Abstract:
A wireless tire pressure sensing system based upon a Schrader valve design is provided. The system includes a valve body, a valve pin and a compression element for sealing engaging the valve body and the valve pin to maintain tire pressure. A pressure sensing device mounted within the valve body and connected to valve pin senses the pressure within the tire and provides its signal to the valve pin. The valve pin is adapted as a component of an antenna that transmits a wireless pressure signal to a remote receiver/transmitter mounted on the vehicle. The receiver/transmitter transmits a corresponding signal to a vehicle control system that generates a warning signal when the tire pressure is below a threshold safety value.

Description:
TECHNICAL FIELD 
   The present invention relates to structure and methods for implementing a Schrader valve that integrates a wireless pressure sensing system. 
   BACKGROUND OF THE INVENTION 
   Vehicles operated with underinflated tires pose a significant safety problem. A major contributor to the high rate of tire failures due to underinflation and gradual pressure loss is that many people do not know whether or not their vehicle&#39;s tires are properly inflated. The U.S. Congress has mandated that automobile manufacturers implement a tire pressure monitoring system by 2007 for automobiles operating the in the U.S. 
   A typical tire pressure monitoring system includes a sensor/transmitter at each wheel to monitor pressure in the tires and a receiver inside the vehicle. Each sensor periodically transmits its unique ID and the pressure in the associated tire to the receiver. The received pressure value is compared to an acceptable pressure level and, if the measured pressure is outside acceptable parameters, a warning is issued to the driver. 
   Tire pressure monitoring systems can be classified as two types: wheel speed based, or indirect, systems and pressure sensor based, or direct, systems. Wheel speed based systems infer tire pressures using the vehicle&#39;s anti-lock braking system wheel speed sensors to measure tire-to-tire differences in rotational velocities that indicate that one tire is at a different pressure from the others. Pressure sensor based systems directly measure tire pressures with pressure sensors mounted either inside the tire or on the stem valve. For a variety of reasons, pressure sensor based systems are preferable. 
   However, currently available pressure sensor based systems do have some disadvantages. For example, pressure sensor based systems that are mounted inside the tire can suffer from signal degradation due to the interference caused by steel elements used in the tire structure. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross section drawing illustrating a conventional Schrader valve. 
       FIG. 2  is a cross section drawing illustrating a wireless pressure sensing system in accordance with the concepts of the present invention. 
       FIG. 2A  is a cross section schematic drawing illustrating an embodiment of a flange/sensor assembly utilizable in accordance with the present invention. 
       FIG. 2B  is a block diagram illustrating a pressure sensor system utilizable in accordance with the present invention. 
       FIG. 3  is a cross section drawing illustrating an embodiment of a pressure sensing device utilizable in accordance with the present invention. 
       FIG. 4  is a block diagram illustrating an alternate embodiment of a pressure sensing device utilizable in accordance with the present invention. 
       FIG. 5  is a cross section drawing illustrating a leaking pressure sensing element utilizable in the  FIG. 4  pressure sensing system. 
   

   DESCRIPTION OF THE INVENTION 
   The present invention provides a wireless pressure sensor based system for monitoring tire pressure that utilizes the general design of a conventional Schrader valve. More specifically, modifications are made to a Schrader that integrate an antenna into the release pin and restoring spring. The force applied to a pressure sensing element mounted on the valve gasket is proportional to the pressure in the tire. This induces a charge across the pressure sensing element that effects the inductive response of the antenna, which a transceiver measures. 
   The features and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description of the invention and the accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
     FIG. 1  shows a conventional Schrader valve  100 . The valve  100  includes a metal valve body  102  that fits into a rubber base (not shown) that is adapted for engagement with a rim hole opening of a tire. The valve body  102  has an axial passage  104  that is reduced at its upper end to provide a pin retaining opening  106 . The valve body  102  includes a central flange opening  108  the edge of which forms a downwardly facing seat  110 . The bottom portion of the valve body  102  forms a spring retainer cup  112  that has a central opening  114  in its bottom wall. A valve pin  116  extends down through the pin retaining opening  106 . The valve pin  116  includes a resilient valve flange  118  which may have a tapered upper surface to assist in centering the valve flange  118  on the seat  110 . An axial spring  120  is disposed in compression in the spring retainer cup  112 . The upper end of the spring  120  engages under the valve flange  118  and surrounds the lower end of the valve pin  116 . The lower end of the spring  120  engages the bottom wall of the spring retainer cup  112 . The upper end of the valve body  102  is typically threaded to allow for threaded engagement with a protective cap (not shown). 
   Those skilled in the art will appreciate that the spring  120  normally maintains the resilient flange  118  in sealing engagement with the valve seat  110 , thus maintaining the pressure within the tire. Force applied to the valve pin  116  opens the valve to allow air to be added to the tire to increase its pressure, or for air to escape from the tire to reduce its pressure. 
   As stated above, the present invention provides a wireless tire pressure sensing system that is based upon modifications to the conventional Schrader valve architecture. An embodiment of a Schrader valve based wireless tire pressure sensing system  200  is shown in  FIG. 2 . 
   The  FIG. 2  wireless tire pressure sensing system  200  includes a metal valve body  202  that fits into sealing engagement with a rubber base  204  that includes a circumferential groove  206  that is adapted for engagement with a rim hole of a tire (not shown) in the conventional manner. The valve body  202  includes an axial passage  208  that is reduced at its upper end to provide an antenna pin opening  210 . The valve body includes a central flange opening  212  the edge of which forms a downwardly facing seat  214 . The bottom portion of the valve body  202  forms a spring retainer cup  216  that has a central opening  218  formed in its lower wall. A valve antenna pin  220  extends down through the pin opening  210 . The valve pin includes a resilient flange/sensor assembly  222 , an exemplary embodiment of which is shown in greater detail in  FIGS. 2A and 2B . The flange/sensor assembly  222  can have any of a number of designs that will be apparent to those skilled in the art. 
   As shown in  FIGS. 2A and 2B , the flange/sensor assembly  222  includes an upper resilient surface  224  adapted for sealing engagement with the valve seat  214 . The assembly  222  also includes a pressure sensor system  226  that includes a pressure sensing device  228  that senses the pressure P t  within the tire and provides a corresponding electrical tire pressure signal  230 . The pressure sensor system  226  also includes driver circuitry  232  connected between the pressure sensing device  228  and the pin antenna  220 . The driver circuitry  232  receives the tire pressure signal  230  from the pressure sensing device  228 , converts it to a tire pressure monitoring signal  234  that is provided to the antenna pin  220 . The antenna pin  220  transmits the tire pressure monitoring signal  234  to a remote receiver/transmitter (not shown) mounted, for example, in the wheel well of the tire. The receiver/transmitter transmits the tire pressure monitoring signal to an on-board processing system that asserts a warning light to the driver if the tire pressure P t  is less than a desired threshold value. 
   With reference to  FIG. 2 , an axial spring  236  is disposed in compression in the spring retainer cup  216  of the valve body  202 . The upper end of the spring  236  engages the flange/sensor assembly  222  and surrounds the lower end of the antenna pin  220 . The lower end of the spring  236  engages the bottom wall of the spring retainer cup  216 . The upper end of the valve body is threaded for engagement with a protective cap (not shown). 
   Those skilled in the art will appreciate that the pressure sensing device  228  and the driver circuitry can be conventional; for example, the pressure sensing device  228  can be a conventional piezoelectric pressure sensing device. 
     FIG. 3  shows an embodiment of a capacitive pressure sensing device  300  that can utilized in the pressure sensing system  226 . The pressure sensing device  300  includes a lower capacitor plate  302  formed at an upper surface of a semiconductor substrate  304 . The lower capacitor plate is electrically coupled, via interconnect structure  303 , to the driver circuitry  232  that is also formed in the semiconductor substrate  304 . A conductive pad  306  is also formed at the upper surface of the semiconductor substrate  304  and is space-apart from the lower capacitor plate  302 . The conductive pad  306  is also electrically coupled to the driver circuitry  232  via interconnect structure  303 . Patterned dielectric material  308  is formed over the upper surface of the semiconductor substrate  304 . The patterned dielectric material  308  has a first opening formed therein to expose the conductive pad  306 . The patterned dielectric material  308  also has a second opening formed therein over the lower capacitor plate  302 . An upper conductive layer  310  is formed over the patterned dielectric material  308 . The upper conductive layer  310  extends into the first opening in the patterned dielectric material  308  to form electrical contact with the conductive pad  306 . The upper conductive layer  310  also extends over the second opening in the patterned dielectric material to define an enclosed pressure cavity  312  between the lower capacitor plate  302  and the upper conductive layer  310 . Further detail regarding the pressure sensing device  300  may be obtained from co-pending and commonly assigned U.S. Application Ser. No. 10/976,449, filed on the same date as this application by Peter J. Hopper et al., titled “MEMS Pressure Sensing Device”, which application is hereby incorporated by reference in its entirety. 
     FIGS. 4 and 5  show an alternate embodiment of a pressure sensing device that can be utilized in the pressure sensing system  226 . 
     FIG. 4  shows a pressure sensor array  400  that includes a plurality of enclosed cavity pressure sensing devices  402  of the type described above with respect to  FIG. 3  and at least one “leaking cavity” device  404  that experiences zero pressure differential across its membrane. 
   With reference to  FIG. 5 , those skilled in the art will appreciate that one way of providing a leaking cavity device  404  is to provide an opening in the upper conductive layer  310  of the  FIG. 3  device  300 . By providing one or more leaking cavity devices in the sensor array  400  to provide a reference pressure signal, the mechanical and thermal properties of the membrane in the closed cavity devices  300  can be backed out through the use of conventional differential capacitive bridge circuitry. Further details regarding the pressure sensing device shown in  FIGS. 4 and 5  may be obtained from co-pending and commonly assigned U.S. Application Ser. No. 10/977,169, filed on the same date as this application by Peter J. Hopper et al., titled, “MEMS Pressure Sensing Array with Leaking Sensor”, which application is hereby incorporated by reference in its entirety. 
   It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the invention as expressed in the appended claims and their equivalents.