Abstract:
A tire information detecting device accurately detects tire pressure and temperature. The device includes a transponder mounted in a tire of a vehicle, and a controller located in the vehicle body. The transponder includes a diode, which modulates and demodulates a signal transmitted to and received from the controller, and a pressure measuring unit, which measures a tire pressure. Also included is a detecting resonance circuit connected between the diode and the pressure measuring unit, which resonates in accordance with a signal from the controller. A resonance circuit resonates in accordance with the signal from the controller and controls a connection between the pressure measuring unit and the detecting resonance circuit.

Description:
[0001]    This application claims the benefit of priority under 35 U.S.C. §119 to Japanese Patent Application No. 2006-124992, filed Apr. 28, 2006, and is hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a tire information detecting device, and more particularly, to a tire information detecting device capable of detecting tire information, including tire pressure and tire temperature. 
         [0004]    2. Description of the Related Art 
         [0005]    Known radio transmission devices wirelessly transmit and estimate measured values, such as a tire pressure, to a controller disposed in a vehicle for the purpose of providing an alarm message for a driver. This is suggested in FIGS. 5 and 6 of U.S. Pat. No. 6,378,360 B1. In such the radio transmission device, a controller ( FIG. 5 ) is provided in the main body of the vehicle, and a measured value transmitter ( FIG. 6 ) is provided inside the tire. 
         [0006]    As shown in  FIG. 5 , the controller includes a carrier oscillator G 1  generating a carrier wave (f 1 ) of 2.4 GHz, a modulator MO 1 , and an oscillator G 2  outputting a modulation oscillation signal. The oscillator G 2  outputs a oscillation signal of frequency (f 2 ), which is similar to a resonance frequency of a resonator of a transponder. A carrier wave from the oscillator G 1  is amplitude-modulated by the oscillation signal of the oscillator G 2 , and an amplitude-modulated high-frequency signal of 2.4 GHz is amplified by an amplifier (not shown), so that it is radiated from an antenna. 
         [0007]    Additionally, the controller includes a switch S 1 , which switches an amplitude modulation by the modulator MO 1 , a receiver E 1 , which receives a high-frequency signal radiated from the transponder and calculates a measured value (V 1 ) of a tire pressure, and a timer T 1 , which controls a switching time of the switch S 1  and a state of the receiver E 1 . An amplitude modulation of the carrier wave is switched by the timer T 1 , and then for a certain period of time, the amplitude-modulated high-frequency signal is transmitted so that a non-modulated carrier wave is transmitted when the amplitude modulation is stopped at a time point t 1 . The receiver E 1  becomes active at a time point t 2  prior to a time= t 1  by about 1 ms, and receives a high-frequency signal via an antenna A 4  from the transponder. 
         [0008]    As shown in  FIG. 6 , the transponder includes a low-pass filter L 1 /C 1 , a varactor diode D 1  (hereinafter, a diode) which functions as a modulator and demodulator, a capacitive pressure sensor SC 1 , which varies with the tire pressure, and a resonator having a quartz-crystal resonator Q 1 , which is excited by a frequency component included in a high-frequency signal from the controller. In the high-frequency signal from the controller, the carrier wave of 2.4 GHz is removed by the low-pass filter L 1 /C 1  and demodulated by the diode D 1 . Accordingly, a signal, which is the same as the oscillation signal of the oscillator G 2 , is extracted. Because the resonance frequency of the resonator is similar to the oscillation signal of the oscillator G 2 , the resonance frequency is excited by a signal generated therefrom. According to such the excitation, the resonance-frequency signal occurs. Additionally, in the resonance frequency of the resonator, since a capacity of the capacitive pressure sensor SCI varies with the tire pressure, the resonance-frequency signal occurring therefrom is influenced by the effect. 
         [0009]    As described above, after the controller transmits the amplitude-modulated high-frequency signal, the controller stops the amplitude modulation and transmits a non-modulated carrier wave. The resonator continuously oscillates for about 1 ms or more even when the amplitude modulation is stopped. Accordingly, the non-modulated carrier wave from the controller is amplitude-modulated with the resonance-frequency signal of the resonator by the diode D 1  and then radiated from an antenna A 3 . In the receiver E 1 , the amplitude-modulated high-frequency signal is received via an antenna A 4  and the resonance-frequency signal is extracted via a demodulator (not shown) so that it is possible to calculate the measured value (V 1 ) of the tire pressure. 
         [0010]    In the radio transmission device disclosed in U.S. Pat. No. 6,378,360 B1, a plurality of resonators are additionally disposed in transponders. It is possible to transmit the signal of measured values of the tire temperature and calculate the measured value in the controller. 
         [0011]    However, in such known radio transmission devices, when multiple resonators are located in a transponder and multiple measured values of a tire pressure, tire temperature, and the like are detected, a temperature characteristic or a temporal degradation characteristic of each of the resonators is different from each other, and thus an error occurs in the measured value. Therefore, the measured value is not detected accurately. 
         [0012]    Specifically, when the tire pressure is measured, a resonance frequency of a resonator for measuring a pressure is influenced by both a pressure and a temperature, whereby the temperature value is obtained from the resonance frequency of the resonator for measuring a temperature in the other side. Although the pressure is obtained without the temperature influence by using the temperature value, when the temperature characteristic or the temporal degradation characteristic of both of the resonators is different from each other, it is impossible to accurately perform the correction. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention solves the above-mentioned problems. It is an object of the invention to provide a tire information detecting device capable of accurately detecting the multiple measured values, including tire pressure and tire temperature. 
         [0014]    According to an aspect of the invention, there is provided a tire information detecting device including a measured value transmitter mounted on a tire of a vehicle, and a controller disposed on a main body of the vehicle so as to transmit and receive a signal to and from the measured value transmitter. The measured value transmitter includes an antenna, a modulator and a demodulator used to transmit and receive signal to and from the controller, a pressure measuring unit for measuring a tire pressure, a detecting resonance circuit connected between the modulator and the pressure measuring unit, which resonates in accordance with the signal from the controller, a connecting resonance circuit, which oscillates in accordance with the signal from the controller, and a control circuit for controlling a connection between the pressure measuring unit and the detecting resonance circuit. 
         [0015]    According to the above-described configuration, a connection between the pressure measuring unit and the detecting resonance circuit is controlled in accordance with a signal from the controller. Accordingly, the connection between the detecting resonance circuit and the pressure measuring unit is switched, whereby the resonance frequency of the detecting resonance circuit is estimated depending on the respective situation by using the controller, whereby plural resonance circuits, for example, quartz-crystal resonators, are not necessary. Thus, a single detecting resonance circuit can calculate both a tire temperature and a tire pressure, thereby accurately detecting the tire pressure and the tire temperature. In addition, the connection between the pressure measuring unit and the detecting resonance circuit is controlled with the resonance of the connecting resonance circuit in the control circuit, whereby the resonance of the connecting resonance circuit is controlled by the controller, thereby controlling the connection between the pressure measuring unit and the detecting resonance circuit. 
         [0016]    The control circuit may include a switching element, which is connected to an input-output terminal of the pressure measuring unit in a first signal line. The switching element enters an ON state so as to connect the pressure measuring unit to the detecting resonance circuit with the resonance of the connecting resonance circuit. Additionally, it is preferable that the switching element enters an OFF state so as to disconnect the pressure measuring unit from the detecting resonance circuit because the connecting resonance circuit does not resonate. In this case, the resonance of the connecting resonance circuit is controlled by the controller so that an ON and OFF state of the switching element is switched and it is possible to control the connection between the pressure measuring unit and the detecting resonance circuit. 
         [0017]    The measured value transmitter may transmit a signal of the resonance frequency of the detecting resonance circuit to the controller as the switching element is an OFF state. Additionally, it is possible to transmit a signal of the resonance frequency of the detecting resonance circuit to the controller as the switching element is an OFF state. In this case, since the resonance frequency, which is transmitted to the controller varies in accordance with the ON and OFF of the switching element, it is possible for a single detecting resonance circuit to calculate the tire temperature and the tire pressure by estimating such the resonance frequency. 
         [0018]    The measured value transmitter may include a second signal line, which bypasses the first signal line between the input-output terminal of the pressure measuring unit side and the input-output terminal. In this case, the ON and OFF state of the switching element can be switched by the connecting the resonance circuit to the second signal line, whereby this does not influence a signal which communicates via the first signal line, thereby controlling the connection between the pressure measuring unit and the detecting resonance circuit. 
         [0019]    The connecting resonance circuit may be an LC resonance circuit. In this case, it is possible to reduce cost. Additionally, the resonance circuit may be a piezoelectric resonator. 
         [0020]    The switching element may be a diode. In this case, it is possible to reduce the cost of the switching element. In addition, the switching element can be an FET. 
         [0021]    The controller may transmit a signal which enables the detecting resonance circuit and the connecting resonance circuit to resonate to the measured value transmitter. Additionally, the resonance-frequency signal of the detecting resonance circuit and the resonance-frequency signal of the detecting resonance circuit are received from the measured value transmitter, and the tire temperature and the tire pressure may be calculated in accordance with the resonance frequency of the detecting resonance circuit extracted from the received signal. In this case, it is possible to calculate both the tire temperature and the tire pressure by using a single detecting resonance circuit, thereby accurately detecting the tire pressure and the tire temperature. 
         [0022]    The controller may calculate the tire temperature in accordance with a difference in frequency between a frequency of a signal which enables the detecting resonance circuit to resonate, and the resonance frequency which is extracted from the received signal in accordance with the signal. 
         [0023]    The controller may calculate the tire pressure in accordance with a difference in frequency between the resonance frequency extracted from the received signal, and the resonance frequency extracted from the received signal in accordance with a signal which enables the detecting resonance circuit to resonate in accordance with the measurement result of the pressure measuring unit. Accordingly, it is possible to calculate the tire pressure except for a temperature influence by using single detecting resonance circuit, thereby more accurately detecting the pressure. 
         [0024]    The detecting resonance circuit may include the quartz-crystal resonator. In this case, an oscillation of a resonance signal becomes stable, and thus it is possible to detect the tire temperature and the tire pressure in a stable manner. 
         [0025]    According to the above-mentioned configurations, it is possible to more accurately detect multiple measured values, including tire pressure and tire temperature. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a diagram illustrating a circuit configuration of a transponder constituting a tire information detecting device according to an embodiment of the invention. 
           [0027]      FIG. 2  is a drawing illustrating a difference in frequency between a resonance frequency extracted from a received signal of the transponder, and a frequency of an osculation signal from a controller. 
           [0028]      FIG. 3  is a chart illustrating a timing diagram. 
           [0029]      FIG. 4  is a drawing illustrating a modified example of the circuit configuration of the transponder. 
           [0030]      FIG. 5  is a schematic circuit configuration of a controller constituting the known tire information detecting device. 
           [0031]      FIG. 6  is a schematic circuit configuration of a transponder constituting the known tire information detecting device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Hereinafter, an embodiment of the invention will be described in detail with reference to the attached drawings. A tire information detecting device according to the embodiment includes a controller disposed on a main body of a vehicle, and a measured value transmitter (hereinafter, “transponder”) disposed inside a tire. 
         [0033]    In the tire information detecting device according to the embodiment, a configuration of the transponder of the invention is different from that of the known tire information detecting devices. The difference with respect to the configuration of the controller will be described with reference to components shown in  FIG. 4 . 
         [0034]      FIG. 1  is a diagram illustrating an example of a circuit configuration of the transponder constituting the tire information detecting device according to the embodiment. 
         [0035]    As shown in  FIG. 1 , a transponder  10  includes an antenna  11  for a transmitting and receiving, and the antenna  11  is connected to a diode  12 . In the diode  12 , an anode is connected to an input-output terminal of the antenna  11 , and additionally a cathode is connected to a ground. Meanwhile, one end of an inductor  13  is connected to the anode of the diode  12 , and the other end thereof is connected to the ground, with a capacitor  14  disposed therebetween. Additionally, the inductor  13  and the capacitor  14  include a low-pass filter. The low-pass filter has a frequency characteristic so as to remove a carrier wave of 2.4 GHz. The diode  12  and the low-pass filter constitute a demodulator. The diode  12  functions as a modulator as well. 
         [0036]    An end of the inductor  13  is connected to a quartz-crystal resonator  16 , which detects tire temperature and tire pressure with a capacitor  15  disposed therebetween. One end of the quartz-crystal resonator  16  is connected to one side of an electrode of the capacitor  15 , and the other end thereof is connected to the ground. A self-resonant frequency of the quartz-crystal resonator  16  is set at 9.800 MHz. In order to adjust resonance frequency of the quartz-crystal resonator  16 , one side of an electrode of a variable capacitor  18  is connected to the quartz-crystal resonator  16 , and the other side of the variable capacitor  18  is connected to the ground. The quartz-crystal resonator  16  and the variable capacitor  18  constitute a detecting resonance circuit  17 . 
         [0037]    One side of an electrode of the quartz-crystal resonator  16  is connected a pressure measuring unit  20  with a diode  19 , which functions as a switching element, disposed therebetween. The pressure measuring unit  20  includes a pressure sensor  22  and two trimmer capacitors  23  and  24 , which smooth a non-uniform detected result of the pressure sensor  22 . The pressure sensor  22  includes a variable capacitor, where the capacitance varies in accordance with a detected pressure. In the diode  19 , an anode is connected to one side of an electrode of the variable capacitor  18 , and a cathode is connected to one end of a resistor  21  and one side of an electrode of the trimmer capacitors  23  and  24 . The other end of the resistor  21  and the other side of an electrode of the trimmer capacitor  23  are connected to the ground. The other side of an electrode of the trimmer capacitor  24  is connected to the ground with the pressure sensor  22  disposed therebetween. The resistor  21  enables the diode  19  to be turned on. 
         [0038]    The anode of the diode  19  is connected to a driving circuit  25 , which enables the diode  19  to be turned on and off. The driving circuit  25  is disposed between one end of the inductor  13  and the anode of the diode  19  and is connected in parallel to the quartz-crystal resonator  16 . In the driving circuit  25 , one end of the inductor  13  is connected to a diode  27  with a capacitor  26  disposed therebetween. In a diode  27 , an anode is connected to one side of an electrode of the capacitor  26 , and a cathode is connected to one end of an inductor  28 . The other end of the inductor  28  is connected to the ground with a capacitor  29  disposed therebetween. The inductor  28  and the capacitor  29  constitute a low-pass filter. The low-pass filter converts a resonance wave acquired from a resonance circuit  30  into a direct current. 
         [0039]    In the driving circuit  25 , a middle point between the capacitor  26  and the diode  27  is connected to the resonance circuit  30 , which functions as a connecting resonance circuit. The resonance circuit  30  includes an inductor  31  and the capacitor  26 , namely, an LC resonance circuit. The middle point is connected in parallel to one end of the inductor  31  and one side of an electrode of a capacitor  32 , and additionally the other end of the inductor  31  and the other side of the electrode of the capacitor  32  are connected to the ground. A parallel resonance frequency  2  of the resonance circuit  30  is set to 10.800 MHz. The resonance frequency of the resonance circuit  30  can be modified if it does not resonate when the quartz-crystal resonator  16  resonates. When the resonance circuit  30  resonates, a signal of the resonance frequency is detected by the diode  27 , and the capacitor  29  charges via the inductor  28 . In addition, the driving circuit  25  and the diode  19  constitute a control circuit, which controls a connection of the pressure measuring unit  20  and the quartz-crystal resonator  16 . 
         [0040]    In the transponder, a resonance frequency of the quartz-crystal resonator  16  is changed in terms of the driving circuit  25  by switching the on and off state of the diode  19 . Specifically, as the driving circuit  25  enables the diode  19  to be turned off, the quartz-crystal resonator  16  forms a resonance circuit together with the variable capacitor  18 . As the diode  19  turns on, the quartz-crystal resonator  16  resonates in a state where the resonance circuit is connected to the pressure measuring unit  20 . In the former, a resonance frequency of the quartz-crystal resonator  16  is influenced by only the tire temperature. On the other hand, in the latter, the resonance frequency of the quartz-crystal resonator  16  is influenced by not only the tire temperature but also the tire pressure. In the controller of the tire information detecting device according to the embodiment, a resonance of the resonance circuit  30  is controlled by switching a oscillation signal performing an amplitude modulation with respect to the carrier wave, whereby an on and off state of the diode  19  is switched so as to detect only the tire temperature or the tire pressure plus the tire temperature. 
         [0041]    The controller according to the embodiment is different from the known tire information detecting devices (see  FIG. 4 ). An oscillator G 2  generates a oscillation signal of a frequency (f 2 ) similar to the resonance frequency of the quartz-crystal resonator  16  and a oscillation signal of a frequency (f 3 ) similar to the resonance frequency of the resonance circuit  30 . An oscillation signal having a mean frequency of 9.800 MHz and a oscillation signal having a mean frequency of 10.800 MHz are generated, and the carrier wave (f 1 ) is amplitude-modulated by those oscillation signals. In addition, a signal of which the carrier wave (f 1 ) is amplitude-modulated forms a signal for allowing the quartz-crystal resonator  16  to resonate by using the former oscillation signal, and the signal of which the carrier wave (f 1 ) is amplitude-modulated forms a signal for the purpose of allowing the resonance circuit  30  to resonate by using the latter oscillation signal. 
         [0042]    In the controller according to the embodiment, it is same as the known controller where an amplitude modulation is switched by a switch S 1 . However, in the controller according to the embodiment, a time between an amplitude modulation by using the oscillation signal of the frequency (f 2 ) and an amplitude modulation by using the oscillation signal of the frequency (f 3 ), is switched. Specifically, an amplitude modulation of the carrier wave is performed by only the oscillation signal of the frequency (f 2 ), and then the amplitude modulation is stopped at a time=t 1  ( FIG. 4 ). Accordingly, a non-modulated carrier wave is radiated. Amplitude modulation is then performed by the oscillation signals of both the frequency (f 2 ) and the frequency (ft), and then the amplitude modulation by using the oscillation signal of the frequency (f 2 ) is stopped at a time=t 3  (not shown in  FIG. 4 ). Accordingly, a high-frequency signal, which is amplitude-modulated by using the oscillation signal of the frequency (f 3 ), is radiated. In addition, the reason that the non-modulated carrier wave is not is radiated is because it is necessary to maintain an ON state of the diode  19  by allowing the resonance circuit  30  to resonate when the tire pressure is measured. 
         [0043]    Next, an operational aspect of the invention will be described. When the tire temperature is measured, in the controller, the carrier wave (f 1 ) of 2.4 GHz is amplitude-modulated by using the oscillation signal (having the mean frequency of 9.800 MHz) of the frequency (f 2 ) generated by the oscillator G 2 . Then, the amplitude-modulated high-frequency signal is radiated from an antenna A 1 . At the time=t 1  ( FIG. 3 ), amplitude modulation is stopped, and at time=t 2 , a receiver E 1  becomes active. Additionally, when the amplitude modulation is stopped, a non-modulated carrier wave is radiated from the antenna A 1 . 
         [0044]    In the transponder  10 , the high-frequency signal of 2.4 GHz, which is amplitude-modulated by using the controller, is detected by the diode  12 , and also the carrier wave of 2.4 GHz is removed by the low-pass filter (coil  13  and capacitor  14 ). Accordingly, a oscillation signal, which is the same as the oscillation signal of the frequency (f 2 ), is extracted. In the quartz-crystal resonator  16 , the resonance frequency is similar to the frequency of the oscillation signal of the frequency (f 2 ), and thus it is excited by the signal generated therefrom. 
         [0045]    Accordingly, a resonance frequency of the quartz-crystal resonator  16  occurs. At this time, the resonance circuit  30  does not resonate due to the high-frequency signal from the controller, so the diode  19  remains in an OFF state. Because of this, the resonance frequency of the quartz-crystal resonator  16  is influenced by only the tire temperature. 
         [0046]    In the controller, when an amplitude modulation is stopped and a non-modulated carrier wave is radiated, in the transponder  10 , the quartz-crystal resonator  16  continuously oscillates for about 1 ms or less from when the amplitude modulation is stopped. Accordingly, the non-modulated carrier wave from the controller is amplitude-modulated by using the diode  12  in accordance with the resonance-frequency signal of the quartz-crystal resonator  16 , and is radiated from die antenna  11 . In the receiver E 1  of the controller, the amplitude-modulated high-frequency signal is received via the antenna A 4 , and the tire temperature is calculated by extracting the resonance-frequency signal via a demodulator (not shown in the drawings). 
         [0047]    In the case where the tire temperature is calculated, in the receiver E 1 , a difference in frequency between the frequency (f 2 ) of the oscillation signal generated from the oscillator G 2  and a resonance frequency (f 2 ) extracted from a received signal of the transponder  10 , is estimated. When the tire temperature varies, a variation of the resonance frequency of the quartz-crystal resonator  16  varies, whereby a difference between the resonance frequency (f 2 ′) and the frequency (f 2 ′) which should be originally detected (Δfa shown in  FIG. 2 ), is estimated. Therefore, it is possible to calculate the tire temperature by using the single quartz-crystal resonator  16 . 
         [0048]    In the case where the tire temperature is calculated, for example, a table illustrating a relationship between a resonance-frequency difference and the tire-temperature variation is preferably used in the calculation. Since the resonance frequency of the quartz-crystal resonator varies with a temperature, when a difference with the oscillation signal increases, an intensity of the received signal may decrease. At that time, it needs to change the frequency of the oscillation signal and measure it again. 
         [0049]    When the tire pressure is measured, the carrier wave (f 1 ) of 2.4 GHz is amplitude-modulated by the oscillation signal (mean frequency of 9.800 MHz) of the frequency (f 2 ) and the oscillation signal (mean frequency of 10.800 MHz) of the frequency (f 3 ), and then the amplitude-modulated high-frequency signal is radiated from the antenna A 1 . Then, at time=t 3  ( FIG. 3 ), amplitude modulation of the oscillation signal of the frequency (f 2 ) is stopped, and at time= t 4 , the receiver E 1  becomes active. Additionally, at the time of the amplitude modulation, by using the oscillation signal of the frequency (f 2 ) is stopped, an amplitude-modulated high-frequency signal of 2.4 GHz is radiated from the antenna A 1  by the oscillation signal of the frequency (f 3 ). 
         [0050]    When measuring the tire temperature, the high-frequency signal of 2.4 GHz, which is amplitude-modulated by using the controller, is detected by the diode  12 , and also the carrier wave of 2.4 GHz is removed by the low-pass filter (coil  13  and capacitor  14 ). Accordingly, a oscillation signal which is the same as the oscillation signal of the frequency (f 2 ) and the frequency (f 3 ) is extracted. 
         [0051]    In the resonance circuit  30 , since the resonance frequency is similar to the oscillation signal of the frequency (f 3 ), it is excited by the signal of the frequency (f 3 ) extracted therefrom. Accordingly, the resonance-frequency signal of the resonance circuit  30  occurs. The resonance-frequency signal is detected by the diode  27 , and is converted into a direct current by the low-pass filter (coil  28  and capacitor  29 ). Accordingly, the capacitor  29  charges. When the capacitor  29  charges to a predetermined level, the diode  19  enters an ON state. Accordingly, the quartz-crystal resonator  16  and the pressure measuring unit  20  are connected to each other, and the resonance frequency of the quartz-crystal resonator  16  which resonates in accordance with a signal of the same frequency as the oscillation signal of die extracted frequency (f 2 ), is influenced not only by the tire temperature, but also by the tire pressure, which is detected by the pressure measuring unit  20 . 
         [0052]    In the controller, when the amplitude modulation is stopped by the oscillation signal of the frequency (f 2 ) and the high-frequency signal, which is amplitude-modulated by the oscillation signal of the frequency (f 3 ), is radiated from the antenna A 1 , the quartz-crystal resonator  16  continues to oscillate for about 1 ms or less from when the amplitude modulation due to the oscillation signal of the frequency (f 2 ). Accordingly, the high-frequency signal, which is amplitude-modulated due to the oscillation signal of the frequency (f 3 ), is amplitude-modulated by using the diode  12  in accordance with the resonance-frequency signal of the quartz-crystal resonator  16 , and is radiated from the antenna  11 . In the receiver E 1  of the controller, the high-frequency signal is received via the antenna A 4 , and the tire pressure is calculated by extracting the resonance-frequency signal via the demodulator (not shown). 
         [0053]    In the case where the tire pressure is calculated, in the receiver E 1 , the difference in frequency between the resonance frequency (f 2 ′) extracted from the received signal out of the transponder  10  and a resonance frequency (f 2 ″) extracted from a received signal out of the transponder  10 , is estimated. When the diode  19  turns ON and the pressure measuring unit  20  is electrically connected to the quartz-crystal resonator  16 , if the tire pressure varies, the resonance frequency of the quart-crystal resonator  16  also varies. Accordingly, as shown in  FIG. 2 , by estimating the difference (Δfb) between the resonance frequency (f 2 ″) and the resonance frequency (f 2 ′) detected when the tire temperature is calculated, it is possible to calculate the tire pressure by using the single quartz-crystal resonator  16 . 
         [0054]    For the purpose of estimation, the known procedure of which a correcting process to correct a portion of a temperature from the measured value can be omitted, the temperature characteristics and a temporal degradation characteristics of the respective quartz-crystal resonators, have little affect. Thus tire temperature and tire pressure can be calculated by using single quartz-crystal resonator  16 . 
         [0055]    In the case where the tire pressure is calculated, for example, a table illustrating a relationship between the resonance-frequency difference and the tire-pressure variation is prepared in advance. Since the resonance frequency of the quartz-crystal resonator varies with the temperature and the pressure, when a difference with the oscillation signal increases, the intensity of the received signal may decrease. At that time, the frequency of the oscillation signal is changed by small amount and is measure again. 
         [0056]    In the transponder  10 , the connection between the pressure measuring unit  20  and the quartz-crystal resonator  16  is controlled in accordance with the signal from the controller by means of a control circuit, which includes the driving circuit  25  and the diode  19 . Accordingly, the connection between the quartz-crystal resonator  16  and the pressure measuring unit  20  is controlled, whereby a resonance frequency of the quartz-crystal resonator  16  is estimated by the controller. Thus, one quartz-crystal resonator  16  can calculate both the tire temperature and the tire pressure without multiple quartz-crystal resonators. 
         [0057]    Additionally, in the driving circuit  25 , as the resonance circuit  30  resonates, the connection between the pressure measuring unit  20  and the quartz-crystal resonator  16  is controlled, whereby the resonance of the resonance circuit  30  is controlled by the controller, thereby controlling the connection between the pressure measuring unit  20  and the quartz-crystal resonator  16 . 
         [0058]    The control circuit includes the diode  19 , which is connected to an input-output terminal of the pressure measuring unit  20  on a first signal line in which the quartz-crystal resonator  16  and the pressure measuring unit  20  are connected to each other. Accordingly, an ON and OFF state of the diode  19  is switched in accordance with the resonance of the resonance circuit  30 , thereby controlling the connection between the pressure measuring unit  20  and the quartz-crystal resonator  16 . 
         [0059]    The transponder  10  includes a second signal line, which bypasses the first signal line between the input-output terminal of the pressure measuring unit  20  side of the diode  12  on the first line and the input-output terminal of the diode  19 , and is connected to an end of the resonance circuit  30  on the second signal line. Accordingly, the ON and OFF state of the diode  19  can be switched by the resonance circuit  30  of which an end thereof is connected to the second signal line, whereby this does not influence the signal which communicates via the first signal line, thereby controlling the connection between the pressure measuring unit  20  and the quartz-crystal resonator  16 . 
         [0060]    The invention is not limited to the above-described embodiment, and may be modified to various forms of the embodiment, if necessary. In the above-described embodiment, the circuit configuration or the like as shown in the attached drawings is not limited to the above-described embodiment, and may be modified. 
         [0061]    For example, in the tire information detecting device according to the above-described embodiment, the transponder includes an LC resonance circuit, which is connected in parallel to the quartz-crystal resonator  16  to form the resonance circuit  30 . But the resonance circuit which is connected in parallel to the quartz-crystal resonator  16 , is not limited to this configuration. For example, the LC resonance circuit may be replaced with a piezoelectric resonator. However, for reasons of cost, the LC resonance circuit as the above-described embodiment may be utilized. 
         [0062]    In addition, in the tire information detecting device according to the above-described embodiment, the diode  19  is used as a switching element which switching the connection between the quartz-crystal resonator  16  and the pressure measuring unit  20 . However, the switching element is not limited to this configuration. For example, the diode  19  may be replaced with an FET. 
         [0063]    Although the detecting resonance circuit comprises the quartz-crystal resonator, the detecting resonance circuit is not limited to this configuration, and may be replaced with a piezoelectric single crystal resonator, made of a single piezoelectric crystal formed of a lithium tantalite (LiTaO 3 ), a lithium niobate (LiNbO 3 ), a lithium borate (Li 2 B 4 O 7 ), a potassium niobate (KNbO 3 ), a langasite crystal (La 3 Ga 5 SiO 14 ), and a langanite (La 3 Nb 0.5 Ga 5.5 O 14 ), a resonance circuit having a ceramic resonator, and an LC resonance circuit. The quartz-crystal resonator is selected because of its precision and stability. 
         [0064]      FIG. 4  shows a modified example of the circuit configuration of the transponder  10 . The transponder  10  includes a MOSFET  33  as a switching element. In the circuit configuration of the transponder  10  shown in  FIG. 4 , the fact that a MOSFET  33  instead of the diode  10  shown in  FIG. 1  is connected and the resistor  21  constituting the pressure measuring unit shown in  FIG. 1  is omitted is different from the circuit configuration in the transponder  10  shown in  FIG. 1 . When the MOSFET  33  is used instead of the diode  10 , when the capacitor  29  charges to a certain amount with response of the resonance circuit  30 , the MOSFET  33  turns ON. Consequently, the tire pressure which is detected by the pressure measuring unit  20  influences the resonance frequency of the quartz-crystal resonator  16 , thereby obtaining the same function and effect as the above-described embodiment.