[Paparazzi-devel] Re: Paparazzi-devel Digest, Vol 33, Issue 1

[Filipe Varela]

Hi

> So I'm putting together a shopping list for a helicopter autopilot  
> based on
> autopilot.sourceforge.net, http://www.sparkfun.com,
> <http://www.sparkfun.com> and anecdotal information on the
> internet.  Sorry but can't find any implementation notes on
> http://savannah.nongnu.org/projects/paparazzi/.
>
> Hovering a helicopter seems to require a 3-axis accelerometer and a
> 2-axis gyroscope.  Autopilot.sourceforge.net claims they hovered a
> helicopter on just this information.

Consider a 6DOF unit (3 axis accels + 3 axis gyros)


> One problem isn't being addressed anywhere:  How do the autopilots
> compensate
> for the drift in the gyroscopes?  Do the accelerometers compensate for
> it?  Do they need a GPS
> to reset periodically?  Do they just have a time limit before  
> requiring
> pilot intervention?

Drift is addressed with a kalman filter. It's a technique whereby you  
take a highly-realiable angle estimate created from accelerometers  
(which is only realiable on the long run, as they don't update  
frequently and suffer interference from linear accelerations) to  
"periodically" reset the body frame state to a point where you can  
trust gyros again.

Say a gyro becomes inacurate after 3 seconds. If you feed the  
accelerometer based attitude estimate to the filter every second,  
your gyros will never become inacurate. They'll be the "only"  
reference you will use between the slower accelerometer estimates.

Mind the "". There are many parameters that can be tuned in such a  
filter. What i wrote is a high-level approach with some inacuracies.  
The purpose is to explain the process, not the actual details.

I suggest you do some googling for complementary filters, kalman  
filtering and study some linear algebra before attempting/spending  
money on such a project, or go for integration only and buy paparazzi  
hardware for your model.

These projects can become pretty expensive and you won't want to  
regret starting one after spending 1000+ EUR/USD would you?


Good luck,
Filipe