Design and Modelling of a Semi-active Helicopter Seat Cushion

Nicole Tiago da Cruznicole.cruz@tecnico.ulisboa.pt

Instituto Superior Tecnico, Lisboa, Portugal

November 2016

Abstract

Helicopters are susceptible to excessive vibratory loads that lead to poor flight ride quality. Themain goal is to help mitigate structural vibration through the performance evaluation of a novel seatcushion. Magnetorheological materials present a promising approach for this purpose. The uniqueproperties of MR materials - their ability to mechanically respond to magnetic fields and their charac-teristic material non-linearity - make designing these applications a challenge. The MR seat cushionconsists of a distributed magnetic material embedded in elastomers and various electromagnetic coils.Two commercial software(COMSOLR© and FEMM), capable of modelling complex electromagneticproblems and non-linear materials, have been used. A detailed study of each material is carried out, interms of design and response as well as the definition of the geometry, properties, boundary conditionsand mesh convergence. The study of analytical and computational models to quantify the performanceof the MR material (response output) has demonstrated that the proposed helicopter seat cushionsystem is effective in reducing the vibration felt by the pilots. Based on all results, a conceptualfinal design configuration is proposed as well as a SimulinkR© block diagram to represent the cushionresponse and a one-degree of freedom control model. This is expected to be the main solution adoptedin the future. The seat cushion design proposed has been performed in collaboration with the NationalResearch Council Canada and it is going to be manufactured and tested.Keywords: helicopter seat cushion, magnetorheological materials, finite element modelling, magneticcircuit, design, control

1. Introduction

Nowadays, the harmful effects on human perfor-mance and health issues caused by undesired vibra-tion transmitted through vehicle seats have beenof increasing concern. Even though the helicopterhas became a versatile mode of aerial transporta-tion, due to its unique capability to take-off andland vertically as well as its ability to hover, therehave been increasing complaints of fatigue, discom-fort and pain during prolonged exposures to thisvibration. This type of exposure interferes with theoperational performance by degrading situationalawareness that may affect decision making.

The sources of this vibration, among all others,are the main hub reactions to the blade passage fre-quency; blade vortex interaction (BVI); gusts andblade stall. Due to the complex coupling betweenthe rotor system, airframe, transmission systemand engine, the vibratory loads and noise energyare transmitted throughout the helicopter structureand contribute to poor ride quality for passengersand crew. Exposure to high intensity, low frequency- 4 to 80 Hz - noise can cause Whole-Body Vibration(WBV), which has become an increasingly signifi-

cant area of concern in helicopter seat design [8].Since there is nothing the pilot could do in flightto minimize the pain, it becomes a distraction thatcould jeopardize the security of the flight, missionor passengers.

Considerable efforts have been undertaken to re-duce helicopter vibration. Many new devices anddesign modifications have been implemented on in-service helicopters, and achieved a few performanceimprovements. However, revolutionary approacheshave to be developed to mitigate helicopter vi-bration. In order to successfully reduce vibrationlevel, vibration cancellation on seats technologiesand techniques (locally) has attracted significant in-terest in recent years, since they have been consid-ered a low-cost strategy for improving comfort andmitigating certain vibration.

This research focuses on the use of magnetorheo-logical materials for enhanced occupant protectionand comfort in order to present a semi-active seatcushion system, which is expected to be the leastcostly change with the least impact in certificationand the most effective, rather than modify or re-place something in the structure itself.

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Magnetorheological Fluid and Elastomer belongto a class of materials that are known as “smartmaterials”. The physical attributes of smart ma-terials can be altered through the application of anexternal stimuli such as stress, electric, magnetic, orthermal stimulation. The term “magnetorheologi-cal” comes from a combination of magneto, mean-ing magnetic, and rheo, the prefix for the study ofdeformation of matter under applied stress.

This project falls within the framework of theNational Research Council Canada and is going tobe a contribution of the University of Victoria, inCanada that will investigate the use of novel semi-active methodologies integrated in helicopter seatsto mitigate the aircrew exposure to high vibrationlevels.

The proposed seat cushion design, in the end, isgoing to be manufactured experimentally tested.

2. Background and Preliminary Design

The author gives a review on magnetorheologi-cal materials (MRE - magnetorheological elastomeror MRF - magnetorheological fluid) and the tech-niques used to control vibrations. Preliminary de-sign is concerned with the initial development ofthe project; what means and what is involved in aseat cushion; the first calculations performed in or-der to help on chosing a viable material to produceand evaluate if the first configuration thought is thebest one or not for this purpose.

2.1. Magnetorheological Materials

The magnetorheological fluids are a type of smartfluid, which change elasticity, plasticity and viscos-ity properties in the presence of a magnetic field,that are composed by ferromagnetic particles (usu-ally iron powder typically 3 to 5 microns) sus-pended in a non-magnetic carrier medium (oil orwater) and a few additives. One great supplier andwidely requested company is LORD Corporation.In recent years, there has been a renewed interestin MR fluid devices, for example, to use in cycling,dampers, seat suspensions and prostheses or evenpolishing optical lenses [9]. MR Fluids can be usedin flow mode, known as valve mode (fluid flowingthrough an orifice) or in shear mode (fluid shearingbetween two surfaces), depending on the functionof the device. The most common model used to de-scribe these materials is the Bingham plastic model:

τ = τy(H) + ηγ (1)

There are several more complex non-linear para-metric models that are used to describe these ma-terials such as Bouc-Wen model. When applyinga magnetic field, the material changes from a fluidstate to a semi-solid state. The fluids particles alignwith the direction of the field, as shown in figure 1.

Figure 1: MR Fluid solidifying in response to mag-netic field

Magnetorheological elastomers are, like the MRfluids, a kind of smart material, which usuallyhas three different components: a rubber or poly-meric matrix (non-magnetic); magnetically polar-izable particles and few additives. There are twopossible types: isotropic MRE (unstructured, curedwithout the presence of an imposed magnetic field)or anisotropic MRE (pre-structured, cured by ap-plying a magnetic field). One usual supplier of thismaterial is Ioniqa Technologies. The MRE has alsothree operation modes: shear mode; squeeze modeand field active mode. Applications for MRE in-clude adaptive tuned vibration absorbers, shock iso-lators, papermaking machine and engine mounts [5].

It has also been investigated a new design con-cept, using a magnetorheological fluid encapsulatedin a passive elastomer matrix. This new designoffers the combined effects of MRF controllabilityand elastomer flexibility so that both the dampingand stiffness properties of the MRF-E alter with thechange in the magnetic field strength. One can con-clude that MRE and MRF complement each other.

2.2. Vibration ControlVibration control is categorized as: active, pas-sive, or semi-active, based on the power consump-tion of the control system. Passive systems, a typeof solution that provides a moderate reduction invibration, comprise a range of materials and de-vices placed at specific points on the structure orequipment for enhancing damping, stiffness, andstrength. The main function of a passive system isdissipating energy and primarily consists of springsand dampers. One of its major disadvantages is thelack of adaptability, which means they only work fora specific frequency and cannot be adapted to dif-ferent environments. Active systems comprise forcedevices and sensors attached at specific points in thestructure or equipment together with controllers toimprove the overall system performance. This tech-nique can usually achieve a high performance foreliminating vibration, but it is too expensive, de-sign intensive, require a great deal of power to op-erate, and adding energy to a system can sometimescause instability. Semi-active solution (in which themechanical properties can be adjusted in real time)possesses the advantages of both passive and active

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solutions. The purpose of semi-active systems is todissipate energy while increasing the durability ofa system. In semi-active control, the properties ofthe actuators are dynamically modified, in order tooptimally damp the vibration of the system.

The control system consists of a structure em-ploying devices and sensors which is exposed todisturbances. Concerning to the area of seat sus-pensions, a number of control approaches have alsobeen proposed and developed to improve the seatsuspension performance. It was brought in a sim-ple, yet effective vibration isolation strategy that isfulfilled by connecting a fictitious damper betweenthe mass and the stationary sky, named SkyhookControl.

2.3. Conceptual DesignThe human body in a seated posture along withthe seat structure were modelled in the vertical di-rection as a mechanical system composed of rigidbodies linked by springs and dampers. A practicalapproach is to consider a Single Degree of Freedom(SDOF) system and to evaluate the stiffness of thesystem and the overall dimensions.

Figure 2: Semi-active vibration system

Natural vibration analysis of a system providesits dynamic characteristics. Based on figure 2, thereis the concept of relative motion that means themovement of the mass relative to the base, whenthis last one is excited by support motion. Theequation of motion of this system can be repre-sented as:

xr = (x− y) (2a)

mxr + cxr + kxr = −my (2b)

Further assuming the pilot to be a rigid body, thenatural frequency f of the pilot-cushion system canbe expressed as:

f =1

2π

√k

m(3)

While not mentioned as a requirement, there is aparticular consideration that has to be taken intoaccount as well during this preliminary design ofthe cushion: minimum required stiffness change.

The initial configuration proposed, in the begin-ning of this work, has a layer with two samples ofMRF and three of MRE (with an angled trapezoidalshape) and two coils in each part surronded by mag-netic material. One of the proposed challenges wasto estimate the stiffness generated from the MREin this kind of configuration. The angled trapezoidsassumed in the calculations act in shear and also incompression due to a vertical load. Therefore, oneshould resolve each angled face into two equivalentsprings.

To achieve the proposed frequency (5.4Hz), itrequires a very soft elastomer (much less than100kPa as Young’s modulus), which is impracti-cal. Some compromises (dimensions and feasabilityof the sample) were made and it ended up beingassumed a desired resonant frequency of 8.1Hz. Ithas resulted in a range of desired sitffness between123kN/m and 255kN/m. The magnetic particlesused for both materials should be soft particles (likecarbonyl iron) due to high permeability and highsaturation magnetization.

To design the different components, it is neces-sary to take into account some details. The con-ceptual design of the cushion was proposed by theNRC but the only constraint indeed is the total sizeof the seat - 50 x 50 cm - that has to fit in thathelicopter (Bell-412); which means that one is al-lowed to explore different inside configurations, di-mensions, placements, results and by the end try tocontribute to a better and new cushion.

This project takes part in the design, evaluationand optimization processes of several seat cush-ions configurations, and as a consequence it is ofthe utmost importance to use a CAD and FEAsoftware that allows the easy and swift modellingand assembly of general configurations. COMSOLMultiphysics R© (released in 2000) is the interactivesoftware for modelling and solving scientific andengineering problems based on partial differentialequations. This environment runs finite elementanalysis together with adaptive meshing. Only forthe magnetic circuit analysis it will be also usedas well the freeware software FEMM, which is apackage for solving problems in 2-D, planar or axialsymmetric that allows quicker simulations, a muchuser-friendly interface and material definition wayand it is the best solution as a beginner.

3. Finite Element Modelling

The finite element method is a numerical methodfor solving problems that involve complex physics,geometry or boundary conditions. This was carriedout using COMSOL Electromagnetic (AC/DC)Module and FEMM (Finite Element ModellingMagnetics) since it is essential that the software isable to calculate problems within the electromag-

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netic field.

3.1. Magnetic Circuit DesignAn optimal magnetic circuit design demands max-imizing magnetic field energy in the fluid gapwhile minimizing the energy losses in the steel fluxpath. The basic theory in the software is based onMaxwell’s equations [2] and its relations were em-ployed:

∇ ·B = 0 (4)

And Ampere’s law:

∇×H = J (5)

Where, H is the magnetic field intensity and J thecurrent density. For the analysis of the magneticfield distribution, it is prudent to also model theair around the component so that any leakage ofmagnetic flux is taken into account.

The main procedure in FEMM is to first definethe type of problem (2-D; planar or axisymmetric),then create the geometry point by point, using coor-dinates and after that assign the different materialsto be used. After that the mesh is generated andthe problem is solved quite quickly. The main pro-cedure with COMSOL is to set up the model (andselect the physics one wants inside the software),create the geometry (based on geometric shapes)and add the materials. Then deal with the meshprocessing, define the input current and other mag-netic field features and, solve the problem.

The electromagnetic coil is an important designparameter, as it is the source of the magnetic cir-cuit. It was based on the Multi-Turn Coil domaininside the software and the coil wire cross-sectionarea could be defined for its dimension, for the SWG(Standard Wire Gauge) number or for the AWG(American Wire Gauge) number. In a perfect sce-nario, higher values for the applied current in thecoil will result in a high magnetic flux on the MRmaterial gap. However, the current that can be ap-plied to the electromagnet coil is limited, which de-pends on the cross-sectional area of the wire and itsmaterial. In the end, a 15 AWG size copper wire isused to wind the coil of the cushions electromagnetwith 600 turns.

Figure 3: CAD model of the coil used for the project

One important factor to be considered is that theoperating point for the MR material has to be se-lected so that it does not fall into the magnetic sat-uration region of each material. A hysteresis loopgives a lot of information on the magnetic proper-ties of a material and it can be represented by therelationship between magnetic flux density B andmagnetic field intensity H and is normally calledthe B −H curve. The magnetorheological materi-als, used in this work, have almost non-hysteresiseffect so that they can be easily demagnetized.

Another main parameter that defines a material’smagnetic characteristics is its permeability µ, whichis the ratio between the applied magnetic field in-tensity H and magnetic flux density B. Permeabil-ity can be thought as being the magnetic conduc-tivity of a material. For design purposes, only theaverage B −H curve, which incorporates magneticsaturation effects, but not hysteresis, is needed oraccepted inside COMSOL.

3.2. Materials Design

The materials used in the cushion have a crucialinfluence on the magnetic circuit. The fast and re-versible reaction of MR fluids offers them to be asuitable candidate to interface between these sys-tems. Two types of hydrocarbon-based magne-torheological fluid will be tested during this work:MRF-132DG and MRF-140CG. Material propertiesfor the MR fluid were chosen based on typical valuesreported in the manufacturer’s provided literatureby LORD Corporation. When encapsulated in aregular elastomer, the matrix material can be, forexample, silicon to avoid the fluid leakage.

There are almost none suppliers of magnetorhe-ological elastomers. The supplier Ioniqa Technolo-gies only provided a rough magnetization curve inimperial units and from that, the author had to con-vert the units of the graph and then calculate thevalue of H because both software only accept theB −H curve of the material.

In order to intensify the field generated by theelectromagnets, high permeability and high satura-tion magnetic material needs to be utilized. Basedon this criterion the AISI 1010 and AISI 1018 willbe tested.

3.3. Mesh Generation

When creating the mesh for the geometry, defaultfree triangular mesh is suitable for the problem, butthe important topic is the refinement of each geom-etrys domain. The free triangular and the quadelements were tested, in order to prove that this isa mesh independent problem. The example of thefiner mesh, with 0.2cm element size will have 224605degrees of freedom solved and the other with 1cmwill have 153537 degrees of freedom. One of the keyconcepts in this section is the idea of mesh conver-

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gence as one refines the mesh, the solution will be-come more accurate. Analyses were conducted withdifferent meshes to determine the ideal element size.

In COMSOL, all exterior boundaries are magnet-ically insulated by default. The main idea is to forcethe vector field component normal to the borderto be zero. Otherwise, like in FEMM, a perimeterboundary condition is applied in order to let thesoftware understand the geometry’s limits.

3.4. ValidationTo validate the method and software used duringthe research, the author tried to reproduce a cou-ple of previous works ([7] and [10]) in order to getfamiliar with the software and verify that the mod-els, units and results were coincident and correctfor this kind of application, so that the readers mayrely on the final results.

3.5. Experimental RigsThere were two rigs to test and identify propertiesof the materials, both in shear or compression mode.The objective from the computational point of viewis to develop models that would represent these rigsand evaluate the magnetic circuit behavior in orderto verify and validate the experimental procedure.The construction of the finite element model beginswith the definition of the geometry and this taskis generally the most time-consuming due to thechallenge posed by the need to create a model thataccurately represents the real system.

The materials are copper for the coil wire, a 15AWG; low carbon steel AISI 1018 for the magneticparts; aluminum for the spacer (placed besides thesample to put the Hall sensor to measure the field)and the MR Elastomer sample.

Figure 4: 2-D model created for shear rig concept

The model was tested to an applied current from0 until 5A and the maximum magnetic flux densityvalue was registered. The output results fluctuatedbetween 50T and 470T .

In the compression rig concept, the 3-D geome-try is much more complex, so the author has de-

cided to reproduce one of the cross-sections of theapparatus that would be the most representative.The lab rig has one coil and the sample apparatusbuilt-in the outer magnetic material, so the MR ma-terial when in compression would benefit the mostfrom the generated magnetic field. The magneticcircuit contains an air gap to separate moving andnon-moving parts, which presents a disadvantage tothis particular rig because it adds a magnetic resis-tance to the circuit, which has to be considered incalculations.

Figure 5: 2-D model created for compression rigconcept

The model was tested for the same applied cur-rent and the obtained results are from 80T to 680T .An extremely important conclusion, based on theseresults, is that it was decided for now, the cushionwould benefit the most only with MR samples incompression because it presents the higher obtainedvalues and are less costly to manufacture. So theangled trapezoidal shape will not be adopted fromnow on.

There is no analytical solution that fully reflectsthe actual problem but some approximations canbe made. For these calculations, the models weredivided into differents parts and it was calculatedthe length, the cross-section area and the relativepermeability value for each one. Based on Hopkin-son’s Law it is possible to have the total reluctanceof the circuit and with that one has the magneticflux, from magnetostatics.

Φ = B·AMRE (6)

N ·I = Φ·Rtotal (7)

R =l

µ0µrA(8)

Where Φ is the magnetic flux; R is the magneticreluctance, l is the mean flux path length, µ is themagnetic permeability and A is the cross-sectionalarea of each part. Based on this information and thegeometry divisions, the calculations were performed

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knowing that for the compression rig there is onlyone coil and for the shear rig there are two coils.

By the end of this procedure, the author askedfor the experimental values and plotted a compari-son graph between those results, the COMSOL andFEMM simulation and, the analytical calculations.

Figure 6: Shear Rig final results

Figure 7: Compression Rig final results

All options produced consistent and almost thesame final results, which means that both modelswere successfully developed and represent indeedthe behavior of the experimental rigs.

The magnetic force is a consequence of the elec-tromagnetic force, one of the four fundamentalforces of nature and it is highly dependent on themagnitude of the magnetic flux density B. To usethis method, one simply has to add a Force Calcula-tion node to the COMSOL model. After solving themodel, forces can be evaluated with Global Evalu-ation node. In order to analytically calculate thesame results, it was used a method of force calcula-tion based on reluctance.

F =B2·A2µ0µr

(9)

A comparison between COMSOL values and ana-lytical calculations was made and the obtained re-sults are extremely consistent and, as expected, themagnetic force increases with the magnetic field.

One future option for the experimental rigs sim-ulation, to get even more precise results, could be

to create the 3-D finite element model of the rigs,in order to consider the tridimensional effects.

4. Parametric Studies and Optimization

Different configurations of MR material cushion sec-tions are used, to identify the parameters that affectthe most the design and shape of the device, in or-der to maximize the magnetic field intensity withinthe gap. After a short optimization example ap-plication and obtaining the final dimensions of thegeometry, the final design configuration of the seatcushion is presented.

4.1. Candidate Configurations

The general concept leaves room for a variety of dif-ferent configurations depending on the choices maderegarding aspects such as the number of coils; thenumber of MR samples; MRF or MRE to be used;the thickness of the gap; magnetic material to beused; size and location of the components and di-mensions of the overall geometry. Four differentconfigurations will be described throughout this sec-tion. The available parameters to test and the ad-vantages or disadvantages of each configuration arementioned as well.

The first chosen design is the simplest one possi-ble, having the MR Fluid or MR Elastomer belowthe coil and a small layer of magnetic material. Thesecond configuration adopted is the one that repro-duces the initial conceptual design. After the con-clusions from the previous designs, it was decided totry to place the MR material sample in the middleof the coil and surrond that with magnetic mate-rial. The fourth configuration was thought in orderto have just one MR material sample, placed in thebest possible location (serving partly as magneticcore). All the dimensions were tested with differ-ent values and having in mind the pros and consof each configuration, it was decided that design 3and 4 would be used for the final configuration ofthe cushion, since they benefit from having the MRmaterial in the middle of the coil.

4.2. Optimization

The first step in this process is the definition ofthe actual problem, that is, to find the best geome-try dimensions (design variables) in order to achievethe highest magnetic flux density. Dimensional op-timization is used as the final step in the designprocess and is carried out once the design is moreor less fixed in terms of the overall shape. In thiscase it is a maximization problem, which is solvedby using the Nelder-Mead method (simplex method[4]) with the aid of the COMSOL Multiphysics R©

Optimization module. With this, the final valueswere used to built the final configuration.

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4.3. Final Design

The CAD drawing and cross-sections of the finalconfiguration are presented, based on the obtaineddimensions.

Figure 8: Final Cushion

The cell 1 is for the MR Fluid and the cell 2 isfor the MR Elastomer.

Figure 9: The two different cross-sections of thecushion

Having the final dimensions and parameters, aCOMSOL magnetic simulation was performed, inorder to check the behavior of the MR material ineach cell with the applied current and the magneticflux density distribution plots are shown.

Figure 10: MRF cell 1 final results

Figure 11: MRE cell 2 final results

Now the seat cushion design is going to be exper-imentally tested.

5. Material Response and Control System

It is fundamental to assure that the cushion is ableto withstand external disturbances such as vibra-tions. In order to verify that, vibration simulationswere performed using Simulink R© to evaluate theMRF performance.

5.1. Mathematical models and validation

The dynamic model of the materials are extremelydifficult to develop due to their non-linear behav-ior and the internal damping. For the MRE thereare no equations or models to represent its behav-ior. There is only a finite-column model proposedto calculate the field induced shear modulus [1]. Forthe MRF, it is used a MRF-E pouch and the equa-tion representing the squeeze mode is based on [3].Squeeze mode arises from vertical motion, whichcreates a pressure in the thin film of material be-tween the two layers. The model is based on theintegration of the total pressure on the surface, andthe final total force equation accounts for the inertiaeffects:

Where h is the gap size, h the velocity of thelayers and the right term on both equations is theinitial fixed spring force due to the initial stiffnessMR pouch has, that depends on the membrane ma-terial type.

It is also possible to differentiate this equation,in order to obtain the stiffness of the material, fordifferent magnetic fields.

Figure 12 and 13 show the force response, usingthe final geometry and the MRF-140CG properties.

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Figure 12: Force versus displacement MR Fluid fi-nal model

Figure 13: Force versus velocity MR Fluid finalmodel

As seen in Fig.5.1 the area on the graph forceversus velocity means the MR Fluid dissipated en-ergy, which means that the system is attenuatingthe effect of the input, from where it is possible toextract the damping coefficient.

5.2. Performed Simulations

Based on all the previous information and under-standing better how the model works, it is possibleto use it into a Simulink model, based on the SDOFprinciple, to study the pilot reaction and responseto an acceleration input.

5.2.1 Acceleration Input

The complete vertical (z-axis) acceleration data foran helicopter flight test has been provided by NRC,through an Excel data file. This data is shown ina time-basis and represents what happens for aninterval of 60s flight test. The input signal has amaximum value of 21.87m/s2 and a RMS value of4.77. One useful analysis of the data is to view theinput signal in terms of frequency and for that weneed a FFT - Fast Fourier Transform, in order totransform the time domain into frequency domain.It gives the input signal frequency spectrum.

Figure 14: Input acceleration signal in frquency do-main

It is possible to verify that the main frequenciesare within the range predicted. There are four ma-jor frequencies (already mentioned) that influenceand represent the system.

5.2.2 Simulink R© System

The author developed a new block diagram modelto incorporate the force equation and the non-linearities. The measured acceleration data wereused to characterize the vibration input from thehelicopter floor to the aircrew body through theseat. Since the force model represents the totalforce, it was created a unique block with both to-gether, having as input the yield stress τ , therelative displacement, velocity and acceleration.The data from MRF-140CG yield strength versusapplied magnetic field intensity is available fromLORD Corporation [6]. The viscosity of the fluid istaken from the data sheet as well and it shows theslope of the graph shear stress versus shear rate.

The parameters used were the initial gap size hof 7.5mm that represents the thickness of the MRFluid; the slope KH that is 0.07; the radius R is2cm; the viscosity is 0.28Pa/s; the density of thematerial is 3640kg/m3 and the mass of the sys-tem (pilot+cushion) is 85kg. The only variable isthe magnetic field, represented by the yield stress.Based on the curve of the material from the datasheet and the COMSOL values, that parameter canvary between 5500 and 58000 Pa.

Figure 15: Final Simulink model

The acceleration felt by the aircrew, after apply-ing the system for example with an input current

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of 0.5A (yield stress of 5500 Pa) is shown in figure16.

Figure 16: Output acceleration example, for 0.5Acurrent

It can be seen that the output is extremely re-duced when compared with the helicopter input,having a reduction of around 97%.

5.3. ControlAfter having the entire system model working well,it is needed to add the controller. A challenge forthe potential applications of MR materials devicesis the development of an appropriate control strat-egy. The main idea is to have a closed-loop controlsystem so that the output has an impact in the con-troller action.

5.3.1 Skyhook Control

The skyhook is based on the mass related to the ab-solute velocity value of the output response. Thenthe signal is affected by a gain value and producesthe desired force for this purpose, Fv. After that,and inside an algorithm (on-off condition) block,it is made a comparison between the desired forceand the force coming from the MR model (dynamicforce) and based on that, a value of yield stress, τ isgiven as output and reintroduced again in the sys-tem. The schematic of this system is reproduced infigure 17.

Figure 17: Schematic of the skyhook control system

The goal is to understand if this type of control,for this particular purpose and configuration, is ap-

propriate and necessary. Based on different valuesfor the gain, several simulations were performed inorder to compare the final obtained result.

One important note is that all these results mustbe tested experimentally, since in theory the reduc-tion seems to be extremely high but in reality itmight not happen.

This semi-active solution would be, ideally, muchmore effective for reduced damping values. Withhigh damping results, as in this case, the use of thissystem is almost unnecessary.

6. Conclusions

This project has a couple of original contributionsand a relevant development regarding MR mate-rials. A magnetorheological seat cushion is pro-posed as a possible solution for the conventionalone. These magnetorheological materials can, infact, mitigate vibrations and are considered a vi-able solution for vibration suppression and control.The research presents a succinct theoretical back-ground. The key throughout this process is to mag-netize them in the right direction and depending onthe particles, the behavior is different. The maingoal was to evaluate the mechanical response of themagnetorheological material in the the presence ofa magnetic field (stiffness and damping control).

A magnetic field analysis was conducted and anelectromagnetic circuit design for a MR cushion wasproposed in this study. Finite element models werecreated and their results prompted important de-sign changes to improve the overall performance ofthe MR material. The definition of the winding inthe coil is of extreme importance, so that the soft-ware may clearly understand how many coils are ineach design. Regarding the finite element software,COMSOL takes a long time to get familiar with,and to understand, all its capabilities and functions,since it is not as user-friendly as FEMM.

After validating the mathematical models, thiswas used to reproduce the response of MR Fluid.With that, a Simulink block diagram model wasdeveloped. The maximum possible displacement is2mm and if the applied current increases, that dis-placement reduces. The acceleration output maxi-mum reduction is around 97%, which confirms thesuitability of the proposed design for vibration at-tenuation (main goal of the project). The proposedcontroller system seems to be almost unnecessarysince high reductions are obtained only with thepassive system.

This project presents the following contributions:a relevant development regarding MR materials; thedesign of an innovative helicopter cushion seat toattenuate vibrations for increased pilot and crewcomfort; and the development of building blocks(like the finite element models, the non-linear anal-

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ysis and simulations and, material response withSimulink implementation) to achieve the proposedconceptual design such as the development of anon-linear material model to simulate the MR elas-tomer.

Future work should start with the optimizationof the model for the MRF material response; thenthe development of a material response model andrespective performance for the MRE and the defini-tion of more adequate and multi-degree of freedomcontrol strategies and comparison with test experi-ments. The implementation of the final solution tocheck the integrability of the semi-active cushion isrequired.

This technology holds great promise for potentialapplications to other vehicle seats.

Acknowledgements

The author would like to thank to the supervi-sors, Professor Afzal Suleman, who provided theopportunity to work in Centre for Aerospace Re-search and Eng.Amin Fereidooni, for their assis-tance and support; to NRC and University of Vic-toria - Canada, for having welcomed me and allow-ing me to perform this research; to all the mem-bers of CfAR, colleagues and professors who helpedme to surpass my limits. A special word to Dr.Luis Falcao da Luz for all his knowledge and guid-ance and, last but not least, I would like to thankall friends and family for the unconditional supportthroughout this journey.

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[2] COMSOL. AC/DC Module–user’s guide.COMSOL, 5:300, 2014.

[3] M. J. Craft, M. Ahmadian, A. Farjoud, W. C.Burke, and C. Nagode. Force characteristics ofa modular squeeze mode magneto-rheologicalelement. Active and passive smart structuresand integrated systems 2010. In Proceedings ofthe SPIE, volume 7643, page 13, 2010.

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