Reconfigurable air bag firing circuit

An air bag firing circuit comprises a firing path which includes in series a safing sensor, a squib, and a FET operated under microprocessor control in response to the output of an electronic crash sensor. A power supply maintains a known voltage across the firing path sufficient to explode the squib upon simultaneous "closure" of both the safing sensor and the FET operated by the microprocessor in response to crash sensor output. Normally, upon detection of a failure in the electronic crash sensor, its supporting electronics, or the FET actuated in response thereto, the microprocessor reconfigures the firing threshold of the safing sensor, as by applying a current to its integral test coil to increasingly bias the sensor's inertial mass away from its switch contacts. However, if a failure of the safing sensor is detected, reconfiguration of its threshold is inhibited notwithstanding the failure of other circuit components to prevent inadvertent deployment of the air bag. Once the safing sensor is reconfigured, the microprocessor turns on another FET to pull one side of the squib to ground, thereby removing the inoperable FET from the firing path and ensuring continued protection of the vehicle passengers until the sensor is serviced or replaced.

BACKGROUND OF THE INVENTION 
The instant invention relates to control circuits for vehicle passenger 
safety restraints, such as air bags, comprising a firing path which 
includes two acceleration sensors whose acceleration-responsive switches 
are connected in series with an explosive squib. 
Known air bag passenger restraint systems employ a control circuit wherein 
a power supply applies a voltage across a firing path which includes in 
series an explosive squib and two acceleration sensors having 
normally-open acceleration-responsive switches therein. The switch of each 
sensor is shunted by a resistor having a nominal resistance substantially 
greater than the internal resistance of the squib. Thus, a small current 
nominally flows through the firing path while the switches of the sensors 
remain in their normally-open positions. The closure of the sensors's 
switches in response to a collision or marked vehicle deceleration causes 
a significant rise in the current flowing through the squib, thereby 
"firing" the squib and triggering deployment of the air bag. See, e.g., 
U.S. Pat. No. 4,695,075, issued Sep. 22, 1987 to Kamiji et al. 
Under the prior art, if the switch of either sensor fails in its closed 
position, or with a propensity to close, the prior art teaches the 
disabling of the entire control circuit to prevent the unintentional or 
premature triggering of the passenger restraint, once again placing the 
passengers at risk. See, e.g., U.S. Pat. No. 3,889,232, issued Jun. 10, 
1975 to Bell, wherein the control circuit shuts down when one sensor 
closes without the corresponding closing of the other sensor. 
Alternatively, in our U.S. Pat. No. 4,958,851 issued Sep. 25, 1990, we 
teach a reconfigurable air bag firing circuit whose firing path comprises 
two acceleration sensors connected in series with an explosive squib. The 
firing circuit further includes means for functionally removing the 
malfunctioning sensor from the firing path by closing or shunting the 
malfunctioning sensor, thereby providing continued protection of the 
vehicle passengers under the control of the remaining, still-operable 
sensor. 
The instant invention is directed to an improved reconfigurable air bag 
firing circuit and an improved method of operating same. 
SUMMARY OF THE INVENTION 
It is an object of the instant invention to provide an improved control 
circuit for vehicle passenger safety restraints which includes two sensors 
whose acceleration-responsive switches are connected in series in the 
firing path thereof, and featuring continuing circuit viability 
notwithstanding the malfunction, or "failure," of the crash-discriminating 
sensor. 
A further object of the instant invention is to provide an improved method 
of operating such a control circuit to provide increased reliability 
notwithstanding a single-point failure therein. 
The improved control circuit for a vehicle passenger safety restraint of 
the instant invention comprises a low-threshold acceleration sensor, or 
"safing sensor," whose acceleration-responsive switch is connected in 
series with an explosive squib and a FET, with the FET closing under 
microprocessor control in response to the output of an electronic sensor 
employing a relatively-higher threshold. Upon the detection of a failure 
of the crash-discriminating electronic acceleration sensor and/or its 
supporting electronics, and after confirmation of continuing safing sensor 
functionality, the circuit reconfigures the firing circuit by raising the 
acceleration threshold of the safing sensor and then, after a suitable 
delay, removing the FET from the firing path by pulling down the side of 
the squib opposite the safing sensor to ground. In the preferred 
embodiment of the invention, the safing sensor is tested, and its 
threshold alternatively raised, by passing a current from a constant 
current source through a test coil integral to the sensor, as controlled 
by an application specific integrated circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
Referring to the drawing, an exemplary air bag firing circuit 10 according 
to the instant invention comprises a firing path 12 which includes, in 
series, the normally-open, relatively-low-threshold 
acceleration-responsive switch 14 of an acceleration sensor (hereinafter 
"safing sensor 16"); and parallel firing path legs 18a and 18b each having 
in series an explosive squib 20 for triggering deployment of a 
driver's-side and a passenger's-side air bag, respectively (both not 
shown), and a FET ("firing FET 22") for pulling down the side of each 
squib 20 opposite the safing sensor 16 to ground when operated by a 
microprocessor 24. The microprocessor 24 is itself responsive to the 
output of an electronic crash sensor integrated within an Application 
Specific Integrated Circuit ("Sensor ASIC 26"), as more fully described 
below. 
A power supply 28 applies a known supply voltage V.sub.s across the firing 
path 12 sufficient to explode each squib 20 upon the simultaneous closure 
of the safing sensor's switch 14 and the firing FET 22 connected to the 
squib 20. The power supply 28 includes a capacitor 30 and charge pump 32 
to maintain the applied voltage V.sub.s if the battery 34 connected 
thereto malfunctions or is otherwise isolated therefrom during a vehicle 
collision. 
The safing sensor 16 is itself schematically represented in the drawing as 
a normally-open acceleration-responsive switch 14 which may be closed 
irrespective of acceleration upon the passage of a current from a constant 
current source 36 through an integral test coil 38. And, by passing the 
current through the test coil 38 in the opposite direction, the nominal 
bias on the acceleration-responsive switch 14 may be increased, whereby a 
higher level of sensed acceleration is required to close the switch 14 and 
fire the squibs 20. A constructed embodiment of the safing sensor 16 is 
taught in U.S. Pat. No. 4,827,091 issued May 2, 1989, to Behr, the 
teachings of which are hereby incorporated herein by reference. 
Similarly, a constructed embodiment of the crash sensor integrated into the 
Sensor ASIC 26 is disclosed in our co-pending U.S. patent application Ser. 
No. 07/413,318 filed Sep. 27, 1989, now U.S. Pat. No. 5,060,504 issued 
Oct. 29, 1991, and entitled "Self-Calibrating Accelerometer," which 
teaching is also hereby incorporated herein by reference. Simply stated, 
the electronic sensor within the Sensor ASIC 26 provides an analog output 
proportional to vehicle acceleration, as through the incorporation of a 
piezoresistive element in the support beam of the sensor's micromachined 
cantilevered inertial mass. After analog-to-digital conversion of the 
electronic sensor's output within the Sensor ASIC 26, the resulting 
acceleration data is communicated to the microprocessor 24 via a Serial 
Peripheral Interface ("SPI 40"), whereupon the microprocessor 24 
determines whether a threshold acceleration has been exceeded, thereby 
indicating a crash condition. If a crash condition is indicated, the 
microprocessor 24 turns on the firing FETs 22 to pull down the side of 
each squib 20 opposite the safing sensor 16 to ground. 
The normally-open switch 14 of the safing sensor 16 and each firing FET 22 
are shunted by a resistor 42 of like nominal resistance. Preferably, the 
nominal resistance of the shunting resistors 42 is several orders of 
magnitude larger than the nominal internal resistance of each of the 
squibs 20. In normal operation, the shunting resistors 42 maintain a 
relatively-low current flow through the firing path 12 and, hence, through 
the squibs 20 thereof. Upon the simultaneous closure of the safing sensor 
16 and the firing FETs 22 in response to an acceleration exceeding the 
respective thresholds of the safing sensor 16 and the electronic crash 
sensor within the ASIC 26 (as determined by the microprocessor 24), the 
shunting resistors 42 are shorted and the current flowing through each 
squib 20 increases to a value above the firing threshold thereof to 
explode same and trigger deployment of each air bag. 
The instant circuit 10 further comprises an Application Specific Integrated 
Circuit ("Diagnostic ASIC 44") for diagnosing a failure of the Sensor ASIC 
26 to properly respond to acceleration, as through interpretation of 
Sensor ASIC data communicated thereto via the SPI 40. An exemplary method 
for testing the integrity of the firing path of an air bag firing circuit 
is taught in U.S. Pat. No. 4,851,705 issued Jul. 25, 1989, to Musser et 
al, the teachings of which are hereby incorporated herein by reference. 
An additional FET ("reconfiguration FET 46") is connected to the firing 
path 12 at points on each leg 18a and 18b between the squib 20 and the 
firing FET 22 thereon via a diode 48, with the reconfiguration FET 46 
being controlled by the Sensor ASIC 26. The reconfiguration FET 46 allows 
the Sensor ASIC 26 to pull the side of each squib 20 opposite the safing 
sensor 16 to ground when the Diagnostic ASIC 44 detects a failure of the 
Sensor ASIC's integral electronic crash sensor or its supporting 
electronics, including failures of the microprocessor 24 or the FETs 22 
controlled by the microprocessor 24. 
Under the instant invention, reconfiguration of the circuit's firing path 
12 is controlled by the two ASICs 26 and 44, the constant current source 
36, and the microprocessor 24, as follows: in the circuit's normal mode of 
operation, the microprocessor 24 initiates firing-path reconfiguration 
through the use of a watchdog timer in the Diagnostic ASIC 44. 
Specifically, the microprocessor 24 periodically resets the timer by 
sending reconfiguration pulses 50 to the Diagnostic ASIC 44. If the 
microprocessor 24 detects a failure of the Sensor ASIC 26, e.g., the 
failure of its electronic crash sensor to properly respond to 
acceleration, or excessive electromagnetic interference ("EMI"), the 
microprocessor 24 stops transmitting reconfiguration pulses 50 to the 
Diagnostic ASIC 44, and the timer runs out to trigger reconfiguration. 
Similarly, the microprocessor 24 will request reconfiguration of the 
circuit's firing path 12 upon detecting a failure of any of the firing 
FETs 22 or the reconfiguration FET 46. A suitable period for the watchdog 
timer is believed to be about 250 msec. 
The Diagnostic ASIC 44 also monitors the microprocessor 24 through a 
deadman timer. In normal operation, the microprocessor 24 periodically 
sends a deadman signal 52 to the Diagnostic ASIC 44 to confirm its 
continuing operability. Upon cessation of the deadman signal 52 or other 
detection of a failure of the microprocessor 24 over the SPI 40, the 
Diagnostic ASIC 44 sends a reset signal 54 to the microprocessor 24 in an 
attempt to return the circuit 10 to full functionality. Should the 
microprocessor 24 fail to respond to a reset, the above-described watchdog 
timer will run out, again causing the Diagnostic ASIC 44 to initiate the 
reconfiguration sequence, as described more fully below. Thus, the use of 
the watchdog timer permits the Diagnostic ASIC 44 to reconfigure the 
firing path 12 notwithstanding the failure of the microprocessor 24, 
thereby enhancing the reliability of the instant circuit 10. 
The Diagnostic ASIC 44 may also conduct periodic dynamic testing of the 
safing sensor 16 under microprocessor control by sending appropriate 
signals 62, 64, and 66 through the PHASE, I.sub.0, and I.sub.1 terminals 
of the constant current source 36, which in turn directs a current 60 in a 
first direction through the sensor's test coil 38 while monitoring the 
voltage at point 58 on the firing path 12. 
Once triggered, the reconfiguration sequence for the instant circuit 10 is 
as follows: the Diagnostic ASIC 44 first determines whether the safing 
sensor 16 has been shorted to ground by monitoring the voltage at a point 
58 on the firing path 12 between the safing sensor 16 and both squibs 20. 
If continuing safing sensor functionality (and firing path integrity) is 
confirmed, the Diagnostic ASIC 44 will send signals 62, 64, and 66 through 
PHASE, I.sub.0 and I.sub.1 terminals of the constant current source 36, 
respectively, whereby the current 60 is directed in a second direction 
through the sensor's test coil 38 to increase its threshold by 
increasingly biasing its switch 14 in the open position. When the current 
source 36 is turned on, the current source 36 also generates current sense 
pulses 68 which are counted by the Sensor ASIC 26. After a suitable number 
of pulses 68 are counted by the Sensor ASIC 26, thereby representing a 
reasonable time delay to permit the reconfigured safing sensor 16 to 
achieve a steady-state heightened threshold, the Sensor ASIC 26 turns on 
the reconfiguration FET 46 to pull down the sides of the squibs 20 
opposite the safing sensor 16 to ground. The firing path 12 of the instant 
circuit 10 is thus reconfigured, with the heightened-threshold safing 
sensor 16 thereafter operating as the circuit's crash-discriminating 
sensor. 
Preferably, the Sensor ASIC 26 counts the current sense pulses 68 only when 
the PHASE input to the Sensor ASIC 26 is high, thereby preventing an 
inadvertent increase in the safing sensor's threshold upon malfunction of 
the current source 36. 
If the monitored voltage at point 58 on the firing path 12 indicates a 
shorted safing sensor 16, the Diagnostic ASIC 44 terminates the 
reconfiguration sequence, since it otherwise might result in inadvertent 
deployment of the air bags if the reconfiguration FET 46 would thereafter 
be turned on. 
Under the instant invention, there is no need to reconfigure the system 
upon single-point failure of either the Diagnostic ASIC 44 or the current 
source 36, as the instant circuit 10 will continue to operate in its 
normal mode through continued operation of the Sensor ASIC 26 and the 
microprocessor 24. And, as noted above, in the event of a failure of the 
Sensor ASIC 26, reconfiguration may be effected by the Diagnostic ASIC 44 
and the microprocessor 24. Similarly, in the event of a failure of the 
microprocessor 24, reconfiguration remains possible by virtue of continued 
operation of the Diagnostic ASIC 44, the current source 36, and the Sensor 
ASIC 26. 
While the preferred embodiment of the invention has been disclosed, it 
should be appreciated that the invention is susceptible of modification 
without departing from the spirit of the invention or the scope of the 
subjoined claims. For example, under the instant invention, the Sensor and 
Diagnostic ASICs 26 and 44 may be repackaged so as to place all 
reconfiguration control in a separate Reconfiguration ASIC which is 
therefore wholly independent from the components providing diagnostic 
capability. Such a reconfiguration ASIC would preferably incorporate 
V.sub.s, current sense, and voltage monitoring inputs; PHASE, I.sub.0, 
I.sub.1, reconfiguration FET control, and reconfiguration pulse outputs; 
and SPI communication with other circuit components regarding electronic 
sensor output, test signal requests, and other circuit component status 
communication.