DJI s900 with 3DR Pixhawk - initial parameters and maiden flight

Hi again,

has been a while since my last blog entry. I already had some succesful flights with my s900 and the Pixhawk. It is a good and reliable team and I'm quite pleased with the performance. Nevertheless I want to go on with my build log to ease the entry steps for other pilots thinking about this combination. 

In this part of the build log I want to give you some hints on setting up some of the more important Pixhawk parameters before you do the first flights.

When you leveled the accelerometers and learned the RC controls in the initial setup, you might have tried to arm the copter already. I had an issue with a missalligned compass, where Pixhawk statet a “Compasses inconsistent“ failure. That means acceleration sensor's forward direction (together with internal compass) and external compass forward direction are off with more than 45 degrees misalignment.
This issue was a bit odd to solve for me. I tried recalibrations of Pixhawk, multiple  alignments of GPS sensor with different combinations of compass direction parameter, but I couldn't solve the problem for two evenings. Maybe I made some other mistakes and I can't tell what did the trick in the end but eventually everything worked out and s900 is performing perfectly. RTL, all flight modes and autonomous functions are working flawless. I find it a bit strange and even scary to fly and have not really figured out why it is working. But I assume there was something like a “Layer-9 issue“ (human failure) with not rebooting Pixhawk after the first correct configuration of compass and flight controller. So my final parameter settings with Pixhawk mounted in backward facing direction and compass mounted in forward facing direction looks like this:

 

So now with everything correctly configured for arming the copter you will most likely face the next special behaviour of s900 and Pixhawk combination. When you arm the copter the motors may not spin at all. The reason is that the DJI ESCs integrated at the end of the arms below the motor mounts are expecting a certain minimal PWM value for start rotating the motors. If you push up your throttle after arming (Be Careful!)  the motors will come to life nonetheless, but I have used to the behaviour that motors start spinning after arming. There are two parameters you will have to tweak for this. The first one is the “Mot_Spin_Armed“ parameter. This was added to APMs/Pixhawks by the developers in recent SW versions. Of course you can also leave that turned off if you like your motors stand still after arming, but find that a security issue as you won't be able to tell if copter is armed or not. OK there is a LED you can check, but this can be failing or covered by obstacles or cables. Six ESCs and motors failing at once seems a bit more improbable to me.
The second step to get your motors spinning when armed is raising the lower threshold of minimal PWM output at ESC connectors of the Pixhawk. The parameter for this is called “Thr_Min“. You will have to play a bit with the values until all motors kick in reliable after arming asthe ESCs seem to differ a bit for low PWM threshold. Even on one and the same copter. Maybe due to some part deviations of some resistors or capacitors on the ESC's input filters. I haven't investigated further. So after all my mot_spin_arm and thr_min parameters look like this:

If your motors all start spinning now after arming then CONGRATULATIONS! Now you are basically ready to start a first flight attempt if you haven't added any further equipment else as battery, Pixhawk, power supplies (UBECs) and receiver. It seems that without any gimbal or other heavy equipment the s900 flies quiet reliable on the Pixhawk's stock PID values.
(Please be aware that these initial values might change over time and in different SW versions!)

I have recorded some footage of the maiden flight for you all with stock PID parameters. You can tell from the video that these are OK for some first circles and testing. But take it easy and don't crash your toy on the first steps (BTW: have I told you already that I'm not responsible for you crashing your equipment when you are following my hints? HA! Now you read it and I'm not to blame for anything anymore ;) )

On last tip for pilots using the pre-3.3 software version for your Pixhawk:
The automatic gear functionality for retractable landing gear is to be implemented in 3.3 and ongoing versions. At least this is my up-to-date knowledge.
I am running version 3.2 when writing this. Call me a scaredy cat, but I am not willing to trying Beta-SW on a machine meant to carry 2000€ of photo equipment and weighing itself in at nearly the same amount of money ;)
So back to adding retractable landing gears to the pre 3.3 SW Pixhawk:
The workaround is, using one of the gimbal controls in the “additional gear" menu of APMPlaner2 (or whatever configuration software you are using). 

Select the according RC channel you want to use (in my case, Channel 8 of my RC transmitter) and select the correct output on the Pixhawks backside output pin bar. Counting of Pixhawks additional outputs starts with number 'RC9' on the right edge of housing (directly viewing on pin bar ). More info on this can be found here: http://copter.ardupilot.com/wiki/landing-gear/
When you have done that, it should look like this:

So that is it for today.

Now you  should have a DJI s900 combined with a Pixhawk that can at least fly and raise its legs. Isn't that a nice starting point for going on to a solid machine for carrying some weight?! In my next post I will show you all my setup for adding an XCAM A10 GoPro gimbal with 360° rotation ability.

Until then: wish you save flying, a nice Christmas and a happy new year if I can't make it to write the next post.
All the best!