Improving the safety of vehicle occupants has gained increasing attention among automotive manufacturers and researchers over the past three decades. Generally, more recent vehicle safety improvement and injury prevention techniques could benefit from accurate knowledge of the occupant presence, characteristics, and/or position within the interior space of the vehicle. There is increased potential for injury mitigation systems to be applied more effectively if the proximity of the occupant to restraint devices is obtained in real-time during vehicle operation. A particular application is the position of the head relative to the head restraint for mitigating neck injuries from rear end impacts, which has led to the development of "active" head restraint systems. The current paper focuses on the head restraint application and initially presents a critical review of the available sensor and measurement technologies for real-time monitoring of the occupant's head position sensing system, including optical and capacitive sensors. To obtain accurate and reliable data, various scenarios related to the driver and passengers are considered in the process of sensor and technology selection, including confounding elements such as their clothing and a changing environment. Based on the results of this critical review, this paper outlines the subsequent research performed to identify the proximity of the vehicle occupant's head relative to the head restraint using a capacitive sensor array. As a prelude to extensive human and sensor system evaluation, the experimental set-up and data collection process for using a real human head are presented and explained, and the results containing the proximity data of the occupant's head relative to the head restraint are reported and discussed. Based on the experiments provided within this paper, the accuracy of head position quantification is less than 0.5 cm.
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