Distributed control system (DCS) is a sophisticated control system for a plant or a process consisting of many controller loops that are usually autonomous controllers distributed throughout the process without any central operator supervisory control.
Table of Contents
The distributed control system is one of the popular control systems specially designed with diagnostic and redundancy capabilities to enhance the controller performance and reliability. This system provides greater flexibility to control operating stations and field devices with high productivity and minimize production costs.
It is the era of technology, and the industrial automation system requires an advanced automation control that should have more control over the old age used complex processes.
The distributed control system is specially designed for the industrial complex, geographically distributed, and extensive processes. In this system, the controllers are distributed throughout the entire plant.
These controllers are connected with the operating PCs and field devices with the help of a high-speed communication system.
Communication buses are used to connect the input devices such as sensors and actuators to the output controller modules such as pumps, heaters, valves, etc. The sensors, actuators, or smart devices can communicate with the other controllers or program logical control systems.
Evolution in process control operations
Many stages have been involved in the process control of large-scale industrial plants. The initial control of process operation was from panels located near the process operation.
There was a considerable workforce and full concentration to communicate, control and monitor the dispersed panels with no overall view of the entire process.
The next development to control the operation was to provide the measurement to a permanently-manned central control room. This system effectively controlled the nearly located process operations through a centralized or localized panel control system.
An inadequate control system was inflexible due to the controller hardware on each loop and continuous operative movement from the control room to the control hardware devices or final control elements to take necessary actions.
With the development in the electronic processor and graphical displays, it becomes easier to combine these discrete controllers with the computer base algorithm with their controlling processor to communicate with the sensor or actuator’s inputs and take their actions to keep the process on the streamline.
The evolution of DCS in the process industry has made many process control easier, user-friendly, and controlled through a specified system.
It is an automated system consisting of sophisticated alarm handling, automatic measuring, controlling the actions of final control elements quickly, reducing many cable runs, and removing the need for the physical record in the form of charts.
The architecture of the distributed control system (DCS)
One of the key advantages of the DCS system is its capability to control a system around nodes. If the control on a process fails, the remaining system keeps working smoothly instead of shutting down the whole system as in the central control system. This process also eliminates the response delay that is observed in the central control system.
This block diagram shows the general model describing a distributed control system’s computerized control functional manufacturing levels.
From the diagram above:
- Level 0 has field devices, including temperature sensors, flow measuring, level indicators, actuators, and final control elements, such as heaters, control valves, etc.
- Level 1 has industrialized output/input based modules and their related electronic processors.
- Level 2 has computers that provide instructions that collect information from the system processor nodes and provide a control screen to the operators.
- Level 3 is a level that monitors the production of a process; it does not control the production, but it provides the production rate concerned with a product’s production targets.
- Level 4 is related to the production schedule.
In traditional DCS, levels 1 and 2 are considered functional levels that include all equipment part of an integrated system.
Technical Points of DCS system
The operator graphical display and the processor nodes are connected with the industry-standard network in a distributed control system. The reliability of this process is enhanced by the dual cabling adjustment from the diverse routes.
The onsite function of I/O modules and their related electronic processor reduce the number of cables in the distributed topology system.
The input sensor and actuator provide information to the processors. The function of the processor is to analyze the input signal and generate the possible output that controls the final control element. For example, 4-20 mA DC or signal either “on” or “off” with the help of a semiconductor switch or relay contacts.
This setpoint control system is used to control the action of final control elements connected with DCS. For this purpose, PID controllers take the input, find the error between the setpoint and current value and take their action for the final control elements.
The large oil refineries and chemical plants have several hundred I/O points data connected with large-scale DCS.
DCS consists of a wide range of control systems that do not only consist of a flow through a pipe or temperature sensing devices, but it is also employed in the cement industry, paper-making machines, speed controller drivers, adjusting the speed of motor drivers that are required in the specialized quality control process and much other application like this.
DCS reliability can also be increased using dual redundant processes that use a “hot switch” that takes action in case of any fault in a control loop.
Although 4-20 mA is the primary field signaling standard, the modern DCS system is equipped with the “Fieldbus” digital protocols, such as Profibus, Fieldbus, HART, etc. Also, it includes other standard Ethernet-based protocols such as ModbusTCP, EthernetIP, and Profinet.
Important features of DSC
Handling Complex processes
Program logical control (PLC) effectively controls high-speed control requirements that monitor and control the process parameters. However, due to a limited number of I/O devices, PLCs cannot handle complex structure control applications.
However, The DCS system can handle complex processes involving multiple products in multiple procedures as in batch process control.
System Redundancy
DCS system provides the facility of redundancy at every point to have more precise control.
Redundancy increases the system reliability by operating the system continuously in a smooth manner even if some abnormalities have occurred in the process operation.
For example, suppose a system restores its steady-state operation after an interval, whether planned or unplanned. In that case, DCS control can handle this situation in a better way as compared to other control systems.
Predefined function blocks
DCS is equipped with the most application libraries, algorithms, and predefined and pre-tested variables to handle complex or uncertain situations to deal with a complex system. It facilitates the system to make it easy and a short time to control.
Program Languages
This system provides more program languages such as functional blocks, ladder, and sequential charts to make the system an effective and user-friendly interface.
Sophisticated Human-machine interface
DCS provides the entire industrial process control in a room as a PC window. In addition, the powerful user interface and graphical representation make the human-machine interface (HMI) user-friendly.
System Security
The structure of DCS is flexible enough to facilitate the system’s scalability by adding more I/O modes in the system without disturbing the primary process control.
Scalability of DCS system
The security of the DCS system facilitates an automated control system. Therefore, it has different levels, such as the administrator, engineer, and operator.
Advancement in DCS
There are new technologies that are the latest development in DCS, which are described as follows:
- Protocols and wireless system
- Data historian, logging, and data transmission
- Control through a mobile interface
- Including web-servers
An excellent DCS trend is becoming centralized at the plant level, monitoring the process through remote equipment.
The opportunity enables the operator to control and monitor the process at an enterprise level (macro) and equipment level (micro), within and outside the plant. Due to this reason, the physical importance of location decreases due to the functioning of wireless access.
Developers have focused on more wireless protocols and refining technology that can be included in DCS.
Currently, DCSs have embedded servers that can provide access through web hosting. As a result, DCS is one of the controls leading the Industrial Internet of Things (IoT) technology.
Currently, many suppliers can control DCS through mobile, both iSO and Android operating systems. It has resolved many issues to control and monitor operations in DCS.
Conclusions
After reading this post, I hope you now understand the fundamentals of VFD and can more easily integrate a VFD into your process.
