Friday, 3 June 2011

ISA106 procedures

Thoughts on the emerging ISA106 standard. And S88 phases.
What can a step in an ISA106 procedure do?
Ask the operator to do something, from setting up equipment to entering data to confirming that a manual or paper procedure has been completed
Request equipment to do something for example run an equipment procedure or set equipment to a state
Set up something such as set points for control loops
Record some data
By the way that looks like just the sort of thing an S88 batch manager recipe procedure step can do - but of course ISA 106 people don't seem to like S88's' batchness'  Comment please.

Let’s look at the second, asking equipment do something and setting equipment states.
What is the difference? If you ask equipment to do something then the equipment might do numerous things and then say Done.
If you set equipment to a state the equipment might still do several things in the process before it says State Established.
There really is no difference in terms of interfacing a procedure with equipment control, but the first of these may run a longer and more process oriented sequence - and in turn set the equipment to states.
Let’s call the 'Do something please' equipment step a phase (following S88 terms).
Because it is run in the equipment - ie by the BPCS (Basic Process Control System ) that controls the equipment , let’s call it an Equipment phase (or if you like an Equipment Procedural Entity) The Equipment phase itself runs a sequence of setting equipment states with transitions (cf feedback) to direct the steps and confirm that the process actions are achieved. 
The Phase could of course set all the individual devices one by one (a common practise). Or it could set a state in a higher level physical entity, provided that entity can in turn set the lower levels devices.
That is what Equipment State Matrices are about – they are deployed in the  Dow/ABB State Based Control paper (see Tables 1 and 2) and by most ControlDraw users - and they are highly advantageous. Why?
If you don’t know what they are you can look for them on google 
They provide logic where all requested states are known easily. And as many states may be used more than once by different steps and even different by procedures they are more efficient. Alarms can be enabled according to the state matrix, Control loops modes set and so on.
In fact part of the Dow/ABB State Based Control paper is about this, and as I blogged before nothing to do with the S88 model where a recipe (cf procedure) speaks to the equipment. And I do mean Speak, since the objective of part 1 was in part to ensure that chemists (and their recipes) could speak to control engineers (and their equipment controls)
I feel that so far S106 is neglecting State Based Control.

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