Tuesday, March 9, 2021

T19 Heavy - 20x95mm MEGA; Mega-Con flywheel system

Ever since the modern (20mm) MEGA caliber and the Centurion was a thing, I have been wanting to build something reliable and magfed to run them full auto. This has only intensified with the advent of special rules for MEGA hits such as defeating shields/armor, killing special zombies, causing double damage, and counting as more points for DTC-like gametypes.

I never ended up doing what I should have back in the dark ages (jam a Zeus cage and a RS pusher box into a hacked up Centurion receiver), so the intent to eventually do a 20mm project followed me all the way to the T19 era, and now I finally got a round tuit to spare and did it.

Hence:


Of course, this starts with a flywheel system. As a starting point for the profile geometry, I loosely scaled up the .50 cal Hy-Con including the 0.56-ish centerline area ratio, yielding 22mm control bore, 22mm rim width, and 15mm gap. I then designed a 67mm centerdistance (jumbo format is probably the best descriptor) system around that, yielding what is more literally a scaled-up Hy-Con than not.

For motors at dev stage, I used Racerstar BR2207S, as I had some 1600kv ones on hand already.









 
This might drive home the scale of this system. .50 cal Gamma cage looks downright small next to it.




Now this needs a breech. I opted for the existing mag standard. It's easy to get caught up in This mag design has X mm of wasted length! and similar griping, and that's how we get crappy, incompatible mags. Some careful design was required to use these mags, including the rear feed ramp to deal with top round positioning issues and the 0 mm cage flange thickness in front of the mag to keep required bolt stroke to a minimum.



Design elements are all the same as other modern T19 breeches. Angle cut, flared magwell, overinsertion stop on feed lips, no fence on the front for comfort, 7mm flanges, ...

This then gets its own mag release, its own side cover set (below), and a stock T19 drivetrain stack with the exception of a slightly longer bolt.



Add in a top rail front segment and one underbarrel gap filler/finisher/hand stop doohickey to polish it off, and we get...


















Filaments in this build: Yoyi translucent red PETG, CC3D bluegrey PETG, Makeshaper orange PETG, Overture white PETG.


Results


Overall excellent. I have had Hasbro mega up to mid 160s fps, and the one used Whirlwind I currently have up to 140-ish, and breakin wasn't even complete when I shot those. Also, this is really easy on ammo compared to a Zeus cage on mega --or a .50 cal T19. Reliability is so far awesome at the 12rps I have it set to max out at. It will feed and shoot just about anything and I have probably shot some of my testing darts 15 times into hard objects by now.


Now about the bad: The motors. I grabbed these because I had them and they would work for a ballistic proof of the cage. This could really use a larger, more modern motor, and also slightly higher kv than 1600, or else 5S or 6S. Spinups to high (which is 18-20k for this) speed are pretty crappy by modern standards, though not unuseful, with the 1600kv BR2207S and 4S. Also, Racerstar motors have quality issues and as usual the dynamic balance on this system is not great. I'll probably go for T-Motor F80 for an upgrade/option for this in the future.


Release


Link


Release notes duplicated here:


###############################################
# T19 MEGA (20x95mm) Initial Release 03-07-21 #
###############################################

Takes Hasbro-style 20mm mags. Mag tolerances are crap on commercial mags,
and some may need hand fitting with a file, particularly clone mags, as well
as release notch issues needing adjustment.

3 mag releases have been provided: the "stock" and -1mm (lower) straight
versions, and the "circular surface" one which is closer to what my hand fit
"stock" release ended up as after adjusting to get easy lockup and minimal play
once locked on all my mags. I recommend trying that one first. Some hand fitting
may be required regardless.

The "MegaMagDropfreeifier" part is a mod for Hasbro (style) 20mm mags which fills
in the second void/notch above the actual release notch, which often causes
trouble with snagging on the release. Print it out and glue it to *one* half of
the mag body (not both). File smooth and/or fill with a little devcon.

Top rail: Uses the T19_TopRail_NewFullLengthRearSeg.stl rear rail segment used
by the new (Gen2) .5 full length breech. Front rail is specific and included.

Bolt is different. All other drivetrain, stock, controls, etc. parts are the
same as a .5 cal T19E1.

Firmware difference is maxRPM=20000 and maxROF=700.

ACE-NX inline version - release; build notes

My current ESC I am building, putting into blaster projects, and am shortly to start selling on Etsy.

Gerbers, BOM, SimonK board definition, precompiled FlyShot firmware: https://drive.google.com/drive/folders/1w04lxvGw-rxBViPkGWJeUk-NnfKFg8dD?usp=sharing

Source: http://oshwlab.com/torukmakto4/ace-inx





Overview:

  • ATmega8 or -8A @16MHz with ceramic resonator
  • Infineon 6EDL04N02 gate driver
  • Nexperia 5x6mm LFPAK56 mosfets
  • LDO logic power
  • Boost converter from 5V rail for 12V gate drive supply
  • 40V mosfet options for proper 6S support, plus 30V ultra-low Rds(on) and high avalanche ruggedness/high Idm options for aggressive higher kv motors
  • 26x46mm, 2 layer, 2oz, 10/10mil trace/space (Fab by any vendor)
  • Minimum 0805
  • Enhanced mosfet footprints for easy hand solderability
  • Busbar ready
  • Designed for FlyShot support and use with tach output
  • ICP signal input
  • New board target: ace.hex (see: ace.inc)
  • SimonK-supported status and warning LEDs





There are no silkscreened component designators on my newer boards because they were frequently not resolvable and/or wouldn't fit anywhere with the density, so here's the usual passive placement guides, pay attention to color code:


The green labelled 50-100k resistors are an optional low side gate pulldown. I have been leaving these out - this was my first design with IC gate drivers and I felt the need to include the footprints for them anyway as I do with discrete boards, but they aren't actually required with the driver.

ISP pads with minified labels: RESET, SCK, MISO on the top row, then GND, MOSI, VCC on the second.

Other unlabeled component placements are obvious/unique. More info is in the BOM on mosfet, etc. options.

Errata

Above three low side gate pulldown resistors are superfluous. They also are not actually in the schematic, they were whacked into the PCB at the last moment anyway.

ISP pads are small and annoying, and could use to be bigger.

Caps for decoupling and so forth are overkill. Several 1206 parts could be 0805, and a few cap footprints could probably be Muntzed or combined into one larger value cap without any ill effect.

I don't particularly like the sense trace length and routing on paper, but these handle startups and sync holding just as well as any other board so I doubt there is a noise issue with that.

The 6EDL04N02PRXUMA1 driver is in the midst of a stock shortage and is unavailable a lot of places right now. Stock should be arriving back sometime this month at most vendors, I believe, but I might design future boards with a different driver in the interest of having those plus this one around in case one driver or another is sold out like this again. I already have a supply of them to use for now.

ATmega8 vs. ATmega8A

The exact MCU part to be used in ATmega8-powered SimonK ESCs is an area of ongoing testing. I have only built one pair of ESCs with the mega8 part so far - but I suspect the mega8 part is a better bet for comparator tolerances than the mega8A part.

Source Project Location Update/Change for DZ Industries Electronics.

Due to an EasyEDA change, the source projects for all DZI blaster management and motor control electronics projects are now found at oshwlab.com/torukmakto4. The new ACE-NX board has had its release created natively in that tool and upon further investigation, the formerly shared EasyEDA projects have also been converted as well. Apologies to anyone finding a broken link in the last few months.


As usual, refer to the release posts here, as some placeholder components have been used (fixes WIP). BOMs posted here are the final ones.