drifting problem

[jiqiu]

Hi,

I have a NI-SCXI-1600 USB DAQ with SCXI 1520 strain analog input. 

I had big problem with drifting. During the test period( about 8 hours), the reading varied about 40 microstrain. See attached.

I did my test in a temperature (65-75F) and humidity (45-55%) control room. I am quite sure there is no problem with my circuit. What else will cause drifting?

Thanks.

[MickeyD]

Hi jiqiu,

The drifting should be fairly minimal, but to tell you for certain if there is a problem in your behavior or not I would need to know the type of strain gauge and the range of inputs it expects and the tolerance it has for error.  Also is there a long lead resistance between your strain gauge and the SCXI 1520.  If you could also let me know what version of DAQmx you are using and how you took this 8 hour acqusition.  What programming language did you use?  that would help us to find a solution to your issue.

Have a great day,

Michael D
Applications Engineering
National Instruments

[jiqiu]

Hi,

Thanks for your reply.

The strain gage is CEA-06-250UW-120. The range of inputs are + 1500 microstrain and the tolerance is 0.3+ 0.2%.

I have 20ft shielded wire between strain gages and the SCXI1520. But the the ground wire are connected to ground.

I am using Labview 8.2. See attached. Totally I have 40 channels. Right now I am acquiring data from 4 channels (full wheatstone bridge for each channel) and the rest are not used. The programming recorded data once every one second. That's all. If you need more information, just let me know. Thanks

[jiqiu]

Hi,

This drifting problem is really frustrated.  I tried different modules and different strain gages. Always the first several hundred readings were stable and correct. After that it began drifting, went up and down.

I tried several things and it still did work:

1. I suspected that it was caused by unterminated inputs floating to the power supply. I connected all unused channels to the ground (by connecting S+ and S- to the ground). It did not work

2. a bias current problem in my connection. I added two resistors as follows. It still did not work.

I do not what i can try next. Please suggest as quick. Thanks

[MickeyD]

Hi,

Looking at the code you posted it seems you are taking the average of a set number of points acquired and when you do that you are getting a range of values of about 40 microstrain.  The Bias current should not be a problem so that would not change the way your measurement behaves.  The drift you are seeing over the course of that time could definitely be related to your strain gauge being heated by the continual excitation it is being exposed to.  This could result in a slight changes in the resistive behavior of the active elements.  Do you know if the Full Bridge configuration is setup in anyway to test for temperature changes in the strain gage element.  Another possiblity would be a large lead distance exposing your single ended signal to large noise issues.  Differential signals do not have this problem, but single ended measurements don't behave well when dealing with large lead lengths.

Looking at the following documents might give us a start on where we could be going wrong.

Field Wiring and Noise Considerations for Analog Signals

This would be a good place to inspect possible noise issues you could be seeing.

The other document has more to do with possible temperature issues with the long term operation of your strain gauge.

Measuring Strain with Strain Gauges

I will keep looking into the issue.  A good way to test out your device would be to connect a known voltage to the channel terminals and see if the voltage reads in appropriatley.  It would be a good step to eliminating any of the internal circutry as responsible for the drift we are seeing.

Have a great day,

Michael D
Applications Engineering

[jiqiu]

Hi Michael,

Thanks for your reply. Here are the results according to your comments

1. strain gauge being heated by the continual excitation it is being exposed to.

I tried to acquire 1 data every 5 minute, so there is no heat accumelated. But I got similar result---drift.

2. a large lead distance exposing your single ended signal to large noise issues

I read Field Wiring and Noise Considerations for Analog Signals. I connected the "S-" to the ground. I got a better results. The drifting was within 10 microstrain over 8 hours (I acquired one reading every 2 minutes). But you can still see it drifted down and there are some small cycles, go up and go down. See attached. I do not know if it is normal or still something is wrong.

Thanks for your assitance.

[jiqiu]

I want to the test---connect a known voltage to the channel terminals and see if the voltage reads in appropriatley

Can you give more details how to do it? My understanding is connecting S+ and S- to a known voltage. But how can I get voltage readins.

Also what do you mean "Full Bridge configuration is setup in anyway to test for temperature changes in the strain gage element"?

Please suggest. Thanks