Fiber optic gyro stabilized by harmonic components of detected signal

• US 4,883,358 A
• Filed: 08/25/1988
• Issued: 11/28/1989
• Est. Priority Date: 09/02/1987
• Status: Expired due to Term

• Current Assignee: Japan Aviation Electronics Industry Limited
• Original Assignee: Japan Aviation Electronics Industry Limited
• Inventors: Okada, Kenichi
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US07/236,435
• Time in Patent Office: 460 Days
• Field of Search: 356/350
• US Class Current: 356/463
• CPC Class Codes: G01C 19/72 with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers

Abstract

Light waves are allowed to pass clockwise and counterclockwise through a circuit optical path of at least one loop, and the light waves having passed through the optical path are interfered with each other. Phase modulator is inserted in one end of the circular optical path, by which the light waves are phase modulated. The intensity of the interference light is detected, as an electric signal, by a photodetector. The fundamental wave component of the modulation frequency of the phase modulator, contained in the electric signal converted by the photodetector, is synchronously detected by a first synchronous detector, from which is output a signal corresponding to an input angular velocity. Second and fourth harmonic components of the modulation frequency of the phase modulator, contained in the electric signal, are synchronously detected by second and third synchronous detectors. The phase modulator is controlled so that the ratio between the outputs from the second and third synchronous detectors is always equal to a preset reference value.

Claims

1. A fiber optic gyro comprising:

a circular optical path of at least one loop;

means for allowing light waves to pass through the optical path in both clockwise and counterclockwise directions;

interference means whereby the light waves having passed through the optical path are made to interfere with each other;

phase modulating means connected in cascade between the interference means and one end of the optical path, for phase modulating both light waves;

photoelectric conversion means for converting the intensity of the interference light into an electric signal;

first synchronous detecting means whereby a fundamental frequency component of a modulation frequency of the phase modulating means, contained in the output signal of the photoelectric conversion means, is synchronously detected and a signal corresponding to an input angular velocity is produced;

second and third synchronous detecting means whereby second and third harmonic components of the modulation frequency of the phase modulating means, contained in the output signal of the photoelectric conversion means, are synchronously detected, respectively; and

means for controlling the phase modulating means so that the ratio between the output from the second synchronous detecting means and the output from the third synchronous detecting means is always equal to a preset reference value.

2. A fiber optic gyro comprising:

a circular optical path of at least one loop;

means for allowing light waves to pass through the optical path in both clockwise and counterclockwise directions;

interference means whereby the light waves having passed through the optical path are made to interfere with each other;

phase modulating means connected in cascade between the interference means and one end of the optical path, for phase modulating both light waves;

a photodetector for detecting, as an electric signal, the intensity of the interference light;

first synchronous detecting means whereby an odd-numbered harmonic component of the modulation frequency of the phase modulating means, contained in the photodetector output, is synchronously detected;

second synchronous detecting means whereby an even-numbered harmonic component of the modulation frequency of the phase modulating means, contained in the photodetector output, is synchronously detected; and

control means responsive to a signal from the second synchronous detecting means to effect control so that an input signal and a reference signal both applied to the first synchronous detecting means become substantially in-phase with each other and an input signal and a reference signal both applied to the second synchronous detecting means are displaced substantially 90 degrees apart in phase.

3. A fiber optic gyro comprising:

a circular optical path of at least one loop;

means for allowing light waves to pass through the optical path in both clockwise and counterclockwise directions;

interference means whereby the light waves having passed through the optical path are made to interference with each other;

phase modulating means connected in cascade between the interference means and one end of the optical path, for phase modulating both light waves;

photoelectric conversion means for detecting, as an electric signal, the intensity of the interference light;

first synchronous detecting means whereby a desired odd-numbered harmonic component of the modulation frequency of the phase modulating means, contained in the output from the photoelectric conversion means, is synchronously detected;

second synchronous detecting means whereby a desired even-numbered harmonic component of the modulation frequency of the phase modulating means, contained in the output from the photoelectric conversion means, is synchronously detected;

third synchronous detecting means for detecting the output of the photoelectric conversion means by a reference signal 90 degrees out of phase with a reference signal of the first synchronous detecting means;

fourth synchronous detecting means for detecting the output of the photoelectric conversion means by a reference signal 90 degrees out of phase with a reference signal of the second synchronous detecting means;

first phase detecting means whereby information on a phase difference between the reference signal and the synchronously detected component of the input signal of the first synchronous detecting means is obtained on the basis of the outputs from the first and third synchronous detecting means;

second phase detecting means whereby information on a phase difference between the reference signal and the synchronously detected component of the input signal of the second synchronous detecting means is obtained on the basis of the outputs from the second and fourth synchronous detecting means; and

control means responsive to the output signals of the first and second phase detecting means to effect control so that the reference signals and the synchronously detected components in the first and second synchronous detecting means are always in phase with each other.

4. The fiber optic gyro of claim 3, wherein the first phase detecting means is means for multiplying the outputs of the first and third synchronous detecting means and the second phase detecting means is a means for multiplying the outputs of the second and fourth synchronous detecting means.

5. The fiber optic gyro of claim 3, wherein the first phase detecting means is means for inverting the polarity of the output of the third synchronous detecting means according to the polarity of the output of the first synchronous detecting means and the second phase detecting means is means for inverting the polarity of the output of the fourth synchronous detecting means according to the polarity of the output of the second synchronous detecting means.

6. The fiber optic gyro of any one of claims 3 to 5, further including:

first range detecting means for detecting that a phase difference between the light waves, depending on an input angular velocity applied to the optical path in its circumferential direction, is in the range of approximately ±

π

/4 about ±

mπ

(where m=0, 1, 2, . . . ), second range detecting means for detecting that the phase difference between the light waves is in the range of approximately ±

π

/4 about ±

(2m+1)·

π

/2 (where m=0, 1, 2, . . . ); and

means responsive to the output from the first range detecting means to apply the output from the second phase detecting means to the control means and responsive to the output from the second range detecting means to apply the output from the first phase detecting means to the control means.

7. A fiber optic gyro comprising:

a circular optical path of at least one loop;

means for allowing light waves to pass through the optical path in clockwise and counterclockwise directions;

interference means whereby the light waves having passed through the optical path are made to interfere with each other;

phase modulating means connected in cascade between the interference means and one end of the optical path, for phase modulating the both light waves;

photoelectric conversion means for detecting, as an electric signal, the intensity of the interference light;

first and second synchronous detecting means whereby adjacent even-numbered harmonic components of the modulation frequency of the phase modulating means, contained in the output from the photoelectric conversion means, are synchronously detected;

third and fourth synchronous detecting means whereby adjacent even-numbered harmonic components of the modulation frequency of the phase modulating means, contained in the output from the photoelectric conversion means, are synchronously detected;

first phase modulation control means whereby a voltage which is applied to the phase modulator is controlled so that the outputs from the first and second synchronous detecting means become equal to each other; and

second phase modulation control means whereby a voltage which is applied to the phase modulator is controlled so that the ratio between the outputs from the third and fourth synchronous detecting means becomes constant.

8. The fiber optic gyro of claim 7, including third phase modulation control means in place of the first phase modulation control means.

9. A fiber optic gyro comprising:

a circular optical path of at least one loop;

means for allowing light waves to pass through the optical path in both clockwise and counterclockwise directions;

interference means whereby the light waves having passed through the optical path are made to interfere with each other;

phase modulating means connected in cascade between the interference means and one end of the optical path, for phase modulating both light waves;

photoelectric conversion means for detecting, as an electric signal, the intensity of the interference light;

first, second and third synchronous detecting means whereby first, second and third harmonic components of the modulation frequency of the phase modulating means, contained in the electric signal converted by the photoelectric conversion means, are synchronously detected;

adding means whereby the output from the third synchronous detecting means is set to a value about 2.2 times larger than the output from the first synchronous detecting means and is added to the latter; and

means whereby the outputs of the adding means and an electric circuit provided at a stage preceding the second synchronous detecting means or the quantity of light reaching the photoelectric conversion means is controlled so that the sum of squared values of the outputs from the second synchronous detecting means and the adding means becomes constant.

10. The fiber optic gyro of claim 9, further including means whereby a deviation of the absolute quantity of the composite vector of the outputs of the adding means and the second synchronous detecting means from a reference value is obtained and the output of the fiber optic gyro is numerically corrected.