Voluntary-Opening
Prehensors with Adjustable
Grip force
Daniel D. Frey, MS
Lawrence E. Carlson, DENG
Vidya Ramaswamy, MS
ABSTRACT
A novel concept for adjusting grip force
in voluntary-opening prehensors is introduced. The vector mechanism allows
the user to rotate a stiff elastic band
within the prehensor to change grip
force substantially with very low energy
input. Two prototype vector prehensors
are described: one modeled after a split
hook, the other modeled after a TRS
Grip III The performances of the prototypes are evaluated and compared
with those of commercially available
prehensors primarily through laboratory testing.
The vector prehensors allow grip
force to be adjusted quickly and easily
among 13 settings ranging from 1/2 to 20
lbs. This added capability was achieved
without adding significant weight or
bulk to the device.
Preliminary field tests demonstrated
the practical feasibility of the device, but
a clinical evaluation has not been performed.
Introduction
Although 85,000-90,000 people in the
United States have lost an upper limb
(1), only about 34,000 (40 percent) use
prosthetic arms or hands (2). The low
percentage of upper-limb amputees using prostheses may be a result of many
factors, including financial limitations
and social reasons, but the authors submit that a major factor is the limited
function and flexibility of the devices
currently available. Therefore, there is
strong motivation for engineering research and development to improve
upper-limb prostheses.
About 90 percent of upper-limb amputees in the United States use body-powered prostheses, which are more reliable, lighter, quieter and less expensive than externally powered prostheses (3). Still, the authors believe there is
considerable room for improvement in
the technology, and advances in body-powered prostheses can have a significant effect on the lives of many amputees.
Among body-powered prehensors,
voluntary-opening (VO) control is used
more widely than voluntary-closing
(VC) control. In a VO prehensor,
springs or elastic bands provide grip
force. This is advantageous because the
amputee may relax cable tension while
maintaining grip. However, a typical
VO prehensor provides only one level
of grip force while the appropriate grip
force for common daily activities (from
holding a paper cup to swinging a hammer) varies widely. In most cases, the
amputee will select a grip force high
enough to allow confident grasp during
the heaviest tasks he/she frequently
performs. However, light grasps are
used much more frequently than heavy
grasps (4) meaning the grip force of any
given prosthesis is usually greater than
necessary. This causes two major difficulties:
- The amputee must exert more energy and apply more force than is required for most tasks. This causes fatigue and discomfort since greater
forces in the cable mean higher pressure applied to the skin through the
harness. Higher cable forces also reduce the service life of prosthetic components such as cables, cable housing
and associated hardware.
- It is very difficult for the amputee
to hold an object gently. The only way
to reduce grip force is to maintain cable
tension, which defeats the primary advantage of VO prehension.
A VO prehensor that allows easy adjustment of grip force over a wide
range would reduce these problems. A
number of attempts have been made to
produce such a terminal device. One
example is the Sierra 2-load voluntary opening hook (see Figure 1
). This design employs two torsion springs: One
is engaged at all times, and the other is
engaged only in the high grip-force setting when a switch is activated. Thus,
the design provides two grip-force settings: 3 1/2 and 7 lbs.
The Sierra 2-load hook is comparable in size and durability to the Hosmer
Dorrance 88X (medium adult) hook
and is easy to adjust between high and
low grip-force settings. (Measured from
the wrist to the hook ends, the Sierra 2-load hook is more than 3/4-inch longer
than the 88X hook, but the actual
hooks on the Sierra 2-load hook are
less than 1/8 -inch longer than those of
the 88X.) Still, the design has not
gained wide acceptance, perhaps due to
its limited range of adjustability, limited
number of settings and relatively high
weight (over twice the weight of the
88X).
Another design that provides variable grip force is the Stanford Children's Hospital adjustable prehension
device (see Figure 2
). It fits onto any
Hosmer Dorrance VO split hook and
allows the amputee to add and remove
one or two modules containing elastic
bands prestretched to the hook. Each
module adds a grip force of 1 1/2 lbs to
the base grip force of about 1 lb. The
amputee must carry modules separately and thus could potentially lose the
modules. Also, a fair amount of effort is
required to adjust the grip force. For
these reasons, this design also failed to
gain wide acceptance and is no longer
commercially available.
Past variable grip VO designs have
failed in the marketplace not from a
lack of need for them, but because they
do not fulfill all of the demands of upper-limb amputees. The goal of this research has been to develop VO prehensors that have a much wider range of
grip force than previously available yet
are quick and easy to adjust and comparable in size and weight to the most
popular prehensors on the market.
The purpose of this article is to present the results of the authors' engineering research and development effort. The concepts behind the devices
are explained, the prototypes are described, and the results of laboratory
testing are discussed. To help guide the
development process, the authors solicited comments from a few individuals who field-tested the prototypes. The
results of the preliminary field tests
provide some anecdotal evidence of
the strengths and weaknesses of the
prehensors developed. However, this
article is not intended to provide a clinical evaluation of the prototypes.
The Vector Prehensor Concept
Figure 3
depicts a split-hook prehensor
and a simplified schematic showing the
forces that make it work. The grip force
applied by the prehensor is proportional to the torque applied by the bands
about the pivot. One obvious way to
change the torque is to vary the band
force. This is the strategy employed by
the designers of the Sierra 2-load hook
and the Stanford Children's Hospital
adjustable prehension device. With this
approach, an additional band or spring
is added for each grip-force setting. The
number of settings must be limited to
avoid excessive bulk or complexity.
Another way to change the band
force is to increase the tension in the
bands. To do so, energy must be added
to the bands, making the procedure
physically strenuous or time-consuming.
To avoid these difficulties, the grip
force can be adjusted by varying the
band angle instead. A vector, such as
band force, is defined by its direction as
well as its magnitude. The concept behind the vector prehensors is to be able
to change the direction of the band
force while leaving its magnitude constant and to be able to swing the band
on an arc without changing its length.
When the band angle is 90 degrees
(as shown in Figure 3
), grip force is
maximized. As the band angle is reduced, the torque applied by the band
about the pivot decreases. Because the
band is not stretched or relaxed, no energy change is required, and the adjustment can be made with minimal effort.
Since the band angle can approach zero, the grip force can be adjusted as low
as desired. Because band angle can
vary continuously, many different gripforce settings can be provided without
added complexity. Since these advantages are afforded by exploiting the
vector nature of the band force, the
prototypes have been named "vector
prehensors."
To demonstrate the utility of the vector prehensor concept, several prototypes were constructed in two basic
configurations: a split hook and a TRS
grip-type prehensor. A VO vector hand
also is possible but has not yet been designed. The next section describes the
two most advanced prototypes developed to date.
Description of Prototypes
The vector hook prototype is shown in
Figure 1
. The body is made of black anodized aluminum. Neoprene-lined aluminum fingers from a Hosmer Dorrance 88X split hook mate with the case
and are screwed in place. The prototype
therefore can be configured as a left- or
right-handed device by replacing the
fingers. The shape, size (5 1/4 inches long)
and weight (4 1/4 oz) of the device are
very close to those of an 88X hook (4 5/8
inches long and 4 oz). Unlike the 88X,
however, the vector hook prototype
does not have a protrusion near the cable attachment.
The vector grip prototype is shown in
Figure 4
. The body is made of a titanium alloy. The molded polyurethane fingers were adapted from a TRS Grip III
prehensor. This device is symmetrical
and therefore can be worn on the right
or left side. It is similar in shape, size
(5.64 inches long) and weight (9.34 oz)
to the TRS Grip III (5.70 inches long
and 9.80 oz). This is one of the first prototypes to employ grip-type prehensile
shapes in a VO prehensor.
In the vector prehensors, adjustment
of grip force is accomplished by depressing and then sliding a rubber-coated button (see Figure 5
).The deliberate
actions of pressing then sliding are designed to minimize the possibility of accidental adjustment. The lowest gripforce setting is nearest the wrist, and
the highest grip-force setting is at the
most distal position. There are a total of
13 settings. The button is wide and accessible so adjustment can be accomplished employing any available surface (e.g., against a countertop or one's
leg). Adjustment can be made without
use of the opposite hand and without
visual cues. The grip force can be adjusted only when the prehensor is fully
closed; when the prehensor is open, the
button cannot be depressed.
The vector prehensors are powered
by two custom-molded elastomer
bands. The best design to date will last
for more than 70,000 cycles at the highest grip-force setting. The bands are
contained within the casing where they
are well-shielded from abrasions, nicks
and sunlight. The casings are interwoven so the bands are covered even
when the hooks are fully open. The de
sign intent is such that the bands should
last the life of the prehensor, but more
thorough field-testing will be required
to establish if the prototype elastomer
bands meet this goal.
With the current design, the elastomer bands can be replaced by a single
individual with Phillips' head and flathead screwdrivers in about 10 minutes.
The prehensor itself acts to pretension
the bands so no specialized tools are required.
Test Methodology
Laboratory testing of the vector prehensors was performed to determine
their grip-force performances at various angles of opening and at all available grip-force settings. The button on
the side of the prehensor was used to
set the grip force to its lowest setting.
Grip force was recorded at opening angles from 50 to nearly 0 degrees. For
every angle examined, the grip-force
setting was increased by one increment,
and the testing was repeated until the
performance at each of 13 settings had
been evaluated.
Grip force was measured by a curved
beam grip-force transducer. The outside of the curved portion of the beam
was fitted with strain gauges for computerized data acquisition (see Figure
6
). As the grip transducer was compressed, the strain gauges were placed
in tension causing their resistance to increase. This change in resistance was
measured and converted to a force
measurement then stored in a computer. Bolts of various lengths were fitted
to the grip transducer to set the angle
of opening of the prehensors to values
from 50 to 10 degrees in 10-degree increments. The opening angle was measured with a protractor.
The technique described did not permit measurement of grip force at opening angles less than 10 degrees due to
the thickness of the grip transducer.
Therefore, to measure the grip force at
a nearly 0-degree opening angle, the
prehensors were pulled open a small
amount with a string (see Figure 7
).The
tension in the string was measured with
a load cell (an electronic device for
measuring forces).
For comparison, commercially available split hooks also were tested. The
grip-force performance of a Hosmer
Dorrance 88X (medium-sized adult
aluminum split hook) was measured
with one, three, eight and 10 standard
prosthetic bands installed. These numbers of bands were chosen to represent
the wide range of grip forces typically
used by upper-limb amputees. A Sierra
2-load hook also was tested to permit
comparison with a currently available
adjustable prehensor. Its grip-force
performance in both the low and high
settings was measured.
Limited field-testing was conducted
by subjects with varying levels of amputation. A transradial, a transhumeral
and a wrist disarticulation amputee (all
unilateral) each wore the vector hook
in place of his/her usual terminal device. Following a test period of several
days, each tester completed a questionnaire eliciting his/her observations on
the performance of the prototype.
The primary purpose of the field-testing was to provide rapid feedback
on the prototypes for continued research and development and to gauge
the economic value of the design concept. The field-testing was not intended
as a scientific clinical evaluation of the
prototype. The appropriate methods
for defining a sample population, selecting subjects, etc., were not employed. Therefore, the authors do not
claim the field-test results scientifically
demonstrate the advantages of the vector prehensor prototypes. The results of
the field tests are offered only as anecdotal evidence of the feasibility of the
vector concept and its current embodiment and to point out areas in need of
further development.
Results
The results of laboratory testing of the
vector hook are shown in Figure 8
. The
grip force as measured at the tips of the
hook fingers is plotted against the
opening angle of the hooks for each
available grip-force setting. In the lightest setting, the grip force remains nearly constant at 1/2-lb throughout the range
of opening. In the highest setting, grip
force begins at about 11 lbs and gradually rises to nearly 20 lbs as the hooks
are opened. The 11 intermediate settings provide a choice of grip-force levels evenly spaced between these two
extremes.
Figure 9
compares the vector hook's
performance with those of similar prehensors. The vector hook at its highest
setting performs roughly the same as
an 88X hook with eight to 10 bands and
provides nearly twice the grip force of
the Sierra 2-load hook at its high setting. The vector hook's lowest setting
was designed as the lightest possible
grip that still allows the hook to close
while pulling 1/16-inch steel prosthetic cable through unlined steel cable housing. In the lowest setting, the vector grip
can hold a Styrofoam coffee cup without crushing it. This is not possible with
the 2-load hook or with an 88X hook
that has only one standard prosthetic
band.
The results of the field-testing, although preliminary, are very encouraging. The vector hook was rated the
same as or superior to standard VO
hooks by each tester in every activity
rated. Each tester responded that
he/she would replace his/her standard
VO hook with a vector hook if it were
available. One tester noted that the
highest position is useful for holding
tools but requires too much force to
open without excessive stress on the cable and other prosthetic components. A
longer lever arm for cable attachment
could be employed to alleviate this
problem. One tester noted that a stainless steel version of the vector hook
would be preferable for performing
tasks that involve heavy work.
Future Work
Because the vector prototypes have
proven promising, an application has
been submitted for a patent on the vector prehensor mechanism. Once the
patent is issued, commercial production
of vector prehensors will commence.
However, before such an effort begins, some engineering challenges must
be met. An elastic element that consistently will last the life of the device
should be developed to eliminate band
replacement. A high fatigue life elastomer with adequate stiffness and
strain to failure must be identified. The
shape of the band should be optimized
to evenly distribute stress. A reliable
process of bonding an elastomer to a
bushing should be developed.
Manufacturing processes that will allow vector prehensors to be produced
at a competitive cost must be identified. Some components of the prototypes may be redesigned for ease of
manufacture and assembly. The prosthetics research group at the University
of Colorado has already begun to address these issues.
Several questions remain to be answered through clinical evaluation. It
remains to be established that use of
vector prehensors in place of standard
VO devices results in measurable improvement in performance of tasks. Also, since lighter settings may be used
frequently and require less cable tension to actuate, one may test the hypothesis that vector prehensors increase the service life of prosthetic
components (such as cable, housing
and other hardware). It may be valuable for future engineering development to establish how the vector prehensors tend to be used (i.e., how often
the lightest and heaviest settings are
employed) and how durable they are in
extended field service.
As vector prehensors are introduced
in the marketplace, it will be important
to determine what training can be provided to the users to allow them to
make the most of their new capabilities.
Over a longer term, the health benefits
(e.g., reduced harness discomfort) resulting from use of vector prehensors
should be investigated.
Conclusion
Laboratory tests demonstrate the prototype vector prehensors provide a
range of grip forces far exceeding those
of any currently available voluntary opening prehensor. They allow grip
force to be varied with minimal effort
among 13 settings ranging from 'A to 20
lbs. The improved function of the prehensors was made without incurring
any substantial penalty in weight or
bulk. Preliminary field tests suggest
vector prehensors will make everyday
tasks easier for upper-limb amputees,
but this claim remains to be substantiated by clinical evaluation.
Acknowledgments
The contributions of Robert Radocy as a
consultant and as a field evaluator are
gratefully acknowledged. We thank
Richard Stewart and Donald Delforge for
field-testing the prototypes. This research
was supported by the NCMRR under
Grant 1-RO1-HD30131-O1.
DANIEL D. FREY MS, is a graduate student in mechanical engineering at the Massachusetts Institute of Technology in Cambridge, Mass.
LAWRENCE E. CARLSON, DENG, is a
professor of mechanical engineering at the
University of Colorado at Boulder in Boulder, Colo.
VIDYA RAMASWAMY MS. is a graduate student at Pennsylvania State University
in University Park, Pa.
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