What is a PLC? A programmable controller, or Programmable Logic Controller (PLC) is a device which is capable of being programmed to perform a controlling function. Before the advent of the programmable controller, the problem of industrial control was usually solved by the use of electromechanical relays or by hardwired solid-state logic blocks. These systems were very flexible in design and easy for maintenance personnel to understand. However, they involved a vast amount of interconnection from relay to relay, and logic block to logic block. In order for the wiring cost to be minimized, relays and logic blocks had to be kept close together. This led to the development of the control panel concept for larger and more complex logic control systems.
Another important factor in the development of the control panel was the ability to check field devices at a common point. A control system with 1000 sensors, limit switches, pushbuttons, and output devices could easily cover a very large area in a manufacturing plant. Thus, it would take a considerable amount of time for a technician to check each device at its location. By having each device wired back to a common point, each device could be checked for operation fairly quickly.
What is a PLC? Programmable controllers were originally designed to provide flexibility in control based on programming and executing logic instructions. Major advantages were realized by adopting the ladder diagram programming language, simplifying maintenance, and reducing the cost of spare parts inventories. In addition programmable controllers allow for shorter installation time and faster commissioning through programming rather than wiring.
In recent years, one of the areas of greatest growth and innovation has been in data communication for PLCs. PLC technology has been adopting the expanding resources of information and communication technology such as wireless local area networks (WLAN). Initially, each manufacturer had their own proprietary system whereby one could remotely interact with the PLC to monitor status and data. These days PLCs are more and more becoming compatible with standards- based communications systems such as Ethernet, Profibus, DeviceNet, and Genius networks. This means that the PLC can be hooked up to any other device on the factory floor that can communicate through these networks.
Ethernet is commonly used for interconnecting information and communication networks in the office, so the PLCs can be monitored and controlled from anywhere in a building. And not only that - they can be accessed from anywhere in the world via modem and telephone line. The newest trend is enabling control and monitoring over the Internet by using a Web browser such as Internet Explorer
What is PLC? Modern PLCs are very similar to personal computers in their size, speed, and memory capacity. Recent innovations in flash memory and LCD-screen technology has greatly improved the memory storage and graphic user interface capabilities of these systems, allowing for a wider range of programming applications and greater flexibility in programming languages. With the IEC-61131-3 standard, it is now possible to program these devices using structured programming languages (such as C), and logic elementary operations. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.
The following points outline some of the other numerous advantages derived from using PLCs:
1. Programmable controllers are easy to program and to install. Most PLCs are provided with quick- release type screw connections for fast wiring of input and output devices. PLC editing features allow program changes, corrections, and loading procedures to be accomplished in a matter of seconds in most cases.
2. The speed with which inner timers operate is much faster than standard time delay relay systems. For example, a standard time delay relay can produce a short delay as fast as 150 milliseconds (ms), with a reset period of 300 ms. A PLC can produce a short delay of 15 ms, with a reset period of approximately 15 ms. In terms of a speed difference between the two systems, a PLC is approximately 13 times faster. An assembly machine using a PLC would as a result have a higher output rate.
3. Access to PLCs is restricted by hardware features such as keylocks, and by software features, such as passwords. Timers and their timing values can also be protected through special commands
4. Problem solving with PLCs is also a major advantage over any other type of control system. Many PLCs are provided with diagnostic indicators which monitor power supply, central processing unit faults, low battery power for memory backup, input and output conditions, forced output conditions, and so on.
5. Programmable controllers can be designed with communications capabilities that allow them to converse with local and remote computer systems or to provide human interfaces. These interfaces range from simple light or annunciator systems with alpha- numeric displays to elaborate video screens that allow the current state of a process to be viewed.
6. PLCs are extremely reliable control devices and can be obtained in forms that can survive and function in harsh conditions. They can meet almost any set of installation code requirements.
What is PLC? While the advantages of using PLCs compared to traditional relay control systems are quite apparent, in recent years one of the main competitors to the PLC has been the use of personal computers in industrial applications. As you will see in subsequent modules, there are a great deal of similarities between PLC and personal computer hardware. However, there are also some distinct advantages associated with PLCs when compared to personal computers and these advantages include:
large base of qualified maintenance personnel knowledgeable in troubleshooting and servicing systems with ladder logic coding, rather than personal computer software languages such as Visual Basic, C++, HTML, etc.
More effective response to power interruptions and failures
Cost advantages for simple machine control applications
Higher reliability that minimizes the expense of shutdown, troubleshooting, repair, & startup
Durable, weather-proof, industrial strength packaging
Long availability and support for product models without the rapid obsolescence common in personal computers.
What is a PLC? A digitally operated electronic apparatus which uses a programmable memory for the inner storage of directions for applying specific functions such as logic, sequencing, timing, counting, and arithmetic, to control, through digital or analog input/output modules, various types of machines or processes.
Another important factor in the development of the control panel was the ability to check field devices at a common point. A control system with 1000 sensors, limit switches, pushbuttons, and output devices could easily cover a very large area in a manufacturing plant. Thus, it would take a considerable amount of time for a technician to check each device at its location. By having each device wired back to a common point, each device could be checked for operation fairly quickly.
What is a PLC? Programmable controllers were originally designed to provide flexibility in control based on programming and executing logic instructions. Major advantages were realized by adopting the ladder diagram programming language, simplifying maintenance, and reducing the cost of spare parts inventories. In addition programmable controllers allow for shorter installation time and faster commissioning through programming rather than wiring.
In recent years, one of the areas of greatest growth and innovation has been in data communication for PLCs. PLC technology has been adopting the expanding resources of information and communication technology such as wireless local area networks (WLAN). Initially, each manufacturer had their own proprietary system whereby one could remotely interact with the PLC to monitor status and data. These days PLCs are more and more becoming compatible with standards- based communications systems such as Ethernet, Profibus, DeviceNet, and Genius networks. This means that the PLC can be hooked up to any other device on the factory floor that can communicate through these networks.
Ethernet is commonly used for interconnecting information and communication networks in the office, so the PLCs can be monitored and controlled from anywhere in a building. And not only that - they can be accessed from anywhere in the world via modem and telephone line. The newest trend is enabling control and monitoring over the Internet by using a Web browser such as Internet Explorer
What is PLC? Modern PLCs are very similar to personal computers in their size, speed, and memory capacity. Recent innovations in flash memory and LCD-screen technology has greatly improved the memory storage and graphic user interface capabilities of these systems, allowing for a wider range of programming applications and greater flexibility in programming languages. With the IEC-61131-3 standard, it is now possible to program these devices using structured programming languages (such as C), and logic elementary operations. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.
The following points outline some of the other numerous advantages derived from using PLCs:
1. Programmable controllers are easy to program and to install. Most PLCs are provided with quick- release type screw connections for fast wiring of input and output devices. PLC editing features allow program changes, corrections, and loading procedures to be accomplished in a matter of seconds in most cases.
2. The speed with which inner timers operate is much faster than standard time delay relay systems. For example, a standard time delay relay can produce a short delay as fast as 150 milliseconds (ms), with a reset period of 300 ms. A PLC can produce a short delay of 15 ms, with a reset period of approximately 15 ms. In terms of a speed difference between the two systems, a PLC is approximately 13 times faster. An assembly machine using a PLC would as a result have a higher output rate.
3. Access to PLCs is restricted by hardware features such as keylocks, and by software features, such as passwords. Timers and their timing values can also be protected through special commands
4. Problem solving with PLCs is also a major advantage over any other type of control system. Many PLCs are provided with diagnostic indicators which monitor power supply, central processing unit faults, low battery power for memory backup, input and output conditions, forced output conditions, and so on.
5. Programmable controllers can be designed with communications capabilities that allow them to converse with local and remote computer systems or to provide human interfaces. These interfaces range from simple light or annunciator systems with alpha- numeric displays to elaborate video screens that allow the current state of a process to be viewed.
6. PLCs are extremely reliable control devices and can be obtained in forms that can survive and function in harsh conditions. They can meet almost any set of installation code requirements.
What is PLC? While the advantages of using PLCs compared to traditional relay control systems are quite apparent, in recent years one of the main competitors to the PLC has been the use of personal computers in industrial applications. As you will see in subsequent modules, there are a great deal of similarities between PLC and personal computer hardware. However, there are also some distinct advantages associated with PLCs when compared to personal computers and these advantages include:
large base of qualified maintenance personnel knowledgeable in troubleshooting and servicing systems with ladder logic coding, rather than personal computer software languages such as Visual Basic, C++, HTML, etc.
More effective response to power interruptions and failures
Cost advantages for simple machine control applications
Higher reliability that minimizes the expense of shutdown, troubleshooting, repair, & startup
Durable, weather-proof, industrial strength packaging
Long availability and support for product models without the rapid obsolescence common in personal computers.
What is a PLC? A digitally operated electronic apparatus which uses a programmable memory for the inner storage of directions for applying specific functions such as logic, sequencing, timing, counting, and arithmetic, to control, through digital or analog input/output modules, various types of machines or processes.
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