design of robotic ankle

Page | 1

DESIGN OF ROBOTIC ANKLE

VISHWAKARMA INSTITUTE OF TECHNOLOGY

DEPARTMENT OF MECHANICAL ENGINEERING

Page | 2

Department of Mechanical Engineering

[2018-2019] Major-project report on subject,

DESIGN OF ROBOTIC ANKLE

C E R T I F I C A T E

This is to certify that Mr.Amit Bhusari[151761] , Mr. Samarth Bhosale [151715], Mr. Rushikesh

Panzade [151100], and Mr.Prasad jadhao [151150], has successfully completed mini project

entitled “DESIGN OF ROBOTIC ANKLE” under my supervision, in the partial fulfilment of

Bachelor of Technology - Mechanical Engineering of University of Pune.

Date : 22/12/2018

Place: VIT PUNE

Prof.R.K.Bhagat Prof. M. B. Chaudhari

Guide's Name Head of Dept.

Page | 3

Guide’s Name

ACKNOWLEDGEMENT

We are very much thankful to Prof. R.K. Bhagat to enculcate scientific views to be used for

common people in us. We acknowledge Prof. R.K. Bhagat for his guidance and technical support

due to which only we could use the operational techniques for useful operations. From bottom of

our heart we want to grace our guide. Due to him we could apply engineering concepts to

fabricate such a model to cater the need of common man and rising industries today.

Page | 4

CONTENT

CERTIFICATE .... 2 ACKNOWLEDGEMENT .... 3

CONTENT .... 5

ABSTRACT .... 6

1. INTRODUCTION .... 8 1.1 Project Background 1.2 Objective 1.3 Problem Statement 1.4 Scope of study

2. LITERATURE SURVEY .... 13

2.1 Basic human leg functioning 2.2 Types of prosthetic or robotic ankle 2.3 Single Axis foot 2.4 Multi axis foot

3. DESIGN .... 19

3.1 Targeted ankle parameters for our model 3.2 Mechanical model 3.3 CAD model 3.4 Components Used

4. WORKING OF PROTOTYPE .... 21

4.1 Steps 4.2 Working specifics

5. METHODOLOGY .... 31

5.1 Calculations of dimensions of gear 5.2 Modeling and Simulating model on ADAMS 5.3 Calculations of equivalent moment of intertia 5.4 Calculation of torque acting on pinion 5.5 Considerations 5.6 System 5.7 Torque and forces considered 5.8 Analysis of Mathematical model using MATLAB

6. MANUFACTURING .... 35

6.1 Discussion 6.2 3D Printing

Page | 5

6.3 PLA (Polylactic acid) 6.4 G-code for 3D Printer

7. CONCLUSION .... 36

7.1 Advantages 7.2 Limitations 7.3 Future scope 7.4 Problem faced 7.5 Balance sheet

8. REFERENCES .... 37

Page | 6

ABSTRACT

Human or Robotic ankle is a series of joints that are highly integrated. A conceptual design of

an ankle joint is presented here to facilitate the terrain adaptability maintaining natural gait

patterns and stability for robot as well as for person with single limb transtibial amputation. The

design consists of physiological ankle movements during walking on flat surface as well as

uneven terrain. A prototype is constructed for the visualization of the conceptual model using

"3-D printing". Until recently, an artificial leg was an inert object—a clunky piece of wood or

plastic that supported a user, but didn’t help much beyond that. But these days artificial limbs

can contain advanced sensors and microprocessors, and their motors can provide a power boost

for each step. Prosthetic feet are devices designed to replace one or more function of the

biological human ankle-foot system. Over the past few years developers have released to the

market a large variety of technologically advanced prosthetic feet, broadening the range of

available devices.

Despite the technological progress, the selection of the most appropriate foot for each person

with amputation is still difficult. Anecdotal evidence indicates that prescription of optimal

prosthetic feet to ensure successful rehabilitation is challenging because there are no generally

accepted clinical guidelines based on objective data. Indeed, despite depending on the patient’s

Medicare Functional Classification Level (MFCL) , the proper choice is often derived from the

particular clinical experience of the prosthetist and the rehabilitation team. Translational

research in this field could provide the scientific evidence required to improve and make more

objective the prescriptions of prosthetic feet to persons with lower limb amputation.

Page | 7

CHAPTER 1

INTRODUCTION

1.1 Project background

Humanoid robots are now used as research tools in several scientific areas. Researchers study

the human body structure and behavior (biomechanics) to build humanoid robots. On the other

side, the attempt to simulate the human body leads to a better understanding of it. Human

cognition is a field of study which is focused on how humans learn from sensory information in

order to acquire perceptual and motor skills. This knowledge is used to develop computational

models of human behavior and it has been improving over time.

Besides the research, humanoid robots are being developed to perform human tasks like

personal assistance, through which they should be able to assist the sick and elderly, and dirty

or dangerous jobs. Humanoids are also suitable for some procedurally-based vocations, such as

reception-desk administrators and automotive manufacturing line workers. In essence, since

they can use tools and operate equipment and vehicles designed for the human form,

humanoids could theoretically perform any task a human being can, so long as they have the

proper software. However, the complexity of doing so is immense.

For better working conditions of a robot, ankle plays a vital role. Without proper walking and

running mechanism, we wont be able to use the humanoids in real life.

Particularly in India, because of growing urbanization, number of people with lower extremity

amputation is increasing due to accident or prevalence of diabetes. Lower limb amputations

account for 91.7% of traumatic amputations among which 53% are transtibial amputation

(below knee amputations) and 33% are transfemoral amputation (above knee). Before 1980,

people having such problem used to wear wooden or metallic structured prosthesis which were

not that comfortable for normal activities.

So for such physically disabled people, there has to be some ankle model which can give these

people a comfortable limb movements with more degrees of freedom.

1.2 Objective

The objective of this project is to design and fabricate a light weight, low cost prototype and test the model.

Page | 8

1.3 Problem Statement

Mechanism for the working of robotic ankle, to achieve the required walking and running

speed. The exisiting models are mostly of two types viz. ‘SHAFT’ type and ‘SPRING’ type. But

these models have two most significant drawbacks viz.

1. Height price

2. Restricted DOF

1.4 Scope of study

The scope of this project is

1. To design a mechanism for the movement of ankle.

2. To design a prototype using 3-D printing

3. To evaluate the mathematical model of a model by using matlab

4. To actually test the working of model.

Page | 9

CHAPTER 2

LITERATURE SURVEY

This chapter includes summaries of existing literature relevant to this thesis. The purpose of the

review is to summarize the existing boundaries of understanding. The chapter is divided into

three sections that will focus on current research on biomechanics of the ankle and lower leg

prosthetics

2.1 Basic Human Leg Functioning

Figure 2.1.1 Gait cycle

Human Ankle Biomechanics during Normal Walking Human gait comprises of two phases i.e.

stance phase and swing phase through which their normal walking can be done . A stance phase

starts with the heel strike and continues till toe-off which contributes around 60-62% of the gait

cycle. . Human gait during normal walking 230 Habib Masum et al. / Procedia Technology ( 2014

) 228 – 235 Remaining 38-40% of gait cycle is in swing phase. The stance phase can be

characterized by the three sub-phases i.e. controlled planter flexion (12% of gait cycle from heel

strike to foot flat), controlled dorsiflexion (38% of gait cycle from foot flat to maximum

dorsiflexion) and powered plantar flexion (10-12% of gait cycle from maximum dorsiflexion to

toe-off) .

From the Fig 2.1.1, it is evident that the entire body weight implies on one foot during the single

limb stance (12% to 50%) of gait cycle(during remaining phases of gait cycles, the body weight

sheared by both the legs or by opposite leg). At the beginning of single limb stance, the CG of

body lies straight on the ankle. Then the CG shifts forward (the distance between line of action

Page | 10

of CG and ankle increases), which maximizes the moments on the ankle. The moment will be

maximum just before the opposite foot strikes. In recent times, various models were introduced

for better ankle mechanism for robots as well as for humans. Every model were using different

mechanisms.

The gait cycle is defined as heel strike to heel strike of the same foot while walking. The two

main phases of the gait cycle are the stance phase and the swing phase. The stance phase is the

time between the heel striking the ground and the toe of the same foot leaving the ground.

The swing phase is defined as the time the toe leaves the ground to the time the heel of the

same foot strikes the ground. Five main functions of the gait cycle outlined by David A. Winter

in The Biomechanics and Motor Control of Human Gait include:

1. Maintenance of support of the upper body during stance

2. Maintenance of upright posture and balance of the total body

3. Control of foot trajectory to achieve safe ground clearance and a gentle heel or toe landing

4. Generation of mechanical energy to maintain the present forward velocity or to increase the

forward velocity

5. Absorption of mechanical energy for shock absorption and stability or to decrease the

forward velocity of the body These five functions will be important in forming some of the

design metrics of the compliant ankle.

2.2 Types of prosthetic or robotic ankles

There are two types of prosthetic or robotic ankles according to the range of our project and

according to today's available mechanisms in market. These two types are as follows.

2.2.1 Pivot or Shaft type ankles

In this model (figure 2.2.1.1), pivot point is used to achieve the required ankle movement. pivot

is an articiulated self aligning hinge. its unique design allows it to be fabricated in exactly the

same manner as the typical flexure joint. pivot provides the mechanism of movements that

gives us the apportunity to control the sagittal plane motion. But its disadvantages are,

• we cant achieve the required angle for motion.

• we can achieve only one degree of freedom which is not suitable for better movements.

• Very high cost

• large torque capacity motor is reqired to achieve the required motion.

Page | 11

Figure 2.2.1.1

2.2.2 spring controlled ankle

In these models (Fig. 2.2.2.1) the required ankle movement is achieved by using springs with

some torsional stiffness. An artificial foot and ankle joint consists of a curved leaf spring foot

member having a heel extremity and a toe extremity, and a flexible elastic ankle member that

connects the foot member for rotation at the ankle joint. An actuator motor applies torque to

the ankle joint to orient the foot when it is not in contact with the support surface and to store

energy in a catapult spring that is released along with the energy stored in the leaf spring to

propel the wearer forward. A ribbon clutch prevents the foot member from rotating in one

direction beyond a predetermined limit position. A controllable damper is employed to lock the

ankle joint or to absorb mechanical energy as needed. The controller and sensing mechanisms

control both the actuator motor and the controllable damper at different times during the

walking cycle for level walking, stair ascent, and stair descent.

Disadvantages:

• only one degree of freedom is achieved by using this mechanism.

• any defect in spring stiffness can cause defect in whole mechanism.

• Very high cost

Page | 12

Figure 2.2.2.1

Prosthetic feet that have movement in two or three axes provide increased mobility at the

ankle, which helps stabilize the user while navigating on uneven surfaces.

2.3 Single-Axis Foot

The articulated single axis foot contains an ankle joint that allows the foot to move up and

down, enhancing knee stability. The more quickly the full sole of the foot is in contact with the

ground, the more stable the prosthesis becomes. This is beneficial for users with higher levels

of amputation (an amputation anywhere between the knee and hip). The wearer must actively

control the prosthesis to prevent the knee from buckling, and the single-axis ankle/foot

mechanism reduces the efforts required to do so. Unfortunately, the single-axis ankle adds

weight to the prosthesis, requires periodic servicing, and is slightly more expensive than the

more basic SACH foot. A single-axis foot may be more appropriate for individuals where

stability is a concern.

Page | 13

2.4 Multi-Axis Foot

Although similar to the single-axis foot in terms of weight, durability and cost, the

multi-axis foot conforms better to uneven surfaces. In addition to the up and down mobility of

the single-axis foot, a multi-axis foot can also move from side to side. Since the added ankle

motion absorbs some of the stresses of walking, this helps protect both the skin and the

prosthesis from wear and tear

Page | 14

CHAPTER 3

DESIGN

3.1 Targeted ankle parameters for our model 1) model should give structural strength to bear the weight of robot (weight of a human body in

prosthetic ankle).

2) The ankle must be capable to deliver required output power and torque .

3) higher degrees of freedom for better ankle movement.

4) reducing the total cost of manufacturing compared to recent models.

3.2 Mechanical model BREIF :

With this model, by using gear meshing assembly we managed to bring out the required

movement in ankle. in this model, we are using bevel gear and 2 spur gears in order to achieve

the required movement of the limb. We are giving the input to the bevel pinion. The bevel gear

and spur pinion and compound gears. The spur gear meshed with pinion is the gear which

rotates the foot attached to this gear. Another spur pinion is meshed with the gear to increase

the inertia.

3.3 CAD Model According to the mathematical, dimensional and structural analysis of out mechanism. The prototype has drawn in CATIA v5 for its 3D visualization.

3.4 Components Used

Page | 15

There are total five different components used in the structure of this prototype of mechanism of prosthetic or robotic ankle to form a perfect gear train. These five numbers of components are as follows

3.4.1 Bevel Gears :

Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of

the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that

are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of

bevel gears is a cone. Bevel gears are used in differential drives, which can transmit power to

two axles spinning at different speeds. Also, bevel gear have higher capacity of torque

transmission.

Differing of the number of teeth (effectively diameter) on each wheel allows mechanical

advantage to be changed. By increasing or decreasing the ratio of teeth between the drive and

driven wheels one may change the ratio of rotations between the two, meaning that the

rotational drive and torque of the second wheel can be changed in relation to the first, with

speed increasing and torque decreasing, or speed decreasing and torque increasing.

3.4.2 spur gear :

Spur gears are the most common type of gears. They have straight teeth, and are mounted on

parallel shafts. Sometimes, many spur gears are used at once to create very large gear

reductions.

Page | 16

3.4.3 servo motor :

Servo motors have been around for a long time and are utilized in many applications. They are

small in size but pack a big punch and are very energy-efficient. These features allow them to be

used to operate remote-controlled or radio-controlled toy cars, robots and airplanes. Servo

motors are also used in industrial applications, robotics, in-line manufacturing, pharmaceutics

and food services.

The servo circuitry is built right inside the motor unit and has a positionable shaft, which usually

is fitted with a gear (as shown below). The motor is controlled with an electric signal which

determines the amount of movement of the shaft.

3.5.3.1 What's inside the servo ?

To fully understand how the servo works, you need to take a look under the hood. Inside there

is a pretty simple set-up: a small DC Motor, potentiometer, and a control circuit. The motor is

Page | 17

attached by gears to the control wheel. As the motor rotates, the potentiometer's resistance

changes, so the control circuit can precisely regulate how much movement there is and in which

direction.

When the shaft of the motor is at the desired position, power supplied to the motor is stopped.

If not, the motor is turned in the appropriate direction. The desired position is sent via electrical

pulses through the signal wire. The motor's speed is proportional to the difference between its

actual position and desired position. So if the motor is near the desired position, it will turn

slowly, otherwise it will turn fast. This is called proportional control. This means the motor will

only run as hard as necessary to accomplish the task at hand.

3.5.3.2 How servo controlled?

Servos are controlled by sending an electrical pulse of variable width, or pulse width

modulation (PWM), through the control wire. There is a minimum pulse, a maximum pulse, and

a repetition rate. A servo motor can usually only turn 90° in either direction for a total of 180°

movement. The motor's neutral position is defined as the position where the servo has the

same amount of potential rotation in the both the clockwise or counter-clockwise direction. The

PWM sent to the motor determines position of the shaft, and based on the duration of the

pulse sent via the control wire; the rotor will turn to the desired position. The servo motor

expects to see a pulse every 20 milliseconds (ms) and the length of the pulse will determine

how far the motor turns. For example, a 1.5ms pulse will make the motor turn to the 90°

position. Shorter than 1.5ms moves it in the counter clockwise direction toward the 0° position,

and any longer than 1.5ms will turn the servo in a clockwise direction toward the 180° position.

Page | 18

When these servos are commanded to move, they will move to the position and hold that

position. If an external force pushes against the servo while the servo is holding a position, the

servo will resist from moving out of that position. The maximum amount of force the servo can

exert is called the torque rating of the servo. Servos will not hold their position forever though;

the position pulse must be repeated to instruct the servo to stay in position.

3.4 ATMEGA16 Controller

ATmega16 is an 8-bit high performance microcontroller from the Atmel’s Mega AVR family.

Atmega16 is a 40 pin microcontroller based on enhanced RISC (Reduced Instruction Set

Computing) architecture with 131 powerful instructions. It has a 16 KB programmable flash

memory, static RAM of 1 KB and EEPROM of 512 Bytes. The endurance cycle of flash memory

and EEPROM is 10,000 and 100,000, respectively. Most of the instructions execute in one

machine cycle. It can work on a maximum frequency of 16MHz. ATmega16 pin diagram should

clarify things a bit.

Page | 19

There are 32 I/O (input/output) lines which are divided into four 8-bit ports designated as PA,

PB, PC and PD. ATmega16 has various in-built peripherals like USART, ADC, Analog Comparator,

SPI, JTAG etc. Each I/O pin has an alternative task related to in-built peripherals.

Page | 20

CHAPTER 4

WORKING OF PROTOTYPE

In this model, we have used the meshing gear mechanism. Our aim is to develop the movement

of an ankle to the required degrees. We are using gears to achieve this movement. This is a

powered ankle model, so we are using motor of low torque to rotate the gears. So achieve this,

we have connected motor to our pinion i.e. bevel gear. That bevel gear is meshing with the first

spur gear. To rotate the second gear, first spur gear is connected to the second spur gear. In

other word, we have used the compound gear concept in which both the gears are on the same

shaft. So the speed of both gears will be same. This second gear is meshing with the third gear.

Due to meshing, second gear will rotate the third gear. To this third gear, we have connected our

foot, for which we want the movement. This foot is connected to the third gear with small shaft.

So the working of this model is as follows:

.

4.1 First step

In first step, we will give input to the motor, which is connected to the pinion i.e. the bevel gear.

We are using servo motor to give the input to the bevel gear. We are using atmega -16 to

control the motor speed. When the input is given to the bevel gear, it will rotate and give the

required input to the first spur gear.

4.2 Second step:

In second step, the input given fom the bevel gear is supplied to the first spur gear. As the

second spur gear is connected to the first spur gear, input is given to the second spur gear.

These two gears are compound gears, both the gears will rotate with the same speed.

4.3 Third step:

In third step, due to meshing, second spur gear will rotate the third spur gear. In this step, the

speed of rotation will reduce to our required speed due to meshing. We have connected one

horizontal foot like structure, in which we want the movement. Due to the rotation of spur gear,

the horizontal foot will perform the required motion of the foot. This moment is proportional to

the given output of the motor. But the speciality of our mechanism is, we are using bevel gear.

And we know that the transmission ratio of bevel gear is very high. So even if we are using the

Page | 21

low power motor, we will achieve the required movement of the foot. So , for the same

required angle required in the foot movement we can achieve with the small torque motor also.

4.4 Working specifics 4.4.1 bevel gear mechanism:

As we know bevel gear have maximum transmission ratio i.e. even if we supply

less input to the bevel gear, we can achieve the required output by using bevel gear.

That is the important reason behind the use bevel gear.

4.4.2 Use of hollow shafts :

As we know , to bear the load of assembly, we require the strong members to support. If the

shafts which are holding the gears are have less resistance to the bear torque, the structure will

collapse. To achieve better resistance to bearing the torque, we have used the hollow shafts for

the gears.

4.4.3 Less torque motor:

Our main aim was to analyze whether we are achieving the required results using less torque

motor. To analyze our model, we have used the less torque valued motor. By using the less

valued motor, we are able to achieve the required results.

Page | 22

CHAPTER 5

METHODOLOGY

5.1 Calculations of Dimensions Of Gears :

First, we have drawn all the gears in the solid works, from which we got the inertia of all the

gears. So by using solidworks we got the dimensions of gears as follows:

1-Bevel pinion

J=7.923*10-3 kg-m2

No. of teeth=14

2-Bevel gear

J=3.42*10-3 kg-m2

No. of teeth=42

3,5-Spur pinion

J=4.55*10-3 kg-m2

No. of teeth=18

4-Spur gear

J=7.25*10-3 kg-m2

No. of teeth=36

Numbering of gears is given as per CAD model drawn in CATIA v5

5.2 Modeling and Simulating Model on ADAMS

Page | 23

Figure 5.2.1 ADAMS Mode

5.2.1 Model details

1) Gears modled by using inbuilt feature of ADAMS.

2) A fixed joint is applied to bevel gear and first spur pinion as it is a compound gear.

3) Torque of 0.25sin(2*π*time) is applied to the bevel pinion.

4) Rotational spring damper is connected to both the spur pinions of particular spring constant.

5) A moment due to external force the spur gear is applied to the gear.

5.2.2 Results

1) Angular velocity of the Bevel pinion(Without external spring torque and moment

Stimulation time-5 sec

Step size-0.01 sec

Page | 24

Figure 5.7.2.1 Graph of Angular velocity vs time

2) Angular velocity of Bevel pinion with external torques Stimulation time-10 sec Step size-0.001 sec


5.3 Calculations of equivalent moment of inertia :

Equivalent moment of inertia is the inertia of the complete gear system.It is the inertia which

the driver(motor or shaft) has to overcome to transmit the torque to the output shaft.

To find equivalent inertia Iet us first assume a system gear system of 1 input pinion and output

Page | 25

gear. In the gear system, the gears are assumed to be discs of radius equal to pitch circle radius.

Following figure describes it :

Figure 5.3.1 Simple Gear System

Ja = Moment of inertia of pinion

Jb = Moment of inertia of gear

ra = Pitch circle radius of pinion

rb = Pitch circle radius of gear

n (Speed Ratio) = ra/rb

The rotational system has rotational stiffness and viscous stiffness. So the mathematical model

of the system should also take into consideration the same. The free body diagram showing the

various torque acting on the pinion and gear is shown-

Figure 5.3.1 (a) Free Body diagram for above system

Ta = Input torque

Here , "1 subscript" describes "pinion" and "2 subscript" describes "gear".

Ɵ = Angular displacement

w = Angular velocity

α = Angular acceleration

K = Torsional stiffness

B = Damping stiffness

Page | 26

Fc = Contact force

5.4 Calculation Torques acting on Pinion :

• Jα = Torque due to inertia

• R1fc = Torque due to contact force

• K1Ɵ1 = Torque due to torsional stiffness

• B1w1 = Torque due to damping coefficient.

Therefore, considering equilibrium and using De-Alembert's law for pinion ,

J1α1+K1Ɵ1+B1w1+FcR1 = T1 (1)

• The torque acting on gear,

J2α2+K2Ɵ2+B2w2+FCR2 = T2 (2)

Now solving (1) and (2) simultaneously for Fc we get,

T1= J1α1 + K1Ɵ1 + B1w1 + (R1/R2 ) (-Tb+J2α2+K2Ɵ2+B2w2)

= J1α1 + (R1/R2 ) (J2α2) + K1Ɵ1 + (R1K2Ɵ2)/R2 + (R2B2w2)/R2 + B1w1 - R1Tb/R2

Here,

α2=nα1

R1/R2=n

Therefore,

J1α1 + (R1/R2) (J2α2) = (J1+n2J2) α1

Here,

( J1 + n2 J2) = EQUIVALENT MOMENT OF INERTIA for pinion-gear system.

Similar condition can be applied for any type of gear systems by taking the square of the

velocity ratio of that gear with respect to pinion.

5.5 Considerations :

Page | 27

1) No torsional stiffness for bevel gears and spur gear

2) Damping coefficient neglected.

5.6 System :

1) The system consists of bevel pinion as driver. The bevel gear and spur pinion are compound

gears. The spur gear meshed with pinion is the gear which rotates the foot i.e the foot is

attached to the this gear.

2)Another spur pinion is meshed with the gear to increase inertia.

3)There are torsional springs attached to the 2 spur pinions to store the energy.

4)This system gives only 1 degree of freedom for movement of the ankle which the rotation on

ankle in vertical direction.

5.7 Torques and forces considered : The torque given to the bevel pinion

(1) is considered to be transmitted to the spur gear.

(4) Without any loss of power. So the torque has to overcome the equivalent inertia of the gear

systems. There is torque generated due to spring force. Also the force acting on the foot

generates a torque which is transmitted to the bevel pinion by the reduction factor.

5.7.1 Torque due to equivalent inertia

Since the Spur pinions(1,5) are identical ,we are multiplying j3 by 2.

Equivalent inertia (J) = J1 + n12

*j2 + n22 *2*j3 + n3

2 *j4

Torque generated = J*α1

5.7.2 Torque generated due to springs

Since there are 2 springs so we are multiplying k by 2

The angle turned by spring is given in terms of input angle considering gear ratio.

Torque generated = (n2*K*Ɵ1) / 3

5.7.3 Torque generated due to external force on foot.

It is assumed that a constant body force F, acts on feet at distance R from centre of gear.

The torque generated due this transmitted to the pinion multiplied by the reduction factor.

Torque generated = n3*M

From De-Alembert's law ,the summation of all these torques is equal to the torque supplied by

Page | 28

the motor -

[J1 + n12*J2 + n22*2*J3 + n32*J4]*α1 + n2*K*Ɵ1/3 + n3*M = T1

5.8 Analysis of our Mathematical Model using MATLAB Simulink

The equation was modeled on Simulink as follows,

Figure 5.7.3.1

The inertia MATLAB function has the equation of equivalent moment of inertia derived . 1)Angular velocity without external torque Solver-45S

Page | 29

Stimulation time-5 sec Step size-0.01s


2) Angular velocity of the bevel pinion with external torques-

Solver-45S

Stimulation time-10s

Step size-0.001

Page | 30


Page | 31

CHAPTER 6

MANUFACTURING

6.1 Discussion

The most crucial and important part of this project is manufacturing. Because there are many types of prosthetic and robotic limbs available in market today. Whose price are very high and the most important factor, they are not light weight as much as one prosthetic limb should be.

While designing this model one of the biggest problem we faced is the "size" of this prototype. Because as designer, we must have to keep our model as close as possible to real life application. So, whole mechanism designed by us should set in area close to the human ankle which is very less practically. To fit whole gear train mechanism in such small area of ankle is near to impossible. But while doing literature survey we came to know about 3D printing which made this design possible to manufacture. We kept some goal while manufacturing this whole mechanism/prototype are as follows: 1) Whole design should be compact in size as much as possible. 2) Ankle should be light weight though strong for real life application. 3) Its cost must be less than the price of available products in market.

To overcome our first goal we designed the mechanism of gears of small size as much as possible using bevel gear which have very high transmission ration.

To overcome our second and third goal we used 3D printing for manufacturing the gears of different size using Material "PLA- Infill 40%" which is very light weight and strong due to its chemical properties. Bevel gear manufactured for our model is only manufactured by 3D printing. Such small sized bevel gears are not directly available in market. The cost for this 3D printing of gear cheap as compared to other.

6.2 3D Printing :

3D Printing also known as Rapid prototyping. The steps of this process of 3D Printing or Rapid

prototyping are as follows:

1. Create a CAD model of the design

2. Convert the CAD model to STL format

Page | 32

3. Slice the STL file into thin cross-sectional layers

4. Construct the model one layer atop another

5. Clean and finish the model

CAD Model Creation: First, the object to be built is modeled using a Computer-Aided Design

(CAD) software package. Solid modelers, such as Pro/ENGINEER, tend to represent 3-D objects

more accurately than wire-frame modelers such as AutoCAD, and will therefore yield better

results. The designer can use a pre-existing CAD file or may wish to create one expressly for

prototyping purposes. This process is identical for all of the RP build techniques.

Conversion to STL Format: The various CAD packages use a number of different algorithms to

represent solid objects. To establish consistency, the STL (stereolithography, the first RP

technique) format has been adopted as the standard of the rapid prototyping industry. The

second step, therefore, is to convert the CAD file into STL format. This format represents a

three-dimensional surface as an assembly of planar triangles, "like the facets of a cut jewel." 6

The file contains the coordinates of the vertices and the direction of the outward normal of

each triangle. Because STL files use planar elements, they cannot represent curved surfaces

exactly. Increasing the number of triangles improves the approximation, but at the cost of

bigger file size. Large, complicated files require more time to pre-process and build, so the

designer must balance accuracy with manageablility to produce a useful STL file. Since the .stl

format is universal, this process is identical for all of the RP build techniques.

Slice the STL File: In the third step, a pre-processing program prepares the STL file to be built.

Several programs are available, and most allow the user to adjust the size, location and

orientation of the model. Build orientation is important for several reasons. First, properties of

rapid prototypes vary from one coordinate direction to another. For example, prototypes are

usually weaker and less accurate in the z (vertical) direction than in the x-y plane. In addition,

part orientation partially determines the amount of time required to build the model. Placing

the shortest dimension in the z direction reduces the number of layers, thereby shortening

build time. The pre-processing software slices the STL model into a number of layers from 0.01

mm to 0.7 mm thick, depending on the build technique. The program may also generate an

auxiliary structure to support the model during the build. Supports are useful for delicate

features such as overhangs, internal cavities, and thin-walled sections. Each PR machine

manufacturer supplies their own proprietary pre-processing software.

Layer by Layer Construction: The fourth step is the actual construction of the part. Using one of

several techniques (described in the next section) RP machines build one layer at a time from

polymers, paper, or powdered metal. Most machines are fairly autonomous, needing little

human intervention.

Page | 33

Clean and Finish: The final step is post-processing. This involves removing the prototype from

the machine and detaching any supports. Some photosensitive materials need to be fully cured

before use. Prototypes may also require minor cleaning and surface treatment. Sanding,

sealing, and/or painting the model will improve its appearance and durability.

6.3 PLA (Polylactic Acid)

Polylactic Acid, commonly known as PLA, is one of the most popular materials used in desktop

3D printing. It is the default filament of choice for most extrusion-based 3D printers because it

can be printed at a low temperature and does not require a heated bed. PLA is a great first

material to use as you are learning about 3D printing because it is easy to print, very

inexpensive, and creates parts that can be used for a wide variety of applications. It is also one

of the most environmentally friendly filaments on the market today. Derived from crops such as

corn and sugarcane, PLA is renewable and most importantly biodegradable. As a bonus, this

also allows the plastic to give off a sweet aroma during printing.

6.4 G-Code For 3D Printing

G1 X108.587 Y111.559 F525 ; controlled motion in X-Y plane


G1 Z0.345 F500 ; change layer



G1 X5 Y5 Z0 F3000.0 E0.02

G20 G0 X7 Y18

G21 G0 X7 Y18

G28.1 X0 Y0 Z0

G0 X10

G91 G0 X10

G21 G90 G17 G0 X6 Y18 G2 X18 Y6 I0 J-12

G0 X-25 Y5

G1 X-25 Y5

Page | 34

G90 M82 M106 S0 M140 S100 M190 S100



G1 Z0.345 F500 ; change layer


G1 X108.532 Y111.472 F525 ; controlled motion in X-Y

G28 ; bring the nozzle to home

M104 S0 ; turn off heaters

M140 S0 ; turn off bed

M84 ; disable motors

Page | 35

CHAPTER 7

CONCLUSIONS

7.1 Advantages of the model

1) Advantage of using bevel gears is that we can use low torque motor to achieve the required

motion as it has high torque transmission ratio .

2)by using this mechanism, we have reduced the cost of robotic ankle upto % compared to

other working model.

7.2 Limitations of the model:

1) It is challenging to reduce the size of this model. so the compactness is at stake.

2) Wear and tear of gears can be problematic for the working of model.

7.3 Future scope

1) This mechanism can be used for designing of prosthetic ankle for human beings with no limb

by using some sensors.

2) We can maximize the degrees of freedom.

7.4 Problems Faced

1) Modeling of gears in adams was difficult as the gears weren’t meshing and transmitting

force.

2) The selection of accurate step size to get the required profile.

3) Finding the correct equivalent moment of inertia of the gear system.

7.5 Balance Sheet

1) Gears 3D printing : 4800 Rs.

2) Outer structure/ body : 1500 Rs.

3) Motor : 800 Rs.

4) Battery (12 V) : 500 Rs.

TOTAL COST = 7600 Rs. (Approximately in range of 7500/- to 8500/-)

Page | 36

REFERENCES

• Williams RJ, Hansen AH, Gard SA.Prosthetic ankle-foot mechanism capable of automatic

adaptation to the walking surface. J Biomech. Eng.2009;131(3):035002.

• Hansen, A. H., D. S. Childress, S. C. Miff, S. A. Gard, K. P. Mesplay. The human ankle

during walking: implications for design of biomimetic ankle prostheses. J Biomech. Engg.

2004; 37(10):1467-74.

• Burgess EM et al.Development and preliminary evaluation of the vaseattle foot. J.

Rehabilitation.

• http://www.ottobockus.com/Prosthetics/Lower-limb-prosthetics.

• Flex-Foot. Ossur [Online]. Available: http://www.ossur.com.

• Powered ankle-foot prosthesis - 1070-9932/08/$25.002008 IEEE

• 2nd International Conference on Innovations in Automation and Mechatronics

Engineering, ICIAME 2014. Conceptual Design of a Powered Ankle-Foot Prosthesis for

Walking

Page | 37

design of robotic ankle

Documents