Programmable Logic Controller (PLC)

Contents Control Process What is PLC? General Architecture of PLC PLC Operating Cycle PLC Hardware Type PLC Architecture Evolution Programming of PLC Basic PLC Advantages Applications

Control Process Physical Quantity is sensed in the form of small current/voltage e.g. Temperature, Pressure, Flow, Level etc (Sensor) This Electrical Signal is amplified to a certain level (Amplifier) Then amplified analogue output is converted into digital form. (Analogue to Digital Converter i.e. A/D) This digital output is fed to the controller of the system to control the various physical quantities with the help of different devices. (PLC, Microcontroller, DSP etc…)

Block Diagram Physical Quantity Amplifier A/D Controller Devices Amplified Analogue output Physical Quantity (Temperature, Pressure etc..) Amplifier A/D Digital Analogue output in mA/mV Controller (PLC, Microcontroller etc…) Control signals Digital /Analogue Devices (Motors etc…)

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What is PLC? A digitally operating electronic apparatus which uses a programmable memory for internal storage of instructions Implement specific functions, such as logic, sequencing, timing, counting and arithmetic Control various types of machines or process through digital or analogue input/output.

Traditional concept of PLC PLC performs relay equivalent functions PLC performs ON/OFF control Designed for industrial environment

General Architecture of PLC

Input of PLC Types of Voltages: DC: 12V, 24V, 48V AC: 120V/220V Input Devices: Pushbuttons Sensors Relay Contacts etc… *Input of a PLC can be Digital as well as Analogue

PLC input is the load in the circuit, sensing if voltage is present. Contd… PLC input is the load in the circuit, sensing if voltage is present.

Output of PLC Types of Voltages: Output Devices: DC: 12V, 24V, 48V AC: 120V/220V Output Devices: Relays (24V DC/AC, 120V/220V AC) Transistors MOSFET (24V DC) Valves Solenoids Light Fan etc…. *Output of a PLC can be Digital as well as Analogue

PLC output is a switch, controlling the current flow to the load. Contd… PLC output is a switch, controlling the current flow to the load.

PLC Operating Cycle

PLC Signal Flow Output

PLC Hardware Type A most basic PLC system is a self contained PLC which has two terminal blocks, one for the Input and other for the Output, called “Micros”. Typically they provide front panel LED status indication of I/O and processor status.

2. Modular Chassis Based PLC The vast majority of PLC’s installed today are modular chassis based PLC consisting of:

3. Modular Chassis-less PLC Systems The advanced PLC’s are chassis-less These are modular PLC systems. These systems also have: -Processor - Power supply -I/O modules -Communication card These components mount directly on a panel to allow easy insertion and removal.

General PLC Blocks Personal Computer 220V, 50/60Hz RS 232 Power Supply CPU Communicat-ion Module Analogue Input Analogue Output Digital Input Digital Output AC to DC conversion (12V or 48V)

Contd…

Contd… 48V DC

Contd… 48V DC

Contd…

Contd… Bus system is

PLC Architecture Evolution

PLC Architecture Evolution

PLC Architecture Evolution

PLC Architecture Evolution

PLC Architecture Evolution

PLC Architecture Evolution

PLC Application Considerations Number of I/O: 10, 16, 20, 32, 156 etc… Memory: -Types: Flash or Battery Backed - Size: 1k, 6k, 12k, 16k, 64k etc… Functions Required: - Instruction Set: Messaging, PWM etc.. - Arithmetic - Communications: Ethernet, Profibus etc… Report Generation

Programming of PLC The purpose of a PLC program is to control the state of outputs based on the current condition of the inputs. Types of programming : Ladder Logic (Analogous to Relay Technology) Function Block (FB’s based on AND-OR logic) Visual Basic (VB based on C language) *DOS based programming (Old Technology)

1. Ladder Logic Most popular and most commonly used PLC language is known as “Ladder Logic” . Ladder Logic purposely resembles relay logic. In order to control a process we try to design the logical continuity from input to output of program.

Ladder Logic Concepts Line Neutral

Ladder Logic Concepts

Ladder Logic Concepts

Read and Write Instruction Read Instruction Write Instruction

Some Important Instructions Latching/Unlatching Instruction: Latch Retain the output energized until reset Unlatch

Retentive timer-on delay(RTO) Timer Instructions Timer ON Delay(TON) Timer OFF Delay(TOF) Retentive timer-on delay(RTO)

Counter Instructions Counter Up (CTU) Counter Down (CTD)

Comparison Instructions Equal (EQU) Not Equal (NEQ) Less Than (LES) Less Than or Equal (LEQ) Greater Than (GRT) Greater Than or Equal (GEQ) Some other instructions are Sequencer Instruction, Shift Register Instruction etc…

Function Block Based on pre-defined blocks of various functions A B AND

Basic PLC Advantages Ease of Programming Ease of Maintenance Designed for Industrial Environment Suitable for Extreme Environmental Conditions Quick Installation Adaptable to Change

Applications