Abstract:
A speed sensor has a stationary electrode and a mobile electrode held normally at a specified distance from each other by means of an elastic supporting device. The speed sensor is attached to one of the wheels of a vehicle such as an automobile. The centrifugal force due to the rotation of the wheel is measured from the displacement of the mobile electrode with respect to the stationary electrode, and the traveling speed of the vehicle is calculated from the measured centrifugal force.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a speed sensor, and more particularly to an electrostatic speed sensor for detecting, for example, the speed of a vehicle such as an automobile traveling on rotating wheels, the detection being based on the centrifugal force caused by the rotation of one of its wheels.  
           [0002]    It has been known to electrostatically detect the speed of an automobile from the centrifugal force caused by the rotation of its tire. Japanese Patent Publication Tokkai 1996-240609, for example, disclosed such a speed sensor, characterized as having a weight attached to the back surface of a mobile electrode because changes in the acceleration as the running speed of the automobile is changed are not sufficient for displacing the mobile electrode of the sensor. A speed sensor of this type is not convenient, however, because the attachment of a weight onto the mobile electrode means an extra component to assemble and an extra work process in its manufacture and also because the finished product has a more complicated structure and the sensor cannot be made compact. In other words, such a prior art speed sensor cannot be easily attached to the tire of an automobile for detecting its speed by means of the centrifugal force thereon.  
         SUMMARY OF THE INVENTION  
         [0003]    It is therefore an object of this invention to provide a structurally simple and compact speed sensor which comprises a reduced number of constituent parts and can be assembled with a reduced number of steps.  
           [0004]    A speed sensor embodying this invention, with which the above and other objects can be accomplished, may be characterized as having a mobile electrode opposite a stationary electrode at a specified distance therebetween and displacing this mobile electrode by a centrifugal force to detect the speed. With a speed sensor thus characterized, there is no need to affix a weight, unlike the prior art technology described above, since the displacement of the mobile electrode is caused by a centrifugal force. As a result, the number of constituent parts and the number of steps required for the assembly process can be reduced, and the sensor as a whole is simpler in structure and can be made compact.  
           [0005]    The mobile electrode may be supported by one or more hinge springs extending inward from a ring-shaped support structure. With the mobile electrode, the support structure and the hinge springs integrated, a compact speed sensor can be provided with a reduced number of components, and such a speed sensor can be easily assembled. Alternatively, the stationary electrode may be provided with a protrusion which penetrates an opening formed through the mobile electrode. With such a structure, even if a large external impulsive force is applied, the protrusion can support the mobile electrode and prevent any plastic deformation of the hinge springs. Such a protrusion may be formed to serve as a terminal for the stationary electrode so that the overall structure of the sensor can be simplified.  
           [0006]    A spacer may be provided between the stationary and mobile electrodes such that they can face each other at a specified distance. This structure is advantageous in that the mechanical precision of the structure is improved, and sensors can be obtained with reduced variations in their operating characteristics. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a sectional view of a speed sensor according to a first embodiment of this invention with a calculating means shown schematically.  
         [0008]    [0008]FIG. 2 is an exploded diagonal view of the speed sensor of FIG. 1.  
         [0009]    [0009]FIG. 3 is a sectional view of a portion of a speed sensor according to a second embodiment of this invention.  
         [0010]    [0010]FIG. 4 is an exploded diagonal view of the speed sensor of FIG. 3.  
         [0011]    [0011]FIG. 5 is a sectional view of a portion of a speed sensor according to a third embodiment of this invention.  
         [0012]    [0012]FIG. 6 is an exploded diagonal view of the speed sensor of FIG. 5.  
         [0013]    [0013]FIG. 7 is a sectional view of a portion of a speed sensor according to a fourth embodiment of this invention.  
         [0014]    [0014]FIG. 8 is an exploded diagonal view of the speed sensor of FIG. 7. 
     
    
       [0015]    Throughout herein, like components are indicated by the same numerals and may not necessarily be explained repetitiously. The circuit for measuring electrostatic capacitance is omitted for the simplicity of disclosure.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The invention is described next by way of examples. FIGS. 1 and 2 show a speed sensor according to a first embodiment of this invention, having a stationary electrode  14  and a mobile electrode  19  disposed inside a housing comprised of a shield cover  10  and a base lid  26  and having rubber rings  13  and  24  and a spacer  16  so as to be kept opposite each other and at a specified distance of separation.  
         [0017]    The shield cover  10  is cross-sectionally U-shaped, or shaped like a cup for containing therein the inner components, to be described more in detail below, and a ventilating opening  11  is provided at the center of its bottom portion. Protruding from the open edge of the side wall portion of the shield cover  10  are a shield terminal  10   a  and a plurality of bendable engagement pieces  12 . The main body of the shield cover  10  may be 16 mm in diameter, and its side wall portion may be 3 mm in height.  
         [0018]    The stationary electrode  14  is set inside the shield cover  10  through the rubber ring  13  and is approximately T-shaped cross-sectionally, having a shaft  15  extending downward from the center of its lower surface to serve as a terminal. The spacer  16 , which serves to position the rubber ring  13  and the stationary electrode  14 , has its outer circumference shaped so as to conform with the inner surface of the shield cover  10 . It has an annular step  17  for engagingly holding the rubber ring  13  and the stationary electrode  14  inside and another annular step  18  for engagingly holding the mobile electrode  19 . These annular steps  16  and  17  are on mutually opposite surfaces of the spacer  16 .  
         [0019]    The mobile electrode  19  is planar and provided with an opening  20  at its center, as can be seen more clearly in FIG. 2. It is supported by a ring-shaped supporting structure (supporter ring  22 ) by way of a hinge spring  21  which is approximately C-shaped, extending inward from the periphery of the supporter ring  22 . The supporter ring  22  has an outer diameter such that it can be engageable with the annular step  18  of the spacer  16 . A terminal  23  for the mobile electrode  19  extends downward from the supporter ring  22 .  
         [0020]    The outer diameter of the rubber ring  24  is such that it can engagingly fit inside the inner peripheral surface of the shield cover  10 . Its inner peripheral surface is provided with a cut  25  for allowing the terminal  23  to pass through.  
         [0021]    The base lid  26  is a circular disk, adapted to engage with the open edge part of the shield cover  10  to sealingly close it. It is provided with an opening  27  for passing the shaft  15  of the stationary electrode  14  and another opening  28  for the terminal  23  for the mobile electrode  19 .  
         [0022]    Numeral  50  schematically indicates a calculating means for measuring the change in the separation between the stationary and mobile electrodes  14  and  19 . Although not separately shown, the speed sensor is adapted to be attached to a tire of an automobile. From the known dimension of the tire and the position of the sensor with respect to the tire on which it is attached, the speed of the automobile can be calculated from the rotational speed of the tire, and the rotational speed of the tire can be calculated in a known manner from the centrifugal force on the speed sensor, or on the mobile electrode  19 .  
         [0023]    For assembling the sensor, the stationary electrode  14  is placed on the rubber ring  13  positioned on the ceiling surface of the shield cover  10 . Next, the spacer  16  is inserted into the shield cover  10 , and the rubber ring  13  and the stationary electrode  14  are properly positioned by engaging them with the annular step  17  of the spacer  16 . After the supporter ring  22  of the mobile electrode  19  is engaged with the annular step  16  of the spacer  16 , the rubber ring  24  is assembled. The shaft  15  and the terminal  23  are passed through the openings  27  and  28  of the base lid  26 , and the bendable engagement pieces  12  are bent after the base lid  26  is engaged with the shield cover  10 .  
         [0024]    The method of using the speed sensor thus assembled will be explained next when it is attached to the wheel of an automobile tire.  
         [0025]    When the automobile is resting stationary, the mobile electrode  19  is not subjected to any centrifugal force and hence is at its normal position opposite the stationary electrode  14 , separated therefrom by a specified distance. As the automobile starts to run and the tire begins to rotate, a centrifugal force operates on the mobile electrode  19 , causing it to move towards the stationary electrode  14  and increasing the electrostatic capacitance therebetween. An increase in the speed of the automobile can be thereby detected by the calculating means  50 .  
         [0026]    As the automobile is decelerated, the centrifugal force on the mobile electrode  19  becomes weaker. As the mobile electrode  19  moves away from the stationary electrode  14 , the electrostatic capacitance therebetween becomes lower, and the deceleration of the automobile is thereby detected similarly.  
         [0027]    [0027]FIGS. 3 and 4 show another speed sensor according to a second embodiment of the invention. For the convenience of disclosure, the calculating means  50  is not shown in these figures. It is similar to the speed sensor according to the first embodiment of the invention described above with reference to FIGS. 1 and 2 but is different in that the positional relationship between the mobile electrode  19  and the stationary electrode  14  is reversed. Thus, the stationary electrode  14  has a terminal  14   a  extending downward from its outer periphery, the rubber ring  24  has two cuts  25  on its inner periphery, and the opening  27  for the terminal for the stationary electrode  14  is provided near the periphery of the base lid  26 .  
         [0028]    The mobile electrode  19  has an opening  20  at its center. The stationary electrode  14  has a protrusion  15 ′ at the center which penetrates the opening  20  through the mobile electrode  19  towards the ventilating opening  11  through the shield cover so as to be serviceable as a terminal.  
         [0029]    In other aspects, the speed sensors according to the first and second embodiments are substantially the same, and hence such aspects will not be described repetitiously.  
         [0030]    This invention relates also to speed sensors of the type adapted to measure also the inner pressure of a sealed container such as the air pressure inside an automobile tire and to detect the speed by measuring the centrifugal force thereon. Such a sensor may be hereinafter sometimes referred to as a pressure-speed sensor for the sake of clarity in description.  
         [0031]    [0031]FIGS. 5 and 6 show such a pressure-speed sensor embodying this invention (or a speed sensor according to a third embodiment of this invention). Disposed inside a housing comprised of a shield cover  110  and a base lid  137 , there are a first mobile electrode  118 , a first stationary electrode  119 , a second mobile electrode  130  and a base member  121  serving as a second stationary electrode. The first stationary and mobile electrodes  118  and  119  are opposite each other, being separated from each other by a specified distance, and together form a pressure sensor. The second mobile electrode  130  and the base member  121  serving as the second stationary electrode are opposite each other, being separated from each other by another specified distance, and together form a speed sensor.  
         [0032]    The shield cover  110  is cross-sectionally U-shaped, or shaped like a cup for containing therein the inner components, to be described more in detail below, having a plurality of discontinuous ventilating openings  111  formed on its ceiling surface in an annular formation. A shallow annular groove  112  is formed on the lower surface of this ceiling surface such that a sectionally square-shaped rubber ring  115  can be engagingly inserted therein. It is also provided with a shield terminal  113  (shown in FIG. 4) and a plurality of bendable engagement pieces  114  protruding from its open edge part.  
         [0033]    The first mobile electrode  118  and the first stationary electrode  119  are disposed inside a standard-pressure chamber  126  by integrally forming a peripheral edge part of a diaphragm  116  with an annular edge part of the base member  121 . This diaphragm  116  has an upwardly swelling form, having a center part  117  with a flat surface which is surrounded by concentrically formed protrusions and indentations. The size of the center part  117  may be varied if necessary. The mobile electrode  118  is directly in contact with the bottom surface of this center part  117 .  
         [0034]    The first stationary electrode  119  is approximately T-shaped cross-sectionally, having a shaft  120  extending downward from the center of its lower surface.  
         [0035]    The base member  121  serving as the second stationary electrode comprises a generally hat-shaped metal piece, having a throughhole  123  formed at the center of its bottom part  122  for passing the shaft  120  of the first stationary electrode  119  therethrough. A fixed electrode terminal  124  is welded integrally to the outer bottom surface of the bottom part  122  of the base member  121 .  
         [0036]    After the shaft  120  of the first stationary electrode  119  is inserted into the throughhole  123  of the base member  121  through a bearing  125  made of a glass material, a sealer is poured in and solidified such that the first stationary electrode  119  is supported by the base member  121  and in a face-to-face relationship with the base member  121  with a specified distance therebetween. As the peripheral edge part of the diaphragm  116  is integrally welded to the open edge part of the base member  121 , the first mobile and stationary electrodes  118  and  119  face each other with another specified distance therebetween.  
         [0037]    As shown in FIG. 5, the throughhole  123  is provided with an annular step  123   a  for engaging the bearing  125  of a glass material having a flange. This serves to prevent the sealing material, when it is poured in, from flowing in excessively and also to reduce the initial floating capacitance by separating the shaft  120  as much as possible from the base member  121 . For this purpose, the portion of the base member  121  adjacent to the shaft  20  of the first stationary electrode  119  is made thinner.  
         [0038]    Numeral  127  indicates a spacer having its outer periphery shaped so as to be engageable with the inner peripheral surface of the shield cover  110 . Its inner peripheral surface is provided with an annular step  128  for properly positioning the base member  121  by engaging therewith and another annular step  129  for properly position the second mobile electrode  130  by engaging therewith.  
         [0039]    As shown in FIG. 6, the second mobile electrode  130  has a throughhole  131  at its center part and is supported by a supporter ring  133  by way of a hinge spring  132  which is approximately C-shaped, extending inward from the periphery of the supporter ring  133 . The supporter ring  133  has an outer diameter so as to be engageable with the annular step  129  of the spacer  128 . A terminal  134  for the mobile electrode extends downward from the supporter ring  133 .  
         [0040]    Numeral  135  indicates a rubber ring for elastically supporting the supporter ring  33  from its backside. This rubber ring  135  has an outer diameter so as to be engageable with the inner peripheral surface of the shield cover  110  and is provided with a cut  136  on its inner peripheral surface for passing the terminal  134  therethrough.  
         [0041]    The base lid  137  is a circular disk, adapted to engage with the open edge part of the shield cover  110  to sealingly close it. It is provided with an opening  138  for the first stationary electrode and openings  139  and  140  for the mobile electrode.  
         [0042]    For assembling this sensor, the first stationary electrode  119  and the base member  121  to become the second stationary electrode are integrated first through the bearing  125 . Next, the peripheral part of the diaphragm  116 , to which the mobile electrode  118  is integrated, is welded to and integrated with the open edge part of the base member  121 . The base member  121  thus integrated is placed on the rubber ring  15  positioned in the shallow groove  112  of the shield cover  110 . Next, the spacer  127  is inserted into the shield cover  110  and positioned by engaging the base member  121  with the annular step  128  of the spacer  127 . After the supporter ring  133  for the second mobile electrode  130  is positioned by engaging with the annular step  129  of the spacer  127 , the rubber ring  135  is inserted into the shield cover  110 . Thereafter, the shaft  120  of the first stationary electrode  119 , the terminal  124  for the second stationary electrode and the terminal  134  for the second mobile electrode are inserted respectively through the openings  138 ,  139  and  140 . After they are thus assembled, the engagement pieces  114  of the shield cover  110  are bent to engage the bottom lid  137 .  
         [0043]    Operations of this sensor as a pressure sensor will be explained next for a situation where the sensor is mounted to the wheel of an automobile tire for measuring the inner pressure of the sealed tire.  
         [0044]    When the internal pressure of the standard-pressure chamber  126  is balanced with the external pressure, the first mobile electrode  118  remains facing the first stationary electrode  119  with the specified distance therebetween. If the external pressure drops, the diaphragm  116  expands, and the mobile electrode  118  moves away from the stationary electrode  119 . As a result, the electrostatic capacitance therebetween is reduced and the lowered external pressure is thereby detected. If the external pressure is increased, on the other hand, the diaphragm  116  is pushed in, and the mobile electrode  118  approaches the stationary electrode  119 , thereby increasing the electrostatic capacitance therebetween. The increase in the external pressure is thereby detected.  
         [0045]    Operation of the same sensor as a speed sensor will be explained next.  
         [0046]    While the automobile remains stationary, there is no centrifugal force operating on the second mobile electrode  130  and hence it faces the outer bottom surface of the base member  121  serving as the second stationary electrode at the specified distance. When the automobile is started and its tires begin to rotate, the second mobile electrode  130  is subjected to a centrifugal force and approaches the outer bottom surface of the base member  121 , thereby increasing the electrostatic capacitance. An increase in the speed of the automobile is thereby detected. As the automobile slows down, the centrifugal force on the second mobile electrode becomes weaker. The second mobile electrode moves away from the base member  121 , and the deceleration of the automobile is thereby detected.  
         [0047]    [0047]FIGS. 7 and 8 show another pressure-speed sensor embodying this invention (or a speed sensor according to a fourth embodiment of this invention). It is similar to the pressure-speed sensor described above with reference to FIGS. 5 and 6 but is different in that the second mobile electrode  130  is disposed inside the standard-pressure chamber  126  and hence faces the inner bottom surface of the base member  121  with a specified distance therebetween. For this reason, the terminal  134  of its second mobile electrode  130  is made to penetrate the bottom  122  of the base material  121  serving as the second stationary electrode through another bearing  141  with a flange and made of a glass material. The terminal  124  for the base material  121  is integrally formed by pressing. The first stationary electrode  119  and the second mobile electrode  130  are disposed separate from each other such that they do not affect each other.  
         [0048]    In other aspects, speed sensors according to the third and fourth embodiments of this invention are substantially the same and hence such aspects will not be described repetitiously. It may be noted, however, that the fourth embodiment is advantageous in that the second mobile electrode  130 , being disposed inside the standard-pressure chamber, is free from the effects of dust or the like and hence malfunctions are not likely to occur. Since the base member  121  is used to function also as the bottom lid, furthermore, the number of constituent parts and the number of production steps can be both reduced and hence the sensor can be made even more compact.  
         [0049]    Many modifications and variations are possible within the scope of this invention. For example, the diaphragm  116  may have its center part  117  function as a mobile electrode. In such an application, the area of this center part  117  may be appropriately enlarged, or a plating process may be effected on the bottom surface of the diaphragm  116  to increase the electrostatic capacitance. Such a modification is advantageous in that the mobile electrode  118  shown in FIGS.  5 - 8  can be dispensed with and hence that the productivity can be further improved. Since the diaphragm  116  becomes lighter and freer to move, the response characteristic of the sensor also improves.  
         [0050]    Although not separately shown, the glass bearing  125  shown in FIGS. 5 and 6 may be replaced by another disposed on the bottom  122  of the base member  121  so as to be sandwiched between the first stationary electrode  119  and the bottom  122  of the base member  121 . This variation is advantageous in that the first stationary electrode  119 , being supported from the backside, is less likely to be deformed. In other words, the sensor becomes less likely to be affected by the external vibrations and impulses and hence more reliable.  
         [0051]    The diaphragms  116  need not necessarily be designed as shown in FIGS.  5 - 8  with concentric protrusions and indentations. A terminal may be extended from the diaphragm  116  for a direct connection with an external circuit.  
         [0052]    The standard-pressure chamber  126  may be filled with a liquid such as a silicon oil. Since a liquid is generally less sensitively affected by a temperature change, it is advantageous in that the sensor becomes even more reliable in view of changes in temperature.  
         [0053]    The embodiments of the invention described above with reference to FIGS.  5 - 8  are a combination of a speed sensor and a pressure sensor. Since many of the constituent parts can be shared commonly between the two sensors, the total number of constituent parts and the number of steps in the production and assembly can be reduced and the sensor as a whole can be made compact.