Driving the field

As fieldbus experience is gained in traditionally demanding applications, the technology is being increasingly applied to simple, everyday uses. This spread of fieldbus know-how is accompanied by a wider availability of fieldbus devices.

As the workplace becomes more automated and the need for real-time, on-line access and information grows, fieldbus technology that allows ac drives to be networked with controllers more easily and cost-effectively is gaining considerable interest.

While the technology for simplifying the way field equipment is linked to the host controller has been around for over a decade, it is only recently that its focus has turned to ac drives.

Part of the delay has been the confusion and uncertainty relating to a firmly established international standard. Because of the complexity of component choice offered by a growing number of international product vendors, it is becoming clear that an industry-wide fieldbus standard is unlikely.

This aside, vendors' products often conform to the requirements of more than one fieldbus technology protocol and, as such, offer end-users more freedom in terms of add-on buying patterns.

This has resulted in many different industrial protocols, with the most recognised being Profibus, Modbus, DeviceNet, InterBus-S, ESP, LonWorks, CanOpen and Ethernet. As well as open fieldbuses there are also many proprietary versions.

The organisations behind the major open fieldbuses are in an era of consolidation, improving the system performance by occasionally adding new features. New open fieldbuses that have appeared lately are ControlNet, Profibus-PA and Fieldbus Foundation H1 (with H2 under development), although the creation of any more seems to have slowed.

The latter two are process fieldbuses of which Profibus-PA is strongly represented in Europe, whereas Fieldbus Foundation H1 is strongly represented in the USA.

What differentiates these fieldbuses are differences in cable lengths and requirements, noise immunity, topology, transmission speed, protocol, number of interconnecting levels and application in different solutions together with management and configuration tools, and the type of administration.

With ac drives, the aim is to define functional profiles within the protocol, which describe how the drives will react on, for example, Profibus, and what information will be transferred to and from the master. All open fieldbus organisations define these requirements.

One new fieldbus system gaining strongly in market share is Industrial Ethernet. This provides proven technology, being commonly used in pc networks and factory automation, but has not yet found an open standard with regard to the data protocol and device profiles.

To date the Ethernet TCP/Modbus interface has been realised by some manufacturers, but without a functional profile. The physical level of this is Ethernet. Embedded in the Ethernet message frame are the transmission control protocol (tcp) frames and the actual data frames, constructed according to the Modbus protocol.

As an interpolation of this, any of the existing open fieldbus data protocols can be used instead of Modbus. If these protocols could additionally be handled by the microcontroller of the Ethernet interface instead of the fieldbus specific ASIC, only one standardised hardware design would be needed. The handling of the data protocols could then be realised in the form of software modules. In this way, the handling of the large variety of fieldbus alternatives would be made much easier.

Cost savings
Traditionally, engineers and managers have accessed data by using a host control system into which all field instrumentation is fed. The downside here is that all devices are attached by complicated, lengthy analogue signal wiring.

Fieldbus technology eliminates the bulk of costly analogue signal wiring by utilising a single cable system onto which field equipment may be hooked for greatly enhanced, digital data retrieval.

There are many other benefits of using fieldbus technology, including the flexibility to extend the net and connect different modules on the same line, the ability to cover greater distances, and the elimination of drift.

The downside of the technology is that users need to know what they are doing and must be prepared for a higher investment in equipment and tools for monitoring. These disadvantages, however, are only temporary, and disappear once experience of the new technology is gained.

Further, it is estimated that traditional analogue signal processing/manufacturing systems cost producers world-wide in excess of $60 billion annually in plant maintenance – a level of spending that is three times greater than capital investment. The use of fieldbus technology in drive installations therefore clearly saves time and money, while providing a versatile and standardised system.

Benefits will continue to accrue after commissioning. For example, the fieldbus cabling enables field device intercommunication that will facilitate bidirectional transfer of data with the host.

It is not only the interface cost that is determining the future for fieldbus. Cost is also associated with the assembly and wiring of the devices. For this reason the fieldbus interface is today usually integrated within the drive.

Choosing a fieldbus
Fieldbus concerns three groups of industrial professionals: users, system integrators and product manufacturers, the requirements of which differ.

For consumers, the fact that the installed system works correctly and cheaply is the main thing; that this happens thanks to a fieldbus may not be important. This group values a high degree of standardisation and versatile control and diagnostics.

An important consideration for system integrators when choosing a fieldbus is how easy it is to install. Therefore, they are interested in equipment to install, configure and monitor the net, and a reduction in wiring implies a direct price advantage. From a system integrator's viewpoint, the main reasons for purchasing fieldbus include the ease of expansion, reduction in space needed and lower power consumption.

Finally, those who design equipment and communication interfaces for a fieldbus are interested instead in the complexity of their task, the availability of specific components and the support for development.

So how do you find the ideal fieldbus for your needs? There is no single fieldbus that can fulfil all requirements for every application. There can be considerable differences in communication requirements for different applications, therefore the fieldbus must be selected according to these requirements.

Most users want to select a simple communication method that fulfils the requirements of their application. The selected standard is meant to have appropriate devices and tools for the application.

Apart from ensuring that the chosen device can accept multi-vendor products, users should consider the following:

• topology;
  
• access to the fieldbus;
  
• physical features;
  
• protocol;
  
• access rights in multimaster systems;
  
• noise immunity;
  
• transmission speed.

• number of nodes;
  
• extent of user data;
  
• modularity, expansion possibilities;
  
• cable requirements;
  
• connection cost;
  
• device replacement without bus interruption;
  
• requirements of the application;
  
• required data transfer capacity;
  
• guaranteed response time;
  
• system safety in a fault situation;
  
• safety of data transfer (error recognition);
  
• special attention towards power supply solutions;
  
• readiness of diagnostic tools.