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Smart materials
Science and technology have made amazing developments in the design of electronics and machinery using standard materials, which do not have particularly special properties(i.e. steel, aluminum, gold).Imagine the range of possibilities, which exist for special materials that have properties scientists can manipulate. Some such materials have the ability to change shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost instantly when near a magnet; these materials are called smart materials.
Smart materials have one or more properties that can be dramatically altered. Most every day materials have physical properties, which cannot be significantly altered; for example if oil is heated it will become a little thinner, whereas a smart material with variable may turn from a liquid state which flows easily to a solid.
Each individual type of smart material has a different property which can be significantly altered, such as viscosity, volume or conductivity. The property that can be altered determines what type of applications the smart material can be used for Varieties of smart materials already exist, and are being researched extensively. These include piezoelectric materials, magnetorheostatic materials, electrorheostatic materials, and shape memory alloys. Some everyday items are already incorporating smart materials (coffeepots, cars, glasses) and the number of applications for them is growing steadily.
Magnetorheological materials (fluids) (MR) are a class of smart materials whose rheological properties (e.g.viscosity) may be rapidly varied by applying a magnetic field. Underinfluence of magnetic field the suspended magnetic particles interact to form a structure that resists shear deformation or flow. This change in the material appears as a rapid increase in apparent viscosity or in the development of a semisolid state.
Advances in the application of MR materials are parallel to the development of new, more sophisticated MR materials with better properties and
stability. Many smart systems and structures would benefit from the change inviscosity or othermaterial properties of MR. Nowadays, these applicationsinclude brakes, dampers, clutches and shock absorbers systems.
1.2 control systems
whenever vibrations are created by some external forces they are controlled on the basis of following control systems
1 passive control system
2 active control system
3 semi-active control system
1.3 Passive Control Systems
Passive control systems alleviate energy dissipation demand on the primary structureby reflecting or absorbing part of the input energy, thereby reducing possible structuraldamage. However, passive control systems are limited in that they cannot deal with the change of either external loading conditions or usage patterns.
1.4 Active Control Systems
Active control systems have the ability to adapt to different loading conditions and tocontrol different vibration modes of the structure. The schematic diagram of an active control system is shown in Fig. 1.2. In this system, the signals sent tocontrol actuators are a function of responses of the system measured with physical sensors.
1.5 Semi-Active Control Systems
A compromise between passive and active control systems has been developed in theform of semi-active control systems, which are based on semi-active devices. A semi-active control device has properties that can be adjusted in real time but cannot inject energy into the controlled system. Frequently, such devices are referred to as controllable passive dampers.
1.6 Mr fluids
Magnetorhelogy :Magnetorheological is a branch of Rheology that dealswith the flow and deformation of the materials underan applied magnetic field. The discovery of MR fluidsis credited to Jacob Rabinow in 1949.Typical magnetorheological fluids are the suspensions ofmicron sized, magnetizable particles (mainly iron) suspended inan appropriate carrier liquid such as mineral oil, synthetic oil,water or ethylene glycol. The carrier fluid serves as a dispersedmedium and ensures the homogeneity of particles in the fluid.
Avariety of additives (stabilizers and surfactants) are used toprevent gravitational settling and promote stable particlessuspension, enhance lubricity and change initial viscosity of theMR fluids. The stabilizers serve to keep the particles suspendedin the fluid, whilst the surfactants are adsorbed on the surface ofthe magnetic particles to enhance the polarization induced in thesuspended particles upon the application of a magnetic field Typically, the diameter of the magnetizable particles rangefrom 3 to 5 microns. Functional MR fluids may be made withlarger particles, however, stable suspension of particles becomes increasingly more difficult as the size increases. Commercialquantities of relatively inexpensive carbonyl iron are generallylimited to sizes greater than 1 or 2 microns. Smaller particlesthat are easier to suspend could be used, but themanufacture of such particles is difficult. Significantly smallerferromagnetic particles are generally only available as oxides,such as pigments commonly found in magnetic recording media.
MR fluids made from such pigment particles are quite stable because the particles are typically only 30 nanometers indiameter. However, because of their lower saturationmagnetization, fluids made from these particles are generallylimited in strength to about 5 kPa and have a large plasticviscosity due to the large surface area. Main parameters of thesefluids are presented in table 1.In the absence of an applied field, MR fluids are reasonablywell approximated as Newtonian liquids. For most engineeringapplications a simple Bingham plastic model is effective atdescribing the essential, field-dependent fluid characteristics. ABingham plastic is a non-Newtonian fluid whose yield stress.
must be exceeded before flow can begin.Thereafter, therate-of-shear vs. shear stress curve is linear. In this model, thetotal yield stress is given by
Lots of modern, complex models of magnetorheologicalfluids are developed.Normally, MR fluids are free flowing liquids having aconsistency similar to that of motor oil.However, in the presence of an applied magnetic field, theiron particles acquire a dipole moment aligned with the externalfield which causes particles to form linear chains aligned to themagnetic field, as shown in Fig. 2.This phenomenon can solidify the suspended iron particlesand restrict the fluid movement. Consequently, yield strength isdeveloped within the fluid. The degree of change is related to the magnitude of the applied magnetic field, and can occur in afew milliseconds. Typical magnetorheological materials can achieve yieldstrengths up to 50–100 kPa at magnetic field strength of about150–250 kA/m. It was found that wall roughness on contactwith the fluid is important for yield strengths, especially in lowmagnetic fields. For low strains prior to yield, the shearmodulus of a MR fluid also shows a very large increase in anapplied magnetic field. MR materials eventually reach asaturation point where increases of magnetic field strength donot increase the yield strength of the MR material. This phenomenon typically occurs around 300 kA/m. The effect ofmagnetic saturation on the strength of MR materials can bestudied by using finite element analysis.
The MR effect is immediately reversible if the magneticfield is reduced or removed. Response times of 6.5 ms havebeen recorded.MR materials that have been already developed are stable intemperature ranges from –50 to 150C. There are slight changesin the volume fraction and hence slight reductions in the yieldstrength at these temperatures, but they are small.Also size and size distribution of the suspended particlesaffect the change in properties of the MR fluid when placed in amagnetic field.
Magnetorheological materials exhibit some advantages overtypical electrorheological materials. In contrast to electrorheologicalmaterials, MR fluids are more useful because thechange in their rheological properties is large, larger that in ERfluids, so an increase of yield stress are 20-50 times stronger.
Unlike ER materials, they are also less sensitive to moisture and contaminants, and
thus MR materials are candidates for use indirty or contaminated environments. They are also unaffected bythe surface chemistry of surfactants as ER materials are. Thepower (50 W) and voltage (12–24V) requirements for MRmaterials activation are relatively small compared with ERmaterials.
A typical MR fluid contains 20–40% by volume of relatively pure, soft iron particles,e.g., carbonyl iron; these particles are suspended in 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. The ultimate strength of an MR fluid depends on the square of the saturation magnetization of the suspended particle.The key to a strong MR fluid is to choose a particle with a large saturation magnetization(Carlson and Spencer 1996a). The best available particles are alloys of iron and cobalt that have saturation magnetization of about 2.4 tesla.The key to a strong MR fluid is to choose a particle with a large saturation magnetization.
The keyto a strong MR fluid is to choose a particle with large saturation magnetization.The best available particles is alloys of iron and cobalt that have saturation magnetization of about 2.4 tesla. Unfortunately, such alloys are prohibitivelyexpensive for most practical applications. The best practical particles are simply pure iron,as they have a saturation magnetization of 2.15 tesla. Virtually all other metals, alloys andoxides have saturation magnetization significantly lower than that of iron, resulting in substantiallyweaker MR fluids.
Typically, the diameter of the magnetizable particles is 3 to 5 microns. Functional MRfluids may be made with larger particles; however, particle suspension becomes increasinglymore difficult as the size increases. Smaller particlesthat are easier to suspend could be used, but the manufacture of such particles is difficult.
Commercial quantities of relatively inexpensive carbonyl iron are generally limited tosizes greater than 1 or 2 microns. Significantly smaller ferromagnetic particles are generallyonly available as oxides, such as the pigments commonly found in magnetic recordingmedia. MR fluids made from such pigment particles are quite stable because the particlesare typically only 30 nanometers in diameter. However, because of their lower saturationmagnetization, fluids made from these particles are generally limited in strength to about 5kPa and have a large plastic viscosity due to the large particle surface area.
1.7 MR fluid components
Magneto rheological (MR) fluids are basically non colloidal suspensions of micro sized magnetisable particles in an inert base fluid along with some additives. Thus there are basically three components in an MR fluid:
A. Base fluid,
B. Metal particles and
C. Stabilizing additives.
A. Base fluid
The base fluid is an inert or non-magnetic carrier fluid in which the metal particles are suspended. The base fluid should have natural lubrication and damping features. For better implementation of MRF technology the base fluid should have a low viscosity and it should not vary with temperature. This is necessary so that MRF effect i.e. variation of viscosity due to magnetic field becomes dominant as compared to the natural viscosity variation. Due to the presence of suspended particles base fluid becomes thicker. Commonly used base fluids are hydrocarbon oils, mineral oils and Silicon oils.
B. Metal particles
For proper utilization of this technology we need such type of particles which can magnetized easily and quickly therefore we use metal particles. Metal particles used in the MR- technology are very small. Size of the particle is approximate of the order of 1 µ m to 7µm. commonly used metal particles are carbonyl iron, powder iron and iron cobalt alloys. Metal particles of these materials have the property to achieve high magnetic saturation due to which they are able to form a strong magnetizing chain. The concentration of magnetic particles in base fluid can go up to 50% (approx.)
C. Additives
It is necessary to add certain additives to MR fluid for controlling its properties. These additives include stabilizers and surfactants. Surfactants serve to decrease the rate of settling of the metal particles. While the functions of additives are to control the viscosity of the fluid, maintain friction between the metal particles and to reduce the rate of thickening of the fluid due to long term use of the fluid thus additives also increase the life of the MR fluid. Commonly used additives are ferrous oleate and lithium stearate. All the three components of an MR fluid define its magneto rheological behavior. Changing any one component will result in change in the Rheological and magneto rheological properties of the MR fluid. An optimum combination of all the three components is necessary to achieve the desirable properties of an MR fluid.
Magnetorheological materials are prepared by adding a total of 117.9 g carbonyl iron powder (MICROPOWDER-S-1640, GAF Chemicals Corporation) to a corresponding carrier fluid as specified in Table 1
The magnetorheological material is made into a homo¬ geneous mixture through the combined use of low shear and high shear dispersion techniques. Specifically, the particles and carrier fluids are initially mixed by hand, and then more thoroughly dispersed using a high speed disperserator equipped with a 16-tooth rotary head. The weight amount of the iron particles present in each of the magnetorheological materials is equivalent to a volume fraction of 0.30. The magnetorheological materials are stored in polyethylene containers.
Properties of MR-fluids
Off-state viscosity :The field-independent viscosity (η) is the most critical off-state property of MR fluids since it has a direct impact on the velocity-dependent minimum output force or torque of a given device in the absence of magnetic field. Further more, this viscosity is also responsible for the temperature dependance of the device output force or torque. The MR-fluid viscosity is mostly influenced by two factors: the intrinsic viscosity of the carrier fluid and the particle volume fraction. The higher the particle volume fraction, the higher the MR-fluid viscosity. At room temperature, most MR-fluid viscosities range from 50 to 200mPas (Carlson, 2009).
Yield stress :The field-dependent maximum yield stress (τy) is the most critical on-state property of MRfluidssince it has a direct impact on the maximum output force or torque of a given device.As already discussed the material of the particles has an impact on the maximumyield stress since its value increases with the square of the saturation magnetization ofthe particles (Carlson and Jolly, 2000).A second factor influencing the maximum yield stress is the particle volume fraction. Rabinow,in 1948, already demonstrated that increasing the particle volume fraction led to anincrease of the output torque of his MR-fluid clutch. Since then, a number of researchershave studied this effect and have shown that the maximumyield stress increases non-linearly with growing particle volume fraction .theoff-state plastic viscosity also increases with particle volume fraction, at an even faster ratethan the yield stress (Figure 2.5b), leading to a decrease of the potential dynamic range (ratiobetween maximum and off-state force or torque) of a device using such fluids.An alternative way to increase the maximum yield stress is to increase the particle size distributioninside the MR-fluid. The advantage of this technique is that it allows the viscosity tobe reduced while maintaining the same particle volume fraction. A particular case of this technique is to use bimodal particle distributions, where two differentsize groups of particles are combinedAs shown in, a substantial increase in yield stress can be achieved by a smallincrease in the proportion of small particles (25% in weight). This effect can be explained byan increased particle packing when chains are formed.shows graphs of the yield stress (τy) versus magnetic field (H) for some typicalMR-fluids from LORD corporation and ISC Fraunhofer Institut Silicatforschung (data obtainedfrom productdatasheets).
1.9 Modes of operation
According to the fluid flow modes, all devices that use MR fluids can be classified ashaving either
(a) a valve mode (flow mode);
(b) a direct shear mode (clutch mode);
© a squeeze film compression mode; or
(d) a combination of these modes.
The schematic of the operational modes are shown in Figure
Valve mode as an operational mode is employed in servo valves, dampers, shock absorbers and actuators. In this mode the magnetic field is perpendicular to the flow of the MR Fluid while the two plates which are electrodes of the magnetic circuit are stationary plates. Since, the size of the gap is significantly smaller than the radius of the valve; the fluid flow is typically modeled by the parallel plate assumption. The developed pressure drop in this mode (for example in a damper) is the summation of the viscous (pure rheological) component due to MR fluid viscosity and the yield stress dependant component due to yield stress of the MR fluid.
Shear mode is typically used in clutches, brakes, chuckling and locking tools. In this mode the electrode plates have relative motion and the magnetic field is perpendicular tothe flow of the MR Fluid. In this operational mode, the total force has also two parts namely viscous and yield stress dependant component.The third operational mode is called squeeze mode which is less well studied than the other modes. Since, it offers the possibility of very large forces; it is used for smallamplitude vibration and impact dampers.
As mentioned before, the magnetic field lines are typically perpendicular to the
fluid flow direction in all modes. The areas where MR fluid is exposed to magnetic flux lines are referred to as “activation regions”.Among these, valve mode is the most widely utilized operational mode.
4MR fluid dampers
Shock absorbers: A shock absorber is basically a hydraulic damping mechanism for controlling spring vibrations. It controls spring movements in both directions: when the spring is compressed and when it is extended, the amount of resistance needed in each direction is determined by the type of vehicle, the type of suspension, the location of the shock absorber in the suspension system and the position in which it is mounted. Shock absorbers are a critical product that determines an automobile's character not only by improving ride quality but also by functioning to control the attitude and stability of the automobile body.
2.5 Shock absorbers offer these advantages:
• Increases operating speed
• Smoother deceleration
• Self-compensates for load changes
• Minimizes shock load to equipment
• Reduces equipment maintenance
• Higher equipment productivity
2.6 Principle Of Operation
The damping mechanism of a shock absorber is viscous damping. Viscosity is the property of a fluid by virtue of which it offers resistance to the motion of one layer over the adjacent on. The main components of a viscous damper are cylinder, piston and viscous fluid. There is a clearance between the cylinder walls and the piston. More the clearance more will be the velocity of the piston in the viscous fluid and it will offer less value of viscous damping coefficient. The basic system is shown below. The damping force is opposite to the direction of velocity.
-CLEARNCE, II-PISTON, III-VISCOUS FLUID
The damping resistance depends on the pressure difference on the both sides of the piston in the viscous medium. The figure shown below shows the example of free vibrations with viscous damping. The equation of motion for the system can be written as mx + cx +kx = 0
A shock absorber is a mechanical device designed to smooth out or dampshock impulse, and dissipatekinetic energy. It is a type of dashpot.
2.7 Vehicle suspension
All vehicles are equipped with some form of suspension damping control. They may use hydraulic fluid, air, gas or be electrically controlled. The two most widely used internal designs are twin-tube and monotube.
The twin tube design is the most common one in use on cars, light trucks, SUV’s and vans. It’s a cost effective unit that provides excellent handling & control characteristics for most driving conditions. The monotube design offers additional performance and can have a more aggressive ride.
In a vehicle, shock absorbers reduce the effect of traveling over rough ground, leading to improved ride quality and vehicle handling. While shock absorbers serve the purpose of limiting excessive suspension movement, their intended sole purpose is to damp spring oscillations. Shock absorbers use valving of oil and gasses to absorb excess energy from the springs. Spring rates are chosen by the manufacturer based on the weight of the vehicle, loaded and unloaded. Some people use shocks to modify spring rates but this is not the correct use. Along with hysteresis in the tire itself, they damp the energy stored in the motion of the unsprung weight up and down. Effective wheel bounce damping may require tuning shocks to an optimal resistance.
Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars are used in torsional shocks as well. Ideal springs alone, however, are not shock absorbers, as springs only store and do not dissipate or absorb energy. Vehicles typically employ both hydraulic shock absorbers and springs or torsion bars. In this combination, "shock absorber" refers specifically to the hydraulic piston that absorbs and dissipates vibration.
2.8 Types of vehicle shock absorbers
Most vehicular shock absorbers are either twin-tube or mono-tube types with some variations on these themes.
mono-tube :The monotube design has a single cylinderThe cylinder is divided into sections: A fluid area and a gas chamber. The piston and shaft move in the fluid portion. It uses a single fluid valve assembly in the piston. The diameter of the single working cylinder and piston valve is larger than in a twin tube even though the outside dimensions of each may the same. There is no need for an air or gas in the fluid area so the valve can operate more responsively and without any aeration or performance fade. The high pressure gas chamber is separated from the fluid area by a floating piston & seal. That provides an expansion area for the excess fluid movement during the compression stroke. On more aggressive movement the floating piston is pushed further into the gas chamber which increases gas pressure quickly and provides additional damping force. Because of its higher performance capabilities, the monotube design is used as original equipment on some vehicles and offered as an upgrade on vehicles that came equipped with the twin tube design.