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Re: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2,

From: Mitchell SLOWER
Subject: Re: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2, only after launch.
Date: Sun, 9 Sep 2012 22:23:30 +0200

Dear all,

I did a flight this morning, it was very similar to Jefferson's flight (holding 15° pitch and 100% throttle after bungee launch, then engaging standby), to see if I can reproduce the behavior Jefferson decribed.

What happened was exactly what Jefferson wrote: the plane hold really well the first part, quickly gaining altitude after bungee lauch, then when it switched to the standby block, plane went into a violent dive of about 40m, then went back in a normal flight.

I had a look at the logs, and linked some screenshots below.

Note about these log screenshots: 
- At 320s: bungee launch
- At 343s: switch to standby block
- At 390s: standard flight (from here, everything is ok,  problem does not happen anymore).

Logs screenshots:

1/ Desired altitude / estimated altitude (z):

Comment: this graph  shows the altitude increase right after bungee, then the dive when switching to standby, and the altitude that decrease at a lower rate then stabilizes and went up again.

2/ Desired climb / Estimated climbrate (zdot) are plotted:

Comment: climbrate goes from 6m/s (setpoint) to 0m/s when standby block engages. It's interesting how the climbrate increases over time, slowly getting back to the setpoint. There might something here to look at.

3/ Desired pitch / attitude (theta) are plotted:

Comment: to Felix: you were right, attitude:theta went up to 40 degrees during bungee takeoff. That's another subject, but interesting to mention here. Otherwise, we clearly see that the control loops are asking for a pitch down when switching to standby block. This is not normal, as altitude setpoint > current altitude and airspeed > airspeed setpoint.

4/ Airspeed / Airspeed setpoint:

As mentioned before: when switching to standby block, airspeed is over the airspeed setpoint, so the plane does not need to pitch down in order to gain airspeed.

To summarize: 

- Problem can be reproduced, on different planes. Both planes were launched using a bungee / catapult.

- It seems not to be AHRS related as plane can hold a steady pitch attitude right after bungee launch.

- It seems to be airspeed specific control loops related, because these are not used when flying at a fixed pitch/throttle (vmode=throttle) but used in the standby block, and we see on the graphs that control loops are asking for a pitch down instead of a pitch up right after bungee launch.

- Can anyone explain the desired climbrate step down (from 6 m/s to 0m/s at 324s), then gradually increase here: ?


Le 9 sept. 2012 03:38, "Romney, Jefferson S" <address@hidden> a écrit :
I have had a similar problem that I think is the same cause. I take off in Auto2 with a catapult and it holds 15 degrees pitch until it gets to altitude. (that means it must not be the AHRS) then as soon as speed control kicks in when it moves to the standby block, it dives very violently it recovers after losing about 10-30 meters altitude, then flies normally. It must be in the airspeed or altitude loops because those are the only ones not active on takeoff when it works just fine. I haven't been able to figure this one out all year so if anyone can, it would be great.

-Jefferson Romney

From: paparazzi-devel-bounces+romneyj=address@hidden [paparazzi-devel-bounces+romneyj=address@hidden] on behalf of Gautier Hattenberger [address@hidden]
Sent: Saturday, September 08, 2012 3:16 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2, only after launch.


Maybe you can look at this
It looks very similar.


Le 08/09/2012 23:10, Mitchell SLOWER a écrit :
I think you are right Eduardo.
The attitude estimation when taking off with bungee might be not perfect, we should try the int_cmpl_quat_gravity_heuristic written by Felix.

As I said before, I thought it was some I-term winding up, so I tried to decrease, even set to zero the two I-terms used in the control loops: v_ctl_auto_pitch_igain and v_ctl_auto_airspeed_igain. No improvement.

Improvement happened when I commented out in the config file the auto pitch inner loop parameters: V_CTL_AUTO_PITCH_PGAIN et V_CTL_AUTO_PITCH_IGAIN. The plane was not diving dangerously anymore, but not getting altitude, just gliding on its track.



2012/9/8 Eduardo lavratti <address@hidden>
Felix, the behavior that mitchel say is completely different than the takeoff with bungee.

BTW, some week ago i tested the auto takeoff using bungee and this behavior not occur.
because of this i think its a integrator problem.

i see int the code that some integrator are reset when manual are engaged.
I not remember if it is a throttle or climb integrator.

Date: Sat, 8 Sep 2012 22:47:53 +0200
From: address@hidden

To: address@hidden
Subject: Re: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2, only after launch.

I'm pretty sure that the AHRS_GRAVITY_UPDATE_NORM_HEURISTIC in 4.0.1 has almost no effect compared to not having it defined.
Whereas the improvements in my int_cmpl_quat_gravity_heuristic clearly should help a lot (if the acceleration of the bungee takeoff is indeed the problem).
At least in simulation I see a large (temporary) pitch error of up to ~20deg before the improvements (of course depends on duration and acceleration in bungee takeoff) before the improvements.

On Sat, Sep 8, 2012 at 10:42 PM, Mitchell SLOWER <address@hidden> wrote:

I did a bungee test in AUTO1 with AHRS_GRAVITY_UPDATE_NORM_HEURISTIC (4.0.1). It worked.

2012/9/8 Felix Ruess <address@hidden>

the pitching down after a bungee takeoff is definitely noticable, however from what you describe I'm not sure if that is the only effect you are seeing.

But please note that AHRS_GRAVITY_UPDATE_NORM_HEURISTIC in v4.0 does not really have a big effect.
As already mentioned previously in
you can test the improved gravity norm heuristic from my int_cmpl_quat_gravity_heuristic branch which is up-to-date with v4.0 otherwise.

Would be great to get some feedback on this as I can't test this with a real bungee takeoff myself.

Cheers, Felix

On Sat, Sep 8, 2012 at 10:19 PM, Eduardo lavratti <address@hidden> wrote:
I have the same behavior.
i am using float_dcm and bungee but i think this is not due bungee.

this behavior occur if i engage auto2 when pitching up.
If i pitch up, stabilize altitude and engage auto2 this behavior not occur.
this seem that some I term are working but i not have time to search the cause.

i have to make this test.
takeoff in manual then engage auto2.
this will reset the integrator.


Date: Sat, 8 Sep 2012 21:57:52 +0200
From: address@hidden
To: address@hidden
Subject: Re: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2, only after launch.

Marc, Christophe: initially, I thought it could be an AHRS related problem, but I'm think it's something else.
(I'm using int_cmpl_quat and the  AHRS_GRAVITY_UPDATE_NORM_HEURISTIC flag).

I think it's not related to AHRS because:
- I had a look at the estimator (theta), and pitch seems to be correct.
- If I lauch with the capatult, fly some time in manual, and engage AUTO2, diving also happens.
- When I disable the airspeed from the airframe config file, everything seems to work (I say "seems" because I did only one try that was successful).


2012/9/8 Christophe De Wagter <address@hidden>
We have implemented the acceleration feedback in 

It has flown several times, including catapult launches but with the complexity of all aspects involved (e.g. gps latency etc.) that is not a guarantee. If you would be so kind to help test it it would be highly appreciated. 

copy the file to your repo, add USE_AHRS_GPS_ACCELERATIONS to your airframe (should pop a message during compile)


On Sat, Sep 8, 2012 at 8:58 PM, Marc Schwarzbach <address@hidden> wrote:
Hi Mitchell,

your problem seems to be an AHRS calculation thing related with the high acceleration at bungee take-off.
See the conversation one month ago: Subject: Re: [Paparazzi-devel] int_cmpl_quat and bungee takeoff

As a short note, I had a similar, but very light behaviour when hand throwing my Easystar also with Lisa/M. It does a little pitch down before starting to climb after the throw...

Probably we should look into the ACC signals for fixed wing..



-------- Original-Nachricht --------
> Datum: Sat, 8 Sep 2012 20:37:18 +0200
> Von: Mitchell SLOWER <address@hidden>
> An: address@hidden
> Betreff: [Paparazzi-devel] Strange behavior: pitch down when engaging AUTO2,  only after launch.

> Dear all,
> I encounter a really strange behavior on a fixedwing.
> I had a look at the logs, made a lot of flights in order to find out
> what's
> wrong, but I really don't understand what's happening.
> So here is the situation:
> First, the setup:
> Lisa/M 2.0, PPZ Version: 4.0.1. I'm using an airspeed sensor,
> I'm launching the plane with a bungee, in manual mode.
> Before launch, current clock is set to circle over home, 150m AGL.
> What's happening:
> After launch, when I change to AUTO2 , the plane does not pitch up with
> motor on as expected in order to reach the desired altitude, but pitches
> down, gaining airspeed and loosing altitude (I checked that airspeed is
> near or over desired airspeed when I switch mode).
> As things get dangerous, I switch to MANUAL, get some altitude, engage
> AUTO2, and the same behavior occurs.
> Then, after a few tries like this, everything works ! And it works really
> fine, as I have a very good airspeed control and climb rate.
> After that, I can switch from MANUAL to AUTO2, and the plane behaves
> normally (get some altitude at the right airspeed and climbrate).
> I did a lot of tests (I initially thought of a I-term winding up), all
> were
> unsuccessful.
> The only clue I found: this problem might be related to the airspeed
> specific control loops, because when I disable the airspeed sensor from
> the
> airframe config file the plane seems to behave correctly (no pitch down
> when engaging auto2).
> Did anyone encounter the same behavior ?
> Do you have any idea about what could be happening ?
> Of course, I can provide further details, setup and logs.
> Regards,
> Mitchell

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