What is a distributed control system (DCS)?
A distributed control system (DCS) is a digital automated industrial control system (ICS) that uses geographically distributed control loops throughout a factory, machine or control area. The goal of a DCS is to control industrial processes to increase their safety, cost-effectiveness and reliability.
A control system is a set of mechanical or electronic devices that regulate other devices or systems through the use of control loops. Control loops are systems composed of all the hardware and software control functions needed for measurements and adjustments in an individual process. Control systems are a central part of industrial and process automation systems.
How does a distributed control system work?
DCSes are made up of control elements distributed throughout a plant or factory. These control elements include computers, sensors and controllers. Each element serves a specific purpose, such as data collection, data storage or process control.
Unlike a centralized control system that operates all machines, a DCS enables each part of a machine to have its own dedicated controller that runs the operation.
A DCS has several local controllers throughout the factory. A high-speed communication network connects the local controllers. While each controller works autonomously, there is central supervisory control run by an operator.
Having the control system architecture distributed around the factory or plant provides an effective control, efficiency and process quality strategy. Additionally, if a piece of the DCS architecture fails, a plant can continue to operate. For these reasons, DSCes are commonly found in industrial fields and processing or manufacturing plants. The market for DSCes continues to grow; it's expected to reach $23.2 billion by 2026 from $17.5 billion in 2021, according to MarketsandMarkets Research.
A DCS is functionally similar to supervisory control and data acquisition (SCADA) industrial control applications. SCADA systems are used in power plants, oil and gas refining, telecommunications, transportation, and water and waste control. They tend to be used in situations where the control center has a more remote location.
Where is a DCS commonly used?
A DCS is commonly used in industrial process industries, including the following:
- chemical plants
- petrochemical and refineries
- nuclear power plants
- water and sewage treatment plants
- food processing
- automobile manufacturing
- pharmaceutical manufacturing
How a DCS is structured
A distributed control system includes both software and hardware elements. The installation of most controllers locally keeps costs low. On-site automated control improves the reliability of these systems, and human oversight is enabled for central control functions and remote control options. Individual processes have their own controllers with separate central processing units (CPUs) so other processes can continue if an individual system fails.
The architectural design of a DCS includes the following:
- An engineering workstation. This is a supervisory controller for the DCS as a whole. The station typically includes configuration tools that let the user complete functions such as creating new loops and input and output (I/O) points, as well as configuring distributed devices.
- An operating station. This station is used for control, operation and monitoring.
- A process control unit. This microprocessor-based controller is designed for automatic and compound loop control.
- A communication system. This system brings data from station to station and is important in distributed control systems. Typical network protocols used include Ethernet, Profibus and DeviceNet.
- Smart devices. These are any smart devices or bus technologies that are used to replace older I/O
In a DCS architecture, sensors gather and process information, which is sent to a nearby I/O module. The data is moved to a process control unit. Smart devices, if used, can also send the data to a process control unit. The data is then further processed and analyzed to produce an output result.
The processed output is based on the control logic used, which is the part of a program that controls program operations. This data is sent to a moving actuator device using another bus. And then the commissioning process occurs. This is where instruments and control systems are verified and documented and where the control logic and implementations are transferred to an engineering station for the operator to view. The operator sends control actions to operation stations.
The devices in this process can be divided into the following five levels:
- Level zero consists of field devices, like sensors and other control elements.
- Level one is the I/O modules and processors.
- Level two is the supervisory computers that collect data from processor nodes.
- Level three focuses on the production control level that monitors production.
- Level four includes production scheduling.
What are the advantages and challenges of a DCS?
Distributed control systems provide the following benefits:
- Complex structures. Unlike comparable programmable logic controllers (PLCs), a DCS can access large amounts of information in a complex environment.
- System redundancy. If a processor fails, the redundancy provided in the DCS ensures that only one section of the plant's processes is interrupted.
- Scalability. More control or process units can be added whenever needed. Adding more I/O modules to a controller also extends I/Os.
- Security. Security and cybersecurity capabilities are enabled at the engineer and operator levels.
Some downsides of the technology include situations where failure of one controller affects more than one loop. DCSes can also increase software development costs, and diagnosing problems can be a complex process.
DCS vs. programmable logic controller systems
PLCs are another type of industrial control technology. They are small, modular computers with customized instructions for performing a particular task. Like DCSes, PLCs are used in ICSes for a variety of industries. PLCs were also designed to replace mechanical relays, drum sequencers and cam timers.
PLCs are useful tools for repeatable processes because they have no mechanical parts. Each CPU continually loops through an input scan, program scan, output scan and housekeeping mode, repeatedly performing a single task while monitoring conditions. The information the controller gathers provides feedback to guide changes and improvements to processes -- some of which can be performed automatically depending on the device's coding.
A PLC controls individual devices, while a DCS can control multiple machines in a plant. And, while PLCs are designed to replace mechanical devices like mechanical relays, DCSes are able to control entire factories and process plants with many interconnected systems.
Other industrial control system technologies include programmable automation controllers, industrial automation and control systems, remote terminal units, control servers, intelligent electronic devices and sensors.
Learn about security challenges commonly found in ICSes and how to overcome them.