Introduction

PAGE 2SEMINAR PAPER PRESENTATION ON MAGNETO RHEOLOGICAL FLUIDS

ABSTRACT

Magneto rheological fluids commonly known as MR fluids are suspensions of solid in liquid whose properties changes drastically when exposed to magnetic field. Magnetorheological (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.

What are MR fluids?

Magnetorheological (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.

Chemical composition

A typical MR fluid consists of 20%40% by volume of relatively pure, soft iron particles, typically 35 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 properties

MR fluids made from iron particles exhibit maximum yield strengths of 3090 kPa for applied magnetic fields of 150250 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.

Theory

In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian liquids. For most engineering applications a simple Bingham plastic model is effective at describing the essential, field-dependent fluid characteristics. A Bingham plastic is a non-Newtonian fluid whose yield stress must be exceeded before flow can begin; thereafter, the rate-of-shear vs. shear stress curve is linear. In this model, the total yield stress is given by:

where:

= yield stress caused by applied magnetic field

= magnitude of magnetic field

= shear rate

= field-independent plastic viscosity defined as the slope of the measured shear stress vs. shear strain rate relationship, i.e., at H=0.

How it works?

Applying a magnetic field to magnetorheological fluids causes particles in the fluid to align into chains.

When some low-density MR fluids are exposed to rapidly alternating magnetic fields, their internal particles clump together. Over time they settle into a pattern of shapes that look a bit like fish viewed from the top of a tank. Such clumpy MR fluids don't stiffen as they should when magnetized. The fish tank pattern is fragile and takes about an hour to fully develop. It

doesn't occur in MR fluids that are constantly mixed and agitated, as in a car's suspension, but it could prove troublesome in other situations.

Above: The structure of particles in an MR fluid gradually changes when an alternating magnetic field is applied. The leftmost picture shows an MR fluid after 1 second of exposure to a fast-changing magnetic field. The suspended particles form a strong, fibrous network. The pictures to the right show the fluid after 3 minutes, 15 minutes and 1 hour of exposure. The particles have formed clumps that offer little structural support.

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 magnetorheological 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 weightof 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 Rabinows 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 described by Rabinow and Winslow are relatively easy to make. The yield strength of the resulting MR fluids is entirely adequate for most applications. Additionally, the stability of these suspensions is remarkably good. It is certainly adequate for most common types of MR fluid application. As early as 1950 Rabinowpointed out that complete suspension stability, i.e. no supernatant clear layer formation, was not necessary for most MR fluid devices. MR fluid dampers and rotary brakes are in general highly efficient mixing devices.

MR fluid in dampers

As motion control systems become more refined, vibration characteristics become more important to a systems 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, theMR 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 heavydutytrucks, 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 offstate.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 grater latitude to formulate a highly stable fluid. MR fluids for these applications are typically formulated as shearing thinning thixotropic gels.

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.

MR fluids on impact and shock loading

Investigations on the design of controllable magnetorheological (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.

MR fluid in automobile clutches

MR fluids are increasingly being considered in variety of devices such as shock absorbers, vibration insulators, brakes or clutches. The activation of MRF clutchs 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.

Double plate MRF clutch design

Bansbach, proposed a double-plate and a multi-plate MRF torque transfer apparatus with a controller that adjusts the input current. The apparatus is proposed to be placed between the engine of a car and its differential. Gopalswamy suggested a MRF clutch to minimize reluctance for fan clutches. Gopalswamy also studied a controllablemulti-plate MR transmission clutch. This clutch was also designed to be placed between the engine and differential. Hampton described a design of MRF coupling with reduced air gaps and high magnetic flux density. Carlson proposed a MR brake with an integrated flywheel.

The figure below shows the prototype of a double plate magneto rheological fluid clutch.

The MR fluid is located in the gap between the input and output plates, with the diameter of 51.94 mm. These plates are connected to 30 mm diameter input and output shafts. The shafts are supported by deep groove ball bearings, which are press-fitted into the side caps. The electromagnet circuit of this clutch consists of an electromagnetic coil, which is wound around an electromagnetic core. This assembly is located inside a 152.4 mm outer diameter casing with 6.35 mm wall thickness, which is also acting as a

return path for the magnetic field. Two O-rings are located in the grooves machined on the circumferences of plates to prevent leakage of MR fluid. The MRF clutch is activated by a power supply connected to two ends of the

electromagnet. The total width of the clutch is 31.75 mm.

The graph above shows magnetic field strength as a function of radius in the MRF section. From the graph it can be observed that the magnetic field increases with increasing radial distance from the rotational axis. This is a desirable outcome since the contribution of the resulting shear yield stress on the torque transmitted increases with increasing radial distance.

The performance of a double-plate magneto-rheological fluid limited slip differential clutch is studied using two types of MR fluids. Theoretical and experimental analyses have illustrated that this MR fluid clutch can transfer high controllable torques with a very fast time response.

MR fluid in automotive suspensions

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.MR fluid controllable damping technology outperforms all existing passive and active suspension systems.

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. The damper consists of a layer of open-celled, polyurethane foam, or other suitable absorbent matrix materials, saturated with ~3 ml of MR fluid surrounding a steel bobbin and coil

Together these elements form a piston on the end of the shaft that is free to move axially inside a steel housing that provides the magnetic flux return path. Damping force is proportional to the sponges active area.

The application of a magnetic field causes the MR fluid in the matrix to develop a yield strength and resist shear motion. The amount of force produced is proportional to the area of active MR sponge that is exposed to the magnetic field. This arrangement can be applied in both linear and rotary configurations wherever a direct shear mode of operation would be used.

MR fluid in washing machines

A good example of unwanted vibratory motion is a washing machine in its spin cycle trying to walk out of the room. MR damping can correct this and other problem vibrations.

The common household washing machine represents a standard compromise between controlling vibration associated with the spin cycle and achieving optimum system performance and efficiency. The tub in a conventional machine is suspended by a number of coil springs that provide mechanical support as well as vibration isolation at high frequency. To prevent potentially damaging vibratory excursions when the drum velocity passes through resonance as it accelerates during the ramp-up to the spin cycle, static vibration dampers are added to the suspension.

Conventional dampers easily control the tubs motion at resonance; they can significantly degrade high-speed vibration isolation. This tendency limits the size of the tub and to some extent dictates the dimensions of the housing that must accommodate the overall motion of the tub.

Because many households have only a washing machine and not a dryer, tub speeds are reaching 2000 rpm, effectively becoming centrifuges that remove almost all the water from the wash load. In fact, manufacturers have had to reduce the size of the drain holes in the tub to prevent extrusion of small items of clothing during the spin cycle.

To achieve this level of performance, manufacturers have incorporated a controllable damping system designed around magnetorheological (MR) fluid.

Conventional springs and magnetorheological dampers work together to stabilize a home washing machine during the spin cycle. The dampers control vibrations as the tub passes through resonance; at the highest speeds the dampers are switched off and vibration isolation is provided by the mechanical springs that support the tub. These can simply be turned off at high spin speeds for an increased degree of vibration isolation.

By activating the damper while the washing machine tub is passing through resonance, a degree of vibration control is achieved not possible with conventional springs alone. The damping mechanism is switched off at the greatest speeds, when the mechanical springs provide vibration isolation.

At high speed, the MR sponge dampers are turned off to enable a high level of vibration isolation. With enhanced vibration control, the drum may be made larger or the housing smaller since it must accommodate less overall tub motion. Ideally, each of a pair of controllable dampers would have to provide 50150 N of damping force when energized and a low residual force of