Fieldbus into the future
FIELDBUS systems are gaining ground as industry’s choice for device to controller communications.
Finding greatest success in greenfield sites, which can specify and install the systems from the start, Fieldbuses require less cabling, less connections and less commissioning time than traditional communication systems. The systems also give access to increased maintenance and performance data, which can be used to reduce operating expenditure.
There are hundreds of different Fieldbus protocols, but everyone is looking for a common single standard.
Currently, Fieldbuses fall into two categories: open standards, such as PROFIBUS which are supported by a large number of Distributed Control Systems (DCS) and PLCs; and proprietary systems which are supported by a single manufacturer and can support only a limited number of devices.
A recent ARC report revealed the Fieldbus market is generally narrowing to a choice between FOUNDATION Fieldbus – H1/HSE and PROFIBUS DP/PROFIBUS PA. But can either of these two protocols, or some other, develop into the standard Fieldbus system being sought?
Both PROFIBUS and FOUNDATION Fieldbus have large installed bases so it is unlikely that either will disappear in the near future, although PROFIBUS is starting to adopt some of the features of FOUNDATION Fieldbus. Both will continue to develop and perhaps converge as they attempt to serve the same applications.
Other protocols such as ASI-BUS, which are suited to simple tasks such as switching and status information will no doubt also survive as there will always be a need for this functionality. However, a lot of protocols will inevitably fade away because they are too specific and are supported by too few companies to produce the flexibility and economies of scale users increasingly demand.
Intrinsic safety
In intrinsically safe applications, an added consideration is the number of Fieldbus devices allowed. In these applications, devices and barriers are designed to eliminate ignition risks from electrical faults.
The type and number of Fieldbus devices permitted in an intrinsically safe area varies according to the type of hazardous atmosphere and whether the ‘ENTITY’ or the Fieldbus intrinsically safe concept (FISCO) intrinsic safety model is used.
The entity model assumes all equipment used in an intrinsically safe environment is approved for that environment, including connecting wire. Although it is recognised worldwide, the model only permits a maximum dc electrical current of 83mA in the wire and a maximum of 18.4V per device.
In contrast, the FISCO model simplifies installation, enabling a greater maximum dc electrical current (256mA gas group IIB) and permitting more devices to be installed per segment.
Despite not being a worldwide standard, FISCO is steadily becoming accepted throughout Europe and is part of the FOUNDATION Fieldbus specification.
Potential Fieldbus users are advised to select the most appropriate protocol for the application or the one supported by the chosen host system.
System Integration
Integration of a Fieldbus with other systems when creating control applications is crucial.
Both FOUNDATION Fieldbus and PROFIBUS provide standard interface files and strategies to allow system integration. Yet, customers are looking to manufacturers for further testing, to give them confidence that products will be compatible with the major protocols.
Both protocols have a high speed backbone and links to the slower speed PA and H1 field segments.
FOUNDATION Fieldbus switched its development of the H2 high speed bus to use high speed Ethernet, while HSE now offers a flexible solution for integration, with several options for redundancy.
PROFIBUS uses RS485 or fibre optic media for its high-speed bus, as this is a well understood technology. This system has perhaps the weakest integration method as it allows for cyclical device access via the GSD file which adds complexity during commissioning.
Device commissioning and parameterisation is also poorly supported, often requiring several external software tools. The open standard of Field Device Tool (FDT) and Device Type Manager (DTM) solves this problem, providing field device support from a single software tool.
DTMs contain full information about devices in the Fieldbus network. A Fieldbus make it easier to interrogate ‘Smart’ devices, and retrieve the status and performance data they contain. This information cannot be accessed via an analogue link, so access through a Fieldbus opens up new possibilities for predictive maintenance, advanced diagnostics and asset optimisation.
FOUNDATION Fieldbus offers tighter integration using DD/CFF/CFH files but this is not as powerful as the DTM for PROFIBUS.
A team is working on an FDT standard for FF and also to enhance the Device Descriptors (DD), a newer competitor to Device Type Managers.
The enhancements will provide graphical data presentation, and improved data organisation and storage. The extensions help developers logically organise parameters in complex devices and allow inclusion of images to help device configuration.
Prospects for Fieldbus
Fieldbus holds the prospect of more low power devices, and devices that store their own documentation, making commissioning easier. Devices with more sophisticated algorithms and improved safety protocols are also on the horizon.
Also, there is scope for devices to become more intelligent their communication of maintenance messages and use of process information when reporting errors.
Wireless access, allowing the use of handheld PDAs for status checking is another prospect. Fibre optic communication would also be useful, preventing interference from radio frequency emissions.
Larger numbers of vendors will lead to greater confidence among users in designing bigger projects, knowing that they are not relying on one vendor to supply all the equipment.
*Bob Curley is the senior vice president-instrumentation for ABB Australia .
29-Apr-2005