Sitting on the fieldbus fence
Andy Verwer and Mustapha Soufian
Programmable automation is constantly developing, generally bringing greater functionality and capability and at the same time becoming cheaper to install,use and maintain. The significance of some developments is, however, not always immediately appreciated by all in the industry. For example, fieldbus technology has been around for many years bringing major benefits and cost savings to users. Yet, there are many installations still being designed that could benefit from using fieldbus but do not take advantage of this technology.
There are many reasons for reluctance in adopting new technology. Engineers rightly prefer to use tried and tested solutions and are hesitant to invest time and money in technology which will not last. One problem is that there are so many different fieldbuses. How do we decide which to use? "I'll wait until I know which fieldbus standard is going to dominate the market" is a comment often heard. In reality, it is most doubtful that one standard will ever totally dominate the fieldbus market. There are so many diverse application areas for fieldbus that many different fieldbuses are sure to survive. Manufacturers of instrumentation and remote I/O often support several fieldbus standards and so it is doubtful that a particular fieldbus system would be limited by lack of equipment availability. Additionally, gateways and couplers are widely available which allow interconnection of one fieldbus system to another.
Another comment often heard, but rarely borne out by hard evidence is"fieldbus devices will be more expensive than conventional analogue devices because of the extra electronics required". This is a fallacy because most modern electronic instruments are internally digital. Analogue operation requires analogue to digital and/or digital to analogue conversion to be added to the device. The cost of a fieldbus interface chip is normally much less than the cost of the analogue converters and high accuracy analogue circuitry which would be required.
The benefits of fieldbus technology to users are well documented. Foremost,fieldbus can result in a significant reduction in cabling and associated termination equipment. As an example, consider an application in which 500 points of I/O are used in an area of 100m x 100m. With conventional multicore cabling well over a tonne of copper signal wire might be installed. Using fieldbus, the amount of cable can be reduced to a few kg. Termination cabinets,junction boxes and cable tray requirements are also significantly reduced. Additional cost benefits are to be found where plant equipment is operating in hazardous environments or explosive atmospheres. This is because some fieldbus standards support intrinsically safe (Exi) protection. Profibus and Foundation Fieldbus, for example, allow many intrinsically safe devices to be protected in one circuit segment. In addition to reduction in wiring, other benefits include rapid installation and system commissioning, reduction in documentation requirements, ease of system expansion, availability of remote diagnostic information and many others.
Most established fieldbus systems are "open" which means that the technical details are published as international standards. User organisations take responsibility for publicising and supporting the fieldbus. Compliance with fieldbus standards is ensured by test centres which check device conformance and operation. The user can therefore be sure that certified equipment from different manufacturers will operate together in a system.
Most fieldbus standards use layers 1, 2 and 7 of the ISO OSI seven-layer model. These layers specify the physical, data link and application level characteristics of the fieldbus communication. Compliance with a particular standard is not the whole story however. At the user level (i.e. above the application layer) the meaning of the information being communicated may be unspecified. User layer specifications or "profiles" standardise the organisation and meaning of the information. Profiles provide interoperability,so that a field device from one manufacturer can be exchanged with a similarly specified device from another without changing the application software. Interoperability allows fieldbus devices to be interchanged in a similar way to the flexibility offered by standardised analogue 4 to 20mA transmission.
Support for fieldbus users is essential. In addition to the support that comes from the vendors, Fieldbus User Organisations publish technical information, case study material, product information and application examples. Help and advice are provided by competence centres that are supported by the user organisations. For example, Manchester Metropolitan University runs the competence centre for the UK Profibus Organisation. The centre runs regular fieldbus workshops for industry, where potential users obtain hands-on experience with real systems. The centre can also provide general support, help,advice and consultancy for users and product vendors. Warwick University operates the Competency Centre for the Device Net Organisation (ODVA), providing a similar level of support.
Fieldbus application areas are constantly expanding; for example, fieldbus is widely used in factory automation, robotics, building automation, process plant,mining and vehicles. Fieldbus solutions are appearing for more difficult areas including safety critical systems and high-speed real-time systems. The introduction of fieldbus technology may in the long term be as significant to the automation industry as the development of PLCs has been. Potential fieldbus users are well advised to stop sitting on the fence and go for fieldbus technology now. One thing is for sure, they will save money and gain flexibility whichever fieldbus standard they adopt.
