Patent Publication Number: US-2023140943-A1

Title: Enhanced front impact detection utilizing pressure tube sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a method and apparatus for controlling a vehicle actuatable occupant protection device and, in particular, to a method and apparatus for detecting vehicle front impacts events and discriminating among several types of vehicle front impact events. The method and apparatus utilizes a front pressure tube sensor to enhance the detection and discrimination of these front impact events. 
     BACKGROUND 
     Actuatable vehicle occupant protection systems, or “vehicle safety systems,” include actuatable restraints, such as actuatable seatbelt retractors and airbags. These vehicle safety systems include a plurality of event sensors, such as accelerometers, and an electronic control unit, referred to herein as an airbag control unit (“ACU”). The ACU monitors signals provided by the sensors and makes determinations on whether to actuate the actuatable restraints. 
     For vehicle safety systems, it is desirable to discriminate amongst the various collision or impact events (“crash events”) in which a vehicle might be involved. Recognizing that real-life crash scenarios are limitless, crash tests have been developed to mimic the most common real-life crash scenarios in terms of crash type and crash severity. If the vehicle safety system can identify and discriminate between the various crash tests and actuate the actuatable restraints in response thereto, then the vehicle safety system will also perform in the real-life crash event. 
     Crash tests can involve a variety of impact types, such as frontal impacts, side impacts, offset impacts, and oblique or angular impacts, each of which is carried out at a prescribed velocity. Vehicle crash events can involve various impacting structures, such as a pole or deformable barrier, the latter of which can be stationary or moving. Each of these impacting structures are specifically designed to represent structures encountered in the real-life crash scenarios. For example, the pole crash test, can implement a pole designed to represent the typical telephone or traffic signal pole, and the deformable barrier can be designed to represent another vehicle involved in the collision. 
     Vehicle safety systems can be configured or adapted to discriminate those crash events for which actuation of the actuatable restraints is desired from those crash events for which actuation of the actuatable restraints is not desired. Crash discrimination therefore entails determining the crash type, e.g., frontal, side, offset, oblique angular, etc. Crash discrimination also entails determining the impacting structure type, e.g., pole or deformable barrier. Crash discrimination also entails determining the crash severity. Crash discrimination further entails misuse conditions, such as vehicle off-road driving, and other conditions, such as animal impacts, where vehicle impacts may be detected, but actuation of the actuatable restraints is not warranted. 
     Vehicle safety systems therefore must determine whether an event is a must-fire (MF) event, a no-fire (NF) event, or a misuse event. Must fire events are those crash events for which airbag deployment must take place. It is for these crash events that further discrimination is desirable, because this further classification allows for tailoring the airbag deployment for the particular classified event. No-fire events are those whose magnitude does not warrant airbag deployment. Misuse events are events where the vehicle is being used in a non-traditional manner, such as off-road driving, and airbag deployment is not desired, even though the magnitude of the detected crash event may dictate otherwise. 
     The National Highway Traffic Safety Administration (“NHTSA”) is a U.S. government agency that governs vehicle safety and assesses new car safety via its New Car Assessment Program (US-NCAP). Through US-NCAP, NHTSA establishes crash tests to establish the crashworthiness of new vehicles and rates those vehicles with a star rating, with a five-star rating being the best. The standards for these tests are issued as Federal Motor Vehicle Safety Standards (FMVSS), which NHTSA issues to implement safety laws passed by Congress. The FMVSS standards describe in detail the precise test procedures used to determine the US-NCAP ratings, which are determined from metrics measured for crash test dummies positioned in the vehicle at the time of the test. 
     The U.S. is not the only country to have its own new car assessment program. Other countries, such as China, Japan, and Australia, and other groups of countries, such as Europe and Latin America, have their own NCAP. While the new car ratings issued by these bodies are similar, some utilize crash tests with slightly different methods. 
     Over the years, safety standards are modified and updated to “push the envelope” when it comes to automotive safety. As a result, in keeping up with the standards, automobile manufacturers are pushed to constantly improve the safety of their products. As the standards become more rigorous, the safety systems adapt and become more complex and capable. Through the evolution of vehicle safety systems, it has been discovered that crash classification is one of the key aspects that helps determine the efficacy of the system. If the safety system can accurately and robustly identify the crash scenario as defined by a safety standard, it can take measures tailored to produce the best results for occupants involved in accidents for which the standard is designed. 
     While vehicle safety systems have been developed with the ability to discriminate a variety of crash events, there exists a continuing need to further classify and discriminate amongst crash events so that the vehicle safety system can take the appropriate responsive action. Among the crash events for which discrimination can be desired are different types of vehicle frontal impact crash events. A rigid frontal barrier is fixed and non-deformable, representing rigid barriers found on the road, such as concrete barriers. 
     One particular frontal crash test is a high speed rigid barrier frontal crash test in which the vehicle impacts the rigid barrier at 56 kph (approximately 35 mph). Because this test involves a vehicle moving at a high rate of speed into a rigid, non-deformable barrier, the crash event develops quickly. Therefore, it is desirable to discriminate this crash scenario as soon as possible in order to allow as much time as possible for the safety system to deploy the necessary safety devices. 
     SUMMARY 
     A method for controlling an actuatable safety device for helping to protect a vehicle occupant includes sensing left-front and right-front pressure values via a pressure tube sensor. The method also includes executing pressure tube metrics that evaluate the left-front and right-front pressure values and selecting switched crash thresholds in response to the pressure tube metrics. The method also includes sensing vehicle acceleration parameters and executing one or more crash metrics that evaluate the vehicle acceleration parameters to determine whether the switched crash thresholds are exceeded. The method further includes controlling deployment of the actuatable safety device in response to determining that the switched crash thresholds are exceeded. 
     According to one aspect, the switched crash thresholds can include no-fire thresholds and misuse boxes. 
     According to one aspect, alone or in combination with any other aspect, the pressure tube metrics can include at least one of pressure tube sensor differential discrimination metrics and pressure tube sensor asymmetric split metrics. 
     According to one aspect, alone or in combination with any other aspect, the pressure tube sensor differential discrimination metrics can evaluate a rate of change in at least one of the left-front and right-front pressure values. 
     According to one aspect, alone or in combination with any other aspect, the pressure tube sensor differential discrimination metric can evaluate a front pressure differential moving average absolute value against an upfront sensor acceleration moving average. 
     According to one aspect, alone or in combination with any other aspect, the pressure tube asymmetric split metrics can evaluate a phase shift in the left-front and right-front pressure values to determine whether a front impact is symmetric, asymmetric-left, or asymmetric-right. 
     According to one aspect, alone or in combination with any other aspect, the pressure tube asymmetric split metrics can evaluate a front-left pressure moving average against a front-right pressure moving average. 
     According to one aspect, alone or in combination with any other aspect, selecting switched crash thresholds in response to the pressure tube metrics can include selecting the switched crash thresholds in response to at least one of:
         The pressure tube sensor differential discrimination metric for either of the left-front and right-front pressure values exceeding an OR threshold.   The pressure tube sensor differential discrimination metric for both the left-front and right-front pressure values exceeding an AND threshold, wherein the AND threshold has a magnitude that is less than a magnitude of the OR threshold.   The pressure tube sensor differential discrimination metric for the left-front pressure value exceeding an OR ASYM threshold and the pressure tube asymmetric split metric indicates an asymmetric-left front impact,   The pressure tube sensor differential discrimination metric for the right-front pressure value exceeding an OR ASYM threshold and the pressure tube asymmetric split metric indicates an asymmetric-right front impact.       

     According to one aspect, alone or in combination with any other aspect, the method can also include determining the occurrence of a high speed rigid barrier collision in response to:
         Selecting switched crash thresholds in response to the pressure tube metrics.   The pressure tube sensor differential discrimination metric for both the left-front and right-front pressure values exceeding a 2 nd  AND threshold.   The pressure tube asymmetric split metric indicates an symmetric front impact.       

     According to one aspect, alone or in combination with any other aspect, the vehicle is an electric vehicle, hybrid vehicle or vehicle equipped with high voltage installation and the method further can include deactivating a high-voltage connection in response to:
         The pressure tube sensor differential discrimination metric for both the left-front and right-front pressure values exceeding an AND HV threshold.   A long moving average metric for both the left-front and right-front pressure values exceeds a FLAT threshold.   At least one of a left low speed rigid barrier suppression metric and a right low speed rigid barrier suppression metric exceeds an HV threshold.       

     According to one aspect, alone or in combination with any other aspect, the left low speed rigid barrier suppression metric can include a left pressure tube sensor differential discrimination metric that evaluates a front-left pressure differential moving average absolute value against one of a CCU X-axis acceleration moving average and an acceleration value obtained from a left up-front acceleration sensor. The right low speed rigid barrier suppression metric can include a right pressure tube sensor differential discrimination metric that evaluates a front-right pressure differential moving average absolute value against one of a CCU X-axis acceleration moving average and an acceleration value obtained from a right up-front acceleration sensor. 
     According to one aspect, alone or in combination with any other aspect, a vehicle safety system can include one or more vehicle safety devices and a controller configured to execute the method as set forth above to actuate one or more vehicle safety devices. 
     According to one aspect, alone or in combination with any other aspect, the vehicle safety system can include a pressure tube sensor (PTS) configured to be mounted at the front of the vehicle. The vehicle safety system can also include an airbag control unit (ACU) configured to be mounted in an instrument panel of the vehicle. The controller can be implemented in the airbag control unit, and the pressure tube sensor can be configured to communicate with the ACU. 
     According to one aspect, alone or in combination with any other aspect, the vehicle safety system can include a left upfront sensor (LT_UFS) configured to be mounted at a front-left corner of the vehicle, and a right upfront sensor (RT_UFS) configured to be mounted at a front-right corner of the vehicle. The left upfront sensor and right upfront sensor can be configured to communicate with the ACU. 
     According to one aspect, alone or in combination with any other aspect, the one or more vehicle safety devices can include at least one of a frontal airbag and a seatbelt pretensioner. 
    
    
     
       DRAWINGS 
       The foregoing and other features and advantages of the invention will become apparent to one skilled in the art upon consideration of the following description of the invention and the accompanying drawings in which: 
         FIG.  1    is a schematic diagram illustrating a vehicle safety system, according to one aspect of the invention. 
         FIG.  2    is a schematic block diagram depicting signal conditioning performed by the vehicle safety system. 
         FIGS.  3 A- 3 C  illustrate pressure tube sensor metrics that can be implemented by the vehicle safety system. 
         FIG.  4    illustrates a Determination and comparison function that can be implemented by the vehicle safety system. 
         FIGS.  5 - 7    illustrate discrimination logic that can be implemented by the vehicle safety system. 
     
    
    
     DESCRIPTION 
     Vehicle Safety System 
     Referring to  FIG.  1   , a vehicle  10  includes a vehicle safety system  100 . The safety system  100  can include a plurality of actuatable vehicle safety devices, which are shown schematically at  110 . The actuatable safety devices  110  can, for example, include airbags (e.g., frontal airbags, side impact airbags, curtain airbags, etc.) and seatbelts, e.g., seatbelt pretensioners. 
     The system  100  further includes a plurality of vehicle-based sensors operatively connected to an airbag control unit (“ACU”)  120 . The ACU  120  is typically mounted between front seats of the vehicle  10 , but could be mounted at alternative locations. The protection devices  110  also are operatively connected to the ACU  120 . The vehicle-based sensors are used for sensing vehicle conditions and crash indications. Since the vehicle-based sensors provide analog signals, the ACU  120  performs analog-to-digital (A/D) conversions of the sensor signals. The ACU  120  includes a controller that is programmed to receive the digitized signals from the vehicle-based sensors, to determine vehicle conditions, such as crash conditions, based on the signals from the vehicle-based sensors, and to control actuation of the safety devices  110  in response to the determined vehicle conditions. 
     The vehicle-based sensors include an ACU sensor  122 , which is integrated into the ACU  120 . The ACU sensor  122  includes a two-axis accelerometer for measuring vehicle accelerations in the direction of the X-axis (X VEH ) and the Y-axis (Y VEH ) and determining values indicative of these sensed vehicle accelerations. CCU_X is a value indicative of vehicle acceleration measured in the direction of the vehicle X-axis at the location of the ACU  120 . CCU_Y is a value indicative of vehicle acceleration measured in the direction of the vehicle Y-axis at the location of the ACU  120 . The ACU  120  can use these signals to determine crashes and to discriminate amongst different crash types. 
     The vehicle-based sensors also include a front pressure tube sensor  140 , referred to herein as the PTS. The PTS  140  is mounted at the front end of the vehicle behind the fascia of the front vehicle bumper  18 . The PTS  140  includes two pressure transducers  142  fluidly connected to a pressure tube  144 . The pressure transducers  142  are mounted at front-left (FR_LT) and front-right (FR_RT) corners, respectively, of the vehicle  10 . 
     The pressure tube  144  is constructed of a flexible, deformable material, such as a vinyl or poly-vinyl chloride (PVC) material. Front impacts with the vehicle deform the pressure tube  144 , causing an increase in pressure inside the tube. The pressure transducers  142  sense the change in pressure and produce left and right pressure signals (PIS_LP and PIS_RP, respectively) that correspond to the pressure change. The PIS_LP and PIS_RP signals are sent to the ACU  120 , which can use the pressure signals to help sense or determine the occurrence of front impacts/crashes with the vehicle  10  and to discriminate amongst different types of front impacts/crashes based on the pressure signals. 
     The vehicle-based sensors can also include a left upfront sensor  146  that is mounted at or near the front-left (FR_LT) corner of the vehicle  10  and a right upfront sensor  148  that is mounted at or near the front-right (FR_RT) corner of the vehicle. The upfront sensors  146 ,  148  are accelerometer sensors that measure vehicle accelerations in the direction of the X-axis. The upfront sensors  146 ,  148  are operable to produce left and right upfront X-axis acceleration signals LT_UFS and RT_UFS, respectively, which are provided to the ACU  120 . The ACU  120  can use these signals to determine crashes and to discriminate amongst different crash types. 
     Signal Conditioning 
       FIG.  2    illustrates block diagrams showing the signal conditioning that some of the sensor data undergoes prior to being implemented in the metrics and algorithms disclosed herein. For example the pressure signals (PIS_LP, PIS_RP) from the pressure transducers  142  of the pressure tube sensor  140  are conditioned via several different functions to produce three different conditioned signals that can be implemented by the metrics and algorithms disclosed herein. Additionally, the central Y-axis acceleration (CCU_Y) from the ACU  120  are conditioned via several different functions to produce a conditioned signal implemented by the metrics and algorithms disclosed herein. 
     As shown in  FIG.  2   , the pressure signals PIS_LP, PIS_RP can undergo a difference calculation (1−z −1  block  152 ) to determine the difference between current and previous data samples. An absolute value determination (Abs block  154 ) produces a positive value for the sensor signal. Low-pass filter (LPF) calculations with different window sizes can be used to smooth the pressure signals. 
     For example, in  FIG.  2   , there are three LPF blocks, each of which have different window size. LPF  1  block  156  calculates a moving average over a predetermined window size, and produces moving average values PIS_LP MA and PIS_RP MA. LPF  2  block  158  calculates a moving average with a wider window size and produces long moving average values PIS_LP MA Long and PIS_RP MA Long. LPF  3  block  160  calculates a moving average with a shorter window size. Because it receives its input from 1−z −1  block  152  and Abs block  154 , short MA block  160  produces moving average values PIS_LP Diff Abs MA and PIS_RP Diff Abs MA. The sample times for each of the MA blocks  156 ,  158 ,  160  can be tunable. 
     Additionally, as shown in  FIG.  2   , the acceleration signal CCU_Y undergoes low pass filtering (LPF block  164 ), which can be used to eliminate high frequency noise from the CCU_Y acceleration signal. An acceleration moving average (AMA block  166 ) calculation with a tunable window size can be used to smooth the filtered acceleration signal. An absolute value determination (Abs block  168 ) produces a positive value for the conditioned signal CCU_Y AMA Abs. 
     Pressure Tube Data and Corresponding Metrics 
     The pressure tube sensor  140  produces pressure data that the ACU  120  can interpret to make early/quick determinations regarding front impacts with the vehicle  10 . Because the pressure transducers  142  are positioned at opposite ends of the pressure tube  144 , certain characteristics of front impacts with the vehicle can be determined by evaluating the pressure data received from the transducers over time and comparing that data with other data, such as acceleration data (ACU_X, ACU_Y) gathered from the ACU  120 . 
     This is illustrated in  FIGS.  3 A- 3 C , each of which shows example plots of pressure vs. time for both the left and right pressure transducer  142  in response to a front impact. Which of the pressure transducers (i.e., left or right) is represented by which of the plots (i.e., solid or dashed) is not important in  FIGS.  3 A- 3 C .  FIGS.  3 A- 3 C  also illustrate example implementations of metrics that can be associated with each of the pressure vs. time plots. 
     Referring to the plot on the left in  FIG.  3 A , one characteristic that can be determined by comparing pressure signals over time for the left and right pressure transducers  142  is amplitude. The amplitude of the pressure signals (PIS_RP, PIS_LP) is indicative of the magnitude or severity of the sensed frontal impact. Because frontal impacts produce high magnitude pressure rises in the pressure tube  144 , sensor clipping can result from all types of events, including must-fire, no-fire, and misuse events. Because of this, the amplitude of the pressure signals (PIS_RP, PIS_LP) does not lend well to crash determination discrimination per se. However, the moving average (MA) of the pressure signals can be used to discriminate short frontal impacts from frontal impacts of a longer duration. An example implementation of a metric associated with this amplitude pressure vs. time plot is shown on the right in  FIG.  3 A . 
     As shown in the metric illustrated on the right in  FIG.  3 A , frontal crashes of a threshold magnitude and duration can be discriminated using the long moving average of the pressure signal (PIS_LP Long MA, PIS_RP Long MA). The crash duration threshold, shown by the dashed line, represents the magnitude, i.e., the amplitude, of the long MA signal that must be maintained over time in order to discriminate the frontal crash condition. The requisite duration can be tunable (e.g., via the moving average window) so that the discriminated frontal crash conditions can match predetermined criteria. 
     Referring to the plot on the left in  FIG.  3 B , another characteristic that can be determined by comparing pressure signals over time for the left and right pressure transducers  142  is the phase shift between the signals. Frontal impacts acting on the pressure tube sensor  140  generate a pressure wave that propagates through the pressure tube  144 . If the impact is centered on the vehicle  10  and, thus, the pressure tube sensor  140 , the pressure wave will travel the tube and reach the pressure transducers  142  at the same time. In this case, the pressure signals (PIS_RP, PIS_LP) will be concurrent, i.e., they will overlie each other in the pressure vs. time plot on the left in  FIG.  3 B . If this were the case, there would be no phase shift between the signals. 
     If, however, the impact is shifted to one side or another of the vehicle  10 , the pressure rise and the resulting pressure signals will be phase shifted due to the differential in time that the pressure wave takes to reach the transducers  142 . This is the case in the example plot illustrated on the left in  FIG.  3 B , in which the magnitude of the phase shift is illustrated generally by the double arrows. Accordingly, it will be appreciated that an impact that occurs toward the left side of the vehicle  10  will produce a phase shift with the left pressure signal occurring first in time, followed by the right pressure signal. Conversely, it will be appreciated that an impact that occurs toward the right side of the vehicle  10  will produce a phase shift with the right pressure signal occurring first in time, followed by the left pressure signal. The amount of shift between the signals can therefore be indicative of the degree of offset between the vehicle and the colliding vehicle/object. An example implementation of a metric associated with this phase shift pressure vs. time plot is shown on the right in  FIG.  3 B . 
     As shown in the asymmetric split metric illustrated on the right in  FIG.  3 B , the phase shift between the pressure transducers  142  can be used to identify the symmetry of a front collision. A symmetrical front collision have the impacting object centered on the vehicle, and left/right asymmetric front collisions have the impacting object shifted left or right on the vehicle. The asymmetric split metric utilizes moving averages of the left and right pressures (PIS_LP MA, PIS_RP MA) to make these determinations. 
     The asymmetric split metric of  FIG.  3 B  implements regions for determining the symmetry of frontal crashes. In  FIG.  3 B , these regions include a symmetric (SYM) region, a left asymmetric (L_ASYM) region, and a right asymmetric (R_ASYM) region. Comparing PIS_LP MA and PIS_RP MA in the asymmetric split metric of  FIG.  3 B , if the metric falls in the SYM region, the frontal collision is determined to be symmetric. If the metric falls in the L_ASYM region, the frontal collision is determined to be left asymmetric. If the metric falls in the R_ASYM region, the frontal collision is determined to be right asymmetric. Through this, the asymmetric split metric can discriminate between symmetric, left asymmetric, and right asymmetric frontal collisions, and this determination can be used to further discriminate frontal vehicle collisions. 
     Because frontal impacts produce high magnitude pressure rises in the pressure tube  144 , sensor clipping can result from all types of events, including must-fire (MF), no-fire (NF), and misuse events. Because of this, the amplitude of the pressure signals (PIS_RP, PIS_LP) itself does not lend well to crash discrimination. The rate of change in the pressure signals can, however, differ in the case of must-fire vs. no-fire front impact events. The rate of change can, therefore, be used to discriminate between those events. 
     Referring to the plot on the left in  FIG.  3 C , the pressure signals can be compared over time to determine the rate of change of the pressure signals (PIS_RP, PIS_LP) from the pressure transducers  142 . The pressure signal rate of change (before clipping) can be indicative of the dynamics of the front impact. The steeper the plot, i.e., the higher the pressure rate of change, the greater the magnitude of the front impact. Accordingly, the pressure rate of change can be used to produce a metric capable of discriminating must-fire (MF) events from no-fire (NF) events. An example implementation of a metric associated with this rate of change pressure vs. time plot is shown on the right in  FIG.  3 C . 
     As shown on the right in  FIG.  3 C , the PTS differential discrimination metric implements various thresholds, indicated by solid lines, for discriminating must-fire events from no-fire events. The different thresholds illustrated in  FIG.  3 C  are used to discriminate particular events, so each can have a threshold magnitude that differs from the others. For example, the OR threshold can have a magnitude that is less than the AND threshold. The threshold indications of the PTS differential discrimination metric are implemented in control algorithms (described below) that are operative to make various determinations regarding the vehicle. 
     The PTS differential discrimination metric utilizes the PIS_LP Diff Abs MA and PIS_RP Diff Abs MA signals which, as described previously is a differential between current and previous pressure samples (see the 1−z −1  block  152  of  FIG.  2   ). The differential signals PIS_LP Diff Abs MA and PIS_RP Diff Abs MA crossing these thresholds determine conditions that warrant a must-fire event if other conditions related to the event are satisfied. The Boolean logic associated with these determinations is described below in the description of the PTS threshold switching algorithms. 
     Determination and Comparison Function 
       FIG.  4    illustrates a determination and comparison function  170  that is implemented in the ACU  120 . The determination and comparison function  170  makes a fire/no-fire determination in response to metrics associated with vehicle collisions. A “fire” determination is a determination that collision metrics have exceeded a threshold for actuating or “firing” the vehicle safety devices  110 . A “no-fire” determination is a determination that collision metrics have not exceeded the firing threshold and, thus, firing the safety devices  110  is not necessary. 
     As shown in  FIG.  4   , an acceleration signal ACU_X (provided by the ACU sensor  122 ) has characteristics (e.g., frequency and amplitude) indicative of the vehicle&#39;s acceleration in a direction substantially parallel to the X-axis of the vehicle  12 . The acceleration signal ACU_X is provided to a low-pass-filter (LPF) function  172  implemented in the ACU  120 . The LPF function  172  filters the acceleration signal ACU_X to eliminate extraneous signal components, such as frequencies resulting from extraneous vehicle operating events and/or from road noise. The signal components removed through filtering are those determined not to be useful in discriminating 1) whether a vehicle crash event is occurring and 2) whether a detected vehicle crash event is one for which actuation of the vehicle safety devices  110  is desired, i.e., a fire event. 
     Vehicle architectures vary from model to model, so the signal components removed through filtering can also vary from model to model. Empirical testing or calculation may be used to determine the signal components useful for discrimination of a vehicle crash condition in a particular vehicle of interest and/or determining whether a detected vehicle crash event is one for which actuation of the vehicle safety devices  110  is desired. It is those signal components that are indicative of a vehicle crash condition and/or that are useful in determining whether a detected vehicle crash event is one for which actuation of the vehicle safety devices  110  is desired that the LPF function  172  outputs for further processing. 
     The filtered output signal from the LPF function  172  is provided to an analog-to-digital (A/D) converter function  174  implemented in the ACU  120 . The A/D converter function  174  converts the analog filtered crash acceleration signal to a digital signal. The output of the A/D converter function  174  may be filtered with an additional filter function (not shown) to eliminate small drifts and offsets associated with the A/D conversion. This additional filter function can be digitally implemented within the ACU  120 . 
     A determination and comparison function  170  implemented in the ACU  120  determines two crash metric values, VEL_REL_X and DISPL_REL_X, from the filtered crash acceleration signal ACU_X. More specifically, the determination and comparison function  170  determines VEL_REL_X, which is velocity in a direction substantially parallel to the X-axis, by integrating the filtered crash acceleration signal ACU_X. The determination and comparison function  170  also determines DISP REL X, which is displacement in a direction substantially parallel to the X-axis, by double integrating the filtered crash acceleration signal ACU_X. 
     In one example implementation, the crash displacement value and crash velocity value can be determined using a virtual crash sensing process fully described in U.S. Pat. No. 6,156,539 to Foo et al. and U.S. Pat. No. 6,036,225 to Foo et al. using a spring mass model of the occupant to account for spring forces and damping forces. A detailed explanation of a spring-mass model can be found in U.S. Pat. No. 5,935,152 to Foo et al. The determination and comparison function  170  compares the value VEL_REL_X against at least one crash discrimination threshold, which comparison is used to discriminate whether a crash event is occurring. 
     In accordance with the example implementation of the invention illustrated in  FIG.  4   , the determination and comparison function  170  compares the VEL_REL_X value as a function of the DISPL_REL_X value against a discrimination threshold, which is identified in  FIG.  4    as a no-fire threshold  180 . If the VEL_REL_X as a function of DISPL_REL_X exceeds the no-fire threshold  180 , a “fire” or “actuate” condition for the safety devices  110  is determined. The determination and comparison function  170  can include a latch function (not shown) that time latches for a predetermined period a determination that the fire/actuate condition has been met. 
     As part of comparing the VEL_REL_X value as a function of the DISPL_REL_X value against the no-fire threshold, the determination and comparison function  170  also ensures that the VEL_REL_X value lies outside of a safing immunity or misuse box  184  before initiating any comparison. The purpose of the misuse box  184  is to filter misuse events and to prevent actuation of the safety devices  110  when crash velocity and/or crash displacement values are within the misuse box values. Misuse events include hammer blows, road bumps, door slams, and other events, which produce output signals from crash acceleration sensors that are not the result of vehicle crash events. 
     The misuse box  184  is represented by values of determined crash velocity and crash displacement below which the safety devices  110  should not be actuated. It is only after the determined crash velocity VEL_REL_X values and/or crash displacement DISPL_REL_X values exceed the velocities and displacements represented by the misuse box  184 , i.e., are outside of the misuse box, that actuation of the safety devices  110  is normally permitted in response to crash metric determinations. 
     The misuse box  184  defines an area bounded by predetermined upper limit values of VEL_REL_X and DISPL_REL_X. When the determined value of VEL_REL_X is within the area of the misuse box  184 , the safing function is OFF or at a digital LOW condition and, therefore, actuation of the safety devices  110  is inhibited and cannot occur. If the value VEL_REL_X is outside of the misuse box  184 , the safing function is ON or at a digital HIGH condition and, therefore, the safety devices  110  can be actuated. 
     If the value VEL_REL_X is outside the misuse box  184  and then enters or re-enters the misuse box, the time period that the safing function is or remains ON can be extended or enhanced. This is referred to as a latched time period. Also, even though only one misuse box is shown in  FIG.  4   , each no-fire threshold can have an associated misuse box. 
     The no-fire threshold  180  implemented by the determination and comparison function  170  can be fixed or variable. Additionally, the determination and comparison function  170  can include more than one no-fire threshold. Multiple no-fire thresholds can be used to provide different no-fire thresholds for different safety devices. For example, one no-fire threshold can be used for airbags, and a different no-fire threshold can be used for seatbelt pretensioners. Additionally or alternatively, multiple no-fire thresholds can be selected or “switched” based on conditions sensed and/or determined from other vehicle safety sensors/systems. In this manner, the no-fire threshold can be further tailored to sensed vehicle conditions at the time of the vehicle crash. 
     PTS Threshold Switching Algorithms 
     Utilizing the metrics described above in reference to  FIGS.  3 A- 3 C , the PTS pressure data obtained from the pressure tube sensor  140  can allow for early and quick discrimination of frontal crash scenarios. This early and quick discrimination allows for switching, i.e., lowering, the no-fire threshold and misuse box of the determination and comparison function  170  ( FIG.  4   ) so that the determination and comparison function can respond quicker to certain discriminated frontal crashes. An example implementation of these switched thresholds is illustrated in  FIG.  4   . 
     As shown in  FIG.  4   , in addition to the no-fire threshold  180 , the determination and comparison function  170  also includes a switched no-fire threshold  182 . The switched no-fire threshold  182  is a no-fire threshold in which the magnitude of the metric required to produce a fire condition is reduced from that of the unswitched no-fire threshold  180 . The determination and comparison function  170  implements the switched no-fire threshold  182  in response to sensed vehicle conditions at the time of a vehicle crash. In the absence of those sensed vehicle conditions, the determination and comparison function  170  implements the no-fire threshold  180 . 
     Also, in addition to the misuse box  184 , the determination and comparison function  170  of  FIG.  4    also includes a switched misuse box  186 . The switched misuse box  186  is a misuse box in which the magnitude of the metric required to indicate a vehicle misuse and inhibit the fire condition is reduced from that of the misuse box  184 . 
     According to one aspect of the invention, whether the determination and comparison function  170  implements the no-fire threshold  180  and misuse box  184  or is switched to the switched no-fire threshold  182  and switched misuse box  186  is determined in response to metrics derived from the pressure tube sensor  140 . A PTS threshold switching algorithm  200  illustrating one example implementation of this is shown in  FIG.  5   . The PTS threshold switching algorithm  200  implements the PTS differential discrimination metric of  FIG.  3 C  and the asymmetric split metric of  FIG.  3 B . As shown in  FIG.  5   , any of the conditions at OR block  208  being satisfied will trigger use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ), as indicated at  220 . These conditions are set forth in the following paragraphs. 
     OR block  208  will be satisfied if the OR threshold of PIS_LP differential discrimination metric  202  is met. Here, the OR threshold is met when the PIS_LP Diff Abs MA crosses the OR threshold line of the metric  202 . The OR threshold is indicative of high magnitude pressure differentials, which indicate a high pressure rate of change. Sensing this high pressure rate of change warrants use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ) and thereby can result in a quicker discrimination of a must-fire front impact event. 
     Similarly, OR block  208  will also be satisfied if the OR threshold of PIS_RP differential discrimination metric  204  is met. Here, the OR threshold is met when the PIS_RP Diff Abs MA crosses the OR threshold line of the metric  204 . The OR threshold is indicative of high magnitude pressure differentials, which indicate a high pressure rate of change. Sensing this high pressure rate of change warrants use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ) and thereby can result in a quicker discrimination of a must-fire front impact event. 
     OR block  208  will also be satisfied if AND block  206  is satisfied. AND block  206  is satisfied when the AND thresholds of both the PIS_LP differential discrimination metric  202  and the PIS_RP differential discrimination metric  204  are met. The AND thresholds are met when the PIS_LP/RP Diff Abs MA crosses the AND threshold line of the respective metric  202 ,  204 . The AND threshold is indicative of high magnitude pressure differentials, which indicate a high pressure rate of change. The magnitude of the AND threshold pressure differentials can be lower than the OR thresholds. Thus, both AND thresholds must be met to trigger switching. Both AND thresholds being met warrants use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ) and thereby can result in a quicker discrimination of a must-fire front impact event. 
     OR block  208  will also be satisfied if AND block  214  is satisfied. AND block  214  is satisfied when the OR ASYM threshold of the PIS_LP differential discrimination metric  210  is met and the asymmetric split metric  212  indicates a left asymmetric (L_ASYM) frontal impact. The OR ASYM threshold is met when the PIS_LP Diff Abs MA crosses the OR ASYM threshold line of the metric  210 . The OR ASYM threshold is indicative of a high magnitude pressure differential, which indicates a high pressure rate of change. The magnitude of the OR ASYM threshold pressure differentials can be lower than the OR and the AND thresholds. Because of this, the OR ASYM is verified by the presence of the L_ASYM indication from the asymmetric split metric  212  at AND block  214 . The L_ASYM is triggered when the phase shift between the PIS_LP MA and the PIS_RP MA indicates a left asymmetric frontal impact (see,  FIG.  3 B ). The presence of the OR ASYM indication and the L_ASYM indications from metrics  210  and  212  met warrants use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ) and thereby can result in a quicker discrimination of a must-fire front impact event. 
     Similarly, OR block  208  will also be satisfied if AND block  218  is satisfied. AND block  218  is satisfied when the OR ASYM threshold of the PIS_RP differential discrimination metric  216  is met and the asymmetric split metric  212  indicates a right asymmetric (R_ASYM) frontal impact. The OR ASYM threshold is met when the PIS_RP Diff Abs MA crosses the OR ASYM threshold line of the metric  216 . The OR ASYM threshold is indicative of a high magnitude pressure differential, which indicates a high pressure rate of change. The magnitude of the OR ASYM threshold pressure differentials can be lower than the OR and the AND thresholds. Because of this, the OR ASYM is verified by the presence of the R_ASYM indication from the asymmetric split metric  212  at AND block  218 . The R_ASYM is triggered when the phase shift between the PIS_RP MA and the PIS_LP MA indicates a right asymmetric frontal impact (see,  FIG.  3 B ). The presence of the OR ASYM indication and the R_ASYM indications from metrics  212  and  216  met warrants use of the switched no-fire threshold  182  and the switched misuse box  186  (see,  FIG.  4   ) and thereby can result in a quicker discrimination of a must-fire front impact event. 
     From the above, it will be appreciated that the vehicle safety system  100  can implement control algorithms for determining when to actuate the safety devices  110 . The control algorithms can implement metrics that utilize data from the pressure tube sensor  140  to improve the responsiveness of the system by realizing a quicker discrimination of certain types of front impacts by switching the no-fire thresholds and misuse boxes. 
     High Speed Rigid Barrier Detection 
     The early and quick discrimination facilitated by the PTS pressure data obtained from the pressure tube sensor  140  also allows for early detection of a high speed rigid barrier frontal impact. The high speed rigid barrier can, for example, be a frontal impact commensurate with 56 kph rigid barrier criteria (e.g., US-NCAP standards). 
     A high speed rigid barrier impact detection algorithm  230  is illustrated in  FIG.  6   . The high speed rigid barrier impact detection algorithm  230  implements the PTS differential discrimination metric of  FIG.  3 C , the asymmetric split metric of  FIG.  3 B , and use switched thresholds and misuse boxes indication  220  of  FIG.  5   . As shown in  FIG.  6   , all of the conditions at AND block  238  being satisfied will trigger a high speed rigid barrier detected block  240 . These conditions are set forth in the following paragraphs. 
     AND block  238  will be satisfied if: 
     The 2 nd  AND threshold of PIS_LP differential discrimination metric  232  is met. 
     The 2 nd  AND threshold of PIS_RP differential discrimination metric  234  is met. 
     Asymmetric split metric  236  indicates a symmetric (SYM) frontal impact. 
     The use switched thresholds and misuse boxes block  220  (see,  FIG.  5   ) is satisfied. 
     The use switched thresholds and misuse boxes block  220  being satisfied indicates that at least one of the conditions at OR block  208  is satisfied. As described above in regard to  FIG.  5   , the conditions that give rise to the use switched thresholds and misuse boxes block  220  being satisfied are based on pressures sensed via the pressure tube sensor  140 . 
     Utilizing the pressure tube sensor  140  is beneficial in the early discrimination of frontal vehicle collisions, especially high-speed frontal vehicle collisions, because the sensor is physically located at the very front of the vehicle where the impact occurs. Because of this, the pressure tube sensor  140  is extremely quick to respond to vehicle deformation resulting from the collision, and can therefore provide a quick and reliable indication of a frontal vehicle collision prior to other sensors, such as accelerometers. 
     The PIS_LP/RP differential discrimination metrics  232 ,  234  compare PIS_LP Diff Abs MA and the PIS_RP Diff Abs MA with LT_UFS_MA and RT_UFS_MA, respectively. LT_UFS_MA and RT_UFS_MA are moving average acceleration values measured via the left upfront sensor  146  and the right upfront sensor  148 , respectively. The 2 nd  AND thresholds of the PIS_LP/RP differential discrimination metrics  232 ,  234  are met when both the PIS_LP Diff Abs MA and the PIS_RP Diff Abs MA cross their respective 2 nd  AND threshold lines of metrics  232  and  234 . 
     The 2 nd  AND thresholds are indicative of high magnitude pressure differentials, which indicate a high pressure rate of change associated with a high speed rigid barrier frontal impact. The SYM indication from the asymmetric split metric  236  indicates a symmetric front impact. Sensing the high pressure rates of change at the front-left and front-right of the vehicle (via the PIS_LP/RP Differential Discrimination metrics  232  and  234 ) and having them verified as symmetrical by the SYM indication from asymmetric split metric  236  warrants the indication at block  240  that a high speed rigid barrier impact is detected. 
     From the above, it will be appreciated that the vehicle safety system  100  can implement control algorithms for determining when to actuate the safety devices  110 . The control algorithms can implement metrics that utilize data from the pressure tube sensor  140  to improve the responsiveness of the system by realizing a quicker discrimination of high speed rigid barrier front impacts. 
     High Voltage (HV) Deactivation 
     The early and quick discrimination facilitated by the PTS pressure data obtained from the pressure tube sensor  140  also allows for early detection of a frontal impact for which deactivation of the electrical system in high voltage (HV) vehicles is warranted. As used herein, HV vehicles can include electric vehicles, hybrid vehicles, or any other vehicle equipped with a high voltage installation. HV deactivation can, for example, cutting off or isolating the vehicle battery from the remainder of the vehicle by, for instance, tripping a switch or circuit breaker. 
     An HV deactivation algorithm  250  is illustrated in  FIG.  7   . The HV deactivation algorithm  250  implements a no-fire/must-fire (NF/MF) discrimination metric  252 , an impact misuse suppression discrimination metric  260 , and a low-speed rigid barrier suppression discrimination metric  270 . As shown in  FIG.  7   , all of the conditions at AND block  278  being satisfied will trigger an HV deactivation block  280 . The conditions at AND block  278  are determined by the discrimination metrics  252 ,  260 , and  270 . These conditions are set forth in the following paragraphs. 
     NF/MF discrimination metric  252  determines whether PIS_LP/RP differential discrimination metrics  254 ,  256  indicate that both the PIS_LP Diff Abs MA and the PIS_RP Diff Abs MA cross their respective AND HV threshold lines. The AND HV thresholds are indicative of high magnitude pressure differentials, which indicate a high pressure rate of change associated with HV deactivation. The AND HV thresholds of both differential discrimination metrics  254  and  256  must be met in order to trigger AND block  278  and HV deactivation block  280 . 
     Impact misuse suppression discrimination metric  260  determines vehicle misuse for which HV deactivation may be desired. As shown, the impact misuse suppression discrimination metric  260  includes a metric  262  that monitors PIS_LP MA Long over time, and a metric  264  that monitors PIS_RP MA Long over time. Both metrics  262  and  264  include a flat threshold triggered when the respective PIS_LP/RP MA Long value exceeds a predetermined threshold value. The flat thresholds of both metrics  262  and  264  must be met in order to trigger AND block  278  and HV deactivation block  280 . 
     Low-speed rigid barrier suppression discrimination metric  270  determines rigid barrier impact for which HV deactivation may be desired. The rigid barrier suppression discrimination can, for example, discriminate a frontal impact commensurate with 27 kph rigid barrier criteria (e.g., US-NCAP standards). As shown, the rigid barrier suppression discrimination metric  270  includes a left HV PTS/X discrimination metric  272  and a right HV PTS/X discrimination metric  274 . Each of these metrics compares PIS_LP Diff Abs MA against either CCU_X AMA or LT/RT_UFS AMA. The HV thresholds of either metrics  272  and  274  must be met (see, OR block  276 ) in order to trigger AND block  278  and HV deactivation block  280 . 
     From the above, it will be appreciated that the vehicle safety system  100  can implement control algorithms for determining when to actuate the safety devices  110 . The control algorithms can implement metrics that utilize data from the pressure tube sensor  140  to improve the responsiveness of the system by realizing a quicker discrimination of HV deactivation events. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. For example, the figures illustrate various metrics that can be implemented in the disclosed vehicle safety system. The thresholds illustrated in these metrics, i.e., the shape of the lines identifying the thresholds, is by way of example only. Those skilled in the art will appreciate that the characteristics of these metrics, i.e., the shapes of the lines identifying the thresholds, can vary depending on a variety of factors, such as the configuration of the particular vehicle in which the safety system is implemented and the design criteria under which the safety system is designed. Such improvements, changes, and/or modifications within the skill of the art are intended to be covered by the appended claims.