PLC (Programmable Logic Controller): A PLC is a digital electronic system designed for industrial applications. It uses a programmable memory to store programs, execute logic operations, sequential control, timing, counting, and arithmetic operations, and control various types of machinery or production processes through digital or analog inputs/outputs. DCS (Distributed Control System): A DCS is a widely used automated high-tech product in industries with many analog loop controls. It disperses the risks associated with control while centralizing management and display functions. A DCS typically consists of five parts: 1. Controller; 2. I/O board; 3. Operator station; 4. Communication network; 5. Graphical and programming software.
I. The Development History of PLC
In industrial production processes, there is a large amount of sequential control of switching quantities. This control involves sequential actions based on logical conditions and interlocking protection actions based on logical relationships, as well as the acquisition of a large amount of discrete data. Traditionally, these functions are achieved through pneumatic or electrical control systems. In 1968, GM (General Motors) in the United States proposed the replacement of relay control devices. The following year, Digital Systems developed a control device based on integrated circuits and electronic technology, which was the first to apply programmed methods to electrical control. This was the first generation of programmable controllers, called Programmable Controllers (PCs).
After the development of personal computers (PCs), for convenience and to reflect the functional characteristics of programmable logic controllers, programmable logic controllers were named Programmable Logic Controllers (PLCs). Today, PLCs are still often abbreviated as PCs.
There are many definitions of a PLC. The International Electrotechnical Commission (IEC) defines a PLC as: A programmable controller is a digital electronic system designed for industrial applications. It uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, and controls various types of machinery or production processes through digital and analog inputs and outputs. Programmable controllers and related equipment should be designed to easily integrate with industrial control systems and facilitate functional expansion.
The period from the 1980s to the mid-1990s was the fastest period of PLC development, with an annual growth rate of 30-40%. During this period, PLCs saw significant improvements in their ability to process analog signals, perform digital calculations, provide human-machine interfaces, and network. PLCs gradually entered the process control field and, in some applications, replaced the DCS system, which had dominated the process control field.
PLCs are characterized by their versatility, ease of use, wide applicability, high reliability, strong anti-interference capabilities, and simple programming. Their role in industrial automation control, especially in sequential control, is irreplaceable in the foreseeable future.
II. PLC Composition
Structurally, PLCs are divided into two types: fixed and modular. A fixed PLC includes a CPU board, I/O board, display panel, memory module, power supply, etc., which are combined into a non-removable unit. A modular PLC includes a CPU module, I/O module, memory, power supply module, and backplane or rack; these modules can be combined and configured according to certain rules.
III. CPU Composition
The CPU is the core of a PLC, acting as its central nervous system. Every PLC has at least one CPU. It receives and stores user programs and data according to the functions assigned by the PLC's system program. It scans and collects status or data from field input devices, storing it in designated registers. Simultaneously, it diagnoses the operating status of the power supply and internal PLC circuits, as well as syntax errors during programming. Once running, it reads instructions line by line from the user program memory, analyzes them, and then generates corresponding control signals according to the tasks specified in the instructions, directing the relevant control circuits.
The CPU mainly consists of an arithmetic logic unit (ALU), a control unit, registers, and data, control, and status buses that connect them. The CPU unit also includes peripheral chips, bus interfaces, and related circuitry. Memory, primarily used for storing programs and data, is an indispensable component of the PLC.
From a user's perspective, a detailed analysis of the CPU's internal circuitry is unnecessary, but a sufficient understanding of the working mechanisms of each part is essential. The CPU's controller manages its operation, reading, interpreting, and executing instructions. Its operating rhythm is controlled by an oscillator signal. The arithmetic logic unit (ALU) performs numerical and logical operations under the direction of the controller. Registers participate in calculations and store intermediate results; they too operate under the control of the controller.
CPU speed and memory capacity are important parameters of a PLC, as they determine the PLC's operating speed, number of I/O operations, and software capacity, thus limiting the scale of control.
IV. I/O Modules
The interface between the PLC and the electrical circuit is achieved through the input/output (I/O) section. The I/O module integrates the PLC's I/O circuitry; its input registers reflect the input signal status, and its output points reflect the output latch status. The input module converts electrical signals into digital signals for the PLC system, while the output module does the opposite. I/O modules include digital input (DI), digital output (DO), analog input (AI), and analog output (AO).
Digital signals are signals with only two states: on and off (or 1 and 0). Analog signals are continuously changing quantities. Common I/O classifications are as follows:
Switching signals: Classified by voltage level, there are 220VAC, 110VAC, and 24VDC; classified by isolation method, there are relay isolation and transistor isolation.
Analog signals: Classified by signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V, -10-10V), etc. Classified by precision, there are 12bit, 14bit, 16bit, etc.
In addition to the general-purpose I/O modules mentioned above, there are also special I/O modules, such as RTD, thermocouple, and pulse modules.
The module specifications and quantity are determined by the number of I/O points. The number of I/O modules can vary, but the maximum number is limited by the basic configuration capability that the CPU can manage, i.e., the maximum number of backplane or rack slots.
V. Power Module
PLC power supplies provide operating power to the integrated circuits of various PLC modules. Some also provide 24V operating power to the input circuits. Power input types include: AC power (220VAC or 110VAC) and DC power (24VAC is commonly used).
VI. Base plate or frame
Most modular PLCs use a baseboard or frame, which serves two purposes: electrically, to enable communication between modules so that the CPU can access all modules on the baseboard; and mechanically, to connect modules so that they form a whole.
VII. Other equipment in the PLC system
1. Programming Equipment: A programmer is an indispensable device for PLC development, application, monitoring, operation, inspection, and maintenance. It is used for programming, setting up the system, and monitoring the working status of the PLC and the system controlled by the PLC, but it does not directly participate in on-site control operations. Small programmers for PLCs are generally handheld, but currently, a computer (running programming software) is generally used as the programmer.
2. Human-Machine Interface: The simplest human-machine interface consists of indicator lights and buttons. Currently, integrated operator terminals with LCD screens (or touch screens) are becoming increasingly widely used, and it is very common for computers (running configuration software) to serve as human-machine interfaces.
3. Input/output devices: used for permanent storage of user data, such as EPROM and EEPROM writers, barcode readers, potentiometers for inputting analog signals, printers, etc.
8. PLC Communication Networking
Advanced industrial network technology enables the rapid and efficient collection and transmission of production and management data. Therefore, the importance of networks in automation system integration engineering is becoming increasingly significant, with some even suggesting that "the network is the controller."
PLCs have communication networking capabilities, enabling them to exchange information with each other, with a host computer, and with other intelligent devices, forming a unified whole and achieving distributed centralized control. Most PLCs have an RS-232 interface, and some have built-in interfaces supporting their respective communication protocols.
PLC communication is not yet interoperable. The IEC has specified a variety of fieldbus standards, which are used by various PLC manufacturers.
For an automation project (especially a medium-to-large-scale control system), network selection is crucial. First, the network must be open to facilitate the integration of different devices and future system expansion. Second, the network type must be selected based on the transmission performance requirements of different network layers, which requires a thorough understanding of the network standard's protocols and mechanisms. Third, specific considerations such as system cost, device compatibility, and suitability for the field environment must be taken into account to determine the network standard used at different layers.
