1. Field of the Invention
The present invention relates to an ignition control method in a passive safety device for a vehicle, and in particular to an ignition control method in a passive safety device having air bags and pretensioners.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Patent No. 2559830, an air bag system having a back-up condenser is known. This air bag system is normally operated with power supplied from an onboard battery. When the battery power supply is shut off, the system is operated with power supplied from the back-up condenser.
If a plurality of air bags are provided on a vehicle, the back-up condenser may be required to supply power to the plurality of air bags after the battery power supply is shut off. Additionally, requests of operation for the plurality of air bags may be generated with certain time lags. Thus, in a vehicle provided with a plurality of air bags, the back-up condenser must have a relatively long back-up time.
When an air bag is inflated, a path may be formed which short-circuits the ignition circuit corresponding to the inflated air bag. If the air bag is continued to be supplied with power in such a situation, the power stored in the back-up condenser is wasted by the inflated air bag. In this case, a sufficiently long back-up time cannot be achieved.
In the above-mentioned conventional air bag system, when electric energy charged in the back-up condenser has decreased to a predetermined extent, it is determined that one of the air bags is supplied with an ignition signal, and a power supply line to that air bag is shut off. Therefore, it is possible to positively prevent the inflated air bag from being supplied with power, so that electric power of the back-up condenser is not wasted. Thus, according to the conventional air bag system, it is possible to achieve a sufficiently long back-up time in a vehicle provided with a plurality of air bags.
However, the back-up time expected in the conventional air bag system is as short as 100 to 200 milli seconds. That is, the conventional air bag system is designed so as to be used to control front and side air bags which must be operated immediately after a phenomenon such as a collision has occurred which phenomenon could cause the shutoff of the battery power supply. Thus, the conventional air bag system can sufficiently satisfy a desired performance with the above-mentioned very short back-up time.
When a roll over of a vehicle has occurred, a passenger's head can be effectively protected if, for example, an air bag is inflated near the passenger's head. A roll over of a vehicle may occur a few seconds after a phenomenon causing the shutoff of the battery power supply has occurred. Thus, when an air bag for protecting a passenger's head (hereinafter referred to as a head-protecting air bag) is provided on a vehicle, a considerably long back-up time is required as compared to a case in which only front and side air bags are provided.
In a large-scale air bag system having the front air bags, the side air bags, and the head-protecting air bags, power consumption of an ignition control device becomes large. In such an air bag system, it is not possible to achieve a sufficiently long back-up time by using the conventional technique, that is, by shutting off power supply to the inflated air bag.