With the introduction of digital communication between the field and process levels and with the shifting of automation functions from process control system (centralised or decentralised) to intelligent field devices, fieldbus technology is assuming more and more importance.
A new chapter has been opened in the development of fieldbus technology. Discussion to date has placed emphasis on the following:
* Serial signal exchange between field units and control systems in the field of industrial process automation as being the better substitute for parallel signal exchange.
* Which types of expenditure can be avoided?
* Which network structures are required for process automation from the point of view of costs and plant availability?
* Protocol definition and standardisation.
* Which units fulfil which protocol standards in order to safeguard the interchanging of third party equipment?
* Which additional facilities are required before a fieldbus system can be operated?
Current discussion is focused on:
* The capacity to supply equipment and peripherals.
* Practical implementation of fieldbus technology in specific plants.
At the present time, neither supplier nor user experience with fieldbus systems is sufficient to warrant the wholesale replacement of current parallel signalling technology with a serial communications network in every process control situation. For a long time the Namur working group has been forecasting substantial savings potential on expenditure for hardware, installation and engineering (see Figure). Suppliers and users alike must take advantage of the practical application phase, just now beginning, in order to enjoy the potential advantages of fieldbus technology within an evolutionary process.
Suppliers and users are still waiting for unified, standardised fieldbus. From the large number of national and de facto standards from manufacturers, two protocol standards have established themselves:
Profibus (process fieldbus) technology
This technology is being developed and maintained by the Profibus User Organisation. The organisation has its headquarters in Karlsruhe and is run by its membership, made up of manufacturers, users and research institutions. Regional user groups support the technology worldwide in 16 leading industrialised countries.
Fieldbus Foundation technology
This technology is being developed and maintained by the Fieldbus Foundation. All important distributed control systems and equipment manufacturers in America, Europe and Asia are members of this organisation which has its headquarters in Austin, Texas and is supported by many user groups worldwide.
The technologies of Profibus and the Fieldbus Foundation differ on a number of important points. The Foundation Fieldbus permits cyclical communication between the field units, thereby enabling decentralised loop-control and calculations to be made, as well as permitting independent alarm signalling by field units. The Profibus master-slave procedure, on the other hand, always requires a cyclical or acyclical accessing by a master unit. Independent action by a slave or the exchange of data between two slaves is not possible without the participation of the master unit.
As a provider of equipment, management and process control systems and analytical engineering for process automation, ABB Automation supports the two technologies mentioned above.
User requirements
The requirements that the fieldbus technology must meet are manifold. The field instrumentation and the associated communications requirements vary according to the kind of process being automated. Furthermore, in the future, not only the digital communication with the field but also the field devices themselves will see changes in their roles and functions in the automation system. It then becomes clear that not only the field level of process automation but also the process and managing levels are going to be affected by the fieldbus technology.
Apart from the technical requirements placed on the fieldbus concept within the framework of complete automation, other general requirements must be met:
* Standardisation of the communications technology.
* Comprehensive product availability both in terms of supplier variety and in terms of required functionality.
* Planning aids, technical training, maintenance and service tools.
On the technical side, the requirements are initially based on all the characteristics of today's analog and parallel methods for signal exchange between the field and the system:
* Reliability in the transfer of information and the time behaviour.
* Intrinsic safety and field unit supply power on signal lines.
* Equipment exchange during running operations and protection from interference from the operated plant.
* Expansion of the process control system and the use of field units, irrespective of manufacturer.
* Standardised interfaces from the fieldbus system to the process and plant operation levels.
User advantages
The advantage to the user is one of the main targets of any development. Seen in this way, the characteristics of a product must directly reflect the requirements of users. This also applies to the fieldbus technology. As a matter of course, as any new technology comes into use, knowledge and experience of both suppliers and users must be collected and put together. This is of special importance for such a complex field as communications technology. Providers and users must, however, face the challenge of utilising this efficient and powerful technology as it evolves.
From the point of view of standardisation, the supplier must be able to make it clear that the user's investment is a safe one ie an investment in a technologically stable future. The supplier industry's preliminary work in the form of R&D investment was made possible by the availability of fieldbus technology, including fieldbus devices and systems.
Looking closely at the advantages of the fieldbus technology, it becomes clear that the main emphasis has been placed on the central user advantage of 'cost reduction'. It would be, however, too simple to see only the reduction of cabling costs. The process of cutting costs is also manifold and needs to be looked at under various aspects, such as 'procurement' and 'operation'.
One main source of cutting costs is in the 'procurement' phase, for:
* Hardware: fewer I/O modules in the process control system, greatly reduced expenses for wiring and explosion proofing, no bulky cable routing making for reduced space requirements in electronics cabinets, switching rooms and cable trays because of the absence of terminal blocks and sub distribution boards.
* Planning/installation/commissioning: less hardware also means less planning, installation and documentation expenditure, less installation time. The configuration of the fieldbus network and unit applications, the simulation of unit, system and process states and diagnostics are all carried out via a centralised engineering station.
This cost reduction potential is continued in the 'operation' phase. At the same time, the availability of the entire plant increases and the flexibility of the user:
* The expense incurred for corrections and retrofits is reduced as fieldbus 'subscribers' from different manufacturers are interchangeable.
* By integrating the digitalised field level into the distributed control system and by centrally accessing all field information, servicing; process: system and unit diagnostics; as well as maintenance, all become easier. The collection of signals within the intelligent field unit to produce error and plausibility information and the processing of the signals of respective field units into process status information by a field management system (Smart Vision) optimise the aforementioned functions in a decisive way.
Because of the innovative potential of the fieldbus technology, it can be assumed that in the course of time and as familiarity and experience grow the value to the user will increase substantially.
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