Abstract:
A SIR system has frontal air bags and side air bags both controlled by the same microprocessor. To guard against spurious deployment of side air bags with minimal software burden, a lateral accelerometer and an arming circuit detect side crash activity and apply an arming signal to a pulse accumulator circuit in the microprocessor which monitors the accumulator state to detect arming, thereby inhibiting deployment when the arming signal is absent. The arming circuit receives the accelerometer signal, removes the dc component which is subject to drift, adds a fixed offset voltage and compares the resultant signal to threshold values to produce an arming signal when a threshold is breached.

Description:
FIELD OF THE INVENTION 
     This invention relates to supplemental inflatable restraint systems and particularly to an arming method and apparatus for verifying a crash event to permit air bag deployment. 
     BACKGROUND OF THE INVENTION 
     Supplemental inflatable restraints (SIRs) are commonly used in motor vehicles to deploy air bags in the event of a crash for occupant protection. A frontal impact sensor, typically an accelerometer, develops a signal in the event of a crash, and a microprocessor evaluates the signal to decide whether to deploy the air bags. An arming circuit independently senses the crash event to verify that a crash is in progress and inhibits deployment in the absence of crash evidence. A firing circuit responds to a deployment command from the microprocessor to cause bag inflation. Typically the bags are in front of the occupants and the impact sensor and the arming circuit are sensitive to acceleration in the longitudinal direction of the vehicle. Now side air bags are employed to protect against side impact; side impact sensors in the vehicle doors send deploy messages to the microprocessor. Another arming circuit including a lateral accelerometer sensitive to acceleration in the lateral direction of the vehicle provides an arming signal to the microprocessor. 
     The microprocessor uses a robust algorithm to assess the frontal impact sensor output and decides whether to deploy, and also processes the frontal arming signal as well as the side deploy messages. To maintain this level of activity and meet the demands for rapid execution of all these functions, it is desirable to avoid any significant processing burden imposed by the lateral arming circuit. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to minimize the impact of a SIR arming circuit on the algorithm throughput of a SIR microprocessor. 
     A SIR circuit includes a microprocessor for processing signals from one or more impact sensors and from arming circuits to determine whether to command air bag deployment. An arming accelerometer produces an output according to acceleration in a particular direction, the output comprising an ac signal on a dc bias voltage which is subject to drift. A signal processing circuit filters out the dc component and then adds a fixed offset voltage to the ac component, and then compares the refined acceleration signal to hardware selectable thresholds to yield an arming signal when the acceleration exceeds a threshold. 
     The microprocessor includes an accumulator circuit which operates independently of the operating software, and is set to accumulate counts when the arming signal is present. When a deployment message is received or developed by the microprocessor the recent accumulator history is sampled and deployment is inhibited if there is no accumulated count and is permitted if there is a count. Diagnostic operations search for a fault in the accelerometer and disable the inhibiting function if a fault is found. These functions are invoked only infrequently and thus add only minimal software burden. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein: 
     FIG. 1 is a block diagram of a SIR system according to the invention; 
     FIG. 2 is a schematic circuit diagram of an arming circuit of FIG. 1 according to the invention; 
     FIGS. 3 and 4 are flow charts of accelerometer diagnostics used in the controller of FIG. 1; 
     FIG. 5 is a flow chart of pulse accumulator management for the controller of FIG. 1; and 
     FIG. 6 is a flow chart of an arming method employed by the controller of FIG. 1. 
    
    
     DESCRIPTION OF THE INVENTION 
     The ensuing description is directed to an arming method and apparatus developed for use with side impact sensing, but it is not limited to that application; it will be evident that the invention applies as well to frontal Impact sensing, for example. 
     Referring to FIG. 1, a SIR controller 10 (or sensing and diagnostic module) includes a microprocessor 12 having inputs connected to a frontal impact sensor 24, a frontal arming circuit 14 (including a longitudinal accelerometer, not shown) a lateral accelerometer 16 and a lateral arming circuit 18, all within the controller 10. External inputs to the SIR controller 10 are a left side impact sensor 20 and a right side impact sensor 22 which are coupled via signal processing hardware, not shown, to the microprocessor 12. The microprocessor 12 has an output directed to the lateral accelerometer 16 and a deploy output coupled to firing circuits 26 which in turn are connected to the left side air bag 28, the right side air bag 30 and the frontal air bags 32 which are selectively deployed according to the deployment command issued to the firing circuit. 
     In operation, acceleration signals from the frontal impact sensor 24 are evaluated by an algorithm in the microprocessor 12 to recognize a crash event and to determine whether and when to deploy the frontal air bag 32. If there is a frontal crash in progress, the frontal arming circuit will issue an arming signal to the microprocessor which allows a deploy command to be issued to the firing circuits 32. If the arming sensor does not sense crash activity, the lack of an arming signal will inhibit a deploy command, thereby providing a check against a false deployment. 
     The left and right side impact sensors 20, 22 differ from the frontal sensor 24 in that they each contain a processor to evaluate whether to deploy a side air bag, and send a deploy message to the microprocessor 12 in the SIR controller. Then the microprocessor 12 has only to verify that the lateral arming circuit 18 permits deployment of a left or right air bag. In the event of a side impact the lateral accelerometer 16 will yield an acceleration signal which is processed by the arming circuit to yield an arming signal which is sent to an accumulator port of the microprocessor 12. An accumulator circuit in the microprocessor can then monitor the arming signal without affecting the algorithm throughput of the microprocessor. 
     Details of the arming circuit 18 are shown in FIG. 2. A nominal 12 v source 40 (battery) supplies voltage to each operational amplifier in the circuit. A 5 v regulated source 42 supplies reference voltages to the circuit. A buffer amplifier 44 having its output coupled to its negative input is connected by its positive input to line 46 which carries a raw acceleration signal from the lateral accelerometer 16. The buffer output is coupled through a capacitor 48 and a resistor 49 to the negative input of an inverting amplifier 50. The capacitor blocks the dc component of the input signal and passes the ac component. A voltage divider 52 comprising equal value resistors is coupled between the 5 v source and ground to produce a constant 2.5 v at the resistor junction. The junction is coupled through a resistor 54 to the positive input of the amplifier 50 so that the amplifier output will be a refined acceleration signal comprising the ac component superimposed on an offset voltage of 2.5 v. The amplifier output is coupled through a feedback resistor 56 to its negative input. The amplifier 50 output is directly connected to the negative input of a comparator 56 and to the positive input of a comparator 58. The negative input of comparator 56 is connected to the junction point of a voltage divider 60 set to provide a threshold voltage slightly higher than the 2.5 v offset, and the positive input of comparator 58 is connected to the junction point of a voltage divider 62 set to provide a threshold voltage slightly lower than the 2.5 v offset. The comparator outputs are joined to provide the arming circuit output on line 64. A diode 66 coupled between the output line 64 and the 5 v source clamps the maximum output voltage to 5 v. The threshold values are selectable by choice of suitable resistors in the voltage dividers 60 and 62. 
     In operation, the voltage offset from the accelerometer 16, which is subject to drifting, is stripped from the signal by the capacitor 48 and a fixed offset of 2.5 v is added to the resulting ac signal by the amplifier 50. The resulting refined acceleration signal is fed to the two comparators 56, 58 which yield a ground level signal as long as the acceleration signal remains between the two threshold values and a high level signal (5 v) when a threshold is breached. This result could not reliably be attained using the raw acceleration signal because the drift in the signal offset would make the comparator function erratic or even void since the offset value itself may change enough to cross a threshold at zero acceleration. Thus the substitution of the fixed offset for the drifting offset is essential to the operation of the arming circuit. 
     The software for accommodating the side arming sensor 16 and circuit 18 is shown in the flow charts of FIGS. 3-6 wherein the functional description of each block in the charts is accompanied by a number in angle brackets &lt;nn&gt; which corresponds to the reference number of the block. FIG. 3 is a lateral accelerometer diagnostic routine which is executed upon power up of the circuitry, when vehicle ignition is turned on. As shown in FIG. 1, a line 68 from the microprocessor 12 extends to the accelerometer 16. This line is used to assert an accelerometer output shift &lt;70&gt;. The accelerometer circuit is normally able to respond to the assertion by shifting the output voltage up. The voltage is sensed on output line 46 which connects to an A/D port of the microprocessor. If the shift does not occur &lt;72&gt; a lateral accelerometer fault is set and latched for the duration of the ignition cycle &lt;74&gt;. Then the line 68 is de-asserted &lt;76&gt;. If however, the shift up does occur &lt;72&gt; the line is de-asserted &lt;78&gt; and if the accelerometer signal shifts back &lt;80&gt; the test is completed, but if it does not shift back the lateral accelerometer fault is set and latched for the duration of the ignition cycle &lt;82&gt;. Thus the initial operability of the accelerometer circuit is assured or the fault is set. Another function occurring during power-up is that the pulse accumulator circuit in the microprocessor is configured to accumulate pulses when the arming signal 64 is high &lt;83&gt;. 
     FIG. 4 shows a method of testing the accelerometer output line 46 for shorts. This occurs every 5 ms triggered by a microprocessor interrupt. If a short to battery &lt;84&gt; or a short to ground &lt;86&gt; is detected, an error counter is incremented &lt;88&gt; and if the count reaches four &lt;90&gt;, a lateral accelerometer fault is set &lt;92&gt;. This result requires four consecutive short detections over 15ms. If however, no short was detected the error counter is cleared &lt;94&gt; and if the last four samples revealed no short &lt;96&gt;, and the fault has not been latched &lt;98&gt; the fault is cleared &lt;100&gt;. Thus if the fault has been latched in the FIG. 3 power up routine it cannot be cleared within the ignition cycle, but if it is not latched, four consecutive-good tests is sufficient to clear a fault. 
     Since the pulse accumulator is configured at power up to accumulate pulses when the arming signal 64 is high, there is no microprocessor activity due to monitoring the arming circuit until the arming signal occurs, except for occasional accumulator updates. As shown in FIG. 5, every 100 ms the microprocessor sets a value &#34;Previous&#34; to the current count (which reflects the recent history of the accumulator) and sets the accumulator count P.A. to zero &lt;104&gt;. 
     When either side impact sensor determines that the air bag for that side should be deployed, it sends repeated deploy messages to the microprocessor 12. This causes an interrupt in the microprocessor program. As shown in FIG. 6, when two deploy messages are received &lt;106&gt;, the state of the lateral accelerometer fault is checked &lt;108&gt;. If a fault is recorded, the deploy circuitry is turned on &lt;110&gt;. If a fault is not recorded, the current count of the pulse accumulator is checked &lt;112&gt; and the Previous count is checked &lt;114&gt;. If either value is non-zero the deploy circuitry is turned on &lt;110&gt;; if both values are zero a fault is set to indicate that there is no signal at the arming circuit &lt;116&gt; and the deploy circuitry is not activated. 
     In this manner the arming circuit inhibits deployment of either side air bag when the lateral accelerometer does not sense lateral crash activity. When the lateral accelerometer is in a failure mode the inhibiting function is disabled. Since the accumulator circuit performs the continuous monitoring of the arming signal along with the 100 ms update, the monitoring requires minimal software burden, and the 5 ms interrupt for checking shorts also adds only minimal burden. The failure check at power up has no-impact on the algorithm throughput. Thus the arming function is accomplished without significantly affecting the main algorithm throughput of the microprocessor. 
     The same arming technique can be applied to the frontal arming sensor which usually has been wholly managed by the software. Thus the software burden can be lightened by adding the simple arming circuit and using an accumulator function of the microprocessor to monitor that arming signal.