RE: [Paparazzi-devel] sky uniformity and roll/pitch bias

[Electro-Technik]

Steve,

It is good the hear about a system going so well.  Are you based in the UK?

Regards.  Alan K.



-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Steve Joyce
Sent: 02 October 2008 16:35
To: address@hidden
Subject: [Paparazzi-devel] sky uniformity and roll/pitch bias

Greetings,

I've been using paparazzi for some time now for some fairly precise 
navigation tasks (photo missions) with quite good results.  I think I have 
my aircraft set up well and the control loops well tuned.

Under some conditions though, the ability to hold course and altitude can 
become poor, even to the point of loss of control.  My conclusion is that 
the distribution of clouds and clear patches in the sky can set up a thermal

bias or "tilted horizon" which gives biased angle estimates.  The effect 
manifests itself by the plane overshooting turns on one heading and 
undershooting them in the opposite heading, and when following lines it may 
never quite reach the desired course, but fly along parallel to the line.

This is a bit different than when you have a roll bias due to misalignment 
between the sensor axis and the aircraft axis:  here it will fly a constant 
offset inside a circle in one direction and outside the circle in the other 
direction, but there are no course dependent effects.  It will also tend to 
turn left or right in AUTO1.

It seems that vertical contrast alone is not such a problem, as long as the 
sky is fairly uniform- it should only affect the resolution you can measure 
the angles.  But the worst case seems to be a mostly clear sky with low dark

(warm) clouds approaching from one direction.

I searched the list and haven't found much discussion about this phenomenon,

except from some people who thought that a low sun could bias the sensors (i

haven't personally found this to be much of a problem).  So I was wondering 
if anyone else could share their experiences, solutions, control system 
changes, or tuning strategies to minimize the effect.  I guess I would like 
to expand the envelope of weather conditions that the plane can navigate 
effectively.

My setup is an electric flying wing with the a stock FMA copilot horizontal 
sensor mounted on top of the fuselage and a vertical sensor mounted near the

back.  The horizontal sensor is oriented at 45 degrees.  I run the sensors 
at 5v to maintain the stock gains but the output is run through a voltage 
divider before being read by the autopilots ADC.

Cheers,
Steve 



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