Patent Application
20070185623
1. Field of the Invention
This invention relates generally to a system and method for detecting vehicle wheel lift and, more particularly, to a system and method for detecting vehicle wheel lift for roll stability purposes, where the system independently determines wheel lift using suspension displacement and wheel speed.
2. Discussion of the Related Art
Stability enhancement systems for vehicles have been offered on various vehicles for many years. Such systems typically control vehicle yaw and side-slip by controlling braking at the wheels of the vehicle. Other systems have been directed to the use of chassis systems, such as active steering and active suspension, to achieve the same objectives. Typically, these control systems do not address vehicle roll dynamics. However, for high center-of-gravity vehicles, such as SUVs, it would be desirable to control the rollover characteristics to maintain vehicle roll stability and to keep all four wheels of the vehicle on the road.
Vehicle rollover control can be achieved using differential braking control, active or semi-active suspension control, rear-wheel steering control, front-wheel steering control, or any combination thereof. In each of these control actions, the controller receives vehicle dynamic information from various vehicle sensors, such as yaw rate sensors, lateral accelerometers, roll-rate sensors, etc., and determines a proper amount of control action to be taken. A balance between controlling the vehicle roll motion and yaw motion needs to be provided to achieve the optimal vehicle response. Therefore, detection of vehicle conditions, especially roll over conditions and stability conditions, is typically viable for the quality of control. A good indication of vehicle roll stability is whether all of the wheels of the vehicle remain in contact with the road surface.
Various methodologies have been developed in the art to detect vehicle wheel lift. One technique compares vehicle lateral acceleration information with a threshold calculated from wheel speed and vehicle speed information. Another known technique for detecting wheel lift uses both passive and active systems to classify a rollover event. In this technique, wheel speed is actively changed by applying a braking torque to a certain wheel to determine if wheel lift occurs at that wheel. Since this technique relies on changing the wheel torque by application of the brakes, it may not be desirable to perform the operation during driver braking or acceleration situations.
In accordance with the teachings of the present invention, a system and method for detecting vehicle wheel lift are disclosed. The system includes wheel speed sensors for measuring the speed of each wheel of the vehicle, and suspension sensors for measuring the position of the vehicle suspension at each wheel of the vehicle. A controller determines whether any of the wheels are off the ground by using a kinematic relationship that uses the wheel speed signals and independently determines whether any of the wheels are off the ground by using damper spring displacement from the suspension sensors.
Additional features of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to a system and method for detecting wheel lift using wheel speed signals and suspension displacement signals is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The present invention proposes a system and method for detecting vehicle wheel lift that uses available vehicle sensor information, such as wheel speed, damper displacement or velocity, yaw rate, vehicle speed, steering angle and lateral acceleration, to detect the state of the vehicle wheels. The vehicle wheel states are then compared to predetermined thresholds to detect the wheel lift. Once wheel lift is detected, the information can be used in a vehicle rollover control system. By using the method of the invention to determine wheel lift, wheel braking is not required to determine wheel lift.
The rollover controller 30 receives various input signals to provide the wheel lift detection, as will be discussed in detail below. Particularly, the vehicle system 10 includes a hand-wheel angle sensor 32 for determining the angle of the hand-wheel 22, a vehicle speed sensor 34 for providing a speed signal Vx indicative of the vehicle speed, a yaw rate sensor 36 for providing a yaw rate signal r of the vehicle yaw rate and a lateral acceleration sensor 38 for providing a lateral acceleration signal Ay of the lateral acceleration or side-slip of the vehicle 12.
The vehicle system 10 also includes a wheel speed sensor 40 that provides a signal Vx,w,rf indicative of the speed of the wheel 14, a wheel speed sensor 42 that provides a signal Vx,w,lf indicative of the speed of the wheel 16, a wheel speed sensor 44 that provides a signal Vx,w,rr indicative of the speed of the Wheel 18, and a wheel speed sensor 46 that provides a signal Vx,w,lr indicative of the speed of the wheel 20. The vehicle system 10 further includes a damper sensor 48 that provides a signal δ indicative of the suspension damper displacement of the suspension damper at the wheel 14, a damper sensor 50 that provides a signal δ indicative of the suspension damper displacement at the wheel 16, a damper sensor 52 that provides a signal δ indicative of the suspension damper displacement at the wheel 18 and a damper sensor 54 that provides a signal δ indicative of the suspension damper displacement at the wheel 20.
As will be discussed in detail below, the wheel lift is detected in two ways, particularly, based on damper displacement and wheel speed. For the damper displacement method,
Equation (1) below can be used to determine whether this condition has occurred by determining if the difference between the displacement signal δ and the critical displacement point δcr is less than a predetermined threshold Δδthreshold at each wheel. Particularly, as the displacement of the suspension damper increases as the vehicle begins to roll, the displacement signal δ increases and approaches the critical displacement point δcr. When the difference between the displacement signal δ and the critical displacement point δcr is small enough, a wheel lift condition is imminent. An example of a critical deflection value for a typical SUV is 60 mm for the front of the vehicle and 100 mm for the rear of the vehicle. These critical deflection values are part of a suspension design parameter and can be easily measured. A typical critical threshold Δδthreshold can be 10 mm.
abs(δ−δcr)<Δδthreshold (1)
If the difference between the displacement signal δ and the critical displacement point δcr is less than the predetermined threshold Δδthreshold then that wheel has lifted off the ground.
In this embodiment, the suspension damper sensors are displacement sensors. However, damper velocity sensors can also be used in the same manner to indicate a potential wheel lift.
From these values and under normal driving condition with no wheel lift, the kinematic relationship between the wheels can be described by equations (2) and (3) below.
rTrf≈Vx,w,rf−Vx,w,lf (2)
rTrr≈Vx,w,rr−Vx,w,lr (3)
This relationship can also be used to determine wheel lift if the difference between the expressions in both equations (2) and (3) is greater than a predetermined threshold ΔVthreshold as provided by equations (4) and (5) below.
abs[rTrf−(Vx,w,rf−Vx,w,lf)]>ΔVthreshold (4)
abs[rTrr−(Vx,w,rr−Vx,w,lr)]>ΔVthreshold (5)
In one example, for a typical full size SUV, the threshold ΔVthreshold can be 1.0 kph if the vehicle is braking or coasting and 3.0 kph if the vehicle is accelerating.
If the vehicle is not cornering where the lateral acceleration signal Ay is below the threshold Ay
If, however, the lateral acceleration signal Ay is less than the lateral acceleration threshold Ay
If the algorithm determines that the lateral acceleration signal Ay is greater than the lateral acceleration threshold Ay
If either of the expressions in equations (4) and (5) is met, then at least one of the wheels is potentially off the ground. The wheel speed algorithm will then increment the WHEEL COUNTER ENTER at box 106. The WHEEL COUNTER ENTER is used so that the wheel speed algorithm does not immediately indicate wheel lift has occurred if equation (4) or (5) is satisfied reducing the chance of a false positive. The wheel speed algorithm must go through several loops of getting the same result that wheel lift is detected before it will output a WHEEL SPEED FLAG of 1, indicating wheel lift is present by the wheel speed method. The specific time frame from when the wheel lift is first detected and when it is output from the wheel speed algorithm is application specific for different vehicles, and would be determined by testing and simulations.
The wheel speed algorithm then determines whether the WHEEL COUNTER ENTER is greater than a second counter threshold at decision diamond 108. If the WHEEL COUNTER ENTER is greater than the second counter threshold at the decision diamond 108, then the wheel speed algorithm sets the wheel speed flag to 1 at box 110 indicating that the wheel speed algorithm has detected a wheel lift. As will be discussed in more detail below, both of the wheel lift detection approaches must indicate a wheel lift before the wheel speed algorithm outputs a WHEEL LIFT FLAG of 1 indicating a wheel lift. If the WHEEL COUNTER ENTER is not greater than the second counter threshold at decision diamond 108, the wheel speed algorithm maintains the WHEEL SPEED FLAG set to 0 at box 112.
If neither of the equations (4) and (5) are met at the decision diamond 104, then the wheel speed algorithm increments the WHEEL COUNTER EXIT at box 114. The same principal applies for exiting the wheel lift detection as for entering the wheel lift detection. In particular, if a previous wheel lift has been detected using the wheel speed signals, the wheel speed algorithm does not want to remove that indication until a suitable period of time has passed. The wheel speed algorithm then determines whether the WHEEL COUNTER EXIT is greater than a third counter threshold at decision diamond 116. If the WHEEL COUNTER EXIT is not greater than the third counter threshold at the decision diamond 116, then the wheel speed algorithm returns to the flow chart diagram 80 by maintaining the WHEEL SPEED FLAG still set to 1. If the WHEEL COUNTER EXIT is greater than the third counter threshold at the decision diamond 116, then the wheel speed algorithm resets the WHEEL COUNTER ENTER and the WHEEL COUNTER EXIT to 0 at box 118. The wheel speed algorithm then sets the WHEEL SPEED FLAG to 0, indicating no wheel lift from the wheel speed detection process at box 120. The wheel speed algorithm then returns to the flow chart diagram 80.
The algorithm is simultaneously determining whether there is wheel lift based on the damper displacement method, as discussed above, at circle 128.
If the measured damper displacement signal δ is greater than the threshold Δδthreshold at the decision diamond 132, then the damper algorithm increments a DAMPER COUNTER EXIT at box 142. The damper algorithm then determines whether the DAMPER COUNTER EXIT is greater than a fifth counter threshold at decision diamond 144. If the DAMPER COUNTER EXIT is not greater than the fifth counter threshold at the decision diamond 144, then the damper algorithm maintains the DAMPER SPEED FLAG set to 1. If the DAMPER COUNTER EXIT is greater than the fifth counter threshold at the decision diamond 144, then the damper algorithm resets the DAMPER COUNTER ENTER and the DAMPER COUNTER EXIT to 0 at box 146 and sets the DAMPER SPEED FLAG to 0 at box 148. The damper algorithm then returns to the flow chart diagram 80.
The algorithm determines whether both of the WHEEL SPEED FLAG and the DAMPER SPEED FLAG are set to 1 at decision diamond 150. If neither of the WHEEL SPEED FLAG and the DAMPER SPEED FLAG is set to 1, or only one of the WHEEL SPEED FLAG or the DAMPER SPEED FLAG is set to 1, then the algorithm sets the WHEEL LIFT FLAG to 0 at box 152 indicating no wheel lift. If however, both the WHEEL SPEED FLAG and the DAMPER SPEED FLAG are set to 1 at the decision diamond 150, then the algorithm sets the WHEEL LIFT FLAG to 1, indicating a wheel lift, at box 154.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
