Abstract:
A capacitive strain sensor for sensing strain of a structure. The sensor includes a first section attached to the structure at a first location and a second section attached to the structure at a second location. The first section includes a capacitor plate electrically isolated from the structure and the second section includes two electrically isolated capacitive plates, both of the plates being electrically isolated from the structure. A flexible connector connects the first section to the second section. The capacitor plate of the first section is separated from the two capacitive plates of the second section by at least one capacitive gap. When strain is experienced by the structure, a change occurs in the capacitive gap due to relative motion between the first and second sections.

Description:
COPENDING APPLICATIONS 
     U.S. patent application Ser. Nos. 12/839,061 and 12/839,170 filed Jul. 19, 2010, which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Aircraft landing gear, amongst other support devices, can experience strains that might lead to catastrophic failure. During landing, a landing gear strut can be deformed due to strain in different directions. Subjective determination of when landing gear should be inspected or replaced may be over- or under-reported, leading to unnecessary inspections or a missed opportunity to inspect. 
     Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length (L):
 
Strain( e )=(Δ L )/ L  
 
     For a polysilicon piezoresistive type of element, the resistance is changed with load applied. When a strain is introduced, the strain sensitivity, which is also called the gauge factor (GF), is given by: 
     
       
         
           
             GF 
             = 
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                   
                   R 
                 
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     L 
                   
                   L 
                 
               
               = 
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                   
                   R 
                 
                 Strain 
               
             
           
         
       
     
     The most popular strain gauges are metal foil elements on polyimide film. Piezoresistive strain gauges have been developed that offer 100× improvement in gauge factor over metal foil elements. These are generally fashioned in the form of a Wheatstone bridge. The ideal strain gauge would change resistance only due to the deformations of the surface to which the gauge is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gauge to the surface, and the stability of the metal all affect the detected resistance. Furthermore, the sensing range of usual strain gauges is limited by maximum stress allowed by the sensing element. For example, the maximum strain limitation of both types of strain gauge and for silicon strain gauges is 3,000 micro-strain.  FIG. 1  shows fatigue limits on foil gauges. Even at 3,000 micro-strain they will start to shift at less than 10,000 cycles. 
     SUMMARY OF THE INVENTION 
     The present invention provides a capacitive strain sensor for sensing strain of a structure. The sensor includes a first section attached to the structure at a first location, a second section attached to the structure at a second location, and a component configured to flexibly connect the first section to the second section. The first section includes a capacitor plate electronically isolated from the structure and the second section includes a capacitor plate electronically isolated from the structure. The capacitor plate of the first section is separated from the capacitor plate of the second section by a capacitive gap. A normal vector to a surface of the capacitive plates is approximately parallel to a longitudinal axis of the structure. 
     In one aspect of the invention, the sensor includes a device that flexibly connects the first and second sections, wherein strain experienced by the structure causes a change in the capacitive gap due to relative motion between the first and second sections. 
     In another aspect of the invention, the first section, the second section, and the device provide a hermetic seal of the capacitive plates. 
     In still another aspect of the invention, the sensor includes a cavity located in at least one of the first or second sections, at least one circuit component located in the cavity, and electrical leads that connect the at least one circuit component to the capacitive plates. 
     In yet another aspect of the invention, the sensor comprises a means for adjusting the gap. 
     These and other examples of the invention will be described in further detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative examples of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is graph of prior art results; 
         FIG. 2  is a perspective view of a landing gear assembly with sensors formed in accordance with the present invention; 
         FIG. 3  is a side view of an exemplary sensor formed in accordance with the present invention; 
         FIG. 4  is a cross-sectional view of the sensor of  FIG. 2 ; 
         FIGS. 5-1  through  6 - 3  illustrate various attachment mechanisms for attaching the sensor to a structure; 
         FIG. 7  is a perspective cross-sectional view of an sensor with an adjustment device formed in accordance with the present invention; and 
         FIG. 8  is an exploded cross-sectional view of the threaded components of the adjustment device shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a capacitance-based strain sensor for application on structures, such as aircraft landing gear—see  FIG. 2 . An exemplary sensor  26  can be connected at various locations on a landing gear assembly  20 , such as a strut piston  22  or a torque linkage  24 . 
     As shown in  FIGS. 3 and 4 , the sensor (or capsule)  26  includes upper and lower end caps  30 ,  32 . The end caps  30 ,  32  are attached to a structure (e.g., external or internal to the strut piston  22  of the landing gear assembly  20 ) where strains are to be measured. Strains from the structure are transferred as motion or displacement to upper and lower segments  52 ,  42 , respectively, located inside the end caps  30 ,  32 , thereby increasing or decreasing the spacing within an internal capacitance structure (described below). 
     The sensor  26  includes a first capacitor plate  54 , which is a metalized surface on the upper segment  52 , which in turn is secured within the upper end cap  30 . The upper segment  52  may be any of a number of insulating materials such as ceramic, glass, plastic, or other such materials. The preferred material may dictate the means of joining to the upper segment  52  or some intermediate element which facilitates the joining. In the present case, the upper segment  52  is a ceramic and is secured to an intermediate metal ring  52 A by means of metallization and brazing. The intermediate metal ring  52 A is secured to an upper capsule element  52 B which, as described later, forms a portion of a capacitive capsule subassembly. The upper capsule element  52 B is secured to the upper end cap  30  by welding, brazing, adhesive bonding, or other metal to metal joining means well known in the art. The first capacitor plate  54  is electrically isolated from the upper end cap  30  by means of the insulating nature of the upper segment  52 . The sensor  26  includes a second capacitor plate  44 , which is a metalized surface on the lower segment  42 . The second capacitor plate  44  is electrically isolated from the lower end cap  32  by a lower segment  42  which may be made of any of a number of insulating materials such as ceramic, glass, plastic, or other such materials. The preferred material may dictate the means of joining to the lower segment  42  or some intermediate element which facilitates the joining. In the present case, the lower segment  42  is a ceramic and is secured to a base plate  40  by means of adhesive bonding. The base plate  40  is secured within the lower end cap  32 . The lower end cap  32  and the base plate  40  are bonded and sealed with a weld, an epoxy bond, or other adhesive means. In one embodiment, the seals between caps and segments are hermetic seals. 
     The in-board ends of the end caps  30 ,  32  are connected to a metal bellows  34 . In one embodiment, an upper capsule element  52 B forms a subassembly with the upper segment  52  and the intermediate ring  52 A and the bellows  34 . Further, the lower plate  40  is connected to the lower segment  42  and to the bellows  34 . All parts  30 ,  52 B,  52 A,  52 ,  34 ,  42 , and  40  are joined in the nature of a capacitive capsule subassembly. This capsule may be welded to end caps  30  and  32  as a final assembly. The metal bellows  34  allows substantial deflection in the axial direction as the upper and lower end caps  30 ,  32  move. The axial direction is parallel to the direction of the measured strain. The axial direction of the assembly is vertical as shown in the illustration in  FIG. 4 . 
     The bellows  34  can be of any compliant material which will permit the necessary extension between the end caps and provide for proper bonding and sealing to the capsule elements  40  and  52 B or to the end caps  30  and  32 . Such materials for the bellows  34  may be metal, plastic, rubber, etc. Such bonding may be by means of welding, brazing, adhesive bonding or other methods suitable to the materials of choice. 
     The upper end cap  30  includes a cavity  48  for receiving various electronic components, such as a discrete circuit board assembly or an application-specific integrated circuit (ASIC)  50 . The cavity  48  may include other components, such as a battery, radio telemetry module (i.e., wireless transmitter), and/or antenna. In one embodiment, the cavity  48  in the upper end cap  30  is sealed from the environment by a cover that is bonded (e.g., welded, epoxied) to an upper surface of the upper end cap  30 . 
       FIGS. 5-1  and  5 - 2  show one method for securing and aligning a strain sensor (not shown) to a structure  84  to be measured for strain. A mounting plate/flange  82  that is directly attached to or integral with one of the end caps includes two or more through-holes with a tapered section  94  at one end of each through-hole. The structure  84  includes two pin-receiving cavities  96 , each having a tapered section  98  flush with the surface of the structure  84 . The pin-receiving cavities  96  line up with the through-holes in the mounting plate/flange  82 . A bolt/pin  86  is received through a washer  88 , the through-hole, then through an eccentric tapered bushing  90  and finally a pin-receiving cavity  96 . The tapered dowels  90  have an approximate 15° taper on both sides, matching the taper in the respective sections  94 ,  98 . The two sides of each tapered dowel  90  are eccentric to each other, equivalent to the possible maximum mismatch in tolerances in machining the mating parts. When inserted in the receiving cavities  94  and  98 , the bushing  90  can be rotated changing the effective center distance of the bushings relative to each other. As the adjusting bushing cams over due to the eccentricity to the two tapers, the receiving holes will settle on the new centers and be in perfect alignment with the bushings. Further, the eccentric tapers are slightly larger in diameter than the tapers in the receiving cavities. This oversized condition generates an effective press fit when the fasteners are torqued into place. One then has two dowel type bushings that are in perfect alignment as well as press fit into the receiving cavities providing a secure locking means to the structure  84 . This secure locking means will transfer any motion perfectly to the mounted plate/flange. 
     Each of the dowels  86  has an Allen hex in the center. This allows rotation of the dowel during installation, providing for the self-centering of the system. 
       FIGS. 6-1  through  6 - 3  show bonded mounting pads  102 ,  104 , used to position the sensor  26  onto a surface, such as the torque linkage  24 . In the example shown, each end cap requires two bushings to maintain squareness of the two ends. In this case, at least one eccentric bushing is required to allow for center distance tolerance mismatch, as shown above. The eccentric bushings are inserted in the receiving cavities. 
     The two mounting pads  102  and  104  with tapered receiving holes are provided. The two mounting pads  102  and  104  are bonded to the torque link  24  or any substrate with an adhesive. To align the mounting pads relative to each other, first a single locating pad  100  is fastened to the mounting pads  102  and  104 . This combined mounting pad assembly is brought into position on the torque linkage  24  or some other substrate. Then, an adhesive is applied between the mounting pads  102  and  104  and the torque link  24 . Once the adhesive is cured, the locating pad  100  is removed and the mounting pads  102  and  104  are ready to receive the strain sensor. The mounting pads  102  and  104  allow the sensor  26  to be properly aligned with the torque link  24 . As shown in  FIG. 6-1 , an exemplary locating pad  100  is trapezoid-shaped to fit in place within the geometry of the torque link  24  or may be aligned with any feature on any substrate. Bolts secure flanges of the upper and lower end caps  30 ,  32  to the surface through the machined holes via the mounting pad holes and tapered bushings (see  FIG. 6-3 ). 
     As shown in  FIGS. 7 and 8 , a microadjustment device  120  allows for very small adjustment of a rotationally fixed shaft  142  in a capacitive sensor, such as the sensor  26  shown in  FIGS. 3 and 4 . Assembly of the sensor may not be as precise as needed for certain applications demanding a fine adjustment be performed after assembly. Similarly in the field, positioning as described in above may not be as precise as is required and may demand similar fine adjustment. Further, such adjustment may be advantageous for recalibrating the sensor should the mounting shift or other conditions change requiring such adjustment. The microadjustment device  120  is used when it is desirable to calibrate in magnitudes of about 0.001 inch travel per one rotation or 0.0001 inch travel or any resolution possible depending on the ratio of thread pitches and thread clearances. 
     The microadjustment device  120  includes a fixed housing  132  (i.e., lower end cap  32 ), a double-threaded nut  140 , and a rotationally fixed shaft  142 . The housing  132 , similar to the lower end cap  32 , is attached to a structure to be measured. The shaft  142  is affixed to a bellows  134  that is attached to an upper capsule element  52 B. The upper capsule element  52 B and shaft  142  support the metallic plates  44  and  54  some distance apart, thereby forming a capacitor. 
     The housing  132  and the outside surface of the double-threaded nut  140  are threaded with the same threads per inch T o . The shaft  142  and the inside of the double-threaded nut  140  are threaded with the same threads per inch T i  where both sets of threads are in the same direction and where T o &lt;T i . So, as the nut  140  is rotated in one direction at 1/T o  inches per revolution, the shaft  142  moves in the other direction at 1/T i  inches per revolution U. Therefore, the actual travel D of the shaft per revolution is the difference of 1/T o  and 1/T i . 
     Example: (where T o =44 and T i =46) 
     
       
         
           
             D 
             = 
             
               
                 
                   1 
                   
                     T 
                     o 
                   
                 
                 - 
                 
                   1 
                   
                     T 
                     i 
                   
                 
               
               = 
               
                 
                   
                     1 
                     44 
                   
                   - 
                   
                     1 
                     46 
                   
                 
                 = 
                 0.001 
               
             
           
         
       
     
     The nut  140  includes a device for restricting its rotational movement after the adjustment has been made. This may be accomplished either permanently, by using a chemical thread lock, or temporarily, by using a nylon plug or set screw  144  on the nut  140 , or other mechanical motion-restricting device. The set screw is threaded into the lower end cap  132 . 
     In another embodiment, the invention is used in a measurement device (not shown) such as a micrometer. The benefits would be a stationary spindle. Resolution of the measurement device could be increased to as fine as one millionth of an inch by incorporating a Vernier scale or digital output as is common in conventional micrometers and by using custom threads. 
     In another embodiment, the nut  140  and other components are made of Invar® or other comparable materials to reduce thermal effects. 
     In another embodiment, a macroadjustment device is created by making the threads on the outside of the nut  140  in the opposite direction from the threads on the inside of the nut  140 . For example, the housing threads are right-hand twist and the shaft threads are left-hand twist causing the shaft to move in the same direction as the nut. This invention can be used in any application that needs adjustments of this magnitude or where two parts (shaft and housing) need to be in a fixed orientation relative to one another, other than the direction of adjustment. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.