Load Cell to NI-9237

[KJM23]

Hello,

I am fairly new to data acquisition in general and LabView. I have a CompactDAQ Chassis connected to a laptop with an NI-9237 Strain/Bridge Card slotted in. The card has RJ50 front connectors with screw terminal adapters. I am wiring a THC-3K load cell to the system and getting a noisy signal in LabView. I have connected the AI+ and AI- terminals to the Signal+ and Signal- wires of the load cell, and the Excitation+ and Excitation- to the corresponding load cell wires (10V internal excitation) in following with the DAQ Assistant connection diagram.

The NI-9237 manual that came with the card showed some wires (the remote sensing wires in particular) jumpered to other terminals. Do I need to jumper any of the other terminals? The load cell also has a shield wire. Where should I connect this wire?

I have also configured the scale in the DAQ Assistant according to the vendor spec sheet that came with the load cell and performed a calibration through the Assistant. The capacity of the load cell is 3000 lbs, but I only had weights up to 40 lbs to use during my calibration. Would this have a big impact on accuracy?

Any help would be greatly appreciated. Thank you.

[Sil.VI]

Hi KJM23, 

My first recommendation here would be to follow the steps described in this tutorial, to make sure you are ok in the connection side of things. You may also want to consider the output response for your loadcell and make sure it fits your needs in terms of accuracy considering the error response and the input range of +-25mV/V.

The remote sensing terminals are optional and can be added to your bridge in order to diminish the gain error like you said the details about this are listed in the manual.

Hope that you can find this information useful! 

[Lysandros]

I agree with Sil.VI

Also, if your load cell is of quality, it should come with a calibration certificate which includes the proportionality and offset factor. You must apply these on the "raw" input signal from the NI-9237 analog input inside a VI by mathematical blocks, because this module does not have many configuration options (basically only excitation voltage). When you are finished, then you apply weights in order to see if the indication matches the weights. Calibrating using weights should be done with multiple weights. Using only 40 lbs for a 3000 lbs cell will produce high errors at higher loads.

Also, take a look at the pinout of the NI-9237.

Best regards

Lysandros

[Cablejon]

when wiring a load cell you should always have the Ex+ and the Sense+ tied together as well as the Ex- and the Sense- tied together. your signal is the Signal+ and Signal-.

The Sense+ and Sense- are for ensuring the excitation voltage stays steady.

[hurch]

Good day
I am to write a code that will read data from two NI 9237 modules (6 channels in total). the modules are to read data from a force balance (strain due to force).

I know its not a matlab community but I could get help, how do I convert strain result to force using a loadcell of specification stated below and the NI 9237. being specific, I need the coefficient to convert strain value given by the daq to force.

Load Cell: STA-7 Aluminum Alloy S Type Low Range Tension/Compression Load Cell

Rated load (kg)                      1, 2, 5, 10, 25, 50
Ultimate breaking load         300% of rated load
Maximum transverse load   <50% of rated load
Output/ Sensitivity                2mV/V (±0.1%)
Zero balance              <±1% of rated load
Excitation voltage                  10vdc recommended, 15vdc maximum

ch6.BridgeMode = 'Full';
ch6.ExcitationVoltage = 10;
ch6.NominalBridgeResistance = 350;

[Cablejon]

The Conversion is done in the mV/V sensitivity. so if you know the range of the load cell and apply the mV/V scaling it will read load directly.  example: 100lbf load cell, 2mV/V output, 10Vdc excitation you would expect a 20mV signal with a force of 100lbf applied to the load cell. The mV/V rating is the output at full scale.

100 lbf applied,  2mV/V x 10Vdc = 20mv output

0 lbf applied, 0mV/V x 10Vdc = 0mV output

HBM has some really good articles on strain and Wheatstone bridge measurements as well as most manufactures of load cells. if your doing a lot of work with load cells I would recommend a book put out by Interface Load cells called the "Load Cell Field Guide" it goes in depth into how to use load cells.