Monday, 18 February 2008

WBF S88 Papers

I am browsing through my large collection of papers presented at WBF conferences over the years. Google Desktop is a wonderful thing as it make it so easy to search them.

There are various types, many are very interesting, and would apply regardless of S88. Great, the WBF is not just S88.

There are also many that claim business benefits as a result of S88. As an engineer, that seems unexciting. For me it is a given that good engineering benefits business!
The most boring are those that claim X% saving here and Y% there, invariably with no evidence that would stand up to the kind of review that scientific work has to.

Some do extend our understanding in ways that are true S88.

But rare are papers that have anything other than positives to say the standard.

S88 did not come out of the blue - many successful systems had been designed according to what we now think of as S88 long before the standard was even in its early drafts. The German NE33 Standard was one of the feeds into S88 Part 1.

And some real problems occurred in the early stages of the standard, you only have to read the paper on CIP that was contributed to the (ancient) EBF - for demonstration imaginary plants, which incidentally, preceded the Part 5 Best Glue Plant by 10 years.

Most of the papers are written by - or at least presented by - people in managerial positions.

But I would really love to hear from people with experience from the Process Controller end - the programmers. So, if you are out there I would love to hear from you and about your experiences with S88 from the control code end.

Saturday, 16 February 2008

Control modules

And how great control modules are pure S88.
Control modules know nothing about the stuff you are making. They do not even know how stuff might be made. Part 1 is clear about that, as the recipe has no interface with control modules.
That does not mean that they cannot do a lot, in fact they are the bedrock of any application. The better they are, the easier the rest is.
'Primitive' Control Modules handle the Inputs and Outputs, from the plant and the operator and from higher levels in the system. They then make sense of it, setting and monitoring the state of the physical equipment that is connected the to IO and driving things like Faceplates.
Primitive Control Modules can be quite intricate, and supposedly simple things like motors can have a surprising number of states and parameters. They are little models in theselves.
Higher level Control Modules such as a PID loop or a one that handles a valve cluster also have states and parameters etc. They may even have sequences to move from one state to another or to perform a cyclic function. Some S88 practitioners choose to all these sequences 'Phases'.
I don't like to, but that is another story that I will return to.
I also use Control Modules for things like resource management - take a look at this Cyclic Request Handler . This is a small simple module to share a common resource among a number of users. These request the resource via a boolean flag, like holding your hand up in class.
They remove that flag when their demand no longer exists. The Module just looks at each in turn, and supplies if the 'hand is up'
The method guarantees First come at most Nth Served where N is the number of users. It is easy state control and avoids needing any form of queue and could easily be implemented in a PLC. Is it a Control Module? If not what is it? I hope Part 5 will find a space for it, and not a procedural one, as it does not need to know anything about making stuff!

Thursday, 14 February 2008

Save the Batch

Or how to be an S88 Purist and how it will improve your controls, be it batch, discrete or continuous.
Often when analysing a process people start by dividing the process into Units and Equipment modules, but without thinking where are the Batches?
It greatly helps if you start by looking for the batches. And Everything can have a batch, even storage tanks.
Once everything has a batch, you can use S88 terms through all your application without confusing the recipe with the equipment.
An important thing about the batches in a plant is that they are not often one for one. For example a pallette of chocolate bars, a tanker full of milk, a delivery of hazel nuts can all be seen as batches.
Drawing a Batch relationship diagram is a good way to crystallise your ideas.
Once that is done you can find the units and the recipes, and decompose them.
Then you can match the procedures to your elemental basic control. Which you can get straight from the equipment for example a P&ID.
Adding higher levels level control modules in Basic Control can makes this far easier.
I will try to extend this in time, but for the present take a look at "Is a storage tank a Unit?" which shows how to run a control recipe in a storage tank just like any other batch control recipe .

On ISA Part 5

The ISA part 5 developments appear to me to be trying to take Part 1 concepts into areas it was never designed for, such as the inside of machines.
In the process of doing this they are inventing new meanings for those Part 1 concepts such as Control Recipe and Procedural Control. This will only confuse, especially the use of terms like Operation and Unit Procedure. They think that an Equipment prefix means that they are not the same thing. I cannot find anywhere in Part 1 that says that - the Equipment prefix adds the ability to direct basic control, but does not remove the Recipe meanings.