The IR sensors used are sensitive to any material of any temperature
above absolute zero. The emitted frequency is dependent on the
temperature of the material and objects of common "earthly" temperatures
typically emit radiation in the range of 8-12µm and therefore are not
affected by objects of higher temperature, such as the sun. This range
of IR passes easily through smoke and rain but can be blocked with a
simple piece of clear tape. We connect the sensors in geometrically
opposed pairs and wire them together backwards - output-to-output, such
that the system will produce 0V if the sensors are of equal temperature,
and any difference between temperatures creates either a positive or
negative voltage the voltage produced is extremely low ~ 40µV/K^4 we
amplify this voltage by a factor of 500x and add a 1.65V DC offset. The
result is that for normal earthly temperatures, the sensor pair will
output a voltage of 0V - 3.3V (the FMC module is 5v) at extreme
temperature differences and 1.65V at equilibrium IR voltage vs. angle
As the Earth emits substantially more IR radiation than the sky, we can
always measure the difference between Earth and sky - even in
cloudy/rainy weather, over snow covered land, or at night. The voltage
produced by the sensor pair is proportional to two factors:
The angle of the craft
The local ground/sky temperature difference
The sensors typically have a 100 degree field of view and produce a
somewhat sinusoidal voltage response relative to their angular
orientation. By combining the data from 3 orthogonal pairs of sensors
(X, Y, Z axes) (The FMC module is only 2 X-Y which is enough for a
straight up rocket. You can add the Z you just need a more complicated
firmware) we can compute the exact bank and pitch angles of the craft in
any weather/terrain condition.
The theory is that at zero angle, the difference in the heat between the
two sensors should be zero, and at 90 degrees it should be maximum. From
this relationship a linear regression is made and angles are calculated
during flight.
Each pair of sensors measures one axis, a minimum of 2 pairs must be
used to measure pitch and roll but best results are obtained thru the
use of a 3rd pair on the vertical axis. Since the output signal from
each sensor pair is proportional to both the attitude and the
weather/terrain, systems with only x-y sensors require a ground
calibration.
Another point is you don't need a recalibration if you use 3 pairs of
sensors so a fault or reboot won't affect your ability to maintain
attitude control if you can recover from the fault or have to reboot.
You can not do that with mems sensors.
That should clear things up.
Monroe
-------- Original Message --------
Subject: [AR] Re: Spin stabilized rocket
From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
Date: Wed, December 26, 2018 9:35 pm
To: arocket@xxxxxxxxxxxxx
Okay, I think I get the idea now. The device has four sensors on it,
all pointing horizontally (or at least that's the attitude it'll be
trying to maintain); you put it in the rocket underneath the nose
cone, in a section of fuselage tube that has about the same diameter
as the device. That's not an area that sees a ton of heating, and
ideally what heating it does see will be equal for the four sensors,
so will cancel out. Still, that's "ideally", not "reliably".
On Tue, Dec 25, 2018 at 08:20:10PM -0700, Monroe L. King Jr. wrote:
I'm telling you it doesn't get hot or (load up) if you place it
properly. And I'm going to present you with that data shortly.
It does indeed work.
Now the ones I used came in the older model of this device and they have
since changed the part number and that newer model is cheaper. I don't
believe this newer part is not useful or may not even be better but I
have not tested the new part number above about 500 mph but they did
work in the stratosphere to fly back a glider that came within 2 MPH of
breaking the model speed record.
The ones that flew on the Q Impulse attempt where the old part number
and did not reach 100kft but no heat soak occurred on that flight. 65kft
I have the data logged I'll have to pull that out of the data but that
is doable.
After I get done with the turbopump project I'll have to get back to the
guidance problems I never solved. I'm not even saying I'll be using the
thermopiles in the end but for straight up I don't think you can beat
them. They are rugged and simple.
Monroe
-------- Original Message --------
Subject: [AR] Re: Spin stabilized rocket
From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
Date: Tue, December 25, 2018 7:54 pm
To: arocket@xxxxxxxxxxxxx
On Mon, Dec 24, 2018 at 09:07:09PM -0500, Henry Spencer wrote:
In fact, they say: "...the reality is that sensor range is probably less
than 1 mile. Therefore, it does not really "see" the horizon. It sees the
average temperature profile all around and inside of 1 mile to determine
level attitude..." So, how well does this work when there's nothing but
air
within 1 mile, or 10 miles, in all directions?
Well, there's still a temperature gradient, so more thermal IR coming
up from below than down from above, so the task is still possible in
theory. (Yes, in the stratosphere, the temperature gradient reverses,
but by the time you get that high the air is so thin that the quoted
mile of range will have vastly expanded.) The earth is still warm
relative to space when you're in orbit. Still, this particular device
is sold for model aircraft use, not for rocketry, so (as Derek already
mentioned) it's unlikely to handle aerodynamic heating well. Even if
it didn't melt, the heating would confuse it.