Acuire data using signal express!

[TVS1]

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@proud NI engineer wrote:

I am not sure why you are seeing extra samples, but using an N-Sample acquisition mode is probably not what you want since there will be gaps in the data.  N-Sample mode re-arms the device each iteration (2048 samples in your case) and while it is doing this, samples will be lost.

Try setting your acquisition mode to be Continuous and see what happens.  You might also want to set the number of samples to be a multiple of the rate so you can get an even 100 seconds of data.

Also, you can try using the logging featues of SignalExpress instead of the Save To ASCII Step.  Recording the data using the logging features provides the following features:

1. You can use a Stop Condition for the Recording Options Tab to specify that the data should be logged for 100 seconds.

2. Once the data is logged you can view the entire log on the data view.

3. You can analyze the logged data using the built-in playback mode and any of the built-in analysis.

4. And if you are not using logging because you desire ASCII logs, you can enable automatic ASCII file generation by selection Tools>>Options from the menu.  Select the Logging Category on the left and enable the Automatically Export Log to ASCII option.

Please let me know if you need further assistance.

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         Hi Proud NI Engineer,

         I had this one last question on this subject. Could you tell me what could be the gap size in the data if one uses the n sample acquisition   mode. This is really kind of an urgent!!

Thanks in advance!

[--Chris]

The size of the gap in the data is very device / computer dependent.  If you log the N-Sample data using the built-in logging and not save to ASCII you will be able to see the gaps if you view the entire log on the data view.  Drag the logged data onto the data viewer and the gaps in the data will show up as empty spaces.  SignalExpress accurately logs the start time of each chunk of data and displays the chunks with the correct spacing on the graph.

N-Sample logging / Save To ASCII will not buffer the data and fill in the gaps since the HW Device did not acquire the data in the gaps.

[TVS1]

Thank you!

[MIJACK]

Hello I am new to Signal Express and have been trying to figure out the relationship of the timing Settings in the Aquire step. Can't find any help or explaination of these settings. What are the limiting factors of these settings when using a USB 6210?

There are three types of Aquistion mode: Continous, N-Sample and On Demand.

I tried following the current thread but it doesn't seem clear to me.

Is there a white paper or help document to determine the calculations and the effect on logging data? Some times I enter numbers and I get an error stating the value is too low?

For example if I am in continous mode and select 2000 samples at 1KHz (1KHz = 1msec) then I should aquire data 1 sample every  500 usec with a sampling rate of the DAC as 1KHz? Is this correct? Or is it .001sec / 2000 s = 1 sample every .5usec?

For example if I use on demand mode and select 20m (20msec) period then I am using the default sampling rate of the DAC but aquiring 1 sample every 20msec.

Can some one point me to some clearity of these settings.

Thanks

[--Chris]

Only the sample rate determines the sampling rate of a signal.  The Samples to Acquire controls the number of samples in the waveform that is acquired.  If you set the sample rate to 1kHz and the number of samples to 2000, you will get new data every 2 seconds where samples are acquired every 1 ms.  The waveform will contain 2000 samples which is 2 seconds of data.

Continuous mode is used to acquire a continuous stream of data from the device.  It will acquire a dynamic signal with no gaps in the data.

N-Sample mode works much like the way a scope does.  It acquires a buffer of data and processes it but each buffer is not contiguous, i.e. there will be gaps in the data.  N-Sampled mode is most useful when you want to trigger each acquired buffer.

1-Sample mode is used when acquiring static signals where a waveform is not needed.  Acquiring things like a DC Voltage, a static strain, or a temperature should use 1-Sample mode.

[MIJACK]

I think you made a mistake? The last line of your post stated that 1-Sample mode should be used for static signals but then you said voltage, strain etc... should use 1-Sample mode. I think you ment to say that dynamic signals should use continuos mode.

Thanks
Steve

[--Chris]

AC Voltage and Dynamic strain definitely use continuous.

If you just measuring a DC Level or a "Static" strain, then 1 Sample is fine.

[rmeh]

Is there any way to coerce signal express to log data at a rate smaller than 1612.9 Hz or dt=620u while using all four channels of NI9237. I need to collect data over several hours and cannot handle such large data sets.

Thanks

[OKors]

I think you would've been fine starting your own thread.

The 9237 has a minimum sample rate of 1.613kS/s, so regardless if you set a rate slower than that the hardware will default to 1.613kS.
This is true even if you have other modules that can sample slower. The 9237 sample rate properties will take priority and upsample everything else.

I recall a similar question being posted a while back and the solution was to down sample using the Subset and Resample step IIRC.