WO2000004227A1

WO2000004227A1 - Method for attenuation of vibrations in a paper machine environment

Info

Publication number: WO2000004227A1
Application number: PCT/FI1999/000617
Authority: WO
Inventors: Jorma Köliö; Jouko Karhunen; Esa Lehtovirta
Original assignee: Valmet Corporation
Priority date: 1998-07-17
Filing date: 1999-07-13
Publication date: 2000-01-27
Also published as: AU5042799A; FI981628A0; FI104208B; FI104208B1

Abstract

A method for damping vibrations in a paper machine environment by means of a dynamic absorber. The vibrations of a vibrating object are measured (6) and an amplified (7) measurement signal is passed to a data processing device (8), in which the measurement signal is analysed. By means of the data processing device (8) a regulation device (9) is controlled by whose means the dynamic absorber attached to the vibrating object is tuned to a desired problematic excitation frequency. The dynamic absorber is tuned in a cyclic circulation to one problematic excitation frequency at a time, to which frequency it is kept tuned for a given period of time, after which it is tuned to the next problematic excitation frequency, etc.

Description

METHOD FOR ATTENUATION OF VIBRATIONS IN A PAPER MACHINE ENVIRONMENT

The invention relates to a method for damping vibrations in a paper machine environment as defined in the preamble of claim 1.

In paper machines and in paper finishing devices, vibrations constitute a major problem that continues to become greater with constantly increasing speeds. There are several possible sources of vibration in paper machines, such as, rolls and cylinders, which comprise a very great mass revolving at a considerable speed. Attempts are being made to eliminate the vibrations of rolls by making them with high measurement precision, and in addition thereto they are also balanced.

However, present-day paper machines and paper finishing devices increasingly employ rolls provided with a soft coating, which rolls may form in operation a significant source of vibration. Such rolls are used, for example, in presses, calenders, coating machines, size presses, supercalenders and equivalent, where said roll provided with a soft coating forms a nip with another roll. A paper web and possibly a felt, wire or equivalent are passed through the nip. When in this kind of nip roll arrangement, a seam of the wire, felt or web, considerable impurities or something else causing a noticeable change in the thickness of the web travelling through the nip, passes/pass through the nip during running, the coating must yield elastically, with the result that the coating serves as a spring that excites vibration.

For example, in a size press and in a coating device of the size press type, a nip is defined by means of two rolls such that one nip roll is mounted by means of bearing housings directly on the frame structure of said device, while the opposite roll is mounted at its bearing housings on loading arms that are attached by means of articulated joints to the frame structure of the machine. In that case, the roll mounted on the loading arms in particular begins to vibrate, in which connection the coating of the soft-faced roll is deformed, with the result that vibration increases and the roll begins to resonate. In such a situation, one vibration frequency usually becomes clearly the most troublesome in its amplitude, in which case the situation may be taken care of by means of a dynamic absorber which is tuned to the problematic excitation frequency in question. The dynamic absorber is capable of damping a very narrow frequency band and the dynamic absorber has no effect on excitations which are outside said narrow frequency band.

In presses where a press nip may be formed between two hard-faced rolls or between a hard-faced and a soft-faced roll, the press nip and a press felt travelling through the press nip may induce problematic vibrations. The seam of the press felt and the water bound to the press felt may produce in the press felt a discontinuity that causes vibration of the press. In that case, in the press structure there are produced a vibration induced by the rolls at a higher frequency and a vibration induced by the press felt at a lower frequency. In such a situation, a dynamic absorber tuned only to one narrow frequency range damps only the problematic excitation frequency situated in said frequency range and other problematic frequencies remain undamped.

The applicant's FI patent application 971864 discloses a method and an apparatus for damping vibration in a paper machine or in a paper finishing device. The method uses a dynamic absorber which comprises an additional weight suspended from a vibrating object by means of a spring. In the method, the vibration frequencies of the vibrating system are measured constantly by means of one or more vibration detectors. The measurement signals of the vibration detector are amplified by means of an amplifier and fed into a vibration analyser which identifies a problematic excitation frequency and converts said problematic excitation frequency into a regulation signal. The regulation signal is fed into a regulation device which changes the spring constant of the spring of the dynamic absorber and/or the mass of the dynamic absorber in order to make the natural frequency of the dynamic absorber substantially equal to the problematic excitation frequency. US patent 4,420,371 discloses a vibration absorber for use in a vibrating headbox. The patent discloses a headbox provided with three separate vibration absorbers. The first vibration absorber fitted at the rear of the headbox damps vibrations in the machine direction, the second vibration absorber mounted on the headbox damps vertical vibrations, and the third vibration absorber mounted on the side of the headbox damps vibrations in the horizontal cross machine direction. The first vibration absorber damping the machine direction vibrations comprises three separate vibration absorbers mounted on a common base, each of said vibration absorbers being formed of a rod attached to said base at both ends thereof and of a weight connected to the middle of the rod, and each of said vibration absorbers being tuned to a different frequency by means of a weight of different magnitude. One vibration absorber intended for said machine direction vibration is tuned to the centre of problematic vibration frequencies, the second vibration absorber is tuned below the centre, and the third vibration absorber is tuned above the centre, in which connec- tion a larger range can be covered than by means of only one vibration absorber.

The object of the present invention is to provide a method by means of which it is possible to damp several problematic vibrations in different frequency ranges by means of one dynamic absorber.

The principal characteristic features of the method according to the invention are set forth in the characterizing clause of claim 1.

In the invention, vibration frequencies of a vibrating system are measured by means of one or more vibration detectors. The measurement signals of the vibration detector are amplified by means of an amplifier and fed into a vibration analyser which identifies the excitation frequencies which are problematic and converts said problematic excitation frequencies into regulation signals which control the dynamic absorber by means of a regulation device.

The invention allows several vibrations situated in different frequency bands to be damped. In implementing the method, an apparatus may be used which is very simple in its construction and mode of implementation and which can be connected by simple operations to existing structures for the purpose of damping vibrations.

In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing.

Figure 1 is a schematic view of one example of an apparatus which is based on a dynamic absorber and by which the method according to the invention may be implemented.

Figure 2 is a schematic view of problematic excitations.

Figure 3 is a schematic view of another example of an apparatus which is based on a dynamic absorber and by which the method according to the invention may be implemented.

In Fig. 1, a dynamic absorber is mounted on a vibrating object, i.e. in this case, on a bearing housing 2 of a roll 1. The dynamic absorber comprises a mass 4 suspended from the vibrating object 2 by means of a spring 3. A rod 3 rigidly mounted in the bearing housing 2 by means of attachment members 5 serves as the spring. The rod 3 is additionally provided with external threads, and it is mounted substantially in a horizontal plane in the bearing housing 2. A disc-shaped weight 4 serves as the mass and comprises a hole provided with internal threads, the weight 4 being disposed on the external threads of the rod 3 by means of said hole. The weight 4 can be displaced in the longitudinal direction of the rod 3 by rotating such that the distance a of the weight 4 from the point of attachment of the rod 4 in the bearing housing 2 can be regulated.

Such a dynamic absorber has one degree of freedom and it can be dimensioned by the basic equation:

k/m = Ω², where k is the spring constant of the spring, i.e. of the rod 3 in this case, m is the mass of the weight 4, and Ω is the angular velocity of the vibrating object, i.e. of the bearing housing 2.

The effect of the dynamic absorber is based on the fact that the natural frequency of the absorber is tuned to be equal to a problematic excitation frequency. In the case of the figure, the spring constant of the rod 3 is inversely proportional to the power of three of the length of the rod 3, and thus the natural frequency of the absorber can be regulated by adjusting the distance a of the weight 4 from the point of attachment of the rod in the bearing housing 2. When the natural frequency of the absorber has been made equal to a problematic excitation frequency by changing the distance a, the bearing housing 2 ceases to vibrate and the weight 4 resting on the rod 3 begins to vibrate, respectively. In that connection, the dynamic absorber generates a force that is in an opposite phase and of equal magnitude to the excitation, whereby the vibration of the machine itself ceases.

In the arrangement of Fig. 1, the controllability of the dynamic absorber is provided such that the bearing housing 2 whose vibration is desired to be damped is provided with a vibration detector 6. The vibration detector 6 transmits a signal that is amplified by an amplifier 7 and passed further to a data processing device 8 serving as a vibration analyser, which filters and analyses the vibration frequencies and locates the problematic excitation frequencies among the frequencies. When the problematic excitation frequencies have been identified, the computer 8 computes the distance a such that the natural frequency of the dynamic absorber coincides with the problematic excitation frequency. After that, the data processing device 8 controls a regulation device 9 connected to the weight 4 of the dynamic absorber, which regulation device moves the weight 4 on the rod 3. The regulation device 9 is advantageously, for example, a stepping motor. The apparatus thus comprises a closed regulating circuit that constantly measures and analyses vibrations and, based on this, regulates the natural frequency of the dynamic absorber. Fig. 2 is a schematic view of vibration that occurs, for example, in a press section. It is seen from the figure that there are several problematic vibration frequencies f _j , f₂,f₃- This may be due to the fact that the vibrating system comprises several devices which vibrate at different frequencies. For example, press rolls in a press section constitute one source of vibration, and the passing of discontinuities of a press felt through a press nip constitute another source of vibration. The vibration frequencies of these differ from one another, and thus there are created several problematic vibration frequencies in the system. For example, in size presses, rolls constitute a considerable vibration source, but the natural frequency of the entire vibrating system is, however, not necessarily equal to a multiple of the rotational frequency of the roll inducing the vibration (in most instances this is not the case).

Fig. 3 shows an example of another apparatus which is based on a dynamic absorber and to which the method according to the invention may be applied. The regulating circuit corresponds to the regulating circuit shown in Fig. 1. The spring 3b of the dynamic absorber comprises a rod which is fitted and attached to a bearing housing 2 in a manner corresponding to the illustration of Fig. 1. The weight 4b of the dynamic absorber comprises a container and a liquid provided therein whose amount is regulated by means of a pump 21 and a valve 22. By pumping more liquid from a storage container 23 into the container forming the weight 4b, the mass of the weight 4b can be increased, and vice versa. The regulation device 9 thus controls, based on a regulation signal received by it, said pump 21 and valve 22 in order to change the amount of the liquid contained in the container associated with the weight 4b. In that case, it is a question of only regulating the mass 4b of the dynamic absorber. In the case of Fig. 3, it may be contemplated that the distance a of the weight 4b from the point of attachment of the rod 3b can also be regulated.

In the situation of Figs. 1 and 3, it may also be contemplated that the spring or rod 3, 3b of the dynamic absorber is made of memory metal. The coefficient of elasticity of memory metal is highly dependent on temperature. In that case, the natural frequency of the absorber can be tuned to a right level by regulating the temperature of the rod 3, 3b. The regulation of temperature can be performed, for example, by means of electric resistors or equivalent heaters. In this kind of arrangement, the additional weight 4,4b can be attached to the rod 3,3b totally rigidly and without a clearance, for example, by welding. The structure may thus be made fairly simple. Regarding memory metals, it may be stated that they are alloys of different metals, of which an alloy of nickel and titanium may be mentioned as one example. The properties of such an alloy may be regulated by introducing into it a sufficient amount of energy in the form of heating, with the result that the crystal structure of the metal alloy can be changed by said introduction of additional energy. Memory metal "remembers" the change which a certain heating operation brings about in the metal alloy.

In the method in accordance with the invention, the dynamic vibration absorbers described above are regulated such that they are tuned in a cyclic circulation to one problematic excitation frequency at a time, to which frequency it is kept tuned for a given period of time, for example, for a few minutes, after which it is tuned to the next problematic excitation frequency, etc. If it is desired to damp, for example, the three problematic excitation frequencies f₁.f2.f₃ illustrated in Fig. 2, the dynamic absorber is tuned first to the frequency f_γ and kept there for some time. After that, the dynamic absorber is tuned to the frequency f₂ and kept again there for some time. After that, the dynamic absorber is tuned to the frequency f₃ and kept there for some time, after which the cycle is started again from the frequency f_j.

Several problematic frequencies that are being excited can be damped in this way. Of course, the dynamic absorber can also be tuned to problematic frequencies in totally arbitrary order. The dynamic absorber can be tuned to problematic excitation frequencies in a systematic cyclic circulation or in a random circulation or based on whichever problematic frequency is being excited at any given time.

Above, the invention has been described by way of example with reference to the figures in the accompanying drawing. The invention is, however, not confined to relating only to the examples illustrated in the figures, but different embodiments of the invention may vary within the scope of the inventive idea defined in the accompanying claims.

Claims

1. A method for damping vibrations in a paper machine environment by means of a dynamic absorber, wherein the vibrations of a vibrating object are measured (6), an amplified (7) measurement signal is passed to a data processing device (8), in which the measurement signal is analysed, and by means of the data processing device (8) a regulation device (9) is controlled by whose means the dynamic absorber attached to the vibrating object is tuned to a desired problematic excitation frequency, characterized in that the dynamic absorber is tuned to one problematic excitation frequency at a time, to which frequency it is kept tuned for a given period of time, after which it is tuned to the next problematic excitation frequency, etc.

2. A method as claimed in claim 1, characterized in that the dynamic absorber is tuned in a systematic cyclic circulation to one problematic excitation frequency at a time, to which frequency it is kept tuned for a given period of time, after which it is tuned to the next problematic excitation frequency, etc.

3. A method as claimed in claim 1 , characterized in that the dynamic absorber is tuned in a random circulation to one problematic excitation frequency at a time, to which frequency it is kept tuned for a given period of time, after which it is tuned to the next problematic excitation frequency, etc.

4. A method as claimed in claim 1, characterized in that the dynamic absorber is tuned to one problematic excitation frequency which is being excited at any given time, to which frequency it is kept tuned for a given period of time, after which it is tuned to the next problematic excitation frequency which is being excited, etc.

5. A method as claimed in any one of claims 1 to 4, characterized in that a rod (3), i.e. a spring, attached at one end thereof to the vibrating object is used as the dynamic absorber, on which rod an additional weight (4) is disposed which can be displaced in the longitudinal direction of the rod, in which connection the dynamic absorber is tuned to a problematic excitation frequency by regulating the distance (a) of the additional weight (4) from the point of attachment of the rod (3), i.e. by changing the spring constant of the dynamic absorber.

6. A method as claimed in any one of claims 1 to 4, characterized in that a rod (3), i.e. a spring, attached at one end thereof to the vibrating object is used as the dynamic absorber, on which rod an additional weight (4) is disposed whose mass can be regulated, in which connection the dynamic absorber is tuned to a problematic excitation frequency by regulating the mass of the additional weight (4).

7. A method as claimed in claim 6, characterized in that, as the dynamic absorber whose mass can be regulated, a container (4b) is used which is suspended on support of the spring (3b) and filled with a liquid whose amount can be regulated.

8. A method as claimed in any one of claims 1 to 7, characterized in that a rod (3) made of memory metal is used as the spring of the dynamic absorber, in which connection the natural frequency of the dynamic absorber is tuned to a right level by regulating the temperature of the rod made of a memory metal material.