Developing a Low Cost and Easily Manufactured Prosthetic
Hand
MaryAnne Haslow-Hall
Scott Pierce Ph.D.
Sweet Briar College
Sweet Briar, VA
ABSTRACT
The loss of a limb can be a life-changing event that can cause grief and decreased
self-esteem. The ability to restore functionality and cosmetic results to a limb deficient
person can be a challenging yet rewarding pursuit. This research seeks to develop a
prosthetic hand that is low cost, functional and aesthetically pleasing. The goal of this
research is to develop a hand that satisfies the criteria mentioned above and enables the
user to pick up and manipulate small objects. Our goal is to develop a lower cost
alternative for those who may be unable to afford expensive, state of the art prosthetics.
INTRODUCTION
The earliest account of the use of prosthetics has been dated as far back as the
Egyptian civilization. In 2000, a tomb near the ancient city of Thebes was discovered that
contained a female mummy wearing a wooden prosthetic toe. [1] This discovery
provided evidence that prosthetics have been used for thousands of years.
The use of prosthetics has, however, changed over time. From the time following the
Egyptian civilization up to the 15th century prosthetics were used in battle and to hide
deformity [2]. The most common prosthetics were the basic peg leg and hand hook. By
the 16th century these devices gained functionality by incorporating springs and gears. It
1
wasn’t until 1861 when the American Civil War began that prosthetics really took off.
After the Civil War the United States government committed to supply prosthetics to all
veterans in need. This commitment changed the goals of prosthetic design to those which
we know today.
Within the past 50 years research has been performed all over the world to help
develop a more state of the art prosthetic hand. Unlike other prosthetics, the prosthetic
hand has yet to catch up to its 20th century counter parts such as the prosthetic leg.
Prosthetic legs have undergone rapid improvement in the past century and have even
given some amputees the ability to participate in Olympic Games with little to no
negative effects from their amputation. Prosthetic hands, however, are improving at a
much slower pace and have not given users the same results as seen with the prosthetic
leg.
The After World War II the America amputee casualty list grew quite large [2].
In response to this, the United States government set out to investigate the state of the art
prosthetic due to complaints from veterans who felt that the current technology was
inadequate. Due to this, the American Orthotics and Prosthetics Association stepped up to
address this need. Since then, many smaller companies and universities have also
stepped up to the goal of developing state of the art prosthetics.
The design discussed in this paper was developed after extensive review of
current state of the art devices. Research was performed to review robotic and prosthetic
hands that have been built or that are being developed. This research allowed for a better
understanding of current technology. After reviewing all information, different design
2
embodiments were developed and after much deliberation a particular embodiment was
chosen to model in Autodesk Inventor modeling software.
PRELIMINARY RESEARCH
The initial stage of this project involved
researching scientific journals and papers that
describe the state of the art in prosthetic hands. Hands
such as the I-Limb Hand (Fig 1) developed by
Fig 1: I-Limb
Scottish company Touch Bionics [3] and the Fluidhand developed by the Orthopedic
University Hospital in Heidelberg [4] are currently considered leaders in the world’s
commercial prosthetic hand market. Academic institutions such as the University of
Iowa, John Hopkins University and Carnegie Mellon have also designed and constructed
prosthetic hands. Research at John Hopkins University was funded by DARPA (Defense
Advanced Research Projects Agency) to construct the prototype prosthetic hand to help
advance the hand prosthetic industry [5]. Carnegie Mellon developed the first
anatomically correct finger and the University of Iowa developed a very unique design in
which the entire hand is constructed out of springs. Interestingly, current state of the art
designs have been developed using both electric and hydraulic power sources. It is clear
that there is still room for new, innovative ideas in the design of upper-limb prosthetics.
During our background research we also focused on electrical and mechanical
components that are commonly used in small robotic devices. This supplied us with a
good knowledge of all possibilities that were available to construct the hand.
3
ARCHITECTURE OF THE HUMAN HAND
The human hand contains 4 fingers, an
opposable thumb and a palm. The hand is made up of
27 bones that form the fingers, thumb and wrist [6]. It
has 27 degrees of freedom: 4 in each finger, 5 in the
thumb and 6 for the rotation and translation of the wrist.
There are tendons and muscles that run along the front
Fig 2: Human Hand
and back side of each digit to control the opening and closing of the hand. The flexor
muscles are located on the underside of the hand and cause the hand to close. The
extensor muscles are on the back of the hand and cause the hand to be opened. The
human thumb has two separate flexor muscles that move the thumb in opposition and
make grasping possible. The joints of each digit are the distal phalange joint (DIP), the
proximal phalange joint (PIP) and the metacarpal joint (MCP).
PRELIMINARY DESIGN IDEAS
Upon completion of the initial research, design goals and constraints for the hand
were formulated. They are:

Reproduce at a low cost

Pick up small objects

Perform multiple grasping functions such as pointing with index finger, grasping
cylindrical objects and picking up small objects using the index finger and thumb.

Device resembles human hand
4
The ability of the device to pick up small objects was one of the most important goals. In
order to translate this goal into design specifications calculations were performed to
determine the average torque on each finger and joint when holding an object with a
weight of 0.91kg (Appendix I). A weight of 0.91kg was chosen as a base measurement
because most small objects that the average person uses everyday do not exceed this
weight. This gave us specifications that we could use to find robotic and mechanical
components that would work and rule out one that would not.
Several brainstorming sessions were held before a final design was chosen. These
sessions resulted in three distinct design approaches, pneumatic, hydraulic and electric
(Appendix II). Each approach utilizes a different source of power to control the motion of
the hand. Design trees were generated for each approach. These trees included various
mounting options for the devices and what type of skeleton the device would employ.
After the design trees were completed a comparison chart of ten design embodiments was
formulated (Appendix III). These ten design embodiments were compared against design
goals such as controllability, force control, packaging and cost. This exercise pointed the
direction for further research that needed to be performed before the selection of a final
design. Once we had most of the required information we began to rate each
embodiment. We gave each embodiment a numerical rating under each goal according to
how well the design satisfied the requirement. In this rating a higher numerical value
corresponds to better satisfaction of the design goal.
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MECHANICAL AND CONTROL SYSTEM DESIGN
Guided by the structured design approach discussed above, we selected a final
design embodiment that utilizes electric cylinders/actuators that are packaged in the palm
and upper wrist of the hand. We selected this design because it has a high goal
achievement score, uses components in which we
have a high level of confidence, and can be fabricated
Fig 3: Firgelli Actuator
in our lab. Significant research went into finding
appropriate electric actuators and motors to control the device. We decided to use
actuators and a controller from a company called Firgelli. This Canadian company is
known for its advanced micro linear actuators. The architecture of these actuators is state
of the art because they incorporate a linear actuator and position feedback into a device
that is small enough to embed in the palm of the hand. Fig 4 & 5 are assemblies of the
index finger positioned on the palm with the Firgelli actuator in place. Fig 4 shows the
Fig 4: Index Finger
Extended
Fig 5: Index
Finger Closed
6
finger in the extended position. Fig 5 shows how the actuator will pull on the tendon
cable and close the finger to the position shown. When the actuator rod retracts in to its
housing it will pull on the tendon wire that is connected to the end of the actuator rod,
runs along the inside of the finger joints, and is attached to the tip of the finger. This
placement of the cable generates a moment about each joint that will act to close the
finger.
Once the actuator was chosen and incorporated in the design, the mechanical
architecture of the hand began to take shape. Another
brainstorming session was held in order to come up
with alternative designs that incorporated the Firgelli
Fig 6:
Torsion
Spring
actuators. Some designs included use of rubber-like materials to bring a finger back to a
straight position and having restricted ball
joints for the finger joints. Other design ideas
used a double cable system to control all
movements of a finger. The design that we
selected utilizes hinge joints (Fig 7) that have
Fig 7:
Joint
Assembly
torsion springs (Fig 6) built into the joint to
return the finger to a straight position. For this
design a tendon cable will run through the
finger itself on the inside of the palm through the rounded holes shown in Figure 7. This
design will keep all mechanical parts located internally for a cleaner more functional
exterior design.
7
Figure 7 is an assembly of one of the index finger joints. It consists of two joint
pieces (blue and green) connected with a pin (red). Located on the pin is a torsion spring
(orange). There are also holes for the nylon tendon to run through. In the index finger
there are three of these joints. Each has one degree of freedom that allows it to move on
one axis. In the human hand the MCP joint (the joint closest to the palm) has two degrees
of freedom; however the lateral degree of freedom does not provide much added
function. For this reason and for the sake of simplicity we chose to utilize only one
degree of freedom in our MCP joint.
Figure 8 shows a picture of the completed index finger. The joints are all made
out of aluminum, the pins connecting the joints are made out of stainless steel and the
”bones” connecting the joints are made out of
carbon fiber. The pins are held in using
retaining push nuts (Fig 12). The design for
the joint has four hinge fingers sticking up
from one joint piece and two hinge fingers
Fig 8: Index
Finger
Assembly
sticking up from the other piece (Figs 9 & 10).
This double hinge design provides housing for
the torsion spring as well as good stability.
These material choices strike a balance between light weight and strength.
8
Fig 9: Final Joint Design
Fig 10: Final Joint Design
This design will be used to construct all the fingers of the hand. Using a single
design for each finger allows for easier machining and a less expensive device. Although
we plan to use the same design for all four fingers, each finger will be set to a distinct
length to allow it to look more like a human hand. These length variations will be
accomplished by varying the length of the carbon fiber bones.
The correct torsion springs were chosen using the moment calculations discussed
above. The calculations took into consideration the weight of each piece of the finger and
the moment it produced on each joint. These calculations were used to select torsion
springs that produce enough torque to cause each joint to straighten-out when there is no
tension on the cable. Upon completion of the calculations the springs were purchased
and the finger assembly was modeled in Autodesk Inventor.
Tendon placement was also discussed and modified. It was determined that the
tendon should run along the palm side of each joint through a rounded hole in each
aluminum joint piece. We chose to run the tendon along the palm side of each joint after
we reviewed documentation on the human hand. In the human hand there are flexor and
extensor muscles and tendons that allow the finger to open and close. The flexor muscles
and tendons are responsible for closing the finger and run along the palm side of the
finger. Our goal is to duplicate the flexor tendons of the human hand by using a tendon
9
cable that has similar placement. The extensor muscles and tendons will be represented
by the torsion springs. We chose to round the holes in the joint pieces to decrease wear of
the wire and avoid breakage. Nylon string was selected as the tendon material in order to
reduce friction and supply a strong light weight material.
Nylon washers and retaining nuts will be placed on the ends of each pin to hold
the joints together and reduce friction (Figure 11 & 12).
Fig 11: Nylon Washers
Fig 12: Retaining Ring
THUMB DESIGN
Upon completion of the index finger design, the design for the thumb began.
Unlike the index finger, the thumb needs to be able to rotate about
multiple axes so that it can move into position to touch any of the
Fig 13:
Spider
Joint
fingers. In humans, the thumb has 5 degrees of freedom: 1 in the
DIP joint, 2 in the PIP joint and 2 in the MCP joint. This allows the
thumb to cross over the palm and touch each of the fingers as well as
change shape for different grips. As discussed above it is difficult to control a single joint
with two degrees of freedom. In our design we utilize a MCP joint with 2 degrees of
freedom while the DIP and PIP joints have only one. This is a reasonable decision
because taking away one degree of freedom from the DIP joint would not hinder the
thumb from making the grasping functions described in our goals. This thumb design will
10
be able to function closely to the human hand and still allow for multiple grasping
positions. The control of the thumb position is simpler if the two axes of rotation of the
MCP joint are coincident. In order to
accomplish this, a spider joint was
designed and attached to the palm (Fig
13). The tendon cable runs through a
Fig 14:
Thumb
Assembly
hole in the center of the spider in order
to keep the DIP and PIP joints from
closing when the spider joint twists. To
open and close the thumb a tendon wire
is run through the digit just as in the fingers. In order to rotate the MCP joint laterally a
Fig 15: Complete Finger, Thumb and Palm Assembly
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small, rotary gear motor is connected to the joint at the base of the thumb. This gear
motor allows the user to determine the position of the thumb ranging from the side of the
palm to touching the pinky finger. Figure 15 shows the solid model of the entire finger,
thumb and palm assembly.
Upon completion of our preliminary hand design, parts were ordered and
drawings of each part of the index finger were assembled and printed. The drawings were
then approved and ready to go to the machine shop in order to begin machining.
CONCLUSION
In this paper we have described the application of a structured design approach to
the design of a prosthetic hand. The goal of this research was to develop a low cost,
functional prosthetic hand. The work described in this paper represents an important
step towards achieving that goal. We estimate that our hand can be manufactured in
small quantities for a cost of about $2000 much less than more complex, state of the art
designs.
This work has allowed us to move towards our goal of helping upper-limb
amputees who cannot afford state of the art prosthetics. Our next goal is to build a
prototype of the index finger, thumb, and palm. This will allow us to test the
effectiveness of our design in delicate grasping tasks. We will then move to the final
design and construction of an entire hand. Finally, we intend to incorporate a myoelectric
sensor system that will allow a user to actuate the hand using signals generated by the
user’s own body.
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Works Cited
[1] Choi, Charles Q. "World's First Prosthetic: Egyptian Mummy's Fake Toe |
LiveScience." LiveScience | Science, Technology, Health & Environmental News. Web.
29 July 2009. <http://www.livescience.com/history/070727_old_toe.html>.
[2] Clements, Isaac P. "HowStuffWorks "The History of Prosthetic Limbs""
Howstuffworks "Health" Web. 29 July 2009.
<http://health.howstuffworks.com/prosthetic-limb1.htm>.
[3] Touch Bionics. Web. 29 July 2009.
<http://www.touchbionics.com/news2.php?section=11&newsid=59&site=/professionals.
php>.
[4] "'Fluidhand': Each finger can be moved separately." PhysOrg.com - Science News,
Technology, Physics, Nanotechnology, Space Science, Earth Science, Medicine. Web. 29
July 2009. <http://www.physorg.com/news128082539.html>.
[5] Campbell, P. "Revolutionizing Prosthetics 2009 Team Delivers First DARPA Limb
Prototype." JHUAPL. Web. 29 July 2009.
<http://www.jhuapl.edu/newscenter/pressreleases/2007/070426.asp>.
[6] Marrero, Ian C. "Hand, Anatomy: eMedicine Plastic Surgery." EMedicine - Medical
Reference. Web. 02 Aug. 2009. <http://emedicine.medscape.com/article/1285060overview>.
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ASME International, 3 Park Avenue, New York, NY, 10016-5990, USA,
[URL:http://www.asme.org]
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International, 3 Park Avenue, New York, NY, 10016-5990, USA,
[URL:http://www.asme.org]
Robot and Human Communication-Proceedings of the IEEE International Workshop,
2000, 405-410
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Figures
Figure 1: I-Limb hand developed by Scottish company Touch Bionics
Figure 2: Human hand
Figure 3: Firgelli micro linear actuator
Figure 4: Index finger extended
Figure 5: Index finger closed
Figure 6: 180 degree torsion spring
Figure 7: Finger joint assembly
Figure 8: Index finger assembly
Figure 9: Final finger joint design
Figure 10: Final finger joint design
Figure 11: Nylon washers
Figure 12: Retaining nuts
Figure 13: Spider joint for MCP joint in thumb
Figure 14: Thumb assembly
Figure 15: Complete hand assembly
14
Appendix I
Moment Calculations
Grasp of a Big Gulp
Room Temp
Big Gulp = 32oz
2 pounds
0.9072 kilograms
F=ma
8.90
Coefficient of Static
Friction = 0.6
(between finger tips
and aluminum)
Moments
Force/
# of Finger
14.833/6
2.47
CSF=F/N
N=8.900/.6
14.83
Newtons
Meters
Nm lbft
lbin
Index Finger
DIP Joint
PIP Joint
MCP Joint
2.47
2.47
2.47
0.02
0.04
0.08
0.05
0.10
0.20
0.04 0.44
0.07 0.88
0.15 1.75
Middle Finger
DIP Joint
PIP Joint
MCP Joint
2.47
2.47
2.47
0.02
0.05
0.09
0.05
0.11
0.22
0.04 0.44
0.08 0.98
0.16 1.97
7.42
7.42
0.02
0.06
0.15
0.41
0.11 1.31
0.30 3.61
Ring Finger
(Same as Index)
Thumb
PIP Joint
MCP Joint
15
Appendix II
Tree Diagrams
Power Source: Electric
Actuator:
Electric Rotary
encoder
force sensor on finger tip
strech sensor
Electric Cylinder
controller/actuator
finger tip
Direct From Motor
force sensor
motor
Motor Mount:
Exo
Exo
Endo
Joints
Palm
Endo
Wrist
Palm
Finger
Endo
Wrist
Palm
Finger
Smart Materials
bend
contract
applied voltage
Power Source: Pneumatic/Hydraulic
Actuator:
Bendy Straw
sensor/control Pressure regulator
position
Ballon Exo
Endo
Built In
Tiny Air Cylinder
Pressure Regulator
Air Motor
Flow Regulator
Exo
Exo
Endo
Joint
Wrist
Endo
Joint
Wrist
16
1
0
2
2
0
3
5
4
4
0
Bendy Straw Hydraulic
Air Cylinder
Hydraulic Cylinder
Hydraulic Motor
Tiny Electric Gear Motor
Electric Rotary in Wrist
Electric Cylinder in wrist
Tiny electric Cylinder in Finger
Smart Materials
Known
Resources
Bendy Straw Pneumatic
Design Embodiments
?
3
3
5
5
3
3
2
3
2
?
1
4
4
1
2
5
4
5
4
?
2
3
3
2
1
3
4
3
3
5
4
4
3
3
4
2
2
4
5
5
4
2
2.5
3
3
2.5
2
5
4
?
4
3
4
4
3
3
4
?
?
Controllability Force
Force
Coolness
(Position)
generation Control Packaging Factor
Cost
WHAT? (Dr. Pierce)
How much force-control architecture
How much force? How to control?
Does a tiny Hydraulic motor exist
If you can't put it at joint forget it
How small/ how much torque?
How Small?
How Small? How much force
Research Bellows and bender
Research Bellows and bender
Plan of Action
Appendix III
Design Embodiments
17