Acq Wheatstone bridge Measurements (Lbs) are negative.

[Vishwas]

I'm absolute novice working with NI tool Labview and NI Hardware. I'm right now trying to learn to use a data acquisition module. I tried running an example design in the Labview tool. The example design is the "Acq Wheatstone Bridge Samples". The results of firing the simulation in the Measurement (lbs) waveform window are -ve. Why is it negative? Also when I vary the strain on the sample "metal" there is change in the acquired values, so I know for sure that the system is capturing some values. I right now have setup a quarter bridge.

[RT4CY]

Hi Vishwas,

Thanks for posting on the NI forums!  I understand that you are trying to acquire data from a quarter bridge strain gage circuit using the Acq Wheatstone Bridge Samples.vi shipping example.  You also mentioned that you seem to be getting negative values for your data.

It would be helpful if you could provide your LabVIEW version, as well as what hardware you are using.  It may be something as simple as having your leads wired backwards.  It is helpful to create the task in Measurement & Automation Explorer (MAX) and examine the Connection Diagram.  Create the task by right-clicking on Data Neighborhood and following the subsequent prompts.

After you are finished creating the task, click on the Connection Diagram tab on the bottom of the configuration window.  This will show you how to physically wire your leads to your device.

[Vishwas]

Hi Rod,

Thanks a lot for your email. I'm using Labview 7.1 and I'm using a SXCI-1520 supported by a SCXI-1314 to implement my Stain guage system. I have implemented a quarter bridge. I do think it is the polarity because I flipped the polarity and the output is still negative.....I also did the measurement and automation explorer annd I check the connections. are fine. +ve to +ve and -ve to -ve.

Also,what is the best to get good at working with NI tools and hardware other than taking a formal training from NI, because that is not an option? Kindly advise.

Thanks,

Vishwas

[RT4CY]

Hi Vishwas,

Are you saying that when you invert the polarity you still get a negative value?  Are you getting the same results when you run a test on the task created in Measurement and Automation Explorer (MAX)?  See my previous post about creating a task in MAX.  You can run a test by simply clicking Run above the graph.

Let’s verify that your strain task is configured properly.  Ensure that you have the proper values for gage factor, gage resistance and excitation voltage.  Also verify that you are using the correct physical channel. 

If you have checked your software settings as well as your physical connections and are still experiencing difficulties, I would begin troubleshooting the 1520 module itself.  Disconnect your strain gage from the 1520.  Apply a known voltage from a source or a battery and connect the positive terminal to S+ and negative terminal to S- on the SCXI-1314 terminal block.  See the following KnowledgeBase  article on how to correctly configure a Voltage task for the 1520 module.

As far as learning LabVIEW, I would recommend this link.  We offer free 3 and 6 hour courses that you can go through at your own pace, as well as several white papers and developer zone articles.  As far as learning DAQmx, I would recommend Getting Started with DAQmx.  Here you can find some great instructional videos to get you started programming with our DAQmx driver.