GB2250273A

GB2250273A - Web tension control system

Info

Publication number: GB2250273A
Application number: GB9125132A
Authority: GB
Inventors: Leonard James Bryant; Derek Henry Dyett; Graham Foster Wright
Original assignee: Molins Ltd
Current assignee: Mpac Group PLC
Priority date: 1990-11-29
Filing date: 1991-11-26
Publication date: 1992-06-03
Also published as: GB9125132D0; GB9026002D0

Abstract

In a cigarette maker paper tension is controlled, particularly upstream of the printer, by passing the paper (2) in a loop (25) around a pulley (24) carried by a pivoted counterbalanced arm (26) so as to impose a predetermined tension on the paper. The size of the loop is controlled by drive means (14) which feeds paper into and/or out of the loop and which is responsive to position of the pulley arm. By use of a high-speed servo-motor (20), the drive can react rapidly to irregularities in the feed of the paper so as to maintain the pivoted arm in a working range in which the predetermined tension is imposed on the paper. <IMAGE>

Description

Web Tension Control Svstem The present invention relates to control of web tension.

In cigarette making machines a paper web is wrapped around a tobacco stream to form a continuous cigarette rod. The web is delivered from a reel of paper on a bobbin and, prior to being wrapped around the tobacco stream, the web is printed at regular intervais (e.g.

with a cigarette brand name). Typically the reels of paper are relatively large (e.g. 450mm in diameter) and the web is delivered at high speed (e.g. 600m/min). At such relatively high speeds small eccentricities in the shape of the reel and other factors can lead to variations in tension in the web as it is delivered through the machine. Relatively small variations in paper tension as it is conveyed through the printer are believed to contribute to small irregularities in the spacing of the print on the paper, and hence to print wander on the completed cigarettes. The present invention is concerned with controlling tension in a moving web, particularly but not exclusively in cigarette making machines.Other examples of where control of web tension may be of importance are found in printing and other processes where the web is required to be treated at regular or predetermined positions.

According to the invention a system for controlling web tension comprises movable guide means around which a loop of the web passes, means for imposing a predetermined load on the guide means which opposes tension of the web in the loop, drive means for the web arranged to convey web into and/or out of the loop, means for sensing the position of the guide means or the size of the loop, and control means for said drive means, said control means being responsive to signals from said sensing means and arranged to vary the web conveyance speed of the drive means so as to maintain said guide means in a working range of positions in which said predetermined load is imposed on the guide means.

The system is particularly useful where a moving web is subject to disturbances tending to cause variations in its tension. Such disturbances may be caused by the effects of web conveying means (other than said drive means) or variations in resistance to web motion, for example.

Ignoring dynamic effects the predetermined load imposed on the guide means will determine the tension in the loop. Generally the ten sion will also be substantially the same in those parts of the web extending beyond the loop (either upstream or downstream) up to the positions at which the drive means (or other drive means) engages the web.

Thus in a preferred construction the drive means comprises a capstan roller or the like arranged immediately upstream of the loop, so that the roller controls speed of the web moving into the loop, and downstream of the loop the web passes over idler rollers or similar guide means which do not substantially affect its tension, up to a position at which the web is printed for example.

In a further preferred construction the drive means is controlled by a high speed servo-motor. It will be appreciated that in order to maintain the guide means within its working range of positions (e.g.

between upper and lower stops) the drive means may have to react quickly to sudden or irregular loads imposed on the web, e.g. by eccentricities in a reel feeding web to the drive means. Thus the drive means must be capable of effectively isolating web in the loop from significant tension variations upstream of (or downstream of) the loop. Ideally the drive means is capable of such isolation up to the breaking tension of the web.

Preferably the system includes a guide means in the form of a pulley. This may be mounted at one end of a pivoted arm, the other end of which carries a counterbalancing weight. In a working range of positions of the arm (e.g. for 65" arc of movement) the tension in the web of the loop is determined by the weight, and is substantially constant.

The control means is preferably effective to cause the drive means to run at a nominal speed related to the required rate of consumption of web by an associated process (e.g. the printer on a cigarette making machine). The control means preferably modifies the speed of the drive means, e.g. to feed more or less web into the loop, in accordance with the sensed position of the guide means.In a typical arrangement in a cigarette making machine we have found that causing the drive means to run at + 10% of current running speed when the guide means is displaced from a nominal central position or range is sufficient to maintain the guide means within its working range not only at nominally constant running speeds of the machine but also when the machine speed is varying (e.g. during start-up), even when tension is varied significantly upstream of the drive means, e.g. due to deliberate disturbance of the rotation of a reel feeding the web to the drive means.

The invention will be further described, by way of example only, with reference to the accompanying diagrammatic drawing, which shows a web tension control system in a cigarette making machine.

A web 2 of cigarette paper is delivered from a reel 4 through a web tensioning device 6 to a printer 8. Beyond the printer 8 the web passes to a rod forming unit (not shown) of the cigarette making machine. The web 2 passes between rollers 10, 12 to a nip between a capstan roller 14 and an associated counter roller 16. The capstan roller 14 is driven through a timing belt 18 by a servo-motor 20. The rollers 10,12 form part of a splicing unit at which the leading end of a web 2A from a new reel 4A is spliced to the trailing end of the web 2 when the reel 4 is near expiry. Subsidiary drive capstans 22 are provided for driving the leading ends of new webs during splicing.

Downstream of the main capstan drive roller 14 the web 2 passes around a guide roller 24 mounted at one end of a pivoted arm 26. At its other end the arm 26 carries a counterbalancing weight 28. The web 2 passes from the guide roller 24 to further, non-driven rollers 30, 32, which direct the web 2 to the printer 8 at which a brand name or the like is printed at spaced intervals on the web so as to appear on individual cigarettes produced on the machine downstream of the rod forming unit.

The pivoted arm 26 is connected to a rotary potentiometer so that an electrical signal indicative of position of the roller 24 can be generated on a line 36. A motor controller 38, which may include processor means, for the servo-motor 20 receives the position signal on line 36, together with a signal on a line 40 relating to the nominal machine speed (and hence the nominal required speed of the web 2). The machine speed signal on line 40 may be supplied by a tacho-generator driven by an appropriate part of the machine. The controller 38 is connected to the servo-motor by a control line 42. It will be appreciated that the illustrated control system represents a simplification of what may be required in practice: for example, the controller 38 may require an input signal indicative of current speed of the servo-motor 20.

In use, the web 2 is driven by the capstan roller 14 at the nominal machine speed as modified by signals received from the potentiometer 34 and processed by the controller 38. The arrangement is such that the capstan roller 14 is driven so as to retain the guide roller 14 in a median position, or at least within a working range where the counter balance weight 28 imposes a predetermined load on the web 2. It will be appreciated that if the capstan roller 14 controls the feed of web into the loop 25 passing around roller 24 so that the length of the loop remains reasonably constant or at least so that the roller is within upper and lower stop positions of the arm 26, the tension imposed on the web 2 in the loop 25 is determined by the moment applied by the weight 28 (ignoring any secondary dynamic effects).Thus the tension in the web 2 downstream of the roller 24, and in particular as it passes through the printer 8, will be substantially constant, irrespective of the tension upstream of the capstan roller 14. In order to achieve this isolation of tension by the capstan roller 14, especially where tension may vary considerably upstream of the capstan roller, e.g. because of a noncircular reel 4 or a braking load imposed on the reel, the response time of the servo-motor 20 (and associated control) needs to be short.In a typical arrangement in a cigarette making machine, where the maximum speed of the web is 600m/min, a system capable of responding in a period of the order of 1 O0msec has been found to be capable of maintaining a substantially constant tension of 235 gms in the loop 25 while tension in the web 2 upstream of the roller 14 varied between 100 and 800 gms. In this arrangement the servo-motor 20 was controlled so as to run at 1 10% of the nominal web speed according to the position of the guide roller 24.

It may be noted that due to the geometry of a pivoting arm such as the arm 26 the static tension imposed by the counterbalance weight 28 will vary slightly depending on the position of the arm. In the arrangement referred to in the preceding paragraph the imposed tension varied by + 1.5% to - 2.6% within a 65" working arc of the arm 26.

A proximity switch (not shown) in the vicinity of the guide roller 24 detects whether web 2 is present, and disables the system if web is not present. If the web breaks during running the system is disabled after a delay of approximately 1 second: the delay allows the capstan roller 14 to continue to feed enough web to facilitate re-threading. The web tensioning system is also disabled when the machine is stationary.

Claims

1. A system for controlling web tension comprising movable guide means around which a loop of the web passes, means for imposing a predetermined load on the guide means which opposes tension of the web in the loop, drive means for the web arranged to convey web into and/or out of the loop, means for sensing the position of the guide means or the size of the loop, and control means for said drive means, said control means being responsive to signals from said sensing means and arranged to vary the web conveyance speed of the drive means so as to maintain said guide means in a working range of positions in which said predetermined load is imposed on the guide means.

2. A system as claimed in claim 1, wherein the drive means comprises a capstan roller or the like arranged immediately upstream of the loop, so that the roller controls speed of the web moving into the loop, and downstream of the loop the web passes over idler rollers or similar guide means which do not substantially affect its tension.

3. A system as claimed in claim 1 or claim 2, wherein the drive means is controlled by a high speed servo-motor.

4. A system as claimed in any preceding claim, wherein the guide means comprises a pulley mounted at one end of a pivoted arm, the other end of which carries a counterbalancing weight.

5. A system as claimed in any preceding claim, wherein the control means is arranged to cause the drive means to run at a nominal speed related to the required rate of consumption of web, said speed being modified in accordance with the sensed position of the guide means.

6. A system substantially as herein described, with particular reference to the accompanying drawings.