1. Overview
In the field of industrial automation, PLC and industrial computer are two types of control equipment that people have to mention. They are the basic equipment of most automation systems. The latest technological development of PLC and industrial PC is the perfect embodiment of engineers' requirements for equipment application performance: the hardware standardization of the controller, and the various control requirements of users are changed through software.
PLC is a general and easy-to-use device designed for sequential control using computer principles. It uses specially designed hardware, and the performance is determined by the control program.
Industrial computer is a convenient control device that uses the PCI bus and PC/104 bus of personal computers and uses functional boards to expand control I/O points to achieve computer control. It has industrial field application characteristics and greatly utilizes the software environment of PCs. Users can easily choose products provided by various manufacturers.
The current technical development level of PLC and industrial computer has greatly exceeded the technical level when they appeared, and each is positioned at different levels. PLC is suitable for low-cost automation projects and as an I/O station for large 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 industrial automation in China will reach RMB 17 billion to 20.7 billion in 2000, of which industrial computers account for about RMB 4 billion and PLCs account for about RMB 3 billion. In addition, PLCs and industrial computers used in DCS systems, FCS systems and NCC systems (accounting for RMB 6 billion) play an important role in the field of industrial automation. These two types of control equipment play an indispensable role in the automation of various fields. For example, the industrial control system of the water plant uses PLC as the I/O control station and industrial computers as the monitoring machine.
In the course of their technological development, PLCs and industrial computers have actively developed technical performance in terms of high reliability, networking and high performance of user-developed software in order to meet the needs of industrial field applications and the needs of user secondary development. The following will focus on the current status and development trends of PLC and industrial computers in terms of hardware, software and network technology applications.
2. Current status and development of hardware technology of PLC and industrial computers
The end users of PLC and industrial computers are metallurgy, mining, cement, petroleum, chemical industry, electricity, machinery manufacturing, automobile, loading and unloading, papermaking, textile, environmental protection and other industries. Their main uses are:
1. Sequential control
Sequential control is the most widely used field. It includes single-machine control, multi-machine group control, automatic production line control, such as injection molding machine, printing machine, bookbinding machine, paper cutting machine, modular machine tool, grinder, assembly machine, packaging production, electroplating assembly line and elevator control.
2. Motion control
Applied in single-axis or multi-axis position control of electric traction system or servo motor.
3. Process control
The analog module can control physical parameters such as temperature, pressure, speed and flow, and provide closed-loop control functions such as PID.
4. Data processing
It can support the control and management of CNC machine tools, multi-axis control, etc.
Due to the increasing requirements of automation systems, traditional PLCs and industrial computers that provide I/O point services can no longer meet complex process requirements. Therefore, PLC and industrial computers have undergone fundamental changes in hardware systems.
The technical development of PLC systems in modules includes:
1. Processor module
Equipped with large-capacity memory, it is optimized to meet the requirements of real-time control. In addition to general I/O scanning and control, remote data exchange, it supports large-scale integrated control, communication, parallel computing, processor independent background programs and processor input interrupts. For example, the ControlLogix processor module of A-B Company has communication functions designed in its core, and with the help of its passive data bus, the bottleneck of the system is eliminated. This flexible structure allows multiple processors, networks and I/O to be used in a rack without restrictions.
2. Information coprocessor module
Read the data table and status file of the main processor, or write data to the main processor through a high-level language program. The program can run alone in the coprocessor in a real-time multi-tasking environment and independently of the PLC processor.
3. High-level language coprocessor
Complex calculations and algorithm implementations are carried out through C and Basic interfaces.
4. Network adapter module
Provide a communication interface between the field bus and the processor so that the PLC processor and the I/O module can exchange data remotely.
5. I/O modules with special functions
For example, A-B Company provides intelligent transmitter modules, temperature control modules, weighing modules, open-loop speed control modules, plastic manufacturing modules, torque control modules, absolute encoding modules, configurable flowmeter modules, current synchronization modules, etc. in its products. The design of these modules takes into account the needs of special industries, so that complex control functions can be solved in a modular way, improving reliability and professional level.
Similarly, industrial computers have also developed rapidly based on I/O boards. The revolutionary development of large-scale integrated circuits and computers themselves has provided a stage for industrial computers. In addition to a full range of I/O boards, the industrial computer series products have also developed integrated workstations, industrial computers with electronic disks, remote RTUs, micro industrial computers suitable for the instrument industry, industrial computers suitable for the video and multimedia industry, industrial computers with monitoring LCD screens suitable for the communication industry, special industrial computers integrated with PLCs, and explosion-proof industrial computers.
The application environment of industrial sites requires PLC and industrial computers to have high reliability, and reliability is guaranteed by electromagnetic compatibility (EMC) and fault-tolerant technology. PLC and industrial computers must undergo strict electromagnetic compatibility testing, such as radiation sensitivity testing, harmonic/voltage fluctuation/voltage sag testing, static electricity/fast pulse/lightning detection, electromagnetic interference detection, etc. EMC ensures the inherent anti-interference characteristics of the equipment. However, it is impossible to ensure that the control equipment does not fail. Therefore, a system with fault-tolerant design is very important for high-reliability systems that cannot be shut down or out of control. At present, important fault-tolerant design technologies include Watchdog and dual-machine hot standby (including hot standby of host, module and communication medium). The work of the hot standby system is transparent to the user: that is, when a fault occurs, all fault point removal and data backup are automatically completed within the shortest control cycle. The completion of this technology includes two aspects: equipment hardware and software. Figure 2 shows the structure of the PLC dual-machine redundant system.
III. Software characteristics of PLC and industrial computers
PLC, industrial computers and fieldbus provide a good underlying control hardware foundation for industrial automation. With the rapid development of software technology, 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, database, control logic operation from the bottom to the high-level man-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 structure of the manufacturing industry, which includes the main functions of enterprise resource planning (ERP) and manufacturing resource planning (MRP), control, human-machine interface and data acquisition and monitoring (SCADA), manufacturing execution system (MES), batch control and equipment interface, etc., but they also increase user costs and the difficulty of sharing data.
Therefore, in 1998, Microsoft introduced the concept of Windows distributed interconnected network application, referred to as Windows DNA. It is a structure that seamlessly connects various business application software within the enterprise. This structure enables software developers in the manufacturing industry to develop strong products with multiple features, and enables end users to integrate their independent manufacturing and business software applications into a digital nervous system.
Control equipment manufacturers such as Rockwell Automation are bringing Windows DNA architecture to manufacturing through their products and collaboration with Microsoft. These companies have their own strategic frameworks for manufacturing software development, such as Rockwell Internet Application Architecture, or RNA, which closely parallels the DNA architecture defined by Microsoft for the enterprise.
Microsoft introduced DNA architecture in 1998, building on Microsoft's Windows NT and Windows 9X operating systems and the user-accepted Microsoft Office suite and Microsoft BackOffice tools. Windows DNA was immediately adopted as a strategic architecture for connecting multiple products in a single enterprise distributed system.
DNA encompasses traditional computer systems and allows for the composition of a variable structure of client/server systems that can improve technology levels and application innovation. Key elements of Windows DNA include tools, integrated storage, business processes, user interface and navigation. In more detail, DNA has the following characteristics:
Common operating system and network;
Common data access;
BackOffice tools and services;
Common application infrastructure;
Integration with Microsoft Office applications;
Powerful development tools;
Inheritable system integration tools;
As defined by Microsoft, Windows DNA for manufacturing includes ERP/MRP, control (whether distributed, PC-based or PLC), HMI and SCADA, MES, batch processing and device interface. Without manufacturing DNA, the gap between these application functions can only be filled by user interface and driver 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 with the upper automation pc system in its version of CIM. However, it is only now that the core technologies of software and hardware to realize the above concepts have matured. They include business applications such as data acquisition and monitoring, human-machine interface (HMI), batch control (Batch), MES, ERP interface, PC bus-based control and communication.
4. Network characteristics of PLC and industrial computers
Fieldbus technology is one of the most profound changes in industrial automation. PLC and industrial panel pc can be easily connected to the DCS system as I/O stations and monitoring stations after adopting fieldbus. Fieldbus is a two-way digital communication technology that replaces the 4-20mA standard and is used to connect intelligent field devices and control devices. Fieldbus is open and interoperable, which allows some control functions to be moved down to field devices.
