state of art of the prosthetic hand
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The state of the art of Prosthesis Hand
A human hand is a complex structure having 21 degrees of freedom (DOF): four DOF per finger
which has three phalanges and one metacarpus ad five DOF for the thumb which has two phalanges
and one metacarpus. It can perform grasping, holding and pinching operations while manipulating
objects of various sizes, weights and shapes. It has 27 bones and a multitude of muscles and tendons, in
addition, each hand has an array of over 17000 tactile sensors.
It is doubtless the most widely versatile machine that has ever existed anywhere.
With existing technology it is near impossible to replicate anything mechanically similar. However,
advances in technology have enabled some considerable improvements in the functionality of aprosthetic hand with an increase in the number of degrees of freedom available through the use of
smaller and lighter motor.
There has been much research towards the creation of a robotic end effector that is similar in function
and appearance, to the human hand. Similarly, in the area of prosthesis design, research is being
conducted towards the creation of a lower arm prosthesis is more like the human hand. The field of
robotic end effector design and the field of lower arm prosthesis design have many parallels. However,
the requirements for producing a mechanical manipulator for use by a laboratory robot are differentthan those for use by an amputee.
A prosthetic hand design must encompass the following properties: Lightweight, any device is worn by the operator on the end of a closely fitting external socket,
hence the weight bears directly onto the skin of the stump. The lever-arm created is therefore large
and the weight can obstruct blood flow in the underlying skin and results in symptoms ranging
from discomfort to skin breakdown. The weight of a human hand is around 1 kg, therefore a
prosthetic hand should satisfy this specification. Compact, the user population varies widely in the length of their residual limb, so any device
should retain all its drives and power sources within as small an envelope as possible, preferably
within the hand profile. Modularity, the use of a modular solution ensures that the largest number of people can use the
device as well as providing simplicity of manufacture of both left and right hands. Low power consumption, to make efficient use of the limited battery energy. Quiet, the purpose of all prostheses is to provide the functional result without attracting undue
attention to the user. The sound of gears and motors or the escape of gas in a pneumatic system istherefore generally unwelcome.
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Appearance- Cosmetic, a hand is considered to be cosmetic in appearance if it is aesthetically
pleasing and looks like the limb its designed to replace in both its lines and colour. Both static and
dynamic cosmesis are important but dynamic one it is more difficult to achieve and can be
enhanced by preserving as much of the persons residual motion as possible. A device can be
considered to be functionally cosmetic if at a glance it is not immediately recognizable as an
artificial hand regardless of whether it is in motion or not or whether it is or is not handlike when
stationary. Price,any device must be useable by a wide range of possible wearers. The prosthetics field is very
price sensitive. This means the device must be produced at a cost that will allow the hand to be
priced competitively.1
In order to reduce the overall weight of the hand an underactuated solution is often used. In
underactuated hands, the number of motors is lower than the number of active joints, so that some
kind of joint motion coupling should be provided. Underactuated hands have the advantage of system
simplicity, i.e., the number of required actuators is reduced while preserving the number of active
joints. Underactuated mechanisms can be used to obtain an adaptive grasp that resembles human
grasping more easily than a hand with completely independent DOFs could achieve. Indeed, the human
hand is also underactuated, as the distal interphalangeal joints of the fingers are not independently
controllable. When applied to mechanical fingers, the concept of underactuation leads to self-
adaptability. Without complex control strategies, self-adaptive fingers will envelope the objects andautomatically adapt to their shape with only one actuator.
The literature shows two different types of underactuated hands, depending on whether a tendon or
link transmission is used. The tendon systems are generally adopted to minimize the transmission
dimensions but are limited to small grasping forces, while link systems are preferred for applications in
which large grasping forces are required.
The following pages present an overview of prosthetic hands, sorted by mechanism used and by date.
The i-Limb is the only hand that fulfils almost the properties described above; in fact is commercially
available while the other hands, described below, are just robotic hand or prototypes. In fact even if
some hands show some relevant innovation, these hands have bulky and heavy housing for the motor,
outside the palm, and so is not possible to consider them like a real prosthetic hand.
1 Some hands, described below, are just prototypes, so it is difficult to establish a price.
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I-Limb Hand (Mead, Shakeshaft, Waddell 2007)
The i-Limb is not only lightweight, but it works and
looks like a human hand. The hand is powered by internal
batteries, fitted in the patients socket, which drive the
five motors of the i-LIMB Hand for a complete day. It is
made from high-strength plastics, and the fingers are
covered by injection-moulded Zytel, a tough nylon that
withstands hot and chemically aggressive environments.
This makes it more realistic, and much more useful and effective in performing everyday tasks. Each i-
LIMB digit is individually powered by a precision direct current micro motor. The motor is short
enough to fit into the phalangeal section next to the knuckle. To make the fingers curve as they grip, a
polyurethane-covered Kevlar toothed belt links each knuckle
joint to the nearest interphalangeal joint. In the i-LIMB
Hand, the interphalangeal joint nearest the fingertip does not
move, but the one nearest the knuckle is controlled by the
motor to make the finger bend and grip.
Using a modular design, the company is able to build
complete hands or partial hands, of different sizes, with the
minimum number of component parts. Modularity alsomeans easy replacement of worn-out parts.
The total cost to the patient is about $50,000 this makes the prosthetic hand more expensive than
traditional myoelectric devices.
Following is a brief description of hands that use a different kind of linkage mechanism.
The Southampton REMEDI hand (Light, Kyberd and Chappell 1994-2001) This hand has six degrees of freedom. The hand
consists of six small electrical motors, two of which are
used to actuate the extension-flexion, and rotation
movements of the thumb with each of the remaining
four motors being assigned to individual fingers. The
modular thumb unit is reversiblein design, so that it
may be used for either a left or right handed
prostheses. Each finger is made from six bar linkages,
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which when extended or flexed curl in a fixed anthropomorphic trajectory, this was designed to
replicate the trajectory of the human finger during natural curling . To ensure a lightweightunit, the
linkage is machined from carbon-fibre epoxy composite and the housing from a polymer thermoplastic
which has a low coefficient of friction.
When the proximal phalanx makes contact with the object, the proximal phalanx stops, and the other
two phalanxes begin rotating and closing on the
object because of the effect of the underactuated
linkages mechanism. By means of the coupling
linkages, the middle phalanx and distal phalanx
rotate at the same time, and the ratio of rotation
angles of two coupling joints--the mid joint and
distal joint-- are about 1:1, which mimic natural human hand movements. Finally, the fingertip or mid
phalanx is in contact with the object and the finger has completed the grasp motion. It shows that the
underactuated linkages mechanism is able to adapt shape with a wide variety of objects.
To reduce the power requiredto hold an object, the fingers are driven via a worm wheel gear
configuration. This also has the additional advantage that it prevents the fingers being back driven after
power is removed from the motor.
The TBM Hand, Toronto/Bloorview MacMillan (Dechev, Cleghorn, Naumann 1999) The TBM hand has five fingers and use a rigid
linkage system for actuating the fingers; each one is
comprised of six links.
The rotation of the thumb is performed manually by
the user. The key to this thumb design is to keep the
drive cable coaxial with the thumb rotational axis so
no matter which angle the thumb assembly is rotated
to, the drive cable will always be able to flex the thumb without slipping off the thumb pulley.
The TBM hand uses a single motor to actuate all
the mechanisms and to do this a novel cylinder
springs is used. Each cylinder spring consists
of a compression spring within a cylinder and
they are linked between link 6 of a finger and a
force plate. When the force plate moves right
along the x-axis, it pulls on the five pistons
distributing the actuators force amongst them.
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HIT/DLR ARHand (Huang, Li Jiang, Y.Liu, Hou, Hegao Cai, Hong Liu 2006)
The HIT/DLR Prosthetic Hand was
developed with the underactuated linkage
based on four- bar linkage mechanisms, placed
between the base joint and the middle joint.
The coupling linkage is passive moving, which
has not been driven by motor, and is
employed between middle and distal joints.
When one of the three fingers touches object
and stops, the other two fingers will continue
moving, until contacting the object. Compared with the tendon transmission, the linkage transmission
has the advantages of high stiffness and
reliability.
The thumb uses a sophisticated transmission
mechanism. The spherical bearing is used which
tends to make the motion track of the thumb
move from the preliminary position to the final
position as a cone surface. Its actuation
mechanism includes the motor, the synchronous pulleys and harmonic gear.
In 2009 the same authors propose the AR III; this hand uses only three actuators to drive five
fingers (total 15 joints).
To reduce system cost and complexity, the
adduction/ abduction and flexion/extension motions
of the thumb are combined together. Furthermore,
the thumb metacarpal is intentionally angled 60 tothe axis of TM joint to make thumb move along with
a cone surface. This configuration gives the thumb
superior grasping ability.
For more humanoid properties, the design of the AR hand III considered the ring and little fingers and
made them move together with the middle finger. These three fingers were mounted parallel on the
base axis, and each is equipped with a torsion spring.
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The SPRING Hand (Carrozza, Massa, Dario, Lazzarini, Vecchi, Cutkosky 2004)
The Spring hand has eight DOFs but only one motor. The
underactuated mechanism includes three cables (for each phalange)
and two compression springs that allow the adaptive behaviour.
The three cables are pulled in unison by means of a linear slider,
the slider is the fundamental element of the transmission system: in
fact it is possible to get the flexion and the extension of the finger
thanks to its two way linear motions. The Spring hand weights
about 400 grams.
The CyberHand (Edin, Cappiello, Micera, Carrozza 2005)
In the CyberHand the three fingered
RTR2 hand has been redesigned; in order
to improve the hand grasp functionality
and its anthropomorphism, all the
phalanges have a cylindrical shape withoutsharp edges. The CyberHand has 16 DoFs
and 6 motors. To reduce the weightthe 5
motors for fingers flexion are housed in a
socket, and the palm is composed by an
outside shell, made of carbon fiber. In order to increase the compliance of the graspinga soft padding
made of silicon rubber can be mounted on the palm. Cable transmissions obviously make it possible to
relocate bulky actuation and avoid problems due to rigid transmissions in articulated mechanism. The
total weight of the hand is about 320 grams, excluding the motors in the forearm.
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The SmartHand (Cipriani, Controzzi, Carrozza 2008)
The 16 DoFs (three for each finger, plus one for the
thumb opposition axis) SmartHand prosthesis is driven
by 4 DC motors. Thumb and index are independently
actuated, whereas the middle, the ring, and the little
fingers are joined together. Another motor is used for
the thumb opposition axis movement, in order to allow
different prehension patterns.
Hiroses soft finger has been selected as underactuated
mechanism for the fingers: the motors by pulling the
tendons which are wrapped along the finger pulleys
located in the joints are employed in the flexion of the
fingers. The reasons for the employment of such a mechanism are: the need for just a single actuator to
allow simultaneous flexion of three phalanxes (thus reducing weight and volumeof the prosthesis), the
simplicity of the control to be implemented and the compliance of the mechanism (related to the
capability of automatically wrap-around objects, allowing multi-contact and therefore stable grasps).
In most research prosthetic hands, non-back-drivability is obtained by means of screw/lead screw pairs;
but its main drawback, is that it is a low mechanical efficiency mechanism. The innovative idea has
been that to develop a high efficiency non-back-drivable miniaturized clutch mechanism. Thismechanism allows the transmission of the rotational motion, when it is originated by the motor shaft,
blocking instead motions originated from the output shaft (connected to a capstan driving the finger
tendon). The capstan has been designed with an eccentric geometry, with the purpose to privilege
strength in the first phase of the grasp, and speed in the last part.
The Manus Hand (Pons, Reynaerts, Saro, Levin, Van Moorleghem 2004)
MANUS-HAND proposes aprosthesis having ten joints of
which three are independently
driven. The fourth and fifth
fingers are provided with a
martensitic structure, this allows a
much higher number of bending
cycles as compared to commonly
used materials, thus improving the
overall reliability of the hand.
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A prototype of ultrasonic motors has been developed with the advantage improved performance and
self braking capabilities in comparison with a commercial DC motor.
There is one single actuator for the thumb motion.
To this purpose, the movements are transformed into
a cycle by means of an intermittent mechanism, the
so-called Geneva wheel. In this cycle two movement
planes are defined: the first plane corresponds to
cylindrical and tip grasps, i.e., thumb is flexed in an
opposition pattern; the second plane, implements
hook and lateral grasps, i.e., thumb is flexed in a non-
opposition pattern. The key position in this cycle is
the neutral position, at which the movement can
change from one plane to the other. The Geneva
wheel is implemented as gear. While the teeth of both gears are engaged there exists a 1:1 coupling
between the two wheels. At a certain
point, the gears are no longer in contact,
but the following wheel is locked in its
position by means of a form closed lock.
The driving wheel can continue rotating and drive another axis while the axis
connected with the following wheel of the
Geneva mechanism is locked.
Two drive paths exist in the Geneva
mechanism: t he first one drives the first axis. It starts at the actuated axis and the motion is transmitted
by means of gear set 4-3
(transmission ratio 1) and
gear set 2-1. The second
drive path also starts at the
actuated axis. This means
that the second axis (lateral
plane) can be coupled to
the driven axis
(transmission ratio 1) or
can be locked. In the firstcase, the opposition axis and gears 2 and 3 rotate at the same speed. Because the first axis (cylindrical
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plane) is mounted on the second axis, there is no relative motion between gears 1 and 2 and the thumb
does not rotate about the first axis. Thus, the thumb moves in the lateral plane. In the second case, the
Geneva mechanism is locked and the second axis does not rotate. Because of this also the housing
stands still and there is a relative motion between gears 1 and 2.
The UB Hand III (Lotti, Tiezzi, Vassura, Biagiotti, Palli, Melchiorri 2005)
In this prototype each finger can have up to 4 degrees of
mobility, obtaining a total number of 20 degrees of mobility
hand, where 16 degrees of mobility are actively actuated
whereas the others are locked or coupled. The internal
articulated structure is designed according the compliant
mechanisms concept so that the mobility of the phalanges
is obtained by means of elastic joints. The compliant
elements are made with close- wound helical springs
that are subjected to bending under the action of
pulling tendons. The fingers obtained by plastic
moulding with inclusion of continuous steel springs.
The actuation tendons are routed across the coiled
springs which form at the same time hinges and therouting paths. This solution allows a simplified
designwith appreciable kinematical propertieslike a
rough kinematical decoupling of the joints. In the upper finger, the yaw joint and the flexural bending
of the proximal phalanges are obtained through two orthogonal single axis hinges, while the articulation
at the base of the thumb is obtained by a single two DOF helicoidal hinges. This last joint is actuated
by means of three cooperating tendons that allow the thumb to bend on a plane having variable
direction. The Brno Prosthetic hand (Zajdlik 2006)
This prosthesis hand has twenty DOFs and
three motors.
The main innovation is in the mechanism
used called "with string and springs". A force
is applied by a string which leads through two
sliders mounted on springs. The first motion
generated when the string is pulled in the
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MCP (metacarpophalangeal) joint. Other joints can be activated only if the slider is below the level of
the appropriate joint. The PIP (proximal interphalangeal) joint moves only when Slider 1 is under its
(PIP) joint and the IP (distal interphalangeal) joint only moves if Slider 2 is below the DIP joint.
Biomimetic Hand (Sung-yoon Jung, Sung-kyun Kang, Inhyuk Moon 2008)
This hand has five fingers
driven by the motor-wire
mechanism, and the degree of
freedom is six. To reduce the
hand weights the fingers
skeleton and the palm was
made of the epoxy resin. To
reduce the number of
actuators this hand has a single
phalange called distal-middle phalange (DMP), and use a link
mechanism between the MCP joint and the PIP joint. To
reduce the friction influence the joint mechanism uses a
small-size bearing between phalanges. In addition, the pulley
mechanism to enable to assist force in finger flexion was also introduced. The wire path betweenpulleys is S shape from the fixed point to the top of the pulley on the PP, and to the bottom of the
pulley on the MB. This wire connection in the pulley mechanism reduces the friction influence, and it
produces a positive force to the direction of the finger flexion.
V-U Hand (Dalley, Wiste, Withrow, Goldfarb 2009)
The V-U hand has 16 joints driven by five independent
actuators.Specifically, to utilizing underactuation to reduce weight, a
hollow structural elements in the hand was developed, and
a space-frame to house the actuation units in the forearm.
By ensuring an efficient transmission (i.e., minimizing
friction in the system) its possible to obtain a low noise
hand. Finally, an anthropomorphic design based on scalable skeletal characteristics of the human hand
has been utilized. In doing this, it is believed that the cosmetic appearance of the device will be as
natural as possible. The skeleton structural components employ a monocoque structure realized in
high-strength nickel coated thermoplastic.
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The underactuation is governed by moment isotropy,
the hand will reach a configurational equilibrium when
all joint moments are equal. This is achieved by a
combination of having the tendon span multiple joints,
and using a pulley differential to split the force of
actuator output equally into two tendons. This design
allows the fingers to move quickly and with sufficient
force.
The last two projects propose other forms of power, instead of the traditional electric motors.
FluidHand (Schulz, Pylatiuk,Bretthauer, Kargov, Werner 2001)
It is based on the so called flexible fluidic actuators. This
actuators are high flexibilitydesigned into their mechanical
construction, realize very complex movements, they are
lightweightand very low manufacturing costs. A single
actuator element consists of a feeding channel for the
pressurized air or liquid and a "chamber" which is connected
to the two movable parts of a joint. During the inflation of the actuator element by air/liquid the volume of the element
expands and the height of the element vertical to the flexible
wall of the chamber increases. By using many fluidic actuator
elements together structures with very complex flexibility can be created. So a total of 18 miniaturized
flexible fluidic actuators are integrated into the mechanical construction of the fingers and the wrist.
The advantage of this design is that the flexible fingers of the hand are able to wrap around objects of
different sizes and shapes; because of the elastic
properties of the actuators the contact force is
spread over a greater contact area. Moreover the
surface of the fingers is soft and the friction
coefficient is increased by the silicone-rubber glove
that covers the artificial hand. The result is a reduced grip force is needed to hold an object. As a side-
effect from the softness and elasticity of the hand it feels more natural when touched than a hard
robotic hand and the risk of injury in direct interaction with other humans is minimized.
Only lightweightmaterials are used for the mechanical construction of the fingers and the wrist so that
each finger weighs less than 20 grams. This makes possible to reduce the mass of a new artificial hand
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to 50% of the mass of a conventional prosthetic hand. The time for a complete flexion and extension
of a finger is less than 100 ms; this is 10 times faster than a conventional prostheses.
Another fluidic hand was developed recently, in 2008 by the same authors.
Eight flexible fluidic actuators of compact design are integrated in the finger joints. Some actuators are
coupled, like the base joints of fingers IVV and both
joints of fingers II and III. The current flexible fluidic
actuator consists of a reinforced flexible bellow which
forms a closed elastic chamber. The axial expansion of
the flexible bellow exerts a pulling force on the joint
fittings, as the actuator is inflated with the fluid.
Low weightand inherent complianceare two major
attractions of this actuator. These materials can
transmit
energy as
well as a cylinder, but they have a higher power-to-weight
ratio at the same pressure and volume. Moreover, the
modular construction allows for the independence of the
actuators, consequently, actuators can be interchangedor
the number of degrees of freedom can be changed. This may be useful when redesigning the endmanipulator for the special application without a reconstruction of the whole system.
Pneumatic Prosthesis Hand (Takeda, Tsujiuchi, Koizumi, Kan 2009)
This five-fingered prosthetic hand using pneumatic
actuator. The finger can operate flexibly because the
pneumatic actuator is implemented directly in the
prosthetic fingers. In the MP joint, there are two
pneumatic actuators for flexion and extension operations.
In the DIP and PIP joints, there are a pneumatic actuator
for extension operation and a rubber gum for flexion
operation. The thumb mechanism is a bit different from
the other fingers; to give the degree of freedom of palmer
adduction, the CM joint has three actuators to rotate the palm direction.
The actuator is composed of a rubber balloon, a net that covers the balloon, and a feeding channel that
injects compressed air into the balloon. Expanding the rubber balloon shortens the net in the
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longitudinal direction and generates force. The expansion
and contraction operations can be controlled by
adjusting the pressure in the rubber balloon.
Even though the hand is only driven by a low-volume of
compressed air, it can generate enough powerto hold an
object that weighs up to 500 g.