Showing papers on "Magnetorheological fluid published in 1997"

Phenomenological model for magnetorheological dampers

Abstract: Semiactive control devices have received significant attention in recent years because they offer the adaptability of active control devices without requiring the associated large power sources. Magnetorheological (MR) dampers are semiactive control devices that use MR fluids to produce controllable dampers. They potentially offer highly reliable operation and can be viewed as fail-safe in that they become passive dampers should the control hardware malfunction. To develop control algorithms that take full advantage of the unique features of the MR damper, models must be developed that can adequately characterize the damper's intrinsic nonlinear behavior. Following a review of several idealized mechanical models for controllable fluid dampers, a new model is proposed that can effectively portray the behavior of a typical MR damper. Comparison with experimental results for a prototype damper indicates that the model is accurate over a wide range of operating conditions and is adequate for control design an...

A comparison of semi-active control strategies for the MR damper

Abstract: Because the behavior of semi-active devices is often highly nonlinear, one of the main challenges in the application of this technology is the development of appropriate control algorithms. In this paper, a number of recently proposed semi-active control algorithms are evaluated for use with the magnetorheological (MR) damper, an innovative semi-active control device that appears to be particularly promising for civil engineering applications. The performance of the resulting control systems are compared through simulation, and the advantages of each algorithm are discussed. The results demonstrate that the performance of the control system is highly dependent on the choice of algorithm employed.

Yield stress in magnetorheological and electrorheological fluids: A comparison between microscopic and macroscopic structural models

Abstract: The yield stress of a magnetorheological suspension is calculated from two different approaches. The first one is based on a mesoscopic description of the structure taking only into account the shape anisotropy of the strained aggregates. The second one is based on a microscopic approach where the interparticle forces, due to the application of the field, are calculated numerically by taking into account the magnetostatics between the particles inside the aggregates. We show that the macroscopic description well applies to suspensions of nonmagnetic particles in a ferrofluid and that a layered structure, consisting of parallel slabs of magnetizable materials should have a yield stress much higher than a structure made of cylindrical aggregates. On the other hand the microscopic approach is appropriated for the description of suspensions of particles of high permeability. In this case, the yield stress is mainly determined by the rupture between pairs of particles and, consequently, it strongly increases with the angle between the line of centers of the pair undergoing the rupture and the field.

Precision optics fabrication using magnetorheological finishing

Abstract: Optical polishing with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the newest process is called magnetorheological finishing (MRF). In MRF, all of the process parameters are controlled by utilizing the state of hydrodynamic flow of a magnetically stiffened magnetorheological abrasive fluid through a converging gap formed by a lens workpiece surface and a moving wall. The shear flow of “plastic” MR fluid results in the development of high stresses in the interface zone and material removal over a portion of the workpiece surface, referred to as the “polishing spot”. The polishing spot is an abrasive-charged, sub-aperture lap that automatically conforms to the local shape of the lens surface. Deterministic finishing is accomplished by mounting a lens on a rotating spindle and sweeping it through the MR fluid with a computer numerical controlled (CNC) machine. A computer program generates both a dwell time schedule for the MRF machine and an accurate prediction of finished surface shape, using a material removal function and initial surface condition information as input. In this paper, we describe the MRF process, a preliminary theory of material removal, properties of the MR fluid, machine configurations, software for finishing, and finishing experiments on a variety of surface shapes (spherical, flat, aspheres) and materials of interest to optics manufacturing. Advantages and current limitations to the process are also described.

On the current status of magnetorheological dampers: seismic protection of full-scale structures

Abstract: Because of their mechanical simplicity, high dynamic range, low power requirements, large force capacity and robustness, magnetorheological (MR) fluid dampers have been shown to be semi-active control devices that mesh well with application demands and constraints to offer an attractive means of protecting civil infrastructure systems against severe earthquake and wind loading. Following an overview of the essential features of MR fluids, this paper discusses the current status of this new technology for seismic protection of structures, including presentation of recent laboratory studies employing a seismically-excited, scale-model building and preliminary analyses of a prototype full-scale MR damper.

Design rules for MR fluid actuators in different working modes

Abstract: The behavior of actuators based on magnetorheological fluids is determined by a variety of parameters. The magnetorheological properties of the MR suspension, the working mode (shear mode, flow mode, squeeze mode) and the design of the magnetic circuit consisting of MR fluid, flux guide and coil all considerably influence the properties of the actuator. This paper presents design rules for MR fluid actuators in different working modes. The behavior of MR fluids in the three working modes was investigated by using a rotational viscometer, a flow mode damper and a new measuring technique working in the squeeze mode. The measurement results for various magnetic flux densities are reported and the results of the different working modes are compared. High dynamic damping forces dependent on the magnetic field can be achieved especially in the squeeze mode. The design of the magnetic circuit of an MR fluid actuator is analyzed by using finite-element-methods. The advantages of integrating permanent magnets into the magnetic circuit of an MR fluid actuator are pointed out. The working point of the actuator can be adjusted by permanent magnets without consuming any power and the maximum power required to drive the actuator can be reduced. From these results design rules for MR fluid actuators are developed.

Chain formation and chain dynamics in a dilute magnetorheological fluid.

Abstract: Magnetorheological fluids are suspensions of magnetizable particles that reversibly change from liquid to solid when subjected to a magnetic field. A field-induced structure of dipolar chains is responsible for these changes. Our work aimed at understanding chain dynamics and the kinetics of chain formation by using dynamic light scattering. Chain length is determined by measurement of the diffusion coefficient. Chain-length growth shows a Smoluchowski behavior.

Magnetorheological fluid fan clutch

Abstract: A magnetorheological fluid fan clutch effects modulated rotational motion transmission between a rotatable input shaft carrying an input clutch plate and a housing having a front cover and a rear cover forming a cavity about the input clutch plate wherein the front and rear covers operate as output clutch plates. A nonmagnetic spacer is positioned between the front and rear covers and radially outside the input clutch plate. A bearing supports the housing on the input shaft through the rear cover so that the input clutch plate is rotatable on the input shaft relative to the housing. Magnetorheological fluid is carried in the cavity, through which torque is variably transferred between the input clutch plate and the housing. A coil is positioned radially outside the nonmagnetic spacer and carries a variable electrical current to effect a variable magnetic field across the input clutch plate and through the magnetorheological fluid providing modulated torque transmission between the input clutch plate and the housing.

Magnetorheological fluid devices exhibiting settling stability

Abstract: A magnetorheological (MR) device ( 20 ) including settling stability. The MR fluid device ( 20 ) includes a housing ( 22 ) including a hollow ( 30 ), a moving element ( 26 ) contained within the hollow ( 30 ), the housing ( 26 ) and moving element ( 26 ) cooperating to form a working section ( 36 ) and a chamber ( 32 ) within the hollow ( 30 ), a MR fluid ( 38 ) contained within the working section ( 36 ) and the chamber ( 32 ), a coil ( 40 ) or the like for generating a magnetic field to act upon the MR fluid ( 38 ) contained within the working section ( 36 ) to cause a rheology change therein, and a magnet ( 25 ) or electromagnet ( 31 ) generating a low-level magnetic field to act upon a substantial portion of the MR fluid ( 38 ) contained in the chamber ( 32 ) to minimize settling of the particles in the MR fluid. Linear and rotary acting embodiments are included.

Passive magnetorheological clutch

Abstract: A passive magnetorheological clutch includes an input member that is constantly driven at engine speed, or some selected proportion thereof. An output member is journaled on the input member so as to be supported thereon in a nonrotating state and to be concentrically rotatable in concert therewith. A space presented between the input and output members contains a quantity of magnetorheological fluid. The space also contains an engagement mechanism including a permanent magnet and a ferromagnetic ring concentrically positioned with an operative gap therebetween, wherein the operative gap's dimension is variable to effect changes in the magnetic field passing between the magnet and the ring. When an actuator effects a reduction in the distance between the magnet and ring, the amount of magnetic field increases in density adjacent the ring. The resultant torque transfer from the input member to the output member is effected across the operative gap and through the magnetorheological fluid. As the operative gap is reduced, the torque transfer increases in a gradual and progressive manner.

Magnetorheological finishing of IR materials

Abstract: Magnetorheological finishing (MRF) is a subaperture lap, deterministic process developed at the Center for Optics Manufacturing. MRF can remove subsurface damage from an optical component while correcting figure errors and smoothing small scale microroughness. The 'standard' magnetorheological fluid for finishing of optical glasses consists of magnetic carbonyl iron and nonmagnetic cerium oxide particles in water. This composition works well for a variety of soft and hard glass types, but it does not perform adequately for certain single crystal materials and polycrystalline compounds used in IR applications. In this paper, we describe modifications to MRF and finishing experiments for LiF, ZnSe, CaF2, AMTIR-1, ZnS, MgF2, sapphire, and CVD diamond.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Magnetorheological fluid clutch with minimized reluctance

Abstract: A magnetorheological fluid fan clutch effects modulated rotational motion transmission between a rotatable input shaft carrying an input clutch plate and a housing having a front cover and a rear cover forming a cavity about the input clutch plate wherein a core carried in the front and rear covers operates as output clutch plates. Bearings support the housing on the input shaft through the front and rear covers so that the input clutch plate is rotatable on the input shaft relative to the housing. Magnetorheological fluid is carried in the cavity, through which torque is variably transferred between the input clutch plate and the housing. A coil is positioned radially outside the non-magnetic spacer and carries a variable electrical current to effect a variable magnetic field across the input clutch plate and through the magnetorheological fluid providing modulated torque transmission between the input clutch plate and the core.

Magnetorheological fluid seismic damper

Abstract: A seismic MR damper (20) for providing controllable damping forces between a first structural member (21) and a second structural member (23) within large structures, such as buildings, bridges, etc. The seismic damper (20) includes a damper body (22) having a hollow cavity (30) and at least one aperture (28), at least one piston rod (24) slidably received within the aperture (28), a piston assembly (26) attached to the at least one piston rod (24) and axially moveable within, and subdividing, the said hollow cavity (30) into a first fluid chamber (32) and a second fluid chamber (34), a controllable passageway (36) for controlling flow between the first fluid chamber (32) and a second fluid chamber (34), a magnetorheological fluid (38) including soft-magnetic particles disbursed within a carrier liquid contained in said controllable passageway (36), first fluid chamber (32), and second fluid chamber (34), and a plurality of alternately-wound coils (40, 40', 40'') for generating a plurality of magnetic fields (39, 39', 39'') which are directed to act upon the magnetorheological fluid (38) contained in said controllable passageway (36) to generate rheology changes, thereby restricting flow of the magnetorheological fluid through the controllable passageway (36).

Magnetorheological liquids, a process for producing them and their use, and a process for producing magnetizable particles coated with an organic polymer

Abstract: Magnetorheological liquids, a process for producing them, their use and a process for producing polymer-coated magnetisable particles used in the magnetorheological liquids.

Compact low-inductance magnetorheological damper

Abstract: According to one aspect of the invention, a magnetorheological fluid (MRF) damping device includes two concentric coils or groups of coils arranged to produce respective magnetic fields that mutually cancel substantially everywhere but within the gap between them The rheological properties of the MRF are magnetically controlled within this gap According to a second aspect of the invention, the piston rod of a MRF damper or other viscous fluid damper communicates its motion to a hollow, capped cylinder Damping fluid within this cylinder communicates via a narrow channel with damping fluid outside of this cylinder When the piston rod undergoes motions directed so as to expel fluid from the interior of the cylinder, resistance within the narrow channel causes pressure to build up within the cylinder, but prevents any substantial amount of pressure from building up without the cylinder, thus isolating sealing devices from the highest resistive forces generated within the damping fluid According to a third aspect of the invention, a piston head for a viscous fluid damper is arranged to pass small-amplitude displacements with reduced damping A floating piston is disposed in a borehole that passes through the piston head This borehole provides a low-resistance pathway for fluid flow when piston displacements are relatively small, but the floating piston closes this pathway when piston displacements are relatively large

A thermodynamic model for magnetorheological fluids

Abstract: An analysis of the dynamic behavior of a magnetorheological (MR) fluid is given in terms of a vectorial internal variable describing the change of the macroscopic average of the relative position vector of suspensions. Under the restriction of the second law, the constitutive equations of the MR fluid for stress, heat flux, magnetization and internal variable can be derived. The related issue of dissipative and energy transfer mechanisms is treated at some length. Studies on the steady shear flow indicate the direction of the internal variable is independent of shear rate. The Bingham-type constitutive equation for shear stress is obtained and endowed with a new meaning. The pressure-driven flow, another significant flow type for the design of MR devices, is also analyzed to study the plug flow region and the relationship between yield stress and flow rate. In addition, a criterion of flow initiated by the applied shear force is proposed based on the saturation of the internal variable and the condition of the equilibrium of forces in the fluid and solid regions.

Manufacturing and characterization of magnetorheological fluids

Abstract: Magnetorheological (MR) fluids are stable suspensions of magnetic particles in a carrying fluid exhibiting controllable rheological behavior in the presence of a magnetic field. Magnetorheological effect represents a reversible increase, due to an external magnetic field, of effective viscosity. MR fluids and devices have the potential to revolutionize the design of hydraulic systems, actuators, valves, active shock and vibration dampers, and other components used in mechanical systems. MR fluids that are currently available suffer from high initial viscosity values and low stability. Hence, there is a compelling need to optimize the MR fluid manufacturing process to produce optimum MR fluid characteristics. The present study proposes to manufacture an optimum composition of a MR fluid in terms of its quality and properties. A high-speed bead mill blending machine is used to manufacture the fluid. Characterization studies are conducted to evaluate the produced fluid.

Some features of the magnetorheological effect

Abstract: The Theological properties of magnetorheological suspensions (MRS) are investigated in a wide concentration range of particles of the disperse ferromagnetic phase in the presence of a magnetic field. It is shown that, along with an increase in the concentration of ferromagnetic particles, the range of control of the viscous stress increment in an MRS can be broadened by changing the size and shape of the ferromagnetic particles, introducing nonmagnetic particles into the dispersion medium as well as by heating the medium to the Curie temperature.

Magnetorheological fluid brake

Abstract: A controllable brake, such as a magnetorheological (MR) fluid brake (20) including a rotor (28) which is preferably disc-like and rotatably supported by a shaft (30) which is rotatably supported by bushings (42) received within a housing (22). A coil assembly (48) is electrically energized to create a changeable magnetic field within a pole piece (32), which is preferably manufactured from a powdered metal material, which most preferably has a density of between about 6.8 and about 7.0 gm/cm3. A magnetically-soft medium, such as an MR fluid (23) which preferably includes magnetically-soft particles disbursed in a carrier liquid is contained within first and second gaps (86 and 88) located between the pole piece (32) and rotor (28). The MR fluid's (23) rheology is controlled by the generated magnetic field to vary the operating torque of the MR brake (20). In one aspect, the brake (20) includes a magnetic saturation zone (25) to reduce the propensity of magnetizing the shaft (30). In another aspect, the pole piece halves (24 and 26) are received in radially spaced pole pockets (37) formed in the housing (22) and preferably include an axial bias spring (70) to ensure intimate contact therebetween. In another aspect, a lip projection (76) traps MR fluid (23) between the lip projection (76) and shaft (30) and minimizes fluid exposure to the shaft seal (44). In another aspect, cooling fins (55) or a projection (53) restrains rotation of the housing (22). In yet another aspect, a spring (46) centers the rotor (28) relative to the housing (22).

Set bearing for motor vehicle

Abstract: The working set bearing (1) has at least one spring element and with fixing elements for connecting at the parts to be located whereby the spring element (2) has a fluid chamber (9), which is filled with a magnetorheological fluid (10). At least one electromagnet (8,12) is arranged outside and/or inside the fluid chamber, whereby combined with a magnetic field created by the electromagnet, the fluid viscosity increases from a liquid to a solid which leads to an increase of spring rate of the set bearing. The fluid without a magnetic field reaction remains liquid but becomes approximately solid when effected by the magnetic field, so that the part of the set bearing, in which the fluid chamber (9) is designed, can no longer be freely deformed.

Magnetic and magnetorheological properties of flowable compositions based on barium and strontium ferrites and iron oxides

Abstract: Magnetic and magnetorheological properties of a number of compositions are examined. The compositions are based on bariumand strontium ferrites, magnetite, and y-Fe203, both commercial and synthesized using specially developed methods, suspended intransformer oil and synthetic binders based on phenol-formaldehyde, epoxyacrylic and pentaphthalic resins. The influence ofnumerous factors, such as the type and magnetic properties of the filler, type of the binder, magnetic properties of the composition onthe orienational effect of the filler in a magnetic field was evaluated. The latter was judged by a magnitude of magnetorheological effect. Keywords: magnetic, magnetorheological properties, barium and strontium ferrites, iron oides. INTRODUCTION A wide range of smart and intelligent materials can be obtained on the basis of fluid compositions with dispersed fillers possessingmagnetic properties. Of special interest are fluid media with inert binders, e.g. oil, and those with polymeric binders. The latter arecapable of forming anisotropic structures in external fields, which may be used as different sensors, actuators, measuring elements,

Modeling and computation of the overall magnetic properties of magnetorheological fluids

Abstract: Magnetorheological (MR) fluids constitute examples of controllable ("smart") fluids, whose rheological properties vary in response to an applied magnetic field. These fluids typically consist of micron-sized, magnetizable particles dispersed in an insulating medium. The essential characteristic of MR fluids is that they may be continuously and reversibly varied from a state of free flowing liquids in the absence of an applied magnetic field to that of stiff semi-solids in a moderate field. The highly nonlinear and oscillatory nature of the magnetization of their constituents provides a challenge for the prediction of the overall magnetic response of MR composites. In this paper we initiate a study of the effective permeability of these materials by appealing to the mathematical theory of homogenization. Within this framework, we consider suspensions that display both linear as well as nonlinear magnetic behavior. For these, we derive analytic formulas for their effective permeabilities. In the case of linear materials comparisons are made with the asymptotic Rayleigh-Maxwell expressions.

A new hybrid model for electrorheological fluid dampers and application to dynamic vibration absorption

Abstract: Dampers incorporating electrorheological or magnetorheological fluid (whose damping parameters can be adjusted by altering an imposed electric or magnetic field, respectively) as part of their structure hold great promise as adaptive‐passive vibration reduction devices. Extant models of electrorheological dampers are insufficient, because of excessive complexity or the lack of broad experimental validation, to allow the prediction of the response of the damper over a wide range of operating conditions. The subject work proposes a new, simple lumped parameter model for electrorheological dampers, validates the new model against empirical data, and explores the application of an electrorheological damper in a tuned dynamic vibration absorber. The vibration absorption capacity of an electro‐ or magnetorheological damper in a dynamic vibration absorber is found to depend on the displacement amplitude and frequency of the vibration, but a substantial reduction in vibration is achievable with only adaptive‐passive control. [Work supported by U.S. Army Research Office.]

A linear motion conveyor having an intermittently actuated damper

Abstract: A linear motion conveyor, of the reciprocating type with a trough (52, fig 2), comprises a drive 56 having a crank shaft 78 which is damped, in order to prevent mechanical knocking, by an intermittently actuated damper assembly 136. Damper assembly 136 may comprise of either a magnetorheological brake or an electromagnetic brake having a partial disc (192, fig 12) disposed between a pair of electromagnets (200, fig 13) . Control means 132 selectively activates the damper assembly 136 when, for example, torque from a drive motor 62 would otherwise go negative. Sensor, eg a switch 130, signals the control 132 to activate the damper assembly 136 at a first time in a cycle and to deactivate at a second time in the cycle. The conveyors speed variations, ie slow forward and fast backwards, may be obtained by using elliptical gears (see fig 4) or a Hooke type joint.