Smart cabin of autonomous vehicle may lead more passengers to recline their seat for comfort. However, reclined seat is disadvantageous for holding occupant in seat when vehicle is subjected to rear-end crash. Under severe rear-end crash and large reclining angle, the backward inertia could completely throw occupant out of seat. Or, even if occupant body can be restrained well by seatbelt, the occupant head could slide out of the head restraint area. Any of these situations may potentially cause severe injuries. To address this safety concern, we developed a sliding seat system designed to enhance occupant retention. Activated by impact inertia from rear-end collision, the system allows the seat sliding backward along its track in a controlled manner, and the sliding stroke is accompanied by a restraint force and absorbs some amount of kinetic energy during the sliding. Meanwhile, the backward sliding of seat, together with backward sliding of occupant, makes the ride-down process more optimal, and thus, occupant retention can be enhanced, and injury risks of head and neck can be reduced.
In this study, we built a MADYMO model and conducted a parametric analysis. The model includes a 50th percentile human model, a vehicle seat, and a seat-mounted three-point seatbelt. Under 56 km/h rear-impact load, we evaluated occupant kinematics and critical injury parameters. A normal upright posture (the baseline) and a relined posture of 45 degrees were compared. The relative displacement between occupant upper torso and seatback was used to measure the distance that occupant slides backward, which is a metric for occupant retention. We investigated four key influencing parameters: seatback stiffness, seatback angle, sliding distance, and sliding force (i.e., restraint force during sliding). Latin Hypercube Sampling was used for experimental design, and Sobol global sensitivity analysis was applied to quantify parameter effects and interactions.
The results have shown that seat sliding distance is the most critical factor for occupant retention, and the longer the sliding distance, the better the retention effect and the lower the injury risk. For a given available sliding distance, there exists an optimal sliding force. In a typical scenario when 200 mm is available for sliding, compared to the traditional fixed seat (no sliding allowed), a Pareto-optimal solution gives: the occupant displacement is reduced by 25% (i.e., better retention), Head Injury Criterion is reduced by 16%, and Neck Injury Criterion is decreased by 39%. For vehicle seat design, using the sliding seat system may help off-load the burden of enhancing recliner stiffness, a critical seat component for maintaining seatback stiffness level in rear-end collisions.