A Constant Accuracy, High Dynamic Range Fiber Optic Gyroscope

Fiber optic gyroscope (FOG) is a new type of angular velocity sensor in the field of inertia technology. Its non-moving parts and all-solid-state characteristics leads to many advantages compared with traditional electromechanical gyro and ring laser gyro. Scale factor is a key parameter of FOG, which affects the angle measurement accuracy. Closed-loop feedback phase modulation is an important method to enhance the scale factor stability in FOG. The multifunctional integrated optical circuit (MIOC) is the actuator of closed-loop feedback, and the mean wavelength stability of transmission spectrum at full temperature affects the scale factor stability of FOG. With the improvement of FOG's accuracy, the effect of the mean wavelength shift on FOG's performance are becoming more and more important, which has been an important factor limiting the scale factor stability enhancement. Based on the optical waveguide theory, we found out alignment deviation (axial gap, transverse dislocation and axial angle deviation) is the main factor of the mean wavelength stability, and verified by coupling experiments. Moreover, the mean wavelength is the most sensitive to the transverse dislocation. Take z=10 μm for example, the mean wavelength increases by 134 ppm relative to the wavelength of 1.31 μm when x increased from 0 to 2 μm. Secondly the axial gap, the mean wavelength increase by 19 ppm when z increased from 0 to 20 μm. Thirdly the angle deviation plays a minor role, and the mean wavelength decreases by 5 ppm when θ increased from 0° to 2°. This study is of great significance to optimize MIOC, and to provide device guarantee for the enhancement of scaling factor stability of FOG.

Draper Laboratory's experience with Interferometer Fiber Optic Gyro (IFOG) technology started via an Internal Research and Development effort in 1978. This work led to developing fiber optic gyros in which significant advances in IFOG component technologies, assembly, integration, and test were achieved. In excess of 30 patents have been issued to Draper as a result of this pioneering effort. More recently, Draper collaborated with JPL to transition their fiber optic breadboard gyro to a space qualifiable instrument for interplanetary long duration missions. During the initial phase, brassboard gyros were designed and fabricated that demonstrated performance that bettered NASA's CRAF-Cassini spacecraft objectives. In the second phase, the Engineering Model gyro was developed to meet mission qualification tests. Concurrent with this later phase, analysis, tests and qualification activities were performed to validate that gyro components would realize the required 16 year life. Two major inventions (patent pending) were conceived; one provides for continuous adaptive scale factor stability and the other integrates the optical source and photodetector into a single component [Source Integrated Detector (SID)], thereby eliminating a coupler and several splices thus reducing loss by 6 db. Using the above technologies and the implementation of common mode rejection of laser noise, Draper has defined a low noise high performance gyro in which rate power spectral density (PSD) is projected to be < 7 X 10-7 (deg/hr)2/Hz. In those applications, where the PSD < 4 X 10-5 (deg/hr)2/Hz is sought, a small size three component (SID, integrated optics circuit and fiber coil) gyro is easily implemented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In modern aerospace technology, strapdown inertial navigation systems (SINS) are widely used, using fiber-optic (FOG) or ring laser (CLG) gyroscopes. During the operation of such systems, the sensitivity axes are rotated relative to the basic coordinate system. The resulting angles between the axes of the base coordinate system and the axes of sensitivity of the navigation system (non-orthogonality) are one of the factors leading to an increase in the measurement errors of the device, which affects the measurement accuracy. During operation, the system is affected by vibrations of various nature, the impact of which can contribute to the appearance of non-orthogonality. The purpose of this work is to determine the maximum permissible vibration amplitudes affecting the SINS body according to the permissible values ​​of the deviation of the FOG sensitivity axes for two variants of the SINS layout. An approach to determining the permissible amplitudes of an external harmonic impact on the unit of a strapdown inertial navigation system based on fiber-optic or ring laser gyroscopes is considered. A design scheme, mathematical and finite element models for calculating natural frequencies and forced oscillations of a strapdown inertial navigation system unit have been developed. In various frequency ranges, numerical calculations have determined the boundary values ​​of the amplitudes of the external harmonic impact on the base of specific configurations of the SINS assembly. It has been established that dangerous states take place in the region of the 1st natural frequency of the system, as well as near higher frequencies. Comparison of the results for design options 1 and 2 allows us to conclude that in order to weaken the effect of vibrations on the accuracy of the SINS unit, it is advisable that the lowest natural vibration frequencies for the SINS assembly be as high as possible (more than 1000 Hz).&#x0D; Key words: vibration; fiber optic gyroscope; strapdown inertial navigation system; finite element method; natural frequencies and modes of vibration.

The DoD goals for inertial sensors have included achieving high accuracy performance and small size at a low cost. This goal has always been a challenging endeavor, since small size and high accuracy have often been costly and technically difficult to achieve. In 1998, the Army patented a fiber sensor coil winding concept that would facilitate the opportunity to make the Fiber Optic Gyroscope (FOG) more competitive in cost with relation to the commonly-used Ring Laser Gyro (RLG). Recent advances in FOG sensor coil winding techniques appear to show great promise in the improved performance. The novel Crossover-Free (CF) winding technique eliminates fiber crossovers and allows the use of inexpensive single-mode fiber (SMF). Experiments were conducted with the use of an analog, open-loop testbed, which was characterized with a 1 km quadrupolar SM sensor coil. Various sensor coil configurations were spliced into the FOG testbed and bias drift tests were conducted. Different fiber lengths, coil diameters, and fiber wind configurations were evaluated. The Crossover-Free sensor coils were precision wound by a semi-automated Fiber Placement Machine (FPM) developed by Stanley Associates. The Crossover-Free sensor coils test results are compared to standard precision wound coils. The bias errors caused by the fiber crossovers in standard SM sensor coils are also discussed and compared to the near elimination of the crossovers in the CF design.

Fiber optic gyroscopes (FOGs) are vital components for manned, unmanned, and autonomous platforms, as well as applications requiring extremely accurate data for stabilization, pointing, orientation of payloads, and navigation and control. FOG-based inertial systems also provide highly accurate data for all types of navigation, positioning, and georeferencing systems. Substantial improvements in the performance of FOGs and FOG-based inertial systems at KVH are directly related to advancements in the design and manufacture of optical fiber, as well as in manufacturing operations and signal processing. Open-loop FOGs, such as those developed and manufactured by KVH Industries, offer tactical-grade performance in a robust, small package. The success of KVH FOGs and FOG-based inertial systems is due to a series of key innovations including the development of proprietary D-shaped fiber with an elliptical core, as well as KVH's ThinFiber. KVH continually improves its FOG manufacturing processes and its signal processing, with the goal of improving accuracies across its entire FOG product line. KVH acquired its FOG capabilities, including its patented E·Core® fiber, when the company purchased the assets of Andrew Corporation's Sensor Products Group in 1997. E·Core fiber is unique in that the light-guiding fiber core — critical to the FOG's performance — is elliptically-shaped. The elliptical core results in an optical path with low loss and high polarization-maintaining ability. In 2011, KVH developed its ThinFiber, a 170-micron diameter dual acrylate-coated fiber that retains the full performance characteristics of E·Core fiber. ThinFiber has enabled the development of compact, high-performance open-loop FOGs, which are also used in a line of FOG-based inertial measurement units (IMUs) and inertial navigation systems (INS).

The use of an optical fiber gyroscope in automotive navigation is reported. The performance requirements for a navigational gyroscope are discussed in terms of the input range, resolution, and bias and scale errors. The OFG-3, an automotive optical fiber gyroscope prototype, is introduced, and its error sources are related to the gyroscope designs. The gyroscope is made of polarization maintaining fibers and incorporates optical phase modulation and synchronous demodulation techniques. The gyroscope electronics restrict the resolution and the bias error, while the optics dominate the scale error. The microcomputer in the electronics compensate for the scale error, and the OFG-3 meets navigational requirements. Automotive navigation has been demonstrated using the gyroscope in combination with a map-matching algorithm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The advantages and the characteristics of the fiber optic gyroscopes (FOGs) for the space application, compared with the traditional mechanical gyroscope and the laser gyroscope, have been mentioned. There are differences of the FOG designing between application in space and other conditions especially the radiation-resistant, low power cost and less weight of the FOG. The key techniques of the fiber optic gyros technology have been proposed and some test results have been discussed.

Development of an automated fiber optic winding machine for gyroscope production

The fiber optic gyroscope (FOG) has a broad application prospect in the field of dynamic angle measurement. The scale factor, which is an important indicator of the FOG, directly determines the angle measurement precision. In this paper, the principles of angular velocity method and angular increment method, which have been commonly used in obtaining the scale factor, are introduced, and the error sources that affect the scale factor in both methods are analyzed in detail. The theoretical analysis results show that the main error sources of the scale factor are the relative error of the angular velocity benchmark generated by the rotary table and the value of the selected angular velocity benchmark in the angular velocity method. Other error sources need to be considered only at small angular velocity benchmark. Instead, the main error of the scale factor comes from the position precision of the rotary table in the angular increment method. Finally, the experiments on the scale factors of the FOG in both methods are carried out, and they are applied to the angle measurement experiments, respectively. The angle measurement results show that the deviation of the angle can be limited in 0.002° when the scale factor obtained by the angular increment method is adopted in angle measurement. It is an order of magnitude smaller than the angular velocity method. We can get the conclusion that the scale factor obtained by the angular increment method is more suitable for angle measurement than the angular velocity method.

Fiber optical gyroscopes (FOGs) have been applied widely in navigation for mobile robots. The precision of outputs from FOGs is affected significantly by uncertain disturbances due to road conditions in practice. It is urgent to eliminate uncertain disturbances from outputs of sensors in harsh environments. This paper proposes an adaptive method of signal processing base on wavelet and fuzzy logic for outdoor mobile robots in different road condition. Property of disturbances in different outdoor road is taken into account to determine filtering thresholds in this research. Corresponding characteristics of real-time data were processed by a fuzzy logic inference system when mobile robots moved, and then they were de-noising by means of wavelet technique. It was proved that this proposed adaptive method can eliminate disturbances due to road irregularities and improve accuracy of surrogate data by experiments.

Gyroscopes and accelerometers are unique devices, the critical input sensors for inertial navigation and surveying systems. Among the many types of gyroscopes only a few have proved adequate for precise inertial applications. These are the ring laser gyroscope (RLG), electrostatic suspended gyroscopes (ESG), air-bearing floated-gimbal spinning mass gyroscopes, and dynamically tuned rotor gyroscopes (DTG). All have been developed to a high art among precision instruments, with supremely low noise and bias errors. Extensive acceptance of inertial navigation systems in commercial flight service, particularly using RLGs, has also required and stimulated development of practical and competitve reliability, cost, and lifetime.A few currently emerging gyroscope technologies might play a role in future inertial systems, but it is not yet clear whether they can advance to be competitive in performance and cost with current gyroscopes. Most notable is the fiber-optic gyroscope (FOG), another form of optical gyroscope, which in the last several years has advanced to a state suitable for engineering prototypes in aircraft attitude heading and reference systems. Another is the hemispherical resonator gyroscope (HRG), which uses the precession of vibrations on the rim of a “wine glass” shaped quartz resonator.

Past and current research efforts initiated in the Government and industry for the development of gyroscopes for tactical weapon system applications are discussed in this paper. Spinning mass gyroscopes were used in the early long-range missiles for precision guidance. During the past two decades, tremendous progress has been made in advancing the performance capability of solid-state optical gyroscope for use in tactical weapon systems. A number of Army missile systems currently use ring laser gyroscopes as part of their navigation systems. A few years, ago, the Government awarded several contrasts to industry to develop small, reliable, low-cost fiber optic gyroscopes (FOG) that can operate in military environments. Recently, the Government awarded several contracts for the development of rugged, micro electromechanical systems (MEMS) gyroscopes to support emerging Army missions, which are moving toward low-cost, small diameter precision guided weapons for use against light targets. The RLG, FOG, and the MEMS-based gyroscopes are traded-off in this paper with respect to cost, size and performance for applications in current and future Army missile systems. The status of ongoing MEMS projects at the US Army Aviation and Missile Command will also be discussed.

Background/Objectives: The main objective of this paper is to perform a comparative discussion in gyro parameters for three cases: (i) V2π (vary)&Vπ/2 (constant), (ii) Vπ/2 (vary) and V2π (constant) and (iii) both V2π and Vπ/2 are varying simultaneously. Methods/Statistical Analysis: Fibre optic gyroscope has to be operated in closed-loop approach to achieve the inertial-grade-performance. The Closed-Loop Interferometric Fibre Optic Gyroscope (CLIFOG) system depends upon ramp voltage (V2π of IOC) and square-wave biasing signal frequency (f bias ). The digital-phase-ramp-function is used as feedback to system and makes gyro to null condition. The biasing signal voltage (Vπ/2) is 1/4 th of ramp voltage (V2π). If there are any variations in ramp and biasing signal voltages, then it introduces variations in gyro performance. Results: The ramp voltage (V2π) and square wave biasing signal voltage (Vπ/2) is increased from 1% to 10%, decreased to 1% to 10% and performed the various tests for three cases. From results, it was noted that: (i) When V2π is varying and Vπ/2 is kept constant, the scale-factor change is increasing as the error in the signal voltage is also increases. (ii) When V2π is kept constant and Vπ/2 is varying, the change in scale-factor is very minute and maintains the scale-factor linearity. (iii) While varying both V2π and Vπ/2 voltages simultaneously, the device is not responding accordingly as the error is increasing and also scale-factor is out of range subsequently. The effects on the CLIFOG system are described with the derived values in terms of bias and scale-factor. The gyro performance is very sensitive with respect to V2π variations and the percentage error is high in gyro output, but very less effect due to Vπ/2 variations when the ramp voltage (V2π) is constant. Conclusion/Application: The Closed Loop Interferometric Fiber Optic Gyroscope (CLIFOG) system requires a proper resetting of ramp voltage (V2π) in order to avoid the bias and scale factor instabilities.

The degradation of optical symmetry caused by the thermal strain of the fiber coil results in the thermal bias drift of fiber optic gyro (FOG). This paper presents a method for quantitative analysis of the optical symmetry of the fiber coil and accurate prediction of thermal bias drift in FOG. Firstly, a thermal strain model is established to illustrate the strain characteristics of the layer structure in the fiber coil under temperature fields, emphasizing the importance of thermal strain on optical symmetry. Then, an equivalent asymmetric index (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α_Tk</i>) is introduced to quantitatively evaluate the optical symmetry of the fiber coil. By deriving equations for optical symmetry and thermal bias drift caused by thermal strain, a novel method is proposed to predict and optimize the thermal bias drift. This method utilizes high-performance OFDR to measure temperature and strain in a 3 km fiber coil to predict thermal bias drift. It also discusses the relationship between the length of the tail fiber and optical symmetry, predicting the precise length required to optimize thermal bias drift. Subsequently, the same fiber coil is placed in the FOG to verify the accuracy of the thermal bias drift prediction. Adjusting the length of the tail fiber reduces the thermal bias drift from 0.33 °/h to 0.13 °/h.

A novel digital compensation scheme is demonstrated to control the gain of the modulation chain and suppress the influence of quantization error on bias. The error produced by the quantization multiplied by the scaling factor is theoretically analyzed. Simulations indicate that the quantization error varies with the input angular velocity and temperature, which is verified by experiments. By switching the integration and compression operations in the modulation chain, this quantization error is reduced, while automatic reset of the digital phase ramp register is achieved. We test the scheme in a fiber optic gyroscope. The test results reveal that the quantization-induced bias is suppressed and the residual bias is two times less than the desired accuracy with data accumulated over one-second sample interval. The scheme is a feasible method to miniaturize fiber optic gyroscope using a totally digital circuit for compensation of the modulation gain.