torsdag den 27. august 2015

DJI s900 with 3DR Pixhawk - wiring main components

Hey there!

It is time to perform the next steps in the build of a DJI s900 Spreading Wings hexacopter with a 3DR Pixhawk as flight controller.
First there were some hints for building up the frame itself. Second I showed you all my solution for mounting the Pixhawk with vibration dampening. In this part of my build log I want to show you what else I have mounted to my new hexacopter and how I wired it all up.

Here is an overview of the components I added to my setup prior to the first flight:

Quite a bit of equipment and for sure not everything teh copter will have to carry in the future.

Let us start with the first part I added to the setup: 180A AttoPilot current and voltage sensor.
Why do I use this part instead of the 3DR Power Module that is already shipped with the Pixhawk? There are two advantages (or better two compelling reasons) and one disadvantage in using an AttoPilot. The disadvantage in using AttopPilot instead of the Power Module is that you don't have the integrated BEC of the PM (Power Module) for directly supplying the Pixhawk. That is also the reason why there are two dedicated BECs in the list above. But more on that later. The two compelling reasons why I have to use the AttoPilot are:
  1. I am using 6S Lipos as battery for the copter and the 3DR PM is not able to handle the high voltage.
  2. The s900 can theoretically draw 6 times 40A of current in peak (6 ESCs with 40A rating). That summs up to a wopping 240 Amps at 25V  (resulting in theorectical 6 KiloWatt max power comsumption! Thats like running 3 hairdryers in full power in parallel. Some really massive power) 
So in the end the 3DR power module would be fried two times with overvoltage and overcurrent (can only handle 60A with a Pixhawk).

The AttoPilot needs to be mounted in the current path of the battery to the power distribution of the s900 (built into the lower plate of the main frame assembly). A description for soldering the board to the battery cables is already on the wiki of Arducopter. The result of this work looks like the following at my copter:

As the AttoPilot sensor range has no built in BEC there comes the first BEC into the setup to combine both at the Pixhawk's PowerModule connector.
There are several information pages on the web were you can gather the wiring information, but for simplicity here is a complete wiring diagram with pinouts:

For this schematic I used the wrong BEC picture. Instead of the shown Hobbywing I used the BECBoy, because 3A is more than enough and 5A of the Hobbywing are better used for servos and so on on the Pixhwak's backward connectors rail.
So I hope this rough schematic makes it a bit easier to setup the battery sensing and the first part of a redundant power supply for the Pixhawk.
Speaking of redundat power supply for the Pixhawk lets have a look at the second BEC out of the above component list. The second BEC is connected in parallel to the same upper XT60 power port of the s900 lower center plate as the first (XT60 in center of picture).

The difference is that the "Hobbywing" BEC (forground of upper picture) is not connected to the Pixhawk's power module connector. Instead it is plugged to the input/output-connectors at the backside of the Pixhawk case (the one for connecting the ESCs of s900 frame). Pixhawk is intelligent enough to include an automatic internal failover when one of the power supplying BECs is failing. Impressive feature in my opinion!
Some more pictures for documentation of BEC mounting:

To finalize the redundant power setup of the copter there is an additional safety feature adviced by 3DR. They say there is a Zener diode required when you power the Pixhawk via the ESC/Servo-Rail. I added another personal touch to the Zener diode. A 1 Microfarad "elko" (tantalum capacitor) and a 22 Picofarad ceramic capacitor in parallel to the diode. The reasons are:
  1. 1 uF Elko: buffering some current for bridging massive power consumptions on the ESC/Servo-connectors
  2. 22 pF ceramic: filtering high frequency noise introduced by bad EMC of connected components. 
The schematic for my "protection, filtering and buffering"-plug is as follows:

The resulting, with heat shrinking tube covered and into a servo plug built, circuit looks like follows:

Output 1 to 6 are s900's ESCs; output 7 is occupied by Zener-Circuit; output 8 is used for 5A 5V Hobbywing BEC power connector.

So with the power circuitry finished, there is still the RC-receiver and the GPS left. The FRSKY X8R is mounted with "doubled sided sticky silicone tape" and some cable ties in upward position to the frame.

-----   Interjection  -----
I really really really love this sticky silicone tape. Have discovered it at the shop when buying the BECBoy supply. You should definetly try that stuff. Best double sided sticky tape for RC I have used. And silicone dampening! Definetly recommending!
-----   Interjection  -----

The two antennas are mounted in a 90 degree angle to the bottom of one carbon fiber tube and one of the aluminium plates of the frame. I know it is not the best decision to mount antennas to conduting (and so shielding) parts, but they are facng downwards and are only shielded in direction of the open sky were no signal is coming from. So I think it is going to be OK.

The 90 degree antenna mounting is for better signal reception and highly recommendet. And as my transmitter antenna als always in a horizontal position, both receiver antennas should also be in a (mostly) horizontal position.
Additionally you can see the Pixhawk safety switch on the last upper picture. I'm not really satisfied with the ziptied switch, but for now it will do. Couldn't drill such a big hole into integrity vital parts of the carbon fiber plates. If someone has a nice solution I'm thankful for hints.

The last component from my list is the GPS. I ordered a dedicated, additional mast from the great bay for mounting, because I do not like the DJI system. The DJI solution has to be glued together (what a nasty solution for a copter like this). And DJI also only includes the bottom folding part of the mount in the delivery. The mast and top mount are missing. So here is a picture of my mount:

Not perfect yet (mounting screws too long), but also OK. And if I have to buy additional parts for GPS I buy and mount the stuff I like. Last hint for GPS: I used countersunk screws and also countersunk my drill holes. So the screws nearly vanish in the top middle carbon plate and there is enough room left for Pixhawk and Pixhawk's top connectors. 

One last thing for wiring which is not on my upper list:
The order for connecting the six ESCs to the Pixhawk's outputs is as follows:

Pixhawk Motor-Out
S900 ESC

I used 4 servo connector wires already delivered with the s900 and adapted the signal wire order. I also adapted the order of the ground wires according to the above list. Not neccessary because all grounds are the same at Pixhawk side. Was in the mood for it :)
The resulting wires look like this:

My final comment on this blog entry:
Maybe you all have noticed that I often use mesh tubing for organizing many of my cables. In fact I always use mesh tubing at my s900 when I have to run two or more singla cables in parallel. This is for a nicer, more professional and organized look. But also for protecting my cables from possible wear out (rubbing). And a professional copter for such a pile of money has deserved that bit of detail.


So again we have finished another part of my build log. Hope you all liked it.

Next up will be the basic software setup of the Pixhawk and some words on "special twists" of the flight controller and frame combination. 
Maybe also some words about my initial PID tunings and the maiden flight ;)

Stay tuned and happy flying!