No abstract

The current paper deals with the elastic and stress analysis of a side-load spring, which is employed in cars belonging to the medium segment of the European automotive market. In the design of this kind of spring both the elastic characteristics and the fatigue strength have to be considered as significant aspects. Side-load springs, indeed, have a thrust axis which results in skew with respect to the damper axis; for this reason they are widely employed in front McPherson automotive suspensions in order to minimize the side force acting on the strut damper, which is responsible for the undesired damper-sticking phenomenon. In addition to this particular elastic property, as a consequence of the research effort in reducing the mass of components typical of the automotive industry, these springs have to face very high working stresses. The structural reliability of the spring must therefore be ensured by high residual stresses, which are induced by presetting and shot peening treatment at the end of the manufacturing process. The evolution of the thrust axis during suspension travel was analysed by a finite element (FE) model of the spring–seats assembly, built starting from a three-dimensional (3D) scan of the spring. In addition an experimental campaign was planned in order to measure the residual stresses at several points along the spring by X-ray diffraction. The actual stresses were then obtained as the superposition of those produced by the external loads, computed by means of the FE model, and the measured residual stresses. The beneficial effect of residual stresses on the structural reliability is discussed with a theoretical fatigue analysis on the basis of different multiaxial criteria. A mechanism of residual stress generation, based on the superposition of stresses coming from presetting and shot peening, is also suggested with the aid of FE simulations.

For the braking test, which has been widely employed in similar forms in the literature, it was shown how the reaction time, with respect to the pedal transition time, can have a higher dispersion due to the influence of external factors. For the lateral skid test, the following measures were identified as the most significant for application studies: the reaction time for the reaction phase, the second peak of the steering wheel angle for the first instinctive response, and the integral of the steering wheel angle for the complete response. The methodology used to analyze the test measures was founded on statistically based and objective evaluation criteria and could be applied to other tests. Even if obtained with a fixed-base simulator, the obtained results represent useful information for applications of the presented PR tests in experimental campaigns with driving simulators.

This paper aims to define a standard procedure for validating a fixed-base driving simulator to be used for road safety studies and in the automotive field for development of new vehicle-subsystems. The driving simulator was developed at the University of Pisa (Italy) - Department of Mechanical, Nuclear and Production Engineering; it is characterized by a static cockpit and a single front projection channel, with vehicle and pedestrian traffic opportunely generated. The validation procedure consisted in a statistical comparison between data recorded by an instrumented vehicle on an urban path and those recorded by the driving simulator on the same path reproduced in virtual reality. A sample of 93 volunteers were submitted to both the drive tests during which several vehicle signals, such as speed data, acceleration, braking action, engine RPM and steering angle were continuously stored. Speed and acceleration data were subsequently analysed through conventional statistical methods (z-test); in order to evaluate differences between real and simulated driving, the statistical analysis was integrated by regression techniques. The analysis allowed to highlight the efficiency of the procedure in both the relative and absolute validation process as well as to evaluate potentials of the specific driving simulator. The procedure has general validity and can be used as a standard procedure for validation of fixed-base driving simulators

In this work, a simulation framework for virtual testing of autonomous driving functions under the influence of a fault occurring in a component is presented. The models consist of trajectory planning, motion control, models of actuator management, actuators and vehicle dynamics. Fault-handling tests in a right-turn maneuver are described, subject to an injected fault in the steering system. Different scenarios are discussed without and with a fault and without and with counteractions against the fault. The results of five scenarios for different criticality metrics are discussed. In the case of a fault without a counteraction, a pronounced lateral position deviation of the ego vehicle from the reference curve is observed. Furthermore, the minimal and hence most critical time-to-collision (TTC) and post-encroachment time (PET) values are calculated for each scenario together with a parameter variation of the initial position of a traffic agent. The minimum TTC values are lowest in the case of a fault without counteraction. For the lateral position deviation and the TTC, the counteractions cause reduced criticality that can become even lower than in the case without a fault, corresponding to a decrease in the dynamic behavior of the vehicle. For the PET, only in the case of a fault without counteraction, a non-zero value can be calculated. With the implemented testing toolchain, the automated vehicle and the reaction of the HAD function in non-standard conditions with reduced performance can be investigated. This can be used to test the influence of component faults on automated driving functions and help increase acceptance of implemented counteractions as part of the HAD function. The assessment of the situation using a combination of metrics is shown to be useful, as the different metrics can become critical in different situations.