Before you actually order any parts, if you don't have a lot of
instrumentation experience, it would be worth it to find some references
on the subject. It's really easy to make a wrong measurement,
especially when dealing with environments that have lots of high
frequency common mode noise such as computers, igniters, etc. Incorrect
grounding, untwisted and unshielded cables, improper antialias
filtering, etc. can all cause mystery offsets and drifts that always
seem to show up at the worst possible time. NI makes quality hardware
(I used to work there) and certainly is good enough for many
applications but I would not put it in the high-end "consumer grade"
category. Professional or aerospace grade instrumentation is a lot more
expensive and typically runs about $1k per channel (that figure is not
necessarily recent) and you really get what you pay for.
In a previous life, we would get instrumentation amplifiers in for
evaluation from different vendors. The easiest way to separate the the
good from the bad is to add 1000 feet of cable between your sensor and
signal conditioner when evaluating it. With just a few feet of cable,
they all meet the specs. However, the first time you accidentally route
a cable too near a PC, motor, or some other source of EMI and screw up
the results for an expensive test, you quickly learn why the expensive
stuff is worth it.
Having said all that, if you pay attention to the specs, use proper
cabling, and understand the limitations of your system, you can get
acceptable results from a $100 DAQ device. For example, using 4-20 mA
sensors can typically provide more noise immunity than a millivolt-level
sensors that have to be amplified. A precision resistor with a low
tempco is a lot cheaper than a signal conditioner and instrumentation
amplifier . However, a lot of the lightweight miniature transducers are
only offered in millivolt level outputs. Amplified sensors that output
0-5V tend to match up nicely with 5V ADC ranges. Speaking of ADC
ranges, try to design your system so the ADC saturates before your
amplifier or sensor does so you know when you're over-ranged.
There are enough inexpensive DAQ options that I would not recommend
building one from scratch unless you're prepared to spend some time on
it. On the bench, you might get away with just connecting the sensor to
the microcontroller ADC but you probably want some signal conditioning
with an op-amp gain stage of some sort and at least a simple RC
antialias filter.. When looking at ADC datasheets, the resolution is
meaningless without also looking at the accuracy. I think the 12-bit
ADC on the CPU I'm using only has an effective number of bits of 9. You
also need to provide a precision reference to the ADC since the typical
default behavior is to use VCC as a reference which probably isn't very
accurate. Digital sensors (I2C, serial, etc.) are always an option as
long as you're not trying to run them very fast and trying to correlate
data between channels.
For S/W, I have a love/hate relationship with LabVIEW. It's great for
whipping something up that has some buttons, control, and graphs. But
as soon as you start doing something serious with it, you find it takes
a lot of effort to not end up with an unmaintainable mess. Every time I
have to manipulate arrays in LabVIEW, I start cursing and wish I was
doing it in C. But then every time I plop down a graph, I remind myself
how much effort that would take to code up with a library.
Good luck!
-Bob
On 01/10/2017 04:52 PM, Graham Sortino (Redacted sender gnsortino for
DMARC) wrote:
I'll preface this by saying this is only tangentially related to rockets so apologies in advance if this is somewhat off topic.
I’m starting to design the sensor acquisition (pressure transducers, flow measurement, thermocouples) and control hardware (servo and solenoid valve actuators) for our 500lbf LOX/Kero test stand. I’m trying to decide how I want to handle data acquisition as well as control and I was curious about other approaches people take.
In the past I’ve done this via a simple Arduino microcontroller w/ support boards for power, servo control, and signal conditioning but I have a bit more budget this time and I’d like to do something more sophisticated. My goals are primarily higher frequency signal acquisition, 12 to 16 bit ADC conversion and some-level of isolation to protect electronics.
From research so far it sounds like there are 2 main approaches to data acquisition / control (1) purchase a professional DAQ such as NI/Labview or (2) mainly build your own using a microcontroller. The prior being very expensive and the latter requiring more time/experience in EE/SW.
I’m starting to lean towards the TI Launched development boards (http://www.ti.com/lsds/ti/tools-software/launchpads/launchpads.page). From what I’ve been reading they seem a lot more sophisticated/professional then an Arduino and it seems like TI is serious about supporting them. I also am getting the feeling that the professional DAQ systems are a tad overpriced for what you actually get and there is more platform lock-in.
Apologies again if this is slightly off-topic but I hope there a few EE/SW folks on this list who find these topics interesting.
All the Best,
Graham
