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An Exercise Device for Upper-
Extremity Sensory-Motor 
Capability-Augmentation 
Based on a Magneto-
Rheological Fluid Actuator
Roman KAMNIK, Jernej PERDAN, Tadej BAJD, Marko MUNIH
Prof. dr. Roman Kamnik, univ. 
dipl. inæ., Jernej Perdan, univ. 
dipl. inæ., prof. dr. Tadej Bajd, 
univ. dipl. inæ., prof. dr. Marko 
Munih, univ. dipl. inæ.; Labora-
tory of Robotics and Biomedical 
Engineering, Faculty of Electri-
cal Engineering, University of 
Ljubljana
Abstract: Resistance exercise has been widely reported to have positive rehabilitation effects for patients with 
neuromuscular and orthopaedic disorders. This paper presents the design of a versatile rehabilitation device in the 
form of a rotation joint mounted on the adjustable arm support that provides a controlled passive resistance dur-
ing the strength training of hand muscles. The resistance is supplied by a rotational magneto-rheological actuator 
controlled by the force-feedback information. The device provides both isometric and isokinetic strength training 
and is reconfigurable for different usage conditions. The experimental evaluation results show that the usage of the 
magneto-rheological actuator is advantageous for electrical motors in cases of passive-resistance-based exercise.
Keywords: rehabilitation, exercise device, magnetorheological fluid
 1 Introduction
In rehabilitation and sports medicine, 
computerized active-exercise de-
vices have been shown to be suitable 
for providing the clinical delivery of 
training of the required intensity [1]. 
An especially challenging aspect is 
the recovery of the hand’s function. 
We have recently developed a novel 
system for hand sensory-motor aug-
mentation [2] that is designed to allow 
the force tracking training of finger 
flexors and extensors and to provide 
objective data on training perform-
ance in isometric conditions. Incor-
porated functional electrical stimu-
lation adds to reduced finger-force 
generation due to injury, thus moti-
vating the user to perform better. The 
system consists of a visual feedback 
display, the hand-force measuring 
device, and the closed-loop control-
led electrical stimulator. The results 
of a pilot therapy study in incomplete 
tetraplegic subjects showed that the 
augmentation of voluntary grip force 
control with the presented system is 
possible. However, the training per-
formed in isometric conditions in 
which at various angular positions 
the external resistance applied to the 
joint is always equal to the force ap-
plied by the patient is not considered 
to be the most efficient.
The isotonic and isokinetic exercises 
are considered to be more efficient. 
The isotonic exercise is performed 
dynamically over a predefined range 
of motion. The resistance applied to 
the joint is either constant or follows a 
ELEKTROREOLOØKI FLUIDI

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Ventil 16 /2010/ 4
predefined pattern as a function of the 
joint’s angular position. This mode of 
exercise is motivated by the length 
tension relationship of the skeletal 
muscle in which the largest force is 
generated when the muscle fibers are 
at their optimal length. The force-pro-
ducing capacity of a muscle changes 
across the range of joint motion and 
is typically highest in the mid range of 
joint motion. Muscle force generation 
during concentric exercise is also in-
fluenced by the contraction velocity, 
as described by the hyperbolic model 
relating the force and velocity dur-
ing contractions. As the contraction 
velocity increases, the muscle force 
decreases. From this relationship 
the isokinetic exercise is motivated, 
which is also performed dynamically, 
but in that case the resistance is ap-
plied to the joint only if a predefined 
angular speed is reached by the joint 
in order to avoid the joint exceeding 
this speed value. This particular exer-
cise mode is the only one that enables 
dynamic training at the maximum 
muscle force over the entire range of 
motion.
Most force-feedback devices that are 
capable of regulating joint motion 
according to the needs of a particu-
lar patient and take muscle and limb 
dynamics into consideration rely on 
electric motors, pneumatics, or some 
other conventional power-producing 
method.
In this paper we present a semi-ac-
tive exercise system based on mag-
neto-rheological fluid (MR fluid) 
actuator [3]. This semi-actively con-
trolled device can be considered as 
one that has properties that can be 
adjusted in real time, but cannot in-
put energy into the system being con-
trolled. Such devices typically have 
very low power requirements and of-
fer the reliability of passive devices, 
while maintaining the versatility and 
adaptability of fully active systems 
[4], [5]. In the second section of the 
paper the principle of the operation 
of the MR fluid actuator is presented. 
The third section presents the design 
of the exercise device based on the 
MR fluid actuator, while the fourth 
section outlines the experimental 
results.
 2 Principle of operation of 
the MR fluid actuator
MR fluids are materials that respond 
to an applied magnetic field with a 
change in rheological behavior. Typi-
cally, this change is manifested by the 
development of a yield stress that mo-
notonically increases with the applied 
field. The MR fluid typically consists 
of micron-sized, magnetically polar-
izable particles dispersed in a carrier 
medium, such as mineral or silicone 
oil. When a magnetic field is applied 
to the fluids, particle chains form, 
and the fluid becomes a semi-solid 
and exhibits viscoplastic behaviour. 
The MR fluid can be readily control-
led with a low voltage (e.g., 12-24 V), 
current-driven power supply output-
ting only 1-2 A.
The behaviour of MR fluids is often 
described as a Bingham plastic mod-
el having a variable yield strength, 
which depends upon the magnetic 
field B. At fluid stresses below the 
yield stress the fluid acts as a viscoe-
lastic material exhibiting Newtonian-
like behavior. At fluid shear stresses 
above the field-dependent yield stress 
τ
yd 
(B) the fluid flow is governed by 
the Bingham plastic equation [9]. 
This behaviour is described by (1):
     (1)
where B is the magnetic field, is the 
fluid shear rate and μ
p
 is the plastic 
viscosity (i.e., the viscosity at B = 0), 
G is the complex material modulus 
(which is also field dependent). τ
yd
 in 
equation (1) is a function of the mag-
netic field B and exponentially in-
creases with respect to the magnetic 
flux density. The relationship is given 
by:
τ
yd 
(B)
 
= kB
β 	
(2)
where the proportional coefficient k 
and the exponent β are intrinsic val-
ues of the MR fluid, which are func-
tions of various factors such as the 
magnetic field, the particle size, the 
particle shape and concentration, 
the carrier fluid, the temperature and 
the magnetic saturation. The applied 
magnetic field B is produced within 
the actuator when a current i is sup-
plied to the electromagnet encircling 
the MR fluid, i.e.,
B
 
= k
k
i
  
(3)
True MR fluid’s behavior exhibits 
some significant departures from this 
simple model. Perhaps the most sig-
nificant of these departures involves 
the non-Newtonian behavior of MR 
fluids in the absence of a magnetic 
field.
In general, the MR devices involve 
either disc-type or valve-type de-
signs. In valve-type designs, the fluid 
is pushed through a narrow channel 
where the magnetic field is applied to 
control the flow rate, and hence the 
applied force. Typically, these designs 
resemble a cylinder-piston assembly 
with the coil on the piston shaft. In 
disc-type designs, the fluid is in a nar-
row gap between a rotating disc and 
a fixed outer casing [6], [7]. The coil 
is positioned close to the outer edge 
of the disc. When the magnetic field 
is applied, the increased yield stress 
of the fluid creates a braking torque 
on the disc. 
The braking torque T
b
 developed by 
the MR fluids in the disc-type actuator 
can be determined as:
      
 (4)
where r
z
 and r
ω
 are the outer and in-
ner radii of the actuator disk, respec-
tively; and γ = r ω / h where ω is the 
angular velocity of the rotating disk 
and h is the thickness of the MR fluid 
gap [10]. Following (2), the equation 
(4) can be rewritten as:
(5)
Integrating (5) and substituting with 
(3) the braking torque developed by 
the MR fluids can be calculated:
     
(6)
τ = 
{
G
γ
τ
yd 
(B) + μ
p
γ
τ < τ
yd
 
τ > τ
yd
 
γ
T
b
 = 2 π 	� τ 	 r
2
dr = 
2 π 	� ( τ
yd
	 + 	 μ
p
γ)r
2
dr
r
z
r
w
r
z
r
w
T
b
 = 2 π 	� ( μ
p
 —+ kB
β
)r
2
dr
r
z
r
w
rω
h
T
b
 = —kk
β 	
(r
3
— rw
3
)i
β 	
+ 
— μ
p
 (r
4
— rw
3
) ω
2π
3
k z
π
2h
z
ELEKTROREOLOØKI FLUIDI

350
Ventil 16 /2010/ 4
Equation (6) shows that the braking 
torque developed in the circular plate 
MR fluid actuator can be divided into 
a magnetic-field-dependent induced 
yield-stress component T
b
 and a vis-
cous component T
μ
:
(7)
 3 MR fluid actuator 
experiments
For actuating the exercise device, a 
rotary MR fluid actuator produced by 
Lord Corporation, USA was used [8]. 
The Lord TFD Device RD-8043-1 is 
of the acquired threshold torque T
S
 
with regards to the input voltage V
C
. 
From the results a nonlinear relation-
ship can be observed.
In the second experiment, the dy-
namic characteristics of the MR fluid 
actuator were measured, evaluating 
the dependence on the motion speed. 
The braking torque was assessed 
during the motion in the forward and 
backward directions, moving with a 
different rotation velocity and with 
a constant MR fluid-actuator input. 
A family of curves was obtained that 
is presented in Figure 2. Each curve 
represents a typi-
cal characteristic 
of the braking 
torque. The pre-
sented values 
sum the yield 
stress compo-
nent T
B
 , the vis-
cous component 
T
μ
, and the static 
friction. From 
the acquired re-
sults a nonlinear 
torque-velocity 
relationship with 
a hysteresis loop 
can be observed 
[11].
 4 Exercise device 
for upper-extremity 
sensory-motor capability 
augmentation
The conceptual scheme of the 
training system for upper-extrem-
ity sensory-motor augmentation is 
presented in Figure 3. The system is 
designed to train the finger or wrist 
flexor and extensor muscles by per-
forming the position-tracking task. 
The reference and actual positions 
are displayed on a visual display as 
two rotational pointers. The MR fluid 
actuator is used as a braking torque 
modulating device that allows exer-
cise under isometric, isotonic or iso-
kinetic conditions. The core of the 
system is a personal computer (PC) 
that is used for reference generation, 
actual hand-force acquisition, visual 
presentation of the reference and ac-
tual position, and control of the MR 
fluid actuator. The software applica-
tion for controlling the system was 
developed in the Mathworks Matlab-
Simulink programming environment 
and it runs in the xPC real-time op-
erating system.
A close-up view of the exercise de-
vice is shown in Figures 4 and 5. The 
device is constructed from aluminium 
strut elements. On the construction, 
the MR fluid actuator is mounted, and 
on its axis an adjustable lever arm with 
a JR3 force/torque sensors (50M31A-
I25; JR3, Inc., Woodland, CA, USA) 
and a finger fixation are fixed. The 
fingers are fixed to the force sensor 
by means of a finger support and a 
Velcro strap. The finger fixation and 
the force sensor enable the acquisi-
tion of the hand forces. To ensure the 
proper position and to prevent the 
arm from moving during the training, 
the forearm is fixed to the arm sup-
port by Velcro straps. The position of 
the force sensor, the actuator and the 
forearm support is adjustable, allow-
T
b
 = T
B 
+ T
μ
 = k
i
i
β
 + k
ω
ω
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
0
0.5
1
1.5
2
2.5
3
3.5
4
V [V] C
|Ts| [Nm]
rotation in clockwise direction
rotation in anti-clockwise direction
Figure 1. Static torque threshold values versus the MR 
fluid excitation
ELEKTROREOLOØKI FLUIDI
-4 -3 -2 -1 0 1 2 3 4 5
-8
-6
-4
-2
0
2
4
6
8
 [rad/s]
M [Nm]
0.0 V
0.3 V
0.6 V
0.9 V
1.2 V
1.5 V
1.8 V
Figure 2. Dynamic characteristics of the MR fluid actuator
capable of producing up to 12 Nm 
of axial torque while it is driven by 
a current-driven power supply with 
current capabilities of up to 1.5 A. 
The device has a position feedback 
sensor integrated, which outputs a 
PWM signal with duty cycle varying 
between 5 and 95%, according to the 
position of the axis.
The torque output was measured us-
ing a test setup with a load cell, a lever 
arm, and a data-acquisition system. 
The braking torque experiment start-
ed with measuring the static torque 
threshold while manually rotating the 
actuator axis, first in the clockwise 
and then in the anti-clockwise direc-
tion. The threshold torque, which is 
actually the sum of the static friction 
and the magnetic-field-dependent in-
duced yield-stress component T
B
, was 
assessed in several repetitions with 
different input voltages. The graph in 
Figure 1 presents the absolute values 

351
Ventil 16 /2010/ 4
ing the accommodation of the meas-
uring setup to each individual, as well 
as to assess either the right or the left 
hand. Two PCI boards were used for 
data acquisition from the force and 
position sensors, and to generate the 
control voltage for the MR fluid ac-
tuator.
 5 Experimental evaluation
To demonstrate the performances 
of the developed exercise device an 
experimental evaluation was made. 
In the position tracking experiment a 
healthy subject was asked to follow 
the reference position, which was 
altered linearly in the range of ±30
o
 
from its center (fingers extended) po-
sition at 180
o
. During the motion, the 
braking torque was modulated by the 
MR fluid actuator according to the 
term:
(8)
in which the parameter α indicates the 
current position of the actuator axis in 
radians, and the sign ± is changed re-
garding the rotation direction (clock-
wise/anticlockwise). According to the 
term above, the braking torque had 
the highest value at the finger-extend-
ed position, while it diminished with 
the displacement from it.
The actual torque was measured 
while the MR fluid actuator activity 
was controlled by a PI controller with 
a feed-forward term according to the 
difference between the actual and the 
-
U/I
MR fluid
actuator
based
exercise device
visual feedback
controller
torque reference
PC - xPCTarget OS
motion reference
torque feedback
position feedback
130 140 150 160 170 180 190 200 210 220 230 0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
T ref[Nm]
alpha
Figure 3. Conceptual scheme of the training system showing its main com-
ponents: exercise device with force sensor and MR fluid actuator, visual 
feedback, and closed-loop controller
Figure 4. Close-up view of the exercise device from the 
left side
0 2 4 6 8 10 12
140
150
160
170
180
190
200
210
220
230
t [s]
? [ ]
anti-
clockwise
rotation
clockwise
rotation
anti-
clockwise
rotation
Figure 5. Close-up view of the exercise device from the 
right side
Figure 6. Motion trajectory in the experimental evaluation 
T
ref
 = ±0.6 ±0.5 
* 
sin(1.8 
* 
(α – 2.269))
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reference values. Figure 6 presents 
the actual motion trajectory achieved 
during the experimental evaluation. 
In Figure 7 the reference T
ref
 and ac-
tual T
act 
torques are shown.
 6 Conclusion
The paper presents the development 
and experimental evaluation of a 
semi-active exercise device for upper-
extremity sensory-motor capability 
augmentation. The device is built on 
the basis of the rotational magneto-
rheological actuator that allows resis-
tive torque modulation. The frame of 
the device is constructed to allow a 
flexible change of the configuration, 
while the controller is implemented 
in the Mathworks Matlab/Simulink 
environment and the real-time xPC 
Target operating system. 
The experimental results show that 
the MR fluid actuator is suitable for 
application in exercise devices as a 
semi-active element providing brak-
ing-torque modulation. On this ba-
sis, several exercise modes can be 
achieved. In comparison to electric 
motor actuators the power-to-weight 
ratio and the need for a power am-
plifier is advantageous in the case of 
MR fluid actuator usage. On the other 
hand, the control is more complex 
since the MR fluid actuator is a highly 
nonlinear device. 
Figure 7. Reference torque tracking of the MR fluid actuator
0 2 4 6 8 10 12
-1.5
-1
-0.5
0
0.5
1
1.5
t [s]
T [Nm]
T
ref
acw
T
ref
cw
T
anti-
clockwise
rotation
clockwise
rotation
act
anti-
clockwise
rotation
The proposed areas of application for 
exercise devices based on MR fluid 
actuators are in rehabilitation and 
sports training.
References
[1] H. I. Krebs, N. Hogan, M. L. Ai-
sen, B. T. Volpe, Robot-Aided 
Neurorehabilitation, IEEE Trans. 
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Augmentation, Neuromodulati-
on, Vol. 11, No. 3, pp. 208-215, 
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[3] D. Senkal, H. Gurocak, Spheri-
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[4] S. Dong, K-Q Lu, J. Q. Sun, K. 
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[6] W. H. Li, H. Du, Deesign and 
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[7] Y. Yan, H. Shi-guo, K. Bo-seon, 
Research on Circular Plate MR 
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Univ. Technol., Vol. 14, suppl. 
1, pp. 257-259, 2007.
[8] M. R. Jolly, J. W. Bender, J. 
D.Carslon, Properties and Appli-
cations of Commercial Magne-
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Passive Damping and Isolation, 
Vol. 3327, pp. 262-275, 1998.
[9] E. Guglielmino, T. Sireteanu, 
C. W. Stammers, G. Ghita, M. 
Giuclea, Semi-active Supsensi-
on Control, London, England: 
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[10] E. J. Park, D. Stoikov, L. Falcao 
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[11] S. Guo, S. Yang, C. Pan, Dyna-
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Naprava za vadbo roke na osnovi mehanskega œlena z 
magnetoreoloøko tekoœino
Razøirjeni povzetek
Vadba z gibanjem proti uporu je uveljavljena v rehabilitaciji in øportu. Pri-
spevek predstavlja sistem za krepitev senzomotoriœnih sposobnosti gornjih 
ekstremitet, ki je razvit na osnovi œlena za zagotavljanje upora s pomoœjo 
magnetoreoloøke tekoœine. Sistem je namenjen vadbi fleksorjev in eksten-
zorjev prstov z izvanjanjem naloge sledenja pozicije. Naprava je zgrajena 
kot prilagodljiva mehanska konstrukcija na katero sta nameøœena opora za 
podlaht ter œlen za zagotavljanje upora z vpetjem za prste in senzorjem 
sile. Med vadbo je upor gibanju zagotovljen glede na povratno informa-
ciji o sili s pomoœjo rotacijskega aktuatorja, ki vsebuje magnetoreoloøko 
tekoœino. Magnetoreoloøka tekoœina (ang. magnetoreological (MR ) fluid) 
je tekoœina, ki ima spremenljive reoloøke lastnosti glede na vpliv magne-
tnega polja. MR tekoœino tvori osnovna nemagnetna tekoœina v kateri se 
nahajajo mikronsko veliki delci, ki se pod vplivom polja polarizirajo. V 
odsodnosti magnetnega polja se ti delci prosto gibljejo, ko pa je tekoœina 
izpostavljena magnetnemu polju, se delci zaœno povezovati v veriæne 
strukture. Veriæne strukture spreminjajo viskozne karakteristike pretoka, 
hkrati pa je zaradi njih od magnetnega polja odvisna tudi meja teœenja 
(ang. yield stress). S pomoœjo mehanskega œlena, v katerem se nahajajo 
rotor v obliki diska, MR tekoœina in elektriœna tuljava, je mogoœe na osi 
ustvariti moment upora, ki je voden z elektriœnim signalom. S pomoœjo 
zaprtozanœnega vodenja glede na informacijo o navoru in pomiku pa je 
lahko tovrstni MR œlen uporabljen za zagotavljanje vadbe v izometriœnih, 
izotoniœnih ali izokinetiœnih pogojih. 
Rezultati vadbe s pomoœjo razvite naprave in MR œlena kaæejo, da je pri 
zagotavljanju pasivnega upora MR œlen moæno uporabiti kot enakovre-
dno zamenjavo elektriœnim, hidravliœnim ali pnevmatskim aktuatorjem. 
Prednosti uporabe so varnost, majhne dimenzije in teæa ter velika ener-
gijska uœinkovitost. Kompleksnejøe je vodenje, saj je MR œlen aktuator z 
nelinerano karakteristiko.
Kljuœne besede: rehabilitacija, naprava za vadbo, magnetoreoloøka 
tekoœina
Acknowledgment
The authors would like to acknowledge the financial support from the 
Slovenian Research Agency (P2-0228), and the Slovenian Technology 
Agency (3211-05000550).
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