Magneto-Rheological (MR) Fluids

Transcription

1 Magneto-Rheological (MR) Fluids Harish Hirani Associate Professor Department of Mechanical Engineering Indian Institute of Technology DELHI Lubrication & Bearings

2 Used in Japan's National Museum of Emerging Science & China's Dong Ting Lake Bridge to counteract vibrations caused by earthquakes and gusts of wind MagnetoRheological Fluids Dr. H. Hirani Mechanical Engnieering, IIT Delhi

3 RHEOS (Greek word) = to FLOW (English word) RheoLOGY= Science of material flow under external load conditions MAGNETOrheological FLUID= Fluid, whose apparent viscosity increases, with application of MAGNETIC field. Liquids that harden or change shape when they feel a magnetic field

4 Soft magnetic particles Particle Sizes!!!!! Carrier liquid Application of magnetic field, polarizes and align magnetic particles. Particle chain formation limits particle movement, which in turn limits the movement of the fluid.

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8 MR Fluids: Consist micron (1-10 μm) sized, magnetically polarizable (soft magnets) dispersed in a carrier liquid such as mineral, silicone oils, kerosene, water. Particles > 10 μm~ unstable against settling Stoke's settling velocity = 2 ( ρ ρ ) P f 9η ga 2 i

9 MR Fluids: Consist micron (1-10 μm) sized, magnetically polarizable (soft magnets) dispersed in a carrier liquid such as mineral, silicone oils, kerosene, water. Particles > 10 μm~ unstable against settling Stoke's settling velocity = 2 ( ρ ρ ) P 9η Particles < 1 μm destabilizing effect of Brownian motion dominates f gr 2

10 Required Particles properties Permeability: degree of magnetization of a material that responds linearly to an applied magnetic field. μ 0 (=4π 10 7 N/A 2 ) is known as permeability of free space.

11 Required Particles properties. Relative Permeability Iron 2000 Nickel 100 Permalloy (78.5% nickel, 21.5% iron) ,000 Mu metal (75% nickel, 2% chromium, 5% copper, 20,000-18% iron) 100,000

12 Required Particles properties. The applied field where the data (called a magnetization curve) crosses zero is the coercivity. Saturation Limit: The limit of applied field at which all the magnetic domains align with the field, and the magnetic-curve flattens out. Coercivities of soft and hard magnets Material Coercivity Permalloy, Ni 81 Fe Co 20 Ni 150 Alnico, a common refrigerator magnet NdFeB 10,000 SmCo 5 40,000

13 Material Approx % Composition Fe Ni Co Mo Other Maximum permeability Iron ,000 21,500 Purified iron ,000 21,500 Permalloy Mn 100,000 10,700 Mu metal ,000 6,500 Saturation flux density B gauss Particle size Permeability Saturation Non-magnetic carrier fluid How many particles???????

14 MR Fluids: 20-50% by vol. Magnetic particles (higher vol. Increase off state viscosity).

15 Yield Stress (kpa) at 100 (1/s) MRF36L MRF36S MRF36M1 MRF36M Magnetic Field, H (ka/m)

16 MR Fluid Properties & Newtonian fluid, τ = ηγ η is plastic viscosity non - Newtonian fluid, τ = τ o + ηγ& In addition to plastic viscosity, elastic viscosity ( τ 0 / & γ ) Apparent viscosity η = τ & γ Fluids do not flow until the applied shear stress crosses a threshold value called the yield stress.

17 % by Volume Iron particle Off-state viscosity of MR fluids (Pas) % by Volume Iron particle Shear Rate (1/s) Special consideration for high shear rate application Apparent viscosity η = τ & γ

18 MR-FLUID.. Make device smart by changing system s properties( stiffness, damping, viscosity, shear modulus) in a desirable manner. S y ~ kpa Useful in active control of vibration & motion, i.e. engine mount, shock absorbers, seat dampers, variable resistance equipment, etc. Motion damping is perhaps the most practical use for MR technology today

19 MR Fluids: times stronger than ER fluids, lower sensitivity to impurities. Property MR fluids ER fluids Max. yield stress τ kpa 2-5 kpa Maximum field ~250 ka/m ~4 kv/mm Apparent plastic viscosity η pa-s pa-s Operable temp. range o C o C Stability Unaffected by most impurities Cannot tolerate impurities Density 3-4 g/cm g/cm 3 Maximum energy density 0.1 Joules/cm Joules/cm 3 Power supply (typical) 2-50 V, 1-2 A V, 1-10 ma

20 Properties of three different types of MR fluids MR fluid MRF-132LD MRF- 240BS MRF-336AG Fluid base Synthetic oil Water Silicone oil Operable temp. range o C Density (g/cc) Weight percent solids 80.74% 83.54% 82.02% Coefficient of thermal expansion Specific 25 o C (J/g o C) Thermal conductivity (w/w o C) Flash point ( o C) > 150 >93 > s -1 /50s -1 (Pa-sec) 0.94/ /

21 Geometries for MR Fluid Most devices that use MR fluids can be classified as having: Fixed poles (Pressure driven flow mode) Servo-valves, dampers and shock absorbers Relatively moveable poles (Direct-shear/sliding mode). Clutches, brakes, chucking and locking devices. Both of these configurations can be described by assuming MR fluid is dispersed between two parallel plates. In the sliding plate (or shear) mode the MRF is stationary and the walls/electrodes move. In the fixed plate (or flow) mode the walls/electrodes are stationary and the fluid moves. Squeeze-film mode Low motion and high force applications

22 Small element of Fluid with sides dx, dy, and dz Force balance: τ p pdy. dz + τ + dy dx. dz = p + dx dy. dz + τdx. dz y x Shear flow mode Pressure flow mode τ = 0 y τ P = y x

23 Application of MR Fluid in Brakes? In friction brakes, two surfaces are pressed together with a normal force to create a friction torque. Disk brake: Flat surface to axis of rotation, and normal force is axial. Drum brake: Cylindrical surface with normal force in radial direction

24 At least one the friction surface is metal (cast iron or steel) and other is usually a high friction material, referred to as lining. Sacrificial. Maximum contact pressure, b p max Maximum bulk temperature, t m, max Coefficient of friction, µ psi kpa F C Friction material a Molded Woven Sintered Metal Cork Wood Cast iron; hard steel a When rubbing against smooth cast iron or steel. b Use of lower value will give longer life

25 Properties of Braking materials Operating in Oil Friction material a Coefficient of friction, µ Molded Woven Sintered Metal Paper Graphitic 0.12 (avg) Polymeric 0.11 (avg) Cork Wood Cast iron; hard steels a When rubbing against smooth steel or cast iron. Brake wiping mechanism!!!!!

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27 MRF Bearing cum Brake Amplification factor RPM 1200 RPM AF = T T i= I i= Current, A 3/14/

28 h r yd ω η τ τ + = dr h r rw dr h r rh T r r yd r r yd = ηω τ π ηω τ π ( ) ( ) ( ) ( ) r r h w r r w r r r r h T yd yd = ηπω τ π ηπω τ π

29 MR Brake Title: Magnetorheological brake operating under shear, squeezing and valve mode. Inventor: H. Hirani and C. Sarkar Application number: 2530/Del/2013 3/14/

30 Application of MR Fluid in Dampers? The practical necessities often require attenuation of the vibrations. Passive Damper Active Damper Semi-active Damper

31 Application of MR Fluid in Engine Mount? Basic Function: To connect the Engine firmly to Chassis / Frame. Vibration isolation, to reduce vibrations transmitted from the engine to the frame. Damper Attacks the source of vibration and reduces the movement of vibration source. Isolator Prevents the transmission of vibration from the vibration source to another part.

32 Mount Advantages Disadvantages Passive Active:Electromecha nical actuators, piezoelectric elements along with closed loop feed back system. Semi-active: combination of the active and passive isolator Easy to design, availability, shows better performance at tuned frequency Ability to adapt to varying operating conditions Optimizing the mount effectiveness under all conditions. Even though the actuation or feed back system fails it functions as a passive mount serving the purpose. Can t change response as per change in input frequency. Costs.Failure of any component brings system to stand still condition. Cost higher than the passive type of mount.

33 Dynamic Stiffness Frequency Dynamic Stiffness of an ideal engine mount

34 Application of MR Fluid in Engine Valves Ex: In most of engines, the valve timings and lift are optimized for one set of operating conditions. engine operates in various load and speed conditions. Consequently, to optimize the engine performance in any condition and any circumstance, a need exists for a device that permits variable valve actuation. Variable lift/timing (VVA) to exploit benefits: Fuel economy Elimination of throttling Reduction in emission. Reference: SAE , SAE , SAE , SAE , SAE ,..

35 Natural frequency rpm Tribo-Pairs: Cam-pushrod pushrod-rocker arm rocker arm-rocker shaft rocker arm-valve valve-valve-guide valve-seat push rod guide

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38 Variable Lift Valve Mechanism Valve lift vs crank rotation Valve lift, mm Amp current 1 Amp current 0 amp Crank rotation, degrees 3/14/2014 Total slides = 27 38

39 Computer Delta converte r Valve Optical Displacement sensor Dc Power supply AC Motor 4 Stroke Diesel engine Electro Magnet Maximum Valve lift(mm) Maximum valve lift Vs Product of speed and Current Experimental values Least square fit Product of Cam speed(rpm) and Current(Amps) Valve_Lift = (N.I) /14/

40 Thank U Variable spring stiffness.

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