Sunday, May 6, 2018

Project T19 Part 11: more parts info, turbo mode demonstration, drivetrain lubrication.

All locked down for combat test at this point. I don't anticipate anything happening, but the old reliable Model Pandora will be on backup duty. I have come to greatly trust that blaster.

So first of all I will hit some quick parts-related things that I didn't cover:

The top rail is a AIM Sports 12x0.40". Amazon carries them. Don't buy the other two thicknesses! (The mounting dimensions are different.) 10-24 socket head cap screws (not button head) are required to mount accurately. Again, watch out for that mounting hole depth in the cage! Too long screw can DENT THE BARREL, DON'T DO THAT.

The bolt motor is a OSM (available from StepperOnline) 17HS16-2004S1. M3 flat head (meaning countersunk) screws are required to mount, and a 10-24 x 3/8" setscrew for mounting the crank on the motor shaft. Shaft must be shortened - use drive spacer as a guide and cut carefully with a zip wheel.

Flywheel motors are Turnigy V-Spec 2205 2350kv. "CW or CCW version?" Doesn't matter, it's a bolt-pattern motor and we don't care about which thread direction of prop adapter is included, which is the only difference, because we don't use prop adapters. Again, very careful with your screw lengths around those stator windings, same as drone applications. M3 and M2 button heads. I use, don't trust these OTOH lengths entirely-- 10mm M3 (mount) and 6mm M2 (flywheel bolt) from Hobbyking, those are kind of soft steel and munchy so maybe not the best.

Flywheel motor controllers need to be able to run SimonK (not BLHeli, I do not use, do not like, and cannot support BLHeli, I have no idea what BLHeli would even do on my high inertia wheels and these motors - probably work, with careful tuning, but with an annoying startup lag and without the right sort of closed-loop mode, and you're on your own with the tuning part!) and have a large enough power stage to run these motors. This is not well defined, but any 20-30A controller should be safe. Look up the datasheets on your mosfets. Notably, I would personally steer clear of the Afro 20A RSM/20A Mini, which has the little SON mosfets and typically are Toshiba TPCC8076/8073 (same as the 12A Ultralite more or less!) rated for only 88A? peak drain current, which is kinda scary for running these with 4S on the bus. A hard lockup from speed is something SimonK does a very good job mitigating damage from, but it is finite, and NOT magic, and there's bound to be a little current excursion associated with that. I personally use Afro full size 20A, which is more or less the same board as the 30A fullsize, just with a bit less actual PCB area, and thermally derated. Perfect for our very transienty (but low continuous current, intermittent duty) app. The 20FS Optos come with Fairchild FDMS8018 mosfets, and that's one beast of a device for that size. Some of the 20FS, 30FS and 30RSM come with Toshiba TPCA8087 and that device seems OK too, though I only have a single pair of those.

Note I tune with Afro 20FS Optos and publish binaries of the T19 wheel drive firmwares for the afro_nfet board target. You need either a USB ESC flash dongle like the corresponding Afro one (if you're sure the SimonK bootloader is already installed) or appropriate AVR ISP gear to flash the controllers, a driver for that, and KKMulticopterFlashTool or just avrdude to do the flashing. If your board is not afro_nfet, you will need to download AVRA and build from source. Or ask me/send me the parameters you want if you know them (I don't expect you to, don't worry) and I will concoct a build for your board. Correspond with me if you have tuning issues/questions, too.

The preliminary Core backplane is built on a 2.0x3.0" Vector perfboard that bolts straight into the drive housing. The processor card is an Arduino Pro Mini (not Pro Micro, that's a different board!), you can use generics or Sparkfun first-source ones. You will need the corresponding FTDI USB serial interface breakout board (I got mine from Sparkfun) and drivers to flash the Pro Mini, but this is what I recommend, because it is rock solid and glitchless; trust me, onboard USB on a dev board like those Pro Micro things, is shit and I will never go back. The bolt driver card is a DRV8825 carrier board; all mine are cheap generics, which are fine, TBH. Other bits you need for the controller build and wiring are 100k and 1k resistors, a small electrolytic cap for the 5V rail, a ~220uF low-ESR electrolytic cap for the 8825 driver's DC bus, and male and female headers, lots of them for socketing both plug-in cards and making every cable connector. Note, your bolt limit switch connector needs to come out the back of the board where the big cutout is in the drive housing.

For logic power, I always advise a buck/boost converter, nothing less, for reliability (brownouts are not fun and can happen, there's a lot of current and noise nearby). I use Pololu S18V20F5. It's a big, expensive, overkill monster, but overkilling these things is not a bad idea, particularly the 35V input rating on that unit. A bad or overapplied logic PSU that shorts and puts unregulated DC bus into your 5V logic stuff will smoke your Arduino, 8825 card, and ESCs instantly and then you are out A LOT of hardware and money (including two increasingly-unobtanium Afros if you're using those) over a stupid fucking pocket change voltage regulator.

The schematic is coming; I promise. But if you look at the firmware... It tells you which pins on the APM board hook up to what. Wire em up. Not rocket science. Switch input circuit for trigger (both throws, so 2 inputs) and the bolt limit switch is a 100k in series with the MCU pin (for input protection) and then on the outside end of that, a 1k pullup to Vcc. Common terminal of the switch goes to ground. Pololu site tells ya how to hook up a 8825 driver - except note from the firmware the numerous pins on the 8825 card that need to be controlled in software in this blaster and should be connected to pins on the APM and not tied high or low. Then, you have Vcc and ground to distribute to every Vcc and ground pin, and raw DC bus to hook to the VM pin on the 8825 with the buscap from that to ground, set Vref on the 8825 to 1.0-1.1V, flash it up, plug it all in and away it should go. Unless your trigger cable is reversed and registers a constant down state when up. Then you won't finish booting and get the selftest growl and flywheel blip (turn it off and flip the trigger cable around and turn it back on).

Now to the part where "SimonK says 'get wrecked'":

This is a 22 round Workermag packed full, on turbo mode, set to 14.2rps and doing well. Shooting garbage darts this fast in long bursts can trip up the magazine on occasion, but the T19 is generally rock solid. The only time it skips steps is when it OUGHT TO skip steps, i.e. when a mag misfeeds catastrophically and a typical DC drive would have crunched the round into a huge folded mess (in which case, you let off the trigger momentarily, the bolt backs up and the malfunction normally self-clears). So that's kind of a thing, isn't it? Not quite the spam level of the DC drive FDL, but close, and it's still direct drive and brushless. Remember when there was skepticism about the stepper motor? Well, I guess there is still skepticism about the stepper motor, but I don't see why.

Now, bugs DID manifest and in this case it was a tribological blunder with the Production's all-printed drivetrain after some use. I didn't lube it at all, initially. I ran it dry like the Model Pandora (n.b.: the Model Pandora's bolt is PVC, so it has a dissimilar-material solution in place). The thrust faces on the side of the bolt and the inside of the spacer rails developed some minor galling with the symptom of random skipped steps after too long a burst was fired (and things heated up and got frictiony). Whoops; that was remarkably stupid to not lube that. The flip side is that (removing the galling and) lubing the drivetrain with white lithium grease resolved all problems and made general operation much smoother, and just messing with some scrap PETG prints, a little grease makes a MASSIVE difference in friction and makes galling almost impossible. Regardless, now that my attention is called to what's getting loaded heavily in this design, I'm definitely going to try out a rollerized bolt with some little bearings to eliminate the sliding friction at that one troublesome side thrust surface. Should help at high ROF and assist with maintaining tolerances without hand-fitting too.

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