In the field of industrial automation, programmable controllers and industrial computers are two types of control devices that people have to mention, which are the basic equipment of most automation systems. The latest technological developments of programmable controllers and industrial PCs are the perfect embodiment of engineers' performance requirements for equipment applications: the hardware standardization of controllers and the various control requirements of users are changed by software.
1 Overview
The programmable controller is a universal, easy-to-use device specially designed for sequential control using computer principles. It uses specially designed hardware, and performance is determined by the control program.
The industrial computer is a convenient control device that utilizes the PCI bus and PC/104 bus of the personal computer and uses the function board to expand and control the I/O point to realize computer control. It has industrial field application features, while greatly utilizing the software environment of the PC, users can easily select the products provided by each manufacturer.
The current state of the art of PLC and industrial computer has greatly exceeded the technical level of its emergence, and each is positioned at a different level. PLC is suitable for low-cost automation projects and I/O stations as large-scale DCS systems. Industrial computer has a good performance-price ratio in medium-scale and small-scale automation projects. Of course, this positioning is not absolute. It is predicted that the market size of China's industrial automation in 2000 will reach 17 billion to 20.7 billion yuan, of which industrial control machines account for about 4 billion, PLCs account for about 3 billion, plus DCS systems, FCS systems and NCC systems (6 billion). The PLC and industrial computer used can be seen to play a pivotal role in the field of industrial automation. These two types of control devices play an indispensable role in the automation of various fields. Figure 1 shows a typical structure diagram of an industrial control system of a water plant, in which PLC is used as an I/O control station and an industrial computer as a monitoring machine.
Figure 1 Typical structure of the industrial control system
Currently, mainstream manufacturers of programmable controllers include AB, Siemens, and Modicon. The mainstream manufacturers of IPCs include Shanghai Contec, Beijing Kangtuo and Advantech, and Axiom.
In the course of its technological development, PLC and industrial computer are actively developing the technical performance of high reliability, networked and high performance user development software in order to meet the needs of industrial field applications and the needs of secondary development of users. The following focuses on the current status and development trends of PLC and IPC technology applications in hardware, software and networks.
2 PLC and industrial computer hardware technology status and development
The end users of PLC and industrial computer are metallurgy, mining, cement, petroleum, chemical, electric power, machinery manufacturing, automobile, loading and unloading, papermaking, textile, environmental protection and other industries. Their main purposes are:
· Sequential control sequence control is the most widely used field, including stand-alone control, multi-group control, automatic line control, such as injection molding machines, printing machinery, bookbinding machinery, paper cutting machines, combination machine tools, grinding machines, assembly machinery, packaging production. , plating line and elevator control.
· Motion Control Apply to single or multi-axis position control of electric drive systems or servo motors.
· Process Control Analog modules can be used to control physical parameters such as temperature, pressure, speed and flow, and provide closed-loop control such as PID.
· Data processing Supports control and management of CNC machine tools, multi-axis control, etc.
Due to the increasing demands of automation systems, traditional PLCs and industrial PCs that provide I/O point services have been unable to meet complex process requirements. Therefore, PLC and industrial computer have fundamental changes in the hardware system.
The technical development of the PLC system on the module is:
· The processor module is equipped with large-capacity memory, optimized for real-time control requirements. In addition to general I/O scanning and control, remote data exchange, it supports large-scale integrated control, communication, parallel computing, and processor independent background programs. And functions such as processor input interrupts. For example, AB's ControlLogix processor module is designed with communication functions in its core. With its passive data bus, the system bottleneck is eliminated. This flexible architecture allows multiple processors, networks, and I/O to be used in a single rack without restrictions.
· The information coprocessor module reads the data table and status file of the main processor, or writes the data to the main processor through a high-level language program. The program can be used in a real-time multitasking environment and a method independent of the PLC processor. Running in the coprocessor.
· High-level language coprocessors Compute complex computations and algorithm implementations through C and Basic interfaces.
· Network Adaptation Module Provides a communication interface between the fieldbus and the processor for remote data exchange between the PLC processor and the I/O module.
· I/O modules with special functions, such as AB, provide intelligent transmitter module, temperature control module, weighing module, open loop speed control module, plastic manufacturing module, torque control module, absolute coding module, Configurable flowmeter modules, current synchronization modules, etc. The design of these modules takes into account the needs of specific industries, allowing complex control functions to be solved in a modular way, improving reliability and professionalism.
Similarly, IPCs are also rapidly advancing on the basis of I/O boards. The revolutionary development of large-scale integrated circuits and computers has provided a stage for industrial PCs. In addition to the full range of I/O boards, the industrial computer series has also developed integrated workstations, industrial computers with electronic disks, remote RTUs, micro-computers for the instrument industry, and industrial PCs for the video and multimedia industries. It is suitable for industrial computer with monitoring LCD screen, special industrial computer with PLC and explosion-proof industrial computer.
The application environment of the industrial field requires high reliability of PLC and industrial computer, and reliability is guaranteed by electromagnetic compatibility (EMC) and fault tolerance technology. PLC and industrial computer must undergo strict electromagnetic compatibility testing, such as radiation sensitivity detection, harmonic / voltage fluctuation / voltage dip detection, electrostatic / fast pulse / lightning strike detection, electromagnetic interference detection. EMC guarantees the inherent anti-jamming properties of the device. However, it is impossible to ensure that the control device does not malfunction. Therefore, systems with fault-tolerant design are important for highly reliable systems that require no downtime and cannot be out of control. At present, the most important fault-tolerant design techniques are Watchdog and dual-system hot standby (including hot standby of host, module and communication media). The operation of the hot standby system is transparent to the user: that is, when a fault occurs, all the removal of the fault point and the backup of the data are automatically completed in the shortest control period. The completion of this technology includes two aspects of equipment hardware and software. Figure 2 shows the structure of a PLC two-machine redundant system.
Figure 2 PLC dual-machine redundancy system structure
3 PLC and industrial computer software features
PLC, industrial computer and field bus provide a good foundation for hardware control of industrial automation. With the rapid development of software technology, the control software is no longer a monotonous digital and menu operation interface, but a graphical human-computer interaction tool. Good industrial control software includes data acquisition from the underlying, database, control logic operations to the high-level human-machine graphical interface (MMI). Here, we introduce the characteristics that PLC and industrial computer application software should have from the perspective of industrial control software architecture rather than individual software functions.
Microsoft has proposed a powerful concept for the distributed network architecture of manufacturing, including the main functions, control, human-machine interface and data acquisition and monitoring (SCADA) of enterprise resource planning (ERP) and manufacturing resource planning (MRP). Manufacturing Execution Systems (MES), batch control and device interfaces, but they also increase user costs and the difficulty of sharing data.
Therefore, in 1998, Microsoft introduced the concept of Windows Distributed Interconnect Network Application, referred to as Windows DNA. It is a structure that seamlessly connects various business applications within an enterprise. This architecture enables manufacturing software developers to develop robust, multi-featured products and enable end users to integrate their separate manufacturing and commercial software applications into a digital nervous system.
Various control equipment manufacturers, such as Rockwell Automation, are introducing Windows DNA structures into manufacturing through their products and cooperation with Microsoft. These companies have their own strategic framework in the manufacturing software development process, such as the Rockwell Internet Application Architecture, or RNA, which is closely related to Microsoft's proposed DNA structure for the enterprise.
Based on Microsoft's Windows NT and Windows 9X operating systems and user-accepted Microsoft Office suites and Microsoft BackOffice tools, Microsoft introduced the DNA structure in 1998. Windows DNA was immediately adopted as a policy structure for connecting multiple products in a single enterprise distributed large system.
DNA encompasses traditional computer systems and allows for the construction of a variable-structured Client/Sever system that increases the level of technology and application innovation. Key elements of Windows DNA include tools, integrated storage, business processes, user interfaces, and navigation.
In more detail, DNA has the following characteristics:
· public operating systems and networks;
· public data access;
· BackOffice tools and services;
· Public application infrastructure;
· Integration with Microsoft Office applications;
· Strong development tools;
· heritable system integration tools;
As defined by Microsoft, Windows DNA for manufacturing includes enterprise resource planning/manufacturing resource planning, control (whether it is distributed, PC-based or PLC), human machine interface and SCADA, manufacturing execution systems, batch processing and equipment. interface. Without manufacturing DNA, the gap between these application functions can only be filled by user interfaces and drive devices. This will increase the cost of the owner's system and make it difficult to share data due to data format issues.
As early as the mid-1980s, Rockwell Automation first identified the need to connect the factory floor to the host computer system in its CIM version. However, the core technologies of software and hardware that have realized the above concepts are mature, including commercial applications such as data acquisition and monitoring, human machine interface (HMI), batch control (Batch), MES, ERP interface, PC-based bus. Control and communication.
Taking the RNA structure as an example, it includes:
(1) Operating system platform It includes the human-machine interface and programming software from the earliest Win3.1 environment to develop a full-featured human-machine interface for Windows NT. With the advent of Windows CE, some companies are planning to offer a full set of CE-based PLC programming, human machine interface and control software.
(2) COM and DCOM
Building COM (Component Object Model) and DCOM (Distributed Component Object Model) make products interoperable, scalable, and flexible. The combination of COM and other industrial control software products has increased the ability to provide integrated package software (ProcessPak and ControlPak), and also increased the modularity of each independent software and promoted the integration of a company's products with other manufacturers' products. For example, Rockwell Software RS SQL is a data entry and transfer processing system that uses COM technology to provide a two-way link between the connection control system and the enterprise database system. It is a version that supports the manufacturing DNA database environment.
(3) Application of Visual Basic Microsoft believes that VB is a key technology for internal integration within the DNA framework. VBA offers three major benefits to the DNA structure. First, it provides end users with a standard way to build their own real-world applications without having to learn a specific language. Second, VBA provides a way to share data or integrate functionality across multiple applications. For example, it allows human interface software to take advantage of Microsoft's Excel spreadsheet capabilities, a system model company's product with computing capabilities and production planning capabilities; and finally, end users such as integrating embedded VBA products into DNA-based products. Manufacturing Information Systems, which enable engineers in the industry to use VB as a rapid development tool for their software products.
(4) ActiveX
ActiveX technology is becoming more and more widely used, and many vendors have supported ActiveX. Rockwell Software first introduced ActiveX technology for manufacturing applications and was the first licensed software developer to embed VBA into its core products.
(5) OPC-Process Control OLE
DDE is a technology developed by Microsoft for front-office applications that shares a small amount of relatively constant data. Unlike DDE, Process Control OLE technology is designed for the control performance, structure, and reliability of manufacturing environments. Microsoft's manufacturing DNA identifies openness and interoperability interface features (OPCs) between devices, control applications, and commercial applications. Rockwell is the OPC organization's developer and main developer. OPC is responsible for establishing COM-based application specifications that allow all applications to access data using the same COM-based interface, simplifying data collection and sharing methods.
Following the above requirements using many technologies for manufacturing DNA, Rockwell also offers integrated software packages for enterprises and industries, such as RSBatch for batch control, RSView32, RSLogix, etc., which can be used to program functional blocks. They also leverage COM integration, VBA and OPC to provide interoperability between applications and other layers of the enterprise. The control software package also provides the same interoperability for controlling I/O, PC bus control, programming, VBA and COM objects.
The highest realm of manufacturing information technology is the idea of ​​seamless connectivity, effective investment, flexible scale, and future development, and sharing data collected from the bottom of the plant within the enterprise. RNA technology ensures that its users use their existing software to implement a DNA-based environment. Only when companies can get enough information to make better decisions, combine their supply channels and operations, can their entire system be closer to users. Although computer hardware and network technology have been greatly developed, there is still much work to be done in the software field.
4 Network characteristics of PLC and industrial computer
Fieldbus technology is one of the most profound changes in industrial automation. The PLC and the industrial computer can be conveniently connected to the DCS system as an I/O station and a monitoring station after using the field bus. Fieldbus is a two-way digital communication technology that replaces the 4-20 mA standard for connecting intelligent field devices and control devices. The fieldbus is open and interoperable, which allows some control functions to be moved down to the field devices.
Fieldbus can also handle complex things: intrinsically safe, dangerous situations, multivariable processes, and regulatoryly demanding environments. The use of fieldbus by PLCs and industrial PCs has the following benefits:
(1) Interoperability Interoperability is defined as the ability of multiple devices from different manufacturers to operate in a single system without any loss of functionality. Interoperability allows plant engineers to integrate different vendors in the same fieldbus. The equipment also allows engineers to replace a fieldbus device with equipment from another manufacturer.
(2) Lower installation costs Fieldbus will greatly reduce the initial cost of the plant, including simplified wiring, fewer terminal blocks, and simplified instrument selection. In addition, the reduced costs include simplified drawings, simplified engineering implementation of the control system, and reduced costs for wiring, wiring cabinets and junction boxes. By transferring control functions to the field, the cost of the control system is also reduced: the space required for DCS is reduced, the number of I/O points in the control system is reduced, and the cabinet space and I/O terminal block are reduced. Fieldbus will provide online remote digital communications for faster commissioning, faster diagnostics, and lower labor costs for installing and commissioning equipment.
(3) Lower maintenance costs Fieldbus will expand the operator's view of the entire process, so that maintenance and process management can be easily and efficiently performed to speed up the process of finding and eliminating faults. Fieldbus enables online diagnostics for easier calibration and predictive maintenance.
(4) Improved performance As control functions are transferred from DCS to separate control loops, system integration and reliability are increased due to faster and more accurate local control. Fieldbus can also take advantage of the benefits of multi-sensor devices because multiple messages are allowed to flow over a pair of wires.
Because fieldbus makes it easier to get device information from the field, plant operators and managers can more tightly control their processes to improve performance and increase process availability and consistency.
Fieldbus is not only a signal communication protocol, but also a new way to control the process. At present, the mainstream fieldbus protocols used in PLC and industrial control are Siemens ProfiBus, Rockwell Automation's ControlNet and DeviceNet, and FOUNDATION fieldbus FF. The following describes the network architecture and communication technology in industrial automation using ControlNet and DeviceNet as examples.
ControlNet is a new real-time fieldbus network for control layer that has been introduced in recent years to provide time-critical I/O data and message data on the same physical media link, including program upload/download. Communication data such as configuration data and peer-to-peer messaging is a highly deterministic, repeatable high-speed control and data acquisition network. I/O performance and end-to-end communication performance are greatly improved compared with traditional networks.
ControlNet is a Producer/Consumer-based network that allows multiple host controllers to coexist on the same link, supporting the transmission of incoming data or point-to-point information, greatly reducing traffic on the network. Improve network efficiency and network performance.
ControlNet is a highly deterministic, repeatable network. The so-called certainty is the ability to predict when data can be reliably transmitted to the target, and the repeatability refers to the ability of the data transmission time to remain constant regardless of the network node add/delete or network busy condition. In practical applications, the selective I/O grouping or interlocking time is set by the network configuration, and these requirements can be further guaranteed. Therefore, ControlNet is very suitable for applications where the control relationship has complex correlations, requiring control information synchronization, coordinated real-time control, and high data transmission speed requirements. Such as the cooperative drive system, welding control, motion control, vision system, complex batch control, process control system with a large number of data transmission requirements, system with multiple controllers and human-machine interface coexistence. ControlNet is ideal for applications where there are multiple PC-based controllers, between different PLCs or between PLC and DCS. ControlNet allows multiple controllers that each have their own independent or shared I/O to communicate with each other or in a flexible interlocking manner. Due to its outstanding real-time, deterministic, and optional intrinsic safety features, it is increasingly used in applications requiring high requirements such as process control.
ControlNet is an open fieldbus. By the end of 1999, it has owned nearly 70 well-known manufacturers including Rockwell Autom-ation, ABB and Honeywell. It is an independent international organization, ControlNet International. Responsible for management, the organization aims to maintain and distribute ControlNet technical specifications and manage the common marketing efforts of member units. At the same time, it provides consistency and interoperability testing services between various vendors' products to ensure the openness of ControlNet.
Control network communication is based on a new communication model: producer/consumer communication mode. Industrial control requires control networks to provide ever-increasing productivity, higher system performance, while providing deterministic, repeatable, and measurable inter-device communication. Simply increasing the baud rate or simply improving the efficiency of the protocol cannot solve the problem fundamentally. The traditional network communication model is a source/destination type or a point-to-point communication method. The advantage of this method is that the content and form of the communication are very clear, and the transmitted source message contains explicit source and address information. However, in the source/destination network mode, when data on the same data source is sent to other nodes on the network, it must be implemented multiple times, which greatly increases the burden on the network and reduces the efficiency of communication. In addition, since the time when data arrives at different network nodes may vary due to the number of nodes on the network, synchronization between different nodes becomes difficult, and the real-time communication cannot be guaranteed.
Unlike previous communication modes, the producer/consumer model allows different nodes on the network to simultaneously access data from the same source. In the producer/consumer mode, data is assigned a unique identifier. According to the specific identifier, multiple different nodes on the network can receive data from the same sender. As a result, the data transmission is more economical. Each data source sends the data to the network at one time. Other nodes selectively collect the data, without wasting bandwidth, improving system productivity, improving communication efficiency, and only need to generate data once, no matter how many nodes need to receive this data. Data is transferred to different nodes at the same time, enabling precise synchronization of communications.
DeviceNet is a fieldbus for connecting industrial equipment (such as limit switches, photoelectric sensors, valves, motor starters, process sensors, bar code readers, inverters, on-screen displays and operator interfaces) to the network. It is characterized by low cost, easy development, support for many manufacturers and does not require expensive connection lines.
DeviceNet is an open protocol originally developed by Rockwell Automation. Currently, DeviceNet technology belongs to the Open DeviceNet Vendor Association and is owned and promoted by the Open DeviceNet Vendor Associaiton. ODVA has more than 300 members of the world's leading automation equipment manufacturers (such as Rockwell Automation, ABB, Omron). China's ODVA organization was led by the Shanghai Electrical Apparatus Research Institute and is actively promoting the technology. As long as the equipment manufacturer participates in the ODVA organization, it can get the relevant DeviceNet protocol standards, and can also join the product discussion group to get help with product development. The development of DeviceNet products is not complicated. Device DeviceNet is a CAN based technology. As a serial communication technology, CAN was developed and rapidly developed in the mid-to-late 1980s to meet the requirements of automotive control networking, and has become an open international standard communication protocol (ISO 11898), including many in industrial automation. The field has been widely used.
DeviceNet was introduced in 1994 and became an open agreement in 1995. It was promoted by ODVA and has been widely used in North America and Asia Pacific. The DeviceNet network media can be a five- Wire cable that includes two signal lines, two 24V power lines, and one shielded line. DeviceNet devices can receive power directly from the network and connect and disconnect devices online from the network.
The application advantages of DeviceNet are:
· It is an open communication standard that supports interoperability of products from multiple manufacturers;
· Can be installed quickly and easily;
· A design that considers the future and can be easily added to the needs of expansion and change;
· Improve operational efficiency by intelligent mechanisms inside the device;
· Provide efficient bandwidth through data generation source/consumer source communication structures;
· The device can be configured and added to the device on-line continuously.
The development of fieldbus network and intelligent equipment and instrumentation will inevitably affect the architecture of DCS. An obvious trend that can be seen now is the further decentralization of DCS. The traditional DCS is still a centralized structure at the I/O control station level. Some systems make the scale of the I/O control station large due to cost or other considerations. This consideration includes: the price of high-performance CPU has dropped very low, in order to give full play to the CPU's ability, you can expand the number of points and loops of an I/O control station to reduce costs. However, this design increases the concentration of danger. If the redundancy measures are added to improve reliability, the system cost will still rise, so it is not an ideal solution. From the current development trend, the use of fieldbus network and intelligent equipment, intelligent instrumentation, together with the general industrial computer can completely form a small DCS, which poses a challenge to the traditional DCS, because based on the field bus network DCS has many advantages, both in terms of system cost, reliability, ease of installation, maintenance, and scalability. So how will the traditional DCS develop to accept this challenge?
The answer is clear. Only the I/O control station is further decentralized. The fieldbus network technology is used to form a distributed control station based on the fieldbus network and composed of intelligent I/O modules. That is to say, the centralized I/O control station in the past DCS becomes a distributed control station, and a layer of on-site network is introduced in the lower layer of the traditional DCS network to form a device-level network, a control-level network and a management level. A three-layer network structure such as the network to meet the ever-increasing application requirements.
After the DCS formed the three-layer network structure, the basic control unit went deep into the equipment control level, and the DCS function extended to the management control level, gradually forming a more complete control and management integration architecture.
Figure 3 shows a three-layer network structure of a typical industrial automation system. The lower layer is the device layer that connects the remote I/O points of the device and PLC with DeviceNet fieldbus. The middle is ControlNet (or DH+, DH485). Etc.) The control layer network that connects the PLC, the industrial computer, and the operator interface, while the upper Ethernet performs the management and information service tasks mainly on the PC or workstation. The three-tier network performs its duties and represents the typical structure of industrial control.
Figure 3 Network structure of a typical industrial automation system
5 Conclusion
With the rapid development of software and hardware technologies and network communication technologies of microcomputers, revolutionary technological progress has taken place in the field of industrial automation. PLC and industrial computer as the basis of industrial control equipment, through the continuous innovation of technology, the position in industrial control is increasingly strengthened, and become the main tool to achieve industrial control technology advancement.
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