DAQ Assistant Accelerometer Parameter Help

[Picaboy]

I am trying to setup a piezoresistive triaxial accelerometer with the daq assistant.  It seems pretty straight forward, but I am having a little trouble determing what Iex Value and dB Reference Value to use.  My particular accelometer's data sheet is here:

http://www.exportlawblog.com/docs/7270A-200K.pdf

I found installation tips (which didn't really help but maybe I missed something) here:

http://www.crash-network.com/Download/TP322.pdf

No where in either literature is there mention of a Iex Source/Value or a dB Reference Value.  I am a little lost, should I just leave them blank?  If anyone could help me on this issue it would be greatly appreciated.

[Izzzzz]

Hello Picaboy,

Thanks for providing the pdfs of your sensor. Your sensor is built of a full bridge strain gage while the accelerometer’s task (in the DAQ Assistant) typically corresponds to sensors built with piezoelectric elements.  With a piezoelectric element values such as Current Excitation Source and Current Excitation Value (Iex) (for your full bridge strain gauge this will be Voltage excitation Vex and Voltage Excitation source) are needed in order to determine the source and value to excite the passive transducer.  dB Reference only applies if you wish to view your results in terms of decibels.  I would recommend choosing a different task i.e.  Voltage, custom voltage with excitation, or a bridge based task and apply a custom scale to convert the voltage units to acceleration.  Also, what hardware are you using in order to acquire acceleration data? 

Best Regards,

Izzy O.

Applications Engineer

National Instruments

ni.com/support

[Picaboy]

Thank you for the speedy reply.  I am using:

cDAQ-9188 Chassis (Ethernet)

NI 9205 for String Potentiometer

NI 9237 for Strain Gauges

NI 9232 for Accelerometers

Would there still be a way to use the accelerometer card (NI 9232) for a piezoresistive gauge?  It says on the data sheet that it can be used with "integrated electronic piezoelectric (IEPE) and non-IEPE sensors".  Would it be possible to set the Iex source to None, the Value to 0 and leave dB reference at 1?  

It also appears as though the bridge circuit is completed on the accelerometer so a NI 9237 wouldn't be much help.  The only other way I see around this is to use the NI 9205 and lose the sensitivity and anti-aliasing features of the accelerometer card, which wouldn't be ideal.

[Izzzzz]

Hello Picaboy,

Yes you can use the NI-9232 with a couple of points to bear in mind.

1. As you have mentioned you will need to set the Iex Source to none and leave the dB reference at 1.
2. You will need to excite your accelerometer with 10Volts of steady excitation as stated in both the PDF and installation guide of your sensor.  If not 10Volts then calibrate the sensor to another voltage excitation source.
3. Ensure that you use DC coupling since the sensor itself is a DC coupled device.

The NI-9237 was designed for ¼, ½, and Full Bridge Strain gages which your device falls under. It has a 24bit ADC similar to the 9232 and an anti-aliasing filter. Here are some pros and cons for this setup.

1. It has voltage excitation so you could utilize this module to excite your accelerometer.
2. You would be taking better advantage of the voltage range. The 9237 has a range of 250mV while the 9232 has a 30V range. If I am reading the specifications sheet correctly (I am making an educated guess that the sensitivity is rated at the recommended excitation value) your accelerometer has a max nominal output of 200mV (100uV/g *2000g). This output fits nicely into the 9237’s range.
3. On the down side the sample rate is 50kS/s compared to the 9232’s 102.4KS/s.

Best Regards,

Izzy O.

Applications Engineer

National Instruments

ni.com/support

[Picaboy]

Thank you for the informative answer and it has definitely helped me greatly!  I will likely use the 9237 to measure my accelerometers, my accelerometer is actually a strain measurement so it makes sense.  Once again, thank you for taking the time to help me with my setup!

[Izzzzz]

No problem. Please post again if you need further assistance with this setup. Good luck with your project.

Best Regards, 

Izzy O.

Applications Engineer

National Instruments

ni.com/support