Introduction to PLC Programming and Implementation (3)


A programmable controller is a powerful machine, but it can only do what it is told to do. It receives all of its directions from the control program, the set of 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 is approached in a systematic manner, the probability of mistakes is less.

The techniques used to implement the control program vary according to the programmer. Nevertheless, the programmer should follow certain guidelines. Table1 lists programming guidelines for new applications and modernizations. New applications are new systems, while modernizations are upgraded existing control systems that have functioned previously without a
PLC (i.e., through electromechanical control or individual, analog, loop controllers).

As mentioned previously, understanding the process or machine operation is the first step in a systematic approach to solving the control problem. For new applications, the strategy should follow the problem definition. Reviewing strategies for new applications, as well as revising the actual method of control for a modernization project, will help detect errors that were introduced
during the planning stages.
The programming stage reveals the difference between new and modernization projects. In a modernization project, the user already understands the operation of the machine or process, along with the control task. An existing relay ladder diagram, like the one shown in Figure1, usually defines the sequence of events in the control program. This ladder diagram can be almost directly translated into PLC ladder diagrams.
New applications usually begin with specifications given to the person who will design and install the control system. The designer translates these specifications into a written description that explains the possible control strategies. The written explanation should be simple to avoid confusion. The designer then uses this explanation to develop the control program.


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