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Safe position monitoring

12 March 2018

Control Engineering Europe looks at the importance of international standards when it comes to the specification of protection devices on machinery.

Machine safety is an integral part of machine design, As Rolf Brunner, technical head of product center safety for Leuze, explains: “In addition to reliability, the ideally fault-free integration of a solution into the processes is decisive for practical use."

Different regions and countries of the world, have different concepts for machine safety and protection of humans interacting with machines. In addition to the different requirements and the evaluation of safety concepts, there are also differences in the responsibilities and in the legal consequences. The following information serves as an introduction to the topic of ‘Safe Positioning’. It does not release the machine designer from performing an in-depth study, or from observing the respective, applicable regional and machine-specific regulations and adhering to the operating instructions of the given devices.

Typical applications where safe positioning or safe movement monitoring is required include, among others, high-bay storage devices which are used in, for example, intralogistics – such as for the positioning of travel and lift axes and the monitoring of the speed at the end of travel paths for collision avoidance with the end-position buffer. With a manned car on a high-bay storage device, it is – depending on the design – only possible to travel at a safely reduced speed.

Automotive industry
Movement monitoring applications in the automotive industry offer another application example. With heavy-load telpher lines, lifting devices are moved up and down when the telpher line is located at a safe position. Skillet systems are not generally directly positioned, but are rather pushed through the production line in a continuous linkage with a single motor. The ‘safe position detection’ will run along in the base of the skid to then raise and lower the lifting device at predefined, safe positions.

The use of standard components – sensors, drive elements and control electronics – in safety applications is fundamentally possible in accordance with EN ISO 13849. This also applies if these components are not designated as safety components in accordance with Appendix V of Machinery Directive 2006/42/EC.

When using industrial standard components that were not specifically designed for use in safety functions, but which contain embedded software, further consideration must be given in accordance with the latest version of DIN EN ISO 13849-1:2016-06.

To achieve Performance Level c or d with standard components, two channels must be used. Furthermore, the components in both of these channels must be based on diverse technologies. The probability of a common cause failure (CCF) is, thereby, almost completely excluded. CCF refers to the failures of redundantly designed units due to a single event with common causes, such as soiling, electromagnetic influences or pressure.

With the bar code positioning systems of the BPS30 and BPS300 series, Leuze electronic is able to offer the diversity demanded, and verified by TÜV, which states that ‘the analysis of the software and hardware of the BPS 3xxi and BPS 3x positioning systems shows that both systems have a different structure with respect to hardware and software design, and requirement for diversity is thereby satisfied for both.’

In man-machine collaboration areas, the requirements of the machinery directive, with respect to safety-related aspects, must be taken into account. In many applications, a position must be safely detected. However, position detection always involves the movements of, for example, high-bay storage devices, telpher lines or machine components in general. In many cases, this can lead to hazards. To avoid these hazards, the position values must be safe. This applies both for the position detection itself as well as for the subsequent position processing. To obtain a safe speed value for an axis, the position changes of the moving axis are calculated per time unit and thereby result in safe speed values. This leads to the question of how position detection can be realised when taking into account safety-related aspects.

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