Abstract:
A system for detecting wheel lift of an automotive vehicle has a speed sensor ( 22 ) coupled to a wheel ( 12 ) of automotive vehicle ( 10 ). A torque control system ( 20 ) is coupled to wheel ( 12 ) to change the torque at the wheel. A controller ( 18 ) is coupled to the torque control system and a speed sensor. The controller ( 18 ) determines lift by changing the torque of the wheel, measuring the change in torque and indicating lift in response to change in torque which may be indicated by wheel speed.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a dynamic behavior control apparatus for an automotive vehicle, and more specifically, to a method and apparatus for determining whether a wheel of an automotive vehicle has lifted from the pavement. 
     BACKGROUND 
     Dynamic control systems for automotive vehicles have recently begun to be offered on various products. Dynamic control systems typically control the yaw of the vehicle by controlling the braking effort at various wheels of the vehicle. By regulating the amount of braking at each corner of the vehicle, the desired direction of the vehicle may be maintained. 
     Typically, the dynamic control systems do not address roll of the vehicle. For high profile vehicles in particular, it would be desirable to control the rollover characteristics of the vehicle to maintain the vehicle position with respect to the road. That is, it is desirable to maintain contact of each of the four tires of the vehicle on the road. 
     Vehicle rollover and tilt control (or body roll) are distinguishable dynamic characteristics. Tilt control maintains the body on a plane or nearly on a plane parallel to the road surface. Rollover control is used to maintain the vehicle wheels on the road surface. 
     Such systems typically use position sensors to measure the relative distance between the vehicle body and the vehicle suspension. One drawback to such systems is that the distance from the body to the road must be inferred. 
     It would therefore be desirable to provide a rollover detection system having reduced costs and increased reliability in predicting the occurrence of a rollover. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the invention to provide a rollover detection system that may be used in conjunction with the dynamic stability control system of the vehicle to determine rollover. 
     In one aspect of the invention, a wheel lift identification system for an automotive vehicle includes a speed sensor coupled to the vehicle producing a wheel speed signal. A torque control system is coupled to the wheel for charging the torque at the wheel. A controller is coupled to the torque control system and the speed sensor. The controller determines lift by changing the torque of the wheel, measuring the change in wheel speed since the torque was changed, and indicating a wheel lift if the change in the wheel speed is greater than a predetermined value. 
     In a further aspect of the invention, a method for determining wheel lift of a vehicle comprises the steps of:
         changing the torque of a wheel;   measuring the change in wheel speed since the step of changing torque; and,   indicating wheel lift if the change in wheel speed is greater than a predetermined value.       

     In a further aspect of the invention, the changing of the torque of the wheel may be performed by increasing the brake pressure for that wheel. When the wheel speed has significant deceleration, a wheel flag is set. When the brake pressure is released and the wheel speed changes greater than a reacceleration threshold, then wheel contact is assumed. If the wheel speed does not increase over the reacceleration threshold within a predetermined time, then wheel lift status is confirmed. As an alternative, driveline torque may be used. 
     One advantage of the invention is that in vehicles employing a dynamic stability control system, additional sensors may not be required. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cutaway view of an automotive vehicle having a wheel lift identification system according to the present invention. 
         FIG. 2  is a flow chart of a wheel lift identification system according to the present invention. 
         FIG. 3A  is a plot of pressure versus time for a wheel lift identification system according to one embodiment of the present invention. 
         FIG. 3B  is a plot of wheel speed versus time for a wheel lift identification system according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described with respect to a wheel lift identification system for an automotive vehicle. Those skilled in the art will recognize that the present invention may be incorporated into a rollover prevention system for an automotive vehicle. 
     Referring now to  FIG. 1 , an automotive vehicle  10  has a plurality of wheels  12 , two of which are shown as elevated above a road plane  14 . A roll control system  16  is included within vehicle  10 . The roll control system  16  is used to counteract the lifting of wheels  12  from road plane  14  as will be further described below. Roll control system  16  includes a roll controller  18  that is preferably microprocessor based. Roll controller  18  may be part of a dynamic stability control system of the automotive vehicle  10 . Roll controller  18  is coupled to a torque control system  20  that is used to control the torque of the wheels  12 . Although torque control system  20  is illustrated as a separate item, torque control system  20  may be included in roll controller  18  which may in turn be included within a dynamic stability control system. Torque control system  20  may act in conjunction with the electronic engine controller, a driveline engagement mechanism or braking system, or a combination of these to control the torque at one or all of the wheels  12 . Torque controller  20  and roll controller  18  may be coupled to wheel speed sensors  22  located at each of the wheels  12 . Wheel speed sensors  22  provide roll controller  18  with a signal indicative of the speed of the individual wheel to which it is attached. Various types of wheel speed sensors including toothed-wheel type systems would be evident to those skilled in the art. 
     Other sensors  24  may be coupled to roll control system  16 . For example, roll angle sensors, steering wheel angle sensors, yaw rate sensors, and other sensors may be incorporated therein. Other sensors  24 , as will be further described below, may be used to identify a condition suitable for the potential of wheel lift. Such a condition may initiate further action by roll control system  16  to verify wheel lift. 
     In the following example, the application of brake pressure is used to provide the change in torque. However, other methods such as applying engine torque may also be used to change the amount of torque at a wheel. Further references to the application of torque to a wheel may include hydraulic or electric brake torque, changes in engine torque or engagement of driveline torque through the use of an electronically controlled transfer case, differential, transmission or clutch. The present invention may also be used to determine if a sensor has failed in the roll control system  16 . That is, if roll is suspected by a particular sensor but all other conditions or sensors indicate otherwise, the sensor may be operating improperly. Also, although speed is used, wheel acceleration may also be used in place of speed as would be evident to those skilled in the art. 
     Referring now to  FIG. 2 , in step  30 , if a roll sensor failure is suspected or in step  32  if wheel lift is suspected by the roll control system  16 , block  34  initiates the wheel lift determination process. In step  36 , torque is applied to the wheel suspected of lifting and the wheel speed at the suspected wheel is stored. In step  38 , the torque is increased by applying a test pulse of torque to the suspected wheel. Torque is applied until a torque threshold (Torque_Max) is achieved. In step  40 , if the torque is greater than the Torque_Max, the torque is held constant in step  42 . In step  44 , if the time as counted by the Build_Counter is greater than a predetermined time, step  46  is executed in which the torque is released and the wheel speed at the initiation of the release of torque is stored. In step  44 , if the counter is not greater than the predetermined hold time, the counter is incremented in step  48 . After step  48  the change in wheel speed is compared to a predetermined change in wheel speed. If the wheel speed change is not greater than a predetermined speed in step  50 , steps  38 - 44  are again executed. If the wheel speed change is greater than a predetermined speed, this indicates a lifted wheel. In this case, step  52  is executed in which a wheel lift status flag is set. After step  52 , step  54  is executed in which the build counter is reset. 
     Referring back to step  40 , if the torque is not greater than the torque threshold then step  50  is executed. 
     Referring back to step  46 , after the wheel speed is recorded after the torque release, step  56  is executed. In step  56  torque is released. After step  56 , step  58  is implemented in which the wheel speed change is compared to a reacceleration threshold. The reacceleration threshold is a predetermined value that corresponds to a wheel speed change that should be achieved should wheel contact be reestablished. The wheel speed change is determined from the time that the torque was released. If the wheel speed change is greater than a reacceleration threshold or if the wheel lift status from steo  52  is zero, wheel contact is assumed. In such a case the traction level may be calculated in step  60 . If the wheel speed does not increase over the reacceleration threshold, then the wheel lift status is confirmed beginning with step  70 . 
     Referring back to step  58 , if the wheel speed is less than the reacceleration threshold, step  62  compares the Dump_Counter to a predetermined dump time. If the predetermined dump time is greater than the Dump_Counter, then the Dump_Counter is incremented in step  64  and steps  56  and  58  are again executed. If the Dump_Counter is greater than the predetermined dump time, then the wheel lift status flag is set in step  66  and the Dump_Counter is reset in step  68 . After step  68 , the process is reinitiated and returns to step  36 . 
     Returning back to step  60 , the traction level is calculated in step  60 . After step  60 , the plausibility of a sensor failure is determined. If, for example, the process was initiated based on the suspicion of a sensor failure from block  30  above and no wheel lift was detected, a sensor failure is indicated in step  72 . For either result, if a sensor failure is indicated by block  70  or not, the build counter and Dump_Counter are cleared in block  74  and the wheel lift status is cleared in block  76 . The end of the routine occurs in block  78 . 
     Thus, as can be seen, the application of torque can be used to first determine whether a suspected wheel has lifted from the pavement. For confirmation, the removal of the torque and the resulting wheel speed change may be used to confirm the initial finding. Advantageously, the system may be implemented in a dynamic stability system of an automotive vehicle without adding further sensors. If rollover is detected, then the rollover can be corrected by applying the brakes or generating a steering correction. 
     Referring now to  FIG. 3A , various lines  90 ,  92 ,  94  are illustrated during the build time to illustrate the variation in pressure of the braking system due to wear and other effects of the brakes. Lines  90 ,  92   94  have little effect on the overall operation of the system. Thus, the thresholds and parameters are selected so that the system is robust to wear and system variation. The maximum pressure P max  is reached and maintained for a hold time (such as set forth in step  42  above) until it is released. 
     Referring now to  FIG. 3B , a plot of wheel speed corresponding to the various times is illustrated. As shown, the wheel speed of a loaded wheel is illustrated by line  96  which is higher than the wheel speed of a lifted wheel illustrated by line  98 . 
     While particular embodiments of the invention have been shown and described, numerous variations alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.