Archive for July, 2012

 

RazEr REV2: Mostly there

Jul 12, 2012 in Project Build Reports, RazEr REV2

Over the past few days I’ve been mostly hanging out at the Georgia Tech Invention Studio. I was nominally there as “guest lecturer”, but I don’t quite think their own 2.00EV is organized yet to the point where I can feel comfortable with that title. All the ‘students’ are actually lab instructors (similar to our MITERS keyholders), so there wasn’t that much ‘teaching’ to do. I did hold some impromptu lecture-like things and generally advised people on their builds where needed (and fixed the waterjet?). Regardless, some… interesting products are coming out of it:


It’s literally twice as long as some of the other scooters.

I’m back now, and one of the first things on the agenda is getting the half-assembled repackage of RazEr up and running. I sort of left this in the middle of construction when I zipped off to Atlanta for the weekend. I had the frame ‘box’ assembled to test fits, but I pretty much had to take it apart again to actually install stuff. My direction was essentially assemble the major subassemblies first (make the fender, reinstall the motor, attach the front end) and then lob the electroncis back in as-is, since it worked fine before.

Here’s the fender in place with its leaf spring installed.The ‘sheet metal work’ was done on a vise, then fitted in place using just tightening screw pressure. 5052 aluminum bends very easily, especially in 1/16″ thickness, so I was literally just leaning on the part to get the bends I wanted. To do the large radius sweep at the top, I bent little by little in ‘facets’ which weren’t drastic enough to be seen as disrete (though you can kind of see it).

Now that I’m a little more comfortable with making sheet metal geometries compatible with other 3d solid parts, I might incorporate it into more builds in the future.

The fender is just mounted on a chunk of 1/4″ threaded rod. Nothing fancy at all this time – no spacers, even. The pressure of the leaf spring alone is enough to keep it in place reasonably.

More progress has been made on frame assembly, with the folding joint  reattached now. I traded the former front end for a new A3 type front that was part of the leftovers from my 2.00EV. It’s substantially less beat to shit and doesn’t wobble as much, and I swear it’s a little taller than the one I had before.

I had a left over new fork from building Straight RazEr (whose wreckage has since been donated to Kramniklabs) which I dug out for this build.

I forgot to take a picture of what’s going on with the 5″ colson wheel, but there is actually a type 1614 bearing bored into each stock Delrin bushing. The Colson comes with a 5/8″ bore bushing  that has a 30mm OD (which presses in to the 30mm bore of the wheel itself). I tried to find a > 30mm bearing that wasn’t of a ridiculous axle diameter so I could bore it into the wheel directly, but gave up and went the other direction instead.

A ‘stock check’ of bearings I had turned up some R8 type and 1614 type bearings. Both were 1 1/8″ outer diameter, which I could bore into the Delrin bushing, but I settled on the 1614 bearing since I easily located a stock 3/8″ bolt to serve as the axle pin.

The job itself was done on tinylathe, which is probably one of the handiest tools I’ve ever worked with.

Moving on to the electronics deck now, I put together the ‘switch panel’ which holds the charge and controller ports as well as the annoyingly bright blue LED endowed power switch. The idea is to have BAT and PWR jumped externally with a Deans ‘patch cable’ so I could jack in a flow-through measuremen device like a Wattmeter if needed. Else, the switch is to serve as the primary turn-on mechanism.

It’s better than the yank-the-battery-connector setup RazEr Rev has used since forever, but I’m wondering how long until this switch falls victim to no-precharge arcing damage like the very first switch arrangement.

The interesting part is on the back. Instead of connecting the switch’s built-in LED to ground directly, I threw a 100 ohm resistor on it. This should prevent the light from exploding right away, as it happens when you try running 12v rated switches on 36 volts… Otherwise, there are just a few select wire jumps which bridge the two Deans ports through the power switch. Note the back-to-back soldered Deans connectors on the right…

With the switch panel done, it’s time to load all the electronics back in. The same shell-less Jasontroller appears, bolted to the aluminum frame directly for some heat sinking. There’s a bit more space for batteries this time, since they can reach all the way under the folding joint, but unfortunately it isn’t enough to actually add more cells – just maybe some padding. If I wanted more battery energy about the only good option is moving to prismatic cells.

With everything wired back to the way it was, I shoved the 3d-printed front endcap on. This was one of those pieces made on the Lab Replicator™.

And the repackaged shot. Unfortunately I gave away my only other black Colson wheel, so it’s gray for now. When I get another one (or get it back), the bearings are transferrable.

This frame rides significantly lower than RazEr Rev – too low, actually. This is likely due to a difference in head tube length between the A3 I got like 5 years ago (for the original RazEr!) and now. RazEr Rev rode slightly ‘nose up’, but this one is definitely nose down. The clearance at the front is about 3/4″, decreasing to less than 1/2″ when I’m actually riding it and the rubber block is compressed.

Not going to work. I’ll compensate by making the two wheel fork sides a little longer. In the mean time, it is rideable, and handles just like it used to except with more stopping. And less exposed wires – check out the 3d printed wire guide at the lower right.

 

RazEr Repackaged?

Jul 08, 2012 in Project Build Reports, RazEr REV2

I mentioned last time during the Great Project Purge that RazEr rEVolution was due for a rebuild very soon. I don’t actually plan on calling it RazEr Repackaged, but that’s pretty much what’s going on here. Like the rebuild of Kitmotter, it’s intended as a literal repackaging of parts I already have – a case mod, I suppose. The goals of this rebuild would be to update the frame to a new construction style that I’m favoring more, as well as to clean up some other design loose ends like adding a brake (mixed-up priorities, anyone?) and building in support for the Jasontroller.

 

The new frame is of roughly the same dimension as RazEr Rev(1?), but it is no longer made of 1/4″ plate for the sidewalls with exposed T-nuts. Instead, the whole structure is 1/8″ aluminum now. Not only does it save weight by reducing unnecessary material use in the side walls, but it opens up the interior volume a little more. The “cavity” for controller and battery is also about half an inch longer. The 1/8″ plates will be attached together with corner blocks similar to those I used in NK.

Additionally, there’s no more structural vs. nonstructural top plate. The black Garolite deck of Revolution is gone in favor of a single top plate and the silvery metal look (changeable with selective application of grip tape or paint).

 

The first subassembly I put details in is the thing that RazEr Rev never had: a fender brake. By that, I mean it neither had a rear fender (until I appended one crudely) nor a mechanical brake. This being the revision that I hope to address shortcomings, it’s going to get a brake.

I finally spent some time to figure out the way that Inventor processes sheet metal geometries so I could make properly mating sheet metal parts. The side of the sheet metal that you make features on really matters, as does the role of sketched bend lines (start-of-bend, centroid of bend, etc.). Yes, I’ve used Inventor for like 7 years without really touching sheet metal features in depth.

Not shown in the above image (but in the one below) is the spring for the fender – it uses a simple bending plate of spring steel instead of a torsion spring due to the limited space under the fender.

There’s other trimmings to be added too. Instead of a Deans shaped hole in the side plate, I’ve just opened up a big rectangle and will be using 3d-printed electrical panels. Right now, the configuration is for two Deans and a switch. One connector is a battery connection and the other goes to the controller – this way I can easily jack in a Wattmeter or similar. Should I decide to change wiring arrangements, the electrical panel is reprintable.

 

The motor wiring will be hidden behind a 3d printed cover. While not Apple-like, this at least cleans up the exterior wiring of the vehicle substantially. I’ve been entertaining the idea of a “kit-class” scooter based off RREV for a while, so maybe this rework will move towards that a little more.

The front fork remains the same from the old frame, since it is a solid design. Here’s the front posed roughly where it should be. One of these days, I sweeeeaaaar I’ll model up a handlebar from a Razor scooter.

 

I cut the frame out of 1/8″ 5052 aluminum. One of the main reasons for moving to all 1/8″ on the frame was the fact that I could get the sheets for much less – they tend to show up more on the surplus channel for one reason or another, 5052 even more so. 5052 is about 2/3rds as strong as 6061-T6, but the vertical height of the material is still more than sufficient to carry the loads I need. 1/4″ 6061 just didn’t make sense any more in the side plates.

This time around, the attachment for the front folding joint is done through a “clip” which makes a material-to-material interface. This opens up space underneath the folding joint which would normally be taken up by a giant nutplate, but this time I can scoot the batteries forward under it. The method is decidedly less stiff, so a “backup plate” of another 1/8″ thickness is also clamped underneath.

1/8″ is too narrow to hold T-nuts directly, so I’m using some 1/4″ to make corner blocks.  I probably didn’t need to use this many either, but it was an easy linear pattern to make. The backup plate is seen at the lower right.

Another Classy Thing I’m putting on this version is a 3d-printed “endcap”, similar to the ones found on stock Razor scooters. For this version, I just used the theoretical outline of the corner blocks and internal plates, which means it doesn’t actually fit if the frame is fully tightened and assembled since these dimensions are compressed a little. It’s not supposed to be waterproof; just a splash guard.

Once the frame is done, I should be able to throw it all back together in a day.

 

 

TinyStar, the 2-Day Dual-Flight-Controller Scale-Model Octorotor

Jul 03, 2012 in Project Build Reports, TinyStar

That might be the most specific descriptors I’ve ever had to heap onto one build. It’s also kind of hard to explain how it got started… which I guess is true for some other stuff I do too. But as a reminder, here’s that mysterious picture that I posted a while back when I was working on Tinycopter:

Why does it have eight propellers!? That’s like, two quadrotors at the same time! And why is it so pointy?

First off, it’s a reasonably accurate scale model of a Cinestar 8. This was 100% intentional – not only am I out to one-up the quadrotor…uhhh, arms race? at MIT, but my buddy Shane Colton is way better at getting free stuff than me and he has a Cinestar 6 frame. But it is used for custom controller testing, so it has a legitimate purpose despite being free (an exercise of the classic industry strategy of “give academia free stuff so they can do weird things to it”). The Cinestars are probably some of the most badass-looking multirotor frames around – totally murdered-out and made of carbon fiber and black GFR nylon, and they kick ass. So much. Also, nice cameras make everything look more epic.

Second, the cool thing about Tinystar is that it uses two independent multirotor controllers (Hobbkying i86, which may be based on KKMulticopter’s firmware, but it’s just different enough that I’m not confident on it) feeding from the same receiver. It is quite literally a Siamese quadrotor twin – one controller is run in “X” mode and controls the 45 degree arms, and the other controls the 90-degree arms in “+” mode. This was a ‘dumb but potentially awesome hack’ idea spurred by discussion at MITERS about how you would go about making an octorotor quickly without having to build a full custom controller, and I’m glad to report that it turned out to be awesome – more on that shortly.

But Tinystar won’t be hauling any FS-100s or RED EPICs. Why?

because it’s tiny and adorable!!!

(Tinycopter is on the left, by the way).

It’s a little over 13″ across and weighs about 280 grams with battery. Now that I’ve shown a picture of the final product, this is how it went down. It literally happened this past weekend.

Strictly speaking, it took more than just 2 days, but sporadically so. The design work was completed in a day, and I intermittently printed the frame components on the Lab Replicator (I can actually say that now – are we living in the future?) over the past week or so. This frame uses the same tactic that I settled upon for Tinycopter v3 – 3D printed joists and clamping components stuck to carbon fiber tubing. Many multirotor kits are made this same way (though with injection-moldings instead of 3D printed parts of course) and the arrangement is sufficiently versatile and adjustable to be able to mess with the design quickly.

The two big round pieces form the main center “hub” of the frame, and the small H-looking things are motor mounts. The landing legs are made of 2mm laser-cut plywood, and will be painted to look more badass.

The arms are made of DragonPlate’s 1/4″ pultruded CF tubing, which I bought a fair amount of for Deathcopter a long time ago, cut to length using one of the small bandsaws.

Here’s the two multirotor control boaards. The mounting plates have mirror-offset holes which can be arranged in a regular octagon using standoffs. The two flight controllers are mounted right above each other, with the height chosen such that one is slightly above the height center of the props and the other is slightly below, for symmetry.

The controllers do not interface with each other at all – they are totally independent; if I had enough thrust, I could fly it as either quadrotor. The “copter mode” is selected by a few DIP switches and the control gains by trim potentiometer. It’s important that these boards are very simple gyroscope-only controllers. They do not help the frame achieve self-levelling due to the lack of accelerometers, so it’s a very different experience flying. I’m used to Tinycopter and its (vaguely) angle-controlled flight, so I’m still not very proficient at flying Tinystar.

Incidentally, a week after the “double quadrotor” discussion and of course right after I had ordered two of the i86 boards, Hobbyking unveiled the KK2 controller which has 8 outputs and performs self-levelling (though not quite the same as angle-control). And a LCD with menu system. AND a little piezo buzzer.

Hmph. Oh well – these things were already in the mail by then, so let’s just press on.

The motors for this thing are the HXT 5 gram outrunner motors – in fact the same ones that 4PCB uses. Tinycopter uses the slightly larger 10 gram motors, but I decided that they were too overkill for this project and could not use the 4 x 2.5″ propellers effectively. The 4 x 2.5 props are even a little small for the 5 gram motors in this application, and in retrospect I should have made the arms slightly longer to use 5 x 3″ props (the same type that Tinycopter uses) for more thrust and payload.

#2-56 screws are used to clamp the blocks to the tubing. I want to go back now and replace every one of these with real steel screws. The few grams I gain in weight will be more than made up by increased stiffness, because these screws can’t apply enough clamping pressure before they strip. Tinystar’s arms need a little adjustment every time I pull a rough landing (which is like, every time).

The frame mid-construction on Saturday…

I assembled the frame as the landing legs’ paint dried. The effect is definitely not carbon fibery, but it’s not bare wood. I am a fan of this style of landing gear and may make some (not so high-heeled ones) for Tinycopter. It’ll definitely fare better than my little 3d printed landing claws on it right now…

Starting to look like something…

The weird arrangement of legs is due to the real Cinestar 8′s need to carry a large camera on a gymbal underneath, with the rearward bias of the landing gear keeping them out of the camera frame further. It also looks more badass. That was all for Saturday – Sunday was reserved for wiring everything up!

oh god

Clearly as the number of rotors increases, wiring messiness increases nonlinearly. This is going to be difficult to arrange elegantly and in a vaguely easy to service fashion…

I guess it didn’t end up that bad looking. I pretty much daisychained each ESC together, so it’s actually quite suboptimal from a power distribution perspective – the last ESC in the chain gets the most screwed in terms of voltage drops and resistance. I would also mount these on the bottom side of the frame if I did it again, for easier access. A tree-style (8-to-4-to-2, for instance) distribution method would have been much better, though a bit more weighty.

Tinystar has noticeable trouble starting all 8 motors at once, something which I attribute largely to the daisychain wiring. Several motors pulsing at once mean the voltage on the positive connection can sag alot while the ground (0v) rises erratically, throwing off the ESCs’ starting routines.

The flight controllers have their own ‘decks’, onto which they are secured with fluffy double-sided tape. This is dramatically less isolation than Tinycopter’s block of memory foam, but it seems to work very well for one reason or another. Perhaps accelerometers really are more trouble than they’re worth…

Powered on! It took a few coin flips to get the motors spinning the correct direction. The controller assumes you have certain motors spinning one way because differential thrust is needed to turn (yaw) and if the props are wrongly-handed the direction of spin for a certain thrust differential will be opposite, leading to hilarity.

I tested one “subrotor” at a time – first, the four 45-degree motors in the “X” configuration, then the 90 degree motors. This was just to make sure I didn’t put everything together only to have something die or not work – I had to go back in after the “X” stage and replace one of the ESCs (Side note: the 6A ESCs come in a old and a new version, the latter of which does not work with 3S lithium batteries but the new one does)

Pretty-shot!

It took me a while to get used to flying in rate mode….

…okay, it actually took me a while to dare fly it at all since it’s too pretty and my past first-flights have all ended in sadness. I still have issues with “station keeping”, or holding it steady in one place, but I also have that issue with Tinycopter and more flight hours can only help it. Rate mode is literally 1 integral away from angle mode, so the joystick movements required are much different. One tactic that has come in handy is briefly flicking the stick in the direction I want to move in- this step-changes the attitude since you command a strong rotation and then none. It’s not the smoothest of movements but it gets the job done.

One thing we noticed immediately is that this thing is so stable. I’m thinking it’s due to two main reasons. First, it has more ‘unit vectors’, so to speak. More directions it can move in while only changing one motor’s speed. A quadrotor moving in a direction that is not parallel to its arms needs to spin up or down at least 2 motors, whereas an octorotor can additionally move in the 45 degree directions without having to do the same. Second, there’s six gyros in total because of the 2 flight controllers, so not only do you get parallel readings, but the flight dynamics make for some mean ‘mechanical averaging’ too – the motors and propellers are on the end of long arms which have some springiness, so noisy command impulses can be absorbed. The propellers themselves are viscous couplings to the air (Thrust is a function of velocity), so it will also tend to damp erratic differnces between the motors.

Basically it was found out that I could set Tinystar flying very level and slow, and actually “dribble” it up and down, pushing down on the battery. It sinks a little, but then recovers.

Then I try flying it again and all hell breaks loose. Hmph.

Anyways, because Tinystar was made in mimicry of something used in cinematography, the test videos are a little more artsy than usual. There are, somehow, already two test videos. The first one was filmed indoors the day of:

And the second one filmed outdoors on the Third Ever MITERS Flight Day with even more arts:


I really should think about making a tiny 3-axis gymbal for this thing… once I switch over to 5 x 3 props so it can lift more than itself.