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This is the fifth in a series of papers on the theory of sensing automobile crashes [1, 2, 3 and 4]. The focus of this paper is to analyze why barrier crash tests are presently conducted and to propose a methodology for determining what crash tests should be run for better overall performance.

In motor vehicle crashes where an occupant has been seriously or fatally injured from a deploying air bag, a common finding has been that the occupant was in close proximity to the air bag (or out-of-position) at the time of deployment. The occupant may have been out-of-position for a variety of reasons including: driver loss of consciousness, pre-impact braking, multiple impacts, rear facing child seat installation, or late firing of the air bag after the occupant has already been forced against the air bag by the crash deceleration. Considerable research has been initiated to develop new or enhanced injury countermeasures to mitigate injuries to persons, particularly children, who are out-of-position at the time of air bag deployment. This paper reports on the development of an occupant position sensor that might be used in conjunction with dual stage or multi-stage inflation technologies for modulating air bag deployment.

In two previous SAE papers (1,2) by the authors, supporting analysis was presented showing the difficulty in achieving a timely response to real-crash events using a single point sensor mounted in the non-crush zone of the vehicle (tunnel, cowl, etc.). The analysis demonstrated the propensity to deploy the air bag(s) late during certain of these events. If a vehicle occupant was not wearing a safety belt, the deceleration forces of the crash could place the occupant out of position and resting against the air bag when it was deployed. In another SAE paper (3) by H. J. Mertz et al, the authors demonstrated that animals, used as surrogates for humans, could be injured if positioned against an air bag at the time of deployment. Arguments are presented here to show that there is insufficient information in the crash pulse as sensed in the non-crush zone to deploy an air bag in time for the unbelted occupant.

In earlier SAE papers the authors demonstrated that the ideal crash sensor for use in the crush zone has a constant velocity change response, and various sensors are now in use which perform as acceleration integrators. Further study of sensor performance, however, has shown that crush zone sensors function by being struck by crushed material which is forced rearward during the crash. This observation has led to the design of an inexpensive sensor which measures crush instead of velocity. The crush of the vehicle is used as an accurate indicator of the severity of the crash. This paper presents the theoretical basis for a sensor which initiates air bag deployment when the crush of the vehicle exceeds a pre-selected amount. In a companion SAE paper, the authors have demonstrated that single point sensing in the passenger compartment may result in late air bag deployment on soft crashes, and, therefore, sensing in the crush zone is required.

Now that airbags are the accepted solution for protecting occupants in frontal impacts, and now that safety sells cars, it is natural to look closely at the second largest killer of automobile occupants, side impacts. This paper develops the theory of sensing side impacts based on the assumption that airbags will soon be used for side impact protection. The trade-offs between the various sensor technologies are discussed including electronic and mechanical sensors. For mechanical sensors, fluid damped, undamped and crush sensing switches are compared. Finally, the requirements for a successful predictive sensor will be presented.

Regardless of whether crash sensors are mounted in the crush zone or non-crush zone, there will always be crashes where the sensors trigger late and the occupant has moved to a position near to the airbag deployment cover where he or she may be injured by the deployment of the airbag. The required sensor triggering time is now determined by assuming that the occupant is a 50% male sitting in the mid seating position. 70% of vehicle occupants are smaller and, on average, sit closer to the airbag and thus are even more likely to be out-of-position. Finally, current sensor systems make no allowance for occupants that are wearing seatbelts, for rear facing child seats located on the front passenger seat or for unoccupied seats. There are thus strong safety reasons for occupant position sensors. This paper discusses the above problems, the difficulties in sensing occupants and objects located in the vehicle and attempts to define the requirements for such devices.

A method of describing crashes using both fixed and moving coordinate systems is presented which leads to a theory as to what is the crush zone of a car and, thus, where forward sensors must be placed. Using a moving coordinate system, a car crash performance index is developed which provides a quick method of determining the severity of the car crash pulse and the difficulty fitting an air bag to a particular vehicle. A two-pulse theory is then developed which shows that the vehicle can be divided into the crush zone and the non-crush zone, and the implications of this theory is developed for mounting sensors in each of these locations. The crush zone is that part of the car which crushes during the crash up to the point that sensor triggering is required and the non-crush zone is the remainder of the vehicle. The implications for sensor placement are that non-crush zone sensors must not become part of the crush zone. Crush zone sensors must be in the crush zone.