1. Field of the Invention
This invention relates generally to a system for reducing vehicle roll and, more particularly, to a system for reducing vehicle roll by controlling the friction-force of a damper at one or more of the wheels of the vehicle.
2. Discussion of the Related Art
Various vehicle control systems are known in the art that improve driver convenience, safety and comfort. One of the areas of control for vehicle control systems is the roll motion of the vehicle.
The centrifugal force on the body of a vehicle induces a vehicle roll during cornering. This vehicle roll may cause driver and passenger discomfort, vehicle instability and possibly even vehicle rollover. To prevent these adverse effects, vehicle roll may be reduced using an active or semi-active roll control system. However, known active vehicle roll control systems typically require a hydraulic system that increases system complexity and cost. A passive anti-roll bar can be used to reduce vehicle roll without a complex hydraulic system, but such an anti-roll bar tends to increase the suspension harshness by transmitting road disturbances, such as from a single wheel pothole or bump.
An attempt to overcome some of the drawbacks discussed above is provided by U.S. Patent Publication No. 2005/0253350, titled Roll Control System, Device and Method for Controlling Vehicle Stability. The system disclosed in this publication uses magnetorheological actuators to engage anti-roll bars to control the roll motion of the vehicle only when needed. However, this system still requires additional actuators to control the anti-roll bars, which also acts to increase system complexity and cost.
In accordance with the teachings of the present invention, a system for providing vehicle roll control is disclosed that controls the friction-force of dampers provided at the wheels of the vehicle. The system includes a lateral acceleration sensor for determining the lateral acceleration of the vehicle, a steering angle sensor for determining the steering angle of the vehicle and a speed sensor for determining the speed of the vehicle. The system calculates a current control signal for the dampers based on the lateral acceleration and/or the steering angle, and uses one or both of the current control signals to control the friction-force of the inside, outside or both of the dampers.
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 providing vehicle roll control using dampers with controllable friction force is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The dampers 20, 22, 24 and 26 can be any damper with controllable friction force suitable for the purposes described herein, such as electrorheological (ER) and magnetorheological (MR) type dampers that can control their friction force including Coulomb friction.
The controller 28 receives various inputs to provide the proper current control signal to the dampers 20, 22, 24 and 26 for roll control. Particularly, the controller 28 receives a lateral acceleration signal ay from a lateral accelerometer 30 indicative of the lateral acceleration of the vehicle 10, a vehicle speed signal v from a vehicle speed sensor 32 and a hand-wheel angle signal δ from a hand-wheel angle sensor 34 providing a steering angle of the vehicle 10 in response to turning of the vehicle hand-wheel 36.
Vehicle roll motion can be modeled as a one degree-of-freedom dynamic model as:
(Ixx+Mshs2){umlaut over (φ)}+br{dot over (φ)}+krφ=Mshsay,m (1)
Where Ixx is the roll moment of inertia of the vehicle body (sprung mass) with respect to the center of gravity, Ms is the mass of vehicle body, hs is the height of the center of gravity from the roll center, br and kr are the roll damping coefficient and the roll stiffness, respectively, and are combined values for a suspension and tire, and ay,m is the measured lateral acceleration, which includes the gravity component due to vehicle roll as well as the true lateral acceleration of the vehicle 10.
When the vehicle 10 is turning, a roll motion is induced on the vehicle 10 which causes the inside of the vehicle body to extend and the outside of the vehicle body to compress. In order to eliminate or reduce this roll motion, the present invention proposes providing a current control signal to the dampers on the inside or outside of the roll motion, or both, to prevent the vehicle 10 from rolling. The roll motion is either detected by the turning radius of the vehicle from the sensor 34 or by the lateral acceleration of the vehicle 10 from the sensor 30. As will be discussed in detail below, the current control signal applied to the particular damper 20, 22, 24 and/or 26 is calculated based on both of these variables. When the current control signal is calculated based on the steering angle, a gain value is determined for the signal using lateral acceleration from the model of equation (1).
For relatively small roll motion, the current control signals applied to the dampers 20, 22, 24 and/or 26 can completely eliminate the roll motion. As the roll motion increases beyond a certain limit based on the vehicle speed and the turning harshness, the limits of the dampers 20, 22, 24 and/or 26 may only be able to partially reduce the amount of roll.
When determining the current control signal for the right or left side of the vehicle 10 to prevent or reduce vehicle roll, a different current control signal may be applied to the front and rear damper, and may be based on vehicle weight.
If the vehicle 10 is not traveling above that speed, then the process ends. If the vehicle speed signal v is greater than the threshold vth at the decision diamond 64, then the algorithm determines whether the absolute value of the hand-wheel angle signal δ from the sensor 34 is less than a predetermined threshold δth at decision diamond 66. The hand-wheel angle signal δ will be either positive or negative depending on which direction the hand wheel 36 is rotated. If the absolute value of the hand-wheel angle signal δ is less than the threshold δth, then the algorithm proceeds. If the hand-wheel angle signal δ is greater than the threshold δth at the decision diamond 66, the algorithm calculates the current control signal Iroll,δ for the dampers 20, 22, 24 and 26 at box 68.
In other embodiments, if the hand-wheel angle signal δ is greater than zero at the decision diamond 74, then the controller 28 may calculate the current control signal Iroll,δ for the left side dampers 20 and 24 at the box 76 or all of the dampers 20, 22, 24 and 26 on both sides at the box 76. Similarly, in other embodiments, if the hand-wheel angle signal δ is not greater than zero at the decision diamond 74, then the controller 28 may calculate the current control signal Iroll,δ for the right side dampers 22 and 26 at the box 76 or all of the dampers 20, 22, 24 and 26 on both sides at the box 76.
In one non-limiting embodiment, the current control signal Iroll,δ applied to the dampers 20, 22, 24 and/or 26 is determined based on the steering angle as:
Iroll,δ=Kδ·|δ| (2)
Where Kδ is a vehicle dependent gain that can be calculated based on the one degree-of-freedom vehicle model shown in equation (1) and the lateral acceleration gain.
When using the lateral acceleration gain concept, which is well known to those skilled in the art, the lateral acceleration signal ay can be calculated as:
Where V is the vehicle speed, L is the vehicle wheel-base and KUS is the understeer gradient. The current control signal Iroll,δ needs to be proportional to the lateral acceleration because the vehicle roll is induced by lateral acceleration, as shown in equation (1). Therefore, the gain Kδ in equation (2) can be calculated as:
Where CKδ is a vehicle dependent parameter and may vary with vehicle speed, roll angle and steering wheel angle.
Returning to
The current control signal Iroll,ay needs to be proportional to lateral acceleration because vehicle roll is induced by lateral acceleration, as shown in equation (1). The current control signal Iroll,ay can be determined based on the lateral acceleration from equation (5) below.
Iroll,ay=Kay|ay| (5)
When using the lateral acceleration signal to determine the current control signal Iroll,ay, the gain Kay is a predetermined value available from look-up tables, and may vary with vehicle speed, roll angle and steering wheel angle.
In another embodiment, the algorithm obtains sensor information at the box 92, determines whether the lateral acceleration signal is positive or negative (left or right) at the decision diamond 94, and calculates the current control signal Iroll,ay for the dampers 20, 22, 24 and 26 at both sides of the vehicle 10.
Returning to
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.
This application claims the benefit of the priority date of U.S. Provisional Patent Application No. 60/916,686, titled Vehicle Roll Control Method Using Controllable Friction Force or MR Dampers, filed May 8, 2007.
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