1

Summer Fellowship Programme 2014

(19-May to 11-July)

Under the Guidance

Of

Dr. Varadhan S.K.M

Neuromechanics Lab

Indian Institute of Technology Madras

By

Jal Panchal

B.Tech Mechatronics Engineering

Manipal Institute of Technology, Manipal

2

Contents

Acknowledgement................................................................................................................................... 3

Overview ................................................................................................................................................. 4

Theory...................................................................................................................................................... 5

Friction: ............................................................................................................................................... 5

Types Of Friction: ................................................................................................................................ 5

Laws of dry friction: ............................................................................................................................. 5

Calculation of Dry Friction: .................................................................................................................. 6

Types of Dry Friction............................................................................................................................ 7

Tribometer: ......................................................................................................................................... 7

Tribometer Setup .................................................................................................................................... 8

Objective: ............................................................................................................................................ 8

Setup Alpha ......................................................................................................................................... 8

General Outline: .............................................................................................................................. 8

Important components of setup: .................................................................................................... 8

Selection and Finalisation of Components ...................................................................................... 8

Setup Beta .........................................................................................................................................17

General Outline: ............................................................................................................................17

Important Components of the Setup: ...........................................................................................17

Selection and Finalisation of Components ....................................................................................17

References and Bibliography .............................................................................................................21

Literature Review: .........................................................................................................................21

Content Referance: .......................................................................................................................21

3

Acknowledgement

I would like to thank Indian Institute of Technology, Madras for giving me this opportunity to be a

summer fellow at the Neuromechanics Lab in the Applied Mechanics Department.

During my stay at IITM, I have acquired valuable academic and non-academic knowledge and skills,

for which I am grateful to Dr. Varadhan SKM, my project guide and mentor. His guidelines and

support, coupled with the brainstorming sessions and the freedom of expression made it the perfect

learning environment.

I also thank my lab mates Meeshwan Marathe, Nayan Bhatt, Rachaveti Dhanush, Saida Naik,

Sampada Gharpure for their invaluable support and help.

4

Overview

The goal of the project was to design and develop a setup to understand the variation of frictional

force between the digits of our hand and contact surface of various materials, and compare he result

obtained with Amonton’s laws of friction.

Work started by reading about friction, its types and properties, factors affecting friction and how to calculate it. Once knowledge about friction was acquired next step was reading research papers by Part et al(Mechanical properties of the human hand digits: Age-related differences) and Ambike et al(Factors affecting grip force: anatomy, mechanics, and referent Configurations). These papers gave an insight about age related factors affecting gripping force in our hands, mechanical enslaving and other factors affecting grip force. With this knowledge, work on designing a setup to test the friction coefficient, began. To calculate the coefficient we needed to know the vertical force and horizontal force at the time of slip. To achieve this, a sensor to calculate force in the required directions and an actuator to cause slip between the finger and the test material are required. ATI sensors already available in the lab were selected for the force sensors. To induce slip, the motion mechanism of the alpha setup was inspired from the linear mechanism of a printer cartridge. This setup was designed and developed. To improve upon a few of the glitches faced in the Alpha, the Beta Setup using a capstan-bowstring mechanism was developed. The control system used Arduino Uno and LabVIEW integration. The final setup prepared Alpha and Beta versions are ready for readings.

5

Theory

Friction: It is the force resisting the relative motion of solid surfaces, fluid layers, and material elements

sliding against each other.

Types Of Friction:

Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is

subdivided into static friction ("stiction") between non-moving surfaces, and kinetic

friction between moving surfaces.

Fluid friction describes the friction between layers of a viscous fluid that are moving relative

to each other.[1][2]

Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.[3][4][5]

Skin friction is a component of drag, the force resisting the motion of a fluid across the

surface of a body.

Internal friction is the force resisting motion between the elements making up a solid

material while it undergoes deformation.

Laws of dry friction: The elementary property of sliding (kinetic) friction were discovered by

experiment in the 15th to 18th centuries and were expressed as three empirical laws:

Amontons' First Law: The force of friction is directly proportional to the applied load.

Amontons' Second Law: The force of friction is independent of the apparent area of

contact.

Coulomb's Law of Friction: Kinetic friction is independent of the sliding velocity.

6

Calculation of Dry Friction:

Coulomb friction, named after Charles-Augustin de Coulomb, is an approximate model used to

calculate the force of dry friction. It is governed by the model:

where

is the force of friction exerted by each surface on the other. It is parallel to the surface,

in a direction opposite to the net applied force.

is the coefficient of friction, which is an empirical property of the contacting materials,

is the normal force exerted by each surface on the other, directed perpendicular

(normal) to the surface.

Normal force

The normal force is defined as the net force compressing two parallel surfaces together; and its

direction is perpendicular to the surfaces. In the simple case of a mass resting on a horizontal

surface, the only component of the normal force is the force due to gravity, where . In

this case, the magnitude of the friction force is the product of the mass of the object, the

acceleration due to gravity, and the coefficient of friction

If an object is on a level surface and the force tending to cause it to slide is horizontal, the

normal force between the object and the surface is just its weight, which is equal to

its mass multiplied by the acceleration due to earth's gravity, g.

Coefficient of friction

The coefficient of friction (COF), often symbolized by the Greek letter µ, is

a dimensionless scalar value which describes the ratio of the force of friction between two bodies

and the force pressing them together. The coefficient of friction depends on the materials used.

For surfaces at rest relative to each other , where is the coefficient of static friction.

This is usually larger than its kinetic counterpart.

For surfaces in relative motion , where is the coefficient of kinetic friction. The

Coulomb friction is equal to , and the frictional force on each surface is exerted in the direction

opposite to its motion relative to the other surface.

7

Types of Dry Friction:

Static friction

Static friction is friction between two or more solid objects that are not moving relative to each

other. For example, static friction can prevent an object from sliding down a sloped surface. The

coefficient of static friction, typically denoted as μs, is usually higher than the coefficient of kinetic

friction.The static friction force must be overcome by an applied force before an object can move.

The maximum possible friction force between two surfaces before sliding begins is the product of

the coefficient of static friction and the normal force:

Kinetic friction

Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub

together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk,

and is usually less than the coefficient of static friction for the same materials.

Tribometer: A tribometer is an instrument that measures tribological quantities, such as coefficient of friction,

friction force, and wear volume, between two surfaces in contact.

Using an analog multidimensional force sensor mounted on a moving block, the coefficient of friction

between the finger surface and test surface can be measured. The sensor will continuously record

the vertical force(Fv) and the horizontal force along the direction of motion(Fh). As there are no other

horizontal forces acting at the sensor, we have Fh = Ff.

So, by measuring the Fv and Ff we can calculate the s and also study the variation of with varying

vertical force applied and material. A motor mechanism will induce the horizontal movement of the

block to cause slip between the finger and the test surface.

8

Tribometer Setup

Objective: To design and develop an equipment to measure and study the coefficient of skin friction between

the finger and test surface. And to verify the result with Amonton’s laws.

Setup Alpha

General Outline:

The test surface is stuck on the sensor surface, which is mounted on a linear guide mechanism. A DC

motor controlled using an Arduino Duo and LabVIEW code, rotated the timing pulley fixed to its

shaft. The rotational motion of the timing pulley is converted to transitional motion using a meshing

timing belt. The translating timing belt is fixed to the slide giving the required horizontal slip force.

The setup was an inspiration from the linear mechanism in a printer, for the movement of the

printer cartridges.

Important components of setup:

Base

Motor and motor driver

Timing pulley and timing belt

Linear guide and slide block

Force sensor

Arduino Uno and LabVIEW

Power suppy

Selection and Finalisation of Components:

1. Base: Due to its durability and workability, 10mm wood sheets were used.

2. Motor: From initial calculations and crude setup design, the approx. torque required to

overcome a max Frictional force of 50N is 150N-cm, which translates to about (15kg-cm,

g=10m/s), with a factor of error of 2, the required torque for the motor was calculated to be

about 30kg-cm. A DC geared motor with low speed and close to 30 kg-cm torque was selected.

9

Motor1:

Speed: 10rpm.

Voltage :12V DC

Torque: 45 Kg-cm

Current drawn: Normal – 0.2A, Peak – 0.6A, Stall – 2A(theoretical).

DC geared motor with side shaft.

This motor gave a linear speed of guide of about 2 cm/s. To obtain higher linear speed and lower

vibration a second motor was selected.

Motor2:

Speed: 60 rpm

Voltage:12V DC

Torque: 45 Kg-cm

Current Drawn: Normal – 0.3A, Peak – 0.8A, Stall – 2A(theoretical)

DC geared side-shaft motor.

Linear speed of guide: 6 cm/s.

This second motor provided sufficient torque and linear speed of slide block, though it generated

nose.

Motor Drivers:

The first setup was controlled using a L293D dc motor controller. Later due to higher current rating

and reliability, L298 was used to control the dc motor.

3. Timing Pulley and Belt.

An Aluminium Timing Pulley of pitch 2mm and diameter 20mm(approx.) was selected to provide the

optimum force transfer from the motor shaft to the belt . A MXL timing belt with appropriate length

and 2mm pitch was selected to obtain 1:1 gear ratio. The pulleys were to be kept at a distance of

150mm apart to obtain a traverse length of the slide block of about 80-100 mm after assembly of

the setup.

Tension in the timing belt was maintained using a sliding mechanism. The dead end pulley was

mounted on the movable slide, after fixing the belt and other parts, this slide was moved and

screwed to maintain optimum tension in the belt.

4. Guideway and slide block:

A Standard linear slide and guide mechanism of length 150 mm manufactured by Precision Bearing

House was identified to provide the required base and linear movement for the sensor. The

mechanism consists of a slide block with double circulating ballscrew mechanism which rolls over a

meshing guide.

10

5. Force Sensors

ATI multidirectional force sensors are used, along with DAQ and LabVIEW to record the force in

vertical and horizontal direction. This plot of continuous force gives us the relation between Fv and

Ff, giving the coefficient of friction.

6. Arduino Uno and LabVIEW Code:

Arduino Uno with Arduino IDE was selected to control the motor. This selection was made due to its

simplicity in using, connecting, coding and also the availability of Arduino Toolkit in LabVIEW with

which it can be integrated and controlled using the graphical VI of LabVIEW.

Initial Versions of Code:

This code caused the motor to run in clock and anticlockwise direction with the press on a switch.

11

This code changed the motor speed and direction change with the press of a button:

12

This code controlled the motor speed and direction using a 10k Potentiometer and a switch.:

After these codes were tried and tested on the Arduino platform, LabVIEW interface with Arduino

was used control the motors using the graphical Vis of LabVIEW and Arduino Uno boards.

13

LabVIEW Vis:

This VI is to switch the motor on and off, using a switch:

This Vi Controls the forward and reverse motion of the motor using a toggle switch and button:

14

This VI Controls the direction and speed of the motor in both the directions:

7. Power Source

During the testing and prototyping phase of the setup, direct RPS supply was used.

The requirements of the power supply are:

Voltage: 12V

Current: 0-2Amps

Power: about 650Watts.

To improve upon this a few ideas have been put forward:

1. A 12V rechargeable Li-ion battery with charger

2. A laptop battery with Adaptor

3. SMPS supply for 12V, 650W.

Finalisation of the power supply is still pending.

15

The Complete Setup Alpha:

16

Sr. No Item Description Place of Use Quantity Source Cost

1 Arduino Board Arduino Uno Control System 1

http://www.rhydolabz.com/index.ph

p?main_page=product_info&cPath=1

52_123&products_id=626 approx. 1700

2 Battery Eliminator upto 15v and 4Amp power the motor 1 lab

3 Belt

2544 MXL timing belt. 0.08"

pitch(2.03 cm), width 6 mm, c-c

145.3mm(details in bill) drive mechanism 1

Best Belts and

Couplings:http://www.bestbelts.in/ 240+ 5%tax

4 Chain Tightening Screw

to maintain tension at the dead

centre 2 Cycle repair Shop 6x2=12

5 Guideway details in prev bill for linear movement 1 http://www.pbh.in/ar_dim.html

6 Motor 10 rpm, 45 kg-cm stall torque. driving the pulley 1

http://www.nex-

robotics.com/index.php?page=shop.

product_details&flypage=flypage.tpl

&product_id=856&category_id=13&o

ption=com_virtuemart&Itemid=45 538

7 Motor Driver TI L293D

motor driver to control current and

voltage 1

http://www.nex-

robotics.com/index.php?page=shop.

product_details&flypage=flypage.tpl

&product_id=48&category_id=14&opt

ion=com_virtuemart&Itemid=45 59

8 Nails 30mm base to support 10 Workshop

9 Nails 15mm various wood pieces 10 Worshop

10 Pulleys

20.19 dia, 32 teeth, double

colar(details in bill) drive mechanism 2

Best Belts and

Couplings:http://www.bestbelts.in/ (760+ 14%tax)x2

11 Screw allen screw size 2.5, length 10mm clamping pulley to motor 1 Lab

12 Screw philips 4mm bore, 10mm length fixing platform to slide 2 Lab(Mechanix set)

13 Screw

size 5 allen screw, approx. 36mm

length fixing pulley at dead centre 1 Worshop

14 Screw philips 35 mmm bore, 25mm length fixing guide to base 3 Lab

15 Sensor+ Force Sensor and fixing allen screw to measure force 1 Available in lab

16 Slide details from previous bill for linear movement 1 http://www.pbh.in/ar_dim.html

17 Wires Jumper Wires Arduino to motorDriver 5

http://www.nex-

robotics.com/index.php?option=com

_virtuemart&page=shop.product_det

ails&flypage=flypage.tpl&product_id

=447&Itemid=45 6x5=30

18 Wires

Single strand connection Wires(<4

Amps) 1m length.

motor to driver and power source

to driver 2 electrical shops

19 Wood Piece 40x35x5 mounting of sensor 1 Workshop -

20 Wood Plank 450x350x10 Base and Alignments 1 Workshop -

21 Zip tags long(200mm) clamp belt to the slide 1 Hardware store 1.5

22 Zip tags short(100mm) clamp belt to the slide 2 Hardware store 1

Component List for Friction Measurement Alpha Setup

The Setup 1 had the following problems which were addressed in the second setup:

High friction between the rotating member and the liner motion member

Maintaining tension in the timing belt.

17

Setup Beta

General Outline:

Two critical problems faced in Setup Alpha were addressed, of high friction in the movement and

improved method for maintaining the tension of the tensile member.

In this setup, a capstan and bowstring mechanism is used to convert the rotational movement from

the motor shaft to the translational motion of the slide block. A capstan is fixed on the motor shaft,

to this a stainless steel rope is wound around and tied to two ends of a movable base, tension in the

rope is maintained using a suitable spring at one end. This movable base is keyed onto the slide

block. The Force sensor is fixed on the slide block similar to the Alpha setup, on the movable block

here. As the motor rotates, the capstan rotates and along with it the bowstring winds and unwinds

on it, this action causes the movable base to move linearly along with the slide block on the guide.

Depending the direction of the motor rotation, the slide block will move towards or away from the

subject.

Important Components of the Setup:

Base

Motor

Capstan, Bow string and Spring

Movable base

Slide block and Guide

Force Sensor

Arduino Uno and LabVIEW

Power Supply

Selection and Finalisation of Components:

1. Base: Due to its durability and workability, 15mm wood sheet was used.

2. Motor: Adapting from Alpha, Beta used the same motor.

18

Motor specifications:

Speed: 60 rpm

Voltage:12V DC

Torque: 45 Kg-cm

Current Drawn: Normal – 0.3A, Peak – 0.8A, Stall – 2A(theoretical)

DC geared side-shaft motor.

Linear speed of guide: 6 cm/s.

The required motor torque in this setup is lower compared to Alpha. We required an approx. torque

of 20 Kg-cm with a factor of error of 2 for an applied Fv of 50N and considering s = 1.

To be able to control he movement of the motor, the motor can be improved upon by using a DC

motor with optical feedback mechanism. The data from the optical feedback can be used to control

the to and fro motion of the slide.

Motor Driver:

The same motor driver L298N and L293D are used for Beta, after being tested on Alpha.

3. Capstan, bowstring and spring:

Bowstring: To hold the estimated tension, and provide the required pull force, a stainless

steel rope of diameter 1.2 mm was selected. Testing can be done with ropes of diameter 0.8,

0.5 and 1mm for durability. The rope used experienced splitting of strands due to wear and

mishandling; this can be taken care by applying a Teflon coat in the rope.

Capstan: A capstan to compliment the bowstring and the motor shaft was designed. A

20mm diameter, V-thread of pitch 1.5mm and depth 1.5 mm with a central bore of 6 mm for

motor shaft, with another bore to key the capstan to the motor shaft was designed.

Spring: A spring at one end is used to maintain the tension in the bowstring to provide the

required motion. The required force on the bowstring is 50N = approx. 5kg, movement of

the slide block is about 3 cm, giving the required force constant = 1.7 = approx. 2 Kg/cm.

4. Movable Base:

The movable base is used to hold the bowstring and transfer the linear motion of the bowstring to

the slide block. Initially the base was designed to be made from Acrylic, but due to inability to bend

Acrylic into the required shape, subject to its brittle nature, a plastic material was used to make the

base.

The current job made causes hindrance to the fingers when placed on the sensor, minor

modifications and design change are required to perfect the design. Possible materials that could be

used:

Glass, by blowing t into shape

3D printed plastic

Wood pieces

Aluminium sheet.

19

Movable Base 1

These Components are similar to Alpha Setup, no changes have been made:

5. Slide block and guide

6. Force sensor

7. Arduino Uno and LabVIEW

8. Power supply.

Capstan 1

Motor 1

20

Sr. No Item Description Place of Use Quantity Source Cost

1 Arduino Board Arduino Uno + Connection Cable Control System 1

http://www.rhydolabz.com/index.ph

p?main_page=product_info&cPath=1

52_123&products_id=626 approx. 1700

2 Allen Screw phillips 4 mm dia, 15 mm length tie bowstring to movable base 2 Workshop tool store

3 Bowstring

1.2/1.0/0.8/0.5 mm dia, about 100

cm

to the tied around capstan and tied

to movable base 1 Ritchie Street 30-1000

4 Capstan

20mm dia, 1.5 mm pitch, 1.5 mm

depth V-thread, with 3mm dia tap

for grub screw

to convert rotational Motion of

motor to linear motion of

bowstring 1 Workshop

5 Grub Screw length 5mm, dia 3mm clamping pulley to motor 1 Workshop Tool Store

6 Motor 60 rpm, 45 kg-cm stall torque. driving the pulley 1

http://www.nex-

robotics.com/index.php?page=shop.

product_details&flypage=flypage.tpl

&product_id=856&category_id=13&o

ption=com_virtuemart&Itemid=45 538

7 Motor Clamps Suitable Motor clamps Clamp motor to base 2 Workshop

8 Motor Driver L298N

motor driver to control current and

voltage 1

http://www.nex-

robotics.com/index.php?page=shop.

product_details&flypage=flypage.tpl

&product_id=343&category_id=14&o

ption=com_virtuemart&Itemid=45 298

9 Movable base As per Design

to connect the capstan mechanism

to the slide block 1 Workshop

10 Screw philips 4mm bore, 1mm length fixing platform to slide 2 Lab(Mechanix set)

11 Screw philips 3.5 mm dia, 25mm length fixing guide to base 3 Workshop, Lab

12 Screw Phillips 4 mm dia, 15 mm length fixing motor clamp to base 6 Workshop

13 Sensor+ Force Sensor and fixing allen screw to measure force 1 Available in lab

14 Slide block and guide details from previous bill for linear movement 1 http://www.pbh.in/ar_dim.html approx 2300

15 SMPS Power supply 12V, 650W. power the motor 1 lab

16 Spring 2-5 Kg/cm spring constant

to maintain tension in the

bowstring 1

Ritchie Street/ Classic Springs

Vellacherry 20-50

17 Wires Connection wires Arduino to motorDriver, Motor driver to power, Motor to Driver2 m(approx)

http://www.nex-

robotics.com/index.php?option=com

_virtuemart&page=shop.product_det

ails&flypage=flypage.tpl&product_id

=447&Itemid=45 50-100

18 Wood Plank 30X30X15 Base 1 Workshop -

Component List for Friction Measurement Alpha Setup

Areas of improvement in Beta:

Use of better motor with less noise and feedback mechanism

Use of a dedicated battery for power supply

Use of bowstring of lesser diameter

Improvement in manufacture of movable base.

21

References and Bibliography

Literature Review:

1. Mechanical properties of the human hand digits: Age-related differences Park, J., Pažin, N., Friedman, J., Zatsiorsky, V. M., & Latash, M. L. (2014). Mechanical properties of the

human hand digits: Age-related differences.Clinical Biomechanics, 29(2), 129-137.

2. Factors affecting grip force: anatomy, mechanics, and referent configurations. Satyajit Ambike, Florent Paclet , Vladimir M. Zatsiorsky, Mark L. Latash(2014)

3. A Friction Differential and Cable Transmission Design for a 3-DOF Haptic Device with Spherical Kinematics. Reuben Brewer, Adam Leeper, and J. Kenneth Salisbury(2011)

Content Referance:

1. www.Google.com

2. www.Wikipedia.com

3. CATIA forum, Dassault Systems

4. Arduino.cc. ,Arduino Forums

5. http://charm.stanford.edu/ME327/AvinashAndChris#toc9