Hybrid Wheelchair

Hybrid Wheelchair Department of Electrical and Mechanical Engineering Faculty of Engineering and Architecture American University of Beirut

Final Year Project Spring 2005-2006 Advisor:

Dr Ahmad Smaili

Co adviser:

Dr Fouad Mrad

Group:

Jackie Fares Elias Achkar Ramzi Stephan Hussein Hajo

Submited on: 23.05.2006

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Dedication We only have to remember the smile on their faces when we met them for the first time. We only have to remember how excited they were when we told them about our FYP. We only have to remember their words “we are here for you; we want you to be here for us”. We only have to remember the staff at “Arc en Ciel” to make our dream came into reality. Members of the staff at “Arc en Ciel” represented for us more than just an inspiration; they taught us that everyone among us can achieve a dream. They helped us dream about tomorrow when we saw their dreams about today. We cannot but offer them our hard work our hopes for success our determination for excellence. We cannot but thank them for what we came up with throughout this year. We cannot but give them our hybrid wheelchair because our dream was theirs. Finally, for our FYP to become an important invention it should go back home, the home where it originally came from, to “Arc en Ciel”.

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Acknowledgement First and foremost, we would like to thank our Final Year Project (FYP) supervisor, Professor Ahmad Smaili, for his support and assistance in guiding us throughout this project. Besides just putting up with us, he was always able to give us insightful feedback, whatever the problem was. We would also like to thank Professor Fouad Mrad for feeding us with constructive suggestions and comments. Special thanks to Dr. Elie Zeitouni for providing us with his laboratory and helping us in designing and implementing the H bridges. Special thanks to both Mr Elie Yaacoub and Mr. Andre Mhewij for granting us with access to their workshops which we used for implementing our wheelchair. A special thanks to our colleague Ali Bazzi for helping us with the H Bridge design. A special thanks to Mr khaled Joujou and Ahmad Farchoukh for their continuous guidance. Warm thanks to “Arc en Ciel” for donating us the chassis of the wheelchair along with the DC motors. Finally, we would like to thank the coordinators of the FYPs, Professor Jean Saade and Professor Marwan Darwish, for heir time in organizing and coordinating the projects. We are also grateful to the departments of Mechanical and Electrical Engineering for making this FYP possible.

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Abstract In our final year project, we are looking at a problem that a small portion of the population faces, this portion represents the “disabled”. Before choosing the topic of our project, the whole group members were determined to make use of our final year project for a good cause. Noticing that the disabled people in Lebanon get very little attention from authorities, we decided to aim our project towards them in order to help as much as we can. Our influence might end up being minimal, but remember that a ten kilometer walk starts with a small step. Disabled people rely heavily on their wheelchairs for transportation. The wheelchair frees them from their burdens and constraints and provides them with mobility. It has become a necessity to all, such that they cannot live without it anymore. For all these reasons, and in order to start a change, we decided to concentrate our effort on pinpointing the weaknesses in wheelchairs and improving them as much as we can. Our project will mainly feature one major idea in accordance with a few minor ones. The major idea that we will be trying to implement is to introduce the hybrid wheelchair for the first time. By hybrid we mean that the wheelchair will be provided by two sources of power, a battery (electric) that works in conjunction with a combustion engine (gasoline) in order to improve efficiency, power output, mileage, and range of the electric wheelchair. This major idea tends to be solving in first place the range that an electric wheelchair is limited by. Knowing that electric wheelchairs work on batteries and have to be recharged continuously, they become bounded with a certain range that they cannot surpass. What we are aiming for is to design a wheelchair that is able to run outdoors, and is also able to run for long hours. In addition to this major idea, we will be working on several issues which include designing and building the whole body of the wheelchair from scratch, and implementing some new, unique, and intelligent control systems that make navigation easier.

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Table of Contents Dedication .......................................................................................................................... 3 Acknowledgement ............................................................................................................. 4 Abstract.............................................................................................................................. 6 Table of Contents .............................................................................................................. 7 Chapter 1 ......................................................................................................................... 11 Introduction....................................................................................................................... 11 1.1 Problem Definition............................................................................................ 12 1.2 Management Summary ..................................................................................... 12 Chapter 2 ......................................................................................................................... 14 Literature Review.............................................................................................................. 14 2.1 Electric Wheelchair........................................................................................... 15 2.1.1 Basic Styles............................................................................................... 15 2.1.2 Indoor vs. Outdoor Use............................................................................. 15 2.1.3 Methods of Propulsion.............................................................................. 15 2.1.5 Electric Wheelchair Battery Types ........................................................... 17 2.1.6 Electric Wheelchair DC Motor Types ...................................................... 18 2.2 Hybrid Vehicles ................................................................................................ 19 2.2.1 Definition ......................................................................................................... 19 2.2.2 How Hybrid Cars Work ............................................................................ 20 2.2.3 Parallel and Series Hybrid ........................................................................ 21 2.2.4 Computer Control ..................................................................................... 22 2.2.5 Battery Charge and Discharge .................................................................. 23 2.2.6 Pros and Cons of Hybrid Vehicle ............................................................. 24 Chapter 3 ......................................................................................................................... 26 3.1 Mechanical Solution ......................................................................................... 29 3.1.1 Choosing the DC Motors .......................................................................... 29 3.1.2 Designing the Chassis for the Electric Wheelchair................................... 33 3.1.3 Choosing the Recharging Technique ........................................................ 34 3.1.4 Picking a Combustion Engine................................................................... 35 3.1.5 Picking the Alternator ............................................................................... 36 3.1.6 Installing the Alternator on the Combustion Engine ................................ 37 3.1.7 Designing the Chassis for the Combustion Engine and the Alternator .. 37 3.1.8 Designing the Chair .................................................................................. 38 3.1.9 Designing a Cover that Provides Insulation to both Sound and Heat....... 38 3.2 Control Design and Implementation................................................................. 39 3.2.1 PWM and Speed Control Principle........................................................... 39 3.2.2 Control Components ................................................................................. 40 3.3 Electrical Solution and Implementation............................................................ 48 3.3.1 The Wheelchair Battery Type................................................................... 48 3.3.2 Batteries Connections ............................................................................... 49 3.3.3 Alternator Connections ............................................................................. 49 3.3.4 Battery Charge Level Detector ................................................................. 49 3.3.5 DC Motor-Driver H-Bridge Circuit .......................................................... 50 7

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3.3.5.1 Definition .............................................................................................. 50 3.3.5.2 Design and Implementation of the H Bridge ........................................ 51 3.3.5.3 Type and Connection of the MOSFETs................................................ 52 3.3.5.4 Need for Drivers ................................................................................... 53 3.3.5.5 Use of Regulators.................................................................................. 54 3.3.5.6 Types of Capacitors Used ..................................................................... 55 3.3.5.7 Choosing the Main Components........................................................... 56 3.3.5.8 Operation of the H Bridge Circuit ........................................................ 57 3.3.6 Main Control Circuit................................................................................. 60 3.3.6.1 Use of 16 MHz Crystal Oscillator ........................................................ 60 3.3.6.2 Use of Regulator ................................................................................... 60 3.3.6.3 AND Gate ................................................................................................. 60 3.3.6.4 Use of Relays ........................................................................................ 61 3.4 Connection of the Electronic Circuits............................................................... 63 3.5 Control Panel .................................................................................................... 64 Chapter 4 ......................................................................................................................... 66 Financial Analysis............................................................................................................. 66 Chapter 5 ......................................................................................................................... 67 Project Tasks..................................................................................................................... 67 Chapter 6 ......................................................................................................................... 68 Conclusion and Improvements ......................................................................................... 68 Glossary ........................................................................................................................... 69 Appendix A ...................................................................................................................... 71 MOSFET Datasheet .......................................................................................................... 72 Driver Datasheet ............................................................................................................... 73 12V Regulator Datasheet .................................................................................................. 74 5V Regulator Datasheet .................................................................................................... 75 AND Gate Datasheet......................................................................................................... 76 Pic Basic Code .................................................................................................................. 78

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List of Figures Figure 1: A proE drawing of our Hybrid Wheelchair....................................................... 11 Figure 2: Electrical Wheelchair available on the market today [12] ................................ 14 Figure 3: Methods of propulsion [1]................................................................................. 16 Figure 4: Parallel hybrid model [5]................................................................................... 22 Figure 5: Series hybrid model [5] ..................................................................................... 22 Figure 6: Parts of our wheelchair...................................................................................... 26 Figure 7: Leroy Somer DC Motor .................................................................................... 33 Figure 8: Main chassis ...................................................................................................... 34 Figure 9: Removed wires and unused items ..................................................................... 35 Figure 10: 250 CC ICE ..................................................................................................... 36 Figure 11: The wheelchair alternator................................................................................ 36 Figure 12: Base of the alternator....................................................................................... 37 Figure 13: ICE cover......................................................................................................... 38 Figure 14: Electrical and Control System......................................................................... 39 Figure 15: PIC16F877A.................................................................................................... 40 Figure 16: Microcontroller Block Diagram ...................................................................... 41 Figure 17: The wheelchair joystick................................................................................... 41 Figure 18: Operational region of the joystick ................................................................... 45 Figure 19: Ramp function of Current Voltage.................................................................. 46 Figure 20: Batteries of the wheelchair .............................................................................. 48 Figure 21: Battery charge level indicator.......................................................................... 50 Figure 22: Simple conceptual schematic of an H-Bridge [8] ........................................... 50 Figure 23: Ceramic capacitors [6]..................................................................................... 55 Figure 24: Chemical capacitors [6]................................................................................... 56 Figure 25: Conceptual design ........................................................................................... 58 Figure 26: H Bridge PCB.................................................................................................. 59 Figure 27: AND Gate conceptual operation ..................................................................... 61 Figure 28: Main control circuit ......................................................................................... 62 Figure 29: Electronic circuits............................................................................................ 63 Figure 30: Control panel design........................................................................................ 64

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List of Tables Table 1: Relation between Joystick, Speed and Direction................................................ 43 Table 2: Primary Required Voltage on the Motors........................................................... 44 Table 3: Added Voltage .................................................................................................... 44 Table 4: Four Useful Connections .................................................................................... 51 Table 5: Four Useful Connections .................................................................................... 56 Table 6: H-Bridge inputs and direction of rotation........................................................... 60 Table 7: Summary Table of AND Gate ........................................................................... 61 Table 8: Cost Analysis ...................................................................................................... 66 Table 9: Project Tasks....................................................................................................... 67

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Chapter 1 Introduction

Figure 1: A proE drawing of our Hybrid Wheelchair

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1.1 Problem Definition Last summer two members of the group were doing their internship at the University of Madison-Wisconsin, and in order to go to work daily, they had to walk down the highway for about fifteen minutes. On their way, they noticed that in the middle of the highway lie two parallel yellow lines about a meter and a half wide. Later on, they saw one day an electric wheelchair passing through these two lines. It turned out that this lane was marked specifically for electric wheelchairs, and that is how all things started. They inquired more about the subject and got to know that these lanes are made specifically for wheelchair users, and run all the way on the highways of the city of Madison. Then one thought led to another, and soon we were criticizing this method and defining problems. We asked ourselves one question: how far can these electric wheelchairs run in the outdoors without being charged, and how can they make use of those long range highways? That is how we formulated our problem. It spurted out of a need, a need which is the desire to use wheelchairs for long hours without being recharged. Asking some disabled people a couple of questions, we concluded that their electric wheelchairs actually do provide them with a long range, but the problem is that they sometimes forget to recharge the batteries, or never go on far trips out of fear that they might face a type of terrain that they cannot surpass. What we want to present for these disabled people is the opportunity to run their wheelchair for days without the burden of thinking about the charge of the batteries. We want to give them peace of mind, freedom, allow them to go on long trips and enjoy their time without having to constantly check their charge and range left. So our major problem and the one that we intend to solve are to remove the range boundaries that keep haunting wheelchair users.

1.2 Management Summary The problem we are facing and trying to solve is the problem disabled people face whenever they get on their wheelchairs. “How much charge is there left in my batteries?” and “What is the range that I can safely travel before I lose power?” These questions describe this problem, which is best defined as the limited range that disabled people face

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when using their wheelchair. What we aimed for through our Final Year Project is to solve this problem by designing and building the first ever hybrid wheelchair. By hybrid we mean that our wheelchair will be powered by two sources of energy: electrical and gasoline. The way we went around in achieving our goal is by placing a 250 cc gasoline engine on a specially made chassis that absorbs vibrations to the back of an electric wheelchair. In addition to absorbing vibrations, the chassis and the engine are fully covered and are well insulated to minimize noise and heat radiation. A 28.4 volts, 100 amps alternator is connected to the engine through a 13 degree conic belt. Thus, when the engine is turned on, the alternator is rotated and power is generated. On the front part of our wheelchair two 12 volt batteries are placed in series. The generated electricity is directed towards the batteries and is made use of to recharge them. The wheelchair is fully controlled by a joystick placed near the left hand rest. Next to the right hand rest there is a control panel that contains all of the switches and battery charge level indicator. So in general, we have achieved our goal of building and implementing our design and as a result, we lay today our hands on the first ever fully functional hybrid wheelchair.

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Chapter 2 Literature Review

Figure 2: Electrical Wheelchair available on the market today [12]

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2.1 Electric Wheelchair Electric wheelchairs appeared in the 1950s. Today's models are better described as electronic chairs rather than electric chairs. Electronic circuitry allows for a control of speed and a precise control of direction.

2.1.1 Basic Styles Many of today's sophisticated electric wheelchairs conform to two basic styles. The first is called the traditional style and consists of a power source mounted behind or underneath the seat of the wheelchair. As the name implies, the traditional unit looks very much like a manual wheelchair. The second design is known as a platform chair. In this design, the seating area, which can often be raised or lowered, sits on top of the power source.

2.1.2 Indoor vs. Outdoor Use There are several groups of powered wheelchairs, based on the intended use. Wheelchairs designed strictly for indoor use have a smaller area between the wheels, allowing them to negotiate the tighter turns and more confined spaces of the indoor world. Other designs allow the electric wheelchair to be used both indoors and outdoors, on sidewalks, driveways, and hard, even surfaces. Finally, some electric wheelchairs are able to negotiate more rugged terrain such as uneven, stony surfaces.

2.1.3 Methods of Propulsion Wheelchairs are classified according to the drive wheel location relative to the system centre of gravity (chair and user). The following are the three basic methods of propulsion:

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Rear Wheel Drive Wheelchair

This is the most common method of drive for an electric wheelchair. The drive wheels are located behind the center of gravity while the front wheels are casters. This method makes the wheelchair fast, but can give a poor turning capability when compared to front and mid wheel drive chairs. •

Center Wheel Drive Wheelchair

This method of drive is the best method of drive for an electric wheelchair. The drive wheels are directly below the center of gravity while the front and rear wheels are casters. The wheelchair can be a little unsteady when starting and stopping but it could not be suitable for uneven surfaces. •

Front Wheel Drive Wheelchair

This method of drive gives a lower top speed than rear wheel drive chairs, but offers a good turning capability. The drive wheels are in front of the center of gravity while the rear wheels are casters. [1]

Figure 3: Methods of propulsion [1]

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2.1.5 Electric Wheelchair Battery Types There are three different battery types used in wheelchair. These are "Wet", "Gel", and the newer "AGM (Absorbed Glass Mat)" types. Their properties are listed below: •

Wet Batteries

Wet batteries use the chemical reaction between lead and sulphuric acid to create electrical energy. These batteries need to be filled with distilled water, and they do have a higher maintenance rate, but are lighter than Gel or AGM batteries. [10]

Positive Aspects Cheaper Less vulnerable to overcharging Great performance with careful maintenance Lighter per Ah compared to most Gel or AGM’s

Negative Aspects Require maintenance Battery acid can leak, causing corrosion and damage to chair and wiring Not approved for airline travel High rate of self-discharge when left sitting (6-7% per month) •

Gel Batteries

Gel batteries contain a mixture of sulphuric acid, fumed silica, pure water, and phosphoric acid, which forms a thixotropic gel. As there is no liquid in the battery, they do not leak or require maintenance like wet batteries. [10] Positive Aspects No maintenance Cannot leak Operate better than wet batteries in low temperatures 17

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Less gas released when charging than wet batteries Approved for air travel Longer life cycle than wet batteries Negative Aspects Expensive More weight per Ah than wet batteries. Susceptible to overcharging •

AGM batteries

AGM batteries have an absorbent glass mat sandwiched between the plates, saturated with acid electrolyte, but with none free to spill. This type of batteries reduces the chance of battery damage caused by vibration and jarring. [10] Positive Aspects No maintenance Can’t spill or leak Shock resistant Minimal gasses released when charging. Low self-discharge rate (3% per month at 77’F) Negative Aspects Highest cost Susceptible to overcharging New technology

2.1.6 Electric Wheelchair DC Motor Types Most power wheelchairs currently utilizes PM motors with iron magnets, brushes and indirect drive trains. Recent innovations within the power wheelchair industry include the use of rare earth magnets; and brushless, gearless, direct drive motors.

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Rare earth magnets support much higher magnetic fields than iron magnets. Motors utilizing rare-earth magnets are smaller and lighter and more powerful than analogous motors with iron magnets. Brushless motors are more efficient than brush motors (brushes introduce electrical power loss). Brushes are also subject to wear and require regular inspection and replacement. Gearing and belts in the indirect drive train are a source of mechanical power loss. Highly efficient, gearless, direct drive motors have recently appeared in the power wheelchair market. These motors can be mounted in close to the drive wheels and allow good access to the under seat compartment. However, these motors tend to be relatively large and expensive. The following list includes some of the motor technologies that have been suggested. •

A brushless, gearless motor, entirely contained within the power wheelchair’s drive wheel.

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Pancake stepping motors efficiently generate high torque, even at high speeds. These motors are durable and reliable.

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Disc-armature DC motors have high power to weight ratio and efficiency.

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Alternating current, three phases, squirrel cage induction motors (SCIM) are inexpensive, efficient, highly reliable, and have a torque speed characteristic very adequate for vehicle propulsion.

2.2

Hybrid Vehicles

2.2.1 Definition A hybrid car is a vehicle that uses a combination of at least two different fuel sources for its propulsion. Although many combinations are possible, generally when people are talking about hybrid cars, they are referring to cars with a combination of

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A gasoline internal combustion engine

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An electric motor

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A battery that powers the electric motor and stores energy for future use.

Hybrid cars may also be called gas-electric hybrids. Some examples of current hybrid cars include the Toyota Prius, Honda Civic Hybrid (HCH), and the Honda Insight.

2.2.2 How Hybrid Cars Work Hybrid cars work by seamlessly integrating a gas engine, an electric motor and a highpowered battery. The battery provides power for the electric motor and is recharged by recapturing energy that would normally be lost when decelerating or coasting. This recapturing of energy is called regenerative braking. If needed, power from the gas engine can be diverted to recharge the battery as well. Because of these charging strategies, hybrid cars never need to be plugged in. [2] To understand how the gas engine, electric motor and battery work together, it is best to divide hybrids into two categories: mild hybrids and full hybrids. Each has its own approach to incorporating the three components. •

Full Hybrid

In full hybrids the gas engine, electric motor, and battery work together. When a full hybrid is started the battery typically powers all the accessories. The gasoline engine is used only if the battery needs to be charged or more power is needed than the battery can supply. At lower speeds the electric motor can operate independently. Then at higher speeds the gasoline engine takes over. During slow acceleration, reversing, and continuous slow speeds the batteries power the electric motor. Only if the battery needs to be charged will gasoline engine be used. When the vehicle is moving at mid-range speeds or accelerating both the eclectic motor and gasoline engine are used. Braking in hybrid vehicles is different than in conventional vehicles. Regenerative braking, as it is called, converts energy that would be wasted in conventional vehicles, to 20

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electricity and stores it in the battery. When slowing the electric motor is reversed so the electricity the rotating wheels are actually turning the motor, rather than vice versa. When the car actually stops, conventional friction brakes are applied where both the gasoline engine and electric motor shut off, though the battery continues to power auxiliary systems. [3] •

Mild Hybrid

In mild hybrids the electric motor depends on the gas engine for power, though the electric motor can generate electricity. When a mild hybrid vehicle is started the gasoline engine warms up. The gasoline engine powers the vehicle when it is moving at a constant rate, such as highway driving. During periods of acceleration the gasoline engine and the electric motor are both utilized to propel the vehicle. Mild hybrid also utilizes regenerative breaking. Mild hybrid systems can be broken down into subcategories: The Stop/Start hybrid system is not true hybrids since electricity is not used to propel the vehicle. However, the electric motor is used when the vehicle is stopped. The Stop/Start hybrid system is not true hybrids since electricity is not used to propel the vehicle. However, the electric motor is used when the vehicle is stopped. The Integrated Starter Alternator with Damping (ISAD) hybrid system allows the electric motors to help move the vehicle in addition to providing stop/start capability. The Integrated Motor Assist (IMA) hybrid system is similar to the ISAD but has a larger electric motor and more electricity can be used to help move the vehicle. [3]

2.2.3 Parallel and Series Hybrid Most hybrids use a combination of mechanical and electrical power to move a vehicle. These two systems work together to deliver energy to the wheels. Manufacturers are using two different design approaches, either a Parallel or Series system. 21

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Parallel Hybrid

In a parallel system the battery and engine are both connected to the transmission. As a result either the battery via the electric motor, or the engine directly to the transmission, or a combination of both, can provide propulsion power.

Figure 4: Parallel hybrid model [5]

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Series Hybrid

In the series type the gasoline engine turns a generator, and the generator can either charge the batteries or power an electric motor that drives the transmission. Thus, the gasoline engine never directly powers the vehicle. However, today’s hybrids are all parallel hybrids.

Figure 5: Series hybrid model [5]

2.2.4 Computer Control For the entire ride, the computer will be calculating when to let the gasoline engine do all the work and how much of a boost it needs from the electric motor. Because of the

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intermittent assist from the electric motor, the gasoline engine can achieve basically the same performance as a conventional car despite its smaller, more efficient size.

2.2.5 Battery Charge and Discharge Where is the electric motor getting its power? It’s actually getting and giving power back and forth from a set of nickel metal hydride batteries. Once again, the computer is playing an important role by knowing when to reclaim excess energy when breaking the wheels with the electric motor (which is now working like a generator). It also knows when to pass power from the battery to the electric motor for acceleration. The computer is monitoring the amount of charge in the batteries, making sure that they never charge over 80% and never under 30% of their capacity. In this way, the batteries will last a couple of hundred thousand miles. •

Recharging Batteries

The batteries found in conventional cars and in hybrid cars are both rechargeable. The difference is in the construction of the batteries interior and the amount of energy the battery can store. A hybrid car uses a conventional lead acid battery. But a hybrid car also has a rechargeable battery, which is constructed quite differently. It is what is called a deep cycle battery. The internal construction of the battery allows it to be fully discharged and recharged over and over again. It is very similar to a battery used in electric vehicles such as GM’s EV1 or a golf cart or new-fangled electric personal scooters. The difference is that electric vehicles need a lot of stored energy, since the stored electrical energy is the only fuel the vehicle has to make it move down the road. These batteries are very large and heavy. For an example, the battery pack in the Electric Ranger battery build by Ford in the late 1990s was 1600 pounds. These batteries carried a

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serious amount of energy. Most of these battery packs are a series of smaller batteries connected together in a series array that adds up to a higher voltage. The hybrid car uses a mixture of today’s gasoline engine and the battery found in electric vehicles (EV), which never found acceptance in the passenger car market. The hybrid battery has evolved a generation or two since the EV days. Today Nickel-Metal-Hydride (NiMH) is being used for hybrid batteries instead of lead acid to reduce the weight and deliver more energy from a smaller package. Because a hybrid also uses a gas engine, the size of the battery is not as large as a pure electric vehicle EV battery. On vehicles such as the Honda Civic and Insight, and the Toyota Prius, the hybrid battery voltages are 300 volts or greater. Where the starter battery in a typical car was measured by cranking amps, hybrid batteries are measured by kilowatt-amp-hours. [4]

2.2.6 Pros and Cons of Hybrid Vehicle Pros of Hybrid Vehicles •

Hybrids emit up to 97% less toxic emissions and half as much greenhousecausing carbon dioxide as the average car.

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Mileage is noticeably higher.

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Hybrids are just as safe as their non-hybrid counterparts. The fact that they run on electricity as well as gas has no bearing on their safety.

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The performance of Hybrids is improving more and more with each passing year, so their efficiency and improvements look bright for the future.

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The Hybrid's battery pack never needs to be charged from an external source - it gets recharged during regenerative braking and by the gasoline engine when necessary.

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Due to a smaller engine, and lightweight materials used in manufacturing Hybrids, they tend to weigh less than their non-Hybrid counterparts.

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Hybrids can run on alternative fuels, decreasing our dependency on fossil fuels, and increasing the fuel options. [5]

Cons of Hybrid Vehicles •

They cost more - anywhere from $2,000 to $5,000 more the non-Hybrid version of the same vehicle.

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Hybrids have a more complex power train, which means more chances for failure and fewer fixes that your typical mechanic could do.

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Parts may cost a bit more and not be as readily available as typical car parts.

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Special high-mileage tires are smaller, but they cost more to replace.

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High-performance electric motors are not yet available in Hybrid vehicles, whose emphasis is on economy not speed. (However, they're getting better each year) [5]

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Chapter 3 Solution and Implementation

Figure 6: Parts of our wheelchair

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After well defining earlier in this report the problem we aim to solve, it is now time to introduce to you the solution. Our solution was divided into three main parts: 1. Mechanical part 2. Control part 3. Electrical part In general, the solution we thought of was creating a hybrid wheelchair, one that is able to generate power within itself and recharge its own batteries. We came across several ideas for creating a hybrid wheelchair. At first we thought of building an electric wheelchair and adding to it an electric generator that will be used only to recharge the batteries. However, we did not agree on implementing this method for the following reasons. •

The generators turn on manually by pulling a lever hard

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We cannot make use of the generators to propel the wheelchair

Because the user of a wheelchair will not be able to pull on a lever to turn on the generator, we wanted to use something that can be turned on automatically, with the press of a button. In other words we needed a combustion engine with an electric start-up mode option. Being limited by the size, we had to choose a relatively small combustion engine that will fit on a wheelchair. Finding the perfect engine was a very hard task. Being limited by lots of parameters, we had to compromise and find the best combination. What we needed was an engine that is: •

Relatively small

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Able to produce enough power to recharge the batteries

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Powered on by an electric switch

After lots of searching we found out that there is no such engine available in the Lebanese market. We tried our luck by surfing the net, but to no avail. A quick decision had to be made, where we had to select 2 out of 3 parameters, and then work out the

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problem of the third parameter by other means. Therefore, we chose to buy a relatively small and electrically powered engine. Dealing with the third parameter that we overlooked when buying our engine, we had to come up with an ingenious way to solve our power problem. Since the alternator embedded in the engine is not big enough to provide enough power to recharge the batteries, we thought of using another alternator to do the job. What we decided to do was to get a big alternator that is keen of recharging the batteries and installing it on the combustion engine. So whenever the engine is turned on, the alternator will turn too and generate power to recharge the batteries. That was a general idea of our solution. We shall indulge ourselves now more specifically in the 3 (mechanical, electrical, and software) aspects of the solution, and describe them in details.

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3.1 Mechanical Solution The mechanical work was basically based on building the whole system. The system includes the actual electrical wheelchair with its chassis, motors, chair, and control panel. In addition to the wheelchair, a combustion engine had to be placed on its own chassis and connected to the wheelchair. Now we shall divide the solution into steps:

3.1.1 Choosing the DC Motors In order to choose the required DC motors that can do the job, we conducted a theoretical study that aims to helping us choose the optimal type and size of DC motors.

Horsepower calculations for a 37˚ incline

R

ƒx W

R: Incline reaction to wheelchair weight. ƒx: friction force. W: wheelchair weight

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Predefined parameters Mwheelchair = 150 Kg This mass accounts for both the mass of the wheelchair approximated to be equal to 70 Kg, and the mass of a standard disabled user which is about 80 Kg. •

g = 9.81m/s2

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Maximum angle of inclination: αmax = 37˚. According to the international laws for transportation the maximum slope angle should not exceed 37˚. We are not sure if this law is respected in Lebanon, but it is definitely respected in the large cities like Beirut, Jounieh and Tripoli.

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Coefficient of friction: µ = 0.5 - 07, we will assume the value µmax = 0.7, to account for the worst possible conditions.

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Wheel Radius: R = 40cm = 0.40 m

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Wheel perimeter: Pwheel = 2.512 m

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Assuming the required acceleration: ax = 1 m/s2

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The average velocity of the wheelchair is: Vav = 5 km/h = 1.39 m/s

Weight of the Wheelchair W= M × g = 150 × 9.81 = 1471.5 N

Reaction of the incline R = W cos(37˚) = 1175.19 N

Friction force ƒx = µmax × R = 0.7 × 1175.19 = 822.6 N

Weight in the direction of the movement Wx = W sin(37˚) = 885.57 N

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At equilibrium ΣFx = F - ƒx - Wx = 0 Î Fmin = ƒx + Wx = 822.6 + 885.57 = 1708.17 N Also: ΣFx = M × ax = F - ƒx - Wx ÎF = 150 + 885.57 + 822.6 Propulsion force F = 1858.17 N

Torque at the wheel T = F × R = 1858.17 × 0.15 = 278.72 N.m

Calculation of rpm V = 5000/ 60 = 83.33 m/min Î rpm = V/ Pwheel = 33.17 rpm T = 278.72 N.m =205 lb-ft Î HP = (T*rpm)/5252; = (205* 33.17)/5252 = 1.29 Horsepower calculations on flat surfaces The wheelchair is going to spend the largest portion of its running time on flat surface. This is why it is necessary to have an approximate idea of the horsepower required to run the wheel chair on this kind of terrain.

Weight of the Wheelchair W= M × g = 150 × 9.81 = 1471.5 N

Reaction of the incline R = W = 1471.5 N

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Friction force ƒx = µmax × R = 0.7 × 1471.5 = 1030 N

At equilibrium ΣFx = F - ƒx = 0 Î Fmin = ƒx = 1030 N Also ΣFx = M × ax = F - ƒx - Wx ÎF = 150 + 1030 Propulsion force F = 1180 N

Torque at the wheel T = F × R = 1180 × 0.15 = 177 N.m

Calculation of rpm V = 5000/ 60 = 83.33 m/min Î rpm = V/ Pwheel = 33.17 rpm T = 177 N.m = 130.54 lb-ft Î HP = (T*rpm)/5252; = (130.54* 33.17)/5252 = 0.82

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The motors we chose are permanent magnet, self braking motors. The motors run on a 24 volts and 13 amps power source. The motors are self braking, but the brakes can be released mechanically and electrically. If the motor is fed by a 24 volt source, then its brakes will be released immediately. These motors reach a peak current during starting equal to 25 amps.

Figure 7: Leroy Somer DC Motor

3.1.2 Designing the Chassis for the Electric Wheelchair Our first step was drawing sketches of what we wanted our chassis to look like, and then we proceeded with drawing these sketches using PRO-E. The challenge in designing was to design a wheelchair that will hold a 250 cc combustion engine and a 100 amp alternator both on one chassis, and connect them to the chassis of the wheelchair. The tricky part comes in centering the weight distribution of the whole system, and in keeping the center of gravity as low as possible for the following reason: Our system is a dynamic model and not static, this means that our wheelchair will be driven around, and we want it to be as steady on the road as possible. After we designed the chassis for the electric wheelchair, we set into building it. However, we did not have to manufacture the chassis on our own because ‘Arc en Ciel’ has donated to us a chassis with two electric DC motors. These are the permanent magnet, self braking motors that I talked about earlier in this report. The picture below shows the chassis with the DC motors and the batteries that we added to it later on. 33

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Figure 8: Main chassis

3.1.3 Choosing the Recharging Technique Knowing that we had to find a way to recharge the batteries, we had to choose which one was the most efficient and applicable of all. In the end all our options were narrowed down to two techniques: •

Electric generator

•

Scooter combustion engine

The advantages of the electric generator are that it is simple to use, provides enough power, is small in size, and is silent. The disadvantages on the other hand are that it cannot be turned on automatically. The advantages of a scooter combustion engine are that it can be turned on with the press of a button, which means that it has an electric start-up option. The disadvantages are that it takes so much space, and it is noisy. Our decision was to go with the scooter combustion engine because we thought that it would be hard on the disabled to pull on a lever arm every time they wanted to turn on the engine, keeping in mind that we will find a solution to the scooter’s disadvantages.

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3.1.4 Picking a Combustion Engine After deciding to proceed with the scooter engine, we had to choose what size of engine we need. The alternators embedded in all sizes of scooters were not big enough to recharge our batteries. As you will see later on, we were obliged to use 24 V batteries, and all alternators in scooters are used to recharge either a 6 V or a 12 V battery. Therefore, we came up with the idea of using the engine to turn another alternator installed on it. Now we had to find an engine powerful enough to turn the alternator and make it generate electricity. The engine that we chose was a 250 cc, 30 bhp, Honda Freeway engine. We took the decision of picking this engine based on the following information. In general, you need a power source equivalent to 7 HP to operate a 150 amp alternator. The combustion engine that we needed was one of a scooter, but we could not find an engine by its own in the market. What we had to do was buy a whole scooter, and then remove the engine from it. So basically, we bought the scooter, which was a Honda Freeway, and we worked on stripping it off till we had the engine with its electrical components in our hands. When we were done taking the scooter apart, we had to build a special chassis for the engine. The chassis that we built is made from iron, and it holds the engine in position while minimizing the vibrations. The reason it minimizes vibrations is because it holds the engine through dampers. The next step was to connect both the wheelchair chassis to the engine chassis.

Figure 9: Removed wires and unused items

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Working some more on the engine, we cleaned it, changed its spark plug for better performance, installed a fuel tank and connected it, removed unnecessary electrical wirings and wrapped the necessary ones tidily, cleaned the carburetor, and disassembled the clutch system.

Figure 10: 250 CC ICE

3.1.5 Picking the Alternator Because we were obliged to use 24 V batteries, we had to pick an alternator that is used to recharge 24 V batteries. Therefore, we bought a 26.8 V, 100 amps alternator. This alternator is usually installed on a 2400 cc diesel engine. This engine is used on trucks, and provides enough power to make the alternator function properly. Our challenge is to install it on a 250 cc engine and make it operate. We will be optimizing with gear sizes in order to find the best combination in terms of speed and torque.

Figure 11: The wheelchair alternator

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3.1.6 Installing the Alternator on the Combustion Engine The alternator had to be connected to the engine in order to operate, and we had to find a way to connect them together. The best way was to connect them by a belt that turned on similar gears on both ends. By similar gears we do not mean in size, in fact we mean that they are similar in the groove in which the belt sits. For more functionality, we placed the alternator on the chassis through a metallic bar that rotates across its hinge axis. This provides the alternator with different levels of installation.

Figure 12: Base of the alternator

3.1.7 Designing the Chassis for the Combustion Engine and the Alternator There was no place for the combustion engine and the alternator on the wheelchair, so we had to build them their own chassis and then connect this chassis to the rear end of the wheelchair. On this chassis both the combustion engine and the alternator will sit, and will be covered and insulated by a plexiglass cover. The horizontal base of the chassis is drilled through in order to make way for the exhaust pipes to pass.

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3.1.8 Designing the Chair The chair was simple and straight to the point. It was composed of four metals welded together to form a horizontal frame. From the both sides of this frame extends (up and down) metallic bars. The bars on the lower side are used to fix the wheelchair on the chassis. The bars on the upper side are used to form a back rest. On this frame sits a piece of wood that is cushioned and covered by leather.

3.1.9 Designing a Cover that Provides Insulation to both Sound and Heat In order to minimize noise and heat, we built a very tight cover that covers up the whole engine and alternator. The cover is made up of 5 mm Plexiglas that is cushioned from the inside walls with a 2 cm layer of Styrofoam. We believe that this combination is the best combination of materials that can be use to insulate against noise and heat at the same time. Of course two holes had to be drilled across two sides of the cover so that the engine receives its need of air.

Figure 13: ICE cover

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3.2 Control Design and Implementation The diagram below shows a summary of the electrical and control system of our wheelchair

Clockwise Counter Clockwise Release Brakes PWM Clockwise Counter Clockwise

H-Bridge

PIC16F877A

Joystick

H-Bridge

PWM

Release Brakes

Figure 14: Electrical and Control System

3.2.1 PWM and Speed Control Principle To control the speed of the DC motors, one needs a variable voltage DC power source. However if you take a 24v motor and switch ON the power to it, the motor will start to speed up: motors do not respond immediately so it will take a small time to reach full speed. If we switch the power off sometime before the motor reaches full speed, then the motor will start to slow down. If one switches the power on and off quickly enough, the motor will run at some speed that is between zero and full speed. Therefore, PWM switches the motor on in a series of pulses. To control the motor speed it varies (modulates) the width of the pulses; from here came then naming: Pulse Width Modulation. The PWM signal is a square wave that has a duty cycle =

time of high in a period × 100 . time of the period

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In essence, as the duty cycle increases the speed of the motor increases. For example, if the duty cycle is 0%, the motor will not rotate. If the duty cycle is 50%, the motor would rotate half its speed. If the duty cycle is 100%, the motor would be rotating at full speed. Since the PWM signal is a square wave, then it should have a frequency. The normal operational frequency for DC motors is between 2 KHz and 6 KHz. By trial and error, we found that for our DC motors, the best frequency is 2 KHz.

3.2.2 Control Components •

Microcontroller

The microcontroller is the brain of the wheelchair; without it, the machine won’t be able to move. Its main job is to take as input some signals from a joystick and generate two PWM signals: one for each motor. These PWM signals enter the H-Bridges in order to be able to control the speed of the rotation of the DC motors. Therefore, the main use of the PIC is for DC motors speed control. In addition to that, the microcontroller should also control the steering mechanism of the wheelchair and should coordinate between the DC motors. Therefore, the microcontroller should control the speed and direction of rotation of each motor. •

PIC16F877A

The decision fell on PIC16F877A because of its capability of generating two PWM signals at the same time. Also, the PIC is a very efficient and powerful microcontroller with so many pins that can be easily programmed. Besides, the components are cheep and do the job perfectly.

Figure 15: PIC16F877A

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Microcontroller I/O

PWM Clockwise Counter Clockwise

PIC16F877A

Joystick

Right motor

Drop Brakes Release PWM Clockwise

Left motor

Counter Clockwise Brake Brakes Release

Figure 16: Microcontroller Block Diagram

Input: the only input to the PIC is joystick. Joystick: The joystick that is used in the project has two potentiometers (forward/Backward, right/left). The voltage of these potentiometers ranges between 0-5V. If the joystick is still (nothing is applied on it), the two voltages would be 2.5V. Also, the joystick can rotate 360o. These facts give the user the ability to go in any direction at different speeds according to the combination to the two voltages across the potentiometers.

Figure 17: The wheelchair joystick

Outputs: There are two sets of outputs; the first is for the right motor, while the second is for the left motor. Each set has 4 outputs: o PWM signal: this is the signal that specifies the speed of the motor

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o Clockwise: if ON, it means that the motor should run clockwise. If OFF, it means

that the motor should NOT run clockwise. o Counter Clockwise: if ON, it means that the motor should run counter clockwise.

If OFF, it means that the motor should NOT run counter clockwise. o Release Brakes: if ON, the brakes of the motor are released. The brakes are

always released whenever the motor should be rotating. In each set, “Clockwise” and “Counter Clockwise” outputs can NOT be ON together. If one of which was ON, the brakes should be released. However, if both were OFF, it means that the motor is OFF and the brakes should be applied. Note that the two DC motors should either be rotating clockwise together or counter clockwise together, or not rotating at all. It is impossible for one to be rotating clockwise while to other is rotating counter clockwise. But it is possible for one to be rotating in a certain direction while the other is not rotating at all: in this case the wheelchair should be steering either to the right or left. •

Control Algorithm

The control starts from the joystick where there the two potentiometers discussed before. The joystick is responsible for generating two output voltages: V1: the voltage across the forward/Backward potentiometer V2: the voltage across the right/left potentiometer.

According to the combination of V1 and V2, the direction of motion is known and hence the duty cycle needed for each H-Bridge (motor). In reality, the voltage across the DC motors will always be either 0V or 24V, but the duration of applying the 24V is what causes the speed variation (PWM main principle). However, note that varying the duty cycle is as if varying the voltage across the motor. Therefore, we will use this note to build upon it the control algorithm.

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In this report, we would be using two kinds of voltage across the motor: The required voltage: it is the voltage level across the motor should be reached after

some time and stay as that level. The current voltage: it is the voltage across the motor at a certain time t.

By trial and error, the DC motors start running at a frequency of 2 KHz when the duty cycle ~ 60 % (around 14V), hence the range of operation of the motors is between 60% (+14V) Æ100% (+24 V) (Similarly for the negative voltages). Note that if any of the required voltages gets above +24V (less than -24V), then the required voltage is set to +24V (-24V). According to the values of V1 and V2 together, the required voltages across the motors can be specified. This is done in two steps: 1. Primary required voltage: is the required voltage that results from V1 (forward/Backward) only. 2. Final required voltage: is the actual required voltage. It is the sum of the primary voltage with an additional value (0Æ10V) that results from the variation in V2. Final required voltage = primary required voltage + added voltage. V1 and V2 range between 0Æ5V, the relative directions are as follows: Table 1: Relation between Joystick, Speed and Direction

V1 (Volts) speed

direction

V2 (Volts) speed direction

0 Æ 0.5

100% backward

0 Æ 0.5

100%

right

0.5 Æ 1

85%

backward

0.5 Æ 1

85%

right

1 Æ1.5

70%

backward

1 Æ1.5

70%

right

1.5 Æ 2

60%

backward

1.5 Æ 2

60%

right

2Æ3

0%

NA

2Æ3

0%

NA

3 Æ 3.5

60%

forward

3 Æ 3.5

60%

left

3.5 Æ 4

70%

forward

3.5 Æ 4

70%

left

4 Æ 4.5

85%

forward

4 Æ 4.5

85%

left

4.5 Æ 5

100%

forward

4.5 Æ 5

100%

left

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The following tables show the primary required voltages and the added voltage to each motor with respect to the variation of V1 and V2 respectively:

Table 2: Primary Required Voltage on the Motors

V1 (Volts)

Primary Required Voltage on the motors Right (Volts)

Left (Volts)

0 Æ 0.5

-24

-24

0.5 Æ 1

-20

-20

1 Æ1.5

-17

-17

1.5 Æ 2

-15

-15

2Æ3

0

0

3 Æ 3.5

15

15

3.5 Æ 4

17

17

4 Æ 4.5

20

20

4.5 Æ 5

24

24

Table 3: Added Voltage

V2

Added Voltage (if 3