JPH05196470A - Optical fiber gyro - Google Patents

Abstract

PURPOSE:To provide an optical fiber gyro requiring no modulation-level control for a phase modulator by periodically changing the amplitude of modulation signal applied to the phase modulator so that it ups and downs through maxi mum point of Bessel function of that order, obtaining the maximum value of synchronized detection output of a photodetector within a specified period to obtain rotational angular velocity. CONSTITUTION:A synchronous signal generator 13 gives both an amplitude modulation circuit 12 and a peak detection circuit 14 the synchronous signal relating with the starting time of one cycle Ts. The generator 13 gives modulation Sin (2pi/Ts) timing of phase modulation amplitude. This specifies the length of the cycle Ts when correct, permitting the detection circuit 14 to obtain the maximum value of those. Ts is mandatory when the circuit 14 resets that value after detecting the maximum value. It is also required so that two maximum values are confirmed within the cycle Ts. Again, the detection circuit 14 requires no synchronous signal if xsi1<xsi<xsi2 proves to be true after amplitude xsin is arbitrarily selected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber gyro for measuring the rotational angular velocity of a moving body such as an automobile, an airplane or a ship.

[0002]

2. Description of the Related Art An optical fiber gyro determines the angular velocity by utilizing the fact that the phase difference of light propagating counterclockwise and clockwise in a fiber coil is proportional to the angular velocity of the coil. The phase modulation method is a method in which a part of an optical fiber near one end of a fiber coil is expanded and contracted to modulate the phase of light propagating therein. FIG. 4 shows a schematic configuration of a phase modulation type optical fiber gyro in which all the optical paths are constituted by optical fibers. Light emitting element 1 as a light source
A monochromatic light is emitted from the optical fiber 2, passes through the optical fiber 2, the first fiber coupler 3, the optical fiber 4, and the second fiber coupler 5, and is incident on both ends of a fiber coil 6 in which a single mode fiber is wound many times. This propagates inside the fiber coil 6 as left-handed light and right-handed light. A phase modulator 7 is provided at one end of the fiber coil 6 to modulate the phase of light as bsin Ωt. The right-handed light and the left-handed light merge at the fiber coupler 5, pass through the optical fiber 4 and the fiber coupler 3, and enter the light receiving element 8. The light receiving element 8 detects the intensity of the interference light of both and converts it into an electric signal. The preamplifier 9 amplifies this and an appropriate harmonic or fundamental wave contained therein is synchronously detected by the synchronous detector 10. The oscillator 11 gives a modulation signal to the phase modulator and gives a synchronous detection signal. Of course there is a suitable divider in between to step down the frequency of the original oscillator.

In this way, the intensity of the interference light is detected by the light receiving element, and the modulation frequency and its harmonic signal are included in this in the form of an expansion equation having a Bessel function as a coefficient. Therefore, a fundamental wave component or an arbitrary harmonic component can be obtained by creating a carrier signal having a modulation frequency or a frequency that is an integral multiple thereof and synchronously detecting the output of the light receiving element. The odd harmonics (including the fundamental wave) can be written as 2E ₁ E ₂ J _{2m + 1} (ξ) sin Δθ (1). However, E ₁ and E ₂ are amplitudes of left-handed light and right-handed light, J _{2m + 1} (ξ) is a (2m + 1) th order Bessel function, and Δθ is a phase difference between left-handed light and right-handed light. This is the object to be sought. When the angular velocity of the rotating body is Ω ₀ , there is a relationship of Δθ = 4πLaΩ ₀ / cλ (2). L is the total length of the fiber of the fiber coil. a is the radius of the fiber coil, c is the speed of light in vacuum, and λ is the wavelength in vacuum. ξ represents the magnitude of modulation, and ξ = 2bsin (LnΩ / 2c) (3). b is the amplitude of phase modulation in the phase modulator, Ω is the phase modulation angular frequency, and n is the refractive index of the fiber. ξ is a term that occurs when the timings of phase modulation in the left-handed light and the right-handed light differ by Ln / 2c. The even harmonics can be written as 2E ₁ E ₂ J _2n (ξ) cos Δθ (4). If the amplitude of light and the magnitude of modulation ξ are stable, the phase difference Δθ can be obtained from only the fundamental wave. That is, assuming that the fundamental wave component is S ₁ , this is equally placed in (1) and Δθ = sin ⁻¹ (S ₁ / 2E ₁ E ₂ J _{2m + 1} (ξ)) (5) Δθ can be obtained.

Some use harmonics rather than the fundamental component. For example, in Japanese Patent Laid-Open No. 60-135816, the fundamental wave component is divided by the direct current component to cancel the influence of the light amount variation. Alternatively, divide the fundamental wave component by the 4th harmonic and tan Δθ
A method of obtaining Δθ in the form of is also proposed (Japanese Patent Application No. 1-
57637). In this way, the amplitude of light is erased, so that the coefficient does not change due to fluctuations in light quantity. In addition, many proposals have been made for stabilizing the scale factor by eliminating fluctuations in light quantity. For example, JP-A-61-1410
No. 6 extracts the direct current component from the output of the light receiving element and controls the power of the light emitting element so that it is constant. Regarding the phase modulation type optical fiber gyro, Japanese Patent Application No.
-57634-37, Japanese Patent Application No. 1-291628-31,
1-295500, Japanese Patent Application Nos. 2-3809, 2-1005
Inventions such as 5, 2-225611 to 19 have been made.

[0005]

The scale factor must be stable in order to be able to accurately obtain the phase difference Δθ by such a method. In other words, it is important that the Bessel function and the amplitude of light are stable. This is because if these change, it is not possible to distinguish between this change and the change in Δθ. As already mentioned, several improvements have been proposed for the problem of light intensity fluctuations. So, here we make the stability of Bessel function a problem. The Bessel function includes the modulation degree ξ as a parameter. This must be constant. ξ is given by (3), but the length L of the fiber, the refractive index n, and the modulation amplitude b change due to temperature fluctuations. In particular, the characteristic change due to the temperature of the piezoelectric element of the phase modulator is large. Therefore, the amplitude b changes significantly. The piezoelectric element is driven near the resonance frequency, but the frequency characteristic of the piezoelectric vibrator changes significantly with temperature. Therefore, even if the voltage is the same, the deformation amount of the piezoelectric element changes depending on the temperature. That is, the phase modulation amplitude b changes greatly with temperature. In the past, great effort was made to control the phase modulator so as to keep ξ constant.

For example, in Japanese Patent Application No. 59-246441, the magnitude of the second harmonic is made constant in order to make the magnitude ξ of the phase modulation constant. Synchronous detection is performed to obtain the magnitude of the second harmonic, and this signal is compared with the reference magnitude.
The amplitude of the phase modulator is controlled so that ₂ (ξ) = 0.3. This is to maximize the fundamental wave component ξ =
We are using a neighborhood of 1.8. At this value, J ₂ (ξ) does not take an extreme value, and as ξ increases or decreases, J ₂ (ξ) also increases or decreases, so that negative feedback control can be performed to make ξ a constant value. Japanese Patent Application 1-
No. 57634 has a phase modulator in which another fiber is wound, and light is passed through the phase modulator so that the light passing through this fiber and the light not passing through are interfered with each other and detected by a light receiving element and the fundamental wave component is kept constant after synchronous detection. The amplitude of the phase modulator is controlled. Japanese Patent Application No. 1-57636 detects that it is stationary and controls the DC component and the second harmonic component at this time to be constant. By keeping the direct current component constant, fluctuations in the light amount can be eliminated, and by making the second harmonic constant, the degree of phase modulation can be controlled to be constant. Both are normal controls using a negative feedback system.

The phase modulator driving circuit may be controlled so that the appropriate even harmonics become zero. Then J _2n (ξ
Ξ is fixed at the zero point of the 2n-order Bessel function for which ₀ ) = 0. Since the zero point of the Bessel function is predetermined, the modulation factor ξ is kept constant by this. For example, the minimum zero of the quadratic Bessel function is 5.13. The minimum zero of the fourth-order Bessel function is 7.59. The inventors have performed control to fix ξ at the zero of the quadratic Bessel function. That is, ξ = 5.13. The above-mentioned Japanese Patent Application No. 1-57637 uses this method to stably control the phase modulation degree. The feedback control for keeping the degree of phase modulation constant must be obtained by synchronously detecting harmonics of orders other than the measurement target. This is compared with a reference value, and if there is a deviation, the amplitude of the phase modulator is increased or decreased in the direction of canceling it. As a matter of course, the structure becomes complicated. This is because a stabilization device for the phase modulator must be provided in addition to the device for stabilizing the light source. Even in such a case, it is difficult to completely make the modulation degree of the phase modulator constant by the feedback system, and the control is performed at the sacrifice of other characteristics such as allowing output drift. However, if you think about it, it is not possible to try to make the modulation constant. It is unavoidable that the modulation factor of the phase modulator changes due to temperature fluctuations. It suffices that the final result is correct even if the modulation degree changes. Therefore, the present invention provides an optical fiber gyro that does not need to control the modulation factor of the phase modulator. This is the purpose.

[0008]

The optical fiber gyroscope according to the present invention has a sine modulation factor (that is, an amplitude b) that includes the maximum value of the Bessel function to be measured without keeping the modulation factor of the phase modulator constant. Vary functionally. The period T _s of variation in modulation degree is larger than the period (2π / Ω) of phase modulation. Then, the maximum value of the synchronous detection output of the desired order within the period T _s is obtained. This is the output signal. Since the amplitude of the phase modulation passes through the maximum value of the Bessel function twice in one cycle T _s , the maximum value should appear twice. This maximum value corresponds to the value of the conventional synchronous detection output.
It is important that the range of amplitude modulation of the phase modulator includes the local maximum of the Bessel function. This can be confirmed by the fact that the coherent detection output increases and decreases twice during one cycle T _s . That is, the cycle of increase / decrease of the synchronous detection output is approximately T _s / 2. If the cycle of increase / decrease of the synchronous detection output is T _s , it means that the fluctuation of the amplitude of the phase modulator does not pass the maximum value of the Bessel function. Therefore, the amplitude is corrected and the synchronous detection output is within T _s . You have to adjust it so that it takes the maximum value twice.

[0009]

The present invention can be applied regardless of whether the phase difference Δθ is obtained by measuring either the fundamental wave component or the harmonic wave component, but here, the (2m + 1) th order harmonic wave is generally explained. S _{2m + 1} = 2E ₁ E ₂ J _{2m + 1} (ξ) sin Δθ (6) ξ = 2bsin (LnΩ / 2c) (7) Conventionally, the amplitude b is set to a constant value, but in the present invention, this is changed at the cycle T _s . Then, ξ fluctuates, but in this fluctuation range, the maximum value of J _{2m + 1} (ξ) is given ξ
_{Include 0} . This is different from phase modulation. This is amplitude modulation of the modulation degree. Don't confuse both. Therefore, the amplitude modulation is called an amplitude sweep here, and the period T _s is called a sweep period. The signal that gives b will be referred to as a sweep signal.

FIG. 2 illustrates the relationship of J _{2m + 1} (ξ) with respect to ξ. The horizontal axis is ξ. An amplitude-modulated signal, that is, a sweep signal is drawn in the vertical direction below. This is ξ = ξ ₁ and ξ =
It is a sinusoidal signal that varies between ξ ₂ . That is, ξ = ξ _m + ξ _n sin (2πt / T _s ) (8) ξ _m = (ξ ₁ + ξ ₂ ) / 2 (9) ξ _n = (ξ ₁ −ξ ₂ ) / 2 (10) It changes more slowly than / Ω. The moving range of this is ξ ₁ <ξ ₀ <ξ ₂ (11). It is assumed that the F point on the ξ axis is ξ ₁ and the G point is ξ ₂ . The output S _{2m + 1} of the (2m + 1) th order harmonic contained in the output of the light receiving element is J _{2m + 1} (
proportional to ξ). Since the time is short, it can be considered that the phase difference Δθ does not change during this period. The maximum value of J _{2m + 1} (ξ) is E, and the value of ξ that gives it is ξ ₀ . Let B be the maximum point and A be the point on the ξ axis where ξ = ξ ₀ . ξ
Is F (ξ before and after the point B that gives the maximum value E of J _{2m + 1} (ξ).
_It vibrates between ₁ ) and G (ξ ₂ ). Let C be the point of J _{2m + 1} (ξ) corresponding to ξ ₁ and D be the point corresponding to ξ ₂ . Since ξ moves between ξ ₁ and ξ ₂ , J _{2m + 1} (
ξ) will move between points C and D. (2m +
1) Since the next synchronous detection output S _{2m + 1} is proportional to this, the output is as shown on the right side of FIG. This is line EH
Is a local maximum value and K is a local minimum value. Of course, the cycle is T _s , but the maximum value is taken twice during this period. If ξ ₂ and ξ ₁ are symmetric with respect to ξ ₀ , K = J. But even then, the waveform is distorted and not sinusoidal. This is because it vibrates around the maximum point. This point is different from the normal control, in which the function is as linear as possible.

The values of ξ ₁ and ξ ₂ change due to temperature fluctuations. Therefore, the lower end F and the upper end G of the sweep signal change. However, even if these changes, the point A corresponding to the maximum point B is F ~
As long as it is included between G, the (2m + 1) th-order synchronous detection output has the same maximum value H. The minimum values K and J change, but the maximum value H does not change. This is because the sweep signal passes through point B. Here, the value H is obtained using the peak detection circuit. This is easily obtained because the maximum value H appears twice in one cycle T _s . Pi - the time constant of the click detection circuit may be set to a degree slightly larger than T _s. Then, the output after passing through the peak detection circuit no longer oscillates and can be obtained as a constant value. Since this is J _{2m + 1} (ξ) = H, the constant (scale) included in the (2m + 1) th-order output is
Factor has been finalized. From this, the value of Δθ can be accurately obtained. For example, in the case of fundamental wave,
Since ξ ₀ = 1.8 takes a maximum value J ₁ (ξ) = 0.582, S in (5) is replaced with the maximum value H, and Δθ = sin ⁻¹ (H / 2E ₁ E ₂ × 0. 582) (12) However, it is necessary to keep the condition of ξ ₁ <1.8 <ξ ₂ . In the case of the third harmonic, ξ ₀ = 4 and the maximum value J ₁ (ξ) = 0.43 is taken, so Δθ = sin ⁻¹ (H / 2E ₁ E ₂ × 0.43) (13) Become. However, the condition of ξ ₁ <4 <ξ ₂ is imposed. The time constant T _p of the peak detection circuit must not be too long. The fluctuation of the synchronous detection output depends on the rotation angular velocity Δ
This is because the fluctuation of θ is also included. Time constant T of change in angular velocity
pin than _a - time constant of the click detecting circuit must be less. The condition T _s <T _p <T _a (14) needs to be satisfied. Of course, the peak detection circuit is not a circuit with a constant time constant, but 1
It may be reset every cycle T _s . In this case, the reset signal must be externally applied to the peak detection circuit. Of course, the scale factor also includes the photoelectric conversion capability of the light receiving element in addition to the above (12), (1
Although not as in 3), these can be considered as constants, so they can be known in advance and can be easily calibrated.

The present invention is characterized in that the degree of phase modulation is not controlled to a constant value in this way, but if so, it may happen that ξ ₀ does not enter between ξ ₁ and ξ ₂ . This can be detected by the fact that the maximum value appears only once within one period T _s of the sweep signal.
FIG. 3 shows an example in which the sweep amplitude deviates from ξ ₀ . In this case, the (2m + 1) th-order synchronous detection output has a period approximately doubled as compared with FIG. 2, and the maximum value appears only once in T _s . In this case, it is necessary to increase the fluctuation range of the phase modulation amplitude. However, if ξ _{1 to} ξ ₂ is set larger than the temperature fluctuation of ξ, such a thing will not occur.

[0013]

1 is a schematic view of an optical fiber gyro according to an embodiment of the present invention. In this, the optical path is composed entirely of optical fibers. Although it is very similar to that of FIG. 4, it is different in that the phase modulation degree is swept. A monochromatic light is emitted from a light emitting element 1 as a light source and passes through an optical fiber 2, a first fiber coupler 3, an optical fiber 4 and a second fiber coupler 5 and enters both ends of a fiber coil 6 in which a single mode fiber is wound many times. To do. This propagates inside the fiber coil 6 as left-handed light and right-handed light. A phase modulator 7 is provided at one end of the fiber coil 6 to modulate the phase of light as bsin Ωt. The phase modulator uses, for example, a piezoelectric element. When an electrode is attached to the inner or outer walls or end faces of a cylindrical or cylindrical piezoelectric element and an AC voltage is applied to this, the element expands and contracts in the radial direction due to the piezoelectric effect, causing the length of the optical fiber to vibrate and the light passing through it. The phase of changes. When the voltage amplitude is increased, the magnitude of the phase change also increases in proportion. The right-handed light and the left-handed light merge at the fiber coupler 5, pass through the optical fiber 4 and the fiber coupler 3, and enter the light receiving element 8. The light receiving element detects the intensity of the interference light of both and converts it into an electric signal. Preamplifier 9
Then, this is amplified and an appropriate harmonic wave or fundamental wave contained therein is synchronously detected by the synchronous detector 10. Oscillator 11
Gives a modulation signal to the phase modulator and a synchronous detection signal. Of course there is a suitable divider in between to step down the frequency of the original oscillator.

The light emitting element 1 is a light source which emits monochromatic light. A laser diode and a super luminescent diode are used. However, the coherence length must be short. Such points are the same as those in FIG.
In addition to this, an amplitude modulation circuit 12 for sweeping the amplitude of phase modulation is provided between the phase modulator 7 and the oscillator 11. This is T
The modulation factor b of the phase modulation is amplitude b = b _m + b _n sin in _s cycles.
Change it to be (2π / T _s ). That is, the amplitude of the voltage applied to the piezoelectric element is changed by such a function type. In this way, ξ changes sinusoidally between ξ ₁ and ξ ₂ . The synchronization signal generator 13 is a circuit for giving the beginning of the cycle T _s . A peak detection circuit 14 is provided subsequent to the synchronous detector 10. The synchronization signal generator 13 has both T-values for the amplitude modulation circuit 12 and the peak detection circuit 14.
Gives a sync signal for the beginning of _s . The synchronization signal generator provides the timing of the modulation of the phase modulation amplitude sin (2π / T _s ).

This, when normal, specifies the length of T _s and allows the peak detection circuit 14 to determine the maximum value of this. This is indispensable when the peak detection circuit detects the maximum value and then resets this value. It must also be confirmed that it has two maxima in T _s . This is also necessary. However, if the peak detection circuit operates with a constant time constant, it is not necessary to specify the start time of T _s . Further, if the amplitude ξ _n is properly selected and it is guaranteed that ξ ₁ <ξ <ξ ₂ , the peak detection circuit 14 does not need such a synchronization signal at all. As the peak detection circuit 14, a commercially available sample hold circuit can be used. Many such products have been manufactured and sold since ancient times. It can also be configured by combining amplifiers. It is preferable to integrate these circuits as an integrated circuit including a preamplifier and the like. An example of the sample hold circuit is shown in FIG. It does a positive peak detection which gives a negative output. The time constant can be specified by C and R. It is also possible to short-circuit the reset terminal and reset every T _s .

[0016]

The degree of modulation of the phase modulator is amplitude-modulated to obtain the maximum value of the synchronous detection output, and the phase difference Δθ is obtained from this. Although a great deal of effort has been conventionally made to keep the phase modulation degree b constant in order to stabilize the scale factor, in the present invention, it is no longer necessary to keep the phase modulation degree b constant. Therefore, a feedback system for keeping this constant is unnecessary. In addition, the drift may not be completely suppressed due to the existence of the feedback system, but the present invention does not have such a problem.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical fiber gyro according to an embodiment of the present invention.

FIG. 2 is a diagram showing fluctuations in amplitude of phase modulation and fluctuations in synchronous detection output when the center of phase modulation is near the maximum point of the Bessel function.

FIG. 3 is a diagram showing a variation in the amplitude of the phase modulation and a variation in the synchronous detection output when the amplitude of the phase modulation deviates from the maximum point of the Bessel function.

FIG. 4 is a configuration diagram of an optical fiber gyro according to a conventional example.

FIG. 5 is a circuit diagram showing an example of a peak detection circuit.

[Explanation of symbols]

 1 Optical element 2 Optical fiber 3 Fiber coupler 4 Optical fiber 5 Fiber coupler 6 Fiber coil 7 Phase modulator 8 Photodetector 9 Preamplifier 10 Synchronous detector 11 Oscillator 12 Amplitude modulation circuit 13 Synchronous signal generator 14 Peak detection circuit

Claims (1)

[Claims] 1. A fiber optic gyro having a principle that light is propagated counterclockwise in a fiber coil to the right and the rotational angular velocity of the fiber coil is obtained from the phase difference between the two lights, which is a monochromatic light as a light source. Light-emitting element, a fiber coil in which a single-mode optical fiber is wound many times, a coupler that connects both ends of the fiber coil to the light-emitting element and the light-receiving element, and propagates in the fiber coil clockwise and counterclockwise. The light receiving element for interfering the generated light and detecting the intensity of the interference light, and the phase modulator for providing the sinusoidal phase modulation to the propagating light provided at one end of the fiber coil are provided. It is incident on both ends and propagates as a right-handed light and a left-handed light in the fiber coil and combines them to detect the intensity of the interference light with the light receiving element. Phase-modulation for synchronously detecting the fundamental wave component or its nth-order harmonic component to obtain the rotational angular velocity by utilizing that the fundamental wave component or the nth-order harmonic component includes the 1st or nth-order Bessel function as a constant. In the optical fiber gyro of the method, the amplitude of the modulation signal given to the phase modulator is changed at a constant cycle so as to go up and down through the maximum point of the Bessel function of the order, and during the constant cycle of the synchronous detection output of the light receiving element. An optical fiber gyro characterized in that the maximum angular velocity is obtained and the rotational angular velocity is obtained using this.