Abstract:
A safety belt assembly for a motor vehicle includes a seat belt tightener for tightening the seat belt upon the deformation of the vehicle chassis. Sensors are located within the chassis that determine when the anchor points of the seat belts move relative to each other, which indicates the chassis is collapsing. When these anchor point move, the seat belt slackens. The seat belt tightener then brings up the slack to prevent the passenger from being thrust from the seat. The assembly also includes a force limiter which prevents the over tightening of the seat belt.

Description:
The invention relates to a safety belt assembly for motor vehicles as described in the claims in view of the specification. 
     BACKGROUND OF THE INVENTION 
     Safety belts for motor vehicles have long been known. They serve the purpose of restraining passengers of a vehicle in the passenger seats in the event of an accident and so of protecting the passengers from uncontrollable movements in the interior of the vehicle. This greatly reduces the risk of injury to passengers in the vehicle. 
     Abrupt vehicle deceleration is known to occur in the event of frontal impact. Since motor vehicle passengers with belts fastened are not rigidly connected to the vehicle or the vehicle seat, there is a certain time delay between vehicle deceleration and passenger deceleration. The greater this time delay, the greater is the stress to which the passenger is subjected. This time delay is determined essentially by slack belt lengths. 
     Various safety belt assemblies are known which have electronically actuatable belt tightening devices for prevention of slack belt lengths. In these assemblies the severity of an accident is detected by means of sensors and is evaluated in a control unit. The belt-tightening device is appropriately activated by the control unit immediately after the beginning of an accident when a specific accident severity value is reached. As a result, the vehicle passenger is immobilized in his seat a few milliseconds after the accident begins. The danger of slipping beneath the pelvic section of the safety belt and striking the head, against the steering wheel for example is also reduced. The faster and more securely the passenger are immobilized in the vehicle seat; the lower is the risk of injury to the passenger. These remarks apply equally to an accident involving side impact. 
     A safety belt normally is fastened to three anchor points on the vehicle body. Generally the position of these anchor points remains unchanged during the entire course of an accident in a frontal or rear impact, unless the vehicle body is damaged in the area of the B columns. 
     The danger that the B column of the vehicle body will be damaged during an accident exists chiefly in the event of a side impact by another vehicle or of a rollover. In these cases the B column often buckles and as a result is displaced relative to the rest of the vehicle. Such displacement is accompanied by change in the relative position of the anchor points of the safety belt, this resulting in recurrence of slack belt lengths. Since the belt-tightening device operates only for a relatively brief period and actuation takes place for the most part immediately at the beginning of an accident, slack belt lengths may recur as the accident proceeds. The vehicle passenger is then no longer immobilized in his seat because of such repeated belt loosening, this leading to the consequence already described that the risk of injury is greatly increased. 
     DE 195 22 684 A I discloses a device for prevention of belt loosening in which device the shoulder belt section of the safety belt is controlled by a guide integrated into the vehicle seat. This guide comprises a clamping mechanism which may be actuated in the event of an accident and when actuated immobilizes the shoulder belt on the vehicle seat. Displacements of the anchor point of the shoulder belt relative to the vehicle seat occurring, as an accident progresses no longer cause belt length loosening with this device. Consequently, the vehicle passenger remains immobilized in his seat. However, this device requires costly modifications of the vehicle seat, which present a disadvantage from the optical viewpoint as well. Vertical adjustment of the shoulder belt is also no longer possible, since the position of the guide in the vehicle seat is assigned in advance as an invariable. 
     Reference DE 42 17 969 C2 discloses a belt tightening device with a multistage ignition drive. This belt tightening device permits delayed actuation of a plurality of explosive charges in succession. The time delay, however, performs the function exclusively of restricting belt restraint force. The second actuation cannot be effected independently of the first; it always occurs immediately and necessarily after the first actuation. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to create a safety belt assembly for motor vehicles which does not present the disadvantages indicated above, which prevent loosening of belt lengths in particular as an accident progresses, an assembly which is simple to produce by cost-effective means. 
     The essential idea of the invention is represented by design of a safety belt assembly such that the safety belt may be tightened twice in succession by actuation of a belt-tightening device. The second actuation of the belt-tightening device prevents loosening of lengths of the belt, which may occur as an accident progresses. 
     Relative displacement of the anchor points of the safety belt is detected by at least one sensor mounted on the B column of the vehicle body. 
     In order to limit for the sake of safety the strain on a passenger in a vehicle by a second actuation of the belt-tightening device, a belt force limiter is provided on the safety belt; it sets a maximum value for belt force. 
     Inasmuch as belt tightening devices which may be actuated only once are highly complicated in structure, and this applies to a far greater extent to belt tightening devices which may be actuated twice, in a simpler embodiment of a safety belt assembly provision is made for replacement of the belt-tightening device, which may be actuated twice with two separate belt-tightening devices of conventional design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is discussed in detail with reference to the drawings, in which 
     FIG. 1 presents a diagram of a safety belt assembly claimed for the invention corresponding to a first embodiment in the interior of a vehicle, and 
     FIG. 2 a diagram of a safety belt assembly corresponding to a second embodiment in the interior of a vehicle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 presents a diagram of the vehicle interior  5  with a safety belt assembly  2  claimed for the invention in the area of a vehicle seat  7  in a motor vehicle  1 . The safety belt assembly  2  comprises a conventional safety belt  12  with a belt tightening device  20  which may be actuated by a control unit  30  to which a plurality of sensors  35   a, b, c  and  d  are connected. The sensors  35   a, b, c  and  d  receive accident-relevant data. The sensor  35   d  mounted in the area of a B column  3  of the body of the motor vehicle  1  detects any deformations of the B column  3  in the course of an accident. 
     A vehicle passenger  9  is restrained in the vehicle seat  7  by means of the safety belt  12 . The safety belt  12  is anchored to the vehicle at three points F 1 , F 2 , and F 3 . A belt fastener  14  (anchor point F 1 ) divides the safety belt  12  into two sections, a lap belt  12   a  and a shoulder belt  12   b.  The shoulder belt  12   b  extends from the belt fastener  14  over the chest area of the vehicle passenger  9  to B column  3 , where its direction is changed by a guide fastening  18  (anchor point F 3 ). The shoulder belt  12   b  extends concealed inside the B column  3  on to the belt-tightening device  20 . The lap belt  12   a  leads from the belt fastener  14  over the pelvic area of the vehicle passenger  9  to a mounting support  16  (anchor point F 2 ) in or on the B column  3 . An example of a slack length of belt  12   c  is indicated by broken lines. 
     FIG. 2 illustrates a second embodiment of a belt-tightening device as claimed for the invention. The numbering corresponds more or less to that selected for FIG.  1 . Unlike the first embodiment, however, the belt-tightening device  20  is provided with a conventional belt tightening device  20   a  and an additional belt tightening device  20   b  on the lap belt  12   a.  The belt tightening device  20   b  may consist, for example, of a connecting rod  50  which may be displaced in the direction of the arrow by means of an actuating device  52 . 
     When the belt tightening device  20   b  is actuated the lap belt  12   a  is guided between guide rollers  54   a,    54   b  and the entire safety belt  12  is tightened or loosening of belt lengths is eliminated. 
     The actuating mechanism  52  is integrated into the vehicle seat  7  so as not to be visible externally. Providing guide rollers  54   a,    54   b  on or inside the vehicle seat  7  rather than inside the B column  3  in order to make the entire belt tightening device  20   b  as inconspicuous as possible is also conceivable. 
     The second belt tightening device  20   b  may also be installed at another point on the safety belt  12 . For example, a belt-tightening device permitting displacement of the belt fastener  12  toward the bottom of the vehicle is conceivable. Another alternative is a conventional belt-tightening device at the end of the lap belt  12   a  or another belt tightening device acting on the belt section extending into the B column  3  of the safety belt  12 . 
     The operation of the invention is described in detail below. 
     The safety belt  12  is normally in position under slight tension on a vehicle passenger  9 . This tension is generated by a pretension belt winder in the belt tightening device  20  or  20   a.  The vehicle passenger  9  can move with relative freedom. His freedom of movement is restricted only slightly by this light belt tension. 
     The sensors  35   a, b, c  and  d  react in the event of an accident. The signals of these sensors  35   a, b, c  and  d  are transmitted to the control unit  30  and are evaluated in this unit. Should these sensor signals exceed specific threshold values, the belt tightening device  20  or  20   a  is activated by the control unit  30 . Belt tightening devices are generally actuated by pyroelectric means. The shoulder belt  12   b  and the lap belt  12   a  are tightened by initial actuation of the belt tightening device  20  or  20   a.  Any slack belt lengths  12  possibly present are eliminated. The vehicle passenger  9  is rigidly immobilized in the vehicle seat  7  a few milliseconds after the beginning of an accident. 
     In the event of a side impact or in a rollover the possibility now exists that the B column  3  will not be damaged immediately at the beginning of an accident but later, after the initial action of the belt-tightening device  20  or  20   a  has “fizzled out.” Damage such as this frequently results in more or less extensive buckling of the B column  3 . The anchor point F 3  is then displaced relative to the other anchor points F 2  and F 1  in the direction of the arrow and the longitudinal distance from F 3  to F 1  decreases. This has the undesirable consequence that shoulder belt  12   b ; tight previously at the beginning of the accident again has a slack belt length  12   c.  It is claimed for the invention that this slack belt length  12   c  is eliminated either by repeated actuation of the belt tightening device  20  or, in the second embodiment, by actuation of the belt tightening device  20   b.  After actuation of the belt tightening device  20  or  20   b  the safety belt  12  is sufficiently tightened and the vehicle passenger consequently again immobilized in his seat. The risk of injury to the vehicle passenger  9  as the accident continues to develop is reduced as a result. 
     Since relative displacement of the anchor points F 1 , F 2 , and F 3  can in theory take place only as a result of damage in the area of the B column  3 , it is claimed for the invention that at least one sensor  35   d  is installed in the B column  3 , defornation of which is detected directly or indirectly by this sensor. In addition to displacement of anchor point F 3 , displacement of anchor point F 2  in the event of side impact is of course also conceivable. An additional sensor would have to be provided for detection of such displacement. 
     In order to prevent strain on the vehicle passenger  9 , repeated actuation of the belt-tightening device  20  should occur in reality only if a slack belt length  12   c  is present. Since a slack belt length  12   c  cannot be measured directly but can be computed from the data of the sensors  35   a, b, c  and  d,  it is logical to provide a belt-force limiting device  60  which restricts the force of the safety belt  12  to a maximum value. Such devices are known in the art and can either be a separate unit from the belt-tensioning devices or incorporated therein. The vehicle passenger  9  is thereby protected from subjection to excessive strain, in particular if a second actuation occurs without noticeable loosening of a belt length  12   c  having occurred. 
     If there is a slack belt length  12   c , the tension of the safety belt  12  generally decreases. Decreases in tension such as this occurring during an accident can be detected by a tension sensor  70  which operates in conjunction with the safety belt  12  and processed in the control unit  30 . Repeated actuation of the belt-tightening device  20  also occurs if the value of the tension signal falls below a specific boundary value during an accident. 
     Sensor  35   d  may be dispensed with in this design, since an effect (slack belt length) can be detected independently of the causes. 
     The invention is not restricted to a driver or passenger seat. An assembly with suitable sensors on other vehicle seats is also conceivable. 
     The mode of operation is similar when two separate belt tightening devices  20   a, b  are used. Repeated actuation of the belt-tightening device  20   b  after initial actuation of the belt tightening device  20   a  is replaced by actuation of the belt tightening device  20   b  by the control unit  30 . 
     The proposed safety belt assembly  2  makes it possible to eliminate slack belt lengths  12   c,  which may occur while an accident is in progress. This results in reduction of the risk of injury to the occupant  9  of a vehicle as the accident progresses.