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METHOD AND CONTROL DEVICE FOR
PAPER WEB PROFILE CONTROL WITH
PLURALITY OF SENSORS
BACKGROUND OF THE INVENTION 5
The invention relates to a control device and a method for the setting/closed-loop control of a cross-machine and/or machine direction profile of a quality feature in the paper manufacturing process.
Control systems which have been used for the crossmachine and/or machine direction profile of a quality feature in the paper manufacturing process. For example, the grammage was controlled as the quality feature. The grammage of the running paper web was measured at the papermaking machine by a sensor, for example a traversing sensor. The measured variable was read into an open-loop/closed-loop control device to control the cross-machine direction profile. Through a control program, the open-loop/closed-loop control device drove actuators to influence the cross-machine
direction grammage profile. An open-loop/closed-loop control device of this type and a method of controlling the grammage and fiber orientation cross-machine direction profile are disclosed in DE-OS 42 39 845. This discloses a method of making adjustments at the headbox of a web- ^ forming machine to provide a web of relatively uniform density and layer height utilizes a headbox with a plurality of transverse sections, each of which is provided with a web material at a variable flow rate and a variable consistency. The method includes the steps of measuring the layer height 3Q profile of the web, measuring the density profile of the web, and comparing a portion of the layer height profile with a corresponding portion of the density profile to determine whether there are corresponding deviations in the profiles. If there are deviations in the layer height and density profiles, 3J the consistency and flow rate of the web material provided to the headbox are adjusted, based on deviations in the layer height profile and stock density profile and whether the deviations correspond with each other, in order to achieve a uniform stock density profile and layer height profile. The 4Q web is preferably dewatered by providing it to a dewatering apparatus, pressed and dried to provide a finished web, and rolled into a roll. The method may be used in connection with a paper machine to form a paper web.
The prior art discloses a flowbox or headbox that is 45 constructed at least partially in sections arrayed in the cross-machine direction. The flowbox influences the paper stock suspension in the individual sections, particularly as to its consistency and its fiber orientation, by feeding dilution water, for example, to control the cross-machine direction 50 grammage profile. Sectional flowboxes are disclosed in allowed U.S. application Ser. No. 08/662,980 now U.S. Pat. No. 5,707,495, and U.S. Pat. No. 5,466,340 whose contents are incorporated by reference herein.
The achievable level of the cross-machine direction pro- 55 file quality feature, the grammage in the present example, using a cross-machine direction control system of this type, was limited, particularly by the fluctuations occurring during paper manufacturing, in the machine direction profile of the quality feature of the paper web. The main cause of machine go direction fluctuations, if the cross-machine direction grammage profile is considered, are volume flow fluctuations and consistency fluctuations in the stock jet.
A traversing sensor that operates point by point, as in DE 42 39 845, to pick up the measured signal relating to the 65 quality feature of the paper web cannot precisely ascertain the cross-machine direction profile of the quality feature.
Instead, it is only possible to determine a signal that is composed of cross-machine and machine direction profile components. This results from the speed at which the paper web runs through the papermaking machine, which is generally greater than 1000 m/min.
The measurement signal, which includes mixed crossmachine and machine direction components, can be used for the control device for the open-loop/closed-loop control, if it is viewed as a cross-machine direction profile signal which is "averaged" over a certain machine direction section of the paper web. However, such averaging over a certain paper section leads to a very "noisy" cross-machine direction profile measured signal. An actual cross-machine direction profile is extracted by filters from this noisy cross-machine direction profile measured signal, and the extracted actual cross machine direction profile is used for control. The severe filtering of the cross-machine direction profile measurement signal enables control interventions of the crossmachine direction profile control system to be performed only infrequently or with too low an amplitude. Thus, control interventions according to the prior art are performed only after a very long time delay in relation to the measurement, for example only after five minutes after picking up the measured signal. At a web production speed of 1500 m/min, for example, this means that the paper web has travelled a distance of 7500 m before a control intervention is performed. This produces problems, for example, the transient behavior of the control system is severely slowed and the attainable cross-machine direction profile is limited by the random component on account of the machine direction fluctuations in the cross-machine direction profile.
In order to solve this problem, German Application DE-AS 20 19 975 proposes using both a stationary sensor and a sensor traversing over the web width for sending the thickness measurement of a web. The two sensors pick up a thickness profile of the web which is passing by. The stationary sensor measures the thickness profile in the machine direction at one point along the web, while the traversing sensor measures the thickness profile on a path running obliquely over the web. The obliquely running profile contains both components of the pure cross-machine direction profile and components of the pure machine direction profile. By subtracting the machine direction profile from the oblique profile, the pure cross-machine direction profile is then determined. Disadvantages of this method or device are that too much time elapses for complete measurement of a cross-machine direction profile and that the thickness of the paper web can only be measured at the end of the papermaking machine. It produces an excessively long or sluggish reaction time for the control system.
SUMMARY OF THE INVENTION
The object of the invention is to provide a control device and a method which overcomes the disadvantages of the prior art.
The invention relates to a control device preferably closed-loop and a method for the control of the crossmachine and/or machine direction profile of at least one quality feature in the paper manufacturing process with the aid of two sensors. The first sensor picks up a first measured signal of the cross-machine and/or machine direction profile of a first quality feature of the paper web produced. The second sensor picks up a further measured signal of a quality feature that differs from that picked up by the first sensor. The sensors transverse the web separately. The sensors act through a controller on actuators leading to the feed line or
lines for the suspension leading to the flowbox for controlling thereby the quality features of the process. In some embodiments, the two sensors measure the same quality feature differently. For example, in measuring grammage, one sensor measures the cross-machine direction profile 5 while the other measures the machine direction profile; or each sensor measures cross-machine, but they traverse the web on respective time paths, e.g. one trails the other, or they move in opposite directions.
By measuring more than one quality feature using more 1Q than one sensor and by using more than one sensor for separate measurements of the moving web being produced, it is possible to now pick up a profile of the web closer to the flowbox supplying the suspension for forming the web and to react more rapidly to fluctuations in the machine direction profile. Furthermore, by using more than one traversing 15 sensor to control the cross-machine and/or machine direction profile of a quality feature of the web, for example, the grammage in the paper manufacturing process, it is also possible to determine the cross-machine direction profile more rapidly from the noisy profile data. The cross-machine 20 direction profile control system can then intervene more frequently and react more rapidly to profile fluctuations. An additional advantage is that the transient behavior of the control system is shortened significantly, which is of decisive advantage, particularly following a grade change or 25 following process disturbances. It is also possible for more than two sensors to be used in a control device of even more complicated construction.
Using the device and the method according to the
invention, it is possible to control the cross-machine and/or machine direction profiles of the widest possible range of quality features in the paper manufacturing process.
Thus, the control of the cross-machine and/or of the machine direction profile of any and all of the following 3J quality features in the paper manufacturing process is enabled without restricting the process:
the formation of the paper web;
the thickness of the running paper web;
the moisture and opacity of the running paper web; 40
the roughness and the mechanical sheet properties of the paper web, such as the modulus of elasticity or the breaking length ratio, for example.
Control over the cross-machine and/or machine direction profile of the grammage and of the fiber orientation of the 45 paper web is particularly preferred. In a particularly advantageous embodiment, a controlled consistency flowbox is used as an actuator for the cross-machine direction grammage profile or the cross-machine direction profile of the fiber orientation. 50
A flowbox of this type is sectioned along the crossmachine direction, and the paper stock suspension consistency and fiber orientation are able to be respectively influenced in the individual sections. This is enabled, for example, by arranging dilution lines, which open into the 55 feed lines, in the region of the feed lines to the respective sections. Each dilution line is equipped with a control valve. Both the consistency and also the fiber orientation in the individual sections can then be influenced via the dilution lines. Flowboxes of this type are disclosed in the above 60 mentioned DE 40 19 593 and, respectively, DE-A43 16 054.
In one embodiment of the invention, a control valve is arranged in the common feed to the flowbox or else in the common return, as the actuator for the machine direction profile control system. 65
It is particularly advantageous if the at least two measured signals from the at least two measuring sensors be fed to a
computer device. At least one measured signal contains both a cross-machine and a machine direction profile component. By means of the computer device, the measured signals can be processed in such a way that a cross-machine direction profile component and a machine direction profile component, as well as a residual component which describes the random disturbances, can be extracted from the measured signals that are picked up. The resulting control signal for the machine direction component is fed to a controller for the machine direction profile control, and the resulting signal for the cross-machine direction component is fed to a cross-machine direction profile controller. The crossmachine or machine direction profile controller then in turn addresses the respective actuators in accordance with its control algorithm. As mentioned above, in controlling the grammage cross-machine and/or the machine direction profile, an advantageous embodiment uses a controlled consistency flowbox, whereas a control valve that is arranged in the common flowbox or, respectively, in the discharge, and that is addressed by the machine direction profile controller, can be provided as the actuator for the machine direction profile.
In a particular embodiment of the invention, the paper stock consistency in the flowbox itself is determined as a further measured signal containing only a machine direction profile component. The paper stock consistency can be measured either in the common feed to the flowbox or else in the individual feed lines.
An alternative embodiment uses the signal from a further sensor which is arranged at the wet end of the papermaking machine as the further measured signal. The further sensor is able to measure the level or depth of the suspension on the fabric or wire of the papermaking machine, which fabric follows the flowbox. A fluctuation in the level of the suspension on the fabric results from fluctuation in the volume of flow. For a constant paper stock consistency, that fluctuation changes the grammage in the corresponding region of the web width. In addition, a paper stock consistency sensor can be provided in the feed lines to the flowbox. With the aid of the latter sensor, in conjunction with the measurement of the level of the suspension, a reliable statement can be made about the amount of paper stock which is being ejected in the corresponding region. The quantity of stock being ejected is directly associated with the grammage at the location and can easily be used as a measure of quality. Conclusions can be drawn about the longitudinal profile of the grammage. Hence, measurement of one feature, e.g. quantity of stock, can be used to adjust another feature, e.g. grammage.
Furthermore, it is also possible to determine the mass flow at the wet end of the papermaking machine, using a radiometric measurement, in particular in the gamma radiation range, and to use this measured variable of the mass flow as a further measured variable to determine the machine direction and cross-machine direction profile of the grammage.
A particular advantage of making the measurement at the wet end of the papermaking machine or, respectively, of measuring the paper stock consistency in the flowbox itself, is that it enables very rapid control to be achieved, because deviations in the flowbox are determined more rapidly than previously by means of a sensor arranged in this area, and this leads to the control system making an intervention. In other words, this means that such an arrangement has a shorter dead time and a shorter controlled system.
In a particularly cost effective and simple embodiment, a sensor arranged outside the flowbox may be of identical construction to the first sensor, to serve as the further sensor.