[tccrockets] Re: The saga of the KISS rocket
- From: David Weinshenker <daze39@xxxxxxxxxxxxx>
- To: tccrockets@xxxxxxxxxxxxx
- Date: Fri, 10 Feb 2012 20:32:16 -0800
James Dougherty wrote:
> Regarding the firmware and how it can fail with harmonic oscillations
> and sensor data, the "Nyquist rate" is the rate in digital signal
> processing which is the upper bound for how much information you can
> process on a bandwidth limited channel. The nyquist rate is 2x the
> true data rate. Nyquist says that if you want 100 samples/second you
> should actually Be sampling 200 samples/second.
> We kind of know that intuitively when we fly altimeters and crank
> up the sample rate as it gives us better graphs As to how the flight
> computers all operate with sensor data (Barometers, Accelerometers,
> IMU's, etc) a rough example of what could be happening is that if
> the A/D samples 50hz, and you oscillate at 20hz, your effective
> sampling rate is 1/2 50hz (25hz) and half the Nyquist rate is 12.5hz,
> the firmware (as it loses efficiency in sensor data) effectively
> "slows down" and apogee may never be seen!
Yeah, half the sample rate is the highest possible frequency that
can be represented in the data; anything higher than that will come
through at the difference between its frequency and the sample rate.
(So at 200 Hz a 190 Hz signal will appear as 10 Hz in the output, and
a 200 Hz input, exactly matching the sample rate, can appear as an
arbitrary DC bias depending on how the sample timing lines up with
the phase of the incoming signal!)
> This is an evil side effect of vibrations with noisy
> sensors driven over A/D
Yeah, this is what we ran into. I think the AltAcc sampled at
something like 16 Hz and I forget what the rate was on the G-Wiz,
but one way or the other we clearly were seeing vibrations with a
lot of high-frequency content well above the "Nyquist" rate of
either system.
The acceleration trace downloaded from the flights with successful
timer or baro deployment - from an R-DAS recording at 200 Hz - was
full of artifacts that looked like "beat frequencies" between the
sampling frequency and higher-frequency stuff in the signal. When
I saw that, and compared it with output from the same unit on solid-
propellant flights, I thought "if I get this mess when sampling at
200 Hz, then no wonder the acceleration-based software on the other
devices, using lower rates, got confused".
> When will you fly your rocket again?
Well, this was back in 2002 as I mentioned; that configuration is
pretty much retired; the next one will be smaller, I think. (KISS
was about 7-9 feet long and 6 inches in diameter, and designed more
for simplicity and robustness than performance.)
> Do you come to TCC launches?
Not as much as I used to, but I still make
it out there every once in a while.
-dw
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