New fieldbus-agnostic approach to safety

EPLsafety is a new bus-independent approach to safety within fieldbus-based systems.

Machines and systems are today stuck in a rigid scheme made up of hardware-based safety functions.

The consequences of this are cost-intensive cabling and limited diagnostic options.

More recent attempts at safety fieldbuses are often characterised by proprietary standards and limited cycle times in the millisecond range.

As a result of the rapidly advancing development of Ethernet solutions for automation, these traditional safety protocols will in future be of little significance.

An analysis of existing safety protocols has shown that they are not suitable as a base for open and real-time-capable Ethernet communication.

It is for that reason that the Ethernet Powerlink Safety Working Group was established on 27th July 2004, within the EPSG.

The core team comprises the companies Bernecker+Rainer, Eckelmann, Innotec, Lenze, KW-Software and Parker Hanifin, other companies extend the team.

Its objective is to define a forward-looking safety protocol for Ethernet-based communication in the millisecond range.

A great deal of intensive work has been carried out since it was established, and the results are worthy of note.

A TUV-certified white paper is already to be presented in the context of this year's SPS/IPC/Drives in November in Nuremberg.

Ethernet Powerlink Safety - or EPLsafety for short - is defined as a bus-independent, autonomous frame, which can in principle also be inserted into other standard protocols than Powerlink.

One of its highlights is the compatibility to CAN messages.

EPLsafety demonstrates its full effectiveness in combination with Ethernet Powerlink.

Safety category SIL3 according to IEC61508 is achieved for the first time with cycle times of up to 100us.

Considering a specific architecture EPLsafety even fulfills the requirements up to SIL4.

The encapsulation of content of EPLsafety enables the standard transport mechanisms of Ethernet Powerlink to be used to the full.

Thanks to the very flexible construction of the frames EPLsafety can be adapted extremely well to various applications such as machines, installations or transport systems.

The frame length is determined by the reference data needed by the application.

EPLsafety stands out with its uniform telegram format for different data purposes (payload data, configuration, time synchronisation).

Each EPLsafety node recognises automatically the data purpose of the telegram.

Thus, the telegram length and type does not need to be configured separately.

As a consequence implementation and handling become much easier.

The flexible standard format permits a bandwidth of between 1 and 32byte (status quo per October 2004) of reference data.

A new technology enables the CRC type to be automatically configured in relation to the length byte.

Its long protocol proves to be a particular advantage - it is also based on the standard format and permits full use of the Ethernet telegram (approximately 1500byte).

Even the most varied requirements present no problem, thanks to the flexible and uniform EPLsafety telegram format (status quo per October 2004).

EPLsafety offers also advantages in the area of Safety Data Transport: EPLsafety embedded in the Powerlink standard; standard PDO mapping; byte offset and length clearly determine the position of the EPLsafety frame; broadcast (1:n); direct sensor-actuator communication possible; look-up table in every safety node (SN) identifies the correct messages; and management node (MN) can be used.

One of the main highlights of EPLsafety is its support of subnetworks within one safety domain.

And EPLsafety can also be used unlimitedly in complex network architectures with safe and nonsafe subnets with their own domains.

As such EPLsafety meets already today the future requirements of distributed automation structures with integrated safety technology.

Safety-directed electronic systems are being used to an ever-increasing extent in practically every area of engineering.

New regulative measures from the legislator, which are intended to ensure the safety and integrity of both people and goods, are leading to a growing need for such systems in complex control and automation systems in all sectors of industry.

One of the most important standards to be met in order to demonstrate the appropriate level of safety is the international standard EN IEC61508.

In reality, safety-related systems in automation continue for the most part to be discreetly wired and electromechanically controlled.

With the spread of fieldbus systems and programmable control systems, the desire to use them in safety equipment is also growing.

Standardised components and networks are expected to deliver considerable cost savings, simplified maintenance and increased flexibility.

EPLsafety is not a standard that is dominated or protected by one manufacturer.

Other manufacturers of electronic safety systems are invited to introduce their requirements into the working group.

Openness is extremely important for the further development of the technology.

For that reason every member of this and further EPSG working groups will have the right to influence and mould the future of Ethernet Powerlink.

Acceptable solutions will be created by balancing the various requirements, which in the end can be implemented rapidly.

The Safety Working Group is led by Volker Sasse of KW-Software in Lemgo.

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