| Fiber optic gyro(FOG)is a new angle velocity sensor with a series of advantages of small size,long life,low cost,high reliability,anti radiation and a wide range of precision,which has a wide range of military and civilian applications in aviation,aerospace,water,marine,land,sea,space et al.FOG is an important element in inertial navigation and its precision directly determines the precision of inertial navigation system.With the in-depth research of FOG technology of many countries and the constantly updated optical devices,the precision of FOG has been increasingly high which leads the result that FOG has exceeded laser gyro in many aspects.This article studies deeply the key technologies of FOG in engineering applications and did some experiments using existing FOG of our lab.This paper introduces the basic principles of fiber optic gyro-Sagnac effect firstly.And then introduces the basic component of digital closed-loop FOG,the way of modulating and demodulating FOG using four state square wave,then derives the transfer function and stable condition of closed-loop FOG.This paper analys the main error sources of FOG and does theoretical analysis on how to eliminate the impact of these errors on FOG precision.Finally,this paper introduces the basic measurement limit of FOG—photon shot noise.Then,this paper introduced a test method of FOG—Allan variance to estimate the error coefficients of FOG.The largest average time is half duration of the sampling data when using Allan variance estimation to estimate the error coefficient,many data were wasted and the confidence of Allan variance estimation is low when correlation time is long.In order to improve the long-term frequency stability,a theoretical 1#variance estimation has been brought out.Theoretical 1#variance estimation has a high confidence when the average time is long,and the average time could be three-quarters of duration of sample data which improves the utilization of data effectively.Allan variance estimation has a higher degree of confidence when the average factor is small and theoretical 1#variance estimation has has a higher degree of confidence when the the average time is long,so using hybrid theory variance estimation to estimate the error coefficients of FOG was put forward.Hybrid Theory is an estimation method-Allan variance estimation when the average is small and unbiased estimation of theoretical 1#variance(theory BR variance)when the average factor is large.Do simulation of Allan variance estimation and hybrid theory variance estimation using FOG of our lab.In the FOG which uses brand band and high output power light source-erbium-doped fiber sourc,the light source intensity noise is much higher than shot noise.So it is necessary to suppress the light intensity noise in order to improve the detection sensitivity and SNR.Analysis of the existing intensity noise suppression method was done and the drawbacks that these methods need to increase a fiber detector and noise signal delay procedure or to add an external intensity modulator circuit which causes large volume and high cost was pointed out.After the analysis of light source intensity noise the fact that polarization degree of light impacts the intensity noise was discovered,so the method of reducing light polizaiton degree to reduce the intensity noise was proposed.Analysis of the structure and parameters of Lyot depolarizer was done,as a result using the Lyot depolarizer to reduce light polarizaiton degree in order to lower the intensity noise was proposed,finally simulation of this method was done.When analyzing Rayleigh backscattering,the fact that modulating the FOG using eigenfrequency square wave will not generate any theoretical Rayleigh backscattering error.Eigenfrequency is an important parameter of modulation,demodulation and sampling which can not be changed when using FOG,so it needs to be measured accurately.At present there are two methods to measure the eigenfrequency-the method based on symmetric square wave modulation and the method based on asymmetric square wave modulation.Measurement method based on symmetric square wave modulation needs to determine the eigenfrequency when photo detector output is a fifty percent duty cycle square wave,measurement accuracy of eigenfrequency depends on the measurement accuracy of duty cycle of photo detector,and it also requires equidistant continuous multi-cycle sampling method to make sure the duty cycle.Measurement method based on asymmetric square wave modulation needs to determine the eigenfrequency according to interference pulse width difference of photo detector output,eigenfrequency can be made sure when the pulse width differency is zero according to extreme search algorithm.The measurement accuracy of both methods is determined by measurement accuracy of the photo detector output signal.This paper proposed two methods—measurement method based on double eigenfrequeny modulation and measurement method based on fourfold eigenfrequeny modulation.The basic principle of both methods is to modulate the Y waveguide by even multiple eigenfrequency square wave,and eigenfrequency can be calculated out when the photo detector output is a straight line without interference pulse.Do experiment of both methods proposed with the existing high-precision FOG of our lab,the accuracy of measurement method based on double eigenfrequency square wave modulation is 0.05KHz and the accuracy of measurement method based on fourfold eigenfrequency square wave modulatio is 0.025KHz.The half-wave voltage is an important parameter in photoelectric modulation and its error could lead to nonreciprocal phase error in FOG,so it needs to measure the half-wave voltage of Y waveguide more accurately.High precision FOG generally uses a closed-loop feedback loop to correct the Y waveguide half-wave voltage in real time,but in low precision FOG there is no second feedback loop to correct the half-wave voltage.So it is necessary to measure the half-wave volgtage more accurately.The half-wave voltage of Y waveguide is proportional to wavelength of the incident light by definition,so it needs to measure half-wave voltage of Y waveguide with the same optical path as FOG.Currently there are many methods to measure half-wave voltage of Y waveguide,but the optical path of Sagnac interferometer method and open-loop search method is the only same path as FOG Using Sagnac interferometer method and open-loop search method,there is no need to build extra measurement path and the accuracy is much higher than other methods.In order to compensate the disadvantages of long measurement time of Sagnac interferometer method and open-loop search method,a brand new method-measurement method based on four state square wave was proposed.Measurement method based on four state square wave is an improvement of open-loop search method,and its speed is eight time as open-loop search method.Experiment was done using high-precision FOG of our lab,and the measurement accuracy is 0.001V.Finally,sampling of some FOG output data under temperature variation was done,and build the model of gyro in order to compensate it using regression analysis method based on least squares estimation.After measuring the FOG output data and bias drift after temperature compensated,a conclusions that the accuracy of FOG could be effectively improved using software modeling and compensation was reached. |
PDF Full Text Request