Why does Labview VI pause intermittently during motor control while loop?

Question

I'm performing some biomechanical testing using Labview to control stepper motor actuators. I have a PCI-7334 controller card and a MID-7604 driver. The testing is run in load control using a while loop that sends a number of steps to the stepper motor that is proportional to the difference between the actual and desired load. The VI intermittently pauses though and sits in active for several seconds (~5 sec to 20 sec). What could cause this problem?
Thank you in advance for your assistance.

Answer 1

Without the subVIs it is hard to see what is going on.
Some general comments:
1. Block diagrams should be limited to the size of one screen.  Yours is over 4 times as wide as my 27" screen.
2. Sequence structures should be avoided.  LabVIEW is a datflow language and only rarely are sequence structures required. There is almost certainly a way to write your program without them.
3. The use of local variables should be avoided.  Again, they have a place but most likely they are not needed in your program. They are prone to race conditions, make extra data copies, and force execution in the UI thread.
4. Floating point data types are not recommended as case selectors. They will be rounded and coerced to integers.
5. Learn about the Producer/Consumer Design Pattern and state machines.  
One possible cause of your slowdown is having the Write to Spreadsheet File.vi inside the while loop. This VI is convenient to use but it opens, writes, and closes the file each time it is called.  I have no way to tell how much data you write on each iteration but as the file grows the OS may need to re-allocate space for the file or fragment the file. These operations can be slow.
Do you really want to Abort the program in the False case?  This will leave any outputs in the last state they had been set. This could have your motor(s) running, clutch engaged, ...  In programs which control physical hardware an orderly shutdown procedure is usually better: set all outputs to a safe condition, close files, close DAQ tasks, and indicate the status to the user.  Calling Stop allows none of this to happen.
Lynn