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Re: [Paparazzi-devel] Using Thermopiles for AHRS on Lisa/M instead of IM

From: Chris Wozny
Subject: Re: [Paparazzi-devel] Using Thermopiles for AHRS on Lisa/M instead of IMU
Date: Thu, 5 Apr 2012 19:34:49 -0700

It does seem silly to use all of them, but I can see some sort of
niche fields where you'd be using both. It makes me wonder whether
there could be some sort of use for Kalman filtering the attitude from
the thermopiles and the IMU and statistically determining which is
better (I believe this statistical analysis is inherently apart of the
Kalman filter from what I've learned in classes.)

Also, thinking about the defines, I'm struggling to think of a reason
you would use IMU and IR together, so maybe just assume that (on top
of the actual define of ADC_CHANNEL_GYRO_P|Q|R) if an IMU is defined
and your AHRS is defined as INFRARED, then activate the gyros (and
maybe magnetometers?). Something like:

#ifdef (ADC_CHANNEL_GYRO_P || ( IMU && AHRS_INFRARED)) //i'm not sure
what the specific defines are for IMU and AHRS as infrared and if you
can even use more complex conditional group preprocessor macros in C

Any thoughts?

- Chris

On Thu, Apr 5, 2012 at 1:46 PM, Felix Ruess <address@hidden> wrote:
> Hi Chris,
> something similar is currently implemented... If you use ahrs type="infrared" 
> and have analog gyros defined then you get something like that (without gyro 
> bias estimation).
> You could pimp sw/airborne/subsystems/ahrs/ahrs_infrared.c to do that...
> To use it with aspirin (and without gyro bias estimation) you would only have 
> to remove the #ifdef ADC_CHANNEL_GYRO_P, etc.
> But we would have to replace that with a nicer way of which gyros are 
> available (if you don't have an IMU with full 3-axis gyros, but just separate 
> ones, maybe only roll, like in the old days...)
> You could also estimate the gyro bias using the infrared sensors as absolute 
> reference... you could even combine it with accels and use IR if they are too 
> bad and vice versa.
> I just didn't think it was worth the effort as you are not likely to use 
> both...
> Cheers, Felix
> On Thu, Apr 5, 2012 at 10:35 PM, Chris Wozny <address@hidden> wrote:
>> Christophe,
>> Something you said made me curious. Is it possible to use the Aspirin
>> IMU solely for gyro stabilization if thermopiles are being used for
>> attitude estimation? I know we've used 3-axis gyros in the past with
>> Tiny v0.99 for roll/pitch/yaw dampening while using IR, but I don't
>> know if this is any different since the gyros are on the Aspirin IMU.
>> Best,
>> Chris
>> On Tue, Apr 3, 2012 at 12:05 AM, Christophe De Wagter
>> <address@hidden> wrote:
>> > Hi,
>> >
>> > The default assignment of adc should be written in
>> > conf/board/lisa-m.makefile
>> >
>> > There might still be the issue that ADC_1 is called 0
>> > USE_0 instead of USE_ADC_1
>> >
>> > For the gyros: the value of the gyro is only distorted a few percent. So 
>> > for
>> > simple stabilization like a roll damper the aspirin is good 3noug even with
>> > vibrations. The problems start when you start integrating, which is not
>> > needed for damping but required for attitude. Then all errors accumulate.
>> > And a gyro error of 1% (sounds like a small error) on a 2000 degree/sec 
>> > gyro
>> > gives 20 degree of additional attitude error PER SECOND. This is corrected
>> > with accelerometers. However the accelerometers also measure vibrations.
>> > These can be several times bigger than gravity. This means that at times 
>> > the
>> > accelerometer even could measure that the earth is up. And low pass
>> > filtering them too much gives latency. And that again gives correction
>> > errors:  e.g. if you suddenly roll 90 degrees, but the accelerometer 
>> > follows
>> > with a delay, initially the filtered accelerometer will measure a huge 
>> > error
>> > and estimate this is due to the gyro bias.
>> >
>> > Did you try to mount the lisa on 4 corners of 3x3x10mm pieces of tempur 
>> > foam
>> > and use very thin flexible cables?
>> >
>> > On Apr 3, 2012 5:06 AM, "Chris Wozny" <address@hidden> wrote:
>> >>
>> >> All,
>> >>
>> >> For reasons outside of my power (trying to dampen vibrations in tiny,
>> >> tiny aircraft leads to massive hair loss and frustration) I've got to
>> >> resort to using IR for the AHRS on our aircraft instead of Aspirin
>> >> v1.0. We're using Lisa/M and I was wondering: are there any default
>> >> assignments for which ports to use for thermopiles like with Tiny? The
>> >> wiki says there should be, but I have a feeling Lisa was designed
>> >> without thermopiles in mind since an IMU would work much better.
>> >>
>> >> Also, why do analog gyros (more specifically Futaba Mini Piezo gyros)
>> >> work great for stabilizing the same plane with the same vibrations
>> >> that cripple the use of Aspirin v1.0 for AHRS?
>> >>
>> >> Best,
>> >> Chris
>> >> _______________________________________________
>> >> Paparazzi-devel mailing list
>> >> address@hidden
>> >>

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