How to scale using an SCC-SG24

[dVaughn]

Hello,

I am having trouble getting my scale correct.

I am using a Tovey Engineering Full Bridge Strain gauge/load cell to measure the press force of an operation. In compression the load cell is said to output negative voltage. I called Tovey Engineering and they told me that  0.1596mV / Klbs/ Volt Excitation would be my output.

I have tried mapping, I have tried linear, I can't figure out what I'm doing wrong. Is it the gain in the SCC-SG24? Any thoughts?

Thanks in advance,

David Vaughn

[Travis G.]

Hello David,

It sounds like you are creating a voltage measurement task in DAQmx and trying to apply your own scale to convert the voltage into a load measurement.  You may want to consider creating a strain measurement DAQmx task instead, which will allow the NI-DAQmx driver to properly scale the voltage measurements for you and report results in units of strain.  You can create a strain task from within Measurement & Automation Explorer (MAX), by right-clicking on Data Neighborhood, select Create New > NI-DAQmx Task.  Select an Analog Input > Strain task, and select the physical channel on the SCC-SG24 module your strain gage is connected to.  Now you can configure this task based upon the gage factor and gage resistance of the strain gage, the bridge configuration you've connected it in, and the excitation provided.  The driver will use these settings to convert the raw voltage measurement from the module to a strain measurement.  Give this a try and let me know if that improves your results.

Regards,
Travis G.
Applications Engineering
National Instruments
www.ni.com/support

[dVaughn]

Thanks Travis. You were right. I was trying to do it using a Voltage. I changed it to a Strain measurement and I am now getting the correct scale. Thank you very much.

Dave

[jwhite]

Hello David,

Did you end up using a custom scale (i.e. linear, map-ranges, etc) to finally get your correct readings?  I am trying to do the exact thing you were, and I cannot get accurate results.  I have set up the strain task for my load cell (0-5lb, 10V excit, and 2mv/v output), but I cannot figure out what scale to use (if any) and what values to input (prescaled and scaled).  Any suggestions?

Thanks,
Jason

[Jennifer O]

Hi Jason,

When you create a strain task the readings you get should already be scaled to the units of strain if you have set the task up correctly with the appropriate values for your strain gage in the settings tab (gage factor, gage resistance etc).  No additional scaling is necessary in the strain task.

Regards,
Jennifer O.

[dVaughn]

Jason,

In response to your reply, I ended up using linear mapping. I did some research and found out that my Strain Gage output a certain mV/ Klbs / Volt of Excitation. Using that number I made the mapping ranges so that 0 - 0 and 1000 - Whatever. The only thing is that that is in units of pounds. I was told to setup a Custom Voltage with excitation task instead of a strain task in order to get Volts that I could then use to get pounds force.

Hope that helps.

David

[jwhite]

Yeah that helps.  I've been trying to set the load cell up as a strain task without any luck so I tried the voltage w/excitation and had much more success.  The problem I am having now is I have two of the same loads cells giving me different readings when I place the same calibration mass on them. Without the mass applied, I can get one sensor to "zero out" but the other still shows a small load of about .02 lbs.  I unplugged the sensors and switched them (thereby using the opposite SCC module) and I get the opposite results.  The one that was "zeroed" now shows a slight load.  I'm thinking it may have something to do with the SG24 module, but I could be wrong.  The other thought is that since one sensor has output of 2 mv/V and the other has 2.0393 mv/V, each sensor has it's own scale but I made sure of this in the setup.  How can I get the other sensor to "zero out"?  Do I need to adjust some settings on the module, or perhaps try the calibration tab in MAX? Thanks for your help...this is so frustrating...

Jason

[Jennifer O]

Hello Jason,

There are a couple of methods to help you to zero out your channels of your SCC-SG24.  They are outlined on page 18 of the User Manual found here.  You should be able to make adjustments to each channel using the trimming potentiometers located on the top of the module.

Have you tried these methods?

Jennifer O.