Abstract:
A transmitter in an apparatus for monitoring a condition of a tire includes a pressure sensor, a transmission circuit, a valve stem, an MR sensor, and a magnetic ring. The valve stem has magnetic pieces. The pressure sensor measures the air pressure in the interior of the tire. The transmission circuit generates a transmission signal containing data that represents the measured air pressure. The magnetic ring magnetizes the magnetic pieces. The position of the magnetic ring relative to the magnetic pieces can be changed. A magnetized state of the magnetic pieces varies according to the position of the magnetic ring relative to the magnetic pieces. The MR sensor detects the magnetized state of the magnetic piece. The transmission signal contains data based on the detected magnetized state in addition to the tire condition data. This is capable of identifying the tire mount position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a transmitter of a tire condition monitoring apparatus that permits a driver in a vehicle passenger compartment to check the conditions of tires, such as the air pressure, and to a tire condition monitoring apparatus. More particularly, the present invention relates to a transmitter of a tire condition monitoring apparatus that can be identified as a data transmission source among a plurality of transmitters of the same type respectively provided in a plurality of tires, and to a tire condition monitoring apparatus using the transmitter. 
     Wireless tire condition monitoring apparatuses that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been used. One such monitoring apparatus includes transmitters, each of which is attached to one of the tires. Each transmitter detects condition, such as the pressure, of the associated tire, and wirelessly transmits data representing the detected tire conditions. A receiver for receiving the data that has been wirelessly transmitted from the transmitters is provided on the vehicle body. 
     The transmitters each transmit data representing a condition of the corresponding tire to the single receiver. When receiving data, the receiver must identify which one of the transmitters has sent the data. To enable the identification, a specific identification data (ID code) is given to each of the transmitters. Each transmitter transmits data on the conditions of the tire together with an ID code given to it. The receiver can therefore identify the transmitter as the transmission source on the basis of the ID code (refer to Japanese Laid-Open Patent Publication No. 2000-103209). 
     To enable the receiver to identify the transmission-source transmitter, it is necessary to register, in advance, in the receiver, the ID codes for the transmitters in the tires mounted on the vehicle. At the time of initial registration, there is also a need to relate the ID codes to the positions at which the tires with the transmitters having the ID codes are mounted. Therefore, initial registration must be performed always when a new tire is mounted on the vehicle or when the mounted positions of the tires on the vehicle are changed. The process of manually performing such initial registration is troublesome and time-consuming. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in consideration of the above-described problem, and objectives of the present invention are:
         (1) to provide a transmitter of a tire condition monitoring apparatus capable of identifying the tire mount position; and   (2) to provide a tire condition monitoring apparatus capable of identifying tire mount positions and requiring no initial registration operation.       

     To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a transmitter in an apparatus for monitoring a condition of a tire of a vehicle is provided. The transmitter is provided in the tire, and includes a measuring device, a transmission circuit, a valve stem, a magnetizing member, and a magnetized state detecting device. The measuring device measures a condition of the tire. The transmission circuit generates a transmission signal containing data that represents the measured tire condition. The tire is filled with air through the valve stem. The valve stem has a magnetic piece. The magnetizing member magnetizes the magnetic piece. The position of the magnetizing member relative to the magnetic piece can be changed. A magnetized state of the magnetic piece varies according to the position of the magnetizing member relative to the magnetic piece. The magnetized state detecting device detects the magnetized state of the magnetic piece. The transmission signal contains data based on the detected magnetized state in addition to the tire condition data. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a diagrammatic view showing a tire condition monitoring apparatus according to one embodiment of the present invention; 
         FIG. 2  is a block diagram showing one of the transmitters shown in  FIG. 1 ; 
         FIG. 3  is a diagrammatic view showing the structure of one of the transmitters shown in  FIG. 1 ; 
         FIG. 4  is a diagram showing the relationship between a pair of pieces of a magnetic material and a pair of permanent magnets in each tire; and 
         FIG. 5  is a block diagram showing a receiver; and 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A tire condition monitoring apparatus  1  according to one embodiment will now be described with reference to the drawings. The apparatus  1  is used in a vehicle such as an automobile. 
     As shown in  FIG. 1 , the tire condition monitoring apparatus  1  includes four transmitters  30  and a receiver  40 . The each transmitter  30  is located in one of the tires  20  of the vehicle  10 . The transmitters  30  are located in the front left tire  20  (FL), the front right tire  20  (FR), the rear left tire  20  (RL), and the rear right tire  20  (RR), respectively. The receiver  40  is located on a body frame  11  of the vehicle  10 . 
     Each transmitter  30  is located in the corresponding tire  20  and is fixed, for example, to a wheel  21  of the tire  20 . Each transmitter  30  measures the condition of the corresponding tire  20 , that is, the pressure of the tire  20 . The transmitter  30  then wirelessly transmits a signal containing air pressure data. 
     The receiver  40  is located at a predetermined position on the body frame  11  and is activated by electricity of a battery (not shown) of the vehicle  10 . The receiver  40  is connected to a reception antenna  41  with a cable  42 . The receiver  40  receives the signal wirelessly transmitted by the transmitters  30  through the reception antenna  41 . 
     A display  50  is located in the view of the driver of the vehicle  10 , for example, in the passenger compartment. The display  50  is connected to the receiver  40  with a cable  43 . 
     As shown in  FIG. 2 , each transmitter  30  includes a transmission controller  31 , which is a microcomputer. The controller  31  includes, for example, a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). A unique ID code is registered in an internal memory, for example, the ROM, of the controller  31 . The ID code is used to distinguish the associated transmitter  30  from the other three transmitters  30 . 
     The tire pressure sensor  32  measures the air pressure in the interior of the associated tire  20  and provides the controller  31  with pressure data, which is obtained from the measurement. Each transmitter  30  is provided with a magnetic resistance (MR) sensor  33 . The resistance value of each MR sensor  33  changes according to changes in the intensity of a magnetic field. The MR sensor  33  outputs a magnetization detection signal based on the magnetic resistance to the transmission controller  31 . 
     Each transmission controller  31  sends data containing the air pressure data and the registered ID code to a transmission circuit  34 . The transmission controller  31  identifies one of the tires  20  to which the corresponding transmitter  30  is attached on the basis of the input magnetization detection signal. That is, the transmission controller  31  determines the position at which the tire  20  having the transmitter  30  is mounted on the basis of the magnetization detection signal. The transmission controller  31  transmits information on the mounted position of the tire  20  to the transmission circuit  34 . The transmission circuit  34  encodes and modulates the data sent from the transmission controller  31  to generate a transmission signal. The transmission circuit  35  then wirelessly sends the transmission signal through the antenna  35 . Each transmitter  30  is provided with a battery  36 . The transmitter  30  is driven by electricity of the battery  36 . 
     As shown in  FIG. 3 , a valve stem  60  has a threaded cap receiving portion  61  and a threaded fixing portion  62 . The fixing portion  62  is threaded with a valve nut  71 . A pair of pieces  62   a  and  62   b  of magnetic material are formed on the threaded fixing portion  62  through plating. The magnetic pieces  62   a ,  62   b  are formed, for example, with iron having a high relative permeability μs (μs=120 to 15000). The plated magnetic pieces  62   a  and  62   b  are separated from each other by an angle of ninety degrees in the circumferential direction of the threaded fixing portion  62 . 
     The valve nut  71  is screwed on the threaded fixing portion  62  to fix the valve stem  60  to the wheel  21  of the tire  20 . A pair of permanent magnets  72   a  and  72   b  are provided in a magnetic ring  72 . The permanent magnets  72   a  and  72   b  are separated from each other by an angle of ninety degrees in the circumferential direction of the magnetic ring  72 . The magnetic ring  72  is detachably fitted to the threaded fixing portion  62 . The magnetic ring  72  is rotatable about the axis of the threaded fixing portion  62 . When the positions of the permanent magnets  72   a  and  72   b  in the circumferential direction correspond to the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62 , the magnetic pieces  62   a  and  62   b  are magnetized by the permanent magnets  72   a  and  72   b . When the positions of the permanent magnets  72   a  and  72   b  in the circumferential direction are away from the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62 , the magnetic pieces  62   a  and  62   b  are not magnetized by the permanent magnets  72   a  and  72   b . The magnetic pieces  62   a  and  62   b  are magnetized or not magnetized depending on the distance between the magnetic pieces  62   a  and  62   b  and the permanent magnets  72   a  and  72   b . A ring nut  73  is screwed on the threaded fixing portion  62  to fix the magnetic ring  72  in the circumferential direction. Each of the valve stem  60 , the valve nut  71 , the magnetic ring  72  and the ring nut  73  is formed of aluminum having a low relative permeability μs (μs=1.000214). 
     The transmitter  30  is accommodated in a casing  80  provided at the lower end of the valve stem  60 . In the casing  80 , the MR sensor  33  is placed immediately below the valve stem  60 . That is, the MR sensor  33  is placed in the casing  80  so as to be closer to the lower ends of the magnetic pieces  62   a  and  62   b , thereby detecting the magnetized states of the magnetic pieces  62   a  and  62   b . The MR sensor  33  outputs the magnetization detection signal to the transmission controller  31  according to the magnetic resistance of the magnetic pieces  62   a  and  62   b . The transmission controller  31  receives four modes of magnetization detection signals. 
     More specifically, it is assumed here that the relationship between the magnetic pieces  62   a  and  62   b  and the permanent magnets  72   a  and  72   b  is determined in advance as shown in  FIG. 4 . 
     Each of the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62  in the tire  20  of the front left wheel (FL) is not close to any of the permanent magnets  72   a  and  72   b  in the magnetic ring  72 . Accordingly, each of the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62  is not magnetized. Consequently, the MR sensor  33  outputs to the transmission controller  31  the magnetization detection signal indicating that each of the magnetic pieces  62   a  and  62   b  is not magnetized. 
     The magnetic piece  62   a  on the threaded fixing portion  62  in the front right (FR) tire  20  is not close to any of the permanent magnets  72   a  and  72   b  in the magnetic ring  72 . On the other hand, the magnetic piece  62   b  on the threaded fixing portion  62  is close to the permanent magnet  72   a  in the magnetic ring  72 . Accordingly, the magnetic piece  62   a  on the threaded fixing portion  62  is not magnetized but the magnetic piece  62   b  on the threaded fixing portion  62  is magnetized. Consequently, the MR sensor  33  outputs to the transmission controller  31  the magnetization detection signal indicating that the magnetic piece  62   a  is not magnetized but the magnetic piece  62   b  is magnetized. 
     Each of the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62  in the rear left (RL) tire  20  are close to the permanent magnets  72   a  and  72   b  in the magnetic ring  72 . Accordingly, each of the magnetic pieces  62   a  and  62   b  on the threaded fixing portion  62  is magnetized. Consequently, the MR sensor  33  outputs to the transmission controller  31  the magnetization detection signal indicating that each of the magnetic pieces  62   a  and  62   b  is magnetized. 
     The magnetic piece  62   a  on the threaded fixing portion  62  in the rear right (RR) tire  20  is close to the permanent magnet  72   b  in the magnetic ring  72 . On the other hand, the magnetic piece  62   b  on the threaded fixing portion  62  is not close to any of the permanent magnets  72   a  and  72   b  in the magnetic ring  72 . Accordingly, the magnetic piece  62   a  on the threaded fixing portion  62  is magnetized but the magnetic piece  62   b  on the threaded fixing portion  62  is not magnetized. Consequently, the MR sensor  33  outputs to the transmission controller  31  the magnetization detection signal indicating that the magnetic piece  62   a  is magnetized but the magnetic piece  62   b  is not magnetized. 
     Therefore, if information on the four modes of the magnetization detection signal is registered on an internal memory, e.g., a read-only memory (ROM) in the transmission controller  31  of the transmitter  30 , the transmission controller  31  can determine to which one of the four tires  20  the transmitter  30  is attached. That is, the transmitter  30  can determine the position of the tire  20  in which the transmitter  30  is installed. Consequently, the transmitter  30  can identify the mounted position of the tire  20 . 
     If the magnetic pieces  62   a  and  62   b  are magnetized during a long time period, residual magnetism exists in the magnetic pieces  62   a  and  62   b  even after moving the permanent magnets  72   a  and  72   b  away from the magnetic pieces  62   a  and  62   b . The intensity of residual magnetism in the magnetic pieces  62   a  and  62   b , however, is lower than that when the magnetic pieces  62   a  and  62   b  are magnetized, as is apparent from the hysteresis phenomenon. Thus, a threshold value selected by considering the residual magnetism in the magnetic pieces  62   a  and  62   b  is set in the transmission controller  31 . More specifically, since the MR sensor  33  outputs the magnetization detection signal to the transmission controller  31  on the basis of the magnetic resistance of the magnetic pieces  62   a  and  62   b , the transmission controller  31  compares the threshold value selected by considering the residual magnetism of the magnetic pieces  62   a  and  62   b  and the magnetization detection signal. On the basis of this comparison, the transmission controller  31  determines whether or not each of the magnetic pieces  62   a  and  62   b  is magnetized, i.e., whether or not each of the magnetic pieces  62   a  and  62   b  is close to the permanent magnet  72   a  or  72   b . Therefore, even in a case where the magnetic pieces  62   a  and  62   b  are magnetized during a long time period, it is possible for the transmission controller  31  to reliably determine whether or not each of the magnetic pieces  62   a  and  62   b  is magnetized. 
     As shown in  FIG. 5 , the receiver  40  includes a reception controller  44  and a reception circuit  45 . The reception controller  44  processes signals received with the reception antenna  41 . The reception controller  44 , which is, for example, a microcomputer, includes a CPU, a ROM, and a RAM. The reception circuit  45  receives signals wirelessly transmitted by the transmitters  30  through the reception antenna  41 . The reception circuit  45  demodulates and decodes the received signals to obtain data, and sends the data to the reception controller  44 . 
     Based on the data from the reception circuit  45 , the reception controller  44  obtains the internal pressure of the tire  20  that is associated with the transmitter  30  that is the source of the received data. The reception controller  44  also causes the display  50  to show data relating to the air pressure. Particularly, when there is an abnormality in the pressure of the tire  20 , the controller  44  accordingly displays a warning on the display  50 . 
     Further, the reception controller  44  stores a unique ID code contained in the received data in its internal memory, e.g., the RAM if a predetermined condition is satisfied. The internal memory has four memory areas each corresponding to the position of one of the four tires  20 . Based on tire position information contained in the received data, reception controller  44  determines the position of the tire  20  that is associated with the transmitter  30  that is the source of the received data. Also, when the predetermined condition is satisfied, the reception controller  44  stores the ID code contained in the received data in the memory area that corresponds to the determined tire position. The predetermined condition is, for example, that the ID code contained in the received data does not match the ID code registered in the memory area that corresponds to the tire position determined based on the received data, and data containing the identical ID code has been received for a predetermined number of times. The predetermined condition is satisfied when, after the position of the tire  20  is changed in relation to the vehicle  10 , the transmitter  30  performs transmission over the predetermined number of times. 
     As described above, in the receiver  40 , the ID codes are registered in the internal memory of the reception controller  44  while being related to the mounted positions of the tires  20 . Thus, unique ID codes representing the transmission-source transmitters  30  are registered in the internal memory of the reception controller  44 . Each ID code is associated with the position of the tire  20  in which the corresponding transmitter  30  is provided. Determination as to coincidence between the ID codes, in other words, whether or not any change has been made in the mounted positions of the tires  20  on the vehicle  10  is executed, for example, during a predetermined time period (e.g., ten minutes) after startup of the receiver  40  with turning-on of a key switch (not shown) of the vehicle  10 . 
     This embodiment has the following advantages. 
     (1) The threaded fixing portion  62  is plated with the pair of pieces  62   a  and  62   b  of magnetic material separated from each other by an angle of ninety degrees in the circumferential direction. The pair of permanent magnets  72   a  and  72   b  are provided in the magnetic ring  72  while being separated from each other by an angle of ninety degrees in the circumferential direction. The magnetic ring  72  is fitted around the threaded fixing portion  62 . Therefore, the magnetic pieces  62   a  and  62   b  are magnetized or not magnetized depending on the remote/close relationship between the magnetic pieces  62   a  and  62   b  and the permanent magnets  72   a  and  72   b . On the other hand, the MR sensor  33  constituting the transmitter  30  is placed immediately below the valve stem  60  in the casing  80 . That is, the MR sensor  33  is placed in the casing  80  so as to be closer to the lower ends of the magnetic pieces  62   a  and  62   b . The MR sensor  33  outputs the magnetization detection signal to the transmission controller  31  on the basis of the magnetic resistance of the magnetic pieces  62   a  and  62   b . The transmission controller  31  can determine to which tire  20  the transmitter  30  is attached on the basis of the magnetization detection signal. That is, the transmitter  30  can determine its mounted position. Consequently, the transmitter  30  can identify the mounted position of the tire  20 . 
     (2) Each transmitter  30  wirelessly transmits to the receiver  40  data including air pressure data, the ID code registered in its internal memory, and information indicating the mounted position of the tire  20 . The receiver  40  registers the ID code in the internal memory of the reception controller  44  by relating the mounted position of the tire  20  and the ID code to each other on the basis of the information indicating the mounted position of the tire  20 . Thus, the receiver  40  can identify the mounted position of the tire  20  and there is no need for performing an operation for initial registration of the ID code. 
     (3) When the key switch (not shown) of the vehicle  10  is turned on, the receiver  40  is activated. Determination is then made during passage of a predetermined time period as to coincidence between the ID codes. This is because the key switch is ordinarily turned off when a new tire  20  is mounted on the vehicle  10  or when a change is made in the mounted positions of the tires  20  on the vehicle  10 . In other words, timing of determination as to coincidence between the ID codes, i.e., whether or not any of the tires  20  has been changed, is selected such that the determination is made only in a particular situation, e.g., a situation where a change is made in the mounted positions of the tires  20 . Therefore, ID codes can be efficiently registered in the internal memory, e.g., the RAM of the reception controller  44 . In other words, each time data wirelessly transmitted from the transmission-source transmitter  30  is received, it is not necessary to again register in the internal memory of the reception controller  44  the ID code contained in the received data. Therefore, it is also possible to reduce the load on the reception controller  44 . 
     (4) Each of the valve stem  60 , the valve nut  71 , the magnetic ring  72  and the ring nut  73  is formed of aluminum. The relative permeability μs (μs=1.00214) of aluminum is significantly small in comparison with the relative permeability μs (μs=120 to 15000) of iron. Each of the valve stem  60 , the valve nut  71 , the magnetic ring  72  and the ring nut  73  is not substantially magnetized by the pair of permanent magnets  72   a  and  72   b . Consequently, the pair of magnetic pieces  62   a  and  62   b  can be efficiently magnetized by the pair of permanent magnets  72   a  and  72   b.    
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     The embodiment has been described with respect to the arrangement in which the ring nut  73  is screwed on the threaded fixing portion  62  of the valve stem  60  to keep the magnetic ring  72  on the threaded fixing portion  62  at all times. However, the magnetic ring  72  may be provided only when necessary by screwing the ring nut  73  on the threaded fixing portion  62  of the valve stem  60 . For example, the magnetic ring  72  is necessary when a new tire  20  is mounted on the vehicle  10  or when a change is made in the mounted positions of the tires  20  on the vehicle  10 . The arrangement may alternatively be such that the magnetic ring  72  and the ring nut  73  are provided on the threaded fixing portion  62  of the valve stem  60  only when necessary. In other words, if the magnetic ring  72  and the ring nut  73  are not always provided on the threaded fixing portion  62  of the valve stem  60 , it is possible to limit the intensity of residual magnetism remaining on the magnetic pieces  62   a  and  62   b . Therefore, there is no need to set in the transmission controller  31  the threshold value selected by considering the residual magnetism. Moreover, since only the valve nut  71  is provided on the threaded fixing portion  62  of the valve stem  60  as in the conventional arrangement, balancing of the tire  20  when the tire  20  is mounted on the wheel  21  can be easily performed. Further, the unsprung weight of the vehicle  10  can be reduced, although the amount of reduction is small. 
     Continuous recesses and projections may be formed in the upper end surface of the valve nut  71  shown in  FIG. 3 , and also continuous recesses and projections may be formed in the opposed lower end surface of the magnetic ring  72 . In this case, the remote/close relationship between the pair of magnetic pieces  62   a  and  62   b  and the pair of permanent magnets  72   a  and  72   b  is reliably maintained. Such an arrangement prevents magnetic ring  72  from rotating by following the rotation of the ring nut  73  when the ring nut  73  is screwed on the threaded fixing portion  62 . Thus, the remote/close relationship between the pair of magnetic pieces  62   a  and  62   b  and the pair of permanent magnets  72   a  and  72   b  is reliably maintained. 
     In the above-described embodiment, the vehicle  10  have four tires  20 . Accordingly, the mounted positions of the tires  20  on the vehicle a 10  are identified by means of the combination of the pair of magnetic pieces  62   a  and  62   b  and the pair of permanent magnets  72   a  and  72   b . Needless to say, the number of magnetic pieces  62   a  ( 62   b ) and the number of permanent magnets  72   a  ( 72   b ) may be changed as desired according to the number of tires  20  mounted on the vehicle  10 . By selecting the numbers of these components, the above-described embodiment can be applied to multi-wheeled buses, trailers, etc. 
     A magnetic impedance sensor, a Hall sensor, flux gate sensor or a coil may be used in place of the MR sensor  33 . That is, any device capable of detecting a change in magnetic resistance of the magnetic pieces  62   a  and  62   b  or a change in magnetism at these magnetic pieces, i.e., whether or not the magnetic pieces  62   a  and  62   b  are magnetized, may be used. 
     While the receiver  40  having one antenna  41  with respect to the four transmitter  30  has been described, the arrangement of the receiver  40  may alternatively be such that four reception antennas  41  are provided in correspondence with the four transmitters  30 . Also, one reception antenna  41  may be provided in each of front and rear portions of the vehicle  10 , that is, two reception antennas may be provided. 
     The above-described embodiment may be applied to other types of valve, e.g., a snap-in type of valve. 
     In addition to the pressure sensor  32 , each transmitter  30  may have another measuring means such as a temperature sensor for measuring the temperature in the tire  20 . In this case, the transmitter  30  wirelessly also transmits the temperature data in the tire  20 . 
     Air pressure data transmitted by the transmitter  30  may indicate the value of the air pressure or whether the air pressure is within a permissible range. 
     Gas filling the tires  20  need not be air, which contains 78% of nitrogen. For example, nitrogen gas (100% of nitrogen) may be used. 
     Other than four-wheeled vehicles, the present invention may be applied to two-wheeled vehicles, such as bicycles and motorcycles, multi-wheeled busses, multi-wheeled towed vehicles, and industrial vehicles, such as forklifts. When the present invention is applied to a trailer, the receiver  40  and the display  50  are provided in the tractor. 
     The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.