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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

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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

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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.

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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.

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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.

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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

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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:

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T T

(a)Section view (b) (c)

Figure5

.

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T T

(a) Section view (b) (c)

Figure 6

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Section view T T

(a) (b) (c)

Figure 7

Here T = the tension in the string.

F = gripping force.

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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‟

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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)

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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

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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.

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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

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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

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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.

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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

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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.

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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,

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T x 0.5 x 10 = 50 x 35.035

Or T = 350.35 N

Figure 14

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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

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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.