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MR fluid IN automotive suspensions

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S7 M7 ROLL NO:20



Vibration control of vehicle suspensions systems has been an active subject of research, since it can provide a good performance for drivers and passengers. Recently, many researchers have investigated the application of magneto rheological (MR) fluids in the controllable dampers for semi-active suspensions. This paper shows the characterization of a damper can be made through the physical characteristics of the MR fluids, current and damper design characteristics. In addition it has been shown that the use of ADAMS software is an Also, a

excellent computational tool to simulate dynamic mechatronics systems.

reconfigurable system is designed to be adjusted according to the circumstances and is able to respond by a position change or by itself just as the MR suspension can do it. Magneto rheological (MR) fluids belong to the general class of smart materials whose rheological properties can be modified by applying an electric field. MR fluids are mainly dispersion of particles made of a soft magnetic material in carrier oil. The most important advantage of these fluids over conventional mechanical interfaces is their ability to achieve a wide range of viscosity (several orders of magnitude) in a fraction of millisecond. This provides an efficient way to control vibrations, and applications dealing with actuation, damping, robotics and mechatronics. However, by use of dynamic simulations software it is possible to analyze the behavior and performance of systems consisting of rigid or flexible parts undergoing large displacement motions. The system uses Magneto-Rheological (MR) fluids and valve-less dampers to provide a wide range of damping force control with unparalleled responsiveness and authority. The MR fluid consists of soft iron particles suspended in a synthetic hydraulic fluid. When current is applied to an electromagnetic coil inside the damper's piston, the resulting magnetic field aligns iron particles, changing the rheology of the fluid i.e., its resistance to flow, and thus produces a mechanically simple but very responsive and controllable damping action without any valves. An onboard controller continually adjusts the damping forces up to once every millisecond based on input from four suspension displacement sensors, a lateral accelerometer and a steering wheel angle sensor.


A typical MR fluid consists of 20-40 percent by volume of relatively pure, 3-10 micron diameter iron particles, 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 to discourage gravitational setting and promote particle suspension, are commonly added to LORD Corporation state-of-the-art MR fluids to enhance lubricity, modify viscosity and inhibit wear. For most engineering applications, a simple Bingham plastic model is effective in describing the essential, field-dependent fluid characteristics. MR fluids made from iron particles exhibit maximum yield strengths of 50-100 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 usage. Further, because the magnetic polarization mechanism is unaffected by temperature, the performance of MR-based devices is relatively insensitive to temperature over a broad temperature range (including the range for automotive use). MR fluids are usually applied in one of two modes. MR fluid operating in valve mode, with fixed magnetic poles, may be appropriate for hydraulic controls, servo valves, dampers, and shock absorbers. The direct-shear mode with a moving pole, in turn, would be suitable for clutches and brakes, chucking/locking devices, dampers, breakaway devices and structural composites. MR fluids exhibit fast response time, high dynamic yield stress, low plastic viscosity, broad operational temperature range, resistance to settling, easy remixing, and excellent wear and abrasion resistance. The magnetic particles, which are typically micrometer or nanometer scale spheres or ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances, as below.


Fig 2.1 MR Fluid

When a magnetic field is applied, however, the microscopic particles (usually in the 0.1-10 µm range) align themselves along the lines of magnetic flux, see below. When the fluid is contained between two poles (typically of separation 0.5-2 mm in the majority of devices), the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity. Importantly, mechanical properties of the fluid in its “on” state are anisotropic. Thus in designing a magneto rheological (or MR) device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the motion to be restricted.

Fig 2.2 Alignment of magnetic particles

It is important to note the difference between MR fluid and ferrofluid. MR fluid particles primarily consist of micron-scale particles which are too heavy for Brownian motion to keep them suspended, and thus will settle over time due to the inherent density difference between the particle and its carrier fluid. The particles in a ferrofluids primarily consist of nano


particles which are suspended by Brownian motion and generally will not settle under normal conditions. As a result, these two fluids have very different 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. 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. 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.

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.


Fig 2.2.1 MR Fluid When Magnetic Field Is Applied

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:

= yield stress caused by applied magnetic field = magnitude of magnetic field 6

The system uses Magneto-Rheological (MR) fluids and valve-less dampers to provide a wide range of damping force control with unparalleled responsiveness and authority. The MR fluid consists of soft iron particles suspended in a synthetic hydraulic fluid. When current is applied to an electromagnetic coil inside the damper's piston, the resulting magnetic field aligns iron particles, changing the rheology of the fluid i.e., its resistance to flow, and thus produces a mechanically simple but very responsive and controllable damping action without any valves. An onboard controller continually adjusts the damping forces up to once every millisecond based on input from four suspension displacement sensors, a lateral accelerometer and a steering wheel angle sensor. And on the Chevrolet Corvette, the driver can select between a tour mode that optimizes ride comfort and a sport mode that enhances driver feedback and handling performance. Military tactical and combat vehicles are being used in more demanding conditions than ever before, and traditional suspensions are struggling to keep up with these changing conditions. Our magneto-rheological (MR) suspension technology provides a solution to these challenges, enabling new levels of performance in military primary suspension systems. Unlike traditional passive suspensions with fixed characteristics, our controllable MR suspensions react to vehicle and terrain conditions thousands of times per second, allowing the suspension to adapt its characteristics to the situation. This provides improved dynamic stability over passively damped systems, which can provide significant benefits, including:
   

Reduced risk of roll-over Improved maneuverability and safe driving speed Reduced occupant fatigue and equipment shock due to vibration Reduced wear and tear on suspension and drivetrain components (particularly in applications where the vehicle is loaded at or beyond the design Gross Vehicle Weight)


This next-generation technology uses a revolutionary damper design that controls wheel and body motion with Magneto-Rheological (MR) fluid in the shocks, and eliminates electromechanical valves. By controlling the current to an electromagnetic coil inside the piston of the damper, the MR fluid's consistency can be changed, resulting in continuously variable "real-time" damping. The system requires no bulky motors or solenoids, nor does one need to make any mechanical adjustments to the suspension components - just sending current to the damper makes the special magnetic fluid do the work. Better still, this design offers superior handling, control and ride quality on the roughest road surfaces because it automatically minimizes damping forces as needed for improved road isolation and ride smoothness. It can respond to inputs in one millisecond, or 10 times faster than systems on the market today. The system uses MR fluid-based monotube shocks, a sensor set and an on-board controller. The MR fluid contains randomly dispersed iron particles. In the presence of a magnetic field, the particles align into structures adopting a near-plastic state, increasing the damping properties while monitoring for changes up to 1000 times per second. Using steering wheel and braking inputs from the driver via an existing set of sensors, MRC isolates and smoothes the action of each tire. That's because electric current is sent to the coils in each damper, changing the flow properties of the damping fluid on an as-needed basis to reduce bouncing, vibration and noise. On bumpy or slick surfaces, the system integrates with the vehicle's traction control unit to assure maximum stability. It also works with ABS and stability enhancement systems to keep the vehicle balanced and poised. As a result, drivers feel a greater sense of security, a quieter, flatter ride and more precise, responsive handling - particularly during sudden, high-speed maneuvers. Ride and handling engineers developing vehicles with MRC can spend time adjusting the algorithms that control the damping responses on a computer and can fine-tune ride and handling characteristics to unprecedented levels of specificity - in less time. Unlike traditional oleo dynamic systems, magneto rheological (MR) fluid suspension systems react instantly to road conditions and driver inputs, thanks to the fact that the dampers use a fluid the viscosity of which is modified by applying an electronically controlled magnetic field.

What this means in practice is greatly improved body control which in turn directly improves handling and road-holding thanks to optimal tyre grip in all road conditions. The result is a much safer and more enjoyable driving experience courtesy of reduced roll and greater


control when accelerating, braking and cornering. This is the very first time the SCM (Magneto rheological Suspension Control) suspension has been used on a high performance sports car.

Performance of a vehicle damper utilizing a Magneto-Rheological (MR) fluid is enhanced through use of a piston having grooves in the walls of a magnetic flux path portion of a fluid flow passage through the piston. The grooves result in a focusing and intensification of the magnetic flux emanating from the corners of the grooves, to thereby intensify the magnetic flux impressed across the fluid flow passage and cause a greater degree of change in viscosity of the MR fluid flowing through the passage than is achievable in prior MR dampers having smooth-walled fluid flow passages. A piston adapted for reciprocating motion along an axis within a working chamber containing a magneto-rheological (MR) fluid therein of a cylinder tube of a damper, the piston comprising: a piston body defining a first and a second face and a fluid flow passage extending through the piston from the first to the second face for directing a flow of MR fluid along a flow direction through the fluid flow passage as the piston reciprocates in the working chamber; and an electromagnetic element within the piston body defining a wall forming a magnetic flux path portion of the fluid flow passage for directing magnetic flux through the wall and the fluid flow passage in a direction generally transverse to the flow direction, the wall including a groove therein extending in a direction generally transverse to the flow direction through the magnetic flux path portion of the fluid passage for intensifying the magnetic flux passing in a direction transverse to the flow direction through the wall and the magnetic flux path portion of the fluid flow passage. Magneto-

Rheological (MR) dampers has typically included a piston that is movable within a working chamber of a cylinder containing the MR fluid. The cylinder is attached to one part of the suspension, and a piston rod extending from the piston and out of the cylinder is attached to another part of the vehicle suspension.


The piston of the MR damper separates the working chamber into a compression chamber and a rebound chamber. The piston is equipped with a sliding fluid seal that prevents leakage of the fluid around the piston, between the piston and the cylinder. The piston also includes one or more smooth-walled flow passages extending though the piston that allow the MR fluid in the working chamber to move between the compression and rebound chambers , as the piston rod and piston are moved in relation to the cylinder of the damper by movement of the vehicle suspension. The flow passages extending through the piston are sized to restrict the flow of MR fluid through the piston, thereby limiting the rate at which the piston can move within the cylinder to be a function of how rapidly the MR fluid can pass through the flow passages. The MR fluid has microscopic particles of a magnetic material suspended in a liquid carrier. When the MR fluid is exposed to a magnetic field of sufficient strength, the suspended particles align with the magnetic field and cause a change in the viscosity of the MR fluid. As the viscosity of the MR fluid changes, the rate at which the MR fluid can flow through the flow passages in the piston is also changed, thereby causing the amount of damping to be changed in a direct relationship to the viscosity of the MR fluid flowing through the flow passages.



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.

Fig 4.1 A shock absorber using MR fluid


Delphi Magne Ride is a semi active suspension control system developed by Delphi Corporation USA. It can respond in real time to road and driving conditions based on input from sensors that monitor body and wheel motion. This production proven system provides fast, smooth continuously variable damping in a cost effective and reliable package. It provides great body motion control and helps to increase tyre to road contact. Delphi Magne Ride is the industry first semi active suspension technology with no electro –mechanical valves and no small moving parts. The system consist of four magneto rheological fluid based mono tube dampers, a sensor set, and an on board electronic control unit (ECU). MagneRide replaces Delphi’s Continuously Variable Real-Time Damping (CVRTD) system, which was previously standard equipment on the Cadillac Seville STS. General Motors markets MagneRide as Magnetic Ride Control on the Cadillac Seville and Magnetic Selective Ride Control on the Corvette.

Fig 4.1.1 Delphi Magne –Ride 12


MagneRide(TM) provides improved comfort, performance and safety from one revolutionary system, through:  Greater sense of safety and security due to improved wheel control;  Minimized vehicle body motion for a flatter ride and more precise, responsive handling;  Reduction of small road disturbances;  Improved load transfer characteristics, providing better roll control and steering precision during sudden, high-speed maneuvers; and  A quieter, more refined ride due to improved road isolation.

In contrast to conventional electro-mechanical solutions, MR technology offers:
      

Quick response time (less than 10 milliseconds) Real-time, continuously variable control of damping Simple design (few or no moving parts) Consistent efficacy across extreme temperature variations High dissipative force independent of velocity Greater energy density Minimal power usage (typically 12V, 1 Amp max current; fail-safe to battery backup, which can fail-safe to passive damping mode)

Inherent system stability (no active forces generated)

In contrast to electro-rheological (ER) fluids, MR fluids:
  

Are 20-50 times stronger; Are significantly less sensitive to contaminants and extremes in temperature; and Can be operated directly from low-voltage power supplies.



Fig 6.1 MR Dampers

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.




In addition to cost-sensitive applications such as washing machines, MR fluid dampers are being used in rotary brakes for exercise equipment and pneumatic systems; in complete semi active damper systems for heavy-duty truck seat suspensions; in adjustable linear shock absorbers for racing cars; and in semi active suspensions for passenger cars. Now under commercial development are very large MF fluid dampers designed for seismic damage mitigation in civil engineering structures such as buildings and bridges. Finally, the technology is being investigated for applications in vehicular steer-by-wire devices and medical equipment such as the joints of prosthetic limbs. The nervous systems of future robots might use MR fluids to move joints and limbs in lifelike fashion. There are many potential applications that make these fluids very exciting." For example, MR fluids flowing in the veins of robots might one day animate hands and limbs that move as naturally as any humans. Book makers could publish rippling magnetic texts in Braille that blind readers could actually scroll and edit. It might even be possible to train student surgeons using synthetic patients with MR organs that flex and slices like the real thing. New developments in MR fluid technology allow the use of permanent magnets which has lots of advantages. The question often arises asking if it is possible to use a permanent magnet to bias a MR fluid valve or device at a mid-range condition. Current could then be applied to the accompanying electromagnetic coil to cancel the magnetic field and open the valve. Alternatively, a reverse current could be applied to the coil to add to the magnetic field taking the device to a higher–range condition. One motivation for creating such a system is to provide a fail-safe mode of operation wherein the device remains in a locked condition when power is lost. Another motivation may be energy conservation in systems intended to remain closed or locked for extended periods of time and then only open momentarily.


     Magneto-liquid mirror telescopes that bend and deform to cancel the twinkling of starlight. Prosthetic limbs for humans (a prosthetic knee based on Lord Corporation MR fluid technology is already available.) Active engine mounts that reduce vibration and quiet noise before it can get into a vehicle. Shock absorbers for payloads in the space shuttle. Active hand grips that conform to the shape of each individual hand or fingers.



Magneto rheological fluids are actually amazing magnetic fluids. MR fluid development is of course a balancing act that is highly coupled with MR device design. MR fluid durability and life have been found to be more significant barriers to commercial success than yield strength or stability. Amenability of a particular MR fluid formulation to being scaled to volume production must also be considered. Challenges for future MR fluid development are fluids that operate in the high shear regime of 104 to 106 sec-1.thus 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.


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