topic Re: Variable and nested loops in LabVIEW

Variable and nested loops

Mike_505 — Fri, 21 Mar 2014 18:34:06 GMT

Hi, I'm a relatively new to LV programming, so be gentle.

In a nutshell, I'm running nested loops that cycles test equipment on and off. The parameters for each loop are input on the front panel by the user. For instance, power supply on time, how often to read in data, power supply off time, and number of on/off cycles. This is a typical task customers ask me to do. Therefore, I decided to make a generic as possible executable vi instead of writing code (in which what I'm asking is trivial), that my technicians can easily use.

I want to put a down-counter in this vi, even a course one. I'll take the on time plus the off time times the number of cycles and display this on the front panel. For some reason I can't figure out a simple way such that after the last cycle completes the display reads zero before the program ends.

For instance, the total time equals 60 minutes and there are 10 minute cycle. I run the program and the display reads 60 during the first cycle, after the first cycle is complete and as the second cycle starts the display changes to 50, and so forth until the last cycle where it reads 10. Then the program ends and the display reads 10. How do I get one last tick out of this so it reads 0 and displays 0 before the program ends?

Thanks!

Re: Variable and nested loops

aputman — Fri, 21 Mar 2014 19:15:02 GMT

You'll need to post some code if you want help.

Re: Variable and nested loops

RavensFan — Fri, 21 Mar 2014 19:34:14 GMT

Wire a zero to a local variable of that indicator so that it will run when the while loop ends.

Re: Variable and nested loops

Mike_505 — Fri, 21 Mar 2014 19:52:15 GMT

A local variable,,,, Exactly what I needed. Thanks!

Re: Variable and nested loops

aputman — Fri, 21 Mar 2014 19:56:15 GMT

....ugh....who needs code anyways.

Re: Variable and nested loops

Mike_505 — Tue, 25 Mar 2014 14:28:56 GMT

Thanks for the help. The local variable worked great. Now I have a couple of follow up questions.

Basically I'm trying to figure out how to make a real time timer.

Using my scenario from above, I'm turning on my power supply for a user set amount of time using a While Loop with a Time Delay in it. I chose this because it allows me to set the length of time the power supply is on and how often during the time it's on I read back data from it.

The issue is if I add a countdown timer it will only update on the panel by the time I have set to read the data back. For example, the power supply on time is set for 1 minute and the readback time is set for two seconds. Therefore, my countdown timer counts down in two second intervals. I tried building a timer by adding another loop within my main loop. However, while it will countdown by the second, which was good, the timing ends being significantly off when it pauses to read the power supply data in. In my saved data file which has an actual time stamp, the interval between reads is ~3 seconds instead of ~2 using the Time Delay.

Here are my questions:

1. Is there a way to make a loop/sequence independent running timer?

2. Is there a way to make an indicator global such that I can write to it in one loop (or sequence) in real time and then when I go to the next loop or sequence keep writing to it in real time?

Thank you!

Re: Variable and nested loops

RavensFan — Tue, 25 Mar 2014 18:14:38 GMT

Yes.

A separate loop can contain the timer. I would use a notifier to send a message to the loop as to whether you need to start or stop, or reset the timer.

Re: Variable and nested loops

Mike_505 — Tue, 25 Mar 2014 18:46:07 GMT

Hi RavensFan,Thanks!, but I'm not sure how that answered my question?

What I'd like to know is if you can define a global variable that can be written to a single indicator? Therefore, if I have a sequence or a loop I can keep writing to the same indicator with data generated in each loop? To help clarify with an example, lets assume I have a Flat Sequence Structure. The first sequence the user controlled counter counts to 5 and is displayed in an indicator on the front panel. We then go to the second sequence, this 5 is passed along and is now added to the user controlled timer in that sequence which lets say is 10. Now the indicator counts up to 15. Repeat this one more time with a user input time of 7 seconds and the indicator counts up to 22 seconds and the program stops. Threre are 3 inputs, a time for each sequence, and one indicator, on the front panel.

If I was writing this in code and had three consecutive loops performing 3 different tests consecutively (I write programs for ATE's), I would define the Time from my example above as a global variable and write it to an output during each loop. Within LV I do not see an easy way to do this. (PS: Time may not be the best example, but it hopefully it gets the point across).

Thanks!

Re: Variable and nested loops

RavensFan — Tue, 25 Mar 2014 19:35:20 GMT

If you have a separate loop that is keeping track of time, then use a notifier to pass data to that loop.

You could also use a local variable of an indicator to update the indicator from different locations. You just have to worry about whether you'd have "race conditions". If you had consecutive loops so that they were fighting at the same time to write to the same indicator, you should be fine.

It is hard to comment further without seeing some code that would implement it.

I never use global variables. Global variables are actually a storage location that can be accessed from multiple VI's, but you would still have the possibility of race conditons if they were used improperly. They dont' have any direct connection to any indicator.

I would tend to use an Action Engine if I ever needed something where a global variable might be used.

Re: Variable and nested loops

Mike_505 — Thu, 27 Mar 2014 14:33:07 GMT

Hi RavenFan,

Sorry for the slow reply and thanks for your support. I pretty much had tried everything you suggested prior to posting. I was hoping for a more eloquent solution.

After getting through writing the VI I was working on I keep wondering why there isn't a scripting language associated with LV that I can use? For instance, I'm looking at how many functions I have in my VI (14 functions!) in order to create an output file with a header and that formats and appends the data after each data collection operation (electrical tests in my case). I have an ATE that runs on VB that honestly, this is trivial to do in and literally takes a few lines of code (openspreadsheet (done once and then updates live afterwards), writedata Row Col, savespreadsheet - that simple) I collect data for each test I'm performing and then call a function to write it.

I'm also curious why there isn't more deterministic timing ability along with the ability to add a countup, countdown, or current time indicator to the front panel that runs independent of everything else in the program.

If there are features like I just mentioned please guide me towards them!

Thanks again!

Re: Variable and nested loops

RavensFan — Thu, 27 Mar 2014 16:00:27 GMT

I would not consider 14 functions a lot.

Anything you can do with text-based code probably takes more lines of code then it does in LabVIEW. If your text-based version of the code is as simple as you say, then the LabVIEW code should not be as complicated as you say.

Everything you are looking to do in LabVIEW can be done. I have a lot of text programming experience, and I would prefer to do just about any of it LabVIEW.

If you haven't taken any courses and tutorials, then do it.

If you have any questions about how to do stuff in LabVIEW, show what you've done and people on the forums will try to help.

Re: Variable and nested loops

Mike_505 — Fri, 28 Mar 2014 15:17:35 GMT

I attached my VI. Please be gentle as it was a learning experience where I started simple and kept adding functionality. I would probably do a few things different...

The code I'm refering to isn't writing from 100% scratch. It was specifically written for a small ATE that besides it's own functionality allows use of other instruments through a GPIB. Like LV, there's a lot of background functionality built into it. However, this looks simpler to me than my 14 functions and easier to follow (granted I'm used to it). While I'm new to LV, I do feel a simple built in scripting language like this would be awesome.

SprdSht.OpenNewSpreadSheet MainWindow, False
'create header
  SprdSht.SetTextRC 1, 1, "Test Program Name: " + prog_name
  SprdSht.SetTextRC 2, 1, Tester
  SprdSht.SetTextRC 3, 2, "Voltage"
  SprdSht.SetTextRC 3, 3, "Current"
  SprdSht.SetTextRC 3, 4, "Ohms"
  SprdSht.SetTextRC 3, 1, "Date"

SprdSht.SaveSpreadSheet 'brings up windows save dialog for user to save file
.
for x = 1 to Z

wait T

NI_GPIB.ibwrt Dev1%,"READ?"

Data = NI_GPIB.ibrd(Dev1%)
.
OutputData ("RESULT", Vout, Iout, Ohm, irow, icol)
next x

Function OutputData (sname As String, Vout, Iout, Ohm, irow, icol)

If sname = "RESULT" Then
  SprdSht.SetNumberRC irow, icol+1, Vout
  SprdSht.SetNumberRC irow, icol+2, Iout
  SprdSht.SetNumberRC irow, icol+3, Ohm
  End If

SprdSht.SetTextRC irow, icol+4, Now 'current date and time to spreadsheet

SprdSht.SaveSpreadSheet 'saves under saved name
irow = irow + 1
End Function

Re: Variable and nested loops

RavensFan — Fri, 28 Mar 2014 22:44:15 GMT

I'm going to take on the challenge of cleaning up this VI.

There are several Rube Goldbergs in it. And other odd things

For instance, there is no need for a while loop where there is terminal + 1 (note there is an increment function that adds 1 to a number without needing to ADD a 1 constant) then stops the loop when 1 is <= that value. i starts as 0, add 1, you now have 1. Guess what 1 equals 1 so it stops after 1 iteration. Always, without fail.

Note that instead of comparing to greater than a zero constant, you can use the >0 function in the boolean pallete. Still one function, but you've eliminated the constant.

It is generally a bad idea to use the Stop sign function. It is the same as just hitting the Abort function. Any shutdown code such as closing a port will never get to run. With a proper architecture, you shouldn't need it.

Sequence structures usually aren't needed (especially ones that have frames where just wires run through it. I think yours can be eliminated entirely.

Arrange your VI so that wires don't need to run backwards. It makes it harder to visualize the order of execution. The Clean Up Diagram button is a good start.

I''m trying to be gentle. Just pointing out some facts. I'll post a cleaned up version after I work on it a bit.

Also, I agree that your VI is a lot more complicated than the text code you posted, but I also think you are doing a lot more actions in the VI. I don't see anywhere nearly as much GPIB writing and text string structuring for those writes in the text code that you are doing in the LabVIEW code, so you really can't compare the two programs.

Re: Variable and nested loops

RavensFan — Sat, 29 Mar 2014 00:05:57 GMT

The simpler task as a comparison is to code your text code into LabVIEW. This took me about 10 minutes.

It creates a tab delimited text file that can be opened in Excel.

Re: Variable and nested loops

Mike_505 — Mon, 31 Mar 2014 14:28:58 GMT

Hi RavensFan,

Thanks a lot!!!

I really didn't want to use the Stops. Those are kind of a test engineering-ism where you want to immediately stop on a failure to prevent (further) damage. I was very careful in when and how they operate such that everything is in a safe state. One of the Stops stops the program before it even really starts. It's basically checking to see if the user input makes sense and therefore allows the programming timing to work properly. I'm sure there's a better way to get to this in this case. I just haven't found it yet. The other case I do want the power supply to shut down immediately to prevent damage to my device, which it does.

I do realize I overengineered it. As I mentioned, I started out just trying to make the supply wiggle and went from there. The add 1 etc was a cut and paste and never changed. I'm sure there's few other functions besides the <0 that will help me simplify that I haven't found yet.

In regards to the sequence structure with the error lines running to them (I may have overcomplicated it!), I could not get the functionality in them to work as I intended otherwise. Those also bothered me. But it worked.

I appreciate the file you uploaded. I will study it well.

Thanks.