cRIO 9074 to read a load cell

[Pritesh]

Hi,

I have a load cell from Transducer Technologies: LBO -500. The specs for the load cell are as follows:

Rated Output (R.O.):

2 mV/V nominal

Rated Output (R.O.):

2 mV/V nominal

Rated Output (R.O.): 2mV/V nominal

Terminal Resistance: 350 ohms nominal

Excitation Voltage: 10 VDC

Safe Overload: 150% of R.O.

I have a cRIO 9074. I also have 2 analog input modules NI 9219 & NI 9237.

My question is which is the appropriate module that I need to use for this application. Also, what code do I need to use in order to get the reading from the load cell.

I am new to DAQ/Labview.... please bare with me.

I also have one more stupid question. I need to get data from about 14 to 15 strain gauges. I will be have around 4 strain gauge rosette setups. I am assuming I will have be using 1 strain gauge per channel.  What is the suggested AI module for this application.

Thank you

Pritesh

[Ben_S]

Pritesh,

Hello!

The NI 9219 will work great for your Load Cell's as it is made as a general purpose AI module, however your Load Cell's probably require excitation and the 9219's only supply up to 2.5V of Excitation Voltage, if your Load Cell's require more than this you'll have to provide it externally.

The NI 9237 is specifically made for straing guages so this will work well for your 14 to 15 strain guages, however you'll need 4 of the modules to read all those strain guages as they only have 4 inputs each.

The LabVIEW Example Finder has a good exampls for the C Series modules which can be found under NI Example Finder >> Hardware Input and Output >> CompactRIO >> Module Specific >> Analog Input>> NI 92xx Getting Started.lvproj. You can get to the Example finder by going to Help >> Find Examples from within LabVIEW

I also want to note that on the cRIO you won't be using the DAQmx Drivers as you would with a cDAQ application, instead you'll either be using FGPA programming or the Scan Mode of the cRIO.

Since you are new to LabVIEW and the cRIO I would suggest taking a look at these documents.

• CompactRIO Developers Guide
• Getting Started with LabVIEW
• Getting Started with CompactRIO and LabVIEW

[Pritesh]

Ben,

Thank you very much for your response.

I have a few follow up questions.

1. For the NI 9237 the min range is +/- 25mV. So does that mean it can read anything above 25mV eg. 26mV, 27mV etc. or can it read on mulitples of 25mV  eg. 50mV, 70mV etc..

I ask this because since my Load cells out put is 2mV/V. This would mean that if my Excitation is 10V. I can have to read upto 20mV max. Bascially my sensor output will be between 0 to 20mV. Is there any NI module that I could possible use to amplify my signal? OR is there any other cheap way that I can amplify the signal. I spoke to the vendor. Transducer Technologies. They have amplifiers for this application but they are selling them for 200$. And since I have 3 Load Cells it will be very expensive.

2.About Strain Gauges. I have to use the same NI 9237 module for them. I want to make sure that I don't run into the same trouble of signal amplification for the output of the bridge. What should I look at to make sure that the strain gauges will work with the NI 9237.

Please explain in as much detail as possible. I have read through all the information on the NI website about strain gauges, c-series modules, analog inputs and a lot of other information. I still have the above mentioned questions.

Thank you very much

Pritesh

[Ben_S]

Pritesh,

The NI 9237 has a maximum input voltage range of +/-25mv. This means that it can read signals in that Range from -25mv to 25mv. So signals greater than 25mV would not be recommended as it would be overloading the device. You say that your max output would be 20mV, this is perfect as it fits in the +/-25mV Range. Your 2mV/V should also have a measure of load to go with that, meaning at 20mV that would represent X amout of Load. You would then use a custom scale to translate the voltage to units of load.

I'm not sure how to answer your second question. Below are some really detailed documents on our website that discuss strain gauges and taking measurements with them. Essentially, you need to make sure that your strain guage is either 120ohms or 350ohms and that you provide the correct excitation voltage such that the output will be between -25mv and +25mv. There is no need to amplify the signal as this will actually cause your readings to be incorrect.

• Measuring Strain with Strain Gages
• Strain Measurements with Strain Gages: How-To Guide (Featuring 9237) 

Also, I found this document which relates to the questions you asked in your first post.

• Advantages of the NI 9237 Over the NI 9219 in Strain Measurements