Introduction to PLC controllers

At the outset of industrial revolution, especially during sixties and seventies, relays were used to operate automated machines, and these were interconnected using wires inside the control panel. In some cases a control panel covered an entire wall. To discover an error in the system much time was needed especially with more complex process control systems. On top of everything, a lifetime of relay contacts was limited, so some relays had to be replaced. If replacement was required, machine had to be stopped and production too. Also, it could happen that there was not enough room for necessary changes. control panel was used only for one particular process, and it wasn’t easy to adapt to the requirements of a new system. As far as maintenance, electricians had to be very skillful in finding errors. In short, conventional control panels proved to be very inflexible.
1.3 Systematic approach in designing an process control system, First, you need to select an instrument or a system that you wish to control. Automated system can be a machine or a process and can also be called an process control system. Function of an process control system is constantly watched by input devices (sensors) that give signals to a PLC controller. In response to this, PLC controller sends a signal to external output devices (operative instruments) that actually control how system functions in an assigned manner (for simplification it is recommended that you draw a block diagram of operations’ flow).
Secondly, you need to specify all input and output instruments that will be connected to a PLC controller. Input devices are various switches, sensors and such. Output devices can be solenoids, electromagnetic valves, motors, relays, magnetic starters as well as instruments for sound and light signalization.
Following an identification of all input and output instruments, corresponding designations are assigned to input and output lines of a PLC controller. Allotment of these designations is in fact an allocation of inputs and outputs on a PLC controller which correspond to inputs and outputs of a system being designed.
Third, make a ladder diagram for a program by following the sequence of operations that was determined in the first step. Finally, program is entered into the PLC controller memory. When finished with programming, checkup is done for any existing errors in a program code (using functions for diagnostics) and, if possible, an entire operation is simulated. Before this system is started, you need to check once again whether all input and output instruments are connected to correct inputs or outputs. By bringing supply in, system starts working.

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