PLC (not done)

PLC

A programmable digital controller specifically intended for automating industrial processes. Can perform logic operations, sequencing, timing, counting and mathematical calculations.

Before PLC

-control tasks were typically implemented using relay logic.

-Controllers were HARD WIRED

-Electricians would wire the system components together based on a circuit diagram developed by the engineer.

-If an error was made, the wires had to be reconnected correctly

-A change in function or system expansion required component changes and rewiring.

PLC Origin

Developed in the implemented systems late 60’s to replace relay-

• Objective was to achieve a more flexible and modular approach to automation compared to hard-wired systems

• Quickly adopted by industry practitioners, was a popular option for automation by the 80’s

• Steady decline in cost and increase in functionality continue to make PLCs very popular

Advantages

Flexible implementation compared to hard- wired systems

• Wiring between devices and relay-contacts not done physically but through the PLC program

• Fast and easy to make changes (no need for physical rewiring)

• Compact modular design

• easy to add, remove or replace modules

• Trouble shooting aids provided to make programming easier and to reduce downtime

• Cost effective

• Reliable components ensure years of continuous performance before failure

Sequential Control

Relay Logic

• Timer Counter Functions

• Auto/Manual Control

Complex Control

• Arithmetic Operations

• Analog Process Control

• Servo/Stepper Motor Control

• Supervisory Control and Data Acquisition

Digital Computer

General purpose device

Optimized for calculation, display and storage

Requires knowledge of computer programming

Processes instructions sequentially

Not suitable for use in harsh environments

PLC

Specifically designed for automation and control

Optimized for industrial control

Knowledge of computer programming not essential

Processes instructions in parallel

Robust and well suited for harsh industrial environments

Input Module

receives digital or analog signals from field devices (sensors, switches) and converts them into logic signals that can be understood by the Processor

Processor

s the “brain” of the PLC, it executes control instructions as specified by the program stored in its memory

Output Module

converts control instructions from the processor into digital or analog signals that can be used to control field devices (actuators)

Programming Device

is used to enter the desired control specifications into the processor’s memory.

Compact type

• Small and compact

• Fix number of I/Os

• Designed for simple applications

• Suitable for stand-alone system

Modular type

• Flexible and versatile

• Huge I/O capabilities

• Designed for complex and sophisticated applications

• Suitable for centralized control system

US

• Allen Bradley

• General Electric

• Westinghouse

Europe

• Siemens

• Festo

• ABB

Japan

• Omron

• Panasonic

• Toshiba

• Mitsubishi

toggle switch

is a device have a plurality of terminals and means operated by a lever for making and breaking electrical connections between pairs of those terminals

limit switch

is a configured to detect when a system’s element has moved to a certain position. A system operation is triggered when a limit switch is tripped.

reed switch

are commonly employed for sensing the position of a mechanism the movement of which is to be monitored, by providing a switched output in response to the passage or approach of an actuating magnet carried by the mechanism

inductive proximity sensor

is an electronic proximity sensor, which detects metallic objects without touching them. It uses the electromagnetic field in order to sense object.

Capacitive proximity sensor

produces an electrostatic field to sense objects. Capacitive proximity switches will sense metal as well as non-metallic material such as paper, glass, liquids, and cloths.

photoelectric sensor

another type of position sensing devices. Photoelectric sensors use a modulated light beam that is either broken or reflected by the target.

Types of Photoelectric Sensor

Diffused reflective

Retro-reflective

Through-beam

Lamps

have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with high melting point such as tungsten and it is usually wound into small coil.

Motors

are machines that produces mechanical movement when supplied by electricity. This motion is caused by the interaction of magnetic fields and current-carrying conductors.

Solenoid Valve

an electromechanical valve that controls the flow of liquid or gas by running or stopping an electric current through a solenoid, which is a coil of wire, thus changing the state of the valve.

Relays

are electro-mechanical switches that are triggered by supplying electricity through its coil. When the relay coil is energized, its contacts will switch to opposite state.

IEC 61131

was published by the International Electrotechnical Commission in 1993 to establish an open standard for PLCs. The latest edition of the IEC 61131 was released in 2013

IEC 61131-3

the third part of IEC 61131 and deals with software architecture and programming

2 graphical and 3 textual programming language standards for PLCs

• Ladder Diagram (LD) – graphical

• Functional Block Diagram (FBD) – graphical

• Instruction List (IL) - textual

• Structured Text (ST) - textual

• Sequential Function Chart (SCF) - textual

Ladder Diagram

Closest to Relay Logic

Functional Block Diagram

Close to logic gate diagram

Instructions List

Translation of Ladder Diagram to Text

Structured Text

Looks like a high level language, makes the programmer think sequentially

Sequential Function Chart

Very high level, can be used to specify both sequential and parallel operations

Rails

There are two rails in a ladder diagram which are drawn as vertical lines running down the far most ends of the page.

Rungs

are drawn as horizontal lines and connect the rails to the logic expressions.

Inputs

are external control actions such as a push button being pressed, or a limit switch being triggered.

Outputs

are external devices that are being turned on and off such as an electric motor or a solenoid valve.

Logic Expressions

are used in combination with the inputs and outputs to formulate the desired control operations.

Address Notation & Tag Names

The address notation describes the input, output, and logic expression memory addressing structure of the PLC. The tag names are the descriptions allocated to the addresses.

Comments

are an extremely important part of a ladder diagram