Monday, 2 March 2009

Why the procedural hierarchy in Equipment?

There seems to be an assumption that equipment procedures must themselves preserve the procedural hierarchy, so if you have an equipment Unit Procedure, that must itself be made up of phases. However I submit that this is not a reasonable assumption, because it makes the recipe have to dive into equipment control when there is no need for it, at least if we are going to preserve equipment - recipe independence.

If you can code a unit procedure in your controller that looks to the recipe like a unit procedure then that is all you have to do, S88 is not a standard for writing control software and should not try to be, that is stated in the standard.

If on the other hand you really want Recipes to reference Unit Procedures or Operations that are designed in your recipe procedure but implemented in the equipment, then I say that this means that you have a recipe controller in your equipment. This contradicts the concept of product independent equipment – I think it is part of the misunderstanding (or at least an alternative understanding to mine) of the original part 1 that the update is now perpetuating

Yes, I can see the need for being able to pass equipment requirements down the levels in a Recipe, but not to pass them across to equipment control.

And I can agree that equipment phase+ control might benefit from structuring into levels, to improve controller code. But hey, real time PLC and DCS control – the sort of thing you use IEC1131/3 languages for is not like recipe control.

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