AIR BAG SYSTEM FOR PROTECTION OF THE OCCUPANTS OF MOTOR VEHICLES

Abstract: An air bag system for protecting the occupants of a motor vehicle in the event of a collision has a plurality of firing circuits comprising bag igniters (3a, 3b, 3c) in series with respective power transistor (T1, T2, T3) which can be triggered in the event of an accident to inflate a corresponding plurality of air bags. A single energy storage capacitor (E) is connected to all of the detonation circuits for supplying energy to activate the bag igniters over the upper common power stage (7) in the event that there is a loss of battery voltage when the system has been actuated. A comparator (9) monitors the voltage drops across respective resistors (R1, R2, R3) in series with the power transistors (T1, T2, T3) and reduces the current through these transistors in the event that an excessive current is detected. Steps are taken to ensure that the period of activation of the current supply to the igniters is limited.

Description: DESCRIPTION AIR BAG SYSTEM FOR PROTECTION OF THE OCCUPANTS OF MOTOR VEHICLES State of the Art The present invention relates to an air bag system for protection of the occupants of a motor vehicle, of the kind described in the precharacterising clause of claim 1. With such systems, it is required that reliable activation of the air bags be ensured in the situation that there is a loss of battery power at a time when such activation is needed and also in the event of shunts or short circuits forming in the driver's and/or front-seat passenger's gas generators. The firing of rear- seat passengers' igniters or squibs and the vehicle crash recorder (when fitted) must also be ensured. In known systems of this kind, it is usual for an individual energy storage capacitor to be associated with each separate air bag firing circuit, both for the processing and/or the digital part of the triggering arrangement. Belt-tightening firing circuits with further individual energy reserves have also been available, at the customer's request. In the aforegoing known systems , each storage capacitor has to be charged by a voltage transformer and individually monitored with respect to its capacitance value. This involves corresponding expenditure on hardware (such as discharge circuits, ADC channels) and expenditure on software (such as ports, ROM space) . It is an object of the present invention to develop a system in which the latter expenditure can be reduced, without sacrificing reliability and safety. The aforegoing object is achieved by adopting the features set forth in the characterising part of claim 1. These have the advantage that a considerable saving on expenditure for circuitry can be obtained in that there is only one single energy storage capacitor to be monitored. Drawing The invention is described further, by way of example only, with reference to the accompanying drawing which is a circuit diagram of one embodiment of an air bag protection system in accordance with the invention for the occupants of motor vehicles. Description of the Exemplary Embodiment The drawing shows part of an air bag protection system wherein a number of firing circuits 1a, 1b, 1c are connected in parallel between a regulated battery voltage U BATT and the earthed side 2 of the vehicle battery. Each firing circuit includes a respective igniter 3a, 3b, 3c connected in series with a respective power transistor T 1 , T , , and a low ohmic resistor R^, Rg, R3. Connected in series with the three resistors ^ , R , 3 is a mechanical switch 6 of known type, which is sensitive to vehicle deceleration and which only closes when a predetermined retardation is experienced (such as would be incurred, for example, in a collision). Thus, in this embodiment, the single mechanical switch is common to all three firing circuits. The three igniters 3a, 3b, 3c are connected via a monitoring switch 7 and diodes D^ and D 2 to the regulated battery voltage Ug A ^ τ . The diodes D-, and D 2 serve as protection against incorrect battery polarisation and the monitoring switch enables the circuit to be selectively tested for correct operation. Also connected to the battery voltage at a poin downstream to the diode D 1 is a single energy storage capacitor E, preferably of the AL-ELCO type, having a large capacitance value, for example 10,000 uF. It will be noted that this single storage capacitor E is commonly connected via the high side monitoring transistor 7 to all three firing circuits 1a, 1b and 1c. The occupant protection system is triggered when, in the event of an accident, a sensor (not shown) is activate and supplies a pulsed signal, for example of 10 ms duratio to turn on the power transistors T 1 , T 2 and T, via driver stages 8 and additionally the high side power stage 7- Provided that the externally acting mechanical retardation on the vehicle has been sufficient to close the arming switch 6, current from the single energy storage capacitor can then pass through the igniters so as to inflate the associated air bags (not shown). In order to limit the current passing through the igniters to an acceptable level, the voltages on the resistors R^, R , ~^ ~ are applied to the negative inputs (-) of respective comparators 9 (only one shown) serving as firing current limiters. The positive inputs (+) of the comparators are provided with reference voltages by passing a current through respective resistors R^ from a constant current source 10 supplied with a stabilised voltage Ug j ^g. Upon the voltage at the negative input exceeding the reference voltage at the positive input of the comparator (corresponding to the current through the resistors R 1 2 3 having exceeded a predetermined value) the comparator outputs a signal to the bases of the transistors T- | , T 2 , , such as to cause the current passing through the collector-emitter paths of these transistors, and hence through the igniters 3~- . 3b, 3c to be reduced to an acceptable level. With limitation of the activation of the output stages in this manner both with respect to time and current, the required energy balance oan be handled by the single capacitor E. By way of example, a 10,000 uF Al-Elco capacitor charged to 16-17 volts is sufficient in the case of two air bags. By using only one "large" capacitor instead of the usual several (usually ) "small" capacitors one obtains, in principle, a capacitor (Al-Elco) with better characteristics (e.g. ESR' durability and reliability). In one type of capacitor used in practice, the C.U product is (approximately) constant for a given size of casing. That is to say, the higher the capacitor of a specific size of casing is charged (the relatively lower is the undervoltage switch-off threshold of the unit), the greater is the amount of energy available. Energy = C (U., 2 -U 2 2 ) 2 where U1 = charging voltage U2 = undervoltage threshold C = capacitance By incorporating output stage current limitation, there is no risk of the firing transistor T 1 , T 2» T being overloaded by current, even when the energy reserve E is charged to a relatively high voltage (e.g. U 1 = 40V). Likewise, the contacts of the mechanical switch 6 are not overloaded if the output stage current is limited as described above. Switches of this type are normally only rated at 10A for 10 ms. Current limitation to, say about 1.5A for each air bag output stage (and for each belt tightener, output stage, if present) wil always keep the operation of the switch 6 within its permitted operational range (over the entire fluctuation of the battery voltage e.g. 7V ^ U BATT ^ 16V). The amount of additional hardware necessary to achieve this system, i.e. the expenditure on circuitry for limiting the firing current per output stage, is relatively small. Furthermore, the overall size and cost of the unit are not increased significantly when using only a single energy reserve capacitor E. Although the number of igniters or squibs in the illustrated embodiment is three, there can be any desired number (e.g. driver and passenger system with belt tighteners for front and rear passengers includes 7 igniters ) .

Claims: CLAIMS 1. System for the protection of the occupants of a motor vehicle, particularly an air bag protection system, having a firing circuit (1) which is located between a battery supply voltage and the vehicle earth and which includes a plurality of bag igniters (3) f each of which is connected in series with a respective triggerable power switch (T 1 ,T 2 ,Tg), and at least one retardation sensitive mechanical arming switch (6), there being associated with the firing circuits energy storage capacitors (E) for supplying initiation energy in the event of a loss of the battery supply voltage when the power switches are actuated, characterised in that there is a single energy storage capacitor (E) associated commonly with all of said firing circuits (1a, 1b, 1c) for supplying initiation energy to those circuits over the upper common power stage (7) in the event of a loss of the battery supply voltage and in that there is provided a means (9) of limiting the current flowing through the individual triggerable power switches ( - Tp . Tg . and hence through the igniters (3), from the energy storage capacitor (E). 2. System according to claim 1, wherein, in addition .to limiting the current which is passed through the power switches by the energy storage capacitor (E) , there is provided a means of limiting the time duration for which the power switches are actuated. 3. System according to claim 1 or 2, wherein the current limiting means comprises a resistor (R^R^R,) in series with each power switch (T 1 ,T 2 ,To), means for detecting the voltage drop across said resistors, comparin same with a reference voltage and in the event that the voltage drop exceeds said reference voltage controlling th power switches (T- T^T to reduce the current therethrou H . System according to claim 3, wherein the power switches are transistors and wherein the voltage drops across the resistors and the reference voltage are compared in one or more comparators, the outputs of which are applied to the transistors such as to reduce the current passing therethrough. 5. System according to claim 2, or claims 3 or -i when appendant to claim 2, wherein the time duration for which the power stages are actuated is limited by using pulsed trigger signals for actuating said power switches.

Inventors: CRISPIN NORBERT, JEENICKE EDMUND, MATTES BERNHARD, SCHUMACHER HARTMUT, CRISPIN, NORBERT, JEENICKE, EDMUND, MATTES, BERNHARD, SCHUMACHER, HARTMUT
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: B60R21
IPC codes: B60R21