DEFINITION OF "PLC"
1. A Programmable Logic Controller, or PLC, is a ruggedized computer used for industrial automation. These controllers can automate a specific process, machine function, or even an entire production line.
OPERATION
1. The PLC receives information from connected sensors or input devices, processes the data, and triggers outputs based on pre-programmed parameters.
2. Depending on the inputs and outputs, a PLC can monitor and record run-time data such as machine productivity or operating temperature, automatically start and stop processes, generate alarms if a machine malfunctions, and more.
3. Programmable Logic Controllers are a flexible and robust control solution, adaptable to almost any application.
KEY FEATURES
1. There are a few key features that set PLCs apart from industrial PCs, microcontrollers, and other industrial control solutions:
2. I/O – The PLC’s CPU stores and processes program data, but input and output modules connect the PLC to the rest of the machine; these I/O modules are what provide information to the CPU and trigger specific results. I/O can be either analog or digital; input devices might include sensors, switches, and meters, while outputs might include relays, lights, valves, and drives. Users can mix and match a PLC’s I/O in order to get the right configuration for their application.
3. Communications – In addition to input and output devices, a PLC might also need to connect with other kinds of systems; for example, users might want to export application data recorded by the PLC to a supervisory control and data acquisition (SCADA) system, which monitors multiple connected devices. PLCs offer a range of ports and communication protocols to ensure that the PLC can communicate with these other systems.
4. HMI – In order to interact with the PLC in real time, users need an HMI, or Human Machine Interface. These operator interfaces can be simple displays, with a text-readout and keypad, or large touchscreen panels more similar to consumer electronics, but either way, they enable users to review and input information to the PLC in real time.
5. Advance Features - In today’s world of the Industrial Internet of Things (iIoT), and Industry 4.0 programmable controllers are called upon to communicate data via Web browser, connect to databases via SQL, and even to the cloud data via MQTT.
PROGRAMING PLC
1. Most PLC programming software offers programming in Ladder Logic, or “C”. Ladder Logic is the traditional programming language. It mimics circuit diagrams with “rungs” of logic read left to right.
2. Each rung represents a specific action controlled by the PLC, starting with an input or series of inputs (contacts) that result in an output (coil).
3. Because of its visual nature, Ladder Logic can be easier to implement than many other programming languages.
4. “C” programming is a more recent innovation.
5. Some PLC manufacturers supply control programming software.
6. In addition to the traditional PLC described above, there are variations, including PLC + HMI controllers.
PLC ARCHITECTURE
1. The architecture of a PLC is based on the same principles of that used in standard computer architecture.
2. However, PLC architecture does differ because the design is based around providing high reliability, immunity to harsh industrial environment, ease of maintenance and access to large amounts of peripheral inputs and outputs.
3. The heart of the PLC system is the CPU (Central Processing Unit).
4. It is made up of a control unit and processor.
5. The CPU control unit manages the interaction between the various PLC hardware components while the CPU processor handles all the number crunching and program (eg ladder logic) execution.
6. Data flow is from the input devices, through the CPU processor and then to the output devices.
7. The CPU processor also exchanges data with the program and data memory.
8. Once all the data is gathered the program (eg ladder logic) is processed in a cyclic fashion.
9. The resulting data flows to the output interface for conditioning and execution of the output devices.
10. The CPU also controls and exchanges data with the communication interface and devices.
11. An addressing system is used for data organization that is shared between the various hardware components.
12. A programming terminal is used to formulate the PLC program (eg ladder logic), load the program into the controller and monitor/control the PLC and its program.
13. The power supply is responsible for supplying and managing the power requirements of the various PLC hardware components.
TYPE OF PLC ARCHITECTURE
1. PLC architecture has advanced tremendously since its inception of the PLC back in the 1960s. Today the options and variations available to the traditional PLC architecture types is almost endless between different PLC manufacturers.
2. The 3 distinct types of PLC architecture available for use in industrial automation are known as fixed, modular and distributed.
3. The terminology surrounding PLC types can vary between PLC manufacturers, especially when talking about fixed PLCs.
4. There is also crossover between PLC types with some fixed type PLCs having modular type features and some modular type PLCs having distributed type features.
5. When considering PLC architecture types we can generally say that fixed PLCs are used for smaller sized, less complex application.
6. Modular PLCs are generally used for medium sized, more complex applications.
7. Whereas distributed PLCs are used for large sized, extensive application spread across multiple location.
FIXED PLC TYPE ARCHITECTURE
1. A fixed PLC employs single unit architecture where all the hardware components are embedded into a single unit.
2. Hardware components such as the power supply, CPU, memory, input, output and communication interfaces are all are built into a fixed PLC.
3. The most common names given to fixed PLCs by different manufacturers are fixed, integrated, nano, micro, compact, small, mini, basic, unitary, standard and brick.
4. A fixed PLC is universally characterized by:
PLC naming convention – Fixed, integrated, nano, micro, compact, small, mini, basic, unitary, standard and brick.
CPU processor – Low powered.
Program and data memory size – Small.
Power supply – Embedded.
Input Interface – Embedded.
Output Interface – Embedded.
Communication interface – Embedded.
Mounting system – Single unit.
Physical size – Small.
Flexibility – Input, output and communication interfaces are fixed.
Expandable – No
Customizable – No
Applications – Basic applications with small number of inputs and outputs.
Cost – Low
ADVANTAGES AND DISADVANTAGES OF FIXED PLCS
1. Fixed PLCs have some great advantages over other types of PLCs. They have been specifically design to cater for smaller, low end automation projects. The advantages of a fixed PLC are:
2. Small in size so they do not take up very much space in an enclosure.
3. Quick and easy to mount.
4. Low in cost so they are an economical solution for basic applications.
5. If you’re considering selecting a fixed PLC to automate your next application its prudent to also consider some of disadvantages to make sure you are selecting the type of PLC that is appropriate for your application. The disadvantages of a fixed PLC are:
6. CPU processing power is low and memory is small so complex tasks can be difficult to realize.
7. Inflexible because the number of input, output and communication interfaces are fixed.
8. Only suitable for basic applications with small number of inputs and outputs.
MODULAR PLC TYPE ARCHITECTURE
1. A modular PLC has a separate module for each of its hardware components.
2. Each PLC module is interconnected using a common mounting system. The mounting system has a certain amount of modules that it can accommodate. This means a modular PLC can be configured to be application specific.
3. A PLC module is a hardware component that carries out a specific function in accordance to the architecture of the PLC system.
4. The main modules used in a modular type PLC are the processor module, power supply module, input modules, output modules and communication modules.
5. The design of these modules will vary for different manufacturers and usually cannot be interchanged between PLC manufacturers.
6. Modular PLCs are used for automating industrial applications where a higher powered processor and large numbers of input and output devices are required.
7. These types of applications that use modular PLCs are usually associated with a higher level of complexity with regards to operation, process control and monitoring.
8. Some examples of industries that readily use modular PLCs are manufacturing, food and beverage, mining and logistics.
9. A modular plc is characterized by:
PLC naming convention – Modular.
CPU processor – Medium to High powered.
Program and data memory size – Medium to Large.
Power supply – Module.
Input Interface – Input Module.
Output Interface – Output Modules.
Communication interfaces – Communication Modules.
Mounting system – Rack, backplane, rail or chassis.
Physical size – Medium to Large.
Flexibility – High
Expandable – Yes
Customizable – Yes
Applications – Medium to High end applications with large number of inputs and outputs.
Cost – Medium to high
ADVANTAGES AND DISADVANTAGES OF MODULAR PLCS
1. Modular PLCs have some great advantages over other fixed PLCs. They have been specifically design to cater for medium to high end automation projects.
2. The advantages of using a modular PLC instead of a fixed PLC are that modular PLCs have lager memory, higher performance processors, larger number of input and outputs, increased communication options, are fully customizable and are easily expanded. This enables the modular PLC to handle larger scale applications and of higher complexity compared to a fixed PLC.
3. Modular PLCs also have the ability to have remotely mounted input and output modules (distributed I/O) that are interconnected using a communication link. This allows for increase number of inputs and outputs, reduced cable requirements and installation flexibility.
4. The modular PLC also has maintenance advantages over fixed PLCs. Each hardware component is separate housed in a module which can be replaced if it is faulty. Whereas a fixed PLC has all its component embedded into a single unit. So when there is a fault the whole unit must be replaced in its entirety and the faulty unit ends up in the trash.
5. If you’re considering selecting a modular PLC to automate your next application it’s wise to also consider some of disadvantages to make sure you are selecting the most suitable PLC for your application. The disadvantages of modular PLC are:
- Large in size so they take up more space in an enclosure than a fixed PLC.
- The mounting system is more complex than a fixed PLC.
-Higher in cost than a fixed PLC so may not be cost effective for smaller applications.
DISTRIBUTED PLC TYPE ARCHITECTURE
1. A distributed PLC is a high end PLC system with modular architecture and the capability to interconnect hardware components across different locations via high speed communication links.
2. Each location in the distributed PLC system contains multiple hardware modules that are housed in a mounting system and are usually called a node, rack or drop.
3. Each drop, node or rack in the Distributed PLC system must have a communication module and can either contain a PLC processor module with input and output (I/O) modules or just I/O modules.
4. When there is a communication module with no PLC processor module and just I/O modules then the node is called distributed I/O or remote I/O.
5. Distributed PLCs are used for large factories and large processing facilities because they are not limited to physical location.
6. They allow hardware components to be located in different locations by utilizing high speed communication links to interconnect processors and distributed I/O.
7. The distributed PLC type is considered to be a site wide process control solution.
8. The biggest differences between distributed PLCs and other types of PLCs is that they contain high performance processors, large memories and are able to handle large volumes of I/O, use higher level programming languages and can handle large amounts of complex process control tasks.
9. In the past a Distributed Control Systems (DCS) was used for large process plants. But the technology driven PLCs of today are performance packed and can handle the heavy demands of a distributed control system.
10. In today’s world of advanced PLCs most modular PLCs have the features of a distributed PLC. Regardless of the terminology used a distributed PLC is universally characterized by:
PLC naming convention – Distributed or Modular with Remote I/O.
CPU processor – High powered.
Program and data memory size – Large.
Power supply – Module.
Input Interface – Input Modules.
Output Interface – Output Modules.
Communication interfaces – Communication Modules.
Mounting system – Rack, backplane, rail or chassis.
Physical size – Medium to Large.
Flexibility – High
Expandable – Yes
Customizable – Yes
Applications –High end applications with large number of inputs and outputs.
Cost –High
ADVANTAGES AND DISADVANTAGES OF DISTRIBUTED PLCS
1. Advantages over other PLC types:
-Plant wide control network with multiple processors and remote I/O drops.
-High performance processor.
-Large program and data memory.
-Able to handle large volumes of I/O.
-Can handle large amounts of complex process control tasks.
-Ease of maintenance.
-Save time and money on installation costs.
2. Because of the high end features that a distributed PLC has they come with some disadvantages that must be considered when selecting the type of PLC that is required for your automation system
-Large in size with bigger installation footprint.
-The mounting system is more complex than a fixed PLC.
-Higher in cost than other types of PLC so they may not be cost effective for smaller less complex applications.
-Higher level programming skills may be required.
Source:https://ladderlogicworld.com/plc-architecture/#:~:text=The%20term%20PLC%20architecture%20refers,used%20in%20standard%20computer%20architecture.
