Programmable Logic Controller Systems

Course information
Catalog description:

Programmable Logic Controllers are now commonly encountered by maintenance electricians and the programme will cover all essential aspects of PLCs as encountered in industry.

Course Objectives:

The objective of the Programmable Logic Controller (PLC) programmed is to equip learners with the skills and knowledge necessary to successfully carry out basic service and maintenance of PLC controlled Industrial Systems in a safe and environmentally sound manner.

Course Learning Outcomes:

On completion of this Course the student shall be able to gain knowledge on Programmable Logic Controllers and understand different types of Devices to which PLC input and output modules are Connected, provide the knowledge about understand various types of PLC registers

The students will be Able to create ladder diagrams from process control descriptions, apply PLC timers and counters for the control of industrial processes and able to use different types PLC functions, Data Handling Function.

Course contents:PLC Systems Overview

Industrial automation began long before PLCs. In the early to mid-1900s, automation was usually done using complicated electromechanical relay circuits. However, the number of relays, wires and space needed to create even simple automation was problematic. Thousands of relays could be necessary to automate a simple factory process! And if something in the logical circuit needed to be changed.

In 1968 the first programmable logic controller came along to replace complicated relay circuitry in industrial plants. The PLC was designed to be easily programmable by plant engineers and technicians that were already familiar with relay logic and control schematics. Since the beginning PLCs have been programmable using ladder logic which was designed to mimic control circuit schematics. The ladder diagrams look like control circuits where power is flowing from left to right through closed contacts to energize a relay coil

PLC Architecture:

The basic architecture of a PLC consists of main components-the processor module, the power supply, and the I/O modules. The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks. The power supply is usually a separate module, and the I/O modules are separate from the processor. The types of I/O modules include discrete (on/off), analog (continuous variable), and special modules like motion control or high-speed counters. The field devices are connected to the I/O modules

The function of the CPU is to store and run the PLC software programs. It also interfaces with the Co-Processor Modules, the I/O Modules, the peripheral device, and runs diagnostics. It is essentially the “brains” of the PLC.

The memory contains the data and the ladder program. It contains an “Executive” program that tells the PLC how to Execute the control Instructions, User’s Program, Communicate with other devices and Other PLCs, Programming devices, I/O devices, etc.

This program is stored in “nonvolatile” memory that meaning that the program will not be lost if power is removed.

PLCs operate by continually scanning programs and repeat this process many times per second. When a PLC starts, it runs checks on the hardware and software for faults, also called a self-test. If there are no problems, then the PLC will start the scan cycle. The scan cycle consists of three steps: input scan, executing program(s), and output scan.

PLC Input/output Modules

The type of input modules used by a PLC depends on the type of input device. For example, some respond to digital inputs, which are either or off while others respond to analog signals. In this case, analog signals represent machine or process conditions as a range of voltage or current values. The PLC input circuitry converts signals into logic signals that the CPU can use. The CPU evaluates the status of inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of outputs.

Output modules convert control signals from the CPU into digital or analog values that can be used to control various output devices. The programming device is used to enter or change the PLCs program or to monitor or change stored values. Once entered, the program and associated variables are stored in the CPU. In addition to these basic elements, a PLC system may also incorporate an operator interface device to simplify monitoring of the machine or process.

The standard PLC module types and their descriptions are as follows:

  • AC Input – Uses AC voltage for input field device status.
  • DC Input (Discrete) – Uses DC voltage for input field device status.
  • DC Input (Analog) – The input is a variable DC signal level.
  • AC Output – Controls the ON/OFF state of AC output field devices such as relays, coils, and solenoids.
  • DC Output (Discrete) – Discrete DC output modules control the ON/OFF states of DC output field devices.
  • DC Output (Analog) – Provides a variable DC level.

PLC Types Modes

There are three types of PROGRAM Mode, MONITOR Mode and RUN Mode.

  • PROGRAM Mode

In PROGRAM mode, the CPU Unit is stopped. User programming can be created or modified, memory can be cleared, programs can be checked, programs can be debugged, and I/O memory tables can be created or modified.

  • MONITOR Mode

In MONITOR mode the CPU Unit is running. I/O is processed in the same way as in RUN mode. The operating status of the CPU Unit can be monitored, bits can be force-set/reset, the set values and present values of timer and counter instructions can be modified, and the present values of word data can be modified. MONITOR mode is used for system adjustments during trial operation.

  • RUN Mode

RUN mode is used for normal system operation. The operating status of the CPU Unit can be monitored; however, bits cannot be force-set/reset, and present values and set values cannot be modified using Programming Devices.

The operating mode can be changed using a Programming Device or by sending a command using Host Link communications.

Ladder Language

One of the best visual programming languages is a PLC programming language. It’s called ladder logic or ladder diagram (LD) and you can learn it very fast.

The smart thing about ladder logic is that it looks very similar to electrical relay circuits. So, if you already know a little bit about relay control and electrical circuits, you can learn ladder logic even faster.

Ladder logic is not only a programming language for PLC’s. It is one of the standardized PLC programming languages.

ladder logic diagrams are drawn vertically. Because:

  • Easier to read

First of all, it makes ladder logic easier to read because it is natural for the eye to go from the left to right and then down to the next line. Just like when you are reading.

  • Drawn on computer

When you draw ladder logic on a computer you will make one line at a time. As you draw more and more lines (in ladder logic called rungs) they will stack on top of each other, making up what looks like a ladder. The best way to look at a large ladder diagram with many lines is to scroll vertically along the screen.

  • Order of execution

The last reason for drawing ladder logic horizontal is to set the order of execution. Order of execution is how the PLC will run your ladder logic. To be more precise in what order your ladder logic instructions will be executed by the PLC. A PLC will always start at the top of your ladder logic and then execute its way down.

This is the most common Symbols using in ladder language

For example:

This is an example PLC ladder logic diagram. The vertical lines on the left and right are called the power rails. The contacts are arranged horizontally between the power rails, hence the term rung. The ladder diagram has three rungs. The arrangement is similar to a ladder one uses to climb onto a roof. In addition, this example introduces the concept of function block instructions. Any instruction that is not a contact or a coil is called a function block instruction because of its appearance in the ladder diagram. The most common function block instructions are timer, counter, comparison, and computation operations. More advanced function block instructions include sequencer, shift register, and first-in first-out operations.

Ladder example for common Logic gates:

  1. Summary:

This course provides a general overview of PLCs and their application in industry. The origins of the PLC and its evolution are covered in detail. The advantages of PLCs are also outlined, and the main components associated with PLC systems are explored. An introduction to ladder logic is presented and the most common types of PLC signals are covered with an emphasis on practical application.

  1. References:

[1]. Programmable Logic Controllers — Principle and Applications by John W Webb and Ronald A Reiss Filth edition, PHI.

[2]. Programmable Logic Controllers — Programming Method and Applications by JR Hackworth and ED Hackworth — Jr- Pearson, 2004.

[3]. Programmable Logic Controllers: An Emphasis on Design and Application by Kelvin T. Erickson, Second Edition, Dogwood Valley Press, LLC. 2011.

Programmable Logic Controller Systems

  1. Course information
  2. Catalog description:

Programmable Logic Controllers are now commonly encountered by maintenance electricians and the programme will cover all essential aspects of PLCs as encountered in industry.

  1. Course Objectives:

The objective of the Programmable Logic Controller (PLC) programmed is to equip learners with the skills and knowledge necessary to successfully carry out basic service and maintenance of PLC controlled Industrial Systems in a safe and environmentally sound manner.

  1. Course Learning Outcomes:

On completion of this Course the student shall be able to gain knowledge on Programmable Logic Controllers and understand different types of Devices to which PLC input and output modules are Connected, provide the knowledge about understand various types of PLC registers

The students will be Able to create ladder diagrams from process control descriptions, apply PLC timers and counters for the control of industrial processes and able to use different types PLC functions, Data Handling Function.

  1. Course contents:

PLC Systems Overview

Industrial automation began long before PLCs. In the early to mid-1900s, automation was usually done using complicated electromechanical relay circuits. However, the number of relays, wires and space needed to create even simple automation was problematic. Thousands of relays could be necessary to automate a simple factory process! And if something in the logical circuit needed to be changed.

In 1968 the first programmable logic controller came along to replace complicated relay circuitry in industrial plants. The PLC was designed to be easily programmable by plant engineers and technicians that were already familiar with relay logic and control schematics. Since the beginning PLCs have been programmable using ladder logic which was designed to mimic control circuit schematics. The ladder diagrams look like control circuits where power is flowing from left to right through closed contacts to energize a relay coil

PLC Architecture:

The basic architecture of a PLC consists of main components-the processor module, the power supply, and the I/O modules. The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks. The power supply is usually a separate module, and the I/O modules are separate from the processor. The types of I/O modules include discrete (on/off), analog (continuous variable), and special modules like motion control or high-speed counters. The field devices are connected to the I/O modules

The function of the CPU is to store and run the PLC software programs. It also interfaces with the Co-Processor Modules, the I/O Modules, the peripheral device, and runs diagnostics. It is essentially the “brains” of the PLC.

The memory contains the data and the ladder program. It contains an “Executive” program that tells the PLC how to Execute the control Instructions, User’s Program, Communicate with other devices and Other PLCs, Programming devices, I/O devices, etc.

This program is stored in “nonvolatile” memory that meaning that the program will not be lost if power is removed.

PLCs operate by continually scanning programs and repeat this process many times per second. When a PLC starts, it runs checks on the hardware and software for faults, also called a self-test. If there are no problems, then the PLC will start the scan cycle. The scan cycle consists of three steps: input scan, executing program(s), and output scan.

PLC Input/output Modules

The type of input modules used by a PLC depends on the type of input device. For example, some respond to digital inputs, which are either or off while others respond to analog signals. In this case, analog signals represent machine or process conditions as a range of voltage or current values. The PLC input circuitry converts signals into logic signals that the CPU can use. The CPU evaluates the status of inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of outputs.

Output modules convert control signals from the CPU into digital or analog values that can be used to control various output devices. The programming device is used to enter or change the PLCs program or to monitor or change stored values. Once entered, the program and associated variables are stored in the CPU. In addition to these basic elements, a PLC system may also incorporate an operator interface device to simplify monitoring of the machine or process.

The standard PLC module types and their descriptions are as follows:

  • AC Input – Uses AC voltage for input field device status.
  • DC Input (Discrete) – Uses DC voltage for input field device status.
  • DC Input (Analog) – The input is a variable DC signal level.
  • AC Output – Controls the ON/OFF state of AC output field devices such as relays, coils, and solenoids.
  • DC Output (Discrete) – Discrete DC output modules control the ON/OFF states of DC output field devices.
  • DC Output (Analog) – Provides a variable DC level.

PLC Types Modes

There are three types of PROGRAM Mode, MONITOR Mode and RUN Mode.

  • PROGRAM Mode

In PROGRAM mode, the CPU Unit is stopped. User programming can be created or modified, memory can be cleared, programs can be checked, programs can be debugged, and I/O memory tables can be created or modified.

  • MONITOR Mode

In MONITOR mode the CPU Unit is running. I/O is processed in the same way as in RUN mode. The operating status of the CPU Unit can be monitored, bits can be force-set/reset, the set values and present values of timer and counter instructions can be modified, and the present values of word data can be modified. MONITOR mode is used for system adjustments during trial operation.

  • RUN Mode

RUN mode is used for normal system operation. The operating status of the CPU Unit can be monitored; however, bits cannot be force-set/reset, and present values and set values cannot be modified using Programming Devices.

The operating mode can be changed using a Programming Device or by sending a command using Host Link communications.

Ladder Language

One of the best visual programming languages is a PLC programming language. It’s called ladder logic or ladder diagram (LD) and you can learn it very fast.

The smart thing about ladder logic is that it looks very similar to electrical relay circuits. So, if you already know a little bit about relay control and electrical circuits, you can learn ladder logic even faster.

Ladder logic is not only a programming language for PLC’s. It is one of the standardized PLC programming languages.

ladder logic diagrams are drawn vertically. Because:

  • Easier to read

First of all, it makes ladder logic easier to read because it is natural for the eye to go from the left to right and then down to the next line. Just like when you are reading.

  • Drawn on computer

When you draw ladder logic on a computer you will make one line at a time. As you draw more and more lines (in ladder logic called rungs) they will stack on top of each other, making up what looks like a ladder. The best way to look at a large ladder diagram with many lines is to scroll vertically along the screen.

  • Order of execution

The last reason for drawing ladder logic horizontal is to set the order of execution. Order of execution is how the PLC will run your ladder logic. To be more precise in what order your ladder logic instructions will be executed by the PLC. A PLC will always start at the top of your ladder logic and then execute its way down.

This is the most common Symbols using in ladder language

For example:

This is an example PLC ladder logic diagram. The vertical lines on the left and right are called the power rails. The contacts are arranged horizontally between the power rails, hence the term rung. The ladder diagram has three rungs. The arrangement is similar to a ladder one uses to climb onto a roof. In addition, this example introduces the concept of function block instructions. Any instruction that is not a contact or a coil is called a function block instruction because of its appearance in the ladder diagram. The most common function block instructions are timer, counter, comparison, and computation operations. More advanced function block instructions include sequencer, shift register, and first-in first-out operations.

Ladder example for common Logic gates:

Summary:

This course provides a general overview of PLCs and their application in industry. The origins of the PLC and its evolution are covered in detail. The advantages of PLCs are also outlined, and the main components associated with PLC systems are explored. An introduction to ladder logic is presented and the most common types of PLC signals are covered with an emphasis on practical application.

References:

    :

[1]. Programmable Logic Controllers — Principle and Applications by John W Webb and Ronald A Reiss Filth edition, PHI.

[2]. Programmable Logic Controllers — Programming Method and Applications by JR Hackworth and ED Hackworth — Jr- Pearson, 2004.

[3]. Programmable Logic Controllers: An Emphasis on Design and Application by Kelvin T. Erickson, Second Edition, Dogwood Valley Press, LLC. 2011.

 

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