Air Bag Control Module: Description and Operation
INTERNAL DESCRIPTION
The Central Airbag (ZAE) module is comprised of several subsystems.
Acceleration Sensor
The acceleration sensor consists of two piezoelectric accelerometers, set at 45 degree forward angles, that convert acceleration (in this case, deceleration) to voltage levels. Unlike the wheelhouse-mounted impact sensors used in the past, this device can not only detect an impact, it can also sense the magnitude and direction of the impact. The Acceleration Sensor is contained within the ZAE module, and replaces the external impact sensors.
Analog/digital Converter
The analog/digital converter uses voltage levels from the Acceleration Sensor and converts them to digital numbers for use by the Processor.
Safing Sensor
The safing sensor is a mechanical impact sensor, with a low threshold. It is monitored in conjunction with the Acceleration Sensor. In order for deployment to occur, the Safing Sensor must detect an impact, and the Acceleration Sensor voltage must be higher than the deployment threshold. In addition, both sensors must "activate" virtually simultaneously, or a diagnostic trouble (fault) code will be set and no deployment will occur. This timing check makes certain that the signals from both the Acceleration Sensor and the Safing Sensor are valid.
Energy
The energy subsystem simply stores enough electrical energy to deploy all Supplemental Restraint System (SRS) devices, even if a power failure of up to 100ms occurs. In addition, there is enough reserve power to ensure that a proper impact message is recorded in the non-volatile memory.
Ignition Trigger(s)
The ignition trigger(s) act as a final stage transistor, directing the Energy supply to the igniter pellets when directed by the Processor. Activation time of the igniter pellets is held for a minimum of 10ms (to ensure deployment even if the impact sensing threshold is exceeded only briefly). The maximum activation time is 50ms, at which point the Ignition Trigger is reset. The Ignition Trigger is activated again if the impact threshold is again exceeded.
Processor
The processor uses all ZAE inputs as well as information from the sensors to control the Ignition Trigger(s), and thereby controls the deployment of airbags. The Processor performs all self-tests, monitoring, and fault indication functions. In addition, the Processor has the ability to disable the Ignition Trigger(s) in the event of certain faults.
Defect Monitoring
Each time the SRS is switched on, the system does a self-test. This is indicated by illumination of the SRS Warning Lamp for five seconds, after which the lamp goes out if no faults are detected. Along with internal checks, the integrity of the following external circuits is checked:
- All igniter circuits for shorts to ground or B+, and resistance range
- SRS Warning Lamp for open or shorts to ground or B+
- Safety Belt Buckle Contacts for open, shorted, and resistance range
- Seat Occupancy Detector (SBE) message validity (when installed)
Recognized Defects
- "Minor" faults (such as seat buckle contact failure) which do not prevent the system from operating. These faults illuminate the SRS warning lamp for about two minutes, and then the lamp goes out.
- "Non-critical" faults (such as igniter resistance) which do not prevent airbag deployment. In this case, the warning lamp is illuminated continuously, but the system is still fully operational.
- "Critical" faults, such as accelerometer failure, which would not allow proper system operation in the event of an impact. The warning lamp is illuminated continuously, the Processor sets a fault and disables the system via the IGNITION TRIGGER SHUTDOWN.
The non-volatile memory has the capacity to store a minimum of 10 different faults, along with corresponding recording of how long each fault was active.
Diagnostic and Programming modes are used with the MoDiC/Service Tester to initiate testing, read out diagnostic trouble (fault) codes, read out ZAE identity, and program (MoDiC Coding 10.0) vehicle-specific information.
EXTERNAL DESCRIPTION
The illustration shows the general appearance of the ZAE module and the single connector. As with past versions of the SRS control module, the connector includes tabs on some of the pin spaces. The tabs break shorting bridges on the female connector when the harness is plugged into the ZAE module. If disconnected, spring contacts in the female connector close, shorting ignition circuits of all the pyrotechnic devices. This reduces the chance of inadvertent deployment during service.
OPERATION STRATEGY
With the use of an accelerometer type impact sensor instead of a contact type, the ZAE SRS module can distinguish between different magnitudes of impact. This information, coupled with inputs from belt latches and the seat occupancy detector (SBE), allows the ZAE module to employ a "tiered" deployment strategy in impact situations.
There are several advantages to this capability. First, in certain situations, unnecessary deployment of the front passenger airbag is avoided. In addition, pyrotechnic safety belt tensioners (if eqquiped) that are not in use (confirmed by latch contacts) are not deployed unless a sensor failure has occurred.
The most important aspect, however, of this "smart" deployment strategy is the ability to raise the airbag deployment threshold (impact tier) if the safety belt is in use. This allows the entire restraint system to respond to an impact appropriately, considering magnitude of impact, the occupancy of the vehicle, and the driver and passenger safety belt use. Note that driver and passenger situations are analyzed and handled independently. The impact tiers and the strategy for the drivers seat is listed above. The table that follows shows the effect of the various possible inputs and impact scenarios for the front passenger
MAINTENANCE
Since the control module conducts regular self-diagnosis and system checks, the only inspection necessary consists of a visual examination of external components (as indicated on the Service Maintenance Checklist), and a verification that the SRS control module memory has no stored diagnostic trouble codes (faults) in memory (as indicated by the SRS warning lamp).