Abstract:
A system for determining a rollover property of a vehicle includes sensors capable of sensing lateral acceleration, a first left tire rotational speed, a first right tire rotational speed, and a mass property. The sensors output a lateral acceleration signal, a first left tire rotational speed signal, a first right tire rotational speed signal, and a mass property signal. A processing unit receives the signals and uses the received signals to determine a rollover property of the vehicle.

Description:
BACKGROUND 
   A rollover accident is an extremely hazardous event for a vehicle operator. The danger of rollover accidents is illustrated by data from the Transportation Research Institute at the University of Michigan (UMTRI), which shows that although rollover occurs in less than 5% of accidents involving tractor-semitrailers, these accidents account for 58% of the fatal injuries suffered by truck drivers. Accordingly, in recent years there has been an increased interest in developing systems that can help avoid vehicle rollovers, especially with respect to heavy duty trucks. 
   Presently known systems for preventing vehicle rollover employ a variety of methods to predict incipient rollover conditions. One known system detects and warns of a rollover condition using sensors to measure the vertical forces acting on each of the vehicle&#39;s drive wheels. Another sensor measures the lateral acceleration of the vehicle. Based on these sensor measurements, the system determines when load is transferred laterally between the wheels of an axle. The system also determines the vehicle&#39;s center of gravity (CG). This information is used to calculate a lateral load transfer ratio (LTR), which corresponds to the actual roll moment acting on the vehicle. LTR is determined based on the difference between the loads acting on the left and right tires as compared to the sum of the loads acting on the tires. If the load on the left and right tires is equal, then LTR=0 and there is no danger of rollover. As one tire begins to lift off of the ground, as in a rollover condition, the load on the other tire increases until LTR=1. When LTR=1, the wheel on one side of the axle is completely lifted off the ground. 
   This presently known system also calculates the approximate height of the vehicle&#39;s CG, which is also an indicator of a potential rollover. A display provides the vehicle operator with an indication of the LTR and the estimated height of the vehicle&#39;s CG to warn the operator of potential rollover conditions. While this system has its advantages, it requires that load sensors be installed at each of the tires for which the forces acting thereon are to be measured. These load sensors would not otherwise be installed on the vehicle and, therefore, represent added cost and complexity for the vehicle. 
   Another known system detects a rollover event by comparing normal loads estimated for each tire with predetermined threshold values. The normal load on a tire is calculated based on the spring constant k and rolling radius r of the tire. The rolling radius R of each tire can be determined in a number of ways using different variables, including the velocity of the car at the car&#39;s CG, sideslip angle, steering angle, length of the wheel track, distance from the axle to the CG, and rotational speed of the tire. However, as most vehicles are not equipped to measure many of these variables, this system also requires that additional sensors be installed on the vehicle. 
   SUMMARY 
   The presently disclosed rollover prediction and warning method is effective for detecting an incipient rollover condition and warning the vehicle operator so that the operator can take corrective action. In one embodiment, the difference between the rotational speeds of the vehicle&#39;s left and right tires is sensed. If the vehicle is turning, part of the difference in rotational speeds is caused by the turn. This difference is calculated and subtracted from the overall difference between the rotational speeds of the tires. The result is approximately equal to the difference between the rotational speeds of the tires caused by load transfer. 
   As a vehicle travels through a turn, the outside tires travel a greater distance than the inside tires in a given amount of time. Therefore, the outside tires have a greater rotational speed than the inside tires. The difference between the rotational speeds of the inside and outside tires due to their different path radii can be determined from the speed V of the vehicle, the radius R of the turn, and the distance d between the inside and outside wheels. The speed V of the truck is determined by averaging the rotational speeds measured at the individual tires. The radius R of the turn is determined from the lateral acceleration and the velocity of the vehicle as it travels through a turn. The calculated value of R is used to determine the difference between the rotational speeds of the inside and outside tires caused by the turn. 
   As a roll moment acts on a vehicle, the load shifts from one side of the vehicle to the other side until the tires on one side carry all of the load, indicating an incipient rollover condition. As the load on a tire increases, the tire deflects, and its effective rolling radius decreases. As a result, the rotational speed of a tire increases as a greater percentage of the vehicle load is carried by that tire. Conversely, as the load applied to a tire decreases, the rolling radius of the tire increases, thereby reducing its rotational speed for a given vehicle velocity. The difference between the rotational speeds of the inside and outside tires is used to determine the differential deflection between the inside and outside tires caused by load transfer. Using a known spring constant K for the tires, the differential loading is determined from the differential deflections. 
   A total load on the wheels is determined by adding the vehicle&#39;s sprung weight, as determined by the air pressure of the vehicle&#39;s air spring suspension, to the vehicle&#39;s unsprung weight, which is fixed. A load transfer ratio (LTR) is calculated from the differential tire loading and the total tire loading. LTR is an indicator of the risk of rollover. Therefore, a warning signal is sent to the vehicle operator when LTR approaches a predetermined threshold, thus allowing the operator an opportunity to take corrective action. 
   Because methods described herein can use existing anti-lock braking system (ABS) sensors to sense the rotational speeds of individual wheels, the only additional sensor required is the lateral acceleration sensor. 
   This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 

   
     DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a rear view of a heavy duty vehicle undergoing a rollover event; 
       FIG. 2  is a plan view of the heavy duty vehicle of  FIG. 1 ; 
       FIG. 3  is a plan view of the heavy duty vehicle of  FIG. 1  turning along a curved path; and 
       FIG. 4  is a flow diagram illustrating a rollover prediction and warning method in accordance with the present disclosure. 
   

   DETAILED DESCRIPTION 
   The disclosed subject matter is directed to a system and method for predicting and warning of a rollover condition in a vehicle without requiring determination of the vehicle&#39;s center of gravity (CG). As a vehicle begins to rollover, the load applied to the wheels on one side of the vehicle are transferred to the wheels of the other side of the vehicle until there is no load applied to the wheels on the first side of the vehicle. At this point, the unloaded wheels are free to lift off the ground, and the likelihood of a rollover is high. Unlike systems that require a measured or calculated CG, the present method uses the relative changes in loading between the left and right sides of the vehicle to determine a rollover parameter. In particular, a vehicle&#39;s lateral acceleration, individual tire speeds, and weight are used to determine the loads on the left and right side tires, F L  and F R , respectively. These loads are monitored to detect any lateral load transfer. Detected lateral load transfer is divided by the overall load to produce a load transfer ratio (LTR). 
   LTR correlates well to roll stability and can be based on loads measured for a single axle, or for any combination of multiple axles. One method of calculating LTR is as follows: 
   
     
       
         
           
             
               
                 LTR 
                 = 
                 
                   
                      
                     
                       ∑ 
                       
                         ( 
                         
                           
                             F 
                             L 
                           
                           - 
                           
                             F 
                             R 
                           
                         
                         ) 
                       
                     
                      
                   
                   
                     ∑ 
                     
                       ( 
                       
                         
                           F 
                           L 
                         
                         + 
                         
                           F 
                           R 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   When the loads on each side of the vehicle are equal, then LTR=0, and there is no predicted rollover danger. At incipient rollover, the load on one side is completely transferred to the other side, and LTR=1. A roll margin is calculated from the LTR using the following formula:
 
Roll margin=1−LTR  (2)
 
   Roll margin is usually expressed as a percentage. Hence, if the vehicle is at incipient rollover, then LTR=1, and the roll margin is 0%. 
   Direct measurement of tire loads requires the use of ground-based scales. Several methods and devices have been used for indirectly estimating tire loads using sensors to variously measure suspension forces, suspension roll angles, axle bending strains, and the like. However, the sensors necessary to utilize these methods and devices are not standard equipment on most vehicles. As a result, currently known methods of predicting vehicle rollover using indirectly estimated tire load data require installing additional sensors at the wheels of the vehicle, which adds cost and complexity to the vehicle. To avoid the added cost and complexity, an alternate method for predicting a rollover condition described herein determines tire loads indirectly, based on differences between the rotational speeds of the vehicle&#39;s tires. 
   A tire acts like a spring, deflecting in response to applied loads. The amount that the tire deflects in response to an applied load depends on the stiffness of the tire and the magnitude of the load. The applied load acting on a tire over the normal range of operating loads is generally proportional to the deflection of the tire and can be calculated using deflection and the radial stiffness of the tire K. When the applied load increases, the tire deflection increases, and the rolling radius of the tire decreases. Consequently, for a vehicle traveling at a given velocity, increasing the load applied to a tire decreases the rolling radius of that tire, which in turn causes that tire to have a higher rotational speed. Conversely, if the load on a tire is decreased, the rolling radius increases and the tire has a lower rotational speed at a given vehicle velocity. Alternately, the applied load can be determined using a database that relates tire deflection to an applied load for one or more given types of tire. 
   For a given vehicle velocity, the rotational speed of a tire is inversely proportional to the effective rolling radius r e  of that tire. The effective rolling radius r e  is the radius of a theoretical rigid tire having the same rotational speed as the actual tire for a given vehicle velocity. The effective rolling radius r e  is not the same at the actual rolling radius r a , which is the distance from the axis of the tire to the ground plane. Where r 0  is the radius of a tire with no load applied, the relationship between effective rolling radius r e  and tire deflection δ can be approximated according to the following equation: 
   
     
       
         
           
             
               
                 
                   r 
                   e 
                 
                 = 
                 
                   
                     r 
                     0 
                   
                   - 
                   
                     δ 
                     3 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Hence, the change in the rotational speed of a tire resulting from a change in radial deflection is about one third the amount that would be predicted from the change in the actual rolling radius r a . 
   Assuming that the stiffness K L  of a vehicle&#39;s left tire is identical to the stiffness K R  of the corresponding right tire, the left and right tires will deflect by the same amount when the vehicle load is distributed equally between the left and right tires. As a result, when differences in tread wear and inflation between the left and right tires are discounted, the rolling radius of the left tire r L  and the rolling radius of the right tire r R  are equal. If r L =r R , then the rotational speed of the left tire ω L  is the same as the rotational speed of the right tire ω R  when the vehicle travels in a straight line. 
   When the vehicle enters a turn, ω L  and ω R  are affected by two factors. The first factor is the difference between the paths traveled by the left and right tires. As shown in  FIG. 3 , at any given point during a turn, a vehicle travels with a velocity V along an arc with a turn radius R. As the vehicle turns to the left, a left tire travels along an arc with a radius R L , which is less than R, while the corresponding right tire travels along an arc with a radius R R , which is greater than R. Where d is a known distance between the left and right tires, R L  and R R  are related to R as follows: 
   
     
       
         
           
             
               
                 
                   R 
                   L 
                 
                 = 
                 
                   R 
                   - 
                   
                     d 
                     2 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
           
             
               
                 
                   R 
                   R 
                 
                 = 
                 
                   R 
                   + 
                   
                     d 
                     2 
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   Thus, when the vehicle is turning to the left, the right tires travel a greater distance in a given amount of time, and ω R  is increased. On the other hand, the left tires travel a shorter distance in a given amount of time, and ω L  is decreased. Conversely, a vehicle turning to the right will experience an opposite result, wherein ω R  is decreased, and ω L  is increased. 
   The second factor affecting ω L  and ω R  is the roll moment generated by the turn. As a vehicle travels through a turn, a centripetal force acts on the vehicle in the direction of the center of the turn. As a result, an apparent force, sometimes referred to as a centrifugal force, acts on the CG of the vehicle in a direction away from the center of the turn, thereby generating a roll moment M. As shown in  FIG. 1 , a vehicle turning to the left has a roll moment M that shifts the load from the left tires to the right tires. As the applied load on a right tire F R  increases, the rolling radius of the right tire r R  decreases, and ω R  increases. At the same time, the applied load on a left tire F L  decreases so that the rolling radius of the left tire r L  increases, and ω L  decreases. Conversely, for a vehicle turning to the right, ω R  is decreased, and ω L  is increased. 
   In order to determine the changes to ω L  and ω R  resulting from load transfer, the changes to ω L  and ω R  caused by turning are calculated and subtracted from the overall changes to ω L  and ω R . Changes to ω L  and ω R  caused by turning are calculated based on the value of turn radius R. Referring to  FIG. 3 , as a vehicle turns with a turn radius R and a velocity V, the vehicle accelerates towards the center of the turn C with an acceleration a N . The radius R of the vehicle turn is related to the velocity V and the lateral acceleration a N  as follows: 
   
     
       
         
           
             
               
                 R 
                 = 
                 
                   
                     V 
                     2 
                   
                   
                     a 
                     N 
                   
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   Referring to  FIGS. 1 and 2 , the lateral acceleration a N  of the vehicle is sensed by a lateral acceleration sensor  30  installed on the vehicle. Vehicle velocity V is determined by averaging the speed at the vehicle tires. Most modern vehicles are equipped with anti-lock braking systems (ABS) that include sensors to continually monitor the speed of individual tires. Thus, the ABS sensors  20  provide an accurate and high sample rate speed signal for each tire without requiring additional sensors. 
   It should be appreciated that the radius R of the vehicle turn can be determined in a number of ways without departing from the scope of the disclosed subject matter. Alternate methods for determining R include sensors to monitor the vehicle steering system or to sense the orientation of the steerable wheels of the vehicle. 
   For a turning vehicle, the change in rotational speed of a left tire due to the turn (Δω tL ) and the change in rotational speed of a right tire due to the turn (Δω tR ) are related to the vehicle turn radius R as follows: 
   
     
       
         
           
             
               
                 
                   Δω 
                   tL 
                 
                 = 
                 
                   
                     
                       ω 
                       tL 
                     
                     - 
                     ω 
                   
                   = 
                   
                     ω 
                     × 
                     
                       ( 
                       
                         
                           
                             R 
                             L 
                           
                           R 
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
           
             
               
                 
                   Δω 
                   tR 
                 
                 = 
                 
                   
                     
                       ω 
                       tR 
                     
                     - 
                     ω 
                   
                   = 
                   
                     ω 
                     × 
                     
                       ( 
                       
                         
                           
                             R 
                             R 
                           
                           R 
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
         
       
     
   
   Subtracting equation (5) from equation (4) gives equation (6), which can be used to determine the difference between rotational speeds of the left tire and the right tire due to the turn Δω t(L−R)  as follows: 
   
     
       
         
           
             
               
                 
                   Δω 
                   
                     t 
                     ⁡ 
                     
                       ( 
                       
                         L 
                         - 
                         R 
                       
                       ) 
                     
                   
                 
                 = 
                 
                   ω 
                   × 
                   
                     ( 
                     
                       
                         
                           R 
                           L 
                         
                         - 
                         
                           R 
                           R 
                         
                       
                       R 
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 9 
                 ) 
               
             
           
         
       
     
   
   Further, replacing the term “R L −R R ” with the known distance d, yields the following: 
   
     
       
         
           
             
               
                 
                   Δω 
                   
                     t 
                     ⁡ 
                     
                       ( 
                       
                         L 
                         - 
                         R 
                       
                       ) 
                     
                   
                 
                 = 
                 
                   ω 
                   × 
                   
                     ( 
                     
                       d 
                       R 
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
   The calculated difference between the rotational speeds of the left tire and the right tire due to the turn Δω t(L−R)  is subtracted from the total difference between the rotational speeds of the left tire and the right tire Δω (L−R) , which is determined from the ABS sensor data. The result is the difference between the rotational speeds of the left tire and the right tire due to load transfer Δω l(L−R) . Having determined Δω l(L−R) , the difference in the left and right rolling radii can be determined. The value of Δω l(L−R)  and the known stiffness K of the tires are used to calculate the difference between the applied loads at the left and right tires F L −F R . 
   LTR calculation requires both the difference of the left and right tire loads F L −F R  and the sum of the left and right tire loads F L +F R . As discussed above, F L −F R  can be determined from changes in the rolling radius of the tires. The sum of the left and right tire loads is calculated by combining the vehicle&#39;s unsprung load with the vehicle&#39;s sprung load. 
   The mass of the vehicle&#39;s unsprung load is generally fixed and can be calculated or determined from manufacturer specifications. In contrast, the vehicle&#39;s sprung load varies with the payload and is detected by a load sensor. In one embodiment, the load sensor detects the pressure of the vehicle&#39;s air suspension. Air suspension is standard equipment for most commercial vehicles for on-highway use, and the pressure of an air spring provides an accurate measure of the sprung load supported by that air spring. Thus, a load sensor measures the air pressure of the air springs and calculates the total sprung load therefrom. By adding the unsprung load to the sprung load, the total ground load for the vehicle ΣF L +F R  is determined. Similarly, the ground load for an individual axle F L +F R  can be determined by adding the sprung load determined from the individual air springs supported by that axle to the unsprung load of the axle. 
     FIG. 4  shows a flow diagram illustrating the presently described embodiment of the rollover prediction and warning method. In steps  102  and  104 , tire sensors sense the rotational speeds of the left tire ω L  and right tire ω R . The average rotational speed of the left and right tires ω is generated in step  106 . Step  110  calculates the turn radius R from the average rotational speed ω and a lateral acceleration a N  sensed in step  108 . The difference between the rotational speeds of the left and right tires due to the turn Δω t(L−R)  is calculated in step  112 . 
   Still referring to  FIG. 4 , the value shown in step  114  is the difference between the rotational speeds of the left and right tires Δω (L−R) , which is calculated by subtracting the rotational speed of the right wheel from the rotational speed of the left wheel. The difference between the rotational speeds of the left and right tires due to the load shift Δω l(L−R)  is shown in step  116 . This value is calculated by subtracting Δω t(L−R)  from Δω (L−R) . In step  118 , the change in difference in the rolling radius between the left and right tires r (L−R)  is calculated from Δω l(L−R) . In step  120 , r (L−R)  and a known radial tire stiffness K are used to calculate the difference between the applied loads at the left and right tires F L −F R . 
   In step  124 , the sprung load of the vehicle is calculated based on the air suspension&#39;s air spring pressure detected in step  122 . The sprung load is added to the unsprung load to provide the total load on the left and right tires F L +F R . 
   LTR is calculated in step  126  by dividing the absolute value of (F L −F R ) by F L +F R . In step  128 , the LTR is compared to an LTR limit value. If the LTR is greater than the LTR limit value, then an incipient rollover condition is present, and a warning is sent to the vehicle operator. If the LTR is less than the LTR limit value, then no rollover condition exists, and no warning is sent. 
   It is noted that in practice, the nominal rolling radii of individual tires will not be identical due to a variety of factors, including tread wear, inflation pressure, tire type, etc. These differences can be detected by sampling the rotational speeds of the tires when the sensed lateral acceleration is approximately zero, i.e. when the vehicle is moving in a straight line. Because the vehicle is moving in a straight line, the difference between the rotational speeds of the tires is generally due to tire differences. By keeping a running average that incorporates multiple samplings, an accurate correction factor can be maintained. 
   While preferred embodiments of the claimed subject matter have been illustrated and described, it should be appreciated that various changes can be made therein without departing from the spirit and scope of the claimed subject matter.