CN106979776B - Digital closed-loop control method of fiber-optic gyroscope based on sigma-delta modulation - Google Patents
Digital closed-loop control method of fiber-optic gyroscope based on sigma-delta modulation Download PDFInfo
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Abstract
本发明提供一种基于西格玛德尔塔调制的光纤陀螺数字闭环控制方法，在光纤陀螺检测角运动输出光相位信号后，进行偏执调制和相位调制；再进行光电转换，得到电信号；进行前置放大，得到放大后的信号；进行A/D转换，得到数字电信号；进行信号解调，得到解调信号；通过西格玛德尔塔控制器，得到二进制比特流序列和反馈信号；二进制比特流序列经过数字滤波得到转速结果；反馈信号经过反馈回路得到相位调制信号。通过设计西格玛德尔塔控制器抑制了极小量级光相位信号的量化死区非线性，提升了信号的信噪比，结合反馈回路实现了有效的光纤陀螺数字闭环控制，可提升光纤陀螺检测角运动的精度。
The invention provides a digital closed-loop control method of fiber optic gyroscope based on sigma delta modulation. After the fiber optic gyroscope detects an angular motion and outputs an optical phase signal, paranoid modulation and phase modulation are performed; then photoelectric conversion is performed to obtain an electrical signal; and pre-amplification is performed. , get the amplified signal; perform A/D conversion to get digital electrical signal; perform signal demodulation to get demodulated signal; get binary bit stream sequence and feedback signal through Sigma Delta controller; The speed result is obtained by filtering; the feedback signal is passed through the feedback loop to obtain the phase modulation signal. By designing a sigma delta controller, the quantization dead zone nonlinearity of the extremely small optical phase signal is suppressed, and the signal-to-noise ratio of the signal is improved. Combined with the feedback loop, an effective digital closed-loop control of the fiber optic gyroscope is realized, which can improve the detection angle of the fiber optic gyroscope. Precision of movement.
Description
Technical Field
The invention relates to the technical field of optical fiber gyroscopes, in particular to a digital closed-loop control method of an optical fiber gyroscope based on sigma delta modulation.
Background
The optical fiber gyroscope is a solid-state sensor instrument with a simple structure and is used for measuring angular velocity. Compared with an electromechanical gyro, the gyro has the advantages of no mechanical and motion wear, long service life, good anti-interference performance, short starting time and large dynamic range. Compared with a laser gyro, the high-precision laser gyro has no high-precision optical processing and mechanical shaking devices, no gas seal and high ignition voltage, and has high reliability. The fiber-optic gyroscope has great military and civil values and wide development prospect, and is widely applied to the fields of inertial navigation systems, attitude measurement systems, petroleum drilling measurement systems, aviation, aerospace, navigation and the like.
The control performance of the closed-loop fiber-optic gyroscope system is an important parameter reflecting the dynamic characteristic of the closed-loop fiber-optic gyroscope system, and the closed-loop fiber-optic gyroscope system not only reflects the tracking capability of the system on angular velocity signals, angular acceleration signals or complex mixed input signals, but also can be used for measuring whether the designed control scheme of the fiber-optic gyroscope system meets the requirements or not. Different application conditions have different requirements on the control performance of the system, and under the condition of higher requirements on precision and real-time performance, if the performance of the designed control system is poorer, a tracking error is generated when the response speed is too slow in tracking an input signal, and even the system cannot normally work in serious conditions, so that the quality of the control performance of the system has great influence on the overall performance. Therefore, it is very important to reasonably design the digital closed-loop control scheme of the fiber-optic gyroscope system.
The digital closed-loop control scheme of the current home and abroad commercialized fiber optic gyroscope is mainly proportional-integral control. The closed loop fiber optic gyro system using the proportional-integral control scheme is an absolutely stable system, but a steady-state error exists when a slope angular velocity signal, an angular acceleration signal or a complex mixed input signal is tracked. The research on the control scheme of the fiber-optic gyroscope is always carried out, the control scheme comprises proportional integral, digital PID, partial differential PID and fuzzy PID, but various PID control schemes are in a simulation analysis stage, and the reason is that although the differential control can accelerate the response speed of a system and give out a large adjusting action in advance, the differential control is not sensitive to high-frequency rapidly-changing noise and signals and cannot generate a corresponding control action, so that various control schemes are not widely applied in actual products, and the proportional integral control cannot be replaced.
The performance of many control schemes is far better than that of proportional-integral control from the viewpoint of automatic control, but because the closed-loop fiber-optic gyroscope system is a weak signal detection system, the Sagnac phase difference signal to be detected is very weak, the signal-to-noise ratio is low, so that the signal is easily submerged by the noise of an optical path and a circuit, and the problem of dead zone nonlinearity exists in the process of carrying out digital-to-analog conversion on the Sagnac phase difference signal. The invention provides a control scheme based on sigma delta modulation to reduce the noise of a fiber-optic gyroscope system, improve the signal-to-noise ratio of the system and improve the tracking performance and the dynamic performance of a closed-loop system.
Disclosure of Invention
In view of the above problems, the present invention aims to provide a digital closed-loop control method for a fiber optic gyroscope based on sigma delta modulation, which has a higher signal-to-noise ratio and overcomes the non-linearity of the quantization dead zone.
The purpose of the invention is realized as follows:
the expression of the photoelectric conversion is
In the formula:
is the photoelectric conversion responsivity, and the unit is A/W; i is0Is a light intensity signal; delta phi is the modulated optical phase signal; i.e. iD(t) is photocurrent;
the expression of the voltage signal is
UD＝Rf×iD(t)
In the formula: rfIs the impedance; u shapeDIs a voltage signal;
step 7, the demodulated signal passes through a sigma delta controller to obtain a binary bit stream sequence and a feedback signal;
the sigma delta controller consists of a controller, a quantization comparator and a data selector; the demodulated signal is processed by a controller to obtain a control signal; the control signal passes through a quantization comparator to obtain a binary bit stream sequence; obtaining a feedback signal through a data selector;
step 8, the binary bit stream sequence is subjected to digital filtering to obtain a rotating speed result;
and 9, the feedback signal passes through a feedback loop to obtain a phase modulation signal.
The present invention may further comprise:
1. the feedback loop consists of a D/A conversion module, a post-amplification module and an integrated optical phase modulator; the feedback signal is converted into an analog feedback signal through the D/A conversion module, the analog feedback signal is amplified through the post-amplification module, and the amplified signal is processed through the integrated optical phase modulator to obtain a phase modulation signal.
2. The process of obtaining the binary bit stream sequence by the demodulation signal through the sigma delta controller is as follows: when the input of the quantization comparator is greater than zero, the output of the quantization comparator is 1; when the input of the quantization comparator is less than zero, the output of the quantization comparator is 0.
3. The process of obtaining the feedback signal by the demodulation signal through the sigma delta controller comprises the following steps: when the output of the quantization comparator is 1, the data selector outputs a positive digital quantity + Dm(ii) a When the output of the quantization comparator is 0, the data selector outputs a negative digital quantity-Dm(ii) a Wherein DmA constant set for the digital selector.
4. The integrated optical phase modulator is a Y waveguide.
5. The digital filtering is integral comb filtering.
The invention has the following beneficial effects:
1. the output of the quantization comparator is regarded as a larger digital quantity calculated by the maximum measurement range of the fiber-optic gyroscope by designing the sigma-delta controller and the closed-loop control loop, the digital quantity is converted into a feedback phase with a larger value by the D/A converter and the phase modulator, and a large feedback quantity is artificially added, so that the phase difference between the input end and the feedback end is increased, the phase difference is larger than the dead zone nonlinearity introduced by the quantizer in the digital-to-analog conversion process, and the influence of the dead zone nonlinearity of the quantizer can be effectively overcome.
2. The method can reduce the noise of the optical fiber gyroscope during measuring the angular motion, improve the signal to noise ratio of the output signal and is beneficial to improving the measurement precision of the optical fiber gyroscope.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a schematic diagram of sigma delta modulation;
FIG. 3 is a graph of ramp tracking signal feedback pulse density obtained by tracking the ramp signal according to the method of the present invention;
FIG. 4 is a graph of the feedback pulse density of a sinusoidal tracking signal obtained by tracking the sinusoidal signal according to the method of the present invention;
FIG. 5 is a graph of a ramp tracking signal obtained by tracking the ramp signal according to the method of the present invention;
FIG. 6 is a graph of ramp tracking error for tracking a ramp signal according to the method of the present invention;
FIG. 7 is a graph of a sinusoidal tracking signal obtained by tracking a sinusoidal signal according to the method of the present invention;
FIG. 8 is a graph of sinusoidal tracking error for tracking sinusoidal signals by the method of the present invention;
FIG. 9 is a graph of a ramp tracking signal for a conventional proportional-integral control scheme for a 10 times noise condition;
FIG. 10 is a graph of a ramp tracking signal for the method of the present invention for a 10 times noise condition;
FIG. 11 is a graph of a sinusoidal tracking signal for a proportional-integral control scheme for a 1000 times noise case;
fig. 12 is a graph of the sinusoidal tracking signal of the method of the present invention for 1000 times noise.
Detailed Description
The method of the present invention is further illustrated with reference to the accompanying drawings.
Fig. 1 shows a flow chart of the method of the present invention, and a digital closed-loop control method of a fiber optic gyroscope based on sigma delta modulation comprises the following steps:
1. detecting angular motion by the fiber-optic gyroscope, and outputting a light phase signal;
2. performing bias modulation and phase modulation;
3. performing photoelectric conversion to obtain an electric signal;
4. pre-amplifying to obtain an amplified signal;
5. carrying out A/D conversion to obtain a digital electric signal;
6. carrying out signal demodulation to obtain a demodulation signal;
7. the demodulation signal passes through a sigma delta controller to obtain a binary bit stream sequence and a feedback signal;
the sigma delta controller consists of a controller, a quantization comparator and a data selector; the demodulated signal is processed by a controller to obtain a control signal; the control signal passes through a quantization comparator to obtain a binary bit stream sequence; obtaining a feedback signal through a data selector;
8. the binary bit stream sequence is subjected to digital filtering to obtain a rotating speed result;
9. the feedback signal passes through a feedback loop to obtain a phase modulation signal;
the feedback loop consists of a D/A conversion module, a post-amplification module and an integrated optical phase modulator; the feedback signal is converted into an analog feedback signal through the D/A conversion module, the analog feedback signal is amplified through the post-amplification module, and the amplified signal is processed through the integrated optical phase modulator to obtain a phase modulation signal.
Specific embodiments are described in connection with laboratory specific equipment parameters.
A forward path portion
A light source in an optical fiber ring of the optical fiber gyro system is shot into the optical fiber ring, the phase difference is changed due to the Sagnac effect generated under the influence of the external angular rate, and a light intensity signal containing phase difference information is detected and demodulated through a circuit part; the design of the light path element is designed on the principle of realizing the minimum interference error, and according to the expression of the Sagnac effect, the link can be regarded as a proportional link with a proportionality coefficient of 2 pi LD/(lambda C)) Wherein, the length L of the optical fiber ring is 1000 m; the diameter D is 0.15 m; light source wavelength lambda is 1.55 × 10-6(ii) a The speed of light in vacuum is c 3 × 108m/s. The value of the proportionality coefficient is 1.29 pi.
The optical signal is converted by the photodetector into an electrical signal that can be processed by the circuit portion. Photocurrent i converted by PIN photodiodeD(t) is:
in the formula:
is PIN photodiode conversion responsivity, the unit is A/W, and the preferred embodiment is
Is 0.9A/W; i is0For the light intensity signal, in the embodiment, let I0Is 10 muW; Δ φ is the phase difference caused by the Sagnac effect. The photocurrent signal is converted into a detection voltage signal U after passing through an amplifierDThe expression is
UD＝Rf×iD(t) (2)
In the formula: rfIs the detector transimpedance in Ω, preferably 100K Ω in the embodiment. The frequency bandwidth of the detector is far larger than the bandwidth of a gyro system, so the process can be equivalent to a proportional link.
Front end amplifier section, probe mid-span impedance RfCan play a role in amplifying signals RfIs limited by the gain-bandwidth product of the detector, and the photocurrent i converted by the PIN photodiode is smallDAnd (t) is very small, so the output voltage of the photoelectric detector is only millivolt level and is not easy to be detected by a signal processing circuit, therefore, a low-noise amplifier is required to amplify the millivolt level voltage signal output by the photoelectric detector, and the bandwidth of the used preamplifier is more than 10 times that of a gyro system, so the part can be equivalent to a proportional link. Front-mounted optical fiber gyroscope for laboratoryThe magnification was set at 78 times.
In the A/D conversion step, the voltage signal output by the preamplifier is analog signal, and the signal processing part of the closed-loop system is performed in FPGA for digital signal processing. It is therefore necessary to use an a/D converter to convert the analog signal into a digital signal that can be processed by the FPGA. If the preamplifier outputs a voltage VoutIn the range of Vmin≤Vout≤VmaxIn which V isminIs a lower threshold voltage, VmaxIs a voltage upper threshold. Because of the high A/D conversion rate, it can also be used as a proportional link and a quantizer, and when N-bit A/D converter is selected, the proportional coefficient K isA/DIs expressed as
The value of the scaling factor is 2047.5 according to equation (3).
After square wave modulation and demodulation, the traditional controller can be regarded as a proportional-integral link.
The sigma delta controller has its basic principle as shown in fig. 2. In FIG. 2, the input signal is Sagnac phase shifted by phis(ii) a Feedback signal of phif(ii) a The integrator is a controller in the original system; the quantization comparator is preferably a zero-crossing comparator, compares the input with 0 or compares the difference between the upper and lower periodic input sampling values, and outputs a binary bit stream sequence; the data selector has different outputs corresponding to different binary inputs. This is a simplified sigma delta controlled system model.
Integrating the demodulated error signal in the original fiber-optic gyroscope closed-loop system to obtain nonreciprocal phase shift, adding a zero-crossing comparator behind the integral controller, wherein the output of the comparator is a large digital quantity +/-DmThis digital quantity is converted into a feedback phase phi with a large value by a D/A converter and a phase modulatorf. By artificially adding a large feedback quantity, the phase difference between the input end and the feedback end is increased and is larger than the dead zone voltage in the A/D conversion process, so that the voltage difference between the input end and the feedback end can be increasedThe influence of quantizer dead zone nonlinearity is overcome. Assuming that the phase difference is greater than zero, the output of the zero-crossing comparator after passing through the integrator is 1, and the positive digital quantity + D is output through the data selectormConverted to + phi phasef(ii) a When phi iss＜ΦfWhen the phase difference is less than zero, the output of the comparator becomes 0, and the output is-D through the data selectormConverted into a phase of-phifThe phase difference becomes again a positive value and the output of the comparator is 1. Cycling on sequentially, the output of the data selector becomes a waveform with alternating positive and negative values. The output of the data selector being passed through + D of the positive digital quantitymAnd a negative digital quantity-DmThe density of the input signal is reflected by the change rule of the input signal. When the input signal is increased, the density of the positive digital quantity should be increased, and the amplitude density should be decreased, and when the input signal is decreased, the density of the negative digital quantity should be increased, and the amplitude density should be decreased, so that the average value of the output waveform is the same as the input phase value, and the output is subjected to low-pass filtering processing to obtain the waveform the same as the input waveform.
Setting a zero-crossing comparator, wherein when the input of the zero-crossing comparator is more than zero, the output of the comparator is 1; when the input of the zero-crossing comparator is less than zero, the output of the comparator is 0; then D is set in the module by the data selectormWherein when the output of the comparator is 1, the output of the data selector is + DmWhen the output of the comparator is 0, the output of the data selector is-Dm。
The sigma delta control scheme requires a data selector to low pass filter the waveform of the output digital quantity. The functions of extraction and low-pass filtering are completed by utilizing an integral comb Filter, and the parameters of the integral comb Filter are designed by a Digital Filter Design tool in Matlab; adjusting the parameters of the filter optimizes the tracking performance of the system so that the same waveform as the input signal is obtained.
Regarding a single value range with symmetry about the zero point, namely the maximum measurement range (single interference fringe) of the gyroscope, taking the phase difference as pi according to the relationship between the Sagnac phase difference and the input angular velocity, and then the corresponding angular velocity is theThe maximum angular velocity that can be measured. The measurable maximum angular velocity is 88.82 degrees/s calculated according to the parameters of the gyroscope in the laboratory, and the feedback phase shift phi is takenfThe corresponding feedback angular velocity is 80 deg/s, and then the conversion relation between Sagnac phase shift and input angular velocity is known, at this moment phifThe value of (D) is 2.82 rad/s.
Second, feedback part
The integrated optical phase modulator plays roles of beam splitting and combining, polarization filtering and phase modulation of light waves in a gyro system, and is a key element of a closed-loop fiber optic gyro. The integrated optical phase modulator is preferably a Y waveguide, which can be regarded as a proportional differential element, and the laboratory prefers that the half-wave voltage of the Y waveguide is 4.1V, so its proportionality coefficient is pi/4.1, and when the input is 4.1V, the output phase of the phase modulator is pi. We need to determine when the phase of the output of the phase modulator is phifThe magnitude of the input voltage is 3.68V, which is determined from the linear relationship between the phase and the voltage.
A D/A conversion part, wherein the D/A converter in the closed-loop control structure of the fiber-optic gyroscope converts the digital feedback compensation amount demodulated by the FPGA into an analog signal in a feedback loop; under normal conditions, the number of bits of a D/A converter is between 12 and 16 bits, the requirement can be met, 16-bit AD768 chips are selected as fiber-optic gyroscope equipment in a laboratory, ideally, the method can be equivalent to a proportional link, and the proportional coefficient is calculated to be 2 pi/65536 through a chip manual.
The post-amplifier amplifies the analog signal after D/A conversion to the Y waveguide, and the part can be equivalent to a proportional link. The post amplification factor of the laboratory fiber optic gyroscope was 16 times. When the D/A converter reaches full scale, that is, the input digital quantity is maximum 65535, the phase modulator generates 2 pi phase, and according to the output voltage of AD768 and the amplification factor of post-amplification circuit, the digital quantity of D/A output when the driving voltage is 3.68V, that is, D output digital quantity can be obtainedmThe value has a size of 29410.
Third, simulation verification
1. Simulink simulation is carried out, representative ramp signals and sine signals are selected as input signals, and signal tracking is carried out by the method.
The amplitude of the related ramp signal is 1, and the frequency is 20 rad/s; sinusoidal signal, amplitude is 1, frequency is 20 rad/s. The maximum step size used by the Simulink simulation system is 0.0001, the variable step size discrete solution method is adopted, and the sampling time is 0.0001 s. The system tracks the feedback pulse density of the ramp signal and the sinusoidal signal as in fig. 3 and 4. The solid line is the fed back pulse density signal and the dashed line is the input signal. The feedback pulse density is subjected to integral comb filtering to obtain the signals shown in fig. 5 and 7, wherein the solid line is the signal after pulse density filtering, and the dotted line is the input signal; the corresponding tracking error is shown in fig. 6 and 8.
3-8, under the condition of noise and modulation and demodulation, the method of the invention can effectively track the ramp signal and the sine signal, the tracking signal is smooth, and the nonlinearity of the quantization dead zone is well inhibited.
2. By setting different noise conditions, the test effect of the traditional proportional-integral control method is compared with that of the method disclosed by the invention.
The noise of the fiber optic gyroscope system mainly comprises the noise of an optical path and a circuit, and the noise of the closed loop detection circuit is mainly considered by the research of the noise of the circuit. The useful signal modulated on the carrier after photoelectric conversion is usually in the level of μ V, the output noise voltage of the photodetector is mV, the noise level of the amplifier and the a/D converter is also mV, the useful signal can be considered to be 1000 times smaller than the noise signal, and these noises in the circuit can be considered as white noises.
Adding-120 dB of white gaussian noise to the model corresponds to adding approximately 10 times more circuit noise to the amplified signal. The results of the conventional proportional-integral control method and the ramp signal tracking of the method of the present invention are shown in fig. 9 and 10, respectively.
Changing the white noise parameter to-160 dB is equivalent to adding nearly 1000 times of noise to the amplified signal, and the results of tracking the sinusoidal signal by the conventional proportional-integral control method and the method of the present invention are shown in fig. 11 and 12, respectively.
As can be seen from fig. 9 to 12, the method of the present invention has a smaller noise amplitude when tracking the ramp signal and the sinusoidal signal, and thus compared with the conventional proportional-integral control method, the method of the present invention can reduce the noise of the whole system and improve the signal-to-noise ratio of the system.
Claims (7)
1. A digital closed-loop control method of a fiber-optic gyroscope based on sigma delta modulation is characterized by comprising the following steps:
step 1, detecting angular motion by the fiber optic gyroscope, and outputting a light phase signal;
step 2, performing offset modulation and phase modulation;
step 3, performing photoelectric conversion to obtain a photocurrent signal;
the expression of the photoelectric conversion is
In the formula:
is the photoelectric conversion responsivity, and the unit is A/W; i is0Is a light intensity signal; delta phi is the modulated optical phase signal; i.e. iD(t) is photocurrent;
step 4, pre-amplifying to obtain an amplified voltage signal;
the expression of the voltage signal is
UD＝Rf×iD(t)
In the formula: rfIs the impedance; u shapeDIs a voltage signal;
step 5, carrying out A/D conversion to obtain a digital voltage signal;
step 6, demodulating the signal to obtain a demodulated signal;
step 7, the demodulated signal passes through a sigma delta controller to obtain a binary bit stream sequence and a feedback signal;
the sigma delta controller consists of a controller, a quantization comparator and a data selector; the demodulated signal is processed by a controller to obtain a control signal; the control signal passes through a quantization comparator to obtain a binary bit stream sequence; the binary bit stream sequence passes through a data selector to obtain a feedback signal;
step 8, the binary bit stream sequence is subjected to digital filtering to obtain a rotating speed result;
and 9, the feedback signal passes through a feedback loop to obtain a phase modulation signal.
2. The method of claim 1, wherein the feedback loop comprises a D/a conversion module, a post-amplification module, and an integrated optical phase modulator; the feedback signal is converted into an analog feedback signal through the D/A conversion module, the analog feedback signal is amplified through the post-amplification module, and the amplified signal is processed through the integrated optical phase modulator to obtain a phase modulation signal.
3. The method of claim 1 or 2, wherein the step of obtaining the sequence of binary bit streams from the demodulated signal via the sigma delta controller comprises: when the input of the quantization comparator is greater than zero, the output of the quantization comparator is 1; when the input of the quantization comparator is less than zero, the output of the quantization comparator is 0.
4. The method for digitally closed-loop controlling a fiber-optic gyroscope based on sigma-delta modulation as claimed in claim 1 or 2, wherein said demodulating signal is fed back to the sigma-delta controller by: when the output of the quantization comparator is 1, the data selector outputs a positive digital quantity, i.e., + Dm(ii) a When the output of the quantization comparator is 0, the data selector outputs a negative digital quantity, i.e., -Dm(ii) a Wherein DmA constant set for the data selector.
5. The method of claim 3, wherein the demodulated signal is sigma delta controlled for digital closed loop control of the fiber optic gyroscope based on sigma delta modulationThe process of obtaining the feedback signal by the device is as follows: when the output of the quantization comparator is 1, the data selector outputs a positive digital quantity, i.e., + Dm(ii) a When the output of the quantization comparator is 0, the data selector outputs a negative digital quantity, i.e., -Dm(ii) a Wherein DmA constant set for the data selector.
6. The method of claim 2, wherein the integrated optical phase modulator is a Y-waveguide.
7. The method of claim 1, wherein the digital filtering is integrator-comb filtering.
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| CN111220142B (en) * | 2020-02-25 | 2020-12-15 | 北京航空航天大学 | Signal processing method and device for a novel ultra-high precision fiber optic gyroscope |
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