I recently finished my quadcopter. It is a big one. 120cm in diameter, about 5kg of weight and it runs 28 inch propellers driven by four Tiger Motor U8-16 100kv motors with a 6 cell LiPo. Im using PixHawk with Arducopter 3.2.1.
It is almost impossible to take off. If I increase the throttle, it tends to flip over instead of lifting itself. Sometimes three legs are above the ground so it rotates in the air around the last leg that remains on the ground, just hovering a few centimeters above the ground but if I increase the throttle further, it starts flipping again.
The strange thing is that it does not flip to the same side every time. One motor is always lacking behind (in terms of rotations per seconds) but it is not the same motor. I could assume that it is always the motor which is located at the most upper part of the terrain (The lawn is not totally flat but it is hard to recognize with the eye). If I just turn the quadcopter so a different motor stands at the position of the formerly slower motor, that motor starts spinning slower as well. So I guess it is not related to ESC calibration error or wrong motor positions.
I have even been able to take off three times (When all motors spun equally) and then the quadcopter behaves not too bad (It drifts a bit but not too much).
So the question is: Why is the quadcopter not adjusting the rotation speed of the slower spinning motor as soon as the other motors lift the quadcopter and the copter is horizontal again? It seems like the motor keeps the same rotation speed, even if the quadcopter is almost flipping over to the direction of the slower motor.
Because the quadcopter is able to fly in general I assume that motors and ESCs work well. It seems to be just the take off which causes trouble. I use stabilize mode during take off. Also tried HoldPos and Loiter but both with same results.
Does anybody have a clue?
(Sorry for cross-posting. I also asked this question in the forum of ardupilot.com I will comment on both threads as soon as any progress is made).
Replies
Michael,
Have you considered increasing the voltage? I'm testing with another big motor/prop combination and get poor control results with 6S and no problems with 12S. I realize this is far from scientific and the U8-16 100kv is specified to work with 6S, but just wanted to share my own experience.
Regards,
Peter
I fixed it. It is difficult to tell what exactly solved it but I assume it is the new version. I formerly used mission planner to flash my Pixhawk to Arducopter 3.2.1.
Now I downloaded the latest version from here:
http://firmware.diydrones.com/Copter/2015-06/2015-06-05-08:06/PX4-q...
That version is definitely newer than 3.2.1 (How can I check the version of the firmware? I guess it is a 3.3 beta)
This version also has a new sonar pre-arm check (Which I personally find useful but I had to search the forum until I found out how to arm it with a working sonar! Hint: The sonar seems to wait for a value between 50cm and 2 meters before the pre-arm test is successfully passed)
Now the UAV behaves normal and is very stable.
Log files should have the version of the firmware loaded on your aircraft.
It will be/was in the early lines of the file.
Sonar? Good luck with that.
In general it is an exercise in noise analysis.
In practice - well, only God can make a bat. They are the most successful application of ultrasonics in nature (discounting multiple varieties of insects and other critters the biology majors will soon correct me on).
-=Doug
I am still running with older code but, does the version you have allow for frame dimensions that are out of 3DR 'normal'?
The system should be calibrated to 'level' before any PID tuning or auto tuning is performed - use a real level tool,not your eyes.
I would go through all the calibrations again, slowly. All my quads were build with 3DR 'C' frame dimensions thus, the base values pretty much worked initially.
It might not hurt to check through my list of reason's why multirotors flip HERE. (replace APM with Pixhawk in your case)
You just might add another one to the list.
-=Doug
Thank you for your answer but I have no idea where to check this. BTW: What is 'normal'. I guess it was somehow related to the version because with the new version it works.
But I also fixed the Pixhawk itself. It could have been possible that there was a very little space in which it was possible for it to move so maybe every time the copter was lifting up, the pixhawk itself might have changed its orientation slightly.