THORACIC INJURY CONTROL - Injury Biomechanics and Control: Optimal Protection from Impact - page 205

decelerating force on the occupant, and reducing the response time of the
belt. Load limiters typically involve an element within the belt retractor
that yields when a predetermined belt tension level is reached. This tension
may range from 2 to 6 kN.
These refinements have been shown to enhance belt performance both in
the laboratory and on the road (Haland and Skanberg, 1989; Adomeit and
Balser, 1987; Foret-Bruno et al., (1998, 2001); Crandall et al., 1997; Kent et
al., 2003b; Petitjean et al., 2002). Continued improvements may be possible
by implementing active control as an integral part of the restraint system.
Active systems (sometimes referred to as “smart” restraints) that adapt to
various inputs are being developed. For example, dual-stage pretensioners
may modulate the magnitude of belt retraction based on the severity of the
collision. Other “smart” aspects of restraint systems have been discussed
by Viano (2003), Miller (1995), Johannessen and Mackay (1995), Fredin
(1995), Bernat (1995), and Andrews (1995). It appears, however, that the
analytical foundation for active control of the restraining force has never
been formulated. The purpose of this chapter is to develop such a foundation
using limiting performance analysis to identify the theoretically optimal
restraint characteristics.
Continuous control of the restraint force may be one way to improve
the restraint system. A concept of such a control that involves moving the
point of attachment of the restraint system to the vehicle or retracting and
releasing the seat belts is proposed. The control design involves the limiting
performance analysis of the isolation of an occupant from the crash impact
and the formation of a feedback to sustain the open-loop control law that
provides the limiting performance.
The limiting performance analysis requires a mathematical model of the
response of the object to be protected (an occupant) to a crash decelera-
tion pulse. The restraint force is regarded as an abstract control variable
that is identified by solving an optimal control problem for the previously
mentioned model. The specific design characteristics of the restraint sys-
tem are not important for the limiting performance analysis and are not
taken into account. The optimal control problem is stated for a performance
index to be minimized. As a rule, the behavior of the system is charac-
terized by several performance criteria and the criteria different from the
performance index are subjected to constraints. An important performance
criterion, which is always considered when dealing with shock/impact iso-
lation, is the displacement of the object to be protected (an automobile
occupant in the case under consideration) relative to the base (a vehicle).
This criterion characterizes the space necessary between the object and the
base to provide the amount of isolation needed to mitigate injuries to the
occupant in a crash. Very often this criterion is used as the performance