Patent Publication Number: US-2023152186-A1

Title: Estimation system, estimation method, and recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2021-187948 filed with Japan Patent Office on Nov. 18, 2021 and claims the benefit of priority thereto. The entire contents of the Japanese patent application are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an estimation system, an estimation method, and a recording medium. 
     BACKGROUND 
     Techniques for estimating the state of tires in a vehicle are known. For example, Japanese Unexamined Patent Application Publication No. 1996-247745 describes a technique for measuring a camber angle of a tire by using two laser devices disposed outside a tire/wheel assembly of an automobile with a space therebetween. Japanese Unexamined Patent Application Publication No. 2019-49488 describes a technique for estimating a load acting on a tire by using an acceleration sensor provided in an inner liner portion of the tire and located at a center in a width direction of the tire. 
     SUMMARY 
     In the technique described in Japanese Unexamined Patent Application Publication No. 1996-247745, since it is necessary to attach the laser devices to the outside of the tire, the device becomes large-scale, and there is a possibility that the device interferes with traveling of the vehicle. In the technique described in Japanese Unexamined Patent Application Publication No. 2019-49488, it is necessary to prepare a dedicated tire in which the acceleration sensor is attached to the inner liner portion. 
     The present disclosure describes an estimation system, an estimation method, and a recording medium capable of estimating a state of a rotating body with a simple configuration. 
     An estimation system according to one aspect of the present disclosure includes: a first sensor that can be disposed between a wheel and a tire mounted on the wheel and outputs a first sensor signal in accordance with a pressing force applied by the wheel and the tire; and a processor that estimates a state of a rotating body including the wheel and the tire based on the first sensor signal. The processor generates a first section signal by dividing the first sensor signal by a specific section and estimates the state of the rotating body based on the first section signal. 
     An estimation method according to another aspect of the present disclosure includes: acquiring a sensor signal in accordance with a pressing force applied by a wheel and a tire mounted on the wheel from a sensor disposed between the wheel and the tire; generating a section signal by dividing the sensor signal by a specific section; and estimating a state of a rotating body including the wheel and the tire based on the section signal. 
     A recording medium according to yet another aspect of the present disclosure is a non-transitory computer-readable recording medium recording an estimation program. The estimation program includes instructions that cause a computer to execute: acquiring a sensor signal in accordance with a pressing force applied by a wheel and a tire mounted on the wheel from a sensor disposed between the wheel and the tire; generating a section signal by dividing the sensor signal by a specific section; and estimating a state of a rotating body including the wheel and the tire based on the section signal. 
     According to each aspect and each embodiment of the present disclosure, a state of a rotating body can be estimated with a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram schematically showing a vehicle equipped with an estimation system according to an embodiment. 
         FIG.  2    is a perspective view of the rotating body shown in  FIG.  1   . 
         FIG.  3    is a configuration diagram schematically showing the configuration of the estimation system shown in  FIG.  1   . 
         FIG.  4    is an exploded perspective view of the sensor module shown in  FIG.  1   . 
         FIG.  5    is a diagram for explaining a force acting on the sensor module shown in  FIG.  1   . 
         FIG.  6    is a diagram for explaining a sensor signal output from the sensor shown in  FIG.  1   . 
         FIG.  7    is a diagram showing an example of a sensor signal during constant-speed travel. 
         FIG.  8    is a diagram showing an example of a sensor signal during acceleration travel. 
         FIG.  9    is a flowchart showing an estimation method performed by the processor shown in  FIG.  1   . 
         FIG.  10    is a diagram for explaining an example of a process of generating a section signal. 
         FIG.  11    is a diagram for explaining another example of a process of generating a section signal. 
         FIG.  12    is a diagram for explaining yet another example of a process of generating a section signal. 
         FIG.  13    is a diagram for explaining yet another example of a process of generating a section signal. 
         FIG.  14    is a diagram for explaining the reaction force from the road surface when the camber angle is 0 degrees. 
         FIG.  15    is a diagram for explaining the reaction force from the road surface in the positive camber. 
         FIG.  16    is a diagram for explaining the reaction force from the road surface in the negative camber. 
         FIG.  17    is a diagram showing an example of a sensor signal for each camber angle. 
         FIG.  18    is a diagram for explaining a slip angle. 
         FIG.  19    is a diagram for explaining a force acting on a sensor module when a slip angle occurs. 
         FIG.  20    is a diagram showing an example of a sensor signal for each slip angle. 
         FIG.  21    is a diagram for explaining a peak-to-peak value and a second peak value. 
         FIG.  22    is a diagram showing the relationship between a peak-to-peak value and a damping ratio when a slip angle, a camber angle, a load and air pressure are changed. 
         FIG.  23    is a partially enlarged view of  FIG.  22   . 
         FIG.  24    is a diagram for explaining an estimation model. 
         FIG.  25    is a configuration diagram schematically showing an estimation system according to another embodiment. 
         FIG.  26    is a diagram showing an example of the arrangement of sensor modules. 
         FIG.  27    is a diagram showing an example of a sensor signal for each camber angle. 
         FIG.  28    is a diagram showing an example of a sensor signal for each slip angle. 
         FIG.  29    is a configuration diagram schematically showing an estimation system according to yet another embodiment. 
         FIG.  30    is a configuration diagram schematically showing an estimation system according to yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Outline of Embodiments 
     An estimation system according to one aspect of the present disclosure includes: a first sensor that can be disposed between a wheel and a tire mounted on the wheel and outputs a first sensor signal in accordance with a pressing force applied by the wheel and the tire; and a processor that estimates a state of a rotating body including the wheel and the tire based on the first sensor signal. The processor generates a first section signal by dividing the first sensor signal by a specific section and estimates the state of the rotating body based on the first section signal. 
     An estimation method according to another aspect of the present disclosure includes: acquiring a sensor signal in accordance with a pressing force applied by a wheel and a tire mounted on the wheel from a sensor disposed between the wheel and the tire; generating a section signal by dividing the sensor signal by a specific section; and estimating a state of a rotating body including the wheel and the tire based on the section signal. 
     A recording medium according to yet another aspect of the present disclosure is a non-transitory computer-readable recording medium recording an estimation program. The estimation program includes instructions that cause a computer to execute: acquiring a sensor signal in accordance with a pressing force applied by a wheel and a tire mounted on the wheel from a sensor disposed between the wheel and the tire; generating a section signal by dividing the sensor signal by a specific section; and estimating a state of a rotating body including the wheel and the tire based on the section signal. 
     In a technique according to the present disclosure including the estimation system, the estimation method, and the recording medium (hereinafter may be simply referred to as a “technique according to the present disclosure”), the sensor (first sensor) disposed between the wheel and the tire is configured to output a sensor signal (first sensor signal) in accordance with a pressing force by the wheel and the tire. A load from the vehicle acts on the sensor (first sensor) via the wheel. A reaction force from the road surface acts on the sensor (first sensor) via the tire. Since these forces can change depending on the state of the rotating body, the technique according to the present disclosure can estimate the state of the rotating body based on the sensor signal (first sensor signal). Therefore, according to the technique of the present disclosure, the state of the rotating body can be estimated with a simple configuration in which the sensor (first sensor) is disposed between the wheel and the tire. 
     In some embodiments, the first sensor may be disposed between a rim included in the wheel and the tire. When the wheel includes a rim, the tire is mounted on the rim. Therefore, the state of the rotating body can be estimated with a simple configuration in which the first sensor is disposed between the rim and the tire. 
     In some embodiments, the rotating body may include a first end and a second end that are both ends in a rotational axis direction of the rotating body. The first sensor may be disposed at a position closer to the first end than a center of the rotating body in the rotational axis direction. When the first sensor is disposed at the center of the rotating body in the rotational axis direction, for example, the first sensor signal changes in the same manner regardless of whether the camber angle changes in the positive direction or the negative direction. On the other hand, in the above configuration, the first sensor signal changes asymmetrically. This makes it possible to improve the estimation accuracy of the state of the rotating body. 
     In some embodiments, the estimation system may further include a second sensor that can be disposed between the wheel and the tire and outputs a second sensor signal in accordance with a pressing force applied by the wheel and the tire. The second sensor may be disposed at a position closer to the second end than the center of the rotating body in the rotational axis direction. The processor may generate a second section signal by dividing the second sensor signal by the specific section and estimates the state further based on the second section signal. In this case, the first sensor and the second sensor are disposed opposite to each other with respect to the center of the rotating body in the rotational axis direction. The first sensor signal output from the first sensor and the second sensor signal output from the second sensor change differently from each other in accordance with a change in the state of the rotating body. Therefore, since the state of the rotating body is estimated using two sensor signals in which different changes occur, it is possible to improve the estimation accuracy of the state of the rotating body compared with a configuration in which the state of the rotating body is estimated using one sensor signal. 
     In some embodiments, the specific section may be a section corresponding to one rotation of the rotating body. When the rotating body rotates, the portion of the rotating body that comes into contact with the road surface changes, and thus the relative positional relationship between the first sensor and the contact portion changes. 
     For this reason, the first sensor signal has a periodicity such that the waveform shape becomes similar every time the rotating body makes one rotation. Therefore, the state of the rotating body can be estimated by analyzing the first section signal corresponding to one rotation of the rotating body. 
     In some embodiments, the processor may estimate the state of the rotating body based on a plurality of waveform characteristics, which are different from each other, calculated from the first section signal. The waveform characteristic calculated from the first section signal can be an index indicating the state of the rotating body. Therefore, by using these characteristics, it is possible to improve the estimation accuracy of the state of the rotating body. 
     In some embodiments, the plurality of waveform characteristics may include a value based on at least one of a maximum value of the first section signal, a minimum value of the first section signal, a difference between the maximum value and the minimum value of the first section signal, a standard deviation of the first section signal, a variance of the first section signal, an average value of the first section signal, a median value of the first section signal, and a value at an inflection point of the first section signal. By using these characteristics, it is possible to improve the estimation accuracy of the state of the rotating body. 
     In some embodiments, the processor may estimate the state of the rotating body by using a machine learning model for estimating the state of the rotating body. In this case, it is possible to improve the estimation accuracy of the state of the rotating body by sufficiently learning the machine learning model. 
     In some embodiments, the state of the rotating body may include at least one of a camber angle, a slip angle, a load applied to the rotating body, and air pressure. A tendency of change in the first sensor signal when the camber angle changes, a tendency of change in the first sensor signal when the slip angle changes, a tendency of change in the first sensor signal when the load changes, and a tendency of change in the first sensor signal when the air pressure changes are different from each other. Therefore, the camber angle, the slip angle, the load, and the air pressure can be estimated separately. 
     In some embodiments, the first sensor and the processor may constitute a sensor module. The sensor module may be provided in the rotating body. The processor may output an estimation result to an external device provided outside the rotating body. In this case, the first sensor signal is processed in the sensor module, and the estimation result is output to the external device. The amount of communication between the sensor module and the external device can be reduced compared with that of a configuration in which the first sensor signal is processed in the external device. 
     In some embodiments, the first sensor may be a piezoelectric element that generates electric energy in accordance with the pressing force. The processor may operate using the electric energy generated by the piezoelectric element. In this case, the processor can operate without receiving electric power from the outside. Accordingly, wiring or the like for supplying electric power from the outside is not necessary, so that the configuration of the estimation system can be simplified. 
     In some embodiments, the first sensor may be a piezoelectric element that generates electric energy in accordance with the pressing force. The processor may estimate the state of the rotating body by using a voltage or an electric current of the electric energy generated by the piezoelectric element as the first sensor signal. In this case, the state of the rotating body can be estimated with a simple configuration in which the piezoelectric element is disposed between the wheel and the tire. 
     Exemplary Embodiments 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are designated with the same reference numerals, and the redundant description is omitted. 
     An estimation system according to an embodiment will be described with reference to  FIGS.  1  to  4   .  FIG.  1    is a diagram schematically showing a vehicle equipped with an estimation system according to an embodiment.  FIG.  2    is a perspective view of the rotating body shown in  FIG.  1   .  FIG.  3    is a configuration diagram schematically showing the configuration of the estimation system shown in  FIG.  1   .  FIG.  4    is an exploded perspective view of the sensor module shown in  FIG.  1   . An estimation system  1  shown in  FIG.  1    is a system that estimates a state of a rotating body  2 . The estimation system  1  can be mounted on a vehicle V, for example. The vehicle V includes the rotating body  2  and is configured to be movable by rotation of the rotating body  2 . Examples of the vehicle V may include an automobile, a bicycle, and a motorcycle. The present embodiment will be described using an automobile as an example of the vehicle V, but the technique according to the present disclosure is not limited to application to an automobile. The vehicle V includes four rotating bodies  2  provided on front, rear, left, and right sides. 
     As shown in  FIG.  2   , the rotating body  2  is an element that is rotatable about a rotational axis AX. The rotating body  2  has an outer end portion  2   a  (first end; see  FIG.  14   ) and an inner end portion  2   b  (second end; see  FIG.  14   ). The outer end portion  2   a  and the inner end portion  2   b  are both ends of the rotating body  2  in a direction in which the rotational axis AX extends (rotational axis direction). The outer end portion  2   a  faces the outside of the vehicle V. The rotating body  2  includes a wheel  21  and a tire  22 . 
     The wheel  21  is a member that transmits rotational force about the rotational axis AX to the tire  22 . The wheel  21  may be made of a member having rigidity. Examples of the constituent material of the wheel  21  may include metal materials such as steel, magnesium, aluminum, and stainless steel, and may include resin materials such as carbon fiber. In the specific example shown in  FIG.  2   , the wheel  21  includes a rim  23  and a plurality of spokes  24 . The rim  23  is an annular member that defines an outer edge of the wheel  21 . The tire  22  is mounted along the outer circumference of the rim  23 . Each of the plurality of spokes  24  extends radially from the center of the wheel  21  to the rim  23 . The rim  23  and the spoke  24  may be integrally formed, or may be formed as separate bodies. 
     The tire  22  is an annular member mounted on the wheel  21 . The tire  22  is provided along an outer circumference (rim  23 ) of the wheel  21 . The tire  22  may be made of a member having flexibility. An example of a constituent material of the tire  22  may include a resin such as rubber. 
     As shown in  FIG.  3   , the estimation system  1  includes a sensor module  3 . The sensor module  3  is a module capable of detecting a pressing force acting on the rotating body  2 . The sensor module  3  is provided in the rotating body  2 . Specifically, the sensor module  3  is disposed between the wheel  21  (rim  23 ) and the tire  22 . For example, the sensor module  3  may be sandwiched between the wheel  21  (rim  23 ) and the tire  22  in the vertical direction. In the present embodiment, a plurality of sensor modules  3  are provided at equal intervals along the outer circumference of the wheel  21  (rim  23 ). Several sensor modules  3  are disposed in the outer end portion  2   a  (outer rim). The sensor module  3  may be disposed in an inner end portion  2   b  (inner rim). 
     The number and positions of the sensor modules  3  provided in one rotating body  2  can be selected as appropriate. In the present embodiment, a plurality of sensor modules  3  are provided in one rotating body  2 . However, for example, one sensor module  3  may be provided in one rotating body  2 . The number of sensor modules provided in one rotating body  2  is not limited to this configuration, and may be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11 or more. For example, the same number of sensor modules  3  as the spokes  24  may be provided in one rotating body  2 . For example, the same number of sensor modules  3  as the interval between two spokes  24  adjacent to each other may be provided in one rotating body  2 . In the case where a plurality of sensor modules  3  are provided in one rotating body  2 , the sensor modules  3  may be arranged at equal intervals along the outer circumference of the wheel  21  (rim  23 ), for example. As another configuration, the plurality of sensor modules  3  may be arranged at different intervals along the outer circumference of the wheel  21  (rim  23 ). As yet another configuration, at least some of the plurality of sensor modules  3  may be arranged at equal intervals along the outer circumference of the wheel  21  (rim  23 ), and the other sensor modules  3  may be arranged at different intervals along the outer circumference of the wheel  21  (rim  23 ). 
     In the specific example shown in  FIG.  4   , each sensor module  3  is configured to be disposed between the wheel  21  (rim  23 ) and the tire  22 . Each sensor module  3  includes a piezoelectric element  31  (first sensor), a back plate  32 , a substrate  33 , a substrate  34 , and a base material  35 . The piezoelectric element  31  is an element that generates electric energy in accordance with an external force such as a pressing force acting on the piezoelectric element  31 . An example of the piezoelectric element  31  may include a piezo ceramic element (piezo element). The piezoelectric element  31  may be formed in a plate shape. 
     The back plate  32  is a plate-like member that protects the piezoelectric element  31 . The back plate  32  may be made of a metal member (for example, stainless steel) or a resin member. The back plate  32  has, for example, a plate-like shape slightly larger than the piezoelectric element  31 . The back plate  32  can also relax the stress of the piezoelectric element  31  by being superimposed on the piezoelectric element  31 . The deformation amount of the piezoelectric element  31  in accordance with the pressing force acting on the sensor module  3  is adjusted by the thickness of the back plate  32 . 
     The substrates  33  and  34  are plate-like members that extract electric energy generated in the piezoelectric element  31  as a sensor signal (first sensor signal). Specifically, the substrates  33  and  34  may extract a voltage or an electric current of electric energy generated in the piezoelectric element  31  as a sensor signal. In the present embodiment, a case where a voltage is handled as a sensor signal will be described as an exemplary case. The substrates  33  and  34  may be flexible printed circuits (FPC). The substrate  33  may be configured to include, for example, a main body portion  33   a  and a wiring portion  33   b . The main body portion  33   a  is a portion forming a laminated structure described later. The wiring portion  33   b  is a portion that connects the sensor module  3  to an external circuit or the like. The substrate  34  includes a main body portion  34   a  and a wiring portion  34   b . The main body portion  34   a  is a portion forming the laminated structure described later. The wiring portion  34   b  is a portion that connects the sensor module  3  to an external circuit or the like. In the present embodiment, the shape of the substrate  33  is substantially the same as the shape of the substrate  34 , but the shape of the substrate  33  may be different from the shape of the substrate  34 . The main body portions  33   a  and  34   a  may be formed to have substantially the same size as the back plate  32 , for example. 
     The base material  35  is a member for attaching the sensor module  3  to the wheel  21 . The base material  35  has a shape following the rim  23 . The base material  35  is provided with a recess  35 a capable of accommodating the laminated structure described later. 
     In the specific example shown in  FIG.  4   , the back plate  32  is superimposed on the piezoelectric element  31 , and the piezoelectric element  31  and the back plate  32  superimposed on each other are sandwiched between the main body portion  33   a  of the substrate  33  and the main body portion  34   a  of the substrate  34 . That is, the laminated structure is formed by laminating the substrate  33 , the back plate  32 , the piezoelectric element  31 , and the substrate  34  in this order, and the laminated structure is accommodated in the recess  35 a of the base material  35 . In this manner, the sensor module  3  is manufactured. The sensor module  3  is disposed at a desired position between the rim  23  and the tire  22 . In the specific example shown in  FIG.  4   , the sensor module  3  may be disposed in the rotating body  2  so that the surface of the base material  35  opposite to the surface on which the recess  35 a is provided is in contact with the rim  23 . In this case, the surface of the main body portion  33   a  of the substrate  33  opposite to the back plate  32  is in contact with the tire  22 . 
     Each sensor module  3  includes, for example, the piezoelectric element  31  as a circuit element. Each sensor module  3  may include, for example, an analog-to-digital (AD) converter  41 , a processor  42 , a communication interface  43 , a power converter  44 , and a power storage device  45  in addition to the piezoelectric element  31 . The AD converter  41 , the processor  42 , the communication interface  43 , the power converter  44 , and the power storage device  45  may be mounted on the substrate  33  or the substrate  34 . 
     The AD converter  41  is a circuit element that converts a sensor signal that is an analog signal output from the piezoelectric element  31  into a sensor signal that is a digital signal. The AD converter  41  outputs the sensor signal that is a digital signal to the processor  42 . 
     The processor  42  is a circuit element that estimates the state of the rotating body  2  based on the sensor signal. The state of the rotating body  2  estimated by the processor  42  includes at least one of a camber angle, a slip angle, a load applied to the rotating body  2 , and air pressure. The processor  42  may output the estimation result to the external device  5  via the communication interface  43 . Details of the processing performed by the processor  42  will be described later. Examples of processor  42  include, but are not limited to, a central processing unit (CPU), a digital signal processor (DSP), an attached support processor (ASP), a microcomputer, a programmable logic controller (PLC), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), and an integrated circuit (IC). The processor  42  may have a multi-core configuration. 
     The communication interface  43  is hardware that enables the sensor module  3  to transmit and receive data to and from the external device  5  via a communication network NW 1 . The communication network NW 1  may be configured as a wired communication network, may be configured as a wireless communication network, or may be configured as a communication network including both of them. Examples of the communication network NW 1  may include one or more of the Internet, an intranet, a wide area network (WAN), a local area network (LAN), Bluetooth (Registered Trademark), a wireless LAN (such as Wi-Fi), a controller area network (CAN), a mobile communication network, and the like. The communication interface  43  may conform to a specific communication protocol, for example. 
     The power converter  44  is a device that converts the sensor signal (voltage) generated by the piezoelectric element  31  so as to be able to charge the power storage device  45 . The power converter  44  is, for example, a power conditioner. As described later, when the sensor signal includes a AC signal that periodically changes, the power converter  44  may include a rectifier circuit. 
     The power storage device  45  is a chargeable and dischargeable device. The power storage device  45  stores the sensor signal generated by the piezoelectric element  31  as electric energy (electric power) and supplies the electric energy to the circuit elements in the sensor module  3 . For example, the processor  42  operates using electric energy generated by the piezoelectric element  31 . Examples of the power storage device  45  may include a storage battery such as a lithium ion battery, and a capacitor. 
     The external device  5  is a device capable of communicating with the sensor module  3 . For example, the external device  5  may be configured to present an estimation result related to the state of the rotating body  2  to a person (occupant) riding in the vehicle V. The external device  5  may be configured to provide the estimation result related to the state of the rotating body  2  to other devices included in the vehicle V, for example. The external device  5  may be configured to provide the estimation result related to the state of the rotating body  2  to a device (for example, a server or the like to which the external device  5  is connectable via a communication line) disposed outside the vehicle V, for example. 
     The external device  5  may be provided outside the rotating body  2  and disposed in the vehicle V. Examples of the external device  5  may include an in-vehicle device and a mobile terminal owned by an occupant. Examples of the mobile terminal may include a smart phone, a tablet terminal, a laptop computer, and the like. The external device  5  may include, for example, a processor  51 , a memory  52 , and a communication interface  55 . The external device  5  may further include, for example, an input device  53 , an output device  54 , and a communication interface  56 . 
     The processor  51  is a circuit element that performs control and computation in the external device  5 . The processor  51  is configured in the same manner as the processor  42 . Examples of the processor  51  include, but are not limited to, a CPU, a DSP, an ASP, a microcomputer, a PLC, an FPGA, an ASIC, and an IC. The processor  51  may have a multi-core configuration. The memory  52  may include a main storage device and an auxiliary storage device. The main storage device is constituted by a random access memory (RAM), a read only memory (ROM), and the like. Examples of the auxiliary storage device include a semiconductor memory and a hard disk device. 
     The input device  53  is a device that receives an input from a user of the external device  5 . Examples of the input device  53  may include a touch panel, a keyboard, and a mouse. The output device  54  is a device that outputs information to the outside of the external device  5 . Examples of the output device  54  may include a display and a speaker. 
     The communication interface  55  is hardware that enables the external device  5  to transmit and receive data to and from the sensor module  3  via the communication network NW 1 . The communication interface  55  may conform to a specific communication protocol, for example. The communication interface  56  is hardware that enables the external device  5  to transmit and receive data to and from devices disposed outside the vehicle V (for example, a server (not shown) or the like to which the external device  5  is connectable via the communication network NW 2 ) via the communication network NW 2 . The communication network NW 2  may be configured by wired communication, may be configured by wireless communication, or may be configured by a combination thereof. Examples of the communication network NW 2  may include one or more of the Internet, an intranet, a WAN, a LAN, Bluetooth (Registered Trademark), Wi-Fi, a mobile communication network, and the like. The communication interface  56  may conform to a specific communication protocol, for example. 
     The processor  51 , the memory  52 , the input device  53 , the output device  54 , the communication interface  55 , and the communication interface  56  may be communicatively connected to each other by, for example, a bus  57 . 
     Next, the sensor signal will be described in detail with reference to  FIGS.  5  to  8   .  FIG.  5    is a diagram for explaining a force acting on the sensor module shown in  FIG.  1   .  FIG.  6    is a diagram for explaining a sensor signal output from the sensor shown in  FIG.  1   .  FIG.  7    is a diagram showing an example of a sensor signal during constant-speed travel.  FIG.  8    is a diagram showing an example of a sensor signal during acceleration travel. 
     In the specific example shown in  FIG.  5   , the sensor module  3  is disposed between an outer flange of the rim  23  and a bead of the tire  22  and is in contact with the flange of the rim  23  and the bead of the tire  22 . The weight W of the vehicle V acts as a pressing force on the piezoelectric element  31  via the wheel  21  (rim  23 ), and the reaction force R from the road surface acts as a pressing force on the piezoelectric element  31  via the tire  22 . The piezoelectric element  31  outputs a sensor signal in accordance with a pressing force by the wheel  21  and the tire  22 . Specifically, the magnitude of the sensor signal changes depending on, for example, the magnitude of the pressing force acting on the piezoelectric element  31  and the amount of change in the pressing force per unit time. In the present embodiment, the sensor module  3  is configured to output a negative sensor signal when the piezoelectric element  31  receives a pressing force, but may be configured to output a positive sensor signal when the piezoelectric element  31  receives a pressing force. As the pressing force acting on the piezoelectric element  31  increases, the absolute value of the sensor signal increases. 
     Specifically, during one rotation of the rotating body  2 , a portion of the rotating body  2  (the outer circumferential surface of the tire  22 ) in contact with the road surface changes, and thus the relative positional relationship between the piezoelectric element  31  and the road surface changes. For example, as the piezoelectric element  31  approaches the road surface, the weight W of the vehicle V acting on the piezoelectric element  31  via the wheel  21  (rim  23 ) increases, and the reaction force R from the road surface acting on the piezoelectric element  31  via the tire  22  increases. When the piezoelectric element  31  comes closest to the road surface, the weight W of the vehicle V acting on the piezoelectric element  31  via the wheel  21  (rim  23 ) becomes the largest, and the reaction force R from the road surface acting on the piezoelectric element  31  via the tire  22  becomes the largest. At this time, the portion of the tire  22  located between the rim  23  and the road surface is compressed to be elastically deformed. When the rotating body  2  further rotates, the weight W of the vehicle V acting on the piezoelectric element  31  via the wheel  21  (rim  23 ) decreases, and the reaction force R from the road surface acting on the piezoelectric element  31  via the tire  22  also decreases. Then, the compressed tire  22  is restored. At this time, elastic vibration may occur in the tire  22 , and in this case, the stress acting on the piezoelectric element  31  is damped while vibrating. 
     In the specific example shown in  FIG.  6   , during one rotation of the rotating body  2 , the sensor signal has a steep peak convex in the negative direction and then has a steep peak convex in the positive direction. The sensor signal then is damped while vibrating. When the vehicle V is traveling at a constant speed, the rotation speed of the rotating body  2  is substantially constant. Therefore, as shown in FIG. 
       7 , the waveform for one rotation is repeated at a constant period. When the vehicle V is accelerating, the rotation speed of the rotating body  2  gradually increases. Therefore, as shown in  FIG.  8   , the period of the waveform for one rotation is shortened. During acceleration travel, the reaction force from the road surface may increase, and in this case, the absolute value of the negative peak gradually increases. 
     An estimation method performed by the processor  42  will be described with reference to  FIG.  9   .  FIG.  9    is a flowchart showing an estimation method performed by the processor shown in  FIG.  1   . The processor  42  may perform the estimation method by, for example, reading out an estimation program stored in a computer-readable non-transitory recording medium and executing the estimation program. Examples of the recording medium may include a ROM that can be accessed by the processor  42 . The series of processes shown in  FIG.  9    is started, for example, every time a certain time elapses. 
     First, the processor  42  acquires a sensor signal from the piezoelectric element  31  (step S 1 ). Specifically, the processor  42  may acquire a sensor signal converted into a digital signal by the AD converter  41 , for example. 
     Subsequently, the processor  42  generates a section signal by dividing the sensor signal by a specific section (step S 2 ). Some examples of a process of generating a section signal will be described below with reference to  FIGS.  10  to  13   , but a process of generating a section signal is not limited to these examples.  FIGS.  10  to  13    are diagrams for describing an example of a process of generating a section signal. The horizontal axes of  FIGS.  10  to  13    represent time. The vertical axis of  FIG.  10    represents voltage. The vertical axes of  FIGS.  11  to  13    represent normalized outputs. The normalized output means a value obtained by dividing the voltage value of the sensor signal by a predetermined voltage value. The sensor signal may include a noise component. Accordingly, the processor  42  may remove the noise component from the sensor signal and generate the section signal using the sensor signal from which the noise component has been removed. 
     As shown in  FIG.  10   , the processor  42  may divide the sensor signal into certain section signals by window control. Specifically, the processor  42  may use a window having a time width and select a portion of the sensor signal included in the window as the section signal. The time width of the window may be a fixed value given in advance or may be a set value that can be changed. The time width of the window is, for example, about 5 seconds. The time width of the window is not limited to this and may be 1 second or more and less than 5 seconds or may be 5 seconds or more. The processor  42  shifts the window by a certain time and selects the portion of the sensor signal included in the shifted window as the next section signal. The time for shifting the window (shift time) may be, for example, about one second. The shift time is not limited to this, and may be a time equal to or shorter than the time width of the window. In the same manner, the processor  42  selects the section signal every time the window is shifted. According to this approach, the processing load of the processor  42  may be reduced by using a fixed window, for example. 
     The time width of the window may be dynamically set. For example, the processor  42  may specify the fundamental period of the sensor signal using a fast Fourier transform (FFT) and set the fundamental period as the time width of the window. 
     As another approach, as shown in  FIG.  11   , the processor  42  may select a section signal using local maximum values and local minimum values included in the sensor signal. Specifically, the processor  42  may specify, for example, a peak in a section exceeding a threshold value for determining a local maximum value in the sensor signal as the local maximum value. For example, the processor  42  may specify a peak (bottom) in a section falling below a threshold value for determining a local minimum value in the sensor signal as the local minimum value. The threshold for determining a local maximum value and the threshold for determining a local minimum value may be set in advance. In a case where the local maximum value and the local minimum value alternately appear in the sensor signal, the processor  42  may specify the local maximum value and the local minimum value by using this fact as a constraint condition. In this case, the processor  42  may set a window to include one local maximum value and one local minimum value that are appear consecutively, and may select a portion of the sensor signal included in the window as the section signal. 
     When the rotating body  2  rotates while being in contact with the road surface, in some situations, a local minimum value of the sensor signal appears when the piezoelectric element  31  comes closest to the road surface, and a local maximum value of the sensor signal appears when the piezoelectric element  31  moves away from the road surface (when the pressing force acting on the piezoelectric element  31  is released). In this case, a section including one local maximum value and one local minimum value that are appear consecutively may correspond to a sensor signal for one rotation of the rotating body  2 . As described above, according to the present approach, even when the rotation speed of the rotating body  2  changes, the sensor signal corresponding to one rotation of the rotating body  2  can be selected as the section signal. In this case, according to the present approach, a section corresponding to one rotation of the rotating body  2  can be selected as the above-described specific section, and the processor  42  can generate section signals by dividing the sensor signal into sections corresponding to one rotation of the rotating body  2 . 
     As yet another approach, as shown in  FIG.  12   , the processor  42  may select a section signal using zero crossing points included in the sensor signal. Specifically, the processor  42  specifies, for example, a zero crossing point when the sensor signal changes from a negative value to a positive value in the sensor signal. Then, the processor  42  sets a section between two continuous zero crossing points as a window, and selects a portion of the sensor signal included in the window as the section signal. 
     When the rotating body  2  rotates while being in contact with the road surface, in some situations, the sensor signal steeply changes from a local minimum value to a local maximum value while the piezoelectric element  31  moves away from the road surface after moving closest to the road surface. In this case, the zero crossing point when the sensor signal changes from a negative value to a positive value appears when the piezoelectric element  31  moves away from the road surface after moving closest to the road surface. Therefore, a section defined by two consecutive zero crossing points may correspond to a sensor signal for one rotation of the rotating body  2 . As described above, according to the present approach, even when the rotation speed of the rotating body  2  changes, the sensor signal corresponding to one rotation of the rotating body  2  can be selected as the section signal. In this case, according to the present approach, a section corresponding to one rotation of the rotating body  2  can be selected as the above-described specific section, and the processor  42  can generate section signals by dividing the sensor signal into sections corresponding to one rotation of the rotating body  2 . 
     Note that the sensor signal may change from a negative value to a positive value other than when the piezoelectric element  31  moves away from the road surface after moving closest to the road surface. In this case, the processor  42  may specify the zero cross point by further using a condition that the change amount (change rate) per unit time of the sensor signal is larger than a predetermined value. 
     As yet another approach, as shown in  FIG.  13   , the processor  42  may select the section signal using an acceleration signal output from an acceleration sensor provided in the wheel  21 . The acceleration sensor may be disposed at the center of the wheel  21 , for example. Specifically, the processor  42  sets one cycle of the acceleration signal as a window, and selects a portion of the sensor signal included in the window as the section signal. 
     Since the direction of the gravitational acceleration detected by the acceleration sensor changes in accordance with the rotation of the rotating body  2 , the acceleration signal has a periodic waveform. For example, when the rotation speed of the rotating body  2  is constant, the acceleration signal becomes a sine wave. In this case, one cycle of the acceleration signal corresponds to one rotation of the rotating body  2 . As described above, according to the present approach, even when the rotation speed of the rotating body  2  changes, the sensor signal corresponding to one rotation of the rotating body  2  can be selected as the section signal. In this case, according to the present approach, a section corresponding to one rotation of the rotating body  2  can be selected as the above-described specific section, and the processor  42  can generate section signals by dividing the sensor signal into sections corresponding to one rotation of the rotating body  2 . 
     The acceleration signal may include a noise component. Accordingly, the processor  42  may remove the noise component from the acceleration signal and practice the approach shown in  FIG.  13    using the acceleration signal from which the noise component has been removed. 
     Subsequently, the processor  42  estimates the state of the rotating body  2  based on the section signal (step S 3 ). As described above, the state of the rotating body  2  estimated by the processor  42  includes, for example, at least one of the camber angle, the slip angle, the load applied to the rotating body  2 , and the air pressure. That is, the parameters representing the state of the rotating body  2  may include the camber angle, the slip angle, the load, and the air pressure. When each parameter changes, the waveform of the sensor signal changes. The influence of each parameter on the waveform of the sensor signal may be different from each other. 
     Hereinafter, the influence of each parameter on the waveform of the sensor signal will be described with reference to  FIGS.  14  to  23   .  FIG.  14    is a diagram for explaining the reaction force from the road surface when the camber angle is  0  degrees.  FIG.  15    is a diagram for explaining the reaction force from the road surface in the positive camber.  FIG.  16    is a diagram for explaining the reaction force from the road surface in the negative camber.  FIG.  17    is a diagram showing an example of a sensor signal for each camber angle.  FIG.  18    is a diagram for explaining a slip angle.  FIG.  19    is a diagram for explaining a force acting on a sensor module when a slip angle occurs.  FIG.  20    is a diagram showing an example of a sensor signal for each slip angle.  FIG.  21    is a diagram for explaining a peak-to-peak value and a second peak value.  FIG.  22    is a diagram showing the relationship between a peak-to-peak value and a damping ratio when a slip angle, a camber angle, a load and air pressure are changed.  FIG.  23    is a partially enlarged view of  FIG.  22   . 
     As shown in  FIGS.  14  to  16   , when the camber angle θ changes, the reaction force received by the rotating body  2  from the road surface RS changes. The camber angle θ can be expressed as, for example, an inclination angle of the rotating body  2  when the vehicle V is viewed from the front. The camber angle θ may be expressed as an inclination of the rotating body  2  with respect to the road surface RS, for example. In the specific examples shown in  FIGS.  14  to  16   , the camber angle θ is expressed as an angle formed by the center axis CX 1  of the rotating body  2  and the normal direction of the road surface RS. The center axis CX 1  is a vertical axis of the rotating body  2 . When the upper end of the rotating body  2  is inclined outward, the camber angle θ is a positive value. This state can be expressed as that the upper end of the rotating body  2  is inclined in the positive direction (positive camber). When the upper end of the rotating body  2  is inclined inward, the camber angle θ is a negative value. This state can be expressed as that the upper end of the rotating body  2  is inclined in the negative direction (negative camber). In the specific examples shown in  FIGS.  14  to  16   , the upper end of the rotating body  2  represents an end portion on the opposite side to the road surface RS. 
     As shown in  FIG.  14   , when the camber angle θ is 0 degrees, a uniform reaction force acts from the road surface RS on the portion of the rotating body  2  (tire  22 ) that is in contact with the road surface RS. As shown in  FIG.  15   , when the camber angle θ is positive, the reaction force from the road surface RS increases toward the outer end portion  2   a  of the rotating body  2 . As shown in  FIG.  16   , when the camber angle θ is negative, the reaction force from the road surface RS increases toward the inner end portion  2   b  of the rotating body  2 . 
     A case where the sensor module  3  is disposed in the outer end portion  2   a  (outer rim) will be described as an exemplary case of the present embodiment. In this case, as shown in  FIG.  17   , as the camber angle θ increases, the reaction force received by the piezoelectric element  31  from the road surface RS increases. Therefore, as the camber angle θ increases, the peak-to-peak value of the sensor signal increases. The peak-to-peak value may be expressed as an absolute value of a difference between a local minimum value in the negative direction and a local maximum value in the positive direction in  FIG.  17   , for example. Further, since the bead of the tire  22  approaches the flange of the rim  23  due to the reaction force received from the road surface RS, the piezoelectric element  31  is pressed against the bead of the tire  22  and the flange of the rim  23 . Therefore, the degree of freedom of the piezoelectric element  31  is reduced. In the present embodiment, the degree of freedom of the piezoelectric element  31  may represent, for example, a degree to which the piezoelectric element  31  is deformable. Thus, as the camber angle θ increases, the vibration after the peak of the sensor signal tends to decrease. 
     On the other hand, as the camber angle θ decreases, the reaction force received by the piezoelectric element  31  from the road surface RS decreases. Thus, as the camber angle θ decreases, the peak-to-peak value of the sensor signal decreases. Furthermore, since the force with which the bead of the tire  22  and the flange of the rim  23  press the piezoelectric element  31  is weakened, the degree of freedom of the piezoelectric element  31  is increased. Thus, as the camber angle θ decreases, the vibration after the peak of the sensor signal tends to increase. 
     As shown in  FIG.  18   , the slip angle φ is an inclination angle of the rotating body  2  when the vehicle V is viewed from above (for example, when the vehicle V or the rotating body  2  present on the road surface is viewed downward). Specifically, the slip angle φ is an angle formed by the direction CX 2  of the rotating body  2  and the traveling direction F of the vehicle V. The direction CX 2  of the rotating body  2  may be, for example, orthogonal to the direction in which the rotational axis AX of the rotating body  2  extends and substantially parallel to the road surface. Hereinafter, for the sake of description, the slip angle φ when the front end of the rotating body  2  is inclined to the outside of the vehicle V with respect to the traveling direction F is expressed as a positive value. The slip angle φ when the front end of the rotating body  2  is inclined to the inside of the vehicle V with respect to the traveling direction F is expressed as a negative value. Note that  FIG.  18    shows the rotating body  2  on the right side of the vehicle V. In the rotating body  2 , a right direction with respect to the traveling direction F is the outside of the vehicle V, and a left direction with respect to the traveling direction F is the inside of the vehicle V. In the rotating body  2  on the left side of the vehicle V, the left direction with reference to the traveling direction F is the outside of the vehicle V, and the right direction with reference to the traveling direction F is the inside of the vehicle V. 
     As shown in  FIG.  19   , when the slip angle φ changes, the degree of freedom of the piezoelectric element  31  is changed. When the slip angle φ is a positive value, the force with which the bead of the tire  22  approaches the flange of the rim  23  increases toward the outer end portion  2   a  of the rotating body  2 . In this case, for example, the pressing force by the tire  22  and the rim  23  in the outer end portion  2   a  (outer rim side) of the rotating body  2  is larger than the pressing force by the tire  22  and the rim  23  in the inner end portion  2   b  (inner rim side) of the rotating body  2 . When the sensor module  3  is disposed in the outer end portion  2   a  (outer rim), the piezoelectric element  31  is pressed by the bead of the tire  22 , and thus the degree of freedom of the piezoelectric element  31  is reduced. Therefore, as shown in  FIG.  20    (particularly, when the slip angle is positive in  FIG.  20   ), the vibration after the peak of the sensor signal is suppressed. 
     On the other hand, when the slip angle ϕ is a negative value, the bead of the tire  22  receives such a force as to be pulled away from the flange of the rim  23  toward the outer end portion  2   a  of the rotating body  2 . In this case, for example, the pressing force by the tire  22  and the rim  23  in the outer end portion  2   a  (outer rim side) of the rotating body  2  is smaller than the pressing force by the tire  22  and the rim  23  in the inner end portion  2   b  (inner rim side) of the rotating body  2 . Accordingly, since the force with which the bead of the tire  22  presses the piezoelectric element  31  is relaxed, the degree of freedom of the piezoelectric element  31  is increased. Therefore, as shown in  FIG.  20   , the vibration after the peak of the sensor signal becomes large. 
     Even if the slip angle φ changes, there is a case where the reaction force from the road surface acting on the piezoelectric element  31  does not change so much. On the other hand, as the slip angle φ increases, the force with which the bead of the tire  22  presses the piezoelectric element  31  may increase. Therefore, in some cases, when the slip angle φ increases, the peak-to-peak value of the sensor signal may slightly increase. 
     When the load changes, the force received by the piezoelectric element  31  from the vehicle body changes. Specifically, as the load increases, the force received by the rotating body  2  from the vehicle body increases. At this time, since the pressing force applied to the sensor module  3  becomes large, the peak-to-peak value of the sensor signal becomes large. On the other hand, as the load becomes smaller, the force received by the rotating body  2  from the vehicle body becomes smaller. At this time, since the pressing force applied to the sensor module  3  decreases, the peak-to-peak value of the sensor signal decreases. That is, since the voltage generated by the piezoelectric element  31  changes due to the change in the load, the waveform of the sensor signal expands and contracts in the vertical axis direction (voltage value). 
     As the air pressure changes, the elastic modulus of the tire  22  changes. Since the tire  22  is less likely to contract as the air pressure is higher, the reaction force acting on the piezoelectric element  31  from the road surface decreases. Thus, the peak-to-peak value of the sensor signal decreases. On the other hand, since the tire  22  is likely to contract as the air pressure is lower, the reaction force acting on the piezoelectric element  31  from the road surface increases. Thus, the peak-to-peak value of the sensor signal increases. That is, since the voltage generated by the piezoelectric element  31  changes depending on the change in the air pressure, the waveform of the sensor signal expands and contracts in the vertical axis direction. 
     The processor  42  estimates the state of the rotating body  2  based on the degree of influence of each parameter on the waveform of the sensor signal. Some examples of a process of estimating the state of the rotating body  2  will be described below, but the process of estimating the state of the rotating body  2  is not limited to these examples. 
     As one approach, the processor  42  may estimate the state of the rotating body  2  based on a plurality of different waveform characteristics calculated from the section signal. The plurality of waveform characteristics include a value based on at least one of a maximum value of the section signal, a minimum value of the section signal, a difference between the maximum value and the minimum value in the section signal (peak-to-peak value), a standard deviation of the section signal, a variance of the section signal, an average value of the section signal, a median value of the section signal, a value at an inflection point of the section signal, and a wavelength of the section signal. For example, as the plurality of waveform characteristics, one or more values among the values exemplified above may be used as they are, a combination of two or more values may be used, or a value calculated from these values by an appropriate calculation formula may be used. 
     For each parameter (camber angle, slip angle, load, and air pressure) representing the state of the rotating body  2 , the relationship between the amount of change in the parameter and the amount of change in each waveform characteristic may be measured and stored in advance. Specifically, for each parameter representing the state of the rotating body  2 , a relationship between an amount of change in the parameter and an amount of change in each waveform characteristic when only the parameter changes may be stored. Here, the number of waveform characteristics used in the process of estimating the state may be equal to or greater than the number of parameters to be estimated among the parameters representing the state of the rotating body  2 . 
     As an example, changes in the peak-to-peak value and the damping ratio with respect to changes in each parameter will be described. The damping ratio is a damping ratio of the waveform of the sensor signal generated when the piezoelectric element  31  moves away from the road surface. The damping ratio is a value obtained by dividing the second peak value by the peak-to-peak value. As shown in  FIG.  21   , as the second peak value, for example, a peak value convex in the positive direction of the voltage occurring after the maximum value in the section signal may be used. When the section signal has no peak value convex in the positive direction other than the maximum value, for example, a value at an inflection point at which the change rate of the slope of the section signal changes from positive to negative after the maximum value may be used as the second peak value. 
     In the example shown in  FIGS.  22  and  23   , the state of the rotating body  2  when the camber angle is 0 degrees, the slip angle is 0 degrees, the load is 5300 N, and the air pressure is 240 kPa is used as the reference state. The reference state represents a state of the rotating body  2  at a certain speed, a certain camber angle, a certain slip angle, a certain load, and a certain air pressure. In the reference state, the peak-to-peak value is 2.9 V and the damping ratio is 0.042. 
     In the examples shown in  FIGS.  22  and  23   , as the camber angle increases, the peak-to-peak value increases and the damping ratio decreases. Specifically, when only the camber angle is changed from −5 degrees to +5 degrees from the reference state, the peak-to-peak value increases from 1.6 V to 3.7 V and the damping ratio decreases from 0.077 to 0.030. As the slip angle increases, the peak-to-peak value increases and the damping ratio decreases. Specifically, when only the slip angle is changed from −1 degree to +1 degree from the reference state, the peak-to-peak value increases from 2.6 V to 3.4 V, and the damping ratio decreases from 0.200 to −0.460. 
     In the examples shown in  FIGS.  22  and  23   , as the load increases, the peak-to-peak value increases and the damping ratio increases. Specifically, when only the load is changed from 3000 N to 7600 N from the reference state, the peak-to-peak value increases from 1.8 V to 3.8 V and the damping ratio increases from 0.037 to 0.052. As the air pressure increases, the peak-to-peak value decreases and the damping ratio decreases. Specifically, when only the air pressure is changed from 160 kPa to 260 kPa, the peak-to-peak value decreases from 2.9 V to 2.6 V and the damping ratio decreases from 0.100 to 0.083. 
     The processor  42  may estimate the state of the rotating body  2  by comparing the actual measurement value with the reference value. The reference value is a value of each waveform characteristic in the reference state of the rotating body  2 . The actual measurement value is a value of each waveform characteristic obtained from the section signal. Specifically, when the actual measurement value is different from the reference value in any one of the waveform characteristics, the processor  42  determines that the state of the rotating body  2  has changed from the reference state. Then, the processor  42  may calculate the value of the parameter to be estimated based on the amount of change in each waveform characteristic. As an example of the state estimation, the processor  42  may estimate the state of the rotating body  2  using the relationship shown in  FIGS.  22  and  23   . In this case, for example, the processor  42  may set two parameters of the state of the rotating body  2  as estimation targets and calculate the values of the two parameters that are the estimation targets from the actual measurement values on the assumption that the parameters other than the estimation targets do not change. 
     The processor  42  may estimate which parameter has changed from the amount of change in each waveform characteristic (the value obtained by subtracting the reference value from the actual measurement value) on the assumption that any one of the plurality of parameters has changed. 
     As another approach, the processor  42  may determine the state of the rotating body  2  using a clustering method such as a k-means method. Specifically, the processor  42  classifies the section signal into one of the clusters set corresponding to each parameter of the rotating body  2  based on the actual measurement value of each waveform characteristic obtained from the section signal. The processor  42  estimates the state corresponding to the cluster into which the section signal is classified as the state of the rotating body  2 . 
     As yet another approach, the processor  42  may estimate the state of the rotating body  2  using the estimation model M. The estimation model M may be, for example, a machine learning model learned to estimate the state of the rotating body  2 . The estimation model M will be described with reference to  FIG.  24   .  FIG.  24    is a diagram for explaining an estimation model. As shown in  FIG.  24   , the estimation model M may be generated by, for example, machine learning using learning data. As an algorithm of the machine learning, an algorithm such as random forest, LightGBM, or deep learning may be used. The estimation model M may be, for example, a classifier that classifies the state of the rotating body  2  into a specific category (for example, a range of camber angles, a range of slip angles, and a range of loads, etc.), or may be a regression model that outputs an estimated value of the state of the rotating body  2 . 
     The learning data may include, for example, a feature vector calculated from a section signal generated from a sensor signal acquired in advance by the sensor module  3 . The feature vector may include values of a plurality of waveform characteristics as elements. The feature vector may include, as elements, one or more of a maximum value of the section signal, a minimum value of the section signal, a peak-to-peak value of the section signal, a standard deviation of the section signal, a variance of the section signal, an average value of the section signal, a median value of the section signal, a value at an inflection point of the section signal (e.g., a second peak value), a wavelength of the section signal, and a value calculated from these values. The feature vector is not limited thereto, and may be, for example, all data itself (for example, a voltage value itself) included in the section signal. A label corresponding to the state of the rotating body  2  may be assigned to the learning data. Examples of labels may include normal travel, changes in camber angle, and changes in slip angle. An amount of change in the parameter may be used as the label. 
     For example, a range of camber angles, a range of slip angles, a range of loads, a range of air pressure, and the like may be used as labels. 
     The estimation model M receives a feature vector calculated from the section signal as an input, and outputs an estimation result. The estimation result is information indicating the state of the rotating body  2 . The estimation result may include information indicating which parameter has changed. The estimation result may include a change amount of each parameter. The estimation result may include a range of each parameter (for example, a range of camber angles, a range of slip angles, a range of loads, a range of air pressure, and the like). 
     In the example of  FIG.  24   , the estimation model M is configured to estimate all states with one model. The estimation model M is not limited thereto, and may include a plurality of estimation models provided for each parameter to be estimated (for example, camber angle, slip angle, load, air pressure, and the like). Each estimation model estimates the state assigned to that estimation model. 
     Subsequently, the processor  42  outputs the estimation result (step S 4 ). In the present embodiment, the processor  42  may output the estimation result to the external device  5  via the communication interface  43 , for example. Upon receiving the estimation result, the external device  5  may present the estimation result to the occupant using the output device  54 , for example. For example, when the output device  54  is a display, the output device  54  displays the estimation result. The external device  5  is not limited thereto, and may provide the received estimation result to another device installed in the vehicle V, for example. For example, the external device  5  may provide the received estimation result to a device disposed outside the vehicle V (for example, a server or the like to which the external device  5  is connectable via the communication network NW 2 ). 
     Thus, a series of processes of the estimation method ends. 
     In the estimation system  1 , the estimation method, and the recording medium described above, the sensor signal in accordance with the pressing force by the wheel  21  and the tire  22  is output from the piezoelectric element  31  disposed between the wheel  21  and the tire  22 . A weight W from the vehicle V (vehicle body) acts on the piezoelectric element  31  via the wheel  21 , and a reaction force R from the road surface acts on the piezoelectric element  31  via the tire  22 . Since these forces can change depending on the state of the rotating body  2 , the state of the rotating body  2  can be estimated based on the sensor signal. Therefore, the state of the rotating body  2  can be estimated with a simple configuration in which the piezoelectric element  31  (sensor module  3 ) is disposed between the wheel  21  and the tire  22 . 
     When the wheel  21  includes the rim  23 , the tire  22  is mounted on the rim  23 . In this case, the piezoelectric element  31  is disposed between the rim  23  and the tire  22 . Therefore, the state of the rotating body  2  can be estimated with a simple configuration in which the piezoelectric element  31  (sensor module  3 ) is disposed between the rim  23  and the tire  22 . 
     In a case where the piezoelectric element  31  is disposed at the center of the rotating body  2  in the direction in which the rotational axis AX extends, for example, the sensor signal changes in the same manner regardless of whether the camber angle changes in the positive direction or the negative direction. On the other hand, in the above-described embodiment, when the piezoelectric element  31  is disposed in the outer end portion  2   a , the sensor signal changes asymmetrically with respect to a change in the camber angle or the like. Here, “the sensor signal changes asymmetrically” means that the sensor signal when the camber angle changes in the positive direction is different from the sensor signal when the camber angle changes in the negative direction. For example, when the camber angle increases in the positive direction, the reaction force from the road surface increases toward the outer end portion  2   a  of the rotating body  2 , and thus the peak-to-peak value of the sensor signal increases. For example, when the camber angle decreases, the reaction force from the road surface increases toward the inner end portion  2   b  of the rotating body  2 , and thus the peak-to-peak value of the sensor signal decreases. That is, by disposing the piezoelectric element  31  in the outer end portion  2   a  of the rotating body  2 , it is possible to improve the estimation accuracy of the state of the rotating body  2 . 
     On the other hand, even if the piezoelectric element  31  is disposed in the inner end portion  2   b , the sensor signal changes asymmetrically with respect to a change in the camber angle or the like. In this case, the reaction force from the road surface in the inner end portion  2   b  of the rotating body  2  changes in accordance with the camber angle. Therefore, even in a configuration in which the piezoelectric element  31  is disposed in the inner end portion  2   b , it is possible to improve the estimation accuracy of the state of the rotating body  2 . 
     As described above, the section signal may be generated by, for example, dividing the sensor signal into sections corresponding to one rotation of the rotating body  2 . When the rotating body  2  rotates, the portion of the rotating body  2  that comes into contact with the road surface changes, and thus the relative positional relationship between the piezoelectric element  31  and the contact portion changes. Therefore, in some situations, the sensor signal has a periodicity such that the waveform shape becomes similar every time the rotating body  2  makes one rotation. In this case, the state of the rotating body  2  can be estimated by analyzing the section signal corresponding to one rotation of the rotating body  2 . 
     The waveform characteristic calculated from the section signal can be an index indicating the state of the rotating body  2 . Therefore, it is possible to improve the estimation accuracy of the state of the rotating body  2  by using a plurality of waveform characteristics, which are different from each other, calculated from the section signal. 
     The maximum value of the section signal, the minimum value of the section signal, the peak-to-peak value of the section signal, the standard deviation of the section signal, the variance of the section signal, the average value of the section signal, the median value of the section signal, and the value at the inflection point of the section signal are values representing the waveform characteristics of the section signal. When the state of the rotating body  2  changes, these values may change. Therefore, it is possible to improve the estimation accuracy of the state of the rotating body  2  by using a value based on at least one of these values. 
     The processor  42  may estimate the state of the rotating body  2  using the estimation model M. In this case, it is possible to improve the estimation accuracy of the state of the rotating body  2  by sufficiently learning the estimation model M. 
     As described above, the tendency of change in the sensor signal when the camber angle changes, the tendency of change in the sensor signal when the slip angle changes, the tendency of change in the sensor signal when the load changes, and the tendency of change in the sensor signal when the air pressure changes may be different from each other. In this case, the camber angle, the slip angle, the load, and the air pressure can be estimated separately. 
     As described above, the piezoelectric element  31  generates electric energy in accordance with a pressing force. For example, the processor  42  may be configured to operate using electric energy generated by the piezoelectric element  31 . According to this configuration, the processor  42  can operate without receiving electric power from the outside of the sensor module  3 . Accordingly, wiring or the like for supplying electric power from the outside of the sensor module  3  becomes unnecessary, so that the configuration of the estimation system  1  can be simplified. 
     As described above, the piezoelectric element  31  and the processor  42  may constitute the sensor module  3 . Such a sensor module  3  may be provided in the rotating body  2 . The processor  42  may be configured to output the estimation result to the external device  5  provided outside the rotating body  2 . In this configuration, the sensor signal is processed in the sensor module  3 , and the estimation result is output to the external device  5 . In this case, the amount of communication between the sensor module  3  and the external device  5  can be reduced compared with that of a configuration in which the sensor signal is processed in the external device  5 . As a result, electric power required for communication can be reduced, so that electric energy generated by the piezoelectric element  31  can be effectively used. 
     An estimation system according to another embodiment will be described with reference to  FIG.  25   .  FIG.  25    is a configuration diagram schematically showing an estimation system according to another embodiment. An estimation system  1 A shown in  FIG.  25    is mainly different from the estimation system  1  in that the estimation system  1 A includes a plurality of sensor modules  3 A and one control module  4  instead of one sensor module  3 . 
     Each sensor module  3 A is mainly different from the sensor module  3  in that the sensor module  3 A does not include the AD converters  41 , the processor  42 , the communication interface  43 , the power converter  44 , and the power storage device  45  as circuit elements. 
     Each sensor module  3 A may, for example, be configured to have a physical structure similar to that of the sensor module  3  and may include the piezoelectric element  31 , the back plate  32 , the substrate  33 , the substrate  34 , and the base material  35 . The plurality of sensor modules  3 A may be provided in the same rotating body  2 , for example. Each sensor module  3 A is disposed between the wheel  21  (rim  23 ) and the tire  22 . Specifically, each sensor module  3 A is disposed between the flange of the rim  23  and the bead of the tire  22  and is in contact with the flange of the rim  23  and the bead of the tire  22 . 
     In the present embodiment, some sensor modules  3 A are disposed in the outer end portion  2   a  (outer rim) and some sensor modules  3 A are disposed in the inner end portion  2   b  (inner rim). The number of sensor modules  3 A disposed in the outer end portion  2   a  may be the same as or different from the number of sensor modules  3 A disposed in the inner end portion  2   b . All of the sensor modules  3 A may be disposed in only one of the outer end portion  2   a  and the inner end portion  2   b.    
     The control module  4  is a module that processes sensor signals output from the plurality of sensor modules  3 A provided in one rotating body  2 . The control module  4  may be provided at the center of the wheel  21 , for example. In the specific example shown in  FIG.  25   , the control module  4  includes an AD converter  41 , a processor  42 , a communication interface  43 , a power converter  44 , and a power storage device  45 . The AD converter  41  and the communication interface  43  may be integrated into the processor  42 . The AD converter  41 , the processor  42 , the communication interface  43 , the power converter  44 , and the power storage device  45  is different from the AD converter  41 , the processor  42 , the communication interface  43 , the power converter  44 , and the power storage device  45  of the sensor module  3  in that signals to be processed are a plurality of sensor signals. 
     Next, an arrangement example of the plurality of sensor modules  3 A will be described with reference to  FIGS.  26  to  28   .  FIG.  26    is a diagram showing an example of the arrangement of sensor modules.  FIG.  27    is a diagram showing an example of a sensor signal for each camber angle.  FIG.  28    is a diagram showing an example of a sensor signal for each slip angle. In the example shown in  FIG.  26   , one sensor module  3 A is disposed in the outer end portion  2   a  (outer rim), and one sensor module  3 A is disposed in the inner end portion  2   b  (inner rim). The sensor module  3 A disposed in the outer end portion  2   a  is referred to as “sensor module  3 Ao”, and the sensor module  3 A disposed in the inner end portion  2   b  is referred to as “sensor module  3 Ai”. 
     Specifically, the sensor module  3 Ao is disposed between the wheel  21  (rim  23 ) and the tire  22  in the outer end portion  2   a . More specifically, the sensor module  3 Ao is disposed between an outer flange of the rim  23  and a bead of the tire  22  and is in contact with the outer flange of the rim  23  and the bead of the tire  22 . The sensor module  3 Ai is disposed between the wheel  21  (rim  23 ) and the tire  22  in the inner end portion  2   b . More specifically, the sensor module  3 Ai is disposed between an inner flange of the rim  23  and a bead of the tire  22  and is in contact with the inner flange of the rim  23  and the bead of the tire  22 . 
     As the camber angle θ increases, the reaction force received by the piezoelectric element  31  of the sensor module  3 Ao from the road surface increases. Thus, as shown in  FIG.  27   , as the camber angle θ increases, the peak-to-peak value of the sensor signal (hereinafter referred to as a “first sensor signal” in some cases) output from the piezoelectric element  31  of the sensor module  3 Ao increases. Further, since the bead of the tire  22  approaches the outer flange of the rim  23  due to the reaction force received from the road surface, the piezoelectric element  31  of the sensor module  3 Ao is pressed against the bead of the tire  22  and the outer flange of the rim  23 . Therefore, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ao is reduced. Therefore, as the camber angle θ increases, the vibration after the peak of the first sensor signal is suppressed. 
     As the camber angle θ decreases, the reaction force received by the piezoelectric element  31  of the sensor module  3 Ao from the road surface decreases. Therefore, as the camber angle θ decreases, the peak-to-peak value of the first sensor signal decreases. Further, since the force with which the bead of the tire  22  and the outer flange of the rim  23  press the piezoelectric element  31  of the sensor module  3 Ao is weakened, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ao is increased. Therefore, as the camber angle θ decreases, the vibration after the peak of the first sensor signal increases. 
     On the other hand, as the camber angle θ increases, the reaction force received by the piezoelectric element  31  of the sensor module  3 Ai from the road surface decreases. Therefore, as the camber angle θ increases, the peak-to-peak value of the sensor signal (hereinafter, referred to as a “second sensor signal” in some cases) output from the piezoelectric element  31  of the sensor module  3 Ai decreases. Further, since the force with which the bead of the tire  22  and the inner flange of the rim  23  press the piezoelectric element  31  of the sensor module  3 Ai is weakened, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ai is increased. Therefore, as the camber angle θ increases, the vibration after the peak of the second sensor signal increases. 
     As the camber angle θ decreases, the reaction force received by the piezoelectric element  31  of the sensor module  3 Ai from the road surface increases. Therefore, as the camber angle θ decreases, the peak-to-peak value of the second sensor signal increases. Further, since the bead of the tire  22  approaches the inner flange of the rim  23  due to the reaction force received from the road surface, the piezoelectric element  31  of the sensor module  3 Ai is pressed against the bead of the tire  22  and the inner flange of the rim  23 . Therefore, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ai is reduced. Therefore, as the camber angle θ decreases, the vibration after the peak of the second sensor signal is suppressed. 
     As the slip angle φ increases, the bead of the tire  22  approaches the outer flange of the rim  23 , so that the piezoelectric element  31  of the sensor module  3 Ao is pressed against the bead of the tire  22 . Therefore, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ao is reduced. Therefore, as shown in  FIG.  28   , as the slip angle φ increases, the vibration after the peak of the first sensor signal is suppressed. As the slip angle φ decreases, the force with which the bead of the tire  22  and the outer flange of the rim  23  press the piezoelectric element  31  of the sensor module  3 Ao is weakened, and thus the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ao is increased. Therefore, as shown in  FIG.  28   , as the slip angle φ decreases, the vibration after the peak of the first sensor signal increases. 
     On the other hand, as the slip angle φ increases, the force with which the bead of the tire  22  and the inner flange of the rim  23  press the piezoelectric element  31  of the sensor module  3 Ai is weakened, and thus the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ai is increased. Therefore, as shown in  FIG.  28   , as the slip angle φ increases, the vibration after the peak of the second sensor signal increases. As the slip angle φ decreases, the bead of the tire  22  approaches the inner flange of the rim  23 , so that the piezoelectric element  31  of the sensor module  3 Ai is pressed against the bead of the tire  22 . Therefore, the degree of freedom of the piezoelectric element  31  of the sensor module  3 Ai is reduced. Therefore, as shown in  FIG.  28   , as the slip angle φ decreases, the vibration after the peak of the second sensor signal is suppressed. 
     As shown in  FIG.  28   , when the AD converter  41  receives the first sensor signal and the second sensor signal, the AD converter  41  may convert the first sensor signal and the second sensor signal into digital signals and output the first sensor signal and the second sensor signal, which have been converted into digital signals, to the processor  42 . When the processor  42  acquires the first sensor signal, which has been converted into a digital signal, from the AD converter  41 , the processor  42  may generate the first section signal by dividing the first sensor signal by the specific section. When the processor  42  acquires the second sensor signal, which has been converted into a digital signal, from the AD converter  41 , the processor  42  may generate the second section signal by dividing the second sensor signal by the specific section. 
     The process of generating the first section signal and the second section signal may be similar to the process of generating the section signal in the estimation system  1 , for example. The processor  42  may estimate the state of the rotating body  2  based on the first section signal and the second section signal. The process of estimating the state of the rotating body  2  may be similar to the process of estimating the state of the rotating body  2  in the estimation system  1 . The processor  42  may output the estimation result. 
     Also in the estimation system  1 A, the same effects as those of the estimation system  1  are obtained in the configuration common to the estimation system  1 . In the estimation system  1 A, the piezoelectric element  31  of the sensor module  3 Ao and the piezoelectric element  31  of the sensor module  3 Ai are disposed opposite to each other with respect to the center of the rotating body  2  in the direction in which the rotational axis AX extends. For example, the piezoelectric element  31  of the sensor module  3 Ao may be disposed in the outer end side with respect to the center of the rotating body  2  in the direction in which the rotational axis AX extends. For example, the piezoelectric element  31  of the sensor module  3 Ai may be disposed in the inner end side with respect to the center of the rotating body  2  in the direction in which the rotational axis AX extends. 
     The sensor signal output from the piezoelectric element  31  of the sensor module  3 Ao and the sensor signal output from the piezoelectric element  31  of the sensor module  3 Ai change differently from each other in accordance with a change in the state of the rotating body  2 . Specifically, as shown in  FIGS.  27  and  28   , the change in the sensor signal output from the piezoelectric element  31  of the sensor module  3 Ao may be opposite to the change in the sensor signal output from the piezoelectric element  31  of the sensor module  3 Ai. In a case where the state of the rotating body  2  is estimated using two sensor signals in which opposite changes occur as described above, an influence due to disturbance or the like can be reduced. As a result, the estimation accuracy of the state of the rotating body  2  may be improved compared with that of a configuration (estimation system  1 ) in which the state of the rotating body  2  is estimated using one sensor signal. 
     An estimation system according to yet another embodiment will be described with reference to  FIG.  29   .  FIG.  29    is a configuration diagram schematically showing an estimation system according to yet another embodiment. An estimation system  1 B shown in  FIG.  29    is mainly different from the estimation system  1  in that the estimation system  1 B includes a sensor module  3 B instead of the sensor module  3  and further includes an external device  5 B. 
     The sensor module  3 B is mainly different from the sensor module  3  in that the sensor module  3 B does not include the processor  42 . In the sensor module  3 B, the AD converter  41  outputs a sensor signal that is a digital signal to the communication interface  43 . The communication interface  43  transmits the sensor signal that is a digital signal to the external device  5 B via the communication network NW 1 . 
     The external device  5 B is mainly different from the external device  5  in that the external device  5 B includes a processor  51 B instead of the processor  51 . The processor  51 B is mainly different from the processor  51  in that the processor  51 B estimates the state of the rotating body  2  based on the sensor signal transmitted from the sensor module  3 B. The processor  51 B may be configured to estimate the state of the rotating body  2 , for example, in the same manner as the processor  42 . The processor  51 B may output the estimation result to the output device  54 , for example. 
     Also in the estimation system  1 B, the same effects as those of the estimation system  1  are obtained in the configuration common to the estimation system  1 . Further, in the estimation system  1 B, the processor  51 B of the external device  5 B estimates the state of the rotating body  2 . In this case, for example, since constraints such as electric power consumption, physical size, and cooling are relaxed, a processor having a higher calculation capability than the processor  42  included in the sensor module  3  can be adopted as the processor  51 B. When such a processor  51 B is adopted, it is possible to shorten the time required for estimating the state of the rotating body  2 . 
     An estimation system according to yet another embodiment will be described with reference to  FIG.  30   .  FIG.  30    is a configuration diagram schematically showing an estimation system according to yet another embodiment. An estimation system  1 C shown in  FIG.  30    is mainly different from the estimation system  1 B in that the estimation system  1 C includes an external device  5 C instead of the external device  5 B and further includes a server  6 . 
     The external device  5 C is mainly different from the external device  5 B in that the external device  5 C includes a processor  51  instead of the processor  51 B. Like the processor  51  of the external device  5 , the processor  51  is a circuit element that performs control and calculation in the external device  5 C. Upon receiving the sensor signal from the sensor module  3 B, the communication interface  55  outputs the sensor signal to the communication interface  56 . The communication interface  56  may be configured to transmit the sensor signal to the server  6  via the communication network NW 2 . 
     The servers  6  may have, for example, a hardware configuration similar to that of the external device  5 C. The processor of the sever  6  may estimate the state of the rotating body  2  based on the sensor signal transmitted from the external device  5 C. In this case, the processor of the server  6  may estimate the state of the rotating body  2  by, for example, processing similar to that of the processor  42 . 
     Also in the estimation system  1 C, the same effects as those of the estimation system  1 B are obtained in the configuration common to the estimation system  1 B. Further, in the estimation system  1 C, the processor of the sever  6  estimates the state of the rotating body  2 . According to this configuration, for example, even in a case where the sensor module  3 B is installed in the rotating bodies  2  of a plurality of different vehicles V, it is not necessary to implement the function of estimating the state of the rotating body  2  in the external device  5  in each of the vehicles V. That is, the server  6  can estimate the state of the rotating body  2  provided in each vehicle V based on the signal collected via the external device  5  provided in each vehicle V. 
     The estimation system, the estimation method, and the recording medium according to the present disclosure are not limited to the above-described embodiments. 
     For example, each of the sensor modules  3  and  3 B, and the control module  4  do not have to include the power converter  44  and the power storage device  45 . In this case, each of the sensor modules  3  and  3 B, and the control module  4  may include a battery or may receive electric power from the outside. 
     In the above-described embodiments, (the piezoelectric element  31  of) each of the sensor modules  3 ,  3 A, and  3 B is disposed in the outer end portion  2   a  or the inner end portion  2   b , but may be disposed at a position depending on the configuration of the wheel  21  and the tire  22 . (The piezoelectric element  31  of) each of the sensor modules  3 ,  3 A, and  3 B may be disposed at a position closer to the outer end or the inner end than the center of the rotating body  2  in the direction in which the rotational axis AX extends. In the example shown in  FIG.  26   , the piezoelectric element  31  of the sensor module  3 Ao may be disposed at a position closer to the outer end than the center, and the piezoelectric element  31  of the sensor module  3 Ai may be disposed at a position closer to the inner end than the center. 
     Any reference to an element using the designations such as “first” and “second”, as used in the present disclosure does not limit the amount or order of the element. Such designations may be used in the present disclosure as a convenient way to distinguish between two or more elements. Thus, references to the first and second elements do not imply that only two elements may be adopted, or that the first element must precede the second element in any way. In the present disclosure, the use of a first element does not imply the assumption of the presence of two or more elements.