jcg1541

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Re: Cliff Diving PID Setting
« Reply #75 on: December 31, 2019, 04:37:30 pm »
What does it do if you leave the normal PID active in the dive?  Oscillate?  Wobble around as if the servo isn't strong enough when you know it is strong enough?  Come straight down and require more pull out pitch than normal?  Is the new servo rated to have at least as much torque as the old servo (maybe the strength is marginal)?
I have a video footage of using normal PIDs in the dive using the new servo. I will post it next. It wobbles around.
The new servo,  eflight s60, is more beefy than my prior servo, emax 9051.  I have developed a "servo saver" modification to prevent gear strips in crashes. The mod weakens the maximum torque of the servo arms, but I am pretty confident that the mod does not impair the precision of the servos. See attached pictures. I use this mod in all my videos, old and new.

jcg1541

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Re: Cliff Diving PID Setting
« Reply #76 on: December 31, 2019, 06:31:52 pm »
What does it do if you leave the normal PID active in the dive?  Oscillate?  Wobble around as if the servo isn't strong enough when you know it is strong enough?  Come straight down and require more pull out pitch than normal?  Is the new servo rated to have at least as much torque as the old servo (maybe the strength is marginal)?

The first dive was with the normal PIDs. The second dive was with 180% high gain mode. The 2 dives were in the same flight back-to-back.
I spun the rotor with consistent constant RPM by using BLHeli MULTI software with PI governor. Both dives had exact same RPM.
The first 5 seconds of the first dive was OK, but I had to abort it after 5 seconds when it swung so much I was not sure what it would crash into.
« Last Edit: January 24, 2020, 07:41:59 pm by jcg1541 »

Re: Cliff Diving PID Setting
« Reply #77 on: December 31, 2019, 10:54:41 pm »
I saw that same circular oscillation (pitch/yaw with roll too) in fast normal flight with normal PIDs.  I am guessing that the fact that the oscillation has a yaw component (with different servo, linkage, servo loading, PID) is saying something?

I would start with Manual mode, making copter adjustments so that it flips (roll and pitch) and pirouettes in alignment with the frame or at very least in alignment with the FC.  Then check that Rate mode flips are also in alignment.

I might try tuning with ZN method like this:  Set I and D to zero and adjust P up till it oscillates.  Cut P in half and adjust I up till it oscillates and reduce I a little.  Leave D set to zero.

jcg1541

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Re: Cliff Diving PID Setting
« Reply #78 on: January 08, 2020, 01:47:28 pm »
Just an observation: Measuring servo error with a fixed servo signal is not the same as measuring the heli leveling error with a stabilizing FC running.  Any slop in the servo or even the linkage is removed by the FC.  Imagine a roll in manual mode with the stick released when it is level.  Ignore inertia and assume that the only issue is slop.  Imagine that with the slop the swash plate never comes all the way back to neutral and so you get a continuing slow roll.  Now imagine what the servo does with the FC in Rate mode when you release the stick when the copter is level.  It continues to drift past level and the FC sees that the rate is not zero and drives the servo far enough past neutral to both counteract the slop and the inertia.  Servo and linkage slop doesn't really matter because the FC takes the slop out.
I see your concerns with the slop play. I want to rest your mind that the problem was investigated and resolved with my 250-gram craft as attached first picture with the 0.1mm patching films (tape). The slop play was a very serious destabilizing problem even for simple hovering without any acrobatics because the airfoil CG shifted as explained in the second and third pictures. I have built the same 250-gram craft 10 times, and I can consistently build stable crafts recently.
I know that symmetrical airfoils have a stable center of pressure CP (but not zero CP shift), while cambered airfoils have a positive feedback CP shift, meaning that a CP shift resulting in more angle of attack change resulting in even more CP shift - an inherent unstable system. 
So, what I learned was
helicopter + small slop = stable
helicopter + large slop = unstable
airplane + any slop = unstable
.
« Last Edit: January 15, 2020, 04:17:58 pm by jcg1541 »