Gap width monitoring

Abstract

The present disclosure relates to a method of monitoring the width (w) of a gap (1) between two grinding discs (2) in a grinding apparatus (10) for treating a fibrous fluid (6). The method includes rotating a first disc of the abrasive discs about an axis of rotation of the first disc relative to a second disc of the abrasive discs. The method further includes obtaining a spectrum of mechanical vibrations in the grinding apparatus caused by rotation of the first disk relative to the second disk. The method further includes determining an occurrence rate of vibration peaks within a predetermined frequency range of the obtained spectrum, the occurrence rate being indicative of the gap width.

Description

Technical Field

[0001] The present disclosure relates to monitoring the clearance width between two grinding discs in a grinding apparatus for processing a fibrous fluid. Background Art

[0002] In the paper industry, equipment such as dispersers, refiners, and defibrators are used to process pulp in the form of a liquid dispersion. The fluid pulp is pressed between two grinding discs that rotate relative to each other about a rotational axis. The clearance width between the discs, i.e., the axial distance between the discs, affects the processing efficiency. Therefore, it is necessary to set an appropriate clearance width during operation.

[0003] Traditionally, before operation, an appropriate clearance width is set by axially pushing the grinding discs towards each other while they are rotating, and the resulting sound heard by the operator defines the zero or minimum clearance width. Starting from this minimum value, the discs can be axially separated by a desired amount, after which grinding can begin. One drawback is that the clearance width is not monitored during operation, and it is necessary to periodically interrupt the operation to recalibrate and find the minimum value. Recalibration is usually required because the grinding discs wear during operation, thereby affecting the minimum value and processing efficiency at a given clearance width.

[0004] US2022 / 0098790 discloses a pulp grinding apparatus in which, in order to determine the minimum distance between substrates, mechanical vibrations are detected on the apparatus, and the distance between the substrates rotating relative to each other is reduced until the frequency and / or amplitude and / or frequency change and / or amplitude change of the vibrations exceed a limit value.

[0005] WO2022 / 043363) discloses sensing vibrations caused by the relative rotation of two processing tools with a processing gap therebetween. The vibration intensity in the frequency range of 5 - 12 kHz is measured over time. When the threshold of the vibration intensity is exceeded, this indicates that the processing tools are in contact with each other, and the processing gap can be increased to prevent further such contact. To avoid false positives, intensity measurements can be performed at several consecutive time intervals.

[0006] US2007 / 125891 similarly discloses measuring the axial vibrations of a pulp refiner over time. When the measured vibrations indicate that the plates are colliding, the pulp flow rate is increased or the plates are further separated. Summary of the Invention

[0007] An object of the present invention is to monitor the clearance width of a grinding apparatus during its operation.

[0008] According to one aspect of the present invention, there is provided a method for monitoring a gap width between two grinding discs in a grinding apparatus for processing a fibrous fluid. The method includes rotating a first grinding disc of the grinding discs about a rotational axis of the first disc relative to a second disc of the grinding discs. The method further includes obtaining a frequency spectrum of mechanical vibrations in the grinding apparatus caused by the rotation of the first disc relative to the second disc. The method further includes determining, within a predetermined frequency range of the obtained frequency spectrum, a prevalence of vibration peaks, the prevalence being an indication of the gap width.

[0009] According to another aspect of the present invention, there is provided a controller including a processing circuit and a storage device storing instructions executable by the processing circuit, whereby the controller is operable to execute embodiments of the methods disclosed herein.

[0010] According to another aspect of the present invention, there is provided a grinding apparatus including an embodiment of the controller disclosed herein, and a first grinding disc and a second grinding disc.

[0011] According to another aspect of the present invention, there is provided a computer program product including computer-executable components for causing a controller to execute embodiments of the methods disclosed herein when the computer-executable components are run on a processing circuit included in the controller.

[0012] By obtaining the frequency spectrum of the mechanical vibrations and determining the prevalence of vibration peaks in the frequency spectrum, in the frequency domain, for example, determining how many peaks above a predetermined vibration amplitude (energy) exist within a predetermined frequency range of the frequency spectrum, an indication of the gap width is provided. A smaller gap width results in a higher prevalence of vibration peaks, indicating more difficult processing of the fibrous fluid (e.g., pulp). The desired prevalence of vibration peaks can be known, for example, from previous operations of the grinding apparatus, typically using the same type of grinding discs.

[0013] By studying the vibration peaks in the frequency domain rather than the time domain, the gap width can be monitored rather than just whether the discs are in contact (colliding) with each other. According to the present invention, the prevalence of vibration peaks in the frequency domain of the frequency spectrum is determined. For example, the prevalence can be defined by the number of vibration peaks within a predetermined frequency range of the frequency spectrum, or by the average distance between adjacent vibration peaks within that frequency range. Thus, the prevalence can be considered as the density of vibration peaks at various different frequencies in the frequency spectrum.

[0014] It should be noted that any feature of any aspect can be applied to any other aspect (where applicable). Similarly, any advantage of any aspect can be applied to any other aspect. Other objects, features, and advantages of the attached embodiments will become apparent from the following detailed disclosure, the appended dependent claims, and the drawings.

[0015] Generally, all terms used in the claims should be interpreted according to their ordinary meaning in the technical field, unless otherwise expressly defined herein. Unless otherwise expressly stated, all references to "an / a / the element, device, component, apparatus, step, etc." should be construed openly as referring to at least one instance of the element, device, component, apparatus, step, etc. Unless expressly stated, the steps of any method disclosed herein need not be performed in the exact order in which they are disclosed. The use of "first", "second", etc. for different features / components of the present disclosure is only intended to distinguish these features / components from other similar features / components, rather than to assign any order or hierarchy to these features / components. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments will be described by way of example with reference to the accompanying drawings, in which:

[0017] Figure 1 is a schematic side view of a longitudinal section of a grinding apparatus according to some embodiments of the present invention.

[0018] Figure 2 is a schematic block diagram of a grinding apparatus according to some embodiments of the present invention.

[0019] Figure 3 is a schematic block diagram of a controller according to some embodiments of the present invention.

[0020] Figure 4 is a schematic flow chart of some embodiments of the method of the present invention.

[0021] Figure 5 is a schematic example of the frequency spectrum of mechanical vibration and the occurrence rate of vibration peaks according to some embodiments of the present invention.

[0022] Figure 6 is a schematic diagram illustrating an example of the occurrence rate of vibration peaks as a function of the gap width according to some embodiments of the present invention. DETAILED DESCRIPTION

[0023] Embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, many different forms of other embodiments are possible within the scope of the present disclosure. On the contrary, the following embodiments are provided by way of example so that the present disclosure will be thorough and complete and will fully convey the scope of the present disclosure to those skilled in the art. Throughout the description, like reference numerals refer to like elements.

[0024] Figure 1The figure illustrates a grinding device 10, such as a disperser, a fine grinder or a refiner. The grinding device is configured to process a fiber fluid, such as a liquid in which fibers are suspended (i.e., a liquid suspension), such as a pulp or a bio-pulp for papermaking. In the case of pulp, the fiber fluid may include cellulose fibers. Thus, in some embodiments, the fibers in the fiber fluid include cellulose or consist of cellulose.

[0025] The grinding device 10 includes a first grinding disk 2a and a second grinding disk 2b, which are arranged to rotate relative to each other about a rotation axis 5. Generally, the first grinding disk 2a and the second grinding disk 2b are arranged in respective planes parallel to each other, and each grinding disk is arranged to be rotationally symmetric about a symmetry line corresponding to the rotation axis 5. For example, the first grinding disk 2a may be included (e.g., mounted) in a rotor 3a, which is arranged to rotate about the rotation axis 5, while the second grinding disk 2b may be included (e.g., mounted) in a stator 3b, which is arranged to be stationary while the rotor 3a rotates. A gap 1 is formed between the first disk 2a and the second disk 2b, and the gap width w is defined by the axial distance between the disks (or between the respective parallel planes in which the disks are arranged). During operation, when the grinding disks 2a and 2b rotate relative to each other, the fiber fluid 6 is squeezed through the gap 1 between the grinding disks 2a and 2b and is processed. For example, as shown in the figure, the fiber fluid 6 may be introduced into the gap 1 from an axial inlet, such as formed in the stator 3b. Thus, as the fluid 6 flows from the axial inlet towards the periphery of the grinding disks, the fluid 6 is processed by the grinding disks 2a and 2b in the gap 1, as indicated by the arrows in the gap 1 in the figure.

[0026] The gap width w can be adjusted by axially shifting at least one of the grinding disks 2a and 2b, for example, by axially moving the rotor 2a towards the stator 2b to reduce the gap width w, or away from the stator 2b to increase the gap width w. However, it can be difficult to know the actual gap width w (i.e., the distance between the disks, in millimeters or other length units) based only on the axial position of the rotor 3a, for example, because the thicknesses of the grinding disks 2a and 2b can vary due to design or wear.

[0027] Figure 2 The figure illustrates a grinding device 10, for example according to Figure 1A grinding device. The grinding device includes a vibration sensor 22 that is used to sense mechanical vibrations in the grinding device, which are generated by the rotation of the disks relative to each other. With the help of the vibration sensor 22, a vibration spectrum can be obtained. The vibration sensor can include an accelerometer, but any other vibration sensor can be additionally or alternatively used, such as including a laser vibrometer. The grinding device 10 can include a housing 21 that at least partially encloses the first grinding disk 2a and the second grinding disk 2b, for example, at least partially encloses the rotor 2a and the stator 2b. For example, when the fluid 6 is squeezed through the gap 1, the mechanical vibrations generated by the rotation of the disks relative to each other can propagate to the housing. Therefore, the vibration sensor 22 can be conveniently arranged to sense the vibrations in the housing 21, for example, by being mounted on the housing 21, especially if the vibration sensor 22 includes an accelerometer.

[0028] The grinding device 10 can also include a controller 20 that is used to control the operation of the grinding device. In some embodiments, the controller 20 controls the rotational speed of the grinding disk(s) and any axial displacement of the grinding disk(s). The controller 20 can, for example, send a control signal to cause the first disk 2a to rotate about its axis of rotation 5. For example, the controller 20 can receive a signal 23 from the sensor 22, and the controller 22 can obtain a spectrum from this signal 23. Based on the vibration spectrum, the controller can also determine the occurrence rate of vibration peaks. In some embodiments, the controller can control the user interface, for example, for communicating with or presenting information to the user / operator (such as a human user) of the grinding device 10.

[0029] Figure 3 The controller 20 is illustrated. The controller 20 includes a processing circuit 31, such as a central processing unit (CPU). The processing circuit 31 can include a processing unit in the form of one or more microprocessors. However, other suitable devices with computing capabilities can be included in the processing circuit 31, such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a complex programmable logic device (CPLD). The processing circuit 31 is configured to run one or more computer programs or software (SW) 33 stored in a storage device 32 of one or more storage units (such as a memory). The storage unit is regarded as a computer-readable device 32, which together with the SW 33 stored on it forms a computer program product as a computer-executable component, and can be, for example, in the form of a random access memory (RAM), a flash memory, or other solid-state memories, or a hard disk, or a combination thereof. As needed, the processing circuit 31 can also be configured to store data in the storage device 32. The controller 20 can also include a communication interface 23, for example, for receiving the sensor signal 23 from the vibration sensor 22.

[0030] Figure 4 Some embodiments of the method of the present disclosure are illustrated. The method is for monitoring the width w of the gap 1 between two grinding discs 2 in a grinding apparatus 10 for processing a fibrous fluid 6. The method includes rotating S1 a first disc 2a of the grinding disc about a rotation axis 5 of the first disc relative to a second disc 2b of the grinding disc. The method further includes obtaining S2 a frequency spectrum 50 of mechanical vibrations caused in the grinding apparatus 10 by the rotation S1 of the first disc 2a relative to the second disc 2b. The method further includes determining S3, within a predetermined frequency range 52 of the obtained S2 frequency spectrum 50, the occurrence rate of vibration peaks 54, the occurrence rate being an indication of the gap width w. In some embodiments of the present invention, the method further includes, during the rotation S1, shifting S4 at least one of the first disc and the second disc 2 along the rotation axis 5. In some embodiments, the shifting S4 is for gradually reducing the gap width w until the determined S3 occurrence rate exceeds a predetermined occurrence rate threshold T, defining a minimum value m of the gap width. In some embodiments, the method may then include monitoring S5 the change of the minimum value over time, the change indicating the progressive wear of the first disc and / or the second disc 2. In some other embodiments, the shifting S4 is for adjusting the gap width w such that the determined S3 occurrence rate is maintained within a predetermined occurrence rate range R.

[0031] Figure 5 The vibration frequency spectrum 50 is illustrated. The frequency spectrum may present vibration amplitudes (energies) at different frequencies, i.e., in the frequency domain rather than the time domain, for example, at any given time or time period, with the frequency f on the X-axis and the vibration amplitude A on the Y-axis, as shown. The frequency spectrum includes or consists of mechanical vibrations in the grinding apparatus 10 (such as its housing 21) caused by the rotation S1 of the first disc 2a relative to the second disc 2b. For example, the mechanical vibrations may be measured over time (during a predetermined time period), for example, by obtaining S2 a sensor signal 23 from a vibration sensor 22. Then, frequency analysis may be performed on the data of the measured vibrations to obtain the frequency spectrum 50. As described herein, the mechanical vibrations depend on the gap width, but also depend on, for example, the rotational speed of the rotor 3a, the number of teeth in the grinding discs 2a and 2b, and the configuration and / or wear of the teeth.

[0032] From the frequency spectrum 50, vibration peaks 54 at respective frequencies can be determined, where at the vibration peak, the specific frequency is defined if the vibration amplitude A at the specific frequency exceeds a predetermined threshold 51. The frequency resolution separating the vibrations at different frequencies may be predetermined, for example, depending on the sampling frequency and the sampling time (i.e., the number of samples).

[0033] When determining the occurrence rate of the vibration peak 54 in the S3 spectrum 50, the vibration peaks within a predetermined frequency range 52 can be considered. The occurrence rate can be regarded as the density of the vibration peaks in the frequency domain of the spectrum, that is, the number of peaks at each different frequency in each predetermined frequency range of the spectrum. In some embodiments, the occurrence rate of the vibration peak 54 is defined by the number of vibration peaks within the frequency range 52. Additionally or alternatively, in some embodiments, the occurrence rate of the vibration peak 54 is defined by the average distance 53 between adjacent vibration peaks 54 within the frequency range 52. Since the average distance 53 between the peaks 54 within the range 52 depends on the number of peaks 54 within the range 52, whether to consider the number of peaks 54 and / or the average distance 53 between the peaks 54 can merely be a matter of easier analysis.

[0034] Figure 6 An example of determining the occurrence rate of S3, such as the number of vibration peaks 54, as a function of the gap width w is illustrated. The gap width w is measured, for example, as the axial position of one of the abrasive disks 2a and 2b, such as the axial position of the rotor 3a. Generally, the occurrence rate increases as the gap width w decreases. When the occurrence rate exceeds a predetermined occurrence rate threshold T, the minimum gap width m, so-called zeroing, can be defined. Thus, the minimum value m of the gap width w can be defined by axially shifting S4 one abrasive disk 2a towards the other abrasive disk 2b until the determined occurrence rate of S3 exceeds the predetermined occurrence rate threshold T.

[0035] In some embodiments, it may be necessary to keep the occurrence rate of the vibration peak 54 relatively constant during the operation of the grinding device 10, and this is generally achieved by axially shifting S4 one of the disks as needed to adjust the gap width w. Thus, in some embodiments, one of the disks can be axially shifted S4 to adjust the gap width w such that the determined occurrence rate of S3 is maintained within a predetermined occurrence rate range R. For example, in some embodiments, during the operation of the grinding device 10, the abrasive disk 2a can be axially shifted S4 from the defined minimum m gap width w to the operating gap width defined by the occurrence rate of the vibration peak 54 within the occurrence rate range R. The occurrence rate at the operating gap width can vary over time, for example, decrease, for example, due to wear of the abrasive disk, which is why the axial shift S4 is performed during operation, such as axially moving the rotor 3a towards the stator 3b, to keep the occurrence rate within the occurrence rate range R.

[0036] The present disclosure has been mainly described above with reference to several embodiments. However, as will be readily understood by those skilled in the art, other embodiments besides those disclosed above are equally possible within the scope of the present disclosure defined by the appended claims.

Claims

1. A method for monitoring the width (w) of a gap (1) between two grinding discs (2) in a grinding apparatus (10) for processing a fibrous fluid (6), the method comprising: rotating (S1) a first disc (2a) of the grinding discs about a rotational axis (5) of the first disc relative to a second disc (2b) of the grinding discs; obtaining (S2) a frequency spectrum (50) of mechanical vibrations in the grinding apparatus (10) caused by the rotation (S1) of the first disc (2a) relative to the second disc (2b); within a predetermined frequency range (52) of the obtained (S2) frequency spectrum (50), determining (S3) an occurrence rate of vibration peaks (54), the occurrence rate being an indication of the gap width (w).

2. The method according to claim 1, wherein each of the vibration peaks (54) in the frequency spectrum (50) is defined as a vibration peak at a specific frequency (f) of the peak by exceeding a predetermined vibration amplitude (A) threshold (51) at the specific frequency of the peak.

3. The method according to any one of the preceding claims, wherein the occurrence rate of the vibration peaks (54) is defined by the number of vibration peaks within the frequency range (52), or by the average distance (53) between adjacent vibration peaks within the frequency range.

4. The method according to any one of the preceding claims, further comprising: during the rotation (S1), shifting (S4) at least one of the first disc and the second disc (2) along the rotational axis (5) to: gradually reduce the gap width (w) until the determined (S3) occurrence rate exceeds a predetermined occurrence rate threshold (T), defining a minimum value (m) of the gap width; or adjust the gap width (w) such that the determined (S3) occurrence rate is maintained within a predetermined occurrence rate range (R).

5. The method according to claim 4, further comprising: monitoring (S5) the change of the minimum value over time, the change indicating progressive wear of the first disc and / or the second disc (2).

6. The method according to any one of the preceding claims, wherein the frequency spectrum (50) is obtained (S2) by means of a vibration sensor (22), the vibration sensor including, for example, an accelerometer.

7. The method according to claim 6, wherein the vibration sensor (22) is arranged to measure vibrations of a housing (21) of the grinding apparatus (10).

8. The method according to any one of the preceding claims, wherein the frequency spectrum (50) is obtained (S2) when the fibrous fluid passes through the gap (1).

9. The method according to claim 8, wherein the fibrous fluid is a slurry or a biopulp.

10. The method according to claim 8 or 9, wherein the fibers in the fibrous fluid comprise cellulose or consist of cellulose.

11. A controller (20) comprising: a processing circuit (31); and A storage device (32) that stores instructions (33) executable by the processing circuit (31), whereby the controller (20) is operable to execute the method according to any one of the preceding claims.

12. A grinding device (10), comprising: The controller (20) according to claim 11; And The first grinding disk and the second grinding disk (2).

13. The grinding device according to claim 12, wherein the grinding device (10) is a disperser, a finisher or a defibrator.

14. A computer program product (32), comprising computer-executable components (33) for causing the controller (20) to execute the method according to any one of claims 1 to 10 when the computer-executable components (33) run on a processing circuit (31) included in the controller.

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