BACKGROUND OF THE INVENTION [0002] Measuring the temperature of a paper web is important, because an incorrect temperature at some stage of the process leads to a deterioration of the paper or board quality, and makes controlling the process difficult. Temperature measurement is needed when optimising many sub- processes of the paper web production process, such as web drying and power usage during drying, solidification of coating paste, and measuring the temperature profile of the web. A paper web is measured according to prior art with a separate temperature measuring arrangement which comprises one or more separate sensors and sensor processing blocks. The sensor is typically sensitive to IR (infrared) radiation, wherefore the electric resistance of the sensor changes in a known manner according to a change in the temperature.

In a prior art measuring system, the temperature sensor is, as a separate measuring apparatus, either fixed close to the paper web or it can move back and forth over it, in which case the entire area of the paper web can be measured. The sensor typically measures the temperature continuously. The measuring information is processed and utilised in the paper-making process.

[0003] One of the prior art paper web temperature measuring methods is disclosed in US patent 5124552, which is incorporated herein by reference. In the measuring method, IR radiation is transmitted through the paper web. The temperature measurement is based on the fact that the wave length dependency of IR radiation absorption changes when the temperature of the paper web changes.

[0004] The number of measuring apparatuses and components in the prior art equipment is high, which increases both the complexity and price of the equipment, requires a lot of space and increases failure probability. A high number of apparatuses also makes it difficult to install the temperature measuring equipment between the other measuring apparatuses. In addition, one problem arises from protecting the separate sets of measuring equipment

in the demanding conditions of a paper machine environment (high temperature, dust, dirt, humidity, etc.). Data transmission between the different sets of measuring equipment also requires more and more electronic devices.

BRIEF DESCRIPTION OF THE INVENTION [0005] It is thus an object of the invention to implement an improved method and an apparatus implementing the method to reduce or eliminate the problems related to the prior art. This is achieved by a method of measuring the temperature of a paper web, which method is characterized in that when measuring one property of the paper web optically by illuminating the surface of the paper web by pulsed optical radiation, the strength of the IR radiation emitted by the paper web itself is, in connection with said measuring, measured on at least one time instant between optical radiation pulses when optical radiation is not directed to the paper web, and the temperature of the paper web is determined by means of the measured strength of the IR radiation.

[0006] The invention also relates to an arrangement for measuring the temperature of a paper web. The arrangement of the invention is characterized in that when measuring one property of the paper web optically by illuminating the surface of the paper web by pulsed optical radiation, the arrangement is, in connection with said measuring, arranged to measure the strength of the IR radiation emitted by the paper web itself on at least one time instant between optical radiation pulses when optical radiation is not directed to the paper web, and to determine the temperature of the paper web by means of the measured strength of the IR radiation.

[0007] Preferred embodiments of the invention are disclosed in the dependent claims.

[0008] The method and system of the invention provide several advantages. The measuring arrangement of the paper web temperature according to the invention makes it possible to monitor the temperature of the paper web while the paper or board is being measured by pulsed optical radiation. A common measuring apparatus structure reduces the number of required apparatuses, which reduces both the complexity and price of the equipment, requires less space and reduces failure probability. In addition, it is easier to protect the measuring equipment from the environment, since separate protection is not needed. Data transmission between the

apparatuses is also easier. By means of the measurements, the paper web can be controlled to make the paper shrinkage even, which reduces folding and allows a high production rate.

BRIEF DESCRIPTION OF THE FIGURES [0009] In the following, the invention will be described in greater detail by means of preferred embodiments and with reference to the appended drawings in which [0010] Figure 1A is a block diagram of an apparatus using one detector, [0011] Figure 1B is a block diagram of an apparatus using two detectors, [0012] Figure 2 shows a chopper disc, [0013] Figures 3A to 3E show the timing of optical rays chopped with the chopper, [0014] Figure 4A shows impinged blowing used in the drying section of a paper machine, [0015] Figure 4B shows through-air-blowing used in the drying section of a paper machine, [0016] Figure 5 shows heat-profile measuring and drying control.

DETAILED DESCRIPTION OF THE INVENTION [0017] The solution of the invention is especially suited for measuring the temperature of a paper web.

[0018] In this application, the NIR (near IR) range of IR (infrared) radiation refers to an electromagnetic spectrum band of 700 nm to 2500 nm.

The MIR (middle IR) range refers to an electromagnetic spectrum band of 2500 nm to 20,000 nm. These definitions correspond to the understanding of a person skilled in the art on NIR and MIR radiation.

[0019] Let us now examine in greater detail a coating measurement for which the solution of the invention is especially suited, without being limited to it, however. In coating measurement, one or more coating components, such as calcium carbonate, kaolin, silicone and water, are measured with MIR radiation, and one or more components, such as kaolin, talcum, gypsum, silicone, water, cellulose and binding agents, such as latex, are measured with NIR radiation. Coating measurement can be performed by one or two detectors in one or more sensors.

[0020] When using one detector in coating measurement, the measurement is made using both MIR and NIR radiation, which are measured at different times. The detector is then an MCT (mercury cadmium telluride) detector or the like. The type of the detector is, however, not essential for the invention, but the essential matter is that the detector is capable of detecting the radiation being measured. From the coating of paper or board, at least one component is measured by MIR radiation, and likewise by NIR radiation. To make the measurement, the method directs IR radiation which is chopped into light pulses by a chopper towards the coating from an optical power source.

For detection, IR radiation reflected from the coating is chopped synchronously with the IR radiation illuminating the coating, the former radiation comprising both pulsed optical radiation originating from the optical power source and the optical radiation emitted by the coating itself. Detection is done in the measurement direction which is other than the secular reflection direction.

[0021] Let us now examine measurement by MIR radiation. From the chopped optical radiation, at least one wavelength band of at least one component is band-pass-filtered, which wavelength is sensitive to the absorption of said at least one component in the MIR range, and the strength of the MIR radiation sensitive to absorption is measured. The strength of the radiation is measured as output or intensity, as is obvious to a person skilled in the art. The point of MIR radiation sensitive to absorption is the maximum absorption point of the component in question. With calcium carbonate, this absorption occurs on an optical band having an average wavelength of approximately 3950 nm. With kaolin, the average wavelength of absorption is approximately 2700 nm. An interference filter is used as the band-pass filter.

Next, at least one absorption strength/level in proximity to the maximum absorption is measured to find out how strong the maximum absorption is in comparison to its environment. This is done as follows. From the chopped IR radiation, one wavelength band of said at least one component is band-pass- filtered, which wavelength is insensitive to the absorption of said one component in the MIR range, and the strength of the MIR radiation insensitive to absorption is measured. After this, the absorption strength/level of one or more components is defined by comparing for each component specifically the strength of the absorption-sensitive MIR radiation with the strength of the absorption-insensitive MIR radiation. At the end of the MIR measurement, the

amount of at least one component of the coating is defined using the MIR absorption strength/level.

[0022] Let us now examine more closely the measurement in the NIR range, which is similar to the measurement done in the MIR range. From the chopped IR radiation, a wavelength band of at least one other component is band-pass-filtered, which wavelength is sensitive to the absorption of said at least one other component in the NIR range, and the strength of the NIR radiation sensitive to absorption is measured. To measure the strength/level of absorption, absorption is also measured on a reference wavelength which is other than that causing the maximum absorption. From the chopped IR radiation, a wavelength band of said at least one other component is then band-pass-filtered, which wavelength is insensitive to the absorption of said at least one other component in the NIR range, and the strength of the NIR radiation insensitive to absorption is measured. After this, the absorption strength/level of said one or more other components is measured by comparing for each component specifically the strength of the absorption- sensitive NIR radiation with the strength of the absorption-insensitive NIR radiation. Finally, the amount of said at least one other component of the coating is defined using the NIR absorption strength/level.

[0023] When using two detectors, of which one measures the MIR radiation and the other the NIR radiation, the measurement is otherwise the same, but both the MIR and the NIR measurement is made at the same time.

NIR radiation can be detected with an InGaAs (indium gallium arsenide) detector, for instance. The type of the detector is, however, not essential for the invention, but the essential matter is that the detector is capable of detecting the radiation being measured.

[0024] Let us now examine coating and temperature measurement by means of Figure 1A. An optical radiation source 100 operating in at least the IR range transmits optical radiation through a chopper 102 to a measurement object 105. The measurement object 105 is paper or board on a paper web, the coating 104 of which will be measured. For temperature measurement, the paper web 103 is formed by the paper or board being measured 105 and a possible coating 104. When measuring coatings, the inventive solution measures the number of components in the coating. The chopper 102 allows optical radiation to momentarily pass through it and part of the time the chopper 102 prevents the optical radiation from passing through

it. The essential thing in the operation of the chopper 102 for measuring coating is thus that during the illumination time, the measurement object 105 is illuminated by IR radiation emitted by the optical power source 100 and at other times, the object being measured is not illuminated by IR radiation emitted by the optical power source 100. This way, the chopper 102 chops the optical radiation into light pulses which hit the object 104 being measured.

From the object 104 being measured, the optical radiation is reflected and scattered into different directions. A part of the optical radiation is reflected towards block 106, in which the optical radiation is filtered and chopped for detection. The optical radiation reflected from the measurement object 104 is, however, not measured from the direction of the specular reflection. In block 106, filtering is done separately with MIR and NIR filters which are preferably interference filters. When measuring the strength of MIR radiation, the radiation emitted from the object 104 being measured is filtered with a MIR filter. When measuring the strength of NIR radiation, the radiation emitted from the object 104 being measured is filtered with a NIR filter. The detection time of coating measurement is the time when MIR or NIR radiation is allowed to pass to a detector 108 for measurement. For coating measurement, detection is performed synchronously at the same time as the object being measured is illuminated with optical radiation. When a paper web being measured is not illuminated by IR radiation, the solution of the invention measures the temperature of the paper web by means of the IR radiation emitted by the paper web itself. The timing of the illumination and detection is described in greater detail in connection with Figure 3. The same detector is used in measuring the paper web temperature as in measuring the coating. The detector thus detects IR radiation in both coating and temperature measurement and converts the strength of the detected IR radiation into an electric signal of corresponding strength. This electric signal is amplified in a pre-amplifier 109 and converted into digital format in an A/D converter 111.

From the digital signal, a digital signal processing block 120 measures the strength of the detected IR radiation, calculates the amount of coating and defines the temperature of the paper web. The temperature is defined by utilising the principle that the strength, i. e. output, of the radiation detected by the detector is in relation to the temperature. The more IR output detected by the detector, the higher the temperature of the paper web. The measuring is

calibrated to display the correct temperature by measuring objects with known temperatures.

[0025] In coating and temperature measurement, the MIR and NIR measurements can thus be performed at different times with one detector 108, from which a measurement signal converted into electric format propagates to the pre-amplifier 109, A/D converter 111 and finally to digital signal processing in block 120.

[0026] Let us now examine coating and temperature measurement by means of Figure 1 B. The solution is in many ways similar to that of Figure 1A, but this solution uses two detectors 108 and 110. The optical radiation detected by the detector 108 is filtered in block 106 in such a manner that only NIR radiation is allowed to pass to the detector 108 while only MIR radiation is allowed to pass to the detector 110. The measurement signal of both detectors is amplified in pre-amplifiers 112 and 116, and the electric measurement signals are converted to digital in A/D converters 114 and 118. The digital signals are processed in the same manner as in Figure 1A in a digital signal processing block 120. When coating paper or board several times, the temperature measurement of the paper web can be performed both before and after the coating.

[0027] Let us now examine the chopper used in the solution of the invention by means of Figure 2. Both chopping the illumination of the paper web with a chopper 102 and chopping the radiation being detected with a chopper 106 is performed with one chopper disc 200. Instead of a disc-like chopper, other optical switches known per se, such as mechanical, electro- optic, magneto-optic, acousto-optical switches, etc., can also be used to chop the optical radiation. The operation of the switches is timed as shown in Figures 3A to 3E. The chopper 200 is thus preferably a disc-like optical radiation chopper comprising teeth 202 for preventing the IR radiation from getting from the optical power source to the surface being measured and gaps 204 between the teeth for allowing the IR radiation from the optical power source to pass to the surface being measured. The disc-like chopper 200 also comprises at least two MIR filters 206,212 which correspondingly allow MIR radiation to pass to the detector on at least two different bands. The MIR filters 206,212 are located on the circumference of the disc-like chopper 200. The maximum absorption point is measured on one MIR filter band and a point outside the maximum absorption point is measured on the remaining one or

more bands. The chopper 200 also comprises at least two NIR filters 208,214 which allow NIR radiation to pass to the detector on at least two different bands. The NIR filters 208,214 are located on the circumference of the disc- like chopper 200. In this case, too, the optical band of one filter allows NIR radiation to pass to the detector at the maximum absorption point of a component of the coating being measured. One or more other NIR filters allow NIR radiation to pass to the detector on other bands than that of the maximum absorption point. In addition, the disc-like chopper 200 comprises filter gaps 210 which prevent IR radiation from propagating to the detectors. The filters 206 and 208 are located so that when optical radiation 230 goes through the teeth 202 on the outer edge of the chopper 200 towards the object being measured, the filters 206 and 208 allow filtered IR radiation 232 and 234 to pass at the same time to the detectors.

[0028] In the solution of the invention, IR radiation emitted by the paper web is allowed to pass to the detector at a filter point 240 for measuring the temperature of the paper web. The measuring can also be performed in such a manner that one or more filters are replaced by an opening 242 in the chopper 200, which allows radiation to pass to the filter when IR radiation 230 is not directed to the paper web from the optical radiation source required for coating measurement. This way, the detector can receive IR radiation emitted by the paper web itself on its entire response band. In temperature measurement, it is advantageous to use a detector detecting MIR radiation. It is also possible to perform temperature measurement when one gap 204 between the teeth is covered and no IR radiation can pass from the optical radiation source to the paper web.

[0029] The chopper disc 200 is preferably run by an electric motor, and when the disc rotates, the teeth and filter gaps chop the radiation passing to the surface being measured and to the detector (s). When there is only one detector in use, as in the case of Figure 2A, the openings 242 and the MIR and NIR filters are located one after the other on the same circumference of the disc-like chopper 200, and alternately allow IR radiation to pass through or filter MIR and NIR radiation for the measurement to one detector (this is not shown in the figure, since it is obvious to a person skilled in the art).

[0030] Let us now examine the timing of illumination and detecting of the object being measured by means of Figures 3A to 3E. Penetration is shown on the Y axis on a free scale, the X axis shows time T, and both curves

are on the same time axis. In Figure 3A, a curve 300 shows the penetration of the chopper between the optical power source and the object being measured as a function of time. The chopper comprises optical filters and possibly also openings for passing optical radiation to the surface being measured and for detection. In Figure 3B, a curve 303 shows the penetration of the chopper between the object being measured and the detector as a function of time on a first wavelength being measured. The initial situation is that the chopper allows optical radiation to pass to both the surface being measured and the detector (curves 330 in Figure 3C and 301 in Figure 3A). The curve 330 is a measurement on a second measured wavelength. The measurement is performed in the same manner on different wavelengths.

[0031] Figures 3A and 3B show coating measurement on the first wavelength. In this situation, the chopper first prevents the first radiation used in the measurement from passing to both the surface to be measured and the detector (curve points 302 and 308). When the chopper between the optical power source and the object to be measured starts to allow optical radiation to pass to the object to be measured (curve point 304), coating measurement can be started on the first wavelength at time instant 316. The chopper no longer allows the first radiation to be measured to pass to the detector at curve point 312.

[0032] Measurement with a third wavelength can be started when the chopper starts to allow radiation to the detector according to the curve 332 of Figure 3D. At point 334, the chopper also allows radiation to pass to the surface to be measured. This way, several components of the coating can preferably be measured on many wavelengths.

[0033] Figure 3E shows temperature measurement of paper web according to the invention. The measurement is performed when the paper web is not illuminated with a pulsed optical radiation at time instants 340 and 342. The measurement times can thus be anywhere between the light pulses.

Radiation emitted by the paper web is allowed to pass to the detector at time intervals 344 and 346. Measurement is also possible when the source emitting optical radiation to the paper web is switched off. Measuring the temperature of the paper web is then always possible when IR radiation emitted by the paper web is allowed to pass to the detector.

[0034] In the solution of the invention, the essential thing is that the temperature of the paper web is measured in connection with another optical measurement of the paper web using chopped optical radiation.

[0035] By means of the measured temperature, it is possible to adjust the temperature of the paper web and to optimize the drying process of the paper web and especially that of coated paper. In IR drying, IR radiation is directed to the paper web from an IR radiation source to raise the temperature of the paper web. Electrically heated or gas-heated radiators, i. e. IR dryers, can, for instance, be used as the IR radiation source.

[0036] Figures 4A and 4B show drying means located in the paper machine drying section for drying the paper being made. In impinged blowing, shown in Figure 4A, hot air is blown from a hood 470 to the paper web 476 on a cylinder or roll, most preferably a dryer roll 472. The air is sucked back to the hood 470 and circulated several times through a burner 378.

[0037] Figure 4B shows a through-air-blowing arrangement which resembles the impinged blowing arrangement. In this case, there are holes on the surface of a cylinder 482 to allow air flow inside the cylinder 482 and exit it.

When hot air is blown from a hood 480 to the paper web 486, the air goes through the web 486 to the cylinder. The blow is controlled by means of temperature measurement by a control block which is preferably a part of the computer-based control system of the paper machine. Temperature can be adjusted in the case of both Figure 4A and 4B by changing the rate and/or temperature of the air flow.

[0038] In the solution of the invention, the temperature of the paper web is preferably measured at several locations in the transverse direction of the web to form a temperature profile of the paper web. Profile measurement can be performed by one or more sensors in such a manner that the sensors are statical or that the sensors traverse, i. e. move back and forth in the transverse direction of the paper web. Figure 5 shows a solution of this type.

The measuring arrangement comprises K sensors 500 measuring the paper web 506, the sensors being functionally connected to a measurement and control block 502, where K is a positive integer. The measurement block 502 preferably controls dryers 504. The dryers 504 can be blow dryers, as shown in Figures 4A and 4B, cylinder dryers operating on stream pressure, or IR dryers. The condition of the dryers can be controlled, optimized and monitored by the solution of the invention. In the coating process, it is possible to prevent

the temperature of the coating paste from rising so high that the surface solidifies. If the surface temperature of the coating paste rises too high and the surface solidifies, water inside the paste boils and explodes the surface. It is also possible to measure the temperature of the paper web at several locations in the machine direction of the web to form a temperature profile for the paper web in the machine direction. By optimising the drying process, it is also possible to save energy. With the arrangement for measuring the temperature of paper web according to the invention, it is possible to ensure that the temperature of the paper web remains even in both transverse and machine direction. This, for its part, ensures that the shrinkage of the paper remains even, which reduces folding and makes a high production rate possible.

[0039] When measuring the temperature of the paper web before the dryer and after it, it is possible to monitor the condition of the dryer by means of the measured temperature and to inform the operator of the paper machine of a possible defect.

[0040] Even though the invention has been explained in the above with reference to examples in accordance with the accompanying drawings, it is obvious that the invention is not restricted to them but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.