[Paparazzi-devel] RE: Paparazzi-devel Digest, Vol 43, Issue 10

[Wagter, C. de]

Hi David,
 
in order to fly in clouds (sometimes people want to do for instance atmospheric 
measurements in the boundary layer) we are trying to by-pass the 
state-estimation problem. Since the small sensors give just a little to much 
noise, and processing is just a little bit too slow we are trying to find 
controllers that use direct sensor-actuator feedback only.
 
The state estimation becomes exponentially more difficult with smaller 
platforms:
 
1) low weight means less weight for shielding, damped mounting, -> more noise
2) small size means everything is packed closer together -> more interference 
problems
3) small vehicles respons quicker -> need faster update rates (>100Hz to 
integrate gyros)
4) small vehicles have bigger turn rates (>500deg/sec) -> need for extra 
measurement resolution (-> 16bits ! / 24 bits ?) 
5) low power/size imposes high processing contraints: fixed-point processing... 
low mem: with 3 Watts of power and 120 grams: the filtering is no problem at 
all... but at 5gram 100 miliWatt... its a little more difficult :-)
 
My experience is that filters that work perfectly on 5kg rotorcraft become 
almost useless in MAV (unless you only fly straight forward at constant speed)
 
So: With a little help from the aerodynamics (relatively stable a/c), this 
direct gyroscope to actuator feedback seems to be very usefull: The (big) 
EasyStar for instance flies very well on 2 gyroscopes and 1 differential 
pressure sensor. So only 3 sensors (just a little more expensive than the IR 
sensors). With this configuration you can fly through any kind of smoke, cloud, 
or in between buildings (which I do not recommend). The pitch loop is a little 
less accurate than with the IR, so I do not recommend flying extremely low (a 
few meters above ground), but the processing even fits in a 8-bit 
microcontroller. (Unlike any 17 state extended kalman filter)
 
Hope this helps,
 
Christophe

David said:
>
>Thank you Simon (and Anton),
>
>But, what if I wish to fly:
> - at night
> - in zero visibility (i.e. fog, rain, smoke)
>
>I have tried for days to google how to determine attitude using different
>sensors. It seems the easy/popular is to use IR. I do now understand the
>principal. I even ordered FMA direct 2-axis and single axis IR sensors to
>use to at least get started.
>
>However, really, wouldn't a MAV truly be very useful for flying in
>conditions nothing else could fly in? Dense smoke? Fog? Moonless Night? I am
>searching google with some luck to research.
>
>I just keep thinking that there must be some way to tell up from down using
>the earths magnetic field or gravity. I'm no unhappy with IR just trying to
>think outside the box if there is a way without thermal limitations..even in
>theory.

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