The Programmable Controller is a Programmable Controller in English. It was referred to as PC in the 1970s and 1980s. Since the 1990s, the development of personal computers, also referred to as PC; coupled with the concept of programmable programming is too large, so the United States AB for the first time to name the programmable controller as a programmable logic controller (PLC- Programmable Logic Controller), for convenience, is still referred to as PLC as a programmable controller. Some people refer to the system composed of programmable controllers as the PCS programmable control system, emphasizing that the programmable controller manufacturers have provided the complete system.
2 PLC development and market situation
2.1 Development history of PLC
In 1968, GM (General Motors) of the United States proposed to replace the relay control device. In the second year, American Digital Corporation developed the first generation of programmable controller to meet the requirements of GM's assembly line. With the development of integrated circuit technology and computer technology, there are now fifth-generation PLC products.
In the manufacturing industry characterized by changing geometry and mechanical properties and in the process industry characterized by the conversion of raw materials into products by physical and chemical changes, in addition to feedback control based on continuous quantities, especially in the manufacturing industry A large number of switching-based open-loop sequential control, which performs sequential action numbers according to logic conditions according to timing actions; in addition, there are control of interlocking protection actions according to logical relationships independent of sequence and timing; and a large number of switching quantities, State quantities such as pulse volume, timing, counters, and analog limit alarms—discrete data acquisition and monitoring. Due to these control and monitoring requirements, PLC has evolved into a product that replaces relay lines and performs sequential control. In the years of production practice, PLC, DCS and IPC have gradually formed a three-pronged trend, and other single-loop intelligent regulators account for a certain percentage in the market.
In the mid-1980s to the mid-1990s, it was the fastest period of PLC development, and the annual growth rate has remained at 30-40%. Due to the advancement of PLC machine processing simulation capability and network function, it has occupied a part of the DCS market (process control) and gradually monopolized the sewage treatment industry, but due to the emergence of industrial PC (IPC), especially the fieldbus technology in recent years. The development of IPC and FCS has also squeezed a part of the PLC market, so the growth rate of PLC in recent years is generally slow. At present, there are more than 200 manufacturers producing more than 300 kinds of PLC products in the world, mainly used in automobiles (23%), grain processing (16.4%), chemical/pharmaceutical (14.6%), metal/mine (11.5%), pulp/paper ( 11.3%) and other industries.
2.2 PLC market situation
There are about 30 domestic PLC production plants, but they do not form a large-scale production capacity and brand-name products. Some of them are produced by imitation, assembly or “brandingâ€. Therefore, it can be said that PLC has not formed a manufacturing industry in China. As a principle, technology and process, there are no cutting-edge technical difficulties. As long as you work hard, you can form a manufacturing industry.
In terms of PLC application, China is very active. In recent years, it has invested about 100,000 sets of PLC products every year, with annual sales of 3 billion yuan, and the application industry is also very wide. However, compared with other countries, the application in machining and production lines needs to increase investment. There are several PLC products popular in China's market:
(1) Schneider Electric, including the early Tianjin Instrument Factory, introduced Modicon's products, and currently has Quantum, Premium, Momentum and other products;
(2) Rockwell's (including AB) PLC products, currently available in SLC, Micro Logix, Co ntrol Logix;
(3) Siemens products, currently available in the SIMATIC S7-400/300/200 series;
(4) GE's products; Japan's Omron, Mitsubishi, Fuji, Panasonic and other company products.
The market potential of PLC is huge. Not only in China, but even in industrially developed Japan, there are surveys showing that the proportion of electromechanical integration products supported by PLC accounts for 42%, and that of relays and contactors is still 24%. Therefore, there are still many occasions where PLCs need to be applied, especially in China.
From the perspective of technological innovation, China's large and medium-sized enterprises must vigorously develop CIMS (Computer Integrated Manufacturing System), and form a FMS flexible manufacturing system in machinery manufacturing plants. PLC is the foundation, so the PLC market is broad.
PLC has the advantages of stable and reasonable price, complete functions, flexible application and convenient operation and maintenance. This is the root cause of its long-term market share. We will focus on several issues and study its development trend.
3 PLC hardware and software
3.1 Classification of PLC
In the 1990s, PLC has formed a variety of small, medium, large, and large PLCs. According to the number of I / O points, can be divided into:
l Micro PLC: 32 I/O
l Small PLC: 256 I/O
l Medium PLC: 1024 I/O
l Large PLC: 4096 I/O
l Giant PLC: 8195 I/O
In recent years, large-scale systems with single-unit support of 300 loops and 65,000-point I/O correspond to medium-sized PLCs, all of which use 16-bit to 32-bit CPUs. Micro and small PLCs originally used 8-bit CPUs. Now, according to communication requirements, some also Switch to 16-bit to 32-bit CPU. Due to I/O
PLC sales below 64 points accounted for 47% of the total PLC, and 64% to 256 points accounted for 31%, accounting for 78% of the total PLC sales. Therefore, more research should be done on micro and small PLCs.
3.2 PLC hardware
The hardware of the PLC controller itself adopts the building block structure, and the product structure of each manufacturer is similar. Take the Japanese Omron C200HE as an example, the bus template frame structure, the CPU frame is installed on the basic frame (CPU motherboard), and other slots are equipped with I/O templates; if the I/O template is long, the CPU motherboard can be I/O. The O expansion cable connects to the I/O expansion motherboard and mounts the I/O template on it; the other method is to equip the remote I/O slave. All of these indicate that the PLC manufacturer develops all hardware components to the user, which is convenient for users to select and configure into PLCs of different scales, and the openness of this hardware configuration is for manufacturers, distributors (agents), system integrators. The end user brings a lot of convenience and brings great convenience to the marketing supply chain. This is a great success experience.
In addition to the general DI/DO and AD/DA templates, I/O templates in PLC have also developed a series of special function I/O templates, which open the way for PLCs in various industries, such as barcodes. Recognized ASCII/BASIC template, PID template for feedback control, high-speed counting template for operation control, machining, single-axis position control template, dual-axis position control template, cam positioner template, radio frequency identification interface template, etc. This will be greatly developed in the future. In addition, the input and output related components, the input of strong interference, the electrical isolation of the output, and the ground isolation are also more perfect.
The CPU in the PLC cooperates with the memory to complete the control function. It differs from the system in which the DCS system processes parameters such as temperature, pressure, and flow. It uses a fast polling cycle, typically 0.1 to 0.2 s, and a faster one, 50 ms or less. It is a digital sampling control system.
3.3 PLC software
In order to complete the control strategy, in order to replace the relay, the user completes the ladder diagram of the control system like the relay circuit, and compiles a set of control algorithm function blocks (or subroutines), called the command system, which is solidified in the memory ROM. The user can call when compiling the application. Instruction systems can be roughly divided into two categories, basic instructions and extended instructions. The instruction system of subdivided general PLC includes: basic instruction, timer/counter instruction, shift instruction, transfer instruction, comparison instruction, conversion instruction, BCD operation instruction, binary operation instruction, increment/decrement instruction, logic operation instruction, Special arithmetic instructions, etc. These instructions are mostly similar to assembly language. In addition, the PLC also increases the internal resources such as sufficient timers, counters, internal relays, registers and storage areas, which brings great convenience to programming.
4 Windows-based programming language standard - IEC61131-3
Due to the differences in the command system of various PLC manufacturers and the different user requirements on the programming method, in recent years, IEC has developed the Windows-based programming language standard IEC61131-3 (1993 IEC issued the international standard IEC1131 for programmable controllers), it Five programming languages ​​are specified: Instruction List (IL), Ladder Diagram (LD), Sequential Function Chart (SFC), Function Block Diagram (FBD), and Structured Text (ST). This includes both textual programming (IL, ST) and graphical programming (LD, FBD), while SFC can be used in both types of programming languages. The IEC Technical Committee (TC65) has come up with the IEC61499 project to extend IEC61131-3, which is a standard for the architecture of modular distributed systems interconnected by communication networks, and will improve IEC61131-3. This is a standardization of the programmable logic control device based on digital technology at a high level, which is a major trend in the development of PLC.
5 PLC network and development trend
One or several PLCs and PCs are connected to the system. The PC functions as the original programmer and the man-machine interface operation station. This new trend in the 1990s brings business opportunities to system integration, while programming software and people. Machine interface software (monitoring software or configuration software) and software interfaces (or driver software) have also been developed.
In recent years, PLC manufacturers have provided the basis of the physical layer RS232/422/485 interface on the original CPU template, gradually added various communication interfaces, and provided a complete communication network. Due to the rapid development of data communication technology in recent years, users have strong requirements for openness, and fieldbus technology and Ethernet technology have also developed simultaneously. Therefore, the PCS system composed of PLC is slightly better than the open state of DCS. At present, Rockwell AB has formed a multi-layer structure system, namely fieldbus such as Ether Net, Co ntrol Net, Device Net and Asi (the original DH+ network is also compatible). In addition to the Profibus-DP communication network and the Profibus-FMS network, Siemens proposed the S7 Routing network, namely Profibus-DP and Industrial Enternet. The network is still developing, and China should have invested in it with a positive attitude.
In 2001, China's machinery industry became a new bright spot in industrial development. The total output value increased by 17.15% over the previous year. The automobile production was the top 10 in the world, and the machine tool output was the fifth in the world. The profit of the machinery industry increased by 33.35%, accounting for more than 60% of the total industrial profits. Exports also showed encouraging growth. Now that the machinery industry proposes to implement networking, PLC practitioners should have a clear understanding of the openness of the network, the performance/price ratio of the network, and the network's reliability, security, and advancement. Work hard.
Network connection is an Internet problem, downlink is a fieldbus problem, and whether the existing network can use Ethernet "e-net" mode, network adopts client/server mode, browser/server mode, producer/consumer The methods and interface software adopt OPC method and other issues have to be further implemented. PLC and intelligent MCC motor control center, NC/CNC numerical control equipment matched with CNC machine tools, and other operational control systems, electronic control equipment, frequency converters and soft starters, etc.; PLC should cooperate with DCS division to act as DCS Remote I/O station, etc.; PLC should cooperate with IPC division of labor, in addition to using IPC as human-machine interface, I/O components for software PLC are also feasible; in addition, PLC and emergency parking safety system (ESD, Emergency Shut) Down Systems) relationships with stereoscopic warehouses, robots, CAD/CAM, etc. In short, PLC is compatible with a variety of new technologies, making PLC a "computer" in the true sense.
The application field of PLC is wide, and there are still many fields to be urgently explored, such as the application for customs transit vehicle certification (Shenzhen Yantian) and automatic drug sales (several Chinese medicine stores). In addition, in discrete event systems, such as road network traffic flow (vehicle counting, passenger counting and dwell time measurement), logistics systems, flexible manufacturing systems (agile manufacturing systems) and all non-standard with service systems, PLC can be used , and then model and take countermeasures and optimize. The future of PLC is beautiful, and all pessimistic arguments are untenable. As for technological advancement, PLC and other technologies merge and disappear, it will take some time!
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