Wednesday, 22 October 2008

PLC is 40 years old!

The PLC is 40 years old, time flies, I first saw one when they were maybe 5. In fact I learned much of my Batch control by programming PLC's. That is where control happens, in the controllers, surprisingly enough. 
While I may have complained that the WBF does not have enough about Control I had not looked though all the papers presented at this years USA Conference. (It is worth joining the WBF for this alone by the way, some are gems) and found one on implementing recipes in a PLC, by Igor Steiner, Janez Tancek, Marko Svetina at INEA d.o.o.
I hope they won’t mind me quoting a bit from their paper
This is about PC based recipe managers versus writing your own in a PLC

High level of abstraction for complexity management
High flexibility&reusability
Rich functionality

Unsatisfactory reliability of the PC platform
Poor adjustment to small and medium projects
Unsatisfactory time behaviour of the PC platform
Too low expressive power of phase behaviour mode
An interesting comparison, I generally agree. The Too low expressive power point is one that I think might be based on an over optimistic view of what a Recipe Phase has to do, for me there is very little it has to do apart from starting and monitoring the execution of an equipment phase, via a phase logic interface. I don’t even believe the ****ing states (such as Pausing!) should exist in the PLI, why does a recipe care about that?
The programming of the equipment phase itself of course can be done in the process controller, PLC in this case, and there you can do what you want using the power of PLC instruction sets, which have high expressive power.
The paper also describes the Concept of Tabular Recipes on a PLC, nothing new, to my knowledge something similar was done in Wigan in the mid 80’s using Siemens S5 PLC’s, but worthy all the same. There is by the way a popular misconception that S88.01 revolutionised batch control, I think that makes the point that it did not. It helped to specify it though, I agree with that, and it raised understanding of the issues.
PS - in fact Tabular recipes have existed in ControlDraw for many years 

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