Abstract:
An airbag deployment rate sensor employs a tape or string that is wound on a spool and connected to the fabric of an airbag cushion so that as the airbag deploys, tape or string is pulled from the spool causing it to rotate. A brake is applied to the spool to prevent the buildup of momentum and so the tape and the spool will come to a rapid stop when the tape is no longer being withdrawn from the spool because the portion of the airbag to which the tape is connected has collided with an object. A sensor is positioned to detect rotation of the spool and so to monitor the rate at which tape or string is being withdrawn.

Description:
FIELD OF THE INVENTION 
     The present invention relates to airbags and sensors used to control airbag deployment in general and to sensors which monitor the actual deployment sequence in particular. 
     BACKGROUND OF THE INVENTION 
     Airbags were originally developed as a passive restraint system, but are known to work best in combination with seatbelts and other safety systems. Although airbags contribute to the overall safety of occupants of an automobile, they can present a danger to an occupant who is positioned too close to an airbag when it deploys. This condition, where the occupant is positioned so that airbag deployment might be dangerous, is referred to as the occupant being “out of position.” Various systems have been developed to detect an “out of position” occupant. Sensor systems designed to detect the occupant&#39;s position often require constant monitoring so that in the event of a crash the occupant&#39;s position is known. Sensor systems designed to detect the position of the occupant have been proposed based on ultrasound, optical, or capacitance sensors. Constant monitoring of sensors, which may have high data rates, requires the design of algorithms which can reduce sensor data to a single condition or a limited number of data conditions which can be used in an airbag deployment decision logic to prevent airbag deployment or for a duel stage airbag to select the level of deployment. Maintaining data integrity between the non-crash positional data, and positional data needed during airbag deployment is complicated by the noisy environment produced by a crash. Dealing with data integrity issues requires increased processor capabilities and algorithm development, which also requires additional testing. 
     Prior art approaches attempt to determine, based on various sensors, the distance between the airbag and a vehicle occupant before the airbag is deployed. In many instances, the vehicle occupant will not be too close to the airbag at the time the decision to deploy the airbag is made, but, because of the rate at which the occupant is approaching the airbag, the occupant will be too close when the airbag is actually deploying. To handle these situations, more sophisticated sensors and algorithms are needed to attempt to predict the occupant&#39;s position when the airbag is actually deployed or nearly completely deployed. The ideal airbag deployment system functions so that the airbag deploys fully or nearly fully before the occupant engages the airbag. Existing systems inhibit airbag deployment when, based on various sensors and algorithms, it is determined that, because of the position of the vehicle occupant, the bag is more likely to harm than to benefit the occupant. Successfully creating a sensor and algorithm system is complicated because there is usually very little delay between the decision to deploy and the actual deployment. Rapid airbag deployment is desirable because the maximum benefit from an airbag is achieved by early deployment. However, more time before deployment maximizes the information available for determining whether deployment is necessary. The desire to maximize effective deployment of the airbag while minimizing unnecessary deployment creates a tension between waiting for more information and deploying immediately. Therefore, once sufficient information is available, deployment typically follows nearly immediately. 
     Therefore, a system which employs occupant position sensors and algorithms must be able to supply at all times an indication of whether airbag deployment should be inhibited so that the inhibit decision can be applied whenever the airbag deployment decision occurs. This means the sensors and algorithms used to develop the occupant position inhibit signal cannot be optimized to deal with a specific time frame in which the actual deployment decision is made. The end result is that such algorithms may be less accurate than desired because they must predict events relatively far in the future—perhaps tens of milliseconds. One known type of sensor shown in European Patent application EP 0990567A1 employs a plurality of tapes which extend between the front of the airbag cushion and a dispensing cartridge mounted on the airbag housing. Tape extraction sensors within the cartridge monitor markings on the tape to determine the rate at which tape is being withdrawn from the cartridge. The tape extraction sensors detect airbag impact with an occupant by a decrease in airbag velocity as measured by the rate of tape withdrawal from the cartridge. Improvements are needed to the known tape cartridges to improve the functionality and reliability of the tape type bag deployment monitoring sensors. 
     SUMMARY OF THE INVENTION 
     The airbag deployment rate sensor of this invention employs a tape or string which is wound on a spool and connected to the fabric of an airbag cushion so that as the airbag deploys, tape or string is pulled from the spool, causing it to rotate. A brake is applied to the spool to prevent the buildup of momentum and so that the tape and the spool will come to a rapid stop when the tape is no longer being withdrawn from the spool because the portion of the airbag to which the tape is connected has collided with an object. A sensor is positioned to detect rotation of the spool and so to monitor the rate at which tape or string is being withdrawn. This provides a measure of the movement of the portion of the airbag to which the tape or string is attached. A braking force is applied to the spool by biasing a shoe against a peripheral rim of the spool, or by biasing a shoe against an upper or lower surface of the spool. In yet another embodiment, the stub shaft about which the spool is mounted is split and biased to supply a braking force against the innermost bearing surface that is engaged with the stub shaft. Rotation rates may be monitored by passing a beam of light through one or more axial openings in the disk of the spool. Alternatively, one or more small magnets may be mounted to rotate with the spool, the magnets being detected by a magnetic flux sensor such as a Hall effect sensor, a GMR sensor, or a reed switch. In another alternative embodiment, a magnet may be positioned above the rotating spool, and the spool may contain magnetic shield elements that pass over a magnetic flux sensor positioned beneath the magnet. If a magnet is mounted on the spool, a simple wire positioned near the spool will experience an induced current. Finally, a spring motor type spring may be positioned between a central stub shaft and the tape or string containing spool to act as a brake. 
     It is a feature of the present invention to provide a means for detecting when a portion of an airbag cushion impacts an object. 
     It is another feature of the present invention to detect the rate at which a portion of an airbag cushion is deploying by monitoring the speed of rotation of a spool from which a tape or string which is attached to the portion of the airbag is withdrawn. 
     It is a further feature of the present invention to provide a sensor for determining the deployment rate of a portion of an airbag cushion that does not require an encoded tape or string. 
     It is a yet further feature of the present invention to provide a tape or string dispenser which can be used with a wide variety of sensors to detect the rate at which tape or string is drawn from the dispenser. 
     Further features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view, partially cut away in section, of the airbag deployment rate sensor of this invention. 
     FIG. 2 is a cross-sectional view of the airbag deployment rate sensor of FIG. 1 taken along section line  2 — 2 . 
     FIG. 3 is a side elevational cross-sectional view of an alternative embodiment rate sensor of this invention. 
     FIG. 4 is a side elevational cross-sectional view of another alternative embodiment rate sensor of this invention. 
     FIG. 5 is a side elevational cross-sectional view of a further alternative embodiment rate sensor of this invention. 
     FIG. 6 is a detail top plan view of the outwardly biased stub shaft of the device of FIG.  5 . 
     FIG. 7 is a side elevational cross-sectional view of still another alternative embodiment rate sensor of this invention. 
     FIG. 8 is a top plan view, partially cut away of the embodiment of FIG.  7 . 
     FIG. 9 is an isometric view, partially cut away in section, of an airbag module as the airbag cushion is deployed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to FIGS. 1-9, wherein like numbers refer to similar parts, an airbag module  10  deploying an airbag cushion  17  is shown in FIG.  9 . An airbag housing  11  contains an igniter  12  and a quantity of gas generant  13  such as 5-aminotetrazole and is mounted behind an instrument panel  14 . A vehicle occupant 15  is seated on a vehicle seat  16  facing the airbag cushion  17 . Strings  34  are fastened to the inside surface  44  of the airbag cushion  17 , and are retained within dispensing cartridges  20  mounted to or behind the airbag housing  11 . The cartridges  20  are mounted fixed with respect to the airbag housing so the relative movement of the airbag cushion  17  can be measured. When the airbag module  10  is activated, the airbag cushion  17  deploys toward the vehicle occupant 15 , and the strings  34  are withdrawn from the cartridges  20 . The purpose of the cartridges  20  and the strings  34  which are withdrawn from the cartridges  20  is to allow the detection of an “out of position” vehicle occupant and adjust or stop the deployment of the airbag cushion in response to detecting the “out of position” vehicle occupant. 
     For simplicity in signal processing, an AC signal is generated by detecting rotation rate of spool  26 , shown in FIG. 2, as string  34  is withdrawn from the dispensing cartridge  20 . The AC signal can be processed and amplified and filtered in a way which may have benefits in terms of overcoming sources of noise, simplicity of processing, and reliability of algorithms. The information from the sensors which detect the rotation of the spool  26  is sent to an electronic control unit which can be used to control vents  18  which may be squib activated, or otherwise activated to let gases out of the airbag housing  11  to slow or stop inflation. Opening vents almost instantaneously reduces the pressure in the airbag cushion  17 . 
     The dispensing cartridge  20  has a housing  22  and a cover  24 . Contained within the housing  22  is the spool  26 , which is mounted for rotation about a central stub shaft  28 , and an axis  29  defined by the stub shaft  28 . The spool  26  has upper  30  and lower  32  spool flanges between which is wound a quantity of lightweight line  34 . The line may be a string, filament, or flattened tape. As used herein and in the claims “string” is understood to refer to a flexible elongated member having any cross-sectional shape, not just a circular cross sectional shape. The line is preferably fixed to the spool to assure rotation of the spool as the line is extracted. The line  34  is wound onto a cylindrical surface  36  extending between the upper spool flange  30  and the lower spool flange  32 . As shown in FIG. 1, one end  40  of the string  34  extends from the spool to an opening  142  in the housing  22  and is attached to an inside wall portion  44  of an airbag cushion  17 . As the airbag cushion  17  is inflated, it draws string  34  from the housing  22  causing the spool  26  to rotate. 
     By monitoring the rate of rotation of the spool  26 , the rate at which string  34  is withdrawn is monitored, and the rate at which string  34  is withdrawn from the housing  22  corresponds with the velocity of the airbag wall portion  44  to which the string  34  is attached. In order to be able to detect when the airbag wall portion  44  decreases in velocity, a brake  48  is provided within the housing  22 , as shown in FIG. 1, which operates against the spool  26  to overcome the momentum of the spool which would keep the spool  26  rotating even when string withdrawal has slowed or stop. The brake  48 , as shown in FIGS. 1-2, is biased by a spring  50  away from a wall portion  52  of the housing  22  so as to engage against the upper and lower spool flanges  30 ,  32 . Friction between the brake  48  and the spool flanges  30 ,  32  is selected so that the string  34  pulled by the expanding airbag cushion  17  is readily extracted from the cartridge  20 , but rotation of the spool  26  is adjusted essentially instantaneously to correspond with the string extraction rate. The forward velocity of the airbag cushion  17  slows in response to impacting an object. Thus, by monitoring the rotation rate of the spool  26 , the impact of the airbag cushion  17  with an “out of position occupant” can be detected. 
     The rate of rotation of the spool  26  is detected by passing light from a light source  54  such as an LED, through openings  56  in a flange  58  which extends between the cylindrical surface  36  and an inner hub  60  surrounding the stub shaft  28 . A light detecting sensor  62  is positioned opposite the light source  54  to receive light passing through the openings  56 . As illustrated in FIG. 1, the flange  58  may have four openings  56  so that light is detected by the sensor  62  four times as the spool  26  rotates once. 
     An alternative embodiment dispensing cartridge  64  is illustrated in the FIG. 3, wherein a light source and a light sensor  66  are combined and positioned below the spool  26 . Through openings  56  in the flange  58  of the spool  26  reduce the amount of light reflected back to the sensor  66  and so it is the absence of light on the sensor  66  which indicates rotation of the spool  26 . An alternative brake  68  is positioned within a cover  70 . The brake  68  is biased by a spring  72  positioned between the cover and the brake, so that the brake pushes downwardly in on the upper flange  30  of the spool  26  to cause a braking friction which overcomes the momentum of the spool and the string  34 . 
     A further alternative embodiment tape dispenser cartridge  74  is shown in FIG. 4. A magnet  76  contained within the cover  78  is positioned above the spool  26  and a magnetic field sensor  80  such as a GMR sensor, a Hall sensor, or a reed switch is positioned below the spool  26 . A magnetic shunt material  82  such as Mu metal or other ferromagnetic material is mounted at discrete locations on the flange  58  to selectively block a magnetic field produced by the magnet  76 . The rotation of the spool  26  and the shunt material  82  mounted thereon causes the magnetic field sensor to be selectively activated. Thus, the rate of rotation of the spool can be detected by the frequency of the output signal of the magnetic field sensor  80 . A brake  84  is incorporated into the housing  86  beneath the spool  26 . A spring  88  biases the brake  84  against the lower flange  32  of the spool  26  to overcome momentum of the spool  26  and tape  90 . The tape  90  may be woven or may be a plastic or metal tape. 
     Yet another alternative embodiment tape dispenser cartridge  92  is shown in FIG. 5. A small high strength magnet  94  such as a neodymium iron boron magnet is positioned within the flange  58  of the spool  26 . Rotation of the magnet can be detected by a magnetic flux sensor  96  which could be as simple as a simple loop of conductive wire in which a current flow is induced by the moving magnetic field caused by the magnet. The magnetic flux sensor  96  can also be a Hall sensor, GMR sensor, or reed switch. The detected rotary motion of the magnet corresponds to rotation of the spool  26 . As shown in FIGS. 5 and 6, the stub shaft  98  is formed in two brake forming parts  100  which are biased against the inside surface  102  of the inner hub  60  to provide a braking action to overcome the inertia of the spool  26  and the string  34 . 
     A still further embodiment tape dispenser cartridge  110  is shown in FIG.  7 . The tape dispenser cartridge  110  has a spool  112  on which a quantity of string  114  is wrapped. The spool  112  is defined by a cylindrical inner portion  116  about which the string  114  is wrapped, and an upper flange  118  and a lower flange  120  which extend radially outwardly from the cylindrical inner portion  116 . The spool  112  is mounted for rotation about a stub shaft  122  by a coil spring  124  that is attached to both the stub shaft  122  and to the spool  112 . The coil spring  124  acts as a brake which resists the string  114  being withdrawn when the airbag to which the string  114  is attached impacts an object and so is no longer moving forward and drawing string from the dispensing cartridge  110 . In order to prevent the spring  124  from rewinding the string  114  onto the spool  112 , a ratchet mechanism is provided consisting of a pawl  126 , and ratchet teeth  128  formed in the peripheral edges of the upper and lower flanges  118 ,  120 . One or more magnets  129  are mounted to the cylindrical portion  116  of the spool  112 , and are detected by a magnetic flux sensor  130  such as a Hall sensor, a GMR sensor, a reed switch, or a loop of wire. The dispenser  110  has a cover  132  which has a downwardly extending lip  134  which can be ultrasonically welded to form a hermetic seal with a conical surface  136  on the housing  138  of the tape dispenser  110 . FIG. 7 is a partly exploded, view before assembly of the cover  132  to the housing  138 . 
     It should be understood that the string  34 ,  114  could be replaced by tape, or the tape  90  could be replaced by a string. The string or tape will preferably be made of high strength lightweight material, for example high-strength &amp; high-modulus polyethylene fiber (HSM-PE fiber) or an aromatic (polyamide) fiber. The outlet  142  of the string  34  as shown in FIG. 1 is sealed with a grommet  140  to prevent moisture and other contaminants from migrating into the interior of the cartridge. The grommet  140  is bonded to the string or tape and may be attached by grooves that fit over flanges which protrude from both sides of the outlet. When the tape is extracted from the cartridge, the grommet  140  moves with the string and pulls away from the outlet  142  that it had previously sealed. Another alternative is a sealing material such as wax or an elastomeric such as rubber that forms a seal that likewise pulls away with the spring upon airbag cushion deployment. The outlet  142  presents a smooth radiused curve surface  144  that is axisymmetric about the string so that, as the string is pulled from side to side during the initial stages of inflating the airbag cushion, the string does not bind. The outlet  142  is radiused so the size of the outlet about the string increase as the string moves out of the outlet, so as to prevent high friction caused by the string being pulled over a sharp corner. If a tape is used, the outlet will be tapered on either side of the tape to accommodate side-to-side motion of the tape. 
     It should be understood that the various braking mechanisms, and the various sensors combined with various light sources or sources of magnetic flux or magnetic shielding could be combined to form additional embodiments of the invention, so that any brake mechanism could be used with any sensor, and vice versa. It should also be understood that for clarity in the illustration the brakes  48 ,  68 ,  84 ,  100  are shown spaced from the spool  26  but in actual practice are engaged with the portion of the spool opposite the brake. In a similar fashion, a gap is shown for clarity between the stub shaft  28  and the inner hub  60  while in practice only as much gap between the stub shaft  28  and inner hub  60  is left as necessary to allow the spool  26  to rotate. Particularly as illustrated in FIGS. 5 and 6, the portions  100  of the stub shaft  98  engage against the inside cylindrical surface  102  of the spool  26 . 
     It should also be understood that the ultrasonic weld illustrated between the cover  132  and the conical surface  136  of the housing  138  could be used with any of the illustrated embodiments. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.