There are several different nodes connected to the dcs communication network, which are usually divided into two categories. One type is the node that is directly connected to the production process through the I/O board. We call it the controller. The controller can be divided into a data acquisition controller and a loop controller according to different functions, and the two can be combined into one node. Another type of node is connected to the human machine, called the human-machine interface. They collect data from the controller through a communication network. According to the function, it is divided into operation stations. This is the main equipment of the factory operation. It needs to read data from the controller and send the operator's intention to the controller, so the data transmission is two-way. The other nodes of the human-machine interface are the engineer workstation, the historical trend station, and the dynamic data server. The engineer workstation usually connects the various algorithms stored in the controller according to the production requirements before the system is put into operation, and fills in the parameters and finally loads them to the controller. It is a configuration tool, and the system can be put into operation. Offline. The work of an engineering station is different from programming in an algorithmic language, called configuration.
There are various control algorithms in the controller's memory, such as: PID, addition, subtraction, multiplication, division, trigonometric function, matrix operation, advanced algorithm Smith predictor, and BASIC, C language interface. If the monitoring software mapping function in the operation station cannot meet the requirements, the dynamic flow chart can also be performed by the engineering station and then loaded to the operation station.
In order to upgrade the controller and operator station of the DCS, the configuration of the controller is read by the reverse engineering station without changing the configuration. Then download to the upgraded controller. Another function of the reverse engineering station is to find faults and read the controller configuration for analysis.
Another important node of the human-machine interface is the dynamic data server, which is the interface between the DCS and the MIS system, and is an isolation device for the DCS and the Web. It is characterized by data that can only go up, not down, and the amount of data stored is massive. A historical trending station is similar to a dynamic data server in that its functionality can be combined into a dynamic data server or it can be a single node.
When the system is just put into operation, network congestion often occurs due to imperfect system configuration. However, after a period of operation, network congestion rarely occurs.
Recently, since the MIS system needs to read production real-time data from the DCS. When the dynamic data server is connected to the network, the network congestion becomes very frequent, and the node crash of various human-machine interfaces is very serious. The severity of network congestion is related to a variety of factors.
When the node is connected to the DCS communication network, there is usually a network interface, and the controller sends the data to the interface. The HMI reads data from the network interface and reads data in accordance with the network communication protocol. The physical structure of the network has two kinds of rings and buses, and the bus network is also logically ring-shaped. The star network is only used for small systems (within 100 I/O points). The commonly used communication protocol is broadcast. At the node on the network, as long as the data is sent to the network, it must continuously broadcast data, and the node that needs the data receives the data. In addition to this method, a broadcast protocol network has a way: a node queries data to other nodes on the network, but if other nodes do not have this data, it repeats the inquiry until it is read. This data. Then if there is no such data on the network, it will cause network congestion.
Operation station crashes, no matter what kind of DCS system, since the advent of DCS in the 1970s, there have been different degrees of crashes. The main software of the operation station is the operating system, monitoring software and driver software of the controller. In the early DCS system, all three of these softwares were developed by DCS manufacturers themselves, or they were dedicated. These three softwares may have a mismatch, so there is a crash at runtime, especially when several special keys are pressed during keyboard operation. Another situation is that some DCS systems and monitoring software are transplanted from other systems, and DCS manufacturers only develop one driver software. In this case, due to the small amount of DCS sales, the problem of cooperation between software is not fully exposed, and the crash is more frequent.
Dynamic data servers are not present when the system is first commissioned. Many DCS users have been added as needed in recent years. When the DCS runs for a long time, the maintenance personnel of the factory are constantly changing, and the configuration of the controller is constantly changing, but there is a phenomenon that the configuration is only added or not, and some configurations are actually not with the real I. /O points are connected. When the dynamic data server is connected, all the data points on the DCS are to be read, and a large number of data points are invalid, which causes network congestion, so the human-machine interface crashes. At this time, you can use the reverse engineer station to read the configuration in the controller, compare it with the contents of the forward engineer station, and delete the invalid points to avoid network congestion and crashes.
Another issue to be aware of is that when accessing a dynamic data server, you should check that the software versions of all interfaces are consistent. Otherwise it will affect the delivery of data.
The third way to solve the network congestion is to use the exception report method to increase the area of ​​the exception report to reduce the network traffic. The exception report means that a point on the site sends data to the network only when it changes, and it will not be transmitted if it does not change. But in order to prevent the point is already bad and the human-machine interface is not known. Therefore, even if the point does not change, it will be reported over a period of time. In order to reduce the number of networks, the two parameters of the exception report are appropriately increased, and the purpose of reducing the amount of data can also be achieved.
The general operating stations introduced in recent years mostly use the NT operating system, and the monitoring software is also used in general, such as FIX, INTOUCH, and so on. Due to the large amount of software sold, there are fewer problems with the software. The open performance of the general operation station is good, and the crash phenomenon is greatly reduced. Regardless of maintenance costs, spare parts procurement is not limited by DCS manufacturers. This is a big step forward for DCS.
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