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
  • G01C19/721—Details, e.g. optical or electronical details

Definitions

• the present invention relates generally to Sagnac interferometers and more particularly to detecting the eigen frequency of a sensing coil of a fiber optic gyroscope.
• the eigen frequency (sometimes called the "proper" frequency) of a fiber optic gyroscope (FOG) sensing coil is an extremely important parameter for the operation of navigation and high performance grade FOGs.
• the eigen frequency is essentially defined by the optical path length of the sensing coil.
• Many sources of rate output errors are reduced or effectively eliminated by operating the bias modulation at the eigen frequency.
• the bias modulation must be operated to within at least a few ppm of the eigen frequency in order for FOGs to meet high performance requirements. New applications are now putting more demanding performance requirements on these types of gyros.
• the current state of the art method of maintaining the bias modulation at the eigen frequency employs a method of indirectly measuring the eigen frequency.
• the eigen frequency is estimated from temperature measurements of the coil and calibration coefficients on a calibrated lookup table. This method has drawbacks in that it requires, among other things, extensive testing for calibration, and, since it is an indirect measurement of the eigen frequency, it has accuracy limitations.
• High performance FOGs for space-based pointing and submarine navigation applications currently use temperature control of the fiber coil to maintain the eigen frequency near a constant value.
• the bias modulation frequency is then maintained near the eigen frequency by deriving the frequency from a temperature controlled or compensated ciystal oscillator.
• This method works well for maintaining gyro operation at the eigen frequency for a relatively short duration of one to two years and under a relatively benign laboratory enviromnent.
• the drift of the crystal oscillator over many years (e.g., 10 or greater) of operation is typically greater than what is required for high performance.
• the coil eigen frequency will significantly drift over many years due to aging effects of the fiber coil itself.
• a better method for controlling the bias modulation to the eigen frequency is to employ an eigen frequency detector and a servo control loop that automatically maintains the gyro operation at the eigen frequency.
• the eigen frequency detector provides an error signal that is null when the bias modulation is at the coil eigen frequency.
• the servo loop maintains the bias modulation at the coil eigen frequency by nulling the error signal from the eigen frequency detector.
• Eigen frequency detection can be accomplished by a number of methods.
• This method adds significant complexity to the optical circuit and introduces additional intensity modulation, which could generate other types of gyro errors.
• Another drawback to this approach is the detection process which involves demodulating the quadrature of the rate signal to extract the eigen frequency error signal. Since the demodulation process of a practical device cannot be made to be exactly in quadrature (90 degrees out of phase) of the rate signal, some crosstalk will occur between the rate and the eigen frequency error signal channels. This crosstalk will limit the performance of the eigen frequency servo. Furthermore, since the eigen frequency demodulator is operating at the same frequency as the rate demodulator, then the eigen frequency demodulator will be sensitive to any interference at the same frequency.
• This interference can be in the form of electrostatic or electromagnetic coupling, or signal currents causing spurious signals due to voltage drops on ground lines or power-supply lines. Signal interference can also limit the performance of this type of eigen frequency servo.
• Another approach is describe in U.S. Patent 5,734,469, which is an improvement over U.S. Patent 5,090,809. This approach also involves demodulating the quadrature of the rate signal. However, the sensitivity of the eigen frequency error signal to changes in eigen frequency is determined and increased by implementing a square-wave bias modulation with a non 50-50 duty cycle. The advantage of this method is that changes in the optical components are not required to enhance the sensitivity of the eigen frequency error signal.
• This present invention proposes an improved method of detecting the eigen frequency, without the drawbacks associated with prior art methods. Specifically, employing the method according to the present invention does not require additional optical components, does not require imperfections in the phase modulator or the bias modulation, and does not involve quadrature demodulation. Furthermore, it has been demonstrated that this new method is capable of detecting 0.3 parts-per-billion changes in the frequency separation of eigen and the bias modulation frequencies. This level of performance is believed to be more than adequate for both navigation and high performance grade FOGs.
• the present invention involves the use of an additional phase modulation applied to the light waves propagating through the sensing coil.
• the additional phase modulation generates an "error" signal that is proportional to the frequency difference between the gyro operating frequency and the eigen frequency.
• a servo loop controls the gyro operating frequency to the eigen frequency by driving the "error" signal to a null.
• the present invention comprises a fiber optic gyro (FOG) that, as is well known, includes a light source, a fiber coupler connected to the light source, an integrated optics chip (IOC) capable of modulating light received from the light source via the fiber coupler, a sensing coil in communication with the IOC, a bias modulation generator for imparting a bias modulation signal to the light, and a photodetector for receiving light returning from the sensing coil that is representative of a rotation rate of the sensing coil.
• FOG fiber optic gyro
• the FOG also includes a second modulation generator for imparting a second modulation, preferably sinusoidal, signal to the light, a high-frequency demodulator in communication with a signal produced, at least indirectly, by the photodetector, and a low-frequency demodulator in communication with the high-frequency demodulator.
• the high-frequency demodulator receives the second, sinusoidal, modulation signal as a reference frequency and the low-frequency demodulator receives the bias modulation signal as a reference frequency, and an output of the low-frequency demodulator represents a magnitude and sign of a frequency difference between a frequency of the second modulation signal and an even-harmonic of the eigen frequency.
• This information can then be used to maintain the operating frequency of the gyro at the eigen frequency of the sensing coil and thereby achieve improved performance.
• the eigen frequency detector can also be used for other types of sensors that employ a Sagnac interferometer.
• an eigen frequency servo can be used to improve the performance of a fiber optic current sensor employing a ring interferometer.
• other types of sensors could be constructed such that the eigen frequency of the coil is made sensitive to the desired measurand.
• Figure 1 shows a sensing coil with markings that help to illustrate the present invention.
• Figure 2 illustrates how the eigen frequency error signal is generated under a particular condition in accordance with the present invention.
• Figure 3 is a functional diagram of an open loop interferometric fiber optic gyro incorporating an eigen frequency detector in accordance with the present invention.
• Figure 4 is a functional diagram of a closed loop fiber optic gyro incorporating an eigen frequency detector in accordance with the present invention.
• Figure 5 shows a block diagram of a digital implementation of the eigen frequency detector in accordance with the present invention.
• the present invention provides an improved method for detecting the coil eigen frequency during normal gyro operation.
• This improved method involves the use of an additional phase modulation applied to the light waves propagating through the sensing coil.
• the additional phase modulation generates an "error" signal that is proportional to the frequency difference between the gyro operating frequency and the eigen frequency.
• a servo loop controls the gyro operating frequency to the eigen frequency by driving the "error" signal to a null.
• Figure 1 shows a diagram of a sensing coil and an integrated optics chip with phase modulators to help describe the concept of an eigen frequency associated with a phase modulator.
• the entire optical loop comprises the optical path from the y-junction, through the integrated optics chip, through the fiber coil, and returning back through the integrated optics chip and to the y-junction.
• the modulated counter-clockwise (CCW) optical wave will have to travel from point ai, through the fiber coil to point bi, and then to the y-junction, whereas the modulated clockwise (CW) optical wave will only have to travel the short distance from point ai directly to the y-junction without travelling through the fiber coil. Therefore, the modulated CCW optical wave will experience a significant delay relative to the modulated CW optical wave.
• the relative time delay between the modulated CCW and CW optical waves is the time ⁇ i that it takes light to travel from point a ⁇ through the coil, to point bi, where points bi and aj have the same optical distance from the y-junction. If the phase modulation is generated in a way that provides a phase shift between the modulation of the CCW and CW optical waves that is an integer multiple of 360 degrees, then the phase modulation on the CW wave will be exactly the same as the phase modulation on the CCW wave when the two waves interfere. Under this condition there will be no phase modulation difference between the counter-propagating optical wave and therefore no resulting intensity variation due to interference between the two waves.
• FIG. 3 shows a functional diagram of a simple open loop IFOG employing the improved eigen frequency detector.
• An open loop IFOG is used here as an example for simplicity. However, the present invention will work equally well with a closed loop IFOG.
• Light from light source 100 passes through a fiber coupler 102 and to an integrated optics chip (IOC) 104 where it is split into two waves by a Y-junction 106.
• the two optical waves counter-propagate through sensing coil 108 and then are recombined at Y-junction 106.
• the recombined waves then propagate back to fiber coupler 102, which redirects a portion of the light to photodetector 110.
• the light intensity at photodetector 110 depends on the phase difference between the counter-propagating waves.
• a phase difference can be created by either rotation along the sensing axis of coil 108 or by applying a time-variant phase modulation to the counter-propagating waves.
• a bias modulation is used to improve the sensitivity of the gyroscope to very low rotation rates.
• a bias modulation generator 112 applies a sinusoidal or square wave drive signal at frequency f 2 to one of the IOC phase modulators 114a. This generates a signal at frequency f 2 at photodetector 110 that is proportional to rotation rate.
• Photodetector 110 converts the optical signal to an electrical signal, which passes through signal conditioning circuits 120, which typically comprise amplifiers and filters.
• the rotation signal at frequency f 2 is then demodulated by gyro signal processing functions 122, which output a dc signal that is proportional to rate.
• a sinusoidal modulation generator 130 produces a sinusoidal signal at frequency fj to drive another IOC phase modulator 114b.
• the sinusoidal modulation generator 130 could be configured to drive phase modulator 114a.
• the present invention would still work if generator 130 is configured to produce non-sinusoidal signals such as a square wave, which may be done to simplify the design of the modulation generator 130).
• the raised cosine curve 1 illustrates the optical intensity at the photodetector as a function of the total phase difference ⁇ between the counter-propagating optical waves that interfere at the photodetector.
• Curve 2 shows the net phase modulation as a function of time.
• the net phase modulation is a composite of the square-wave bias modulation and a sinusoidal modulation.
• the interferometer intensity output at the detector, curve 3, can be found by translating the points in curve 2 onto the curve 1.
• the bias modulation is a square wave which is alternating between ⁇ /2 and - ⁇ /2 in phase, which is a typical amplitude for bias modulation in practice. When the bias modulation switches from ⁇ /2 and - ⁇ /2, the slope of curve 1 switches from negative to positive.
• the signal generated by the sinusoidal phase modulation at frequency fi is modulated by the bias modulation at f 2 . Therefore, the error signal will occur as side-bands about the frequency fj.
• the side-bands will be at frequencies fi +/- n* f 2 , where n is an integer.
• the photodetector signal is demodulated by two phase-sensitive demodulators. First, the photodector signal is demodulated at frequency fi by a high-frequency demodulator 132. The output of high-frequency demodulator 132 is then demodulated at frequency f 2 by a low frequency demodulator 134.
• the output of the "low-frequency" demodulator is the eigen frequency error signal, which represents the magnitude and sign of the frequency difference between the sinusoidal drive and the even-harmonic of the eigen frequency.
• the order of the two demodulators can be switched as long as the low- frequency demodulator is configured in a way that it will pass signals at the higher frequency fj . Care must be taken when selecting the frequencies fi and f 2 . Poor performance of the eigen frequency detector and the gyro rate output can occur if frequency fi is set to be an exact even integer of frequency f 2 .
• the preferred method is to choose a frequency difference ⁇ f such that, when the sinusoidal modulation frequency fi is at an even integer of the eigen frequency f el , then the bias modulation frequency f 2 is at the eigen frequency f e2 . This way, gyro rate errors associated with operation off the eigen frequency will be eliminated or minimized when the eigen frequency error signal is zero.
• the frequency ⁇ f is detennined by the optical path length between sinusoidal and bias phase modulators and the even harmonic number m that is chosen.
• the sensitivity of the eigen frequency detector is proportional to the even harmonic number m. Therefore, the sensitivity of the eigen frequency detector can be increased without increasing the amplitude of the sinusoidal modulation by choosing a larger m, as long as the gyro electronics can pass, with little attenuation, an error signal at frequencies about f]. Having the ability to increase the sensitivity by many factors is a major advantage that the present invention has over prior art. For a typical high performance FOG having a coil of 4km of fiber, m is about 64 and ⁇ f is about 24Hz.
• Another major advantage the present invention has over prior art is the asynchronous double demodulation process.
• the double demodulation process is made immune to many sources of errors.
• One example is an error caused by signal interference.
• the first demodulator will reject any signals that are not occurring at odd integers of the frequency fj and the second demodulator will reject any signals that are not occurring at odd integers of frequency f 2 .
• the combined demodulators will only pass signals having the frequencies fi +/- n* f 2 , where n is an integer.
• Most sources of signal interference will not be at frequencies fi +/- n* f 2 because a non-linear process is required to mix signals at harmonics of fj with signals at harmonics of f .
• phase modulators Two separate phase modulators are not required for implementing the present invention. Having two sets of phase modulators that are in different points of the optical loop requires a relatively long integrated optics chip. Some applications require FOGs to be a small as possible. These smaller FOGs do not have enough room within its package to accommodate the relatively long integrated optics chips with two sets of separated phase modulators and thus have integrated optics chips with only one set of phase modulators. For this type of FOG, the sinusoidal modulation is applied to the same phase modulator as the bias modulation. It is still important to maintain the same frequency relation shown in Equation 3 with ⁇ f not equal to zero.
• the eigen frequency detector output will still be zero when the sinusoidal modulation frequency fi is at an even integer of the eigen frequency f e2 .
• the bias modulation frequency will no longer be exactly at the eigen frequency f e .
• Rate errors associated with operating away from the eigen frequency can be minimized by choosing a frequency difference ⁇ f that is relatively small, but not zero.
• An example is a FOG with a 1km coil.
• the eigen frequency in this case would be about 100kHz. If m is chosen to be 64 and ⁇ f is chosen to be 16Hz, then the bias modulation frequency will be only 2.5 ppm away from the eigen frequency, which is more than adequate for most applications involving a smaller FOG.
• U.S. Patent 5,781,300 which is incorporated herein by reference.
• the scheme described in U.S. Patent 5,781,300 involves adding sinusoidal modulation for reducing bias errors due to backscatter. This scheme has since been found, however, to be an excellent way of detecting the eigen frequency in accordance with the present invention. More specifically, it has been determined that if the sinusoidal modulation frequency is set to an even harmonic of the eigen frequency that satisfies a special condition, then the main gyro signal processing will be insensitive to the residual signal.
• the special condition is determined by how the main gyro signal processing is done.
• the spurious signal is at a frequency such that there are not integer number of cycles of the spurious signal that occur within the gate period, then a rate error will result from the spurious signal. However, if the spurious signal is at a frequency such that there are integer number of cycles of the spurious signal that occur within the gate period, then a rate error will not occur.
• the special condition for the sinusoidal phase modulation is to pick an integer m, such that an integer number of sinusoidal phase modulation cycles fit within the gate period when ⁇ f is zero. As long as ⁇ f is kept small enough, this rate error due to the small deviation from this special condition will still be insignificant.
• Figure 4 shows a functional diagram of a closed-loop FOG employing the improved eigen frequency detector in a servo loop that controls the operational frequencies of the gyro.
• sinusoidal modulation is the same modulation (error suppression modulation) that is used for suppressing various types of gyro errors.
• a direct digital synthesizer (DDS) circuit 210 generates the error suppression modulation.
• a band-pass filter 212 is used to remove spurious signals from the error suppression modulation.
• FOGs employ digital signal processing.
• the photodetector signal is amplified, filtered and digitized by analog front-end electronics 220.
• the digital signal processing is typically done in a field programmable gate array (FPGA) 230.
• FPGA field programmable gate array
• Figure 4 shows the digital signal being split into two paths 232, 234, one for the rate loop processing and the other for the eigen frequency detector.
• a master clock DDS circuit 240 derives the master clock frequency from a crystal oscillator 242.
• the master clock frequency is used to clock the logic functions in FPGA 230.
• the bias modulation frequency is generated by FPGA 230 and is derived from the master clock frequency.
• the digital eigen frequency detector of the present invention preferably comprises two digital demodulators 132, 134 and, in addition, a sample register 236.
• First demodulator 132 demodulates at frequency fi and second demodulator 134 demodulates at frequency f 2 .
• the output of sample register 236 is the output of the eigen frequency detector.
• a servo function 238 maintains the error suppression frequency at the even harmonic of the coil eigen frequency f e ⁇ by adjusting the frequency of master clock 240. This servo loop automatically maintains the bias modulation frequency at the coil eigen frequency f e2 since both the error suppression and the bias modulation frequencies are derived from the master clock frequency.
• Figure 5 shows a block diagram of a digital implementation of the eigen frequency detector in accordance with the present invention.
• This design uses continuous asynchronous double demodulation.
• the eigen-frequency detector logic uses the rising edge of a master clock frequency , f mclk , input as the synchronous clock input by which all registers are clocked.
• the f mc i k/2 input is synchronous to the fmcik signal and is equal to f mc ik/2.
• the clocks fi and f 2 are preferably asynchronous fifty percent duty cycle square wave inputs, where f ⁇ is equal to m times f 2 minus a small frequency difference and m is an integer.
• the clock fi is received and sent to phase adjust control logic 239 to produce the phase adjusted fj signal, fi ⁇ .
• Phase adjust control logic 239 provides for a programmable phase adjustment of the fi input in increments 360/2 k degrees, where k is an integer, using the ⁇ (k:0) bus input producing f j ⁇ .
• First demodulator 132 sums over the positive half of the phase adjusted fie signal, and subtracts the ⁇ over the negative half of fie producing ⁇ ⁇ .
• the content of first demodulator 132 , ⁇ ⁇ is transferred to second demodulator 134 and is then subsequently reset to zero.
• Second demodulator 134 sums ⁇ ⁇ over the positive half of f 2 and subtracts ⁇ over the negative half of f 2 producing ⁇ ⁇ . This operation runs continuously.
• the sample signal, f s is generated from rising edge detection logic 310.
• Rising edge detection logic 310 receives a positive pulse, f , with a period greater than 1 /f mc i producing an active high sample signal, f s , having a pulse width period of 1/ f mc i k .
• the contents of second demodulator 134, ⁇ ⁇ is transferred to sampling register 236 producing ⁇ ⁇ (n), where n is an integer.
• second demodulator 134 is subsequently reset to zero. This operation runs continuously.
• Gating control logic 312 provides for an omission of demodulation transfers from first demodulator 132 to second demodulator 134.
• a logic high gate signal, fg ate input with a period greater than f m ci k/2 generates f gj causing an omission of modulation cycle transfers from first demodulator 132 to second demodulator 134 for a period of fg ate plus four times the f mc i k2 time period.

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