Abstract:
The present invention is directed to a system for determining lift-off or wheel departure of one or more wheels associated with a vehicle from a road. The system includes a sensor that measures the wheel speed of at least one wheel, and a controller that calculates the resonance frequency of the at least one wheel, calculates variations in the resonance frequency, compares the variations with a threshold, and indicates lift-off of the wheel from the road if the variations exceed a threshold.

Description:
BACKGROUND  
       [0001]     The present invention generally relates to rollover protection systems.  
         [0002]     Dynamic control systems have been recently introduced in automotive vehicles for measuring the body states of the vehicle and controlling the dynamics of the vehicle based on the measured body states. For example, certain dynamic stability control systems know broadly as control systems compare the desired direction of the vehicle based on the steering wheel angle, the direction of travel and other inputs, and control the yaw of the vehicle by controlling the braking effort at the various wheels of the vehicle. By regulating the amount of braking torque applied to each wheel, the desired direction of travel may be maintained. Commercial examples of such systems are known as dynamic stability program (DSP) or electronic stability control (ESC) systems.  
         [0003]     Other systems measure vehicle characteristics to prevent vehicle rollover and for tilt control (or body roll). Tilt control maintains the vehicle body on a plane or nearly on a plane parallel to the road surface, and rollover control maintains the vehicle wheels on the road surface. Certain systems use a combination of yaw control and tilt control to maintain the vehicle body horizontal while turning. Commercial examples of these systems are known as anti-rollover prevention (ARP) and rollover stability control (RSC) systems.  
         [0004]     Typically, such control systems referred here collectively as dynamic stability control systems use dedicated sensors that measure the yaw or roll of the vehicle. However, yaw rate and roll rate sensors are costly. Therefore, it would be desirable to use a general sensor to determine, for example, the rollover propensity of the vehicle, that is, a sensor that is not necessarily dedicated to measuring the roll of the vehicle. The invention may also augment a system that includes yaw and/or roll rate sensors.  
       SUMMARY  
       [0005]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a system and method for determining lift-off or wheel departure of one or more wheels associated with a vehicle from a road. The system includes a sensor that measures the wheel speed of at least one wheel, and a controller that calculates the resonance frequency of the at least one wheel, calculates variations in the resonance frequency, compares the variations with a threshold, and indicates lift-off of the wheel from the road if the variations exceed a threshold.  
         [0006]     Further features and advantages of this invention will become apparent from the following description, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  depicts a vehicle with a tire lift off detection system in accordance with the invention;  
         [0008]      FIG. 2  is a schematic of a system model for determining wheel lift-off in accordance with the invention;  
         [0009]      FIG. 3  is a flow diagram of a sequence of steps to determine wheel lift-off;  
         [0010]      FIG. 4A  illustrates the frequency response of a front inner wheel during a double fishhook maneuver;  
         [0011]      FIG. 4B  illustrates the frequency response of a rear inner wheel during the double fishhook maneuver;  
         [0012]      FIG. 4C  illustrates the frequency response of a front outer wheel during the double fishhook maneuver;  
         [0013]      FIG. 4D  illustrates the frequency response of a rear outer wheel during the double fishhook maneuver;  
         [0014]      FIG. 5A  illustrates the frequency response of a front outer wheel during a fishhook maneuver;  
         [0015]      FIG. 5B  illustrates the frequency response of a rear outer wheel during the fishhook maneuver;  
         [0016]      FIG. 5C  illustrates the frequency response of a front inner wheel during the fishhook maneuver; and  
         [0017]      FIG. 5D  illustrates the frequency response of a rear inner wheel during the fishhook maneuver.  
         [0018]      FIG. 6A  illustrates the frequency response of a front outer wheel during another fishhook maneuver;  
         [0019]      FIG. 6B  illustrates the frequency response of a rear outer wheel during the fishhook maneuver;  
         [0020]      FIG. 6C  illustrates the frequency response of a front inner wheel during the fishhook maneuver; and  
         [0021]      FIG. 6D  illustrates the frequency response of a rear inner wheel during the fishhook maneuver.  
     
    
     DETAILED DESCRIPTION  
       [0022]     Referring now to  FIG. 1 , a vehicle  10  includes a rollover control system  12  embodying the principles of the present invention. The system  12  identifies dynamic characteristics and conditions of the vehicle  10  to reduce the rollover propensity of the vehicle  10  in actual driving conditions. In certain implementations, the system  12  may be a component of a dynamic stability control system.  
         [0023]     The system  12  includes a controller  14  and various sensors  16  associated with the wheels  18 . In the present embodiment, the sensors  16  measure the speed of the respective wheels. This information is transmitted to the controller  14  which analyzes the information to estimate the vertical load on the tires. Specifically, each wheel  18  has a tire  22  mounted on a hub  20  and is modeled as a second order spring-mass-damper model as shown in  FIG. 2  to determine a resonance frequency, ω n , of the wheel, according to then system of equations: 
 
Θ 1 −Θ 2 =Θ s  
 
 J   1 {dot over (ω)} 1   =−KΘ   s  
 
 J   2 {dot over (ω)} 2   =KΘ   s   +T   L   +T   d    (1) 
 
 where 
 
         [0024]     Θ 1  is the rotational angle of the wheel  
         [0025]     Θ 2  is the rotational angle of the tire  
         [0026]     Θ s  is the difference between Θ 1  and Θ 2    
         [0027]     J 1  is the rotational moment of inertia of the hub  
         [0028]     J 2  is the rotational moment of inertia of the tire  
         [0029]     K is a spring constant  
         [0030]     ω 1  is the rotational velocity of the hub  
         [0031]     ω 2  is the rotational velocity of the tire  
         [0032]     {dot over (ω)} 1  is the rotational acceleration of the hub  
         [0033]     {dot over (ω)} 2  is the rotational acceleration of the tire  
         [0034]     T L =F x R is the longitudinal torque on the tire  
         [0035]     F x  is the longitudinal force on tire  
         [0036]     R is the radius of the tire (from center of hub)  
         [0037]     T d  are road disturbances (i.e. “Noise”)  
         [0038]     To linearize and simplify the system Eq. (1), a perturbation of T L  at an operating point S v =S vO  is derived to yield  
                         Δ   ⁢           ⁢     T   L       =         ∂     T   L         ∂     S   v         ⁢     |       S   v     =     S     v   ⁢           ⁢   O           ⁢     Δ   ⁢           ⁢     S   v                     =     α   ⁢           ⁢       R   2     ⁡     (       Δ   ⁢           ⁢     V   /   R       -     Δω   2       )                       ⁢                   (   2   )             
 
 where 
 
         [0039]     S v =V−ωR is the slip velocity  
         [0040]     ω is the angular velocity of the wheel  
         [0041]     V is the velocity of the vehicle  
         [0042]     α is the extended brake stiffness defined as the gradient of F x  at S v =S vO ,  
         [0043]     and ω n  is the natural frequency of the model  
         [0000]     Since the inertia of the vehicle is significantly larger than that of the wheel, the assumption |Δω 2 |&gt;&gt;|ΔV/R| is made such that Eq. (2) simplifies to 
 
Δ T   L   =−αR   2 Δω 2    (3) 
 
         [0044]     From the perturbation of system (1) and Eq. (3), transfer function from the road disturbance ΔT d  to wheel speed Δω 1  is obtained as follows:  
               H   ⁡     (   s   )       =     K         J   1     ⁢     J   2     ⁢     s   3       +       J   1     ⁢   α   ⁢           ⁢     R   2     ⁢     s   2       +       K   ⁡     (       J   1     +     J   2       )       ⁢   s     +     K   ⁢           ⁢   α   ⁢           ⁢     R   2                   (   4   )             
 
 Since the target of the estimation is α, the second order system is enough as a vibration model. To reduce order, the 3rd order term of H(s) is estimated:  
               G   ⁡     (   s   )       =       K         J   1     ⁢   α   ⁢           ⁢     R   2     ⁢     s   2       +       K   ⁡     (       J   1     +     J   2       )       ⁢   s     +     K   ⁢           ⁢   α   ⁢           ⁢     R   2           =       b   2         s   2     +       a   1     ⁢   s     +     a   2                   (   5   )             
 
 where  
         a   1     =       K   ⁡     (       J   1     +     J   2       )           J   1     ⁢   α   ⁢           ⁢     R   2             
         a   2     =     K     J   1           
         b   2     =     K       J   1     ⁢   α   ⁢           ⁢     R   2             
 
 such that the resonance frequency, ω n , is  
         ω   n     =       1     2   ⁢   π       ⁢       K     J   1               
     or     
         ω   n     =       1     2   ⁢   π       ⁢       α   2             
 
 and the strength of the resonance depends on the extended brake stiffness α and the tire-road friction. 
 
         [0045]     Thus, the system  10  uses a second order spring-mass-damper model as shown in  FIG. 2 , and the wheel speed of the wheel  20  to estimate a resonance frequency, ω n , is estimated using a filter, such as a RLS or Kalman filter. The resonance frequency, typically in the range between about 30 and 60 Hz, is correlated to the vertical force on the tire.  
         [0046]     Shown in  FIG. 3  is a preferred process  100  that illustrates the operation of the system  10 . A measured signal  102 , such as the wheel speed, may be pre-filtered  104  before the resonance frequency is estimated to remove noise and unwanted information from the signal  102 . A RLS or Kalman filter  108  receives the signal from the pre-filter  104  and employs a model  106 , for example, as described by the system of Eq. (1), to calculate the parameters of interest α 1  and α 2  which module  110  employs to calculate the resonance frequency ω n . If variations in the resonance frequency exceeds a threshold, the system  10  can indicate to the roll over prevention system or the driver that a wheel has lifted off the ground.  
         [0047]     The pre-deviations in the estimated resonance frequency during a validity window indicates a deviation of the vertical load on the tire. When the resonance frequency reaches a pre-determined threshold, the algorithm indicates a wheel lift status condition. The status of all four wheels can be monitored continuously. Alternatively, only the outer wheels can be monitored to conserve processing resources. The wheel lift status condition from the monitored wheels can be combined to provide more detailed wheel lift indication, such as no-wheel-lift, single-wheel-lift, two-wheel-lift, or single-wheel-lift with impending two-wheel-lift.  
         [0048]     FFT processing of wheel speed data from an implementation of the system  10  are illustrated in the following examples.  FIG. 4  shows the characteristics of a vehicle in a double-fishhook maneuver. In these figures, the x-axis is the frequency spectrum and the y-axis is the signal power.  
         [0049]      FIGS. 4A and 4B  show the behavior of the front inner wheel and the rear inner wheel, respectively, and  FIGS. 4C and 4D  show the behavior of the front outer wheel and the rear outer wheel, respectively. As shown in  FIGS. 4A, 4B , and  4 C, the symbols FIG, RIG, and FOG represent the data for when all the tires are grounded (i.e. when some load greater than zero is being applied to the tire) prior to the test event for the front inner wheel, the rear inner wheel, and the front outer wheel, respectively. The symbols FIL, RIL, and FOL represent the data for the tires during the test event when one or more tires is lifted off the ground (i.e. when a zero normal load is being applied to the tire) for the front inner wheel, the rear inner wheel, and the front outer wheel, respectively. As shown in  FIG. 4D , the difference between the lifted and the grounded data for the rear outer wheel is barely perceptible. Thus,  FIG. 4  demonstrates that the system  10  can detect the difference between the characteristics of a lifted tire and that of a grounded tire.  
         [0050]      FIGS. 5 and 6  illustrate the characteristics of a vehicle in two different fishhook maneuvers.  FIGS. 5A and 6A  refer to the front outer wheel and  FIGS. 5B  and  6 B refer to the rear outer wheel in the two tests, which show that there is minor difference between the lifted and grounded characteristics of the outer wheels. Referring to  FIGS. 5C, 5D ,  6 C, and  6 D, the symbols FIG and RIG indicated that the signals for the front inner grounded and the rear inner grounded wheels are barely perceptible. On the other hand, the symbols FIL and RIL indicated that there are noticeable spectra for the front inner lifted and the rear inner lifted wheels during the fishhook maneuver.  
         [0051]     In other embodiments, other conditions may be monitored, including the wheel speed, suspension travel, and sidewall torsion (i.e. smart tire). When the suspension travel is measured, a quarter-car model describing the suspension-tire characteristic and the measured suspension travel is employed to estimate the resonance frequency of the tire, which is correlated to the vertical force on the tire.  
         [0052]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of an implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.