The control system means that the machine, mechanism or others can be maintained and changed in the desired wayequipmentAny amount of interest or variability within.The control system is also designed to make the controlled object reach the presetidealState.The control system makes the controlled object tend to a desired stable state.
For example, suppose you have a cardriving system , the speed of the car is itsacceleratorFunction of position.The desired speed can be maintained (or the desired speed change can be achieved) by controlling the pressure of the accelerator pedal.thisAutomotive drive system(acceleratorCarburetorAnd engine vehicles) form a controlsystem。
classification
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Classification according to whether there is feedback
No feedback is calledOpen loop control system(open loop control system), the input of this system is directly supplied to the controller, and the controller controls the controlled object.Its main advantages are simple structure, low price and easy maintenance;The disadvantage is low precision and easy to be interfered by environmental changes (such as power fluctuations, temperature changes, etc.).
The closed loop control system is called when there is feedback. The difference between the input and the feedback signal (i.e. deviation signal) is added to the controller, and then the output of the controlled object is adjusted to form a closed loop control loop.Therefore, closed-loop system is also called feedback control system, and this feedback is called negative feedback.Compared with the open loop system, the closed loop system has outstanding advantages, including high precision, good dynamic performance, strong anti-interference ability, etc.Its disadvantages are complex structure, expensive price and high requirements for maintenance personnel.
Classified according to the signal processing technology used
Analog control system, the control system that uses analog technology to process signals is called analog control system.
Digital control system refers to the control system that uses digital technology to process signals.
Classification according to whether the input quantity is constant
The input is constant. This control system is generally called constant value control system, such as constant speed motor, constant temperature hot stove, etc.
The output changes with the input, and this control system is called a servo system.For example, the missile automatic aiming system.
working principle
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Detect the actual value of output quantity (controlled quantity);Compare the actual value of output quantity with the given value (input quantity) to obtain the deviation;
The deviation value is used to generate control and regulation function to eliminate the deviation, so that the output can maintain the expected output.
performance requirement
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In order to realize the basic task of automatic control, it is necessary to put forward requirements for the behavior of the system in the control process.The basic requirements for the control system are usually met by the response of the system to specific input signals.For example, it is expressed by the transition process of unit step signal and some eigenvalues of steady state.On the premise of ensuring stability, the system shall have good dynamic and steady performance, namely:
Control system has been widely used in various fields of human society.In industry, various physical quantities encountered in the production process of metallurgy, chemical industry, machinery manufacturing, etc., includingtemperature、flow、pressure、thickness、tension、speed、position、frequency、phaseThere are corresponding control systems.On this basis, by adoptingdigital computerIt has also established a digital control system with better control performance and higher degree of automation, and a process control system with dual functions of control and management.Applications in agriculture include automatic water level control system, automatic operation system of agricultural machinery, etc.
military technique
The application examples of automatic control include various types of servo systems, fire control systems, guidance and control systems, etc.In aerospace, aviation and navigation, in addition to various forms of control systems, the application fields also include navigation systems, remote control systems and varioussimulator 。
other
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stayOffice Automation, book managementtraffic controlEven in daily household affairs, automatic control technology has practical applications.With the development of control theory and control technology, the application field of automatic control system is still expanding, almost involvingBiology、Medical Science、ecology、Economics、SociologyAll fields.
Distributed control
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development history
In fact, the control system has been used since the 1940s. The early field based instruments and the later relays formed the predecessor of the control system.withPLCandDCSAs a representative, since its application in 70 years, it has achieved rapid development in industrial process control in metallurgy, electric power, petroleum, chemical industry, light industry, etc.Since the beginning of the 1990s, fieldbus control systems and PC based control systems have emerged in succession. This paper will briefly introduce various common control systems, and analyze the evolution process and development direction of the control system.
In the mid-1970s, due to large-scale equipment, high requirements for continuity of process flow, increased process parameters to be controlled, harsh conditions, centralized display and operation requirements, the popular electric unit combination instrument could not fully meet the requirements.In this case, the manufacturers in the industry determine the development ofDCSThe products shall be mainly based on analog feedback control, supplemented by switching value sequence control and analog switching value mixed batch control, which can cover most industries such as oil refining, petrochemical, chemical, metallurgical, electric power, light industry and municipal engineering.
Before and after 1975Direct digital controlDevice（DDC）Based on the automatic control and computer technology ofdistributed control system （DCS）。Because computers were not popular at that time, it should be emphasized that users can use DCS without understanding computers when developing DCS;At the same time, the development of DCS should also emphasize the provision of the entire system to users.In addition, the developed DCS shall have the same technical conditions as the conventional instruments in the central control room to ensure reliability and safety.
In the next 30 years, DCS will be first matched with complete sets of equipment, and then gradually expanded to process unit transformation. At the same time, it will also be divided into large DCS and small and medium-sized DCS products, making their performance price ratio more competitive.Although there is not much breakthrough in the principle of DCS products, there are three generations of DCS products due to technological progress, changes in the external environment and changes in demand.It was the first generation product from 1975 to the early 1980s, the second generation product from the mid 1980s to the early 1990s, and the third generation product from the mid 1990s to the early 21st century.
control station
In DCS system, the control station is a complete computer, and its main I/O equipment is field input and output processing equipment, as well as process input/output (PI/O), including signal transformation andSignal conditioning，A/D、D/Atransformation.The control station is the foundation of the whole DCS, and its reliability and safety are the most important. The phenomenon of crash and control failure is absolutely not allowed, and redundancy, power failure protectionanti-interferenceThe explosion-proof system should be effective and reliable to meet user requirements.
About DCS control stationsystems software, includingreal-time operating system , programming language andCompiling system, database system, self diagnosis system, etc.Second generation DCScontrol stationAt firstProcess orientedLanguage and high-level language;The system software of the third generation DCS control station can be completedOffline configurationAnd online modification of control strategy.In order to complete the control strategy, for sequence control and batch controlconfigurationProgramming, various DCS control stations adopt different methods.
DCS operation station
DCS operation station has operator function, engineer function, communication function and high-level language function, among which engineer function includes systemconfigurationSystem maintenance, system general（Utility）Functions include system configuration, operation marking, trend recording, historical data management, general picture configuration, control station configuration, process unit or area configuration, etc.
Control system
Actual DCSOperation stationIs a typical computer, which is similar tocontrol stationDifferent, it has rich peripheral equipment and human-computer interface.In terms of human-computer interface, it gradually transited to GUIgraphical user interfaceIt is a platform and uses a mouse. During configuration, the flow chart and control circuit diagram are made using menus, windows, etc., so that the man-machine interface is friendly.The third generation DCS operation station ispersonal computer（PC）AndWindowsThe popularity of the operating system and the general monitoring graphic software have been born on the basis of commercialization.DDEorOPCInterface technology,EthernetThe interface is connected to the management network.DCS system configuration, operation station configuration and control station configuration are all equipped with corresponding software to provide engineering designers of DCS users with human-computer interfaces.Some DCS adopt general monitoring graphic software, or take such software as the core for secondary development.
Data communication and network
Because data communication standards involve network structure, communication medium (channel)communication protocol、IEEE802.4Token bus Transmission mode and IEEE802.5Token Ring NetworkThe communication protocol of transmission mode is the most widely used in DCS system. Whether it can become the communication standard of DCS in the future remains to be seen.
Development issues
DCSThe existing problems mainly focus on three aspects:
1. That is, system openness;
2. Wiring problems with field sensors, transmitters and actuators;
3. The price is expensive [1].
These problems have been solved in the third generation DCS.In the 21st century, the new generation of DCS should meet the needs of users.
In terms of the distribution of DCS application industries, such as the transformation of DCS products, it can maintain its position in these industries.
Programming control
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Birth and development of PLC
In 1968, the oil pressure department of General Motors established the firstProgrammable controllerTheir goal is to eliminate both complex and expensive relay control systems.This design specification requires solid state system and computer technology, and requires that it can survive in an industrial environment, be easily programmed, and be reused.By 1969, the firstPLCbe born.At that time, it was called programmable controller, which was called Programmable Controller in English, abbreviated as PC.Because the first generation PLC was designed to replace the relay, ladder diagram language was used as the programming method, forming the programming standard of the factory.These early controllers met the initial requirements and opened the door to the development of new control technologies.In a short time, PLC rapidly expanded tofood、Drinks、Metalworking、manufactureandpapermakingAnd other industries.
PLC is generally divided into micro PLC, small PLC, medium PLC, large PLC, PC card type PLC and PC compatible PLC according to the scale of I/O points carried by CPU.Various scale classification standards are shown in the attached table.
A typical PLC usually includes CPU module, power module and some input/output modules, which are inserted on a backplane.If the configuration is increased, it may include an operator interface, monitoring computer, communication module, software and some optional special function modules.Programmable controllerIt is not only easy to install, small in space and energy consumption, but also has diagnostic indicators to help with fault diagnosis and can be reused in other projects.Although the functions of PLC, such as running speed, interface type and data processing ability, have been greatly improved, PLC has always maintained its original design principle, that is, simplicity first.
Today's PLC
PLC technology has been developing since its birth.These developments not only improved the design of PLC, but also changed the design concept of control system.In the past, PLC was suitable for discrete process control, such as switching, sequential action execution and other places. But with the growing function of PLC, PLC also began to enter the field of process automation.
Control system
PLC classification
PLC type appearance typical I/O point range typical application
Micro PLC fixed I/O point, brick type<32 point replacement relay, distributed I/O
Small PLC brick type, modular 33-128 point industrial machine switch control and commercial use
Medium PLC modular, small rack 129-512 point complex machine control and somedistributed system
PC card type PLC ISA or PCI bus card type>129 points machine control, monitoring system
PC compatible controller modular, large or small rack>129 points machine control, monitoring system
Hardware development of PLC
The hardware progress of PLC is listed below:
Coke dry quenching control system
·Adopt new and advancedmicroprocessorAnd electronic technology to achieve fast scanning time·Small and low-cost PLC can replace four to ten relays;
·High density I/O system provides space saving interface at low cost;
·Microprocessor based intelligent I/O interface extends the distributed control capability. Typical interfaces are as followsPID，network，CAN bus，field bus ，ASCIIsignal communication,location, host communication module and language module (such asBASICPASCALC), etc;
·Including the improvement of the structure design of input and output modules and terminals to make the terminals more integrated;
·Special interface allows some devices to be directly connected to the controller, such as thermocouple, thermal resistance, stress measurement, fast response pulse, etc;
·External equipment has improved the operator interface technology, and the system document function has become the standard function of PLC.
These hardware improvements have led to the enrichment and development of PLC product series, from the smallest micro PLC with only 10 I/O points to the large PLC with 8000 points.These product families use common I/O systems and programmingperipheral, can form a local area network, andOffice networkto be connected.The whole PLC product series concept is a very cost saving control system concept for users.
Software Development of PLC
Similar to the development of hardware, PLC software has also made great progress, greatly strengthening the functions of PLC:
·PLC introduces object-oriented programming tools, and forms a variety of languages according to the IEC 61131-3 standard of the International Electrotechnical Commission;
·Small PLC also provides powerful programming instructions, and therefore extends the application field;
·High level languages, such as BASIC and C, can already be implemented in some controller modules, providing greater programming flexibility in external communication and data processing;
·Ladder logic can realize advanced function block instructions, and users can use simple programming methods to realize complex software functions;
·The function of diagnosis and error detection extends from simple fault diagnosis of system controller to process and equipment diagnosis of controlled machines and equipment;
·Floating point arithmetic can carry out complex calculations involved in measurement, balance and statistics in control applications;
·Data processing instructions are simplified and improved, and complex control anddata acquisitionAnd processing functions.
Although PLCs are much more complex than before, they still maintain surprising simplicity. For operators, today's high-performance PLCs are as easy to operate as those 30 years ago, or even simpler.
Future development of PLC
The future development of PLC depends not only on the development of the product itself, but also on the integration of PLC with other control systems and plant management equipment.PLC is integrated into computer integrated manufacturing through network（CIM）In the system, their functions and resources areNumerical control technologyRobot technologyCAD/CAMTechnology, personal computer system, management information system and layered software system will play an important role in the future development of the factory.New PLC technology advances include better operator interface,graphical user interface（GUI），interfaceIt also includes interfaces with equipment, hardware and software, and supports artificial intelligence such as logical I/O systems.The software development will use widely used communication standards to provide the connection of different equipment. The new PLC instructions will be based on increasing the intelligence of PLC. The knowledge-based learning instructions will also be gradually introduced to increase the system capacity.It is certain that PLC will play an important role in the future factory automation. Control strategies will be intelligently distributed rather than centralized. Super PLC will be used in applications that require complex computing, network communication and monitoring of small PLC and machine controllers.
Common control system
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Fieldbus control system
Control system
1. Characteristics of Fieldbus
The outstanding feature of fieldbus is that it combines centralized and decentralized DCSDistributed controlThe structure becomes a new type of fully distributed structure, and the control functions are completely decentralized to the site, and the basic control functions are realized by the on-site intelligent devices themselves.The characteristics of fieldbus mainly include the following aspects:
The 4~20mA analog signal of traditional DCS is completely replaced by digital signal, and the signal is transmitted in two directions.A pair of twisted pairs or a cable can usually be connected to multiple devices, so the consumption of cables, terminals, trays and trays is greatly reduced.At the same time, the communication bus extends to the field sensors, transmitters, controllers and servo mechanisms. The operator can realize the online monitoring, diagnosis, calibration and parameter setting of the field equipment by the main control system in the control room, saving the hardware quantity and investment.
In terms of structure, the fieldbus has only two levels: field equipment and operation management station. The I/O control station of the traditional DCS is incorporated into the field intelligent equipment, and the I/O module is canceled. The field instruments are built-inmicroprocessorThe output result is directly sent to the adjacent control valve, which does not need to go through the main control system of the control room, and realizes complete dispersion in structure.
(3) The bus network system is open
Give the power of system integration to users, who can form systems of different scales with products from different suppliers according to their own needs and considerations.The field instruments of different manufacturers can be used to replace the field instruments of another manufacturer in case of failure.
2. Current Fieldbus Standards
Fieldbus technology is a network communication technology born in the late 1980s. After more than ten years of development, several representative fieldbus standards and several series of products have emerged in the world, including:
In the process of research and formulation of fieldbus standards, there have been many enterprise groups or organizations. Through constant competition, by 1994, two camps had basically formed internationally, led by ISP protocol, and the World FIP protocol was formulated jointly with 150 European companies.The two groups merged in 1994 to establish the Fieldbus Foundation (FF), which is committed to developing the internationally unified fieldbus protocol.FF protocol conforms to IEC1158-2 standard, also known as SP50 standard.
It is a field bus standard as a German national standard and a European national standard.Jointly launched by the Institute.It uses the OSI modelphysical layer、data link layer。Fieldbus message specification (FMS) type is only hiddenOSIThe third to sixth layers of the standard adoptapplication layer。
(3) LonWork (Local Operating Network) fieldbus
It was officially launched in 1990.It adopts all 7-layer protocols of the ISO/OSI model, and adoptsobject-orientedThe design method ofNetwork CommunicationsThe design is simplified to parameter setting, with the maximum transmission rate of 1.5Mbps and transmission distance of 2700m,Transmission mediumIt can be twisted pair, optical cable, radio frequency, infrared ray, power line, etc.
It is used for data communication between internal measurement and execution parts of the vehicle.The CAN structure model adopts the layer 1, 2 and 7 protocols of the ISO/OSI model, namelyphysical layer、data link layerandapplication layer。
In addition, it is worth mentioning that the Highway Addressable Remote Transducer (HART) protocol, the first one introduced, is compatible with 4~20mAanalog signalAnd modulating digital signals.Its digital communication adopts modulation/demodulation mode, which belongs to the analog systemdigital systemTransition products in the process of transformation, so they have strong competitiveness in the current transition period and have developed rapidly.
3. The Future of Fieldbus
The field bus master line will become a revolutionary leap in the development of industrial control, and the reports about field bus will emerge one after another. Among them, the focus of attention is whether there is a unified field bus standard in the world.
Different fieldbuses have different functions and have their own applications. It seems unrealistic to rely on one kind of fieldbus to win the world.Of course, on the other hand, quite a few fieldbuses are similar and very similar in technology. In the environment of survival of the fittest, similar buses will eventually become unified.However, fierce competition in commercial interests is the main reason for this problem. Automation manufacturers that have a great impact on fieldbus are unwilling to give up their own fieldbus because they want to maintain their own position and interests as much as possible;Therefore, in a long period of time in the future, the field bus standard debate will become more and more intense.
Open control system
Control system
1. Concept of open control system
For a long time, most of the control systems used by manufacturing and production enterprises are dedicated and closed architecture, and the hardware constituting the system is customized according to their own standards.Whether DCS, PLC or FCS, although they have the advantages of simple structure, mature technology, large batch of products and so on, their inherent shortcomings are increasingly exposed relative to the ever-changing production requirements.In many cases, when users want to expand or change functions, they must turn to the system provider. It is difficult to integrate special requirements into the control system.This not only increases the cost of manufacturing enterprises, but also becomes the "bottleneck" of control system upgrading.
Due to the increasingly fierce market competition, manufacturers are required to have strong market adaptability. This trend has led to the emergence of a new concept, that is, modular, reconfigurable, and extensible hardware and software systems, which is open control systems.This system provides the possibility for manufacturers to integrate their technology with the technology or products of any third party.
2. Open route of international control system manufacturers
According to the international authority American Automation Market Research CompanyARC(Automation Research Corp.) estimates that in Asia, the sales of open systems such as PC based control systems and Ethernet I/O modules are expected to increase by 145% in the past two years. Therefore, it can be seen that the growth rate of open systems in the control system market is far higher than that of traditional control systems.
Almost all manufacturers have realized that open systems will be the future of automation systems.The word "open" can be seen in the new product samples of any control system manufacturer.However, their openness is usually to implement local interconnection and communication by adding some interfaces to the upper layer of the original system, which is not really open.Not to mention the bottom layer of the controllerOperation stationAt this level, they have many difficulties in communicating with the outside world.This situation is not due to their inability to develop fully open systems, but their unwillingness and inability to do so due to their product strategy.Although the market growth rate of open control system is relatively high, manufacturers are unwilling to give up the existing market of dozens of times because the absolute value is still small.
But these companiesOpen systemThe market is not taking a stand by attitude.All companies are considering using existing product combinations to develop formal "open" systems.Siemens PCS7 and WinAC, for example, actually useIPCThe conceptual system consisting of S7 PLC typically shows that on the one hand, it does not want to lag behind others in the field of open systems, and on the other hand, it does not want to lose the complex complex of old users.Similarly, Rockwell AB's ControlLogic, Fisher rosemount's Delta-V, etc., all label the original dedicated system "open".Their goal is the same, that is, not to missOpen systemThis ship can sell the original system as many as possible.Instead, some relatively small emerging companies, such as Opto22, SoftPLC, Controlsoft in the United States and Transmitton in the United Kingdom, are fully committed to the development of truly open systems.These companies do not have the burden of the past, and the market target of the products they develop does not have the problem of grabbing their jobs, so they can promote without worries.Since their products fully consider openness, they will become a new force of open control systems in the future.
3. Problems in International Open System Research
First of all, the concept of open control system is not clear, and the platform problem of open control system has not been solved.Used by each systemArchitectureInconsistency, lack of compatibility and interchangeability between each other, so the software and hardware of each system do not have portability andInteroperability。
Secondly, the image likeWindows、UNIX、OS/2And other new operating systems.Software development ideas and technologies are backward, always at or belowStructured ProgrammingLevel.Not taking full advantage of object-orientedsoftware reuse New theories and technologies in software engineering, which are the key to the realization of open control systems.
In addition, the upgrading, updating, modification and maintenance of products still depend on the manufacturer, and the corresponding development tools and environment are not provided. Users cannot integrate their own ideas or products or those of any third party into the system.
PC-BASED control system
Historically, VME bus industrial controller has always been the preferred model for many embedded applications [6].In 1981, MostekMotorola、PhilipandSigneticsThe company inventedVMEBus, a new standard in 1996VME64(ANSI/VITA1-1994) Increase the bus data width to 64 bits, and the maximum data transmission speed is 80Mbps.The VME64x bus specification formulated by Force Computers has increased the bus speed to 320Mbps.vmebus IPCReal timeControl platform, most of which runreal-time operating system And provided by the OS manufacturerSoftware development toolsDevelop applications.The latest VME bus product has adopted a 500MHz Pentium III processor.
Because users want to use the same operating system and development tools as familiar desktop PCs, the direct application of open desktop PCs in industrial environments has resulted.In addition to VME bus industrial control computers, a series of PC based embedded industrial control computers compatible with ISA/PCI bus standards have been produced, among which the most representative are CompactPCI/PXI bus, AT96 bus, STD bus, STD32 bus, PC/104 and PC/104 Plus bus embedded industrial control computers.
With the evolution of Intel CPU and Microsoft DOS/WINDOWS architecture into a de facto standard (Wintel), the ISA/PCI bus ruggedized industrial PC (IPC) began to penetrate into the industrial field.However, although reinforcedIPCThe desktop computer based on the large motherboard has been industrialized, but its backplane technology still has many shortcomings: poor thermal design, poor connection mode, non-standard template size and limited number of PCI slots (maximum 4).So.
In June 1995, PCI SIG officially announcedPCILocal busSpecification version 2.1, and PICMG launched the first standard PCI/ISA passive backplaneBus standard。To apply the PCI bus specification of PCI SIG toindustrial control computer System, PCI November 1995Industrial computerThe Manufacturers' Federation (PICMG) has issued the CompactPCI specification version 1.0.Due to CPCI busIPCThe problem of reliability and maintainability is well solved, and the price of software and development tools based on Microsoft is relatively low, so CPCI industrial personal computer can quickly enter the embedded product market.However, compared with the PCI/ISA ruggedized IPC, the CompactPCI IPC has not been actually applied in the industrial process control field, but has been widely used in the telecommunications market, due to the high overall cost, great difficulty in technology development, and the incomplete and unified definition of the passive backplane, resulting in poor template matching and high requirements for electromagnetic compatibility design.
electric control system
The electrical control system is generally called the secondary control loop of electrical equipment. Different equipment has different control loops, mainly including the control mode of high-voltage electrical equipment and low-voltage electrical equipment.
Control method
According to different requirements of equipment, there are many electrical control methods, including the following aspects of control:
1. Controlled by relay contactor
2. Contact control
3. Contactless logic control
4. Programmable control
5. Controller control
Design scheme of electrical control system
Control requirements: the electrical control system shall be designed to fully meet the needs of the site for control equipment. The main design shall take into account the characteristics of simplicity, reliability, economy and applicability, so as to ensure that the control mode is compatible with the control needs, the degree of generalization, and fully meet the process requirements, with good versatility and flexibility.
The first control system
Single chip microcomputer+PC control system.Is the cost of SCM lower?But lighter?More flexible embedded in the hardware cutting machine.However, the biggest disadvantage of SCM is that its processing power is not strong enough, so many complex control programs must be completed jointly by SCM and PC. That is to say, for those responsible operations or very complex cutting patterns, it is still difficult to rely on SCM alone.
The second control system
Control system with PC as processing platform.This kind of control system is the easiest to design and implement, and has high flexibility. The reason is that the controller based on PC as the processing platform can have rich and familiar software and hardware resources, so this control platform is very powerful in design and operation?If we can make full use of PC software and hardware resources, it is easy to design flexible and complex control programs.However, the disadvantage is that this control system cannot run independently from the PC, so the cost is relatively more expensive than other methods
Control system expansion
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Intelligent Vehicle Control System Based on Electromagnetic Sensor Path Recognition
