ROAD SURFACE CONDITION ESTIMATION METHOD AND ROAD SURFACE CONDITION ESTIMATION DEVICE

Abstract

A device for estimating a condition of a road surface on which a tire is travelling, the device including: an acceleration sensor 11 that detects acceleration in a tire radial direction, an acceleration waveform extracting unit 12 that extracts an acceleration waveform from the acceleration, a differential waveform calculating unit 13 that calculates a differential waveform of the acceleration waveform, a rotation time calculating unit 14 that calculates a rotation time of the tire from the differential waveform, a normalized acceleration waveform generating unit 15 that generates a normalized acceleration waveform by using the rotation time, and a road surface condition estimating unit 16 that determines whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.

Description

TECHNICAL FIELD

The present invention relates to a method and a device for detecting infiltration of water between a tire and a road surface before shifting to a hydroplaning condition.

BACKGROUND

There has been known that, when a tire travels on a wet road surface and if water infiltrates between the tire and the road surface, a gripping power of the tire is lowered since a part of the tire is caused not to be in contact with the road surface. When a water infiltration amount increases and the tire is caused to completely float, a hydroplaning phenomenon occurs, thus it becomes impossible to control a vehicle.

Conventionally, as a method for detecting a hydroplaning condition, there has been proposed a method in which a strain sensor is embedded in a tire tread to detect a vertical compressive stress σ₂ acting on a block having the strain sensor embedded therein, and a feature amount (1−S/S₀) that characterizes a strength of the hydroplaning condition is calculated from a time change of the vertical compressive stress σ₂ (see, for example, Patent Document 1).

CITATION DOCUMENT

Patent Document

Patent Document 1: Japanese Patent No. 5259245

SUMMARY OF THE INVENTION

Technical Problem

However, there has been a problem that, in the method described in Patent Document 1, even though the strength of the hydroplaning condition can be detected, it is difficult to predict a precursor stage before entering the hydroplaning condition.

The present invention has been made in view of the conventional problem and aims at providing a method and a device for detecting a water infiltration condition between the tire and the road surface before entering the hydroplaning condition.

Solution to Problem

The inventor has found, as a result of earnest examinations, that even though a water film W exists on a road surface R as shown in FIG. 10 (a), if there is no water infiltration between a tire 20 and the road surface R (within a ground contact area), the tire 20 can exhibit a sufficient gripping power. Thus, an acceleration waveform in a tire radial direction detected by an acceleration sensor (not shown) installed in the tire 20 is substantially the same to that of a case when traveling on a dry road surface. However, as shown in FIG. 10(b), when a vehicle speed or a water depth increases, water infiltrates between the tire 20 and the road surface R, and a part of the tire 20 is caused not to be in contact with the ground, which results in change in a shape of a ground contact part of the acceleration waveform. Therefore, if information of this acceleration waveform is used, it is possible to detect the water infiltration condition between the tire 20 and the road surface R before entering the hydroplaning condition, and thus the inventors have reached the present invention.

Namely, the present invention provides a method for estimating a condition of a road surface on which a tire is traveling. the method including: a first step of detecting an acceleration in a tire radial direction to be input to the tire by an acceleration sensor installed in the tire; a second step of extracting, from the acceleration, an acceleration waveform that is a time-series waveform of the acceleration in the tire radial direction; a third step of obtaining a differential waveform of the acceleration waveform; a fourth step of calculating a rotation time of the tire from the differential waveform; a fifth step of generating, by using the rotation time, a normalized waveform that is formed by normalizing the acceleration waveform or the differential waveform; and a sixth step of determining, from the normalized waveform, whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.

The present invention also provides a device for estimating a condition of a road surface on which a tire is traveling, the device including: an acceleration sensor that is installed in the tire and that detects acceleration in a tire radial direction; an acceleration waveform extracting means that extracts, from the acceleration, an acceleration waveform that is a time series waveform of the acceleration in the tire radial direction; a differential waveform calculating means that calculates a differential waveform of the acceleration waveform; a rotation time calculating means that calculates a rotation time of the tire from the differential waveform; a normalized acceleration waveform generating means that generates, by using the rotation time of the tire, a normalized acceleration waveform that is formed by normalizing the acceleration waveform; and a road surface condition estimating means that determines whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition, in which, the road surface condition estimating means defines, as a determination area, an area that is 30% or more and 90% or less of a ground contact area in the normalized acceleration waveform, and determines, from the normalized acceleration waveform in the determination area, whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

The summary of the invention does not enumerate all the features required for the present invention, and sub-combinations of these features may also become the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a road surface condition estimation device according to a first embodiment of the present invention;

FIG. 2 is a diagram illustrating a mounting example of an acceleration sensor;

FIGS. 3(a) to 3(c) are diagrams illustrating examples of an acceleration waveform in the tire radial direction and a differential waveform;

FIG. 4 is a diagram illustrating a normalized acceleration waveform and a method for calculating a detection parameter;

FIG. 5 is a flow chart illustrating a road surface condition estimation method according to the present invention;

FIG. 6 is a diagram illustrating a configuration of a road surface condition estimation device according to a second embodiment of the present invention;

FIG. 7 is a diagram illustrating a normalized differential waveform and a method for calculating a detection parameter;

FIG. 8 is a diagram illustrating another example of the detection parameter in the normalized differential waveform;

FIG. 9 is a diagram illustrating another example of the detection parameter in the normalized differential waveform; and

FIGS. 10(a) and 10(b) are diagrams illustrating a change of the acceleration waveform in the tire radial direction caused by decrease in a gripping power due to water infiltration between the tire and the road surface.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG. 1 is a functional diagram illustrating a configuration of a road surface condition estimation device 10 according to a first embodiment of the present invention. In FIG. 1, the reference sign 11 denotes an acceleration sensor, 12 denotes an acceleration waveform extracting means, 13 denotes a differential waveform calculating means, 14 denotes a rotation time calculating means, 15 denotes a normalized acceleration waveform generating means, and 16 denotes a road surface condition estimating means.

The acceleration waveform extracting unit 12 to the road surface condition estimating means 16 are each configured, for example, by computer software and a storage device such as a random access memory (RAM).

The acceleration sensor 11 is, as illustrated in FIG. 2, disposed on an inner liner portion 21 of the tire 20 at a central portion in a tire width direction so that a detection direction becomes the tire radial direction, to thereby detect acceleration in the tire radial direction input from the road surface to a tire tread 22.

The acceleration waveform extracting means 12 extracts the acceleration waveform that is a time-series waveform of the acceleration in the tire radial direction output from the acceleration sensor 11.

FIG. 3(a) is a diagram illustrating an example of the acceleration waveform (a case where there is no water infiltration between the tire and the road surface), in which the horizontal axis is time [sec.] and the vertical axis is the radial direction acceleration A[G]. The acceleration waveform has a peak before a step-in point P_f and a peak after a kick-out point P_k, respectively, and has a feature that a magnitude of acceleration becomes substantially zero in the vicinity of a ground contact center. The step-in point P_f is a point where inclination between two peaks becomes minimum (negative and an absolute values is maximum) and the kick-out point P_k is a point where the inclination becomes maximum. Incidentally, positions (time) of the step-in point P_f and the kick-out point P_k are normally obtained from the differential waveform to be described later.

The differential waveform calculating means 13 obtains, by calculation, the differential acceleration waveform in the tire radial direction that is the differential waveform of the acceleration waveform (hereinafter referred to as differential waveform). The differential waveform has, as illustrated in FIG. 3(b), a negative peak appearing at the step-in point P_f and a positive peak appearing at the kick-out point P_k, and has a feature that the inclination becomes substantially zero in the vicinity of the ground contact center. Incidentally, the vertical axis of the differential waveform is the radial direction differential acceleration DA=dA/dt.

The rotation time calculating means 14 calculates, from the differential waveform obtained by the differential waveform calculating means 13, a rotation time T of the tire, which is a time required for the tire 20 to rotate one rotation.

As shown in FIG. 3 (c), the rotation time T of the tire is obtained from an interval between two adjacent step-in points P_f, or from an interval between two adjacent kick-out points P_k of the differential waveform.

The normalized acceleration waveform generating means 15 generates, using the rotation time T of the tire 20 calculated by the rotation time calculating means 14, a normalized acceleration waveform that is formed by normalizing the acceleration waveform extracted by the acceleration waveform extracting means 12.

More concretely, as shown in FIG. 4, by normalizing the time of the horizontal axis (time) by the rotation time T, as X=t/T, the acceleration waveform is converted into a measurement position waveform whose horizontal axis corresponds to a position of the acceleration sensor 11. Incidentally, FIG. 4 illustrates the acceleration waveform in a case where water is infiltrating between the tire and the road surface.

For example, if a measurement position of the step-in point P_f is X_f1=t_f1/T=1, a measurement position of a next step-in point P_f,+1 becomes X_f2=(t_f1+T)/T=1+1=2.

Further, if a time interval between the step-in point P_f and the kick-out point P_k is CT and CT/T=CL, a measurement position of the kick-out point P_k becomes X_kn=t_kn/T=X_fn+CL. Furthermore, a measurement position of the ground contact center becomes X_cn=X_fn+CL/2 (n=1, 2, 3, . . . ).

Hereinafter, an explanation of the suffix n will be omitted.

Incidentally, the ground contact section becomes [X_c−CL/2, X_c+CL/2].

Also, with respect to the vertical axis, the acceleration A(G) is multiplied by the square of the rotation time T. That is, because the acceleration A is a temporal differentiation of the speed, it is proportional to the square of the rotation time T. Therefore, if the vertical axis is defined as GT2=AT², GT2 becomes an amount which does not depend on the speed (vehicle speed).

As described above, if the acceleration waveform is normalized by using the rotation time T of the tire 20, the vertical axis and the horizontal axis can take values that do not depend on the vehicle speed.

The road surface condition estimating means 16 includes a determination section setting unit 16a, a detection parameter calculating unit 16b and a road surface condition determining unit 16c, and determines, from the normalized acceleration waveform generated by the normalized acceleration waveform generating means 15, whether or not water infiltration condition between the tire 20 and the road surface R is in a condition to be shifted to the hydroplaning condition.

The determination section setting unit 16a sets a determination section that is a section of the normalized acceleration waveform to be used for determination. In the present embodiment, the center of the determination section is set to a ground contact center X_c and, a section width is set to DL=C·CL.

CL is a time width of the ground contact area illustrated in FIGS. 3 (a) and 3(b), and C is a constant that satisfies 0.3≤C≤0.9.

Therefore, the determination section becomes [X_c−DL/2, X_c+DL/2].

C is set to 0.9 or less and 0.3 or more because, in either of cases where C is greater than 0.9 and where C is less than 0.3, a difference caused by the water infiltration condition in detection parameters to be described later becomes small.

The detection parameter calculating unit 16b calculates, from the normalized acceleration waveform in the determination section, a detection parameter for determining whether or not the condition of the road surface R is in a condition to be shifted to the hydroplaning condition.

In the present embodiment, as shown in FIG. 4, the detection parameter is a degree θ of a tilt angle of a straight line passing through the point P_a at the end portion (X_a=X_c−DL/2) on the step-in side and the point P_b at the end portion (X_a=X_c+DL/2) on the kick-out side of the normalized acceleration waveform.

θ can be expressed by the following formula (1).

θ=tan⁻¹(ΔGT2/DL)  (1)

where, DL is the section width and ΔGT2 is a difference |GT2 (P_b)−GT2 (P_a)| between a normalized acceleration GT2 (P_a) at the point P_a and a normalized acceleration GT2 (P₂) at the point P_b.

The road surface condition determining unit 16c compares the degree θ of the tilt angle with a preset threshold value θ_h, and in a case where θ>θ_h, determines that the road surface condition is in a condition to be shifted to the hydroplaning condition, and in a case where θ≤θ_h, determines that the road surface condition has not reached the condition to be shifted to the hydroplaning condition.

Next, the road surface condition estimation method according to the present invention will be described with reference to the flowchart of FIG. 5.

First, detecting by the acceleration sensor 11, the acceleration in the tire radial direction, which is input from the road surface to the tire tread 22 (step S10), then extracting the acceleration waveform from the detected acceleration in the tire radial direction (step S11).

Next, obtaining, by calculation, a differential waveform of the acceleration waveform (step S12), and from an interval between two step-in points P_f of the differential waveform, calculating the rotation time T of the tire 20, which is a time required for the tire 20 to rotate for one rotation (step S13).

Then, generating, by using the rotation time T of the tire 20 calculated in the step S13, the normalized acceleration waveform that is formed by normalizing the acceleration waveform extracted in the step S11 (S14).

As described above, the horizontal axis of the normalized acceleration waveform is a measurement position X=t/T of the acceleration sensor, and the vertical axis is the normalized acceleration GT2=AT².

Next, after setting a determination section that is a section to be used for determination of the normalized acceleration waveform (S15), calculating the detection parameter (step S16).

In the present embodiment, as the detection parameter, the degree θ of the tilt angle of the straight line m passing through the point P_a at the end portion on the step-in side and the point P_b at the end portion on the kick-out side of the normalized acceleration waveform, was used.

Finally, comparing the degree θ of the tilt angle with the preset threshold value θ_h, and in the case where θ>θ_h, determining that the road surface condition is in the condition to be shifted to the hydroplaning condition, and in the case where θ≤θ_h, determining that the road surface condition has not reached the condition to be shifted to the hydroplaning condition (S17).

Second Embodiment

FIG. 6 is a functional block diagram illustrating the configuration of a road surface condition estimating device 30 according to a second embodiment. The road surface condition estimating device 30 includes the acceleration sensor 11, the acceleration waveform extracting means 12, the differential waveform calculating means 13, the rotation time calculating means 14, a normalized differential waveform generating means 35 and a road surface condition estimating means 36, and determines, by using the normalized differential waveform that is formed by normalizing the differential waveform, whether or not the road surface condition is in a condition to be shifted to the hydroplaning condition.

Explanations of the acceleration sensor 11 and the acceleration waveform extracting means 12 to the rotation time calculating means 14 are omitted, because the elements denoted by the reference signs same with those of the first embodiment have the configurations identical to those of the first embodiment.

The normalized differential waveform generating means 35 generates, by using the rotation time T of the tire 20 calculated in the rotation time calculating means 14, the normalized differential waveform that is formed by normalizing the differential waveform obtained by the differential waveform calculating means 13.

Specifically, as illustrated in FIG. 7, by normalizing the time of the horizontal axis (time) by the rotation time T, as X=t/T, the differential waveform is converted into a measurement position waveform, the horizontal axis being in correspondence with the position of the acceleration sensor 11, and the differential acceleration DA (G/sec.) of the vertical axis is multiplied by the cube of the rotation time T.

Incidentally, FIG. 7 illustrates the differential waveform in a case where water is infiltrating between the tire and the road surface.

Since the differentiation DA is the time differentiation, the differentiation DA is proportional to the cube of the rotation time T. Therefore, if the vertical axis is set to GT3=AT³, GT3 becomes an amount that does not depend on the speed (vehicle speed).

As such, if the differential waveform is normalized by using the rotation time T of the tire 20, the vertical axis and the horizontal axis can have values that do not depend on the vehicle speed, as similar to the normalized acceleration waveform of the first embodiment.

The road surface condition estimating means 36 includes a determination section setting unit 36a, a detection parameter calculating unit 36b and a road surface condition determining unit 36c, and determines, from the normalized differential waveform generated by the normalized differential waveform generating means 35, whether or not the water infiltration condition between the tire 20 and the road surface R is in a condition to be shifted to the hydroplaning condition.

The determination section setting unit 36a sets a determination section that is a section of the normalized differential waveform to be used for determination. In the present embodiment, as similar to the first embodiment, the determination section is set to [X_c−DL/2, X_c+DL/2]. Here, X_c is the ground contact center and DL is the determination section width.

The detection parameter calculating unit 36b calculates, from the normalized differential waveform in the determination section, the detection parameter for determining whether or not the condition of the road surface R is in a condition to be shifted to the hydroplaning condition. In the present embodiment, as illustrated in FIG. 7, the integrated value size S from the end portion (X_a=X_c−DL/2) on the step-in side to the end portion (X_b=X_c+DL/2) on the kick-out side of the normalized differential waveform in the determination section was defined as the detection parameter. When water infiltrates between the tire 20 and the road surface R, a minus amount of the integration in the determination section increases, and thus the integrated value size S increases. Accordingly, by comparing the integrated value size S with a present threshold value S_h, it is possible to estimate a degree of the water infiltration between the tire 20 and the road surface R.

The road surface condition determining unit 36c determines that the water infiltration condition between the tire and the road surface is in a condition to be shifted to the hydroplaning condition, when the integrated value size S is larger than the preset threshold value S_h.

Although the present invention has been described using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It is apparent to those skilled in the art that various modifications and improvements may be added to the above-described embodiments. It is also apparent from the claims that embodiments with such modifications or improvements may belong to the technical scope of the present invention.

For example, in the first embodiment, the detection parameter is set to θ=tan⁻¹(ΔGT2/DL), however, the difference of the normalized acceleration |GT2 (P₂)−GT2 (P₁)| may be used.

Further, in the first embodiment, GT2=AT² was set by normalizing the vertical axis A[G] by the rotation time T. However, if the threshold θ_h is obtained for each rotation time T or if a map representing a relationship between the rotation time T and the threshold θ_h is obtained in advance, it is not necessary to normalize the vertical axis. The same applies to the second embodiment.

Moreover, in the second embodiment, the integrated value size S of the normalized differential waveform in the determination section was defined as the detection parameter. However, as illustrated in FIG. 8, when water infiltrates between the tire 20 and the road surface R, a distance d=X_z−X_a between the end portion X_a on the step-in side and a zero-cross point X_z in the determination section of the normalized differential waveform increases. Thus, it is possible to determine, by using this distance d as the detection parameter, whether or not the water infiltration condition between the tire 20 and the road surface R is in the condition to be shifted to the hydroplaning condition.

Instead of the determination section, a distance d′=X_z−X_f between the end portion X_f on the step-in side and the zero-cross point X_z in the ground contact section may be used as the detection parameter.

Alternatively, as illustrated in FIG. 9, when water infiltrates between the tire 20 and the road surface R, an absolute value level |GT3 (X_f)| of the peak P_f on the step-in side of the normalized differential waveform decreases. Therefore, it is possible to determine, by using the absolute value level |GT3 (X_f)| as the detection parameter, whether or not the water infiltration condition between the tire 20 and the road surface R is in the condition to be shifted to the hydroplaning condition. In this case, the determination is made within the ground contact section.

Though the present invention has been described as above, the present invention can also be described as follows. That is, the present invention provides a method for estimating a condition of a road surface on which a tire is traveling, the method including: a first step of detecting an acceleration in a tire radial direction to be input to the tire by an acceleration sensor installed in the tire; a second step of extracting, from the acceleration, an acceleration waveform that is a time-series waveform of the acceleration in the tire radial direction; a third step of obtaining a differential waveform of the acceleration waveform; a fourth step of calculating a rotation time of the tire from the differential waveform; a fifth step of generating, by using the rotation time, a normalized waveform that is formed by normalizing the acceleration waveform or the differential waveform; and a sixth step of determining, from the normalized waveform, whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.

As such, since the water filtration condition between the tire and the road surface is estimated from the feature of the tire radial direction acceleration waveform normalized by using the rotation time or from the feature of the differential waveform that is formed by differentiating the tire radial direction acceleration waveform, it is possible to precisely predict whether or not the road surface condition is in the condition to be shifted to the hydroplaning condition, that is, a precursor condition before entering the hydroplaning condition.

Incidentally, the rotation time of the tire can be obtained from an interval between adjacent peaks on the step-in side or from an interval between adjacent peaks on the kick-out side of the differential waveform.

Further, the sixth step includes defining, as a determination area, an area that is 30% or more and 90% or less of a ground contact area, where the precursor condition before entering the hydroplaning condition is specifically prominent and determining, from the acceleration waveform or the differential waveform in the determination area, whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition. Thus, it is possible to effectively predict the precursor condition. In the meantime, the ground contact area means an area between the peak on the step-in side and the peak on the kick-out side of the differential waveform (or the normalized differential waveform),

Further, if the detection parameter is set to either one of, or a plurality of, or all of a degree θ of a tilt angle of a straight line passing through a point at an end portion on a step-in side and a point at an end portion on a kick-out side in the determination area of the normalized acceleration waveform, a distance d of a zero-cross point of the normalized differential waveform from the end portion on the step-in side in the determination area, and an integrated value size S from the end portion on the step-in side to the end portion on the kick-out side in the determination area of the normalized differential waveform, it is possible to precisely and certainly predict the water infiltration condition between the tire and the road surface.

Further, the same effect can be obtained even by using, as the detection parameter, the size of the peak value on the step-in side of the differential acceleration waveform.

Furthermore, the present invention provides a device for estimating a condition of a road surface on which a tire is traveling, the device including: an acceleration sensor that is installed in the tire and that detects acceleration in a tire radial direction; an acceleration waveform extracting means that extracts, from the acceleration, an acceleration waveform that is a time series waveform of the acceleration in the tire radial direction; a differential waveform calculating means that calculates a differential waveform of the acceleration waveform; a rotation time calculating means that calculates a rotation time of the tire from the differential waveform; a normalized acceleration waveform generating means that generates, by using the rotation time of the tire, a normalized acceleration waveform that is formed by normalizing the acceleration waveform; and a road surface condition estimating means that determines whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition, in which, the road surface condition estimating means defines, as a determination area, an area that is 30% or more and 90% or less of a ground contact area in the normalized acceleration waveform, and determines, from the normalized acceleration waveform in the determination area, whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

By employing the configuration described above, it is possible to realize the device for estimating the condition of the road surface that can precisely predict the precursor condition before entering the hydroplaning condition.

Incidentally, by using the normalized differential waveform that is formed by normalizing the differential waveform, instead of the normalized acceleration waveform that is formed by normalizing the acceleration waveform, it is possible to precisely predict the precursor condition before entering the hydroplaning condition.

REFERENCE SIGN LIST

10: Road surface condition determination device, 11: Acceleration sensor, 12: Acceleration waveform extracting means, 13: Differential waveform calculating means, 14: Rotation time calculating mans, 15: Normalized acceleration waveform generating means, 16: Road surface condition estimating means, 16a: Determination section setting unit, 16b: Detection parameter calculating unit, 16c: Road surface condition determining unit, 20: Tire, 21: Inner liner portion, and 22: Tire tread.

Claims

1. A method for estimating a condition of a road surface on which a tire is traveling, the method comprising: a first step of detecting an acceleration in a tire radial direction to be input to the tire by an acceleration sensor installed in the tire;a second step of extracting, from the acceleration, an acceleration waveform that is a time-series waveform of the acceleration in the tire radial direction;a third step of obtaining a differential waveform of the acceleration waveform;a fourth step of calculating a rotation time of the tire from the differential waveforma fifth step of generating, by using the rotation time, a normalized waveform that is formed by normalizing the acceleration waveform or the differential waveform; anda sixth step of determining, from the normalized waveform, whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.

2. The method for estimating a condition of a road surface according to claim 1, wherein, the sixth step includes defining, as a determination area, an area that is 30% or more and 90% or less of a ground contact area in the normalized waveform, and determining, from the normalized waveform in the determination area, whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

3. The method for estimating a condition of a road surface according to claim 2, wherein, in a case where a degree θ of a tilt angle of a straight line passing through a point at an end portion on a step-in side and a point at an end portion on a kick-out side in the determination area of the acceleration waveform that is normalized in the fifth step is greater than a preset threshold value θh, determination is made that the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

4. The method for estimating a condition of a road surface according to claim 2, wherein, in a case where a distance d of a zero-cross point of the differential waveform that is normalized in the fifth step from an end portion on a step-in side in the determination area is greater than a present threshold value dh, determination is made that the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

5. The method for estimating a condition of a road surface according to claim 2, wherein, in a case where an integrated value size S from an end portion on a step-in side to an end portion on a kick-out side in the determination area of the differential waveform normalized in the fifth step is greater than a preset threshold value Sh, determination is made that the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

6. The method for estimating a condition of a road surface according to claim 1, wherein, from a size of a peak value on a step-in side of the differential waveform, determination is made as to whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

7. A device for estimating a condition of a road surface on which a tire is traveling, the device comprising: an acceleration sensor that is installed in the tire and that detects acceleration in a tire radial direction;an acceleration waveform extracting means that extracts, from the acceleration, an acceleration waveform that is a time series waveform of the acceleration in the tire radial direction;a differential waveform calculating means that calculates a differential waveform of the acceleration waveform;a rotation time calculating means that calculates a rotation time of the tire from the differential waveform;a normalized acceleration waveform generating means that generates, by using the rotation time of the tire, a normalized acceleration waveform that is formed by normalizing the acceleration waveform; anda road surface condition estimating means that determines whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.wherein, the road surface condition estimating means defines, as a determination area, an area that is 30% or more and 90% or less of a ground contact area in the normalized acceleration waveform, and determines, from the normalized acceleration waveform in the determination area, whether or not the water infiltration condition between the tire and the road surface is in the condition to be shifted to the hydroplaning condition.

8. A device for estimating a condition of a road surface on which a tire is traveling, the device comprising: an acceleration sensor that is installed in the tire and that detects acceleration in a tire radial direction;an acceleration waveform extracting means that extracts, from the acceleration, an acceleration waveform that is a time series waveform of the acceleration in the tire radial direction;a differential waveform calculating means that calculates a differential waveform of the acceleration waveform;a rotation time calculating means that calculates a rotation time of the tire from the differential waveform;a normalized differential waveform generating means that generates, by using the rotation time, a normalized differential waveform that is formed by normalizing the differential waveform; anda road surface condition estimating means that determines whether or not a water infiltration condition between the tire and the road surface is in a condition to be shifted to a hydroplaning condition.