27-12-2012, 01:52 PM
MR-fluid brake design and its application to a portable muscular rehabilitation device
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Abstract
Many devices are available on the market for the evaluation and rehabilitation of patients
suffering from muscular disorders. Most of them are small, low-cost, passive devices based on
the use of springs and resistive elements and exhibit very limited (or even not any) evaluation
capabilities; extended muscular force evaluation is only possible on stationary, expensive,
multi-purpose devices, available only in hospitals, which offer many exercise modes (e.g. isokinetic
mode) that are not available on other devices.
The objective of this thesis is to make the functionalities currently only implemented on bulky
multi-purpose devices available at a lower cost and in a portable fashion, enabling their use
by a large number of independent practitioners and patients, even at home (tele-medecine
applications).
In order to achieve this goal, a portable rehabilitation device, using a magneto-rheological
fluid brake as actuator, has been designed. This particular technology was selected for its
high level of compactness, simple mechanical design, high controllability, smooth and safe
operation. The first part of this thesis is devoted to the design of MR-fluid brakes and their
experimental validation. The second part is dedicated to the design of the rehabilitation
device and the comparison of its performances with a commercial multi-purpose device (CYBEX).
Introduction
Muscular rehabilitation devices: market overview
The market of muscular rehabilitation devices is composed of three major categories (Figure
described in further details in the following subsections.
Category 1:v Multi-function deices
These are computer controlled devices that can be used for muscular evaluation and exercise
of almost any body joint. Data acquired during evaluation is displayed to the patient, to increase
his motivation, and recorded for further analysis by the practitioner. All these devices
offer a large number of exercise modes.
Multi-function devices are large, bulky and not portable. Furthermore, they exhibit low repeatability
for the evaluation of small body joints (such as wrist) and a lack of precision in
measurements (Chan and Maffulli, 1996) (Leclerq, 1999). Finally, changing the configuration
of the device for the evaluation and exercise of one joint to another is highly time-consuming.
Such devices are thus not suited for small practices where two successive patients rarely suffer
from disfunction of the same joint of the body.
Major players in this category include CSMI with the Humac Norm (former CYBEX),
BIODEX, BTE Technologies with the Primus and CONTREX (Figure 1.2). All these manufacturers
are based in the US, except for CONTREX which is a Swiss company. These
devices are priced between 45000 and 90000 euros, depending on the accessories, making
them affordable only for clinics and hospitals.
Motivations and market positioning
From the market analysis described above, we can conclude that there is no portable multifunction
muscular evaluation device available on the market that may be affordable to a large
number of independent practitioners. Furthermore, some exercise modes are currently only
available on the the costly multi-function devices, designed to accommodate all body joints,
leading to a lack of accuracy and repeatability in measurements conducted on small joints.
Moreover, the bulkiness and complexity of existing devices prohibits their use by the patients
without the assistance of a practitioner.
There is thus a need for a device that would offer similar functionalities (and exercise modes)
as those implemented on multifunction devices but in a much more compact design, portable,
affordable for independent practitioners (price of about 5000 euros), well suited for small
joints evaluation and that could be used by the patient in an autonomous way, possibly at
home (telemedecine applications).
The aim of this thesis was thus to develop such device. Magneto-rheological (MR) fluid brake
actuation was selected thanks to its high level of compactness, high controllability and safety
of operation as compared to alternative technologies. The prototype described in this work
was designed for the joints of the forearm but could be easily extended to other joints such
as hand, ankle... However, it should be noticed that using a controllable brake (semi-active
actuator) introduces a limitation as compared to existing multi-function devices which are
based on active actuators (CYBEX, BIODEX..). Indeed, a limited number of exercise modes
can not be implemented with such type of actuator, since the torque can only be opposed
to motion. Figure 1.5 compares the various characteristics of the prototype described in this
thesis with the various categories of commercially available devices. It can be observed that it
combines the advantages of all categories except for the versatility of exercise modes limited
by the use of a brake as actuator.
Carrier fluid
Carrier fluids are selected based on their intrinsic viscosity, their temperature stability and
their compatibility with other materials of the device. The most common carrier fluids are
hydrocarbon oils, which can either be mineral oils or synthetic oils (or a combination of both),
thanks to their good lubrication, durability and the availability of a large range of additives.
Silicon oils can be used instead in order to achieve a broader operating temperature range
or due to compatibility issues with other materials of the device (e.g. rubber seals). (Jolly
et al., 1998) gives guidelines regarding the temperature range of some MR-fluid formulations
as well as their compatibility with typical seals materials.