John Harding - Insert Catchy Title Here

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Sunday, June 07, 2015

250 Mini Quad Build Log

I like my 3D Robotics Y6 - very stable, good flight time, easily carries the gimbal, and by now I've got it tweaked the way I like it (gimbal and LEDs hard-wired, velcro for easy mounting of components, etc).  I took it on vacation last Summer and got some fun videos but it was a lot to pack - it plus other gear took up half a large suitcase.  So I thought I'd try building a smaller 250 Mini.


I was eyeing the QAV250 but couldn't justify spending that kind of money before knowing if I'd actually like/use the smaller size.  So I opted for the HobbyKing FPV250LH ($20).  At first, I thought it would be nice to have the extra cage space for mounting components, but at this point, it doesn't seem worth it.  The electronics can all mount on the top/bottom of the main plates.  I've had trouble with camera mounting - the fatshark camera doesn't work well, there aren't great holes for mounting a camera stand, I tried a board camera but it needs 12V, etc.  I plan to try removing the lower cage.

Motors & Props
For motors, I went for the Sunnysky X2204S 2300kV motors ($22).  A bit pricey at $22/each (looks like $24 now), but I'd been burned by crappy motors on my first build and everyone seems happy with these.  I've been happy, too.  I believe you can now get them in CW/CCW sets.

I got both 5" and 6" gemfans (and even some 5" 3-blades I haven't tried yet).  The 6" props fit, but they're so long they don't leave much clearance for cabling around the APM, GPS, etc.  The first time I tried spinning up the props, I chopped through a bunch of cables.

At this point, I've gotten used to APM and ArduCopter, so wanted to stick with that platform.  Poking around on rcgroups, people seemed reasonably happy with the ReadyToFlyQuads MiniReadyToFlyer , so I went for it:

If I had to do it again, I'd get the power module from somewhere else, due to the backwards pin layout used on the RTFQ module.  I'm also not sure about the ESC - it's convenient to use with the Mini APM which as a single connector/cable for the motor outputs.  But it was a bit tricky to solder with quite small pads and I'm not great at soldering.

Battery: 3S 1300mAH ($9)
FatShark 250mW Video Transmitter (re-using from my Y6, which got upgraded to 600mW)
HobbyKing MinimOSD ($21)
Camera: undecided
FrSky D4R-II ($21)

The Build

It's been a while and I didn't take great notes, sadly.

Frame Assembly
The frame assembled pretty easily - no problems with alignment, missing pieces, etc.  I didn't both assembling the lower cage until after I had everything working.  I did have to drill holes for mounting the electronics, using stacked nylon spacers.

When I did eventually put the lower cage on, it holds OK in normal use but comes off pretty easily in a crash.  As I said before, not really sure it's worth it.

With the ESC, APM and GPS all stacked up, the small cage that's meant to mount on top obviously doesn't fit.  I've seen some folks mount it on spacers, but I've just got it open to the air.

I had a lot of issues with the electronics.  I was ambitious and decided to solder motors directly to the 4-in-1 ESC.  If you look carefully at the ESC, you can see motor numbers screened onto it.  I goofed up at first and assumed the motor connections were sequential; they're not.  Fortunately, I was able to just move the motors around (rotation direction was correct).  The quad would still flip if I throttled it up, so I did a motor test in mission planner and found they were still wrong.  It turns out the APM->ESC cable that came with everything is wired strangely.  I emailed Paul to ask about it and he said it's intentional.  For this to work, you'd have to ignore the motor numbers screened on the ESC.  My solution was to swap the wires in the connector to make it pass-through (1->1, 2->2, 3->3, 4->4).

The GPS/Compass was a little tricky as well.  There were mixed reports of how to hook it up.  RCGroups suggests using A-port but I'm using B-port with no problem.  With the antenna facing up, you'll want the connectors facing the left side of your quad.  I've also had problems with interference on the cable (I initially tucked the slack under the ESC) so you'll have to keep that tidy.

I also wasn't getting a current reading from the power module - for a while, the RTFQ XT60 power modules had the connectors backwards, which prevents the current sensor from working.  I wrote up a whole post on power modules, but the fix here was to swap out for a standard power module and then re-order the cable to match.

End Result

So far, I'm pretty happy with it.  Just getting started trying to fly FPV, which takes some getting used to.  I also need to find a better camera setup.  I had been velcro-ing in the FatShark camera; the small size naturally angled it up to fit in the camera opening, but it wasn't that stable.  I tried a few more cameras:
SurveilZone CC1333 ($26) - Should fit the camera mount if you trim the border, but requires 12V power supply.  I should probably wire up a tap for this, but haven't gotten around to it.
SurveilZone HS1177 ($35) - I thought the stand would work well but there are no holes to screw it into; would likely need to use mounting tape on the stand.  Unfortunately, I just get a black screen from this camera and need to return it.
Currently trying a Hero3 Black velcro'd onto the front.  We'll see how that works out.

Monday, June 01, 2015

APM Power Modules

I was having a problem with my 250 Mini Quad (need to write up the build log at some point) where I wasn't getting current measurements from the power module.  Most of the parts for my 250 came from, including the mini APM and power module.  I had checked and double-checked everything, but no luck.  Some extensive digging on rcgroups turned up this post confirming that the ReadyToFlyQuads have the XT60 connectors on backwards, with the result that it can't measure current properly.  

No problem, I thought - I'll just swap one in from one of my other drones.  I grabbed the HobbyKing power module I use on my 450 and hooked it up.  But when I plugged in the battery, the ESCs didn't give their final chime indicating a signal from the flight controller.  After banging my head for a while, I noticed that the APM wasn't powering up.  This was strange - the mini APM uses a different layout for the PM connector on the board itself, but I assumed the power module layout would be standard (because they're also used with full-size APMs). But no.

Standard power module layout (looking at the receptacle as you'd plug it in with the retaining tabs facing up), a standard power module puts out:
V   V   Is   Vs   G   G 
The ReadyToFlyQuads power module puts out:
G   G   Vs   Is   V   V

[Vs and Is are measured Voltage and Current, respectively]

I re-configured the cable from the mini APM for a standard power module layout and everything now works like a charm.  Hopefully google search will pick this up in case others run into the same problem I did.

Since I needed another power module (figured it was cheaper to buy another one than the hassle of re-connecting the XT60 connectors in order to get current sensor working), I tried going super-cheap and ordered a power module from eBay for $8 shipped.  Standard layout, though the voltage out is slightly lower (5.37v vs 5.40v on the other two).  Will see how it holds up.  On the plus side, it comes with a 6-pin DF13->JST-SH cable for use w/ the mini APMs, including the correct wire order.  I need to put a load on it in order to confirm the current sensor works.

Update 6/6/2015:
I ordered another power module from ReadyToFlyQuads because, why not?  This one came with the XT60 connectors on the right way but still the backwards layout for the power module connector.