Method and controller for detecting vibrations in creping blades

Abstract

The present application relates to a method for detecting vibrations in a creping blade of a tissue machine, which tissue machine is provided with one or more vibration sensors which are arranged to detect vibrations of the creping blade, the method comprising the steps of receiving vibration signals from the vibration sensors, which vibration signals are indicative of vibrations of the creping blade, monitoring (302) the vibrations signals within one or more resonance frequency ranges, identifying (304) one or more peak frequencies within the resonance frequency ranges, monitoring (306) the amplitude of the one or more peak frequencies, comparing (308) the amplitudes of the one or more peak frequencies with a respective of one or more pre-determined levels, and generating (310) a chatter alert if one or more of the monitored amplitudes are above said respective pre-determined level.

Description

[0001] METHOD AND CONTROLLER FOR DETECTING VIBRATIONS IN CREPING BLADES

[0002] TECHNICAL AREA

[0003] The present application relates to paper creping processes and the occurrence of vibrations in creping blades, in particular chatter vibrations.

[0004] BACKGROUND OF INVENTION

[0005] When producing tissue paper, one vital component is the crepe blade that is in contact with a yankee cylinder. The yankee cylinder is used for drying a fibre web and in order for the web to be held on the surface of the yankee cylinder, different types of chemicals are used as coating layers on the yankee cylinder. The dried web is removed from the yankee cylinder by the crepe blade. During the removal of the web, the web is creped for producing soft tissue.

[0006] There are many factors that interact with each other during creping for producing an end product with the desired properties, such as for instance the yankee cylinder, the coating layer and the crepe blade. During the creping process, the crepe blade is exposed to vibrations. This is due to periodic changes in the friction between the crepe blade and the yankee cylinder. The vibrations of the crepe blade causes it to bounce in the coating, creating an uneven surface. If the bouncing reaches high levels, there is a risk that the crepe blade cuts through the coating and comes in contact with the surface of the yankee cylinder which causes damage to the yankee cylinder in the form of chatter marks. This is more common when the crepe blade becomes worn. A new crepe blade will cut effortlessly into the coating but as the crepe blade is worn, the cutting ability is reduced and any irregularities in the coating will increase, which leads to an increase in vibrations.

[0007] Chatter marks on a yankee cylinder is something that should be avoided because a yankee cylinder with chatter marks has to be reground, which is costly and also causes production stops. Therefore, methods and systems have been developed for monitoring vibrations in crepe blades in order to detect increased vibrations that could lead to chatter marks. Vibration sensors are attached to the crepe blade holder and the signals from the sensors are analysed. A common analysis approach is to use acceleration as measure and since raw vibration signals are complex, a root mean square (RMS) average is calculated. However, with RMS, it is difficult to decisively identify chatter frequencies and in particular, it is difficult to set limits when measures have to be taken, such as replacing a worn crepe blade.

[0008] There is thus room for improvements regarding preventing chatter marks and the like damage to yankee cylinders.

[0009] BRIEF DESCRIPTION OF INVENTION

[0010] The aim of the present application is to improve monitoring and detection of chatter frequencies in creping blades in order to avoid serious damages to a yankeecylinder and possibly other equipment used in the creping process. This aim is solved with the features of the independent patent claim. Preferable embodiments form the subject of the dependent patent claims.

[0011] According to a main aspect, a method for detecting vibrations in a creping blade of a tissue machine is provided, which tissue machine is provided with one or more vibration sensors which are arranged to detect vibrations of the creping blade.

[0012] The method comprises the steps of receiving signals from the vibration sensors, which signals are indicative of vibrations of the creping blade, monitoring the vibration signals within one or more resonance frequency ranges, identifying one or more peak frequencies within the resonance frequency ranges, monitoring the amplitudes of the one or more peak frequencies, comparing the amplitudes of the one or more peak frequencies with a respective of one or more pre-determined levels, and generating a chatter alert if one or more of the monitored amplitudes are above said respective pre-determined level.

[0013] One advantage of the above method is that it focuses on specific resonance frequency ranges of the vibration spectrum incoming from the vibration sensors as opposed to conventional methods where a much larger extent of the vibration spectrum is used and wherein these signals are subjected to RMS average. With the present method, chatter frequencies that may lead to damages of the yankee cylinder may be detected much more precisely and specifically and therefore enables setting of levels that the peak amplitudes should not exceed.

[0014] Preferably, the growth over time of the amplitude of the one or more peak frequencies is monitored in order to follow the behaviour of the creping blade.

[0015] Regarding frequency ranges used with the method, a first resonance frequency range is 10 - 20 KHz. This first frequency range corresponds to creping blade chatter frequencies in directions of vibration generally coinciding with the breadth of a creping blade and transversal to a longitudinal direction of the creping blade.

[0016] Further regarding frequency ranges used with the method, a second resonance frequency range is 1 - 6 kHz. This second resonance frequency range corresponds to creping blade chatter frequencies in directions of vibration generally transversal to the breadth of a creping blade and transversal to a longitudinal direction of the creping blade.

[0017] According to another aspect of the method, vibration sensors may be arranged on both sides of the creping blade and wherein signals from both sensors are indicative of vibrations of the creping blade, and obtaining one or more active resonance frequencies from of the creping blade within resonance frequency ranges.

[0018] This is an advantage if the creping blade is long and chatter may occur at different positions along the creping blade. If one sensor is used, it may be far from chatter position and may then not give the intended vibration levels to the sensor. This risk is reduced by using vibration sensors on both sides. In this regard, it is possible to arrange a plurality of vibration sensors along the extension of the creping blade.

[0019] Regarding the vibration sensors, they may be acceleration sensors and wherein the amplitudes are power peaks. Further, regarding generating an alert signal, this may comprising the steps of generating a warning signal should one pre-determined level be exceeded and generating an alarm signal should an even higher pre-determined level be exceeded. With two, or even more, levels of alerting that something is not normal regarding the creping blades, operators of the tissue creping machine may be provided with more information and decision points as to what measures that need to be taken should a level be exceeded.

[0020] According to a further aspect of the application, a controller is provided, comprising program instructions which, when the program is executed by the controller, causes the controller to carry out the method according to the application. Moreover, computer-readable storage medium may be provided, comprising program instructions which, when executed by a controller, cause the controller to carry out the method according to the application.

[0021] These and other aspects of, and advantages with, the present invention will become apparent from the following detailed description of the invention and from the accompanying drawings.

[0022] BRIEF DESCRIPTION OF DRAWINGS

[0023] In the following detailed description of the invention, reference will be made to the accompanying drawings, of which

[0024] Fig. 1 is a schematic view of a yankee cylinder provided with a doctor blade for a creping process,

[0025] Figs. 2 and 3 show different types of chatter vibrations in a doctor blade, and

[0026] Fig. 4 is a flow chart of a method according to the application.

[0027] DETAILED DESCRIPTION OF THE INVENTION

[0028] The present application relates to tissue paper making machines and in particular creping blades 10 that are used in conjunction with yankee cylinders 12 in the tissue making process for creping the paper web, schematically shown in fig. 1 . In order to monitor the behaviour of the creping blade 10 during creping and in order to detect the presence of chatter vibrations in the creping blade 10, vibration sensors 14 are positioned in relation to the creping blade. Preferably, vibration sensors 14 are positioned at both ends of the blade as seen in Fig. 1 , i.e. on the drive side and on the tending side, for instance or preferably on the creping blade holders. The vibration sensor may only be positioned on one side, but with very long creping blades, chatter may occur far from the sensor and may then not give the intended vibration levels to the sensor. In this regard, it is possible to provide several vibration sensors along the length of the creping blade. The vibration sensors 14 may for instance be acceleration sensors.

[0029] The vibration sensors 14 are operationally connected to a controller 16 via suitable communication channels 18. The controller is provided with I / O elements for receiving signals from the vibration sensors as well as data storage elements for storing data from the vibration sensors and processor elements programmed and capable of processing data from the vibration sensors 14.

[0030] The controller may comprise a computer which may take the form of substantially any suitable type of hardware or hardware / firmware device implemented using processing circuity such as, but not limited to, a processor, Central Processing Unit (CPU), a controller, an arithmetic logic unit (ALU), a digital signal processor, an Application Specific Integrated Circuit (ASIC), a circuit for digital signal processing (digital signal processor, DSP), a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, an application-specific integrated circuit, or any other device capable of electronically performing operations in a defined manner, or other processing logic that may interpret and execute instructions. The herein utilised expression “computer” may represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above.

[0031] The controller may further comprise a memory unit, wherein the computer may be connected to the memory unit, which may provide the computer with, for example, stored program code and / or stored data which the computer may need to enable it to do calculations. The computer may also be adapted to store partial or final results of calculations in the memory unit. The memory unit may comprise a physical device utilised to store data or programs, i.e., sequences of instructions, on a temporary or permanent basis. According to some embodiments, the memory unit may comprise integrated circuits comprising silicon-based transistors. The memory unit may comprise e.g. a memory card, a flash memory, a USB memory, a hard disc, or another similar volatile or non-volatile storage unit for storing data such as e.g. ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), EEPROM (Electrically Erasable PROM), etc. in different embodiments.

[0032] When in operation, the controller 16 obtains signals from the vibration sensors 14. The software for detecting chatter is programmed to handle certain frequency ranges in the acceleration power spectrum in which active resonance frequencies from the creping blade occur. It has been identified that these ranges are 10 - 20 kHz for linear vibrations of the creping blade, as shown in Fig. 2 where the direction of the vibrations generally coincide with the breadth of the creping blade, i.e. between the base and the tip of the blade, and 1 - 6 kHz for bending vibrations of the creping blade, as shown in Fig. 3 where the direction of the vibrations is generally transversal to the breadth of the creping blade and transversal to the longitudinal direction of the creping blade. Regarding the above frequency spectra, it is to be understood that depending on operational parameters, geometries of the creping blade, and other factors that affect the creping process, the ranges may vary somewhat, and it might therefore be necessary to adjust the ranges upwards or downwards along the frequency spectrum.

[0033] When a power peak occurs within these ranges, it is an indication of the resonance frequency of that particular creping blade. Continuous data from the sensor signals are monitored and in particular the highest peak of resonance frequency detected within the ranges. This frequency may move somewhat up or down during operation, but not to a great extent. The controller continuously monitors the power peak level throughout the life of the creping blade and the amplitudes and their growth over time are monitored continuously. The controller may in advance be provided with limits that the peaks should not exceed, which otherwise may result in chatter vibrations that can damage the yankee cylinder. The limits may be set depending on several operational parameters that may be connected to the actual tissue machine. Regarding the creping blade, parameters like the geometry of the creping blade holder, the blade angle and its stick-out as well as the bevel angle may be important. Further, the end product may also affect due to requirement of more aggressive workload for certain products in order to produce the desired paper properties as well as the mix and amount of coating used.

[0034] In this regard, there may be more than one limit that the controller is provided with and against which the power peak level is compared. There could for instance be a lower limit and if the peak exceeds that limit, the controller may issue a warning signal that something is not normal, but that may not require drastic actions such as shutting down the machine. In addition, there may be a higher limit and if the peak exceeds that limit, the controller may issue an alarm signal that something is seriously wrong, which may require immediate shutdown of the machine in order not to damage the yankee cylinder.

[0035] The controller is as mentioned above provided with program codes to perform the method steps 300 according to the present application. This is shown schematically in Fig. 4.

[0036] In step 302, the controller receives vibration signals from the vibration sensors, wherein the vibration signals are indicative of vibrations of the creping blade.

[0037] In step 302, the controller monitors the vibration signals within one or more resonance frequency ranges. Depending on the type of resonance frequencies, either linear or bending vibrations, two different types of frequency ranges are provided to the controller. For linear vibrations, the frequency range provided is between 10 to 20 kHz and for bending vibrations, the frequency range is between 1 to 6 kHz. The controller is set to analyse frequencies within these ranges. In step 304, the controller is programmed to identify one or more peak frequencies within the resonance frequency ranges. That is, frequencies within the resonance frequency ranges that have a much higher amplitude are targeted. In most cases, there is one small frequency span that provides these power peaks. In the further step 306, the amplitudes of the one or more peak frequencies are monitored by the controller, which now are identified as important for detecting chatter vibrations of the creping blade. In step 308, the amplitude of the monitored peak frequencies are compared with a respective of one or more pre-determined levels, which may be set depending on several factors affecting the creping process. The above steps are performed continuously during operation. As an example, the sampling of signals from the vibration sensors may be performed about every third minute or so, which may be important for the controller in order to monitor the growth of the amplitudes.

[0038] In step 310, the controller is programmed to generate a chatter alert signal if one or more of the monitored amplitudes are above said respective pre-determined level. As a sub-step 312 to 310, the controller may be programmed to generate a warning signal should one level be exceeded and then generate an alarm signal should an even higher level be exceeded.

[0039] It is to be understood that the embodiment described above and shown in the drawings is to be regarded only as a non-limiting example of the invention and that it may be modified in many ways within the scope of the patent claims.

Claims

PATENT CLAIMS1 . A method for detecting vibrations in a creping blade (10) of a tissue machine, which tissue machine is provided with one or more vibration sensors (14) which are arranged to detect vibrations of the creping blade, the method comprising the steps of:- receiving vibration signals from the vibration sensors, which vibration signals are indicative of vibrations of the creping blade,- monitoring the vibration signals within one or more resonance frequency ranges,- identifying one or more peak frequencies within the resonance frequency ranges,- monitoring (306) the amplitude of the one or more peak frequencies,- comparing (308) the amplitudes of the one or more peak frequencies with respective of one or more pre-determined levels, and- generating (310) a chatter alert if one or more of the monitored amplitudes are above said respective pre-determined level.

2. Method according to claim 1 , wherein the growth over time of the amplitude of the one or more peak frequencies is monitored.

3. Method according to claim 1 or 2, wherein a first resonance frequency range is 10 - 20 KHz.

4. Method according to claim 3, wherein said first frequency range corresponds to creping blade chatter frequencies in directions of vibration generally coinciding with the breadth of a creping blade.

5. Method according to any of the claims 1 to 4, wherein a second resonance frequency range is 1 - 6 kHz.

6. Method according to claim 5, wherein said second resonance frequency range corresponds to creping blade chatter frequencies in directions of vibrationgenerally transversal to the breadth of a creping blade and transversal to a longitudinal direction of the creping blade.

7. Method according to any of the preceding claims, wherein vibration sensors are arranged on both sides of the creping blade and wherein signals from both sensors are indicative of vibrations of the creping blade, and obtaining one or more active resonance frequencies from of the creping blade within resonance frequency ranges.

8. Method according to any of the claims 1 to 6, wherein a plurality of vibration sensors are arranged along the extension of the creping blade.

9. Method according to any of the preceding claims, wherein said vibration sensors are acceleration sensors and wherein the amplitudes are power peaks.

10. Method according to any of the preceding claims, further comprising the steps of generating (312) a warning signal should one pre-determined level be exceeded and generating (312) an alarm signal should an even higher predetermined level be exceeded.11 . Method according to any of the preceding claims, wherein the vibration signals comprise one or more active resonance frequencies from of the creping blade.

12. A controller (16) comprising program instructions which, when the program is executed by the controller, causes the controller to carry out the method according to claims 1 - 11.

13. A computer-readable storage medium comprising program instructions which, when executed by a controller, cause the controller to carry out the method according to claims 1 - 11.

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