Patent Publication Number: US-2023158837-A1

Title: Tire wear measuring device and power generating device

Description:
CLAIM OF PRIORITY 
     This application is a Continuation of International Application No. PCT/JP2021/027841 filed on Jul. 28, 2021, which claims benefit of Japanese Patent Application No. 2020-141508 filed on Aug. 25, 2020. The entire contents of each application noted above are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tire wear measuring device that detects wear of a tire based on a magnetic field from a magnet embedded in the tire and relates to a power generating device. 
     2. Description of the Related Art 
     The progression of wear of a tire reduces grip capability in traveling on a road and water discharge capability for discharging water between the tire and a road in traveling on a wet road. A driver or a fleet manager visually inspects treads of tires for wear and replaces a worn tire to ensure safety before the tire is excessively worn and unsafe for use. For example, slip signs provided in grooves on a tire are used for visual inspection. An inspection operation is complicated. A wear state may be wrongly determined. Some users may never perform inspection. If whether a tire is worn is wrongly determined, the tire having reduced capabilities may be continuously used, which is not preferable in terms of safety. 
     A sensor module has been developed to measure the degree of tire wear by using a method other than visual observation. For example, Japanese Unexamined Patent Application Publication No. 2019-203831 discloses a sensor module including a magnetic sensor that detects the magnetic flux density of a magnetic field in a direction along the radius of a tire in a tread portion. The sensor module is configured to measure wear of the tread portion based on the magnetic flux density detected by the magnetic sensor. 
     Japanese Unexamined Patent Application Publication No. 2011-239510 discloses an in-tire power generating device that supplies power to other devices in a tire. The power generating device includes rotary members each including a magnet, a base supporting the rotary members, and coils located in the base. The magnets rotate with rotation of a tire, thus generating electric power. 
     The sensor module disclosed in Japanese Unexamined Patent Application Publication No. 2019-203831 is intended to measure the strength of a magnetic field changing due to wear in order to always accurately determine a wear state of a tire. Accordingly, Japanese Unexamined Patent Application Publication No. 2019-203831 does not describe a configuration intended to reduce the size and weight of the sensor module. The sensor module is mounted on a tire in use. A large size or a heavy weight of the sensor module may cause the tire to be out of balance when rotating. Therefore, the sensor module is required to be as small and light as possible. The device disclosed in Japanese Unexamined Patent Application Publication No. 2011-239510, which is an elaborate power generating system, tends to increase in weight. A tire with such a device may be out of balance when rotating. 
     SUMMARY OF THE INVENTION 
     The present invention provides a tire wear measuring device that is capable of accurately detecting tire wear and achieves a reduction in size and weight and a power generating device that is mountable in a tire. 
     The present invention provides a tire wear measuring device that detects wear of a tire based on a magnetic field from a magnet embedded in the tire, the device including at least one magnetic sensor and a magnetic collecting member capable of transmitting the magnetic field from the magnet, the magnetic collecting member having an outer edge from which the magnetic field from the magnet is emitted as an emission magnetic field. The at least one magnetic sensor is disposed in a position where the emission magnetic field is detectable by the at least one magnetic sensor. 
     In the above configuration, the emission magnetic field from the magnetic collecting member is detected. Such a configuration eliminates the need for disposing the at least one magnetic sensor in proximity to the magnet embedded in the tire. This leads to higher flexibility in designing the at least one magnetic sensor. This allows simplification of a structure of the tire wear measuring device, resulting in a reduction in size and weight of the device and higher detection accuracy. 
     Preferably, the tire wear measuring device further includes a magnetic-field guiding member that guides the emission magnetic field. In this case, the magnetic collecting member may be a coin-type battery. Preferably, in plan view in a direction along a normal to an electrode surface of the coin-type battery, an end of the magnetic-field guiding member that is adjacent to the at least one magnetic sensor is disposed outside the outer edge of the coin-type battery, and the at least one magnetic sensor is disposed between the coin-type battery and the end of the magnetic-field guiding member. 
     Such a configuration allows the emission magnetic field guided by the magnetic-field guiding member to be efficiently detected by the at least one magnetic sensor, resulting in improved detection accuracy of the at least one magnetic sensor. Specifically, the emission magnetic field guided by the magnetic-field guiding member is formed between the coin-type battery and the end of the magnetic-field guiding member. The emission magnetic field guided by the magnetic-field guiding member allows an increase in magnetic flux density. Therefore, the at least one magnetic sensor disposed between the coin-type battery and the end of the magnetic-field guiding member can accurately detect the emission magnetic field. 
     Preferably, the electrode surface of the coin-type battery faces toward the magnet. Such a configuration allows the entire outer edge of the coin-type battery to emit the emission magnetic field. The at least one magnetic sensor disposed in proximity to the outer edge can detect the emission magnetic field. 
     The magnetic-field guiding member may be an antenna that serves as a waveguide and that emits and receives electromagnetic waves. The use of an antenna for communication as the magnetic-field guiding member enables a reduction in size and weight of the tire wear measuring device. 
     A direction in which the emission magnetic field is detectable by the at least one magnetic sensor may be parallel to the electrode surface of the coin-type battery. The end of the magnetic-field guiding member and the at least one magnetic sensor may be arranged on the same plane parallel to the electrode surface of the coin-type battery. 
     Such a configuration allows the emission magnetic field that is detected by the at least one magnetic sensor to contain more components parallel to the electrode surface of the coin-type battery, resulting in efficient detection of the emission magnetic field. 
     The at least one magnetic sensor may include a first sensor and a second sensor. The first sensor may be disposed at one side of the coin-type battery in a direction parallel to the electrode surface of the coin-type battery, and the second sensor may be disposed at the other side of the coin-type battery in the direction. Wear of the tire may be detected based on an output of the first sensor and an output of the second sensor. 
     In this case, preferably, in plan view in the direction along the normal to the electrode surface of the coin-type battery, the electrode surface of the coin-type battery has a center aligned with the magnet embedded in the tire and located on a straight line connecting the first sensor and the second sensor. 
     The outputs of the first and second sensors can be used to cancel out the influence of noise, such as an external magnetic field. A magnetic field at one side of the coin-type battery and a magnetic field at the other side thereof are oriented in opposite directions. For example, using the difference between the outputs provides an output whose magnitude is approximately two times the magnitude of an output of one magnetic sensor, leading to improved detection accuracy. 
     In the tire wear measuring device including a magnetic-field guiding member that guides the emission magnetic field, the at least one magnetic sensor may include a first sensor and a second sensor, the magnetic-field guiding member may have a first end and a second end that are adjacent to the at least one magnetic sensor, the magnetic collecting member may be a coin-type battery, the first sensor and the first end may be arranged at one side of the coin-type battery in a direction parallel to an electrode surface of the coin-type battery, and the second sensor and the second end may be arranged at the other side of the coin-type battery in the direction, and wear of the tire may be detected based on an output of the first sensor and an output of the second sensor. 
     Such a configuration allows the emission magnetic field guided by the magnetic-field guiding member to be efficiently detected by the at least one magnetic sensor, leading to improved accuracy of measurement of tire wear. 
     The tire wear measuring device may further include a coil disposed within a range of the magnetic field from the magnet, and an induction current that is generated in the coil due to rotation of the tire may be available as an operational power source. In this case, preferably, the coil is disposed between the magnetic collecting member and the magnet. 
     As a relative positional relationship between the magnet and the coil changes with rotation of the tire, the density of a magnetic flux passing through the coil changes. Disposing the coil between the magnetic collecting member and the magnet increases the density of the magnetic flux passing through the coil, leading to improved power generation efficiency. Therefore, while the tire wear measuring device is kept small in outer dimensions (size), electric power generated by the coil can be used for operation of the tire wear measuring device. 
     The present invention further provides a power generating device including a magnet embedded in a tire and a coil disposed within a range of a magnetic field from the magnet. In this device, a relative positional relationship between the magnet and the coil changes with rotation of the tire, and power is generated due to a change in density of a magnetic flux passing through the coil caused by a change of the relative positional relationship. Preferably, the power generating device further includes a magnetic member, and the coil is disposed between the magnetic member and the magnet. 
     Such a configuration enables an induction current that is generated due to a change in density of the magnetic flux passing through the coil caused by rotation of the tire to be used for power generation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view of the configuration of a tire wear measuring device according to an embodiment of the present invention; 
         FIG.  2 A  is a vector map showing a magnetic field in the tire wear measuring device of  FIG.  1   ; 
         FIG.  2 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  1   ; 
         FIG.  3 A  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  1   ; 
         FIG.  3 B  is a contour map showing sensor-detectable components of a magnetic field in a tire wear measuring device of  FIG.  8   ; 
         FIG.  4    is a schematic diagram illustrating the positional relationship between a magnet, a coin-type battery, magnetic sensors, and ends of an antenna in the tire wear measuring device of  FIG.  1   ; 
         FIG.  5    is a schematic cross-sectional view of the configuration of a modification of the tire wear measuring device; 
         FIG.  6 A  is a vector map showing a magnetic field in a tire wear measuring device of  FIG.  5   ; 
         FIG.  6 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  5   ; 
         FIG.  7    is a schematic sectional view of the tire wear measuring device mounted on a tire; 
         FIG.  8    is a schematic cross-sectional view of the configuration of a related-art tire wear measuring device; 
         FIG.  9 A  is a vector map showing a magnetic field in the tire wear measuring device of  FIG.  8   ; 
         FIG.  9 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  8   ; 
         FIG.  10    is a schematic cross-sectional view of the configuration of another modification of the tire wear measuring device; 
         FIG.  11    is a schematic cross-sectional view of the configuration of a power generating device according to an embodiment of the present invention; 
         FIG.  12 A  is a perspective view illustrating the geometry of tire wear measuring devices of Example 1 and Example 2; 
         FIG.  12 B  is a perspective view illustrating the geometry of a tire wear measuring device of Comparative Example 1; and 
         FIG.  13    is a graph showing the ratios of outputs in Examples 1 and 2 to an output in Comparative Example 1. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. The same components are designated by the same reference signs in the figures, and redundant description is omitted as appropriate. 
       FIG.  7    is a schematic cross-sectional view of a tire wear measuring device according to an embodiment of the present invention mounted on a tire. As illustrated in  FIG.  7   , a tire wear measuring device  10  is disposed on an inner surface  21  of a tire  20 , and a magnetic object  30  is embedded in a tread portion  23  at an outer surface  22 . As the magnetic object  30  wears down together with the tread portion  23 , a magnetic field M, which is indicated by broken lines in  FIG.  7   , from the magnetic object  30  changes. The tire wear measuring device  10  can detect a wear state of the tread portion  23  by measuring the magnetic field M. For example, a wear state of the tire  20  can be measured based on a table previously storing changes of the magnetic field M associated with wear of the magnetic object  30  and a measured value of the magnetic field M. 
       FIG.  8    is a schematic cross-sectional view of the configuration of a related-art tire wear measuring device. As illustrated in  FIG.  8   , a tire wear measuring device  100  includes a coin-type battery  101 . Because the coin-type battery  101  includes a package made of a high-permeability soft magnetic material, a magnetic field created by the magnetic object  30  embedded in the tread portion  23  of the tire  20  tends to be guided to the coin-type battery  101 . For this reason, typically, the coin-type battery  101  is not disposed between the magnetic object  30  and magnetic sensors  102 A and  102 B. The magnetic sensors  102 A and  102 B are arranged closer to the magnetic object  30  than the coin-type battery  101 , and are positioned sufficiently away from the coin-type battery  101 . In other words, a distance D 1  from the magnetic object  30  to the coin-type battery  101  is greater than a distance D 2  from the magnetic object  30  to the magnetic sensors  102 A and  102 B so that the coin-type battery does not affect magnetic detection by the magnetic sensors  102 A and  102 B. Such arrangement of the coin-type battery  101  and the magnetic sensors  102 A and  102 B in  FIG.  8    leads to a complicated structure of the tire wear measuring device  100 , causing an increase in size of the device. 
       FIG.  9 A  is a vector map showing a magnetic field in the tire wear measuring device of  FIG.  8   .  FIG.  9 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  8   . The vector map shows the directions of magnetic field components and magnetic flux densities by using vectors. The contour map shows the magnetic flux densities of components of the magnetic field in a direction along the X axis as shades of gray. A darker shade of gray indicates a higher magnetic flux density. 
     As illustrated in  FIG.  9 A , in the related-art tire wear measuring device  100 , a high-magnetic-flux-density region is formed near the magnetic object  30 . The magnetic sensors  102 A and  102 B are arranged in this region to detect a magnetic flux from the magnetic object  30 . However, it has been found that a region in which the magnetic flux density of the components in the direction along the X axis is high is formed in proximity to an outer edge  101   e  of the coin-type battery  101  opposite from the magnetic object  30 , as illustrated in  FIG.  9 B . A magnetic field in this region is an emission magnetic field that is the magnetic field from the magnetic object  30  guided to the coin-type battery  101  and emitted through the coin-type battery  101 . The emission magnetic field changes as the magnetic field from the magnetic object  30  changes. Therefore, measuring the emission magnetic field enables detection of a change in magnetic field from the magnetic object  30 . 
     Specifically, the coin-type battery  101  can be disposed closer to the magnetic object  30  than the magnetic sensors  102 A and  102 B, and the magnetic sensors  102 A and  102 B can be arranged in a position where the emission magnetic field emitted from the outer edge  101   e  of the coin-type battery  101  is detectable by the sensors. Such a configuration enables measurement of a change in magnetic field from the magnetic object  30 . This configuration allows the coin-type battery  101  to be used as a magnetic collecting member (pseudo yoke), resulting in a simplified structure of the tire wear measuring device  100 . This enables a reduction in size and weight of the tire wear measuring device  100 . Embodiments of the present invention will now be described. 
       FIG.  1    a schematic cross-sectional view of the configuration of the tire wear measuring device according to the embodiment of the present invention. As illustrated in  FIG.  1   , the tire wear measuring device  10  according to the embodiment includes a magnetic sensor  12 A, a magnetic sensor  12 B, and a coin-type battery  11 , and detects a magnetic field from the magnetic object  30  embedded in the tire  20  to measure wear of the tire  20 . 
     The coin-type battery  11  is disposed in a position where the coin-type battery  11  can transmit the magnetic field from the magnetic object  30 . The coin-type battery  11  has an outer edge  11   e  from which the magnetic field from the magnetic object  30  is emitted as an emission magnetic field. The embodiment will describe an example in which a commonly used coin-type (button-type) battery is used as a power source of the tire wear measuring device  10 . A magnetic collecting member is not limited to the coin-type battery  11 . Any magnetic collecting member capable of transmitting a magnetic field from the magnetic object  30  can be used. As used herein, the term “magnetic collecting member capable of transmitting a magnetic field” refers to a component, such as a battery, that allows a magnetic field from the magnetic object  30  to be emitted as an emission magnetic field therefrom and that includes a portion made of a high-permeability soft magnetic material. The coin-type battery  11  includes a package (exterior) continuously extending from an electrode surface  11   d  to the outer edge  11   e , and the package is made of a soft magnetic material. When disposed in a position affected by the magnetic field from the magnetic object  30 , the coin-type battery  11  transmits and allows the magnetic field from the magnetic object  30  to be emitted, as an emission magnetic field, from the outer edge  11   e  located remote from the magnetic object  30 . 
     In this embodiment, the term “position affected by the magnetic field from the magnetic object  30 ” refers to a region where the magnetic flux density of the magnetic field from the magnetic object  30  is detectable. Although a magnetic flux density of more than 0 mT can be detected, a lower magnetic flux density is susceptible to noise. For example, it is preferred that the difference between multiple detection results be obtained to cancel out the influence of noise. As used herein, the term “being disposed in the position affected by the magnetic field from the magnetic object  30 ” refers to a state in which a magnetic-field transmitting portion made of a soft magnetic material is disposed in the position affected by the magnetic field. The coin-type battery  11  transmits and allows the magnetic field to be emitted, as the emission magnetic field, from the outer edge  11   e  as long as a portion of the electrode surface  11   d  that is made of the soft magnetic material is located in the position affected by the magnetic field. For this reason, the whole of the portion made of the soft magnetic material does not have to be disposed in the position affected by the magnetic field from the magnetic object  30 . 
     The electrode surface  11   d  of the coin-type battery  11  may face toward the magnetic object  30 . In other words, the electrode surface  11   d  is disposed to face toward the inner surface  21  of the tire  20  such that the direction (normal direction) of a normal  11 L to the electrode surface  11   d  is along the Y axis. Thus, the coin-type battery  11  can efficiently transmit the magnetic field from the magnetic object  30  and allow the magnetic field to be emitted, as the emission magnetic field, from the outer edge  11   e.    
     The magnetic sensors  12 A and  12 B are arranged in a position that is adjacent to a surface  11   f  of the coin-type battery  11  opposite from the magnetic object  30  and where the emission magnetic field emitted from the outer edge  11   e  is detectable by the sensors. The magnetic sensors  12 A and  12 B are arranged on the same plane. The emission magnetic field is emitted obliquely upward in  FIG.  1    from the outer edge  11   e . A region with a higher magnetic flux density than that of its surrounding region is formed in proximity to the outer edge  11   e . In other words, arranging the magnetic sensors  12 A and  12 B in the position where the emission magnetic field emitted from the outer edge  11   e  is detectable enables accurate detection of a change in magnetic field. To detect the emission magnetic field through the magnetic sensors  12 A and  12 B, for example, a magnetic flux density in a direction in which the emission magnetic field is detectable in the position of each of the magnetic sensors  12 A and  12 B is preferably 0.4 mT or more. In this embodiment, the term “magnetic flux density” as used herein refers to a magnetic flux density that is detected when the tire wear measuring device  10  is mounted on a new tire  20  prior to use. 
     In the use of the magnetic object  30  generating a magnetic field with a surface magnetic flux density of, for example, 26 mT, if the coin-type battery  11  is disposed in a position where the distance D 1  from the magnetic object  30  to the electrode surface  11   d  is in the range of approximately 10 to approximately 20 mm, the coin-type battery  11  can transmit the magnetic field. To accurately detect the emission magnetic field from the outer edge  11   e  through the magnetic sensors  12 A and  12 B, the magnetic sensors  12 A and  12 B are arranged such that a distance (LX) along the X axis from the outer edge  11   e  of the coin-type battery  11  is 2.8 mm or less, preferably 2.5 mm or less, more preferably 2.3 mm or less. For the same purpose, the magnetic sensors  12 A and  12 B are arranged such that a distance (LY) along the Y axis from the outer edge  11   e  of the coin-type battery  11  is 3.2 mm or less, preferably 2.9 mm or less, more preferably 2.7 mm or less. 
     As illustrated in  FIG.  1   , the distance D 2  along the Y axis between the magnetic object  30  and the magnetic sensors  12 A and  12 B is greater than the distance D 1  between the magnetic object  30  and the coin-type battery  11 . Accordingly, the magnetic sensors  12 A and  12 B do not directly detect the magnetic field from the magnetic object  30 . The magnetic sensors  12 A and  12 B detect the emission magnetic field transmitted through the coin-type battery  11  and emitted from the outer edge  11   e . In the embodiment, the term “distance between components” as used herein refers to a distance between portions of the components that are closest to each other. 
     For the magnetic sensors  12 A and  12 B, which measure the emission magnetic field from the outer edge  11   e , magnetoresistive elements each having a resistance that changes depending on the direction and strength of a magnetic field are used. Examples of the magnetoresistive element include a giant magnetoresistive (GMR) element and a tunneling magnetoresistive (TMR) element. Measurement by the magnetic sensors  12 A and  12 B does not have to be continuously performed in real-time, and may be intermittently performed at regular time intervals. Alternatively, measurement may be performed in response to an external instruction received through a radio communication unit (not illustrated). Measurement at regular time intervals or based on an instruction results in less power consumption than that in continuous measurement. Hall elements may be used to measure a change in strength of a magnetic flux, instead of the magnetoresistive elements as the magnetic sensors  12 A and  12 B. Magneto-impedance elements may be used as the magnetic sensors  12 A and  12 B to measure a change in impedance caused by a change in magnetic field. 
     The magnetic sensors  12 A and  12 B, which are configured to detect a magnetic flux density in the direction along the X axis, can accurately detect the emission magnetic field from the outer edge  11   e . The detection direction is not limited only to the direction along the X axis. The magnetic sensors  12 A and  12 B may be configured to detect magnetic fields along three axes (i.e., the X axis, the Y axis, and the Z axis) orthogonal to each other. In this case, each of the magnetic sensors  12 A and  12 B may include three sensor elements each detecting a magnetic field along one axis. In the embodiment, the magnetic sensors  12 A and  12 B are GMR sensors each including a GMR element in a mold package. 
     The tire wear measuring device  10  may output information on wear of the tire  20  based on magnetic-field measurement by the magnetic sensors  12 A and  12 B to an in-vehicle device through, for example, the radio communication unit. The tire wear measuring device  10  can transmit information on measurement results of the magnetic sensors  12 A and  12 B to the in-vehicle device and receive information from the in-vehicle device through the radio communication unit. The transmission and reception of information through communication between the tire wear measuring device  10  and external devices is controlled by a central processing unit (CPU) (not illustrated). 
     The tire wear measuring device  10  may include an antenna  13  for external communication. The antenna  13  has opposite ends  13   a  and  13   b  positioned in proximity to the magnetic sensors  12 A and  12 B, respectively, such that each end is disposed in a position where the emission magnetic field from the outer edge  11   e  can be guided to the end. The antenna  13  may serve as a waveguide, and emit and receive electromagnetic waves. The antenna  13  functions as a magnetic-field guiding member (yoke) that guides the emission magnetic field from the outer edge  11   e  of the coin-type battery  11 . The ends  13   a  and  13   b  of the antenna  13  are arranged in proximity to the outer edge  11   e  of the coin-type battery  11 , and each function as a magnetic-field guiding member (yoke) that guides the emission magnetic field. The magnetic sensors  12 A and  12 B are arranged between the outer edge  11   e  and the ends  13   a  and  13   b.    
       FIG.  2 A  is a vector map showing a magnetic field in the tire wear measuring device of  FIG.  1   .  FIG.  2 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  1   . As illustrated in  FIGS.  2 A and  2 B , the tire wear measuring device  10  is configured such that the coin-type battery  11  is disposed in a position where the coin-type battery  11  can transmit the magnetic field from the magnetic object  30  and such that each of the magnetic sensors  12 A and  12 B is disposed in a position where the emission magnetic field from the outer edge  11   e  of the coin-type battery  11  is detectable by the sensor. Such a configuration allows the coin-type battery  11  to function as a pseudo yoke and also allows the magnetic sensors  12 A and  12 B to detect the emission magnetic field from the outer edge  11   e . Disposing the coin-type battery  11  between the magnetic object  30  and the magnetic sensors  12 A and  12 B in the above-described manner enables a reduction in size and weight of the tire wear measuring device  10  and accurate measurement of a magnetic field from the magnetic object  30 . 
       FIG.  3 A  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  1   .  FIG.  3 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  8   . 
     As illustrated in  FIG.  3 A , in the tire wear measuring device  10  according to the embodiment, regions in which the magnetic flux density of components in the direction along the X axis is high are formed between the outer edge  11   e  of the coin-type battery  11  and the ends  13   a  and  13   b  of the antenna  13 . These regions are formed by the emission magnetic field guided from the outer edge  11   e  of the coin-type battery  11  to the ends  13   a  and  13   b  of the antenna  13 . The arrangement of the magnetic sensors  12 A and  12 B each having a sensitivity axis in the direction along the X axis in these regions enables accurate detection of the magnetic field from the magnetic object  30 . Furthermore, since the magnetic sensors  12 A and  12 B are arranged between the outer edge  11   e  of the coin-type battery  11  and the ends  13   a  and  13   b  of the antenna  13 , each of the magnetic sensors  12 A and  12 B is reliably disposed in a position where the emission magnetic field is detectable by the sensor. 
     As illustrated in  FIG.  3 B , in the related-art tire wear measuring device  100 , the magnetic sensors  102 A and  102 B are arranged closer to the magnetic object  30  than the coin-type battery  11  and directly detect the magnetic field from the magnetic object  30 . Such a configuration makes it difficult to reduce the size and weight of the measuring device. 
       FIG.  4    is a schematic diagram illustrating the positional relationship between the magnetic object  30 , the coin-type battery  11 , the magnetic sensors  12 A and  12 B, and the ends  13   a  and  13   b  of the antenna  13  in the tire wear measuring device of  FIG.  1   .  FIG.  4    schematically illustrates the positional relationship in plan view in the direction along the Y axis, or the direction along the normal  11 L (refer to  FIG.  1   ) to the electrode surface  11   d  of the coin-type battery  11 . In other words, the electrode surface  11   d  faces away from the viewer of  FIG.  4   . As illustrated in  FIG.  4   , preferably, the end  13   a  of the antenna  13  adjacent to the magnetic sensor  12 A and the end  13   b  thereof adjacent to the magnetic sensor  12 B are arranged outside the outer edge  11   e  of the coin-type battery  11 , or so as not to overlap the coin-type battery  11 . Preferably, the magnetic sensor  12 A is disposed between the outer edge  11   e  of the coin-type battery  11  and the end  13   a  of the antenna  13 , and the magnetic sensor  12 B is disposed between the outer edge  11   e  of the coin-type battery  11  and the end  13   b  of the antenna  13 . 
     Referring to  FIG.  4   , in plan view in the direction along the Y axis, the outer edge  11   e  of the coin-type battery  11 , the magnetic sensor  12 A, and the end  13   a  of the antenna  13  are arranged so as not to overlap each other, and the outer edge  11   e  of the coin-type battery  11 , the magnetic sensor  12 B, and the end  13   b  of the antenna  13  are arranged so as not to overlap each other. The arrangement of  FIG.  4    is an example. The magnetic sensors  12 A and  12 B only have to be arranged between the outer edge  11   e  of the coin-type battery  11  and the ends  13   a  and  13   b  of the antenna  13 . For example, in plan view in the direction along the Y axis, the magnetic sensor  12 A, the outer edge  11   e , and a part or whole of the end  13   a  may overlap each other, and the magnetic sensor  12 B, the outer edge  11   e , and a part or whole of the end  13   b  may overlap each other. 
     The tire wear measuring device  10  may include the magnetic sensor  12 A and the magnetic sensor  12 B. The magnetic sensor  12 A may be disposed at one side of the coin-type battery  11  in the direction along the X axis parallel to the electrode surface  11   d  of the coin-type battery  11 , and the magnetic sensor  12 B may be disposed at the other side thereof. In the embodiment, as illustrated in  FIG.  4   , the electrode surface  11   d  of the coin-type battery  11  may have a center O aligned with the magnetic object  30  embedded in the tire  20  and located on a straight line L connecting the magnetic sensor  12 A and the magnetic sensor  12 B. 
     The straight line L connecting the magnetic sensor  12 A and the magnetic sensor  12 B is parallel to the X axis. Each of the ends  13   a  and  13   b  of the antenna  13  is located on the straight line L. The magnetic sensors  12 A and  12 B are symmetrically arranged with respect to the center O of the electrode surface  11   d  of the coin-type battery  11 . The ends  13   a  and  13   b  of the antenna  13  are symmetrically arranged with respect to the center O of the electrode surface  11   d  of the coin-type battery  11 . 
     The magnetic sensor  12 A and the end  13   a  may be arranged at one side of the coin-type battery  11 , and the magnetic sensor  12 B and the end  13   b  may be arranged at the other side thereof. The coin-type battery  11  is superposed on the magnetic object  30  as viewed in the direction along the Y axis. 
     The above-described configuration causes an emission magnetic field Ma detected by the magnetic sensor  12 A and an emission magnetic field Mb detected by the magnetic sensor  12 B to have the same magnetic flux density and be opposite in orientation to each other. Therefore, wear of the tire  20  can be detected based on an output of the magnetic sensor  12 A and an output of the magnetic sensor  12 B, resulting in improved redundancy of the tire wear measuring device  10 . 
     The magnetic sensors  12 A and  12 B are similarly affected by an external magnetic field, serving as noise in measurement. For this reason, the difference between the outputs from these two sensors can be used to eliminate the influence of the external magnetic field. Since the outputs from the two sensors are based on the magnetic fields oriented in opposite directions, the use of the difference between the outputs provides an output whose magnitude is two times the magnitude of an output from one sensor in addition to elimination of the influence of noise. Therefore, the influence of noise, such as an external magnetic field, can be eliminated, and an output can be increased in magnitude, thus achieving accurate measurement of wear of the tire  20 . 
     The magnetic object  30  includes a polymeric material and a hard magnetic particulate material (magnetic particles) dispersed in the polymeric material and magnetized in one direction. The magnetic object  30  is embedded in the tread portion such that the direction of magnetization is aligned with a radial direction of the tire. For the polymeric material, for example, a rubber material having the same formulation as that of a tread rubber composition for the tread portion is preferably used. 
     The magnetic object  30  preferably has a magnetic flux density of 1 mT or more at the surface thereof. In terms of achieving reliable measurement of the magnetic flux density of the magnetic object without being affected by the magnetism of the earth, the magnetic object  30  has a magnetic flux density of preferably 0.05 mT or more, more preferably 0.5 mT or more at measurement locations where the magnetic sensors  12 A and  12 B are arranged. 
     In terms of keeping a magnetic force from the magnetic object  30  from adversely affecting, for example, other in-vehicle electronic devices, the magnetic object  30  preferably has a surface magnetic flux density of 600 mT or less. In terms of keeping the magnetic object  30  from attracting a piece of metal, such as a nail, on a road in traveling on the road, the magnetic object  30  more preferably has a surface magnetic flux density of 60 mT or less. The surface magnetic flux density of the magnetic object is a value measured by a tesla meter in direct contact with the magnetized magnetic object  30 . 
     Modification 
       FIG.  5    is a schematic cross-sectional view of the configuration of a modification of the tire wear measuring device according to the embodiment. A tire wear measuring device  50  of  FIG.  5    is configured such that the magnetic sensors  12 A and  12 B and the ends  13   a  and  13   b  of the antenna  13  are arranged on a surface  51   d , which faces the coin-type battery  11 , of a substrate  51 . The tire wear measuring device  50  differs from the tire wear measuring device  10  in the above-described configuration. In other words, the tire wear measuring device  50  includes the magnetic sensors  12 A and  12 B and the ends  13   a  and  13   b  of the antenna  13  located on the same plane of the substrate  51  parallel to the electrode surface  11   d  of the coin-type battery  11 . 
       FIG.  6 A  is a vector map showing a magnetic field in the tire wear measuring device of  FIG.  5   .  FIG.  6 B  is a contour map showing sensor-detectable components of the magnetic field in the tire wear measuring device of  FIG.  5   . As illustrated in  FIGS.  6 A and  6 B , such a configuration, in which the magnetic sensors  12 A and  12 B and the ends  13   a  and  13   b  of the antenna  13  are located on the same plane, allows an emission magnetic field from the outer edge  11   e  of the coin-type battery  11  to be guided to the ends  13   a  and  13   b  of the antenna  13  and be oriented in the direction along the X axis in which the emission magnetic field is detectable by the magnetic sensors  12 A and  12 B on the substrate  51 . This leads to higher detection accuracy of the tire wear measuring device  50 . 
       FIG.  10    is a schematic cross-sectional view of the configuration of another modification of the tire wear measuring device according to the embodiment. As illustrated in  FIG.  10   , a tire wear measuring device  60  includes a coil  61  disposed within the range of a magnetic field from the magnetic object  30  and can use, as an operational power source, an induction current generated in the coil  61  by rotation of the tire  20 . The tire wear measuring device  60  differs from the tire wear measuring device  10  (refer to  FIG.  1   ) in the above-described configuration. The magnetic object  30  embedded in the tread portion  23  of the tire  20  is displaced due to deformation of the tire  20  associated with its rotation. The tire wear measuring device  60  uses the displacement of the magnetic object  30  for electric power generation. 
     The coil  61  may be disposed between the coin-type battery  11 , which also functions as a magnetic collecting member, and the magnetic object  30  embedded in the tread portion  23  of the tire  20 . The magnetic flux density of a magnetic field from the magnetic object  30  passing through the coil  61  changes due to deformation or vibration of the tire  20  associated with rotation of the tire  20 . A change in magnetic flux density in the direction along the Y axis, indicated by an outlined double-headed arrow in  FIG.  10   , causes an induction current in the coil  61 , which in turn produces an electromotive force. The electromotive force is used as a power source of the tire wear measuring device  60  for, for example, activation of the tire wear measuring device  60  or communication. 
     In the tire wear measuring device  60 , a change in magnetic flux density of the magnetic field from the magnetic object  30  associated with rotation or vibration of the tire  20  is used to generate an induction current in the coil  61 , thus generating electric power. Therefore, the tire wear measuring device  60  can be kept small and lightweight, and generated electric power can be used for various applications. As described above, a change of the relative positional relationship between the magnetic object  30  and the coil  61  caused by rotation of the tire  20  is converted into and used as electrical energy. This reduces a power consumption burden on the coin-type battery  11 . 
     For a magnetic field between the magnetic object  30  and the coin-type battery  11 , the surface of the coin-type battery  11  collects the magnetic field, thus increasing the magnetic flux density of components in the direction along the Y axis of the magnetic field. For a typical magnetic flux passing through a coil, as the magnetic flux density of components (components in the direction along the Y axis in  FIG.  10   ) of the magnetic flux that are orthogonal to the coil is higher, the magnetic flux through the coil changes more significantly, so that a larger induction current is generated in the coil. For this reason, when viewed from the magnetic object  30 , the coil  61  is disposed adjacent to the coin-type battery  11  and is positioned closer to the magnetic object  30  than the coin-type battery  11 . This increases an induction current that is generated in the coil  61  with rotation of the tire  20 , thus enabling efficient electric power generation. 
     An electromotive force produced in the coil  61  changes depending on the magnitude of or degree of change in the magnetic flux density of a magnetic field from the magnetic object  30 . In other words, an electromotive force produced in the coil  61  reflects a state of the tire  20 . For this reason, an electromotive force produced in the coil  61  may be used to detect a state of the tire  20 . For example, as the magnetic object  30  embedded in the tread portion  23  decreases in size due to wear of the tire  20 , the magnetic flux density of the magnetic field from the magnetic object  30  decreases, so that a change in magnetic flux density of the magnetic field from the magnetic object  30  associated with rotation of the tire  20  also decreases. Therefore, an electromotive force (induction current) produced in the coil  61  changes depending on the degree of wear of the tire  20 . For this reason, wear of the tire  20  can be detected based on an electromotive force produced in the coil  61 . In this case, the tire wear measuring device  60  may exclude the magnetic sensors  12 A and  12 B in  FIG.  10   . 
     The coil  61  is electrically connected to a power-storing component (not illustrated). The power-storing component includes a rectifying circuit and a charging circuit, and allows an induction current generated due to a change in magnetic flux density to be charged in a capacitor. For the rectifying circuit, for example, a rectifying element including a circuit that rectifies an induction current (alternating current) generated in the coil  61  can be used. For the charging circuit, for example, a capacitive capacitor that stores electric charge of an induction current can be used. Using electric power stored in the capacitor for activation of the tire wear measuring device  60  or communication reduces the burden on the coin-type battery  11 , thus extending a period (service life) during which the tire wear measuring device  60  can be continuously used. If the coin-type battery  11  is a secondary battery, the coin-type battery  11  may be used as a power-storing component, and the above-described capacitor may be omitted. In this case, the power-storing component includes a charging and discharging circuit instead of the charging circuit. 
       FIG.  10    illustrates an exemplary configuration in which the magnetic object  30  deforms as the tire  20  deforms. The configuration is not limited to this example. The tire wear measuring device  60  may have any configuration as long as deformation of the tire  20  causes a change in magnetic flux density of a magnetic field from the magnetic object  30  in the coil  61 . For example, the tire wear measuring device  60  may have a configuration in which deformation of the tire  20  causes not the magnetic object  30  but the coil  61  to be deformed to cause a change in magnetic flux density of the magnetic field from the magnetic object  30  in the coil  61 . 
       FIG.  11    is a schematic cross-sectional view of the configuration of a power generating device according to an embodiment of the present invention. As illustrated in  FIG.  11   , a power generating device  70  includes the magnetic object  30  embedded in the tire  20  and the coil  61  disposed within the range of a magnetic field from the magnetic object  30 . A relative positional relationship between the magnetic object  30  and the coil  61  changes with rotation of the tire  20 . A change of the relative positional relationship causes a change in magnetic flux density of the magnetic field passing through the coil  61 , thus generating electric power. 
     The power generating device  70  further includes the coin-type battery  11  of a secondary battery type, serving as a magnetic member. The coil  61  is disposed between the coin-type battery  11  and the magnetic object  30 . Such a configuration can increase an induction current that is generated in the coil  61  with rotation of the tire  20 , resulting in efficient power generation. 
     Although the antenna  13  is illustrated in  FIG.  11   , the antenna  13  may be removed from the configuration if it is not required. For example, the power generating device  70  may supply power to a sensor separate from the power generating device  70 , and the sensor may have an antenna for communication. 
     The above-described embodiments are intended for easy understanding of the present invention and are not intended to limit the scope of the present invention. Therefore, the components disclosed in the above embodiments are intended to be construed as including all design changes and equivalents belonging to the technical scope of the present invention. For example, the power generating device  70  of  FIG.  11    may include a measurement unit that measures a change in amplitude or frequency of an electromotive force produced in the coil  61  and may detect a tire state including a wear state based on a signal obtained by the measurement unit. If an electromotive force produced in the coil  61  is a signal source to be measured by the measurement unit, the power generating device  70  may serve as a detection device that outputs information on a tire state. Although the coin-type battery  11  functions as a magnetic member in the power generating device  70 , another magnetic member different from the coin-type battery  11  may be provided. If the power generating device  70  functions as a detection device, the coin-type battery  11  may be excluded. 
     EXAMPLES 
     To determine how much an output in the tire wear measuring device  10  according to the embodiment having the configuration illustrated in  FIG.  1    and an output in the tire wear measuring device excluding the antenna  13  are higher than that in the related-art tire wear measuring device  100  of  FIG.  8   , the outputs were measured and compared. The devices each included the magnetic object  30  having a surface magnetic flux density of 26 mT. 
       FIG.  12 A  is a perspective view illustrating the appearance and schematic internal structure of tire wear measuring devices of Example 1 and Example 2.  FIG.  12 B  is a perspective view illustrating the appearance and schematic internal structure of a tire wear measuring device of Comparative Example 1. Table 1 and  FIG.  13    illustrate the measured outputs. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Comparative 
               
               
                   
                 without yoke 
                 with yoke 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 D1 (mm) 
                 16.0 
                 16.0 
                 20.7 
               
               
                 D2 (mm) 
                 21.2 
                 21.2 
                 17 
               
               
                 Size (mm) 
                 28 × 36 × 15 
                 28 × 36 × 15 
                 36 × 47 × 24 
               
               
                 Volume Ratio (%) 
                 37.2 
                 37.2 
                 100.0 
               
               
                 Weight (g) 
                 21 
                 21 
                 42 
               
               
                 Weight Ratio (%) 
                 50.0 
                 50.0 
                 100.0 
               
               
                 Output Ratio (%) 
                 123.0 
                 145.7 
                 100.0 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 1 and  FIG.  13   , the configuration in which the coin-type battery transmits a magnetic field from the magnetic object and the magnetic sensors detect an emission magnetic field emitted from the outer edge of the coin-type battery allows a remarkable reduction in volume and weight as compared with the related-art configuration, and also achieves improved detection accuracy due to higher output. 
     The present invention is applicable to a tire wear measuring device capable of measuring a wear state of a tire without visual observation.