01-11-2010, 03:48 PM
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MAGNETORHEOLOGICAL FLUID DAMPER
INTRODUCTION
Vibration isolators or dampers are commonly used to mitigate vibrations in structures or machines. The isolators are typically passive and they are designed to reduce the vibration of the most undesired frequency. However, in many applications, the excitation frequency varies across a large range. In a semi-active device, the stiffness or damping can be adjusted during operation. Changing the stiffness of the support device can be exploited by moving the eigenfrequency of the system to bypass the resonance. Reliable control of the support device requires monitoring of a critical point of structure and knowledge about the frequency response of the system. This kind of adaptive isolator can change operation conditions according to dominant loading resulting in improved vibration isolation capability compared to passive systems. Undesired vibrations are reduced in different loading conditions, i.e. a large frequency range is covered.
Several concepts have been studied recently to utilize smart materials in adaptive structures. Magnetorheological (MR) fluids and elastomers, shape memory alloys (SMA) and piezoelectric materials comprise a class of smart materials whose properties can be controlled. The constitutive behaviour of smart materials couples their mechanical response (stress and strain) with other physical fields like magnetic or electric field or heat, which makes it possible to develop an adaptive structure without complicated mechanisms.
Magnetorheological materials (MR) are a class of materials whose rheological properties are rapidly varied by applying a magnetic field. This change is in proportion to the magnetic field applied and is immediately reversible. MR-materials exhibit rapid, tuneable and reversible transition from a free-flowing state to a semi-solid state upon the application of an external magnetic field. MR-material provides simple, quiet and rapid-response interface between the electronic control and mechanical system to mitigate the vibration of the host structure (Li et al., 2000). MR-materials are useful in many applications because the change in their material properties is large.