Basics of PLC.

What is PLC ?

PLC is a Solid state / computerized industrial computer that performs discrete or sequential logic in a factory environment its call PLC.Its Was originally develop to replace mechanical relay,timer and counter.

What is a Discrete:- When you press one switch then start all lights its mean discrete.

What is a Sequential:- When you press one switch then lights on one by one its mean sequential

PLC System Functional Block diagram.

       

Fig of PLC Block diagram

Programmable Logic Controllers continuously monitors the input values from various input sensing devices (e.g. accelero meter, weight scale, hardwired signals, etc.) and produces corresponding output depending on the nature of production and industry. A typical block diagram of PLC consists of five parts namely.

1. Rack or chassis

2. Central Processing Unit (CPU)

3. Power Supply Module

4. Input  Module

5. Output Module

6. Communication Interface Module  

7. Programming Device. 

1) Rack or Chassis:-

In all PLC systems, the PLC rack or chassis forms the most important module and acts as a backbone to the system. PLCs are available in different shapes and size. When more complex control systems are involved, it requires larger PLC racks.

Small sized PLC is equipped with fixed I/O pin configuration. So, they have gone for modular type rack PLC, which accepts different types of I/O modules with sliding and fit in concept. All I/O modules will be residing inside this rack / chassis.                                                                      

       

Fig of Rack type PLC

2) CPU Module and Memory:-

CPU module has a central processor, ROM & RAM memory. ROM memory includes operating system, driver and application program. RAM memory is used to store programs and data. CPU is the brain of PLC with an octal or hexagonal microprocessor. Being a microprocessor based CPU, it replaces timers, relays, and counters. Two types of processors as a single bit or word processor can be incorporated with a PLC. One bit processor is used to perform logic functions. Whereas word processors are used for processing text, numerical data, controlling and recording data. CPU reads the input data from sensors, process it and finally the command to controlling devices. DC power source, as mentioned in the previous discussion is required voltage signals. CPU also contains other electrical parts to connect cables used by other units.

CPU consist of following blocks 

• ALU(Arithmetic Logic Unit)  
• Programming memory.        
• Process image memory.  
• Internal timer and counter.
• Flags or status bit filter.
• CPU required 5v.                          

3) Power Supply Module:-

This module is used to provide the required power to the whole PLC system.

It converts the available AC power to DC power which is required by the CPU and I/O module. PLC generally works on 24V DC supply and Rack type PLC work on 230/220Vac. Few PLC uses an isolated power supply.

4) INPUT Module:-

Input module act as an interface between the field control input and the CPU voltage/current generated by the sensors,transducer,limit switch,push button,etc.

Input module converts the field signal into standard control signal for processing by the PLC.

The input module of PLC does four main functions.

1. Input module interface receives the signal from process devices at 220 V AC

2. Converts the input signal to 5 V DC that can be used by PLC

3. Isolator block is used to isolate/prevent PLC from undergoing fluctuation

4. After which the signal is sent to the output end i.e the PLC

 5) OUTPUT Module:-

Output Module acts as link between the CPU and the output devices located in the field devices could be relays,contaector,lamps,motorized actuators, solenoid valves,etc.These device  actually control the process.

6) Communication Interface Module:-

To transfer information between CPU and communication network, intelligent I/O modules are used. These communication modules help to connect with other PLCs and computers which are placed at a remote location.

7) Programming Device:-

A personal computer(PC) is the most commonly used programming device.The software allows users to create,edit,store and troubleshoot programs.

PLC Scan Cycle. 

The working of a PLC can be easily understood as a cyclic scanning method known as scan cycle. 

PLC Scan Cycle

•  The operating system starts cycling and monitoring of time. 

• The CPU starts reading the data from the input module and checks the status of all the inputs. 

• The CPU starts executing the user or application program written in relay-ladder logic or any other PLC- programming language. 

• Next, the CPU performs all the internal diagnosis and communication tasks.

• According to the program results, it writes the data into the output module so that all outputs are updated. 

• This process continues as long as the PLC is in run mode.  

Signal OF INPUT/OUTPUT Module

There is more then types of input and output module in PLC and please show below Block Diagram of Signal of I/O Module.

Fig signal of INPUT/OUTPUT Module

•  There is Two Types of Signal I/O module 1) Discrete Types and 2) Analog Type
•  In this PLC have two module digital and analog module show here with explain which device to connect to Discrete module or Analog module.
•  Discrete type and Analog type have also two types 1) Input device and 2) output device and also show in figure which type of model have connect device.

Types of PLC. 

 

           

Small Type PLC:-I/O Module CPU all as a one unit all module fixed wit CPU

Modular Type PLC:- I/O module can be expand but up to some limit.

Rack Type PLC:-

          -Components include rack type PLC.

          (1) Rack,                       (2) Slot,

          (3) Modules/card,         (4) Channels.

General I/O module available in 2,4,8,16,32 channels as per application.

PLC Programming Language. 

When using a PLC, it’s important to design and implement concepts depending on your particular use case. To do this we first need to know more about the specifics of PLC programming. A PLC program consists of a set of instructions either in textual or graphical form, which represents the logic that governs the process the PLC is controlling. There are two main classifications of PLC programming languages, which is further divided into many sub-classified types.

1. IL     (Instruction list)

2. LD   (Ladder Diagram)

3. FBD (Function block diagram)

4. SFC (Sequential function chart)

5. ST   (Structured Text)   

PLC Advantages:-

- Reliability   

- Flexibility in programming and reprogramming

- Cost effective for controlling complex systems.

- Small Physical size, shorter project time.

- High Speed of operation.

- Ability to communicate With computer systems in the plant.

- Ease of maintenance / troubleshooting.

- Reduced space.

- Energy Saving

PLC Disadvantages:-

- PLC device are proprietor it means that part or software of one manufacturer can't be used in combination with parts of another manufacturer.

- Limited design and cost option.

- Fixed Circuit operations.

- PLCs manufacturers offer only closed architectures.

PLC Applications.

PLCs have a variety of applications and uses, including:

1. Process Automation Plants (e.g. mining, oil &gas)

2. Glass Industry

3. Paper Industry

4. Cement Manufacturing

5. In boilers – Thermal Power Plants