10 April 2012
There are large number of fieldbus protocols available today. While each has distinct technical features, their relative advantages can be fairly marginal to the user. Rather than wait for a universal fieldbus protocol to emerge, users and equipment suppliers are now focusing on the provision of rich services – enhancements to a fieldbus protocol through the creation of new messaging formats and diagnostic capabilities – essentially getting the network to carry more of the information that matters.The rise in fieldbus popularity, since its first arrival in the 1970s is largely due to the creation of ‘open’ systems which allowed for the creation of multi-vendor systems. Today, many diverse protocols exist, each meeting different demands for speed, connectivity, synchronisation and other requirements. As each of these protocols has been developed to meet a specific set of requirements, it might seem optimistic to ever have expected a universal fieldbus standard to develop.More recently, Industrial Ethernet has developed as a variety of the familiar office-based infrastructure. The advantages are familiar standards and inexpensive components – a part can often be picked up at the nearest computer store for a modest cost.Rich services The main fieldbus themes emerging at present are the development of rich services, real-time synchronisation and increased data transfer. There is a demand for more flexibility, utilising the fieldbus network for programming intelligent devices, and for detailed information gathering, particularly for energy monitoring purposes. In response, enhancements to the protocols in the form of additional services are often implemented, hence the term ‘richer’.Equipment manufacturers need to be able to use these capabilities to meet customer demands. For example, ABB’s latest drives include an energy saving feature for pump applications which can show how much CO2 is saved by using variable speed drives compared to traditional methods of control and the energy cost savings in the local currency. This information can be monitored in real-time from a central location.Today’s drives include many diagnostic and supervisory capabilities and a fieldbus network makes this data more easily available, providing a useful tool for evaluating process performance and for preventative maintenance.One particularly useful feature of fieldbuses is the facility to remotely monitor intelligent devices. A UK water industry company, for example, has recently connected their pump drives via an Ethernet network, using a browser-based tool that enables parameters to be changed or pumps to be monitored from a central location. Checking pumps this way saves time as well as money, as many of these pumps are in locations far from the centre of the operation. Another advantage of using fieldbus is that it usually results in ‘spare’ hardwired digital and analogue I/O on the drive that can be utilised for other purposes, such as receiving feedback of pressure or flow data, for pump protection, or for alarms. The I/O points of the drive are effectively used as distributed I/Os in the automation system, potentially saving I/Os on the PLC and reducing wiring that would otherwise have gone back to the PLC.Improved reliabilityFieldbus systems can now offer reliability, speed and real-time performance - qualities that have, traditionally, been seen as the preserve of hardwired solutions. This does not, however, herald the death of hardwiring. It is still used for backup protection in the event of fieldbus network failure in many industries, for example. It may also be required when adding new equipment to existing automation systems utilising ‘closed’ or less popular fieldbus protocols, where the vendor does not offer a fieldbus interface for their device. However, PLC control by hardwire does look set to reduce in the years to come. Meeting the need for greater flexibility it may be possible that, in the future, we could see drives which support two different fieldbus protocols operating simultaneously. This would allow redundant operation or enable the master controlling device to communicate to the drive through one protocol, while another device such as a HMI or browser based remote monitoring network, uses a different protocol. There could be all sorts of interesting combinations.
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