Safety fieldbus; the time is right

Designing and building a prototpye safety fieldbus is easy; gaining the necessary approvals to allow the system to be used is a lot harder

Fieldbuses approved for safety-related applications are perhaps one of the most controversial developments in industrial networking in recent years. Several developers have already found that designing and building a prototype system is easy, but gaining the necessary approvals to allow the system to be implemented in safety-critical applications is far harder. There then follows a process of education to encourage customers that fieldbuses (albeit specialist ones) can be used for safety systems, although in the past the only acceptable solution was hard-wiring.

Meanwhile, the debate is ongoing over whether it is acceptable to use the same cables for power and data, and there is a parallel debate continuing over whether to use one common network for safety data as well as non-safety data relating to conventional actuators and sensors. Nevertheless, there is now a reasonably good choice of systems available, and care needs to be taken to ensure that one is chosen that best suits the application. Some are viewed - fairly or unfairly - as little more than a convenient means of linking multiple light curtains, while others can be used to carry huge volumes of safety-related data the length of a production line. Other alternatives are available that, if you can tolerate a reduced response time, may be used over distances measured in kilometres.

Although the technical debates are likely to continue into the future, few people would deny that fieldbuses for safety-related data have their attractions. A major advantage is that diagnostic data is carried as well as safety data so that, for instance, if an emergency stop switch is operated, the machine controller's display can indicate which switch has been actuated, so the operator can go immediately to the relevant station on the machine. For a long production line, this can save many minutes of costly downtime. Moreover, if a suitable control system is also being used, such as the Pilz PSS (Programmable Safety System), it can be arranged that there are defined zones on the machine, so that only parts of the line are shut down if an emergency stop or other safety device is activated - yet only one controller is required. Other advantages of safety fieldbuses are similar to those for conventional fieldbuses: they save on the initial wiring costs (in terms of both materials and labour); they are faster to commission; and they are easier to modify.

Although safety fieldbuses are not yet widespread throughout manufacturing industry, they are already being used in some plants and it is likely that they will be installed at an accelerating rate as confidence in the technology increases. Pilz is not the only supplier working in this field: networks based on Interbus, Profibus and AS-Interface are now available, as well as the Safenet and Esalan systems. Another that has only recently been announced is based on DeviceNet.

The DeviceNet Safety system, in common with Profisafe and Interbus Safety, will allow users to mix conventional and safety devices on the same network. Clearly there is no industry consensus yet on whether this is a good or bad practice, and there have not been enough systems installed to be able to tell which approach will be preferred by customers. Many of the various safety fieldbuses have the support of some of the largest suppliers of automation and control equipment - the development costs will have been enormous - so it is unlikely that any of these will disappear in a hurry. Users may have to work through a complex selection process for many years to come.

The debate continues over whether it is acceptable to use the same cables for power and data, or for the same network to carry safety and non-safety data, but few would deny that safety fieldbuses have their attractions