Magic Karpet Alex Dual Degree Thesis IIT Madras

7/31/2019 Magic Karpet Alex Dual Degree Thesis IIT Madras

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Design and development of an intuitively controlled

personal transport device based on a skateboard platform

A REPORT

Submitted in partial fulfillment of the requirements

for the award of the degree of

Master of Technology

(Automotive Engineering)

and

Bachelor of Technology

(Engineering Design)

By

Alex J Vazhatharayil

Under the guidance of

Dr. Sandipan Bandyopadhyay

Department of Engineering Design

Indian Institute of Technology Madras

June 2012


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THESIS CERTIFICATE

This is to certify that the report titled Design and development of an intuitively

controlled personal transport device based on a skateboard platform submitted by

Alex J Vazhatharayil, to the Indian Institute of Technology Madras, Chennai for the

award of the degree ofBachelor of Technology and Master of Technology, is a bona

fide record of the research work done by him under my supervision. The contents of this

report, in full or in parts, have not been submitted to any other Institute or University for

the award of any degree or diploma.

Dr. Sandipan Bandyopadhyay

Assistant Professor

Department of Engineering Design

Indian Institute of Technology Madras

Place: Chennai

Date: 21st

June, 2012


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ACKNOWLEDGEMENTS

I am very thankful to my guide Dr. Sandipan Bandyopadhyay who believed in me and

encouraged me to pick up a product design project as my M.Tech project. I am also

grateful to him for having provided support whenever necessary while giving me the

freedom to innovate. I am extremely grateful to IC&SR for having funded this project

under Student Innovative Project. This level of work could not have been possible

without the financial support. I would like to thank the people at the institute workshop

who helped fabricate the truck. I am extremely thankful to Mr. Ranganathan who did

the mechanical fabrication of most of the components of the final prototype. Hissuperior understanding and work quality has helped speedup the project. I am ever

grateful to the creators of Arduino which made writing microcontroller codes insanely

simple. I thank the developers of open source software-Processing. I thank Eagle for

providing a free version of their circuit building software for hobby use, which was

extensively used in this project. Last but not the least I would like to thank my parents

for all the support they have provided.


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ABSTRACT

In this project, an attempt was made to make a practical product that a user would want to

use. The product attempted to build is meant to address the problem of personal transport for

short distances of approximately 5km to 10km.

In a country like India with a large population, transportation from point to point is a major

problem. With almost always crowded roads and the risks/tension involved in driving to

work, people are increasingly tending to use the well established public transport system. The

problem with public transport is that they are crowded at many times, but with limited space

that we have in our cities, when more people use public transport, less is the traffic on road

and more will be the space for improving public transport. One major factor that prevents

people from using the public transport is the walk involved to and from a public transport

station. The intention of this project is to create a device that saves the user from short

distance walks(less than 5km) which can also be carried by the user on a public transport

system. If successfully deployed this device is to promote the usage of public transport and

enhance user travel experience.

This product is to be first deployed in a controlled environment like IIT Madras campus

where the travel within the campus is less 5 km but needs to be made on a daily basis. This

new innovative product with electric batteries will be far more efficient that gas based two-

wheelers and four-wheelers. It can also be used in large housing colonies to commute within

the housing colonies. Once the roads are made better, soon they should be usable on any

pothole free commutable roads.


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LIST OF FIGURES

Figure 1.1: The three most common use-case of the device ..................................................... 2Figure 1.2: Segway the human transporter (reprinted from www.segway.com/) ..................... 3Figure 1.3 A typical electric skateboard available in the market (reprinted from

www.altered.com) ..................................................................................................................... 3Figure 1.4: Uno dicycle (reprinted from en.wikipedia.org/wiki/Uno_(dicycle)) ..................... 4Figure 1.5: Honda UX-3 (reprinted from world.honda.com/U3-X/) ........................................ 5Figure 1.6: Left - 250W electric skateboard, right 800W electric skateboard (reprinted from

www.alteredelectricskateboards.com) ...................................................................................... 6Figure 1.7 : Zboard (reprinted from zboardshop.com) ............................................................. 6Figure 1.8: Different kinds of decks (reprinted from www.texaslongboards.com/) ............... 12Figure 1.9: A typical skateboard truck (reprinted from www.skaterevolution.com/)............. 13Figure 1.10: Drop-deck longboard (reprinted from www.muirskate.com/) ........................... 13Figure 1.11: Typical electric skateboard trucks (reprinted from

fastestelectricskateboard.co.uk/) ............................................................................................. 13Figure 1.12: 10 inches wheels used in an electric skateboard (reprinted from

www.brolive.org/) ................................................................................................................... 14Figure 1.13: Model of the user standing on the deck .............................................................. 15Figure 1.14: Model of the rider and device when going up a ramp ........................................ 16Figure 2.1: The completed deck with trucks and wheels attached. The arch shape of the deck

is noticeable in this figure. ...................................................................................................... 19Figure 2.2: Sandpaper stuck on the top to increase friction on the top of the deck ................ 20 Figure 2.3: Painting the deck to prevent the deck from decay due to moisture ................... 20Figure 2.4: The underside of the deck after paint job ............................................................. 21Figure 3.1: Parts of a truck (reprinted from skaterevolution.com) ......................................... 24Figure 3.2: Side view of the truck model when the tilt angle b is equal to 0 ........................ 25Figure 3.3: Front view of the truck showing angle b............................................................. 26Figure 3.4: The top view of the truck showing the angle t ................................................... 27Figure 3.5: Axial view of the truck showing angle r ............................................................ 28Figure 3.6: Plot ofb with r................................................................................................... 30Figure 3.7: Plot oft with r ................................................................................................... 31


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Figure 3.8: Plot oft with b ................................................................................................... 31Figure 3.9: Plot of height of deck CG with r when l1/l2 is 0.1 .............................................. 32Figure 3.10: A solid model of the truck developed ................................................................ 34Figure 3.11: The truck prototype mounted to deck version 2 ................................................. 35Figure 4.1: FSR-Force Sensing Resistor (reprinted from www.sparkfun.com/) .................... 37Figure 4.2: Loadcell sensor used in digital bathroom scale (reprinted from sparkfun.com) .. 38 Figure 4.3: Two loadcells used in parallel to create the bridge .............................................. 40Figure 4.4: Two loadcells used in anti-parallel to create the bridge ....................................... 41Figure 4.5: 1000 ohm resistors and a trimpot used to balance the bridge with one loadcell .. 42Figure 4.6: Schematic of the loadcell board. .......................................................................... 43Figure 4.7: Gain trimpot and bridge trimpot on the loadcell circuit ....................................... 44Figure 5.1: Joystick - used for testing and simulations ........................................................... 50Figure 5.2: Data from the device being plotted live as it is tested .......................................... 53Figure 5.3: Schematic circuit for adjusting Vref...................................................................... 54Figure 6.1: 250W motor and 500W motor .............................................................................. 57Figure 6.2: Plot of torque output at the motor shaft vs. RPM of 500W motor at 24V ........... 58 Figure 6.3: Plot of power output with RPM of 500W motor at 24V ...................................... 59Figure 6.4: Plot of efficiency of the 500W motor at different RPM at 24V ........................... 59Figure 6.5: Transmission system where the motor is connected to a gear box which

connected to the wheel ............................................................................................................ 60Figure 6.6: Minimum torque required at the wheel at various speeds .................................... 63Figure 6.7: Torque available at 24V (thick) and torque required (dashed) at motor with

sprocket ratio of 3:1. ............................................................................................................... 64Figure 6.8: Fullymax 4900 mAh and 2700mAh battery ......................................................... 65Figure 6.9: Sabertooth motor driver (left) and electric scooter driver (right) ......................... 68Figure 6.10: Analog accelerometer (left) and digital accelerometer (right) (reprinted from

rhydolabz.com) ....................................................................................................................... 69Figure 7.1: Acrylic loadcell mount used for testing, with all 4 loadcells in place .................. 71Figure 7.2: Complete test assembly with the glass placed over 4 loadcell on the acrylic mount

connected to the circuit implemented on breadboard ............................................................. 71Figure 7.3: load cell mount ..................................................................................................... 72Figure 7.4: Loadcell, loadcell mount and loadcell circuit board ............................................ 73


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Figure 7.5 : Exploded view of double layered deck assembly ............................................... 74Figure 7.6: Exploded view of the wheel assembly ................................................................. 75Figure 7.7: Free wheel hub ..................................................................................................... 76Figure 7.8: Powered wheel hub .............................................................................................. 76Figure 7.9: Motor mount attached to the motor ...................................................................... 77Figure 7.10: Exploded view of the motor mount .................................................................... 78Figure 7.11: Basic device diagram, showing the longitudinal axis(X) and lateral axis(Y) and

the four loadcell mount points ................................................................................................ 79Figure 7.12: Control strategy version 1 .................................................................................. 80Figure 7.13: Control strategy version 2 .................................................................................. 81Figure 7.14 : Control strategy version 3 ................................................................................. 83Figure 7.15: Modified control strategy version 3 ................................................................... 84Figure 7.16: The completed prototype to test intuitive control .............................................. 85


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TABLE OF CONTENTS

ACKNOWLEDGEMENT ....................................................................................................... i

ABSTRACT ............................................................................................................................ ii

LIST OF FIGURES ................................................................................................................ iii

TABLE OF CONTENT ......................................................................................................... vi

CHAPTER 1 Establishing project goals and specification ....................................................... 1

1.1 Introduction ..................................................................................................................... 1

1.2 Objectives and motivation ............................................................................................... 1

1.3 Survey of similar products in the market ........................................................................ 3

1.3.1. Devices based on inverted pendulum platform ....................................................... 3

1.3.2. Devices based on skateboard platform .................................................................... 5

1.4 Projects goals and specifications ..................................................................................... 6

1.4.1. Mission statement .................................................................................................... 6

1.4.2. Product description .................................................................................................. 6

1.4.3. Key business or humanitarian goals ........................................................................ 7

1.4.4. Primary market ........................................................................................................ 7

1.4.5. Secondary market .................................................................................................... 7

1.4.6. Assumptions ............................................................................................................ 7

1.4.7. Avenues for creative design .................................................................................... 7

1.4.8. Scope limitations: .................................................................................................... 7

1.4.9. Technical Questioning ............................................................................................. 8

1.4.10. The product/solution Intuitively controlled electric skateboard. ......................... 9

1.4.11. Identification of potential Customers .................................................................... 9

1.4.12. Anticipated Customer Requirements ..................................................................... 9

1.4.13. Functional requirements ...................................................................................... 11

1.5 System description ........................................................................................................ 11

1.5.1. Deck ....................................................................................................................... 11

1.5.2. Truck ...................................................................................................................... 12

1.5.3. Wheels ................................................................................................................... 14

1.6 Mathematical Model ..................................................................................................... 14


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1.6.1. Using the mathematical model .............................................................................. 17

1.7 Conclusion ..................................................................................................................... 18

CHAPTER 2 Designing the deck ............................................................................................ 19

2.1 Introduction ................................................................................................................... 19

2.2 Building skateboard deck version 1 .............................................................................. 19

2.3 Building Skateboard deck version 2 .............................................................................. 21

2.4 Conclusion ..................................................................................................................... 22

CHAPTER 3 Designing the truck ........................................................................................... 23

3.1 Introduction ................................................................................................................... 23

3.2 Building Skateboard Truck ........................................................................................... 23

3.3 Parameters of the new truck design ............................................................................... 24

3.4 Modeling the truck ........................................................................................................ 28

3.5 Parametric study of the new truck design ..................................................................... 30

3.5.1. Variation of height of the deck with various parameters ....................................... 32

3.6 Prototyping the new design ........................................................................................... 34

3.7 Conclusion ..................................................................................................................... 35

CHAPTER 4 Designing and prototyping the CG sensor ........................................................ 36

4.1 Introduction ................................................................................................................... 36

4.2 Concept selection .......................................................................................................... 36

4.2.1. Concept 1 Pressure pads ..................................................................................... 36

4.2.2. Concept 2 Loadcells ........................................................................................... 36

4.2.3. Concept 3 Force sensing resistors ...................................................................... 37

4.2.4. Concept selected Loadcells ................................................................................ 37

4.3 Designing and building the load cell sensor .................................................................. 38

4.3.1. Selection of Loadcell ............................................................................................. 38

4.3.2. Designing the Loadcell Circuit board .................................................................... 39

4.3.3. Concept Selection .................................................................................................. 39

4.4 Adjusting gain and balancing the bridge ....................................................................... 44

4.5 Designing the Loadcell Filter Board ............................................................................. 45

4.6 Conclusion ..................................................................................................................... 45

CHAPTER 5 Designing the Main Controller ......................................................................... 46


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5.1 Introduction ................................................................................................................... 46

5.2 Design Requirements of the Controller ......................................................................... 46

5.3 Design Constraints ........................................................................................................ 47

5.4 Designing the Main Board ............................................................................................ 48

5.4.1. Tilt Sensor.............................................................................................................. 48

5.4.2. Motor controller output ......................................................................................... 48

5.4.3. Joystick .................................................................................................................. 49

5.4.4. SD Card ................................................................................................................. 50

5.4.5. Live piling of data on a remote computer .............................................................. 50

5.4.6. Using the live data ................................................................................................. 52

5.4.7. Communication with secondary controller boards ................................................ 53

5.4.8. Adjusting loadcell reference voltage ..................................................................... 54

5.5 Conclusion ..................................................................................................................... 54

CHAPTER 6 Component Selection and Specification ........................................................... 55

6.1 Introduction ................................................................................................................... 55

6.2 Motor ............................................................................................................................. 55

6.2.1. Motor 250W .......................................................................................................... 57

6.2.2. Motor 500W .......................................................................................................... 57

6.2.3. Chosen motor ......................................................................................................... 58

6.3 Transmission ................................................................................................................. 60

6.3.1. Option 1- Using a Gear box ................................................................................... 60

6.3.2. Option 2 Use chains or belts ............................................................................... 61

6.3.3. Selecting the chain ................................................................................................. 61

6.3.4. Calculating minimum torque requirement of the device ....................................... 62

6.3.5. Selecting and manufacturing the Sprocket ............................................................ 63

6.4 Batteries ......................................................................................................................... 64

6.5 Wheels ........................................................................................................................... 66

6.6 Motor Controller ........................................................................................................... 66

6.6.1. Cheap motor controllers that are used in electric scooters .................................... 66

6.6.2. Sabertooth motor controller ................................................................................... 67

6.6.3. Choosing motor driver ........................................................................................... 68


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6.7 Accelerometer ............................................................................................................... 68

6.8 Conclusion ..................................................................................................................... 69

CHAPTER 7 Prototyping to test intuitive control .................................................................. 70

7.1 Introduction ................................................................................................................... 70

7.2 Testing the loadcell interface ........................................................................................ 70

7.3 Designing the loadcell mount used in the prototype ..................................................... 72

7.4 Designing the deck ........................................................................................................ 73

7.5 Designing the wheel mounts ......................................................................................... 74

7.6 Designing the Wheel Hub ............................................................................................. 75

7.7 Designing the sprocket .................................................................................................. 76

7.8 Designing the motor mounts ......................................................................................... 77

7.9 Design of Control Algorithm ........................................................................................ 78

7.9.1. Testing the loadcell and motor .............................................................................. 79

7.9.2. Evolution of the control algorithm ........................................................................ 80

7.9.3. Control algorithm version 1 ................................................................................... 80

7.9.4. Control strategy version 2 ...................................................................................... 81

7.9.5. Control strategy version 3 ...................................................................................... 81

7.10 Field testing and Results ............................................................................................. 84

7.10.1. Going over bumps and ramps .............................................................................. 85

7.11 Conclusion ................................................................................................................... 86

CHAPTER 8 Conclusion ........................................................................................................ 87

8.1 Summary ....................................................................................................................... 87

8.2 Future Work .................................................................................................................. 88

8.2.1. To make the full featured prototype ...................................................................... 88

8.2.2. To convert the full featured prototype to a product ............................................... 89

CHAPTER 9 References ......................................................................................................... 90


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CHAPTER 1ESTABLISHING PROJECT GOALS AND SPECIFICATION

1.1IntroductionPeople always need to move from one place to another. The means of transportation depends

on the distance of travel and the infrastructure that is available. Single person transport i.e., a

personal transport device was never given much of a priority since designing a vehicle for

one person would not make economical sense. However in the past decade, more and more

personal transport devices are being made. Two of the most popular personal transportation

devices are Segway [1] and skateboard. Segway is a standing platform on two coaxial wheels.The rider standing on Segway acts as an inverted pendulum [2] and Segway actively balances

the user. Skateboard on the other hand is manually powered and is a device which has four

wheels. Over a period of time skateboarding has become a sport where the skateboarder does

stunts with the skateboard. While skateboards are rarely used for transport, rather another

variation of a skateboard called the longboard [4] is used to take a user from point to point.

Longboards are specifically designed to for personal transport.

There have been many personal transport devices that have been inspired by Segway and

skateboards. Segway has inspired many devices because they are very intuitive to control,

while skateboards have inspired many devices because it is a stable and fun device to ride on.

Many personal transportation devices can either be linked to a Segway or to a skateboard.

1.2 Objectives and motivationWhen a person wants to travel, there are a lot of ways this can be done depending on what is

available and the distance to be covered. The person can choose to use his private vehicle to

travel the entire distance or could choose to travel via public transport or could choose a

combination of public transport and private transport. There are many

advantages/disadvantages for either private or public transport. In this case we are

considering a typical urban commute and the important points to be noted are:

Private transportation is always available at owners disposal It takes effort to use private transport(learn to use, exhausting to use) Public transport is cheaper and cleaner for the environment


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Private transport usual

The motivation of this projec

urban commute, so that we hcome up with product which

while being more convenient

Figure 1.1 shows the most co

Figure 1.1:

Case 1 is when the user uses

uses private transport (car) to

there is no parking space clos

all the cases the user needs to

Case 1 the commute from

walk. Case 2 the commute

walks all the way from home

the bus stop from home or fr

transport.

The product designed in this

personal transportation devic

portable. Intuitive control of t

intuitive, the learning barrier

makes the ride more fun and

was give utmost importance.Segway but is based on a s

2

ly requires additional facilities like car par

is to promote more usage of public transpo

ve a cleaner and greener city. The objectiveis a solution that will enable more usage

to the traveler.

mon urban travelers use case scenario

he three most common use-case of the devi

public transport (bus) to get to office. Case

get to office, but has to park his car some

to the office. Case 3 is when the user walks

commute small distance mostly by walkin

ome to bus stop and from bus stop to offic

from parking space to office, the user walks

to office. Particularly in case 1 the thought o

m bus stop to office can deter someone fro

project is an attempt to provide a very in

that can be used to travel short distances

he device was given a lot of importance bec

to use the product is drastically reduced. Al

less effort. The product being intuitive, safe

t was desired that the new product has the iateboard platform. The product being on s

ing etc.

rt in a typical daily

of this project is toof public transport

e

2 is when the user

here else because

/cycles to office. In

g the distance, e.g.,

, the user needs to

. Case 3 the user

f having to walk to

m using the public

tuitive, safe to use

(5km) and is also

use if the device is

so intuitive control

and being portable

tuitive control of aateboard platform


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gives significant advantage as the skateboard platform on 4 wheels is by itself stable and far

cheaper than Segway.

Figure 1.2: Segway the human transporter (reprinted from www.segway.com/)

Figure 1.3 A typical electric skateboard available in the market (reprinted fromwww.altered.com)

1.3Survey of similar products in the marketAlthough an intuitively controlled skateboard is a unique product that has not been made

before, there are many products that are very close in functionality. Most intuitivelycontrolled devices are designed on two platforms- inverted pendulum and skateboard.

Majority of the devices in the market are based on inverted pendulum platform. Few of the

products are mentioned here.

1.3.1.Devices based on inverted pendulum platformThese devices are modeled as an inverted pendulum where the rider and the device form the

inverted pendulum. The device actively balances the invented pendulum to stay stable. In this

process of stabilizing the rider, the device moves. Hence if the rider can create appropriate


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disturbances in the system then the device moves in the necessary direction. It so happens

that intuitive motions of the user are the right disturbance to the inverted pendulum system.

1.3.1.1SegwaySegway is the first commercial personal transportation device based on an inverted pendulum

mechanism. Segway has inspired the development of a number of devices. It is the leader of

intuitively controlled devices. However Segway has a fundamental problem. Inverted

pendulum has to be actively stabilized since it is a naturally unstable device. This has lead to

the use of expensive sensors and redundancies that make it expensive (cheapest model for

$5000). Figure 1.2 shows a Segway.

1.3.1.2UnoThis is also a single person transportation device. Uno is a dicycle [4] and it works exactly

like a Segway. In a Segway the rider stands on the device but on Uno the rider sits on the

device like on a bike. Uno is also an intuitively controlled device that is based on an inverted

pendulum mechanism. It has all the features and disadvantages of a Segway. Latest version of

this device is a transformer where the device can alter between a normal bike and a dicycle.

Figure 1.4: Uno dicycle (reprinted from en.wikipedia.org/wiki/Uno_(dicycle))

1.3.1.3Honda UX-3Honda UX-3 is also based on inverted pendulum concept. Like in Uno the rider sits on UX-3.

While Uno is primarily designed for outdoor use, UX-3 is designed for indoor use and hence

it is compact. Unlike the other inverted pendulum devices UX-3 is designed as an inverted

pendulum along two axes. Hence the device can move sideways also.


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Figure 1.5: Honda UX-3 (reprinted from world.honda.com/U3-X/)

1.3.2.Devices based on skateboard platformAlthough skateboards are not powered, there are considered as a very intuitive transportation

device. Intuitive in skateboards refer to how turns are made. However in order to use a

skateboard, the rider needs learn how to balance on a skateboard. Following are some

examples of devices build on skateboard platform.

1.3.2.1Electric skateboardElectric skateboards were in the market for a long time. They are basically a skateboard /

longboard with a motor attached to them. The motor is controlled using a hand held controller

that is either wireless or wired. Electric skateboards are rated based on the power of the motor

they use. Presently electric skateboards are available from 100W to 800W. Typically low

power electric skateboards are designed to be light and portable while the high power

versions are designed for speed, acceleration and to be used off-road. Figure 1.6 shows a

medium powered skateboard and a high powered skateboard.


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Figure 1.6: Left - 250W electric skateboard, right 800W electric skateboard (reprinted fromwww.alteredelectricskateboards.com)

1.3.2.2ZboardThis is an intuitive controlled electric skateboard. This product was launched in 2012 after

this project was started. Zboard [5] is functionally same as the objective of this project.

However Zboard used front and read pressure pads to collect controller information from the

user. This is not completely intuitive. Zboard is like a controller with buttons that the user

needs to step on to accelerate / decelerate. The pressure pads cannot be used to measure

weights. They can only be used to detect weight.

Figure 1.7 : Zboard (reprinted from zboardshop.com)

1.4 Projects goals and specifications1.4.1.Mission statementThe mission of this project is to design a personal transportation device that is to be used for

short distances and is portable (can be carried around, taken in a car or a bus).

1.4.2.Product description A device that will help users commute small distances of the order of 1-5km to be

built on an electric skateboard platform that is intuitively controlled by the user


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1.4.9.Technical QuestioningTechnical questioning is a set of questions that needs to be answered to understand the

system/product being developed. It also acts as a reminder for the designer to stay on target.

1. What is the problem really about?a. A low cost, personal transportation device for short distances that is also

portable in convention modes of transport like a bus or car.

2. What implicit expectations and desires are involved?a. It has to small and light enough to be portable in a bus or a carb. It has to be cheap enough to adopted by the target customersc. It has to be a fun device at the minimum.d. It has to be based on a skateboard platform

3. Are the stated customer needs, functional requirements, and constraintstruly appropriate?

a. This is a device that is a variant design where intuitive control is added to anelectric skateboard

b. Local customers are not used to a skateboard as such, so customer needs andfunctional requirements generated is meaning less

4. What avenues are open for creative design and inventive problem solving?a. The truck designb. Selection of wheel( pneumatic or polyurethane)c. Design of the deck(to accommodate the user and to isolate road vibrations to

the user)

d. Mount design (vibration isolation)e. Battery pack design (partially swappable battery)f.

Intuitive control design( how to control the device intuitively by shiftingweight on the device )

g. Control Strategy (strategies to avoid bad situation based on control input)5. What avenues are limited or not open for creative design? Limitations on scope?

a. Restriction on importing and using high end components. Limited exposure toa skateboard or a longboard

6. What characteristics/properties must the product have?a. Indoor and outdoor useb. Easy to drive


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c. Deployable in a campus like IIT Madrasd. Fun device to usee. Adequate safety features

7. What characteristics/properties must the product not have?a. Should not be bulky or too heavyb. Should not be too expensive

8. What aspects of the design task can and should be quantified now?a. Average size and weight of a driverb. Dimension of the user

9. What are the technical and technological conflicts inherent in the design taska. Cost vs. battery performanceb. Battery performance vs. weightc. Redundancy

1.4.10.The product/solution Intuitively controlled electric skateboard.This product will be electric powered. This device will not have a controller. The user

himself standing on the device will be the controller. The user is to control the direction of

travel of the device by shifting his weight in the corresponding direction. For example if the

user wishes to accelerate, he needs to lean forward and to brake, he needs to lean backward.

1.4.11.Identification of potential CustomersThe potential customers for this device will be

Cyclist Bus commuters People who drive a car just because the walk to the bus stop or train station is too

much

For those who love skateboarding and wants to take it to the next level1.4.12.Anticipated Customer RequirementsThese requirements identified are the anticipated customer requirements because the local

customers/ target market does not understand the new product under development. The

customer segment considered is residents of IIT Madras campus. Almost none have actually

tried riding a skateboard. The number of people who have tried an electric skateboard can be

safely assumed to be zero for practical purpose. And the product that we are designing is a

variant design of an electric skateboard, or a concept that does not exist. So extracting useful


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data from a customer study in India would be impractical. This customer requirement was

later validated by students who started picking up skateboarding skills while the project was

running.

1.4.12.1Mandatory requirement Usable on IIT Madras campus Should easily fit into the trunk of a small car Should be portable on a train Should be safe to ride - has to be safer than a skateboard User controlled emergency hand brakes On-Off switch for the skateboard When the device is switched off, the device should be capable of working as a normal

longboard

Braking mechanismo No physical breakso The motors will be used for braking

Range of 5kms to 20km Speeds of around 30kmph Maximum speed - within the safe speed of a normal skateboard Sensors

o To detect the CG of the user on the skateboard1.4.12.2Preferred requirements

Should work for normal paved roads and streets in India Should also be portable on buses

Either two wheel drive or four wheel drive Sensor

o To detect the acceleration of the deviceo To detect velocity of the device

Wheel speed sensor(preferably motors with encoders) Separate wheel speed sensors on non powered wheels for more

accurate measurements

o Angle of incline of a slopeo To detect drag


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1.4.12.3Nice to have requirements Should work off-road - as in on unpaved almost flat terrains Reflectors for night usage

1.4.13.Functional requirements1. Detect the user input/intention2. Identify necessary actions to balance the user3. Drive the motor4. Display the battery status, speed and acceleration5. Emergency override

1.5System descriptionThis project was decided to be implemented on a skateboard platform. Typically a skateboard

consists of the following parts:

Deck Truck Wheels

1.5.1.DeckDeck is the platform on which the rider stands on. Decks are usually made of wood but are

available in fiberglass etc. There are many variant for deck design. Depending on the use of

the deck, the flexibility and the shape of the deck vary. Decks of skateboards which are used

primary in skate rings for tricks and stunts are hard, light and rigid. These decks are also

curved upwards. While in longboards the decks are designed for comfort riding. Longboard

decks are longer for comfort, heavier for stability, and flexible for suspension properties.

Even within longboards, the decks vary based on what they are used for. Longboards used for

curving has different decks from longboards used for riding downhill.


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Figure 1.8: Different kinds of decks (reprinted from www.texaslongboards.com/)

1.5.2.TruckTruck is the part of the skateboard on to which the wheels are mounted. The geometry of thetruck is what helps the skateboard turn. The truck consists of two major parts the hanger

and the base plate. The hanger forms the axle to mount the wheels. The base plate is attached

to the deck. The hanger is attached to the base plate with a rotary joint. A typical skateboard

truck assembly is shown in Figure 1.9.

There is a special variant of truck called drop-deck truck. In this kind of truck, the truck goes

through the deck and the truck is attached to the top surface of the deck. The primary reason

to use such trucks is to lower the CG of the board. Drop-deck longboards are easier to push

since they are closer to ground. Since the truck has to pass though the deck, it weakens the

deck.


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Figure 1.9: A typical skate(reprinted from www.skatere

In electric skateboards, th

attached to hanger of th

Figure 1.1

13

board truckvolution.com/)

Figure 1.10: Drop-dec

from www.m

e motor is attached to one of the trucks. Spec

e truck. A typical electric skateboard truck s

1.11.

1: Typical electric skateboard trucks (reprintfastestelectricskateboard.co.uk/)

k longboard (reprinted

irskate.com/)

ifically the motors are

t is shown in Figure

d from


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1.5.3.WheelsSkateboard wheels come in a wide variety. Tricks and stunts skateboards use small, hard and

light wheels while longboards use larger, wider and softer wheels for smoother ride. Mostly

the wheels are made of polyurethane. The density of polyurethane is varied to achieve

different hardness for the wheels. Typically the diameter of a longboard wheel is around

70mm, while skateboard wheels are around 50mm. Low powered electric skateboard wheels

use 70mm longboard wheels, while 800W electric skateboards use 10 inches pneumatic

wheels.

Figure 1.12: 10 inches wheels used in an electric skateboard (reprinted fromwww.brolive.org/)

1.6Mathematical ModelIn order to detect the user inputs, a parameter that reflects the intuitive motions of the rideron the deck was required. Since Center of Gravity (CG) location of the user reflects the

motion of the user on the deck it was chosen as a parameter to measure, to identify user input

to the device. A mathematical model was developed to understand how the CG locations

depended on external forces.

The following is the mathematical model that is assumed to represent the user on the

skateboard as closely as possible.


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Figure 1.13: Model of the user standing on the deck

The user standing on the deck is modeled as a mass kept on top a table which is in turn

placed on top of the deck of the skateboard. The Center of Gravity (CG) of the user is what

the mass in Figure 1.13 represents. The legs of the table represent the legs of the user and are

assumed to be massless. The front 2 legs of the table combined represent one leg of the user

and the back 2 legs of the table represent the other leg of the user.

This model is chosen over standard human models because the way human behaves on a

skateboard is different from his/her actions elsewhere. This model allows the user to stand on

the skateboard in any manner he sees fit as long as he has one leg in the front and one leg

toward the rear of the deck.


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Figure 1.14: Model of the rider and device when going up a ramp

On a standard skateboard there are 3 parameters that the user controls in order to balance on a

skateboard.

Position of legs both front and rear

Height of the CG from the ground Weight distribution Shifting the CG to different position on a skateboard

The above mentioned parameters can be modeled as follows

Position of legs of user- position of legs of the table (d1 and d2) Height of CG from ground (h) by varying the lengths of the legs of the table Weight Distribution - by shifting the location of the mass on the table


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Center of mass of a standing person is located just below the belly button. Any standing

posture that the user takes is represented as motion of the mass on the table top. The model

also shows that in the absence of aerodynamic force (FA) and pseudo force (ma) the user can

only lean so much, such that the users CG stays within his base area. In this case it means

that the mass on that table cannot be taken beyond the legs in the absence of aerodynamic

force (FA) and pseudo force (ma). When the user bends his knees while on the board, the

situation is modeled by decreasing the height of the table.

Ultimately the objective is to give the user as much freedom to do whatever he/she pleases on

the board but at the same time measure one parameter that can be used to control the device.

The user can achieve the same parameter reading in multiple body configurations.

1.6.1.Using the mathematical modelBased on the above model in Figure 1.14, a mathematical simulation was done on

Mathematica. There are only three forces that the user experiences while on a skateboard

inertial force, gravitational force and the aerodynamic force. These forces were estimated and

F1 and F2 were calculated by force and moment balance.

CG location along longitudinal axis was calculated from F1 and F2. A positive CG readingmeans the projection of CG along the longitudinal axis is on the front side of the board. CG

reading increases when the user leans forward and decreases when the user leans backwards.

The following observations were made:

Drag and inertial acceleration has similar effects on the CG location. Both of themshift the CG location backwards. The force on the front foot decreases and the force

on the back foot increase.

Force exerted by the legs of the user can never be negative (i.e., if that happens theuser will topple over). Given this condition, when there is no drag force the center of

mass of the user has to be in the base area (between legs).

In the presence of drag force the user can lean far more such that the CG position isahead of the front leg.


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The presence of an aerodynamic force or inertial force reflects in the reading as aproportional decrease in the CG reading. That means more the acceleration or drag

less is the reading, meaning the system gets a feed back to decelerate.

With a positive (climbing up a ramp) the force on the front leg becomes less thanthe force on the back leg. With a negative (coming down a ramp) the force on the

front leg becomes more than the force on the back leg. Since the force F 1 and F2 are

normal forces their magnitude is mostly the highest when is zero.

On a positive slope the CG reading shifts backwards and just like drag force andacceleration it adds a linear shift to the CG location.

The presence of linear shift is advantageous because in the presence of drag oracceleration or positive slope the CG reading increases. i.e., if the user wants to

maintain constant speed he will need to lean forward. Leaning forward is also

necessary for the user to not topple over. Hence the CG reading supports and mimics

the natural position of a user.

Based on the above observations, CG of the rider along the longitudinal axis was selected as

the parameter to measure in order to build an intuitive control for the electric skateboard.

1.7ConclusionSince skateboards were a device that is not common in India, a skateboard was purchased

from abroad to understand the mechanism and the working principles. Once the mechanisms

and the working principles of the skateboard were understood, stages for development of the

device were planned. Since most of the parts of the skateboard are not available in India, it

was decided that each part of the skateboard be independently prototyped to test for

manufacturability. Deck and trucks was prototyped. Wheels were bought off the shelf.

Since deck, truck, CG sensor and the main controller were the main parts of this device the

next four chapters are dedicated to describing how they were designed and prototyped.

Chapter 6 is dedicated to component selection and Chapter 7 is about bringing it all together

and building the intuitive controlled prototype to test intuitive control.


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CHAPTER 2DESIGNING THE DECK

2.1IntroductionDeck is the platform on which the rider stands on while skating. Since a skateboard/

longboard were something that was not familiar to Indian context, and since

skateboard/longboard decks were not available in India, custom build deck was made. A

deck can be designed depending on their usage.

Skateboard decks are designed to be used for tricks and stunts- Hence they are light, rigid

and have curved edges due to which tricks can be performed.

Longboard decks on the other hand does not have curved edges, they are flexible to give a

more comfortable ride and are usually heavier. Longboards are designed for cruising.

2.2Building skateboard deck version 1Four layers of 6mm plywood of the dimension 1 foot x 4 feet was stacked together and stuck

with fevicol wood glue. This created a single deck of thickness 24mm and of the dimension 1

foot x 4 feet. The glue was given 2 days to dry. While the glue was drying a perpendicular

load was applied on the surface of the deck in order to pre-stress the deck. Because of this

when the glue hardened, the deck was pre-stressed and had a nice arch shape.

Figure 2.1: The completed deck with trucks and wheels attached. The arch shape of the deckis noticeable in this figure.


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Trucks and wheels of a longboard were attached to the newly created deck as shown in

Figure 2.1 and test run. The deck was found to be acceptable. The arch was noticed to give

added strength to the deck. The deck has been used and rigorously tested for more than a year

and it still retains its integrity.

Lessons learned from building skateboard deck version 1:

Four layers of 6mm plywood were not necessary as it makes the board too thick, rigidand heavy

The deck needs to flex to give an even smoother comfortable ride. A rough surface was needed on the top to increase friction sandpaper with the

course side up was glued to the deck - See Figure 2.2.

Making the deck hence forth was no longer a problem. All the fine details in makinga deck were understood.

Figure 2.2: Sandpaper stuck on the top to increase friction on the top of the deck

Figure 2.3: Painting the deck to prevent the deck from decay due to moisture


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Figure 2.4: The underside of the deck after paint job

2.3Building Skateboard deck version 2The first version of the skateboard deck was designed such that the trucks were mounted to

the bottom. The second skateboard deck was designed to be used as a drop deck. Drop deck

is a deck used for longboards where the truck goes through the deck and is fasted with the topsurface of the deck as shown in Figure 1.10. On a normal deck, the truck is attached to the

bottom surface of the deck. The first deck was designed to be a normal deck. The second

deck was designed to be a drop deck. While the second deck was build, it was also decided to

decrease the thickness and weight of the deck and to experiment with new materials.

The new deck was build with three layers of 4mm plywood. In comparison with the previous

deck the second deck is 1 layer less and is 2 mm less per layer. This resulted in significant

reduction in weight and thickness of the deck. The weight was reduced by 25% and the

thickness was reduced from 24mm to 14mm. But this also made the deck substantially

weaker particularly at the place where the truck was mounted. Three layers of fiberglass

mesh were added to the top and the bottom surface of the deck. In addition to this, extra

layers of fiberglass mesh were added to the region where the truck was to be mounted. The

resin used was LY556 and hardener used was Araldite HY951.


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The new deck was substantially thinner and lighter than the first deck. Also the new deck

was flexible. The deck could flex but not break. This is due to the presence of fiberglass. This

flex resulted in better ride comfort.

Lessons learned building skateboard deck version 2:

Fiberglass allows the deck to flex Flex deck are more comfortable to ride on

2.4ConclusionIn this chapter we presented the evolution of the deck design. Deck version 1 was built to be

hard and rigid but was designed with an arch. Deck version 2 was built to be flexible and

new materials were experimented. Having done two prototypes of the deck, the concepts

involved in deck design has been completed tested and understood.


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CHAPTER 3DESIGNING THE TRUCK

3.1IntroductionAn off-the-shelf truck and wheel were used with skateboard deck version 1. To build an

intuitively controlled skateboard, a motor needs to be mounted on the skateboard to power it.

The motor can only be mounted on the truck because otherwise transmission is not possible.

Hence the skateboard truck had to be custom designed.

3.2Building the Skateboard TruckA skateboard truck was reverse engineered and a model was developed and implemented in

Mathematica. The truck was designed in such a fashion that the geometry of the design

enabled the wheel axle to turn when the deck is tilted with respect to ground. Although the

geometry of the truck made it compact and turned the axle when the deck tilted, the design

was inherently unstable. When the user stands on the deck without tilting his deck he was at

the highest position possible. If the deck was tilted in any direction the CG of the user will

lower. Hence the deck will never return to center. In order to return the tilt of the deck back

to center a bushing was used in commercial skateboard, which exerts force in the opposite

direction bringing the deck to level position. This gave the deck a sense of stability.

The custom build truck was designed to be inherently stable. That is when the rider tilts the

deck, the truck turns the wheel axles in the appropriate direction but in addition to that, it also

raises the users CG. This way the rising CG will automatically bring the deck back to center

where the CG of the rider is at the lowest. This new design involved change is dimension of

certain parts of the truck- length l1 and l2 as marked in Figure 3.2. In standard trucks the

length l2 is always lesser than l1 and in most cases l2 is zero. This reduces the mass of the

truck. In the modified design, the truck was designed such that the ratio l1/l2 is always less

than 1. Also in the new design a rotary joint is used instead of a pin joint. These design

changes increase mass and decrease the strength of the truck but it allows for natural return to

center mechanism.


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Figure 3.1: Parts of a truck (reprinted from skaterevolution.com)

3.3Parameters of the new truck designAngle of the truck , length l1 and length l2 is marked in Figure 3.2. These are the 3important design dimensions of the skateboard. The angle is as shown in the Figure 3.2 as

if the truck is mounted to a perfectly horizontal deck. If truck is mounted at an angle, then the

angle is measured with the horizontal. In this new design, it is the ration of l 1/l2 that makes

the truck self center.


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Figure 3.2: Side view of the truck model when the tilt angle b is equal to 0

Angle b is the angle the deck makes with the horizontal. This is the input that the usercontrols to turn the skateboard. It is marked in Figure 3.3.


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Figure 3.3: Front view of the truck showing angle b

The angle t is the angle by which the truck turns with the vertical. This is the output angle of

the truck. The rider by changing the angle of the deck b, controls t .It is this angle t that

forms the turn geometry of the skateboard. The angle t is marked in Figure 3.4.


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Figure 3.4: The top view of the truck showing the angle t

The truck consists of two parts that move with respect to each other- one that attaches to the

deck and the other part to which the wheels are mounted. The motion of these two parts with

respect to each other is purely rotational in nature. Angle r is a measure of this angle of

rotation between these two parts. This angle does not hold any physical significant but it is

used to parametrically represent other angles.


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Figure 3.5: Axial view of the truck showing angle r

3.4Modeling the truckIn a truck there are four parts that can move with respect to each other. They are:

1. Base plate2. Hanger3. Wheel one4. Wheel two

In a real skateboard truck, both the wheels are attached to the hanger and are free to rotate

about their axis. The motion of the wheels is irrelevant in this model and hence the rotary

degree of freedom of the wheels is ignored. Both the wheels are considered to be part of the

hanger. Therefore in this model there are only two parts that can move with respect to each

other the base plate and the hanger. The hanger is attached to the base plate with a rotary

joint. See Error! Reference source not found..


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The hanger consists of a strut of length l2, the wheel axle and the wheels. Since the hanger

was modeled as a single rigid body in a plane, if any two points were known, every other

point on the hanger could be derived. The geometry of the hanger can be seen in Figure 3.2 -

Figure 3.5.

Similarly the base plate was also modeled as a single rigid body consisting of a rectangular

plate and a triangular plate placed perpendicular to each other. The rectangular plate attaches

the truck to the deck. One side of the plate forms the rotary axis of the hanger. The geometry

of the base plate can also be seen in Figure 3.2 -Figure 3.5.

Both the hanger and the truck have been defined in their own reference frames. The hangerwas translated such that the free end of the strut now coincides with the side of the triangular

plane of the base plate. The hanger was also rotated about the same free end such that the

strut was perpendicular to a side of the triangular plate. This side of the triangular plate was

defined as the axis of rotation of the hanger with respect to the base plate angle r, which is

zero when the strutis in the same plane as that of the triangular plate. Angle r is shown in

Figure 3.5.

The base plate and the hanger are in the same reference frame and any point on either thebase plate or the hanger can now be calculated. Since the hanger can be rotated with respect

to the base plate, any point on the hanger is dependent on the angle r.

The current reference frame has the truck completely stationary and the hanger in pure rotary

motion. This reference frame is not of much interest to us. Ground reference frame is defined

in such a way that the Z coordinate of the lower most point of the wheels are zero. Also in an

actual skateboard any longitudinal line drawn on the deck / base plate will always remain

parallel to the ground. It means that, the longitudinal sides of the rectangular plate of the base

plate needs to be parallel to the ground. Therefore ground reference frame was defined by

incorporating these constraints. Since the lower most point of the wheels was dependant on

r, the ground reference frame is also dependant on r. Therefore with respect to the new

reference frame any point on the base plate or the hanger was dependant on r. In the ground

reference frame, the tilt of the deck was defined as the angle that the lateral sides of the base

plate make with the horizontal angle b, and the turn of the truck was defined as the angle

that the axle turns about the vertical axis angle t.


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3.5Parametric study of the new truck designA geometric model for the truck was simulated. The truck primarily consists of two parts

the base plate and the hanger and the joint between them is a rotary joint. The relationship of

each point with respect to each other was defined and using translation and rotation matrixes

the model was created. The relationship of certain variable was plotted. In order to

understand the importance and function that each variable play in the design of the truck, a

parametric study was done. The following observations were made.

Figure 3.6: Plot ofb with r

The variation of deck slant angle b with r is almost linear in nature. We also notice that as

we increase the characteristic angle of the truck, the slope of the graph decreases.


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Figure 3.7: Plot oft with r

The variation of wheels turn angle t with r is almost linear in nature. Also we notice that as

we increase the characteristic angle of the truck, the slope of the graph increases.

Figure 3.8: Plot oft with b

Plotting t (turning angle) vs. b (deck slant angle) we again get an almost linear plot. And as

we increase the characteristic angle of the truck, the slope of the graph keeps on increasing.


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3.5.1.Variation of height of the deck with various parametersThe variation in height of the CG of the deck determines whether the truck will self center or

not. If the CG of the deck raises when the deck is tilted, then the truck will self center since

the only way to bring the CG down is to center-align the deck. Experimenting with values on

the skateboard truck model, it was noticed that when the l1 was smaller than l2, the plot of the

height of the deck with the angle of tilt of the deckr was cup shaped. This means that when

the l1 was smaller than l2, CG of the deck is at the lowest position when the tilt of the deck is

equal to zero. This can be seen in Figure 3.9.

Figure 3.9: Plot of height of deck CG with r when l1/l2 is 0.1

It can be seen in Figure 3.9 that even when l1 is one by tenth of l2 the variation in height of

the deck CG is only marginal - 3mm. This height difference is not noticeable by human eye.

In order to make this variation in height substantial either l1 should be very small or l2 should

be very large. Due to constraints in mounting mechanism l1 cannot be to too small. The

length l2 cannot be made too long as it affects the strength the truck and also obstructs with

the deck. Typically a ratio of 0.1 to 0.3 is achievable. The thickness of the deck adds to l 1. In

order to prevent the thickness of the deck adding to l1 the truck has to be mounted as a drop

deck.

Trucks specs chosen to prototype

Characteristic angle of the truck : 45degrees


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Length l1 : 10mm(drop deck) Length l2 : 50mm

Deck and other specs

Deck length : 920mm to 1000mm Deck width : 280mm Wheel diameter : 65mm to 75mm Wheel width :50mm Wheel material : Polyurethane

In this new design, the trucks are so designed such that the CG of the deck is at the highest

position at the extremes. This means that without the bushes or centering mechanism, the

skateboard self centers. But when a person stands on top of the deck, the CG of person is so

high that it shifts the entire system CG to the skaters CG. Due to this extra height, the self

centering property of the skateboard is lost. This is on the assumption that the rider tilts to the

same degree as the board does, but in practice it can be noted that although the rider tilts, he

does not tilt as much as the deck but uses his ankle to tilt the deck. If we approximate the tilt

angle of the rider to be zero, the self-centering of the deck is not lost. So this was to be tested

in a real use case scenario.

With this mechanism the center of the deck rises as the deck tilts. So the deck byitself is stable.

The deck + human is considered statically unstable if we assume that the human alsotilts by the same angle the deck tilts there by reducing the CG of the human by a

larger degree than the rise in CG because of rise of the deck.

Although thats the preliminary assumption, human body is a really complex systemand it is possible for us to shift our weights to the toes or to the heals while still

maintaining upright position. That being the case if we assume that the CG of human

does not tilt with the deck then we do have a static stable equilibrium. When board is

in motion, it appears that we can compare a skateboard to a cycle where there is

dynamic stability (because ultimately the skateboard is suspended on 2 points even

though it has 4 wheels).


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With the above assumptions, the design proposed(without centering mechanism)should be superior to existing designs (with centering mechanism)

3.6Prototyping the new designIn order to test the new truck design, a prototype was made. This prototype was made as per

calculations. The prototype was made1 out of mild steel. Ideally the truck should be cast but

in order to save time and since it was to be a prototype to test the design, the truck was made

as an assembly. An assembly for the truck would not be ideal because when the truck is used,

due to vibrations the assembly could come loose.

Figure 3.10: A solid model of the truck developed

1

The prototype was at the Central Workshop, IIT Madras.


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Figure 3.11: The truck prototype mounted to deck version 2

The truck was fabricated and mounted to the deck version 2. The truck was tested and it was

clearly visible that the truck could self center. However there was one problem deck

version 2 could flex. Due to this the angle of the truck with the horizontal changes as the

deck flexes. It was noticed that when the deck flexes the self centering of the truck was lost.

As the deck and truck was tested extensively, the deck began to sag a little. This resulted in

few degrees variation in the angle of the truck with the horizontal due to which the natural

self centering was not observable anymore.

From using the prototype it was clear that relying on the self centering was not a good idea

since it was affected by slight changes in parameters and the effect was lost when the deck

flexes. So the self centering is not a viable option. However it was noticed that the dynamic

stability was available. Just like a cycle is dynamically stable, the skateboard truck is also

dynamically stable. The truck would self center when in motion. Since the truck was noticed

to be dynamically self centering, the truck design was not abandoned. This dynamical self

centering coupled with electric differential would be ideal for the final device.

3.7ConclusionIn this chapter we presented the design evolution of the truck. A model to understand the

parameters was created and it was noticed from the parameter study that keeping l 1/l2 less

than one, gave some advantages to the truck. This truck was prototyped and the new design

was verified.


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CHAPTER 4DESIGNING AND PROTOTYPING THE CG SENSOR

4.1IntroductionIn order to implement intuitive control, a sensor mechanism that can detect the users input

was required. The most intuitive motion to control a skateboard is to lean forward to

accelerate and to lean backward to decelerate. A sensor mechanism that can detect the riders

lean was required and the following concepts were explored. Other concepts considered were

more intrusive and not practical to use from a user perspective, hence was dropped.

4.2Concept selectionIn order to create a CG sensor, the following 3 concepts was considered.

4.2.1.Concept 1 Pressure padsUse pressure pads that detect a split of weight. This would be simple to use but the pressure

pads are usually used to detect pressure or force rather than to measure it. Without measuring

the pressure, user posture cannot be detected.

4.2.2.Concept 2 LoadcellsUse loadcells this will mean that the deck will have two layers with the loadcells in

between the layers. That or the trucks have to be designed in such a way so as to

accommodate the loadcells in them. Loadcells can give accurate reading which can be used

to estimate the users posture but mounting loadcells will be a problem.

Advantages

Accurate Loads within range

Disadvantage

Pretty bulky Need to make the skateboard 2 layers to accommodate the loadcells Or the trucks have to be redesigned to keep the boards single layered


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4.2.3.Concept 3 Force sensing resistorsUse force sensing resistors (FSR) This comes as thin layers whose resistance changes with

force applied. They are really compact and can be stuck to the top surface of the board.

Multiple sheets can be placed adjacent to each other to measure the load and also measure

approximate position.

Advantages

Simple and compact The design of the skateboard will be straight forward without much of changes

Disadvantages

The small sheet does not give information on point of application of force. It willhave to be approximated as the geometric center of the sheet.

Multiple sheets laid out adjacent to each other can give a significant understanding ofposition.

They creep over time They are not designed to continuously measure load in the order of magnitude of a

human body. Typical measurement rages are up to 1-5kg.

Figure 4.1: FSR-Force Sensing Resistor (reprinted from www.sparkfun.com/)

4.2.4.Concept selected LoadcellsConcept 1 was dropped because it can only be used to detect weight and not measure.

Concept 3 was the least intrusive and easiest to setup but FSR do not give accurate reading.

FSR are typically used to detect weight and not to measure it. Hence measuring the split of

weight was difficult. Concept 3 was tested out and since measurements were difficult it was

dropped. Concept 2 of the other hand was much difficult to implement since the loadcells

needs to be embedded into the deck. A double layered deck was necessary for this. The

measurements from the loadcells were accurate and split of weight could be quantified with


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significant resolution and accuracy. Hence concept 3 was chosen as a mean to measure the

user input to the device.

4.3Designing and building the load cell sensorLoadcells are used to measure the force/weight. In order to detect weight shift of the user the

projection of CG of the rider along at least one axis (along the longitudinal direction) is

required. The projection of the CG of the rider along the lateral direction is an optional input.

It could be used as an input for steering if electrical differential drive was to be used. Since at

least 3 points are required to perfectly balance a body it was decided that the load will be

transferred via 4 loadcells placed on the extremities of the deck. These 4 load points will

form the base area for rider to stand on.

4.3.1.Selection of LoadcellA loadcell that has a measurement range of 100kg with a resolution of at least 0.1kg was

preferred. After searching on the internet for loadcells, many manufacturers and distributors

were found with the above specification. Upon enquiring for price, it was found that all these

loadcells were above Rs 8,000. These loadcells had well defined mount mechanisms to attach

top surface and the bottom surface. But the mount mechanisms make the loadcells tall by at

least 50mm. While searching for cheaper alternatives, it was noticed that there were cheap

digital scales that used a simple half bridge loadcell to measure weight. Sparkfun was selling

the same for Rs 500 per piece. Since an inexpensive digital scale costs Rs 500 and contained

4 loadcells in them, it was purchased and the loadcells were extracted from it.

`

Figure 4.2: Loadcell sensor used in digital bathroom scale (reprinted from sparkfun.com)


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4.3.2.Designing the Loadcell Circuit boardVery little documentation was available about the loadcell. Since there are 3 wires from the

loadcell, the loadcell is a half bridge loadcell. The specifications of the loadcell are as shown

in Table 4.1.

Table 4.1: Specifications of the loadcell used.

Property Units Value

Capacity Kg 40-50

Comprehensive Error mV/V 0.05

Output Sensitivity mV/V 1.00.1

Nonlinearity %FS 0.03

Repeatability %FS 0.03

Hysteresis %FS 0.03

Creep (3min) %FS 0.03

Zero Drift (1min) %FS 0.03

Temp. Effect on Zero %FS/10 1

Temp. Effect on Output %FS/10 0.05

Zero Output mV/V 0.1

Input Resistance 100020

Output Resistance 100020

Insulation Resistance M 5000

Excitation Voltage V 10

Operation Temp Range 0--+50

Overload Capacity %FS 150

4.3.3.Concept SelectionThe following concepts were considered for building the loadcell circuit

4.3.3.1Concept 1Two loadcells used in parallel to form a complete bridge [6], where only one loadcell is

loaded while the other once is kept as a dummy. In this configuration the both the white

wires are connected together and the both the black wires are connected together. In this

configuration the effective output of the bridge is a linear function of load on loadcell1 minus

load on loadcell2. This means that if each loadcell is equally loaded the effective output will


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be zero. The only way to

configuration is as shown

Figure 4.3

4.3.3.2Concept 2Two loadcells used in

configuration the white w

black wire of loadcell 1

effective output of the bri

In this configuration the

Hence both the loadcells

when tested, in this confi

zero error when converted

full scale was 50kg. Thi

configuration is as shown

40

et a useful reading will be to keep one loadc

in Figure 4.3.

: Two loadcells used in parallel to create the

anti-parallel configuration to form a co

ire of loadcell 1 is connected to black wire

s connected to white wire of loadcell 2. In

ge is a linear function of load on loadcell1

output is the function of the sum of loads

can take the load. Although this might see

guration, the full bridge formed was not p

to kg values was a significant error, approxi

was not acceptable unless the zero error

in Figure 4.4.

ll as a dummy. The

bridge

plete bridge. In this

f loadcell 2. Similarly

this configuration the

plus load on loadcell2.

on both the loadcells.

m as ideal, practically

rfectly balanced. This

mately 20kg, when the

can be rectified. This


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Figure 4.4:

4.3.3.3Concept 3Use only one loadcell per

the bridge a couple of resi

of exact 1000 ohms were

commonly available resis

bridge. Alternatively one

is possible but practicall

resolution of the trim pot

bridge but a better balanci

in series with the trim p

1000ohms) was used the

shown in Figure 4.5.

41

wo loadcells used in anti-parallel to create t

full bridge. In this case instead of using anot

stors and a trim pot was used to balance the

available it would have sufficed to build an

ors have a 10% error, they cannot be used

rim pot alone can be used to balance the bri

, perfectly balancing this bridge would b

will not suffice. It would be possible to app

ng was desired. The solution was to use two

ot in between the resistors. If a trim pot

bridge could be very accurately balanced.

e bridge

her loadcell to balance

bridge. If two resistors

external bridge. Since

directly to balance the

dge. Theoretically this

difficult because the

oximately balance the

resistors and a trim pot

f 100 ohms (10% of

This configuration is


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Figure 4.5: 1000 ohm r

4.3.3.4Finalizing concepThere are four load point

of eight loadcell will be

four of them placed as du

If concept 2 was impleme

two load reading. This m

This will work but the pr

is acceptable but a better s

With four load point an

give four load reading. Thlateral direction can be ob

successfully tested, conce

The maximum voltage th

available 5V used in micr

the micro-controller boar

bridge. The loadcell giv

42

sistors and a trimpot used to balance the brid

t

. To implement this with the first concept

equired where four of them are mounted at

my. Hence concept 1 was rejected.

nted with each loadcell mounted at each load

ans that only the projection of CG along on

jection of CG along the lateral axis is also d

olution was desired.

mounting the loadcells at these load point

is means that the projection of CG along thetained. Since balancing the bridge using resi

t 3 was chosen as it was superior to concept

t can be applied to the bridge is 10V. Ther

-controller board and 22.2 V from the batte

was less than 10 volts, 5V was used as the

s an output of 1mV/V at full scale. Sinc

ge with one loadcell

ould mean that a total

these load points and

point, it can give only

axis can be obtained.

esired. So this concept

s using concept 3 will

longitudinal as well asstors and trim pot was

1 and concept 2.

are 2 power voltages

y. Since only 5V from

voltage applied to the

5V was the voltage


54/101

43

applied, 5mV is the full scale output. This means that when 50 kg was applied to a load cell

the load cell bridge would output 5mV. This was far too low a voltage to be directly read by

Analog to Digital Converter (ADC).

This output from the bridge had to be amplified by 500-1000 times to be used in a standard

5V ADC. Amplification of this magnitude can only be done using an instrumentation

amplifier. Instrumentation Amplifier AD620 and INI114 were considered. These amplifiers

were shortlisted based on availability and ease of use. Both these amplifiers have identical

pinout and can be interchanged in a circuit that is designed for one of them. INI114 cost Rs

490 and AD620 costs Rs 140. Although INI114 claims better performance, in practice no

difference was noticed between AD620 and INI114. Due to the significant cost advantage

AD620 was chosen. At any point it can be swapped with INI114 if a better performance is

required.

Figure 4.6: Schematic of the loadcell board.


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44

4.4Adjusting gain and balancing the bridgeDetermining the gain required- A gain of 1000 will convert the full scale reading of 5mv to

5V, while a gain of 500 will amplify a full scale reading of 5mV to 2.5V. Instrumentation

amplifier has a reference voltage which can be set by the user. The amplifier provides the

output with respect to this reference voltage. It means that if the reference voltage is 1V then

with a 500 gain the output will range from 1V to 3.5V, 1V when there is no load and 3.5V

when there is a 50kg load (full scale load). After testing the instrumentation amplifier it was

observed that, for a bridge voltage

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