Abstract:
This invention relates to an improved method and mechanism for eliminating acceleration-dependent errors and loop interaction effects in gain-stabilized, closed-loop, interferometric fiber optic gyroscopes. Such method and mechanism include enabling the modulation gain accumulator and disabling the rate accumulator on every occurrence of ramp flyback if the gyroscope is operating below a certain velocity threshold and enabling the modulation gain accumulator and disabling the rate accumulator on one out of every N occurrences of ramp flyback if the gyroscope is operating above a certain velocity threshold. Such method and mechanism also includes use of proportional circuitry to scale the magnitude of the bias modulation component to be a percentage of the serrodyne ramp voltage and the DC component to be zero volts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to &#34;Fixed Interval Background Auto-Alignment For Closed Loop Interferometric Fiber Optic Gyroscopes&#34; application Ser. No. 09/002,086 filed Dec. 31, 1997 by James Joseph Jaklitsch and Jay Michael Markey, commonly owned, and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved means for aligning a closed loop interferometric fiber optic gyroscope. Fiber optic gyroscopes are used to sense rotation. Closed loop interferometric fiber optic gyroscopes (IFOGs) sense angular rate by propagating light in counter-rotating directions around a fiber optic spool as shown in FIG. 1. The fiber optic spool is wound around the axis in which rotation is to be sensed. A broadband light source (1) injects light through a coupler (2), into an integrated optics chip (IOC) (3). In the IOC, the light is split into two paths and injected in counter-rotating directions through the fiber spool (5). The optical fiber has a typical length of 100 to 2,000 meters. Rotation of the fiber optic spool around the axis in which rotation is to be sensed causes an effective optical path length increase in one direction with a corresponding path length decrease in the other direction. Consequently, the light propagating through the fiber spool is shifted in phase in accordance with a phenomena known as the Sagnac Effect. The light wave traveling in the rotation direction acquires a phase lead, while the opposite traveling wave acquires a phase lag. The composite phase shift is proportional to the rotation rate that is desired to be measured. Small rotation rates produce small phase shifts, while large rotation rates produce large phase shifts. The light wave is then routed back through the IOC (3) and the coupler (2) to an interferometric phase detector. The counter-rotating light beams produce an optical interference pattern at the phase detector (6), with phase proportional to the rotation rate of the fiber spool (5). 
     As shown in FIG. 3(b), the interference pattern at the phase detector follows a raised-cosine pattern. This interference pattern is caused by the difference in phase of the two light waves. The difference in phase is proportional to the rotation rate. That is, the output of the phase detector is proportional to the cosine of the phase difference between the two light waves. Consequently, the phase detector output has a small slope at small phase differences, small phase differences correspond to small rates of rotation. This means that the rate of change of the cosine function near zero will be very small. As a result, the output function provides very low sensitivity for low rotation rates. 
     Therefore, it is desirable to move the operating point of the phase detector from 0° to ±90° (±Π/2 Rad), where the detector has high sensitivity. This is accomplished by adding a phase shift to one of the light waves. That is the function of the phase modulator (4). The phase modulator (4) (see, e.g., FIG. 2) is a serrodyne device that generates a phase shift proportional to the change in applied voltage at its input. The applied voltage is the sum of the voltage produced by the bias modulator and the serrodyne ramp generator. 
     The purpose of the biasing modulator is shown in FIG. 3(b). It is used to shift the voltage sampling point out to approximately ±90° (±Π/2 Rad) where the voltage slope is maximized and, consequently, the sensitivity of the modulator is maximized. This allows the phase of the interference pattern to be accurately computed by differencing the voltage measured at the positive and negative bias points (which equals the slope of the raised-cosine curve at Π/2 Rad). Therefore, small variations in rotation rate can be accurately detected and will vary linearly since the slope of the curve around ±Π/2 Rad is linear. 
     The biasing modulator delays one of the waves by Π/2 Rad with respect to the other. It does this by adding or subtracting a voltage corresponding to Π/2 to the input of the phase modulator (4). As a result, the detector has its maximum sensitivity for low rotation rates. In addition, this sensitivity is close to being linear. 
     However, as the rotation rate increases, the response of the detector moves away from Π/2, where the curve becomes less and less linear. In addition, the curve&#39;s slope flattens, thereby reducing the detector&#39;s sensitivity. 
     Therefore, a second modulation component is added. The other modulation component is the serrodyne ramp modulator, which applies the phase shift necessary to hold the gyro in a rebalanced condition. It provides sufficient phase change so as to cancel the phase shift difference generated by the opposite traveling light waves due to rotation. 
     As shown in FIG. 3(c), in order to apply a constant phase offset, the control voltage input to the phase modulator (4) must be applied as a stepped ramp which changes, each Eigen period, by an amount proportional to the desired phase shift. Since the required re-balance phase shift can be large or small, positive or negative, depending on the gyroscope&#39;s motion, the stepped ramp is correspondingly fast or slow, increasing or decreasing, as required to produce the phase shift needed to rebalance the gyroscope. 
     The ramp can not increase indefinitely and, at some point, must fly-back to zero. The flyback is made transparent to the gyroscope&#39;s phase modulator (4) by scaling the magnitude of the flyback voltage to correspond to the desired step size plus or minus 360° (step size ±2Π Rad). This produces the same phase shift during the fly-back as is produced by the other voltage steps in the ramp, effectively making a continuous phase shift. 
     The composite phase modulation signal is illustrated in FIG. 3(d). This signal is the sum of the biasing modulation (a) and the serrodyne ramp modulation (c). 
     Therefore, closed loop operation is obtained by measuring the detector phase (6) and driving a matching phase shift into an electro-optic phase modulator (4), thus re-balancing (i.e., zeroing) the detected phase shift. 
     The phase shift necessary to hold the gyroscope in a rebalanced condition is directly proportional to angular velocity of the fiber spool, and the processing electronics (7) derive the rate output (19) of the gyroscope from the commanded phase shift. FIG. 2 illustrates the best prior art for the closed loop processing electronics (7). The phase detector (6) output (which in a preferred embodiment is an analog square wave) is first amplified (9) and then digitized with an A/D converter (10). The digitized output of the A/D converter is a modulated signal, which is de-modulated (11) to recover the detector phase error. (In a preferred embodiment the demodulator (11) is a square wave detector). The phase error is then integrated in a digital accumulator (12) or integrator to compute the present gyro rate. The output from the accumulator or integrator (12) corresponds to the rotational rate of the gyroscope and drives both a serrodyne modulated ramp generator (13), and the gyroscope rate output (19). 
     The serrodyne modulation ramp generator (13) produces a stepped digital waveform in which the step size corresponds to the amount of phase shift required to hold the phase detector (6) output at null (gyroscope phase held in a re-balanced condition). The digital output of the ramp generator (13) corresponds to the rotational rate of the gyro and is converted to a stepped analog voltage ramp by a D/A converter (14). The voltage ramp is summed (18) with a biasing modulation voltage to form a composite phase modulation signal, which is amplified (15) and used to drive the phase modulator (4). The phase modulator adds a phase shift to one of the light waves to re-balance the gyro for closed loop operation. 
     In a preferred embodiment, the biasing modulation voltage is generated with a square wave generator (17), which is slaved to the same bias modulation timing (16) that provides a reference to the demodulator (11). 
     The components of the composite phase modulation signal are illustrated in FIG. 3. As shown in FIG. 3(a), in a preferred embodiment, the biasing modulation is a square wave, switched at the gyro Eigen period. The Eigen period is simply the time required for light to propagate around the fiber spool. 
     Since the commanded re-balance phase shift is the gyroscope output, the linearity and stability of the transfer function between the digital phase command at the ramp generator (13) and the actual phase shift produced by the modulator (4), directly impacts gyro performance. In order to produce a high performance IFO gyroscope, this phase command transfer function must be held stable despite changes in environmental factors such as temperature, humidity, power supply voltage, etc. 
     The present state-of-the-art for stabilizing the phase command transfer function is embodied in a technique that utilizes a gain scaling control loop to regulate the transfer function and hold it constant despite environmental disturbances. This technique consists of regulating the gain of the serrodyne ramp modulation so as to hold the fly-back transition equal to exactly ±360° (plus the appropriate phase step for re-balance). 
     As shown in FIG. 2, the D/A converter (14) for the serrodyne ramp modulator has its gain controlled by a separate modulation gain-scaling D/A converter (21). This D/A (21) scales the magnitude of the serrodyne ramp and can scale the ramp magnitude such that the ramp Fly-back voltage produces either greater or lesser than 360° of phase shift at the phase modulator (4). 
     The gain-scaling D/A (21) is controlled by a modulation gain accumulator (20) which accumulates the phase error (6,9,10,11) produced when ramp flyback occurs. The modulation gain accumulator (20) is normally held at its accumulated value, and is enabled to run only when the ramp generator (13) signals that a ramp flyback is about to occur. Once enabled, it samples phase error (output of demodulator (11)) for two Eigen periods, so that both phases of biasing modulation are accumulated. 
     The gain scaling control loop formed by the modulation gain accumulator (20) and the modulation gain-scaling D/A (21) operates to stabilize the phase command transfer function by adjusting the gain of the serrodyne ramp modulation D/A (14) until the phase error measured during a fly-back event is zero, which corresponds to a phase shift of ±360°. If the gain (20,21) is slightly in error, then the fly-back phase shift will not be precisely ±360°, and a non-zero phase error will be accumulated (20). This corrects the ramp magnitude (21,14), driving it to a balanced condition in which the ramp flyback is forced to a precise ±360°. 
     LIMITATIONS OF PRIOR ART 
     There are several significant error sources in the prior art technique of using a gain-scaling control loop to stabilize the phase command transfer function. These include: (1) loop interactions (acceleration-dependent errors); and (2) non-constant relation between serrodyne and biasing modulation. 
     The prior art allows the phase-rebalance control loop to run simultaneously with the gain-scaling control loop. Normally, the phase-rebalance control loop is active, accumulating measured phase error in the rate accumulator (12) while the gain-scaling control loop is disabled, with the gain accumulator (20) held at its previous value. This allows the gyroscope to be actively rebalanced while the modulation gain (20,21) is held static. 
     When a ramp fly-back condition occurs, however, the gain accumulator (20) is enabled and the measured phase error is accumulated into both the rate accumulator (12) and the gain accumulator. When this happens, both the phase-rebalance control loop and the gain scaling control loop attempt to provide corrections based on the measured phase error. If the gyroscope acceleration is very low (phase error near zero), this may not be a significant error. When the acceleration increases, however, the phase error is non-zero and both loops attempt to correct simultaneously. This produces instabilities and interactions between the loops that dissolve into chaotic, random behavior as the gyro is accelerated rapidly. The net effect is acceleration-dependent errors that prevent the gyro from operating reliably under conditions of non-constant velocity. 
     Another error source in the prior art is caused by holding the magnitude of the biasing modulation fixed while the serrodyne ramp modulation is adjusted by the gain scaling control loop. This causes the equivalent phase of the biasing modulation to change with respect to the equivalent phase of the serrodyne ramp modulation. In addition, the DC offset component of the serrodyne ramp can change as the gain scaling D/A (21) is changed. Both of these effects contribute parasitic error terms that tend to degrade stability and linearity. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the performance of high quality fiber optic gyroscopes by eliminating acceleration-dependent errors caused by interaction of the phase re-balance and gain scaling control loops, and to eliminate parasitic errors caused by changes in the relative magnitudes of the three components of the composite phase modulation signal. 
     It is an another object of the present invention to disclose a method of eliminating acceleration-dependent errors and loop interaction effects in gain-stabilized, closed loop, interferometric fiber optic gyroscopes, said gyroscopes being characterized by having a phase re-balance control loop for closed loop operation, and a separate gain-scaling control loop for stabilizing the phase modulator transfer function. 
     It is a further object of the present invention to use arbitration logic (22) to provide time-sliced, multiplexed control of said phase re-balance Loop and said gain scaling control loop and, by so doing, eliminate interactions between said loops which cause acceleration-dependent errors. 
     It is still a further object of the present invention to use arbitration logic (22) to enable either the rate accumulator (12), or the modulation gain accumulator (20), but never both simultaneously. 
     It is an additional object of the present invention to use arbitration logic (22) to enable the rate accumulator (12), and disable the modulation gain accumulator (20) except during occurrences of a ramp flyback condition. 
     It is still another additional object of the present invention to use arbitration logic (22) to enable the modulation gain accumulator (20), and disable the rate accumulator (12), on every occurrence of a ramp flyback condition when the gyroscope is operating below a certain velocity threshold, and to enable the modulation gain accumulator (20), and disable the rate accumulator (12), on every one out of N occurrences of a ramp flyback condition when the gyroscope is operating above a certain velocity threshold. 
     It is still a further object of the present invention to disclose a method for improving linearity and stability of gain-stabilized, closed loop, interferometric fiber optic gyroscopes, said gyroscopes being characterized by having a phase re-balance control loop for closed loop operation, and a separate gain-scaling control loop for stabilizing the phase modulator transfer function. Said method is comprised of using proportional compensation circuitry to maintain an exact proportional relation between the three principal components of the composite phase modulation signal, said principal components consisting of serrodyne ramp, biasing modulation and DC offset. 
     It is still another additional object of the present invention to use proportional compensation circuitry to adjust the magnitude of the biasing modulation signal component so as to maintain said magnitude at a constant proportion (approximately 25%) of the magnitude of the serrodyne ramp component as the magnitude of said serrodyne ramp component is adjusted by said gain scaling control loop. 
     It is still a further object of the present invention to use proportional compensation circuitry to force the DC offset component of the DC-coupled composite modulating wave form to zero and maintain said DC component at zero magnitude as the magnitude of the serrodyne ramp component is adjusted by said control loop. 
     It is another additional object of the present invention to provide an alternate method for improving linearity and stability of gain-stabilized, closed loop, interferometric fiber optic gyroscopes, said gyroscopes being characterized by having a phase re-balance control loop for closed loop operation, and a separate gain-scaling control loop for stabilizing the phase modulator transfer function. Said alternate method is comprised of using digital or analog circuitry to construct a scaled composite modulating waveform, said waveform consisting of a serrodyne ramp component of unit magnitude, a biasing modulation component at approximately 25% fixed relative magnitude, and a DC component of zero; and using a variable gain linear amplifier (15) to adjust the magnitude of the composite modulating signal, while holding a fixed relation between the signal components, under control of a gain-scaling control loop in accordance with existing art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a basic block diagram of a closed loop interferometric fiber optic gyroscope; 
     FIG. 2 is a block diagram of existing prior art for closed loop interferometric fiber optic gyroscopes; 
     FIG. 3a-d show the components of a composite phase modulation signal; 
     FIG. 4 shows the improvements made to the existing prior art by the invention; 
     FIG. 5 is a detailed schematic of the arbitration logic; and 
     FIG. 6 is a detailed schematic of the proportional compensation circuitry. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The improvements to prior art are highlighted in FIG. 4. Arbitration logic (22) provides time-slice multiplexing between the rate accumulator (12) and the modulation gain accumulator (24), enabling one or the other (23,24), but never both simultaneously. Except during occurrences of a ramp flyback condition, the rate accumulator (12) is enabled (23), accumulating phase error and driving the serrodyne ramp generator (13) with a commanded phase shift as necessary to hold the gyroscope in a re-balanced condition. 
     When a ramp flyback condition is reported by the serrodyne ramp generator (13), the arbitration logic (22) response depends on the gyroscope angular rate. If the gyroscope rate is below a certain threshold (7.5°/sec in a preferred embodiment), the arbitration logic (22) disables (23) the rate accumulator (12) and enables (24) the modulation gain accumulator (20) for two Eigen periods, thus allowing the gain scaling control loop to adjust without interacting with the phase rebalance control loop. 
     If the gyroscope rate is above the threshold, the arbitration logic allows the rate accumulator (12) to remain enabled (23) for most of the ramp flyback events, disabling the rate accumulator (12) and enabling the modulation gain accumulator (20) on a one out of N basis, where N is 8 in a preferred embodiment. This action ensures the gyroscope remains stable and rebalanced at high rates, in which the ramp flyback event can occur as frequently as every two Eigen periods. In other embodiments, the rate accumulator is disabled and the modulation gain accumulator is enabled over a range from one out of every four flybacks to one out of every sixteen flybacks. 
     A detailed schematic of the arbitration logic is shown in FIG. 5. U1, U2, and U3 generate the MODE signal as a function of the velocity input V 18:14!. In a preferred embodiment, V 18:14! is a two-s-complement value where V14 represents 7.5°/sec. When the velocity is positive (V18 is logic low) U1 will go active only if the velocity is below 7.5°/sec. When the velocity is negative (V18 is logic high) U2 will go active only if the velocity is below -7.5°/sec. Thereby, the MODE signal will be logic low if the velocity is below 7.5°/sec or greater. In other embodiments, the threshold can be as high as 30°/sec. 
     In a preferred embodiment, U4-U10 is a divide-by-8 up counter. It is incremented during every flyback (controlled by U11). U12, U13, and U14 form a 2-to-1 multiplexer that is controlled by the MODE signal. When MODE is logic low, U13 will force the UPDATE signal active during every flyback. When MODE is logic high, U12 will force the UPDATE signal active only when the divide-by-8 counter is at state 7 (Q2=Q1=Q0-high), thus UPDATE is active only one out of every eight flybacks. In other embodiments, the UPDATE signal is active from one out of every four flybacks to one out of every sixteen flybacks. The UPDATE signal represents a modulator gain accumulator update cycle. 
     U15-U20 is a state machine that generates the HOLD signal during every update. The HOLD signal is used to hold the value of the RATE accumulator constant while the modulator gain accumulator is updated. HOLD is active for a fully modulation period to avoid accumulating a DC bias error. U21 inverts the HOLD signal to create the rate accumulator enable signal. U22 synchronizes the modulator gain accumulator enable signal (GAIN --  EN) with the rate accumulator enable signal (RATE --  EN). 
     Input signal description: 
     CP--one-clock wide timing pulse used to indicate when valid data has been acquired 
     FCLK--master clock 
     FLYBACK--indicates a Serrodyne Modulation Ramp Generator flyback 
     MR--master reset 
     V 18:14!--velocity, where, in a preferred embodiment, V14 represents 7.5°/sec 
     Output signal description: 
     GAIN --  EN--modulator gain accumulator enable 
     RATE --  EN--rate accumulator enable 
     A detailed schematic of the proportional compensation circuitry is shown in FIG. 6. D/A converter U2 produces the serrodyne ramp modulation voltage. The serrodyne ramp data is supplied to U2 as a digital value, and is converted to a stepped analog current. U2 contains an input op-amp stage which holds the voltage at VR+ to zero (virtual ground). The current I in  is multiplied by N/256, where N is the digital value of the Ramp data. The current I out  is converted to a voltage in output amplifier U3 to produce a stepped voltage ramp at TP3, which is output to the gyroscope&#39;s phase modulator (4). 
     If the voltage at TP1 is zero, I in  is established by resistor R1. In the present implementation, V ref  is 2.5V, and I in  is nominally 1.25 ma. This generates a peak amplitude (ramp magnitude) at TP2 of 1.25 V, which is amplified by a factor of 3.43 in amplifier U3. 
     D/A converter U1 provides the modulation gain-scaling function by providing, in a preferred embodiment, ±20% trim to the nominal value of I in . U1 generates a voltage between ±2.5V (V ref ) TP1, with zero being the nominal value. When TP1 is zero, the current flow through R2 is zero, since the VR+ input of U2 is a virtual ground. This sets I in  to its nominal value of 1.25 ma, as established by R1. As TP1 develops a non-zero voltage the current flow through R2 adds to (or subtracts from) I in  to adjust the ramp magnitude. In a preferred embodiment, full scale voltage at TP1 provides a ±20% trim to I in  due to the 5:1 ratio between R2 and R1. In other embodiments, full scale voltage at TP1 can provide up to a ±50% trim. 
     In a preferred embodiment, the resistors R4 &amp; R3 and R6 &amp; R7 have the same 5:1 ratio as R2 and R1, and thus provide the same maximum ±20% trim to the other signal components as the ramp magnitude is adjusted. Resistors R3 &amp; R4 control the magnitude of the biasing modulation, which is developed by switch U3 as a square-wave signal, then AC coupled through C1 to a summing node at amplifier U3. The 4:1 ratio between R8 and R7 sets the magnitude of the biasing modulation to be 25% of the serrodyne ramp modulation (If the Ramp is defined to be 360°, the biasing modulation will be proportionally ±90°). In other embodiments, the magnitude of the biasing modulation is 20% to 30% of the serrodyne ramp modulation. 
     Resistors R5 &amp; R6 inject a current into the summing node of amplifier U3 to offset the DC bias of the voltage at TP2, and restore the average value of the signal at TP3 to zero volts. The ramp at TP2 runs from zero to nominally 1.25V, with an average value of 1/2 the peak voltage. The 2:1 ratio between R5 and R1 provides an off-setting current at 50% of the value of I in , thus canceling the DC bias at TP2. 
     Proportional compensation is provided by the matching ratio of resistors R2:R1, R4:R3, and R6:R5. As the serrodyne ramp magnitude is adjusted by a change in voltage at TP1, the magnitude of the biasing modulation and the DC offset compensation are adjusted in exact proportion. This allows the phase shift produced by the serrodyne ramp flyback to be adjusted without injecting spurious effects caused by altering the relative phase significance of the biasing modulation, and without incurring parasitic nonlinearities by allowing the DC offset of the composite modulation signal to change. 
     While the invention has been disclosed in this patent application by reference to the details of preferred embodiments of the invention, it is to be understood that the disclosure is intended in an illustrative rather than in a limiting sense, as it is contemplated that modifications will readily occur to those skilled in the art, within the spirit of the invention and the scope of the appended claims.