PATENT ABSTRACT
A strain sensor device for measuring loads on aircraft landing gear. This is done by measuring strains in the lower end of the strut, by which we infer the loading in the entire landing gear structure. These strains can be very large (as high as 10,000 microstrain) and can be imposed in numerous random directions and levels. The present invention includes a removable sensor assembly. An electromechanical means is presented that can accommodate large strains, be firmly attached to the strut, and provide good accuracy and resolution.

PATENT DESCRIPTION
COPENDING APPLICATIONS 
     U.S. patent application Ser. Nos. 12/839,061 and 12/839,401 filed Jul. 19, 2010, which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Aircraft landing gear, among 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-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 silicon 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 capability of the sensing element. For example, the maximum strain limitation of these types of strain gauges is 3,000 microstrain.  FIG. 1  shows fatigue limits on foil gauges. Even at 3,000 microstrain they will start to shift at less than 10,000 cycles. High strength steels can exceed 6,000 microstrain. 
     SUMMARY OF THE INVENTION 
     The present invention provides a monitor for measuring loads on aircraft landing gear. This is done by measuring strains in the lower end of the strut, from which we infer the loading in the entire landing gear structure. These strains can be very large (as high as 10,000 microstrain) and can be imposed in numerous random directions and levels. The present invention includes a removable sensor assembly. An electromechanical means is presented that can accommodate large strains, be firmly attached to the strut, and provide good accuracy and resolution. 
    
    
     
       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 a graph showing results of some prior art; 
         FIG. 2  is a perspective view of a landing gear assembly with a strain sensor device formed in accordance with the present invention; 
         FIG. 3  is a perspective view of an exemplary strain sensor device; 
         FIG. 4  is a partial perspective view of the device shown in  FIG. 3 ; 
         FIG. 5-1  is perspective cutaway view of the device of  FIG. 3  attached to a landing gear assembly; 
         FIG. 5-2  illustrates an alternate embodiment for a ring/spline; 
         FIG. 6  is a perspective view of a strain sensor device formed in accordance with an alternate embodiment of the present invention; 
         FIG. 7  is a partial view of the device of  FIG. 6 ; 
         FIG. 8  is a partial perspective view of a strain sensor device formed in accordance with an alternate embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of the device of  FIG. 8 ; 
         FIG. 10  is a side view of an alternate sensor included in the device of  FIG. 3 ,  7 , or  9 ; 
         FIG. 11  is a schematic diagram showing exemplary sensor distribution for any of the devices shown in the previous FIGURES; and 
         FIGS. 12-1  and  12 - 2  illustrate an exemplary system for determining force imbalances using one of the sensor devices of the other FIGURES. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is a perspective view of a landing gear assembly  20  that includes a strut piston  22 . A sensor ring assembly  24  is mounted to the strut piston  22 . The sensor ring assembly  24  includes one or more sensors that provide signals of stress or strain experienced by the strut piston  22 . Exemplary sensors are described in copending U.S. patent application Ser. Nos. 12/839,061 and 12/839,401 filed Jul. 19, 2010. 
     As shown in  FIGS. 3-5 , an exemplary sensor ring assembly  24 - 1  includes two independently mounted rings  30 ,  32  that are secured to respective guide/anchor rings/splines  42 . Each of the two rings  30 ,  32  includes a pair of joined C sections (halves). The joined C sections form a complete ring encircling the landing gear strut piston  22 . The anchor rings/splines  42  may be an integral feature (spline) machined into the structure (the strut piston  22 ) or attached as separate bonded rings where close diametral tolerances are held. In one embodiment, the anchor rings/splines  42  include a raised feature (mesa) that engages the rings  30 ,  32 . The rings  30 ,  32  include a matching negative feature (groove), which mates securely with the raised feature. The rings  30 ,  32  include a groove matching the ring/spline  42  where the flanks of the groove and the ring/spline  42  have matching angles. Angled contact allows a wedging effect where minimal clamping load will be translated into higher anchoring forces. The anchor rings  42  are adhesively bonded to the lowest portion of the strut piston  22  where close diametral tolerances are held. 
     Ends of the C sections of each of the rings  30 ,  32  include flanges. The flanges are machined to allow a predefined clearance, thus allowing the bolts to draw the rings  30 ,  32  tight to the anchor ring/spline  42  with a specific torque that defines the force of engagement. The matching angle may be 30 degrees; other angles may be used, depending on the application. In another embodiment, a separate ring (not shown) is fabricated and bonded to the strut piston  22  according to a locating fixture to guarantee proper spacing. 
     In a further embodiment, the spline or ring  50  may be segmented as shown in  FIG. 5-2  to allow for local engagement of individual sensors. 
     The sensor ring groove engages the segments in the same way as the complete spline described above. The segmentation is intended to localize the displacement input to the individual sensors. Isolation between the sensors provides accurate sensing of local strain under the sensor. The flexures in the sensor rings are intended for the same purpose. The wedging action into the groove is the same as the solid ring. 
     Each ring  30 ,  32  (upper and lower) secures one or the other end of one or more capacitive capsule sensors  40 . The capsule sensors  40  are secured at one end to the upper ring  30  and at another end to the lower ring  32 . Top and bottom ends of the capsule sensors  40  are connected to each other through a welded metal bellows, thereby allowing substantial deflection as the upper and lower rings  30 ,  32  move with respect to one another. Exemplary sensors  40  are described in copending U.S. patent application Ser. Nos. 12/839,061 and 12/839,401, which are hereby incorporated by reference. 
     The capsule sensor  40  is welded to the upper and lower rings  30 ,  32  or is machined from the same material (i.e., integral) as the rings  30 ,  32 . Other attachment means may be used. 
     Landing gear struts are hydraulically damped as a shock absorber. In some landing gear struts the hydraulic fluid can extend into the region where the sensor rings are mounted. On heavy loading, the hydraulic pressure can generate large radial deformations. Preliminary modeling indicates that at least one such strut piston  22  sees radial swelling of several thousandths of an inch due to the hydraulic pressure in addition to the axial strains due to axial loading. These significant radial strains result in large Poisson strains in the axial direction. These Poisson strains will be “compressive” in the sense that the piston becomes shorter by 30% of the radial strain. This, plus the large axial load on the end of the strut piston  22  itself, results in a compounded contraction of the space between the two rings  30 ,  32 . 
     The other strains induced in the system are bending strains. Bending of the strut piston  22  will induce compressive and tensile strains in the sides opposite each other. Hence, fore and aft sensors  40  (relative to the strain) in the assembly  24 - 1  will see a differential strain (expanding and contracting) due to fore and aft bending. Similarly, port and starboard sensors  40  (relative to the strain) will see differential strains due to sideways loading of the landing gear  20 . The induced Poisson strains, as well as any imposed axial loading, affect all sensors identically (same sign). This axial strain becomes a common mode effect. This allows mathematical discrimination of sensor output due to axial strains and bending strains. 
     A rigid ring may be overstressed by the radial expansion of the strut. Also, there may be non-uniform strains around the strut which a rigid ring will not discriminate. For this reason a flexible member is introduced into the ring structure. 
     The radial strains are accommodated by serpentine flexures  34 . The flexures  34  are stiff enough so that the rings  30 ,  32  remain seated on the strut piston  22  under high-G loads and shock loads but remain within the yield strength of the ring material. In other words, the serpentine flexures  34  allow radial expansion yet provide good stiffness in the other load directions. In one embodiment, the flexures  34  are fabricated using wire electric discharge machining (EDM), laser cutting, or waterjet cutting. 
     Stresses and strains are never totally uniform in a complex structure such as strut  22 . If each sensor were rigidly connected to the next, the differences in strain would be eliminated due to the stiffness of the ring assembly  32 . In the case of the segmented spline/ring as described in paragraph 0021, the anchoring feature would only engage the ring groove structure under each of the sensors allowing the intermediate segments between the flexures to move independently, minimizing crosstalk between the sensors. The serpentine flexures  34  provide 95% attenuation of crosstalk. 
     The serpentine flexures  34  include cutouts that have a large radius at the bottom of each cut, thereby reducing stress concentrations in the ring structure. The rings  30 ,  32  are fabricated from a corrosion-resistant and heat-treatable material, such as 17-4 or 15-5 PH stainless steel or any material with high strength and adequate corrosion resistance. 
       FIGS. 6 and 7  show an alternate ring assembly  24 - 2  that includes ring components  66  that include one or more cavities  72  in which a capsule sensor(s) (not shown) resides. Wires (not shown) are received within channels  74  in the ring components  66 . The wires are routed to an electronics bay  78  located at an end of one of the ring components  66 . The channels  74  and the cavities  72  are (hermetically) sealed by a cover  64  that is welded to the ring components  66 . The cover  64  environmentally protects the channels  74  and cavities  72  in the ring components  66 . The electronics bay  78  includes electronics necessary for conditioning signals from the capsule sensors. Such electronics may also include batteries, radio telemetry modules, and antennae for wireless communications. 
     The ring components  66  include flanges  80  that allow reception of securing devices, such as bolts, for connecting to another ring component  66  around the strut piston  22 . An inner radius of the ring components  66  includes machined flexible cantilevered fingers  70 . The radial strains are accommodated by the flexible cantilevered fingers  70  shown in  FIG. 7 . The fingers  70  are stiff enough so that the ring components  66  and the cover  64  remain seated on the strut piston  22  under high G loads and shock loads but remain within the yield strength of the ring material. 
       FIG. 8  shows a capacitive strut ring sensor  120  formed in accordance with an alternate embodiment for measuring overload/hard landing of an aircraft. The capacitive strut sensor  120  includes two separated rings  124 ,  126  that can be attached to the bottom of the strut piston  22 . Each ring  124 ,  126  has two C sections. Multiple capacitive plates  130 ,  132  (any number may be used) are placed around each ring  124 ,  126 . The capacitive plates  130  in the lower ring  124  are radially aligned with capacitive plates  132  in the upper ring  126 . 
     Under strain, the capacitance change of one side of the sensor  120  may be different from the capacitance change on the radially opposite side of the sensor  120 . The differential output of the sensed capacitive changes provides information on the relative angular displacement of the two rings which result from differential strains on opposites sides of the strut  22 . As in the previous embodiments, axial displacements of the strut will result in a common mode change in capacitance in all the capacitor plate pairs. 
     In one embodiment, the capacitive plates  130 ,  132  are uniformly distributed on an insulator plate  136  that may be integral to the ring body (e.g. sputter deposited on an insulating layer) or may be a separate planar element secured (e.g., epoxied) to the respective ring  124 ,  126 . The capacitive plates  130 ,  132  are separated within a respective ring  124 ,  126  by a circuit component  138 . The circuit component  138  is electrically connected to one or more of the capacitive plates  130 ,  132  and to an external communication component  139  via wires or electrical traces (not shown). The external communication component  139  is mounted to an exterior surface of one of the rings  124 ,  126 . The external communication component  139  includes a wireless transmitter for sending and/or receiving signals and may include a device similar to those included in the electronics bay  78 . 
     The rings  124 ,  126  are attached to the strut piston  22  by an adhesive or by a clamshell mounting device, such as that described in  FIG. 5  above.  FIG. 9  shows a cross-sectional view of the sensor  120 . The distance between upper ring  126  and lower ring  124  is fixed. The upper ring  126  can be a piece of metal that acts as a ground plane. The readings from capacitive plates  130 ,  132  are relatively unaffected by a thermal mismatch between strut material and bonding material. Thermal deformation causes the capacitive plates  130 ,  132  to shift radially. This deformation should have minimum impact on capacitance value. The thermal expansion of the strut piston  22  in the axial direction will introduce a minor temperature coefficient of capacitance as the spacing between the rings  124 ,  126  changes with thermal expansion of the strut/piston material. This can be compensated by incorporating a temperature measurement device which provides a signal to the compensation circuitry. 
     The rings  124 ,  126  are connected with flexible seals  140 ,  142 . The seals  140 ,  142  allow the rings  124 ,  126  to move, while keeping particulates from contaminating the capacitive gap. 
     In one embodiment, each ring  124 ,  126  includes two halves (C sections) that are bolted (or other means of fastening) together, placing the rings  124 ,  126  in radial compression over the strut (not shown). In one embodiment, a spline (not shown) or other type of protrusion such as the spline  42  shown in  FIG. 5-1  are tangentially disposed around the strut. The spline mates with a matching cavity  137  for allowing positive anchoring of the rings  124 ,  126  to the strut. 
     Sensitivity of the sensor ring assemblies  24 - 1 ,  24 - 2  and  120  depends on the mounting distance (L) between the lower and upper rings  30 ,  32 ,  66 , or  124 ,  126  and the initial gap (d) between the capacitive plates  130 ,  132  or the plates held within 40. If the gap (d) between capacitive plates is large, rest capacitance will be lower and changes in spacing will have less effect on capacitance value. If the gap is small, rest capacitance will be larger and changes in spacing will cause large changes in capacitance. If L is large, there will be more movement between the plates  130 ,  132  due to a larger ratio between L and d. The larger the L, the more output one can get. In one embodiment, the capacitance value of each capacitor should be within 1 pF to 10 pF or 2 pF to 250 pF when a particular commercially available capacitance-to-voltage converter application specific integrated circuit (ASIC) is used. The capacitance range can be extended to any range if using a specially designed circuit. 
     For the capacitive sensors and electronics employed, sensing resolution can be better than one microstrain and full-scale input range can be 10,000 microstrain. Accuracy is usually at least 10 times the resolution, and the system accuracy (composite of all sensors in the system) may be estimated at about 50 microstrain or 100 microstrain providing a percent accuracy of 0.5 to 1%. 
     Unlike foils train gages which conventionally are not useful beyond 3,000 microstrain, this design can sense strains well beyond 3,000 microstrain without fatigue of the sensing element. Strains of up to 6,600 microstrain, the usual fatigue limit of metals, and up to 10,000 micro-strain, the maximum ultimate strength of high strength materials, can be measured. The current embodiment is displaced by 0.010″ at 10,000 microstrain. This displacement is dependent on the starting spacing of the splines. A 0.010″ displacement over a 1″ spline spacing is equivalent to 10,000 microstrain. A 0.5″ spline spacing would displace 0.005″ at 10,000 micro-strain and so forth. This tolerance of large strains is due to the flexible bellows in the sensor structure which are part of the overall ring assembly. 
     Because a typical landing gear does not take any torsion in the lower portion of the strut piston  22 , there may be other locations or other applications where torsional strain may be measured. However, there is interest in measuring side load on the strut which results in a lateral (port and starboard) shearing load on the piston. In order to measure side shear of the strut, either a sensor as disclosed in copending U.S. patent application Ser. No. 12/839,061 to lateral (perpendicular to the piston axis) is employed, as shown in  FIG. 10 . Shear loading is indicated by the arrow. A slot  150  is integrally cut into the upper ring  126  and capacitive electrode plate  156  is placed vertically on a protrusion that extends from the lower ring  124 . A second capacitive electrode  158  is mounted on a wall of the slot  150  opposite the plate  156 , thereby forming a capacitor. The lower ring  124  also includes a slot  152  for receiving a capacitive electrode  160  mounted on a protrusion from the upper ring  126 . A capacitive electrode  162  is mounted to the wall within the slot  152  that is opposite the capacitive electrode  160 . The capacitive electrodes  160 ,  162  are electrically isolated from the rings  126 ,  124  by being separately bonded to an insulating material or an insulating layer applied to the ring surfaces prior to metallization. Thus, two vertical capacitors are formed in the same axis on the each side of the strut. The capacitive electrodes  156 ,  160  are isolated from rings  126 ,  124 . The rings  126 ,  124  are common or case ground. Or, there may be two isolated plates independently bonded to other gaps. When a lateral load occurs, one capacitance value will increase and the other value will decrease; therefore, a differential capacitive output is obtained. In this way, the twist deformation of the strut piston  22  will be accurately detected. 
       FIG. 11  shows an example of a sensor layout for use in any of the embodiments described above. Four sensors  200  are placed at the center of each quadrant, relative to aircraft orientation and one sensor  200  either fore or aft which serves as an additional reference as needed. The four sensors  200  detect bending in the strut  210  as the opposite sides of the strut  210  go into compression or tension. By calculating the resulting vector direction and amplitude, one can get a reading of maximum strain regardless of sensor orientation to the load. Vertical load on the strut  210  causes a common mode compression or tension on all five sensors  200 . This can be mathematically calculated as well to get a reading of weight and balance. A shear sensor is placed in either the fore or aft position providing the lateral shear reading required. 
     An overall capacitive strut sensor measurement system is shown in  FIGS. 12-1  and  12 - 2 . The five strain capacitors and the one lateral capacitor are connected to respective capacitance-to-voltage converters  258  (such as MS  3110  or AD  7746  or ZMD or any other equivalent circuit).  FIG. 12-1  illustrates the various sensors in the system as an example of a serially connected system. The connections can also be parallel or be any convenient arrangement depending on proximity of the various sensors and the preferred cable routing. The block diagram  FIG. 12-2  indicates voltage outputs of the converters  258  are sent to a microcontroller  260 . The microcontroller  260  converts the analog signal into digital data. A software algorithm executed by the microcontroller  260  calculates the total load and imbalanced load, as well as twisting load. All of this load information is then sent to a receiving system which stores or further analyzes the data. An alternative embodiment may include a radio frequency (RF) chip  264  and an antenna and a remotely located data collector (RDC) which acts as the receiving system and storage device. 
     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.