Patent Publication Number: US-2023140829-A1

Title: Vehicle-mounted system

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-177904 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 vehicle-mounted system. 
     Description of the Background Art 
     In a tire pressure monitoring system (TPMS) in a vehicle, a detector has conventionally been attached to each of a plurality of tires. The detector attached to each of the plurality of tires transmits pneumatic pressure information to a processing device such as an ECU attached to a vehicle body. 
     Some TPMS’s are provided with an auto location function to automatically determine to which tire among a plurality of tires a detector is attached. 
     Japanese National Patent Publication No. 2013-514934 describes an apparatus for localizing installation positions of vehicle wheels including a rotation speed sensor that detects a rotation speed of a vehicle wheel such as an ABS rotation speed sensor and a wheel electronics unit disposed in the vehicle wheel. The wheel electronics unit is allocated to each tire and transmits information on an angle of rotation of the tire and information on a pressure of the tire to a vehicle side. Each vehicle wheel is provided with ninety-six edges in total per revolution, and the rotation speed sensor counts the number of times of passage of a reference position over the edge as a result of rotation of the vehicle wheel to detect the rotation speed of the vehicle wheel. 
     A unit on the vehicle side in Japanese National Patent Publication No. 2013-514934 obtains the angle of rotation of the tire detected by the wheel electronics unit and the rotation speed of the vehicle wheel detected by the rotation speed sensor and localizes the installation positions of the vehicle wheels based on relation between the angle of rotation of the tire and the rotation speed of the vehicle wheel that have been obtained. 
     SUMMARY OF THE INVENTION 
     In order to derive relation between the angle of rotation detected by the wheel electronics unit and the rotation speed detected by the rotation speed sensor, the apparatus for localizing installation positions of vehicle wheels in Japanese National Patent Publication No. 2013-514934 obtains information on a plurality of angles of rotation from the wheel electronics unit. Therefore, after a plurality of times of transmission and reception between the wheel electronics unit and a vehicle-side unit, the unit on the vehicle side determines the installation positions of the vehicle wheels. 
     When a frequency of transmission of the information on the angle of rotation is low, it takes time to collect information necessary for determination of the installation positions of the vehicle wheels, and consequently, a total time period for determination of the installation positions of the vehicle wheels may become long. On the other hand, when a frequency of transmission of information on the angle of rotation is high, transmission and reception are frequent also when tires are not rotated, and consequently, power consumption by the wheel electronics unit may increase. 
     The present disclosure was made to solve the problem described above, and an object thereof is to shorten a time period required for determination of a tire position while increase in power consumption by a tire detector attached to the tire is prevented in a vehicle-mounted system that determines a tire position after a plurality of times of transmission and reception of data between the tire detector and a control device on a vehicle side. 
     A vehicle-mounted system according to one aspect of the present disclosure is a vehicle-mounted system provided in a vehicle including a tire, and includes a detector attached to the tire, the detector transmitting a detection signal, and a monitoring unit that determines a position of the tire based on a plurality of detection signals received from the detector. The detector determines a direction of revolution of the tire, and when the detector determines change of the direction of revolution of the tire, the detector switches a frequency of transmission of the detection signal from a first frequency to a second frequency higher than the first frequency. 
     According to the aspect above, whether or not tires have been rotated is determined based on change in direction of revolution of the tire. When it is determined that the tires have been rotated, a frequency of transmission of a detection signal is increased. Thus, time required for determination of a tire position can be shortened while increase in power consumption by the tire detector is prevented. 
     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 to which a vehicle-mounted system according to the present embodiment is applied. 
         FIG.  2    is a block diagram showing an exemplary configuration of a tire detector. 
         FIG.  3    is a diagram showing an exemplary appearance of the tire detector. 
         FIG.  4    is a diagram for illustrating an exemplary method of determining a tire position in the present embodiment. 
         FIG.  5    is a transition diagram of arrangement of the tire detector when a tire on a front right side revolves. 
         FIG.  6    is a diagram showing the tire before tire rotation. 
         FIG.  7    is a diagram showing exemplary tire rotation. 
         FIG.  8    is a diagram showing the tire after tire rotation. 
         FIG.  9    is a flowchart showing processing for switching a frequency of transmission of a UHF signal. 
         FIG.  10    is a flowchart showing tire position determination processing based on a direction of revolution of the tire. 
         FIG.  11    is a flowchart showing tire position determination processing on the premise that the tire rotation in  FIG.  7    is carried out. 
     
    
    
     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. 
     Overall Configuration 
       FIG.  1    is a diagram schematically showing a configuration of a vehicle  100  to which a vehicle-mounted system according to the present embodiment is applied. 
     Vehicle  100  according to the present embodiment is a vehicle including tires  11  and  12  as front wheels which are steering wheels and tires  13  and  14  as rear wheels which are non-steering wheels . Vehicle  100  is of a front-wheel drive type. Vehicle  100  may be of a rear-wheel drive type or an all-wheel drive type. A direction FR shown in  FIG.  1    represents a direction of forward travel of vehicle  100 . 
     In the description below, a vertical direction when vehicle  100  is arranged on the plane is defined as a “Z-axis direction,” a direction perpendicular to the Z-axis direction, in the direction of forward travel of vehicle  100 , is defined as a “positive direction along an X axis,” and a direction perpendicular to an X-axis direction is defined as a “Y-axis direction.” Hereafter, a positive direction along a Z axis in each figure may be referred to as an upper side and a negative direction along the Z axis may be referred to as a lower side, the positive direction along the X axis may be referred to as a front side and a negative direction along the X axis may be referred to as a rear side, and a positive direction along a Y axis may be referred to as a right side and a negative direction along the Y axis may be referred to as a left side. 
     Vehicle  100  includes a system that monitors a pneumatic pressure of each tire (TPMS). Specifically, vehicle  100  includes a plurality of tire detectors  31  to  34  each detecting a tire pressure, revolution speed sensors  81  to  84  detecting revolution speeds of revolution bodies  61  to  64  attached to the tires, respectively, and a TPMS receiver  40 . Tire detectors  31  to  34  are attached to wheels of tires  11  to  14 , respectively. Tire detectors  31  to  34  may each be formed integrally with a valve for intake of air into each tire. Tire detectors  31  to  34  may each be formed separately from the valve. 
     Each of tire detectors  31  to  34  is activated when a prescribed activation condition is satisfied, and detects a pneumatic pressure of each tire and transmits 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 to be satisfied regularly or irregularly. Tire detectors  31  to  34  are thus intermittently activated at timings different from one another and transmit UHF signals. 
     The UHF signals outputted from tire detectors  31  to  34  include information indicating specific ID numbers for identifying at least respective tire detectors  31  to  34 . Specifically, the UHF signal outputted from tire detector  31  includes “01” as the ID number and the UHF signal outputted from tire detector  32  includes “02” as the ID number. The UHF signal outputted from tire detector  33  includes “03” as the ID number and the UHF signal outputted from tire detector  34  includes “04” as the ID number. 
     The UHF signals outputted from tire detectors  31  to  34  each include information representing a tire pressure in addition to the information indicating the ID number. As TPMS receiver  40  receives the UHF signal outputted from each of tire detectors  31  to  34 , it can monitor a pneumatic pressure of each tire. 
     Tires identical in specifications and construction are employed as tires  11  to  14  for allowing tire rotation. Therefore, tire detectors identical in configuration are adopted also for tire detectors  31  to  34 . When tires  11  to  14  do not have to be described as being distinguished from one another, tires  11  to  14  are simply referred to as a “tire 10” below. When tire detectors  31  to  34  do not have to be described as being distinguished from one another, tire detectors  31  to  34  are simply referred to as a “tire detector  30 .” 
     TPMS receiver  40  is provided on a vehicle body side of vehicle  100 . TPMS receiver  40  includes a monitoring unit  45  that monitors a pneumatic pressure of each tire. Monitoring unit  45  includes a storage  46 , a processing unit  47 , and an antenna A 1 . Antenna A 1  is configured to receive a UHF signal transmitted from tire detector  30 . Monitoring unit  45  accepts the UHF signal received by antenna A 1 . 
     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 tire detector  30  is attached and information indicating a tire pressure are stored in storage  46  as being brought in correspondence with an ID number of each tire detector  30 . In the present embodiment, four tire positions (a front left side, a front right side, a rear left side, and a rear right side) in total are set by a tire position determination method which will be described later, and an ID number of each tire detector  30  is brought in correspondence with any one tire position. 
     Specifically, the tire position “front left side” is brought in correspondence with the ID number “01” and the tire position “front right side” is brought in correspondence with the ID number “02”. The tire position “rear left side” is brought in correspondence with the ID number “03” and the tire position “rear right side” is brought in correspondence with the ID number “04”. When tires are rotated and monitoring unit  45  detects attachment at different tire positions, monitoring unit  45  updates relation between the ID number and the tire position. 
     When monitoring unit  45  receives a UHF signal, it checks the ID number included in the UHF signal against the ID number stored in storage  46 , and obtains the tire position brought in correspondence with the ID number. Monitoring unit  45  updates the pneumatic pressure at the obtained tire position with the tire pressure included in the UHF signal. 
     For example, when monitoring unit  45  receives the UHF signal including the ID number “01”, it refers to correspondence between the ID number “01” and the tire position stored in storage  46 . In storage  46 , the “front left side” is brought in correspondence with the ID number “01” as the tire position. Monitoring unit  45  updates the pneumatic pressure on the “front left side” 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  52 . Display  52  is arranged at a position where a driver can visually recognize the same. Display  52  is arranged, for example, in an instrument panel within the vehicle. 
     Monitoring unit  45  performs tire pressure determination processing for determining whether or not the tire pressure included in the received UHF signal is equal to or lower than a low-pressure threshold value. When the tire pressure is equal to or lower than the low-pressure threshold value, monitoring unit  45  has the tire position where the tire pressure has become the low-pressure threshold value shown on display  52  together with a warning. TPMS receiver  40  performs tire pressure determination processing each time it receives the UHF signal and monitors each pneumatic pressure of the 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. 
     Revolution speed sensors  81  to  84  are each a vehicle wheel speed sensor included, for example, in an antilock brake system (ABS). Revolution bodies  61  to  64  that revolve in synchronization with tires  11  to  14  are attached to tires  11  to  14 , respectively. Examples of revolution bodies  61  to  64  include a wheel, an axle, and a gear. When revolution speed sensors  81  to  84  do not have to be described as being distinguished from one another, revolution speed sensors  81  to  84  are simply referred to as a “revolution speed sensor 80” below. When revolution bodies  61  to  64  do not have to be described as being distinguished from one another, revolution bodies  61  to  64  are simply referred to as a “revolution body 60” below. 
     Revolution body  60  is provided with a detection mechanism such as a plurality of (for example, forty-eight) magnets or teeth. Revolution speed sensor  80  transmits a pulsed signal of rectangular waves to TPMS receiver  40  with the use of the detection mechanism such as the magnets or the teeth provided in revolution body  60 . Revolution speed sensor  80  detects both of a rising edge and a falling edge of the pulsed signal and detects ninety-six pulses (counter value: 0 to 95) per revolution of the tire. Revolution speed sensor  80  starts counting with an angle of revolution of tire  10  at the time of ignition ON being defined as a reference point (0). A counter value of revolution speed sensor  80  increases from 0 to 95 in one revolution of tire  10  and returns to 0 when tire  10  makes one revolution. 
     Each revolution speed sensor  80  and the tire position are stored in storage  46  as being brought in correspondence. Specifically, the “front left side” is brought in correspondence with revolution speed sensor  81  and the “front right side” is brought in correspondence with revolution speed sensor  82 . The “rear left side” is brought in correspondence with revolution speed sensor  83  and the “rear right side” is brought in correspondence with revolution speed sensor  84 . 
     Configuration of Tire Detector  30   
     An exemplary configuration of tire detector  30  will be described below with reference to  FIGS.  2  and  3   .  FIG.  2    is a block diagram showing an exemplary configuration of tire detector  30 . As shown in  FIG.  2   , tire detector  30  includes a controller  35 , a pressure sensor  38 , an acceleration sensor (G sensor)  39 , an antenna A 2 , and a transmission circuit CT. 
     Controller  35  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. 
     In storage  36 , an ID number specific for each tire detector  30  shown in  FIG.  1    is stored. In storage  36  of tire detector  31 , “01” is stored as the ID number, and in storage  36  of tire detector  32 , “02” is stored as the ID number. In storage  36  of tire detector  33 , “03” is stored as the ID number, and in storage  36  of tire detector  34 , “04” is stored as the ID number. 
     Controller  35  controls transmission circuit CT to transmit a UHF signal from antenna A 2 . Controller  35  outputs the UHF signal at timing when a prescribed activation condition is satisfied. Tire detector  30  is provided with a not-shown battery, and operates with electric power supplied from the battery . This battery is constructed not to readily externally be charged. Therefore, in tire detector  30  in a first embodiment, desirably, operating time is minimized to suppress power consumption by tire detector  30 . 
     From this point of view, the “prescribed activation condition” is set in advance to suppress a frequency of activation of tire detector  30  as much as possible. For example, the prescribed activation condition may include such a timer-based activation condition that a timer has counted lapse of prescribed timer time since previous stop and such an acceleration-based activation condition that a result of detection (which is also referred to as an “acceleration G” below) by acceleration sensor  39  has attained to a specific value (for example, a maximum value or a minimum value). 
     The “timer time” used as the timer-based activation condition described above may be set to a fixed value or a variable value that varies with acceleration G. For example, controller  35  may determine whether or not a tire is revolving based on acceleration G which represents the result of detection by acceleration sensor  39  and change the set timer time. 
     In the vehicle-mounted system in the present embodiment, for a tire position determination method which will be described later, a “prescribed activation condition” in travel of vehicle  100  is determined. Tire detector  30  transmits a UHF signal, with the fact that a gravity component included in acceleration G has attained to a specific value after lapse of a prescribed transmission interval since last transmission of the UHF signal being defined as the “prescribed activation condition.” 
     Acceleration sensor  39  detects an acceleration generated in a biaxial direction in tire detector  30  and outputs a result of detection to controller  35 . Pressure sensor  38  detects a tire pressure and outputs a result of detection (which is also referred to as a “tire pressure P” below) to controller  35 . 
     The UHF signal includes information indicating acceleration G and information on time of detection of acceleration G in addition to the ID number stored in storage  36  and information indicating tire pressure P. Tire detector  30  may further include a temperature sensor that detects a tire temperature in addition to pressure sensor  38  and acceleration sensor  39 . 
       FIG.  3    is a diagram showing an exemplary appearance of tire detector  30 . Tire detector  30  is attached as being fixed to a wheel WH of tire  10 . A position of tire detector  30  changes with revolution of tire  10 . 
       FIG.  3    shows a revolution axis direction D 1 , a revolution circumferential direction D 2 , and a revolution diameter direction D 3  of wheel WH when tire  10  revolves. Acceleration sensor  39  of tire detector  30  is a biaxial acceleration sensor having revolution diameter direction D 3  and revolution circumferential direction D 2  as detection directions. In other words, acceleration sensor  39  detects an acceleration generated in revolution diameter direction D 3  and an acceleration generated in revolution circumferential direction D 2 . The acceleration in revolution diameter direction D 3  is used for a prescribed activation condition for transmission of a UHF signal. The acceleration in revolution circumferential direction D 2  is used for determination as to whether or not tires have been rotated. 
     As to Tire Position Determination Method 
     The vehicle-mounted system in the present embodiment determines a tire position based on a detection value from each revolution speed sensor  80  and a detection value from acceleration sensor  39  of each tire detector  30 .  FIG.  4    is a diagram for illustrating an exemplary method of determining a tire position in the present embodiment. 
       FIG.  4    shows as waveforms, each of exemplary transition of an acceleration of gravity generated in revolution diameter direction D 3  of tire  12  on the front right side during forward travel of vehicle  100 , exemplary transition of a counter value of revolution speed sensor  82  on the front right side, and exemplary transition of a counter value of revolution speed sensor  81  on the front left side. 
     Controller  35  of tire detector  32  obtains over time, an acceleration generated in revolution diameter direction D 3  from acceleration sensor  39 . The acceleration generated in revolution diameter direction D 3  includes a centrifugal component resulting from revolution of tire  10  and a gravity component (acceleration of gravity). 
     A gravity component (acceleration of gravity) generated in revolution diameter direction D 3  will be described below with reference to an example shown in  FIG.  5   . 
       FIG.  5    is a transition diagram of arrangement of tire detector  32  when tire  12  on the front right side revolves.  FIG.  5    shows transition of arrangement of tire detector  32  when tire  12  is viewed from the positive side (outer side of vehicle  100 ) in the Y-axis direction. 
       FIG.  5    shows arrangements 1h to 12h as twelve patterns of exemplary arrangement of tire detector  32 . Arrangement 12h of tire detector  32  is such an arrangement that tire detector  32  is located in revolution diameter direction D 3  extending from a central point CP1 of tire  12  in the positive direction along the Z-axis. Arrangement 1h represents arrangement of tire detector  32  when tire  12  revolves by θ degrees clockwise from the state of arrangement 12h. Arrangement 2h represents arrangement of tire detector  32  when tire  12  revolves by θ degrees clockwise from the state of arrangement 1h. 
     As shown in  FIG.  5   , in arrangement 12h (0 degree) or arrangement 6h (+180 degrees) of tire detector  32 , an absolute value of the acceleration of gravity in the detection direction is largest. In the example in  FIG.  5   , the detection value from acceleration sensor  39  in revolution diameter direction D 3  when tire detector  32  is in arrangement 12h is +1 G. The detection value from acceleration sensor  39  in revolution diameter direction D 3  when tire detector  32  is in arrangement 6h is -1 G. Depending on a direction of attachment of tire detector  32 , positive and negative signs of the acceleration of gravity as the detection value shown in  FIG.  5    may be reversed. It is noted that 1 G is equal to 9.8 m/s 2 . 
     The gravity component (acceleration of gravity) generated in revolution diameter direction D 3  thus changes depending on arrangement of tire detector  32 . While tire  12  is revolving, transition of the acceleration of gravity generated in revolution diameter direction D 3  exhibits a sinusoidal wave as shown in  FIG.  4   . The sinusoidal wave shown in  FIG.  4    is illustrated as a sinusoidal wave having a constant period for the sake of simplification of description. When a revolution speed of tire  12  is not constant, the period of the sinusoidal wave may constantly vary. 
     Controller  35  calculates a centrifugal component based on transition of the acceleration in revolution diameter direction D 3  detected by acceleration sensor  39 . Controller  35  desirably removes the calculated centrifugal component from the acceleration in revolution diameter direction D 3  detected by acceleration sensor  39  to extract only the gravity component (acceleration of gravity). 
       FIG.  4    shows transition of the gravity component (acceleration of gravity) after the centrifugal component is removed from the acceleration generated in revolution diameter direction D 3 . Tire detector  30  can thus determine timing when the position of tire detector  30  is in arrangement 12h based on the calculated acceleration of gravity generated in revolution diameter direction D 3 . Tire detector  30  in the present embodiment transmits the UHF signal at timing when the position of tire detector  30  is in arrangement 12h. 
     More specifically, tire detector  30  transmits the UHF signal when the acceleration of gravity attains to +1 G after lapse of a prescribed transmission interval Dr since last transmission of the UHF signal. For example, transmission interval Dr is set to one minute. Tire detector  30  should only be able to transmit the UHF signal at the acceleration of gravity at which arrangement of tire detector  30  can uniquely be specified and may transmit the UHF signal when the acceleration of gravity attains to -1 G. 
     Tire detector  30  transmits the UHF signal by being triggered by the fact that it is located in arrangement 12h based on a revolution position of tire  10  after lapse of one minute since last transmission of the UHF signal. When data on last transmission of the UHF signal is not stored in storage  36 , tire detector  30  transmits the UHF signal by being triggered only by the fact that it is located in arrangement 12h. The waveform shown in  FIG.  4    may include the centrifugal component. 
     In the example in  FIG.  4   , at timing t1, tire detector  32  is located in arrangement 12h and the acceleration of gravity attains to +1 G. At timing t1, tire detector  32  transmits the UHF signal. In other words, at timing t1, the UHF signal is transmitted and received between tire detector  32  and monitoring unit  45 . Monitoring unit  45  receives the UHF signal including the ID number “02”. Tire detector  32  starts counting of the timer for counting transmission interval Dr from timing t1 of transmission of the UHF signal. 
     In the example in  FIG.  4   , a period from timing t1 to timing t2 corresponds to transmission interval Dr. When the acceleration of gravity attains to +1 G after timing t2, regarding the prescribed activation condition as having been satisfied, tire detector  32  transmits the UHF signal. In other words, at timing t3, tire detector  32  transmits the UHF signal. Thereafter, while vehicle  100  is traveling, tire detector  32  transmits the UHF signal each time the prescribed activation condition is satisfied. 
     Monitoring unit  45  refers to a detection value (counter value) from each of revolution speed sensors  81  to  84  at timings t1, t3, and t5 of reception of the UHF signal. In the waveforms showing exemplary counter values of revolution speed sensors  81  and  82  shown in  FIG.  4   , the ordinate represents the counter values of revolution speed sensors  81  and  82  and the abscissa represents elapsed time. The counter values of revolution speed sensors  81  and  82  increase with forward travel of vehicle  100  and revolution of tires  11  to  14 . 
     Each of tires  11  to  14  provided in vehicle  100  independently revolves for the purpose of prevention of slip or the like. In other words, tires  11  to  14  are different from one another in period of revolution. Therefore, the counter values of revolution speed sensors  81  and  82  increase at rates different from each other also in the exemplary waveforms shown in  FIG.  4   . 
     Monitoring unit  45  obtains the counter value from each revolution speed sensor  80  at timing t1 of reception of the UHF signal. Monitoring unit  45  obtains a counter value X 1  from revolution speed sensor  82  on the front right side. Monitoring unit  45  obtains a counter value X 3  from revolution speed sensor  81  on the front left side. Though  FIG.  4    shows only revolution speed sensors  81  and  82  for the sake of simplification of description, monitoring unit  45  obtains a counter value from each of revolution speed sensors  83  and  84 . 
     Then, monitoring unit  45  obtains the counter value from each revolution speed sensor  80  at timing t3 of reception of the UHF signal from tire detector  32 . Monitoring unit  45  obtains counter value X 1  from revolution speed sensor  82  on the front right side. Monitoring unit  45  obtains a counter value X 2  from revolution speed sensor  81  on the front left side. 
     Monitoring unit  45  can determine that revolution speed sensor  81  different in counter value between timing t1 and timing t3 and tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached are not in synchronization with each other. Monitoring unit  45  can thus determine that the tire position of tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached is not the front left side. 
     On the other hand, monitoring unit  45  can determine that revolution speed sensor  82  identical in counter value between timing t1 and timing t3 may be in synchronization with tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached. Monitoring unit  45  can thus determine that the tire position of tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached may be the front right side. The period of the counter value from revolution speed sensor  82  and the period of one revolution of tire  12  are in synchronization with each other. Therefore, when the position of tire detector  32  is located in arrangement 12h, the counter value from revolution speed sensor  82  is uniquely determined. 
     For tire positions brought in correspondence with revolution speed sensors  83  and  84  as well, monitoring unit  45  determines whether or not tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached may be arranged. Since each tire  10  independently revolves, the counter values from revolution speed sensor  80  not in synchronization may accidentally be the same between timing t1 and timing t3. 
     Monitoring unit  45  repeats determination by obtaining the counter value from revolution speed sensor  80  until it can uniquely specify the tire position of tire  12  to which tire detector  32  that has transmitted the UHF signal including the ID number “02” is attached. Thus, in the vehicle-mounted system in the present embodiment, the tire position is determined after transmission and reception of data a plurality of times between tire detector  30  and monitoring unit  45 . In other words, monitoring unit  45  determines the tire position of tire  10  to which tire detector  30  is attached based on a plurality of UHF signals received at different timings from tire detector  30 . The counter value of revolution speed sensor  80  may correspond to the “first revolution angle information” in the present disclosure. The acceleration of gravity generated in revolution diameter direction D 3  of tire detector  30  may correspond to the “second revolution angle information” in the present disclosure. 
     The tire position determination method as shown in  FIG.  4    is by way of example and the tire position determination method in the present embodiment should only be a method of determination of a tire position after transmission and reception of data a plurality of times between tire detector  30  and monitoring unit  45 . For example, tire detector  30  may be configured to transmit the UHF signal at transmission intervals Dr different for each tire position and monitoring unit  45  may determine the tire position based on the number of times of reception of the UHF signal per unit period. 
     The tire position determination method may be a method of determination based on reception intensity of the UHF signal. For example, TPMS receiver  40  is arranged at a position where distances between TPMS receiver  40  and tires  10  are different from one another. TPMS receiver  40  thus receives UHF signals at different reception intensities from tire detectors  30 . TPMS receiver  40  determines the tire position of tire  10  to which each tire detector  30  is attached based on an average of reception intensities of UHF signals received from tire detectors  30 . 
     Detection of Tire Rotation 
     Tire detector  30  in the present embodiment detects tire rotation based on a detection value from acceleration sensor  39 . When tire detector  30  detects tire rotation, it shortens transmission interval Dr of the UHF signal. 
     Exemplary detection of tire rotation by tire detector  30  will be described below with reference to  FIGS.  6  to  8   .  FIG.  6    is a diagram showing tires  13  and  14  before tire rotation. As described with reference to  FIG.  1   , tires  13  and  14  are attached to vehicle  100  as rear wheels. When a design surface of wheel WH that can visually be recognized from the outside of vehicle  100  is defined as a front surface, a surface FP of wheel WH of each of tires  13  and  14  is a rear surface on the back of the design surface. In other words, surface FP is a surface of the wheel that is difficult to visually recognize from the outside of vehicle  100  and a surface a direction of normal to which is oriented toward the inside of vehicle  100 . Tires  13  and  14  are fixed with surfaces FP of wheels WH thereof face each other. 
     As described with reference to  FIG.  3   , acceleration sensor  39  detects also the acceleration generated in revolution circumferential direction D 2  in addition to revolution diameter direction D 3 . Tire detector  30  (tire detector  33  and tire detector  34 ) is fixed to each wheel WH so as to detect, as a positive value, the acceleration in revolution circumferential direction D 2  during acceleration in revolution in a first revolution direction which is a counterclockwise direction when tire  10  is viewed from surface FP and detect, as a negative value, the acceleration in revolution circumferential direction D 2  during acceleration in revolution in a second revolution direction which is a clockwise direction when tire  10  is viewed from surface FP. While vehicle  100  travels forward, tire  14  revolves in the first revolution direction and tire  13  revolves in the second revolution direction. 
     Therefore, when vehicle  100  accelerates in the direction of forward travel, tire detector  34  of tire  14  on the right side of the vehicle detects a positive acceleration in revolution circumferential direction D 2 , whereas tire detector  33  of tire  13  on the left side of the vehicle detects a negative acceleration in revolution circumferential direction D 2  equal in magnitude to the acceleration detected by tire detector  34 . When vehicle  100  decelerates while it is traveling forward, tire detector  34  of tire  14  on the right side of the vehicle detects a negative acceleration in revolution circumferential direction D 2 , whereas tire detector  33  of tire  13  on the left side of the vehicle detects a positive acceleration in revolution circumferential direction D 2  equal in magnitude to the acceleration detected by tire detector  34 . 
       FIG.  7    is a diagram showing exemplary tire rotation. As described with reference to  FIG.  1   , vehicle  100  is of the front-wheel drive type. In general, in tire rotation in a vehicle of the front-wheel drive type, tires are attached as being interchanged in position in such a manner that the tire on the front left side is moved to the tire position on the rear left side, the tire on the rear left side is moved to the tire position on the front right side, the tire on the front right side is moved to the tire position on the rear right side, and the tire on the rear right side is moved to the tire position on the front left side. The tire which was attached to the rear side is attached to the front side with the left and right sides being interchanged. 
       FIG.  7    shows a diagram after tire rotation of tire  10  attached to vehicle  100  described with reference to  FIG.  1   . Tire  14  which was attached on the rear right side in  FIG.  1    is attached to the front left side, and tire  13  which was attached on the rear left side is attached on the front right side.  FIG.  8    is a diagram showing tires  13  and  14  after tire rotation. 
     As shown in  FIG.  7   , when tires are attached with the left and right sides being interchanged, the direction of revolution of the tire when vehicle  100  travels forward is different between before and after tire rotation. Specifically, when vehicle  100  accelerates in the direction of forward travel after tire rotation, tire detector  33  of tire  13  attached on the right side of vehicle  100  detects a positive acceleration in revolution circumferential direction D 2 , whereas tire detector  34  of tire  14  attached on the left side of vehicle  100  detects a negative acceleration in revolution circumferential direction D 2  equal in magnitude to the acceleration detected by tire detector  33 . When vehicle  100  decelerates while it is traveling forward, tire detector  33  of tire  13  attached on the right side of vehicle  100  detects a negative acceleration in revolution circumferential direction D 2 , whereas tire detector  34  of tire  14  attached on the left side of vehicle  100   detects a positive acceleration in revolution circumferential direction D 2  equal in magnitude to the acceleration detected by tire detector  33 . 
       FIGS.  6  to  8    illustrate that there are tires  10  attached with the left and right sides being interchanged, in general tire rotation in vehicle  100  of the front-wheel drive type. There are tires  10  attached with the left and right sides being interchanged also similarly in a vehicle of the rear-wheel drive type or the all-wheel drive type. 
     In tire rotation in the vehicle of the rear-wheel drive type or the all-wheel drive type, in general, the tire on the rear left side is attached at the tire position on the front left side, the tire on the front left side is attached at the tire position on the rear right side, the tire on the rear right side is attached at the tire position on the front right side, and the tire on the front right side is attached at the tire position on the rear left side. The tires which were attached on the front side are attached on the rear side with the left and right sides being interchanged. In other words, the direction of revolution in forward travel of the vehicle, of the tire moved from the front side to the rear side is different between before and after tire rotation. 
     Thus, in general tire rotation in the vehicle of any of the front-wheel drive type, the rear-wheel drive type, and the all-vehicle wheel drive type, there are tires attached with the left and right sides being interchanged. The technique described in the present embodiment is applicable to a scene where the left and right sides of positions of attachment along a direction of the entire length of the vehicle, of at least one set of (that is, at least two) tires are interchanged. Each tire  10  to which the vehicle-mounted system in the present embodiment is applied is a tire without a directional pattern. 
     Tire detector  30  in the present embodiment determines whether or not tires have been rotated based on the acceleration in revolution circumferential direction D 2 . As described with reference to  FIG.  3   , acceleration sensor  39  of tire detector  30  detects the acceleration in revolution circumferential direction D 2 . Whether the acceleration in revolution circumferential direction D 2  is detected as the positive value or the negative value is determined based on the direction of revolution of tire  10  and whether vehicle  100  is accelerating or decelerating. 
     Tire detector  30  in the present embodiment calculates a ratio between the positive value and the negative value of the acceleration in revolution circumferential direction D 2  over a time period of travel of vehicle  100 . In a general vehicle, a time period for which a brake is driven is shorter than a time period for which an accelerator is driven. Therefore, when an acceleration period and a deceleration period in a forward travel period of vehicle  100  are compared with each other, the acceleration period is longer than the deceleration period . In other words, a period for which acceleration sensor  39  detects the acceleration in revolution circumferential direction D 2  as a positive value is longer than a period for which acceleration sensor  39  detects the acceleration in revolution circumferential direction D 2  as a negative value. 
     Tire detector  30  calculates the ratio between the positive value and the negative value of the acceleration in revolution circumferential direction D 2  over the time period of travel of vehicle  100  and determines whether the direction of revolution of tire  10  in forward travel of vehicle  100  is the first revolution direction or the second revolution direction. Tire detector  30  determines that the direction of revolution of tire  10  in rearward travel of vehicle  100  is the direction reverse to the direction of revolution in forward travel. Processing for determining whether the direction of revolution of tire  10  in forward travel of vehicle  100  is the first revolution direction or the second direction of travel is referred to as processing for determining the direction of revolution of tire  10  below. 
     Each time a predetermined condition is satisfied, tire detector  30  performs processing for determining the direction of revolution of tire  10 . When tire detector  30  determines that the direction of revolution of tire  10  in forward travel of vehicle  100  has changed, it determines that tires have been rotated. This is because the direction of revolution of tire  10  in forward travel of vehicle  100  changes only when tire  10  is attached on a different side of the left and right sides. Tire detector  30  may perform processing for determining the direction of revolution of tire  10  not based on the acceleration generated in revolution circumferential direction D 2 . For example, tire detector  30  may perform processing for determining the direction of revolution of tire  10  based on the acceleration generated in revolution diameter direction D 3 . 
     When tire detector  30  in the present embodiment determines that vehicle  100  remains stopped for a prescribed period or longer, it performs processing for determining the direction of revolution of tire  10 . Tires are rotated while vehicle  100  remains stopped. Therefore, by performing processing for determining the direction of revolution of tire  10  after vehicle  100  remained stopped for the prescribed period or longer, the determination processing can be performed at appropriate timing and processing load imposed on processing unit  37  can be lessened. Tire detector  30  determines travel or stop of vehicle  100  based on whether or not the detection value from acceleration sensor  39  is changing. 
     Shortening of Interval of Transmission of UHF Signal 
     When tire detector  30  in the present embodiment determines that tires have been rotated, it shortens transmission interval Dr. In other words, tire detector  30  increases a frequency of transmission of the UHF signal. For example, tire detector  30  shortens transmission interval Dr from one minute to fifteen seconds. Monitoring unit  45  can thus receive a plurality of UHF signals transmitted from tire detector  30  in a shorter period. 
     Since monitoring unit  45  in the present embodiment can receive the UHF signal for specifying the tire position in a short period, time required for determination of the tire position can be shortened. Since transmission interval Dr is shortened upon detection of tire rotation, operating time of tire detector  30  can be short when tires are not rotated, and power consumption can be suppressed. 
     Tire detector  30  sets transmission interval Dr back to the original transmission interval based on the fact that the number of times of transmission of the UHF signal reaches a predetermined defined number of times after transmission interval Dr is shortened. In other words, tire detector  30  extends transmission interval Dr from fifteen seconds to one minute. The vehicle-mounted system thus prevents increase in power consumption due to transmission interval Dr kept shortened. The predetermined defined number of times is the number of times sufficient for monitoring unit  45  to determine the tire position, and may be determined in experiments or the like. 
     The frequency of transmission of the UHF signal at transmission interval Dr (for example, one minute) before tire rotation may correspond to the “first frequency” in the present disclosure. The frequency of transmission of the UHF signal at transmission interval Dr (for example, fifteen seconds) after tire rotation may correspond to the “second frequency” in the present disclosure. 
     Flowchart 
       FIG.  9    is a flowchart showing processing for switching a frequency of transmission of the UHF signal. The flowchart shown in  FIG.  9    is performed by tire detector  30 . Tire detector  30  determines whether or not a prescribed period or longer has elapsed since stop of vehicle  100  (step S 101 ). When the detection value from acceleration sensor  39  does not change for a predetermined period or longer, tire detector  30  determines that vehicle  100  remains stopped. 
     When the prescribed period or longer has not elapsed since stop of vehicle  100  (NO in step S 101 ), tire detector  30  repeats processing in step S 101 . Tire detector  30  determines the direction of revolution of tire  10  in forward travel of vehicle  100  (step S 102 ). Tire detector  30  determines whether or not the direction of revolution of tire  10  in forward travel of vehicle  100  has changed (step S 103 ). Tire detector  30  determines whether or not the direction of revolution of tire  10  has changed from the direction of revolution before stop for the prescribed period or longer. 
     When tire detector  30  determines that the direction of revolution of tire  10  in forward travel of vehicle  100  has changed (YES in step S 103 ), it shortens transmission interval Dr (step S 104 ). In other words, regarding tires as having been rotated, tire detector  30  increases the frequency of transmission of the UHF signal. When tire detector  30  determines that the direction of revolution of tire  10  in forward travel of vehicle  100  has not changed (NO in step S 103 ), the process returns to step S 101  with tires being regarded as not having been rotated. 
     After transmission interval Dr is shortened, tire detector  30  determines whether or not the number of times of transmission of the UHF signal has reached the defined number of times (step S 105 ). Tire detector  30  counts the number of times of transmission of the UHF signal after transmission interval Dr is shortened. When tire detector  30  determines that the number of times of transmission of the UHF signal has not reached the defined number of times (NO in step S 105 ), it repeats processing in step S 105 . 
     When tire detector  30  determines that the number of times of transmission of the UHF signal has reached the defined number of times (YES in step S 105 ), it sets transmission interval Dr back to the original transmission interval (step S 106 ). In other words, transmission interval Dr is set to the transmission interval before it is shortened. 
     First Example of Determination of Tire Position by Monitoring Unit  45   
     As described above, in vehicle  100  of the front-wheel drive type, tire rotation shown in  FIG.  7    is carried out. When monitoring unit  45  receives the UHF signal at shortened transmission interval Dr from any one of tire detectors  31  to  34 , it can determine positions of other tires based on the direction of revolution of tire  10 . 
       FIG.  10    is a flowchart showing tire position determination processing based on the direction of revolution of tire  10 . The flowchart shown in  FIG.  10    is performed by monitoring unit  45 . Monitoring unit  45  determines whether or not it has received the UHF signal at shortened transmission interval Dr (step S 201 ). When monitoring unit  45  does not receive the UHF signal at shortened transmission interval Dr (NO in step S 201 ), it repeats processing in step S 201 . 
     When monitoring unit  45  has received the UHF signal at shortened transmission interval Dr (YES in step S 201 ), it obtains tire  10  that revolves in the direction of revolution the same as the direction of revolution before change, of tire detector  30  that has transmitted the UHF signal at shortened transmission interval Dr (step S 202 ). An example in which the UHF signal is received at shortened transmission interval Dr from tire detector  34  attached to tire  14  rotated from the tire position “rear right side” to the “front left side” will be described below with reference to the example in  FIG.  7   . 
     Monitoring unit  45  can obtain the direction of revolution of tire  10  from each tire detector  30 . As described above, tire detector  30  can determine the direction of revolution when surface FP of attached tire  10  is two-dimensionally viewed, based on the detection value from acceleration sensor  39  during the acceleration period and the deceleration period of vehicle  100 . In the example in  FIG.  7   , before tire rotation, tire  14  on the rear right side and tire  12  on the front right side revolve in the same direction of revolution when surface FP of each of them is two-dimensionally viewed. In other words, monitoring unit  45  obtains tire  12  that revolves in the direction of revolution identical to the direction of revolution before change, of the direction of revolution of tire  14  to which tire detector  34  that has transmitted the UHF signal at shortened transmission interval Dr is attached. 
     Monitoring unit  45  determines that the position of tire  12  has been interchanged in a front-rear direction (step S 203 ). Specifically, monitoring unit  45  determines that the position of tire  12  has been changed from the “front right side” to the “rear right side.” Since monitoring unit  45  can determine that the position of tire  12  that has been identical in direction of revolution to tire  14  the position of which has been interchanged in a lateral direction has been interchanged only in the front-rear direction, it can determine that the position of tire  12  has been changed from the “front right side” to the “rear right side.” 
     Thus, the vehicle-mounted system in the present embodiment can determine the position of tire  12  without receiving the UHF signal from tire detector  32  attached to tire  12  and can shorten the time period required for determination of positions of tire  12  and tire  14 . 
     Second Example of Determination of Tire Position by Monitoring Unit  45   
     In vehicle  100  of the front-wheel drive type, as described above, tire rotation shown in  FIG.  7    is carried out. When monitoring unit  45  receives the UHF signal at shortened transmission interval Dr from any one of tire detectors  31  to  34 , it can determine the tire position of tire  10  to which each of tire detectors  31  to  34  is attached. 
       FIG.  11    is a flowchart showing tire position determination processing on the premise that tire rotation in  FIG.  7    is carried out. The flowchart shown in  FIG.  11    is performed by monitoring unit  45 . Monitoring unit  45  determines whether or not it has received the UHF signal at shortened transmission interval Dr (step S 301 ). When monitoring unit  45  does not receive the UHF signal at shortened transmission interval Dr (NO in step S 301 ), it repeats processing in step S 301 . 
     When monitoring unit  45  has received the UHF signal at shortened transmission interval Dr (YES in step S 301 ), it determines that the tire position of tire detector  30  that has transmitted the UHF signal at shortened transmission interval Dr has been interchanged in the front-rear direction and the lateral direction (step S 302 ). Hereafter, as in  FIG.  10   , an example in which the UHF signal is received at shortened transmission interval Dr from tire detector  34  attached to tire  14 , the position of which has been changed from the tire position “rear right side” to the “front left side,” will be described. 
     Specifically, in step S 301  in  FIG.  11   , monitoring unit  45  receives the UHF signal at shortened transmission interval Dr from tire detector  34 . Monitoring unit  45  obtains the “rear right side” from storage  46 , as the tire position of tire  14  which was determined with the tire position determination method described above. Monitoring unit  45  changes the tire position of tire  14  from the “rear right side” to the “front left side” based on reception of the UHF signal at shortened transmission interval Dr from tire detector  34  attached to tire  14 . In other words, monitoring unit  45  determines that the tire position of tire  14  has been interchanged in the front-rear direction and the lateral direction. 
     Then, monitoring unit  45  determines that the tire position of tire detector  30  which was attached at the tire position determined in step S 302  has been interchanged in the front-rear direction (step S 303 ). Specifically, monitoring unit  45  determines that the tire position of tire  11  that was attached at the tire position “front left side,” which is the position of rotated tire  14 , has been changed from the “front left side” to the “rear left side” and updates storage  46 . 
     Then, monitoring unit  45  determines that the tire position of tire detector  30  that was attached at the tire position determined in step S 303  has been interchanged in the front-rear direction and the lateral direction (step S 304 ). Specifically, monitoring unit  45  determines that the tire position of tire  13  that was attached at the tire position “rear left side,” which is the position of rotated tire  11 , has been changed from the “rear left side” to the “front right side” and updates storage  46 . 
     Then, monitoring unit  45  determines that the tire position of tire detector  30  that was attached at the tire position determined in step S 304  has been interchanged in the front-rear direction and the lateral direction (step S 305 ). Specifically, monitoring unit  45  determines that the tire position of tire  12  that was attached at the tire position “front right side,” which is the position of rotated tire  13 , has been changed from the “front right side” to the “rear right side” and updates storage  46 . 
     Thus, the vehicle-mounted system in the present embodiment can determine the tire position of each tire  10  based on reception at shortened transmission interval Dr from any one of tire detectors  31  to  34 . The vehicle-mounted system in the present embodiment can thus shorten the time period required for determination of the tire position of each tire  10 . 
     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 a modification thereof described above are specific examples of aspects below. 
     (1) A vehicle-mounted system according to one aspect of the present disclosure is a vehicle-mounted system provided in a vehicle including a tire, and includes a detector attached to the tire, the detector transmitting a detection signal, and a monitoring unit that determines a position of the tire based on a plurality of detection signals received from the detector. The detector determines a direction of revolution of the tire. When the detector determines change of the direction of revolution of the tire, the detector switches a frequency of transmission of the detection signal from a first frequency to a second frequency higher than the first frequency. 
   According to the aspect above, the vehicle-mounted system determines whether or not tires have been rotated based on change of the direction of revolution of the tire, and when it determines that the tires have been rotated, it increases the frequency of transmission of the detection signal. The vehicle-mounted system can thus shorten the time period required for determination of the tire position while it prevents increase in power consumption by the tire detector.   
   (2) In one aspect, when the detector determines stop of the vehicle for a prescribed period or longer and when the detector determines change of the direction of revolution of the tire from a direction of revolution before stop for the prescribed period or longer, the detector switches a frequency of output of the detection signal from the first frequency to the second frequency. 
   According to the aspect above, since the vehicle-mounted system performs processing for determining the direction of revolution of tire  10  only when it is highly likely that the tires have been rotated, processing load can be lessened.   
   (3) In one aspect, the detector determines the direction of revolution of the tire in forward travel of the vehicle based on a direction of an acceleration in a revolution circumferential direction of the tire. 
   According to the aspect above, tire detector  30  can determine the direction of revolution of tire  10  with the use of acceleration sensor  39 .   
   (4) In one aspect, a revolution body sensor that detects as first revolution angle information, an angle of revolution of a revolution body that revolves in synchronization with revolution of the tire is further provided. The detection signal includes second revolution angle information on which determination of an angle of revolution of the tire can be based. The monitoring unit determines a position of the tire based on the first revolution angle information and the second revolution angle information. 
   According to the aspect above, the vehicle-mounted system can determine the tire position based on the angle of revolution of the tire and the angle of revolution of the revolution body.   
   (5) In one aspect, the tire is a tire without a directional pattern. 
   According to the aspect above, tire detector  30  is attached to the tire the direction of revolution of which is highly likely to change in forward travel of vehicle  100  when tires have been rotated.   
   (6) In one aspect, the tire is attached to the vehicle as one of a front wheel and a rear wheel of the vehicle. When the monitoring unit receives the detection signal from the detector at the second frequency, the monitoring unit obtains information of another tire the direction of revolution of which is identical to the direction of revolution before change when viewed from a prescribed surface of the tire, and determines that a position of the another tire has changed from the other to one of the front wheel and the rear wheel of the vehicle. 
   According to the aspect above, by receiving the detection signal at the second frequency from one detector, monitoring unit  45  can determine the position of the tire that revolved in the direction of revolution identical to that of another tire to which the detector has been attached, and can shorten processing for determining the position of each tire  10  provided in vehicle  100 .   
   (7) In one aspect, the tire is attached to the vehicle as one of a right wheel and a left wheel with respect to a direction of an entire length of the vehicle and one of a front wheel and a rear wheel of the vehicle. When the monitoring unit receives the detection signal from the detector at the second frequency, the monitoring unit determines that a position of the tire has changed from one to the other of the right wheel and the left wheel with respect to the direction of the entire length of the vehicle and has changed from one to the other of the front wheel and the rear wheel of the vehicle, and determines that a position of another tire that was attached to the other of the right wheel and the left wheel with respect to the direction of the entire length of the vehicle and to the other of the front wheel and the rear wheel of the vehicle has changed from the other to one of the front wheel and the rear wheel of the vehicle. 
   According to the aspect above, by receiving the detection signal at the second frequency from one detector, monitoring unit  45  can determine the position of another tire and hence can shorten processing for determining the position of each tire  10  provided in vehicle  100 .   
   

     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.