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Emerald Group Publishing Limited
Copyright © 2001, MCB UP Limited
Fieldbus: where are we now?
Fieldbus: where are we now?
Keywords: Fieldbus, Safety
Another revolution in automation is happening, with greater opportunities for those prepared to act quickly. Fieldbus adoption has been painfully slow. A faster pace of change on the plant floor can be assured as Ethernet promises to be the backbone of the sensor to boardroom enterprise, with many automation manufacturers having positioned themselves to provide this shop floor to top floor transparency. With the introduction of IEC 61508 (functional safety of electrical/electronic/programmable electronic safety-related systems) safety-related technologies are no longer held back, allowing the development of machine safety fieldbus. With similar benefits to conventional fieldbus technology, it will fundamentally change machine safety systems.
Just as people wrestle with the suitability of Ethernet for the plant floor, the arguments often stated are the quality of components, connectors and determinism (forget the cultural differences between the IT department and control engineers for a moment, although the IT department's knowledge of firewalls will prove most valuable to the control engineers). Industrial strength Ethernet components are available, with a number of companies announcing hardened connectors and complete Ethernet equipment lines. The question of determinism can be tackled in two ways adding switches to reduce the number of nodes on a segment and therefore collisions; or simply increasing the data rate ten fold, from 10 to 100Mbs, network loading at 10Mbs will be insignificant at 100Mbs. The question is now which version of Ethernet, or is it?
With the possibility of the now standardized protocols losing out, there is a race to be the protocol of choice on Ethernet, with all the old favourites entering the fray (including Ethernet I/P – DeviceNet/ControlNet, Interbus (hybrid), Modbus/TCP and Profibus). Invested interests ensure all your needs will be catered for should you wish to take the plunge and try Ethernet. There are others promoting open systems; the Industrial Automation Open Networking Alliance (IAONA) is a relatively new trade group that intends to encourage the growth of open networking in industrial automation. There is also the Interface for Distributed Automation organisation (IDA – technology originally developed by Jetter), whose aim is to distribute intelligence, enabling greater modularity, horizontal and vertical integration.
After IEC 61158, the "eight-headed monster", this maybe the closest we get to a universal fieldbus. It will look the same whatever colour cable you choose, but remember the flavour of the protocol will not be, so (PC-based) gateways to other protocols are likely to proliferate, as did hardware versions for fieldbuses.
The next race, your choice for safety fieldbus has a small number of front runners now, with many more due on the market before the end of 2001. Users will have the choice to choose systems that are suited to small or large scale implementations, with the benefit of separation, combination or interfacing with existing conventional fieldbus devices.
Safety-related fieldbus offers significant advantages over traditional hardwired safety systems. Wiring complexity is reduced, along with the associated design, commissioning and installation costs. Distributed intelligence down at the device level allows rapid fault diagnosis and rectification. With such facilities built in, the need for diagnostics to be designed in is avoided, saving significant additional engineering expense. Further savings are likely as the range of safety devices increases from I/O, emergency stops and light curtains to include safety-drives and robot interfaces.
The plant floor automation hierarchy will collapse as more devices have Ethernet connectivity. This will result in a situation that would have been recognizable a few years ago with PLCs with Ethernet and proprietary networks. The difference being the openness, choice and the greater ease with which systems can be integrated. Will Ethernet be used for safety-related applications? Many of the existing protocols are being developed with an additional safety layer, whilst the same protocols are also being developed for Ethernet. A combination of the two is inevitable. Ethernet will increase the impact of PC control, but will not necessarily be recognizable as an industrial PC or white box, allowing the use of Web browsers for HMI applications for instance and the ability to extract shop floor data easily into e-business applications utilising OPC. As one vendor insists, "the network is the controller".
© R. Piggin (Previously published in part in the IEE Review)