1. Field of the Invention
The present invention relates to a system and method for automatically holding vehicle speed at a cruising speed and automatically controlling the vehicle speed so as to follow a preceding vehicle at a safe inter-vehicle distance, and specifically relates to a system and method therefore which can appropriately track the preceding vehicle along a curved road.
2. Description of the Prior Art
Along with the recent development of automatic cruise control systems which move the vehicle at a constant, set speed, various systems have been proposed for controlling vehicle speed so as to automatically track a leading veicle at a safe inter-vehicle distance when the automatic cruise control system is actuated in order to prevent a rear-end collision of the preceding vehicle and to improve the drivability of the vehicle.
Conventional systems as described above are exemplified in Japanese Patent Application Open No. 58-43009 now U.S. Pat. No. 4,519,469.
The structure of the system disclosed in the above-identified document is shown in FIG. 1. In FIG. 1, a radar unit 1 monitors the distance between the vehicle (hereinafter referred to as the first vehicle) and an obstacle preceding and moving essentially parallel to the first vehicle. A vehicle speed sensor 3 monitors the vehicle speed. An arithmetic operation circuit 5 controls the vehicle speed via a throttle valve opening angle control circuit 7 on the basis of the distance detected by the radar unit 1 and the vehicle speed detected by the sensor 3 such that the vehicle automatically follows the obstacle at a safe intervehicle distance. In addition, the arithmetic operation circuit 5 controls the vehicle speed so as to hold the vehicle constant speed when the circuit 5 receives a signal from a curved road detection circuit 11 indicating that the vehicle is travelling on a curved road. The curved road detection circuit 11 recognizes that the vehicle is moving along a curved road when a steering angle sensor 9 outputs a signal indicating that a steering wheel is rotated through greater than a predetermined angle.
Since the directivity of the radar unit 1 is limited to the direction of travel, the system must take measures to prevent erroneous vehicle speed control due to erroneous results from the radar unit 1 which may not be measuring the distance from the vehicle to the preceding vehicle S, but rather to a reflector mounted on a guard rail or the like, as shown in FIG. 2(a).
However, the system disclosed in the above-identified document still has a problem in that since the vehicle speed is held just as it was when the first vehicle was trailing another vehicle, the vehicle speed is not suitably controlled in cases where the other vehicle approaches or pulls away from the first vehicle as it passes through a long curve. That is to say, in a specific case in which the first vehicle Z abruptly approaches the preceding vehicle S as they move through a curve as shown in FIG. 2(b), the vehicle driver will need to control the vehicle speed manually to decelerate the vehicle in accordance with the decrease in the intervehicle distance.