first detector with respect to changes in the basis weight of the paper sheet. When the outputs of the two detectors are combined correctly, these types of moisture sensors can be used to measure the moisture content of the paper web or the percentage moisture of the paper web so that changes in the basis weight of the sheet 35 do not affect the moisture measurement.

However, the intensity of the transmitted infrared beam does not only depend on the moisture content and surface weight of the paper web. The infrared absorption of a wet paper web also varies with the wavelengths of the infrared rays. Water and the fibers of the paper web absorb certain wavelengths of the infrared spectrum more efficiently than other wavelengths, so there are absorption peaks and valleys at different wavelengths in the infrared absorption spectrum. These peaks and declines shift to short wavelengths as the paper web temperature rises and to longer wavelengths as the paper web temperature decreases. However, the infra-10 red humidity sensors described above are not able to compensate for variations in the infrared absorption spectrum caused by temperature changes in the paper web. The humidity measurements of these devices therefore involve a significant error.

The present invention comprises an apparatus for determining the moisture content of a paper web by measuring the infrared transmittance of the paper web in two different wavelength ranges of the infrared frequency range, and a method for adjusting the moisture of the web based on such measurement. The apparatus and method are primarily, but not exclusively, for the continuous measurement and control of moisture in a moving paper web of a paper machine. The moisture sensor of the present invention can in this case be allowed to be swept in the width direction of the reciprocating paper web (i.e., in the transverse direction of the paper web) so that the moisture of the paper web could be measured at different points in the width and length of the paper web. The message process connections and computer programs associated with this sensor automatically compensate for the effects of moisture measurement based on the infrared transmittance of the paper web, as well as changes in basis weight when the sensor is insensitive to the paper web temperature. The features of the moisture sensor and the web moisture control method according to the invention appear from the appended claims.

The infrared radiation humidity sensor of the present invention includes an infrared radiation source. From this infrared radiation source, an infrared beam is transmitted through a moving paper web.

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As the beam passes through the paper web, the water and fibers of the paper web absorb some of the energy of the infrared rays. The infrared beam contains a wide range of wavelengths. However, the moisture-containing paper web primarily absorbs 5 infrared rays of certain wavelengths.

The humidity sensor also includes an infrared receiver. This receiver is located on the opposite side of the paper web when viewed from an infrared source and measures the infrared spectrum of the transmitted infra-10 red bundle in two separate wavelength ranges. The receiver comprises a beam splitter and two infrared detectors. The beam splitter directs a portion of the infrared beam to each detector. A separate bandpass filter is located before the detector. Each of the in-15 infrared detectors thus measures only a portion of the transmitted beam that falls within the bandpass range of that filter.

One of the infrared bandpass filters transmits only 20 infrared radiation with wavelengths around a water absorption peak of about 1.93 microns. The output of the detector associated with this first filter therefore depends primarily on the moisture content of the paper web.

25 The second bandpass filter passes only wavelengths in a region that primarily corresponds to the absorption region of the paper fibers. The infrared beam measured by the detector behind this second filter primarily provides an indication of the basis weight of the paper web. The bandpass range of this second filter 30 may be in the range corresponding to the infrared transmission of the paper fibers, e.g., about 1.83-1.85 microns. By setting the bandpass range of this filter to this transmittance range, variations in infrared intensity measurements caused by temperature variations can be minimized. As will be seen below, the output of this detector is used to correct the moisture measurements of the first detector for variations in the basis weight of the sheet. The present invention thus provides a message, 5 97571, which provides information on the moisture content of a paper web in spite of changes in basis weight and essentially regardless of the temperature of the paper web.

According to the present invention, the outputs of the two detectors are combined by means of an empirically determined formula so as to obtain an output which gives an indication of the moisture content of the paper web. As described above, the output of the detectors of the second receiver of the present invention will depend primarily on the amount of moisture in the paper web between the infrared beam source and the receiver. Therefore, this detector alone cannot give correct information about the percentage moisture of the paper web, as the detector receives less infrared radiation when (1) the percentage moisture li-15 increases or (2) when the percentage moisture of the paper web remains unchanged but the basis weight of the paper web increases. By monitoring the printout of only one of these detectors, the paper mill operator (or paper mill process control computer) is unable to distinguish between the increase in sheet moisture 20 and the increase in basis weight. However, the filters included in the second detector are selected so that the output of the detector varies mainly according to surface weight variations. By combining the output of the first detector according to the weighted average with the outputs of the second detector, the resulting combination can be used to estimate the moisture content of the paper web so that this estimate is independent of both paper web temperature and basis weight variations.

The actions of the formula used to combine the detector outputs depend, for example, on the exact locations of the bandpass filters in the absorption spectrum and the width of these bandpass regions. The exact values of these coefficients can be determined empirically by measuring the outputs of each detector at a variety of surface pressures, after which 35 coefficients are selected for a formula that combines these outputs to minimize the temperature sensitivity of the resulting weighted average.

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Instead of utilizing two separate infrared detectors and a beam splitter, the receiver side of the humidity sensor may consist of a single infrared detector and a mechanism by which each of the band-pass sensors described above is in turn placed in front of a single detector. This configuration determines the output of the detector at different times, which correspond to the times that each filter is in the path of the infrared beam. The outputs of the detector corresponding to the times each filter is located in front of the detector are combined in a manner similar to the two detector embodiment described above to obtain a moisture measurement sensor that is not sensitive to paper web temperature changes.

Figure 1 shows a humidity sensor according to the present invention which is not sensitive to the temperature of the paper web.

Figure 2 shows the infrared transmission spectrum of a moisture-containing, medium weight paper web.

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Figure 3 shows the infrared transmission spectrum of a wet, heavy paper web.

The following is a description of the presently preferred practice of using the invention. The purpose of the description is to illustrate the general principles of the invention at the same time without, however, limiting the invention. The object of the invention is best understood from the appended claims. 1 2 3 4 5 6 The present invention comprises an infrared humidity sensor 10 shown in Figure 1. This device is used to measure the moisture of the paper web 3 12 and to automatically compensate for this measurement with respect to variations in the basis weight of the paper web 12. The device shown in Fig. 1 5 uses a sensor 14, 6 to carry out the measurement, which can be imagined to consist of two parts, a transmitter part 16 and a receiver part 18, located on opposite sides of the paper web 12.

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The infrared source 16 uses an incandescent lamp 20 and an oval reflector 22 to guide the infrared beam 24 through the moving paper web 12. It is preferred, but not essential to the invention, that the amount of infrared radiation 5 from the source 16 and the radiation to the paper web 12 be modulated to a certain frequency. This modulation can be implemented with any available device. For example, as shown in Figure 1, the tips 28 of the voice iron 26 may be placed in the path of the infrared beam 24. The oscillating tips 28 10 modulate the infrared beam 24 as the tips 28 move either in front of or out of the path of the beam 24. Alternatively, a transparent plate (not shown) having a plurality of evenly spaced radial slits may be used and rotated through the path of the beam so that the beam either passes through the slits or impinges on the transparent portion of the plate. In the case of both devices, the beam 24 is modulated with a known frequency. The reason for modulating the beam is explained below.

In the embodiment shown 20 in Figure 1 is preferred, but not essential for the invention that the beam can be reflected back and forth in the sensor 14, the source-side 16 and the on-probe-side range 18 before it arrives at the receiver 18, the infrared beam 24 repeated reflection of the source in the range of 16 to 25 and the receiver 18 so that the the beam passes through the paper web 12 numerous times, providing certain advantages in measuring the moisture of very light papers, such as tissue paper, and very heavy grades of paper. The advantages of multiple reflection are discussed in more detail in U.S. Patent No. 3,793,524, incorporated herein by reference, which is incorporated herein by reference. Alternatively, the beam 24 may be directed directly as a single beam from the source 16 to the receiver 18 so that the beam 24 passes through the paper web 12 only once.

The receiver part 18 of the present invention comprises a beam splitter 30 which divides the beam 24 from the infrared source 16 into two beams, 32 and 34. Each beam is applied to separate bandpass filters 40 and 42 located on the path of each of the two beams and immediately before the lead sulfide. 38. Each filter 40 and 42 is selected to transmit a portion of infrared radiation in a portion of the infrared band. Radiation outside the bandpass range of filters 40 and 42 is reflected through these filters back to beam splitter 30 (shown in reference numerals 44 and 45 in the figure) and thus does not reach these reference or measurement sensors 36 and 10 38. Thus, in the humidity sensor of this invention a single beam is applied to receiver 18. the optics divide the beam into two separate beam 32 and 34, each measured at a different infrared detector 36 and 38.

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The bandpass range of each filter 40, 42 is selected so that the wavelength of the radiation entering each detector 36, 38 and thus the detector message also depend on the basis weight of the paper web and the moisture content of the paper web. Using these two messages and the equations described below, the process control computer 50 periodically calculates the moisture of the paper web 12. In addition, when the sensor 14 is allowed to swipe back and forth over the movable paper web 12 produced in the paper mill, the device of the present invention can generate messages that indicate the moisture of the paper web 12 at various points along its length and width. These moisture measurements can be utilized in the selective control of various known devices 51 to increase and decrease the moisture of various parts of the paper web 12, thereby obtaining a paper web 12 having a uniform moisture in both the width and length directions.

Based on the results of the humidity measurements, the humidity correction can be performed manually. However, several modern paper-35 mills are highly automated. In these mills, messages from sensor 18 of the present invention are fed to a centralized process control computer 50, which calculates the moisture profile of the paper web based on messages received from infrared sensors 9, 97571, 36, 38 and, depending on the result, selectively activates one of a number of known devices 51 to change paper. the moisture content of the parts. Several devices can be used to change the moisture profile of the paper web, including devices such as selectively controlled water jets to increase the humidity of certain portions of the paper web 12 and infrared heaters to selectively dry certain portions of the paper web 12.

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Figures 2 and 3 show the infrared transmission spectra of the moisture-containing medium and heavy weight paper webs 101. As best shown in Figure 2, the absorption of infrared radiation at certain wavelengths is more efficient than at other wavelengths. The absorption at shorter wavelengths of the spectrum, i.e. less than about 1.9 microns, depends mainly on the absorption of the fibers of the paper web. In contrast, at longer wavelengths of the spectrum, greater than about 1.9 microns, water absorbs infrared radiation more efficiently than paper web-20 nan fibers. The moisture content of the paper web thus mainly affects the part of the spectrum consisting of longer wavelengths. For example, a water absorption peak (corresponding to the lowest infrared transmission) is reached at a wavelength of about 1.93 microns. However, the water in the paper web absorbs 25 over the entire infrared spectrum, at least to some extent.

The infrared absorption spectrum of water and paper is special in that the absorption properties of the whole spectrum shift to shorter wavelengths as the paper web temperature rises and to longer wavelengths as the paper web temperature decreases. The infrared spectrum of a wet sheet of paper at high temperatures is illustrated by dashed line 102 in Figures 2 and 3. The continuous line in these figures depicts the same absorption properties, but at longer wavelengths because the paper web temperature is lower than the spectrum depicted by dashed line 102.

97571 The technique of the present invention, infrared bandpass filter 42 (Figure 1) and detector 38 have a bandpass area 112 around the absorption peak 106 (1.93 microns). In this case, the message of the measuring detector 3 8 depends to a large extent on the humidity of the paper -5 web 12, but is substantially independent of the web temperature. The detector message (hereinafter referred to as the MES message) gives a rough estimate of the moisture content of the paper web.

As mentioned above, the basis weight of the paper web 12 also affects the infrared absorption spectrum. In order to obtain a message which is mainly dependent on the basis weight of the paper web 12, a bandpass filter 40 is placed before the comparison message detector 36. The bandpass area 110 (Figs. 2 and 3) of this filter 40 is located in an area of the infrared transmission spectrum 101 where the paper web fibers are mainly absorbed. . For example, in this preferred embodiment, the bandpass filter 40 has a bandpass range of wavelengths between about 1.7 and 1.9 microns. A detector 36 associated with this filter 20, called a reference message detector, provides a print whose amplitude depends primarily on the mass of paper fibers per unit area of the paper web 12 through which the infrared beam 24 passes. As the basis weight of the paper web increases, the amount of infrared radiation passing through the paper-25 web 12 decreases. The message received from this reference detector 36 (hereinafter referred to as a "REF" message), as described below, is used to compensate for the measured moisture value with respect to changes in the basis weight of the paper web 12. It is essential that the bandpass region of the filter 30 40 be located in a portion of the infrared absorption spectrum that is predominantly affected by the paper web fibers (i.e., less than 1.9 microns) but is not sensitive to paper web temperature variations, e.g., near the transmission peak 107 shown in Figure 2-3. about 1.83-1.85 mik-35 rons.

As previously mentioned, the tips of the voice iron 26 modulate the infrared energy of the infrared source 16 into a sinusoidal shape.

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The output of each of the infrared detectors 36, 38 is thus modulated sinusoidally at the same frequency and in the same phase as the measured infrared beams 32, 34. The paper web 12 itself and other external sources (not shown in Fig. 5a) also provide infrared energy to the detectors 36, 38. component.

The outputs of both detectors 36, 38 are passed to a message processing circuit 52. This circuit is designed 10 to filter out the DC component from the detector messages.

The filtered detector messages are passed to a phase synchronizing demodulation circuit included in the message processing circuit 52. The purpose of the phase synchronizing demodulator is to filter out changes in the messages of the detectors 36, 38 that are not due to varying infrared absorption of the paper web 12. For example, as a 60 Hz line hum in detector messages, it is filtered out in the demodulator circuit, as described below.

The signal wave oscillator 54 drives the tips of the iron 26 at the resonant frequency of the iron 28. The output of this signal wave oscillator 54 is first converted into a rectangular wave having the same frequency and phase as the signal waves using the sound iron 26. The rectangular wave output 25 53 is fed to the phase synchronizing demodulator portion of the message processing circuit 52 along with the messages received from each of the infrared detectors 36, 38. The messages of the infrared detectors 36, 38 are, of course, modulated at the same frequency and in the same phase as the output of the oscillator 54. By demodulating 30 the outputs of each detector 36, 38 with a rectangular wave having the same frequency and phase as the output of the sonic iron oscillator 54, and averaging the multiple demolished outputs, the humidity sensor 10 of the present invention filters out changes in detector messages due to external infrared intensity changes or external messages such as 60 Hz mains voltage. This filtering technique using a phase synchronizing demodulation circuit is well known 12,97571. Of course, changes in the intensity of infrared energy from external sources or mains voltage to the detectors 36, 38 would result in erroneous humidity measurement.

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The average amplitude of each demodulated message from each detector provides an indication of the amount of infrared radiation passing through the paper web within the bandpass range of the filter associated with each detector. The amplitudes of the average amplitudes and the demodulated detector messages are measured in the message processing circuit, digitized, and input to a process control computer 50. This computer 50 calculates the moisture of the paper web 12 using the equations and methods described below. The moisture value of the paper web obtained as a result of the calculation gives an indication of the moisture of the paper web 12 regardless of the temperature and surface weight of the paper web.

The computer combines the digitized detector amplitude messages 20 to obtain a moisture indicating printing of the paper web 12 according to one of several equations. These equations combine the values indicating the amplitudes of each detector message to obtain a weighted average of these three values. The weighting coefficients of these equations depend on the width and location of the bandpass region of each filter in the absorption spectrum 101. The values of the coefficients are chosen empirically to obtain a weighted average of detector message values that is as insensitive to temperature and basis weight variations. One current cost-benefit calculation is made as follows:

The water ratio R1, which is a rough estimate of the weight of water per unit area of the paper web, is given by the formula R1 = REF / MES. The REF or reference value, as mentioned above, is mainly sensitive to the basis weight of the paper web, while the MES or measurement value is mainly sensitive to the moisture content of the paper web. The ratio R1 indicates the moisture content of the paper web, but is not at all sensitive to the temperature of the paper web at 13,97571. In the case shown in Figure 2, the REF filter is located near 1.85 microns and the MES filter is located near 1.93 microns.

5 The process control computer 50 can calculate the weight of water per unit area according to any of the above formulas using the linear equation WW = A * R1 + D, where A and D are the slope and height difference of the curve defined by the linear equation and WW is the weight of the sheet water per unit area per 10 den. The percentage moisture of the paper sheet can be calculated from this last equation using the auxiliary measurement of the weight per unit area of the paper web, BW. Thus, the percentage moisture of the paper web is calculated by the process control computer 50 by multiplying 100 * WW / BW.

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As mentioned, in the invention, the first bandpass region associated with the measurement detector is selected to transmit infrared radiation in that portion of the infrared spectrum centered approximately at the infrared absorption peak 20 of the water, i.e., at about 1.93 microns. For example, as the temperature of the paper web increases, the intensity of the measured infrared radiation increases on the long wavelength side of the filter bandpass region, while the measured infrared radiation decreases by approximately the same amount on the short wavelength side opposite the bandpass region. Thanks to this technology, the amount of infrared radiation entering the entire measuring detector, and with it the MES message, is relatively insensitive to the temperature of the paper web. However, the reference detector is used as described above to receive a message to the process control computer indicating the amount of paper fiber in the paper web per unit area.

By means of the invention, the process control computer of a modern automated paper mill can measure the moisture of the moving paper web 35 at various points in the length and width direction of the paper web. Moisture measurements are compensated for paper web basis weight variations. The paper mill process control computer can selectively activate various prior art devices to increase or decrease the moisture of various portions of the paper web to produce high quality paper with uniform moisture using these basis weight compensated measurements. As mentioned above, for example, water jets and infrared lamps can be used to control the moisture of the paper web.

One skilled in the art will appreciate from the foregoing that various modifications may be made to the optical system and processes described herein without departing from the spirit and scope of the invention.

Claims (4)

15 97571 A paper web moisture sensor which is not sensitive to the temperature of the paper web and which comprises an infrared radiation source (16) for guiding an infrared beam (24) through the paper web (12) 5, characterized in that the sensor further comprises a detector (18; 30, 40, 42, 48 36, 38, 46) for detecting the amount of infrared radiation passing through the first (112) and second (110) wavelength regions of the infrared spectrum (101), said first region (112) being located around the infrared absorption peak (106) of the water 10, and is selected therefrom; that as the temperature of the web changes, the opposite changes in the amount of longer-wave infrared radiation from the top of the region and the amount of shorter-wave infrared radiation from the bottom of the region substantially offset each other, the intensity of the infrared radiation passing through form small uu infrared radiation, which is mainly sensitive to the amount of paper fiber in the path of the infrared beam (24). 20 A paper web moisture sensor according to claim 1, characterized in that the first region (112) is located around 1.93 microns and the second region (110) is located in the wavelength range of about 1.83 to 1.85 microns. 25 Paper web moisture sensor according to claim 1, characterized in that it comprises means (50) for calculating the moisture content of the web (12) using the amount of infrared radiation detected from the first (112) and second (110) regions of the infrared spectrum (101). A method for controlling the humidity of a paper web (12), comprising (a) passing an infrared beam (24) through the paper web (12) 35, and (b) moving the beam (24) relative to the web (12), characterized in that 97571 16 (c) measuring, at a plurality of locations on the paper web (12), the amount of infrared energy passing through the paper web (12) in the first (112) and second (110) separate regions of the infrared spectrum (101), the first region (112) being located at the water infrared absorption peak (106). ) and the upper and lower limits of the first region wavelengths are selected so that as the web temperature changes, the opposing changes in the amount of longitudinal infrared radiation from the top of the region and the amount of shorter wave infrared radiation from the bottom of the region substantially offset each other. the amount of infrared energy transmitted is independent of the temperature of the paper web, and of which a the second region (110) is located in a portion of the infrared spectrum (101) that is substantially absorbed by the paper fibers 15; (d) calculating, by means of a computer (50), the moisture at various points in the paper web (12) using infrared transmission measurements of the first (112) and second (110) regions of the infrared band; and (e) changing the moisture 20 (51) of the at least one portion of the paper web (12) based on the moisture contents calculated for the various locations on the paper web.