Processing arrangement for optical rate sensor

• US 5,262,843 A
• Filed: 04/28/1992
• Issued: 11/16/1993
• Est. Priority Date: 04/21/1986
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Lear Siegler, Inc. (LSS Holdings LLC)
• Inventors: Sugarbaker, Daniel L., Perfitt, Thomas E., Wallen, Gary A.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US07/875,589
• Time in Patent Office: 567 Days
• Field of Search: 356/350, 372/94
• US Class Current: 356/464
• CPC Class Codes: G01C 19/72 with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers

Abstract

An optical rate sensor (300) is disclosed for use in inertial reference systems to measure rates of angular rotation with substantially high resolution and sensitivity. The sensor (300) comprises a passive ring Sagnac interferometer (302) with a multiturn fiber optic ring (308) through which counter-propogating optical waves are transmitted. A non-reciprocal phase modulation is applied to the waves, and the waves are combined to produce a "zero order" fringe pattern having an intensity varying in accordance with the relative phase between the counter-propogating waves. The phase modulation is under control of modulated control signals which are functionally related to measured values of the angular rotation rate. The phase modulation thereby produces an intensity signal which tends to alternate between values of substantially maximum change with respect to given changes in phase shift resulting from the non-reciprocal phase modulation. The optical rate sensor (300) also includes an analog integrator (364) and a central processing unit (384) which provides for optimal control and filtering for determination of the rate of angular rotation and appropriate control signals to be applied to the non-reciprocal phase modulator. The central processing unit (384) also includes an arrangement whereby a relatively wide dynamic range is achieved with substantially minimal errors in rate determination and with the use of a physically realizable digital-to-analog converter through an arrangement whereby the most significant bits of the determined control signals are "stripped off" prior to application through a digital-to-analog converter (392).

Claims

1. An optical rate sensor for use in inertial reference systems to detect angular rotation and measure magnitude and directional sense of the rate of angular rotation, said rate sensor comprising:

a passive ring Sagnac interferometer wherein a pair of counter-propagating optical waves are transmitted in opposite directions through a ring path and emerge therefrom with a relative Sagnac phase shift indicative of said rate of angular rotation, said interferometer comprising combining means for recombining said counter-propagating waves wherein the resultant low order fringe pattern is representative of the relative phases of said counter-propagating waves;

phase modulation means connected to said interferometer for externally applying to said counter-propagating waves a non-reciprocal phase shift;

circuit means connected to said interferometer for generating an intensity signal indicative of said resultant low order fringe pattern; and

measuring means connected to said circuit means for measuring said intensity signal and generating an output signal corresponding to said rate of angular rotation;

characterized in that;

said measuring means comprises;

demodulation means responsive to said intensity signal for demodulating said intensity signal and for generating a phase error signal;

integration means connected to said demodulation means for integrating said phase error signal and for generating a corresponding integrated phase error signal;

analog-to-digital converter means for converting said integrated error signal into a digital error signal corresponding thereto;

central processing means responsive to said digital error signal and to clock signal to estimate the true value of said integrated phase error signals by combining past and present measurements of said signals, such that mean square errors between true and expected values thereof are minimized, and for generating a control signal representative thereof;

modulation means for modulating said control signal so as to apply a level adjusted AC signal to said phase modulation means, and for generating a modulated reference signal;

adder means for summing a period time-variant AC bias signal with said modulated reference signal, and for generating a modulator control signal, said control signal representative of the relative Sagnac phase shift of said pair of counter-propagating waves.

2. An optical rate sensor in accordance with claim 1 characterized in that the period of said AC bias signal is approximately twice the ring transit time for an optical wave traversing said ring path.

3. An optical rate sensor in accordance with claim 1 characterized in that the amplitude of said time-variant feedback signal will vary and substantially correspond to a non-reciprocal phase shift equal and opposite to the then-current relative Sagnac phase shift of said counter-propagating waves.

4. An optical rate sensor for detecting angular rotation, comprising:

a ring interferometer wherein a pair of counter-propagating optical waves are transmitted in opposite directions through a ring path and emerge therefrom with a relative Sagnac phase shift indicative of the rate of angular rotation, the interferometer comprising an optical combiner which combines the counter propagating waves emerging from the ring into a combined optical wave wherein a resultant low-order fringe pattern is representative of the relative phases of the counter propagating waves,

a detector coupled to the interferometer for generating an intensity signal indicative of the resultant low-order fringe pattern;

control circuitry connected to the detector and including a digital processor and responsive to the intensity signal in each of a plurality of predefined time periods to store data derived from a measurement of the intensity signal and to generate a phase modulator control signal derived from the data stored in a previous time period and a measurement of the intensity signal in the current time period; and

a phase modulator connected to the control circuitry and the interferometer and responsive to the control signal to apply to the counter-propagating waves a phase shift corresponding to the phase modulator control signal.

5. The sensor in accordance with claim 4 wherein the control circuitry is responsive to the intensity signal in each of the time periods to compute from a measurement of the intensity signal an estimate of the value of the measurement of the intensity signal in the next time period and wherein the data stored in each time period comprises the computed estimate.

6. An optical rate sensor for detecting angular rotation, comprising:

a ring interferometer wherein a pair of counter-propagating optical waves are transmitted in opposite directions through a ring path and emerge therefrom with a relative Sagnac phase shift indicative of the rate of angular rotation, the interferometer comprising an optical combiner which combines the counter propagating waves emerging from the ring into a combined optical wave wherein a resultant low-order fringe pattern is representative of phase difference of the counter propagating waves;

detector circuitry coupled to the interferometer for generating a detector output signal representative of the phase difference;

control circuit including a digital processor and responsive to the detector output signal to generate a square wave phase error control signal having a plurality of pulses with predetermined starting times and indicative of a detected phase error;

a phase modulator connected to the control circuitry and the interferometer and responsive to the phase error control signal to apply to the counter-propagating waves a phase shift corresponding to the detected phase error; and

interconnection circuitry connected to the detector circuitry to selectively apply the detector output signal to the control circuitry only during selected periods of time having starting times related to the predetermined starting times and having a duration substantially equal to a period of time T required by the optical waves to traverse the ring path.

7. The sensor in accordance with claim 6 wherein the square wave phase error control signal has a periodic rate which is a function of the time period T required by the light waves to traverse the ring path.

8. The sensor in accordance with claim 7 and further comprising timing circuitry for generating periodic timing pulses and wherein the interconnection circuitry is connected to the timing circuitry and responsive to the timing pulses to selectively apply a detector output signal representative of the phase difference to the control circuitry only at certain periods of time defined as a function of the periodic rate of the square wave phase error control signal.

9. The sensor in accordance with claim 8 wherein the interconnection circuitry applies detector output signals to the control circuitry at periods of time having starting times delayed from the predetermined starting times.

10. The sensor in accordance with claim 8 and further comprising a square wave generator for generating a square wave phase shift signal comprising a series of pulses of predetermined magnitude and having a periodic pulse rate defined as a function of the time period T required by the light waves to traverse the ring path and circuitry transmitting a combined output signal obtained by combining the phase error control signal and the phase shift signal to the phase modulator.

11. The sensor in accordance with claim 10 wherein the duration of one period of the periodic pulse rate of the phase shift signal is less than the time period T required by the light waves to traverse the ring path.

12. In an optical rate sensor comprising control circuitry and a ring interferometer wherein a pair of counter-propagating optical waves are transmitted in opposite directions through a ring path and emerge therefrom with a relative Sagnac phase shift indicative of the rate of angular rotation and wherein the counter-propagating waves emerge from the ring in a combined optical wave in which a resultant low-order fringe pattern is representative of phase difference of the counter-propagating waves, a method for controlling operation of the optical rate sensor comprising the steps of:

monitoring the combined optical wave and applying to the control circuitry a periodic electrical output signal which is representative of predetermined portions of the low-order fringe pattern occurring only in certain periods of time;

generating in the control circuitry and on the basis of the periodic control signal representative of the predetermined portions of the low-order fringe pattern a periodic phase error control signal indicative of a detected phase error;

periodically applying a phase shift to the counter-propagating waves proportional to the generated periodic phase error control signal, wherein a phase shift is applied to the counter-propagating waves proportional to a control signal generated on the basis of only predetermined portions of the low-order fringe pattern while other portions of the fringe pattern are blanked out.

13. The method in accordance with claim 12 wherein the periodic output signal has a periodic rate which is a function of the time period T required by the light to traverse the ring path.

14. The method in accordance with claim 12 wherein the step of generating a phase error control signal includes the step of generating a square wave phase error control signal having a periodic rate which is a function of the time period required by the light waves to traverse the ring path.

15. The method in accordance with claim 14 in which the step of generating a square wave phase error control signal comprises the step of generating such a control signal having a plurality of pulses having predetermined starting times and wherein the step of generating a periodic output signal comprises the step of generating such a signal having a plurality of pulses occurring in synchronism with the pulses of the phase error control signal and having starting times delayed from the starting times of the phase error control signal pulses.

16. The method in accordance with claim 15 and further comprising the step of generating a square wave phase shift signal comprising a series of pulses of predetermined magnitude and having a period pulse rate defined as a function of the time period T required by the light waves to traverse the ring path and the step of combining the square wave phase error control signal and the square wave phase shift signal prior to the step of applying the phase shift to the counter-propagating waves.

17. The method in accordance with claim 16 wherein the step of generating a square wave phase shift signal comprises the step of generating such a signal wherein the duration of one period of the periodic pulse rate is less than the time required by the light waves to traverse the ring path.

18. An optical rate sensor for detecting angular rotation, comprising:

a ring interferometer wherein a pair of counter-propagating optical waves are transmitted in opposite directions through a ring path and emerge therefrom with a relative phase shift indicative of the rate of angular rotation, the interferometer comprising an optical combiner which combines the counter-propagating waves emerging from the ring into a combined optical wave wherein a resultant low-order fringe pattern is representative of the relative phase shift;

detector circuitry coupled to the interferometer for generating a detector output signal representative of the phase shift;

a phase modulator connected to the interferometer and responsive to a phase control signal to apply to the counter-propagating waves a phase shift defined by the phase control signal;

timing circuitry for generating periodic timing pulses defining predetermined time intervals;

control circuitry including a digital processor responsive to the detector output signal to generate a phase control signal indicative of a detected phase error;

interface circuitry connected to the control circuitry and the phase modulator and responsive to the timing pulses to selectively apply the generated phase control signal to the phase modulator only during selected ones of the time intervals defined by the timing circuitry;

interconnection circuitry connected to the detector and the control circuitry and responsive to the timing pulses to selectively apply the detector output signal to the control circuitry only during the selected time intervals, whereby only selected portions of the detector output signal representative of the phase shift of the counter-propagating waves are applied to the control circuitry.

19. The sensor in accordance with claim 18 wherein the interconnection circuitry applies the detector output signal to the control circuitry at a periodic rate which is a function of the time period T required by the optical waves to traverse the ring path.

20. The sensor in accordance with claim 19 wherein the periodic rate is a function of 1/nT, and n is an integer.

21. The sensor in accordance with claim 18 wherein the interconnection circuitry comprises a switch responsive to the timing pulses to selectively connect the detector output signal to the control circuitry only during the selected time periods.