Unitary: – A Unitary PLC is a simple form of controller. This contains all the basic system components in a single box. Unitary PLC’s are mainly attached directly to the device or application system which is being controlled. Unitary PLC components include processor, which run the software program, also allows ports for input and output connection. Micrologix 1000 is a type of unitary PLC which is most commonly used, developed by Allen Bradley. The Micrologix 1000 includes on-board memory for storing programs, 32 digital input and output ports and communication port used to program the unit. . This setup is typical of many unitary systems.
Advantages of Unitary PLC: – There’re reasons why people choose the unitary PLC. First of all, this system is suitable for small to medium scale business. It’s compact and it’s very simple. Second, because of this special trait, it can be attached directly to the system being controlled. The simple and basic design allows portability and easy access. With such flexibility and easiness, the users won’t have to fumble with messy applications or difficult procedures. It’s true that the unitary PLC may be very simple and basic, but it’s a perfect choice for small business with fixed type of operation.
Disadvantages of Unitary PLC: – Limited function, poor interfaces, limited communication and data types, limited memory and program size, fixed inputs and outputs.
Examples of Unitary PLC: – Allen-Bradley Micrologix 1000
Modular: – A modular PLC contains several different modules that can be coupled together to build a customized controller. Additional modules, including analogue to digital signal converters or additional outputs, can be added to this core unit as needed. This modular design allows a PLC to be customized and changed easily. Typically, a base module contains core functions such as electrical power regulation, the computer processor, and input connections. The Allen Bradley Micrologix 1200 is the most commonly used example of the modular PLC type. This unit is able to handle between 23 and 40 inputs and outputs. The actual number of connections can be expanded easily by adding modules. This provides a wide range of flexibility and is typical of a modular PLC.
Advantages of Modular PLC: – The modular PLC has the capacity to accomplish more complex processes making it more advantageous. Modular PLCs were designed for that purpose, to expand and customize processes for seamless growth. Modular PLCs have far more memory and has the capability to store a higher volume of information.
Disadvantages of Modular PLC: – Modularity decreases space efficiency. Modularity can increase diagnostic labour when modules fail. Modularity increases complexity of configuration. Most (not all!) modular PLCs have low IP ratings, so you need enclosures. More spare stock to maintain in site stores. I/O racks encourage centralized instead of distributed I/O, which is certainly not best practice these days.
Example of Modular PLC: – Allen-Bradley (Rockwell) PLC-5
Rack Mounting: – The rack mounting type of PLC is more similar to the modular concept, but is implemented differently. Whereas each module in a modular PLC connects to the base unit directly, a rack mounting PLC keeps each module separate. All extra modules are connected through a network, and modules are held in organized racks. This approach allows for larger systems to be built without becoming overly cluttered and complicated. Modules are well organized on the rack and can be removed and reinserted as needed. The commercial unit SLC 500 is an industry-standard example of the rack mounting PLC type. There are essentially no limits on the number of modules that can be added to this system, each mounted on a standard rack chassis. This setup allows large, scalable automation solutions to be built and is common in factory settings.
Advantages of Rack Mounting: – They have expandable memory and they come in cards which just slot into a mounted part. Allow large extensions to the programme. Operates/runs and scans programme more quickly and efficiently due to quick scan time. Adaptability, Can easily be replaced, contains its own power source, Slot and memory cards can be interchangeable or re programmed and easily replaced. Integrated power supply and can be fed with analogue inputs.
Disadvantages of Rack Mounting: – Overwhelming initial cost
Example: – Mitsubishi Q Controller Base Unit 12 Slots, DIN Rail Mount
The voltages applied to the inputs of a PLC: –
Nominal current per input: – This value specifies the minimum input current that the input device must be capable of driving to operate the input circuit.
Nominal input voltage: – The AC or DC value specifies the magnitude and type of voltage signal that will be accepted.
Ambient temperature rating: – This value specifies what the maximum temperature of the air surrounding the I/O Module should be for best operating condition.
On-stage input voltage: – This value specifies the voltage at which the input signal is recognized as being absolutely ON.
Typical Switching Voltages: –
5 Vdc (TTL)
Pressure switches: – A pressure switch for sensing fluid pressure contains a capsule, bellows, Bourdon tube, diaphragm or piston element that deforms or displaces proportionally to the applied pressure. The resulting motion is applied, either directly or through amplifying levers, to a set of switch contacts. Since pressure may be changing slowly and contacts should operate quickly, some kind of over-centre mechanism such as a miniature snap-action switch is used to ensure quick operation of the contacts. The pressure switch may be adjustable, by moving the contacts or adjusting tension in a counterbalance spring. Industrial pressure switches may have a calibrated scale and pointer to show the set point of the switch.
Level Switches: – A float switch is a device used to detect the level of liquid within a tank. The switch may be used in a pump, an indicator, an alarm, or other devices.
Proximity Sensors: – A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal.
Photoelectric sensor: -A photoelectric sensor, or photo eye, is an equipment used to discover the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are largely used in industrial manufacturing.
PH sensor: – A pH Meter is a scientific instrument that measures the hydrogen-ion concentration (or pH) in a solution, indicating its acidity or alkalinity. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode.
IR(infra-red) sensor: – An infrared sensor is an electronic instrument which is used to sense certain characteristics of its surroundings by either emitting and/or detecting infrared radiation. Infrared sensors are also capable of measuring the heat being emitted by an object and detecting.
Temperature sensor: – The most commonly used type of all the sensors are those which detect Temperature or heat. These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water heating system to highly sensitive semiconductor types that can control complex process control furnace plants.
In computer program development, a panel is a representation of what information will be sent to a user’s display screen in given circumstances. Typically, when designing a program, the user interface is specified by portraying what information (text and pictures) will be presented to the user at different stages of using the program. For example, each menu, help page, or other form of content constitutes a panel of information that is to be implemented by developers and tested by early users.
Twisted Pair Cable: – Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of cancelling out electromagnetic from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighbouring pairs.
Advantages of Twisted Pair Cable: – Cheaper and far easier to splice. Less susceptible to electrical interference caused by nearby equipment or wires. In turn are less likely to cause interference themselves. Because it is electrically “cleaner”, STP wire can carry data at a faster speed.
Disadvantages of Twisted Pair Cable: – STP wire is that it is physically larger and more expensive than twisted pair wire. STP is more difficult to connect to a terminating block.
Coaxial Cable: – Coaxial cables are a type of cable that is used by cable TV and that is common for data communications. Taking a round cross-section of the cable, one would find a single centre solid wire symmetrically surrounded by a braided or foil conductor. Between the centre wire and foil is an insulating dielectric. This dielectric has a large affect on the fundamental characteristics of the cable. Data is transmitted through the centre wire, while the outer braided layer serves as a line to ground. Both of these conductors are parallel and share the same axis. This is why the wire is called coaxial.
Advantages of Coaxial Cable: – Sufficient frequency range to support multiple channels, which allows for much greater throughput. Lower error rates. because the inner conductor is in a Faraday shield, noise immunity is improved, and coax has a lower error rates and therefore slightly better performance than twisted pair. Greater spacing between amplifiers coaxial cable shielding reduces noise and crosstalk, which means amplifiers can be spaced farther apart than with twisted pair.
Disadvantages of Coaxial Cable: – More expensive to install compare to twisted pair cable. The thicker the cable, the more difficult to work with.
Fibre Optics Cable: – An optical fibre is a flexible, transparent fibre made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibres are used most often as a means to transmit light between the two ends of the fibre and find wide usage in fibre-optic communications, where they permit transmission over longer distances and at higher bandwidths (data rates) than wire cables.
Advantages of Fibre Optic: – Immune to electrical interference. Immune to corrosion
Disadvantages of Fibre Optic: – The complex electronics at both ends of the line tends to be more expensive. Needs specialist expertise to lay fibre and many installers are more familiar with copper cable
Networks: – This is the PLC vendor’s way of connecting their brand of PLCs together to share information. As each PLC vendor has their own protocol, programming references, addressing, and instructions. Some vendors differ in these protocols amongst their own models. Most PLC vendors have one proprietary network that works on all models of their own PLCs. Allen Bradley has two basic networks that move can be connected to more advanced networks.
Advantages of Network: – Files can be stored on a central computer (the file server) allowing data to be shared throughout an organisation. Files can be backed up more easily when they are all on a central fileserver rather than when they are scattered across a number of independent workstations. Networks also allow security to be established, ensuring that the network users may only have access to certain files and applications.
Disadvantages of Network: – The main disadvantage of networks is that users become dependent upon them. For example, if a network file server develops a fault, then many users may not be able to run application programs and get access to shared data. To overcome this, a back-up server can be switched into action when the main server fails
A Programmable Logic Controller (PLC) is simply a specialized type of computer for controlling industrial processes. It usually is set to store programs in a battery-backed electronic memory, or non-volatile RAM.
To execute a program, a computer needs to be powered up and started. Once this happens the CPU loads an instruction from a portion of the memory space occupied by ROM (read-only memory) where a loader program resides and also the basic control programs for the busses that lead to input-output registers and buffers. Once the basic hardware is initialized to a known state the program in ROM executes a jump to the program in RAM.
Instructions in RAM are loaded into the CPU and processed. The CPU and the IO Registers have flags that specify the outcome of certain instructions or results from the Arithmetic and Logic Unit (ALU) or certain external events (Transmission complete output register empty, or Input waiting, for example) and either the program polls for these events or the events interrupt the CPU which then stores its current state and loads from an interrupt vector to call specialized code to deal with the event.
That is pretty much the course without getting into specifics on the PLCs on the market. A PLC is basically worthless without sensors and switches connected to it to actually control something.
Finally, the PLC is in most ways very much like the personal computer. The PC fetches its initial program from ROM or EPROM, then gathers a saved state (settings for various options such as the device to boot from first) from battery-backed non-volatile RAM before seeking to load the operating system and its programs from some boot device. The PLC has non-volatile RAM as its boot device in many cases.
ALU: – An arithmetic logic unit is a digital electronic circuit that performs arithmetic and bitwise logical operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on floating point numbers. An ALU is a fundamental building block of many types of computing circuits, including the central processing unit (CPU) of computers, FPUs, and graphics processing units (GPUs). A single CPU, FPU or GPU may contain multiple ALUs.
ROM: – Memory programmed at the factory that cannot be changed or altered by the user. It can be read but not written to, and it retains its content when the power is turned off. The ROM contains the basic input/output systems that allow you to access I/O devices.
RAM: – Read/write memory. Memory used to store computer programs and interact with them. It loses its content when the power is turned off. The more RAM memory held by a computer the larger the application program you can run.
BUS: – A group of binary data lines or connecting cables that provides a means of sending and receiving information between different parts of the computer. You can think of a bus as a highway on which data travels within a computer. All buses consist of two parts: an address bus and a data bus. For example, a 16-bit bus can transmit 16 bits of data.
AND gate: – The AND gate is named, if ‘0’ is said to be false and ‘1’ is said to be true, the gate acts in same way as the logical “and” operator. The output true when ’both’ the input are ‘true’. Otherwise the output is ‘false’.
OR gate: – The output is true if either or both of the input is true. It both input are false then the output is false.
NOT gate: – A logical inverter, sometimes called a NOT gate, to differentiate it from other types of electronic inverter devices, it has only one output.
Mechanical diagram of connection of PLC:
Electrical diagram of connection of PLC:
Unitary PLC: The Main Basic Types
Besides the unitary PLC, there’re two other types of PLC system: the modular and the rack mounted one. The unitary type is considered the most basic and simplest kind of PLC system. It consists of one box only, which accommodates processor, software, and output-input connectors. The modular type is the one consisting of several modules or several programs that can be used to adjust the users’ preference and need. This type of PLC system is suitable for complicated processes and operation, where the overall program is more lenient and flexible; not fixed. The rack mounted type is basically almost the same with the modular one, but the implementation is different. In modular type, different modules will be connected directly to the base unit. In rack mounted type, however, the modules remain separated. People have their own reasons why they choose the unitary PLC or the other types.
Unitary PLC: the benefits of using it
The benefit of unitary PLC is, the system is suitable for all small to medium scale industries. It’s compact and simple. It can be attached directly to the systems being controlled. With such flexibility and easiness, the users no need to fumble with messy application or difficult procedures. It’s perfect for small business with fixed type of operations.
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