Measuring Force with Strain Gauge (Gage?)

[DailyDose]

Note: For the sake of this discussion, I will use the spelling 'gage' unless someone corrects me.

The hardware I am using is an SCXI 1520 Strain Gage.  The chassis is 1001 although I believe that is irrelevant.  It's all hooked up correctly talking to a PCI.

The client wants to measure Force and has been measuring Force.  So... I made the mistake of making my program using the Force DAQmx VIs because to me, it made sense.  Force can still be measured using a Full-Bridge, so why not?  Well, having no hardware to test on, I delivered and it didn't work (naturally).  After some tedioius tinkering, eventually figured out you can't use Force DAQmx with the SCXI 1520, you have to use AI->Strain.  Which I guess is a duh moment seeing as how the 1520 specifically says Strain Gage, but whatever.

Anyways, the client has been using this exact hardware for the same application to measure Force.  They just wanted updated software.  So now the engineering piece comes into play.  How to go from strain to Force.  I've been doing some research to see if there's some formula that will take into consideration things like X mV/V and a 2-point linear scale.  So how does one go about resolving this?

Is there a formula?  Do I create a scaling system in NI-DAQ?  Should I be using playin AI and not Strain so I can simply go from a voltage to Force?  Any help here would be hot.  Thank you.

[DailyDose]

One more thing...
I continue to get a 200088 Error when trying to do an Offset Null Calibration.  NI Error chart says the following: "Task specified is invalid or does not exist."  Now, I know the task exists, so I'm guessing I fall under the invalid category.  Does this VI not work with SCXI?  I am seeing absolutely no documentation on this.  Maybe someone out there has experience with this b4.

And to my previous post, I have found this white paper and am reading through it.  I just like to let people know I am indeed trying to help myself.  

[t@nim@t0R · Accepted solution]

In terms of the engineering side of things, there is a fundamental relationship between strain and force that is described by Hooke's Law. Hooke's Law relates stress and strain by bringing in material constant, and then the stress can be converted to a force through an understanding of how you are loading your sample in question. These fundamentals are talked about in more detail here: https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=me&chap_sec=01.3&page=theory

In terms of your application, however, it may be easier to read voltage and scale it to force with some testing. To do this, follow the steps outlined in this KnowledgeBase article: 
Reading a Voltage with the SCXI-1520: http://digital.ni.com/public.nsf/websearch/7A84A5BAA867779886256A71006DFA0E?OpenDocument

In regards to the offset nulling, this can only be done in a Voltage Task. The steps to utilize the offset null are described here: 
Can I use the SCXI-1520 to Null the Offset of a Voltage Signal: http://digital.ni.com/public.nsf/allkb/F1A1655B76A64E5286256A77006AF2C2

Regards,

Tanim Islam

Applications Engineer | National Instruments

ni.com/support

[DailyDose]

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t@nim@t0R wrote:

In terms of your application, however, it may be easier to read voltage and scale it to force with some testing. To do this, follow the steps outlined in this KnowledgeBase article: 
Reading a Voltage with the SCXI-1520: http://digital.ni.com/public.nsf/websearch/7A84A5BAA867779886256A71006DFA0E?OpenDocument

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This is what I was thinking that I have no choice but you measure the Voltage and convert to Force.  Which I guess doesn't really seem too tedious.  This would mean for a load of 100 lbs and a ratio of 10mV/V, each mV is a lb assuming my excitation is 10V?  Which leads to the next question, how does one enable the excitation voltage when only using the AI Voltage DAQmx VIs?

[DailyDose]

I used AI Custom Voltage With Excitation.  That seems to work.  And then I'll just have to do the mV/V --> lbs conversion with the voltage I read.

[t@nim@t0R]

Glad to hear you were able to configure the voltage. Yes, the scaling will need to be configured, and can be tested using known forces. 

Good luck!

Regards,

Tanim Islam

Applications Engineer | National Instruments

ni.com/support