PLC
by Shivajirao Jadhav Mumbai
Marks: 08 (12)
A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.
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.
Principle of PLC:
A programmable controller, as illustrated below, consists of two basic sections:
• the central processing unit
• the input/output interface system
The central processing unit (CPU) governs all PLC activities. The major components of the CPU are the processor, the memory system and the system power supply.
The input/output (I/O) system is physically connected to the field devices that are encountered in the machine. These field devices may be discrete or analog input/output devices, such as limit switches, pressure transducers, push buttons, motor starters, solenoids, etc. The I/O interfaces provide the connection between the CPU and the information providers (inputs) and controllable devices (outputs).
During its operation, the CPU completes three processes: (1) it reads, or accepts, the input data from the field devices via the input interfaces, (2) it executes, or performs, the control program stored in the memory system, and (3) it writes, or updates, the output devices via the output interfaces. This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.
Block diagram/Architecture 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.
Applications of PLCs:
Residential elevators
Automotive industry
Car washer
Waste water treatment
Air compressor
Oxygen refilling equipment
Textile equipment
Advantages of PLC:
No hard wire required.
High reliability.
High flexibility.
Low cost.
Fast response time.
Easy trouble shooting.
No moving parts.
Functions of PLC:
Sequential relay control
Motion control
Process control
Data handling
Storage processing
Communication capabilities
Difference between PLC and PC:
Specifications of PLC:
Power supply voltage
Power consumption
Noise immunity
Vibration/shock immunity
Weight
Memory
Scan time
Accuracy
Efficiency
Types of PLC modules:
CPU Module: CPU module consists of a central processor and its memory. The Processor is responsible for doing all the necessary computations.
Power Supply Module: These modules supply the necessary power required for the whole system by converting the available AC power to DC power required for CPU and I/O modules.
Digital I/O module: These are used to connect the sensors and actuator that are of digital in nature
Analog I/O modules: These are used to connect the sensors and actuators that provide the analog electric signals
Communication Interface Modules: These are intelligent I/O modules that exchange the information between a CPU and communication network.
Questions
Marks wise Questions: 1M(2), 3M(2), 4M(1)
1. Fill in the blanks with given alternatives.
-------- is specialized computer used to control machine process.
------- logic is used to program PLC.
----------industry is largest user of PLC.
PLC has no --------
Input source of PLC converts analog to ---------.
Ans: 1. PLC 2. Ladder 3. The automotive 4. Keyboard 5. Digital.
2. Answer the following.
What is PLC?
Draw architecture of PLC.
With block diagram explain working of PLC.
What are the functions of PLC?
Differentiate between PLC and computer.
Explain working principle of PLC.
Write advantages of PLC.
Write applications of PLC.
Write different types of PLC modules.
