Abstract:
A tire monitor radio circuit includes a control circuit delivering a binary digital baseband signal, a modulation circuit having an oscillation circuit generating a carrier wave and switched between an oscillatory and a non-oscillatory states, an antenna resonance circuit having a coil and a capacitor both connected together, a resistance damping circuit having a switching element and a resistance and connected to the antenna resonance circuit, the resistance damping circuit being switchable between an operative and an inoperative states, and a compensation circuit putting the resistance damping circuit into the operative state when the modulation circuit is switched to the non-oscillatory state, the compensation circuit returning the resistance damping circuit to the non-operative state when or before the modulation circuit has been or is switched to the oscillatory state. When he resistance damping circuit is on the operative state, resonance current of the damped oscillation in the antenna resonance circuit is applied to the resistance, and the resistance serves as a damper thereby to reduce statically determinate time of damped oscillation. As a result, the digital signal can accurately be detected from the carrier wave at the reception side. Consequently, reliability in the communication can be improved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    The present invention relates to a tire monitor radio circuit for performing radio transmission to a tire pressure detector mounted on a wheel of a vehicle and a tire monitor system provided with the radio circuit. 
         [0003]    2. Description of the related art 
         [0004]    Tire monitor systems of the above-described type generally comprise a tire monitor device mounted on a body of a vehicle and tire pressure detectors mounted on respective tires of the vehicle. Information about inner pressure of each tire is transmitted by radio between the vehicle body and each wheel. JP-A-2005-119370 discloses one of such tire monitor systems. In one of known communication manners, a tire monitor device provided at the vehicle body side delivers a trigger signal by radio. In reply to the trigger signal, a tire pressure detector transmits results of detection of tire pressure by radio. In this case, the tire monitor device carries out an amplitude shift keying (ASK) modulation based on a digital baseband signal. 
         [0005]    Furthermore, there is a time when a reception level of radio transmission from a first communication device is excessively high at a second communication device. In this case, for the purpose of improving reliability of radio communication accompanied by ASK modulation and demodulation, a technique is known which informs by radio the first communication device of the excessively high reception level so that transmission output of the first communication device is lowered. JP-A-2005-45451 discloses one of the above-described techniques. 
         [0006]    The ASK modulation and demodulation result in the following problem. To modulate a digital baseband signal W 10  as shown in  FIG. 9 , for example, a carrier wave is imparted to an antenna resonance circuit when the digital baseband signal W 10  has turned to “1.” When the signal W 10  has turned to “0,” impartment of carrier wave to the antenna resonance circuit is stopped. As a result, a modulated signal W 11  shown in  FIG. 9  is delivered from an antenna. 
         [0007]    However, damped oscillation after stop of carrier wave impartment to the antenna resonance circuit is superimposed on the modulated signal W 11 . Accordingly, the modulated signal W 11  contains an amplitude component S 10  of damped oscillation and an amplitude component S 11  corresponding to “1” of the digital baseband signal W 10 . When the signal is demodulated at the reception side, the amplitude component S 10  is equated with the amplitude component S 11 , and a demodulated signal W 12  is generated. Consequently, the demodulated signal W 12  differs from the digital baseband signal W 10 , and accurate information cannot be transmitted. In the aforesaid conventional technique, however, a transmission output level is merely reduced. Accordingly, the difference between the amplitude component S 11  corresponding to “1” of the digital baseband signal W 10  and amplitude component S 10  of damped oscillation cannot be rendered distinct. Moreover, since reduction in the transmission output renders the signal weaker against noise, accurate information still cannot be transmitted. 
       SUMMARY OF THE INVENTION 
       [0008]    Therefore, an object of the present invention is to provide a tire monitor radio circuit which can improve communication reliability and a tire monitor system provided with the tire monitor radio circuit. 
         [0009]    The present invention provides a tire monitor radio circuit performing radio transmission to a tire pressure detector mounted on a wheel of a vehicle. The tire monitor radio circuit comprises a control circuit delivering a binary digital baseband signal, a modulation circuit having an oscillation circuit generating a carrier wave, the modulation circuit switching between an oscillatory state where the carrier wave is delivered and a non-oscillatory state where the carrier wave is not delivered, based on inversion of a binary value of the digital baseband signal, an antenna resonance circuit which has a coil and a capacitor both connected together and is capable of resonating in response to the carrier wave delivered by the modulation circuit, a resistance damping circuit which has a switching element and a resistance and is connected to the antenna resonance circuit, the resistance damping circuit being switchable between an operative state where a resonance current of the antenna resonance circuit can be applied to the resistance and an inoperative state where the resonance current of the antenna resonance circuit cannot be applied to the resistance, and a compensation circuit putting the resistance damping circuit into the operative state when the modulation circuit is switched from the oscillatory state to the non-oscillatory state, the compensation circuit returning the resistance damping circuit to the non-operative state when or before the modulation circuit is switched from the non-oscillatory state to the oscillatory state. 
         [0010]    In the tire monitor radio circuit of the present invention, the modulation circuit switches between the oscillatory state and the non-oscillatory state according to inversion of the binary value of the digital baseband signal delivered by the control circuit, whereby the carrier wave is modulated in response to the digital baseband signal. The resistance damping circuit provided with the switching element and the resistance is connected to the antenna resonance circuit. When the modulation circuit is switched from the oscillatory state to the non-oscillatory state, the resistance damping circuit is switched to the operative state, whereby resonance current of damped oscillation in the antenna resonance circuit is supplied to the resistance. In this case, the resistance serves as a damper thereby to reduce statically determinate time of damped oscillation. As a result, the digital signal can accurately be detected from the carrier wave at the reception side. Moreover, the resistance damping circuit is returned to the inoperative state when or before the modulation circuit has been or is switched from the non-oscillatory state to the oscillatory state. Accordingly, transmission output is not reduced, either. Consequently, the reliability in the communication can be improved by the above-described arrangement as compared with the conventional arrangement. 
         [0011]    In a preferred embodiment, the coil of the antenna resonance circuit has two terminals, and the resistance damping circuit comprises the switching element and the resistance series-connected between both terminals of the coil. In this case, the resistance is conductively connected between the terminals of the coil of the antenna resonance circuit when the modulation circuit has been switched from the oscillatory state to the non-oscillatory state. As a result, the resonance current due to the damped oscillation flows to the resistance, whereupon electric energy is consumed and accordingly, statically determinate time of damped oscillation is reduced. 
         [0012]    In another preferred embodiment, the capacitor of the antenna resonance circuit has two terminals, and the resistance damping circuit comprises the switching element and the resistance connected in series between both terminals of the capacitor. The resistance is conductively connected between the terminals of the capacitor of the antenna resonance circuit when the modulation circuit has been switched from the oscillatory state to the non-oscillatory state. In this case, too, the resonance current due to the damped oscillation flows to the resistance, whereupon electric energy is consumed and accordingly, statically determinate time of damped oscillation is reduced. 
         [0013]    In further another preferred embodiment, the capacitor and the coil of the antenna resonance circuit are connected in series into an inductance-capacitance (LC) series circuit having two terminals, and the resistance damping circuit comprises the switching element and the resistance series-connected between both terminals of the LC series circuit. A closed circuit including the coil, capacitor and resistance is established in synchronization with the switch of the modulation circuit from the oscillatory to the non-oscillatory state. Consequently, the resonance current due to the damped oscillation flows to the resistance, whereupon electric energy is consumed and accordingly, statically determinate time of damped oscillation is reduced. 
         [0014]    The invention also provides a tire monitor system which includes a tire pressure detector mountable on a wheel of a vehicle and a tire monitor device mountable on a vehicle body and comprising the above-described tire monitor radio circuit. 
         [0015]    In the foregoing system, the tire monitor mounted on the vehicle body performs radio transmission to the tire pressure detector mounted on the wheel. The tire pressure detector responds, transmitting result of detection of tire pressure. The tire monitor device thus receives the detection result. Consequently, the tire pressure can be monitored at the vehicle body side. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other objects, features and advantages of the present invention will become clear upon reviewing the following description of the preferred embodiment, with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a conceptual view of a tire monitor system in accordance with an embodiment of the present invention; 
           [0018]      FIG. 2  is a sectional view of a wheel and tire pressure detector; 
           [0019]      FIG. 3  is a block diagram showing an electrical arrangement of the tire monitor system; 
           [0020]      FIG. 4  is an electrical circuit diagram of a tire monitor radio circuit; 
           [0021]      FIG. 5  is a time chart showing a modulated signal and a demodulated signal for a digital baseband signal; 
           [0022]      FIG. 6  is an electrical circuit diagram of an antenna resonance circuit in a second embodiment; 
           [0023]      FIG. 7  is an electrical circuit diagram of an antenna resonance circuit in a third embodiment; 
           [0024]      FIG. 8  is an electrical circuit diagram of an antenna resonance circuit in a fourth embodiment; and 
           [0025]      FIG. 9  is a time chart showing a modulated signal and a demodulated signal for a digital baseband signal in the use of a conventional tire monitor radio circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    A first embodiment of the present invention will be described with reference to  FIGS. 1 to 5 .  FIG. 1  illustrates a tire monitor system  10  in accordance with the first embodiment of the invention. The tire monitor system  10  comprises, for example, four tire pressure detectors  30  provided on respective wheels  13  of a vehicle  11  such as automobile and a single tire monitor device  50  provided on a body  12  of the vehicle  11 . Only one of the wheels  13  is shown in  FIG. 1 . 
         [0027]    Each wheel  13  is comprised of a tire wheel  14  having a rim  14 A and a tubeless tire  15  fitted with the rim  14 A. The rim  14  is formed with a valve mount hole  14 C as shown in  FIG. 2 . A tire valve  16  is inserted through and fixed to the valve mount hole  14 C. The tire valve  16  comprises a cylindrical valve stem  16 A with two open ends and a valve core  17  with a check-valve structure disposed in the valve stem  16 A. The tire valve  16  has a distal end protruding to an inner circumferential surface side of the rim  14 A. On the other hand, the tire valve  16  has a proximal end protruding to an outer circumferential surface side of the rim  14 A and disposed in the tire  15 . The tire valve  16  has an outer surface formed with a male thread  16 B. A cap  18  is in threaded engagement with a distal end of the male thread  16 B. 
         [0028]    The tire pressure detector  30  mounted on each wheel  13  is fixed to the proximal end of the tire valve  16 . The tire pressure detector  30  comprises a case  37  fixed to the tire valve  16 . A circuit board  38 , a button cell  39  and the like are accommodated in the case  37 . On the circuit board  38  are mounted a control circuit  31 , a low-frequency (LF) receiving circuit  32 , a radio frequency (RF) transmission circuit  33 , a memory  34 , a pressure sensor  35  and a temperature sensor  36  as shown in  FIG. 3 . Furthermore, the memory  34  stores identification data set for every tire pressure detector  30 . The LF receiving circuit  32  and the RF transmission circuit  33  are provided with respective antennas  40  and  41 . 
         [0029]    When receiving an external command through the LF receiving circuit  32 , the control circuit  31  is triggered and carries out a predetermined program, loading results of detection by the pressure sensor  35  and temperature sensor  36 . The control circuit  31  adds identification data to detection data, transmitting the data to the RF transmission circuit  33  by radio. 
         [0030]    The tire monitor device  50  is disposed, for example, in the rear of a dashboard (not shown) mounted on the vehicle body  12  and is connected to and supplied with electric power by a vehicle battery (not shown). The tire monitor device  50  includes a control circuit  51 , a low-frequency (LF) transmission circuit  52 , a radio-frequency (RF) receiving circuit  67 , an input key circuit  68 , a memory  80 , a display circuit  69  and a display  70 , as shown in  FIG. 3 . The LF transmission circuit  52  and the RF receiving circuit  67  are provided with respective antennas  42  and  43 . 
         [0031]    The LF transmission circuit  52  delivers a radio signal, and each tire pressure detector  30  transmits results of detection of an inner pressure and temperature of a tire by radio in reply to the signal. The RF receiving circuit  67  receives the results of detection of tire pressure and tire temperature. Furthermore, the control circuit  51  determines whether the tire pressure and tire temperature are unusual, thereby driving the display circuit  69  so that warning or the like is displayed on the display  70 . 
         [0032]    A tire monitor radio circuit  53  comprises the control circuit  51  and the LF transmission circuit  52 . The control circuit  51  is composed of a digital circuit provided with a central processing unit (CPU; and not shown). The control circuit  51  is provided with a serial output port  51 S from which a binary digital baseband signal is delivered to each tire pressure detector  30 . When the binary digital baseband signal is at “0,” the potential difference between the serial output port  51 S and a ground (GND) is zero. When the binary digital baseband signal is at “1,” the potential difference between the serial output port  51 S and the GND is at a predetermined level. 
         [0033]    The LF transmission circuit  52  includes a modulation circuit  54  and an antenna resonance circuit  60 . The modulation circuit  54  includes an oscillation circuit  55 , an AND circuit  56  and an amplifier circuit  57 . The oscillation circuit  55  generates and supplies carrier waves to the AND circuit  56 . More specifically, the AND circuit  56  includes a pair of input terminals. The oscillation circuit  55  is connected between one of the input terminals of the AND circuit  56  and the GND, changing a potential difference between the input terminal of the AND circuit  56  and the GND at predetermined intervals ( 125  kHz, for example). Furthermore, the serial output port  51 S of the control circuit  51  is connected to the other input terminal of the AND circuit  56 . As a result, when the digital baseband signal is at “1” and the amplitude of the carrier wave is at or above a predetermined potential difference relative to the GND, output of the AND circuit  56  is switched to an on-state. The output of the AND circuit  56  is switched to an off-state in other cases. As a result, when the digital baseband signal W 1  becomes “1,” a carrier wave is substantially delivered from the AND circuit  56 . When the digital baseband signal becomes “O,” output of the carrier wave from the AND circuit  56  is stopped. The output of the AND circuit  56  is amplified by the amplifier circuit  57  to be supplied as output of the modulation circuit  54  to the antenna resonance circuit  60 . 
         [0034]    The antenna resonance circuit  60  is connected to the output side of the modulation circuit  54  and includes a capacitor  58  and a coil  59  connected in series between output of the modulation circuit  54  and the GND. The antenna  42  has a coil  42 C connectable to the coil  59  of the antenna resonance circuit  60  by electromagnetic induction. 
         [0035]    A resistance damping circuit  62  is connected in parallel to an inductance-capacitance (LC) series circuit  60 A between output of the modulation circuit  54  and the GND. The LC series circuit  60 A comprises the capacitor  58  and the coil  59  both constituting the antenna resonance circuit  60 . The resistance damping circuit  62  comprises a resistance  64  and a transistor  63  serving as a switching element. The resistance  64  and the transistor  63  are connected in series to each other. The transistor  63  is, for example, an NPN bipolar transistor and has a collector connected to output of the modulation circuit  54  via the resistance  64  and an emitter connected to the GND. Furthermore, the transistor  63  has a base  63 B serving as an on-off control terminal in the invention. Between the base  63 B and the serial output port  51 S of the control circuit  51  is connected a NOT circuit  61  serving as a compensation circuit in the invention. 
         [0036]    The tire monitor system  10  arranged as described above will operate in the following manner. When an ignition key switch of the vehicle  11  is turned on or regularly while the vehicle  11  is moving, the tire monitor device  50  asks each tire detector  30  of each wheel  13  for the results of detection of tire pressure and temperature. For this purpose, the tire monitor device  50  delivers a radio signal as a trigger for operating each tire pressure detector  30 . More specifically, the control circuit  50  of the tire monitor device  50  delivers a binary digital baseband signal W 1  (see  FIG. 5 ) containing predetermined information to each tire pressure detector  30 . When the binary digital baseband signal W 1  becomes “1, ” the modulation circuit  54  is, in response, switched between an oscillatory state where the modulation circuit  54  delivers a carrier wave to the antenna resonance circuit  60  and a non-oscillatory state where the modulation circuit  54  does not deliver the carrier wave. When the modulation circuit  54  is in the oscillatory state, the antenna resonance circuit  60  resonates so that radio waves are transmitted from the antenna  42 . On the other hand, when the modulation circuit  54  is in the non-oscillatory state, the resonance of the antenna resonance circuit  60  is stopped, whereby transmission of radio waves from the antenna  42  is stopped. Thus, a modulated signal W 2  (see  FIG. 5 ) according to the digital baseband signal W 1  is delivered from the antenna  42 . 
         [0037]    In response to the signal transmitted from the tire monitor radio circuit  53  of the tire monitor device  50 , each tire pressure detector  30  delivers by radio the results of detection by the pressure and temperature sensors  35  and  36  from the RF transmission circuit  33 . The tire monitor device  50  receives the detection results, determining defect in each tire and each tire pressure detector  30 . Thus, according to the tire monitor system  10  of the invention, abnormality of each tire  15  can be monitored at the vehicle body side  12 . 
         [0038]    In order that information may accurately be transmitted from the tire monitor radio circuit  53  to each tire pressure detector  30 , the following process is carried out in the tire monitor system  10 . The resistance damping circuit  62  is switched between an operative state and a non-operative state in synchronization with the switching between an oscillatory state and a non-oscillatory state of the modulation circuit  54  during radio transmission by the tire monitor radio circuit  53 . More specifically, when the digital baseband signal W 1  becomes “0,” output of the NOT circuit  61  is turned to the on-state, whereupon the transistor  63  is turned on and accordingly, the resistance damping circuit  62  is turned to the operative state. When the digital baseband signal W 1  becomes “l,” output of the NOT circuit  61  is turned to the off-state, whereupon the transistor  63  is turned off and accordingly the resistance damping circuit  62  is turned to the non-operative state. When the resistance damping circuit  62  is turned to the operative state, a closed circuit is established by the coil  59  and capacitor  58  of the antenna resonance circuit  60  and the transistor  63  and the resistance  64  of the resistance damping circuit  62 . When resonance current due to damped oscillation of the antenna resonance circuit  60  flows through the closed circuit, the resistance  64  serves as a damper, thereby reducing statically determinate time of the damped oscillation. 
         [0039]    Consequently, a modulated signal W 2  to be delivered from the antenna  42  is steeply switched between an amplitude state and a non-amplitude state in synchronization with inversion of the digital baseband signal W 1  from “1” to “0” as shown in  FIG. 5 , whereby a demodulated signal W 3  demodulated by each tire pressure detector  30  based on the modulated signal W 2  can accurately match the digital baseband signal W 1 . Moreover, when the modulation circuit  54  has been switched from the non-oscillatory state to the oscillatory state, the resistance damping circuit  62  is returned to the non-operative state, whereupon the transmission output is not turned down. Thus, the tire monitor system  10  of the embodiment can improve the reliability in the radio communication as compared with the conventional systems. 
         [0040]      FIG. 6  illustrates a second embodiment of the invention. The second embodiment differs from the first embodiment mainly in the arrangement of a resistance damping circuit  71 . More specifically, the resistance damping circuit  71  is connected in series with the switching element  71 B and the resistance  71 A between both terminals of the coil  59  of the antenna resonance circuit  60 . The switching element  71 B is designed to be on-off controlled in response to a signal from the control circuit  51 . In this respect, a circuit connecting the control circuit  51  and the switching element  71 B serves as a compensation circuit in the second embodiment of the invention. 
         [0041]    According to the second embodiment, when the modulation circuit  54  has been switched from the oscillatory state to the non-oscillatory state, the resistance  71 A is conductively connected between both terminals of the coil  59  of the antenna resonance circuit  60 . Accordingly, resonance current due to damped oscillation flows into the resistance  71 A, whereby electric energy is consumed. Consequently, statically determinate time of damped oscillation can be reduced. 
         [0042]      FIG. 7  illustrates a third embodiment of the invention. The third embodiment differs from the first embodiment mainly in the arrangement of a resistance damping circuit  72 . More specifically, the resistance damping circuit  72  is connected in series with the switching element  72 B and the resistance  72 A between both terminals of the capacitor  58  of the antenna resonance circuit  60 . Other arrangements of the third embodiment are same as those of the second embodiment. The third embodiment can achieve the same effect as the second embodiment. 
         [0043]      FIG. 8  illustrates a fourth embodiment of the invention. The tire monitor system of the fourth embodiment has an arrangement achieved by combination of the resistance damping circuits  71  and  72  of the second and third embodiments. Accordingly, the fourth embodiment can achieve the same effect as the second and third embodiments. 
         [0044]    The invention should not be limited to the foregoing embodiments but encompasses the following modified forms. Furthermore, the invention can be modified in various forms in practice without departing from the gist thereof. Firstly, the transistor  63  is turned off in synchronization with the switch of the modulation circuit  54  from the oscillatory state to the non-oscillatory state in the first embodiment. However, for example, time measurement may start upon switch of the digital baseband signal W 1  from “1” to “0.” The transistor  63  may be turned on after a lapse of predetermined time so that the transistor  63  is turned off before the modulated circuit is switched from the oscillatory state to the non-oscillatory state. Secondly, although the transistor  63  is a bipolar transistor (see  FIG. 4 ) in the first embodiment, the transistor  63  may be a field effect transistor (FET), instead. 
         [0045]    The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.