Abstract:
Substantially hemispherical concave first and second surfaces of substantially equal radius and surface area face each other about a proof mass supported for movement between the surfaces. The surfaces and proof mass have electrically conductive portions allowing assessment of differential capacitance for measurement of acceleration. Electrically conductive portions are connected to a conditioning circuit in an embodiment.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to capacitive accelerometers, and, more specifically, to accelerometers tolerant of extreme conditions, particularly of high temperatures. 
     Accelerometers are used to measure acceleration and come in several forms. Capacitive accelerometers have a conductive mass, called a seismic or inertial proof mass, suspended on a spring of some sort between two conductive plates. A gas, such as air or a non-reactive gas, occupies the space between the mass and the plates. The arrangement forms two capacitors, one between each plate and the proof mass with the gas acting as a dielectric. When the device experiences an acceleration, the proof mass moves closer to one plate or the other, reducing the gap between the proof mass and one plate while increasing the distance between the proof mass and the other plate, changing the capacitance in the capacitors. By comparing the capacitances, the direction and magnitude of the acceleration can be determined. However, most capacitive accelerometers can only measure acceleration along one axis because of their structure. To measure acceleration along more than one axis, one accelerometer must be provided for each axis of interest, which can become complicated. In addition, because of their materials and construction, capacitive accelerometers tend to be susceptible to failure at high temperatures, such as might be experienced in a gas turbine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an embodiment, an accelerometer has concave first and second surfaces of substantially identical surface area that face each other around a proof mass supported for movement between the surfaces. The surfaces and proof mass have electrically conductive portions allowing assessment of differential capacitance for measurement of acceleration. 
     Another embodiment has a system including an accelerometer with concave first and second surfaces, the first surface facing the second surface, each having at least one electrically conductive region on a respective portion of each of the first and second surfaces. The accelerometer also has a proof mass supported between the first and second surfaces for movement with at least a portion of the proof mass being electrically conductive. The system includes a conditioning circuit connected to each electrically conductive region of the first and second surfaces and the electrically conductive at least a portion of the proof mass and configured to provide a respective signal indicative of capacitance between each electrically conductive region of the first and second surfaces and the electrically conductive at least a portion of the proof mass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a perspective view of an accelerometer according to an embodiment as disclosed herein. 
         FIG. 2  is a perspective exploded view of an accelerometer according to an embodiment as disclosed herein. 
         FIGS. 3-10  are schematic views of hemispherical surfaces of accelerometers according to embodiments as disclosed herein. 
         FIGS. 11-13  are schematic views of conditioning circuitry according to embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the accompanying FIGS., examples of an accelerometer according to embodiments of the invention are disclosed. For purposes of explanation, numerous specific details are shown in the drawings and set forth in the detailed description that follows in order to provide a thorough understanding of embodiments of the invention. The details shown and described are examples and are not limiting on the scope of the invention. It will be apparent, however, that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing 
     As seen in  FIGS. 1 and 2 , an accelerometer  10  according to an embodiment includes hemispherical concave first and second surfaces  11 ,  12  formed in first and second plates  13 ,  14 , the first surface  11  facing the second surface  12 . While hemispherical surfaces are shown in the example embodiments, other configurations could be used within the scope of embodiments, such as, but not limited to, polygonal surfaces, elliptical surfaces, hyperbolic surfaces, and parabolic surfaces, so long as the first and second surfaces have substantially identical profiles and surface area. In the example embodiment shown, the first and second surfaces  11 ,  12  are of substantially identical radii of curvature, surface area, and other dimensions such as, but not limited to, depth, and perimeter at the surfaces of the plates  13 ,  14 . Each of the first and second surfaces  11 ,  12  has at least one portion that is or carries an electrically conductive region, each electrically conductive region being centered about the surficial center of its respective portion. A seismic or inertial proof mass  20  is suspended between the first and second surfaces  11 ,  12  by a spring-like element  30 , such as a flexure plate, so that the proof mass  20  can engage in motion in at least one dimension. While a spherical proof mass  20  is shown in the example embodiments, other configurations or profiles could be used within the scope of embodiments, such as, but not limited to, polygonal, elliptical, hyperbolic, and parabolic profiles. The spring-like element  30  in an embodiment constrains the proof mass  20  to motion along the longitudinal axis of the accelerometer, while in other embodiments, the spring-like element  30  allows motion in two or three dimensions. As is known, the motion of an object in multidimensional spaces can be described by components of its motion along axes. For example, two dimensional motion can be described by components of motion along two mutually perpendicular axes. Similarly, for example, three dimensional motion can be described by components of motion along three mutually perpendicular axes. 
     At least a portion of the proof mass  20  is electrically conductive. The space  25  between the proof mass  20  and the first and second surfaces  11 ,  12  is filled with a gas that acts as a dielectric. In an embodiment, the gas is non-reactive, or has low reactivity, at high temperatures, such as helium, though other gases can be used as long as they have suitably low reactivity for the purpose disclosed herein. To allow for deflection of the spring, each of the first and second plates of an embodiment include a cut-out, such as a frustroconical cut-out  15 ,  16  around the opening of the respective hemispherical surfaces  11 ,  12 . The remaining portions of the engaging surfaces  17 ,  18  of the first and second plates  13 ,  14  engage and retain the flexure plate. In embodiments, the first and second plates  13 ,  14 , the spring-like element  30 , and the proof mass are made of a non-conductive material. The electrically conductive portions of the first and second plates  11 ,  12  and proof mass  20  in an embodiment are formed by applying metal coatings to the first and second surfaces  11 ,  12  of the plates  13 ,  14  and the proof mass  20 . Such coatings will have a thickness, but so long as the thickness is substantially uniform, the effect of the thickness is balanced and/or negligible. In high temperature application embodiments, the metal coatings of embodiments are made from a metal with a high melting point so that the coatings are unperturbed at the high temperatures to which the accelerometer is to be exposed. 
     The material from which first and second plates  13 ,  14  of high temperature application embodiments are made in is a ceramic material that tolerates high temperatures. Other materials can be employed for other applications as appropriate within the scope of the invention. While the plates  13 ,  14  are shown in  FIGS. 1 and 2  as being cylindrical, other shapes can be employed in embodiments. Similarly, while the cut-out portions  15 ,  16  that allow motion of the flexure plate  30  are shown as frustroconical, other shapes can be applied as long as they accommodate the appropriate motion of the flexure plate  30 . The material from which the seismic or inertial proof mass  20  is made is also a ceramic material in embodiments, particularly in high temperature application embodiments, though other suitable materials can be used. 
     As shown in  FIG. 1 , the flexure plate  30  is attached to the proof mass  20  so that the inner periphery of the flexure plate  30  moves with the proof mass  20 . At its outer periphery, the flexure plate  30  is held by the ends  17 ,  18  of the upper plate  13  and the lower plate  14 . An outer housing  40  can be included to hold the assembly together and to provide a sealed chamber to hold the dielectric gas. The outer housing can be stainless steel, ceramic, or another suitable heat-tolerant material for high temperature application embodiments. The outer housing  40  shown in  FIG. 1  is only an example, and other shapes, sizes, types, and assemblies of outer housings can be used without departing from the scope of embodiments. 
     As seen in  FIGS. 3 and 4 , each hemispherical surface  11 ,  12  of an embodiment has one portion carrying a respective electrically conductive region  310 ,  320 . In the embodiment shown in  FIGS. 3 and 4 , each entire surface  11 ,  12  is coated with a metal to form the electrically conductive regions  310 ,  320 , but smaller coated areas can be employed to form the electrically conductive regions  310 ,  320  as long as they are the same size and the centers of the electrically conductive regions  310 ,  320  coincide with surficial centers of the respective surface to which they are applied. Each electrically conductive region  310 ,  320  forms a capacitor with the proof mass  20 . When a single coated portion  310 ,  320  is on each surface, the accelerometer  10  can measure a component of acceleration along a single axis by virtue of the change in capacitance induced by deflection of the proof mass  20  when the accelerometer  10  experiences acceleration, or at least a component thereof, along the axis. For example, as seen in  FIG. 3 , the single axis of measurement, labeled the z-axis, passes through the centers of the hemispheres. 
     As seen in  FIGS. 5 and 6 , an embodiment employs four portions in each hemispherical surface  11 ,  12  that are or carry electrically conductive regions  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 , and  480 . Combinations of the regions are employed to measure acceleration or components thereof in three dimensions. Each of the eight electrically conductive regions  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 , and  480  are electrically isolated from adjacent ones of the electrically conductive regions. Electrical isolation is achieved, for example, by leaving portions of the surfaces uncoated to form borders between the regions. Each electrically conductive region effectively forms a capacitor with the spaced-apart proof mass. Opposed pairs of the regions form opposed capacitors that can measure acceleration, or at least components thereof, along axes passing through the respective opposed pairs. In addition, opposed pairs of sets or groups of the regions can be used to measure acceleration, or at least components thereof, along axes passing through the respective opposed pairs of groups of the regions. For example, in embodiments, using combinations of the eight electrically conductive regions as seen in  FIGS. 5 and 6  allows measurement of acceleration in space by, for example, measuring components of the acceleration along the three mutually perpendicular Cartesian axes. Measurement of the component of acceleration along the y-axis as labeled in  FIG. 6  can be achieved by comparing the total capacitance of a first set of regions  410  and  450  to the total capacitance of a second set of regions  430  and  470 , along the x-axis as labeled in  FIG. 6  by comparing the total capacitance of a third set of regions  420  and  460  to the total capacitance of a fourth set of regions  440  and  480 , and along the z-axis by comparing the total capacitance of a fifth set of regions  410 - 440  to the total capacitance of a sixth set of regions  450 - 480 , for example. Other sets and orientations can be used in embodiments to provide similar results. 
     In an embodiment, as with the single electrically conductive region per hemispherical surface embodiment discussed above, the electrically conductive regions can be smaller than the respective portions of the surface to which they are applied as long as the center of each electrically conductive region coincides with the surficial center of the respective portion to which it is applied. In addition, the electrically conductive regions should all be the same size in embodiments. 
     A more general principle according to embodiments is that opposed pairs of capacitors formed by opposed electrically conductive regions or opposed symmetric sets or groups thereof and the proof mass can measure accelerations along an axis passing through the centers of the opposed electrical portions and the center of the proof mass in its initial position. Thus, embodiments are not limited to one or four electrically conductive regions per hemispherical surface as described above, but can have two, three, five, or more electrically conductive regions as appropriate for a given situation. Current manufacturing practices render some embodiments too difficult and/or costly to employ, but it is expected that refinements and improvements in manufacturing will enable easier and less costly manufacture of many, if not all, embodiments that might be useful. 
     For example, as seen in  FIGS. 7 and 8 , an accelerometer that can measure acceleration in a plane is shown according to an embodiment. Each hemispherical surface  11 ,  12  has two electrically conductive regions  510 ,  520 ,  530 ,  540  electrically isolated from each other. As above, the electrical isolation is achieved by simply leaving portions of the surfaces uncoated to form the desired borders between the regions. Each region effectively forms a respective capacitor with the spaced-apart proof mass  20 , and opposed pairs  510 / 540 ,  520 / 530  of the regions can measure components of acceleration along axes passing through the respective opposed pairs simply by comparing the capacitance of one region of a pair to that or the other region of a pair. Measurement of acceleration in the x-z plane formed by the x- and z-axes shown in FIGS.  3  and  5 - 9  can be achieved if the accelerometer, or at least the hemispherical surfaces, are properly oriented. Thus, measurement in the x-z plane can be achieved by comparing the total capacitance of the top regions  510 ,  520  to the total capacitance of the bottom regions  530 ,  540  for the z-axis, and comparing the total capacitance of the regions of one side  510 ,  530  with the total capacitance of the other side  520 ,  540  for the x-axis. 
     As seen in  FIGS. 9 and 10 , an alternate three axis accelerometer according to an embodiment employs six electrically conductive regions  610 ,  620 ,  630 ,  640 ,  650 ,  660  electrically isolated from each other, three in each hemispherical surface  11 ,  12 . Again, the electrical isolation can be achieved by leaving portions of the surfaces uncoated to form borders between the regions. The hemispheres  11 ,  12  have substantially identical arrangements of their electrically conductive regions, but one is rotated 180 degrees relative to the other in the x-y plane to place the regions  610 ,  620 ,  630  of the upper hemisphere  11  opposite the proof mass  20  from corresponding regions  640 ,  650 ,  660  of the lower hemisphere  12 . Each region effectively forms a respective capacitor with the spaced-apart proof mass  20 , and opposed pairs  610 / 660 ,  620 / 650 , and  630 / 640  of the regions can measure acceleration, or at least components thereof, along axes passing through the centers of the regions of respective opposed pairs. Using three opposed pairs as seen in  FIGS. 9 and 10  allows measurement along three axes so that components of acceleration along the three Cartesian mutually perpendicular axes can be measured with proper orientation of the accelerometer  10  and/or appropriate correction factors applied in conditioning circuitry and/or processing hardware and/or software. 
     As with the embodiments discussed above employing one and four electrically conductive regions per hemispherical surface, the electrically conductive regions of embodiments can be smaller than the respective portion of the surface to which they are applied as long as the center of each electrically conductive region coincides with the surficial center of the respective portion to which it is applied. In addition, the electrically conductive regions should all be the same size in embodiments. 
     In embodiments, as discussed above, the proof mass and opposed pairs of capacitors or opposed symmetric sets or groups of capacitors are connected to conditioning circuitry. The conditioning circuitry monitors capacitance between each capacitor or group of capacitors and the proof mass and provides a respective signal, such as a voltage, indicative of a differential capacitance between opposed pairs or opposed symmetric sets or groups of capacitors. Each signal is an indication of a magnitude and/or direction of acceleration experienced by the accelerometer as a function of a degree of change of the monitored capacitances from an initial value. 
     Thus, as seen in  FIG. 11 , a single axis measurement of acceleration can be achieved by connecting a first capacitance  1110  comprising a first capacitor or set of capacitors and a second capacitance  1120  comprising a second capacitor or set of capacitors  1120  to conditioning circuitry  1130 . The conditioning circuitry  1130  provides a signal  1140  indicative of a differential capacitance between the first and second capacitances  1110 ,  1120 . The signal  1140  is indicative of a magnitude and/or direction of acceleration experienced by the accelerometer as a function of a degree of change of the first and second capacitances  1110 ,  1120  from initial values. For example, as seen in Table 1, the conditioning circuitry  1130  could be applied to: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 1 st  capacitance 
                   
               
               
                   
                 1110 formed by proof 
                 2 nd  capacitance 1120 formed by 
               
               
                 Embodiment 
                 mass 20 and electrically 
                 proof mass 20 and electrically 
               
               
                 of FIGS.: 
                 conductive regions: 
                 conductive regions: 
               
               
                   
               
             
             
               
                 3 and 4 
                 310 
                 320 
               
               
                 5 and 6 
                 410-440 
                 450-480 
               
               
                 5 and 6 
                 410, 450 
                 430, 470 
               
               
                 5 and 6 
                 420, 460 
                 440, 480 
               
               
                 5 and 6 
                 410, 440, 450, 480 
                 420, 430, 460, 470 
               
               
                 5 and 6 
                 410, 420, 450, 460 
                 430, 440, 470, 480 
               
               
                 7 and 8 
                 510, 520 
                 530, 540 
               
               
                 7 and 8 
                 510, 530 
                 520, 540 
               
               
                 9 and 10 
                 610 
                 660 
               
               
                 9 and 10 
                 620 
                 650 
               
               
                 9 and 10 
                 630 
                 640 
               
               
                   
               
             
          
         
       
     
     The sets in each example can be switched between the first and second capacitances  1110 ,  1120  as appropriate for a particular situation in an embodiment. 
     Similarly, as seen in  FIG. 12 , a two-axis measurement of acceleration can be achieved by connecting first, second, third, and fourth capacitances  1210 - 1240  to conditioning circuitry  1250 . As above, each capacitance can be a single capacitor or a set of capacitors. The conditioning circuitry  1250  provides a first signal  1260  indicative of the differential capacitance between the first and second capacitances  1210 ,  1220  and a second signal  1270  indicative of a differential capacitance between the third and fourth capacitances. For example, as seen in Table 2, the conditioning circuitry  1250  could be applied to: 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 1 st /3 rd   
                   
               
               
                   
                 capacitance 1210/1230 
               
               
                   
                 formed by proof 
                 2 nd /4 th  capacitance 1220/1240 
               
               
                 Embodiment 
                 mass 20 and electrically 
                 formed by proof mass 20 and 
               
               
                 of FIGS.: 
                 conductive regions: 
                 electrically conductive regions: 
               
               
                   
               
             
             
               
                 5 and 6 
                 410-440 
                 450-480 
               
               
                 5 and 6 
                 410, 450 
                 430, 470 
               
               
                 5 and 6 
                 420, 460 
                 440, 480 
               
               
                 5 and 6 
                 410, 440, 450, 480 
                 420, 430, 460, 470 
               
               
                 5 and 6 
                 410, 420, 450, 460 
                 430, 440, 470, 480 
               
               
                 7 and 8 
                 510, 520 
                 530, 540 
               
               
                 7 and 8 
                 510, 530 
                 520, 540 
               
               
                 9 and 10 
                 610 
                 660 
               
               
                 9 and 10 
                 620 
                 650 
               
               
                 9 and 10 
                 630 
                 640 
               
               
                   
               
             
          
         
       
     
     The sets in each example can be switched between the first/third capacitance  1210 / 1230  and second/fourth capacitances  1220 / 1240  as appropriate for a particular situation in an embodiment. 
     Similarly, as seen in  FIG. 13 , a three-axis measurement of acceleration can be achieved by connecting first, second, third, fourth, fifth, and sixth capacitances  1310 - 1360  to conditioning circuitry  1370 . As above, each capacitance can be a single capacitor or a set of capacitors. The conditioning circuitry  1370  provides a first signal  1380  indicative of the differential capacitance between the first and second capacitances  1310 ,  1320  and a second signal  1385  indicative of a differential capacitance between the third and fourth capacitances  1330 ,  1340 , and a third signal  1390  indicative of a differential capacitance between the fifth and sixth capacitances  1350 ,  1360 . For example, as seen in Table 3, the conditioning circuitry  1370  could be applied to: 
     
       
         
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 1 st /3 rd /5 th  capacitance 
                   
               
               
                   
                 1310/1330/1350 
                 2 nd /4 th /6 th  capacitance 
               
               
                   
                 formed by proof 
                 1320/1340/1360 formed by 
               
               
                 Embodiment 
                 mass 20 and electrically 
                 proof mass 20 and electrically 
               
               
                 of FIGS.: 
                 conductive regions: 
                 conductive regions: 
               
               
                   
               
             
             
               
                 5 and 6 
                 410-440 
                 450-480 
               
               
                 5 and 6 
                 410, 450 
                 430, 470 
               
               
                 5 and 6 
                 420, 460 
                 440, 480 
               
               
                 5 and 6 
                 410, 440, 450, 480 
                 420, 430, 460, 470 
               
               
                 5 and 6 
                 410, 420, 450, 460 
                 430, 440, 470, 480 
               
               
                 9 and 10 
                 610 
                 660 
               
               
                 9 and 10 
                 620 
                 650 
               
               
                 9 and 10 
                 630 
                 640 
               
               
                   
               
             
          
         
       
     
     The sets in each example can be switched between the first/third/fifth capacitance  1310 / 1330 / 1350  and second/fourth/sixth capacitances  1320 / 1340 / 1360  as appropriate for a particular situation in an embodiment. 
     By applying embodiments as disclosed herein, a compact and robust accelerometer is provided. In particular, a multi-axis accelerometer that can withstand extreme temperatures, such as are present in a gas turbine, is provided. 
     While the instant disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope thereof In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.