Override PLC Values

[Squirrell]

I have my VI set up to monitor flow rates of surgical tubes.  Everything in this giant automated piece of equipment is controlled by an Allen Bradley PLC, and I am using RSLinx to interface with LabView. 

One station on this machine cuts the tubing to a certain length to achieve the desired flow rate (since the ID is consistent to 0.0001").  This cut length is controlled by an operator, and some basic math is required to calculate the proper cut length.  Unfortunately, small variances in the tubing requires the operator to change the cut length at least twice per hour; time that can be better used doing other things.

I would like to have the cut length controllable through LV.  So far, I have set up quite an array of gauges and status bars, and I have an incremental dial for controlling the cut length (and it's bound to the proper tag with write/read).  Unfortunately, when I run the VI and change the cut length value, the PLC does not take the new value. 

Any ideas on this?

Secondly, is it possible to take an average of 16 tag values, calculate the proper cut length (very simple equation) and automatically change it? 

As always, help is greatly appreciated!

[Squirrell]

Update:  Is there some sort of command to update values? 

[Squirrell]

Still no ideas?  I thought this would be simple. 

[Squirrell]

Basically, I just want to be able to change variables in a PLC using LabView. I know this is possible.  Can anybody help?

[Ben]

"and it's bound to the proper tag with write/read).  Unfortunately, when I run the VI and change the cut length value, the PLC does not take the new value.  "

Use the utilities that come with the AB OPC to monitor your tag and makes ure it is being updated.

It sounds like the PLC code is stepping on your update. If so change the PLC to only read from the tag you are writting.

Ben

[Squirrell]

Well, here's the thing:

Operators currently use a touch-screen panel:  PanelView Plus by Allen Bradley.

Is there a way to "squeeze" in new values without affecting the PLC code?   I can't get the original programmer back in here unless something breaks.  I also don't want to stop production.  The machine runs 24/7 Sunday-Friday

[Ben]

Can you run this question past your PLC programmer?

I can only speculate otherwise.

Ben

[Squirrell]

Good idea.

I'm positive that he uses RSView32 Works.  I tried using that program, but the data logging feature did not have the flexibility that I needed.  That's why I'm evaluating LabView- I see that there's a lot more flexibility.

[Brian_Beal]

Ben is probably correct in that your PLC code is overwriting the value.  The real question is are you changing the right variable?  Typcially, if a variable is set by the touchscreen, it probably is updating an intermediate variable, perhaps scaling it, then writing it to the final  memory location.  Also, variables controlled by the touchcreen usually don't get re-written until the value is changed by something.  In other words, the touchscreen is probably not updating the value, but something in the program is.  Getting into the PLC code is really the only way to find out.

[Squirrell]

You know, I thought that for a while.

I'm trying to change the cut-length on one station (there's multiple stations).

I sifted through thousands of tags looking for anything to do with cut-length.  I then ran a data-monitor on that tag in RSLinx.  Sure enough, I'd change the tag on the touch-screen and it would not register until I pressed "OK."  At that moment, RSLinx noticed the change.

I'll call the programmer and ask him about this.  I don't have access to the PLC ladder logic program.  Besides, if anything goes wrong and stops production while I'm doing this...

There is a clipboard set of variable/tags in the PLC unit.  We use them for different "recipes."  There are hundreds of parts; the machine processes them all.