step response using USB daq 6215

[samtha]

Hi there

I am using DAQ 6215 (USB) to perform step response of a device, i am using transition and pulse measurement function to determine the rise time and fall time 

but it doesnt seem to work for the frequencies below 0.5hz how can i solve this issue is there any other way and also how can i determine the settling time of the response signal please suggest me something i am attaching my VI just check it out thank you........

[Doug_L._Bear]

You seem to be missing two of your attached VI's, the ones that configure your tasks so I can't tell how your timing is setup.  Are you getting an error occuring?  I was able to perform a rising edge time analysis at 0.25 Hz without issue so long as I was taking a large enough chunk of the waveform so that the analysis VI could see the rising edge.

[samtha]

Hi Doug,

Sorry for missing out the sub VI's here i am attaching them.....thank you for the reply 

[samtha]

Hi Doug,

I have set the sampling rate (o/p) and samples to write to 800000 similarly sampling rate (i/p) and samples to read to 250000 it (loopback) seemed to work i.e it fast and the transition measurement block doesnt seem to bother me at low frequencies but i still have to test it real time i dont have the device now once i do the test i will let you know if it is really working thank you......bye.....

[samtha]

Hi Doug,

With the above mentioned settings there is another problem at frequency 0.1hz the frequency i detected using extract single tone measurement indicates 99.7k and it fluctuates between this value and 2.07its not constant it is the same case till 2hz i dont understand the problem pls check this i am attaching my VI with slight changes thank you bye......

[Doug_L._Bear]

The problem is not with the VI, it is with the timing parameters you are using.  You are acquiring 250,000 samples at a time at 250,000 samples/second.  Meaning that you are acquiring (and analyzing) 1 second of data at a time.

To be able to perform a single tone extraction properly you need at least double the period of the waveform, this is why the issue resolves itself at 2 Hz (or with a period of 0.5 seconds).  If you are acquiring and analyzing less waveform data than this, you will not read the correct frequency as there will not be enough data.

[samtha]

Hi Doug,

 I am able to determine the overshoot, rise time and fall time but how can i determine the settling time of the response signal can you give me a suggestion bye.........

[Doug_L._Bear]

Settling time is generally accepted to be within 2% of the desired amplitude.  There is no VI or anything that I can find to do this in LabVIEW.  What I would recommend is that you have to just do a manual compare of the signal you are reading to the desired amplitude and flag the time at which it gets within 2%.

http://en.wikipedia.org/wiki/Settling_time