external time base changes with velocity

[wayuu]

Hi everybody
I am using output channel A of a quadrature TTL encoder as external time base of the acquisition of three analog input signals. I acquire a total of 1024 scans, one scan for one pulse of the encoder. I use this system to acquire signal from an internal combustion engine. When the velocity of the engine is 1700 rpm, my system acquires the number of cycles of the signals that I expect. But when the velocity is 2000 rpm (or more) my system acquires more cycles altough the number of scans don´t change. It is like the board (or anything) doesn´t read all the pulses of the encoder). I use LabView 5.0 and the example: "Acquire N Scans ExtScanClk D-Trig.vi" for my acquisition. My board is PCI 6023-E.
Have anybody any idea w
hat should I do? I appreciate your help.

[Jorge M.]

Hello Wayuu,

what I'd do is to analyze the encoder signal to see if it's a clean signal at that speed. Depending on the equipment around and the working conditions, chances are that your signal degrades at that speed.
Another thing I would definitely try is to generate a TLL pulse train with a function generator, and feed the board with that clock. By doing this you could check various clock frequencies and verify that the board is working properly.

Hope this helps !

[wayuu]

Hello Jorge and everybody

First of all, thank you for your help.
I tried what you suggested and these are the results:
-The encoder signal is clear at 2000 rpm.
-Using a function generator, the problem comes back.

Something new that I saw is that when I acquired only one channel, my system acquires the number of cycles that I expected (also at 2000 rpm), but If I acquire two channels the problem comes back.

I don´t know what to do. I really appreciate if somebody can help me.

Kind regards.
Wayuu

[Jorge M.]

Wayuu,

according to what you say, it seems fairly clear that the board is not missing pulses and that those pulses are a clean signal.
What comes to my mind now is... Why do you say that you get more cycles? I mean, if you always get the number of samples you request, what's the problem? Maybe a delay between channels?
If that's the case, then it may have something to do with the interchannel delay parameter.
You can change the value of this parameter with the AI Config vi.
I would first check these links below that talk about it, and see if the behavior you are seeing is just an effect of your board having a unique ADC and hence a delay between samples of different channels.

http://digital.ni.com/public.nsf/websearch/9AE87416C8792FC286256
D190058C7D3?OpenDocument

http://zone.ni.com/devzone/devzone.nsf/webcategories/E8C00D6CC2BDD44386256882001425DA?opendocument&node=

Regards,

[wayuu]

Hello everybody

Jorge, thank you so much for your help.

I tried changing interchannel delay and the acquisition of two analog channels was all right (at 2000 rpm), altough this not worked acquiring three channels.
I calculated the delay this way:

(60/rpm)*(1rev/1024S)*(1/number of channels).

Is this estimate all right? Have I to change another parameter (such as interchannel delay)?

Thank you and regards.

[Jorge M.]

Hello,

the interchannel delay is the time between samples (of different channels). Example: If you acquire 3 channels, and want to sample every channel every 3 seconds, the maximum interchannel delay you could use is just below 1 second.
The formula is here:

http://digital.ni.com/public.nsf/websearch/9AE87416C8792FC286256D190058C7D3?OpenDocument

This can also be seen more clearly on this link's picture. The scan clock would pulse in my example every 3 seconds while the interchannel delay is the time between one sample of channel 1 and one sample of channel 2.

http://zone.ni.com/devzone/conceptd.nsf/webmain/7D8EAEA1D87CFB2686256802007B8C52?opendocument

I'would try to play with the interchannel delay in order to get c
orrect measurements.
Hope this helps!