PROGRAMMABLE CONTROLLERS AND THEIR PROGRAMMING
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PROGRAMMABLE CONTROLLERS AND THEIR PROGRAMMING
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Programmable controllers are known as the most widely used electronic devices in the control of production and assembly process in most automated factories due to its simplicity and versatility. A programmable controller is a user-friendly, microprocessor-based, specialized computer carrying out control functions of many types and levels of complexity in industrial applications.
In the coming sections the introduction to company and company profile can be overviewed. This work discusses the overview of programmable controllers and their programming.
ABTRACT
TABLE OF CONTENT
- INTRODUCTION
- BACKGROUND OF THE STUDY
- AIM OF THE STUDY
- SIGNIFICANCE OF THE STUDY
- ADVANTAGES OF PROGRAMMABLE CONTROLLER
- DISADVANTAGES OF PROGRAMMABLE CONTROLLER
- APPLICATIONS OF PROGRAMMABLE CONTROLLERS
- PROGRAMMABLE CONTROLLER FEATURES
- PROGRAMMABLE CONTROLLERS BASICS
- HISTORY BACKGROUND OF PROGRAMMABLE CONTROLLER
- EARLY METHODS OF PROGRAMMING
- ARCHITECTURE OF PROGRAMMABLE CONTROLLER
- PROGRAMMING
- PROGRAMMING DEVICE
- LADDER PROGRAMMING
REFERENCES
CHAPTER ONE
1.0 INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Programmable Controller is a microprocessor based system that uses programmable memory to store instructions and implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes (Dunn, 2009).
Programmable controllers can range from small modular devices with tens of inputs and outputs (I/O), in a housing integral with the processor, to large rack-mounted modular devices with thousands of I/O, and which are often networked to other PLC and SCADA systems (Maher, 2019).
They can be designed for many arrangements of digital and analog I/O, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory (Maher, 2019).
Programmable controllers were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers to replace hard-wired relay logic systems. Since then, they have been widely adopted as high-reliability automation controllers suitable for harsh environments. This study discusses an overview of the programmable controllers and their programming (Maher, 2019).
1.2 AIM OF THE STUDY
The main aim of this study is to carry out a research on a programmable controller and their programming. The objectives are:
- To have good knowledge of programmable controllers
- To study the programming of programmable controllers
1.3 SIGNIFICANCE OF THE STUDY
Therefore, the essence of this work is to help the student develop a good general working knowledge of programmable controllers with concentration on relay ladder logic techniques and how the programmable controller is connected to external components in an operating control system.
1.4 ADVANTAGES OF PROGRAMMABLE CONTROLLER
- Very fast
- Easy to change logic i.e. flexibility
- Reliable due to absence of moving parts
- Low power consumption
- Easy maintenance due to modular assembly
- Facilities in fault finding and diagnostic
- Capable of handling of very complicated logic operations
- Good documentation facilities
- Easy to couple with the process computers
- Analog signal handling and close loop control programming
- Counter, timer and comparator can be programmed
1.5 DISADVANTAGES OF PROGRAMMABLE CONTROLLER
1. There's too much work required in connecting wires.
2. There's difficulty with changes or replacements.
3. It's always difficult to find errors; And require skillful work force.
4. When a problem occurs, hold-up time is indefinite, usually long.
1.6 APPLICATIONS OF PROGRAMMABLE CONTROLLERS
The batch process in chemical, cement, food and paper industries are sequential in nature, requiring time or event based decisions. Programmable controllers are being used more and more as total solutions to a batch problem in these industries rather than just a tool. In batch process savings are developed principally from reduced cycle time and scheduling. Cycle automation provides rigid control enforcement to eliminate human errors and to minimize manual interventions. Increased efficiency in scheduling is to be expected with maximum utilization of equipment and reduction of fluctuating demands on critical equipment. In large process plants programmable controllers are being increasingly used for automatic start up and shutdown of critical equipments. A PLC ensures that an equipment cannot be started unless all the permissive conditions for safe start have been established. It also monitors the conditions necessary for safe running of the equipment and trip the equipment whenever any abnormality in the system is detected. The programmable controllers can be programmed to function as an energy management system for boiler control for maximum efficiency and safety. In the burner management system it can be used to control the process of purging, pilot light off, flame safety checks, main burner light off and valve switching for changeover of fuels (Byres, 2018)..
1.7 PROGRAMMABLE CONTROLLER FEATURES
Programmable controllers features can be concluded in the following:
- They are rugged, withstand industrial environment, such as heat, humidity, mechanical shocks and vibrations
- The interfacing for inputs and outputs is inside the controller
- They are easily programmed
- Programmable controllers are capable of both logic and PID control.
1.8 PROGRAMMABLE CONTROLLERS BASICS
Programmable controllers were invented by Dick Morley in 1964. Since then programmable controllers has revolutionized the industrial and manufacturing sectors. There is a wide range of programmable controllers functions like timing, counting, calculating, comparing, and processing various analog signals.The main advantage of programmable controllers over a “hard-wired” control system is that you can go back and change a programmable controllers after you’ve programmed it, at little cost (just the cost of the programmer’s time). In a hard-wired control system, you’re essentially having to rip out wires and start from scratch.
1.9 HISTORY BACKGROUND OF PROGRAMMABLE CONTROLLER
Invention and early development
Programmable controllers originated in the late 1960s in the automotive industry in the US and were designed to replace relay logic systems (Wayand, 2012). Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers (Wayand, 2012).
The hard-wired nature made it difficult for design engineers to alter the process. Changes would require rewiring and careful updating of the documentation. If even one wire were out of place, or one relay failed, the whole system would become faulty. Often technicians would spend hours troubleshooting by examining the schematics and comparing them to existing wiring (Laughton & Warne, 2012). When general-purpose computers became available, they were soon applied to control logic in industrial processes. These early computers were unreliable and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality (Laughton & Warne, 2012)
The programmable controllers provided several advantages over earlier automation systems. It tolerated the industrial environment better than computers and was more reliable, compact and required less maintenance than relay systems. It was easily extensible with additional I/O modules, while relay systems required complicated hardware changes in case of reconfiguration. This allowed for easier iteration over manufacturing process design. With simple programming language focused on logic and switching operations, it was more user-friendly than computers using general-purpose programming languages. It also permitted for its operation to be monitored. Early programmable controllers were programmed in ladder logic, which strongly resembled a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other programmable controllers used a form of instruction list programming, based on a stack-based logic solver (Kenney, 2020).
Today, programmable controllers maintain the same core functionality and simplicity that originally made them so popular with manufacturers. However, thanks to continued advancement in processor and memory technology, programmable controllers continue to shrink in size while growing in power and speed. These technological advancements have led to new capabilities, such as vision system integration, motion control, and support for multiple communication protocols. Modern programmable controllers also seamlessly integrate with ERP and MES systems, as well as powerful machine monitoring software and SCADA, providing new ways for manufacturers to drive improvements to their operations’ efficiency and performance through machine data analysis.
1.10 EARLY METHODS OF PROGRAMMING
Many early programmable controllers were not capable of graphical representation of the logic, and so it was instead represented as a series of logic expressions in some kind of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more common for ladder logic to be used, because it was a familiar format used for electro-mechanical control panels. Newer formats, such as state logic and Function Block (which is similar to the way logic is depicted when using digital integrated logic circuits) exist, but they are still not as popular as ladder logic. A primary reason for this is that programmable controllers solve the logic in a predictable and repeating sequence, and ladder logic allows the person writing the logic to see any issues with the timing of the logic sequence more easily than would be possible in other formats (Bolton, 2015).
Up to the mid-1990s, programmable controllers were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of programmable controllers programs (Dunn, 2009). Some proprietary programming terminals displayed the elements of programmable controllers programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to a lack of memory capacity. The oldest programmable controllers used non-volatile magnetic core memory.
1.11 ARCHITECTURE OF PROGRAMMABLE CONTROLLER
A programmable controller controls the manufacturing processes for integrated production lines and equipment. Programmable controllers were designed to replace the need for a large bank of relays or timers in facilities with numerous inputs and outputs. Due to their durability and ability to automate multiple processes, programmable controllers have become a staple in modern manufacturing. Below is the basic architecture of programmable controllers. Programmable controllers consist of hardware and software, and they are discussed as below:
Hardware
The main components of a programmable controllers consist of a central processing unit (CPU), power supply, programming device, and input and output (I/O) modules.
CPU
The CPU is the brain of the programmable controllers and carries out programmed operations. These operations or outputs are executed based on signals and data provided from connected inputs.
I/O Modules
The input module connects the input terminals to the rest of the system. Each terminal is usually electrically isolated from the internal electronics by OPTO ISOLATORS. This is a way of passing on the status of the input (on or off) by use of a light emitting diode and phototransistor. A typical opto isolator is shown. They have the advantage of reducing the effects of spurious pulses generated from electromagnetic sources. It is also a safety feature to prevent live voltages appearing on the input lines in the event of a fault.
Internally a computer usually operates at 5 V DC. The external devices (solenoids, motor starters, limit switches, etc.) operate at voltages up to 220V AC. The mixing of these two voltages will cause severe and possibly irreparable damage to the programmable controllers electronics. Less obvious problems can occur from electrical noise introduced into the programmable controllers from voltage spikes on signal lines, or from load currents flowing in AC neutral or DC return lines. Differences in earth potential between the programmable controllers cubicle and outside plant can also cause problems.
The question of noise is discussed, but there are obviously very good reasons for separating the plant supplies from the PLC supplies with some form of electrical barrier. This ensures that the PLC cannot be adversely affected by anything happening on the plant. Even a cable fault putting 415 V AC onto a DC input would only damage the input card; the PLC itself (and the other cards in the system) would not suffer. This is achieved by optical isolators, a light-emitting diode and photo-electric transistor linked together.
Protection of the PLC from outside faults. The programmable controllers supply L1/N1 is separate from the plant supply L2/N2 switch on. Because there are no electrical connections between the diode and the transistor, very good electrical isolation (typically 1 and 4kV) is achieved.
A DC input can be provided. When the push-button is pressed, current will flow through D1, causing TR1 to turn on, passing the signal to the PLC internal logic. Diode D2 is a light-emitting diode used as a fault-finding aid to show when the input signal is present. Such indicators are present on almost all programmable controllers input and output cards.
The resistor R sets the voltage range of the input. DC input cards are usually available for three voltage ranges: 5V (TTL), 12V 24V, 24V 50V.
A possible AC input circuit. The bridge rectifier is used to convert the AC to full wave rectified DC. Resistor R2 and capacitor C1 act as a filter (of about 50ms time constant) to give a clean signal to the programmable controllers logic. As before, a neon LP1 acts as an input signal indicator for fault finding, and resistor R1 sets the voltage range. The isolation barrier and monitoring LEDs can be clearly seen. This card handles eight inputs and could be connected to the outside world.
Programmable controllers input modules connect various external devices, such as sensors, switches, and push buttons to the programmable controllers to read various digital and analog parameters, such as temperature, pressure, flow, speed, etc.
Following are the some of the Input devices.
• FIeld Transmitters for Pressure, Temperature, Flow, Level etc
• Analyzers (pH, DO, Conductivity, SOX, NOX, etc)
• Switches and Push buttons
• Sensing Devices
• Limit Switches
• Photoelectric Sensors
• Proximity Sensors
• Condition Sensors
• Pressure Switches
• Level Switches
• Temperature Switches
• Vacuum Switches
• Float Switches
• Encoders
Inputs and Outputs connected to programmable controllers
Outputs Modules:
Outputs to actuators allow a programmable controllers to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity.
Solenoid Valves - Logical outputs that can switch a hydraulic or pneumatic flow.
Lights - Logical outputs that can often be powered directly from programmable controllers output boards.
Motor Starters - Motors often draw a large amount of current when started, so they require motor starters, which are basically large relays.
Servo Motors - A continuous output from the programmable controllers can command a variable speed or position.
Outputs from programmable controllers are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters.
Output cards again require some form of isolation barrier to limit damage from the inevitable plant faults and also to stop electrical noise corrupting the processor’s operations. Interference can be more of a problem on outputs because higher currents are being controlled by Computers and industrial
There are two basic types of output card. Eight outputs are fed from a common supply, which originates local to the programmable controllers cubicle (but separate from the supply to the programmable controllers itself). This arrangement is the simplest and the cheapest to install.
Each output has its own individual fuse protection on the card and a common circuit breaker. It is important to design the system so that a fault, say, on load 3 blows the fuse FS3 but does not trip the supply to the whole card, shutting down every output. This topic, called discrimination, is discussed further in Chapter 8. A PLC frequently has to drive outputs which have their own individual supplies.
A typical example is a motor control centre (MCC) where each starter has a separate internal 110-V supply derived from the 415-V bars. The card arrangement could not be used here without separate interposing relays (driven by the PLC with contacts into the MCC circuit). An isolated output card, has individual out-puts and protection and acts purely as a switch.
This can be connected directly with any outside circuit. The disadvantage is that the card is more complicated (two connections per output) and safety becomes more involved. An eight-way isolated output card, for example, could have voltage on its terminals from eight different locations.
Relay outputs can be used (and do give the required isolation) but are not particularly common. A relay is an electromagnetic device with moving parts and hence a finite limited life. A purely electronic device will have greater reliability.
Less obviously, though, a relay-driven inductive load can generate troublesome interference and lead to early contact failure. Optical isolation is again used to give the necessary separation between the plant and the programmable controllers system.
Diode D1 acts as a spike suppression diode to reduce the voltage spike encountered with inductive loads. The output state can be observed on LED1. If NPN transistors are used, a current sinking card can be made. AC output cards invariably use triacs, a typical circuit being.
Triacs have the advantage that they turn off at zero current in the load, which eliminates the interference as an inductive load is turned off. If possible, all AC loads should be driven from triacs rather than relays
An output card will have a limit to the current it can supply, usually set by the printed circuit board tracks rather than the output devices. An individual output current will be set for each output and a total overall output. Usually the total allowed for the card current is lower than the sum of the allowed individual outputs.
Power Supply
The power supply provides power to the programmable controllers by converting the available incoming AC power to the DC power required by the CPU and I/O modules to operate properly. Common voltage levels required by the programmable controllers (with and without the power supply) are 24Vdc, 110Vac, 220Vac.
The programmable controllers manufacturer typically determines programmable controllers development software. Allen Bradley, Siemens, and GE each have their own software development platforms for programming their programmable controllers models. Once the platform is determined, the actual programming of the programmable controllers logic can be done in a few different methods. The most common methods of programmable controllers programming include Ladder Logic, Function Block, and structured text.
Ladder Logic
Ladder Logic is a graphical programmable controllers programming language and is the most common method of programming. Ladder Logic can be used to execute tasks such as sequencing, counting, timing, data manipulation, and more. Ladder Logic is structured similarly to relay logic; however, the physical switches and coils used in relay logic are replaced by the programmable controllers’ memory locations and I/O.
Structured Text
Structured text is a text-based programmable controllers programming language and is similar to Python, Visual Basic, or C coding languages. Programming with structured text has multiple advantages, such as the program requiring less space due to being text based instead of graphic based. Additionally, the structured text can be combined with other programming languages, such as creating function blocks containing functions written in structured text.
Function Block
Function block programmable controllers programs are represented in the form of graphical blocks. Signals or data flow into the function block from inputs connected to the programmable controllers. When the incoming signals or data triggers the function block’s pre-programmed function, the programmable controllers executes one or more outputs. Function blocks can have standard functions such as timers, counters, calculating min and max values, obtaining averages, and more.
1.12 PROGRAMMING
Programmable controllers are intended to be used by engineers without a programming background. For this reason, a graphical programming language called Ladder Diagram (LD, LAD) was first developed. It resembles the schematic diagram of a system built with electromechanical relays and was adopted by many manufacturers and later standardized in the IEC 61131-3 control systems programming standard. As of 2015, it is still widely used, thanks to its simplicity (Keller, 2014).
As of 2015, the majority of programmable controllers systems adhere to the IEC 61131-3 standard that defines 2 textual programming languages: Structured Text (ST; similar to Pascal) and Instruction List (IL); as well as 3 graphical languages: Ladder Diagram, Function Block Diagram (FBD) and Sequential Function Chart (SFC). Instruction List (IL) was deprecated in the third edition of the standard (Lin, 2011).
Modern programmable controllers can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C.
While the fundamental concepts of programmable controllers programming are common to all manufacturers, differences in I/O addressing, memory organization, and instruction sets mean that programmable controllers programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible (Kenney, 2020).
1.13 PROGRAMMING DEVICE
Programmable controllers programs are typically written in a programming device, which can take the form of a desktop console, special software on a personal computer, or a handheld programming device. Then, the program is downloaded to the programmable controllers directly or over a network. It is stored either in non-volatile flash memory or battery-backed-up RAM. In some programmable controllers, the program is transferred from a personal computer to the programmable controllers through a programming board that writes the program into a removable chip, such as EPROM.
Manufacturers develop programming software for their controllers. In addition to being able to program programmable controllers in multiple languages, they provide common features like hardware diagnostics and maintenance, software debugging, and offline simulation (Harms, 2019)
A program written on a personal computer or uploaded from programmable controllers using programming software can be easily copied and backed up on external storage.
Functionality
The main difference from most other computing devices is that programmable controllers are intended-for and therefore tolerant-of more severe conditions (such as dust, moisture, heat, cold), while offering extensive input/output (I/O) to connect the programmable controllers to sensors and actuators. programmable controllers input can include simple digital elements such as limit switches, analog variables from process sensors (such as temperature and pressure), and more complex data such as that from positioning or machine vision systems (Maher, 2019) programmable controllers output can include elements such as indicator lamps, sirens, electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple programmable controllers, or the programmable controllers may have external I/O modules attached to a fieldbus or computer network that plugs into the programmable controllers (Kenney, 2020).
The functionality of the programmable controllers has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking. The data handling, storage, processing power, and communication capabilities of some modern programmable controllers are approximately equivalent to desktop computers. Programmable controllers -like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some programmable controllers in certain applications. Desktop computer controllers have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than programmable controllers, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in programmable controllers. Operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the controller may not always respond to changes of input status with the consistency in timing expected from programmable controllers. Desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical (Kenney, 2020).
Basic functions
The most basic function of a programmable controller is to emulate the functions of electromechanical relays. Discrete inputs are given a unique address, and a programmable controllers instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of "examine if on" instructions will energize its output storage bit if all the input bits are on. Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a "rung" of a "ladder diagram ", and this concept is also used to describe programmable controllers. Some models of programmable controllers limit the number of series and parallel instructions in one "rung" of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an "internal coil" with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a programmable controllers program.
Some programmable controllers enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which, in a sufficiently complex circuit, may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these "sneak paths" is either a bug or a feature, depending on programming style.
More advanced instructions of the programmable controllers may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variables internally that may not correspond to discrete logic.
Communication
programmable controllers use built-in ports, such as USB, Ethernet, RS-232, RS-485, or RS-422 to communicate with external devices (sensors, actuators) and systems (programming software, SCADA, HMI). Communication is carried over various industrial network protocols, like Modbus, or EtherNet/IP. Many of these protocols are vendor specific.
Programmable controllers used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.
Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.
User interface
Programmable controllers may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the programmable controllers connected via a communication interface.
1.13 LADDER PROGRAMMING
The form of programming commonly use with programmable controllers is ladder programming. Each program task is specified as though a rung of a ladder.
Thus a rung could specify that the state of switches A and B be examined and if both A and B are closed then a solenoid, the output is energized (Kenney, 2020).
Figure 21.4 (a), (b) Alternative ways of drawing an electric circuit, (c) comparable rung in a ladder program
The sequence followed by a programmable controllers when carrying out a program:
1- Scan the inputs associated with one rung of the ladder program 2- solve the logic operation involving those inputs
- Set/ reset the outputs for that rung
3- move on to the next rung and repeat operations 1, 2, 3....and so on until the end of program with each rung of the ladder scanned in turn.
- The PLC then goes back to the begining of the program and starts again
- The ladder diagram consists of two vertical lines representing the power rails.Circuits (rung) are connected as horizontal lines,
REFERENCES
Wayand, Ben. www.mroelectric.com. MRO Electric https://www.mroelectric.com/blog/what-is-a-plc/. Retrieved 11 May 2021. Missing or empty |title= (help)
Laughton & Warne 2012, p. 16/3: "The first industrial computer application was probably a system installed in an oil refinery in Port Arthur USA in 1959. The reliability and mean time between failure of computers meant that little actual control was performed."
Kenney, Muirae (2020). "The Basics of Ladder Logic". Automation Industrial. Retrieved 2020-11-24.
Dunn, Alison (2009). "The father of invention: Dick Morley looks back on the 40th anniversary of the PLC". Manufacturing Automation. Retrieved 2020-02-23.
Strothman, Jim (2003-08-01). "Leaders of the pack". ISA. Archived from the original on 2017-08-08. Retrieved 2020-02-24.
Brier, Steven E. (2018). "O. Struger, 67, A Pioneer In Automation". The New York Times. Retrieved 2020-02-24. Dr. Odo J. Struger, who invented the programmable logic controller, which makes possible modern factory automation, amusement park rides and lavish stage effects in Broadway productions, died on December 8 in Cleveland. He was 67.
Byres (2011). "PLC Security Risk: Controller Operating Systems - Tofino Industrial Security Solution". www.tofinosecurity.com.
Keller, William L Jr. Grafcet, A Functional Chart for Sequential Processes, 14th Annual International Programmable Controllers Conference Proceedings, 1984, p. 71-96.
Lin, Sally; Huang, Xiong (9 August 2011). Advances in Computer Science, Environment, Ecoinformatics, and Education, Part III: International Conference, CSEE 2011, Wuhan, China, August 21-22, 2011. Proceedings. Springer Science & Business Media. p. 15. ISBN 9783642233449 – via Google Books.
Harms, Toni M. & Kinner, Russell H. P.E., Enhancing PLC Performance with Vision Systems. 18th Annual ESD/HMI International Programmable Controllers Conference Proceedings, 1989, p. 387-399.
Maher, Michael J. Real-Time Control and Communications. 18th Annual ESD/SMI International Programmable Controllers Conference Proceedings, 2019, p. 431-436
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