02-07-2019 08:14 AM - edited 02-07-2019 08:43 AM
Dear NI
usually I do conversions in my own LabVIEW vis , however today I tried to create an acceleration task in MAX. DAQmx 18.0.0f0 (you can reproduce it with a simulated USB 4432 or so).
After creating the Task I was a little upset, because the sensitivity scaling doesn't support SI units, only something funny like mV/g or V/g ... but offers the correct scaling units m/s²
I haven't tested if a conversion to SI units will happen ... will it?
So no valid SI unit's are offered for the sensitivity! (I think it will scale to g_n but not even the scaling factor is noted, or is it g_local ?? 😉 )
OK, lets do a calibration:
UUps WTF (I haven't supplied correct reading (SIM device) however what scaling is it??)
OK, lets have a look at the value table:
OK 🙂 g seems to be equal to m/s² 😄 😄
and back at the task:
What will be the output??
Finally, let's have a look at the Help link for acceleration:
Sorry, german, while I can install LabVIEW in englisch, I can't do it for MAX, DAQmx... would ease a lot
However I assume in all language versions the unit for acceleration is wrong. g is no unit, g_n is a value valid at less than 1% on earth.
I had customers asking if they should use g_local (they had measured) instead ...
If you want to process acceleration values you have to convert to SI units anyway.
So please fix the bug(s) and convert to SI unit.
Even if you change it now, the units will not change after 20. of Mai (start of the new SI) 😉
More help for the use of the SI units :
https://www.nist.gov/pml/special-publication-811
02-07-2019 10:29 AM
It's not a "bug", it's a (careless!) feature. I think it comes from accelerometer manufacturers -- one of my student teams was building a medical device that incorporated a triaxial accelerometer-on-a-chip, and everything was calibrated in mV/g (or V/g). In one sense, this made it easy -- we calibrated it by putting it face-up, measuring the voltage, putting it face down, measuring the voltage, then they worked for a half-hour figuring out how to compute "gain" and "offset" (I exaggerate slightly -- it didn't take quite that long). Note -- the values quoted by the manufacturer said "± 10%", and I had to convince them that calibration was necessary to get meaningful data ...
Bob Schor
02-07-2019 11:06 AM - edited 02-07-2019 11:32 AM
Bob, if it's only the support and use of g_n I would say OK, If they don't know it better ...
and g_local (if you know it) can be used as a primary calibration source 🙂
but the assistant and task generator is buggy by mixing up the units!
AND at least the SI unit for acceleration should be available. (Should I note that as an idea?)
Sadly enough I often see missinterpretations even from people who should know it.
02-07-2019 02:27 PM
I didn't say it was a good (or proper, or admirable) feature ... You are absolutely right, proper units are important. And if NI wants to allow acceleration in "g" units, that should be, at best, an option in addition to the proper SI units.
Bob Schor
02-07-2019 04:57 PM
The scary part is, that this task generator/assistant show that inside of NI there is no (or seems to be no) commitment to SI units (or the quality/software rules haven't been checked)...
Just checked it with LV2012... different .. but also buggy.