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.
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:
(Please be aware that these initial values might change over time and in different SW versions!)
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).
When you have done that, it should look like this:
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!