‫بسم هللا الرحمن الرحيم‬

‫‪PLC Course‬‬
‫مقرر الحاكمات المنطقية المبرمجة‬

‫‪Lec 2‬‬

‫‪DR. M. Arafa‬‬ ‫‪22/2/2023‬‬

 The basic elements of a control system

Physically, a mechatronic system is composed of four basic elements:

1) sensors (sensor is a device that detects and responds to some type of input from the

physical environment like temperature, speed, pressure, light,…etc.)

2) Actuators (actuator is a device that makes something move or operate)

3) Plant / System (Robot, Self Autonomous Vehicle, CNC Machine, Vehicle

engines, Conveyor systems, Assembly systems, Cranes, Air craft engines,…, etc.)

4) Analog Controllers / Digital controllers (Computer /Microprocessor /

Microcontroller / PLC ,……, etc.)

 For example, the washing machine as a control system:

1) Sensors
• Sensors that detect conditions such as water level and temperature sensors.

2) Actuators
• The motor that spin the tub

• The electric water valve to fill it with the water.

3) Plant
• The machine body that contains the inner drum to be loaded with clothes and water

4) Controller
• Embedded controller (microprocessor/microcontroller based) on an integrated circuit

• It tells the washing machine what to do, based on the selections you make, such as the
wash cycle and water temperature.
 The electric water valve

The Plant

The Controller
 What Is A Programmable Logic Controller
(PLC)?

➢ A programmable logic controller (PLC) is an industrial

computer control system (microprocessor-based controller)
with a programmable memory.

➢ It continuously monitors the states of input devices and

makes logic-based decisions using a custom program
to control the states of output devices.
Box Type Modular or Rack Type
 PLC Applications

The common applications of PLCs:

➢ Production lines

➢ Elevators

➢ Escalators

➢ Traffic Control

➢ Robot:
▪Toxic chemical materials spraying robot

▪Skyscrapers glass washing robot

Traffic Control
Escalator

Production Line Elevator

 Escalator

➢ Most of the time the escalator in high traffic works at the normal rated operating
state. In the absence of passengers, it is still working at the same rated speed
operation with energy consumption (also the life time is reduced).
➢ PLC controls the speed of the escalator when no one uses the escalator to save
energy and increase its life time.
Production Line
Production Line
Washing Robot Spraying Robot
 Common PLC’s Manufacturers

➢ Siemens

➢ ABB

➢ Schneider (Modicon)

➢ Rockwell (Allen-Bradley)

➢ LG

➢ Mitsubishi

➢ Omron

➢ Toshiba
Mitsubishi Schneider Siemens

LG Allen-Bradley ABB
 The Considerations of Choosing a PLC

➢ Will the system be powered by AC or DC voltage?

➢ Does the PLC have enough memory to run the user program?

➢ Does the system run fast enough to meet my application’s

requirements?

➢ What is the type of software that is used to program the PLC?

➢ Will the PLC be able to manage the required number of inputs and
outputs of the application?

➢ Do you need network connectivity and can it be added to the PLC?

 PLC Operation

➢ During the PLC operation, the CPU completes three sequential

processes: Read (Input Scan), Execute (Program Scan), and Write
(Output Scan)

(2) Execute
(1)Read (3)Write
 PLC Operation

➢ (1) Read (Input Scan):

Detects the state of all input devices that are connected to the PLC
➢ (2) Execute (Program Scan):
Executes the user control program that stored in its memory
➢ (3) Write (Output Scan):
Updates (energizes or de-energize) all output devices that are
connected to the PLC.

❑ This process of sequentially reading the inputs, executing the program

in the PLC memory, and updating the outputs is known as scanning
Process.
PLC Programming Languages
 PLC Programming Languages
There are four programming languages that are used to program a
PLC:
1. Ladder Diagram (LD)

2. Function Block Diagram (FBD)

3. Structured Text (ST)

4. Instruction List (IL)

➢ The commonly used programming language is Ladder Diagram.
 PLC Programming Languages

1. Ladder Diagram (LD)

A graphical programming language. Initially programmed with

simple contacts that simulated the opening and closing of
switches and relays, Ladder Logic programming has been
expanded to include such functions as counters, timers, shift
registers, and math operations.
 PLC Programming Languages

2. Function Block Diagram (FBD)

A graphical programming language for describing signal and

data flows through re-usable function blocks. FBD is very useful
for expressing the interconnection of control system algorithms
and logic.
 PLC Programming Languages

3. Structured Text (ST)

A high level text language that supports structured programming.

It has a language structure (syntax) and supports a wide range of
standard functions and operators.
 PLC Programming Languages

4. Instruction List (IL)

A low level language “like assembly language” that is based on

similar instructions list languages found in a wide range of
today’s PLCs.
Instruction Description
LD : load input
LDI : load inverse input
OR : or with = branching = parallel
ORI : or inverse with
AND : and with = series
ANI : and inverse with
OUT : out to
 PLC Programming Languages

4. Instruction List (IL)

LD X1
OR X2
ANI X3
OUT Y

Ladder diagram
Instruction List
Example: A PLC program using different
Programming Languages
Main Components of PLCs
 Main Components of PLCs

There are a wide range of PLCs with different sizes, but all contain six basic
components:

1) Input Module (receive signals from input devices = sensors of the PLC)

2) Output Module (send the control action to output devices = actuator of the
PLC)

3) CPU (Central Processing Unit)

4) Memory

5) Power Supply

6) programming unit, device, panel or PC/software

 Memory

➢ The most important characteristic of a PLC is the user’s ability to change

the control program quickly and easily.

➢ The memory is the area of the PLC where all the sequences of instructions,
or programs, are stored and executed by the processor to provide the desired
control of field devices.

➢ The memory sections that contain the control programs can be changed, or
reprogrammed, to suite the manufacturing line procedure changes or new
system start-up requirements.
 Memory

➢ The total memory in a PLC is actually composed of two different

memories:

1) Executive Memory (System Memory)

2) Application Memory (User Memory)

➢ Data Table Area

➢ User Program Area

 (1) Executive Memory (System Memory)

➢ The executive memory is a collection of permanently stored programs

that are considered part of the PLC itself. These supervisory programs
direct all system activities, such as execution of the control program and
communication with peripheral devices.

➢ The executive section is the part of the PLC’s memory where the system’s
available instruction software is stored (i.e., relay instructions, block
transfer functions, math instructions, etc.).

➢ This area of memory is not accessible to the user (ROM Memory).

➢ The executive memory is called the system memory

 (2) Application Memory (User Memory)

➢ The application memory is also called user memory.

➢ According to its function, the application memory is classified to two types:

(1) Data Table Area (2) User Program Area

➢ The User Program Area provides a storage area for the user-programmed
instructions that form the application control program.

➢ The Data Table Area is composed of several areas, each having a specific
function and usage as illustrated in the following figure.
 The usage of Data Table Area

➢ Stores the status of digital inputs connected to

1
the PLC’s input interface.
1

2 ➢ Stores the status of the control actions of the

3 2 digital output devices that are connected to
the PLC’s output interface.

➢ Contains storage bits that are referred to as

either internal outputs, internal coils (control
relays), or internals.
➢ These internals provide an output, for
3 interlocking purposes, of ladder sequences in
the control program.
➢ Internal outputs do not directly control
output devices because they are stored in
(2) Application Memory addresses that do not map the output table
and, therefore, any output devices.
 Power Supply
➢ Its responsibility is not only to provide internal DC voltages to the system
components (i.e., processor, memory, and input/output interfaces), but also
to monitor and regulate the supplied voltages and warn the CPU if
something is wrong.

➢ The power supply, then, has the function of supplying well-regulated power
and protection for other PLC system components as it contains (low
voltage transformer - bridge rectifier circuit – filter – voltage regulator).

➢ A bridge rectifier circuit is used to convert an AC power to a DC power.

The output of the rectifier circuit is not pure DC as some of AC pulses ,
known as ripples, remain and must be removed by a filter.
 Power Supply

➢ The filter in the power supply circuit is used to remove the remaining AC
pulses or ripples.

➢ A voltage regulator is a circuit that creates and maintains a fixed output

voltage, irrespective of changes to the input voltage or load conditions.

➢ Most PLCs, however, require a 120 VAC or 220 VAC power source, while
a few controllers will accept 24 VDC.
 Input Devices

➢ Toggle Switch
➢ Push button Switch
➢ Selector Switch
➢ Temperature Switch
➢ Limit Switch
➢ Liquid Level Switch
➢ Relay Contacts
➢ Photoelectric Sensors (Photo Sensors )
➢ Pressure Switch
Toggle Switch Push Button Switch Selector Switch
Temperature Switch Limit Switch Liquid Level Switch
 Photo Sensor

Pressure Switch
 Input Devices

➢ Temperature Switch:

• An inexpensive temperature-sensing mechanism is the bimetallic strip.

• bimetallic strip is a thin strip consisting of two different metals, joined

face to face, each metal having a different rate of thermal expansion.

• When the strip heats or cools, different rates of thermal expansion

between the two metals causes it to bend.

• The bending of the strip can then be used to actuate a switch contact
mechanism.
 Input Devices

➢ Limit Switches:
• These limit switches closely like toggle or selector hand switches fitted
with a lever pushed by the machine part.
• Often, the levers are tipped with a small roller bearing, preventing the
lever from being worn off by repeated contact with the machine part.
• Most applications use the limit switch to detect the position.
 Input Devices

➢ Liquid Level Switch:

• A floating object can be used to actuate a switch mechanism when the

liquid level in a tank rises past a certain point.

• Level switches can also be designed to detect the level of solid materials
such as wood chips, grain, coal, or animal feed in a storage silo, bin, or
hopper.
 Input Devices

➢ Pressure Switch:

• Gas or liquid pressure can be used to actuate a switch mechanism if that

pressure is applied to the switch , it converts pressure to mechanical force.

• The mechanical force can then be used to actuate a switch contact

mechanism.
 Input Devices

➢ Pressure Switch: