Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
  • G01C19/58—Turn-sensitive devices without moving masses
  • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
  • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
  • G01C19/721—Details, e.g. optical or electronical details
  • G01C19/722—Details, e.g. optical or electronical details of the mechanical construction
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
  • B64G1/00—Cosmonautic vehicles
  • B64G1/22—Parts of, or equipment specially adapted for fitting in or to, cosmonautic vehicles
  • B64G1/24—Guiding or controlling apparatus, e.g. for attitude control
  • B64G1/28—Guiding or controlling apparatus, e.g. for attitude control using inertia or gyro effect
  • B64G1/283—Guiding or controlling apparatus, e.g. for attitude control using inertia or gyro effect using reaction wheels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
  • B64G1/00—Cosmonautic vehicles
  • B64G1/22—Parts of, or equipment specially adapted for fitting in or to, cosmonautic vehicles
  • B64G1/24—Guiding or controlling apparatus, e.g. for attitude control
  • B64G1/36—Guiding or controlling apparatus, e.g. for attitude control using sensors, e.g. sun-sensors, horizon sensors
  • B64G1/369—Guiding or controlling apparatus, e.g. for attitude control using sensors, e.g. sun-sensors, horizon sensors using gyroscopes as attitude sensors
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T74/00—Machine element or mechanism
  • Y10T74/12—Gyroscopes
  • Y10T74/1218—Combined

Definitions

• the present invention relates generally to an attitude reference system. More specifically, the present invention relates to an integrated reaction wheel assembly (RWA) and fiber optic gyro (FOG) that may be used in the aerospace industry.
• RWA reaction wheel assembly
• FOG fiber optic gyro
• a FOG operates on a principal known as the Sagnac effect, which describes what happens when two beams of light travel in opposite ' directions around the same closed path. If the path is not rotating, the two beams complete their circuit and arrive back at the starting point at the same instant. When the structure is rotating about an appropriate axis, however, the two beams traverse different path lengths, i.e. the beam traveling in the direction of rotation travels a longer path and vice versa. This difference is small, but it can be detected and measured to indicate rotation rate.
• a FOG has a long physical path length defined by many turns of optical fiber wound into a coil. The beam is divided and injected into opposite ends of the coil, makes a single pass through it, and comes out the other end. Then the two beams are recombined, forming an interference pattern. Since the beams trace exactly the same physical path, although in opposite directions, the interference should be fully constructive in the absence of a Sagnac effect. Rotation changes the path lengths and the phases of the interfering beams, thus producing changes in the fringe intensity that are proportional to the rotation rate.
• the aerospace market has a need for a low inertial noise attitude reference system that offers long life and increased inertial performance (lower noise) while minimizing mass.
• Current systems utilize a separate inertial reference unit (IRU) assembly, which can add additional mass to a satellite.
• IRU inertial reference unit
• a reaction wheel assembly and fiber optic gyro device may include a reaction wheel assembly having a reaction wheel assembly housing, a fiber optic gyro coil integrated with the reaction wheel assembly housing, and a fiber optic gyro electronics integrated with the reaction wheel assembly housing.
• the fiber optic gyro coil may be wound around the reaction wheel assembly housing.
• the gyro coil may also be located within the reaction wheel assembly housing.
• Fig. 1 is a perspective view of an integrated RWA/FOG device
• Fig. 2 is a schematic diagram of the components of a FOG.
• FIG. 3 is a cross-sectional view of an alternative embodiment of an integrated RWA/FOG device. DETAILED DESCRIPTION OF THE INVENTION
• bias stability or the stability of the output when the sensor is exposed to a constant input rate.
• Bias stability is measured in degrees per hour.
• Two factors for achieving good bias stability in a FOG are maintaining the reciprocity of the optical path and controlling polarization errors. Maintaining reciprocity is a major goal of the entire optical design. Suppressing polarization errors calls for some additional steps.
• a second trick is to install segments of birefringent fiber ahead of critical cross-coupling sites.
• the two polarization waves propagate at different speeds in the birefringent material resulting in the beams being temporally separated and hence decorrelated to each other upon exiting the fiber.
• Another step is to hold very tight tolerances on the angular alignment of splice joints to minimize the magnitude of the cross-coupled signal.
• polarizers with a very high extinction ratio block as much light as possible in the off-axis polarization mode. Combining all these measures is advantageous in achieving the 0.01 degree per hour bias stability needed in a navigation-grade gyro.
• Random noise is the second major performance parameter. It is white noise with equal content of all frequencies superimposed on the output signal.
• the customary unit of measure for the noise is degree/root hour. Since the noise is white, it tends to cancel itself out when it is averaged or integrated over a long period. Thus, in principle, it can be reduced to an arbitrarily low level given enough time. The existence of this noise means there is some uncertainty in any measurement of the rotation rate. The uncertainty is smaller when the signal is averaged over a longer period of time.
• the major sources of random noise in a FOG are backscattered light from fiber imperfections, photon shot noise in the detector, electronic thermal noise, and intensity fluctuations in the light source.
• a backscattered beam is one that goes partway around the loop, say, in the clockwise direction and then after reflection suddenly appears to be part of the counterclockwise beam. Spurious interference from many such reflected beams would overwhelm the genuine gyroscopic signal. But, with a short coherence length light source, backscattered beams will reach each the detector in correlation with the primary beam only if they happen to be scattered within a few micrometers of the center of the coil. Before short coherence length sources were tried, noise contamination in FOGs was so severe that they could not reliably detect the Earth's rotation.
• Photon shot noise is caused by the random statistical nature of photons impinging on the detector and releasing electrical charge carriers. Its magnitude decreases in proportion to the square root of the optical power reaching the detector. Photon shot noise sets a fundamental lower limit on the noise budget of the sensor and in many FOG products is the dominant noise source. In some high- performance gyros, a high-power light source reduces photon shot noise to a very low level. When this is done, fluctuations in light source intensity can become the dominant noise source. Because this form of noise is a characteristic of the light source fluctuations, it can be measured and canceled or subtracted out.
• Scale-factor accuracy Another of the three aforementioned performance parameters is scale- factor accuracy. Ideally, a gyro would exhibit a perfectly linear relationship between input rotation rate and output pulse rate. Scale-factor accuracy describes departures from this ideal. Errors are characterized by deviations from the perfect linear graph, expressed in units of parts per million. Major sources of scale-factor error are temperature-dependent variations in the wavelength of the light source and uncertainties and nonlinearities associated with the signal processing electronics.
• the wavelength of a semiconductor light source typically changes by about 300 parts per million per degree Celsius of temperature change. Any wavelength change has a one to one effect on the scale factor. Gyro designers must either characterize and compensate for these errors or else control the temperature of the light source. Until fairly recently, the above solutions limited FOG scale- factor performance to no better than 30 parts per million over a full range temperature environment. The adoption of doped-fiber light sources has significantly improved FOG scale-factor accuracy. These devices are 50 times less sensitive to temperature changes. Combining them with localized temperature control and compensation can yield one part per million performance.
• a FOG/RWA combination that performs to the levels necessary for use in navigation, but does so in a smaller package and without significantly adding weight to the satellite.
• Such a new multifunction device may be realized in accordance with the present invention by integrating a "long-life" FOG with a RWA.
• the resulting product offers both reaction torque and attitude information without the need for a separate IRU assembly, thus reducing weight to the satellite.
• the FOG may achieve lower inertial noise by increasing the fiber LD product without significant impact to the RWA footprint or envelope and also eliminates the traditional IRU package from the craft.
• improved FOG performance or increased LD product resulted in increased mass.
• the present invention leverages the RWA envelope and packaging to minimize impact to the spacecraft mass.
• satellite manufacturers presently install RWAs, up to four or more, on the spacecraft to impart rotational rates about the satellite's three axes.
• the spacecraft guidance and control processor typically closes all attitude control loops using information to and from both the IRU and the RWA devices.
• the integrated RWA/FOG of the present invention offers a prealigned sensor/effector solution that simplifies integration. The output data, however, may be used in much the same way as the traditional solution.
• FIG. 1 depicts an integrated RWA/FOG device 100 according to one embodiment of the present invention.
• RWA/FOG 100 includes a housing 102 similar to that used with a traditional RWA. Housing 102 is modified in the present invention to include a FOG coil 104 along with the associated FOG electronics 106. By including the FOG componentry as an integrated portion of the RWA, less space may be consumed in the spacecraft.
• FOG coil 104 comprises a coil of fiber optic cable wound around a portion of the RWA housing. As with traditional FOGs, coil 104 is connected to a light source 108 as shown schematically in Fig. 2.
• Light source 108 produces light at wavelengths in the near-infrared region, for example between about 0.83 micrometers and about 1.55 micrometers.
• One important specification for the light source is its coherence function, which describes the ability of two beams of light derived from the same source to create an interference pattern when the are recombined. Users of laser light sources may want to maximize the coherence function so that waves remain correlated over long distances or long intervals. In a FOG, the ideal is just the opposite. A short coherence helps to eliminate spurious interference from waves that have traveled non reciprocal paths.
• Light source 108 may be specially designed and fabricated to limit the coherence length to tens of micrometers, or less than fifty times the wavelength of the light emitted.
• a detector 110 is included to convert an optical interference signal from the gyro to the electrical domain, where it can be processed and interpreted. Variations in brightness as the interference fringe passes across detector 110 become variations in voltage or current. Silicon photodetectors work well at shorter wavelengths, while more exotic quaternary compounds such as indium gallium arsenide phosphide (InGAsP) respond to the less energetic photons at longer wavelengths. Important specifications include the device's quantum efficiency (which designates the percentage of photons absorbed that create charge carriers in the semiconductor) and the frequency characteristics of the detector and its associated circuitry.
• InGAsP indium gallium arsenide phosphide
• Coil 104 is wound from the center of the fiber, keeping pairs of points that are equidistant from the center adjacent to one another. The idea is to create a thermal symmetry between the coil halves to cancel environmentally introduced nonreciprocal errors.
• the fiber of coil 104 may be made of fused silica of about 80 or 125 micrometers in diameter, for example.
• the fiber may also have a protective organic coating on the surface.
• Light is not conducted throughout the bulk of the fiber, but only through an optical core or waveguide between approximately five and ten micrometers in diameter.
• the fiber may be a single- mode fiber, meaning that light in the waveguide is confined to a single spatially guided mode having two possible polarization states.
• Some gyros may require polarization-maintaining fiber, in which one axis of the fiber is stressed during manufacture to create a material where the two polarization modes propagate at different velocities. This special fiber helps to suppress unwanted mixing between polarization states.
• Other gyros may be built with inexpensive single-mode fiber similar to that used in telecommunications lines.
• the FOG electronics 106 include a fiber optic coupler 112 that is functionally equivalent to the half-silvered mirror that serves as a beam splitter in an optical system built from discrete components. Light entering coupler 112 is split into two beams traveling in the same direction, but exiting through separate ports. Coupler 112 may be made by closely bonding two lengths of fiber so that light leaks from one core into the other. Optical loss and the ability to maintain the entering polarization distribution in the exit legs may be important characteristics of coupler 112.
• Light emitted from light source 108 also passes through other components of the FOG electronics 106 including a spatial filter 114, a polarizer 116, and a phase modulator 118, which usually has a bandwidth or frequency response greater than 500 megahertz.
• These devices may be fabricated in a crystal of lithium niobate, for example, a material having a strong electro-optic coefficient. Waveguides may be created on the surface of this chip to distribute optical signals and electrodes may be deposited where electrical excitation would induce phase modulation.
• FOG electronics 106 are included in a base portion of the RWA/FOG device 100.
• the FOG/RWA assembly may be calibrated based on wheel speed feedback in order to filter the gyro output to account for the RWA induced motion.
• the FOG may actually serve as a sensor to diagnose health of the RWA before wheel current changes become obvious, which is currently an indicator of RWA health. This would allow more advance notice to ground operations for reconfiguring a spacecraft.
• the FOG may also be able to detect if any balance shift occurs over the life of the RWA. This could be sensed by the momentum vector shift and may be detected by the FOG at low frequencies.
• utilizing RWA housing 102 as a winding core for coil 104 may simplify the FOG production.
• Another advantage achieved by having an integrated RWA/FOG device is the ability to sense vibrations in all axes coming out of the RWA array. This allows for actively detuning the RWAs to run at speeds that do not beat with each other, thus minimizing detrimental effects on a spacecraft. Because the FOG is co-located with the RWA, such vibration sensing may be achieved.
• Fig. 3 shows an alternative embodiment of an integrated FOG/RWA device 200.
• a smaller gyro 204 for example one having a diameter of approximately four inches, is included in the base of RWA housing 202.
• the gyro electronics 206 are included on the opposite side of housing 202.
• This embodiment may be advantageous in that RWA housing 202 may not need to be modified to accommodate a FOG device.
• the foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.

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• Engineering & Computer Science (AREA)
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• Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Aviation & Aerospace Engineering (AREA)
• Optics & Photonics (AREA)
• Electromagnetism (AREA)
• Power Engineering (AREA)
• General Physics & Mathematics (AREA)
• Gyroscopes (AREA)

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Citations (2)

• 2003
  • 2003-07-23 US US10/624,688 patent/US7057734B2/en not_active Expired - Fee Related
• 2004
  • 2004-07-20 WO PCT/US2004/023218 patent/WO2005052513A1/en not_active Ceased
  • 2004-07-20 EP EP04817727A patent/EP1646843A1/en not_active Withdrawn
  • 2004-07-20 JP JP2006521173A patent/JP2006528358A/ja not_active Withdrawn

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