Design of Mechanical Gripper

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 1

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

2

3

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 cooperation and help.

4

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.

5

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.

6

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

7

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.

8

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, nonindustrial 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.

9

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, multitask 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 10

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.

11

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 biologicallyinspired 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 rareearth 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.

12

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 noncontact 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.

13

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.

14

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

15

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.

16

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.

17

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.

18

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.

19

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:

20

`

T (a)Section view

T

(b)

(c) Figure5

.

21

T (a) Section view

T (b)

(c)

Figure 6

22

Section view

T

(a)

T (b)

(c)

Figure 7 `

Here T = the tension in the string. F = gripping force.

23

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‟ 24

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)

25

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

26

Or

X=d+L

Or

X=L

[since d is considered to be very small or tending towards zero] ........(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.

27

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

28

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

29

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.

30

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

31

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) (ii)

The sine component of the tension force increases. 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.

32

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, 33

T x 0.5 x 10 = 50 x 35.035 Or

T = 350.35 N

Figure 14

34

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

35

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)

36

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

V 1 = r 1 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

37

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

V 2 = r 2 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)

38

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 mm2 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 39

Diameter of the shaft is taken as d = 4 mm. Hence area = π x d2/4 = 12.57 mm2 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).

40

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

=2xπxrx2

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.

41

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

=2xπxrx2

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.

42

(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.

43

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.

44

(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.

45

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.

46

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

47

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

48

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

49

Figure 23

50

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.

51

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.

52

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.

53

Links:

Figure26

Figure 27

Figure 28 54

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.

55

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

56

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

57

Figure 35

58

Figure 36 The palm (base) on which the fingers and the thumbs were mounted could not be fabricated due to time constraints.

59

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.

60

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.

61

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.

62

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.

63

UNIT: 9 BIBLIOGRAPHY

1)

IMAGE TABLE LINKS

IMAGES Image1 Image 2 Image 3 Image 4 Image 5 Image 6 Image 7 Image 8 Image 9 Image 10 Image 11 Image 12 Image 13 Image 14 Image 15

http://www. reports/pptsc_lg.asp.htm http://www.shadowrobot.com/reports_es.htm http://www.megabots_reports/grippers.html http://mindtrans.narod.ru/hands/pictures/openarm_v2 http://www. magnum.htm http://www.shadowrobot.com http://www.shadowrobot.com http://www.h-e-i.co.jp/products/e_m_g/ph_sh_2_004.html http://www.kk-dainichi.co.jp/e/gifuhand.html http://www.robotiq.com/en http://www.kineadesign.com/portfolio/prosthetics/#rp2009team http://www.kineadesign.com/portfolio/prosthetics/#rp2009en http://www.dist.unige.it/cannata/machand.html http://www.graal.dist.unige.it/facilities/ 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.

64