Optical fiber gyro with memory storing data measured under application of angular velocity

A signal processing circuit of an optical fiber gyro comprises a memory for storing data measured in the signal processing circuit in accordance with angular velocities applied to an optical fiber sensing loop at preparatory stage, and the applied angular velocities. In operation, the memory is accessed in accordance with data obtained in the signal processing circuit at an operation stage, so that an angular velocity is read from the memory without complicated calculation.

FIELD OF THE INVENTION 
The invention relates to an optical fiber gyro, and more particularly to, 
the improvement on response speed and measuring precision in an optical 
fiber gyro using a phase modulating method. 
BACKGROUND OF THE INVENTION 
A first conventional optical fiber gyro comprises an optical fiber sensing 
loop through which lights radiated from a laser light source propagate in 
the clockwise (CW) and counter clockwise (CCW) directions, a phase 
modulator for modulating the light to be propagated through the sensing 
loop, a light detector for receiving the light propagated through the 
sensing loop to generate an electric signal, and a signal processing 
circuit for processing the electric signal to calculate an angular 
velocity of a rotating member on which the sensing loop is mounted. 
In operation, a light radiated from the laser light source is divided into 
lights to be propagated through the sensing loop in the CW and CCW 
directions. Then, the lights propagated through the sensing loop in the CW 
and CCW directions are coupled to be supplied to the light detector, from 
which an electric output signal corresponding to the received light is 
supplied to the signal processing circuit. 
In the signal processing circuit, an angular velocity of the rotating 
member is detected in accordance with Sagnac phase difference .phi.s which 
is generated in the CW and CCW propagating lights by Sagnac effect. The 
Sagnac effect is described in, for instance, the U.S. Pat. No. 5,272,516, 
and the relation between the angular velocity .OMEGA. and the Sagnac phase 
difference .phi.s is defined below. 
EQU .OMEGA.=(1/a).phi.s (1) 
where a is a constant. 
The output signal supplied from the light detector to the signal processing 
circuit comprises a DC (direct current) component, a fundamental wave 
component, a duplicate harmonic wave component, a triplicate harmonic wave 
component, a quadruple harmonic wave component, etc. which are defined by 
output signals corresponding to the CW and CCW lights, triangular 
functions of the Sagnac phase difference and the phase modulating 
frequency, and respective orders of Bessel functions based on a phase 
modulating degree. The output signal is synchronously detected in the 
signal processing circuit to generate a fundamental wave component 
S.sub.1, a duplicate harmonic wave component S.sub.2, and a quadruple 
harmonic wave component S.sub.4. 
In accordance with the synchronous detection of the output signal, the 
angular velocity is detected as explained below. 
(1) When the phase modulation of the CW and CCW lights is carried out to 
make the ratio S.sub.4 /S.sub.2 constant, the ratio of the first and 
second order Bessel functions is constant. Thus, the angular velocity 
.OMEGA. is detected by using an Arctan (tan.sup.-1) function based on the 
detected ratio s.sub.1 /S.sub.2. 
(2) When the phase modulation of the CW and CCW lights is not carried out 
to make the ratio S.sub.4 /S.sub.2 constant, the ratio of the first and 
second order Bessel functions is calculated in accordance with the ratio 
of the second and fourth order Bessel functions which is calculated from 
the non-constant ratio S.sub.4 /S.sub.2. Thus, the angular velocity 
.OMEGA. is detected by using an Arctan (tan.sup.-1) function based on the 
detected ratio S.sub.2 /S.sub.1 and the calculated ratio of the first and 
second order Bessel functions. 
In general, the fundamental wave component S.sub.1 is multiplied in a 
pre-amplifier by an amplication factor K. Thus, a multiplied value S 
(=S.sub.1 .times.K) is obtained to enhance the detecting sensitivity of an 
angular velocity, and the multiplied value S is used to determine as to 
whether a measuring range should be changed over. In such a case, the 
value S/K is used as a fundamental wave component S.sub.1 to calculate an 
angular velocity .OMEGA.. 
Other than the above described conventional optical fiber gyro, the 
Japanese Patent Kokai No. 4-231814 which was laid open on Aug. 20, 1992 
describes a second conventional optical fiber gyro which comprises a 
memory for storing a relation of output signals and angular velocities in 
the form of a polygonal line, and characteristic amounts of the polygonal 
line. 
In operation, an angular velocity is detected by reading the angular 
velocity from the memory based on a detected output signal in 
consideration of the characteristic amounts. 
In the first conventional optical fiber gyro, however, there are 
disadvantages in that a response speed is low, because using an Arctan 
(tan.sup.-1) function takes a long time, and detecting/precision is not 
high, because (1) a result of the Arctan (tan.sup.-1) function and a 
detected value do not coincide to each other in accordance with the 
non-linear characteristics of an optical system and an electric circuit 
system in the optical fiber gyro, (2) a measuring range is changed over by 
the value S (=S.sub.1 .times.K), thereby shifting a measuring range 
changing point in accordance with the change in output power of a light 
source, so that the state is changed between calibration and operation, 
(3) the divisional calculation S/K (=S.sub.1) is carried out, and (4) the 
detected angular velocity includes an error caused by the rotation of the 
earth. 
The second conventional optical fiber gyro has a disadvantage in that 
detecting precision is not high, because a deviation amount between a 
value of an Arctan (tan.sup.-1) function and a detected value is different 
for each optical fiber gyro, in addition to the same disadvantages as 
discussed in the first conventional optical fiber gyro. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an optical fiber 
gyro in which a response speed is high. 
It is another object of the invention to provide an optical fiber gyro in 
which detecting precision is high. 
According to the invention, an optical fiber gyro, comprises: 
means for providing two lights; 
means for modulating a phase of at least one of the two lights; 
an optical fiber sensing loop for propagating the two lights in clockwise 
and counter clockwise directions, the optical fiber sensing loop being 
mounted on a rotating member, and the at least one of the two lights being 
supplied from the modulating means; 
an optical coupler for coupling the two lights propagated through the 
optical fiber sensing loop and supplied therefrom to provide a coupled 
light; 
a light detector for receiving the coupled light to generate an output 
signal; and 
a signal processing circuit for processing the output signal to detect an 
angular velocity of the rotating member; 
wherein the signal processing circuit, comprises: 
means for generating data dependent on an angular velocity of the rotating 
member by processing the output signal; 
a memory for storing angular velocities applied at a preparatory state on 
the rotating member, and data generated in the generating means when the 
angular velocities are applied at the preparatory state on the rotating 
member; and 
means for reading an angular velocity corresponding to data generated in 
the generating means at an operation state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing an optical fiber gyro in a preferred embodiment according 
to the invention, the aforementioned first conventional optical fiber gyro 
will be explained in FIG. 1. 
The first conventional optical fiber gyro comprises a laser light source 1, 
a polarizer 2, an optical fiber sensing loop 3, a light detector 4, a 
preamplifier 5, optical couplers 6a and 6b, a phase modulator 7, a signal 
processing circuit 8 including a synchronous detection circuit 9, an 
analog to digital (A/D) convertor 10, a CPU 11, and an oscillator 12, a 
digital to analog converter 13, and a low pass filter (LPF) 14. 
In the first conventional optical fiber gyro, a laser light radiated from 
the light source 1 is divided into two lights by the optical coupler 6a, 
and one of the divided lights is supplied via the polarizer 2 and the 
optical coupler 6b to the sensing loop 3 to be CW and CCW lights 
propagating through the sensing loop 3. One of the CW and CCW light is 
modulated in phase by the phase modulator 7 prior to the propagation of 
the sensing loop 3. The CW and CCW propagated lights are coupled in the 
optical coupler 6b to be supplied via the polarizer 2 and the optical 
coupler 6a to the light detector 4. 
When the sensing loop 3 is rotated on a rotating member (not shown) by an 
angular velocity .OMEGA., a Sagnac phase difference .phi.s occurs between 
the CW and CCW propagated lights by "Sagnac effect". The phase difference 
.phi.s and the angular velocity .OMEGA. have a proportional relation which 
is represented by the equation (1), as explained before. 
EQU .OMEGA.=(1/a).phi.s (1) 
where a is a constant. 
In accordance with the equation (1), the angular velocity .OMEGA. is 
obtained by detecting the phase difference .phi.s. 
An output signal which is obtained by converting a light signal received in 
the light detector 4 to an electric signal is supplied to via the 
pre-amplifier 5 to the signal processing circuit 8. 
The phase modulator 7 applies a phase modulation of a phase modulating 
degree m on one of the CW and CCW propagating lights by receiving a 
sine-wave signal of a constant frequency fm supplied via the D/A converter 
13 and the LPF 14 from the oscillator 12 in the signal processing circuit 
8. As a result, the output signal obtained in the light detector 4 
includes a signal component of a phase modulated frequency (fundamental 
wave) and plural components of harmonic waves. 
In general, an instantaneous value P of the output signal is represented by 
the equation (2). 
##EQU1## 
where P.sub.L and P.sub.R are amplitudes (maximum values) of the CW and 
CCW lights, m is a phase modulating degree, fm is a phase modulating 
frequency, J.sub.0 (m) to J.sub.4 (m) are Bessel functions, and .phi.s is 
a Sagnac phase difference. 
The output signal is supplied via the pre-amplifier 5 to the synchronous 
detection circuit 9 in which synchronous detection is carried out for each 
predetermined frequency component in accordance with a synchronous signal 
supplied from the oscillator 12. 
A fundamental wave component S.sub.1, and duplicate and quadruple harmonic 
wave components S.sub.2 and S.sub.4 are represented by the equations (3) 
to (5). 
##EQU2## 
The equation (1) is modified to define the equations (6) to (8) in 
accordance with a ratio S.sub.1 /S.sub.2 between the fundamental wave 
component S.sub.1 and the duplicate harmonic wave component S.sub.2 which 
are obtained in the equations (3) and (4). 
##EQU3## 
The equation (9) is defined in accordance with the equations (4) and (5). 
##EQU4## 
Thus, the angular velocity .OMEGA. is detected as explained before, and 
discussed again below. 
(1) When the phase modulator 7 modulates one of the CW and CCW lights to 
make the ratio S.sub.4 /S.sub.2 constant, the angular velocity .OMEGA. is 
obtained in accordance with the measured value S.sub.1 /S.sub.2 by using 
the Arctan (tan.sup.-1) function (the equation (8)) into which the 
constant value J.sub.2 (m)/J.sub.1 (m) is substituted. 
(2) When the phase modulator 7 does not modulates one of the CW and CCW 
lights to make the ratio S.sub.4 /S.sub.2 constant, the angular velocity 
.OMEGA. is detected by calculating the ratio J.sub.2 (m)/J.sub.1 (m) based 
on the ratio J.sub.4 (m)/J.sub.2 (m) calculated from the ratio S.sub.4 
/S.sub.2, and substituting the ratios J.sub.2 (m)/J.sub.1 (m) and S.sub.1 
/S.sub.2 into the Arctam (tan.sup.-1) function (the equation (8)). 
Next, an optical fiber gyro in the preferred embodiment according to the 
invention will be explained in FIG. 2, wherein like parts are indicated by 
like reference numerals as used in FIG. 1. 
In addition to the structure as shown in FIG. 1, the optical fiber gyro 
comprises an EEPROM (electrically erasable and programmable read only 
memory) 15 included in the signal processing circuit 8. 
FIG. 3 shows an apparatus for calibrating the optical fiber gyro as shown 
in FIG. 2. The calibrating apparatus comprises a turn table 23 for 
rotating the optical fiber gyro 22, a driver 24 for driving the turn table 
23 to rotate with a predetermined rotative velocity, and a personal 
computer 25 for controlling the driver 24 to drive the turn table 23. 
In the calibrating apparatus, an angular velocity .OMEGA. applied to the 
optical fiber gyro 22 is changed by driving the turn table 23 with a 
rotative velocity controlled via the driver 24 by the personal computer 
25. Then, data X measured in the optical fiber gyro 22 are supplied to the 
personal computer 25, in which a table comprising angular velocities 
.OMEGA. and measured data X is prepared. 
Where a phase modulating degree m of the phase modulator 7 is controlled to 
be constant, the measured data X is S/S.sub.2 or K.multidot.S.sub.1 
/S.sub.2, and where the phase modulating degree m is not controlled to be 
constant, the measured data X is [J.sub.2 (m)/J.sub.1 
(m)].multidot.[S/S.sub.2 ] or [J.sub.2 (m)/J.sub.1 
(m)].multidot.[K.multidot.S.sub.1 /S.sub.2 ]. The selections of a 
measuring range for S and that for S.sub.1 are automatically carried out 
in the CPU 11 of the optical fiber gyro 22 in accordance the value 
S/S.sub.2. 
In the table composed of the angular velocities .OMEGA. and the measured 
data X, an angular velocity .OMEGA.e of the earth at a position where the 
calibration of the optical fiber gyro 22 is carried out is added to the 
measured angular velocities .OMEGA.to prepare a new table. 
The angular velocity .OMEGA.e of the earth is represented by the equation 
(10). 
##EQU5## 
Where .phi. is a latitude which is positive in the clockwise direction. 
The table thus prepared is a function of the measured data X relative to 
the angular velocities .OMEGA.. Then, the table is converted to a table 
for the angular velocities .OMEGA. relative to the measured data X. The 
interval of the measured data X is preferably constant in processing the 
measured data X. Therefore, it is preferable to use linear interpolation 
in converting the table. 
The calculated table data are stored in the EEPROM 15 in the signal 
processing circuit 8, so that the CPU 11 in the optical fiber gyro 22 
accesses to the EEPROM 15 to read the angular velocities .OMEGA. 
therefrom. It is preferable to use the linear interpolation even in 
calculating the angular velocities .OMEGA.. 
In operation, the optical fiber gyro 22 is mounted on the turn table 23, 
and an angular velocity applied to the sensing loop 3 of the optical fiber 
gyro 22 is to be detected. The applied angular velocity and data detected 
in the signal processing circuit 20 are stored into the EEPROM 15 in which 
a table is prepared. The table is accessed in the signal processing 
circuit 8 by the CPU 11. Consequently, an angular velocity is obtained in 
a short time as compared to the conventional optical fiber gyro in which 
an Arctan (tan.sup.-1) function is used. Further, the shift of a measuring 
range changing point caused by the change of power in an output light of 
the laser light source 1 is avoided by using a ratio between a value 
obtained by amplifying a fundamental wave signal (a phase modulating 
frequency signal) and an even harmonic wave signal, in case where a 
measuring range is changed, and the measuring precision of angular 
velocities is improved, because data obtained by multiplying an 
amplification factor K of the pre-amplifier 5 to the fundamental wave 
component S.sub.1 which is synchorously detected is data for detection of 
an angular velocity. 
Next, examples of tables will be explained. 
It is presumed that the data X.sub.1 is S/S.sub.1 or [J.sub.2 (m)/J.sub.1 
(m)].multidot.[S/S.sub.2 ], and the data X.sub.2 is K.multidot.[S/S.sub.2 
] or [J.sub.2 (m)/J.sub.1 (m)].multidot.[K.multidot.S/S.sub.2 ]. 
FIGS. 4 and 5 show tables including the results of angular velocities 
measured by rotating the optical fiber gyro 22 on the turn table 23. The 
tables include angular velocities .OMEGA.of the turn table 23 and the 
measured values X.sub.1 and X.sub.2. 
FIGS. 6 and 7 show tables in which the data in FIGS. 4 and 5 are corrected 
in consideration of the angular velocity of the earth. 
In FIGS. 6 and 7, .OMEGA. is a value which is obtained by adding the 
angular velocity .OMEGA.e of the earth to the angular velocity .OMEGA.a of 
the turn table 23, and the latitude .phi. is 90.degree., wherein FIG. 6 
corresponds to FIG. 4, and FIG. 7 corresponds to FIG. 5, respectively. 
In FIGS. 6 and 7, the tables thus prepared include the angular velocities 
.OMEGA. which do not have an equal interval between each adjacent two 
values. Actually, the A/D converter 10 is saturated in the range where the 
values of the angular velocities .OMEGA. are small and large. For this 
reason, tables as shown in FIGS. 8 and 9 are prepared in accordance with 
the correction of the tables in FIGS. 6 and 7 by using the linear 
interpolation. 
In FIGS. 8 and 9, the values X.sub.1 are amplified ones, so that the A/D 
converter 10 is saturated beyond lower and upper angular velocities 
designated by "CHANGE", because a converting voltage range of the A/D 
converter 10 has an operative limitation, as shown in FIG. 8. For this 
reason, the range is changed to the values X.sub.2, as shown in FIG. 9, 
before an angular velocity will be the constant value. 
In the preferred embodiment, the duplicate and quadruple harmonic wave 
signals are used as even harmonic wave signals. On the other hand, other 
harmonic wave signals may be used in the invention. 
In the preferred embodiment, the values X.sub.1 and X.sub.2 are separated 
in the different tables. On the other hand, the values X.sub.1 and X.sub.2 
may be listed to be values X in one table, where no shift of the measuring 
range changing point caused by the change of power in the output light of 
the light source is observed, and an amplification degree is corrected by 
software. In such a case, the capacity of the memory is decreased. 
As described above, the optical fiber gyro in the invention includes a 
memory for storing angular velocities applied at the calibration stage to 
the sensing loop and data measured in the signal processing circuit, and 
an access circuit (CPU) for reading the stored angular velocities in 
accordance with the measured data in actual operation. For this structure, 
there are expected advantages in the invention in that response is fast, 
and precision is high. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be construed as embodying all modification 
and alternative constructions that may occur to one skilled in the art 
which fairly fall within the basic teaching herein set forth.