Abstract:
A crew protection apparatus includes a DC power source; a squib connected in series with the DC power source; a plurality of switch circuits inserted between the DC power source and the squib or between the squib and the ground side; an acceleration sensor for detecting an acceleration caused by a collision; a collision determination circuit for, when receiving an acceleration signal from the acceleration sensor, determining the scale of a collision on the basis of the acceleration signal and outputting an ignition control signal in coincident with the timing supplied to the switch circuit and further outputting a current control signal in synchronism with the ignition control signal when it is determined that the collision is a serious collision; and a current limit circuit for limiting the magnitude of an ignition current flowing through the squib in accordance with the ignition control signal and the current control signal from the collision determination circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a crew protection apparatus which inflates an air bag at the time of collision of a vehicle or the like to protect a passenger from the collision. 
     2. Description of the Related Art 
     Such an example of conventional passenger protection apparatus will be explained with reference to FIG.  3 . 
     In this figure, a reference numeral  1  depicts a vehicle-mounted battery,  2  an ignition switch and  3  a DC/DC converter for boosting the output voltage of the vehicle-mounted battery  1  and outputting the boosted voltage. A reference numeral  4  depicts a current limit circuit formed by a first field effect transistor  7 , a current detection resistor  9 , a comparison circuit  10 , a constant current source  11 , a resistor  12 , a charge pump circuit  13 , a switch circuit  14  or the like. Further, a reference numeral  5  depicts a reverse-current prevention diode,  6  a backup capacitor, and  8  a second field effect transistor. 
     The backup capacitor  6  is charged by the DC/DC converter  3  through the reverse-current prevention diode  5 . The second field effect transistor  8  has a drain side connected to the non-grounded side terminal of the backup capacitor  6  and a source side connected to a percussion cap or squib  15  described later. A current of about  2  amperes corresponding to 99%, for example, of the ignition current flowing into the squib  15  flows through the transistor  8  as an ignition current. 
     The current limit circuit, or a squib drive control circuit  4  will be explained below. 
     The first field effect transistor  7  is an N channel type with a small capacity for shunting the current flowing from the backup capacitor  6  and the reverse-current prevention diode  5  in order to control the current flowing through the second field effect transistor  8 . The first field effect transistor  7  is connected at its drain side to the drain side of the second field effect transistor  8  and connected at its source side to the source side of the second field effect transistor  8  through the current detection resistor  9  with a small allowable power. A small current of several milli-amperes (corresponding to the remaining 1%, for example, of the ignition current flowing into the squib  15 ) flows through the first field effect transistor  7 . 
     The comparison circuit  10  has a non-inverted (+) input terminal supplied with a reference voltage generated by the constant current source  11  and the resistor  12  connected in series and has an inverted (−) input terminal supplied with the voltage generated by the current detection resistor  9 . The output terminal of the comparison circuit is connected to the output terminal of the switch circuit  14  and to the gates of the first and second field effect transistors  7 ,  8 . The comparison circuit  10  changes its output into a high level when the reference voltage is larger than the input voltage and into a low level when the reference voltage is not larger than the input voltage. 
     A reference numeral  18  depicts an acceleration sensor for detecting an acceleration signal which is generated at the time of the collision of a vehicle. A reference numeral  19  depicts a microcomputer which determines the scale of the collision on the basis of the acceleration signal from the acceleration sensor  18  and supplies an ON signal to the switch circuit  14  when it is determined that the collision is a serious accident. The microcomputer  19  supplies a trigger signal to the charge pump circuit  13  when a power source is turned on. 
     The charge pump circuit  13  will be explained in detail with reference to FIG.  4 . 
     The charge pump circuit  13  includes a voltage doubler rectifier circuit formed by an oscillation circuit  13   g , an inverter  13   a , diodes  13   d ,  13   e , capacitors  13   c ,  13   f  and a resistor  13   b . When the oscillation circuit  13   g  is supplied with the trigger signal, for example, the signal which becomes high level at the time of the turning-on of the power source from the microcomputer  19  described later, the voltage doubler rectifier circuit generates a voltage twice the amplitude of the voltage (+V) of the power source (double-amplitude voltage) only during the period where the trigger signal is supplied thereto. The voltage doubler rectifier circuit supplies the double-amplitude voltage thus generated to the first field effect transistor  7  in order to drive the first and second field effect transistors  7 ,  8  thereby to set the gate voltages of the transistors  7 ,  8  higher than the drain side voltages thereof. 
     The squib  15  is connected at its one end to the output side of the squib drive control circuit  4  and at it&#39;s the other side to the ground through a reverse current prevention diode  16  and an acceleration switch  17  connected in series. The microcomputer  19  determines the state of the collision on the basis of the acceleration signal from the acceleration sensor  18  for detecting the collision of the vehicle. When the microcomputer determines that it is necessary to operate the air bag or the like, the microcomputer supplies the ON signal to the switch circuit  14  to turn it on and simultaneously supplies the trigger signal to the charge pump circuit  13 . 
     The operation of the aforesaid arrangement of the conventional crew protection apparatus will be explained. 
     (a) When the power source is turned on, the microcomputer  19  supplies the trigger signal of a high level to the charge pump circuit  13  thereby to continuously operate the oscillation circuit  13   g  and hence always charge the second capacitor  13   f , whereby the charge pump circuit  13  outputs the double-amplitude voltage. 
     (b) In this state, if the microcomputer  19  does not output t he ON signal to the switch circuit  14 , the s witch circuit  14  is kept in an off state, so that the first and second transistors  7 ,  8  are maintained in an off state. 
     (c) In contrast, when the microcomputer  19  determines due to the occurrence of a serious accident that the collision occurred is a serious accident on the basis of the output from the acceleration sensor  18 , the microcomputer  19  outputs the ON signal to the switch circuit  14  thereby to turn on the switch circuit  14 . As a consequence, the first and second field effect transistors  7 ,  8  are supplied at the gates thereof with the voltage signals of a high level larger than the voltages of the source sides of the first and second field effect transistors  7 ,  8 , respectively, so that the first and second field effect transistors  7 ,  8  start to operate in an active area. 
     Thus, the ignition current flows into the squib  15  through the first and second field effect transistors  7 ,  8 . The magnitude of the shunt current of the ignition current at this time is detected by the current detection resistor  9 , and the detection voltage of the current detection resistor  9  is supplied to the inverted (−) input terminal of the comparison circuit  10 . As a result, when the voltage of the inverted (−) input terminal of the comparison circuit  10  becomes larger than the reference voltage, the comparison circuit changes its output level into a low level to lower the gate voltages of the first and second field effect transistors  7 ,  8  thereby to shift the operation states thereof toward the non-conductive states. 
     However, when the first and second field effect transistors  7 ,  8  approach toward the non-conductive states, the voltage of the positive voltage side of the current detection resistor  9  decreases. When the voltage of the positive voltage side of the current detection resistor  9  becomes smaller than the reference voltage applied to the comparison circuit  10 , the output of the comparison circuit  10  becomes high level, so that the output voltage of the charge pump circuit  13  is outputted again through the switch circuit  14 . Accordingly, the gate voltages of the first and second field effect transistors  7 ,  8  increase and shift again toward the conduction states in the active areas. 
     Hereinafter, the aforesaid operation is repeated during the period where the switch circuit  14  is turned on so that the constant current flows into the second field effect transistor  8 . As a consequence, the constant current is supplied to the squib  15 . Of course, the acceleration switch  17  is turned on in this state. 
     However, according to the aforesaid conventional passenger protection apparatus, there is a possibility that the microcomputer may be accidentally damaged simultaneously with the turning-on of the mechanical type acceleration switch  17 , so that the switch circuit  14  may be turned on. 
     Also, in the case where a current limiter circuit or the second field effect transistor  8  is turned on in order to conduct failure diagnosis, when the acceleration switch  17  is turned on, or a panel connected to the squib  15  is grounded accidentally, there is a fear that the ignition current flows into the squib  15 . 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in view of the aforesaid conventional problem and intends to control the output of an ignition signal to a squib by using a plurality of output signals from a microcomputer with a simple configuration. 
     In order to achieve the aforesaid object, a passenger protection apparatus according to the present invention comprises a DC power source; a squib connected in series with the DC power source; a plurality of switch means connected to a positive voltage side and a negative voltage side of the squib; an acceleration sensor for detecting an acceleration; a collision determination means for determining a scale of a collision on a basis of an acceleration signal from the acceleration sensor and outputting a current control signal together with an ignition control signal when it is determined that the collision is a serious collision; and a current limit circuit for turning on each of the plurality of the switch means to flow an ignition current through the squib and limiting a magnitude of the ignition current in accordance with the ignition control signal and the current control signal from the collision determination means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit block diagram showing the arrangement of a crew protection apparatus according to an embodiment of the present invention; 
     FIG. 2 is a circuit block diagram showing the arrangement of a crew protection apparatus according to another embodiment of the present invention; 
     FIG. 3 is a circuit block diagram showing the arrangement of a conventional crew protection apparatus; 
     FIG. 4 is a circuit diagram showing the arrangement of a charge pump circuit in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a description will be given in more detail of preferred embodiments of the invention with reference to the accompanying drawings. 
     Embodiment 1. 
     FIG. 1 shows the arrangement of a passenger protection apparatus according to an embodiment of the present invention. In FIG. 1, like or same parts as those explained in the prior art of FIG. 3 are designated by the same reference numerals and the detailed explanation thereof is omitted. 
     In FIG. 1, reference numerals  18 ,  20  depict first and second acceleration sensors for detecting the acceleration at the time of collision. The first and second acceleration sensors  18 ,  20  have the same efficiency and detect the acceleration to the same direction. A reference numeral  21  depicts a microcomputer formed by a comparison circuit (comparison means)  22 , a collision determination/failure diagnosis circuit (collision determination/failure diagnosis means)  23 , a switch circuit (switch means)  24  or the like. The comparison circuit  22  compares the magnitude of an acceleration signal supplied from the second acceleration sensor  20  with a reference value, and when the magnitude of the acceleration signal exceeds the reference value, the comparison circuit  22  determines that a collision has occurred and supplies a switch signal to the collision determination circuit  23 . 
     The collision determination/failure diagnosis circuit  23  receives an acceleration signal supplied from the first acceleration sensor  18  and the switch signal from the comparison circuit  22 . When the collision determination/failure diagnosis circuit  23  determines that a collision is a serious collision, the collision determination/failure diagnosis circuit  23  outputs a high level signal (current limit signal) to the switch circuit  24 , outputs a high level signal (ignition control signal) to a transistor  25   a  of a drive control circuit  25  thereby to turn off a transistor  25   b , and further supplies a high level signal to a switching transistor  26 . The function of the mechanical type acceleration switch  17  shown in FIG. 3 is formed by one of collision determination functions of the second acceleration sensor  20 , comparison circuit  22 , switching transistor  26  and collision determination/failure diagnosis circuit  23 . 
     The switch circuit  24  is normally in an off state. At the time of the occurrence of a serious collision, the switch circuit  24  becomes low level at its output in response to the high level signal from the collision determination/failure diagnosis circuit  23 . 
     A reference numeral  27  depicts a constant-current control circuit formed by a comparison circuit  28 , a control transistor  29 , a reference current detection resistor  30 , a first current adjustment resistor  31 , a second current adjustment resistor  32 , a comparison circuit  33 , drive transistors (switch means)  34   a ,  34   b , a current detection resistor  35  or the like. The comparison circuit  28  compares a set reference voltage with the non-grounded side voltage VO of the first current adjustment resistor  31 . The comparison circuit  28  holds the voltage VO of the non-grounded side terminal of the first current adjustment resistor  31  constant and so a value of a constant current Ia is determined by the first and second current adjustment resistors  31 ,  32 . 
     The constant current Ia is converted into a voltage by the reference current detection resistor  30  and the voltage thus converted is supplied to the non-inverse side (+) input terminal of the comparison circuit  33  through a signal line A. The comparison circuit  33  compares the voltage thus converted with the collector voltage of the drive transistor  34   a . The comparison circuit  33  functions such that the voltage across the reference current detection resistor  30  and the voltage across the current detection resistor  35  at the negative potential side, respectively, become identical with each other, whereby the current Ib which is (a value of the current detection resistor  30 /a value of the current detection resistor  35 )-times as large as the collector current Ia of the drive transistor  34   b  flows into the drive transistor  34   b . The drive transistor  34   a  has a current capacity of about 100 times as large as that of the drive transistor  34   b . The current which flows between the collector and the emitter of the drive transistor  34   a  is about 10 times as large as a current value Ib which flows into the drive transistor  34   a.    
     The output side of the comparison circuit  33  is connected to the collector of the transistor  25   b  of the drive control circuit  25 . When the transistor  25   b  is in an off state, the comparison circuit  33  compares the current value Ia which is set by the first and second current adjustment resistors  31 ,  32  with the collector current Ib of the drive transistor  34   a  thereby to control the on-state of the drive transistor  34   a.    
     In contrast, when the transistor  25   b  is in an on state, since the output terminal of the comparison circuit  33  is fixed at a low level, both the drive transistors  34   a ,  34   b  are not controlled in an on state. 
     The function of the aforesaid arrangement of the passenger protection apparatus according to the embodiment will be explained. 
     (1) In the case where the collision determination/failure diagnosis circuit  23  does not determine that a collision has occurred. 
     Since the switch circuit  24  is in an off state, the value of the current flowing through the first and second current adjustment resistors  31 ,  32  is a set current value, that is, a small current value which is insufficient to ignite the squib  15 . Thus, even if the microcomputer  21  causes the program error or crash and so the collision determination circuit  23  supplies the high level signal to the drive control circuit  25  and the switching transistor  26 , only a small current flows into the squib  15 , so that the squib  15  can not be ignited. 
     Further, the probability of occurrence of such a phenomenon is small that the collision determination/failure diagnosis circuit  23  changes its output signal supplied to the switch circuit  24  into a high level thereby to change the voltage of the connection point between the first and second current adjustment resistors  31 ,  32  into a low level. That is, the probability of occurrence of such a phenomenon is very small that the three output terminals of the collision determination circuit  23  simultaneously change into a state for flowing an ignition current into the squib  15 . 
     (2) In the case where the collision determination/failure diagnosis circuit  23  determines that a collision has occurred. 
     Since the switch circuit  24  changes its state into an on state, the connection point between the first and second current adjustment resistors  31 ,  32  is grounded. Accordingly, the current Ia of a large value flows through the resistor  31 , then the current Ia is detected by the current detection resistor  30  and the voltage corresponding to the detected current is supplied to the comparison circuit  33 . In this case, since the drive control circuit  25  is in an off state and the switching transistor  26  is in an on state, both the drive transistors  34   a ,  34   b  are turned on and hence the ignition current is supplied to the squib  15 . 
     (3) In the case of conducting failure diagnosis of the constant current control circuit. 
     The collision determination/failure diagnosis circuit  23  has a failure diagnosis function in addition to the collision determination function, and the failure diagnosis function starts instead of the collision determination function. The failure diagnosis function is conducted by temporarily sampling the non-grounded side potential of the squib  15  while a low-level signal is supplied to the switching means  24  by the collision determination/failure diagnosis circuit  23 , that is, while a node of the first and second current adjustment resistors  31  and  32  is in a non-contact state. 
     That is, while the switching means  24  is off (while the ignition current is not outputted), the node of the first and second current adjustment resistors  31  and  32  is brought in the non-grounded state so that the amplitude of the current Ia flowing in the reference current detection resistor  30  is adjusted so as to be a micro-current. 
     While the ignition current is not outputted (while a low-level signal is supplied to the switching means  24 ), the collision determination/failure diagnosis means  23  makes the transistor  25   b  of the drive control circuit  25  intermittently turn off, thereby turning on the drive transistor  34   b  so that a micro diagnosis current flows through a diagnosis resistor not shown. In this situation, with a potential developed at the non-grounded side of the squib  15  being inputted to the collision determination/failure diagnosis means  23 , the drive transistors  34   a  and  34   b  are subjected to failure diagnosis, and if occasions demand, the diagnosis result is displayed in a display unit. 
     With the above structure, even if, for example, harnesses to the squib  15  (corresponding to a line connecting the drive transistor  34   b  and the squib  15  and a line connecting the squib  15  and the switching transistor  26 ) are grounded, since the two first and second current adjustment resistors  31  and  32  are in the non-grounded state, and a current flowing in the drive transistor  34   b  and the squib  15  is adjusted to be a micro diagnosis current, there is no case where the ignition current flows. 
     Embodiment 2 
     In a second embodiment, the constant-current circuit  11  in FIG. 3 may be structured by a circuit shown in FIG.  2 . 
     In FIG. 2, a comparison circuit  28 , a control transistor  29 , current adjustment resistors  31 ,  32  and switching means  24  are connected in the same manner as the circuit structure shown in FIG. 1, and a phase inversion circuit  35  is connected between the control transistor  29  and the power line, that is, at the positive potential side of the control transistor  29 . 
     In this case, the switching circuit  24  turns on in response to a high-level signal (current limiter signal) from the microcomputer  19 , and the ignition control signal in FIG. 1 corresponds to an on-signal which is supplied to the switch circuit  14  from the microcomputer  19 . 
     As was described above, the present invention can reduce a probability of occurrence of such a phenomenon that an ignition current flows into the squib due to the crash of the microcomputer. 
     Also, since the switch means is comprised of transistors, it is inexpensive more than the mechanical switch. 
     Further, since the current limit circuit is comprised of a resistor group, the circuit is manufactured inexpensively. 
     Further, since a group of resistors are connected in series, the structure is simple. 
     Further, since a constant-voltage is applied to one end of the resistor group, a constant current can be readily obtained.