Functional safety requirements are ever increasing in the industrial automation.
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.
This paper presents a complete analysis for CANopen communication, based on the most commonly supported services without dedicated safety extensions. The analysis for CAN communication is based on widely accepted equations and parameter values. In addition to the CAN communication, effect of the most commonly supported CANopen communication services will be analyzed. Some improving factors needed to be neglected to keep the analysis understandable. Main result is that CANopen offers significant improvement in dependability of the communication by filling the gaps of CAN layer. CANopen provides several magnitudes higher dependability than the analog instrumentation. After analysis, some solutions to reduce effectiveness of residual errors are listed, most of which are introduced in various device profiles.
The capability of a machine control system to perform a safety function is expressed using performance levels (PL). This paper presents the results of a study where PL estimation was carried out for a safety function implemented using three different distributed control system structures. Challenges relating to the process of estimating PLs for safety related distributed machine control functions are highlighted. 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 safety function used as a generic example in PL calculations relates to a mobile work machine. It is a safety stop function where different technologies (electrical, hydraulic and pneumatic) can be utilized. It was detected that by replacing analogue cables with digital communication the system structure becomes simpler with less number of failing components, which can better the PL of the safety function.
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