Archive for June, 2014


Beyond Unboxing: The Great Cambridge Chainsaw Massacre; Ryobi RY40511 Cordless Chainsaw

Jun 23, 2014 in Beyond Unboxing, Reference Posts

Hello everyone,

I’ve decided that I need a career switch. After 2-3 years of being a shop ninja, I’ve decided to move on and become a…

…Chainsaw murderer. I’ll see you shortly.

This is probably the shortest turnaround time for a Beyond Unboxing post ever. Around 2 in the afternoon on Sunday, I received a tip from Shane and the motley crew at Freefly about this device and how it might be worth looking into.  An hour later, I was blitzing down Storrow Drive heading due west towards the Home Depot in Watertown, MA on a war path to obtaining one. Just to check it out. And an hour after that, it was parted out completely on my shop bench.

Why the hell do I find tearing apart consumer products so amusing? Probably because I know both how the sausage is made as well as how the sausage will be cooked and eaten. As with other Beyond Unboxing posts, the focus is how to divert these products into small electric vehicle or robotics applications, in the context of teaching newbie builders and hackers to be resourceful and to see parts everywhere.

From the past few years in meddling with lithium ion batteries and brushless motors, I’m always giddy to see them used in more and more tools and common implements, but they’ve been primarily in the domain of the ultra-high-end until recently: Spending $300-400 on a drill for parts, to me, is not really worthwhile when you can engineer around the same problem using other commercial parts, or even custom-made ones, for much less. This is part of the appeal of things like mini-jasontrollers and angle grinder gearboxes: To duplicate the functionality would imply spending an order of magnitude or more, and the chopped up commercial product would be workable for all but the most stringent and demanding of applications (Former students, I’m counting on you to put T-nuts and Jasontrollers in space).

I’m glad to see these lithium ion and brushless systems coming down in price and going up in power. The advent of the Inexpensive Chinese Brushless Motor has been beneficial to many industries. First they started small with compact drills and drivers, then moved up to saws, and now bigger power tools whose domains were previously dominated by gas engines. Once you get to things with power levels in the hundreds and thousands of watts, you can ride them. Perhaps the advent of this tool is the closing of a circle of life for vehicle builders: Years ago, kids would build shoddy vehicles powered by gasoline engined chainsaws and leafblowers. Now, they can finally do so with electric. And have it not suck.

Here’s what’s inside a Ryobi model RY40511 “40v” cordless chainsaw. I’m surmising I know (directly or through only 1 or 2 degrees of separation) the intern who first conceived this and put it together, because as you’ll see, the hardware is quite familiar….

First off, this is what the box looks like. GAS-LIKE POWER! I can’t believe you guys trademarked that. I mean, it’s basically in the vein of “polypropylene-like” modeling resins for expensive-ass 3D printers (sorry, Objet!) or “fat-like” additives for greasy fast food.

…and would it kill you to use a longer box?! What is this, Minecraft? How many of these get bent up and broken in shipping?

Putting a picture right on the box of an outrunner. Damn, I wish I paid more attention to the aisles of Home Depot now! Maybe I could have gotten on this months before!

Step 1: unboxing. This is pretty much one of the only normal pictures this post will contain, so savor it. Included is the saw unit, a battery, and a charger.

The battery in all its glory. I’d wager by the oblong shape of things, it’s an 18650 pack made with 1.5-1.8Ah “power” cells laid flat in a “W” pattern. You’ll see later that this wasn’t a bad guess – being a little familiar with the state of the industry helps (and some math: 55Wh / 40V is about 1.4, figure in some overhead).

The battery has two terminals besides the + and – called T1 and T2. Most lithium packs have a connection to the battery management system or similar, and this format varies by manufacturer.

While I was plotting the deconstruction of the saw, I put the battery on its charging dock. It’s a fast charger – 1 hour or less.

I’m extremely happy that lithium ion power tools are becoming the norm, because that means an end to the era of those irritating as fuck “Revive your battery!!!” kits and guides sold by industrial charlatans all over eBay and the like, because those clutter up every possible search you can make with power tools terms. That only “works” for nickel cells (it’s called Zapping, and it doesn’t actually fix much in the long term), and if you tried that with a lithium battery, congratulations on your newly made IED.

Taken out of the box. Where a 40-50cc gas engine would go is a big hollow battery cavity.

Hmm, weird. The tabs inside the tool would only connect to what is called T2. This will come into play later.

And here is the whole device. It weighs about 10 pounds without the battery, which is way lighter than a gas powered saw. Chain bar oil container at the front, variable squeeze handle at the back. I liked how it felt – it’s definitely designed from the ground up to be an electric saw, and not a crude rush to market retrofit of a gas saw.

That’s all the generic unboxing post you’ll get from me.

Break out the wrenches and hex keys! The majority of the body screws on this thing are T25 torx. First to come off is the chain cover, with four screws.

The two M8 nuts come off also, and the drive sprocket and tensioner assembly are revealed.

The chain and bar assembly come off easily once the nuts are removed.

To remove the drive sprocket and flange, you’ll need a snap ring plier, or some tiny screwdrivers and a bucket of patience. Retaining rings used to be my most hated component when it came to taking something apart in my middle school and high school years, because I didn’t have retaining ring pliers!

But now I love them. Welp.

The smaller case screws take T20 torx. There’s three on this side of the saw.

However, I found that the side plates were keyed into the gray top. There’s more T25 screws on the top plate, and then it lifts off pretty easily:

…to reveal the Jasontroller… uhhh, okay, Jasontroller. Two 470uF bus capacitors and discrete FETs, two per leg of the three-phase bridge, is all I can see at this point. The FETs are Alpha-Omega (what kind of semiconductor company name is that!?) AOT470 type – kind of low end of the market, but they’ll get the job done I suppose.

The board is actually quite well designed and marked. Hell, everything is labeled already! Even the motor hall sensors and the trigger switch wiring harness. The ‘throttle’ is 5 pins labeled S, S-, S+, -, and +. The “communication” wire leads to terminal T2 on the battery.

If I had to guess, this assembly contains a “throttle switch” that tells the controller you’ve hit the trigger (before the signal changes), and the S, -, and + are what would be a 3-pin Potentiometer sensing element or Hall sensor sensing element. I haven’t verified this on the actual throttle, by the way, but it makes sense.

Back to the saw! This assembly in front of the motor appears to be a bar oil pump driven from the motor, or perhaps feeding the motor. It has to come out for the motor to be removed. Two T25s screw it into the motor.

But that wasn’t all. To remove the motor, I had to be able to take the other half of the plastic shell off. This took a longer T25 bit than I had in the form of that green set. It was 5:45 PM on Sunday.

I’m proud of the Commonwealth of Massachusetts and the City of Boston for not supporting the usage of speed cameras, and of Mikuvan for not…. uhh, exploding. The closing staff at HF were waiting for me at the front door when I pulled in. Thanks guys!  This is a T-handle Torx set, 42926.

(The observant would note that the local Home Depots close at 9pm, but…)

With my newfound long-reach Torx set, the case screws come off.

The goods are within sight. Check out that motor – it’s like a pointier, more gritty version of a Hobbyking outrunner!

Hell, it even says the size on there. However, this motor house, whoever they are, seems to label their motors properly: By stator diameter and stator length! Hobbyking would actually call this, in all likelihood, a 63-54 type motor. The can length compares.

The motor is attached to the controller through these terminal block things. I guess they wanted something to distinguish them from an R/C outrunner. Should have used 4mm Bullet connectors!

The motor sits over a Hall sensor board. This board uses 120-degree oriented Hall effect sensors, but what is interesting is that they’re surface mount devices, and oriented vertically. Having a custom motor without a nosecone, unlike the R/C outrunners, would let you do that. Having the sensors in such close proximity to the magnets’ ends, not sensing through a steel can, surely contributes signficantly to their ability to not drift significantly out of time at high speeds (That video is the reason why we made those Hall sensor boards slotted and adjustable).

Removing those two T25 screws on the oil pump reveals the pump mechanism. Now, this thing is clever. So I’ve never taken apart a chainsaw before this point, so I’m not sure if this is in common use, but it’s cool anyway. The little gear inside the pump body is engaged by a helical spring slipped over the motor shaft (more visible in later pictures). This spring is a slumlord worm gear, meshing with the pump’s own gear.

I didn’t get a picture of the right side of the pump where it connects to the black rubber tube, but it’s a lobe looking thing that acts like a one-rotor gear pump. It’s enough to squirt a little chain bar oil onto the blade steadily, nothing special.

Continuing the motor removal with 3 more T25 screws:

And the system is out! I love this motor. Not only does it have a big fan cast into the end, but it has a normal shaft. It’s 12mm diameter, 10mm across flats. You can straight up slam a belt cog or chain sprocket over this and use the flats with some giant set screws and be done with it. Or, you know, just use the friggin’ chain and make a killer ice-racing machine.

I studied the plastic parts for a while. They’re mostly useless for our purposes, but looking at them is a great manufacturing study. The injection molding crew spent some time on this! And imagine those molds being made.

Here’s a close up of the controller. The construction is “jasontroller-like”. The gate drive circuitry is entirely discrete. Hell, the microcontroller is a STM8S, a staple for Chinese motor controllers. I bet this pinswaps for a generic Chinese e-bike controller pretty well.

I decided to take apart the battery next to see what’s inside. They put a bit more effort into this one: It needs a T15 security bit (with the center pin). Luckily, I had such a thing.

Four screws and the lid pops off. Holy crap, that’s a ton of electronic jibba-jabba for a battery. On the left, a big fat current sensing shunt. On the right, the battery management circuitry and charge balancing circuitry. On the very left, edge, buried under the board, were two semiconductors of some sort – I think they might be TVS clamping diodes. I decided against unsoldering each and every cell connection to try and find out, but again, it would make sense.

To lift the battery pack out, there’s two tiny hidden screws on the bottom, right where it says RYOBI. I’m going to guess that placing one of them right under the “O” was intentional.

To find hidden screws, I usually drag a screwdriver in a grid pattern around the tops of stickers and badges until it “sinks”. It is a very common tactic to hide screws under labels, stickers, and those little rubber bumpy feet if your appliance has them. Sadly, not everyone loves giant-ass cap screw heads sticking up every which way on their products like me. I’m a mechano-aesthetic snob.

A close-up look at the power side of the BMS. The 1 milliohm current sense shunt resistor has some little analog filtering passives growing on it which may or may not lead to the “T1″ and “T2″ terminals. Again, to find this out for certain is your job required unsoldering 12 cell tabs, and I wasn’t going to delve in that deep yet.

Again, if I had to guess, these do not (or do not only) lead to the T1 and T2 terminals, but is also used by the BMS to determine if it has to cut off the battery pack current.

BAM! Who was spot on about the battery? Here’s a LG Chem 21865 1.5Ah “power” cell. I say “power” because there’s often 2 ways you can optimize your lithium ion battery: for power or for energy. It all depends on how thin your terminal plates and separator is (the thinner, the more volume is occupied by active anode and cathode material, but the more resistance and hence less ability to dump amps). Most power tools are going to use “power” cells, most cell phones and low drain devices “energy” cells. For instance, in the 18650 (one size smaller) size, 3Ah “energy” cells are becoming common, and these are the next larger size and only 1.5Ah. But as will be seen, they will push current.

(Today when I learned about the world of customizing electronic cigarettes. What the great carbide-tipped fuck? Seriously, it’s like the old adage of racing – “if it moves, race it”: If it’s a electromechanical device, mod it.)

With the saw fully vivisectioned (never before has this term worked so well, since everything is still working!), let’s start doing some science.

The “throttle” is a plunger switch – on the side, it says it’s a Defond model EGA (see page 44 of this catalog). As suspected, it’s an ‘enable’ switch, plus a 3 pin potentiometric element. Which pins are which, I haven’t dug into.

First, the motor running at top speed (it sounds awesome) draws anywhere between 4.2 to 4.4 amps at the battery voltage measured at this instant in time (39.2v).

No-load waveform of the motor, showing voltage (39.2V) and electrical frequency (1,389Hz)

Scoping two of the phases tells me that at full throttle (100% pwm – no switching waveform seen, just the commutation frequency cycling of the FETs), the motor spins at 1,389Hz, or about 12,000 rpm. To get from electrical Hz to mechanical RPM, you take this waveform frequency ( “electrical cycles per second” ), multiply by 60 (“electrical cycles per minute”), then divide by the number of pairs of magnetic poles of the motor. For this motor and many R/C outrunners like it, it’s 14 magnets, so 7 pole pairs. You arrive at 11,905 RPM, with some amount of error nobody cares about outside of an instrumentation course at MIT.

At this speed, with the Hall sensor timing given, the motor’s Kv (“volts per RPM”) is approximately 300: 11,900 RPM / 39v. This is fast for a motor of this size, so to be useful in a vehicle, it will require significant gearing.

I next measured the low end – what the controller will run the motor at if you tell it to do so at the lowest possible speed. What I noticed right away is that there is a speed-dependent cut-out, presumably to prevent saw damage in the event of the chain getting stuck. Let’s find out what this is:

Cut-out speed of the controller, under load, 66 eHz or 500 RPM

Often, these motor controllers will hold the motor spinning down to a lower speed than the minimum speed they will start up at, termed the cut-out speed. For this controller, it was 66 eHz, or roughly 500 mechanical RPM.

By the way, I tested all these speeds by grabbing the motor while it was spinning, wearing leather welding gloves. At one point, the glove started smoking when I did a haul all the way out to about 65 amps. Do not do this. Ever.

If you do, do not grab the pointy fan bit.

Cut-in speed (minimum speed) of the controller, approximately 350 eHz or 3000 RPM

However, the minimum cut-in speed of the motor was about 3000 rpm. This implies the following:

  • The motor must be able to start up with relatively small load and get to this 3000 rpm speed quickly
  • Once there, it will happily be loaded to any degree provided the speed does not fall below 500 rpm.
  • I wasn’t able to accurate characterize the startup torque available during the startup routine, because that required too many hands.

This may ruin the usefulness of this setup for go-karts and related (e.g. no foot-starting available), but should be still useful for scooters (push starts with feet is part of the game). I try to tell people to model controllers/motors with low speed lockout similar to sensorless motors: the motor shouldn’t be able to tell that you are there, and this is accomplished with sufficiently high gearing.

With a 300 RPM/V motor, you need plenty of it anyway!

I was curious about what the T2 pin connection on the saw’s connector led to. Scoping it, I saw something which was basically an analog voltage that varied with the current being drawn. Zooming in close enough to the scope showed the PWM frequency, so I’m pretty sure this is just a straight tap from the T2 terminal of the battery pack, possibly corroborating both tool and BMS having access to the current sensing shunt in the battery.

I couldn’t tell what the sensitivity was, given that I was out of hands. It seemed nonlinear: 4 to 6 amps occupied a 1-volt division, but so did ~20 to ~35 amps (did I say something about the glove smoking… I was dumping 900 watts into myself here). So there may be something else at play.

Next, let’s find out the resistance line-to-line of the motor. This is the determinator of how much the motor can pull, current-wise. Using the 4-wire method, I got that it was about 29 milliohms. Solid – this is on par with an R/C outrunner of similar size.

(The 4-wire method is basically: Two terminals whose ONLY JOB is powering the device under measurement, two terminals whose ONLY JOB is to measure something. This isolates the effects of parasitic resistance in the power wires from the sensing wires.)

By the way, in 2.00gokart, every student characterizes a motor for Kv and R in the fashion just presented.

One last thing I wanted to find out is if the controller was stupid flexible enough to run without the battery and its sensing circuitry. Hey, the absence of the battery current sense line would mean it’s drawing 0 amps all the time so it shouldn’t ever turn off, right!?

The answer was yes, it will run off just any old power supply. No hackery needed. Supply 38 volts, turn motor.

It will in fact run down to 20 volts.


So what do we have here?

The Ryobi 40V cordless chain saw is:

  • A 300 rpm/V, 14 pole, 63xx class, 29 milliohm brushless motor with a 12mm flatted shaft and beefy aluminum mount
  • A controller with Hall sensors, a free plunger throttle (add your own lever/knob), and maybe an irritating speed-dependent cutout that will cause loads slower than 500 rpm to not work.
  • A battery and charger with internal battery management circuit that can dump at least 60+ amps for some time.

Gee, that sounds like a go-kart or scooter to me. Thanks, Ryobi!

I invite anyone else who is curious to pick one of these up and maybe try running it in something. I’d say you would be hard pressed to find matched components of this power level for less than $200, battery included. The downside is that a spare battery is $100+ by itself, but as can be seen, the controller will happily use any ol’ battery. I’m curious to see how the speed-dependent cutout affects real world operation, since I couldn’t get a good handle on how much available torque it has between 0 and 3000 RPM, the controller’s “minimum speed”.

As technology marches on, so the spoils of yesterday’s cutting edge shall become available to the homebuilder/hacker. Hopefully this series will contain some more interesting tools and equipment in the future!


Chibi-Mikuvan The Extended Universe Edition: Water Cooling Loop and Digital Dash

Jun 21, 2014 in Chibi-mikuvan

It must be awkward observing your own viscera from the perspective of your own husk.

Now that the summer season here in the shop has settled down and people are well on their way to wrecking my laser cutter, I’m advancing on the checkoff list before the Detroit Maker Faire and its associated PPPRS race. I’m like, signed up noncommittally and everything. Through a web form. That’s some serious confidence right there.

water cooling

Last time, I mentioned the water cooling circuit and electronic dashboard. Recall that the motor I ended up investigating and deciding on is a water-cooled boat motor, and I’ve been running it without any water cooling. This has been somewhat tolerable – it is actually quite efficient, and even during the hard garage runs, I was able to do 2 or 3 back to back and not feel that the motor was nearing burnout temperature. However, all of this is was done in mild weather, at night. I’m sure that if the daytime temperatures were much higher, it would be a greater risk to run the motor that hard. Bottom line is, why buy a water cooled motor and not water-cool it!? I could have sprung for its longer air-cooled cousin if I didn’t want to mess with that.

The water cooling circuit I had imagined was going to be quite simple. This wasn’t going to have to dispose of a kilowatt or more. I ran some rough numbers and expected that at 100 amps continuous (all day, which will never happen), the motor is going to only dissipate about 150 watts. A modern workstation PC’s graphics card along can shit more than that at any point in time. The plan was to base this system off a computer liquid cooling rig. I scouted eBay for used water cooling kits for a little while before Mike handed me a Powermac G5 pump and radiator, first revealed a few posts back.

So that was taken care of! The only missing from this kit was a reservoir. Now, I could have been crafty and just made a whole closed loop using the pump and radiator alone, but a reservoir would be helpful for topping off the system and allowing expansion space (I have zero cred here – real-Mikuvan’s coolant expansion tank is the hose looped back onto itself and pointed away from the battery so any spurting pressurized coolant goes straight down…)

So that’s how I went to the grocery store, bought a jar of pickled olives, and ate the whole thing. That was a very uneasy evening.

I made some “panel mount” barb fittings for 6mm ID Tygon tubing and attached them to the can lid using my favorite adhesive/sealant, Goop. It’s rubberized, so it’ll be both watertight and somewhat flexible. The other end of these fittings end in a non-barbed tube that I extend down into the jar as a pickup and……dropoff? tube.

It took a while to come up with a component placement, since the only left over space was immediately behind the seat and in front of the electronics case, which is not a very good airflow spot. So I did what I could for the location. The big board it all sits on is 6mm hardboard. I purchased several (dozen) sheets from a local wood products distributor as prototyping material for 2.00gokart and the shop in general, so plenty was left over. I’m just going to use this hardboard as-is. It’s not very afraid of water (unlike MDF), and is quite rigid.

The jar mounting bracket was produced on an Up Mini, one of the handful of hobby-class 3d printers accessible by students in my shop. I designed it while shuffling component placement in my head – I knew this bracket would be needed, just didn’t know exactly where yet.

Installing the tubing was quite straightfoward. Near the motor, the 6mm tubing steps down via a custom 3d printed adapter to 4mm tubing to attach to the motor. Hello, flow restriction. Designing these adapters was a fun excursion into the world of fluid systems design. They were made using the lab Objet (high resolution resin printing) machine.

McMaster wanted to charge me $7 each for these damn things, so I figured having a $33000 state of the art rapid prototyping machine knock one out …uhh, I guess I really have no case here. I think these fittings will still work if made with a hobby FDM printer, but there would likely be sealant involved to fill the filament low spots.

Testing on a power supply, I found out that the pump wanted 0.4 amps at 12 volts by itself. Yikes – the little hacked 5V BEC units I was using to produce 12V (I have hogsheads of these) can realistically only sustain about 1 amp when jacked to their new output voltage. I was already needing 1 amp for the contactor.

Solution: Just whip up another one in like 5 minutes. AUX runs the pump, LOGIC is dedicated to the Arduino, contactor, and sensors. I have 0.5 amps left to add gaudy lighting.

Here’s the completed water cooling loop! After a little leaking, all the O-rings seem to have found their seats and I haven’t noticed any more drops on the ground.

I ran this thing around the shop “track” (meaning the conveniently rectangular closed loop building at night when everyone’s gone) for half an hour, doing 55 lap or so (I may have lost count). I ran the battery straight down, and the motor could best be described as “lukewarm”. Maybe warm water for hand and face washing temperature. This was with no additional airflow to the radiator besides what leaked behind the seat through that cutout in the mounting plate. Without power data to back it up, I can’t really draw firm scientific conclusions, but yes.

As the weather gets hotter here, I’ll most likely set up the 2.00gokart test track one weekend and just try and keep running as long as possible. I think the addition of this system will help the reliability greatly.

the digital dash of science

One thing that has been bugging me about Chibi-Mikuvan is indeed the utter lack of science on it. Because of my use of the 150A Anderson connectors, I don’t have any instrumentation that can jack in the middle between the battery and motor controller. I mean, besides making yet another why-did-you-even-bother-adapter.

I conceived the idea of rolling my own wattmeter, or perhaps Anal Cyclist Cycle Analyst, after finding the datasheet for the current sensors that are integrated inside the Ford Fusion hybrid battery, the LEM DHAB S/34 series. It was easy enough to read, as an isolated Hall Effect loop sensor, and I could put it any where. It’s also easy to rig a voltage divider to sense the battery voltage. With those two pieces of information, I was basically satisfied (I could go totally insane and add way too many features and datalogging to boot, but…)

First off, removing the ridiculous connector shell on the current sensor and attaching a pigtail of wire to it. Protip: The cheapest source of 4 conductor shielded high quality cable is old USB cables. Almost all of the signal cabling on this thing is made of hacked up computer cables!  8 conductors? Ethernet. 9 conductors? DB-9… nobody uses those any more. USB has 4 conductors and an outer shield that could be another.

Next, I swung by Microcenter and got a Sparkfun serial LCD kit. Imagine my surprise when I found out it was not one of their integrated one-board sLCDs, but a cleverly packaged Arduino. Well hell, I could have made that with the onboard Arduino. Oh well.  I was looking for a sLCD in particular because I wanted the simpler interface, and not actually having to resort to using all 8 pins of an Ethernet cable to run the otherwise parallel signals.

I designed this mounting bracket in about half an hour, to be slowly worked on by the Up Mini, while I wrote LCD code. For some reason, I’ve been all about those flexural snap fits lately.

Here it is being produced. I’m basically an Up salesman, so I’ll take this opportunity again to plug these machines. They just work. Extremely well – they’re still the only machines I’ve dealt with which produce truly breakaway support lattices. Even an old Dimension BST machine.

I have a personal Up Plus, and the shop has an Up Mini.

Getting this LCD up and running was quite easy. I basically followed the Sparkfun datasheet and instructions. I couldn’t get it to change baud rates, however, despite trying that command a few times. So for now, it’ll just talk slowly. (I typically run my Arduino-related serial things at 57.6Kbaud).

I used the remainder of the USB cord to run the LCD from the handlebars to the electronics box. This wiring job is almost approaching unserviceable – I’m strongly considering a full refactor into something more accessible and serviceable prior to the Detroit Maker Faire, now that I know what connections need to be made and what could be done better. For instance, I have a few 12V, 10A DC/DC converter modules that could replace the two smaller hacked ones and run all the gaudy lighting I want. 

On the breadboard are some of the signal interface circuitry for the current sensors and voltage sense. Just simple R’s and C’s, they form low pass filter networks to help smooth the sensor readings. I’m not sampling thousands of times a second here…

With two cables now snaking up to the handlebars, I used some left over wire-twizzlers (proper name: spiral wire or cable loom) to constrain them. It looks almost professional!

Here’s the end result!

I spent way too much time in the code measuring the length of the digits to keep them at the correct offsets such that the units’ letters (V, A, W) didn’t move. The digits have enough space in front to add a negative sign for regenerative braking. Volts have 2 digits, Amps has 3, and Watts has 4. The LCD refreshes 5 times a second, which makes it a bit flickery, but I still can catch glimpses of the levels that the measured physical variables hit.

I’d say this setup is definitely more dashboard than instrumentation. Instrumentation would imply I investigated and know the level of precision attainable with the parts and the potential error and uncertainty in each one of those indicated measurements. Which, no. This was calibrated with a Harbor Freight voltmeter. There is literally nothing between it and just flat out guessing.

Here is the current version of the Chibi-Mikuvan running code. I’m posting it primarily because of the LCD interface code – the rest of it is a big fancy servo tester knob (I have fathoms of those).

I’ve been thinking of a potential next step with this code, now that I have the ability to read the battery current directly. The idea is to use something similar to RageBridge’s current limiting scheme in order to follow the time-current curve of the PRS series fuse. The fuses specified, MIDI style fuses from Littelfuse, are slow-blow ones which give you a moment of surge current capability, so I’m thinking about a way to try and push that envelope with current limiting – no matter how hard you punch the throttle, it will stay (ideally, just barely) within the transient limit of the fuse.

Something I have to think about now, though, is how to make a “Chibi-Mikuvan adapter” to hitch it to real-Mikuvan, though if we have enough go-karts coming from here, it’s easier to just borrow or rent a trailer.

Chibi-Mikuvan: More Stickers is More Power, Right?

Jun 04, 2014 in Chibi-mikuvan


The semesterly action has died down to the extent that I no longer know what to do with myself. This is the gap of three-odd weeks between the end of the spring semester and the beginning of the summer action. As usual, the IDC is hosting mobs of Singaporean students as well as MIT students involved in The Collaboration. Like last year, I’m about to indoctrinate a fair percentage of their current freshman class with the Church of Chibikart. The specific implementations of this summer’s Silly Go-Kart Church Camp will be discussed soon. There are omniwheels.

During this break, I am of course going to be working my various problem children who need my attention. Most of this will involve facilities upgrades and improvements of the current robot or silly go-kart fleet.

  • Changing Überclocker over to easily-swappable Banebots wheels – I talked about this near the end of the Motorama event report. The 40A durometer “McMasterbots” wheels are very great for traction, but they wear out quickly and I have to individually machine each wheel to fit my hubs. Hypothetically, once switched to fast-swap hex hubs, I can replace an entire wheel set in 5 minutes flat.
  • Finishing Chibi-Mikuvan’s liquid cooling loop and creating the telemetry system / electronic dashboard.
  • Maybe even working on repairing the 1lb and 3lb bots (yes, including Colsonbot) for the Bot Blast event this coming July!
  • Maybe maybe some more real-van bodywork.

Truth be told, I’ve actually been spending most of the past week, and likely much of the coming weeks, designing something which I think is gonna be pretty damn interesting. This is a far out enough concept that I want to flesh it out concretely before taking it public, so you’ll have to deal with this artificial cliffhanger for a bit longer.

In between, Chibi-Mikuvan has been accruing more stickers. It is a well known principle in auto racing that stickers make you faster!

Well, perhaps a different type of sticker. They most likely didn’t mean….

Miku stickers!

It’s amazing the huge range of quality you are presented with when you search for “miku sticker”. I gradually homed in on one sticker/decal house with an eBay store, based in Malaysia, that had the highest count and variety of Miku stickers. The characteristic I was seeking the most, though, was huge. Loads of stores had the tiny notebook sticker or cell phone decal, but only this place had them in 33″ tall, and in high quality with the transfer layer.

Granted, the shell isn’t that tall, so some parts of Miku will necessarily be lost. I decided this was fine, though, as most itasha decor seems to focus on making the most recognizable parts of the character as large as possible (I need LED rims). I think for this version I’ll spare the fully custom paint job – it’ll be stickers on white to start with.

What followed was days of arduous experimental Miku-placement, researching and answering the toughest of unresolved issues in this field such as “Is it tacky to have more than one Miku per side?” (The above example would indicate no, it isn’t).

Dry erase marker and masking tape was the layout method of choice. I scribbled roughly where body lines were going to go in order to come upon a satisfactory placement.

This one, at least, was fairly obvious. It was of the exact correct height and aspect ratio to take a ride in the rear window impression.

And on the left side.

I was conflicted on whether to use only the two largest ones, or also include the others. The two in question are of the same art style and by the same artist (KEI), giving a more cohesive look, but of course had poor coverage. The two art styles shown here definitely clash a little.

After placement was finalized, I rough-trimmed the stickers to shape, applied them, then very carefully precision-trimmed the bottoms with a craft knife. In all, the results were quite good.

I actually ordered another pack of stickers – this time, with more logos and banners (the single lone Hatsune Miku logo wasn’t enough!). They should arrive soon.

The right side, for now, will lay a little more bare. Maybe all the new ones will go on this side!

I moved onto the issue of car number. The two numbers traditionally associated with Miku – “01″ and “39″, are both taken already! Sadness. Well, I’m certainly not going to try and claim the 01 number from the team until I beat them.

So at least for now, let’s roll with “3939″.

The door decals were printed on sticker vinyl by my good buddy Ted, who knows graphic things and stuff and whatever. I tried printing them, but my poor CMYK only printer can’t handle the BRIGHT NEON MAN-GENTA coloration of Miku’s graphic logo (the 初音ミク bit) – it kept coming out orange-red. Apparently, professional printers just have a BRIGHT NEON MAN-GENTA color channel. The lettering itself was made using Inkscape.

I carefully knifed the number out by hand (and ruler) – no cut templates were made. I supposed I could have fed this into the vinyl cutter if I had planned this better. In ambient room lighting, it looks tacky because the background stands out, but in bright outdoor light, it’s not as noticeable.

replacing the Trackstar

Little known fact: For one reason or another, after the previous garaging video, the Trackstar 200A controller died. It didn’t die in flames – rather, one phase simply stopped driving. So in fact I could ‘skateboard’ Chibi-Mikuvan up to 5-6mph, hop in, and it would actually still keep on driving. However, with a dead phase, it couldn’t start or run at low speeds. Inertia! It works!

Having one of the student teams this past spring suffer a Trackstar failure in almost exactly the same fashion, I began to suspect that the up and down jostling and high frequency impacts of being used in a vehicle was causing the signal board to come loose inside. As documented in One Thousand and One Hobbyking Amps, the signal board on these things is held on by 1 small header, and allegedly retained by rubber foam tape. However, when I pulled that unit apart, the board came off really easily, with not much compression of the foam tape visible.

It’s my hunch that impacts shake the signal board header loose just enough to cause one bank of FETs to possibly become disconnected from the driver chips, and since the power board doesn’t appear to have any gate protection hardware (pulldown resistors and Zener diodes), that phase probably died instantly from cross-conduction or dV/dt damage to the gates.

I decided that the power board was not worth saving for a few reasons. First, it’s a massively heavy (6 ounce or heavier) board with copper conductor bars soldered to it – rework would have been borderline impossible. Second, it’s potted for waterproofing, as far as I can tell. The FETs are sandwiched right next to the conductor bars.

Hence, I gave in and just ordered a replacement. Damn you, Hobbyking!

This time, the first thing I did was to crack it open. Shown on the right is the number of tools I needed to use to do this. I have no clue what the hell these screws are supposed to be, but some prefer a 2mm hex, some a T9 (!?) Torx, and two of them Phillips.

Next, I took out my favorite contact cement & robot wheel retread compound & waterproofing coating & what have you. E6000, and its related adhesive Goop, are some of my favorite adhesives. In this case, I’m using it far more as a structural filler. I put 3 big globs of it on the near two corners and far edge of the board.

I’m basically resigning this controller to be never repaired, but that’s fine.

After replacing the controller, I joined in another MITERS silly vehicle proving session in the garage. This time, I couldn’t get the dashcam to stick to the helmet any more, so had to rely exclusively on random people filming, which always ends so well. The art and science of holding a camera steady is one that is only developed through a habit of documentation – most people can’t do it. This is all the useful video I was able to get that wasn’t jittery, out of focus, or cut too short.

That sound at 0:16 which is oddly reminiscent of a gear falling out of a gearbox is…. yeah. Under heavy motor braking, once again the pinion got yanked off its taper by the action of the helical bevel gear. It proceeded to screw itself back in once I hit the throttle really hard. I need to remake that entire input shaft at this point, since it’s doubtlessly been gouged up and damaged by the slipping gear. I’m also going to reduce the maximum motor braking force back down to 25% – I left it at 50% for this test – for actual racing, since it seems to be a point of reliability trouble.

This run, I beat my previous time (57 seconds) by 2 seconds since I carried more speed through the end turns. I’ve figured out how to get this thing to not understeer near top speed – the answer is lean all the way forward. I believe this puts me within 2 seconds of the all-time garage record by Tinykart.

Left to do on Chibi-Mikuvan is making the water cooling circuit, for which I have all the parts for already. The next thing to do after that is to make spare everythings: battery packs and entire motor-gearbox assemblies are first. This way, if I have a motor or gearbox failure in battle during the race, it’s easy to swap and get going again. Eventually, I want to set up the 2.00gokart course again and run one full simulated race scenario – as many laps as possible on one battery, a battery change, and a tire change.

I’ve signed up for the Detroit Maker Faire race, which is a feasible one to get to, so shit will get real soon!