Patent Publication Number: US-2023160920-A1

Title: Tire position determination system

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-188357 filed with the Japan Patent Office on Nov. 19, 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 tire position determination 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&#39;s are provided with an auto location function to automatically determine to which tire among a plurality of tires a detector is attached. For example, Japanese Patent Laying-Open No. 2019-48547 discloses a tire state information detection system that determines to which tire of double tires used in a truck and the like a detector is attached. 
     Among such TPMS&#39;s, there is a system including an initiator. The initiator transmits a command signal to a tire at a prescribed tire position. The detector transmits a response signal to a processing device such as an ECU provided on a vehicle body side based on reception of the command signal from the initiator. The processing device determines attachment of the detector that has transmitted the response signal to a tire at the prescribed tire position. 
     SUMMARY OF THE INVENTION 
     When the initiator is provided at each of a plurality of tire positions, however, cost may increase. On the other hand, when a single initiator is used to transmit a command signal to a plurality of detectors, the processing device provided on the vehicle body side may not be able to determine from which detector it receives the response signal. 
     The present disclosure was made to solve the problem described above, and an object thereof is to determine a tire position of each of a plurality of tires with the use of a single initiator. 
     A tire position determination system according to one aspect of the present disclosure is a tire position determination system provided in a vehicle including a first tire and a second tire different from the first tire. The tire position determination system includes an initiator that transmits a command signal, a first detector attached to the first tire, the first detector transmitting a detection signal when the first detector receives the command signal, a second detector attached to the second tire, the second detector transmitting a detection signal when the second detector receives the command signal, and a monitoring unit configured to receive the detection signal. A first distance between the first tire and the initiator is equal to or shorter than a second distance between the second tire and the initiator. Each of the first detector and the second detector includes an acceleration sensor that detects an acceleration in a direction orthogonal to a revolution axis direction. The detection signal includes a detection value from the acceleration sensor. When the monitoring unit receives the detection signal from the first detector or the second detector, the monitoring unit performs determination processing for determining whether a detector that has transmitted the detection signal is the first detector or the second detector based on positional relation between the detector that has transmitted the detection signal and the initiator estimated from the detection value from the acceleration sensor included in the received detection signal. 
     According to the aspect above, with the use of a value of the acceleration detected by each detector in addition to signal intensity of the detection signal received from each detector, a larger number of statuses of revolution of each tire can be specified. As variations of the statuses of revolution are wider, a larger number of tire positions can be specified. The tire position determination system thus determines a tire position of each of a plurality of tires with the use of a single initiator. 
     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 tire position determination system according to a first embodiment is applied. 
         FIG.  2    is a side view of the vehicle. 
         FIG.  3    is a block diagram showing an exemplary configuration of a tire detector. 
         FIG.  4    is a diagram showing an exemplary appearance of the tire detector. 
         FIG.  5    is a transition diagram of arrangement of the tire detector when a tire revolves. 
         FIG.  6    shows a graph of exemplary attenuation when radio wave intensity T1 is set as transmission intensity. 
         FIG.  7    is a diagram illustrating the graph shown in  FIG.  6    by referring to an initiator and an appearance of the tire. 
         FIG.  8    is a flowchart showing exemplary tire position determination processing in the first embodiment. 
         FIG.  9    is a diagram for illustrating arrangement of the initiator in a modification of the first embodiment. 
         FIG.  10    shows a graph showing exemplary attenuation when radio wave intensity T2 is set as transmission intensity. 
         FIG.  11    is a diagram illustrating the graph shown in  FIG.  10    by referring to the initiator and an appearance of the tire. 
         FIG.  12    is a flowchart showing exemplary tire position determination processing in a second embodiment. 
         FIG.  13    is a diagram for illustrating arrangement of the initiator in a modification of the second embodiment. 
     
    
    
     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. 
     First Embodiment 
     &lt;Overall Configuration&gt; 
       FIG.  1    is a diagram schematically showing a configuration of a vehicle  100  to which a tire position determination system according to a first embodiment is applied. 
     Vehicle  100  according to the first embodiment is a vehicle including tires  11  and  12  on a front side which are steering wheels and tires  13  to  16  on a rear side which are non-steering wheels. Tires  11  to  16  are each in such a form that a single tire is attached at a single tire attachment position. A direction FR shown in  FIG.  1    represents a direction of forward travel of vehicle  100 . Tires  13  to  16  may each be such double (twin or dual) tires that two tires are attached at a single tire attachment position. 
     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  36  each detecting a tire pressure, initiators  61  and  62 , and a TPMS receiver  40 . Tire detectors  31  to  36  are attached to wheels of tires  11  to  16 , respectively. Tire detectors  31  to  36  may each be formed integrally with a valve for intake of air into each tire. Tire detectors  31  to  36  may each be formed separately from the valve. 
     Each of tire detectors  31  to  36  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  36  are thus intermittently activated at timings different from one another and transmit UHF signals. 
     The UHF signals outputted from tire detectors  31  to  36  include information indicating specific ID numbers for identifying at least respective tire detectors  31  to  36 . Specifically, the UHF signals outputted from tire detectors  31  to  36  include ID numbers “01” to “06”, respectively. 
     The UHF signals outputted from tire detectors  31  to  36  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  36 , it can monitor a pneumatic pressure of each tire. 
     Tires identical in specifications and construction are employed as tires  11  to  16  for allowing tire rotation. Therefore, tire detectors identical in configuration are adopted also for tire detectors  31  to  36 . When tires  11  to  16  do not have to be described as being distinguished from one another, tires  11  to  16  are simply referred to as a “tire  10 ” below. When tire detectors  31  to  36  do not have to be described as being distinguished from one another, tire detectors  31  to  36  are simply referred to as a “tire detector  30 .” Tires  11  to  16  are identical in tire diameter. 
     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 first embodiment, six tire positions (a front left side, a front right side, a rear first-row left side, a rear first-row right side, a rear second-row left side, and a rear second-row right side) in total are stored in correspondence with the respective ID numbers of tire detectors  30 . 
     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 first-row left side” is brought in correspondence with the ID number “03” and the tire position “rear first-row right side” is brought in correspondence with the ID number “04”. The tire position “rear second-row left side” is brought in correspondence with the ID number “05” and the tire position “rear second-row right side” is brought in correspondence with the ID number “06”. 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  determines 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  determines the tire pressure 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. 
     Initiators  61  and  62  for activating tire detectors  33  to  36  on the rear side are electrically connected to TPMS receiver  40 . Initiator  61  is arranged in the vicinity of tire  13  on the left side in the rear first row and used for activation of tire detectors  33  and  35 . Initiator  62  is arranged in the vicinity of tire  14  on the right side in the rear first row and used for activation of tire detectors  34  and  36 . 
     Initiators  61  and  62  identical in configuration are adopted. When initiators  61  and  62  do not have to be described as being distinguished from each other, they are also denoted as an “initiator  60 ” below without being distinguished from each other. 
     Initiator  60  includes a not-shown antenna, and is configured to output a radio signal in a low frequency (LF) band (which is also simply referred to as an “LF signal” below) from the antenna. Initiator  60  transmits the LF signal to tire detector  30  based on an instruction from monitoring unit  45 . The LF signal is a command signal for instructing tire detector  30  to perform a specific operation. 
     Each tire detector  30  can receive the LF signal from initiator  60 . Each tire detector  30  is configured to output a UHF signal when the prescribed activation condition described above is satisfied. The “prescribed activation condition” in the first embodiment includes reception of the LF signal. In other words, each tire detector  30  transmits the UHF signal on condition that it receives the LF signal. 
     Relation between each tire position and the position of initiator  61  or  62  is stored in storage  46 . For example, a tire position closest to initiator  61  being “rear first-row left” and a tire position second closest thereto being “rear second-row left” are stored in storage  46 . 
       FIG.  2    is a side view of vehicle  100 .  FIG.  2    shows vehicle  100  when viewed from a side of the negative direction of the Y axis. Initiators  61  and  62  are arranged on a side of the positive direction along the X axis of tires  13  and  14  on the rear side. In other words, tire  13  is arranged between initiator  61  and tire  15 . Tire  14  is arranged between initiator  62  and tire  16 . 
     &lt;Configuration of Tire Detector  30 &gt; 
     An exemplary configuration of tire detector  30  will be described below with reference to  FIGS.  3  and  4   .  FIG.  3    is a block diagram showing an exemplary configuration of tire detector  30 . As shown in  FIG.  3   , tire detector  30  includes a controller  85 , a pressure sensor  38 , an acceleration sensor (G sensor)  39 , antennas L 1  and A 2 , a reception circuit CR, and a transmission circuit CT. 
     Controller  85  includes a storage  86  and a processing unit  87 . Processing unit  87  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  87  are described in the program stored in the ROM. 
     In storage  86 , an ID number specific for each tire detector  30  shown in  FIG.  1    is stored. In storages  86  of tire detectors  31  to  36 , “01” to“06” are stored as the ID numbers, respectively. 
     Antenna L 1  receives the LF signal transmitted from initiator  61  or  62 . Controller  85  accepts the LF signal received by antenna L 1  through reception circuit CR. Reception circuit CR detects reception intensity of the LF signal received by antenna L 1 . 
     Reception circuit CR outputs a voltage in accordance with radio wave intensity (received signal strength indicator (RSSI) signal) of the inputted LF signal. Controller  85  obtains intensity (which is referred to as an “RSSI value” below) of the received signal (radio wave) resulting from A/D conversion of this voltage. The RSSI value in the first embodiment is obtained as a voltage ratio [dBμV] to 1 μV. The unit of the RSSI value may be a voltage [V] or power [W]. Reception circuit CR is configured not to receive the LF signal lower than radio wave intensity M but to receive the LF signal equal to or higher than radio wave intensity M. 
     Controller  85  controls transmission circuit CT to transmit a UHF signal from antenna A 2 . Controller  85  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 the 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  85  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 tire detector  30  in the first embodiment, the prescribed activation condition includes reception of the LF signal from initiator  60 . Tire detector  30  transmits a UHF signal including detection information representing an ID number, a tire pressure P, acceleration G, and an RSSI value to monitoring unit  45  by being triggered by reception of the LF signal. 
     Pressure sensor  38  detects a tire pressure and outputs a result of detection (which is also referred to as “tire pressure P” below) to controller  85 . Acceleration sensor  39  detects an acceleration in a uniaxial direction generated in a direction orthogonal to a revolution axis direction of tire  10  and outputs a result of detection to controller  85 . Acceleration sensor  39  in the first embodiment has a revolution circumferential direction of tire  10  as a detection direction. 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.  4    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.  4    shows a revolution axis direction RD1, a revolution circumferential direction RD2, and a revolution diameter direction RD3 of wheel WH when tire  10  revolves. As described above, acceleration sensor  39  of tire detector  30  in the first embodiment is the uniaxial acceleration sensor having revolution circumferential direction RD2 as the detection direction. 
     &lt;Detection Value from Acceleration Sensor  39 &gt; 
       FIG.  5    is a transition diagram of arrangement of tire detector  33  when tire  13  revolves.  FIG.  5    shows transition of arrangement of tire detector  33  when tire  13  is viewed from the negative side (outer side of vehicle  100 ) in the Y-axis direction.  FIG.  5    shows the detection value from acceleration sensor  39  while vehicle  100  remains stopped. 
       FIG.  5    shows arrangements  1   h  to  12   h  as twelve patterns of exemplary arrangement of tire detector  33 . Arrangement  12   h  of tire detector  33  is such an arrangement that tire detector  33  is located in revolution diameter direction D3 extending from a central point CP3 of tire  13  in the positive direction along the Z-axis. Arrangement  12   h  is referred to as arrangement at “0 degree” or “+360 degrees” below. 
     Arrangement  1   h  represents arrangement of tire detector  33  when tire  13  revolves by 0 degrees clockwise from the state of arrangement  12   h.  0 degrees in  FIG.  5    is set to thirty degrees. Arrangement  1   h  is referred to as arrangement at “+30 degrees” below. Arrangement  2   h  represents arrangement of tire detector  33  when tire  13  revolves by 0 degrees clockwise from the state of arrangement  1   h . Arrangement  2   h  is referred to as arrangement at “+60 degrees” below. 
     Arrangement  3   h  represents arrangement of tire detector  33  when tire  13  revolves by 0 degrees clockwise from the state of arrangement  2   h . Arrangement  3   h  is referred to as arrangement at “+90 degrees” below.  FIG.  5    thus illustrates twelve patterns of exemplary arrangement of tire detector  33  at 0 degree (360 degrees), +30 degrees, +60 degrees, +90 degrees, +120 degrees, +150 degrees, +180 degrees, +210 degrees, +240 degrees, +270 degrees, +300 degrees, and +330 degrees. 
     As described with reference to  FIG.  4   , acceleration sensor  39  of tire detector  33  is a uniaxial acceleration sensor that detects an acceleration only in one direction and has a tire circumferential direction (revolution circumferential direction RD2) as the detection direction. Therefore, as shown in  FIG.  5   , an acceleration of gravity in the detection direction is highest at arrangement  3   h  (+90 degrees) or arrangement  9   h  (+270 degrees) of tire detector  33 . 
     In the example in  FIG.  5   , tire  33  is attached such that the detection value from acceleration sensor  39  at arrangement  9   h  is +1 G. In other words, the detection value from acceleration sensor  39  when tire detector  33  is at arrangement  3   h  is −1 G. 
     When tire detector  33  is at arrangement  8   h  or arrangement  10   h  the detection value from acceleration sensor  39  is +√ 3/2 G. When tire detector  33  is at arrangement  7   h  or arrangement  11   h , the detection value from acceleration sensor  39  is +½ G. When tire detector  33  is at arrangement  12   h  or arrangement  6   h , the detection value from acceleration sensor  39  is 0 G. 
     When tire detector  33  is at arrangement  1   h  or arrangement  5   h , the detection value from acceleration sensor  39  is −½ G. When tire detector  33  is at arrangement  2   h  or arrangement  4   h , the detection value from acceleration sensor  39  is −√ 3/2 G. Depending on a direction of attachment of tire detector  33 , positive and negative signs of the acceleration of gravity as the detection value shown in  FIG.  5    may be reversed. 
     Tire detector  33  transmits the UHF signal including the detection value from acceleration sensor  39  to monitoring unit  45 . Monitoring unit  45  can estimate arrangement of tire detector  33  based on the detection value from acceleration sensor  39 . As shown in  FIG.  5   , the detection values from acceleration sensor  39  are in line symmetry with respect to the Y axis that passes through central point CP3. 
     Estimation of arrangement of tire detector  33  based on the detection value from acceleration sensor  39  in the first embodiment will more specifically be described. As described with reference to  FIGS.  1  and  2   , initiator  61  is arranged on the side of the positive direction along the X axis of tire  13 . With a straight line in revolution diameter direction RD3 extending from central point CP3 of tire  13  shown in  FIG.  5    toward the positive direction along the Z-axis being defined as a boundary, a region of tire  13  can be divided into a region on a side close to initiator  61  and a region on a side distant from initiator  61 . In the example in  FIG.  5   , arrangement  7   h  to arrangement  11   h  are arrangements in the region on the side close to initiator  61 . Arrangement  1   h  to arrangement  5   h  are arrangements in the region on the side distant from initiator  61 . 
     Initiator  61  is arranged on the side of the positive direction in the X-axis direction of tire  13 . When arrangement of tire detector  33  is on the side close to initiator  61 , the detection values from acceleration sensor  39  are all positive. When arrangement of tire detector  33  is on the side distant from initiator  61 , the detection values from acceleration sensor  39  are all negative. By determining whether the detection value from acceleration sensor  39  included in the received UHF signal is positive or negative, monitoring unit  45  can determine whether tire detector  33  is arranged on the side close to or distant from initiator  61 . 
     Monitoring unit  45  can obtain at least two arrangements as arrangement candidates for tire detector  33  based on the detection value from acceleration sensor  39 . For example, when monitoring unit  45  finds the detection value from acceleration sensor  39  as +√ 3/2 G, tire detector  33  obtains arrangement  8   h  and arrangement  10   h  as arrangement candidates. Alternatively, when monitoring unit  45  finds the detection value from acceleration sensor  39  as −½ G, tire detector  33  obtains arrangement  1   h  and arrangement  5   h  as arrangement candidates. 
     When the detection value from acceleration sensor  39  is +1 G, monitoring unit  45  estimates that tire detector  33  is in arrangement  9   h . When the detection value from acceleration sensor  39  is −1 G, monitoring unit  45  estimates that tire detector  33  is in arrangement  3   h . For tire detector  35  attached to tire  15  as well, relation between arrangement of tire detector  35  and the detection value from acceleration sensor  39  is similar to relation between the arrangement of tire detector  33  and the detection value from acceleration sensor  39  described with reference to  FIG.  5   . 
     Acceleration sensor  39  of tire detector  30  in the first embodiment may detect the acceleration generated in revolution diameter direction RD3 (centrifugal direction). When the acceleration generated in revolution diameter direction RD3 is detected, detection values from acceleration sensor  39  are in line symmetry with respect to the Z-axis that passes through central point CP3. Therefore, monitoring unit  45  is unable to determine whether or not tire detector  30  is arranged in the region close to the initiator only based on the positive and negative signs of the detection value from acceleration sensor  39 . 
     When acceleration sensor  39  detects the acceleration generated in revolution diameter direction RD3, monitoring unit  45  detects the acceleration consecutively two times at intervals at least shorter than a period of revolution of tire  10  by ninety degrees. Since monitoring unit  45  can thus uniquely determine arrangement of tire detector  30  based on the detection value from acceleration sensor  39 , it can determine whether or not tire detector  30  is arranged in the region close to the initiator. 
     &lt;As to Attenuation of Radio Wave Intensity&gt; 
       FIG.  6    shows a graph of exemplary attenuation when radio wave intensity T1 is set as transmission intensity. The abscissa in  FIG.  6    represents a distance (unit: m) of radiation of the LF signal from initiator  61  and the ordinate represents radio wave intensity (unit: W) of the LF signal. As the distance is longer, an amount of attenuation is larger and radio wave intensity of the LF signal gradually becomes lower. As the distance of radiation of the LF signal emitted from initiator  61  is longer, the amount of attenuation of radio wave intensity of the LF signal is larger.  FIG.  6    shows radio wave intensities L and H. Radio wave intensity H is higher than radio wave intensity L. 
     Radio wave intensity H may correspond to the “first threshold value” in the present disclosure. Radio wave intensity L may correspond to the “second threshold value” in the present disclosure. 
       FIG.  6    shows an example in which initiator  61  transmits the LF signal with radio wave intensity T1 being set as transmission intensity. Radio wave intensity at the time of transmission from initiator  61  is referred to as “transmission intensity” below. On the other hand, radio wave intensity at the time when tire detector  30  receives the LF signal with radio wave intensity thereof being attenuated is referred to as “reception intensity.” Radio wave intensity T1 represents radio wave intensity at which both of tire detectors  33  and  35  can sufficiently receive the LF signal even though the LF signal attenuates. 
     A width Wd3 represents a width of a range of possible distances between tire detector  33  and initiator  61 . A width Wd5 represents a width of a range of possible distances between tire detector  35  and initiator  61 . Arrangement of tire detectors  33  and  35  changes with revolution of tires  13  and  15 . Therefore, distances between tire detectors  33  and  35  and initiator  61  change within the ranges of width Wd3 and Wd5. Width Wd3 and Wd5 can be estimated from arrangement of initiator  61  and tires  13  and  15  and the tire diameter of tires  13  and  15 . 
     As shown in  FIG.  6   , radio wave intensity H corresponds to a distance at the center of width Wd3. Radio wave intensity L corresponds to a distance at the center of width Wd5. Radio wave intensities H and L are stored in storage  46  of monitoring unit  45 . 
     Monitoring unit  45  in the first embodiment determines the tire position of tire  10  to which tire detector  30  is attached based on the RSSI value. More specifically, when monitoring unit  45  receives the UHF signal including the RSSI value equal to or higher than radio wave intensity H, it determines that the UHF signal has been transmitted from tire detector  33  attached to tire  13  close to initiator  61 . When monitoring unit  45  receives the UHF signal including the RSSI value lower than radio wave intensity L, it determines that the UHF signal has been transmitted from tire detector  35  attached to tire  15  distant from initiator  61 . 
     When monitoring unit  45  receives the UHF signal including the RSSI value equal to or higher than radio wave intensity L and lower than radio wave intensity H, it may not be able to determine the tire position of tire detector  30  that has transmitted the UHF signal. An example in which the monitoring unit is unable to determine the tire position will be described by referring to a distance D1 and a distance D2. 
     Distance D1 is a distance which is included within width Wd3 and relatively close to width Wd5. Distance D2 is a distance which is included within width Wd5 and relatively close to width Wd3. According to the graph shown in  FIG.  6   , reception intensity at distance D1 is radio wave intensity R1 and reception intensity at distance D2 is radio wave intensity R2. 
     Though radio wave intensity of the LF signal attenuates with the graph shown in  FIG.  6    being defined as the reference, an error may be caused by various factors. Specifically, the amount of attenuation of radio wave intensity of the LF signal is affected by an ambient environment and radio wave intensity may change by an error from the graph in  FIG.  6   . Reception intensity at distance D1 may thus attain to radio wave intensity R2 and reception intensity at distance D2 may attain to radio wave intensity R1. 
     When the RSSI value included in the UHF signal is equal to or higher than radio wave intensity L and lower than radio wave intensity H, in consideration of the error, monitoring unit  45  is unable to determine the tire position only based on the RSSI value. When monitoring unit  45  in the first embodiment receives the UHF signal including radio wave intensity equal to or higher than radio wave intensity L and lower than radio wave intensity H, it determines the tire position with a tire position determination method which will be described later. 
       FIG.  7    is a diagram illustrating the graph shown in  FIG.  6    by referring to initiator  61  and an appearance of tires  13  and  15 .  FIG.  7    shows initiator  61  and tires  13  and  15  when viewed from the side of the negative direction along the Y axis as in  FIG.  2   . 
     A distance D3 represents a distance between tire detector  33  at arrangement  3   h  and tire detector  33  at arrangement  9   h . Since tire  13  and tire  15  are identical in tire diameter, distance D3 is defined also as a distance between tire detector  35  at arrangement  3   h  and tire detector  35  at arrangement  9   h . A distance D4 represents a distance between tire detector  33  at arrangement  3   h  and tire detector  35  at arrangement  9   h.    
     Though distances D3 and D4 in  FIG.  7    represent only the distance along the X-axis direction for the sake of convenience of illustration, distances D3 and D4 are each a distance in three dimensions. Since distance D4 at the time when tire detector  33  and tire detector  35  are closest to each other is thus short, probability of reception of the LF signal by both of tire detector  33  and tire detector  35  as a result of an error in the amount of attenuation of the LF signal becomes high. In other words, monitoring unit  45  may not be able to determine the tire position only based on the RSSI value. In the first embodiment, distance D4 is shorter than half distance D2. 
     A line LnL is a line that shows in a simplified manner, a boundary at which reception intensity attains to radio wave intensity L when it is assumed that the amount of attenuation follows the graph shown in  FIG.  6    without consideration of the error. A line LnH is a line that similarly shows in a simplified manner, a boundary at which reception intensity attains to radio wave intensity H when it is assumed that the amount of attenuation follows the graph shown in  FIG.  6    without consideration of the error. 
     A distance between a central point CP1 of initiator  61  and central point CP3 of tire  13  may correspond to the “first distance” in the present disclosure. A distance between central point CP1 of initiator  61  and a central point CP5 of tire  15  may correspond to the “second distance” in the present disclosure. 
     Tire Position Determination in First Embodiment 
     When the tire position determination system in the first embodiment receives the UHF signal including radio wave intensity equal to or higher than radio wave intensity L and lower than radio wave intensity H, it determines the tire position based on arrangement of tire detector  30  estimated from the detection value from acceleration sensor  39 . 
       FIG.  8    is a flowchart showing exemplary tire position determination processing in the first embodiment. Monitoring unit  45  determines whether or not vehicle  100  has stopped traveling (step S 101 ). Monitoring unit  45  determines whether or not vehicle  100  has stopped with the use of a not-shown vehicle velocity sensor. When vehicle  100  has not stopped traveling (NO in step S 101 ), monitoring unit  45  repeats processing in step S 101 . 
     When vehicle  100  has stopped traveling (YES in step S 101 ), initiator  61  is instructed to transmit the LF signal at radio wave intensity T1 (step S 102 ). Initiator  61  receives the transmission instruction and transmits the LF signal at radio wave intensity T1. Each of tire detectors  33  and  35  transmits the UHF signal in response to reception of the LF signal. 
     Monitoring unit  45  receives the UHF signal (step S 103 ). Monitoring unit  45  determines whether or not the RSSI value included in the received UHF signal is equal to or higher than radio wave intensity H (step S 104 ). When the RSSI value is equal to or higher than radio wave intensity H (YES in step S 104 ), monitoring unit  45  determines that the UHF signal has been transmitted from tire detector  33  of tire  13  attached at the tire position close to initiator  61  (step S 105 ). In other words, monitoring unit  45  determines that the tire position of tire detector  30  that has transmitted the UHF signal received in step S 103  is “rear first-row left.” 
     When the RSSI value is lower than radio wave intensity H (NO in step S 104 ), monitoring unit  45  determines whether or not the RSSI value included in the received UHF signal is lower than radio wave intensity L (step S 106 ). When the RSSI value is lower than radio wave intensity L (YES in step S 106 ), monitoring unit  45  determines that the UT-IF signal has been transmitted from tire detector  35  of tire  15  attached at the tire position distant from initiator  61  (step S 107 ). In other words, monitoring unit  45  determines that the tire position of tire detector  30  that has transmitted the UHF signal received in step S 103  is “rear second-row left.” 
     When the RSSI value is not lower than radio wave intensity L (NO in step S 106 ), monitoring unit  45  determines whether or not the detection value from acceleration sensor  39  included in the UHF signal received in step S 103  is positive (step S 108 ). In other words, monitoring unit  45  determines whether or not tire detector  30  is arranged in the region in tire  10  close to the initiator based on the detection value from acceleration sensor  39 . Monitoring unit  45  can thus estimate positional relation between tire detector  30  and initiator  61  based on the detection value from acceleration sensor  39 . 
     When the detection value from acceleration sensor  39  is positive (YES in step S 108 ), monitoring unit  45  determines that the UHF signal in step S 103  has been transmitted from tire detector  35  of tire  15  at the tire position distant from initiator  61  (step S 109 ). 
     As shown in  FIG.  7   , between line LnL and line LnH, tire detector  35  of tire  15  is arranged in the region close to initiator  61  in tire  15 . On the other hand, between line LnL and line LnH, tire detector  33  of tire  13  is arranged in the region distant from initiator  61  in tire  13 . Therefore, monitoring unit  45  can determine that the tire position of tire detector  30  that has transmitted the UHF signal received in step S 103  is “rear second-row left.” 
     When the detection value from acceleration sensor  39  is not positive (NO in step S 108 ), monitoring unit  45  determines whether or not the detection value from acceleration sensor  39  included in the UHF signal received in step S 103  is negative (step S 110 ). In other words, monitoring unit  45  determines whether or not tire detector  30  is arranged in the region distant from the initiator in tire  10  based on the detection value from acceleration sensor  39 . Monitoring unit  45  can thus estimate positional relation between tire detector  30  and initiator  61  based on the detection value from acceleration sensor  39 . 
     When the detection value from acceleration sensor  39  is negative (YES in step S 110 ), monitoring unit  45  determines that the UHF signal in step S 103  has been transmitted from tire detector  33  in tire  13  at the tire position close to initiator  61  (step S 111 ). Monitoring unit  45  can determine that the tire position of tire detector  30  that has transmitted the UHF signal received in step S 103  is “rear first-row left.” 
     When the detection value from acceleration sensor  39  is not negative (NO in step S 110 ), monitoring unit  45  quits the process without determining the tire position. This is because, even based on the detection value from acceleration sensor  39 , monitoring unit  45  is unable to determine the tire position when tire detector  30  is in arrangement  12   h  or arrangement  6   h.    
     Thus, in the first embodiment, even when monitoring unit  45  receives the UHF signal including the RSSI value lower than radio wave intensity H and equal to or higher than radio wave intensity L, it can determine the tire position based on arrangement of tire detector  30  estimated from the detection value from acceleration sensor  39 . The tire position determination system in the first embodiment can thus determine the tire position of each of tire  13  and tire  15  with the use of single initiator  61 , without providing the initiator for each of tire  13  and tire  15 . 
       FIG.  8    illustrates a configuration in which monitoring unit  45  has initiator  61  transmit the LF signal when vehicle  100  stops. In one aspect, monitoring unit  45  may have initiator  61  transmit the LF signal also while vehicle  100  is traveling. In this case, monitoring unit  45  obtains only the acceleration of gravity by removing centrifugal force generated by travel of vehicle  100  from the detection value from acceleration sensor  39 . Monitoring unit  45  calculates centrifugal force generated by travel of vehicle  100  in accordance with the velocity of vehicle  100  received from a not-shown speedometer. 
     Modification of First Embodiment 
     In the first embodiment, tire  13  is arranged between initiator  61  and tire  15 . Initiator  61 , however, is not necessarily arranged at the position shown in  FIG.  7    and may be arranged at various positions. 
       FIG.  9    is a diagram for illustrating arrangement of initiator  61  in a modification of the first embodiment. As shown in  FIG.  9   , initiator  61  is arranged on the side of the positive direction along the Z-axis of tire  13 . Concentric circles shown with dashed lines around the transmission circuit of initiator  61  represent radio wave intensities of the LF signal. 
     The tire position determination system in the modification of the first embodiment determines radio wave intensities L and H defined as the threshold values in accordance with arrangement of initiator  61  and tires  13  and  15 . As shown in  FIG.  9   , in the tire position determination system in the modification of the first embodiment, line LnH represents a boundary line in contact with tire  15 . In other words, radio wave intensity H is set such that tire  15  is not included but at least a part of tire  13  is included in a range equal to or higher than radio wave intensity H. 
     In the tire position determination system in the modification of the first embodiment, line LnL represents a boundary line in contact with tire  13 . In other words, radio wave intensity L is set such that tire  13  is not included but at least a part of tire  15  is included in a range lower than radio wave intensity L. 
     In the modification of the first embodiment, the region close to initiator  61  in tire  13  is a region where radio wave intensity is equal to or higher than radio wave intensity H and the region distant from initiator  61  in tire  13  is a region where radio wave intensity is lower than radio wave intensity H. The region close to initiator  61  in tire  15  is a region where radio wave intensity is equal to or higher than radio wave intensity L and the region distant from initiator  61  in tire  15  is a region where radio wave intensity is lower than radio wave intensity L. 
     Thus, even in an example in which initiator  61  is arranged as in  FIG.  9   , monitoring unit  45  can determine the tire position based on whether tire detector  33  or  35  is arranged in the region close to or distant from initiator  61  when the RSSI value is lower than radio wave intensity H and equal to or higher than radio wave intensity L. The tire position determination system in the first embodiment is thus applicable without limitation being imposed by arrangement of initiator  61 . 
     In the modification of the first embodiment, in the tire position determination system, initiator  61  may be arranged equidistantly from the tire center of tire  13  and the tire center of tire  15 . In this case, in the tire position determination system, when a combination of the RSSI value in the signal received from tire detector  33  and the detection value from acceleration sensor  39  of tire detector  33  is identical to a combination of the RSSI value in the signal received from tire detector  35  and the detection value from acceleration sensor  39  of tire detector  35 , monitoring unit  45  may discard data representing the RSSI value and the detection value from acceleration sensor  39 , and when the combinations of the RSSI value and the detection value from acceleration sensor  39  are different from each other, monitoring unit  45  may specify the tire position. 
     Second Embodiment 
     In the first embodiment described above, an example in which radio wave intensity T1 at which both of tire detectors  33  and  35  are able to sufficiently receive the LF signal even in consideration of attenuation of the LF signal is set as transmission intensity is described. In a second embodiment, an example in which initiator  61  sets as transmission intensity, radio wave intensity T2 at which only tire detector  33  is able to receive the LF signal in consideration of attenuation of the LF signal will be described. In the second embodiment, description of the configuration similar to that in the tire position determination system in the first embodiment will not be repeated. 
       FIG.  10    shows a graph of exemplary attenuation when radio wave intensity T2 is set as transmission intensity.  FIG.  10    shows radio wave intensity M. Radio wave intensity M refers to radio wave intensity indicating a boundary beyond which tire detector  30  is unable to receive the LF signal. Specifically, tire detector  30  is configured to be able to receive the LF signal equal to or higher than radio wave intensity M but not to be able to receive the LF signal lower than radio wave intensity M. Reception circuit CR of tire detector  30  is configured to receive the LF signal equal to or higher than radio wave intensity M when antenna L 1  senses that LF signal. Radio wave intensity M may correspond to the “third threshold value” in the present disclosure. 
     As shown in  FIG.  10   , radio wave intensity T2 which is transmission intensity is set such that radio wave intensity M defined as a reception boundary corresponds to a distance longer than a maximum distance of width Wd3 and shorter than a minimum distance of width Wd5 due to attenuation of the LF signal. Thus, when various factors such as an ambient environment are not taken into account, only tire detector  33  is able to receive the LF signal. As described above, however, the amount of attenuation of radio wave intensity of the LF signal may be changed by the ambient environment. Therefore, depending on arrangement of tire detector  35 , tire detector  35  may receive the LF signal at transmission intensity of radio wave intensity M. 
       FIG.  11    is a diagram illustrating the graph shown in  FIG.  10    by referring to initiator  61  and an appearance of tires  13  and  15 . A line LnM is a line that shows in a simplified manner, a boundary at which radio wave intensity of the LF signal attains to radio wave intensity M due to attenuation. As shown in  FIG.  11   , radio wave intensity M is set such that line LnM is arranged between tire  13  and tire  15 . When tire detector  35  is arranged in the region close to the initiator such as arrangement  9   h  and when the amount of attenuation of radio wave intensity of the LF signal decreases due to the ambient environment, tire detector  35  receives the LF signal. In  FIG.  11   , in response to reception of the LF signal by tire detector  35 , tire detector  35  transmits the UHF signal. 
     When tire detector  35  receives the LF signal due to an error caused in the amount of attenuation, tire detector  35  is arranged in the region close to initiator  61 , and in this case, both of tire detectors  33  and  35  may receive the LF signal. When tire detector  35  is in the region distant from initiator  61 , it is arranged at a position distant from line LnM. Therefore, even when an error is caused in the amount of attenuation, tire detector  35  is unable to receive the LF signal. In this case, only tire detector  33  receives the LF signal. 
     In the second embodiment, a method of determining a tire position based on a detection value from acceleration sensor  39  even when tire detector  35  receives the LF signal due to an error as shown in  FIG.  11    is described. Radio wave intensity T2 may correspond to the “first radio wave intensity” in the present disclosure. 
     As to Tire Position Determination in Second Embodiment 
     The tire position determination system in the second embodiment has the LF signal transmitted with transmission intensity thereof being set to radio wave intensity T2, and determines the tire position based on arrangement of tire detector  30  estimated from the detection value from acceleration sensor  39 . 
       FIG.  12    is a flowchart showing exemplary tire position determination processing in the second embodiment. Monitoring unit  45  determines whether or not vehicle  100  has stopped traveling (step S 201 ). When vehicle  100  has not stopped traveling (NO in step S 201 ), monitoring unit  45  repeats processing in step S 201 . 
     When vehicle  100  has stopped traveling (YES in step S 201 ), initiator  61  is instructed to transmit the LF signal at radio wave intensity T2 (step S 202 ). Initiator  61  receives the transmission instruction and transmits the LF signal at radio wave intensity T2. When each of tire detector  33  and tire detector  35  receives the LF signal, it transmits the UHF signal. 
     Monitoring unit  45  receives the UHF signal (step S 203 ). Monitoring unit  45  determines whether or not the detection value from acceleration sensor  39  included in the received UHF signal is negative (step S 204 ). In other words, monitoring unit  45  determines whether or not tire detector  30  that has transmitted the UHF signal received in step S 203  is arranged in the region distant from the initiator in tire  10 . 
     When the detection value from acceleration sensor  39  is not negative (NO in step S 204 ), monitoring unit  45  quits the process. When the detection value is not negative, tire detector  30  that has transmitted the UHF signal in step S 203  is arranged in the region close to initiator  61 . As described above, when an error is caused in the amount of attenuation and when tire detector  35  is arranged in the region close to initiator  61  in tire  15 , tire detector  35  may receive the LF signal and transmit the UHF signal. 
     Therefore, when monitoring unit  45  receives the UHF signal from tire detector  30  arranged in the region close to initiator  61 , it is unable to determine the tire position and quits the process as shown in  FIG.  12   . In other words, monitoring unit  45  discards data on the UHF signal received in step S 203 . Monitoring unit  45  may have data on the UHF signal stored in storage  46 , instead of discarding the same. 
     When the detection value from acceleration sensor  39  is negative (YES in step S 204 ), monitoring unit  45  can determine that the UHF signal received in step S 203  has been transmitted from tire detector  33  and determines that the tire position is “rear first-row left” (step S 206 ). As shown in  FIG.  11   , tire detector  30  that can receive the LF signal the transmission intensity of which has attained to radio wave intensity M while it is arranged in the region distant from initiator  61  is only tire detector  33 . At this time, monitoring unit  45  has the tire position “rear first-row left” and the ID number included in the UHF signal received in step S 203  stored in storage  46  in correspondence with each other. 
     Then, monitoring unit  45  has initiator  61  transmit the LF signal the transmission intensity of which is set to radio wave intensity T1 (step S 207 ). Monitoring unit  45  determines whether or not it receives the UHF signal with an ID number different from the ID number included in the UHF signal received in step S 203  (step S 208 ). When monitoring unit  45  does not receive the UHF signal including the different ID number (NO in step S 208 ), the process returns to step S 207  and monitoring unit  45  has initiator  61  transmit the LF signal again. 
     When monitoring unit  45  receives the UHF signal including the different ID number (YES in step S 208 ), it can determine that the UHF signal with the different ID number has been transmitted from tire detector  35  and determines that the tire position is “rear second-row left” (step S 209 ). Monitoring unit  45  has the tire position “rear second-row left” and the ID number received at a branch in step S 208  stored in storage  46  in correspondence with each other. 
     Thus, the tire position determination system in the second embodiment can determine a plurality of tire positions with the use of single initiator  61  based on the detection value from acceleration sensor  39 , also when it has initiator  61  transmit the LF signal the transmission intensity of which is set to radio wave intensity T2. 
       FIG.  12    illustrates a configuration in which monitoring unit  45  has initiator  61  transmit the LF signal when vehicle  100  stops. In one aspect, monitoring unit  45  may have initiator  61  transmit the LF signal while vehicle  100  is traveling, by removing centrifugal force in accordance with a velocity of vehicle  100  as in  FIG.  8   . 
     Modification of Second Embodiment 
     Tire  13  is arranged between initiator  61  and tire  15  also in the second embodiment. Initiator  61 , however, is not necessarily arranged at the position shown in  FIG.  7    and may be arranged at various positions. 
       FIG.  13    is a diagram for illustrating arrangement of initiator  61  in a modification of the second embodiment. Initiator  61  in  FIG.  13    is arranged at a position similar to the position in  FIG.  9   . In the modification of the second embodiment, as shown in  FIG.  13   , line LnM is arranged between line LnL and line LnH. In other words, radio wave intensity M has a value calculated by adding a half value of a value calculated by subtracting radio wave intensity L from radio wave intensity H, to radio wave intensity L. 
     Thus, even in an example where initiator  61  is arranged as in  FIG.  13   , monitoring unit  45  can determine the tire position when transmission intensity of the LF signal is set to radio wave intensity T2. In other words, the tire position determination system in the second embodiment can also determine a plurality of tire positions with the use of single initiator  61 . The tire position determination system in the second embodiment is thus applicable without limitation being imposed by arrangement of initiator  61 . 
     Though monitoring unit  45  determines the tire position based on whether radio wave intensity is equal to or higher than radio wave intensity H or lower than radio wave intensity L in the first embodiment, it may determine the tire position based on whether radio wave intensity is higher than radio wave intensity H or equal to or lower than radio wave intensity L. 
     &lt;Modification in Connection with the Number of Tires&gt; 
     In the first embodiment and the second embodiment, a configuration including one axle in front and two axles in rear in which two tire positions of tires  13  and  15  aligned in the X-axis direction are determined is described. Vehicle  100 , however, may be constructed to include three or more axles in rear. Monitoring unit  45  in the first embodiment can determine three more tire positions by newly setting a threshold value in addition to radio wave intensity H and radio wave intensity L. 
     More specifically, in the tire position determination system, when a further tire is arranged on the side of the negative direction along the X axis of tire  15  shown in  FIG.  7   , radio wave intensity corresponding to a boundary line that passes through a central point of the tire arranged on the side of the negative direction along the X axis is set as a new threshold value. When the UHF signal includes the RSSI value at radio wave intensity lower than radio wave intensity set as the new threshold value, monitoring unit  45  can determine that the UHF signal has been transmitted from the tire arranged on the side of the negative direction along the X axis of tire  15 . 
     When monitoring unit  45  receives the UHF signal including the RSSI value at radio wave intensity equal to or higher than radio wave intensity set as the new threshold value, it can determine from tire detector  30  of which of tire  15  and the tire arranged on the side of the negative direction along the X axis of tire  15  the UHF signal has been transmitted, based on the detection value from acceleration sensor  39 . Thus, even in an example where the number of tires is increased to three or more, by increase of the threshold value, monitoring unit  45  can determine the tire position. 
     It should be understood that the embodiments disclosed herein are 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 embodiments and the modifications thereof described above are specific examples of aspects below. 
     (1) A tire position determination system according to one aspect of the present disclosure is a tire position determination system provided in a vehicle including a first tire and a second tire different from the first tire. The tire position determination system includes an initiator that transmits a command signal, a first detector attached to the first tire, the first detector transmitting a detection signal when the first detector receives the command signal, a second detector attached to the second tire, the second detector transmitting a detection signal when the second detector receives the command signal, and a monitoring unit configured to receive the detection signal. A first distance between the first tire and the initiator is equal to or shorter than a second distance between the second tire and the initiator. Each of the first detector and the second detector includes an acceleration sensor that detects an acceleration in a direction orthogonal to a revolution axis direction. The detection signal includes a detection value from the acceleration sensor. When the monitoring unit receives the detection signal from the first detector or the second detector, the monitoring unit performs determination processing for determining whether a detector that has transmitted the detection signal is the first detector or the second detector based on positional relation between the detector that has transmitted the detection signal and the initiator estimated from the detection value from the acceleration sensor included in the received detection signal. 
     According to the aspect above, when the monitoring unit is unable to determine the tire position only based on reception intensity, it can determine the tire position based on whether or not the tire detector is arranged in the region close to the initiator. The tire position determination system can thus determine the tire position of each of the plurality of tires with the use of a single initiator. 
     (2) In one aspect, the detection signal further includes reception intensity of the command signal from the initiator. The monitoring unit has the initiator transmit the command signal. When the reception intensity included in the received detection signal is equal to or higher than a first threshold value, the monitoring unit determines that the detector that has transmitted the detection signal is the first detector. When the reception intensity included in the received detection signal is lower than a second threshold value, the monitoring unit determines that the detector that has transmitted the detection signal is the second detector. When the reception intensity included in the received detection signal is lower than the first threshold value and equal to or higher than the second threshold value, the monitoring unit performs the determination processing. 
     According to the aspect above, even when the distance between tire detector  33  and tire detector  35  is short in tire  13  and tire  15 , the tire position determination system can determine the tire position by estimating arrangement of tire detector  33  and tire detector  35 . 
     (3) In one aspect, the first threshold value and the second threshold value are determined in accordance with arrangement of the initiator, the first tire, and the second tire. 
     According to the aspect above, the tire position determination system can determine an appropriate threshold value based on arrangement of the initiator, the first tire, and the second tire. 
     (4) In one aspect, the first threshold value is set such that the second tire is not included but at least a part of the first tire is included in a range where attenuated intensity of the command signal is equal to or higher than the first threshold value, and the second threshold value is set such that the first tire is not included but at least a part of the second tire is included in a range where attenuated intensity of the command signal is lower than the second threshold value. 
     According to the aspect above, radio wave intensities L and H can be determined based on the distances between initiator  61  and tires  13  and  15  and attenuation of the LF signal. 
     (5) In one aspect, the monitoring unit has the initiator transmit the command signal at first radio wave intensity. When the monitoring unit estimates based on the detection value from the acceleration sensor included in the received detection signal that the detector that has transmitted the detection signal is not arranged in a region close to the initiator in the first tire or the second tire, the monitoring unit determines that the detector that has transmitted the detection signal is the first detector. When the monitoring unit estimates based on the detection value from the acceleration sensor included in the received detection signal that the detector that has transmitted the detection signal is arranged in the region close to the initiator in the first tire or the second tire, the monitoring unit discards the detection signal. The first radio wave intensity is set such that the second tire is not included but the first tire is included in a range where attenuated intensity of the command signal is equal to or higher than a third threshold value, and the first detector or the second detector is configured to be unable to receive the command signal at reception intensity lower than the third threshold value and to be able to receive the command signal at reception intensity equal to or higher than the third threshold value. 
     According to the aspect above, the tire position determination system can determine the tire position by causing transmission of the LF signal transmission intensity of which is set to radio wave intensity T2. 
     (6) In one aspect, the first tire is arranged between the initiator and the second tire. 
     According to the aspect above, the region close to initiator  61  and the region distant from initiator  61  in tires  13  and  15  can suitably be determined. 
     Though embodiments of the present invention have been described, it should be understood that the embodiments disclosed herein are 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.