Systems and methods to reduce differential harmonics of resonance tracking modulation in a resonant fiber optic gyroscope

Patent metadata

Field	Value
Publication number	US-11204246-B1
Application number	US-202016900419-A
Country	US
Kind code	B1
Filing date	Jun 12, 2020
Priority date	Jun 12, 2020
Publication date	Dec 21, 2021
Grant date	Dec 21, 2021

Abstract

Systems and methods are provided to reduce at least one differential harmonics of a resonance tracking modulation in a resonant fiber optic gyroscope (RFOG). The fundamental frequency of the resonance tracking modulation of each of the clockwise and counter clockwise optical signals is substantially identical; however, the amplitude and phase of the Nth harmonic of a clockwise (CW) resonance tracking modulation and the Nth harmonic of a clockwise (CCW) resonance tracking modulation may differ due to non-linearities in the RFOG. Embodiments of the invention diminish, e.g., reduce to zero such vectoral difference. Differential harmonics may be generated at one or more harmonics.

First claim

The invention claimed is:

1. A system for diminishing differential harmonics of common resonance tracking modulation in a resonant fiber optic gyroscope (RFOG), comprising:
beat note servo circuitry configured to receive an electrical beat note signal and to generate at least one differential harmonic alternating current (AC) error signal, where the at least one differential harmonic AC error signal is generated using the electrical beat note signal, where the electrical beat note signal is created from a beat note of a first optical signal generated by a first optical laser and a second optical signal generated by a second laser, where the first optical signal and the second optical signal are modulated with the common resonance tracking modulation, where the first optical signal circulates in a first direction in a resonator of the RFOG, and where the second optical signal circulates in a second direction in the resonator of the RFOG that is opposite the first direction;
differential harmonic correction circuitry configured to generate at least one error cancellation signal, where each error cancellation signal is generated using a corresponding differential harmonic AC error signal, and where each error cancellation signal and its corresponding differential harmonic AC error signal correspond to a differential harmonic selected to be diminished; and
adder circuitry configured to add the at least one error cancellation signal to an offset frequency signal to the differential harmonics, where the offset frequency signal comprises a frequency used to tune a carrier frequency of one of the first optical signal and the second optical signal to a resonant frequency in respectively one of the first direction and the second direction;
wherein each differential harmonic of the common resonance tracking modulation is a vectoral difference between a harmonic of the common resonance tracking modulation of the first optical signal and a same harmonic of the common resonance tracking modulation of the second optical signal, and where a harmonic may be a fundamental frequency.

2. The system of claim 1 , wherein the beat note servo circuitry is further configured to generate a beat note frequency.

3. The system of claim 1 , wherein the beat note servo circuitry comprises:
beat note (BN) analog to digital converter (ADC) circuitry comprising an input and an output, where the output of the BN ADC circuitry is configured to receive and digitize the electrical beat note signal;
BN digital mixer circuitry comprising a first input, a second input, and an output, where the first input of the BN digital mixer circuitry is configured to receive the electrical beat note signal, which has been digitized, from the BN ADC circuitry, and where the output of the BN digital mixer circuitry is configured to generate the at least one differential harmonic AC error signal;
digital loop filter circuitry comprising an input and an output, where the input of the digital loop filter circuitry is coupled to the output of the BN digital mixer circuitry; and
a BN numerically controlled oscillator (NCO) comprising an input and an output, where the input of the BN NCO is coupled to the output of the digital loop filter circuitry, and where the output of the BN NCO is coupled to the second input of the BN digital mixer circuitry.

4. The system of claim 3 , wherein the output of the digital loop filter circuitry is configured to be generate a beat note frequency.

5. The system of claim 1 , further comprising a transimpedance amplifier coupled to an input of the beat note (BN) servo circuitry and configured to convert the electrical beat note signal from a current signal to a voltage signal.

6. The system of claim 1 , wherein the differential harmonic correction circuitry comprises at least one error reduction circuit configured to generate an error cancellation signal using a differential harmonic AC error signal for a differential harmonic selected to be cancelled; and
wherein each error reduction circuit comprises:
sine-cosine signal generator (SCG) circuitry comprising a first output configured to provide a sine signal and second output configured to provide a cosine signal at a same frequency;
first digital mixer circuitry comprising a first input, a second input, and an output, where the first input of the first digital mixer circuitry is configured to receive the at least one differential harmonic AC error signal, and where the second input of the first digital mixer circuitry is coupled to the first output of the SCG circuitry;
second digital mixer circuitry comprising a first input, a second input, and an output, where the first input of the second digital mixer circuitry is configured to receive the at least one differential harmonic AC error signal, and where the second input of the second digital mixer circuitry is coupled to the first output of the SCG circuitry;
first accumulator circuitry comprising an input and an output, where the output of the first digital mixer circuitry is coupled to the input of the first accumulator circuitry;
second accumulator circuitry comprising an input and an output, where the output of the second digital mixer circuitry is coupled to the input of the second accumulator circuitry;
first digital multiplier circuitry comprising a first input, a second input, and an output, where the first input of the first digital multiplier circuitry is coupled to the output of the first accumulator circuitry, and where the second input of the first digital multiplier circuitry is coupled the first output of the SCG circuitry;
second digital multiplier circuitry comprising a first input, a second input, and an output, where the first input of the second digital multiplier circuitry is coupled to the output of the second accumulator circuitry, and where the second input of the second digital multiplier circuitry is coupled the second output of the SCG circuitry; and
signal combiner circuitry comprising a first input, a second input, and an output, where the first input is coupled to the output of the first digital multiplier circuitry, and where the second input is coupled to the output of the second digital multiplier circuitry.

7. The system of claim 6 , wherein the differential harmonic correction circuitry further comprises adder circuitry configured to combine the output of each error reduction circuit.

8. A method for diminishing differential harmonics of common resonance tracking modulation in a resonant fiber optic gyroscope (RFOG), comprising:
modulating the common resonance tracking modulation on a first optical signal and a second optical signal;
receiving an electrical beat note signal, where the electrical beat note signal is created from a beat note of the first optical signal generated by a first optical laser and the second optical signal generated by a second laser;
generating at least one differential harmonic alternating current (AC) error signal;
generating at least one error cancellation signal, where each error cancellation signal is generated using a corresponding differential harmonic AC error signal, and where each error cancellation signal and its corresponding differential harmonic AC error signal correspond to a differential harmonic selected to be diminished; and
adding the at least one error cancellation signal to an offset frequency signal to the differential harmonics, where the offset frequency signal comprises a frequency used to tune a carrier frequency of one of the first optical signal and the second optical signal to a resonant frequency in respectively one of a first direction and a second direction;
wherein each differential harmonic of the common resonance tracking modulation is a vectoral difference between a harmonic

Assignees

• Honeywell Int Inc

Inventors

• Strandjord Lee K
• Tarleton Norman Gerard
• Croker Chuck

Classifications

• G01C19/721

  Details, e.g. optical or electronical details · CPC title

• G01C19/727 Primary

  using a passive ring resonator · CPC title