XuGong v2 Pro and 3DR Pixhawk - build and thoughts

I had dream: to have foldable multirotor vehicle with gimbal and GoPro on it.
I've built Tarot 650 already and it's still 650mm when folded and not encourage to carry in backpack.
After poking around internet I found four options worth to look in details.

• Black Snapper, which slowly became Black Snapper Pro L from Globe Flight.
• AD1 from ActionDrone.
• QU4D from SteadiDrone, now they have variety foldable quads.
• XuGong v2 PRO from ImmersionRC.

In a beginning I bounced back and forth between Black Snapper and AD1 with leaning toward German creation. Snapper is quiet small when folded but heavy, total weight with battery and gimbal for ~15minutes flight time ends up close to 2.2-2.6 kg. This is pretty heavy for taking it for hiking, biking or any other non-motorized adventure. And I'm not counting remote and at least one spare battery to carry. Bad thing is that Snapper does not offer theirs nice front attached gimbal any more, they moved to DJI Zemuse hanging down option which made it less attractive for me.

AD1 is a little longer when folded, weight and capabilities are similar. California team offering combo with nice gimbal based on BaseCam SimpleBGC controller. When folded it's quiet long - 510mm and doesn't seem very portable. Just 140mm shorter than Tarot 650. And fully loaded weight is more than 2kg.

QU4D was nice and it was first foldable and really portable quad I saw on the market these days. Cannot say any more details. Its successors are good, but they are all in 2.2kg+ category.

Now I'm looking at XuGong v2 PRO. Light, small and advertising ~20 minutes flight time. Down sides: small compartment to put electronics. Frame is designed for DJI NAZA, power distribution board and OSD all tuned for this FC. And I'm APM person, especially because of guided flight modes. To imagine how small compartment is I can say that AMP 2.6 with headers facing to sides will not fit. There is option to use FC with connectors facing up and I thought about it.

How to fit Pixhawk?

Here I hit my first problem - I never was able to make APM to work with 3DR GPS/compass module. May be because my APM is a RCTimer clone, I don't know. I decided to move to Pixhawk FC. It's a little longer than APM, so need to do something with it.

First I removed existing headers and cleaned up solder.

Second I soldered new straight connectors. Don't look at gaps, since it's quadcopter, there is no need for all eight servo connectors. Also one of AUX pin was scarified.

After such vandalism act survived Pixhawk FC fits nicely in small compartment and all connectors are accessible.

There are round holes on side walls where safety switch goes as this place was make exactly for it :)

At this point we have main part assembled. In next step everything needed to be connected.
One thing: USB port is completely blocked. Thinking to get remote one from 3DR.

How to control gimbal from FC

Here is one more caveat with OSD/Camera control. Remember - everything here is for DJI NAZA. Gimbal pitch control and camera switch are implemented on PDB. It requires PPM input from receiver and uses CH7 for pitch and CH8 for camera switch. Chanel selection can be adjusted in GUI from ImmersionRC. Caveat is that PDB reads PPM from middle point before it hits FC.

First. Need to "improve" PDB for our needs. To do so, I removed two 100 Ohm resistors R11 and R12 to disconnected IC on PDB from gimbal connector.

And soldered single servo wire to three point: brown-GND, red and orange are PWMs for pits and roll from FC. I forgot to document which of them is what, but it's easy to figure out.
Note: on other end of wire you need two servo connectors, not one.

But this is not enough to make gimbal behave as wished. You need to change firmware to [Rocker Position Mode] G3 2-Axis Gimbal Firmware V2.13 version or later, where "Position Mode" is essential. Firmware can be found here under G3 2-Axis For Aircraft.

Results

Here is maiden:

Pictures above shows Multistar 4006 700KV motors on it with 10" props. Later they were replaced with Tmotor AirGear set. Power consumption in both cases is approximately the same.

Bad things

1. Because the main body is hanging down on 8 rubber dumpers system is susceptible to oscillations especially with high D-rates.
2. Gimbal is hard mounted to frame, as a result any oscillations coming to frame go directly to camera and video. This is visible even on 60 FPS footage when windy or flying fast.
3. Arm locks are the weakest point in whole design. Any impact will slice a little plastic notch and arm become freely rotating. In video below copter hit tree and then fell down from ~7 feet -- two locks were lost. It was third minor crush and I used all spare locks.
4. Gimbal itself is slow in response. In video below you can see that camera is tilting too slow to catch person movements.

There is a thread on FPVLabs where there is drawings for bigger frame plates.

Bottom line

 Nice, light and compact frame but fragile and not good for missions where you may not be able to revoke control back and save craft.