What is a PLC?
A Programmable Logic Controller (PLC) is a digital electronic system specifically designed for industrial environments. It uses a programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations. Through digital or analog inputs and outputs, it controls various types of machinery or production processes.
What is the function of a PLC?
PLCs are a core component of most industrial control and factory automation systems, often referred to as the "brain" of the factory. PLCs achieve a wide range of functions by receiving and processing data from sensors and machines, and then use this data to control and operate external equipment. In addition, a successful design must possess reliability, system stability, and 100% interoperability with connected automation systems.
In modern industry, programmable logic controllers (PLCs) are now quite similar to lightweight computers. There are even programmable automation controllers (PACs) that integrate personal computers (using embedded operating systems) and PLC architectures. These PACs are electronic systems that can control machinery, manufacturing processes, and other control modules through digital or analog input/output modules.
1. Used for on/off control
Currently, the primary goal of PLCs, and one that other controllers cannot match, is their ability to conveniently and reliably control on/off signals. There are numerous examples of using PLCs for on/off control, in industries such as metallurgy, machinery, light industry, chemicals, and textiles—almost all industrial sectors require them.
2. Used for analog quantity control
Analog quantities, such as current, voltage, temperature, and pressure, change continuously. Industrial production, especially continuous production processes, often requires the control of these physical quantities. It is entirely possible to implement analog quantity control using a PLC.
For PLCs to perform analog signal control, they need to be equipped with A/D and D/A units that convert between analog and digital signals. These are also I/O units, but special types.
3. Used for motion control
In addition to switching and analog quantities, actual physical quantities also include motion control. PLCs can receive counting pulses, and with their data processing and calculation capabilities, and if equipped with appropriate sensors (such as rotary encoders) or pulse servo devices, various controls can be achieved based on the principles of NC.
4. Used for data acquisition
With the development of PLC technology, its data storage area is becoming increasingly larger. A PLC can also be configured with a small printer to periodically print out the data from the DM (Data Management) area. The PLC can also communicate with a computer, allowing the computer to read the data from the DM area and process it. In this case, the PLC becomes a data terminal for the computer.
5. Used for signal monitoring
PLCs have many self-test signals and numerous internal components, yet most users do not fully utilize their capabilities. In fact, they can be used to monitor the PLC's own operation or to monitor the controlled object.
6. Used for networking and communication
PLCs have strong networking and communication capabilities, and new networking structures are constantly being introduced. PLCs can be connected to personal computers for communication, allowing computers to participate in programming and control management of the PLC, making PLCs more convenient to use.
Composition of PLC programmable controller
A PLC (Programmable Logic Controller) consists of a CPU, instruction and data memory, input/output interfaces, power supply, and digital-to-analog converters. Essentially, a programmable logic controller is a computer specifically designed for industrial control. Its hardware structure is largely the same as that of a microcomputer, and its basic components are described in detail below:
1. Power supply
The power supply is used to convert AC power into DC power required by the PLC. Currently, most PLCs use switching power supplies.
2. Central Processing Unit
The central processing unit (CPU) is the control center of a PLC and its core component; its performance determines the performance of the PLC.
3. Memory
A memory is a semiconductor circuit with memory function. Its purpose is to store system programs, user programs, logic variables, and other information. The system program is the program that controls the PLC to perform various functions. It is written by the PLC manufacturer and stored in read-only memory (ROM), which cannot be accessed by the user.
4. Input Unit
The input unit is the input interface connecting the PLC to the controlled device. It acts as a bridge for signals to enter the PLC, and its function is to receive signals from the master control element and the detection element. Input types include DC input, AC input, and AC/DC input.
5. Output Unit
The output unit is also the connection component between the PLC and the controlled device. Its function is to transmit the PLC's output signals to the controlled device, that is, to convert the weak electrical signals sent by the central processing unit into level signals to drive the actuators of the controlled device. Output types include relay outputs, transistor outputs, and thyristor outputs.
In addition to the components mentioned above, PLCs, depending on the model, also have various external devices. These devices assist in programming, monitoring, and network communication. Commonly used external devices include programmers, printers, cassette recorders, and computers.
