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Re: [Paparazzi-devel] Project planning


From: Stephen Dwyer
Subject: Re: [Paparazzi-devel] Project planning
Date: Fri, 18 May 2012 00:13:41 -0600

Hello,

I would suspect the ParkJet should be fine, seems rather similar to a
Multiplex FunJet, which is a very common Paparazzi airframe. I would
agree with Hector though, if you don't have experience with flying,
then choose a more stable, trainer-like airframe.

As Hector mentioned, IMU drift can be caused by several things. One
thing is that some AHRS algorithms do not take into account the
centripetal acceleration in turns (faster, more agile aircraft will
experience greater acceleration), which will cause improper attitude
estimates. Most of the AHRS code in Paparazzi uses GPS velocity
measurements to correct for this, so it shouldn't be an issue. Perhaps
someone with more knowledge and experience can elaborate.

Regarding IR vs IMU, generally a well designed IMU provides better
dynamic response due to the time constant of the thermopile
measurements with the IR system. In agile aircraft, IR systems are
sometimes supplemented with gyros. To continue Hector's list, another
consideration between the two can be vibration (IMUs can be very
sensitive to this). Shouldn't be an issue in a small electric though
(compared to say a glow or gas engine).

Hope that helps!

Thanks,
-Stephen Dwyer

On Tue, May 15, 2012 at 1:26 AM, Hector Garcia de Marina
<address@hidden> wrote:
> On Tue, May 15, 2012 at 7:30 AM, Stuart MacIntosh
> <address@hidden> wrote:
>>
>> Hello and thank you to everyone who has helped this project become what
>> it is!
>>
>> At the moment I'm planning how to best mate my airframe and a paparazzi
>> system, and I have a few questions.
>>
>> I'm hoping to fly a blended wing body (Hobbyking ParkJet) and it's a
>> pretty fast wee thing, although it has a light wing loading and still
>> glides with a modest stall speed. Does the problem of drift in IMU's
>> become more apparent with an increase in velocity? (ie; is this thing
>> going to be too fast?)
>
>
> You have to look at the curves of your accelerometers provided by the
> manufacture,
> but the great issue here is the temperature.
> The MEMS gyros can change their reading depending on the acceleration, not
> on
> the velocity, but as before, the issue is the calibration over temperature.
>
>>
>>
>> Would thermopile-style attitude measurement be more suited to a faster
>> vehicle, and what are the dis- and advantages to these two attitude
>> measurement techniques?
>
>
> If you have a clear sky, and you are not flying over water, it depends on
> the bandwidth of your thermopile. Some users here can tell you that
> this sensors depends highly on your weather, terrain, etc. The great
> advantage is that it does not depend in any non-inertial effect.
>
> An IMU can not measure the attitude by itself, it needs a fusion algorithm
> such as DCM or Kalman Filter. The big issue here are the drifts,
> calibration, etc
> (you have to calibrate your thermopile as well). The good news are that IMU
> does not depend on the terrain or weather.
>
>
>>
>>
>> Should the IMU be located at the airframe's center-of-gravity?
>> (experience with race-car datalogging would suggest so) and if it cannot
>> be; is a correction factor needed?
>
>
>
> Yes, you should, otherwise, your acceleration are not with respect the CoG,
> you should
> remove them with the known distance to the CoG and the angular velocity.
>
>>
>>
>> This is a fun way to learn about control systems, navigation and flight
>> dynamics!
>
>
> If this is your first experience flying (and moreover with UAVs) I do not
> recommend you
> going so fast and start with a well-known trainer :P
>
>
>>
>>
>> Cheers,
>> Stuart.
>>
>> _______________________________________________
>> Paparazzi-devel mailing list
>> address@hidden
>> https://lists.nongnu.org/mailman/listinfo/paparazzi-devel
>
>
>
>
> --
> Héctor
>
>
>
> _______________________________________________
> Paparazzi-devel mailing list
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>



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