What is Distributed Control System, DCS?
Distributed Control System (DCS) is a sophisticated computerized control system for a process or plant used to control complex, large, and geographically distributed applications. In this system, autonomous controllers are distributed throughout the entire plant area, but there is no central operator supervisory control. A high-speed communication network connects these distributed controllers to field devices and operating PCs. By localizing control functions near the process plant with remote monitoring and supervision, the distributed control system (DCS) concept improves reliability and reduces installation costs. In recent times, the DCS concept is widely used in continuous process plants requiring high reliability and security. A distributed control system is also widely used in many other industrial fields like chemical plants, metallurgical process plants, food processing units, water management systems, nuclear power plants, automobile industries, petrochemical plants, etc.
The DCS is the control system that continuously monitors and controls the process during normal plant operation and may provide three different types of safety functions:
- Continuous control action, keeping the process at set point values within the normal operating envelope. It helps in preventing the progression of an abnormal scenario following an initiating event.
- Identify process excursions beyond normal boundaries and provide this information (typically, as alarm messages) to the operator, who is expected to take a specific corrective action (control the process or shut down).
- State controllers (logic solver or control relays), take automatic action to trip the process, rather than attempt to return the process to within the normal operating envelope.
The architecture of Distributed Control System
Distributed Control System has three important attributes. The first quality is that it distributes various control functions into relatively small subset systems. These sub-system sets are semiautonomous, and a high-speed communication bus interconnects them. Functions like process control, process supervision, data acquisition, data presentation, reporting information, storing, information retrieval, etc are performed by these sets.
Automation of the manufacturing process by integrating advanced control strategies is the important second quality of DCS. The third attribute is the proper arrangement of things as a system.
Distributed control systems, by organizing the entire control structure as a single automation system, are unified through a proper command structure and information flow. The basic elements of a DCS consist of an engineering workstation, process control unit or local control unit, operating station or HMI, smart devices, and communication system.
Working Principle of DCS System
Sensors in a DCS control system sense the process information and then send it to the local I/O modules. The actuators connected with these modules control the process parameters. The collected information is used by the process control unit via field bus.
This information is processed further and analyzed to produce the output results. The control logic implemented in the controller is used in this process. The control actions as per the output results are then carried to the actuator devices via field bus. At the engineering station, the DCS configuring, commissioning, and control logic implementation are carried out.
Basic Distributed Control System Elements
Upon receipt of information from the operator, the DCS interacts with the processes continuously. The distributed control system also facilitates the opening and closing of valves using operator manual control. To achieve all these functions DCS consists of various elements like
- Engineering PC or controller or Engineering Workstation
- Distributed controller or Local control unit
- Operating station or HMI
- Communication system media and protocol
- Smart or Intelligent Devices
DCS component failures
The DCS consists of many hardware and software components and relies heavily of communication equipment to access and display process information. The Control functions are composed of many components that can fail. Examples of control function failure are:
- Field sensor fails
- Inadequate control loop tuning
- Input/outputs (I/O) fail to change states
- The main processor ceases processing information
- Inadequate or slow data acquisition (e.g., sluggish response under upset conditions)
- Positioner fails
- The final control element fails
- Loss of the operator HMI information
- Disrupted communication with field devices.
The DCS control loop is a relatively weak barrier, as there is usually
- little redundancy in the components,
- limited built-in testing capability, and
- limited security against unauthorized changes to the internal program logic.
The limited security arrangements are particularly important when considering the effectiveness of the DCS. Human error (in modifying logic, bypassing alarms and interlocks, etc.) can significantly degrade the anticipated performance of DCS systems if security is not adequate.
Characteristics of Distributed Control System
There are various important features of DCS as listed below:
- DCS can handle complex processes using dedicated controllers.
- It provides a system redundancy feature that increases system reliability.
- To deal with large and complicated systems DCS offers many algorithms, pre-tested and pre-defined functions, more standard application libraries, etc.
- The distributed control system provides highly sophisticated HMI to control and monitor processes.
- Depending on the number of I/O’s the CDS structure is scalable.
- Also, a DCS design system offers a highly secured system at different levels of operation.