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Safety: is it time to jump?
Fieldbus continued to evolve, with further developments in the
use of fieldbus for safety-related applications. Offering similar
benefits to conventional fieldbus technology, safety-related fieldbus
has fundamentally changed machine safety systems, and research undertaken
by the National Institute for Industrial Safety in Japan using SafetyBUS
p and Pilz safety controllers suggests total cost of ownership savings
in excess of 50% against the traditional relay based approach.
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 new standards, one
of which is IEC 61508, and revision of others (IEC 60204-1 and EN
954-1, with EN 954-1 to be superseded by ISO 13849-1), safety-related
systems have been able use 'state of the art' electronic, programmable,
and network-based technologies in machine safety. Formerly, standards
placed reliance upon hardwired electromechanical components. This
is a major change and the impact is no less significant than when
early PLCs replaced relay logic in standard control systems.
This pace of change, both technological and standards based, has
led to a more cautious and evolutionary approach by some, in contrast
to early adopters like the automotive sector which has already embraced
the benefits of a safety bus. Clearly, safety-related fieldbus can
offer advantages over traditional hardwired safety systems, with
reduced wiring complexity and lower design, commissioning, installation
and reconfiguration costs. Where intelligence is distributed down
to device level, comprehensive diagnostics are available to speed
up fault rectification, making maintenance faster and easier. Programmable
safety controllers and a new breed of safety-related devices bring
additional functionality and flexibility that makes possible integration
with robots and safety-related drives. In safety-networked motion
control, including start/stop, synchronisation and positioning,
safety-related fieldbus brings far greater opportunities than just
the replacement of parallel hardwiring and safety relays.
The question with fieldbus remains: which should you choose, or
should you wait? Hesitation equals lost opportunity, since obvious
benefits are being realised now. However, what are the factors to
be considered?
In determining which safety system is going to be used, decisions
need to be made about physical size: what maximum bus lengths are
supported, what is the input/output count and how many devices and
network media can be supported? Is there a need for centralised
or distributed safety outputs, or are both required? Functionality,
flexibility and the extent of diagnostics depend on how much intelligence
there is going to be within individual networked devices, network
bandwidth and the extent to which the control system is programmable.
The range of current and future devices available will also be a
factor as this determines how the system can be developed in the
future.
Traffic and loading has performance implications for those considering
the combination of non-safe devices with safety-related fieldbus.
The potential for undesirable interaction between safety and non-safety
devices on the same network not only has implications for network
and device design, but also for installation and use under IEC 61508.
Separation of safety networks from conventional fieldbus avoids
performance issues that could otherwise arise because of the higher
bandwidth demands of safety devices for error detection and avoidance.
Additionally, stringent EMC requirements for safety-related systems
may determine whether standard fieldbus devices may be used in combined
systems. IEC 61508 seeks to address failures in safety-related systems
that could occur throughout a system's life cycle and as these principles
are implemented in other standards they will apply to all but the
least complex safety systems. The justification required for compliance
with IEC 61508 is likely to be assisted by clearly distinguishing
between safety-related and conventional fieldbus systems, putting
the higher SIL (Safety Integrity Level) functions on to the safety
bus.
Safety-related fieldbus systems will allow users to choose from
safety networks that are suited to small or large-scale implementations,
with the benefit of separation, combination and interfacing with
existing conventional fieldbus networks. As with any technology,
choose carefully, ensure it is appropriate and has performance suitable
for safety, and then prepare to reap the benefits.
SafetyBUS p Club
q101@industrialnetworking.co.uk
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