METHOD FOR EXECUTING GRADE CHANGE ON A PAPER MACHINE

Abstract: The invention relates to a method and system for executing grade change in a paper machine. In the method, a controller (120) controls at least two input variables affecting the paper machine process, which variables affect the same output variable which represents the property defining the paper grade. The controller (120) controls at least one input variable toward the setting of the second grade before the grade change while the first grade is being manufactured. The controller (120) compensates the impact of said input variable on the output variable with at least a second input variable to keep the output variable substantially unchanged until the end of the manufacture of the first grade. Corresponding action can also be taken at the end of the grade change.

Description: METHOD FOR EXECUTING GRADE CHANGE ON A PAPER MACHINE FIELD OF THE INVENTION The invention relates to a method for executing grade change on a paper machine, in which method the paper grade is changed from a first grade to a second grade. BACKGROUND OF THE INVENTION Grade change on a paper machine refers to the changing of the paper grade being run to another paper grade. Grade change is made by changing various process variables, such as the basis weight and moisture, simultaneously to correspond to the target values of a new grade. The aim is to make the change as quickly as possible while the web is running, because the product produced during grade change usually ends up as broke and the aim is to keep the amount of broke as small as possible. Due to the complexity of the process and the cross-effects, grade change is an extremely difficult operation. The batch sizes of paper grades are currently quite small, which results in more frequent grade changes. The operating speeds of paper machines increase continuously and new and faster machines are being built, which necessitates minimization of the time used for a grade change. Also, grade changes must not cause breaks in the web. In prior art, a grade change is made by directing the grade variables used as the input variables in the process toward the settings of the new grade. In prior art, grade change can be made using an open control, a closed control, a grade change based on simulation or a grade change based on multi-variable control, for instance. In an open control, quality controls, such as basis weight, ash and moisture controls and closed loop control, are deactivated during grade change. Lower level controls, including stock flow, steam pressure of cylinders, etc. are, however, activated. In an open control, ramp speeds, ramp start-up times and settings for lower level controls after a grade change are defined by means of examples, experience and/or calculation formulas. The lower level settings are ramped with a separate ramp program. In a closed control, the quality controls are activated and the settings of the quality controls are being ramped. Otherwise, a closed control is mainly the same as an open control. US Patent 3 886 036, which is incorporated herein by reference, discloses both an open and a closed control grade change. For grade change in an open control, target ramps are defined in advance for the input variables of the process, such as machine speed, stock flow, headbox pressure and steam pressure, and the grade change is executed according to these target ramps. However, defining the target ramps requires the development of process models. Another problem is that the process models are extremely dependent on the assumptions made during modelling, which means that the grade change may fail. A closed control grade change is executed in the publication by keeping the steam pressure constant during grade change and by adjusting the machine speed, i.e. the only variable to be changed during grade change is the basis weight. This kind of a grade change is called a dryer limited grade change. A change in the basis weight is also made to cause a corresponding change in the slice opening of the headbox. The machine speed is adjusted to maintain the moisture at a desired value. Such closed loop grade changes do not operate smoothly, which means that a grade change takes too much time. A simulated grade change utilizes either an open or a closed control, and the input variables are ramped in advance and the most suitable ramps are selected for each situation on the basis of the simulation result. One solution of this kind is disclosed in US Patent 5 718 060, which is incorporated herein by reference. In this solution, too, the duration of a grade change is too long. In a multi-variable control, several input variables are processed at the same time and new settings are entered for several input variables directly at the time of the grade change and setting trajectories are defined. One known multi-variable control is the Horizon Multi-Variable Predictive Control (HMPC). In this solution, too, the duration of a grade change is too long. BRIEF DESCRIPTION OF THE INVENTION It is an object of the invention to implement a method and a system according to the method in such a manner that the above problems can be solved. The invention relates to a method for executing grade change on a paper machine, in which method the paper grade is changed from one grade to another and in which method at least two input variables affecting the paper machine process are controlled, which variables affect the same output variable which represents the property defining the paper grade. The invention is also characterized in that at least a first input variable is directed toward a setting of a second grade before a grade change when the first grade is being made; and the impact of said at least one input variable on the output variable is compensated with at least a second input variable to keep the output variable within the acceptable range of the first grade until the end of its manufacture. The invention also relates to a system for executing grade change in a paper machine, which system controls the grade change from a first grade to a second grade and the system is adapted to control at least two input variables affecting the paper machine process, which variables affect the same output variable which represents the property defining the paper grade. Further, the system of the invention is adapted to direct at least a first input variable toward a setting of a second grade before grade change; and the system is adapted to compensate the impact of said at least first input variable on the paper making process by controlling said at least one first input variable in such a manner that the output variable stays within the acceptable range of the first grade until the end of its manufacture. The method and system of the invention provide several advantages. Grade change becomes faster. Less energy is wasted and more stock saved since the amount of broke created during grade change is reduced as a result of the faster grade change. Paper quality becomes better, because there is less broke than before in the paper (broke is mixed with the paper being made). BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in greater detail in connection with preferred embodiments and with reference to the attached drawings in which Figure 1 shows a block diagram of a paper machine, Figure 2A shows examples of control ramps of input variables, Figure 2B shows an output variable, Figure 2C shows examples of target ramps of input variables, Figure 3A shows the dryer section of a paper machine, Figure 3B shows a control arrangement of the cylinder steam pressure, Figure 3C shows an impingement blowing arrangement, Figure 3D shows a through-air-blowing arrangement, Figure 4 shows an exemplary diagram on utilizing grade change models, and Figure 5 shows the principle of multi-variable control. DETAILED DESCRIPTION OF THE INVENTION The solution of the invention is particularly well-suited for paper machines in which grade changes are executed during an uninterrupted run. Let us first take a look at the basic equations related to the control and system engineering of a paper machine. In control and system engineering, the operation of a system can be described with differential equations. To determine the operation of a system in a specific situation, the differential equation must be solved. Several solutions exist for solving differential equations. One way to present and solve differential equations is to use convolution integral transformations, such as the Laplace transformation. In this application, other convolution integral transformations with which the inventive solution can be described are considered equivalent to the Laplace transformation. These include the Fourier transformation and the Z transformation. The mathematical presentations of the Laplace transformation and Laplace inverse transformation are as follows: L {f(t)} = F(s) = \e- st f(t)dt, o co 0 where is the Laplace transformation operator, L is the Laplace inverse transformation operator, s is the s variable of an imaginary s space in a Laplace transformation, t is the time variable, f(t) is a function with variable t, F(s) is a Laplace-transformed function with variable s. The Laplace transformation is generally used to transform the time variable functions of the input and output variables in the process, and the s space is the frequency dependency of the input and output variables. A typical simple transfer function is Y(s) Ke ~Td s G(s) = -^- = —- , X(s) 1 + τs where G(s) is the transfer function of the process, K is the process gain, Td is the process dead time and τ is the process time constant, X(s) is the input variable and Y(s) is the output variable. The descriptor of the input variable function X(s) is, in this case, a ramp function describing in a simple manner the most typical input variable functions affecting the paper technology processes. In this application, the time constant τ plays an important part. The ramp speed is also important and, thus, in the solution of the invention, the synergy of the time constant τ and the ramp speed is essential, because this synergy determines how quick an impact an input variable has on the operation of the process. When the ramp speed is substantially slower than the time constant τ (usually a time several orders longer than the time constant τ), the ramp speed mainly decides the impact of the input variable X(s) on the output variable Y(s). Usually, it is, however, the time constant τ that determines the impact of the input variable X(s) on the output variable Y(s). Typical input variables include the steam pressure of the cylinder in the dryer section of the paper machine and the impingement blowing (the blowing rate), which affect the moisture of the paper, which is the output variable. Controlling the steam pressure is slower than controlling the impingement blowing, i.e. the time constant of the steam pressure is longer than that of the impingement blowing. The grade change model, which is presented utilizing the above transfer function, is one application of dynamic models and, in particular, state models, wherein the grade change model used is intended to describe the dynamic behaviour of the process sufficiently accurately during the grade change. Figure 1 shows a general structure of a paper machine. One or more types of stock is fed into the paper machine through a wire pit silo 100 which is usually preceded by a blending chest and machine chest (not shown in Figure 1 ). The stock is metered into a short circulation controlled by a basis weight control or a grade change program. The blending chest and the machine chest can also be replaced by a separate mixing reactor (not shown in Figure 1 ) and stock metering is controlled by feeding partial stocks separately by means of valves or some other type of flow control means 122. In the wire pit silo 100, water is mixed into the stock to achieve the required consistency for the short circulation (dashed line from a former 110 to the wire pit silo 100). Sand (centrifugal cleaners), air (deculator) and other coarse material (pressure filter) are removed from the thus obtained stock using cleaning devices 102 and the stock is pumped by a pump 104 to the headbox 106. Before the headbox 106, a filler TA, such as kaolin, calcium carbonate, talc, chalk, titanium dioxide, diatomite, and a retention aid RA, such as inorganic, inartificial organic or synthetic water-soluble polymers, are added to the stock in a desired manner. The purpose of the filler is to improve the formation, surface properties, opacity, lightness and printing quality as well as to reduce the manufacturing costs. Retention aids RA, for their part, improve the retention of the fines and fillers while speeding up dewatering in a manner known per se. From the headbox 106, the stock is fed through the slice opening 108 of the headbox to the former 110 which is a fourdrinier in slow paper machines and a gap former in fast paper machines. In the former 110, water drains out of the web, and ash, fines and fibres are led to the short circulation. In the former 110, the stock is fed as a fibre web onto a wire, and the web is preliminarily dried and pressed in a press 112. The fibre web is primarily dried in dryers 114 and 116. In addition, there usually is at least one measuring beam 118 for measuring for instance the moisture MOI of the fibre web, the ash content ASH and basis weight BW of the paper being made. The paper machine, which in this application refers to both paper and board machines, also comprises a reel and size presses or a calender, for instance, but these parts are not shown in Figure 1. The operation of a paper machine is known per se to a person skilled in the art and need, therefore, not be presented in more detail in this context. Figure 1 also shows a control arrangement of a paper machine for maintaining a uniform quality in the grade and for enabling grade change. Factors affecting the quality and grade change are the number and proportion to each other of partial stocks, quantity of filler, quantity of retention aid, machine speed, quantity of white water and dewatering capacity. A controller 120 controls the metering of the partial stocks by means of valves 122, the metering of the filler TA by means of a valve 126, the metering of the retention aid RA by means of a valve 124, adjusts the size of the slice opening, controls the machine speed in the former 110, controls the quantity of white water and the dewatering process in blocks 114 and 116. Instead of the valves 124 and 126, also other known control and measurement means affecting the flowing and metering of the retention aid RA and filler TA, such as a metering pump, can be used. The controller 120 also utilizes a measuring device 118 to monitor the control measures, quality and/or grade change. The controller 120 preferably also measures the properties of the paper web elsewhere (e.g. at the same locations where controls are made). The controller 120 is part of the control arrangement based on automatic data processing. Figure 2A and 2B illustrate the principle of the solution of the invention on an XY axis. Both axes are positioned freely on a selected scale, the value of the variable is the Y axis and the time T is the X axis. Grade change begins at a time instant T1 and ends at a time instant T2. Thus, the first grade is manufactured until T1 , a grade change is executed between T1 and T2, and the paper manufactured at T2 fulfils the requirements of the second grade. Figure 2A has two input variables 200 and 202 which affect the same output variable and are controlled differently before grade change. The input variable 200 has a shorter time constant than the input variable 202. The input variable 200 can also generally be ramped faster than the input variable 202. In the solution of the invention, the operation is per ormed in such a manner that at a time instant T01A, which is substantially before the start T1 of the actual grade change, the change of the input variable 202 having the longer time constant from the control level (setting) of the grade 1 toward the control level (setting) required by the grade 2 is started. So as to maintain uniform grade 1 quality in the final product being manufactured, which in the solution of the invention is paper, until the start of the grade change at T1 , the change of the input variable 202 is compensated by changing the input variable 200 at T02A to an opposite direction with regard to the input variable 202. Time instants T01A and T02A can be the same instant or a different instant and are dependent on at least the paper grade, the grade change and the control variables. Thus, no essential change is effectively directed to the product being manufactured, i.e. paper, through the synergy of the input variables 200 and 202. At the grade change time instant T1 or slightly before it at a time instant T03A, the input variable 200 is quickly ramped toward the control level required by the grade 2 by the time instant T2. Even though the ramping of the input variable 200 toward the grade 2 is started slightly before the actual grade change instant T1 , the paper grade remains as the grade 1 until the grade change time instant T1 , i.e. the output variable 204 remains within tolerance limits 206 and 208 and the paper grade remains within its own tolerance limits as the grade 1. When the input variable 200 is impingement blowing or through-air-blowing and the input variable 202 is the cylinder steam pressure, the time instant T01A is, for instance, a few seconds before the grade change T1 and the time instant T03A is in the range of tens of seconds before the grade change T1. When filler content, retention aid content, stock, machine speed, headbox slice opening, for instance, are controlled by the input variables, T01 A is in the range of 30 min or less before the grade change T1 and T03A is then in the range of a few minutes before the grade change T1. The ramping is preferably executed with the fastest possible change rate determined by the other limitations in the process. Thus, the grade change can be executed in the shortest possible time (T1 to T2). After the grade change has occurred by the time instant T2, the input variables 200 and 202 are adjusted to the optimum run conditions related to the grade 2. An approved run of the grade 2 begins at T2 which is determined by the instant when the output variable 204 remains between the tolerance limits 206 and 208. The invention is thus characterized by directing the process well before a grade change into a state advantageous for the grade change, and directing the process during a grade change into a state which allows the grade change to be completed quickly while taking care that the quality criteria of the different grades are met both before and after the grade change. Figure 2B shows the behaviour of the output variable 204 on a time axis equal to that of Figure 2A (excluding dead time). The output variable 204 must remain between the predefined lower limit 206 and higher limit 208 at least during the running of either of the grades 1 and 2. When the change made before the grade change to the input variable 202 is compensated with the input variable 200, the output variable 204 stays within the limits allowed for the grade 1. Similarly, a well balanced transfer after the grade change to the optimum operating conditions for the grade 2 keeps the output variable 204 within the required limits and the quality of the final grade 2 product good. In the solution of the invention, the second input variable 200 is used to compensate the impact of the first input variable 202 on the output variable 204 until the first and the second input variable have reached the settings of a grade 2 normal run. Figure 2C shows one more example on how the input variables can be ramped according to the inventive idea. In this example, the input variable 214 having a longer time constant than the input variables 210 and 212 is, before the grade change, directed at maximum rate or a transfer rate slower than the maximum rate toward a setting specific to the grade 2 from a time instant T01 C onward. As in the case of Figure 2A, the second input variables 210 and 212 are directed toward another direction than the input variable 214 to keep the quality of the final product uniform before the grade change from the time instants T02C and T03C onward. The time instants T01C, T02C and T03C can be the same instant or at least partly different instants. During the grade change at T1 , both input variables 210 and 214 are preferably directed at maximum rate toward the setting specific to the grade 2. Any exceeding input variable 210 and 214 values are corrected at the end of the grade change at T2 and the process transfers to optimum operating conditions. In the solution of the invention, the number of input variables is not limited and the only condition is that the input variables have different time constants, i.e. ramping speeds, and that different input variables affect the same output variable, in which case the impact of the other input variables on the output variable can be compensated with at least one input variable until the grade change is started. Changes to the input variables before the grade change are typically made for instance 30 min (T1 to T01C) before the grade change. The length of the time (T1 to T01 C) depends, however, on the grade change, the input variables, etc. A maximum change rate is defined for each input variable. This change rate may differ in different ramping directions. For instance, when raising the steam pressure P, the ramping rate is typically faster than when the steam pressure P is lowered. Let us now take a look at the solution of the invention by means of Figure 3, in which the first input variable is the steam pressure of the cylinders 330 in the dryer section of the paper machine, which cylinders 330 are used to dry the paper web, and the second input variable is the effective drying capacity of the impingement blowing and/or the through-air-blowing, with which the paper web is also dried. Figure 3 shows a greatly simplified diagram of the dryer section of a paper machine comprising one or more blocks 300 to 304 with cylinders 330 (all cylinders are not separately numbered), the blocks corresponding to block 108 of Figure 1 , and one or more impingement blowing blocks 306 and through-air-blowing blocks 308, which correspond to block 110 in Figure 1. The temperature of the cylinders 330 is controlled by means of control blocks 312 to 322. The control blocks 312 to 322 receive their control commands from a grade change controller 310. The impingement blowing unit 306 comprises a cylinder or roll, preferably a suction roll, 340 and a hood 342. The blowing rate is controlled with a regulator 344 and the air (or another gas- like substance) temperature is controlled with a regulator 346. The through-air- blowing block 308 is similar to the impingement blowing block 306. The through-air-blowing block 308 comprises a cylinder 348, hood 350 and a control block 352 which controls the rate and temperature of the air (gas-like substance) flowing through the paper being manufactured. In cylinder drying, the surface temperature of the cylinder 330 is a function of the steam temperature inside the cylinder 330. The steam temperature inside the cylinder depends on the steam pressure, and, thus, the cylinder temperature can be changed by changing the steam pressure, which is obvious per se to a person skilled in the art. Figure 3B describes the principle of drying based on the steam pressure of the cylinder. Steam, usually hot water vapour, is fed to a cylinder 360 through a cylinder axle 362 and valve 364 from a steam source. Condensate is removed from the cylinder 360 through siphons. In impingement blowing, described in Figure 3C, a hot air flow is directed from a hood 370 to a paper web 376 on a suction roll 372. The air is sucked back into the hood 370 and recycled several times through a burner 378. The air is heated with the burner 378. T = 350°C and v = 90 m/s are typical blowing values. The running of the paper web 376 is controlled by means of auxiliary cylinders 374. The drying capacity of impingement blowing can be changed by changing the temperature or, especially, the flow rate. The system-engineered time constant of impingement blowing, used as the first input variable and with which the impingement blowing affects the moisture content of the paper web, which is the output variable, is shorter than the cylinder steam pressure time constant used as the second input variable. Thus, in the solution of the invention, the steam pressure of one or more cylinder groups in blocks 300 to 306 is controlled before the grade change like the curve 202 in Figure 2A (or curve 212 in Figure 2C), i.e. well before the grade change, the steam pressure is directed toward the settings of the new (second) grade while compensating for the impact caused by the steam pressure change with impingement blowing. The impingement blowing is controlled in one or more blocks 306 like the curve 200 in Figure 2A (or the curve 210 in Figure 2C). Control block 314, which preferably is part of the computer-based control system of the paper machine, is responsible for the control. Figure 3D shows a through-air-blowing arrangement resembling the impingement blowing arrangement. In this case, the surface of the cylinder 382 has holes, through which air flows in and out of the cylinder 382. When hot air is blown from the hood 380 to the paper web 386, the air flows through the web 386 to the cylinder. The time constant of through-air-blowing is shorter than the time constant related to the control of the steam pressure. In the solution of the invention, the cylinder steam pressure, which is the input variable, is, before grade change, directed toward the setting of the second grade and the temperature change caused by the cylinder steam pressure is compensated with the through-air-blowing, which is the second input variable, and the web moisture, which is the output variable, is kept substantially unchanged before the grade change. Thus, in the solution of the invention, the steam pressure of one or more cylinder groups in blocks 300 to 306 is controlled before the grade change like the curve 202 in Figure 2A (or curve 212 in Figure 2C), i.e. well before the grade change, the steam pressure is directed toward the settings of the new (second) grade while compensating for the impact caused by the steam pressure change with through-air-blowing. The through-air-blowing is controlled in one or more blocks 306 like the curve 200 in Figure 2A (or the curve 210 in Figure 2C). Control block 314, which preferably is part of the computer-based control system of the paper machine, is responsible for the control. In a preferred embodiment of the invention, the filler TA, which is the first input variable, is directed substantially before the grade change toward the settings of the second grade and the change caused by the filler TA to the ash content ASH, which is the output variable, is compensated with the retention aid RA, which is the second input variable. When adding more filler TA, the amount of ash ASH increases. This increase in ASH can be prevented before the grade change by reducing the input of the retention aid RA into the stock. Correspondingly, by reducing the amount of filler TA, the amount of ash ASH would decrease, unless the input of the retention aid RA was increased. When the grade change begins, the input of the retention aid RA is ramped toward the setting of the new (second) grade. In a preferred embodiment of the invention, the filler TA and retention aid RA, which are the input variables, are, before the grade change directed toward the settings of the second grade and the change caused by the filler TA and retention aid RA to the basis weight BW, which is the output variable, is compensated with the stock KM, which is the second input variable. Adding filler TA and/or retention aid RA increases the basis weight BW. This increase in the basis weight BW can be prevented before the grade change by reducing the input of stock KM into the wire pit silo. Correspondingly, by reducing the amount of filler TA and/or retention aid RA, the basis weight BW would decrease, unless the input of stock KM into the wire pit silo was increased. When the grade change begins, the input of the stock is ramped toward the setting of the new (second) grade. In the solution of the invention, the stock KM, which is the input variable, is directed toward the settings of the second grade before the grade change. The headbox slice opening, which is the second input variable, is directed in a compensatory manner to keep the basis weight BW, which is the output variable, substantially unchanged. The time constant of the stock KM is longer than the time constant related to the control of the headbox slice opening. Adding more stock KM increases the basis weight BW. The increase in basis weight BW can, however, be prevented before the grade change by making the headbox slice opening smaller. When grade change begins, the slice opening is changed toward the setting of the new (second) grade. Let us now take a look at the factors affecting grade change in known grade change solutions to which the solution of the invention can be applied. Figure 4 describes an example of utilizing grade change models in grade changes. In block 400, estimates are calculated for basis weight and moisture using grade change models before the ramping is started. After the ramping is started, the values of the output variables can be estimated by means of the grade change models until the end of the grade change. In addition, the trend of moisture and basis weight are monitored during the entire grade change. If no corrections need to be made during the grade change, which data is obtained in block 410, the grade change is deemed successful in block 402 and the data and trends related to the grade change are saved into the database for successful grade changes. If a re-ramping has been made, the grade change data is saved into the database for re-ramped grade changes. The operator acknowledges the grade change completed when the key variables of the process are within the new limits. If after the ramping and when the quality controls are activated, the grade change has not been acknowledged completed, the grade change has not been successful. Modelling and update of the grade change models is performed in block 404. The grade change models described in Figures 2A and 2C can be calculated again using a known modelling method either at specific times or for instance when the operating point of the paper machine is detected to have changed, i.e. for instance when the machine speed has changed to a substantially higher speed than before. The modelling may be a complete one or only a few variables can be modelled. Typically, these variables are coefficients in the moisture model. The modelling is normally started by categorising the material being modelled according to the changes made and the operating point used. Typically, only grade changes saved in the database for successful grade changes are accepted as material for modelling. The database in block 406 contains all necessary grade change models, their parameters and information concerning their operating areas. These grade change models are based on using the operating area of the old grade as the starting point for the model and the more specific definition of the model is made according to the change from the old grade to the new grade. Block 408 describes the start-up and control of the grade change. When a predefined amount of the old grade run remains, typically less than 30 minutes (see Figures 2A and 2B), preparation for the grade change is begun, wherein the ramping of the input variable having the longest time constant toward the settings of the second grade is started. However, at the same time the system makes sure that the grade does not change before the grade change by compensating the change with the second input variable in accordance with the solution of the invention. At this stage, the old grade (first grade), the new grade (second grade) and all other data related to the new grade are known. Several variables are processed in block 410 and the data is used in blocks 408 and 412. For instance, the ramps according to Figures 2A and 2C are calculated in block 414 using the models obtained from blocks 408 and 412. Figure 5 shows a block diagram of multi-variable control. Multi- variable control is disclosed in greater detail for instance in the article James River Cuts Grade Change Time with Automated, Predictive Controls: D. McQuillin, P.W. Huizinga, pages 143 to 146, Pulp & Paper, September 1994, which is incorporated herein by reference. The paper machine 500 is controlled using a multi-variable controller 502 having as input data the target values (e.g. settings of the current or the next grade), the measuring data (output variables) of the paper being currently made in the paper machine, which are fed back at desired stages of the paper-making process, and the disturbances. The multi-variable controller 502 enters into the paper machine 500 the input variables so as to direct the paper machine to make the current paper grade or to change to another paper grade according to the solution of the invention. The multi-variable control optimizes the control changes to be made in making paper. With multi-variable optimization, two input variables can preferably be selected to control one output variable according to the solution of the invention. The solution of the invention can generally be applied to various known controls, such as an open control, a closed control, a grade change based on simulation or a grade change based on multi-variable control. Even though the invention has been explained in the above with reference to examples in accordance with the attached drawings, it is obvious that the invention is not restricted to them but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.

Claims: CLAIMS 1. A method for executing grade change on a paper machine, in which method the paper grade is changed at least from a first grade (1) to a second grade (2), and in which method at least two input variables (200, 202, 210, 212, 214) affecting the paper machine process are controlled, which variables affect the same output variable (204) which represents the property defining the paper grade, characterized in that at least one first input variable (202, 214) is directed toward a setting of the second grade (2) before the grade change when manufacturing the first grade (1 ); and the impact of said at least one input variable (202, 214) on the output variable (204) is compensated with at least one second input variable (200, 210, 212) to keep the output variable (204) within the accepted limits of the first grade (1 ) until the end of the manufacture of the first grade (1 ). 2. A method as claimed in claim 1, characterized in that two most important input variables (200, 202) are selected using multi-variable optimization, of which the first input variable (202) is, before the grade change, directed toward the setting of the second grade (2) and the output variable (204) is compensated with the second input variable (200) to remain substantially unchanged until the grade change. 3. A method as claimed in claim 1, characterized in that two input variables (200, 202) are used to control the output variable (204), of which the first input variable (202) affects the operation of the process slower than the second input variable (200), and, before the grade change, the control of the first input variable (202) toward the setting of the second grade (2) is compensated with the second input variable (200). 4. A method as claimed in claim 2 or 3, characterized in that during the grade change, the first (202) and the second (200) input variable is directed in the same direction toward the settings of the second grade (2). 5. A method as claimed in claim 4, characterized in that during the grade change, both input variables (200, 202) are directed in the same direction at the fastest possible change rate. 6. A method as claimed in claim 1, characterized in that to complete the grade change, the input variables (200, 202, 210, 212, 214) are directed to settings specific to the second grade (2). 7. A method as claimed in claim 6, characterized in that the to complete the grade change, the impact of the first input variable (202) on the output variable (204) is compensated with the second input variable (200) until said first and second input variable (200, 202) have reached the settings of the second grade (2) normal run. 8. A method as claimed in claim 1, characterized in that the control of at least one input variable (202) toward the setting of the second grade (2) is started at a predefined time (T01) before the starting time (T1) of the grade change. 9. A method as claimed in claim 8, characterized in that the predefined starting time (T01) of control of each input variable (200, 202, 210, 212, 214) and the ramping speed depends on the grade, the grade change and the control variables. 10. A method as claimed in claim 1, characterized in that cylinder (330) steam pressure, which is the first input variable (202), is, before the grade change, directed toward the setting of the second grade (2) and the change in the web moisture caused by the cylinder steam pressure is compensated with impingement blowing, which is the second input variable (200), and the web moisture, which is the output variable (204), is kept substantially unchanged until the grade change. 11. A method as claimed in claim 1, characterized in that the cylinder (330) steam pressure, which is the first input variable (202), is, before the grade change, directed toward the setting of the second grade (2) and the temperature change caused by the cylinder steam pressure is compensated with through-air-blowing, which is the second input variable (200), and the web moisture, which is the output variable (204) is kept substantially unchanged until the grade change. 12. A method as claimed in claim 1, characterized in that the filler, which is the first input variable (202), is, before the grade change, directed toward the setting of the second grade (2) and the change in the ash content, which is the output variable (204), caused by the filler is compensated with the retention aid, which is the second input variable (200). 13. A method as claimed in claim 1, characterized in that the filler and the retention aid, which are the first input variables (202), are, before the grade change, directed toward the settings of the second grade (2) and the change in the basis weight, which is the output variable (204), caused by the filler and the retention aid is compensated with the stock, which is the second input variable (200). 14. A method as claimed in claim 1, characterized in that the stock, which is the first input variable (202), is, before the grade change, directed toward the settings of the second grade (2) and the change in the basis weight, which is the output variable (204), caused by the stock is compensated by the headbox (106) slice opening (108), which is the second input variable (200). 15. A system for executing a grade change in a paper machine, which system controls the grade change from a first grade (1) to a second grade (2) and which system is adapted to control at least two input variables (200, 202, 210, 212, 214) affecting the paper machine process, which variables affect the same output variable (204) which represents the property defining the paper grade, characterized in that the system is adapted to direct at least one first input variable (200, 202, 210, 212, 214) toward the setting of the second grade (2) before the grade change; and the system is adapted to compensate with the control of at least a second input variable (200, 212, 214) the impact of said at least first input variable (200, 214) on the paper-making process in such a manner that the output variable (204) stays within the acceptable range of the first grade (1) until the end of its manufacture. 16. A system as claimed in claim 15, characterized in that the system is adapted to use multi-variable optimization which is adapted to select the two most important input variables (200, 202), by means of which the system is adapted to keep the output variable (204) substantially unchanged until the grade change. 17. A system as claimed in claim 15, characterized in that the system is adapted to control two input variables (200, 202) in such a manner that the input variable (202), which has a slower impact on the operation of the process, is, before the grade change, directed toward the set value of the second grade (2), and the input variable (200), which has a faster impact on the operation of the process, is used in a compensatory manner so as to keep the output variable (204) substantially unchanged before the grade change. 18. A system as claimed in claim 17, characterized in that during the grade change, the system is adapted to direct both input variables (200, 202) into the same direction. 19. A system as claimed in claim 17, characterized in that during the grade change, the system is adapted to direct both input variables (200, 202) into the same direction at the fastest possible rate. 20. A system as claimed in claim 15, c h a ra cte rize d in that to complete the grade change, the system is adapted to direct the input variables (200, 202, 2210, 212, 214) to settings specific to the second grade (2). 21. A system as claimed in claim 15, characterized in that to complete the grade change, the system is adapted to compensate with the second input variable (200) the impact of the first input variable (202) on the output variable (204) until said first and second input variable (200, 202) have reached the settings of the second grade (2) normal run. 22. A system as claimed in claim 15, characterized in that the system is adapted to direct the first input variable (202) toward the settings of the second grade (2) at a predefined time (T01) before the grade change. 23. A system as claimed in claim 22, characterized in that the predefined starting time (T01) of control depends on the grade, the grade change and the control variables. 24. A system as claimed in claim 15, c h a ra cte rize d in that the system is adapted to direct the cylinder (330) steam pressure, which is the first input variable (202), toward the setting of the second grade (2) before the grade change, and the system is adapted to control the impingement blowing, which is the second input variable (200), in the impingement blowing block (308) in a compensatory manner to keep the web moisture, which is the output variable (204), substantially unchanged until the grade change. 25. A system as claimed in claim 15, characterized in that the system is adapted to direct the cylinder (330) steam pressure, which is the first input variable (202), toward the setting of the second grade (2) before the grade change, and the system is adapted to control the through-air-blowing, which is the second input variable (200), in the through-air-blowing block (308) in a compensatory manner to keep the web moisture, which is the output variable (204), substantially unchanged until the grade change. 26. A system as claimed in claim 15, characterized in that the system is adapted to direct the filler, which is the first input variable (202), by means of a valve (126) toward the settings of the second grade (2) before the grade change, and the system is adapted to control the retention aid, which is the second input variable (200), by means of a valve (124) in a compensatory manner to keep the ash content, which is the output variable (204), substantially unchanged until the grade change. 27. A system as claimed in claim 15, characterized in that the system is adapted to direct the filler and the retention aid, which are the first input variables (202), by means of valves (126 and 124 respectively) toward the settings of the second grade (2) before the grade change, and the system is adapted to control the stock, which is the second input variable (200), in a compensatory manner by means of at least one valve (122) to keep the basis weight, which is the output variable (204), substantially unchanged until the grade change. 28. A system as claimed in claim 15, characterized in that the system is adapted to direct the stock, which is the first input variable (202), by means of at least one valve (122) toward the settings of the second grade (2) before the grade change, and the system is adapted to control the headbox (106) slice opening (108), which is the second input variable (200), in a compensatory manner to keep the basis weight, which is the output variable (204), substantially unchanged until the grade change. 29. A system as claimed in claim 15, c h a racte rize d in that the system comprises a controller (120, 310, 502) based on automatic data processing, which is adapted to control the grade change. 30. A system as claimed in claim 29, characterized in that target ramps according to which the input variables are ramped during the grade change are defined in advance for the input variables of the process in the controller (120, 310, 502).

Inventors: HUHTELIN TAISTO, HUHTELIN, TAISTO
Applicants: HUHTELIN TAISTO, VALMET AUTOMATION INC, HUHTELIN, TAISTO, VALMET AUTOMATION INC.
Application year: 1999
Main classification: D21G9
IPC codes: D21G9