vgwit

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Oscillation during descendings.
« on: October 16, 2018, 11:09:20 am »
Hi guys,

Still can not overcome oscillation of my 250mm quad on descendings. It presents even in Rate mode. Tried to suppress it with TPS but w/o success. FC is Revo with default PIDs.
By the way, why TPS says to increase PIDs in this case? I thought that oscillation is a result of excessive PIDs. Am I wrong? Here is last fligth FPV:

https://drive.google.com/open?id=1YFM1RRFO2DSvbQgm-lv4kPhKiYSYa_WC



Re: Oscillation during descendings.
« Reply #1 on: October 16, 2018, 05:41:11 pm »
This particular issue is not actually a normal oscillation and is made better by increasing PIDs (at least at low throttle, as long as you don't cause actual oscillations at mid to high throttle).  The default curve for TPS increases PIDs at lower throttle and decreases PIDs for higher throttle which is exactly what is needed to fix this.  You may want to drag the curve (line) to make it steeper (still keep it crossing in the middle though, technically it should cross at the hover throttle) if it doesn't give you enough correction.

Since you have a Revo class FC, you should know that AutoTune produces tighter PIDs that helps this.  Be careful.  AutoTune doesn't always work well for all models and/or you may want to run it a few times.  Some models need System->Settings->SystemIdentSettings->DerivativeFactor reduced from 1.0 to something smaller, say 0.5 or even 0.25
Description: Multiplicative factor. If you have D term oscillations, reduce it toward zero and it will reduce PID D terms.
https://librepilot.atlassian.net/wiki/spaces/LPDOC/pages/25657377/AutoTune

f5soh

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Re: Oscillation during descendings.
« Reply #2 on: October 16, 2018, 06:52:19 pm »
A perfect vertical descent is never easy, due do the vortex.
A correct descent can be done while going forward.

Schugy

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Re: Oscillation during descendings.
« Reply #3 on: November 11, 2018, 01:46:49 pm »
Luckily my PNP-quad has ESCs that could be converted from older SimonK firmware to BLHeli 14.9 firmware (also two years old already). With regenerative braking (called "damped light" in BLHeli or "complement pwm" in SimonK) the propellers are not freewheeling but actively controlled during descent. Still not perfect because of the vortexes but much better.
On 3S/12.6V with 30V/38A burst FETs I have enough headroom for the voltage spikes that can occur when all motors brake violently.

Re: Oscillation during descendings.
« Reply #4 on: November 11, 2018, 06:38:02 pm »
Perhaps even more important is that you have the PIDs tuned tightly (high), which is really the way they should be anyway.

For Revo class FCs, it's amazing how much difference you get for this problem if you run AutoTune.

Schugy

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Re: Oscillation during descendings.
« Reply #5 on: November 11, 2018, 07:33:52 pm »
Took my old UOV-values and when I enter them in LpTune and change the settings from "Rapid ESC (PWM sync)" to "Oneshot125 (active braking)" the resulting PIDs are a lot higher. Flies like a hole new quad. Also took my quad out today and I could hardly make it feed energy back into the battery(-3A/-36W). Straight descend was almost as fast as ascend (-5.9 m/sec), height was only 20 m. As always take numbers with a grain of salt.
« Last Edit: November 11, 2018, 07:38:28 pm by Schugy »