Basics of Programmable Logic Controller and Its Functionality

Basics of Programmable Logic Controller and Its Functionality

“PLC” means “Programmable Logic Controller”. The word “Programmable” differentiates it from the conventional hard-wired relay logic. It can be easily programmed or changed as per the application’s requirement. The PLC also surpassed the hazard of changing the wiring. The PLC as a unit consists of a processor to execute the control action on the field data provided by input and output modules. In a programming device, the PLC control logic is first developed and then transferred to the PLC.

FUNCTIONS and Features OF PLC

• It can perform relay-switching tasks.

• It can conduct counting, calculation and comparison of analog process values.

• It offers flexibility to modify the control logic, whenever required, in the shortest time.

• It responds to the changes in process parameters within fractions of seconds.

• It improves the overall control system reliability.

• It is cost effective for controlling complex systems.

BASIC BLOCK DIAGRAM OF THE PLC

As shown in the Fig, the heart of the “PLC” is the center, i.e., the Processor or CPU (Central Processing Unit).

• The CPU regulates the PLC program, data storage, and data exchange with I//O modules.

• Input and output modules are the media for data exchange between field devices and CPU.

• It tells CPU the exact status of field devices and also acts as a tool to control them.

• A programming device is a computer loaded with programming software, which allows a user to create, transfer and make changes in the PLC software.

• Memory provides the storage media for the PLC program as well as for different data.

COMPONENTS OF THE PLC SYSTEM

CPU or processor: The main processor (Central Processing Unit or CPU) is a microprocessor-based system that executes the control program after reading the status of field inputs and then sends commands to field outputs.

I/O section: I/O modules act as “Real Data Interface” between field and PLC CPU. The PLC reads the real status of field devices, and controls the field devices by means of the relevant I/O cards.

Programming device: A CPU card can be connected with a programming device through a communication link via a programming port on the CPU.

Operating station: An operating station is commonly used to provide an "Operating Window" to the process. It is usually a separate device (generally a PC), loaded with HMI (Human Machine Software).