Abstract:
A mechanical acceleration switch  12  acting at the time of collision tends to occur chattering easily, thereby lowering reliability in operation due to insufficient application of ignition current to an igniter of an airbag. When the mechanical acceleration switch  12  is closed and first-stage igniters  4  and  7  are ignited, a forced igniter  115  is turned on. Even if chattering occurs in the mechanical acceleration switch  12,  the first-stage igniters  4  and  7  and a backup capacitor  11  are completely conducted by turning on a driving transistor  31,  and an electric current is sufficiently delivered to the igniters  4  and  7.  As a result, it is possible to improve reliability in operation. It is no longer necessary to increase capacity of the capacitor  11  and decrease chattering of the mechanical acceleration switch  12.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an air bag starter that is installed in a vehicle and protects driver and other occupant inside the vehicle from a shock at the time of occurring any accident such as collision, and to a backup circuit that is used in the air bag starter to accurately back up inflating operation of the air bag.  
           [0003]    2. Description of Related Art  
           [0004]    A so-called air bag system that is installed in a vehicle (such as car) and inflated by, for example, an igniting powder loaded in the system or breaking a plug (stopper) of a high-pressure cylinder upon occurring any collision or the like in order to protect driver and other occupant inside the vehicle from a shock has been put into practical use. A system for ignition of the air bag including an electric circuit is hereinafter referred to as an air bag starter or simply a starter.  
           [0005]    [0005]FIG. 5 shows an example of a construction of a conventional starter, for example, a starter disclosed in the International Application No. PCT/JP00/07051. Referring to FIG. 5, reference numeral  1  is a battery installed in a vehicle, and numeral  2  is an ignition switch for starting an engine of the vehicle. Numeral  3  is a driver&#39;s seat air bag disposed in the driver&#39;s seat, and numeral  4  is an igniter for a first-stage inflator (detonating powder or the like not shown in the drawing) for inflating the driver&#39;s seat air bag  3 . Numeral  5  is an igniter for a second-stage inflator that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.  
           [0006]    Numeral  6  is an assistant driver&#39;s seat air bag disposed in the assistant driver&#39;s seat, and numeral  7  is an igniter for a first-stage inflator for inflating the assistant driver&#39;s seat air bag  6 . Numeral  8  is an igniter for a second-stage inflator (detonating powder or the like not shown) that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.  
           [0007]    Numeral  9  is an air bag control unit including an electric circuit for transmitting an electric signal to each of the foregoing igniters and igniting them. Numeral  10  is a DC-to-DC converter boosting input voltage supplied from the battery  1  installed in the vehicle and outputting the voltage, and numeral  11  is a backup condenser (feeding means) charged with a power outputted by the DC-to-DC converter  10 . Numeral  12  is a mechanical acceleration switch arranged to close when a decelerating acceleration of the vehicle exceeds a predetermined level. The mechanical acceleration switch  12  is arranged so that an electric current is not sent to the inflators owing to any malfunction of any driving transistor and prevents the inflators from ignition caused by the malfunction. Such a mechanical acceleration switch  12  is disclosed in, for example, the Japanese Patent Publication (unexamined) No. 211023/1997. The mechanical acceleration switch disclosed in this Japanese Patent Publication (unexamined) No. 211023/1997 has a construction in which the switch is closed upon occurring a weight shift owing to so-called a colliding acceleration at the time of a collision, and the switch is opened upon returning the weight to a predetermined position by spring force when the acceleration disappears. Numeral  13  is an acceleration sensor that measures an acceleration of the vehicle and outputs a signal conforming to the acceleration. Numeral  14  is ignition-judging means that makes a judgment on ignition of the first-stage inflator and the second-stage inflator of the driver&#39;s seat air bag  3  as well as the first-stage inflator and the second-stage inflator of the assistant driver&#39;s seat air bag  6  on the basis of the acceleration signal inputted from the acceleration sensor  13 . This ignition judging means  14  turns on related driving transistors (described later) and at the same time outputs a signal X (shown in FIG. 5) synchronizing with “on” of the driving transistor.  
           [0008]    Numeral  15  is a second-stage forced igniting means (forced igniting means) that is disposed in parallel to the mechanical acceleration switch  12 , connects the backup condenser  11  with a common connecting portion where the first-stage igniters  4  and  7  and the second-stage igniters  5  and  8  are connected, and supplies an ignition current to the second-stage igniters  5  and  8  at the time of forcedly igniting the second-stage inflators in response to the signal from the ignition judging means  14 . Numeral  31  is a driving transistor (switching means for forced ignition or a switch) disposed in parallel to the mechanical acceleration switch  12  between the backup condenser  11  and the common connecting portion where the first-stage igniters  4  and  7  and the second-stage igniters  5  and  8  are connected in parallel.  
           [0009]    Numeral  32  is closure detecting means that detects the mechanical acceleration switch  12  being closed and holds a closure signal (hereinafter referred to as signal A for convenience of explanation) output for a predetermined time after the mechanical acceleration switch  12  is once closed and then opened. Numeral  33  is a two-input AND gate (logic means) in which two input terminals are connected to the ignition judging means  14  and to (the signal A of) the closure detecting means  32  respectively, and an output terminal is connected to a gate (control terminal) of the driving transistor  31 .  
           [0010]    The driving transistor  31 , the closure detecting means  32 , and the AND gate  33  constitute the second-stage forced igniting means  15 . Numerals  16  and  17  are driving transistors (first switching means) that controls turning on and off of an electric current for igniting the first-stage inflator of the driver&#39;s seat air bag  3 . Numerals  18  and  19  are driving transistors (second switching means) for the second-stage inflator of the driver&#39;s seat air bag  3 . Numerals  20  and  21  are driving transistors (first switching means) for the first-stage inflator of the assistant driver&#39;s seat air bag  6 . Numerals  22  and  23  are driving transistors (second switching means) for the second-stage inflator of the assistant driver&#39;s seat air bag  6 . The ignition judging means  14  is connected to gates (control terminals) of the driving transistors  16 ,  17 ,  18 ,  19 ,  20 ,  21 ,  22  and  23 , and controls “on” and “off” of the driving transistors  16 ,  17 ,  18 ,  19 ,  20 ,  21 ,  22  and  23  in response to the acceleration signal inputted from the acceleration sensor  13 .  
           [0011]    Now, described below are operations of the air bag system shown in FIG. 5. FIGS. 6 and 7 are timing charts showing various aspects of igniting operation in the air bag system shown in FIG. 5.  
           [0012]    When a key not shown is turned to a position for starting the engine and the ignition switch  2  is closed, the DC-to-DC converter  10  boosts a direct-current power outputted from the vehicle-mounted battery  1 , and the backup condenser  11  is charged with the power.  
           [0013]    In case of occurring any collision, when a decelerating acceleration caused by the collision exceeds a predetermined acceleration, the mechanical acceleration switch  12  is closed. In this process, the closure detecting means  32  detects the mechanical acceleration switch  12  being closed, converts the output signal (the signal A) delivered to the AND gate  33  from L level to H level, and holds the signal level.  
           [0014]    On the basis of the acceleration signal inputted from the acceleration sensor  13 , the ignition judging means  14  chooses a proper inflation form among four forms of the driver&#39;s seat air bag  3  and the assistant driver&#39;s seat air bag  6  described below.  
           [0015]    Inflation form A: the air bags are not inflated.  
           [0016]    Inflation form B: only the first-stage inflators are ignited, and the air bags are inflated to a proper degree.  
           [0017]    Inflation form C: The first-stage inflators are ignited and then the second-stage inflators are ignited with a predetermined delay, thus the air bags are inflated strongly to a certain degree.  
           [0018]    Inflation form D: The first-stage inflators and the second-stage inflators are ignited at the same time, thus the air bags are inflated strongly.  
           [0019]    If either the inflation form B or C is chosen, the ignition judging means  14  turns on the driving transistors  16 ,  17 ,  20  and  21  in order to ignite the first-stage inflators of the driver&#39;s seat air bag  3  and the assistant driver&#39;s seat air bag  6 . As a result, an electric current necessary for the ignition is conducted to the first-stage igniters  4  and  7  to ignite the first-stage inflators, thus the driver&#39;s seat air bag  3  and the assistant driver&#39;s seat air bag  6  are inflated to the proper degree. In this case, as shown in FIG. 6, the mechanical acceleration switch  12  is supposed to be closed at the time when the driving transistors for igniting the first-stage inflators are turned on, and it is therefore possible to ignite the first-stage inflators.  
           [0020]    Next, if choosing the inflation form B, it is not preferable to leave the second-stage inflators without igniting them in view of safety, and therefore the second-stage inflators are forcedly ignited immediately after the collision. In this case, a time lag from the ignition of the first-stage inflators to the ignition of the second-stage inflators is, for example, T=100 milliseconds. As shown in FIG. 6, whether the mechanical acceleration switch  12  is opened or closed is not always clear at the time when the second-stage inflators are ignited. To cope with this, when the driving transistors  18 ,  19 ,  22  and  23  are turned on, the output signal X to the AND gate  33  is converted from L level to H level at the same time. The signals of H level (i.e., the signal A and the signal X) are inputted to both two input terminals of the AND gate  33  and, thus, the signals of H level are outputted and the driving transistor  31  is turned on. As a result, the common connecting portion where the first-stage igniters  4  and  7  and the second-stage igniters  5  and  8  are connected in parallel and the backup condenser  11  are electrically connected, and therefore the backup condenser  11  supplies a current to the second-stage igniters  5  and  8 , thus the second-stage inflators are ignited.  
           [0021]    According to the foregoing construction, when the signal X that commands the second-stage inflators to ignite is outputted by the ignition judging means  14 , the signal A indicating that the mechanical acceleration switch  12  is closed is also outputted by the closure detecting means  32 . As a result, the driving transistor  31  is turned on and the second-stage igniters are ignited.  
           [0022]    However, collision of a vehicle includes extremely indefinite factors, and therefore operation waveform of the mechanical acceleration switch  12  is not always stable unlike FIG. 6. FIG. 7 is a timing explanation diagram for explaining timing and shows a waveform that may actually occur. At the moment of collision, the mechanical acceleration switch  12  presents a so-called chattering phenomenon such as irregular vibration. As a result, a first-stage ignition current delivered from the condenser  11  to the air bag  3  or  6  has a waveform that oscillates as indicated by numeral  99  in FIG. 7, and, for example, average value of the electric current is lowered and the ignition becomes uncertain, eventually resulting in lowering of reliability in operation.  
           [0023]    To overcome the mentioned disadvantage, in the conventional air bag starter, in addition to adoption of any arrangement for reducing the chattering in the mechanical acceleration switch  12 , it is necessary to improve reliability in ignition by making the time for turning on the first-stage ignition driving transistors longer than a minimum time necessary for ignition of the inflators and enlarging a capacity of the condenser  11  as accordingly so that the current is applied for a longer time. Alternatively, it is necessary to prevent influence of chattering by boosting a voltage for charging the condenser  11  and shortening a rise time of the current.  
           [0024]    The foregoing explanation describes about a case of a two-stage ignition type air bag system, and it is a matter of course that the same problems may occur in a one-stage ignition type air bag system. The first-stage ignition type air bag system is not provided with the forced igniting means  15  in FIG. 5, and the air bags are not provided with any portion related to the second-stage inflators. However, the operation for ignition of the first-stage igniters  4  and  7  is the same, and this system likewise has the foregoing problems caused by chattering of the mechanical acceleration switch  12  in the same manner as in the case of the two-stage type air bag system.  
           [0025]    As described above, in the conventional air bag starter, if any chattering takes place in the mechanical acceleration switch at the time of turning on the driving transistor and delivering the ignition current, the ignition current is not sufficiently delivered to the driver&#39;s seat air bag and/or the assistant driver&#39;s seat air bag, and in some cases, the inflators do not ignite. As a result, a problem exists in that reliability in operation is lowered.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention has been made to solve the above-discussed problems and has an object of providing an air bag starter in which ignition current is not lowered even if any chattering occurs in the mechanical acceleration switch, thereby reliability in igniting operation being secured.  
           [0027]    An air bag starter according to the invention includes a mechanical acceleration switch, closure detecting unit, ignition judging unit, logic unit, and a switch. The mechanical acceleration switch  12  is connected to an igniter of an air bag and closes in response to acceleration of a vehicle. The closure detecting unit detects the mentioned mechanical acceleration switch being closed and outputs a signal A. The ignition judging unit detects an acceleration of the vehicle, judges acceleration level, decides a method of igniting the mentioned igniter, and outputs a signal B. The logic unit outputs a forced ignition signal to forcedly ignite the mentioned igniter on the basis of AND of the mentioned signal A and signal B. The switch  31  is connected in parallel to the mentioned mechanical acceleration switch  12  and controlled by the mentioned forced ignition signal.  
           [0028]    In the air bag starter of above construction, upon starting the mechanical acceleration switch, the forced igniting unit connected in parallel to the mechanical acceleration switch is closed. As a result, even if any chattering occurs in the mechanical acceleration switch, electric current is sufficiently delivered to the igniter to be ignited first, and the air bags are inflated without fail. This improves reliability in operation. Furthermore, it is no longer necessary to enlarge capacity of the capacitor and take any measures for improving accuracy in operation of the mechanical acceleration switch.  
           [0029]    The foregoing and other object, features, aspects and advantages of the present invention will become more apparent the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a circuit block diagram of an air bag starter according to Embodiment 1 of the present invention.  
         [0031]    [0031]FIG. 2 is a timing chart to explain operation of the air bag starter in FIG. 1.  
         [0032]    [0032]FIG. 3 is a circuit block diagram of an air bag starter according to Embodiment 2 of the invention.  
         [0033]    [0033]FIG. 4 is a timing chart to explain operation of the air bag starter in FIG. 3.  
         [0034]    [0034]FIG. 5 is a circuit block diagram of a conventional air bag starter.  
         [0035]    [0035]FIG. 6 is a timing chart to explain operation of the air bag starter in FIG. 5.  
         [0036]    [0036]FIG. 7 is a timing chart to explain defects in operation of the air bag starter in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    Embodiment 1.  
         [0038]    [0038]FIG. 1 is a circuit diagram of an air bag starter according to Embodiment 1, in which a two-stage type air bag system is taken as an example. In the drawings, the same numerals indicate same parts as those in the drawings to explain the conventional air bag starter, and detailed explanation of them is omitted herein.  
         [0039]    Now referring to the drawings, reference numeral  1  is a battery installed in a vehicle, and numeral  2  is an ignition switch for starting an engine of the vehicle. Numeral  3  is a driver&#39;s seat air bag disposed in the driver&#39;s seat, and numeral  4  is an igniter (detonating primer) for a first-stage inflator (detonating powder or the like, though not shown in the drawing) to inflate the driver&#39;s seat air bag  3 . Numeral  5  is an igniter for a second-stage inflator that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.  
         [0040]    Numeral  6  is an assistant driver&#39;s seat air bag disposed in the assistant driver&#39;s seat, and numeral  7  is an igniter for a first-stage inflator to inflate the assistant driver&#39;s seat air bag  6 . Numeral  8  is an igniter for a second-stage inflator (detonating powder or the like not shown) that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.  
         [0041]    Numeral  109  is an air bag control unit including an electric circuit for transmitting an electric signal to each of the foregoing igniters to ignite them. Numeral  10  is a DC-to-DC converter for boosting input voltage supplied from the battery  1  installed in the vehicle and outputting the voltage, and numeral  11  is a backup condenser (feeding means) charged with power outputted by the DC-to-DC converter  10 . Numeral  12  is a mechanical acceleration switch arranged to close when a decelerating acceleration of this vehicle exceeds a predetermined level. Numeral  13  is an acceleration sensor that measures an acceleration of this vehicle and outputs a signal corresponding to the acceleration. Numeral  114  is ignition-judging means that makes a judgment on ignition of the first-stage inflator and the second-stage inflator of the driver&#39;s seat air bag  3  as well as the first-stage inflator and the second-stage inflator of the assistant driver&#39;s seat air bag  6  on the basis of the acceleration signal inputted from the acceleration sensor  13 . This ignition judging means  14  turns on related driving transistors (described later) and, synchronously with this turning on, outputs a signal B1 for first-stage forced ignition and a signal B2 for second-stage forced ignition.  
         [0042]    Numeral  115  is a forced igniting means that is disposed in parallel to the mechanical acceleration switch  12 , forcedly ignites the second-stage inflators in response to the signal from the ignition judging means  114 . This forced igniting means  115  assists, serving as a backup, first-stage ignition to ignite the first-stage inflators without fail as described later. Numeral  31  is a driving transistor (switching means for forced ignition) disposed in parallel to the mechanical acceleration switch  12 . Numeral  32  is closure detecting means that detects the mechanical acceleration switch  12  being closed and outputs a closure signal (hereinafter referred to as signal A) for a predetermined time after the mechanical acceleration switch  12  is once closed and then opened.  
         [0043]    Numeral  133  is a three-input-two AND gate in which three input terminals are connected respectively to a signal B1 terminal of the ignition judging means  114 , a signal B2 terminal of the ignition judging means  114 , and a signal A of the closure detecting means  32 . The three-input-two AND gate is logic means that outputs an “on” signal on condition that, among three inputs A, B1, and B2, the inputs A and B1 are both “on” or the inputs A and B2 are both “on”. An output terminal of the three-input-two AND gate  133  is connected to a gate (control terminal) of the driving transistor  31 . For convenience of explanation, numeral  133  is hereinafter referred to as two/three AND gate.  
         [0044]    The driving transistor  31 , the closure detecting means  32 , and the two/three AND gate  133  constitute the forced igniting means  115 . Numerals  16  and  17  are driving transistors (switching means) for controlling an ignition circuit of the first-stage inflator of the driver&#39;s seat air bag  3 . Numerals  18  and  19  are driving transistors (switching means) for the second-stage inflator of the driver&#39;s seat air bag  3 . Numerals  20  and  21  are driving transistors (switching means) for the first-stage inflator of the assistant driver&#39;s seat air bag  6 . Numerals  22  and  23  are driving transistors (switching means) for the second-stage inflator of the assistant driver&#39;s seat air bag  6 . In addition, the ignition judging means  114  is connected to gates (control terminals) of the driving transistors  16 ,  17 ,  18 ,  19 ,  20 ,  21 ,  22  and  23 . The ignition judging means  114  on/off controls the driving transistors  16 ,  17 ,  20  and  21  or the driving transistors  18 ,  19 ,  22  and  23  in response to the acceleration signal inputted from the acceleration sensor  13 . The ignition judging means  114  outputs the signal B1 synchronously with “on” of the driving transistors  16 ,  17 ,  20  and  21 , and outputs the signal B2 synchronously with “on” of the driving transistors  18 ,  19 ,  22  and  23 . In other words, the signal B2 is equivalent to the signal X in the foregoing description of the conventional air bag starter.  
         [0045]    [0045]FIG. 2 is a timing chart to explain various situations in igniting operation of the two-stage type air bag starter shown in FIG. 1.  
         [0046]    Upon occurrence of any collision, during the period when acceleration caused by the collision exceeds a predetermined acceleration level, the mechanical acceleration switch  12  is closed with chattering. At this time, the closure detecting means  32  detects the mechanical acceleration switch  12  being closed, converts the signal A for the two/three AND gate  133  from L level to H level, and holds the foregoing signal level.  
         [0047]    The ignition judging means  114  judges the signal of the acceleration sensor  13  and decides the igniting method as described in the foregoing conventional example. The ignition judging means  114  turns on the driving transistors  16 ,  17 ,  20  and  21  in order to ignite the first-stage inflators of the driver&#39;s seat air bag  3  and the assistant driver&#39;s seat air bag  6  according to the decided igniting method. At the same time, the output signal B1 for the two/three AND gate  133  is converted from L level to H level. The signal B1 is substantially synchronized with closing operation of the mechanical acceleration switch  12 , but is not always synchronized with the closing operation.  
         [0048]    The two/three AND gate  133  outputs a signal of H level and turns on the driving transistor  31  (shown as a first-stage forced ignition signal in the drawing) when signals of H level are inputted to two among the three input terminals. Accordingly, the first-stage igniters  4  and  7  and the backup condenser  11  are completely conducted by turning on the driving transistor  31 . As a result, it is possible to prevent reduction in electric current caused by chattering of the mechanical acceleration switch  12 , and it is possible to ignite the air bags without fail.  
         [0049]    The subsequent operations of igniting the second-stage igniters are the same as those described referring to FIGS. 5, 6 and  7  of the conventional starter, and further explanation thereof is omitted herein. When a predetermined time has passed since the first-stage igniting operation, the ignition judging means  114  turns on the second-stage driving transistors  18 ,  19 ,  22  and  23 , and outputs the signal B2 at the same time. Then, the logic circuit  133  turns on the forced igniting means  31  on the basis of AND of the signal A and signal B2.  
         [0050]    Since chattering of the mechanical acceleration switch does not influence the first-stage ignition current, it is not necessary to take measures to reduce chattering in constitution of the mechanical acceleration switch  12 .  
         [0051]    Furthermore, it is not necessary to take measures such as boosting voltage with which the condenser  11  is charged and expanding capacity of the capacitor. As a result, it is possible to avoid increase in cost and enlargement in configuration.  
         [0052]    Although two air bags are shown for the driver&#39;s seat and the assistant driver&#39;s seat in FIG. 1, it is also possible to dispose only one air bag or more than two air bags without changing the fundamental operation as a matter of course.  
         [0053]    Although the foregoing description takes the case of a two-stage type air bag system, in case employing an air bag system with more than two stages, operations and advantages thereof essentially remain unchanged. That is, the ignition judging means outputs plural signals B (B1 . . . n) and the semiconductor switch  31  is turned on by a forced ignition signal outputted on the basis of a AND of the signal A and any of signals B1 . . . n, thereby overcoming the problem of chattering in the mechanical acceleration switch  12  in the same manner as in the two-stage type air bag system.  
         [0054]    Embodiment 2.  
         [0055]    Now, a case of one-stage type air bag system is hereinafter described with reference to FIG. 3. The fundamental constitution is the same as that in FIG. 1, and detailed explanation is omitted herein. Air bags  3  and  4  are not provided with any second-stage igniter, and moreover any driving transistor to be connected to such second-stage igniter does not exist. The forced igniting means  115  is provided with an AND gate  33  that acts upon receipt of the signal A outputted by the closure detecting means  32  and the signal B outputted by the ignition judging means  114 .  
         [0056]    Operation of the air bag starter in FIG. 3 is hereinafter described with reference to a timing chart of FIG. 4.  
         [0057]    Upon occurrence of any collision, during the period when acceleration caused by the collision exceeds a predetermined acceleration level, the mechanical acceleration switch  12  is closed with chattering. At this time,the closure detecting means  32  detects the mechanical acceleration switch  12  being closed, converts the signal A for the two/three AND gate  133  from L level to H level, and holds the foregoing signal level.  
         [0058]    The ignition judging means  114  judges the signal of the acceleration sensor  13  and decides the igniting method. The ignition judging means  114  turns on the driving transistors  16 ,  17 ,  20  and  21  in order to ignite the inflators of the driver&#39;s seat air bag  3  and the assistant driver&#39;s seat air bag  6 . The output signal B for the AND gate  33  is converted from L level to H level at the same time.  
         [0059]    When signals of H level are inputted to both of the two input terminals, the AND gate  33  outputs a signal of H level and turns on the driving transistor  31  (shown in FIG. 4 as a forced ignition signal). The igniters  4  and  7  and the backup capacitor  11  are completely conducted by turning on the driving transistor  31 . As a result, it is possible to prevent reduction in electric current caused by chattering of the mechanical acceleration switch  12 , and it is possible to ignite the air bags without fail.  
         [0060]    Embodiment 3.  
         [0061]    In the foregoing Embodiments 1 and 2, a starter to be connected to the air bags is specifically described in detail. However, it is to be noted that the most essential part of the invention is a backup circuit. This backup circuit is arranged to back up the mechanical acceleration switch  12  and avoid influence of chattering thereon by turning on the transistor  31  connected in parallel to a contact of the mechanical acceleration switch  12  when a detection signal of the acceleration sensor  13  disposed separately from the mechanical acceleration switch  12  exceeds a predetermined level.  
         [0062]    Specifically, the backup circuit is constituted as a device including the forced igniting means  115 , the ignition judging means  114 , and the acceleration sensor  13  in FIG. 1, for example. Therefore, the same advantage is achieved even in case of constituting, for example, the back up circuit alone. In this constitution, the DC-to-DC converter  10 , the condenser  11 , the mechanical acceleration switch  12 , the first-stage and second-stage driving transistors  16  to  23 , etc. are disposed outside separately from the starter, and subsequently, the backup circuit is connected to a separately disposed starter.  
         [0063]    In the case of the two-stage type air bag system, it is possible to use in common the second-stage igniting means and the forced igniting means connected in parallel to the mechanical acceleration switch, thus a more economical air bag system is obtained.