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What\u2019s involved in APC 2.0 and why<\/h2>\n\n\n\n
APC 2.0 incorporates many changes and lessons-learned that serve this end. APC agility means (among other things) costs that fall within normal operating budgets, so that it doesn\u2019t break the bank or have to wait for funding; it means time frames that allow new opportunities to be captured and problems to be solved as they arise within a reasonable operational time frame \u2013 not a year or two down the road; it means APC work that can be included within projects, not as separate follow-on projects, without becoming critical path items; it means applications that are set-and-forget and do not require continuous support \u2013 or that fall apart without continuous support; and it means technology that can be effectively owned and operated by on-site control personnel without dependence on off-site specialists.<\/p>\n\n\n\n
In APC 2.0, these characteristics of agility come about from a combination of lessons learned and new ideas that are sometimes novel and sometimes just common sense from process control and process operation points of view. The figure illustrates many of the main aspects of APC 2.0 that will be discussed in upcoming installments of this series.<\/p>\n\n\n\n
In the APC 2.0 era, the central focus of APC will become closing multivariable loops, just like the focus of base-layer control has always been closing single loops. It\u2019s fundamentally the same. If console operators have to intervene frequently to adjust outputs or setpoints \u2013 perhaps so frequently that they are sometimes overwhelmed \u2013 then something is missing. It could be a problematic single loop or \u2013 quite often \u2013 the root cause will be found to be a missing multivariable loop, so that the operator is actually serving as a manual multivariable controller, a job that could be automated via APC to close the related multivariable loops. Closing loops brings the universal automation benefits of more timeliness and consistency, fewer alarms and trips and greater optimization.<\/p>\n\n\n\n
Are you intervening too often with control loops?<\/h2>\n\n\n\n
In APC 2.0, this fundamental aspect of operation \u2013 how often console operators have to intervene in control loops \u2013 is readily measured and managed using loop intervention analysis and the loop intervention metric. The loop intervention metric is also called the bad actor loop metric, because it is similar to the bad actor alarm metric from alarm management. If operators have to intervene too often, it needs to be addressed to assure safe and effective operation. In many or most cases, as the series intends to explain, an automated multivariable controller, that is APC, is the appropriate solution.<\/p>\n\n\n\n
What\u2019s in a name? APC, MPC, MIMO and other terms to know<\/h2>\n\n\n\n
Probably the easiest and most common way to talk about advanced process control is to call it \u201cAPC\u201d and by that to mean \u201cmultivariable control and optimization.\u201d Many other terms have been used, such as MPC (model-predictive control), MIMO (multiple-input-multiple-output), and MLC (multi-loop control). But APC is the most commonly used term.<\/p>\n\n\n\n
Within APC, the terms manipulated variable (MV) and controlled variable (CV) are almost universally used, even if they can be counterintuitive at first. MVs are the control system \u201chandles\u201d that are directly manipulated by APC. Usually they are single-loop controller setpoints or outputs. CVs are the control system dependent variables that are indirectly controlled, by virtue of the MVs, such as various temperatures, pressures or stream qualities.<\/p>\n\n\n\n
APC is shorthand for multivariable constraint control and optimization. In APC, MVs are manipulated within limits to keep CVs within limits (to the extent possible). This is multivariable constraint control. Secondarily, after constraint control, MVs are further manipulated to bring MVs and CVs to their most optimal values (to the extent possible). This is optimization.<\/p>\n\n\n\n
Allan Kern<\/strong> is principal APC Consultant with Lin & Associates Inc. Edited by Mark T. Hoske, editor-in-chief, Control Engineering, <\/em>WTWH Media,<\/em> mhoske@wtwhmedia.com.<\/p>\n\n\n\n <\/p>\n\n\n\n CONSIDER THIS<\/p>\n\n\n\n Are you attending to control loops too often in your processes? Perhaps you need APC 2.0.<\/p>\n\n\n\n ONLINE<\/p>\n\n\n\n For more insights from Allan Kern, see:<\/p>\n\n\n\n