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Reykjavik, Iceland

Gudmundsson K.H.,University of Iceland | Jonsdottir F.,University of Iceland | Thorsteinsson F.,Ossur Inc. | Gutfleisch O.,Leibniz Institute for Solid State and Materials Research
Journal of Intelligent Material Systems and Structures | Year: 2011

The study investigates the field-induced shear yield stress and the off-state viscosity of selected unimodal and bimodal magnetorheological (MR) fluids. Five grades of commercially available iron powder are used to prepare unimodal MR fluids, one for each powder grade, and bimodal MR fluids, using two grades of micron-sized powder. The iron powder contains particles with a mean diameter ranging from 1 to 8 μm. The solid loading of all fluids is held to a constant value while varying the particle size and the ratio between the coarse and the fine powders. All fluids employ a perfluorinated polyether base fluid whose qualities are described. The bimodal MR fluids are compared to their corresponding unimodal fluids. Results show the bimodal fluids to have a lower off-state viscosity than their corresponding unimodal fluids. An application in prosthetic devices is introduced in which the yielding and the off-state characteristics of the MR fluid are of equal importance. In this application, the shear yield stress determines the rigidness while the off-state viscosity determines the flexibility in the absence of a magnetic field. For this particular application, prominent MR fluids are selected based on the ratio between the on-state yield stress and the off-state viscosity. © 2011 The Author(s).


Gudmundsson K.H.,University of Iceland | Jonsdottir F.,University of Iceland | Thorsteinsson F.,Ossur Inc.
ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2011 | Year: 2011

The study presents an experimental investigation into the trade-offs between field-on versus field-off rheological characteristics of magnetorheological (MR) fluids. This is relevant in a particular application in prosthetic devices where field-off characteristics are of equal importance to the field-on rheological characteristics. The paper introduces a biomechanical prosthetic knee joint that uses an MR fluid to actively control its rotary stiffness while an amputee walks. The knee is a synergy of artificial intelligence, advanced sensors and MR actuator technology. The knee joint is equipped with an MR rotary brake, utilizing the fluid in direct-shear mode. The MR fluid has response time in the order of milliseconds, making it possible to vary the knee's stiffness in real-time, depending on sensors data. The field-on characteristics of the employed MR fluid define the rigidness of the knee joint while the field-off characteristics define its flexibility in the absence of a magnetic field. Five MR fluid compositions are prepared, each with a different solid loading ranging from 0.25 to 0.35, by volume. All fluids employ a commercially available carbonyl iron powder and a base fluid. The MR fluids are experimentally evaluated in a rheometer, where both field-off and field-on characteristics are measured. An MR fluid figure of merit function is introduced which is used to rate the selected MR fluids for a potential application in the MR prosthetic knee. An MR fluid composition is sought with the highest ratio between the field-on shear yield stress and the off-state viscosity. The research shows the off-state viscosity to decrease faster than the field-on shear yield stress when reducing the solid loading from 0.35 to 0.25. This suggests that an optimum solid loading exists with regards to the defined merit function. The off-state viscosity of suspensions is known to be exponentially dependent on solid loading while the field-on shear-yield stress is known to sub-quadratically dependent on solid loading. Field-on and field-off models are presented from literature. The models compared to the experimental data and used to theoretically predict the optimum solid loading with regards to field-on shear yield stress and off-state viscosity. As a result of the experimental and the theoretical analysis, a prominent MR fluid composition is selected for a potential application in the MR prosthetic knee. This has been shown to help in the development of prosthetic devices and furthering the success of an MR prosthetic knee joint. © 2011 by ASME.


Jonsdottir F.,University of Iceland | Gudmundsson K.H.,University of Iceland | Dijkman T.B.,University of Twente | Thorsteinsson F.,Ossur Inc. | Gutfleisch O.,Leibniz Institute for Solid State and Materials Research
Journal of Intelligent Material Systems and Structures | Year: 2010

Motivated by the use of magneto-rheological (MR) technology in prosthetic devices, the goal of this study is to develop a MR fluid composition that is tailored for the requirements of a prosthetic knee actuator. A MR fluid composition is sought with a suitable balance between the field-induced shear stress, the off-state viscosity, and sedimentation stability for the proposed application. Rheological characteristics are investigated for samples with monodisperse micron-sized particles and bidisperse fluids with a mixture of micron- and nanosized particles. Two types of nanosized particles are used. All fluid samples employ a novel perfluorinated polyether oil as carrier liquid which enhances stability. The samples are investigated with respect to both field-induced and off-state characteristics. The results are compared to analytical and empirical models that exist in the literature. The monodisperse fluids are shown to give a favorable trade-off between field-induced strength and off-state viscosity. The addition of a small concentration of nanoparticles is found to moderately increase the field-induced shear-yield stress. However, for a larger concentration of nanoparticles, the yield stress begins to decrease. Nanoparticles exhibit an undesirable effect on the off-state viscosity. The results reveal valuable information for the designers of MR fluids and designers of prosthetic actuators. © The Author(s), 2010.


Gudmundsson K.H.,University of Iceland | Jonsdottir F.,University of Iceland | Thorsteinsson F.,Ossur Inc.
Journal of Intelligent Material Systems and Structures | Year: 2011

This study investigates the field-induced rheological characteristics of PFPE-based MR fluids. Four PFPE-based MR fluid samples were prepared, each with a different solid loading. All samples contain a single grade of carbonyl iron powder with an average particle diameter of 2 μm. The PFPE base fluid enhances sedimentation stability and its composition and qualities are introduced in the article. The magnetization characteristics of the iron powder and a fluid sample are measured. Its magnetic properties are compared to known models from the literature. The shear-yield stress in all fluid samples is investigated experimentally, as a function of solid loading and the magnetic flux density. The results are compared to established shear stress models for MR fluids. Model parameters are set to conform to the PFPE-based MR fluids. The PFPE-based MR fluids are shown to have a comparable shear-yield stress, when compared to commercially available MR fluids with a comparable particle loading and particle size. An application for the PFPE-based MR fluids is introduced. This application is an adaptive MR prosthetic knee, utilizing the fluid in shear mode. © 2011 The Author(s).


Jonsdottir F.,University of Iceland | Gudmundsson K.H.,University of Iceland | Hreinsson E.,University of Iceland | Thorsteinsson F.,Ossur Inc. | Gutfleisch O.,Leibniz Institute for Solid State and Materials Research
Electro-Rheological Fluids and Magneto-Rheological Suspensions - Proceedings of the 12th International Conference | Year: 2011

The performance of most commercial MR fluid devices depends on the shear strength of the MR fluid. The field-induced yield stress can be accurately characterized experimentally by a magneto-rheometer. However, as a number of factors, for example, particle volume ratio and particle size, are known to affect the rheological properties of the fluid, such measurements can be very time consuming. Hence, computational models represent a valuable tool in the design of MR fluid devices. This study provides a computational model to quantitatively predict the shear yield stress for MR fluids. The configuration is a representative unit cell in the form of a cube of a visco-plastic material which contains a number of rigid spherical particles. The cube undergoes a simple shear deformation and the mean induced shear stress is calculated. The model is used to study the effect of particle size and volume ratio on the dynamic yield stress of MR fluids.

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