Has it really been 2 whole months since I shoved LBS under a bench and sort of forgot about it? That warrants a break in my project timeline!

Since then, it’s been slowly migrated all over MITERS as people make space to work on things. My box of hardware and components has collected a fine coat of grinding dust, other people’s projects, and assorted unwanted parts. More than a few tours of the place in the mean time have been given with the parting promise that it will be done “Soon”.

Well, soon is ideally in the next two weeks. I’m picking the completion effort back up again since I want to be tooling around on it come June 3rd.  Control issues aside, the most important unfinished detail is the replacement of the melons (which was the underpinning of its internal reporting name, Melontank) with Plain Ol’ DC motors. The new drive motors are CIM motors, the same kind used in both Segfault and something like a quarter million FIRST robots. So does this mean I have to call it “CIMtank” now?

The CIM motors are much higher speed motors, so I needed a “preduction” to get the speed of the vehicle back down to the range it was in with brushless power. As detailed last time, I was going to do this using a shady little e-bike planetary gearbox and some crafty arrangement of rotating shafts.

This shaft-mounted speed reduction solution has been affectionately named “CIMulink”, after everyone’s favorite MATLAB simulation toolbox. The aluminum sprocket adapter bolts onto the (rotating) planetary carrier, and it spins on bearings which just ride directly on the motor shaft.

Like so. These are the solid part geometries turned from some 2.25″ aluminum stock. I milled a D flat onto the CIM motor shafts so they directly engage the Currie gearbox input. The output “bump” sits in the adapter’s bearing indentation, though it’s the motor shaft itself which contributes most of the alignment in the system.

I removed the sprockets from the Turnigy motors and bored them out to 7/8″ to fit the adapters. These sprockets were some kind of horrible sintered steel that machined like garbage, and would spark if I fed too hard. They also finished poorly and also smelled really bad. What, I can’t even get real steel in my power transmission components any more?!

Anyways, the 7/8″ boreout  left too little “thread” in the hub to tighten a set screw, so the adapter had a blind hole drilled at the set screw location for the screw to seat in. It functions more as a pin in this capacity.  The flange holes in the aluminum adapter were finished using my indexing fixture on the mill.

With some 10-32 low-head cap screws, the adapters were bolted to the Currie gearboxes. The gearbox didn’t have a bolt circle in it originally – the four holes through which it was riveted for structure were drilled out to a depth of 1/4″, then tapped with a 10-32 bottoming tap.

This is what the CIMulink looks like mounted to the mot…

Wait, there isn’t a mount there…

That’s because during all this time, Make-a-bot was faithfully printing out the motor mounting bracket seen in the initial CAD image. Each one of those took about 1.5 hours…. during which I probably could have just straight up machined it, but I’m both lazy and didn’t have stock of the right size to start with.

Oh, there it is.

The mount is a total of 12mm thick and is printed from white ABS plastic. It’s 90% filled, so it will be more than strong enough for the application. Four 8-32 bolts retain the Currieboxen to the mount, and the ears are through-bolted to the frame.  I couldn’t find 3 inch long bolts to connect the new (thicker overall) motor module, so I had to use 3.5″ long bolts for now. They stick out quite a ways, so I’ll either cut them down or maybe just turn them around later.

I’m also going to change the tensioner arrangement – right now, the chain slings under the white tensioner sprocket. I’m finding that I can’t expand the tensioner diameter any further without it interfering with the teeth of the treads, and the chain is still a bit loose. Moving the chain to over that standoff decreases the tension roller diameter needed, but I do lose a tooth or two of sprocket contact. With sufficient added tension, though, I think this should be fine.

With the drivetrain swap completed, LBS looks…about the same it has been for the past four months or so. Well, it’s already 5 months late, so why do I care?!

The track pods draw about 8 to 10 amps no-load per side at 24 volts on a power supply test.

At this point, I could actually just drop the control rig from Überclocker into it or something and be done. However, that’s simple, realistic, and stands a chance of working, so we can’t have that.

In the interest of eventually pursuing the dual-glove “fingerless” control using XBee radios, I’ve elected to use a 2.007 Arduino Carrier board, a wonderful robot motherboard-like device created specifically for the class this year. It even comes with an XBee socket already. But for the next week and a half, which is less time than I can foresee me designing and ordering boards and parts for the wrist controller, the interim solution for control will be just using the Arduino Carrier to interpret throttle and steering signals from a shady 2.4ghz hand-held radio.

I have, of course, prepared a custom motor control solution for it too.

This is a little logicless power amplifier board similar to the Segtroller boards I made for Segfault. The difference is that it isn’t locked-antiphase, just has two independent PWM inputs and a master disable. Gate drive voltage is derived directly from the power rails by a single 15v regulator. So basically, it’s a Small Cute Full Bridge.

Distinct from all the other random motor control modules I seem to make, though, is the fact that it has an ACS714 Hall current sensor in line with the power inputs so it can sense DC bus current. I’m going to try and make LBS current controlled so it doesn’t jerk around. Current control directly dictates the torque a motor can produce, so it would be like setting a maximum acceleration. A current sense output pin is broken out on the header row so I can feed it back into the Arduino board.

These boards are currently out for fabrication, so they should arrive by the end of next week. That’s a little too close, though, so who knows – maybe I will just pitch a robot controller in it!

No, not to someone who wants to mass produce it for the world (for better or worse), or to mass media (yet), but to brushed DC motors, just like I said I wouldn’t do last time. I should consider a career in politics or something. Anyways, out of practicality reasons and an attempt to not to try and address 5 design flaws at once, a change from my usual build tactic, I’ve elected to switch LBS to a POD (Plain Old DC) system. DC motors don’t have blank sensor states, and controlling them doesn’t involve a state machine (or nested loops with rotor position estimators and current sensors and matrix transforms). In the space available for drive motors I won’t be able to mount anything as powerful as the rewound 80/85s, but judging by how well I was able to stay on the thing during the preliminary trial, I don’t really have a problem with this. My main goal is to get the fingerless-control designed and built so I can test if it’s worthy of further investigation.

By the way, here’s an interesting build from a few years ago that accomplishes everything LBS was supposed to do, and in a better form factor. See, I should have just done this straight away.

The concept is pretty close of the Boolean union of Snow Scooter and LBS.

Oh, also, here’s a sneak peek of Chuckranoplan 0003:

No more Delta wing?! I’ve been eyeing different wing planforms since I got this book (which is awesome but clearly shows the imperfections of being a first print edition). The authors break down the cladistics of GEVs more finely than I do, separating the field into five categories based from my understanding on operating altitude and the ability to statically hover. Of particular interest to me was everything that wasn’t the Lippisch delta wing. The real reason I don’t like them as much despite their clear advantages in ground effect is that I currently have no clue how to build a tapered wing For Real. Unless, perhaps, I build a garage-sized 3D printer, which of course is not out of the question.

And they look derpy.

I’ve been looking into some of the Chinese GEV designs recently, since they have all featured square wings with small vessel-length-to-wing-chord ratios (the wings are very long compared to the vehicle length) and aspect ratios (almost square or even rectangular the wrong way). A square wing is much more in my comfort zone of things I could conceivably build without getting too deep into specialized materials, tools, and techniques. This is important, since a 20 foot Megachuckranoplan is still lurking in the back of my mind.  One example is the Tianyi 1. This vessel (and its forerunners) are the subject of some pretty intensive analysis in the book. They represent a middle ground between the more ship and hovercraft-like types which can only skim over calm water (such as the Aquaglide) and the Legit Ekranoplans of yore which could operate at higher altitudes…. and by higher I mean like 10 feet.

To prevent this from turning into a Chuckranoplan post, I’ll leave it at that. This model will still be 3D printed hollow to the best of my ability, but the square airfoils mean that I could easily make a more conventional model aircraft wing out of some laser-cut balsa wood. Weigh estimates put the above at 220 grams dead empty, and only if MaB gets it right.

oh yeah, melontank.

This thing here is going to save my day.

What IS that?

It’s a weird little integrated 4:1 planetary gearset from some kind of Currie electric bicycle. Around 2 years ago, they started showing up on the surplus channel, but it looks like they’ve been around before that, just more expensive. The construction is fairly “discount EV” standard – sintered steel gears, cast aluminum carriers, and rivets. What’s cool about them is that they have inbuilt support rollers coaxial with the planet gears that turn the whole thing into a really big and shitty roller bearing, so they can take some moment load. Presumably in the bicycle, there was no other bearing for the chain sprocket.

I have a little bit of history with these, as they were first discovered and used for the Greenwheel project. But more recently, they’ve been ingeniously used to great effect on the ExkateCD project as a “preduction” stage to replace an existing drive motor with a faster one but retaining roughly the same output speed after the belt drive.

Hey, sounds like something I need to do. The motor they used was a stock CIM motor, the same kind used on FIRST robots, but the motor I’m going to use is a…

…

… Okay, it’s a CIM motor. Whatever. Stop judging me.

Incidentally, it’s the same motor that powers Segfault. They’re made in the tens of thousands each year for FIRST robots, are rated for a solid half horsepower each (which really means you can punch 2 or 3 into them for a very short amount of time), and aren’t too obnoxiously sized. Oh, yeah, they’re cheap, about $30 new, which makes the price to performance comparison very good for a stock janky DC ferrite magnet brush motor.

So the plan for LBS is to down-convert the 5000 RPM @ 12v by 4 first, using the Shady CurrieBoxen, and then run the stock 5:1 chain drive that’s in place now. Essentially a direct swap-in of the two brushless outrunners that are in there now. The combined 20:1 reduction might actually be too much, so I also have the option of going to 4:1 chain drive with the alternate sprocket set I have. Doing this would mean that I’d have to drop the 36v power system for LBS and go to at most 24v, which will be plenty anyway.

This will be accomplished using the unique mounting configuration for the Currieboxen that ExkateCD explored. The ring gear is bolted to the motor mount, the output is taken through the carrier plate, but the output completely floats on the motor shaft. The CIM motor shaft is just the right length to stick out past the gearbox enough to put a ball bearing on. The output coupler can ride on this bearing while being attached to the carrier plate at the same time, which is firmly held in place axially by the built-in rollers. The assembly is made stiffer than the stock (very loose) output because of the addition of the coupler-side bearing. And of course the motor mount has the same bolt pattern as the 80/85 motors.

Here’s how the assembly goes together. The ring gear attaches to the motor mount through the perimeter bolt holes, which appear to clear #8 machine screws.

And the assembly from the back, showing the coupler with 8mm ball bearings.

Here’s the assembly dropped into the existing motor mount on LBS for a fit test. The new motor setup is longer, extending all the way to the other side of the track pod, but it’s smaller in diameter. Overall, it’s not a painful conversion. I just have to sit on my ass now and wait for the motors, supplementary Currieboxen, and random hardware to ship from all over the place.

In the mean time, my focus will be on implementing the fingers-free controller. The spring demo season (where I spam exhibitions, fairs, and events of all kinds with everything I’ve built ever) is under way, so I also need to get some of the other projects running again. The poor RazErBlades, for instance, are out one bettery pack (and the replacement I bought doesn’t fit because fuck you Hobbyking for changing the dimensions of products without updating the website and picture!).  Ideally, Melontank will be ready….

by Commencement.

[dun DUN DUUUUNNNNNN]