1. Field of the Invention
The present invention relates to a method and apparatus for estimating a road surface condition under a traveling vehicle.
2. Description of the Related Art
To enhance the travel stability of a vehicle, it is desired that the road surface condition or the ground contact state of the tire is estimated with accuracy and the data thus obtained is fed back to vehicle control. If the road surface condition or the ground contact state of the tire can be estimated in time, then it will be possible to operate such advanced control as ABS (antilock braking system) braking before taking any danger avoidance action such as braking, accelerating, or steering. With such facility, there will be a marked boost in the safety of vehicular operation.
There are a number of proposed methods for estimating a road surface condition. In one of such methods (see Patent Document 1, for instance), a tire used for the estimation of road surface conditions has deformable structural areas, each including sipes, formed at a specific pitch P circumferentially in the shoulder region of the tire. And the vibrations of the running tire tread are detected by an acceleration sensor to acquire a vibration spectrum, and at the same time the wheel speed is measured by a wheel speed sensor. Then the road surface condition is estimated from the vibration level caused by the deformable structural areas at the detected frequency calculated from the wheel speed measured by the wheel speed sensor and the pitch P.
Also, there are methods proposed for estimating the road-hugging performance of a wheel, using a kinetic state quantity of the wheel (see Patent Document 2, for instance). As described in Patent Document 2, an FFT analysis is performed on the vibration components in the unsprung resonant frequency range extracted from a wheel speed. Then the maximum value Gv of gain of the obtained frequency component in the analysis is compared with the predetermined reference value Gv0. And if the maximum value Gv of gain is greater than the reference value Gv0, the road-hugging performance is determined to be poor, and if the maximum. value Gv of gain is smaller than the reference value Gv0, the road-hugging performance is determined to be satisfactory.
Also, there are various road surface shape detecting apparatuses disclosed. In one of them (see Patent Document 3, for instance), the vibrations of unsprung parts of a vehicle when the vehicle goes over a road surface bump are detected by acceleration sensors attached to the unsprung parts of the right and left wheels. At the same time, the difference Ap-p between the maximum value and the minimum value of the unsprung vibrations is calculated. Then the road surface bump height X1 and the road surface bump width X2 are estimated using the difference Ap-p between the maximum value and the minimum value and a multiple regression equation representing a predetermined relationship between the road surface bump height X1 and road surface bump width X2 and the difference Ap-p between the maximum value and the minimum value. Note here that the multiple regression equation is determined for each of wheel speeds.