Abstract:
A capacitive pressure sensor for measuring a pressure applied to an elastic member includes a capacitive plate disposed adjacent to the elastic member so as to define a gap between a planar conductive surface of the elastic member and a corresponding planar surface of the capacitive plate. The gap, capacitive plate and elastic member together define a capacitor having a characteristic capacitance. The sensor further includes an elongated electrical conductor characterized by an associated inductance value. The conductor is fixedly attached to and electrically coupled with the capacitive plate. The gap between the capacitive plate and the elastic member varies as a predetermined function of the pressure applied to the elastic member so as to vary the characteristic capacitance. The capacitor and the electrical conductor together form an electrical resonator having a characteristic resonant frequency. Varying the capacitance of this tank circuit varies the resonant frequency of the tank circuit. Thus, the resonant frequency of the tank circuit is indicative of the pressure applied to the elastic member. The close physical proximity of the capacitor and the electrical conductor equalizes the effects of environmental influences such as temperature variations, vibration and shock, thus making such effects more predictable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     1. Field of the Invention 
     The present invention relates to a pressure sensor, and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations. 
     2. Background of the Invention 
     Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displacable (relative to the fixed element) conductive member, such as a metal diaphragm, or a plated non-conductive member, such as a metalized ceramic diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Because the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance C of the sensor may be approximated by the equation:              C   =       ε                 A     d             (   1   )                                
     where ε is the permittivity of the material between the parallel plates, A is the surface area of the parallel plate and d represents the gap between the plates. The characteristic capacitance is inversely proportional to the gap between a central portion of the diaphragm and the conductive surface of the fixed element. In order to permit a pressure differential to develop across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side. 
     In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of the interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor capacitor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. Regarding equation (1), if ε and A are held constant, the greatest slope of the d verses C plot occurs when d is small. Thus, for the greatest sensitivity, the gap is made as small as possible when the device is in equilibrium and the sensor is designed so that the gap d changes as pressure is applied. The multiplicative effect of ε and A increases the sensitivity of the d to C relationship, so ε and A are maximized to achieve the highest possible sensitivity. 
     In a typical prior art embodiment, the sensor capacitor formed by the fixed conductive surface and the diaphragm is electrically coupled via conductors to an oscillator circuit. The oscillator circuit typically includes an inductor that forms a tank circuit with the remotely located sensor capacitor. This LC tank circuit provides a frequency reference for the oscillator circuit; the output frequency of which is a direct function of the resonant frequency of the tank circuit. The resonant frequency of the tank circuit is in turn a direct function of the inductance L of the inductor and the capacitance C of the sensor capacitor. It is well known to those in the art that the resonant frequency ω 0  of a simple LC tank circuit is given by            ω   0     =       1     LC       .                                         
     As long as the values of the inductor and the capacitor both remain fixed, the output frequency of the oscillator circuit remains constant. However, since the capacitance of the sensor capacitor varies as a function of the pressure applied to the diaphragm, the output frequency of the of the oscillator circuit also varies as a direct function of the applied pressure. 
     Such a configuration produces a signal whose frequency is indicative of the pressure applied to the remote sensor. One disadvantage to this configuration is that having the capacitive sensor located remotely can introduce environmentally induced errors in the expected resonant frequency of the tank circuit. For example, it is well known to those in the art that the inductance value L of an inductor and the capacitance value C of a capacitor are each temperature dependent to some extent, depending upon the design of each particular physical component. The effect of the temperature on the capacitance or inductance of a particular component is often quantified as the “temperature coefficient” associated with that component. It is possible to design a component so as to minimize the temperature coefficient, thus rendering the value of the device relatively insensitive to temperature, but commercially available components typically do have a measurable temperature coefficient which affects the component performance. It is also possible to choose components whose temperature coefficients are complementary, such that the net effect of a temperature change to the components together is nominally zero. However, when two components are not located together, such as the capacitive sensor and the inductor in the oscillator circuit, the ambient temperatures are often different, and complementary temperature coefficients do not produce a nominally zero sensitivity to temperature changes. 
     Another disadvantage to having a remotely located capacitive sensor is that the conductors used to electrically couple the sensor to the oscillator circuit introduce stray capacitances and inductances to the basic LC tank circuit. This disadvantage could be mitigated and thus acceptable if the stray values remained constant, but the stray values can change with environmental factors, physical movement of the conductors, etc. 
     It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     The foregoing and other objects are achieved by the invention which in one aspect comprises a capacitive sensor for measuring a pressure applied to a conductive, elastic member, or a plated non-conductive elastic member, having at least a first substantially planar surface and being supported on at least one edge. The sensor includes a housing for supporting the elastic member by its edge, thereby forming (i) a controlled pressure chamber disposed on the side of the elastic member corresponding to the first planar surface, and a variable pressure region disposed on the side of the elastic member opposite said first side. The sensor also includes a capacitive plate disposed substantially adjacent to the elastic member so as to define a gap between the first planar surface and a corresponding planar surface of the capacitive plate. The gap, capacitive plate and elastic member together define a capacitor having a characteristic capacitance. The sensor further includes an elongated electrical conductor characterized by an associated inductance value. The conductor is fixedly attached to and electrically coupled with the capacitive plate. The gap between the capacitive plate and the elastic member varies as a predetermined function of the pressure applied to the elastic member so as to vary the characteristic capacitance. The capacitor and the electrical conductor together form a tank circuit having a characteristic resonant frequency; varying the capacitance of this tank circuit varies the resonant frequency of the tank circuit. Thus, the resonant frequency of the tank circuit is indicative of the pressure applied to the elastic member. 
     In another embodiment of the invention, the pressure applied to the elastic member is generated by a pressure differential across (i) the first planar surface of the elastic member and (ii) a second planar surface of the elastic member disposed substantially parallel to the planar surface. In one embodiment, this pressure differential is the result of a constant, controlled environment being in contact with the first planar surface, along with a fluid under pressure being in contact with the second planar surface of the elastic member. 
     In another embodiment, the electrical conductor is disposed in a spiral configuration within a plane substantially parallel to the capacitive plate. 
     In a further embodiment, the sensor further includes an insulator disposed between the capacitor plate and the electrical conductor. The insulator may be fixedly attached to either the capacitor plate, the electrical conductor, or both. 
     In another embodiment, the sensor further includes a stiffening element fixedly attached to the capacitive plate and the conductive element. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 shows a sectional view of one preferred embodiment of a capacitive pressure sensor; 
     FIG. 2 shows the capacitive sensor of FIG. 1 with a higher pressure in the variable pressure region than the controlled pressure region; 
     FIG. 3A shows a bottom view of the capacitor plate; 
     FIG. 3B shows a top view of the inductor coil; 
     FIG. 4A shows the capacitor and the inductor coil connected as a series resonant tank circuit; 
     FIG. 4B shows the capacitor and the inductor coil connected as a parallel resonant tank circuit; 
     FIG. 5 shows the tank circuit of FIG. 4A connected to an oscillator circuit; 
     FIG. 6 shows a closing-gap embodiment of the pressure sensor of FIG. 1; 
     FIG. 7 shows the sensor of FIG. 1 including a stiffening element attached to the electrode assembly; 
     FIG. 8 shows an alternate, multiple layer embodiment of the inductor coil from the sensor of FIG. 1; 
     FIG. 9 shows another view of the multiple layer inductor coil shown in FIG. 8; and, 
     FIG. 10 shows another embodiment of the sensor shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a sectional view of one preferred embodiment of a capacitive pressure sensor  100  constructed in accordance with the present invention, which produces a characteristic capacitance proportional to a pressure (e.g., pressure via a fluid medium) applied to the sensor  100 . Sensor  100  includes an electrically conductive, elastic member  102  that forms a physical boundary between a variable pressure region  104  and a controlled pressure region  106 . FIG. 2 shows the capacitive sensor of FIG. 1 with a higher pressure present in the variable pressure region  104  than the controlled pressure region  106 . The elastic member  102  is supported at its periphery 108 by a support member  110 . The support member  110  may include, or be integral with, the pressure sensor  100  housing, as is disclosed and described in detail in U.S. Pat. No. 5,442,962, assigned to the assignee of the subject invention and is hereby incorporated by reference. 
     In this embodiment, the planar surface of the elastic member  102  is substantially circular, although alternate embodiments may incorporate other shapes. A connection post  112  for supporting an electrode assembly  114  is fixedly attached to the elastic member  102 . The connection post  112  may be attached to the elastic member  102  by brazing, soldering, welding, gluing, press fit, stud mount, or by other securing methods known to those in the art. The cross section of the elastic member  102  (shown in FIG. 1) is somewhat greater (i.e., thicker) at the center, as compared to the perimeter, to provide a foundation for attaching the connection post  112 . Other elastic member  102  cross sections may be used to provide similar results. Similarly, the electrode assembly  114  may be attached to the connection post  112  by brazing, soldering, gluing, press fit, stud mount, or by other methods of securing components known to those in the art. 
     The electrode assembly  114  includes a capacitor plate  116 , an insulator  118  and a planar inductor coil  120 . The capacitor plate  116 , a bottom view of which is shown in FIG. 3A, is shaped, sized and contoured to substantially match the planar surface of the electrically conductive elastic member  102 . In a preferred embodiment, the capacitor plate  116  includes a sheet of copper, silver or gold bonded to an insulating base  117  such as fiberglass, polyimide, glass, or ceramic, although other electrically conductive materials and other insulating materials known to those in the art may be used to form the capacitor plate  116  and the insulating base  117 , respectively. Alternately, the capacitor plate  116  may be etched from a copper-clad substrate, or screened and fired using thick-film techniques, using procedures well known for the fabrication of printed circuits. 
     The insulator  118  may include a separate piece of insulating material bonded to and contiguous with the capacitor plate  116  and the inductor coil  120 , or it may include an extension of the insulating base from the capacitor plate  116 . The insulator  118  may include fiberglass, polyimide, ceramic, or other insulating materials known to those in the art. 
     A preferred embodiment of the inductor coil  120 , a top view of which is shown in FIG. 3B, includes an elongated electrical conductor wound in a spiral form within a plane that is substantially parallel to the capacitor plate  116 . As with the capacitive plate  116 , the inductor coil  120  may be etched from a sheet of conductive foil bonded to an insulator  118 , using printed circuit board techniques well known to those in the art. Alternatively, the coil may be screened and fired using thick-film techniques well known to those in the art. In other embodiments, the coil  120  may include a single long conductor, wound in the shape shown in FIG.  3 B and bonded to an insulator  118 . Other methods of fabricating the coil  120  known to those in the art (e.g., vapor deposition, photoetching, etc.) may also be used, as long as the resulting coil  120  provides the inductive properties described herein. The end of the coil  120  shown in FIG. 3B is electrically coupled to a plated through-hole  128  that passes through the insulator  118 . The plated through-hole  128  is also electrically coupled to the capacitor plate  116 ; the coil  120  is thus electrically coupled to the capacitor plate  116 . In alternate embodiments, this electrical coupling between the coil  120  and the capacitive plate  116  may be accomplished by an electrical conductor passing through the insulator  118 , by a conductor wrapping around the side of the insulator  118 , or by other methods known to those in the art. 
     The capacitive plate  116 , the conductive elastic member  102  and the gap  126  formed between the capacitive plate  116  and the elastic member  102  form a capacitor  130  having a characteristic capacitance. In general, the characteristic capacitance of such a structure is directly proportional to the areas of the capacitive plate  116  and the elastic member  102 , and inversely proportional to the distance between the capacitive plate  116  and the elastic member  102 . 
     In a preferred embodiment of the invention, the pressure sensor  100  senses a pressure applied to the elastic member via a fluid medium present in the variable pressure region  104 . The pressure in the controlled pressure region  106  may be ambient atmospheric pressure (i.e., simply exposed to the “open air”) or it may be more precisely controlled with respect to a constant pressure reference. A difference in pressure across the two regions  104  and  106  produces a net differential pressure  124  on the elastic member  102 . When the variable pressure region  104  is greater than the controlled pressure region  106 , the direction of the elastic member displacement is from the variable pressure region  104  to the controlled pressure region  106 , as shown in FIG. 2. A change of ambient pressure in the variable pressure region  104  produces a corresponding change in the amount of displacement of the elastic member  102 . FIG. 1 shows the elastic member  102  in a neutral displacement position; i.e., when the differential pressure across the elastic member  102  is substantially zero. In the neutral displacement position, a substantially uniform gap  126  exists between the capacitive plate  116  and the elastic member  102 . FIG. 2 shows the elastic member  102  displaced toward the controlled pressure region  106 , such that the elastic member  102  presents a convex surface in the controlled pressure region  106 . In this convex displacement position, a non-uniform gap  126  exists between the capacitive plate  116  and the elastic member  102 . The width of the non-uniform gap  126  near the connection post  112  is substantially the same as the uniform gap  126  in the neutral displacement position, and the width of the non-uniform gap  126  increases as the distance from the post  112  increases. The increase in the gap  126  distance as the elastic member  102  displaces toward the controlled pressure region  106  produces a decrease in the characteristic capacitance. Thus, the characteristic capacitance of the capacitor  130  formed by the capacitive plate  116 , the conductive elastic member  102  and the gap between them is inversely proportional to the magnitude of the differential pressure  124  applied to the elastic member  102 . 
     In one embodiment of the invention, the capacitor  130  is electrically coupled in series to the inductive coil  120  so as to form a series resonant tank circuit  132  having a resonant frequency          ω   0     =     1     LC                              
     as shown schematically in FIG.  4 A. 
     Alternately, the capacitor  130  may be electrically coupled in parallel to the inductive coil  120  so as to form a parallel resonant tank circuit  132  having a resonant frequency          ω   0     =     1     LC                              
     as shown schematically in FIG.  413 . In either case, the tank circuit ( 132  or  134 ) is electrically coupled to an oscillator circuit  136  that uses the tank circuit  132  as a frequency reference, as shown in FIG. 5 for a series resonant tank circuit  132 . The oscillator circuit  136  is electrically coupled to the tank circuit  132  via conductors electrically coupled to inductor terminal  129  and capacitor terminal  131 . The output of the oscillator circuit is a signal S OUT  having a frequency of            ω   OUT     =     1     LC         ,                          
     thus the capacitance C is a function of the frequency; i.e.,        C   =       1       ω   OUT   2        L       .                            
     Since the characteristic capacitance of the capacitor  130  is directly proportional to the magnitude of the differential pressure  124  applied to the elastic member  102 , the frequency ε OUT  of the output signal S OUT  is also a function of the magnitude of the differential pressure  124 . The close mutual proximity of the inductive coil  120  and the capacitor  130  ensures similar environmental conditions for both components of the tank circuit  132 . 
     A closing-gap embodiment of a pressure sensor  200 , shown in FIG. 6, includes an electrically conductive elastic member  202  secured about its perimeter  208  by a housing  210 . In this form of the invention, the housing  210  includes an upper portion  210   a  and a lower portion  210   b , and the elastic member  202  is secured between the two portions at its perimeter  208 . The elastic member may be secured by a bonding technique known in the art such as brazing, welding, gluing, etc., or the elastic member may be secured by pressure (i.e., clamping) between the upper portion  210   a  and the lower portion  210   b  of the housing  210 . As with the embodiment shown in FIG. 1, the elastic member  202  forms a physical boundary between a variable pressure region  204  and a controlled pressure region  206 . In the closing-gap embodiment, however, the electrode assembly  214  is not mechanically coupled to the elastic member  202  via a connection post. Rather, the electrode assembly  214  is suspended from the housing  210  by a suspension post  212 , such that the electrode assembly  214  is disposed substantially adjacent to the elastic member  202 . Because the electrode assembly  214  is not attached to the elastic member  202  in this embodiment, the cross section of the elastic member  202  can be relatively uniform as shown in FIG. 6, as opposed to the non-uniform cross section (i.e., thicker at the center and tapering out toward the perimeter) of the elastic member  102  shown in FIG.  1 . 
     The construction of the electrode assembly  214  in this embodiment is essentially the same as for the form of the invention shown in FIG. 1; the electrode assembly  214  includes a capacitor plate  216 , an insulator  218  and a planar inductor coil  220 . The inductor coil  220  and the capacitor plate  216  are electrically coupled via the plated through-hole  228 . A capacitor  230  having a characteristic capacitance C is formed by the capacitor plate  216 , the conductive elastic member  202  and the variable gap  226  formed between the plate  216  and the member  202 . Since the areas of the capacitive plate  216  and the elastic member  202  do not vary, the characteristic capacitance C varies only as a function of the gap  226 . As a differential pressure  224  is applied to the elastic member  202  in a direction from the variable pressure region  204  toward the controlled pressure region  206 , the elastic member deflects toward the electrode assembly  214 , so as to be substantially convex in the controlled pressure region. This pressure induced deflection toward the electrode assembly closes the variable gap  226 , thereby increasing the characteristic capacitance C. The characteristic capacitance C is thus directly proportional to the magnitude of the differential pressure  124  applied to the elastic member  102  for this embodiment of the invention. Electrical access to the capacitor  230  is gained by a first electrical terminal  229  and a second electrical terminal  231 . In one preferred embodiment, the first electrical terminal  229  is electrically coupled to the inductor coil  220  through an electrically conductive suspension post  212 , and the second electrical terminal  231  is electrically coupled to the elastic member  202  at its perimeter  208 . 
     In one embodiment, the electrode assembly  214  includes a stiffening element  140  as shown in FIG.  7 . The stiffening element  140  prevents flexure of the overall electrode assembly, which in turn maintains the capacitor plate  116  within its nominal plane  142 . The stability of capacitor  130  of FIG. 1, formed in part by the variable gap  126 , is dependant upon the capacitor plate  116  being substantially planar. Flexure of the plate  116  due to temperature variations or other environmental forces (such as vibration and shock) may corrupt the measured value of the characteristic capacitance of the capacitor  130 . Any corruption of the characteristic capacitance translates directly to a corruption of the resonant frequency too of the tank circuit  132  and thus to a corruption of the measurement of the differential pressure  124 . The stiffening element  140  may include ceramics or other materials that are known to exhibit small amounts of expansion or contraction with respect to ambient temperature variations. 
     In another embodiment of the invention, the inductor coil  120  of FIG. 1 may include a multi-layer inductive coil. The coil  150  shown in FIG. 8 includes two layers of electrical conductor electrically coupled in series via a plated through-hole  152 , although alternate embodiments may include any number of layers. The two layers of electrical conductor are bonded to opposite sides of an insulating layer  154 , similar to the construction of a multi-layered printed circuit board. One utility of a multiple layer inductive coil  150  is a higher characteristic inductance value due to the increase in the length of the conductor. Another utility of the multiple layer inductive coil  150  is the ability to compensate a variation of the coil&#39;s characteristic inductance with respect to temperature variations. It is well known to those in the art that as a planar spiral coil  150  expands in its spiraling plane and the distance d 1  between adjacent turns of a single coil increases, the characteristic inductance L of the coil increases (see FIG.  9 ). It is also well known that as the distance d 2  between two coils increases, the characteristic inductance L of the coils decreases. An expansion of the insulating layer due to a temperature change results in a corresponding increase in both d 1  and d 2 . By choosing the appropriate initial dimensions d 1  and d 2 , and by choosing a material for the insulating layer  154  having an appropriate expansion coefficient (with respect to temperature), the changes in characteristic inductance of the coil  150  due to the changes in d 1  and d 2  can be made to cancel. 
     In yet another form of the invention, as shown in FIG. 10, the capacitor  330  portion of the electrode assembly  314  is located within the housing  310 , formed by upper portion  310   a  and lower portion  310   b , while the insulator  318  and the inductor  320  portions are disposed outside of the housing  310 . An electrically conductive post  312  extends through the upper portion  310   a  of the housing  310 , and is secured in place by a non-conductive sleeve  322 . This sleeve  322  electrically isolates the conductive post from the housing  310 . Electrical access to the resonator formed by the inductor  320  and the capacitor  330  is gained via a first terminal  329  and a second terminal  331 . The first terminal  329  is electrically coupled to the diaphragm  302  at the perimeter  308 . The second terminal  331  is electrically coupled to a first end of the inductor  220 . The second end of the inductor  220  is electrically coupled to the conductive post  320 , as is the capacitive plate  316 . Thus, the conductive post serves not only to support the capacitive plate  316  and the inductor  320 , but also to electrically couple the inductor  320  to the capacitor  330 . 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.