Patent Publication Number: US-2006001533-A1

Title: System and method for monitoring tire pressure

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a system and method for monitoring a tire pressure.  
      A tire pressure monitoring system (hereinafter referred to as “TPMS”), which timely calls a driver&#39;s attention to a decrease in a tire pressure of a traveling vehicle, has already been required for a new car as a compulsory system in North America. In addition, there is an indication that this system is becoming widespread in other regions. TPMS are categorized into a direct type (sensor type) which directly detects a tire pressure by an air pressure sensor, and an indirect type which estimates a decrease in a tire pressure based on a difference in a rotational speed of a tire. In the present specification, a TPMS is meant to represent a direct type of TPMS.  
      Generally speaking, a TPMS includes sensor units, a monitoring unit and a display unit. Each sensor unit is installed in a tire attached to a wheel. The monitoring unit is mounted on a vehicle body. And the display unit is installed in a dashboard. A sensor unit, which has an air pressure sensor and a wireless communication device at the minimum, detects an air pressure within the tire, transmitting a signal of the air pressure to the monitoring unit. The monitoring unit monitors the signal and sends a signal indicative of a decrease in the air pressure to the display unit if the air pressure falls below a predetermined value.  
      A permanent sensor identification, which makes it possible to distinguish the sensor units, is given to each unit during its fabrication. When the sensor unit transmits a signal of tire pressure to the monitoring unit, the sensor identification is carried with the signal. In this way, the monitoring unit is able to determine which air pressure sensor has transmitted the received signal of the tire pressure.  
      The monitoring unit makes a correlation between a sensor identification (ID) and a location of a tire (including a spare tire) in which a sensor unit having this sensor ID is installed. This correlation is called registration of a sensor ID. The monitoring unit, which registers sensor IDs, is able to determine the location of a tire whose air pressure is decreasing, indicating it on the display unit. In this connection, identification of the location of a tire is divided into a case where all tires, a forward right tire, a forward left tire, a rear right tire, a rear left tire and a spare tire, are distinguished, and the other case where only a rolling tire and a spare tire are distinguished. A TPMS is required to distinguish a rolling tire from a spare tire at the minimum.  
      Registration of a sensor ID is generally carried out when a tire is attached to a wheel at a maintenance shop and the like, where a dedicated apparatus or process for charging air is employed. Because this type of registration uses the dedicated apparatus, it has been regarded as a burdensome and time consuming method. Furthermore, when a driver replaces a worn tire with a spare tire or make a rotation of tires including a spare tire, the registration made at the maintenance shop will be invalid accordingly.  
      A method for automatically registering a sensor ID has been proposed recently. For example, there is a method, in which communication antennas are prepared in the vicinity of tires including a spare tire, respectively, and a monitoring unit receives radio waves transmitted by sensor units with the antennas. The monitoring unit determines a sensor unit from which an antenna has received the strongest radio wave. Accordingly, the monitoring unit is able to know a sensor ID of the sensor unit which lies closest relative to the antenna. In this way, it is possible to carry out registration of a sensor ID without manual operation.  
      However, because a spare tire is stowed near rear tires, the radio waves transmitted by sensor units mounted on the spare tire and the rear tires tend to overlap, interfering with each other. In addition, it may be that a radio wave transmitted by another vehicle traveling in parallel has an adverse effect on radio communication. As a result, the monitoring unit is unable to properly recognize a reception level of radio wave received by each antenna, possibly leading to an erroneous registration of a sensor ID.  
      There is another method for automatically registering a sensor ID, which utilizes a command indicator of a low frequency electromagnetic wave that is used for transmitting signals for on-off control for a battery of a sensor unit and the like. This type of command indicator is called initiator. According to this method, initiators are positioned near tires including a spare tire, respectively. The monitoring unit transmits a control signal through an initiator which is assigned to a tire, receiving a signal transmitted by a sensor unit in response to this control signal. Because a sensor ID of the responded sensor unit is carried in the received signal, the monitoring unit is able to know the sensor ID of the sensor unit which is located closest to the initiator, thereby registering the sensor ID.  
      However, this method has the similar problem attributed to the fact that the spare tire is stowed near rear tires. For example, a control signal transmitted by an initiator positioned near a rear tire often activates not only a sensor unit for the rear tire but also that for the spare tire. In this connection, the fact that a low frequency wave is typically used for the control signal is also contributes to the anomaly described above. If the anomaly occurs, the monitoring unit simultaneously receives response signals from two sensor units, which results in a trouble that the monitoring unit can not distinguish the rear tire and the spare tire. As a result, the monitoring unit is not able to make correct registration of a sensor ID.  
      The technical problems described above are all ascribed to the fact that a spare tire is stowed in a vehicle. In order to solve these problems, patent documents 1 and 2 disclose a method for distinguishing a rolling tire from a non-rolling tire (spare tire). The method, which detects a temperature of air within a tire, determines whether or not a tire is rolling based on an increase in the temperature. This method takes into account the phenomenon that an air temperature within a tire mounted on a rolling wheel rises, which is continuously deformed as a result of receiving a force exerted by a vehicle body and a road surface while a vehicle is traveling.  
      Patent document 1: 2004-82853 (paragraphs 0064-0096 and  FIGS. 2-9 )  
      Patent document 2: 2003-154824 (paragraphs 0020-0060 and  FIGS. 1-8 )  
      However, the method described above has a drawback that it takes long to know whether or not a tire is rolling based on the temperature increase. The patent document 1 discloses data that a temperature of air within a tire increases by 5 degrees Celsius when a vehicle travels at 100 km/h for 20 minutes. Generally speaking, the faster travels a vehicle, the more will be an increase rate in a temperature of air within a tire. When a vehicle travels at lower speed, for example 50 km/h, it is estimated accordingly that it takes more than 40 minutes to observe a temperature rise of 5 degrees Celsius. Judging from this period of time it is concluded that the method is far from being applicable to a practical use.  
     SUMMARY OF THE INVENTION  
      In view of the problems described above, the present invention seeks for a system and method for monitoring a tire pressure.  
      It is an aspect of the present invention to provide a system for monitoring a tire pressure, which comprises a sensor unit and a monitoring unit. The sensor unit is mounted on a tire of a vehicle to detect an air pressure in the tire. The monitoring unit is mounted on the vehicle to monitor a signal indicative of the air pressure detected by the sensor unit so as to watch a decrease in the air pressure. The sensor unit comprises an accelerometer so as to detect an acceleration acting on the tire. And the monitoring unit classifies the tire as one of a rolling tire and a spare tire based on a signal indicative of the acceleration transmitted by the sensor unit.  
      The classification is made in the following manner. If a tire is mounted on a rolling wheel, an acceleration acts on a sensor unit as a result of rotation of the wheel. An accelerometer is installed in the sensor unit so as to detect an acceleration induced by rotation of the wheel. A monitoring unit compares the detected acceleration with a predetermined value, which is, for example, selected to be greater than an acceleration induced by vibration of the vehicle. The monitoring unit determines that the tire is mounted on a rolling wheel if the acceleration is greater than the predetermined value. The monitoring unit otherwise determines that the tire is mounted on a spare wheel.  
      Because the acceleration is used in the system described above, it is possible to promptly determine rolling or non-rolling of a tire if the vehicle starts traveling to reach a predetermined speed.  
      It is another aspect of the present invention to provide a system for monitoring a tire pressure, which comprises a sensor unit and a monitoring unit. The sensor unit is mounted on a tire of a vehicle to detect an air pressure in the tire. The monitoring unit is mounted on the vehicle to monitor a signal indicative of the air pressure detected by the sensor unit so as to watch a decrease in the air pressure. The sensor unit comprises an accelerometer so as to detect an acceleration acting on the tire. When the vehicle travels at not less than a predetermined speed, the monitoring unit determines whether or not the tire is rolling based on a signal of the acceleration and a signal of a sensor identification which are transmitted by the sensor unit. And the monitoring unit comprises a memory module which stores a correlation between the sensor identification and the tire.  
      If the predetermined speed is selected in such a manner that an acceleration induced by rotation of the wheel traveling at this speed is sufficiently greater than an acceleration induced by vibration of the vehicle, it is possible to reliably determine rolling or non-rolling of a wheel. The monitoring unit registers a correlation between a sensor ID and a tire, which has been classified as a rolling or spare tire.  
      Because the monitoring module registers a sensor ID in the memory module, the monitoring module is able to determine if a tire experiencing an air decrease is mounted on a rolling or spare wheel by making access to the memory module, even if the vehicle travels at a lower speed or comes to a stop.  
      It is still another aspect of the present invention to provide a system for monitoring a tire pressure, in which the monitoring unit further comprises an alarm module. The alarm module raises an alarm with information on the tire classified as one of a rolling tire and a spare tire, when the monitoring unit detects a decrease in the air pressure in the tire.  
      When the monitoring unit detects the decrease in the air pressure in the tire, the monitoring unit is able to promptly and reliably determine whether the tire is mounted on a rolling or spare tire. In order to make this determination, the monitoring unit compares the acceleration with the predetermined value, or makes access to the memory module according to the sensor ID. The monitoring module delivers an alarm including information which tells whether the decrease in the air pressure occurs in a rolling or spare tire.  
      It is yet another aspect of the present invention to provide a method for monitoring a tire pressure with a system which comprises a sensor unit that is mounted on a tire of a vehicle and a monitoring unit that is mounted on the vehicle. The method comprises the following steps: detecting an air pressure in the tire and an acceleration acting on the tire; monitoring a signal indicative of the air pressure detected by the sensor unit so as to watch a decrease in the air pressure; receiving a signal of the acceleration and a signal of a sensor identification assigned to the sensor unit which are transmitted by the sensor unit; when the vehicle travels at not less than a predetermined speed, determining whether or not the tire is rolling based on the signal of the acceleration and the signal of the sensor identification, and registering a correlation between the sensor identification and the tire; and when a decrease in the air pressure in the tire is detected, classifying the tire as one of a rolling tire and a spare tire, and raising an alarm.  
      The predetermined speed is selected in such a manner that an acceleration induced by rotation of a wheel at the location of a sensor unit is sufficiently greater than an acceleration induced by vibration of a vehicle. In this way, the method enables the monitoring unit to register a sensor ID with the memory module without erroneously classifying a tire as a rolling or spare tire.  
      Furthermore, a driver is able to know simultaneously to which type of tire, a rolling or spare tire, the tire belongs, which has experienced a decrease in tire pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram illustrating a system setup for a vehicle on which a system for monitoring a tire pressure according to the present invention is mounted.  
       FIG. 2  is a block diagram showing structure of a sensor unit according to the present invention.  
       FIG. 3  is a schematic diagram illustrating a configuration of data which a sensor unit transmits.  
       FIG. 4  is a functional block diagram illustrating a monitoring unit according to the present invention.  
       FIG. 5  is a flow chart showing a flow executed by a monitoring unit according to the present invention.  
       FIG. 6  is a graph showing the relationship between rotational acceleration generated by rotation of a wheel and speed of a vehicle.  
       FIG. 7  is a schematic diagram illustrating the configuration of a module for storing registered sensor ID.  
       FIG. 8  is a flow chart showing steps carried out by a module for determining decrease in tire pressure and a module for generating alarm.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An embodiment of the present invention is now described with reference to the accompanying drawings.  
      As shown in  FIG. 1 , a vehicle  1 , whose forward portion is shown at the top of  FIG. 1 , has four rolling wheels, namely a forward right wheel  2 FR, a forward left wheel  2 FL, a rear right wheel  2 RR and a rear left wheel  2 RL. In addition, the vehicle  1  has a spare wheel  2 SP mounted on its rear portion. Sensor units  3 FR,  3 FL,  3 RR,  3 RL and  3 SP are installed in tires attached to the wheels  2 FR,  2 FL,  2 RR,  2 RL and  2 SP, respectively.  
      In description hereinafter, rolling wheels  2  is meant to represent all of the wheels  2 FR,  2 FL,  2 RR and  2 RL, so that they are distinguished from the spare wheel  2 SP. In contrast, sensor units  3  is meant to represent all sensor units  3 FR,  3 FL,  3 RR,  3 RL and  3 SP, not distinguishing the sensor unit  3 SP installed in the tire attached to the spare wheel  2 SP from other sensor units.  
      In the upper right portion of  FIG. 1 , a sensor unit  3  is schematically illustrated. A lower surface of the sensor unit  3 , which faces downward in  FIG. 1 , is adapted to be curved so that it fits to a surface of a rim. In an upper surface of the sensor unit  3 , which faces upward in  FIG. 1 , an air hole  38  and a sensor hole  39  are made. The air hole  38  is directly communicated with a tire valve  37 . On the other hand, the sensor hole  39  is communicated with a place where an air pressure sensor and an accelerometer (both not shown) are installed. In this way, the sensor unit  3  detects an air pressure within a tire and acceleration, outwardly transmitting signals indicative of the detected data through a transmitter and antenna (both not shown).  
      A monitoring unit  5 , which is mounted on the vehicle  1 , receives wireless signals with an antenna  7 , which are transmitted by a sensor unit  3  installed in a tire attached to each wheel, thereby acquiring its tire pressure and acceleration. The monitoring unit  5  inspects if there is a decrease in tire pressure by making a comparison between the acquired tire pressure and a predetermined value. If the monitoring unit  5  detects a decrease, the monitoring unit  5  displays it on an indicator  4  disposed in a display unit positioned in front of a driver&#39;s seat.  
      The monitoring unit  5 , to which a speed sensor  6  is electrically connected, receives a signal indicative of a vehicle speed. For example, it may be possible to incorporate a rotational speed of a drive shaft, which lies in a backward stage of a transmission, as the vehicle speed. Furthermore, it may be possible to utilize a sensor provided for each wheel so as to implement an Antilock Brake System (ABS) instead of the speed sensor  6 . Though an average speed is typically adopted for the vehicle speed in this case, it may be alternatively possible to select a different type of speed other than the average speed.  
      Description is given of structure and function of the sensor unit  3  and the monitoring unit  5  with reference to  FIGS. 2-4 .  
      As shown in  FIG. 2 , the sensor unit  3  includes a micro processor  31 , to which a transmitter  32  with an antenna  33  in addition to a pressure sensor  34 , a temperature sensor  35  and an accelerometer  36  are electrically connected. The micro processor  31  has an arithmetic processor  311  and a memory  312 . The memory  312 , which typically includes a Random Access Memory (RAM) and Read Only Memory (ROM), has a sensor ID memory  3120  which occupies a portion of this ROM. The sensor ID memory  3120  stores a sensor ID so that a sensor unit  3  is given its individual identification.  
      The micro processor  31  has a timer (not shown) so as to collect data such as a tire pressure detected by the pressure sensor  34 , temperature detected by the temperature sensor  35  and acceleration of a tire detected by the accelerometer  36 , at regular intervals, each 10 minutes, for example. The micro processor  31  assigns a sensor ID, which it reads out from the sensor ID memory  3120 , to these air pressure, temperature and acceleration, generating a data unit  30 . The micro processor  31  transmits the data unit  30  outside the sensor unit  3  with the transmitter  32  and antenna  33 .  
      As shown in  FIG. 4 , the monitoring unit  5  includes a module  51  for receiving signal of sensor unit, a module  52  for entering signal of speed sensor, a module  53  for determining wheel movement, a module  54  for registering sensor ID, a module  55  for determining decrease in tire pressure, a module  56  for generating alarm, a module  57  for storing registered sensor ID and a receiver  58 . In the monitoring unit  5 , these modules except for the receiver  58  are configured in a computer (not shown) having a processor and memory. More specifically speaking, the memory provides an area for the module  57 , and the processor executes computer programs stored in the memory so as to implement functionalities associated with the modules  51 ,  52 ,  53 ,  54 ,  55  and  56 .  
      Description is given of operation for each of the modules  51 - 56  with reference to  FIGS. 5-8  in addition to  FIG. 4 .  
      As shown in  FIG. 5 , the module  51  receives a signal indicative of a data unit  30  transmitted by a sensor unit  3  with the receiver  58  and the antenna  7  (step S 61 ). As shown in  FIG. 3 , the data unit  30  includes a sensor ID identifying which sensor unit  3  has detected the data unit  30 .  
      The module  52  incorporates a signal indicative of vehicle speed from the speed sensor  6  (step S 62 ).  
      The module  53  determines whether or not the tire to which the sensor unit  3  is attached is rolling based on an acceleration included in the data unit  30  received by the module  51  and a vehicle speed incorporated by the module  52 .  
      Generally speaking, when a wheel rotates, it imposes a centrifugal force on a sensor unit  3  attached to a tire. As a result the sensor unit  3  detects an acceleration. A rotational speed of a wheel, namely a speed of the vehicle  1  (vehicle speed) and acceleration induced by the centrifugal force relate as shown in  FIG. 6 . For example,  FIG. 6  shows that when a vehicle speed is 32 km/h, an acceleration of 9 G acts on the sensor unit  3 . In this connection, it is known that an acceleration induced by vibration of the vehicle  1  while running does not exceed an upper limit of 4.4 G, approximately.  
      Accordingly, when the sensor unit  3  detects an acceleration of not less than a predetermined value, 9 G for example, it is possible to determine that a tire, to which the sensor unit  3  is attached, is rolling. It may be alternatively possible to select a smaller value than 9 G as long as it is greater than 4.4 G. The value of 9 G is selected as an example, taking into account a maximum error of 3 G for an accelerometer  36 . The acceleration 9 G also includes a margin. Because the sensor unit  3  accordingly keeps the margin of 1.6 G, it is possible to reliably distinguish an acceleration induced by rotation of a wheel from that by vibration.  
      As shown in  FIG. 5 , the unit  53  determines whether or not the vehicle speed received in step S 62  reaches a predetermined speed, for example 40 km/h (step S 63 ). This step is carried out so as to provide more reliable determination of rolling or non-rolling for a tire. As shown in  FIG. 6 , if a vehicle speed is equal to or more than 40 km/h, an acceleration including the error will not fall to 9 G. Accordingly, if the vehicle speed is equal to or more than 40 km/h (Yes in step S 63 ), the unit  53  conducts determination of rolling/non-rolling for a tire. Otherwise (No in step S 63 ), the unit  53  terminates processing without carrying out the determination.  
      When the vehicle speed is equal to or more than 40 km/h (Yes in step S 63 ), the unit  53  determines whether or not an acceleration, one of the signals received in step S 61 , is equal to or more than 9 G (step S 64 ). If the acceleration is equal to or more than 9 G (Yes in step S 64 ), the module  54  determines that the tire is rolling and temporarily registers a sensor ID correlated with the sensor unit  3  as a rolling wheel  2  (step S 65 ). Making a temporary registration of a received sensor ID as a rolling wheel  2  is meant to store the sensor ID into a memory area  571  for rolling wheel provided in the module  57 , as shown in  FIG. 7 .  
      When the acceleration is less than 9 G (No in step S 64 ), the module  54  determines that the tire is not rolling, registering temporarily the sensor ID as a non-rolling wheel (step S 66 ). Making a temporary registration of a received sensor ID as a non-rolling wheel is meant to store the sensor ID into a memory area  572  for spare wheel provided in the module  57 , as shown in  FIG. 7 .  
      Next, the module  54  checks whether or not determination of rolling/non-rolling wheel has been carried out based on the data unit  30  for not less than 4 out of 5 tires including the spare wheel  2 SP (step S 67 ). If the determination has been done for not less than 4 tires (Yes in step S 67 ), the module  54  checks if four sensor IDs have been registered as rolling wheels  2  (step S 68 ). If the four sensor IDs have been temporarily registered as the rolling wheels  2  (Yes in step S 68 ), the module  54  registers not only them as the rolling wheels  2  (step S 69 ), but also a sensor ID, which has been temporarily registered as a non-rolling wheel, as a spare wheel  2 SP (step S 70 ).  
      If the module  54  has not checked the data unit  30  for not less than 4 wheels (No in step S 67 ), or the four sensor IDs have not been temporarily registered as the rolling wheels  2  in step S 68  (No in step S 68 ), the module  54  returns to step S 61 , carrying out step S 61  and subsequent steps for another data unit  30 , which has not yet undergone processing.  
      Registration of a sensor ID is meant to store a sensor ID into one of the memory area  571  for rolling wheel and the memory area  572  for spare wheel provided in the module  57 . As registration is first carried out in steps for making temporary registration (steps S 65  and S 66 ) in the present embodiment, the registration of a sensor ID in steps S 69  and S 70  is actually meant to confirm completion of temporary registration and to notify that information in the module  57  has been available for steps shown in  FIG. 8  to be described later.  
      Logic applied to steps S 67  and S 68  may create a case where a sensor ID is not registered in the memory area  572  for spare wheel. However, even in this case, a sensor ID of a sensor unit  3  attached to a tire, which is classified as a rolling wheel  2  by the steps shown in  FIG. 5 , is registered in the memory area  571  for rolling wheel. When a TPMS gives an alarm only for a tire which is mounted on a rolling wheel  2 , no problems will occur. Furthermore, when the TPMS separately gives an alarm for a rolling wheel  2  and a spare wheel  2 SP, no problems will occur, either. The reason for this is explained as follows. When a sensor ID is not registered in the memory area  571  and the memory area  572  is blank (no registration), it is possible to conclude that a data unit  30  given this sensor ID has been transmitted by a sensor unit  3 , which is attached to a spare wheel  2 SP.  
      As described above, after the completion of registration of a sensor ID, the monitoring unit  5  checks a decrease in a tire pressure. If the monitoring unit  5  detects a decrease, it generates an alarm.  
      As shown in  FIG. 8 , the module  55  for determining decrease in tire pressure (see  FIG. 4 ) receives a data unit  30  (step S 81 ), which is transmitted by a sensor unit  3  attached to each tire. As shown in  FIG. 3 , the data unit  30  includes a tire pressure and a sensor ID. The module  55  determines whether or not the received tire pressure has fallen below a predetermined value (step S 82 ). If the tire pressure has not fallen below the predetermined value (No in step S 82 ), the process is automatically terminated.  
      In contrast, if the tire pressure has fallen below the predetermined value (Yes in step S 82 ), the module  55  makes access to the module  57  so as to identify a wheel with which the received sensor ID is correlated (step S 83 ). If the wheel is classified as a rolling wheel  2  (Yes in step S 84 ), the module  56  sends the indicator  4  an alarm calling attention to a decrease in tire pressure (step S 85 ). If the wheel is not classified as a rolling wheel  2  (No in step S 84 ), the module  56  terminates steps without sending an alarm.  
      Though the module  56  does not give an alarm for the spare wheel  2 SP in the flow shown in  FIG. 8 , it may be alternatively possible that the module  56  gives an alarm for the spare wheel  2 SP. In this case, step S 84  can be eliminated. It may be necessary instead that an alarm generated in step S 85  should be configured so as to tell to which category a wheel experiencing a decrease in tire pressure belongs, a rolling wheel  2  or a spare wheel  2 SP.  
      In  FIG. 1  an example of display  40  is shown, which is provided by the indicator  4  so as to give an alarm for a decrease in tire pressure. It is assumed that the example of display  40  includes display of an alarm for a spare wheel  2 SP. When an alarm is given for a rolling wheel  2 , an icon  41  symbolizing vehicle and an icon  43  symbolizing an unusual tire are highlighted. In contrast, when an alarm is given for the spare wheel  2 SP, an icon  42  symbolizing spare tire and the icon  43  are highlighted. An icon  44 , which symbolizes an unusual TPMS system, is highlighted when an anomaly occurs in the TPMS system. If an alarm is not required for the spare wheel  2 SP, it may be possible to eliminate the icons  41  and  42 .  
      The embodiment described above, in which the accelerometer  36  in the sensor unit  3  detects an acceleration induced by rotation of a wheel, determines whether or not the wheel is rolling. In this way, it is possible to determine whether or not a tire is rolling by assessing acceleration, which is detected while the vehicle  1  is traveling at not less than a predetermined speed (40 km/h, for example), based on a threshold of predetermined acceleration, 9 G for example, which is greater than an upper limit of acceleration (4.4 G) induced by vibration of a vehicle.  
      Next, description is given of a modification for the embodiment described above.  
      It may be possible to carry out comparison of acceleration with a predetermined value (9 G for example) in a sensor unit  3  instead of a monitoring unit  5 . In this case, a micro processor  31  of the sensor unit  3  makes the comparison, and the sensor unit  3  transmits only the result to the monitoring unit  5 . More specifically speaking, the sensor unit  3  transmits “true” or “false” signal, namely “1” or “0” signal. Though this requires modification for some of the steps in the monitoring unit  5 , the same method is basically inherited.  
      Though the embodiment described above does not have a function of displaying the location of a tire attached to a rolling wheel  2 , which experiences a decrease in tire pressure, it may be possible to implement this function by adding a conventional initiator, for example. More specifically speaking, an initiator, which is able to appoint a sensor unit  3  to respond, is placed near each rolling wheel  2 . In this way, a monitoring unit  5  acquires the function described above. As described in “BACKGROUND OF THE INVENTION”, addition of initiators alone does not enable distinguishing rear tires from a spare tire. If information stored in a module  57  for storing registered sensor ID according to the present invention is introduced in addition to the initiators, it is possible not only to distinguish the rear tires from the spare tire, but also to correlate a sensor ID with a tire attached to each rolling wheel  2 . As a result, it is possible to give the location of a rolling tire in displaying an alarm calling attention to a decrease in tire pressure.  
      It should be noted that the present invention is applicable to both types of spare tires such as an emergency tire, a tempa spare tire, for example, and a normal tire.  
      Foreign priority document, JP 2004-198129 filed on Jul. 5, 2004, is hereby incorporated by reference.