For automated driven vehicles with a driving automation level above two, the driver is not available immediately as fallback when the automated driving system fails. Therefore, a redundant design for each automated driving system (e.g. the automated steering system) is a central safety requirement. The grade of redundancy, i.e. if it has to be fully fail operational or just a certain level of fail degraded, depends on the definition of the safe state in case of a failure and on the way how to reach it. The safe state itself depends on the driving situation respectively the type of road, where the automated vehicle is driving. The goal of this article is to determine the amount of steering power and energy required in different use cases and road types to reach the safe state.

Therefore, a definition of the safe state for automated driving trucks is determined using the ISO 26262 and existing definitions. With the help of German national road construction guidelines for highways, rural roads and urban roads, the safe state and the necessary driving maneuvers to reach it are determined for different defined road types. A 12-t two-axle truck has been equipped with measurement equipment as test vehicle. The determined driving maneuvers to reach the safe state are driven with the test vehicle and the required steering power and steering energy are measured.

The results of this investigation are the minimum required steering torque, power and energy for each tested driving maneuver. The minimum redundancy requirements to the automated steering system for a specific use case of automated driving, such as fully automated highway driving, are determined considering all driving maneuvers to reach the safe state in the worst case. Depending on the intended use cases for the automated vehicle, different fallback requirements are determined for the redundant automated steering system. Although the achieved results of this contribution are only representative for the used test vehicle, they are still helpful to get an impression and some real data for the required fallback steering torque, power and energy. It has to be considered that the required steering power and steering energy are highly influenced by the front axle load and thus by the load of the vehicle and by the steering and axle geometry of the vehicle as well. However, based on the findings of this article, the fallback concepts of future redundant active steering systems for highly and fully automated driven trucks can be developed according to the intended use cases.

The requirements for the mentioned exemplary use case of fully automated driving on highways with hard shoulders are very low, thus it should be possible to realize the steering redundancy with low effort. However, for other use cases the redundancy requirements are much higher.