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having a size on the order of 0.01 to 0.1 ptm and volume
fraction of the particles between 20% and 60% [9,10]. The
electro-rheological effect arises from the difference in the
dielectric constants of the fluid and particles. In the presence
of an electric field, the particles, due to an induced dipole
moment, form chains along the field lines. The induced
structure changes the ERF's viscosity, yield stress, and other
properties, allowing the ERF to change consistency from
that of a liquid to something that is viscoelastic (such as a
gel) at response time on the order of milliseconds. ERF
properties of high yield stress, low current density, and fast
response (less than 1 ms) offer essential characteristics for
the construction of rehabilitation devices. ERFs can apply
very high electrically controlled resistive forces while their
size (weight and geometric parameters) can be very small.
ERFs are not abrasive, non-toxic, and non-polluting (meet
health and safety regulations).
Control over a fluid's rheological properties offers the
promise of many possibilities in engineering for control of
mechanical motion. The use of ERFs for tactile sensing in
robotic fingers was proposed in [11]. Based on that work,
several researchers proposed the use of ERFs in tactile
arrays used to interact with virtual environments [12] and
also as assistive devices for the blind to read the Braille
system [13]. A 5x5 ERF tactile array was developed in [14].
An ERF-based planar force-feedback manipulator system
that interacts with a virtual environment was studied in [15].
An ERF-based force-feedback joystick has been developed
in the Fraunhofer-Institut in Germany [16]. The use of ERF
resistive elements and brakes in rehabilitation has been very
limited. The few rehabilitations devices employing ERF
elements that have been developed so far were fixed based,
non-portable, non-wearable systems [17-19].
Very similar to ERFs are the Magneto-Rheological
Fluids (MRFs) whose rheological properties change with
variations of a magnetic field instead of an electric field.
Several MRF based haptic / force resistive systems have
been described in [20-23]. Lower limb prosthetic systems
using MRF brakes have been developed [24-26]. Some
rehabilitation devices using MRF brake / damper systems
have also been proposed [27, 28].
In this paper we present a novel ERF based active knee
rehabilitation orthotic device that presents high portability
and offers very accurate computer control and monitoring of
resistive torque and motion.
III. AKROD DESIGN AND PROTOTYPE
AKROD is composed of the following main
components: a) ERF-based brake; b) brace and gear
assembly and c) sensors. To set the design goals for the knee
device, peak torque during extension in isometric exercises
was used as the benchmark. This has been shown to be on
average, 172 Nm for healthy men and 112 Nm for healthy
women [29]. Since the torques necessary for walking are
less than these maximum capabilities, 172 Nm at 5 kV was
set as the designed torque output for the knee device (Note:
5kV is the maximum output voltage from the power supply
that is used). Therefore, for an operating maximum voltage
of the ERF at 3kV, the torque capabilities of the AKROD
are equal to 78 Nm. With the knee moment assumed to be
75 Nm/kg, AKROD is able to support an inpidual of 104
kg (approximately 229 lbs). This is sufficient to be used
with most of the stroke population. Below we provide
design details for AKROD's components and we present the
first prototype developed.
ERF- Brake
The ERF brake uses a resistive smart fluid element
(RSFE) to modulate the device resistive torque. The RSFE