Abstract:
An improved method of selectively suppressing deployment of a vehicular inflatable restraint utilizes both static and dynamic variation in the measured capacitance of a vehicle occupant in a complementary manner that reduces overall system variability. The decision to allow or suppress deployment of the restraint is determined based on a comparison of the static capacitance reading with at least one threshold, and the dynamic variation of the measured capacitance and other parameters such as seat belt tension, seat humidity and seat temperature are used to adjust the threshold in a direction to minimize the overall variability of the system. In a preferred embodiment, the variation of the measured capacitance with respect to the variation in vertical acceleration of the vehicle is used to estimate the free capacitance of the occupant, and such free capacitance is used to adjust the threshold.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a method of allowing or suppressing deployment of an inflatable restraint based on sensed occupant capacitance, and more particularly to a method of taking into account both static and dynamic capacitance data.  
         BACKGROUND OF THE INVENTION  
         [0002]    Vehicle occupant capacitance sensing systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of characterizing the occupant for purposes of determining whether to allow or suppress deployment of the restraints. For example, it is generally desired to allow deployment for an adult, and to suppress deployment (or reduce deployment force) for a child. However, it has been found that a child occupant can produce a static capacitance reading similar to that of a small adult (5 th  percentile female) due to variations seat belt tension, the orientation of the occupant on the seat, and the usage of aftermarket seat covers or pads, for example. In other words, the static capacitance readings for a child occupant and a small adult occupant statistically vary over first and second ranges due to system variability, and there can be some amount of overlap between the first and second ranges under certain conditions. Accordingly, what is needed is a method of more reliably distinguishing between those occupants for whom deployment should be suppressed and those for whom deployment should be allowed.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is directed to all improved method of selectively suppressing deployment of a vehicular inflatable restraint in which both static and dynamic variation in the measured capacitance of a vehicle occupant are used in a complementary manner that reduces overall system variability. The decision to allow or suppress deployment of the restraint is determined based on a comparison of the static capacitance reading with at least one threshold, and the dynamic variation of the measured capacitance and other parameters such as seat belt tension, seat humidity and seat temperature are used to adjust the threshold in a direction to minimize the overall variability of the system. In the preferred embodiment, the variation of the measured capacitance with respect to the variation in vertical acceleration of the vehicle is used to estimate the free capacitance of the occupant, and such free capacitance is used to adjust the threshold. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a system diagram illustrating a passenger seat of a vehicle equipped with a capacitance sensor, a passenger occupant detection electronic control unit (PODS ECU), an airbag control module (ACM), and vehicle acceleration sensors for characterizing an occupant of the seat according to this invention.  
         [0005]    [0005]FIGS. 2, 3 and  4  depict a flow diagram representative of a software routine executed by the PODS ECU of FIG. 1 in carrying out the method of this invention.  
         [0006]    [0006]FIG. 5 is a flow diagram illustrating an alternate embodiment of the routine portion depicted in the flow diagram of FIG. 4. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]    The present invention is disclosed in the context of a restraint system for an occupant of a vehicle passenger seat  10 , where the occupant capacitance is sensed based on a capacitance sensor  12  (or array of capacitance sensors) in a seat cushion  16 . In general, however, the present invention also applies to other types of occupant capacitance sensing systems, such as systems that sense occupant capacitance in a seat back, headliner or any other vehicle location.  
         [0008]    Referring to FIG. 1, the vehicle seat  10  is supported on a frame  14 , and includes foam cushions  16  and  18  on the seat bottom and back. The capacitance sensor  12  is disposed in or under the foam cushion  16  substantially parallel with the central seating surface, and provides an output signal on line  20  indicative of the capacitance of a seat occupant. A relative humidity sensor  22  and a temperature sensor  24  are located in proximity to the capacitance sensor  12 , and provide electrical output signals oil lines  25  and  26  indicative of the seat humidity SEAT_HUM and seat temperature SEAT_TEMP.  
         [0009]    As also shown in FIG. 1, the seat  10  is equipped with a conventional shoulder/lap seat belt  28  anchored to the vehicle floor (not shown) and B-pillar  30 . In use, the belt  28  is drawn around an occupant or through the frame of a child or infant seat, and a clip  32  slidably mounted on the belt  28  is inserted into the buckle  34  to fasten the belt  28  in place. A retractor assembly (not shown) mounted in the B-pillar  30  maintains a desired tension on the belt  28 , and locks the belt  28  in place when the vehicle experiences significant deceleration. A belt tension sensor  36  detects the tension applied to seat belt  28 , and provides an electrical signal (BTS) indicative of the tension magnitude on line  38 . The tension sensor  36  may be located in the B-pillar  30  as shown, near the floor on the outboard side of seat  10 , or in any other convenient location, and may be constructed as disclosed, for example, in Research Disclosure No. 41402. October, 1998, Page 1304, incorporated herein by reference.  
         [0010]    The capacitance, humidity, temperature and seat belt tension signals on lines  20 ,  25 ,  26  and  38  are provided as inputs to a passenger occupant detection system electronic control unit (PODS ECU)  40 , which in turn, is coupled to an airbag control module (ACM)  42  via bi-directional communication bus  44 . The ACM  42  may be conventional in nature, and operates to deploy one or more airbags or other restraint devices (not shown) for vehicle occupant protection based on the vertical and/or horizontal acceleration signals obtained from vertical acceleration sensor (V)  48  and horizontal acceleration sensor (ii)  46 , and occupant characterization data obtained from PODS ECU  40 . In general, ACM  42  deploys the restraints if the acceleration signals indicate the occurrence of a severe crash, unless the PODS ECU  40  indicates that deployment should be suppressed. Of course, other more sophisticated controls are also possible, such as controlling the deployment force of the restraint devices based on the occupant characterization data provided by PODS ECU  40 . Also, ACM  42  communicates the suppression status to a driver display device  50  to enable the driver to verify that the system has properly characterized the seat occupant.  
         [0011]    In the illustrated embodiment, the primary function of PODS ECU  40  is to determine whether deployment of the inflatable restraints should be allowed or suppressed based on the various input signals mentioned above. However, it is known that in actual vehicle usage, the sensed capacitance for a given seat occupant will vary to some extent even under static conditions due to differences in temperature, humidity, seat belt tension, and seat cover tension. Of course, vehicle movement and occupant movement can cause significant variation of the sensed capacitance. Thus, it is difficult to accurately distinguish a child occupant from a small adult occupant under all circumstances by simply comparing the sensed capacitance to a fixed threshold. Accordingly, an important aspect of the present invention involves identifying major factors influencing the sensed capacitance, and using such factors to adjust the threshold in a direction that minimizes the overall system variability. In the illustrated embodiment, the major factors that can be identified include the free capacitance of the occupant, the seat belt tension, the seat humidity and the seat temperature. For example, if the seat belt tension (as measured by the sensor  36 ) is relatively high, the sensed capacitance will be skewed higher than would otherwise be expected, and the capacitance threshold is adjusted upward in relation to the measured tension to minimize variability of the occupant status determination due to higher-than-normal seat belt tension. Likewise, if the relative humidity (as measured by sensor  22 ) is higher than normal, the output capacitance will be skewed higher than would ordinarily be expected, and the capacitance threshold is adjusted upward in relation to the amount by which the measured humidity deviates from a normal humidity range. And finally, the capacitance threshold is adjusted upward when the occupant free capacitance is estimated to be relatively low, and downward when the occupant free capacitance is estimated to be relatively high; this also minimizes variability by increasing the likelihood that deployment will be allowed for an adult (high free capacitance) occupant, and suppressed for a child (low free capacitance) occupant.  
         [0012]    According to another aspect of this invention, the free capacitance of the seat occupant is estimated by considering the variation of the capacitance signal output (that is, ΔCAP) with respect to variations in acceleration measured by vertical acceleration sensor  48 . Considering the above-mentioned factors that affect the capacitance sensor output signal variability, the overall variability (ΔCAP) may be considered as the sum of several components, as follows:  
         Δ CAP=ΔCAPzero+ΔCAPsb+ΔCAPfc+ΔCAPenv+ΔCAPom   (1)  
         [0013]    where ΔCAPzero is the variation in the capacitance signal that occurs even when the seat  10  is empty due to seat cover tension and so forth, ΔCAPsb is the variation in the capacitance signal due to seat belt tension, ΔCAPfc is the variation in the capacitance signal due to the effects of vehicle movement on the occupant free capacitance, ΔCAPenv is the variation in the capacitance signal due to environmental conditions such as temperature and humidity, and ΔCAPom is the variation in the capacitance signal due to occupant movement. Since CAXPzero, CAPsb and CAPenv are substantially constant for at least short time intervals, the components ΔCAPzero, ΔCAPsb and ΔCAPenv will be small, and ΔCAP may be considered as the sum of ΔCAPfc and ΔCAPom over a suitably short interval. Also, the component ΔCAPom can be minimized by ignoring (filtering) substantial excursions of the output signal, since occupant movement typically results in output signal shifts that are much higher than signal shifts due to vertical acceleration of the vehicle. With these assumptions, the overall variability in the capacitance sensor output (ΔCAP)) may be expressed as:  
         Δ CAP=ΔCAPfc=ΔACCEL *( FCc/s+FCoc )  (2)  
         [0014]    where ΔACCEL is the change in vertical acceleration of the vehicle, FCc/s is the combined capacitance of the cushion  16  and sensor  12 , and FCoc is the free capacitance of the seat occupant. Thus, the combined free capacitance (FREE_CAP) of the occupant, the cushion  16  and the sensor  12  may be estimated as:  
           FREE   —   CAP=K ( ΔCAP/ΔACCEL )  (3)  
         [0015]    where K is a constant, and FCoc may be estimated as:  
           FCoc=K ( ΔCAP/ΔACCEL )− FCc/s   (4)  
         [0016]    The flow diagrams of FIGS.  2 - 5  illustrate a software routine periodically executed by the PODS ECU  40  for carrying out the above-described method. The flow diagram of FIG. 2 represents a main or executive routine, whereas the flow diagrams of FIGS.  3 - 5  detail occupant status determination.  
         [0017]    At the initiation of each period of vehicle operation, the PODS ECU  40  executes an initialization routine as indicated by block  90  of FIG. 2 for initializing various registers, parameters and flags to zero or some other default setting. In the case of this invention, for example, the suppression status (STATUS) may be initialized to a default setting, or to a setting determined in the previous ignition cycle, and the threshold adjustment THR_ADJ may be initialized to zero. A similar initialization also occurs in the event of a dynamic reset. Following initialization, the blocks  100 - 126  are repeatedly executed as shown. The blocks  100  and  102  read all of the sensor information mentioned above in reference to FIG. 1 and perform diagnostic testing of the system and components. If the diagnostic testing detects a fault, the block  104  is answered in the affirmative, and the block  106  sets the status message to FAULT. Otherwise, the block  108  is executed to determine occupant status as detailed in the flow diagrams of FIGS.  3 - 5 . If the occupant status is OCCUPIED INHIBIT, as determined at block  110 , the block  112  sets the status message to SUPPRESS FOR CHILD. If the occupant status is OCCUPANT ALLOW, as determined at block  114 , the block  116  sets the status message to ALLOW FOR ADULT. And if the occupant status is EMPTY SEAT, as determined at block  118 , the block  120  sets the status message to SUPPRESS BECAUSE EMPTY. If blocks  110 ,  114 , and  118  are answered in the negative, the block  122  sets the status message to INDETERMINATE. The block  124  then sends the determined occupant status message to ACM  42 , and the block  126  checks for removal of system power. When system power is removed, the block  128  is executed to perform shut-down tasks, and the routine is exited.  
         [0018]    Referring to FIG. 3, determining the suppression status generally involves comparing, a filtered version of the capacitance sensor output (FILT_CAP) to various thresholds. Initially, the blocks  130  and  132  are executed to determine if there has been a driver-override of the occupant sensing system or if system initialization has not been completed. In either case, the block  148  is executed to set STATUS to INDETERMINATE, and the routine is exited. Usually, however, blocks  130  and  132  will be answered in the negative, and the block  134  is executed to update the threshold adjustment THR_ADJ based on FREE_CAP, SEAT_HUM, SEAT_TEMP and SBT, as detailed in the flow diagrams of FIG. 4 or  5 . FIG. 4 illustrates an embodiment where the vertical acceleration signal ACCEL is available to PODS ECU  40 , whereas FIG. 5 illustrates an embodiment where the vertical acceleration ACCEL is unavailable.  
         [0019]    If FILT_CAP is less than or equal to a predetermined low threshold Kempty indicative of an unoccupied seat, the blocks  136  and  138  detect the condition and set STATUS to EMPTY, completing the routine. If FILT_CAP is greater than Kempty, block  140  compares FILT_CAP to an adjustable threshold defined by the sum (Kadult+THR_ADJ), where Kadult is a default value of the adjustable threshold, and indicative of large child occupant (that is, an occupant slightly smaller than a 5 th  percentile adult female, for example). The term THR_ADJ is determined at block  134  as mentioned above, and may be either positive or negative in sign to increase or decrease the value of the sum (Kadult+THR_ADJ). If block  140  is answered in the affirmative, the occupant is considered to be a child for whom deployment of the restraints should be suppressed, and block  142  is executed to set STATUS to OCCUPIED INHIBIT. If FILT_CAP exceeds the sum (Kadult+THR_ADJ) and is less than an unrealistically high threshold Khigh_pr, as determined at block  144 , the occupant is considered to be an adult for whom deployment of the restraints should be allowed, and block  146  is executed to set STATUS to OCCUPIED ALLOW. If FILT_CAP exceeds Khigh_pr, a reliable indication of occupant position cannot be obtained, and the block  148  is executed to set STATUS to INDETERMINATE. Although not shown, the routine will preferably include a degree of hysteresis to prevent STATUS from toggling between two different states; once STATUS stabilizes in a given state, the hysteresis values can be increased to reduce sensitivity to road noise, occupant movement, and so on.  
         [0020]    Referring to FIG. 4, updating the threshold adjustment THR_ADJ initially involves determining if RUN_TIME (that is, the time elapsed during the current driving cycle) exceeds a reference tine REF_TIME, such as two minutes. If block  150  is answered in the negative, the vehicle is considered to be primarily stationary; in this case, dynamic variation of the capacitance signal CAP is significantly influenced by occupant movement, seat adjustment, etc., and the block  152  is executed to update THR_ADJ based on SEAT_HUM. SEAT_TEMP and SBT. As indicated above, THR_ADJ is increased in relation to SBT if SBT is higher than would ordinarily be expected, since such tension has the effect of skewing CAP higher than would occur with normal seat belt tension. In this case, increasing THR_ADJ increases the sum (Kadult+THR_ADJ), which proportionately increases the likelihood that the occupant will be characterized as a child (i.e., that STATUS will be set to OCCUPIED INHIBIT). The same is true of the measured relative humidity SEAT_HUM. On the other hand, THR_ADJ is decreased (adjusted in the negative direction) in relation to the deviation of SEAT_TEMP below a normal range of temperatures, since the cold temperature has the effect of skewing CAP lower than would occur in the normal temperature range. In this case, decreasing THR_ADJ decreases the sum (Kadult+THR_ADJ), which proportionately increases the likelihood that the occupant will be characterized as an adult (i.e., that STATUS will be set to OCCUPIED ALLOW).  
         [0021]    Once RUN_TIME exceeds REF_TIME, the blocks  154 ,  156 ,  158  and  160  are executed to identify the free capacitance FREE_CAP the occupant based on a detected variation of the sensed capacitance CAP with respect to variation of the measured vertical acceleration ACCEL. The block  154  determines if the capacitance signal value CAP_SAMPLE determined at block  100  is within a predetermined percentage (20% in the illustrated embodiment) of a running average CAP_AVG of the capacitance signal. If not, the unusually high or low value of CAP_SAMPLE is considered due to occupant movement, and is ignored for purposes of estimating the occupant free capacitance. However, if block  154  is answered in the affirmative, the block  156  is executed to update the acceleration variance ACCEL_VAR, and to use CAP_SAMPLE to update the capacitance signal variance CAP_VAR. This involves summing filtered values of the capacitance sensor output signal CAP and the vertical acceleration sensor output signal ACCEL, and calculating a sum of squares SQUARE_SUM cap , SQUARE_SUM accel  for each of the signals, as follows:  
           SQUARE   —   SUM   cap   =[SQUARE   —   SUM   cap   +CAP   f   2   ]/REF 1  (5)  
           SQUARE   —   SUM   accel   =[SQUARE   —   SUM   accel   +ACCEL   r   2   ]/SAMPLES   (6)  
         [0022]    where SUM cap  is the summation of the capacitance signal values, SUM accel  is the summation of the acceleration signal values, SAMPLES is the number or summed values, and the subscript f indicates a filtered value. Then, the variance CAP_VAR of the capacitance sensor signal and the variance ACCEL_VAR of the acceleration sensor signal are calculated as follows:  
           PS   —   VAR=SQUARE   —   SUM   cap −( SUM   cap   /SAMPLES ) 2   (7)  
           ACCEL   —   VAR=SQUARE   —   SUM   accel −( SUM   accel   /SAMPLES ) 2   (8)  
         [0023]    The block  158  then estimates the combined free capacitance FREE_CAP of the occupant, cushion  16  and sensor  12  according to the ratio CAP_VAR/ACCEL_VAR, using equation (3) above. As a practical matter, the execution of block  158  should be skipped if ACCEL_VAR is a very low value in order to avoid dividing by a small number, and also since the free capacitance estimate will be less reliable. So long as FREE_CAP is within a normal rang)e of values determined by the reference values REF_MIN and REF_MAX, the block  160  will be answered in the affirmative, and block  162  will be executed to update THR_ADJ based on FREE_CAP, SEAT_HUM, SEAT_TEMP and SBT. As indicated above, THR_ADJ is adjusted upward in relation to the amount by which FREE_CAP is below a range of values that ordinarily occur with a small adult, and downward in relation to the amount by which FREE_CAP is above such range of values. This has the effect of minimizing system variability by increasing the likelihood that that STATUS will be set to OCCUPIED ALLOW for an adult (high free capacitance) occupant, and that STATUS will be set to OCCUPIED INHIBIT for a child (low free capacitance) occupant.  
         [0024]    As indicated above, the flow diagram of FIG. 5 illustrates an alternative UPDATE THR_ADJ routine (designated by the reference numeral  134 ′) where the vertical acceleration ACCEL is unavailable. In this case, the threshold adjustment term THR_ADJ is updated based on CAP_VAR instead of FREE_CAP so long as CAP_VAR is within a window defined by REF_MIN and REF_MAX, as indicated at blocks  178  and  180 . Of course, computation of the acceleration variance term is eliminated, at indicated at block  176 . Also the predetermined percentage of block  174  (corresponding to block  154  of FIG. 4) is enlarged from 20% to 40% in order to reliably discriminate signal variation due to occupant movement from signal variation due to vehicle movement. The blocks  170  and  172  correspond directly to the blocks  150  and  152 , respectively, of FIG. 4.  
         [0025]    In summary, the method of this invention provides a simple and effective way of utilizing both static and dynamic occupant capacitance-responsive data in a complementary fashion to more reliably determine if deployment of inflatable restraints should be allowed or suppressed. While illustrated in reference to the illustrated embodiment, it is expected that various modifications will occur to persons skilled in the art. Accordingly, it should be understood that occupant characterization methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.