FR2675251A1 - Fibre-optic gyrometer - Google Patents

Abstract

The invention relates to a fibre-optic gyrometer comprising a light source (1), an optical fibre loop (2), a circuit (6) splitting the light beam from the source into first and second propagation beams which are sent into the optical fibre loop along mutually opposite directions, an optical detection circuit (8, 9) used to inject the first and second light beams into the waveguide after passage through the loop and to detect them, a dynamiser modulator (16) used to phase shift the first and second beams, a phase-shift detector (2, 7, 10, 11, 12, 13) used to detect the phase shift between the first and second beams on the basis of the output signal of the optical detector and thus to calculate the angular speed of rotation on the basis of the phase shift. This gyrometer is noteworthy in that it comprises a delivery circuit (17, 20) used to deliver the output signal of the phase-shift detector for a constant time duration, synchronously with the period of the dynamiser modulator, a circuit (18) used to integrate the output signal of the delivery circuit, and a circuit (15, 19, 14) used to alter the frequency of the dynamiser modulator as a function of the output signal of the integrator.

Description

 The present invention relates to a fiber optic gyro, which is used to measure the rotational angular velocity and rotation angle of navigation vehicles, such as aircraft, ships, automobiles, etc.

 Recently, fiber optic gyrometers using the optical Sagnac effect have been developed and realized in practice, which constitutes a small and reliable detection apparatus for determining a rotational angular velocity or similar magnitude. Of the fiber optic gyrometers, fiber optic gyrometers that calculate the rotational angular velocity from the phase shift between light beams propagated clockwise and counterclockwise within a long loop of optical fiber are called gyrometers. fiber optic interference type. Among the interference-type optical fiber gyrometers, the optical fiber gyrometer of the phase-modulation system and the fiber-optic gyrometer of the transit-time modulation system, also known as dynamisation, have more particularly been developed.

 Figure 1 of the accompanying drawings shows an example of a fiber optic gyrometer of the phase modulation system according to the prior art.

 As shown in FIG. 1, a laser light beam delivered by a light source 1, for example a semiconductor laser, a light-emitting diode, or the like, is aerated under the action of an optocoupler , and one of the laser light beams is caused to affect a polarizer 4. The light beam coming from the polarizer 4 is separated under the action of a photocoupler 6 so as to produce a light beam of clockwise direction and a beam These light beams are caused to affect an optical fiber loop 3, which is formed by winding a single optical fiber several times, from one of its respective ends, they travel through the fiber loop. 3 and they are reintroduced into the photocoupler 6 as light beams emitted from the respective other end. These incident beams are mixed by the photocoupler 6 and are delivered, via the polarizer 4, the photocoupler 5 and a photoreceptor 2, to a voltage converter (C / V) 7, which then produces an output signal in the form of voltage.

When an angular rotational velocity Q is applied to the optical fiber loop 3, the Sagnac effect occurs in the light beams propagating inside the optical fiber loop 3 in the mutually opposite directions, so that that a phase shift AO proportional to the angular rotation speed Q is produced during the emission time of the light beams. The phase shift AO is expressed by:
4sLR
AO = Q where R is the radius of the optical fiber loop 3, L is the length of the optical fiber loop 3, X is the wavelength of the light emitted by the light source 1, and C is the speed light.

Moreover, in this optical fiber gyrometer of the phase modulation system, a phase modulator 8 is placed at one end of the optical fiber loop 3, the phase modulator 8 being excited by a signal coming from a generator of signals 9 so as to phase modulate the light beams propagating within the optical fiber loop 3 in mutually opposite directions. If op is assumed to be the angular frequency of the signal applied to the phase modulator 8 , i.e. the signal from the signal generator 9, then the output signal I of the voltage converter 7 is expressed by:
I = K {1 + cosAO (Jg (x) - 2J2 (x) cos2o) pt + ...) - Sir0 (2J1 (s) cosopt -.

... (2) where x is the degree of phase modulation, Jo, J1, J2> ... are the functions of
Bessel, K is a constant of proportionality, and t is time. A synchronization detector (sync) 10 synchronously detects the component of the angular frequency o> p among the components of the angular frequency of the output signal I, relative to the signal having the angular frequency op which comes from the signal generator 9, to produce an output signal (2KJ1 (x) sine0) which is proportional to sinua0.

 The Fiber Optic Gyrometer of the Transit Time Modulation System, or Dynamizer System, has been developed to provide a wider dynamic range than that available with the Fiber Optic Gyroscope of the Phase Modulated System. . Figure 2 of the accompanying drawings shows a particular arrangement of the optical fiber gyro dynamizer system according to the prior art. In Figure 2, like parts corresponding to those of Figure 1 are designated by the same references.

 As can be seen in FIG. 2, the fiber optic gyroscope of the dynamiser system comprises, in addition to the elements of the optical fiber gyro of the above-described phase modulation system, a modulator 16 of the transit time, or modulator dynamiser, which is disposed at a first end, or input end, of the optical fiber loop 3 and which serves to superimpose a sawtooth wave (energizing wave) to the light beams propagating inside the optical fiber loop 3 in mutually opposite directions, an integrator 11 for integrating the phase shift between the clockwise and counterclockwise light beams which are detected through the photoreceptor 2, the C / V converter 7 and the detector 10, another integrator 12 for integrating the output signal of the integrator 11, a reference signal generator 2st for producing a signal reference 2x, a repositioning circuit 14 for comparing the reference signal 23t and the output signal of the integrator 12 so as to produce a repositioning signal under the effect of which the output signal of the integrator 12 becomes a sawtooth wave, and a counter 13 for counting the energizing wave coming from the integrator 12.

 The operation of the dynamic fiber-optic gyroscope will be described below.

Since the energizing modulator 16 is placed at the first end of the optical fiber loop 3, the light beams moving respectively clockwise and counterclockwise have their phases vary at different time positions, as shown in FIG. 3A. Consequently, the phase shift appearing on the photoreceptor 2 is as shown in FIG. 3B, where Os is the phase shift between the clockwise and anticlockwise light beams and Ts is the period of the energizing wave. If we assume that X represents the time difference determined by the length of the optical fiber loop 3, then the phase shift Os is expressed by: 2 #
Tus bone T (3)
Then, the output signal of the CN converter 7 is expressed by the preceding equation (2) where AO has been replaced by (AO + Os). In addition, the output signal of the synchronization detector 10 giving the interference is expressed by 2KJl (x) sin (AO + 0s). This interference signal supplied by the synchronization detector 10 is integrated by the integrators 11 and 12 and is sent back to the dynamiser modulator 16 in order to modify the period of the energizing wave so that the phases of the emitted light beams become mutually coincident that is sin (O + Os) = 0, ie AO = s. Since the time difference Os is proportional to the period of the energizing wave, AO also becomes proportional to the energization period. Therefore, the angular velocity of rotation Q can be determined by counting the frequency of the waveform. In this way, assuming that fs = 1Jfs represents the frequency of the energizing wave, then the rotational angular velocity Q can be expressed by
#C
# = 2LR #. fs ... (4)
However, with conventional fiber optic gyrometers, the amplitude of the energizing wave, the repositioning time (ie the so-called return time) with respect to the reference signal 2s, or a similar signal, limit the performance of fiber optic gyrometers and, consequently, the error of the gyrometer scale factor increases. Especially, if the amplitude of the energizing wave is not 2 # (which will be indicated as constituting the error 2z) or if there is a return time, then the relation AO + Os = 0 is not established, so that the output signal of the synchronization detector 10 is not at zero, which causes an error. The adjustment for adjusting the amplitude of the energizing wave so as to reduce the time to return to zero and thus solve the problems indicated above is very difficult. On the other hand, the detection to determine the error 2st and the value of the return time used to correct the error 2 # is also very difficult.

 It is therefore an object of the invention to provide an improved optical fiber gyro in which the above-mentioned disadvantages and disadvantages found in the prior art can be eliminated.

 More specifically, an object of the invention is to provide an optical fiber gyro which can be prevented from being affected by the error 23t and the return time and which can detect a correct rotational angular velocity.

 Another object of the invention is to produce an optical fiber gyro in which the error of the scale factor is extremely small.

 Another object of the invention is to produce an optical fiber gyro having an extremely simplified circuit configuration.

 A further object of the invention is to produce a fiber optic gyroscope which is intended to be used with navigation vehicles, for example aircraft, ships, automobiles, etc.

 According to one aspect of the invention, the optical fiber gyroscope comprises a light source for emitting a light beam, an optical fiber loop, a circuit for dividing the light coming from the source into a first and a second propagation beam and propagating the first and second propagation beams in the optical fiber loop in mutually opposite directions, an optical detection circuit for introducing the first and second propagation beams, after passing through the optical fiber loop, into the same waveguide and detecting thereof, a dynamiser modulator for phase shifting the first and second propagation beams, a phase shift detector for detecting the phase difference between the first and second propagation beams from the output signal of the optical detector so as to calculate the rotational angular velocity from the phase shift. This fiber gyro differs in that it comprises a delivery circuit for delivering the output signal of the phase-shift detector for a constant duration in synchronism with the cycle of the energizing modulator, a circuit serving to integrate the output signal of the delivery circuit, and a circuit for varying the frequency of the energizing modulator according to the output signal of the integrator.

The following description, designed as an illustration of the invention aims to provide a better understanding of its features and advantages; it is based on the accompanying drawings, in which like reference numerals are used to designate like or similar parts in the different views, and where:
FIG. 1 is a block diagram showing the mounting of an optical fiber gyro of the phase modulation system according to the prior art;
FIG. 2 is a block diagram showing the mounting of a fiber optic gyro of the dynamisation system according to the prior art;
FIGS. 3A and 3B are simplified diagrams respectively for explaining the operation of the dynamiser optical fiber gyro which is shown in FIG. 2;
FIG. 4 is a block diagram showing a first embodiment of an optical fiber gyro according to the invention;
FIG. 5 is a block diagram showing an example of the circuit configuration of a gate signal generating circuit used in the optical fiber gyro of FIG. 4;
FIGS. 6A to 6G are timing diagrams respectively serving to explain the action of the gate signal generating circuit of FIG. 5;
FIGS. 7A to 7C are diagrams serving respectively to explain the invention;
FIG. 8 is a block diagram showing a second embodiment of the optical fiber gyro according to the invention;
Fig. 9 is a block diagram showing an example of a gate signal generator circuit used in the second embodiment of Fig. 8; and
FIGS. 10A to 10E are simplified diagrams respectively serving to explain the operation of the second embodiment of FIG. 8.

 A first embodiment of an optical fiber gyro according to the invention is represented by the block diagram of FIG. 4. In FIG. 4, identical parts corresponding to those of FIGS. 1 and 2 are designated by the same references and will not be described.

 As can be seen in FIG. 4, the optical fiber gyrometer according to this embodiment comprises, in addition to the elements of the conventional examples of FIGS. 1 and 2, a synchronization gate 17 intended to allow the passage of the output signal of the synchronization detector 10 for a constant duration in synchronism with the period of the energizing wave, a gate signal generator circuit for producing a gate signal to be output to the synchronization gate 17, an integrator 18 for integrating the signal from the synchronization gate 17, and a subtracter 19 for subtracting the output signal of the reference signal generator 2s from the output signal of the integrator 18, to provide the repositioning circuit 14 with a signal subtracted.

 The gate signal generating circuit 20 generates, as a function of the output signal of the repositioning circuit 14 and the energizing excitation signal from the integrator 20, a square wave signal which goes high for a constant duration of time. sychronism with the revitalization cycle. An example of the circuit configuration of the gate signal generator circuit will be described below in connection with FIG. 5.

 As can be seen in FIG. 5, the gate signal generating circuit 20 comprises a peak hold circuit MAX, a peak hold circuit MIN, a calculation circuit 33, a comparator 34, a delay circuit 35, rocker circuits of the RS type 36 and 37 and a delay circuit 38.

 The peak hold circuit MAX detects and maintains the maximum value of the energizing excitation signal supplied to it. Peak retaining circuit 31 resets the maximum value for the time during which it is supplied by means of the output signal R of the RS flip-flop circuit 37.

 The peak hold circuit MIN detects and maintains the minimum value of the energizing excitation signal supplied to it. The MIN peak hold circuit 32 repositions the minimum value for the time during which it is supplied by means of the output signal R of the RS flip-flop circuit 37.

 The calculation circuit 33 multiplies the difference between the output signals of the peak hold circuit MAX and the peak hold circuit MIN by the value 0.8 and adds the value multiplied to the output value of the holding circuit peak MIN 32.

 The comparator 34 compares the output signal of the calculation circuit 33 and the energizing excitation signal in order to produce a high level output signal when the energizing signal becomes greater than the output signal of the calculation circuit 33.

 The RS flip-flop circuit 36 produces a high level output signal at its terminal Q in response to a positioning signal S which results from the application to the repositioning signal from the repositioning circuit 14 (see FIG. a predetermined delay provided by the delay circuit 35 and further produces a low level output signal at its terminal Q in response to the reset signal R which is the output signal of the comparator 34.

 At the same time, the output signal of the comparator 34 is divided to produce a signal which is applied to the input of the RS flip-flop circuit 37 as a positioning signal and a signal which is delayed by a duration of predetermined delay provided by the delay circuit 38 and is applied to the input of the RS flip-flop circuit 37 as a repositioning signal. The output signal R coming from the RS flip-flop circuit 37 becomes a signal which has a delay time determined by the delay circuit 38 and, during this time, the peak hold circuit MAX 31 and the peak hold circuit MIN 32 are put in the repositioning state.

 The action of the gate signal generating circuit 20 will be described in relation to FIGS. 6A to 6G constituting time diagrams.

At the instant t'0, from the moment when the output signal of the comparator 34 begins to rise, the peak hold circuit MAX and the peak hold circuit MIN are simultaneously repositioned, as shown in FIGS. 6B and 6C and start the peak hold operation after the delay provided by the delay circuit 38 has elapsed. At the next instant tl, the peak hold circuit MAX holds the value
V1 and, at the instant t2 following, the peak retaining circuit 32 MIN maintains the value V2.

 Therefore, the calculating circuit 33 produces a value output signal (V1 - V2) x 0.8 + V2 for the duration of time t2 to instant t'2, as shown in FIG. 6D. .

 If we compare the output signal of the calculation circuit 33 and the dynamisation signal Sg appearing in FIG. 6A, as is done in the comparator 34, we note that the output value of the calculation circuit 33 and the value The output of the energizing excitation circuit is reversed when the energizing excitation signal becomes greater than 0.8 times the peak value. Therefore, the output of the comparator 34 goes high at time t'2, as shown in Figure 6E.

 In response to the delivery of a high level output signal from the comparator 34, the repositioning signal from the RS flip-flop circuit 37 is supplied to the MAX peak hold circuit 31 and the MIN 32 peak hold circuit and is also sent to the RS 36 flipper circuit.

 Once the delay determined by the delay circuit 38 has passed, the peak hold circuit MAX and the peak hold circuit MIN cause the peak hold operation to start in order to detect and maintain maximum and minimum values, V3 and V4, of the next energizing excitation signal, then sends the latter to the calculation circuit 33.

 Meanwhile, the repositioning signal shown in Fig. 6F, which comes from the repositioning circuit 14, is delayed by the delay time predetermined by the delay circuit 35 and is applied to the input of the RS flip-flop circuit 36 as a signal. positioning. The RS flip-flop circuit 36 goes high in response to the positioning signal and goes low in response to the front edge of the output signal of the comparator 34, as shown in FIG. 6G, so as to produce a square wave signal. which is at the high level for a constant duration in synchronism with the period of dynamisation. This square wave signal is supplied to the synchronization gate 17 as a gate signal.

 While the moment at which the gate signal goes to the low level set to the moment when the energizing signal becomes equal to 0.8 times the peak value, this magnitude is not limited to 0.8 and can be chosen from among other appropriate values, for example 0.7 or 0.9, or a similar value.

 In addition, it is preferable that the time which is delayed by the predetermined duration with respect to the time when the gate signal goes high, that is to say its delivery of the repositioning signal by the repositioning circuit 14, be set so as to exceed the return time, if this is possible.

 We then return to Figure 4, to describe the role played by the first embodiment as a whole, also in relation to Figures 7A-7C.

 If there is a 2s error or a return error, the output of the sync detector 10 is not reset (see Fig. 7A).

Depending on the gate signal from the gate signal generator circuit, the output of the synchronization detector 10 is allowed to pass through the synchronization gate 17 (see Fig. 7B). Then the output signal of the synchronization gate 17 is integrated by the integrator 18, then the integrated output signal of the integrator 18 and the output signal of the reference signal generator 23r are subtracted from one of the other by the subtractor 19.
Because of the subtraction result obtained in the subtracter 19, the reference voltage of the repositioning circuit 14 increases or decreases, so that the amplitude and frequency of the energizing wave increases or decreases so as to adjust to zero the phase shift between the first and second propagated light beams. At this time, the influence of the 2x error and the return time are eliminated from the energizing wave, so that the fiber optic gyrometer of this embodiment can to deliver a correct rotational angular velocity.

 Figure 8 shows another embodiment of the invention.

The second embodiment differs from the first embodiment, shown in FIG. 4, simply because a gate signal generating circuit 21 produces a gate signal as a function of the repositioning signal coming from the repositioning circuit 14, the signal of FIG. energizing excitation from the integrator 12 and the output signal of the subtractor 19.

 An example of the circuit configuration of the gate signal generating circuit 21 is shown in FIG. 9, while time diagrams of this circuit are shown in FIGS. 10A-10E.

 As can be seen in FIG. 9, the gate signal generator circuit 21 comprises a multiplier 41, a comparator 42, a delay circuit 43 and an RS rocker circuit 44.

 The multiplier 41 multiplies the reference signal 23t coming from the subtracter 19 (see FIG. 8) by 0.8 and gives the result of the multiplication to the comparator 42. The comparator 42 compares a level 41a, which is equal to 0.8 times the reference signal 2x, with the energizing excitation signal Sg as shown in FIG. 10A, and produces a high level when the energizing excitation signal So becomes greater than 0.8 times the level of the reference signal 2x, as shown in Figure 1013.

The repositioning signal (see FIG. 10C) coming from the repositioning circuit 14 of FIG. 8 is delayed by a predetermined delay time supplied by the delay circuit 43 and is then delivered to the rocker circuit
RS 44 as positioning signal, as shown in Figure 10D. The RS flip-flop circuit 44 goes high in response to the positioning signal and goes low in response to the leading edge of the output signal of the comparator 34, as shown in Fig. 10E, so as to produce a square wave signal which goes high for a predetermined time in synchronism with the dynamisation period. This square wave signal is supplied to the synchronization gate 17 (see FIG. 8) as a gate signal.

 While the time at which the gate signal goes low is fixed at the instant at which the energizing signal becomes equal to 0.8 times the peak value, this magnitude is not limited to 0.8 and can be selected from other appropriate values, such as 0.7 or 0.9, or another value.

 In addition, it is preferable that the time delay of the predetermined duration with respect to the moment at which the gate signal goes high, that is to say the delivery of the repositioning signal by the repositioning circuit 14 is set to exceed the return time, if possible.

 With the second embodiment of the invention, the peak hold circuit, or similar circuit, is not necessary, so that the circuit configuration of the gate signal generating circuit 21 can be simplified to the extreme.

 According to experimental results obtained according to the invention, the error of the scale factor can become less than 10 ppm, so that excellent results can be obtained.

 According to the invention, since the output signal of the photodetectors 2, 7 and 10 (the receiver 2, the voltage converter 7 and the synchronization detector 10) is delivered for a constant duration in synchronism with the period of the modulator of 16 and that the amplitude and the frequency of the dynamisation modulator 16 varies as a function of the output signal of the integrator 18 which integrates the output signal thus obtained, it is possible to zero the phase difference between the first and second light beams. propagated. Therefore, it is possible to obtain a fiber optic gyro that can be prevented from being affected by the error 2s and the return time and in which the error of the scale factor is extremely small.

 The constant duration during which the output signal of the photodetector is obtained in synchronism with the period of the energizing modulator must not be strictly determined. Thus, even if this constant time varies, only the gain changes and no polarization error occurs, which results in the configuration of the fiber optic gyroscope circuit being greatly simplified.

 Of course, those skilled in the art will be able to imagine, from the device whose description has been given merely by way of illustration and by no means as a limitation, various variants and modifications that are not outside the scope of the invention. .

Claims (5)

 integration.  dynamiser modulation according to the output signal of said means  issue; and (i) means (15, 19, 14) for varying the frequency of said means of  in synchronism with the period of said dynamiser modulation means; (h) means (18) for integrating said output signal of said means of  output of said phase shift detection means for a constant duration  phase shift; characterized by (g) delivery means (17, 20; 17, 21) for outputting the signal of  optical system, in order to calculate the rotational angular velocity from said  propagation light from the output signal of said detecting means  detect the phase difference existing between said first and second beams  first and second propagation light beams; (f) phase shift detection means (2, 7, 10, 11, 12, 13) for  loop of optical fiber, in the same waveguide and to detect them; (e) dynamizer modulation means (16) for phase shifting said  second light beam propagation, after passing through said  mutually opposed; (d) optical detection means (8, 9) for introducing said first and  propagation in said optical fiber loop in directions  and propagating said first and second light beams of  bright in a first and a second light beam of propagation  An optical fiber gyro, comprising: (a) a light source (1) for emitting a light beam; (b) an optical fiber loop (3); (c) means (6) for dividing the light beam from said source  An optical fiber gyroscope according to claim 1, characterized in that said delivery means is formed of a synchronization gate (17) to which the output signal of said detecting means is provided and of a generator (20; 21) which generates and outputs a gate signal to said synchronization gate.  An optical fiber gyroscope according to claim 1, characterized in that said frequency variation means is constituted by a reference signal generator (15) for generating a reference signal, a subtractor (19) for subtracting the an output signal of said integrating means (18) of said reference signal, and a repositioning circuit (14) to which the subtraction result from said subtractor is provided.  An optical fiber gyroscope according to claim 3, characterized in that said gate signal generator (20) produces the gate signal in dependence on the output signals of said repositioning circuit and said phase shift detecting means.  An optical fiber gyroscope according to claim 3, characterized in that said gate signal generator (21) generates the gate signal in dependence on the output signals of said repositioning circuit, said phase shift detection means and said subtractor.