Numerical Simulation of Piezoelectric Sensor Used in a New Pedestrian Protection System

In recent years, pedestrians have been involved in many accidents. Statistics show that in Europe pedestrians are involved in 12% of the accidents and 20% of road users who lose their life in accidents are pedestrians. Authorities and automobile manufacturers are interested to reduce the physical damages of these users. The EU requires a very accurate design of new car front-end, because this is the first part involved in the car-to-pedestrian impact. Some EU organisations as EEVC (European Enhanced Vehicle-Safety Committee- Working Group 17) and ACEA (Association des Constructeurs Europeens d’Automobiles) have decided the directives on the design of the new car front-end. These documents settle on the experimental methods and the biomechanical parameters that have to be considered combining the front-end aggressiveness with the crash test dummies damages. Moreover, many car and system manufacturers are studying new pedestrian protection systems as the active hood that raises instantly when a pedestrian is hit: a sensor system placed in the vehicle’s front bumper sends a signal a two steel pistons below which lift the rear part of the hood making the pedestrian’s head hit a deformable and flexible surface instead of a hard and rigid one. These systems will help car manufacturers to meet the new EU test requirements that the European car manufacturers accepted in 2001 and that will become effective in July 2005. This paper describes an activity on car-to-pedestrian impact carried on by the Universita` degli Studi di Firenze in co-operation with Centro Ricerche FIAT. This activity concerns the study of legform FEM model for LS-Dyna software and the numeric simulation of a sensor system placed in the vehicle’s front bumper. The sensors are piezoelectric stripes stuck on the bumper according to opportune geometries. These sensors were characterised through experimental analysis changing deflection and impact speed; the relationship between the numeric parameters and the electric signal was determined working on easy tests. It was subsequently simulated the impact between the bumper and a legform FEM model developed by Universita` degli Studi di Firenze working on LS-Dyna software. The legform model respect the structural characteristics required by EEVC WG17 report. The legform model was validated by experimental analysis carried on at Centro Sicurezza FIAT AUTO. Thanks to simulations it has been possible to optimise the position of the sensors on the bumper and also to judge the minimum number of sensors, which can distinguish between the different impacts.