Control Engineering

"Mode" is too simple a word. I have found that we need to add adjectives in our use of standards. Unfortunately, I only discovered this simple truth of standards after we finished the ISA 88 standard. The MODE in ISA 88 is really at least two different things, one is the PROCEDURAL CONTROL MODE (this is the one with semi-automatic) and it defines how the a recipe's procedural element is controlled (including the equipment phase). The other mode is the EQUIPMENT MODE, which is not really defined very well in ISA 88. The mode as described in the Make2Pack has been variously defined as "Unit Mode, Operational Mode, Procedural Mode, and Mode Of Control. Randy Dwiggins has proposed "calling the S88.01 meaning in relation to procedural control the “Procedural Mode”, the S88.01 meaning in relation to basic control the “Control Mode”, and the packaging meaning the “Operational Mode”." These are good starting points, but we may want to call them "PROCEDURAL CONTROL MODE", "BASIC CONTROL MODE" and "OPERATIONAL MODE". The more adjectives we add the less chance there is for confusion.

Someone asked if it's possible to get an alert when something is posted on this blog. Yes, via an RSS feed. Here's a link to instructions on how to setup an RSS feed: www.controleng.com/learnrss . I know RSS readers vary in how they're set up and what they do. Here's how I subscribed to this blog (and other areas of Control Engineering) on my Google page (which can serve as an RSS reader). --I clicked into Add stuff, then I... --Dragged the XML symbol onto the page, which works for some readers. That didn't seem to work for this one, so then I... --Clicked Add stuff by URL. Then... --From another browser, I clicked onto the XML feed button (the orange XML button is to the right of the blog) so I could get the RSS feed URL. Then I... --Pasted that URL into the Add by URL field and hit Add. Now the blog posts appear on my Google RSS feed (which also doubles as my Google search page). I know there's a way to get an email alert using this or other tools and will look into that, unless someone else explains first. Of course you also can save the blog page among your favorites and check in once in awhile. -Mark T. Hoske, Control Engineering editor in chief

Modes of procedural (or sequence) operation are different from modes of Control (Analog / Digital)Operation. I have worked with designing ''Operator Guide Mode'' of Start-up / Shut-down sequence of Power Plants. This mode is to assist a new-guy in getting to run through the sequence operation and understand various commands & check-backs at every step with the associated wait-time & step-time but with no commands being issued to the real-world (you can say this is similar to the simulation mode for a control loop or a digital drive module). Automatic, Semi-automatic and Operator-guide modes are "sequence or procedural" modes. In this context, a ''Manual'' mode is sometimes referred to as ''OFF'' meaning the sequence mode is ''switched off'' meaning it is neither in auto or semi-auto but operations are fully at the equipment level with operator fully in-charge of running the sequence himself / herself. To me, mode refers to that operational aspect that explains "how" an equipment or group of equipments or a process Unit is being run and NOT the "what" as referred in earlier comment posted here.

The Procedural States given as an example in S88 are imho misconceived, and their literal interpretation by batch product suppliers has held back the progression of procedural control software products and even basic control. I hope that this S88 development work will not be constrained by these products, rather that it will help to improve future ones. The reason I make the comment is that I think that the ideal of separating recipes from equipment is fundamental and right, but has been contaminated by the inclusion within the ‘example states for procedural elements’ (Fig 18 in S88.01) states that are nothing to do with the product, ie starting, holding, stopping . To me these states are that belong to the physical model and are best looked after there. Consequently, in every real batch operation and phase I have seen the majority of the procedural logic is concerned with physical considerations. This would not be the case if recipes were truly separate from equipment. I suggest that modes are set points for control elements that determine their behaviour, Auto and Manual for example determine whether an operator, or some calculation, sets the required outputs. States reflect the actual or required settings of ''objects'' In the case of equipment objects there can be many states, for example the permutations of the control module states valve positions around an equipment entity. In the case of Procedural elements the states should be few and simple, with only the minimum required to interface with another procedural element. Why does an equipment independent recipe need to be able to do more than start an element (phase, operation etc) and know when it is finished so it can tell the next to start? Francis

I am in agreement with what Francis is stating. And, his comments should be taken into consideration.

If you are a causal visitor to this blog, just checking in to see what is going on, but not signed up to the mail list, then you are missing out. There has been a lively debate! Perhaps the editor would like to summarize it.