Patent Description:
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): <MAT>.

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:
<MAT>.

The most popular strain gauges are metal foil elements on polyimide film. Piezoresistive silicon strain gauges have been developed that offer 100x 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 <NUM>,<NUM> microstrain. <FIG> shows fatigue limits on foil gauges. Even at <NUM>,<NUM> microstrain they will start to shift at less than <NUM>,<NUM> cycles. High strength steels can exceed <NUM>,<NUM> microstrain.

The present invention provides a device configured to mount to a strut piston as defined in claim <NUM>, the device comprising: a first ring comprising a first half, a second half and an inner wall, wherein the first ring is configured to mount to the strut piston by fastening the first and second half together thereby placing the first ring in radial compression over the strut piston; a second ring comprising third half, a fourth half and an inner wall, wherein the second ring is configured to mount to the strut piston by fastening the third and fourth half together thereby placing the second ring in radial compression over the strut piston a plurality of pairs of capacitive plates integral with the first and second rings respectively, the capacitive plates being mounted to a surface that is approximately perpendicular to the first and second inner walls, wherein the capacitive plates are electrically isolated from their respective rings.

One or more optional features of the device are also provided according to the dependent claims.

Preferred and alternative examples of the present invention are described in detail below with reference to the following drawings:.

<FIG> is a perspective view of a landing gear assembly <NUM> that includes a strut piston <NUM>. A sensor ring assembly <NUM> is mounted to the strut piston <NUM>. The sensor ring assembly <NUM> includes one or more sensors that provide signals of stress or strain experienced by the strut piston <NUM>.

As shown in <FIG>, an exemplary sensor ring assembly <NUM>-<NUM> includes two independently mounted rings <NUM>, <NUM> that are secured to respective guide/anchor rings/splines <NUM>. Each of the two rings <NUM>, <NUM> includes a pair of joined C sections (halves). The joined C sections form a complete ring encircling the landing gear strut piston <NUM>. The anchor rings/splines <NUM> may be an integral feature (spline) machined into the structure (the strut piston <NUM>) or attached as separate bonded rings where close diametral tolerances are held. In one embodiment, the anchor rings/splines <NUM> include a raised feature (mesa) that engages the rings <NUM>, <NUM>. The rings <NUM>, <NUM> include a matching negative feature (groove), which mates securely with the raised feature. The rings <NUM>, <NUM> include a groove matching the ring/spline <NUM> where the flanks of the groove and the ring/spline42 have matching angles. Angled contact allows a wedging effect where minimal clamping load will be translated into higher anchoring forces. The anchor rings <NUM> are adhesively bonded to the lowest portion of the strut piston <NUM> where close diametral tolerances are held.

Ends of the C sections of each of the rings <NUM>, <NUM> include flanges. The flanges are machined to allow a predefined clearance, thus allowing the bolts to draw the rings <NUM>, <NUM> tight to the anchor ring/spline <NUM> with a specific torque that defines the force of engagement. The matching angle may be <NUM> 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 <NUM> according to a locating fixture to guarantee proper spacing.

In a further embodiment, the spline or ring <NUM> may be segmented as shown in <FIG> 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 <NUM>, <NUM> (upper and lower) secures one or the other end of one or more capacitive capsule sensors <NUM>. The capsule sensors <NUM> are secured at one end to the upper ring <NUM> and at another end to the lower ring <NUM>. Top and bottom ends of the capsule sensors <NUM> are connected to each other through a welded metal bellows, thereby allowing substantial deflection as the upper and lower rings <NUM>, <NUM> move with respect to one another. Exemplary sensors <NUM> are described in copending.

The capsule sensor <NUM> is welded to the upper and lower rings <NUM>, <NUM> or is machined from the same material (i.e., integral) as the rings <NUM>, <NUM>. 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 <NUM> 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 <NUM>% of the radial strain. This, plus the large axial load on the end of the strut piston <NUM> itself, results in a compounded contraction of the space between the two rings <NUM>, <NUM>.

The other strains induced in the system are bending strains. Bending of the strut piston <NUM> will induce compressive and tensile strains in the sides opposite each other. Hence, fore and aft sensors <NUM> (relative to the strain) in the assembly <NUM>-<NUM> will see a differential strain (expanding and contracting) due to fore and aft bending. Similarly, port and starboard sensors <NUM> (relative to the strain) will see differential strains due to sideways loading of the landing gear <NUM>. 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 <NUM>. The flexures <NUM> are stiff enough so that the rings <NUM>, <NUM> remain seated on the strut piston <NUM> under high-G loads and shock loads but remain within the yield strength of the ring material. In other words, the serpentine flexures <NUM> allow radial expansion yet provide good stiffness in the other load directions. In one embodiment, the flexures <NUM> arc 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 <NUM>. If each sensor were rigidly connected to the next, the differences in strain would be eliminated due to the stiffness of the ring assembly <NUM>. In the case of the segmented spline/ring as described in paragraph <NUM>, 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 <NUM> provide <NUM>% attenuation of crosstalk.

The serpentine flexures <NUM> include cutouts that have a large radius at the bottom of each cut, thereby reducing stress concentrations in the ring structure. The rings <NUM>, <NUM> are fabricated from a corrosion-resistant and heat-treatable material, such as <NUM>^<NUM> or <NUM> PH stainless steel or any material with high strength and adequate corrosion resistance.

FIGURES <NUM> and <FIG> show an alternate ring assembly <NUM>-<NUM> that includes ring components <NUM> that include one or more cavities <NUM> in which a capsule sensor(s) (not shown) resides. Wires (not shown) are received within channels <NUM> in the ring components <NUM>. The wires are routed to an electronics bay <NUM> located at an end of one of the ring components <NUM>. The channels <NUM> and the cavities <NUM> arc (hermetically) sealed by a cover <NUM> that is welded to the ring components <NUM>. The cover <NUM> environmentally protects the channels <NUM> and cavities <NUM> in the ring components <NUM>. The electronics bay <NUM> 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 <NUM> include flanges <NUM> that allow reception of securing devices, such as bolts, for connecting to another ring component <NUM> around the strut piston <NUM>. An inner radius of the ring components <NUM> includes machined flexible cantilevered fingers <NUM>. The radial strains are accommodated by the flexible cantilevered fingers <NUM> shown in <FIG>. The fingers <NUM> are stiff enough so that the ring components <NUM> and the cover <NUM> remain seated on the strut piston <NUM> under high G loads and shock loads but remain within the yield strength of the ring material.

<FIG> shows a capacitive strut ring sensor <NUM> formed in accordance with an alternate embodiment for measuring overload/hard landing of an aircraft. The capacitive strut sensor <NUM> includes two separated rings <NUM>, <NUM> that can be attached to the bottom of the strut piston <NUM>. Each ring <NUM>, <NUM> has two C sections. Multiple capacitive plates <NUM>,<NUM> (any number may be used) are placed around each ring <NUM>, <NUM>. The capacitive plates <NUM> in the lower ring <NUM> are radially aligned with capacitive plates <NUM> in the upper ring <NUM>.

Under strain, the capacitance change of one side of the sensor <NUM> may be different from the capacitance change on the radially opposite side of the sensor <NUM>. 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 <NUM>. 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 <NUM>, <NUM> are uniformly distributed on an insulator plate <NUM> 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 <NUM>, <NUM>. The capacitive plates <NUM>,<NUM> are separated within a respective ring <NUM>, <NUM> by a circuit component <NUM>. The circuit component <NUM> is electrically connected to one or more of the capacitive plates <NUM>, <NUM> and to an external communication component <NUM> via wires or electrical traces (not shown). The external communication component <NUM> is mounted to an exterior surface of one of the rings <NUM>, <NUM>. The external communication component <NUM> includes a wireless transmitter for sending and/or receiving signals and may include a device similar to those included in the electronics bay <NUM>.

The rings <NUM>, <NUM> are attached to the strut piston <NUM> by an adhesive or by a clamshell mounting device, such as that described in <FIG> above. <FIG> shows a cross-sectional view of the sensor <NUM>. The distance between upper ring <NUM> and lower ring <NUM> is fixed. The upper ring <NUM> can be a piece of meta) that acts as a ground plane. The readings from capacitive plates <NUM>, <NUM> are relatively unaffected by a thermal mismatch between strut material and bonding material. Thermal deformation causes the capacitive plates <NUM>, <NUM> to shift radially. This deformation should have minimum impact on capacitance value. The thermal expansion of the strut piston <NUM> in the axial direction will introduce a minor temperature coefficient of capacitance as the spacing between the rings <NUM>, <NUM> 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 <NUM>, <NUM> are connected with flexible seals <NUM>, <NUM>. The seals <NUM>, <NUM> allow the rings <NUM>, <NUM> to move, while keeping particulates from contaminating the capacitive gap.

In one embodiment, each ring <NUM>, <NUM> includes two halves (C sections) that are bolted (or other means of fastening) together, placing the rings <NUM>, <NUM> in radial compression over the strut (not shown). In one embodiment, a spline (not shown) or other type of protrusion such as the spline <NUM> shown in <FIG> are tangentially disposed around the strut. The spline mates with a matching cavity <NUM> for allowing positive anchoring of the rings <NUM>, <NUM> to the strut.

Sensitivity of the sensor ring assemblies <NUM>-<NUM>, <NUM>-<NUM> and <NUM> depends on the mounting distance (L) between the lower and upper rings <NUM>, <NUM>, <NUM>, or <NUM>, <NUM> and the initial gap(d) between the capacitive plates <NUM>, <NUM> or the plates held within <NUM>. 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 <NUM>, <NUM> 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 <NUM> pF to <NUM> pF or <NUM> pF to <NUM> 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 <NUM>,<NUM> microstiain. Accuracy is usually at least <NUM> times the resolution, and the system accuracy (composite of all sensors in the system) may be estimated at about <NUM> microstrain or <NUM> microstrain providing a percent accuracy of <NUM> to <NUM> %.

Unlike foils train gages which conventionally are not useful beyond <NUM>,<NUM> microstrain, this design can sense strains well beyond <NUM>,<NUM> micrstrain without fatigue of the sensing element. Strains of up to <NUM>,<NUM> microstrain, the usual fatigue limit of metals, and up to <NUM>,<NUM> micro-strain, the maximum ultimate strength of high strength materials, can be measured. The current embodiment is displaced by. <NUM>" at <NUM>,<NUM> microstrain. This displacement is dependent on the starting spacing of the splines. <NUM>" displacement over a <NUM>" spline spacing is equivalent to <NUM>,<NUM> microstrain. A <NUM>" spline spacing would displace. <NUM>" at <NUM>,<NUM> 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 <NUM>, 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, a sensor as shown in FIGURE <NUM> is used. Shear loading is indicated by the arrow. A slot <NUM> is integrally cut into the upper ring <NUM> and capacitive electrode plate <NUM> is placed vertically on a protrusion that extends from the lower ring <NUM>. A second capacitive electrode <NUM> is mounted on a wall of the slot <NUM> opposite the plate <NUM>, thereby forming a capacitor. The lower ring <NUM> also includes a slot <NUM> for receiving a capacitive electrode <NUM> mounted on a protrusion from the upper ring <NUM>. A capacitive electrode <NUM> is mounted to the wall within the slot <NUM> that is opposite the capacitive electrode <NUM>. The capacitive electrodes <NUM>, <NUM> are electrically isolated from the rings <NUM>, <NUM> 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 <NUM>, <NUM> are isolated from rings <NUM>,<NUM>. The rings <NUM>,<NUM> 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 <NUM> will be accurately detected.

<FIG> shows an example of a sensor layout for use in any of the embodiments described above. Four sensors <NUM> are placed at the center of each quadrant, relative to aircraft orientation and one sensor <NUM> either fore or aft which serves as an additional reference as needed. The four sensors <NUM> detect bending in the strut <NUM> as the opposite sides of the strut <NUM> 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 <NUM> causes a common mode compression or tension on all five sensors <NUM>. 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 <FIG> and <FIG>. The five strain capacitors and the one lateral capacitor are connected to respective capacitance-to-voltage converters <NUM> (such as MS <NUM> or AD <NUM> or ZMD or any other equivalent circuit). <FIG> 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> indicates voltage outputs of the converters <NUM> are sent to a microcontroller <NUM>. The microcontroller <NUM> converts the analog signal into digital data. A software algorithm executed by the microcontroller <NUM> 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 <NUM> and an antenna and a remotely located data collector (RDC) which acts as the receiving system and storage device.

Claim 1:
A device configured to mount to a strut piston (<NUM>), the device comprising:
a first ring (<NUM>, <NUM>) comprising a first half, a second half and an inner wall, wherein the first ring (<NUM>, <NUM>) is configured to mount to the strut piston (<NUM>) by fastening the first and second half together thereby placing the first ring (<NUM>, <NUM>) in radial compression over the strut piston (<NUM>);
a second ring (<NUM>, <NUM>) comprising third half, a fourth half and an inner wall, wherein the second ring (<NUM>, <NUM>) is configured to mount to the strut piston (<NUM>) by fastening the third and fourth half together thereby placing the second ring (<NUM>, <NUM>) in radial compression over the strut piston (<NUM>);
a plurality of pairs of capacitive plates (<NUM>, <NUM>) integral with the first and second rings (<NUM>, <NUM>) respectively, the capacitive plates (<NUM>, <NUM>) being mounted to a surface that is approximately perpendicular to the first and second inner walls,
wherein the capacitive plates (<NUM>, <NUM>) are electrically isolated from their respective rings (<NUM>, <NUM>).