Software/hardware structure in a distributed control system - Database & Sql Blog Articles

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1, the distributed control system dcs consists of 4 parts

The distributed control system DCS consists of four parts: I/O board, controller, operator station (human machine interface (EMI)), and communication network. The technical level of each DCS manufacturer in the I/O board and controller is not far from each other. If there are some differences, there are more or less algorithms in the controller, and the combination of algorithms is different. The difference between I/O boards lies in Some have intelligence, some do not, but the controller must read all LO data in a cycle within Is; the difference between the operating stations is relatively large, the main difference is whether to choose PC or small machine, UNIX or Windows operating system, Using dedicated or general-purpose monitoring software, the operating system and monitoring software can reduce the crash when the better cooperation; the biggest difference is the communication network, the worst is the polling method, the best is the exception reporting method, the rate difference is different Big.

The I/O board is directly connected to the production process through the terminal board to read the signal from the sensor. There are several different types of 20 boards, and each type of I/O board has a corresponding terminal board. Such as analog input, 4 ~ 20mA standard signal board and millivolt signal board for reading thermocouple; 4 ~ 16 channels; analog output, usually 4 ~ 20mA standard signal, generally it The channel is relatively small, 4 to 8 channels; the switch input, 16 ~ / 1 / 1JL one channel; the switch output, the switch input and output are also divided into different voltage levels of the board, such as DC 24V, 125V; AC 220V or 115V, etc., 8 to 16 channels are different; pulse volume input, used for acquisition rate signals, 4 to 8 channels; fast interrupt input; HART protocol input board; field bus LO board.

Each LO board is connected to the I/O bus. For the safety and integrity of the signal, the signal should be refurbished before entering the LO board. The above lower limit check, temperature compensation, and filtering can be done on the terminal board or separately. The board for signal conditioning is called signal conditioning. board. The I/O bus is connected to the controller.

As a DCS controller, it must have various function blocks: the function block connected to the hardware is usually 4 blocks, which is related to the type of the input board; including the analog input function block, the analog output function block, and the digital input. Function block, digital output function block. Each function block must be connected to a specific terminal block. If there is a signal to receive the fieldbus, a function block for receiving the fieldbus signal is also required. In addition, there are 4 functional blocks connected to the network, which are: analog network input, analog network output, switch network input, and switch network output. Followed by PID function block, station function block, followed by arithmetic operations (addition, subtraction, multiplication, division), then function calculation (primary filter, lead lag, two-dimensional curve, etc.), triangular geometry operation (sine, Cosine, tangent, cotangent, etc.) and three-dimensional matrix operations. Some advanced operations, such as fuzzy logic, model control, etc., are more or less, but they are the criteria for judging the strength of DOS systems.

In order to increase the number of I/O points of the controller and increase the distance from the controller to the field, the field bus and the controller can be connected, such as the LonvJorks bus. At this time, there should be a function block in the controller when connecting to Lonworks. The modules on the Lonworks bus have separate I/O boards and computing boards, and there are a small number of function blocks in the computing board.

The controller is the core component of the DCS, which is equivalent to a PC. Some Das controllers are PCs themselves. It mainly has CPU, RAM, F2PROM and ROM chips, and two interfaces, one receiving the signal from the I/O bus downward, and the other interface is sending the signal up to the network and connected to the man-machine interface. The ROM is used to store control algorithms that perform various arithmetic functions (some Das are called function block libraries). Store function blocks in the library, such as control algorithm PID, with dead zone PID, integral separation PID arithmetic operation add, subtract, multiply, divide, square, square, function operation primary filter, sine, cosine, X-Y function generator Advanced-lag; more advanced algorithms include Smith estimation, C language interface, matrix addition, matrix multiplication; logical operations have logical AND, logical OR, logical non, logical and non-etc. Usually the station function block not only combines the analog quantity and the switching quantity, but also connects with people. The more function blocks, the easier it is for the user to write the application (ie configuration). According to the process requirements, the function blocks are connected to form a control scheme. The control scheme is stored in the E2PROM. Because the E2PROM can be erased and the configuration changes as the process changes, the configuration is stored in the E2PROM. Different users have different configurations. During configuration, the user selects the desired function block from the function block library, fills in the parameters, and connects the function blocks. The formation control scheme is stored in the E2PROM. At this time, the controller becomes the operation mode after the configuration mode is put into operation, and the operating system, the function block configuration software and the communication software are installed in the controller. In order for the system to operate safely, the closed-loop controller must be redundant, one for each, and hot standby.

The communication network connects the process station and the human-machine interface into one system. Communication networks come in several different forms, such as bus, ring, and star. The bus type is also logically ring-shaped. The star type is only suitable for small systems. Whether it is a ring type or a bus type, it is generally broadcast. Other protocols have been used less. The speed of the communication network is around 10M and about 100Mb/s.

The closed-loop controller, analog data collector and logic operator can be directly connected to the human-machine interface on the communication network, and each different controller on the network acts as an independent node on the network. Each node performs a different function. They should all have a network interface. In order to save the network interface, some DOS devices, such as a closed-loop controller, an analog data collector and a logic operator, are pre-connected to the control bus, which is called a process control station. This not only increases the number of LO points that the process control station can receive, but also saves the interface. Then connect to the network through the interface and connect to the human-machine interface.

The HMI has four different forms of nodes, which are operator stations, engineer workstations, historical trend stations, and dynamic data servers. The operating station is equipped with operating system, monitoring software and controller driver software to display the system's label, dynamic flow chart and alarm information. A Das system can have several operating stations, each of which can display the same content or different content. DOS manufacturers can be equipped with dedicated operation stations or general operation stations. The engineer workstation is configured for the controller (CAD) and can also be configured for the operator station (for dynamic flow diagrams). If the monitoring software is very capable of drawing, the drawing work can be done independently by the monitoring software. Another function of the engineering station is to read the configuration of the controller for controller upgrades and to find faults. We call it the reverse engineer station. Historical trend stations are used to store historical data. The dynamic data server is the interface between Das and MIS systems, and is also the isolation of Das and Web.