Design of a Magnetorheological Fluid for an MR Prosthetic Knee Actuator with an Optimal Geometry

112 Pages·2011·2.45 MB·English

Design of a Magnetorheological Fluid for an
MR Prosthetic Knee Actuator with an
Optimal Geometry

Ketill Heiðar Guðmundsson

FACULTY OF INDUSTRIAL ENGINEERING,
MECHANICAL ENGINEERING AND COMPUTER SCIENCE
UNIVERSITY OF ICELAND
2011

Design of a Magnetorheological Fluid for
an MR Prosthetic Knee Actuator with an
Optimal Geometry

Ketill Heiðar Guðmundsson

A thesis submitted to the Faculty of Industrial Engineering, Mechanical
Engineering and Computer Science, University of Iceland for the degree
Philosophiae Doctor in Mechanical Engineering

Thesis Supervisor
Dr. Fjóla Jónsdóttir

Doctoral committee
Dr. Fjóla Jónsdóttir (chairman)
School of Engineering and Natural Sciences, University of Iceland.

Dr. Halldór Pálsson
School of Engineering and Natural Sciences, University of Iceland.

Dr. Hilmar Bragi Janusson
Ossur Inc., Iceland.

Dr. Oliver Gutfleisch
Leibniz Institute for Solid State and Materials Research Dresden,
Germany.

Opponents

Professor Antonio José Faria Bombard
Department of Physics and Chemistry, Federal University at Itajubá,
Brazil.

Professor Faramarz Gordaninejad
Department of Mechanical Engineering, University of Nevada, Reno.
Design of a Magnetorheological Fluid for an MR Prosthetic Knee Actuator with an
Optimal Geometry
A thesis submitted to the School of Engineering and Natural Sciences, University of
Iceland for the degree Philosophiae Doctor in Mechanical Engineering

Copyright © 2011 Ketill Heiðar Guðmundsson
All rights reserved

Faculty of Industrial Engineering, Mechanical Engineering and Computer Science
School of Engineering and Natural Sciences
University of Iceland
VR II, Hjarðarhaga 2-6
107 Reykjavík
Iceland

ISBN 978-9979-9935-5-1

Prentun: Háskólaprent
Reykjavík, May 2011
Ágrip (in Icelandic)
Kveikjan að rannsókninni er rafsegulvökva gervihnjáliður sem þarfnast betrumbóta.
Gervihnjáliðurinn er aðlögunarhæfur, í rauntíma, með hjálp rafsegulvökva. Vandamál hafa
komið upp þegar gervihnjáliðurinn er notaður í krefjandi aðstæðum, eins og fjallgöngu sem
dæmi. Markmið verkefnisins er tvíþætt: að auka bremsuvægi hnjáliðarins undir áhrifum
segulsviðs og samtímis að minnka bremsuvægi hnjáliðarins þegar segulsviðs nýtur ekki
við. Til að ná markmiðunum tveimur er notuð samþættuð aðferð vökvahönnunar og
rafsegulvökvavirkjarahönnunar. Rafsegulvökvi er hannaður, sérsniðinn fyrir notkun í
gervihnjálið. Samhliða vökvahönnuninni, er rafsegulvökva snúningsbremsa hönnuð fyrir
gervihné.
Við rafsegulvökvahönnunina er notuð aðferð tilrauna. Tuttugu og tvær vænlegar
rafsegulvökvablöndur eru metnar í mælitækjum, til notkunar í gervihnjáliðnum. Vökvar
þessir eru blandaðir og þeim gefin einkunn samkvæmt hönnunar markmiðum
gervihnjáliðarins. Vænlegar rafsegulvökvablöndur eru meðal annars; eintoppa
rafsegulvökvar með breytilegri agnastærð og agnamagni, tvítoppa rafsegulvökvar með
blöndu af tveimur gerðum af ögnum í míkrómetra skala, og tvítoppa rafsegulvökvar með
ögnum í nanómetra skala. Mælikvarði á gæði vökva er skilgreindur sem hlutfallið milli
skerspennu í vökvanum, þegar hann er undir áhrifum segulsviðs, á móti seigju vökvans,
þegar segulsviðs nýtur ekki við. Sá vökvi sem hefur hæsta hlutfallið mun leiða til víðs
sviðs bremsuvægis í gervihnjáliðnum.
Við hönnun rafsegulvökvavirkjarans eru notaðar aðferðir líkanagerðar og bestunar.
Líkön eru smíðuð fyrir bremsuvægi gervihnjáliðarins, með og án segulsviðs, þar sem
segulsviðslíkanið byggir á smábútagreiningu. Hönnunarmarkmið gervihnjáliðarins eru: hátt
bremsuvægi undir áhrifum segulsviðs, lágt bremsuvægi þegar segulsviðs nýtur ekki við og
litla þyngd. Málamiðlanir milli hönnunarmarkmiða eru rannsakaðar með fjöl-markmiða
bestun. Greiningin leiðir í ljós betrumbætur á hönnun sem hefur verið nýtt í framleiðslu á
nýrri og bættri útgáfu af rafsegulvökva gervihnjáliðnum.

Abstract
The motivation for this study is a magnetorheological (MR) prosthetic knee joint. The MR
prosthetic knee is adaptive in real-time via an MR fluid. Problems have been experienced
when the prosthetic device is used in demanding situations, like hill-climbing for example.
The goal of this project is twofold; to increase the on-state torque output of the prosthetic
device and, at the same time, to decrease the off-state torque. To achieve the two goals, a
combined MR fluid design approach and an MR actuator design approach is adopted. An
MR fluid is designed that is tailored for this specific application. Parallel to the fluid
design, an MR rotary brake actuator is designed for the prosthetic knee.
The MR fluid design is approached by experimentally evaluating twenty-two
potential MR fluid compositions for the proposed device. These fluids are mixed and rated
against the design objectives of the prosthetic device. Potential MR fluid compositions
include; unimodal MR fluids, with a varying particle size and a variable solid loading,
bimodal MR fluids with two grades of micron-sized particles, and bimodal MR fluids with
nanoparticles. A fluid figure of merit is defined that is the ratio between the on-state shear
yield stress and the off-state viscosity. The fluid with the highest ratio will result in the
widest torque range for prosthetic device.
The MR actuator design is approached by building torque models of the device and
optimizing the models. An off-state and an on-state model are developed where the on-
state model is based on a magnetic finite element analysis. Design objectives for the device
are: high on-state braking torque, low the off-state rotational stiffness and low weight.
Trade-offs between the design objectives are explored with a multi-objective optimization
technique. The analysis suggests design improvements that have been realized in a newly
built and an enhanced version of the MR prosthetic knee.

Preface
This dissertation has been prepared in partial fulfillment of the requirements for a Ph.D.
degree in Mechanical Engineering at the University of Iceland. The research was carried
out at the Department of Industrial Engineering, Mechanical Engineering and Computer
Science at the University of Iceland, in the testing laboratories of Ossur Inc. in Iceland and
at the Leibniz Institute of Solid State and Materials Research Dresden, in Germany.
The work was funded by the University of Iceland Doctoral research fund and
RANNIS grant no. 090035021. The work was supported by Ossur Inc.