The technical talking point in the industrial automation marketplace is about networking communications on the factory floor and in the process environment. All businesses, both large and small, are looking for a solution to connect sensors to actuators, field-level instruments to controls, including PLCs, DCSs, computers, scada, alarms, displays, recorders, controllers –in fact all the periphery that completes the loops.
There is an expectation from these operators that one all-encompassing, internationally approved Fieldbus standard will emerge. End users anticipate that the primary benefit of the standardisation will be 'interoperability', that is the ability for a transmitter from vendor A to connect seamlessly to a PLC from vendor B, which communicates with a valve from vendor C, and should any of these components need to be replaced, this can be done with a substitute from vendor D. But this still remains elusive as each supplier drives his or her proprietary 'increased functionality' into the market. At the basic functionality level, full interoperability is possible, using protocols such as Profibus PA, but to access the vendor-specific 'added value', true interoperability is still a myth and will remain so.
Further, even if or when a Fieldbus standard is complete (IEC 61158) this will not eliminate the need for other industrial networks to co-exist for a variety of reasons, including cost, speed of transmission, complexity, compatibility with the already installed device base and future expansion plans. What is the solution? Do the end users understand what they need? Who has the competence to bring together all these technologies and considerations into a user-friendly solution? Are the requirements of large and small businesses the same?
Will there be a single bus solution?
A wide variety of networks are available in the industrial automation environment. However, because their capabilities and functionalities overlap, confusion arises. At the lowest level are the sensor and device networks that were originally designed for digital (on/off, open/closed) interfaces. These are fast and effective but with only limited applications beyond relatively simple machine control. Industry-accepted protocols such as actuator sensor interface (ASi); lnterbus S and DeviceNet dominate this sensor level.
One level up, Profibus-PA provides improves performance at the Fieldbus level for sophisticated continuous (the older 4 to 20 mA) instruments and controls, replacing and supplementing the features and functions provided by HART (originally developed for transmitter calibration and diagnostics). The primary differences are increased speed, reduced complexity and improved transmission over long distances.
Foundation Fieldbus overlaps somewhat with Profibus-PA and this is the primary area of contention in the ongoing IECIISA SP50 committee discussions and confusion in the recently accepted IEC 61158 guidelines. At the next level up in the hierarchy we have the control networks which include ControlNet (developed by Allen-Bradly and used by Honeywell) clearly overlapping with some of the functionality and performance provided by Profibus - DP and the (yet to be accepted) Foundation Fieldbus H2 protocol.
The highest level of Fieldbus concerns information and Internet connectivity, for which Ethernet TCP/IP may be the accepted standard. During the past year, Ethernet has gained wide acceptance, even at the lower levels, because its performance is well established at the higher end, while commercial proliferation and plummeting prices have extended usage at all levels. Today, Ethernet is not practical at the Fieldbus or sensor levels since this protocol cannot supply power, for the 'historical' loop powered devices, and does not comply with the intrinsic safety requirements in hazardous areas.
One size fits all?
The overlap between the various bus standards is evident and the details are still subject to dispute. The combination of commercial viability and practical time to market far outweighs theoretical considerations. The choice of Fieldbus is dictated by 110 count, physical distances, speed of response, hardware configuration, software requirements, cost and - perhaps most importantly - interfaces with other equipment and systems, either existing or anticipated future. Typically, a choice of one particular bus results in a proliferation of that technology until limitations (speed, 110 count) are exceeded. This is because the 'cost-of-change' is high, both from the aspects of technology, and training.
What is becoming very clear is that no single Fieldbus product will emerge as the universal standard in the industrial automation environment. It is far more likely that several standards will emerge at the various levels of performance and complexity, in different applications and environments. Small and medium-size companies are likely to have different needs and expectations and the requisite standards to satisfy their needs.
Interoperability
Because interoperability between equipment from multiple vendors is a major factor for many users, all the major open-bus organisations offer certified interoperability, typically through independent third-party test establishments such as the Profibus Competence Centres. This directly addresses the key point: whether 110 from vendor A will operate with sensors from vendor B and controllers from vendor C. Typically, the products are certified to work together. However, there may still be differences in speed of operation and other technical features which still provide proprietary differentiation.
The standardisation debate
A Fieldbus standard benefits end users, not vendors. Certainly, it is a reasonable and justified objective for any vendor to provide differentiated functionality and to offer products that 'road test' better than their competitors. After all, increased sales and profits are the purpose of any business. Expecting vendors to support a standard that eliminates their advantage is futile. The suppliers all support interoperability within the standard, and the accredited associations sponsor interchangeability among products from different vendors through independent testing. However, the incompatibility between vendor and user objectives will inevitably prolong the confusion until one network - or more likely a few - attains de facto leadership.
What is the reality?
Today in Africa, at the Fieldbus level, this leadership role is filled by Profibus-PA and Profibus DP at the control network level. The extensive installed base, end-user acceptance and ongoing enhanced functionality has cemented this technology in the market but the Foundation Fieldbus philosophy is attracting renewed interest since, at least on paper, it appears to be a more elegant solution. As with all new technologies, there will be pioneers, evangelists and followers. The reality is that the 4 to 20 mA era is over and end-users must now decide to embrace the new generation of plant optimisation tools since the utopia of one Fieldbus will not be realised.
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