Steve Jones, General Manager of the CC-Link Partner Association (CLPA), reviews the principles behind fieldbus communications and looks at the advantages.
It is rare to find a control or automation system in which all of the components are from a single supplier, so there is a need to get the diverse products to communicate with each other effectively, elegantly, securely and deterministically. Industrial fieldbuses offer a means by which this can be achieved.
A fieldbus is a field- or device-level network that provides high-speed, deterministic communication for controlling and monitoring a wide range of automation equipment such as drives, PLCs, HMIs, motors and sensors over a single cable. Significantly a field bus is "˜open', that is to say it can accept data signals from devices made by many different companies, mixing and matching these as required.
The need to develop fieldbus technologies has arisen because automation systems are seldom consist of equipment from a single supplier. The few mono-vendor systems that are installed tend to lose their purity fairly quickly, as the inevitable upgrades and redevelopments bring in components from other vendors.
Until the mid-1990s most automation equipment manufacturers seemed to ignore the fact that their products could not communicate with other manufacturers' products. Perhaps they thought that this helped them secure areas of the market and freeze the opposition out. End-user engineers had the choice of working hard to maintain their mono-vendor systems, or working hard to develop interfaces that would allow them operate different brands of equipment on the same system.
This all began to change about 10 or 15 years ago. I do not know whether vendor engineers came to their senses and decided to become more flexible and responsive towards their customers, or whether end-user engineers rose up in revolt and forced their suppliers to start co-operating a bit more. It does not really matter how it happened; that is history. What matters is the result: the fact that we can now 'mix and match' makes of equipment fairly freely, which removes a major headache and aids the advancement of automation through a wide range of industries.
Today open fieldbuses provide the necessary multi-vendor communications backbone for machine, cell and process control industries ranging from semiconductors to food and beverage, automotive to pharmaceutical, materials handling to building automation.
Their installation and operational simplicity has also encouraged other sectors to take up automation for the first time. Now, for instance, tractor-towed machines are planting or harvesting produce far faster and more accurately than farm hands, while scientists have been able to develop more sophisticated experiments and pilot studies. They are used in subsea cable laying vessels, on the Eurostar's passenger comfort systems and in many energy-saving systems developed in recent years.
There are now three major globally accepted fieldbuses in the industrial automation sector, of which CC-Link is the youngest: CC-Link, which was developed in Japan and remains the dominant choice throughout Asia-Pacific and for those companies exporting to these regions; DeviceNet, developed in America and often the preferred fieldbus there; and Profibus, of European origin and found on much equipment built in Germany. Counting up the number of installed nodes suggests that CC-Link is rapidly gaining ground due to its ease of use. CC-Link is becoming one of the globally dominant fieldbuses.
Significantly, although these systems are not compatible, it is not unusual to find two or even all three functioning side by side in the same machine or plant. If such an installation had been a greenfield development, designed and built in one go, then it is likely that only one fieldbus would have been used; when two or more fieldbuses are found together it is a sure sign of retrofitting and staged developments of the system - one of the primary benefits of fieldbuses.
There are a number of core benefits to fieldbus networks, beyond the original objective of allowing the free development of multi-vendor automation systems. Perhaps the feature that best illustrates the new paradigm that fieldbuses have created is that CC-Link networks can be up to 13.2km long with no degradation of communications speed.
Because CC-link and other fieldbuses systems are arranged as a single network that provides both communication and power, the amount of wiring required is reduced massively (often by an order of magnitude). But the materials cost saving is dwarfed by the cost of installing the wiring, and fades to practically nothing when you consider the cost of ownership of a plant over five, 10 or 20 years, in which maintenance, upgrades and reconfigurations have to be carried out.
Communication over CC-link is fast and does not slow down even when the volume of data is very high. Crucially it is also deterministic for real-time control; that is to say, data is transferred in an accurate and predictable time, and its arrival is assured. (In non-deterministic communications - such as standard Ethernet - this is not the case; this may be completely unacceptable in a high-speed or safety-critical installation.)
There are many more benefits to CC-link and fieldbus communications relating to reliability, reconfigurability and the ease with which automation information can be processed into production data, management information, commercial and financial forecast. With CC-Link Safety, users can leverage traditional CC-Link cables, and communications between existing CC-Link remote I/O and remote device nodes are supported in addition to CC-Link Safety nodes. Thus, safety control systems are implemented with reduced wiring, and maintainability is improved through centralised configuration and monitoring of network devices from a master node.
Since the 'Fieldbus Wars' of the 1990s, the automation fieldbus has led a quite revolution that has changed control engineering for ever, making it easier to implement and develop control systems as conditions dictate. What makes fieldbuses interesting is that the market and technology is far from static. We are seeing some bus networks being integrated into others, the inevitable increased interest in industrial Ethernet for some applications and, to support intelligent devices on the network, FDT/DTM and EDDL
There are many benefits of open networks: they simplify the integration of equipment plant-wide; improve the plant control and efficiency; reduce downtime; and improve the information flow to management. The important thing is to encourage more equipment suppliers to integrate the protocol into their product. In the UK, for example, one of CLPA's members - Mitsubishi Electric - is a leading supplier of automation equipment, so it makes sense for third-parties to integrate CC-Link into a product. The benefit to that supplier is not only easier access to sites using that control supplier, but CLPA also actively supports them by including their products in partner catalogues and circulates their information worldwide.