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Re: [Paparazzi-devel] Critical conditions for IR thermopiles


From: Reto Büttner
Subject: Re: [Paparazzi-devel] Critical conditions for IR thermopiles
Date: Mon, 23 Aug 2010 21:18:20 +0200

Hello all,

thanks for all your field reports. The variety of conditions paparazzi
has been tested in is impressive!

As summary I learn that the IR thermopiles are great when there is no
precipitation and one does not intend to enter heavy clouds. An IMU
will still work unter these conditions, but is more difficult to tune
and filter. As I do not intend to fly in preciptiation or heavy
clouds, I probabely will stick to the IR thermopiles. I take the risk
of not being able to fly or having control degradation in some exotic
meteorogical situations like "hot-air weather front during or after
long light (hot) rain" or "one side of the sky is rainy and another
clear".

Cheers, Reto

2010/8/23 Christophe De Wagter <address@hidden>:
> We have experienced (and sometimes searched) the limits of the IR quite
> often...
> above water, above ice, in mountains, above clouds, inside thin clouds, in
> rain, in fog, ... we have done all at least once the latest years...
> our conclusion:
> most of the above can work, so the thermopiles are certainly useful...
> however all of the above situations do "reduce the IR contrast". This means:
> the measurement is (slightly) less nice (less resolution and more
> calibration issues). It depends on the type of airframe and correctness of
> e.g. your biases whether you will "notice" anything...
> known to work:
> -funjet/microjet above ice and water and above clouds
> -easystar/minimag inside relatively thin clouds and fog
> known to give slight offsets
> -sunset: often circles become ovals when the plane looks straight into the
> rising sun
> known not to work well:
> -rain (water is not good for electronics in general but for sensitive
> temperature sensors it's worse)
> -hot-air weather front during or after long light (hot) rain
> -thick clouds (this is presumed not to work but never seen any flight yet:
> we might check that soon with our IMU plane)
> -Combining things, like in clouds over water... probably will not be nice
> either. (have to check)
> But you can measure the contrast before flying, so usually weather can be
> know in advance. When things go wrong... you will see too sharp or too
> shallow turns, but not necessarily big trouble. e.g. Our EasyStar has once
> succesfully flown through significant clouds, but since the infrared
> measured no difference between left and right, and the plane is stable, it
> kept flying relatively well.
> The IMU has other advantages and disadvantages like filter stability,
> temperature changes causing gyro bias changes causing attitude drift,
> vibrations causing bias changes and gyro bias changes in presence of
> accelerations, startup/warmup times, calibration and alignment (especially
> magnetic) issues etc...
> What to prefer... depends on your application: if you like the ease,
> vibration-proof, instant-boot and fool-proofness of the infrared, or go into
> IMU and Barometer and learn about alignment, drift, filter initialization
> etc...
> In general one can say that the IMU version is required if your platform is
> so unstable that a human can not fly it, (e.g. quadrotors will not fly
> Infrared only) ...
> PS: There are a lot of "IMU" boards out on the market. While most can be
> used to stabilize a quadrotor, do not forget that the filter problems are
> much smaller for hovering or slowly moving quadrotors than in fixed wing
> aircraft as they can keep their nose pointing in the same direction and can
> not fly as fast (acceleration errors). Illustration: if you have a 10degree
> offset in you roll angle and then make a 360 degree turn, that means that
> about 10% of the heading gyro will be erroneously added to your pitch.... =
> over 30 degrees of error. So a constant, accurate and fast source of
> feedback is needed (Accelerometer / magnetometer + GPS). This why many very
> nice IMU boards do not work readily with airplanes... unless you add some
> good code and calibration parameters :-)
>
> On Mon, Aug 23, 2010 at 4:37 PM, Reto
> Büttner <address@hidden> wrote:
>>
>> Hello all,
>>
>> has anyone experienced problems with IR thermopiles in flight?
>>
>> I am thinking of exotic meteorological conditions where the IR
>> contrast between sky and earth might be to small. Or when flying in
>> fog or clouds. Or when flying in mountains close to terrain. My
>> paparazzi planes with IR thermopiles have worked fine until now, but I
>> just have been flying in nice weather conditions. I have read a lot
>> how robust IR thermopiles are to various meteorological conditions,
>> including flights in the arctic.
>>
>> On the other hand many people in the community are not quite sure,
>> prefer IMU solutions and there are rumors of projects that have not
>> worked due to IR thermopile issues. I appreciate the efforts of
>> numerous people to integrating an IMU into paparazzi, but for my
>> project I am not sure if it is worth the effort.
>>
>> Has anyone experienced the limitations of IR thermopiles?
>>
>> Reto
>>
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>
>
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