Abstract:
An improved method of selectively suppressing deployment of a vehicular inflatable restraint utilizes dynamic variation in the apparent weight of a vehicle occupant to infer a free mass of the seat occupant. The free mass of the occupant is inferred by filtering out portions of a weight-responsive signal due to occupant position adjustment and inferring the occupant free mass based on the variation of the apparent weight with respect to the variation in vertical acceleration of the vehicle. The decision to allow or suppress deployment of the restraint is determined based on a comparison of the static weight reading with at least one threshold, and the occupant free mass is used to adjust the threshold in a direction to minimize the overall variability of the system. Measures of the seat belt tension and the seat temperature are also be used to adjust the threshold in a direction to minimize system variability.

Description:
TECHNICAL FIELD 
   This invention relates to a method of allowing or suppressing deployment of an inflatable restraint based on sensed occupant weight, and more particularly to a method of taking into account both static and dynamic weight data. 
   BACKGROUND OF THE INVENTION 
   Vehicle occupant weight 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 weight reading similar to that of a small (5 th  percentile) female adult due to variations seat belt tension and the orientation of the occupant on the seat, for example. In other words, the static weight 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. Fortunately, the range of variability is substantially reduced in the case of dynamic weight readings (i.e., the variation of the weight reading), and the dynamic variation has been used to more reliably characterize the seat occupant for purposes of determining whether to allow or suppress restraint deployment. For example, in the U.S. Pat. No. 6,246,936 to Murphy et al., issued on Jun. 12, 2001, and assigned to the assignee of the present invention, the dynamic variation is used to distinguish a tightly cinched child seat from an adult; and in the U.S. patent application Ser. No. 09/895,742, filed on Jul. 2, 2001, and assigned to the assignee of the present invention, the dynamic variation is normalized with respect to acceleration of the vehicle to compensate for the effects of operating the vehicle on a rough road surface. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved method of selectively suppressing deployment of a vehicular inflatable restraint in which both static and dynamic variation in the apparent weight of a vehicle occupant are used in a complementary manner that reduces overall system variability. The free mass of the seat occupant is estimated based on the variation of the sensed weight with respect to the variation in vertical acceleration of the vehicle, ignoring signal variations due to occupant position adjustment. The decision to allow or suppress deployment of the restraint is determined based on a comparison of the static weight reading with at least one threshold, and the occupant free mass is used to adjust the threshold in a direction to minimize the overall variability of the system. In the preferred embodiment, measures of the seat belt tension and the seat temperature are also be used to adjust the threshold in a direction to minimize system variability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a system diagram illustrating a passenger seat of a vehicle equipped with a fluid-filled bladder, 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. 
       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. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is disclosed in the context of a restraint system for an occupant of a vehicle passenger seat  10 , where the occupant weight is sensed based on the fluid pressure in a seat cushion bladder  12 . In general, however, the present invention also applies to other types of occupant weight sensing systems, such as systems that sense the strain in a seat frame element, or systems that include a network of pressure sensitive cells distributed over the seating area. 
   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 bladder  12  is disposed in or under the foam cushion  16  substantially parallel with the central seating surface, and preferably contains a fluid such as silicone which is non-corrosive, and not subject to freezing at extreme ambient temperatures. In addition, a semi-rigid back-plate may be placed under the bladder  12  to provide a suitable reaction surface, as disclosed for example in the U.S. patent application Ser. No. 09/311,576, filed May 14, 1999, assigned to the assignee of the present invention, and incorporated herein by reference. Alternatively, the bladder  12  may be placed between two semi-rigid back-plates to provide reaction surfaces on both sides of the bladder. 
   The bladder  12  is coupled to a pressure sensor  20 , which provides an electrical output signal on line  22  indicative of the fluid pressure in the bladder  12 . A temperature sensor  24  located in proximity to the bladder  12  provides an electrical output signal on line  26  indicative of the bladder and foam temperature. The sensor  24  can be provided as a separate sensor as indicated in  FIG. 1 , or may be integrated with the pressure sensor  20 . 
   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 sensor (not shown) within the buckle  34  detects insertion of the clip  32 , and provides an electrical signal on line  36  indicative of the seat belt latch status (i.e., buckled or unbuckled). Additionally, a belt tension sensor  37  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  37  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. 
   The electrical pressure, temperature, seat belt latch status and seat belt tension signals on lines  22 ,  26 ,  36  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 (H)  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. 
   In the illustrated embodiment, the primary function of PODS ECU  40  is to estimate occupant weight based on the various input signals mentioned above, and to determine whether deployment of the inflatable restraints should be allowed or suppressed. The relationship between occupant weight and the output of pressure sensor  20  can be empirically determined or calibrated in a factory setting by placing a known weight on the seat  10  under a given set of environmental conditions. However, it is known that in actual vehicle usage, the sensed pressure 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 covering tension. Of course, vehicle movement and occupant movement can cause significant variation of the sensed pressure. Thus, it is difficult to accurately distinguish a child occupant from a small adult occupant under all circumstances by simply comparing the sensed pressure to a fixed threshold. Accordingly, an important aspect of the present invention involves identifying major factors influencing the sensed pressure, 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 mass of the occupant, the seat belt tension and the cushion temperature. For example, if the seat belt tension (as measured by the sensor  37 ) is relatively high, the sensed pressure will be skewed higher than would otherwise be expected, and the pressure 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 cushion temperature (as measured by sensor  24 ) is colder than normal, the output pressure will be skewed lower than would ordinarily be expected, and the pressure threshold is adjusted downward in relation to the amount by which the measured temperature deviates from a normal range of temperatures. And finally, the pressure threshold is adjusted upward when the occupant free mass is estimated to be relatively low, and downward when the occupant free mass is estimated to be relatively high; this also minimizes variability by increasing the likelihood that deployment will be allowed for an adult (high free mass) occupant, and suppressed for a child (low free mass) occupant. 
   According to another aspect of this invention, the free mass of the seat occupant is estimated by considering the variation of the pressure signal output (that is, ΔPS) with respect to variations in acceleration measured by vertical acceleration sensor  48 . Considering the above-mentioned factors that affect the pressure sensor output signal variability, the overall variability (ΔPS) may be considered as the sum of several components, as follows:
 
Δ PS=ΔPS zero+Δ PSsb+ΔPSfm+ΔPSenv+ΔPSom   (1)
 
where ΔPSzero is the variation in the pressure signal that occurs even when the seat  10  is empty due to seat cover tension and so forth, ΔPSsb is the variation in the pressure signal due to seat belt tension, ΔPSfm is the variation in the pressure signal due to the effects of vehicle movement on the occupant free mass, ΔPSenv is the variation in the pressure signal due to environmental conditions such as temperature and humidity, and ΔPSom is the variation in the pressure signal due to occupant movement. Since the components ΔPSzero, ΔPSsb and ΔPSenv are relatively constant for at least short time intervals, ΔPS may be considered as the sum of ΔPSfm and ΔPSom over a suitably short interval. Also, the component ΔPSom 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 pressure sensor output (ΔPS) may be expressed as:
 
Δ PS=ΔPSfm=ΔACCEL *(MASS c/b +MASS f )  (2)
 
where ΔACCEL is the change in vertical acceleration of the vehicle, MASSc/b is the combined mass of the cushion  16  and bladder  12 , and MASSf is the free mass of the seat occupant. Thus, the combined free mass (FREE_MASS) of the occupant, the cushion  16  and the bladder  12  may be estimated as:
 
FREE_MASS= K (Δ PS/ΔACCEL )  (3)
 
where K is a constant, and MASSf may be estimated as:
 
 MASS f=K (Δ PS/ΔACCEL )−MASS c/b   (4)
 
   The flow diagrams of  FIGS. 2-4  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. 3-4  detail occupant status determination. 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  18  is executed to determine occupant status as detailed in the flow diagrams of  FIGS. 3 and 4 . 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. 
   Referring to  FIG. 3 , determining the suppression status generally involves comparing a filtered version of the pressure sensor output (FILT_PRESSURE) 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 estimated occupant free mass, seat belt tension SBT and seat temperature SEAT_TEMP, as detailed in the flow diagram of FIG.  4 . If FILT_PRESSURE 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_PRESSURE is greater than Kempty, block  140  compares FILT_PRESSURE 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_PRESSURE 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_PRESSURE 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. 
   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 time 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 pressure signal PS is significantly influenced by occupant movement, seat adjustment, etc., and the block  152  is executed to update THR_ADJ based on SEAT_TEMP and seat belt tension 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 PS 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). Similarly, 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 PS 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). 
   Once RUN_TIME exceeds REF_TIME, the blocks  154 ,  156 ,  158  and  160  are executed to identify the free mass FREE_MASS of the occupant based on a detected variation of the sensed pressure PS with respect to variation of the measured vertical acceleration ACCEL. The block  154  determines if the pressure signal value PS_SAMPLE determined at block  100  is within a predetermined percentage (5% in the illustrated embodiment) of a running average PS_AVG of the pressure signal. If not, the unusually high or low value of PS_SAMPLE is considered due to occupant movement, and is ignored for purposes of estimating the occupant free mass. However, if block  154  is answered in the affirmative, the block  156  is executed to update the acceleration variance ACCEL_VAR, and to use PS_SAMPLE to update the pressure signal variance PS_VAR. This involves summing filtered values of the pressure sensor output signal PS and the vertical acceleration sensor output signal ACCEL, and calculating a sum of squares SQUARE_SUM ps , SQUARE_SUM accel  for each of the signals, as follows:
 
SQUARE_SUM ps =[SQUARE_SUM ps   +PS   f   2   ]/REF 1     (5)
 
SQUARE_SUM accel =[SQUARE_SUM accel   +ACCEL   f   2 ]/SAMPLES  (6)
 
where SUM ps , is the summation of the pressure signal values, SUM accel  is the summation of the acceleration signal values, SAMPLES is the number of summed values, and the subscript f indicates a filtered value. Then, the variance PS_VAR of the pressure sensor signal and the variance ACCEL_VAR of the acceleration sensor signal are calculated as follows:
 
 PS   —   VAR =SQUARE_SUM ps −(SUM ps /SAMPLES) 2   (7)
 
 ACCEL   —   VAR =SQUARE_SUM accel −SUM accel /SAMPLES) 2   (8)
 
The block  158  then estimates the combined free mass FREE_MASS of the occupant, cushion  16  and bladder  12  according to the ratio PS_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 mass estimate will be less reliable. So long as FREE_MASS is within a normal range 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_MASS, SEAT_TEMP and seat belt tension SBT. As indicated above, THR_ADJ is adjusted upward in relation to the amount by which FREE_MASS is below a range of values that ordinarily occur with a small adult, and downward in relation to the amount by which FREE_MASS 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 mass) occupant, and that STATUS will be set to OCCUPIED INHIBIT for a child (low free mass) occupant.
 
   In summary, the method of this invention provides a simple and effective way of utilizing both static and dynamic occupant weight-responsive data in a complementary fashion to more reliably determine if deployment of inflatable restraints should be allowed or suppressed. The static data is compared to a threshold for purposes of determining if deployment should be allowed or suppressed, and dynamic information is utilized along with vehicle acceleration information to estimate the free mass of the occupant for the purpose of adjusting the threshold in a direction to minimize variability of the allow/suppress decision due to environmental and other factors. The threshold is also adjusted for other factors that can be specifically identified, including seat belt tension and seat cushion temperature. While illustrated in reference to the illustrated embodiment, it is expected that various modifications will occur to persons skilled in the art. For example, this invention is not limited to pressure based bladder systems, and may be applied equally as well to other occupant weight sensing systems, as indicated above. 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.