Field of the Invention

[0001] The present invention relates generally to magneto-rheological elastomers (MREs) and more particularly to an MRE vibration isolation apparatus, system and method.

[0002] The invention has been developed primarily for use as part of a base isolation system for buildings to minimise damage during seismic activity and will be described predominantly in this context. It should be appreciated, however, that the invention is not limited to this particular field of use, being potentially applicable to a wide variety of other applications. Such applications include foundations for other architectural or civil engineering structures such as bridges and towers, mountings for motors, engines or other vibrating machine components, supports for isolated control rooms and other substructures in marine vessels, vehicular suspension systems, mountings for scientific, electronic or optical instruments and the like, in which adaptive damping or vibrational isolation characteristics may be beneficial.

Background of the Invention

[0003] The following discussion of the prior art is intended to place the invention in an appropriate technical context and allow the potential benefits of it to be properly understood. However, any reference to prior art throughout the specification should not be construed as an express or implied admission that such art is widely known or is common general knowledge in the field.

[0004] The most widely utilised seismic protection system for buildings is base isolation. This technique is intended to mitigate the effects of earthquakes, tremors and the like by essentially isolating or decoupling the building structure and its contents from potentially dangerous ground motion, especially in the frequency range in which the building would be most adversely affected.

[0005] Applications of such base isolation systems have been relatively successful in many seismically active nations including the United States, China, Japan and New Zealand. However, despite improved earthquake performance relative to conventional building foundation techniques, due to their passive nature, conventional base isolation systems can only be designed for a specific target earthquake type, in terms of seismic frequency range and amplitude.

[0006] Consequently, a conventional passive base isolation system may be moderately effective against one earthquake, but quite ineffective (or even produce adverse effects) against another earthquake involving seismic vibrations in a different frequency or amplitude range. In particular, recent research has shown that a base isolation system optimised for relatively low frequency seismic activity may become even more vulnerable to high frequency seismic activity, and vice versa. These limitations are exacerbated by the fact that the specific frequency of seismic vibrations can be both highly unpredictable and highly variable even in the same geographical area, due to a number of complex variables including fault proximity. Hence, traditional base isolation systems can be vulnerable in a range of circumstances including both near-fault and far-fault earthquakes.

[0007] As a potential solution to these problems, the use of magneto-rheological elastomers (MREs) has been contemplated. These are essentially elastomeric composite materials, in which the effective stiffness or shear modulus can be altered in real time to a limited degree, in a controlled manner, by varying the strength of an applied magnetic field. To date, however, significant limitations have been encountered in device design, particularly in terms of the ability to generate sufficiently high magnetic fields in MREs to produce effective changes in lateral stiffness across a sufficiently broad operational range, while also providing high vertical stiffness and load-bearing capability, both of which are critical in practical applications.

[0008] These sorts of practical difficulties, coupled with limitations in the extent to which the damping characteristics can actually be varied, have impeded the translation of MREs from laboratories into practical engineering applications, particularly in the context of relatively large-scale installations such as base isolation in building structures where heavy loads and large forces are involved.

[0009] A further difficulty in providing an adequate range of control over MRE damping characteristics in practical applications has been found to arise from difficulties in generating and maintaining a sufficiently strong, uniform, consistent and controllable magnetic field through the MRE material with relative efficiency under normal operating conditions.

[0010] It is an object of the invention to overcome or substantially ameliorate one or more of the limitations of the prior art, or at least to provide a useful alternative.

Summary of the Invention

[001 1] Accordingly, in a first aspect, the invention provides a vibration isolation assembly adapted to be positioned between first and second bodies, the vibration isolation assembly including:- a top plate adapted for connection to the first body,

a bottom plate adapted for connection to the second body,

a vibration damping core disposed between the top plate and the bottom plate, the core including a plurality of damping layers formed from an MRE material having variable damping characteristics responsive to variation in strength of an applied magnetic field, and a plurality of support layers of relatively greater stiffness and relatively lower magnetic reluctance interposed respectively between the damping layers to form a laminated central core section;

an electromagnetic coil extending around the core between the top plate and the bottom plate, to generate a magnetic field of variable strength passing generally axially through the core; and

a yoke extending generally around the coil and adapted to accommodate relative lateral displacement between the top plate and the bottom plate;

the assembly being adapted for use with a control system configured to regulate the strength of the magnetic field generated by the coil, thereby to alter the effective damping characteristics of the core, so as in use to provide adaptively controllable damping of relative vibrational displacement between the first and second bodies.

[0012] Preferably, the vibration damping core is configured, in use, to serve as the core of a magnetic circuit, generated by the coil. [0013] In some embodiments, the core includes a top block and a bottom block, preferably formed from relatively thick plates of steel or other suitable ferric material and disposed respectively above and below the laminated central core section, to increase the overall magnetic permeability of the core.

[0014] Preferably, the top plate, bottom plate, yoke and core are disposed in close proximity, so as to form a substantially closed generally toroidal pathway with minimal air gaps for the magnetic flux defining the magnetic field.

[0015] Preferably, the assembly is able to vary the effective lateral stiffness and/or damping characteristics by a predetermined minimum extent, in response to controlled regulation of the strength of the magnetic field generated by the coil, to enable the assembly to shift the effective isolation sufficiently to isolate dominant frequencies from a relatively broad range of earthquake types. Preferably, the predetermined minimum extent of variation in lateral stiffness or damping is at least 20%. More preferably, the predetermined minimum extent of variation is at least 30%, and most preferably it is at least 50%. In some embodiments, a variation of 100% or more may be achievable with suitable MRE materials and optimal device configurations.

[0016] In some embodiments, the damping layers are formed predominantly from substantially the same MRE material, of substantially the same thickness. In other embodiments, however, combinations of different MRE materials may be used, optionally in conjunction with other design parameters including layer size, shape, geometry, thickness and orientation.

[0017] In one preferred embodiment, the composition of the MRE material comprises a mixture of 70% carbonyl iron particles, 10% silicone oil and 20% silicone rubber (all percentages by weight). Another preferred formulation comprises 70% carbonyl iron particles, 15% silicone oil and 15% silicone rubber (again by weight). However, it should be understood that a wide variety of MREs may be used, according to the particular engineering application, design considerations and required performance parameters.

[0018] In some embodiments, the support layers are formed from substantially the same material, such as steel, and of substantially the same shape and thickness. Again, however, different combinations and permutations of support layer materials, thicknesses and geometries are envisaged to facilitate optimisation of performance characteristics in particular operating environments.

[0019] In some embodiments, the preferred material for the support layers includes a ferric material such as iron, low-carbon steel, or silicon steel. However, other materials, preferably with relatively low magnetic impedance, may additionally or alternatively be used, including but not limited to soft iron, rare earth magnets, magnetic alloys, sintered metals or composites, injection moulded composites of resins and magnetic powders, paramagnetic materials, and the like.

[0020] In some embodiments, some or all of the support layers may be configured to include MRE elements and hence may also perform a partial vibration damping or isolation function. Similarly, some of the damping layers may be configured to include structural support elements and hence may also perform a partial reinforcing or structural support function.

[0021] Advantageously, it has been found that the laminated structure provides relatively large compressive load capacity and stiffness in the axial direction as the steel support layers prevent excessive lateral bulging of the MRE layers, while lateral flexibility is provided by the controllable magnetic field-dependent deformation in shear of the MRE layers, which are less constrained in the lateral direction by the support layers.

[0022] A synergistic effect is preferably also achieved, because the support layers, when formed from a structural material with relatively high magnetic permeability such as steel, substantially increase the overall magnetic conductivity of the core, noting that the MRE layers themselves will typically have relatively low magnetic permeability. The top and bottom blocks, preferably also formed as solid as steel cylinders above and below the central laminated core section, further enhance the magnetic permeability of the core.

[0023] Preferably, the respective MRE layers are bonded or vulcanised to the adjacent support layers. Chemical bonding, vulcanising or glueing may also be enhanced by mechanical means such as surface etching, or complementary interlocking surface formations such as dimples, grooves, keyways or the like on the respective layers. [0024] In one preferred embodiment, the vibration isolation assembly is generally cylindrical in configuration, comprising a substantially cylindrical core, a coil with a substantially annular cross-sectional profile extending around the core, and a yoke also with a substantially annular cross-sectional profile, of marginally larger diameter, extending around the coil. It should be understood, however, that in other preferred embodiments, the overall configuration or cross-sectional profile be assembly may alternatively be square, elliptical, rectangular, pentagonal, hexagonal or any other suitable geometrical configuration - including irregular configurations adapted for specific installations.

[0025] In one preferred embodiment, the core is adapted to transfer the primary structural load axially between the first and second bodies, while a marginal axial clearance is provided between the yoke and either the top or the bottom plate, to allow relative vibrational displacement in the lateral direction.

[0026] In an alternative preferred embodiment, the yoke is adapted to transfer the primary structural load axially between the first and second bodies, while the core is adapted predominantly to generate controlling shear forces in the lateral direction. In this case, a sliding bearing may be provided between the yoke and either the top or the bottom plate, to accommodate a limited degree of lateral displacement, while supporting the primary axial load between the first and second bodies.

[0027] In one preferred embodiment, the vibration isolation assembly takes the form of a base isolation unit for a building structure, wherein the first body is the building structure and the second body is a supporting foundation or footing for the building structure. In this context, it will be understood that a typical base isolation system for building structure will utilise a series of base isolation units disposed in spaced apart relationship, preferably under common control.

[0028] In one embodiment, the assembly includes an active or semi-active control system adapted to sense vibrational inputs, and actively regulate the strength of the magnetic field in response to those inputs according to predetermined control parameters, so as adaptively to optimise the damping characteristics of the core in real time. The control parameters may be based on predetermined control algorithms, "look-up" data tables, feed-forward or feed-back control loops with PID type or various other control parameters, or any suitable combination of such control methodologies. [0029] In another embodiment, the assembly includes a passive or "smart passive" control system in which the device is energised by electric current produced from vibration of the structure to be controlled, for example by means of an induction circuit adapted to generate electric currents directly in response to relative displacement of electromagnetic control elements, and to deploy those electric currents to effect suitable variations in the magnetic field strength generated by the coil, according to a predetermined passive control logic. Such systems may also optionally utilise supplementary power supplies in order to amplify the electric currents generated by the induction circuits or similar mechanisms, and thereby broaden the range of effective control. Various combinations of active, semi-active and passive control inputs are also envisaged in other embodiments.

[0030] In a second aspect, the invention provides a vibration isolation system including: at least one vibration isolation assembly as previously defined, disposed effectively as a bearing element between the first body and the second body;

at least one vibration sensor adapted to generate sensor signals indicative of actual or potential relative vibrational displacement between the first and second bodies; and

a controller adapted to generate control signals in response to the sensor signals, so as to regulate the strength of the magnetic field generated by the coil and thereby adaptively to control the damping characteristics of the vibration isolation assembly according to a predetermined process control strategy.

[0031] In some embodiments, non-linear control strategies may be employed, for example whereby the rate of increase in the level of adaptive stiffness or damping is proportional to displacement from a neutral position. Control algorithms may also be programmed specifically to avoid predefined resonant frequencies, to maintain accelerations below predetermined threshold levels, to confine lateral displacements within predetermined limits, and so on.

[0032] In one preferred embodiment, the first body is a building structure, the second body is a fixed foundation or footing for the building structure, and a plurality of the vibration isolation assemblies are disposed in spaced apart relationship to support the building structure on the foundation or footings.

[0033] In yet another aspect, the invention provides a method of base isolation in a structure subject to vibration, the method including the steps of:- providing at least one vibration isolation assembly as previously defined;

positioning the at least one vibration isolation assembly between the structure and a base;

providing at least one vibration sensor adapted to generate sensor signals indicative of actual or potential relative vibrational displacement between the structure and the base;

providing a controller responsive to the sensor signals;

generating control signals in response to the sensor signals by means of the controller on the basis of a predetermined process control strategy;

regulating the strength of the magnetic field generated by the coil in the at least one vibration isolation assembly in response to the control signals; and

thereby adaptively controlling the damping characteristics of the vibration isolation assembly according to the predetermined process control strategy in order to minimise or modify the impact of the vibration on the structure.

[0034] In one preferred form of this aspect, the structure is a building, the base is a foundation for the building, and the process control strategy is adapted to reduce the impact of seismic vibration on the building.

Brief Description of the Drawings

[0035] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

[0036] Figure 1A is a diagrammatic cross sectional side elevation showing a multi-storey building structure with conventional passive foundations, according to the prior art;

[0037] Figure 1 B shows the typical displacement of the building structure shown in Figure 1A, in response to seismic activity;

[0038] Figure 2A is a diagrammatic cross sectional side elevation, similar to Figure 1A but incorporating an adaptive base isolation system including MRE vibration isolation assemblies in accordance with the present invention; [0039] Figure 2B shows the typical displacement of the building structure shown in Figure 2A, in response to seismic activity;

[0040] Figure 3 is a perspective view of an adaptive MRE vibration isolation assembly according to a first embodiment of the invention;

[0041] Figure 4 is an enlarged cross sectional side elevation view, showing a vibration isolation assembly of Figure 3, according to a first embodiment of the invention;

[0042] Figure 5 is a cross sectional side elevation view, showing a vibration isolation assembly according to a second embodiment of the invention;

[0043] Figure 6 is a cross sectional side elevation view of a third embodiment of the invention, also depicting the typical profile of magnetic flux through the apparatus;

[0044] Figure 7 is a diagrammatic side elevation view of an experimental test set-up, for a vibration isolation assembly according to the invention;

[0045] Figure 8 is a chart representative of test data from the experimental test set-up of Figure 7, showing a typical response of force versus displacement at different levels of applied current, at a frequency of 0.5 Hz and a base displacement of 5 mm; and

[0046] Figures 9A to 9D show a series of force-displacement loops for the test set-up at an amplitude of 5 mm and frequency of 0.5 Hz.

Preferred Embodiments of the Invention

[0047] Referring to the drawings, Figure 1A shows a conventional multi-storey building structure 1 , incorporating multiple levels 2 and foundations comprising a base slab 3, and conventional passive footings 4. In response to vibrational input, for example, as a result of seismic activity, the building structure will typically be displaced as indicated by arrows "A" with respect to reference line 5 in Figure 1 B. The amplitude of vibrational displacement in the building structure typically increases with height and can readily exceed the maximum design strength of the building, particularly if accelerations are excessive or if resonant frequencies are encountered. [0048] As is well known, in earthquake conditions this can lead to serious damage or total collapse of buildings with catastrophic consequences, in both financial and human terms. Even if the footings 4 include passive base isolation elements, for example through the use of rubberised damping blocks (not shown), stresses and strains in the building structure can nevertheless accumulate to damaging or catastrophic levels if the seismic vibrational input frequencies are outside the optimal design range for the base isolation system.

[0049] Figure 2A shows the same building structure 1 , incorporating a base isolation system 10 comprising a series of spaced apart adaptive vibration isolation assemblies 12 in accordance with the present invention. By allowing a limited degree of resilient, damped lateral displacement of the foundations in response to seismic activity, the magnitude of lateral displacement of the building structure with respect to the reference line 5 is substantially reduced, as is the variation in displacement between the lower and upper levels of the building, as indicated by arrows "A" in Figure 2B. Moreover, the base isolation system of the present invention allows the damping characteristics of the vibration isolation assemblies to be selectively varied. This in turn allows the frequency response of the building to be adjusted in real time and optimised so as to avoid harmonic resonances and minimise damage across a broad range of seismic frequencies and amplitudes, as described more fully below.

[0050] Figures 3 and 4 show the vibration isolation assembly 12 in more detail, in accordance with a first embodiment of the invention. The assembly includes a bottom plate 15 adapted for connection to a first body, which in this application corresponds to a concrete footing 16 connected directly with the ground beneath the building structure (see Figure 2A). A top plate 18 is similarly adapted for connection to a second body, which in this application corresponds to the base slab 3 connected to the building structure. The top and bottom plates 18 and 15 perform a dual function of providing structural support and forming part of a magnetic circuit for the device. They are therefore formed from a suitable material such as steel or other relatively strong ferric metal or metal alloy.

[0051] The assembly further includes a vibration damping core 20 disposed between the top and bottom plates. The core includes a plurality of damping layers 21 formed from an MRE material having variable damping characteristics responsive to variation in strength of an applied magnetic field. A plurality of support layers 22, each formed from a sheet of structural material of relatively high magnetic permeability such as steel, are interposed respectively between the damping layers 21 , to form a central laminated section 25 of the core. The core 20 also includes a top block 26 and a bottom block 28, preferably formed from a relatively thick plate of steel or other suitable ferric material, to further increase the magnetic permeability of the core.

[0052] An electromagnetic coil 30 extends around the core between the top and bottom plates 18 and 15, to generate a magnetic field of variable strength in response to an applied electrical current variable strength, as described more fully below. A yoke 32 extends around the coil, between the top and bottom plates. The particular embodiment illustrated is substantially cylindrical in configuration, such that the core, coil, yoke, top plate and bottom plate are all substantially cylindrical and coaxial. It should be understood, however, that other configurations of the apparatus including square, rectangular, elliptical and polygonal configurations are also envisaged.

[0053] As best seen in Figure 4, the bottom plate 15, top plate 18, core 20 and yoke 32 are disposed in close relative proximity, so as to induce a substantially closed generally toroidal pathway for the magnetic flux defining the magnetic field through the assembly. In the embodiment of Figure 4, a small air gap 34 is defined between the top surfaces of the yoke and the coil, and the underside of the top plate 18, to permit a limited degree of relative lateral displacement of the top plate, as indicated by arrows "A".

[0054] In this embodiment, it will be appreciated that the core itself is a primary structural element, adapted to transfer compressive load axially between the top and bottom plates, while as previously noted, a marginal axial clearance 34 is provided between the yoke and the top plate, to allow a limited degree of relative vibrational displacement in the lateral direction.

[0055] An alternative embodiment is shown in Figure 5, in which similar features are denoted by corresponding reference numerals. In this case, the yoke 32 is the primary structural element, adapted to transfer a majority of the structural compressive load axially between the top and bottom plates, while the core 20 is adapted predominantly to support shear forces in the lateral direction. To enable this, a sliding bearing assembly 37 is interposed between the yoke 32 and the top plate 18 (or alternatively between the yoke and the bottom plate). This bearing assembly accommodates a limited degree of lateral displacement, while fully or at least predominantly supporting the primary axial compressive load between the first and second bodies.

[0056] Figure 6 shows a third embodiment of the invention, which is similar in configuration to the first embodiment shown in Figure 4 and wherein again similar features are denoted by corresponding reference numerals. In this case, it will be noted that the solid steel top and bottom blocks 26 and 28 of the core are substantially thicker so as to further enhance the magnetic field strength and flux density through the core. This figure also shows a diagrammatic representation of the flux lines 40, indicating the general shape and configuration of the magnetic field through and around the apparatus, in use. It will be appreciated that the embodiments shown in Figures 4 and 5 comprise similar magnetic circuits and hence would produce magnetic fields of similar shape and configuration.

[0057] The assembly is adapted for use with a control system 35 incorporating sensors 36 (see Figure 2A). The control system is configured to regulate the strength of the magnetic field generated by the coil, thereby to alter the effective damping characteristics of the core, so as in use to provide adaptive control, in real time, over the vibrational damping characteristics of the core.

[0058] Due to the field-dependent shear modulus and hence variable damping characteristics of the MRE material, the base isolation assembly has controllable shear stiffness and damping across a broad operating range, dramatically surpassing the performance of traditional passive base isolators in terms of effectiveness and functionality for seismic protection in civil engineering structures.

[0059] Turning now to describe the constituent components of the assembly in more detail, as previously noted, the core consists of multiple layers of thin MRE sheets 21 sandwiched between and bonded to adjoining layers of thin steel plates 22. The steel plates 22 provide the required vertical load bearing capacity and stiffness, while also preventing lateral budging of the MRE material. The requisite flexibility in the lateral or horizontal direction is provided by the deformability in shear of the MRE layers 21 , and shear stiffness can be varied substantially and instantaneously under the influence of the applied magnetic field. [0060] In one preferred form, the matrix material for the MRE is formed from a combination of room temperature vulcanising (RTV) silicone rubber (sourced from H B Fuller Company, Germany) and silicone oil. The volume fraction ratio of silicone rubber and silicone oil in the matrix is 1 :1 , although this may be varied significantly to achieve desired material properties and performance characteristics for particular applications. The magnetic particles dispersed through the matrix are composed of carbonyl iron, with an average diameter of around 5 μηι (sourced from BASF Company, Germany). The volume fraction of carbonyl iron particles in the matrix material is approximately 30%. Overall, in terms of weight percentages, the formulation comprises approximately 70% carbonyl iron particles, 10% silicone oil and 20% silicone rubber. Again, however, this may be varied significantly to achieve particular performance characteristics. The mixture is ideally placed in a vacuum case initially to remove air bubbles and then poured into a mould. The mixture is then preferably cured for approximately 24 hours at room temperature under a constant magnetic flux density of 1.0 Tesla.

[0061] This formulation produces a soft rubber-like consistency with approximately 100% shear modulus change under a magnetic field of 0.7 Tesla, as compared with the shear modulus at 0.0 Tesla (or the absence of a magnetic field). For 10% shear strain, the shear moduli were 0.1 MPa and 0.2 MPa with magnetic field strengths of 0.0 Tesla and 0.7 Tesla, respectively.

[0062] It should be appreciated, however, that depending upon the intended application and the relevant design constraints, a wide variety of different MRE formulations and materials may alternatively or additionally be used. For example, another suitable MRE formulation is composed 70% carbonyl iron particles, 15% silicone oil and 15% silicone rubber (all weight percentages). This formulation has been found to produce substantially higher increases in shear modulus, in even lower strength magnetic fields, as compared with the formulation previously described. It should also be understood that the various MRE damping layers need not be of the same size, shape or material composition, and the same applies for the intermediate support layers.

[0063] In the construction of the core itself, because the magnetic permeability of the MRE is relatively low, as well as providing structural integrity the supporting layers 22 are formed from steel sheets having high magnetic permeability so as to substantially increase the overall magnetic conductivity of the core. To further improve the permeability of the core, the top and bottom blocks 26 and 28, composed essentially of solid steel or iron cylinders of substantially the same dimensions, are added respectively to the top and bottom of the central laminated core section 25.

[0064] With the device assembled, the bottom cylinder 28 is in direct contact with the bottom plate 15 and the top cylinder or block 26 is in close proximity to (in the embodiment of Figure 4) or directly in contact with (in the embodiments of Figures and 6) the top plate 18. In conjunction with the coil 30, this arrangement forms a substantially closed toroidal path (with small air gaps in some embodiments) for the magnetic flux - with the steel yoke 32 surrounding the coil and completing the flux path. This arrangement provides a relatively uniform and concentrated magnetic field through the entire MRE core structure, as best seen in figure 6.

[0065] Preliminary modelling and testing indicates that in order to provide relatively high and uniform field strength through the core, the optimal ratio between the overall length of the core 20 including the top and bottom blocks 26 and 28 and the central laminated section 25 of the core (the "length ratio" for the core) will vary according to a range of design parameters. These parameters include the material type and properties of the specific MRE and steels (or other materials) utilised, the maximum magnetic field strength, and the overall size and proportions of the device, and the like. However, for practical purposes and considering other competing design constraints, a useful and effective length ratio can be anywhere in the range from 1.1 to around 5.0, and is ideally in the range of 1.5 to around 4.0. In some particularly preferred embodiments, the length ratio is between 2.0 and around 3.0. It should also be understood that in order to optimise or customise the magnetic field geometry, the top and bottom blocks may be formed in different thicknesses, materials and/or geometries with respect to one another. One or more intermediate blocks may also be provided, between the top and bottom blocks, so as to divide the central laminated core structure into multiple core segments.

[0066] In the embodiment illustrated in figure 2A, the building structure incorporates four levels 2 above the ground level, and sensors 36 are positioned on each level. These may take the form of accelerometers, position transducers, location sensors, or a combination of these or other sensing mechanisms. Additional sensors may be positioned in the foundation structure, in the substrata underlying the building structure, in the base isolators and also at remote locations such as earthquake monitoring stations, as required. The network of sensors may also include stress or strain gauges in structural elements of the building.

[0067] In the appropriate configuration and combination, the senses are typically arranged to detect vibrational displacement or acceleration and to provide corresponding sensor signals to the computerised controller 35, as part of an adaptive feedback control system. The controller in turn generates control signals according to a predetermined control strategies and algorithms, which are amplified and fed to the electromagnetic coils in the respective base isolators 12. In this way, the control system is adapted to respond dynamically, in real time, to modify the stiffness and damping characteristics of the base isolators.

[0068] The control algorithm will typically be designed to avoid resonant or harmonic frequencies in the building structure, reduce vibrational accelerations in the building structure to levels that can be absorbed without catastrophic damage, and to confine lateral displacement or "drift" of the building structure within limits to avoid peripheral damage such as dislodgement of walls or fracturing of pipe work for building services.

[0069] In one relatively sophisticated implementation, an active or semi-active control system is utilised, based for example on predetermined control algorithms, "look-up" data tables, feedforward or feedback control loops with various tailored control parameters, or suitable combinations of such control methodologies. However, a more basic implementation that is envisaged makes use of a more passive or reactive control system adapted to generate control signals in direct response to the vibrational displacement itself. For example, an electromagnetic induction circuit may be utilised to generate electrical currents directly in response to the relative displacement of electromagnetic elements in the building, and a controller may be configured to deploy those induced electric currents directly (with appropriate amplification or smoothing if required) to effect variations in the magnetic field strength generated by the coil in each of the base isolators, according to a predetermined passive control logic. The key aspect of this system is that the energy supplied to the coil is related directly to the external motion of the building structure.

[0070] In some installations, a passive control system may be deployed in combination with an active system, so a degree of adaptive dynamic control is maintained as part of a backup or fail safe strategy, even in the event of power failure. Experimental results

[0100] A test apparatus was constructed, using a vibration isolation assembly 12 in the configuration shown in Figure 4. In the test model, the central laminated section 25 of the core comprises forty-six layers of steel sheet, each layer with a thickness of 1 mm, and forty-seven layers of MRE sheet, each with a thickness of 2 mm. The laminated core section has a diameter of 140 mm, and the cylindrical steel top and bottom blocks 26 and 28 each have the same diameter and a height of 70 mm. The cylindrical yoke 32 has a height of 271 mm and a wall thickness of 10 mm. The yoke is firmly attached to the outer surface of the coil. The clearance space 34 between the top of the yoke and the underside of the top plate 18 is approximately 5 mm.

[0101] In the prototype device, the electromagnetic coil is cylindrical in configuration with an internal diameter of 196 mm and an external diameter of 232 mm. The coil comprises 3100 turns of copper wire, having a diameter of 1.2 mm and a total length of 2, 100 m. The resistance of the coil is 32.3 Ω. The coil is supported by an outer cylindrical non-magnetic casing formed from an epoxy resin formulation, with an inner diameter of 192 mm and a height of 276 mm. The radial clearance space between the core and the coil enables the isolation assembly to accommodate a maximum lateral deformation between the top and bottom plates of 26 mm, which is equivalent to the maximum design shear strain of 27.7%. The magnetic field strength inside the MRE layers was monitored utilising a magnetic field sensor from Integrity Design and Research Corporation, USA.

[0102] To evaluate the performance of the isolation unit, a testing apparatus including a shake table was utilised, as shown in Figure 7. The testing apparatus comprises a reaction floor 50, a load cell 52, and a shake table 54 adapted to oscillate in the direction of arrows B. In this setup, the bottom plate 15 of the isolator is connected to a base plate 55 on the shake table, while the top plate 18 is connected to an upper mounting plate 57, which is connected to the reaction floor 50 via the load cell 52. In this way, the shake table was used to apply a horizontal shear load, in either static or sinusoidal form, to simulate seismic activity while the load cell measured the lateral load applied to the isolation assembly. During the test, the top plate and the load cell remained stationary so as to eliminate inertial forces that would otherwise arise from the motion of the top plate. A DC power supply with a capacity of 200 V and 8 A was utilised to provide a DC current to energise the electromagnetic coil. [0103] Since the dominant frequencies of earthquakes are generally below 5 Hz, the frequency range of the input sinusoidal waveforms was set from 0.5 to 3.0 Hz in the dynamic tests. A series of harmonic excitations, at frequencies of 0.5 Hz, 1.0 Hz and 3.0 Hz were thus generated by the shake table, to allow the behaviour of the vibration isolation assembly to be studied and modelled. Two discrete amplitudes for the sinusoidal excitation were used, being 5 mm and 10 mm, respectively. During the test, the MRE isolation assembly was energised with various electrical currents at 0.0 A, 3.0 A and 5.0 A, respectively.

[0104] Figure 8 is a chart showing the typical force-displacement relationship of the prototype MRE vibration isolation assembly at a constant frequency of 0.5 Hz and amplitude of 5 mm, subjected to various applied currents at 0.0 A, 3.0 A and 5.0 A, respectively. The shake table motion is also plotted in this Figure. It should be noted that the amplitude of the shake table motion as shown in the chart was scaled up in order to be more clearly presented.

[0105] As can be seen, there was a significant increase in the lateral shear force generated by the MRE base isolator as the applied current increased from 0.0 A to 5.0 A. For each applied current, the shear force ramped up almost immediately and reached its stable maximum value after only one cycle. The force-displacement loops of the isolator at a frequency of 0.5 Hz and amplitude of 5 mm are shown in Figures 9A to 9C, at 0.0 A, 3.0 A and 5.0 A respectively These loops are superimposed in Figure 9D. Additional test data is set out in the tables below.

[0106] Table 1 shows the maximum lateral force (N) supported by the MRE vibration isolation unit, under various loading conditions.

Table 1 : Maximum force [N] of the MR elastomer Base isolator under various loading conditions

Amplitude D = 5 mm Amplitude D = 10 mm

Frequency Applied current [A] Increase Frequency Applied current [A] Increase

[Hz] 0.0 3.0 4.0 % [Hz] 0.0 3.0 5.0 %

0.5 102.05 1 17.12 148.29 45.31 0.5 199.65 224.54 271 .53 36.68

1 .0 107.13 124.08 154.76 44.46 1 .0 214.47 235.1 1 286.77 33.71

3.0 122.65 142.96 175.32 42.94 3.0 246.06 270.49 327.64 33.15

[0107] Table 2 shows the effective stiffness (kN/m) of the MRE vibration isolation unit, under various loading conditions.

Table 2: Effective stiffness [kN/m] of the MR elastomer Base isolator under various loading conditions

Amplitude D = 5 mm Amplitude D = 10 mm

Frequency Applied current [A] Increase Frequency Applied current [A] Increase

[Hz] 0.0 3.0 5.0 % [Hz] 0.0 3.0 5.0 %

0.5 19.73 22.24 27.13 37.49 0.5 18.58 21 .62 25.59 37.75

1 .0 20.91 23.61 28.59 36.72 1 .0 19.53 22.77 26.92 37.83

3.0 24.22 27.38 32.51 34.27 3.0 23.42 26.03 30.40 29.80

[0108] Table 3 shows the damping coefficient (kN*s/m) of the MRE vibration isolation unit, under various loading conditions.

Amplitude D = 5 mm Amplitude D = 10 mm

Frequency Applied current [A] Frequency Applied current [A]

[Hz] 0.0 3.0 5.0 [Hz] 0.0 3.0 5.0

0.5 0.98 1 .23 1 .55 0.5 1 .07 1 .25 1 .55

1 .0 0.56 0.69 0.87 1 .0 0.63 0.68 0.86

3.0 0.24 0.29 0.37 3.0 0.24 0.29 0.36

[0109] As will be seen from the test data, the increases in force for each testing set vary from 33% to 45%. For a given frequency at constant amplitude of 5 mm, the increases in stiffness vary from over 34% to 37%. For an amplitude of 10 mm, the increases in stiffness vary from around 30% to 37%. When the loading frequency increases, the extent of increase in stiffness is marginally lower, in comparison with lower loading frequencies. It will also be observed that, for two different loading amplitudes with the same loading frequency and the same input current, the effective damping coefficients remain substantially the same. The values of damping coefficient for low loading frequencies are higher than those under higher loading frequencies.

[0110] These results indicate that the MRE vibration isolation assembly can be used as a controllable stiffness device, with effective increases in sheer stiffness from 30% to 38% for various loading conditions. The damping in the device is relatively low with no applied magnetic field, but increases substantially with increasing magnetic field strength.

[011 1] For example, the damping ratio can increase from 0.24 kN*m/s at a loading frequency of 3.0 Hz (no magnetic field) to 1.5 kN*m/s with the magnetic field generated by a 5.0 A input current.

[0112] The device is capable of instantly changing its shear stiffness and damping properties, in response to energisation of the magnetic field. Hence, the isolation assembly is able to provide a substantial degree of controlled adaptability in real time, in response to variable vibrational inputs. It is also envisaged that with more responsive MRE formulations, it will be possible to obtain significantly wider ranges of adjustability in damping modulus.

Numerical Modelling

[0113] To enable reasonably accurate prediction of the dynamic behaviour of the MRE vibration isolation apparatus, a Kevin model was proposed, which includes a spring element and a damping element:

f (x) = kx + cx

where k and c are the stiffness and damping coefficients respectively.

[0114] Comparison between the experimental data and the proposed Kevin model showed substantial conformity and hence the Kevin model is able to effectively capture the performance dynamics of the system in a controlled testing environment, enabling reliable modelling and computer simulation.

[0115] The invention, at least in its preferred embodiments, thus provides an improved MRE vibration isolation assembly and associated system that can be utilised to provide dynamically adaptable vibration isolation and damping characteristics in a wide variety of applications, including particularly in high-load applications such as base isolation for building and other civil engineering structures.

[0116] The system is relatively cost-effective to manufacture, reliable, easy to control, highly responsive and flexibly programmable. Another particular advantage of the configuration of the invention is that a strong, consistent and relatively uniform magnetic field can be maintained through the core, thereby providing improved control over the MRE material. As more sophisticated control algorithms are developed, they can be readily integrated into the control system, without requiring the base isolation units themselves to be substituted or modified, although this can also be readily done. As more advanced MRE materials are developed, the electromagnetic cores can also be readily upgraded or replaced for enhanced performance, if required. In these and other respects, the invention represents a practical and commercially significant improvement over the prior art.

[0117] Although the invention has been described with reference to specific examples, it should be understood that these are not intended to be limiting in any way, and it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.