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Chapter 1

Introduction

1.1 Movement: Wheeled, Tracked and Legged

Almost all animals and humans as well have legs for locomotion or movement. But

historically the very first vehicles developed by human were run with wheels. And even now

through the industrial revolution and technological progress wheeled locomotion has

remained the most widespread technology of transportation.

The wheeled locomotion is most advantageous in predetermined and plane surfaces.

However, it is disadvantageous in uneven and smooth terrains. The tracked locomotion has

been developed in order to cope with this problem. However, tracked locomotion is also

troublesome, since it destroys the terrain on which the vehicle is moving.

Legged locomotion is mechanically superior to wheeled or tracked locomotion over a variety

of soil conditions and certainly superior for crossing obstacles. This advantage of legged

locomotion is mostly due to the fact that legged systems use isolated footholds. Due to this

fact as an alternative to both wheeled and tracked forms, legged locomotion is being

developed by imitating the legged animals in nature.

1.2 Why Legged Mechanism?

The main advantage of legged robots or moving machine is their ability to access places

impossible for wheeled machines. US army investigation reports that about half the earth’s

surface is inaccessible to wheeled or tracked vehicles, whereas this terrain is mostly exploited

by legged animals.

Wheeled and tracked systems follow the surface in a continuous manner and consequently

their performance is spoiled by the worst parts on the terrain. A legged system, on the other

hand, can choose the best places for foot placement. These footholds are isolated from the

remaining terrain. Hence the performance of the legged system is boosted by the best

footholds. Besides using isolated footholds, the legged system can provide active suspension,

which does not exist in wheeled or tracked systems. A legged system can be arranged (leg

lengths lengthened and shortened) according to the level alteration and they can jump over

obstacles or holes. Therefore, the body can be moved in a desired course, for example

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horizontally, whatever the terrain is. The legged locomotion also inflicts the least damage to

the surface compared to wheeled and tracked systems.

The most forceful motivation for studying legged moving machine is:

• To give access to places which are dirty

• To give access to places those are dangerous

• To accomplish jobs which are highly difficult for wheeled or tracked vehicles

1.3 Multi-Legged Walking

Many of the animals in nature have adopted legs for various environmental conditions.

Centipedes, spiders, cockroaches, cats, camels, kangaroos, and human are among those,

either with different number of legs or with different kind of walking. People turned their

attention to these walking animals, after it was recognized that the wheeled and tracked

systems did not satisfy all the needs. The basic requirements of any walking mechanism can

be acquired from these biological examples. The plan of walking, namely the “gait pattern”,

determines the sequence of stepping of legs with their stance and swing durations in each

step. Gait pattern generation is the generation of rhythmic plan that determines which leg will

be in power or return stroke in any time. A coordination and control unit is also needed that

controls the legs according to the gait pattern. Another requirement is stability: both static

and dynamic.

There are different types of legged walking robots. They are roughly divided into groups

according to the number of legs they possess. Bipeds have two legs, quadrupeds four,

hexapods six and octopods have eight legs. Biped walking mechanisms are dynamically

stable, but statically unstable, they are harder to balance, and dynamic balance can only be

achieved during walking. Hexapods or six legged robots, on the other hand, have advantages

of being statically stable. During walking they can move three legs at a time, thus leaving

three other legs always on the ground forming a triangle.

1.4 Six Legged Walking Mechanism

The proposed Six Legged Walking Mechanism has three pairs of legs: three attached to the

front shaft and three more to the rear shaft. The mechanism is a lower pair mechanism with

only pin joints or revolute pairs. The legs are made by extending the couplers of a four bar

RRRR mechanism. All the four bar mechanisms are of crank-rocker type and the crank is

connected to the motor. A pair of legs (front and rear) share the same frame and a common

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pivot. The motion and the gait is obtained by keeping suitable angular lag between the cranks

connected to the various legs. While the mechanism is in motion, at any instant of time three

legs keep contact with the ground forming a triangle and making the mechanism dynamically

stable.

1.4.1 Applications

1. It would be difficult to compete with the efficiency of a wheel on smooth hard

surfaces but as condition increases rolling friction, this linkage becomes more viable

and wheels of similar size cannot handle obstacles that this linkage is capable of. Toys

could be developed that would fit in the palm of your hand and just large enough to

carry a battery and a small motor.

2. Six leg mechanical spiders can be applicable for the making of robots. It has a wide

range of application in the manufacturing of robots. A large version could use existing

surveillance technology to convert your television into a real-time look at the world

within transmitting range.

3. It would also relay commands from the remote to the spider bike additional

frequencies could be used to operate manipulators for retrieving the mail during

unfavorable weather or taking the dog out.

4. It can also be used for military purpose. By placing bomb detectors in the machines,

we can easily detect the bomb without harmful to humans. It can be used as heavy

tanker machines for carrying bombs as well as carrying other military goods.

5. It is also applicable in the goods industries for the small transportation of goods inside

the industry. The mountain roads or other difficulties where ordinary vehicles cannot

be moved easily can be replaced by our six-leg mechanical spider.

6. Heavy loads can be easily transported if we made this as a giant one. It has got further

application for the study of linkage mechanism and kinematic motions. The geometry

and conditions can be changed according to application needs. It can travel in rough

surfaces very easily, so this machine can be used in rough surfaces were ordinary

moving machine cannot travel.

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Chapter 2

Literature Review

2.1 History of Legged Mechanism

The scientific study of legged locomotion began just very a century ago when Leland

Stanford, then governor of California, commissioned Edward Muyridge to find out whether

or not a trotting horse left the ground with all four feet at the same time (). Stanford had

wagered that it never did. After Muybridge proved him wrong with a set of stop motion

photographs that appeared in Scientific American in 1878, Muybridge went on to document

the walking and running behavior of over 40 mammals, including humans. His photographic

data are still of considerable value and survive as a landmark in locomotion research. The

study of machines that walk also had its origin in Muybridge’s time. An early walking model

appeared in about. It used a linkage to move the body along a straight horizontal path while

the feet moved up and down to exchange support during stepping. The linkage was originally

designed by the famous Russian mathematician Chebyshev some years earlier. During the 80

or 90 years that followed, workers viewed the task of building walking machines as the task

of designing linkages that would generate suitable stepping motions when driven by a source

of power.

Many designs were proposed but the performance of such machines was limited by their

fixed patterns of motion, since they could not adjust to variations in the terrain by placing the

feet on the best footholds. By the late 1950s, it had become clear that linkages providing

fixed motion would not suffice and that useful walking machines would need control. One

approach to control was to harness a human. Ralph Mosher used this approach in building a

four-legged walking truck at General Electric in the mid-1960s. The project was part of a

decade-long campaign to build advanced operators, capable of providing better dexterity

through high-fidelity force feedback. The machine Mosher built stood 11 feet tall, weighed

3000 pounds, and was powered hydraulically. Each of the driver’s limbs was connected to a

handle or pedal that controlled one of the truck’s four legs. Whenever the driver caused a

truck leg to push against an obstacle, force feedback let the driver feel the obstacle as though

it were his or her own arm or leg doing the pushing. After about 20 hours of training, Mosher

was able to handle the machine with surprising agility. Films of the machine operating under

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his control show it ambling along at about 5 MPH, climbing a stack of railroad ties, pushing a

foundered jeep out of the mud, and maneuvering a large drum onto some hooks. Despite its

dependence on a well-trained human for control, this walking machine was a landmark in

legged technology.

2.2 Different Types of Legged Mechanisms

Normally six bar mechanism is chosen for moving leg robot because of its superior force-

transmission angle and bigger oscillating angle in comparison with other types such as the

four-bar mechanism. Force transmission is very important for leg mechanisms, because of the

point contact with the ground. The leg mechanism itself has one DOF for lifting, whilst the

base of mechanism has another DOF for swinging. The body size and link dimensions are

determined from the maximum swing and lift angles. Each link is created by entering its

shape and reference coordinates. To mate the contact surfaces of the parts, the assembly bar

of the assembly mating menu is used. Then the component is rotated around an axis,

specifying the desired axis and rotation for the selected surfaces.

Yoseph Bar-Cohen, in his report named -Biomimetics: mimicking and inspired-by biology,

discussed that how the evolution of nature led to the introduction of highly effective and

power efficient biological mechanisms. Imitating these mechanisms offers enormous

potentials for the improvement of our life and the tools we use [2]. Shibendu Shekhar Roy,

Ajay Kumar Singh, and Dilip Kumar Pratihar, in their paper, highlighted the analysis of Six-

legged Walking Robots and the attempt made to carry out kinematic and dynamic analysis of

a six-legged robot. A three-revolute (3R) kinematic chain has been chosen for each leg

mechanism in order to mimic the leg structure of an insect [3]. Patil Sammed Arinjay and

Khotin developed Bio-Mimic Hexapod and explained Dynamic Modeling and Control in

Operational Space of a Hexapod Robot and comments the real times application of hexapod

robot for control. Based on an operational trajectory planner, a computed torque control for

the leg of hexapod robot is presented. This approach takes into account the real time force

distribution on the robot legs and the dynamic model of the hexapod. First, Kinematic and

dynamic modeling are presented. Then, a methodology for the optimal force distribution is

given. The force distribution problem is formulated in terms of a nonlinear programming

problem under equality and in equality on straits. The friction on strains is transformed from

nonlinear inequalities into a combination of linear equalities and linear inequalities.

Simulations are given in order to show the effectiveness of the proposed approach [4].

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Fig.2.2.1- ASV Six Legged Walking Machine

Fig.2.2.2- OSD Six Legged Walking Robot

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Fig.2.2.3- John Dear Walking Robot

Fig2.2.4- Forest Walking Machine

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Chapter 3

Methodology

3.1 Problem Statement

Literature review reveals that the main concern with the moving leg mechanism is the

number of links involved in the design of the structure. If there are more number of links it is

very tedious to design and operate. For the mechanism to move in a smooth manner the

dimensions of the pieces should be precise. More priority should be given to maintain the

exact lengths of the links which is determined by the position of the holes to be drilled. Also

the movement depends on the amount of power or motion which is transferred to the

locomotive parts with respect to the position of the drills. The key to success for this

mechanism is for the designer to make it locomotive even in the roughest of terrains. An

objective is set to develop a six- legged mobile robot in this project.

3.2 Methodology

The objective is to be achieved through the following steps which defines the methodology

adopted in this project work:

Literature Review

Problem

Design

Fabrication of Project

Testing and Alingment

Conclusion

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Chapter 4

Plan of Work

A work plan is an outline of a set of goals and processes by which a team and/or person can

accomplish those goals, and offering the reader a better understanding of the scope of the

project. Work plans, whether used in professional or academic life, help you stay organized

while working on projects. Through work plans, we break down a process into small,

achievable tasks and identify the things we want to accomplish.

In this semester, we have to learn about objectives and then determining the most effective

activities of our project Six Legged Walking Mechanism and draw a success full design of all

parts and components. So, that at first, we have a knowledge about mechanism and

principles.

Our concept of project based on the walking mechanism of six legged animal like Spider,

Crab etc. so we have to learn walking mechanism of six legged animal and construct a

mechanical design which can run using these mechanism. The working schedule of our

project are listed below step by step,

Step 1: Identify the purpose of our project.

Step 2: Making a knowledge about working mechanism.

Step 3: Draw some designs of parts and components.

Step 4: Purchase material and accessories.

Step 5: Construction of all parts.

Step 6: Assembling all parts and components.

Step 7: Testing of Product.

In this semester, we have to make on first three steps that gather a knowledge and make some

designs, it helps to done our project success fully.

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Chapter 5

Design of the Six Legged Walking Mechanism

5.1 Design of Leg

Leg structure is the core component of multi-legged walking machines which features

determine the basic properties of multi-legged walking machine. Leg structure of this

walking machine use six linkage. Crank is the antriebstange, bracing swing around sleeve.

When crank fixed to the shaft at one end, link 3 swings around the joint C and Joint D back

and forth, preparing the institution for preliminary leg open and contractile function. When

the joint B in the sleeve, joint B subject constraints from sleeve rather than turning, so joint D

and joint C hold up the machine body, link 3 ‘s swing make the body moving forward.

Crank rotating 360° around the drive shaft center completes an operation cycle. In the first

180° of crank rotation, as joint B in the sleeve, link 1 and 2 is equivalent to an overall, link

3‘s swing makes the body moving forward. In the last 180°of crank rotation, joint B leaves

sleeve, goes into rotating pair. Under the action of joint B, C and D and link 4, 3 lift off the

ground, no longer support the body, achieve the movement. This leg structure can convert

single rotary motion into leg walking exercise directly and has similar moving gesture with

quadruped mammals.

5.1.1 Gait Selection

Gait is the process that single leg put up and down according to a certain order and trajectory.

Gait choice should base on the leg structure. Suitable gait is the basis of the walking machine

to realize continuous stability walking motion. Currently, most of the six-legged walking

machine using tripod gait. Triangle gait refers to three pairs of legs are divided into two

groups, with alternating triangular support structure to move forward, number 1, 3, 5 legs for

a group, No. 2, 4, 6 legs for the other group, thereby formed two triangular brackets.

Tripod gait time plan for each leg; the gray portion represents contact with the ground. When

all the legs of one triangular bracket lift off the ground, the other triangular bracket’s legs

support the body and provide the driving force for forward movement. Two groups of legs

touch the ground alternately. The machine center of gravity projection is always falls in the

triangular bracket projection which has very good stability.

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5.2 Design of transmission system

There are many aspects should be considered in transmission system design, including leg

arrangement, motor position and transmission accuracy. It plays a vital role in body stability,

reliability and scalability.

This walking machine use rectangular body, both left and right sides of the machine arranged

three legs, respectively. Leg connects to body through a joint shaft. We use the symmetrical

arrangement to ensure the machine’s center of gravity coincides with the geometric center. In

order to ensure the six-legged gait coordination in walking, we use a single motor to drive 6

legs movement synchronously. System power pass through the chain, specific delivery path

are as follows: Motor output power pass through the sprocket 1, sprocket 2 to the shaft 2,

then drive shaft 2 and shaft 3, respectively. In order to ensure the same speed of shaft 1, 2 and

3, the transmission ratio both of sprocket 3 and sprocket 5, sprocket 4 and sprocket 6 choose

1 preferably, use the same specifications of the chain to achieve constant speed drive.

Fig.5.2.1. Six leg kinematic moving machine

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Fig.5.2.2- Simplified front view of the machine

The six legged moving machine shown in the figure.2 consists of flat aluminium bars, a chain

and sprocket arrangement, motor connected to give the required motion. This figure shows

the simplified diagram of the project drawn using AutoCAD.

5.3 Assessment of virtual prototype

Virtual prototype is built on prototype systems or subsystems of the computer models.

Compare to physical prototype, it have functional fidelity to a certain extent. In this study, we

use virtual prototypes to replace physical prototypes to test and evaluate the design

characteristics of the walking machine.

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5.4 Design and Drawing

Fig.5.4.1- Design of 12.5 mm Bush

Fig.5.4.2- Design of 58 mm Bush

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Fig.5.4.3- Design of 15 mm Bush

Fig.5.4.4- Design of 55 mm Bush

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Fig.5.4.5- Design of 51 mm Bush

Fig.5.4.6- Design of M6 Bar

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Fig.5.4.7- Design of 39 mm Bush

Fig.5.4.8- Design of Moving Link

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Fig.5.4.9- Design of Leg

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Fig.5.4.10- Design of 23 Teethed Sprocket

Fig5.4.11- Design of Fixed Link

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Fig.5.4.12- Design of assembled Six legged walking mechanism

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Chapter 6

Conclusion And Future Work

In this semester, a six legged walking mechanism is designed. Careful attention is paid to the

main design issues and constraints that influence the technical feasibility and performance of

these systems. A design procedure is outlined in order to systematically design a six-leg

walking machine. In particular, the proposed design procedure takes the mechanical structure

and leg configuration, the actuating and drive mechanisms, payload, motion conditions, and

walking gait, providing a useful tool for the systematic choice of main design characteristic.

Due to making this project we learnt that the mechanical walking concept of the six legged

animal like crab and spider. By using this mechanism, we have to make this project and

gather a concept on kinematic moving mechanism, 4 bar mechanism, and leg structure.

In next semester we make a mechanical structure by using these design and test the load

carrying capacity and its performance. The scope of our project is to make this machine walk

on any Surfaces with a stiff gait slow and carries a limited load. It could be used for

surveillance in sewer maintenance.

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REFERENCES

References of our project are listed below,

1. http://mechanicalprojects101.blogspot.in/2011/10/mechanical-projects_03.html By

Jagan, Praveen G, Prabhakar Narayanan, Pradyumn.

2. A project report on Inversion of 4 bar mechanism for kinematic motion by Dhaval

Patel, Government Polytechnic Chhotaudepur, mechanical department.

3. Fabrication of six legged kinematic moving mechanism by R.Arjunraj, A.Arunkumar,

R.Kalaiyarasan, B.Gokul, R.Elango, Department of Mechanical Engineering Nandha

Engineering College, Erode-638052, Tamilnadu, India.

4. https://en.wikipedia.org/wiki/Kinematics kinematic mechanism.

5. Design and Fabrication of Six leg kinematic moving machine. By Mahesh waran,

Kaviarasan, Muthu Kumar, Elavarasan.

6. Six leg kinematic moving machine- Mechanical engineering project from

http://www.hatchedeggs.com/blog/education/mechanical-engineering-project-six-leg-

kinematic-moving-machine

7. KinematicWalker from http://mechanicalprojects101.blogspot.in/2011/10/mechanical-

projects_03.html