Initially industries used relays to control the manufacturing processes. The relay control panels had to be regularly replaced, consumed lot of power and it was difficult to figure out the problems associated with it. To sort these issues, Programmable logic controller (PLC) was introduced.

What is PLC?
Programmable Logic Controller (PLC) is a digital computer used for the automation of various electro-mechanical processes in industries. These controllers are specially designed to survive in harsh situations and shielded from heat, cold, dust, and moisture etc. PLC consists of a microprocessor which is programmed using the computer language.
The program is written on a computer and is downloaded to the PLC via cable. These loaded programs are stored in non – volatile memory of the PLC. During the transition of relay control panels to PLC, the hard wired relay logic was exchanged for the program fed by the user. A visual programming language known as the Ladder Logic was created to program the PLC.
PLC Hardware
The hardware components of a PLC system are CPU, Memory, Input/Output, Power supply unit, and programming device. Below is a diagram of the system overview of PLC.

  • CPU – Keeps checking the PLC controller to avoid errors. They perform functions including logic operations, arithmetic operations, computer interface and many more. 
  • Memory – Fixed data is used by the CPU. System (ROM) stores the data permanently for the operating system. RAM stores the information of the status of input and output devices, and the values of timers, counters and other internal devices. 
  • I/O section – Input keeps a track on field devices which includes sensors, switches. 
  • O/P Section - Output has a control over the other devices which includes motors, pumps, lights and solenoids. The I/O ports are based on Reduced Instruction Set Computer (RISC). 
  • Power supply – Certain PLCs have an isolated power supply. But, most of the PLCs work at 220VAC or 24VDC.
  • Programming device – This device is used to feed the program into the memory of the processor. The program is first fed to the programming device and later it is transmitted to the PLC’s memory. 

System Buses – Buses are the paths through which the digital signal flows internally of the PLC. The four system buses are:
·         Data bus is used by the CPU to transfer data among different elements.
·         Control bus transfers signals related to the action that are controlled internally.
·         Address bus sends the location’s addresses to access the data.
·         System bus helps the I/O port and I/O unit to communicate with each other.

 

How does a PLC work?

      The PLC receives information from connected sensors or input devices, processes the data, and triggers outputs based on pre-programmed parameters. Depending on the inputs and outputs, a PLC can monitor and record run-time data such as machine productivity or operating temperature, automatically start and stop processes, generate alarms if a machine malfunctions, and more. Programmable Logic Controllers are a flexible and robust control solution, adaptable to almost any application.


What-is-PLC-programmable-logic-controller

    There are a few key features that set PLCs apart from industrial PCs, microcontrollers, and other industrial control solutions:
• I/O – The PLC’s CPU stores and processes program data, but input and output modules connect the PLC to the rest of the machine; these I/O modules are what provide information to the CPU and trigger specific results. I/O can be either analog or digital; input devices might include sensors, switches, and meters, while outputs might include relays, lights, valves, and drives. Users can mix and match a PLC’s I/O in order to get the right configuration for their application.
• Communications – In addition to input and output devices, a PLC might also need to connect with other kinds of systems; for example, users might want to export application data recorded by the PLC to a supervisory control and data acquisition (SCADA) system, which monitors multiple connected devices. PLCs offer a range of ports and communication protocols to ensure that the PLC can communicate with these other systems.
• HMI – In order to interact with the PLC in real time, users need an HMI, or Human Machine Interface. These operator interfaces can be simple displays, with a text-readout and keypad, or large touchscreen panels more similar to consumer electronics, but either way, they enable users to review and input information to the PLC in real time.

Advanced PLC Features

     In today’s world of the Industrial Internet of Things (iIoT), and Industry 4.0 programmable controllers are called upon to communicate data via Web browser, connect to databases via SQL, and even to the cloud data via MQTT.

The All-In-One PLC

      An All-in-One PLC integrates the controller with the HMI panel, creating a compact, easy-to-use automation solution. Users no longer need to establish PLC to panel communications and can program both the Ladder Logic and HMI design in a single software environment. An all-in-one approach saves time, reduces wiring, and cuts the cost of purchasing multiple devices.

How is a PLC Programmed

      A PLC program is usually written on a computer and then is downloaded to the controller
      Most PLC programming software offers programming in Ladder Logic, or “C”. Ladder Logic is the traditional programming language. It mimics circuit diagrams with “rungs” of logic read left to right. Each rung represents a specific action controlled by the PLC, starting with an input or series of inputs (contacts) that result in an output (coil). Because of its visual nature, Ladder Logic can be easier to implement than many other programming languages.
     “C” programming is a more recent innovation.
      Some PLC manufacturers supply control programming software.

What are the different types of PLC?

     In addition to the traditional PLC described above, there are variations, including PLC + HMI controllers.

Programmable Logic Controller by Unitronics

     Unitronics is a pioneer in the manufacture and design of Programmable Logic Controllers with integrated HMI panels and built-in I/O. They launched the very first All-in-One PLC on the market and have continued to improve the technology based on market feedback and industry advancements.
    Unitronics now offers several rugged PLC product lines to support a broad range of system requirements. Compactly designed, these game-changing controllers create immediate savings by eliminating time-intensive tasks such as PLC-Panel wiring and communication setup. Unitronics supports the All-in-One concept with revolutionary software that enabled Ladder control, HMI design, and all hardware and communication configuration to be programmed in a single, simple software environment.

Advantages & Disadvantages

Advantages
  • PLCs can be programmed easily which can be understood clearly well.
  • They are fabricated to survive vibrations, noise, humidity, and temperature. 
  • The controller has the input and output for interfacing.
Disadvantages
  • It is a tedious job when replacing or bringing any changes to it. 
  • Skilful work force is required to find its errors.
  • Lot of effort is put to connect the wires.
  • The hold up time is usually indefinite when any problem arises.