WO2000050865A1

WO2000050865A1 - Method and apparatus for determining non-linear shrinkage on the basis of the tension profile

Info

Publication number: WO2000050865A1
Application number: PCT/FI2000/000141
Authority: WO
Inventors: Juha KNIIVILÄ; Jorma O. Virtanen
Original assignee: Metso Paper Automation Oy
Priority date: 1999-02-23
Filing date: 2000-02-22
Publication date: 2000-08-31
Also published as: FI990385A7; FI990385L; FI990385A0; AU2808600A

Abstract

The invention relates to a method and an apparatus for determining the non-linear shrinkage profile of a moving web. The invention utilizes data obtained on the web (3) tension profile from a device (5) for measuring the web (3) tension, the data being used to determine the non-linear shrinkage profile of the web (3).

Description

METHOD AND APPARATUS FOR DETERMINING NON-LINEAR SHRINKAGE ON THE BASIS OF THE TENSION PROFILE

The invention relates to a method of determining a non-linear shrinkage profile of a moving paper or board web, or a change therein, in which method a moving web is directed past at least one measuring device 5 which measures the properties of the web.

The invention further relates to an apparatus for determining a nonlinear shrinkage profile of a moving paper or board web, or a change therein, the apparatus comprising at least one measuring device for measuring the properties of the web, and means for processing measurement data obtained

10 from the measuring device.

In the continuous manufacture of paper or board, the paper or board machine comprises, in the cross-machine direction, a high number of actuators, each of which can be adjusted separately. The actuators are used to adjust the properties of the web in the cross-machine direction. Measuring

15 devices that measure the aforementioned adjustable properties are usually situated downstream of the actuators. The properties are measured from several different points of the web in the cross-machine direction, but the number or location of the measuring points do not usually correspond to the number or location of the actuators in the cross-machine direction. The

20 dimensional changes occurring in the cross direction of the web between the actuators and a measuring point can be divided into two groups. Firstly, the web width or the distance of the web edges from one another changes. In most cases the web shrinks or the distance between the web edges decreases. However, the web width can also increase or stay constant. The

25 change in the web width is referred to as 'total shrinkage', regardless of whether the web width actually decreases, increases or stays the same. Secondly, cross-directional shrinkage of the web is usually not the same or equal in every part of the web in the cross direction, which means that the local changes in the web width vary in the web's cross direction. Furthermore,

30 the local cross-directional shrinkage is almost always non-linear, wherefore the phenomenon is referred to as 'non-linear web shrinkage' or 'shrinkage profile'. The actual dimensional changes included in the non-linear web shrinkage can be either shrinkage or expansion, or alternatively, there are no changes within the limits of the measuring accuracy. The aforementioned changes in the web

35 properties are not usually constant in time, but they vary. All in all, the combined effects of the total web shrinkage and the non-linear shrinkage result in a complicated and often non-linear correlation between the actuators and the measurement results of the measuring systems for the profile control. However, this correlation, which is referred to as 'actuator control mapping', must be determined in order to enable correct adjustment of the actuators and to obtain desired cross-directional properties of the web.

There are several methods and apparatuses known per se for measuring the total web shrinkage, which do not interfere with the web in any way. On the other hand, the non-linear shrinkage of the web is typically measured by means of alignment tests conducted on the web. US Patent 5,658,432 discloses an arrangement where the actuator is provided with marking means which mark the web moving past the actuator with a suitable dyed liquid. The marks form parallel arrays in the moving web in the machine direction downstream of the actuator. Non-linear shrinkage of the web changes the location of the arrays, and these changes can be used to conclude the mapping of the profile control.

US Patent 5,400,258 discloses an arrangement where at least one actuator acting on the cross direction profile of the web is moved from its normal position, whereafter the web's cross direction profile is measured and the measurement result is used to determine the part of the web that was affected by the change in the actuator's position. On the basis of data obtained from several changes in the actuator's position it is possible to determine the profile control mapping by means of algorithms which are often very complicated.

The aforementioned arrangements for measuring and modelling the web's non-linear shrinkage profile are based on operations that are aimed at the web or the product. These operations deteriorate the web properties, wherefore the web will not be a useful product. Further, they decrease the degree of utilization of the machine and cause financial losses. The losses increase with the rising speeds of production of paper and board machines. Furthermore, it is difficult to carry out the determinations since they are not usually automatic. Also, the aforementioned methods require rather massive, complicated and expensive special equipment.

An object of the present invention is to provide a method and an apparatus eliminating the aforementioned drawbacks. The method according to the invention is characterized by determining the web tension profile or a change therein, and determining the non-linear shrinkage profile of the web or a change therein on the basis of the web tension profile or the change therein.

The apparatus according to the invention is characterized in that the measuring device and the means for processing the measurement data obtained from the measuring device are arranged to determine the web tension profile or a change therein, and that the apparatus comprises means for determining the non-linear shrinkage profile of the web or a change therein on the basis of the tension profile or the change therein.

A basic idea of the invention is that the non-linear shrinkage of the web, or a change therein, is determined on the basis of the web tension profile. Furthermore, the idea of a preferred embodiment is that the actuator control mapping of the web is determined on the basis of measurement data on the web tension profile and the total web shrinkage.

An advantage of the invention is that when the non-linear shrinkage of the web is measured and the mapping of the actuator control is determined, the web is not subjected to any additional operations that would deteriorate the product properties of the web. The shrinkage profile can be measured without complicated and expensive special devices arranged in the paper or board machine, since the conversion of the tension measurement data into a shrinkage profile requires only simple calibration. The determination of the shrinkage profile does not necessarily require any special arrangements or stages of operation, but the profile can be obtained automatically, if desired.

The invention will be described in greater detail in the accompanying drawings, in which Figure 1 is a schematic top view of a part of a paper or board machine,

Figure 2 shows schematically a typical web tension profile, Figure 3 shows schematically a web shrinkage profile derived from the tension profile of Figure 2, Figure 4 shows schematically a typical web shrinkage profile obtained in laboratory measurements,

Figure 5 is a general view of the correlation between the web shrinkage and tension measurement and the actuator mapping in a paper or board machine, Figure 6 shows schematically a partly sectional side view of an apparatus for measuring the web tension, to be applied in determination of the web's non-linear shrinkage,

Figure 7 shows schematically a partly sectional view of another apparatus for measuring the web tension, to be applied in determination of the web's non-linear shrinkage, viewed in the machine direction, and

Figure 8 shows schematically a partly sectional side view of a third apparatus for measuring the web tension, to be applied in determination of the web's non-linear shrinkage. Figure 1 shows schematically a simplified top view of a part of a paper or board machine. The machine comprises a headbox 1 , from which pulp is supplied to a former 2. In the former 2 the pulp is formed into a web 3, which propagates in a direction denoted by an arrow. The headbox 1 comprises actuators 1 a placed in parallel in the cross machine direction and used to set the height of the slice opening and to adjust the cross-directional properties of the web 3. The web 3 is dried in a dryer 4, whereafter the web 3 travels past measuring devices 5 and 22. Measuring device 5 is provided with measuring means for measuring the web 3 tension, and the data on the web tension is used in a data processing unit 6 to determine the web tension profile or the cross direction profile for the web tension. Measuring device 22 comprises measuring means for measuring such web properties as tension, moisture, basis weight and thickness. These are measured from several points in the cross direction of the web in real time. The measurements are used to determine the profile of each web property in the data processing unit 6. The data obtained from measuring device 22 is supplied to the data processing unit 6, which controls and adjusts the actuators of the machine, such as the actuators 1a of the headbox 1 , on the basis of the data. For the real-time determination of the total shrinkage of the web 3, the machine is provided with measuring means, preferably camera means 21 , for measuring the change in the width. The measurement data obtained by the means is supplied to the data processing unit 6 to be used in the control and adjustment of the actuators of the machine. There may be more than one measuring device 5 and 22 in the paper machine, placed for example in the dryer 4 or positioned before and after the calendering. The use of several measuring devices 5 provides data on the change in the web tension, which can be used to determine the changes in tension profiles and thus the changes in shrinkage profiles. The paper machine also comprises for example a press and a reel and possibly size presses or a calender, which are not shown in the accompanying figure for the sake of clarity. Furthermore, the operation of the paper machine is fully known per se to those skilled in the art, wherefore it will not be described in greater detail herein.

Figure 2 shows schematically a typical web tension profile measured with measuring means provided in the measuring device 5 and determined based on the measurements. The web tension changes non- linearly such that it reaches the maximum values in the middle of the web and correspondingly the minimum values in the web edges.

Figure 3 shows schematically the non-linear shrinkage profile of the web determined on the basis of the measurement results shown in Figure 2. The shrinkage profile is obtained by filtering out high frequencies from the measurement results and by calibrating the remaining measuring data in a manner known per se. The measurement data can be filtered for example in a measuring device, in a data processing unit, in an mapping process of an actuator or in some other suitable stage.

Figure 4 shows schematically a typical web shrinkage profile determined by means of laboratory measurements. It can be seen that the shrinkage profile is similar in shape to the tension profile of Figure 2 and the non-linear shrinkage profile of Figure 3. It has been observed in tests and measurements that there is a high correlation between the tension profile values and the shrinkage profiles: when the web tension increases, the inverse value of the web shrinkage also increases, in other words the web shrinkage decreases. Therefore, further calibration of the measurement results for the tension profile provides the web shrinkage profile in a simple manner, as shown in Figure 3. In principle, further calibration only requires that suitable values are determined for the constant and slope of the conversion equation in order to calculate the shrinkage from the tension measurement values. The calibration can be carried out in several different manners. The web can be divided suitably, in the cross-machine direction, into areas or zones to be calibrated separately, each of which can be subjected to appropriate calibration operations. The calibration can also be implemented by determining a web-specific profile for the initial value of calibration in the cross direction, whereafter a positional additional factor for calibration is determined at each point of the web on the basis of the initial value profile. The most appropriate calibration operation can be determined for example by means of laboratory tests, which is evident to those skilled in the art and will not be discussed in greater detail herein. It is also possible to determine a basic form of the web shrinkage profile for example by means of laboratory measurements or in some other manner. In such a case changes detected in the tension profile during the measurement reveal the process dynamics or the changes in the shrinkage profile. The shrinkage profile and the total web shrinkage can be used further to determine the mapping of the actuators in a manner known per se. Figure 5 shows generally the correlation between the web shrinkage measurements, tension measurements and the actuator mapping in the paper or board machine. Measuring means which measure the web width supply measurement data 15 on the web edges to means 16 that determine the total shrinkage. The means 16 determine the total web 3 shrinkage on the basis of the data 15. In a preferred embodiment the measuring means are camera means 21 , but they can also be other devices known per se to those skilled in the art. To determine the total web 3 shrinkage at a certain interval of the paper machine, data is required on the web 3 width at the beginning and end of the interval. At the wet end of the paper machine the web 3 is cut into a particular width, and data on the web 3 width is obtained for example from the distance between the cutting means. Further, the web 3 width in the point concerned can be determined by means of an optical measuring device, for instance. At the end of the interval under observation, for example at the dry end of the paper machine, the web 3 width can be determined by an optical measuring device, for instance. The means measuring the web tension profile provide measurement data 17, which is subjected to the calibration operations described in connection with Figure 4 in order to determine the shrinkage form of the web in means 18 for determining the shrinkage profile. Data about the total shrinkage and the shrinkage profile is supplied to a data processing unit 19, which combines all the measured data about the changes in the web shrinkage and produces this data 20 to be used in the mapping of the actuators. For the sake of clarity, the means are shown in the figure separate from one another, but naturally the means can also be positioned together, or the different data processing operations can take place in one and the same data processing unit. Figure 6 shows schematically a partly sectional side view of an apparatus for measuring the web tension, to be applied in determination of the web shrinkage. The apparatus comprises a beam-like measuring device 5', which extends over the entire width of the web 3. The web 3 travels over the measuring device 5', supported by an air cushion 8 formed of an air flow carried by the web 3. The measuring device 5' is provided substantially along its entire width with openings 9, which are connected to pressure sensors 10 for measuring the local pressure of the air cushion 8 that is proportional to the local web tension. The separating capacity of the measurement can be improved by increasing the number of the openings 9 and the sensors 10. The measurement data obtained from the pressure sensors 10 is subjected in the data processing unit 6 to operations that convert the data into tension profile values for the web 3. The data processing unit 6 also comprises means 7 for converting the tension profile values into values of the web 3 shrinkage profile. In a preferred embodiment the means 7 also comprise means for filtering out high frequencies from the measurement data. In another preferred embodiment the means 7 further comprise means for determining the mapping of the actuators 1a on the basis of measurement data on the web 3 tension profile and total shrinkage. Besides the determination of the web mapping, the shrinkage profile values can also be used for other purposes, which will not be described in greater detail herein as they are evident to those skilled in the art. The measuring means and the data processing equipment required for determining the shrinkage profile are already present in the paper or board machine, and the profile determination does not interfere with the process in any way. It does not cause losses nor does it require additional stages of operation, but it is a rapid, reliable and low-cost method.

Figure 7 shows schematically a partly sectional view of another apparatus for measuring the web tension, to be applied in determination of the web shrinkage. The apparatus is viewed from the direction of travel of the web 3. The apparatus comprises a measuring device 5", which has a similar cross- sectional shape as the measuring beam 5' shown in Figure 6. An air cushion 8 is formed between the measuring device 5" and the moving web 3 from the air carried by the web 3, in the same manner as described in connection with Figure 6. The pressure of the air cushion 8 acts on the measuring device 5", and the force produced by the pressure is measured by strain gauges 11 provided in the device 5". Measurement results provided by several strain gauges 11 arranged all along the measuring device 5" can be used to determine the non-linear tension profile of the web 3 and further the shrinkage profile, as described in connection with Figure 5. The measuring apparatus differs from the apparatus of Figure 6 in that the means 7 are separate from the data processing unit 6. The separating capacity of the measurement can naturally be improved by increasing the number of the strain gauges 11 provided in the measuring device 5".

Figure 8 shows schematically a partly sectional side view of a third apparatus for measuring the web tension, to be applied in determination of the shrinkage of a travelling web 3. A measuring device 5'" of the apparatus comprises several parallel means 12 that come into contact with the web 3. The means 12 are subjected to a force which is proportional to the web 3 tension and which causes a movement that is transmitted via a lever 13 to a detector 14. The means 12 can also have some other shape than the one shown in Figure 8, such as a cylinder, in which case they are possibly arranged to be rotated by the web 3. The measurement results are used to determine the web shrinkage profile and the mapping of the actuator control as described in connection with Figure 5.

The drawings and the related description are only intended to illustrate the inventive idea. The details of the invention can vary within the scope of the claims. Therefore, the non-linear shrinkage profile of the web can also be determined from measurement results obtained by means of for example an acoustic method or some other method of measuring a property that is proportional to the tension of the moving web. The determination of the non-linear shrinkage profile according to the invention does not require the measurement data that is measured from the web to be utilized for any other purpose.

Claims

1. A method of determining a non-linear shrinkage profile of a moving paper or board web, or a change therein, in which method a moving web (3) is directed past at least one measuring device (5, 5', 5", 5'") which measures the properties of the web (3), characterized by determining the web (3) tension profile or a change therein, and determining the non-linear shrinkage profile (3) of the web or a change therein on the basis of the web (3) tension profile or the change therein.

2. A method according to claim 1, characterized by determining suitable values for the constant and slope of a conversion equation in order to determine the shrinkage profile from the tension profile.

3. A method according to claim 1 or 2, characterized by determining the total web (3) shrinkage, and by determining web (3) mapping by means of the non-linear shrinkage profile of the web determined from the web (3) tension profile, and the total web (3) shrinkage.

4. A method according to claim 3, characterized by using camera means (21) in the determination of the total web (3) shrinkage.

5. A method according to any one of the preceding claims, characterized by filtering out high frequencies from the determined web (3) tension profile before the non-linear shrinkage profile of the web (3) is determined.

6. A method according to any one of the preceding claims, characterized by determining the web tension profile by means of web tension values measured by the measuring device (5, 5').

7. A method according to any one of the preceding claims, characterized by measuring the web (3) tension by means of at least two pressure sensors (10) arranged in the measuring device (5') to measure the pressure of an air cushion (8) that is proportional to the local tension of the web (3).

8. A method according to any one of claims 1 to 6, characterized by measuring the web (3) tension by means of at least two strain gauges (11) arranged in the measuring device (5") to measure a force resulting from the web (3) tension.

9. A method according to any one of claims 1 to 6, char- acterized by measuring the web (3) tension by means of at least two contact means (12) arranged in the measuring device (5'") to substantially come into contact with the web (3) to measure a force resulting from the web (3) tension.

10. An apparatus for determining a non-linear shrinkage profile of a moving paper or board web, or a change therein, the apparatus comprising at least one measuring device (5, 5', 5", 5'") for measuring the properties of the web (3), and means (6) for processing measurement data obtained from the measuring device (5, 5', 5", 5'"), characterized in that the measuring device (5, 5', 5", 5'") and the means (6) for processing the measurement data obtained from the measuring device (5, 5', 5", 5'") are arranged to determine the web (3) tension profile or a change therein, and that the apparatus comprises means (7) for determining the non-linear shrinkage profile of the web (3) or a change therein on the basis of the tension profile or the change therein.

11. An apparatus according to claim 10, characterized in that the apparatus is provided with means for determining the total web (3) shrinkage and means for determining web (3) mapping by means of the nonlinear shrinkage profile of the web (3) determined from the web (3) tension profile, and the total web shrinkage.

12. An apparatus according to claim 11, characterized in that the means for determining the total web (3) shrinkage comprise camera means (21).

13. An apparatus according to any one of claims 10 to 12, characterized in that the means (7) for determining the web (3) shrinkage profile comprise means for filtering out high frequencies from the tension profile.

14. An apparatus according to any one of claims 10 to 13, characterized in that the measuring device (5') is provided with at least two pressure sensors (10) arranged to measure the pressure of an air cushion (8) that is proportional to the web (3) tension, in order to measure the web (3) tension.

15. An apparatus according to any one of claims 10 to 13, characterized in that the measuring device (5") is provided with at least two strain gauges (11) arranged to measure a force resulting from the web (3) tension, in order to measure the web (3) tension.

16. An apparatus according to any one of claims 10 to 13, characterized in that the measuring device (5'") is provided with at least two contact means (12) arranged to substantially come into contact with the web (3) and arranged to determine a force resulting from the web (3) tension in order to measure the web (3) tension.