Method and system for estimating the state of a road surface

Abstract

Method and system for estimating the state of a road surface (40) wherein it is provided to associate a monitoring unit (2) to a tyre (11) of a vehicle, said monitoring unit (2) comprising at least one detecting element (3) adapted to measure a quantity descriptive of a deformation of the tyre (11); fitting said tyre (11) to a wheel (20) of said vehicle and operating said vehicle so as to cause rotation of said tyre (11) on a road segment having said road surface (40) wherein, due to said fitting and operating, said tyre (11) is deformed in a deformation zone (46, 42, 47, 44). For each revolution of the tyre (11), it is also provided for: obtaining a signal representative of said quantity measured during the rotation of the tyre (11); frequency filtering said signal in a first frequency band so as to obtain a first filtered signal and in a second frequency band, different from said first frequency band, so as to obtain a second filtered signal; processing the first filtered signal so as to obtain a value of a first parameter indicative of the variability of said first filtered signal in said first frequency band, said processing being carried out at a portion of the first filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is located in said deformation zone (46, 20 42, 47, 44) of the tyre (11); processing the second filtered signal so as to obtain a value of a second parameter indicative of the variability of said second filtered signal in said second frequency band, said processing being carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is in said deformation zone (46, 42, 47, 44) of the tyre (11).

Description

[0001] Method and system for estimating the state of a road surface

[0002] DESCRIPTION

[0003] The present invention concerns a method and system for estimating the state of a road surface.

[0004] In particular, the present invention concerns a method and system for estimating the state of a road surface by means of a monitoring unit associated with a tyre of a vehicle.

[0005] In the course of the following description and claims, the following definitions apply.

[0006] The terms "axial", "axially", "radial", "radially", "longitudinal", "longitudinally", "circumferentially" and "circumferentially" are used with reference to the tyre.

[0007] In particular, the terms "axial" and "axially" are meant to indicate references / quantities arranged / measured or extending in a direction substantially parallel to the rotation axis of the tyre.

[0008] The terms "radial" and "radially" are meant to indicate references / quantities arranged / measured or extending in a direction perpendicular to the rotation axis of the tyre, i.e. in a direction intersecting the rotation axis of the tyre and lying in a plane perpendicular to that rotation axis.

[0009] The terms "longitudinal" and "longitudinally" are meant to indicate references / quantities arranged / measured or extending tangentially to the tyre and substantially perpendicular to the axial direction and the radial direction (i.e. in the advancing direction of the tyre / vehicle).

[0010] The terms "circumferential" and "circumferentially" are meant to indicate references / quantities arranged / measured or extending along a circumference that extends about the rotation axis of the tyre.

[0011] The term "contact area" referred to a tyre is meant to indicate the portion of the tyre in contact with a surface when the tyre is fitted on a rim of a wheel and a predetermined vertical load is exerted on the tyre. The contact area has typically an angular extension comprised between 5° and 60°. A value equal to 0 generally refers to a situation in the absence of vertical load on the tyre.

[0012] The term "area of entry to the contact area" or simply "entry area" referred to a tyre are meant to indicate a portion of the tyre that is contiguous with the contact area and precedes it with reference to a rolling direction of the tyre. The area of entry to the contact area typically has an angular extension comprised between 5° and 60°.

[0013] The term "area of exit from the contact area" or simply "exit area" referred to a tyre is meant to indicate a portion of tyre that is contiguous with the contact area and that comes after it with reference to a rolling direction of the tyre. The area of exit from the contact are typically has an angular extension comprised between 5° and 60°.

[0014] The term "deformation zone" referred to a tyre fitted on a rim of a wheel is meant to indicate a portion of the tyre that undergoes a deformation on a surface due to the effect of the vertical load acting on the tyre and due the fact that the tyre is formed of elastically deformable material. The deformation zone may, for example, comprise or correspond to at least one of: contact area, area of entry to the contact and area of exit from the contact area. The deformation zone typically has an angular extension comprised between 5° and 180°.

[0015] While the tyre rolls on a surface, the contact area, the entry area, the exit area and, in general, the deformation zone move circumferentially along the tyre itself.

[0016] The term "state" referred to a road surface is intended to indicate conditions of the road surface in terms of its surface profile, for example with reference to the presence and extent of local or more or less persistent deviations in the surface profile with respect to a reference flat surface.

[0017] The Applicant has observed that knowledge about the state of a road surface is very useful for monitoring and controlling the performance of a vehicle and of a tyre, for example with regard to handling, fuel consumption, rolling resistance, friction, adhesion and wear of the vehicle and the tyre.

[0018] This knowledge is also very valuable for monitoring the performance of the road surface itself, for example in terms of noise emissions and surface damage or deterioration, in order to be able to verify the compliance of the road surface with predetermined standards and the need to carry out specific road maintenance interventions.

[0019] In this context, the international standard ISO 13473-1 divides the texture of the road profile into four main categories depending on the spatial wavelength of the texture, i.e. the distance between periodically repeated portions of the road profile: • microtextures: spatial wavelengths of the texture less than 0.5 mm

[0020] • macrotextures: spatial wavelengths of the texture between 0.5 mm and 50 mm

[0021] • megatextures: spatial wavelengths of the texture between 50 mm and 0.5 m

[0022] • uneveness: spatial wavelengths of the texture between 0.5 m and 50 m (or up to 100 m).

[0023] In general, micro- and macrotextures mainly affect the tyre / road friction and consequently the grip and wear of the tyres.

[0024] The macro- and megatextures mainly act on road and tyre noise and on the tyre rolling resistance.

[0025] In turn, the megatextures and unevenness generally affect the driving comfort, handling, fuel consumption and wear of the vehicle.

[0026] It is known to use electronic monitoring devices inside tyres including sensors and other components suitable for obtaining information regarding various quantities of a tyre such as, for example, temperature, pressure, acceleration, number of revolution of the tyres, vehicle speed, load acting on the tyre and the like.

[0027] The tyre of a vehicle can also be used as a tool to obtain information regarding quantities external to the tyre, such as, for example, quantities relating to the surrounding environment and road infrastructures.

[0028] EP 1 678 019 describes a method and a system for determining the roughness of a tyre rolling surface by means of a sensor device associated with the tyre. The sensor device comprises an accelerometer which provides a first acceleration signal representative of the acceleration of a point of the tyre during rolling on the surface. The first acceleration signal is frequency filtered with a bandpass filter to extract a second signal representative of motion components of said point due to deformations undergone by the tyre during rolling. The second filtered signal is processed at a portion thereof that corresponds to a rolling step of the tyre in which said point is in a zone contiguous with a contact zone of the tyre with the rolling surface. This processing is carried out so as to provide a current roughness parameter indicative of the roughness of the rolling surface on which the tyre is rotating. The parameter indicative of the roughness thus obtained is compared with reference curves that represent the trend of the roughness parameter as the angular speed of the tyre varies for different classes of reference roughness. The comparison is made taking into account the current angular speed assumed by the tyre during the measurement of the current roughness parameter.

[0029] In this context, the Applicant has perceived the need to provide a method for estimating the state of a road surface in a reliable and versatile manner.

[0030] The Applicant has understood that the aforesaid need can be met by monitoring a quantity descriptive of a deformation undergone by the tyre during rotation on a road segment having said road surface.

[0031] In greater detail, the Applicant has understood that from a signal representative of said quantity it is possible to estimate the state of the road surface based on two values of two distinct parameters, respectively representative of the variability of said quantity at two distinct frequency bands that can be referred to the deformation undergone by the tyre in a predetermined deformation zone in association with two predetermined ranges of spatial wavelengths relative to the road surface.

[0032] In accordance with a first aspect thereof, the invention concerns a method for estimating the state of a road surface.

[0033] Preferably, it is provided to associate a monitoring unit with a tyre of a vehicle.

[0034] Preferably, said monitoring unit comprises at least one detecting element adapted to measure a quantity descriptive of a deformation of the tyre.

[0035] Preferably, it is provided to fit the tyre on a wheel of said vehicle.

[0036] Preferably, it is provided to operate said vehicle so as to cause rotation of said tyre on a road segment having said road surface.

[0037] Preferably, due to said fitting and operating, the tyre is deformed in a deformation zone.

[0038] Preferably, for each revolution of the tyre, it is provided to obtain a signal representative of said quantity measured during the rotation of the tyre.

[0039] Preferably, for each revolution of the tyre, it is provided to frequency filter said signal in a first frequency band so as to obtain a first filtered signal.

[0040] Preferably, for each revolution of the tyre, it is provided to frequency filter said signal in a second frequency band different from said first frequency band so as to obtain a second filtered signal. Preferably, for each revolution of the tyre, it is provided to process the first filtered signal so as to obtain a value of a first parameter indicative of the variability of said first filtered signal in said first frequency band.

[0041] Preferably, said processing is carried out at a portion of the first filtered signal that corresponds to a rolling step of the tyre wherein said monitoring unit is located in said deformation zone of the tyre.

[0042] Preferably, for each revolution of the tyre, it is provided to process the second filtered signal so as to obtain a value of a second parameter indicative of the variability of said second filtered signal in said second frequency band.

[0043] Preferably, said processing is carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre wherein said monitoring unit is in said deformation zone of the tyre.

[0044] Preferably, for each revolution of the tyre, it is provided to identify within a two- dimensional space of parameters, the position of a point having the value of the first parameter and the value of the second parameter as coordinates.

[0045] Preferably, for each revolution of the tyre, it is provided to determine the state of the road surface based on the position of said point within said space.

[0046] In accordance with a second aspect thereof, the invention concerns a system for estimating the state of a road surface.

[0047] Preferably, the system comprises a monitoring unit configured to be associated with a tyre of a vehicle.

[0048] Preferably, said monitoring unit comprises at least one detecting element configured to measure a quantity descriptive of a deformation of the tyre.

[0049] Preferably, when said tyre is fitted to a wheel of said vehicle and said vehicle is operated so as to cause rotation of said tyre on a road segment having said road surface, due to said fitting and operating the tyre is deformed in a deformation zone.

[0050] Preferably, the system comprises at least one processing unit comprising software modules.

[0051] Preferably, for each revolution of the tyre, the software modules are configured to obtain a signal representative of said quantity measured during the rotation of the tyre.

[0052] Preferably, for each revolution of the tyre, the software modules are configured to frequency filter said signal in a first frequency band so as to obtain a first filtered signal.

[0053] Preferably, for each revolution of the tyre, the software modules are configured to frequency filter said signal in a second frequency band different from said first frequency band so as to obtain a second filtered signal.

[0054] Preferably, for each revolution of the tyre, the software modules are configured to process the first filtered signal so as to obtain a value of a first parameter indicative of the variability of said first filtered signal in said first frequency band.

[0055] Preferably, said processing is carried out at a portion of the first filtered signal that corresponds to a rolling step of the tyre wherein said monitoring unit is located in said deformation zone of the tyre.

[0056] Preferably, for each revolution of the tyre, the software modules are configured to process the second filtered signal so as to obtain a value of a second parameter indicative of the variability of said second filtered signal in said second frequency band.

[0057] Preferably, said processing is carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre wherein said monitoring unit is in said deformation zone of the tyre.

[0058] Preferably, for each revolution of the tyre, the software modules are configured to identify within a two-dimensional space of parameters the position of a point having the value of the first parameter and the value of the second parameter as coordinates.

[0059] Preferably, for each revolution of the tyre, the software modules are configured to determine the state of the road surface based on the position of said point within said space.

[0060] The Applicant has found that starting from the aforesaid values of the first parameter and of the second parameter it is possible to reliably estimate the state of the road surface and that this estimate can be made independent of the working conditions of the tyre such as, for example, the radius and angular speed of the same.

[0061] By appropriately selecting the first frequency band and the second frequency band based on a first range of spatial wavelengths and a second range of spatial wavelengths relative to the road surface it is also possible to monitor and control different performance relative to the vehicle, tyre and road surface. For example, if it is of interest to monitor and control the pneumatic / road friction and consequently the grip and wear of the tyres, the first frequency band and the second frequency band may be chosen based on a first and a second range of spatial wavelengths corresponding to the micro- and macrotextures. If, on the other hand, it is of interest to monitor and control the noise of the road and of the tyre and / or the rolling resistance of the tyre, the first frequency band and the second frequency band may be chosen based on a first and a second range of spatial wavelengths corresponding to the macro- and megatextures. In addition, if it is of interest to monitor and control the driving comfort, handling, fuel consumption and wear of the vehicle, the first frequency band and the second frequency band may be chosen based on a first and a second range of spatial wavelengths corresponding to the megatextures and unevenness.

[0062] Overall, this achieves the above objectives of reliability and versatility.

[0063] Preferably, said deformation zone of the tyre has an angular extension of about 40°.

[0064] In a preferred embodiment, the deformation zone of the tyre corresponds to an area of entry to the contact area.

[0065] Alternatively, the deformation zone of the tyre may correspond to the contact area between the tyre and the road surface or to an area of exit from the contact area.

[0066] Alternatively, the deformation zone may comprise the entry area, the contact area and the exit area. In this case, the deformation zone may have an angular extension of 180°.

[0067] Preferably, said first frequency band is determined based on a first range of harmonics of said signal that refer to the deformation undergone by said tyre in association with a predetermined first range of spatial wavelengths relative to the road surface.

[0068] Preferably, said second frequency band is determined based on a second range of harmonics of said signal that refer to the deformation undergone by said tyre in association with a predetermined second range of spatial wavelengths relative to the road surface.

[0069] Preferably, the first range of spatial wavelengths and the second range of spatial wavelengths correspond to two different categories of textures of road surfaces profiles. Preferably, the first range of spatial wavelengths and the second range of spatial wavelengths are different from each other and selected from : a range of spatial wavelengths comprised between 0.05mm and 0.5mm; a range of spatial wavelengths comprised between 0.5mm and 50mm; a range of spatial wavelengths comprised between 50mm and 0.5m and a range of spatial wavelengths comprised between 0.5m and 100m.

[0070] In a preferred embodiment, the first range of spatial wavelengths is comprised between 0.5mm and 50mm and the second range of spatial wavelengths is comprised between 50mm and 0.5m.

[0071] In a preferred embodiment, the first range of harmonics and the second range of harmonics are determined based on a Fourier transform of said signal.

[0072] Preferably, the first range of harmonics and the second range of harmonics are different from each other.

[0073] Preferably, the first range of harmonics and the second range of harmonics are contiguous with each other.

[0074] In a preferred embodiment, the space of parameters is defined by two perpendicular axes respectively representative of values of the first parameter and of the second parameter.

[0075] For example, the value of the first parameter and the value of the second parameter can be obtained by performing a root-mean-square or standard deviation or arithmetic mean calculation on appropriate samples of the first filtered signal and, respectively, of the second filtered signal.

[0076] Preferably, the value of the first parameter and the value of the second parameter are obtained by normalizing based on one or more quantities. Preferably, these quantities are selected so as to make the value of the first parameter and the value of the second parameter independent of specific working conditions such as, for example, the angular speed of the tyre and the type of the tyre.

[0077] Preferably, the value of the first parameter and the value of the second parameter are obtained by normalizing with a factor proportional to the radius of the tyre.

[0078] Preferably, the quantity measured during the rotation of the tyre is an acceleration component to which said monitoring unit is subjected during the rolling of the tyre. In a preferred embodiment, the acceleration component is radial.

[0079] In a preferred embodiment, when the quantity measured during rotation of the tyre is a radial acceleration component, the value of the first parameter and the value of the second parameter are obtained by normalizing with a factor proportional to a reference acceleration value of the tyre.

[0080] Preferably, said reference acceleration value corresponds to a value of the radial acceleration component far from the contact area, i.e. at an angular position that is at least 50° away from the centre of the contact area. In other words, the reference acceleration value corresponds to a value of the radial acceleration component outside the deformation zone of the tyre.

[0081] Alternatively, the acceleration component may be longitudinal or axial.

[0082] Preferably, for each revolution of the tyre, the state of the road surface is determined by comparing, within said space, the position of said point with respect to one or more predetermined clouds of points, each cloud of points being representative within said space of the state of a reference road surface.

[0083] Preferably, for each revolution of the tyre, the state of the road surface is determined by comparing, within said space, the position of said point with respect to one or more reference points, each reference point being representative within said space of the state of a reference road surface.

[0084] Preferably, it is provided to collect the points identified at many revolutions of the tyre carried out on said road segment in a first time period and to determine a first trend of the state of the road surface on said road segment analyzing, within said space, the position of the points collected.

[0085] Preferably, it is provided to collect the points identified at many revolutions of the tyre carried out on said road segment in a second time period and to determine a second trend of the state of the road surface on said road segment analyzing, within said space, the position of the points collected.

[0086] Preferably, the second time period is temporally subsequent to the first time period.

[0087] Preferably, it is provided to compare the first trend and the second trend so as to monitor possible changes in the state of the road surface between the first time period and the second time period.

[0088] Preferably, said at least one detecting element comprises an accelerometer configured to measure at least one acceleration component to which said monitoring unit is subjected during the rotation of the tyre.

[0089] Further characteristics and advantages of the present invention will become clear from the following detailed description of some exemplary embodiments thereof, provided solely by way of non-limiting examples, which description will be made with reference to the accompanying drawings, in which:

[0090] - figure 1 schematically shows a system for estimating the state of a road surface according to an embodiment of the invention;

[0091] - figure 2 shows a radial section of a tyre to which a monitoring unit in accordance with an embodiment of the present invention is associated;

[0092] - figure 3 schematically shows a monitoring unit according to an embodiment of the invention;

[0093] - figure 4 shows schematically an example of a deformed tyre;

[0094] - figure 5 shows an example of a curve representative of a radial acceleration component that can be measured, during a revolution of the tyre, by an accelerometer of a monitoring unit as a function of the angular position 0of the accelerometer in a measurement zone centred around the contact area and having an angular extension equal to 180°;

[0095] - figure 6 shows a Fourier transform in the domain of the harmonics of the signal represented in figure 5;

[0096] - figure 7 shows a first signal filtered according to the method of the invention;

[0097] - figure 8 shows a second signal filtered according to the method of the invention;

[0098] - figure 9 shows a space of parameters OLUR.r and OLURn* with a point P and three clouds of points Nl, N2, N3 representative of the state of three reference road surfaces;

[0099] - figure 10 shows the space of parameters of figure 9, wherein the point P is positioned differently;

[0100] - figure 11 shows experimental results obtained by the Applicant where the values of a proximity parameter Pr obtained on many revolutions of a tyre on two reference road surfaces associated with two clouds of points N3 and N2 are indicated in the ordinate; - figure 12 shows experimental results obtained by the Applicant where it is indicated the probability value ("prob" in the ordinate) with which a certain value of the proximity parameter Pr (indicated in the abscissa) was obtained in two time periods T1 and T2;

[0101] - figure 13 shows experimental results obtained by the Applicant where the values of a proximity parameter Pr obtained on many revolutions of a tyre on two reference road surfaces associated with two clouds of points N3 and N2 are indicated in the ordinate and the values of a parameter MPD associated with these two surfaces are shown in the abscissa;

[0102] - figure 14 shows a space of parameters OLURi and OLURII with three clouds of points Nl, N2, N3 representative of the state of three reference road surfaces.

[0103] Figure 1 shows a system 30 for estimating the state of a road surface according to an embodiment of the invention.

[0104] The system 30 comprises four monitoring units 2 and a central processing unit 31 external to the four monitoring units 2.

[0105] In the embodiment illustrated, the system 30 is implemented in a vehicle (not illustrated) provided with four tyres 11, to each of which a respective monitoring unit 2 is associated. The vehicle may for example be a car. However, the present invention also applies to other types of vehicles, such as two- or three-wheeled scooters, motorcycles, tractors, buses, trucks or light trucks, i.e. to vehicles with two, three, four, six or more wheels distributed on two or more axles.

[0106] The monitoring units 2 are in communication with the central processing unit 31.

[0107] The central processing unit 31 may be part of an on-board vehicle control computer (not shown) or be in communication with such on-board computer and / or other remote units (e.g. part of a road infrastructure).

[0108] Typically, the communication between the monitoring units 2 and the central processing unit 31 is a wireless communication (e.g. a Bluetooth communication).

[0109] The central processing unit 31 is external with respect to the tyres 11 in which the monitoring units 2 are fixed. Said central processing unit 31 can be positioned anywhere in the vehicle where it can be reached by the wireless signal (for example Bluetooth) transmitted by the monitoring units 2. For example, the central processing unit 31 may be a box integrated in the vehicle. In another embodiment, the central processing unit 31 can be a mobile personal device of the driver of the vehicle (for example a smartphone or a tablet), provided with suitable applications and / or software modules configured at least for communication with the monitoring units 2, as well as for processing the data received from the monitoring units 2.

[0110] According to one embodiment, each monitoring unit 2 is fixed at an inner surface of the respective tyre 11.

[0111] With reference to this embodiment, figure 2 shows a section of a wheel 20 of the vehicle comprising the tyre 11 and a support rim 12. This tyre 11 is of the type known as "tubeless", i.e. without an inner tube. The tyre 11 is inflatable by means of an inflation valve 13, positioned, for example, on a channel of the support rim 12. The tyre 11 includes a carcass structure 16, not illustrated in detail, shaped according to a substantially toroidal configuration and terminating in two beads 14, 14' each formed along an inner edge of the carcass 16 for fixing the tyre 11 to the support rim 12. The beads 14 and 14' comprise respective annular reinforcing elements 15 and 15', called bead cores.

[0112] A belt structure 17 comprising one or more belt strips is applied to the carcass structure 16 in a radially outer position.

[0113] A tread band 18 is superimposed on the belt structure 17, in a radially external position, on which longitudinal and / or transverse recesses are typically shaped, arranged to define a desired tread pattern.

[0114] The tyre 11 also comprises two sidewalls 19, 19' applied in axially opposite positions on the carcass structure 16.

[0115] The inner surface of the tyre 11 is typically covered by a sealing layer 111, so- called "liner", comprising one or more layers of air-impermeable elastomeric material adapted to guarantee the hermetic seal of the tyre 11 itself.

[0116] Preferably, as illustrated in figure 2, the monitoring unit 2 is arranged on an inner wall of the tyre 11 (in particular on the liner 111) which is opposite to the tread band 18. Even more preferably, the monitoring unit 2 is arranged substantially at an equatorial plane of the tyre 11.

[0117] The monitoring unit 2 is fixed on the inner wall of the tyre 11 by means of a suitable fixing element 332. According to an alternative embodiment, the monitoring unit 2 may be incorporated within the structure of the tyre 11 in the region of the tread 18 and, for example, within the tread band 18 itself or between the belt 17 and the tread band 18.

[0118] As illustrated in figure 2, the following directions can be defined for the tyre: radial direction Z, longitudinal (or advancing) direction X and axial (or lateral) direction Y.

[0119] In the embodiment illustrated in figure 3, each monitoring unit 2 comprises a detecting section 10, a battery 8, a local processing unit (or CPU) 6 associated with a memory (not shown), a transceiver 7 and an antenna 9.

[0120] The monitoring unit 2 may be of the type currently available on the market generally comprising temperature and / or pressure sensors and an accelerometer, or other inertial sensors.

[0121] In the embodiment illustrated, the detecting section 10 of the monitoring unit 2 comprises an accelerometer 3, in particular a radial accelerometer, oriented inside the monitoring unit 2 so as to have an axis substantially orthogonal to the inner surface of the tyre 11. The accelerometer 3 is configured to output an acceleration measurement descriptive of the deformations in radial direction that the tyre 11 undergoes during rolling. Other detecting elements suitable for measuring physical quantities descriptive of the deformations of the tyres 11 could be used, such as longitudinal accelerometers, axial accelerometers, strain gauges, etc.

[0122] In the embodiment illustrated, the detecting section 10 of said monitoring unit 2 further comprises a pressure sensor 4 configured to provide a measurement of the pressure inside the tyre 11 and a temperature sensor 5 configured to provide a measurement of the temperature of the tyre 11.

[0123] The measurement provided by the accelerometer 3 is provided to the local processing unit 6.

[0124] The local processing unit 6 is configured, through appropriate software and / or firmware modules, to receive, from the detecting section 10, the measurements made by the radial accelerometer 3 and by the temperature and pressure sensors 4, 5 and process them.

[0125] In particular, the local processing unit 6 is configured, by means of appropriate software and / or firmware modules, to process the quantity measured by the accelerometer 3 in order to implement in its entirety a method for estimating the state of a road surface according to the invention. Alternatively, the local processing unit 6 can be configured, by means of appropriate software and / or firmware modules, to implement only a part of the estimation method according to the invention and then send, via the transceiver 7 and the antenna 9, the partial results of the processing carried out to the central processing unit 31 which will complete the implementation of said method.

[0126] Ultimately, the choice of distributing the processing between the monitoring units 2 and the central processing unit 31 to implement the estimation method of the invention is a compromise between several constraints to be balanced, such as: hardware complexity, battery consumption, cost, processing power available for the local processing units 6 of the monitoring units 2, etc.

[0127] The transceiver 7 is configured to implement a bidirectional communication via the antenna 9 with the central processing unit 31 specifically configured for communication with the monitoring units 2 comprised within the tyres 11. In preferred embodiments, the transceiver 7 comprises a Bluetooth Low Energy (BLE) module.

[0128] The battery 8 directly or indirectly supplies electric energy to the various components of the monitoring unit 2. In preferred embodiments, it may be a rechargeable battery with power absorbed by the mechanical energy caused by the rotation of the tyre 11.

[0129] According to the estimation method of the invention, by operating the vehicle, the tyres 11 fitted to respective wheels 20 of the vehicle are rotated on a road segment having a road surface 40.

[0130] As schematically shown in figure 4, as a consequence of said fitting and operating, the tyre 11 is deformed in a deformation zone 44.

[0131] In particular, by virtue of the load acting on the tyre 11 (represented in figure 4 by the arrow Fz perpendicular to the road surface 40) and of the fact that the tyre 11 is formed by elastically deformable material, the tyre 11 undergoes a deformation.

[0132] This deformation affects a circumferential region 44 of the tyre 1, defined between two circumferential ends 44a, 44b, in which the shape of the tyre 11 deviates from a substantially circular shape (shown in hatching in figure 4).

[0133] The circumferential region 44 is positioned in a lower portion of the tyre 11 facing the road surface 40. The circumferential region 44 comprises a contact area 42, defined between two circumferential ends 42a, 42b, an area of entry 46 to the contact area 42 defined between two circumferential ends 44a and 42a and an area of exit 47 from the contact area 42 defined between two circumferential ends 42b and 44b.

[0134] The two regions 46, 47 are contiguous with and external to the contact area 42. In particular, the entry area 46 is contiguous with the contact area 42 and precedes it with reference to a rolling direction of the tyre 11, indicated by the arrow d in figure 4. In turn, the exit area 47 is contiguous with the contact area 42 and comes after it with reference to the rolling direction d of the tyre 11.

[0135] The deformation region 44 as a whole, the contact area 42, the entry area 46 and the exit area 47 have an angular extension that can vary depending on the dimensions of the tyre 11, the inflation pressure and the load acting on the tyre 11.

[0136] Typically, the deformation region 44 has an angular extension 5 less than or equal to 180°; the contact area 42 has an angular extension a comprised between 5° and 60°; the entry area 46 has an angular extension p comprised between 5° and 60°; and the exit area 47 has an angular extension / comprised between 5° and 60°.

[0137] According to the method of the invention, for each monitoring unit 2, once the vehicle is operated, it is provided to start measuring the quantity descriptive of deformation of the tyre 11 which, in the embodiment described, is the radial acceleration component measured by the accelerometer 3 which, in turn, corresponds to the radial acceleration component to which the monitoring unit 2 is subjected during the rolling of the tyre 11.

[0138] Preferably, in order to limit energy consumption, for each revolution of the tyre 11, the measurement is carried out only during the passage of the monitoring unit 2 at the deformation area 44 or, in general, at a measurement zone centred around the contact area 22 and having an angular extension of less than or equal to 180°, for example of about 160° or 180°.

[0139] By way of example, figure 5 shows a curve representative of a radial acceleration component that can be measured, during a revolution of the tyre 11, by the accelerometer 3 of each monitoring unit 2 as a function of the angular position 6 of the accelerometer 3 in a measurement zone centred around the contact area 22 and having an angular extension equal to 180°. In figure 5, 6=90° indicates an angular position of the accelerometer 3 at the centre of the contact area 42, 6>90° indicates angular positions after the centre of the contact area 42, and e<90° indicates angular positions before the centre of the contact area 42. In addition, 6=0 and 0=180° indicate two angular positions of the accelerometer 3 diametrically opposite with respect to the centre of the contact area 42.

[0140] As can be noted, this curve has a region in which the value of the radial acceleration tends to oscillate around an almost constant value (indicated in the figure with aP) and a region in which the value of the radial acceleration varies abruptly. The value aPcorresponds to an average value of the acceleration measured by the accelerometer 3 when it is far from the contact area 42 (i.e. outside the deformation area 44, in an angular position that is away from the centre of the contact area 42 by at least 50°), where the tyre is substantially undeformed. The region in which the radial acceleration value varies abruptly corresponds to the situation in which the accelerometer 3 approaches, enters, transits, exits and moves away from the contact area 42. This region substantially corresponds to the deformation region 44 (see figure 4) in which the tyre undergoes deformation due to contact with the road surface 40 and to the crushing imposed by the load Fz. The curve in figure 5 is therefore representative of the deformation undergone by the tyre 11.

[0141] According to the estimation method of the invention, for each revolution of the tyre 11, the operations described below are carried out.

[0142] Firstly, a signal is obtained in a generic unit of measurement, for example electrical, representative of the radial acceleration measured by the accelerometer 3 during the rotation of the tyre 11.

[0143] Preferably, as already described above, for each revolution of the tyre 11, the measurement is carried out only during the passage of the monitoring unit 2 at the deformation area 44 or, in general, at a measurement zone centred around the contact area 22 and having, for example, an angular extension of 180°.

[0144] The signal thus obtained (which will have a trend similar to that shown in figure 5) is processed so as to obtain a Fourier transform of the same in the domain of the harmonics.

[0145] By way of example, figure 6 shows a curve representative of a Fourier transform of the signal of figure 5 as a function of the number of harmonics (h) of that signal. In this curve it is possible to identify a first region I and a second region II at respectively a first range of harmonics and a second range of harmonics of the signal, distinct from the first range of harmonics. The first range of harmonics is connected to the deformation undergone by the tyre 11 in association with a predetermined first range of spatial wavelengths relative to the road surface 40. In turn, the second range of harmonics is connected to the deformation undergone by the tyre 11 in association with a predetermined second range of spatial wavelengths relative to the road surface 40.

[0146] By appropriately selecting the first range of spatial wavelengths and the second range of spatial wavelengths it is possible to monitor different performance relative to vehicle, tyre and road surface.

[0147] For example, in a preferred embodiment of the invention, the first range of spatial wavelengths corresponds to a range of megatextures relative to the road surface 40 (i.e., it is comprised between 50 mm and 0.5 m) while the second range of spatial wavelengths corresponds to a range of macrotextures relative to the road surface 40 (i.e., it is comprised between 0.5 and 50 mm).

[0148] This advantageously allows to monitor and control the noise of the road surface 40 and of the tyre 11 and the rolling resistance of the tyre 11.

[0149] Once the first range of spatial wavelengths and the second range of spatial wavelengths have been selected, the extremes of the first range of harmonics and of the second range of harmonics can be derived starting from the extremes of the first range of spatial wavelengths and of the second range of spatial wavelengths using the relationship: h= (K* R) / A, where h represents the number of harmonics under examination, R the radius of the tyre and the spatial wavelength under examination relative to the road surface 40.

[0150] Once the first range of harmonics and the second range of harmonics of the signal are thus identified, a first frequency band and, respectively, a second frequency band corresponding to them are derived.

[0151] The extremes of the first frequency band and of the second frequency band can be derived starting from the extremes of the first range of harmonics and of the second range of harmonics using the relationship: where f represents the frequency under examination, h the number of harmonics under examination, R. the radius of the tyre 11 and aPthe centripetal acceleration of the tyre (i.e. the radial acceleration measured by the accelerometer 3 far from the contact area 22 or, in other words, outside the deformation area 44) at the revolution under examination of the tyre 11.

[0152] Once the first frequency band and, respectively, the second frequency band have been obtained, the signal (such as that in figure 5) representative of the radial acceleration measured by the accelerometer 3 during the revolution under consideration of the tyre 11 is frequency filtered in the first frequency band so as to obtain a first filtered signal and in the second frequency band, different from the first frequency band, so as to obtain a second filtered signal.

[0153] By way of example, figures 7 and 8 respectively show the first filtered signal and the second filtered signal obtained by filtering the signal of figure 5 in the first frequency band and in the second frequency band, which are determined based on the first range of harmonics and the second range of harmonics identified by the Fourier transform of the signal shown in figure 6.

[0154] Subsequently, the first filtered signal is processed so as to obtain a value of a first parameter OLURi indicative of the variability of said first filtered signal in said first frequency band. In other words, the first parameter OLURi is indicative of the variability induced on the signal of radial acceleration by the spatial wavelengths relative to the road surface 40 belonging to the first range (e.g., the megatextures).

[0155] As schematically shown by a box in figure 7, said processing is carried out at a selected portion of the first filtered signal that corresponds to a rolling step of the tyre 11 wherein said monitoring unit 2 is located at the entry area 46. In the example, an angular extension of the entry area 46 equal to 40° was considered (with 6 ranging from 30° to 70°). This angular extension can be fixed (that is, selected a priori) or it can be selected as a function of the shape of the signal (such as that in figure 5) representative of the radial acceleration measured by the accelerometer 3 and, therefore, of the operating conditions of the tyre 11.

[0156] In turn, the second filtered signal is processed so as to obtain a value of a second parameter OLURII indicative of the variability of the second filtered signal in the second frequency band. In other words, the second parameter OLURII is indicative of the variability induced on the signal of radial acceleration by the spatial wavelengths relative to the road surface 40 belonging to the second range (e.g., the macrotextures).

[0157] As schematically shown by a box in figure 8, said processing is carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre 11 wherein said monitoring unit 2 is located at the entry area 46. In the example, an angular extension of the entry area 46 equal to 40° was considered (with 6 ranging from 30° to 70°). This angular extension can be fixed (that is, selected a priori) or it can be selected as a function of the shape of the signal (such as that in figure 5) representative of the radial acceleration measured by the accelerometer 3 and, therefore, of the operating conditions of the tyre 11.

[0158] According to one embodiment, the value of the first parameter OLURi can be obtained by performing a root-mean-square calculation on appropriate samples of said selected portion of the first filtered signal.

[0159] For example, the value of the first parameter OLURi can be obtained using the following relationship: where co represents the angular speed co of the tyre 11; co2R represents the centripetal acceleration of the tyre 11; R is the radius of the tyre 11; ai,i represents the value of the first filtered signal at the i-th sample and n represents the number of samples considered within the entry area 46 shown in the box in figure 7. The centripetal acceleration of the tyre 11 can correspond to the radial acceleration measured by the accelerometer 3 outside the deformation area 44 or can be estimated as angular speed co squared for the radius R, where the angular speed co can be determined according to techniques known in the art.

[0160] According to one embodiment, the value of the second parameter OLURII can be obtained by performing a root-mean-square calculation on appropriate samples of said selected portion of the second filtered signal.

[0161] For example, the value of the second parameter OLURII can be obtained using the following relationship: where co represents the angular speed co of the tyre 11; co2R represents the centripetal acceleration of the tyre 11 as described above; R is the radius of the tyre 11, am represents the value of the second filtered signal at the i-th sample and n represents the number of samples considered within the entry area 46 shown in the box in figure 8.

[0162] Preferably, for the purposes of calculating the values OLURi and OLURII, the first filtered signal and the second filtered signal are sampled at each revolution of the tyre 11 with a sampling frequency that is adjusted as a function of the rolling speed of the tyre 11 for the revolution under examination in order to guarantee the acquisition of the same number n of samples within the entry area 46 regardless of the rolling speed of the tyre 11.

[0163] Advantageously, in the calculation of the values OLURi and OLURII, normalization factors are used (specifically o2R and R) selected so as to make the value of the first parameter OLURi and the value of the second parameter OLURII independent of specific working conditions such as, for example, the angular speed co of the tyre and the radius of the tyre R.

[0164] In the specific case, following experimental tests, the Applicant considered to use two different normalization factors for OLURi and OLURII (specifically, c2R and2R).

[0165] Once the two values OLURi and OLURII have been calculated as described above, through a series of post- processing operations such as for example scaling and roto-translation, detailed below, it is provided to obtain a pair of values OLURi* and OLURII* .

[0166] Once the two values of OLURi* and OLURII* have been calculated, it is provided to identify within a two-dimensional space of parameters the position of a point P having the value OLURi* of the first parameter and the value OLURII* of the second parameter as coordinates.

[0167] The state of the road surface 40 is finally determined based on the position of said point P within said space of parameters OLURi* and OLURII* .

[0168] Preferably, for each revolution of the tyre 11, the state of the road surface 40 is determined by comparing, within said space, the position of said point P with respect to one or more predetermined clouds of points, each cloud of points being representative within said space of the state of a reference road surface.

[0169] Figure 9 shows a space of parameters defined by two perpendicular axes respectively representative of values OLURi* of the first parameter (in abscissa) and of values OLURII* (in ordinate) of the second parameter.

[0170] By way of example, figure 9 shows three clouds of points Nl, N2, N3 respectively representative of the state of a first, a second and a third reference road surface having different states, i.e. levels of evenness / unevenness significantly different from each other. In particular, the first, second and third reference road surface have gradually decreasing levels of unevenness so that the first reference road surface represents a much more uneven surface while the third reference road surface represents a much more even surface.

[0171] In the example considered, the three clouds of points Nl, N2, N3 were experimentally obtained by the Applicant by rolling the tyre 11 on the three reference road surfaces having distinct values of the parameter MPD ("mean profile depth") which is an indicator known in the art to define the level of texture of the surfaces and is defined by ISO 13473-1 standards. For example, the second reference surface corresponding to the cloud of points N2 had a parameter MPD of about 0.8 mm while the third reference surface corresponding to the cloud of points N3 had a parameter MPD of about 2.85 mm.

[0172] During the rolling of the tyre 11 on the three reference surfaces, using the relationships described above, a pair of values OLURi and OLURII was calculated for each revolution of tyre and the points corresponding to each pair of values were reported in a space of parameters defined by two perpendicular axes respectively representative of values OLURi of the first parameter (in abscissa) and of values OLURII (in ordinate) of the second parameter so as to obtain three clouds of points Nl, N2, N3 as schematically shown in figure 14.

[0173] As visible from figure 14, the three clouds of points Nl, N2, N3 are well distinguishable from each other.

[0174] Once the three clouds of points Nl, N2, N3 were thus obtained in the space of parameters defined by OLURi and OLURII, a series of post-processing operations were performed including, for example, value scaling operations (for example in a range of values comprised between -1 and 1), identification of separation lines between the clouds of points Nl, N2, N3 by means of special classification algorithms (such as, for example, of the SVM or "Support Vector Machine" type), identification of a point of origin 0 of intersection of said separation lines, identification of medoids Ml, M2, M3 associated with the three clouds of points and roto-translation of the clouds around the point of origin 0 so that a line passing through said point of origin 0 and the medoid M3 coincides with an axis OLURII* =0.

[0175] Following these post- processing operations, for each pair of calculated values OLURi and OLURII, a pair of values OLURi* and OLURII* was obtained and a space of parameters defined by the values OLURi and OLU II (shown in figure 14) shifted to the space of parameters defined by the values OLURi* and OLURII* (shown in figures 9 and 10).

[0176] In the space of parameters defined by the values OLURi* and OLURII* (shown in figures 9 and 10), the values OLURi* and OLURII* assume negative values following the aforesaid post- processing operations.

[0177] As schematically shown in figures 9 and 10, for each cloud of points Nl, N2, N3 three points Ml, M2, M3 representative of such clouds were identified. In the illustrated example, the three points Ml, M2, M3 were obtained by determining the medoids of the three clouds of points Nl, N2, N3.

[0178] In the example of figure 9, the state of the road surface 40 for the revolution of tyre 11 under examination can therefore be calculated based on the position of a line passing through the point of origin O and the point P with respect to lines passing through the point of origin O and the medoids Ml, M2, M3.

[0179] For example, the state of the road surface 40 can be determined by determining a proximity parameter Pr indicative of the closeness / distance of the line passing through the point of origin O and the point P with respect to the lines passing through the point of origin O and the three medoids Ml, M2, M3.

[0180] In the case of figure 9 where the point P is under a line passing through a point of origin O (where OLURi* = OLURII* = 0) and the medoid M2, this proximity parameter Pr can be determined according to the following relationship:

[0181] M30P Pr = - - -

[0182] M3OM2 where the numerator represents the angle formed by a line passing through the point of origin O and the medoid M3 and the line passing through the point of origin O and the point P while the denominator represents the angle formed by the line passing through the point of origin 0 and the medoid M3 and the line passing through the point of origin 0 and the medoid M2.

[0183] The proximity parameter Pr will be equal to 0 in the event that the point P is on the line passing through the point of origin 0 and the medoid M3 while Pr will be equal to 1 in the event that the point P is on the line passing through the point of origin 0 and the medoid M2.

[0184] In the event that the point P is below the line passing through the point of origin 0 and the medoid M3, the proximity parameter Pr will be negative.

[0185] In the event that, on the other hand, the point P is - as schematically shown in figure 10 - above the line passing through the point of origin 0 and the medoid M2, the proximity parameter Pr can be determined according to the following relationship: where the numerator of the fraction represents the angle formed by the line passing through the medoid M2 and the point of origin 0 and the line passing through the point P and the point of origin 0 while the denominator represents the angle formed by the line passing through the medoid M2 and the point of origin 0 and the line passing through the medoid Ml and the point of origin 0.

[0186] The proximity parameter Pr will therefore be:

[0187] - comprised between 1 and 2 in the event that the point P is between the line passing through the point of origin 0 and the medoid M2 and the line passing through the point of origin 0 and the medoid Ml;

[0188] - equal to 2 in the event that the point P is on the line passing through the point of origin 0 and the medoid Ml;

[0189] - greater than 2 in the event that the point P is above the line passing through the point of origin 0 and the medoid Ml.

[0190] A value of the proximity parameter Pr close to 0 will therefore be indicative of a state of the road surface 40 similar to that of the third road surface (third cloud of points N3); a value of the proximity parameter Pr lower than 0 will be indicative of a state of the road surface 40 less uneven than that of the third road surface (third cloud of points N3); a value of the proximity parameter Pr close to 1 will be indicative of a state of the road surface 40 similar to that of the second road surface (second cloud of points N2); a value of the proximity parameter Pr close to 2 will be indicative of a state of the road surface 40 similar to that of the first road surface (first cloud of points Nl) and a value of the proximity parameter Pr higher than 2 will be indicative of a state of the road surface 40 more uneven than that of the first road surface (first cloud of points Nl).

[0191] The value of the proximity parameter Pr thus calculated therefore allows to determine how much the state of the road surface 40 is or is not similar to the state of one of the three reference road surfaces.

[0192] This value of the proximity parameter Pr can then be better defined by collecting the values obtained for each revolution of tyre from each of the four monitoring units 2 associated with the vehicle.

[0193] Furthermore, it is possible to determine the state of the road surface by calculating the values of the proximity parameter Pr on many revolutions of the tyre and analyzing the values obtained to identify whether there is a road segment with a more or less homogeneous road surface state. For example, in the case of a road segment with a homogeneous road surface state, it will be possible to identify a trend value on which all calculated values tend to stabilize.

[0194] This trend value can then be better defined by collecting the values obtained from each of the four monitoring units 2 associated with the vehicle.

[0195] By means of experimental tests performed on the two road surfaces corresponding to the clouds N2 and N3, the Applicant found that the values of the proximity parameter Pr calculated as described above on many revolutions of the tyre on the same road surface correctly stood at a trend value close to 0 for the surface corresponding to the cloud of points N3 and to a trend value close to 1 for the surface corresponding to the cloud of points N2.

[0196] The results obtained from these experimental tests are shown schematically in figure 11 where the values of the proximity parameter Pr obtained on many revolutions of the tyre at the third surface and the second reference surface associated with the clouds of points N3 and N2 are indicated in the ordinate.

[0197] As can be noted, the more even surface (corresponding to the cloud of points N3) has a value of the proximity parameter Pr that tends to 0 and a lower dispersion of the measured values while the more uneven surface (corresponding to the cloud of points N2) has a value of the proximity parameter that tends to 1 and a greater dispersion of the measured values due to the greater unevenness of its texture.

[0198] The Applicant has also correlated the values of the proximity parameter Pr obtained on many revolutions of the tyre at the third surface and the second reference surface associated with the clouds of points N3 and N2 with the values of the parameter MPD of these surfaces, indicators of the level of texture of the surfaces according to the aforementioned ISO 13473-1 standards.

[0199] The results of this correlation are illustrated in figure 13 where a good correspondence can be seen in general between the values of the proximity parameter Pr obtained and the value of the parameter MPD associated with the surface under examination. In particular, it can be noted that the surface corresponding to the cloud of points N3, having an MPD of about 0.8 mm (indicative of a generally even surface), has a value of the proximity parameter Pr that tends to 0 while the surface corresponding to the cloud of points N2, having an MPD value of about 2.85 mm (indicative of a more uneven surface) has a value of the proximity parameter that tends to 1.

[0200] The values of the proximity parameter Pr obtained for the two surfaces are therefore consistent with the values MPD and allow to provide reliable indications on the state of a road surface.

[0201] In a preferred embodiment, according to the estimation method of the invention it is also provided to collect the points P identified at many revolutions of the tyre 11 carried out on the same road surface 40 in a first time period T1 and to determine a first trend of the state of the road surface 40 analyzing, within said space of parameters, the position of the points collected.

[0202] After that, it is provided to collect the points P identified at many revolutions of the tyre 11 carried out on the same road surface 40 in a second time period T2, following the first time period Tl, and to determine a second trend of the state of the road surface analyzing, within said space of parameters, the position of the points collected.

[0203] It is then provided to compare the first trend and the second trend so as to monitor possible changes in the state of the same road surface 40 between the first time period Tl and the second time period T2. This advantageously allows to monitor the state of the road surface at different times and to check for the presence of possible deteriorations or improvements that have occurred over time. By means of experimental tests carried out on the same road surface 40, the Applicant has calculated the values of the proximity parameter Pr as described above on many revolutions of the tyre 11 in a first time period T1 and, subsequently, in a second time period T2 (a few months apart from each other).

[0204] The results of these tests are shown in figure 12 where the probability value ("prob" in the ordinate) with which a certain value of the proximity parameter Pr (indicated in the abscissa) was obtained in the two time periods T1 and T2 is indicated. The central zone indicated with the references Tl, T2 indicates an overlapping zone between the results obtained in the two time periods Tl, T2.

[0205] As can be noted, in the first time period Tl, the values of the proximity parameter Pr stood at a value close to 0.8 while in the second time period T2 the values of the proximity parameter Pr stood at a greater value, around 1.1, with a greater dispersion. The tails appeared on the left in the second time period T2 show the possible presence of reworked road surface segments (and therefore associated with lower values of the proximity parameter Pr) while the tails appeared on the right in the second time period T2 show the possible presence of deteriorated road surface segments (and therefore associated with higher values of the proximity parameter Pr). In this context, reworked road segments are meant to indicate road segments on which rework has been carried out between the two time periods Tl and T2 whereby in the second time period T2 these segments generally show a more even state of the surface. In turn, deteriorated road surface segments are meant to indicate road segments that between the two time periods Tl and T2 have undergone an increase in unevenness (for example, due to wear or weather events or road accidents or other).

[0206] In the light of the present description it will be clear that the present invention allows to estimate the state of a road surface in a reliable and versatile way.

[0207] Thanks to the normalization factors used in the calculation of the first parameter and of the second parameter, this estimate can be made independent of working conditions of the tyre such as, for example, its radius and angular speed.

[0208] The present invention also allows to keep track of the state of the road surface and to check if improving or worsening changes have occurred over time.

[0209] In addition, by appropriately selecting the first range of spatial wavelengths and the second range of spatial wavelengths relative to the road surface it is possible to monitor and control different performance relative to the vehicle, the tyre and the road surface.

[0210] For example, by selecting the first and second range of spatial wavelengths so as to correspond to the macro- and megatextures, it is possible to monitor the performance of the road surface in terms of noise emissions and surface damage or deterioration, in order to be able to verify the compliance of the road surface with predetermined standards (for example, noise emissions) and the need to carry out specific road maintenance interventions.

[0211] The tyre is therefore used as a tool to obtain information regarding quantities external to it, such as, for example, the performance of a road surface segment in terms of noise and integrity.

[0212] The information processed according to the estimation method of the invention may then be communicated externally to the vehicle (for example to a remote server) to be used by a special road infrastructure management and maintenance system.

Claims

CLAIMS1. Method for estimating the state of a road surface (40) comprising:- associating a monitoring unit (2) with a tyre (11) of a vehicle, said monitoring unit (2) comprising at least one detecting element (3) adapted to measure a quantity descriptive of a deformation of the tyre (11);- fitting the tyre (11) to a wheel (20) of said vehicle and operating said vehicle so as to cause rotation of said tyre (11) on a road segment having said road surface (40) wherein, due to said fitting and operating, the tyre (11) is deformed in a deformation zone (46, 42, 47, 44); wherein, for each revolution of the tyre (11), the method comprises:• obtaining a signal representative of said quantity measured during the rotation of the tyre (11);• frequency filtering said signal in a first frequency band so as to obtain a first filtered signal;• frequency filtering said signal in a second frequency band different from said first frequency band so as to obtain a second filtered signal;• processing the first filtered signal so as to obtain a value of a first parameter indicative of the variability of said first filtered signal in said first frequency band, said processing being carried out at a portion of the first filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is located in said deformation zone (46, 42, 47, 44) of the tyre (11);• processing the second filtered signal so as to obtain a value of a second parameter indicative of the variability of said second filtered signal in said second frequency band, said processing being carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is in said deformation zone (46, 42, 47, 44) of the tyre (11);• identifying, within a two-dimensional space of parameters, the position of a point (P) having the value of the first parameter and the value of the second parameter as coordinates;• determining the state of the road surface (40) based on the position of said point (P) within said space.

2. Method according to claim 1, wherein the deformation zone (46, 42, 47, 44) of the tyre (11) corresponds to a contact area (42) between the tyre (11) andthe road surface, to an entry area (46) to the contact area (42) or to an exit area (47) from the contact area (42).

3. Method according to claim 1 or 2, wherein:- said first frequency band is determined based on a first range of harmonics of said signal that refer to the deformation undergone by said tyre (11) in association with a predetermined first range of spatial wavelengths relative to the road surface (40); and- said second frequency band is determined based on a second range of harmonics of said signal that refer to the deformation undergone by said tyre (11) in association with a predetermined second range of spatial wavelengths relative to the road surface (40).

4. Method according to claim 3, wherein the first range of spatial wavelengths and the second range of spatial wavelengths correspond to two different categories of textures of road surfaces profiles.

5. Method according to claim 3 or 4, wherein the first range of spatial wavelengths and the second range of spatial wavelengths are different from each other and selected from: a range of spatial wavelengths comprised between 0.05mm and 0.5mm; a range of spatial wavelengths comprised between 0.5mm and 50mm; a range of spatial wavelengths comprised between 50mm and 0.5m and a range of spatial wavelengths comprised between 0.5m and 100m.

6. Method according to any one of the previous claims, wherein the quantity measured during the rotation of the tyre (11) is an acceleration component to which said monitoring unit (2) is subjected during the rolling of the tyre (11).

7. Method according to any one of the previous claims, wherein, for every revolution of the tyre (11), the state of the road surface (40) is determined by comparing, within said space, the position of said point (P) with respect to one or more predetermined clouds of points (Nl, N2, N3), each cloud of points (Nl, N2, N3) being representative within said space of the state of a reference road surface.

8. Method according to any one of the previous claims, wherein, for every revolution of the tyre (11), the state of the road surface (40) is determined by comparing, within said space, the position of said point (P) with respect to one or more reference points (Ml, M2, M3), each reference point (Ml, M2, M3) beingrepresentative within said space of the state of a reference road surface.

9. Method according to any one of the previous claims, wherein it is provided to collect the points (P) identified at many revolutions of the tyre (11) carried out on said road segment in a first time period (Tl) and to determine a first trend of the state of the road surface (40) on said road segment analyzing, within said space, the position of the points (P) collected.

10. Method according to claim 9, wherein it is provided to collect the points (P) identified at many revolutions of the tyre (11) carried out on said road segment in a subsequent second time period (T2) and to determine a second trend of the state of the road surface (40) on said road segment analyzing, within said space, the position of the points (P) collected.

11. Method according to claim 10, wherein it is provided to compare the first trend and the second trend so as to monitor possible changes in the state of the road surface (40) between the first time period (Tl) and the second time period (T2).

12. System (30) for estimating the state of a road surface (40) comprising:- a monitoring unit (2) configured to be associated with a tyre (11) of a vehicle, said monitoring unit (2) comprising at least one detecting element (3) configured to measure a quantity descriptive of a deformation of the tyre (11), wherein, when said tyre (11) is fitted on a wheel (20) of said vehicle and said vehicle is operated so as to cause the rotation of said tyre (11) on a road segment having said road surface (40), as a consequence of said fitting and operating, the tyre (11) is deformed in a deformation zone (46, 42, 47, 44);- at least one processing unit (6, 31) comprising software modules that, for each revolution of the tyre (11), are configured for:• obtaining a signal representative of said quantity measured during the rotation of the tyre (11);• frequency filtering said signal in a first frequency band so as to obtain a first filtered signal;• frequency filtering said signal in a second frequency band different from said first frequency band so as to obtain a second filtered signal;• processing the first filtered signal so as to obtain a value of a first parameter indicative of the variability of said first filtered signal in said first frequency band, said processing being carried out at a portion of thefirst filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is located in said deformation zone (46, 42, 47, 44) of the tyre (11);• processing the second filtered signal so as to obtain a value of a second parameter indicative of the variability of said second filtered signal in said second frequency band, said processing being carried out at a portion of the second filtered signal that corresponds to a rolling step of the tyre (11) wherein said monitoring unit (2) is in said deformation zone (46, 42, 47, 44) of the tyre (11); • identifying, within a two-dimensional space of parameters, the position of a point (P) having the value of the first parameter and the value of the second parameter as coordinates;• determining the state of the road surface (40) based on the position of said point (P) within said space.

13. System (30) according to claim 12, wherein said at least one detecting element comprises an accelerometer (3) configured to measure at least one acceleration component to which said monitoring unit (2) is subjected during the rotation of the tyre (11).

Images