The advantages of PLC control
Any control system goes through four stages from conception to a working plant. A PLC system brings advantages at each stage. The first stage is design; the required plant is studied and the control strategies decided. With conventional systems design must be complete before construction can start. With a PLC system all that is needed is a possibly vague idea of the size of the machine and the I/O requirements (how many inputs and outputs).
The input and output cards are cheap at this stage, so a healthy spare capacity can be built in to allow for the inevitable omissions and future developments. Next comes construction. With conventional schemes, every job is a one-off with inevitable delays and costs. A PLC system is simply bolted together from standard parts. During this time the writing of the PLC program is started (or at least the detailed program specification is written).The next stage is installation, a tedious and expensive business as sensors, actuators, limit switches and operator controls are cabled. A distributed PLC system (discussed in Chapter 5) using serial links and pre-built and tested desks can simplify installation and bring huge cost benefits. The majority of the PLC program is written at this stage.
Finally comes commissioning, and this is where the real advantages are found. No plant ever works first time. Human nature being what it is, there will be some oversights. Changes to conventional systems are time consuming and expensive. Provided the designer of the PLC system has built in spare memory capacity, spare I/O and a few spare cores in multi core cables, most changes can be made quickly and relatively cheaply. An added bonus is that all changes are recorded in the PLC program and commissioning modifications do not go unrecorded, as is often the case in conventional systems.
There is an additional fifth stage, maintenance, which starts once the plant is working and is handed over to production. All plants have faults, and most tend to spend the majority of their time in some form of failure mode. A PLC system provides a very powerful tool for assisting with fault diagnosis. A plant is also subject to many changes during its life to speed production, to ease breakdowns or because of changes in its requirements .A PLC system can be changed so easily that modifications are simple and the PLC program will automatically document the changes that have been made.
