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What is PLC?

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Programmable logic controllers are now the most widely used industrial process control technology. A programmable logic controller (PLC) is an industrial
grade computer that is capable of being programmed to perform control functions. The programmable controller has eliminated much of the hardwiring associated with conventional relay control circuits.

Other benefits include fast response, easy programming and installation, high control speed, network compatibility, troubleshooting and testing convenience, and high reliability.

1. Characteristics of PLC control system:

+ Increased Reliability.

+ More Flexibility.

+ Lower Cost.

+ Faster Response Time.

+ Communications Capability.

+ Easier to Troubleshoot.

+ Reduce 80% number of extension cord.

+ PLC power consumption is very low.

+ Easy to learn programming languages> easy programming and changing programs.

+ Adaptation in harsh environments: Temperature, humidity, voltage fluctuations …

2. Structure of PLC

2.1 PLC has two main types of design:

+ Fixed I/O: Is typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together, and the I/O terminals will have a fixed number of connections built in for inputs and outputs. The main advantage of this type of packaging is lower cost. The number of available I/O points varies and usually can be expanded by buying additional units of fixed I/O. One disadvantage of fixed I/O is its lack of flexibility; you are limited in what you can get in the quantities and types dictated by the packaging. Also, for some models, if any part in the unit fails, the whole unit has to be replaced.

+ Modular I/O is divided by compartments into which separate modules can be plugged. This feature greatly increases your options and the unit’s flexibility. You can choose from the modules available from the manufacturer and mix them any way you desire. The basic modular controller consists of a rack, power supply, processor module (CPU), input/output (I/O modules), and an operator interface for programming and monitoring. The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack. The PLC processor is also connected
to the backplane and can communicate with all the modules in the rack.

2.2 PLC is composed of the following main components:

+ The power supply supplies DC power to other modules that plug into the rack. For large PLC systems, this power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies. For some small micro
PLC systems, the power supply may be used to power field devices.

+ CPU: Is the “brain” of the PLC. A typical processor usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing user program instructions, numerical values, and I/O devices status.

+ Memory (RAM / ROM) : Used to store programs and data. Most PLCs have to use batteries to feed memory, but newer PLCs do not need batteries to store programs and data.

+ The I/O system forms the interface by which field devices are connected to the controller. The purpose of this interface is to condition the various signals received from or sent to external field devices. Input devices such as pushbuttons, limit switches, and sensors modifying programs, and transferring programs to multiple machines.

+ Other Modules : There are many modules with special functions that help the PLC to communicate with peripheral devices Enthernet Module, CC-Link Module, RS232/485 Module, Position Module, Modbus Module , EtherCAT Module, …

2.3 Principles of Operation

The signals from external devices (sensors, contacts, etc.) are first fed to the CPU via the input module. After receiving the input signal, the CPU will process and send the control signals through the output module to external controlled devices according to a preprogrammed program.
A cycle consists of reading input signals, executing programs, internal communications, checking errors, sending updates to the output signal called a scan cycle or a scan cycle (Scan Cycle). Usually a scan takes a very short time (from 1ms to 100ms). The time taken for this scan depends on the speed of the PLC’s command processing, the short duration of the program, the communication speed between the PLC and peripheral devices.

3. Application of PLC

From the above advantages, PLC has now been applied in many different fields in civil and industry such as:

+ Transport lifting system.

+ Packaging line.

+ The robot assembles the product.

+ Pump control.

+ Chemical processing line.

+ Paper production technology.

+ Glass production line.

+ Cement production line

+ Food processing technology.

+ Semiconductor components manufacturing line.

+ TV assembly line.

+ Control the traffic light system.

+ Automatic parking management.

+ Alarm system.

+ Industrial sewing line.

+ Elevator control.

+ Car production line

+ IC manufacturing.

+ Check production process.

+ …

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