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Product category: Machine Safety Monitoring and Control
News Release from: Schmersal UK | Subject: Fieldbus safety
Edited by the Engineeringtalk Editorial Team on 08 March 2005

Safety and the fieldbus - the
specialist's view

Terry Hayward argues that the extension of fieldbuses to integrate people and machine safety should remain the focus of specialist safety experts and not automation experts.

The safety of people and machinery has long been the single focus of specialist machinery safety manufacturers such as Schmersal, just as fast and reliable control systems have been the focus for automation manufacturers It was, therefore, natural for the evolution of networked systems and fieldbuses to be pioneered by automation manufacturers seeking to reduce cycle times and provide greater flexibility of the control process

However, according to some commentators, there is widespread reluctance to accept that, as far as safety is concerned, a network-based system can be as reliable as a dedicated hard-wired installation.

And when lives are at risk users are unwilling to put their faith in software and networking protocols.

To overcome this reluctance, Terry Hayward of Schmersal UK argues that the extension of these networks to integrate people and machine safety should remain the focus of specialist safety experts and not the automation experts that pioneered the control systems.

With the exception of those connected to automation manufacturers, these safety specialists have no allegiance to a particular network but are working with all bus system.

Schmersal, for example, is actively working to enable safety devices to be bus connected, both through its own proprietary fieldbus systems and AS-I Safe, Profibus and DeviceNet.

The advantages of having a networked or fieldbus solution are generally well known.

Savings on cabling and installation time are significant, and due to their diagnostic capabilities, such systems can reduce downtime and therefore maximise the efficiencies of a machine.

Fieldbus is more often applied to major automation projects, where the network installation is now almost the chosen route.

Many engineers working on smaller projects are still using discreet I/O and PLC structures with spider cluster wiring to control devices, and are only just beginning to implement network solutions.

Against this background it is important to consider not simply what is right for fieldbus, but what is also right for safety?.

Should we argue that the most important factor is the total reliability of the system - and confidence in the system - rather than commercial gains from reduced wiring, maximum uptime etc?.

Since the earliest days of fieldbus, manufacturers and engineers have pondered the "safety" issues: How should safety components be incorporated into production lines? How is existing machinery best accommodated? Is it easier to design-in safety on new machinery? Should I go for a "proprietary" or "open" system?.

At this stage discussion is academic, as new and emerging Euronorms and IEC documents do not yet allow networked and fieldbus safety technologies to be deployed.

Discussions are taking place, but there is a considerable and ongoing debate to determine how machine safety standards should be modified to accommodate networks and fieldbuses.

A number of interested parties are working to resolve the questions and, ultimately, a suitable standard will emerge.

When that happens, an engineer wanting to take advantage of the savings using safety networks will, not surprisingly, be faced with choices.

There is the discreet approach - using a network of safety devices, perhaps with its own "safety" PLC, but separate from the main automation system.

This approach is available from a number of manufacturers.

Schmersal has the Esalan system, which is both proprietary and open and operates independent of the operational control.

All commercially available safety switching devices can be connected to it and connections are made to the input and output stations of the system.

There are of course arguments against such systems.

They are product specific and not open.

They are stand-alone, separate from the machine's systems.

They are not covered by any Euronorm standards.

And you don't necessarily gain all the benefits from the installed wiring costs and diagnostics because you still have to run a completely separate wiring system for the safety components.

The automation sector, on the other hand, has long promoted the advantages of a true open system that would allow safety products from any manufacturer to fit into an existing automation system to create a unified network or fieldbus.

Safety is added as an overlay to the wiring already installed and does not have to be increased.

Such an installation would ideally be covered by Euronorm standards and, although not certified, would be considered compliant with the agreed philosophy or protocol.

Within the open network option the engineer is still faced with choices, this time from the various schemes already nominated.

Examples include the high-volume, bit-based Actuator Sensor Interface (AS-i) system, Profibus P, which is endemic in Europe and DeviceNet, which is endemic in the USA.

The latter two are both byte-based systems.

As a global leader in machine safety components, Schmersal's strategy does not support any one fieldbus system, preferring instead to integrate with all approaches.

Take AS-i as an example.

AS-i is the most widely used of the automation fieldbuses because it is at the Actuator Sensor Interface, where there are more actuators and sensors than drives or motor control sensors - and therefore, more kilometres of wiring.

This system now also features an additional element - AS-i Safety - a master/slave system created by a consortium of open manufacturers, including Schmersal, who have pooled their technology and designed a low-level safety interface.

The principle of AS-i Safety is to put the basic safety devices onto a network and connect them, using an AS-i ribbon cable, via a safety monitor and the main motor controller.

The system delivers the typical fieldbus benefits - reduced wiring, increased diagnostic capability, easier programming and reduced down-time.

AS-i Safety, which does not require a safety-PLC, allows the integration of safety components such as emergency stops, safety switches, solenoid interlocks, magnetic safety sensors, etc.

It also permits mixed operation of standard and safety devices and diagnostics with standard-master/PLC.

It offers a maximum response time of 35ms, accommodates a maximum of 31 safety slave units, uses and is fully compatible with the standard AS-i protocol and achieves Control Category 4 under EN954-1.

AS-i also achieves stop category four, a response time of 35ms and compatibility with existing AS Interfaces, which means no new wiring has to be added to a scheme.

At present, the AS-i Safety concept is also approved by the TUV and BIA, although there is work still to be done in standards committees to ensure that it is covered completely by Euronorms.

The safety monitor (the device that is expected by many to supersede the safety relay) can have one or two pairs of enabling paths and multiple monitors can be incorporated in one AS-i system.

Diagnostics capabilities are available via the AS-Interface and configuration software, which also controls all monitor functions.

Parameters can be set for contactor monitoring, stop category 0 and 1, restart inhibit, local restart function and hold to run control.

Schmersal has accelerated the integration of AS-i technology into its range of E-stops, safety switches, solenoid interlocks and magnetic safety sensors.

Examining the other options begs the question - where is the future?.

DeviceNet and Profibus are both expected to be strong players, DeviceNet particularly in the North American markets.

As byte networks they can of course, carry more traffic and more data.

This makes them more suitable for complex process controls - featuring speed controllers, drives, mixers, peer to peer, etc - and less suited to simple master/slave arrangements.

Schmersal is already developing its range of safety products with the necessary CAN chips and protocols to suit DeviceNet and Profibus.

Looking to the future Schmersal has already produced devices that offer yet another possibility - electronic technology integrated into the device itself, to make it more intelligent yet still able to be hard wired.

Safety devices with a bus interfaces offer a number of advantages including reduced parallel wiring, easy and low-cost installation, easy replacement in case of failure, embedded technology, control Category 4, a combination of safe and nonsafe signals, improved diagnostics, conformance testing and increased machine availability.

An example is the Schmersal BZ16 key-operated safety switch, which has coded safety relay technology built in and also features nonmechanical contact.

For many engineers the ideal will be an open interface that allows them to plug in components from any manufacturer and know that they have an absolutely safe system - one that has been reviewed by experts and can be described, in the context of the Euronorm, as setting the required levels of integrity and safety.

Their very real concerns about conformity to Euronorms has fuelled a continuing debate that is questioning exactly how fieldbus should proceed in order to be integrated within safety standards.

And there are signs that engineers may lose confidence in products or manufacturers who do not instigate a network solution.

With so many vested interests there is the potential for fieldbus to become a battle for the safety industry and one which specialists like Schmersal must win.

If we don't, safety becomes an addon for the automation manufacturer and not the focus of a safety specialist.

At every stage it is important for engineers and manufacturers to understand the implications.

For example, the adoption of any safety network or Fieldbus heralds the demise of the safety relay, which is currently matched to the safety switch and forms part of a hard-wire system.

(A system may, for example, have two safety switches or an E-stop reporting to one safety relay whose output contacts are then controlling safety interlocks).

When a Fieldbus approach is adopted one master safety controller acts as a safety relay across the network, taking inputs from 12 or 15 safety devices.

Safety relays become redundant and the safety controller takes over.

This highlights an example of the conflict of interest that can arise.

Because specialist relay manufacturers have a vested interest and will loose their market because they sell nothing else, they may encourage customers to adopt the proprietary approach rather than an open approach.

Hopefully, the final choices will lie with the wider engineering community and manufacturers must be careful not to believe that there is one panacea or simple solution.

There is certain to be considerable debate before a fieldbus leader emerges and engineers finally define which products meet the specification, which systems give them confidence and what are the really important advantages and disadvantages?.

Which is why safety component manufacturers need to listen to engineers.

In the current climate automation and safety manufacturers can go off in many different technological directions.

But whether we feel that AS-i, Profibus or DeviceNet is the fieldbus of choice, we must ensure that we meet the expectations of the engineers and safety specifiers - indeed all people - responsible for safety.

And we must ensure that they are happy with the choices and fully understand the future of fieldbus.

Meanwhile, by listening, Schmersal will continue to develop components for proprietary networks such as Esalan and AS-i Safety.

And we will offer products for DeviceNet and Profibus.

And we will have individual intelligent devices.

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