by Distributed Control System (DCS)
Introduction
DCS is a control system which collects the data from the field and decides what to do with them.
Data from the field can be:
- Stored for future reference
- Used for simple process control
- Use in conjunction with data from another part of the plant for advanced control strategies
[You must be registered and logged in to see this link.]
DCS architecture
- Operator Console
- Engineering Station
- History Module
- Data Historian
- Control Modules
- I/O
Operator Console
- Like the monitors of computers
- Provide the feedback of what they are doing in the plant as well as the command we issue to the control system
- These are also the places where operators issue commands to the field instruments
Engineering Station
- Stations for engineers to configure the system and also to implement control algorithms.
History Module
- Like the harddisk of PCs
- Store the configurations of the DCS as well as the configurations of all the points in the plant
- Store the graphic files that are shown in the console and in most systems these days they are able to store some plant operating data
Data Historian
- Software that are dedicated to store process variables, set points and output values
- They are usually of higher scanning rates than that available in the history module.
Control Modules
- These are like the brains of the DCS
- Specially customized blocks are found here
- These are customized to do control functions like PID control, ratio control, simple arithmetic and dynamic compensation
I/O
- Manage the input and output of the DCS
- Input and output can be digital or analogues
- Digital I/Os are those like on/off, start/stop signals
- Most of the process measurements and controller outputs are considered analogue
- These are the points where the field instruments are hard-wired to.
[You must be registered and logged in to see this link.]
Information displays
Various types of display:
- Group displays
- Overview displays
- Detail displays
- Graphic displays
- Trend displays
group display
Shows the operating parameters of group of control loops such as four, eight, twelve, or sixteen control loops, arranged in rows so that they look like the faces of instruments on an instrument panel.
group display
[You must be registered and logged in to see this link.]
Overview Displays
- Shows the bar essentials of a number of groups, each group in a separate rectangle
- Digital conditions can also be displayed on an overview display
Detail Displays
- Specific to a single loop or control function
- Includes additional information defining constants, limits, and other characteristics of the function
Graphic Displays
Graphic display capability allows a picture to be drawn on the screen so that the operator can look at a portion of the process more realistically than by watching a row of bar graphs
Trend Displays
- Trend displays are the distributed control system equivalents of chart records.
- Shows change in process value that have taken place over a period of time
[You must be registered and logged in to see this link.]
DCS Software Configuration
- Operating System Configuration
- Algorithm Libraries
- Controller Function Configuration
Operating System Configuration
- Configured from the operator station, sometimes using a special keyboard
- Master program opens and closes the files to use information as it is required
- Configuring is usually done off-line
Algorithm Libraries
Algorithms are used to satisfy three distinct process needs
- Analog control
- Digital control
- Sequential control
Controller Function Configuration
Algorithm library includes advanced control strategies such as
- Dynamic compensation lead/lag blocks for feed-forward
- External feedback for anti-wind up
- Self-tuning
- Algorithms for analog input
- Statistical process control
- Fuzzy logic
- Model-based optimization
DCS Communication
DCS are interfaced with:
- Supervisory computers and PLCs Smart transmitters
- Microprocessor based analyzers and other instruments in the system
International Field-bus Standards
German standard called a profi-bus
- Suppliers - Rosemount, Fisher Controls, Siemens, Endress & Hauser, Yokogawa etc
Another standard, formed from the alliance of the standards groups Inter-Operable Systems Project (ISP) and WorldFIP North America, combined together
- Suppliers - Honeywell and Telemechaniques
Third standard protocol, called the Controller Area Network (CAN)
- Suppliers - Alien-Bradley, ABB, Numatics, Medicon, Por-Log
Data Highway Designs
- Communication device that allows a distributed control system to perform the distribution of the controlling function through out a large plant area
- Highway length can be more than 8 kilometers
Data Highway Designs
Popular medium for the data highway:
- Coaxial cable
- Fiber glass cables
Data Highway Designs
Fiber optics is attractive for use as a data high because of the following advantages:
- It eliminates problems of electromagnetic and radio frequency interference, ground loops, and common mode voltages.
- It is safe in explosive or flammable environments
- It can carry more information that can copper conductors.
- It is inert to most chemicals.
- It is lighter and easier to handle than coaxial cable.
On-line Information System
On-line information system may include any or all of the following functions:
- Data collection, checking, and verification
- Data reconciliation
- Data storage and retrieval
Features of DCS
DCS can perform the following functions:
- Monitor and manipulate the process
- Retrieve historical data
- Configure the system
- Build schematic displays
- Develop control programs
- Diagnose system failures
Advantages of DCS
- Overall cost of the installation is lower
- Interface with the process is improved
- More reliable
- Flexible and relatively easy to expand
- Programming required to tailor the system can be done without knowing a high-level programming language
