load cell ni9237

[dav78]

Hi All,

I am trying to test if my NI9237 and Load Cell LCM900 are corrected wired and configurated prior to take them to the measuring site.

My load cell is ranged from 0-500KN, i am appying 100 Kg that wil give me 0.0026 mV, with an excitation of 3.3 V. Is any minimum voltage that NI9237 can measure.

Checking the NI9237 says that i must connect AI+ and AI-, EX+ and EX-, and RS+ and RS-. I have from the sensor:

  A Green +OUT

  B White -OUT

  C Black -INPUT

   D Red +INPUT

Can i connect this (Terminals NI9237)?

  Terminal 2 (AI+) - Green

   Terminal 3:(AI-) White

   Terminal 6: (EX+) Red

   Terminal 7: (EX-) Black

 There was a voltage drop when I supplied 5 V, i guess that is because of exceeding the 150 mW.

Please, I will appreciate any advise and/or confrimation that connections are correct.

Many Thanks,

[Petru.T]

Hi dav78,

You can have a look at the specifications page for the 9237 here: http://sine.ni.com/nips/cds/view/p/lang/en/nid/208791

I am not an expert in strain gauges, but you might find useful the Resolution, Maximum Voltage Range and Maximum Voltage Range Accuracy figures. This should help answer your first question.

In terms of the wiring, the 9237 manual ( http://www.ni.com/pdf/manuals/374186e.pdf ) mentions

"When you connect a half bridge to the NI 9237, you must connect 

the AI+, EX+ and EX–, and RS+ and RS– signals, but you do not
need to connect the AI– signal because it is connected internally.
When you connect a full bridge, you must connect the AI+ and
AI–, EX+ and EX–, and RS+ and RS– signals. Refer to Figure 3
for an illustration of how to connect half and full bridges to the
NI 9237."

I think you might have known this already, but maybe it helps.

[dav78]

Hi Petru,

Thank you for your reply.

I have wired using a NI9949 screw terminal. For the pins 5 RS- and 7 EX- we connected with a short wire making common terminal, and the same for pins 4 RS+ and 6 EX+. However this way i am omitting the Rl (wire resistance of the sensor which is 5 m length), which i consider nearly 0 Ohms, otherwise I have to wire from 4/5 pin to the strain gauges with a L m wire as the Rl and that would be the same resistance.

In order to create a half bridge, I need two strain gauges. I am short of strain gauges for my tests. Do you think I could use a resistor of equal resistance to the strain gauges i.e. 120 Ohms 1%, and screw it to the 6 and 2 pins.?

I can not easily find the resolution, is that 24 bits, and the minimum and maximum voltage. NI has given me a figure of 0.038 mV range sensitivity, which i can not find from the manual.

Looking at the +-25 mV/V. I have sensors 2m/V, which i suppose are in the range and other which is 100 mV output, which I think that with 4 V excitation voltage i will cover 100 mV. However I am thinking to provide 3.3 V for all the channels in  order to avoid the 150 mW issue, so not expecting measurements in the higher range.

Please correct me if any assumption is wrong.

Thank you again for your help,

Kind Regards,

David,