needed inputs for measurements using NI9205

[kmmlvswnth]

Hi,

I am using c series NI9263 AO module and NI9205 AI module in a NIcDAQ 9174 chassis to send signal (send command displacement in form of voltage) to an external MTS controller and measure the actual displacement, force from MTS (closed loop feedback). I am using Labview 2010 to perform this. I am having trouble with couple of issues as below

1. What is the best way to eliminate noise in the measured signal?  My measured signal has lot of fluctuation (noise). I am using differential measurement system, range of +- 10V, have grounded NIcDAQ chassis( grounding screw). I am trying to use lowpass butterworth ptbypt filter, however I am not clear what should be my cutoff frequency and the order?
2. What is the reasonable range one can expect for a given command? I mean if I am sending 4.00V then what could be the reasonable fluctuation range? what is the ideal level of noise for my Voltage output/measured range.

I appreciate any help in this regard, since from civil background I am facing it hard to figure out. Thanks.

[Michelle_N]

Hi - I would start with this:

Noisy Readings on the NI 9205

http://digital.ni.com/public.nsf/allkb/62F7C1C5DD187E58862576B200700323?OpenDocument

In general here are some links for noise reduction:

Five Techniques for Reducing Measurement Noise

http://zone.ni.com/devzone/cda/tut/p/id/6830

[kmmlvswnth]

Hi Michelle,

Thanks a lot for the speedy inputs. I need some help on the grounding issue, I am really confused

1. What is the difference between NicDAQ 9174 grounding screw, and NI9205 module AIgnd (terminals COM 10 & 21 fig 2 in NI9205 operating instructions)? Currently I am grounding the 9174 chassis (grounding screw) to the MTS controller ground, so both source and measurement system  have common ground (Here since I am using chassis ground, I am presuming all the cDAQ modules(9205 & 9263) connected to this chassis USB ports also have the same ground, and each individual module need not be explicitly grounded). Am I correct or wrong?  And is there is any chance that the there could be voltage difference between chassis ground and individual module ground and a possibility of ground loop current flow?
2. Or is it a safe idea to ground all the ground terminals available (like chassis ground screw, NI9205 COM terminals or AI sense(which is correct), NI9263 I am not sure how to ground it)

Technically which is a good idea? Your help is really appreciated. Thanks in advance.

[kmmlvswnth]

Hi Michelle,

Thanks a lot for the speedy inputs. I need some help on the grounding issue, I am really confused

1. What is the difference between NicDAQ 9174 grounding screw, and NI9205 module AIgnd (terminals COM 10 & 21 fig 2 in NI9205 operating instructions)? Currently I am grounding the 9174 chassis (grounding screw) to the MTS controller ground, so both source and measurement system  have common ground (Here since I am using chassis ground, I am presuming all the cDAQ modules(9205 & 9263) connected to this chassis USB ports also have the same ground, and each individual module need not be explicitly grounded). Am I correct or wrong?  And is there is any chance that the there could be voltage difference between chassis ground and individual module ground and a possibility of ground loop current flow?
2. Or is it a safe idea to ground all the ground terminals available (like chassis ground screw, NI9205 COM terminals or AI sense(which is correct), NI9263 I am not sure how to ground it)

Technically which is a good idea? Your help is really appreciated. Thanks in advance.

[Michelle_N]

I assumed you were only getting noise on the AI of the 9205. If this is the case, you will need to connect the COM on the 9205 explicitly to ground. The module is isolated, so when doing an AI you need to connect COM to gnd. The link above shows the different signal connection configurations but the important thing for you is that the all involve connecting COM to ground. Try this and let me know if it clears up your signal.

[kmmlvswnth]

Hi Michelle,

Thanks for the help. Now I have grounded all the COM terminals and my results are good, consistent. The noise levels reduced to a range of +- 8mV. I am still working them to bring in to range of +-8mV. I would like to know is there any relation between AD accuracy (16 bit which mean 0.3mV) and noise levels??

Also I observed one more issue, when I am reading some unwired channels in my Lab-view program it is reading voltage which is progressively increasing (Ideally it should read 0V with some noise), and when I checked with multimeter, the reading  goes back to zero as soon as I bring multimeter terminal ends in touch with the AI9205 Input channel terminals. The technician in my lab advised me that there is some charge being developed, some capacitance effect, may be because of poor wiring, and it may or may not contribute to noise. Can you suggest me some thing here? Is it a serious concern, and does it anyway can contribute to noise?? Any suggestions how to deal with this. Thanks in advance.

[kmmlvswnth]

Hi Michelle,

I would be glad if you could help me how to get accurate results with very low level voltage commands, I mean if +-10V equal to +-10mm and then if I command 0.003mm, which mean 0.003V, I am unable to capture my signal correctly. I am reading some noise.

Also what type of filter would be ideal choice in my case.Thanks in advance. 

[kmmlvswnth]

Hi Michelle,

I would be glad if you could help me how to get accurate results with very low level voltage commands, I mean if +-10V equal to +-10mm and then if I command 0.003mm, which mean 0.003V, I am unable to capture my signal correctly. I am reading some noise.

Also what type of filter would be ideal choice in my case.Thanks in advance. 

[Kyle_A.]

Hi Vishh,

If you are reading a DC signal, a low pass filter either externally or in LabVIEW would be best for you.  Noise is most often high frequency, so a cutoff frequency set relatively low will help clean up your signal.  I don't know what frequency you are sending (if more than DC) but if you are updating DC voltages you can set the filter low to around 15 Hz or so, but any value that eliminates noise in your system should work.  There are filter VI's you can use or you can try the Filter Express VI.  This is all done post processing, which means you will have to acquire the noisy signal and bring it into LV, and then filter the result.  External filters would clean up the signal previous to acquisition.

For low voltage readings, you want to be sure to set your input range to be the ±200mV range on the module.  This gives you 400mV / (2^16 - 1) = 6.10uV per step resolution. 

As far as your signal, there are various ways you can reduce noise, such as shielding your cables, using short cable lengths, eliminating noise sources, and taking differential measurements like you're already doing.  I hope this is helpful.

[kmmlvswnth]

Hi Kyle,

Thanks a lot for the mail.  I am not clear on, what do you mean by DC frequency, I am using two timed loops in Lab view, one to update the AO (NI9263) output module 100 times a second and one more to read signal from AI (NI9205 module) with the same frequency. And I am using Butter worth PtByPt Low pass filter with low cutoff frequency of 10Hz.  

Does this seem good? Or may I  need to change filter properties? I don't know how a filter works, so I was just playing with parameters. Thanks.