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Proposed Design of Mechanical grippers
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1
DESIGN, ANALYSIS AND DEVELOPMENT OF
MECHANICAL GRIPPER
Submitted By -
SWASTIK BHATTACHARYA 200815059
SUBHANKAR DAS 200815055
TANMAY ROY 200815061
Under The Guidance Of
SHRI MK PATHAK, SCIENTIST D,
SHRI ANUPAM BANSAL, SCIENTIST B,
Research & Development Establishment (Engineers), Pune
For Summer Training, May – June, 2011
Department Of Mechanical Engineering
Sikkim Manipal Institute Of Technology
Majitar, East Sikkim – 737136
Under
SIKKIM MANIPAL UNIVERSITY
2
Contents
Page No
I) Certificate 3
II) Acknowledgement 4
II) Abstract 5
Unit – 1 Problem statement 6
Unit – 2 Introduction 7
Unit – 3 Literature & survey 10
Unit – 4 Concept 17
Unit – 5 Design 24
Unit – 6 Prototype development 56
Unit – 7 Conclusion 60
Unit - 8 Utility, Limitations, Future Aspects 61
Unit – 9 Bibliography 64
3
4
Acknowledgement
We would like to thank Shri MK Pathak, Scientist D and Shri Anupam Bansal, Scientist B, Research
& Development Establishment (Engineers), Pune for guiding us throughout the project especially with
the design calculations and analysis. Their valuable guidance and advice has made it possible for us to
complete this project on time. We would also like to thank the Prototype Development for developing
our prototype in rapid prototyping machine. We would also like to give our sincere gratitude to Shri
Alok Mukherjee, Scientist F, Head Robotics, R&DE Pune whose encouragement and advices helped
us greatly.
We also thank Research & Development Establishment (Engineers), DRDO for giving us an
opportunity to do the project under their reputed organisation.
Last but not the least we would like to thank all the members of the ROBOTICS UNIT and all
the staffs of Research & Development Establishment (Engineers), Pune for their valuable co-
operation and help.
5
Abstract
The aim of this project was to design a mechanical gripper capable of gripping a body of given load
and dimension. The design was required to be a light weight, cost effective and capable of gripping
irregular bodies easily.
The problem statement is discussed in Unit 1. The basic idea about grippers and type of gripper
currently available is discussed in Unit 2. The study done on various type of hand gripper and its
characteristic has been included in Unit 3. The design requirement, its concept with working principle
has been discussed in Unit 4. Unit 5 includes the theoretical analysis on kinematics, dynamics and
stresses involved. The design has been done using UG (UNIGRAPHICS) NX5 and SOLID WORKS
2010.The prototype development has been included in unit 6. The utility of the design and its future
aspects has been included in Unit 8.
6
UNIT : 1
PROBLEM STATEMENT
Research and Development Establishment (Engineers), an organisation under The Defence Research
and Development Organisation, wanted us to design, analyse & develop a mechanical gripper capable
of lifting objects of given load and dimensions. The design of the gripper must be such that it can
easily hold irregular bodies and the entire gripper and the body must be stable after the gripping is
done. It should be of lightweight. It should not be costly and should be fabricable with easily available
resources.
7
UNIT - 2
Introduction
A gripper is a device that holds an object so it can be manipulated. It has the ability to hold and
release an object while some action is being performed. The fingers are not part of the gripper, they
are specialized custom tooling used to grip the object and are referred to as "jaws." It is an important
component of industrial robots because it interacts with the environment and objects, which are
grasped for manipulative tasks. Usually, a gripper of industrial robots is a specialized devise, which is
used to grasp one or few objects of similar shape, size, and weight in repetitive operations. There are
different types of gripper. Some common type of gripper is illustrated below:
1. Parallel gripper
A gripper mechanism is designed so that the gripper faces are parallel when the mechanism moves
together and apart. The parallel movement of the jaws is
generated by a rack/pinion drive. By application of
pressure of two opposite pistons the jaws move
synchronously towards each other. They are very compact
by virtue of the fact that the drive is integrated into the
housing and are low weight due to the use of high-strength
material (e.g. aluminium).High gripping force through
wedge and hook principle is achieved.
Parallel Gripper offers:
2 jaw parallel motion
Durability – designed for use in very dirty or
severe environments
Double seals to protect the gripper from
environmental contamination that could lead to
failure
2. Three jaw gripper
Depending on the operation of the gripper, the jaws are
pulled in or out via the slots. This allows cantered
gripping. Designed for applications requiring three
points of contact and, due to its high durability, works
particularly well in harsh environments (for example
grinding and deburring).
The 3-Jaw gripper offers:
3 jaw parallel motion
Flexibility of stroke
Self-centring of parts
High grip force to moment ratio
Positive pick & place
High clamping force for rapid part transfer
8
3. Pneumatic gripper
A pneumatic gripper is a specific type of pneumatic actuator that typically involves either parallel or
angular motion of surfaces, A.K.A “tooling jaws or fingers” that will grip an object. It is the most
widely used pneumatically powered gripper; it is basically a cylinder that operates on compressed air.
When the air is supplied, the
gripper jaws will close on an object
and firmly hold the object while
some operation is performed, and
when the air direction is changed,
the gripper will release the object.
Typical uses are to change
orientation or to move an object as
in a pick-n-place operation. Linear
motion pneumatic components are
double acting cylinders that require
a dry air supply. The synchronized,
true parallel motion of the fingers
is generated by a pinion
mechanism powered by a double
acting piston. The jaws are
supported by a T-SLOT way. The
advantage of this design is that the
jaw support is greatly increased.
The gripping force can be adjusted
by varying the supplied air pressure.
The pneumatic parallel jaw offers:
Jaws are T-Slot bearing supported to prevent jaw breakage and offer superior load bearing
performance.
High gripping force to weight ratio.
Compact design with long stroke.
True parallel jaw motion for easy tooling.
4. Hydraulic gripper
The movement of the jaw is generated
by a piston driven by hydraulic power.
It basically a cylinder that operates on
compressed liquid. When the liquid is
supplied, the gripper jaws will close on
an object and firmly hold the object
while some operation is performed,
and when the liquid is taken out, the
gripper will release the object.
9
The hydraulic gripper offers:
Since hydraulic operates at high pressure than pneumatic therefore gripping force achieved is
more.
Since liquids can fit into any shape container, this makes it easier to construct a compact
motor, as the liquid used to force pressure does not need to be contained in a casing that
requires a certain size. Therefore gripper can be easily constructed.
Hydraulic systems require fewer parts, making them more durable. Hydraulic systems can be used
over long distances or periods of time with little wear due to their comparatively fewer moving parts.
So less maintenance is required.
5. Fingered gripper
Robotic end effectors are the "hand"
of the robot's arm. By attaching a
tool to the robot flange (wrist), the
robotic arm can then perform
designated tasks. Such a robot
system which is designed to support
humans in non-specialized, non-
industrial surroundings like these
must, among many other things, be
able to grasp objects of different
size, shape and weight. And it must
also be able to fine-manipulate a
grasped object. Such great
flexibility can only be reached with
an adaptable robot gripper system, a
so called multifingered gripper or robot hand. Examples of robotic end-effectors include robotic
grippers, robotic tool changers, robotic collision sensors, robotic rotary joint, robotic press tooling,
compliance device, robotic paint gun, robotic deburring tool, robotic arc welding gun, robotic
transgun, etc.
The fingered gripper offers:
Better flexibility in griping an object.
Independent movement of the finger assists in gripping a thing properly.
No shape restriction is there.
10
UNIT - 3
LITERATURE
The description of many other gripper can be found in which they fall mainly in two categories i.e.,
industrial and anthropomorphic designs. The manipulative operations are usually performed by using
two-finger grippers, which are powered and controlled for the grasping action by one actuator only. In
addition, two-finger grippers are used both for manipulation and assembling purposes since most of
these tasks can be performed with a two-finger grasp configuration. However a two fingered
configuration would not ensure a safe grasp as sideway slip can easily occur if any irregularities are
present on the object‟s surface or the object is hold in the way that the centre of gravity does not
become collinear with the forces applied by the gripper‟s fingers. Since a gripper gives a great
contribution to practical success of using an automated and/or robotized solution, a proper design may
be of fundamental importance. The design of a gripper must take into account several aspects of the
system design together with the peculiarities of a given application or a multi-task purpose. Strong
constraints for the gripping system can be considered for lightness, small dimensions, rigidity, multi-
task capability, simplicity and lack of maintenance. These design characteristics can be achieved by
considering specific end-effectors or gripper‟s strength. Most studies of gripper design have
proceeded under the assumption that the frictional force will be large enough to keep the object from
sliding in the fingers, however in practice it is very difficult to ensure that the frictional forces
between the finger tips and the object are sufficiently high to hold the object. Other grippers, which
have more than two fingers use motors on each joint of the finger, which decreases the load holding
capacity of the gripper due to self-weight of the motors. Moreover they have some gear arrangements
to provide interlocking at the joints which not only decreases the load holding capacity but also
increases the probability of mechanical failure at any joint. Basic features for a gripper depend
strongly of the grasping mechanism. Thus, factors can be considered before choosing a grasping
mechanism as following:
• Characteristics of the gripper, which include maximum payload, dimensions, orientations, number of
the composed links;
• Characteristics of the objects, which include weight, body rigidity, nature of material, geometry,
dimensions, condition, position and orientation, contact surfaces, forces acting on the object and
environmental conditions;
• Gripper technology, for the construction of components (Mechanism links and finger parts) with
proper Manufacturing and materials;
• Flexibility of the gripper, whether it allows rapid replacement, or easy adjust and external
modification, or adaptation to a family of objects that are contained within a range of specifications;
• Cost for design, production and application to robot operation and maintenance.
Most of industrial grippers are actuated by a linear actuator. However, two actuators can be useful
when the fingers can operate independently with a symmetric or unsymmetrical behaviour. Many
others types of gripper mechanisms are used in order to achieve suitable mechanical design with
grasping efficiency, small size, robust design, light and low-cost devices. The mechanical design
determines the fundamental „dexterousness' of the hand, i.e. what kind of objects can be grasped and
what kind of manipulations can be performed with a grasped object. In fact, those characteristics are
fundamental from a practical viewpoint for the grasping purpose, since they may describe the range of
11
exerting force on the object by the fingers, the size range of the objects which may be grasped and a
particular manipulation type. Thus, a dimensional design of gripper mechanisms may have great
influence on the maximum dimensions of the grasped object by a gripper, and on the grasping force,
since the mechanism size may affect the grasp configuration and transmission characteristics. These
peculiarities can be considered well known when it is taken into account the great variety of
mechanisms which have been used.
12
Survey on fingered gripper
1) Shadow Robot Company Ltd.
Model Smart Motor Hand (C6M) uses Shadow's electric “Smart
Motor” actuation system, rather than the pneumatic Air Muscle
actuation system of other Dextrous Hand systems. The Hand is
driven by 20 Smart Motor units mounted below the wrist which
provide compliant movements. Following the biologically-
inspired design principle, a pair of tendons couple each Smart
Motor to the corresponding joint of the Hand. Integrated
electronics in the Smart Motor unit drives a high efficiency rare-
earth motor,and also manages corresponding tendon force
sensors. The Hand system (hand, sensors, and all motors) has a
total weight of 4 kg.
2) Prensilia Srl
The EH1 Milano Hand is a programmable anthropomorphic
human-sized hand able to grasp a variety of objects and to
sense them through multiple force and position sensors.
Modular actuation units are placed in flanges customized for
the application, and string transmission allows for remote
actuation, thus enabling the employment of low payload
robotic arms. The hand alone weighs just 250g. Each actuator
contains a CPU, firmware, sensor acquisition electronics,
communication electronics, servo-controllers, and one
brushed DC motor. The hand communicates through RS232
or USB and is ready to be easily integrated with your
application within multiple research scenarios ranging from
prosthetics, neuroscience, human-robot interaction,
rehabilitation, etc.. The EH1 Milano series firmware routines
allow to perform grasps automatically, by just sending a
single byte from your application. Alternatively advanced
users may implement completely customized control schemes,
taking advantage of the embedded 1 kHz servo-control loops.
13
3. Elumotion Ltd.
The Elu2-Hand is a human-scale anthropomorphic
robot hand able to approximate real hand
movements at humanlike speeds. The Elu2-Hand
has 9 DOFs that are servo actuated within the
hand‟s volume. Whilst originally designed to fit
onto the Elu2-Arm the compact Elu2-Hand design
means it may be fitted onto many different robot
arms. The Elu2-Hand hand has large soft pad areas
that aid the hand manipulate objects and provide
the potential for tactile sensing. Each degree of
freedom has the potential for ultra reliable non-
contact absolute sensing and limit switches
providing extra positioning redundancy for safety
critical applications
4. NASA
Each hand has a total of 14 degrees of freedom. It
consists of a forearm which houses the motors and
drive electronics, a 2 degree of freedom wrist, and a 5
finger, 12 degree of freedom hand. The forearm,
which measures 4 inches in diameter at its base and is
approximately 8 inches long, houses all 14 motors, 12
separate circuit boards, and all of the wiring for the
hand.
14
5. Artificial Intelligence Laboratory, University of Zurich
Robotic hand inspired by the muscle tendon system of the
human hand. The robotic hand has 13 degrees of freedom,
and each finger has been equipped with different types of
sensors (flex/bend, angle, and pressure). The same robotic
hand has been used as a prosthetic device. EMG signals
can be used to interface the robot hand non-invasively to a
patient and electrical stimulation can be used as a
substitute for tactile feedback.
6. California Institute of Technology
The Harada hand has four fingers and a thumb built to
approximate dimensions of the human hand. Each of the
four fingers has three links and three revolute joints to pitch
the finger forward out of the plane of the palm. The thumb
has two links with two revolute joints. All motors and
gearing are located within the rigid palm. They are
controlled through a computer interface which takes TTL
level inputs representing commands for finger contraction
and extension, and converts them to drive signals for each
motor. Control inputs can also be generated from muscle
activity recorded with EMG electrodes placed on a human
forearm, and processed by a custom pattern recognition
circuit built into the robot forearm cavity.
15
7. Robotics and Mechanisms Laboratory at Virginia Tech
RAPHAEL (Robotic-Air Powered Hand with Elastic
Ligaments) is a humanoid robotic hand that utilizes
corrugated tube actuation with compressed air. Unlike
electromechanically actuated hands, thanks to the natural
compliance, Raphael can mimic the grasping capability of a
human hand more accurately. By changing the pressure of
the compressed air, the amount of applied force can also be
controlled.
8. Mechatronics and Automatic Control Laboratory
(MACLAB)
MAC-HAND is a four fingered anthropomorphic robot
hand. Each finger has three DOFs and is actuated by four
independent tendons driven by DC motors. The four
fingers are identical, and consist of two phalanges. Each
finger is independently actuated by four motors. The
control is performed by four microcontrollers, one for each
finger, Finally the coordinated control of the hand is
demanded to a supervision computer connected through a
CAN bus link.
9.Dainichi Company, Ltd. Kani, Japan, Kawasaki &
Mouri Lab
Gifu hand form is approximate for the human hand to
not only size but also motor function like geometrically
in order to realize grasp and operation of the object by
changing human. The index is 5, and joint number and
degree of freedom equal to the human finger joint have
been established. The thumb has 4 degrees of freedom 4
joint
16
Inferences of the survey
From the survey of all the above grippers it is found that the human hand configuration is the most
flexible one and can be manipulated very easily for grasping objects of different size and shape within
the specified weight.
17
UNIT: 4
CONCEPT
4.1 - DESIGN REQUIREMENTS FOR THE GRIPPER
It should be able to grip a body of mass 8 kg with a safety factor of 1.25.
It should be able to grip cylindrical object of diameter upto 230mm
It should be able to grip a cube of minimum length 10mm.
It should be able to grip a sphere of minimum diameter 10mm.
There should be no backlash.
It should provide free movement of the string.
There should be stability in the gripper and the body (load) after the gripping is done.
It should be precisely controllable through computer operations or manually.
It should be of lightweight.
It should be easily fabricated with easily available resources.
It should be a low cost solution.
It should be the closest imitation of the human hand.
18
4.2 - DESIGN OF THE GRIPPER
It is well known that minimum three points are required to hold any object. In this work, a five-finger
gripper each finger with three links has been designed to hold irregular objects as this can be used for
both force and form closure purpose. In comparison to gripper with single link, parallel jaw, etc where
it may fail if the friction force is not sufficient, here the presence of the three link will augment the
friction force and will help in firmly gripping the object.
Figure 1(a): Fingers with single link , W: weight of object.
`
Figure 1(b) Fingers with three links, N: frictional force.
19
The gripper consists of a base, five fingers with three links each (figure ) with five motors placed in
the palm. In order to control the three links of the fingers, one motor is required. For non
synchronizing motion of five fingers, five actuators has been used to grip the object. Here motor is
connected with the pulleys of the fingers through a string.
pulley Link A
Link B
Link C
Figure 2
Figure 2
The basic components of a five-finger gripper are given in (Fig.15,16). Fingers are the elements that execute the
grasp on objects, finger tips are directly in contact with a object. Grasping mechanism is the transmission
component between the actuator and the fingers; actuator is the power source for the grasping action of a
gripper.
Fixed point
String
Figure 3
Each finger stemming from the palm can be modeled as an open chain linkage system stemming from
a fixed point. Figure shows a schematic of this model. Actuators located at the joints adds weight
to the system, causing actuators to exert more force or torque which leads to less than optimal
efficiency. Taking that into account, there are no actuators at the joints.
20
4.3 - String and Pulley Driven Actuation
The control method for this robotic hand uses strings fixed at each joint which are connected
to a motor placed inside the palm. The control string for each finger segment is threaded through the
hollow spaces in each subsequent finger link and down through the palm itself. All the strings are
connected to a single shaft which is driven by a motor. The strings controlling the finger are
connected to a motor. By rotating the motor, one side pulleys rotate in one direction and the other side
pulleys in the other. With this, the thread gets wounded on side pulleys and relaxed on the other. The
side on which it gets wounded becomes the inner side of the folding finger.
4.4 - Working Principle
This gripper has five fingers with three links which will augment the friction force and will help in
firmly gripping the object. The fingers and the thumb move independently by five motors that are
mounted on the base. The fingers have links and each of the links have pulleys mounted on it. These
pulleys are mounted on to a shaft of the link. Strings are provided which passes over the pulleys to a
fixed point provided in the link1.The string is directly connected to a D.C motor and direct torque is
transmitted throughout the pulley which in turn moves the link. Since string is directly connected to
the D.C motor so the tension throughout the pulley remains same in every point. The different finger
position when tension is applied in the string is shown below:
21
T T
(a)Section view (b) (c)
Figure5
.
22
T T
(a) Section view (b) (c)
Figure 6
23
Section view T T
(a) (b) (c)
Figure 7
Here T = the tension in the string.
F = gripping force.
24
UNIT: 5 DESIGN
5.1 - CALCULATION OF THE LENGTH OF EACH LINK IN A FINGER
Figure 8
Let
Distance between the shaft of the finger and the edge of the thumb be X,
Distance between the shafts of the thumb be Y,
Length of each link be L.
Maximum dimension of the body to be held by the gripper between the finger and the thumb be „D‟
= 120 mm
Minimum dimension of the body to be held by the gripper be „d‟
25
Let us consider the configuration of the gripper for holding the body of maximum dimension.
From the above figure we have,
Lcosα1 + Lcos(α1+β1) + Lcos(α1+β1+ɣ1) + X = D ........(1)
For proper gripping without slipping, α1 + β1 + ɣ 1 = 90°
Hence, from equation (1) we have,
Lcosα1 + Lcos(α1+β1) + Lcos(90) + X = D
Or Lcosα1 + Lcos(α1+β1) + X = 120 ........(2)
Now since the gripper is symmetrical i.e. the motion of each link in a finger is equal,
α1 = β1 = ɣ 1
But α1 + β1 + ɣ 1 = 90°
Hence α1 = β1 = ɣ 1 = 30°
Hence from equation (2), we have
Lcos30 + Lcos(60) + X = 120
Or 1.366L + X = 120 ........(3)
26
Now let us consider the configuration of the gripper for holding the body of minimum dimension.
Figure 10
From the figure 10 we have,
Lcosα2 + X = - Lcos(α2 + β2) + d – Lcos(α2 + β2 + ɣ2 ) ........(4)
For proper gripping of the body of minimum dimension, α2 + β2 + ɣ 2 = 180°
Hence, from equation (3) we have,
Lcosα2 + X = - Lcos(α2 + β2) + d – Lcos(180)
Or Lcosα2 + X = - Lcos(α2 + β2) + d + L ........(4)
Now since the gripper is symmetrical i.e. the motion of each link in a finger is equal,
α2 = β2 = ɣ 2
But α2 + β2 + ɣ 2 = 180°
Hence α2 = β2 = ɣ 2 = 60°
Hence from equation (4), we have,
Lcos60 + X = - Lcos(120) + d + L
27
Or X = d + L [since d is considered to be very small or tending towards zero]
Or X = L ........(5)
Using equation (5) in equation (3), we have
1.366L + L = 120
Or 2.66L = 120
Or L = 120/2.66
Or L = 50.71
For convenience we take L = 50mm
Hence from the above calculation we get length of each link in a finger to be 50mm.
28
5.2 - DETERMINATION OF DISTANCE BETWEEN THE SHAFT OF THE FINGER
AND THE EDGE OF THE THUMB (X)
As discussed earlier X is the distance between the shaft of each finger and the edge of the thumb
facing the shaft i.e, the distance AB in figure 10
From equation (5), we have,
X = L = 50
Hence X = 50mm
Figure 10
29
5.3 - DETERMINATION OF DISTANCE BETWEEN THE SHAFTS OF THE
THUMB (Y)
As discussed earlier Y is the distance between the shaft of the two thumbs.
Figure 11
From the figure above, we have
Lcosα3 + Y + Lcosθ3= - Lcos(α3 + β3) – Lcos(α3 + β3 + ɣ3 ) – Lcos(θ3 + Φ3 + Ψ3) –Lcos(θ3 + Φ3)
........(6)
The two thumbs should not curl completely before the meet each other. They should come in contact
with each other at an angle of 180°.
Hence θ3 + Φ3 + Ψ3 = 180°
Since the two thumbs move symmetrically,
α3 = θ3 , β3 = Φ3 , ɣ3 = Ψ3
and also since each link moves equally,
α3 + β3 + ɣ3 = θ3 + Φ3 + Ψ3 = 180°
or α3 = θ3 , β3 = Φ3 , ɣ3 = Ψ3 = 60°
putting in equation (6), we have,
Lcos60 + Y + Lcos60= - Lcos(120) – Lcos(180) – Lcos(180) –Lcos(120)
Or Y = 2L
Or Y = 100mm
30
Variation of X, Y and L with respect to the angles α1, β1, ɣ 1, α2, β2, ɣ2
The above table shows the variation of X,Y and L with the variation of the angles α1, β1, ɣ 1, α2, β2,
ɣ2 for the gripper to be capable of holding a body of maximum dimension of 120mm and a minimum
dimension nearly equal to zero.
31
5.4 - CALCULATION OF GRIPPING FORCE
The load to be lifted by the gripper is 10kg.
L = 10kg
Therefore weight of the load = 10 x g , where g is the acceleration due to gravity
= 10 x 9.81 N
= 98.1 N
This load will be distributed between the thumb side and the finger side.
Hence load on each side = 98.1/2 N = 49.05 N
The coefficient of friction between the rubber and metal block is 0.7.
Therefore,
μ x Normal reaction force = Load on each side
or normal reaction force = 49.05/0.7 = 70.07 N
This is the total reaction force on the thumb side as well as the finger side.
Since our design comprises of 3 fingers this normal reaction force will be divided between the 3
fingers equally.
Therefore gripping force on each finger is
F1 = 70.07/3 = 23.357 N
Again since our design comprises of 2 thumbs, the normal reaction force will be divided between the
2 thumbs equally.
Therefore gripping force on each thumb is F2 = 70.07/2 = 35.035 N
32
5.5 - DETERMINATION OF THE DISTANCE OF THE POINT FROM THE
CENTRE OF THE PULLEY WHERE THE STRING IS TO BE PIVOTED
Figure 12
From the figure we can see that the string is a tangent to the pulley. The line joining the centre of the
pulley to the point makes an angle θ with the string. The sine component of the tension force is
responsible for the gripping action and it contributes to achieving the required gripping force.
Now as we increase the angle θ, two things take place.
(i) The sine component of the tension force increases.
(ii) The distance k decreases.
Now the distance k must be such that there is enough clearance between the circumference of the
pulley and the point where the string is fixed. But as we increase the distance, θ decreases and hence
the sine component of the force.
After checking the value of k and Tsinθ for a variety of values of θ we see that for θ = 30°, the
clearance k is 10 mm for a pulley radius of 5 mm. Also the Tsinθ component for θ = 30° is
considerable. Hence for our design we take the value of θ as 30° and the value of k as 10mm.
33
5.6 - CALCULATION OF TENSION IN THE STRING
Let
T= the tension in the string.
L= the length of each link
F= the gripping force
R= the radius of the pulley
The gripping force will be acting on the tip of the link which is at a distance of „L‟ from the shaft axis.
But the tension force of the string will be acting at the point where the thread is pivoted which is at a
distance of 10mm from the shaft axis.
Figure 13
Normal force on the pin where the string is fixed = Tsinθ
Since θ = 30°,
Hence normal force = Tsin30°
From the figure we have,
Tsin30° x 10 = L x F
As calculated earlier,
L =50mm ,
F for finger side = F1 = 23.357 N
Ffor thumb side = G2 = 35.035 N
Hence for the finger side,
T x 0.5 x 10 = 50 x 23.357
Or T = 233.57 N
And for the thumb side,
34
T x 0.5 x 10 = 50 x 35.035
Or T = 350.35 N
Figure 14
35
5.7 - CALCULATION OF THE TORQUE OF THE MOTOR
Since the tension force will be constant throughout the entire length of the thread, this force will act
tangentially on the pulley mounted on the shaft of the motor.
FINGER SIDE
The torque on the motor driving the fingers is
Torque = T x radius of the pulley
Since radius of the pulley used is 5 mm
Tension (T) = 233.57 N
Hence
Torque = 233.57 x 5
= 1167.85 Nmm
= 1.1675 Nm
THUMB SIDE
The torque on the motor driving the thumbs is
Torque = T x radius of the pulley
Since the radius of the pulley used is 5 mm
Tension (T) = 350.35 N
Hence
Torque = 350.35 x 5
= 1751.75 Nmm
= 1.7517 Nm
36
5.8 - KINEMATIC ANALYSIS
Now we shall analyse the motion a finger considering it as a 4 bar open chain mechanism.
Let
The angular velocity of link A about point R be Ѡ.
The tangential velocity of link A about point R be V.
The angular velocity of link A about point Q be Ѡ1.
The tangential velocity of link A about point Q C be V1.
The angular velocity of the link A about point P be Ѡ2.
The tangential velocity of the link A about point P be V2.
α, β, ɣ, r1, r2, θ1, θ2 are as depicted in the figures.
Now let us consider the motion of the link A about link B
Figure 15
We know,
V = L x Ѡ, where L is the length of each link.
........(7)
37
Now let us consider the motion of the link A about link C.
Figure 16
From geometry we have
θ1 = ɣ/2
From the above figure, we have
V1 = V x cosθ1
Or V1= L x Ѡ x cosθ1
But V1 = r1 x Ѡ1
Hence,
r1 x Ѡ1 = L x Ѡ x cosθ1
Ѡ1 = (L x Ѡ x cosθ1) / r1
Where r1 = 2 x L x cosθ1 and θ1 = ɣ/2
38
Considering the motion of link A about the shaft of the fixed base
Figure 17
From geometry we have
Θ2 = tan-1
(Lcos α/( r2 + Lsin α))
From the above figure, we have
V2 = V1 x cosθ2
Or V2 = V x cosθ1 x cosθ2
Or V2 = L x Ѡ x cosθ1 x cosθ2
But V2 = r2 x Ѡ2
Hence
r2 x Ѡ2 = L x Ѡ x cosθ1 x cosθ2
Ѡ2 = (L x Ѡ x cosθ1 x cosθ2)/ r2
Where r2 = r1 x cosθ2 + L x cos( ɣ/2 + β - θ2)
39
5.9 - STRESS ANALYSIS
We shall be discussing the stresses acting on the following points:
(i) String (ii) Pulley (iii) Shaft connecting two links (iv) Link
(i) String:
The tension in the string is
T = 233.57N for finger side &
T = 350.35N for thumb side
Due to the tension of the string, the shaft over which the string is wound will experience a shear force.
Since the string is fixed to the shaft of the link A, it will undergo shear. Since the string is has very
little contact with the shafts of link B and link C, the shear in their case will be negligible.
Shear stress for the finger side
Shear stress = τ = tension force/area
Tension force for the finger side = 233.57 N
Area = cross sectional area of the shaft
Diameter of the shaft is taken as d = 4 mm.
Hence area = π x d2/4 = 12.57 mm
2
Therefore, τ = 233.57/12.57 = 18.58 N/mm2
Shear stress for the thumb side
Shear stress = τ = tension force/area
Tension force for the finger side = 350.35 N
Area = cross sectional area of the shaft
40
Diameter of the shaft is taken as d = 4 mm.
Hence area = π x d2/4 = 12.57 mm
2
Therefore, τ = 350.35/12.57 = 27.87 N/mm2
(ii) Pulley:
The string is wound into a full circle over the pulley before it leaves the pulley. Due to the tension
force on the string, the pulley will be subjected to 2 tension forces as shown in the figure. There will
be 2 components of each force. One in the radial direction and another perpendicular to the radial
direction. The components of the 2 force perpendicular to the radial direction cancel each other. The
radial component of the 2 forces adds up.
Figure 19
Hence from geometry we see that the radial component of each force is
equal to
T x cos(60 - α/2) [for the pulley placed between link C and the base]
Where T is the tension in the string.
Hence the total radial component on the pulley is 2T cos(60 - α/2).
41
Shear stress on the pulleys on each finger
Shear stress on the pulley between link A and link B:
Force = 2T cos(60 - ɣ/2)
or force = 2x 233.57 cos(60 - ɣ/2)
[ since tension T for finger = 233.57N]
or force = 467.14 cos(60 - ɣ/2)
Area = 2 x π x r x 2
Since radius of the pulley is 5mm
Therefore,
Area = 62.832 mm2
τ = force/area
= 467.14cos(60 - ɣ/2)/62.832 N/mm2
= 7.435 cos(60 - ɣ/2) N/mm2
Where ɣ is the angle between link A and B.
Similarly,
Shear stress on the pulley between link B and link C:
τ = 7.435 cos(60 - β/2) N/mm2
where β is the angle between link B and C.
Shear stress on the pulley between link C and the base:
τ = 7.435 cos(60 - α /2) N/mm2
where α is the angle between link C and the base.
42
Shear stress on the pulleys on each thumb
Shear stress on the pulley between link A and link B:
Force = 2T cos(60 - ɣ/2)
or force = 2x 350.35 cos(60 - ɣ/2)
[ since tension T for finger = 350.35N]
or force = 700.7 cos(60 - ɣ/2)
Area = 2 x π x r x 2
Since radius of the pulley is 5mm
Therefore,
Area = 62.832 mm2
τ = force/area
= 700.7 cos(60 - ɣ/2)/62.832 N/mm2
= 11.152 cos(60 - ɣ/2) N/mm2
Where ɣ is the angle between link A and B.
Similarly,
Shear stress on the pulley between link B and link C:
τ = 11.152 cos(60 - β/2) N/mm2
where β is the angle between link B and C.
Shear stress on the pulley between link C and the base:
τ = 11.152 cos(60 - α /2) N/mm2
where α is the angle between link C and the base.
43
(iii) Shaft connecting two links:
Due to the tension of the string which passes over the pulley, a force will be exerted on the shaft on
which the pulley is mounted. As discussed in case of the pulley, only the radial component of this
tension force will be acting on the pulley. This will cause a shear in the shaft.
Shear stress on the shafts connecting two links on each finger
Shear stress on the shaft between link A and link B:
Force = 2T cos(60 - ɣ/2)
or force = 2x 233.57 cos(60 - ɣ/2)
[ since tension T for finger = 233.57N]
or force = 467.14 cos(60 - ɣ/2)
Area = π x r2
Since radius of the pulley is 2mm
Therefore,
Area = 12.566 mm2
τ = force/area
= 467.14cos(60 - ɣ/2)/ 12.566 N/mm2
= 37.174 cos(60 - ɣ/2) N/mm2
Where ɣ is the angle between link A and B.
Similarly,
Shear stress on the shaft between link B and link C:
τ = 37.174 cos(60 - β /2) N/mm2
where β is the angle between link B and C.
Shear stress on the shaft between link C and the base:
τ = 37.174 cos(60 - α /2) N/mm2
where α is the angle between link B and C.
44
Shear stress on the shafts connecting two links on each thumb
Shear stress on the shaft between link A and link B:
Force = 2T cos(60 - ɣ/2)
or force = 2x 350.35 cos(60 - ɣ/2)
[ since tension T for finger = 233.57N]
or force = 700.7 cos(60 - ɣ/2)
Area = π x r2
Since radius of the pulley is 2mm
Therefore,
Area = 12.566 mm2
τ = force/area
= 700.7cos(60 - ɣ/2)/ 12.566 N/mm2
= 55.76 cos(60 - ɣ/2) N/mm2
where ɣ is the angle between link A and B.
Similarly,
Shear stress on the shaft between link B and link C:
τ = 55.76 cos(60 - β /2) N/mm2
where β is the angle between link B and C.
Shear stress on the shaft between link C and the base:
τ = 55.76 cos(60 - α /2) N/mm2
where α is the angle between link B and C.
45
(iv) Link:
The force acting on the link will be the same as that acting in case of the pulley and also in case of the
shaft connecting the two links. This is because the same component of the tension force will be
transmitted through the pulley, shaft connecting the two links and finally to the link. This force will
cause tearing of the link at the circular portion.
Tearing stress on the link on each finger:
Figure 20
Tearing stress between link A and link B
Force = 2T cos(60 - ɣ/2)
Or Force = 467.14 cos(60 - ɣ/2) [since T= 233.57 on the finger side]
Area = thickness x length of tear
= 30 x 6 = 180 mm2
Ϭt = force/area = 467.14 cos(60 - ɣ/2) / 180
= 2.6cos(60 - ɣ/2) N/mm2
Where ɣ is the angle between link A and B.
Tearing stress between link B and link C
Ϭt = 2.6cos(60 - β/2) N/mm2
Where ɣ is the angle between link B and C.
Tearing stress between link C and the base
Ϭt = 2.6cos(60 - α /2) N/mm2
Where ɣ is the angle between link C and the base.
46
Tearing stress on the link on each thumb:
Tearing stress between link A and link B
Force = 2T cos(60 - ɣ/2)
Or Force = 700.7 cos(60 - ɣ/2) [since T= 350.35 on the finger side]
Area = thickness x length of tear
= 30 x 6 = 180 mm2
Ϭt = force/area = 700.7 cos(60 - ɣ/2) / 180
= 3.9cos(60 - ɣ/2) N/mm2
Where ɣ is the angle between link A and B.
Tearing stress between link B and link C
Ϭt = 3.9cos(60 - β /2) N/mm2
Where β is the angle between link B and C.
Tearing stress between link C and the base
Ϭt = 3.9cos(60 - α/2) N/mm2
Where α is the angle between link C and the base.
47
5.10 - CALCULATION OF THE CHANGE IN LENGTH OF THE STRING
REQUIRED FOR MAXIMUM MOVEMENT OF A FINGER
Figure 21
The configuration of the finger in its ideal state is shown in the figure.
The string goes around all the pulleys and shafts as shown in the figure.
The length of the string L1, Lw1, L2, Lw2, L3, Lw3 are as shown in the figure.
From geometry the values of L1, Lw1, L2, Lw2, L3, Lw3 are found and are as follows:
L1 = 6mm
Lw1 = 48.826mm
L2 = 6mm
Lw2 = 1.855mm
L3 = 30
Lw3 = 43.26
L = the extra length of the string taken for for winding it around the motor and pulleys attached to the
motor = 20mm
48
Therefore total length of the string = 3* L1 + 2* Lw1 + 2* L2 + 4* Lw2 + 2*L3 + Lw3 + L
= 3*6 +2*48.826 + 2*6 + 4*1.855 + 2*30 +43.26 + 20
= 258.332mm
Now let us assume that the maximum angular deflection of one link with respect to another is 90°.
For such a configuration of the grippers(figure ), the total length of the string.
We observe that on the value of Lw1 changes and all other lengths remains the same.
Therefore total length of the string = 3* L1 + 2* Lw1 + 2* L2 + 4* Lw2 + 2*L3 + Lw3 + L
= 3*6 +2*39.35 + 2*6 + 4*1.855 + 2*30 +43.26 + 20
= 239.38mm
Hence change in length of the string = 258.332mm – 239.38mm = 18.952mm
Hence the change in length of the string required for maximum movement of a finger = 18.952mm
49
5.11 - SHAPE DETERMINATION
Based on the above calculations, the following few conceptual designs were put forward. They are
shown in the figures below:-
Figure 22
50
Figure 23
51
Figure 24
Among the above mentioned conceptual designs, the concept 2 (figure 23) was chosen due to its
better resemblance with the human arm and its capability of holding irregular bodies being better than
the others.
52
The components of the designed gripper are described below.
Pulley:
Figure 24
Figure 24 shows the pulley used in the gripper. The pulleys used have an effective diameter of 10mm
and an external diameter of 15mm. The thickness of the pulley is 3mm.
53
Shaft:
Figure 25
The figure 25 shows the shaft used in the gripper. The diameter of the shaft used are 4mm and are
35mm in length.
54
Links:
Figure26
Figure 27
Figure 28
55
Figure 29
The above figures show the dimension of each link. Each finger consists of 3 links. The links have a
shafts to shafts distance of 50mm. The distance between the two shafts present in the link over which
the string passes is 30mm.
56
UNIT: 6
Prototype Development
The finger segments and hand base were solid modelled in Rapid protyping. Rapid protyping is the
automatic construction of physical objects using additive manufacturing technology. Today, they are
used for a much wider range of applications and are even used to manufacture production-quality
parts in relatively small number. The use of additive manufacturing for rapid prototyping takes virtual
designs from computer aided design (CAD) or animation software. In the manufacture of the
prototype ABS is used.
Acrylonitrile Butadiene Styrene (ABS) - This material is a terpolymer of acrylonitrile, butadiene and
styrene. Usual compositions are about half styrene with the balance divided between butadiene and
acrylonitrile.
Features of ABS:
1. Flame Retardant.
2. High Heat Resistance
3. Good Impact Resistance
4. High Impact Resistance,
5. High Flow General Purpose,
6. Good Flow
7. Good Process-ability,
8. High Gloss Good
9. Dimensional Stability
57
The following assembly was required to be fabricated.
Figure 31
The three fingers and the two thumbs were fabricated and tested successfully. The figure below shows
the design of a finger.
Figure 32
58
Figure 35
59
Figure 36
The palm (base) on which the fingers and the thumbs were mounted could not be fabricated due to
time constraints.
60
UNIT: 7
CONCLUSION
The objective of this robotic hand is to achieve an easily controllable and energy efficient system
incorporating a majority of movements seen in daily life. Previous works in the field of robotic
grippers are typically too bulky to be used in practical applications. By observing human hand
postures researchers concluded that a large percentage of hand positions can be approximated by a
simple grasping motion. Taking human hand tissue structure into account, this motion has been
reconstructed using a system of pulleys and strings driven motors.
61
UNIT: 8
8.1 - APPLICATION
As the five fingers of the gripper move independently, it provides a better gripping of irregular bodies
over parallel gripper and three jaw gripper. Since it is string driven and it does not involve any gear
arrangements so it is light weight and portable. It can be used for gripping operation in robots which
performs grabbing and releasing of hazardous materials from one place to another provided the
gripper is installed with an arm. As the design involves arrangement of pulleys and gears so it is easy
and cheap to manufacture.
62
8.2 – LIMITATIONS
1) As it is string driven so there is chance of failure of the gripper due to weir of the string.
2) As single direction movement of each finger is controlled by a single string so weir of the string
will lead to collapse of the movement of the finger in that direction.
3) The gripper cannot be used for carrying loads exceeding 8kg.
4) The gripper cannot hold objects below 6 mm in dimension.
63
8.3 - FUTURE ASPECTS
1) Sensors may be mounted which can sense the gripping force required for a given load and flexibly
adjust its gripping power.
2) Pitch, Yaw and roll movement can be given to the gripper to enhance its degree of freedom.
3) It can be used in bomb detection and diffusion robots provided adequate control system is installed.
4) When integrated with proper sensors they can be used in debris clearing and recovery vehicles.
64
UNIT: 9
BIBLIOGRAPHY
1) IMAGE TABLE
IMAGES
LINKS
Image1 http://www. reports/pptsc_lg.asp.htm
Image 2 http://www.shadowrobot.com/reports_es.htm
Image 3 http://www.megabots_reports/grippers.html
Image 4 http://mindtrans.narod.ru/hands/pictures/openarm_v2
Image 5 http://www. magnum.htm
Image 6 http://www.shadowrobot.com
Image 7 http://www.shadowrobot.com
Image 8 http://www.h-e-i.co.jp/products/e_m_g/ph_sh_2_004.html
Image 9 http://www.kk-dainichi.co.jp/e/gifuhand.html
Image 10 http://www.robotiq.com/en
Image 11 http://www.kineadesign.com/portfolio/prosthetics/#rp2009team
Image 12 http://www.kineadesign.com/portfolio/prosthetics/#rp2009en
Image 13 http://www.dist.unige.it/cannata/machand.html
Image 14 http://www.graal.dist.unige.it/facilities/
Image 15 http://en.wikipedia.org/wiki/Rapid_prototyping
2) REFERENCES-
[1] Kinematics and Linkage Design – HALL
[2] Open Hardware definition, http://www.opencores.org/OIPC/def.shtml
[3] Shadow Open Hardware, http://www.shadow.org.uk/projects/openhardware.html
[4] Ashish Singh, Deep Singh and S.K. Dwivedy. “
Design and Fabrication Of A Gripper For Grasping Irregular Objects”. Indian Institute of
Technology, Guwahati.
[5] Sarah Jane Wikman.” INTER-FINGERCOORDINATED DC MOTOR DRIVEN
GRASPING ROBOTIC HAND”. Massachusetts Institute of Technology, June 2009.