I have been trying to tune my quad for the first time and I've run into a slight issue: As I increase the throttle during takeoff, the quadrocopter starts to shake violently while still on the ground. However, the oscillations stop as soon as it is is airborne and the quad feels very stable and responsive in flight.
My PIDs are:
STAB_P: 3.25
RATE_P 0.37
RATE_I 0.2
RATE_D 0.017
If anyone can give me an idea on what's causing this I would much appreciate it.
Daniel
Replies
@Doug
I am very new to the world of multirotors / RC flying in general so I really appreciate your explanation! The quad does indeed lift off with less shaking if I raise the throttle quickly. However, I am still not sure if the shaking I am seeing is entirely due to the ground effect. The quad oscillates rapidly (I'd say at around 10Hz) and quite violently as long as the landing legs are still touching the ground. Wouldn't I see more of a "wobbling about" if the effect was caused only by turbulent air / prop wash?
With regard to your question, I also normally fly in Stabilized mode. My quad flies ok with the default PID values but I wanted to experiment a little bit . Better is the enemy of good, etc. ;)
@Dave
I started over from scratch and arrived at much more reasonable values. RATE_P is now 0.189, RATE_I 0.61, and RATE_D 0.0167. I tried following your guide (thank you very much, btw!) and was a bit confused when my quad was not oscillating wildly in the rig / while test flying even with the parameters way outside your suggested range.
I am still a bit lost on how to interpret the graph below (I've graphed Roll In against Roll). Am I seeing it correctly that there is still quite a bit of overshoot in Acro mode? Does the graph look ok to you apart from that? Thank you for your help!
Regards,
Daniel
likeseasons_quad_061013.jpg
Forgive me if you have a good amount of flight experience and my reply is known to you.
Often during lift-off, or as the aircraft approaches a condition of 'neutral buoyancy -- negative gravity', there is a ground effect that causes turbulence and this can look like the aircraft is experiencing control/stability issues. In one sense it does but once you have lifted beyond this variable area, the aircraft settles in. That sounds like what you are describing.
To test this hypothesis, you could suspend or otherwise elevate your aircraft well above the flat ground surface and attempt to lift off,observing if the situation changes.
All of these words assume you have no major imbalance in motors and propellers and that all motors are identical and all the ESCs are the same.
As such, once you are confident in the aircraft, most flyers simply lift up quickly to clear the area and move away from that zone of disturbance. The 4 swirling, rotating columns of air generated by the quad certainly interact and play havoc until clear air is reached.
One other question, are you flying in Stabilized mode?
I always fly in Stabilized mode and have, as yet, to mess with any PID variable.
-=Doug
Rate_P is very high!,
Normally it should be between 0.08 and 0.18 depending on the power of your machine.
Are you sure this flies ok once it's in the air? Doesn't it overshoot your stick inputs?
A small amount of wobble on take off is normal, its due to the disturbed air close to the ground (ground effect).
But that rate_P is the highest I've seen, even on my test rig with a huge weight hanging out the front and back we can only go to 0.25...