JP3390943B2 - Optical interference angular velocity meter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical interference gyroscope, and more particularly to an optical interferometer that reduces the influence of aliasing noise when AD conversion is performed on a detection signal output from a light receiver to reduce random noise of a gyro output. Interference gyroscope.

[0002]

2. Description of the Related Art A conventional example will be described with reference to FIG. Figure 4
Shows a conventional example of a closed loop type optical interference gyro.
In FIG. 4, the light emitted from the light source 1 passes through the optical coupler 2, the polarizer section 31 of the optical integrated circuit 3, and the Y branch section 32, and is turned clockwise (CW) to both ends of the optical fiber coil 4 as an optical path.
Light) and counterclockwise light (CCW light). The CW light and the CCW light propagating through the optical fiber coil 4 are phase-modulated by the optical phase modulator 33 and the optical phase modulator 34 arranged at one end of the Y branch section 32. That is, the optical phase modulator 3
A phase modulation signal is applied to 3 to phase-modulate the CW light and the CCW light at one end of the optical fiber coil 4.
On the other hand, a feedback phase signal is applied to the optical phase modulator 34 so that the phase difference between the two lights at the other end of the optical fiber coil 4 is always canceled to zero. The CCW light is incident on one fiber end of the phase-modulated CW optical fiber coil 4, while the CCW light is incident on the other fiber end of the optical fiber coil 4. The CW light propagates in the clockwise direction in the optical fiber coil 4 and exits through the other fiber end, and is sent to the optical integrated circuit 3 again. The CCW light also propagates through the optical fiber coil 4 in the counterclockwise direction, exits through one of the fiber ends, and is sent to the optical integrated circuit 3 again. Here, when an angular velocity is input with respect to the central axis of the optical fiber coil 4, a Sagnac phase difference is generated between the CW light and the CCW light based on the Sagnac effect. The CW light and the CCW light sent to the circuit 3 interfere with each other in the Y branch section 32 of the optical integrated circuit 3 to generate interference fringes. This interference light is received by the light receiver 5 which is a photoelectric converter via the polarizer section 31 and the optical coupler 2,
The interference light intensity of the interference fringe is converted into an electric signal. Based on this electric signal, the angular velocity input around the central axis of the optical fiber coil 4 can be detected. This optical interference angular velocity meter gives a feedback phase to CW light and CCW light in a direction to cancel the Sagnac phase difference generated based on the input angular velocity, and always controls the phase difference between both lights of the optical fiber coil 4 to zero. Closed loop control is performed, the amount of feedback given is measured, and this is detected as the input angular velocity. As a result, the closed-loop optical interference gyro is hardly affected by fluctuations in the amount of light emitted from the light source, fluctuations in transmission loss of the optical section, and other fluctuations.

Here, how to provide the feedback phase will be described. After the photoelectrically converted electric signal corresponding to the input angular velocity is amplified to an appropriate value in the photodetector 5,
It is supplied to the D converter 6. From the digital electric signal AD-converted by the AD converter 6, a synchronous detection circuit 72 of the digital signal processing unit 7 extracts a control error signal in a closed loop.

The feedback signal output from the synchronous detection circuit 72 is integrated by the integrator 73, the integrated output is input to the digital phase ramp generating circuit 74, and a stepped sawtooth wave having a repeating frequency corresponding to the integrated output is generated. Is generated. Next, the stepped sawtooth wave is DA converted by the DA converter 9 and applied as an analog feedback phase signal to the optical phase modulator 34 installed in the optical integrated circuit 3 so as to be applied between the left and right lights of the optical fiber coil 4. A feedback phase difference Δφ _f is generated.

As the feedback phase signal, the phase difference of the light from the optical fiber coil 4 is 2 as shown in FIG.
A stepped sawtooth wave (digital phase ramp) that is reset at π is used. The width of one step of this stepped sawtooth wave is set in accordance with the transit time τ of the light propagating through the optical fiber coil 4, and the height of one step appears in proportion to the input angular velocity. Since the phase shifts of the stepwise sawtooth waves received by the left and right lights propagating through the optical fiber coil 4 are deviated from each other by τ time, a feedback phase difference Δφ _f equal to the height of one step is given between the two lights. To be done. This feedback phase difference Δφ _f appears in the closed loop operating state as having the opposite polarity and the same absolute amount as the Sagnac phase difference Δφ _s . on the other hand,
The repetition frequency f of the stepwise sawtooth wave is: f = 2R · Ω / nλ (1) where R: radius of the optical fiber coil n: refractive index of the optical fiber λ: wavelength of light The sawtooth wave shown in FIG. 6 can also be used as the feedback phase signal. The repetition frequency of the sawtooth wave is the same as the expression (1), and is proportional to the input angular velocity Ω. Therefore, the pulse output of this frequency can be taken out as the gyro output.

Applying the phase difference biasing will be described with reference to FIG. In the above closed-loop optical interference gyro, a square wave with a pulse width of τ time gives + π /
Alternating phase modulations of 4 and -π / 4 are applied. Focusing on the points of + π / 2 and −π / 2, which are the maximum rates of change of interference light, + π / 2 and −π / 2 alternate between both lights, as shown in FIG. 7B. A given rectangular wave phase difference biasing is applied.

The area I in FIG. 7B shows that the control error in the closed loop control is 0, that is, the phase difference between the two lights is 0. Region II shows the case where a control error occurs and a phase difference Δφε (= Δφ _s −Δφ _f ) occurs between the two lights. As a result, the interference light I reaching the photodetector 5 in FIG. 4 has the phase difference biasing Φ ₁ , Φ ₂ , ...
.., .PHI. ₈ and appear as I ₁ , I _2, ..., I ₈ . I _1, corresponding to the [Phi ₂ I _2, I ₃ corresponding to the [Phi ₃ in the region of I corresponding to the Φ _1, Φ ₄
There is no level difference between I ₄ and I ₄ , which correspond to Φ ₅ , but in the region of II, there is a level difference between I ₅ and I ₇ corresponding to Φ ₅ and Φ ₇ and I ₆ and I ₈ corresponding to Φ ₆ and Φ _8. Occurs.

Here, the interference light output corresponding to I ₁ , I ₃ , I ₅ , and I ₇ is I _A , and the interference light output corresponding to I ₂ , I ₄ , I ₆ , and I ₈ is I _B. , I _A and I _B can be expressed by the following equations with reference to the graph of the amount of received light in FIG. _{I A = - (P 0/} 2) sinΔφε ········ (2) I B = (P 0/2) sinΔφε ········ (3) the difference between I _A and I _B When seeking _{ΔI ΔI = I a -I B =} -P 0 sinΔφε ················ (4) where, P _0: maximum interference light reaching the light receiver Value Δφε: Control error (phase = Δφ _s −Δφ _f ) Δφ _s : Sagnac phase difference Δφ _f : Feedback phase difference. Equation (4) indicates that the control error Δφε can be known by knowing ΔI. The interference light is photoelectrically converted in the light receiver 5, amplified by the amplifier 50, and then input to the AD converter 6. In the AD converter 6, the electric signals corresponding to the interference lights I ₁ , I ₂ , ... Are sequentially read and converted into digital signals. The synchronous detection circuit 72 sequentially subtracts the encoded digital signal and outputs a digital amount corresponding to the equation (4).

[0009]

An AD converter 6 converts the photoelectrically converted analog signal corresponding to the detected interference light.
In the case of inputting to and sampling in, the sampling frequency f _s is usually set to f _s > 2f _c with respect to the maximum frequency f _c of the original analog signal in accordance with the sampling theorem because the original analog signal needs to be restored. .

However, in the case of the conventional example of the above-mentioned optical fiber gyro, as shown in FIG. 7, I ₂ , I ₂ , ...
When reading the level of the received light amount indicated by, it is necessary that the rectangular wave analog signal is not deformed as much as possible. To this end, the cutoff frequency f _LPF of a low-pass filter (LPF) (not shown) inserted from the photodetector 5 to the front stage of the AD converter 6 is set sufficiently higher than the sampling frequency f _s , and f _LPF > 10 f _s . I'm designing. Therefore, the analog signal including noise has a frequency component higher than the sampling frequency f _s . Therefore, the frequency components higher than the sampling frequency f _s are in the undersampling state, which causes aliasing noise. The generation of this aliasing noise causes an increase in random noise as the gyro output.
Here, when this low pass filter is configured by a low pass filter with a first-order delay, the equivalent noise band f
_n is expressed as follows.

F _n ≈1.6 f _LPF (5) On the other hand, the noise output from the light receiver 5 is changed to V _n [V _rms / √
(Hz)], the random noise component V _n1 [V _rms ] included in the analog signal input to the AD converter 6
Is expressed by the following equation. V _n1 = V _n √ (f _n ) = V _n √ (1.6f _LPF ) ... (6) Here, when f _LPF = 10 f _s , the formula (6) is V _n1 = 4V _n √ ( f _s ) ... (7) Incidentally, the conditions of the limit causing no aliasing noise, because it is _{f c = (1/2) f s} , the random noise component V when no aliasing noise
_From the equation (6), _n2 becomes V _n2 = V _n √ (f _n / 2) ... (8). By the way, the maximum frequency component f _{c of} random noise
Is the same as the equivalent noise band f _n of the low pass filter.

Here, the ratio of the noise level in the undersampling state and the noise level in the absence of aliasing noise will be taken. V _n1 / V _n2 = 4√ (2) = 5.66 (9) That is, the conventional example of the optical fiber gyro is compared with the random noise included in the original optical fiber gyro. 5.66
This means that double noise is generated by undersampling, and the generation of this aliasing noise is an obstacle in improving the accuracy of the optical fiber gyro. And
In the analog signal processing unit 5 ′ including the light receiver 5 and the amplifier 50, when the amplification degree of the amplifier 50 is increased, noise is naturally amplified correspondingly, and the input range of the amplifier 50 and the AD converter 6 is limited. In some cases, the necessary and sufficient amplification degree cannot be obtained. In particular, when the amount of light reaching the light receiver 5 is small, the necessary and sufficient amplification degree cannot be obtained.

There are the following cases where it is necessary to suppress the amount of light reaching the light receiver 5 to a small extent. When the optical fiber gyro is operated at a high temperature or is used for a long life, the light amount of the light source of the optical fiber gyro is narrowed down and used. Light is distributed from one light source to a multi-axis optical fiber gyro for use.

The above-mentioned conventional examples of the optical fiber gyro have been difficult to be used for these and other cases. The present invention provides an optical interference angular velocity meter that solves the above problems.

[0015]

According to a first aspect of the present invention, there is provided an optical interference angular velocimeter for detecting a rotational angular velocity input around an axis of an optical fiber coil by an intensity of interference light of left and right circulating lights propagating in opposite directions in the optical fiber coil 4. In the optical fiber coil 4, the optical phase modulator 33 for alternately applying the optical phase difference biasing of mutually opposite polarities between the left and right circulating lights, and the optical receiver 5 and the optical receiver 5 for detecting the interference light intensity are provided. An analog signal processing unit 5'comprising an electric filter 51 for inputting the obtained electric signal and extracting a frequency component in a specific range, and a peak sampling circuit 52 for sampling positive and negative peak values of an analog output signal of the electric filter 51. And an AD converter 6 for converting the output signal of the analog signal processing unit 5 ′ into a digital signal.
An optical interferometer angular velocity meter having a digital signal processing unit 7 for processing the digital output signal of the D converter 6 and extracting the output signal corresponding to the input angular velocity is constructed.

According to a second aspect of the present invention, in the optical interference gyro according to the first aspect, the electrical filter comprises an electrical bandpass filter 51 and the electrical bandpass filter 5
The center frequency of 1 constituted the optical interference gyro which coincided with the frequency of the optical phase difference biasing. Further, claim 3:
In the optical interference gyro according to any one of claims 1 and 2, in the optical phase difference biasing by the optical phase modulator 33, an optical phase difference of ± π / 2 is alternately applied between both orbiting lights. An optical interferometer gyro which is the one to be used was constructed.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment will be described with reference to FIG. In FIG. 1, members common to those in FIG. 4 are designated by common reference numerals. In the embodiment of FIG. 1, the analog signal processing unit 5 ′ in FIG. 4 is connected to an electric bandpass filter 51 after the amplifier 50, and a peak sampling circuit 52 is connected after the electric bandpass filter 51. Equivalent to a thing. The output terminal of the peak sampling circuit 52 is connected to the input terminal of the AD converter 6. This electrical bandpass filter 51
Has its center frequency f ₀ set equal to the phase modulation frequency f _p . The phase modulation frequency f _p is normally set to f _p = 1 / 2τ because it is necessary to generate a rectangular wave phase difference biasing of ± π / 2. The reason is ± π /
In order to apply the phase modulation signal of No. 4 to the phase modulator and to generate the rectangular wave-like biasing with the phase difference of ± π / 2 in the left and right light, it is necessary that the width of the rectangular wave is τ. . However, the time during which the left and right light propagates in the optical fiber coil: τ = nL / c, n is the refractive index of the optical fiber coil, c is the speed of light, and L is the length of the optical fiber coil. Here, referring to FIG. 8, FIG. 8B shows a case where the width of the rectangular wave is not τ. In this case, the phase difference biasing is discontinuous where it is surrounded by the dotted circles, and does not result in a rectangular wave as a whole. FIG. 8C shows a case where the width of the rectangular wave is τ. In this case, the phase difference biasing becomes a rectangular wave as a whole as shown by the solid line.

The gain characteristic of the electric band pass filter 51 can be designed as shown in FIG. Selectivity: If Q = f ₀ / (f ₂ −f ₁ ), f _s = f ₀ , and Q = 5, the equivalent noise band B of the electric bandpass filter 51 is set.
Becomes 1.2f _s /5=0.24f _s . This sufficiently satisfies the condition of f _s > 2f _c at which aliasing noise occurs.

Here, referring also to FIG. 3, the photoelectrically converted analog signal corresponding to the interference light is shown in FIG.
The signal as shown in FIG.
After being amplified to a necessary voltage in, the electric bandpass filter 51 is allowed to pass. The output signal that has passed through the electrical bandpass filter 51 has a sinusoidal waveform as the intensity of the interference light changes, as shown in FIG.
This sinusoidal waveform has a peak sampling circuit 52
Entered in. In the peak sampling circuit 52, at the midpoint of the time of each phase biasing of the optical phase modulation corresponding to the interference light outputs I ₁ , I ₂ , ...
The waveform of (b) is sampled, the positive peak value and the negative peak value of this waveform are acquired, and this is sample-held. FIG. 3C shows a clock pulse train to be encoded. The positive and negative peak values sampled and held by the peak sampling circuit 52 are then
It is input to the AD converter 6 and AD-converted.

[0020]

As described above, according to the present invention, the equivalent noise band can be significantly reduced by providing the electrical bandpass filter in the preceding stage of the AD converter. As a result, it is possible to reduce the influence of aliasing noise generated by data sampling, and eventually contribute to suppressing the output noise of the optical interference gyro as small as possible. Then, by reducing the size of noise included in the analog signal, the amplification degree of the analog signal processing unit can be increased as required, and even when it is necessary to suppress the amount of light reaching the light receiver to be small. The amplification degree can be increased to meet the demand.

As described above, the present invention eliminates aliasing frequency noise in an optical fiber gyro that performs digital phase modulation signal processing. However, a peak sampling circuit is provided and an analog electrical bandpass is provided. By performing peak sampling of the sinusoidal signal obtained through the filter, ringing due to digital discontinuous optical phase difference biasing, noise generated at the photoreceiver, spike noise from the outside, and other noise However, it has the effect of effectively removing it.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an example.

FIG. 2 is a diagram showing a gain characteristic of an electrical bandpass filter.

FIG. 3 is a diagram illustrating an analog signal processing unit.

FIG. 4 is a diagram illustrating a conventional example.

FIG. 5 is a diagram illustrating a feedback phase signal.

FIG. 6 is a diagram illustrating another feedback phase signal.

FIG. 7 is a diagram illustrating phase difference biasing.

FIG. 8 is a diagram for explaining the reason why the phase modulation frequency is set to 1 / 2τ.

[Explanation of symbols]

1 light source 2 Optical coupler 3 Optical integrated circuit 31 Polarizer part 32 Y branch 33 Optical phase modulator 34 Optical Phase Modulator 4 Optical fiber coil 5 Light receiver 5'analog signal processing unit 50 amplifier 51 Electrical bandpass filter 52 Peak sampling circuit 6 AD converter 7 Digital signal processor 72 Synchronous detection circuit 73 Integrator 74 Digital Phase Ramp Generator 9 DA converter

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01C 19/64-19/72

Claims (3)

(57) [Claims] 1. An optical interference angular velocimeter for detecting a rotational angular velocity input around an axis of an optical fiber coil based on an interference light intensity of left and right circulating lights propagating in opposite directions to each other in the optical fiber coil. It is equipped with an optical phase modulator that alternately applies optical phase difference biasing of opposite polarities between orbiting light, a photodetector that detects the intensity of coherent light, and an electrical signal obtained from the photoreceiver is input to input frequency components in a specific range. And an analog signal processing unit including a peak sampling circuit for sampling positive and negative peak values of an analog output signal of the electric filter, and an AD for converting an output signal of the analog signal processing unit into a digital signal It is equipped with a converter and processes the digital output signal of the AD converter to obtain the output signal corresponding to the input angular velocity. Optical interference gyro which is characterized by comprising a digital signal processing unit to issue. 2. The optical interference gyro according to claim 1, wherein the electric filter is an electric bandpass filter, and the center frequency of the electric bandpass filter is equal to the frequency of the optical phase difference biasing. An optical interference angular velocity meter characterized by the above. 3. The optical interference angular velocity meter according to claim 1, wherein the optical phase difference biasing by the optical phase modulator is an optical phase difference of ± π / 2 between both orbiting lights. An optical interference angular velocimeter characterized by applying alternately.