strain gage experiment

[mekdeng]

Hello All,

I am working on strain measurement experiment using strain gages. I am using SCXI-1000 signal conditioner, SCXI-1317 connector block. I have a plate supported on 4 sleeves. 2 strain gages are attached to each sleeve. When I connect 4 strain gages one from each sleeve  to the connector block and apply a force of 200lbs on the plate and do a test run in labview, the strain values I am getting are way off. Here the details of inputs

Lead Resistance: 0
Gage Resistance: 120
Gage factor: 2
Strain range max = 1M
      min = -1M
V(initial) : 0
V(exe) : 5V
V(exe) Source: External
Srian gage config: Quarter Bridge 1

The Strain values I expected to be around 50 microstrain.
The vlaues I am getting are 180, 520, 370 microsrtain, fluctuating +/- 50 microstrain to the above vlaues. The fourth strain gage is reading -75microstrain and is stable. I tried changing V(exe) but nothing really chaged in readings except the fourth strain gage but it always was a negative valve.

I could not figure what am I doing wrong. If anyone in the group has any experience with this please help me out.

Thank you

Mekdeng

[devchander]

Hi Mekdeng,

I will attempt to answer your question, but i also advise that your queries on SCXI in the Signal conditioning  Discussion forum board.

Here it looks like you are using External Excitation instead of internal excitation that your SCXI 1521 module is capable of sourcing.

so, check how you have wired on your sensor to the SCXI 1317 Terminal block. 

plus your max min range settings 1M ( is it 1000000 micro strain?) seems to be too high, so check it.

Regards

Dev

[mekdeng]

Thank you Dev,

Yes, I also have SCXI-1521 module along with SCXI-1317 and SCXI-1000. When I select internal source I get a error message that source is not connected so I selected external source and did a trial run.

My strain gages have 3 lead wires and I connected then to the connector block as per the manual. 2 wires coming out of one leg of the strain gage were connected to the AI and QTR, and one wire coming out of the other leg to the Ex+ just as per the SCXI-1317 manual.

I'll decrease the max and min range of the input and try the experiment.

Thanks once again.

Hari

[Logan_K]

Hari,

Are you using a LabVIEW example for reading SCXI-1521 strain gages? What are the details of the error that you get when you try to use the internal excitation? This module is designed to use internal excitation, so your strain measurements won't work unless you select the internal option.

Are you or the exmple that you are using perform an Offset Null and Shunt Calibration? These are important for making sure that your strain measurements are accurate. When you perform these calibrations you must ensure that your strain gage is in it's resting state.

I hope this helps.

-Logan

[mekdeng]

Hari,

Are you using a LabVIEW example for reading SCXI-1521 strain gages? What are the details of the error that you get when you try to use the internal excitation? This module is designed to use internal excitation, so your strain measurements won't work unless you select the internal option.

Are you or the exmple that you are using perform an Offset Null and Shunt Calibration? These are important for making sure that your strain measurements are accurate. When you perform these calibrations you must ensure that your strain gage is in it's resting state.

I hope this helps.

-Logan

Thank you Logan,

 Iam using LabVIEW for reading strain. I could not read anything of my strain gages when I select internal excitation. But when I select external I could read some values though they are not what I am expectiing.

I tried to calibrate my strain gages using Measurement and Automation Explorer, but when I go to step where I have to select DAQ hardware , the program not detecting SCXI-1521. So unable to calibrate my strain gages.

If you know how to make Measurement and Automation Explorer detect SCXI-1521 please help me out

Thank you

Mekdeng

[devchander]

Hi Mekdeng,

Try configuring/adding the SCXI chassis to the DAQ card Properly.

These links should tell you how.

http://digital.ni.com/public.nsf/websearch/5902345227DFCA1286256EDA0063F50C

http://digital.ni.com/public.nsf/allkb/445C4DF1A24A9B8B86256D8F0078E5C2?OpenDocument

http://www.ni.com/support/scxi/scxidaq/soft.htm

Then, see if your SCXI 1521 and its terminal block SCXI 1317 are detected by your MAX

Then do a self test for chassis on max itself.

If all this goes fine, you should solve the problem you mentioned.

Hope this Helps

Regards

Dev

[Logan_K]

Mekdeng,

Those documents that Dev pointed you to are good, but in your situation I personally think the best place to get started with configuring your SCXI system in MAX is the SCXI Quick Start Guide, which should have come in the box that your SCXI module and chassis were shipped in. Here is the online link to that document if you need it:
http://digital.ni.com/manuals.nsf/websearch/A0FB7849526E33E086256F64006F628E
If you follow the steps in the quick start guide and still are having problems detecting your SCXI-1521 in MAX, then let us know the specific details of the problems.

Good Luck!
Logan

Message Edited by Logan K on 12-02-2005 08:55 AM

[mekdeng]

Thank you Logan,

I followed the steps in SCXI Quick start Guide and configured my hardware im MAX and worked fine. Now that I configured my harware I tried to calibrate my strain gages and the error I got is attached with this reply. I figured out that I got this error may be bacause I am trying to use 120 ohm strain gages with SCXI-1521 which works only with 350ohm resistors as per NI application engineers. Do I really have to get a SCXI-1521B module which works 120ohm resistors to do my experiment or get new set of 350ohm strain gages?

One thing I am amazed of is why does NI makes different modules to go with different capacity resistors. I only know about 1521 and 1521B modules and what if anybody uses 240ohm or 1000ohm resistors.. which module should they use?...

I am wondering if anyone know of any work around for me with out changing my module or strain gages.

Thank you

Mekdeng

[Logan_K]

The short answer is that you are correct. The SCXI-1521B must be used for 120ohm strain gages, and the SCXI-1521 must be used for 350ohm strain gages because they have fixed quarter bridge completion resistors built into them. If you have a non-standard or less common strain gage like a 1000ohm or 240ohm, then you must use a module which enables you to customize your quarter-bridge completion resistor, like the SCXI-1520 with the SCXI-1314 terminal block.

So, unfortunately there is not a good workaround for you with this module and I appologize for your inconvenience.

Regards,
Logan K