How do I control a data log session with period and sample time?

Question

I need a data logging system where the operator can select 2 logging parameters: Log Period and Sample Time. I also need a START and STOP button to control the logging session. For example, set the log period for 1 hour and the sampling time for 1 second. (I may be using the wrong jargon here.) In this case when the START button is clicked, the system starts logging for 1 second. An hour later, it logs data for another second, and so on until the operator clicks the STOP button. (I will also include a time limit so the logging session will automatically stop after a certain amount of time has elapsed.)
It’s important that when the STOP button is clicked, that the system promptly stops logging. I cannot have the operator wait for up to an hour.
Note that a logging session could last for several days. The application here involves a ship towing a barge at sea where they want to monitor and data log tow line tension. While the system is logging, I need the graph X-axis (autoscaled) to show the date and time. (I’m having trouble getting the graph to show the correct date and time.) For this application, I also need the system to promptly start data logging at a continuous high rate during alarm conditions.
Of course I need to archive the data and retrieve it later for analysis. I think this part I can handle.
Please make a recommendation for program control and provide sample code if you can. It’s the program control concepts that I think I mostly need help here. I also wish to use the Strip Chart Update Mode so the operator can easily view the entire logging session.
DAQ Hardware: Not Selected Yet
LabVIEW Version: 6.1 (Feel free to recommend a v7 solution because I need to soon get it anyway.)
Operating System: Win 2000
In summary:
How do I control a graphing (data log) session for both period and sample time?
How do I stop the session without having to wait for the period to end?
How do I automatically interrupt and control a session during alarm conditions?
Does it make a difference if there is more than one graph (or chart) involved where there are variable sample rates?
Thanks,
Dave

Answer 1

Hello Dave,
Sounds like you have quite the system to set up here. It doesn�t look like you are doing anything terribly complicated. You should be able to modify different examples for the different parts of your application. Examples are always the best place to start.
For analog input, the �Cont Acq&Chart (buffered).vi� example is a great place to start. You can set the scan rate (scans/second) and how many different input channels you want to acquire. This example has its own stop button; it should be a simple matter to add a manual start button. To manually set how long the application runs, you could add a 100 ms delay to each iteration of the while loop (recommended anyway to allow processor to multi-task) and add a control that sets the number
of iterations of the while loop.
For logging data, a great example is the �Cont Acq to File (binary).vi� example.
For different sample rate for different input lines, you could use two parallel loops both running the first example mentioned above. The data would not be able to be displayed on the same graph, however.
If you have more specific questions about any of the different parts of your application, let me know and I�ll b happy to look further into it.
Have a nice day!
Robert M
Applications Engineer
National Instruments
Robert Mortensen
Software Engineer
National Instruments