(1) Field of the Invention
This invention relates to a roll-over suppressing control apparatus for a vehicle for suppressing overturning of the vehicle while the traveling performance of the vehicle is secured.
(2) Description of the Related Art
When a vehicle turns, depending upon the traveling speed or the steering situation of the vehicle, a moment in the rolling direction is generated on the vehicle and sometimes degrades the stability of the vehicle. In such an instance, if the moment in the rolling direction increases excessively, then there is the possibility that the vehicle may be placed into roll-over (overturning). Therefore, various control apparatus have been proposed conventionally which take a braking step before the vehicle is placed into roll-over.
For example, Japanese Patent Laid-Open No. 2000-104582 (hereinafter referred to as Patent Document 1) discloses a vehicle behavior control apparatus having such a configuration that, if it is decided that the vehicle behavior is unstable, then fuel supply to an engine is blocked and, after the vehicle behavior is placed back into a stable state, the fuel supply to the engine is re-started in response to the value of the lateral acceleration (lateral G) of the vehicle. According to the configuration, in a state wherein the vehicle behavior is unstable, the engine torque is reduced so that the wheels may grip the road surface well, and after the vehicle is placed back into a stable state, the control of reducing the engine torque is ended in response to the value of the lateral acceleration of the vehicle to restore a stable state wherein no hunting of the control occurs.
While the technique disclosed in Patent Document 1 takes a braking step of reducing the propelling force of the vehicle through control of the engine torque to suppress the vehicle speed, also another technique of applying braking force to the wheels to brake the vehicle has been proposed.
For example, Japanese Patent Laid-Open No. Hei 10-24819 (hereinafter referred to as Patent Document 2) discloses a braking force control apparatus having such a configuration that it includes a first control mode wherein it controls the vehicle behavior based on a turning characteristic (magnitude of the yaw rate) of the vehicle and a second control mode wherein rolling (overturning) is controlled based on an excessive roll index (lightness or decreasing speed of the load to a wheel) of the vehicle. In the first control mode, a difference in braking force is provided between turning inner wheels and turning outer wheels to control the yaw moment of the vehicle in the turning direction or turning round direction, but in the second control mode, braking force is applied to the wheels and the fuel amount to be supplied to the engine is reduced to control the vehicle speed. According to the configuration, when the vehicle is in the first control mode, recovery of the vehicle behavior can be achieved, and when the vehicle is in the second control mode, the centrifugal force to act upon the center of gravity of the vehicle can be reduced to prevent rolling and the traveling stability can be maintained well.
In this manner, according to the apparatus disclosed in Patent Document 2, in order to prevent rolling of the vehicle, the control of applying braking force to the wheels is carried out simultaneously in addition to the control of decreasing the engine torque disclosed in Patent Document 1.
Incidentally, turning of a vehicle is divided into non-steady (transient) turning upon lane change, emergency avoiding steering and so forth wherein the turning direction is changed over suddenly and steady turning represented by turning along a loop bridge or the like along which turning in the same direction continues.
In the non-steady turning, a high lateral acceleration does not appear immediately after a changeover of the turning direction, but a lateral acceleration appears rather later. However, a roll rate appears immediately after a changeover of the turning direction and generally is less liable to be controlled by the driver. If the roll rate becomes excessively high during such turning, then the vehicle may possibly turn over to its side even if the lateral acceleration remains low.
On the other hand, upon steady turning, a high roll rate does not occur with the vehicle except when the turning behavior of the vehicle changes such as upon starting of turning or upon ending of turning, and the vehicle can be controlled readily by the driver. However, a lateral acceleration in the turning outer side direction normally appears. Then, if the lateral acceleration becomes excessively high during such turning, then the vehicle may possibly turn over to its side even if the roll rate remains low.
Taking notice of such different types of turning of a vehicle as described above, if the control of applying braking force to the wheels as in the technique disclosed in Patent Document 2 and the control of reducing the engine torque are carried out simultaneously, then upon non-steady turning such as upon emergency avoiding turning, the vehicle can be slowed down sufficiently to suppress the roll-over and can be turned safely. However, upon steady turning, since the control of the vehicle during turning by the driver is comparatively easy, the vehicle is sometimes slowed down excessively by the driver. Further, since the engine output power is suppressed, the intention of the driver to raise the vehicle speed or to keep the vehicle speed becomes less likely to be reflected on the vehicle speed, and there is a subject also that the driving feeling is deteriorated.