Reducing Lumbar Spine Vertebra Fracture Risk With an Adaptive Seat Track Load Limiter

Abstract

In future fully automated vehicles, sleeping or resting will be desirable during a drive. While a horizontal position currently appears infeasible, a relaxed seating position with a reclined seatback and an inclined seat pan which enables a safe, comfortable position for sleeping or resting is possible. However, the inclined seat pan increases the forces and moments acting on the lumbar spine of the occupant and thereby the risk of lumbar vertebra fractures in a frontal crash. An energy management system integrated into the longitudinal seat adjustment (a seat track load limiter: STLL) that can reduce this risk should be investigated. When evaluating the injury reduction potential of a new restraint system such as a STLL it is important to include variations in both occupant size and crash severity. Otherwise, there is a risk of sub-optimizing, that is, the restraint system is only working for a limited number of situations. The restraint systems addressing these variations are normally referred to as adaptive restraint systems. The first objective of the study is to develop an activation strategy (adaptive release time of the STLL) for different crash severities and occupant sizes, making full use of the available stroke distance without bottoming out the STLL. The second objective is to evaluate the potential of the adaptive STLL to reduce the risk of lumbar vertebra fractures by comparing it to 1) a fixed seat and 2) a passive version of the STLL. Simulated frontal impacts were performed with two male SAFER human body models (HBMs) as occupant surrogates: mid-sized (80 kg and 1.8 m) and large (130 kg and 1.9 m). Three crash pulse severity levels were evaluated: low (40 km/h), medium (50 km/h), and high (56 km/h) impact speeds. The fracture risk was evaluated for the five lumbar vertebrae (L1–L5) in three different seat conditions: 1) a seat fixed to the sled, 2) a passive STLL that moves when a given force is exceeded, and 3) an adaptive STLL which moves at a time that depends on the occupant mass and crash pulse severity. The risk for lumbar vertebra fracture increased with crash pulse severity, while HBM size had no effect on risk. For all conditions, the passive STLL reduced injury risks compared to the fixed seat, and the adaptive STLL reduced risk even further.

Development of a Restraint System for Rear-Facing Car Seats

“…The force versus deformation behaviour of the constant-force deformation element is given in Figure 7. Constant-force deformation elements are commonly used in aviation [32,33] and automotive industries [34,35] to cushion the impact. The deformation element is plastically deformed and its elastic rebound is minimal; nevertheless, the energy-absorbing device applies high damping (30 kNs/m) against seat pan motion in the reverse direction (towards the rear of the car) to limit uncontrolled motion of the seat in the rebound phase of the impact during frontal crashes.…”

“…Although HS( 35) is a rear impact pulse, it is still applied to the seat considering that the proposed rear-facing seat with torso plates can be reversed to be used as a forward-facing seat. The crash pulse HS (35) is applied horizontally to the proposed rear-facing seat. Table 4 includes changes in velocities (delta-V), means and peak accelerations of the crash pulses along the x, y, and z axes of the inertial frame.…”