cRIO-9073 AI signal noise

[smartwater]

Hi,

I am integrating several meters with 4-20mA signals into a LabVIEW program for simple data acquisition. I am consistently seeing a +/- 0.6mA oscillation around the actual value. This holds true for 3 types of meters (pH, temperature, and quanta) so it is not an issue with the meters/transmitters. The signal coming from the meters is grounded.

I am working with an NI cRIO-9074 chassis and a 9203 AI module. The chassis programming mode is set to scan mode. I have tried (2) different AI modules and get the same problem each time. I have a 9073 chassis I could try, if its possible the chassis is the problem.

I have found several threads in this forum regarding similar issues and most of these were resolved using the FPGA capabilities of the chassis and applying a filter to smooth out the AI signal. I don't have the dev time required to learn the ins and outs of FPGA so my question(s)...

- What could be causing the oscillations in the AI signal? I.e.. what can I do to further reduce the noise I am seeing

- Is it even possible to get a steady signal using my equipment in scan mode? If not then I may be forced to utilize FPGA...

If there's a thread I missed that covers this then please refer me to it. Thanks in advance for any help!

[LVbum42]

Are the wires running next to any power sources?  The oscillation tells me it may be environmental noise from an induced electric field.  You can try running an FFT on the signal to see what frequency the oscillations are occurring.  If it's around 60Hz, it is most likely environmental.  If this is the case you could try using shielded wires or positiioning the wires further away from any electrc fields they may be close to.  Otherwise, the FPGA solution would be to implement a notch filter for the frequency of the oscillating noise.

[johnsold]

The inputs to the 9203 are ground referenced. You indicate that the signal from your meters is grounded. I do not understand how you can have a grounded source and a grounded AI device and expect to measure current.  How do you have things connected?  What is the path for the current?

Lynn

[smartwater]

Isaac - The wires run next to the cRIO units power supply as well as a larger power supply. We are currently using shielded wire (2 stranded w/ 3rd shield strand). I'll try moving the larger power supply if this could help, though.

I have not done a Fourier transformation since college...I know you can do this in excel but have not had much luck with it so far. I'll keep trying but do you know of (or can you reference me to) a good source of how to do this?

Lynn - you are correct, my lingo was bad. I have one of the two strands connected to an AI spot and the other connected to the common ground spot on the AI module.

The 'grounding' I was referencing was with the 3rd shield strand. I have attached the 'AI module-end' of this cable to a nail in the wall. The other end of this is not connected to anything (trying to avoid a ground loop)

Thanks for the responses! My electronics background is not all that strong, so thanks for sorting through my incorrect lingo and situation.

[smartwater]

A quick update...moving the cRIO unit and transmitters away from one of the power sources resulted in a drop in the noise level to about +/-0.4mA for 5/6 of the signals. The 6th signal has minimal noise. This has been the case since before I moved anything. Does it make sense that this is because this single signal is isolated from any electrical fields? I'm not quite sure how to explain this...

I’m going to further isolate the cRIO from all power sources when I'm back on site tomorrow. Thanks again for the recommendation!

[Wayne.C]

smartwater,

I have experienced similar problems with the 9203 module.  Something about it's design makes it more susceptible to certain kinds of electrical noise.  Sometimes several turns of signal cable put through a toroid will kill the high frequency hash. 

[johnsold]

It sounds like your wiring is picking up interference from other devices. That is about all it is possible to say without detailed knowledge of all of the devices and wiring. Interference can be coupled capacitatively, inductively, or through shared impedance. Often the coupling is difficult to determine even when the system is well documented.

Try to see if there is any correlation between the interference and the behavior of the external devices. Does it go away when they are off, in standby, or operating in certain modes? Does anything they do make it worse?  Does the interference have the same frequencies as signals produced or used by the external device (but not present in the desired signal)?

Are grounds wired sequentially, in star fashion, or in parallel?

Are all "grounded" points at the same potential?

Lynn

[smartwater]

Wayne - I tried looping several turns of wire through a toroid...this did not help. This may be because the signal wire I am using is shielded.

Lynn - grounds are being wired in parallel. I would assume they are at the same potential...could I check this with an ammeter? I'm going to take a closer look at the frequency of the noise and experiment with different settings. So far the noise is unaffected by whether the transmitter is on/off. It is even noisy when the channel is not connected to any wires.

What's weird is that I get a steady mA value when I check the transmitter AND AI module end with an ammeter. Does this mean that the noise is being generated between the AI module and the cRIO controller?

Thanks again,

Jeff

[johnsold · Accepted solution]

Jeff,

It is hard to accurately measure ground currents and differences in ground potential.  Just connecting the instruments sometimes changes the distribution of the currents.

The star configuration is the best grounding method for reducing noise. It assures that no ground currents are common to multiple conductors. Un fortunately in most systems it is impossible to connect the grounds that way.

Your ammeter may have a slow enough response time to effectively filter out the variations.  An oscilloscope is much better for tracking interference.

What is your sampling rate? What does the signal look like on a graph? Can you post your VI with some typical data saved as default?

Lynn

[smartwater]

Hi Lynn,

I did some rewiring and was able to ground the shield wires together in a star fashion. Unfortunately, this had no effect on the noise. I also do not have access to an oscilloscope. I've replaced both the cRIO unit and the module so I don't think its faulty equipment.

I've attached the VI we are using and some sample data. The first data column is pH and the 2nd is temperature (in A, not scaled). The sample rate is 5s. The scan rate is 10ms. Manipulating either of these has no effect on noise.

I've started looking into anti-aliasing filters. Do you (or anyone else) have any experience using these/could you share? Are there other ways to reduce noise aside from those you've previously recommended? I'm willing to try anything at this point. Thanks for your continued help!

- Jeff