Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
1999-09-15
2001-01-30
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
C356S464000
Reexamination Certificate
active
06181428
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention related to closed-loop fiber optic gyros. More particularly this invention pertains to a method for compensation for temperature-dependent drift changes in a fiber optic gyro, and to a fiber optic gyro that operates according to that method.
2. Description of the Prior Art
In fiber optic gyros, two light beams that originate from a single light source are injected in opposite directions into a fiber coil. Once they have passed through the coil in opposite directions, they are recombined to produce an interference pattern on a detector. If the coil is rotated about its axis, the Sagnac effect generates a non-reciprocal phase shift between the two light beams that produces a shift in the interference pattern. The intensity and direction of the interference shift is proportional to ration direction and rate of the coil about its axis.
When a fiber optic gyro is reset, the detector output signal is processed by electronic control loops to form a non-reciprocal phase shift that is applied to a phase modulator (generally located at one end of the fiber coil) to compensate for the Sagnac phase shift (produced by rotation) between the two counterpropagating light waves. In such fiber optic gyros, the non-reciprocal phase shift to compensate for the interference shift produced by rotation of the coil is dependent upon rotation W in accordance with the following equation:
&phgr;=S·W,
where S defines the scale factor. The scale factor S is dependent upon changes in the length of the coil fiber. EP 0 245 118 B1 teaches stabilization of scale factor S by measurement and correction of the fiber length in a gyro. To do this, means are provided for comparing the detector output signal with a signal derived from the phase modulation. This results in the production of a signal that is proportional to the current optical length of the fiber coil. Such signal is employed to control the frequency f
M
of the phase modulator such that:
f
M
=½&tgr;=c/2nL
In this case, &tgr; is the time for light to propagate through the coil, c is the speed of light, n is the refractive index of the fiber and L is the length of the fiber on the coil. The term f
M
is thus a measure of the optical fiber length of the coil. If the refractive index n is constant, the regulated modulation frequency can be employed (as is known) for scale factor correction to avoid sudden major intensity changes at the detector. If, on the other hand, the refractive index n changes, such scale factor correction is unusable as f
M
is dependent on it, but the scale factor S is not.
The above-referenced patent document also fails to address the fact that changes in temperature cause drift changes over time in the fiber gyro that differ for heating and cooling. This behavior, which is known in principle as the Shupe effect, depends indirectly upon the temperature and its time derivatives and—as more detailed investigations show—is influenced by:
the coefficients of expansion of the cladding and core material of the optical fiber, the coating, the adhesive layers and the coil former;
the variation of refractive index with temperature; and
the change in the internal pressure in the coil, produced by the materials, mentioned above, being of different coefficients of expansion, which also acts on the refractive index and on the increase in length.
The Shupe effect can be reduced by so-called quadrupole coil winding. With regard to residual drift, which can still be observed, multiple influencing parameters imply that a temperature model dependent upon the temperature T and its time derivatives T′ and T″, is complex and nonlinear. Repeatability and long-term stability are, therefore, particularly poor since internal pressure differences can be equalized in the long term, particularly in the organic materials employed. Furthermore, temperature sensors cannot be placed in the fiber winding of the coils. Due to the poor thermal conductivity of fiber coils, the termperature measurements generally do not provide a good representation of the coil windings' temperature. Such temperature models for limiting the Shupe effect are known, but offer no satisfactory solutions.
SUMMARY AND OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a method for compensation for temperature-dependent Shupe effect drift changes in a fiber optic gyro.
It is yet another object of the invention to provide a fiber optic gyro with compensation of temperature-dependent Shupe effect drift changes.
The present invention addresses the preceding and other objects by providing, in a first aspect, a method for compensation for temperature-dependent drift changes in a fiber optic gyro which are caused by changes of the optical coil length. Such method includes the steps of measuring the optical path length of light through the fiber coil and generating a model of the drift. The source of the drift and therefore the model is dependent upon the variation of the optical pathway of light through the optical fiber coil of the fiber optic gyro. Thereafter, the drift changes in the output signal of the fiber optic gyro are compensated on the basis of the model and of the measurement of the optical path length.
A another aspect, the invention provides an interferometric fiber optic gyro. Such gyro includes an optical fiber which is wound to form a coil. A light source has an output light beam split in a coupler into at least two beam elements that counterpropagate through the fiber. Means are provided for recombining the beam elements to form an interference pattern once the beam elements have exited the optical fiber. A detector is provided to which the interference pattern is applied to produce a signal that indicates the intensity of the interference pattern. An evaluation unit determines the rotation acting on the fiber coil as a function of the output signal of the detector. Means are additionally provided for compensating for temperature-dependent drift changes of the gyro as a function of the change of the optical length of the fiber.
The preceding and other features of the present invention will become further apparent from the detailed description that follows. Such description is accompanied by a drawing figure. Numerals of the drawing FIGURE correspond to those of the written description with like numerals referring to like features throughout both the drawing figure and the written text.
REFERENCES:
patent: 5329349 (1994-07-01), Patterson et al.
patent: 5430544 (1995-07-01), Poisel et al.
patent: 6028668 (2000-02-01), Rider
patent: 401124704A (1989-05-01), None
Handrich Eberhard
Kemmler Manfred
Kramskry Elliot N.
Litef GmbH
Turner Samuel A.
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