Evaluating performance levels of machine control functions

According to the basic machinery safety standard ISO 13849-1, the capability of a machine control system to perform a safety function is expressed using performance levels (PL). This paper brings out some challenges in the process of evaluating performance levels for safety related machine control functions. One of these examines the use of different cabling schemes in the implementation of a safety function and its effect on the PL evaluation. The challenges are highlighted using a generic example of a safety function relating to a mobile work machine where different technologies (electrical, hydraulic and pneumatic) can be utilized. A safety stop function with different structures was used as an example in PL calculations. Analogue cables in mobile work machines can cause a remarkable risk, since they are vulnerable to disturbances and failures. A part of these failures may be difficult to detect. In this study it was detected that by replacing analogue cabling with digital communications the reachable PL can be increased.