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ELB Design Review

Jed Storey, MIT „13

Franco Montalvo, MIT „13

Last updated: 5/23/2010

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Table of Contents

Overview: p.2

Foreword: p.2

I. Longboards: p.3

II. Hub Motor Design: p.4-17

III. Motor Controllers: p.18-19

IV. Batteries: p.19-22

V. Deck: p.22

VI. Battery Box: p.22-23

VII. Trucks: p.23-24

VIII. Radio Control: p.25

IX. Safety: p.25-26

X. Goals: p.26

XI. Sponsors: p.26

Appendix: p.27

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Overall Concept

The 4WD electric longboard was inspired by the BWD Scooter, which was designed and

built over the summer of 2007 at the Edgerton Center by a team of high school students under

the guidance of Shane Colton: http://web.mit.edu/first/scooter/ . Longboards are simply a

variation of skateboards, which are made up of a deck (wooden plank to stand on), two trucks (to

support the deck off the ground), and four wheels (see Section I). The plan is to have a 3-phase

brushless DC (BLDC) electric hub motor in every wheel of a longboard (see Section II). The

four motors will be powered by two of Shane‟s 3PH Duo controllers (see Section III) and lithium

polymer batteries (see Section IV). Control will be accomplished via a handheld, wireless

controller similar to a 1 joystick radio control (R/C) transmitter (see Section VIII). Safety is also

a key concern and will be discussed in detail in Section IX. The goals of the electric longboard

project are discussed in Section X.

Foreword/Notes/Need Suggestions

I need advice on:

II.2.1.i, page 3: I need ideas on how to bore out a precise inner diameter of my wheels to

create tires. See the referenced section for why this might be a difficult problem. (I got

one possible solution from Steve Banzaert, which is discussed in that section).

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I. Longboards

1.1. General Longboard

Unlike skateboards, longboards are typically tailless, or have only one tail, and

are therefore directional. Although the name implies that it is a longer board, it is not

necessarily the case. The term longboard refers to the shape of the board more than

anything else. These types of boards, although able to be use for tricks, are meant more

for cruising, high speeds, and carving/sliding into turns. For this reason, their decks are

usually more flexible than regular mainstream skateboard decks, have wider more

sensitive trucks for turning and stability at higher speeds, and much larger, softer wheels

for comfort while riding. Larger wheels are also safer since they are less likely to be

jammed by a small pebble or debris on the road. Longboards vary depending on the

requirements of the user; if they are meant for commuting, therefore requiring stability

and speed with rare turning, an optimal board would have a minimal amount of flex,

extremely soft and large wheels (most likely above 80 millimeters in diameter, below 70a

in standard skate-wheel polyurethane hardness grade, a rough surface for grip when

riding at high speeds), and stiff trucks (to support the weight of the rider).

1.2. Powered Longboard

To extend the example of a longboard meant for commuting from the previous

section to a powered longboard, it would also be a great idea to have the board lifted

higher off of the ground by either using risers, having larger wheels, or flexing the deck

upwards to allow for more ground clearance under the battery box (see Section IV).

Risers seem like the most implementable of these three.

Fig. 1: An example of a riser. This one is from my old mountain board. Note the many screw holes for the various types of mounting

conventions.

Risers are pieces of plastic, the size of the footprint of a truck, that get screwed

between the trucks and the board. The problem with risers on regular longboards is that

the rider experiences discomfort from having to crouch and push on the ground to propel

the board. Because the board will be powered, the main issue with being higher off the

ground is eliminated.

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II. Hub Motor Design

2.1. Restrictions

i. Wheel sizing

The limiting factor for the size of the hub motors is the size of the longboard

wheels because they must fit inside the wheels. Longboard wheels are made of

plastic with varying grades of hardness depending on the style and preferences of

the rider. They are typically anywhere between 70-100mm in diameter.

Fig. 2: Examples of typical longboard wheels.

The idea is to turn out (using a boring bar) the inner plastic, resulting in a tire for the

rotor to glue into. Assuming at least 5mm of tread, 5mm for a rotor, 3mm for

magnets, and a .5mm airgap, 27mm (13.5*2) off of the diameter of the wheel is the

maximum possible diameter for the stator. The diameter of the stator needs to be as

large as possible for voltage and rotational speed considerations (see Section

II.2.3.ii). As a result, the largest possible longboard wheels are required. The largest

ones found were 110mm (~4.3 inches) in diameter, 63mm wide wheels, and four

were purchased.

Fig.3: One of the purchased wheels.

These are hard plastic hubs coated in rubber, instead of solid plastic like most

longboard wheels. These wheels are relatively hard; they didn‟t come with a

manufacturers hardness rating, but I would estimate they are around 90a, which is

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higher than ideal (see Section I.1.1.i). An inner diameter of 82.5mm will be turned

out, including the entire 60mm inner hub (see Fig.3). However, the fact that it is

rubber poses a problem with the lathing idea. Without any inside support, the lathe

clamps may deform the rubber. It turns out that there is inside support, though. The

plastic hub actually has a ring around it that extends to approximately 90mm in

diameter and 40mm wide that is hidden by the rubber. I found it by poking a very

thin wire into the rubber tread and measuring where it stopped. Thus, I believe these

wheels can be bored out on a lathe. Steve Banzaert had a pretty clever idea on how

to do this: Loosely clamp the wheel in the lathe chuck and use a giant centering

cone to square it up. Then tighten the chuck teeth down onto the wheel and start

boring. Other ideas on how to bore out a precise inner diameter are welcome.

ii. Mountain board wheel considerations

Mountain board wheels were also considered as candidates for the wheels. The

advantages to mountain board wheels are: they are larger in diameter (8 inches on

average), possibly resulting in a larger stator, and they have air filled tires, resulting

in a smoother ride.

Fig.4: Typical mountain board wheel.

Unfortunately, they will not work, and the above picture shows why. The three bolts

shown in the middle of the blue hub hold the two halves of the hub together. They

lie in the diameter that would need to be bored out. So why not make a single piece

hub? Because having a hub that can split into two halves allows for the tires/tubes to

be replaced. Without a split hub, tube replacement would be impossible. Because of

the thickness of the tires/tubes, there is not enough room to make a split hub with

screws out farther than the minimum boring diameter, either. Thus, mountain board

wheels were ruled out.

There is another reason that mountain board wheels were ruled out that is not

apparent in the above picture. Inflatable tires require a fill valve, which must poke

through the inside of the hub. Since these hub motors will fill the entire hub, having

a fill valve would be impossible.

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iii. Stator sizing

Stators are stacked laminations of very thin, stamped silicon steel. The stators for

the ELB were difficult to find. The BWD scooter had the luxury of donated, custom

cut laminations. Due to expense, this was not an option, so I turned to harvesting.

Harvesting stators is the act of searching through various sorts of scrapped

machines for stators approximately the size you need. After you find them, you strip

them of the windings, carefully take them off whatever mount they are on, and

combine or cut them to get the required length. Standing copiers, treadmills, power-

tools, car/motorcycle starters, etc are all good stator sources. The size and shape of

the wheels put a cap on the size of the stator at about 70mm. Copiers typically have

50 or 70mm stators, with 50mm being most common (and too small, unfortunately).

I felt it would have been a waste of time trying to tracking down tons of scrapped

copiers in order to find the ones with identical 70mm stators. I spent many hours

looking online for other sources of stators. I found many brushless motors on the

American Science and Surplus website, a cheap surplus industrial parts website, so I

bought a few to examine. One of them has 15mm of 70mm, 12 tooth (12t) stator

laminations. Success. I was tipped off about another stator that might work on

GoBrushless.com (GB).

Fig. 5: The GB 18t, 65mm stator. It is made of .2mm laminations and is 34mm long. Notice the green epoxy coating. I have

no idea what the notch is for - any thoughts on this would be welcome.

It is an 18t, 65mm stator that they sell in various lengths. It also comes with a thick

epoxy coating to electrically isolate the steel from the magnet wire, which is a major

plus (the 70mm has a flimsy plastic shell). The 18t allows for more flexibility when

it comes to magnets. It also turns out that the American Science and Surplus stators

are more expensive per unit length. However, the GB stator is slightly smaller in

diameter.

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Fig. 6: Top: the 70mm stator with 1 tooth unwound. Bottom right: the 65mm stator. Bottom left: the wheel.

After weighing the advantages and disadvantages, (and doing some calculations-

see Section II.2.3) I decided to purchase four 34mm long GB stators.

More recent issues: At the time of this most recent updating, 3 of the purchased

stators are still on backorder. That brings the lead time up to 4 months, which is

simply ridiculous. This is the last time I buy outsourced stators that aren‟t already in

stock (make sure you email the stock provider for confirmation of stock status…GB

didn‟t warn me about them being out-of-stock). Another problem I‟ve had with

these stators is the epoxy coating. The epoxy at the heads of some of the teeth was

thin and proceeded to chip off upon winding. While not too detrimental, it was

annoying to have to find all the chips/weak spots and 5-minute epoxy them.

2.2. Pre-design education

I knew a little about BLDC motors and how they work, but nowhere near enough to

build a basic one, let alone a hub motor. Thus, I needed to learn a lot about

brushless motor theory before I could start designing the motors. Shane helped a lot

with this process. I do not think it is necessary for me to recount everything I

learned due to length considerations, and because I would basically be paraphrasing

my resources. I will, however, mention my resources.

i. Online resources

Extensive time was spent reading up on the BWD build, as well as Charles

Guan‟s scooter builds: http://www.etotheipiplusone.net/ ,

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

I got a general since of how brushless hub motors work, and how to build one.

ii. Electric Motors and Drives by Austin Hughes

At the suggestion of Shane, I purchased this book. It has been an incredible

resource for basic brushless motor theory. I strongly suggest buying it.

iii. Shane Colton‟s blog: http://scolton.blogspot.com/

An amazing resource for everything electric motor.

2.3. Initial calculations

I did a few initial calculations to help me get a general sense of the motors. The

characteristics I was trying to set were: maximum current, required battery voltage,

number of turns, strength of the magnets, and length of the stator. I followed the

same formulas the BWD scooter team used,

http://web.mit.edu/first/scooter/motormath.pdf, to get an estimate of torque and

maximum speed. While I understood the theory, this was a very confusing process

for me, as everything intertwined and there are multiple ways to influence speed

and torque. As a general guideline, I was shooting for approximately the same

torque and top speed as the BWD scooter. My method was largely educated

guessing; I would pick a combination of the previously mentioned traits, run the

calculations, and then see which ones I could vary to get a more favorable outcome.

At the time I was doing this, I was still undecided between the 70mm and 65mm

stators, so I did calculations for both.

i. Torque

The BWD has a total torque of approximately 10N, so that was my goal. Torque

is proportional to the current you run the motor at, the length of the stator, and the

number of windings. The equation for torque is as follows:

𝑇 = 𝑟 𝐼 𝑊 𝑙 2 𝐵 , where:

𝑇 = 𝑡𝑜𝑟𝑞𝑢𝑒 𝑁𝑚

𝑟 = 𝑟𝑎𝑑𝑖𝑢𝑠 𝑡𝑜 𝑎𝑖𝑟𝑔𝑎𝑝 𝑚

𝐼 = 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝐴

𝑊 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔𝑠

𝑙 = 𝑙𝑒𝑛𝑔𝑡𝑕 𝑜𝑓 𝑡𝑒𝑒𝑡𝑕 (𝑚)

𝐵 = 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑓𝑖𝑒𝑙𝑑 (𝑇𝑒𝑠𝑙𝑎)

The “2” multiplier in the equation comes from the fact that each tooth has two sides.

W is calculated by multiplying the number of windings/tooth by the number of

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teeth/phase by the number of phases active at any given time (which is 2). I

estimated the magnetic field at the airgap would be about 1T (it turns out that this

estimate was right on-see Section II.2.4.ii). Here is an example calculation for the

34mm long, 65mm diameter, 18t stator:

𝑇 = . 0325𝑚 10𝐴 15 ∗ 6 ∗ 2 = 180 . 034𝑚 2 1𝑇 ≈ 4𝑁𝑚

That is the torque for a single motor. However, it does not take into account

electrical, magnetic, or mechanical losses, which greatly decrease this value. I

assumed the losses would approximately halve this value, and FEMM (see Section

II.2.4.ii) supports my estimate.

ii. Top speed

BackEMF is generated when the magnets pass over the coils, and it counters the

applied voltage. Maximum speed of a motor occurs when the backEMF equals the

applied voltage. BackEMF is proportional to the speed at the airgap, number of

windings, and the length of the stator.

𝑉 = 𝑙 𝑊 𝑆𝑎𝑔 (2 ∗ 𝐵), where

𝑉 = 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = 𝑏𝑎𝑐𝑘𝐸𝑀𝐹 @ 𝑡𝑜𝑝 𝑠𝑝𝑒𝑒𝑑

𝑙 = 𝑙𝑒𝑛𝑔𝑡𝑕 𝑜𝑓 𝑡𝑒𝑒𝑡𝑕 𝑚

𝑊 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔𝑠

𝑆𝑎𝑔 = 𝑎𝑖𝑟𝑔𝑎𝑝 𝑠𝑝𝑒𝑒𝑑 (𝑚)

𝐵 = 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑓𝑖𝑒𝑙𝑑 (𝑇𝑒𝑠𝑙𝑎)

The airgap speed is calculated by multiplying the required ground speed times the

ratio of the airgap diameter to the diameter of the wheels. Thus, the smaller the

diameter of the wheels, the higher the necessary rev/sec, and the higher the

backEMF. This is why having smaller wheels requires operating at a higher voltage.

I wanted a top speed of about 15mph, which translates into about 3.96m/s at the

airgap. Using the same values from the torque calculations above:

𝑉 = . 034𝑚 180 3.96𝑚 𝑠 2 ∗ 1𝑇 = 48𝑉

It seems that a 48V system will work well.

iii. Efficiency and other concerns

48V is boarding on high voltage. I do not think that it is necessarily dangerous,

assuming appropriate safety precautions are taken (see Section IX). Despite this, I

did consider various ways to get the operating voltage down. One way to get the

operating voltage down, and maintain the same speed and torque, would be to

decrease the number of turns/tooth while increasing the current. The problem with

this approach has to do with losses, specifically power lost as heat generated by a

lot of current being forced through the small motor wires. The efficiency of the

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motor would drop as a result. Another issue is that higher current can be just as

dangerous, or more so, as higher voltage (pick your poison). Also, I designed the

motors with Shane‟s controller (see Section III) in mind- pushing more than 30A

per channel makes me uneasy, especially since the controllers will be sealed in a

box (see Section VI), and thus will receive poor cooling. Another way it would be

possible to run at a lower voltage, while maintaining speed and torque, is to

decrease the length of the stator or decrease the strength of the magnetic field. Since

the magnetic field is an intrinsic property of the magnets, that cannot easily be

changed. I say easily because Shane‟s motor controllers are capable of

implementing something called “field weakening”. However, it is potentially

dangerous due to the high currents involved to counter the magnets‟ fields and the

fact that if it messes up while I‟m on the board, the motors could seize up, causing

me to fly. If length of the stator is decreased, torque is decreased, so current would

have to increase; this results in the same problems as decreasing the number of

turns.

Another way that a decrease in voltage might be reasonable is if the diameter of

the wheels is increased. This would decrease acceleration (unless current, and thus

torque, is increased), but increase ground speed for the same airgap speed.

However, I‟ve already shown that mountain board wheels will not work (see

Section II.2.1.ii), and I am not aware of any other possible wheels. Learning how to

cast urethane to make my own tires is not within my grasp time-wise.

There are magnetic losses, too. These result from saturating the steel and eddy

currents. Having the stator made up of laminations attempts to minimize the latter.

Steel saturates around 1.6T; driving the motor substantially above that results in

heat losses. Also, having teeth with smaller cross sectional areas results in

“compressed” flux lines, causing an increase in the magnetic field in that region.

This is another reason to have as large a stator as possible- there is usually less steel

saturation. Knowing these things helped me size the magnets (see Section II.2.6).

iv. Conclusions

These various initial calculations suggested the following motor setup: use the

18t, 34mm long GB stator, 15 windings/tooth, 10A, and a 48V battery system.

2.4. FEMM Analysis

FEMM (Finite Element Method Magnetics) is a program that can be used to

calculate the theoretical torque of a motor design to a relatively good degree of

precision via finite element analysis. It can also calculate many other useful

characteristics, such as magnetic flux density (field). The goal of using FEMM was

to have some reliable data to compare the results of my initial calculations to,

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decide on the number of poles I would use, optimize magnet size, optimize rotor

thickness, and optimize airgap size. Essentially, my method consisted of importing

a DFX file from Solidworks, preparation work in FEMM, assigning various values

to different blocks of the drawing, running the finite element analysis, and checking

the results for magnetic flux density in certain regions, as well as torque. Each

simulation iteration took approximately 30 minutes. I ran a lot (I lost count at 20) of

different simulations before I settled on a motor that I liked.

i. Poles

At this point, I started considering the number of poles I would use. I was still

undecided on which stator I would use, too. The 12t stator is best used with 10 or 14

poles. The 18t stator, however, has many options; 12, 14, 16, and 20 poles work

well depending on the wiring scheme. The number of poles (number of magnets)

can loosely be thought of as the “gearing” of a hub motor- the more poles, the lower

the “gear ratio”. This was another advantage of the 18t stator; I could get a “lower

gear ratio” thanks to the ability to use 20 poles. Note: I acknowledge the fact that

the “gear ratio” analogy is widely disputed.

ii. Results

I ran simulations for both the 12t and 18t stators. There was no appreciable

advantage to either, so for other previously mentioned reasons, I finally settled on

the 18t stator. I settled on the following characteristics in addition to the ones from

2.3.iv: 3mm thick, N40 grade NiFeB magnets, .5mm airgap, 82.5mm rotor, and 20

poles.

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Fig. 7: FEMM output/flux density scale for the chosen motor.

The airgap magnetic field is about 1T, which was my estimate from Section II.2.3.

The magnetic flux density is never above 2.5T, which is good for saturation issues.

The calculated torque is around 3.1Nm, which is very close to what I was shooting

for. I expect the final result to be slightly lower due to mechanical losses.

iii. Recent concerns

I will likely make a 1mm airgap. The reason for the increase is that I am worried

about collisions between the stator and the magnets due to the large impulse forces

that the motors will experience. I am not confident in the polycarbonate hubs being

able to absorb that much force without flex. The rotor‟s inner diameter and outer

diameter will be increased by 1mm. This small change in diameter should not affect

the mounting of the magnets.

2.5. Wire

i. General

Magnet wire is standard gauge copper coated in a tough enamel coating. The

coating prevents the coils from shorting out.

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ii. Wire calculations

I did various calculations to determine gauge and number of possible parallel

coils. The goal was to minimize resistance by packing in as much copper (large

gauge wire or multiple smaller strands in parallel) as possible to the slots between

the teeth of the stator. I started by picking some gauges to test and finding their

diameter and resistance/km. I measured the distance from tooth head to base. I

divided that number by the various diameters and rounded down. That number is

the number of possible wraps on the first layer. The number of layers that can fit is

dependent on how tall the wire stack is when it packs (I calculated the ratios for 2,

3, and 4 layers), and how much room there is. From those numbers, I would

calculate the total possible number of turns that could fit on a tooth. I would then

divide that number by 15 to get the number of possible parallel coils. The more

parallel coils, the less the resistance. Also, the thicker the gauge, the less the

resistance, but less parallel coils. So the goal was to find the wire gauge that

resulted in the lowest net resistance. 16 AWG magnet wire was the result.

Fig. 8: Two spools of Essex magnet wire I bought. Left: 4lbs of 22AWG. Right: 6lbs of 16AWG.

iii. Winding

The motors will be wound AaABbBCcCAaABbBCcC. Each letter stands for a

tooth, where upper case is clockwise and lower case is counterclockwise.

iv. Recent Issues

I attempted to wind a stator with 16AWG wire. I was able to get only 14

turns/tooth. This experience taught me a lot, e.g. it‟s hard to pack turns tightly

enough to match your math. I‟m going to go with 17AWG wire instead. Also, the

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epoxy coating on the teeth chips off pretty easily, resulting in shorted phases. Thus,

I may use an idea developed by the BWD; electrically isolating fiberglass end

laminations to isolate the wire from the sharp ends of the stator.

2.6. Magnets

i. Type and Properties

By far, the most common type of motor magnet is NiFeB, or “rare earth”,

magnets. They have many advantages over iron magnets, including high flux

densities, and they can be made into almost any shape. However, when heated too

high, they can lose their magnetism. While this isn‟t a concern for the current

design, if I were to drive the motors at very high currents, I could conceivably heat

the magnets up enough to cause them to demagnetize. The magnets will be secured

with JB Weld because of its heat resistant properties.

ii. Finding Motor Magnets

Finding arc magnets for a given motor is impossible unless you get them custom

made. That is why the BWD scooter used bar magnets, which are more readily

available. I decided to go with custom arc magnets, though, partially for wow

factor, but mostly because they offer a smoother ride and because they aren‟t too

much more than bar magnets. I purchased 80 arc magnets from

http://www.supermagnetman.net/ .

2.7. Solidworks

i. Final Drawings

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Fig. 9: Final rendering of one of the hub motors. Note: that is not how the actual tread will look.

Fig. 10: Exploded view of final motor design.

In Fig.9, notice that the hubcap has to stick out of the wheel. This is due to the

available bearing sizes and the Hall Board.

ii. Mechanical Components (See Appendix for all CAD drawings)

A. Axles

The axles are steel, slightly less than 3/8” in diameter, 90mm long, and have

3/8-24 threads on ¾” of the ends. They are cast into the trucks (see Section VII).

The axles will thread into the hubs. A flat small flat spot will be ground down on

two sides of the axle to accommodate grub screws from the hub. These grub screws

will prevent the hub from unscrewing from the axle.

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B. Hubs

These are the most complicated part of the wheel assembly. They will be

lathed out of T3-2024 aluminum stock, which was chosen for its high sheer

strength. The stators will be press fit onto the 28mm x 34mm section. The three

phase wires will terminate on the outer-side (where outer-side stands for side away

from the deck) of the stator and will be routed through three small holes in the hub.

The holes extend past and under the inner-side bearing, so the wires end up outside

the wheel. Another method for wire-routing is to flatten a spot on the axle under the

bearing for the wires to go through. However, I didn‟t want to do this because it

weakens the axle. A larger hub and bearings were the results. The hubs also have

small slots for snap rings that act as bearing retainers. Threading the hubs was

difficult. The tap size hole was drilled through the hub. Then a close-axle-fit bit was

picked and the hubs were bored up to ½” from the outer-side. This ½” was then

threaded from the outer-side with a 3/8”-24 tap. This allows for the hub to sheath

over the axle and screw onto the threads.

C. Hall Board

The hall-effect sensor board is a small piece of acyllic that will clamp around

the hub next to the stator. It will hold the 3 hall effect sensors at the correct spacing,

which is

360

# 𝑜𝑓 𝑝𝑜𝑙𝑒 𝑝𝑎𝑖𝑟𝑠 = 10

3= 12𝑜 apart for this motor design. (There are other

possible placements for the sensors, too. If you place each of the three sensors on

the first tooth of each phase (120 electrical degrees apart), it is guaranteed to work

for any motor. They don‟t have to be on a separate board either; instead, they can be

embedded between stator teeth- if you do the math, it turns out that this spacing is

ok, too.) The thickness of this part may shrink - I‟m not sure exactly what thickness

it needs to be. It will likely depend on the size of the hall sensors.

D. Spacers

These will be acrylic or polycarbonate rings that go between the rotor and

hubcaps. They prevent the stator windings from interfering with the bearings. The

outer-side one is wider to accommodate the hall board inside it. The thickness of

these may vary (I just need to build a motor and see how much space I have to work

with).

E. Hub Caps

The hubcaps will be made of two different thicknesses of polycarbonate,

depending on whether it‟s an inner or outer-side hubcap. They will contain the

bearings and bearing seats and will act as shields. A lot of the impact forces will be

transferred to the axles through these. They will also have a groove cut in them that

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will have a 1/16” o-ring seal in it. This ring will make a face-seal with the spacer,

forming a water tight seal.

F. Bearings

The outer-side bearing will be 5/8” ID x 1 3/8” OD. The inner-side bearing

will be 1” ID x 2” OD. The relatively large sizes are a result of the hub having to

sheath around the axle, wire routing, and availability. Thin bearings of the sizes that

I‟m looking for cost upwards of $60 a piece, so I‟m stuck with the thicker bearings.

The large thickness also results in wide hub caps and is partially responsible for the

hub caps extending past the planes of the tire.

G. Bolts/Pins

In one side of the motor, 5 of the 10 holes thought the hubcaps, spacers, and

rotor will be threaded for 4-40 bolts. The other 5 holes will be precision machined

for 1/8” steel pins. Bolts allow for some wiggle room, which is why alignment pins

will supplement them. The bolts will likely be hex head cap screws, and will be

slightly less than half the total width of the motor. There will be 10 screws per

motor, 5 per side, and 10 pins per motor, 5 per side.

H. Rotor

The rotors will be made of low carbon steel, specifically 1020 grade. Low

carbon content is key for magnetic properties.

Fig. 11: The rotor stock I bought. It will have to be bored out and cut to correct dimensions.

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III. Motor Controllers

3.1. General

The ELB was designed with Shane‟s 3PH Duo controller in mind:

http://web.mit.edu/scolton/www/3phduo.pdf . Two will be used, one for the front motors

and one for the rear motors.

3.2. Performance

Depending on the MOSFETs, current sensors, and various other elements of the

motor controller, Shane‟s Duo platform is very versatile. The scooter has IXYS GWM

90A/100V MOSFETs, but the current sensors are limited to 60A. I plan on running at

around 10A/per motor and 48V (so 20A/48V), which are nowhere near the limits of this

controller.

3.3. Recent issues

i. The IXYS GWM MOSFETs used in the original Duo are not longer in stock

anywhere. This leaves two options. 1. Order the minimum quantity (16=$300) and have

to wait 4 months for them to come in. 2. Find a substitution. Shane found a substitution

and designed an adapter board: http://www.irf.com/product-

info/datasheets/data/irf7759l2pbf.pdf . Luckily, they will be able to fit in the same

footprint as the GWM MOSFETs.

Fig. 12: Diagram showing the new FETs (white outlines) fitting in the same footprint as the IXYS ones. Picture courtesy of Shane.

Unluckily, the adapter boards are $30 each, meaning $120 increase in price (two boards

per controller, two controllers = four adapter boards). This isn‟t too bad, though, and is

better than waiting 4+ months for IXYS chips.

Update: The IXYS modules randomly appeared in stock on a website and Shane

bought them up, so now I have the four I need!

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ii. The BWD scooter was recently behaving oddly, possibly due to a component on the

controller board being shaken loose. Regardless of whether or not this is the case,

vibration isolation is a good idea for any electronics system in a high vibration

environment. The challenge is that the heatsinks on the MOSFETs need to be mounted to

a large piece of metal (like the chassis on the scooter) for heat exchange. However, I

can‟t use the battery box because it will be rigidly attached to the deck, and therefore

vibration prone. One possibility is to have the heatsinks attached to loops of copper sheet

that are attached to the skid plate; the loops would act like springs. Another possibility is

to have a finned heatsink on the MOSFET heatsink and the motor controller suspended

from the deck on rubber mounts. Then have heatsinks and small CPU fans throughout the

battery box. The latter is the solution I plan on pursuing.

IV. Batteries

4.1. General Information and Discharging

The batteries will be six 6S1P-5000mAh lithium polymer (LiPo) battery packs.

When referring to individual packs, the “6S” means the number, in this case 6, of lithium

polymer cells in series. Since each cell has a nominal voltage of 3.7V, this translates to

22.2V. The “1P” means the number, in this case 1, of cells in parallel inside the pack. If

this number was, say 2, then there would be 6 sets wired in series of 2 cells in parallel =

12 total cells. The 5000mAh is the total rated capacity of the pack. The 6 packs will be

wired in a 2S3P configuration (2 sets wired in series of 3 in parallel each); this effectively

gives a 12S3P pack, or 44.4V-15000mAh pack.

Fig. 13: Battery pack configuration. The two motor controllers will be wired in parallel to the final terminus shown.

The large capacity is for run time, which I estimate will be 15-20 minutes at full throttle.

Assuming that‟s 15mph, my range between charges should be at least 3 miles.

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Voltage for LiPo packs is a little different than other batteries because LiPo

voltages change a lot as the pack is being discharged. The range for a single cell (1S) is

4.2V at full charge to 3.3V at full discharge; traditionally, 3.7V is used to calculate pack

voltage because it is a midpoint. This means, for a 12S pack like the one I‟m planning on

using, the voltage range will be 50.4V - 39.6V. That range is acceptable. Because of the

massive capacity, the maximum discharge current is, depending on the exact battery

packs I buy, at least 225A (if I buy higher performance batteries, that threshold could go

as high as 450A). However, I don‟t expect to be running at currents anywhere near 225A

(more like 80A max total for all 4 motors).

4.2. Charging - General Safety

Charging LiPo packs is different than other batteries. LiPo‟s must be charged with

a balancer. A balancer is an electronic device that controls how much power each series

cell is getting; another way of thinking about it is that it keeps the voltages of each series

cell the same. The balancer is able to do this because LiPo packs are built with “taps” or

wires connected to each individual series cell.

Fig. 14: Example LiPo pack. These specific taps are “JST-XH” brand.

If a cell were to get “out of balance” (too high or too low voltage) with the other

cells, it could cause a fire. Remember, 1 series cell can contain multiple parallel cells;

however, this does not matter because all the parallel cells in a single series cell must be

at the exact same voltage. LiPo cells must never be over-charged. If they are, they puff up

and then explode. Thus, it is important to always be present while LiPo‟s are charging.

4.3. Charging – ELB

Most chargers can only charge up to 6S. Therefore, I‟m going to have to charge

the two parallel stacks separately (each is a 6S3P, or 22.2V-15000mAh). To accomplish

this, I plan on parallel wiring the charging taps of the three parallel packs.

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Fig. 15: Wiring the taps together of three 6S1P packs to create a 6S3P pack. The same wiring scheme as shown by the brown wires

will be replicated over the six remaining pins. Then the whole process will be repeated for the other 6S3P pack.

HobbyKing sells a 4 X 50W charger/balancer (balancer is built into the charger)

that can charge up to 6S LiPo on each channel. The 50W limits the charging current to

~2A. Charging 15Ah at a rate of 2A will take about 7.5 hours.

I also plan on being able to charge the batteries without having to remove the skid

plate. This means that the two final male taps will be mounted in an aluminum end plate

of the battery box. Having external charging taps also allows for easy connection of cell

monitoring equipment (see next section).

4.4. More Safety

I‟ve already mentioned why it is important to balance and watch the cells/packs

while they are charging, but it is also important to watch the packs while they are

discharging. While the cells won‟t come out of balance over a single discharge cycle,

LiPo‟s can be over-discharged. I‟m not talking about discharge rate (I already mentioned

that I won‟t be running at currents anywhere near that threshold), but the “depth” of the

cycle, or how far the pack is discharged. A LiPo is over-discharged when the individual

series cells drop below 3.3V. This can cause permanent damage to the LiPo cells, shorten

the lifespan of the pack, or possibly (if the discharge rate is high enough) cause

catastrophic/explosive failure/detonation. Thus, a cell voltage monitor is a must. One type

of cell voltage monitor plugs into the taps of a pack and displays the voltages of the all of

the cells. Another type is just wired into the main battery power lines and indicates when

the pack voltage drops below 3.3V * (# of series cells); this only works for well balanced

packs. Another type is like the first one, in that it plugs into the taps, but like the second

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one in that it is just and indicator, i.e. an indicator light turns on whenever a cell drops

below a certain voltage (3.3V usually). Any of these types are acceptable for my

application. I purchased one of the first (for occasionally check-ups) and two of the

second (for continuous monitoring).

V. Deck

The deck was salvaged from an old mountain board I have.

Fig. 16: The deck.

It is made of formed/shaped plywood. The black coloring is from grip tape. The eight

large holes were for bindings - these will have to be filled-in/waterproofed. The eight

smaller holes are for mounting the trucks.

VI. Battery Box

6.1. General

The purpose of the battery box is to create a waterproof environment for the lithium

polymer batteries, wiring, safety equipment, and motor controllers. It will be constructed

out of aluminum and a steel skid plate, then screwed/epoxied to the deck.

6.2. Specifications

The curve of the deck means that, in order to maximize space usage and ground

clearance, the battery box will have to follow the curve of the deck.

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Fig. 17: Rendering of an earlier battery box design. The red boxes represent the volume of the 6 LiPo packs. The green boxes

represent the motor controllers. Note: The curve, length, and width of this deck are identical to the real one.

The largest components (the six LiPo‟s) will be mounted in the middle of the box to take

advantage of the larger volume there. The aluminum side pieces will be milled from 3”

wide x ¼” thick 6061 stock. They will be screwed/epoxied together. Then they will be

bolted to the deck, and the gap between the sides and the deck will be sealed with

Permatex Blue RTV silicone gasket sealant or Plumber‟s Goop, both favorites of ROV

hobbyists. The forces will be greatest on the end pieces; specifically, the force from the

arch of the deck attempting to compress will transfer into a sheer force directly into the

bolts holding the end pieces to the deck. Therefore, as many high-strength steel screws as

can be fit into the length of the end piece will be used. The skid plate will be made of thin

steel and screwed (and countersunk) into the bases of the side pieces. There will be a

neoprene gasket between the skid-plate and sides that provides a watertight seal. Access

will be via unscrewing and taking the skid plate off. There will be holes (not shown)

milled in the side and end pieces that will have charging/balancing taps glued in; these

holes will have water proof covers while not charging. Holes will be drilled in the ends

for the motor wires and Hall Effect sensor wires; these will be sealed around the wires

with one of the above-mentioned sealant products. Holes will also be drilled in the sides

for wires for two giant safety fuses. Cooling fans for air circulation and heatsinks will be

mounted on the interior.

VII. Trucks

7.1. The Problem

The hub motors need to be easily removable from the trucks for servicing.

“Easily” means “requiring only basic hand-tools”. There are two ways to do this: Make

the trucks‟ axles removable, or make the hub that the stator is mounted on removable.

7.2. The Removable Axle Attempt

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We bought some cheap skateboard trucks to test out our removable axle idea on.

The plan was to cut off the old axles flush with the end of the trucks, then bore out and

thread the truck itself. Then a threaded steel axle could be screwed in (and screwed out),

thus making a removable axle. However, we found out that all skate (and reasonably

priced longboard) trucks were made from non-machineable/structural metals (Zinc being

the most common). Therefore, our plan was not viable. The only way to have removable

axles would be to either make or purchase “precision” trucks. Precision trucks are

“precision” machined, rather than cast. They almost always have threaded, removable

axles. A set of these costs anywhere between $270 and $400, depending on brand. This

puts them out of our price range. The other option is to design and machine our own.

While this would be cool, it could take months to design, test, and iterate to find a

suitable and durable truck. The high-grade aluminum alloy stock that would be required

would also be very expensive, especially if the necessary prototype trucks are made.

Therefore, other ideas were pursued.

7.3. Threaded Hubs

Another option to making removable wheels is to have the hub (that the stator is

mounted on) threaded to the axle, thus making the hub removable and leaving the axle

mostly intact. This involves a complicated hub design (see Section II.2.7.ii.B), but it

would still significantly cheaper and easier than custom designed trucks. This solution

also allows the use of some trucks I currently own. The trucks used were salvaged from

an old mountain board I have.

Fig. 18: The mountain board trucks we will be using. Note the brackets on the bottom one, which will require slightly shorter hubs.

The hubs will sheath over the entire axle (the black parts in the above picture), and then

thread onto the position the nuts in the above picture currently occupy.

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VIII. Remote Control System (Franco‟s Contribution)

The radio control system will be based on 2.4 GHz, as opposed to an AM or FM

radio wave pattern. This is because, compared to an analog signal, a digital signal may be

more easily controlled and moderated to manage the output of the system. This will be

necessary in order to weed out any incorrect or harmful commands sent to the system, i.e.

a small electrical shock or having another interfering signal. For example, if we were

using analog radios, the motor would have no choice but to accept any and all commands

it would receive, no matter the source or command, but if we are using a digital radio, we

are able to check the signal for reasonability. This is extremely important on our case

particularly because if any stray signal reached our controller, the rider would be in

danger of experiencing an immediate unwanted movement and would potentially be

harmed. Restrictions on the radio transmitter which will have to be taken into account in

the design are: short range (to minimize sending or receiving stray signals), extremely

simple (one channel), handheld, and possibly tethered to board to avoid loss or theft. At

the moment I am talking with upperclassmen, particularly Shane Colton, in order to learn

more about radio controllers and find the most feasible options for our project.

IX. Safety

9.1. Electronic safety

A contactor is basically a high power relay; when a low power coil is energized, the high

power circuit is completed. The contactor that will be used is an Albright normally open

(NO) SU60. It is rated for 48V and 100A, and will go in-line with the positive main

battery line. It will have a 48V coil (the coil only draws a few mA) so that I can just tap

the main batteries to power the contactor‟s coil circuit, eliminating the need for a

secondary battery pack.

Fig. 19: Albright SU60 contactor.

The contactor coil circuit will consist of a tap from the positive side of the main batteries,

a magnetic safety switch, the contactor coil, and a return tap to the negative side of the

main batteries. The magnetic switch will likely be custom fabricated. It will work by

having a magnet strap on my shoe. When I step on the board, the magnet on my shoe

attracts a magnet in the battery box that is attached to a contact. That contact touches

another contact resulting in a completed coil circuit and a closed contactor. In addition to

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the contactor, fuses will be placed external to the battery box in the positive lines of the

3P pack configurations.

9.2. Investment protection

I will likely incorporate an A-GPS (assisted-GPS) tracking system eventually. It would

have to be an activate-only-when-needed system, which are available.

X. Overall Goals for the ELB

Transport student (Jed) and school materials through campus and Cambridge area

o Handle 300 pounds

o Comfortable /smooth ride

o Transportable (light enough to carry around)

o Reasonable battery life

o Components able to withstand vibration/shock

o Waterproof

o Easy accessibility to components

o Modular

XI. Sponsors

The Edgerton Center: http://web.mit.edu/edgerton/

S-electronics: http://scolton.blogspot.com/

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Appendix Note: Dimensions are in inches.

A. Front Hub

B. Rear Hub

28

C. Inner-side Hubcap

D. Outer-side Hubcap

29

E. Inner-side Spacer

F. Outer-side Spacer

30

G. Hall board

H. Rotor

31

I. Battery Box