Abstract:
An airbag deployment sensor has a cartridge containing a quantity of tape one end of which is attached to the inside surface of an airbag cushion. Deployment of the cushion pulls tape from the cartridge at a rate that is monitored by transmitting light through the tape, or by detecting the presence of metalized, or magnetic shielding portions, of the tape.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to monitoring of airbag deployment with a tape, and methods and circuits for processing a stream of data received from a sensor that monitors the rate at which the tape is being withdrawn from a cartridge.  
         BACKGROUND OF THE INVENTION  
         [0002]    Experience has shown that airbags work best in combination with seat belts and other safety systems. Although airbags contribute to the overall safety of occupants of an automobile, they can present a danger to a vehicle occupant who is positioned too close to an airbag when it deploys. This condition, where the vehicle occupant is positioned so that airbag deployment might be dangerous, is referred to as the vehicle occupant being “out of position.” Various systems have been developed to detect an “out of position” vehicle occupant. Sensor systems designed to detect the vehicle occupant&#39;s position often require constant monitoring so that in the event of a crash the vehicle occupant&#39;s position is known. Sensor systems designed to detect the position of the vehicle occupant have been proposed based on ultrasound, optical, or capacitance sensors.  
           [0003]    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 are used in an airbag, deployment decision 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.  
           [0004]    Prior art approaches attempt to determine, based on various sensors, the distance between the airbag and the passenger 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 vehicle occupant is approaching the airbag, the vehicle occupant will be too close when the airbag is actually deploying. To handle these situations, more sophisticated sensors and algorithms are needed in order to attempt to predict the vehicle occupant&#39;s position when the airbag is actually deployed or nearly completely deployed. In other words, the ideal airbag deployment system functions such that the airbag deploys fully or nearly fully before the vehicle 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 vehicle occupant.  
           [0005]    Successfully creating a sensor and algorithm system is complicated because there is usually very little delay between the decision to deploy and actual deployment. This is so because the maximum benefit from an airbag is achieved by early deployment, and at the same time, more time before deployment maximizes the information available to determine 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.  
           [0006]    A system which employs vehicle 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 vehicle 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.  
           [0007]    One known type of sensor shown in EP 0990567A1, employs a plurality of tapes which extend between the front of the airbag and a tape dispensing cartridge mounted on the airbag housing. Tape extraction sensors within the cartridge monitor the rate at which tape is withdrawn from the cartridge and thus can detect airbag impact with a vehicle occupant by a decrease in airbag velocity. This type of sensor which can monitor the way an airbag is actually deploying solves the problem of predicting whether a vehicle occupant will be out of position at time of airbag deployment. In this arrangement the airbag is deployed, and if it encounters a vehicle occupant before it has reached a certain stage of deployment the airbag is vented which effectively removes the airbag. Several tapes and tape dispensing cartridges are used to monitor different portions of the bag so that if any portion of the bag contacts a vehicle occupant, the fact of contact can be detected and the bag vented to prevent injury to the out-of-position occupant. To be practical, this type of sensor—which monitors actual deployment—needs simple but robust techniques for monitoring the rate at which tape is withdrawn from the cartridge.  
         SUMMARY OF THE INVENTION  
         [0008]    The airbag deployment sensor of this invention has a cartridge in which-a quantity of tape is stored. One end of the tape is attached to the inside surface of an airbag cushion so that when the cushion is deployed it pulls tape from the cartridge. The rate at which the tape is pulled from the cartridge is monitored by transmitting light through the tape, or by detecting the presence of the metalized or ferrous portions of the tape.  
           [0009]    In a first embodiment a tape ½ mm by 5 mm constructed of black polyethylene has 2 mm diameter holes spaced 5 mm on center extending along the length of the tape. An infrared light emitting diode is positioned on one side of the tape and a phototransistor is positioned opposite the light emitting diode. The phototransistor is connected to a comparator circuit with hysteresis that provides a clean digital output proportional to the rate at which the holes formed in the tape are pulled past the phototransistor. Alternatively, an infrared transparent tape on which an infrared opaque pattern has been printed may be used.  
           [0010]    In a second embodiment, a tape ½ mm by 5 mm has 5 mm regions that are spaced 5 mm apart, which have been metalized. For example, a metal film may be deposited on Mylar® tape and selectively etched to form metalized regions or metalized paint may be used on film or cloth. The metalized regions may be detected by one of three methods. The first method employs two closely spaced contacts that are connected by the metalized regions as they pass over the contacts. This type of detector may also be connected to a comparator circuit with hysteresis to provide a digital outlet. The second method for detecting the passage of the metalized regions employs a capacitive plate as a sensor. The capacitive plate is part of an oscillator circuit where the frequency of the oscillator circuit is controlled by the capacitance of the capacitive plate. As the metalized regions move opposite the capacitor plate, a variable capacitor is formed so that the amount of capacitance in the circuit changes. With this varying capacitance, the frequency of the oscillator increases and decreases as the metalized regions pass the capacitor plate. A third method of detecting the rate at which a tape with metalized regions is pulled from the cartridge employs an amplitude modulated signal. An oscillator of a few hundred kHz to about 1 MHz is connected into a first electrode. A second electrode spaced from the first electrode is connected to an amplification circuit. The metalized region forms a capacitive link between the first electrode and the second electrode that efficiently transmits the oscillator signal to the amplifier. Therefore as the metalized regions pass the first and second electrodes, the signal received by the amplifier circuit varies in amplitude. The output of the amplifier is rectified, producing a pulsed DC output.  
           [0011]    If the metalized region is formed from a ferromagnetic alloy, movement of the ferromagnetic region can be used with a permanent magnet to affect a magnetic field sensor such as a Hall effect sensor, a GMR sensor, or even a simple conductor loop or coil. The permanent magnet is positioned opposite the magnetic field sensor, and the ferromagnetic metalized region acts as a magnetic shield selectively blocking magnetic field lines from the permanent magnet to the magnetic field sensor.  
           [0012]    It is a feature of the present invention to provide a tape that is drawn from a cartridge to detect airbag cushion employment rate that is constructed to reliably affect a sensor.  
           [0013]    It is a further feature of the present invention to provide methods for detecting the velocity of a tape being pulled from a cartridge by an airbag cushion.  
           [0014]    It is another feature of the present invention to provide a tape sensor combination that employs detecting a change in capacitance.  
           [0015]    It is a still further feature of the present invention to provide a tape, sensor combination that employs detecting a change in magnetic field strength.  
           [0016]    It is another feature of the present invention to remind a tape sensor arrangement that can accommodate variations in sensor performance due to device to device variation and aging effects.  
           [0017]    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  
       [0018]    [0018]FIG. 1 is a schematic plan view of a tape cartridge employing the infrared sensor arrangement of this invention.  
         [0019]    [0019]FIG. 2 is an electrical schematic drawing of a circuit used in the tape cartridge of FIG. 1 to provide a clean digital output from the infrared sensor.  
         [0020]    [0020]FIG. 3 is a top plan view of a tape for use in the tape cartridge of FIG. 1.  
         [0021]    [0021]FIG. 4 is a top plan view of an alternative tape for use within the tape cartridge of this invention.  
         [0022]    [0022]FIG. 5 is a schematic plan view of an alternative tape cartridge of this invention employing a capacitive sensor.  
         [0023]    [0023]FIG. 6 is an electrical schematic drawing of a circuit used in the tape cartridge of FIG. 5 to provide a frequency modulated output signal proportional to the speed of the tape leaving the tape cartridge.  
         [0024]    [0024]FIG. 7 is another electrical schematic drawing of a circuit used in the tape cartridge of FIG. 5 to provide an amplitude modulated output signal proportional to the speed of the tape leaving the tape cartridge.  
         [0025]    [0025]FIG. 8 is a schematic plan view of a tape cartridge of this invention employing a magnetic field sensor.  
         [0026]    [0026]FIG. 9 is an electrical schematic drawing of a circuit used in the tape cartridge of FIG. 8.  
         [0027]    [0027]FIG. 10 is a schematic plan view of a yet further tape cartridge of FIG. 1 employing a contact sensor.  
         [0028]    [0028]FIG. 11 is a schematic cross sectional view of an airbag module incorporating the tape cartridge of FIG. 1.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring more particularly to FIGS. 1-11, wherein like numbers refer to similar parts, an airbag housing  12  with an attached folded airbag cushion  13  is shown in FIG. 11. A gas generator  14  is mounted to the airbag housing which incorporates a valve  15  which can be used to stop the inflation of the airbag cushion  13  by venting gas from the gas generator  14 . As shown in FIG. 1, a cartridge  20  containing a length of tape  22  is mounted to the housing  12 . As shown in FIG. 11, one end of the tape  22  is attached to the inside surface  18  of the airbag cushion  13  so that when the gas generator  14  is activated and the airbag cushion  13  is deployed, tape  22  is withdrawn from the cartridge  20 .  
         [0030]    As shown in FIG. 3, the tape  22  is a black polyethylene strip about 5 mm in height and about ¼ mm thick. The tape  22  may alternatively be formed of opaque Mylar® oriented polyester film, or metallic high temperature film. The tape  22  has a series of holes  24  that are 2 mm in diameter and spaced about 5 mm apart. An infrared light emitting diode  26 , as shown in FIGS. 1 and 2, is positioned opposite a phototransistor  28 . When a hole  24  is positioned between the diode  26  and the phototransistor  28 , infrared light passes from the diode to the phototransistor causing it to turn on. The use of a transmission sensor produces a more reliable detection of tape movement which is substantially insensitive to variation in component properties, whether variations between components or variations in a component due to temperature or time.  
         [0031]    The use of infrared light is advantageous because the light is less subject to scattering due to dust between the light source and the light detector. However, other wavelengths of light could be used. As shown in FIG. 2, a comparator circuit  30  based on operational amplifier  32  is designed with hysteresis so that a clean digital pulse is produced for each hole  24  that passes between the diode  26  and the phototransistor  28 . The comparator circuit  30  with hysteresis eliminates multiple pulses due to noise during the switch transition. Filtering and wave shaping circuitry may be added to further tailor the signal. The resulting output  34  is a digital waveform with a frequency proportional to the tape speed and the pulse width inversely proportional to tape speed. Although shown as discrete components, the circuit  30  could be on a single chip.  
         [0032]    Another tape  36  is shown in FIG. 4. The tape  36  is formed of transparent material such as Mylar® oriented polyester film to which has been applied rectangular areas  38  of opaque paint or a layer of metallization. Metallization provides a tape  36  that has first portions which are electrically conductive and second portions which are not electrically conductive serially positioned along the tape. The Mylar® film may have dimensions similar to that of the black polyethylene tape  22  shown in FIG. 3, with the rectangular areas  38  being about 5 mm square and spaced about 5 mm apart. The tape  36  may also be used in the cartridge  20  where the transparent spaces transmit light and the rectangular areas block the transmission of light.  
         [0033]    An alternative approach of detecting a tape  36  such as the one shown in FIG. 4, in which the rectangular areas  38  are metalized, is-illustrated in FIGS. 5, and  6 . A tape cartridge  42  employs a fan fold tape storage technique with a spring biased brake  44 . The tape cartridge  42  uses a capacitor based sensor  46 . The sensor  46  may be used with an oscillator circuit  48 , such as the one shown in FIG. 6, to frequency modulate a base frequency as the tape  36  passes the sensor. The oscillator circuit  48  may be a simple relaxation oscillator circuit using an operational amplifier  50  and several discrete components. It should be noted that many types of oscillators may be used, as long as the oscillation frequency can be tuned by using a small capacitive element. A 555 timer circuit would be another implementation which requires no inductor.  
         [0034]    In the circuit of FIG. 6, the tape forms a capacitive element C 2  that is in parallel with the capacitor C 1 . This series combination of R 3  and C 1  and C 2  sets the oscillation period which may have a mid frequency of about 300 kHz. R 1  and R 2  set the threshold switching voltage. If R 1 =R 2  this voltage is ½ VCC. When the circuit is powered up, the operational amplifier  50  rails to either the plus VCC or minus VCC output state. The parallel combination of C 1  and C 2  is then charged to plus ½ VCC or minus ½ VCC through the resistor R 3 , at which point the operational amplifier rails in the opposite direction. As the metalized area  38  on the tape increases the value of C 2 , the base frequency of the oscillator decreases. This increase in capacitance is followed by a decrease in capacitance as the tape  36  moves to where there is no metalized area  38  opposite the two plates  54  and the frequency of the oscillator increases. Thus an FM signal is generated which is dependent upon tape speed. This FM signal may be demodulated to provide an output frequency corresponding to tape speed.  
         [0035]    The oscillator circuit  48  is based on an operational amplifier  50  wherein the mid frequency of the oscillator is about 300 kHz. The capacitor C 1  controls the frequency of the amplifier output  52 . Two metal plates  54  are connected in parallel with the plates of the capacitor C 1  so that when a rectangular metalized area  38  is positioned opposite the two metal plates  54  a second capacitor C 2  is formed that increases the capacitance of capacitor C 1 .  
         [0036]    As shown in FIG. 8, yet another approach to detecting the speed of the tape  36  as it is withdrawn from the cartridge  42  is based on amplitude modulation. An amplitude modulation circuit  56 , shown in FIG. 7, has an oscillator circuit  58  that has an oscillation frequency of, for example, 300 kHz to 1 MHz. The signal generated by the oscillator circuit  58  is coupled through a capacitor C 3  formed out of two metal plates  54  and a metalized area  38  on the tape  36 . Thus the two metal plates  54  and the metalized area  38  of the tape form a capacitive couple between the oscillator  58  and the output  64 . When the metalized area  38  completely overlaps the two metal plates  54 , the signal is most efficiently transmitted between the oscillator circuit  58  and the positive input  60  of the operational amplifier  62  of the circuit  56 . When a metalized area  38  only partially overlaps the metal plates  54  or is completely absent, the transmitted signal decreases or reaches a minimum. Thus the amplitude of the signal received from the oscillator circuit  58  varies with the speed at which the tape is moving past the capacitor C 3 . The output  64  of the operational amplifier  62  is rectified by diode D 1  supplying a pulsed DC output which has frequency which is directly proportional to the speed at which the tape  36  is being withdrawn from the cartridge  42 .  
         [0037]    Still another approach to detecting the speed of the tape  36  as it is withdrawn from a cartridge  65  is based on the metalized regions  38  being formed of a magnetically impermeable material such as iron, nickel, cobalt, or alloys based on them which have an effective amount of one or more of the ferromagnetic metals. Mu-metal, a nickel-iron alloy (77 percent Ni, 15 percent Fe, plus Cu and Mo), is particularly effective at shielding magnetic fields and also may be used. The metalized regions  38  act as magnetic shunts and prevent the magnetic lines of force from a permanent magnet  66 , as shown in FIG. 9, from reaching and affecting a Hall effect sensor  68  which forms part of an integrated circuit which has a unipolar Hall sensor with the open collector output. The integrated Hall device  70  may perform other functions such as temperature compensation, a comparator with hysteresis, and a voltage regulator. The Hall device  70  generates a digital output when the magnetic field to which the Hall effect sensor  68  is exposed exceeds the predetermined switch point.  
         [0038]    Another approach to detecting the passage of the tape  36  with metalized regions  38  is illustrated in FIG. 10. The cartridge  72  has two spaced apart electrical contacts  74  that successively engage the tape  36  against a supporting member  73 . When a metalized region  38  bridges the electrical contacts  74  a circuit, not shown, provides a voltage or current output which is not present when a metalized region  38  is not connecting the contacts  74 . A comparator circuit (not shown) with hysteresis removes any contact bounce and provides a clean and digital output which has a frequency which is proportional to the speed at which the tape  36  is withdrawn from the cartridge  72 .  
         [0039]    It should be understood that the tape  22  or  36  can be used with various methods of storing the tape within the cartridge, for example: wrapped around the central post, or wrapped around a rotatable spool, or simply formed in a coil or fan fold arrangement. It should be understood that tape  22  or  36  could be a metal tape with holes formed therein. It should be understood that the metallization could be by any technique which forms a conductive film on a base film and could include plating, flame spraying, vacuum depositing, adhesive bonding, or painting the conductive regions on to a tape substrate. The tape substrate is not intended to be limited to a film but could include a woven material or fabric. Moreover, the tape material may be high temperature film, a woven cloth or any other material capable of sustaining inflator temperatures and having the necessary tensile strength  
         [0040]    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.