Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
  • G01C19/58—Turn-sensitive devices without moving masses
  • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
  • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
  • G01C19/726—Phase nulling gyrometers, i.e. compensating the Sagnac phase shift in a closed loop system

Definitions

• the present invention relates to fiber optic gyros and more particularly to a device for measuring a non-reciprocal phase shift generated in an optical ring interferometer of the gyrometer, also called SAGNAC interferometer.
• SAGNAC interferometer mainly comprises a light energy source generally constituted by a laser; an optical device consisting either of a certain number of mirrors, or of an optical fiber wound on itself, this device forming a waveguide; a device for separating and mixing light and a device for detecting and processing the detected signal.
• a fundamental property of ring interferometers is reciprocity, which can be expressed as follows: any disturbance in the optical path similarly affects the two waves, although these two waves are neither subjected to it at exactly the same instant, nor in the same direction.
• the phase difference (which will be called in the following ⁇ ) between the two waves which recombine in the separation and mixing device after having traversed the optical path is zero.
• the detection and processing device detects signals representing the optical power of the composite wave obtained after recombination. This power can be broken down in the interferometers of the known art into two components: a constant component and a component proportional to Cos ( ⁇ ), this component only existing at the appearance of "non-reciprocal" disturbances.
• the sensitivity is maximum since the term to be measured is proportional to Cos ( ⁇ + ⁇ / 2), that is to say to sin ( ⁇ ).
• the instability of these devices is generally of the same order of magnitude as the variations of the quantity to be measured.
• This method is based on the property that the SAGNAC interferometer has of producing the equivalent of a discrete temporal derivation.
• a phase modulation being produced at one end of the fiber loop one of the waves undergoes the modulation at the moment when this is produced, while the other wave undergoes it with a delay equal to the propagation time in the fiber.
• the "natural frequency" of the interferometer is (1 / 2.t 0 ), and represents the modulation frequency at which the two waves undergo two phase shifts in phase opposition.
• phase shift between the two optical waves is therefore equal to the difference S (t) -S (t -t 0 ), where S (t) is the signal applied to the phase modulator. It is therefore seen that, if the half-period of the modulation signal is t 0 , the phase shift at the output of the interferometer is equal to twice the phase shift applied. It is this method which is used to create the bias giving the operating point of the interferometer.
• phase shift ⁇ 0 due to the non-reciprocal effect, in this case due to the rotation if it is not zero.
• a more precise method avoiding errors due to possible drifts of the various elements used, for example optoelectronic elements, consists of an indirect method or "zero method". According to this method, this difference in phase shift is compared with ⁇ ⁇ / 2 radians by generating an additional phase shift, equal in absolute value to the amplitude of the phase shift due to the non-reciprocal effect and of opposite sign, so that l 'to cancel.
• this method involves two separate operations: phase modulation and the generation of a feedback signal.
• the proportionality factor or scale factor is not linked to that used for the modulation of ⁇ ( ⁇ / 2) radians.
• phase "ramp" cannot be infinite, that is to say that the signal, which in practice consists of a control voltage of a phase modulator, cannot increase above d 'a determined threshold.
• a usable method is to generate sawtooth phase-shift control signals of peak to peak amplitude 2 ⁇ radians, the mathematical functions involved being periodic and of period 2 ⁇ radians. It follows the problem of precisely determining this amplitude of phase shift equal to 2 ⁇ radians.
• the phase “ramp” consists of a digital signal.
• the phase modulation, also in digital form, and this phase “ramp” are combined into a single signal and converted into an analog signal for controlling a phase modulator arranged in the ring.
• the invention to overcome the drawbacks of the prior art which have just been mentioned, proposes a fiber optic gyrometer comprising a Sagnac interferometer using two light waves circulating in opposite directions in a ring waveguide, comprising a photodetector delivering an electrical signal representing the light intensity of the interference between the two waves, and optical phase shifting means of the waves controlled by a slot modulation signal capable of controlling a variation of optical phase at a frequency FO substantially equal to 1/2.
• t 0 is the travel time of a wave in the guide
• the photodetector being connected to at least first and second sampling circuits controlled in phase opposition by a clock at frequency FO and providing two samples at each period respectively on a first and a second input of a differential amplifier, an analog-digital converter at the output of the differential amplifier and an adder / subtractor to accumulate the digital values successively supplied by the analog-digital converter, l adder / subtractor providing content representing a parameter of the gyrometer rotation measurement, characterized in that means are provided for reversing, at a frequency f much lower than the frequency FO, the phase of the clock, by so as to alternate, at frequency f, the direction of the difference in samples at the output of the differential amplifier, and in this that the adder / subtractor is also controlled by the frequency f, to operate alternatively as an adder or as a subtractor.
• FIG. 1 represents a ring interferometer of the known art
• FIG. 2 represents the variation of the optical power P s in an output branch of the interferometer of FIG. 1.
• FIG. 3 shows a block diagram of a gyrometer according to the invention.
• FIG. 4a represents a modulation signal Um of the phase modulator of the gyrometer of FIG. 3.
• FIG. 4b represents the control signal from an inverter of the complementary states of a clock of the gyrometer of FIG. 3.
• - Figure 5 shows a variant of the gyrometer of Figure 3 according to the invention correcting the scale factor of the phase ramp controlling the modulator.
• Figure 1 schematically illustrates the architecture of a ring interferometer as described in this patent.
• a laser source S produces a beam of parallel rays 1, towards a separating device constituted, for example, by a blade or a semi-transparent mirror M optically coupled to the ring 2 of the interferometer.
• This ring 2 can be produced for example using a single mode optical fiber wound on itself. Indeed, the sensitivity of the measurement is increased thanks to the use of a long optical path, proportional to the number of turns.
• This ring 2 is looped back onto the separating device M which also plays the role of a mixing device and thus defines an output branch 3.
• the ring is therefore traversed by two waves propagating in opposite directions: one in the direction clockwise S1 the other anticlockwise S2 These two waves recombine on the separating plate M. The result of this recombination can be observed in the output branch 3 using the photodetector 4.
• ⁇ 0 be the phase difference between the two waves propagating in opposite directions in the ring and Ps the optical output power that can be measured in the output branch 3. In the absence of "non-reciprocal" disturbance ⁇ 0 is zero.
• L the length of the course optical
• ⁇ the wavelength of the light emitted by the laser source S
• C the speed of light in the ring 2.
• the sensitivity of the interferometer is very low if the phase difference ⁇ is little different from zero. This is the case in a gyrometer if one wishes to measure low rotational speeds ⁇ .
• a constant “non-reciprocal” bias can be introduced in the phase of the two waves traveling in opposite directions so as to move the operating point of the interferometer.
• a phase modulator 5 is introduced on the wave path in the ring 2, bringing into play a reciprocal effect.
• the phase modulator 5 (FIG. 1) is excited so as to create a phase variation ⁇ (t) of the wave passing through it.
• This variation is periodic, its period being equal to 2.t o , t 0 being the travel time of a wave in the ring.
• FIG. 3 represents the architecture of a gyrometer according to the invention implementing the ring interferometer of FIG. 1.
• An electronic device 20 for operating the interferometer receives electrical information from the photodetector 4 optically coupled to channel 3, at the output of the separating blade M of the interferometer of FIG.
• a processor 28 manages the entire electronic operating device 20 of the gyrometer according to the invention.
• the control signals Ca and Cb necessary for the control of the synchronous detector 26 which can be, for example, the copying of impulse signals from a clock 30 driven by a quartz oscillator 32 of the processor 28.
• the synchronous detector 26 essentially comprises a first 34 and a second sampler / blocker 36 controlled by the clock 30, through an inverter 38 of the complementary logic states H and R supplied by the clock 30, so that the signal Ud at the output of the adapter 24 is sampled, by the first blocker sampler 34, for a half-period t 0 of the phase modulation of the optical signal in one direction, then, by the second blocker sampler 36, during the another half-following period of the phase modulation of the optical signal in the other direction, the two directions corresponding to the synchronous phase modulations in + ⁇ / 2 and - ⁇ / 2 (sum of the physical phase shifts according to S2 of + and- ⁇ / 4 to T- 1 0 and according to S1 from - and + ⁇ / 4 to T).
• the amplitude of this modulation is extracted by the synchronous detector 26 which supplies an analog voltage Us corresponding to the phase variation.
• Analog voltage Us after digitization by a analog / digital converter 42 is applied to a digital control circuit 44 generating a composite signal for modulating the phase modulator 5.
• the analog / digital converter 42 is controlled by a clock at frequency FO.
• the purpose of the digital control circuit 44 is to develop a digital ramp and to combine it with the digital phase modulation signals.
• the digital control circuit 44 includes an adder / subtractor 46 receiving on inputs the digital signals at the output of the analog-digital converter 42 and an accumulation order at the frequency FO and supplying an output with digital information to an integrator 48 responsible for producing a digital ramp whose slope is a function of the speed of rotation of the gyrometer.
• the digital output of the integrator 48 drives a digital / analog converter 50 generating, through a power amplifier 52, the analog modulation voltage Um of the phase shifter 5 disposed in the path of the light waves of the interferometer.
• the operating device comprises means for reversing, at a frequency f much lower than the frequency F0, the phase of the clock 30, so as to alternate, at this frequency f , the meaning of the difference in samples.
• the adder / subtractor circuit 46 is also controlled by the frequency f, to operate alternately as an adder (+1) or as a subtractor (-1).
• the states H and H supplied by the clock 30 are inverted by the inverter 38, at the rate of the frequency f applied to a control input 54 of the inverter 38.
• the sampler-blockers 34, 36 perform the sampling of the signal Ud at the output of the adapter 24 of the photodetector 4.
• the signal Ud represents the optical power resulting from the interference between the two light waves Si and S2 propagating in the fiber optics of the interferometer.
• Each of the outputs 60, 62 of the sample-and-hold units 34 and 36 drives one and the other of the two inputs (+, -) of a differential amplifier 64 supplying at its output the voltage Us representing the difference between two consecutive samples taken in either half period of the frequency FO of the optical power signal Ud at the output of the photodetector 4.
• Each of the sampler-blockers of the synchronous detector has control inputs Ea, ⁇ a and Eb, Eb controlled by the clock 30 through the inverter 38 as described below.
• the output Ca of the inverter is connected respectively to the input Ea of the first blocker sampler 34 and to the input Eb of the second 36 and the output Cb of the inverter 38 is connected to the input Ea of the first sampler- blocker 34 and at the input Eb of the second, thus, in a known manner, the sample maintained is the analog value present at the input on the rising edges for example, of the inputs Ea and Eb.
• the signal presented at the input of the analog / digital converter 42 is the difference between the values sampled during the last rising edges of the signals at the inputs Ea and Eb.
• the inverter 38 receives by its control input 54 a command signal Co of inversion, at the frequency f, having for a half-period of duration 1 / 2.f, a high state then during the following half-period, of the same duration, a low state.
• the inversion control signal is, for example, in the high state
• the states H and H are transmitted respectively to the outputs Ca and Cb of the inverter 38, the output Ca transmitting the state H of l clock and the output Cb state H
• the inversion control signal is in the low state
• the states H and R are inverted at the outputs Ca and Cb of the inverter 38, the output Ca transmitting the state R of the clock and the output Cb state H.
• the differential amplifier 64 presents at its output a voltage corresponding to a series of differences of two consecutive samples A p and B (P + i) taken respectively during one and the other phases of modulation of the light signals (+ ⁇ / 2 and - ⁇ / 2).
• the signal Us at the output of the differential amplifier representing the difference of the samples (A p - B (p + i ) ) during a period 2to, is applied, after digitization by the analog-digital converter 42, to the adder / subtractor 46.
• the adder / subtractor 46 is controlled by the processor 28, while the control signal from the inverter 38 is in the high state, so as to effect a positive accumulation (+1).
• the states H and R are reversed at the outputs Ca and Cb of the inverter 26, the output Ca transmitting the state R of l 'clock and output Cb state H reversing the logic states at the respective inputs Eb and Eb and Ea and ⁇ a of the sample and hold units.
• the samples taken by the first blocker sampler 34, when the control signal Co of the inverter was in the high state during a phase variation of the light signals in one direction, are taken by the second sampler. blocker 36 when the control signal Co of the inverter 38 is in the low state, and vice versa.
• the processor 28 reverses the command of the adder / subtractor 46 to effect a negative accumulation (-1) and thus maintain the same direction of the phase ramp.
• FIG. 4a represents a modulation signal Um of the phase modulator 5 of the gyrometer of FIG. 3 according to the invention and FIG. 4b the control signal Co of the inverter 38 of states of the clock 30.
• the signal of Um modulation applied to phase modulator 5 generates the phase ramp, with a slope proportional to ( ⁇ 0/1 0 ) and peak-to-peak amplitude equal to 2 ⁇ radians, combined with the phase modulation signal of + ⁇ / 4 and - ⁇ / 4 at the frequency 1/2 t 0 .
• adder / subtractor 46 will for example perform the following sum:
• the servo function is not modified since the sign of the cumulative phase error signal in the first accumulator is not affected by the double inversion. Only the sign of the error of the offset of the electronics voltages is alternated because this error is not modified by the first inversion (phase inversion) whereas it is not the second inversion (of sign).
• the operation sequence performed by the adder / subtractor 46 shows that at each transition of the clock f, a sample x of the signal Ud has been omitted.
• the average rate of operation of the adder / subtractor 46 is equal to FO-f. This can be translated in the event of constant sign rotation by a systematic error of relative value f / FO. In order to correct this error, it is possible to store the average value of addition / subtraction preceding the transition of the clock f and by an additional circuit or logic operator to add / subtract half of this average value to compensate the missing half sample.
• the operating device of the gyrometer according to the invention makes it possible both to use an analog-digital conversion which is not necessarily at the frequency of the optical phase modulation, which reduces the consumption of electronics, improves noise immunity and compensates for the offset faults, or “offset” in English, of the analog part of the electronics, in particular the voltage shifts of the differential amplifiers.
• FIG. 5 shows the block diagram of a variant of the gyrometer of FIG. 3 according to the invention correcting the scale factor of the phase ramp of the phase modulator 5.
• the operating device of the gyrometer comprises four sampler-blockers.
• a first group of two blocker samplers 70, 72 drives the two inputs of a first differential amplifier 74, the assembly forming a first synchronous detector 75, the first differential amplifier 74 providing the difference of the samples taken by the first group of sample and hold.
• the first and the second group of sampler-blockers are respectively controlled by a first 82 and a second 84 clock through respective inverters 86, 88 of the states of the clocks according to the operation described in the case of the gyrometer of FIG. 3.
• the first and the second group of sampler-blockers operate as a synchronous detector in the same way as in the case of the gyrometer in FIG. 3 described above.
• the processor 28 controls simultaneously depending on whether one operates with a phase modulation deviation of ⁇ / 2 or 3 ⁇ / 2:
• a first switch 88 selecting either the output of the first differential amplifier 74 from the first group of sample and hold units, or the output of the second differential amplifier 80 from the first group of sampler-blockers, for driving the analog-digital converter 42 of the operating electronic device;
• a second switch 92 of the same type as the first switch 88 providing information to the digital / analog converter 50 on the basis of the information G1 and G2 at the output of the first and of the second group of sampler-blockers.

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• 2001
  • 2001-05-15 FR FR0106396A patent/FR2824905B1/fr not_active Expired - Fee Related
• 2002
  • 2002-04-30 CA CA002446706A patent/CA2446706A1/fr not_active Abandoned
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