Abstract:
Impending rollover events are detected based on vehicle roll rate, roll angle and lateral acceleration, and an assessment of the relative polarities of vehicle roll rate and lateral acceleration. When the vehicle roll rate and lateral acceleration are opposite in polarity, the roll rate vs. roll angle thresholds used to distinguish between rollover events and non-rollover events are more sensitive than for conditions for which the vehicle roll rate and lateral acceleration are of the same polarity. Additionally, the roll rate vs. roll angle thresholds are adaptively modified based on roll angle and lateral acceleration to provide increased detection sensitivity under conditions that typically precede a rollover event, and reduced detection sensitivity under conditions for which a rollover event is unlikely.

Description:
TECHNICAL FIELD 
   The present invention relates to rollover detection in motor vehicles, and more particularly to a detection method that provides early and reliable discrimination between rollover events and non-rollover events. 
   BACKGROUND OF THE INVENTION 
   Various rollover detection methodologies have been developed for activating electrically deployed rollover safety devices such as air bags, side curtains, seat belt pretensioners and pop-up roll bars. Additionally or alternatively, the system may activate visual, auditory or haptic warnings. A representative algorithm for detecting an impending rollover event is disclosed in the U.S. Pat. No. 6,542,792 to Schubert et al. In the disclosed algorithm, an angular rate sensor measures the vehicle attitude rate of change or angular roll rate, and an impending rollover event is detected based on a comparison of the roll rate and the corresponding roll angle. Other input signals pertaining to vehicle speed, steering wheel angle, yaw rate and side-slip angle may also be utilized. With any such algorithm, the objective is to discriminate between rollover events and non-rollover events as early as possible and as reliably as possible so that the warnings are issued and the safety devices deployed in a timely fashion, and only when actually needed. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved method of detecting an impending rollover event based on vehicle roll rate, roll angle and lateral acceleration, and an assessment of the relative polarities of vehicle roll rate and lateral acceleration. The method distinguishes between conditions where the vehicle roll rate and lateral acceleration are of opposite polarity and conditions where the vehicle roll rate and lateral acceleration are of the same polarity. When the vehicle roll rate and lateral acceleration are opposite in polarity, the likelihood of an impending rollover event is significantly higher, and the roll rate vs. roll angle thresholds used to distinguish between rollover events and non-rollover events are more sensitive than for conditions for which the vehicle roll rate and lateral acceleration are of the same polarity. Additionally, the roll rate vs. roll angle thresholds are adaptively modified based on roll angle, time and lateral acceleration to provide increased detection sensitivity under conditions that typically precede a rollover event, and reduced detection sensitivity under conditions for which a rollover event is unlikely. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a rollover detection system including a microprocessor-based control unit (MCU) for carrying out the method of this invention; 
       FIG. 2  graphically depicts a prior art grey zone of roll rate vs. roll angle operation for purposes of impending rollover detection; 
       FIG. 3  is a diagram of a vehicle encountering a curb trip condition, possibly triggering a rollover event; 
       FIG. 4  graphically depicts a first adaptive threshold adjustment according to this invention; 
       FIG. 5  graphically depicts a second adaptive threshold adjustment according to this invention; and 
       FIG. 6  is a flow diagram representative of a software routine executed by the MCU of  FIG. 1  for carrying out the method of this invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , the method of this invention is illustrated in the context of a rollover detection system  10  including a microprocessor-based control unit (MCU)  12 , an angular rate sensor  14 , a vertical accelerometer  16  and a lateral accelerometer  18 . The angular rate sensor  14  is responsive to the time rate of angular roll about the longitudinal axis of a vehicle in which the system  10  is installed, and it will be understood that a pitch angular rate sensor may also be included for the purpose of detecting a pitch-over condition. The MCU  12  is coupled to a power and communication interface module  20  and various restraint control modules (RCM)  22 ,  24 ,  26  by a bi-directional data communications bus  28 . General vehicle sensor data available on serial data bus  30  is captured for MCU  12  by the interface module  20 , and MCU  12  outputs a rollover deployment command signal on communications bus  28  when an impending rollover event is detected. The rollover deployment command signal activates the restraint control modules  22 ,  24 ,  26 , which deploy various rollover restraints such as seat belt pretensioners, and side curtain airbag and a pop-up roll bar. 
   As described in detail in the aforementioned U.S. Pat. No. 6,542,792, incorporated herein by reference, MCU  12  integrates the roll rate measured by angular rate sensor  14  to determine the accumulated roll angle, and utilizes the combination of the measured roll rate and the determined roll angle to detect an impending rollover event. The rollover detection technique is graphically illustrated in  FIG. 2 , where the determined roll rate vs. roll angle operating point is compared to a grey zone  30  defined by various thresholds, including an all-deploy threshold  32 , a no-deploy threshold  34 , a minimum roll angle  36  and a minimum roll rate  38 . In general, MCU  12  issues a rollover deployment command signal when the determined roll rate vs. roll angle operating point lies on or above the all-deploy threshold  32 . When the roll rate vs. roll angle operating point lies below the all-deploy threshold  32  but within the grey zone  30 , a potential rollover condition is indicated, and steps may be taken to increase the detection sensitivity if the potential rollover condition persists for at least a calibrated time interval. For example, the all-deploy threshold  32  can be temporarily adjusted downward toward the no-deploy threshold  34  as indicated by the successive threshold lines  32   a ,  32   b ,  32   c ,  32   d , thereby increasing the likelihood of rollover safety device deployment. In the same way, the minimum roll angle threshold  36  may be temporarily increased as indicated by the threshold line  36   a  during events such as the return-to-ground after a near rollover to reduce the likelihood of rollover restraint deployment. 
   The present invention recognizes that most rollover events are preceded by a period during which the roll rate and the lateral acceleration of the vehicle are opposite in polarity. This principle is illustrated in  FIG. 3  by the diagram of a receding vehicle  40  sliding sideways as indicated by the arrow  42 . The vehicle body  44  is coupled to wheels  46   a ,  46   b  by a set of suspension members  48 , and at some point, the right-hand wheels  46   b  contact an edge-of-roadway barrier  50  such as a curb. The impact with the barrier  50  imparts lateral acceleration to the vehicle  40  as indicated by the arrow  52 . At the same time, the vehicle  40  experiences a clockwise rotational movement about the barrier  50  as indicated by the arrow  54  since the vehicle center of mass (COM) is higher than the barrier height. The lateral acceleration is considered to be negative in sign because it opposes the prevailing forces accelerating the vehicle  40  toward the barrier  50 , while the clockwise rotational movement is considered to be a positive roll rate because it is consistent with the forces accelerating the vehicle  40  toward the barrier  50 . Using the same polarity conventions, the roll rate and lateral acceleration are also opposite in sign when sideways sliding causes the left-hand wheels  46   a  to contact a barrier. 
   The method of the present invention utilizes the above-described phenomenon to classify potential rollover events into one of two categories: those for which the roll rate and lateral acceleration are of opposing polarity, and those for which the roll rate and lateral acceleration are of the same polarity. The roll rate vs. roll angle thresholds of  FIG. 2  are individually calibrated for each category, providing increased detection sensitivity when the propensity for rollover is highest, as indicted by opposing polarity roll rate and lateral acceleration. 
   The method of the present invention preferably also includes adaptive downward adjustment of the calibrated minimum roll rate threshold (i.e., the threshold  38  of  FIG. 2 ) if the roll angle continuously exceeds a reference roll angle for at least a calibrated period of time. The adjustment may be a fixed, one-time adjustment, a graduated adjustment, or a staged adjustment, as desired. In any case, downward adjustment of the minimum roll rate threshold  38  increases the likelihood that the roll rate vs. roll angle operating point of the vehicle will fall within the grey zone  30 , leading to earlier rollover detection in cases where the vehicle is being operated on a steep slope or where the driver makes a sudden and severe steering correction after drifting off a roadway toward a ditch or embankment.  FIG. 4  graphically illustrates a one-time adaptive adjustment of the threshold  38 , with the lowered threshold being designated by the reference numeral  38 ′. 
   The method of the present invention preferably also includes adaptive upward adjustment of the calibrated all-deploy threshold (i.e., the threshold  32  of  FIG. 2 ) whenever the roll angle and roll rate exceed minimum deploy thresholds but the lateral acceleration is below a reference acceleration value. This can occur in cases which do not present a high likelihood of rollover, such as where a vehicle gradually drifts off a roadway with little or no corrective effort by the driver.  FIG. 5  graphically illustrates a one-time adaptive adjustment of the all-deploy threshold  32 , with the raised threshold being designated by the reference numeral  32 ′. 
   The flow diagram of  FIG. 6  represents a portion of a software routine executed by MCU  12  for carrying out the method of this invention. Initially, the block  60  is executed to read sensor information including the roll rate sensed by angular rate sensor  14  and the lateral acceleration (ACCEL_LAT) sensed by lateral accelerometer  18 . The roll angle is then updated based on the current roll rate data, as indicated at block  62  and explained in the aforementioned U.S. Pat. No. 6,542,792. The block  64  then determines if ACCEL_LAT and the measured roll rate are of opposite polarity as defined above in respect to  FIG. 3 . If so, the block  66  selects a relatively sensitive roll rate vs. roll angle calibration set corresponding to the grey zone  30  of  FIG. 2 ; if not, the block  68  selects a less sensitive roll rate vs. roll angle calibration set. As explained above, this enables earlier detection of an impending rollover event under conditions most conducive to rollover without sacrificing reliability. 
   The blocks  70 - 80  determine if the minimum roll rate (i.e., the threshold  38  of  FIG. 2 ) of the selected roll rate vs. roll angle calibration set should be lowered for increased sensitivity. The block  70  determines whether the roll angle magnitude is greater than a calibrated reference angle (REF_ANGLE) such as 34-39 degrees. Ordinarily, block  70  is answered in the negative, and the blocks  72  and  74  are executed to set a counter (CTR) to zero and to retain or restore the calibrated minimum roll rate. However, whenever the roll angle magnitude exceeds REF_ANGLE, the blocks  76  and  80  are executed to increment CTR and compare its value to a calibrated reference count (REF_COLNT), corresponding to a time interval such as 600-900 msec. If the roll angle magnitude remains above REF_ANGLE for the entire time interval, the block  80  is executed to adjust the minimum roll rate downward relative to the calibrated value as described above in reference to  FIG. 4 . This has the effect of increasing the likelihood of rollover detection, thereby enabling earlier detection of an impending rollover event under high roll angle operating conditions without sacrificing reliability under other conditions. Lowering the minimum roll rate may directly cause the roll rate vs. roll angle operating point to exceed the all-deploy threshold  32 , but may more likely cause the operating point to fall within the grey zone  30 , contributing to adaptive downward adjustment of the all-deploy threshold  32  as described in the aforementioned U.S. Pat. No. 6,542,792. 
   The blocks  82 - 88  determine if the all-deploy threshold  38  of the selected roll rate vs. roll angle calibration set should be raised for reduced sensitivity. The block  82  determines whether the absolute magnitudes of the roll rate and roll angle exceed respective minimum values defined by the selected roll rate vs. roll angle calibration set, and the block  84  determines if ACCEL_LAT is below a reference acceleration (REF_ACCEL) such as 0.5-0.6 g. If blocks  82  and  84  are both answered in the affirmative, the block  86  is executed to adjust the all-deploy threshold  32  upward relative to the calibrated value as described above in reference to  FIG. 5 . If block  82  is answered in the negative, the routine is exited and then repeated; if block  84  is answered in the negative, the block  88  retains or restores the calibrated all-deploy threshold  32 . The blocks  82 - 88  have the effect of reducing the likelihood of rollover detection in cases where rollover is considered to be unlikely without sacrificing rollover detection reliability under other conditions. 
   Finally, the block  90  compares the roll rate vs. roll angle operating point with the all-deploy threshold  32  (or  32 ′ if adaptively adjusted). If the operating point is above the all-deploy threshold, and block  92  verifies that the magnitude of the roll angle is increasing, block  94  is executed to issue a deployment command for the rollover restraints. If the operating point is below the all-deploy threshold, or the roll angle magnitude is not increasing, the routine is exited and then repeated. 
   In summary, the method of the present invention provides faster and more reliable detection of an impending rollover event through selection of roll rate vs. roll angle thresholds that are tailored to specified vehicle operating conditions. In this way, rollover detection criteria may be more sensitive when operating conditions are conducive to rollover or consistent with a rollover event, and less sensitive when operating conditions are inconsistent with a rollover event. This simplifies the calibration effort while providing ample ability to tailor the deployment thresholds for optimal reliability and timing. 
   While the method of the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the fixed threshold adjustments of the illustrated embodiment may be replaced with variable adjustments depending on the degree to which a parameter exceeds respective reference value, and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.