nuckollsr(at)cox.net
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 Posted: Sun Nov 12, 2006 4:14 pm Post subject: The po' boy's data acquisition system. |
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At 08:16 AM 11/12/2006 -0500, you wrote:
| Quote: | I have just finished rereading the Connection and it is amazing what you
learn when you go through another cycle of education. Of particular
interest to me this time through was a comment Bob made at the end of the
Temperature Chapter regarding using a switch box and multiple home made
thermocouples to measure critical temperatures during flyoff. There were
several items that he suggested that I had not even considered. As an
example: alternator stator temp, alternator regulator temp are two he
suggested. I quickly extrapolated to all the things I would like to know
like electronic ignition coil temp, and then there is the in and out oil
temp and air temp to evaluate my oil cooler installation. Next I thought of
the intercooler in and out charge and cooling air temps. The list for my
turbocharged engine quickly rose to thirty some odd items to evaluate
temperature on during the test flight phase. I did some research in the
AeroElectircic archives and found a few suggestions for PC data collection
interfaces but when I looked at the cost per port my data collection system
was easily over a thousand dollars with an assembeled price point of
$30/port.
So now my design problem became how do I reduce the price per port for a 30
port temperature data collection device? I refined what I wanted to know to
be three phases of temperature:
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<snip>
First, you're to be commended for having the curiosity to go find
things out and convert them to numbers for the benefit of yourself
and others with whom you share the data.
The box of "tinker-toys" is huge and the tradeoffs options not much
smaller. From my own perspective, the commodity that drives most of
my decisions is time-to-first data. I DO fabricate a lot of specialized
fixtures and circuits. However if I can buy some pre-fabricated
modules and perhaps supplement them with fabricated interfaces, some
combination of these represents the most economical approach.
First, allow me to suggest that while your three-light approach is
certainly better than nothing, it's like working in a base ten
world where only the digits 0, 3 and 7 are permitted. Further, it
presupposes that you have a good idea of where the switch points
for each of the lights should be placed.
If I were doing this for RAC and needed to be up and running
in a few days, I'd consider getting 1 to 4 a/d modules from
http://www.weedtech.com/
Check out the 8 channel, 12-bit analog input module with 4.095
volts full scale input with 1 mv resolution. This critter
is $69 for a $8.50/channel cost. Team these modules with a
thermocouple to analog conditioner board using AD597ARZ chips.
The artwork for a mating board is at:
http://www.aeroelectric.com/PPS/ECB_Artwork/9804_320-1A_KTC_Board
This board is designed to "piggyback" onto a Weeder analog board
using the same hole pattern as the Weeder's d-sub mounting screws.
So, for about $130 in materials, one can have an 8-channel, thermocouple
interface module that is easy to talk to with a laptop temperature
recording utility. You can calibrate your installation to plus/minus
1 degree C or so. With just one module and as many TC harnesses as
you wish, you can change from one group of 8 temperatures for one
phase of the testing to another group of 8 just by unplugging and
re-plugging the appropriate harness.
The TC chip module teamed with the Weeder board and a cheap
PC now joins in your arsenal of investigative tools with a
capability of recording and sharing real numbers. I use out-
of-date lap tops running Power Basic under DOS 6.22. The last one
I bought cost me about $100 off ebay.
I've not proofed the ECB yet . . . but if you're interested,
I'll order some and get one stuffed up. I've been intending
to add this board to my own toolbox for some time to replace
a couple of older, hand-wired TC signal conditioners I've been
using over the years. You can see one of my last projects at:
http://www.aeroelectric.com/Pictures/Data_Acquisition
The forth picture in the series shows the hand-wired thermocouple
conditioners that I want to replace with the surface-mount,
ECB assembly.
This is perhaps a bit more expensive than your proposal
. . . but $time$ to first-data will be shorter.
Better yet, the data will be RECORDED for later analysis
and sharing. Further, the Weeder a/d board channels can be
pressed into service for measuring and recording lots of other
interesting numbers about how your airplane operates. Even
if you need to go find a laptop, you should be able to assemble
this suite of test equipment for under $300.
Bob . . .
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