Can anyone help? I'm trying to take the dynamic signal from a single
strain gauge and input it into my computer so I can record and analyze
the data (maybe dump it into Excel). I can't affort to spend a lot on
data acquisition hardward, so I'm wondering if there is a way I could
do this for less than $200.
Greetings,
There are several low-cost A2D products, most using USB Try a google
search on USB data acquisition. One such is
http://www.labjack.com/downloads.html
(which I have not used) I see on their FAQ that have a
utility/demonstration app that dumps data into an excell file. I
don't know if this'd work for you, but it (a) sounds reasonably close
and (b) since this prod has interface software avail for multiple
languages (assuming the SW works) you'd have more flexibility. These
would give you multiple channels of analog input + misc other handy
I/O abilities. Another mfgr. is
http://www.measurementcomputing.com/
they've got both USB and serial modules in your pricerange. I've used
boards from them, but not these modules. YMMV.
Since sound cards are not well spec'd re low-frequency response,
acquiring data via something that goes down to DC may save you time
and hair vs. the soundcard approach.
This doesn't yet address getting from a single gauge to a usable
signal. Some suggestions:
If you possibly can, mount a second gage -- ideally such that it
strains in the opposite sense for the loads of interest. Use these in
a half bridge config. (Even if you can only mount a second gage to
respond only via poisson coupling, or no strain at all, you'll at
least gain on thermal stability, which can be a big factor, even for
foil gages.) Whether you end up with 1 or 2 gages bonded, complete
the bridge with precision (1% of better), low tempco resistors,
located as close to the gage(s) as you can (to help with common-mode
noise). You can use higher value resistors (in parallel) to trim out
the passive bridge elements to better than their nominal resistor
tolerance.
Avoid connectors if you can (gold plated contacts if not, and
mechanically secure them!); and *shield* your wires. Make sure your
excitation is low-noise and can drive the bridge without sag. (Remote
sensing/control of the excitation would help, but may be more
complexity than you want to take on.)
Choose the excitation voltage so that the mounted gages do NOT feel
notably warm to the touch, even after say 10 minute warm-up. A linear
supply with remote sensing is a quick hack for consistent excitation,
if you have one handy at a voltage that doesn't heat your gages too
much.
I suggest using an instrumentation amplifier, preferably one with
single-resistor gain setting. FYI, digikey stocks several. I used
the Burr-brown (now part of TI) INA-2126 (a dual) in my last
strain-gage project. Several companies (incl Melexis, I think) are
offering single-chip solutions that include excitation and (I think)
A2D, These might be even easier, but I haven't yet tried them.
HTH,
Larry
PS I post via google, and I cannot figure out how to get google to
post my posts with an obfuscated address (along the lines of
[email protected]) Google seems to require a valid
address, which it makes visible.) How are you folks doing your
posting?
