usb-6009 reference trigger using loops

[Ianrosey]

Hi,

I am a student studying for a Beng. As my final project I am looking to create a fault recorder for electrical motors. The basic premise is that it sits and waits for a trip signal, at which point it saves the analogue waveforms of voltage and current for a pre-defined length of time leading up to the trip.

I selected the USB-6009 as the DAQ, mainly based on price. At the time I didn't realise the importance of the pre-trigger (reference trigger) function which the 6009 does not support.

I am sure that there must be a way of using the computer memory to create a buffer using a continuous loop that keeps re-writing to a TDMS file, however I am really struggling to implement this code.

I know the ideal solution would be to use a better suited device, however having already purchased the device and stated it in my project specification this is not possible.

If anyone is able to offer any advice of a possible implementation of this solution and a potential example of a similar program, I would be forever grateful!

[johnsold · Accepted solution]

1. Do continuous acquisition. Keep the data in a circular buffer large enough to store the amount of pre-trigger data you want plus all the post-trigger data which may be acquired before the processing is complete.

2. When the trip signal occurs, either extract the pre-trigger data for stroage or store a marker which tracks where the pre-trigger data starts.

3. After the post-trigger data is acquired, write all the relevant data to the file.  Unless you have very long pre- or post-trigger data blocks, TDMS will not be required.  The USB-6009 is slow enough that you could keep a few minutes of data in memory before running into memory management problems.  That should be much more than you need for a motor fault recorder.

Consider using an Action Engine (search for Ben's Nugget on the topic) for the buffer. 

Lynn

[Ianrosey]

Dear Lynn,

Many thanks for your quick response, I really appreciate it.

Would it be possible for you to offer me some advice as to how to set up the circular buffer for use with the data aquisition device, I have been trying to experiment with this since your reply and haven't been able to get any results.

Please understand that I am very much a novice to Labview, I undertook the project hoping that I would be able to get a good understanding of the program, however I am fast feeling that I have bitten off more than I can chew!

Thanks for your help.

Ian.

[johnsold]

A quick search for "LabVIEW circular buffer" found these:  Circular buffer using functional globals Queue based circular buffer Array as circular buffer and many others.

Lynn 

[Ianrosey]

Hi Lynn,

Thanks for all your help, having done some reading and playing around i've managed to get to the bottom of the problem, I was confusing myself by not realising that the output of the DAQ device is actually just an N dimensioned array.

Ian.