Timing and triggering with several outputs and inputs

[MathieuSG]

Hello,

We recently bought a cDAQ 9172, with a 9233 IEPE for acoustic/vibration measurement, a NI9215 for analog voltage measurement and a 9263 module for analog output.

The 9215 module is used in order to perform acoustic measurements but with small condenser microphones (used in mobile phone for example).

Do you know a method to test if my outputs, IEPE inputs and analog inputs are synchronised and start all together?

I used this method at the moment : http://zone.ni.com/devzone/jsp/largeImage.jsp?imagename=/cms/images/devzone/tut/a/60319f241026.gif&l...

It doesn't work for the 9233. By reading helps, I understood it is due to "delta sigma technology" and anti-aliasing filtering, but I don't know how synchronize all my modules. Do you have an idea or advice to solve my problem ??

Furthermore, I would say that this example synchronizes the samples recording but I am not sure that the output task and input task start simultaneously (because there is no triggering VI). Is it right ?

How could I make them starting simultaneously, for example if the user push on a start button ?  

In the future, I also would like to use 2 synchronized outputs : 1 is used for the swept sine generation and the 2nd is used to drive a turntable, in order to do directivity measurements. For example, if the 2nd output generates 0,1 V, the turntable is at angle 10°, if voltage is 2,5 Volt, then angle is 250°....

Do you have an idea/advices to help me ? Do I need to use trigger, event stucture, flat structure, timed stuctured ???? What would be the easiest way for a beginner like me ??

Thanks a lot if you can help me.

Mathieu

[dousley]

Hi Mathieu,

I found this knowledge base article: 43J65CAY which is about compensating for ADC group delay in sigma delta converters like the ones on the 9233.

http://digital.ni.com/public.nsf/allkb/CAE12AA9BCAA51A48625722C004866F8

So if they are all sharing a sample clock, but the 9233 appears to be lagging behind, it sounds to me like you are right. It has to do with the architecture of the ADC on the 9233 (similar to what would be found on other 24-bit audio type devices). Putting in a delay like the knowledge base article describes ought to improve synchronization.

As to the analog output, I have some questions for you. Do you need to hardware synchronize your analog signal generation to the turntable or would software sequencing be sufficient? I'm imagining that you have a table that you set to a particular position, run a test, and then turn the table to a new position, and run another test, and so on. Is that the case? If so, you may not need to synchronize the two outputs. You could just make sure that your sequence in software allows for sufficient delay between when you turn the table and when you run a test. 

Although there is more than one way to do it, you could definitely use a sequence structure (flat or stacked) to ensure the order of each task. 

Hope that helps.

[MathieuSG]

Hi Dousley,

thanks a lot for your answer. I'm going to study your article concerning delay compensation.

Concerning the measurements with the turntable, you are right: the table is at 0°, a measurement is done, then it turns to 15°, a 2nd measurement is done, ....

I 'll try a sequence structure. In fact, I had imagined a much more complicated system 🙂

Thanks for your advices.

Mathieu