Distributed control systems use networked sensors and controllers to monitor and regulate geographically separated processes.
History and Development
Early process control systems used fixed wired networks to link centralized controllers and instrumentation together to regulate flows and operations in plants and factories. However, as processes grew in scope and complexity, the limitations of centralized architectures became apparent. Distributed control systems addressed these challenges by leveraging modular, decentralized architectures.
In the 1970s, work began on protocols to communicate between dispersed regulators over shared communication networks. Fieldbus standards emerged in the 1980s allowing sensors and actuators to connect to programmable logic controllers using digital buses. This paved the way for distributed sensor/controller architectures. Major advances occurred in the 1990s as Ethernet and TCP/IP networks enabled truly distributed sensing, control and operator interfaces across sites. Today multinational plant operations integrate thousands of distributed controllers over secure IP networks.
Architectures and Components
A Distributed Control Systems consists of a network of autonomous, intelligent controllers linked by digital communication protocols. Sensors gather data from the physical process and transmit values over the network to controllers. Controllers execute algorithms to analyze data, determine appropriate responses and command actuators accordingly. Control functions are dispersed among modules rather than centralized in one location.
Controllers have embedded intelligence, control logic and I/O interfaces. Programmable Logic Controllers (PLCs) are commonly used controllers, designed for industrial environments. They execute ladder logic or structured text programs to monitor inputs and control outputs. Distributed controllers communicate over robust digital fieldbus or Ethernet networks to coordinate actions and share information across the whole system in real time.
Human-machine interfaces (HMIs) allow operators to monitor the system, change setpoints and configure controllers as needed from centralized control rooms. Historians log and archive process data for performance analysis and regulatory compliance. Engineering workstations are used to program controllers and develop the overall control strategy.
Advantages of Distributed Control
Distributed systems overcome the limitations of centralized architectures. By dispersing controllers close to the processes and endpoints, response times are faster. If a controller fails, the rest of the system can potentially still operate normally. Modular addition of controllers enables incremental capacity growth.
Networked sensors provide more visibility into plant operations through real-time streaming of data to multiple points. Control algorithms can be tailored for specific process sections or equipment. Integrated HMIs give centralized oversight while retaining benefits of localized control. Configuration changes to controllers don't require shutdowns as with single centralized units. Overall reliability, flexibility and maintainability are higher in distributed models.
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