RU2523759C1 - Angular velocity range extension for open-circuit fibre-optic gyro - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: fibre-optic gyro comprises fibre ring interferometer and electronic data processing unit. The latter comprises sync detector to isolate spinning signal and electronic device for accumulated data division at sync detector output to permanent signal component at sync detector input. Mismatch signal zeroing feedback circuit includes auxiliary FM voltage generator. Angular velocities are measured at Sagnac phase difference variation in the range of ±[(0÷π/4)] rad; ±[(3π/4+2πn)÷(5π/4+2πn)] rad; ±[(7π/4+2πn)÷(9π/4+2πn)] rad, where n=0, 1, 2…. Ring interferometer beam phase difference modulation is used as a intermittent series of rectangular pulses with amplitude of ±π/2 rad and ±3π/2 rad. Angular velocities are measured at Sagnac phase difference variation in the range of ±[(π/4+2πn)÷(3π/4+2πn)] rad and ±[(5π/4+2πn)+(7π/4+2πn)] rad. Ring interferometer beam phase difference modulation is used as a intermittent series of rectangular pulses with amplitude of 0 rad and ±π rad.

EFFECT: extended range of measurements.

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Description

The invention relates to the field of fiber optics and can be used in the design of fiber-optic gyroscopes and other sensors of physical quantities based on single-mode optical fibers.

The fiber optic gyroscope (hereinafter referred to as FOG) contains a fiber ring interferometer (FRI) and an electronic information processing unit. The FRI contains a source of optical radiation, a fiber divider of radiation power, an integrated optical circuit (hereinafter - IOS), a multi-turn sensitive coil and a photodetector. IOS contains in its composition a Y-divider of the power of optical radiation based on polarizing channel waveguides and a phase modulator located on the output arms of the Y-divider. Channel waveguides of the Y-divider are formed in a lithium niobate substrate using proton-exchange technology, which allows the waveguides to acquire polarizing properties. On the output channel waveguides is a phase modulator of optical rays passing through the channel waveguides. The phase modulator is a metal electrode deposited on both sides of the channel waveguides. When an electric voltage is applied to the electrodes due to the electro-optical effect in the material of the channel waveguides, the refractive index changes, which leads to the effect of phase modulation of the optical rays propagating along the channel waveguides. The ends of the optical fibers of the sensitive gyro coil are docked to the output waveguides of the Y-divider.

An interference pattern is formed on the FRI photodetector, formed by two optical beams that have passed the sensitive coil of the gyroscope in two mutually opposite directions. When the ring interferometer rotates between these two beams, due to the Sagnac effect, a phase difference arises, which is expressed as follows:

F _c = [4πRL / λc] × Ω,

where R is the radius of the sensitive coil of the gyroscope;

L is the fiber length of the coil;

λ is the central wavelength of the radiation source;

c is the speed of light in vacuum;

Ω is the angular velocity of rotation of the gyroscope.

Thus, the optical radiation power at the photodetector can be represented as:

P _f = ½P ₀ (1 + cos Φ _s ),

where P ₀ is the power of the rays interfering at the photodetector.

To increase the sensitivity of VOG near zero angular velocities, auxiliary phase modulation is used. To achieve the effect of phase modulation of rays in a ring interferometer using a phase-modulated IOS, the time delay of the fronts of rays interfering on the photodetector is used when passing through the phase modulator of the IOS. This time delay is equal to the travel time of light rays along the light guide of the FRI sensitive coil and is:

, $$\tau =\frac{L{n}_0}{c}$$

,

where n ₀ is the refractive index of the material of the fiber of the sensitive coil.

When applying voltage pulses to the phase modulator that follow with a frequency of 1 / 2τ, the photodetector current can be represented as:

I _f = ½P _o η _f (1 + cosθ _m sosθ _c + sinθ _m + sinF _c ),

η _f - current / sensitivity of the photodetector,

θ _m is the amplitude of the auxiliary phase modulation.

Next, the signal from the photodetector is fed to the input of the photodetector current amplifier, at the output of which there is a voltage proportional to:

U = ½P _о η _f R _H (1 + cosθ _m cosθ _c + sinθ _m sinФ _c ),

where R _H is the load resistance of the current amplifier of the photodetector.

After applying voltage to the phase modulator in the form of a sequence of pulses following with a frequency of 1 / 2τ and with amplitudes introducing the phase difference of the rays (π-Δ) and (π + Δ) radians, the voltage at the output of the photodetector current amplifier can be represented as [1 ]:

U = ½P _about η _f R _n (1-cosΔcos _{c c} ± sinΔsin _{c c} ).

Further, the voltage is supplied to one of the two inputs of the differential amplifier [2]. The voltage from the output of the differential amplifier is fed to the input of the ADC and then to the digital part of the circuit of the electronic unit. The digital part of the circuit is performed either on a programmable logic integrated circuit (FPGA) or on the basis of a microprocessor. Using the synchronous detection, the rotation signal is isolated in the digital part of the circuit, as well as the constant component of the common signal, which are fed to the input of the signal division circuit and when Δ = π / 2 radians, a signal of the form is formed at its output:

U _d = 2tg (π / 4 + δΔ) × sinФ _c ,

where δΔ is the instability of the amplitude of the phase modulation.

It follows from the expression for the voltage at the output of the signal division circuit that the open-loop FOG coefficient does not depend on fluctuations in the amplitude of the rotation signal, but the dependence on the instability of the phase modulation amplitude (π-Δ) and (π + Δ) radians remains.

To stabilize the amplitude of the auxiliary phase modulation, a feedback loop is used, which includes a third synchronous detector to isolate the error signal, which is formed on the photodetector when the efficiency of the phase modulators changes. The signal from the output of this synchronous detector is fed to the input of the voltage amplitude regulator of the auxiliary phase modulation (VFM), which changes the voltage so that the signal at the output of the synchronous detector is kept equal to zero. This means that the amplitude of the WFM is stable, and in this case, at the output of the synchronous detector of the rotation signal, the signal takes the form:

U _d = 2 × sinF _s .

A disadvantage of the known open-loop VOG information processing scheme is the small measurement range of angular velocities. When the Sagnac phase difference is greater than π / 4 radians, the FOG sensitivity begins to fall, and when the phase difference is π / 2 radians, it becomes equal to zero.

The aim of the present invention is to expand the range of measurement of angular velocities.

This goal is achieved by the fact that for measuring angular velocities when changing the Sagnac phase difference in the ranges ± [(0 ÷ π / 4)] radians; ± [(3π / 4 + 2πn) ÷ (5π / 4 + 2πn)] radians; ± [(7π / 4 + 2πn) ÷ (9π / 4 + 2πn)] radians, where n = 0, 1, 2, 3, ..., use the phase difference modulation of the rays of the ring interferometer in the form of a periodic sequence of rectangular pulses with amplitudes ± π / 2 radians and ± 3π / 2 radians, and for measuring angular velocities when changing the Sagnac phase difference in the ranges ± [(π / 4 + 2πn) ÷ (3π / 4 + 2πn)] radians; ± [(5π / 4 + 2πn) ÷ (7π / 4 + 2πn)] radians; they use modulation of the phase difference of the rays of the ring interferometer in the form of a periodic sequence of rectangular pulses with amplitudes of 0 radians and ± π radians.

The extension of the range of measurement of angular velocities is achieved through the use of two types of WFM with different amplitudes, which allows measurements of angular velocities in a wide range of changes in the Sagnac phase difference without loss of FOG sensitivity.

The invention is illustrated by drawings. Figure 1 shows the structural diagram of the VOG with an open circuit. Figure 2 shows the types of output characteristics of the VOG with an open circuit in different sub-ranges of the Sagnac phase difference. Figure 3 shows the voltage of the WFM and the law of the change in the phase difference of the rays of the FOG ring fiber interferometer when using the first type of WFM. Figure 4 shows the formation of the rotation signal of the VOG when using the first type of VFM. Figure 5 shows the formation of the VOG mismatch signal when using the first type of VFM. Figure 6 shows the structure of the common signal at the output of the current amplifier of the VOG photodetector when using the first type of VFM. Figure 7 shows the voltage and phase difference of the optical beams of the FOG ring fiber interferometer when using the second type of VFM. On Fig shows the formation of the rotation signal of the VOG when using the second type of WFM. Figure 9 shows the formation of the VOG mismatch signal when using the second type of VFM. Figure 10 shows the structure of the common signal at the output of the current amplifier of the VOG photodetector when using the second type of VFM.

Figure 1 shows the structural diagram of the VOG with an open circuit. Radiation from a light source 1 with a short coherence length, for example an erbium fiber superluminescent source (EMCI), is fed to the input of an optical fiber fiber divider 2. As a fiber splitter, a three-port fiber splitter or fiber circulator can be used. Next, the radiation from the output of the radiation divider is fed to the input of an integrated optical circuit (IOS) 3. IOS is performed on a substrate of lithium niobate having an electro-optical effect. A proton-exchange Y-splitter of optical radiation is formed in the substrate. Channel waveguides of a Y-coupler have a polarizing effect. A phase modulator is formed on the output arms of the Y-coupler by applying a phase modulator on both sides of the channel waveguides of the metal electrodes. The output waveguides of the Y-coupler are connected with adhesive joints to the two ends of the optical fiber of the sensitive 4 FOG coil. Thus, the light beam from the EVSI passes through the fiber radiation divider and enters the input of the Y-divider of the IOS. The Y-divider divides the optical beam into two beams that pass through the fiber of the sensitive VOG coil in two opposite directions, then these two beams are combined by the IOS Y-splitter and, passing the fiber divider, get to the area of the photodetector 5, where they form the interference pattern of the rays transmitted sensitive coil clockwise and counterclockwise. The output of the photodetector is connected to the input of the current amplifier of the photodetector 6, then the voltage from the output of the current amplifier of the photodetector is supplied to one of the two inputs of the differential amplifier 7. The voltage from the output of the differential amplifier is fed to the input of an analog-to-digital converter (ADC) 8. The signal from the ADC is input digital information processing board 9. The signal from the ADC output is fed to the inputs of synchronous detectors 10, 11, which select the amplitude of the rotation signal and the constant component of the signal, respectively. The signal from the output of the synchronous detector to isolate the constant component of the signal is fed to the input of circuit 12 with a transmission coefficient 1 / (1-α), where α <1. Next, the signal from the output of the circuit with the transmission coefficient 1 / (1-α) is fed to the input of the circuit with the transmission coefficient 1 / G 13, where G is the gain of the differential amplifier. The signal from the output of this circuit through a digital-to-analog converter (DAC) 14 is fed to an electric voltage divider 15 with a transmission coefficient a and then to the second input of a differential amplifier. Thus, at the output of the differential amplifier there is a signal of the form:

U _du = ½P _about η _f R _n G × [(1-cosΔcos Φ _c ) × (1-α) ± sinΔsin Φ _c ].

Thus, for α <1, only a part of the constant component of the common signal at the photodetector passes through the differential amplifier.

Next, the signal from the output of the circuit with the transmission coefficient 1 / (1-α) is fed to the input of the division circuit 16. At the input of the division circuit, the signal has the following form:

U _const = ½P _о η _f R _n G × [(1-cosΔcos Φ _c ).

The second input of the division circuit also receives a signal from the output of the synchronous detector of the rotation signal. At the output of the synchronous rotation signal detector there is a signal of the form:

U _Ω = P _o η _f R _n G × (sinΔsin Ф _c ).

Thus, the signal at the output of the division circuit can be represented as:

U o _out = [2sinΔ / (1 + cosΔcos Ф _c ) × sin Ф _c .

To exclude the influence of the angular velocity of rotation on the FOG scale factor with an open loop, Δ = π / 2 radians is selected, and then the signal at the output of the division circuit can be represented:

U _O = 2ctg (Δ / 2) × sinF _c.

It follows from the above relation for the VOG output signal with an open circuit that its scale factor depends on the stability of the amplitude of the WFM. The instability of the amplitude of the WFM is mainly determined by the instability of the effectiveness of the phase modulators of the IOS (instability of their half-wave voltage). To ensure the stability of the WFM in the digital part of the circuit, a closed feedback loop is organized. For this purpose, a third synchronous detector 17 is used to extract the error signal [1]. The mismatch signal appears in the structure of the general signal when the efficiency of the phase modulators of the IOS changes. After the synchronous detector, the error signal is supplied to the regulator 18, which controls the amplitude of the voltage pulses of the WFM generated by the generator 19. The voltage of the WFM through the DAC 20 and the operational scaling amplifier 21 is supplied to the electrodes of the phase-modulated IOS. The regulator controls the amplitude of the voltage pulses in such a way as to keep the signal at the output of the detector of the error signal equal to zero. In this case, for the signal at the output of the signal division circuit, you can write:

U _out = 2 × sinF _c.

Thus, the FOG scale factor with an open circuit does not depend on a change in the amplitude of the rotation signal, or on the instability of the efficiency of phase modulators of IOS.

The sin function has the maximum derivative in the range of the Sagnac phase difference in the range ± π / 4 radians, and hence the maximum sensitivity to angular velocity. With a further increase in the Sagnac phase difference, the sensitivity of the open-loop FOG begins to decrease, and at ± π / 2 the radian generally becomes equal to zero. Figure 2 shows a view of the FOG output characteristic with an open circuit in different subranges of the Sagnac phase difference, namely, the sin 22 and cos 23 curves. It can be seen that when the sin curve has zero sensitivity to angular velocity, the cos curve, on the contrary, has maximum sensitivity to angular velocity and vice versa. Thus, to ensure maximum sensitivity of the open-loop VOG to angular velocity, it is necessary in the subranges of the Sagnac phase difference to vary 24-25; 26-27; 28-29, as well as in subbands 24-30; 31-32; 33-34; use the sin function to measure angular velocity. The measured Sanka phase difference in this case can be represented as ± [(0 ÷ π / 4)] radian; ± [(3π / 4 + 2πn) ÷ (5π / 4 + 2πn)] radians; ± [(7π / 4 + 2πn) ÷ (9π / 4 + 2πn)] radians, where n = 0, 1, 2, 3. In the remaining sub-ranges of the Sagnac phase difference, it is necessary to use the cos function to measure the angular velocity to ensure maximum sensitivity VOG. The measured Sagnac phase difference in this case can be expressed as follows, ± [(π / 4 + 2πn) ÷ (3π / 4 + 2πn)] radians; ± [(5π / 4 + 2πn) ÷ (7π / 4 + 2πn)] rad.

For use in different subranges for measuring the angular velocity of the sin and cos functions, two types of WFM are used. Figure 3 shows the voltage 35 of the WFM of the first type and the corresponding change in the phase difference of the rays of the ring interferometer 36. For Δ = π / 2 radian, the phase difference of the rays of the ring interferometer is a pulse sequence with the amplitude of the pulses of the phase difference ± π / 2 radians and ± 3π / 2 rad. Figure 4 presents the formation of the rotation signal when using the WFM of the first type. The figure shows the change in the Sagnac phase difference in the range ± π / 4 radians. When superimposing the phase difference determined by the WFM on curve 37 and at a nonzero angular velocity (the shift of the phase difference of the WFM at a nonzero angular velocity is shown by dashed line 38), a rotation signal 39 is present on the VOG photodetector.

Figure 5 shows the formation of the rotation signal when using the first type of VFM. The dashed line 40 shows the change in the amplitude of the WFM with an increase in the efficiency of the phase-modulated IOS, and a mismatch signal of type 41 is generated on the photodetector. Figure 6 shows the structure of the common signal on the photodetector when using the first type of WFM. The common signal contains a rotation signal at a frequency of 6 / τ, where τ is the optical path along the fiber of the sensing coil, a mismatch signal at a frequency of 3 / τ, and a constant component 42. When using the first type of WFM, the signal at the output of a synchronous detector of a rotation signal can be represented as:

. $${\text{U}}_{\text{âûõ}}^{\text{one}}=\text{2}\times {\text{sinФ}}_{\text{c}}$$

.

7 shows the voltage 43 of the second type of VFM and the corresponding change in the phase difference of the rays of the ring interferometer 44. The phase difference of the rays of the ring interferometer contains pulses, each of a duration multiple of τ, and having an amplitude of 0 radians and ± π radians. On Fig shows the formation of the rotation signal in the measuring range of the Sagnac phase difference from π / 4 radians to 3π / 4 radians when using the second type of WFM. Dashed line 45 shows the shift of the WFM phase difference with increasing angular velocity, as a result of which a rotation signal 46 is generated at the photodetector. Figure 9 shows the formation of the error signal when using the WFM of the second kind. The dashed line 47 shows the increase in the amplitude of the pulses of the WFM phase difference with an increase in the efficiency of the phase-modulated IOS, as a result of which a signal of the form 48 is generated at the photodetector. The signal contains a rotation signal at a frequency of 1 / 4τ, and a mismatch signal at a frequency of 1 / 2τ and a constant component U _p 50. The signal at the output of the division circuit in the case of using the second type of VFM can be represented as:

.

.

The type of the FOG output characteristic with an open circuit and the methods of its linearization are considered in [3].

Literature

1. A.M. Kurbatov, R.A. Kurbatov. "Ways to improve the accuracy of fiber optic gyroscopes." "Gyroscopy and Navigation." UDC 531.383 No. 1 (76). 2012, pp. 102-121.

2. A.M. Kurbatov, R.A. Kurbatov. “Electronic circuit of the primary signal processing of the photodetector of a fiber-optic gyroscope”, application No. 20111132181, priority from 07.29.2011

3. N.J. Frigo. "A constant accuracy, high dynamic range fiber optic gyroscope." Proc. SPIE, v. 719, p. 155, 1986.

Claims (1)

A method for expanding the angular velocity measurement range of an open-loop fiber-optic gyroscope containing a fiber ring interferometer and an electronic information processing unit, which in turn contains a synchronous detector for isolating the amplitude of the rotation signal and an electronic device for dividing the accumulated information at the output of the synchronous detector by the constant component of the signal at the input of the synchronous detector, as well as the feedback loop for zeroing the error signal and containing the generator voltage of auxiliary phase modulation, when applied to the phase modulator, the phase difference between the beams of the ring interferometer is formed in the form of a periodic sequence of rectangular pulses, a synchronous detector for isolating the error signal when changing the efficiency of the phase modulator and the amplitude modulator of the phase modulation pulses, characterized in that for measurement angular velocities when changing the Sagnac phase difference in the ranges ± [(0 ÷ π / 4)] radians; ± [(3π / 4 + 2πn) ÷ (5π / 4 + 2πn)] radians; ± [(7π / 4 + 2πn) ÷ (9π / 4 + 2πn)] radians, where n = 0, 1, 2 ..., use the phase difference modulation of the rays of the ring interferometer in the form of a periodic sequence of rectangular pulses with amplitudes ± π / 2 radians and ± 3π / 2 radians, and for measuring angular velocities when changing the Sagnac phase difference in the ranges ± [(π / 4 + 2πn) ÷ (3π / 4 + 2πn)] radians; ± [(5π / 4 + 2πn) + (7π / 4 + 2πn)] radians; they use modulation of the phase difference of the rays of the ring interferometer in the form of a periodic sequence of rectangular pulses with amplitudes of 0 radians and ± π radians.

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