From what have seen, the GND rail is common to bother sides of the Pixhawk. You can easily take an Ohm-meter and check for continuity across the pins. You do have a ground loop, however what you have illustrated is how I have my Pixhawk wired (with two BECs), and I have not had any operational issues.
Keep in mind the MinimOSD also has a ground connection that goes on the comm cable over to the Pixhawk. The GND wires are short enough it should not be a problem (as indicated on various Wiki's). What you wouldn't want is ground current from your servos (or ESCs) coming back on comm / signal lines. You will get a little current over these lines, but as long as the 6V UBEC has a short wire connection to the Servo rail on the Pixhawk, the loop current should be small.
This is the Wiki I have referenced in the past:
I have tried using 6vdc UBECs with the Pixhawk servo output power rail and I do not recommend it. Since the Pixhawk will use the power on the servo output power rail as a secondary power source, I recommend keeping the voltage below 5.5vdc and if you are running servos on the servo output I recommend installing the 5.6vdc zener diode fix Link
Voltage spikes above 5.5vdc on the servo output power rail can cause the Pixhawk to reset and a subsequent loss of control.
You're right, I missed thinking about the voltage rating of the 6V UBEC. I am using a 5V UBEC in the same configuration as the Anaconda Diagram (except I am not running servos). And so you are also right about the Zener diode installation when using servos. Takes a village!
Ok, so I just need to make my BEC wire small as possible. I will also set the bec to 5v instead of 6v
I would not change the size of the UBEC wire. Keep it the same size as the (+) side.
Also, you could provide (+) leads from your UBEC directly to each servo, take the (-) side to the Pix Servo Rail, and then route the (S) and (Gnd) leads between your Pixhawk outputs and each servo (do this by lifting the tabs that hold the pins in the servo leads at the Pixhawk end of the wire) - you just won't have secondary power to the Pixhawk (or you could / should install the Zener Diode as described in TCoyle's post and bring the positive lead to the Pix).
As T Coyle mentions, servos can create voltage spikes on the servo rail (from back-fed voltage). This is the reason for adding the Zener Diode when using servos.
Which way is the diode installed?
See attached. Red lead is (+) on servo rail, Black is (-) on servo rail.
Ok I went to a electrical component store.
I bought some zener diode
It's a 5.6v zener diode. But the person said it was 1W. Will it work? I bought 5 since they cost little or nothing.
The documentation wiki says to use a zener diode 1n5339 , which after googling says 5W.
Since there is no current limiting resistor in series with the zener diode the zener will have to dissipate the over voltage spikes by itself which can cause a peak power dissipation in the zener over and over. Therefore the higher wattage (5w) zeners will stand a better chance of surviving the over voltage power dissipation transients over time than smaller wattage zeners.
I have never seen any two zener diodes conduct at exactly the same voltage so one or the other will conduct before the other one does and take the brunt of the transient.
However, a 1w zener is better than no zener until you get a 5w zener to replace it.
Any idea abut where to buy read made secondary power cable? I'm not really keen on soldering and all that, i would like to purchase something ready made. Any suggestions sir??? I have been looking on Hobbyking, could not find.
Thanks in advance.