plc is programmable control system which reduced complex hardwired relay logic. Programmable logic controller (PLC) store instructions, like sequencing, timing, counting, arithmetic, data manipulation, communication etc. to control industrial machines, systems and processes. Controlling process is depends on inputs, outputs condition of system. plc have basic features. plc is used for the purpose of efficiency and economy. It reduced human error and provides continues result of process or system. plc save fuel, cost, processing material, labor and increased production. below are some common question for plc asked in interview.
Programmable Logic Controller (PLC) is a microprocessor or microcontroller based industrial computer used for automate processes by receiving inputs from sensors, processing data based on a program, and controlling outputs.
in plc redundancy Two PLC CPUs (primary and backup) operate in tandem. plc redundancy means providing an alternative control system in the fault condition. This will provide reliability to the process control system as well as reduce downtime during a failure.in redundancy we implement a backup plc to ensure continuous operation of an automated process in case of primary plc failure.
following are basic component of plc
Sensors are devices that convert one form of energy into another. They measure physical properties, such as temperature, light, motion, pressure, humidity, or sound, and convert them into electrical signals or data that can be interpreted, analyzed, or used by other systems, devices, or applications.
PLC programming languages are standardized by IEC 61131-3, which defines following five primary languages used for programming PLCs.
ladder diagram is a graphical based programming language for plc. It visually resembles a ladder, with two vertical rails representing power supply and horizontal rungs representing control logic.
Structured text programming language is a high level text based programming language for plc. it is a combination of statement and conditional statement where you define your function to run a process.
functional block diagram is graphically blocks based plc programming language. these graphical blocks represent logic and functions. functional block diagram have graphical block like logical blocks, timer and counter block, arithmetic and comparison blocks.
CFC (Continuous Function Chart) is a graphical programming language used in industrial automation for programming PLC and DCS systems. It allows the user to create control logic by arranging and interconnecting functional blocks freely on a workspace. Unlike other structured languages, CFC does not impose a strict top-to-bottom or left-to-right flow, providing flexibility in logic design.
rack or chassis is physical structure for mounting and connecting all necessary modules within the PLC, such as input, output, CPU, power supply, and communication modules etc.in plc rack power supply module supply necessary power to the different input-output modules through the backplane bus system. rack or chassis used in modular plc system.
communication protocol used to communicate plc with other devices like hmi, scada, development software, programming station etc. some of the standard common protocol for plc
4-20 mA signal represent analog signal standard in plc. 4-20 mA signal used for transmitting analog sensor data to plc or from the PLC to field devices by using analog input or analog output module.
analog devices represent physical quantities that can have an infinite number of values. Analog input modules normally have multiple input channels that allow 4, 8, or 16 devices to be interface to the PLC. The two basic types of analog input modules are voltage sensing and current sensing.
PLC DIGITAL MODULE This type of interface connects field input devices of the ON/OFF nature such as selector switches, pushbuttons, and limit switches. Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that require simple ON/OFF switching.
The scan cycle is the time taken by a PLC to continuously monitor its inputs, execute its logic, and control outputs in real-time. This cycle repeats and is continuously executed for the duration of the operation.
Data Types Represent Values Such As numbers, real numbers, strings, and Boolean values, Each data type has a specific range of values and operations that can be performed on it. All Data Type Have Some Reserved Memory For Storing Value In That Memory. following are data types used in different type of plc
following are most common plc manufacturer