Abstract:
A tire pressure reporting and warning system employs low-cost passive magnetically coupled pressure senders within the tires. These senders employ permanent magnets that rotate in response to pressure and may conveniently be mounted on the valve stem. A sender comprises a high-permeability helical ribbon that translates in response to pressure and penetrates a magnetic circuit. The magnetic circuit rotates into alignment with the helical ribbon. A novel feature of this invention is the dual-purpose use of the magnet both as a means for producing rotation in response to pressure and simultaneously for producing the remotely sensed external magnetic field. The direction and strength of the external field depends both on the rotation of the magnet with respect to the tire and on the overall orbital motion as the tire rotates. Remote pressure readers at each wheel respond to the magnetic field components and interpret the response asymmetry in terms of tire pressure by continuously calculating response skew as the tires rotate. Analyzing skew obviates the need for tire rotation sensing and timing and eliminates magnetic strength effects. No special alignment is required between senders and readers, so the readers may be mounted rather arbitrarily nearby the vehicle wheels.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of Provisional Patent Application Ser. No. 60/529,211, filed Dec. 12, 2003. 
   This Application is related to application Ser. No. 09/922,395 (now U.S. Pat. No. 6,520,006, issued Feb. 18, 2003), to application Ser. No. 09/927,736 (now U.S. Pat. No. 6,647,771, issued Dec. 18, 2003), and to application Ser. No. 10/191,612. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Not applicable. 
   BACKGROUND 
   1. Field of Invention 
   This invention relates specifically to vehicle tire pressure sensing and in general to remote pressure sensing. 
   2. Description of Prior Art 
   The U.S. Government has passed a law, known as the TREAD Act, requiring in-dash tire pressure reporting or warning systems for all new vehicles. U.S. Pat. No. 6,662,642 to Breed, et al, provides a good summary of the present art. Two main types of systems have emerged to meet this requirement—indirect and direct pressure sensing systems. The first, or indirect, type measures differential tire rotation speed to detect an anomalous rate for one tire, indirectly indicating under- or over-inflation. The advantage is passive operation with no in-tire components but the disadvantage is the inability to detect anomalous pressure in all tires. The second, or direct, type typically involves placing battery-operated transmitters within tires (possibly attached to or part of the tire valve stem) to transmit pressure readings to external receivers. While this permits sensing the pressure in all tires, the in-tire unit is relatively large due to the requirement for a battery. Another disadvantage is the periodic need to dismount tires to replace batteries. Alternative systems either try to generate sufficient electrical power internally through various means or transmit sufficient power into tires from external sources. 
   U.S. Pat. No. 6,520,006 to Burns discloses another direct approach. Here, a remote vehicular tire pressure reporting system comprises (1) an in-tire magnetic pressure sending apparatus wherein a permanent magnet is rotated mechanically in response to tire pressure plus (2) a magnetic pressure reading apparatus mounted on the vehicle containing sensors responsive to magnetic field direction. Advantages include passive operation that eliminates the need for battery replacement and sensing the direction rather than strength of a distant magnetic field. Magnetic field direction is more accurately controlled and measured than magnetic field strength. However, this system has the disadvantage of requiring coaxial alignment between the sender and receiver, at least once per wheel rotation, which limits location possibilities for both the sender and the reader. In the present improved invention, a method for reading the orientation of the sender field pressure reader that exploits wheel rotation during all or part of a rotation cycle lifts the restriction of coaxial alignment. 
   U.S. Pat. No. 6,647,771 also to Burns discloses another magnetically coupled tire pressure reporting system based on a novel magnetooptic display attached to the outer tire wall. However, this system does not meet the requirements for an in-dash pressure display. 
   U.S. Pat. No. 4,866,982 to Gault teaches a tire pressure monitoring system where a stationary Hall-effect sensor measures tangential spacing between a fixed magnet and a second magnet moveable in response to a linear pressure actuator. Changes in tangential spacing between the two magnets affect the timing between features in the combined magnetic field patter. Variations in timing are determined from the signal waveform generated as the spaced magnets, rotating with a wheel, sweep by a stationary sensor. U.S. Pat. No. 4,807,468 to Galan describes a similar system. Both Gault and Galan teach close coupling between magnet and sensor and an externally mounted magnetic sender requiring penetration into the pressurized interior of the tire and rim by a pressure line. 
   U.S. Pat. No. 5,814,725 to Furuichi et al, discloses a mechanism that penetrates a tire rim wherein a piston-driven screw rotates a permanent magnet Magnetic field strength is measured by a stationary Hall-effect sensor that is mounted transversely to the magnet rotation axis. U.S. Pat. No. 6,182,514 to Hodges also discloses a magnet in a bellows that moves to change the magnetic field strength at an external magnetic intensity sensor or magnetic switch. U.S. Pat. No. 4,667,514 to Baer describes a similar arrangement. These types of device typically share the same problems as the other devices that depend on sensing magnetic field strength rather than direction. 
   U.S. Pat. No. 3,807,232 to Wetterhorn teaches a self-contained gauge comprising a permanent magnet attached in place of the conventional dial pointer of a Bourdon tube pressure gauge so that it rotates with pressure. A magnetic compass sensor is coaxially aligned to detect the rotational direction of the magnet and hence the pressure. U.S. Pat. No. 6,499,353 to Douglas et al. discloses a virtually identical Bourdon tube and coaxial magnetic compass apparatus to that of Wetterhorn wherein the sender and compass are separated by and are perpendicular to the wall of the pressure vessel. However, Bourdon tubes are complex, bulky, and are too fragile for road tire use. Bourdon tube forces are also weak. Bourdon tubes further lack the ability to support and rotate the larger magnets required for vehicular application. Furthermore, the requirement for coaxial alignment perpendicular to the tire wall is unacceptable to vehicle designers. 
   Several mechanisms besides Bourdon tubes have been disclosed for converting translational pressure or force urging into rotary motion via mechanical coupling. U.S. Pat. No. 5,103,670 to Wu describes the use of a screw to convert linear displacement from a conventional bellows to actuate a directly viewed rotary dial or pointer. U.S. Pat. No. 6,082,170 to Lia et al. describes a blood pressure apparatus that uses a diaphragm bellows and a compressible helical ribbon spring to rotate a dial pointer. None of these types of device employs magnetic coupling for remote sensing. 
   U.S. Pat. No. 2,722,837 to Dwyer teaches a pressure dial apparatus comprising a magnetic circuit with a permanent magnet translated by pressure coupled through a diaphragm along a high permeability helix. The helix and attached dial pointer rotate in accordance with the longitudinal position of the magnetic circuit along the helix. Various improvements and variations of this basic system are disclosed in a series of later patents assigned to Dwyer Instruments, Inc., etc. (e.g., U.S. Pat. No. 4,374,475 to Hestich, U.S. Pat. No. 4,890,497 to Cahill, U.S. Pat. No. 4,938,076 to Buchanan, etc.) None of these disclose rotating a magnet in response to pressure or employing a rotated magnetic field for remote pressure sensing. 
   Numerous devices include mechanisms moving a permanent magnet in response pressure or other force to induce a sensed effect in a material responsive to variation in magnetic field strength. For example, U.S. Pat. No. 4,006,402 to Mincuzzi, U.S. Pat. No. 4,843,886 to Koppers et al. and U.S. Pat. No. 4,627,292 to Dekrone, each teach a device based on either magnetoresistance and magnetic saturation. U.S. Pat. No. 4,339,955 to Iwasaki discloses a mechanism that exploits variation in the incremental permeability of a magnetically soft material. These devices sense field strength instead of direction. Devices based on the sensing the strength of a magnetic field rather than field direction typically require a narrow spacing between the sensing means and the translated magnet. They are very sensitive to changes in spacing, small misalignments, and extraneous magnetic fields. Accordingly, such devices generally require careful and extensive calibration before measurements are made, and are generally unacceptable for tire pressure reporting. 
   Still other concepts of remote pressure sensing involve a change the state indicator responding a preset pressure level. For example, U.S. Pat. No. 3,946,175 to Sitabkhan teaches switching a magnetically susceptible reed in response to pressure actuated displacement of a magnet. U.S. Pat. No. 5,542,293 to Tsuda et al. describes a conventional bellows actuated mechanism that uses a fixed and a moveable magnet to switch the orientation of a third magnet. U.S. Pat. No. 5,717,135 to Fiorefta et al. discloses use of magnetic coupling to switch the state of a transducer from producing to not producing a signal. These types of mechanisms do not produce a continuous output responsive to pressure. 
   SUMMARY 
   Passive pressure senders placed within pressure vessels, such as a pneumatic vehicle tires, employ permanent magnets that rotate in response to pressure within vehicle tires. The direction and strength of the external magnetic field depends both on the rotation of the magnet with respect to the tire and on the overall orbital motion as the tire revolves. Remote pressure readers respond to the direction of the magnetic field and interpret the field direction changes in terms of tire pressure. In accordance with the present invention a magnetically coupled pressure sender comprises a pressure responsive bellows that translates a high-permeability helical ribbon element. In the preferred embodiment, the helical ribbon penetrates a central bore in a permanent magnet. Pole pieces at the end of the magnet that engage the helical ribbon across narrow gaps comprise, along with the magnet, a magnetic circuit free to otherwise rotate. The magnetic circuit will rotate to a position corresponding to minimum magnetic energy, which depends on the translated position of the helical ribbon. Thus a novel feature of this invention is the dual-purpose and simultaneous use of the sender magnet both as a means for producing magnet rotation in response to pressure and as a means for providing the remotely sensed distant magnetic field. Another novel feature of this invention is elimination of the previous requirement for special (i.e., coaxial) alignments of the remote pressure reader with the pressure sender, at least once per wheel revolution. The invented pressure reader continuously analyzes the asymmetry, or skew, of changes induced in the sensed magnetic field during the wheel revolution. Analyzing skew obviates the need for wheel rotation sensing and timing and eliminates magnetic strength effects. Each pressure reader associated with a particular wheel provides signals indicative of the associated tire pressure for separate display and warning. Conveniently, the pressure sender may be mounted on or part of the tire valve stem. Air passages bypassing the standard Schrader valve may implement the pressure sender as a gauge pressure sensor. 
   OBJECTS AND ADVANTAGES 
   Accordingly, objects and advantages of the present invention are:
         (a) to provide a passive means from within a vehicle tire for reporting pressure;   (b) to provide a continuous reading of tire pressure;   (c) to provide a tire pressure reporting system with an indefinite lifetime;   (d) to eliminate any need for batteries within tires;   (e) to eliminate any need for power generation within tires:   (f) to eliminate any need for sources transmitting power into tires;   (g) to provide a low cost means for reporting tire pressure;   (h) to provide a compact and lightweight pressure sensor internal to tires;   (i) to provide a means for reporting tire pressure that does not require timing or sensing of wheel rotation;   (j) to provide independent tire pressure reporting for all vehicle wheels;   (k) to provide accurate tire pressure reporting;   (l) to provide tire pressure reporting referenced to a “cold” tire;   (m) to provide design flexibility for the installation of a tire pressure reporting system;   (n) to provide a gauge pressure reading;   (o) to provide an absolute pressure gauge;   (p) in general, to enhance vehicle safety.       

   Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings. 

   
     DRAWING FIGURES 
       FIG. 1  shows an automobile with magnetically coupled pressure senders mounted inside the tires, pressure readers outside the tires, signal cables, and a pressure display unit. 
       FIG. 2A  is a cross section of a magnetically coupled pressure sender wherein the sending magnet is rotated by a translating helical member. 
       FIG. 2B  is a cross section of the magnetically coupled pressure sender of  FIG. 2A  perpendicular to the axis of rotation of the magnet. 
       FIG. 3A  shows a cross section through an alternative magnetically coupled pressure sender wherein the pressure bellows is inside the rotated magnet. 
       FIG. 3B  shows a cross section through an alternative magnetically coupled pressure sender wherein the translating helical ribbon is coupled to the pressure bellows by means of a temperature sensitive spiral member. 
       FIG. 3C  shows a cross section through a magnetically coupled pressure sender wherein the translating helical ribbon engages a slot. 
       FIG. 4A  is an elevation view illustrating the geometry of a magnetically coupled pressure sender inside a wheel and a pressure reader on the vehicle. 
       FIG. 4B  is a vertical cross section through the wheel of FIG.  4 A and part of a vehicle. 
       FIG. 5  is a cross section of a magnetically coupled pressure sender integrated into a valve stem. 
       FIG. 6A  plots examples of the radial magnetic fields at a magnetically coupled pressure reader versus tire revolution. 
       FIG. 6B  plots the tangential magnetic fields at a magnetically coupled pressure reader versus tire revolution. 
       FIG. 7  plots the skew of the radial and tangential magnetic fields at a magnetically coupled pressure reader plus the ratio of radial to tangential skew versus pressure angle. 
       FIG. 8  is an electronic block diagram of a magnetically coupled pressure reader. 
   

   REFERENCE NUMERALS IN DRAWINGS 
   
     
       
             
             
             
             
           
         
             
                 
             
           
           
             
               10 
               magnetic pressure sender 
               11 
               magnetic field 
             
             
               12 
               magnetic pressure reader 
               13 
               wheel 
             
             
               14 
               tire 
               15 
               rim 
             
             
               16 
               signal cabling 
               18 
               display device 
             
             
               20 
               pressure vessel 
               22 
               bellows 
             
             
               24 
               first end cap 
               25 
               second end cap 
             
             
               26 
               annular magnet 
               28 
               annular bearing 
             
             
               30 
               helical ribbon 
                   30A 
               helical ribbon 
             
             
               31 
               bearing 
               32 
               magnetic pole piece 
             
             
               33 
               spiral ribbon 
               34 
               pressure port 
             
             
               36 
               housing 
               37 
               cam follower 
             
             
               38 
               annular magnet 
               39 
               slot 
             
             
               40 
               bellows 
               42 
               bellows end cap 
             
             
               44 
               helical ribbon 
               46 
               housing end cup 
             
             
               47 
               bearing 
               48 
               magnetic pole piece 
             
             
               49 
               port 
               50 
               pressure vessel 
             
             
               51 
               wheel axle 
               52 
               body member 
             
             
               53 
               wheel rim 
               54 
               valve stem 
             
             
               56 
               Schrader valve 
               58 
               port 
             
             
               60 
               air passage 
               62 
               field sensor module 
             
             
               64 
               amplifier and conditioner 
               66 
               ADC 
             
             
               68 
               microcontroller 
             
             
                 
             
           
        
       
     
   
   DESCRIPTION 
   Preferred Embodiment of the Overall Vehicle Tire Pressure Sensing and Reporting System 
     FIG. 1  depicts the installation of the invented magnetically coupled tire pressure reporting system in a vehicle. Related arrangements are shown in U.S. Pat. Nos. 6,520,006 and 6,647,771 to Burns. The disclosures of these patents are incorporated herein by this reference. In particular, this invention relates to improvements to U.S. Pat. No. 6,520,006 to Burns. Burns teaches a method for externally sensing pressure of a fluid within a pressure vessel (e.g., a vehicle tire) comprising: (a) rotating a sender magnet inside of the pressure vessel in response to pressure within the vessel; (b) sensing the orientation of the rotated magnetic field outside the pressure vessel; and, (c) correlating the sensed orientation of the external magnetic field to pressure within the pressure vessel. The specific improvements of this invention are: (1) rotating the internal sender magnet by means of magnetic coupling to a helical element translated by pressure; and, (2) resolving the magnet orientation by analyzing properties of the sensed external magnetic field over all or part of a vehicle wheel rotation cycles. 
   Referring to  FIG. 1 , magnetically coupled pressure senders  10  are mounted inside pneumatic tires  14  mounted on wheel rims  15 . Wheels  13  comprise rims  15  and tires  14 . Pressure within each tire independently rotates magnets within a sender  10 . Senders  10  provide associated external magnetic fields  11 , each of whose orientations is fixed relative to its associated vehicle wheel  13 . Said relative orientation of each said magnetic field  11  is uniquely determined by the associated tire pressure. Senders in other tires (not shown) produce similar magnetic fields. Independent of the magnet and magnetic field orientation relative to a tire and urged by tire pressure, each of said provided magnetic fields  11  further revolves as the associated sender  10  orbits around the axle of corresponding wheel  13 . Magnetically coupled pressure readers  12  contain internal sensors (not shown) to sense the provided magnetic fields  11  from associated senders  10 . Thus the magnetic field at each magnetically coupled pressure reader  12  changes in both direction and strength independently (1) in response to tire pressure urging and (2) in response to the revolution of the corresponding wheel  13 . Magnetically coupled pressure readers  12  are mounted on the vehicle nearby the corresponding wheels  13 . Readers  12  also analyze waveforms produced by the internal magnetic field sensors to recover and report the pressure in tires  14  to display devices, such as display device  18 . 
   Magnetic field sensors within readers  12  may be essentially of the types disclosed in Burns, U.S. Pat. No. 6,520,006 and U.S. Pat. No. 6,647,771. Magnetic field sensors within readers  12  may comprise one or more magnetoresistive, magnetooptic, or Hall-effect sensors responsive to magnetic field direction and strength. Alternatively, magnetic field sensors within readers  12  may comprise one or more induction coils that are responsive to the time rate of change of magnetic field component Tire revolution produces time varying magnetic fields at the locations of readers  12 . Signal cables  16  couple the outputs of readers  12  to a pressure display or warning device  18 . Alternatively, the functions of display device  18  may be incorporated into or combined with other vehicular instrumentation and display means. 
   Preferred Embodiment of the Magnetically Coupled Pressure Sender 
     FIG. 2A  shows a cross section of the preferred embodiment of magnetically coupled pressure sender  10 . Bellows  22 , first end cap  24 , and second end cap  25  comprise a flexible and substantially cylindrical sealed pressure vessel  20 . Vessel  20  is optionally penetrated by pressure port  34 . Bellows  22  is compressed or expanded longitudinally according to the difference between external pressure P A  and internal pressure P B  communicated via port  34 . In this first case sender  10  is a differential or gauge pressure sender. Optionally, port  34  may be deleted and vessel  20  evacuated, so that internal pressure P B  is substantially close to zero. In this second case sender  10  is an absolute pressure sender. 
   Vessel  20  contains cylindrical annular magnet  26 . Magnet  26  is supported by non-magnetic narrow cylindrical annular bearing (or sleeve)  28  and is mechanically free to rotate around bearing  28 . Alternatively, magnet  26  may be attached to bearing  28  so that both rotate together with respect to vessel  20 . Magnet  26  is magnetized in a direction transverse to its axis as shown by the arrows. ( FIG. 2B  shows a transverse cross section A—A through sender  10 ). Helical ribbon  30  is attached to end plug  26  substantially concentrically with the axis of magnet  26 . Helical ribbon  30  is preferably composed of material with high magnetic permeability and may easily be manufactured by twisting a flat ribbon. Helical ribbon  30  passes through the center bore of annular bearing  28  and is free to slide longitudinally within bearing  28 . In the preferred embodiment, bearing  28  is partly supported by and is also free to rotate around helical ribbon  30 . Bearing  28  is also supported by a boss or other means on end cap  24 , and optionally may freely rotate with respect to end cap  24 . Bearing  28  may be restrained mechanically by various means to prevent significant longitudinal movement of itself or magnet  26 . Alternatively, all or part of end cap  24  may be composed of high magnetic permeability material so that magnetic attraction by and to magnet  26  prevents significant longitudinal movement. 
   Magnetic pole pieces  32  are attached to an end of magnet  26 . Pole pieces  32  are preferably composed of high magnetic permeability material and serve to concentrate magnetic flux in a narrow region along and across helical ribbon  30 . Magnet  26  will tend naturally to rotate to a position of minimum magnetic energy. Said minimum energy will occur when pole pieces  32  are in substantial alignment with the wide transverse dimension of helical ribbon  30 . Transverse motion of helical ribbon  30  in response to pressure change acting on bellows  22  thus produces proportional rotary motion of magnet  26 . In turn, said rotary motion of magnet  26  will alter the direction and strength of the distant magnetic field sensed by readers  12 . Reference is made to certain previous art, as exampled by U.S. Pat. No. 2,722,837 to Dwyer, wherein a magnetic circuit is translated past a similar type of helix, causing a gauge dial pointer attached to the helix to rotate. That is, the helix is held in place longitudinally while being free to rotate whereas the magnetic circuit is prevented form rotating while being free to move longitudinally. In the claimed invention, the actions of the magnetic circuit and helix are now reversed. There is no dial pointer, and the magnet is free to rotate while being held longitudinally and the helix is free to translate longitudinally while prevented from rotating. Furthermore, rotating the magnet rather than the helix provides a new functionality: Additional rotation and change of the distant magnetic field sensed remotely. Thus magnet  26  simultaneously serves the novel dual purposes of (1) producing the distant magnetic field sensed remotely by readers  12 , and (2) to convert translation of helical ribbon  30  due to pressure urging into rotary motion. 
   Alternate and Additional Embodiments of the Magnetically Coupled Pressure Sender 
     FIG. 3A  shows a cross section of an alternative magnetically coupled pressure sender  10 . This alternative is useful when small bellows are required, as in situations where the sensed pressure is high and a compact sender  10  is desired. In  FIG. 3A  bellows  40  is contained within the center bore of mechanically freely rotating annular magnet  38 . Magnet  38  is magnetized transversely to its axis as shown by the arrows. Magnetic pole pieces  48  are attached to one end of magnet  38 . Bearings  47  attached to housing  36  support the assembly comprising magnet  38  and pole pieces  48 , permitting free rotation of the entire assembly. Housing  36  optionally may be hermetically sealed. 
   Optional ports  49  permit pressure communication into bellows  40 , into housing  36 , or into both. Helical ribbon  44  is attached to bellows end cap  42  and extends past pole pieces  48  into housing end cup  46 . Helical ribbon  44  slides and rotates freely within end cup  46 . The assembly comprising magnet  38  and pole pieces  48  aligns itself rotationally with the wide dimension of helical ribbon  44  at the longitudinal position of the pole pieces. Bellows  40  expands and contracts longitudinally in accordance with pressure. Expansion and contraction of bellows  40  translates helical ribbon past pole pieces  48 . In response, the assembly comprising magnet  38  and pole pieces  48  rotates to maintain alignment locally between pole pieces  48  and the wide dimension of the helical ribbon. Rotation of magnet  38  in response to pressure urging changes the direction of the magnetic field additionally at reader  12  and thereby communicates pressure information. As with the preferred embodiment, magnet  38  simultaneously serves the dual purposes of (1) producing the magnetic field sensed remotely by readers  12 , and (2) to convert translation of helical ribbon  44  into rotary motion. In an additional variant of this embodiment, the assembly comprising end cap  42 , helical ribbon  44 , end cap  46 , and pole pieces  48  may be contained substantially inside bellows  40 , providing a more compact sender  10 . 
     FIG. 3B  shows a cross section through another alternative magnetically coupled pressure sender, which additionally provides compensation for tire temperature. Tire pressure is supposed to be measured when the tire is “cold”. After running, tires heat up, increasing the internal pressure, which produces a misleading pressure reading. Adding a temperature sensitive member such as spiral ribbon  33  alters the magnetic field rotation to produce a fictitious pressure reading that corresponds to the pressure that would have been measured in a cold tire. In  FIG. 3B  helical ribbon  30  is attached to a bearing  31  that is supported by and is further free to rotate with respect to end cap  25 . Spiral ribbon  33  is attached at one end to bearing  31  and at the other end to bellows  22 . Spiral ribbon  33  resembles a torsion spring and may be comprised of a bimetallic strip. Elongation and change in the average radius of curvature of spiral ribbon  33  with increasing temperature produces rotation of helical ribbon that in urges less rotation of magnet  26  and external field  11 . 
     FIG. 3C  shows a cross section through another alternative magnetically coupled pressure sender wherein the rotation of magnet  26  is urged mechanically by means of a helical ribbon  30 A acting as a cam slidably engaging slot  39  in cam follower  37 . Helical ribbon  30 A and cam follower  37  are preferably composed of non-magnetic material. A high pitch helix is required to permit sliding motion against friction and to avoid back driving bellows  22 . This alternative is useful in more static applications where potential wear affecting lifetime is not an issue and back driving must be avoided. 
   Details of the Preferred Embodiment of the Tire Pressure Sender 
     FIG. 4A  is an elevation view of a vehicle wheel  13  comprising tire  14  and rim  15 . 
     FIG. 4B  is a cross section view through tire  14 , rim  15 , and vehicle body member  52 . Tire  14  and rim  15  form a pressure vessel  50 . Within vessel  50  magnetically coupled pressure sender  10  is attached to valve stem  54 . Alternatively, sender  10  may be part of valve stem  54 . In response to the pressure within vessel  50  sender  10  produces a magnetic field  11  oriented at a characteristic “pressure angle” θ with respect to radius R s  of sender  10  from axle  51  of wheel  15 . Magnetically coupled pressure reader  12  is located at radius R r  from the axle of wheel  15 . Sender  10  need not be in the same plane perpendicular to axle  51  of wheel  13  as the plane with reader perpendicular to axle  51  of wheel  13 . Furthermore, the orientation of axis of sender  10  may be arbitrary with respect to axle  51 . 
     FIG. 5  shows a cross section of a magnetic pressure sender  10  combined with valve stem  54 . Valve stem  54  contains a conventional Schrader valve  56  (for clarity, not shown are certain other standard components such as springs typically comprising a conventional Schrader valve). One or more ports  58  allows passage of compressed air into tire  14  for filling. One or more air passages  60  permits communication of ambient atmospheric pressure Pa into the interior of pressure sender  10 . Bellows  22  compresses and expands in accordance with the difference between tire pressure P t  and ambient pressure P a . Magnet  26  rotates in response to translation of helical ribbon  30  attached to bellows end cap  25 . 
   As wheel  13  revolves, pressure angle θ remains constant (assuming no pressure change) while projected angle φ between sender  10  and reader  12  steadily increases. Projected angle φ measures wheel revolution. Wheel revolution produces changes in the strengths of the radial (B R ) and tangential (B T ) magnetic field components (with respect to the orientation of axle  51 ) of magnet  26  at the location of reader  12 .  FIGS. 6A and 6B  plot examples of the radial and tangential magnetic field components as functions of tire revolution angle for several values of pressure angle θ. An induction coil will respond to the rate of change, or derivative, of a field component plotted in  FIGS. 6A and 6B , with respect to time or wheel revolution angle. The signal produced by a radially oriented induction coil is similar to the signal produced by a tangentially oriented magnetic field sensor and conversely the signal produced by a tangentially oriented induction coil is similar to the signal produced by a radially oriented magnetic field sensor. An important feature of the field components plotted in  FIGS. 6A and 6B  (and their derivatives) is significant asymmetry. Asymmetry may be measured by calculating the third moment, or skew, of a magnetic field component or time rate of change of magnetic field components. Other and alternative means for calculating asymmetry will be apparent to those familiar with signal processing theory. 
   Skew of a continuous and periodic time varying quantity x is the third moment of that quantity, defined mathematically by the following equation: 
         SK   x     =     ∮         (       x   -     μ   x         σ   x       )     3     ⁢     ⅆ   t             
 
   The basic integration is taken over one wheel rotation. The first (mean, μ x ) and second (standard deviation, σ x ) moments are also calculated over the cycle of one tire rotation. Skew is a dimensionless, ratiometric quantity that is independent of magnetic field strength and tire revolution rate.  FIG. 7  plots the skews of the radial component (SK R ) and tangential component (SK T ) versus pressure angle θ corresponding to the example shown in  FIGS. 6A and 6B . The basic integration interval is one wheel revolution cycle, but may be extended or limited to multiple and partial wheel rotations. Independent of wheel revolution rate, each wheel revolution cycle produces the same expected value for skew (so long as the tire pressure remains constant). Averaging multiple cycles improves the signal/noise ratio. Averaging over many revolution cycles also reduces any effects caused by including fractional wheel rotations in the skew calculation. 
   Also plotted in  FIG. 7  is the ratio of tangential to radial skew, which exhibits a linear relationship to pressure angle over a significant and useful span. 
     FIG. 8  is an electronic or signal-flow block diagram of a pressure reader  12  comprising magnetic field sensor module  62 , signal conditioning and amplifying module  64 , analog-to-digital converter  66 , and microcontroller  68 . Sensor module  62  preferably comprises one or more induction coils oriented in radial or tangential directions, or both, with respect to the axle of wheel  15 . Alternatively, sensor module  62  comprises one or more magnetoresistive, magnetooptic, or Hall-effect sensors responsive to magnetic field direction and strength. Conditioning and amplifying module  64  amplifies signals developed by module  62  and drives analog-digital converter (ADC)  66 . Module  64  also filters the sensed signal to remove responses from extraneous fields, including AC magnetic fields. ADC  66  is preferably free running or self-triggered. Optionally, ADC  66  may be triggered by wheel rotation. Thus, pressure reader  12  may also serve as a wheel rotation rate sensor in a anti-lock braking system (ABS) Microcontroller  68  calculates the skews of the magnetic field components sensed by module  62 . Microcontroller  68  further communicates corresponding tire pressure values or signals to external devices for warning and display. Preferably, microcontroller  68  performs a running average of radial and tangential magnetic field component skews, which eliminates any requirement for tire revolution indexing or timing. In addition, microcontroller  68  preferably calculates a filtered average of skew that gives more weight to the more recent outputs of sensor module  62  than to earlier outputs. Reader  12  can run continuously without any regard to the phase of wheel revolution. 
   CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
   Accordingly, it can be seen that I have provided a method and apparatus for sensing tire pressure using passive senders within vehicle tires. 
   Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various other embodiments and ramifications are possible within its scope. For example, the bellows may be replaced by a diaphragm, a bladder, other elements capable of producing translation urged by pressure change, or a combination of these elements. In addition, the annular magnet may be replaced by a hollow magnet or magnetic circuit. A separate magnet from that used to provide the external magnetic field may be employed to urge rotation of the external magnetic field. Furthermore, the helical ribbon may be replaced by other members or elements having similar magnetic responses and properties. 
   Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.