ARRANGEMENT FOR DAMPING OF STRUCTURAL RESONANCE

Abstract: A hull (11) has mounted thereon a number of accelerometers (20) arranged to provide a data signal corresponding to movement or vibration of the hull (11) to which each is fixed. The hull (11) and/or a thrust block (18) has mounted thereon a number of mass dampers (21) that are controlled by a detector (23) arranged between the accelerometers (20) and mass dampers (21). The detector (23) calculates from the data signal generated by the accelerometers (20) different modes of resonance so as to determine the onset of the excitation of a particular resonance mode and controls the mass dampers (21) so as to generate forces to selectively dampen the resonance mode.

Description: ARRANGEMENT FOR DAMPING OF STRUCTURAL RESONANCE This invention relates to a selective damper system for a structure, in particular to a selective damper system for a marine vessel. It is considered conventional in some areas of marine engineering to mount machinery, for example components of a propulsion unit, to a hull portion of the marine vessel through a thrust block such that thrust force generated by the propulsion unit acts on the hull portion through the thrust block to drive the marine vessel in a direction opposite to that in which the thrust force is moving. However, because the propeller of the propulsion unit used to generate the thrust force is itself acting on turbulent water flowing around the hull of the vessel, the thrust force generated is not steady but comprises fluctuations in the magnitude of thrust force. If the frequency of these fluctuations should coincide with a structural resonance in the vessel, this resonance will be excited and will greatly amplify the thrust force fluctuations thus generating significant noise levels and subjecting the structure of the vessel to substantial stresses. In addition the machinery in the vessel can also generate fluctuating forces and again, if the frequency of these fluctuating forces coincides with a structural resonance in the vessel, any resonance will be excited and will greatly amplify the force fluctuations, thus generating significant noise levels and subjecting the structure of the vessel to substantial stresses. The transmission of force fluctuations through the hull of a marine vessel often means that passengers carried by the marine vessel are not able to comfortably occupy certain areas within the vessel. This means that valuable space is lost within the hull of the marine vessel. One technique for cancelling resonance is to generate an exactly equal and opposite force, i.e. 180 degrees out of phase to the force exciting the resonance and to apply this generated force to the structure to directly cancel the excitation force so as to prevent the resonance being excited. The present invention arose during consideration of means to dampen those resonances that otherwise would be excited in order to reduce the associated noise and stress levels in the structure of the marine vessel. According to the present invention a selective damper system for a structure comprises at least one accelerometer mounted to the structure, each accelerometer being arranged to generate a data signal indicative of local motion of the structure at its mounting position, a detector arranged to receive the data signal from each accelerometer and to generate a control signal in response to at least one resonance being generated in the structure, and at least one mass damper mechanically fixed to the structure and arranged to receive the control signal so as to generate a force to damp the resonance of the structure. In this manner, resonance vibration generated by a source can be sensed by accelerometers mounted to the structure and a detector can determine a force that a mass damper should create in order to selectively dampen a particular resonance in the structure, such that the system extracts energy from the vibrating structure so as to reduce the amplitude of the resonance. This in effect is making the structure insensitive to excitation of resonance rather than, as in prior art systems, attempting to exactly cancel the forces exciting the resonance. Accordingly, less force may be required to dampen a resonance than the force required to excite the resonance, unlike in prior art systems, wherein the force required to cancel the exciting force must be exactly equal and opposite to the exciting force. It will be understood that in this system possible resonance vibration is detected and a suitable force applied so as to inhibit true resonance actually developing, that is the onset of resonance is detected and inhibited before it grows in amplitude, whereas, in prior art system, true resonance has to occur before an appropriate equal and opposite force can be applied so as to exactly cancel the developed resonance. Preferably, the structure may be a hull of a marine vessel. In this manner resonances that would otherwise be generated by a propulsion unit carried by the marine vessel can be selectively prevented from building up into significant noise sources. This can be particularly useful in a marine vessel where certain areas of the vessel are not suitable for use by passengers because of excessive resonance generated noise, thereby allowing more use of a marine vessel. In a system employing a plurality of accelerometers, the accelerometers may be distributed at various locations on the structure. It may be that accelerometers already exist on the structure for another purpose, in which case use of these accelerometers will reduce the need for providing routing paths for electrical connections. The detector may be arranged to determine at least one mode of resonance of the structure and to generate a control signal when that mode of resonance has been detected. In this manner, the detector can determine which mode of resonance is building up in the structure and apply appropriate control signals to operate each mass damper so as to selectively dampen this particular mode of resonance. Each mass damper may be arranged to generate a force substantially to dampen each mode of resonance. The detector may comprise a modal matrix decomposition function arranged to identify at least one resonance mode of the structure from the data signal and an inverse modal matrix function to generate an appropriate control signal dependant on each identified resonance mode. Preferably, the mass damper may comprise a mass supported from a housing that is fixed to the structure and a pair of electromagnets operable to move the mass in an axial direction to generate a reaction force substantially to oppose the vibration of the structure. A diaphragm arrangement may support the mass from the housing. Each accelerometer may be mounted to the structure in a location to detect the onset of each selected resonance and each mass damper may be mounted to the structure in a location to substantially to damp the motion of each selected resonance mode. The location can be determined in advance of locating each device either using analysis or empirical processes or may be determined once each device is in situ. This invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a schematic plan view of a hull and propulsion mechanism of a marine vessel incorporating the invention; Figure 2 is a detailed elevation view of Figure 1; Figure 3 is a block diagram of a detector comprising modal transform apparatus for converting seven accelerometer inputs into three resonance outputs, and Figure 4 is a schematic diagram of a mass damper. In Figure 1 a marine vessel 10 comprises a hull 11 housing a propulsion unit 12 comprising a drive means 13 arranged to provide power to rotate a propeller shaft 14 through a coupling 15 so as to rotate a propeller 16 in water 17 surrounding the hull 11 of the marine vessel 10. A force generated by action of the propeller 16 through the water 17 is arranged to act through the propeller shaft 14 on a thrust block 18 which is securely mounted to a portion of hull 11 by bulkhead 19. Thereby force created by the propeller 16 acts to move the hull 11 in a direction opposite to that of the generated force. In Figure 2, the same references have been used to indicate similar integers to those used in Figure 1. Figure 2 illustrates a hull 11 having mounted thereon a number of accelerometers 20 which provide a data signal corresponding to movement or vibration of the hull 11 to which each is fixed. It will be understood that the term accelerometer 20 relates to any sensor which may detect either the motion or the proximity of the structure, in this case the hull 11 , to which it is mounted and produce a data signal corresponding to the motion or proximity or change in motion or proximity of the structure to which it is mounted. The hull 11 and /or thrust block 18 or any other suitable surface has mounted thereon a number of mass dampers 21, which are described in more detail below, but may be electromagnetic shakers. However, as a general description, each mass damper 21 comprises a mass 22 arranged to be moved in a lateral direction in response to a control signal produced by a detector 23 connected between each accelerometers 20 and each mass damper 21. The detector 23 is arranged to create a control signal from a data signal generated by each accelerometer 20. In this manner, resonance created or transmitted by the propeller shaft 14 or any other part of this propulsion unit 12 can be dampened by moving each mass 22 such that its movement dampens the resonance created by the propellor shaft 14 or any other part of the propulsion unit 12 to the hull 11 and/or thrust block 18. In Figure 3, the same references have been used to indicate similar integers to those shown in Figures 1 and 2. Figure 3 shows a detector 23 which is located between each accelerometer 20 and each mass damper 21. In this case, an array of seven accelerometers 20, have been distributed about the hull 11 and/or thrust block 18 to calculate three different modes of resonance so as to control five mass dampers 21, which have also been distributed about the hull 11 or thrust block 18. Each accelerometer 20 produces a data signal 24 which is received by a modal transform matrix 25 which calculates the amplitude of different modes of resonance . The modal transform matrix 25 is arranged to produce an excitation output 26 for each mode of resonance to be detected. In effect, the modal transform matrix 25 spatially filters out particular natural resonant modes of the hull 11 and comprises a model of the properties of the hull 11. Each excitation output 26 is used to feed a damping algorithm 27, one for each mode of resonance to be detected, that are arranged to generate a modal control demand force 28. Each modal control demand force 28 is transformed by an inverse modal transform matrix 29 into actual mass damper demand force control signal 30, which is used to control the mass dampers 21. In effect the inverse modal transform matrix 29 selects the best configuration to dampen a selected resonance. It will be understood that although seven accelerometers are used to provide a data signal 24 to control six mass dampers 21, the numbers of accelerometers 20 or mass dampers 21 may be varied depending on the application and that this in turn effects the size of modal transform matrix 25, the size of inverse modal transform matrix 29, and the number of damping algorithms 27 required. The construction of modal transform matrix 25, algorithms 27 and inverse modal transform matrix 29 and their coefficients are dependent on the application and may be determined from analysis or empirical methods prior to mounting the accelerometers 20 and mass dampers 21 or once accelerometers 20 and mass dampers 21 have been mounted within the marine vessel 10 . It should be understood that the accelerometers 20 may be fixed to the hull 11 or to any surface in which resonant vibration can build up. It will also be understood that the mass dampers 21 may be mounted on the thrust block 18 or to any surface that the transmission of vibration is required to be dampened. In the case when the marine vessel 10 already has accelerometers 20 distributed about the hull 11 for a different purpose, these accelerometers could be used to determine the different modes of resonance, thereby a saving can be made on the requirement to provide and mount accelerometers 20 to the hull 11 and to provide routing paths for electrical connections between the accelerometer 20 and the detector 24. Using selective damping the onset of the excitation of a particular resonance mode can be detected and forces can be created immediately and selectively to start damping that resonance mode. This action can prevent a selected resonance mode from building in amplitude. Since the phase of the control forces can be controlled relative to the excitation of the resonance mode, the damping can be achieved in such a way that there is a reduced or negligible shift in the natural resonant frequency of the mode. Figure 4 shows a typical mass damper 40 which comprises a mass 41 suspended by a pair of diaphragms 42 between fixing portions 43 of a housing 44. Electromagnets 45 and 46 are arranged at either end of the mass 41 and are operated by control means, not illustrated, to move the mass 41 laterally between the electromagnets 45 and 46. The electromagnets 45 and 46 are also mounted to the housing 44. In operation each electromagnet 45, 46 exerts an attraction force on the mass 41 and by varying the force exerted by each electromagnet 45, 46, using the control means, the mass 41 can be moved in a lateral direction. The pair of diaphragms 42 serve to suspend the mass 41 between fixing portions 43 and to assist in returning the mass 41 to a central position or to retain the mass 41 in a central position when electromagnets 45, 46 are not in operation. It will be understood that alternative arrangements for mass dampers 40 exist, for example, electromagnets 45, 46 may be substituted with permanent magnets and the mass 41 may comprise an electromagnet arrangement. In this arrangement the control means is arranged to energise the electromagnet so as to move the mass 41 laterally between the permanent magnets. It will be understood that it is necessary to provide mass dampers 21 arranged in such an orientation so as to oppose a resonance vibration in the structure to which it is attached. Although a lateral type mass damper 21 has been described, it is envisaged that a mass damper may be employed that dampens resonant vibration in the hull 11 in a plurality of directions. Although the invention has been described with reference to a marine vessel 10, it should be understood that the invention can be applied to any structure in which resonance vibration needs to be dampened. An advantage of using the invention in a large structure is that the force required to dampen a resonance vibration can be less than the force exciting this resonance, thereby smaller devices can be used to dampen resonance vibration in larger structures.

Claims: CLAIMS 1. A selective damper system for a structure, comprising at least one accelerometer mounted to the structure, each accelerometer being arranged to generate a data signal indicative of local motion of the structure at its mounting position, a detector operably arranged to receive the data signal from each accelerometer and to generate a control signal in response to at least one resonance being generated in the structure, and at least one mass damper mechanically fixed to the structure and arranged to receive the control signal so as to generate a force to damp the resonance of the structure. 2. A vibration damper system, as in Claim 1, wherein the structure is a hull of a marine vessel. 3. A vibration damper system, as in Claim 1 or 2, including a plurality of accelerometers, wherein the accelerometers are distributed at various locations on the structure. 4. A vibration damper system, as in any preceding claim, wherein the detector is arranged to determine at least one mode of resonance of the structure and to generate a control signal when that mode of resonance has been detected. 5. A vibration damper system, as in Claim 4, wherein each mass damper is arranged to generate a force substantially to dampen each mode of resonance. 6. A vibration damper system, as in any preceding claim, wherein the detector comprises a modal matrix decomposition function arranged to identify at least one resonance mode of the structure from the data signal and an inverse modal matrix function to generate an appropriate control signal dependant on each identified resonance mode. 7. A vibration damper system, as in any preceding claim, wherein the mass damper comprises a mass supported from a housing that is fixed to the structure and a pair of electromagnets operable to move the mass in an axial direction to generate a reaction force substantially to oppose the vibration of the structure. 8. A vibration damper system, as in Claim 7, wherein a diaphragm arrangement supports the mass from the housing. 9. A vibration damper system, as in any preceding claim, wherein each accelerometer is mounted to the structure in a location to detect the onset of each selected resonance and each mass damper is mounted to the structure in a location to substantially to damp the motion of each selected resonance mode. 10. A vibration damper system substantially as illustrated in and/or described with reference to the accompanying drawings.

Inventors: JOHNSON FREDERICK ALISTAIR, SEXTON DAVID CHARLES, SWINBANKS MALCOM ALEXANDER, JOHNSON, FREDERICK, ALISTAIR, SEXTON, DAVID, CHARLES, SWINBANKS, MALCOM, ALEXANDER
Applicants: BAE SYS ELECTRONICS LTD, JOHNSON FREDERICK ALISTAIR, SEXTON DAVID CHARLES, SWINBANKS MALCOM ALEXANDER, BAE SYSTEMS ELECTRONICS LIMITED, JOHNSON, FREDERICK, ALISTAIR, SEXTON, DAVID, CHARLES, SWINBANKS, MALCOM, ALEXANDER
Application year: 2001
Main classification: F16F15
IPC codes: B63B17, B63B43, B63H21, F16F7, F16F15