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| PLC - Programmable Logic Controller
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The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968, when the company were looking for an alternative to replace complex relay control systems
The new control system had to meet the following requirements:
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Simple programming |
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Program changes without system intervention (no internal rewiring) |
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Smaller, cheaper and more reliable than corresponding relay control Systems |
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Simple, low cost maintenance |
Areas of application of a PLC
Every system or machine has a controller. Depending on the type of technology used, controllers can be divided into pneumatic, hydraulic, electrical and electronic controllers. Frequently, a combination of different technologies is used. Furthermore, differentiation is made between hard-wired programmable (e.g. wiring of electro-mechanical or electroniccomponents) and programmable logic controllers. The first is used primarily in cases, where any reprogramming by the user is out of the question and the job size warrants the development of a special controller.Typical applications for such controllers can be found in automaticwashing machines, video cameras, and cars.However, if the job size does not warrant the development of a special controller or if the user is to have the facility of making simple or independent program changes, or of setting timers and counters, then the use of a universal controller, where the program is written to an electronic memory, is the preferred option. The PLC represents such a universal controller. It can be used for different applications and, via the
program installed in its memory, provides the user with a simple means of changing, extending and optimising control processes.
The original task of a PLC involved the interconnection of input signals according to a specified program and, if "true", to switch the corresponding output. Boolean algebra forms the mathematical basis for this operation, which recognises precisely two defined statuses of one variable: "0" and "1" (see also chapter 3). Accordingly, an output can only assume these two statuses. For instance, a connected motor could therefore be
either switched on or off, i.e. controlled. This function has coined the name PLC: Programmable logic controller, i.e. the input/output behaviour is similar to that of an electromagnetic relay or pneumatic switching valve controller; the program is stored in an electronic memory.However, the tasks of a PLC have rapidly multiplied: Timer and counter functions, memory setting and resetting, mathematical computing operations
all represent functions, which can be executed by practically any of today`s PLCs.
PLC Hardware
Hardware Components of a PLC System are
Central Processor unit (CPU),
Memory,
Input/Output,
Power supply unit,
Programming device, and other devices.
Central Processing Unit (CPU)
CPU Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.
CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.
System Busses
The internal paths along which the digital signals flow within the PLC are called busses.The system has four busses:
- The CPU uses the data bus for sending data between the different elements,
- The address bus to send the addresses of locations for accessing stored data,
- The control bus for signals relating to internal control actions,
- The system bus is used for communications between the I/O ports and the I/O unit.
Memory
System (ROM) to give permanent storage for the operating system and the fixed data used by the CPU.
RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM`s that can be programmed and then the program made permanent.
I/O Sections
Inputs monitor field devices, such as switches and sensors.
Outputs control other devices, such as motors, pumps, solenoid valves, and lights.
Power Supply
Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module.
Programming Device
The programming device is used to enter the required program into the memoryof the processor.
The program is developed in the programming device and then transferred to the memory unit of the PLC.
PLC Operation
Input Relays
These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
Internal Utility Relays
These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays.
Counters
These do not physically exist. They are simulated counters and they can be programmed to count pulses.
Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include highspeed counters that are hardware based.
Timers
These also do not physically exist. They come in many varieties and increments.The most common type is an on-delay type.
Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
Output Relays
These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc.
They can be transistors, relays, or triacs depending upon the model chosen.
Data Storage
Typically there are registers assigned to simply store data. Usually used as temporary storage for math or data manipulation.
They can also typically be used to store data when power is removed from the PLC.
PLC Communications
Extension modules
PLC I/O number can be increased through certain additional modules by system extension through extension lines. Each module can contain extension both of input and output lines.
Extension modules can have inputs and outputs of a different nature from those on the PLC controller.
Remote I/O connections
When there are many I/O located considerable distances away from
the PLC an economic solution is to use I/O modules and use cables to
connect these, over the long distances, to the PLC.
Remote PLCs
In some situations a number of PLCs may be linked together with a master PLC unit sending and receiving I/O data from the other units.
Cables
Twisted-pair cabling, often routed through steel conduit. Coaxial cable enables higher data rates to be transmitted and does not require the shielding of steel conduit.
Fiber-optic cabling has the advantage of resistance to noise, small size and flexibility.
Parallel communication
Parallel communication is when all the constituent bits of a word are
simultaneously transmitted along parallel cables. This allows data to be transmitted over short distances at high speeds. Might be used when connecting laboratory instruments to the system.
Parallel standards
The standard interface most commonly used for parallel communication is IEEE-488, and now termed as General Purpose Instrument Bus (GPIB).
Parallel data communications can take place between listeners , talkers , and controllers. There are 24 lines: 8 data (bidirectional), 5
status & control, 3 handshaking, and 8 ground lines.
Serial communication
Serial communication is when data is transmitted one bit at a time. A data word has to be separated into its constituent bits for transmission and then reassembled into the word when received. Serial communication is used for transmitting data over long distances. Might be used for the connection between a computer and a PLC.
Serial standards
RS-232 communications is the most popular method of plc to external device communications. RS 232 is a communication interface included
under SCADA applications. Other standards such as RS422 and RS423
are similar to RS232 although they permit higher transmission rates and longer cable distances.
There are 2 types of RS-232 devices:
DTE (Data Terminal Equipment and a common example is a computer.)
DCE ( Data Communications Equipment and a common example is a modem.)
ASCII
ASCII is a human-readable to computer-readable translation code
(each letter/number is translated to 1's and 0's). It`s a 7-bit code, so we can translate 128 characters (2^7 is 128).
Protocols
It is necessary to exercise control of the flow of data between two devices so what constitutes the message, and how the communication is to be initiated and terminated, is defined. This is termed the protocol.
One device needs to indicate to the other to start or stop sending data.
Interconnecting several devices can present problems because of compatibility problems.
In order to facilitate communications between different devices the International Standard Organization (ISO) in 1979 devised a model to be used for standardization for Open System Interconnection (OSI).
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