Re: [Paparazzi-devel] (no subject)

[antoine . drouin]

Quoting xx xx <address@hidden>:


> > What kind of mouse are you talking about ? sloted disk and infrared fork ?
> > How do you use this ?
> Sloted disk, i have found a similar modification on a internet page, i can
> seek it out or make some images from one of my modifications.
> Basically, you can modify such a mouse to be a really accurate roll/pitch
> sensor. The tilt sensor is used to calibrate the mouse and to map the
> relative information of the sloted disk to a fixed 12 bit position.

That is very clever. I would appreciate to see a pic or plan.

But I think that what you are measuring is acceleration (~forces). 
It can indicate attitude (pitch + roll) only when the plane is flying a level
straight line at constant speed (uniform mouvement?).
For example, when the plane is flying a stabilized symetrical curve, the
acceleration is exactly perpendicular to the plane, just as if it was level.

Am I missing something ?


> If i see this correctly, you can archive this cheaper using cheap CDS 
> photo sensors with a polarisation filter.

I don't know. The Melexis chips are non contact temperature sensor. There are
details on these components in this paper
http://www.ctie.monash.edu.au/hargrave/horizon_sensing_autopilot.pdf 

If you try with cd sensor let me know :)


> Can you tell me more about the fine tuning and calibration .

We get the attitude of the plane using difference of temperature.

We have two pair of sensors, one "looking" front and back, the other one left
and right.

T1 is the difference of temperature between right and left
T2 is the difference of temperature between front and back

roll = K * T1
pitch = K * T2

We get an initial K from a contrast measurement before the flight. Then, during
the flight
we compute a roll from the GPS heading variation rate, assuming we are flying
symetrical curves.

We use a least square algorithm to fine tune K.

(this is in lls.c)

This is not very important for just stabilizing the plane, as the low level loop
is very tolerant regarding gain setting. But the upper level navigation loop is
far less tolerant and this really improves things. It also adjusts for fast
changing contrast conditions, like sunrise.


Hth

Antoine