PLCs typically operate using a cyclic scanning mode, with some large and medium-sized PLCs incorporating an interrupt mode. After the user has debugged the user program, it is written into the PLC's memory using a programmer. Simultaneously, the field input signals and the controlled actuators are connected to the input terminals of the input modules and the output terminals of the output modules. The PLC is then set to run mode, and the subsequent tasks are performed by the PLC according to the user program. The overview diagram is a block diagram of the PLC execution process. During operation, the PLC primarily processes six modules.
1. Power Supply Components: The power supply component provides the power required for PLC operation, converting external power into power suitable for the PLC's internal components and control panel. 2. Microprocessor (CPU) and Memory Components: The microprocessor (CPU) is the core component of the PLC. CPUs vary between manufacturers; some use commercially available standard chips, while others use dedicated chips for programmable controllers. Memory components come in two types: ROM and RAM. 3. Input and Output Components
The input and output components of a PLC controller serve as the interface for data exchange between the PLC and the industrial production site. Unlike ordinary computers, PLCs operate in harsher environments and require strong anti-interference capabilities; the input and output components are designed for this purpose. This section introduces relevant knowledge about PLC control systems. So, what exactly is a PLC?
First, the Chinese name for PLC is Programmable Logic Controller, which is used as a digital operating electronic system in industrial applications. It uses programmable memory to internally store and execute logic operations, sequential control, arithmetic operations, and other operational instructions. It achieves remote centralized control of various mechanical equipment production processes through digital or analog inputs and outputs.
A PLC consists of functional units such as a CPU instruction and data memory, input/output interfaces, power supply, and analog-to-digital converter. Whether in a computer-controlled system, a centralized distributed control system (DCS), or a fieldbus control system (FCS), a large number of various PLC controllers are always used. Almost all hardware manufacturers involved in industrial automation produce PLC products, with Siemens, Schneider Electric, Mitsubishi, and Delta being among the most widely used.
In short, a PLC is a technology that improves work efficiency and facilitates the automatic control of mechanical equipment. Due to its advantages of being easy to understand, easy to learn, stable, and reliable, it is widely used in various industries, from FA (factory automation) to PA (process automation).
PLC control systems are a new generation of industrial control equipment formed by introducing microelectronics, computer technology, automatic control technology, and communication technology on the basis of traditional sequential controllers. The purpose is to replace relays with PLCs for executing logic, timing, and sequential control functions such as counting, and to establish flexible remote control systems. They are characterized by strong versatility, ease of use, wide adaptability, high reliability, strong anti-interference capabilities, and simple programming. The use of PLC control systems on the control panel of a unit unit (LCU) is very common.
The internal operating mode of a PLC control system typically employs a cyclic scanning mode, while some large and medium-sized PLC control systems incorporate an interrupt mode. After the user completes the debugging of the user program, the program is written into the PLC memory using a programmer. The field input signals and controlled actuators are then connected to the input terminals of the input module and the output terminals of the output module, respectively. The PLC operating mode is then selected as the running mode, and subsequent tasks are performed by the PLC according to the user program.
PLC, or Programmable Logic Controller, holds a very important position in the industrial field. To enhance everyone's understanding of PLCs, this article will introduce them based on two points: 1. What is a PLC control system? 2. What are the differences between safety PLCs and ordinary PLCs? If you are interested in PLCs, feel free to continue reading with me.
I. What is a PLC control system?
Since the introduction of Programmable Logic Controllers (PLCs) in the United States in the 1960s to replace traditional relay control devices, PLCs have experienced rapid development and have been widely used around the world. Simultaneously, the functions of PLCs have been continuously improved. With the continuous development of computer technology, signal processing technology, control technology, and network technology, and the increasing demands of users, PLCs have added analog signal processing and motion control functions to their existing switch signal processing capabilities. Today, PLCs are no longer limited to logic control; they also play a vital role in motion control, process control, and other fields.
As the preferred product for discrete control, PLCs experienced rapid development in the 1980s and 1990s, with an annual growth rate of 20% to 30% worldwide. With the continuous improvement of factory automation and the expansion of the PLC market, the growth rate of PLCs in industrialized countries has slowed down in recent years. However, in developing countries such as China, the growth of PLCs has been very rapid. According to relevant data, global PLC sales revenue was approximately US$10 billion in 2004, occupying a very important position in the automation field.
PLCs evolved from relay control principles. In the 1970s, PLCs only supported on/off logic control, initially used in the automotive manufacturing industry. They store instructions for performing logic operations, sequential control, timing, counting, and calculations; and control various machines or production processes through digital input and output operations. User-written control programs express the technological requirements of the production process and are pre-stored in the PLC's user program memory. During runtime, the stored program is executed line by line to complete the required operations. The PLC's CPU contains a program counter that indicates the address of each program step. During program execution, this counter automatically increments by 1 for each step. The program executes sequentially from the starting step (step number zero) to the final step (usually an END instruction), then returns to the starting step in a loop. The time required for the PLC to complete one loop operation is called a scan cycle. Different PLC models have scan cycles ranging from 1 microsecond to tens of microseconds. PLCs use ladder logic programming, exhibiting a fast advantage in logic solving; at the microsecond level, solving a 1K logic program takes less than 1 millisecond. It treats all inputs as digital signals, with each 16-bit (or sometimes 32-bit) signal representing an analog signal. Large PLCs use a separate CPU to perform the analog signal calculations and then send the results to the PLC controller.
Over the past decade, with the continuous decrease in PLC prices and the expanding user demand, more and more small and medium-sized equipment have begun to adopt PLC control, resulting in rapid growth in PLC applications in my country. With the rapid development of China's economy and the continuous improvement of basic automation levels, PLC usage in my country will continue to maintain a high growth momentum for some time to come.
When a general-purpose PLC is used in specialized equipment, it can be considered an embedded controller. However, a PLC offers higher reliability and better stability compared to a typical embedded controller. In practice, some users who previously used embedded controllers are now gradually replacing them with general-purpose or custom-designed PLCs.
What are the three main categories of PLC control systems?
One of the three major categories of PLC control systems – stand-alone systems
A stand-alone system is the simplest type of PLC, meaning that one PLC controls one piece of equipment or a simple production line, requiring relatively few I/O points and storage capacity.
The second of the three major categories of PLC control systems – Centralized control systems
A centralized control system refers to a system where one PLC controls multiple devices or production lines. These devices or production lines are located close to each other and are interconnected. However, this also means that in a centralized control system, a failure in one device or production line will affect the entire system. Therefore, it requires a high number of I/O points and sufficient storage capacity.
The third of the three major categories of PLC control systems – Distributed Control Systems
Distributed control systems, also known as distributed control systems, require a single PLC with communication capabilities to control multiple controlled objects. These controlled objects are geographically dispersed and cover a wide area. However, a key advantage of distributed control systems is that a failure in one controlled object or the PLC itself does not affect the operation of the entire system. PLC development is progressing rapidly. Besides the three main types of PLC control systems mentioned above, some now propose integrated PLC and EIC control systems, which integrate electrical control, instrumentation control, and computer control. When considering which control system to use, companies should consider how to adapt to the characteristics of these systems and how this will facilitate further development of system functions.
