Abstract:
A system for controlling air bag deployment in vehicles includes the capability of accommodating varying inductants values in the air bag circuit. Wire harnesses that couple air bag components have varying characteristics, such as length, which affect the inductants value of the circuit. The inventive arrangement utilizes charge and discharge times of at least one capacitor for measuring the inductants of a particular circuit. The system includes a controller that accommodates the inductants by introducing an appropriate delay in the firing circuit signal to operate the air bag.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application No. 09/257,122, filed Feb. 25, 1999, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to air bag systems for use in automotive vehicles and more particularly to an AC firing circuit for triggering various types of air bag systems. 
     BACKGROUND OF THE INVENTION 
     It is known in the art relating to vehicle air bag systems to provide a system that includes a firing circuit that applies energy to a firing element, or squib, to cause inflation of an air bag. The firing circuit is controlled by a deployment command signal which is sent by a microprocessor when the microprocessor has determined that a crash situation exists requiring deployment of the air bag. 
     A DC or AC firing circuit may be used to trigger the firing element. One example of an AC firing circuit includes a capacitor coupled in series with the firing element. The capacitor has a relatively small capacitance value such that a direct voltage applied to the firing element would be insufficient to fire the firing element. Only after a series of AC current pulses will sufficient energy be transferred to the firing element to cause deployment of the air bag. 
     One disadvantage of such an AC firing circuit is that it requires a tuned frequency to supply maximum energy to the firing element. Variations in the value of inductance result in different levels of energy being transferred to the firing element. Wiring harnesses connecting various air bags within the vehicle to a firing circuit have different values of inductance depending on the harness length and its routing. Also, whether the air bag requires a clockspring will affect the value of inductance. Thus, it is desirable to have a firing circuit that supplies maximum energy to the firing element over a wide range of inductance. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and universal firing system for providing sufficient energy to trigger a firing element causing deployment of various types of air bags within a vehicle. 
     An object of the present invention is to provide an AC firing circuit for supplying maximum energy to trigger a firing element to cause deployment of various types of air bags over a wide range of inductance. 
     Another object of the present invention is to provide the ability to trigger the firing element even if there is a short to ground or the power supply within the firing circuit. 
     The universal firing system of the present invention includes an acceleration sensor for producing an output signal indicative of the vehicle&#39;s acceleration. From the output signal of the acceleration sensor, an air bag controller determines whether a crash condition exists requiring deployment of an air bag. If deployment of the air bag is necessary, the controller sends a firing command signal to a power switching circuit which supplies charge and discharge current pulses of alternating polarity to a firing loop having a firing element in series with an unknown inductance and a capacitor. In order to provide maximum current to the firing element, a delay circuit is connected to the power switching circuit to control and adjust the frequency of the charge and discharge current pulses applied by the power switching circuit to the resonant frequency of the firing loop. 
     The delay circuit detects a charge or discharge period of the capacitor and delays the charge or discharge current pulse of the series for a delay period proportional to the detected charge or discharge period. Then, the delay circuit drives the power switching circuit to apply a next current pulse of the series in an opposite direction of the present current pulse. After the series of current pulses, a sufficient amount of energy is transferred to the firing element causing a chemical reaction which generates a gas and causes the air bag to inflate. 
     The method of the present invention includes the steps of receiving a firing command signal from an air bag controller indicative of the existence of a crash condition requiring deployment of an air bag. In response to the fifing command signal, a power switching circuit applies a series of charge and discharge current pulses of alternating polarity to a firing loop having a firing element coupled in series with unknown inductance and a capacitor. During the application of the current pulses, the charging and discharging of the capacitor to a predetermined low voltage level is detected and measured. This measured charge or discharge period, which provides an indication of inductance, is used to calculate a delay period for delaying the start of the next current pulse in the series to adjust the frequency of the current pulses to the resonant frequency of the firing loop. Thus, by maintaining the current at resonant frequency, maximum current is provided to the firing element. 
     These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with a general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic diagram of a universal firing system for various types of air bag systems in accordance with the present invention; 
     FIG. 2 is a graph of a voltage across a firing loop vs. time where the firing loop has an inductance equal to 4 μH and; 
     FIG. 3 is a graph of a voltage across a firing loop vs. time where the firing loop has an inductance equal to 14 μH. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1 of the drawings in detail, numeral  10  generally indicates a universal firing system for triggering various types of air bag systems. The firing system  10  includes a firing circuit  12  which triggers a firing element  14 , or a squib, to cause inflation of an air bag (not shown). To determine if the air bag needs to be deployed, an acceleration sensor  16  monitors the vehicle acceleration and produces an output signal indicative of the acceleration. An air bag controller  18  coupled with the sensor  16  analyzes the output signal from the sensor  16  and determines whether a crash condition is present requiring deployment of the air bag. If a crash condition exists, the controller  18  sends a firing command signal via signal lines  20  and  22  to the firing circuit  12 . The signal lines  20  and  22  preferably represent input to and output from the controller  18 . 
     The airbag controller  18  preferably is a programmable microprocessor. Given this description, those skilled in the art will be able to develop the specific software code necessary for achieving the results provided by this invention using commercially available microprocessors or custom design circuitry, for example. While the controller  18  is shown with certain inputs and outputs, the use of the controller is nonspecifically limited to the schematic lines shown in FIG.  1 . Only specific connections have been shown for simplicity. 
     The firing circuit  12  applies a series of AC charge and discharge current pulses of alternating polarity to a firing loop  24  sufficient to fire the firing element. The firing loop  24  includes the firing element  14 , a capacitor  26  and unknown inductance  28  in series. The capacitor  26  preferably has a relatively small capacitance value to prevent a direct current from inadvertently triggering the firing element  14 . Thus, only after a cumulative series of current pulses will sufficient energy be transferred to the firing element  14  to ignite and cause inflation of the air bag. 
     The firing circuit  12  includes a power switching circuit  30  which supplies the charge and discharge current pulses to the firing loop  24  and a delay circuit  32  that controls the power switching circuit  30  and adjusts the frequency of the current pulses to provide maximum current to the firing element  14 . The frequency of the current pulses needs to be adjusted because of the unknown inductance  28 . The inductance  28  varies from one air bag to another because of a wiring harness (not shown) that connects the firing circuit  12  to the firing element  14  which generally is at a location remote from the air bag controller  18 . The wiring harness has an inherent inductance and the value of the inductance depends on the length and route of the harness. Variations in inductance will affect the amount of energy transferred to the firing element  14  ensuring that sufficient energy is transferred to ignite the firing element  14 . 
     The power switching circuit  30  preferably includes four electronic switches  34 , 36 , 38 , 40 , such as field effect transistors (FET), connected in an H-Bridge configuration with the firing loop  24  connected between the two pairs of charge and discharge switches  34 , 36  and  38 , 40 . A first end  42  of the firing loop  24  is connected to one half of the H-Bridge and a second end  44  of the firing loop  24  is connected to the other half of the H-Bridge. The H-Bridge configuration allows current to flow bidirectionally and thus one direction of current flow charges the capacitor  26  and the other direction of current flow discharges the capacitor  26 . Also, by using the H-Bridge configuration instead of just a pair of switches, the firing element  14  will still fire even though a short exists from either side of the firing element  14  to ground or to a voltage power supply  46 . 
     The voltage power supply  46  is connected with the power switching circuit  30  in order for the switching circuit  30  to apply current to the firing loop  24 . Each half of the bridge includes two switches  34 , 40  and  36 , 38 . Charge Switches  34 , 40  act as current sources between the power supply and the switching circuit  30  and supply current to charge the capacitor  26  and the discharge switches  36 , 38  act as current sinks, which couple a grounded return path to the wiring harness to discharge the capacitor  26 . Each pair of switches  34 , 40  and  38 , 36  operate in a push-pull arrangement which means that one switch,  34  or  38  conducts during one half of a cycle and the other switch  36  or  40  conducts during the other half of a cycle. 
     The delay circuit  32  controls the four switches  34 , 36 , 38 , 40  between conductive and non-conductive states to apply charge and discharge current pulses at maximum current to the firing loop  24 . The delay circuit  32  preferably includes comparators  48 , 50 , 52 , 54 , AND gates  56 , 58 , delay means  60 , 62  and a flip-flop  64 . The controller  18  also controls delays within the circuitry. Comparators  48 , 52  are connected to the source of switch  34 , the drain of switch  36  and the second end  44  of the firing loop  24 . Comparators  50 , 54  are connected to the source of switch  44 , the drain of switch  40  and the first end  42  of the firing loop. 
     The outputs of the comparators  48 , 50  are applied to the AND gate  56  to determine when to start discharging the capacitor  26 . The outputs of comparators  48 , 50  are logically HIGH, when a voltage at the first end  42  of the firing loop  24  is greater than a predetermined low voltage level while a voltage at the second end  44  of the firing loop  24  remains above a predetermined high voltage level. When the outputs of comparators  48 , 50  are logically HIGH, the output of the AND gate  56  is logically HIGH, otherwise the output of the AND gate  56  is logically LOW. When the AND gate  56  is logically HIGH, this indicates that the direction of the current pulse should change to allow the discharging of the capacitor  26 . 
     The outputs of the comparators  52 , 54  are applied to AND gate  58  to determine when to start charging, the capacitor  26 . The outputs of the comparators  52 , 54  are logically HIGH, when the voltage at the second end  44  of the firing loop  24  is greater than the predetermined low voltage level while the voltage at the first end  42  of the firing loop  24  is above the predetermined high voltage level. The output of AND gate  58  is logically HIGH when comparators  52 , 54  are logically HIGH, otherwise the output of AND gate  58  is logically LOW. When the AND gate  58  is logically HIGH, this indicates that the direction of the current pulse should change to allow the charging of the capacitor  26 . 
     The first delay means  60  is connected between the output of AND gate  56  and a reset input of the flip-flop  64 . A second delay means  62  is connected between the output of AND gate  58  and a set input of the flip-flop  64 . The illustration includes delay means  60 , 62  but the delay may be provided by the air bag controller  18  without separate circuitry. The controller  18  preferably is programmed to operate the circuitry of each embodiment to achieve the desired timing characteristics. In one example, the functions of the illustrated logic components are all in the controller  18 . 
     The controller  18  preferably is programmed to measure a charge or discharge period which equals the time required for the first  42  or second  44  end of the firing loop  24  to be greater than the predetermined low voltage level. Then, the controller adds a delay period that is proportional to the measured charge or discharge period to the output of the AND gate  56  or  58  depending on which AND gate  56  or  58  is logically HIGH. The size of the delay preferably is based upon data stored in memory in the controller  18 . Given this description, those skilled in the art will be able to determine the appropriate delay times to accommodate the inductance of a given system to meet the needs of their particular situation. 
     The delay period adjusts the frequency of the present series of current pulses such that the next current pulse series applied to the firing loop  24  will be at the resonant frequency of the firing loop  24 . In this manner, the inductance value is accommodated so that the system  10  is capable of efficiently operating a variety of air bag systems. The outputs of the delay means  60 , 62  are applied to the flip-flop  64 . The flip-flop  64  changes the outputs from the delay means  60 , 62  to their opposite states. The outputs of the flip-flop  64  are applied to the gates of the switches  34 , 36 , 38 , 40  to drive the switches between conductive and nonconductive states depending on the desired current pulse direction for charging and discharging the capacitor  26 . 
     In operation, the air bag controller  18  sends a firing command signal to initiate firing the firing element  14  via signal lines  20  and  22 . Switches  34 , 40  are in a conductive state and switches  36 , 38  are in a nonconductive state. The charge current pulse flows from the second end  44  of the firing loop  24  to the first end  42  of the firing loop  24 , charging the capacitor  26 . The voltage at the second end  44  of the firing loop  24  is pulled high and the voltage at the first end  42  of the firing loop  24  is pulled low. As the current increases through the firing loop  24 , the voltage at end  44  will decrease and the voltage at end  42  will increase. When the voltage at end  42  reaches the predetermined low voltage level and the voltage at end  44  remains above the predetermined high voltage level, AND gates  56 , 58  change states, indicating the desire to change the direction of the current pulse to discharge the capacitor  26 . Stated differently, when a voltage across the firing loop reaches a threshold voltage level, the AND gates  56 , 58  change states. 
     The delay means  60 , 62  hold the output signals from the AND gates  56 , 58  for the delay period to adjust the frequency of the present current pulse to the resonant frequency of the firing loop  24 . The output signals of the delay means  60 , 62  are applied to inputs of the flip-flop  64 . The flip-flop  64  changes the out put signals of the delay means  60 , 62  to their opposite states. The outputs of the flip-flop  64  are applied to the gates of the switches  34 , 36 , 38 , 40 . 
     Now switches  38 ,  36  are in a conductive state and switches  34 ,  40  are in a nonconductive state, causing a discharge current pulse to flow in the opposite direction through the firing loop  24  to discharge the capacitor  26 . The voltage at the first end  42  is pulled high and the voltage at the second end  44  is pulled low. As the current increases through the firing loop  24 , the voltage at end  42  will decrease and the voltage at end  44  will increase. When the voltage at end  44  reaches the predetermined low voltage level and the voltage at end  42  is still above the predetermined high voltage level, AND gates  56 , 58  change states, indicating a desire to change direction of the current pulse direction to charge the capacitor  26 . Stated differently, when the voltage across the firing loop reaches the threshold voltage level, AND gates  56 , 58  change states. 
     The delay means  60 ,  62  hold the output signals from the AND gates  56 , 58  for the delay period to adjust the frequency of the current pulses to the resonant frequency of the firing loop  24 . The output signals of the delay means  60 , 62  are applied to inputs of the flip-flop  64 . The flip-flop  64  changes the output signals of the delay means  60 , 62  to their opposite status. The outputs of the flip-flop  64  are applied to the gates of the switches  34 , 36 , 38 , 40 . Now, switches  34 , 40  are in a conductive state and switches  36 , 38  are in a nonconductive state, causing a charge current pulse to flow in the opposite direction through the firing loop  24 . The whole cycle begins again and current is continued to be applied to the firing loop  24  until the firing element  14  is ignited, causing the air bag to inflate or the controller  18  interrupts the series of current pulses. 
     The controller  18  may override the delay of  60  and  62  by providing the necessary delay directly via signal lines  20  and  22  or by controlling the flip-flop output directly. The resistors  75  facilitate permitting the controller  18  to override the flip-flop output. 
     FIG. 2 is a graph of a voltage  100  across a firing loop vs. time where the firing loop has an inductance equal to 4 μH, illustrating a relatively low inductance in an air bag system. This illustration shows the results of introducing a delay of 25 μs. 
     FIG. 3 is a graph of a voltage  100 ′ across a tiring loop vs. time where the firing loop has an inductance equal to 14 μH, illustrating a relatively high inductance in an air bag system. This example shows the preferred effect of introducing a 45 μs delay. The longer delay accommodates the resonant frequency of the larger inductance valve FIGS. 2 and 3 show that an AC-firing circuit of the present invention supplies maximum current to the firing loop over a wide range of inductance in the air bag system. Given this description, those skilled in the art will be able to determine the appropriate delay time to address the inductance of a system to meet the needs of their particular situation. 
     Although the invention has been described by reference to a specific embodiment, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiment, but that it have the full scope defined by the language of the following claims.