Capstone project: Design conversion mid drive motor kit for MTB (mountain bikes) to eMTB (electric mountain bikes)

Luận văn này trình bày quá trình thiết kế và chuyển đổi bộ kit động cơ truyền động giữa cho xe đạp địa hình (MTB) thành xe đạp điện địa hình (e-MTB). Nghiên cứu bao gồm các bước từ khảo sát nhu cầu thị trường, lựa chọn các phương án thiết kế cơ khí và điện tử, đến thử nghiệm và đánh giá hiệu suất. Kết quả đạt được bao gồm thiết kế hộp số, hệ thống tay quay, mạch cảm biến Hall, cảm biến nhiệt, và cải tiến thiết kế mạch điều khiển của VESC Open Source. Dự án kết thúc với các thử nghiệm thực nghiệm để đánh giá khả năng hỗ trợ đạp và điều khiển dòng điện, từ đó đưa ra kết luận và triển vọng phát triển sản phẩm trong tương lai. Mô tả luận văn: Luận văn này là dự án tốt nghiệp của sinh viên Nguyễn Công Hiếu, thuộc ngành Kỹ thuật Cơ điện tử, Đại học Bách Khoa Thành phố Hồ Chí Minh. Mục tiêu của dự án là thiết kế bộ kit chuyển đổi động cơ truyền động giữa để biến một chiếc xe đạp địa hình thông thường thành xe đạp điện địa hình, đáp ứng nhu cầu di chuyển bền vững và thuận tiện cho người dùng trong điều kiện địa hình khó khăn. Nội dung chính của luận văn bao gồm: -Tổng quan và nhu cầu thị trường: Trình bày lý do thị trường cần e-MTB và bộ kit chuyển đổi, cùng với công nghệ điều khiển mô-men và đo lường mô-men cho xe đạp điện. -Thiết kế cơ khí: Bao gồm thiết kế hộp số, hệ thống tay quay, và các bộ phận cơ khí khác. -Thiết kế hệ thống điện tử: Bao gồm thiết kế mạch cảm biến Hall, cảm biến nhiệt, và mạch điều khiển. -Thử nghiệm thực nghiệm: Lắp ráp hệ thống cơ khí và điện tử, đánh giá khả năng hỗ trợ đạp và điều khiển dòng điện. Luận văn kết luận với những kết quả đạt được và những hạn chế còn tồn tại, cùng với những triển vọng phát triển trong tương lai. Dự án này không chỉ cung cấp giải pháp kỹ thuật mà còn đóng góp vào xu hướng phát triển bền vững trong lĩnh vực phương tiện cá nhân.

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

FACULTY OF MECHANICAL ENGINEERING

CAPSTONE PROJECT

DESIGN CONVERSION MID-DRIVE MOTOR KIT FOR MTB (MOUNTAIN BIKE) TO E-MTB

(ELECTRIC MOUNTAIN BIKE)

Student’s name: Nguyen Cong Hieu Student ID: 1952048

Instructor: MSc Luong Thanh Nhat

Ho Chi Minh City, 2024

No.: _/ĐHBK – CK

FINAL YEAR PROJECT PROPOSAL

(This form must be appeared at the first page in the report of the final year project)

Students’s name: Nguyen Cong Hieu Student’s ID: 1952048 Program: Mechatronics Engineering Class: CC20COD1

- Research overview of problems related to e-MTB:

+ Reasons why the market needs e-MTB, especially e-MTB conversion kits + Explore torque control technologies for e-Bikes

+ Explore torque measurement technologies for e-Bikes

+ Propose and analyze feasible options and select the appropriate solutions for whole conversion kit for MTB to e-MTB

- Mechanical design

+ Design the gearbox for the motor + Design the crankset system - Electrical system design

+ Hall effect sensor circuit design for the motor

+ Temperature sensor circuit design for the motor and MOTFETs + Modify the design of the controller circuit of VESC Open Source - Experimental

+ Assemble mechanical and electrical system + Experimental evaluation of current controller + Experimental evaluation of pedal assist ability - Technical drawings 5, including:

+ 1 A0 drawing, about: Conceptual design + 1 A0 drawing, about: Mechanical design

Faculty: Department Date of defense Evaluation grade: Archived place:

First and foremost, we extend our deepest thanks to our parents and family members for their unwavering trust, encouragement, and support throughout our educational journey

I wish to convey my sincere appreciation to MSc Luong Thanh Nhat, who imparted not only the essential knowledge but also the professional demeanour required of an engineer His patience and enthusiasm in pointing out deficiencies in technical drawings and practical machining techniques significantly broadened our understanding and enriched our experience for our future endeavours

I am also profoundly grateful to VIEROBOT Co., Ltd for their invaluable assistance in providing ideas, equipment, research space, and funding for our project Special thanks go to the electrical, mechanical, and embedded programming engineers who supported us throughout the implementation process

Lastly, I extend our gratitude to the teachers of the HCMUT for their instruction and assistance, enabling us to complete the Mechatronics Bachelor Program I am also thankful to our dear friends for their companionship, support, and collaboration on various assignments and projects I hope to have the opportunity to work together again in the future

1.1.1 Growing demand for sustainable transportation source 1

1.1.2 The best solution for developing sustainable personal transportation 1

1.1.3 Definition of e-Bikes and e-MTB 2

1.1.4 The development potential of e-MTB and the need for e-MTB conversion kits 2

1.2 Literature review 3

1.2.1 Domestic research 3

1.2.2 International research 4

1.3 Necessity of product research and development 5

1.4 Research and implementation range 6

Criteria 1: Working in Off-Road condition & Require power 6

Criteria 2: Pedal assistance 6

Criteria 3: Lower limit for maximum speed at crankset 7

1.5 Structure of the thesis 7

CHAPTER 2: SELECTION METHODS 8

2.1 Mapping selections 8

2.2 Priority selections 9

2.2.1 Position of actuator & working principal 9

2.2.2 Battery cover selection 12

2.3.1 Torque sensor selection 21

2.3.2 Temperature sensor selection 22

2.3.3 Rotor position sensor selection 22

2.3.4 Select ESC open source 23

2.3.5 Select BMS & Cell pins 24

CHAPTER 3: MECHANICAL DESIGN 25

3.1 Working principle 26

3.1.1 Method to measure torque from both legs 26

3.1.2 Method motor assist the system 26

3.2 Motor selection and transmission ratio 27

3.2.1 Calculate the power for the motor and motor selection 27

3.2.2 Transmission ratio distribution 29

3.2.3 Technical specification of the transmission 31

3.3 Chain drive design 31

3.4 Gearbox design 36

3.4.1 Transmission gears: sun gear and planetary gear 36

3.4.2 Transmission gears: planetary gear and ring gear 39

3.4.3 Shaft design 42

3.4.4 Key selections 46

3.4.5 Bearing selection 49

3.5 Crankset design 51

3.6 Holder motor design 51

CHAPTER 4: ELECTRICAL – ELECTRONIC DESIGN 54

4.1 Controller hardware module 54

4.2 Hall effect sensors module 56

4.1.1 Position arrangement for hall effect sensors 56

4.1.2 Schematic 56

4.3 Temperature sensors 59

4.4 Bluetooth module 60

CHAPTER 5: CONTROL – ALGORITHM DESIGN 61

5.1 Current reference generator 62

5.2 Current controller 63

CHAPTER 6: EXPERIMENTAL RESULTS AND EVALUATION 67

6.1 Mechanical processing and assembly 67

6.1.1 Gearbox 67

6.1.2 Crankset 69

6.1.3 Overall assembly 71

6.2 Electronic processing and assembly 74

6.2.1 Controller and driver 74

6.2.2 Hall effect sensors 75

INDEX OF FIGURE

Figure 1.1: Compare the carbon footprints of the different vehicles 1

Figure 1.2: Growth Trends & Forecasts of e-MTB market size (2024 - 2029) 3

Figure 1.3: Growth Trends & Forecasts of e-Bikes market size (2024 - 2029) 3

Figure 1.4: A prototype mid-drive conversion kit to e-Bikes in IndieGoGo 5

Figure 2.1: Priority selections 8

Figure 2.2: Secondary selections 8

Figure 2.3: Mid-drive motor e-Bikes 9

Figure 2.4: Hub-drive motor e-Bikes 9

Figure 2.5: The way to measure torque of the right leg 10

Figure 2.6: Proposed principle diagram 11

Figure 2.7: The system working without assistance 12

Figure 2.8: The system working with assistance 12

Figure 2.9: Product drawing from the RYOBI manufacturer 13

Figure 2.10: How to measure the current of the phases in FOC 15

Figure 2.11: Error between measured current vector and desired current vector 15

Figure 2.12: Converting to the fixed coordinate 16

Figure 2.13: Converting to the synchronous rotation coordinate 16

Figure 2.14: States of converting 3-phase to controllable 2-phase in FOC 16

Figure 2.15: Controllers for id and iq 17

Figure 2.16: States of converting controlled 2-phase to 3-phase in FOC 17

Figure 2.17: Additional reluctance torque of Toyota/ Prius Hybrid THS II Motor [25] 18

Figure 2.18: Stationary frame state observer for a salient machine [26] 18

Figure 2.19: Product drawing from the manufacturer 20

Figure 3.1: Working principle & main blocks for mechanical design 25

Figure 3.2: How the system measures the torque of the left leg 26

Figure 3.3: How the system measures the torque of the right leg 26

Figure 3.4: How does the system separate the torque of the motor and the cyclist 27

Figure 3.5: How does the system assist the cyclist 27

Figure 3.6: Tab Design of Chain Design in Autodesk Inventor 32

Figure 3.7: Tab Calculation of Chain Design in Autodesk Inventor 33

Figure 3.8: Tab Selection Chain of Chain Design in Autodesk Inventor 34

Figure 3.9: Working principle of the planetary gearbox with fixing the ring gear 36

Figure 3.10: Tab Design of Gear Design (Sun + Planet gears) in Autodesk Inventor 36

Figure 3.11: Tab Calculation of Gear Design (Sun + Planet gears) in Autodesk Inventor 37

Figure 3.12: Tab Design of Gear Design (Ring + Planet gears) in Autodesk Inventor 39Figure 3.13: Tab Calculation of Gear Design (Ring + Planet gears) in Autodesk Inventor 40

Figure 3.14: Arrange the distance on the shaft 42

Figure 3.15: Tab Design of Shaft Design in Autodesk Inventor 43

Figure 3.16: Analyzing the force acting on the shaft 43

Figure 3.17: Tab Calculation of Shaft Design in Autodesk Inventor 44

Figure 3.18: Shear Force Graph, YZ Plane 44

Figure 3.19: Shear Force Graph, XZ Plane 45

Figure 3.20: Bending Moment Graph, YZ Plane 45

Figure 3.21: Bending Moment Graph, XZ Plane 45

Figure 3.22: Ideal Diameter of Shaft 46

Figure 3.23: Tab Design of Key (Chain Sprocket) in Autodesk Inventor 46

Figure 3.24: Tab Calculation of Key (Chain Sprocket) in Autodesk Inventor 47

Figure 3.25: Tab Design of Key (Chain Clutch) in Autodesk Inventor 48

Figure 3.26: Tab Calculation of Key (Chain Clutch) in Autodesk Inventor 48

Figure 3.27: Tab Calculation of Bearing Selection in Autodesk Inventor 50

Figure 3.28: Put the force the chain system exerts on the shaft in Autodesk Fusion 52

Figure 3.29: Define the material for objects being analyzed 52

Figure 3.30: Displacement of the Motor after solving in Autodesk Fusion 53

Figure 3.31: Safety factor of the Motor after solving in Autodesk Fusion 53

Figure 4.1: Reference schematic design of Open source VESC 54

Figure 4.2: General schematic of the controller hardware 55

Figure 4.3: 3D images of the controller hardware 56

Figure 4.4: Hall effect sensors arrangement 56

Figure 4.5: Prototype schematic of the hall effect sensors module 57

Figure 4.6: Hall effect sensors module signal at the prototype version 58

Figure 4.7: Final schematic of the hall effect sensors module 58

Figure 4.8: Hall effect sensors module signal at the final version 59

Figure 4.9: Reference design for NTC sensors 59

Figure 4.10: Wiring diagram of temperature sensors inside of the ESC (left side) and motor (right side) 60

Figure 4.11: Schematic of the Bluetooth module 60

Figure 5.1: General block diagram of motor support 61

Figure 5.2: Block diagram of current control by the FOC 61

Figure 5.3: The current control system is divided into two blocks 64

Figure 5.4: Block diagram after prediction and reduction 64

Figure 5.5: Pole suppression plan in current control 65

Figure 5.6: Transfer function prediction results in VESC Tool 65

Figure 6.1: Some images about components of the gearbox are 3D printed 67

Figure 6.2: Some images after machining and assembly the gearbox 68

Figure 6.3: The prototype version of the crankset 69

Figure 6.4: Version 1 of the crankset with changing to block aluminium 70

Figure 6.5: Freewheel damage when going off-road for over 60km 70

Figure 6.6: The crankset version 2 has replaced the freewheel with the bearing 71

Figure 6.7: Several images after whole assembly 72

Figure 6.8: Several render image of mechanical system Version 2 73

Figure 6.9: Moisture damage, particularly those subjected to significant loads and high temperatures 74

Figure 6.10: A layer of nylon is applied to the torque sensor to enhance its durability and resistance to moisture 74

Figure 6.11: Component replacement process 74

Figure 6.12: The PCB circuit after processing 75

Figure 6.13: Mounting the PCB into the stator of the motor 75

Figure 6.14: Sensor signal when they are placed in the wrong face angle 76

Figure 6.15: Sensor signal when they are far the rotor 77

Figure 6.16: The usable signal of hall effect sensors 77

Figure 6.17: Some temperature sensor installation locations 78

Figure 6.18: Reading the temperature signal 78

Figure 6.19: Some images about installation the Bluetooth module 79

Figure 6.20: The experimental setup for recalibrating the cadence speed 80

Figure 6.21: Packaging a battery 80

Figure 6.22: Balance the battery cells before packaging the battery 81

Figure 6.23: Balance the battery cells after packaging the battery 81

Figure 6.24: Preparation for the current control experiment 82

Figure 6.25:Experiment result with the step inputs (8A to 19A, each step 1A) 83

Figure 6.26: Experiment result with the step input (0A to 10A, one step 10A) 83

Figure 6.27: Motor speed work no load at sensor mode 84

Figure 6.28: Motor speed work no load at sensorless mode 84

Figure 6.29: Motor speed work on the heavy load at sensorless mode 85

Figure 6.30: Motor speed work on the heavy load at sensor mode 85

Figure 6.31: The motor operates under heavy load in sensor mode when the ERPM is below 5000, and transitions to sensorless mode when the ERPM exceeds 5000 86

Figure 6.32: Setup motor before the pedal-assist experiment 87

Figure 6.33: Logging data at Tour Mode 88

Figure 7.1: Total distance traveled under assistive mode 90

INDEX OF TABLE

Table 1.1: Constraints for Criteria 1 in summary 6

Table 2.1: Comparison between Mid-drive and Hub-drive 9

Table 2.2: Comparison between current control methods 14

Table 2.3: Comparison between types of clutches in gearbox 19

Table 2.4: Comparison between types of clutches in crankset 20

Table 2.5: Comparison between types of torque sensors 21

Table 2.6: Specification of T13 torque sensor 22

Table 2.7: Comparison between types of rotation position sensors 22

Table 2.8: Comparison between two popular FOC open sources 24

Table 3.1: Specification of the motor 29

Table 3.2: Select transmission ratio for planetary gear box 30

Table 3.3: Transmission ratio distribution 31

Table 4.1: Specifications of the A3144EUA hall effect sensor 57

Table 6.1: Compare upgrades in two gearbox versions 68

Table 6.2: Compare upgrades in two crankset versions 71

Table 6.3: Compare other upgrades in two versions 71

CHAPTER 1: OVERVIEW 1.1 Problem statement

1.1.1 Growing demand for sustainable transportation source

Mobility plays a vital role in the current society that we live in With the growing concern for preserving and sustaining it for future generations, mainly because of the increasing scarcity of natural resources and environmental concerns, protecting the environment poses a significant challenge to society and governments worldwide

Owing to this, some international organizations worldwide are implementing strict criteria for vehicles For instance, In January 2020, the European Union implemented Regulation (EU) 2019/631, setting CO2 emission performance standards for new passenger cars and vans The average CO2 emissions from new passenger cars registered in Europe have decreased by 12% compared to the previous year, and the share of electric vehicles tripled

1.1.2 The best solution for developing sustainable personal transportation

It’s a common knowledge that petrol cars are one of the most harmful means of transport to the planet As a result, several new technologies have been proposed to reduce CO2 emissions, such as hybrid and electric cars Yet electric bikes remain the most sustainable means of transport, which is reliable, and eco-friendly (Figure 1.1)

Figure 1.1: Compare the carbon footprints of the different vehicles

e-Bikes offer significantly extended travel distances compared to traditional bikes, thanks to the motor's power assistance Research findings indicate that both walking and cycling (with bike riders and pedestrians) possess the potential to reduce car-related CO2 emissions by 8.5 million tonnes annually, equivalent to the energy consumption of 971,309 households over a year In contrast, e-Bikes demonstrate a net emissions

reduction capability of 16 million tonnes per year, nearly double the impact and surpassing the energy usage of 1.8 million homes for the same duration

Additionally, e-bikes present another dimension of sustainability They boast a lower carbon footprint than conventional bikes, as they require less food consumption to support pedalling Traditional bikes have a carbon footprint of 21g of CO2 per kilometre, with a significant portion stemming from the additional food needed for cycling and the remainder originating from the manufacturing process [1] The emissions from food production result from various factors, including deforestation for land use, livestock emissions, transportation, fertilizer production, and waste generation 1.1.3 Definition of e-Bikes and e-MTB

An electric bicycle, commonly known as an e-Bikes, is a bicycle equipped with an electric motor that assists the rider in propulsion There are 2 main types of electric bicycles classified according to the electric control signal:

• Pedal-Assist (No Throttle): Motor provides assistance only when the rider is pedalling

• Throttle-Assist (No Pedalling Required): Motor can be activated by a throttle without pedalling and Pedalling is optional

Before mentioning e-MTB, let talk about the mountain bikes (MTB) MTB are a bicycle designed for off-road cycling Mountain bikes share some similarities with other bicycles, but incorporate features designed to enhance durability and performance in rough terrain, which makes them heavier, more complex, and less efficient on smooth surfaces [2]

Therefore, e-MTB are a combination of the ruggedness of MTB and the electric motor of e-Bikes

1.1.4 The development potential of e-MTB and the need for e-MTB conversion kits Over the years, the sales of e-Bikes have increased rapidly due to the rise in fuel costs, which has led to the growth of the implementation of electric bicycles as a daily means of transport

The electric MTBs market is a small part of the entire e-Bike market (5.77B vs 34.99B) (Figure 1.2 & 1.3) However, while the e-Bikes market already has many giants, especially Chinese companies, the e-MTB market still has opportunities for Vietnamese companies to develop Besides that, although the market share is not large, the estimated Compounded Annual Growth Rate (CAGR) of e-MTB is 12.56% [3] , which is 1.5 times higher than that of e-Bikes (8.16%) [4]

Figure 1.2: Growth Trends & Forecasts

The advantages of e-MTB include [5]: • Pedal assistance for longer distances • Optimal off-road capability

• Higher speeds, more fun • Everyday practicality

• Flexibility for different types of riders

However, there are disadvantages of e-MTB like [5]: • High cost

• Heavy weight

• Dependence on battery life

And the e-MTB conversion kit can efficiently deal with these three existing disadvantages of e-MTB on the market

1.2 Literature review

1.2.1 Domestic research

At the current time, in May 2024, we have not found any research papers on domestic e-Bikes or e-MTB design However, there are some domestic companies that have been developing their own lines related to e-MTBs, such as:

• The Edge TX product line by Robot Viet Limited Liability Company:

mid-drive conversions kit for MTB to e-MTB in prototype process

• The Stella product line by Robot Viet Limited Liability Company (or

VierCycle): hub-drive conversions kit for bikes to e-bikes in commercial state

• The MB1 product line by MET EV Company: e-Bikes in prototype process

1.2.2 International research

The studies

• In 2021, a group of students from the Maritime University of Raja Ali Haji in Indonesia designed and tested an electric-assisted bicycle with the aim of providing an alternative to traditional fossil fuel powered transportation for residents on a remote island [6] The bicycle included components such as a 24 VDC 10Ah DC power source, a motor driver, a speed controller, a unidirectional freewheel in the front sprocket, a throttle, a 24 VDC 350 Watt brushed DC motor, and no torque sensor The DC motor was mounted on the bicycle frame in an A-shaped arrangement, and the power source was placed on the saddle By using a tachometer sensor and a voltmeter to measure wheel speed, motor voltage, and current consumption, the research group demonstrated a linear relationship between motor input voltage, motor current, and wheel speed Real-world testing showed that users could ride the electric bicycle without pedalling, and the motor temperature increased from 30.6°C to 56.8°C within 21 minutes However, the DC motor eventually burned out, and the research group attributed this to long-term motor usage and resulting overheating

• A research article published in December 2022 summarized the characteristics and capabilities of Mid-drive e-Bikes and delved into the analysis of different motors for various electric bicycle applications [7] The article highlighted the current trend of using permanent magnet synchronous motors (PMSM) powered by AC as the most suitable motor for e-Bikes due to their ability to generate high torque, especially at low speeds The article also provided a Fuzzy control algorithm flowchart for controlling an e-Bikes in the presence of various disturbances such as uneven terrain and gusty winds

The commercial products

• There are many famous and long-standing manufacturers of e-MTB products, the most typical are: BOSCH, YAMAHA, GIANT, TREK, SCOTT, Moreover, there are also several manufacturers of mid-drive conversion kit for bikes to e-Bikes from China, the most typical is BAFANG

• There are also several commercial projects in the process of calling for capital on IndieGoGo since 2020 [8] related to (Figure 1.4)

Figure 1.4: A prototype mid-drive conversion kit to e-Bikes in IndieGoGo

However, the conversion kit for MTB to e-MTB being commercialized on the market comes from only the company CYC-Motor in Taiwan (Researched in 2024

The price of an unibody e-MTB is not affordable with almost buyers The average price of the best e-MTB of 2020 at €8,121 and increased to €8,846 for 2021 Moreover, survey participants spent an average of €4,593 on their current e-MTB while planning to spend €4,953 [10]

Without an official distributor and the market is not large enough, domestic maintenance, repair and replacement will certainly be very difficult

1.4 Research and implementation range

This thesis presents the research and design of a conversion kit for MTB into MTB, with the following design criteria

e-Criteria 1: Working in Off-Road condition & Require power

Assists cyclists on off-road terrain, with 10% gradients at speeds of 10-12 mph The weight of the cyclist is 75kg and the weight of the bike is 20kg The average power for the e-MTB effort is about 220 watts (Table 1.1)

• Average speed = 10 -12 Mph and power of biker = 220 watts, which is equal to the average bike speeds [14] and average power consumption of cyclists using MTB without electricity [15] This means that when the power assist mode is turned on, the pedaller’s speed and feeling can remain unchanged even though it is in tough biking conditions

• Slope = 10% in off-road condition, which is arranged in difficult levels and only for mountain bikes Not suitable for children under 11 years old Requires a high level of competency in bike control and a high level of physical fitness Technically challenging with features such as tight turns, small rock steps, narrow boardwalk sections, and may cross steep exposed side slopes [16]

• The weight of the cyclist is about 75kg which is obtained from test methods section 6.1 TCVN 7448:2004 "Electric bicycles - General safety requirements and test methods"

• The weight of the bike is 75kg which is obtained from several previous prototypes

Table 1.1: Constraints for Criteria 1 in summary

Biker’s power consumption ≤ 220𝑊

Criteria 2: Pedal assistance

The system has to support that the power is equal to the power consumed by the biker times the defined gain coefficient in real time

Criteria 3: Lower limit for maximum speed at crankset

Motor speed at crankset is not slower than cadence of bikers Avoid the situation where the biker pedals faster than the motor pulls

1.5 Structure of the thesis

The thesis is organized as follows:

Chapter 1: Giving reasons why e-MTB is needed in green mobility and defining the design criteria

Chapter 2: Choose main components/ parts/ modules to design and build a complete e-MTB

Chapter 3: Calculation and mechanical design for transmission elements Chapter 4: Calculation and electronic design for several modules

Chapter 5: Design controller for the Field Oriented Control (FOC) method and build the concept for pedal assist

Chapter 6: Experimental results and evaluation Chapter 7: Summary and development orientations

CHAPTER 2: SELECTION METHODS 2.1 Mapping selections

Figure 2.1: Priority selections

Figure 2.2: Secondary selections

ESC Open Source Rotor Position

Sensor Torque Sensor

Temperature Sensor

BMS & Cell PIN

Bluetooth Communication

Module

Following design criteria rooted in the interplay among system components, the

selection process is bifurcated into two distinct parts: Priority Selections and Secondary

Selections Priority Selections wield significant and direct influence over the system

Conversely, Secondary Selections, while crucial, offer multiple alternatives or can be

omitted without compromising the system integrity

2.2 Priority selections

The selection process adheres to a hierarchical order, commencing with options characterized by minimal input arrows and progressing towards options influenced by a greater number of input arrows

2.2.1 Position of actuator & working principal

Motors mounted on e-Bikes are categorized into two primary types: Hub-Drive and Mid-Drive The figures below (Figure 2.3 & 2.4) illustrate these two layout options more explicitly, with the yellow circle representing the motor

To elucidate why the majority of e-MTBs on the market adopt mid-drive configurations, the table below (Table 2.1) [17] will analyse the advantages and disadvantages of these two types of e-Bikes

Table 2.1: Comparison between Mid-drive and Hub-drive

MID - DRIVE

Power at the Crank:

This central placement delivers power directly to the bike's chain and subsequently the rear wheel, which closely mimics the natural pedalling experience

Efficient Power Distribution:

Provide more efficient power distribution, as they leverage the bicycle's gears

Better Handling and Balance:

Typically offer improved handling and balance because the motor's weight is centralized, resulting in a more natural weight distribution

Ideal for Off-Roading:

Costlier and Complex:

It is higher cost and complexity Installation and maintenance might be more involved and costly compared to hub drive motors

Figure 2.3: Mid-drive

It is often favoured for off-road and mountain biking, where power and control are essential Their ability to adapt to different terrains and inclines makes them an excellent choice for adventurous riders

HUB - DRIVE

Simplistic Integration:

It is integrated into the bicycle's wheel hub, either in the front or rear wheel This design is simple and relatively straightforward, making hub motors an attractive choice for those who want a hassle-free e-Bikes experience

Low Maintenance:

There are fewer moving parts compared to mid-drive motors This can result in lower long-term ownership costs

Suitable for Everyday Commuting:

Hub drive e-bikes are well-suited for urban and city commuting, as they provide consistent power delivery on flat terrain They are also quieter than mid-drive motors, which can be an advantage in residential areas

Limited Efficiency on Inclines:

Hub drive motors lack the gear-shifting capability of mid-drive motors, making them less efficient when tackling steep hills

Weight Distribution Concerns:

Since the motor is located in the wheel hub, hub drive e-bikes can exhibit imbalanced weight distribution This may affect handling and control, especially on rough terrain

With Criteria 1: Working in Off-Road condition, easily select mid-drive

transmission for this conversion kit

Drawing reference from various mid-drive principal diagrams available on the market, including patents, enables a comprehensive understanding of the design principles and operational mechanisms underlying mid-drive e-Bikes [18] (Figure 2.5)

Figure 2.5: The way to measure torque of the right leg

1: Motor – 2 & 3: Clutch transmission device – 4: Axle – 5: Chainwheel – 6: Torque detection unit – 7: Hosing – 11 & 12: Gears – 41: Hollow tube – 42a & 42b: Cranks

The proposed principle diagram (Figure 2.6) must adhere to the following criteria: 1 The sensor position solely measures the power exerted by the cyclist and does

not gauge the total power applied to the vehicle

2 The moments generated by the pedal and the motor, acting on the vehicle (rear wheel), remain independent of each other Specifically:

• The diagram must prevent motor torque from being transmitted to the pedal to ensure safety when only the motor is in operation

• Prevents pedal torque from acting on the motor, thereby ensuring that the pedaller does not expend additional effort to rotate the motor when electric assistance is not engaged

Figure 2.6: Proposed principle diagram

The system will have 2 operating states: Without power assistance (Figure 2.7) and

with power assistance (Figure 2.8) The image below (Figure 2.9 &2.10) will more

clearly describe the torque line (Red line) and how the sensor measures the runner's torque

2.2.2 Battery cover selection

Due to running in relatively harsh hilly terrain, priority is given to choosing an aluminium protective cover for the battery, with the aim of reducing weight, providing better protection and finally good heat dissipation Like other electric vehicle manufacturers, aluminium is widely used in the construction of battery boxes

There are aluminium protective covers for electric vehicles available on the market and have a bottle design [19] The capacity of the battery depends on the cells we choose to package However, the general configuration of the battery is 10s2p or 10s3p and the rated voltage is 36V

2.2.3 Motor selection 2.2.3.1 Select motor type

Modern electric bicycles (e-Bikes) predominantly utilize brushless DC (BLDC) motors, a significant advancement over the older brushed motor technology The primary distinction between these two types of motors lies in the method of current direction alternation In brushed motors, mechanical brushes are used to switch the direction of the current flowing to the motor, a process that inherently reduces efficiency

and leads to wear and tear over time In contrast, BLDC motors achieve this alternation electronically, eliminating the need for brushes and thereby enhancing efficiency and longevity This technological shift has led to the widespread adoption of BLDC motors in e-Bikes over the past decade, solidifying their status as the industry standard due to their superior performance and durability [20] In addition, their electronic commutation not only enhances efficiency and durability but also allows for more precise control compared to traditional brushed motors Consequently, BLDC motors have become the dominant choice in the design and development of contemporary electric bicycles 2.2.3.2 Motor selection & specifications

Calculate power to satisfy Criteria 1 by OMNI Calculator [21] the calculation

formula is based on analysis and data collection of millions of people around the world The results have shown that a total of 𝑃𝑑𝑒𝑠𝑖𝑟𝑒 ≈ 727𝑊 is needed, where power of biker: 𝑃𝑏𝑖𝑘𝑒𝑟 ≤ 220𝑊 then power of motor: 𝑃𝑚𝑜𝑡𝑜𝑟 ≥ 𝑃𝑑𝑒𝑠𝑖𝑟𝑒− 𝑃𝑏𝑖𝑘𝑒𝑟 ≥ 507𝑊 The detail result in section 3.2.1 Calculate the power for the motor and motor selection

To meet the proposed capacity requirements, the operating voltage is set at 36V However, considering factors such as domestic availability and the need for a compact

design, the Genuine Ryobi RY18LMX40A Electric Motor emerges as a suitable

choice This motor, with a rated capacity of approximately 600W, not only aligns with the voltage specification but also offers a reliable and efficient solution that is readily accessible in the domestic market Its compact design further enhances its suitability for the intended application, ensuring both performance and practicality

Figure 2.9: Product drawing from the RYOBI manufacturer

Φ Φ Φ Φ

From the manufacturer's motor specifications (Detail specification in section 3.2.1 Calculate the power for the motor and motor selection, max speed at rated torque at the rated voltage 36V:

Max Speed @ Rated Torque (Rpm) = 3230 rpm 2.2.4 Control method selection

2.2.4.1 Compare pros and cons of control methods

Nowadays, there are many control strategies for PMSM drives, including oriented control (FOC), direct flux control (DFC), and direct torque control (DTC) [22]

field-To choose the most suitable control type, we will look at the following set of three main factors: Adaptation with load variation, EV applications, large user community

Table 2.2: Comparison between current control methods

ADAPTATION WITH LOAD

very large just

research some practical projects

2.2.4.2 Working principle of FOC

The main idea of vector control is to control not only the magnitude and frequency of the supply voltage but also the phase In other words, the magnitude and angle of the space vector are controlled [24] The steps to implement FOC can be divided into four main steps:

Step1: Measure current already flowing in the motor

Just measure two phases and can interpolate the remaining phase using Kirchhoff's current law (Figure 2.10)

Figure 2.10: How to measure the current of the phases in FOC

Step2: Compare the measured current (vector) with the desired current (vector), and generate error signals

Since a synchronous motor, if we know the angle of the rotor, we can know the angle of the rotor flux The error signal is indicated in the figure below (Figure 2.11)

Figure 2.11: Error between measured current vector and desired current vector

Put is & Rotor flux axis into new coordinate systems (Figure 2.12 & 2.13) The purpose of this is to convert from sine form to linear form, it is a lot easier to regulate

Figure 2.12: Converting to the fixed coordinate

Figure 2.13: Converting to the synchronous rotation coordinate

The above is an explanation in terms of graphics (Figure 2.14), but in terms of mathematics, we have the operation to convert the coordinate system:

Figure 2.14: States of converting 3-phase to controllable 2-phase in FOC

Forward Clark transformation, converse 3-phase to 2-phase 𝑖𝛼 =3

2𝑖𝑎𝑖𝛽 =√3

2 𝑖𝑏 −√3

To maximize torque and efficiency, the vector 𝑖𝑑 has to become zero It causes the vector 𝑖𝑑 will lie on the q axis which is orthogonal with the rotor flux axis (Figure 2.15)

Step 4: Modulate the correction voltages onto the motor terminals (Figure 2.16)

Figure 2.16: States of converting controlled 2-phase to 3-phase in FOC

Reverse Park transformation:

𝑣𝛼 = 𝑣𝑑𝑐𝑜𝑠𝜃𝑑− 𝑣𝑞𝑠𝑖𝑛𝜃𝑑𝑣𝛽 = 𝑣𝑑𝑠𝑖𝑛𝜃𝑑+ 𝑣𝑞𝑐𝑜𝑠𝜃𝑑Reverse Clark transformation:

𝑣𝑎 =23𝑣𝛼𝑣𝑏 = −1

3𝑣𝛼 +1√3𝑣𝛽

𝑣𝑐 = −13𝑣𝛼−

In addition, in FOC there are also many other control techniques, that can operate such as observer, tracking and torque optimization (Figure 2.17)

Figure 2.17: Additional reluctance torque of Toyota/ Prius Hybrid THS II Motor [25]

FOC can also operate in sensor-less mode, which means that without a rotor

position sensor, an observer can still predict rotor position for Step 2, yet this is not

recommended for high load cases or the initial state of the motor This diagram will illustrate (Figure 2.18) the way for estimating rotor position 𝜃̂

Figure 2.18: Stationary frame state observer for a salient machine [26] (LPF: Low Pass Filter)

2.2.5 Gearbox selection 2.2.5.1 Select gearbox type

Criterion 3 combined with experimental data and collected from sources on the

Internet [27], the maximum pedalling cadence of a cyclist: ncadence ≤ 130 rpm The transmission ratio of the entire system can be calculated using the formula:

𝑢𝑠𝑦𝑠𝑡𝑒𝑚 ≤ 𝑛𝑚𝑜𝑡𝑜𝑟𝑛𝑐𝑎𝑑𝑒𝑛𝑐𝑒 =

2.2.5.2 Clutch in gearbox selection

As mentioned in the principal diagram, in section 2.2.1 Position of actuator & working principal, there needs to be a clutch inside the gearbox to prevent the cyclist from pulling the motor when running in unassisted mode

Table 2.3: Comparison between types of clutches in gearbox

W One-way bearing Freewheel Sprag Clutch LOAD

CAPACITY 40%

slip at heavy load broken at heavy

1 There are commercially available sprockets on the market With large chain sprockets according to BCD 104 standards (For MTB), the tooth number ranges from 32 to 46 teeth With small sprockets specialized for e-Bikes with 9/13 teeth From there, the external transmission ratio can be calculated from 32/13 to 46/9

SCORE 0.825 0.35 0.85

Sprag Clutch is an optimal choice Furthermore, some types of Sprag Clutch combined with planetary gears are also available on the market (Max torque is 30Nm)

Figure 2.19: Product drawing from the manufacturer

2.2.6 Crankset system selection 2.2.6.1 Select drive transmission type

For use in applications requiring very large loads and high transmission ratios, be sure to choose a chain drive instead of a belt drive

2.2.6.2 Clutch in crankset selection

This clutch has the function of ensuring safety when the engine is operating and must have it so that the torque of the engine and the driver can be separated, from which the torque/ power of the operator can be measured cyclists

Unlike the clutch in a gearbox, the priority criteria for selecting this clutch will be different For example, the load capacity will not have a large weight because the torque that this clutch must load is only the pedal's torque, it does not include the motor's power

Table 2.4: Comparison between types of clutches in crankset

W One-way bearing Freewheel Sprag Clutch DIFFICULTY

a lot of choices a lot of choices no diversity

VIBRATION 20% very low 1 high 0.5 very low 1 LOAD

CAPACITY 10%

slip at heavy load broken at heavy

SCORE 0.875 0.85 0.75

It can be said that using a one-way bearing as a clutch is the most suitable, although there are difficulties in design because the bearing size is often large and can cause entanglement in the pedal probe

2.3 Secondary selection

2.3.1 Torque sensor selection

The first wave of e-bikes relied on simple cadence-based control resulting in quite unpredictable handling when cornering and during start/stop Complementing this with a torque sensor to sense the force put into the pedals is a significant improvement

On an e-bike, the torque sensor measures the applied pedalling torque of the rider and sends it to the controller of the e-bike along with cadence information Based on this information an algorithm controls the motor speed and power The performance of the e-bike as perceived by the rider depends heavily on the precision and accuracy of the motor control

Table 2.5: Comparison between types of torque sensors

W Strain GaugeContactless Slipring Strain GaugeIndirect Strain GaugeSHAFT

OTHER

only hub-drive; no belt drive; limited dynamic

range

As evident from the overview table, the contactless strain gauge sensor is a quite favourable solution for many e-bike applications The main disadvantage is that the rotating part needs additional electronics

The T13 Torque Sensor from Shanghai Moreway International Trade Company Limited was chosen because it is a contactless strain gauge In addition, this

sensor has a cadence signal (one phase in clockwise) to calculate the pedal's power, available stock and reasonable price

Table 2.6: Specification of T13 torque sensor

2.3.2 Temperature sensor selection

The only criteria for choosing a temperature sensor is that it must be very small because it needs to be placed in the PCB circuit and in the motor stator Therefore, NTC 10k resistant sensor is chosen

2.3.3 Rotor position sensor selection

Table 2.7: Comparison between types of rotation position sensors

1 Bottom Bracket (BB) length 83 𝑚𝑚

4 Max power consumption < 0.5𝑊

5 Speed signal Forward rotation: 18 pulse/ rotation

Backward rotation: no speed signal output 6 Torque output range 1.5 ~ 3.0 𝑉𝐷𝐶

HALL SENSOR ABI SENSOR SINUSOIDAL SENSOR

ADVANTAGES

- Simple and small construction

- Working in harsh environments - Low cost

- Relatively high

resolution - High resolution and accuracy compared to incremental

encoders

continuous

feedback without discrete pulses

DISADVANTAGES

- Low resolution

compared to other type encoders - Susceptible to interference from external magnetic - Limited accuracy and precision, especially at high speed because of a small wrong in any phases

- High cost

- Requires more space for sensor placement

- High cost

- Requires more sophisticated signal processing - Requires accurate design for sensor mounting location

For durable, economic and streamlined design, I choose an option using hall sensor for Rotation position sensor

Problem solving plans:

• Just using the hall sensor for motors at low speed to reduce error caused by phase difference

• Using capacitors for filtering noise Hall sensor selection criteria:

• Type unipolar switching output instead of analog voltage output • High-temperature operation ≥ 80°C

• Ease to buy and low cost

• Output rise & fall time ≪ phases circle time 2.3.4 Select ESC open source

Several well-known brands provide Field-Oriented Control (FOC) controllers for various applications, including motor control, inverters, and other electronic systems Some common examples include:

• Texas Instruments: Texas Instruments offers FOC controllers for electric motor applications, including both direct and indirect control

• Infineon Technologies: Infineon provides FOC controllers for a wide range of applications from electric motors to inverters

• STMicroelectronics: STMicroelectronics offers FOC control solutions for electric motors and various other applications

• NXP Semiconductors: NXP provides FOC controllers for motor and inverter applications

• Microchip Technology: Microchip offers FOC control solutions for diverse applications ranging from electric motors to low-power electronics

Additionally, there are many other companies such as Bosch, Mitsubishi Electric, and Toshiba that also provide FOC controllers for the market However, to easily modify the program to suit the e-MTB application, two common open-source FOC controllers are considered: VESC and SimpleFOC

Table 2.8: Comparison between two popular FOC open sources

EASE TO PROGRAM 25% using ChibiOS for 0 1

running multi-thread framework Arduino

DEEP EMBEDDED 25%

all firmware is public,

user can modify a library on framework Arduino cant modify

2.3.5 Select BMS & Cell pins

To achieve the highest capacity, INR18650-35E (3500mAh) cells from Samsung or INR18650-35V (3500mAh) cells from EVE are used Both types of cells offer a maximum discharge current of approximately 10A While the price of each EVE cell is two-thirds that of a Samsung cell, their quality is generally comparable The battery configuration consists of two blocks connected in parallel, resulting in a peak current output of 20A Consequently, a Battery Management System (BMS) rated at 10s 20A is selected to accommodate this configuration and ensure optimal performance and safety

CHAPTER 3: MECHANICAL DESIGN

The figure below (Figure 3.1) shows the mechanical principal diagram of the entire system, which was shown in section 2.2.1 Position of actuator selection & working

chain-drive, crankset, and motor holder Each section will detail the specific design considerations and methodologies used to optimize the performance and integration of these components within the overall system

Figure 3.1: Working principle & main blocks for mechanical design

3.1 Working principle

3.1.1 Method to measure torque from both legs

The operating principle of the bottom bracket in this design is to measure the total torque generated by both legs of the rider This is achieved through the integration of torque sensors within the bottom bracket assembly

Figure 3.2: How the system measures the

3.1.2 Method motor assist the system

The motor in this e-MTB design can operate in two scenarios: either faster than the pedal foot or at the same speed as the pedal foot Due to Criterion 3 , there is no scenario where the motor operates slower than the pedal foot The crankset clutch plays a crucial role in ensuring the safety and functionality of the system Its primary functions are twofold:

• Protection of Pedal Legs: The crankset clutch safeguards the rider's legs from the impact of the motor By engaging or disengaging as needed, it prevents the motor from transmitting excessive force to the pedals, thereby reducing the risk of injury or discomfort to the rider (Figure 3.4)

• Accurate Torque Measurement: The crankset clutch also ensures that the torque sensors only measure the rider's pedaling force, not the motor's torque When the clutch is disengaged, it decouples the motor's torque from the crankset, allowing the sensors to always measure only the rider's input (Figure 3.5)

Figure 3.4: How does the system separate the torque of the motor and the

cyclist

Figure 3.5: How does the system assist the cyclist

3.2 Motor selection and transmission ratio

3.2.1 Calculate the power for the motor and motor selection The system efficiency to crankset, not to rear wheel

𝜂 = 𝜂𝑔𝑒𝑎𝑟2 × 𝜂𝑐𝑙𝑢𝑡𝑐ℎ× 𝜂𝑐ℎ𝑎𝑖𝑛 × 𝜂𝑏𝑒𝑎𝑟𝑖𝑛𝑔3𝜂 = 0.972× 0.99 × 0.93 × 0.993 = 0.84 Where: 𝜂𝑔𝑒𝑎𝑟 = 0.97: gear efficiency 𝜂𝑐𝑙𝑢𝑡𝑐ℎ = 0.99: clutch efficiency 𝜂𝑐ℎ𝑎𝑖𝑛 = 0.93: chain efficiency

𝜂𝑏𝑒𝑎𝑖𝑟𝑛𝑔 = 0.99: 𝑎 pair of bearing efficiency

The required power to satisfy Criterion 1 is calculated using the formula form Omni Calculator [21]

• Resisting force due to gravity

𝐹𝑔 = 𝑔 × sin(arctan(𝑠𝑙𝑜𝑝𝑒)) × (𝑀 + 𝑚) 𝐹𝑔 = 9.807 × sin (arctan(10%) × (75 + 20) 𝐹𝑔 = 92.701𝑁

Where: 𝑔 = 9.807 𝑚/𝑠2: gravitational acceleration 𝑠𝑙𝑜𝑝𝑒 = 10%: slop of hill

𝑀 = 75 𝑘𝑔: biker′s weight

𝑚 = 20 𝑘𝑔: bike′s weight • Rolling resistance

𝐹𝑟 = 𝑔 × cos(arctan(𝑠𝑙𝑜𝑝𝑒)) × (𝑀 + 𝑛) × 𝐶𝑟𝑟

𝐹𝑟 = 9.807 × cos(arctan(10%)) × (75 + 20) × 0.0253 𝐹𝑟 = 23.453𝑁

Where: 𝑔 = 9.807 𝑚/𝑠2: gravitational acceleration 𝑠𝑙𝑜𝑝𝑒 = 10%: slop of hill

𝐶𝑟𝑟 = 0.0253: coefficient at knobby tires & offroad surface • The aerodynamic drag

𝑃 =(𝐹𝑔+ 𝐹𝑟 + 𝐹𝑎) × 𝑣1 − 𝑙𝑜𝑠𝑠

𝑃 =(92.701 + 23.453 × 7.714) × 5.5561 − 4%

𝑃 = 716.886𝑊

Where: 𝐹𝑔 = 92.701𝑁: Resisting force due to gravity 𝐹𝑟 = 23.453𝑁: Rolling resistance force 𝐹𝑎 = 7.714𝑁: Aerodynamic drag

𝑣 = 5.556 𝑚/𝑠: biking speed

𝑙𝑜𝑠𝑠 = 4%: percentage loss in power at chain not well − oil The require power for motor:

𝑃𝑚𝑜𝑡𝑜𝑟 =𝑃 − 𝑃𝑐𝑦𝑐𝑙𝑖𝑠𝑡𝜂

Pmotor ≥716.886 − 220

Where: 𝑃 = 716.886𝑊: Power to satisfy design criteria 𝑃𝑐𝑦𝑐𝑙𝑖𝑠𝑡 ≤ 220𝑊: Power the cyclist wants to expend 𝜂 = 0.84: the system efficiency

As the previous preliminary calculation in section 2.2.3 Motor selections, the

engine selected is Genuine Ryobi RY18LMX40A Electric Motor with specification

table (Table 3.1)