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Product category: Machine Safety Monitoring and Control
News Release from: Pilz Automation Technology | Subject: SafetyBus p
Edited by the Engineeringtalk Editorial Team on 03 September 2004

Decentralised safety delivers user
benefits

The use of PLCs and fieldbuses has grown rapidly for automation tasks, and now we are seeing a similar pattern for safety-related control functions, says Dipl Ing Holger Bode.

Decentralised control concepts have increasingly become established within automation Input and output modules are installed directly on the machine and are connected to the central controller via a fieldbus system

This concept is also suitable for safety-related signals.

SafetyBus p from Pilz is an example of this type of safe, open bus system.

Just as with conventional bus systems, safety-related networking offers users the same benefits, such as less wiring, savings on control cabinet space, universal diagnostics, greater flexibility and higher availability.

The need for safety-related functions in mechanical engineering is continually growing.

The general rule-of-thumb used to be that approximately 5 to 10% of all inputs/outputs were concerned with safety-related functions, but this figure has now clearly increased in favour of safety.

The reasons are obvious: the growing level of automation reduces the work involved for personnel but requires increased personal protection - particularly during maintenance or servicing work.

The complexity of the plant or machinery carries the risk that operators will not fully understand the wide variety of machine status conditions.

So protection of personnel becomes increasingly important and the number of safety-related components rises.

However, if you study the situation from the viewpoint of the operator and legislation, it is clear that manufacturers are obliged to build "safe machines" and, in doing so, are bound by laws.

Directives and standards help, providing manufacturers with suggestions and guidelines for achieving their objectives.

In the past, safety functions may have mainly concerned emergency stop or safety gate guarding, for example, but demands will continue to grow on manufacturers both now and in the future.

Increasingly it will not just be static signals such as those from an emergency stop pushbutton that are required, but "dynamic" signals will also need monitoring.

This is similar to the evolution of PLC technology.

As well as protecting personnel, safety-related components are also being used more and more to protect machinery itself.

On the one hand there is a need to protect the enormous investments made in plant and machinery, and, on the other hand, downtime for necessary repairs causes considerable expense on account of the high level of automation.

From both aspects it is clear that safety is becoming increasingly significant.

Until recently the safety of a plant or machine could be covered centrally via a PSS (programmable safety system), but now the decentralisation of safety is making its mark due to the increased complexity described.

The process is similar to the one experienced years ago when networking the (standard) control section of a plant or machine.

The driving forces are primarily a reduction in wiring, increased flexibility and comprehensive diagnostics.

The SafetyBus p safe, open bus system makes the decentralisation of PSS programmable systems a reality.

The various safety-related functions, such as emergency stops or safety gate switches, are wired locally on I/O modules, which are themselves safe.

The low granularity of 16 I/O points per decentralised module ensures that cable routes to the sensor and actuator remain short.

The modules themselves are connected serially to the programmable safety system via SafetyBus p.

The safe bus system goes beyond this form of pure I/O decentralisation: as it is possible to interconnect several programmable systems safely, safety-related "intelligence" can be distributed.

This is particularly necessary on complex plant or machinery, which is divided into virtually independent sections.

Each of these substructures may have its own secure SafetyBus p network with programmable safety system and decentralised I/O modules, which, for their part, will either be connected via SafetyBus p or via a safe gateway.

In this way it is possible to remove certain sections from the plant or machine network, to undergo servicing or maintenance work for example, without adversely affecting the operation of the remaining plant or machinery.

The sustainability of users' interest in comprehensive solutions can be seen in the wide range of applications implemented within a very short timescale: quite clearly, a baggage handling system at a large airport has little in common with either the manufacture of concrete blocks or a press line at a car plant; at best, perhaps with the high-rise storage plant connected to the press line.

And yet the safe bus system is used on all these plants.

Baggage handling systems are characterised by their substantial length.

The safety function is confined to linking the signals from the local emergency stop pushbuttons to the programmable safety system via SafetyBus p, using decentralised I/O modules.

The eight, largely independent conveyor areas each have one programmable safety system and up to 25 decentralised I/O modules.

The safety system's integral standard Interbus interface is used for universal diagnostics across the entire network.

Using SafetyBus p not only reduces the substantial wiring considerably, but the system's flexibility also helps to separate the planning of the electrical installation from that of the mechanics.

Machines used to manufacture concrete blocks or for woodworking illustrate the wide range of safety functions.

Global emergency stop pushbuttons are connected to SafetyBus p in exactly the same way as local safety gates or light guards.

The plants have one programmable safety system and around ten decentralised I/O modules over a network extension of approximately 500m.

The onboard I/Os on the programmable safety system are used for local control cabinet requirements.

Press lines have a totally different topology, which is best explained through the "distributed intelligence" described above.

Each individual press - and potentially its transfer robot too - is controlled by its own PSS.

All the external safety devices over the entire press line, such as emergency stops, safety gates or light guards at the loading and unloading ends, are assigned to their own programmable safety system using decentralised I/O modules, via SafetyBus p.

The individual "press safety systems" are connected to the external SafetyBus p network.

This topology guarantees that individual presses can be removed from the network at any time - for tool changes, for example - while the rest of the press line remains in operation.

The variety of applications illustrated in these three examples demonstrates the wide range of the safe bus system SafetyBus p.

Nevertheless, safety is more than just a product.

That is why Pilz offers additional services alongside the technical support available for its own products when implementing an application.

These services include safety advice, risk assessments, creation of safety concepts, configuration and implementation of entire systems, as well as performing safety-related assessments.

As the demands made of a safe bus system can vary tremendously, many companies are promoting the further development of the bus system under the umbrella of the SafetyBus p Club International.

One objective on the user side, for example, is for cross-industry standardisation, where possible, of safety-related devices that have a direct connection to SafetyBus p.

Another objective is to establish a uniform profile for directly connected safety devices.

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