Characterisation and Utilisation of Steering Feel in Heavy Trucks

Abstract: Steering is next to braking the most important control feature of a road vehicle. The driver gives input through the steering wheel and the vehicle reacts in a certain way, which results in a transfer function between input and output. The transfer function can be modified by means of an actuator in the steering system to improve safety, handling and steering feel. To be able to use active steering systems to improve the steering feel, we need to understand how steering feel comes about. There have been several investigations to find out how drivers experience a change in vehicle steering and handling behaviour and how a change in vehicle handling behaviour affects the driver. However, as yet, there is no standardised way to find mutual corresponding measurements, assessments and ratings, nor is there a consistent definition of steering feel. An important part of investigating steering feel concerns how to measure what drivers feel. One of the essential prerequisites in the present research work is that steering feel, as perceived by human beings, can be allotted in dimensions. To define this non-instrumental space, a method to find the dimensions that people use to perceive and describe steering feel has been developed. It is shown that it is possible to extract up to nine dimensions describing the steering feel of road vehicles. This was experimentally evaluated using a driving simulator. In the test, drivers assessed truck steering system settings that differed in friction, damping, inertia and stiffness, due to five dimensions of steering feel. The same steering system settings were also tested in accordance with ISO standards for vehicle handling to acquire characteristic instrumental quantities. The instrumental measurements and the non-instrumental assessments were then analysed with respect to their correlation with each other. The results show that there are indeed correlations and also which of the handling quantities influence which dimension of steering feel.One possible use of the increased knowledge of steering feel evaluation is to influence the driver's behaviour by a directed change of steering feel. In a track test the steering wheel torque of a truck was modified depending on the lateral dynamic vehicle driving state. During the experiment the cornering behaviour of truck drivers was evaluated regarding lateral acceleration, which is related to rollover accidents. Statistical evaluation showed a decrease of maximum lateral acceleration values while cornering when the steering wheel torque was decreased at high lateral acceleration. There are also more possibilities to modify the transfer function between driver input and vehicle response. Artificial understeering and yaw rate gain acceleration are two functionalities that were developed during this work and were evaluated by simulation. Thus, based on the knowledge about steering feel, an application-oriented hypothesis could be formulated and evaluated. The fundamental part of this thesis contributed a puzzle piece to the mapping of steering feel while the advanced part established ties to future applications with active and semi-active steering as well as driver assistance systems.