NI-9218 Accelerometer Setup

[DT18]

I am having difficulty setting up some sensors with a NI-9218 c module. I'm trying to retool a impact test system to use two different sensitivity accelerometers which I already have. The PCB brand accelerometers are listed below.

3651A122KG

3741E12100G

When I try to setup the accelerometers using the Acceleration option in Ni-Max, the options available don't make a whole lot of sense to me given the sensor specs, and I don't receive any sort of feedback when testing the sensors. Admittedly, this is my first first big Labview project and I don't have much experience with this instrumentation type so the sensors' specification callouts and how they incorporate into Labview/Ni-Max is a bit overwhelming. Any input or tips is much appreciated. Thanks!

[Anthony.M]

DT18,

Could you include an image of how you are setting it up in MAX? That will help to understand.

[Henrik_Volkers · Accepted solution]

Not only the MAX setup is important!

The two sensors are of different type and need different exitation!

Please also send a schematic of how you wired the sensors to the DAQ.

The type 3651A is a piezoresistive bridge. Calibration data (you havn't attached) is only valid for 10V (my guess ;)) exitation. the 9218 doesn't have 10V bridge exitation (3.3 or 2V)  so you have to ask PCB how big the influence of that change is to the sensitivity (normal bridge sensors scale linear with exitation, piezo resistive can act different*)  That's why the cal sheet just note exitation voltage and sensitivity in mV/(m/s²)   instead of (mV/V)/(m/s²)

The two sensors not only have different sensivity, also the bandwidth differs , so expect different results from the same impact shock!

*) I work in a national cal lab, so maybe I'm a bit too picky with specs 😉 for +-10%  I wouldn't care 😄

[DT18]

Henrik,

Sorry for the delayed reply. You make some great points. I think part of the problem is I've been trying to get it to work with the "Accelerometer" setting in MAX, which only allows current excitation. However, that's kind of a moot point since the calibration sheets do indeed call out 10V and as you pointed out that is not available for internal excitation.

I've sent an e-mail off to PCB to get their opinion on the effect of a lower excitation voltage. I have a feeling, I'm going to have to bite the bullet and either pick up a new sensor, a different signal conditioner, or another power supply for external excitation. Thanks for helpful information.

[Henrik_Volkers]

A lot of accelerometers in the field are of IEPE (or CCLD , constant current line drive)  type , needing an exitation current. But you have different sensors.

You can still use the piezoresistive sensor, in MAX it will be a full bridge sensor and you just enter the sensitivity divided by the exitation voltage. (about 0.02 (mV/V)/g ). You (maybe) just need a new calibration .. migth be cheaper than a sensor 😉

For the 3741 : This sensor needs a 6-30 Voltage supply and the output is a differential signal (+-2V) with 2.5V_common (bias). (as it seems to me)

At a first glance the 9218  can provide that, however a second look (at the getting started guide) it doesn't seem to be possible with the standard modes. Seems that they only do a fully differential measurement in bridge modes....  with 12V exitation the AI- seems always to be connected to EX- internally 😞

Contact NI (via phone/email, but maybe they answer here )  and ask if the 9218 can be configured to provide 12V exitation AND do a fully differential measurement in the 64mV and/or 16V range.

Otherwise DAQ and (both) Sensors don't really match...   There are tricks and hacks to do measurements, but if your setup is for a traceable standard impact test ..