I. Introduction:
The history of the automation control system is like a history of the Spring and Autumn Period and the Warring States Period. It has formed many genres under the influence and impact of electronics, software, PC, and network technology. Especially after entering the 1990s, the various schools have merged and merged. You have me in you, and I have you in me. Even the heads of the various schools have a little description of what the unique skills of our school are. , What is the difference with Shaolin Wudang.
I often hear some confused questions from friends who have been in the industry for many years. What is the difference between PLC, DCS, PAC, FCS, and NCS? What are the selection criteria? Will the future control system be more specialized or more general? Will these terms be used in the future?
Every time at this time, the professionals of various manufacturers always vaguely say: "Ah, this problem is very complicated. It cannot be described clearly in one word. It was OK before, but now it is becoming more and more unclear. , I'll talk to you in detail when I have time."
Today, taking advantage of the world’s treasured land of instrumentation and automation, these issues are discussed and discussed here. On the one hand, they will attract more experts to pay attention to this topic. On the other hand, they will also give themselves a chance to organize their ideas. Look at the direction of the research and development of the year. (Deweisen Technology's automation business has been transferred to Shenzhen Rectangular Technology in early 2008)
Every time I want to rectify the history of the control system, it’s a big deal, because if you want to make it clear, you need a very clear picture and detailed text. And I’m a very lazy person. Typing is fine, if you want to do it. A very beautiful picture will have a headache, but if there is no picture, I can’t explain it clearly, so I still reluctantly draw a picture. Please forgive me for not being very precise.
Through the above picture, you can find that the entire history of industrial control has been fully affected by PC technology, and the characteristics of industrial control make it impossible to completely unify with PC thinking.
1. PLC history
As can be seen from the above figure, as early as the 1960s, the main control system was mainly composed of various relay groups and instruments. The relay group was mainly used to complete logic and digital control, and the instrument was mainly used to complete analog loop control. . There is a clear difference between the two.
Before the 1960s, the automatic control systems of automobile production lines were basically composed of relay control devices. Every modification of the car at that time directly led to the redesign and installation of the relay control device. With the development of production, the renewal cycle of automobile models has become shorter and shorter. In this way, the relay control device needs to be redesigned and installed frequently, which is very time-consuming, labor-intensive, and material-consuming, and even hinders the shortening of the renewal cycle.
In order to change this situation, General Motors of the United States made a public bidding in 1969, requiring new control devices to replace relay control devices, and proposed ten famous bidding indicators, namely:
1) Easy to program, program can be modified on site;
2) Modular structure is adopted for easy maintenance;
3) The reliability is higher than the relay control device;
4) The volume is smaller than the relay control device;
5) The data can be directly sent to the management computer;
6) The cost can compete with the relay control device;
7) The input can be 115V AC;
8) If the output is AC 115V 2A or more, it can directly drive solenoid valve contactors, etc.;
9) Only minor changes to the original system during expansion;
10) The user program memory capacity can be expanded to at least 4K.
At this time, the first 8-bit CPU was born a year away, so the colleagues of DEC later adopted a special bit block processor 2901. This architecture satisfies GE's requirements very well, but this It has also led to decades of disputes over PLC architectures, and mainstream PLCs are still using this architecture.
Many friends have misunderstood this issue, thinking that such an old technology is still in use, but it is not. The bit block processor is actually an architecture, which is the same as the old Harvard architecture system is still used in the latest PC CPU.
Because the initial positioning of the PLC is the processing of bit logic, if you have developed the underlying software and hardware, you will understand how slow it is to use a general-purpose CPU to process bit operations. You need to pass and operate the bits in a word first. Shift again, after the calculation, it needs to be written back after the reverse operation, and the positioning description of the bit in the word is also required. On the contrary, if there is a dedicated processor for processing, it will become much faster and simpler. This kind of architecture is quite convenient when dealing with bit logic and has a far-reaching impact. Today, most new generation high-speed PLCs still use dedicated bit block processors.
With the development of PLC technology, logic processing alone is far from enough. Therefore, most mid-to-high-end PLCs now have two CPU cores, one is a general-purpose processor, and the other is a dedicated processor (such as rectangular technology’s V80 and PPC11 series, its dedicated processor single chip with ladder diagram dedicated processor, operation control processing unit, communication management unit, IO interface, expansion bus interface, etc.).
Of course, the new generation of bit block processors is far from comparable to the original 2901. In addition to completing the analysis of ladder diagram instructions and the calculation of logic instructions, and through a series of optimizations, the stability, reliability, and reliability of the entire system are improved. Real-time performance has been greatly improved, so this bit block processor has actually become the core device in mid-to-high-end PLCs, and is also the core technology of major PLC manufacturers.
Simply summarize the system characteristics of traditional PLC: program cycle cyclic scheduling mechanism + interrupt scheduling mechanism; incremental fieldbus communication method; ladder diagram programming method; single machine operation mainly;
2.DCS
Since the birth of the first PLC, industrial controllers have received a profound and significant impact from PC technology, the first being DCS.
Distributed Control System (DCS, Distributed Control System): In fact, it is not a single product, but a system of a series of software and hardware product combinations. The hardware part can be a PLC or an IPC-based control system. The early DCS manufacturers Using a closed software and hardware platform, but soon it was affected by the PC. Many manufacturers began to use PLC as the hardware platform, using standard computers plus commercial operating systems as the software and hardware platforms for the operator station, engineer station, and history server. What's more, a third-party configuration software is used as the monitoring software. In particular, the new generation of DCS has completely become a large integration of software and hardware, and only some of the above algorithms and optimization programs are provided by the manufacturers themselves.
"Rectangular company DCS hardware system structure diagram"
A simple summary of the unique characteristics of most DCS can be summarized as: network as the main structure; program timing scheduling mechanism (10mS~NS or time slice scheduling mechanism); distributed system structure, each DPU is interconnected through the network; based on Distributed real-time database for full communication; programming habits of function diagrams; (this summary is not strict and cannot be recognized by all DCS vendors, but many mainstream DCS currently use this structure); general or dedicated operating station software and History server software;
3.PC_BASE
1) Advantages of PC_BASE:
When PC_BASE appeared, it caused great repercussions in the industrial control industry. The controllers of that era were quite expensive, and the low cost of PC software and hardware was quite attractive to everyone. At that time, many scholars believed that PC_BASE was the future. The mainstream direction of large-scale industrial control systems. Engineers at the time were divided into two groups. One group believed that PCs were developed for commercial use and the control community could only absorb its useful technology, while the other group believed that PC technology was widely used and there were so many software and hardware resources available. It has great benefits for the standardization of the controller and reducing the cost.
In this process, domestic industrial control manufacturers including DCS, PLC and various special-purpose controllers have widely adopted the PC_BASE structure to develop new products. Most of them use 386, 486, and 586. Among them, ICOP's 386X_M6117D and MAPLE's 486DX4-100M are One of the better industrial-grade X86 chips. (PPC22 and PPC31 of Shenzhen Rectangular Technology are the control system of PC_BASE platform)
2) The dilemma of the PC_BASE control platform: (Only comment on the situation where PC_BASE acts as a control system, and does not involve other application methods)
PC_BASE has encountered some difficulties in its development in recent years. At the beginning, everyone chose PC_BASE because of the convenience of development, especially in the era of DOS and X86 compatibility. You can write a very complex control program in just one week. When there is a network, everyone can communicate with each other through BBS. At that time, I felt that everything was under control.
Software part:
Now that there are fewer and fewer DOS users, many manufacturers are guiding engineers to use the WINNT platform, and WINNT’s shielding of the bottom layer makes the majority of low-level software development engineers feel quite depressed, because the WDM driver development of the WINNT system requires the use of DDK and other complexities. Tools, even if you use simple development tools such as XTOOLS, it will make people feel like a shoe. The complex interface of the new generation of PC hardware also makes everyone unwilling to spend too much time on it, making PC_BASE open and convenient Sex is greatly offset. At the same time, the low reliability of the WINNT system makes most automation engineers stop.
WINCE before 2.0 is also a maddening software. Not only is the reliability poor, but the real-time performance is also quite poor. It makes people suspect that this thing can only be used as a display screen. Later, after wince2.0 came out, it was better, but personally, WINCE There are still conflicts. It may be that I have suffered a lot at the beginning. I always think that an industrial control product is not suitable for using WINCE as an operating system, because the advantages of WINCE's system structure including compatibility, openness, and graphics are all aimed at handheld consumer products. , Such as PDA, for the high real-time and high reliability required by industrial control, it is a bit of a failure. In this regard, Linux is worse, because Linux is developed for commercial computers. Many companies are reducing weight for Linux and forcibly adding preemptive scheduling mechanisms to Linux, so that Linux can be used in embedded environments, but WINCE also has its shortcomings, and at the same time it is more serious, so it is not a good choice. In terms of operating systems, in fact, VXWORKS and NECLUES may be a good choice.
Because of these requirements of PC_BASE, some professional soft logic companies have been derived, mainly based on European and American companies. They provide a complete set of software for IPC manufacturers, so that users can use IPC like PLC, thus simplifying the software. Development work.
The soft logic vendors represented by KW, Infortem, and Isagraf combined with the OPEN PLC organization have greatly promoted the development of the IEC61131 standard, but also because of this, its development idea of ​​using PC_BASE as the hardware platform has led to the current IEC61131-3 part of the The expenditure of software and hardware resources is too large, which greatly limits the promotion of IEC61131-3 among traditional leading controller manufacturers. Until now, other traditional controller manufacturers except soft logic manufacturers only support IEC61131, but in fact, they support IEC61131. The standards are far from each other, and even the basic first-level compatibility cannot be reached. Last time the chairman of the OPEN PLC organization came to China, and he admitted quite frankly about this issue. At present, there are still very few manufacturers supported by IEC61131, and there are many different opinions on the standard.
Hardware parts:
What’s more terrible for the PC_BASE system is that the development speed of industrial control software and hardware is far behind the development speed of civilian products. Often this CPU of civilian products has been discontinued, and the industrial control industry has only begun to try it out. At present, low-end X86 supporting chips are available. The discontinuation of production, including DRAM, makes it impossible for everyone to continue to use the 386, 486, and 586. The few chips that are being produced are also difficult to accept due to price issues. At the same time, Intel and Microsoft have completely abandoned the downward compatibility rule, making the significance of PC_BASE's existence diminish day by day. No industrial control manufacturer has kept up with the update speed of these two big brothers. When your product is ready to go on the market, you will find that all the devices you use have been discontinued, and you can only go to the second-hand market to buy disassembled parts.
At the same time, most mainstream X86 chips are now high-density packaging, including a large number of high-density interface definitions. It is difficult to deal with dustproof, anti-vibration, and heat dissipation issues in an industrial environment. It appeared in aerospace systems not long ago. Reinforced computers crashed due to an earthquake. From the perspective of research and development, these problems are difficult to solve, because the entire system is not designed for industry. Before 586, all of this can be tolerated, but now the system is difficult to achieve. The way to deal with it, especially when it is made into a tight-playing control system, this problem is more serious.
This is also the source of software and hardware technology and market power for industrial computer manufacturers to switch to software PLC and PAC platforms. By breaking the software and hardware architecture of PC_BASE, coupled with third-party logic software, a good control system can be realized.
4. Fieldbus technology and FCS
Soon after the emergence of soft PLC, a new technology wave hit the industrial control industry severely. This is the field bus. At the same time, the concept of FCS (a fully distributed field bus-based control system) was derived based on the field bus.
At the beginning, the field bus only provided a networking function for PLC and field instruments, but soon someone thought that if all the control logic is distributed to the field, there is no need to pull all the cables to the control room, which saves money. A large amount of cost, and with the decentralization of control, its safety can be greatly improved, and local failures will no longer cause the collapse of the entire system. This is the basic theory and origin of FCS.
Eight years ago, I was also a supporter and actual developer of FCS. I am convinced that the current communication capabilities, chip capabilities are getting stronger, and the price is getting lower and lower. Today, FCS is the future control system. It will replace PLCs. , All control systems including DCS. But in the actual development and application process, we found that after some problems are fully dispersed, not only the cost increases, but also the maintenance becomes more difficult, because all nodes rely on the network, and the reliability of the network becomes a bottleneck. . With such a long network cable, any short circuit or open circuit in any section will cause fatal damage. And the decentralized logic will cause a major error in the entire system due to the failure of an intermediate node. Of course, if the user has a deep understanding of the distributed control concept, there is no problem, but in fact, it allows the user engineer to understand such a complicated topology. Is unrealistic.
Unless there is a new development in the future neuron network chip research and development (not the 3150 of the LON bus), it can be automatically replaced by another logic node after a certain logic operation node is damaged. At the same time, better network-based logic programming software is required. This software can perform reasonable logical segmentation of the distributed controller, and can have a reasonable treatment method for the situation that occurs after any node is damaged, whether it is protected or ignored. The ultimate ideal situation of FCS development is that there are only sensors and actuators without separate controllers. All sensors pass their own parameters to the required actuators. Each actuator calculates and controls according to the parameters obtained by the network, and at the same time calculates itself. The obtained intermediate value is passed to other actuators. Because of the intermediate value problem, the entire control network will become quite complicated. Every point with intermediate value must have a reasonable processing strategy. Ideally, when there is a problem with the intermediate logic point, it can be controlled by another A logical point to replace, or a reasonable protection strategy. In the foreseeable time, it will be difficult for us to develop FCS that meets all requirements, so we have reason to believe that FCS will only be applied in a part of the market.
However, the concept of FCS has deeply affected the PLC industry. At present, most PLCs have fieldbus interfaces. In fact, after the FCS debate, PLCs already have the hardware requirements to form FCS, such as the V80 series of Rectangular Technology. A thousands-point FCS system is formed through the field bus. The consequence of this is that it is difficult to have an independent DAS system market space in the future. IO modules including various fieldbuses will be replaced by some PLCs, because the cost of the two is the same, and the functions of the PLC are much stronger.
After several years of controversy and research, many researchers at home and abroad have formed a temporary consensus, that is: According to the actual situation of the scene, choose whether to distribute or concentrate. In many cases, a method of overall dispersion and partial concentration is comparison. suitable. For example, in the metallurgical industry, many sites use PPC11 as the main station and V80 as the sub-stations. The sub-stations are distributed on-site, and each sub-station is responsible for a specific control object or loop. In this way, when there is a problem with the network, each sub-station can handle its own tasks well, and at the same time, the distance between each sub-station and the device is reduced to less than 100 meters, making wiring and maintenance relatively simple.
There is a very interesting situation in the technical wave of fieldbus, that is, the formulation of IEC61158. This process fully reflects the conflicts of the major interest groups in the international industrial control industry. In order to protect their own interests, they have been in the process of protecting their own interests for as long as 15 years. Failed to reach a truly meaningful agreement, the final result was the coexistence of 8 standards, and later expanded to 13 (there were 14 standards, but one withdrew), the scope of the standard also from the initial coverage process , Buildings, electricity, etc. have been retreated to only include process control. In fact, I personally think that it is not realistic to make a unified standard to include all industries. Each industry has different concerns about cost and performance. For general process control, you may choose PROFIBUS, etc. For building control, you can choose LONWORKS, data acquisition and MODBUS can be selected for communication between devices. However, in the same industry, there should be a unified standard. Many of our engineers will be troubled by the V80 and Siemens or AB control system. Therefore, there is no need to follow the trend. Domestic manufacturers have more exchanges, more communication and more compatibility, and strengthen the interconnection of various domestic products. This will be the key for domestic industrial control manufacturers to win from small to large. I hope that the experts of the Fieldbus Fund Committee can be more pragmatic. Promote the intercommunication of domestic industrial control manufacturers under the current situation, especially for newly developed systems, instead of putting too much energy on translating foreign standards and some purely academic research.
The influence of the Internet on industrial control is no less than that of a fieldbus. Its role will be fully reflected in the next few years, but it will not be in the form of industrial Ethernet, but in the form of a website. Its purpose is not only to provide a communication link and a human-machine interface, but to better integrate equipment into our information world. At present, the more common applications include environmental protection monitoring, hydrological monitoring, street lamp energy-saving management, and public transportation. Management, remote device monitoring, etc. Nowadays, many people use GPRS plus standard configuration software or develop PC software by themselves, but this is just the beginning. From a technical point of view, a completely open website-based configuration software platform is provided ( Runs on the website, for many customers to maintain and manage by themselves), for users to take up the device data they care about, and perform related display, storage, and processing. It is completely achievable, including future smart homes and smart homes. Equipment management and remote maintenance by equipment manufacturers can all be done on the same equipment network platform.
I believe that the biggest user of the Internet in the future must not be people, but equipment, switches, and each light bulb. When the network enters the equipment and home, it is completely achievable that the cost of a node with communication is less than 50 yuan. The most profitable network in the future is not GOOGLE or the like, but may be a little-known device network. If there is a device network that dominates the rivers and lakes (providing users with interfaces between people and devices), then it will be the largest market. The equipment network must be the biggest business in the industrial control industry in the future, because it will be the ultimate goal of the equipment industry development. It uses services as a means of charging instead of selling products. It hides too many services to be discovered, which will generate nuclear power for the equipment industry. Explosive impact.
5.PAC
The concept of PAC has been controversial in China since its emergence. What is PAC, and is it necessary for PAC to exist independently? I have argued with some colleagues, and these seniors in Duomen are broad-minded and didn't care about me. Later, everyone came to have a deep exchange with me. Therefore, after the argument, I have also thought about these issues, and I will show my ugliness here to see if I can give you some inspiration.
As mentioned earlier, in the past, PLC, DCS, FCS, operation control, IPC (only refers to the controller with soft logic), these control systems are clearly defined, each with its own traditional application fields, and each with its own technology Genre, but since the late 1980s, due to the influence of the development of PC and network technology, this boundary has become more and more blurred. Now even industry professionals can't explain the point.
From a market perspective, customers still choose PLC, DCS, or IPC from the corresponding manufacturer according to their own industry habits, but from a research and development perspective, these control systems are much the same on the technical platform, with only a few differences in details. For this reason, manufacturers in the industry are also very distressed. I often see that some PLC manufacturers are hindered by industry habits when they enter the traditional DCS industry. It takes a lot of time to explain: "In fact, our PLC+ configuration software is completely capable. To meet your control needs, PLC can now not only execute logic, but also has strong loop control. In fact, there is no difference between PLC and DCS in terms of hardware and underlying software."
Unclear, this is a very embarrassing question. After years of development, PLC, DCS, IPC, and FCS are no longer the original things. There is no clear difference between them. In fact, these control systems have penetrated each other. However, it is difficult for customers to accept this concept. For this reason, since the 1990s, when major manufacturers developed their own new generation of control systems, they began to deliberately or unintentionally put forward some new terms to replace the original control system. Some manufacturers have even developed a new generation of general control platforms that integrate DCS, PLC, IPC, and operation and control, such as integrated control systems from companies such as Toshiba.
For this reason, everyone thinks that there is no need to divide the general control system into traditional functions, but only according to the computing power and the number of points. Therefore, a new definition is urgently needed to clarify the name of this type of control system. In order to compete for the naming right of the next-generation general control platform, major manufacturers have begun a competition. ECS, NCS, PAC, NLC, ALC, etc., many new terms have followed one after another. Because there is no clear qualitative change process, this issue has been very confusing, and many new terms have not been recognized by customers, but have become vague. And because of the publicity offensive of NI, GE and major IPC manufacturers, PAC has received a lot of attention in China.
PAC was proposed by Craig Resnick, a senior researcher of ARC Consulting Group in 2001. Because it is not very clear, various manufacturers have defined PAC according to their own needs. Therefore, you can see many versions of the definition on the Internet, most of which are from various manufacturers. Modified according to the characteristics of one's own products.
I copied the definition from the industrial control website as follows:
1) Multi-domain functions (logic, motion, drive and process)-this concept supports multiple I/O types. The integration of logic, motion and other functions is the requirement of ever-increasing complex control methods
2) A single multidisciplinary development platform-a single development environment must be able to support various I/O and control schemes
3) A software tool used to design applications that run through multiple machines or processing units-this software tool must be able to adapt to distributed operations
4) With an open and modular system structure, it is suitable for unit operations from factory automation to process industry
5) Open and modular architecture-design and technical standards and specifications must be open, modular and combinable in implementation
It can be seen from the above that the definition of PAC is very vague. Basically, PLC, DCS, and IPC control systems on the market can satisfy several items more or less, and even many products support more than the above definition. Therefore, PAC is more a summary of the current generation of control systems, and more of a summary of the functions of a new generation of PLC, rather than a newly created product. At the same time, PAC has also pointed out a development direction for various control system manufacturers. .
Therefore, if you are a user, you don’t need to care about these terms. It’s more to see if this system can provide you with the best price/performance ratio. On the contrary, if you are a system integrator or an engineer, you can use different terms to describe your product according to your customers' preferences.
This may make some PAC supporters dissatisfied, but this is indeed my understanding, because products that fully meet the above requirements appeared in the 1990s, but at that time, people called these products based on the X86 platform more. Soft PLC, such as PPC31 series of Shenzhen Rectangular Technology, adopts X86 real-time multitasking platform, supports five languages ​​including IEC61131-3, JAVE, C++, built-in WEB server, and supports WEB-based remote HMI with sending and Receive E-Mail function to automatically transmit production and maintenance data, support complex motion control functions, support various open interfaces such as electronic hard disk, Ethernet, fieldbus, etc., from any angle, it is satisfied with the definition of PAC, but we It is still called PLC.
Three, the development direction of the control system
In the previous chapter, we spent a lot of time talking about the ins and outs of various control systems, and thought that it is no longer appropriate to organize the control system in the traditional way, so we have the concept of PAC. So how should the future control system be divided from the perspective of application? In order to simplify the description, we still focus on the PLC to avoid too wide a range.
1. From a market perspective, the general control platform in the future is first divided according to the type of application. We are accustomed to dividing the control system into three categories: 1) Engineering project type-engineering automation projects, such as metallurgy, papermaking, electric power, petrochemicals, etc.
There are several mainstream architectures for general control systems: 1) General CPU + interpretation mode execution; represented by DCS and some small PLCs, soft PLCs, and PACs, the disadvantage of this method is that the speed is quite slow.
2) General CPU+compilation mode execution; some small PLCs, soft PLCs, and PACs are technical methods such as rectangular technology V60. The speed of this method is more than 20 times faster than the interpretation method. The disadvantage is the unpredictability and unpredictability of the generated code. The disadvantage that the lower-level computer program cannot be uploaded makes its security inherently problematic.
3) General-purpose CPU + special processor; medium and high-end PLCs use this kind of technology, such as AB SLC500, ControlLogix, Schneider Quantum, Siemens 300, 400, Mitsubishi Q series, rectangular technology V80, PPC11, etc., all adopt this structure . The principle reliability and speed of this method are the fastest, more than 20 times faster than the compilation method, and can be easily improved. The disadvantage is that the cost is higher than the first two. Because of its safety, our current military series adopts this architecture, which has been fully tested on many guns, vehicles, ships, and ammunition. At the same time, we are currently developing a space-specific control platform on this platform.
4) SOC dedicated chip, which integrates general-purpose CPU, dedicated processor, and peripheral functions in the same chip. This has many applications in the new generation of PLC, and its cost performance is relatively high. For example, rectangular technology is developing N80 series. The performance of this mode is consistent with mode 3, and the batch cost is much lower than mode 3, but the development cost and development difficulty are relatively high.
Which kind of technology is better and more advanced, can only be said to choose the most suitable technology according to different application fields. In the 1990s, because of the rapid development of electronic technology, the speed of CPU was more than enough for industrial control requirements, so there was a wave of soft PLC and PC_BASE technology booms. At that time, everyone believed that this would be the future development. Direction, but what is surprising is that the rapid development of electronic technology makes the PC_BASE technology designed for civilian use not suitable for industrial applications (mainly the upgrading of devices and several architectures, this situation is intensifying). Therefore, the soft PLC technology is separated from the PC_BASE platform and more adopts the MCU platform, such as ARM. At the same time, the current bottleneck is not in speed, but in functions such as communication and operation control. These functions are difficult to be strengthened by software. Therefore, in general, current mid-to-high-end soft PLCs will increase the performance of this block by adding FPGAs.
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