Cap Kart Summer Rebuild 1

The Cap Kart rebuild has begun in earnest, so I'll try to post semi-frequent updates on the progress. It's a summer-long project, hopefully culminating in some new test drive video and data by August. The static website will eventually be updated as well.

First, the kart diff. This is a real mechanical test for us. In 2008 we chose to invest in an engine-less racing kart specifically so that we would not have to mess with any critical chassis or drive mechanics, short of adding a chain and sprocket for the electric drive motor. Now we're essentially tearing apart the rear axle to add a custom-built axial differential. It's a lot of heavy-duty steel fabrication...

Serious Business.

 ...for a group that's used to designing aluminum parts to be cut on an abrasive waterjet and carefully assembled. But rest assured we can bring some of our skills to bear on the project. Namely, making it look good.

Photo-render courtesy of DiffMaster Max H.

Now might be a good time to explain how it works. Like a more conventional bevel gear differential, the axial differential allows a single input (the chain drive) to supply torque to both rear wheels, while still allowing them to rotate at different speeds. This is most obviously necessary when turning, since the outside wheels must travel faster than the inside wheels. An "open" differential like this follows a simple rule: the torque on each wheel must be the same.

Formula SAE cars often use a chain-drive differential (though probably not axial) on the rear axle. In fact I'm sitting right next to one so why don't I take a picture?

 See it?

It's inside that aluminum can.

It's not obvious from the picture, but the brake disk and sprocket are both fixed with respect to the differential case. This way, braking torque is applied at the input and split to the rear wheels in the same manner as accelerating torque. We'll be pursuing a similar design in ours, mounting the brake disk to the hub opposite the new sprocket.

It will be fun to assemble. For now, though, it's just lots and lots of fabrication:

 Gear stock is cut to length, then bored out.

Hubs are formed out of aluminum disks. 

 

Ready to add holes for the gear posts.

Nearly completed hub.

And the gears actually do mesh...so far.

 While the mechanical side is progressing, I've been trying to remember how exactly the controller works. (I mean, conceptually I remember what it does, but the details of the wiring have been lost a little bit to time.) While we're rebuilding and adding new lithium-ion batteries, the plan is to update the power electronics as well. The only substantive change, though, will be replacing the contactor/diode switching circuit that enabled the ultracapacitor boost and brake modes with semiconductor switches. Here's what I mean:

If you're looking at this schematic and thinking "WTF?" then you're in good company. It took us some time and some trial-and-error to settle on this particular method of using the Maxwell ultracapacitor module, and even longer to recognize that it actually does have some significant advantages over other methods. All that is part of Cap Kart v1 development history, though, so I won't focus on it here. If you stare at the circuit for long enough, it might make sense.

The only change is that where there was a contactor and diode before, there is now a second high-power half-bridge. This bridge will only be used to turn the capacitor boost on and off, but in theory it could also be PWMed to modulate how much capacitor assist to use. That's too fancy for us, though. The purpose of this upgrade is mainly to take out one extra diode drop from the circuit.

We'll also be exploring new control modes for the separately excited motor to squeeze every ounce of power out of the system. The new batteries, though much lighter, do have a relatively low peak power and one of the challenges will be to make the most out of it. In some sense, this will give us more of an opportunity to demonstrate the advantages of the ultracapacitor assist. (Before, the oversized AGM batteries could have sourced or sinked enough power to make the capacitor somewhat unnecessary.) 

More to come...

HexBridge Shield BLDC and this silly thing from Texas Instruments

First, the Arduino HexBridge Shield BLDC challenge has been answered. In this post, Charles shows that it's a relatively simple matter to write control code for the Arduino that will turn the HexBridge into a viable brushless DC motor controller. As I mentioned (but was too lazy to prove) in the original post about the HexBridge concept, BLDC control involves setting one phase as high (or PWM), one as low, and one as off/floating. By cycling through all 3P3 = 6 permutations of this, you get rotational motion. The trick is linking this to the Hall effect sensors (or something else) that determine which state should be active. Here's the light load test video.

Dual BLDC control (when would you ever need that?) is simply replicating this on the second set of three half-bridges. Or you can run one BLDC controller and one reversible brushed DC controller using two out of the three remaining outputs. Or...well, you get the idea. Configuration flexibility proven, the next step is probably heavy load testing to see its limits. So far, I've tested to 20A. How about 50A?

Now for a bit of a rant...

This is the new Texas Instruments MSP-EXP430G2 LaunchPad. It's a $4.30 (clever) development board for the MSP430 value line devices. Does it look like a red Arduino? Can you imagine the developers' meeting? I can:

Hey guys, we're getting killed in the entry-level market by this Arduni thing. How are we going to get new engineers and embedded developers to use our product in the future?

How about we like...create something that looks like it, but with our cheapest chip. And we'll make it red. And we'll sell it at a loss, to get people hooked.

Now, I love the MSP430 microcontroller line. I use the MSP430F2274, specifically. It is significantly better in many ways than the ATmega328 used by the Arduino. But the chips they use on this board, the MSP430 value line, are actually somewhat lacking in features. For example, they have only one timer (vs. the ATmega328 with three). Timers are critical for the kind of stuff I do, like motor control, so that is a little disappointing. But I guess for the entry level, at $4.30, it's still a good way to get people engaged.

Except...do people use the Arduino because it's cheap? Or has a convenient form factor? Or is it because it's easy to program? I'm tempted to say it's the latter. And the MSP430 line, while being a pretty standard application of C programming, does not have the convenient (did I really just say that?) simplified programming language or the backing of a giant user group that Arduino now has. So I'm not sure. Maybe if they had come up with the idea of making an easy to use MSP430 dev board a few years ago, it would have caught on and been developed more.

Oh, wait.

How is that quarter not shorting out the 3V3 and GND pins?

That's right. Cam and I thought of putting an MSP430 chip on a dev board in 2007. (Seriously, look at the date on the board.) The thought process was essentially:

Hey check out this new Arduino thing. I wonder if we could make a better one.

How about using an MSP430F2274, and making it small enough to fit on a standard breadboard? And integrating an XBee header. And, heck, let's just make it USB and wirelessly programmable too.

We called it the 430stamp, or, informally, the wootstick (short for Wireless bOOTloading...sort of). It is arguably the most useful thing I have ever made. Seriously, I use it for everything. Doesn't it look familiar? It's the signal board for the 3ph Duo controller, where it executes simultaneous field-oriented control of two brushless motors. It's also the transmitter AND receiver for my RC car. There's one for programming the Cap Kart wirelessly. They were briefly used in 2.007. In other words, they work well enough that I rely on them. Very few things I've ever made have gotten to that status.

So, will I be buying any MSP430 LaunchPads? Probably not. So many people have told me about them now that I might just buy a few out of curiosity. But I already have a good, reasonably cheap (~$20) MSP430 solution. I wouldn't mind seeing more people use these as opposed to Arduino, just because they will have to learn how lower-level embedded programming works. (Not assembly...but low-level C. Fewer user-friendly libraries.) Overall, a $4.30 dev board from TI is a good thing, but I'm not necessarily impressed.

Revenge of the Cap Kart

The Cap Kart; the most ambitious and the most unique vehicle in the fleet of the Edgerton Center Summer Engineering Workshop, an ad-hoc group that for the last three years has taken on projects "that you can ride." We built it to reduce our dependence on foreign natural resources go fast. The kart can't rightfully be called our flagship; that distinction belongs to a different vehicle. But it's the fastest and most powerful by far, and after sitting on the bench (literally) for a year, it's going to exact its revenge.

The kart gets its name from its defining feature, a capacitor of 110F/16V (yes, F) that stores braking energy and puts it at the driver's fingertips in the form of a boost button, similar to the push-to-pass KERS setup. It's made by Maxwell Technologies specifically for automotive applications, and it looks like this:

The "Cap" part of the Cap Kart actually worked more-or-less on the first try. In fact, the kart as a whole worked well enough to get in a few test drives, plus some good flywheel test data. But it was still a rough prototype. It was also very impractical to test, for a number of reasons: It was too heavy, too wide, and stored in a building with no ramp access. Moving it involved taking off all the batteries, going sideways through a door, up an elevator, out another door, and down a flight of steps.

Now that we have some new space with double-doors, wide hallways, a freight elevator, and loading dock access, many of these problems have gone away. In fact, it is feasible to take the kart out without ever lifting it now, which also means the batteries could stay on. Then again, of all the things on the kart, the most obvious target for improvement are the batteries. We started with three 79Ah deep cycle marine batteries (lead acid), each weighing a whopping 53lbs. That's 159lbs worth of batteries...or roughly half the weight of the kart without driver. And one of the three original batteries has since died. So, we're ready to move on.

 

...but don't worry, the old batteries have found a new purpose.

We talked about lithium-ion batteries back in 2008 when we made the Cap Kart, but at the time there were two big reasons why we opted not to use them. One: They were (and still are) expensive. The price has come down a good deal, even in just two years, but they are a big investment. Two: We did not have the experience working with lithium batteries to justify the investment. Lead acid batteries can take more abuse. Now, though, we're more confident in our ability to handle a low-voltage lithium-ion pack, so we're going to take it on.

Our cells of choice: the Thundersky TS-LFP40AHA. These are the same brand that the Mythbusters used on their electric kart. The cells range from about $40-80 each, depending on the source. We opted to get a kit, which includes pressure banding, bus bars, balancers, and a 15A charger. This was from Elite Power Solutions. They come in a nice thick cardboard box, shipped regular-old UPS (not freight or hazmat).

Cute.

We never really had a range issue, so the minor hit in total battery capacity will not be a problem. However, the peak current output of this pack is probably a bit lower than what we've become accustomed to. I measured the internal resistance of these cells to be about 3.5mΩ, which agrees with The Internet. That puts the realistic current output capability at something like 120-200A (3-5C). Our motor/controller was quite happy (or unhappy) with 300A. It would be nice to match the two current ratings a bit better. (Though at low duty cycle it doesn't matter.) There may need to be some...buffering, of sorts. And there's always that capacitor thing, too. Getting the most bang for the buck has become one of our themes, and we're pretty good at it.

The very reasonable goal is to cut the battery weight by 2/3. In other words, all of the new batteries will weigh as much as one of the old ones. Since the batteries are half the weight of the kart, that decreases the kart weight by 1/3. The drivers weigh the same, though, so really it's more like 1/5. Decreasing mass by 20% leads to a 25% increase in acceleration (because math is funny like that), all else being equal. Hey now, wouldn't it be nice to have some better handling to go with that extra acceleration?

Here's the rear of the kart as-is. It's a live axle: the wheels and rear axle are one rigid body. This is great for traction...not so great for tight turning, especially on asphalt. One wheel has to slip, which can be made easier if it is lifted off the ground by a combination of weight distribution and chassis flex. But we added a ton of weight to the back and welded solid steel battery trays across the entire frame, so so much for that. Instead, we're going to add a differential. And hey, it gives us another chance to rip off Charles Guan.

The last target for improvement will be the control(ler). That is, both the physical and the software side. As much fun as it was to blow up TO-220 MOSFETs over and over and over, I think we're ready for something a little more serious. And to go with it, a new control algorithm that takes full advantage of the separately-excited DC motor in the field-weakening (again?) region. It should be able to more closely approximate the constant-torque, then constant-power curve for which electric motors are so well suited. 

Oh, and the best part is that I won't be writing the software this time! Nor will I be designing the differential. Nor will I be cleaning up the wiring. (Okay, maybe I will a little because I have OCD.) But yeah, my favorite project team is on it, so I get to mostly just chill. Oh and figure out where the heck we're gonna test this thing...

Arduino HexBridge Shield v2.0

This is what I meant to do the first time.

Recap: I have lots of motor controllers. Some of them work, others do not. But they are almost all based on a combination of MSP430 microcontrollers and a totally isolated, totally modular gate drive solution. Recently I've been feeling the pressure to create a more...populist...solution, which basically means putting it on an Arduino shield. So that's what I did.

Even though v1.0 was a total failure, it gave me a chance to refine the design in ways other than making it not go crazy as soon as it was under any load. I had some extra time to sit around and think about exactly what I would want in an Arduino-based motor controller. Well, not what I would want, but what one would want...you know. ;) Here is the important functionality that I was targeting:

Six semi-independent MOSFET half-bridges. I say semi-independent because each set of three shares a single PWM output. So, at most it can control two motors. But these could be brushless (3-phase) or brushed motors (H-bridge). You can also put channels in parallel for higher current capacity. There should be a simple state selection for each bridge: PWM, low, or float:

This leaves a ton of flexibility, and only occupies one I/O pin per phase, plus the two PWM pins. The "float" state is important for BLDC control or for disabling a phase entirely. I talked about some of the possible configurations in the previous post. On to the new board:

The FETs are the same (IRFS3107 or IRFS3207 or any other D2PAK), but they are arranged differently this time. All the high-side FETs are on the top, and all the low side FETs are on the bottom. This let me move the entire power section out off the side of the board, away from the Arduino and the logic. To conserve horizontal space, I switched to 12-13mm vertical capacitors. They can face up , or down into what will likely be dead space next to the Arduino main board.

Speaking of dead space, I stuffed the DC/DC converter input and output capacitors in the gap between the arduino USB port and power jack and moved everything out of the way of a potential ProtoShield above the whole stack:

Tetris win. Isn't the Arduino supposed to disappear now or something?

Also, the whole board is 2.1" wide now...the width of the Arduino main board. So it is much more compact than the original version. Here's what it looks like all wired up:

Form factor aside, DOES IT WORK? I made some major changes to the gate drive, switching over to the IR21844 integrated half-bridge driver. This is a wonderful chip that automatically handles synchronous rectification with shoot through protection. It also has separate signal and power grounds that can float within ±5V of each other. Because of this, I abandoned optical isolation of the PWM signals for the first time in two years. This worried me, but it seems to work just fine. The ancillary benefit is that the old gate drive, which cost about $15 per half-bridge, is replaced by a $2.50 chip.

To execute the truth table I specified, it needs a bit of supporting logic, in the form of AND gates, comparators, and inverters, but these are dirt cheap. The cool part is the high-Z detector, a window comparator that checks to see if each selector pin is being driven:

If the input pin (which enters from the top of that schematic) is not being driven high or low, it will be pulled to 2.5V by the matched resistors. This is between the threshold of the window comparator, so the output will indicate the high impedance state, and the gate drive for that phase will be disabled. To get to this state from the Arduino, you just set that selector pin as an input.

I started by testing it as an H-bridge with a low-current DC motor, but I quickly got bored and scaled up:

 

This is the SepEx configuration that I described in the previous post. Three of the six outputs are tied together to power the high-current armature winding. Two of the remaining outputs are configured as an H-bridge to control the field. Reversing the field changed the direction of rotation, so the armature need not be reversed. The test motor is from the Cap Kart, which is getting a makeover soon. It's a wheels-up test, so the loads are fairly light (20A armature, 10-15A field current). None of the FETs got warm over the course of testing. If I had a legitimate way of doing current feedback, I would actually put it on the ground and drive it. But for now it's just a demo to show the flexibility afforded by having six output channels.

So, since I have no intention of making more or selling these it's up to you to make them! Here is probably everything you need:

HexBridge Shield Design Files (ZIP, 779kB)

In that folder you will find the schematic, EAGLE files, bill of materials, Gerber files, state table, and sample Arduino code for running the SepEx motor. I would especially like to see a brushless motor controller next...and I have a feeling it will happen very soon.

Arduino Hex-Bridge Shield: Concept and v1.fail

So, I guess I am finally coming to terms with living in a world where Arduino rules. If you've been living under a rock, an Arduino is an AVR-based microcontroller development board with an array of peripherals, a huge library of code, and a large user base. They have become a staple of DIY projects and "Arduino compatibility" of DIY hardware is now a legitimate concern.

I'm faced with two options, then. One: I could continue to rant about how I designed most of my current TI MSP430-based hardware to do everything the Arduino does, but better, and how the Arduino is not a replacement for learning how to design circuit boards and program microcontrollers directly. Or, two: I could drink the Kool-Aid and make something useful that also happens to sits on top of an Arduino.

Cue the Hex-Bridge Shield. (Scientific name: Large Arduino Switching Shield with Field Effect Transistors.)

Here are some pictures of v1.0, a failure for reasons which I will discuss below:

Hex-Bridge, Top Side: MOSFETs.

Sidecap.

Hex-Bridge Bottom Side: Gate Drive.

Okay, so calling it a shield is a bit of a stretch. It doesn't really sit on the Arduino so much as the Arduino sits under it. (Can you see where?) It's pretty compact for what it is, though: six semi-independent, high-power MOSFET half-bridges, with all the gate drive circuitry included. Each bridge was a version of my modular half-bridge design, which I've used in every motor controller I've built for the last two years.

The gate drive is based on a design we originally did for the Cap Kart long ago. Each FET gets its own 2.5A gate drive optocoupler (HCPL-3120). An optocoupler passes a signal through an LED and photosensor (packaged in a single chip) to electrically isolate a circuit, which is useful for protecting the microcontroller from high power switching noise. These particular optocouplers are designed to drive the gate of a MOSFET, on when the LED is on, off when the LED is off. By arranging two LEDs in anti-parallel and including a low-pass filter, passive shoot-through protection for each bridge is achieved.

One high-level half-bridge schematic.

Notice how the LEDs are arranged so that only one can be on at any given time. Also notice that the low-side gate drive optocoupler is fed by an inverted signal. This creates synchronous rectification, mirroring the PWM on the low-side to minimize diode losses and allow regenerative braking. This is just a high-level schematic, leaving out details such as the shoot-through delay filter and the gate resistors and protection components. But it should give the general idea. One down side of this method is that it requires a separate high-side supply. This makes it expensive (about $15 per bridge for all the gate drive components, which is more than the FETs themselves). Integrated half-bridge drivers such as the IR21844 provide a less expensive solution. v2.0 will use these.

The MOSFETs are IRFS3107 surface mount packages. They have a typical Rds of 2.5mΩ. (For reference, that's about the same as one foot of 14AWG copper wire.) These are monstrous 75V FETs that can, with proper heat sinking, probably handle "high-double-digit" currents. Throw a fan on and I don't even want to speculate how much current you could pass through it.

IRFS3107 Output Characteristic. 

0.2V at 85A...way better than an IGBT.

Everything on the board is sized for an input voltage of 18 to 48V. On the input, there is 990μF worth of 63V electrolytic capacitors. (These could easily be replaced with 50V or 35V capacitors of higher capacitance for lower-voltage, higher-current applications.) An LM2575HV switching regulator provides 15V @ 1A to power the Arduino and the gate drivers.

An here's where the failure comes in. Take a look at an Arduino. (I know you have one.) All of the digital signal pins are on the top row of headers, but the +5V, VIN, and GND pins are on the bottom row. I made the horrible mistake of running lone +5V and VIN lines from the LM2575 and the logic ICs, around the entire high current switching board, to the bottom headers. Bad bad bad bad bad bad. I can't even describe how bad it was.

By eliminating those traces and running an external connection for +5V, GND, and VIN, I was able to eliminate at least the most dramatic of the failure modes. While in this hacked state, it was able to run 20A continuously through a single phase with just natural convection. The resulting FET temperature was about 90ºC. Extrapolate as you see fit for a current rating with paralleled FETs, heat sinking, and/or fans.

So it has the potential to work, but in terms of a board design, this one is total scrap. Version 2.0, which is already in the works, will be better in every way. (Smaller, cheaper, better mechanical and thermal layout, easier to stack additional shields on.) But since I have nothing much to show for it yet, I can only tell you the design intent:

It would be great if the Arduino had six completely independent high-resolution PWM channels like some chips do. Then you could control two brushless motors. You could even do sinusoidal field-oriented control! Well, maybe not. But anyway, it would be cool if this shield could be used to control two brushless DC motors using square wave drive, at least. With six phases to play with, that should be possible. The trickiness is in multiplexing the PWM signal from Arduino pin 9 and 10 out to each set of three phases. Thinking about NAND gates makes my head hurt, but fortunately I got some help in coming up with a creative solution using an analog switch:

Chuxxorplexer

Actually, the real key is using one selector pin to completely control each bridge, including being able to disable it. This is where tri-stating comes in. If the selector pin is driven high, the relevant PWM from pin 9 or 10 is ANDed in and the bridge executes synchronous rectification. If the pin is driven low, the low-side FET is steady on, grounded that phase. If the pin is set as an input (high impedance), neither FET is on and the phase floats. This allows full four-quadrant control while still preserving the "off" state. Don't ask me how the analog switch accomplishes all that in one shot...but it does. Just stare at it until it make sense. That part totally works. This is what comes out of the magic box:

Digital pins 2-7 are used to select a state for each of the bridges. The bridges are labeled {A, B, C, U, V, W} from left to right. Each bridge can have one of three states:

• When its state pin is written HIGH, the bridge executes synchronous rectification based on the PWM present on pin 9 for A, B, and C, or pin 10 for U, V, and W. This means the high side FET will be PWMed with a duty cycle set by analogWrite(9 or 10), while the low side FET does the opposite.
• When its state pin is written LOW, the low side FET of the bridge will be steady on and the high side will be off. This is effectively like grounding that phase. The PWM does not matter.
• When its state pin is set as an INPUT using pinMode(), it will turn off both FETs and the phase will float. This is useful for disabling the controller or for the undriven phase in BLDC control. Note that even in this state, diode flyback can occur.

Unfortunately, the Chuxxorplexer won't make it into v2.0, which uses a different gate drive configuration entirely, but the functionality will be preserved using a much less elegant brute force set of logic chips. The important thing is that the three-state control in the table above will be preserved. This is, IMO, the cool part, giving almost complete flexibility of bridge configuration for one or two motors.

Here are a few examples:

 Monster H-Bridge

The monster H-bridge can be achieved by setting {A,B,C}={1,1,1} and {U,V,W}={0,0,0} for forward, or {A,B,C}={0,0,0} and {U,V,W}={1,1,1} for reverse. This parallels three bridges to make a super half-bridge, then combines two super half-bridges to make the full H-Bridge. This gives the highest current reversible control with regenerative braking.

Dual H-Bridge

The dual H-Bridge, also known as the drive controller. This can be used to drive the left and right sides of a tank steer robot. Since each bridge only uses one FET per leg now, the current capability is lower than the single monster H-bridge. To get this configuration, {A,B,C} = {1,0,X} or {0,1,X} for forward and reverse and {U,V,W} = same idea. X means that the unused phase is set as an INPUT, left floating. Hey, why not use the two unused phases to make a third H-bridge? Because there are only two PWM channels. Sorry. But the next one should be obvious...

Hey now...

The dual-channel BLDC controller, something I've seen before somewhere. Basically, you would cycle through every permutation of {1,0,X}, one phase driven, one grounded, and one floating. Except now you're on your own for how to achieve electronic commutation. One idea would be to use the floating phase and an analog pin to do back EMF-based sensorless control. Another idea would be to use Hall effect sensors and interrupts.

Monster Half-Bridge

The monster half-bridge, also known as the weapon or spin controller. It can only go in one direction, but it parallels all the MOSFETs for the highest possible current capability. This is actually not a "safe" configuration, since it is possible (and likely) to shoot-through from ABC to UVW if the PWMs are not in sync. To get this configuration, use only one PWM, 9 or 10. Set the other as an input and tie them together with a resistor. If you don't do this, instant destruction will occur. Along the same vein, there is also a monster BLDC controller configuration that ties A to U, B to V, and C to W.

Lastly, my favorite configuration: the SepEx controller:

SepEx Controller

This combines a super half-bridge with a regular H-bridge. The super half-bridge controls the high-current armature winding of a separately-excited DC motor. (By the way, don't bother reading the description of the SepEx motor's regenerative braking method on that page...it's totally wrong.) The smaller H-bridge controls the low-current field windings, allowing the field to be varied and reversed. Now, I don't want to pretend that this controller is powerful enough to drive a D&D SepEx motor under full load...it's not. But conceptually it can be configured for the SepEx by using {A,B,C} = {1,1,1} and {U,V,W} = {1,0,X} or {0,1,X}. Actually, don't I have a SepEx motor somewhere that I can try this with?

Oh right, there it is.

I'll leave off with a teaser for v2.0:

 Can you spot the layout improvements?

Pirate Radio Control: HK123's and Field Weakening

It's nice out, and I'm done with finals, so I took out the RC car again and made a few improvements. Then I crashed it and undid some of those, but don't worry it's not too bad!

First, I changed the batteries. I had been using A123 26650 cells. These are LiFePO4 cells that are most abundantly found in 36V DeWalt drill batteries. In fact, since they are virtually unavailable to the general public, the cheapest way to get them is actually to take them out of drill batteries. I'm sure there are significantly more 36V DeWalt drills than there are batteries for them at this point. Each pack contains 10 cells, and can be found on eBay for roughly $100, so about $10 per cell. Wasteful...and still expensive.

A less expensive solution is lithium polymer. Sites like Hobby King sell these pouch-like batteries for more like $5 per cell, for an equivalent capacity. They're 3.7V nominal, instead of 3.3V, and they come in prismatic packs, so overall they have a higher energy density than LiFePO4. The 30-40C discharge versions are comparable in power density as well. The one downside is that they tend to burst into flames when abused...

A compromise, then. Hobby King also makes LiFePO4 prismatic batteries. (We call them HK123s now.)

HK123

For $70, you get 12 cells. That's just under $6 per cell. Now, these packs are decidedly hit-or-miss, based on the reviews. They might arrive with one or more cells completely dead. In fact, the one I ordered had two dead cells. :( But since I was going to cut it open anyway, I wasn't too upset. Taking the packs apart is a real pain, but I don't imagine it is significantly harder than taking apart a DeWalt battery. One snag I ran into was that the positive terminals are aluminum crimped to nickel tabs. I ripped off the nickel, not realizing its importance, and had to resort to aluminum soldering to connect the cells. (Yes, you can solder aluminum.)

Questionable soldering job.

I rearranged the cells into 6S1P, same as the old RC car pack. Putting it together is easier than putting together cylindrical cells, which almost makes up for the difficulty of taking it apart. All the balance and power leads come out of the same side, and it makes a nice compact package that is easy to heat shrink. Or, if you don't have any heat shrink, caution tape...

Notice the size difference.

The cells themselves are almost the same weight as A123 cells. (They should be, being the same chemistry and capacity.) So I only saved 122 grams (6% of the total car weight), mostly in packaging. But since they are prismatic, they stack nicely and take up much less volume. I measured their internal resistance to be about 13mΩ, which is a little higher than an A123 cell but not much. I fully expected it to be able to handle the 40A peak current on the car. Here's what it looks like on the car:

 

 

Step two in the RC car upgrade was implementing field weakening control. Setting up the full explanation of field weakening would take one or two whole posts, so here I will just get straight to the point. The car uses half of my 3ph Duo brushless ESC, which is a fully functional AC controller that treats everything as a vector in the rotor frame of reference. The voltages applied to the motor are balanced three-phase sine waves, which leaves only two degrees of freedom: magnitude and phase. 

Under normal operating conditions, the controller sets the phase wherever it has to to place the current vector right between the magnets, using feedback control. This is the point of optimum torque production, the least amount of current for the most amount of torque, and varies with speed and load due to inductance. The magnitude then just sets the magnitude of torque.

In field weakening, the controller sets the phase ahead of the normal operating point. There are many ways to think about what this does. In one way of thinking, part of the current vector is now opposing the permanent magnets a little, weakening the field that the other part of the current vector interacts with to produce torque. This reduces the back EMF of the motor, allowing it to spin faster at a given voltage magnitude, but with less torque. It effectively changes the motor constant, almost like an overdrive gear. Hey now, where have I seen this before?

I wish I could provide a more thorough analysis of the field weakening capabilities of this controller, but in all honesty it gets a little messy. For one, the unit frequency that the controller uses to do all its calculations is the PWM frequency, which is 14.5kHz. With the motor now spinning at 45,000rpm, or 750Hz, the resolution of everything is shot. Furthermore, lag in the measurements starts to become significant and the rotor frame may not even be in the right place anymore. This controller was designed for direct-drive scooter motors that spin at 1,000rpm...so I'm happy if it doesn't go unstable at this speed.

Time for testing! Here's the baseline, a normal "full-throttle" acceleration with no field weakening:

It gets from 0-30mph in 1.9 seconds, pushing about 34A (peak) to the motor. The top speed is 34mph and the peak power output (to the ground, m*a*v) is about 200W. Notice that by the time the motor has hit top speed, the voltage phase is a full 42º ahead of where it would be at no load. This keeps the current right between the magnets, lined up with the back EMF, for optimum torque. At this phase angle, the power in to the terminals of the motor is about 330W. So from the motor terminals to the ground, a system efficiency of 61% at peak load.

Now with field weakening:

The acceleration profile up to 30mph is identical, taking about 1.9s. Then, at 30mph I hit the "boost" button. The controller tries to put some current on the axis that fights the magnets (negative d-axis). It does so by advancing the phase to 75º, sacrificing current on the q-axis (torque producing current) to get there. So, the acceleration slows, but it can get to a higher top speed of 44mph. Maybe you just need to see it:

 

That ant-looking thing is the car. The sound is probably the more useful measure. With this kind of motor, field weakening is fairly limited, but a 30% increase in top speed without changing anything other than software is still much appreciated. That is...until you hit a pebble at 42mph...

Unfortunately, I do not have video of the crash, but I do have the car's last transmission:

Black box.

Clearly I had half a second to attempt to slow down and regain control. I probably needed another half second to do so, though, because it still hit the curb at 27mph. This time, something happened to the radio before the battery came out. (You can tell because the voltage never drops.)

Yep, antenna got ripped out. 

 That might be fixable with some solder. The brand new battery did indeed fly out as well, landing several feet away. Here's the aftermath:

Not right.

But even the bashed-in cell is okay.

That's a big check in the durability box for the HK123's, if you ask me. The only thing that really broke was a tab on one of the cells. And it wasn't the aluminum solder joint; the tab just sheared off completely. Good thing I have four spare cells from the original pack.

The car itself was totally fine.

So in summary:

• HK123s are a viable and fairly rugged LiFePO4 solution, if you don't mind disassembling and reassembling a pack. More compact and less expensive than A123.
• Field weakening is cool, until you hit a rock.
• I should find a wider test location.

Super Mean Well: The Scooter Fast Charger

Disclaimer: Do not try this unless you are very comfortable with the concepts and dangers of battery charging. I am pursuing this as an experiment, not advocating it for general use. Attempt at your own risk.

A while ago, I had the idea of repurposing large LED power supplies as battery chargers. The premise is that both use the same output characteristic. That is, they will apply a constant current or a constant voltage, whichever is lower. For batteries, the constant current is applied first and supplies most of the charge. Then, when the applied voltage reaches a certain threshold, the voltage remains constant and the current ramps down. If you don't understand how this works, you probably don't want to work with batteries anyway.

Mean Well makes inexpensive 100W LED power supplies (presumably for commercial lighting) under the PLC-100 line. They are roughly the size and weight of laptop power supplies and are available in a range of voltage outputs, up to 48V. They have a constant-current and constant-voltage trim pot that allow adjustment within a limited range around the nominal output. They retail for about $65 and are available from several electronics distributors including Mouser.

I have been using the PLC-100-36 to charge the batteries on the BWD scooter for some time now. The scooter battery pack is lithium iron-phosphate. (Note that these are generally safer than other lithium-ion chemistries, which is why I am more comfortable using a non-standard charger.) It's a 33V, 4.4Ah pack (equivalent to two 36V DeWalt drill batteries). I set the constant current to 2.2A, so it's a C/2 charge, meaning it would take approximately two hours to fully charge the pack.

There is obviously no balancing circuit (though I did try to make one). It's important that the cells be well-balanced before charging and that the voltages be monitored in case one starts to peak early. Cell voltage monitors are widely available, for example this one and this one that also balances. They will not automatically cut off the charger, though, so constant observation is required. (Generally a good idea for battery charging anyway.) For general use, an integrated charger/balancer such as this takes care of all the monitoring and cutoffs. The downside is that for most of these RC chargers, you still need a bulky front-end DC power supply.

Though it is compact, inexpensive, and plugs directly into the wall, the 100W Mean Well supply falls short of the RC chargers in output power. So I was very glad when I came across the new HLG-240 line. This is a 240W version of the Mean Well LED power supply that retails for about $130. It comes in a compact (but dense) aluminum enclosure with wires already attached. Here's the HLG-240-36:

And for scale, here it is next to the scooter:

It's not as light as the 100W version, but it's still something you could see carrying in a backpack. And just like the 100W version, it plugs straight into the wall. This particular model has a voltage trim range of 33.5-38.5V and a current trim range of 3.3-6.7A, a good match for the scooter battery pack. Other models offer other ranges. (Be careful: Only the model with the "A" suffix are adjustable!) And here it is doing its thing:

With a maximum output current of 6.7A, this supply can now charge the scooter pack in about 45 minutes (~1.5C). Whether or not this is practical or necessary, I don't know. I've never had a case where I absolutely needed to charge it in less than an hour. But it's interesting to know that the capability exists for about the same cost and size as a laptop power supply. (Okay, this is a little bigger than a laptop supply.) I still don't recommend it for casual use, since it doesn't implement balancing or voltage monitoring. But if somebody made an small adapter that did these things, I could see it being a good, inexpensive solution to small EV charging.