Strain Gauge Problem

[James P. Martin]

Hello All,

I am using SCXI 1520, Terminal Block 1314 and DAQ 6035 E Card, for Strain Gauge Measurement.

We have got a Strain Gauge of 120 Ohm with the NI Hardware which was connected to CH02(One of the 8 channels provided), Now my problem is that if I connect the Strain Gauge to some other channel(except Channel 2) I am not able to get any readings , and if I reconnect it back to Channel 2 it gives prorper results.

I have connected the strain Gauge in a Quarter bridge Configuration(obviously for a Single Strain Gauge) as per the circuit diagram provided correctly.

I have Pentium -4 , 256MB RAM, Win XP SP2 machine.

Could anybody please try to explain where I am going wrong.

With Regards

[Jim Kring]

it could be a problem with your terminal block. they sometimes go bad.

[Neville D]

Hello All,

I am using SCXI 1520, Terminal Block 1314 and DAQ 6035 E Card, for Strain Gauge Measurement.

We have got a Strain Gauge of 120 Ohm with the NI Hardware which was connected to CH02(One of the 8 channels provided), Now my problem is that if I connect the Strain Gauge to some other channel(except Channel 2) I am not able to get any readings , and if I reconnect it back to Channel 2 it gives prorper results.

I have connected the strain Gauge in a Quarter bridge Configuration(obviously for a Single Strain Gauge) as per the circuit diagram provided correctly.

I have Pentium -4 , 256MB RAM, Win XP SP2 machine.

Could anybody please try to explain where I am going wrong.

With Regards

Have you tried re-initializing your SCXI modules and DAQ card through NI-MAX?

Sometimes things go funny with SCXI and re-initializing seems to help.

NI-MAX>Devices & Interfaces>DAQmx Devices>SCXI module <right click and select initialize for each of the modules and the DAQ card as well.

Hope this helps.

Neville.

[James P. Martin]

Hello

Thank you all who replied to this topic.

My problem is solved actually I was using 120ohm strain Guage and in 1314 there are 350 Ohm completion resistors on all channels except Channel 2.

But Now As I am configuring full bridge , so I dont have to worry about completion resistors and I am getting output

Here I wanted to know that the result(Output) which I am getting , what to infer from it??(Is it strain reading or just some voltage??) and Do I need any kind of calibratrion to get some meaningful measurement.

As I am a student please forgive me if my query is not good enough.

Thanks & Regards

James

[pallen]

Here I wanted to know that the result(Output) which I am getting , what to infer from it??(Is it strain reading or just some voltage??) and Do I need any kind of calibratrion to get some meaningful measurement.

I'm using a similar setup currently.

The raw values you're seeing are "meaningful". But you may want to scale them into units such as Newtons or Pounds.

This can be done several ways. Search "Wheatstone Bridge" in the examples.

The way I scaled my load cells was use the manufacturers supplied calibration documentation, and apply that to a Map Ranges Scale I configured in MAX.

Using the supplied Full Scale Output values and my excitation values, I was able to set Min and Max values in pounds for my load cells. Super easy! And each of them bang on when I verified.

Good luck.

[Daniel B]

I'm using a similar setup currently.

The raw values you're seeing are "meaningful". But you may want to scale them into units such as Newtons or Pounds.

This can be done several ways. Search "Wheatstone Bridge" in the examples.

The way I scaled my load cells was use the manufacturers supplied calibration documentation, and apply that to a Map Ranges Scale I configured in MAX.

Using the supplied Full Scale Output values and my excitation values, I was able to set Min and Max values in pounds for my load cells. Super easy! And each of them bang on when I verified.

Good luck.

James,

I have been trying the same measurment with my SCXI 1520, Terminal Block 1314 and DAQ 6036 E Card using LV 8.0.1 daq-mx, etc...

Setting up the strain measurments with a MAX "strain" task (check in the generatre daq-mx tasks) was the easiest way I found to properly scale the outputs. There is a strain task with all the necessary scaling info built in. This task could be wired into a quick express VI for waveform display or an indicator, etc.

As for wiring the bridge, I have 350Ohm gages and used factory installed resistors the quarter bridge type one configuration. I got it working by follwoing the example wiring diagram in MAX, but had trouble when wiring it up as shown in a similar diagram in the 1520/1314 manuals.

Also, 2 of my 3 1314 termal blocks has lost a channel, check it for voltage with a DMM, if you turn the task on in max, the P+, P- circuit should power up on the selected channel.

-DRB

============================

[Bill Gilbert]

I suppose it's a little late to be chiming in, but depending on how you configure the gauges, you will(hopefully) get a voltage proportional to the strain and the level of applied excitation. As you are finding out, there are lots of ways to configure and apply the gauges.

If you are doing experimental stress analysis, the manufacturers of the gauges will have lots of reference materials and calculators you can use to come up with likely values of voltage per unit microstrain. If you are experimenting with transducer design, can use the experimental stress formulas to guess at the ouput of your design, then probably run a calibration using a deadweight tester or comparison to a trusted transducer.

If you were using off the shelf load cells or pressure transducers etc., you would be given a full scale output value in millivolts output per volt of applied excitation , mV/V. They are generally temperature compensated as well.

-BG

[James P. Martin]

I suppose it's a little late to be chiming in, but depending on how you configure the gauges, you will(hopefully) get a voltage proportional to the strain and the level of applied excitation. As you are finding out, there are lots of ways to configure and apply the gauges.

If you are doing experimental stress analysis, the manufacturers of the gauges will have lots of reference materials and calculators you can use to come up with likely values of voltage per unit microstrain. If you are experimenting with transducer design, can use the experimental stress formulas to guess at the ouput of your design, then probably run a calibration using a deadweight tester or comparison to a trusted transducer.

If you were using off the shelf load cells or pressure transducers etc., you would be given a full scale output value in millivolts output per volt of applied excitation , mV/V. They are generally temperature compensated as well.

-BG

Hi

I am doing starin gage measurement for experimental setup in my Lab , now I have successfully employed the strain gauges on the beam and connected them in a full bridge configurations.

Actually I am wondering about the fact that as I am using SCXI 1520 and Block 1314 which are meant for strain gage measurements , now when I run the strain gage measurement program provided by NI in examples so what I am gettining voltage coressponding to strain or Strain itself.

Another query is that what kind of calibration I will be needing in doing so, or why I need calibration when I will be developing my own program for strain gage measurement.

I think its a bit basic questions to be asked but everyone has to start from the scratch.

Thanking you

James.