Paul Lamar wrote:
> We [Paul Lamar & Tracy Crook --RAK] discussed how to measure the
> pressure wave in the p-port intake manifold as a function of
> e-shaft angle during one cycle per revolution of the e-shaft.
> The max pressure will occur at different times depending on the
> RPM and the length and diameter of the runners. So far suggestions
> have been: feeding the output of the high speed pressure sensor
> into a PC audio input card or the microphone jack of a note book
> computer. Tracy pointed out the problem with that are those are
> AC inputs and we need absolute numbers from the pressure sensors.
I'm not certain I completely agree with Tracy's position. It is true
that the EC2 needs to know the absolute pressure in order to determine
the appropriate dwell time for the injectors. But I believe that we are
interested in the shape of the pressure graph in order to learn about
the pressure wave in the intake manifold at various RPMs. In that case,
an AC coupled sensor would work just fine. So for your tests I think
the audio card idea might have been ok, except that I don't know that
the audio card would work very well because of the sampling rate. At
7500 RPM, the e-shaft is generating a 125 Hz TDC signal. If you try to
take a reasonable number of sample in a revolution (like 100) to get a
good idea of the pressure graph, then you're talking 100 * 125 Hz, or
12.5 KHz. And that 100 is really coarse; I'd rather have 200 samples
and that's 25KHz.
I have a dated PC with an ISA slot that I don't mind loaning along with
the digidata recorder. This system can record 1-16 channels at 333KHz
(and lower rates as well), DC coupled. Would be great for ground
testing, but obviously not in flight. This would be the quickest &
cheapest route.
As far as recording pressure transducers AC coupled, I have to agree
with Russell. We measure dynamic pressure magnitudes such as this all
the time with space shuttle engine testing using kistler pressure
transducers, AC coupled. Using the power spectral density, magnitude
units in V^2 Hz, I can use composite RMS techniques to determine RMS
energy, calculated block by block over the sequence of data. Those
units are easily converted to engineering units, or MAP in this case.
Initially, it wasn't clear to me what your goal was. You want to measure
points along the curve, not the conglomeration of pressure pulses.
Therefore, this technique would not be very useful.
There is another route you can take that would only require 3-5 bucks in
parts:
If you want the static (un-changing) pressures on an AC coupled
recorder, I can whip up a VCO (voltage controlled oscillator),
demodulate this in software, and write out your clean signal to file.
My goal here was that I can just send you software to record your
signals (using your soundcard) with no hardware purchase necessary.
However, you already mentioned that you need more than 2 analog inputs
so a soundcard may not be a good route.
National Instruments makes some great stuff, including a usb based
acquisition system, starting ~$150. I can dig around and see if I can
get something on loan from a comrade.
Sounds like we have a very strong need here for an inexpensive data
acquisition system. Since something like this would better the rotary
aviation cause and improve installation quality, I'm in. It would help
everyone here in several areas (taking radiator deltaP and deltaT
measurements in flight, measure injector pulses using non intrusive
current sensors, etc). I'll look into what it would take of me to punch
one out, preferably FLASH memory recording, post test reading via the
USB. Anything with real time viewer just makes it more complicated, so
no promises here. I'm not interested in making $ or marketing, I'd just
send out microchip software, simple programmer, parts list, and gerber
files (or Eagle PCB layout files) for everyone to solder their own PCB.
Marc Pedings
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