Archive for June, 2012

 

Return of the Kitmotter

Jun 30, 2012 in Kitmotter!, Project Build Reports

Kitmotter, the concept that never quite was.

Originally, I built Kitmotter 0001 to be used on a little display stand in order to show the concept of laminated (coarsely-layered, anyway) rotors for potential hub motor or custom BLDC motor applications. The concept is a direct knock of the laminated hub motors of B.W.D Scooter which was the first to explore the idea of using waterjet-cut steel rings with magnet “indents” allowing for easy placement and waterjet-cut plastic (or metal) endcaps – basically allowing a brushless motor to be constructed without any intensive machining work. At least, it gets rid of the need for a large machine to turn the steel rotors that my designs all feature – I can make a whole post about all the random workarounds that we’ve tossed around for building whole motors. The same idea has been used on a few vehicles besides B.W.D – the Pneu Scooter and its close design relative picofahrrad, and a non-motor application, just to name some examples.

Kitmotter-on-a-stick lasted for several demo/exhibition type events until I took it to Singapore…


Poor Kitmotter.

The two little necks of 1/4″ acrylic just did not like a kilogram of motor hanging off it as it was bounced around my luggage. And with that, the base was retired and Kitmotter was relegated to a fairly simple life of “static item I would occasionally grab of the table to show people”.

That is until I dropped it one day.

With its main source of attention payments (from curious freshmen and ambitious motor builders) gone, Kitmotter was forced to live in the slums of my multistorey handcart of parts and stuff for many months (picture from before it suffered its unfortunate luggage incident)


Kitmotter could never afford to live in one of those new parts drawers.

As my cleanups and project purges were happening, so the slums were being cleared, demolished to make way for new expensive high-rise plastic sorty-bin developments. The first tenants of the new development were the fasteners, who pretty much lived on a level above the rest of the parts beforehand anyway (literally). Kitmotter was temporarily forced to stay with friends on another shelf in MITERS. The future seemed bleak for Kitmotter – forgotten, broken, and tossed away, an embodiment of ideas whose time had come to pass.

Until today, when I found 3 sheets of almost pristine 6mm acrylic in the laser cutter scraps pile at the Media Lab. Almost, meaning some UROP most likely took 1 part out of one of the corners then left the rest of the material hidden in the pile hoping nobody would find it because he’s too damned lazy to drag it back upstairs. I swear I haven’t done this before during my undergrad adventures, people.

Well, no name means no claim, so I quickly whipped up a new display case design while a job for the lab was processing:


Yes, white on white. I know.

This box gets rid of the mounting ‘stick’ as well as removing a very obvious pinch point in the original design. Instead the motor is first mounted to a reinforcing cross, then screwed to the case. There’s more acrylic to crack… not that it won’t be any harder, since acrylic. The case is also shorter than the first

Here’s the parts of the new box after cutting. I picked some scrap dark green acrylic to make the spacer rings from this time. The “KITMOTTER” is actually vector-etched into the top plate on a setting just fast enough to break the white plastic coating. Normally all of this is peeled off, but the separated letters remained white, making for a good contrast.

But before any exterior remodeling, I first went in and fixed one thing about Kitmotter that has been wrong since the beginning: THE SENSORS ARE IN THE WRONG SLOTS. Actually, even worse – they have been in the wrong slots for every motor I’ve built which has internal sensors.

I have previously put the Hall sensors into the slot between two teeth of the same phase i e.g. between A and a, or b and B. The rationale being that when a magnet transition (edge between N and S poles) happens, that set of teeth will “pull” the magnets above it into direct alignment.

I’m fairly certain this belief was just carried over from 2007-2008 when i was first learning How Moter, and then never validated or refuted. It took several very brain-twisting discussions and whiteboard sessions before I finally saw the fundamental error in judgement. I still can’t quite explain it in diagrams or short technical sentences – this post by Amy might be the closest thing. Bottom line is, the concept is true (magnets being pulled into alignment) but the magnitude of the movement required is 60 electrical degrees, and it is only possible if the Hall sensors are placed between two teeth of DIFFERENT phases (different “letters” in the conventional notation). It is still possible to find a ‘combination’ of sensor and phase connections which resulted in cyclic commutation, but the timing would always be too far advanced by 30 degrees (or too far retarded). Either way, not good, and it explained why 1. Kitmotter always sounded like a moped engine, and 2. why Tinytroller had issues with running it because it had no ability to compensate for sensor timing.

The fix involved just shifting the sensors over one slot. Because Kitmotter was never built to be actually used in a vehicle, I just heat-gunned the hot-glue-retained sensors and squished them down over another slot. I placed them between the ab, BC, and Ca teeth this time, like I was supposed to.

With its new green trim rings (for green energy and transportation!!!)*, I closed Kitmotter back up again, with fresh and unbent bolts too.

*shoots self

The new case completed. The large hole on the top plate is designed to clear the motor wires, and the motor itself is mounted only to the cross.

Replacing the motor controller was a straightforward deal since the wiring remained the same – another example of my project “case mods” this summer, I suppose. The controller is a sensored Jasontroller – this one I actually bought from Jason himself in Singapore last January. These are, unlike the eBay controllers, sensored-only.

I can already tell that the sensors are actually correct this time – Kitmotter no longer runs shittily in both directions! It’s much smoother, and the current draw is lower at 36 volts. Previously I was getting 4-5 amps of current draw, which I attributed to the bearings (huge and greased) dragging it down…all, you know, 160W of it. The no-load current has now decreased to only 1.3 amps at 36 volts, which makes way more sense. The minimum loaded motor speed before it starts ‘bouncing’ due to the timing error is also eliminated – switching too soon would cause the motor to jump back and forth as it doesn’t quite have enough torque to overcome the load before the phases switch again.

With this new discovery, I now trust sensored commutation a little more again. And all this time I thought it was just sensored being terrible.

Here’s a short video of the new home of Kitmotter 0001!

But wait, that’s not all.

son of kitmotter

Everyone wants their offspring to have a better life than they do, and Kitmotter is no different.

To fit a hub motor in a wheel, the wheel must not have a center. One of the issues that caused the project to stall out initially was the lack of a “ring tire” or definitive way of turning a stock wheel into one. We bounced all kinds of ideas around, such as wrapping urethane tread around the steel can like B.W.D (later attempted by Jedboard), but decided it was very troublesome and difficult to reproduce. The compromise idea seems to be Pneu Scooter’s “sidemotor” arrangement where the hub motor isn’t concentric exactly with the wheel, but offset from it in order to use its existing bearings.

It’s difficult to explain how some times solutions to problems seem to pop up with no warning in your head. I had originally thought of using a hole saw to clear out the center of a wheel a long time ago, but I quickly scrubbed the idea because how the hell are you going to keep it centered without a mill?

What I had forgotten back then was that hole saws generally have pilot drills in the center. I’m used to seeing hole saws being used in strange milling machine fixtures to make “fishmouth” joints for future welded tube frames. In this case, the pilot drill is not used.

It was during a conversation with Jamison about his latest sidemotor build that the thought of using a reduced-shank pilot to use the existing wheel bearings of a caster wheel as the centering mechanism suddenly dawned upon me. Now that I have hindsight, duh, it was so obvious. It doesn’t even need to be a drill bit – there’s nothing to drill. It literally can be a pin that is 1/4″ in diameter on one end, since most hole saw arbors take 1/4″ pilot drills.

Buying a 1/4″ reduced-shank drill on Mcmaster-Carr, though, was the quickest solution.

So here’s everything. Most of the skate and scooter wheels that I deal with have an 8mm bearing. 5/16″ is literally a hair smaller than 8mm – 0.3125 vs. 0.3149 (for normal hairs measuring about 0.0025″ diameter). It would be terrible if it were the other way around. You can reasonably use a 5/16″ rod as an 8mm axle – which is exactly the intention here. The drill bit would not have a very exciting existence during this operation, since it would just be spinning inside a bearing.

The inside “corable” diameter of the 125mm skate wheels I use (“YAK” type 12-spoke wheels) is about 3.25″ or 82.5mm. This is conveniently a size for which they make hole saws.

I replaced the pilot drill with the 5/16″ reduce-to-1/4″-shank drill bit. The alignment seems to be spot on here. Because the pilot drill needs to go past the wheel’s bottom, I elevated the whole thing on a piece of scrap wood. I clamped the edges of the wheel through to the workbench to stabilize it – a similar procedure will probably need to be done for drill press jobs. I’m actually not sure why I went for the hand drill this time – probably due to torque concerns from the ~1HP DeWalt drill versus a wimpy 1/3HP drill press motor, but the alignment and precise feed control could have made alot of difference.

Alot of positive feedback induced jamming later (note to self: drill press.), and  it worked!

After vacuuming out the swarf, the result is quite splendid indeed. The interior finish is clearly not as refined and clean-shaven as a lathe boring job, but who cares?

Basically, my assessment of this process is that one of the last missing pieces of a fully-accessible Kitmotter has been realized. I’m now really kicking myself for not having thought of this earlier. The major problems with Kitmotter were the lack of consistent stators (an issue that is solved by consistent copiers and laser printers), stator-to-bore adapter solutions (solved by 3d printable nylon hubs you can get from Shapeways or other 3DRP vendors), and… tires.

The same type of 12-spoke YAK wheels also comes in 100/110mm size which I’ve confirmed to be borable out to about 2.375 (2 3/8″, or 60mm). Guess what – they make hole saws for that size too. This can get exciting.

The hole saws actually appear to cut a little oversize. There’s two contributions to this (rather massive) overcut. First, the saws themselves are a little bigger than nominal dimension, and second, I could not hold this thing straight at all manually. It seems like a drill press is pretty much mandatory. Each time I twisted the saw, it would take a bigger bite out of one side. It tended to jam on the thin plastic spokes (a finer tooth saw would also mitigate this), so some times the twisting was fairly severe.

However, taking into account a straight cut, the diameter is probably going to be still 3.26″ to 3.27″ anyway – which is pretty much 83mm even. Sloppily using an Inch tool to make a metric dimension….

So, what’s the next step? I need to reduce Kitmotter 0001′s 3.5″ outer diameter down another 1/4″. This is much more difficult than it sounds, because the magnets need to come down in thickness (probably to 1/8″) and the can must get substantially thinner radially, and I could run into trouble with the case fastening screws. The screws will most likely need to be moved ‘external’ to the steel ring, sitting in circular grooves.

Alright, enough of this “future talk”, here is Kitmotter 0002, coming soon to a…nother demo stand? near you.

So what’s going on here?

  • The 7 #4 bolts have been turned into 14 #2 threaded studs. This diameter change was absolutely necessary to thin the can down down to 3.25″ – there was no other way.
  • The radial grooves seat the screws and secure the layered can, while the endcaps have fully enclosed holes to keep the studs on-dimension
  • The magnet thickness is 1/8″ instead of 1/4″, again to bring down the diameter. I’ll definitely lose a little torque from this.
  • The bigger endcap has a flange that is supposed to be used as a guide to drill into the wheel, in order to retain it. I might add more holes for more strength since the threads will be in soft gooey scooter plastic.
  • The axle is a stock 5/8″ keyed shaft of a 3″ stub length that McMaster sells directly. The hub to the stator is 3d printed. All other trimmings are left up to the user.

I’m going to build a prototype of Kitmotter 0002 in the next 2 weeks to validate this model, and I consider it right now to be prime Instructables fodder if it ends up working out. Essentially the last missing link in an “accessible” hub motor vehicle has been solved – where accessible means hypothetically buildable without access to heavy machinery like lathes and mills.

All Good (And Poorly-Maintained) Things Must Come To An End: The Great Project Purge of 2012

Jun 26, 2012 in Chuckranoplan, Cold Arbor, Nuclear Kitten 5, Project Build Reports, RazEr rEVolution, Test Bot 4.5 SP1

i swear to god i will fix this later

At some point, I need to stop telling myself that. It’s well known that my stuff isn’t exactly world-class in terms of reliability and Six Sigma class in quality, but even I can get sick enough of it to declare it a loss and start over. Over the past few months (and years) of neglect, quite a few of the robots and silly vehicles have become damaged and non-operational. I kept Swearing That I’ll Fix It Soon, Guys, but my shelf of stuff is long past overflowing with parts and project detritus and some of them contain good parts that I don’t want to keep buying. With my general shift of operations towards the newly opened IDC space just up the Z-axis from MITERS, tearing down some of the old derelicts and returning their parts to the Earth (/my storage bins) became more appealing – especially as I started collecting more stuff, most of it landing on my fresh new corner desk.

So it is with great sadness (and hidden catharsis) that I must announce the decomissioning of…

Cold Arbor

Cold Arbor never really worked – the frame was too flexible to accommodate the huge teeth of the saw. After Motorama 2010 and Dragon*Con’s Robot Battles ’10, CA pretty much only ventured off my shelf for the occasional demo – it illustrated, visually, what a “combat robot” was very well. Pretty much everyone’s first reaction at the word “Battlebot” is “You should put a saw on it to cut through the other robot!”, and CA is…. well, pretty much a saw. It never really stopped driving, but then the saw actuator broke so it couldn’t do the extending thing any more. Arbor, being the biggest lead weight I had on my shelf, was therefore the first to go.

But before I tore it totally down, I decided to use it still-functional and very smooth drive base as a test dummy.

Last year in the Austrailian robot fighting circle (did you know that Australia has a very active robot combat scene too?), one of the builders began to modify Hobbyking brushless controllers to act as H-bridges for DC drive motors, utilizing 2 of the 3 half-bridges available on the average BLDC controller. I’ve been advocating something like this for a while – use the cheap hardware base that is Chinese brushless motor controllers instead of custom-developing an expensive niche robot controller solution. The choices in robot controllers these days are either said niche and expensive but generally reliable controllers, or these one-tiny-FET-per-leg overfeatured doodads that I’ve literally had zero success rate with. Or you straight build your own and have them work, but I’ve also not successfully managed that yet. There’s nothing on the market right now which is just a bucket of large FETs like the old Victor 883s (which you can still buy, but they’re now a design so old it can almost drive).

That aside, I have also never bothered to schematic-trace the brushless ESC boards or learn & put up with enough raw Atmel C to reflash the microcontrollers (though I suppose I could have flashed Arduino onto them…). So, a ton of hot air rage on my end, but lots of action in the 40+page thread over on the Robowars forum, which has seen all of the cheap common ESCs reverse engineered and firmware implemented for – up to and including its own confusing beepy configuration menu.

They’ve now started selling them (when I say ‘they’, I really mean like one dude), and I took the chance to get some modified “85A” units based off this Hobbyking ESC.

First, I had to remove most of Arbor’s existing electronics. Okay, so my success rate with the Sabertooth controllers isn’t zero – Arbor runs two of the closely related SyRen controllers, but $75 for 25 amps is stupid these days, and I’m also royally undersizing their loads – one is running a little Speed 400 class motor and the other is running a drill type 550 motor which sees about a 10% duty cycle on raising and lowering the saw.

Way cleaner wiring and layout with the ESCheap85 in – I could easily see a robot with a whole rack of these next to eachother. The massive spam of SMT FETs technique used to great success by cheap Chinese controllers is an acceptable compromise, in  my opinion, between one-tiny-SMT-FET per leg used by the Sabertooth and Roboclaw and other most-likely-designed-by-newly-graduated-college-students controllers, and the one-huge-nice-FET approach I usually take. It keeps the board size down, too.

After hooking this up, Arbor was taken on several somewhat strenuous (and absurd) test drives.

None of it was very scientific, nor was there really enough space to seriously stress the bot out. I’m going to have to use these in battle myself before I’m fully sold on the idea, but based on the reports of the substantial number of Australian users, they’re pretty bulletproof, and a few American users have already run 18v DeWalt drills in drivetrains using them (the same motors that Clocker uses). The 85A type has been praised as a “Victor replacement”, but its more limited voltage range (30V fets and 35v capacitors) doesn’t quite convince me it can be swapped directly into a native 24v (up to 28v fully charged and more during dynamic braking) system. I fully agree with the concept, though, and for about $1 per amp I don’t have any complaints past my own reservations.

That doesn’t mean I’m no longer going to attempt my own controllers – I have yet to successfully execute a small current-controlled vehicle H-bridge, of which robot controller is a simpler subset. But that’s for another post.

At the end of it all, here’s Arbor mid-scrapping:

Scrapping is such a negative word. It took me a while to crack open that weapon drive gearbox, since I sealed it up so well at the start – and some of the bolts were bent.

Here’s everything I ended up keeping from Arbor. All of the motors, pretty much all of the drive mechanics (especially those delicious custom gearboxes, which were one of my first good ones), and of course the saw and worm drive in case I rebuild it all. The VictorHVs and Sabertooth controllers were also kept and filed in my robot controllers bin.

prospect for rebuilding: slim

Arbor was a very complicated robot with lots of moving parts – it’s something which is more difficult to get right, and it’s usually more disappointing (to watch as well as to operate) when it doesn’t work. Arbor’s build was rather rushed and many details weren’t completely thought out. I’m more likely to build a 30lb bot that is either more plainly functional or spend alot more time to build a complex but well-designed and tested robot before trying to compete with it.

Going down the line, next I pulled out…

nuclear kitten 5

NK5 was heavily damaged last Robot Battles, and ever since then has been sitting on the shelf. However, the disc motor still works great – and I can make spare discs, so that’s definitely being reused on something. The controllers and motors were also potential salvage items.

NK5 was the last robot I built before I converted fully over to “T-nut” style construction, visible in pretty much all my stuff from 2009 onwards. The design actually dates from late 2008 – my first major t-nutted endeavor was the ill-fated 2.007 robot. The frame has these wonderful corner bars that I machined for this application, but it seems like now you can buy everywhere. I really liked these, so I went ahead and saved them. Tapping into real metal is way better than t-nuts at any rate.

Here’s NK’s remnants pile. The frame materials were just not worth keeping, but I kept the motors – the gearboxes are not stripped, but one of the pinions fell off (but is intact). They might become donor parts for future gearboxes. I am a fan of these little 25mm metal gearboxen: while they are not planetary, they’re big and chunky inside to make up for it, and fairly cheap at $10-15 each.

prospect for rebuilding: hell yeah

I can’t guarantee when, but D*C 2012 is likely because I pretty much have everything-minus-frame. The disc is up for some revision, though. Big tall vertical disc spinners are no longer in vogue, being replaced by small, low bricky drum things with built-in motors (of which there are now like 50).

Next up is my pride and joy,

test bot 4.5 MCE

Really? The bot that made it to real-deal-Battlebots-IQ, then Motorama 2008 and back? The first thing I ever worked on at MITERS? Yep, since its default parking spot since Moto 2008 has been in Clocker’s lifter when it’s not doing other things.

TB certainly has the most grime of any of the bots, and the lifter was pretty much utterly trashed – it took a direct from the vertical disc bot Igoo at Motorama 2008 (that video is slightly painful to watch).

This is one of my first drill motor hacks. I did a few in 2006 for the original TB version 4, but they were either terrible or dismantled very quickly. This thing predates my entire website, pretty much. The extension shaft with the pinion was added when I redesigned the lifter for Moto 2008. It had an additional outboard support, but since it was made of UHMW, the whole gearbox still flexed too much to keep the gears in mesh, and so the pinion stripped very quickly in battle.

After I took the damaged arm parts off, I realized that TB’s drive base was actually in very good mechanical shape. I still love those gearboxes, too: they are super special 12:1 drill box hacks that I made with mating the salvaged 18 tooth planet gear and 9 tooth pinion gear of the first stage of a drill motor with an intact output stage. Coupled with the extremely overvolted 9.6v drill motors, this made the bot have a rather zippy top speed of 14mph. The first version of this gearbox predates the website (again) – this version at least had the luck of being milled, so things actually lined up!

I briefly entertained throwing the BotBitz ESCs in the frame just to drive it around again, but decided against it for the time being. It’s sure been a long time since I’ve had a 4WD drill-powered box.

So I closed it right back up again. Only the damaged arm and wedge parts were scrapped – otherwise, I think I can put something interesting in this bot again, or at least give it a better sendoff at a serious combat event later on, as the most honorable fate for a combat bot is still, in my opinion, being thoroughly vaporized into a cloud of small particles.

prospect for rebuilding: not for Robot Battles

TB4′s design was optimized for “arena” combat which has more guaranteed smooth floors and a more pressing need for huge, thick angled armor. The RB stage is purposefully left fallow to discourage pure wedges – a passive aggressive attempt at encouraging more robot creativity, which I contend has been successful in the past few years even though it kind of locks me out from competing in 12lbers again there with this design. Maybe Motorama 2013….

Finally, a project that I hate to see get tossed so early, but…

razer revolution

It’s lived a decadent life of being a demo attention whore as well as occasionally coming in handy when Melonscooter was on blocks, and has seen 4 different motor controllers (Double DEC’er, Melontroller, Tinytroller, and Jasontroller!), but recently RazEr Rev has become kind of a wreck.

I donated the front end to another MITERS scooter effort after the new battery got 2 dead cells after only a few weeks – definitely a case of bad initial conditions. Since then, it’s been sort of chilling in a corner, slowly being eroded away by the tides of cruft and dead power supplies that ebbs and floes around the shop.

The Jasontroller works great, the battery can be surgically corrected (I’m literally going to scalpel/X-acto knife the dead cells out and make it into a 10S pack), and the Dual Non-Interleaved Razermotor is a little rattly in the bearings but otherwise functional.

So that’s pretty much all I kept. Oh, and the extra heavy duty generation 2 Razor handlebar, after they moved away from welded-to-frame folding joint but before cost cutting made the joint like 24 gauge steel. This front hinge is massive – the steel is something like 0.13″ thick.

The reason I decided to scrap RREV now is because I’ve become dissatisfied with the frame design. It uses a design which I now consider inferior to other similar scooters in the way it’s put together. Starting over with the frame will be a great way to optimize the design towards less material use (like giant plates of 1/4″ aluminum) and make it simpler to assemble in addition to making dedicated space for the battery and Jasontroller, both of which were “aftermarket” additions. It should end up lighter for the same performance, but I don’t see it getting any smaller. Sorry Jamo, but Razor Wind is a little on the small side for my tastes now.

All this talk of what I’m gonna do means the

prospect for rebuilding: immediate

I already ordered some more giant aluminum plates (…sigh) and will probably be redesigning the frame this week. I’ve already got the changes planned out – they’ll just need execution. Like NK5, it will just be a matter of moving old parts over to a new chassis – there’s otherwise not much about RREV that I’m unhappy about. It’s definitely going to get a stock fender.

other stuff

I didn’t take any pictures, but all the Chuckranoplans have been parted out and recycled too. I’m probably not going to be touching this for a while until I stop being afraid of foam so I can build meaningful scale models. 3D printer models were fun for design practice, but are too heavy to work.

Alright, now that I’ve eaten half my offspring, I can start considering rebirthing them again!

Return to the Copters: Global Flying Things Update

Jun 19, 2012 in Ballcopter, Chibicopter, Project Build Reports, Tinycopter

Whoa, this site still exists.

I’ve been primarily working on a Silly Media Lab Vehicle project for the past 2 weeks or so, in the spirit of me having done so during all 4 years of my undergraduate career, which is why I haven’t been posting anything. It was actually kind of refreshing to work on a silly vehicle for someone else again. Since it’s not really my project to publicly expose all the fun innards, I’ll refrain from doing so for now. It’s not that fancy, however.

Anyways, with that project squared away for now, I’m gonna take a bit of time off… by returning to my rag-tag fleet of flying things. Whatever happened to Chibicopter anyway? The last update on that was like… mid-April. So it’s on first:

Chibicopter

Chibicopter was the project I settled on to complete for the Media Lab’s DIY Manufacturing class. Admittedly, as I usually tend to do, I treated the project far more as a personal project than a means to take the class seriously, and this was reflected in the reviews I received at the end of the class.  I ended up half-assing or straight up skipping several of the assignments at the beginning because I was far more interested in seeing it work. If anything, having to do it for class made me take it less seriously – it’s an interesting psychological effect that I see in many project based classes at MIT, where once your own stake is reduced in the project, you begin turning away from it or needing to push yourself through it.

Seeing several of my undergrad peers push themselves through the same project classes this past year that I went through (namely 6.115 and 6.131) with high aspirations at the beginning of the final project push made me realize one of those everything-in-moderation things: that if you try to do an epic project for a class, you probably will end up getting sick of it at the end, because the agenda is no longer fully yours to keep. My 6.115 and 6.131 projects were rather tame in scope by comparison, and my most “epic” class project, Segfault, was actually 90% done already by the time the class started, and I pretty much only built the analog PI controller.

But that’s besides the point. The real point is that Chibicopter is gonna need alot of rethinking before it will actually fly.

Here’s essentially what it ended up as:

Previously, I had appended a FTDI header for easier (okay, possible) programming – never really having explore the wireless bootloading any further. I’m also unhappy about going with XBee control now, because it limits the command input interface to something which can talk to an XBee – which i took care of with XBYPASS this time around, but that’s unrealistic for a product or even for my own amusement in the future since it involves two $20 pieces of hardware (XBees are expensive!). And then, in the middle of term, Hobbyking, as always, came out with a solution to my problems:

Well then.

This thing seems to work fairly well – it acts as a WiFi access point, so you literally connect to its network and transmit packets with an IP socket. It already had its data format decyphered, and another enterprising MITERS member therefore wrote an iPad app which used the iPad’s internal accelerometer to control a quadrotor using tilt alone (the stock Hobbyking provided app using virtual touchscreen joysticks which were kind of annoying to use). So really if I were to revamp Chibicopter (for another product design class?) I would just lob one of these things on it.

Feeding control inputs to it was never really the hard part. I had feared that the system dynamics of Chibicopter, being so tiny, will be faster than what I could stably control with only 50Hz command refresh rates (the servo pulse repetition rate). As a result, I “overclocked” the Arduino servo library to 100hz, the maximum speed that it can do with 4 servos.

The reason it’s limited is because the servo library starts each pulse sequentially – one has to finish before the other starts. Other approaches such as the KK Multicopter controller (which Hobbyking has a version of and which most of the MITERcopters use) start all the pulses at once and end them according to desired pulse length. This is how they achieve up to 495 Hz control – a 2000us pulse has a frequency of 500Hz, and a very small dead time is used between pulses. It’s a more complex approach and needs two timers on the ATMega chips, so you pretty much only have to be making a flight controller for it to make sense – maybe not a general-use Servo library.

I think 100hz is fast enough for Chibicopter, but the rest of the problem was mechanical. The arms are really floppy – Shapeways’ “White, Strong, and Flexible” sintered nylon is really all three of them. The props are also not very balanced, and because they are so small, were hard to balance. As a result, Chibicopter tended to resonate strongly, which was most likely overwhelming my IMU.

Oh, did I mention I soldered the IMU directly to the board, which is mounted directly to the frame? From past copter experience, I really should have seen this coming as very bad news from the start. Even worse was that it was soldered at one end so it was really its own stiff pendulum – if there is one thing your inertial measurement unit should not have, is its very own not-very-inertial dynamics.

Basically, what it comes down to is needing to totally redesign the thing for easier communication, more stiffness, and higher bandwidth. The final “test video” has already been out for a while – I got it to a condition which I was satisfied with for now in time for the end of term:

As can be seen, it’s pretty much fundamentally unstable at the moment, tending towards divergent oscillations. But it’s very cute while doing so.

I might not rebuild Chibicopter immediately, but when I do so again, I think it will move towards an all-PCB construction like the other very small copters that exist now. I’ve purchased a different style of motor which has real mounting holes (not like the mounting sticks of the 2 gram HXT motors).

tinycopter

Poor tinycopter.

Tinycopter has actually been remarkably reliable, though in various states of disrepair, since its last update in February. It doesn’t have much test video past that unfortunate first video because I would usually just fly it around without thinking about it. It’s been to several demo events since then, and is very stable and easy to fly.

One of the things I wanted to change about the design was the fact that the entire board was sitting on a giant block of memory foam. This seemed to be a great idea until the foam started disintegrating where I had glued it to the frame. I had to compensate by adding more CA glue, so eventually the foam became a stiff block in places. Usually you’d put just the IMU on foam or other shock-mounting substance. Another undesirable trait of the foam block was that if I crashed for any reason, the whole thing might shift angularly because one bundle of wires was pressing on a corner more, or something, the end result being that Tinycopter never really flies the same way twice. It was a bit unpredictable and the trim angles changed constantly.

Near the end of term, it was also starting to fall apart – the glue joint on the crossed Lincoln-log carbon fiber rod frame was coming apart, I had broken off one of the standoff landing legs, and one of the motors was temperamental. So I decided to rebuild the frame this past week and roll up all of the changes I wanted to make.

I decided to construct the frame using 3D printed joists for carbon fiber tubes (heeeeeeeeeey, that reminds me of something). The center cross piece holds the long tube and two short ones in an X shape, and allows me to clamp tightly on them with bolts. The outer ‘landing legs’ are intended to bolt through the motors’ mounting flange and use it as a giant meta-washer.

Here’s the frame fitted together, without any other hardware yet. I’m hoping this build isn’t going to be heavier than the current one – while the 3DP plastic adds a bit of weight, there are many other places on the current iteration which can afford to lose some. I ordered smaller (6A) controllers which will save a gram or two each, and alot of the big servo cable wiring will disappear, as will the chunk of dense foam.

Beginning the decommissioning of the old frame…

I decided to go for a more integrated wiring approach this time. Underneath the board is a ring of wiring that distributes battery voltage to the four controllers, with the control electronics in the middle. The IMU is now on a little block of foam (which has been made into a “foam flexure” through selective cutting, not really visible in the picture). It will be wired using tiny 30ga wirewrapping wire to further isolate it from vibrations.

Signal side wiring complete and board installed… See that there’s only one connector for each controller?

That’s why. I put the power and R/C signal wires next to eachother on a header so I can keep the wire lengths short and have one thing to plug in. The ESCs are of a much better form factor this time, and mount cleanly on the sides of the frame, secured with a zip tie through the board mounting standoff slots. The motor wires exit at the place they are needed.

I like this arrangement alot – pretty much only full integration of the ESCs onto the board is better for wiring cleanliness, but if I do that, then Tinycopter becomes a 5pcb.

And it’s back! Now with landing lights!

Besides accidentally wiring the motors up sideways (rotationally confusing which motor was which), I had to do relatively little tuning to get it flying again, since the hardware is pretty much the same. The gains were turned up some, since these ESCs appear to exhibit much better linearity than the previous ones.

Now to remember to take more video before I blow it up again – I’ve already succeeded in busting off all 4 landing leg things at least once each (but don’t worry, they are both gluable and easily remakeable!)

ballcopter?

I’m getting an urge to try this thing again. The previous attempts ended in dismal failurenearly a year ago (what actually happened at the end of that post was it not working at all and then biting Shane’s finger). These things are probably being mass produced by Sony now, or something, and have been built many times by other model hobbyists. But I still want to try my hand at it since I haven’t been able to produce a working one yet.

Since last year, I’ve figured out that my control approach was incorrect. I was trying to control the angle of tilt of the thing using the upper flaps. Really angle is controlled by the lower flaps and the upper set is used for translation. All flaps are used for rotation. It came to me that this was the proper method after I watched one of Ryan’s Giant 3D Foamies do a… I don’t know what you call it, but statically hover point straight up, like an airplane burnout – something planes should not be doing, but anyway.


Excuse the…uhhh…. bloodstain?

The ballcopter is exactly a small plane, flying straight up hard enough to offset its own weight, in a little hamster ball. Like how planes pitch up and down with the elevators on the tail, so ballcopters tilt and roll using their lower flaps. A ballcopter flying horizontally should reduce to the case of two orthogonal little planes.

I still have like 20 square feet of foamcore, so I might just go for trying the old design again with my New and Improved Control Solution. I’ve been recently more favoring carbon fiber hoops for a frame with 3D printed joists

and one more thing

No, that’s not actually a Cinestar 8.

D. P. R. Chibikart Garage Hoonage

Jun 04, 2012 in Chibikart, D.P.R. Chibikart, Project Build Reports

Over the weekend, I took Chibikart (and a few tagalongs) to the Ol’ Silley Vehicule Proving Grounds and took a few metered runs up:

It was actually slower than Chibikart1 by a fair margin, hitting only a 72 second best time, compared to Chibikart 1′s best of 62 seconds. On the whole, though, it was more efficient, consuming only 11Wh of battery during that run. The best product score was 784.8 Wh*s.

Neither result – that it’s slower but more efficient overall – is surprising. First, we already know that hub motors are less efficient than indirect drive systems – they have to pull more current, generally, to perform the same amount of work and being would for high torque also necessarily increases the motor resistance (for the same form factor).

But DPRC is slower because the Turnigy 5065 motors have a much lower torque produced per amp even after accounting for the 2.5:1 geardown between them and the wheels. From my adventure building the new motors, I know their torque constant Kt to be roughly 0.12 Nm/A. For the Turnigy motor, at 236 RPM/V, that translates to a Kt of 0.04 [1] – multiply by the 2.5:1 torque increase of the chain drive that that comes right out to 0.1 Nm/A.

This different alone isn’t enough – Chibikart 1 has four motors, for a grand total (lumped parameter) of 0.48 Nm/A, whereas DPRC only has 2 motors for a total of 0.2 Nm/A. Given that the 350W Jasontroller is safely limited to 25A output in all cases, DPRC can only produce half of the acceleration of Chibikart 1. But most of the garage race is spent flooring it, or at near-constant velocity, so only significant speed changes will contribute to the time. Hence why the discrepancy isn’t, say, 50% slower or something.

I have a feeling that Chibikart 1 on 2 motors will get a much worse result than DPRC – it’s only a ~16% time gain (7/6ths) for half of the available torque!

 

D. P. R. Chibikart: The Everything Update + Instructables

Jun 01, 2012 in D.P.R. Chibikart, Project Build Reports

Kind of like Kim Jong-Il, nobody really knows when the Democratic People’s Republic of Chibikart was born, or where. I think it was some time on  Wednesday, actually, but I haven’t put up an update on it here because I’ve been plowing through the Instructable that I promised pretty much every day since then. This thing is a book. It’s 46 Instructables steps long, but each one has on average 5 or 6 “substeps” because otherwise I was facing the prospect of a 200+ step Instructable. But more on that later.

First, a recap of… what essentially is Wed. night, I suppose. (The prior update which include some of the frame construction is here)

First off is the electronics deck. Compared to Chibikart, wiring this thing was a breeze. I gave the Jasontrollers a well-deserved haircut since I was not using any of their auxiliary functions, and that instantly made the wiring like 10 times cleaner.

DPRC features a real terminal block which serves all the power distribution and signal connections, so the plan itself was more open to begin with.

I do like this arrangement of parts – it’s fairly clean, and the plate is entirely under the seat so you don’t really see any of it from a standing position. The plate can be pushed forward if I ever want to switch to the bigger 500W class Jasontroller, but in my mind this is not really worthwhile.

If those Jasontrollers look a little familiar, almost like I used them for something else before, that’s because…

Poor Chibikart.

Well, my next batch of Jasontrollers didn’t come on Wednesday, and last time I pledged that

If they don’t come by Wednesday, I might actually knock two Jasontrollers off Chibikart for now just to get it over with. Because I want to ride it. Badly.

-me, a few days ago

Chibikart still runs fine on 2 motors, though! In fact, for a while, it totally did. The acceleration is a little less glamorous.

Here’s the electronics deck installed in DPRC with wire extend-o-splices already made. The e-deck drops onto the frame from the bottom – the whole frame is turned over, the e-deck bolted on, and then it’s turned back over to finish wiring the switch and other parts. The seat is off this whole time. It was alot more elegant than trying to jiggle all the components while the seat was already covering them, like I had to do for Chibikart1.

And here is the completed Pretty Shot!

This build is way cleaner, and also much lighter. We weighed Chibikart versus DPRC, and Chibikart actually weighs almost exactly 50 pounds. This was over my estimates, but Chibikart also has an unnecessarily huge battery and much heavier motors.

DPRC weighed in at only 36 pounds, with everything on it. I swear Melonscooter is about that heavy…

Compared to Chibikart 1. They’re the exact same outer dimensions and exact same height otherwise. Seeing Chibikart1 weigh so much really makes me want to downgrade the A123 module to some A123 12V7 bricks or the equivalent K2 bricks. I do have a few more A123 bricks left over, so perhaps it’s time for an Alphanumeric Battery Company (ABC) shootout with Chibikarts!

Instructable

As promised in the original mission statement, I’ve finished the Instructable document. In it, you will find the above build progress pics and MANY MANY MANY more. The “Instructable pictures” folder in my DPRC build pics folder has 298 items in it. I don’t even have a word count, but I am positive it is well over 9000. I’ve entered it into the “Make It Real” contest, because the line of 3d printers in the IDC shop needs expanding – either the Objet or the PP3dP Up would be a fun addition to the lineup in the minishop.

hoonage video

This is probably the part that people actually care about – the test video! We made use of the convenient closed loop found in the building architecture yet again. I used Chibikart1 to film a few people driving around the… uhhh…. course.

Additionally, the first spinup video is here – it was linked in the Instructable too.

Merry Chibiiing.