RE: [Paparazzi-devel] barometer or not?

[gisela.noci]

Hi Vassilis.

I have been following your efforts with interest. We have a 40 and 60 size
highwing trainer that we have been flying extensively with the ap. Autopilot
stuff. The 40 trainer has I guess about 60 or 70 flights and the system
works great. We have however developed our own ground station system since
we have specific needs for payload management, flight plans management, and
we wish to certify some of this for with our regulatory bodies hear in
Namibia. 

We have been flying now for some months and many flights, using pressure Alt
as our Altitude control sensor, rather than the GPS slow update. We sample
the sensor and compute Alt at 60Hz ( the cycle at which all our sensors are
sampled and computations done ) , and then do the actual altitude command
updates ( ie, the pitch from altitude loop). We have modified the Pap. Way
of doing things considerably and do not control altitude via airspeed, but
have adopted more conventional control, with elevator from pitch command,
aileron from roll command,  throttle from airspeed command as our inner
loops.
 
I am not sure what you mean by 

"to get the full benefits of  
the barometric sensor we should update at 10Hz or more"

Do you mean to update the inner loops at 10 Hz or just sample the sensor and
compute the result (Alt) at 10Hz, ensuring that when the inner loop command
update occurs, that it is using the very latest sensor status (a latency of
100ms max)? Updating the inner loops at 10Hz would seem excessive, as the
elevator, etc, could hardly be expected to respond much in 100ms, in fact,
most cheap analog servos are as slow as this anyway.

We have achieved very good flight results, with altitude control within 3 to
4 meters of setpoint, flying at 22m/s, with wind speed of 8 to 11m/s.
Horizontal control is equally good, with circles and figure eight patterns
yielding track errors of less than 3meters in similar winds. We do auto
take-of from standstill and it works perfectly. We are now working on the
autoland, using ultrasound sensors for the AGL sensing, and the autoland
sequence (spiral till 50m AGL, speed = 20m/s, the exit on the spiral tangent
to the specified touchdown point, as airspeed of 18m/s, then kill throttle
at 4m AGL and proceed to flare point at touchdown) works very well - have
done many 'virtual landings' at 20meters up, just need good AGL sensing! I
have, at the virtual touchdown point, switched to manual (throttle closed)
and just held the A/C till the flare point and touched down with ease, so
soon-soon!

Anyway, Baro-Alt is the way to go; 200ms for alt update is not great, and
who know what data lag is in the GPS internal computation of the Alt
anyway..

Have fun

Joe

-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Vassilis V.
Sent: Sunday, November 01, 2009 9:49 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] barometer or not?

My 2c for the barometer, I used the ETS sensor and not the SCP1000 but  
they should behave similarly.
- You will have to update the Kalman parameters very likely, the ones  
used for the SCP1000 did not work for me (too much filtering and lag).  
I updated SVN with what currently works fairly well for my setup (see  
R and SIGMA2 constants in baro_ets.c).
- The altitude loops are updated at 4Hz, to get the full benefits of  
the barometric sensor we should update at 10Hz or more. I have not  
made that change yet in the code, I am still running at 4Hz.

Vassilis


On Nov 1, 2009, at 12:19 PM, antoine drouin wrote:

> On Sun, Nov 1, 2009 at 6:04 PM, Chris <address@hidden> wrote:
>> I just finished rewriting all the SCP1000 files including the use  
>> of the
>> barometer as an alternative altimeter for the autopilot.
>> The files affected are the baro_scp.x ,estimator.x and main_ap.x
>> In simulation it works fine but i was wondering if the reading of  
>> this
>> barometer are more accurate than the gps altitude especially know  
>> with EGNOS
>> support.
>> Any opinions?
>
> It's not so much about accuracy than latency. GPS is filtered, so
> lagging badly and also low bandwidth.
> You have some GPS/baro data recorded from a couple of years ago in the
> svn in the sw/in_progress/barometer directory if you wanna plot or
> play with filters
>
>> I am also writing some new files for the  SRF08  for use in auto  
>> landings
>> now that after my heart problems i got me a nice two months leave.
>
> I never got anything interesting from a ultrasonic sensor on fast
> moving vehicles  (read fixed wing)
>
> Regards
>
> Poine
>
>
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