Monday, 30 March 2009

More on State Based Control

I spent a lot more time going through the ABB paper on the benefits of State Based Control
And in order to understand it better I have spent some time turning their example into a ControlDraw model. It was very interesting, first finding how easily the example can be modelled (using standard ControlDraw features such as Equipment State Matrices)  and how similar the ABB/Dow method is to the way ControlDraw can handle such requirements. One example (of many)  is that ControlDraw supports what you might call cascaded State matrices, where the state in a higher level matrix (for example a Unit State) sets the state in a lower level one such as an Equipment State. 
Please note - I do not mean the states like Starting, Holding etc in S88 Parts 1 and 5, I mean states like the ones the paper uses like Total Reflux, Empty etc. Well done ABB.
I also looked at some models in my archive where I found examples of Units that have 50 or more states and 20 or so equipment modules as compared with the 9 states and 6 EM's in the paper. Some of these date from 2001. So it really is not new. But nonetheless it is good approach, and as my examples show it is also highly scalable.
The paper says
"One last benefit of SBC is that it is an implementation of process control based on the principles outlined in ISA S88 Part 5"
I cannot see that at all. If anything the paper presents something better than Part 5 has offered, but it is really quite unrelated. I cannot either find any indication that ABB has been involved in Part 5 - I may be wrong, but if I am right, please ABB get involved, and don't make such statements for the sake it of it. 

One negative - a detailed review showed how poor the spreadsheet examples in the paper are.
The problem with using spreadsheets for this kind of application are numerous. Typical problems include:
  1. Complex formatting, which while it may make table look pretty actually makes it very difficult to extract the data, for example to help with generating the application.
  2. Multiple lines in the same cell are often a problem
  3. If a tagname changes then there is a lot of work to correct the spreadsheet
  4. You need a spreadsheet for each Unit, and there is nothing to handle higher levels like process cells
Of course these problems disappear if you use ControlDraw !
By the way, the same paper is now available from, and you do not have to register.

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