Issues in the selection of DCS - Database & Sql Blog Articles

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After the process is determined, the control system is also determined. Which control system to choose is an important issue, which is mainly based on project size and investment budget. Compared with analog instruments, dcs is more complex and requires higher technical performance. In addition to control hardware, it also involves communication protocols, monitoring software, and data storage connected to it. After the emergence of the new system, combined with the Internet technology of the Internet, it also involves industrial connection software and optimization control software, real-time database, connection with factory relational database, and interoperability of traditional DCS and plc.
1. According to the investment budget, if the control system has more control loops, the analog acquisition is larger, and the thermal resistance and thermocouple are collected more, DCS should be used. Which DCS to use should be considered from the following aspects:
At present, it is the handover period between the new and the old systems, DCS software and hardware are being dedicated to the general purpose. Choose a newer type of system, the price is relatively low, the spare parts are better to buy, and the maintenance cost will be greatly reduced. Interconnecting with other systems is easy and inexpensive. System integration is the trend in the next decade and is a new stage in the development of computer technology.
DCS manufacturers tend to be more specialized in a particular field. Such as Honeywell TDC2000, TDC3000, TPS and PKS systems, Yokogawa CS1000, CS3000 systems are mainly used in the petrochemical sector, because its closed-loop control loop is more, analog control is more convenient. The analog and digital control are completely separate and are not as convenient as other DCS in terms of switching control.
BAILEY N90, INFI90, Westinghouse WDPFI, WDPFII, OVATION systems are mainly used in power systems. These two systems are better at combining analog and switching quantities. The OVATION system is a new soft DCS system and one of the best-selling systems available today.
Fisher-Rosmount and Westinghouse were acquired by Emerson. The old systems PROVOX and RS3 are mostly used in chemical systems, and their batches and formulas are very distinctive. Now it has been basically migrated to the new system Delta V, the new system is compatible with the fieldbus and Hart protocol.
Different processes have special requirements, such as power plant equipment (DEH) and event sequence recording (SOE). The ratio of switch quantity to analog quantity in cement industry and metallurgical industry is about 6:1, and there are many pure lag links, sometimes control compensation, etc. These factors should be considered when selecting the type. Some DCS system switching logic control configuration is not convenient, then consider the use of DCS and PLC mixed. For example, DCS is used in power plant boiler control, and PLC is used for logic control such as coal handling and water treatment and ash removal.
Economics should be considered from the perspective of DCS prices and expected benefits. In the same grade, the imported DCS control function is stronger. Imported products have some advanced control algorithms, such as Smith's estimation, three-dimensional matrix calculations, etc., because of the long sales time, the software assurance system is more perfect. Domestic DCS prices are much lower, and can also meet basic technical requirements. Due to late development, some technologies are more advanced than foreign ones, such as Ethernet applications, rail-mounted embedded PC-based controllers, and already fieldbuses. Interface, etc. From a structural point of view, China's DCS is more reasonable than some imported DCS.
The controllers of traditional DCS manufacturers in foreign countries are not much different, but the operation stations are quite different. It is divided into two categories based on PC and minicomputer. The price of minicomputers is much higher than that of PCs, and many models have been discontinued. The PC operating system uses NT, although the stability is not good, but it is enough for the operation station. The SCSI of the minicomputer interface, the transmission rate is theoretically 20 times serial. With the NT operating system as the operating station, such as the serial connection, the number of points (number of tags) can be less, which is related to the hardware configuration. Sometimes due to poor hardware and software, it will crash frequently. Domestic PCs are cheaper, using PC operating stations, can use the latest models and monitoring software, and the development of computer technology is basically synchronized. Field technicians are also more familiar with the installation, commissioning, networking and development of software. The monitoring software is divided into dedicated and general purpose. The general monitoring software has good open performance and can exchange data in DDE or OPC mode in Ethernet. The dedicated monitoring software script is not rich and cannot be connected with third-party devices, so it is about to exit the market at present, but viruses and hackers are difficult to invade; the fatal shortcoming is poor open performance.
The traditional DCS is about to exit the market, and the new system can be integrated or supplied by the DCS manufacturer as a system. For the integrated system, the operation station can be used universally, and the controller has many interfaces, which are used in conjunction with the PLC and the Web. Users can integrate through engineering companies and should choose according to the size and budget of the project. Thanks to the general monitoring software, programming software according to IEC61131-3 standard, embedded system or soft DCS system can be selected, the integrated control system is likely to achieve the highest cost performance.
The system size is mainly determined by the number of input and output points of the system. This includes the number of input and output points required for interlocking and special control requirements. The model of the DCS is selected depending on the amount of work for determining and maintaining the funds. Some DCSs require maintenance, and the control part basically does not require maintenance.
When selecting a model, consider whether redundancy is required. The communication network of the new DCS is redundant, and the distance between nodes is related to the total length of the DCS network and the geographical configuration of each node. General Ethernet, network card and other prices are very low, and the dedicated network interface is very expensive. Economics involves many factors. It is best to contact similar units in the country for technical training or to consult experienced experts to solve the difficulties.
The technical strength of the project contractor. It is necessary to select a unit with strong technical strength as the overall scheme, and the specific programming or configuration can be completed separately. Foreign DCS manufacturers and technicians are unlikely to come to China for a large number of services. They are not familiar with our application, and it is even more impossible for any system to be clear. Therefore, the specific implementation is better for domestic technicians.
If the imported DCS is used, technical support from some foreign industrial development software companies can be obtained, and the system can be better. Chinese technicians have also been improved. Although China's technology has a certain distance compared with foreign countries, domestic technicians have a better understanding of the domestic situation, and with the support of large foreign companies, the engineering will achieve better performance.
After sales service. Foreign manufacturers usually have high prices for spare parts and spare parts, and they cannot be provided in time. Users should choose manufacturers with strong manufacturers, strong technical strength and good technical support in China. Manufacturers will continually update their products, and compatibility between new and old products will be better. Domestic DCS manufacturers supply and spare parts supply is timely, and after-sales service is better. Many third-party manufacturers have also developed many inexpensive products, which are available to users.
The advancement of technology. Computer technology is developing rapidly, and the development of DCS is relatively stable, which involves the application of new technologies. It should be noted that some foreign manufacturers often push the less mature products to the third world in order to seize the market. The sales time of domestic DCS is not long enough, and the testing conditions of some software are not perfect. In some extremely special cases, there is not enough evidence to cause problems, so sometimes the failure will be unclear.
System integration is mature and can be used to save money. Should learn from ABB, its industrial IT is the integration of many products. In a similar way, not only can a good system be integrated, but also a technical team.
In summary, the selection should not only consider the project size and investment budget, but also consider a range of other factors. System integration is the future direction, not only meets the control requirements, but also has good performance and low capital.
2. Determine the model of the operator station controller and the number of controllers and the number of I/O points to be processed according to the number of input/output points. The system must first decide how many controllers and man-machine interfaces to use. The system consisting of loop controllers is relatively simple, and the number of IO points provided is determined. It is possible to use the number of I/O points and loops that are full (except for the fieldbus connected to the controller). After the brand of the purchased product is determined, it is necessary to determine the model of the PLC or DCS, how many controllers to purchase, and how many I/O cards are required for one controller. This issue is important to the end user.
Hardware configuration. After the model numbers of DCS and PLC are determined, the number of cards of various types of I/O is calculated. This is called hardware (or hardware configuration) configuration. There are three aspects to the hardware configuration: first, the location of the I/O device associated with the controller location. The remote I/O system has advantages in terms of installation and wiring costs. After the local I/O or remote I/O is selected, the module model is selected. Usually only one type of I/O module is required. After the model is selected, count the number of I/O modules per model. The statistics are divided according to the field sensor used, the working principle of the control device and the voltage amplitude. According to the (input/output) function, voltage, and working principle, the number of interface modules is counted.
Software configuration. After the above work is completed, the software configuration is started, that is, the DCS adopts the function block configuration, and the PLC compiles the ladder diagram. On-site installation and commissioning, and finally make the system work well. In addition to the number of I/O modules, consider the processor memory used by the controller logic program, and finally consider the processor instruction system.
Regarding DCS, the controller I/O point configuration is more complicated than the PLC. Because DCS mainly considers analog processing, it accounts for more processor memory. Therefore, after the selection, you must also determine the point margin. Based on experience, assuming that there are actually 100 I/O points, a controller that recognizes 300-400 I/O should be used.
Suppose a DCS controller can connect 64 I/O cards. When actually connecting, it is suitable to connect more than 20 I/O cards. Each card is 8 or 16 channels. If it is a switch, the number of blocks can be more appropriate; the number of analog blocks is less.
If a DCS controller can complete 120 PID control loop tasks, it is better to make more than 40 loops when doing configuration, plus some switch and analog acquisition, the controller can run normally. Do not be too full, otherwise it will not only affect the operation of the controller, but also affect the DCS communication network.
A system needs to use several controllers. When each controller is configured as a hardware, the load should be balanced with each other. The number of input/output points of each controller should be equivalent. How many PID loops each controller has to configure is not a big deal, as long as the number of I/O points is about 30-40% of the maximum capacity. If the input/output points of the two controllers are not exceeded, but the difference is too far, for example, one is more than 1000 points, and the other is only more than 100 points. At this time, the communication network is not working properly.