I am trying to fine tune PID's on my quad and have two questions:
1. In Acro mode does raising the D value make it less likely to overshoot when coming out of forward flight? The wiki seems a little ambiguous on this one. It is entirely possible I don't understand what is written here, but raising the D term seems to make the quad transition out of forward flight much smoother (in ACRO mode)
" A negative D value is used to help the ArduCopter change faster to a level position after forward flight."
AND
"Once you are happy with hovering performance, try moving the ArduCopter into forward flight, then stop to a hover. Does the ArduCopter dip too much when transitioning from forward flight to a hover? If it does, the P gain may be overcompensating (too much motor power) when transitioning to a hover. To reduce this, apply a negative D value (try decreasing it by decrements of -5) to reduce this effect."
It looks like this might be an old out of date webpage that is apparently still live. I thought it was pretty useful however.
2. In Stable mode when transitioning out of moderate or greater forward flight (by simply letting go of the stick) the quad tilts totally the opposite direction and comes to a grinding halt. I would like it to smoothly level itself and continue to glide forward (like a normal real RC helicopter) What should I change?
Thanks!
Ted
Attached is my current PID config screen
Replies
Hey guys,
So glad I found this thread - I knew there's was something odd about how acro mode stabilizes itself. I also come from a heli background and after years on the sticks there - watching the quad snap back to level in acro mode just left me baffled. I've spent the last 3 flying sessions fighting this and although I've gotten it close to where I want it for the type of flying I was expecting - I still fight it on the turns and during stops.
I realized it wasn't me after I saw this video:
http://www.openpilot.org/pid-tuning/
If you skip to 6:00 he does a demo where he locks one of the axis. He's running a split screen with throttle on the left and ael/ele on the right with his quad tied to two chairs. At one point he pushes forward then lets go of his stick, and you can clearly see the quad still holding it's forward angle. That's precisely the behavior I'm used to in helicopters.
Anyway - the bird's flyable, just not what I was expecting for acro mode. What I can say though is that 2.8's running very smooth for me generally speaking, moreso than the previous release, so the improvements definitely helped.
My setup:
HT FPV
NTM 2826 1200s
8x4 CF props
3300 3S batteries
AUW: 1350g
My latest PIDs
STAB
P: 3.5
I: .010
D: 001
ACRO
P: .063
I: .020
D: .002 <--- (FYI .008 nearly flipped keep this number low!)
AXIS_ENABLED = 1
I tried lowered Rate P to mitigate the recovery snap, all it ended up doing was making all the controls sluggish. Guess we'll see where this goes.
I just flew today and can't come back with good results. I have my quad flying wonderful in most respects However, it still pitches opposite the direction of movement, its does it worst forward and back. Worse yet, if I keep it moving slowly for an extended period (I like to fly back to my house, about a hundred yards and walk behind it) thus meaning 2-3 min of slight forward pressure, if I let go it pitches back significantly, far more than it was pitched forward, then quickly levels. If you are trying to fly FPV this is a no-go item. I would like to just push my trim forward when FPV flying but can't because the Arducopter fights these long term inputs (stabilze mode).
I am convinced this must be in the code, I have ramped my Stab and Rate I from 0 - 4 with no discernible difference in flight characteristics. I have lowered and raised my imax values from 0-5 with the I term at 0-4 with no flight changes. Are these values commented out in the latest release or something? I have changed the Stabilize D term from 0 - 4 with no changes, I have adjusted the "Stabilize D schedule" from 0 - 100 with no change (cant find documentation on that setting). Maybe this isn't a problem, maybe it is designed to work this way in the code, but I would prefer we had a mode that did not do this.
Just from my observations, the Stabilize code seems to operate as follows:
-A stick input commands a specific angle.
-If the input is held, the angle slowly scales down.
-If the stick is suddenly let go (a zero angle command) the scale addition still exists, and the quad is actually commanded in the opposite direction, which then ramps down to zero.
-When a tilt is commanded the motors automatically ramp up to prevent the helicopter from descending. This commonly causes my quad to rocket climb when letting go of the sticks, I have to bring the throttle way down until the scale command drops and then bring the throttle back up.
**Josh: have you seen this adjustment to acro mode (axis enable)? It basically looks like the quad will fly around in rate based mode with a slight bias towards leveling itself. (perfect!)
http://code.google.com/p/arducopter/wiki/AC2_AcroMode
It looks like something that you and I might like flying in, but I can't find the settings for it in the mission planner...
Increasing my D values for both acro and stable mode seems to help a lot....but the value is VERY VERY sensitive. I've been creeping up by .001 at a time.
Sometimes its hard to tell if its the wind or the Ardupilot causing the quad to tilt when transitioning out of forward flight. I have 12" props on a very light helicopter, so the wind really gets under there. Either way, increasing D is helping, I'm going to keep on creeping it up.
Anyone? This seems like a normal question, am I in the wrong forum?
How does it behave in the non-stabilized manual mode?