topic Re: data acquisition in Multifunction DAQ

data acquisition

kgy — Fri, 15 Aug 2008 20:12:56 GMT

I have USB6289. 'm trying to use DAQ assistant to do data acquisition in labview. (the DAQ assistance is directly linked with Write Labview Measurement File to save to a outside exl. file). For the acquisition mode, I want continuous recording until stop. I select acquisition mode to be continuous samples, but how to set the "samples to read" and rate? It seems it won't accept large numbers, e.g. 1G. In fact, I may collect data for long time(more than half an hour). What's the best way for setting?

In singal experess, I have similar problem for setting the Timing settings, so that I can collect all the data until physically stopped. How should the setting be in signal express?

Re: data acquisition

Hillman — Mon, 18 Aug 2008 13:53:54 GMT

Hi kgy,

Heres an example code that should work. I am not aware of any issues with 1G samples to read, unless the DAQmx read timesout or you cant buffer enough data.

Let me know how you get on,

Regards,

Re: data acquisition

Paul_C. — Mon, 18 Aug 2008 16:04:48 GMT

Hi Kgy,

The reason samples to read cannot be set to 1G is mostly likely because you cannot allocate enough memory to handle that much data. If you set the samples to read to 1G, your computer has to be able to hold all of those samples in memory at one time. If you need to collect a large amount of data, I would recommend acquiring it continuously in smaller sets of data and just controlling how long it runs. For example, if you need 1G of samples at 1KHz. Then you can read a 1000 samples at a time and transfer those to hard disk each iteration of a for loop. From there you can control the number of iteration to just acquire the 1G samples. For my above example, it would need to iterate 1M times. Below is some code that demonstrates the behavior I'm talking about.

In LabVIEW SignalExpress, you can do the same thing by placing your DAQmx acquire and write to file steps inside a conditional repeat for the number of iterations you want. This will again acquire smaller number of samples each time. It is important also to understand that in both examples the task is not stopping. It is acquiring data continuously and just fetching data from the buffer in smaller chunks that your memory can handle.

Note: Make sure you run once and not continuously in SignalExpress.

I hope this helps,

Paul C.

Message Edited by Paul C. on 08-18-2008 11:04 AM

Re: data acquisition

kgy — Mon, 18 Aug 2008 20:13:44 GMT

Many thanks, Hilman and Paul. Unfortunately, I can't open the file sent by Hilman due to version problem. I'm using version 7.1.

Paul,

I've made the same program as you gave. Unfortunately, the while loop does not seem work.It stoped after collecting the number of sample I specified. please refer to attachment for my setting. Is there anything wrong? Or did I miss any settings in the DAQ assistant?

Re: data acquisition

Paul_C. — Tue, 19 Aug 2008 15:38:29 GMT

Hi Kgy,

You have to set the DAQ Assistant to acquire data continuously. Here is a picture of how I had mine configured.

In addition, you may want to consider wiring the error clusters so it will stop the loop if there are errors.

Regards,

Paul C.

Message Edited by Paul C. on 08-19-2008 10:38 AM

Re: data acquisition

kgy — Wed, 20 Aug 2008 18:30:06 GMT

Thanks a lot. I found the error. It's in the write to measurement file. I set enable at "i==1"; then it only write to the file once. Now it works fine.

Re: data acquisition

kg_yan — Mon, 25 Aug 2008 21:44:11 GMT

Paul:

What is the best way to real-time display all the acquired data into a graph? Now the waveform graph is placed into the loop, so each time only part of the acquired data is shown in the graph. How to real-time display all the data?

Re: data acquisition

S_Hong — Tue, 26 Aug 2008 16:04:22 GMT

Hi kg_yan,

Unfortunately, there is no real means of of viewing absolute real-time data on a non real-time target. However, by placing the waveform graph in the while loop as you did and by carefully selecting the number of samples to read, you should be able to get fast updates on your graph. For example, for large read samples, your DAQmx read will wait until the required amount of sample points are acquired in the buffer before returning the data to be displayed on your graph. So depending how long it takes to acquire this data, the graph might update slowly. On the other hand, if you set this number too low, you may not be emptying your buffer fast enough and therefore you might run into buffer overflow problems. If you simply want to empty all contents in the buffer at every read, which I recommend, you could wire a -1 constant in your number of samples per channel. Like this, the graph will update as fast as it can with whatever data in your buffer.

I hope this helps,

Re: data acquisition

S_Hong — Tue, 26 Aug 2008 16:19:46 GMT

Hi kg_yan, I believe I have perhaps misread your question. I believe you can simply use a waveform chart instead of a waveform graph. You will then be able to view all your past data on one graph. You may also enable the X-scrollbar by right-clicking on your chart in the front panel and going to Visible Items » X-Scrollbar. Sorry for the confusion.

Re: data acquisition

kg_yan — Wed, 27 Aug 2008 20:58:14 GMT

It works! Many thanks! However, there is one issue. Each time when I restart a new data collection, the old data from previous collection were still displayed there. How to remove the old data and restart with an empty graph only showing all the data in current loop of collection?

Re: data acquisition

devchander — Thu, 28 Aug 2008 05:24:42 GMT

Either at the end of the iteration or at the beginning of the iteration, put a property of for the Waveform chart, Set its property as>>Histroy data and wire an empty array to it

See attached VI

Re: data acquisition

kgy — Thu, 28 Aug 2008 13:33:18 GMT

Thank you so much.! But would you please give me an image instead of vi file? I can't open it because of version problem. Thanks!

Re: data acquisition

S_Hong — Thu, 28 Aug 2008 21:05:29 GMT

Hi kgy,

I figured I just open the VI and give you a snapshot of the block diagram.

Message Edited by S_Hong on 08-28-2008 04:05 PM

Re: data acquisition

devchander — Fri, 29 Aug 2008 07:10:30 GMT

Thanks S_Hong

Re: data acquisition

kgy — Fri, 29 Aug 2008 13:04:47 GMT

Thanks a lot, S-Hong!

Re: data acquisition

kgy — Fri, 29 Aug 2008 18:58:43 GMT

oh, I still have problem wiring the array with the history properties. Please see attachment. How should I modify?

Re: data acquisition

devchander — Tue, 02 Sep 2008 06:20:01 GMT

kgy,

Do not connect a dbl array

Instead, right click on the property node>> pop down menu>>create>> constant

Now, you would have wired a blank 1D array of Dynamic data

Look at attached pic

Re: data acquisition

kgy — Tue, 02 Sep 2008 19:29:37 GMT

I think something is wrong here. I can't select creating a constant here because it is greyed out. Please check : right-click on the waveform chart to create a property node - click to select "history data"- right-click "change to write",-right-click, create-"constant"(but unfortunately it is greyed out).

For wiring it with the loop, I create another history data element, and then change it to read before wiring it with the while loop. In this case, will the waveform chart be refreshed at the beginning of the loop or at the end? What I am trying to do is to have an empty chart when starting the data collection. Then the chart displays all the data collected until stop. Please advice. Many thanks.

Re: data acquisition

S_Hong — Tue, 02 Sep 2008 23:02:12 GMT

Hi kgy,

I am not exactly sure why you are unable to create a constant for your History Data. Could you try creating a control instead and see what kind of data type you get?

As for wiring the property node to the while loop, the reason we do this is to ensure that this property gets executed before entering the loop. As with any structure in LabVIEW, the loop structure will only start once all its inputs have been acquired, in this case, the error handle.

Re: data acquisition

kgy — Wed, 03 Sep 2008 15:15:46 GMT

It works now! Thanks a lot!