WO2004035443A1

WO2004035443A1 - Web tension control

Info

Publication number: WO2004035443A1
Application number: PCT/FI2003/000768
Authority: WO
Inventors: Topi Tynkkynen; Matti Innala; Teppo Kojo; Tatu PITKÄNEN; Petteri Lannes
Original assignee: Metso Paper, Inc.
Priority date: 2002-10-17
Filing date: 2003-10-16
Publication date: 2004-04-29
Also published as: FI20021852A0; FI20021852A; AU2003268996A1

Abstract

The invention relates to a method and an apparatus for controlling the tension of a moving web during the manufacture of a web-like material, whereby a moving drawing means (2) is pressed against the web (1) and an electrostatic charge is generated in the web and/or the driving means in order to change the friction between the web and the driving means. In accordance with the invention, the tension of the web (1) is measured and the charge to be generated is adjusted on the basis of the measurement result.

Description

Web tension control

Technical field

The invention relates to the manufacture of a web-like material, such as paper, and is directed to the control of the tension of a moving web. The invention can be used with drawing rollers and nip rollers, for instance.

Technical background

Paper is manufactured by continually condensing a cellulose fibre suspension into an even web. The web is conveyed by means of rollers or wires, for instance. The required web tension is produced by means of drawing rollers, for instance. The grip required for these is provided in the compression between the wire and the cylinder. Suction rollers can also be used, whereby the web is sucked by underpressure against the roller. It may be difficult to achieve adequate grip especially at high speeds. One reason for this is air carried between that reduces friction. The negative impact of air can be diminished with the aid of a grooved roller surface. However, at low running speeds, grooving is hardly useful.

Patent specification FI 104645 discloses an arrangement for improved grip by means of drawing rollers. In this specification, the drawing rollers are connected to a low potential and electrodes connected to a higher potential are disposed in the vicinity of the rollers. Thus, the electric field generated between the rollers and the electrodes attaches the web to the drawing rollers with stronger hold. With the web more intimately adhered to the roller under the electric force of attraction, the amount of intermediate air consequently decreases.

Summary of the invention

A method and an apparatus for web tension control have now been invented as defined in the independent claims. The dependent claims present some preferred manners of implementing the invention.

In accordance with the invention, the web is tightened by means of a driving means such as a roller, and an electrostatic charge is generated in the web or the driving means, so that the web, owing to the electric attractive force, tends to adhere to the driving means or inversely, to reject it, the web tension is measured and the generated charge is adjusted on the basis of the measurement result. Preferably, the charge is generated in the web and the driving means is at the earth potential. The invention can be used especially in order to increase the grip between the web and the driving means.

Charge can be generated especially by means of electrodes. The electrode is preferably detached from the web and located at the side of the web opposite to the driving means. The driving means can act also as a counter-electrode. When the driving means is at a different potential, especially earthed, the web is pressed against the driving means under the electrostatic force. The electrode may be especially an ion blast electrode.

The electrode is preferably out of contact with the web. It may have a width equal to the entire web, for instance, or only to part of the width of the path (to contribute to web feeding, for instance).

The charge to be generated can be generated by changing the current, voltage or power fed to the electrode, or by varying the distance of the electrode from the web or the driving means.

In accordance with the invention, an excellent grip is achieved even by means of a relatively low static voltage.

The invention can be used, for instance, with a drawing roller, a driving nip or a drawing roller group without a wire, for instance, in a coating machine or in a drying cylinder set. The invention is especially applicable to path feeding in a coating machine.

The invention can be carried out with a simple and relatively small-sized apparatus having low energy consumption. The service requirement is also small. The apparatus causes very low noise. Yet it is easy to adjust and adjustment can be performed by means of an exact algorithm. The adjustment is thus also easily automated. If desired, the tension can also be adjusted in different ways at different locations of the width of the path (profiling). The desired grip is also achieved at low speeds. In this manner, tension will be well controlled in all situations, also as changes occur. There will also be smaller risks of breakage.

In accordance with the invention, the roller or wire grip can be adjusted so as to be constant, even though the friction coefficient of the web would change (e.g. coated/uncoated grade). It is also possible to develop an expert system to support the automation system for optimising the grip of the driving means or the web feed belts with minimised risk of breaking as a result.

The invention allows the number of rollers in drawing roller groups to be reduced and possibly also the suction rollers to be replaced.

Drawings

The accompanying drawings pertain to the written description of the invention, and they illustrate some manners of implementing the invention.

Figure 1 shows a drawing roller of the invention, and its control.

Detailed description of the invention

In a web driving means, a moving surface is pressed against the web, the web adhering to the surface by friction. The driving means may especially be a roller. There may be a plurality of such successive drawing rollers, which form a so called drawing roller group. There may also be a second roller disposed against the roller so that a drawing nip is formed.

Electrostatic charge can be generated in the web by means of a suitable charging device, such as a friction device, by the touch or induction of a charged electrode or by an ion blast electrode. Owing to the electrostatic potential difference, the other side of the web forms a grip point together with the surface of the driving device. Preferably, a method not touching the web is used for charging, such as ion blast. Adequate charge can be achieved with power low enough to readily allow for safety aspects. Commercially available suitable electrodes can be used in the system, such as ion blast electrodes. Generally, charge generated at one location will prevail at the web surface over a sufficient period. After the grip area, the charge may be eliminated if necessary, for example by bringing the surface into contact with an earthed surface, such as the surface of an earthed roller.

The driving means surface facing the web is preferably earthed. Earthing can be carried out for example by means of an electrically conductive coating. Thus, for instance, the roller coating can be made of polymer or any suitable metal, such as aluminium or steel. The surface of the driving means can also be charged to potential of the same sign as the web or to opposite potential.

Owing to the electrostatic attractive force, the web is pressed more tightly against the surface of the driving means, thus also reducing the access of air to the space between the web and the driving means. However, if necessary, to further reduce the action of air, the roller surface can be grooved, for instance.

An ion blast apparatus has a peak-shaped electrode and a large flat counter- electrode, between which a strong electric field is generated. Then the electric field forms a conical corona starting from the tip of the peak-shaped electron. Usually there is a plurality of peak-shaped electrodes so that a desired kind of coverage is achieved. The counter-electrode (in this case preferably a driving means) is usually at the earth potential. At the tip of the peak-shaped electrode, a corona discharge is produced, which ionises the surrounding gas. Then the formed ions having a sign opposite to that of the electrode charge and particles adhered to these pass in the conical field towards the counter-electrode. In this way a flow called ion blast is generated. Preferably the peak-shaped electrode voltage has a negative voltage, because the gas molecules in air naturally form negative ions. This allows a regular and controllable ion flow to be readily obtained. The peak-shaped electrode may have an absolute value voltage e.g. in the range from 30 to 1000 kV, typically in the range from 80 to 160 kV. The distance of the electrodes to the counter-electrode may be e.g. 2 to 2000 mm, typically 10 to 100 mm.

In the arrangement of the invention, with the use of ion blast, the web is conveyed across the field, whereby charged particles adhere to the web. Ion blast is easy to construct and control. Patent specification FI 104645 discloses a number of methods for charging a web, by means of ion blast techniques, among other things.

The system comprises at least one charging device, by means of which a charge is generated in the web. The charge can be generated over the entire width or only part of the width. The system may comprise a plurality of charging devices in the transverse direction.

The charging device is controllable to allow variation of the generated charge.

The surface of the drawing device preferably constitutes the counter-electrode of the ion blast electrode. In that case, the drawing device can earthed.

In electrode charging, the pressure p generated by the electrode in the web can be calculated by the formula p = F/A = I /(ε-v ), in which I is the current supplied to the electrode (actually the current per width unit of the web, yet in the practice one may refer to current if the width remains constant), v is the web speed, ε is permittivity (air permittivity being approximately the same as vacuum permittivity ε₀), F is the active electrostatic force and A is the area on which the force is exerted. Consequently, the pressure is most advantageously regulated by altering the current. The current can thus be defined by the formula I = v-(pε)^/2. The current depends on the electrode voltage and the discharge resistance, which, in turn, depends on the electrode size, shape and similar matters, which should preferably be maintained constant.

The arrangement of the invention allows the control of electrostatic forces applied in conjunction with a moving web by means of an electric current. The current required can be calculated by the formula above. The desired pressure (or force) is given as a set value, the permittivity is indicated as a constant and the speed can be obtained from the machine (the automation system, for instance). The arrangement may include back coupling from a controllable system, so that an electric current is directed to the electrodes on the basis of the current value measured for the web tension.

The system can be implemented using ion blast electrodes and charging generators. The generators can be controlled by programmable logic. The generators can be set to act as current sources, and then the current intensity can be adjusted to the desired value via an automation system, for instance.

The web tension can be measured at one or more locations as desired. In one embodiment, the tension is measured after the driving means. The tension may also be determined e.g. directly on the basis of the moment of the drive of the driving means and/or the subsequent or the preceding driving means. The tension may also be measured before the driving means or at the driving means.

In a normal drawing roller, which does not provide a particularly good grip, the web tension is approximately the same before and after the roller. In accordance with the invention, the grip can be adjusted so that the web tension before the roller is substantially higher or lower than after the roller as desired. The tension can be measured before and after the drawing roller, on the basis of the difference between the moments of the preceding roller and the subsequent roller, and the traction or the braking and also the supply to charging electrodes can be adjusted accordingly.

In one embodiment, there are several charging devices in the transverse direction. The devices can also be separately adjusted. Similarly, the tension can be determined at several parallel locations, thus providing the tension profile for the adjustment of the charging devices.

In one embodiment, the grip of the drawing means is adjusted so as to achieve the desired slippage. This allows particularly a reduction of the risk of breakage as changes occur, such as the web-feeding band breaking because of a pull. Also, this is of particular benefit in accelerations and during braking of offline-machines. The control of the charges of the driving means and the web allows an increase or a decrease in the grip whenever necessary. The exemplified charges may even make the web reject the drawing means.

In the drawing roller design of figure ϊ, the paper web 1 is tensioned by means of the roller 2 under the web. At the roller draw area, above the web, an electrostatic charging electrode 3 (ion blast electrode) extending across the web has been placed, the voltage of the electrode being supplied by a generator 4. The roller surface at least is electrically conductive and earthed. The electrode charges the upper surface of the web electrostatically, so that the electric traction force presses the web under a given pressure against the roller, thus increasing friction. A device 5 for measuring the web tension is placed after the roller 2. The value provided by this is compared with the target value in comparison unit 6, and the calculating unit 7 calculates the voltage value required for the generator 4 on the difference. The value of the web speed v is also fed to the calculating unit.

As an assumed example, the device of figure 1 comprises

Path speed v = 2500 m/min Current I = 20 mA/m

Voltage U = 60 kV

Permittivity ε₀ = 8.85 • 10^"12F/m

Roller diameter d = 0.8 m

Refraction angle in the drive area = 30 °

Friction coefficient μ = 0.2

This yields

Length of drive area 1 = 0.209 m

Pressure (=(ι²/ε₀-v²)) p = 2.6 • 104 Pa

Path tension (= μ-l-p) T = 1090 N/m

Required power P = 1200 W

Claims

1. A method for controlling the tension of a moving web in the manufacture of a web-like material, whereby a moving drawing means (2) is pressed against the web (1) and an electrostatic charge is generated in the web and/or the driving means in order to change the friction between the web and the drawing means, characterised in that the web (1) tension is measured and the charge to be generated is adjusted on the basis of the measurement result.

2. A method as defined in claim 1, in which the web tension is measured after the driving means (2).

3. A method as defined in claim 1 or 2, in which the charge is generated by means of an electrode (3), such as an ion blast electrode.

4. A method as defined in claim 3, in which the charge to be generated is adjusted by varying the current, voltage or power supplied to the electrode (3).

5. A method as defined in claim 3 or 4, in which the charge to be generated is adjusted by varying the distance between the electrode (3) and the web or the drawing means, respectively.

6. A method as defined in claim 4, in which the current I is adjusted according to the formula I = v-(pε)¹ , in which v = web speed, p = the pressure generated by the electrode in the web and ε = permittivity of the medium between the electrode and the web.

7. A method as defined in any of the preceding claims for increasing the friction between the web and the drawing means.

8. An apparatus for controlling the tension of a moving web in the manufacture of a web-like material, the apparatus comprising a moving drawing means (2) pressed against the web (1) and an electrode (3), by means of which an electrostatic charge is generated in the web or the driving means, characterised in that the apparatus comprises additionally a means (1) for measuring the tension of the web (1) and an adjusting device (3) for adjusting the current supplied to the electrode on the basis of the measurement result.