Method for estimating road surface state

1. A method for estimating a road surface state, the method estimating the current road state on which a vehicle is running at one of predetermined road states based on a sound detection signal from a ground contact surface sound detection unit for detecting a sound in the vicinity of the ground contact surface of the tire of the running vehicle, comprising:

calculating a wave profile of frequency dispersion as said sound detection signal by executing a 1/N octave analysis of said detected sound during the vehicle being running, and determining whether said calculated wave profile of frequency dispersion (hereinafter referred to as a “measured wave profile”) satisfies predetermined determination conditions of the respective road surface states to estimate the road surface state,

wherein a microphone arranged in a container is used to detect said sound, and said determining step is adapted to apply said determination conditions to said measured wave profile including a resonance caused by the container, and

wherein a first determination condition is such that a minimum value of a sound pressure of a band having higher frequency than a band including a frequencies of said resonance caused by the container (hereinafter referred to as a “container resonance frequency band”) is larger than a minimum value of a sound pressure of a band having lower frequency than the container resonance frequency band,

a second determination condition is such that five frequency bands including a band where the sound pressure is the smallest out of bands having lower frequency than the container resonance frequency band contain two bands where the sound pressure is minimal or minimum,

a third determination condition is such that a sound pressure of a band having a second lowest frequency is larger than sound pressures of any other band having lower frequencies than the container resonance frequency band,

a fourth determination condition is such that the container resonance frequency band is referred to as a band X, a band having the lowest frequency among bands having higher frequencies than the container resonance frequency band having a lower sound pressure than a minimum sound pressure of the bands having lower frequency than the container resonance frequency band is referred to a band Y, and all bands between the band X and the band Y have higher sound pressures than a straight line connecting the sound pressures of the bands X and Y, and

the road surface state is estimated with at least one of the first to fourth determination conditions.

2. The method for estimating a road surface state according to claim 1 , wherein a sound from anterior of a rear wheel of the vehicle is used as said sound to be detected.

3. The method for estimating a road surface state according to claim 1 , wherein, when the measured wave profile satisfies the first determination condition, the road surface corresponding to the measured wave profile is estimated at in a wet state.

4. The method for estimating a road surface state according to claim 1 , wherein, when the measured wave profile satisfies the second determination condition, the road surface corresponding to the measured wave profile is estimated at in a frozen state.

5. The method for estimating a road surface state according to claim 1 , wherein, when the measured wave profile satisfies at least one of the third and fourth determination conditions, the road surface corresponding to the measured wave profile is estimated at in a dry state.

6. The method for estimating a road surface state according to claim 1 , wherein, when the measured wave profile satisfies none of the first to fourth determination conditions, the road surface corresponding to the measured wave profile is estimated at in a slushy or snowy state.

7. The method for estimating a road surface state according to claim 1 , further comprising:

comparing, in a first step, the measured wave profile with said first determination condition, and, when the measured wave profile satisfies the first determination condition, estimating the road surface corresponding to the measured wave profile at in a wet state;

when the measured wave profile does not satisfy the first determination condition in the first step, subsequently comparing, in a second step, the measured wave profile with said second determination condition, and, when the measured wave profile satisfies the second determination condition, estimating the road surface corresponding to the measured wave profile at in a frozen state;

when the measured wave profile does not satisfy the second determination condition in the second step, subsequently comparing, in a third step, the measured wave profile with said third and fourth determination conditions, and, when the measured wave profile satisfies at least one of the third and fourth determination conditions, estimating the road surface corresponding to the measured wave profile at in a dry state; and

when the measured wave profile does not satisfy the third and fourth determination conditions in the third step, estimating the road surface corresponding to the measured wave profile at in a slush or snowy state.