Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicHIGHLIGHTSThe implementation of a control program requires complex organizationaland analytical skills, which change depending on the application. Becausethey are so varied, we cannot explain how to solve every specific control task.Nevertheless, we can provide you with techniques and guidelines for completingthis problem-solving process. In this handbook, we will introduce astrategy for implementing a control program, which includes program organization,system configuration, and I/O programming. These strategies alsoapply to PLCs with the IEC 1131-3 programming standard. Additionally, wewill present both simple and complex PLC programming examples. After youfinish, you will be ready to learn how to document the PLC system—the laststep in implementing the control program.1 CONTROL TASK DEFINITIONA user should begin the problem-solving process by defining the controltask, that is, determining what needs to be done. This information providesthe foundation for the control program. To help minimize errors, the controltask should be defined by those who are familiar with the operation of themachine or process. Proper definition of the task is directly related to thesuccess of the control program.Control task definition occurs at many levels. All of the departmentsinvolved must work together to determine what inputs are required, so thateveryone understands the purpose and scope of the project. For example, ifa project involves the automation of a manufacturing plant in whichmaterials will be retrieved from the warehouse and sent to the automaticpackaging area, personnel from both the warehouse and packaging areasmust collaborate with the engineering group during the system definition.Management should also be involved if the project requires data reporting.If the control task is currently done manually or through relay logic, theuser should review the steps of the manual procedure to determine whatimprovements, if any, can be made. Although relay logic can be directlyimplemented in a PLC, the procedure should be redesigned, when possible,to meet current project needs and to capitalize on the capabilities of programmablecontrollers.2 CONTROL STRATEGYAfter the control task has been defined, the planning of its solution can begin.This procedure commonly involves determining a control strategy, thesequence of steps that must occur within the program to produce the desiredoutput control. This part of the program development is known as thedevelopment of an algorithm. The term algorithm may be new or strange tosome readers, but it need not be. Each of us follows algorithms to accomplishIndustrial Text & Video Company www.industrialtext.com 1-800-752-83984
Introduction to PLC Programming and Implementation—from relay logic to PLC logiccertain tasks in our daily lives. The procedure that a person follows to gofrom home to either school or work is an algorithm—the person exits thehouse, gets into the car, starts the engine, and so on. In the last of a finitenumber of steps, he or she reaches the destination.The PLC strategy implementation for a control task closely follows thedevelopment of an algorithm. The user must implement the control from agiven set of basic instructions and produce the solution in a finite number ofsteps. If developing an algorithm to solve the problem becomes difficult, heor she may need to return to the control task definition to redefine theproblem. For example, we cannot explain how to get from where we are toBullfrog County, Nevada unless we know both where we are and whereBullfrog County is. As part of the problem definition, we need to know if aparticular method of transportation is required. If there is a time constraint, weneed to know that too. We cannot develop a control strategy until we have allof this problem definition information.The fundamental rule for defining the program strategy is think first,program later. Consider alternative approaches to solving the problem andallow time to polish the solution algorithm before trying to program thecontrol function. Adopting this philosophy will shorten programming time,reduce debugging time, accelerate start-up, and focus attention where it isneeded—on design when designing and on programming when programming.Strategy formulation challenges the system designer, regardless of whetherit is a new application or the modernization of an existing process. In eithercase, the designer must review the sequence of events and optimize controlthrough the addition or deletion of steps. This requires a knowledge of thePLC-controlled field devices, as well as input and output considerations.3 IMPLEMENTATION GUIDELINESA programmable controller is a powerful machine, but it can only do what itis told to do. It receives all of its directions from the control program, the setof instructions or solution algorithms created by the programmer. Therefore,the success of a PLC control program depends on how organized the user is.There are many ways to approach a problem; but if the application isapproached in a systematic manner, the probability of mistakes is less.The techniques used to implement the control program vary according to theprogrammer. Nevertheless, the programmer should follow certain guidelines.Table1 lists programming guidelines for new applications and modernizations.New applications are new systems, while modernizations are upgradedexisting control systems that have functioned previously without aPLC (i.e., through electromechanical control or individual, analog, loopcontrollers).Industrial Text & Video Company www.industrialtext.com 1-800-752-83985
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