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Re: Re: [Paparazzi-devel] thermopile / rocket

From: Don Shrum
Subject: Re: Re: [Paparazzi-devel] thermopile / rocket
Date: Thu, 25 Jun 2009 09:07:13 -0400

It sounds like I may be able to build simple circuits that detect a voltage greater than X and then open a circuit to actuate a servo.  This would make for a very simple autopilot.  Is there perhaps a way to integrate what I want to do with an existing autopilot?  My thought was that a circuit that actuates based on small voltage changes would make for a much faster response time. 

I am running a space program out of my garage.  My wife is thrilled :)

High is between 35 and 100KM thus the angle of the horizon will be 6 to 10 degrees below.

I suspect the sensor will work just fine even at that altitude.

On Thu, Jun 25, 2009 at 4:02 AM, <address@hidden> wrote:
Hi Don,

> I'm interested in using the Horizontal IR sensor from the Paparazzi
> 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
> axes.  Is there a document that explains that in detail?  Is this
> constantly supplied?  Could someone confirm this is the case?

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
> 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
> 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

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.


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