Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Patent
1998-04-03
1999-09-28
Cuchlinski, Jr., William A.
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
280735, B60R 2132
Patent
active
059579889
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a control circuit for controlling power supplied to an igniter element and an igniter system.
BACKGROUND OF THE INVENTION
Igniter devices are used commonly in applications such as airbag deployment and seat belt pretensioner activation in cars. The igniter device comprises an igniter element which converts electrical energy to heat. Typically, the igniter element comprises a hot wire bridge which is heated by a current of 1 Amp (A) or more. In, for example, airbag applications, the heat generated in the igniter element ignites a pyrotechnic material adjacent the heater element which burns a propellant. This produces gas to inflate the airbag.
A particular concern to automotive manufacturers is the possibility of igniter devices firing inadvertently due to a fault. For example, inadvertent firing of an airbag igniter device could have serious consequences for a driver whilst driving. Two of the major type of faults which can occur in igniter devices are: 1) improper connections to either side of the igniter element; and 2) faulty igniter elements themselves. It is therefore common to have both high and low side switches coupled to the igniter element by wires to ensure that a fault, such as improper connection to the igniter element, does not fire the igniter device.
However, the igniter device may still be inadvertently fired due to Electrical Static Discharge (ESD) and/or RF energy induced in the long wires between the high and low side switches and the igniter element. This problem can be mitigated by using a hot wire bridge igniter element which has a low resistance of 2 ohms and 1.5 Amps (A) for 3 ms firing conditions. That is, 13 milli-Joules (mJ) of energy is required to fire the hot wire bridge. The required firing energy is thus relatively high compared to the typical ESD and RF induced energies and thus this solution provides some protection against induced currents. However, in view of the increase in RF sources such as GSM radios, repeaters and electromagnetic interferences, automotive manufacturers consider that this form of protection is not sufficient.
A further disadvantage of using a low resistance igniter element is that the high and low side switches have to supply a current of 1 A or more and therefore require the use of oversized MOSFET power transistors which have an on-resistance R.sub.dson in the range of 2 ohms, like the igniter element itself. Such high and low side switches are therefore expensive and require large die area. Furthermore, about 66% of the available energy is lost through the high and low side switches which means that the efficiency of the firing loop is relatively low. Under these conditions, the level of energy that must be stored in the reservoir capacitors, which are used to fire an airbag igniter element should the battery be disconnected or shorted during a crash, is very high.
Airbag igniter devices are safety devices which are not intended to explode in normal driving conditions but must explode when fired due to an accident. It is therefore necessary to make regular diagnostics to ensure that the igniter is able to work in case of an accident over the given lifetime of the airbag equipment (typically 15 years). In fact, airbag manufacturers are now requiring that diagnostics be provided for all airbag igniter devices.
A typical diagnostic system uses current limited voltage sources or current sources to test for improper connections to either side of the igniter element. A diagnostic current which is less than the current required to fire the igniter element is applied and the voltage at one or the other end of the igniter element is measured to check for shorts to battery, shorts to ground and open firing loops. Typical diagnostic currents are in the range of 15 to 30 mA. By measuring the diagnostic voltage across the igniter element for a given diagnostic current, it is also possible to determine the resistance of the igniter element and whether the resistance changes. If the resistance of the igniter
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Conraux Jerome
Osajda Marc
Cuchlinski Jr. William A.
Dover Rennie W.
Hernandez Olga
Motorola Inc.
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