DCS

Distributed Control System

Why DCS

Ø  For Total Plant Automation

Ø  For Higher Productivity

Ø  For Optimal Process Control

Ø  For Advance Process Control

Ø  For Regulatory Compliance

Ø  For Management information System

Ø  In Tune with Global Requirement

What is DCS System

A distributed control system (DCS) is a specially designed automated control system that  geographically and functionally distributed all  over the plant or control area. It is specially designed with redundancy and diagnostic capabilities to improve control reliability and performance. It gives greater flexibility to control distributed discrete field devices and its operating stations

Distributed Control System is a specially designed control system used to control complex, large and geographically distributed applications in industrial processes. In this, controllers are distributed throughout the entire plant area. These distributed controllers are connected to both field devices and operating PCs through high speed communication networks as shown in figure.

It differs from the centralized control system wherein a single controller at central location handles the control function, but in DCS each process element or machine or group of machines is controlled by a dedicated controller. DCS consists of a large number of local controllers in various sections of plant control area and are connected via a high speed communication network.

In DCS control system, data acquisition and control functions are carried through a number of DCS controllers which are microprocessor based units distributed functionally and geographically over the plant and are situated near area where control or data gathering functions being performed as shown in the figure above. These controllers able to communicate among themselves and also with other controllers like supervisory terminals, operator terminals, historians, etc.

Distributed individual automatic controllers are connected to field devices such as sensors and actuators. These controllers ensure the sharing of gathered data to other hierarchal controllers via different field buses. Different field buses or standard communication protocols are used for establishing the communication between the controllers. Some of these include Profibus, HART, arc net, Modbus, etc.

DCS is most suited for large-scale processing or manufacturing plants wherein a large number of continuous control loops are to be monitored and controlled. Nowadays, distributed control system has been found in many industrial fields such as chemical plants, oil and gas industries, food processing units, nuclear power plants, water management systems, automobile industries, etc.

Features of DCS:

 

Ø  Reduce System Failure: Control function is distributer among multiple CPUs (Field Control Stations). Hence failure of one FCS does not affect the entire plant.

 

Ø  Sophisticated HMI: Use color CRT with outstanding resolution. Trending, logging and graphical representation of the HMI’s give effective user interface.

Ø  Redundancy is available at various level.

 

Ø  Superior Reliability: Due to redundancy and partitioning it ensures an outstanding level of reliability, performance and expandability.

Ø  Broad Flexibility and expandability: Structure of DCS can be scalable based on the number of I/O’s from small to large  system by accommodating additional stations.

Ø  System security: Security is also provided at different levels such as engineer level, entrepreneur level, operator level, etc.

Ø  Interlocks: Instruments and interlocks are created by software. Generation and  modification of the interlocks are very flexible and  simple

Ø  Easy system maintenance: In the unlikely event of failure, a self diagnostic function  quickly locates the source of the problem, Modular component design permits easy replacement .Maintenance and troubleshooting become very easy.

Ø  Minimize production cost: To increase reliability, productivity and quality while minimizing the production cost.

 

Ø  Powerful alarming system of DCS helps operators to respond more quickly to the plant conditions

 

Ø  Cost effective in the long run.

Ø  Information regarding the process is presented to the user in various format.

Ø  Field wiring is  considerably less

 

Working & Operation of DCS System

 

The operation of DCS goes like this; Sensors senses the process information and send it to the local I/O modules, to which actuators are also connected so as to control the process parameters. The information or data from these remote modules is gathered to the process control unit via field bus. If smart field devices are used, the sensed information directly transferred to process control unit via field bus.

The collected information is further processed, analyzed and produces the output results based on the control logic implemented in the controller. The results or control actions are then carried to the actuator devices via field bus. The DCS configuring, commissioning and control logic implementation are carried at the engineering station. The operator able to view and send control actions manually at operation stations.

Architecture of DCS:

Architecture of DCS are almost same for all manufactures with a little difference. Here is  a architecture of DCS from Yokogawa model CENTUM CS 3000

Human Interface Station (HIS)

HIS is the operating station, is used to operate, monitor and control plant parameters. It can be a PC or any other monitoring device that has a separate software tool on which operator can view process parameter values and accordingly to take control action.

Operating stations can be a single unit or multiple units where a single unit performs functions like parameter value display, trend display, alarming, etc. while multiple units or PCs performs individual functions such as some PCs display parameters, some for trend archives, some for data logging and acquiring, etc.

Engineering Station (ENG)

Engineering station is a PC with engineering capabilities used for system configuration and system maintenance.

It is the supervisory controller over the entire distributed control system. It can be a PC or any other computer that has dedicated engineering software

This engineering station offers powerful configuration tools that allow the user to perform engineering functions such as creating new loops, creating various input and output points, modifying sequential and continuous control logic, configuring various distributed devices, preparing documentation for each input/output device, etc.

Field Control Station (FCS)

Field control station is the control unit for plant process control. Varying with the application capacities and application usages, there are several types of field control stations in the lineup. Controllers are distributed geographically in various section of control area and are connected to operating and engineering stations which are used for data monitoring, data logging, alarming and controlling purpose via another high speed communication bus.

These communication protocols are of different types such as foundation filed bus, HART, Profibus, Modbus, etc. DCS provides information to multiple displays for user interface.

Safety Control Station (SCS)

Safety control station is a controller of the ProSafe-RS TÜV SIL3 certified safety instrumented system and it can be configured on the same network with CENTUM CS 3000 enabling HIS to monitor status of ProSafe-RS.

I/O Modules

Varieties of Fieldnetwork I/O (FIO) are available in compact sizes to convert process signals for FCS formats.  Remote I/O are for remote input and output that passes the field signals to FCS control unit via remote buses.

Bus converter (BCV)

Bus converters are used to link multiple domains.

System Integration OPC Station (SIOS)

SIOS serves as a gateway for connecting CENTUM CS 3000 R3 system and the OPC server to the third-party PCS. It exchanges data, and acquires alarms and events through the OPC server.

Control Network (Vnet/IP)

A real-time control network for linking the stations such as FCS, HIS, and BCV.

Field Digital Network

CENTUM CS 3000 R3 supports field digital protocols such as　FOUNDATION fieldbus, HART, PROFIBUS, DeviceNet, Modbus, Modbus/TCP, EtherNet/IP and ISA100 wireless.

Controller

Yokogawa controllers are designed for controlling and monitoring industrial plants which must keep running non-stop in stabilized status with high reliabilities.

DCS Systems from Different Vendors

Some of the available DCS systems include

Ø  ABB- Freelance 800F and 800xA, Freelance 2000

Ø  Yokogawa  - Micro Excel , Centum Excel, Centum CS 1000, CS 3000, Centum VP

Ø  Honeywell  -TDC 3000, Experion PKS, TPS, GUS

Ø  Emerson  - Delta V Digital Automation

Ø  Siemens  - Simatic PCS 7

Ø  Allen Bradley  - NetLinx

Ø  Foxboro –I/A Series

Ø  Moore – APACS

PLC and DCS a Relative study:

Ø  DCS was developed as a replacement of ‘PID controller’, whereas PLC was developed for the replacement of ‘Relay Logic’.

Ø  PLC is mainly used for small application, whereas DCS is used for large application.

Ø  Both PLC and DCS are used for ‘logic’ and ‘continuous controller’. But PLC is mainly used for Logic where as DCS is mainly used for continuous control.

Ø  Scan time of PLC is less than DCS. So PLC is faster than DCS.

Ø  DCS must have redundancy but PLC might not.

 

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Field Control Station (FCS)

A cabinet type and a rack-mount type of FCS are available from Yokogawa. Both Vnet/IP and V net control networks can be used for configuring the system networks. The dual-redundant design of the FCS processor modules, power supply, I/O modules, and I/O networks achieve high availability of 99.99999%. The pair & spare architecture is unique to Yokogawa that improves the stability of process control.

Process I/O Modules

Fieldnetwork I/O (FIO)
Yokogawa offers compact, cost-effective, and reliable I/O devices, targeted as the industrial standard.

Subsystem Integration

In order to operate and monitor the subsystems of the auxiliary devices controlled by programmable logic controller (PLC), various types of analyzers and other instruments for the integrated plant automation, FCS subsystem communication capability enables connection with the subsystems, and also supports dual-redundant configuration.

The subsystems include Mitsubishi MELSEC, Allen Bradley PLC-5 and SLC500, Modbus compatible devices, Siemens SIMATIC, Omron SYSMAC, Yokogawa FA-M3, Darwin, and DAQSTATION and so on.

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Human Machine Interface

Human Interface Station (HIS)

Operators access production control system via CENTUM CS 3000’s human interface station (HIS) to make fast and intelligent decisions that maximize performance and minimize risks.

Based on the ergonomic design concept, the HIS can be selectable from desk top, open display style console, and enclosed display style console types. Multiple monitors can be set, and each one of the monitor displays multiple operation windows.

Dedicated Operation Keyboard

The operation keyboard is unique to Yokogawa. It consists of alphanumeric keys and special operation keys which are assembled with the dustproof and drip-proof flat keys. By assigning graphics to each button, operators can call up a graphic by one touch.

Operation Windows

There are different window like Graphic window, Control window, Overview window, Trend window

Screen Modes

The ‘Multiple-window mode’ like an office PC and the ‘Full-screen mode’ like the legacy operator console are available for operation and monitoring environment.

Open Interface

The process control industry standard open interface OPC (OLE for Process Control, or open productivity & connectivity) is provided for collecting the process data, trend data and messages.

Access Control

To prevent operation errors and other problems and ensure the safety of the system, such as prohibiting operations by unauthorized persons, limiting the ranges of operation and monitoring, and restricting operator actions to the system, extensive security functions are applied.

 

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Process Control Unit of DCS

It is also called as a local control unit, distribution controller, or process station. A distributed control system can consists of one or more process stations that can be extended with different types of I/O units. These controllers consist of a powerful CPU module, field bus or communication module with extended field bus capability and either direct or remote connected I/Os.

The field devices like sensors and actuators are connected to I/O modules of this unit. Some field devices can be directly connected to field bus (such as Profibus) without any I/O module, which can be termed as smart field devices.

These units acquire the information from various sensors via input module, analyze and process it based on the control logic implemented and sends the output signals via output modules to have control on actuators and relays.

This acts as process station, which is responsible for acquiring and controlling the data from the process. This unit consists of a power supply along with CPU section, Ethernet section, Profibus section and remote communication interface unit for I/Os interfacing as shown in the figure where first module is AC 800F unit and other one is remote I/O (also called as communication interface module).

Communication System

The communication medium plays a major role in the entire distributed control system. It interconnects the engineering station, operating station, process station and smart devices with one another. It carries the information from one station to another. The common communication protocols used in DCS include Ethernet, Profibus, Foundation Field Bus, DeviceNet, Modbus, etc.

It is not mandatory to use one protocol for entire DCS, some levels can use one network whereas some levels use different network. For instance, consider that field devices, distributed I/Os and process station are interconnected with Profibus while the communication among engineering station, HMI and process station carried though Ethernet as shown in the figure below.

The major advantage of DCS is the redundancy of some or all levels of the control area. Most of the cases critical processes are installed with redundant controllers and redundant communication networks such that problem in main processing line should not affect the monitoring and control functions because of the redundant processing section.

 

Smart or Intelligent Devices

The intelligent field devices and field bus technology are advanced features of DCS technology that replaces traditional I/O subsystems (I/O modules). These smart devices embed the intelligence required for simple sensing and control techniques into the primary sensing and actuating devices. And hence it replaces the need for a DCS controller to perform routine sensing and control process.

These field devices can be directly connected to field bus so that sourcing of multiple measurements to the next higher level control station is possible via digital transmission line by eliminating extraneous hardware such as local I/O modules and controllers.

Difference between SCADA and DCS (DCS vs SCADA)

Although both DCS and SCADA are monitoring and control mechanisms in industrial installations, they have different goals. There exist some commonality between DCS and SCADA in terms of hardware and its components, however, there are certain requirements by the end applications that separates a robust and cost-effective DCS from the viable SCADA system. Some of the differences between DCS and SCADA are listed below.

1.      DCS is process oriented, whereas SCADA is data-gathering oriented. DCS emphasizes more on control of the process and it also consists of supervisory control level. And as a part of doing so, it presents the information to the operator. On the other hand, SCADA concentrates more on acquisition process data and presenting it to the operators and control centre.

2.      In DCS, data acquisition and control modules or controllers are usually located within a more confined area and the communication between various distributed control units carried via a local area network. SCADA generally covers larger geographical areas that use different communication systems which are generally less reliable than a local area network.

3.      DCS employs a closed loop control at process control station and at remote terminal units. But in case of SCADA there is no such closed loop control.

4.      DCS is process state driven where it scans the process in regular basis and displays the results to the operator, even on demand. On the other hand, SCADA is event driven where it does not scan the process sequentially, but it waits for an event that cause process parameter to trigger certain actions. Hence, DCS does not keep a database of process parameter values as it always in connection with its data source, whereas SCADA maintains a database to log the parameter values which can be further retrieved for operator display and this makes the SCADA to present the last recorded values if the base station unable to get the new values from a remote location.

5.      In terms of applications, DCS is used for installations within a confined area, like a single plant or factory and for a complex control processes. Some of the application areas of DCS include chemical plants, power generating stations, pharmaceutical manufacturing, oil and gas industries, etc. On the other hand SCADA is used for much larger geographical locations such as water management systems, power transmission and distribution control, transport applications and small manufacturing and process industries.

In spite of these major differences, the modern DCS and SCADA systems come with common standard facilities while dealing process plant automation. However, the choice between DCS and SCADA depends on its client and end application requirement. But if the client choice between these two, by gaining equal requirement from the process, DCS is the economical choice as it help to reduce the cost and offer better control.

 (https://www.electricaltechnology.org/2016/08/distributed-control-system-dcs.html)

Foxboro I/A Series

The following figure is the basic architecture of Foxboro I/A Series system

Siemens PCS7

The following figure is the basic architecture of Siemens PCS7

Honeywell Experion PKS

The following figure is the basic architecture of Honeywell Experion PKS

Honeywell TDC3000

The following figure is the basic architecture of Honeywell TDC3000

Honeywell Total Plant Solutions (TPS)

The following figure is the basic architecture of Honeywell TPS

Yokogawa Centum CS3000

The following figure is the basic architecture of Yokogawa Centum CS3000

What is Distributed Control System

7 Important features of DCS

• To handle complex processes:

In factory automation structure, PLC-Programming Logic Controller is used to control and monitor the process parameters at high speed requirements. However due to limitation of number of I/O devices, PLC’s cannot handle complex structure.

System redundancy:

System Redundancy

CENTUM CS3000 R3

Application of Windows Technology

With Windows Remote Desktop capability, plant operation, monitoring, and engineering can be performed from a personal computer in your office or at a remote field location without any additional software.

Remote Operation and Monitoring
The same HIS displays in the control room can be shown on the PC in your office. For production facilities in remote locations around the world, remote operation and monitoring can be simply structured.

Remote Engineering
The engineering work for modification can be performed remotely via a network, eliminating the need for dispatching engineers and reducing both maintenance and engineering cost.