A Practical Guide to Programmable Logic Controllers: Principles and Applications by John W Webb
Programmable Logic Controllers: Principles and Applications by John W Webb
If you are interested in learning about programmable logic controllers (PLCs), one of the most widely used technologies in industrial automation, you might want to check out the book Programmable Logic Controllers: Principles and Applications by John W Webb. This book is a comprehensive and practical introduction to PLCs that covers both fundamental and advanced topics. It combines clear explanations with a wealth of industry examples that make complex concepts easy to understand. It also provides exercises and problems that help you test your knowledge and skills.
Programmable Logic Controllers Principles And Applications By John W Webb Pdf Free 16
In this article, we will give you a brief overview of what this book covers and why it is a valuable resource for anyone who wants to learn more about PLCs. We will also discuss some general aspects of PLCs such as their definition, importance, working principle, types, programming languages and applications. By the end of this article, you will have a better idea of what PLCs are and what they can do.
What are programmable logic controllers (PLCs)?
A programmable logic controller (PLC) is a specialized computer that is designed to control industrial processes and machines. Unlike general-purpose computers that can perform various tasks depending on the software installed, a PLC is programmed to perform a specific set of operations based on the input signals it receives from sensors and switches. A PLC can also output signals to actuators such as motors, valves, relays, etc. to control the behavior of the process or machine.
PLCs are widely used in industrial automation because they offer several advantages over conventional hard-wired control systems. Some of these advantages are:
Flexibility: A PLC can be easily reprogrammed to change or modify the control logic without changing the hardware or wiring.
Reliability: A PLC can operate reliably in harsh environments such as high temperature, humidity, vibration, noise, etc. A PLC can also detect and correct errors and faults in the system.
Modularity: A PLC can be composed of different modules that can be added or removed as needed. This allows for easy expansion and maintenance of the system.
Cost-effectiveness: A PLC can reduce the cost of installation, operation and maintenance of the system by reducing the number of components, wires and labor required.
Communication: A PLC can communicate with other devices and systems such as computers, human-machine interfaces (HMIs), supervisory control and data acquisition (SCADA) systems, etc. This allows for remote monitoring and control of the system.
Why are PLCs important in industrial automation?
Industrial automation is the use of machines and systems to perform tasks that were previously done by humans. Industrial automation can improve the efficiency, quality, safety and productivity of various processes and industries. Some examples of industrial automation are:
Manufacturing: The use of robots, conveyors, assembly lines, etc. to produce goods and products.
Energy: The use of smart grids, renewable energy sources, etc. to generate and distribute electricity.
Transportation: The use of trains, planes, cars, etc. to transport people and goods.
Water treatment: The use of pumps, filters, valves, etc. to treat and supply water.
PLCs are important in industrial automation because they are the main devices that control the logic and sequence of these machines and systems. PLCs can receive input signals from various sensors and switches that monitor the status and conditions of the process or machine. PLCs can also output signals to various actuators that manipulate the process or machine according to the programmed logic. PLCs can also communicate with other devices and systems that provide data, feedback, instructions, etc.
By using PLCs, industrial automation can achieve higher levels of performance, accuracy, consistency, safety and flexibility than manual or hard-wired control systems. PLCs can also adapt to changing requirements and conditions by modifying the control logic through reprogramming.
How do PLCs work?
A PLC consists of four basic components: power supply, input/output (I/O) modules, central processing unit (CPU) and memory. The following diagram shows a simplified representation of a PLC:
+-----------------+ Power Supply +-----------------+ +-----------------+ +-----------------+ Input Output Module Module +-----------------+ +-----------------+ +-----------+-----------+ +-----------------+ +-----------------+ CPU Memory +-----------------+ +-----------------+
The power supply provides the necessary voltage and current to operate the PLC and its connected devices. The input module converts the input signals from sensors and switches into digital signals that can be processed by the CPU. The output module converts the digital signals from the CPU into output signals that can control actuators such as motors, valves, relays, etc.
The CPU is the brain of the PLC that executes the programmed logic and controls the operation of the system. The CPU consists of three subcomponents: arithmetic logic unit (ALU), control unit (CU) and registers. The ALU performs arithmetic and logical operations on data. The CU controls the sequence and timing of operations. The registers store data temporarily during processing.
The memory is where the program and data are stored in the PLC. The memory consists of two types: read-only memory (ROM) and random access memory (RAM). The ROM stores the permanent information such as the operating system and firmware of the PLC. The RAM stores the temporary information such as the user program, variables, timers, counters, etc.
The operation of a PLC can be summarized by a simple cycle called scan cycle or scan time. During each scan cycle, the PLC performs three main tasks:
Read inputs: The PLC reads the input signals from sensors and switches through the input module and stores them in memory.
Execute program: The PLC executes the user program stored in memory using data from inputs and outputs. The program consists of a series of instructions that define the control logic for the system.
What are the different types of PLCs?
PLCs can be classified into different types based on various criteria such as size, functionality, input/output capacity, communication capability, etc. Some of the common types of PLCs are:
Fixed or compact PLCs: These are PLCs that have a single unit that contains the power supply, CPU, memory and input/output modules. They have a fixed number and type of inputs and outputs that cannot be expanded or changed. They are suitable for small and simple applications that require low cost and minimal wiring.
Modular PLCs: These are PLCs that consist of separate modules that can be connected together to form a customized system. They have a separate power supply module, CPU module, memory module and input/output modules that can be added or removed as needed. They have a flexible number and type of inputs and outputs that can be increased or decreased according to the application. They are suitable for medium and large applications that require high performance and scalability.
Rack-mounted PLCs: These are a type of modular PLCs that have a rack or chassis that houses the modules. The rack provides a common communication bus and power supply for the modules. The modules can be easily inserted or removed from the rack without affecting the other modules. They are suitable for large and complex applications that require high speed and reliability.
Distributed PLCs: These are PLCs that have multiple controllers that are connected by a network. Each controller has its own power supply, CPU, memory and input/output modules. The controllers can communicate with each other and share data and commands. They are suitable for distributed and decentralized applications that require high flexibility and coordination.
What are the principles of PLC programming?
PLC programming is the process of creating a program that defines the control logic for a PLC system. A PLC program consists of a series of instructions that tell the PLC what to do with the input signals and how to generate the output signals. A PLC program can be written in different languages depending on the preference and application of the programmer.
Some of the common languages for PLC programming are:
Ladder logic is the most widely used PLC programming language. It is a graphical language that uses symbols to represent electrical components such as contacts, coils, timers, counters, etc. The symbols are arranged in a ladder-like structure that resembles an electrical schematic diagram. Each rung of the ladder represents a logical expression or condition that controls an output device.
Ladder logic is easy to learn and understand because it mimics the traditional hard-wired control systems. It is also compatible with most PLC models and manufacturers. However, ladder logic can be difficult to use for complex and sequential processes that require many rungs and symbols.
An example of ladder logic is shown below:
+----[ ]----+----( )----+ Input 1 Output 1 +----[ ]----+----( )----+ Input 2 Output 2 +----[/]----+----( )----+ Input 3 Output 3 +-----------+-----------+
This ladder logic program has three rungs that control three output devices. The first rung turns on output 1 when input 1 is on. The second rung turns on output 2 when input 2 is off. The third rung turns on output 3 when input 3 is off.
Function block diagram
Function block diagram (FBD) is another graphical PLC programming language. It uses blocks to represent functions such as arithmetic operations, logical operations, timers, counters, etc. The blocks have inputs and outputs that can be connected by wires to form a network of functions. Each block performs a specific operation on the input data and generates an output data.
FBD is more suitable for complex and mathematical processes that require many calculations and operations. It is also easier to reuse and modify existing blocks than ladder logic symbols. However, FBD can be less intuitive and harder to troubleshoot than ladder logic.
An example of FBD is shown below:
+-----+ +-----+ A B +--+--+ +--+--+ +----+-----+ v +-----+ + +--+--+ v +-----+ C +--+--+ v +-----+ > 10 +--+--+ v +-----+ D +-----+
This FBD program has four blocks that control one output device. The first block adds the values of inputs A and B. The second block assigns the result to output C. The third block compares the value of output C with 10. The fourth block turns on output D if the value of output C is greater than 10.
Structured text (ST) is a textual PLC programming language that uses statements to define the control logic. It is similar to high-level programming languages such as C, Pascal, Basic, etc. It uses variables, data types, operators, expressions, assignments, conditions, loops, functions, etc. to create a program.
ST is more powerful and flexible than graphical languages because it can perform complex and sequential processes that are difficult to represent by symbols or blocks. It is also more compact and concise than graphical languages because it can use fewer lines of code to achieve the same functionality. However, ST can be more difficult to learn and understand than graphical languages because it requires more syntax and logic.
An example of ST is shown below:
IF Input_1 THEN Output_1 := TRUE; ELSE Output_1 := FALSE; END_IF; IF NOT Input_2 THEN Output_2 := TRUE; ELSE Output_2 := FALSE; END_IF; IF NOT Input_3 THEN Output_3 := TRUE; ELSE Output_3 := FALSE; END_IF;
This ST program has three statements that control three output devices. The first statement turns on output 1 if input 1 is on. The second statement turns on output 2 if input 2 is off. The third statement turns on output 3 if input 3 is off.
Instruction list (IL) is a low-level PLC programming language that uses mnemonics to represent instructions. It is similar to assembly language that is used for microprocessors. It uses operands, operators, labels, jumps, etc. to create a program.
IL is the most basic and efficient PLC programming language because it directly corresponds to the machine code of the PLC. It can execute faster and use less memory than other languages. However, IL is the most difficult and tedious PLC programming language because it requires more instructions and details to achieve the same functionality as other languages.
An example of IL is shown below:
LD Input_1 ST Output_1 LDN Input_2 ST Output_2 LDN Input_3 ST Output_3
This IL program has six instructions that control three output devices. The first instruction loads the value of input 1 into the accumulator. The second instruction stores the value of the accumulator into output 1. The third instruction loads the inverted value of input 2 into the accumulator. The fourth instruction stores the value of the accumulator into output 2. The fifth instruction loads the inverted value of input 3 into the accumulator. The sixth instruction stores the value of the accumulator into output 3.
Sequential function chart
Sequential function chart (SFC) is a graphical PLC programming language that uses symbols to represent sequential processes. It is similar to flowcharts that are used for algorithm design. It uses steps, transitions, actions, branches, etc. to create a program.
SFC is more suitable for sequential and event-driven processes that require synchronization and coordination of multiple tasks. It is also easier to visualize and document than other languages. However, SFC can be less efficient and flexible than other languages because it requires more memory and processing time.
An example of SFC is shown below:
+-----+ +-----+ +-----+ Step1----->Step2----->Step3 +-----+ +-----+ +-----+ A=0 A=A+1 A=A*2 +-----+ +-----+ +-----+ ^ ^ ^ v v v +-----+ +-----+ +-----+ What are the applications of PLCs in different industries?
PLCs are widely used in various industries to automate and control different processes and machines. PLCs can improve the efficiency, quality, safety and productivity of these industries by providing reliable and flexible control solutions. Some of the examples of PLC applications in different industries are:
In the manufacturing industry, PLCs are used to control various aspects of the production process such as assembly, packaging, quality control, testing, sorting, etc. PLCs can coordinate and synchronize multiple machines and devices to perform complex and precise tasks. PLCs can also monitor and adjust the parameters and conditions of the process such as speed, temperature, pressure, level, etc. PLCs can also collect and store data from the process for analysis and optimization.
An example of a PLC application in the manufacturing industry is an automatic bottle filling system. This system uses a PLC to control the sequence and timing of filling bottles with liquid products such as water, juice, soda, etc. The PLC receives input signals from sensors that detect the presence and position of bottles on a conveyor belt. The PLC also outputs signals to actuators that control the valves, pumps, motors, etc. that fill the bottles with the desired amount and type of liquid. The PLC also monitors and regulates the flow rate, pressure and temperature of the liquid. The PLC also records and displays the data such as the number of bottles filled, the amount of liquid used, etc.
In the energy industry, PLCs are used to control various aspects of the energy generation and distribution systems such as solar panels, wind turbines, hydroelectric dams, power plants, substations, transformers, etc. PLCs can optimize and regulate the power output and quality of these systems by adjusting the parameters and conditions such as voltage, current, frequency, power factor, etc. PLCs can also protect and isolate these systems from faults and disturbances by detecting and reacting to abnormal situations such as overloads, short circuits, surges, etc. PLCs can also communicate and coordinate with other devices and systems such as smart meters, smart grids, SCADA systems, etc.
An example of a PLC application in the energy industry is a wind turbine system. This system uses a PLC to control the operation and performance of a wind turbine that converts wind energy into electrical energy. The PLC receives input signals from sensors that measure the wind speed, direction, temperature, humidity, etc. The PLC also outputs signals to actuators that control the pitch angle of the blades, the yaw angle of the nacelle, the rotational speed of the generator, etc. The PLC also monitors and regulates the voltage, current and frequency of the electrical output. The PLC also records and transmits the data such as the power generated, the wind conditions, etc.
In the water treatment industry, PLCs are used to control various aspects of the water treatment process such as pumping, filtering, disinfecting, etc. PLCs can monitor and adjust the parameters and conditions of the water quality such as pH, turbidity, chlorine, etc. PLCs can also protect and maintain the water treatment equipment and facilities by detecting and responding to faults and alarms. PLCs can also communicate and coordinate with other devices and systems such as sensors, valves, meters, SCADA systems, etc.
An example of a PLC application in the water treatment industry is an automatic control system for chemical makeup water treatment of boiler. This system uses a PLC to control the sequence and timing of adding chemicals to the raw water to make it suitable for boiler use. The PLC receives input signals from sensors that measure the water level, temperature, pressure, conductivity, etc. The PLC also outputs signals to actuators that control the pumps, valves, heaters, etc. that add and mix the chemicals with the water. The PLC also monitors and regulates the parameters and conditions of the water quality such as pH, hardness, alkalinity, etc. The PLC also records and displays the data such as the amount of chemicals used, the water quality indicator