Abstract:
An external mount tire pressure sensor system which substantially reduces the effect of centrifugal force on tire pressure measurements. A sensor has a main guide body portion with two parallel leg portions each having a central bore. A slider element is slidably mounted in each leg portion, and the two slider elements are mechanically connected for translatory movement in unison. A spring urges the slider elements to a neutral position. The sensor attaches to a tire valve stem. Gas from a tire encounters one of the slider elements and urges it in opposition to the spring force. The differential interconnection between the two slider elements cancels out th effect of the centrifugal force when the wheel rotates. A stiff flexible wall section enables the axis of the sensor to be aligned with the wheel radius to optimize performance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to automotive tire pressure sensors. More particularly, this invention relates to a method and system for monitoring internal tire pressure of vehicles using an externally mounted sensor.  
           [0002]    Tire pressure sensor systems are known and are commonly used to monitor the internal air pressure in individual pneumatic tires of a vehicle and to provide a warning signal to the driver whenever the internal air pressure in one or more of the vehicle tires is dangerously low or high. The warning signal is typically generated by an r.f. signal generator controlled by a microprocessor connected to the tire pressure sensor whenever the internal tire pressure measured by the sensor lies outside a predetermined normal operating range. This r.f. signal is transmitted to a vehicle-mounted receiver, which uses the warning signal to alert the driver either visually (by activating a warning lamp or display) or audibly (by activating an audible alarm) or both.  
           [0003]    Known tire pressure systems typically employ one of two basic design configurations. The first type of design configuration employs an internal arrangement in which all components comprising the signal transmitting portions of the tire pressure sensor system are positioned within the tire casing. In a typical installation of this type, the pressure sensor, r.f. generator, the microprocessor, and th D.C. battery power source are physically installed inside the pneumatic tire casing prior to inflation of the tire, usually by attaching these components to an inner surface of the wheel. Design configurations of this first type suffer from several disadvantages. Firstly, the installation of the system components within the tire casing is not simple, requires careful attention to the component location and mounting, and increases the manufacturing cost of the entire automobile. In addition, many vehicles use steel-belted tires, which interact in a detrimental manner with the r.f. signals generated internally of the tire. Further, when the battery or one of the other system components fails, replacement of the defective component requires that the affected tire be removed from the wheel before replacement can be done, which is costly and time consuming.  
           [0004]    The second type of basic design configuration used for current tire pressure sensor systems employs components which are mounted in a housing which is threadably attached to the exposed outer end of the valve stem of the tire. The sensor system components include a spring-biassed slider element translatably mounted in a guide which is threadably attached to the tire valve stem. The slider is exposed to the internal gas pressure within the tire when the guide is attached to the valve stem. The slider typically includes one or more contact elements which can interact with circuit contact elements mounted at preselected points within the guide. As the internal gas pressure increases within the tire, the slider is translated in a first direction within the guide. If the internal gas pressure reaches a maximum threshold value, the contact elements adjacent one end engage the guide circuit contact elements, which activates an r.f. generator and causes a high pressure warning signal to be generated. As the internal gas pressure decreases, the slider is translated in the opposite direction. If the internal gas pressure reaches a minimum threshold value, the contact elements adjacent the other end of the slider engage the other guide circuit contact elements, which activates the r.f. generator and causes a low pressure warning signal to be generated.  
           [0005]    While this design arrangement avoids the disadvantages noted above associated with internally mounted tire pressure monitoring systems, the accuracy of such external mount sensors is adversely affected by the centrifugal forces associated with the rotating wheel. In particular, as the angular velocity of the wheel changes, the centrifugal force acting on the slider element within the sensor also changes, causing additional force to act on the slider. Thus, the linear position of the slider within the guide is a function of both the internal tire gas pressure and the centrifugal force acting on the slider. As a result, the sensor can easily produce fals warning signals, and can also fail to generate a valid warning signal. This problem is compounded by the fact that the sensor guide may not be mounted exactly radially of the wheel, but at an angle with respect to the radial direction. Consequently, it is difficult to design a sensor in such a manner as to compensate for the centrifugal force component acting on the sensor slider. Efforts to provide a simple yet accurate and durable tire pressure monitoring system using an external valve stem mounting configuration have not been successful to date.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention comprises a method and system for monitoring internal vehicle tire pressure employing externally-mounted components which are relatively simple and inexpensive to manufacture and install on a vehicle, which provide accurate internal tire pressure readings, which can easily and inexpensively be replaced in case of failure, and which substantially reduce or entirely eliminate inaccurate pressure readings due to the effect of centrifugal forces on the sensor components.  
           [0007]    In a broadest aspect, the invention comprises an external mount tire pressure sensor system having a main guide body member with a pair of laterally spaced essentially parallel leg portions joined by an intermediate portion, each leg portion having a slider element bore with an essentially linear section. The intermediate portion of the main guide body member preferably has an arcuate shape. A first one of the leg portions has an open end attachable to a tire valve stem, preferably by means of an internally threaded wall portion adjacent the open end. The other one of the leg portions has a closed end.  
           [0008]    A pair of slider elements are each translatably located in a different one of the leg portions, and the pair of slider elements is mechanically coupled together for translatory motion in unison so that a superior force acting on one of the slider elements causes both to move by the same amount. The two slider elements are preferably mechanically interlinked by a stiff, flexible member. The slider element in the first one of the leg portions has a fluid seal, preferably an O-ring seal, about the periphery.  
           [0009]    A bias element, preferably a spring, is positioned between the closed end of the other leg portion and a free end of the slider element in the other one of the leg portions for urging the pair of slider elements to a neutral position.  
           [0010]    A fluid entrance element is included in the first leg portion adjacent the open end for communicating internal gas pressure from a vehicle tire mounted on a wheel to a confronting surface of the slider element in the first one of the leg portions so that gas pressure from the tire can produce a translatory force on the slider element in the first leg portion in opposition to the bias element. The fluid entrance element preferably comprises an apertured partition positioned in the slider element bore in the first leg portion adjacent the open end and a plunger member extending toward the open end for engaging the plunger of a tire valve when the sensor system is installed on a tire valve.  
           [0011]    The sensor system further includes electrical position contacts mounted in the slider element bore for enabling determination of the translatory position of at least one of the slider elements.  
           [0012]    The sensor system is installed on the valve stem of a tire by engaging the internal threads of the main guide body member with the external threads of the tire valve stem and rotating the main guide body member until the two are fully engaged. When fully engaged, the sensor plunger retracts the tire valve stem plunger to admit gas from the tire to the interior of the sensor where it applies a force to the slider element located in the first leg. If this force is greater than the spring force, both slider elements are displaced in their respective bores, and the electrical position contacts register this displacement. Any influence of centrifugal force due to rotation of the wheel is cancelled out by the differential interconnection of the two slider elements.  
           [0013]    The main guide body member is provided with a stiff flexible adjustment wall section for enabling the spatial attitude of the main guide body member to be manually adjusted after the sensor system is initially attached to a tire valve stem. The adjustment wall section is preferably located adjacent the open end inboard of the fluid entrance element. The adjustable wall section enables the installer to manipulate the main guide body portion to align the longitudinal axis of the sensor with the radius of the wheel. This optimizes the sensor position with respect to the wheel radius so that any centrifugal force will act equally on each slider element.  
           [0014]    The invention provides a convenient solution to the problem of monitoring internal tire pressure in vehicles equipped with pneumatic tires when using an externally mounted pressure sensor attached to the tire valve stem. The system can be installed either during assembly of a new vehicle or as an aftermarket item. Further, existing vehicles without tire pressure sensor systems can easily be retrofitted with a state-of-the-art system at relatively low cost. This is particularly beneficial in jurisdictions which mandate low tire pressure warning devices on all road vehicles. Most importantly, the invention affords a low cost, effective solution to the problem of unwanted centrifugal force contributions to the positioning of the slider element in external mount tire pressure systems.  
           [0015]    For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic sectional view of a valve stem mount external single tire pressure sensor according to the prior art;  
         [0017]    [0017]FIG. 2 is a schematic sectional view of a differential tire pressure sensor according to the invention;  
         [0018]    [0018]FIG. 3 is a schematic view illustrating initial installation of the sensor according to the invention on a tire valve; and  
         [0019]    [0019]FIG. 4 is a schematic view illustrating the installation of the sensor according to the invention after proper final adjustment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Turning now to the drawings, FIG. 1 is a schematic sectional view of a typical prior art external valve stem mounted tire pressure sensor. As seen in this Fig., the known tire pressure sensor system includes an electromechanical sensor generally designated with reference numeral  10  having a main guide body member  12  fabricated from a durable metal, such as steel, aluminum, or the like. Guide body member  12  has a longitudinally extending central bore  15  which is closed at one end by an integral end wall portion  16 . The other end of guide body member  12  is open and the internal wall portion of this open end is provided with internal threads  17  of size and pitch to sealingly engage the external threads of a conventional tire valve stem (not shown). A bore partition  19  is arranged within bore  15  adjacent the open end, and includes a centrally positioned outwardly extending plunger  21  which engages the tire valve stem plunger when the sensor is threadedly attached to the tire valve stem so that gas pressure within the tire is communicated into the internal bore  15  via partition apertures  22 .  
         [0021]    A slider element  25  is located within bore  15  in the hollow region between bore partition  19  and end wall portion  16 . A fluid seal  26 , usually an O-ring, is positioned between the external surface of slider element  25  and the internal wall surface of bore  15  so that gas pressure from the tire acts essentially on the end wall surface  27  of slider element  25 . A bias spring  29  is coupled between end  31  of slider element  25  and end wall portion  16  to provide a centering counter force on slider element  25  which urges slider element  25  to a neutral position in bore  15 .  
         [0022]    A plurality of position sensor contacts  33  is arranged along the inner wall surface of bore  15 . Contacts  33  serve to identify the linear position of slider element  25  within bore  15  and are electrically connected to a transmitter in a transmitter and antenna module  35 . A battery  37  provides electrical power necessary for the operation of the sensor system. Module  35  and battery  37  are housed in a cover member  39  attached to the main guide body member  12 .  
         [0023]    In operation, sensor system  10  is attached to a tire valve stem by engaging the internal threads  17  with the external tire valve stem threads and rotating the main guide body member  12 . As plunger  21  advances into the tire valve stem, the free end of plunger  21  engages the free end of the tire valve stem plunger, thus releasing internal tire gas pressure into bore  15 . The pressure of the gas acts on end wall  27  of slider element  25 , which is translated within bore  15  by an amount determined by the force on end wall  27  due to the gas pressure and the opposing spring force produced by spring  29  in the opposite direction. As slider element  25  is translated along bore  15 , position sensor contacts  33  convey position information to the transmitter in module  35 . As the internal tire gas pressure varies, slider element  25  is re-positioned accordingly. If the gas pressure rises to a maximum threshold value, the transmitter generates a high pressure warning signal, which is broadcast by the antenna to the associated receiver, and a high pressure warning indicator is activated in the driver&#39;s compartment. If the gas pressure falls to a minimum threshold value, the transmitter generates a low pressure warning signal, which is broadcast by the antenna to the associated receiver, and a low pressure warning indicator is activated in the driver&#39;s compartment.  
         [0024]    While the above-described prior art tire pressure sensor system is capable of providing accurate warning signals to the driver while the vehicle is stationary, once the vehicle is in motion the accuracy is severely impaired by the action of centrifugal force on the slider element  25 . This force, generated by the whel rotation, directly influences the position of slider element  25  within the bore  15 . The magnitude of the centrifugal force actually acting on the slider element  25  is a function of the angular velocity of the wheel and the solid angle subtended by the longitudinal axis of main guide body member  12  and the wheel radius. Since the position of main guide body member  12  depends on the angle at which the tire vale stem emerges from the rim of the wheel, which varies from tire-to-tire and wheel-to-wheel, it is impossible to calculate with any accuracy the solid angle subtended by the sensor body axis and the wheel radius in advance of installation. In addition, the actual position of the sensor during wheel rotation may change with the angular velocity of the wheel and the centrifugal force acting on the sensor body and valve stem. Thus, until the advent of the invention described below, it has been at least extremely complicated and practically impossible to devise a compensation structure to eliminate the effect of centrifugal force on the accuracy of known external valve stem mounted tire pressure sensor systems.  
         [0025]    [0025]FIG. 2 is a schematic sectional view illustrating the preferred embodiment of the invention which substantially reduces or entirely eliminates the influence of centrifugal force on the accuracy of an external valve stem mounted tire pressure sensor system. As seen in this Fig., the invention  40  has a main guide body member  42  with two linear leg portions joined by a curved intermediate portion. Like the prior art sensor described above, the open end of main guide body member  42  has internal threads  47  for attaching the device to a tire valve stem, and a central partition  49  with a plunger  51  for engaging the tire valve stem plunger when the device is installed on the tire valve stem. Partition  49  has fluid communication apertures  52  to enable the internal tire gas pressure to be admitted to the entry portion of an internal bore  45 .  
         [0026]    Positioned within one leg portion of bore  45  (the leg portion adjacent partition  49 ) is a first slider element  55  arranged for linear translation along the straight portion of that part of the bore  45 . A fluid seal, such as the O-ring  56  illustrated, ensures that the gas pressure admitted through apertures  52  acts ssentially one end wall surface  57  of slider element  55 . A second slider element  55   a  is positioned within the other leg portion of bore  45  (the leg portion adjacent the closed end wall  46 ) and is also arranged for linear translation along the straight portion of that part of the bore  45 . A spring  59  is interposed between end wall surface  61  of slider element  55   a  and the inner wall surface of end wall  46  and applies a biasing force to slider element  55   a.  Slider elements  55  and  55   a  are rigidly interconnected together by means of a flexible connector member  58  which interlinks translatory motion of each slider element so that both slider elements move in tandem, regardless of which of the two slider elements currently experiences the superior force. Thus, if the force due to the gas pressure on wall surface  57  of slider element  55  is greater than the spring force exerted on slider element  55   a,  both slider elements will be translated an equal amount in the direction inward of the main guide body member  42 . Conversely, if the spring force is greater than the force due to the gas pressure, both slider elements will be translated an equal amount in the opposite direction. Flexible connector member  58  may be fabricated from any suitable material capable of providing the necessary rigidity to prevent compression or expansion of the member  58 , while having sufficient flexibility to follow the arcuate path presented by the curved intermediate portion of main guide body member  42 . Some metal materials, such as stainless steel rod; as well as some synthetic materials such as Kevlar and polycarbonate in rod form, may be used. Other materials will occur to those skilled in the art.  
         [0027]    Slider element  55   a  coacts with position sensor contacts  63  in the same manner as that described above for the prior art device of FIG. 1. In turn, position sensor contacts  63  convey slider element position information to a transmitter in a transmitter and antenna module  65 , which broadcasts appropriate warning signals via the antenna in module  65  to an associated receiver. A battery  67  provides appropriate electrical power to the circuits. Module  65  and battery  67  are both housed in a cover  69  attached to main guide body member  42 .  
         [0028]    In use, the invention of FIG. 2 is initially installed by engaging the internal threads  47  with the external threads of the tire valve stem and rotating the main guide body member  42  about the tire valve stem until the two members are fully engaged. As plunger  51  advances into the tire valve stem, the free end of plunger  51  causes the tire valve stem plunger to retract, thus admitting the internal gas into bore  45 , through apertures  52  and onto surface  57  of slider element  55 . As the gas pressure varies, this variation causes translation of both slider elements  55 ,  55   a  in unison. When the vehicle starts to move, and centrifugal force is created by the angular velocity of the wheel, the centrifugal force acting on slider element  55  also acts on slider element  55   a.  Since this force at any given instant is unidirectional, it acts equally on slider elements  55 ,  55   a,  but with opposite effect. For example, if the centrifugal force is from the right as viewed in FIG. 2, this force acts on slider element  55  to urge this element to the left and also acts on slider element  55   a  to urge this element to the left. Since both slider elements are rigidly linked together by connector  58 , neither will move in response to the centrifugal force. This differential interconnection thus cancels out the effect of centrifugal force on the linear position of the slider elements. Thus, regardless of the magnitude, direction, and manner of variation of the centrifugal force produced by the rotation of the wheel, the differential interconnection of the two slider elements eliminates any influence of this force on the accuracy of the pressure measurements obtained.  
         [0029]    As noted above in connection with the description of the prior art device of FIG. 1, one of the factors affecting the magnitude of the centrifugal force which influences a tire pressure sensor slider element is the solid angle subtended by the axis of the main guide body member and the radius of the wheel. In order to minimize the uncertainty attendant upon the exact magnitude of this angle, the invention is provided with a feature which allows the longitudinal axis of each parallel leg portion of the main guide body member  42  to be manually adjusted during installation of the sensor  40  on the tire valve stem so that these axes are essentially parallel to the radius of the wheel. As seen in FIG. 2, the portion of the main guide body member adjacent the partition  49  has a stiffly flexible wall section  70 . This wall section  70  is constructed in a manner similar to the flexible wall section of a conventional oil or hydraulic fluid funnel so that the spatial attitude of the main guide body member  42  to the right of the flexible wall section  70  can be selectively manipulated with respect to that portion of the main guide body portion to the left of wall section  70 , all as viewed in FIG. 2. Given the stiffness of wall section  70 , however, after manual adjustment, the relative position of main guide body member  42  will remain fixed over a wid rang of centrifugal forces up to a magnitude which will only be achieved under the most extreme circumstances. The manner in which this is accomplished will now be described with reference to FIGS. 3 and 4.  
         [0030]    [0030]FIG. 3 illustrates in schematic form the spatial attitud of the sensor  40  after initial installation on the valve stem of a tire on a wheel  72 . As seen in this Fig., the sensor  40  has achieved a random spatial attitude with respect to the radius R of wheel  72 , due to the random nature of the angle of emergence of the tire valve stem from the wheel  72 . If left in this random attitude, it is highly likely that the pressure measurements obtained even by the differential configuration of the sliders according to the invention will be less than completely accurate due to the unequal centrifugal forces acting on slider elements  55  and  55   a.  The magnitude of this inequality is a factor of the slightly different solid angles subtended by the two leg portion axes with respect to the radius R of the wheel.  
         [0031]    [0031]FIG. 4 illustrates in schematic form the spatial attitude of the sensor  40  after manual adjustment by the installer. As seen in this Fig., the sensor  40  has both main guide body member leg portion axes essentially aligned with the radius R of the wheel  72 . Consequently, the centrifugal force acting on the slider elements  55  and  55   a  positioned in each leg portion will be essentially the same, thereby eliminating the effect of the centrifugal force on the tire pressure measurement.  
         [0032]    While the preferred embodiment has been thus-far described as a single unit for one tire, in practice each tire of a vehicle will be equipped with a tire pressure sensor system  40 . Various encoding arrangements can be made to uniquely identify each individual sensor, and the warning indicator can be configured to identify the particular tire which is currently under-inflated or over-inflated.  
         [0033]    As will now be apparent, the invention provides a simple, low cost internal tire pressure sensor system which substantially reduces or entirely eliminates the effect of centrifugal force on the tire pressure reading. In addition, the sensor system according to the invention is relatively simple to install on a vehicle tire without requiring the removal of the tire from the wheel. Moreover, the tire pressure sensor according to the invention can be installed on the vehicle wheel during mounting of the tire on the whel. Further, replacement of the battery or other failed system components can be done without the ned for removing the tir from the whel, which simplifies repair or replacement of the entire system and thus lowers the cost of maintenance. Lastly, the invention provides an accurate and reliable system for monitoring tire safety on all vehicles using pneumatic tires.  
         [0034]    While the invention has been described with reference to a particular preferred embodiment, various modifications, alternate constructions, and equivalents may be employed, as desired. For example, different flexible wall section constructions may be used for wall section  70  than that described with reference to the preferred embodiment. Also, other position sensor contact arrangements may be used, such as a limit stop contact pair on each end of the permitted linear travel range of one or both slider elements  55 ,  55   a.  Further, position sensor contact elements  63  may be arranged in the bore region of slider element  55 , or in the bore regions of both slider elements  55 ,  55   a.  Therefore, the above should not be construed as limiting the invention, which is defined by the appended claims.