Redundant Steering System forHighly Automated Driving of Trucks

Abstract

The development of automated driving (AD) from partially automated driving (AD2-) to highly automated driving (AD3+) is not only in the focus of the passenger car industry, but also in the commercial vehicle (CV) industry, especially in the development of trucks. There is already a lot of research work on AD3+ for passenger cars. However, in this area there is still a great need for research for trucks, particularly in the area of steering systems suitable for AD3+, since the requirements of these, especially with regard to the maximum required steering forces and steering powers, are much higher for AD3+ trucks than for AD3+ passenger cars. Therefore, the subject of this thesis is to develop a concept of an active steering system for AD3+ trucks by means of a deductive methodology and a systematic analysis of the solu-tion space. The development is based on the frame requirements for truck steering systems with regard to assembly space, interfaces, energy supply and axle loads as well as on the operational and redundancy requirements determined in this thesis. On the basis of these requirements, a redundant electric power steering system is excluded from the solution space due to the insufficient electrical power available on board and a redundant hydraulic power steering system for efficiency reasons. With today's actuators, the solution space is limited to combinations of electric and hydraulic power steering, the so-called hybrid steer-ing systems, for which the possible different functional structures are derived. These are evaluated on the basis of requirements from a safety analysis, whereby the solution space is limited. The developed concept, which meets all requirements, is a redundant active steering system (RASS) with an electric subsystem and a hydraulic subsystem, which is equipped with an active steering valve that can be controlled by the driver as well as by an electrical signal. The RASS provides a so-called fail-degraded functionality whose degree of degradation was determined by the determined redundancy requirements. The double controllable steering valve is designed in such a way that the driver is able to override the automatic system at any time and that the required steering torque can be distributed arbitrarily be-tween the electric and the hydraulic subsystem within the torque and power limits of the electric subsystem. This functionality is usable to increase efficiency compared to conven-tional truck steering systems. An operating strategy is developed for the various system states of the RASS which, tak-ing into account the driver's state, the required steering torques and possible system faults, controls the power steering state in such a way as to increase the efficiency of the steering system, controls the transitions between manual and automated driving and provides fallback strategies in the event of a fault. The result is an innovative steering concept that meets all the requirements of today's trucks and is suitable for AD3+.