Of course the first thing I did after making sure the motor worked was throwing it on RazEr. Then, over the past week, I have been beating the crap out of it by commuting everywhere – when I could. The weather has been abhorrent over the weekend.

I’m proud to say that everything has been working flawlessly. The scooter is almost excessively stealthy due to its low profile nature. When fully loaded, the motor makes a very attenuated “brushless whistle” that’s just enough to cause people in front of me to move out of the way instinctively, but they’re not really sure what on earth it is.

Well, until I fly by.

So let’s backtrack and see what happened.

Once again, I start with the finished product. Remounting the motor into the scooter was not a difficult affair, since the center hub and shaft was the same. Otherwise, it just involved hooking up a few wires again. It was nice to see something working after it had been sitting idly on a hook for a year.

Here’s a shot of the business end. Those side rails which form the wheelie bar have been around since the very first wheelmotor iteration!

It turns out that having the motor oriented towards the right side was not exactly a good design choice. I should have installed the can the other way – when torque is applied externally to this orientation, the wheel tries to unscrew the locking ring. This happened a few times in testing, so I ended up Loctiting the ring to the can threads.

Such a reversible process.

Here’s the important parts of the vehicle. If you have never seen this before, there are two 5AH LiPoly packs which form most of the belly volume. The remaining volume up front holds a big model airplane motor controller and a servo tester to convert an analog throttle voltage to R/C signal PWM.

Also, a bunch of LEDs.

Here’s a closeup of the flip side of the motor, the removable faceplate. Technically this should have been on the wire exit side, but it was 5 in the morning when I installed everything and I don’t feel like pulling it apart again.

This was actually the most fun part of the rebuild. I didn’t have another resistive throttle available, because they had all began malfunctioning. The cheapo servo tester I used actually doesn’t take a voltage and turn it into a pulse; it performs a timed discharge of a capacitor through a resistance. This was fine and all when the servo tester was used with the knob it came with, but it meant that I could only use a resistive method of interfacing the throttle. A standard cheap Hall Effect throttle puts out a voltage and won’t let a cap discharge through it.

So I did what any desperate engineer would do – I whipped up a slumthrottle out of some aluminum bits, a potentiometer, and a long extension spring used in torsion. It works better than it should. Using this, I recalibrated the throttle endpoints of the controller and also changed a few settings such as timing and startup.

With this, I went rocketing down the hallway a few times. Because I’m here writing this, you know it worked.

And boy did it work. Here’s the high score of the day, about 1,200 watts on a good launch. I’ve since pulled almost 1,400 by bringing my ‘kick start’ speed closer and closer to the controller’s minimum pickup speed.

The controller, being sensorless and aircraft-optimized, has a minimum speed below which it thinks the motor is stalled and will refuse to start. The “base speed” for Razer is about 5 miles per hour, below which the motor will not actually produce torque when commanded.

And here is RazEr after the “maiden IRL voyage” back at East Campus.

I’ve been using the scooter to commute every change I’ve gotten, just to put as many miles on it as possible. Nothing has yet broken, nor started shaking apart. I’ve been purposefully using sidewalks and cobblestone paved pathways whenever possible just to see what WOULD shake apart first, but the motor and other systems have remained steadfast.

The total mileage on this motor is probably 3 or 4 by now. A single cross campus trip consumes about 0.25 amp hours, and the longest trip so far has consumed 1.3 amp hours.

Here’s a Google map of the most recent “long haul”. I began with a cross campus round trip, then quickly followed with an continuous loop around the eastern half (third?) of campus. The distance was 1.89 miles, so given the 1.3 AH consumption, we can figure that RazEr has a “mileage” of about 25 watt hours per mile.

I’ll try to time a “campus loop” now that the weather is nicer. I’ve monikered the continuous strip of sidewalk and bike path bounded by Massachusetts Avenue, Vassar Street, Main Street, and Ames Street as the “MITburgring Ostschleife”, after the Nurburgring.

It’s been hanging on a utility hook since the last controller fire. Everything works and the batteries are still charged, so all I need is a BLDC motor controller. Since everything still technically “works”, I don’t intend to touch the scooter that much, if at all. Any work on it will be replacing the shell of the wheelmotor with something more substantial (and better engineered, and more reversably built).

Well, it’s STILL hanging on a hook – almost a whole year now.

I can’t really remember when the “last voyage” of RazEr was, except that the hub motor was already on its way out. By that time, it wouldn’t even start from standstill and made ugly grinding noises when the controller finally did start it. The bearings were trashed, and I had filled the cavity between the wheel and motor can with hot glue so things would stay together.

After the motor just totally locked up one day, I put RazEr back on the hook and pretty much forgot about it. But since then, I had been slowly revising the motor design as a background process – when nothing else is on deck, I’d open up the model files and mess around with things.

I was specifically targeting a few shortcomings of the existing motor in the redesigns.

• The tire should be removable without taking the whole motor apart. The existing version only had two solid end plates securing everything, and if I ever wanted to change the wheel, I would actually have to disassemble the motor down to the wound stator.
• I should not have to modify the tire itself past just cutting the center out. The fact that the side plates were attached with through-bolts meant that I had to cut bits out of the rigid plastic rim of the scooter wheel. This made the wheel very floppy and nonstructural.
• No more protrusions outside the can! Combined with the nonstructural wheel, it meant that the through bolts were used in heavy cyclic bending – this lead to them backing out, fracturing, stripping… everything.
• The 6802 type bearings were wimpy and very underrated for the loads that the motor had to bear.

The last problem had to be held off until a complete blank-slate redesign because I could not otherwise recycle the core, including the stator, which had been designed around 5mm wide bearings and was well-epoxied in place.

The theme of the redesigns drifted towards some form of removable rings around the perimeter of the motor which retained the wheel between them.  I bounced around between two means of “removable” – either giant threaded rings:

… or radial screws, in the same manner that I would later execute on Deathrunner:

Threaded collars offer a way to actually tighten to the wheel – retaining it by compressive friction, but I consider threading a nontrivial operation. Additionally, there would still be a need to assemble the motor’s bearing endcaps that was nonpermanent, a difficult affair unless I either wanted to thread over crossdrilled holes or make the endcaps themselves an additional threaded mount. Either way, lots of threads, and lots of chance to bum it up.

On the other hand, some radially bolted collars are simpler to make, but don’t allow setting the ‘grip’ on the wheel – if i’m a millimeter short, then the wheel wouldn’t be actually retained under torque, and so on. They also allow easier installation or removal of the endcaps, as I saw on Deathrunner.

So what’s the decision here, then?

Well, an exact 50/50 compromise. On one side, radial case screws drilled into the magnet can (which is now much thicker and structural, and features a solid integrated collar) retain a bearing endcap. This is the “service entrance” since these parts are not permanently joined.

The other side features a removable threaded collar that mates with fine threads cut into the steel can. I included some spanner wrench holes to aid in tightening and removing. The bearing cap on this side is “permanent” – it will be aluminum, but slam-fitted into the can.

This was the design I settled on. It achieves a good balance of manufacturing convenience with serviceability.  Now I needed the stator out of the now retired motor. I disassembled RazEr to have a look at the motor insides after the failure.

Nothing prepared me for the utter horror that was the motor internals.

It’s already looking bad. Bent bolts, stripped threads and heads, and clumps of torn up urethane and hot glue debris.

Where’s the tire?

Oh, yeah. I had to cut that off because the screws were too broken to be extracted from the endcap, and the whole thing was probably friction welded together by now.

I finally knock one endcap off with a large screwdriver and mallet and

AAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHH

What HAPPENED?

The entire interior surface of the motor was completely caked in some black powdery mess. It looks like something either got inside, or otherwise managed to interfere with the stator-rotor airgap, and subsequently got ground to bits.

This probably explained the inability of the motor to start from standstill because of the massive friction.

Through some investigation of the surface conformity of the magnets, I discovered these two loose and displaced magnets which bear clear scraping marks. The black powder is probably corroded magnet bits – both dust from their ceramic bulk and the nickel-copper plating. There’s probably stator steel particles involves also. Rust on the interior tells me that water got into the motor, probably from riding around all winter.

The motor casing, after separation.

I feared for the stator’s integrity, but it was electrically sound  – no burns or broken wires. The loose magnet had carved a very impressive gouge across half its thickness. The uniformity of this gouge tells me that the magnet has been dragging like this for a very long time.

Seriously – that’s a wear pattern you usually see on DC motor commutators.

The damage isn’t permanent or serious, so the stator will be reused.

Alright, so usually this is where I make about 4 build reports detaining every minor machining step in the process of making this thing. But I think we’ve heard enough about day to day machine technique, so I’ll start saving some words and just show the interesting steps.

I got up one day and decided to just blitz everything in one shop spree. I got most of the way there, too.  The components above are “protoforms” – they don’t have holes drilled and tapped yet, but the basic shape of the motor is done and the threads have even been cut into the motor can!

Making the can’s  3 1/4″-24 GYF (Girl You Fine) thread went quicker and was less disasterous than I had anticipated. Fortunately, my toy indexable cutter set came with a carbide tipped threading tool. Regardless, the finish left alot to be desired. I ended up running a needle file through the threads to clean them up – otherwise, the steel appeared more liable to tearing and smearing than.. cutting.

I suspect it’s just a matter of technique, though. That and tool quality – something mitr0nz lacks a bit.

Hurrr. A bit.

Making the threaded external collar, though, was a adventure in a first. I had to make internal threads that had to mate with an external one.

A long time ago, I anticipated eventually having to deal with this problem (or was just tool shopping) and picked up some internal threading tools, both left hand and right handed. I finally was able to whip one out and use it on the aluminum threaded collar.

Here they are, the most hideous threads known to man.

Actually, they are not that bad. Aluminum machines like a blaze with carbide, and most of the gunk in the threads is just particles mixed in cutting oil.

And the motor can screws on!

I decided to leave the inner diameter a little loose. This made the collar wobble slightly on the motor can, but I figured that while removing metal is always easier than putting it back, trying to line up the lathe’s leadscrew and threading feed with something it already made is not so easy.

A bit of shaping and parting later, there was a threaded collar.

And the collar threaded onto the can…

And everything test-fitted together.

After making sure the fits were correct, I put in the new motor magnets. They are SuperdupermagnetGeorge‘s M2515. For all your stock and custom magnet needs, see Supermagnet George. He’s supplied the mags for literally ALL the motors I have ever built, including the customs for Deathrunner!

There are very few magnet arrangements that yield an almost 100% fill with flat magnets, but somehow, during the builds of this motor, I managed to hit two already. 28 magnets complete a circle in this redesigned can with a gap the size of a few stacked sheets of paper left over!

It could have been totally closed if I had machined the can just 0.005″ smaller.

With the motor assembled, I cored out my last 125mm scooter wheel.

And here is the motor in the test fixture, ready for a whirl. I ended up being too lazy to actually put fastening features on the ring, and ended up just cranking it with a set of vise grips.

I can’t say that I’m satisfied with the machining tolerances (there’s a very small amount of wobble and the bearing fits are suboptimal), but that’s what I get for blitzing everything.

Oh, by the way, here is a picture of this motor’s back-EMF profile.

For the Mechanical Engineering Department’s Measurement and Instrumentation course that I’m taking this Spring, I’m messing around with a few ways of predicting the torque a motor will make, without actually… you know, measuring the torque.

Analyzing the BEMF profile (the voltage profile put out by a motor when driven by an external velocity source is one way to estimate the torque constant of the motor. The short story is that in SI units, a motor’s BEMF constant (Volts / (rad/s), or just volt*seconds) is equal to its torque constant (Newton-meters / amp). Given volts out and speed, you have the kernel of the motor characterized.

So how do you extract this info from a neato wave thing? The “volts” part is easy.

• It’s just what the oscilloscope says on the cursors! Actually, there’s a catch. I am measuring the motor line to line (i.e. ground clip to one phase wire, signal input to another, and the third one left loose). The oscilloscope references all its grounds to earth ground, whether or not the ground clip is actually connected to “zero volts”. Therefore, instead of getting the voltage out referenced to 0 volts, I’m essentially getting the voltage referenced to the opposite of itself because I am measuring an AC voltage source (the spinning motor) via two terminals. Result: It’s not 20.5 v I’m observing, it’s actually 10.25v.
• The Tek 2445 scope can’t compensate for 10x input probes. I found that using 1x inputs, the scope was overwhelmed when I spun the motor quickly (using an R/C car wheel attached to a frackin’ pneumatic die grinder – it goes FAST). So, I used 10x attenuation on the probe without changing any scope settings. The voltage you are seeing on is 102.5 volts

One hundred and two volts.  Yeah, this thing was being spun FAST. How fast? The frequency of the sine wave will reveal the motor speed.Reading the timebase and lining the sine wave up peak-to-next-peak,

• We know that every time the wave comes back around, the motor completes one electrical cycle. This is approximately 4.4  divisions of 500 microseconds each, or 2.2 milliseconds per electrical cycle.
• LRK motors take 7 electrical cycles to complete one mechanical cycle. That’s .0154 seconds per mechanical cycle.
• We need to convert (s / 2pi radians) to rad/s. So divide by 2pi to get rid of the factor, then invert the whole thing.  The motor speed is 407 rad/s.
  

That’s it. Razer’s motor has a Km (motor BEMF constant) of 102.5v / 407 rad/s = 0.251 Vs

By direct conservation of power, the torque constant is 0.251 Nm/A. Or damn close to it – nonidealities always decrease the motor constants, resulting in a faster, less torquey, less efficient motor.

I can’t quite sanity check this yet, but given that Razer runs a 28v electrical system, the motor should run approximately (1 / 0.251) rad/s / V , times 28V = 111.5 rad/s or about 1000 RPM unloaded. RazEr has 5 inch wheels, and ran about 10-12MPH on a good day, which is about 800 RPM. Not unreasonable for a light loaded cruise, so I would unscientifically say Km = 0.251 is a reasonable estimate.

The real test would be a dynamometer…

So, the motor’s done. But where’s RazEr? I still need to actually put the thing back together and check everything for running order…

Überclocker Remix

Let’s start with some pictures of epic motor ownage. I cracked open the toasted HTI gumball machine motors out of curiosity after removing them from the bot. What awaited me inside was a scene of utter devastation.

That doesn’t look very healthy. It appears the commutator decided to just melt off the backing material. This motor actually still ran, just throwing blazing white sparks everywhere. The discoloration of the copper next to the crater attests to the extreme heating that occured.

The brush cap from the left side, which simply failed open circuit at the event. Well, now the reason is clear why it failed open. Half of the brush conductor spring just sort of flew off and melted itself into the other side of the plastic brush holder.

The carbon brush itself was bouncing around inside the motor.

Another view. That bit of spring must have been pretty hot to instantly melt itself into the plastic.

And another view of the copper droplet that is the commutator. Oddly enough, the windings themselves seemed for the most part to be just fine.

Here’s Überclocker looking decrepit on a table. Since a robot with no motors is akin to a dog with no legs, or a fish with no fins, I began the quest to search for a… well, more legitimate motor. That’s when I remember that I found these, from Way Back.

DeWalt drills are classic musclebot motors. Sadly enough, these were of different voltages (!?), which not only surprised me as to how on earth their previous user expected their creation to move in a straight line, but saddened me because I… well, want mine to.

It was enough to perform a fit test and draw up plans to modify the gearbox to accept these motors while UPS channels their Brownian Motion to get a matched set of motors to me. They are the “new” DeWalt motors, where “new” is relative to 2003 or so. These drills have 3 speeds and are infinitely more of a bitch to mount. So I will only be using the motors in my custom frakenb0xen.

Fit test. The good news is that these motors are roughly the same length as the 700-size HTI motors, but a little fatter. No issue, considering the gearboxes have plenty of wiggle room.

The gearbox modified to accept a DeWalt motor, with its Alien Technology Motor Pinion of neither metric nor Imperial tooth pitch. Needless to say, this will be removed and the HF motor pinion crammed on in place.

So am I over-motoring the HF drill gearbox parts by putting a real motor on them? Perhaps. However, I think it’s a legitimate move in a 30 pound robot, because the laws of physics dictate that I can only put so much power to the ground. I’m mostly after the “real motor” bit, not so much increased drivetrain power, because the robot doesn’t have the traction to use it.

With motors now on the way, this conversion ought to go quickly since I’ve already drilled the new mounting holes to accommodate them. Überclocker should then be able to attend more events.

The (Possibly?) Final Chapter in the Tragedy of the LOLrioKart

So by now all ya’ll have probably heard of this.

While the details surrounding the citation were totally illegitimate and imply a degree of recklessness that was not present at all, the bottom line is that the kart is not going on any more open road adventures until it’s legit. And by legit, I mean registered and insured and fully street legal.

Whatever measure this takes, it will happen. It will simultaneously the most confusing and most glorious thing on the planet.

But the good news is that through two weeks of intense demos and driving during Orientation, the kart didn’t explode. The motor controller, version 6, is more or less stable. That’s huge. That’s like, me doing something right in electronics for once.

Of course, if I actually run the numbers on the electrical characteristics of the power converter, it would probably make real EEs run away to vomit. But the kart has survived more than twenty power cycles without misbehaving, save for the flakey DC-DC converter that caused the initial failure of version 6. A replacement module with better-designed (read: existent) filtering solved the problem.

So I’m satisfied. There will be little active work on LOLrioKart this term, with most of the fleeting effort concentrating on the battery system. After said weeks of operation, two cells in the battery pack are now just resistors. I regularly saw the voltage dipping under 45 volts on acceleration, which is concerning to say the least. Battery management solutions are condensing around me, so I may make the jump to lithium iron phosphate cells.

Now let’s move onto the new shiat.

This is a Xootr Street push scooter.

Gee, that looks kind of like every other push scooter on the planet. You know, like a Razor scooter. I thought you already had one of those? With like… a motor on it, right? That you built? I heard you built a motor. Can you show me how that works? Can you build me one?

… </average_miters_visitor>

Oh, that’s the difference.

As much as I love RazEr when it works, it’s time for me to realize that it’s too freakin’ small. I’ve managed to hit the tiny-but-functional goal, and at the same time the maximum recommended rider weight a few times. With some more scrupulous design, I could probably fit more batteries in there, but otherwise all the useful space is essentially occupied. And while 5 inch wheels are great for shoving your average copier motor core into, they are not great for shoving into your average pothole.

I need something bigger. More legit™. So thanks to MITERS for coming up with an engineering sample of the Xootr Street. I won’t actually be making any mods to this one, since it’s … not mine, and stuff.

This thing measures a bit over 3 feet long when fully deployed. The wheels are 7 inches in diameter and cast aluminum. It’s the smoothest thing ever on the ground because of the large wheel-to-bearing diameter, which minimized rolling friction. And the deck is absolutely enormous….

… and HOLY MAGNESIUM JESUS ON A STICK. It is in fact CNC machined from billet aluminum. These guys are just like me, except with infinitely more style. A scooter? Made from real metal?

And the 10 center-side pockets are just big enough to comfortable seat two A123 26650 cells apiece! How about that. 20 cells yields almost 150 watthours of battery pack energy.

Ground clearance check. The deck height, oddly enough, is almost the same height of the Razor A3 frame. The wheel line is just an inch and a half or so higher to fit the 7″ wheels. Overall, as can be seen, there is about 1.25″ of “fiddle space” from the bottom of the deck (not including the pockets) to the top of the 1.5″ parallel.

This is good, because hiding all the goodies under the vehicle frame contributes to vehicle aesthetics and the illusion that something which is not supposed to be motorized is moving under the directive of an unknown force.

There are no motor drawings or plans for this yet, but the profile of the wheels and their spacious internal diameter make them amenable to stuffing axial flux coreless motors inside, maybe even one per wheel. I’ve been itching to build a real surface-wound (no iron core) pancake motor for a while, but have been put off by their complexity in manufacturing.

As more details condense from the bot-aether, I’ll give this project its own page, category, and possibly a snappy and witty name. This is not a high priority project, as I don’t even have the vehicle yet, and it might spill over into Spring term.

Spring is a better time to blaze around anyway.

Now Announcing Project SEGFAULT

Segways.

A bad pun on the word segue. A fundamentally unstable faceplant-waiting-to-happen of an inverted pendulum. A cool exercise and great demonstration of basic control theory.

DIY balancing personal transporters have been attempted and perfected many times before. It’s almost passè. There’s even instructions on how to do it and code for your choice of microprocessor. All you need is two fat motors, a rate gyroscope, an accelerometer, and determination.

The whole thing about “microprocessors” is what has been putting me off. I like to think I’m familiar with mechanical engineering principles. I’m shake on electronics and EE. But I’m the last person you want to ask about anything software related. I hate software. With a passion. Even though I use it ALL day, I shudder to see what goes on under the glitzy Web 2.0 interface, or under the ultrasonically-welded sealed cap of an Atmel chip.

…so that’s why I want to do it all in ANALOG ELECTRONICS.

That’s correct. Op amps, comparators, linear components, passives… it’s a 6.002 (or 6.101) paradise. I stochastically arrived upon this idea near the beginning of the term, but it took a few weeks before I took it upon myself to do some research on gyros and accelerometers, and sketch out a rudimentary control network composed primarily of rail-to-rail op amps.

Then I remembered that Dale had built an analog balancing robot, so naturally I read the site and discovered I was doing it totally wrong.

I have a sneaking suspicion that a relatively non-chaotic differential equation like the one that governs inverted pendulums can be pretty easily translated to a continuous time control system (analog, as opposed to a discrete time digital control system). The idea as a whole is to have a purely analog, continuous-time front end controlling a Class D amplifier, also known as a switching amplifier or if the output is bidirectional a locked antiphase amplifier. Basically this just means your transducer wiggles back and forth really quickly… but some times more in one direction than another, so the summation of the movements is a velocity.

But Charles, isn’t a switching amplifier a digital thing?!

Yeah, if I implemented a real linear motor driver, I would have a battery life of 30 seconds and require heat sinks the size of Hannah Montana. Sssshh…. don’t tell anybody.

With the plan now more grounded (HURRRRRRRRRR PUN) than before, I’m moving forward with the mechanical details, as I always tend to do first. Once I have a rolling frame, I could conceivably roll analog or digital, or mixed-signal. As always, this entails a trip to MITERS and a few hours of mining for parts.

…

Yeah, so it’s nothing much yet. I grunged these 9″ pneumatic tires for the project as they were the only two matching wheels in MITERS that weren’t already on something.

9 inches? Isn’t that a bit small (&thats_what_she_said;) ? It is, but there’s nothing fundamentally wrong with having smaller wheels on such a machine. It just makes obstacle negoatiation tougher. If anything, I can get away with having less torquey motors because of the increased mechanical advantage.

The design work continues! After I get my control theory a bit more in line, I’ll sketch up a schematic of what I think should work. There are endless supplies of linear circuit components at MITERS and kicking around the EE labs for experimentation. I have accelerometers and gyros on the way from Sparkfun for experimentation.

…Oh, that’s the other cheat here. Real, modern MEMS sensors. I’m going for the analogginess here, not period-realism.

SEGFAULT will get its own page and category as it develops. This is my number one goal for the end of the term, and I’m actually going to try to get the controller graded. Here goes… something!

Video below, build reports, project page.

While I seem to be in “build season” mode year-round, it is during long breaks with little in the way of academic or life obligations that I get the most done. Last summer, I began work on LOLrioKart and built Überclocker, Pop Quiz 2, and Nuclear Kitten for Dragon*Con.

… which sort of sucked horribly for everything. Except NK, but only by about *this* much.

So what’s coming down the projectubes this summer?

Mostly the same thing. D*C is my biggest bot-celebration of the year, so once again the combat robot fleet takes high priority. Since there’s really just one robot that needs rebuilding, I also have the usual pile of small electric vehicle projects, of which only one is actually urgent.

Übercløcker RЭmiχ

I started redesigning Uberclocker some time in the fall of last year, hoping to get it done by Motorama 2009. Of course, due to scheduling concerns and logistics, this didn’t happen. But what that presented me with was the chance to put it away and not look at it for several months.

This is pivotal. The basic design has already been hashed out, but now I get to return to it after not thinking about it for a while. I am now in the process of analyzing the 3d model for any “impossible objects” that I might have included, or Really Bad Ideas. Such design flaws plagued the real life Uberclocker 1.0 at D*C last year.

Planned upgrades from 1.0? Well, besides EVERYTHING, the primary focus is on drivetrain reliability, center of gravity, and the upper clamp arm.

As a member of the pushybot school of combat robotic thought, I value maneuverability and driving above jawesometacular weaponry. Uberclocker 1.0 had a strange serpentine timing belt setup that seemed like a really awesome idea at 5 in the morning, but… wasn’t.

The robot also suffered from “centrally located center of gravity” syndrome at the event. While a CoG near the geometric centroid of the robot is good in practically every other case, the fact that the bot’s sole purpose was to grab another opponent and lift it off the ground meant that it just sort of faceplanted every time I attempted a lift. Not a very impressive show. The redesign lengthened the wheelbase of the bot, and selective weight reduction moved the CoG back about 4 inches, without additional ballast.

Oh, that’s right, Uberclocker 1.0 weighed in at an incredible 22.5 pounds out of 30 at the event. I’ll fix that too.

What I didn’t really get to (properly, anyway) in the redesign was the upper clamp arm. The previous arm was both weak and structurally unsound. While I think I took care of the “unsound”, I still have my doubts as to the clamp mechanism’s effectiveness.  In the past, clampbots have used pneumatics to actuate the upper half of the clamp. This is advantageous because a pneumatic actuator requires no “holding power”, unlike an electric motor, which has to be continually powered to produce torque. Pneumatics also have a certain amount of spring-back ability that a solidly coupled electric actuator doesn’t.

But robot-heaven forbid that I make Überclocker even more complicated by incorporating a pneumatics system for the one actuator that might need it. Thus, I’m still partial to a (spring-coupled) leadscrew-type mechanism, over the current design candidate’s motor-on-a-weird-gear. Except this time it won’t be driven by a beetleweight motor.

I intend to keep the “Chinese puzzle” frame, and will be refining it for ease of assembly. I devoted a few weeks to just fabricating the frame parts last time – no, never again. That’s what computer-controlled machine tools are for.

Pop Quiz 2√2

Incidentally, 2√2 is about 3. Not quite there, which also describes this planned rebuild of Pop Quiz 2. It’s not quite a complete conceptual revision, but there will be significant upgrades all around.

PQ2 is one of the (if not the) flattest 1lb class robots around that has an active weapon. It hits lower than some undercutters. The problem is that going the extra 1/8″ down in this current design meant that I had to ditch practically all the well-known, battle-proven parts – Sanyo gearmotors, SPEKTRUM 2.4ghz receivers, etc.

It was a fun thought experiment come to life, but the robot had a horrific reliability record, almost no reception due to the FM ground-band receiver, and a 5 minute chopped hack of a master power switch that ended up disintegrating after exactly 1 hit at D*C 2008. Pop Quiz had about 15 seconds in the arena.

Not cool. For ’09, I am INCREASING the height of the bot. Me, making a robot taller. How many times does THAT happen?

The robot height will be increased to about .400″, enough to cram in a set of real Sanyo micro gearmotors. The rest of the robot’s electrical system is sound, and so is the weapon motor. I’ll most likely end up reusing the electronics anyway, minus the cheesy little FM park flyer receiver. Instead, it will be swapped out with the latest Spektrum DSM offering, and I will run one transmitter between all the robots.

There’s no current virtual model for PQ2.8284171, but just imagine the current bot 0.025″ thicker.

Nuclear Kitten 5.1 Digital Surround Sound Edition

I’m actually satisfied with the performance of one of my combat ‘bots for once. NK needs very minor rework to take another run at D*C. The weapon motor needs some magnet reglued, and the weapon pod pivot axle is slightly bent and needs to be made better anyway. Past that, I have a spare blade to replace the faceplant-into-steel-bumper bent blade.

The only point of concern with NK is the drivetrain. Despite having a mechanically isolated weapon, I’m still blowing drive gearboxes, just because the bot is that much more powerful. I might switch to something like the 50:1 Copal motors || redesign the motor mount || use softer wheels.

No frame changes are necessary, since the bot escaped D*C rather unscathed.

LOLrioKart

Since I discovered that the main battery pack was leaking voltage all over the place (somehow, through an eighth inch of rubber?), I stripped down the entire electrical system and tested all the batteries. It turns out that the steel casings of the cells are live, something which I’m fairly certain should never be the case. While it’s fairly common for the battery negative terminal to also be the casing, the errant voltages are always somewhere between 0 and 1 volts.

This case voltage doesn’t seem to have negatively affected the cells, but I’m fairly certain it’s the culprit behind stray frame voltages. Somehow.

The focus for LOLrioKart work will be the electrical system. I intend to complete and test the ginormoFET controller and possibly implement dynamic (or regenerative!) braking using the upper leg of the half-bridge. Mechanically, the kart is fine.

Well, except for the brakes, but they’ve always been undersized and insufficient.

Ultimately the goal is to run it for longer than 1 minute on all 54 volts, or the full pack voltage of whatever eventual power system I might come into. I’m heavily considering crating up LOLrioKart and shipping it down when Dragon*Con comes around, so I can drive it in the parade. This could possibly be the worst idea I have ever thought of.

Project RazEr

It’s been hanging on a utility hook since the last controller fire. Everything works and the batteries are still charged, so all I need is a BLDC motor controller. Since everything still technically “works”, I don’t intend to touch the scooter that much, if at all. Any work on it will be replacing the shell of the wheelmotor with something more substantial (and better engineered, and more reversably built).

Time to get crackin’.

I am now thoroughly convinced beyond any doubt that power electronics just plain hate me and want to see me suffer.

Last week, the 5v switching regulator on RazEr went out with no explanation. Fortunately, it just stopped switching, and didn’t jam 35 volts through all my control logic like last time.

Today, still depressed from the failure of LOLrioKart and wanting at least one working vehicle, I cracked it open and found out that the (poorly) reflow-soldered output capacitor fell off. After soldering it back on, and still have it be dysfunctional, I made a 30-to-5-volt linear regulator from a 78T05 chained to a 78T12.  One 7805 cannot handle the drop from 30 volts to 5, so the 7812 is an intermediate stage. This is an arrangement that’s worked for me before.

After installation, the headlight, servo driver, and ESC fire right up. I was able to reset my throttle endpoints as usual. Then I pushed on the handlebar throttle, and

FML.

(Oddly enough, the headlight and servo driver  stayed on through the entire ball of fire. That means my regulator works.)

Back to DC motors and relays…

So, here’s an epicly long build report all the way from start to finish. The whole process wasn’t long and drawn out this time, mostly because the fabricated components were conceptually simple. The majority of work was done in a few evenings at MITERS (as usual.)

Here’s the plan. Unlike version 1 of RazEr, the module is one-piece, not a weird jumble shoved into the tube frame of the scooter and an add-on pack underneath. This time, I intended to cut out the entire bottom of the scooter in order to fit everything in.

Benefit: A vast increase in usable, contiguous space. That’s the biggest difference. While I only gain maybe 2 cubic inches from version 1 (in the form of the aluminum bottom of the scooter), that 2 cubic inches of metal divided the battery pack, electronics, and everything else from eachother. Losing this barrier meant I could use bigger battery packs for MOAR P0WAR!. Not only that, but I could use stock R/C hobby lithium polymer batteries.

Like this 5AH, 4S1P battery from UnitedKingoftheHobbyCity (or whatever the hell they call themselves now). I got two and intend to run them in series for 5AH, 29.6v nominal – or almost 150 watt hours of battery power, quite substantial for something this small.

One pain with using Li chemistry batteries is that they require balancing. Unlike Nickel chemistry batteries, you can’t just let a Li cell bake off an overcharge while weak cells in the pack catch up. Usually, that involves catching fire.

Having more contiguous cubic inches in the design meant that I could use more real electronics, such as this 5S balancer module. The plan is to mount two inside, each balancing a single 4S pack. There was no danger of ground-looping a pack because these balancers have no other physical connection to the world.

Alright, so let’s see how everything fits. So far, so good. It’s hard to get a sense of physical size on a 2D computer screen. When am I going to get fully-immersive 3D holographic CAD programs?!

This is “The Stack” of electronics. Of course, the end result will be more organized, and use actual mounting screws, standoffs, and the like.

The 100A 42V brushless controller is on the bottom – the intention is to strap the MOSFET side directly to the thick aluminum for heatsinking. On top of that go the balancer cards. On top of that still is the 5V switching regulator that will power all the logic (no more linear regulator dropping 35 volts to 5!). Barely visible, between the balancer cards, is a servo tester. I decided to just buy one since they make them smaller, better, and more durable than I ever can – come on, it fits *between* the two balancer cards.

Alright, enough visualizing. Let’s begin.

What happens when you need 18 inches of a 3″ square aluminum tube? You buy 6 feet, because McMaster-Carr says so. I had to split the enormous 6 foot stick into two three foot sticks, then cut the 18 inches I needed off that.

So I lied – I only needed half of that 18 inches.

Except it’s half in that other way. With  the new SLITTING SAW!!!!! I was able to take a clean cut down the longitudinal plane to separate the tube into 2 U-channels.Two little clamp fingers set into the T-slot and dialed in provided a flat stop to clamp the tubing against.

18 inches is a long way to crank a mill manually – this and many other operations were facilitated by the mill power feed.

Wait, what power feed? Charles, the MITERS Bridgeport doesn’t have an X-axis table feed.

No, but I have a cordless drill with a socket wrench that fits the handle nut. Worked beautifully.

Finishing the U-channel sides to proper height. I dug up all the random clampy things we had for these operations – I think we need more.

After making a bunch more profile-shaping cuts and drilling holes, it’s time to throw it on the body as a test!

Note that the wheelmotor is missing at this point. I still needed some form of transportation – walking just seems so inefficient after putzing around on wheels – and the hub motor, while smooth, still added massive drag compared to a plain skate wheel. So I dismounted the drive motor and threw in a scooter wheel while construction was going on.

As can be plainly observed, I have a few pebbles worth of ground clearance. Surprisingly, it’s still more than the average 90/100mm-wheeled Razor scooter.

Here’s one result of planning ahead and designing things before building them. This is the front endcap of the module (the rear is similar, just without the holes). Eventually, when everything is mounted, I’ll go around the edges with silicone sealant and make this sucker splash-resistant.

The “window” is laser-cut acrylic, done on the Media Lab’s GIANT LAZER.

Throwing everything into the unfinished module. Hmm, space is a little tighter than I had imagined up int he front.

Well, the important bits fit, so it’s time to commit the changes to the scooter body.

I set the aluminum extrusion frame of the Razor scooter up in the mill and carved out the entire bottom. After this, the frame is no longer structural, and the module itself has to be attached with more security since it has to take a load. That was my concern with switching to the current design. We’ll see how it goes, I suppose.

The no-longer-tube frame cleaned up. I guess now it’s more of a double-T shape, or a U-channel with a fat bottom end.

Let’s see how the back end fits. Verdict: Pretty well. I was afraid of misjudging the height of the interior as well as that radius at the interior top of the frame, but it seems to be correct.

Nothing a giant bead of sealant can’t remedy.

So, the bounding dimensions are correct. Time to add the details!

Milling the angle at the front. I could have left it square and gotten a little more space, but the angle complements the existing angle on the scooter frame, and doesn’t make the whole thing look like a box.

Also, I drilled/milled holes and slots for the eventual battery plugs.

Test fitting after milling the angle.

So what are those five little holes in the front for?  Well, I figured as long as I was making a cap for the module, I might as well add some spiffiness to the whole thing. So, the five holes are for superbright white LEDs, which cast a low-angle light in front of the vehicle to act as a sort of headlight.

The entire physically finished frame. Essentially, this module is an extension of the internal cavity of the scooter. If it weren’t for my obnoxios bad-setup-induced chatter marks, it would almost look like it came this way.

Now that I was mounting a bunch of electronics right under the folding joint, the large conductive cylindrical protrusions (otherwise known as screws) had to go. Simple – attach the joint, then cut off the screws flush with the mounting plate so they don’t stick out any further from the bottom.

Making a precisely (okay, semi-precisely) engineered frame extension module for a scooter may be easy enough, but now how am I going to securely attach it to the original frame? The plan called for simple set screw pressure (the six holes in the sides), but I pretty much knew from the start this wasn’t going to hold anything.

Set screws by themselves exert strong point pressures, but this can easily exceed the yield strength of the metal and cause epic joint strength fail because the connection point simple deforms away from the screw.

This is why removing anything from a shaft that has a slipped-set-screw ring cut into it is horrible and usually involves a giant press or grinding wheel.

How to overcome this? While reading about how to adjust the cross-slide gibs on the MITERS lathe, I realized that if I slipped a soft piece of into the gap (between my module sidewall and the original scooter frame), and then applied the set screw pressure to that, the pressure gets distributed by the soft metal into a friction force that covers a wide area, binding the module to the frame.

This is the same mechanism that some small hobby lathes and mills adjust their slide gibs with, except, of course, you don’t want to go as far as to bind the sliding surfaces together.

So notice the strip of aluminum crammed into the gap. When all six set screws are tightened against it, the whole assembly is rock solid.

I christen this the “Ludicrous Gibs” mounting method for attaching two channel-shaped objects together. ROTT was a great game, by the way.

Wiring work begins. Note the neat laser-cut Deans connector mounts. Each one snugly holds a Deans female plug, aided by a drop of thin CA glue. In turn, they are screwed into the side of the channel.

The left plug is the battery, and the right plug leads to the controller and other electronics. This will be bridged by a removable power link, like how I make most of my robots’ main power switches.

It’s kind of silly to try and aim a plug into a socket as an EV switch, but I couldn’t find a switch of the proper rating (or form factor) and invented this idea in a few minutes. However, the benefit is that I have easy access to both sides – battery OR vehicle, should I want to run experiments or fit a power meter between them to take runtime data.

Dropping the components in… There’s no turning back now.

Looking pretty good so far. After mounting the batteries (read: tacking them down with foamy double-sided tape), I performed a quick fit check.

Again, pretend the cruddy chatter marks don’t exist. That was the result of “Hmm, I think clamping on a 1/8″ thick section of material and then face-cutting 2 inches away is a good idea”.

In a moment of disappointment, I discover that plugs do indeed have a real volume – not complex, nor purely imaginary.

Instead of re-engineering everything to fit in the balancer cards, I decided to go dig up a 9-pin connector of some sort to let me access the cells individually from the outside.

It ended up that the battery balancer cards weren’t going to work anyway. They begin working whenever they are connected to a battery, and are always on, even if not balancing. Therein lay the problem – I couldn’t just reach in and turn them off. That means once connected, they will always be draining the battery with the minuscule current needed to run themselves.

It isn’t much, but is enough to discourage me.

I found this neat 9-in single-row header in a box of cables. A .1″ male header row fits it, so I was in luck if I need to make a cable.

I did have to make a slot to mount it, however. So I threw the module onto the mill with giant, live lithium batteries strapped in and cut the slot.

As to not cause heart palpitations in safety freaks, I’ll refrain from posting the picture.

Anyways, magic happens and it works. Here’s the finished shot! No external electronics this time – the box mounted on the steering column is empty. And I even have a real thumb throttle.

The front is covered in electrical tape because I didn’t want wheel grunge fouling up the shiny new acrylic window. Since this picture, it’s been pretty well dirtied up.

Weigh gain over version 1 is approximately 1.5 pounds, mostly in form of SERIOUS FUCKING \M/ETAL from the aluminum module frame, and slightly larger batteries.

Still light, however. The lightest around, as far as I know.

The question that everybody asks after “What the hell is THAT?” is “How fast does it go?”. I’ll need camerafolk to answer this, but the answer is “At least 15MPH on flat ground and fresh batteries”. It’s hard to gauge speed when you’re on it, just like 60mph  seems much slower in a car than standing on the side of the road.

However with a properly calibrated controller (such that it recognized me not as a stuck motor), I metered 1.2kW on good launches. The wheelmotor actually has substantial torque – just from the feel, I would rate it around half that of Snuffles the First, i.e. “it will fly out from under you if you’re not paying attention”, which is what happened to me the first time after calibrating the ESC.

I need to actually to “back-run” the numbers here to see if they match up with my initial predictions before building the motor.

So that wraps it up. Now that the power electronics side of the project is done, I need to rebuild the wheelmotor, which is falling apart day-by-day and is only running at the moment because I pumped all the empty space between the tire and rim with hot glue.

Yeah, I hot glued my motor together.

And here’s a shot of the headlight working.

i can haz 5-minute cross-campus commutez back plz?

Partial electrical system meltdown combined with salty winter slush means that the electrical system has gone haywire.  As in, something is bridging my battery V+ connection, the frame, and the BEC. That’s pretty intense right there – flipping the power switch to OFF does not actually turn the BEC off. Additionally, I ran down my partial charge, and now have no safe means to recharge the cells.

The underside, and part of the inside by the rear wheel, is also pretty well caked in grunge.

So I’m taking the scooter offline for now, and will be going on a fast rebuilding process. The list of updates include

• One-piece underside attachment that mounts all the batteries and control electronics. On the inside. This means cutting out the bottom of the scooter and using that new underside attachment as a stressed structural element.
• Waterproof.
• Minimize custom electronics. Easier on me, more idiot(me)proof. Plus, you can get a servo tester for 5 bucks now..
• Waterproof.
• Internal battery balancing. Little dedicated balancers are only a few square inches in area and cost 15 dollars. Well worth the extra cost, in my opinion.
• Waterproof.
• All main battery wiring will be heavy gauge, and there will be one charge port.
• Waterproof.
• No-through-bolt wheelmotor design. Can actually change tires!
• Waterproof.
• Stock LiPoly or LiFePO4 packs, not custom assembled jobbles.
• Waterproof.
• External routing of motor wiring. for easier service.
• Waterproof.
• Did I mention waterproof?
• Waterproof.

1. When you charge your 120 watt-hour lithium ion polymer battery at 6 amps, please make sure your internal charge-balance wiring is not made of 24 gauge wire.

2. If they are, and you should choose to run 6 amps through them, please make sure they are not tensioned against a rough edge in your vehicle’s all-aluminum frame.

3. Should they be so situated, please at least make sure the impending insulation meltdown and dead-shorting of the lithium batteries occurs more than half an inch away from the aforementioned batteries.

If all of these failed to be true, then welcome to my life.

I’m glad that said 24 gauge wire burned through its plastic connector housing before Bad happened.

A few minutes after setting up the charge, I heard my charger beep furiously, indicating a premature charge termination (that’s what she said?).

I turn around and an enormous white smoke cloud is hovering above the scooter back end. Fearing the worst, I grab the thing, bust through the nearest non-emergency door and pitch the whole vehicle into a snow pile.

The heat was intense enough to melt the acrylic connector mounts  and completely vaporize the smaller balancing connectors. The large Deans connectors were fine, because the short occurred through the small wire.  Very fortunately I got it out of there before the ass end of the lithium cells overheated, because angry Li cells are not to be dealt with lightly.

Combine with the very close packing of the cells in the scooter chassis and it could have been.. well, more interesting.

Anyone know what the plastic is that most R/C hobby stuff and electronic casings are made of? Whatever it is, it burns leaving a hideous, acrid, obnoxious smell that can only be described as one part lifelong chain-smoker, one part wet decomposing grass clippings, and one part burnt garlic toast. It also covers the surrounding area in a sticky black oil-like substance.

And it does not ever come out of things. It’s the same stuff which they make power MOSFETs out of, apparently, since those smell just as bad.

The batteries seem to be fine, but the back two cells in the belly pack may have localized thermal damage. Since I don’t like playing lithium polymer games, I might replace those two cells. This is also an opportunity to rethink my battery strategy. The electricals of RazEr are a complete pitch-together hack made of double-sided tape, Goop, zip ties, and heatshrink.

tl;dr use thicker wire 4 batts

LOLrioKart

Over the weekend, I was using my charger to recondition some found SLAs in the great MITERS lead-acid battery pile, before the really dead ones (including 10 car batteries) were sent for disposal.

I remembered I had one of these. And this. Thus, on a whim, we haphazardly taped together a rudimentary electrical system for LOLriokart out of some of those found batteries.

It was just like the first RazEr test run – a knob with no spring return in an awkward position requiring a delicate balance of dexterity and madness to operate. Fortunately, with a 4 wheeled vehicle, no balance was required.

To my surprise, the sensorless ESC was able to get the kart moving pretty adeptly. I guess that “12mhz CPU” is good for something. (Also, there’s much backlash in the chain drive, so the motor can probably move enough for the ESC to pick up the switching sequence before it hits a load.)

A test video is here.  The 24 volts of SLAs were sagging to under 18 volts loaded.  I estimate the speed at maybe 10MPH, +/- some. Still, in close quarters like the N52 hallway, it was mildly exciting. Obstacles included night janitors, that fire extinguisher, several polished wood and glass art display cabinets, and the MIT Outing Club championship canoe.

And the very well-placed panel of plywood at the end.

See? I promised I’d get the kart moving before February! I just didn’t say how moving!

When the waterjet opens again, and I get a larger sheet of aluminum to finish the battery basket, then the *REAL* fun can begin.

Speaking of the battery basket, here’s the concept.

Made of 1/8″ and 1/4″ thick aluminum, it will support the batteries (bounding-box outlines in clear gray) with room to add some shock-absorbing rubber or foam padding. The mounts will clip onto the chassis (bottom halves of the clamp mounts not shown), and be on adjustable-width sliding mounts. The adjustableness compensates for the fact that I don’t actually know how wide the kart, and these compliant mounts allow me to move the batteries slightly if something turns out to be in the way.

I decided to go for the 4-across mounting style just because it leaves more usable (continguous) volume under the basket.

The top plate will be made of whatever nonmetal I find when I cut everything else out. I have it spec’d out as wood, but it could be fiberglass, MDF, Lexan… etc. It will be spring-loaded to the top of the batteries by the corner mounting holes. It will also double as the electronics mount.

Combining the topic of electronics mounting and Conveniently-placed Plywood Planels of Kart-stopping (+1), and continuing my everlasting quest to engineer my way around simple and reliable solutions, I have thinking about giving LOLrioKart power brakes.

Using a beefy servo mounted on the brake mount on each front wheel,  and some interfacing with a foot pedal (you know, like a servo tester, or a microcontroller interface that also handles other vehicle auxilary functions), just use the servos to yank on the levers. With such an interface, I could actually adjust brake balance, timing, bias, and that stuff.

Wait, can’t you just run some cables? MITERS has bins full of bike brake parts I could just pull.

Yeah, but I’m lazy. I would much rather rebuild the brake mounts to include mounting provisions for a servo, then interface with the brake pedal using a clumsily-built and possibly unreliable electronic interface. It’s all the rage these days, like aluminum billet where a simple clamp-and-weld would have sufficed. Besides, since this is an incredibly bad idea to begin with, I might as well add another layer of bad-idea.

(It is indeed easier, faster, and better to route two Bowden cables – don’t get me wrong.)

Work on LOLrioKart will probably taper off a bit as the semester begins.

Speaking of semester, today is Registration day (as well as Techfair), and I need to wake up before sunset.

Over the past week, I’ve been riding the scooter around to get to class and run errands. It attracts a fair share of stares and questions, since nobody really expects to see a Razor scooter cruising at 10mph. Yes, I mounted the real e-bike throttle onto the handlebars, so it’s actually controllable. No, I have not went back and fixed that terrifically beautiful hack of a signal interface.

I have also been pretty good at “path look-ahead” to avoid potentially lethal potholes, but have yet to attempt a flying leap over the railroad tracks.

The range from a full change has been confirmed to be over 3.5 miles. This is quite in line with the ~4 mile calculation, and seems to include all of the inefficiencies that I did not include in the approximation (rolling resistance, terrain variation, wind resistance, etc). This is perfect. Why? Because it lets me make several cross-campus trips per day – e.g. to get to class and back, to get food, and to get to MITERS and bumble for hours on end. Here’s an example.

Yeah, all the blue lines are on top of eachother. This consists of the following trips from today:

1. To class in the morning (Ames St. @ Amherst to Mass Ave. @ Memorial Drive)
2. From class to the Media Lab (Mass Ave @ Mem Drive to Ames St.)
3. From the Media Lab to more class (Ames St to Mass Ave @ Mem Drive
4. Class back to dorm, through the north building cluster (Mass Ave. to Ames St. via Stata Center)
5. Dorm to Student Center for some dinner (Ames St. to Mass Ave)
6. And back.
7. Dorm to 7-Eleven (Ames St. to Main @ Portland)
8. And back again.

That about covers my needs, really. 3.5 miles and back is also enough to make it to area hardware stores, which is of course a priority.

However, I have also been constantly maintaining the motor to keep it running. There’s nothing electrically wrong with it, but the design of the motor and wheel interface is that

1) it tends to force the motor apart, since the inside of the wheel has a chamfer on both sides and I have a matching outward chamfer on the motor endcaps, and

2) the wheel itself tries to torque the endcap tie screws out. How the hell does that happen? I had to cut indents into the tire’s plastic rim to pass those tie screws. They are circular in profile, and my guess is that with every compression cycle of the tire as it rolls on the ground, the indent sort of cams up against the screw and torques out a bit.What then happens is the screw sticks out far enough to bang against the aluminum frame, making a very audible click that tells me to stop immediately and ride unpowered the rest of the way.

This phenomenon has even defeated red Loctite.

I’m working on a design revision of the outer can that doesn’t rely on those screws to keep everything together. Either they will be routed internally, or the can itself will have some other method of fastening things while still allowing the tire to be changed. For now, the stator won’t change, since I can’t find another giant copier to rip another stator out of.

Oh, and I have also let all the Li cells go horribly out of balance. I haven’t made a charge plug for the DB9  charge/balance port – rather I have just been attaching alligator clips from my charger to wire leads shoved into the main + and – pins. Safe, eh?But now I have discovered that the cells have up to a quarter volt disparity, which is too much to let the whole pack charge at once. I’ll get around to making the balancer plug and cycling the cells appropriately.

The bottom line is that IT WORKS and IT’S AWESOME.