Abstract:
A system for monitoring tire pressure and producing a first signal in the event that a low tire pressure condition has been sensed and a second, different signal in the event that a low tire pressure condition has not been sensed and a third signal in the event that the system is not operable. The system comprises a sensor for each tire and an associated transceiver antenna coil. Each sensor comprises a pressure switch and a circuit that has a first resonant frequency when the pressure switch is in a first state and a second, different resonant frequency when the switch is in a second state. An excited circuit associated with each transceiver antenna coil generates an AC electromagnetic field across the transceiver antenna coil and a detector circuit is operable to demodulate information communicated passively by the sensor that reflects its resonant frequency.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention is a system that provides for the remote sensing of a low vehicle tire pressure condition and the generation of a signal indicating the presence of the low pressure condition. This invention is also a sensor for use in the system. 
   2. Description of the Prior Art 
   Systems for remotely monitoring vehicle tire pressure were developed many decades ago and inventors have been improving on them ever since. Tire pressure monitoring systems will be required in passenger vehicles sold in the US beginning with model year 2008. The requirements for those systems are spelled out in the TREAD Act and have been known for some time. 
   SUMMARY OF THE INVENTION 
   The present invention is based on the discovery of an elegantly simple and reliable system for monitoring tire pressure and providing a signal in the event that a low tire pressure condition has been sensed or that a tire pressure sensor is not functioning. The system comprises a sensor for mounting in each tire to be monitored and an associated transceiver antenna coil that is mounted near each monitored tire. Each sensor comprises a pressure switch and a circuit that includes an antenna coil and a reference capacitor which establish a reference resonant frequency for the circuit. The circuit also includes an additional capacitor, hereinafter referred to as a condition capacitor, that is inactive when the pressure switch is in a first state and is actively connected in parallel with the reference capacitor when the pressure switch is in a second state. When the pressure switch is in the first state and the condition capacitor is inactive, the sensor circuit will have a first resonant frequency and, when the pressure switch is in the second state and the condition capacitor is actively connected in the sensor circuit, it will have a second resonant frequency that is different from the first resonant frequency. Each transceiver antenna coil is operatively associated with an exciter circuit that generates an AC electromagnetic field across the transceiver antenna coil and with a detector circuit that is operable to demodulate information communicated passively by the sensor that reflects its resonant frequency. Preferably, each transceiver antenna coil is sequentially activated so that a single sensor is interrogated or polled at a given time. In this way, a signal may be correlated with a particular tire reflecting the state of the pressure switch in the sensor on that tire. 
   Accordingly, it is an object of this invention to provide a reliable tire pressure monitoring system comprising a sensor and a transceiver antenna for each tire. 
   It is another object of the invention to provide a wheel mounted sensor that operates reliably and without power internal to the sensor such as might be provided by batteries. 
   It is yet another object of the invention to provide a tire pressure monitoring system that includes a sensor that cooperates with a transceiver to produce a first signal indicating that the system is operable and a second signal that indicates either that a low pressure condition has been sensed or that the system has somehow become inoperable. 
   It is a further object of this invention to provide a sensor including a pressure switch that is bi-stable and will change from a first state to a second state when a low tire pressure condition is detected and will remain in the second state until there is a substantial increase in the tire pressure. 
   These and other objects and advantages of the invention will be apparent from the following detailed description of the invention including the preferred embodiments, reference being made therein to the attached drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a sensor and an associated transceiver according to the present invention. 
       FIG. 2  is a generalized graph showing the resonant frequency of the sensor when the pressure switch is in a first state and the resonant frequency of the sensor when the pressure switch is in a second state. 
       FIG. 3  is a schematic representation of the system applied to a four wheeled vehicle in a configuration that will identify the condition of each sensor independently of the condition of the other three sensors. 
       FIG. 4  is a perspective view of a sectioned in-line embodiment of a sensor according to the present invention. 
       FIG. 5  is another perspective view of the in-line sensor shown in  FIG. 4 . 
       FIG. 6  is a perspective view of a multiple chambered embodiment of a sensor according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now in more detail to the drawing figures,  FIG. 1  shows a schematic diagram of a tire pressure monitoring system according to the invention, indicated generally at  10 , for a single wheel. The system  10  comprises a transceiver  12  and a sensor  14 . The transceiver  12 , or a portion of it, is adapted to be mounted on the vehicle, for example, in or adjacent to the wheel well associated with the tire in which the pressure is to be monitored. The sensor  14  is adapted to be mounted inside of a tire or at least associated with the tire so that a portion of the sensor is exposed to the ambient pressure inside of the tire. 
   The transceiver  12  comprises an exciter circuit  16  that is operable to generate an AC electromagnetic field across an antenna coil  18 . The transceiver  12  further comprises a detector circuit  20  that is operable to demodulate signals produced or induced in the sensor  14 . 
   The sensor  14  has a circuit that comprises an antenna coil  22  and a reference capacitor  26 . When alternating current passes through the transceiver antenna coil  18 , the sensor antenna coil  22  will be inductively coupled to the transceiver antenna coil  18  and alternating current will be induced to flow through the sensor antenna coil  22 . The circuit comprising the sensor antenna coil  22  and the reference capacitor  26  will have a resonant frequency which is referred to herein as a reference resonant frequency. 
   The sensor circuit also includes a condition capacitor  28  and a pressure switch  30 . The pressure switch  30  is bi-stable meaning that it is only stable in two distinct conditions or positions and not in between those two positions or conditions. This should not be construed to mean that the switch might not be a tri-stable switch that is stable in only three positions or a switch that is stable in more than three positions. It does mean that when the switch changes from a first state to a second state that it will remain in the second state despite relatively minor fluctuations in the ambient pressure. 
   When the pressure switch  30  is in a first state, the condition capacitor  28  is inactive and inoperable in the sensor circuit and does not affect the resonant frequency of the sensor circuit. In this embodiment, when the pressure switch  30  is in the first state, the resonant frequency of the sensor circuit will be a condition resonant frequency. When the pressure switch  30  is in a second state, the condition capacitor  28  is actively connected in parallel with the reference capacitor  26 , the resonant frequency of the sensor circuit will be the reference resonant frequency and that will be different than the condition resonant frequency. This is shown, in general terms, in the plots in  FIG. 2 . The solid line plot is a stylized representation of the impedance of the sensor circuit with the pressure switch  30  in the second state and the condition capacitor  28  in parallel with the reference capacitor  26 , at various frequencies. The sensor circuit with the condition capacitor  28  connected in parallel with reference capacitor  26  has a Reference resonance frequency R(RF), which is the frequency at which the impedance of the circuit is highest. This Reference resonant frequency C(RF) is detectable by the transceiver  12  and provides a positive indication that the transceiver  12  and the sensor  14  are operative and that the pressure switch  30  is in the second state. When the pressure switch  30  is in its first state and the condition capacitor  28  is out of the sensor circuit and not in parallel with the reference capacitor  26 , the resonant frequency of the sensor circuit will be, for example, as shown in  FIG. 2 , a Condition resonant frequency R(RF) shown in a dashed line in  FIG. 2 . The Reference resonant frequency R(RF) will be lower than the Condition resonant frequency C(RF) in the case where the pressure switch  30  is open and the condition capacitor  28  is not in the sensor circuit in parallel with the reference capacitor  26 . A frequency detector in the detector  20  can detect the frequency of the signal resulting from the induced current flowing in the sensor circuit and thereby provide a signal indicating whether the resonant frequency coincides with the Reference resonant frequency or the Condition resonant frequency. 
   Referring now to  FIG. 3 , a system, indicated generally at  32 , for monitoring the tire pressure in four tires of a vehicle is illustrated schematically. First, second, third and fourth tire sensors  34 ,  36 ,  38  and  40  corresponding generally with sensor  14  ( FIG. 1 ) are provided in the system  32 . Each sensor is associated with a corresponding transceiver antenna coil. First sensor  34  is adjacent to a first transceiver antenna coil  42 . Second sensor  36  is adjacent to a second transceiver antenna coil  44 . Third sensor  38  is adjacent to a third transceiver antenna coil  46 . Fourth sensor  40  is adjacent to a fourth transceiver antenna coil  48 . The transceiver antenna coils  42 ,  44 ,  46  and  48  are operably and, preferably sequentially, connected through a multiplexer  50  to a transceiver detector  52  and an exciter  54 . The multiplexer  50  is operable to selectively and sequentially connect the detector  52  and the exciter  54  to the various transceiver antenna coils  42 ,  44 ,  46  and  48  under the control of a selector  56 . The multiplexer  50  provides the capability for a single sensor to be activated or interrogated or polled at a time so that a sensor signal received by a given transceiver antenna can be correlated and associated with a particular tire. 
   Referring now to  FIGS. 4 and 5 , an embodiment of a sensor according to the present invention is indicated generally at  58 . The sensor  58  comprises an antenna coil  60  mounted on a printed circuit board  62  which, in turn, is mounted at one end of a sensor body  64  The printed circuit board  62  includes at least a first capacitor (not shown) and a second capacitor (not shown) although, if desired, one or both capacitors could be mounted outside of the printed circuit board  62 . The coil  60  is physically and electrically connected to the printed circuit board  62  at junctions  66 . When a pressure switch in the sensor  58  is in a second state, the first capacitor, which corresponds with the reference capacitor  26  ( FIG. 1 ) and the second capacitor, which corresponds with the condition capacitor  28  ( FIG. 1 ), are electrically connected, parallel with each other and in series with the coil  60 . When the pressure switch is in a first state, just the first capacitor is electrically connected in series with the coil  60  in the sensor circuit. 
   The sensor  58  includes a pressure switch comprising a pressure membrane  68  that is mounted in a cavity in the sensor body  64  between two seals comprising a first O-ring  70  and a second O-ring  72 . The membrane  68  and the O-rings  70  and  72  are held in place by an externally threaded ring  74  which cooperates with internal threads  76  in the sensor body  64 . The membrane  68  together with the O-ring  72  and an adjacent portion of the sensor body  64  define a reference pressure chamber  78 . A central passageway  80  in the sensor body  64  houses a central conductor  82  which is subject to electrical contact with the membrane  68 , at one end, and is electrically connected at the other end to the printed circuit board  62  through a junction  84 . The central passageway  80  is sealed by or around the central conductor  82  so that the pressure chamber  78  is a sealed chamber except for an initialization passageway  86  that extends from the pressure chamber  78  to the outside of the sensor body  64 . Before the sensor  58  is ready for use, the pressure chamber  78  can be pressurized to a reference pressure, through the initialization passageway  86 , and the passageway  86  can then be sealed so that the reference pressure is maintained in the pressure chamber  78 . 
   The membrane  68  is a bi-stable, snap action diaphragm membrane, so-called because of its properties when it is exposed to pressure differentials on either side of it. In service, the membrane will be exposed, on one side, to the pressure in the reference pressure chamber  78  and, on the other side, to ambient pressure prevailing inside of a tire. The membrane  68  has a central region  88  that is generally flat and is surrounded by an extremely shallow, conically-shaped region  90 . Outside of the region  90 , there is another, generally flat, ring-shaped region  92 . The membrane is preferably made of a conductive material and, preferably, a springy, corrosion-resistant material such as stainless steel having a minimal thickness, such as about two thousandths of an inch, so that it is very flexible. With the perimeter of the membrane  68  constrained between the O-rings  70  and  72 , the membrane  68  will try to assume one of two neutral positions or states for it. One neutral position or state, referred to herein as the first state, is shown in  FIG. 4  where the central region  88  is spaced from the central conductor  82 . In the other neutral position or state, referred to hereinafter as the second state, the central region  88  of the membrane  68  is in contact with the central conductor  82 , as shown in  FIG. 5 . 
   It is preferred that the membrane  68  be conductive, as shown in  FIGS. 4 and 5 . The membrane  68  cooperates with the sensor circuit to determine whether or not the condition capacitor (not shown) will be in or out of the sensor circuit. The membrane  68  is electrically connected to the printed circuit board  62  through a conductor  94  that extends from a printed circuit board junction  96  to the membrane  68 . When the pressure membrane  68  is in the second state, it contacts the central conductor  82  and the printed circuit board junctions  84  and  96  are electrically connected. In this case, the condition capacitor will be in the sensor circuit in parallel with the reference capacitor. Pressure inside of the tire acting on one side of the membrane  68 , when high enough, will maintain the membrane  68  in the second position or state. When the pressure in the tire is no longer high enough to maintain the membrane in the second position or state, the membrane  68  will snap into the first position or state, electrically disconnecting junctions  84  and  96  and preventing the condition capacitor from acting in parallel with the reference capacitor, thereby changing the resonant frequency of the sensor circuit from the reference resonant frequency to the condition resonant frequency. The sensor  58  can be designed so that the membrane  68  will snap from the second position or state into the first position or state at a desired threshold, for example, 75 percent of the recommended tire pressure as required under the TREAD Act. One important characteristic of the sensor  58  and, specifically, the membrane  68  in the sensor  58 , is that once the membrane  68  snaps into the first position or state, it will maintain that position or state despite fluctuations in the pressure inside of the tire. In other words, oscillation of the membrane between positions or states, at or near the threshold pressure, is positively avoided by the snap action membrane. It can be designed to stay in the first position or state even when the pressure in the tire increases by one, two, three or more psig. There would be some advantage if the sensor membrane was designed to snap back from the first position or state to the second position or state when the pressure in the tire reaches the recommended tire pressure, thereby changing the sensor  58  back to the second state indicating that the tire pressure is okay. 
   An alternative embodiment of a sensor according to the invention is indicated at  100  in  FIG. 6 . The sensor  100  comprises a housing  102  with a central aperture or bore  104  for housing a valve (not shown) for inflating a tire associated with the sensor. A valve would extend through the bore  104 , downwardly in  FIG. 6 , and extend out of the rim on which the sensor was mounted so that the housing  102  would be positioned inside of the rim. The sensor  100  further comprises a pressure switch indicated generally at  106  and comprising a snap action pressure membrane  108 . A sensor antenna coil  110  is mounted on the opposite side of the central aperture  104  from the pressure switch and it is electrically connected to a reference capacitor  112 . A condition capacitor (not shown) is housed within the housing  102  and is connected in the manner described above with reference to  FIGS. 4 and 5  for the condition capacitor. In fact, the sensor  100  operates just the same way as the sensor  58  ( FIGS. 4 and 5 ). 
   It will be appreciated that various changes and modifications are possible from the specific details of the invention shown in the attached drawing figures and described above with reference thereto, and such changes and modifications can be made without departing from the spirit thereof as defined in the attached claims. For example, in place of a snap action membrane, a pressure switch membrane might be stable in three positions and the third position might enable or disable an additional circuit component to provide a further signal indicating the state of the pressure switch. For example, a second condition capacitor might be employed in a sensor circuit in a sensor having a tri-stable pressure membrane. Further, the sensor can take other forms not specifically described herein. The sensor can be mounted on the rim of a tire, on or with a valve for the tire or otherwise so long as ambient pressure inside of the tire is in communication with one side of the pressure membrane.