Distributed control as applied to industrial processes is a relatively young technical field, tracing its beginnings back to the invention of the microprocessor and the development of the first systems in the early 1970s. There is no question that the market introduction of the first commercial distributed control system in late 1975 was a milestone that marked a new era in industrial control. However, even in the short time since then, the field has undergone a significant amount of change that makes the early systems look primitive in the light of technology of the 1980s. Some examples of this evolution are as follows:
• Local control units that once were no more than replacements for a few PID loops now are able to execute a combination of continuous control, sequential control, data acquisition, and batch control functions. Accomplishing each of these functions no longer requires the use of separate hardware.
• Configuring these control functions once meant an endless amount of button-pushing, with constant references to a stack of manuals. The development of CAD technology now allows the user to draw, edit, and duplicate control configurations directly on a CRT screen and then download them directly to the controllers.
• The bandwidth of the shared communications facilities in a distributed system has increased by a factor of 40 or more. The use of fiber optics as a transmission medium is common.
• CRT displays that once only mimicked simple panelboard instrumentation devices have been supplanted by colour graphics displays that provide the operator with an easily understandable view of plant operations, from the highest level of supervisory control down to the individual loop level.
CONCLUSION
CONCLUSION
I did a detailed study on Distributed Control System which is a microprocessor based system. I studied the basic components of Distributed Control System, their working and about the different shared communication facilities used in Distributed Control System.
While going through all these things, I find certain areas in Distributed Control System which are going to be dramatically changed in near future due to the pace of technological progress. Likely trends of development will be in the areas of system architecture, control capabilities, operator interfaces, and apllications. These are summarized below:
-Trends in System Architecture
One major architectural trend that already is under way is the expansion in scope and the corresponding integration of functions of the distributed control system in the industrial plant. Horizontal expansion is occurring in the sense that many more of the plant instrumentation and control functions previously performed by separate, independent subsystems are now being integrated into a single plantwide system.
Examples of these independent subsystems are programmable controllers used for safety and interlock functions, thermocouple data loggers, and sequence-of-events monitors. Of course, a degree of separation in function will remain when needed for redundancy and safety reasons. However, the similar hardware will be used to implement the functions, thus reducing the amount of personal training required and the number of different spare parts kept on hand. Another factor encouraging horizontal expansion is the continuing emphasis on optimizing plantwide production operations and minimizing overall energy usage . This can only be accomplished through the plantwide integration of the monitoring and control functions.
These influences toward complete integration of plant operation will also encourage the vertical expansion in scope of the distributed control system. Currently, a different vendor often supplies the hardware at each level of information and control
in the plant: from the sensors and analyzers that obtain the basic measurement information in the plant, through the first level of regulatory control, and up to the higher levels of plant production computer system and corporate information systems.
The task of providing interface is simplified significantly if most of the layers are implemented with distributed control hardware that has been designed from beginning to support an integrated database. The information in this database can then be used at the highest levels in the control hierarchy to provide on-line plant performance information and an analysis of alternative plant operating strategies.
Trends in Controller Capabilities
The continuing increase in power and capacity of the microprocessors, memories, and associated support chips that make up a distributed control system is a trend that will have a significant impact on the capabilities of the LCUs in such a system. One impact is to increase the speed of execution of the algorithms only two or three times a second. This sampling and processing rate is too slow to handle fast control loops such as compressor surge controls and boiler windbox pressure control. Hardware advances will make it possible for controllers to execute their control algorithms at a rate ten to 50 times per second. This will allow digital controllers to be used in the full range of applications implemented today with analog controllers.
Trends in Operator Interface
The current designs of operator interfaces in distributed control systems have been influenced greatly by the historical use of panelboard instrumentation as the primary operator interface in the control of industrial processes. To encourage plant operators to accept the new hardware, designers of CRT-based interfaces have used many of the characteristics of panelboards in the layout of displays and keyboards in the new equipment. Clearly, some immediate work is required to incorporate current knowledge of what constitutes a good interface design into standard display formats and keyboard and touch screen layouts. However, the general acceptance of VDUs as the primary operator interface mechanism in the distributed systems has opened the door to developing new concepts in the interfaces between humans and machines.
Application Knowledge- the Key Trend
As distributed control systems become more standardized and the major vendors, systems become saturated with the most recent technology, a major factor in evaluating and designing these systems will be the amount of knowledge and experience that the vendor has in the user’s intended area of application. This knowledge will be especially important as horizontal and vertical expansion in scope of the distributed control system continues. To provide a system that is effective in a particular application, the vendor designing that system must be aware of :
1. The requirements on and characteristics of the various subsystems that make up the total instrumentation and control system for the application.
2. The specialized sensors and analyzers that are used in that application.
3. Any specialized control hardware and control or computing requirements for that application.
4. The modes of plant operation in that application, including the methods used in plant startup, shutdown, and change in operating modes.
This knowledge will not replace the expertise of the user, who knows the plant better than anyone else. However, if the vendor does not have this application background and experience, it is likely that the distributed control system the vendor supplies will not have the hardware interfaces and the control and computational packages needed to help the user put together a system that works well in the application.