Theory

👉 Definition of PLC

     A programmable logic controller is currently defined by the NEMA (National Electrical Manufactures Association) as a “Digital electronic device that uses the programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting and arithmetic operations to control machines and processes.” 

     Older PLCs were capable of handling only discrete inputs and outputs (i.e. ON-OFF type signals), while today’s system can accept and generate analog voltages and currents as well as wide range of voltage levels and pulsed signals. Its purpose is to monitor crucial process parameters and adjust process operations accordingly.

     The PLC is basically a programmed interface between the field input devices like push-buttons, limit switches, sensors etc. and final control elements like actuators - motors, solenoids, valve dampers, drivers - or status indicating output devices like lamps, LEDs, hooters etc.

👉 Types of PLC

There are two types of PLC according to their constructions

➤ Compact PLC:-

• In Compact PLC CPU unit, Communication Port, Power Supply unit, Inputs and Outputs all are mounted on a single board.
• I/O capacity of Compact PLC is fixed and less than I/O capacity of Modular PLC.
• Generally used for small applications such as Hydraulic press machine, any Special Purpose machine (SPM), etc.

➤  Modular PLC:-

• In Modular PLC consisting separate parts as described below:-

1.  Base Rack,
2.  Power Supply Module,
3.  CPU Module,
4.  Input Modules,
5.  Output Modules,

• I/O capacity of Modular PLC is expandable and more than I/O capacity of Compact PLC.
• Generally used for large Application Such as Plant Automation, Power Plant Handling, etc.

👉 Block Diagram

A simplified block diagram of a PLC shown in above Fig.

It has three major units/sections.

1.  I/O (Input/Output) Modules 
2.  CPU (Central Processing Units)
3.  Programmer / Monitor

(1)   I/O (Input/Output) Modules 

     The I/O section establishes the interfacing between physical devices in the real world outside the PLC and the digital arena inside the PLC. 

     The input module has bank of terminals for physically connecting input devices, like push buttons, limit switches etc. to a PLC. The role of an input module is to translate signals from input devices into a form that the PLC’s CPU can understand. 

     The output module also has bank of terminals that physically connect output devices like solenoids, motor starters, indicating lamps etc. to a PLC. The role of an output module is to translate signals from the PLC’s CPU into a form that the output devices can use.

(2) CPU (Central Processing Units) 
     The central processing unit, the brain of the system, is the control portion of the PLC. It has three sub-parts: 

       (a) Power supply
       (b) Processor
       (c) Memory system 

(a) Power Supply
     The power supply provides power to memory system, processor and I/O Modules.

     It converts the higher level AC line Voltage to various operational DC values for electronic circuitry.
     It filters and regulates the DC voltages to ensure proper computer operations.
(b) Processor
     The processor, the heart of CPU is the computerized part of the CPU in the form of Microprocessor / Micro controller chip.
     It supervises all operation in the system and performs all tasks necessary to fulfill the PLC function.

• It reads the information i.e status of externally connected input devices with input module. 
• It stores this information in memory for later use. 
• It carries out mathematical and logic operations as specified in application program. 
• After solving the user's program, it writes the result values in the memory. 
• It sends data out to external devices like output module, so as to actuate field hardware. 
• It performs peripheral and external device communication. 
• It performs self-diagnostics.

(c) Memory system

     The memory is the area of the CPU in which data and information is stored and retrieved. The total memory area can be subdivided into the following three Sections.

• System Memory

     It is used to store an executive program or system software. An operating system of the PLC is a special program that controls the action of CPU and consequently the execution of the user's program.

• DATA Memory

     It is used to store numerical data required in math calculation, bar code data etc.

     The input image memory consists of memory locations used to hold the ON or OFF states of each input field devices, in the input status file.
     The output status file consists of memory locations that stores the ON or OFF states of hardware output devices in the field. Data is stored in the output status file as a result of solving user program and is waiting to be transferred to the output module's switching device. 

• Program Memory

     It contains user's application program.

(3) Programmer/Monitor 

     The Programmer/Monitor (PM) is a device used to communicate with the circuits of the PLC. The programming unit allows the engineer/technicians to enter the edit the program to be executed.

👉 Scan Cycle

     A PLC program is generally executed repeatedly as long as the controlled system is running.

• The status of physical input points is copied to an "I/O Image Table". (This memory area accessible to Processor so it can read Input status of the Inputs)
• The program is then run from its first instruction rung down to the last rung.
• It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.

    

     "This scan time may be a few milliseconds (3ms to 10ms) for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100ms) to execute the program." 

👉 Communication Protocol

     "It is a set of rules for data transmission when PLC is connected to Network. Cable is used to connect different external device”.

     Types of the different communication Protocols are as under:-

• RS 232 (Serial Communication)
• RS 422 (Serial Communication)
• RH 485 (Serial Communication)
• CAN BUS
• MOD BUS
• DEVICE NET
• PROFIBUS-PA
• PROFIBUS-DP
• ETHERNET
• ETHERCAT
• INTER BUS-S
• SERIPLEX

     Out of this first 10 are most commonly used protocols. Every protocol has a different "Baud rate".

Baud Rate:- "Rate of data transmission on network."
Unit:- Bits/Second.
Range:- 120 bits/sec To 100 Mbps.

👉 Programming Language

• Ladder Diagram. (Note: - Mostly use everywhere.) 
• Function Block Diagram.(FBD) 
• Sequential Function Chart.(SFC) 
• Structured Text Language.(ST)

 👉 Advantages

• Grate life and Reliability.
• Tremendous Flexibility.
• Shorter Project time.
• Ease of maintenance.
• Economical.
• Easier Storage and Documentation.
• Energy Saving.
• Required less space.

👉 Features

• Sequential Logic Solver.
• Bit Operation.
• Data Transfer.
• Text handling.
• PID Calculation.
• Subroutines.

 👉 Leading Vendor of PLC

• Siemens
• Allen Bradley.
• Schneider.
• Beckhoff.
• Mitsubishi.
• Messung.
• GE-Fanuc.
• Omron, etc.

👉 PLC Selection Criteria

• Inputs and Outputs Requirement.
• CPU Memory.
• Compatible Protocol.
• Integration of High-Level Application.
• Reliability & Flexibility.
• Spare Parts and Maintenance.
• Cost of Hardware.
• Local Support Availability.

👉 Types of INPUT and OUTPUT

Digital		Analog
Input	Output	Input	Output
Push Button	Lamp	RTD	Actuator
Selector Switch	Indication	Pressure Sensor	Flow Meter Control
Limit Switch	Solenoid Coil	Flow Sensor	Analog Speed Control
Reed Switch	Buzzer
Proximity Sensor

• In digital I/O the signal condition is ON / OFF (1 / 0).
• In analog I/O the signal condition is either 0 to 10V or 4mA to 20mA.

👉 Sink / Souce Concept

     “Sinking” and “Sourcing” terms are used to describe the direction of current flow through a field device in relation to the power supply and the associated I/O point in I/O modules. Sinking or sourcing is of utmost importance when a PLC interfaces solid state field devices.

• Fig. shows a battery, switch and a load (Bulb). As shown in first Fig., the current flows from positive terminal of the battery to the switch and then to the (bulb) load and returns to the negative terminal of the battery. In this circuit the switch is said to be in the sourcing mode, and the (bulb) load is in the sinking mode. 
• In second Fig., the current from battery first passes though the (bulb) load and then through switch before returning to the battery. In this circuit the (bulb) load is sourcing and switch is sinking.
• The sink and source connection with respect to PLC are shown in below Figure.