Brief introduction of PLC

We all know full form of PLC is PROGRAMMABLE LOGIC CONTROLLER. PLC is a device which is use for automation such as industrial process, machine automation and controlling for various type of equipment. A PLC is a user-friendly, microprocessor based computer that controls many types of functions and levels of complexity.

Why we use PLC?

                Before PLC we used the microcontroller and Relay logic for controlling and automation. If we use microcontroller, it have highly breakdown and highly failure due to shorting and atmospheric conditions. But PLC is safe from shorting and all atmospheric conditions. Ladder programming uses for PLC programming. Ladder programming contain readymade block for any execution. But in microcontroller we have to create logics for every block. Ladder programming is very easy and easily understood able as compare to microcontroller programming. It can also be programmed, controlled and operated by a UNSKILLED person

Flexibility

  • Before PLC, each production machine required its own electronic controller for example 5 machines might require 5 different controllers.
  • Now using PLC, we can control all 5 machines by a single PLC.

Modification and correction errors

  • With a wired relay-type panel, if we want modification or correction in machine, it require more time to rewiring of panel.
  • But by using PLC, we can modify program easily with in minute time without wiring changes.

Simulation

  • We can check program or logic in lab using simulator facility.

Visual observation

  • We can check input and output easily either it is coming or not.

Reliability and Maintainability

  • PLC contains solid state device. That’s why maintenance cost is very low or reliable as compare mechanical timer, relay and contactor.

Block diagram of PLC.

Block diagram of PLC- Programmable logic controller
Block diagram of PLC- Programmable logic controller

PLC contains mainly three unit CPU, INPUT and OUTPUT.

CPU:-CPU contains a processor. CPU read and executes programming instruction which is programmed by programmer. CPU controls all activity by receiving input, and as per program control all output. Using communication device we can communicate CPU with other devices like personal computer and others. Using communication devices we can download program in memory and CPU can also read other device’s memory.

INPUT:-PLC can read both type of input (digital and analog input). Digital input means on/off or discrete signal. Analog input means variable voltage/current or data input. Digital input connected to CPU with optocoupler device as per following figure:-

input block diagram of PLC
input block diagram of PLC

Here photo coupler is used for safety purpose. If any abnormality comes from input section, photo coupler shut off and no signal goes to CPU unit.

DIGITAL INPUT– Push-buttons, selector switches, joy sticks, relay contacts, pressure switches and proximity switches, sensor etc.

ANALOG INPUT– Potentiometers, temperature transducers, level transducers, pressure transducers, humidity transducers etc.

We show input as X0, X1, X2,….. I0, I1, I2,…..

OUTPUT:- Output is not directly connected to cp. Between CPU and output section there is relay or transistor. We show output as Y0, Y1, Y2,….. Q0, Q1, Q2,……

Output block diagram of PLC
Output block diagram of PLC

How to connect input to plc

                There are several types of input like push button, limit switch, pressure switch and sensor. Here we connect one push button to PLC.

Sink and source concept:-

There is a common point (S/S) in PLC for several input point. We connect input to PLC in two types PNP and NPN. PNP and NPN connection also called SINK & SOURCE connection. Sink and source connection depend on current flow between common point and input point. If current flows from common point to input point, this is called as SOURCE or NPN connection.  If current flows from input section to common point, this is called as SINK or PNP connection.

SOURCE CONNECTION:- In this type of connection +VDC supply connect to S/S point and -VDC supply use for input connection. Whether we give input to plc than we use –VDC supply for input supply. In this connection current flow from s/s point to input point in plc. But in field side current flows from input point to s/s point.

PLC source block diagram - Programmable logic controller
PLC source block diagram – Programmable logic controller

SINK CONNECTION:-In this type of connection -VDC supply connects to S/S point and +VDC supply used for input connection. Whether we give input to PLC then we use +VDC supply for input supply. In this connection current flows from input point to S/S point in PLC. But in field side current flows from S/S point to input point.

PLC sink block diagram - Programmable logic controller
PLC sink block diagram – Programmable logic controller

How to connect output with PLC:-

There is com point in output section. This point is common point of all output. Whatever supply is given to com point, when output is high then that’s supply (com supply) comes out from output point.

Let’s take an example to understand output circuit deeply.

PLC Output circuit block diagram
PLC Output circuit block diagram

Here we connect one relay with output section. We connect +24V dc power supply to com point. When output Y0 is high +24v supply comes out from Y0 point. That Yo point connects to 24V operated relay.

PLC Output circuit with relay block diagram
PLC Output circuit with relay block diagram

HOW WE PROGRAM A PLC IS MY NEXT TOPIC

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