Fiber optic rotation sensor

Abstract: A spool for receiving a fiber optic sensor coil (64) includes a single, substantially-planar mounting flange (68) and a central hub (66). The coil (64) can be directly wound upon the hub (66). The coil (64) is mounted traverse to the plane of the mounting flange (68) and is unconfined in that direction as the surface of the hub is substantially non-adhesive with respect to the inner layer of the coil. This allows the axial coil to expand in response to increases in temperature without generating gyro bias errors. The device is also substantially free from vibration-induced bias errors due to the fact that the integral spool-plus-coil structure has a relatively high resonant frequency lying above the frequencies of typical environmental vibrations.

Description: The present invention relates to a fiber optic rotation sensor, in particular for use in a fiber optic gyroscope. A fiber optic gyroscope comprises the following main components: (1) a light source, (2) a beamsplitter (either a fiber optic directional coupler or an integrated-optics Y-junction), (3) a fiber optic coil, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the beamsplitter into copropagating and counterpropagating waves that travel through the sensing coil. Associated electronics measures the phase relationships between the two interfering, counterpropagating beams of light that emerge from the opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed. Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. These are both time-varying and unevenly distributed throughout the coil and induce variations in the index of refraction and the length that each counterpropagating wave encounters as it travels through the coil. The phase shifts imposed upon the two waves due to environmental factors can be unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal. One approach to reducing the sensitivity arising from environmental factors has involved the use of symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the winding is located at the innermost layer while the two ends of the coil are located at the outermost layers. N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities in "Compensation of Linear Sources of Non-Reciprocity in Sagnac Interferometers", Fiber Optics and Laser Sensors I , Procs. SPIE, v. 412, p. 261 (1989). Furthermore, United States patent US-A-4793708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by duopole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding. United States patent US-A-4856900 of Ivancevic entitled "Quadruple-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereof" teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent to end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns that climb between connecting layers. Both of the aforementioned United States patents are the property of the assignee herein. United States patent No. 5333214 of Huang et al. entitled "Apparatus For Reducing Magnetic Field Induced Bias Errors in a Fiber Optic Gyroscope" addresses the suppression of bias errors induced by the Faraday effect in a sensor coil exposed to a magnetic field. The invention disclosed in that application (property of the assignee herein) teaches the use and design of compensator loops for counteracting the effects of both radially and axially-directed magnetic fields. In either case, a predetermined degree of twist of a preselected fiber twist mode is imposed upon the compensator loop to create a counteracting-corrective Faraday effect. United States patent No. 5371593 of Cordova et al. entitled "Sensor Coil For Low Bias Fiber Optic Gyroscope", also property of the assignee herein, addresses additional problems related to environmental factors. While acknowledging that the design of the sensor coil can impact the gyro's random walk, bias stability, bias temperature sensitivity, bias temperature-ramp sensitivity, bias vibration sensitivity, bias magnetic sensitivity, scale factor temperature sensitivity, scale factor linearity and input axis temperature sensitivity, the device disclosed in that application discloses a coil for which windings are potted in an adhesive material of a predetermined composition. Careful selection of the potting material (particularly in terms of modulus of elasticity) results in reduction of vibration-induced bias, coil cracking, degradation of h-parameter and temperature-ramp bias sensitivity. The coil is formed on a spool of carbon composite material whose coefficient of thermal expansion approximates that of the overlying fiber windings. In addition, this application discloses that the close matching of the thermal expansion characteristics of the spool and the fiber windings as well as proper selection of the coil potting material will minimize the Shupe-like bias caused by thermal stress that would be otherwise exerted by a standard metallic spool. While the use of adequate potting and spool materials will tend to minimize bias environmental sensitivities, conventional support and spool designs, which feature a substantially-cylindrical hub sandwiched between a pair of end flanges, are difficult to "match" to the potted coil. This is due to the asymmetry of expansions of such coils in response to temperature change. The coefficient of thermal expansion of a potted coil in the axial direction is often on the order of one-hundred (100) times that of the radial direction. Unfortunately, a corresponding asymmetry does not exist with regard to the supporting spool. Rather, spools of conventional design and material composition exhibit isotropic thermal expansion characteristics. This relative imbalance introduces bias errors through coil stressing and creates bonding and cracking problems. For example, in a spool-and-coil arrangement in which the material of the hub of the spool closely approximates the radial coefficient of thermal expansion of the potted coil, the axial expansion of the coil will exceed that of the hub. As a result, significant axial compression of the coil can occur when the temperature rises since axial expansion of the potted coil is limited by a relatively "fixed" separation distance between the spool's end flanges. Further, the stressing due to differential thermal expansion coefficients at the coil-hub interface can result in either rupture or in coil cracking. On the other hand, in a spool fabricated of material closely matching the axial coefficient of thermal expansion of the potted coil, one may expect the relatively-greater radial expansion of the hub in response to temperature change to degrade performance by squeezing the fiber of the coil whose radial dimension is relatively fixed. United States patent No. 5818590 of Patterson entitled "Flange-Supported Sensor Coil For a Fiber Optic Gyroscope", property of the assignee herein, discloses a spool designed to address the thermally-induced Shupe bias that results from the abovementioned thermal incompatibility of conventional spool designs with the asymmetric radial and axial thermal expansion coefficients of potted sensor coils. That application discloses a spool that consists of a single mounting flange and an interior hub. The coil is mounted upon with axis transverse to the plane of the flange. In an alternative embodiment, the coil is split into sections that lie atop and at the bottom of the flange. The coil is free to expand axially because the interior of the coil is separated from the hub by a finite distance. The much smaller radial coefficient of thermal expansion of the coil assures that the separation from the hub needn't be excessive and that undue stressing is not experienced at the coil-flange interface. While the above device exhibits good thermal performance characteristics, it is subject to vibration-induced bias effects which can become particularly acute in an environment that includes vibrations at the resonant frequency of the potted coil. European patent application no.0521400 discloses an optical fibre coil unit for a fibre optic gyro in which the coil is housed within an annular case. According to the present invention there is provided a rotation sensor comprising: a spool, the spool comprising a hub and a flange; and a coil of optical fibre encircling the hub; characterised in that: the coil is embedded in a potting material; one end of the coil is fixed to a surface of the flange; and the coil is supported in a non-adhesive manner by the hub such that the coil may expand or contract axially relative to the hub. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a perspective view of a prior art sensor coil of the single flange type wherein the axis of the coil is orthogonal to the plane of the single mounting flange; Figure 2 is a perspective view of a prior art coil of the above-described type according to an alternative arrangement for addressing vibration-induced bias error effects; Figure 3 is a side elevational view in cross-section of a prior art sensor coil in accordance with the preceding figure mounted for use in an inertial navigation system ("INS") ; Figures 4(a) and 4(b) are bottom plan and side elevational views illustrating details of a prior art spool for supporting a split sensor coil as illustrated in Figures 2 and 3; Figure 5 is a perspective view of a sensor coil mounted upon a spool in accordance with one embodiment of the invention; Figure 6 is a cross-sectional of a spool-and-coil taken at line 6-6 of Figure 5; Figure 7 is a partially broken away detailed perspective view of the spool of Figures 5 and 6; and Figure 8 is a detailed sectional view taken at the interface of the sensor coil of Figure 7. Figure 1 is a perspective view of a sensor coil 10 comprising a plurality of coaxial turns of an optical fiber 12 engaged to a spool in accordance with a prior art sensor coil of the type wherein the axis of the coil 10 is orthogonal to the plane of a single mounting flange 14. The spool comprises the mounting flange 14 in combination with a central cylindrical hub 16. The coil 10 is bonded solely to the single flange 14 and separated from the central hub 16 by a finite distance "d". The free-standing sensor coil 10 is bonded by means of a conventional adhesive layer 18 to the disk-like mounting flange 14 of the spool. The above-described configuration is accomplished by first winding the continuous optical fiber 12 in a predetermined configuration upon another spool of conventional configuration. During the winding process or thereafter the coil 10 is preferably impregnated with a potting material. After such potting material has cured, the wound coil 10 may then be removed from the winding spool, achieving a free-standing configuration for fixation to the spool of Figure 1. The arrangement shown in Figure 1 reduces Shupe effect bias that results from temperature-induced stressing of the fiber 12. Such bias is attributed in part to the anisotropic thermal expansion characteristic of potted coils when combined with a spool of conventional configuration comprising a central cylindrical mandrel terminating in opposed end flanges. While the coil is anisotropic with respect to its axial and radial thermal expansions, the spool is isotropic. The anisotropy of the coil is well recognized and follows from the differential stiffnesses of the coil geometry in the axial and radial directions. The coil may be analyzed as a series of concentric rings. When subject to heating, the law of conservation of volume directs the bulk of coil expansion along the coil axis of rotation. The configuration of Figure 1 limits the designer's task of matching thermal expansions to consideration of the relationship between the radial coefficient of thermal expansion of the coil 10 and that of the isotropic mounting flange 14 since the spool and the coil are not in axial contact due to the separation d from the hub 16. As such, the coil 10 may expand axially without constraint, eliminating the axial compression-induced stressing that can occur in a spool fixed to a conventional spool. Such stressing results from the substantially larger coefficient of expansion of the coil 10 in the axial direction (relative to the coefficient of the hub or mandrel). Further, by separating the hub 16 from the innermost coil layer, the axial expansion of the coil 10 cannot create stresses at the coil-to-hub "anchor points" that could cause both delamination of the coil from the hub and cracks that propagate through the coil in prior art designs at a forty-five (45) degree angle. Such cracking is observed when the internal stress exceeds the adhesive strength of the bond between the jacket of the fiber coil and the potting material (the axial expansion of a fiber coil mounted to a conventional aluminum spool can create stresses above 3.5 MPa (500 p.s.i.) Despite the advantages, in terms of thermal bias sensitivity, realized with a mounting arrangement in accordance with Figure 1, bias errors are induced in a free-standing coil 10 as in Figure 1 from another environmental source, vibration. This issue is addressed in the prior art in accordance with the argument disclosed in Figure 2, a perspective view of an alternative mounted sensor coil. As before, a cylindrical central hub 20 is separated from the inner layer of a potted coil 22 by a finite distance d. The mounting flange 24 of the alternative embodiment is located at an intermediate position along the length of the hub 20 so that the coil 22 may be arranged into a back part 25 and a front part 26 (the division of the coil could be into two equal portions or two unequal portions) being mounted to opposed surfaces of the mounting flange 24. It is a central conceptual advantage of the configuration of Figure 2 that it alleviates vibration effects resulting from the absence of axial support. The coil 22 (which is somewhat flexible due to the character of known potting materials) is subject, in an axially unsupported arrangement, to stressing in response to unavoidable environmental vibrations. As the coil-and-mounting flange form a cantilevered system, the effects of environmental vibration become most pronounced when the frequency of vibration coincides with the natural or resonant frequency of the cantilevered coil. The split coil arrangement of Figure 2 addresses the problem of vibration induced bias. Splitting the coil 22 increases the resonant frequency of each half beyond a threshold (about 2000 Hz) where environmental disturbances are minimal. Generally, for certain applications the power spectral density of environmental vibrations is a maximum in the region below 1100 Hz and decreases thereafter. It has been found that a 1 km sensor coil of the type shown in Figure 1 wound in an orthocyclic pattern and potted in a conventional adhesive material possesses a natural frequency that approximates 1100 Hz. In the case of the above-referenced 1 km sensor coil, such splitting into parts 24 and 26 of lesser lengths (each comprising about 500 meters of optical fiber), two coil segments are created, each of which possesses a resonant frequency greater than 2400 Hz that removes the coil 22 from the range of significant stressing from environmentally-induced mechanical vibrations. Figure 3 is a side elevational view in cross-section of a split coil-and-mounting flange arrangement engaged to a pedestal 28 for operation. As can be seen, the device is contained within a µ-metal shield 30 that interlocks with a base plate 32. Within the casing formed by the shield 30 and the base plate 32, the pedestal 28, preferably formed of stainless steel, receives a bolt 34 that forms a portion of an inertial navigation system (INS). The bolt 34 permits alignment with a predetermined axis that is intended to serve as the input axis of the inertial measurement unit. Various optical and electrical components are also located within the casing formed by the shield 30. The split sensor coil 36 is divided into a front part 38 and a back part 40, each comprising a plurality of turns of a continuous optical fiber. The coil 36 is mounted upon a disk-like member 42 of the mounting flange. An encircling shoulder 44 of the stainless steel pedestal 28 supports a central hub 46 of the spool. The thinner wall thickness at the lower half of the hub 46 provides added flexibility or "give" in the region of abutting pedestal and spool materials of differing coefficients of thermal expansion to protect from angular misalignment of the disk-like portion 42 when the device is subject to "excessive" thermally-induced radial expansion of the stainless steel pedestal to abut the front half wall of the hub. A retainer plate 48 sits atop the back or "thick" half of the hub 46 of the coil mounting spool. The entire assembly is, in turn, secured by means of a second bolt 50 that completes the "sandwich" arrangement including the retainer plate 48, the pedestal 28 and the base plate 32. The splitting of a sensor coil to overcome vibration effects introduces complexities that increase device costs while reducing yield. The unavoidable fiber transition from one side of a mounting flange to the other complicates both flange design and the coil winding process. The fiber "transition " (i.e. the process of exiting the last turn of the winding pattern at one side of the flange until beginning the first turn of the winding pattern at the opposed side) must be gradual to minimize the introduction of microbends into the coil that can produce optical bias effects. Preferably, the transitions should take place over 3/4 of a fiber turn from the end of winding of one part of the split coil to the beginning of winding of the other part of the coil. Figures 4(a) and 4(b) are bottom and side elevational views, respectively, of a spool for supporting a split configuration. As can be seen, the spool 50 generally comprises a disk-like mounting flange 52 joined to a central hub 54 that extends therethrough. As mentioned above, the inner wall thickness of the hub 54 preferably varies along its axial length, reflecting the coupling of the hub 54 to the pedestal (not shown in Figures 4(a) and 4(b)). Slots 56 and 58 and communicating arcuate grooves 60 and 62 are carefully machined at the periphery of the disk-shaped member 52 to permit a gradual transition of the fiber between the front and back sections of the sensor coil. The grooves 60 and 62 are provided in the surface of the member 52 upon which the second half of the split coil is wound to guide the fiber to the center of member 52. The split coil may be fabricated or wound upon the mounting flange 50 by first providing a take-up spool that is coaxial with and of greater diameter than the outside diameter of the hub 54. Preferably, a quadrupole winding pattern is employed. After one half of the split coil has been wound upon one side of the disk-like member 54, one must then proceed to the other side to wind the other half of the split coil. Alternatively, the split coil can be wound inwardly from the outer periphery of the disk-shaped member 52. The edge slots 56 and 58 and the arcuate grooves 60 and 62 are preferably located, dimensioned and designed so that the aforesaid 3/4 turn transition can occur. While a spool including a single mounting flange, rather than a conventional arrangement in which a hub or mandrel and, hence, the coil, is sandwiched between a pair of end flanges, addresses bias errors associated with the coil's anisotropic thermal characteristics, the solution to the attendant vibration-induced bias issues raises other problems. The split coil concept complicates the spool structure by requiring the careful machining of grooves at the periphery of the mounting flange. Moreover, it requires a winding process that cannot be readily adapted to automation. This is due to the care with which the fiber must be seated within the peripheral grooves of the mounting flange. As a result, significant yield limitations are associated with split coil configurations. Moreover, the split coil concept is limited to applications for which the input vibration spectrum lies below approximately 2400 Hz. The inventors herein have addressed the foregoing shortcomings of the prior art and have as a result devised coils having the thermal performance advantages of an axially-unconstrained coil in designs that are readily subject to automation and thereby enjoy attendant economies. Furthermore, such new designs are particularly useful for applications for which the input vibration spectrum exceeds 4000 Hz (and is typically below 5000 Hz.) Figure 5 is a perspective view of a sensor coil 64 mounted upon a spool in accordance with one embodiment of the invention and Figure 6 is a cross-sectional view thereof taken at line 6-6 of Figure 5. Referring to Figures 5 and 6, the spool of this embodiment of the invention comprises a central cylindrical hub 66 in combination with a planar mounting flange 68. A layer 69 of an appropriate material (for example, an adhesive) holds one end of the coil 64 to the upper surface of the mounting flange 68. In contrast to free-standing coil arrangements, the innermost layer 70 of the coil 64 is wound upon and in contact with the outer surface 72 of the hub 66. As a consequence, the sensor coil 64 may be formed directly upon the hub or mandrel 66 of the spool of the invention. This simplifies coil winding vis-a-vis arrangements that include a separation distance d between the outer surface 72 of the hub 66 and the innermost layer 70 of the sensor coil 64. In addition, the coil 64 is not free-standing, being fully supported by the central hub or mandrel 66 of the spool. As a result, the coil 64 does not in and of itself constitute an independent structure subject to vibrations near or at its resonant frequency. Rather, it is "anchored" to the spool both axially (through intimate contact) and radially to form a combined composite structure whose resonant frequency is much higher than that of a free-standing potted coil. The resonant frequency of the composite structure lies well above the range of environmental vibrations previously discussed. In this particular embodiment, it lies well above 4000 Hz. The spool of this embodiment of the invention achieves the aforesaid advantages without incurring bias error effects due to differential radial and axial expansions of the potted sensor coil in the presence of temperature change. This is accomplished by providing a non-adhesive coating at the outer surface 72 of the central mandrel or hub 66 of the spool. Figure 7 is a partially broken away perspective view of a spool 74 in accordance with the embodiment of the invention. The interior of the cylindrical hub or mandrel 76 (and, for that matter, the mounting flange 78), may for example, be formed of a molded carbon composite material. Alternatively, titanium is, for example, suitable for high accuracy gyro designs. The exterior of the hub 76 is preferably coated with a layer of material 80 that is non-adhesive (i.e. is characterized by a low coefficient of friction) with respect to the sensor coil wound thereon. Such material may comprise, for example, a polymer such as a mixture of fluorinated ethylene propylene ("FEP") and EPOXY (coefficient of friction approximately .12) or tetraflouro ethylene ("TFE") mixed with EPOXY or an equivalent resin (yielding a coefficient of friction of approximately .08). The selection of an appropriate coating material 80 will depend upon the characteristics of the composition of the spool as is well understood in the art. Conventional coating processes, including baking, may be employed to provide a sufficiently non-adhesive coating to enable axial expansion of the sensor coil 64 without generating stresses at the interface between the innermost layer 70 of the sensor coil 64 and the outer surface 72 of the central hub or mandrel 66. Figure 8 is a detailed sectional view of a portion of the interface between the coil windings 70' of the innermost layer 70 of the potted sensor coil 64 of Figure 5 and the outer surface of the hub 66 comprising a non-adhesive layer 80. The hub 66 is preferably formed of low-thermal expansion material such as titanium or an appropriate carbon composite. As can be seen, the non-adhesive layer 80 provides a relatively friction-free surface upon which the innermost layer of the sensor coil 64 is free to slide to accommodate its significant axial expansion when subject to heating. The disparity in axial and radial thermal expansions of a potted coil reflects the volumetric expansion and contraction that one may expect to occur as the sensor coil undergoes temperature cycling. The low-thermal expansion material of the spool approximately matches the radial thermally-induced expansion of the potted sensor coil 64 (on the order of 3 to 5 ppm/degree Centigrade). Unlike designs that include a separation distance, the present invention, by providing a non-adhesive surface at the coil-to-hub interface, drastically increases the resonant vibration frequency of the coil structure well beyond the range of vibration inputs, without sacrificing the ability to absorb vastly different degrees of thermal expansion in the radial and axial directions. Thus it is seen that the embodiments of the present invention provides an easy-to-manufacture device that is readily amenable to mass production. A sensor coil mounted to a spool in accordance with the embodiment of the invention is substantially free of bias errors due to the changing temperatures and the vibration present in the surrounding environment. The simplicity of the spool design, in addition to facilitating automation of the coil winding process, yields a spool that is simple to manufacture and thereby further increases the yield, and lowers the cost, of the resultant device.

Claims: A rotation sensor comprising: a spool, the spool comprising a hub (66) and a flange (68); and a coil of optical fiber (64) encircling the hub (66), characterised in that : the coil (64) is embedded in a potting material; one end of the coil (64) is fixed to a surface of the flange (68); and the coil (64) is supported in a non-adhesive manner by the hub (66) such that the coil (64) may expand or contract axially relative to the hub (66). A rotation sensor according to claim 1, characterised in that the optical fiber wound on the hub (66) to form the coil (64) comprises a plurality of layers of coaxial turns. A rotation sensor according to claim 1 or 2, characterised in that the interior of the coil (64) is in contact with the outer surface of the hub (66). A rotation sensor according to any of the preceding claims, wherein the hub (66) is coated with a layer of material that is non-adhesive. A rotation sensor in accordance with any one of the preceding claims, characterised in that the hub (66) includes a coating layer of material having a coefficient of friction below 0.20, and preferably below 0.12. A rotation sensor in accordance with claim 5, characterised in that the coating layer comprises a polymer. A rotation sensor in accordance with claim 6, characterised in that the polymer is a fluorinated polymer. A rotation sensor in accordance with claim 7, characterised in that the fluorinated polymer is polytetrafluoroethelyne. A rotation sensor in accordance with any one of the claims 6 to 8, characterised in that the polymer is mixed with a curable resin. A rotation sensor in accordance with claim 9, characterised in that the curable resin is an epoxy resin or an equivalent resin. A rotation sensor in accordance with any one of the preceding claims, characterised in that the hub is formed of material having a thermal expansivity of 5 ppm per degree Celsius or less. A rotation sensor in accordance with claim 11, characterised in that the hub is of moulded carbon composite material. A rotation sensor in accordance with claim 11, characterised in that the hub is of titanium.

Inventors: BILINSKI DONALD J, CHIN GENE H, CORDOVA AMADO, FERSHT SAMUEL N, BILINSKI, DONALD J., CHIN, GENE H., CORDOVA, AMADO, FERSHT, SAMUEL N.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1995
Main classification: G01C19
IPC codes: G02B6, G01C19