1. Field of the Invention
Modern passenger cars are equipped with various active and passive safety devices. Active safety devices are designed to prevent accidents, whereas passive safety devices, like seat belts and air bags are present to mitigate the effects of an accident on the occupant.
2. Description of the Related Art
This invention relates to seat belts, especially to their behaviour in frontal crashes. When a frontal crash occurs, the occupant tends to move forward within the occupant's cell and to hit the steering wheel or dash board or other hard parts of the car's interior. The reason for this tendency of the occupant to move forward is the vehicle's high deceleration, which is characteristic for a frontal crash.
The main purpose of a seat belt system is to prevent the aforementioned forward motion of the occupant. Moreover, in an early stage of the crash, most seat belt systems activate so-called pyrotechnic belt pre-tensioners to increase the force by which the seat belt restrains the occupant.
Once the vehicle's deceleration gets too severe, the seat belt itself may cause harm to the occupant, since the belt's force that acts on the occupant becomes too high (causing rib fractions or similar injuries). In order to prevent such injuries, modern seat belt systems provide a certain controlled belt pull-out. This reduces/limits the belt force and thus the occupant's injury risk.
An advanced method to provide this belt pull-out during an accident is described in EP 1 778 527 B1, where belt pull-out is provided in sole dependency of the vehicle's deceleration (as opposed to most seat belt systems, where belt pull-out is provided in dependence of the belt force in the first place). The advantage of the former method is due to the fact that the seat belt's protective effect does not depend on the occupant's weight (or to be more exact: on the occupant's mass).
A mechanical realization of the method described in EP 1 778 527 B1 can be achieved by a spring-mass-system, fixed to the occupant cell that emulates the crash behaviour (i.e. the kinematics) of an optimally restrained occupant. These kinematics of the so-called reference mass (or reference body) are then transferred to a release and blocking device, which again transfers the motion to the seat belt, which finally transfers the motion to the occupant. By use of this, for example, a constant deceleration of the occupant's thorax may be achieved.
The term ‘spring’ used here subsumes springs with all kinds of force-travel-characteristics (progressive, digressive, constant etc.). Also, the terms acceleration and deceleration shall here refer to the same physical variable, regardless of its algebraic sign.
Typically, the driver's thorax can move approximately up to 300 mm forward before hitting the steering wheel or instrument panel. An optimal occupant restraint system should take full advantage of this given space for the occupant's forward displacement. This means that the above-mentioned reference body should be able to travel at least the same way relative to the car's occupant cell. This again means that the seat belt system already described, installed in a seat belt retractor in the car's B-pillar needs at least 300 mm of installation space within the car's longitudinal direction, maybe even more. Such an amount of space is, however, typically not available where the seat belt retractor is installed. It would therefore be advantageous to reduce the travelling space of the reference body and to use a transmission to compensate for the reduction of displacement of the reference body in the actual belt pull-out.
It is known that a transmission can be used to change the ratio between the displacement of the reference body and the actual belt pull-out in order to compensate for the seat belt geometry (this is due to the fact that a typical seat belt geometry does not automatically ensure a 1:1 translation of belt pull-out to the occupant's thorax; typically the ratio is closer to 1:1.5). A transmission can, however, only compensate for a constant ratio between the travel-time-function of the reference body and the travel-time-function of belt pull-out. Hence the travel-time-function of the belt pull-out that results from the travel-time-function of the reference body shows the same characteristics as the travel-time-function of the reference body, except for a constant transmission ratio.
The travel-time-function of belt pull-out is an important characteristic in an occupant restraint system which must meet certain requirements. Hence, when the travelling space of the reference body is scaled down, the characteristics of travel-time-function of the belt pull-out should not be altered, except for a constant scaling factor, which can be compensated for by a transmission. Keeping on the one hand certain characteristics of the travel-time-function of the belt pull-out while, on the other hand, downscaling the necessary travelling space of the reference mass means that the downscaling of the necessary travelling space should not alter the characteristics of the travel-time-function of the reference body, except for a constant scaling factor.
However, it is not trivial to achieve a travel-time-function of the reference body for a given vehicle deceleration which differs from the original travel-time-function by a constant scaling factor only. In other words, if for a given vehicle deceleration a(t), where t is the time, the travel distance of the reference body results in a function s(t), a method is now needed to achieve a new travel distance snew(t) where X·snew(t)=s(t) and X is a constant scaling factor.
Accordingly, it is a first object of the present invention to provide a method of downscaling the travel-time-function of a reference body which allows for a reduced travel space of the reference body in a spring-mass-system of a vehicle occupant restraint system.
It is a second object of the invention to provide a method of establishing belt pull-out with a specified travel-time-function in a vehicle occupant restraint system by using a reference body in a spring-mass-system, which method allows for a reduced travel space of the reference body.
It is a third object of the invention to provide a spring-mass device with a spring and a movable reference body which allows for a reduced travel space for the reference body.
It is a fourth object of the invention to provide an occupant restraint system comprising a spring-mass device with a spring and a movable reference body which allows for a reduced travel space for the reference body.