Programmable Logic Controllers (PLCs): Structure and Function
The term PLC, widely available in the middle, means Programmable Logic Controller. It was originally called PCs (Programmable Controllers), but with the arrival of the IBM PC, to avoid confusion, we used the acronym PLC definitively. In Europe, the same concept is called PLC. The more appropriate definition is: Industrial System Automatic Control working under a sequence stored in memory, logic instructions. It is a system because it contains everything needed to operate, and industrial by having all the records necessary to operate in the harsh environments found in industry.
Historical Presentation of PLCs
was first introduced in 1960 in the industry because of the need to eliminate the large outlay which was produced by replacing the complex system of control based on relays and contactors.
PLC (Programmable Logic Controller) means Programmable Logic Controller.
A PLC is a device used for control. This control is made based on a logic defined by a program.
Structure of a Programmable Logic Controller
PLC is an industrial hardware, which is used for data collection. Once obtained, the bus passes through (e.g., by Ethernet) to a server. Its history goes back to the late 1960s when the industry looked for new electronic technologies for a more efficient solution to replace the control systems based on circuits with relays, switches, and other components commonly used to control combinational logic systems.
Today, PLCs not only control the operating logic of machines, plants, and industrial processes, but also can perform arithmetic operations, manage analog signals for control strategies, such as proportional-integral-derivative (PID) controllers.
The current PLC can communicate with other controllers and computers in local area networks and are an essential part of modern distributed control systems.
There are several programming languages; traditionally, the most used are Ladder Diagram (Ladder), preferred by electricians, instruction list, and programming by state, although more intuitive languages have been incorporated that allow implementing complex algorithms by simple flowcharts easier to interpret and maintain. A more recent language preferred by computing and electronics professionals is the FBD (Function Block Diagram) which uses logic gates and blocks with different connected functions.
In programming, you can include different types of operands, from the simplest, such as Boolean logic, counters, timers, contacts, coils, and mathematical operators, to more complex ones, such as handling tables (recipes), pointers, PID algorithms, and communication functions that enable multiprotocol interconnect with other devices.
A programmable logic controller consists of a set of cards or printed circuit boards on which electronic components are located.
The programmable controller has the typical structure of many programmable systems, such as a microcomputer. The basic hardware structure of a programmable console is constituted by:
– Power Supply
– Central Processing Unit (CPU)
– Input/Output (I/O) Interface Modules
– Memory Module
– Programming Unit
In some cases where the work to be performed by the controller is more demanding, intelligent modules are included.
Central Processing Unit (CPU)
It is the most complex and essential to the controls.
It is the most complex and essential programmable controller, which in other terms could be considered the brain of the controller. The central unit is designed based on microprocessors and memory and contains a control unit, the programmer’s RAM, timers, counters, internal relay-type memories, images of the process inputs/outputs, etc. Its mission is to read the states of input signals, run control, and govern the outputs; the process is continuous at high speed.
A programmable logic controller consists of a set of cards or printed circuit boards on which electronic components are located.
The programmable controller has the typical structure of many programmable systems, such as a microcomputer. The basic hardware structure of a programmable console is constituted by:
– Power Supply
– Central Processing Unit (CPU)
– Input/Output (I/O) Interface Modules
– Memory Module
– Programming Unit
In some cases where the work to be performed by the controller is more demanding, intelligent modules are included.
POWER SUPPLY
The function of the power supply controller is supplying power to the CPU and other cards as configured in the PLC.
+ 5 V to feed all the cards
+ 5.2 V to power the programmer
+ 24 V for 20 mA current loop channels.
Central Processing Unit (CPU)
It is the most complex and essential programmable controller, which in other terms could be considered the brain of the driver.
The central unit is designed based on microprocessors and memory and contains a control unit, the programmer’s RAM, timers, counters, internal relay-type memories, images of the process inputs/outputs, etc. Its mission is to read the states of input signals, run control, and govern the outputs; the process is continuous at high speed.
I/O INTERFACE MODULES
They provide the link between the controller and the CPU system’s field devices. Through them, the exchange of information originates, either to acquire data or to command machine control of the process.
Types of Input and Output Modules
Because there are a variety of external devices (sensors and actuators), we find different types of input and output modules, each of which serves to handle a certain type of signal (discrete or analog) at a given value of DC or AC voltage or current.
Discrete Input Modules
Discrete Output Modules
Analog Input Modules
Analog Output Modules
MEMORY MODULES
They are devices to store information temporarily or permanently.
It has two types of memory:
Volatile (RAM)
Non-volatile (EPROM and EEPROM)
PROGRAMMING UNIT
Programming terminals are the means of communication between humans and machines. These devices are made up of keyboards and display devices.
There are three types of programmers: handheld manuals, video type (PC), and the computer.