MODELING, TESTING AND EVALUATION OF MAGNETO-RHEOLOGICAL ... · PDF file71 Int. J. Mech. Eng. &...

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Int. J. Mech. Eng. & Rob. Res. 2013 Ashwani Kumar and S K Mangal, 2013

MODELING, TESTING AND EVALUATION OFMAGNETO-RHEOLOGICAL SHOCK ABSORBER

Ashwani Kumar1 and S K Mangal1*

*Corresponding Author: S K Mangal, [email protected]

This paper presents design, testing and evaluation of a Magneto-Rheological (MR) shockabsorber. The devices, which fill the gap between purely passive and fully active control systems,offer a reliability of passive systems and yet it maintains the versatility and adaptability of the fullyactive devices. These devices are popularly known as semi-active control systems. In thesedevices, after application of a the magnetic field, the fluid changes from liquid to semi-solid statein milliseconds thus results in an infinitely variable, controllable shock absorber capable ofgenerating large variable damping forces. The advantage of the MR shock absorbers over theconventional fluid shock absorbers are many, e.g., simple in construction, needs little power,quick in response, few moving parts, etc. In this paper the basic theory behind the MR Shockabsorbers, its testing and evaluation is carried out and its use in vibration control is also studied.For this purpose, a MR shock absorber from Lord Corp. USA is bought and tested in theLaboratory using an electrodynamics vibration shaker and associated data acquisition system.Its performance is then studied in the form of damping force, displacement with respect to time.

Keywords: Magneto-rheological fluid, MR shock absorber, Semi-active control system

INTRODUCTIONSome smart materials have the ability tochange from a liquid to a solid almost instantlywhen placed near a magnet. These materialshave multiple properties, e.g., electrical,magnetic, mechanical and thermal and cantransform energy that can be altered usingsome external f ields. The Magneto-Rheological (MR) fluids are field responsiverheology where their fluid and other properties

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Int. J. Mech. Eng. & Rob. Res. 2013

1 Mechanical Engineering Department, PEC University of Technology, Chandigarh, India.

are controlled by varying the external magneticfield. The discovery of MR fluids is credited toRabinow (1948) at the US National Bureau ofStandard. Typical MR fluids are thesuspensions of micron sized, magnetizableparticles (mainly iron) suspended in anappropriate carrier liquid such as mineral oil,synthetic oil, water or ethylene glycol, etc. Thecarrier fluid serves as a dispersed mediumand ensures the homogeneity of particles in

Research Paper

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the fluid. A variety of additives (stabilizers andsurfactants) are also used in the MR fluid toprevent gravitational settling and to promote astable particles suspension, to enhancelubricity and to change initial viscosity of theMR fluids (Ashwani and Mangal, 2012). Thestabilizers serve the purpose to keep theparticles suspended in the fluid, whilst thesurfactants are absorbed on the surface of themagnetic particles in order to enhance thepolarization induced in the suspendedparticles on application of the magnetic field.The size of the magnetizable particle rangesfrom 3 to 5 microns in the MR fluid used for theshock absorber application.

When no magnetic field applied on the MRfluid, the ferrous particles are randomlydispersed in the medium (Figure 1a). In thepresence of a magnetic field, the particles startto move in order to align themselves along thelines of magnetic flux, Figure1b. Figure 1cshows the formation of chains of ferrousparticles, creating more viscosity andincreased shear strength (Ashwani andMangal, 2010a). As this change occurs almostinstantly, the MR fluids are attractive solutionsfor real-time control applications, e.g., shockabsorber, brakes, clutches, engine mounts,valves, etc. The changes of liquid-solid-liquid

state or the consistency or yield strength of theMR fluid can be controlled precisely andproportionally by altering the strength of theapplied magnetic field.

In the absence of an applied magnetic field,the MR fluids are reasonably well approximatedas Newtonian liquids. For most engineeringapplications, a simple Bingham plastic modelis effective to describe its essential, field-dependent fluid characteristics. A Binghamplastic is a non-Newtonian fluid whose yieldstrength must be exceeded before the flowbegins (Siginer, 1999). Typical MR fluid canachieve yield strengths up to 50-100 kPa atmagnetic field strength of about 150-250kA/m. These MR fluids are stable intemperature ranges from—50 °C to 150 °C.

In this paper, the basic theory behind theMR Shock absorbers, its testing andevaluation is carried out and its use invibration control is also studied. For thispurpose, a MR shock absorber from LordCorp. USA is bought and tested in theLaboratory using an electrodynamicsvibration shaker and associated dataacquisition system. Its performance is thenstudied in the form of damping force,displacement with respect to time.

Figure 1: Illustration of Activation of the MR Fluid

(a) No Magnetic Field Applied (b) Magnetic Filed Just Applied (c) Ferrous Particles Aligned

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MAGNETO-RHEOLOGICAL(MR) SHOCK ABSORBERA typical MR shock absorber consists of acylinder, piston, excitation coil and the MR fluidwhich is enveloped in the cylinder. As thisdevice act on the magnetic effect, the MRshock absorber should have optimized designto fulfill its magnetic performance andeventually to achieve a satisfactory semi-activecontrol device. The MR shock absorber offersa highly reliable operation and can be viewedas fail-safe device if the control hardwaremalfunctions and the absorber turns to passiveone. Figure 2 shows schematic example of theMR fluid shock absorber. The operation of theMR device is fundamentally different from thatof MR brakes and clutches (Ashwani andMangal, 2010b).

does have the departures from this modeleven then this gives a good reference for thebehavior of the fluid (Carlson and Jolly, 2000;Poyner, 2001; Seval, 2002; and Ashwani andMangal, 2010b). The shear stress associatedwith the flow of the MR fluid can thus bepredicted by the Bingham equations(Spencer et al., 1996). In this model, the totalfluid shear stress is given by:

Hy y ...(1)

where y is the yield stress [Pa], H is the

Magnetic field strength [A/m], is the

Plastic viscosity [Pa.s] and is the Shear

strain rate [s–1].

For the total fluid shear stresses below y,

the MR Fluid behaves as visco-elastic materialand is given as:

G y ...(2)

where the G is the complex material modulusand is the fluid shear strain.

The MR shock absorbers generally use theflow mode of the fluid. In the flow mode, thepressure has two components of the pressuredrop, i.e., pressure loss due to the viscousdrag and pressure loss due to the fielddependent yield stress (Carlson and Jolly,2000). The total pressure drop (P) in the fluidcan be given as (Spencer et al., 1996).

g

Lc

wg

QLPPP y

n

3

12...(3)

where P is the viscous pressure loss, P

is the field dependent pressure loss, is thefluid viscosity, Q is the flow rate, L is the polelength, w is the pole width, g is the fluid gap,

y

is the field dependent yield stress and c is

Figure 2: Schematic Representationof MR Shock Absorber

In the absence of an applied field, MR fluidsare reasonably well approximated asNewtonian liquids, i.e., off-state. In the onstate (when activated under magnetic field)the fluid behaves as a Bingham plastic withvariable yield strength. All though the fluid

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constant. Its value varies between 2 to 3 whichdepends upon the ratio of field dependentpressure loss to viscous pressure loss.

The volume of fluid exposed to the magneticfield controls the desired MR effect (Spenceret al., 1996; and Vibration and Seat Design,2001) and is known as active fluid volume. Theabove equations can be manipulated todetermine the active fluid volume (V) and isgiven as:

my

WkV

2 ...(4)

where k is a constant and is the desiredcontrol ratio required to achieve a specifiedmechanical power, W

m (Spencer et al., 1996).

These variables can be defined as:

PQWP

P

ck m

;;12

2 ...(5)

The Equation (4) can be further solved toprovide constraints and aspect ratios for anefficient use of the MR fluid (Spencer et al.,1996) as:

Qc

wgy

122...(6)

DESIGN CONSIDERATIONOF MR SHOCK ABSORBERA magnetic field in the flow path of the MR fluidneeds to be generated to use the MR shockabsorber effectively. The magnetic field isapplied by using a copper coil which iswounded around the piston body as shown inFigure 3 with hatching. The leads of the coilare taken out through the piston rod to givethe variable current to the coil in order togenerate the variable magnetic field which inturn produces variable damping effect. Theconstruction of the shock absorber is simpleand easy to manufacture.

Figure 3 shows the conceptual design ofthe MR shock absorber. The Spool of magnetwire generates magnetic flux within the lowcarbon steel piston. The flux in the magneticcircuit flows axially through the piston core ofdiameter D

c, through the piston poles of length

Lp and through a MR fluid gap of thickness t

g

and axially through the cylinder wall of thicknesstw. The MR shock absorber design involves

different dimensions of the piston, cylinderassembly, i.e., diameter of the cylinder bore(D

b), the diameter of the piston rod (D

r), the

thickness of the cylinder wall (tw), the diameter

Figure 3: MR Shock Absorber

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of the piston core (Dc), the outside piston

diameter (Dp), the pole length (L

p) and the

thickness of the MR fluid gap (tg). The total

pressure drop across the piston can bedetermined by substituting the appropriatesymbols of the Figure 3 in the Equation (3).This is given as:

g

y

ggp

pwn t

c

ttD

LLQPPP

3

212

...(7)

The force generated in the MR shockabsorber is the pressure drop times the pistoncross section area. It can be given as:

4

22rgp DtD

PF

...(8)

MR SHOCK ABSORBER ANDEXPERIMENTATIONA MR shock absorber as shown in Figure 4has been bought from the LORD Corporation

USA, i.e., RD-8040-1 (short stroke). In this MRshock absorber, MR fluid flows from a highpressure chamber to a low pressure chamberthrough an orif ice in the piston head(www.lord.com). The MR shock absorber iscompact and is suitable for industrialsuspension applications. Continuouslyvariable damping is achieved by developinga variable magnetic f ield strength(www.lord.com). The technical specificationsof the shock absorber (www.lord.com) is asunder

1. Stroke 55 mm

2. Extended Length 208 mm

3. Body Diameter 42.1 mm

4. Shaft Diameter 10 mm

5. Tensile Strength 2000 N

6. Shock absorber Forces (Peak to Peak)

a) 5 cm/sec @ 1 A >2447 N

b) 20 cm/sec @ 0 A <667 N

7. Operating Temperature range 0 to 71 °C

The shock absorber is tested to check itsperformance. The setup of the lab as shown inFigure 5 is used for this purpose. In the setup,an Electro-Dynamic Vibration (EDV) shakerhaving a rated sine force of 200 kgf with afrequency range between 1 to 3500 Hz is used.The shaker has a maximum displacement(peak to peak) of 20 mm. The Shaker can beused for both horizontal and vertical vibrationalanalysis. The shaker is controlled by PC BasedDigital Vibration Controller cum analyzer withbuilt-in signal conditioner unit. The controllerunit of the shaker is a close loop controlsystem. The set-up has a compatible dataacquisition and instrumentation system which

Figure 4: MR Shock Absorber

Source: www.lord.com

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gives the data in the form of force, velocity,displacement and acceleration with respect totime. On the shaker system, the MR shockabsorber is mounted. The test was performedfor number of cycles for different current values,i.e., 0 A, 0.1 A and 0.2 A across the coils at afrequency of 1 Hz. The force experienced bythe piston rod, which was prevented from

motion, was sensed by a load cell fixed at thetop of the MR shock absorber while thedisplacement is recorded throughdisplacement sensor.

RESULTSFigure 6 shows the variation of damping forcev/s displacement of the shock absorber for

Figure 5: Experimental Setup for the Testing of MR Shock Absorber (Mounted)

Figure 6: Force vs. Displacement Curves with Different Levels of Current

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one cycle. The Figure shows that as the valueof current increases the damping forceincreases and the displacement of the pistonis decreased. The damping force is low forzero current and it increases gradually as thecurrent is increased. The variation of forcewith displacement are re-plotted in Figure 7for each current setting, i.e., 0.0, 0.1 and 0.2

A for large number of cycles. The Figure 8shows the variation of displacement with timefor a large number of cycles for the samecurrent settings. The Figure 9 shows thevariation of damping force with respect topeak-peak displacement of the shockabsorber at the above mentioned threecurrent setting.

Figure 7: Force vs. Displacement Curves at (a) 0.0 A, (b) 0.1 A and (c) 0.2 A

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Figure 7 (Cont.)

Figure 8: Displacement vs. Time Curves at (a) 0.0 A, (b) 0.1 A and (c) 0.2 A

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Figure 8 (Cont.)

Figure 9: Damping Force vs. Peak to Peak Displacement Curves with Different Levelsof Current

CONCLUSIONIn this paper, the basic theory behind the MRShock absorbers, its testing and evaluationis carried out. A MR shock absorber fromLord Corp. USA is bought and tested in thelaboratory. The results presented in this papershow the good efficiency of vibrationdamping. The above results show that the

user can have a very good control over thedamping force of the MR shock absorber byvarying the input current which is supplied tothe magnetic coil of the absorber. This is anecessity for a semi-active vibration controlsystem. The result further showed that thedamping force is not zero at zero input current,i.e., at off-state. It is because of the fact that

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fluid viscosity alone is responsible for thedamping effect in the off-state. It is equivalentto failure/mal-function of the electro-magneticcoil or fail-safe condition.

REFERENCES1. Ashwani Kumar and Mangal S K (2010a),

“Comparative Study of Vibration-ControlSystems”, On-CD, in the InternationalConference on Engineering Innovations:A Fillip to Economic Developments, Heldat Continental Group of Institutes,Jalvehra, Fathegarh Sahib, Punjab Heldon February 18 to 20.

2. Ashwani Kumar and Mangal S K (2010b),“Mathematical Modeling of ER/MRDamper Systems Using Herschel-BulkleyModel”, in the National Conference onGlobal Trends on MechanicalEngineering, held at R.B.I.E.B.T., MohaliCampus, Punjab from April 16 to 17,pp. 123-128.

3. Ashwani Kumar and Mangal S K (2012),“Properties and Applications ofControllable Fluids: A Review”,International Journal of MechanicalEngineering Research, Vol. 2, No. 1,pp. 57-66.

4. Carlson J D and Jolly M R (2000), “MR Fluid,Foam and Elastomer Devices”,Mechatronics, Vol. 10, Nos. 4-5, pp. 555-569.

5. Poyner C James (2001), “InnovativeDesigns for Magneto-RheologicalDampers”, MS Thesis, Advanced VehicleDynamics Lab, Virginia Polytechnic Instituteand State University, Blacksburg, VA.

6. Rabinow J (1948), “The Magnetic FluidClutch”, Transactions of the AIEEE,Vol. 67, pp. 1308-1315.

7. Seval G (2002), “Synthesis andProperties of Magneto Rheological (MR)Fluids”, Ph.D. Thesis, University ofPittsburgh.

8. Siginer D A (1999), “Advances in the Flowand Rheology of Non-Newtonian Fluids”,Elsevier.

9. Spencer B F Jr, Dyke S J, Sain M K andCarlson J D (1996), “PhenomenologicalModel of a Magneto-RheologicalDamper”, ASCE Journal of EngineeringMechanics, pp. 1-23.

10. “Vibration and Seat Design” (2001), LordCorporation White Paper, Thomas LordResearch Center, Cary, NC.

11. www.lord.com

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