A Programmable Logic Controller (PLC) is a type of programmable memory used to store programs and execute user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations. It controls various types of machinery or production processes through digital or analog inputs/outputs. Its full English name is Programmable Logic Controller, and its full Chinese name is 程序逻辑控制 (Programmable Logic Controller). It is defined as a digital electronic system designed specifically for industrial applications. A Distributed Control System (DCS) is a distributed control system widely used in industries with many analog loop controls. It aims to disperse the risks associated with control while centralizing management and display functions. A DCS generally 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 extensive sequential control of switching quantities. This control involves sequential actions based on logical conditions, interlocking protection actions based on logical relationships, and the acquisition of a large amount of discrete data. Traditionally, these functions were achieved through pneumatic or electrical control systems. In 1968, GM (General Motors) in the United States proposed replacing 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 (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.
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.
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.
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.
