fpga - NI 9215 C Series

[KarthikSrinivasa]

Hi,

I'm a Mechanical Engineer new to LabVIEW Real-Time and FPGA. I have been trying to connect an accelerometer sensor to the NI 9215 'C' Series Module in the cRIO 9024 controller chassis but experiencing a hard time in finding the right connections especially with the "GROUND". I would very much appreciate any effort in giving me more insight on the problem that I'm experiencing and help me acquire a voltage signal. I went through various white papers but none of them explained in detail.

[GerdW]

Hi Karthik,

which "GROUND" are you talking about?

The sensor gives you a voltage signal: it usually provided by two wires. You have to connect those two wires to the contacts of one input channel!

The module comes in two versions: BNC or springclamp. Each version has two contacts per channel as written on the body on the module - and in it's manual...

[KarthikSrinivasa]

Hi,

Thanks for the reply. I appreciate your effort in helping me out. I'm using the NI 9215 with screw terminal which has 4 AI+ and 4 AI- and NC and COM terminals. Consider for example connecting a potentiometer which has 3 wires. How do you propose me to connect it to the NI 9215 with screw terminal in order to acquire a voltage signal? I can wirte a LabVIEW VI that acquires voltage. But, I'm stuck with the hardware connection. I went through RSE, NRSE and other pages, but couldn't get the clear picture on connecting the ground/"COM" terminal according to the terminology.

Regards,

Karthik

[0utlaw]

Hello KarthikSriniva,

A three-wire potentiometer is usually a simple voltage divider ( http://en.wikipedia.org/wiki/Voltage_divider ).  You typically connect it to ground and a voltage supply and the third wire gives you a variable voltage output that scales with the resistance of the sensor.  Since the 9215 is a differential sensor, you'll need to connect the third (variable output) wire to either the AI+ or AI- terminal and either bridge the GND or wire the supply voltage to the unused AI terminal. The signal you get will be proportional to Z1 or Z2 (in the linked article) depending on how you wire the DAQ.

Depending on what you're using for a power supply be very careful how much current you attempt to sink to or source from the module.

Regards,

[KarthikSrinivasa]

Hi Tom,

Thanks for the reply. From what I have understood after reading your message is that if I connect the variable output(third wire which scales the resistance) to AI+, I should connect the negative terminal of the supply voltage to the AI- terminal and vice versa? In that case, nothing goes into the "COM" in the NI 9215 with screw terminal?

By the way, I'm using the NI power module to power up the cRIO 9024 controller and a battery to supply voltage.

Regards,

Karthik

[0utlaw · Accepted solution]

Hi KarthikSrinva,

What you've described might work, but remember that the 9215 has a maximum range of +/- 10V from its own ground reference (COM). 

Since the battery is a floating (unreferenced) voltage source, you'll want to bridge the AI- and COM (common ground) terminals to ensure that you have a common reference and the differential signal doesn't float out of the range of the DAQ.  This is described in section three of the Field Wiring and Noise Considerations guide I believe you were referring to earlier:

Field Wiring and Noise Considerations for Analog Signals

http://www.ni.com/white-paper/3344/en/#toc3

Regards,

[KarthikSrinivasa]

Thanks for the insight Tom