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1 April 2005

Fieldbus flexes for safety networks

By Richard Piggin

Developments in safety-related fieldbus are transforming the way we engineer safety in the plant. Engineers can no longer meet the requirements of reliability, flexibility, and comprehensive diagnostics with conventional relay-based safety systems. Safety-related networks need to fulfill essential needs traditional systems cater to while supporting new functionality for specialized and future safety system demands.

The changing characteristics of manufacturing processes and increasing demands mean more flexible approaches to safety, which we cannot address with traditional methods. Conventional safety relay technology has also restricted safety systems' functionality, particularly in terms of flexibility and diagnostics. We now widely use fieldbus networks for transmitting control data, but not safety-related data. Machine safety systems will benefit from the simplification fieldbus can provide, along with other generic benefits such as easy maintenance, faster installation, and reduced downtime. We can also realize more advantages when using fieldbus in a sophisticated manner, where the developments in safety-related technology enable flexible approaches to safety engineering.

A specifically designed safety-related fieldbus must meet new international standards, such as IEC 61508—Functional safety of electrical/electronic/programmable electronic safety-related systems—or the sector-specific/horizontal implementations. IEC is preparing standards based on 61508 for the process and machinery sectors (IEC 61511 and IEC 62061 respectively), and the safety functionality of electrical power drive systems (IEC 61800-5-2). These new standards enable engineers to use safety technology developments, whereas some standards have previously restricted their use. Safety standards in the past have not reflected the state of the art in programmable control and data communications and have encouraged the use of electromechanical components instead.

These new standards and revisions of others will reflect the significant developments in safety-related automation. The removal of parallel hardwiring in controls using conventional fieldbus affords design flexibility, modularity, testing, and maintenance; you can realize the same benefits in safety circuits with safety-related fieldbus. Safety circuits become less complex, with far fewer cables and connections. This will improve reliability and help maintenance, simplifying reconfiguration over the lifetime of the system. Safety fieldbus architectures with bridges, routers, gateways, and various media options will meet the needs of most applications and provide flexibility to support future upgrades.

Safety fieldbus

Conventional fieldbus networks are not suitable for safety-related controls since they require additional error detection and avoidance mechanisms. While conventional networks have appropriate error detection and correction methods, without modification they lack the ability to independently and rapidly detect network, cable, or safety device failures. An independent safety layer is necessary to detect connection or device failures and implement the required emergency shutdown action to avoid danger.

The additional safety protocol layers must detect and provide protection against errors, such as repetition, loss, insertion, incorrect sequencing, message corruption, delay, and the coupling of safety and standard data. You need to implement at least one measure as a defense against each error. These include a running number sequence, watchdog timer, reception acknowledgement, data integrity assurance, redundancy, and different data integrity assurance systems for safety and standard messaging.

One of the principal benefits of safety fieldbus is the simplified connectivity of devices and equipment, just as many have experienced with conventional fieldbus. Initial fieldbus devices provided I/O interfacing, integrating a range of equipment developed afterwards. The same applies to safety fieldbus. Established safety network technologies are now providing additional benefits with increasing connectivity to a variety of safety-related products. These include more traditional light curtains, scanners, and emergency stops, and more sophisticated devices, such as robots, safety drives, safe pneumatics, and various wireless devices.

Behind the byline

Richard Piggin is chairman of the SafetyBUS p Club International e.V and employed at Pilz, Northamptonshire, United Kingdom.

Adding safety layers to fieldbus