RU2500989C2 - Electronic unit for fibre-optic gyroscope - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention can be used when designing fibre-optic gyroscopes and other physical quantity sensors based on single-mode light guides. Reduction of the parasitic zero offset of fibre-optic gyroscopes and increase in stability of the scaling factor is achieved by stabilising the amplitude of rotation and mismatch signals under the effect of external destabilising factors by dividing the variable parts of rotation and mismatch signals by their constant component.

EFFECT: invention enables to eliminate dependency of zero offset and scaling factor of fibre-optic gyroscopes on change in power of interfering beams in a ring interferometer, caused by changes in ambient temperature, output power of the radiation source, as well as vibration loads and radiation effects.

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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 closest in technical essence to the proposed invention is a fiber-optic gyroscope (hereinafter referred to as FOG), described in [1]. VOG 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 = (4π RL / 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 _Ф = 1 / 2P ₀ (1 + cosϕ _S )

where P ₀ is the total 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 an IOS phase modulator, we use the time delay of the ray fronts interfering on the photodetector while passing through the IOS phase modulator. This time lag 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 voltage with a frequency of 1 / 2τ to the phase modulator, the photodetector current can be represented as:

I _f = _^1/2 P _{0 f} η (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.

The variable component of the signal after the current amplifier of the photodetector is fed to one of the two inputs of the differential amplifier. Its second input receives the constant component of the common signal after the photodetector current amplifier and, thus, the differential amplifier isolates and amplifies the variable component of the common signal at the photodetector. The signal from the output of the differential amplifier goes to the input of an analog-to-digital converter and then to the input of the first demodulator of the gyroscope rotation signal, which is contained in the variable part of the general signal at the frequency of the auxiliary phase modulation. At the output of the first demodulator, a voltage is allocated whose magnitude is proportional to the angular velocity. Integrated optical phase modulators have a finite passband due to the presence of electrode capacitance. The band is also limited by the passband of the photodetector current amplifier. From the limited bandwidth of the modulator-amplifier, the edges of the pulse rotation signal are pulled after the current detector of the photodetector, and therefore, when detecting the rotation signal by the first demodulator, an error in measuring the angular velocity occurs. To eliminate this error, an electronic key is used, which resets the rotation signal for the duration of tightening of its fronts, and thereby the error in measuring the angular velocity is minimized.

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 stepwise sawtooth voltage (SPN). As a result, the expression for the photodetector current takes the following form:

I _f = _^1/2 R 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.

The step-like sawtooth voltage so that there are no errors in the measurement of angular velocity should have an amplitude at which the phase of the rays changes by 2π radians. The duration of each step is τ, where τ is the travel time of the light rays along the fiber of the sensitive coil. The phase difference between the beams of the ring interferometer, which is introduced using the SPD, is proportional to the voltage difference in the adjacent steps of the SPN. The voltage from the first demodulator is fed to the input of the first regulator, which changes the voltage difference in the adjacent steps in such a way that the voltage at the output of the first demodulator becomes zero. In this case, the Sagnac phase difference, determined by the magnitude of the angular velocity, is ΔU × η = ϕ _S = φ _K , where ΔU is the voltage difference in the adjacent steps of the SPN, and η is the efficiency of the phase-modulated IOS. Thus, the value of AU determines the measured angular velocity. Given that in the compensation mode ϕ _S -φ _K ≈ 0, the photodetector current can be represented as:

I _f = _^1/2 P ₀ η _f {1 _{+ cosφ m ± sinφ m · sin} (φ S -φ K)}

Thus, the signal at the output of the photodetector contains both a constant component and a variable. 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;

U _P - the amplitude of the SPN;

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 value:

MK = 4πRL / (λc × ηU _p τ _st )

If we choose τ _st = τ and provide ηU _П = 2π, then the expression for the scale factor of the gyroscope takes the following form:

MK = 2R / λ n 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 in the information processing scheme with a closed feedback loop, it is necessary to ensure the amplitude of the SPD, which introduces a phase difference between the FRI rays equal to 2π radians. If this condition is not met, then when the SPN is reset, a light pulse appears on the photodetector, which is then amplified by the current amplifier of the photodetector and leads to an error in measuring the angular velocity. The spurious pulse when resetting the maximum value of the SPD in the case when its amplitude is such that ηU _{П is} not equal to 2π radian, the spurious pulse present on the photodetector is a mismatch signal. Thus, the variable part of the signal at the photodetector contains both a rotation signal and a mismatch signal. The mismatch signal is allocated by the second demodulator, the signal from the output of which then goes to the second controller. The second regulator changes the amplitude of the SPN so that the voltage at the output of the second demodulator is equal to zero.

One of the main disadvantages of the well-known FOG electronic unit is the dependence of the offset of the zero signal on changes in the amplitude of the rotation signal. The amplitude of the rotation signal depends on the stability of the output power of the optical radiation source, the transmission coefficient of the electronic path depending on changes in temperature, changes in the optical signal due to vibration and radiation exposure, etc. For example, when a gyroscope is exposed to sinusoidal vibration with a certain frequency and amplitude, the parasitic zero shift increases linearly depending on the amplitude of the vibrations [2]. When exposed to sinusoidal vibration at a frequency (the willow is modulated according to a sinusoidal law, both the phase difference between the beams of the ring interferometer ΔФ (ω _в ) × cos (ω _in t) and the power of the interfering rays. When analyzing the signal, it contains terms that can be described as follows expression [3]:

Ω _p ~ ΔI (ω _in ) × ΔФ (ω _in ) × cos ² (ω _in t)

where ΔI is the amplitude of the modulation of the intensity of the interfering rays,

Ω _p - spurious offset of the zero signal of the gyroscope.

Thus, when the vibration frequency changes and its amplitude increases, the stray offset of the zero signal changes. To eliminate this effect, it is necessary to achieve the following conditions either ΔФ (ω _в ) = 0, or ΔI (ω _в ) = 0. The fulfillment of the first condition is associated with the improvement of the manufacturing technology of the sensitive coil, and the second condition can be fulfilled by stabilizing the amplitude of the rotation signal in the frequency band significantly exceeding the gyroscope vibration frequency. The instability of the zero bias is further enhanced due to the instability of the FOG scale factor due to changes in the amplitude of the error signal. The source of instability of the amplitude of the mismatch signal, as well as instability of the amplitude of the rotation signal, are the same changes, for example, the output power of the radiation source, as well as the change in the optical power loss of the interfering rays when the optical path of the VOG passes through external destabilizing factors.

The aim of the present invention is to reduce parasitic bias of zero and increase the stability of the scale factor of FOG due to changes in the amplitude of the rotation signal and the error signal when exposed to FOG external destabilizing factors.

This goal is achieved by the fact that as a pre-amplifier of the photodetector current, an amplifier with a controlled gain is used, and the signal from the ADC output goes to the input of a digital electronic key, then the signal goes to the inputs of the first demodulator and the second demodulator, and the signal is allocated at the first output of the first demodulator rotation of the gyroscope, which is fed to the first input of the first signal division circuit, and at the second output of the first demodulator, a constant component of the rotation signal is allocated which then goes to the input of the circuit with a signal transmission coefficient 1/1-α, where α <1, after which the signal from its output goes to the second inputs of the first and second signal division circuits, and the signal goes to the first regulator from the output of the first division circuit wherein the mismatch signal from the output of the second demodulator is fed to the first input of the second signal division circuit, the output of which is fed to the input of the second controller, and the signal from the output of the circuit with a transmission coefficient of 1/1-α also goes to the input of the circuit with a signal transmission coefficient la 1 / G, where G is the gain of the differential amplifier, the field of which the signal from its output goes to the input of the digital-to-analog converter and then to the control circuit of the gain of the photodetector current amplifier, as well as to the input of the circuit with the transfer coefficient α, from which the signal arrives to the second input of a differential amplifier.

Reducing the parasitic zero offset of the FOG and increasing the stability of the scale factor is achieved by stabilizing the amplitude of the rotation signals and the mismatch when exposed to external destabilizing factors by dividing the amplitudes of the rotation signals and the mismatch by their constant component.

The invention is illustrated by drawings. Figure 1 shows the structural diagram of a known fiber optic gyroscope. Figure 2 shows the formation of the rotation signal of a fiber optic gyroscope. Figure 3 shows the formation of the error signal when changing the scale factor of the fiber optic gyroscope. Figure 4 shows the structure of the signal at the output of the current amplifier of the photodetector of a fiber optic gyroscope. Figure 5 shows the structural diagram of the electronic unit of a fiber optic gyroscope.

Figure 1 shows a structural diagram of a known [1] VOG. VOG consists of a FRI and an electronic unit for processing information from an interferometer. The FRI contains a source of broadband optical radiation 1, a fiber splitter 2, an IOS 3, a sensitive coil 4, and a photodetector 5. The photodetector current is converted into an electrical signal using an amplifier 6. The signal from the output of the photodetector current amplifier is transmitted to an electronic switch 7. The key is intended to cut out the voltage spikes that are formed in the gyro signal due to the capacitance of the electrodes of the phase modulators of the IOS. Further, the signal from the output of the key goes to the first input of the differential amplifier 8, and its second input receives the signal from the output of the integrator 9, with the help of which only the variable part of the general FOG signal is extracted after the differential amplifier. This is necessary in order to amplify the variable part of the signal to improve the dynamic characteristics of FOG. The variable component of the signal is then fed to the input of an analog-to-digital converter (ADC) 10. From the output of the ADC, a variable signal amplified by a differential amplifier is fed to the input of a digital circuit, which includes a demodulator of the gyroscope rotation signal 11 and a mismatch signal demodulator 12. The signal from the signal demodulator output rotation is fed to the input of the regulator 13, and from the output of the demodulator of the error signal to the input of the regulator 14. The signals from the regulators are fed to the digital board 15, which includes auxiliary phase modulation (VFM) voltage generator and SPN generator to compensate for the Sanka phase difference. The signal from the regulator 13 controls the step of the sawtooth voltage, and the signal from the regulator 14 the amplitude of the SPN.

Using the WFM voltage generator, a voltage of type 16 is formed (Figure 2), while the phase difference between the FRI beams varies according to the law 17. The phase modulation amplitudes have four fixed values ± (π-Δ), and ± (π + Δ) radians, where Δ = π / 2 ⁿ , with n = 1, 2, 3 ... The intensity at the site of the photodetector is determined by the cosine function 18, the argument of which is the phase difference of the optical beams that have passed the FRI sensitive coil clockwise and counterclockwise. If there is an angular rotation speed depending on its sign, the phase difference between the rays of the ring interferometer is shifted relative to the cosine curve either to the right or to the left, as a result, the gyroscope rotation signal 19 is generated on the photodetector. The rotation signal has a constant component β and a variable component α ¹ . When the circuit operates in closed loop feedback mode, the value of β can be represented as:

β = (1-cosΔ)

And the value of the variable component of the rotation signal in the form:

α ¹ = sinΔ × sin (ϕ _S -φ _K ) = sinΔ × Δφ

where Δφ is a small value in the compensation mode.

Figure 3 shows the formation of the error signal. When changing the efficiency of the phase modulator of the IOS, the amplitude of the phase modulation, depending on the sign of the change in efficiency, either increases or decreases, which leads to the formation of a mismatch signal 20 on the photodetector.

A mismatch signal with an amplitude of α ² is formed against the background of a constant component, as is the gyroscope rotation signal. The amplitude of the error signal is proportional to:

α ² ~ sinΔ × U _2π × Δη

where U _2π is the voltage when applying it to the phase modulator with an efficiency η between the FRI rays, a phase difference of 2π radians is introduced.

Δη - change in the efficiency of the phase modulator of IOS. When zeroing the output of the second demodulator of the error signal, the code of the voltage levels of the auxiliary phase modulation is determined, that is, voltage codes that provide the amplitudes of the auxiliary phase modulation (π-Δ) radians and (π + Δ) radians. By adding these codes, the voltage code of the amplitude of the SPN, which compensates for the Sagnac phase difference, is determined.

In the presence of uncompensated rotation and misalignment signals, the general form of signal 21 (Figure 4) at the output of a photodetector current amplifier with a gain G can conditionally be represented as:

U = G × U ₀ × (β ± α ¹ ± α ² )

where U ₀ is the voltage proportional to the power of the interfering rays of the FRI.

GU ₀ β = U _cp is the constant component of the signal at the output of the photodetector current amplifier;

GU ₀ α ¹ = ΔU ₁ is the voltage amplitude of the rotation signal of the VOG.

GU ₀ α ² = ΔU ₂ is the voltage amplitude of the VOG error signal.

Under external influences on the FOG (temperature change, vibration loads, radiation exposure, etc.), the value U ₀ changes, which is a common factor of the constant and variable parts of the signal at the photodetector.

Figure 5 presents the structural diagram of the electronic unit of a fiber optic gyroscope. On the board 22 is the analog part of the circuit of the electronic unit. The signal from the output of the current amplifier of the photodetector 23 with a controlled gain goes to the first input of the differential amplifier 24 with a gain G, then the signal from its output goes to the input of an analog-to-digital converter (ADC) 25, after which the signal from its output goes to the input of the board 26, which is a digital part of an electronic unit circuit. The digital part of the circuit can be built both on the basis of a microprocessor and using a programmable logic integrated circuit (FPGA). The signal from the ADC output is sent to a digital key, designed to cut out the spikes in the rotation and mismatch signals, which are formed due to the presence of the capacitance of the electrodes of the phase modulator of the IOS and the limited bandwidth of the optoelectronic path. After the key, the signal is fed to the input of the first demodulator of the gyroscope rotation signal 28. The rotation signal demodulator performs two functions and has two outputs. At the first output, a voltage proportional to the amplitude of the rotation signal is allocated, and at its second output, a voltage proportional to the constant component of the common signal at the photodetector. Thus, at the first output of the demodulator there is a signal of the form:

U _BP = 2U ₀ × α ¹ × G

The constant component signal from the second output of the first demodulator passes device 29, which has a signal transmission coefficient, the value of which can be represented as follows:

K _p ²⁹ = 1/1-α

where α <1, thus, at the second input of the first division circuit there is a signal of the form:

U _cp = 2U ₀ × β × G

Further, a voltage proportional to the amplitude of the gyroscope rotation signal and a voltage proportional to the constant component of the general signal from the output of the device 29 are supplied to the first and second input of the first signal division circuit 30, respectively, and then to the input of the first controller, which controls the value of the STS step generated by the SPS generator in digital parts of the circuit. The signal at the output of the first division circuit can be represented as:

U _o ³⁰ = ctg (Δ / 2) × Δφ

Thus, the voltage proportional to the amplitude of the rotation signal becomes independent of the change in the power of the interfering rays at the FRI photodetector. In this case, when the power of the rays changes due to changes in the output power of the radiation source, exposure to vibrational loads, radiation exposure, etc. A parasitic zero shift of VOG does not occur. A parasitic zero offset can occur if the amplitude changes when exposed to external destabilizing factors of the error signal.

The signal from the digital key output also goes to the input of the second demodulator - the demodulator of the error signal 31. A voltage proportional to the amplitude of the error signal is allocated at its output. The signal at the output of the demodulator can be represented as:

U _out ³¹ = G × U ₀ × sinΔ × U _2π × Δη

The voltage from the output of the second demodulator is supplied to the first input of the second division circuit 32, and the second input of the division circuit receives voltage from the second output of the first demodulator of the rotation signal. At the output of the second division circuit, the voltage is proportional to:

_O U ³² = _^1/2 ctg (Δ / 2) × U _2π × Δη

Here, also, the voltage proportional to the amplitude of the error signal does not depend on the change in the power of the interfering rays in the FRI. Next, the voltage from the output of the second division circuit is fed to the input of the second regulator, which controls the amplitude of the SPN formed by the generator in the digital part of the electronic unit.

A voltage proportional to the constant component of the common signal at the photodetector from the output of the device 29 is supplied to the input of the device 33 with a transfer coefficient, which can be expressed as follows:

K _n ³³ = 1 / 2G

Next, the signal is fed to the input of a digital-to-analog converter (DAC) 34, from its output to the input of a circuit 35, which controls the gain of the photodetector current amplifier. Using this feedback loop for controlling the gain of the photodetector current amplifier, stabilization of both the constant component of the signal at the output of the photodetector current amplifier and its variable components is achieved at a constant angular velocity of the device and with a constant change in the efficiency of the phase-modulated IOS. The signal from the output of the DAC also arrives at the input of the circuit 36 with the transmission coefficient, which is expressed as follows:

K _p ³⁶ = α

Next, the signal from the output of the circuit enters the second input of the differential amplifier. Thus, at the first input of the differential amplifier, there is a signal containing both the variable components of the signal U ^per = U ₀ × (± α ² ± α ² ) and its constant component U _post ¹ = U ₀ × β. At the second input of the differential amplifier there is only a constant component, which can be represented as U _post ² = U ₀ × β × α. As a result, the output of the differential amplifier is allocated and amplified as variable components of the signal, as well as part of the remaining constant component of the signal. The voltage at the output of the differential amplifier, thus, can be represented as:

U = G × U ₀ × {β (1-α) ± α ¹ ± α ² }

By choosing the coefficient a, it is possible to significantly reduce the resolution of the ADC for the electronic unit of high-precision FOG. For FOG with FRI with a total turn area of 100 m ² , a wavelength of the radiation source λ = 1550 nm, an amplitude of auxiliary phase modulation of 7/8 × π and 9/8 × π radians to achieve an accuracy of 0.001 deg / h at α = 0.99 it is enough to use a 16-bit ADC in the electronic circuit.

Literature

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

[2] T. Greening "Digital intensity suppression for vibration and radiation intensitivity in a fiber optic gyroscope" US Patent No. US 2008/0079946 AL, Apr.3, 2008.

Claims (1)

The fiber optic gyroscope electronic unit containing a preliminary photo-detector current amplifier, an electronic switch, a differential amplifier of a variable component of a rotation signal and a mismatch signal, the output signal of which is input to an analog-to-digital converter, as well as a first gyro rotation signal demodulator and a second mismatch signal demodulator and two regulators that change the voltage of the step of the stepped sawtooth voltage and its amplitude, respectively, for zeroed rotation and mismatch signals, characterized in that a pre-amplifier of the photodetector current is used with an amplifier with a controlled gain, and the signal from the ADC output is fed to the input of a digital electronic key, then the signal is fed to the inputs of the first demodulator and second demodulator, and at the first output of the first demodulator, the gyroscope rotation signal is allocated, which is fed to the first input of the first signal division circuit, and the constant constant is allocated at the second output of the first demodulator an avalanche of the rotation signal, which then goes to the input of the circuit with a signal transmission coefficient 1/1-α, where α <1, after which the signal from its output goes to the second inputs of the first and second signal division circuits, and the signal comes from the output of the first division circuit to the first controller, while the mismatch signal from the output of the second demodulator is fed to the first input of the second signal division circuit, the output of which the signal is fed to the input of the second controller, and the signal from the output of the circuit with a transmission coefficient of 1/1-α is fed to the input of the circuit with to 1 / G signal transfer coefficient, where G is the gain of the differential amplifier, the field of which the signal from its output goes to the input of the digital-to-analog converter and then to the control circuit of the gain of the photodetector current amplifier, as well as to the input of the circuit with the transfer coefficient α, from the output of which the signal goes to the second input of the differential amplifier.

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