1st Annual IEEE Healthcare Innovation Conference of the IEEE EMBS
Houston, Texas USA, 7 - 9 November, 2012
Force Characterization of a Body-Powered Hybrid
Upper Extremity Prosthesis
John A. Martinez, Melissa A. Saucedo, Mark Gorelick, Stephen A. Wallace, Ozkan Celik, Member, IEEE
Abstract— In this study, we present the results of an experimental force characterization protocol conducted for a bodypowered hybrid upper extremity prosthesis that is able to
operate in both voluntary closing (VC) and voluntary opening
(VO) modes. Globally, body-powered prostheses are still the
most preferred alternative after almost two centuries from
their first implementation. They commonly constitute the best
available option, especially in resource-limited settings. The
purpose of this study is to obtain the relationship between the
cable tension applied at the shoulder level and the resulting grip
force for a body-powered hybrid prosthesis, under quasi-static
conditions. The understanding of this relationship will aid in
the development of human subject experiments examining the
usability of a concurrent VO/VC switching mechanism. Results
of the force characterization protocol indicate that the mapping
between grip force and cable tension does not significantly
depend on the aperture size and that there is non-linear friction
in force transmission that affects the repeatability of forces.
Index Terms— Upper extremity prosthesis, body-powered
prosthesis, force characterization
I. I NTRODUCTION
The loss of a hand or arm is clearly a traumatic event usually resulting in profound physical, psychological, emotional
and vocational consequences. There are approximately 1.7
million people with amputations living in the United States
and of those, well over 500,000 involve the upper extremity
[1]. While the number of cases due to trauma and cancer are
leveling off or declining, amputation due to vascular disease,
particularly diabetes mellitus, are on the rise and likely to
double by the year 2050.
To maintain function in activities of daily living, many
people with upper extremity amputations choose to wear
a prosthetic limb. While the use of, and satisfaction with,
upper extremity prostheses have been thoroughly investigated [2], far fewer studies have examined how the upper
extremity prosthesis is controlled by the user (see [3]–
[5]). Complicating this observation is the fact that there are
several types of prostheses available, each type requiring
presumably different motor control strategies for effective
use. While there is a variety of upper-extremity prostheses,
for several reasons including cost, ease of operation and
ease of maintenance, the body powered prostheses are still
the most preferred alternative after almost two centuries
from their first implementation. Body powered prostheses
J. A. Martinez, M. A. Saucedo, O. Celik are with the School of
Engineering and M. Gorelick, S. A. Wallace are with the Department of Kinesiology, San Francisco State University, San Francisco, CA
94132 (e-mails: john1111@mail.sfsu.edu, msaucedo@mail.sfsu.edu, gorelick@sfsu.edu, saw@sfsu.edu, ocelik@sfsu.edu)
Fig. 1. San Francisco State University (SFSU) Hybrid Prosthesis and the
experimental rig used in force characterization of the prosthesis. FSR: force
sensing resistor.
commonly constitute the best available option, especially in
resource-limited settings.
In body powered devices, the prosthesis is controlled by
the action of several muscles, for example a shoulder or arm
muscle, which is transferred to the prosthesis by means of
a cable. There are two major types of upper extremity body
powered prosthesis, voluntary opening (VO) and voluntary
closing (VC). The VO prosthesis is held closed by means of
springs or rubber bands, and the action of the user’s muscle
opens the hand. The advantage of the VO is that once an
object is grasped, the user can relax and does not have to
exert any effort at maintaining the grip on the object. The
disadvantage is that the grip force of the hand is limited
by the strength of the springs or bands and there is an
incompatible relationship between the muscular effort used
and the resulting grip force [5]. The VC prosthesis is held
open by springs or bands and is closed by action of the
user. The advantage of the VC prosthesis is that its function
more closely resembles that of the anatomical hand, in which
there is a compatible relationship between muscular effort
used and the resultant grip force. The disadvantage is that
the user must apply continued effort to maintain a grip on
an object.
A person awaiting a new prosthesis currently has to
decide between being fitted with a VO or a VC prosthesis.
The choice they make represents a long-term commitment,
involving not only the expense of the prosthesis, but the time
and cost of training and rehabilitation with the device. It is
our view that making this choice is unnecessarily restrictive.
For this reason, as part of an earlier project, a prototype of a
new, body-powered prosthesis, the San Francisco State Uni-
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versity (SFSU) Hybrid (VO/VC) Prosthesis was designed,
developed and built (see Fig. 1). The SFSU Hybrid Prosthesis
is designed to allow the user to utilize the advantages of the
VO and VC prehensors in one prosthesis [6].
A model, or more specifically, a force input and output
characterization of the prosthesis is highly desirable to inform design of human subject experiments that will focus
on quantifying the advantages and disadvantages imposed
by using VO and VC mechanisms. The purpose of this
article is to present the results of force input and output
characterization tests of the SFSU Hybrid Prosthesis. These
tests consisted of two parts: a calibration stage that involved
calibration of force sensing resistors (FSRs) versus a high
precision force sensor by application of controlled loads; and
a mechanical testing stage which involved use of calibrated
FSRs in measuring grip force at the prehensor end and of a
load cell in measuring cable tension at actuation end.
The scope of the characterization methods in this project
has been limited to quasi-static loads (steady state relationships between input and output) and to VC operation only.
The quasi-static relationship between the cable tension and
grip force at the actuator is determined by the lever arm
and internal gear ratio, as well as the spring force that
returns the lever arm to its normal position and friction
in cables, gears and bearings. Although it is possible to
account for the effects of some of these components on the
force input-output relationship in calculation, effects of other
components such as friction can accurately be determined via
only experimental testing.
The paper is structured as follows: Section II describes
the experimental setup, protocol, and calibration methods.
Section III summarizes the results of the calibration and force
characterization experiments and includes a discussion of the
results.
II. M ETHODS
A. Calibration of Force Sensing Resistors
The first part of the experimental protocol involved obtaining voltage-to-force calibration relationship for the FSRs
Although FSRs provide a cost-efficient way to measure
contact forces, their outputs can vary significantly based on
the mechanical properties of the material they are placed
on and the contact area. Hence, they do not come with a
universal calibration and have to be calibrated under the
conditions of a specific task.
In the calibration protocol, we placed the FSR on an
aluminum plate (thickness: 3mm) and guaranteed a minimal
contact area (point contact) by using a wooden ball (see Fig.
2). This was to establish equivalent conditions to the second
part of the experimental protocol, in which the prosthetic
finger pressed on the FSR via a point contact.
An ATI Industrial Automation Nano17 6-axis
Torque/Force Sensor (for the axis used, sensitivity:
1/160N, range: 35N) was used for obtaining accurate and
reliable force readings. To gather the calibration data, an
increasing series of weights ranging from 100g to 3800g
were placed in-line with the FSR and the ATI force sensor
Fig. 2. Experimental setup used in the calibration of the force sensing
resistor (FSR) via force readings from an ATI Nano17 force/torque sensor.
The wooden ball helps maintain a point contact on the FSR, to provide an
equivalent condition to the prosthetic finger’s point contact in characterization tests.
sustained by the experimental setup as seen in the Fig. 2. A
relationship was attained between the force applied to the
FSR and its voltage output. These results were scrutinized
to determine whether the FSR is a reliable source of data to
determine the gripper force vs. cable tension relationship,
the criteria for this being repeatability. Also, to test whether
small variations in the position of the applied force would
drastically alter the results, a separate set of tests were
conducted where the applied force was offset from the
center in four different directions.
B. Force Characterization
The second part of the experimental protocol built on
top of the satisfactory repeatability and consistency of the
FSR calibration data as reported in Section III. The second
experiment aimed to obtain the mapping between the tension
on the actuating cable and the grip force applied at the
gripper end. To find this relationship, the FSR was placed
in a pinch grip at the prehensor as varying levels of tension
were applied to the cable. The tension on the cable was
measured using an in-line load cell (Interface SML-100,
sensitivity: 11.35 mV/N, range: 444 N), which, including the
FSR, were connected to a Noraxon Telemyo data acquisition
system. A target tension was applied to the actuating cable
manually using the tension adjustment lever (see Fig. 1) and
readings were recorded after 7 seconds, which was chosen as
a sufficient settling time. Three tests were completed in this
fashion for each aperture size, which varied among small,
medium, and large cases as made possible via the aluminum
plate and wooden blocks (see Fig. 1). The opening at the
pinch grip was 3mm, 18mm and 36 mm for the small,
medium and large blocks, respectively.
III. R ESULTS AND D ISCUSSION
A. Calibration of Force Sensing Resistors
Fig. 3 summarizes the results of the calibration tests in
log-log (base 10) plots. Data sets collected for both the
centered condition (Fig. 3(a)) and the offset condition (Fig.
3(b)) indicate that the variation among different tests and
conditions are sufficiently small for obtaining a satisfactory
calibration. The individual line fits (and line fit parameters)
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Test 1 Test 2 Test 3 Test 4 Line Fit (Test 1) Line Fit (Test 2) Line Fit (Test 3) Line Fit (Test 4) 1.8 Force Sensor (Log Newtons) 1.6 1.4 1.2 2 Down Offset Up Offset 1.8 Right Offset 1.6 LeB Offset 1.4 Line Fit (Down Offset) y = 4.150x -­‐ 0.75 R² = 0.992 Line Fit (Up Offset) y = 4.114x -­‐ 0.722 R² = 0.983 Force Sensor (Log Newtons) 2 y = 4.216x -­‐ 0.8377 R² = 0.991 y = 4.956x -­‐ 1.2152 R² = 0.989 y = 4.335x -­‐ 0.9185 R² = 0.975 y = 5.643x -­‐ 1.6759 R² = 0.98 Line Fit (Right Offset) y = 3.610x -­‐ 0.5167 R² = 0.881 1.2 1 0.8 Line Fit (LeB Offset) y = 3.813x -­‐ 0.6173 R² = 0.969 1 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 0.1 0.2 0.3 0.4 0.5 Force Sensing Resistor (Log Voltage) 0 0.6 0 0.1 0.2 0.3 (a)
Fig. 3.
0.4 Force Sensing Resistor (Log Voltage) 0.5 0.6 (b)
Results of the calibration tests for cases (a) force applied to the center of the FSR (b) force applied with a small offset in reported direction.
30 30 25 25 Grip Force (Newtons) 35 Grip Force (Newtons) 35 20 20 Test 1 Test 2 Test 3 15 Test 4 Test 5 15 Test 6 10 10 5 5 0 0 0 20 40 60 80 100 Cable Tension (Newtons) 0 120 20 40 60 80 Cable Tension (Newtons) (a)
100 120 (b)
35 30 Grip Force (Newtons) 25 20 Test 7 Test 8 15 Test 9 10 5 0 0 20 40 60 80 Cable Tension (Newtons) 100 120 (c)
Fig. 4. Results of force characterization experiments, summarizing the relationship between cable tension at shoulder level and corresponding grip forces
produced at the prehensor. (a) Force relationship for small aperture size (b) Force relationship for medium aperture size (c) Force relationship for large
aperture size.
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show sufficiently small deviations with respect to each other
justifying the repeatability of the sensor output. Based on
this data, we concluded that the FSRs can be used to
determine the contact force exerted by the gripper of the
prosthetic simulator. The data points from all eight test were
combined in obtaining a single power function calibration
curve between FSR’s voltage output and corresponding force
measurements. This calibration equation of the FSR was
determined as
Force = 0.159VFSR 4.136 .
(1)
B. Force Characterization
Results of the force characterization tests are summarized
in Fig. 4. It is observed that although the aperture size has
an effect on the relationship between cable tension and grip
forces, this effect is very small and may be negligible in
certain experimental scenarios. Increasing aperture size leads
to a decreasing sensitivity of grip force as can be observed
from the decreasing slope of the data points from Fig. 4(a) to
Fig. 4(b) and from Fig. 4(b) to Fig. 4(c). Another important
observation is the dead zone for the grip force for all three
aperture sizes. The grip force does not exceed 3N until the
cable tension reaches 60N for small and medium aperture
sizes, and until it reaches 45N for the large aperture size.
Although the general shape of the force relationship is an
S-curve or resembles a sigmoid function, it seems possible
to assume a linear relationship past the dead zone in the low
force region, especially for the large aperture size.
Although the force relationships are mostly repeatable
across three test runs, there are non-linearities in the force
transmission path causing significant variations at the cable
tension, for obtaining a specific grip force. These variations
take a maximum value of approximately 10N for the small
and medium aperture size, and of approximately 8N for
the large aperture size. These variations are well above
the human force control resolution of 0.4N at shoulder [7]
and hence can lead to problems in usability or in future
human subject experimental testing protocols, by making it
difficult to generate a target grip force in a consistent fashion
via shoulder actuation. This indicates that either the human
subject experiment protocol needs to be designed so that
the target grip forces lie in a range where force relationship
is relatively consistent, or that the non-linearities within
the device need to be eliminated or reduced for improved
repeatability.
It is also worth discussing the underlying mechanisms
behind the variations in force mapping function, or the
specific non-linearities of the device. We believe that these
variations can mostly be attributed to the frictional forces
in the gear train, based on the observation that the gear
train induces considerable changes in the device’s force
output which exhibits configurational dependence on the gear
positions. Hence we believe that the repeatability of the force
mapping can be improved by reducing the friction in the gear
train.
IV. C ONCLUSION
We conducted a force characterization of a body-powered
hybrid upper extremity prosthesis to reveal the mapping
between the cable tension applied at the shoulder level and
the resulting grip force, under quasi-static conditions. A
calibration protocol was completed for force sensing resistors
(FSRs) by using a highly accurate and precise force sensor.
Calibrated FSRs were then used to measure grip forces in
a second experiment, while a linear load cell was used
to monitor cable tension which was manually varied. The
characterization revealed that the mapping between grip force
and cable tension only slightly change for varying aperture
sizes and that there are non-linear friction components,
mostly due to friction in gears, that affects the repeatability
of the force mapping.
V. ACKNOWLEDGEMENTS
We gratefully acknowledge Mechatronics and Haptic Interfaces Laboratory at Rice University and Dr. Marcia K.
O’Malley for the ATI force/torque sensor used in this study.
R EFERENCES
[1] K. Ziegler-Graham, E. J. MacKenzie, P. L. Ephraim, T. G. Travison, and
R. Brookmeyer, “Estimating the prevalence of limb loss in the united
states: 2005 to 2050,” Archives of Physical Medicine and Rehabilitation,
vol. 89, no. 3, pp. 422–429, 2008.
[2] L. E. Pezzin, T. R. Dillingham, E. J. MacKenzie, P. Ephraim, and
P. Rossbach, “Use and satisfaction with prosthetic limb devices and
related services,” Archives of Physical Medicine and Rehabilitation,
vol. 85, no. 5, pp. 723–729, 2004.
[3] D. L. Weeks, S. A. Wallace, and D. I. Anderson, “Training with
an upper-limb prosthetic simulator to enhance transfer of skill across
limbs,” Archives of Physical Medicine and Rehabilitation, vol. 84, no. 3,
pp. 437–443, 2003.
[4] D. L. Weeks, D. I. Anderson, and S. A. Wallace, “The role of
variability in practice structure when learning to use an upper-extremity
prosthesis,” JPO: Journal of Prosthetics and Orthotics, vol. 15, no. 3,
p. 84, 2003.
[5] S. Wallace, D. I. Anderson, M. Trujillo, and D. L. Weeks, “Upper
extremity artificial limb control as an issue related to movement and
mobility in daily living.” Quest, vol. 57, no. 1, p. 14, 2005.
[6] T. Sullivan and K. S. Teh, “Design and fabrication of a hybrid bodypowered prosthetic hand with voluntary opening and voluntary closing
capabilities,” in Proc. ASME International Mechanical Engineering
Conference and Exposition (IMECE), 2011.
[7] H. Z. Tan, M. A. Srinivasan, B. Eberman, and B. Cheng, “Human
factors for the design of force-reflecting haptic interfaces,” Dynamic
Systems and Control, vol. 55, no. 1, pp. 353–359, 1994.
272