FINAL DESIGN REPORT

SPRING 2015

Wheelchair Modification - Haim Baruh

John Adelhoch

David Haaf

Adam Kaufman

John Kelly

John Popivchak

Chris Poresky

Dan Roberts

B3

MAE RUTGERS UNIVERSITY SCHOOL OF ENGINEERING

Submitted to— Linda Kurth, Sunny Wong, and Haim Baruh

TABLE OF CONTENTS

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Contents Page No.

Executive Summary 2

Statement of Problem 2

Design Choice 3

Detailed Design 5

Engineering Analysis 10

Materials 13

Manufacturing Process 14

Manufacturing/Assembly Overview 16

Operation 17

Testing 18

Weaknesses/Unresolved Problems 18

Conclusion 19

References 19

EXECUTIVE SUMMARY

Our team designed and manufactured a kit which attaches to a manual wheelchair frame and

converts it into an electrical one. The kit is composed of a motor and control system which allows the user

to maneuver with minimal physical strength and/or coordination. A budget of $750 was allotted to our

group to select the necessary materials, however we managed to spend a total of $716.79. which included

the components of the motor and control system, the batteries, and various items which connect our

system to the wheelchair. We tested the individual components of our assembly separately (the motor

assembly and the controls assembly) but did not test the interaction of the two before the completion of

this report. Barring significant losses due to a sum of small inaccuracies or unforeseen design flaws in the

final connection, our wheelchair kit should function appropriately on Rutgers Day. The design of our

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product was nearly flawless, but the manufacturing methods used were time-consuming. Also, the

machined components were not perfectly precise or accurate, but this could be corrected with practice.

STATEMENT OF THE PROBLEM

Wheelchairs are most commonly used by individuals who have an injury, illness or disability that

prevents them for being able to walk on their own due to pain, discomfort or paralyzation. A wheelchair

will allow the individual to regain a sense of independence by allowing them to move around on their

own without the aid of others. Our product is meant to help alleviate some of these issues by providing a

safer, cheaper, and easier to use alternative to the current market for similar products.

The main goal of our design was to provide a safe way for individuals with low strength to get

around on their own. This includes individuals who may experience fatigue while manually operating a

wheelchair, who have arm or hand related disabilities or disorders, or who experience reduced energy due

to old age. Having difficulties getting around on your own is a frustrating experience, more so when you

have to rely on someone else to help you to move at all. The user will be able to propel him/herself

forward at a safe speed of 2 feet per second at low power on a flat surface, or 4 feet per second at high

power on a flat surface or 2 feet per second at high power up a typical wheelchair ramp. The controls

were designed such as that only intentional operation will generate motion. The buttons require enough

force to operate the motors, but not so much as to defeat the purpose of providing less stress to the user.

Aside from operation, our goal was also to provide a product that could be readily accessed by

anyone who currently has need of a powered wheelchair, but may be unable to afford it. The average cost

of powered wheelchairs can run up to around $7,000 with some going as high as $30,000. Even products

with similar goals as ours can run up to around $3,000. We were able to provide a safe and cheap

alternative after spending only $716.79.

Overall our product is capable of travelling at a safe speed comfortable to the user, while being

easy to use without the worry of accidentally causing unwanted motion, all at a low cost.

DESIGN CHOICE

Fall (Initial) Design:

Our project timeline was composed of the fall semester, where a majority of our conceptual

design efforts were established and the spring semester, where the design was finalized and molded in

order to produce a functioning end product.

Initial fall semester design ideas centered around a wheelchair with large paddles that came up

just above the armrests of the wheelchair. These paddles, when pushed, would propel the wheelchair

forward. This design would be marketed toward patients with arthritis. The large paddles would be easily

operable for people with these conditions. Many issues had arisen when this idea was considered,

however. For one, while the operator could easily move in the forward direction with these paddles,

moving backwards would be an extreme challenge. The whole concept behind this design was to help

arthritic patients with trouble grasping a control joystick. In order to move backward, the paddles would

have to be grasped and pulled toward the person using it. This type of movement would be more

demanding than other methods, and therefore not feasible. The design would also make precise movement

of the chair very tough. The maneuverability of the chair would be limited. Turning precisely to operate

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through a home or crowded store would not be an option, making the use of this type of wheelchair

limited.

After the paddle idea was thwarted, thoughts quickly moved toward designing a kit that could be

applied to any common manually powered wheelchair that would essentially convert it into an electrically

powered wheelchair. This conversion kit would be much cheaper than current electric wheelchairs, which

can cost as much as a car, if not more. The initial design of this kit consisted of two common DC motors,

a simple battery platform, and a roller-ball control system. The DC motors would be mounted on the

frame of the wheelchair, underneath the seat. Each motor would essentially operate the movement of one

wheel. The two motors would be tied together through the control system so that when told to go forward,

both motors would operate in sync with one another. While continuing to attempt to cater to arthritic

patients, ideas for the user-controlled input system which incorporated a roller-ball design seemed

feasible. A ball mounted to the wheelchair and located within reach of the user could be rolled, and this

motion would be converted to the movement of the motor. The entire system would be operated by a

battery. A simple, solid plastic platform would be attached to the chair, connected from one side of the

chair frame to the other, as a location for the battery. With this overall kit concept, we started to develop

more of an understanding of what we wanted to accomplish through our efforts. The motors and control

system would make the operation simple for patients with severe disabilities, while the overall concept

was known to be a good idea, a few flaws of this particular design were found. For one, the chair would

not be able to be folded. Typical manual powered wheelchairs have about 2.5” of clearance between the

left and right frames. The bulky DC motors would prohibit any chance of folding from taking place. In

addition to the motors, our battery platform featured a one piece construction, making folding impossible

unless it was removed. The removal of the platform every time the wheelchair was to be folded was a

poor design as it would take a lot of time and would be extremely tough for disabled patients. The rolling

ball user control method would also be difficult to implement. In operation, this type of movement could

lead to inconsistent, jerky movements of the wheelchair.

As these design concepts were improved through iterative thinking, the final design concept of

the fall was similar to the second design concept, but with a few improvements. First, the wheelchair

would be powered by a motor, but a brushless hub motor would be created and used instead of a

traditional DC motor. The hub motor was a good choice due to its low size profile and customizability, as

it would need be created from scratch to fit the specific parameters. We aimed to mount the motor to the

outside of the wheelchair tire rim, allowing the wheelchair to still fold as originally designed. This motor

would have a shaft attached to the frame, causing the wheel to rotate as current is applied to it. With the

folding abilities not compromised, the wheelchair would have a much more attractive design than most

electric wheelchairs due to the fact that it could be easily transported. Once attached, there would be no

need to remove the motors. The small profile would allow them to stay attached through transportation.

Also, the overall width of the chair when in use would not be significantly increased. This was important

for use in homes not created for handicapped access due to the fact that the chair would still fit through

normal doorways and halls easily.

The battery platform described above was a necessary component, but this too added a constraint

that would make the folding of the wheelchair tough, if not impossible, unless it is removed. For this

reason, the platform was modified so that it would be split in two, and the individual section would then

fold up with the rest of the wheelchair. This split platform design would be created from a lightweight

plastic, and the two sections would interlock similar to the lid of storage containers used by popular

pharmacy stores. The interlocking feature would be able to support the weight of the batteries and control

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circuitry. With a fixed distance between the frame sections when the wheelchair is completely open, the

precise manufacturing of the two segments would ensure the soundness of the platform under a load.

Should the two piece platform not hold up, supports could be attached from the frame to the individual

platform sections, guaranteeing that the platform would support this weight.

Finally, to conclude our initial fall design concept, the user controlled input for the motors would

be accomplished using large buttons that are often used for gaming controllers. The large size of these

buttons would allow patients with extreme disabilities, such as advanced arthritis, to easily control the

wheelchair. The buttons themselves are also relatively inexpensive. This system was preferred, even

compared to a joystick or trackball, since it required no grasping or significant hand movements. These

controls could easily be operated with just the push of a palm or finger. These buttons would

communicate with and control the motors using an arduino system. An arduino is a microcontroller

prototyping platform that can be used in circuits and programmed to interpret signals from input devices

and send outputs to other devices. The buttons would function as inputs to the arduino, which interpret the

signal from each button as high or low. When depressed, the buttons send a high signal. There would be

five buttons, one to go forward, one to rotate left, one to rotate right, one to go in reverse, and a stop

button. Only one button push could interpreted at a time (with the exception of the stop button which

overrides any other buttons), and pressing multiple buttons simultaneously would produce no effect. The

arduino would take the signals from the buttons, and directs the motor action based on its coding.

Reversing the directions of the motors for the purposes of moving straight in reverse or turning would be

accomplished through the use of an h-bridge, which would be placed in the circuit of the control system

between the arduino and the motors. A rough circuit diagram including the major components can be seen

in Figure (1):

Figure (1): Initial Control System Diagram

Using a multi-way switch, it would also be possible to send a signal to the arduino to adjust the speed of

the wheelchair between slow, normal, and slightly faster speeds. This would be accomplished through

arduino coding adjusting the speed of the motors depending on the input signal coming from the switch.

All of these speeds would be relatively slow to ensure the safety of the user. There would also be a

position on the switch that turns the controls off so that directions would not be accidentally pressed when

the system was not in use. A simple LED circuit could also be used to monitor the battery level, so that

users would know when the battery level was beginning to get low. This monitor could be made using a

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few LEDs and a zener diode which would respond and would stay lit depending on the voltage level in

the battery. All of these components would be housed within a plastic enclosure that could be mounted

onto either arm of the wheelchair using straps that would go around the arm. The plastic enclosure would

be between one and two inches in thickness so that the control panel would not extrude uncomfortably

outward from the arm.

Spring (Final) Design:

With the design process above finalizing the fall semester, we began fabrication of our product

during winter/early spring. As parts were created and assembly began to take place, various components

of the system needed to be either adjusted or completely redesigned. The efforts during the redesign

stages will be described below:

The entire design and fabrication process was dependent on the parameters of the motor’s stator.

The prefix of stator implies it will remain stationary as the motor operates. The stator purchased for this

project had 12 legs or, slots for the magnet wire to wrap around. It also had a length of 1 ⅜”. The number

of wraps corresponded to different power and torque parameters and was determined through calculations

which are included in the Engineering Analysis section of the report. The stator was then wrapped to

these determined specifications. A larger stator was preferred, but these specialized items were difficult to

purchase. A larger stator would have made the wrapping process easier and less time consuming, but time

constraints and market availability led to the need to purchase the ones that were available. The

parameters of the other project parts were designed with a dependence on the stator’s size and how it was

wrapped.

The center shaft of the motor ,shown in Figure (2), was the next part designed. The center shaft

replaced the axle of each wheel, which was previously attached by a simple nut and bolt. The new center

shaft was designed to fit into the wheelchair unit with no modifications needed. The center shaft is

symmetrical from the midpoint. From middle to outer edge, the diameter of the shaft progressively

decreases in diameter across the entire 6” length. The middle section of the center shaft was 1 11/16” long

with a ¾” diameter. This segment attached to the stator. It was passed through the ¾” center of the stator.

The top of the stator was aligned with the outer edge of this cylindrical segment. The stator was affixed to

the shaft by a high strength, temperature resistant epoxy. Since the height of the stator was only 1 ⅜”, the

center shaft had additional length untouched by the stator which allowed a hole to be drilled through

perpendicularly. This hole created an escape pathway for the wires which connected the power supply and

control system to the stator. The diameter of the segment length of the shaft adjacent to the middle section

was called the bearing seat, since it is where the bearings were placed. The diameter of the center shaft at

this section was decreased to 11/16” to match the inner diameter of the bearings. This adjacent section

was milled to a length of ⅜”. The precise fit of the bearing onto the shaft eliminated any unwanted

movement of the bearing during motor operation. The bearing also has limited movement in the axial

direction due to the backstop created by the larger diameter of the center shaft on one side, and by the

endcaps on the other side, which will be discussed further on. The final segment of the shaft, the end

segment, was 1 ¾” long with a 7/16” diameter. These ends were designed to fit through the frame of the

wheelchair on one end and through the bearings of the wheel on the other. The ends of the shaft were

threaded to fit a 5/16-18” bolt. The bolts were fitted with a lock washer to prevent any loosening during

the potentially rough operation of a moving wheelchair. The shaft and stator were therefore designed to

be affixed to the frame of the chair with no rotation. The center shaft was made from alloy 6061 aircraft

aluminum to ensure minimum bending when under the load of the entire system and a passenger.

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Aluminum was the preferred material for its light weight, machinability, low cost, and high strength.

Other aluminum alloys were considered, but they did not have a comparable cost to strength ratio that we

found with the 6061.

Figure (2): Center Shaft Engineering Drawing

Next, the design of the rotor, shown in Figure (3), and the magnet system ensued. The rotor is

coaxial with the shaft and stator. The prefix of rotor implies that it will rotate as the motor operates. The

rotor was designed to be the same length as the stator, 1 ⅜”. Magnets were then selected in accordance to

the rotor size. The magnets selected were 1” in length, ¼” wide, and 0.1” thick. These fit well into the

rotor. They would be glued into place using a high strength, temperature resistant epoxy adhesive to

prevent any shifting during operation. The inner diameter of the rotor selected so that an approximately ½

mm air gap existed between the stator and the magnets. An air gap of this size would provide optimal

performance. The rotor was further designed to have 6 bolts inserted perpendicular on each end for

endcap attachment. The bolts used were 6-32 x 1 ¼”. The six bolt locations were evenly spaced

circumferentially and were drilled and tapped to the required specifications and were about 0.6” deep.

The rotor was made from low carbon steel tubing to ensure strength and temperature resistance. The idea

of creating the rotors from a plastic additive printing method was considered, but it was found that this

would not hold up under potential applied forces. The final product selection settled on a 3” outer and

2.5” inner diameter cold drawn low carbon steel tubing.

Figure (3): Rotor Engineering Drawing

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The endcaps, shown in Figure (4) and (5), were the final motor component designed. The endcaps

were of vital importance to the system due to their role in both anchoring the components of the motor

together and by creating a means of transferring power from the spinning rotor to the wheelchair wheels.

The endcaps were also designed to contain the components of the motor, and hence there was an endcap

on either side of the rotor. These two components were very similar except the one adjacent to the

wheelchair wheel had holes for attachment to the wheelchair while the other endcap, more so located

“inside” the wheelchair solely focused on attaching to the rotor. The endcaps were designed with the

same outer diameter as the rotor. This created a smooth transition between the rotor and endcaps to

prevent entanglement of any dangling items as the motor spins. They also had six holes which aligned

with the six bolt holes of the rotor. Bolts passed through the end caps and anchored into the thick steel

rotor, securing the endcaps as tight as possible to the rotor. Maximum strength of the endcaps was

necessary. A recessed divot allowed the bearings to be fully supported around a full 360 degrees of

rotation. Holes centered in the endcap allow for the shaft bolts to pass through the endcap and thread into

the shaft uninhibited. Ventilation spaces were also designed into the endcap to allow heat produced by the

running motor to escape into the atmosphere. Another set of 9 holes were added in the middle of the

endcaps. Bolts passed through these holes to the rim of the wheel. The wheelchair wheel rims had

matching holes drilled into them which corresponded to those of the endcap, which allowed the motor to

be attached to the wheelchair. This attachment ensured the wheel would rotate along with the

endcap/rotor assembly. When power will be applied to the motor, the rotor, endcap, and wheel will all

rotate together as the stator remains stationary. The endcaps were created by an additive printing process

(3d printing) due to the low cost and quick production time. Aluminum endcaps were preferred over the

plastic printed endcaps, but budget constraints limited our options. The aluminum endcaps would also be

much more difficult to manufacture, and time constraints led to the decision to have them printed. The

plastic components were durable and easy to machine for a more precise fit.

Figure (4): Inner Endcap Engineering Drawing

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Figure (5): Wheel Side Endcap Engineering Drawing

The battery platform was redesigned from the fall semester. Initially, the platform was a two

piece system that would allow the chair to fold when the batteries were removed. After analyzing the 3D

drawings of the proposed system, which were going to be fabricated by an additive printing process, two

issues were found. For one, the pieces were too large to be created by the printing options that were

financially feasible for the project. Secondly, the folding design would not be strong enough to support

the weight of two 25 pound batteries. Due to these findings, the battery platform was then made from an

aluminum block. The aluminum was desirable for its low weight and high strength. The final platform

was 24” wide, 8” long, and ½” thick 6061 aluminum alloy.

For the control system, although many aspects of the final design remained similar to the initial

one, new knowledge gained along the way as well as time and cost constraints required the design to be

changed during construction. The large buttons, which were one of the main points of the system, were

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still incorporated. However, the main thing that changed was the use of electronic speed controllers

instead of an h-bridge in order to control the speed and direction of the motor’s rotation. It was

determined that an h-bridge would not be able to handle the levels of current and voltage required to drive

the motors. Additionally, the arduino and h-bridge alone cannot precisely or reliably control the speed at

which the motors rotate. Thus, in the final design the arduino drives the motors through the use of two

electronic speed controllers, which take control inputs from the arduinos as well as power from the

batteries to direct the motion of the motors. The speed controllers allow the arduino to control the speed

and direction of the motors based on the frequency of the signal that is sent to them. To make the chair go

forward, both motors are rotated forwards. To make the chair go backward, both motors are rotated

backwards. To make the chair rotate left, the left motor is rotated backward and the right motor is rotated

forward. Lastly, to make the chair rotate right, the left motor is rotated forward and the right motor is

rotated backward. A stop button overrides any other combination of button presses to make both motors

stop. Due to time constraints and the steep learning curve that has been involved in the various electronic

components associated with this project, only one low speed could be used on our prototype.

Additionally, the box containing the control systems is larger than was originally planned in the fall due

to the new electronics components that had to be added to the system in order to ensure functionality. An

updated basic circuit diagram showing the major system components can be seen below in Figure (6):

Figure (6): Final Control System Diagram

ENGINEERING ANALYSIS

For our design of a cost effective power-assisted wheelchair modification kit we had to design a

motor and control system that would give us the power we need, while remaining light-weight and

aesthetically pleasing. Described below are the calculations, completed in the fall semester, which were

used to decided component specifications:

For the design of an electric brushless DC hub motor, we first had to figure out what sort of

power we needed so that the hub of the motor could transmit enough power to the wheelchair wheels to

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assist the user. In order to obtain the desired power output we had to figure out the appropriate

components associated with power for a rotating set of wheels. For this we used the following equation

(1):

(1) 𝐻𝐻 = 𝐻∗𝐻𝐻𝐻

5252

Where HP is the power is in units of horsepower, T is torque in foot-pounds, rpm is rotations of the wheel

per minute, and the constant is a conversion factor for rpm and foot-pounds/min to horsepower. For

Torque we used the following equation (2)

(2) 𝐻 = 4𝐻𝐻𝐻0 (𝐻

𝐻+𝐻) 𝐻𝐻

Where m is teeth per phase of the stator, N is complete turns of the wire per phase, B0 is the magnet

surface remanence rating, t is the magnet thickness, g is the air gap width, L is the length of the magnets,

and R is the motor radius.This was the key equation for determining the quality of the motor needed to

suit our needs.

Starting with the easiest portion of this equation (1) we wanted to find how many revolutions of

the wheel we desired per minute, and for that we needed to know how fast we wanted our power-assisted

wheelchair to travel when activated. Keeping in mind that our project was being designed for those with

special needs in situations where safety is our first priority, we decided to go with a speed of around 2

feet/second. In order to convert this to rpms we used the following equation (3):

(3) 𝐻𝐻𝐻 = 120𝐻𝐻

𝐻

Where 120π is a conversion factor for the full rotation of a circular object times the number of seconds in

a minute, 𝑉 is our desired speed in feet/second and r is the radius of the wheel on our wheelchair in feet.

For our project we had a wheelchair with one foot radius wheels. For wheelchairs with other

wheel sizes the power of the motor would be slightly different and may require some modification. Since

the overall power equation is factored in by 1/radius, a smaller wheel radius would require a higher power

if the torque remained the same, while a larger wheel radius would have a lower power requirement.

However, a smaller person would typically use a smaller wheelchair which would have a smaller wheel

radius. A smaller body would mean less weight which would result in a lower torque requirement which

would lower the power requirement. In our calculation of required torque we used data collected in a

previous study of required torque of wheelchair users over different surfaces. The table below shows the

average values of required torque per wheel for a person weighing between 150 and 200 lbs on varying

surfaces in Table (1):

Table (1): Required Torque for Different Surfaces

Torque (ft-lbs): 1.475 ± 0.738 1.9177 ± 0.885 2.2864 ± 1.18 2.5815 ± 1.03

Surface Type: wood, tile, carpet,

smooth concrete

(reference surface)

interlocking

(uneven) concrete

pavers

grass, dirt 5 degree concrete

ramp

The reference surface was smooth concrete and required an average torque of about 1.5 foot-

pounds. Wood, tile and carpet all produced similar torque requirements. While traveling over interlocking

concrete pavers of non-uniform height the required average torque became about 1.9 foot-pounds. Dirt

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and grass are even more random in height and friction and so the required average torque became about

2.3 foot-pounds. As expected, the highest torque requirement is found when an opposing force is applied

with a ramp that has a 5 degree slope. The average required torque is about 2.6 with as high as 3.6 foot-

pounds.

Using the calculations for torque and rpm it was possible to find the power we wanted our motors

to operate at. Table (2) shows some of the ranges for power based on the desired speed of 2 feet/second,

or 754 rpms, as well as the torque values from the previous table. Adjustments were also made to

compensate for the added weight of the batteries.

Table (2): Power Ranges

Torque, (ft-lbs): RPM Power, (HP):

Mean Value: (Adjusted Value:)

1.7997 ± 0.443 (2.3139 ± 0.443) 754

.2584 ± 0.0636 (.3322 ± 0.0636)

Max. Value: (Adjusted Value:)

3.6115 (4.5144)

754

0.5185 (.6481)

Min. Value: (Adjusted Value:)

0.737 (.9827) 754

0.1058

(.1333)

Now that the physical power required to operate our power-assisted wheelchair were found, this

has to be converted into electrical power so that the appropriate battery source to supply the motors can be

found. This was done using equation (4):

(4) 𝐻𝐻 = 𝐻𝐻

746𝐻

Where HP is the power in horsepower, V is the voltage in volts, I is the current in amps, 𝜂is the motor

efficiency which is a non-dimensional scalar used to account for possible power losses in the motor, and

746 is a conversion factor for horsepower to watts.

Battery choices can then be considered where cost, duration, and weight are taken into account.

Since our project is being designed for power-assistance as needed, we are able to cut the cost while

purchasing batteries, which is usually the largest cost sink for powered wheelchairs. Our primary concern

for the battery is then going to be its weight. A 12 volt, 55 amp-hour, rechargeable, sealed, lead acid

battery weighs 39.7 lbs and a 12 volt, 35 amp-hour, rechargeable, sealed, lead acid battery weighs 22.5

lbs. A typical powered wheelchair uses two of these batteries to obtain a voltage of 24 volts, with some

going as high as 88 amp-hours which costs upwards of $600. There are newer batteries that weigh much

less, but the cost skyrockets for these and for us it will be easier and cheaper to compensate for an

increase in weight by modifying the torque requirement. The adjusted torque values in table EA.2 shown

are to compensate for the addition of about 50 lbs of extra weight since it is likely we will use a battery

combination that will add around 40 to 50 lbs of additional weight. For simplicity and due to time

constraints we will be using a simpler battery setup for demonstration purposes.

Combining everything above we should now have everything we need to design a motor that will

operate at about 2 feet per second at max power while climbing a 5 degree ramp and about 2 feet per

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second at low power, or about 4 feet per second at max power, on most typical surfaces for a person

weighing between 150 and 200 pounds.

MATERIALS

The materials selection for the motor in our design was dependent on three main factors: safety,

performance, and durability. Safety was ensured through the selection of materials that would be able to

handle significant loads and stresses well outside of expected typical operating conditions in order to

account for unforeseen circumstances. Performance is affected by the magnetic properties, physical

dimensions, and configuration of the objects in space, which are all limited and defined by material

selection. Finally, materials for each part were selected to best fit their exposure to stresses and conditions

so as to ensure durability. Below is a list of the materials selection for each of the main components:

● Stator - The maximum diameter stator available was chosen due to its direct relationship with

torque provided by the motor. The stator consists of layers of sheet metal with a powder coating

in order to protect wire wrappings from wear and tear.

● Wire - Enamel-coated copper wire was chosen in order to prevent it from puncturing the coating

on the stator. Thin wire allowed for easier winding of the motor with a higher number of turns to

increase motor torque. The benefits of a smaller diameter were balanced with the added internal

resistance of more turns to find an optimal value.

● Magnets - The magnets with the highest available strength (N52) as well as high heat resistance

were chosen in order to maximize motor torque and to provide a failsafe to prevent melting even

if overheating due to friction induced by a foreign object were introduced into the motor internals.

● Rotor - Low carbon, mild, seamless steel was chosen for the rotor because its material does not

greatly affect the motor performance. It was most important that it has a relatively smooth surface

so as to allow for easy mounting of the magnets. A large outer diameter was chosen for structural

integrity and better torque transmission.

● Endcaps - The endcaps were manufactured using rapid prototyping technology because they had

some intricate features. Their strength was less of an issue which allowed for low fidelity 3D-

Printing using basic material.

● Bearings - Double sealed bearings were chosen so as to completely prevent sand, water, or other

foreign material from entering the bearing and interfering with motion. The bearings are steel ball

bearings which have a strength far above the expected stresses in their current configuration.

● Shaft - Aluminum 6061 was chosen for the shaft in order to maximize strength, minimize weight,

and prevent interference with the electromagnetic internals of the stator-magnet-motor

configuration.

● Magnet Glue - An epoxy with operable temperatures from -65 to 350 deg. F and high strength

was chosen in order to keep the magnets in place as they operate within strong magnetic fields

and various temperatures.

● Battery Platform - Aluminum 6061 was also chosen for the battery platform in order to

maximize strength while minimizing added weight.

● 22 AWG Wire - For the buttons and arduino, 22 gauge copper wire is used as it is appropriate for

the levels of current and voltage being passed between the controls and the arduino

● 10 AWG Wire - For the ESC’s and the motors, the level of current required a larger gauge wire,

and thus 10 gauge copper wire was selected

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MANUFACTURING PROCESS

As was stated prior, the manufacturing process began during late winter, and required trial and

error to finally arrive at a satisfactory product.

The stator and wire were the first materials delivered and since wrapping the stator by hand was

considered perhaps the toughest fabrication aspect, this was the first manufacturing aspect attempted by

our group. Initially, our group selected a wire which had a diameter of 0.0253” (22 gauge), but this

proved to be too thick, because the amount of wire turns around each stator leg were a significant fraction

of what we had intended and this would’ve greatly jeopardized the torque output. Thinking fast, the

student in charge of hand-wrapping the stator found a new batch of wire with a diameter of 0.008” (32

gauge) and this proved to effectively wrap around the stator to meet the design specifications. The

thinness of the wire and the exactness necessary for wrapping required a patient and detail-oriented

attitude. Considering the wire broke halfway through the wrapping process twice, a total of seven hours

were spent until completion. Typically, stators are wrapped by a machine, but due to the limited budget of

our project, such a method proved to be too costly. The hand wrapping of the stator proved to be one of

the finest feats of our project fabrication.

Within this time, one foot of rotor tubing was delivered, and so other members of our group

picked up this hunk of steel and headed to the machine shop. First, the proper lengths of the rotors were

marked (1.5 inches), and a cross-sectional cut was made through the steel tubing using a bandsaw. The

length of the rotor was fined tuned using a lathe, which slowly chiseled away the steel millimeter by

millimeter until the proper design lengths were achieved. Since our kit requires two rotors, this process

was completed for two lengths of steel tubing. For the first attempt, our group actually adjusted the inner

diameter of the steel tubing using a lathe, but later, as we were about to glue the magnets to the rotor, this

initial inner diameter caused a large air-gap between the magnets and stator. Hence, we had to return to

the machine shop and use our excess steel tubing to cut new steel-tubing lengths without adjusting the

inner diameter, since the original inner diameter later had the most optimal air gap given the

circumstances.

Our designed center shaft, as detailed above, has length-dependent diameter values. Again, since

our kit has two motors, two center shafts were necessary to fabricate. The 18 inch aluminum beam was

delivered, and our group picked it up and headed back to the machine shop to complete the fabrication of

this part. The designed length of the center shafts were 6’’, and so a bandsaw was initially used to cross-

sectionally cut the shafts to their general length. The initial diameter of the aluminum beam was ¾ inch,

and our design required a smaller diameter towards the outer ends of the beam. The proper adjustments of

the beam diameters were completed using a lathe, which again shaved off excess diameter of the

aluminum beam until the proper diameter was achieved for the given beam section. A hole was drilled

through half of the center shaft using a drill press to create an escape route for the stator magnet wire. The

inside bolt was also drilled out to allow the wires to pass through. Wires were then insulated to prevent

any abrasion on the wire.

Once the rotors were cut to the correct length and the magnets were received the gluing process

began. The spacing layout for the magnets were found using a rotor calculator which can be found in the

references section. This application offered a detailed image of the magnet layout. This image was then

set to the exact inner diameter of the rotors. The printed layout was then used to align the magnets as they

were attached one by one. The magnet layout can be seen in Figure (7):

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Figure (7): Magnet/Rotor Layout

As the magnets were attached, a 72-hour high strength epoxy was applied. The same epoxy was later used

to glue the stator to the center shaft for each of the motors. The stator was clamped down and one side

was sealed with tape. The mid section of the center shaft was designed with a diameter about 2 mm less

than the stator so that a 1 mm gap existed between the surface of the stator and center shaft. This left

room for the epoxy to fit while gluing. The center shaft and inside of the stator were then coated with the

epoxy. Finally the center shaft was inserted into the stator and the excess epoxy was removed.

The endcaps were designed using a 3D drawing software, Solid Edge, and the completed designs

were saved as a .stl file onto an SD card. This SD card was then taken to Rutgers Makerspace, which is a

free additive printing (3D printing) service ran by Rutgers students. The process was relatively simple as

one of our group members handed the SD card to a Makerspace volunteer and showed which .stl files

needed printing. There were four parts total which were manufactured since one endcap is composed of

two parts that are screwed into both sides of the rotor. Since there are two rotors, two endcaps were

needed and hence four parts total were printed. The manufacturing of the first part ran smoothly, but the

machine broke after this and delayed the printing of the other parts. The final endcap products were

accurate and attached by bolt into the rotors with ease.

The manufacturing of the control system took place gradually over a few weeks. It was a learning

process, as our group had never really worked with electronics for a project of this level of complexity

before. It was through construction and experimentation that we gradually learned of the various

components and procedures necessary to construct our circuit. Wiring the components in the circuit was

difficult at first, but gradually became easier with practice.

MANUFACTURING/ASSEMBLY OVERVIEW

The assembly of all of the motor components is straightforward and can be completed with

simple hand tools. An exploded view of the motor is shown in Figure (8):

Figure (8): Exploded views of Motor Assembly

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As you have read, our product is very convenient for the user because it requires minimal

mechanical knowledge and only simple hand tools in order to install at home. The shaft of the mechanical

wheelchair is simply removed by unscrewing the bolts on either end. Our product is then inserted where

the old shaft once was and the included bolts are used to screw into the shaft. Holes were drilled through

the rim of the wheelchair wheel, the only modification to the wheelchair necessary. From there, the wires

need only to be inserted into their respective connections as will be shown in the instruction manual.

This simplicity on the end-user side is achieved through careful and precise manufacturing and

assembly of the parts prior to their packaging and shipping for delivery to the customer. There are three

main groups of techniques employed to completely construct the manual-to-electric wheelchair

conversion kit:

1. The metal pieces including the shafts, rotors, and battery platform are machined.

2. The endcaps are 3D-Printed using rapid prototyping machines.

3. The stator winding, circuitry, and connections are constructed by hand (and later, using precise

machines) in order to ensure accuracy.

The endcaps are unique in that they have depressions designed as features which allow the motor

internals to fit into them snugly. The shafts have varying diameters so as to accommodate the pieces that

fit onto each section and to prevent them from moving axially along the rod. Finally, the shafts were

designed to fit perfectly into the existing standardized American wheelchair structure to optimize

compatibility and versatility of our product.

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OPERATION

Our product offers a very simple control system that requires very little movement. This is a

result of our solution to the original problem; to provide a safer, cheaper and easier alternative to the

current market for similar products. One would operate the wheelchair using the four button control

system that is easily connectable to either wheelchair armrest. This allows the user to gently rest their arm

down and use only one or two fingers to move. The up arrow moves the chair forward, while the down

arrow moves it in reverse. When the user hits left or right on the buttons, only one wheel motor is

activated which turns the chair in the respective direction. If the user would like the wheelchair to be

folded for easy transportation or storage, the batteries and battery platform could be removed, and the

wheelchair will fold according to its original design.

As stated before, the design of our product was intended for users confined to a wheelchair with

limited mobility. Our product is targeted to help those who do not have enough strength or mobility to

push the wheelchair on their own. The specific targeted users for our product are patients with arthritis,

scoliosis, or other mobility limiting diseases, as well as the elderly who simply do not have the strength to

operate manual wheelchair. Ideally, our product would be sold to hospitals, or assisted living

communities; along with individual clients.

Some constraints that we may run into when trying to market this product is a size constraint. The

addition of the motors to each wheel makes the wheelchair about 3.5 inches wider overall. This can make

the chair difficult to maneuver through doorways and hallways if those passages are too narrow. Another

size constraint that we would experience selling our product is that not every manual wheelchair is the

exact same size, or has a frame that would match the connections designed for our product. When

designing we tried to choose a wheelchair that best represents the norm, however, some adjustments may

have to be made to the wheelchair kit depending on the client. A third and final size constraint that our

product may experience is a user weight limit. Our original designs were intended for patients over 200

pounds. Due to size limitations when purchasing the stators, the maximum weight the wheelchair can

operate with is 130-150 pounds. This causes problems when a heavier client wants our product. A heavier

client would cause the need of a larger stator therefore, larger motors. This may interfere with the

wheelchairs ability to enter doorways even more than the current design.

Other constraints that would limit the success of our product are geological and environmental

constraints. A client that lives in a very mountainous region may have difficulty operating our product. A

steep incline required more torque from the motors, in combination with a heavier user can cause the

product to fail. Steep hilly areas can also be a safety hazard if the wheelchair gains too much speed. In

addition, a user that lives in a very wet and rainy region may also experience problems. Slippery

conditions can create a safety issue with the wheelchair sliding.

TESTING

Our testing procedure for the control system involved an investigation into the proper frequency

inputs requisite to produce the desired current and torque. We also needed to ensure that the buttons,

when pressed, would produce the intended effect.

Simultaneously, we tested the mechanical interactions taking place inside the motor assembly.

We made sure that the rotational motion proceeded smoothly and had different group members watch

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different components during operation in order to quickly spot and resolve any unwanted contact or

interference.

At the time of writing this report, the entire wheelchair assembly with the control system and

motor functioning together has not been tested in a practical situation where the wheelchair would move.

Because the control system and motor work individually, however, we are confident that the system will

be ready for demonstration on Rutgers Day.

WEAKNESSES/UNRESOLVED PROBLEMS

While fabricating our motors, we ran into a few problems that had to ultimately go unresolved.

The first issue was that stator was too small. For our project, we originally tried to get a stator that would

be larger so that it would provide more torque. Additionally, if we had a larger stator, winding it with the

copper wire would have been much easier. Due to the limited number of vendors that supply stators, we

had to go with a stator with an outer diameter of 53mm. A larger stator would also have been able to

handle turns of larger gauge wire, which would be able to handle higher levels of current to reliably

produce more torque on the chair. A second issue that ended up going unresolved was the fact that the

steel tubing we used for the rotors had an inner diameter that was slightly too large. When designing our

motor, we strived for a 0.5 mm air gap between the stator and the rotor. This air gap would have provided

a stronger force for the motor due to to the reduced distance between the magnetic pole and the magnet.

Unfortunately, the rotor we used had an air gap of about 2.5 mm which reduces the magnetic force

between the magnets and the wire. Another unresolved issue that had arisen resulted from our completion

of gluing the stators to the center shaft. Slight error occurred while gluing since the stator was attached

off-center, which causes a slight wobble during rotation. This also creates an unevenly distributed

magnetic pull.

The full original concept for our control system was not realized due to time constraints and the

steep learning curve for the electronic components involved. We originally planned to install a battery

monitor using zener diodes and LED’s, but this feature had to be cut in the final prototype. The monitor

would be a useful feature for the chair to possess, and thus would be one of the first priorities for further

development. Multiple speed settings were also planned for the final product, but could not be

implemented due to time constraints. Some additional electronic components and greater programming

expertise could be used to implement this feature. The last issue we encountered was with the power

supply. Originally, we intended on using a 12V lithium ion battery for each rotor but making these

batteries compatible with the ESC’s and the system as a whole would have required more time, learning,

and resources. The ESC’s are designed to work with small lithium polymer batteries, and thus, for the

purposes of demonstration during our presentation, these smaller LiPo batteries will be used. They can

deliver the necessary amperage for a limited period of time, which will be enough for our presentation.

CONCLUSION

This project provides a cheaper alternative for a power wheelchair. Given the fact that powered

wheelchairs can cost thousands of dollars, our project already trumps the affordability of these products as

only $716.79 was spent. For future product development a few changes could be made. For one, the

stator and rotor sizes would probably perform better if they were larger. The materials we used to make

these motors will work for a weight of 130-180 lbs. Any heavier and the torque from the motors will not

be enough to turn the wheels. By using a larger stator and larger rotor, we would have more room for

larger magnets. The result would be a stronger magnetic force and therefore more torque. This would

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however require more power to drive the motor. Now knowing the amounts of materials needed for

construction, we could be more efficient with budget to delegate for other use (motor materials, higher-

quality electronics components). The only part that would be troublesome to duplicate would be the

wrapped stators. This could be done by a machine but for the purpose of our project we did it manually. If

this were to be made available for commercial use, a stator wrapping machine would be necessary.

REFERENCES

● http://www.instructables.com/idReferences:/Make-Your-Own-Miniature-Electric-Hub-Motor/

● http://www.instructables.com/id/ESC-Programming-on-Arduino-Hobbyking-ESC/

● https://dronesandrovs.wordpress.com/2012/11/24/how-to-control-a-brushless-motor-esc-with-

arduino/

● Koontz, A. (2005). A kinetic analysis of manual wheelchair propulsion during start-up on select

indoor and outdoor surfaces. Journal of Rehabilitation Research and Development,42(4), 447-

458.

● http://www.gobrushless.com/shop/index.php?app=ccp0&ns=display&ref=rotorcalc&Stator_dia=

53&Rotor_dia=63.5&Mag_width=6.35&Mag_thick=2.54&num_holes=0&FS_airgap=4.2891&di

m_holes=0&num_poles=14&submit=Calculate+Now