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What's Setpoint Weight
#1
Hey guys, I've just written this beginner guide to explain what Setpoint weight and transition are, what do you guys think? anything that needs correcting?
https://oscarliang.com/setpoint-weight-t...asurement/
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[-] The following 2 users Like Oscar's post:
  • sirdude, kaitylynn
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#2
I think the explanation is very good. But I have already watched a bunch of videos prior to reading your article. To me, everything seems correct and made sense. Good job.
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#3
(08-Feb-2018, 06:02 PM)voodoo614 Wrote: I think the explanation is very good. But I have already watched a bunch of videos prior to reading your article. To me, everything seems correct and made sense. Good job.

thanks Voodoo!
Don't be a LOS'er, be an FPV'er :)  My Gear - Facebook - Instagram - Twitter
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#4
Nicely done Oscar. Cleared up a few thing I was in the dark about. Thank you!
"Damn the torpedoes!!!  Full speed ahead!!!"
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#5
I have seen those settings and had no idea, so left them alone. Now that you have helped me understand them, there is something to check out! Thank you, Oscar Smile
SoCal Kaity :D
OMG, no one told me it would be this much fun!  Addicted :)
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#6
You wouldn't even know English isn't your first language Oscar. Nice job. Well written and informative with simple to understand explanations.
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#7
Your article still implies that D-Setpoint Transition can turn off D-Setpoint Weight when returning the sticks towards center, --> "From my understanding, Setpoint Transition decreases the influence of Setpoint Weight when the stick is returning back to center".

This doesn't appear to be the case. The pid.c code does appear to have a confusing term, "smartFeedforward", that can turn off the D-Setpoint Weight, but I'm fairly sure this is not what we're talking about here. (???)

Also, from my personal (but not necessarily easier-to-understand) perspective, I'd frame the difference between Error and Measurement in terms of either (1) Damping or (1) Angular Acceleration Tracking.

For instance, when D-Setpoint Weight is zero, the D-term is just
-Kd*filtered_gyro/DT ...in other words, just damping; every bit of angular acceleration goes into reducing the motor commands.

When D-Setpoint Weight is equal to 1.0, the Kd terms turns into more like a acceleration tracking term,
+Kd*(feed_forward_rate - filtered_gyro)/DT ...in other words, now the Kd term is now like a proportional feedback gain on angular acceleration.

When it's neither zero nor one, then,
+Kd*(d_setpoint_weight*feed_forward_rate - filtered_gyro)/DT ...in other words, it's either over-tracking or under-tracking angular acceleration. But with, d_setpoint_weight>1, there's the potential for positive feedback. But assuming it doesn't go unstable, just means faster response with more overshoot.
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#8
erikspencer Wrote:[...] The pid.c code does appear to have a confusing term, "smartFeedforward", that can turn off the D-Setpoint Weight, but [...]

Found a discussion here that explains the code implementation well: https://www.rcgroups.com/forums/showthre...st39667667

Turns out that the confusing "smartFeedforward" term has nothing to do with turning off D-Setpoint Weight, it has to do with turning off the P-term if the feed-forward ("Kd*d_setpoint_weight*feed_forward_rate") is already larger than it.

I misspoke when I mentioned the potential for positive feedback.  The feed-forward can cause overshoot and behavior similar to instability--known as derivative kick--but not instability in the classic off-to-infinity controls sense, since d_setpoint_weight is not at all involved in the feedback term.
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