Can I acquire and analyse in real time with regular Labview?

Question

I have to acquire samples (which vary cyclically in a roughly sinusoidal fashion) from a sensor, and check every sample to see if it is the minimum (the valley) of a cycle. If it is, and it does not fall within an expected range, I have to take a corrective action that involves rejecting the part that was just measured as well as a 30 more parts (to make sure that the defective part has been rejected). The signal from the sensor is not very noisy, but beause of the nature of the measured object, there could be local minima and maxima. To guard against that, a point is considered to be a valley only if subsequent readings deviate above that point by a certain amount. If a part is indeed defective,
a digital out put has to issued to reject that part.
Can all these be done using regular Labview (not RT)? I tried it out with a proto-type VI, using DAQmx vis, continously acquiring samples, but reading one sample at a time from within a loop (the VI I used is attached with this question). The result has been disappointing, since each time the loop executes there is a delay that keeps building up. Finally, even after the part feed has been turned off I can see Labview processing signals from parts that have long since gone past the measuring head.
Another perplexing thing I found is that the time taken to execute the while loop in the vi is not consistent; it takes anything from 6 to 50ms to execute.
I will need atleast 8-12 samples from a part to build its profile, and the feed rate is about 3000 parts per minute. I am using Labview 7.0 with an NI-6013 card in a Windows 2000 environment.
Thanks for any suggestions / recommendations.
Attachments:
find_trough_2.vi ‏378 KB

Answer 1

Hello,
Thank you for your suggestions; I had already resigned myself to going for a Real Time system, your answer convinced me to commit myself to it!
That said, your reply leads to a couple of (related) questions...
1. Your point regarding the use of Local Variables is well taken; I have been repeatedly told at various training sessions how the necessity of updating the LVs during each loop iteration slows the computation time. However, what alternative do we have when there are several controls to which we have to write AND read data multiple times during a loop iteration, and perform different computations based on the value held by these controls? (You have seen the VI I attached with the original question). Some of these conditional
computations further change the value of the controls. Does Labview have any other mechanism to store and manipulate the intermediate value from a computation?
2. I did a simple experiment to determine the average loop time, and the results were surprising. I placed the entire content of the VI I used (Find the Valley in cycle.vi) in a stacked sequence structure, and wired the index counter "i" to a control to count the number of iterations the loop executes. I placed a frame before this with a tick count instruction to get the start time of the loop, and a frame after this to get the end time of the loop. Dividng the difference of these with the number of iterations, I got the average loop time to be around 1.2ms! Am I interpreting my results incorrectly?
Thnaks once again for your response. I would really appreciate your views on the questions I have raised in this comment
Regards
Arun P. Madangarli