Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
2000-12-20
2003-06-24
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
Reexamination Certificate
active
06583882
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to interferrometric fiber optic gyros (IFOG), and more particularly to lowering the total gyro noise in an IFOG.
2. Description of the Related Art
There is a growing demand for high accuracy gyros for satellite pointing applications. In order to improve gyro sensitivity, it is necessary to lower the total gyro noise. In typical satellite pointing applications, an interferrometric fiber optic gyro (IFOG) is employed. In the IFOG, noise elements arise from both the optical and electrical elements.
A closed loop IFOG is illustrated in FIG.
1
. An IFOG generally includes a light source
10
, a coupler
20
, an integrated optics chip
30
, and a fiber coil
40
, which comprise the optical circuit
5
. The fiber coil
40
provides the rotation-sensitive interferometer. The processing electronics
45
of the IFOG generally comprise a photodetector
50
, an amplifier/filter
60
, an analog-to-digital converter (A/D)
70
, a digital signal processor (DSP)
80
, a digital to analog converter (D/A)
90
and amplifier
95
.
The processing electronics
45
function to provide a feedback phase shift in the optical circuit
5
which effectively nulls out a rotation-induced phase shift sensed in the fiber coil
40
. The magnitude of the feedback phase shift corresponds to the rotation rate. The photodetector
50
converts an optical power output from optical coupler
20
to a corresponding voltage. The corresponding voltage is processed by amplifier/filter
60
and translated to a digital signal by A/D converter
70
. The corresponding feedback signal is calculated in DSP
80
, and fed back into the gyro with a modulating square wave via D/A converter
90
and amplifier
95
.
In a square wave modulated IFOG, a rotation about a rate input axis
41
of the fiber coil
40
produces input signals at the photodetector
50
, which are denoted as “A” and “B” in
FIG. 1A. A
difference A−B corresponds to the sensed input rotation rate and is designated D
error
. The input signal characteristics result from interference patterns of the counter propagating light waves that travel in the fiber optic coil
40
. The sharp spikes present in the waveform are a result of the modulated signals driving the interference patterns through the peak of the interference curves.
An accurate measurement of the magnitude of the A and B levels of the photodetected signals is crucial to the overall performance of the IFOG. However, this measurement accuracy of the low level “A” and “B” signals is compromised by the presence of noise and the high level signal generated by the optical spikes. A high gain is required to maintain the required closed loop performance, but results in signal distortion due to saturation effects from the high optical spike signals in the signal processing electronics
45
, most notably in the photodetector
50
and amplifier/filter
60
.
Prior art signal processing methods utilize very fast D/A converters
90
and amps
95
in modulation schemes to minimize the width of the optical spikes. The photodetector
50
is typically comprised of a photodiode
52
and a wide bandwidth transimpedance amplifier
54
to maintain signal fidelity. The amplifier/filter
60
is typically comprised of several stages of high gain amplification with clipping circuitry and filters to extract the desired A and B signals. The wide bandwidth of the photodetector
50
and amplifier/filter
60
make them susceptible to noise pickup.
Additionally, digital techniques using multiple samples or oversampling are typically employed to average the noise in each A and B sample. The use of oversampling increases system speed requirements and requires the use of high-speed logic and a fast DSP
80
. The fast logic and higher clock speeds increase the likelihood of noise coupling into the high gain wide band photodetector
50
and amplifier/filter
60
, which ultimately degrades IFOG performance.
Therefore, an improved apparatus and method for signal conditioning to reduce IFOG noise are needed.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus for reducing an overall IFOG noise.
It is another object of the present invention to provide an improved method of signal conditioning in an IFOG.
To achieve the above objects, a signal conditioning circuit to reduce an interferrometric fiber optic gyro (IFOG) noise is provided in accordance with the present invention, the IFOG including a light source, IOC and coupler to output an optical power signal corresponding to a rotation-induced phase shift in a fiber coil of the IFOG. The signal conditioning circuit comprises a photodiode that converts the optical power signal to a corresponding electrical compensation signal and at least one digital controlled (switched) integrator that processes the electrical compensation signal. Selectively switching the integrator or integrators between a plurality of modes effectively produces a compensation signal with lower noise characteristics.
A method of signal conditioning in an IFOG to reduce IFOG noise is also provided in accordance with the present invention. The method comprises the steps of converting the optical power signal to a corresponding electrical compensation signal by selectively integrating a current of the electrical compensation signal using at least one switched integrator.
REFERENCES:
patent: 4717256 (1988-01-01), Ensley et al.
patent: 5262843 (1993-11-01), Sugarbaker et al.
patent: 5430545 (1995-07-01), Kovacs
patent: 5467190 (1995-11-01), Desmarais et al.
patent: 5926275 (1999-07-01), Sanders et al.
Kovacs Robert A.
Scruggs Michael K.
Wall Peter A.
Yu Ming-Hsing
Honeywell International , Inc.
Turner Samuel A.
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