SMMA Fall Technical Conference - Wrap-Up

Well, I don't really have any new pictures, so here's some dessert.

Yesterday wrapped up my trip to the SMMA Fall Technical Conference and I failed to update the "live blog" (which I have decided never to do again) mostly because I spent many hours in the airport and on a plane. But anyway, I am back in Cambridge now. The trip was definitely a good one for me. I expected to learn a lot, and I did. Here's a few things that I came away with:

• The electric motor industry is very big, reaching out into many markets. The biggest ones seems to be industrial (big constant duty induction motors) and automotive (little accessory PM motors, not traction). But both of those markets took a big hit. The "new markets" are renewable generation (wind) and automotive traction. Almost everyone identified these markets, but few have stepped into them yet, it seems.

• There was a very interesting idea proposed by EMERF for a pre-competitive research consortium that could maybe focus on some of these new markets, or on PM issues. The thing that comes to mind for me is the FEMMulator. Here's what I mean: The "industry" is slow, and built on a large pool of experience and expertise and data from existing designs. And then there's the FEMMulator...a free program that one person wrote to run an iterative FEA design simulation on a brushless motor. Here's the "disruptive technology," except instead of a technology it's really more of a disruptive methodology. The idea that anyone can get access to enough information and computing power to design a motor these days means that the process is different...and a collaborative research intiative seems to make sense, IMO.

• On the technical side, there are a lot of things I know now that I will have to look at more closely. For one, segmented drive PWM. I thought I knew all there was to know about PWM in an h-bridge or inverter, but apparently not. So I will look into the more interesting methods and see if they could be applied to my controller at all. Also, there is a commercial sensorless zero-speed-start BLDC controller available call the DPFlex, from Agile Systems. It uses variations in inductance to measure starting position of the rotor. I knew this was possible and had been researched, but it's interesting to see one in production. I wonder if I could get my hands on one for cheap...or else it's good at least to know that it exists.

• There were also some ideas that were met with a bit of skepticism. There was QM Power, which, well I understand where the skepticism comes from. I actually thought their presentation was pretty good, but after going back and reading their website...their idea is either ridiculous or they do have a clever breakthrough and need to fire their entire marketing/promotional team for making it sound ridiculous. There's also a clever idea for a harmonic drive motor, which is like a harmonic drive gearbox but without the strain wave generator...you use electromagnetic forces to move the spline. That one is really clever, and it would certainly work. The question is how efficient it would be. But it would be great for robot actuators, etc.
  

• I got some good feedback on the axial motor concept. It's fairly unique even among axial motors, and most people think it could get very good power/torque density. Interesting to see how it works out.

That's pretty much the wrap-up. Lots of new contacts and lots to think about. It was an good trip for me and I think the presentation part went pretty well. (Nobody asked about the segway...so that's good...) As promised, a link to the presentation I did (minus videos) and also to the paper. (Overview of the Summer Engineering Workshop with emphasis on the BWD Scooter project, with cool pictures of course.)

SMMA Fall Technical Conference - Day 2

Day 2 at the SMMA Fall Technical Conference.

Today was the first day of the conference proceedings. That meant tabletop sessions and parallel presentation sessions from 8AM to 7PM, followed by a nice dinner. The tabletops were very interesting. I didn't get a chance to wonder around much because I had to man my table, but I was right next to the bearing guys:

Bearing sea.

So one of life's mysteries (How do you assemble a ball bearing?) has been answered for me. The answer, it turns out, is far less magical than I thought. Anyway, that was fun. Proto Laminations also had a tabletop across the way with a very flashy poster. Here's a fun picture of our matching tables:

How is it standing up?

That, by the way, is Wally Rippel from AC Propulsion. He was also responsible for the Great Transcontinental Electric Car Race between Caltech and MIT...in 1968. Yes, 1968. Long before Tesla or the EV1, they raced electric cars across the country in 210 hours, recharging from the grid along the way. Caltech won by 30 minutes, but both cars finished the race...in 1968. So what exaclty have we been doing since then? (Besides landing on the moon and inventing the internet.) Well, batteries are a lot better these days. As are power electronics. Maybe time for another race?

One observation I got from today was that the motor manufacturing industry, as a whole, is relatively conservative. (They describe themselves this way...it doesn't have the same connotation as it does back in Boston...it's more a deliberate, careful way of doing business, not a political/social position.) But since the economic downturn, things are being shaken up and the big shaker-uppers are energy and transportation. So a lot of the manufacturers need to decide whether they want to take big risks by heading into these markets.

One big deal topic was permanent magnets. Some went as far as to say that they are avoiding permanent magnets altogether as too risky an investment for motors. The logic, which eludes my own sense of the workings of a global economy, is as follows:

1. China has all the rare earth ore.
   
2. They want to make magnets instead of exporting ore.
   
3. More value added = more profit for China, less for the US.

Okay, but doesn't that just mean that you get your finished, ready-to-install motor magnets from China instead of from the US? They would probably be cheaper, if anything! It's not like oil...there is no impending shortage. Maybe I just don't understand the market. But here's the next step in the logic I've heard that is the real interesting part to me:

• If China wants to make magents instead of exporting ore, then they will eventually want to make motors instead of magnets.

Well now I can see why that would be a problem for the motor manufacturers. But I still can't see how that justifies moving away from PM if it is clearly the better design choice for certain types of motors. Isn't the end result there that China makes better motors and we still lose market share? Wouldn't be the first time we've gone down that road.

Interesting, interesting stuff. But a bit outside my expertise, so I'll stick to the technical stuff.

Day 3 coming up.

SMMA Fall Technical Conference - Day 1

Today was the pre-conference workshops day, and that meant a four-hour course on "Fundamentals of Brushless Motor Control" for me.

Oh boy.

Despite the 8AM-ness of it, I got a lot out of it. It was just the right level for me...stuff I had seen before but don't full understand yet. Hence the reading material. That's the kind of thing that will keep me entertained for quite a while.

For one, I learned that there are more ways than I knew of to drive a PWM signal in an h-bridge or 3-phase bridge. I don't really understand some of the more clever ways yet, or the differences in performance, but it's something to look into in the controllers I build.

Other than that, today was Essex Active plant tour day. I learned what a slot wedge is:

A slot wedge.

It's a wedge that goes in slots. (Motor slots, of course.) It can be made of many things, including Nomex. The windings presumably go inside the wedge. And presumably the wedge is a tad longer than the slot. So the windings never touch steel. Genius. I wish I had some when we were winding the scooter motors. Oh well.

My table-top demo is ready and I will be manning the post early tomorrow morning. Then in the afternoon I present, right before AC Propulsion... Moderately frightening. More pictures tomorrow when that's all over with and I can relax a bit.

Live from Chicago: SMMA Fall 2009 Technical Conference

Here goes my first-ever attempt to use the blog in semi-real-time.

This week, I am in Chicago for the SMMA Fall 2009 Technical Conference.

The SMMA is "The Motor and Motion Assocation." (The 'S' is silent.) Actually it used to stand for the Small Motor Manufacturer's Association. I wonder if I can get it changed to the "Scooter Motor Manufacturer's Association." Probably not. But anyway, you can find more information on the SMMA, which serves as the trade association for the electric motor and motion control industries, here: http://www.smma.org.

I am participating in the conference thanks to support from the Electric Motor Education & Research Foundation (EMERF) which, as the name might suggest, is right up my alley. This foundation supports and promotes academic, pre-commercial electric motor R&D, as well as educational initiatives in the field of electric motors. More information is available here: http://www.emerf.org.

This all has to do with the BWD Scooter project. Since we had the completely original, not ripped-off at all idea of making brushless hub motors for a kick scooter, I made a contact at Proto Laminations who is also a member of the SMMA/EMERF. Having now actually made brushless hub motors for a scooter, I am going to present it, or, I guess, present the Summer Engineering Workshop and its fleet (yes, fleet now) of vehicles. Of course, the scooter is the only one that's in any way portable:

I did say "in any way."

And since it is generally not acceptable to ship/check/carry batteries that you made yourself, the A123 pack has to stay at home. But cordless drills (and their batteries) are totally fine:

All Tech men carry batteries...and cordless drills.

Ok TSA? So back off. Or don't:

Not surprising.

Actually, the trip out went smoothly. Actually, very smoothly. Actually, it was the single most coordinated thing I have ever seen at an airport. A full 50 minutes before the scheduled departure time, we were told to board the aircraft as quickly as possible because they were going to try to beat a storm. ~40 people got on and sat down, the jets spooled up, and we did a rolling takeoff 20 minutes early. Amazing. I honestly don't see what all the complaining is about air travel...

As for the scooter, it survived. Or wait, did it!? Why does the rear motor feel like it's got Jello in it? This is usually a sign of a phase-to-phase short, which, if you've ever shorted the leads on a PM motor, you know is electronic braking. Well, this was a scary few minutes of wondering if the phase windings somehow shorted during transport. But the problem was a lot simpler apparently:

?

Well, actually, I have no idea what the problem was. But I know what the solution was:

1. Remove strain relief zip-tie.
2. Replace strain relief zip-tie.

Problem solved. I don't really want to know what is going on. At least it's not inside the motor. I just have to not mess with it for the next three days. Did I mention that I hate live demos? This is why I obsessively video tape everything while it's working. And why I brought a soldering iron...just in case. Also, the non-functional switch broke:

:(

But since it was non-functional, I guess it's okay.

Anyway, tomorrow I am taking an early-morning brushless motor control course where I might learn just how wrongly I've been doing it. So sleep may be in order. My presentation is on Wednesday, and I will post the slides right after. (But by now, you probably know what to expect.) And, of course, I will get to see all the other demos and presentations. Pictures and updates to come.

Updates: 3ph Duo and Epic Axial Motor

Being midway through the fall semester means starting to make preparations for 2.007, the annual MIT MechE robotics class/contest that happens in the spring term. While this involves a lot of manual labor in the form of building the contest tables, I can't actually show any pictures of that because it would give away the game. Besides the woodwork, there are also some more boring but necessary tasks to be done, such as testing new batteries:

What? Don't give me that look.

I'm not a very good multitasker, but if one of the tasks is watching a battery heat up a cup of water, I can do it. So here are some project updates and pictures in the meantime:

The 3ph Duo controller, the latest in my line of increasingly functional motor controllers, is finished. As in done, built, tested, working as planned. Here are a few pictures from the assembly, which, though not perfect, went pretty well:

The IXYS GWM 100-01X1, a wonderful 100V, "90A" three-phase MOSFET bridge package, was the key to this new controller, enabling two-channel, 1kW per channel BLDC control in a small form factor. Unfortunately, they only seem to be available from one place (Arrow) and only in the straight lead "-SL" version. This required some interesting bending, cutting, forcing, and soldering to get the chip to fit on my new circuit board:

It's not quite through-hole. Consider it...semi-surface mount. Same goes for the current sensors, which are designed to be through-hole but have their leads cut to length so they can sit flush with the sensor opposite the IXYS package. Not the cleanest assembly procedure ever, but it works. The IXYS package heat tab gets thermal pasted to a heat sink, and 5mm plastic stand-offs set the height correctly.

All of the other IC components (HCPL-3120 optocoupled gate drivers, DCP021212 isolated high-side power supplies, LM2575 switching regulator, MC7805 linear regulator, OPA2340 R2R op-amps, and 74HC04 hex inverter) are now surface mount. The capacitors are still through-hole because I hate surface mount caps. But at least they are in the right place now.


The bus bar / heat sinks are gone and instead the power wires get soldered directly to surface-mount-style pads the board. This is something I saw in another ultra-thin controller. The signal connectors are still the same (Molex 0.1" R/A keyed).

These are all mostly form factor changes, but there were a few significant functional improvements as well. First, both motors now run on a single MSP430 microcontroller. Turns out this 16-bit, 16MHz chip can output enough PWMs on enough pins to pull this off. It processes twice the number of hall-effect interrupts as before, but some testing shows that even with two motors, the main loop execution breaks down at 1.6 million electrical RPM (228,000 mechanical RPM, or Mach 4.5, for the scooter). Basically, it makes the Arduino look silly.

Second, the current sensors now sense phase current, through a convoluted single-sensor synchronous measurement method that I wish I had never thought of. (It is a real pain to code, and is relatively noisy compared to sensing DC-link current, but at least it is a direct measure of actual motor current.) The current data looks a little noisy (about +/-4A noise on top of a 20A signal), so I've been working on software filtering mostly to clean it up even more for analysis, but the actual torque control feels smooth enough. The other data acquisition functions (all wireless) work great. Here's a cool test of the different motor constants measured cleanly at no-load:


This definitely shows the difference between the front motor (blue) with 60 windings per phase and the rear motor (green) with 90 windings per phase. Other interesting things I've done with the data acquisition: measured motor resistance (0.275-Ohm front, 0.355-Ohm rear) measured battery internal resistance (~0.075-Ohm for a 10S2P A123 26650 pack), measured top speed (~18mph), tested PI control of current (not very much benefit).

Oh, and most importantly, it fits in the deck:

So there. Everyone who asked why the original version was so big can be quiet now. I will be testing and writing this controller up very thoroughly, since it's in a state now to which few of my projects ever get. I'm not sure how useful it will be, since there is still a lot of "custom" in it, especially on the microcontroller side, but hopefully people find the bits and pieces useful at least. Somebody can design an Arduino adaptor for the power stage... For now, here is a link to the schematic, board files, source code, and other random bits and pieces for anyone who is interested.

Next up, the Epic Axial Motor. This is the beast. Although just 4" wide and 9" diameter, it will produce multiple tens of kW. I'd estimate it at around one horsepower per pound, minimum. Well, the simulation says so anyway.

Flux.

But I don't know, I don't really trust simulations yet. Although it did very well for the scooter motors. The goal here is to build the motor in stages, testing along the way to double-check or refine the simulation. The axial-flux design with two rotors and separable tapered roller bearings lends itself to "easy" assembly and disassembly for prototyping. "Easy" if you can safely accommodate for the 500 lbs or so of finger-nomming axial force. So anyway it's time to build.

Yes, I actually went into the machine shop and fabricated a metal part. Aren't you proud of me? I've been doing EE-type projects for so long that I forgot what it's like to go home with aluminum shaving in my hair. What's that? There are waterjet-cut parts in the background? Nonsense. I made those by hand. Okay fine, I got those from Big Blue Saw, which now has a low-taper waterjet! As for the rotor, Mike "Master of the EZ-Trak" Nawrot is going to produce the spinning magnets of doom. (It would take me a month...)

After that, it's on to winding a test set of stator segments. And by winding, I really mean folding, or something, since we are using concentric layers of 15-mil copper shim stock for massive current-carrying capability. I'm actually fairly excited to see how this works. It could have significant advantages over large-gauge magnet wire...or it could be a total failure. We'll see.

Next week = SMMA Fall Technical Conference in Chicago where I get to meet the pros.

Last-In, First-Out: Charger / Balancer

I normally don't wrap-up projects in reverse order from when I start them, but the charger/balancer is going to be an exception for a few reasons. One: It works okay as-designed, since it's really simple. Two: It is still annoying to use and in all honestly I probably won't.

The point was to solve two problems with charging a medium-sized lithium iron phosphate battery pack. The biggest problem is the lack of small, inexpensive constant-current/constant-voltage power supplies that also plug into the wall. (Think laptop adapter size.) Thanks to the wonderfully cheap Meanwell PLC-series LED supplies, this was easy to solve. I got a 100W version, which can put out 2.65A (regulated to this current at anywhere from 30-36V), or about 0.6C for my battery pack. This means a < 2-hour charge, which is pretty good for a very portable charger that plugs straight into the wall. There's just one small problem:

Why are my ears ringing at night?

For whatever reason (cheap electronics) the Meanwell LED supply goes into a very noisy switching routine in constant-current mode, and the frequency of switching happens to be about 1,700Hz. Fun fact: The human ear is most sensitive to frequencies around 2kHz and three times more sensitive to this frequency than to middle C (262Hz). Basically, if I wanted to torture someone with a particular tone, this would be it. The good news is that you can tell when it switches over to constant-voltage mode, because the 1,700Hz fades away leaving only a faint and relatively pleasant high-frequency voltage regulation circuit.

The second part of this project was the balancer. The balancer was a very cool idea. It used the LTC-1440, a comparator with a built in voltage reference, to detect overvoltage and trigger a power draining transistor. Said power transistors were 44H11s, which are very solid NPN transistors with h of well over 100. The base circuit is described in more detail in my last post. And, well, it works just fine:

What would I do without MATLAB?

The current starts to flow through the transistor at about 3.55V and reaches a peak value (set by a resistor) at about 3.60V. The peak value in this case was set to just about 2.2A, which is what the Meanwell supply was to be set to. At such a current, the transitor would be required to dissipaye 2.2A*3.6V=7.9W. That's a lot, but not unreasonable with a heat sink and fan:

Baby heat sinks.

The boards stack nicely and a modestly powerful 120mm case fan blows air across them. Tested at 2.2A for over 30 minutes, the steady-state temperature of the transistor's tab settled at about 55ºC. That suggests it could be pushed a bit harder, probably all the way to the 2.65A limit on the Meanwell supply. This would just require changing the 220-ohm base resistor to something a bit smaller.

So what's the problem? (There must be one.) Well for one, something is killing the LTC-1440s. My best guess is it's the fan or the 12V switching regulator that powers the fan, or some combination of the two. With the fan disconnected or running from a separate 12V supply, the problem does not present itself, even after multiple connect-disconnect cycles. The symptoms are: only the circuits connected to the top three or bottom two cells fail, and they stop producing a reference voltage or an output after some small number of connect-disconnects. Could be momentary overvoltage, made possible somehow by the switching regulator connected across the whole cell stack and depending on which side (high or low) makes contact first.

I really don't know. Normally that would bother me, but as I was testing this thing I realized that I also don't care. The biggest problem is that it's too damn big. The whole portability aspect is gone, and forget about taking it through an airport (or out in public in Boston) with all the circuitry and wires. Not much I can do about that...a cell balancer needs as many wires as there are cells (plus one). And the transistors definitely need heat sinks and a fan to balance at any reasonable current. So forget it! For now anyway, I can live without it.

"But how do you keep the cells balanced?" I don't. Every once in a while I might drain or charge ones that are way off with a power supply.

"But how do you keep the cells from overcharging?" I feel like if I explain the method, somebody will try it and blow up their cells... It involves setting the charger CV so that, worst-case, no single cell can exceed a certain voltage. For example, if the pack is at 33.50V, and the highest cell is at 3.37V (measured), I set the charger to 33.50 + (4.20V - 3.37V) = 34.33V. This is more than enough voltage overhead to put it into CC mode, where it sits happily for a long time before I have to readjust the CV based on a new worst-case measurement. If none of that made sense to you, you should probably invest in a user-friendly charger/balancer. :)

Anyway, back to other things:

Going to the SMMA 2009 Fall Technical Conference in two weeks. Mildly frightening, but I am looking forward to the immersive motor experience. Definitely a post in the works after that.

The 3ph Duo controller is DONE and WORKING and I am starting the long process of writing it up. An extensive post will follow.

The Epic Axial Motor is...in progress.

What's Next: Charger / Balancer

Not ideal.

Charging LiXX batteries is an annoying, time-consuming process and can easily result in a dead battery pack, or in the case of Lithium Polymer batteries, massive destruction. There are two main problems:

1. They need a constant-current, constant-voltage (CC/CV) supply. You can't just hook them up to an AC adaptor or a variable-voltage power supply. The internal resistance is so low that even a small difference in voltage between the supply and the pack will cause a large current to flow.
2. Cells need to be protected from overcharging on an individual basis. Unlike lead-acid, they will not self-balance and are not tolerant of temporary overcharing. The voltage of every cell should be kept below a certain value at all times.

There are plenty of solutions to Problem 1. Most lead-acid battery chargers have a CC/CV strategy of some sort. Unfortunately, they only make them in increments of 12V (14V cutoff). So for my target application of a 33V pack with a 36V cutoff, I'm out of luck.

The more expensive option is an adjustable charger such as the Astroflight 109. This is basically a lab-style power supply in a small package. It will automatically monitor and adjust voltage and current, taking whichever hits a limit first as its operating point. So for a fully dischaged, 30V pack, it may only need to put out 31-32V to supply 5A. But as the pack voltage rises, it will adjust its own voltage to maintain 5A until the upper limit of 36V is reached. Fancy. But it also has problems. The biggest of which is that it runs on a 12V supply with no provisions for plugging into the wall. So for $120, you still need an AC adaptor. And it still doesn't do balancing.

Under ordinary circumstances this frustrating lack of good options would be the cue for me to build something from scratch. One idea I had was to use a Dell notebook adaptor as the front-end, since I carry one (or two) of those around on a regular basis anyway. But that still means I would have the build the CC/CV stage AND the balancer. And I don't want to make a big project out of this...I just need something convenient. So I started looking for hybrid off-the-shelf/custom solution and stubled upon this:

It means well.

The Mean Well PLC-100 is a 100W power supply module "suitable for LED lighting and moving sign applications." Clearly, exactly what you need for charging batteries... But actually LED drivers tend to be current-controlled devices, and in fact this power supply does have a constant-current output from 75% to 100% of its voltage rating. So the 36V version can provide a stable, adjustable current of about 2-2.65A from 27-36V. Perfect for a 33V battery pack, which should rarely drop below 27V anyway. 100W will charge the pack in under two hours. The cut-off voltage is also adjustable to between 85-100% of the rating. So the 36V version can be adjusted from 30.6-36V. For different-sized packs, there are versions for 12, 15, 20, 24, 27, 36, and 48V, all with a similar 85-100% trim range.

That takes care of the front-end supply. Now all that's left is the monitoring and balancing part. It would be excellent if this part didn't require a microcontroller or any programming. It would be even more excellent if each cell had its own, independent, floating circuit that handled balancing. Turns out this isn't actually that hard:

The monomer, if you will. (Click for a clearer picture.)

This circuit runs entirely off the voltage of a single battery cell. The core is a voltage comparator, the LTC1440, which has a built-in 1.182V reference and optional hysteresis setting (not used in the circuit above). The comparator check the voltage of the cell, through a 1% resistor divider. When the cell voltage reaches 3.6, it will turn on its output. The output drives an indicator LED and a transistor which bleeds away power. If the transistor can bleed away more current than the charger can supply, it can even be used to balance and the end of a charge. In this case, that would mean a transistor that can sink 3.6V*2A=7.2W. That's a pretty beefy heat sink, but not unheard of.

The boards will look something like this:

Don't ask why it only has five cells.

You can chain them together indefinitely, or use only a fraction of a board. No programming. No extra power supplies. Very simple.

When combined with the Meanwell front-end, the whole thing will be small enough to fit in a backpack and comparable in price to a Li+ charger/balancer/AC adapter combo. I'm doing it more for the convenience of not having to look around for power supplies, but I never can pass up the opportunity to use things for other-than-their-intended purposes.