Introduction to PLC

 

Introduction to PLC

 

An industrial digital computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, robotic devices, or any activity that requires high-reliability control and ease of programming and process fault diagnosis is known as a programmable logic controller (PLC), also known as a programmable controller.

They were initially created in the automotive sector to provide adaptable, durable, and simple program controllers to swap out hard-wired relays and timers. Since then, they have gained widespread use as tough environment-friendly high-reliability automation controllers. A PLC is an example of a "hard" real-time system because it must respond to input conditions with output results within a finite amount of time or else an unintended operation will occur.

History of PLC

The logic was typically represented as a series of logical expressions in a Boolean format in early PLCs because they were frequently incapable of displaying the logic graphically (akin to Boolean algebra).

Ladder logic was a familiar format used for electro-mechanical control panels, so as programming terminals developed, it became more widely used. Although there are more recent formats available, such as state logic and Function Block diagrams, ladder logic remains the most widely used. This may be because programmers favor ladder logic over structured text programming due to its greater visual appeal.

PLCs were programmed using proprietary programming panels or special-purpose programming terminals up until about the mid-1990s. These devices frequently had dedicated function keys that represented the various logical components of PLC programs.

Plain ASCII code representations of contacts, coils, and wires were common, but some proprietary programming terminals displayed PLC program elements as graphic symbols.

Architecture

 

 

I/O Section:

The I/O section creates the interface between actual hardware found outside the PLC and the digital environment found inside the PLC.

Push buttons, limit switches, and other input devices can be physically connected to a PLC using the input module's bank of terminals. An input module's job is to convert input device signals into language that the PLC's CPU can understand.

A bank of terminals on the output module is also used to physically connect a PLC to output devices like solenoids, motor starters, indicating lamps, etc. A PLC's output module's job is to convert signals from the CPU into forms that the output device can understand.

The tasks of the I/O section can be classified as:

·       Conditioning

·       Isolation

·       Termination

·       Indication

If necessary, an electronic system can be added to link I/O modules to distant I/O devices. The distance between the actual operating process and the CPU and its I/O modules under PLC Control can be thousands of feet.

CPU Section:

The control portion of the PLC is located in the Central Processing Unit, which is the system's brain. There are three subsections.

·       Memory System

·       Processor

·       Power Supply

Memory System:

Data and information are stored and retrieved from the CPU's memory. The following four sections sake up the entire memory area.

I/O Image Memory:

In the input status file, each input field device's ON or OFF state is stored in memory locations known as the input image memory.

The ON or OFF states of field-installed hardware output devices are stored in memory locations that make up the output status file. After the user program is solved, data is saved in the output status file and is awaiting transfer to the switching device of the output module.

Data Memory:

Data memory is used to store numerical data, such as bar code information, which is necessary for mathematical calculations.

User Memory:

 User memory is where the user's application program is stored.

Executive Memory:

Executive memory is used to store system software or executive programs. A PLC's operating system is a unique program that regulates CPU activity and, in turn, the user's program's execution. An operating system for a PLC is provided by the PLC manufacturer and is stored in memory permanently. It is designed to scan image memory, interpret user programs' instructions stored in main memory, and execute user application programs.

Processor:

The processor, also known as the CPU's "heart," is the computerized portion of the device, which is represented by a Microprocessor/Microcontroller chip. It oversees all system operations and carries out all duties required to carry out the PLC function.

It uses the input module to read data, such as the status of externally connected input devices. This data is kept in memory for future use.

According to the application program's instructions, it performs mathematical and logical operations. The result values are stored in memory after the user's program has been solved. To activate field hardware, it transmits data to external devices like the output modules. It communicates with external and peripheral devices. It carries out self-diagnosis.

Power Supply:

The memory system, processor, and I/O Modules are all powered by the power supply.  For use in electronic circuitry, it transforms the higher level AC line voltage into a range of useful DC values.

Programmer/Monitor:

The Programmer/Monitor (PM) is a tool for interacting with the PLC's circuits. Engineers and technicians can enter and edit the program that will be executed using the programming unit.

It can be a handheld device with a membrane keypad for entering programs and an LCD or LED display for viewing program steps or functions in its most basic form.

A separate industrial terminal or personal computers with typewriter-style keyboards and CRT monitors are used in more advanced systems. It enables programmers to write, view, and edit the program and download it into the PLC with the aid of proprietary software. Additionally, it enables users to keep an eye on the PLC as it executes the program. With the help of this monitoring system, it is possible to check the proper operation of internal coils, registers, timers, and other items that are not readily apparent from the outside. Additionally, if necessary, internal register data can be changed. to optimize program performance while debugging A cable attached to a unique programming port on the PLC is used for communication between the PM and PLC. A dedicated card installed in the computer or a serial port can be used to connect to the personal computer.

FEATURES OF PLC :

PLC control systems use specialized I/O modules to create the hardware of the control system with a minimal amount of measurement and peripheral circuitry, enabling complete system control through programming.

1. High Reliability

The most crucial characteristics of PLC are its strong anti-interference quality and extremely high reliability. to enable PLC to function reliably under highly disruptive conditions. In PLC, numerous techniques are used. Software control, as opposed to relay control mode, can reduce faults caused by a malfunctioning original electric contact spot outside.

         Industrial-grade components created using cutting-edge processing techniques can withstand interferences, and self-diagnosis features of memory protection watchdog circuits can significantly boost PLC performance.

2. Good Flexibility

Ladder diagram, SFC, STL, ST, and other programming languages are available for PLCs. The operator can effectively operate a PLC if he is proficient in just one of the programming languages. Anyone who wants to use PLC has many options. According to engineering practice, the number of modules can be increased to increase capacity and function, giving PLC good flexibility.

3. Quality of Strong Easy-Operating

The PLC programs can be easily edited and changed using a computer, either offline or online. By displaying the information about the fault and the Self-Diagnosing Function, it is very simple to determine where the fault is, which simplifies PLC maintenance and repair. The modularization, standardization, and serialization of PLC make configuration very simple.