JPH087070B2 - Phase modulation optical fiber gyro - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber gyro for detecting a rotational angular velocity mounted on a moving vehicle or the like, and more specifically, a light source and light from the right and left sides. An optical system including an optical path propagating around, a phase modulator provided in the optical system and driven at a predetermined frequency, a photoelectric conversion unit converting an optical output signal from the optical system into an electric signal, and the photoelectric converter. The present invention relates to a phase modulation type optical fiber gyro including a signal processing unit that extracts a predetermined frequency component from an output signal of a conversion unit and performs angular velocity calculation.

[Prior Art] An optical fiber gyro of this type of phase modulation system detects a rotational angular velocity based on the following principle.

That is, when the optical system is rotated at an angular velocity Ω,
Due to the Sagnac effect, a phase difference Δθ occurs in the light propagating in both the left and right directions in the optical path. At this time, the relationship between the angular velocity Ω and the phase difference Δθ is Δθ = (8πA / λc) Ω (1) A: Area surrounded by optical path C: Light velocity in vacuum λ: Wavelength in vacuum

When the modulation voltage f (t) by the phase modulator is f (t) = bsin ω _mt (2) b: modulation degree ω _m : modulation angular frequency, the light passing through the optical path is clockwise or counterclockwise. The phase of the phase modulation deviates by φ.

φ = ω _m τ = n lω _m c = 2πf _m n / c (3) τ: light transit time l: fiber length n: refractive index f _m : modulation frequency The optical output of the optical system is electrically converted by the photoelectric conversion unit. When converted to a signal, S (Δθ, t) = | E ₁ sin {ωt + Δθ / 2 + bsin (ω _m + φ / 2)} + E ₂ sin {ωt−Δθ / 2 + bsin (ω _m −φ / 2 )} | ² = 1/2 (E ₁ ² + E ₂ ² ) + E ₁ E ₂ J ₀ (2bsin (φ / 2)) cos △ θ (DC level signal) + 2E ₁ E ₂ J ₁ (2bsin (φ / 2)) sin △ θ cosω _m t (fundamental frequency component S ₁ ) ＋ 2E ₁ E ₂ J ₂ (2bsin (φ / 2)) cos △ θcos 2ω _m t (second high frequency component S ₂ ) ＋ 2E ₁ E ₂ J ₃ (2bsin (φ / 2)) sin △ θcos3ω m t ( third high-frequency component _{S 3) + 2E 1 E 2} J 4 (2bsin (φ / 2)) cos △ θcos4ω m t ( fourth high frequency component S ₄₎ + ...... ( 4) Where Jn is the Bessel function.

Δθ can be obtained by extracting the fundamental frequency component S ₁ and the second high frequency component S ₂ from the equation (4).

△ θ = tan ^-1 [J ₂ (ξ) / J ₁ (ξ) · S ₁ / S ₂ ] (5) ξ = 2bsin (φ / 2) According to the equation (5), the value of △ θ is indefinite. But S ₁ and S ₂
It can be specified by examining the sign of.

And, conventionally, the second harmonic component S ₂ and the fourth harmonic component
The ratio to S ₄ S ₂ / S ₄ ＝ J ₂ (ξ) / J ₄ (ξ) By controlling ξ ＝ 2bsin (φ / 2) so that (6) is constant, the constant term in Eq. (5) J ₂ (ξ) / J ₁ (ξ) was kept constant.

That is, the fundamental modulation frequency component and a plurality of higher-order modulation frequency components are extracted from the output signal from the optical system, and the drive frequency of the phase modulator is set so as to converge the amplitude ratio between the plurality of components to a predetermined value. By controlling, the fluctuation of the scale factor (the proportional coefficient between the input phase difference and the output voltage) due to the fluctuation of the polarization state in the optical fiber due to the environmental changes such as temperature and pressure is compensated.

Then, in order to extract the basic modulation frequency component and a plurality of higher-order modulation frequency components, a plurality of synchronous detection circuits (referred to as lock-in amplifiers) corresponding to the respective frequency components are provided in the subsequent stage of the photoelectric conversion unit. However, the outputs of the synchronous detection circuits are input to a digital signal processing unit that performs angular velocity calculation, control of the drive frequency of the phase modulator, and the like via an A / D conversion unit that sequentially converts the outputs into digital signals.

The synchronous detection circuit receives a basic modulation frequency signal and a plurality of higher order modulation frequency signals as reference signals, respectively, and outputs a spectrum for each modulation frequency component of the output signal of the photoelectric conversion unit as a DC voltage analog signal processing Circuit.

[Problems to be Solved by the Invention]

According to the above-described circuit configuration of the conventional technique, the stability of the analog signal processing unit, particularly the synchronous detection circuit, has a great influence on the accuracy in compensating the fluctuation of the scale factor.

That is, in the synchronous detection circuit, since the analog signal is handled, there is a possibility that the compensation accuracy may be deteriorated due to the fluctuation of the output voltage based on the temperature characteristic or the change over time. The adjustment error required in the circuit has a great influence on the compensation accuracy.

Further, not only the number of such synchronous detection circuits is required, but not only the cost of parts is increased, but also the assembling / adjusting time is lengthened due to the complicated adjusting process, and the assembling / adjusting cost is increased. Is also invited.

Therefore, without using a synchronous detection circuit that is an unstable analog circuit, an A / D conversion unit that directly converts the output signal of the photoelectric conversion unit into a digital signal, and extracts a predetermined frequency component from the digital signal. It is conceivable that a signal processing unit that performs angular velocity calculation is provided and decomposed into a basic modulation frequency component and a plurality of higher-order modulation frequency components without performing synchronous detection.
In order to obtain the harmonic components, it is necessary for the A / D converter to sample at a frequency that is eight times the fundamental modulation frequency f _m of the output signal of the optical system based on the sampling theorem (sampling frequency f _s = 2.4f _m ), there is a problem that such a high-speed A / D conversion element is very expensive.

 An object of the present invention is to eliminate the above-mentioned conventional disadvantages.

[Means for Solving the Problem] In order to achieve this object, the phase modulation type optical fiber gyro according to the present invention has a characteristic configuration in which the signal processing unit is a sampling pulse for one cycle of an output signal of the photoelectric conversion unit. Equivalent time sampling pulse generator for repeatedly generating a time shift over a plurality of cycles, and based on the sampling pulse from the equivalent time sampling pulse generator, converts the output signal of the photoelectric conversion unit into a predetermined number of digital signals A It is composed of an / D conversion section and a digital signal processing section that performs the angular velocity calculation after restoring the predetermined number of digital signals into one cycle signal and extracting a predetermined frequency component.

[Work]

That is, in the signal processing unit, after directly converting the output signal of the photoelectric conversion unit into a digital signal in the A / D conversion unit, the digital signal processing unit in the subsequent stage converts the digital signal into a basic modulation frequency component and a plurality of components. Derivation of angular velocity by decomposing it into high-order modulation frequency components and applying a predetermined calculation, and
Since the driving frequency of the phase converter is controlled, the signal processing unit does not need a synchronous detection circuit as an analog signal processing unit.

Moreover, since the A / D conversion unit converts the output signal of the photoelectric conversion unit into a predetermined number of digital signals based on the sampling pulse from the equivalent time sampling pulse generation unit, for example, the equivalent time over three cycles By performing the sampling, the sampling frequency _{f s = (2 · 4/3)} f
_m and 1/3 of the originally required sampling frequency f _s = 2 · 4f _m suffices, which means that it is possible to use an inexpensive low-speed A / D conversion element. Here, the equivalent time sampling pulse generated by the equivalent time sampling pulse generating section may be a plurality of cycles in which the output signal of the photoelectric conversion section can be regarded as a substantially periodic signal. It may be.

〔The invention's effect〕

Therefore, according to the phase modulation type optical fiber gyro according to the present invention, the output signal of the photoelectric conversion unit is directly converted into a digital signal by the A / D conversion unit and input to the digital signal processing unit. Therefore, compensation accuracy may deteriorate due to fluctuations in output voltage due to temperature characteristics and changes over time, and there is no need to use a synchronous detection circuit that requires various adjustment processes, resulting in reduction in component cost and assembly cost. In addition to being able to do so, it has become possible to ensure the desired performance in a stable manner over a long period of time.

Furthermore, the equivalent time sampling enables the use of an inexpensive low-speed A / D conversion element, resulting in a significant cost reduction effect.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a phase modulation type optical fiber gyro includes a light source (2) using a laser diode or the like and an optical path (3) for propagating light from the light source (2) in both left and right directions.
An optical system (1), a phase modulator (6) provided at one end of the optical path (3) and driven at a predetermined frequency, and an optical output signal from the optical system (1) converted into an electrical signal. And a signal processing unit (8) that performs angular velocity calculation and the like based on the output signal of the photoelectric conversion unit (7).

The optical path (3) is composed of an optical fiber loop formed by winding a polarization-maintaining single-mode optical fiber in a coil shape, and the light from the light source (2) is a polarization-maintaining optical coupler (4).
(') And the polarization filter (5). That is, the light source (2) and the polarization maintaining optical coupler (4),
The optical system (1) is constituted by (4 '), the polarization filter (5) and the optical path (3).

The phase modulator (6) uses an electrostrictive element (PZT) to connect the optical fiber forming the optical path (3) to one end of the optical path (3).
It is configured to be wound around the optical fiber and is driven by a signal from a drive unit (11) described later to drive the electrostrictive element at a predetermined frequency to expand or contract the optical fiber to propagate the light in the optical path (3). Modulate the phase of.

The photoelectric conversion unit (7) is a photoelectric conversion unit (7a) that converts the optical signal transmitted from the polarization-maintaining optical coupler (4) that propagates in the optical path (3) in both left and right directions into an electric signal. And a preamplifier (7a) for amplifying the minute electric signal converted by the photoelectric conversion unit (7a) to an appropriate level.

The signal processing unit (8) is an A / D conversion unit (9) for converting the output signal from the preamplifier (7b) into a digital signal.
And a digital signal processing unit (10) for performing angular velocity calculation, feedback control to the phase modulator (6), etc. based on the output signal from the A / D conversion unit (9), and the phase modulator (6) From a digital signal processing unit (10), a drive unit (11) for driving a signal, a signal generation unit (12) for transmitting a predetermined signal to each of the A / D conversion unit (9) and the drive unit (11), The feedback data of the above to the drive unit (11)
It is composed of a D / A converter (13) and the like.

The A / D converter (9) collects an analog signal sampled and held by a sample and hold circuit that takes in an output analog signal from the preamplifier (7b) in synchronization with a sampling pulse described later. Yes constituted by an a / D converter for converting a digital signal, the output signals from said optical system (1), the fundamental modulation frequency component (frequency f _m), the second high-frequency component (frequency 2f _m), And the fourth harmonic component (frequency 4f _m ) is captured.

The sampling pulses, the signal generator (12) a reference signal generating unit divider a reference signal of a frequency f _r output from (12a) (12b) obtained by dividing the frequency f _s at signal As shown in FIG. 2, the frequency f _s = 8 so that the sampling timing for one cycle of the fundamental modulation frequency f _m of the output signal of the optical system (1) is repeatedly generated by shifting the sampling timing over three cycles. It is divided into / 3 · f _m .

In other words, the output signal of the optical system (1) should be the same as the output signal of the optical system (1) where a frequency non-sampling pulse of 8f _m (2 × 4f _m ) or more is originally required in order to capture the fourth harmonic component. Is approximately constant over 3 cycles, the equivalent time sampling method is used to sample the number of samplings required for 1 cycle over 3 cycles, and the low-speed and high-accuracy A / D converter with relatively low cost is used. It can be used.

The digital signal processing unit (10) is composed of a high-speed digital signal processor (DSP) and a signal input / output interface circuit.
A signal restoration processing unit that rearranges the sampling data converted into a digital signal over one cycle as time-series sampling data for one cycle, and the frequency components (S ₁ , S shown in equation (4)) from the restored data. ₂ , S ₄ ),
The sign of the fundamental frequency component S ₁ and the second harmonic component S ₂ is checked on the basis of the input of the frequency-divided phase reference signal to the frequency divider (12b), and the phase difference Δθ is obtained according to the equation (5). An angular velocity calculation unit for substituting the equation (1) into the rotational angular velocity,
It comprises a modulation voltage control unit for controlling the modulation voltage represented by the equation (2) so as to keep the ratio of the second harmonic component and the fourth harmonic component by the equation (6) constant.

It should be noted that the output of the modulation voltage control unit outputs the D / A conversion unit (1) so that the modulation degree by the phase modulator (6) becomes constant.
It is input to the drive unit (11) via (3).

 Another embodiment of the present invention will be described below.

[1] In the previous embodiment, the sampling pulse was generated by dividing the output signal of the reference signal generator (12a), but the sampling pulse is the modulated signal of the phase modulator (6). It may be generated by using a synchronized phase locked loop circuit.

[2] In the above embodiment, the sampling data required for one cycle is sampled over three cycles as the equivalent time sampling method, but the invention is not limited to this, and it is necessary for one cycle. The sampling data may be sampled over n cycles (n> 1).

[3] In the previous embodiment, the data sampling by the A / D converter (9) is performed by the equivalent time sampling method using a single A / D converter, but as shown in FIG. , A plurality of A / D converters are used and the data sampling by each A / D converter is a frequency that satisfies the sampling theorem over one period of the fundamental frequency f _m (for example, in the above embodiment, f _s = 8f _m ) Sampling pulse frequency fs ₁ ~ fsn
The frequency may be divided into two and supplied to different A / D converters at different timings.

In this case, there is an advantage that the dynamic range becomes large because all the sampling is completed in one cycle even though a low-priced low-speed A / D converter is used.

[4] The equivalent time sampling method according to the above embodiment may be adopted in the plurality of A / D converters described in [3].

[Brief description of drawings]

FIG. 1 shows an embodiment of a phase modulation type optical fiber gyro according to the present invention. FIG. 1 is a block diagram and FIG.
FIG. 3 is a timing chart showing sampling timing by the D conversion unit, and FIG. 3 is a block configuration diagram of a signal processing unit showing another embodiment. (1) …… optical system, (2) …… light source, (3) …… optical path,
(6) …… Phase modulator, (7) …… Photoelectric conversion unit, (8)
…… Signal processor, (9) …… A / D converter, (10) …… Digital signal processor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-138812 (JP, A) JP-A-61-71499 (JP, A) JP-A-62-51317 (JP, A)

Claims (1)

[Claims] 1. An optical system comprising a light source and an optical path for propagating light from the light source in both left and right directions, a phase modulator provided in the optical system and driven at a predetermined frequency, and an optical output from the optical system. A phase modulation type optical fiber gyro comprising a photoelectric conversion unit that converts a signal into an electric signal and a signal processing unit that extracts a predetermined frequency component from the output signal of the photoelectric conversion unit and performs an angular velocity calculation. Section, an equivalent time sampling pulse generator that repeatedly generates a sampling pulse for one cycle of the output signal of the photoelectric converter by shifting the time over a plurality of cycles, and based on the sampling pulse from the equivalent time sampling pulse generator, An A / D conversion unit that converts the output signal of the photoelectric conversion unit into a predetermined number of digital signals, and the predetermined number of digital signals into one cycle signal An optical fiber gyro of a phase modulation type, which comprises a digital signal processing unit for performing the angular velocity calculation after restoring and extracting a predetermined frequency component.