Manual scaling of the load cell in RT

[haavissa]

Dear readers,

I would need to use a load cell connected trough a NI9218 using a full bridge screw terminal to my cRIO (9067) in fpga mode. I have the following calibration information on the load cell:

Excitation voltage used in the calibration 10 V

Maximum load 0.6 kg

Rated output 0.8859 mV/V

Zero balance 0.002 mV/V

I was able to get correct readings from the load cell with cDAQ and DAQmx using the strain measurement and the excitation voltage of 3.3 V provided by the module. The rated output and zero balance had to be inserted in the bridge scaling as a two point scale. (http://www.ni.com/tutorial/7130/en/)

How should I do the same thing in my cRIO host code? I was able to set the NI9218 module properties to +/- 22mV/V and 3.3 Vex. How do I calculate the load from the measured signal?

Thanks in advance.

[CollindeWit]

Hi!

I am Collin de Wit, an Application Engineer at National Instruments. I read your question and if I understand correcty you want to know how to use the DAQ code on a RIO system? First of all check that you have the correct software installed on your computer, see this link for more information on that: http://www.ni.com/getting-started/install-software/compactrio

I think the following link is very usefull to test and understand good how the FPGA environment works so you know best which solution works for you and how to use code on the FPGA. http://www.ni.com/tutorial/14532/en/

About the 9218 module, the excitation current level is on a per channel basis. Each channel can supply 17.8 or 10.1mA depending on the output voltage level set. Typical strain gauges come in 120ohm or 350 ohm. To ensure the NI 9218 can excite your sensor, perform the following actions:

1. Determine the nominal output resistance the NI 9218 will see based on the bridge configuration.
2. Use this nominal output resistance and calculate the current needed based on the Strain Excitation voltage level set in software.

For example, a 120 ohm Full Bridge Strain Gauge will have a nominal output resistance of 120 ohms and need 11.66mA when powered with a 2V supply or 27.5mA when powered by a 3.3V supply. The NI 9218 is limited to 10.1mA at 3.3V so you will need to set the excitation level to 2V. Exceeding the output current will fault the excitation lines and cause the channel to cut power to the sensor.

Also please take a look at this discussion forum where someone also found information on a similar topic. http://forums.ni.com/t5/Real-Time-Measurement-and/Front-panel-control-on-compact-rio/m-p/2478304

Please let me know after this information if any further assistance is needed and if your question were responded.

Best regards,

Collin de Wit

[Benson19]

For loading cell conneceted through normally you'ld setup an experiment with min and max weights,observe the analog output going to the PIC ADC.A couple of mid weights would be good,incase output is not 100% linear. Once you satisfied with the output range'get max bits for max weight',etc.,then cut code as required.