Logic forControlling ComplexIndustrial Systems
Douglas S. Shukert · 2005
Traditionally, flowcharts havebeenusedto describe industrial control systems. Flowcharts provide a nicestep-by-step orderliness to complex systems; butunfortunately, real systems do notoperate step-by-step, theyoperate concurrently. Inreal systems there will bemany actions happening simultaneously. Trying touse flowcharts todescribe concurrent systems becomes adifficult andcomplex task. Trying to write programs fromsuchflowcharts iseven moreofa chore, andsuchprograms willnot usually runefficiently. Nowadays, there isalotoftalk about describing complexindustrial systems asState-Action Machines. Usingstate-action machines isabig step intheright direction, butmanyprogrammers continue totrytouseflowchart techniques to describe state-action machines. Thispractice leads todiagrams that areinsomecases more complex thantraditional flowcharts. Thispaperdescribes awaytocreate complex control systems asinteracting sets ofState-EventAction(SEA)modules. The SEA method involves dividing anymachine, assembly line, or process intodistinct modules. Eachmodule progresses through its ownparticular setofstates, andresponds withspecific actions toexternal and internal events according toits state. InSEAprogramming, theflowcharts that are typically usedtodescribe machine operation are reduced tosetsoftables, onetable foreach module. Therowheadings ofthetable list all the states ofthemodule, andthecolumn headings list alltheevents thataffect themodules. Each state/event pairresults ina particular action, whichareallentered inthebodyofthetable. To determine howa modulewouldrespond toa particular event, onemerely needs tolookalong therowofthemodule's state andreadtheaction that corresponds tothat event. Thebiggest advantage oftheSEAmethod isits simplicity. Itiseasytounderstand andeasyto program. Programs created fromSEAtables are easytocreate andrunmoreefficiently than programs madefromflowcharts. Inaway,SEA programs mimichumanbehavior inthat humans alsorespond toevents (stimulus) according to their state orcondition.