inFlux: A Magneto-rheological MaterialInterface

Daniel FitzgeraldMIT Media Lab75 Amherst StreetCambridge, MA 02142, USAdjfitz@media.mit.edu

Andres CalvoMIT Media Lab75 Amherst StreetCambridge, MA 02142, USAandresc@media.mit.edu

Udayan UmapathiMIT Media Lab75 Amherst StreetCambridge, MA 02142, USAudayan@media.mit.edu

Julia Litman-CleperMassachusetts Institute ofTechnology75 Amherst StreetCambridge, MA 02142, USAjulialc4@mit.edu

Harpreet SareenMIT Media Lab75 Amherst StreetCambridge, MA 02142, USAsareen@media.mit.edu

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AbstractWe present a stiffness-changing interface based on a magneto-rheological (MR) fluid. The device consists of a materialsurface with elecro-magnetically induced visco-elasticity,which acts as a proxy for stiffness during tangible interac-tion with the material. We present several advantages ofthis enabling technology and outline potential applicationsand routes for future development.

Author KeywordsMaterial; Interface; Magnetic; Ferrofluid; Stiffness; Magne-torheological; Electromagnet

ACM Classification KeywordsH.5.2 [Information interfaces and presentation]: HapticsDe-formable Input

IntroductionTangible interfaces and haptic devices have focused onshape and form change of materials. However, few inter-faces have attempted to modulate the mechanical proper-ties of the material. These properties provide feedback tothe user during interaction that can be used to convey addi-tional information in addition to the shape of the surface.

The present devices [1, 2] are unable to modify stiffnesswithout volumetric change due to pneumatic actuation be-

Figure 1: Flux Tangible Interface Device

ing employed. However, in some applications changingshapes to achieve stiffness control is not practical. WithFlux interface, we are able to maintain the same volumewhile still achieving variation in stiffness and an analog con-trol. We present scenarios where such a material will proveuseful.

Related workIn HCI, jamsheets [4] explored tunable stiffness using pneu-matic particle jamming to control the flexibility of sheetmaterials. Shape displays [2] have been used to rendershapes, but only recently to simulate mechanical propertiesof materials, and in these cases only indirectly by replicat-ing the associated resulting bulk behavior. Few interfaces

are capable of dynamically controlling the stiffness of ma-terial via "smart materials." Such materials, including fer-rofluid, have been demonstrated in sensing systems asinputs, but less often as outputs. Haptic interfaces utiliz-ing magneto-rheological (MR) fluid have been introducedby Jansen et al. [1] but not explored further, despite sev-eral promising advantages. Najmaei et al. [3] proposed theuse of MR fluids in actuators for haptic devices. The fieldof robotics has explored MR fluids in applications requiringcontrollable adhesion [5, 6].

Principle of OperationWhen MR fluid is exposed to magnetic fields, the micro-particles align along the magnetic field lines in long chains.These chains resist perturbations, and therefore prevent theflow of the carrier fluid, resulting in an overall increase inapparent viscosity. At a high level, this increase in viscositycauses the material to become more stiff.

Figure 2: MR Fluid Off State

Figure 3: MR Fluid On State

Tunable Material PropertiesCompared to other methods to achieve tunable stiffness,MR fluids provide several advantages:

• Instantaneous Change Stiffness changes can beachieved with a negligible delay, making this tech-nology suitable for real-time and interactive stiffnesscontrol. Furthermore, haptic effects can be achievedby rapidly toggling stiffness, creating a "vibratory" ef-fect, but only during dynamic interaction. Importantly,stiffness cannot be perceived without applying forceto the material, which means the user must deformthe material to feel these effects.

• Continuous Stiffness Regime Because the viscos-ity of the material is directly related to the appliedmagnetic field, which can be continuously varied, theresulting stiffness can be set anywhere between acompletely "off" state, which is the normal viscosity ofthe material, and a full "on" state, which is the viscos-ity under the full power of the electromagnet.

• Direct Local Control The material only changes stiff-ness where under the influence of a magnetic field.Since magnetic fields can be directed towards certainareas, this property allows us to change the stiffnessof precise regions of material while leaving adjacentmaterial unaffected.

• Shape-Independent Stiffness Control Altering thestiffness of the material does not result in any sig-nificant volumetric or shape change. Thus, materialproperties can be controlled independently of form.Furthermore, this operation is silent and impercep-tible through any method other than physical touch.We see this as an advantage for potential applica-tions in discrete interfaces when only the intendeduser should be able to perceive the transduced infor-mation.

• Simple Design The operation of the device and thecontrol and actuation system are very straightforward,so the design and prototyping of interfaces based onthis technology can be rapid and inexpensive.

Prototype and Technical DesignA basic material element prototype was created consist-ing of three primary components: the material surface, theactuating electromagnet, and a micro-controller and driv-ing circuitry to control the actuator. The components aremounted inside a housing and additional buttons were inte-grated to control the material stiffness.

MaterialThe magneto-rheological (MR) fluid used consists of ferrousmicro-particles, in this case iron oxide powder (FE304),suspended in a carrier fluid, in this case common vegetableoil. The viscosity of the mixture depends on the proportionof powder to oil, which was adjusted such that the mate-rial has an "off state" viscosity similar to ketchup ( 100,000cps). A pouch was fabricated by sealing a flocked elasticmembrane to a plastic sheet backing. Approximately 20mlof the mixture was injected into the pouch.

ElectromagnetA 50mm diameter 24V cylindrical electromagnet was usedto generate a magnetic field to actuate stiffness changein the material. With approximately 0.3A at full power, thisresults in over 7W dissipated into the electromagnet. How-ever, with a heat-sink and air cooling system, the resultingheat is not felt by the user.

Electronic ControllerTo control the magnetic field, a micro-controller (ArduinoUno), is used to send a Pulse-Width-Modulated (PWM)signal to power MOSFET that drives the electromagnet.

The Arduino also reads input signals from the adjustmentknob and controls status lights.

Application ScenariosThere are several immediate applications for this material.Existing interfaces are capable of rendering dynamic, high-resolution visual information (such as common computerscreens), and dynamic medium resolution forms and topol-ogy (such as Shape Displays). However, interfaces capableof dynamically rendering material properties are uncom-mon. The ability to instantaneously change the material’sstiffness makes it useful for objects that could benefit fromrapid changes of rigidity. For instance, a knee pad made outof the material could become stiff only as users are fallingdown. The rest of the time, the knee pad could remain flexi-ble to improve users’ comfort. Sensors such as accelerom-eters could detect when users are falling and transform thestate of the material before they hit the ground.

Another example application is a knife with a blade thatcontains the material. The blade becomes stiff to cut ob-jects such as food. When users are about to accidentallyslice their fingers, the blade automatically becomes flexibleto prevent injury. The rapid change of stiffness of the mate-rial minimizes the risk of injury.

Applications in which rendering arbitrary material propertiescould be desirable include: online shopping for certain prod-uct classes such as pillows, beds, cloths, etc., haptic feed-back for teleoperation, especially in robotic surgery where avariety of different tissues must be discriminated against bycontact feedback.

Future DevelopmentThere are two obvious avenues of future development forthis type of tunable stiffness technology: large scale sin-

gle element interfaces and interfaces consisting of many ofsmaller elements.

Stiffness Tunable Support SurfaceOne promising application of a larger version of this ele-ment is in tunable stiffness support and manipulation sur-faces, including pillows, beds, chairs, armrests, mobile de-vice cases, etc. Such devices could conform to the userlike a water bed, but stiffen to give adjustable support orergonomic form like a gel or memory-foam. Another appli-cation utilizes the instantaneous response of the material toprotect the user or device from impact damage. For exam-ple, a helmet or knee pad could allow flexibility and comfortduring normal use, but instantly stiffen to provide a energy-absorbing cushion when impact is detected.

Stiffness Element Array - Material DisplaysProducing very large magnetic fields is difficult becausefield strength drops with the cube of distance from the source.It is therefor easier to scale the material element down thanup. With many small elements arranged in an array, a mate-rial surface could be created with higher resolution, similarto Mudpad.[1]

Magnetorheological ElastomersAnother direction for investigation is magneto-rheologicalelastomers, which consist of ferrmoagnetic microparticlesdispersed in an elastic matrix material. Unlike in MR fluids,the material can not flow, so "viscosity" does not changeand the particles return to their original relative positionsafter deformation. However, the stiffness of the materialcan still be modulated because the particles still try to formchain structures which resist perturbation. Furthermore,the force exerted on the particles by magnetic fields couldinduce shape changes, including contraction, expansion,and bending.

Future Vision - multiple property controlThe long term vision for magnetically actuated tangible in-terfaces is to achieve control over several mechanical prop-erties of the materials involved, including viscosity, stiffness,density, etc. and to equip these devices with sensors forbidirectional material interaction. When combined in mul-titudes, such material elements would form a high resolu-tion material display, capable of rendering different materialproperties in different places and times in highly dynamicways, all independently of the shape of the material (whichcan either be completely passive, or actuated separately, asin the contrasting Shape Displays.)

Results & ConclusionWe demonstrated a tangible interface element capable ofdynamically changing material stiffness using an electro-magnetically activated magneto-rheological fluid. The mate-rial can change stiffness rapidly, silently, reversibly, througha continuous range, and independently of shape. We out-line several potential applications and routs for future devel-opment in this area, including material rendering, conformalsupport and manipulation surfaces, and material displays.

AcknowledgementsWe would like to thank the Teaching Assistants for MAS.S834:Tangible Interfaces: Ken Nakagaki, Viirj Kan, and Luke

Vink, as well as Professor Hiroshi Ishii for their support andfeedback during the development process.

References[1] Yvonne Jansen. Mudpad: Fluid Haptics for Multitouch

Surfaces.[2] Daniel Leithinger, Sean Follmer, Alex Olwal, and Hi-

roshi Ishii. Shape Displays: Spatial Interaction withDynamic Physical Form.

[3] Nima Najmaei, Peyman Yadmellat, Mehrdad R.Kermani, and Rajni V. Patel. 2014. Application ofMagneto-Rheological Fluid based clutches for im-proved performance in haptic interfaces. In ICRA.IEEE, 832–837.

[4] Jifei Ou, Lining Yao, Daniel Tauber, JÃijrgen Steimle,Ryuma Niiyama, and Hiroshi Ishii. jamSheets: ThinInterfaces with Tunable Stiffness Enabled by LayerJamming.

[5] Masaaki Watanabe, Nicholas Wiltsie, Anette E. Hosoi,and Karl Iagnemma. 2013. Characteristics of control-lable adhesion using magneto-rheological fluid and itsapplication to climbing robotics. In IROS. IEEE, 2315–2320.

[6] Peyman Yadmellat and Mehrdad R. Kermani. 2013.Adaptive hysteresis compensation for a magneto-rheological robot actuator. In IROS. IEEE, 4900–4905.