Method and system for correcting random walk errors induced by rate reversals in a dithered ring laser gyroscope

Abstract

A correction to the output angle of the ring laser gyroscope is calculated as a function of the phase difference and the magnitude of coupling between the two counterpropagating beams when the dither oscillations change direction. A pair of heterodyne detectors produce heterodyne signals indicative of the interference pattern between the counterpropagating beams. A signal indicative of the sum of the separate beam intensities is demodulated with the heterodyne signals to determine the magnitude of coupling between the beams in the ring laser gyroscope.

What is claimed is: 
     
       1. A method for correcting random walk errors caused by coupling between two counterpropagating light beams in a ring laser gyroscope that is body dithered at a frequency ω D  about a sensor axis at a dither depth B m , the ring laser gyroscope forming a beat signal by interference of the two counterpropagating light beams to indicate rotation by an angle Ψ about the sensor axis, comprising the steps of: (a) producing a pair of heterodyne signals that are indicative of the phase difference between the two counterpropagating light beams;   (b) sampling the heterodyne signals to determine the phase difference Ψ T  =φ H  +ε between the two counterpropagating light beams at rate reversals of the ring laser gyroscope;   (c) adding signals indicative of the intensities of the two counterpropagating light beams to produce an intensity sum signal;   (d) demodulating the intensity sum signal with at least one of the sampled heterodyne signals from step (b);   (e) processing the demodulated intensity sum signal to determine the magnitude B L  of coupling between the two counterpropagating light beams; and   (f) calculating a correction to the angle of rotation as a function of the coupling B L  between the two beams and the phase difference Ψ T  =φ H  +ε between the two counterpropagating light beams at rate reversals of the ring laser gyroscope.   
     
     
       2. The method of claim 1, further including the step of determining whether the change in direction at rate reversals is counterclockwise to clockwise or clockwise to counterclockwise. 
     
     
       3. The method of claim 1, wherein the step of determining the magnitude of the coupling between the beams includes the steps of: detecting changes in direction of the dither oscillations;   measuring the intensity of a first one of the beams;   measuring the intensity of the other of the beams; and   adding signals indicative of the intensities of the two beams to form a sum signal.   
     
     
       4. The method of claim 3, further including the step of: forming a heterodyne signal indicative of the beat frequency produced when the counterpropagating beams interfere with one another; and   demodulating the sum signal with the heterodyne signal.   
     
     
       5. The method of claim 1 wherein the step of calculating a correction to the angle of rotation includes calculating a correction angle according to the equation ΔΨ correction  =B L  [2π(B m  ω d ) -1  ] 1/2  sin (φ H  +ε±π/4). 
     
     
       6. A system for correcting random walk errors caused by coupling between two counterpropagating light beams in a ring laser gyroscope that is body dithered counterpropagating light beams in a ring laser gyroscope that is body dithered at a frequency ωhd D about a sensor axis at a dither depth B m , the ring laser gyroscope forming a beat signal by interference of the two counterpropagating light beams to indicate rotation by an angle Ψ about the sensor axis, comprising: means for producing a pair of heterodyne signals that are indicative of the phase difference between the two counterpropagating light beams;   means for sampling the heterodyne signals to determine the phase difference Ψ T  =φ H  +ε between the two counterpropagating light beams at rate reversals of the ring laser gyroscope;   means for adding signals indicative of the intensities of the two counterpropagating light beams to produce an intensity sum signal;   means for demodulating the intensity sum signal with at least one of the heterodyne signals corresponding to the most recent rate reversal of the ring laser gyroscope;   means for processing the demodulated intensity sum signal to determine the magnitude B L  of coupling between the two counterpropagating light beams and;   means for calculating a correction to the angle of rotation as a function of the coupling B L  between the two beams and the phase difference a rate reversals of the ring laser gyroscope.   
     
     
       7. The system of claim 6, further including means for determining whether the change in direction at rate reversals is counterclockwise to clockwise or clockwise to counterclockwise. 
     
     
       8. The system of claim 6, wherein the means for determining the magnitude of the coupling between the beams includes: means for detecting changes in direction of the dither oscillations;   means for measuring the intensity of a first one of the beams;   means for measuring the intensity of the other beams; and   means for adding signals indicative of the intensities of the two beams to form a sum signal.   
     
     
       9. The system of claim 8, further including: means for forming a heterodyne signal indicative of the beat frequency produced when the counterpropagating beams interfere with one another; and   means for demodulating the sum signal with the heterodyne signal.   
     
     
       10. The system of claim 6 wherein the means for calculating a correction to the angle of rotation includes means for calculating a correction angle according to the equation   ΔΨ.sub.correction =B.sub.L [2 π(B.sub.m ω.sub.d) .sup.-1 ].sup.1/2 sin (φ.sub.H +ε±π/4).     
     
     
       11. A system for correcting random walk errors in a ring laser gyroscope that is body dithered at a frequency ω D  and a dither depth B m  about a sensor axis, a beat signal formed by interference of two counterpropagating light beams indicating rotation of an angle Ψ about the sensor axis, comprising: means for producing a pair of heterodyne signals indicative of the light intensity resulting from interference of the two light beams;   a turnaround detector connected to the means for producing a plurality of heterodyne signals, the turnaround detector including means for detecting phases of the heterodyne signals when the dither oscillations change direction and including means for determining whether the change in direction of the dither oscillations is clockwise to counterclockwise or counterclockwise to clockwise;   means for producing an intensity sum signal indicative of the sum of the intensities of the two light beams; and   an intensity demodulator means for demodulating the intensity sum signal with the heterodyne signals output from the turnaround detector, the intensity demodulator including means for determining the magnitude of coupling between the two light beams and for determining the phase difference between the heterodyne signals and the intensity sum signal.   
     
     
       12. A method for reducing the random walk error of a ring laser gyroscope being body dithered at a frequency ω D  and having a dither depth B m  about a sensor axis with a beat signal formed by interference of two counterpropagating light beams indicating rotation of an angle Ψ about the sensor axis, comprising the steps of: measuring the phase of optical signals input to a pair of heterodyne detectors which detect the interference of the two counterpropagating beams;   producing an intensity sum signal indicative of the sum of the intensities of the two counterpropagating light beams;   measuring temperature-induced changes in phase of the intensity sum signal; and   processing the temperature-induced changes in phase of the intensity sum signal to calibrate the output of the ring laser gyroscope to compensate for temperature-induced phase changes in the two counterpropagating beams.   
     
     
       13. The method of claim 12, further including the steps of: accumulating the temperature-induced phase difference as integer multiples of 2π; and   associating each complete 2π change in the temperature-induced phase difference with a one count error in the output of the ring laser gyroscope.   
     
     
       14. A system for reducing the random walk error of a ring laser gyroscope being body dithered at a frequency ω D  and having a dither depth B m  about a sensor axis with a beat signal formed by interference of two counterpropagating light beams indicating rotation of an angle Ψ about the sensor axis, comprising the steps of: means for measuring the phase of optical signals input to a pair of heterodyne detectors which detect the interference of the two counterpropagating beams;   means for producing an intensity sum signal indicative of the sum of the intensities of the two counterpropagating light beams;   means for measuring temperature-induced changes in phase of the intensity sum signal; and   processing the temperature-induced changes in phase of the intensity sum signal to calibrate the output of the ring laser gyroscope to compensate for temperature-induced phase changes in the two counterpropagating beams.   
     
     
       15. The method of claim 14, including: means for accumulating the temperature-induced phase difference as integer multiples of 2π; and   means for associating each complete 2π change in the temperature-induced phase difference with a one count error in the output of the ring laser gyroscope. .Iadd.   
     
     
       16.  A method for producing a signal for measuring rotation of a ring laser gyroscope about a sensing axis in which two waves propagate in opposite directions in a closed cavity and interfere to form an interference pattern that is indicative of the rotation rate of the ring laser gyroscope, comprising the steps of: producing a pair of heterodyne signals that are indicative of the phase difference between the two counterpropagating light beams;   producing an intensity sum signal indicative of the sum of the intensities of the two waves;   high pass filtering the intensity sum signal;   demodulating the intensity sum signal with the heterodyne signals to produce a demodulated intensity sum signal; and   processing the demodulated intensity sum signal to calculate a correction term that compensates for errors in the rotation rate indicated by the interference pattern at rate reversals of the ring laser gyroscope. .Iaddend. .Iadd.   
     
     
       17.  The method of claim 16 including the steps of: sampling the heterodyne signals at rate reversals in motion of the ring laser gyroscope about the sensing axis; and   processing the demodulated intensity sum signal to determine the magnitude of coupling between the two waves at rate reversals in motion of the ring laser gyroscope about the sensing axis. .Iaddend. .Iadd.18. The method of claim 17 including the steps of:   processing the heterodyne signals to detect rate reversals of the ring laser gyroscope;   determining the phase difference between the two counterpropagating light beams at rate reversals of the ring laser gyroscope; and   calculating a correction to the angle of rotation as a function of the coupling between the two beams and the phase difference between the two counterpropagating light beams at rate reversals of the ring laser gyroscope. .Iaddend. .Iadd.19. The method of claim 18 including the step of providing a velocity window to avoid making corrections to the angle of rotation at spurious rate reversals in the motion of the ring laser gyroscope about the sensing axis. .Iaddend. .Iadd.20. A system for producing a signal for measuring rotation of a ring laser gyroscope about a sensing axis in which two waves propagate in opposite directions in a closed cavity and interfere to form an interference pattern that is a function of the rotation rate of the ring laser gyroscope, comprising:   means for producing a pair of heterodyne signals resulting from interference of the two waves;   means for producing an intensity sum signal indicative of the sum of the intensities of the two waves;   means for high pass filtering the intensity sum signal;   means for demodulating the intensity sum signal with the heterodyne signals to produce a demodulated intensity sum signal; and   means for processing the demodulated intensity sum signal to calculate a correction term that compensates for errors in the rotation rate indicated by the interference pattern at rate reversals of the ring laser gyroscope.   
     
     
        .Iaddend. .Iadd.21.  The system of claim 20 including: means for sampling the heterodyne signals at rate reversals in motion of the ring laser gyroscope about the sensing axis; and   means for processing the demodulated intensity sum signal to determine the magnitude of coupling between the two waves at rate reversals in motion of the ring laser gyroscope about the sensing axis. .Iaddend. .Iadd.22. The system of claim 21 including:   means for processing the heterodyne signals to detect rate reversals of the ring laser gyroscope;   means for determining the phase difference between the two counterpropagating light beams at rate reversals of the ring laser gyroscope; and   means for calculating a correction to the angle of rotation as a function of the coupling between the two beams and the phase difference between the two counterpropagating light beams at rate reversals of the ring laser gyroscope. .Iaddend. .Iadd.23. The system of claim 22 including means for providing a velocity window to avoid making corrections at spurious rate reversals in the motion of the ring laser gyroscope about the sensing axis. .Iaddend.