Patent Publication Number: US-2023139686-A1

Title: Tire revolution direction determination system

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-177599 filed with the Japan Patent Office on Oct. 29, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a system that determines a direction of revolution of a tire attached to a vehicle. 
     Description of the Background Art 
     A direct tire pressure monitoring system (TPMS) has conventionally been available as one of tire pressure monitoring systems. In the TPMS of this type, a detection device provided with a sensor such as a pressure sensor is directly attached to a vehicle wheel side where a tire is attached. On a vehicle body side, an antenna and a receiver are provided. When a detection signal from the sensor is transmitted from the detection device on the vehicle wheel side, the receiver receives the detection signal through the antenna and a tire pressure is detected. 
     For such a direct TPMS, various apparatuses with a function to determine in which tire of a vehicle a detection device attached to the tire is provided have been developed. For example, Japanese Patent Laying-Open No. 2019-48547 discloses a system that determines to which tire of double tires employed in a truck and the like a detection device is attached. This system includes a sensor that detects an acceleration in a tire revolution circumferential direction, determines a direction of revolution of the tire based on a result of detection of the acceleration in the tire revolution circumferential direction, and determines in which of the double tires a detector is provided based on a result of determination of the direction of revolution of the tire. 
     SUMMARY OF THE INVENTION 
     The system disclosed in Japanese Patent Laying-Open No. 2019-48547 determines the direction of revolution of the tire where the detector is arranged based on the result of detection of the acceleration in the tire revolution circumferential direction. The acceleration in the tire revolution circumferential direction, however, cannot be detected unless the vehicle is accelerating. In other words, the system disclosed in Japanese Patent Laying-Open No. 2019-48547 is unable to determine the direction of revolution of the tire unless the vehicle is accelerating. 
     The present disclosure was made to solve the problem described above, and an object thereof is to enable determination of a direction of revolution of a tire even when a vehicle is in a constant-velocity traveling state in which the vehicle is traveling at a constant velocity. 
     A tire revolution direction determination system according to one aspect of the present disclosure is a tire revolution direction determination system that determines a direction of revolution of a tire attached to a vehicle, and the tire revolution direction determination system includes a detection device arranged in the tire and a determination device configured to obtain information from the detection device. The detection device includes a first sensor arranged in the tire, the first sensor detecting a first acceleration in a tire diameter direction, and a second sensor arranged in the tire, the second sensor detecting a second acceleration in the tire diameter direction. The tire includes a wheel portion including a first-side surface and a second-side surface on a rear side of the first-side surface. With a direction of clockwise revolution when the tire is viewed from the second-side surface being defined as a reference direction, the first sensor is arranged in front of the second sensor in the reference direction. When the determination device receives the first acceleration and the second acceleration from the detection device while the vehicle travels forward, the determination device specifies change over time of the first acceleration and change over time of the second acceleration, and determines whether the direction of revolution of the tire where the detection device is arranged is the reference direction based on whether change over time of the first acceleration is more advanced than change over time of the second acceleration. 
     According to the aspect above, whether or not the direction of revolution of the tire is the reference direction is determined based not on the acceleration in a tire revolution circumferential direction but on the acceleration (the first acceleration and the second acceleration) in the tire diameter direction. Therefore, even when the vehicle is in the constant-velocity traveling state, the direction of revolution of the tire can be determined. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram schematically showing a configuration of a vehicle. 
         FIG.  2    is a block diagram showing a configuration of a detection device. 
         FIG.  3    is a diagram for illustrating a direction of detection of an acceleration by a first detector and a second detector. 
         FIG.  4    is a diagram schematically showing a waveform of a component of an acceleration of gravity superimposed on a first acceleration G 1  and a second acceleration G 2  when a vehicle is traveling forward at a constant vehicle velocity. 
         FIG.  5    is an exploded perspective view of double tires on a rear right side. 
         FIG.  6    is a flowchart showing an exemplary procedure of processing by a monitoring unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present disclosure will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     &lt;Overall Configuration&gt; 
       FIG.  1    is a diagram schematically showing a configuration of a vehicle  10  to which a tire revolution direction determination system according to the present embodiment is applied. 
     Vehicle  10  according to the present embodiment is a vehicle including a single tire as a front wheel which is a steering wheel and double (twin or dual) tires as a rear wheel which is a non-steering wheel. The single tire refers to a form of attachment of a single tire at one tire attachment position. The double tires refer to a form of attachment of two tires of the same size coupled to each other at one tire attachment position. The double tires are mainly adopted in a large-sized vehicle such as a truck or a bus. 
     Specifically, vehicle  10  includes front tires  11  and  12  and rear double tires  21  and  22 . 
       FIG.  1    illustrates an example in which vehicle  10  is a rear-wheel drive type. Rear double tires  21  and  22  are attached to an axle R 1  on a rear left side and an axle R 2  on a rear right side, respectively. Vehicle  10  is not limited to the rear-wheel drive type but may be a front-wheel drive type or an all-wheel drive type. 
     Double tires  21  on the rear left side include a tire  21   a  on a vehicle inner side and a tire  21   b  on a vehicle outer side. Double tires  22  on the rear right side include a tire  22   a  on the vehicle inner side and a tire  22   b  on the vehicle outer side. 
     Vehicle  10  further includes a system that monitors a pneumatic pressure of each tire (TPMS). Specifically, vehicle  10  includes a detection device  30  arranged in each of six tires  11 ,  12 ,  21   a,    21   b,    22   a,  and  22   b  and a TPMS receiver  40 . For example, detection device  30  may be formed integrally with a valve for intake of air into each tire. 
     Detection device  30  is activated when a prescribed activation condition is satisfied, and detects a tire pressure and outputs a radio signal in an ultra high frequency (UHF) band (which is also simply referred to as a “UHF signal” below) that includes a result of detection. The “prescribed activation condition” is set in advance for each detection device  30  to be satisfied regularly or irregularly. Detection devices  30  can thus intermittently be activated at timings different from one another. 
     The UHF signal outputted from each detection device  30  includes not only information indicating a tire pressure but also information indicating a specific ID number for identifying at least each detection device  30 . As TPMS receiver  40  receives the UHF signal outputted from each detection device  30 , it can monitor a pneumatic pressure of each tire. 
     Tires identical in specifications and construction are employed as tires  11 ,  12 ,  21   a,    21   b,    22   a,  and  22   b  for allowing tire rotation. Therefore, detection devices identical in configuration are adopted also for detection devices  30 . 
     Each detection device  30  includes a first detector  31  and a second detector  32 . A configuration of detection device  30  will be described in detail later. 
     TPMS receiver  40  is provided on a vehicle body side of vehicle  10 . TPMS receiver  40  includes an antenna A 1  and a monitoring unit  45 . Antenna A 1  is configured to receive a UHF signal transmitted from each detection device  30 . Monitoring unit  45  monitors a pneumatic pressure of each tire based on the UHF signal received by antenna A 1 . Monitoring unit  45  includes a storage  46  and a processing unit  47 . 
     Processing unit  47  includes a processor such as a not-shown central processing unit (CPU), a memory, and an input and output buffer. The memory includes a read only memory (ROM) and a random access memory (RAM). The processor develops a program stored in the ROM on the RAM and executes the same. Various types of processing performed by processing unit  47  are described in the program stored in the ROM. 
     Information indicating a position of a tire where each detection device  30  is arranged and information indicating a tire pressure are stored in storage  46  as being brought in correspondence with an ID number of each detection device  30 . In the present embodiment, six tire positions (a front left side, a front right side, a rear left inner side, a rear left outer side, a rear right inner side, and a rear right outer side) in total are set in advance and an ID number of each detection device  30  is brought in correspondence with any one tire position. 
     When monitoring unit  45  receives a UHF signal from each detection device  30 , it specifies the tire position based on the ID number included in the UHF signal by referring to the information stored in storage  46  and updates the pneumatic pressure at the specified tire position with the tire pressure included in the UHF signal. 
     TPMS receiver  40  can have information on correspondence between the tire position and the tire pressure stored in storage  46  shown on a display  60 . Display  60  is arranged at a position where a driver can visually recognize the same. Display  60  is arranged, for example, in an instrument panel within the vehicle. 
     When the tire pressure included in the received UHF signal is equal to or lower than a low-pressure threshold value, monitoring unit  45  has the tire position where the tire pressure is equal to or lower than the low-pressure threshold value shown on display  60  together with a warning. TPMS receiver  40  performs processing for determining the tire pressure for each received UHF signal and monitors each pneumatic pressure of each tire. The driver can thus recognize in real time the position of the tire the tire pressure of which has become equal to or lower than the low-pressure threshold value. 
     &lt;Configuration of Detection Device  30 &gt; 
       FIG.  2    is a block diagram showing a configuration of detection device  30 . Detection device  30  includes first detector  31  and second detector  32  as described above. First detector  31  and second detector  32  are electrically connected to each other through a connection line L 1 . 
     First detector  31  includes a controller  35 , a pressure sensor  38 , an acceleration sensor (G sensor)  39 , an antenna A 2 , and a transmission circuit CT. 
     Pressure sensor  38  detects a tire pressure and outputs a result of detection to controller  35 . Acceleration sensor  39  is a uniaxial acceleration sensor that detects an acceleration in one direction. Acceleration sensor  39  detects an acceleration applied to first detector  31  and outputs a result of detection (which is also referred to as a “first acceleration G 1 ” below) to controller  35 . 
     Detection device  30  may further include a temperature sensor that detects a tire temperature in addition to pressure sensor  38  and acceleration sensor  39   
     Controller  35  of first detector  31  includes a storage  36  and a processing unit  37 . Processing unit  37  includes a processor such as a not-shown CPU, a memory, and an input and output buffer. The memory includes a ROM and a RAM. The processor develops a program stored in the ROM on the RAM and executes the same. Various types of processing performed by processing unit  37  are described in the program stored in the ROM. 
     Information indicating an ID number specific for each detection device  30 , a result of detection by pressure sensor  38 , and a result of detection (first acceleration G 1 ) by acceleration sensor  39  of first detector  31  is stored in storage  36  of first detector  31 . 
     Second detector  32  includes controller  35  and acceleration sensor (G sensor)  39 . Second detector  32  is a detector without antenna A 2 , transmission circuit CT, and pressure sensor  38  provided in first detector  31 . 
     Similarly to acceleration sensor  39  of first detector  31 , acceleration sensor  39  of second detector  32  is also a uniaxial acceleration sensor that detects an acceleration in one direction. Acceleration sensor  39  of second detector  32  detects an acceleration applied to second detector  32  and outputs a result of detection (which is also referred to as a “second acceleration G 2 ” below) to controller  35  of second detector  32 . 
     Controller  35  of second detector  32  includes storage  36  and processing unit  37  similar to those of controller  35  of first detector  31 . Information indicating a result of detection (second acceleration G 2 ) by acceleration sensor  39  of second detector  32  is stored in storage  36  of second detector  32 . 
     Controller  35  of second detector  32  is configured to output information on second acceleration G 2  stored in storage  36  of second detector  32  to first detector  31 . 
     Controller  35  of first detector  31  controls transmission circuit CT to output a UHF signal from antenna A 2 . The UHF signal includes an ID number stored in storage  36 , information indicating a tire pressure, information indicating first acceleration G 1 , and information indicating second acceleration G 2 . 
     &lt;Direction of Detection of Acceleration by First Detector  31  and Second Detector  32 &gt; 
       FIG.  3    is a diagram for illustrating a direction of detection of an acceleration by first detector  31  and second detector  32 .  FIG.  3    illustratively shows arrangement of first detector  31  and second detector  32  when tire  22   b  on the rear right outer side is seen through from the vehicle outer side. Arrangement of first detector  31  and second detector  32  is the same also in tires other than tire  22   b.    
     First detector  31  and second detector  32  are fixed to an outer circumferential surface of a wheel WH of each tire. First detector  31  and second detector  32  are arranged at positions distant by a prescribed angle of revolution θ from each other. 
     Each of first detector  31  and second detector  32  detects an acceleration (a centrifugal acceleration) in a tire diameter direction. Acceleration sensor  39  of first detector  31  and acceleration sensor  39  of second detector  32  are configured to detect an acceleration applied in a direction away from a revolution center of the tire as a positive value and to detect an acceleration applied in a direction toward the revolution center of the tire as a negative value. In this case, each of accelerations G 1  and G 2  detected by acceleration sensors  39  has a value resulting from superimposition of a component of an acceleration of gravity that varies with an angle of revolution of the tire on a centrifugal acceleration of the tire. 
     For example, when first detector  31  is located at a position at “twelve o&#39;clock” as shown in  FIG.  3   , the component of the acceleration of gravity superimposed on first acceleration G 1  detected by first detector  31  is “−1 G” (G: acceleration of gravity) and the component of the acceleration of gravity superimposed on second acceleration G 2  detected by second detector  32  is “−1 G● cos θ”. 
     Though  FIG.  3    shows an example in which first detector  31  and second detector  32  are arranged on the outer circumferential surface of wheel WH of each tire, first detector  31  and second detector  32  should only be arranged at positions where the acceleration (centrifugal acceleration) in the tire diameter direction can be detected, and the positions of arrangement of first detector  31  and second detector  32  are not necessarily limited to positions on the outer circumferential surface of wheel WH. For example, first detector  31  and second detector  32  may be arranged on an inner wall of a rubber portion of each tire or an outer surface of the rubber portion of each tire. 
       FIG.  4    is a diagram schematically showing a waveform of a component of an acceleration of gravity superimposed on first acceleration G 1  and second acceleration G 2  when vehicle  10  is traveling forward at a constant vehicle velocity. As shown in  FIG.  4   , while vehicle  10  is traveling forward at a constant vehicle velocity, that is, while the tire is revolving in a forward revolution direction at the constant velocity, the component of the acceleration of gravity superimposed on first acceleration G 1  and second acceleration G 2  has a sinusoidal waveform with a time period for one revolution of the tire being defined as one period P. 
     Change over time of the component of the acceleration of gravity superimposed on first acceleration G 1  and change over time of the component of the acceleration of gravity superimposed on second acceleration G 2  deviate from each other by a time difference Q corresponding to a prescribed angle of revolution θ. 
     Though a waveform outputted from each acceleration sensor  39  has a value calculated by adding the centrifugal acceleration to the component of the acceleration of gravity, the centrifugal acceleration does not periodically vary in one revolution of the tire. Therefore, change over time of the component of the acceleration of gravity superimposed on first acceleration G 1  is also simply referred to as “change over time of first acceleration G 1 ” below and change over time of the component of the acceleration of gravity superimposed on second acceleration G 2  is also simply referred to as “change over time of second acceleration G 2 ” below. 
     Prescribed angle of revolution θ should only be smaller than 180° and more preferably smaller than 90°. In the present embodiment, prescribed angle of revolution θ is set to approximately 30°. By thus setting angle of revolution θ, monitoring unit  45  or controller  35  can know relation (retardation and advance) between change over time of first acceleration G 1  and change over time of second acceleration G 2  as will be described later. 
     &lt;Construction of Double Tires&gt; 
     Vehicle  10  according to the present embodiment includes double tires  21  and  22  as the rear wheels which are non-steering wheels as described above. 
       FIG.  5    is an exploded perspective view of double tires  22  on the rear right side. An exemplary construction of double tire  22  will be described with reference to  FIG.  5   . Double tires  21  on the rear left side are also similar in construction to double tires  22 . 
     Double tires  22  include tire  22   a  on the vehicle inner side (which is also referred to as an “inner tire  22   a ” below) and tire  22   b  on the vehicle outer side (which is also referred to as an “outer tire  22   b ” below). Wheel WH of each of tires  22   a  and  22   b  includes a flat portion FP that protrudes on the outer side relative to a side surface portion (a sidewall portion) of each tire. Wheel WH includes a first-side surface on a side where flat portion FP protrudes and a second-side surface on a rear side of the first-side surface. 
     Inner tire  22   a  is fixed to axle R 2  by being fastened by an inner nut NIN by insertion of a bolt BT of a hub H 2  of axle R 2  into a hole provided in flat portion FP of wheel WH. A tip end side (vehicle outer side) of inner nut NIN is threaded. 
     Outer tire  22   b  is fixed to inner tire  22   a  by being fastened by a wheel nut NW by insertion of the thread on the tip end side of inner nut NIN into the hole provided in flat portion FP of wheel WH. Inner tire  22   a  and outer tire  22   b  are fixed while flat portions FP of wheels WH face each other. Inner tire  22   a  and outer tire  22   b  are thus connected to axle R 2  as being coupled back-to-back. 
     Therefore, with a clockwise direction when each wheel WH is viewed from the side of the second-side surface being defined as the “reference direction,” when vehicle  10  travels forward, the direction of revolution of outer tire  22   b  is the same as the reference direction whereas the direction of revolution of inner tire  22   a  is the direction reverse to the reference direction. 
     As shown in  FIG.  5   , in each of inner tire  22   a  and outer tire  22   b,  first detector  31  is arranged in front of second detector  32  in the reference direction. Therefore, while vehicle  10  travels forward, in outer tire  22   b  that revolves in the reference direction, change over time of first acceleration G 1  is more advanced than change over time of second acceleration G 2 , whereas in inner tire  22   a  that revolves in the direction reverse to the reference direction, change over time of first acceleration G 1  is lagged behind change over time of second acceleration G 2 . 
     The “reference direction” described above may be set to the clockwise direction when each wheel WH is viewed from a side of the first-side surface, and in that case, relation between the reference direction and the direction of revolution of each of tires  22   a  and  22   b  is opposite. 
     So long as three conditions below are satisfied, a condition of arrangement of first detector  31  and second detector  32  is not necessarily limited to the condition of arrangement shown in  FIGS.  3  and  5   . 
     (Arrangement Condition 1) 
     When each wheel WH is viewed from the side of the first-side surface (the side where flat portion FP protrudes), a positive direction of the acceleration is defined in the same direction. 
     (Arrangement Condition 2) 
     When each wheel WH is viewed from the side of the first-side surface, the detectors are attached to the same surface. 
     (Arrangement Condition 3) 
     When each wheel WH is viewed from the side of the first-side surface, positional relation with respect to the reference direction is unified. 
     &lt;Determination of Direction of Revolution and Determination as to Inner Side or Outer Side of Double Tires&gt; 
     As described above, while vehicle  10  travels forward, in outer tire  22   b,  change over time of first acceleration G 1  is more advanced than change over time of second acceleration G 2 , whereas in inner tire  22   a,  change over time of first acceleration G 1  is lagged behind change over time of second acceleration G 2 . 
     In view of this fact, each detection device  30  according to the present embodiment is configured to output to monitoring unit  45 , a result of detection of first acceleration G 1  and second acceleration G 2  at a plurality of (at least two) consecutive timings. Then, when monitoring unit  45  receives the result of detection from detection device  30 , it performs processing for determining whether that detection device  30  is arranged in the inner tire or the outer tire of the double tires (which is also referred to as “determination as to the inner side or the outer side of the double tires” below) in a manner as below. 
       FIG.  6    is a flowchart showing an exemplary procedure of processing when monitoring unit  45  makes determination as to the inner side or the outer side of the double tires. This flowchart is performed while vehicle  10  travels.  FIG.  6    shows an example of determination as to the inner side or the outer side of double tires  22  on the rear right side. 
     Monitoring unit  45  determines whether or not it has received the result of detection of first acceleration G 1  and second acceleration G 2  at a plurality of (at least two) consecutive timings from detection device  30  arranged in any one of double tires  22  on the rear right side (step S 10 ). 
     When monitoring unit  45  makes determination as NO in step S 10 , it is unable to specify change over time of first acceleration G 1  and change over time of second acceleration G 2 , and hence it skips subsequent steps and quits the process. 
     When monitoring unit  45  makes determination as YES in step S 10 , it can accurately specify change over time of first acceleration G 1  and change over time of second acceleration G 2 , and hence it specifies change over time of each of accelerations G 1  and G 2  based on the result of detection received in step S 10  (step S 12 ). 
     Then, monitoring unit  45  determines whether or not change over time of acceleration G 1  is more advanced than change over time of acceleration G 2  (step S 14 ). As described above, in the present embodiment, prescribed angle of revolution θ (a difference between an angle at which first detector  31  is arranged and an angle at which second detector  32  is arranged) is set to approximately 30° as shown in  FIG.  3   . Monitoring unit  45  can thus accurately determine whether or not change over time of acceleration G 1  is more advanced than change over time of acceleration G 2 . In other words, for example, when angle of revolution θ is set to 180°, the difference between change over time of acceleration G 1  and change over time of acceleration G 2  is comparable to 180°, and retardation and advance of change over time cannot accurately be determined. In the present embodiment, however, since angle of revolution θ is set to approximately 30°, such a problem can be solved. 
     When change over time of acceleration G 1  is more advanced than change over time of acceleration G 2  (YES in step S 14 ), monitoring unit  45  determines that detection device  30  that has outputted the result of detection received in step S 10  revolves in the reference direction (step S 16 ). Then, in view of the fact that it is outer tire  22   b  that revolves in the reference direction in forward travel of the vehicle in double tires  22  on the rear right side (see  FIG.  5   ), monitoring unit  45  determines that detection device  30  that has outputted the result of detection received in step S 10  is located in outer tire  22   b  (step S 18 ). 
     When change over time of acceleration G 1  is lagged behind change over time of acceleration G 2  (NO in step S 14 ), monitoring unit  45  determines that detection device  30  that has outputted the result of detection received in step S 10  revolves in the direction reverse to the reference direction (step S 20 ). Then, in view of the fact that it is inner tire  22   a  that revolves in the direction reverse to the reference direction in forward travel of the vehicle in double tires  22  on the rear right side (see  FIG.  5   ), monitoring unit  45  determines that detection device  30  that has outputted the result of detection received in step S 10  is located in inner tire  22   a  (step S 22 ). 
     As set forth above, the tire revolution direction determination system according to the present disclosure includes detection device  30  arranged in each of tires and monitoring unit  45 . Detection device  30  includes first detector  31  that detects first acceleration G 1  in the tire diameter direction and second detector  32  that detects second acceleration G 2  in the tire diameter direction. First detector  31  is arranged in front of second detector  32  in the reference direction. When monitoring unit  45  receives first acceleration G 1  and second acceleration G 2  from detection device  30  while vehicle  10  travels forward, it specifies change over time of first acceleration G 1  and change over time of second acceleration G 2  and determines whether or not the direction of revolution of the tire where detection device  30  is arranged is the reference direction based on whether or not change over time of first acceleration G 1  is more advanced than change over time of second acceleration G 2 . 
     According to the aspect above, whether or not the direction of revolution of the tire is the reference direction is determined based not on the acceleration in the tire revolution circumferential direction but on the acceleration (first acceleration G 1  and second acceleration G 2 ) in the tire diameter direction. Therefore, even when vehicle  10  is in the constant-velocity traveling state, the direction of revolution of the tire can be determined. 
     [First Modification] 
     In the embodiment above, an example in which a tire position determination manner according to the present disclosure is applied to determination as to the inner side or the outer side of the double tires coupled back-to-back is described. 
     The tires to which the tire position determination manner according to the present disclosure is applicable, however, should only be two tires arranged back-to-back in the vehicle, and not necessarily limited to double tires. For example, the tire position determination manner according to the present disclosure may be applied to front tires  11  and  12  back-to-back arranged in vehicle  10  so as to determine in which of front tires  11  and  12  detection device  30  is arranged. 
     [Second Modification] 
     In the embodiment above, an example in which two acceleration sensors  39  are arranged as being aligned in the reference direction in each tire is described. The number of acceleration sensors  39  arranged as being aligned in the reference direction in each tire, however, is not limited to two, and three or more acceleration sensors  39  may be provided. 
     [Third Modification] 
     In the embodiment above, an example in which monitoring unit  45  provided on the vehicle body side of vehicle  10  determines relation between change over time of first acceleration G 1  and change over time of second acceleration G 2  based on first acceleration G 1  and second acceleration G 2  received from detection device  30  and determines the direction of revolution of the tire based on the result of determination is described. 
     The determination device that determines relation between change over time of first acceleration G 1  and change over time of second acceleration G 2  and determines the direction of revolution of the tires, however, is not necessarily provided on the vehicle body side, and may be arranged in each tire. For example, controller  35  of first detector  31  arranged in each tire may determine relation between change over time of first acceleration G 1  and change over time of second acceleration G 2  based on first acceleration G 1  and second acceleration G 2  obtained from acceleration sensor  39  of first detector  31  and acceleration sensor  39  of second detector  32 , and determine the direction of revolution of the tires based on the result of determination. 
     It should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims rather than the description above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     The illustrative embodiment and the modifications thereof described above are specific examples of aspects below. 
     (1) A tire revolution direction determination system according to the present disclosure is a tire revolution direction determination system that determines a direction of revolution of a tire attached to a vehicle, and the tire revolution direction determination system includes a detection device arranged in the tire and a determination device configured to obtain information from the detection device. The detection device includes a first sensor arranged in the tire, the first sensor detecting a first acceleration in a tire diameter direction, and a second sensor arranged in the tire, the second sensor detecting a second acceleration in the tire diameter direction. The tire includes a wheel portion including a first-side surface and a second-side surface on a rear side of the first-side surface. With a direction of clockwise revolution when the tire is viewed from the second-side surface being defined as a reference direction, the first sensor is arranged in front of the second sensor in the reference direction. When the determination device receives the first acceleration and the second acceleration from the detection device while the vehicle travels forward, the determination device specifies change over time of the first acceleration and change over time of the second acceleration, and determines whether the direction of revolution of the tire where the detection device is arranged is the reference direction based on whether change over time of the first acceleration is more advanced than change over time of the second acceleration. 
     According to the aspect above, whether or not the direction of revolution of the tire is the reference direction is determined based not on the acceleration in a tire revolution circumferential direction but on the acceleration (the first acceleration and the second acceleration) in the tire diameter direction. Therefore, even when the vehicle is in a constant-velocity traveling state, the direction of revolution of the tire can be determined. 
     (2) In one aspect, the first sensor is arranged at a position distant by a prescribed angle of revolution in the reference direction from the second sensor. The prescribed angle of revolution is set to a value smaller than 180°. 
     According to the aspect above, the determination device can determine whether or not change over time of the first acceleration is more advanced than change over time of the second acceleration more accurately than in an example where the prescribed angle of revolution is set, for example, to 180°. 
     (3) In one aspect, the detection device is configured to transmit to the determination device, a result of detection of the first acceleration and the second acceleration at a plurality of consecutive timings. 
     According to the aspect above, the determination device can accurately specify change over time of the first acceleration and the second acceleration. 
     (4) In one aspect, when change over time of the first acceleration received from the detection device is more advanced than change over time of the second acceleration, the determination device determines the direction of revolution of the tire where the detection device is arranged as the reference direction. When change over time of the first acceleration received from the detection device is lagged behind change over time of the second acceleration, the determination device determines the direction of revolution of the tire where the detection device is arranged as a direction reverse to the reference direction. 
     According to the aspect above, whether the direction of revolution of the tire where the detection device is arranged is the reference direction or the direction reverse to the reference direction can be determined based on relation (retardation and advance) between change over time of the first acceleration and change over time of the second acceleration. 
     (5) In one aspect, the determination device is arranged in the tire. 
     According to the aspect above, the determination device arranged in the tire can determine the direction of revolution of the tire. 
     (6) In one aspect, the detection device is arranged in each of two tires attached back-to-back to the vehicle. The determination device determines in which of the two tires the detection device is arranged based on a result of determination of the direction of revolution of the tire where the detection device is arranged. 
     According to the aspect above, in which of two tires attached back-to-back to the vehicle the detection device is arranged can be determined. 
     (7) In one aspect, the two tires are double tires coupled in such a manner that the first-side surfaces of the two tires face each other. While the double tires are attached on a right side of the vehicle, when the direction of revolution of the tire where the detection device is arranged is the reference direction, the determination device determines that the detection device is arranged in a tire on a vehicle outer side, of the double tires, and when the direction of revolution of the tire where the detection device is arranged is a direction reverse to the reference direction, the determination device determines that the detection device is arranged in a tire on a vehicle inner side, of the double tires. While the double tires are attached on a left side of the vehicle, when the direction of revolution of the tire where the detection device is arranged is the reference direction, the determination device determines that the detection device is arranged in a tire on the vehicle inner side, of the double tires, and when the direction of revolution of the tire where the detection device is arranged is the direction reverse to the reference direction, the determination device determines that the detection device is arranged in a tire on the vehicle outer side, of the double tires. 
     According to the aspect above, in which of the double tires the detection device is arranged can be determined. 
     Though an embodiment of the present invention has been described, it should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.