Profibus has been described as Europe's leading industrial fieldbus. So what is the current status of the version approved for use with safety-related qqqdata? Jon Severn investigates.

According to a recent report by Frost & Sullivan, the market analysts, Profibus is Europe's leading industrial network. It is also now widely recognised that fieldbuses for conveying safety-related data are a viable proposition, whether or not they carry control-related data as well. What seems to be more difficult to comprehend, however, is why the version of Profibus designed for use with safety-related data, known as Profisafe, has been slow to come to fruition, and why the corresponding Profisafe-compatible products have not been coming on to the market more quickly.
The actuator-sensor interface (Asi) fieldbus standard has proved popular with the likes of machine builders, and Asisafe was quickly adopted when the necessary approvals were granted (see panel). Asi is often seen as the little sister of Profibus in that it is similar in concept, compatible - via appropriate gateways - but suitable only for smaller, more compact, less sophisticated installations. One of the advantages of Profisafe is always said to be that local Asisafe networks can easily be linked to a larger Profisafe system via gateways.
Nevertheless, despite repeated announcements about Profisafe, it is only very recently that the system has been certified by T†V and become available on the market. And now that it is available, users can benefit from a level of functionality that is simply not available from Asisafe; effectively there are no restrictions regarding baud rate, number of bus subscribers or the transmission method, providing the required response times of the automation tasks allow. Furthermore, the level of diagnostic information that is generated is far superior to that from Asisafe.
It is easy to understand why it has taken so long for approvals to be granted, given the critical nature of the system. Indeed, approvals for other safety fieldbuses have also taken longer to gain than the developers anticipated; delayed availability is not new in this market.
It has also been suggested that manufacturers of safety components have held back from developing products, waiting for Profibus International to generate interest in the market and, hence, demand for the products. This is also understandable on two fronts: first, there are already many competing safety fieldbuses and no manufacturer would wish to develop a new interface for all of its product ranges without being certain that there will be demand for those products; and second, because the Profibus (and Profisafe) architecture allows systems to be configured that are physically much larger, it has not been clear what will be the most popular way for users to integrate Profisafe-compatible products.

Remote I/O modules

One of the new products launched at Hannover this year was Erwin Sick's UE 4150 IP67 remote I/O module (Fig.1). Available with up to eight Category4 safety inputs or 16Category2 safety inputs, these can be linked directly to a Profisafe fieldbus to allow conventional safety components to be used. Additional enhanced safety inputs enable sophisticated devices such as light curtains and laser area scanners to be connected.
The logic behind this approach is that this is the way in which, say, an automotive welding cell would currently be wired. A Profibus network may run the entire length of a body-in-white assembly line but, at each welding station, local I/O modules would be used to connect the various sensors and actuators that are situated around the cell. In other words, there is no local fieldbus around the cell, just individual components that are all wired back to the point at which the I/O module links to the global fieldbus. Similarly, the safety I/O modules allow the various guard switches, emergency stop buttons, light curtains and other safety devices to all be wired back to a single point, from which they can be connected directly to the Profisafe network. Being sealed to IP67 means that the I/O modules can be installed in most cases without the need for additional cabinets. By taking this approach, Erwin Sick has avoided having to develop a new range of products that is identical to its existing range in all respects other than specially-developed, costly Profisafe interfaces.
Not surprisingly, another manufacturer of safety components has also adopted a similar approach; Turck used the Hannover fair to launch its FSDP series IP67 remote I/O modules. These are suitable for use with safety systems requiring safety integrity level2 or 3 and are available with a range of different I/O configurations including up to 16 digital inputs.

Direct connection

Meanwhile, other companies have developed products that can be connected directly to a Profisafe network. Siemens, with its historical links to Profibus, has been at the forefront of such developments. In addition to the ET200M and ET200S remote I/O systems, Siemens has recently launched the ET200iS system. Although very similar to the standard components, the 'i' versions suit hazardous areas where intrinsically safe components must be used (Fig.2).
For control, Siemens offers a choice of two families of programmable logic controller (PLC) that can be used as masters. For medium-sized applications, the Simatic S7-315F is available while, for more sophisticated projects, the Simatic S7-414F/FH and S7-417F/FH are appropriate. Later in 2002, a lower-specification processor may be launched that could significantly reduce the cost of processing and make Profisafe a viable option for smaller applications.
Other products are also due for launcher later this year, and thes will be suitable for connecting directly to a Profisafe network. One of these, the Siguard Laser Scanner LS4 will transmit the protection field status and warning field status as well as basic diagnostic data via Profisafe signals (Fig.3). It will also be possible to transmit information relating to up to four different protection fields from the PLC to the scanner. One of the main attractions for the scanner, however, is that should a device need to be replaced, perhaps for example if it is damamged by a forklift truck, the scanner configuration is automatically uploaded from the controller to the replacement scanner.
A fail-safe motor starter, light barriers and a safety version of the Sinumerik 840D numerical control system are also due to be launched this year.
Once these products are available, and it is believed that various others are also anticipated from yet more manufacturers, engineers considering the use of a Profisafe system will have a reasonable range of components from which to assemble a system. If the system architecture is such that there is a Profisafe spine or loop with localised connection of several devices, the use of Profisafe-compatible remote I/O modules could provide a convenient, cost-effective means of building a system that predominantly uses standard (non-Profisafe) safety devices.
Only time will tell whether Profisafe becomes as commonplace throughout Europe as Profibus has, but considering the companies backing the system, it stands a good chance of being a winner.