Fieldbus: the technology moves on in line with industrial safety concerns

Fieldbus is expected to become more popular as systems offer compliance with the standard IEC 61508. This applies to safety-related control systems and will increase the safety aspect of control and monitoring in line with today's increased focus on industrial safety. Eric Russell reports.

Fieldbus is undoubtedly an effective way to connect control room to factory. It is based on a light, low cost two-wire cable run from the controlling computer to each device in turn, whether it be a pump, valve, sensor, contact breaker or motor. And if these devices have information to send back to the control room, from intelligent valves or condition monitoring sensors, for example, that can come back along the same two-wire cable.

It is also very simple to wire in new plant when the factory layout changes for new products, for example. This is a long way from conventional control system wiring, which needs a separate run of cable to each device and another run if information has to come back to the control room.

Electronic address

With a fieldbus system, although the plant devices appear to be wired together in parallel, each is programmed with its own electronic address. When a command is sent from the control room it is preceded by the address of the device that it is meant for. This wakes up the required device and causes it to respond while the command is ignored by all the other devices.

There are many standards of fieldbus, each reflecting the thinking and needs of the manufacturer or organisation that initiated the standard. But the key difference between them is the speed at which they operate.

Data transmission rates

The fastest, such as Ethernet, offer the highest data transmission rates and are suitable for detailed data communications between control room and processing line.

The lowest level are designed just for switching actuators and motors, as on special production machinery.

But speed costs money. Systems which can transfer large amounts of processing data from the factory into management information systems are over-engineered if the production line consists only of automatic machines performing routine operations. On the other hand, a device-level fieldbus can be expensive to upgrade if it is needed to perform higher level tasks in the future.

Increased popularity

Fieldbus is expected to become more popular as systems offer compliance with the standard IEC 61508. This applies to safety-related control systems and will increase the safety aspect of control and monitoring in line with today's increased focus on industrial safety.

The standard applies to systems that incorporate electrical and/or electronic and/or programmable electronic (E/E/PE) devices. It covers possible hazards caused by failure of the safety functions to be performed by the E/E/PE safety-related systems, as distinct from hazards arising from the equipment itself, such as electric shock.

It is generically based and applicable to all E/E/PE safety-related systems irrespective of the application.

Systems such as these include emergency shut-down procedures, fire systems, gas burner management, crane automatic safe-load indicators and machinery guard interlocking.

Parts 1, 2, 3 and 4 of IEC 61508 are designated as IEC basic safety publications. This means that IEC Technical Committees will have to use these parts in the preparation of each of their own sector standards that has E/E/PE safety-related systems within its scope. IEC 61508 will therefore have far reaching implications across all IEC application sectors.

A safety-related system comprises everything necessary to carry out safety functions. This includes hardware, software and human elements where failure of the function would cause a significant increase in the risk to the safety of people or the environment.

Such a system can comprise stand-alone equipment dedicated to perform a particular safety function, such as a fire detection system, or can be integrated into process plant, such as motor speed control.

The new standard should encourage a higher level of safety in food processing systems.

Firstly, because it reduces the cost of implementing a safe system and, secondly, because it will become accepted practice in time as part of a food processor's due diligence strategy when considering safety of employees.

Safety systems that are designed as such tend to be inflexible so they are not easily amended when processing lines change. They are also proprietary, which ties the user into that manufacturer when modifications are required.

A fieldbus system that is compliant to IEC 61508 will still provide the high degree of flexibility of two-wire control systems while devices can be added from a wide range of manufacturers.

After all the sections of IEC 61508 have been published the standard goes into what is termed maintenance. This ensures the standard is proactively monitored so it continues to be fit for purpose.

Some parts of the standard have been published and are available from most national health and safety organisations. It is in seven parts in total and was ratified by the CENELEC Technical Board in July 2001. The current date for completion of the whole standard is August 2002.

Founded in 1906, the IEC, International Electrotechnical Commission, is the global organisation that prepares and publishes international standards for all electrical, electronic and related technologies. Members include the world's major trading nations and a growing number of industrialising countries.

Around the suppliers

A-B says the Open DeviceNet Vendor Association (ODVA) recently developed and introduced DeviceNet Safety, an open fieldbus system that uses DeviceNet as its base. It addresses the requirements outlined in

EN 954-1, DIN 19250 and IEC 61508.

DeviceNet Safety is designed to meet the requirements of demanding machinery-shutdown and process-sector-availability applications. It also allows standard DeviceNet hardware and safety devices to operate on the same network.

End users can take advantage of DeviceNet's cost savings and flexibility for safety applications, without having separate networks for safety and standards control.

Pilz Automation Technology develops, manufactures and supplies products for process and automation control. Over the past 10 years it has taken a leading role in educating the market with regard to safety legislation through seminars and software packages. It also publishes a Guide to Machinery Safety and another for Programmable Safety Systems.

By Dr Richard Piggin says conventional fieldbus technology is generally prohibited for safety-related use unless the bus system is designed to meet the requirements of a safety system.

With the introduction of IEC 61508 new safety-related technologies are no longer held back, allowing the utilisation of machine safety fieldbus, such as

SafetyBUS p from Pilz.

He says that conventional fieldbus networks are not suitable for safety-related controls, since additional error detection and avoidance mechanisms are required. While conventional networks have appropriate error detection and correction methods, they lack the ability to independently and rapidly detect network, cable or safety device failures.

An independent safety layer is then necessary for risk detection and to implement a close down if necessary.

Safe communications

Siemens says ProfiSafe enables astandard' and asafe' communication over the standard Profibus. The new safety-related profile ProfiSafe will in future allow safe communications as well as normal communications over standard Profibus components. This will smooth the way to simple integration of safety-related applications into the overall automation landscape.

Triconex Europe is a major supplier of fault tolerant systems and has developed its Triple Modular Redundant architecture.

The company is particularly involved in International Standards evolution and designs its applications to comply with the major international safety standards including IEC 61508. p

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