01-04-2011, 10:58 AM
Presented by:
ARUN . B
[attachment=11473]
INTRODUCTION
A magneto rheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity.
The upshot of which is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.
HISTORY
Approximately sixty years ago,in the1940s,JacobRabinov discovered the MRF effect at the US National bureau of Standards. At the same time W. Wislow was working on a competitive field called Electro-Rheological Fluid.The electro-Rheological effect depends on an electrostatic field, and the magneto-rheological (MR) effect depends on a magnetic field.
MRF technology advantages have created a very high level of interest to introduce products based on MRF technology during the most recent couple of years.
CHEMICAL COMPOSITION
A typical MR fluid consists of 20%–40% by volume of relatively pure, soft iron particles, typically 3–5 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol.
A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear.
PHYSICAL COMPOSITION
MR fluids made from iron particles exhibit maximum yield strengths of 30–90 kPa for applied magnetic fields of 150–250 kA/m .
MR fluids are not highly sensitive to moisture or other contaminants that might
be encountered during manufacture and use. Further, because the magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives.
The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles
BEHAVIOR
reach a maximum point after which increases in magnetic flux density have no further effect, as the fluid is then magnetically saturated. The behavior of a MR fluid can thus be considered similar to a Bingham plastic, a material model which has been well-investigated.
However, a MR fluid does not exactly follow the characteristics of a Bingham plastic. For example, below the yield stress , the fluid behaves as a viscoelastic material, with a complex modulus that is also known to be dependent on the magnetic field intensity. MR fluids are also known to be subject to shear thinning, whereby the viscosity above yield decreases with increased shear rate. Furthermore, the behavior of MR fluids when in the "off" state is also non-Newtonian and temperature dependent, however it deviates little enough for the fluid to be ultimately considered as a Bingham plastic for a simple analysis.
APPLICATIONS
MR fluids find a variety of applications in almost all the vibration control systems. It is now widely used in automobile suspensions, seat suspensions, clutches, robotics, design of buildings and bridges, home appliances like washing machines etc.
IN CLUTCH PLATES :
The activation of MRF clutch’s built-in magnetic field causes a fast and dramatic change in the apparent viscosity of the MR fluid contained in the clutch. The fluid changes state from liquid to semi-solid in about 6 milliseconds. The result is a clutch with an infinitely variable torque output.
MR technology enables new levels of performance in automotive primary suspension systems. Shock absorbers incorporate magneto rheological fluids to provide real-time optimization of suspension damping characteristics that improve ride and handling
ADVANTAGES
The MR fluid sponge damper requires neither seals nor bearings, and uses the same inexpensive components found in existing passive dampers, but with a few important modifications.
At high speed, the MR sponge dampers are turned off to enable a high level of vibration isolation.
The power requirements for controllable MR fluid dampers are so low that a net energy saving might be realized.
LIMITATIONS
One major limitation of these MR dampers are the high cost required for the installation. This can be neglected taking into account the considerable increase in the efficiency of the associated machine.
MR dampers are now using temporary magnets which require an applied magnetic field of 150–250 kA/m. Latest technologies permits the use of permanent magnets also.
CONCLUSION
MR fluids are actually amazing magnetic fluids. MR fluid durability and life have been found to be more significant barriers to commercial success than yield strength or stability. MR fluids can be considered as a better way of controlling vibrations. The key to success in all of these implementations is the ability of MR fluid to rapidly change its rheological properties upon exposure to an applied magnetic field.ampers are now using temporary magnets which require an applied magnetic field of 150–250 kA/m. Latest technologies permits the use of permanent magnets also