PLC ( Programmable Logic Controller ) : Introduction, Use, Example with Block diagrams

Brief introduction of PLC

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

Why do we use PLC?

                Before PLC we used the microcontroller and Relay logic for controlling and automation. If we use a microcontroller, it has high breakdown and high 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 contains a readymade block for any execution. But in the microcontroller, we have to create logic for every block. Ladder programming is very easy and easily understood able as compared to microcontroller programming. It can also be programmed, controlled, and operated by an 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 the machine, it requires more time to rewire of the panel.
• But by using PLC, we can modify the program easily within minute time without wiring changes.

Simulation

• We can check programs or logic in the lab using a simulator facility.

Visual observation

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

Reliability and Maintainability

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

Block diagram of PLC.

PLC contains mainly three units CPU, INPUT, and OUTPUT.

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

Also Check: 9 Tips to Learn Programming Faster

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:-

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

DIGITAL INPUT– Push buttons, selector switches, joysticks, relay contacts, pressure switches and proximity switches, sensors, 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 a relay or transistor. We show output as Y₀, Y₁, Y₂,….. Q₀, Q₁, Q₂,……

How to connect input to plc

                There are several types of input like a 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 points. We connect the input to PLC in two types PNP and NPN. PNP and NPN connection is also called SINK & SOURCE connection. The sink and source connection depend on the current flow between the common point and the input point. If current flows from the common point to the input point, this is called a SOURCE or NPN connection.  If current flows from the input section to a common point, this is called a SINK or PNP connection.

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

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

How to connect output with PLC:-

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

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

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

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