Method and apparatus for controlling a moving paper web

Method and apparatus for controlling moving paper web during manufacture or subsequent processing, wherein a web elongation profile and a moisture profile are measured. The web elongation is governed by at least two CD moisture actuators, which are situated at different stages in the web manufacturing process or subsequent processing. Means for measuring the web elongation profile and the moisture profile are situated after the CD actuators.

FIELD OF THE INVENTION
 The invention relates to a method of controlling a moving paper web during
 manufacture or subsequent processing. The invention further relates to an
 apparatus for controlling a moving paper web during manufacture or
 subsequent processing.
 BACKGROUND OF THE INVENTION
 The paper web deforms geometrically during manufacture, or during
 subsequent processing operations, especially operations involving wetting
 or drying of the paper web. This deformation includes shrinking as its
 water content falls below about 35% by mass, and stretching or elongation
 due to applied forces. Forces are applied especially in the direction of
 movement of the web, including a tension in the plane of the web. The
 tension is generally not uniform, since the shrinkage and elongation are
 not uniform, and these three properties are related to each other.
 However, shrinkage and elongation are properties of the paper web, while
 tension is a property of the process. Nonuniformity in the web tension
 leads to problems in operating the paper making or paper processing
 equipment, including web breaks which cause production interruptions.
 Nonuniformity in the elongation leads to a variety of problems, including
 sheet breaks, caused by the fact that the paper web is longer in some
 place than in others.
 U.S. Pat. No. 5,694,448 discloses a solution for regulating the web tension
 profile. That method requires measurement of the caliper and grammage
 profiles in addition to the web tension profile. Further, it achieves its
 aim of controlling the web tension profile by manipulating the actuators
 which are also used to control the profiles of caliper and grammage. Thus,
 improvements in the web tension profile are generally achieved at the cost
 of effects in the caliper and grammage profiles, and a compromise solution
 is obtained. Further more, the solution enables the regulation of the web
 tension profile only, and does not consider the web elongation profile.
 An article "Optimize or compromise? The art of former section tuning".
 Odell M. 51 Appita Annual General Conference 1997 Proceedings, Volume 1
 discloses controlling the web tension profile by using a steam box in the
 press section to adjust the shape of the moisture profile going into the
 dryers. By making the edges drier and thus changing the drying rate
 profile it is possible to reduce the tension profile error at the reel.
 The moisture profile at the reel is corrected using rewetting in the
 dryers. In this case too, only the web tension is adjusted, and does not
 consider the web elongation profile. In failing to consider the web
 elongation, control of the web tension can lead to an inferior solution
 with undesirable effects on the web elongation profile.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a method and an apparatus, in
 which the above drawbacks can be avoided.
 The method of the invention is characterized by comprising the steps of
 measuring a web elongation profile and a moisture profile and controlling
 the web elongation by at least two actuators which affect the cross
 machine direction (CD) moisture profile, said actuators being situated at
 different stages in the web manufacturing process, and means for measuring
 the web elongation profile and the moisture profile being situated after
 the CD moisture actuators.
 Further, the apparatus of the invention is characterized by comprising
 means for measuring a web elongation profile and a moisture profile, at
 least two actuators, which affect the CD moisture profile and means for
 controlling the web elongation by said CD moisture actuators, said CD
 moisture actuators being situated at different stages in the web
 manufacturing process, and the means for measuring the web elongation
 profile and the moisture profile being situated after the CD moisture
 actuators.
 The basic idea of the invention is that the web elongation is controlled by
 use of at least two actuators which affect the CD moisture profile, said
 actuators being situated at different stages in the web manufacturing
 process. Preferably, a first CD moisture actuator is situated in the wet
 press or elsewhere near the wet end of the paper machine, and a second CD
 moisture actuator is situated near the reel or elsewhere near the dry end
 of the paper machine. A first moisture profile measurement device and a
 first web elongation profile measurement device are situated after the
 second moisture profile actuator. The moisture and web elongation profile
 measurements need not be at exactly the same location, but preferably
 there are no wetting or drying processes between them.
 The elongation profile can be measured directly for example by using a
 method disclosed by K. R. Wadhams et alii, "The measurement of
 differential CD shrinkage", Paper Technology, January 1991. Alternatively,
 the elongation can be inferred tram a known tension profile and a known
 shrinkage profile. The tension profile can be measured for example by
 using the method and equipment presented in U.S. Pat. No. 5,052,233. The
 shrinkage profile can be measured for example by using the method
 disclosed by E. DiMauro et alii, "New on-line sensor for paper shrinkage
 measurement", Proc Control Systems '94 (May 31-June 2, Stockholm, Sweden).
 Measurements of shrinkage or elongation may be provided from laboratory
 analysis of samples taken from the paper, or may be provided using on-line
 measurements. Since tension is a property of the process, it must be
 provided from on-line measurements
 Cellulose fibers swell when wetted, and shrink when water is removed. This
 dimensional change is less along the axis of the fiber than across the
 axis. Paper is formed from an aqueous suspension in the wet end and
 dewatered through pressing and drying to a moisture level of typically
 5-7%. As a result, it shrinks by several percent. No shrinkage occurs in
 the initial dewatering in the forming section and presses, as the water
 content is too high and fiber bonding is weak. As soon as the sheet has
 sufficient fiber bonding strength to support itself, shrinkage will
 accompany further dewatering.
 However, the paper web is dimensionally restrained for part of its journey,
 being supported and transported by fabrics which impose frictional
 constraints on shrinkage. It is completely supported through the forming
 and pressing section, but open draws occur thereafter. When subsequent
 dewatering occurs to these fabrics, the sheet receives an increment of
 strain, which can be relaxed only when it is in an open draw, where it is
 less constrained in dimension. If the sheet is sufficiently wet, the
 strain is generally plastic (no stress is induced), or viscous (stress
 depends on rate of deformation, rather than on amount). This is the case
 in the forming and pressing sections, and in the initial drying stages.
 Strain due to constrained shrinkage when the sheet is drier is generally
 elastic--it induces a stress or tension. The elastic strain will be
 maintained if suitable tension is applied, and will relax otherwise.
 The sheet is under machine direction (MD) tension in open draws, to support
 the mass of the web, stabilize its path, provide forces needed to detach
 it from fabrics and cylinders, and to overcome aerodynamic drag. Local MD
 tension prevents relaxation of strain in the sheet in the MD. However, if
 the strain can relax in the CD, this leads to local Poisson or
 Poisson-like elongation in MD, and results in a lower local MD tension.
 There is no externally applied CD tension per se, but the sheet is
 dimensionally restrained in CD at each end of the draw, and the length of
 the draw is typically much shorter than the sheet width. As a result,
 there is a CD tension profile induced in the open draw as a result of the
 CD straining due to dewatering. This CD tension is much less than the MD
 tension, and falls essentially to zero at the sheet edges, which are less
 restrained than the sheet center. Thus, although the sheet stretches in
 the MD both plastically and elastically, more than countering the effect
 of shrinkage in MD, it shrinks overall in the CD, and the shrinkage is
 greater at the edges than in the middle of the sheet.
 Local MD dimensional change is the sum of local elongations due to CD
 shrinkage, local plastic strain, and local elastic strain. The amount of
 local elastic strain largely determines the local web tension (together
 with the local elastic modulus of the sheet). In an open draw, the total
 MD dimensional change is the same at all locations across the sheet, and
 reflects the difference in speed of the upstream and downstream rolls. The
 MD tension profile will reflect the balance at each point between
 cumulative MD plastic strain, cumulative MD elongation due to CD
 shrinkage, current plastic or viscous straining, and cumulative elastic
 straining at a given speed difference.
 In the initial drying, where strain tends to be viscous or plastic rather
 than elastic, nonuniform MD elongation due to nonuniform CD shrinkage does
 not affect the tension profile. In later drying, where strain tends to be
 elastic, nonuniformity in MD elongation causes a tension profile, due to
 the processes described above. There is a gradual transition between these
 two regimes, occurring during the initial stages of drying. In the presses
 and before the first open draw, strain is plastic in both MD and CD, and
 no dimensional change occurs.
 The basic principle of web elongation control according to this invention
 is to change the evolution of the CD shrinkage profile, so that the CD
 shrinkage nonuniformity occurs mainly in the initial drying, where plastic
 strain of the sheet will match MD elongation due to CD shrinkage.
 Shrinkage in later drying is then more nearly uniform, so that the MD
 elastic strain (and hence MD tension) are more uniform.
 Since the evolution of the shrinkage profile is determined by the evolution
 of the moisture profile, web elongation control is carried out by
 nonuniformly dewatering the sheet in the wet end. The intention is that
 parts of the sheet which tend to shrink more in the CD are dewatered more
 in the plastic regime. Ideally, this is achieved using a steam box or
 other suitable CD moisture actuator in the forming section or presses.
 Since this also causes a nonuniform moisture profile entering the dryers,
 it may adversely affect the moisture profile at the reel. Preferably,
 there is also another CD moisture actuator situated close to the reel to
 correct the moisture profile. If this latter actuator is close to the
 reel, then its effect on subsequent shrinkage, elongation, and tension is
 small. However, if it is distant from the reel, its effect on elongation
 and tension may be significant.
 It must be noted that this control strategy will often produce high rewet
 flows in the same regions as high steam box pressures. This is the
 opposite of the conventional dual-actuator moisture control, which
 commonly attempts to avoid that situation in order to economize on energy
 usage. Said strategy is far from optimal with respect to the resulting
 elongation and tension profiles. Controlling the elongation and/or tension
 profiles in addition to the moisture profile leads to a particular range
 of energy usage, limiting scope for minimization of energy usage.
 Manipulating the average level of steam box and rewet actuators without
 compromising their profiling can still be used for energy optimization.
 Note also, that it is impossible to achieve flat CD profiles of MD
 elongation or tension. This is because the CD profile of CD tension falls
 to zero at the sheet edges in an open draw. As a result, elastic MD strain
 is lower at the edges, even with zero shrinkage, as the sheet is free to
 contract elastically in CD to relax elastic MD strain. This is a
 consequence of standard membrane mechanics. Thus, neither the elongation
 nor the tension can be uniform, and they will be related through the CD
 shrinkage.
 Moreover, by dewatering nonuniformly in the presses and in the first open
 draw sections, the overall shrinkage profile Itself may be changed. This
 is because dewatering in the elastic strain regime is reduced for those
 parts of the sheet which previously exhibited the most CD shrinkage.
 Dewatering in a plastic regime with open draws (as in the dryers) is
 accompanied by CD shrinkage. However, MD elongation is uniform across the
 sheet, since the straining in the first few open draws is viscous, rather
 than elastic. Additionally, dewatering in a plastic regime with a fully
 restrained sheet (as in the presses) causes plastic CD strain instead of
 CD shrinkage, and no MD elongation.
 An advantage of the invention is that the web elongation can be adjusted
 simply, accurately and in a versatile manner. A further advantage of the
 invention is that actuators for control of neither the caliper profile nor
 the grammage profile need be used for control of web elongation. These
 actuators may thus be freely used to control their designated profile
 properties, including correcting for any effects caused by the web
 elongation control of this invention. As a result, the control of
 elongation does not disrupt control of the caliper or grammage profiles,
 and no compromise is required.
 Furthermore, since the invention uses two CD moisture actuators, it is
 utilizing a common redundancy which exists in many paper machines. As a
 result, although the web elongation is controlled using a CD moisture
 actuator, and this will affect the moisture profile, a second CD moisture
 actuator is available to compensate for these effects. Thus, both web
 elongation and moisture profiles can be controlled without compromise.
 In this disclosure the tern `paper` also refers to board and tissue.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 schematically shows a paper machine. The paper machine comprises a
 headbox 1, from which pulp is fed into a former 2, where a fiber web 3 is
 formed of the pulp. The web 3 is conveyed to a press 4 and further to a
 dryer unit 5. From the dryer unit 5 the web 3 is conveyed to a reel 6. The
 paper machine may also comprise, for example, size presses or a calender,
 which are not illustrated in the Figure for the sake of clarity.
 Furthermore, the function of the paper machine is known per se for those
 skilled in the art, and will therefore not be further explained in this
 context.
 The web elongation is controlled by use of at least two actuators which
 affect the CD moisture profile, said actuators being situated at different
 stages in the web manufacturing process or subsequent processing such as
 coating, calendering or supercalendering, and printing, especially
 operations involving rewetting or drying of the web, or operations
 involving stretching of the paper web in the direction of the movement of
 the paper web. For example, the first CD moisture actuator could be a
 sectionally-controlled steam box 7 in the wet presses 4, and the second CD
 moisture device could be a sectionally-controlled rewet spray device 8
 near the reel 6. Note that the CD moisture actuators can be of types other
 than those depicted, such as sectional infra-red, sectional pocket
 ventilation, and the like. Similarly, the two CD moisture actuators can be
 situated at other locations in the paper machine, provided they are
 separated by at least one processing operation, such as a group of dryer
 cylinders. A measurement beam 9 comprises a web elongation measurement
 device, which is used for measuring the elongation profile, and a moisture
 profile measurement device, which is used for measuring the moisture
 profile.
 FIG. 2 is a block diagram showing a solution of the invention. In one
 embodiment of the invention, the first CD moisture actuator is governed to
 control the web elongation profile measured by the web elongation
 measurement device, and the second CD moisture actuator is governed to
 control the moisture profile measurement by the first moisture device.
 These can be two independent Single Input Single Output (SISO) profile
 controllers, or they can have provision for cooperation or decoupling.
 FIG. 3 schematically depicts a variation on this embodiment, which employs
 a steam box and a rewet actuator, and in which two optional enhancements
 are shown. First, the control of the steam box employs an optimization,
 which includes the operating state of the rewet actuator. Thus, control of
 web elongation using the steam box actuator is not allowed to force the
 rewet actuator outside set limits. Hence, the moisture profile is given
 control precedence over the web elongation profile in a simple manner.
 Second, a web elongation target manager is included for the web elongation
 profile. This allows the elongation target shape to be adapted to the
 current elongation level, and/or provides a library of target profiles for
 web elongation in different operating conditons.
 FIG. 4 shows a solution, where the elongation profile can be determined on
 the basis of the tension profile. An elongation estimation profile can be
 provided by correcting the tension profile with process measurements. The
 process measurement comprises, for example, total draw, evolution of draw
 and speed measurement of the paper machine. Since CD shrinkage and MD
 elongation strongly correlate, the shrinkage profile can also be taken
 into account when determining the elongation estimation profile. The
 elongation estimation profile may thus be used, for example, instead of
 the elongation profile in the solution shown in FIG. 3.
 In another embodiment, the first and second CD moisture actuators are each
 governed to control both the web elongation and the moisture profiles.
 Each of the controllers govern a single CD moisture actuator to control a
 combination of the web elongation and the moisture profiles. For example,
 the two controllers may employ optimizations, which need not both use the
 same weighting factors on the two profiles. FIG. 5 schematically depicts a
 variation on this embodiment, which employs a steam box and a rewet
 actuator, and in which some optional enhancements are shown. First, bath
 the steam box control and the rewet control employ optimizations. Second,
 each optimization includes both the web elongation and the moisture
 profiles, and includes the operating state of the other actuator. Third, a
 web elongation target manager is included, as described in an earlier
 embodiment.
 In yet another embodiment, the first and second CD moisture actuators are
 governed in combination to control both the web elongation and the
 moisture profiles. FIG. 6 depicts a variation on this embodiment, in which
 a Multiple Input Multiple Output (MIMO) CD controller employs an
 optimization to control both the web elongation and the moisture profiles
 by governing a steam box and a rewet actuator.
 The embodiments described above all relate to a paper making machine with
 two CD moisture actuators and with a single measurement of the web
 elongation profile and a single measurement of the moisture profile. The
 depictions of embodiments all relate to the example layout of FIG. 2, in
 which the CD moisture actuators are a steam box and a rewet. However,
 these embodiments are illustrative of the Idea of the invention, and do
 not limit its scope. The disclosed invention anticipates and encompasses
 other embodiments of the idea of the invention which, for example,
 additionally (i) utilize further measurements of web elongation and/or
 moisture profiles at other locations, or (ii) utilize measurements of
 other profiles at the same or different locations, or (iii) govern further
 CD moisture actuators, or (iv) govern other CD actuators in addition to CD
 moisture actuators, or (v) control other properties in addition to web
 elongation and moisture profiles. The present invention thus anticipates
 and encompasses combination with control art for profiles of gloss and/or
 caliper and/or grammage and/or ash and/or coat weight and/or sheet
 composition and/or fiber orientation and/or opacity and/or color, and with
 the CD actuators governed to regulate those profiles. The form of
 combination is a MIMO controller or a set of coordinated or coupled SISO
 controllers which control at least one other profile in addition to
 moisture and web elongation, and which govern at least one other CD
 actuator in addition to the first and second CD moisture actuators,
 As one example, additional measurement devices, for example a second
 measurement beam 10 shown in FIG. 1, for moisture and/or web elongation
 profiles may be situated before the second moisture actuator. The control
 of web elongation and moisture profiles may additionally employ these
 measured profiles in its calculations, especially by including them.
 As another example, one or more additional CD moisture actuators may be
 located between the first and second CD moisture actuators. The control of
 web elongation and moisture profiles can govern these CD moisture
 actuators in addition to the first and second CD moisture actuators.
 An embodiment exemplifying the use of a second moisture profile measurement
 and a third CD moisture actuator is depicted in FIG. 7. In the illustrated
 case, the third CD moisture actuator is of a different type to the other
 two being for example an infra-red heater 11 shown in FIG. 1, since this
 is commonly encountered in papermaking, but does not need to be different.
 FIG. 8 shows an embodiment exemplifying the use of a different profile
 measurement and a different CD actuator in combination with the
 measurements of web elongation and moisture profiles and the two CD
 moisture actuators. In this case the fiber orientation angle profile is
 controlled in conjunction with the web elongation and moisture profiles,
 by governing the slice lip of a headbox in addition to the steam box and
 rewet actuators. This is particularly valuable in preventing dimensional
 problems in the paper sheet during manufacture or use. As noted earlier,
 shrinkage is greatest across the fibers, so that the local fiber
 orientation distribution determines the local axis of maximum shrinkage.
 Nonuniform orientation combined with nonuniform shrinkage and tension can
 lead to planar and a planar deformations of the sheet during drying and
 during later operations such as printing or converting. The illustrated
 embodiment employs a MIMO CD optimization, but obviously other
 configurations can also be used, such as coupled SISO CD controllers.
 The drawings and the description thereof are merely intended to illustrate
 the inventive idea. The details of the invention may vary within the scope
 of the claims. The blocks in the block diagrams shown in the drawings also
 depict means producing or providing said function.