Abstract:
A passenger protecting apparatus for inflating an air bag or tightening a seat belt is provided that is smaller in size and more reliable than any previously known passenger protecting apparatus. The passenger protecting apparatus has two switches connected in series with a squib. The switches are controlled by a control unit using independent program routines in a redundant manner. The control unit combines front collision and side collision detection in one unit. The control unit controls the switches based on the outputs of several acceleration sensors, by comparing their respective outputs to threshold values.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a passenger protecting apparatus, which inflates an air bag for protecting the passenger in an instant, when a vehicle has an accident such as a collision. More specifically, the present invention relates to a controlling unit of a squib for inflating an air bag. A “squib” is herein defined as a detonator for inflating an air bag and/or for tightening the seat belt, in this specification and Claims. 
     2. Description of the Prior Art 
     FIG. 8 shows a block diagram of a controlling unit of a squib, in the prior art, of a squib in such a passenger protecting apparatus. 
     An ignition switch  102 , a DC/DC converter  103 , a diode  104  for counter-current prevention, a first switching transistor  106  (first switching circuit), a squib  107 , a second transistor  108  (second switching circuit) are connected in series between both of the terminals of a battery  101 . The line connecting the diode  104  and the first switching transistor  106  is connected through a back up capacitor  105  with large capacity to the ground line. 
     In a collision, first and second acceleration sensors  109 , 111  generate acceleration signals. The acceleration signal from the first acceleration sensor  109  is provided to a collision judging means  110 , and the acceleration signal from the second acceleration sensor  111  is provided to a comparing means  112 . When the acceleration signal from the second acceleration sensor  111  exceeds a standard value, a high level signal is provided to an input terminal of an AND gate means  113  and to the collision judging means  110  as a switch signal. 
     The procedure of the collision judging algorithm in the collision judging means  110  begins at the moment, when a trigger signal, which corresponds to the standing up edge of the high level signal, is given. It estimates the accident whether the accident is so grave that it leads to an injury of the passenger(s). When it estimates that the accident can lead to a grave injury of the passenger(s), the collision judging means  110  supplies a high level signal to the other input terminal of the AND gate means  113 . 
     The base terminal of the first switching transistor  106  is connected with the output terminal of the AND gate means  113  through an inverter  114 , and the base terminal of the second switching transistor  108  is connected with the output terminal of the AND gate means  113  directly. 
     When the accident is estimated to be grave, the AND gate means receives high level signals from the collision judging means  110  and comparing means  112  simultaneously. As a result, the first and second switching transistors  106 , 108  turn on. The back up capacitor supplies the electric energy stored therein to the squib  107 , then the squib actuates the air bag to inflate. 
     In the aforementioned passenger protecting apparatus, only one AND gate means  113  is used, and both switching transistors  106 , 108  are driven, depending on the output of the AND gate means. 
     There is a problem in constructing the controlling unit by a micro computer containing a CPU, and a soft ware program equivalent to the collision judging unit  110 , the AND gate  113 , the inverter and the comparing means  112 . 
     When the output of the AND gate means becomes to a high level, due to malfunction in the soft ware program or due to a false signal caused by external noise, the first and second switching transistors connected to the both sides of the squib turn on simultaneously. This can lead to an erroneous operation of the squib. 
     FIG. 9 is a block diagram of another example of a controlling unit of a squib in a passenger protecting apparatus that is disclosed in a Japanese Patent Application, which has not yet been published. 
     The controlling unit comprises a controlling unit for a front air bag  201  (it is called also as a pre-tensioned unit), a controlling unit for a side air bag at the conductor&#39;s seat  202  and a controlling unit for a side air bag at the seat next to the driver  203 , which communicate to each other using a multi-superposition communication system, using the power supply wire as a signal line. 
     At first, the controller unit for the front air bag is explained. 
     First, second and third voltage increasing circuits  206 ,  207 ,  208  convert the input voltage, which is supplied from a battery  204  through an ignition switch  205 , to a higher voltage and charge a first, second and third back up capacitor  209 ,  210 ,  211 , which are respectively connected. 
     The input terminals of a first, second and third switching circuit  212 ,  213 ,  214  are connected respectively with the output terminal of the corresponding first, second and third voltage increasing circuits  206 ,  207 ,  208 , and their output terminals are connected respectively with a corresponding squib  215 ,  216 ,  217 . 
     When the ignition switch  205  is put on, the first, second and third back up capacitor  209 ,  210 ,  211  are charged from the battery  204  through the first, second and third voltage increasing circuits  206 ,  207 ,  208 , and a micro computer  220  in the controlling unit for front air bag  201  starts its procedure. Simultaneously, the voltage regulating circuit  223  begins to supply electric power to the controlling units for the side air bag at the driver&#39;s seat  202  and the controlling unit for side air bag at the seat next to the driver&#39; seat  203 . 
     When an igniting signal is provided to the input terminal of one of the first, second and third switching circuits  212 ,  213 ,  214  from the micro computer  220 , an ignition current from the corresponding back up capacitor  209 ,  210 ,  211  flows through the corresponding first, second and third squibs  215 ,  216 ,  217 . 
     Reference numeral  218  denotes a mechanical acceleration sensor, which is connected with the first squib  215  in series and turns on when an acceleration over a predetermined value is detected. On the other hand, the longitudinal direction acceleration sensor  219  is an electric acceleration sensor, which detects the longitudinal acceleration at an collision of vehicle. 
     Reference numeral  220  denotes a micro computer, having a collision judging function. It estimates the accident as to whether it is grave or not, on the basis of the acceleration signal from the longitudinal acceleration sensor  219 . When it estimates that the accident is grave, it turns on the first switching circuit  212 . Hence, the charge stored in the first back up capacitor  209  flows as an ignition current through the first squib  215  and the mechanical acceleration sensor  218 , which are connected in series. As a result, the front air bag for a front collision of vehicle is inflated, or the pre-tensioned unit is actuated. In this manner, the air bag and so like is inflated protecting the passenger(s) at the front collision of vehicle. 
     The controlling unit for the side air bag at the driver&#39;s seat  202  comprises of a first transverse acceleration sensor  230 , a micro computer  231 , an acceleration switch  232 , a second sending circuit  233 , a resistor  234  and a second receiving circuit  235 . On the other hand, The controlling unit for the side air bag at the seat next to the driver&#39;s seat  203  comprises a second transverse acceleration sensor  240 , a micro computer  241 , an acceleration switch  242 , a third sending circuit  243 , a resistor  244  and a third receiving circuit  245 . The corresponding components in both of the units are identical. 
     The transverse acceleration sensors  230 ,  240  have an identical structure as the longitudinal acceleration sensor  219 , however, they are configured so as to detect the acceleration in the direction differed 90 degrees from the detection direction of the longitudinal acceleration sensor  219 , namely they detect the acceleration in the transverse direction of vehicle. They provide their outputs to the micro computer  231 ,  241 , respectively. 
     The micro computer  231 ,  241  have a collision judging functions as the micro computer  220  has, and estimate the accident on the basis of the acceleration signals provided from the second or third transverse acceleration sensors  230 ,  240  and the switch signal provided from the acceleration switches  232 ,  242 . 
     The voltage regulating circuit  223  supplies a regulated voltage through the resistor  224  to the controlling units for the side air bag at the driver&#39;s seat  202  and for the side back at the seat next to the driver&#39;s seat  203 . 
     A signal demand signal from the micro computer  220  to the controlling units for the side air bag at the driver&#39;s seat  202  or to the controlling unit for the side back at the seat next to the driver&#39;s seat  203  is provide to the first sending circuit  225  via a signal line X. The first sending circuit  225  superimposes it on the power supply line A. 
     On the other hand, when the micro computer  231  in the controlling units for the side air bag at the driver&#39;s seat  202  estimates that the accident is grave, on the basis of the switch signal from the first acceleration switch  232  and the acceleration signal from the first transverse acceleration sensor  230 , it sends a corresponding signal, as a responding signal to the signal demanding signal from the micro computer  220 , through the second sending circuit  233 , the resistor  234  and the power supply line A, which are connected in series. 
     In the same manner, when the micro computer  241  in the controlling units for the side air bag at the driver&#39;s seat  203  estimates that the accident is grave, on the basis of the switch signal from the second acceleration switch  242  and the acceleration signal from the second transverse acceleration sensor  240 , it sends a corresponding signal, as a responding signal to the signal demanding signal from the micro computer  220 , through the third sending circuit  243 , the resistor  244  and the power supply line A, which are connected in series. 
     The first receiving circuit  226  receives the responding signal from the controlling unit for the side air bag at the driver&#39;s seat  202  or from the controlling unit for the side air bag at the seat next to the driver&#39;s seat  203  through the power supply line, which functions here as a communication line. And it sends the responding signal to the micro computer  220  through a communication line Z. 
     When the micro computer  220  receives a signal, indicating that the accident is grave, from the micro computer  231  or  241  through the communication line Z, it actuates the corresponding second or third switch circuit  213 ,  241  to turn on. Hence, the charge stored in the second or third back up capacitor  210 ,  211  flows through the second or third squid  216 ,  217 . As a result, the side air bag at the driver&#39;s seat of at the seat next to the driver&#39;s seat is inflated. In this manner, the air bag is inflated for protecting the passenger from the side collision of vehicle. 
     The rigidity of an old type vehicle is strong in the longitudinal direction, and is relatively weak in the transverse direction of vehicle. Accordingly, the controlling unit for a front collision  201  is located at the intermediate portion of the driver&#39;s seat and the seat next to the driver&#39;s seat. And the controlling units for a side collision  202 ,  203  are located separately at a lower portion of the center pillars beside the driver&#39;s seat and the seat next to the driver&#39;s seat. This is in order to assure the protection of the passenger from both front and side collisions. Due to this structure, fabrication and installing costs are rather large. 
     Recently, however, vehicles have the improvement of having high rigidity against external forces in the transverse direction. Thus the necessity to arrange the controlling units separately has decreased. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a passenger protection apparatus, in which the control of squib is realized by a micro computer and a soft ware computer program, which can avoid mis-functioning of the squib. 
     Another object of the present invention is to provide a passenger protection apparatus, in which the transverse acceleration sensors are incorporated into a control unit for a front collision so as to unify the control units, and to improve the S/N ratio of the acceleration signal. 
     The first object is attained by the passenger protection apparatus according to claim  1 , more specifically, it is attained by a passenger protection apparatus comprising of: 
     a first acceleration sensor; 
     a second acceleration sensor, the polarity of the output of the second acceleration sensor being opposite to that of the first acceleration sensor; 
     a comparing means, which compares the output of the second acceleration sensor with a predetermined standard value, and generates a switch signal, when the output exceeds the standard value; 
     a collision judging means, which estimates whether the collision is grave or not, on the basis of the output of the first acceleration sensor; 
     a first AND gate means, which receives the output of the comparing means calculated for the first AND gate means, and the output of the collision judging means calculated for the first AND gate means; 
     a second AND gate means, which receives, independently from the first AND gate means, the output of the comparing means calculated for the second AND gate means, and the output of the collision judging means calculated for the second AND gate means; 
     a squib; 
     a first and second switching circuit, which are connected with the squib in series and are controlled so as to turn on, according to the outputs of the first and second AND gate means, respectively; 
     wherein the comparing means, the collision judging means, and the first and second AND gate means are realized as a software computer program routine. 
     In an embodiment, the outputs of the collision judging means appear at different output ports of the micro computer, through the first and second AND gate means. 
     In an embodiment, the first and second acceleration sensors have same characteristics. 
     In an embodiment, one of the first and second acceleration sensors detects the acceleration in higher range, and the other detects the acceleration in lower range. 
     The second object is attained by the passenger protection apparatus according to claim  4 , more specifically, it is attained by a passenger protection apparatus comprising of: 
     a squib for inflating an air bag; 
     a first and second acceleration sensors, the sensing direction of which is opposite to each other; 
     a comparing means, which compares the output of the second acceleration sensor with a predetermined standard value, and generates a switch signal, when the output exceeds the standard value; 
     a collision judging means, which estimates whether the collision is grave or not, on the basis of the output of the first acceleration sensor, and generates an ignition signal, according to the estimation; 
     means for controlling the squib, which generates a signal to blow out the air bag, on the basis of the ignition signal from the collision judging means and the switch signal from the comparing means; 
     wherein the collision judging means calculates the subtraction between the outputs of the first and second acceleration sensors, and estimates the collision, on the basis of the calculated subtraction. 
     In an embodiment, two sets of the first and second acceleration sensors are installed therein, one of them is orientated along the longitudinal direction, and the other is orientated along the transverse direction of the vehicle. 
     In an embodiment, the first and second acceleration sensors of each set, for detecting the acceleration along the longitudinal direction or along the transverse direction of the vehicle, have same characteristics. 
     In an embodiment, the collision judging means estimates whether the collision is a front collision or a side collision, on the basis of the polarities of the outputs of the first and second acceleration sensors. 
     In an embodiment, the collision judging means estimates whether the collision is grave or not, on the basis of thee outputs of the first and second acceleration sensors, and generates an ignition signal to actuate the squib, on the basis of the estimation. 
     In an embodiment, the first acceleration sensor detects a large acceleration, the second acceleration sensor detects a small acceleration with high sensitivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of electric circuit of a passenger protection apparatus according to the first aspect of the present invention. 
     FIGS. 2A-B show an example of a set of signals for explaining the functioning of the apparatus in FIG.  1 . 
     FIG. 3 shows a block diagram of electric circuit of a passenger protection apparatus according to the second aspect of the present invention. 
     FIG. 4 shows a block diagram of a passenger protection apparatus according to the third aspect of the present invention. 
     FIG. 5 shows a flow chart of the micro computer  20  in the apparatus of FIG.  4 . 
     FIGS. 6A-C show an example of a set of signals for explaining the functioning of the apparatus in FIG.  5 . 
     FIG. 7 shows a block diagram of a passenger protection apparatus according to the fourth aspect of the present invention. 
     FIG. 8 shows a block diagram of a passenger protection apparatus in the prior art. 
     FIG. 9 shows a block diagram of a passenger protection apparatus in another prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Aspect of the Invention 
     The elements in FIG. 1, which are identical or equivalent with elements in FIG. 8, are referred by the same reference numerals, and their explanation is omitted. Only the points different from the apparatus in the prior art in FIG. 8 are explained, here. 
     The first and second acceleration sensors  109 ,  111  are so arranged that the polarities of their output are opposite, namely one is positive and the other is negative. For example, they are arranged back to back and are so configured that their sensing directions are different to each other by 180 degrees. In this embodiment, the characteristics of these sensors  109 ,  111  are the same. 
     The micro computer  116  is comprised of the collision judging means  110 , the comparing means  112 , a first and second AND gate means  113 ,  115 . Each is realized as a software program routine. The routines corresponding to the collision judging means  110 , the comparing means  112 , the first AND gate means  113  and the second AND gate means  115  are executed sequentially and repeatedly. 
     The collision judging means  110  starts its procedure to estimate the accident, on the basis of an output of the first acceleration sensor  109 , when a trigger signal, which is an output of the comparing means  112 , is supplied. 
     The collision judging means  110  supplies the signal, which represents the estimation of the accident, to one input terminal of each first and second AND gate means  113 ,  115 . The comparing means  112 , which compares the output of the second acceleration sensor  112  and a predetermined standard value, supplies a switch signal to the other input terminal of each first and second AND gate means  113 ,  115 . 
     The output terminal of the first AND gate means  113  is connected with the base terminal of the first switching transistor  106 , which is the signal input terminal of the transistor, through an inverter. The output terminal of the second AND gate means  115  is connected with the base terminal of the second switching transistor  108 . The output terminals of the second AND gate means and the inverter are realized by the output ports of the micro computer  116 , here. 
     Using this circuit, the influence of external noise is reduced as explained below. 
     When an accident takes place, the first acceleration sensor  109  outputs a signal, for example as shown by a solid line in FIG.  2 (A). And the second acceleration sensor  111  outputs a signal, for example as shown by a solid line in FIG.  2 (B), their polarities are opposite to each other. When a common noise (E) is superimposed on these signal, due to the back to back configuration of the acceleration sensors. The output of the first acceleration sensor  109  becomes as shown by the broken line (H) in FIG.  2 (A). And the output of the first acceleration sensor  109  becomes as shown by the broken lines (F) in FIG.  2 (B). 
     In section T in FIG. 2, where noises are superimposed, the value of the acceleration signal to be supplied to the collision judging means  110  is increased in the direction to exceed a threshold value G as shown in FIG.  2 (A), on the contrary, the value of the acceleration signal to be supplied to the comparing means  112  is decreased in the direction to the zero level, namely the opposite direction to exceed the threshold value G. In other words, when common noises are added, the output of one of the first and second acceleration sensors  109 ,  111  increases in the direction to exceed a threshold value G, the other decreases in the opposite direction to exceed the threshold value G, namely the direction not to actuate the squib  107 . Thus the protection false signals created by noise is improved. 
     It shall be remarked that the first and second AND gate  113 ,  115  receive independently outputs from the collision judging means  110  as well as from the comparing means  112 , additionally, their outputs are independently supplied to the corresponding first and second switch circuit  106 ,  108  as shown in FIG.  1 . Namely, the first and second AND gate means receive signals from the collision judging means  110  and the comparing means  112  independently from each other, and execute their logic independently using independent program routines from each other, and they output their result from different output ports. 
     The signals to be supplied from the collision judging means  110  and the comparing means  112  to the first and second AND gate means  113 , 115  are, in principle, the same. However, in this aspect of the invention, the signals for the first and second AND gate  113 ,  115  are independently calculated and are independently outputted through different output ports. As a result, even if the micro computer  116  is malfunctioning, the possibility that the values in both of the output ports will simultaneously reach a high level is small. Consequently, the possibility of erroneous functioning of the squib decreases. 
     Preferably, the first and second acceleration sensors are the same. However, acceleration sensors of different characteristics may be used, according specific cases. For example, when this aspect of the invention is applied to a passenger protection apparatus against the front collision of vehicle, one can select an acceleration sensor of nominal range of 50 G as a first acceleration sensor  109  for detecting a large acceleration, and can select an acceleration sensor of nominal range of 30 G as a second acceleration sensor for detecting a small acceleration. 
     This aspect of the invention has following advantages. The reliability of the control of the squib can be improved, because the squib is controlled according to results obtained independently in the micro computer. Because the first and second switch circuit can be controlled by the independent output ports of the micro computer, the possibility that the output ports become simultaneously a high level due to noises etc decreases. When the acceleration sensors have an identical characteristics, and they are arranged so that their output polarity are opposite to each other, the possibility of erroneous estimations of accident decreases. 
     Second Aspect of the Invention 
     In this aspect of the invention, the apparatus has an acceleration level judging means  118 , which receives the output of the first acceleration sensor  109 , as shown in FIG.  3 . The acceleration level judging means  118  judges as to whether somewhat a collision happened, on the basis whether the output of the first acceleration sensor  109  exceeded a predetermined level or not. When it judges that a collision happened, it supplies a trigger signal to the collision judging means  110 . The collision judging means, in turn, starts its collision judging algorithm program, when it receives the trigger signal. 
     Also, in this aspect of the invention, each of the blocks indicated by the reference numerals  110 ,  112 ,  113 ,  115 ,  116 ,  118  is realized by a software program routine, and they are executed one after another, and are repeated. 
     When the first and second acceleration sensors  109 ,  111  are installed on electric circuits in the apparatus, they shall be arranged adjacent to each other and the circuit shall have identical circuit patterns so that their noise circumstances are the same, and their output lines suffer a common noises. 
     Third Aspect of the Invention 
     The elements in FIG. 4, which are identical or equivalent with elements in FIG. 9, are referred by the same reference numerals, and their explanation is omitted. Only the points different from the apparatus in the prior art are explained, here. 
     The control unit for the front air bag  201 ′ in FIG. 4 is a unit incorporating the functions of the first and second transverse acceleration sensors  230 ,  240 , the first and second acceleration switches  232 ,  242 ′, micro computers  231 ,  241  in FIG. 9 into the control unit for the front air bag  201 . 
     The second transverse acceleration sensor  240  and the acceleration switch  242 ′ in the control unit for the air bag at the seat next to the driver&#39;s seat  230  is incorporated in the control unit for front air bag as a unit. A second transverse acceleration sensor  242   a  and a comparing means  242   b  are connected with the micro computer  220 ′. A switching transistor  236 ′ is connected in series with the squibs  216  and the second switching circuit  213 . Another switching circuit  246  is connected in series with the squib  217  and the third switching circuit  214 . The switching transistors  236 ′,  246  are ON-OFF controlled by the micro computer  220 ′ in synchronism with the switching circuits  213 ,  214  respectively. 
     In this aspect of the invention, communication between the micro computer  220  in the control unit for a front air bag  201  and the micro computers  231 ,  241  in the control units for side air bags  202 , 203  is not necessary. Only a supply of ignition current to the side air bags is needed. 
     The following functions are added into the micro computer  220 ′ in the control unit for front air bag  201  compared with the prior art. 
     A first and second transverse acceleration sensors  230 ′,  242   a , which have the same characteristics, are arranged so that their sensing direction is different by 180 degrees, for example, back to back to each other, 
     When an acceleration from a same direction is applied to the first and second transverse acceleration sensor  230 ′ and  242   a , the polarities of their outputs are opposite, namely one is positive and the other is negative, as shown in the collision section in FIG.  6 . It will be understood by comparing each pair of solid lines and the broken lines. 
     When a front collision occurs, the first and second transverse acceleration sensors  230 ′,  242   a  output signals, having signal shapes shown by the rigid lines in FIG.  6 (A), (B). On the contrary, when a side collision occurs from the seat next to the driver&#39;s seat, the first and second transverse acceleration sensors  230 ′,  242   a  output signals, having opposite polarities, as shown by dotted lines in FIG.  6 . 
     As shown here, the acceleration signals provided into the micro computer  220 ′ may have different polarities, depending upon the collision direction. 
     When the sensors have this configuration, electric noises appear in the signal lines as common noises, as shown in FIG.  6 . They are shown in the noise sections in FIG.  6 (A), (B). Namely the noise on both of the signal lines has the same wave shape. When the micro computer  220 ′ calculates the subtraction between the output of the first transverse acceleration sensor  230 ′ and the second transverse acceleration sensor  242   a , their noise components disappear and the acceleration signal appears as a signal multiplied by two, as shown in FIG.  6 (C). 
     The procedure in the micro computer  220 ′ is explained below, referring the flow chart in FIG.  5 . 
     ST 109 : When the power supply is put on, the routine starts and ST 110  follows. 
     ST 110 : It is determined whether a signal is provided from the longitudinal acceleration sensor  219 . When it is determined that the acceleration signal is not provided, ST 120  follows. When it is determined that it is provided, ST 130  follows. 
     ST 120 : The subtraction between the outputs of the first and second transverse acceleration sensors  230 ′,  242   a  is executed to obtain an acceleration signal with good S/N ratio. It is determined whether an acceleration signal is actually provided, on the basis of the obtained acceleration signal. When it is determined that such an acceleration signal is not provided, the routine returns to the step ST 110 . When the level of the obtained acceleration signal exceeds a predetermined standard value, the routine ST 170  follows. 
     ST 130 : The signal of longitudinal acceleration is read out. 
     ST 140 : It is determined whether it is a grave collision or not. When it is determined that it is not a grave collision, the routin returns to the step ST 110 . When it is determined that it is a grave collision, ST 150  follows. 
     ST 150 : The first switching circuit  212  is put to turn on, so that the squib  215  is actuated to inflate the front air bag, ST 160  follows. 
     ST 160 : The procedure ends. 
     ST 170 : It is determined whether a switch signal is provided from the comparing means  242   a . When it is determined that such a signal is not provided, the procedure returns to the step ST 110 . When it is determined that the signal is provided, ST 180  follows. 
     ST 180 : The subtractions between the outputs of transverse acceleration signals, each of them is provided sequentially, are carried out to obtain a transverse acceleration signal with large S/N ratio. Then the routine proceeds to ST 190 . 
     ST 190 : It is determined whether the collision is grave or not, on the basis of the obtained transverse acceleration signal. Simultaneously it is determined whether the collision is a front collision or a side collision, on the basis of the shapes of the signals from the first and second transverse acceleration sensors  230 ′,  242   a , namely, according to their polarities. In other words, it is determined whether the set of the signals is better matched to the solid line set or the dotted line set, shown in FIGS.  6 (A), (B). When it is determined that it is not a grave accident, the routine returns to the step ST 110 . When it is determined to be a grave accident, the routine proceeds to ST 200  follows. 
     ST 200 : The second or third switching circuit is put to turn on, so as to actuate the squib  216  or  217 . Then the side air bag at the driver&#39;s seat or at the seat next to the driver&#39;s seat inflates. Then the routine ST 210  follows. 
     ST 210 : The routine ends. 
     Steps ST 110 , ST 130 , ST 140 , ST 150  belong to a common flow chart. The steps ST 120  to ST 200  belong to a new one. 
     Fourth Aspect of the Invention 
     This aspect is explained below, referring to FIG.  7 . 
     In this aspect, the mechanical switch  218  in FIG. 9 is disappeared with, and in its place a second longitudinal acceleration sensor  219 , switching transistors  236 ,  246 ,  247  are installed in the control unit for front air bag  201 ′. The second longitudinal acceleration sensor  243  has, preferably, the same characteristics as that of the first longitudinal acceleration sensor  219 . The second longitudinal acceleration sensor  243  is arranged in a back to back configuration with the first acceleration sensor, in the same as the first and second transverse acceleration sensors  230 ′  242   a.    
     The micro computer calculates the subtraction between the outputs of the first and second longitudinal acceleration sensor  219 ,  243  to obtain an acceleration signal with a high S/N ratio, as is calculated regarding to the first and second transverse acceleration signals. For this purpose, a comparing means  244  for making a switch signal, similar to the comparing means  242   b  in the third aspect, is arranged between the second longitudinal acceleration sensor  243  and the micro computer  220 ′, and a procedure similar to the flow chart in FIG. 5 is carried out. The explanation of the procedure is omitted, to avoid redundancy. 
     In the explanation of the aforementioned aspect of the invention, the first and second longitudinal acceleration sensors  219 ,  243  have the same characteristics. However they can have different characteristics. For example, it is possible to select a wide range for the sensor  219  and a small range for the sensor  243 . More specifically, a sensor of a nominal maximum acceleration of 50 G for the sensor  219 , and a sensor of a nominal maximum acceleration of 30 G for the sensor  243  can be selected. In this case, the S/N ratio of the output of the sensor  243  is rather high. Thus, it is possible to predetermine a smaller acceleration for the standard in the comparing means  244 . 
     As mentioned above, the third and fourth aspects of the invention allow the simplification of the structure of the passenger protection apparatus, which contains a plurality of air bags. Thus it is possible to decrease fabrication costs. 
     When all of the sensors have the same characteristics, the control of storing the necessary parts for the fabrication is easy, And it is possible to treat electric noise as being substantially identical, because substantially same noise appears in sensors of a similar kind. 
     On the other hand, when two kinds of sensors, a high cost sensor for a wide sensing range and a low cost sensor for a small sensing range, are used, the fabrication costs of a passenger protection apparatus can be decreased.