Today (september 2003) the
standard method used to fire an airbag is egnitting a thin metal filament.
The current needed typically is about 2A. The resistance of most filament
used is about 2 Ohm.
To guarantee that the airbag can be fired even if the battery is already disconnected by the impact of the crash the firing energy is taken from a capacitor buffer. Usually the capacitors used are several hundred microfarad electrolytic capacitors. Sine the capacitors (especially at cold) have a non negligible equivalent series resistance, the semiconductor switch has a resistance and the battery voltage might be very low the voltage in the capacitors is boosted to a typical value of 25V to 30V. So most systems consist of a switch mode power supply charging the capacitors, the switches to fire the filament and extensive diagnosis features to verify that the system is ready and operatable.
Fig. 1: Concept of an Airbag System
The system consists of the following functions:
1. The filament used to ignit
the airbag (SQUID)
2. The switches to fire the airbag (M11 and M12)
3. The energy reservoir (C1)
4. The charger for C1 (L1, D1, M1, OP1, R1..R3,PWM)
5. Monitoring of the charge state of C1 (Comp1)
6. Monitoring of the ignitter and the switches (I11..I14, V11, AMP11, R11..R14, ADC)
7. The control logic
Check of total path resistance:
AMP11 is used to measure the resistance of the filament SQUID. To verify the resistance the amplifier must be operated in a propper operating point. So before measuring the logic turns on V11 and one of the current sources I11 or I12. The test currents are choosen low enough to be sure the airbag will not fire.
Using low currents (about 10mA) the voltage drop measured by the instrumentation amplifier (AMP11, R11 to R14) is close to the offset voltage of the amplifier AMP11. Therefore the measurement is done with two different currents. The resistance of the filament then calculates as:
RSQUID = (V2-V1) / ((I12-I11)*gain)
"gain" is the gain of the
instrumentation amplifier. To exploit the dynamic range of the ATD the
control logic can adjust the gain of the instrumentation amplifier. (What
is shown here as an instrumentation amplifier can of course be implemented
totally different. The concept shows an instrumentation amplifier because
this is the most commonly known structure used for differential measurements.)
The calculation is performed by the control logic.
Detection of shorted cables:
To detect short circuits the small current generators I11 and I13 are used.
With V11 disconnected and I12 and I13 enabled the voltage of both nodes of the filament must go high (current of I12 is higher). If either one of the wires is shorted to ground this can be detected. (Some systems use dedicated comparators. In the circuit shown the instrumentation amp can be reconfigured making R11 low resistive and R13 high resistive such that AMP11 operates as a non inverting amplifier with a gain close to 1 w.r.t. ground.)
If I12 and I14 is on the voltage will go low (current of I14 is higher). This configuration is used to detect short circuits to a supply voltage (connecting battery to a cable or bridging M11).
Note that always two current sources with different currents are used to prevent missmeasurements resulting from possible floating nodes and capacitive coupling of wires.
Once it is certain that the system has no shorted cables the switches M11 and M12 can be tested.
To test M11 the logic will turn on M11 and I13. The expected behavior is that both cables go high.
To test M12 the logic will turn on M12 and I11. Now the expected behavior is tha both cables go low.
The tests MUST be performed in the sequence shown here. The switches may not be tested before a short circuit of either one of the cables connecting the SQUID to one of the supply rails is excluded. (If a short is present during the switch test the airbag will fire!).