Patent Publication Number: US-6905573-B2

Title: Method and equipment for cleaning and maintaining rolls

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a U.S. national stage application of International Application No. PCT/FI01/00775, filed Sep. 6, 2001, and claims priority on Finnish Application Nos. 20001966, filed Sep. 6, 2000; 20002001 filed Sep. 12, 2000; and 20002002 filed Sep. 12, 2000, the disclosures of each application are incorporated by reference herein. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY 
     SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and equipment for cleaning and maintaining the rolls of paper or board machines, and especially the rolls of finishing equipment. 
     Calendars comprise two or more hard and/or soft-surfaced calender rolls, which together form a calender nip or nips through which the paper web to be treated is made to pass. Calender rolls, especially soft-surfaced rolls such as a supercalender&#39;s paper rolls or corresponding, and rolls equipped with a so-called soft calender&#39;s soft cover, especially a polymer cover, are sensitive to damage. The cause of damage is often impurities, such as local fiber agglomerations, that cause a pressure impact while passing through the nip, which loads the soft cover of the calender roll causing it first to heat (the so-called hot spot phenomenon) and in time causing permanent deformation of the cover and damage. Corresponding deformations and damage can also appear in metallic calender roll surfaces and in the surfaces of belts that pass through the calendering nips. 
     The damage-induced heating of a calender roll is especially harmful for rolls with polymer covers. Due to the polymer&#39;s large heat expansion coefficient and extremely poor heat conductivity, even a small damaged area expands quickly and continues to heat to such high temperatures that it becomes deformed. If a covered roll is made of thermosetting resin, it loses its original properties at the same time as it melts again. The above-described heating reaction can be triggered, for example, by a small piece of paper stuck on the surface of the roll, a fiber agglomeration or a spot released from a paper or board cover which, when it enters a calender nip, causes greater flex than in its immediate surroundings in the cover, which heats the roll cover unevenly. 
     The prevention of local soiling by, for example, continuous doctoring is not usually feasible financially or with respect to the best quality of the paper or board, because most of the polymer covers in use are not very friction-resistant, so that preventive cleaning may itself wear out the cover more than the actual calendering. 
     If soiled areas could be detected early enough, it would be possible to use, for example, a cleaning doctor or other device for cleaning the roll surface for short periods without the roll cover being damaged by continuous or often repeated doctoring. Thus, the operating life of soft-surfaced calender rolls could be notably lengthened. 
     As is already known, thermometers traversing in a crosswise direction with respect to the machine, that monitor the temperature of the cover, are used to monitor the condition of calender rolls, especially soft-surfaced rolls. Problems arise in the said temperature monitoring application and in other corresponding already-known systems, due to the fact that the flexible roll cover whose temperature is being monitored is usually at least somewhat dielectric. Thus, the partially frictional contact between the paper web and the cover produces quite high charges of static electricity on the surface of the cover and the somewhat dry paper web. These potential differences attempt to discharge through the available routes of smallest resistance. The thermograph often has to be installed externally in the support structure, in which case the said charges of static electricity find the thermograph to be the easiest outlet for discharge, thus causing the sensitive electronics of the thermograph to be exposed to quite high voltages and to require protection against them. 
     Even though monitoring the temperature of the calender roll surface usually makes it possible to detect excessive loading on the cover or a local temperature rise due to local internal non-homogeneity at a sufficiently early stage, it requires installation of quite heavy, expensive and space-taking equipment in the proximity of the roll to be monitored. Especially the lack of space causes great difficulties in connection with multi-roll finishing equipment, in which each device that is not part of the actual paper web finishing process complicates maintenance and repair of the equipment. 
     Another known method of detecting the soiling of a calender roll surface is a measuring method based on roll vibration, which is presented in the applicant&#39;s patent application FI 974255. In this method, one or more vibration sensors that indicate and localize the vibration point caused by soiling are inserted in the calender roll structure. Soiling can also be detected by a fault detection system that monitors the state of the paper web. This system detects holes and coating color streaks in the paper web. Other known systems that detect roll soiling include a machine condition monitoring system, a quality measurement system, web moisture measurement and the above-explained web temperature measurement. 
     When the soiling of a roll has been detected, an attempt is made to remove the soiling as quickly as possible to prevent damage to the surface of the roll and to minimize the amount of rejected product. Most commonly, the dirt is removed by using a conventional blade doctor, in which the doctoring blade extends for the whole length of the calender roll and usually continuously removes, i.e. doctors, the soiling and impurities. Other known doctoring methods include a washing doctor that directs a liquid jet on the soiled part, and a flow doctor that directs an air jet on the part to be cleaned. A set of calender rolls can also be supplemented with a cleaning nip, which removes the worst coat weight deviations that come with the paper web before they are transferred to the set of calender rolls. 
     The problem of the conventional blade doctor is that the doctoring function is applied to the roll surface continuously, which leads to unnecessary wearing of the roll, thus shortening the roll&#39;s operating life. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention disclosed herein is to create a new method and equipment for cleaning and maintaining the rolls of paper or board machines, and especially the rolls of finishing equipment. 
     A special goal of the invention is to create a new method and equipment, with which the soiling of finishing equipment rolls is detected by on-line measurement and an immediate doctoring operation is applied to the soiled part. 
     The purpose of the invention is also to present a simpler method for roll surface maintenance, which method can be used to maintain the different rolls in paper or board production and finishing processes while the roll is running in the process. 
     In addition, the purpose of the invention is to present an efficient method by means of which the condition of finishing equipment rolls can be monitored especially in on-line calendering after the coating station, and perform efficient maintenance during operation of the calender at the same time as the paper web is run through the calender. The invention is also suitable, however, for use in off-line calendering if maintenance problems occur in it. 
     An additional purpose of the invention is to create a new method and equipment in which the doctoring operation is performed for several rolls by a common doctor actuator. 
     The method that is the object of the invention aims at the rapid localization and removal of dirt particles, coating color streaks and other impurities traveling with the web before they enter the nip of the finishing equipment rolls, for example, the calender. Thus the expensive damaging of roll covers can be prevented. The method attempts especially to prevent roll damage in on-machine applications, in which the calender is located after the coating machine. 
     In the method according to the invention the soiling of the finishing equipment rolls, for example a calender roll, is detected and localized by a known method, such as a measurement based on roll vibration, information given by a fault detection system, a thermograph measurement, information obtained from a condition monitoring system or other known method, or alternatively by the new method presented in this application in which soiling is detected and localized by measurement based on laser radiation. 
     An advantageous embodiment of the invention is based on the notion that the soiling of the roll in contact with the paper web, for example in a coating station or calender, can be prevented by spraying liquid momentarily on the surface of the web before it comes into contact with the roll and/or on the surface of the roll when a deviation is detected on the surface of the web or the roll. A deviation on the surface of the web, for example a hole, can cause an extra substance agglomeration on the web or roll, or, an extra substance agglomeration, for example extra coating color on the web or soiling of the roll surface, has already occurred in fact. The liquid can prevent extra substance agglomerations in advance, or extra substances such as extra amounts of coating color end up in the web due to the liquid. The part of the paper web which contains the liquid sprayed on the web or the roll is in most cases removed as broke. 
     According to the invention disclosed herein, the soiling is removed by a new kind of doctoring method in which the actuator of the doctor operating as a cleaning device is directed to clean only the detected soiled part of the roll. The cleaning device is controlled by the information obtained from the soiling detection and localization system. For example, a blade doctor extending the whole length of the roll a zone doctor of limited length, a washing doctor, a blow doctor or a laser doctor according to the invention can be used as the cleaning device. In one advantageous embodiment of the invention, each side of the roll stack of a finishing machine, for example a multi-nip calender, has only one cleaning device, which is movable so that it is able to reach and clean all of the roll surfaces in turn. 
     A device according to one advantageous embodiment of the invention uses two doctor blades. A liquid that-prevents or reduces the sticking of dirt is sprayed between the doctor blades and/or before the first blade in the roll rotation direction. The liquid is sprayed continuously or at certain time intervals depending on how much dirt from the paper grade being run sticks onto the roll surface. If the roll needs only light cleaning, the doctor blades can be used separately as well as simultaneously, i.e. one blade can be out of use when the roll surface is being cleaned with the other blade. 
     The equipment can have a fault detection device which supplies information, on the basis of which both doctor blades are placed, if necessary, in the working position and the spraying of liquid is began, if it is not desired to keep the doctor blade and the feeding of liquid in continuous operation. Especially for faults coming from coating, both blades should be in use, because the coating color is relatively difficult to detach from the rolls. 
     The fault detection device consists of a detector device, which is arranged to monitor the soiling of the roll surface. The observations of the detector device go for processing to a data processing device, which gives an automatic operating command to the roll cleaning device when there is need for cleaning. The number of doctor blades taking a working position and/or the surface material of the doctor blade chosen for the work and/or the amount of liquid to be sprayed depends on the degree of soiling detected by the detector device. The fault detection device can also include other functions than the ones mentioned above. 
     The benefit of the invention disclosed herein is, for example, that a better and faster cleaning result is achieved using a doctor actuator according to the invention. Thus, the operating life of the calender rolls can be lengthened and the amount of broke caused by soiling can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail in the following with reference to the accompanying drawings, in which: 
         FIG. 1  presents a measuring and adjustment principle according to the invention. 
         FIG. 2A  shows the use of a laser detector according to the invention for indicating and detecting soiling. 
         FIG. 2B  presents the operating principle of a laser doctor according to the invention. 
         FIG. 3A  presents a control mechanism according to one embodiment of a blade doctor according to the invention. 
         FIG. 3B  presents a control mechanism according to a second embodiment of a blade doctor according to the invention. 
         FIG. 3C  presents a control mechanism according to a third embodiment of a blade doctor. 
         FIG. 4  presents the principle of the method according to the invention as a side view of the equipment handling the paper web. 
         FIG. 5  presents the principle as a cross-section across the running direction of the web. 
         FIG. 6  presents one embodiment of the invention as a side view in connection with the calender. 
         FIG. 7  presents a second embodiment of the invention as a side view in connection with the coating station. 
         FIG. 8  presents a third embodiment of the invention as a side view in connection with the blade coating station. 
         FIG. 9  is a schematic drawing of a side view of a monitoring system related to the invention. 
         FIG. 10  presents the system of  FIG. 9  as a plan view. 
         FIG. 11  presents a block diagram of an advanced system for performing detection. 
         FIG. 12  presents a side view of one application of a device according to the invention. 
         FIG. 13  presents a cross section of a doctor blade and its surrounding structure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  presents an example of an operating principle of an embodiment of the method according to the invention. In  FIG. 1 , the web W comes from the coating machine  10  and passes through the detector device  21  of the fault detection system  20  to the calender roll stack  50 . The fault detection system  20  detects the holes and coating color streaks on the web W coming from the coating machine  10 , and reports the detected faults to the control system  41  of the doctor actuator  40 . Thus, soiling caused by coating color streaks and holes can be prevented and removed before the impurities move to the nip of the calender roll. The coating color streaks can be removed by, for example, a cleaning nip (not shown) located before the calender roll stack  50 . Dirt particles that reach the surface of the rolls of the calender roll stack  50  are detected by the vibration measurement system  60 , or by CCD camera measurement (not shown), or by thermograph measurement (not shown) or by soiling monitoring equipment which consists of a measuring unit  31  and a control unit  35 . 
     The fault detection system  20 , the soiling monitoring equipment  31 ,  35  or the vibration measurement system  60  gives information on the detected soiling, on the basis of which the doctor actuator&#39;s control system  41  activates the doctor actuator  40  which acts as a cleaning device. The activation takes into account different time delays which comprises, for example, measuring delays, electronics and software delays and the delay of the doctor actuator  40 . In addition, the web velocity and the distance of the fault detection system  20  from the doctor actuator  40  is taken into account, thus determining the exact moment of time when the detected damage or dirt in the web will reach the calender roll stack  50 . 
     On the basis of the information received from the fault detection system  20 , the soiling monitoring equipment  31 ,  35  or the vibration measurement system  60 , the detected soiling can be classified according to its degree of seriousness. The doctoring method to be used, the duration of doctoring and the adjustment of the controlling quantities of the doctor actuator can be chosen based on the classification, and thus the doctoring can achieve the best possible result with the fastest possible response time. 
       FIG. 2A  shows the detection of soiling by means of a laser measurement system  30  according to the invention. The laser measurement system  30  consists of the measuring unit  31  and the control unit  35 . The measuring unit  31  comprises a laser light source  32 , a lens  33 , and a detector  34 . A laser beam is directed from the laser light source  32  through the lens  33  onto the cover  52  of the roll  51 , on which a dirt particle P is detected. Part of the laser beam is reflected from the dirt particle P on the cover  52  of the roll  51  onto detector  34 , which gives information on the detected dirt particle P and its location to the control unit  35  of the laser measurement system  30 . The control unit  35  analyzes the measurement signal received from the detector  34 , carries out a power adjustment and direction steering with respect to the laser light source  32 , and gives an alarm to the control system  41  of the doctor actuator  40 . The doctor actuator  40  is, for example, the laser doctor  410  according to the invention presented in  FIG. 2A , which is described further in  FIG. 2B , or some other type of doctor actuator such as a blade doctor, a laundering doctor or a fan doctor. Doctor actuator  40  may comprise one doctor actuator or a combination of several doctor actuators. 
     The detecting and localizing of dirt particle P can be performed not only by detector  34 , but also by a CCD camera, in which case a laser beam is swept over the roll surface and the changes appearing in the sweeping area, which indicate soiling, are detected by the CCD camera. The laser light source  32  of the measuring unit  31  of soiling monitoring equipment can be, for example, a HeNe laser whose advantage is a small divergence of the beam, a stable wavelength and a narrow spectrum. There can be one or more units monitoring the roll. The units can be fixed or movable. The units can be moved either by servomotors or by a pneumatic, hydraulic or mechanical actuator or mechanism. Alternatively, the laser beam can be aimed at different parts of the roll by using, for example, a robot and laser combination known from the car industry, with which a 3-dimensional operating area is possible. 
     Localizing dirt particle P by means of a laser beam can be based on different kinds of methods known as such. In measurements based on phase comparison, the radiation received from the laser light source is modulated, sent to the measurement object and the part returning from the object is detected by the detector. Based on the phase difference between the signal received from the receiver and the sent signal, it is possible to determine the distance traveled by the beam to the measurement object on the basis of multiples of the wavelength. In a triangle measurement, on the other hand, the laser light source, the object to be measured and the detector form a triangle, in which information on the shape of the surface, the distance and other properties is based on how the radiation sent to the surface is imaged on the surface of the detector. In this method, the surface being studied is swept with a narrow laser beam, in which case the path of the light point received from the detector combined with known measurement geometry provides information on the shape of the swept surface. The presence of soiling can be concluded from the detected unevenness of the surface. 
       FIG. 2B  shows an example of a doctor actuator operating as a cleaning device, which, in this example, is the laser doctor  410  according to the invention. In laser doctor  410  a laser beam of the laser light source  411  is aimed through lens  412  onto dirt particle P on cover  52  of roll  51 , which dirt particle has been localized in the above-mentioned way. The laser beam is aimed at such an angle α onto the surface of the polymer roll that the radiation power to be absorbed by the dirt particle P is directed only at the dirt particles, and the heat wave of the laser beam penetrates only to a predetermined depth. The intensity, pulse duration and wavelength of the laser source  411  is adjustable. In state-of-the-art devices the area of influence of the laser beam can be aimed with at least ±2 μm precision, which is sufficient for this application, because the thickness of the dirt particles left on the roll surface is normally at least 100 μm. A laser doctor according to the invention is especially suitable for cleaning soft rolls with polymer covers. Because the polymer cover of a calender roll and the dirt particles found on it absorb electromagnetic radiation, in this case laser radiation, in a different way, the laser radiation can be aimed to affect selectively only the dirt particles, in which case the polymer cover of the roll is not harmed. 
       FIG. 3A  presents an example according to the invention of a second embodiment of a doctor actuator operating as a cleaning device. Doctor actuator  420  consists of a doctor blade  421 , oscillating device  422 , frame part  423 , support mechanism parts  424 ,  425 ,  426  and the adjustment mechanism of the doctor blade  427 . The motion of the doctor blade  421  of the doctor actuator  420  in the direction of the tangent of the surface of the roll  51  to be doctored or in the direction of the roll radius is carried out by means of a servomotor or electromagnet, or alternatively by means of a pneumatic, hydraulic or mechanical actuator or mechanism. The motion of the blade  421  can also be carried out by using a magnetostrictive material in either the blade or the support structures of the blade. 
       FIG. 3B  presents a detail of the adjustment mechanism of the doctor blade  421  presented in FIG.  3 A. In this embodiment, the short motion of the doctor blade  421  is carried out, with respect to support point X, by controlling the pressures in chambers  427   1  and  427   2 , which are formed from, for example, a hose. In addition, by controlling the chamber pressures it is possible to adjust the doctoring pressure, i.e. the force that the doctor blade  421  exerts on the roll cover. 
       FIG. 3C  presents an example in which the length of the doctor blade  432  is adjusted to create the motion of the doctor blade in the direction of the tangent of the roll surface. The doctor mechanism  431  consists of a doctor blade  432 , air containers  433 ,  434 , piston  435 , cylinder  436  and frame part  437 . With precise control of the motion and stroke length of the doctor blade and the correct choice of doctor blade material, the doctoring can be performed without damaging the polymer cover. Because air containers  433 ,  434  are situated in the immediate proximity of the piston  435 , the doctor blade  432  can be moved fast. 
     The mechanical structure of the doctor actuator operating as a cleaning device ensures that when the doctor actuator is in the ready position it can be moved quickly enough to the working position, i.e. in contact with the surface to be doctored. For example, when the doctor actuator comprises pneumatic actuators, then the working cylinder, hose or other corresponding actuator can be pre-pressurized, in which case, on the basis of the information received from the soiling monitoring device equipment  31 ,  35  or the fault detection system  20 , the rapid magnetic valve is opened/closed. 
       FIG. 4  presents the principle of implementing a method according to the second embodiment of the invention as a side view of the web handling equipment. Web W is in contact with roll  1 , either by passing over it in a certain sector or by touching one point of the roll&#39;s outer shell (broken line). The former case may concern, for example, the counter roll of a coating station which, from the back side, guides the web before coating color is spread on the outer surface of the web at the roll by some coating method. An example of the latter case is a calender in which web W can be in contact, at minimum, only at the point where it is pressed between roll  1  and the opposite roll in the calender nip. Thus, both cases concern a station in which a surface treatment for finishing a web made of fiber stock—the treatment being coating in the former case and calendering in the latter case—is performed on a continuous web arriving from previous treatment phases and usually moving at a running velocity of over 1000 m/min. 
     Due to substances coming mainly with the web, but also possibly from elsewhere in the environment, extra substances may remain on the surface of roll  1 . These substances can be detected by a detector device D 1  that inspects the surface of the roll, the alternatives of which are presented hereinafter. It is also possible that already earlier such deviations are detected in web W that will cause problems at a point or area where the web W will come in contact with the surface of roll  1 . This detector device, which is to be located a sufficient distance before roll  1  in the running direction of the web, is indicated with the reference indicator D 2 . In  FIG. 4 , the spraying device  3  is situated before the detector D 1  in the rotation direction of the roll, but it can also be situated after the detector D 1 . 
     The observations of detector device D 1  or D 2  are processed in the data processing device  2 , which automatically gives the operating command to spraying device  3  to clean the surface of roll  1  or to prevent soiling (sticking of extra substances). Spraying device  3  can spray liquid for a sufficiently long time onto the part of the shell of roll  1  that is not covered by web W, with the result that the extra substance agglomerations detach from the roll surface under the influence of the liquid, or their adhesion to the roll surface decreases to the extent that, when they come into contact with web W, they stick to the web. Web W thus operates as a kind of “cleaning felt”, by which the roll surface can be cleaned, and the part of the web which is thus used for cleaning purposes can later be removed as broke. It is also possible that the sticking of foreign substances on the roll from the web can be prevented, depending on the type of substance and the liquid. Thus, the detector device D 2 , situated before the roll, is used so that such operations can be begun before the substance reaches the roll. 
     According to the second alternative, presented by a broken line in  FIG. 4 , the liquid can be sprayed from spraying device  3  before roll  1  onto that surface of web W that comes into contact with the roll shell, in which case the dampened web is able to take with it the extra substances stuck onto it from the surface of the roll, and this part of the web can later be removed as broke. Thus, the operation of spraying device  3  is determined according to the observations of detector device D 1  monitoring the surface of roll  1 . 
     In addition, it is possible that the cleaning liquid sprayed on the surface of the web is of such type that it prevents the substance on the web from sticking to the surface of the roll. Thus, the detector D 2 , situated before the roll and the spraying point, is used as the detector. 
     The process control system can be used to determine which part of the web is now broke due to the liquid addition, and in which location the said part of the web is moving, so that it can be removed. 
     It is also possible that the cleaning by spraying is performed as a routine operation at certain phases of the finishing process of paper and board, for example, when the product will become broke in any case. In this case, the detector devices are not necessarily used, but the spraying is performed by the command of the production process control system, irrespective of whether there are extra substances on the roll surface or not. Thus, the finishing process control system can give the command for cleaning suitably timed for a certain process phase, such as during a roll change or a grade change. Another alternative is to time the cleaning to be performed according to a previously determined schedule, for example, at certain time intervals. 
     It is also possible that cleaning in a certain predetermined phase or during a certain predetermined moment of time is performed only if the detector device D 1  monitoring the roll surface detects that there is a need for it. Thus, the cleaning command can be confirmed by comparing it to the information given by the detector device D 1 . 
       FIG. 5  presents the situation in  FIG. 4  seen as a plan view of the situation opposite the shell of roll  1 . Here, mainly the various alternatives of how to carry out the spraying are presented. As spraying device  3  is able to spray along the whole width of web W or length of roll  1 , it may consist of a series of nozzles  3   a  placed side-by-side which cover the whole width of the web or one traversing nozzle that is able to spray along the whole width of the web by moving across the machine. Full-width spraying is performed when it is desired to clean roll  1  as a routine procedure, i.e. during suitable stages of paper or board finishing processes or at certain time intervals according to the above-mentioned principle. The conduits for bringing the liquid to the nozzle/nozzles  3   a  are not presented as they can be implemented and the spraying can be controlled (activated/stopped) by known solutions. 
     The spraying devices in  FIG. 5  can also spray only part of the web width, in which case all the nozzles  3   a  are not in operation, but the spraying is performed only in a certain place by one or several nozzles  3   a  situated across the machine, depending on the location of the problem point. With a traversing spraying device  3 , this is done by moving the nozzle across the machine to the corresponding point. 
       FIG. 6  presents the application of a spraying method according to the invention in connection with a calender. The calender roll  1  to be cleaned is a so-called soft roll with a polymer surface and the web passes through calender nip N between this roll  1  and the other, adjacent, roll  4 . According to the principle presented in  FIGS. 4 and 5 , spraying device  3  is situated either opposite to the outer perimeter surface of roll  1 , for example so that its area of influence is in the latter half after the nip N in the direction of the roll rotation, i.e. in the 180° sector after the nip, most suitably in the 120° sector after the nip. Calender roll cleaning that is performed by spraying the roll surface is more advantageous than cleaning performed by spraying the web surface. This latter alternative, in which the nozzles of spraying device  3  are aimed at the surface of the web W along the direction of motion of the web, before the nip N, is marked with a broken line. 
     In the alternative in  FIG. 6 , the spraying device  3 , with nozzles  3   a  divided along the width of the machine or a traversing nozzle in the cross direction, can be used.  FIG. 6  also presents the detector devices D 1  and D 2 . Detector device D 1  monitors the surface of roll  1  and the data processing device  2  in connection with it processes the observed data and gives a spraying command to spraying device  3  when needed. Correspondingly, it is possible that the spraying command is given at certain phases of the process, according to the commands of the control system of the finishing process, irrespective of whether there is a need for cleaning. This need can be ensured on the basis of the information given by detector device D 1  monitoring the surface of roll  1 , as is presented above. 
     When moving to spraying by spraying device  3 , the calender also performs other operations, like moving the doctor blade, marked in the figure with reference number  5 , to a point in contact with the calender surface located after the spraying point in the rotation direction, and reducing the nip load of the calender nip N. 
     These operations are performed just a moment before beginning the spraying. The purpose of doctoring is to remove substances possibly sticking to the roll and to remove the possible extra liquid, but not to prevent the flow of the liquid to nip N on roll  1 . 
     The deviations detected by detector device D 2  on the web W before the calender nip N are such that they can cause the quality of the surface of roll  1  to deteriorate. Typically these types of deviation are holes, around which agglomerations of coating color have built up in the earlier coating process and have not yet dried up. Additionally, other deviations that can be visually detected by detector device D 2 , such as dirt spots etc., can cause a spraying command. The soiling of the calender roll can be prevented by a detector device which detects deviations in the web before the web comes in contact with the roll. If the liquid sprayed on the surface of the web and/or roll contains chemicals that prevent the wet coating color or dirt from sticking to the roll, the soiling of the roll can be prevented in advance. 
     The alternative of  FIG. 6  is especially suitable for use in such paper or board finishing lines in which the web, constantly traveling at the running speed, is coated and calendered consecutively, with the calender being situated after the coating station. In this type of off-machine line, in which the machine rolls obtained from the paper or board machine are unwound one after the other and spliced to each other, the problems mentioned in the introduction appear, due to the coating color coming to the point of the continuation splices. In addition, this invention is suitable for such finishing lines in which both the coating station and the calender are situated after the production line, i.e. when an on-line coating machine and on-line calender are part of the same continuously operating production line at the end of the paper and board machine. These production lines do not have problems related to continuation splices, but instead, in these cases, the faults of the papermaking process, such as holes, reach the coating station and the calender. 
     The invention can be applied in such calenders in which the nip is formed between two rolls, in which case such-roll pairs can exist two in succession, or in multi-nip calenders in which two or more nips in succession are formed between the rolls of a roll stack. In a multi-nip calender the cleaning operations can be advantageously aimed at the first rolls, for example, the first soft-surfaced roll. 
       FIG. 7  presents a spraying arrangement according to the invention, applied in the coating station. In the coating station the web W moves in a certain sector guided by the counter roll  1 , in which a coating color is spread on its outer surface by coating device  6 , which in  FIG. 7  is a blade coating device. Coating device  6  doses, in a way known as such, a certain amount of coating color onto the surface of the passing web, per surface area unit of the web. 
     Before web W comes into contact with counter roll  1 , the surface of web W is monitored at a sufficient distance from the coating station by detector device D 2 , which detects the holes that cause problems in the coating station in the way presented in the introduction of this application. When a hole is detected, spraying device  3  sprays liquid on the surface of counter roll  1  at a suitable stage. Consequently, a coating color patch, which would cause spots on the backside of the web at intervals along the length of the outer perimeter of the roll is not left on the surface of the counter roll. Instead, the liquid dissolves the coating color off and it is transferred rather soon to the web passing by. The spraying can be begun, for example, just before the hole passes through the coating device  6  or immediately after it, when the hole has passed through the coating device  6 . If the spraying is timed so that there is liquid on the roll already as the hole comes to the surface of the roll, the sticking of the coating color patch on the wet surface of the roll at the point of the hole can be prevented. 
     Another possibility is to observe the surface of counter roll  1  with detector device D 1  and if coating color patches or other impurities are detected on it, the surface of counter roll  1  is sprayed with liquid by spraying device  3 , as a result of which the coating color patch can be transferred to the web. Cleaning performed with both methods produces broke, the amount of which is relatively small, however. It is possible by means of the process control system to determine which part of the web is broke due to this operation, and when it will arrive at a certain point in later processing devices, in which it can be removed so that it does not become part of the final product Detector device D 1  or D 2  is in contact with data processing unit  2 , which handles the information from the detector device and gives commands to the spraying device. 
     Here also a detector device located before the roll gives the possibility of preventing the soiling of the roll by means of preparatory procedures (spraying). 
     Also in this alternative, it is possible to spray liquid on the backside of web W instead of on the surface of roll  1  in a situation in which a hole passes through the coating station, in which case the coating color patch can thus be transferred to the dampened web when the corresponding surface comes into contact with the surface of counter roll  1 . This spraying device  3  is marked in  FIG. 7  with broken lines. 
     The liquid to be sprayed on the surface of counter roll  1  can contain chemicals that help in repelling the wet coating color patch so that it does not stick to the surface of the counter roll. 
     Using the same principles as mentioned above in connection with the calender, the cleaning or preventive action can here also be directed at a certain zone in the web in the width direction either by activating only one or more nozzles  3   a  of the nozzle series or by bringing a traversing nozzle to the right place in the cross direction of the machine. 
     The surface of the web W that comes against counter roll  1  can already have been coated in a previous coating station in the situation in  FIG. 7 , in which case the station in  FIG. 7  is the second coating station. The spreading of liquid in a sufficient quantity ensures that the coating does not stick to the roll, because a thick enough liquid layer prevents the sticking. 
     Also in the situation in  FIG. 7  it is possible to perform whole-width cleaning by spraying liquid at such a phase that is appropriate for the process, i.e. when the paper or board passing through roll  1  at the moment of cleaning will end up as waste in any case. 
       FIG. 8  presents a side view of a coating station in which the spraying is applied in a different way. In this case, a liquid is sprayed onto the side of the web W that will be coated later by coating device  6 . In this case, the uncoated web W is inspected by the detector device D 2  before the coating point and/or the coated web W is inspected by the detector device D 3  after the coating point. The spraying device  3  is located before the coating device  6  and is pointed towards the surface of the web that is to be coated If the detector device D 2  is used before the coating point, the spraying device  3  will be located after this detector device D 2 . 
     When a deviation is detected in web W, which, detected by detector device D 2  before the coating point will be a deviation disturbing the coating process, or detected by detector device D 3  after the coating point will be a sign of a continuous disturbance at the coating station, the spraying device  3  will start spraying on the still uncoated surface of the paper. This exploits the well-known phenomenon that coating color will not stick if the base paper is wet. If the deviation detected by the detector D 2  before the coating point is a hole or, in the case of an off-line coating machine, also a splice, liquid can be sprayed onto this point so that the coating color does not stick to the point in question and cause the problems presented in the introduction to this application as it passes through the calender, for example. If, on the other hand, a continuous disturbance is detected by the detector device D 3  after the coating point, such as a blade streak resulting from a particle left under the coating blade, the spraying will be continued until the particle has been washed away, which can be detected by the same detector device D 3 . In the case of holes, streaks and other deviations restricted to a certain point in the cross direction of the web, restricted spraying can be applied by the spraying device  3  to the corresponding point, in which case one or more switched-on nozzles  3   a  in the nozzle series can be used, or a traversing nozzle can be driven to the corresponding point in the cross direction of the machine. In the case of a whole-width splice, whole-width spraying is used simultaneously. Thus, a nozzle series covering the whole web width is the more economical option, because it allows the implementation of both restricted spraying in a certain zone and spraying that is simultaneously directed at the whole web width, in accordance with the nature of the deviation. Spraying can also be carried out according to a profile by controlling adjacent nozzles  3   a  separately. 
     The liquid used can be water, by which it is possible to prevent the wet coating color from sticking to the paper or to dissolve away a particle from under the blade. As there is liquid on the surface of the paper, the blade will not bum at the uncoated point during blade coating and the web tension will not be changed. 
     The dirt that collects on the roll is moistened, in one advantageous embodiment of the invention, by a liquid whose boiling point is higher than the roll temperature in process conditions. Thus, the temperature cracking of the roll is avoided. The dirt on the roll is then removed by methods known as such, which is explained in this application. 
     The points treated in the above-mentioned ways can be removed from production as broke at a later stage of the finishing process, and the travel of these web areas can be monitored by means of a process control system. 
     Furthermore, the detector devices D 2  and/or D 3  in the alternative depicted in  FIG. 8  are connected to the data processing unit  2  which processes the information from the detector device and issues instructions to the spraying device  3  to ensure its correctly-timed functioning. 
     The most suitable device for use as the detector device D 1 , D 2  or D 3  will be a device that is capable of visually observing a certain-sized area on the surface of roll  1  or web W and storing the observations for realtime processing. The device can be a camera equipped with an image sensor, that senses electromagnetic radiation, such as a CCD camera. A detector device can also be an IR camera that measures infrared wave lengths, whereas on the other hand a device that makes visual observations will measure the visible light if one detector device is not sufficient to observe the whole width of the web or the roll sufficiently accurately, a number of devices can be arranged side by side, so that each will observe a zone of a certain width A detector device is not necessarily based on visual detection, but the presence of a deviation on the surface of a roll or web can be identified by some other fault detection method, for example by web fault detection. 
     The foregoing has dealt with the detection of deviations and the taking of measures by spraying performed on the web or the roll, focusing on different spraying alternatives and spraying timing with respect to the paper finishing process in general. The following description gives a more detailed explanation, with reference to  FIGS. 9 and 10 , of the possibilities of performing observation of the surface of roll  1 , for example a calender roll. 
     The detector device D 1  can be a CCD camera that measures the polymer surface of the calender roll. The camera continuously measures the intensity of the light reflecting from the roll surface, which light can be produced by a special light source  7 .  FIG. 10  shows a number of detector devices D 1 , each mounted so as to observe a roll zone of a certain width; i.e. the devices are located at suitable intervals in the axial direction. Thus, good resolution is obtained and even small deviations can be detected. Correspondingly, there are several light sources, for example, each detector device D 1  has its own, which illuminate the observation zone of each detector device. 
     As data on the roll rotation speed is brought to the CCD camera processor by, for example, pulse sensor  8 , the camera can be used to make a map in the processor memory of the roll surface during one rotation of the roll, i.e. a two dimensional image. During each roll rotation or at certain intervals, e.g. after ten rotations, this map is compared in the data processing unit  2  with the rotation map obtained in the previous rotation. Thus, it is possible to notice if some point on the roll suddenly changes its color due, for example, to a coating color patch or a piece of paper. 
     This detection principle can be used not only in the calender roll, but also in that alternative of the embodiment of  FIG. 7  in which the surface of the counter roll  1  is inspected in order to detect coating color patches. 
     The data processing unit  2  has a data transfer connection with the spraying device  3  in the manner referred to in connection with  FIGS. 6-8 . 
     As has been mentioned in the foregoing, in on-line processes in which the calender is in the same production line as the paper machine it is not possible to eliminate web faults in the rewinder. 
     The following is an explanation of an alternative designed for this kind of production line, for observing the surface of a roll or web in a calender, for example. 
     The use of a web fault detection system in roll protection is complicated by the fact that it is not exactly known what the received fault data tells us about how harmful the phenomenon observed on the web will be to the roll surface. The lack of this information easily leads to a situation in which the calender&#39;s control system over-reacts to the web&#39;s fault data, so that the calender&#39;s possible protective measures (cleaning, rapid openings, etc.) are activated nearly the whole time and disturb the actual production, or, as in this invention, the spraying is activated unnecessarily often. In a second extreme case there is also the possibility that the calender&#39;s control system will not react at all to the fault information, in order not to disturb production, in which case there is in practice no roll protection. 
     The problem related to the condition monitoring of roll  1  in roll protection is detection after the event. The event leading to damage should begin before it can be detected. In such a case, it can happen that when the roll protection measures begin, the cover has already been damaged, and even rapid measures cannot do more than save from additional problems (production quality problems, safety risks). 
       FIG. 11  is a schematic presentation of the principle of the method. The method exploits the above-described system used for web fault detection and systems used in the condition monitoring of the roll cover which are similar to present systems. The innovation in the invention is the combining of these systems into an automatically functioning, deductive machine which functions as an automatic monitoring data processing unit  2 . The deductive machine comprises equipment and a learning deductive or computation algorithm which is taught to predict the data for the roll cover condition monitoring system on the basis of the data obtained from the web fault detection system: whether, on the basis of the web fault data, it is possible to predict, for example, the vibration measurement “hot spot” observation made by observing the surface of roll  1 , the band-like heated area on the roll cover observed by the IR camera or the soiling on the roll surface witnessed by the CCD camera, or whether the run will probably flow without cover monitoring alarms. The teaching of the deductive machine takes place by some known algorithm or a new algorithm to be used in the implementation of machine intelligence. 
     After the learning stage, a system according to the invention knows which fault information obtained from the web fault detection system may cause real harm to the roll cover. A taught system can initiate protective measures already on the basis of classified fault data obtained from the fault detection system, without needing to wait for observation of the beginning of roll damage or soiling by the detector device carrying out the condition monitoring of the rolls. The measures in this invention are spraying by spraying device  3  on the surface of web W or roll  1 . The protective measures are begun only if necessary. If the deductive machine has not learnt that the type of fault observed is normally followed by an alarm concerning the roll condition monitoring, the calender control system does not begin protective measures. 
     Thus it will be possible to avoid the unnecessary disturbance of production caused by protective measures. 
     At the learning stage of the system, information is gathered on the influence of web faults in the creation of phenomena causing roll cover faults. Then possibly the creation of some cover faults begins to occur. The development of these faults can be stopped, however, at their early stage because the cover condition monitoring is on and triggers the roll protective measures, even if late in comparison with the detection of web faults. 
     The leaning can be implemented either as a separately selected learning mode or as continuous learning. In the latter case, the system can continually improve its performance by learning from new situations. 
     In the invention, a system designed for web fault detection, similar to present systems, can be improved if necessary so that the detector device established for the reception of electromagnetic radiation will use not only the visible light (e.g. CCD cameras) but also infrared wave lengths—then its possibilities of observing damp coating color patches will improve. 
     In monitoring the soiling of rolls (in imaging by the CCD camera) it is possible to exploit the fact that wet coating color sticks on the calender rolls most likely in the first nips. Then, after the pigment coating of the paper, CCD camera monitoring is needed only in the calender&#39;s first nips. If the locations of the cameras can be chosen suitably so that the image contains not only the roll but also the nip area, the invention can exploit both the roll surface image and the web break automatics—if the image processing algorithm does not observe the web in the nip area, a web break has occurred. 
     The invention is not limited only to the embodiments presented in  FIGS. 4-11 , but it can be adapted within the framework of the inventive idea presented in the patent claims. The calenders in which the invention can be used are not limited to the calender presented in FIG.  6 . Blade coating is presented in  FIGS. 7 and 8 , but the invention can be used also in connection with other coating methods, such as film transfer coating, even if blade coating is more problematic due to the coating principle, such as substances remaining on the blade which can cause streaks. The type of detector is not limited by the foregoing, although devices observing the surface without touching it and based on the measurement of electromagnetic radiation (visible light, IR area), which are able to monitor large surfaces simultaneously and are equipped with good image processing possibilities are indeed more advantageous. Then the detection is based on the reception of visible light or the reception of other electromagnetic radiation grades such as IR. For example, information on deviations in the calender roll can be gained by analysis based on vibration measurements carried out by acceleration sensors. 
     In the spreading of liquid, spraying is advantageous as a non-contact spreading method, in which case a certain amount of the liquid is dosed from the nozzles onto the surface of the roll or web, but it could be possible to conceive of the spreading on the surface of roll  1  occurring also by an element which touches the surface of the roll and transfers liquid onto the surface of the roll. 
       FIG. 12  presents roll  1 , the surface of which is cleaned with two doctor blades. Doctor blades  72  can be either similar to or different from each other. Doctor blades  72  can also be in use simultaneously or separately, in other words, the motion of a doctor blade in/out of contact with the surface of the roll is controllable independently of the other doctor blade. Doctor blades  72  can also be loaded with loads of different magnitude in order to achieve the wanted result. Doctor blade  72  continues advantageously along the whole width of the roll and is attached to the blade holder  75 . 
     The stem  80  of the doctor holds a liquid container, through which a liquid that prevents or reduces soiling can be pumped onto the surface of roll  71  from feed members  73 , that can be, for example, nozzles made for spraying liquid. The nozzles are situated in the frame of the doctor and are directed so that they spray liquid on the surface of the roll in the direction of rotation, before the corresponding blade. The direction of rotation is indicated with an arrow in the figure. 
     The sprayed liquid can be such that changes the surface of roll  71  to be more hydrophilic compared to what it was before treatment with the said liquid. The sprayed liquid is chosen so that a minimum of adhesion force is reached between the surface of roll  71  and the substance attempting to stick onto the surface, in which case soiling is notably reduced or entirely prevented. 
       FIG. 13  presents doctor blade  72  used for cleaning the roll and the structure surrounding it as a cross-section. Doctor blade  72  is attached to blade holder  75 . Frame  80  of the doctor holds liquid container  74 , through which a liquid that prevents or reduces soiling can be pumped to be sprayed onto the surface of the roll by feed members  73 . Frame  80  of the doctor is attached to a base  76  in such a way that the frame  80  can be adjusted to a suitable angle by controlling members  77  and  78  and bearings  79 . Controlling member  77  is for loading pressure and controlling member  78  is for opening pressure. 
     EXAMPLE 
     The surface material (Duraheat, Durasoft) of the roll was treated with aqueous solutions of surface active substances. The experiments used a cationic surface active substance (Quadrilan AT), anionic surface active substance (Perlankrol EP 36) and non-ionic surface active substance (Monolan 8000/E80). In all cases the cleaning result was good. 
     While performing tests in laboratory conditions on the surfaces treated in the above-mentioned way, it was discovered that their angle of contact was about 50°, in which case the roll surface had become somewhat more hydrophilic. 
     The invention is not limited to what is stated above, but it can vary in accordance with the Claims. The materials of the doctor blades can be different. The liquid sprayed on the surface of the roll can be any kind of liquid substance, which eases cleaning the surface of the roll and/or changes the surface properties of the roll in the desired direction. The main point in this invention is, that by using at least two doctor blades the cleaning result of the calender roll can be adjusted to be suitable. 
     The following presents the Claims, within the inventive idea of which the details can vary or deviate from the above-presented demonstrative examples.