Title:
Measurement method and apparatus in the manufacture of paper or paperboard
Kind Code:
A1


Abstract:
In a paper or paperboard machine measurements are conducted with at least one measurement means attached to an element in the paper or paperboard machine. The measurement means has two surfaces made of electrically conductive material and has insulating material between them. The change in the capacitance between the surfaces made of conductive material is measured.



Inventors:
Pitkanen, Tatu (Jarvenpaa, FI)
Tynkkynen, Topi (Vantaa, FI)
Rata, Ilkka (Jyvaskyla, FI)
Seppa, Heikki (Helsinki, FI)
Innala, Matti (Jarvenpaa, FI)
Lannes, Petteri (Jokela, FI)
Kauhanen, Risto (Espoo, FI)
Application Number:
10/957073
Publication Date:
05/12/2005
Filing Date:
10/01/2004
Assignee:
PITKANEN TATU
TYNKKYNEN TOPI
RATA ILKKA
SEPPA HEIKKI
INNALA MATTI
LANNES PETTERI
KAUHANEN RISTO
Primary Class:
Other Classes:
700/127, 162/263
International Classes:
D21F3/06; D21F11/00; D21G1/00; D21G9/00; G01L5/00; G01L; (IPC1-7): D21F11/00
View Patent Images:



Primary Examiner:
HUG, JOHN ERIC
Attorney, Agent or Firm:
STIENNON & STIENNON (MADISON, WI, US)
Claims:
1. A measurement method in a paper or paperboard machine or finishing apparatus for paper or paperboard, comprising the step of taking measurements by means of at least one measurement means attached to at least one element in the paper or paperboard machine or finishing device for paper or paperboard, wherein the measurement means comprises two surfaces made of conductive material and having insulating material between them, and that the capacitance and/or the change in the capacitance between the conductive material surfaces is measured, wherein the measurement means is positioned between separate layers of a coating of the at least one element.

2. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a measurement means in which at least one conductive material surface and/or insulating material is positioned in a roll and/or roll coating in the paper or paperboard machine.

3. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a measurement means in which at least one conductive material surface and/or insulating material is positioned in one of the following: a belt, a wire, a coating blade or rod, a doctor blade or a guide rod.

4. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a measurement means in which at least one conductive material surface and/or insulating material is positioned in the roll shell or in the roll coating.

5. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a measurement means that is provided with a substantially strip-like shape.

6. The method according to claim 5, wherein the capacitance and/or the change in the capacitance is measured by means of a strip-like measurement means that is positioned in the element in such a manner that in the longitudinal direction of the measurement means different sections of the same enter the measurement point, such as a nip, alternately as a result of the movement of the element.

7. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with at least one measurement means which is composed of substantially separate fragments and has two layers made of conductive material and having insulating material between them.

8. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a perforated measurement means.

9. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured with a measurement means that is positioned at least partly as a spiral around the roll.

10. The method according to claim 1, wherein the total linear load and/or nip pressure profile between two elements pressed together in the paper or paperboard machine or finishing device for paper or paperboard is measured on the basis of the capacitance and/or the change in the capacitance.

11. The method according to claim 10, wherein the total linear load and/or nip pressure profile of at least one roll nip is determined on the basis of the capacitance and/or the change in the capacitance.

12. The method according to claim 1, wherein at least one of the following is determined on the basis of the capacitance and/or the change in the capacitance: moisture content, temperature or tension of the web.

13. The method according to claims 1, wherein the capacitance and/or the change in the capacitance is measured with at least two measurement means coupled together by means of a half-bridge coupling, and the bridge is measured by means of alternating voltage.

14. The method according to claim 1, wherein the capacitance and/or the change in the capacitance is measured when the roll is rotating.

15. A measurement apparatus in a paper or paperboard machine or finishing apparatus for paper or paperboard, in which apparatus at least one measurement means is attached to at least one element in the paper or paperboard machine or finishing device for paper or paperboard, wherein the measurement means comprises two surfaces made of conductive material and having insulating material between them, and that the measurement means is arranged to measure the capacitance and/or the change in the capacitance between the conductive material surfaces, wherein the measurement means is positioned between separate layers of a coating of the at least one element.

16. The apparatus according to claim 15, wherein in the measurement means at least one conductive material surface and/or insulating material is positioned in a roll and/or roll coating in the paper or paperboard machine.

17. The apparatus according to claim 15, wherein in the measurement means at least one conductive material surface and/or insulating material is positioned in one of the following: a belt, a wire, a coating blade or rod, a doctor blade or a guide rod.

18. The apparatus according to claim 16, wherein roll has multiple layers of coating thereon, and the measurement means is positioned between the coating layers of the roll coating, and intermediate layer formed of an adhesive substance, overlies the measurement means.

19. The apparatus according to claim 15, wherein the measurement means has a substantially strip-like shape.

20. The apparatus according to claim 19, wherein the strip-like measurement means is positioned in the element in such a manner that in the longitudinal direction of the measurement means, different sections thereof enter the measurement point, such as a nip, alternately as a result of the movement of the element.

21. The apparatus according to claim 15, wherein at least one measurement means is substantially a separate fragment composed of two layers made of conductive material and having insulating material between them.

22. The apparatus according to claim 15, wherein the measurement means is perforated.

23. The apparatus according to claim 15, wherein at least one measurement means is positioned at least partly as a spiral around the roll.

24. The apparatus according to claim 15, wherein the apparatus comprises means for determining the total linear load and/or nip pressure profile between two elements pressed together in a paper or paperboard, machine or finishing device for paper or paperboard on the basis of the measured capacitance and/or the change in the capacitance.

25. The apparatus according to claim 24, wherein the apparatus comprises means for determining the total linear load and/or nip pressure profile of at least one roll nip on the basis of the measured capacitance and/or the change in the capacitance.

26. The apparatus according to claim 15, wherein the apparatus comprises means for determining on the basis of the measured capacitance and/or the change in the capacitance one of the following: moisture content or tension of the web.

27. The apparatus according to claim 15, wherein the apparatus comprises means for coupling at least two measurement means together by means of a half-bridge coupling, as well as means for measuring the bridge by means of alternating voltage.

28. A measurement method in a paper or paperboard machine or finishing apparatus for paper or paperboard, comprising the step of taking measurements by means of at least one measurement means attached to at least one element in the paper or paperboard machine or finishing device for paper or paperboard, wherein the measurement means comprises two surfaces made of conductive material and having insulating material between them, and that the capacitance and/or the change in the capacitance between the conductive material surfaces is measured, wherein the capacitance and/or the change in the capacitance is measured with a measurement means in which the paper or paperboard web forms at least part of the insulating material.

29. A measurement apparatus in a paper or paperboard machine or finishing apparatus for paper or paperboard, in which apparatus at least one measurement means is attached to at least one element in the paper or paperboard machine or finishing device for paper or paperboard, wherein the measurement means comprises two surfaces made of conductive material and having insulating material between them, and that the measurement means is arranged to measure the capacitance and/or the change in the capacitance between the conductive material surfaces, wherein in the measurement means the paper or paperboard web forms at least part of the insulating material.

Description:

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority on Finnish Application No. 20031440, Filed Oct. 3, 2003, the disclosure of which is incorporated by reference herein.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to a measurement method and a measurement apparatus in a paper or paperboard machine.

In modern paper machines it is necessary to measure characteristic variables both from the paper and the parts and partial processes of the machine itself to monitor the quality of the paper to be manufactured and the different parts of the paper machine, as well as to ensure the operation of the machine. On the basis of these variables it is possible to control the machine and its parts to ensure uniform quality of the produced paper.

One factor affecting the quality of paper are the numerous roll nips formed in the paper machine by two rolls rotating against each other, through which nips the paper web to be formed travels at different stages of papermaking. Such nips exist for example in presses, calenders, reel-ups, coating stations and slitter-winders. The nip force prevailing in the nip in the axial direction of the rolls, i.e. the linear load that presses the paper web or another medium traveling through the nip substantially affects the quality of the paper to be produced. To make the paper uniform in quality within its entire width, it is of primary importance to measure the linear load profile prevailing in the nip, so that the process can be adjusted to attain the desired quality for the end product.

U.S. Pat. No. 5,048,353 discloses a method for measuring the nip profile of a reeling nip, in which a strip containing piezoelectric measurement elements, or separate piezoelectric measurement elements are attached on the surface of a reeling cylinder extending in the form of a spiral extending from one end of the cylinder to another. On the basis of the pressure message given by the elements it is possible to determine the nip pressure profile prevailing in the nip in the axial direction of the cylinders. In U.S. Pat. No. 5,562,027 the piezoelectric measurement elements are placed on the surface of the roll in one or several rows in parallel to the shaft of the roll. U.S. Pat. No. 5,383,371 discloses placing a film-like piezoelectric measurement element or measurement elements underneath or inside the roll coating in one row in parallel to the shaft of the roll. A problem with film-like piezoelectric sensors is that their measurement accuracy is poor, because they are sensitive to temperature, which causes creeping of the signals given by said sensors. When using point-like sensors that are attached on the surface of the roll, problems are caused by the fact that the sensors leave marks on the paper. Furthermore, it is difficult to attach them.

Furthermore, in the measurement of the linear load profile, it is known to measure the pressure prevailing in the nip by means of strain-gauge sensors placed on the surface of the roll. One problem of this method is for example the difficulty of installing the sensors.

SUMMARY OF THE INVENTION

Thus, it is an aim of the present invention to attain a measurement method in a paper or paperboard machine that eliminates the above-described problems and makes it possible to conduct measurements in the paper or paperboard machine reliably without leaving marks in the paper to be produced. Furthermore, it is an aim of the invention to provide an apparatus implementing the aforementioned method.

The measurement method is intended to be used in numerous locations in the paper or paperboard machine or in the finishing apparatus for paper or paperboard, when the aim is to measure linear load, linear load profile, temperature, moisture content and tension. Possible targets of application include for example measurement of the nip profile of the roll nip and numerous other targets that will be discussed later.

The invention is based on the idea that in the manufacture or finishing of paper or paperboard web the properties of the partial processes or parts of the paper or paperboard machine are measured during the run of the paper or paperboard machine or finishing device by means of measurement sensors attached to an element in a paper or paperboard machine or finishing device. The measurement sensors are substantially capacitive measurement sensors.

The measurement sensors are composed of at least one capacitor that is formed of two conductive material surfaces, for example metal films having an insulating material between them. When for example a radial force is exerted on an element in the paper and paperboard machine, the distance between the surfaces changes, which causes a change of capacitance between the surfaces, on the basis of which it is possible to determine the linear load exerted on the element. In the measurement of the change in capacitance it is possible to use for example a capacitive half-bridge coupling and measurement of the bridge can be made by means of alternating voltage.

The capacitance of a plate capacitor can be determined by means of the following formula: C=ɛ×AS(1)

    • in which
    • C=capacitance,
    • ε=the permittivity of the insulating material,
    • A=effective surface area between the metal surfaces and
    • S=distance between the plates.

Thus, the change in the capacitance can be caused by a change in any factor. Thus, by means of the change in the capacitance caused by the change in the distance S between the metal surfaces it is possible to determine the force exerted on the films, i.e. the tension of the web or wire or for example the nip pressure between the rolls, and on the basis of the change in the capacitance caused by the change in the permittivity it is possible to determine the moisture content or temperature of the material, for example the roll coating. On the basis of the change in capacitance caused by a change in the effective surface area A between the film-like metal surfaces it is possible to determine the shearing force. The effective surface area A between the films can change for example so that the conductive films are placed in the roll coating, and they are capable of moving with respect to each other. The measurement method can be implemented for example for the measurement of both in the travel direction and cross-direction of the paper web effecting local elongations exerted on the paper web in the nip, thus making it possible to recognize local flaws in the paper or paperboard web or reasons for a web break. The measurement of the shear force can be applied especially for monitoring the condition of roll coatings. Thus, on the basis of the change in the shear force it is possible to detect permanent shift of the conductive films with respect to each other, which results for example from the loosening of the roll coating or a part of it, wherein the manufacturing process can be stopped before the coating or a part of it becomes permanently detached. The insulating material has a crucial significance in the measurement of different variables. By selecting suitable materials as insulating materials, it is possible to measure the desired property at a given time. For example for the measurement of the moisture content of the roll coating a hygroscopic insulating material is selected, which material absorbs moisture, thus creating a change in permittivity, which can be seen in the capacitance.

To implement the invention it is possible to apply several different measurement sensor structures. The sensor can, for example be integrated in the coating of an element in a paper or paperboard machine, such as a roll, a coating blade or the like, in such a manner that insulating material used in the coating is used as an insulating substance and the conductive surfaces of the capacitors are metal films evaporated or glued on the layers of the coating. It is also possible to form the sensor with its conductive surfaces and insulating substances into a strip-like or net-like structure or to form individual point sensors that are placed inside the coating. Such a structure integrated in the coating is simple and easy to install in the element in connection with the coating process. Furthermore, it is affordable and durable, and does not cause marks in the paper to be manufactured. It is also possible to use the frame parts of the element, such as the shell of the roll as the conductive surface of the sensor.

It is also an advantage of the measurement solution according to the invention that by using the capacitive measurement method it is possible to easily protect the measurement sensors and conductors from electrical disturbances. Other advantages of the measurement method include good resolution and good tolerance to disturbances. Furthermore, it is possible to modify the measurement method and apparatus according to the object of measurement in question by selecting the sensor material, i.e. the insulation and the conductive layers so that they are suitable for each object of measurement and coating. By means of the bridge coupling it is possible to improve the stability and temperature compensation of the sensor. In addition, it is easy to manufacture the sensors in different sizes and shapes depending on the object of measurement and the property to be measured.

Another advantage of the invention is that it enables reliable measurement of the cross-profile of nip pressure profiles of an element equipped with a sensor and the nips, for example roll nips of another machine element pressed against the same in the machine direction.

When a measurement sensor integrated in the roll coating is used, the measurement sensors are attached to the roll at the manufacturing stage of the roll, and it is dependent on the property to be measured where the roll containing the measurement sensors is positioned in the paper machine. By means of a roll equipped with sensors it is possible to measure the force or linear load profile between said roll and another, structural element extending longitudinally in the lateral direction of the paper machine and being pressed against the roll. During the measurement there may be a moving paper web or another medium between the roll equipped with sensors and the structural element pressed against the same. The roll equipped with sensors is thus suitable for measurement of total linear load and/or nip pressure profile of roll nips formed by rolls in different parts of the paper or paperboard machine, wherein the nip is formed for example by a roll equipped with sensors and another roll, for example the roll of a press, a calender, a reel-up or a slitter winder, and the paper web travels through the nip. The roll equipped with sensors is also suitable for measurement of linear load profiles of backing rolls in coating stations, and blade and rod load profiles of the coating blades and coating rods in coating devices, wherein the coating blade or rod is pressed against the roll equipped with sensors. In the paper machine it is also possible to measure the loads of doctor blades in different locations by means of the roll equipped with sensors according to the invention.

Furthermore, the roll equipped with sensors according to the invention is suitable for example for measurement of the temperature distribution of coatings on calender rolls, as well as for example for the measurement of the moisture content of the coating on a polyurethane-coated roll.

In a corresponding manner it is possible to place measurement sensors in other longitudinal elements in the lateral direction of the web in the paper or paperboard machine or finishing apparatus, for paper, such as doctor blades, or coating blades or rods. It is also possible to place measurement sensors in an endless element moving in the travel direction of the web and forming a closed loop, such as a wire, a felt, or a belt of an extended nip calender. Furthermore, it is possible to place measurement sensors in an element substantially narrower than the width of the paper or paperboard machine, for example in a texture used in threading or a wheel guiding said texture.

An element, such as a beam or a rotating or non-rotating roll extending longitudinally in the lateral direction of the web produced in a paper or paperboard machine is also especially suitable for the measurement of the tension of a paper or paperboard web and/or a texture, such as a wire or felt in a paper or paperboard machine. Thus, the web or the texture is conveyed over the element equipped with measurement sensors in such a manner that the web or the texture touches said element and the tension and/or tension profile of the web or texture is measured.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to the appended drawings.

FIG. 1 shows a cross-section of the coating layers of a roll equipped with measurement means according to the invention.

FIG. 2 shows a perspective view of a roll according to the invention equipped with a measurement sensor.

FIG. 3 shows a roll shell equipped with measurement sensors according to the invention, spread out as a plane and seen from above.

FIG. 4 shows a measurement sensor according to the invention formed over the web.

FIG. 5 shows schematically a measurement arrangement composed of separate sensors.

FIG. 6 shows a perspective view of a third roll equipped with measurement sensors according to the invention.

FIG. 7 shows a circuit diagram of the electronics according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a preferred embodiment of placing a measurement means according to the invention, i.e. a sensor 1 between separate layers of a coating of an element of a paper and paperboard machine, which is a roll in this case. The bottom layer 6 of the roll coating is laminated on the shell 5 of the roll 2, and the sensor 1 is attached on the same. On the sensor 1 there is an intermediate layer 7 of the coating and on that a surface layer 8 of the coating. The intermediate layer 7 typically consists of an adhesive substance, the purpose of which is to fasten the bottom layer 6 on the upper coating layers of the roll. The materials of the bottom layer and the intermediate layer, i.e. the adhesive substance and the surface layer are selected according to the given use of the roll. Roll coating materials and their selection and the roll coating process are obvious for anyone skilled in the art, and thus they will not be discussed in more detail in this context.

The sensor 1 is composed of an insulating film 3 on both sides of which a conductive film, such as a metal film 4 is attached by means of glueing, painting or evaporating. The conductive film can be for example a copper film or it can be made of conductive polymer. The insulating film 3 is typically made of plastic, and it is selected according to the variable to be measured by means of the sensor. It is also possible to select one of the coating materials used in the roll as an insulating film. The sensor can be easily manufactured in connection with the coating of the roll by evaporating, painting or glueing two metal films between the coating layers in such a manner that the sensor is formed. The sensor 1 can also be manufactured beforehand either by printed-circuit technology or by evaporating, or by attaching the metal film in some other way on both surfaces of the insulating material, and attaching the thus formed sensor between the coating layers of the roll when coating the roll. Thus, it is also possible to select one of the materials used in the roll coating as an insulating material. The sensor can also have a perforated or net-like structure, wherein it is possible to affect the elastic properties of the sensor in such a manner that the sensors inside the roll coating do not affect the behavior of the coating during pressing, for example in nip contact. Preferably the metal films of the sensor are as thin as possible and the insulating layer therebetween is of the same material as the roll coating.

FIG. 2 shows a paper machine roll 2 according to the invention equipped with measurement sensors 1, said roll being suitable especially for measuring the nip force of a nip formed by two rolls positioned against each other. The roll 2 can be used analogously for measurements with any counter element that forms a nip with the roll 2. In FIG. 2 the roll 2 forms a nip N with the roll 17. The sensor 1 is formed as a strip for example in such a manner that on top of a metal film of 50 μm in thickness, for example a copper or aluminium foil, a plastic having a thickness of 50 to 100 μm is laminated and on top of the same another metal film of 50 μm is laminated. The overall thickness of such a structure is under 200 μm. Single sensor strip is for example 50 mm wide. It is possible to make holes therein, for example round holes having a diameter of 5 mm, or square-shaped holes of 5 mm×5 mm in size. The number of the holes in the strip can vary, but an advantageous structure is such where there are a large number of holes, for example in such a manner that the section of the strip remaining between the holes is only 1 mm wide. The sensor 1 is attached on the surface of the roll between the coating layers of the roll as disclosed above, i.e. on top of the intermediate layer of the coating, wherein the fastening of the sensor between the coating layers takes place easily. The holes improve the fastening of the strip. The sensor 1 is attached on the surface of the roll into a spiral that winds from one end of the roll to the other. When nip force is measured while the roll 2 is rotating, the sensor 1 measures the cross-profile of the nip force once per revolution. It is also possible to manufacture the strip-like sensor by means of printed circuit technology, wherein it is also possible to integrate the signal conductors in the structure. Such a single channel measurement is easy to install, it is wear-resistant and affordable. For example a thin coaxial cable can be used as a sensor, wherein sensor capacitance is formed between the intermediate conductor and the protective shell.

The use of a capacitive bridge coupling requires two identical sensors attached to the roll 2. FIG. 3 shows the shell of such a roll spread out as a plane. Both sensors wind half the length of the roll shell. By means of such a roll it is possible to measure the linear load of one nip in the formation of which the roll participates. In such a case where the measurement roll forms nips with two rolls, two strips are used, both of which wind only a quarter of the length of the roll shell. Thus, both capacitances of the bridge can function as active measurement elements, wherein it is possible to compensate for example the changes in the temperatures occurring in the object to be measured. The other branch of the bridge can also be used as a separate temperature/moisture reference. The capacitive bridge coupling can also be used when only one sensor and by other means formed reference capacitance are in use.

FIG. 4 shows an embodiment of the invention in which the measurement sensor 1 is formed over the nip N in such a manner that the web 35 traveling in the nip forms a part of the insulating material 3. In other words, capacitance is measured over the paper web. Thus, the conductive surfaces are positioned in the rolls 2 forming the nip in such a manner that both rolls contain one conductive surface. The conductive surface can be either a roll shell 5 or above-described conductive film 4 arranged in the roll coating. Thus, the measurement can take place either when the roll shells 5 of both rolls 2 function as conductive surfaces, or when the conductive films 4 in the roll coatings of both rolls 2 function as conductive surfaces or in such a manner that the roll shell 5 of one roll and the conductive film 4 of the other roll function as conductive surfaces. Instead of the rolls 2 the web can also be supported by an endless metal loop on both sides of the paper web, said metal loop functioning as a conductive surface. Such a structure can also be implemented for example in extended nip calenders. The paper web can also function as a part of the insulating material in a measurement solution in which the nip N is formed by the loop of an endless supporting element, such as a wire or a belt that travels via guide rolls and is equipped with a conductive surface, and a roll 2 whose shell and the film positioned in the roll coating function as a conductive surface.

The sensors 1 can also be formed of separate fragments. Thus, they can be formed of separate insulating fragments metallized on both sides, for example of substantially square-shaped insulating fragments. Such a structure is shown in FIG. 5. This embodiment comprises two sensors 1 having coupled to their lower metal surfaces, i.e. metal surfaces closer to the roll shell, conductors 9 and 10, and having coupled to the upper metal surfaces closer to the outer surface of the roll coating conductors 111 and 12. Thus, the sensors form a pair, wherein the lower metal surfaces of the sensors are coupled together by means of a conductor 13, and the upper metal surfaces of the sensors are coupled to conductors 14. One advantage of the structure is, for instance that because the sensor elements are located close to each other, they are at the same temperature, and thus, creeping of the signal does not occur in the measurement. The placement of individual sensors on the surface of the roll is shown in FIG. 6.

The size of individual sensors is determined on the basis of the property to be measured. If an individual sensor or sensors forming the sensor pair have a large surface area, i.e. they extend in the machine direction over the nip width, they measure force, i.e. the total linear load prevailing in the nip. The linear load is typically indicated as a force prevailing in the direction of the loading, i.e. in this case in the machine direction, divided with the nip width (extension in the cross-machine direction), and the unit used for the same is typically kN/m. If, on the other hand, the sensor or sensors have a small surface area, they measure pressure, and thus it is possible to determine the pressure profile of the nip in the machine direction. By means of the invention it is also possible to simultaneously measure both the total linear load prevailing in the nip and the nip pressure profile, wherein a measurement method is utilized in which two sensor-conductor combinations are installed around the roll in a spiral-like manner according to FIG. 6. Sensors 1′ having a large surface area and measuring primarily force are attached to the conductor 15, and sensors 1″ having small surface area and measuring primarily pressure are attached to the conductor 16. The conductors 15 and 16 are composed of the above-described three conductors, one conductor 13 and two conductors 14.

The roll coating wears when the roll is used in a paper or paperboard machine or in a finishing apparatus for paper or paperboard. The effect of the wearing of the roll coating on the sensor signal is corrected by means of a mathematical model.

FIG. 7 shows an example of the measuring principle of a bridge-coupled nip force meter according to the invention. The roll 2 is equipped with two strip-like measurement sensors 1. In both strips the upper, closest to the outer surface of the roll located metal film 4 is energized, the voltage being brought thereto via a conductor 18. The alternating voltage attained from a voltage source 21 is conveyed via an amplifier 22 and an inverter 23 to the primary coil 24 of a transformer. The secondary coil 25 of the transformer is grounded to the earth of the roll from the center of the secondary coil 25. If the coupling coefficient of the transformer is close to one, the arrangement guarantees that the same voltage is attained in both upper metal films 4 and the phase difference of the signals is π. The lower metal films 4 located closer to the shell of the roll are connected and they are coupled to a current amplifier 20 by means of a conductor 19, said current amplifier forcing the potential of the lower metal films to the same level with the potential of the shell of the roll 2. The current amplifier 20 can be formed either by using a current-type operation amplifier or a voltage amplifier of which a current amplifier is formed by means of negative feedback coupling. To maximize the current signal, a coil 27 is positioned in the input of the amplifier, said coil tuning the measurement capacitances. After the amplification the signal is multiplied by control voltage in a phase detector 28, said control voltage being phase shifted 90 degrees to detect the change in the capacitance. The signal is either positive or negative, depending on which one of the strip-like measurement sensors 1 is pressed as a result of the linear load. After the phase detector, the signal is guided via a filter 29 and an analog-to-digital converter 30 to a transmitter 31. The above-described electronics are positioned in the roll 2, for example to its end. By means of the transmitter 31 the signal is transmitted wirelessly to a receiver 32 located close to the roll, from which it is transmitted further to be processed in a computing and data processing device 33. In the computing and data processing device the desired total linear load, linear load profile or nip pressure profile is calculated from the measured capacitance and/or capacitance change signal. The position of the measurement sensors 1 with respect to the rotation of the roil 2 is measured by means of an angular sensor 34, said position data being also conveyed to the data processing device 33. If the sensors are arranged in such a manner that there is an area in each revolution where a force is not exerted on either of the strips, this point can be used as a reference. If desired, it is also possible to provide the circuit with a varactor which can be adjusted to balance the bridge by means of the reference point.

If desired, the above-described technique necessary for the electrification and processing of the measurement data can be positioned entirely outside the roll 2, wherein the variable measured by the sensors is transmitted as an electric signal outside the roll for example by means of slide rings. Preferably a major part of the electronics is positioned at the end of the roll 2, close to the roll shaft, wherein wireless data transmission can be utilized. Thus, a transmitter, for example a telemetry transmitter is positioned for example at the end of the roll for transmitting the measurement data for example by means of an inductive link to a telemetry receiver positioned close to the roll.

The voltage required by the sensors can be produced in many different ways, for example by conveying it to the roll by means of an inductive coupling or a slide conductor positioned for example at the end of the roll. It is also possible to integrate an accumulator or a battery in the measurement apparatus or for example at the end of the roll, which accumulator or battery can be loaded during maintenance of the machine when the roll is stopped. The invention is not intended to be limited to the embodiments presented above as examples, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. Capacitive sensors can be used analogously in all structures extending in the cross-machine direction that are subjected to forces resulting from the process. The sensors can also be integrated in elements moving in the machine direction, such as belts or wires, for example in belts of a shoe calender. In static elements that do not move in accordance with the web speed separate sensors are advantageously used, if the aim is to measure loading at different points in the cross-machine direction, such as doctor blades.