RU2512598C1 - Method of reducing precision readiness time of fibre-optic gyroscope - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to fibre optics and can be used in designing fibre-optic gyroscopes. The gyroscope includes two feedback loops, the first of which is used to provide linearity of the output characteristic via Sagnac phase difference compensation by applying stepped saw-tooth voltage to phase modulators of an integrated optical system. The second feedback loop is used to provide scaling coefficient stability by stabilising the auxiliary phase modulation amplitude. The code of the output signal of the gyroscope is corrected using the code of the amplitude of the auxiliary phase modulation voltage. Efficiency of phase modulators of the integrated optical circuit is stabilised by stabilising temperature. The code of the output signal of the gyroscope is corrected by scaling changes in the code of the amplitude of the auxiliary phase modulation voltage based on changes in the code of the output signal of the gyroscope.

EFFECT: high accuracy of the gyroscope by reducing the precision readiness time by correcting the output characteristic thereof.

2 cl, 10 dwg

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. To the output channel waveguides of the Y-divider, the ends of the optical fibers of the sensitive gyro coil are docked.

An interference pattern is formed at 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:

ϕ _S = [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, at the photodetector, the intensity of optical radiation can be represented as

I _Φ = 1 / 2P ₀ (1 + cosϕ _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 amounts to

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

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

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

I _f = P ₀ η _f [1 + cosϕ _m · cosϕ _S ± sinϕ _m · sinϕ _S ],

η _f - current sensitivity of the photodetector,

ϕ _m is the amplitude of the auxiliary phase modulation.

To ensure a large dynamic range of measuring angular velocities and to obtain a high linearity of the FOG output characteristic in the optoelectronic information processing circuit, the so-called compensation method for reading the phase difference of the rays is used, the essence of which is that a compensating phase difference is supplied to the phase modulator along with the voltage of the auxiliary phase modulation Sagnac sawtooth step voltage [1]. As a result, the expression for the photodetector current takes the following form:

I _f = P ₀ η _f {1 + cosϕ _m · cos [ϕ _S -ϕ _K) ± sinϕ _m · sin [ϕ _S -ϕ _K ]},

where φ _K is the phase shift introduced by the sawtooth voltage to compensate for the Sagnac phase difference.

Considering that in the compensation mode ϕ _S -ϕ _K ≈ 0, and then the photodetector current can be represented as

I _f = P ₀ η _f {1 + cosϕ _m ± sinϕ _m · sin [ϕ _S −ϕ _K ]}

The accuracy of the FOG is also determined by the stability of the scale factor. For the output signal of the gyroscope operating according to the compensation scheme in the closed feedback loop mode, the following relation is valid [1]:

$$f_n(t)=\frac{\mathrm{four}\pi RL}{\lambda c}\cdot \frac{\mathrm{one}}{\eta U_P{\tau }_{\mathrm{from}t}}\Omega (t)$$

where f _n (t) is the frequency of the compensating phase saw;

Ω (t) is the angular velocity of rotation of the gyroscope;

η is the phase modulator efficiency;

U _P - peak voltage value of the compensating saw;

τ _article - the duration of the steps of the compensating saw.

From this expression it follows that the scale factor of VOG in this case is the quantity

MK = 4πRL / (λc × ηU _p τ _st ).

If we choose τ _article = τ and provide ηU _П = 2π, then the expression for

the scale factor of the gyroscope takes the following form:

MK = 2R / λn ₀ .

In order of magnitude, the stability of the scale factor is most affected by ηU _П due to the large instability of the efficiency η of the phase modulator. Therefore, in order to stabilize this quantity, and ultimately stabilize the scale factor, a second feedback loop is organized in the information processing circuit with a closed feedback loop, which allows us to stabilize the value of U _P η at 2π radians for any changes in the efficiency of the phase modulator η [one]. When the efficiency of the IOS phase modulator changes when the maximum value of the step-like sawtooth voltage is reset to its minimum value, a spurious optical pulse with a duration of τ is formed on the photodetector [1]. By changing the sweep amplitude of the sawtooth step voltage, this pulse disappears, which means that ηU _П = 2π radian, and thus stabilization of the scale factor occurs.

Another way to stabilize the scale factor is to stabilize the amplitude of the auxiliary modulation (WFM). The WFM is used, in which the phase difference of the rays of the fiber circular interferometer (FRI) FOG is a sequence of rectangular pulses of duration multiple of τ and amplitudes (π ± Δ), where Δ = π / 2 ⁿ , with n = 1, 2, 3 ... . When the efficiency of the phase modulators η changes, a mismatch signal appears on the photodetector [2]. The mismatch signal is extracted using a second synchronous detector (mismatch signal demodulator). The output of the demodulator of the error signal is connected to a controller (integrator), which controls the amplitude of the voltage of the WFM. Using the regulator, the signal at the output of the demodulator becomes zero, which means that the stability of the amplitude of the WFM is restored. After that, the WFM voltage is scaled to determine the sweep voltage of the sawtooth step voltage U _P , at which ηU _P = 2π radian. Thus, the stabilization of the scale FOG coefficient occurs through stabilization of the amplitude of the WFM.

One of the main problems of a fiber-optic gyroscope is to ensure a short accuracy readiness time when it is turned on “cold”. The initial drift of the VOG output signal is associated with changes in the amplitude of the rotation signal that carries information about the angular velocity [1]. But the drift of the VOG output signal is also associated with changes in the parasitic initial bias that occurs at the output of the rotation signal demodulator.

The aim of the present invention is to increase the accuracy of a fiber optic gyroscope by reducing its accuracy readiness time.

This goal is achieved by the fact that the output signal code of the gyroscope is corrected using the voltage amplitude code of the auxiliary phase modulation, while ensuring the stability of the efficiency of the phase modulators of the integrated optical circuit by stabilizing its temperature, and also when scaling the code output signal of the gyroscope, the operation of scaling code changes is used the voltage amplitude of the auxiliary phase modulation by changes in the code of the output signal of the gyroscope.

2. The method for reducing the accuracy readiness time of a fiber-optic gyroscope according to claim 1, characterized in that when the temperature of the phase modulators of the integrated optical circuit is changed, the voltage code of the auxiliary phase modulation is further scaled, while the readings of a temperature sensor measuring the temperature of the phase modulators of the integrated optical circuit.

Improving the accuracy of a fiber-optic gyroscope is achieved by reducing the accuracy readiness time of the device when it is “cold” turned on. The reduction of the instrument readiness time is achieved by reducing the parasitic bias of the zero signal of the gyroscope by mathematical correction of its output characteristic by changing the code of the voltage amplitude of the auxiliary phase modulation.

The invention is illustrated by drawings. Figure 1 shows a functional diagram of a fiber optic gyro with two feedback loops. Figure 2 shows the voltage of the auxiliary phase modulation and the law of change of the phase difference of the rays of the ring interferometer fiber optic gyroscope. Figure 3 shows the formation of the rotation signal of a fiber optic gyroscope. Figure 4 shows the formation of the error signal of the fiber optic gyroscope. Figure 5 shows the structure of the common signal at the output of the current amplifier of the photodetector of a fiber optic gyroscope. Figure 6 shows the drift of the output signal of the fiber optic gyroscope and the amplitude code of the stepped sawtooth voltage for 12 hours. Figure 7 shows the Allan variation of the output of a fiber optic gyroscope. The maximum drift was 0.038 deg / h, the minimum drift was 0.0126 deg / h, the noise spectral density was 0.0024 deg / h ^1/2 . On Fig shows graphs of unadjusted and adjusted drift of the output signal of a fiber optic gyroscope using a step-like sawtooth voltage amplitude for correcting the code. Figure 9 shows the Allan variation of the corrected drift of the output signal of a fiber optic gyroscope. The maximum drift was 0.0108 deg / h, the minimum drift was 0.0066 deg / h, the noise spectral density was 0.0024 deg / h ^1/2 . Figure 10 shows graphs of the adjusted and uncorrected drift of the output signal of the fiber optic gyroscope during the first 15 minutes.

Figure 1 shows the structural diagram of a fiber optic gyroscope. It contains a radiation source 1, a three-port optical beam splitter 2, an integrated optical circuit 3, and a fiber sensitive coil 4. An erbium-doped superluminescent source with a short radiation coherence is usually used as a radiation source to minimize polarization error in a fiber ring interferometer (FRI). The three-port optical radiation splitter can be either a fiber alloy splitter or a fiber optical radiation circulator. The integrated optical circuit contains a Y-divider of optical radiation and two phase modulators formed on the output arms of the Y-divider. The fiber sensitive coil, depending on the accuracy of the FOG, contains from several hundred meters to several kilometers of fiber, which preserves the polarization of radiation. The light beam from the radiation source enters the first input of the radiation divider, then it enters the input of the Y-splitter of the integrated optical circuit (IOS) and is divided into two beams that pass through the fiber of the sensitive coil in two opposite directions, after which these two beams are combined Y- IOS splitter in one beam, then passing a three-port splitter, the beam hits the site of the photodetector 5, where they form the interference pattern. The current of the photodetector is amplified by amplifier 6. Next, the signal from the output of the current amplifier of the photodetector is fed to the first input of the differential amplifier 7. The second input of the differential amplifier receives the signal of the constant component of the common signal at the output of the current amplifier of the photodetector, extracted using a low-pass filter 8. As a low-pass filter Frequency commonly used integrator. Thus, the differential amplifier amplifies only the variable component of the overall signal. The variable component of the signal contains voltage peaks that occur in the structure of the overall signal due to the finite bandwidth of the electronic path. The spikes are removed using the electronic key 9. From the output of the electronic key, the signal goes to the input of the digital-to-analog converter (DAC) 10 and then to the input of the digital part of the processing electronics. The digital part of electronics is built either on the basis of a programmable logic integrated circuit (FPGA), or on the basis of a microprocessor. The signal from the ADC output is fed to the inputs of two synchronous detectors (demodulators) 11, 12, which emit rotation and misalignment signals, respectively. The signal from the first demodulator that selects the amplitude of the rotation signal is fed to the input of the regulator 13, which generates the voltage code of the step of the sawtooth step voltage generated by the generator 14. Using the sawtooth step voltage, the Sagnac phase difference between the FRI beams is compensated. The signal after the second demodulator, which allocates the amplitude of the voltage of the error signal, also enters the input of the controller 15, which regulates the code of the amplitude of the WFM voltage generated by the generator 16. At the output of the second demodulator, by changing the code of the amplitude of the voltage of the WFM, the phase modulation amplitude between the FRI beams is stabilized. The condition for the stability of the amplitude of the phase modulation is the absence of a mismatch signal at the output of the second demodulator. Further, the signals from the outputs of the step-like sawtooth voltage generator and the VFM voltage generator are fed to the input of the adder 17, and from its output, to the input of an analog-to-digital converter (DAC) 18. From the output of the DAC, the step-like sawtooth voltage and phase modulation voltage are fed through a scaling amplifier 19 to electrodes of phase modulators IOS. Figure 2 shows the voltage of the phase modulation 20, which is supplied to the electrodes of the phase modulators of the IOS, while the phase difference of the FRI beams varies according to the law 21. The duration of each step of the voltage of the phase modulation is equal to the travel time of the rays along the fiber of the sensitive FRI coil τ. The value Δ is selected from the series π / 2 ⁿ , where n = 1, 2, 3 ....

Figure 3 shows the formation of the rotation signal VOG. Changes in the radiation intensity at the photodetector are described by a cosine curve 22. The presence of a gyroscope rotation, depending on the sign of the rotation speed, leads to a shift of the phase difference curve between the FRI rays relative to the cosine curve. The dashed lines 23 show the shift of the Sagnac phase difference curve between the FRI rays relative to the cosine curve, resulting in a signal 24 present at the photodetector whose amplitude is proportional to the angular velocity of rotation. The amplitude of the rotation signal is allocated at the output of the first demodulator. Then, using the regulator, the corresponding value of the sawtooth voltage step is selected, with which the signal at the output of the demodulator is reset. The first rotation signal demodulator is part of the first FOG feedback loop, with which the angular velocity of rotation is measured. By compensating for the Sagnac phase difference with a sawtooth step voltage when closing the first feedback loop [1], the linearity of the gyroscope output characteristic is achieved in a wide range of angular velocity measurements.

Figure 4 shows the formation of the error signal. The mismatch signal is generated on the photodetector when the efficiency of the phase-modulated IOS changes. In this case, the change in the phase modulation efficiency is shown by dashed lines 25. Thus, a mismatch signal 26 is generated at the photodetector. The amplitude of the mismatch signal is extracted using the second demodulator. Using the second regulator, the amplitude of the VFM voltage is changed so that the signal at the output of the second demodulator is zero. Then, using the code conversion of the amplitude amplitude of the phase modulation voltage, the amplitude of the sawtooth step voltage is determined so that the phase difference between the FRI beams introduced by the phase modulators with the minimum value of the sawtooth step voltage and its maximum value is, for example, 2π radian [1]. This is necessary so that when resetting the maximum value of the sawtooth step voltage to its minimum value (resetting the sawtooth voltage), no changes in optical power occur on the photodetector. The phase difference of the rays of the ring interferometer during the discharge of the sawtooth voltage can take other values [2]. Thus, the second mismatch signal demodulator is included in the second feedback loop, which is designed to stabilize the FOG scale factor [1]. Figure 5 presents the structure of the common signal at the photodetector, which contains the rotation signal 27, the error signal 28 and their constant component 29.

Figure 6 presents a graph of the 12-hour drift of the output signal of the VOG 30 and the amplitude code of the sawtooth step voltage 31, which is used to compensate for the Sagnac phase difference between the rays of the FRI VOG. The vertical axis of the graph shows the measured projection of the angular velocity of the Earth's rotation in degrees / hour. The graph shows that when the instrument is turned on “cold”, a slow drift of the gyroscope output signal is observed for three hours, and then a stable measurement mode is reached within 9 hours of the projection of the angular velocity of the Earth. In this case, the first 3 hours should be considered the time of the accuracy of VOG readiness, which in many cases of VOG applications is completely unacceptable. It is known that a possible cause of the initial drift of the FOG output signal is changes in the amplitude of the rotation signal and the presence of a stray zero offset at the output of the rotation signal demodulator. In this case, the expression for the error of compensation of the Sagnac phase difference compensation can be represented as

Ψ _k = Φ _s -θ ₁ / I ₀ η _f Gsinφ _m ,

where θ ₁ is the spurious zero offset at the output of the rotation signal demodulator, Φ _c is the Sagnac phase difference, θ ₁ , I ₀ η _f Gsinφm is the amplitude of the rotation signal, where I ₀ is the intensity of the interfering rays at the photodetector, η _f is the current sensitivity of the photodetector, G is the gain of the current amplifier of the photodetector and differential amplifier, φ _m is the amplitude of the WFM of the FRI rays. 7 shows a variation of Allan 32 drift of the VOG output signal. From this graph it follows that the minimum drift of the output signal is 0.0126 deg / h, the noise pattern is “white”, the noise spectral density is 0024 deg / h ^1/2 . The above mechanism also leads to instability of the FOG scale factor through a parasitic initial zero offset of 62 at the output of the mismatch signal demodulator. Thus, the cause of the drift of the VOG output signal when it is turned on is a change in the stray zero offset at the output of the demodulators θ ₁ and θ ₂ and changes in the amplitudes of the rotation and mismatch signals. The expression for the error in maintaining the amplitude of the Sanyak phase difference compensating sawtooth step voltage ΔU _k on the modulator at the required level in the presence of a stray bias θ ₂ at the output of the mismatch signal demodulator can be represented as

ΔU _k = θ ₂ / I ₀ η _f Gsinφ _m η ₀ ,

where η ₀ is the efficiency of rad / V phase modulators of the integrated optical circuit;

I ₀ η _f Gsinφ _m is the amplitude of the error signal.

As can be seen in the graph, the VOG output signal code and the amplitude code of the step-like sawtooth voltage correlate with each other. This is the expected result, since the amplitudes of the rotation and mismatch signals change synchronously, and the stray zero offsets at the outputs of the demodulators of these signals can also be rigidly coupled. Based on this, a mathematical correction of the FOG output characteristic was carried out using the voltage code on the phase modulators of the IOS with the corresponding operation of its scaling. The operation of scaling the amplitude code of the sawtooth step voltage is as follows. The maximum change in the code of the output signal of the device when it is “cold” turned on is determined, and the maximum change in the code of the amplitude of the step-like sawtooth voltage on the modulator is also determined. After that, the corresponding coefficient for converting the change of the amplitude code of the sawtooth step voltage to the change of the output code of the gyroscope is determined and then this coefficient is used to correct the output signal of the FOG by subtracting the scaled code of the amplitude of the step-like sawtooth voltage from the code of the output signal of the gyroscope. On Fig shows graphs of the corrected drift of the output signal of the VOG 33 and the uncorrected drift superimposed on each other. Line 34 characterizes the average value of the output signal during the last 9 hours of its drift. Figure 9 shows the Allan variation of the corrected drift of the VOG 35 output signal. The minimum drift value was 0.0066 deg / h, the noise pattern remained “white” and the noise spectral density was 0.0024 deg / h. It can be seen from the graph of the corrected drift that when the output characteristic is corrected, the time to reach the stable mode (accuracy readiness time) is significantly reduced. Figure 10 shows graphs of uncorrected 36 and adjusted drift 37 during the first 15 minutes. From the graphs it can be seen that the accuracy of the readiness of the VOG was reduced from 3 hours to 3.5 minutes. The above description of the correction of the output characteristic using the scaled code of the amplitude of the step-like sawtooth voltage is valid if the efficiency value of the phase-modulated IOS is stable. In this case, it is necessary to maintain the IOS temperature unchanged, for example, using a thermal stabilization system. If the thermal stabilization of the IOS cannot be carried out, the step voltage amplitude code can be further scaled by measuring the IOS temperature using a temperature sensor. It is known that the electro-optical coefficients of lithium niobate, which is the IOS substrate and determining the efficiency of phase modulators, have a linear temperature dependence. In this case, the sawtooth step voltage code ΔU _k when changing the temperature of the phase-modulated IOS is further scaled by the temperature change as follows:

ΔU _k = θ ₂ / I ₀ η _f Gsinφ _m η (T ₀ ),

η (T ₀ ) = η (T) -α (T-T ₀ ),

where T is the current value of the IOS temperature, measured by a temperature sensor; α is the temperature coefficient of change in the efficiency of phase modulators; T ₀ is the temperature of the phase modulators of the IOS, at which the initial scaling of the amplitude code of the step-like sawtooth voltage was performed according to the output signal code of the gyroscope, that is, by the angular velocity.

Literature

[1] G. A. Pavlath, "Closed-loop fiber optic gyros" SPIE v. 2837, 1996, pp 46-60.

[2] A.M. Kurbatov, R.A. Kurbatov “Ways to improve the accuracy of fiber-optic gyroscopes” No. 2, 2012. “Gyroscopy and navigation”.

Claims (2)

1. A way to reduce the accuracy readiness time of a fiber-optic gyroscope containing two feedback loops, the first of which is used to provide linear output characteristics by compensating for the Sagnac phase difference by applying a step-like sawtooth voltage to the phase modulators of the integrated optical circuit, and the second loop feedback is used to ensure the stability of the scale factor, which is achieved by stabilizing the amplitude of the auxiliary phase module features characterized in that the output signal code of the gyroscope is corrected using the voltage amplitude code of the auxiliary phase modulation, while ensuring the stability of the efficiency of the phase modulators of the integrated optical circuit by stabilizing its temperature, and also, when changing the output signal code of the gyroscope, the operation of scaling code changes the voltage amplitude of the auxiliary phase modulation by changes in the code of the output signal of the gyroscope. 2. The method for reducing the accuracy readiness time of a fiber-optic gyroscope according to claim 1, characterized in that when the temperature of the phase modulators of the integrated optical circuit is changed, the voltage code of the auxiliary phase modulation is further scaled, while the readings of a temperature sensor measuring the temperature of the phase modulators of the integrated optical circuit.

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