Rocket Science

2 months...ouch

2012-01-29T20:09:00.002-05:00

Needless to say (but I will say it anyways), I've been busy. Leading MIT's DBF team has taken up every bit of my spare time since november. Well, almost...

I got the hubcaps and spacers for LiteBoard's hub motors machined:

I did the spacers on the lathe, but the hubcaps I did entirely in the mill. I really like the mill method, rather than going from the lathe to the mill. It took a lot less time. I didn't check one of my drill bits though, and being a student shop, it was damaged, so the tip broke off in my part. I was lucky that it had gone most of the way through before breaking off, though, so I was able to get the tip out. I also decided to try loctite bearing compound this time, so I made the bearing holes slightly oversized.

Even though I haven't been building much, I've been gathering supplies: 2 hobbyking 80A car esc's, 2 more 6S 5000mAh packs (in addition to the 2 I have left over from ELB), all of the metal stock I'll need, 4 1.25"x4" colsons, a massive lipo charger and power supply, a $15 hobbyking 2.4GHz 2 channel radio, etc.

The next machining I'll do is probably coring out the colsons.

Left to do:

core out colsons
machine axle adapters and hubs
wind stators
order rotor plates
find/buy hall effect sensors
assemble motors
test motors/esc's
assemble massive lipo pack (6S4P)
laser cut grip tape design
assemble board
add lights
+more

Wow that's a long list :/ . I hope I have time this semester to work on it. I'll only be taking 3.5 classes to give me time for DBF and this, so we'll see how it goes.

Here we go!

2011-11-14T20:46:00.000-05:00

I’ve been busy, but I’ve somehow found time to work on LiteBoard. Oh right…what is LiteBoard?

Now Presenting LiteBoard:

But first, some history: I want a light*, fast way to get around campus, which is the same premise and idea that evolved into ELB. Unfortunately, ELB turned out to be more of a prototype and a learning experience. Now I want a light*, fast, and polished product to get around campus. Sure, there are commercially (and cheaper) available options, such as a bicycle (can’t pick it up and take it to class…also not original and would get stolen), roller blades (no way…I’d kill myself on those), kick scooters (lammeee), electric scooters (lammeerrr), hub motor scooters (so not original)…but I wanted a longboard. So I came up with the EHB concept, and started acquiring parts for it. But once I did a weight build-up, I realized that EHB wouldn’t actually be much lighter than ELB, so I suspended work on it. I’ve been riding around on the mountainboard I bought for EHB for awhile now…I have to say, it is a great way to get around. It’s just slow: I can only maintain about 10mph.

LiteBoard is the V3 concept that I came up with after completing ELB. V2 is/was EHB, but even though it would address most of the problems ELB has, it didn’t address one very important one: weight. ELB weighs 40lbs. EHB is predicted to weigh 35lbs. Liteboard: 25 lbs.

Not enough time to do a rendering=screen shot.

Lite will be a 2WD hub motor powered electric longboard (actually a longboard this time, and not a mountainboard). Each motor will be ~1100W, producing ~3.9Nm of torque each. Lite’s estimated top speed will be ~25mph. The basis of Lite is this longboard:

Pending cool grip-tape design

Randal II 50deg 180mm trucks, ABEC11 97mm Flywheels, and a bamboo/maple deck I got off of ebay (it was incredibly cheap, yet very high quality. ebay=good source for longboard parts). The deck needed some modifications to use the massive 97mm Flywheels:

Initial cutout lines.

1st try: didn't remove enough.

Second try: Now the wheels don't bite the deck when I turn.

I think it looks awesome...

Why did I need 97mm Flywheels? …

Detailed Motor design:

I couldn't get a good angle of the internals. The axle extension is not round, but keyed to fit with the hub that is pressed into the stator.

Since ELB ended up with about twice as much torque as I’d ever need, I decided to shoot for the same amount of torque/motor for LiteBoard, but cut the number of motors in half. I followed the same design process I used to design ELB's and EHB's motors: Start with simple calculations, move on to SolidWorks CAD drawings, import DXF's into FEMM, modify CAD, repeat...

FEMM pre-diameter shrinking.

The motor underwent one large design change as a result of FEMM. Turns out that pretty much the whole stator was saturated, so I wasting a ton of energy. I shrunk the magnets from 3mm to 2mm thick, and shrank the whole OD of the motor by 4 mm, and only took a .1Nm hit in the process.

Estimated Motor Specs:

Specification	Value
# Phases	3
Power	1100W
Voltage	6S Lipo
Max current	50A+
Torque (FEMM)	3.9Nm
No-load speed	30mph
Stator dia	55mm
Stator length	35mm
Teeth	18
Turns/tooth	4
Rotor OD	70mm
Magnet thickness	2mm
Magnet grade	N40
# Poles	20
Motor width	60mm
Motor OD	100mm

The high current, low voltage method was picked in order to use HobbyKing’s 80A car ESC, which is about 1/3 of the cost of an equivalent Kelly Controller, and about ¼ the size. It also has a sensored option that I plan on using. I will likely get some pretty serious I^2R losses, but I’ll try to keep wires as short as possible and pack the stator with multiple parallel strands of magnet wire (thinking 22Ga). 8 parallel 22ga stands has same resistance/m as one 13ga strand…idk, I’ll see how many strands I can fit.

The stators are Scorpion Power Systems 5535 18T stators. They come in their S5535 motor series, which are about $400 a piece…which is why I bought the stators directly from them again ($140 for both, free DHL express shipping). Yet again, they are very high quality:

I officially love these guys' work.

I decided to bite the bullet and go with BigBlueSaw for waterjetting my rotor plates. Turns out that 11*.125” =35mm to within a fraction of a mm…I got lucky there. 22 plates will run my $250…ouch! But I did the math, and if I spent the amount of time working that it would take me to machine the rotors, I would come out ahead if I just have them cut for me (and waterjetting them here is a no-go…it’s freakin’ expensive).

Batteries: While I thought about trying A123 cylindrical cells for this one (specifically a 7S7P pack made of their 26650 cells), it turns out that it won’t fit under my deck. So I went with lipo again. Specifically, I’m going to buy 2 more 6S 5000mAh 25C packs from HobbyKing, and using the leftover packs from ELB, I’m going to make my own, slim 6S4P pack (22.2V, 20Ah).

Control: At first I’ll use a $15 hobbyking 2Ch. 2.4GHz system. But I’m planning on doing something like this eventually.

Tires: So my last post suggested a Colson 4”x7/8” rubber wheel was my best bet. But that was before I shrunk the OD of the motor from 74mm to 70mm, which puts it out of the range of the 7/8” one. So I’m going to use two 4”x1.25” ones.

From left to right: 4x7/8 turned out, 4x7/8, 4x1.25, 4x1.5,4x2

Note the huge void.

After digging a bit deeper: MORE huge voids.

Next I tried the 1.5" wide one:

Only 1" wide of plastic inside (after turning out visible part of hub). Guess I'll be using the 1.25" wheels (more plastic for press fit).

I was able to cut the rubber on these.

Again: another void...

Here's a table of 4" Colson rubber wheel data for your reference.

Safety: I’ll have the same setup as I did on ELB. A contactor sitting in the main battery line that is actuated by a magnetic reed-switch from a magnet strapped to my shoe. This will act as a fail-proof way to make sure that if I’m not on the board (anymore…), it doesn’t keep going. I’m also planning on wearing a helmet with it.

The ultimate goal is have a very sleek, finished product that achieves all of my criteria (safe, light, reliable, fast, etc…)

Anyways, that’s the plan so far. I should be able to get a lot of it done over IAP…we’ll see.

Oh, and to leave you with a cliffhanger: it’s called “Lite” for more than one reason ;) .

Wheel Study

2011-10-27T20:25:00.000-04:00

I'll explain why I'm studying wheels in my next post. But for now: I studied some wheels.

More precisely, I bought a whole bunch of small rubber-treaded wheels from McMaster so I could measure them (McMaster doesn't have CAD of their wheels, which is annoying). I needed to be able to core out ~73-75mm of the hub.
I bought these: 2337T35 , 2337T42 , 2337T37 , 2829T68, 2829T56 , 2439T42 , 2243T22

All 4in wheels of varying widths and materials. I got them in and measured their hubs and 5 had hubs that were large enough, and only 3 of those were reasonably priced or would work for me (2337T42,2337T35,2829T68). Interestingly, the wider the wheel got, the smaller the diameter of the hub...no idea why. I like the super soft tire (2243T22), but it was really rounded, and I'm planning on putting multiple tires on *ahem* something.

2 of the 3 wheels are mcmaster's "Rubber Wheel" series. They are black phenolic? hubs with black rubber tread. The other one is "Performance Rubber-Tread"...actually Colson Casters! Nylon or polypropylene? hub with grey tread.

The phenolic ones are easy to machine...not nice, but easy. Phenolic is probably the WORST smelling material I've ever tried to cut...mix of shit and rotting vegetation. In fact, I gave up after a few passes. Even though the wheels are a lot cheaper, it isn't worth dieing for...

So I tried the Colson. MUCH nicer. Super easy to cut, and NO smell. Found my wheel!

More EHB stuff

2011-10-01T19:30:00.000-04:00

While I really didn't have the time today, I decided to do a little work on EHB.

I finished designing the battery box:

I used SolidWorks' sheet metal tools to design it. It's contoured to fit my deck. I plan on waterjetting it out of 1/16" 6061 aluminum. The battery packs are 4s1p 5000mAh Zippys.

I also did a pretty detailed mass calculation. It comes out to about 35lbs...which is annoying because I was shooting for < 30lbs. The motors are ~13lbs, and the batteries and battery box are ~5lbs. Unfortunately, the four kelly controllers weigh in at ~7lbs. Add in the deck and the trucks, and I get ~32lbs. Grrr...

So at this point, I have 2 options: Keep the current heavy configuration, or give in and make it 2WD (soo not as cool...). 2WD will save me about 9lbs, which brings the total down to ~26lbs.

I guess another option would be to suspend work on EHB and start LiteBoard. Though that would mean my $500+ investment in EHB parts would just be sitting around until LiteBoard is done...

We'll see what happens. Heck knows, at the rate things are going (I'm swamped), neither may get done by the time I graduate.

Update

2011-09-24T15:06:00.000-04:00

Well...not much has happened. I've been swamped (work, classes, DBF, etc...).

I got my hands on a pair of Ground Industries Bionic Spring trucks...arguably the best trucks out there (too bad GI seems to be going out of business).

I also bought a bunch of metal stock (for hub motors and battery box). EHB will happen eventually...just not as fast as I'd originally hoped...so no, you can't ride it yet.

EHB updates

2011-08-30T11:35:00.000-04:00

I shrunk the innerside bearing in the CAD. I decided that a 1x2x.5 inch bearing is a ~~little~~ lot ridiculous, so I downsized it to a 25x37x7mm bearing (which happens to have about the same load carrying capacity as the 12x28x8 stock bearings). This shrunk the size of the width of the motor by about 4mm, freed up some space around the coils inside, and shaved a few ounces off the estimated weight of the motors, which is hovering right around 1kg each...less than half the weight of ELB's motors (5lbs each!).

Another advantage of shrinking the motor by this much is that it allows me to use MBS Matrix trucks (in addition to the Bionic trucks I currently plan on using). They have the same size axles, except the 1/2" section on the Matrix trucks is 3mm shorter than on the Bionics. If I end up breaking the Bionics, and Ground Industries really does go out of business, then I'll have an alternative. I could also just buy Matrix Wides, which have an axle even longer than the Bionics, but smaller motors are more covert :) . There's actually another difference between the MBS and GI trucks... the GI axles are held to much tighter tolerances than the MBS ones. In fact, much of GI's machining seems to be of higher quality than MBS's. Too bad they seem to be going out of business.

Unlike ELB, I'm trying very hard with EHB to design for flexibility. ELB's motors were designed only to work with the MBS skate-style trucks, only to have polyurethane strip tires (I got lucky and found an alternative), designed never to be opened once they're closed, etc... I'm addressing all of these and other issues with EHB's motors.

Despite the cost, and previous conclusions, I'm leaning towards having the rotors waterjet cut by Big Blue Saw. It'll be about $260 for twelve 1/4" thick rotors, which seems absolutely insane to me. I re-ran the numbers, though, and if I was getting paid for the time I'd spend cutting (along with material and operating costs), it actually about evens out. In other words, waterjetting is expensive unless you have free access to one.

The other large expense will be the motor controllers. I really wish I had time to learn how to design brushless PM motor controllers, but there's no way in heck it'd be cost efficient for me to do so. I'm thinking 4 Kelly KBS controllers, which is ~ $320 with the water proofing option. OUCH.

As you can probably tell, I did a preliminary cost estimate. The total cost for EHB is looking to be about $1600 (not counting labor of course). That's a shit ton of money. I already have $345 invested in it (stators and board).

I had a realization: 2 motors will be a lot cheaper than 4. Probably not half, because the 2 motors will have to be larger, but at least 30% cheaper...I'll keep that in mind for V3. Of course, with two motors, I lose 4WD, which is necessary for the wintery conditions I hoped to drive ELB in and hope to drive EHB in.

Classes start in a week.

Moving forward

2011-08-20T20:35:00.000-04:00

Moving forward with EHB (as I've now decided to call HeavyBoard) will be a much delayed process. While much of the design is done, fabrication can't start until I have some time...ha...ha...hahhaa...hahahahahaha...time... that's a good one. I'm right on the edge of work week, orientation, rush, the start of classes, the start of clubs, and the restart of work (working at the same place I did this summer). So time will be VERY hard to come by...expect delays.

As for ELB, I've decided to fix the 2 major mechanical issues: axle-hub slippage and the risers. This should make it useable as a demonstrator. I might throw a fan or two and some heat-sinks inside the battery box, which might actually make it street ride-able if the steering problem is fixed by new risers. There's another issue though...one that I can't fix: weight. ELB weighs 40 pounds...ugh...lugging it around is a huge pain, so riding it to class is a non-starter. I'll probably just ride my current mountainboard around for now. At this point I should note that the new "HeavyBoard" will actually be lighter than ELB (The "heavy" stands for "heavy duty").

I determined for sure that the risers are the reason the turning sucks. I set the board upside down on a bench and torqued the trucks by hand and could see the rubber risers deflecting by 1/2" + . I bought some stiff plastic ones to replace them...I'll lose all of what vibration damping I had left, but at least I'll be able to steer.

2 more lessons learned:

Wire stretch: make sure that there's enough slack in the motor wires for the trucks to turn for V2. I got lucky this time and there is just barely enough slack for the trucks to turn all the way.
Buy set screws that are harder than my axles. The axles are harder than the set screws...so big surprise they weren't working well. I'm going to flatten some spots on the shaft and get harder steel knurled cone set screws.

Sorry it's fuzzy. You can sorta see the flattened cup.

Even flatter cups.

Random picture of super sketchy charging.

Hall Effect Sensor Placement for Permanent Magnet Brushless DC Motors

2011-08-20T16:08:00.000-04:00

This is a very confusing topic.

I spent a few hours today re-teaching myself the theory for hall effect sensor locations, then even longer trying to come up with a clear way to present it. It was a combination of gathering information from forums and looking back at my old notes (which were derived from Shane's expertise). The goal of this post is to gather all of that information on one webpage and relay it in as clear a format as I can.

Disclaimer: I am not an electrical engineer, so some of this may not be accurate. That being said, I'm 90% confident that it is.

Legend:

edeg : electrical degrees
erot: electrical rotation. 1 erot = 360 edeg
mdeg : mechanical degrees
mrot: mechanical rotation. 1 mrot = 360 mdeg
pp : number of magnet pole pairs . 1 pp = 2 magnets (1 north, 1 south)
s : number of slots (in the stator)

This post will cover how to place hall effect sensors onto 3 phase motors being run by 60 edeg and 120 edeg hall position controllers (motor controllers that expect the hall sensors to be placed 60 and/or 120 edeg apart). While it is feasible to design a motor controller to expect the hall sensors to placed some other number of edeg apart, I have never seen or heard of one (there just aren't any common commercially available ones that accept anything other than 60 and/or 120 edeg hall sensor placement). I believe the reason for this is that it makes the code and following math more complicated, though I could be wrong as I have never designed my own motor controller. I'm only going to cover 3 phase motors in this post because they are the most common type, though the following equations could be extended to any number-of-phase motors with minor modifications.

We need to figure out where to place the 3 hall effect sensors. Let's start with some math:

The first thing you want to find is the number of mdeg per erot . In other words, the number of mechanical degrees the rotor spins to make one complete electrical rotation.

Equation 1: (360 mdeg / pp) = n mdeg per erot =n mdeg per 360 edeg

Note: don't confuse this with the equation mrpm * pp = erpm, which is useful for finding electrical rpm given the mechanical rpm of your motor.

Now, let's say you want to use a motor controller that requires 120 edeg hall effect sensor placement. You need to find the number of mdeg per 120 edeg. So you just divide the above equation by 3.

Equation 2: (360 mdeg / 3*pp) = m mdeg per 120 edeg.

This value, m, gives the minimum number of mdeg that you can space each of the hall effect sensors apart and still achieve 120 edeg spacing.

At this point, you need to choose whether you want to go with mounting the hall effect sensors on an internal board 1 2, outside of the motor (usually on some sort of jig / board (scroll down 2/3 page) positioned so that it can pick up the magnetic flux leaking out of the motor), or inside the stator slots 1 2 3 (note: if you mount them on the side of the coils like I did, make sure you make them as close to the magnets as possible) on the coils. The advantage of the former two options is that the board can be rotated to retard or advance the timing of the motor (adjustable timing). The only way to adjust the timing of the third option is in software.

NOTE: It is VERY important to place the hall effect sensors as precisely as possible. Being off by a few mechanical degrees can put you off by many tens of electrical degrees.

If you want to mount the hall sensors on some sort of jig/board (internal or external), then you're done with the math! The above value, m, gives you the number of mechanical degrees that you should space each hall effect sensors apart (for 3 hall effect sensors, that's a total arc of 2*m mdeg). If m is too small for your liking, you can multiply it by any integer value, e.g. 2, 3, 4 etc..., to get other spacings that will work with 120 edeg motor controllers. (While the hall effect sensors will no longer be exactly 120edeg apart, they will be a multiple of 120 edeg apart, which will work, too).

If you want to mount the hall sensors in the stator slots, then you need to find the number of mdeg per slot:

Equation 3: (360 mdeg / s) = x mdeg per slot

Now you need to multiply m from Eq. 2 by integers until you find an integer, i , that gives you a number divisible by x . m*i gives you the number of mdeg you should space the hall effect sensors apart, and:

Equation 4: ((m*i) / x) = # of slots between hall effect sensors.

There are likely multiple choices for i , especially as the number of slots and poles in a motor increases. As long as the above equations are satisfied, then a motor controller that wants the hall effect sensors 120 edeg apart will work.

_______________________________________________

Now for motor controllers that requires 60 edeg hall effect sensor placement. Equation 1 still applies, but Equation 2 now becomes:

Equation 2': (360 mdeg / 6*pp) = m mdeg per 60 edeg.

This value, m, gives the minimum number of mdeg that you can space each of the hall effect sensors apart and still achieve 60 edeg spacing. Following the logic from the 120 edeg spacing case above, you can multiply m by any integer and still maintain 60 edeg spacing. You can then directly transfer this number of mdeg to a board/jig for mounting the hall sensors to.

Or you can mount the hall sensors in the stator slots. Doing this is identical to the 120 edeg spacing case; Equations 3 and 4 stay the same for this case.

Note: It is interesting, and logical, that you will obtain all of the 120 edeg spacing multiples in the 60 edeg spacing case (120 is a multiple of 60).

***Note2: You have to be careful with your winding scheme. Winding schemes can affect which mdeg spacings work and which don't. Sometimes you'll have to flip a hall sensor over (see Ex 4 below). For simplicity, you should place the hall effect sensors in sensible locations (first one on or between teeth), despite the fact that it often doesn't matter as long as they're spaced correctly (I say "often" because if the sensors are rotated together, you can adjust the timing of the motor, and thus it's performance and characteristics). ***

__________________________________________________

Time for some EXAMPLES!

Ex 1: ELB's motors with internal hall effect sensors mounted on a rotatable "hall board" for 120 edeg controllers.

ELB's motor is a 18 slot, 20 pole brushless outrunner with winding scheme AaABbBCcCAaABbBCcC. At first, I wanted to have the sensors on a hall board that I could rotate about the axle in order to easily adjust the timing. So I did the math:

Equation 1: (360 mdeg / 10 pp) = 36 mdeg per erot =n mdeg per 360 edeg

Equation 2: (360 mdeg / 30) = 12 mdeg per 120 edeg.

So I spaced the hall effect sensors 12 degrees apart, for a total arc of 24 degrees, which made for a nice, small hall board. (I laser etched the degree lines on the hall boards I cut out, which proved to be super nice for aligning the sensors). This worked. Unfortunately, the little hall boards were very flimsy, and I really didn't have enough room for a hall board inside the motor (or outside), so I went to gluing the sensors into the stator slots...see next example.

Ex 2: ELB's motors with internal hall effect sensors glued into the stator slots for 120 edeg controllers.

Time for more math:

Equation 3: (360 mdeg / 18) = 20 mdeg per slot

Equation 4: ((m*i) / x) = ((12* 5 ) / 20) = 3 slots between hall effect sensors.

So the hall effect sensors needed to be spaced 60 mdeg apart (600 edeg), or one every 3 slots. Which is exactly what I did, and it works great. i = 10 also works, and places the hall sensors 120 mdeg apart, or evenly around the stator. In fact, 120 mdeg works for many common slot/pole combinations...so you could just skip all of this math and do it that way.

I will not be doing a 60 edeg spacing controller in-slot sensors example for the ELB. It turns out that the only mdeg hall spacings that work for 60 edeg controllers with a 18s, 20 pole motor are the same as the 120 edeg spacing mdeg multiples. In other words, the hall sensors end up in the same place as with the 120 edeg spacing case. But don't take my word for it, try the math!

Ex 3: EHB's motors with internal hall effect sensors glued into the stator slots for 120 edeg controllers.

EHB's motors will be 12 slot, 14 pole brushless outrunners with winding scheme AacCBbaACcbB.

Equation 1: (360 mdeg / 7 pp) = 51.4 mdeg per 360 edeg

Equation 2: (360 mdeg / 7*3) = 17.14 mdeg per 120 edeg.

Equation 3: (360 mdeg / 12) = 30 mdeg per slot

Equation 4: ((m*i) / x) = ((17.14* 7 ) / 30) = 4 slots between hall effect sensors.

The first multiple i that works is 7. It turns out that the only way to place the sensors in the stator slots when using a motor controller that expects 120 edeg spacing of the sensors, is to place the sensors 120 mdeg apart (spaced equally around the motor).

This is not to say that you couldn't mount the sensors on some sort of jig 17.14 mdeg apart...you can. But if you want the stators in the slots on this type of motor, you have to space them 120 mdeg apart.

Red dots indicate slots that the sensors should be placed in.

Ex 4: EHB's motors with internal hall effect sensors glued into the stator slots for 60 edeg controllers.

Let's take the same motor as in Ex 3, but now the motor controller is expecting 60 edeg hall effect sensor spacing.

Equation 1: (360 mdeg / 7 pp) = 51.4 mdeg per 360 edeg

Equation 2: (360 mdeg / 6*7) = 8.57 mdeg per 60 edeg.

Equation 3: (360 mdeg / 12) = 30 mdeg per slot

Equation 4: ((m*i) / x) = ((8.57* 7 ) / 30) = 2 slots between hall effect sensors.

Now the hall effect sensors can be placed closer together. However, there is a catch. Since the hall sensors are placed like this: A(sensor)ac(sensor)CB(sensor)baACcbB , the second (C-phase) sensor needs to be flipped over because the magnetic field is reversed in that slot because that slot is wound the other direction compared to the first and third sensors' slots. This is why you have to be careful with winding schemes.

Blue dot indicates slot where hall sensor should be flipped over.

________________________________________________

Notes on hooking up the controller to your motor. You will have spend some time testing to see which hall effect sensor corresponds to which phase. And unless you have the ability to modify the code in the motor controller, you will have to play with wire combinations in order to get the correct one. Having a 2 channel scope helps a lot. Since there are many topics on endless-spheres about doing this, and it's dependent on the type of motor you have, I will not go into detail.

The Press

2011-08-17T19:47:00.000-04:00

ELB made it onto Hack-A-Day and MakeZine Blog !! Awesome!

Win!

2011-08-15T21:52:00.001-04:00

The ELB successfully ran for the first time yesterday, after a year and a half of work. WOHOO!!!!

Let me apologize in advance for the shitty pictures...still using my cellphone camera.

But back up a day:

I modified the pistol grip AM r/c car controller I bought off of ebay to be more useful for controlling a longboard. In otherwords, I took it to a bandsaw:

Unscrewed.

Random PCB that didn't have anything connected to it...

The control board.

post-bandsaw action

Post dremel action. That bit is amazing for carving plastic. The white shiny stuff on the right is just bad lighting.

It fits perfectly!

Wired up.


I only needed 5 AA's to power the board, so I cut the other 3 off.

I also cut out small delrin plates to cover up all of the holes in the controller, but didn't put them on yet in case we needed access to the transmitter board (which we did).

I needed to finish pressing on the scooter tires next. I used the exact same process as last time.

I made a boo-boo milling this one, and a approximately 20 degree arc ended up too large in diameter. I just pressed a piece of aluminum in the gap after I put the tire on.

It looks awesome.

Re-wired.

After a few hours of shane being awesome and tinkering with software...

IT RUNS!!!!!!!!!!


All four motors spinning.

The thing is a beast. It sounds almost like a jet turbine spinning up. Unfortunately, there is a lot of mechanical friction, especially in one of the wheels (too tight of a press fit on the large bearing), which results in about 300W total at full speed no load. So the mechanical/electrical/magnetic losses are eating about 2/5 hp at full throttle, which sucks.Probably a combination of machining errors, press-fit errors, and really cruddy bearings (VXB's cheapest bearings).

Lessons learned up to this point:

Scooter wheels make awesome looking tires.
Mechanical accuracy is essential for efficiency.
Shitty bearings are shitty.
Two motors is probably more efficient despite increased current/windings necessary to maintain torque.
Find a better way to have charge leads and battery disconnects outside. The current setup is wayy to sketchy. *This is the part that I am totally at a loss on and could use advice.*

Here is a video of it running:

Things to listen for: the awesome spin-up, the horrible sounds of the shitty bearings, the slipping of the hubs on the axles (the "cogging") , the battery box rattling like a shopping cart.
Things to watch for: The magnets taped to our feet. The really cruddy turning. You can see attempts to turn; the board is even all the way rolled at points, but you can see that the risers are giving way and the trucks aren't torquing.

Video

We definitely got the breaking right, haha. Thanks to Shane for taking video!

Post run.

You can tell by the wear patterns on the tires that the axles are slightly swept.

Results:

The steering sucks....bad. Like 15 ft turning radius bad, which was really disappointing because I'm so used to the awesome trucks on my current longboard/mountainboard thing. Part of it is the trucks, I think. But the majority of the problem is coming from the giant stack of risers you can see in the above picture. The risers are there for 3 reasons.
1. Vibration/shock absorbtion. Mountainboards usually rely on the deck springyness to absorb shocks, but I bolted a 1/4" aluminum battery box to the bottom, which makes it super stiff.
2. 2 of the risers on each side are angled, which gives me an extra 15ish degrees of truck tilt, which should aid steering.
3. They raise the ground clearance under the battery box enough in order to turn. Without them, the battery box will scrape the ground. And I can't mount larger tires without significant ($ and time) overhaul.

The last reason is the most detrimental...it means that I really need that much riser. But why are they a bad thing? Well, when I lean out to turn, the deck rolls, causing a torque on the trucks, which causes them to pivot. The rubber (which makes for a very flimsy/flexible mount to the deck), just flexes in the opposite direction, instead of forcing the trucks around into the turn. I totally should have (but didn't) foreseen that.

Another problem came up: Even with the axle set screws as tight as I could make them, the hubs were still slipping around the axles.

There's also a weird control issue. While the transmitter is transmitting a slight breaking command at the neutral point, and 3 of the 4 motors are listening, one of the motors actually accelerates at neutral. We have no idea why. The same motor also oscillates while it's doing this.

It's also blatantly obvious that it's wired too much for torque and not enough for speed.

Possible Solutions:
1. Not care and move on to V2 - HeavyBoard. Leave ELB 90% complete
2. Fix/finish ELB. This involves machining a stiff plastic riser to replace the rubber ones (which might not even fix the turning issue), grinding flat spots in the axles for the set screws to set, fixing the weird control issues (not sure if possible), implementing the field oriented control, adding a fan circulation system, adding lights, waterproofing...probably another 60 hours worth of work for a board that won't be very comfortable to ride.

While it would be nice to finish ELB, after spending thousands of dollars and so many hundreds of hours working on it, it's probably a better move practically to leave it alone and start V2. On this note, I'll part with ELB for $3000 (controllers and battery charger not included) if anyone is interested, haha.

Lessons learned:

Folded sheet aluminum battery box. Lighter and more flexible.
Don't use skateboard style trucks on a mountainboard.
Avoid massive risers if possible.
Figure out a better hub-to-axle interface.
Follow all lessons learned from ELB on V2.

Christmas in August

2011-08-08T19:57:00.003-04:00

I got some presents (from myself)!

First, a pistol-grip controller:

It's in way to good of shape for what I'm planning on doing to it. I'm planning on gutting it/hacking it apart and using it to replace the Wii Nunchuck controller I put together a few weeks ago. I also got a couple of 2 channel receivers with it...not sure what I'll do with those.

Second, I got the scorpion stators in!

They shipped them DHL Express, which was nice.

They even included fiberglass end plates and coil protector sheets. But how do the stators look?

Pretty good. The laminations are not perfectly aligned, but they're close. The result is a slightly larger maximum diameter of 70.15mm, instead of 70mm. Many of the dimensions were off by about .05mm, which is probably about the accuracy of the stamp tooling.

I believe the wire was installed to keep them from breaking apart during shipment.

Too bad they didn't cut them slightly oversized. I'll have to use the wire protector sheets. The double key slot is a nice feature.

Both 70mm x 50mm stators. I plan on cutting them down to four 70mm x 0.75in .

Not bad for $200 with shipping.

The next step is to update the CAD files with the exact dimensions, but that'll have to wait until after I finish ELB. Only 2 weeks left O.o .

T-minus 2 weeks

2011-08-04T17:24:00.000-04:00

Success!

Turns out that none of the Edgerton Lathes could handle the scooter wheels, either, so I had to mill them.

First step: clamp wheel to table. Second step: gauge (0,0)

3rd step: mill circle at slightly less than press fit radius

They look like mini plastic stators, haha.

4th step: finishing pass at press fit radius. 1.622" in this case. Step 5: repeat.

Then press fit the hubs on. I got lucky and found a PVC fitting that works perfectly. Note: it looks angled because of the camera angle

I press fit the first too a little too far, but luckily the press was strong enough to push all 3 wheels.

Ta-da!

Massive alien hubmotor!

It looks really sick on the board:

The polycarbonate hubcap-inserts make it look even more alien.

I didn't have time to finish milling out the wheels (I got 1/3 of them done), so the other wheels aren't done yet. I should be able to finish the tires sometime next week.

Bummer

2011-07-30T15:35:00.000-04:00

None of the lathes I have access to on the weekends have chuck jaws long enough to hold the scooter wheels I bought. They just pop out when I try to tighten the jaws. The wheels are only 1 inch wide, but they're 4.9" in diameter, so I have to grip them by the reversed side of the jaws, which are <1/2" on most lathes. The Edgerton Shop has some chucks that would work, but it was closed the only Saturday this summer I needed to use it...go figure, right?
Next thing I tried was the laser cutter. My plan was to cut out a hole for the wheel in a piece of plywood, then set the wheel in it, and cut the ID I needed. Well, it was sucking particularly bad today. In the past, I was able to cut through 1/2" plywood in one pass (nice plywood, too). This time I couldn't even cut through 1/4" shitty plywood in one pass. And yes, I cleaned both mirrors and the lenses. So I have no idea what's wrong with it. And as you can imagine, the wheel didn't get perfectly lined up, so when the laser went over it the first time, it didn't cut a concentric circle, so I gave up on that method.
I also maybe could have cut it out on a mill. But (yet another but) the MITERs mill vise only opens to 5", so I couldn't use angle blocks, so I'd have to make some sort of jig. It would probably end up being about as accurate as the laser all said and done...so not worth it at all.
So I went on a hunt for some 4.9in pipe to press the wheel into while I clamp the pipe in the jaws. Turns out you just can't find the stuff. PVC is made in 4" and 6" (I found plenty of both of those). Even Mcmaster doesn't carry plastic pipe 4 7/8" ID (well, they have acrylic...but that's useless because it'll just break). So I would have to buy some undersized pipe, turn it out to the right ID, then press a wheel in, bore it out, take it out, press a new wheel in, etc...huge pain in the ass.

So, four fails later, I'm just going to listen to the MechE gods and give up. I'll take a few hours off of work this week and go and use the Edgerton lathes.

R&D is expensive

2011-07-24T17:54:00.000-04:00

Yet another lesson learned from this project: R&D is very expensive.

The tire glue didn't work. It might have been because the metal was too smooth (it really wasn't that smooth...), or wasn't properly prepared. But for whatever reason, as soon as I took the hose clamps off, I got to watch the polyurethane slowly peel back from the edges. Damn.

Note the bubble made it into the steel.

20 minutes later...

Apparently, urethane strip's natural position when wrapped around something is warped:

, which is probably why it peeled up. I doubt any amount of glue will be able to hold that down in the long run. So, options:

1. Try prepping the surface better and re-glue.
2. Try thinner polyurethane strips.
3. Cast tires (so the warping problem goes away)
4. Cast tires with attachment mechanisms embedded in them (solves both warping and mounting problems)
5. Find a wheel to bore out and glue on (so I don't have to cast tires)
6. Screw on the polyurethane strip (see below).

I seriously doubt 1 and 2 will work judging by how poor the bond was this time. 3 and 4 require expensive casting equipment (hey, R&D is expensive...). That leaves options 5 and 6.

Let's start with option 6.

The idea is to have 8 of these "tire-retainer" rings waterjetted. They would bolt to either side of the motor using the existing bolt hole patterns. The extra set of bolt holes would have #4 sheet metal/wood/self-tapping/self-threading/sharp-pointy-things go through them and into the sides of the polyurethane tires (that are hoseclamped on to the wheel during this process). Screwing into the polyurethane works great:

Unfortunately, having the 8 plates waterjetted would cost me $120...ouch...again, R&D is expensive. Let's try to find something cheaper- on to option 5!

Now, it just so happens that I made the OD of the hubmotors 3.25"...which happens to be exactly the maximum ID you can bore out 125mm scooter wheels to. Check it out:

Alien Wheel!

I'm going to try to press fit three scooter wheels per motor. I bought 12 off of ebay for $65 with shipping. We'll see what happens.

Money pains

2011-07-23T23:51:00.001-04:00

Just checked Big Blue Saw for water jet cutting costs. My rotors will cost almost $17 a piece for regular cutting and over $20/piece for low-taper cutting...and that's without the bolt holes, which add another $6/part (and cutting 24 out of 1/8" makes the prices worse). I'd have to do the bolt holes on the mill anyways, because of the waterjet's taper, so I took those out. At first, I thought this was insanely expensive. Then I realized that if I did my own waterjetting, it would probably take 4 minutes/part, and at $3/minutes, that's $12/part in just cutting costs. Add $5/part in steel, and that's the same price as Big Blue Saw. Wow, waterjetting is expensive.

In contrast, I can turn four 3/4" long rotors out of steel tube for $65 and a few hours. But I won't have the nice magnet notches.

Waterjetting Pros: Very handy little magnet notches that conform to the magnet geometry. No effort on my part.
Waterjetting Cons: $200+ dollars. Over triple the cost of turning rotors. I have to put 12 parts in a mill instead of 4.

Turning Pros: Cheap: $65. I only have to put 4 parts in a mill afterwards (instead of 12).
Turning Cons: I have to spend probably 3-4 hours turning (even if I take time off work to do this, it's still cheaper than waterjetting). No little magnet notches, so add an hour to magnet gluing time.

I'm fairly certain I can glue the magnets in the right place without the notches (I've done it before). And I REALLY don't want to spend $200 on the rotors, when I'm already spending $220 on the stators. I'll sleep on it.

But speaking of stators. The stators I'm getting are from Scorpion Power Systems . They started out by producing lower cost brushless outrunner motors for the R/C aircraft market, but are now expanding to larger motors and wind generators. They're currently developing the "65" (65mm) and "85" series motors. You can purchase the 65mm stators from GoBrushless.com (they are the ones I used in ELB). Apparently they're developing a 70mm and 130mm line, too, or at least the R&D guy I talked to suggested that by letting me purchase a couple stators from them. So far, they are the only source I know of for stators in the famed "70mm-130mm" gap (where copier motors and turnigy motor stators are on one end, and alternators and ceiling fans are on the other end).

T-minus 4 weeks

2011-07-23T20:15:00.002-04:00

I can't believe the summer is 2/3 over. Between working at Aurora, this longboard, designing the next longboard, rocket team, house managing (work week is in 4 weeks O.o ) , designing a laser light show, planning classes, etc... I've been insanely busy with projects. It almost feels like school, minus the little voice in the back of my yelling at me to start my psets.

Anyways, progress report.
All controllers working: check.
All motors working: check.
All essential electronics in box: check.

That means it's time to glue tires! Last time I mentioned a suggestion I got about how to fix my shearing problem when wrapping the tires around the rims. Well...

It works! I tried various ways to clamp the angle, but eventually gave up; the glue is too slick for clamping. However, it was plenty strong for butt-joints, even unclamped. Thus, I hammered in the rest of the motors' steel alignment pins, tightened the screws down, wiped off the motors with rubbing alcohol, and starting gluing on tires.

Steps for gluing on polyurethane sheets/strips to round objects:
1. Acquire a couple packages of 3M Scotch-Weld Urethane Adhesive DB-640.
2. Acquire some plungers from used tubes of epoxy (the cheap double plunger kind). This prevents you from having to buy 3M's overpriced cartridge gun.
3. Acquire a bunch of hose clamps/ low profile zip ties (low profile because they're easier to pull on).
4. Tools: An electric driver or screwdriver, a point-ish glue application device (sharpened stick), rubbing alcohol, paper towels, disposable GLOVES (you'll see why this in caps in a second), something to mix the 2-part glue in.
5. I'm assuming you already have the shape of polyurethane you need cut (bandsaws and hacksaws work well for harder urethanes). Test wrap around part to make sure it'll fit.
6. Wipe off the part with alcohol.
7. Mix up some glue and spread a thin film over the part. Don't use a thick film or it will squeeze out all over the place when you tighten the hose clamps.
8. Spread some glue on the joint ends
9. Carefully wrap the urethane around part and hold with one hand while slipping hose clamps on/over it one by one with the other hand.
10. Grab driver and tighten hose clamps until they're sorta snug (not all the way).
11. Your urethane has probably moved around by now, so reposition it.
12. Continue successively tightening hose clamps until they're all tight.
13. Wipe up excess glue with paper towels/alcohol.
14. Wait 24 hours for it to cure.

This glue is INCREDIBLY messy. Like, 10x worse than the messiest epoxy I've every used, so wear gloves. I didn't wear gloves and got it all over my hands. Alcohol seemed to get some of it off, but not all. It's a really weird feeling: it seems to have increased my hands' coefficient of static friction, without affecting the coefficient of sliding friction, and without being sticky to the touch. Very odd sensation...

The mess. The other side was even worse.

Mostly clean.

I only did one wheel today as a test. I'll see how it turns out tomorrow evening.

Left to do:
1. Finish gluing on tires (4 hours)
2. Figure out/make new package for hand controller (3 hours)
3. Re-time and program motor controllers (6 hours)
4. Finish mounting motor controllers (45 minutes)
5. Test drive (1 hour)
6. Install fans (5 hours)
7. Install lights (20 hours) (optional)
8. Waterproofing (4 hours + drying time)

Estimated work time left on project: ~44 hours (including lights). I'd call it 90% done at this point. Wohoo!

Now presenting the ELB-Heavy
ELB-Heavy will be an electric longboard powered by four custom 500W brushless sensored electric hubmotors and controllers. These new motors will incorporate all of the lessons I've learned so far to make a much lighter, faster, and more robust electric propulsion system.

You can see the coils through the polycarbonate if you zoom in.

One of the goals of this board is to make the motors much more covert. I believe this hub motor design does that nicely. It even uses half the stock hub.

Things to note: red fiberglass stator protector (like BWD), stock outer-side hubcap, wheel restrained by stock nut, stock axle, stock bolt holes, rotor laminations.

Things to note: hole for valve-stem, massive 1" inner-side bearing, through-hub holes for wire routing, odd bolt patterns (actually a 7 hole pattern for steel alignment pins that stop at the seal disk, and the stock hub's 4 bolt pattern).

The polycarbonate seal disk has its own thin bearing. The axle really does step like that...it looks like they take 1/2" steel stock and turn it down to 12mm for 36mm of the axle.

The donor board is a brand new Ground Industries (which might unfortunately being going out of business, if rumors prove true) "Patrol" that I got off of ebay for a steal ($140 shipped). I replaced the off-road tires with some MBS street tires. I'm using the board to ride around Cambridge/MIT while I finish ELB. Speaking of this board...I have a set of brand new GI foot bindings and like-new Bionic Trucks (with sets of T4 and T1 cubes) that I'd like to sell if anyone is interested.

Sorry for all the fuzzy cell phone shots. I still haven't gotten in the habit of carrying around my camera with me.

Motor Specifications
Power: 500W
Operating current: 18A
Poles: 14
Slots: 12
Phases: 3
Windings/phase: 84
Winding style: AacCBbaACcbB (dLRK)
Stator: 0.75in x 70mm, 0.2mm M19 silicon steel laminations (by Scorpion)
Rotor: Three 0.25in laminations of waterjetted low-carbon steel
Magnets: 28 3/4x1/4x1/8" N40 NdFeB bar magnets
Air-gap: 0.9-ish mm
O.D. : 92.1mm
Width: 54mm
Estimated torque (FEMM): ~3.15 Nm.

Other Specs
Controllers: ??
Battery: 8S LiPo (or possibly 9S LiFePo4)
Estimate top speed: 30mph
Tires: Pneumatic 200x50

FEMM simulation. Lots of saturation...oh well.

So, if you've been following ELB from the beginning, then you may have noticed that these motors produce the same amount of torque (in FEMM) as the ELB's motors (ELB was at 3.1Nm in FEMM and ~2.9Nm in real life). Here is my reasoning for this: ELB ended up with twice the amount of torque I'd ever really need at 10A, and I'm doubling the current for these motors. However, I'm also doubling radius and halving the stator width. Therefore, 2 x 2 x 1/2 x 1/2 = 1 . It's very hand wavey, but I think it should get me reasonable torque numbers. Worse comes to worse, I can always drive them at more current (18A is pretty low as far as PEV's go).

I haven't picked a motor controller yet. Hey shane...want a test bed for your new sensorless controllers? haha

I decided to go with bar magnets this time (instead of custom arc magnets) because of the sky-rocketing price of neodymium. There will be 2 magnets per pole as a first order simulation of an arc (an angle).

I guess one question to ask is, why make a "Heavy" version? The basic answer is that Cambridge streets are awful...and having pneumatic tires is amazing because of that. And Cambridge streets get 3x worse in the winter (which is half the year), so an electric board that can have off-road tires, drive through slush, and you can pick up and take to class is a nice thing to have. Ironically, this one will likely be lighter than the original ELB...

I don't think I'll be able to get started on it this summer, but we'll see.

Fast progress as unusual

2011-07-18T16:52:00.000-04:00

The controllers are all working now thanks to Shane. And the problems were, drumroll please... 100% my fault! I guess I shouldn't be surprised. This was my first time hand-soldering (w/o a reflow oven) tiny surface-mount components. And the second board only had 1 issue, so I was obviously getting better at it (I'm still bad at it though...need more practice). Basically just a bunch of poor solder joints, crossed pads, and toasted resistors caused a few components to die - luckily nothing major. The next step is re-timing them (because we messed with the timing so much trying to troubleshoot them). After that, it's programming the handheld controller.

I'm testing the polyurethane glue's ability to make strong butt-joints right now...we'll see how it turns out in a few hours. 5-minute epoxy was too brittle and just snapped at the joint when I tried to bend it. The whole point of this is to see if I can cut the / off of one end of my tire strips and make a /\ on one end and a |\/| on the other (thanks for the tip, josh!). The V joint should get rid of the sliding problem when gluing on the tires. If it doesn't work, I'll have wasted $160 worth of polyurethane sheet, and instead of buying another sheet of it, I'll probably invest in casting materials and cast the right shape tires. Too bad my design doesn't allow me to retrofit mountainboard tires to the wheels.

Speaking of mountainboard wheels, here's a tease frame of what is to come:

"Hmm...looks like a normal mountainboard wheel." Can you spot the difference?

Slow progress as usual

2011-07-10T16:37:00.000-04:00

More electrical issues...two of the phases on MC3 (the trouble child) are now screwy. So we're just going to wipe that side of the board clean and start over.

Actual progress:
Tires:
I cut out and shaped all the tires. They're cut out of a sheet of 3/8" 70A polyurethane rubber (which was insanely expensive).

I first started with strips, then cut 45 deg angles on the ends.

I tried wrapping a tire around the wheel and holding it with zip-ties.

Unfortunately, I need more coverage to hold the gap closed.

I bought a bunch of hose clamps and massive zip ties to try next. The hose clamps held the gap together well, but another problem came up. The natural position for the urethane is warped when it is curled, which makes the angled-cut edges not want to line up. This means that I can't hold it in place while tightening the hose clamps (which requires two hands). Currently I have no solution to this problem. Maybe a combination of zip ties and hose clamps...I don't know. I can't just glue the edges together, because no fast setting glue is strong enough (it takes a lot of force to hold the edges together). I may end up saying screw it and buying polyurethane casting resin and just casting my own tires.

Shielding sensor wires:

I bought some sweet 5 conductor shielded wire off of ebay. It's kapton coated, which is a nice feature.

Replaced the green mess of wires.

I can't open the motors anymore without significant effort, so I just spliced wires.

The shield is connected to the sensor ground.

Nunchuck:
I mostly finished the Wii Nunchuck wireless controller (Shane did the electronics and programming...Thanks Shane!)

It's ugly, I know. I need to figure out a different package. I was too optimistic thinking that I could jam it in the nunchuck.

Heat sinks:

I soldered a little copper sheet to the 5V regulator (center) on the boards because they get pretty hot.
The MOSFET package on the left (no heatsink) is going to be removed, along with most of the components on MC3.

And that's about it for now. I'm not planning on gluing tires until we resolve these controller issues- they're what's holding up progress at the moment.

Last two weeks

2011-06-25T20:18:00.000-04:00

Well...it happened. I was 99% sure it would happen and it did: Electronics problems. Here's what happened:

About three months ago, all 4 motors and controllers were working perfectly. Two weeks ago, all 4 motors and 3.667 of the motor controllers were working perfectly. We (Shane and I) discovered that one of the phases was messed up for no apparent reason. Anyways, I put the board under the work bench and got it out 2 days later. Literally nothing was different and suddenly NONE of the controllers were working properly. This is why I hate electronics. After many hours of trouble shooting (mostly shane being a genius), he came up with some stuff to try. We think the noise from the sensor wires being so close to the power wires is messing everything up. So I bought some shielded 5 conductor wire (to be installed next week) for the sensors, and Shane added some capacitors to the sensors. This cleared up the problem for 3 out of the 4 MC's. However, the one that was having phase issues still didn't work...and when one of the gate drivers started smoking...well, yeah. You get the idea. My shitty soldering job on a single resistor caused one of the gate drivers to smoke after awhile. The controller requires replacing 3 IC's, after which it should* (cross fingers) be good to go.

So as usual, it's not the electrons' faults that it didn't work...

Anyways, other stuff that's been accomplished: The Wii Nunchuck has been hacked apart so that a Xbee + board can fit. The trucks and risers have finally been properly bolted on. I used every single riser pad I bought to get enough ground clearnence for turning. Besides that, nothing has happened really.

Left to do:
1. Finish Wii-nunchuck controller (8 hours?)
2. Test and program motor controllers (8 hours)
3. Finish fixing electronics problems (10 hours)
4. Finish mounting motor controllers (45 minutes)
5. Test ride around a little inside (10 min)
6. Cut/Glue polyurethane tires on (24 hours)
7. Test drive
8. Install fans (5 hours)
9. Install lights (20 hours) (optional)
10. Waterproofing (4 hours + drying time)

Estimated work time left on project: ~80 hours (including lights).

One month down, two months to go. O.o

Weekly update

2011-06-12T00:58:00.003-04:00

The motor controller mounts have been installed. I made them from 1/16" polycarbonate strips: measured the height the controller needs to sit, heated up the plastic with a heat gun until it was soft, then used some square blocks to form it into a little bracket. The fore and aft mounting brackets have a little hole drilled in them that allows a 4-40 screw to go through and into a mounting hole in the MC. Small pieces of super vibration dampening rubber will be placed between the mounting brackets and the MCs.

The little white wires running from one end of the board to the other in the picture below are wires from a magnetic deadman's switch that controls the contactor coil. A magnet on my foot closes the switch, thus activating the contactor...so when I fall off (not if), the contactor will open, and the board won't keep going.

Below are pics of the partially assembled board.

It's starting to look like a longboard.

Hell yeah. Note: BWD is in the background

I tried standing on it and turning, but discovered that there's not enough ground clearance (I get edge bite). Luckily, I bought a ton of riser pads and installed all of them...it looks kinda funny, but that got the ground clearance up to a more reasonable 1 3/8 inches. It'll also give me more suspension, which is much needed with such a stiff deck.

While all of this was going on, I was balancing/charging my giant lipos. It turns out that one of the cells in one of the packs was much lower than the others. At first I thought it might be a bad cell, but the charger has been able to get it up from 3.4v to 3.94 at the time of writing this. I also learned that putting the charger on top of the power supplies is a bad idea (I know...duh...). I started smelling something awful about 15 minutes into charging and found the duct-tape that I used to hold the power supplies together in a block melting/burning...they got really freakin hot. Unfortunately, despite the separate power supplies, each individual charger maxes out at 50W (~2A for these packs), which implies that it is software limited and not power supply limited. I was hoping to get 5A out of them so I could charge the packs in under an hour...oh well.

Compare the 2 motors' wiring. Also note the massive stack of risers.

I also did a bunch of wire routing. I'm about 3/4 the way done with getting all the motor wires routed. All of the essential inner wiring is complete (in other words, everything but the fans and lights). The nice sleeving stuff I bought works like a charm. Something I thought of is that I could have used ribbon cable or something like ethernet cable for the sensor wires, instead of running 5 separate strands...doing that probably would have made my life easier (yet another lesson learned).

I really really REALLY hope that none of the electronics have decided to stop working. Debugging this system takes forever, and taking apart a motor is a whole day (and destructive) ordeal. We'll see soon enough (queue ominous background music).

Left to do:
1. Finish wiring (3 hours)
2. Design/Make Wii-nunchuck controller (15 hours?)
3. Test and program motor controllers (8 hours)
3.5. Fix electronics problems (I'm 99% sure something will go wrong...it always does)
4. Finish mounting motor controllers (45 minutes)
5. Test ride around a little inside (10 min)
6. Cut/Glue polyurethane tires on (24 hours)
7. Test drive
8. Install fans (5 hours)
9. Install lights (20 hours) (optional)
10. Waterproofing (4 hours + drying time)

Estimated work time left on project: ~80 hours (including lights)

June work

2011-06-05T02:55:00.001-04:00

Well, bad news...I've decided to suspend work on the ELB indefinitely.
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Just kidding.

I've actually just restarted work on it. I spent about 9 hours figuring out how I'm going to do all the wiring. The following picture shows about half of the result.

You can see the 4 blue lipos in the middle of the deck. Instead of linking the batteries together and running only a few plugs out of the box, I decided to go the "run-every-single-plug-out-of-the-box" route. While this meant more plugs, more drilling, and more soldering, I really wanted the ability to separate all of the packs (safety, maintenance, etc). Having each pack's terminal and taps also has a charging advantage: Since my modified HobbyKing 4 port charger (see previous post) can charge 4 packs at a time at 200W each (I think...) , I can charge faster than if I was charging the packs in parallel.
Anyways. It'll be apparent what the mess of wires is doing when it's done. The black thing on the right is a MAXI blade fuse holder. The black in the box is an Albright SU-60 contactor.

Left to do:
1. Design/make/instal mounts for motor controllers (6 hours)
2. Finish wiring (5 hours)
3. Install fans (5 hours)
4. Install dead man's switch (3 hours)
5. Blue-RTV holes (2 hours)
6. Mount trucks with risers (2 hours)
7. Design/Make Wii-nunchuck controller (15 hours?)
8. Mount ESC's and motors. Test and program motor controllers (8 hours)
8.5. Fix electronics problems (I'm 99% sure something will go wrong...it always does)
9. Test ride around a little inside (10 min)
10. Cut/Glue polyurethane tires on (24 hours)
11. Test drive
12. Finish waterproofing (2 hours)

Estimated time left on project: ~72 hours
I'd say ELB is about 80-85% complete at this point.

So, in accordance with the laws of project-dom, I've started designing my next projects before finishing this one. 3 more electric hubmotor powered longboards: a "light" version, a "heavy" version, and a secret version. "oooo, I wonder what it is..." :-P . I should have some preliminary CAD drawings by the end of the summer for "heavy". So, by extrapolating my current project work schedule, I should be done with the next three longboards by...oh...say...2020? 2025? haha.

In other news:
MIT Rocket Team update: We got 2nd place at USLI!
DBF: Top 10!
I won the Unified Engineering Flight Competition! Our plane carried a record breaking 941g of payload.

I'm really happy with the way the semester turned out...even though I was stressed beyond belief (and couldn't find time to work on ELB :( ) . I'm interning at Aurora Flight Sciences this summer designing/building UAV's...it's really awesome. I'll be doing ELB work some nights and on the weekends, so look for weekly updates.

Update

2011-04-06T09:55:00.001-04:00

Yeahh...I've been crazy busy. The 80% finished ELB has been sitting in the Edgerton Center workshop for the last few months. Why? USLI Rocket Team activities have picked up in preparation for competition week next week.

Our Rocket Carrying a UAV Payload

After that, I may have time to work on the ELB...no guarantees though :/ .

Good news though! I'll be at MIT this summer, so I should be able to finish the ELB this summer! Stay tuned for that. (only a year late...haha)

-Jed

They run!

2011-02-18T08:21:00.000-05:00

If the video wasn't taken with a cellphone, you would be able to see the coils pretty clearly while it's spinning.

Anyways, the new hubcaps are all on the motors.They all work as they should now. Unfortunately, due to USLI with the MIT Rocket Team, DBF (oh, and classes...) demanding massive time commitments, this project has slowed wayyy down. It's very unfortunate, but that's life: never enough time to do everything you want to do.

Pics:

Coils!!

The test rig setup.

That's all for now. I'll try to get more of it finished for the energy showcase coming up, but no guarantees.

Pics!

2011-01-31T17:08:00.000-05:00

Damn my internet...typed up this entire post and lost it all. I'm not going to rewrite it all.

I finally sucked it up and rebuilt the eXKateCD (see previous posts for its story of misfortune). It has a Kelly KDS Controller, 9 new Lipos, wired hand throttle, and a KEYED battery selector that should prevent anyone we don't want from turning it on or plugging the battery in backwards. Check it out:

The KDS controller

The new LiPo array.

As it turns out, the KDS series of Kelly Controllers doesn't have Regenerative Braking, even though it is perfectly capable of doing regen. They just didn't include it in the software so they could charge twice as much for the series with regen....or they are so incompetent that they didn't realize they could easily make it do regen. Either way, the board doesn't have brakes, which is bad. A new motor controller will be designed/built for it (probably not by me considering all the other stuff I'm working on) that'll have wireless control via Xbee radio modules.

ELB Pics:
I thought I had more pics than this, but I couldn't find them.

Right: outer-side hubcap with polycarbonate inserts after 3000 grit lapping paste. Left: The same as right, but after heat treated with a heat gun. This melted the surface of the plastic, leaving it crystal clear.


Pile of ~~crap~~ inferior, busted polycarbonate hubcaps.

Update: The motors are all but completed. Just waiting on more screws to come in from McMaster. I hope I didn't damage anything when taking them apart. One thing I did learn when taking them apart pertains to ball bearings. When designing an assembly with ball bearings, make sure to only have the bearing press-fit into one side of the assembly if the assembly is ever to be taken apart. Furthermore, make the press-fit the part of the assembly you want. Sounds obvious, but I never had to think about it before this. Two of the 2" bearings were semi-press fit onto the hub-shafts, making it impossible to get them off, thus making it impossible to press-fit them into the hubcaps. So I had to take the hubcaps to the machine shop and take a few thousandths off the bearing surface to make them a close fit instead of a press fit. Of course, none of this would have happened if I had just gone with aluminum hubcaps in the first place...

To-Do list:
1. Glue tires onto wheels (2 hours)
2. Component layout inside of battery box (8 hours)
3. Figure out trucks risers (1 hour)
4. Make hand-held remote control (2 hours)
5. Program motor controllers to take throttle commands from hand-held remote control (2 hours)
6. Test (1 hour)
7. LED lights (4 hours)
8. Waterproof / Loctite (1 hour)

I'd say this project is 80-85% complete at this point. But with DBF, Rocket Team, and classes in full swing, the last little bit might take awhile.

News

2011-01-30T17:19:00.001-05:00

Latest Lesson learned:
Designing something that might work is significantly easier than building something that will work.

In other news:
I co-authored with Shane and Charles a paper that was accepted to the EVER '11 conference in Monaco!!!!!!!!!!

In other other news:
We got the 4th motor calibrated for ELB finally, after I replaced the sensors. Still no idea what was wrong with them. Anyways, this means I can start replacing hubcaps and finally finishing the motors. I'll have some pics of stuff when I find my microSD to SD card adapter.

That's all for now.

Back

2011-01-11T07:25:00.000-05:00

Back in Cambridge. List of stuff I need to get done:

1. Fix sensors and calibrate motor 4
2. Finish the layout design of the battery box
3. Place all components
4. Waterproofing
5. Make handheld controller
6. etc...

Still a lot left to do. This month is the 1 year anniversary of the beginning of this project. It shoulda been done 6 months ago, but I've been too busy and have had too many setbacks. Oh well.

Crap

2010-12-10T09:02:00.001-05:00

We got three out of four hub motors calibrated, which is awesome. The current problem is that the fourth hub motor's sensors decided to die...no idea why or how, because they sometimes work, and sometimes not. It can't be a magnetic field issue because a. they are mounted in the exact same locations on all motors. b. they do change state...except sometimes to the WRONG state. WTF, that shouldn't even be possible. I even tested them with a small magnet and an oscilloscope before assembling the motor, and they worked fine. And we determined it wasn't a problem with the MC board because we ran a different hub motor on it just fine. Oh well. I've ripped the sensors out and am going to put three new ones in at some point. Then reassemble and test again. Then put NEW HUBCAPS on! Wohoo.

Lessons learned over the past month or so:
1. Sensors can fail even when taking every precaution
2. Don't use steel alignment pins (see earlier posts) until the final motor integration onto the board. Pulling those things out sucks
3. Make your hub motor rotors with a water-jet, like this.
4. I should have wound the motors hotter (or just made smaller motors). At 10A, I'll have more torque than I could ever use/stay on the board with. In fact, this board would have no trouble at all running on two motors. Something I will consider if weight ends up being an issue.

Mid-November

2010-11-21T21:32:00.000-05:00

I'll know I'm doing too much stuff when these posts start coming at more than 1 month intervals.

Anyways, time for another ELB update!
I finished cutting and rough sanding the last 40 polycarbonate inserts for my new hubcaps.

It's INCREDIBLY tedious work to polish these things. The above is after cutting, 220 grit, and 400 grit. There's still 600, 1200, 1500, and 3000 (lapping paste) to go. Anyways, this is the point where I got bored and decided to work on something else.

This is my charger: Turnigy 4x6S . It can charge four 6S packs independently. I figured I could use it to charge my 12S ELB pack (just link two of the ports together). But...
Each charging port is NOT independent, despite what you may have heard about it, or may assume about it, or read about in the description ("All ports work independently of each other."). From the outside, all you see is a single input for 12Vish DC and 4 charging ports. Internally, there are 4 separate chargers. Sounds ok, right? However, all of the chargers' inputs (+ and - ) are linked by traces that go all the way around the board. This wouldn't be a problem if they were high quality (isolated input/output) chargers. But this is a Chinese Cheap Charger (CCC). The negative/ground wire is connected straight through from the input to the output (battery ground). This means that all of the outputs are at the same potential, which in turn means you can't link ports and charge anything over 6S. I need 12S. There goes $100. OR:

Doctorbass to the rescue. Check out his hack for this charger...genius. He made it into a single 24S charger by isolating the four chargers by cutting the traces that link them, and then buying 4 small (cheap) switching power supplies, 1 for each charger. His application is a little different than mine, so he did some other cool modifications, but I'll just show how I did it.

Take it apart

Each charger has two holes like this for wires to go into. It's like they were originally going to manufacture it to run off of four independent power supplies (like it should have been), but they instead just added a trace all they way around to save money.

Cut traces on each side of each set of input holes. Dremel worked well.

Cut the bottom traces, too. Be careful not to cut into other traces, just the power traces.

Back side was easier. Could just slit the board without hitting any components.

The four 12V, 5A power supplies. You can get these on eBay for <$40.

Cut the DC plug off the power supplies and string the wires through the wire hole in top plate of the charger. (De-solder the existing input wires first.)

Solder the new inputs to the circuit board.

Yep, definitely isolated now.

Finish soldering all the input wires. Route them as you see fit.

Semi-finished. They all work!

Removed excess wire. That's better.


Double-stick tape/duct tape the power supplies together. I velcroed the charger to the power supplies, too.

Now I have one big block of a 24S, 200W charger. Not too bad for $130 and 2 hours.

In addition to these things, Shane's been helping me calibrate the motor controllers to my motors (well, he's been doing most of it).


Note the old hubcaps with chunks of missing polycarbonate....why I'm making new ones.

I've got updated data for the motors now, too. Before, using a Turnigy controller and running the motors sensorless, we calculated a top speed of about 30mph. Now, using sensored sinusoidal control and about 15 degree advanced timing, the top speed is around 20mph. Not too bad. The really impressive thing is the torque. I'm looking at getting 50lbs of force at the ground..assuming 200lbs of mass (rider+board), that translates into 1/4G of acceleration, which is nuts...especially for something without a seat or handlebars. I probably should have wound the motors a little less for torque and a little more for speed (less windings, or thinner stator, etc), but I think 20mph is plenty.

That's all for now.

The other Electric Longboard

2010-11-21T20:39:00.000-05:00

Well, bad news for the other electric longboard (the one made during the Edgerton Center Summer 2010 Engineering and Design Class).

Remember the eXKateDB (drill battery) hack?:

I shoulda seen it coming, but see those little connectors? They can be plugged in backwards...yeah...bad stuff happened. Turns out the Exkate controller doesn't have reverse polarity protection. I actually found this out the hard way at the end of the class this summer. I plugged the battery in backwards and it fried some components. Luckily, Shane was there to help me fix it. This time (wasn't me) however, more things fried:

Blown up transistor and resistor. Another transistor is fried on the board.


You can see the diode we replaced before. It was fine, but the transistor below it wasn't too happy.

Assuming more stuff wasn't burnt, we probably could have fixed it, but the whole thing was turning into a hack at this point. So....new controller! Brushed Kelly Controller to be specific. It has better specs than the Exkate controller, for a lot less. However, it doesn't have a built in radio system. So....new radio system! A Hobbyking 2 channel 2.4GHz radio was purchased ($17). We'll have to figure out some way to translate the PWM servo signal to a 0-5VDC signal for the Kelly throttle input, but that shouldn't be too difficult. Hmm...what to do with the extra channel.

A new set of Lipos are here (I need to find time to install them :/ ). To prevent what ultimately caused all of this trouble in the first place, a keyed battery selector will also be added. After that, the longboard should pretty much be set and idiot-~~proof~~ resistant.

About the Edgerton Center at MIT

2010-11-10T08:37:00.002-05:00

http://techtv.mit.edu/videos/9e150e3e855438bb92984ae2e87081cecc19684a/private

Check it out.

Polycarbonate Inserts

2010-11-06T21:39:00.000-04:00

Time to suck it up and polish some polycarbonate.
I bought 220, 400, 600, 1200, and 1500 grit sandpaper, as well as some 3000 grit lapping compound, a felt bob, and a cotton bob. With my polishing kit ready, I cut ten pieces of the 1/2" dia. PC and ten of the 1/4" dia. PC with a band-saw. Then I started with the 220 and progressed to the finer grits. To remove the burr raised from sanding, I put each stud in a drill press and filed down the burr. Then I flipped the stud over, and did it again.
For the buffing, I put the felt bob (I found it worked better than the cotton one) in a drill press, applied some of the lapping compound, turned it on, and buffed the ends of the studs. They came out much clearer than before, but still kinda hazy.
So then I took the heat gun and carefully heated up the each end, making sure to take the heat off before the PC bubbled. The final result were ~.45" long PC studs that you can read text through. Then I pressed them into a hubcap with an arbor press. Check it out:


Random sheet of paper I found. You can see the circuit diagram through the PC.

However, it wasn't nearly as easy as I just made it sound. The whole process took 4+ hours (yes, I'm blowing off work). That's just for 1...I still need to do 3 more. Ahhh, not enough time.

October ELB

2010-10-28T14:21:00.002-04:00

Despite a crazy work load, I was able to get a lot done this month. The machining on the new AL hubcaps is complete. They're made out of 2024 alloy, which is very tough as aluminum alloys go. Check it out:

(Ignore the chuck changes...these images are from different days.)

The apparent warping is from the thick coat of oil covering the part.

TA-DA:

Lathe work done

Sorry, fuzzy.

Done

Scale: these are 3.25" in diameter.

24ish hours total spent in the machine shop. Polycarbonate plugs will be press-fit into the big holes to maintain waterproof-ness (remember, one of the goals is to have a waterproof board I can ride in the rain/snow). You can sorta see the ring I left inside the bearing pocket- that's for spacing out the bearing so the inner bearing race doesn't rub on the outer wall (learned that from the first set of hubcaps). The large holes in the inner side hubcaps (the ones that don't look like swiss cheese), are for the massive 2" bearings I bought. See previous posts for why I'm using 2" bearings. Anyways, they came out really great.

The next step is to figure out how to make the polycarbonate plugs. I bought some 1/4" and 1/2" PC rods and plan to cut them into 80 plugs. The big problem is figuring out how to polish the ends so that they are see-through. There are a number of ways to polish polycarbonate:

Potentially the best way is methylene chloride vapor polishing. This method basically eats the top surface (and thus the scratches) of the PC part, leaving a perfectly smooth (and thus clear) finish. The problem with this is that methylene chloride is nasty stuff, and I don't have access to a vent-hood. So that won't work for me.

Another method is dipping PC parts (or any clear plastic parts apparently) in certain types of floor was (like Future) and letting the wax dry on the part. Apparently, this results in a perfectly clear part (I guess the wax fills the scratches?). I know R/C plane hobbyists use this method to refinish their cockpits. My problem is that my motors will give off heat, possibly melting the wax.

Another way is to just sand the ends with steadily finer and finer sand paper, and then use polish (like jewelers rouge) on a buffer. I tried this, but the ends were still very hazy. I'm going to try again with finer grit sandpaper (like, 800 if I can get it local).

Yet another way is to heat the surface of the PC part to the point where the surface melts. Unfortunately, you have to make sure you are evenly heating the surface, and only the surface. If you overheat parts of the PC, it'll bubble. I tried this method, too, with a heat gun, but the edge got a lot hotter than the center, causing bubbling. However, the parts that weren't bubbled were significantly clearer than the polished ones, so if I can figure out some other heat source, I'll try this method again. Results:

The outer two are sanded and polished (note haze). The middle one was heated (note bubbles).

Anyone have a good way to polish polycarbonate?

Here's a sneak peak:

Beginning the electronics layout.

Note the old hubcaps.

My activities this term

2010-09-22T19:38:00.000-04:00

Dear readers,

You may have noticed that the Electric Longboard project pace has slowed down significantly since the summer. This is due to the following activities:

1. Fall term Classes. I'm taking the Aero/Astro Core class(es) called Unified, which is basically one GIANT 48 unit ~~bootcamp~~ course covering thermo, fluids, structures, and signals/systems. This is in addition to other classes, including an advanced composites course (16.202) and German 2.

2. Research. I'm starting a UROP (undergraduate research opportunities program) in the Space Propulsion Lab. I'm working with a senior on a new kind of L.E.O. ion thruster. VERY cool.

3. Rocket Team. The MIT Rocket Team is doing NASA's University Student Launch Initiative (USLI) this year.

4. Design/Build/Fly. I'm doing DBF again. This is always a huge time commitment. Last year, while it only counted as a 6 unit UROP, I spent probably 18 units worth of time or more on it.

5. House Manager for my fraternity. Takes LOTs of time and effort.

Yeah....I'm busy. So I'll fit the ELB in when I can.

9/20/10 Update

2010-09-20T20:42:00.000-04:00

ELB: The motors are in the process of getting internal Hall effect sensors. The method seems to be working well, so I plan on doing it to all of them, reassembling temporarily with the semi-busted polycarbonate hubcaps, testing/calibrating, then replacing the hubcaps with aluminum ones. The aluminum stock is in, so I should be able to start machining those soon.

eXKateCD: It is now the eXKateDB (exkate drill batteries). Let me explain. Someone (no idea who) came into the lab, saw it under the desk, probably tried to play with it, and left the Lipos plugged in. This drained the ENTIRE pack down to about 1V...yeah, Lipos don't like that; they're dead.

Wall of Shame.

So while we wait for $160 worth of new batteries to come, I rigged this up:

2 18V drill batteries in series. Not pretty, but no noticeable power loss, which is nice. Plus, we don't have to worry about these being drained all the way down (NiCD's actually like that). Our plan to prevent this from happening again is to have a keyed battery selector and hiding the key. I never wanted to have to do the 4 hours worth of soldering, etc. that it took to put the original Lipos in again, so I definitely don't want to have to do it a third time.

That's all for now.

9/5/10 Motor Update

2010-09-05T03:25:00.002-04:00

Good news: I took the sensor board out and tested it with a small magnet. The sensors were fine, they just weren't close enough to the magnets/coils. So I'm going to stick them in the slots of the stator and get rid of the sensor board. Even though they won't have the same angular separation as on the board, it will be a combination of slots that works, such as A(sensor)aAB(sensor)bBC(sensor)cCAaABbBCcC. I'm pretty sure I did a previous post on calculating hall effect sensor positions, or it's in my design review for this project, so look that up if you want to know where to place hall effect sensors. Anyways, this means that the sensor board cannot be adjusted, which is nice to have if you want to mess with timing. Oh well.

picture added 8/20/11

Bad news: Chips of the polycarbonate hub caps/spacers fell out. However, it wasn't the total disintegration I thought it would be. In fact, it's salvageable, so I may just put them back together the way they were. (I'll at least do one to test the sensors in the slots idea.) On the other hand, sidewalk cracks and other bumps might finish the disintegration process, which would be really bad while riding.

Another option is to make new hubcaps/spacers out of aluminum, but it wouldn't be see-through anymore. :(
I could waterjet new polycarb spacers and hubcaps, but I really don't want to deal with that again. Especially because it'll be impossible to get rid of every little molecule of loctite.

Another option is to shove the rotor into an aluminum pipe and make aluminum hub caps for the pipe, eliminating the spacers. Something like this:

I got this idea from Charles if you can't tell.

Cool. The cylinder in the background is the tire.

The holes are filled with glued or pressed-in polycarbonate disks. I really like the way this looks, and I think it'll be much stronger than the current design (though the current design is plenty strong...). The larger diameter will give a slightly higher topspeed. One draw back with this is that the outer edge of the hub cap is now an important surface and has to be a very close fit for weather-proofing. In fact, the hubcap-outer tube interfaces would probably have to be sealed with silicon. Drilling radial holes is no easy task, and getting it apart would also be a pain.

Here's a combination of the above two options:

Aluminum hubcaps with polycarbonate inserts in the outer-side one. Keeps the current bolt pattern. Scraps the useless o-ring seal (explained in an earlier post). This allows partial see-through without the draw backs of polycarbonate (can't loctite, not as stiff).

I'm leaning towards the last one.

Sigh...

2010-08-31T01:17:00.001-04:00

Bad news. A bunch of my sensors aren't functioning. This means I'll have to break the motors open and either a. fix broken wire connections, b. reposition the sensors, or c. both.

That doesn't seem that bad, except the polycarbonate hubcaps have spiderweb cracks from the loctite and parts of them might disintegrate...which would be very bad. It would pretty much mean a redesign of the motor casings.

Since I don't really have time for the next few weeks to work on it directly, I've started designing custom LED lights for it.

That's all for now

eXKateCD

2010-08-28T19:40:00.000-04:00

The build log is this Instructable. Feel free to ask questions about it in comments here or there.

Video links:
1
2
3
The whole album is here

Other projects included:

Overhaulin' this custom built Segway
Making a Quadrotor from scratch (an instructable should be up soon, so please don't ask me about it)
Pong from scratch (and by scratch, I mean wires, LED's, and microcontrollers. I didn't work on it, so again, please don't ask me about it)

Here are some pics of those projects and of the Edgerton Center Summer 2010 Engineering and Design Class in general:

Parts of the quadcopter

Believe it or not, the quadcopter actually flew.


Pong from scratch. Tilting the controllers causes the paddle to move.

The old segway.

Clear deck

Oh yeah, LED's


The eXKateCD team

Class photo

Shane's summary is here.

Done for the Summer

2010-08-08T21:13:00.002-04:00

Here are all the polycarb parts laid out in order (insert rotor in middle):

The waterjet didn't cut as cleanly as the lathe (duh), thus the cloudy white appearance. You can also sorta see in the 3/8" (left) spacer how nasty the small thru-holes are. I undersized all the holes in my waterjet drawing because I knew it couldn't cut them perfectly and I was planning on drilling them out later. Instead of cutting nice straight holes, though, it kinda just bubbled up inside and threw up some plastic out the otherside. Next time I need small holes cut in a piece I'm waterjetting I'll do the holes on a mill.

Anyways, here are all the polycarb parts stacked up. The waterjetted ones need to be drilled and cleaned up a bit, but that wasn't too bad.

Next came skid plate work. I cut the basic size with a band saw, used a leveling surface and gauge to score lines parallel to the factory edges at the length/width I wanted, and then used a belt sander to even it all out. Then I marked all the screw hole locations.

Next came drilling. I used a drill press because I was too lazy to set up a mill. I ended up off by a tiny bit on a few of them, so I just took an endmill and routed out the holes a little in the right direction. Then I countersunk the holes.

Countersinking


Fully assembled battery box. The extra skidplate lip was intentional.

It turns out I was a little off (max. 3/16") on the curves in the CAD drawing I used for cutting out the sides:

Oh well, that can easily be filled with Blue RTV.

The next things that needed to be done were gluing/wiring the hall effect sensor boards:

Win for polycarbonate.

I used epoxy to glue down the hall effect sensors, so I scored the surface. You can also see where I scratched in the 12 degree angles for spacing.


4 motors, 4 boards.

Since each sensor had a power, ground, and phase pin, I could wire all the powers and grounds together and only have 5 wires to deal with. I did this by first bending the pins as in the picture below, and cutting to short pieces of solid copper wire:

Then I soldered all the power pins to the bar:

And all the ground pins to the next bar:

Then I just soldered on the wires:


See the Blue RTV (aka. smurf blood) in my nails?


All 4 wired. The old first one is above.

Next came inserting all the stators into the rotors:

Then putting on the bearings and hall boards.

Cool reflection of the coils in the bearing.

I needed to glue the 1/16" in-board hubcap bearing retainer plates to the inner-board hubcaps. I though that super glue would work...

Well, it worked. But it turned cloudy and purplish when it dried. Oh well, at least I got them on there.

The next thing I made were the o-ring seals. I cut some 1/16" o-ring cord stock to length and superglued the ends together in the groove. Then greased them. This worked great. Unfortunately, I'm not really sure how useful they'll be. A strip of urethane tire will be glued to the outer surface of the motor, sealing the interface between the hubcaps and spacers. If water gets under the tire, sure it won't get into the motor between the hubcaps and spacers, but the interface between the rotors and spacers won't have a seal...so it looks like the o-rings were kinda extraneous (they look cool though):


O-rings

Time to start assembling:

1st big problem: It turns out that the "1/2" inch sheet of polycarbonate I bought (and made the rear hubcaps out of) was closer to 7/16". This meant that the 1/2" wide in-board bearing had to stick out past the plane of the motor...which was now blocked by the superglued bearing retainer plates. Whoops...guess I shouldn't assume the thicknesses of my materials. grrr.... This meant I had to pry/chisel/sand/cut the stupid plates off:

Now they look even uglier.

Resistance is useless!

Back to assembling:

Do you see the 2nd big problem? It has to do with that double hole I painstakingly drilled because I broke off a tap in the original hole in the rotor. See the dark smudge in the left (second) hole? Yeah...I broke the screw off in it. WTF. That's irony if I've ever seen it. It's ok though; 4 screws should be fine.

Ok, how about the 3rd big problem? It's visible in the pic above, but I'll give you a hint with the next pic:

Polycarbonate and Loctite do NOT mix. I knew that loctite and acrylic didn't mix, but I had no idea about polycarb. I noticed these spiderweb cracks when I was halfway through the assembling the third motor. Better late than never, I guess... Yet another advantage to Charles' metal can methods. Any future hub motors I build will likely be some variation on his style. The motors should still be strong enough with the cracks because the screws are mostly providing compression, while the steel pins do most of the work.

Guess what's wrong now...

At this point, the 4th big problem popped up. 2 of the motors had full shorts. Ahhhhhhh. I checked them all before I assembled them! Dis-assembly and inspection revealed the same problem in the same place in both. Epoxy on a sharp edge had chipped away underneath a wire causing the wire short to the stator. Luckily it was in an accessible place. I cut the wires, slid some heat shrink on, and re-soldered. It only took 3 hours...
I really really dislike these stators now. Aside from taking 4-5 months for them to come, the epoxy coating really sucks. Using fiberglass insulating end plates (like on the BWD scooter) would have prevented the problem, and I'll likely use that method with any future motors I build.

Next, I did some work on the MC and soldered the sensor connectors on:

Then Shane and I (mostly Shane) tried to match the motors to the controllers (takes a lot of trial and error with phase and sensor combinations). In comes big problem 5: Turns out one of the sensor wires in the first motor we tested has a bad connection...crap. I get to take it apart again...

Next came mounting the battery box to the deck. This required drilling 14 clearance holes for #10 screws in precise locations...on a curved deck. Yeah, I ended up drilling 1/4" holes as close to the right locations as I could get. It turned out well:

Drilled and countersunk.

I decided that securing the sideplates directly to the deck with screws was unnecessary. I figured the screws in the endplates would be plenty.


Screwed on.


Skid plate removed for electrical component locating.

And that's it for the summer. I got as far as I could, but I will have to continue in the fall.
Lessons learned:
1. Don't do small holes with a waterjet.
2. Measure all material...don't assume thicknesses.
3. Don't use superglue on polycarbonate if you want to see through it.
4. Loctite doesn't mix with polycarb or acrylic.
5. Use fiberglass endplates for stators when winding. Epoxy sucks.

Stuff left to do:
1. Figure out battery box wiring/component locations.
2. Mount/wire components in battery box.
3. Drill holes in battery box for various wires.
4. Fix motor sensor(s).
5. Finish motor testing.
6. Cut/Glue tires.
7. Replace truck bushings.
8. Seal/waterproof everything (with smurf blood).
9. Loctite
10. Lights.
(11. Lighter battery box?)

In the meantime, I will be writing an Instructable for the ExcateCD (a modified Excate longboard that actually works...as opposed to the stock Excates that really don't work). It was one of the projects of the Summer 2010 Edgerton Engineering and Design Class (video)....yes, I have been working on TWO electric longboards for the past month. I'll also likely write up a few things here about the class. I didn't so much under-estimate the amount of time it would take me to finish my longboard, as underestimate how much time being a teacher of the class would take.

Now I'll leave you with a couple pictures of Shane destroying/performing surgery on/obliterating a MOSFET with a sawzall:

2010-08-03T22:00:00.000-04:00

I took a break from motor work to waterjet stuff.


Battery box sides.

Remember how I hate acrylic?

Busted.

I redid everything with polycarb:


Polycarbonate hall boards.


Inner-side hub cap caps.


Inner and outer side spacers. Polycarb>acrylic.

I cut more alignment pins, too:

Dull dremel bit = burnt steel.

I also press-fit the rest of the hubs/stators:


The two shorter ones are for the rear truck. See Design Review Document for why.

Next, I did the mill work on the battery box sides and end pieces. This included lots of holes:

Note: a lot of the dimensions are wrong...old pic.


Both side pieces clamped together and clamped to a support beam.

The twist clamps got in the way, so...


...Much better.


Tapping.

Attempting to put holes in the end of a side piece. The drill bit tracked like crazy.

In retrospect, it would have been much easier to machine the battery box if the end pieces were between the sides, instead of the way it is now. The reason I designed it this way was so any frontal impact forces would not put sheer force on the bolts holding the ends/sides of the battery box together. Drilling the holes into the ends of the side pieces was far from trivial. The drill press table had to be swiveled to the exact location, then the piece had to be squared up and tightened down. Even then, the drill bit tracked badly, I think because the sides of the drill press table are not square. Anyways, it should be good enough...

Skid plate material.

I had two choice for skid plates. I order a 12X24" sheet of 1/8" aluminum and of .09" steel from Speedy Metals. After doing some math, it turned out I'd save about 2.5lbs by going with the aluminum. While the steel would have been a lot stronger, I thought the aluminum would be sufficient; if I were to hit something requiring the extra strength of the steel, I'd probably be in pretty bad shape anyways.

Battery box!

Stuff left to do on the battery box:

1. Drill and countersink the skid plate.

2. Tap lots of holes.

3. Figure out all component locations.

4. Mount to deck.

5. Seal/waterproof.

I finished winding the rest of the stators and gluing magnets into the rest of the rotors. I couldn't get the packing as tight with these three because their epoxy jobs were worse than the first one

Work left for motors:

1. Wire hall boards.

2. Route wires.

3. Assemble.

4. Test.

5. Configure to motor controllers (time consuming).

6. Seal.

7. Glue tires.

So much time and so little to do...stop, wait, reverse that.

Lots of Snapping

2010-07-25T15:00:00.000-04:00

Let's just get down to business. Time to wind a stator. I haven't changed the electrical design of the motor since the Design Review . Each tooth gets 15 coils of 17AWG wire, for a total of 270 coils per motor (about 18 meters of copper). My winding pattern was AaABbBCcCAaABbBCcC, as described by this table (fyi, that's a great website if you haven't found it yet, granted it's in Polish...).

1st phase wound. Note the careful packing - it was the only way I could fit 15 turns on each tooth.

It took approximately 5 hours to wind the whole motor. Not bad, considering the gauge and my lack of experience. I have wound motors before, but they were those tiny little brushed kits with like, 28AWG wire and 3 teeth. Anyways:

You can see the permanent marker (smudges) that I used to mark the teeth numbers.

Success! No shorts and I didn't wind a phase backwards! As I mentioned earlier, these GoBrushless (actually Scorpion, and actually actually Chinese) stators have a unsatisfactory epoxy coating that can result in shorted phases. I could tell that the extra epoxy I added to thin/chipped parts of the coat really helped. If you use these stators, I strongly suggest doing this. Moving on:

See the gap? I had these custom arc magnets made 1mm-ish too thin to allow for gaps between the magnets (it turns out that complete magnetic filling actually hurts performance). The problem is, these magnets really like to snap together, so I had to figure out some way to space them. And not only space them, but space them the exact right amount...hmmm. While I figured that out, I realized I needed some super glue and micro-balloons (already had JB weld) to glue the magnets with, so I put in an order to towerhobbies. While that was coming in, I finished these:

Sorry for the bad pic quality, I'm still using a cellphone camera.


Much more badass with the MOSFETs.

Also during that time, I took a few days off to try to find a water-jet for the battery box sides. The Aero/Astro one will be down until at least Tuesday, and I'd have to attend an orientation during a time I can't make in order to use the HobbyShop one (and pay a membership fee...). Yeah, this sucked. It put me behind schedule on the battery box. I could have used the Edgerton/Archie laser to cut a template, which I could use to mark my aluminum, which I'd cut out on the band saw and spend an hour with the belt sander...but it turns out that I still couldn't use the laser for various computer glitchy reasons. *Sigh... oh well. So I was out a water-jet and a laser.

Luckily the towerhobbies order came in. I found this, which allows you to print out a diagram of your magnet placement. Of course, I spent an hour trying to get a 1:1 copy to come out of the stupid Athena printers, but I eventually got one close enough.

So I figured out the placement issues, but what about the magnet-snap issues? Super glue and a smart placing pattern to the rescue. I placed/glued every other magnet first. The super glue is great because it wicks right in underneath the magnets. Credit goes to Charles for giving me that idea.

Anyways, I still had the problem of putting in the other magnets in such a fashion that they wouldn't snap together (the nice diagram I printed out couldn't help me here). Through trial and error, I eventually figured out exactly how much spacing I needed between magnets...

...and it turns out this thin wire was perfect. After I got the middle magnet to this point, I would carefully remove one of the wires, put in a drop of super glue, let it set for a few seconds, then do the same for the other wire. It worked great:

Now I just needed to fill the gaps between the magnets with epoxy to make sure they don't pop out. This is where the microballoons come in.


My setup.

You can mix microballoons (or silica, or a number of other things) with epoxy to make it thicker (or give it other properties). In this case, I mixed some with JB weld until I got a consistency that wouldn't run. Then I went about cramming it between magnets as best I could. The alcohol was for clean up (it does a nice job of dissolving/cleaning up epoxy); I soaked a couple paper towels and ran them around the inside of the magnet ring to wipe up any excess epoxy. You can also use MEK, Acetone, and various other chemicals to clean up epoxy. Anyways, results:

It worked great! The dark patches between the magnets in the above pictures are the iron/steel filings from the JB Weld (JB Weld actually has metal in it). The magnets were strong enough to partially suck them out of solution! While that dried, I finished up wiring the stator:


Note the small twist on the bottom left: I terminated this motor in "Wye", as opposed to "delta".

Oh, so THAT's what the through-holes in the CAD drawings of his hubs are for...Yes, instead of running the wires through the middle of the hub (which is filled by axle in my case), or running them under a bearing via a flat spot in the hub (which sucks because you end up with 3/4 of a bearing surface), I ran the wires like this. But what's the 4th hole for? You'll see.

I took a break to work on some of the mundane stuff, like making alignment pins:

Yes, I could have bought them pre-made, but that costs a fortune. Instead, I bought a couple rods of precision ground 1/8", O1 tool steel, cut them down with a dremel, and then beveled the ends. I made these all about a 1/4" to long, but oh well.

Then I got the laser working!!


Cutting the hall-effect sensor board.

Cutting some spacers.


Finished product: laser cut acrylic spacers and hall-board.

The three lines etched in the hall-board are the angle lines for hall-sensor placement. Also, see the notch in the hall-board? The next thing to do was to drill a cross hole in the bottom of the hall-board for a screw that would tighten it onto the hub. Much easier said than done:


Oops.

So I went back, redrew the part with thicker edges, and tried cutting another one:

Can you tell I'm a noob with the laser cutter? This is what happens when you pick the wrong color mapping (this laser works by assigning colors to an AutoCad file that translate into various cutting powers).

Take 2:

Much better, I even got the screw in without breaking it.


Epoxied hall effect sensors.

Time for some sensored motor theory: To figure out the angle needed between hall effect sensors, you can follow this tutorial , or you can just do this: (360/# poles) / 3. Credit goes to Shane for that one. That get's you the number of mechanical degrees between two sensors, 12 in my case. Apparently, you can also just place them between the stator teeth. It's all mostly equivalent electrically (I'll let you work out the geometry), just differently mechanically.

Now I had all the major components made to start final assembly!


Everything laid out.

Of course I break off the hall sensors when wiring them...


RE-epoxying the hall sensors.

Oh, so THAT's what the 4th hole in the hub is for...sensor wires. I just fed the wires through, jammed the board up against the coils, tightened the screw...and SNAP. Crap. See the crack in the above pic? I officially disliked acrylic at this point (hating it came a few minutes later).


Lowering in the stator.

I used a vise and a drill press to slowly and carefully lower the stator into the rotor. I didn't want them snapping together and destroying each other. Next, I put the snap ring bearing retainers on, put the bearings on, put the spacers on, and put the hub caps on. Then after hammering in 10 alignment pins and putting in 9 screws:

TA-DA! The 10th screw is missing because it started to cross-thread for some unknown reason.

I really hate acrylic; every single hole in the spacers cracked, either from drilling them out (I didn't trust the laser to cut perfect holes, so I undersized them), or from assembly. I'm just going to have to suck it up and make polycarbonate spacers and hall-boards. Another thing to note from this pic: the screws are too long...or I didn't thread enough of the holes in the steel...either way, I'm getting shorter screws. Oh, and as noted previously, the alignment pins are too long. I knew I would have to rebuild the motor anyways, so I didn't bother grinding them shorter. I also didn't bother with the o-ring seals.

The 1/16" polycarb inner-side bearing retainer plate thing isn't on yet (I haven't made them yet...need water-jet or 5 hours with a mill).

Despite all the minor problems, it turns incredibly smoothly.

BEAST

Time for testing! Shane helped me with this part.


Here's the mess when we thought we could use a radio control Tx/Rx.

The drill batteries supplied 36V, the motor controller is a Turnigy 100A sensorless (no sensors for this test), and the scope is for measuring pulse width, i.e. speed. We were originally going to use a radio control setup for control, but it turns out all of the R/C stuff at the Edgerton Center is broken (and it's not user error...I've been driving/flying R/C stuff for years). Time to make a bigger mess:

The power supply is providing 5V for the R/C input and the function generator is providing the would-be-receiver output (pulse) signal. Changing the duty cycle is equivalent to throttle. Go Shane for figuring all that out.

It worked! Yay! Here are some specs:

2 phase resistance: 0.269Ohms (very consistent over all combinations ...within 1%)

kV: 50.5 rpm/volt

kT: 0.189 Nm/A

So at 10A per motor, I'm looking at a total of 8 Nm of torque, which is plenty. FEMM calculated about 2.3Nm per motor...not bad. This is the crazy part: at 48V, with 4in diameter wheels, this board will be going close to 30mph! Uhh...maybe I should rename it "DeathBoard".

Now I just have to make 3.5 more hub motors, including a rebuild of this one. Oh boy. LOTS of good news, though! I have most of the parts made for the other motors already, all of the parts I designed worked perfectly (I only had materials issues), and my torque estimates were right on the money. In other words, I think I did a good job engineering the motors. The only thing off target was my speed goal...I was aiming for about 15mph. I'm still not sure how I was off by a factor of 2.

Here's another pic:

See the ploycarb flakes in the bearing pocket? That's from the bearing race rubbing. I'll add that to the list of stuff to fix.

Next week: More motor building and (hopefully) battery box construction.

Hub Caps

2010-07-17T21:01:00.000-04:00

The next job was to machine the polycarbonate hubcaps.

The first step was to cut out a somewhat circular shape on the bandsaw. Then I center drilled it and and put it in the lathe for pressure turning. Pressure turning is when you use a centering bit/cone and press the workpiece against the chuck:

Doing this allowed me to round the edge and get a precise diameter.

Note the scratches from the chuck teeth- I didn't have enough pressure on them.

The next step was to take it out of the lathe, change the chuck, and grasp the outer circumference so that I could bore out the bearing surface. It turns out that I needed to take off about 8 thousandths less than the bearing OD for a nice fit. That seems like a lot, but polycarb is significantly softer than metal.

The four outer hubcaps.

Next, I needed to do the inner side hubcaps. These required a 2in bearing surface hole all the way through. So the first thing I did was take a 1.75" hole saw to them:

The hole saw was a pain because of the massive amount of friction it made and the tiny amount of surface area I could clamp (all of them spun in the clamps at least once). Then I put them on the lathe and cleaned up the inner surface.

Next came mill work. I needed to bore 10 holes (5 for an alignment pin and 5 for 4-40 screws) and cut a 3/32" o-ring groove. EZ-Trak to the rescue:

Note the ultra-thin parallels.

It worked pretty well.

ooo...Shiny

All that's left for these is cutting the 1/16" polycarb endplates for the inner-side hubcaps and threading the 4-40 holes.

All that's left to do for a test motor is:

1. The above

2. laser cutting acrylic spacers and hall-effect board

3. winding a stator

4. cutting alignment pins

5. gluing magnets

6. assembling

Oh and:

soldering lots of surface mount stuff on two S-Electronics 3PH Duo's. Oh boy.

Money

2010-07-14T00:34:00.002-04:00

Oh, and I've officially passed the $2K mark. OUCH. Luckily, I don't think there's much else to buy.

Rotors Part 2

2010-07-13T22:19:00.000-04:00

It turns out that drill bits track slightly when drilling. This means that drilling all the way through (34mm) my rims for the alignment pins and 4-40 screws is a bad idea. This also means that I had to drill halfway and flip the part over (doubling the number of holes I had to drill to 80...). I also knew there'd be no way to get a perfect alignment, so I just offset the two sets of holes.

The mill work went smoothly. Just put them in the EZ-Trak, found center with a gauge thingy, programmed a drill circle, did a pass with a center drill, a pass with an 1/8" skipping every other, and the other holes with a #43. Then stop, flip, and repeat. The angle holder thing brought deflection down to only a few thousandths.

I probably made over 1000 pecks over the course of all 4 rotors...so repetitive and soooo dull. But the results were pretty dang good.

Next came tapping the 40 4-40 holes.

So, 40 holes with a small tap...guess what happens.

Oops.

Oh well, that's ok. At least I only snapped one. I just moved the hole over a little.

Drilling into the magnetic flux ring seems like a bad idea in terms of magnetics and saturation...and it is, but luckily the holes will be (mostly) filled with steel pins/screws. After spending 16ish hours on these things, I'm starting to like the can threading method better.

Next Step

The next step is to get that rotor to fit in that tire. This requires boring out the inside of the wheel. The only way without making some time consuming compression jig was to put the tire in a giant chuck, use a boring bar to try and bore out the inside, and hope the thing doesn't deform. Oh, and hope that the boring bar can cut the rubber.

So far so good

The hard plastic cut like a dream. But when I got to the rubber...

...small chunks of rubber started flying out. Then it started to deform, which is when I stopped. I did NOT want this thing flying at my face.

What a mess. Total fail. It turns out that instead of a hard plastic ring directly underneath the rubber tire, there are large plastic teeth, such that, when they molded the tire onto the plastic, rubber would go between the teeth. So when I cut the bases of the teeth off by boring out the inside, the teeth turned into plates that would hinge on the rubber between...resulting in a very flexible and totally worthless part for turning. But couldn't I just make a jig for it? I could, that would stop the deformation...but it turns out that turning rubber this soft is impossible without constantly freezing it with liquid nitrogen. Now that would be cool, but I'm not about to set it up.

In summary: This won't work. Crap.

So now what? I basically have two options:

1. Learn polyurethane molding. I've always wanted to learn how to do it, but I'm not sure I have the time right now. My basic plan would be to make a silicon mold from these tires or some large cylinder, then cast them in urethane.

2. Find some urethane strip and glue it on the rotors, kinda like this (scroll down).

We'll see how it goes. There's always a bright side, though:

Hell yeah.

Rotors Part 1

2010-07-10T18:29:00.000-04:00

I'm back at MIT for a month to help teach the Edgerton Center's Engineering and Design Class, so I'm going to attempt to finish the ELB in my spare time.

Time for rotor work. I used the cold saw to cut a 2" piece of my 1020 low carbon steel stock. Mark gave me these cool lathe spider things that fit around the chuck teeth and provide a stand off from the chuck face (I didn't have enough stock to cut longer pieces).

After facing, I used a big boring bar to remove about 3/8" of from the inner diameter:

...and I found out how steel wool is made:

1 of 2 arm-fulls. For perspective, that's a full size trashcan.

...a lot of it.

After that, I simply flipped the piece around and cut off the extra.

Ta Da:

It really wasn't that easy (took me about 7 hours to do all 4), mostly because it's steel and I had to do a ton of tool passes (because I ordered stock that was too small ID...oops).

Next: Mill work, aka putting 80 holes in these for screws/alignment pins.

Well dang. Laser Update

2010-07-09T00:08:00.000-04:00

So I went through all the time and effort to fix the Laser and make mounts for it...and the mast holder collapses in <2 minutes. It's totaled...it had been repaired before Dad bought it for the same problem, so no surprise. Oh well.

Pics from Southern Thunder

2010-06-22T12:33:00.000-04:00

Ready to go for Level 1 cert. flight

On the pad. I couldn't get any pics of launches with my cruddy cellphone camera. However, I did get my L1!

Our prep area

Ready for L2

Let's play: "Find the L2 rocket in the 70ft trees!" Yeah, this sucked. I paid an arborist to climb the tree and get it out. I got my L2, though! Been waiting 10 years to be able to that...

Dad's Maniac. To give perspective, he's 6'5"

Very pretty day. Very hot day.

A "P" motor. Damn that's big. (Pepsi can for scale)

Well, that's probably about it for rocket stuff this summer due to bad timing with being out-of-town. Back to other projects.

Rockets

2010-06-15T16:47:00.000-04:00

Finally, a post inline with the name of this blog! I've been working on a rocket for Southern Thunder 2010 for the past week or so. I plan on certifying Level 1 and 2 with it.

Note: This build log is written for people with some previous knowledge of high-power model rocketry.

This rocket is a Blackhawk R&D 3" Stinger kit. It came as a kit with pre-cut fins and tubes, which is very convenient. I cut my own fins/tubes for many of my rockets, and it can be a real pain.
The first steps were to rough up the 3" body tubes and glass them (I'm going to put a J engine in it afterall).

Fiberglassing setup. The tubes are slid on here and laid up. Note the wax paper on the floor.

Glassed tubes.

Leftover resin formed to the shape of a cup it was in.

I used 4 oz. glass and West Systems 105/205 resin. You can see the rings I trimmed off the ends of the tubes in one of the above pics. The discoloration is where the resin soaked into the tubes differently. I then had to cut out the fin slits.

Then I soaked the ends of the tubes in superglue and sanded them to keep them from burring.

The dark ring is superglue.

The next step was to make the motor mount (the thing that holds the rocket engine).

Here's the forward centering ring epoxied on to the motor tube.

Kevlar is used for the first part of the shock cord because it's fire-resistant (there's a black-powder ejection charge that fires to deploy the drogue or main chute depending on setup) and won't wear out as fast as a regular shock cord. This will be attached to a shock cord, which will be attached to the bulkhead.

While that was drying, I started working on the payload section:

Here's the bulk head epoxied in. I used some Kevlar pulp in the epoxy to strengthen it. Another common filler is short carbon fiber threads; I couldn't find our bag of them, so I just went with the Kevlar. Silica/micro-balloons are other common fillers.

Filleting the other side of the bulk head with epoxy.

Then the motor mount was shoved into the aft end of the body tube and glued with some 5-minute and filleted:

The shiny stuff inside the tube is a fillet of about 30 grams of West Systems mixed with Kevlar pulp poured in. That centering ring is a good 18 inches down the front end of the body tube, so the only way I could get the epoxy in there is to aim, pour, and hope I didn't make too big of a mess.
While that was setting, I worked on the back centering ring. I didn't glue the two aft centering rings in (so only the one forward ring got glued in) when I glued the motor mount; you'll see why in a moment. The back centering ring needs motor retention screw mounts:

Motor retention screw mounts epoxied in.

While all that was drying, I worked on the fins. They are laser cut out of 3/16" plywood- very nice by my standards. The edges needed to be tapered/sanded, so I went to the belt sander:

A rounded rear fin.

I also drilled 1/16" holes in the fins near the body tube interface.

Forward fins. The fin tabs were cut a little short, so I glued a then strip of balsa to the bottoms.

These holes will act like rivets when filled with epoxy when filleting. The next step was to glue in the forward fins.

First fin glued in.

All forward fins glued in. Note the aft centering ring: it's not glued in, just holding the motor tube centered.

After gluing the fins in, they needed to be filleted. I used West Systems/Kevlar pulp again. Some people do tip to tip fin glassing at this point. I didn't feel that it was necessary with this rocket. My view is that it is only necessary for rockets flying L motors and over, or anything going Mach.

I normally fillet the inside of the fins while doing the outside fillets, but I couldn't on the forward fins because of their displacement from the rear end of the rocket (you'll see in a moment how I did it on the aft fins). So I needed some other type of inside strengthener. Expanding foam to the rescue!

The setup. Rocket, expanding foam, and MEK for clean up (lost the acetone).

I enlisted dad's hands for this part because foam is messy stuff. We attached some straw extensions to the foam nozzle in order to reach all the way down to the forward centering ring. Then I sprayed a bunch of the stuff in there. The stuff expanded a bit more than we thought, so I grabbed the middle centering ring and shoved it in there to stop the expansion. You can still see some of the mess after the first few rounds of clean up in the following pics:

The straw extension.

Overall, this method worked very well. The next step was gluing in the back fins and filleting them, outside and inside.

Note the inner fillets on the motor tube and body tube.

Instead of inner fillets, many people put sections of fiberglass on the inside. Again, I didn't think it was necessary on this rocket.

Then the aft centering ring was epoxied in and filleted.

And that's it for the structure! Next, primering and painting.

I sprayed two coats of heavy filler primer, then wet sanded.

Much smoother!

However, this primer is also really good at filling in the spots between grains in sandpaper. So I went through a lot of sandpaper:

Overall, I did three primer coats. Next came painting. I forgot to paint the tube white before laying down a coat of yellow on the gray primer, so I ended up having to do three coats of yellow...oh well.

Painting the fins black.

Painting black stripes.

Finished paint job! Not my best work, but ok. Time for decal work.

Post-clear coat. Done!

I now have a rocket that is basically ready for an H and J motor at Southern Thunder. It's missing an altimeter bay for dual deployment (necessary for the 7000ish feet high that it will go on the J), but that's a short/easy project.
It's also long enough for a smallish, e.g. I, hybrid engine, so that might happen if I get my cert flights in in time.

The pile of hybrid engine parts.

I'll post a few pics of the launch next week.

Laser Sailboat trailer holders

2010-06-15T14:46:00.000-04:00

I've been fixing up our HobieCat 16 and its trailer for the past few weeks in preparation for a roadtrip. I thought I had just finished up the trailer when Dad randomly picks up a Laser at a yardsale for super-cheap.

We want to tow on our current boat trailer between the Hobie's hulls. So I made some adapter/holders for it.

The first step was to measure out the Hobie trailer, specifically the relative positions of the three cross-members that would have the holders bolted to them. After I had those measurements, I determined the position the Laser would have under the Hobie and made wire-profiles of the Laser's hull at the points of the cross-members.

I started with some stiff wire and laid it on the Laser's hull at the position I wanted a profile. Then I bent and rebent and rerebent (etc) until they matched the profile. The next step was to transfer these to some cardboard.

These are then transferred to some pressure-treated 2x10's and cut out. They are only half profiles because I found that it was easier to mark half and flip them over to do the other half than to have a full carboard profile; doing it this way also more-or-less guaranteed symmetry (the wire profiles were far from perfect).

My setup. The jig-saw was used to cut the curve. The circular saw was for relief cuts. Wasp spray was for wasps.

In retrospect, a jigsaw is not the right tool for this. They just aren't meant to cut anything over an inch thick. It was incredibly frustrating and slow, and I broke three blades on this project. Unfortunately, we didn't have any other tool that could cut curves. Here's the result after much sanding:

Profiles in order from left to right = aft to fore. Spacing not to scale.

Unfortunately, the wire must have bent slightly inward from handling, and they all ended up not fitting right. I had to go back with the jigsaw and widen up the curves.

The next step was to attach carpeting to protect the boat's hull. I cut up an old weather-proof rug into strips and shaped/stapled/nailed the fabric to the holders. Here's the final result:

Not bad.

The next step is to bring them to the trailer, mark/drill holes, and bolt them to the trailer. Due to weather, I haven't been able to do this.

TV harvest

2010-06-15T13:49:00.001-04:00

CRT TV's are great sources for bunches of electronic components. We recently had one die from a lightening strike, so I thought I'd take it apart.

Our dead 27" CRT TV.

I wasn't sure what fried on it, and still don't know even after taking it apart. It would only show a single horizontal line in the middle of the screen, as if the entire picture had been compressed into that line, so maybe the vertical deflector coil went bad (it looked ok though...). The sound worked fine, despite the picture, but I thought that having a 75 pound, 6W stereo was pretty useless, so I started disassembling/breaking apart.

I read this instructable before proceeding. It was pretty useful, especially the safety stuff. The CRT acts as a capacitor and needs to be discharged. So I put the TV on my skateboard and rolled it outside:

The clip on the screwdriver shoved in the ground was part of my grounding rig. I attached the other end of the red wire to another screwdriver and shoved it under the suction-cup like thing on the top of the TV to discharge the CRT (I heard a tiny "pop"). The shiny thing in bottom center is one of the 3W speakers. You can see the circuit board (and the flyback transformer!) to the left of it.

Close up of the CRT.

Some people salvage the CRT for stuff. I have no idea what I'd use it for, so I didn't mess with it. I was after the flyback transformer ( ;) watch for a plasma speaker future project). Here are a bunch of pics (click them to enlarge):

Close up of circuit board.

Close up of flyback transformer. Sorry about the bad picture quality- I should be getting a real camera, i.e. not a cell phone camera, sometime soon.

I always wondered what was inside of an S-video thingy. Turns out there are a lot of tiny coils.

Circuit board post-harvesting. I basically took everything that wasn't a tiny resistor, possibly fried, or generally useless.

Here are some pics of all the stuff that came out of this TV:

Transformers, crystals, relay, semiconductors, multi-watt resistors.

Capacitors.

I don't know what these are. Some seem to be inductors of some type.

Moral of the story: Never throw out your TV, and before you go recycle it, steal all the useful components.

Magnetohydrodynamic Propulsion Drive

2010-06-04T23:36:00.003-04:00

Cool name, right? Magnetohydrodynamic drives use a crossed electric and magnetic field to propel ions, such as salt water ions. That simple. Think the caterpillar drive from the Hunt for the Red October, or the Japanese boat, "Yamoto". I first came accross this idea at "Evil Mad Scientist Laboratories" 3 years ago. I found this article shortly afterwards and decided to build one! I apologize for the cruddy pictures-it's a cell phone camera.

The gray strips on the inside are aluminum electrodes. The big bricks on the left and right side are 1" x 1/2" x 4" NIB magnets. The magnet field on the inside maxed out my sensor, which goes up to 1000 Gauss. I'm guessing the inner gap is around 1T, considering the size of these things. The middle box is made of fiberglass; it needs to be to hold the magnets apart.

The black coating is "Plasti-Dip", possibly one of my favorite things. It's a liquid rubber that drys to form a rubberized coating on pretty much any surface. It's used by R/C hobbyists to waterproof electronics, such as servos and receivers. I used it to coat the MHD drive.

It comes in both a can and a spray. See the big spill on the can? Funny story...so I bought this thinking it would be easy to just dip the magnets in one at a time and get a nice coat on them. Turns out the can is steel. The magnet just jumped into the can and made a huge mess- my hands were black for about a week. (That's when I bought the spray.)

My setup was a tub of salt water and a 13.8V 25A DC power supply. Here are some videos of it in action:

Here are the links if the videos aren't working: #1, #2.

The effects of electrolysis are pretty obvious; I can't think of any way to counter this. The aluminum electrodes wear out pretty quickly. Reversing the electric field (and thus the flow) every now and then seems to help with electrode wear. Titanium or platinum electrodes would probably help a lot, but are also expensive. It was a cool project. Future work will include:
1. Incorporating a similar (probably bigger/longer magnets) into a custom R/C boat or submarine.
2. Using onboard power. I'm thinking LiFePO4 power cells- the high discharge rate would be very beneficial. I think I'll stay away from LiPo's this time. Considering all the H2 and O2 produced, I think there will be enough flammable stuff around.
3. Experiment with ways to reduce the electrolysis and increase electrode life.

Learning Lessons

2010-06-04T13:46:00.000-04:00

Well, after over 4 months since ordering, the remaining 3 stators are here. And what did I get for my trouble? Express shipping to the wrong address and a 15% coupon. Wow, gee, thanks...

Yeah, I'm pissed off. The express shipping meant that they couldn't change the address before it was delivered. Now I might not be able to get them for another 3 months. Anyone know what MIT does with mail sent to closed dorm addresses over the summer?

Needless to say I am most dissatisfied with GoBrushless. One of the most incompetent merchants I've ever dealt with. However, it wasn't completely their fault because their supplier in China is mostly to blame. Though the shipping error was completely their fault.

Anyways, I'll get them eventually. I'm glad I didn't have to do a total redesign, or cut up a motor ((like one of these) for a stator, the latter of which would have resulted in at least a slight redesign.This experience taught me the value of flexibility; unfortunately, brushless motors aren't very flexible. I still should have designed for more available parts (stators); GoBrushless is currently the only source for 65mm stators. Believe me, I know. I've spent hours and hours scouring Google for another source- nothing.

In summary, here are the lessons I've learned:
1. Always ask about stock status of a part before ordering.
2. Stators (or anything) from China could take many months.
3. Design for available parts/design for flexibility.
4. For what you pay for custom USA parts, you get back in time. For example, if I had gone with ProtoLam, I could have designed exactly the stator I wanted and I probably would have had all four of them in less than a month, but it probably would have cost four times as much. (There's also a quality advantage to USA parts, but that's a different story.) However, in the real engineering world, the net cost of waiting four times as long would have outweighed the higher production costs.
5. Repeatedly to the point of exhaustion remind the merchant of the correct shipping address.

I found out something else, too. These stators, and some other ones GoBrushless sells, are the same (as in from same supplier) as the ones in Scorpion motors. However, Scorpion doesn't officially have a 65mm motor out yet, though apparently that's coming soon along with 85mm ones. Scorpion is also developing wind generator motors, so they could potentially be a source for large stators. I bet they'd sell you stators directly if you contacted them.

Finally

2010-06-03T10:46:00.001-04:00

I HAVE RECEIVED WORD THAT THE STATORS ARE IN THE USA!!!!

That is all.

ELB mid/late May

2010-05-24T13:43:00.002-04:00

The first thing you should notice (well, at least if you go to MIT) is that mid/late May is when I should be studying for finals. Hmmm, evil finals or fun machine work... Finals won out eventually, but until then, I put off studying as long as possible to finish these hubs.

Now on to the mill work. I needed to put a round groove into the press-fit surface that would accept a 3mm rollpin. So I used a ball end mill. I had no idea people made these things; they're pretty cool.

I don't have pictures, but during this process I did a tool change on every part in order to drill and power-tap 10-32 grubs screw holes for axle locking. The next step was to flip the part upright and bore the four wire holes. This required an extra long drill because the regular length drill couldn't get the last 3/4" or so.

Test gauge used to zero the X and Y axis. Mark has a bunch of these.

Really long drill bit. I'm not sure why it looks like I'm drilling into the vice here...weird camera angle I guess.

I got two finished this day before the shop closed.

The 4 completed hubs. Note the two shorter ones for the rear.

Now I just need to press fit them. Unfortunately, I didn't have all four stators in (grrr...I still don't) yet, so I only did one. First, I had to machine a little tube that would provide a good surface for pressing. You can see it underneath the stator in this picture:

Ta-Da! One finished.

You can see the rollpin in place in the above picture. That little bugger was hard to get in...it kept bending. Anyways, that's it for now. I'm taking June off, but I'll be back to work on this in July as a side project to teaching high school kids about engineering through an Edgerton Center class. I'm hoping to get the whole board finished by early August!

Stuff left to do:

1. YELL AT GoBrushless UNTIL THEY GIVE ME MY OTHER STATORS

2. Wind

3. Solder the motor controller boards

4. Machine the rotors

5. Plastic mill/laser work (hubcaps/spacers/hall effect sensor board)

6. Bore out the tires

7. Assemble a wheel and test it

8. Mill/waterjet the battery box components, assemble, and test waterproof-ness

9. Find out what Franco did with the radio control

10. Wire everything up

11. Don't kill myself the first time I ride it.

Yikes, that's a lot of work left.

ELB early May

2010-05-24T13:06:00.001-04:00

The hubs are the parts that adapt from the axle to the stator. Some people combine the axle and the hub into the same piece (think BWD), but I couldn't do that because longboards have trucks. Trucks are somewhat complicated hunks of metal that allow skate/long/mountain-boards to turn. They also have axles embedded in them meant for non-hub motor wheels, which means I have to have an adapter if I want to use stock trucks. I probably could have made my own, but that would have required a lot of design time and money (large chunks of aluminum are expensive), and I'm a firm believer in "simpler is better" when it comes to engineering. Read the design review for more details on the trucks I used, but for now, here's a picture:

Anyways, back to hubs:

Front hub design. The rear differs only in length thanks to some stupid brackets welded on the axles I'm using- see the pic of the trucks above.

They are made out of T3-2024 aluminum, which has high shear strength. The hub slips over most of the axle and then threads onto the last half inch. Two grub screws bite into two flat spots ground onto the axle, thus keeping the hub from unscrewing and falling off. The largest diameter surface is a press-fit for the stator, and the groove in that surface is for a rollpin (Ohhh! That's what the round groove in the inner surface of the stators is for...). The thin grooves on the bearing surfaces are for snap rings, i.e. bearing retainers. The four holes that go through the thickest part of the hub are my way of avoiding having to grind a flat spot in the hub under a bearing for wires to route through. The drawback of this plan is that I have to have a massive bearing on the deck-facing side of the motor. Oh well, at least the larger bearing will handle shocks from the horrible Cambridge streets/sidewalks better.

I began by cutting down the stock into pieces slightly longer than the total length of the front hubs. The plan was to make four identical hubs and then cut off some of the end on two of them to make the rear hubs.

My mess. The second step was to do the boring and threading. Two are finished with that step in this picture.

Mark (the Edgerton Student Shop guy) wanted to use the CNC lathe for the outer surfaces, so I of course agreed (soo much less work and more precise). After about three minutes of CAM, the lathe was ready to go. Check it out:

One of the half finished hubs. S'up Mark.

Four half done! This took maybe 30 minutes. It would have been much quicker if I didn't have to be so careful about not taking too much off of the bearing surface.

Next step was to flip them around and do the other half.

Left: one of the massive 1" x 2" bearings. Top: One hub with the surfaces done.

4 with finished surfaces. You can see the inner threads in this pic.

The next step was cutting snap-ring grooves, one 5/8" and one 1" in each. Mark has an awesome bookcase full of engineering reference guides, and one of them happened to have everything you can imagine about snap-rings in it. He also has some cutting tools specifically designed to cut very very thin grooves.

So shiny... Note the snap-ring groves.

The next steps were all mill work, and were done later in May.

ELB April

2010-05-24T10:23:00.001-04:00

Due to other school projects, I didn't really start building until April. I began by practicing winding the stators.

My winding station. Giant vice with paper for protecting the epoxy. The piece of PVC is how I get leverage.

The stators are GoBrushless 18T, 65mm x 34mm. They have a green epoxy coating for insulation and wire-enamel protection. Unfortunately, that epoxy likes to chip off the tips of the teeth, which means 5-minute to the rescue. So strike two against these stators. What was strike one you ask? The fact that I ordered four of these back in January and STILL haven't gotten the other three in. GRRRRR

It was at this point that I realized that the giant spool of 16AWG wire I bought wasn't going to work because I couldn't fit 15 turns of it on a tooth (I got 14! so close...). So I went to 17AWG:

I tried a bunch of different schemes to get the best possible packing, including that ugly loop sticking out the side. Needless to say I abandoned that idea.

Success! That's 1/2 of 1 phase wound with 15t of 17AWG wire per tooth. These pics were the product of a couple hours of trial and error (including messing around with 16AWG), but now I know how to wind these motors.

The next step was working on the hubs, probably the most complicated part of this whole project.

Electric Longboard - Pre-blog work

2010-05-23T23:15:00.001-04:00

About the project: While the name of this blog is Rocket Science, it was originally created as a build-log for the Electric Longboard (ELB) project. Inspired by the hub motor stylings of the BWD Scooter, one of Shane's builds, and Charles' many builds, the ELB is a 4WD longboard with custom in-wheel hub motors. I came up with the general idea in November and completed the bulk of the design work over IAP, MIT's awesome-we-don't-have-real-classes-in-January period. I turned the project into a personal (and later, a duo with Franco M.) research project through the Edgerton Center.

Check out the updated design review!