Has anyone tried messing with the center of gravity not beeing in the center of thrust?
I want to try an experiment. I am building an octaquad having its battery pack (2000g) center of mass 6,5mm back from the drones center of gravity. I intend to mount the gimbal with a camera (954g) 172mm in front of the center of thrust. The octaquads diagonal is about 880mm and it is a symetrical X having about 62cm (excluding props) from front to back and from side to side. From what I calculate the center of mass will be about 53mm in front of center of thrust.
How will it affect the PIDs? Would I be able to compensate for it by changing the params INS_POS_X?
You will need enough overhead in the battery/ESC/motor/prop combination to allow for the unevenly distributed weight. I see you spending lots of time in ecalc.
I would start by tuning (and Autotune) at the minimum take off weight and with the mass as close to centre of thrust as possible to get a good baseline.
Then you could see how it behaves by adding the payload and de-rating these:
ATC_ACCEL_P_MAX x (min_TOW / max_TOW)
ATC_ACCEL_R_MAX x (min_TOW / max_TOW)
ATC_ACCEL_Y_MAX x (min_TOW / max_TOW)
Or if you want to be more technically correct, make up a dummy payload to match your camera and gimbal that’s fixed very well and wont move or vibrate, then Autotune again with that in place.
Dont Autotune with a real gimbal and camera fitted.
I wouldnt be manually changing PIDs unless really necessary. Use transmitter-based tuning and a dummy payload fitted. I think of the PIDs as being about control of the ESCs/motors/props (there’s some other aspects too, but bare with me…) and those things are not changing just because you added the payload you always planned for. Selection of well matched, good quality parts will be essential.
Thanks for your help. I always manually change PIDs as I feel quite good at PID tuning.
I have tested my theory and the results are really interesting. It looks like the first thing that was limiting the change of CG was the motor power. Although the drone has a TWR of 3:74 without payload after mounting a 900g dummy payload 177,296mm in front of the CG the motor power started to lack, as during the automated breaking in loiter mode two of the front motors had to max out their RPM.
As for the PIDs of course I see a slight degradation in the logs, but tbh I havent noticede it in flight.