In a DCS (Distributed Control System), multiple nodes are connected to the communication network, typically categorized into two types. One group is directly linked to the production process through I/O boards and is known as controllers. These controllers can be either data acquisition or loop controllers, and sometimes they are combined into a single node. The other type of node connects with the human-machine interface (HMI), which collects data from the controller via the network. HMIs include operation stations, engineer workstations, historical trend stations, and dynamic data servers.
Operation stations serve as the main equipment for factory operations, allowing operators to monitor and control processes. They must both read data from the controller and send operator inputs back, making the data transmission two-way. Engineer workstations are used before system deployment to configure algorithms, set parameters, and load them into the controller. This configuration process differs from traditional programming and is more about setting up the system to function properly.
The controller's memory includes various control algorithms such as PID, basic math functions, trigonometric operations, advanced methods like Smith predictors, and even support for languages like BASIC and C. If the built-in monitoring software doesn't meet requirements, the engineering station can create custom dynamic flowcharts and load them onto the operation station.
To upgrade controllers and operation stations, reverse engineering stations can extract the current configuration without altering it, then download it to the new hardware. These stations also help in diagnosing issues by analyzing the controller’s configuration.
Dynamic data servers act as an interface between the DCS and the MIS system, ensuring secure data transfer. They only allow data to flow upward and store large volumes of information. Similar to dynamic data servers, historical trend stations can either operate independently or be integrated into the server.
When a system is first commissioned, network congestion may occur due to incomplete configurations. However, this issue usually diminishes over time. Recently, as the MIS system needs real-time data from the DCS, frequent congestion has been reported, especially when the dynamic data server is active. This leads to crashes in HMIs.
Each node on the network has a network interface, allowing the controller to send data to the HMI. Data is transmitted using broadcast protocols, where all nodes receive the data. If a node requires specific data, it may query others, potentially causing congestion if the data isn't found.
Operation station crashes have been common since the 1970s. Early systems had compatibility issues between operating systems, monitoring software, and drivers, leading to instability. Some systems used transplanting software, which caused further problems due to limited testing.
Dynamic data servers were not initially present but have been added over time. As the system runs, configurations change, and some points may no longer be valid. When the server is connected, it reads all points, many of which are invalid, causing congestion and crashes. Using a reverse engineering station can help identify and remove these invalid points.
It's important to ensure that all software versions match when accessing a dynamic data server, as mismatched versions can affect data transmission. Another method to reduce congestion is the exception report technique, where data is sent only when changes occur. To prevent missing updates, periodic reports are also sent, even if no changes are detected.
Recent operation stations often use the NT operating system with general-purpose monitoring software like FIX or INTOUCH. These systems are more stable due to widespread use and better compatibility. Open systems also reduce maintenance costs and dependency on original manufacturers, marking a significant improvement in DCS technology.
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