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SACE outline

Identify potential AS hazardous failures at tier n

The proposed design of the AS at each tier shall be analysed to identify the potential hazardous failures that could arise as a result of that design. Although an AS has been developed to satisfy the identified safety requirements, it still may be the case that the AS may be capable of doing something else, under certain conditions, that may be hazardous. Such hazardous failures can often not be identified until detail of the design solution, and in particular the characteristics of the system components is understood.

Potentially hazardous failures are identified by considering possible deviations from intended behaviour that may arise for the AS. Analysis shall be undertaken based upon the information regarding the design at the current tier under consideration ([VW]). The analysis shall consider information about the particular components proposed as part of that design. In particular the known limitations of the components, their known failure modes, and the conditions under which the components may fail, or under which their performance may deteriorate, shall be considered in the analysis.

Example 25 - Autonomous passenger shuttle Automotive

The cameras chosen to be used on an autonomous passenger shuttle to determine its distance from the footpaths adjacent to the road are found to not function well in conditions that present sunlight that is both of high intensity, and at an acute angle to the road. This is a potentially hazardous failure since it may mean that the shuttle drives too close to the footpath.

Analysing the AS design for possible deviations must be undertaken predictively at higher levels of abstraction where specific component solutions have not been chosen. For this analysis, a technique such as HAZOP [17] that uses a set of defined guidewords applied to elements of the design in order to prompt the identification of possible deviations, can be applied. For more detailed levels of design, once the properties of the actual components used are known a more specific deviation analysis techique such as failure modes and effects analysis (FMEA) [6] can be used, perhaps in combination with Fault Tree Analysis [26] in order to understand the causal path. Safety analysis such as this forms part of traditional safety engineering processes. For AS it is important that deviations are specifically identified relating to the autonomy of the system (particularly understanding and decision making deviations).

Note 24 - HAZOP analysis

Although a fairly traditional HAZOP analysis may be applicable to AS, particular attention must be paid to subtle deviations when interpreting the guidewords. For example, when analysing a proposed perception component, careful consideration must be given to how the following guidewords may be defined and interpreted:

  • More (more than one object detected when only one is present)
  • Less (less objects are detected than are actually present)
  • As well as (an extra area is classified as navigable as well as the intended route)
  • Part of (only part of an object is detected)
  • Other than (an object is classified other than what it is)

It may also be the case that additional guidewords are required when considering AS such as:

  • Intermittent (considers intermittent detection and/or classification)
  • Erroneous but credible (elicits failure conditions relating to information that is incorrect but ‘believable’)

Having identified the possible deviations, the hazardous failures are determined by considering which of the deviations, if they occurred in the AS, could result in a hazardous outcome.

The results of the analyses, including the identified hazardous failures shall be documented in the safety analysis report ([BB]). The safety analysis report shall also provide a justification for the sufficiency of the analysis approach used, including the suitability of the approach for the particular tier under consideration and the appropriateness of any modifications made to existing techniques to consider autonomy related deviations.

Continue to: Activity 20. Define mitigations for identified hazardous failures

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