søndag den 23. august 2015

DJI s900 with 3DR PixHawk - Mounting the Pixhawk


So Sebastian here again with the second part of my build log for a DJI s900 with a 3DR Pixhawk as flight controller. In the first part I gave some tips for building the standalone frame of the Spreading Wings multirotor. This time I want to show you all my solution for mounting the flight controller to the frame with added anti vibration silicone mounts.

But before we come to the hard work, I want to answer the question why I chose 3DR components instead of DJI's?
(Of course you can skip the next passages if you don't want to hear religious jibber jabber about different system architectures again and again ;) )
As you may have already found out, DJI is selling some capable and astonishingly expensive flight controllers for this frame. The Wookong series (maybe a little bit outdated?), the Naza flavored controllers (ment for smaller crafts as far as I learned) and last the state of the art, professional A2 flight controllers. I definitely did not want to start with Wookong or Naza controllers for several reasons. The only possible choice from DJI would have been the A2. It seems to be very capable and all the DJI add-on equipment seems very thought through. Cables with already fitted connectors and meassured lengths. Prepared mount locations for everything. Available tuning values for PID stabilization algorithm. And many things more. The only thing you have to do is buy, mount and fly.
Buy a lot. Buy expensive. Buy the golden Apple-style bird cage.
No open documentation about protocols and hardware. No open software for flight controller. DJI in control about where you fly if they want. And so on.
For my taste, I do not appreciate dongled systems. I do not like somebody to tell me what I can or can not do with the stuff I bought from my hard earned money. As mechatronics/electronics engineer I like to know what parts of my systems do things how they do them. How they communicate. How they work. And DJI does simply not offer this kind of information and openness. So not my cup of tea. And definitely not my cup of tea when the flight controller plus GPS is as expensive as the main frame itself. If components were competitively priced to similar parts or even if DJI would be more open minded, I would be tempted to buy DJI. But with all the negative points I was nagging about, 3DR and their (or better ETH Zürich's flight controller), the PixHawk, seems a lot more sexy for a tinkering guy like me.

So let's start discussing about mounting the controller to the frame.

First decision: vibration damping or not?
I don't know if one can get away with plain double sided sticky tape or anything like that for vibration damping with this copter. The propulsion systems are prebalanced in factory, so there should be less vibration from the start on, compared to a complete DIY copter. But with a 1300€ frame ment to carry the same money in camera systems some day, you should not rely on a exhausted factory worker having balanced your frame/motors/props after a 10 hour shift. So additional dampening is a must have. But what dampening system is the one to choose? Well that depends on where you want to mount your controller to.

So the really first decision: where to mount the Pixhawk to the frame?
DJI is mounting their A2 in an upward position with the upper case side facing in forward flight direction to an easy to reach plate at the front left of the frame. Quite special. But there is a second DJI-box called the "IMU". It is responsible for meassureing acceleration and rotation rates. But even this box is not mounted to any point near the center of the frame. Not in the center of gravity nor near the turning axis of roll, pitch or yaw. If it works for DJI then so be it. But in general not the most advisable positions for a Pixhawk, as far as I know. Although I think it would be possible to follow DJI's solution for a mounting position (Pixhawk supports 26 plus mounting directions).
I chose a more common position which also shelters the flight controller a little better. The flat plastic dome mounted on the lower frame plate, which is isolating and housing the power connectors.
Sadly I have no good picture to show you the virgin state of my mounting position, but you can see it in this assembly video from DJI.

Benefits of this mounting position:
  • It is nearly in center of all turning axis. 
  • Nearer to the center of gravity. 
  • In the event of a crash, which will of course never happen, the controller is located at the safest position there is. So you can at least rescue the Pixhawk and the logs to investigate why there was (of course not) a crash.
Disadvantages:
  • not much space above the Pixhawk where a lot of connectors and cables must be (and that gets even worse if you plan vibration damping). 
  • Not very accessible. 
  • Quite some work to fit Pixhawk in there.
So how to mount the Pixhawk in this location?
This question leads us back to "mounting with vibration protection?" discussion. And I think I have made it clear enough that you should never mount a flight controller without at least a little bit of dampening.
So what option to use with Pixhawk and s900 frame? Well if you buy a genuine flight controller from 3DR there are some double sided sticky foam pads included in the package. They are ment to be mounted at the four corners on the bottom of the flight controller. The only problem is: they are way to fat.
Second option: I had bought one of the anti vibration mounts for APM and Pixhawk controllers. The ones with the two carbon fiber plates and four silicone vibration cylinders connecting the plates in the corners (quite similar to these ones). Problem: the mount is also way to high for the limited space within the s900 frame.

 
Third solution: using one of the O-Ring-style or ear-plug vibration mounts. Problem: I don't want to trust some thin silicone rings only secured in a small slot between a screw and some spacing standoffs. Or some plates with ear-plugs. So just some plates with holes and memory foam. Too much money flying around with this specific build.
Fourth solution: after searching the web and looking for some finished product, I came to the conclusion that there is no "ready to use" solution for mounting a pixhawk to a s900. So I thought: "if there is none, just build one".
Mounting the anti vibration mount from option two completly into the frame was no option as already stated. But as it was already bought and available, I wanted to use at least the silicone dampeners and maybe the carbon fiber boards. After some thinking and even asking my wife for ideas, I settled for a mix of solutions from my (our) brain storming sessions:


This design offers some advantages in my opinion:
  • dampening via the silicone cyclinders
  • very flat setup because pixhawk is sitting low between the dampeners (height is even adjustable via washers/screws)
  • Steal screws, washers and nuts give a bit more mass for better dampening (what does a mechanical engineer do if there are too many vibrations? -> Adding more mass)
I started the construction of the mount with "machining" the small carbon fiber arms connecting the silicone dampeners with steal screws. For this I repurpesed the upper (smaller) plate of the not usable APM mount. I cut off the edge arms of the smaller plate and grinded them to the same shape with my Proxxon drill tool. It turned out that it was a good idea to fasten two of the cut mounting pieces with a 3mm screw and a nut together for grinding. So at the end you get two identical shaped pieces for the front and two identical pieces for the back of the mount:


Afterwards I drilled some 3mm holes for the connecting screws between mount pieces and bigger CF plate of the former APM mount.
With the mounting arms finished, they could be connected to the bigger, lower CF plate via the screws and washers to form the inner part of the new dampening mount.
Next step was drilling the mounting holes for the silicone dampeners into the s900's dome cover. For this I added some millimeters in direction to the outer rims of the dome so the silicone dampeners have a bit of pretension in all directions. I used a 3,2mm drill from a Proxxon drill set I bought some time ago. But I think a standard 3mm drill will also finish the job.
The silicone dampeneres are a bit tricky to install at the end, but with a small screw driver or flattened tooth pick and a bit of caution you can stuff them in there.

The final result looks like this:



And in the end there is enough space between the Pixhawk and the s900's top carbon fiber plate left for connecting all the cables.


I can't offer any meassurements for the dampening characteristics of the mount, yet. But I will extend the blog post as soon as I have some graphs and results from flying.

Please be aware I have mounted the Pixhawk facing in backwards direction so the outputs for ESCs and servos are facing in forward direction of the frame. With this it is much more easier to connect the ESCs (6 servo cables) and you can use the servo cables already delivered with the s900 package. These are very decent quality and very soft.
For mounting the Pixhawk facing backwards you will have to adjust the mounting parameters within the parameters list of the flight controller to 'YAW = 180°' (using Mission Planner or whatever configuration tool you like). The correct parameter setting is:

AHRS_ORIENTATION = '4'

What I also did to improve the charateristics of my Pixhawk regarding the barometer: add some more foam material to the inner housing of the Pixhawk.
When you open the case and unscrew the Pixhawk PCB you will find a small mounting compartment for foam in the lower case. This is directly underneath the barometer chip. With adding an additional chunk of foam prior to mounting the flight controller into the frame you can make sure to block gusts of air from the props reaching the barometer. My additional chunk was about 2cm thick. Normal barometric changes due to rise or fall of the copter during flight should have no trouble reaching the barometer. Even through you now have a much denser packed foam compartment.


A last tip for reducing possible vibrations because of the thin plastic of the s900's dome cover:
The dome is reinforced with some thin support beams on the lower side. But in general it is still a bit weak and wobbly for my taste. So what I did to reduce vibrations furthermore is adding a block of 'moon gel' right in the middle between dome and the cover of the battery terminals at the lower frame plate.

 If somebody does not know 'moon gel' yet: It is a gelly like material with very good vibration dampening properties. It is very soft and originally used by drummers (music band) to dampen high frequencies at their drums. It influences the sound by absorbing harmonics of the drum skins. My moon gel blocks are from a music instrument store around the corner and my pads are called 'Black Mamba Damper Pads'.  Just ask some employee for gelly damper pads for drums. He will most likely know what you are looking for :)






This stuff is also very good for dampening your GoPro or wahtever action camera during flight.

So this is it. My solution for a Pixhawk dampening mount in a DJI s900 SpreadingWings hexacopter.
Hope this may be helpful for you all, even if you try this mount style with another copter, plane or what ever vehicle :)

All the best, Sebastian