Re: [Paparazzi-devel] =?ISO-8859-1?Q? thermopile / rocket?=

[martin]

Hi Don,

> I'm interested in using the Horizontal IR sensor from the Paparazzi
project
> in an amateur rocket.

sounds interesting.

> I am assuming that pins 1 and 4 supply a positive or negative variable
> voltage that relates to the direction and angle of bank on each of the
two
> axes.  Is there a document that explains that in detail?  Is this
voltage
> constantly supplied?  Could someone confirm this is the case?

The theory can be found at
http://paparazzi.enac.fr/wiki/Theory_of_Operation#Infrared_Sensors

The Eagle schematics are in the svn. The thermopiles are connected
head-to-head on the board so that they give a zero voltage if they see
the same IR radioation or a positive/negative if different. The output
is amplified with an op-amp. As we do not have a negative supply voltage
we operate with a virtual ground at Vcc/2. So you will get Vcc/2 (e.g.
1.65V with 3.3V supply) at same radiation and Vcc or GND at the maximum.
The thermopiles have some 20-30ms delay in reacting to changed
radiation. Not sure at what speed you need to run a control loop on
board a rocket.

> I have been told these sensors may not work at high altitudes.  I
assume
> this might be because the 'image' seen by the sensors will become
> proportionally more sky as altitude increases and thus, the difference
> between the opposed sensors might become less.  Is this the case?  Is
there
> some other reason the sensors become less effective at altitude?

How high is high? We have operated Paparazzi aircrafts successfully (in
a cleared airspace) at 3500m altitude above ground. The portion of sky
will not change proportional. You can calculate the angle of the horizon
line:

alpha = arccos( r / (r+h) )

r = 6366km (radius of earth)
h = altitude in km

In 3.5km you get a 1.9° lower horizon, at 30km it will be 5.5°. You will
still have a balanced view at this altitude and you will get less IR
radiation from humidity in the sky part of your view as air will have
lower pressure and contain less water.

Martin