The most effective way to log data and read simultaneously (DAQmx, PDM) high data rates

Hello
 
I want to acquire the data of several Modules cDAQ using several chassis to
high data rates (100 k samples per second if possible). Let's say the measurement time is 10 minutes and we got a large number of channels (40 for example). The measured data is written to a PDM file. I guess, the memory or the HARD disk speed is the limits. For the user, there must be a possibility to view the selection of channels in a graph during the measurement.

My question: what is the best and most effective way to save and read data at the same time?

First of all, I use an architecture of producer-consumer and I don't want to write and display the data in the same loop. I expect two possibilities:

[1] to use the 'DAQmx configure logging.vi' with the operation 'journal and read' to write the data to a PDM file. To display the data in a second loop, I would create a DVR samples documented and 'sent' the DVR for the second loop, where the data will be displayed in a graph (data value reference). This method has the disadvantage that the data of all channels is copied into memory. Correct me if I'm wrong.

[2] use 'DAQmx configure logging.vi', but only with the "journal" operation to write the data to a PDM file. To view the selected data, I had read a number of samples of the TDMS file in the second loop (I'm currently writing the TDMS file). In this case, I have only one copy data from the selected channels (not), but there will be more HARD drive accesses necessary.

What is the most effective and efficient solution in this case?

Are there ways to connect and read data with high frequencies of sampling?

Thank you for your help.

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You say that the measurement time is 10 minutes. If you have 40 channels and you enjoy all CHs at 100 kHz, it is quite a number of values.

In this case, I always try to approach under the conditions of use. If a measure is only 10 minutes, I just connect all PDM data and create a graphic module that could be in the same loop of consumers where connect you the data. You can always work on the raw data files big offline afterwards, the extraction of all the information you need (have a look at the product called NI DIAdem: http://www.ni.com/diadem/)

The main issue is that the user needs to see in the graph (or perhaps a chart can be useful too). Lets say that the graph is 1024 pixels wide. It makes no sense to show multiple data to 1024 points, Yes? Every second will produce you 100 data points k per channel. What is the useful information, which should see your username? It depends on the application. In similar cases, I usually use some kind of data reduction method: I use a moving average (Point by point Mean.VI for example) with a size of the interval of 100. This way you get 100 data points of 1000 per channel every second. If you feed your graph every second with these average values, it will be able to data points in 1024 of the store (as a default) by channel (curve), which is a little more than 10 minutes, so that the user will see the entire measurement.

So it depends on the frequency at which you send data to the consumer. For example, collect you values 1024 by iteration of the producer and send it to the consumer. Here you can make a normal means calc or a bearing (according to your needs) and he draw a graphic. This way your chart will display only the values of the last 10 seconds...

Once I programmed some kind of module where I use a chart and not a graph, and the user can specify the interval of the absolute timestamp that is traced. If the data size is larger than the size of the chart in pixels, the module performs an average calculation in order to reduce the number of data points. Of course, if you need to see the raw data, you can specify an interval that is small. It all depends on how you program zoom functions, etc... In my case I hade a rate of 1 Hz, so I just kept all data in RAM limiting the berries to keep 24 hours of data, so that technicians could monitor the system. In your case, given the enormous amount of data, only a file read/write approach can work, if you really need access to all of the RAW data on the fly. But I hope that the values of working capital means will be enough?

Tags: NI Software

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