15-10-2016, 09:36 AM
1459102608-SeminarReport.pdf (Size: 1.3 MB / Downloads: 5)
ABSTRACT
Magneto rheological fluids commonly known as MR fluids are
suspensions of solid in liquid whose properties changes drastically when exposed to
magnetic field. Magneto rheological (MR) fluids are materials that respond to an
applied field with a dramatic change in their rheological behavior. The essential
characteristic of these fluids is their ability to reversibly change from a free-flowing,
linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds
when exposed to a magnetic field.
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.
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.
Introduction
Magneto rheological fluids commonly known as MR fluids are suspensions of solid in
liquid whose properties changes drastically when exposed to magnetic field. It is this property
which makes it desirable to use in different vibration controlling systems.
1.2 What are MR fluids?
Magneto rheological (MR) fluids are materials that respond to an applied field with a
dramatic change in their rheological behavior. The essential characteristic of these fluids is their
ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with
controllable yield strength in milliseconds when exposed to a magnetic field.
1.3 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.
1.4 Physical properties
MR fluids made from iron particles exhibit maximum yield strengths of 30–90 kPa for
applied magnetic fields of 150–250 kA/m (1 Oe . 80 A/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, it is a relatively straightforward matter to stabilize MR
fluids against particle-liquid separation in spite of the large density mismatch. The ultimate
strength of the MR fluid depends on the square of the saturation magnetization of the suspended
particles.
What makes a good MR fluid?
The most common response to the question of what makes a good MR fluid is likely to be
"high yield strength" or "non-settling". However, those particular features are perhaps not the most
critical when it comes to ultimate success of a magneto rheological fluid. The most challenging
barriers to the successful commercialization of MR fluids and devices have actually been less
academic concerns.
As anyone who has made MR fluids knows, it is not hard to make a strong MR fluid. Over
fifty years ago both Rabinow and Winslow described basic MR fluid formulations that were every
bit as strong as fluids today. A typical MR fluid used by Rabinow consisted of 9 parts by weight
of carbonyl iron to one part of silicone oil, petroleum oil or kerosene.1 To this suspension he would
optionally add grease or other thixotropic additive to improve settling stability. The strength of
Rabinow’s MR fluid can be estimated from the result of a simple demonstration that he performed.
Rabinow was able to suspend the weight of a young woman from a simple direct shear MR fluid
device. He described the device as having a total shear area of 8 square inches and the weight of
the woman as 117 pounds. For this demonstration to be successful it was thus necessary for the
MR fluid to have yield strength of at least 100 KPa.
MR fluids made by Winslow were likely to have been equally as strong. A typical fluid
described by Winslow consisted of 10 parts by weight of carbonyl iron suspended in mineral oil.2
To this suspension Winslow would add ferrous naphthenate or ferrous oleateas a dispersant and a
metal soap such as lithium stearate or sodium stearate as thixotropic additive. The formulations
Application of MR-Fluids
4.1 Applications of MR fluids
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. Before discussing the above
said applications in detail it is desirable to go through the behavior of MR fluids on different types
of loading and what are the design considerations provided to compensate this.
4.1.1 MR fluid in dampers
As motion control systems become more refined, vibration characteristics become more
important to a system’s overall design and functionality. Engineers, however, have tended to look
at motion control and vibration as separate issues. Motion control, it might be said, presents fairly
familiar design engineering problems while vibration suggests more subtle problems. Few design engineers have either the hands-on experience or the training to address both sets of problems in a
single design solution.
Most devices use MR fluids in a valve mode, direct-shear mode, or combination of these
two modes. Examples of valve mode devices include servo valves, dampers, and shock absorbers.
Examples of direct-shear mode devices include clutches, brakes, and variable friction dampers
. In valve mode when the piston in a MR fluid damper moves, the MR fluid jets through
the orifices quite rapidly causing it to swirl and eddy vigorously even for low piston speed.
Similarly, the shear motion that occurs in a MR brake causes vigorous fluid motion. As long as
the MR fluid does not settle into a hard sediment, normal motion of the device is generally
sufficient to cause sufficient flow to quickly remix any stratified MR fluid back to a homogeneous
state. For a small MR fluid damper two or three strokes of a damper that has sat motionless for
several months are sufficient to return it to a completely remixed condition.
Except for very special cases such as seismic dampers, lack of complete suspension
stability is not a necessity. It is sufficient for most applications to have a MR fluid that soft settles
– upon standing a clear layer may form at the top of the fluid but the sediment remains soft and
easily remixed. Attempting to make these MR fluids absolutely stable may actually compromise
their performance in a device. One of the areas where MR fluids find their greatest application is
in linear dampers that effect semi-active control. These include small MR fluid dampers for
controlling the motion of suspended seats in heavy duty trucks, larger MR fluid dampers for use
as primary suspension shock absorbers and struts in passenger automobiles and special purpose
MR fluid dampers for use in prosthetic devices.
In all of these devices one of the most important fluid properties is a low-off state viscosity.
While in all of these examples having a MR fluid with a high yield strength in the on-state is
important, it is equally important that the fluid also have a very low off state. The very ability of
an MR fluid device to be effective at enabling a semi-active control strategy such as “sky -hook”
damping depends on being able to achieve a sufficiently low off-state. Care must be taken in
choosing fluid stabilizing additives so that they do not adversely affect the off-state viscosity.
Earthquake dampers and other some other special applications in which the device will sit
quiescent for very long periods of time represent special cases where fluid stability issues may
have overriding importance. Because of the transient nature of seismic events these dampers never
see regular motion, which can be relied on to keep the fluid mixed. This lack of motion also has it
benefit. Unlike dampers used in highly dynamic environments, seismic dampers do not need to
sustain millions of cycles. The fact that durability and wear are not issues gives the fluid designer
greater latitude to formulate a highly stable fluid. MR fluids for these applications are typically
formulated as shearing thinning thixotropic gels.
4.1.2 MR fluids on impact and shock loading
Investigations on the design of controllable magneto rheological (MR) fluid devices have
focused heavily on low velocity and frequency applications. The extensive work in this area has
led to a good understanding of MR fluid properties at low velocities and frequencies. However,
the issues concerning MR fluid behavior in impact and shock applications are relatively unknown.
To investigate MR fluid properties in this regime, MR dampers were subjected to impulsive
loads. A drop-tower test facility was developed to simulate the impact events. The design includes
a guided drop-mass released from variable heights to achieve different impact energies. The
nominal drop-mass is 55 lbs and additional weight may be added to reach a maximum of 500 lbs.
throughout this study; however, the nominal drop-mass of 55 lbs was used. Five drop-heights were
investigated, 12, 24, 48, 72 and 96 inches, corresponding to actual impact velocities of 86, 127,
182, 224 and 260 in/s.
Two fundamental MR damper configurations were tested, a double-ended piston design
and a mono-tube with nitrogen accumulator. To separate the dynamics of the MR fluid from the
dynamics of the current source, each damper received a constant supply current before the impact
event. A total of five supply currents were investigated for each impact velocity.
After reviewing the results, it was concluded that the effect of energizing the MR fluid only
leads to “controllability” below a certain fluid velocity for the double-ended design. In other
words, until the fluid velocity dropped below some threshold, the MR fluid behaved as if it was
not energized, regardless of the strength of the magnetic field. Controllability was defined when
greater supply currents yielded larger damping forces.
For the mono-tube design, it was not possible to estimate the fluid velocity due to the
dynamics of the accumulator. It was shown that the MR fluid was unable to travel through the gap
fast enough during the initial impact, resulting in the damper piston and accumulator piston
traveling in unison. Once the accumulator bottomed out, the fluid was forced through the gap.
However, due to the energy stored in the accumulator and the probable fluid vaporization, it was
impossible to determine the fluid velocity and in many cases the damper did not appear to become
controllable.
In conclusion, the two designs were compared and general recommendations on designing
MR dampers for impulsive loading were made. Possible directions for future research were
presented as well.