Abstract:
A method for controlling an airbag system is provided. The airbag system includes an airbag module which may be deployed at different rates to optimize performance of the airbag module to a given impact. The airbag system includes a control module, an impact sensor, a seatbelt restraint sensor and an airbag module. The airbag module includes an inflator having two independent squibs, a first one of the squibs being coupled to a first charge and a second one of the squibs being coupled to a second charge. The method monitors various vehicle dynamics and controls the generation of first and second squib pulses which cause the deployment of the first and second charges so as to optimize the rate at which the airbag is deployed for an impact of a given magnitude. The method also monitors the integrity of each squib circuit and when a fault in a squib circuit is detected, the deployment algorithm is modified to ensure that the airbag will deploy.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to occupant restraint systems and more particularly to a method for controlling the deployment of an airbag. 
     2. Discussion 
     Modern airbag systems generally include an airbag control module (ACM) and one or more single-stage airbag modules. The ACM typically includes software and hardware to diagnose the airbag system, one or more accelerometers to sense acceleration or deceleration, and a software algorithm to ascertain the severity of an impact and decide whether or not to deploy an airbag module. 
     Generally, when an airbag module is deployed, the ACM generates an electrical signal through the vehicle wire harness to a squib which causes the rapid combustion of a pyrotechnic charge, producing gases which inflate the corresponding airbag. Modern production vehicles do not use multiple inflation rates in deploying the airbags due to the complexity and cost of the prior art systems. Consequently, the rate in which the airbags are deployed is not controlled to optimize the performance of the airbag system to the magnitude of a given impact. As the speed with which the vehicle occupants move relative to the vehicle during a collision depends upon the speed of the collision, optimization of the rate of deployment can be utilized to tailor the deployment of an airbag to minimize risk of injury from contact between the vehicle occupant and the vehicle as well as between the vehicle occupant and the airbag. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide an airbag system which can vary the rate of deployment of an airbag according to the magnitude and severity of an impact. 
     It is a more specific object of the present invention to vary the rate of airbag inflation through the use of an inflator having two independent squibs. 
     It is another aspect of the preset invention to provide an airbag system having an improved ability to deploy an airbag where a fault in a squib circuit has been detected. 
     To achieve these objects, a method for controlling an airbag system is provided. The airbag system includes an airbag module which may be deployed at different rates to optimize performance of the airbag module to a given impact. The airbag system includes a control module, an impact sensor, a seatbelt restraint sensor and an airbag module. The airbag module includes an inflator having two independent squibs, a first one of the squibs being coupled to a first charge and a second one of the squibs being coupled to a second charge. The method monitors various vehicle dynamics and controls the generation of first and second squib pulses which cause the deployment of the first and second charges so as to optimize the rate at which the airbag is deployed for an impact of a given magnitude. The method also monitors the integrity of each squib circuit and when a fault in a squib circuit is detected, the deployment algorithm is modified to ensure that the airbag will deploy. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram of an airbag system according to the preferred embodiment of the present invention; 
     FIG. 1B is a schematic diagram of the electrical pulses generated by the control module to initiate combustion of the first and second charges in the inflator; 
     FIG. 2 is a flowchart detailing the airbag deployment method of the present invention; 
     FIG. 3 is a flowchart detailing the airbag deployment method of an alternate embodiment of the present invention; 
     FIG. 4 is a flowchart for a diagnostic subroutine for detecting squib circuit faults. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1A of the drawings, an airbag system according to a presently preferred embodiment of the present invention is schematically indicated by reference numeral  10  and is shown operatively associated with vehicle  12 . Airbag system  10  is shown to include a control module  14 , an impact sensor  16 , a seatbelt restraint sensor  18  and an airbag module  20 . Impact sensor  16  is operable for generating an impact signal in response to a collision between vehicle  12  and another object. Impact sensor  16  may be an accelerometer  22  and preferably produces an impact signal which continuously varies in response to the magnitude of the collision. Control module  14  includes a microprocessor  24 , memory  26 , and a timer  28 . Control module  14  receives the impact signal from impact sensor  16  as well as other signals relating to the vehicle impact dynamics in order to determine the severity of a collision. The other impact dynamics could include, for example, a flag generated by the seatbelt restraint sensor  18  indicating that the vehicle occupant is restrained by a seatbelt. 
     Airbag module  20  includes an inflator  40 , first and second squib circuits  42  and  44 , respectively, and an airbag  46 . First squib circuit  42  includes a negative circuit element  50 , a positive circuit element  52  and a first squib  54  which is coupled to inflator  40  proximate a first charge  55 . Negative and positive circuit elements  50  and  52  electrically couple first squib  54  to control module  14 . Second squib circuit  44  includes a negative circuit element  60 , a positive circuit element  62  and a second squib  64  which is coupled to inflator  40  proximate a second charge  65 . Negative and positive circuit elements  60  and  62  electrically couple second squib  64  to control module  14 . 
     When it becomes necessary to deploy airbag  46 , control module  14  generates a series of pulses according to the method of the present invention discussed in detail, below. First and second squib pulses  66  and  68  as shown in FIG. 1B are representative of the type of electrical signals produced by control module  14 . Generation of first squib pulse  66  causes first squib  54  to heat and ignite first charge  55 . Combustion of first charge  55  produces a first volume of gas which is directed into airbag  46  causing it to inflate at a first rate. Generation of second squib pulse  68  causes second squib  64  to heat and ignite second charge  65 . Combustion of second charge  65  produces a second volume of gas which is directed into airbag  46  causing it to inflate at a second rate. As one skilled in the art may appreciate, the generation of first and second squib pulses  66  and  68  may be controlled by control module  14  so as to optimize for a given collision the timing of the airbag inflation, as well as the rate at which the airbag  46  is inflated. As one skilled in the art may also appreciate, first and second charges  55  and  65  may be sized differently (i.e., non-equally) to further optimize the performance of airbag module  20 . 
     Referring now to FIG. 2, the method for controlling the deployment of airbag  46  will be discussed. The methodology enters the routine at block  100  and progresses to decision block  104  where the methodology determines whether the vehicle occupant is using their seatbelt restraint. If the seatbelt restraint is being used, the methodology proceeds to block  108  where a first set of inflation parameters is stored in memory  26  for subsequent use. This first set of inflation parameters includes predetermined threshold levels for judging the magnitude of the collision which may also include parameters relating to the duration of the impact. For the purposes of this example, it will be assumed that there are three predetermined impact severity levels associated with the first set of inflation parameters and that these parameters are set to values HIGH 1 , MEDIUM 1  and LOW 1 , respectively. The values of HIGH 1 , MEDIUM 1  and LOW 1  are stored in memory  26  in registers HIGH, MEDIUM and LOW, respectively. The methodology then proceeds to decision block  120 . If the seatbelt restraint is not being used, the methodology proceeds to block  112  where a second set of inflation parameters is stored in memory  26  for subsequent use. The second set of inflation parameters is similar in content to the first set of inflation parameters except that the magnitude of the various parameters reflects desired differences resulting from differences between restrained and unrestrained vehicle occupants. As with the first set of inflation parameters, the second set of inflation parameters will also be assumed to have three predetermined impact severity levels. These severity levels are set to values of HIGH 2 , MEDIUM 2  and LOW 2 , respectively. The values of HIGH 2 , MEDIUM 2  and LOW 2  are stored in memory  26  in registers HIGH, MEDIUM and LOW, respectively. The methodology then proceeds to decision block  120 . 
     At decision block  120 , the methodology determines whether an impact exceeding a predetermined level, TIMPACT, has occurred. The magnitude of TIMPACT typically corresponds to a change in acceleration of approximately 2-3 g. If such an impact has not been detected, the methodology returns to decision block  104 . If an impact exceeding TIMPACT has occurred, the methodology proceeds to block  124  where timer  28  is reset to zero (0) and started. The methodology then proceeds to decision block  128 . 
     At decision block  128 , the methodology again determines whether the magnitude of the impact still exceeds TIMPACT. If the magnitude of the impact is equal to or less than TIMPACT, the methodology will return to decision block  104 . If the magnitude of the impact still exceeds TIMPACT, the methodology will proceed to decision block  132  where the magnitude of the impact is compared to the LOW threshold (i.e., LOW 1  or LOW 2 ) stored in memory  26 . If the magnitude of the collision does not exceed the LOW threshold, the methodology returns to decision block  128 . The methodology will continue in the loop between decision blocks  128  and  132  until such time that the magnitude of the impact diminishes below TIMPACT or exceeds the LOW threshold. If the magnitude of the impact exceeds the LOW threshold, the methodology proceeds to decision block  136 . 
     In decision block  136 , the methodology next determines whether the value in timer  28  exceeds T (HIGH) . In the example described, T (HIGH)  is the time in which an unrestrained 50 th  percentile male dummy moves five inches forward relative to vehicle  12  in a 30 mile-per-hour flat frontal impact. It will be understood, however, that the criteria on which T (HIGH)  is based may differ depending upon a diverse number of variables, including the design of the vehicle, airbag deployment rate and other design criteria unique to a given application. Accordingly, the above mentioned criteria for establishing T (HIGH)  is provided for purposes of illustration and not meant to be limiting in any manner. Referring back to decision block  136 , if the value in timer  28  is less than T (HIGH) , the methodology proceeds to decision block  140 . If the value in timer  28  is greater than T (HIGH) , the methodology proceeds to decision block  156 . 
     In decision block  140 , the methodology next compares the magnitude of the impact to the HIGH threshold. If the magnitude of the impact exceeds the HIGH threshold, the methodology proceeds to block  144  wherein control module  14  generates first and second squib pulses  66  and  68  simultaneously, or at least in immediate succession, so as to inflate airbag  46  as quickly as possible. Inflation of airbag  46  at this maximum rate is desirable due to the speed of the vehicle occupant relative to vehicle  12 . The potential to cause injury to the vehicle occupant by inflating airbag  46  at this maximum rate is substantially reduced due to the use of T (HIGH) . The methodology prevents deployment of airbag  46  at the maximum rate if the value in timer  28  equals or exceeds T (HIGH) . This minimizes the risk that the vehicle occupant will be too far forward relative to vehicle  12  when airbag  46  is deployed and thereby substantially reduces the risk that the vehicle occupant will be injured by the deploying airbag  46 . 
     If the magnitude of the impact does not exceed the HIGH threshold, the methodology proceeds to decision block  148  where the magnitude is compared to the MEDIUM threshold. If the magnitude of the impact does not exceed the MEDIUM threshold, the methodology proceeds to block  150  where a flag, MEDDEPLOY, is set to “false” (i.e., MEDDEPLOY=0). The methodology then returns to decision block  136 . If the magnitude of the impact exceeds the MEDIUM threshold, the methodology proceeds to block  152  where the MEDDEPLOY flag is set to “true” (i.e., MEDDEPLOY=1). The methodology then returns to decision block  136 . The methodology continues in the loop between decision block  136  and blocks  150  and  152  until the magnitude of the impact exceeds the HIGH threshold or the value in timer  28  equals or exceeds T (HIGH) . 
     Operation in this loop, therefore, permits airbag  46  to be deployed at any time prior to the time at which the vehicle occupant has moved too far forward relative to vehicle  12 . As such, the methodology guards against the risk that airbag  46  will be inflated at a less-than-optimal rate for impacts exceeding the HIGH impact magnitude threshold. 
     At decision block  156 , the methodology evaluates the MEDDEPLOY flag. If the flag is set to true, the methodology proceeds to block  164  wherein control module  14  generates first squib pulse  66  so as to initiate the combustion of first charge  55  to begin the inflation of airbag  46 . After a predetermined time interval, control module  14  generates second squib pulse  68  so as to initiate the combustion of second charge  65  to further inflate airbag  46 . The combustion of first and second charges  55  and  65 , respectively, in this manner causes airbag  46  to inflate at an intermediate rate. Testing has shown that the predetermined time interval between the initiation of first and second charges  55  and  65 , respectively, may range between 10 to 30 seconds. However, the magnitude of the predetermined time interval between the initiation of first and second charges  55  and  65 , respectively, varies from application to application based upon a number of criteria, including the specific design of the air bag and the vehicle in which it is used. Referring back to decision block  156 , if the MEDDEPLOY flag is set to false, the methodology proceeds to decision block  168 . 
     At decision block  168 , the methodology compares the value in timer  28  to T (MED) . In the example discussed, T (MED)  is similar to T (HIGH)  in that it is obtained from empirical testing and is representative of the position of the vehicle occupant relative to vehicle  12 . T (MED)  may therefore be the time in which an unrestrained 50 th  percentile male dummy moves five inches forward relative to vehicle  12  in a 14 mile-per-hour flat frontal impact. It will be understood, however, that the criteria on which T (MED)  is based may differ depending upon a diverse number of variables, including the design of the vehicle, airbag deployment rate and other design criteria unique to a given application. Accordingly, the above mentioned criteria for establishing T (MED)  is provided for purposes of illustration and not meant to be limiting in any manner. Referring back to decision block  168 , if the value in timer  28  is less than T (MED) , the methodology proceeds to decision block  172  where the magnitude of the impact is compared to the MEDIUM threshold. If the magnitude exceeds the MEDIUM threshold, the methodology proceeds to block  164  and airbag  46  is deployed at the intermediate rate as discussed above. If the magnitude does not exceed the MEDIUM threshold, the methodology returns to decision block  168 . The methodology continues in the loop between decision blocks  168  and  172  until such time that the magnitude of the impact exceeds the MEDIUM threshold or the value in timer  28  equals or exceeds T (MED) . If the value in timer  28  equals or exceeds T (MED) , the methodology proceeds to block  184  wherein control module  14  generates first squib pulse  66  so as to initiate the combustion of first charge  55  and inflate airbag  46  at a low rate. 
     Referring now to FIG. 3, the methodology for controlling the deployment of airbag module according to an alternate embodiment of the present invention will now be discussed. Note that as significant portions of this methodology are identical to the methodology described in conjunction with FIG. 2, only those portions which differ will be discussed. 
     One such difference is the use of a diagnostic sub-routine to determine whether the first and second squib circuits  42  and  44  are functional. After determining whether the vehicle occupant is using their seatbelt restraint and selecting the corresponding set of inflation parameters, the methodology proceeds to block  216  where a diagnostic sub-routine is performed. The diagnostic sub-routine is shown in detail in FIG.  4 . 
     Referring now to FIG. 4, the methodology enters the diagnostic sub-routine at block  300  and progresses to block  304 . At block  304 , the control module  14  measures the voltage present at the negative and positive terminals  70  and  72 , respectively. The methodology then proceeds to decision block  308  where the voltage measured at positive terminal  72  is compared with a predetermined value, VP 1 SHORT. 
     If the voltage measured at positive terminal  72  is less than VP 1 SHORT, positive circuit element  52  may have been shorted to ground. The methodology then proceeds to block  312  where the SQUIB 1 FAULT flag is set. The methodology then advances to block  340 . Returning to decision block  308 , if the voltage measured at positive terminal  72  is not less than VP 1 SHORT, indicating that positive circuit element  52  has not shorted to ground, the methodology proceeds to decision block  316  where the voltage measured at negative terminal  70  is compared with a predetermined value, VN 1 SHORT. 
     If the voltage measured at negative terminal  70  is greater than VN 1 SHORT, negative circuit element  50  may have been shorted to the vehicle power supply (not shown). The methodology then proceeds to block  312 . If the voltage measured at negative terminal  70  is not greater than VN 1 SHORT, indicating that negative circuit element  50  has not shorted to the vehicle power supply, the methodology proceeds to block  320 . 
     In block  320 , control module  14  applies a predetermined current to positive circuit element  52  and measures the voltage at negative and positive terminals  70  and  72 , respectively. The methodology then proceeds to decision block  324 . 
     In decision block  324 , the methodology will compare the measured voltage at positive terminal  72  to the measured voltage at negative terminal  70 . If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  70  and  72 , respectively, is greater than a predetermined value, VS 1 OPEN, first squib circuit  42  is presumed to be electrically open (i.e., will not conduct electricity) and the methodology will proceed to block  312 . If the absolute value of the voltage difference at negative and positive terminal  70  and  72 , respectively, is not greater than VS 1 OPEN, the methodology proceeds to decision block  328 . 
     In decision block  328 , the methodology compares the absolute value of the voltage difference between the voltage measured at negative and positive terminals  70  and  72 , respectively, to a predetermined value, VS 1 SHORT. If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  70  and  72 , respectively, is less than VS 1 SHORT, first squib circuit  42  is presumed to have electrically shorted and the methodology proceeds to block  312 . If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  70  and  72 , respectively, is not less than VS 1 SHORT, first squib circuit  42  is presumed to be operational and the methodology proceeds to block  340 . 
     At block  340 , the control module  14  measures the voltage present at the negative and positive terminals  80  and  82 , respectively. The methodology then proceeds to decision block  344  where the voltage measured at positive terminal  82  is compared with a predetermined value, VP 2 SHORT. 
     If the voltage measured at positive terminal  82  is less than VP 2 SHORT, positive circuit element  62  may have been shorted to ground. The methodology then proceeds to block  348  where the SQUIB 2 FAULT flag is set. The methodology then advances to block  366 . Returning to decision block  344 , if the voltage measured at positive terminal  82  is not less than VP 2 SHORT, indicating that positive circuit element  62  has not shorted to ground, the methodology proceeds to decision block  352  where the voltage measured at negative terminal  80  is compared with a predetermined value, VN 2 SHORT. 
     If the voltage measured at negative terminal  80  is greater than VN 2 SHORT, negative circuit element  60  may have been shorted to the vehicle power supply. The methodology then proceeds to block  348 . If the voltage measured at negative terminal  80  is not greater than VN 2 SHORT, indicating that negative circuit element  60  has not been shorted to the vehicle power supply, the methodology proceeds to block  356 . 
     In block  356 , control module  14  applies a predetermined current to positive circuit element  62  and measures the voltage at negative and positive terminals  80  and  82 , respectively. The methodology then proceeds to decision block  360 . 
     In decision block  360 , the methodology will compare the measured voltage at positive terminal  82  to the measured voltage at negative terminal  80 . If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  80  and  82 , respectively, is greater than a predetermined value, VS 2 OPEN, second squib circuit  44  is presumed to be electrically open (i.e., will not conduct electricity) and the methodology will proceed to block  348 . If the absolute value of the voltage difference at negative and positive terminal  80  and  82 , respectively, is not greater than VS 2 OPEN, the methodology proceeds to decision block  362 . 
     In decision block  362 , the methodology compares the absolute value of the voltage difference between the voltage measured at negative and positive terminals  80  and  82 , respectively, to a predetermined value, VS 2 SHORT. If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  80  and  82 , respectively, is less than VS 2 SHORT, second squib circuit  44  is presumed to have electrically shorted and the methodology proceeds to block  348 . If the absolute value of the voltage difference between the voltage measured at negative and positive terminals  80  and  82 , respectively, is not less than VS 2 SHORT, second squib circuit  44  is presumed to be operational and the methodology proceeds to bubble  366  where the diagnostic subroutine terminates. The methodology then proceeds to decision block  120  as shown in FIG. 3 where the magnitude of the impact is compared to a predetermined value. 
     Another difference with the method of the alternate embodiment concerns the generation of the first and second squib pulses  66  and  68  to inflate airbag  46  at either the intermediate or low rates. With regard to the methodology subsequent to the decision to deploy airbag  46  at the intermediate rate (i.e., MEDDEPLOY=true at decision block  156 , or the magnitude of the impact exceeds the MEDIUM threshold at decision block  172 ), the methodology first inquires at decision block  260  as to the status of the SQUIB 1 FAULT flag. If the SQUIB 1 FAULT flag has been set to true indicating that first squib circuit  42  is not operational, the methodology proceeds to block  262  where control module  14  generates second squib pulse  68  to initiate the deployment of airbag  46  at the alternative low rate. Alternative low rate may have a rate which is slightly greater or lesser than low rate depending on the difference in the sizes of first and second charges  55  and  65 , respectively. While deployment of airbag  46  at the alternative low rate is not optimal, it nonetheless ensures that airbag  46  will be deployed and as such, provides the vehicle occupant with a measure of restraint which may not have been available if the methodology had attempted to deploy airbag  46  at the intermediate rate. 
     Returning to decision block  260 , if the SQUIB 1 FAULT flag has not been set to true, the methodology will proceed to decision block  263  where the status of the SQUIB 2 FAULT flag is checked. If the SQUIB 2 FAULT is set to true indicating that second squib circuit  44  is not operational, the methodology proceeds to block  184  and deploys the airbag at the low rate. While deployment of airbag  46  at the low rate is not optimal, it nonetheless ensures the deployment of airbag  46  and provides the vehicle occupant with a measure of restraint which may not have been available if the methodology had attempted to deploy airbag  46  at the intermediate rate. 
     If the SQUIB 2 FAULT flag has not been set to true, both first and second squibs  54  and  64 , respectively, are presumed to be operational and the methodology proceeds to block  164  where airbag  46  is caused to deploy at the intermediate rate as previously explained. 
     With regard to a decision to deploy airbag  46  at the low rate (i.e., the value of timer  28  is not less than T (MED)  in block  168 ), the methodology first inquires at decision block  280  as to whether the SQUIB 1 FAULT flag has been set to indicate that first squib circuit  42  has a fault. If the SQUIB 1 FAULT flag has been set, the methodology proceeds to block  262  and control module generates second squib pulse  68  to initiate the deployment of airbag  46  at the alternative low rate. While deployment of airbag  46  at the alternative low rate is not optimal, it nonetheless ensures the deployment of airbag  46  and provides the vehicle occupant with a measure of restraint which may not have been available if the methodology had attempted to deploy airbag  46  at the low rate. If the SQUIB 1 FAULT has not been set to true, the methodology proceeds to block  184  where airbag  46  is caused to deploy at the low rate as previously explained. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.