Digital Design of Low-cost 3-DOF Prosthetic Hand

Proceeding of the IEEE
International Conference on Information and Automation
Shenzhen, China June 2011
Digital Design of Low-cost 3-DOF Prosthetic Hand
Xi Tang 1,2 , Changjie Luo 1 , Kai He 1
1
Shenzhen Institutes of Advanced Technology, Chinese
Academy of Sciences�Shenzhen, Guangdong Province,China
2
University of Science and Technology of China
Hefei , Anhui Province, China
xi.tang@siat.ac.cn
(changjie.luo & kai.he)@siat.ac.cn
ABSTRACT-This paper presents a low-cost 3-DOF (Degree
Of Freedom) prosthetic hand and developes a kind of digital
design software. The prosthetic hand has 3 movements including
fingers opening and closing, wrist swinging and wrist rotating.
Structure and dimensions are defined by the shape features and
physical functions of real hands. A digital design software is
developed to assist the design of prosthetic hand parts. Equations
of profile curves are defined before the digital design. With the
equations and Solid-Works API (Application Programming
Interface), a 3D part could be generated by coding in VB
(Visual Basic). The software has two function modules, quick
design module and customizable design module. As for the quick
design module, it is designed for the disabled with one hand.
Scan the healthy hand of the disabled, critical dimensions will be
extracted and 3D drawing of related parts can be generated. The
customizable design module is developed for special order.
According to the order, the designer can design prosthetic hand
quickly using the digital software.
Keywords—prosthetic hand,3-DOF, digital design
�.INTRODUCTION
Hands are one of the most complex and sophisticated
tools human beings depend on to live and work. For the upper
limb disabled, it means the loss of the hand function and hand
function recovery is the goal of researchers. Intelligent
prosthetic hand technology had been developed in the past 20
years, its main characteristic was to complete the appropriate
action according to the directives issued by brain. And for
technical reasons, most mature intelligent prosthetic hand was
controlled by electromyographic (EMG). However, there is
still much room for improvement of the structural design of
prosthetic hand. In addition, there was no professional digital
design software for prosthetic hand. The typical products of
prosthetic hand at home and abroad in recent years adopted
two transmission types, tendon drive and connecting rod drive.
A typical tendon drive five fingers prosthetic hand is
developed by Stanford University[1] and a typical connecting
rod drive prosthetic hand is developed by US IOWA State
Ruxu Du 3
3
The Chinese University of Hong Kong
Hong Kong,China
rdu@mae.cuhk.edu.hk
University[2].Also there are some other advanced prosthetic
hands in abroad, such as the Dexterous Hand developed by
NASA[3], the DLR-�,� Dexterous Robert Hand developed
by German Aerospace Center[4-5], and TBM prosthetic hand
in Canada[6]. But the problem of these products was that it
was more complex in the structure and difficult to control
because of the over-emphasis on the adaptability of the
fingers. Moreover, some other functions of prosthetic hand
were limited. Combining the advantages of these two kinds of
prosthetic hand, a new type of 3-DOF prosthetic hand system
was developed. This prosthetic hand realized fingers opening
and closing, wrist rotating and wrist swinging. The software
includes two function modules, quick design and
customizable module, which helps designers save a lot of
work and shorten the order cycle.
�PROSTHETIC HAND MODEL
As shown in Fig.1, the designed prosthetic hand has 3
components, it can achieve 3 kinds of movements separately,
fingers opening and closing, wrist swinging and wrist rotating.
Component1 is the actuating mechanism for fingers opening
and closing, Component2 is the wrist swinging actuating
mechanism and Component3 is the wrist rotating actuating
mechanism.
Fig.1 3-DOF prosthetic hand model
This work is partially supported by a direct grant from SIAT (O945102001) and a grant from Shenzhen Key Laboratory of Precision Engineering
CXB201005250018A
978-1-61284-4577-0270-9/11/$26.00 ©2011 IEEE 309

Fig.2 Prosthetic hand fixed in a tube
To make the prosthetic hand practical and easy to control,
we used the gear drive to achieve prosthetic hand movements.
Two pairs of gears were used to control fingers opening and
closing, wrist swinging respectively. In addition, a separate
motor is used to control wrist rotating. The Size of fingers and
wrist were designed on the basis of anthropometric data [7].
Two position limit switches for every DOF are installed to
control the movement range. As shown in Fig.2, the prosthetic
hand is fixed in a tube and to reserve room for the tube, the
shorter prosthetic hand is designed the wider universality it
has. But it can’t be too short, the parts need to occupy
necessary space to ensure the realization of the functions. The
wrist swing movement ranges from 0° to 115°, inward swing
ranges from 0° to 70°and outward swing ranges from 0° to
45°.The prosthetic hand has three fingers, thumb, index finger
and middle finger. According to the anthropometric data, the
sizes of forearm and fingers are described in TABLE1-3 [8-9].
TABLE1.
Sizes Of Forearm
Gender
Sizes/mm
Male
(ages from 18 to
50)
Female
(ages from 18
to 55)
Forearm length 206 ~268 185~242
Wrist width 52~65 48~60
Wrist thickness 32~42
TABLE 2.
Male Finger Sizes
30~35
Sizes of
fingers(mm)
Thumb Index
finger
Middle
finger
Total length 46.3~62.3 60~79 65.4~87.4
TABLE 3.
Female Finger Sizes
Sizes of
fingers(mm)
Thumb Index
finger
Middle
finger
Total length 45.4~61.4 57~76 62.8~84.8
The gear transmission ratio of fingers opening and
closing movement was set about 100 as well as the wrist
swinging movement .The designed grip strength was 3kg.
310
The angular velocity of the fingers is 60-80rpm, the
transmission ratio is set about 100, so we choose 8000rpm
motor. Similarly, the angular velocity of the other two motors
is also 8000rpm.
Also we have done some necessary classification of the
parts before developing the digital design software. Gears and
fingers belong to one class, these parts have close relationship
with the real hand thus we need to extract size information
from the real hand graphics. Human hands’ physical function
includes grab, pinch and push. Because the fingers of the
prosthetic hand don’t have the adaptation of position
adjustment (adjust the position of each joint of fingers to grip
objects of various shapes better), we bend the fingers to
maintain a certain angle, so that the hand can well hold
objects of common shapes. The pinch action is mainly for
some small objects, plane or curved objects, it’s easy to
complete when the fingers closed. The maximum opening
angle of thumb and index finger is about 120° while there’s
no external interference. 0° to 80 ° is most commonly used
when we grab objects. Based on this, we define the ultimate
opening angle as 100 °. Another important dimension is the
distance from the thumb root to the wrist, it relates to the size
of the internal gear drive mechanism. The size of the gear
drive to control fingers opening and closing should be less
than or equal to the dimension. Here we assume the distance
from index finger root to the part between thumb and index
finger is L. Since the total transmission ratio is about 100,
Gear1 would be very small and difficult to manufacture, so
we make it constant. Fig.3 depicts the gear drive mechanism
which is used to control fingers opening and closing.
Fig.3 Fingers opening and closing gear transmission
As shown in Fig.4, the total center distance of the gear
transmission is a, and a=L. The center distance of Gear1 and
Gear2 is a1.The center distance of Gear3 and Gear4 is a2.

Fig.4. a=a1+a2
Gear1 dimensions are shown in TABLE4.:
TABLE4
Gear1 Parameters
Modulus m1 0.4
number of teeth z1 7
normal profile angle
∂
20°
addendum coefficient ha* 0.8
The transmission ratio of gear.1 and gear.2 is 10. The
transmission ratio of gear.3 and gear.4 is 11.Gear.2
dimensions can be concluded through calculations, which are
shown in TABLE5.
TABLE5
Gear.2 Parameters
modulus m2 0.4
number of teeth z2 70
normal profile angle
∂
20°
addendum coefficient ha* 0.8
′
Then, a1
μd
a1
= +
μ + 1 2
1
1
′
a1
μd1
a2
= L − ( + ) �
μ1
+ 1 2
Gear.3 and gear.4 dimensions can be defined by
calculation.
TABLE6
Gear.3 Parameters
modulus m3 0.5
number of teeth z3 8
normal profile angle
∂
20°
addendum coefficient ha* 0.8
TABLE7
Gear.4 Parameters
modulus m4 0.5
number of teeth z4 88
normal profile angle
20°
addendum coefficient ha* 0.8
∂
311
Similar method can be used to get the dimensions of
wrist rotation gears. Sleeve, covers, bolts and other standard
parts belong to another class. These parts can be classified as
universal parts which can be found in database.
�.DIGITAL DESIGN SOFTWARE
A digital design software is designed to associate part
design. The software has two functional modules, quick
design module and customizable module. The software was
developed on the basis of Solid-works and was integrated into
a plug-in, using the API function to generate parts feature
curves and finish extension. All of these features were
expressed with a series of equations. In the quick design
module, real hand’s 3D graphic is inputted, the gears, fingers
will be outputted in Solid-works interface. Any of the gears
and fingers’ 3D drawing would be generated if chosen. Those
parts of the prosthetic hand can ensure that the shape and size
of prosthetic hand similar to the real one. The interface of this
module is very brief, only contains one input. This module
can greatly reduce the work of designers, shorten order
cycle.The other module is designed for special order, because
there may be the minority whose hand sizes are special. In
this module, the interfaces are some complex. There are
several inputs in an interface, and this module is more
professional, designers are needed. Entering all the parameters,
then parts would be generated. This module can also reduce
the drawing work. The framework of the digital design
software is shown in Fig.5.
Input real hand 3D
drawing
Choose the part
Generate the
drawing
Open Solid-works
Quick design Customizable
Run plug-ins
Gears design
Parts design
Gear shaft Driven gear Fan gear 1 Fan gear 2
Finish drawing Finish drawing …… .
Fig.5 Software framework
Other parts
Other fingers The thumb
�.DEMOS
Feature curve equations are needed before drawing the
parts.Fig.6 and Fig.7 show the characteristics of the tooth
profile curves of modified gear, drawing the tooth profile

according to a series of equations [10-11].
Fig.6 reflects the relationship between locking angle and
modification coefficient.
Fig.6 Curves of locking angle and modification coefficient [12]
Fig.7 shows the tooth profile of a modified gear in
processing, there are 3 curves in one side of a profile.
Fig.7 Curves of tooth profile of modified gear tooth in processing
The left side of a single tooth is described in Fig.8.
Fig.8 The left curve of a single tooth
The parameters and formulas of modified gear are
described as follows:
Radius of cutting pitch circle (r):
mZ
r =
2
locking angle : ( ∂ �): 2x
inv ∂′ = tan∂
Z
center distance alternating coefficient:(y): cos∂
y = Z(
−1)
cos∂′
addendum alternating coefficient( Δ y )� y xn
y − = Δ 2
312
addendum:( a
dedendum( h ): f
outside radius( r ): a
fillet radius( r ): f
m: modulus
Z: number of teeth
h ): ha = ( ha
* + xn
− Δy)
m
h =( ha * + c*
−c
)m n
f
f
r = r + h
a
r = r − h
x n : modification coefficient
addendum coefficient: h * = 0.
8
clearance coefficient: c * = 0.
25
angle of pressure: ∂ = 20 °
Curvilinear equations of the profile:
1 2 p p C
:
x l cos 12
l12
= T θ + S sinθ
+ r(sinθ
−θ
cosθ
)
y l sin 12
l12
θ =
= −T
θ + S cosθ
+ r(cosθ
+ θ sinθ
)
C :
p2
p3
π
+ tan( − ∂)
S
2
r
Tl12 l12
x A cos 23
A23
= T θ + S sinθ
+ r(sinθ
−θ
cosθ
)
y A sin 23
A23
= −T
θ + S cosθ
+ r(cosθ
+ θ sinθ
)
TA S A tanθ
23 23
θ
r
+
=
C :
p3
p4
x l cos 34
l34
= T θ + S sinθ
+ r(sinθ
−θ
cosθ
)
y l sin 34
l34
θ =
= −T
θ + S cosθ
+ r(cosθ
+ θ sinθ
)
T l34
r
[12]
a
f
a
Modified gears can be drawn through above methods.
And as shown in Fig.9, some of the parameters should be
inputted in the customizable design module.

Fig.9 Gear shaft design interface
Inputting all the parameters in Fig.9, the 3D drawing of
gear shaft would be generated, which is shown in Fig.10.
Fig.10 Gear shaft drawing in Solid-works
Fig 11 to Fig 14 show the quick design module and
Fig.15 to Fig.17 show the customizable design module. Only
some interfaces are listed. In the quick design module, a real
hand 3D graphic is inputted, the software calculate the
fingers’ size, the distance from index finger root to the part
between thumb and index finger and wrist sizes and then the
gear and finger sizes can be sure. Finally, the parts can be
generated in Solid-works with a few equations.
Quick design module:
Fig.11 The initial interface of software
Choose “Quick design” and click “next”, we can a part to
be drawn., which is shown in Fig.12.
313
Fig.12 Quick design module
In this interface, any listed parts can be generated by
inputting the 3D graphic of a real hand.
Fig.13 A 3D real hand Fig.14 The thumb
The thumb is drawn by extracting the size information of
the real hand.
Customizable design module:
This module is designed for special order, designers
should define the parameters of a part first and then entering
the parameters, the 3D drawing of a part can be generated.
This work can shorten a lot of time drawing.
Fig.15 The initial interface of software
Four kinds of gears are listed in Fig.16.

Fig.16 Gears design
Fig.17 is the fan gear design interface. The 2D graphic
shows the location of dimensions.
Fig.17 Fan gear1
According to the design, a prototype has been made as
shown in Fig.18, and the test results show the prototype works
well.
Fig.18 Prosthetic hand prototype
�.CONCLUSIONS
This paper presents a low-cost 3-DOF prosthetic hand
and a kind of digital design software. The structure and sizes
of the prosthetic hand are defined with reference to
314
anthropometry data. The developed software has two function
modules, quick design module and customizable design
module. The quick design module shows how the parts of
prosthetic hand can be designed by inputting a graphical hand.
The customizable design module is designed for special case.
Designers can use it to design kinds of prosthetic according to
the needs of users. A preliminary study of assembling parts
has been carried out. Further work is to optimize the design
and the digital software.�
ACKNOWLEDGEMENT
The authors wish to thank Professor Guanglin Li, Mr
Long Yu and Liang Chen, Neural Engineering Research
Center, Shenzhen Institutes of Advanced Technology, Chinese
Academy of Sciences. They give us a lot of advice and
technical support.
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