Fiber-Optic Gyros and MEMS Ac LITEF GmbH Freiburg, P. 0. Box 774, 79007 Freiburg, Germany rasch.

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Transcript of Fiber-Optic Gyros and MEMS Ac LITEF GmbH Freiburg, P. 0. Box 774, 79007 Freiburg, Germany rasch.

  • Fiber-Optic Gyros and MEMS Accelerometers

    Andreas Rasch, Eberhard Handrich, Giinter Spahlinger, Martin Hafen, Sven Voigt, and Michael Weingartner

    LITEF GmbH Freiburg, P. 0. Box 774, 79007 Freiburg, Germany

    Fiber-optic gyros (FOGs) and micro-electro-mechanical-systems (MEMS) ac- celerometers are used today in inertial strapdown systems for medium ac- curacy and expanding into high-performance strapdown navigation systems in competition with ring laser gyros (RLGs), whereas from the low-accuracy side, MEMS gyros are used for expanding to the medium accuracy ranges. The FOG principle is based on constant light velocity. This results in a phase difference of lights which are propagating through a fiber coil in clockwise ( cw) or counterclockwise ( ccw) directions if a rate is applied. The phase difference is proportional to the rate. The FOG technology has been developed from an open-loop design- still used in some market niches- to closed-loop design with high bandwidth and random phase modulation technique. The first gen- eration of FOG systems uses one light source split by a 3x3 coupler to three fiber coils. More than 15.000 FOGs for such triad systems have been produced and delivered. Typical applications are Attitude and Heading Reference Sys- tems or Land Navigators, which are described. The second generation of FOG systems uses single-axis FOGs with internal processors. A large quantity of these fiber-optic rate sensors (J.t-FORS) can be easily calibrated separately and later assembled to modular systems. The features of the J.t-FORS family for bias values from 6° /h down to 0.05° /h are given. The different bias values are realized by adapting the fiber length on the coil. The other optical parts and the electronics are unchanged. One main feature for the common electronics is the tracking of the modulation frequency to the actual fiber length.

    The MEMS accelerometers are still mechanical sensors built by the micro- machining technique. The technology, the mechanical sensor, and the electron- ics are described on the example of the B-290 Triad, which is a typical MEMS accelerometer product. Test data for bias repeatability and stability and scale factor accuracy before and after temperature compensation are presented.

    J. Jahns et al. (eds.), Microoptics © Springer Science+Business Media New York 2004

  • 276 Andreas Rasch et al.

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    1990 1995 2000 2005 v .....

    Fig. 1. Evolution of gyro technologies.


    Fig. 2. Block diagram of the closed-loop FOG.

    1 Basic Fiber-Optic Gyro Technology

    If we look at FOGs, there exist the open- and the closed-loop designs. The major advantage of the closed-loop FOG is the high linearity of the scale factor and its insensitiveness against environment, especially against vibration.

    The evolution of the most important gyro technologies in terms of accu- racies over the years is shown in Figure 1.

    Today, the high-accuracy end is still occupied by RLGs. The moderate accuracy range from 0.01° /h to 30° /h is mainly covered by FOGs, and MEMS gyros are rising up from the low end accuracy.

    LITEF produces a closed-loop design only. The block diagram of the closed-loop FOG is shown in Figure 2. The heart of the FOG is the multi- function integrated optic chip (MIOC). The MIOC realizes the polarizer, the main coupler, and the modulator in one chip, which is shown in Figure 3. The MIOC production is done on LiNbOa wafers by forming proton-exchanged waveguides and sputtered electrodes. Thirty-two MIOCs are diced out of one wafer, which is shown in Figure 4.

    The advantage of this technology is the high extinction ratio of the po- larizing waveguides formed by proton exchange [1]. The drawback of this technology is the high up-front investment for an independent in-house pro-

  • Fiber-Optic Gyros and MEMS Accelerometers 277

    . - -· ----; -

    Fig. 3. Multifunction integrated optic chip (MIOC) .

    Fig. 4. Lithium niobate technology steps: 3-in. wafer with 32 MIOCs, subwafers, and single chips.

    duction for the MIOC and the challenge of the new technology, which requires clean rooms and equipment for lithography, proton-exchange baths, annealing ovens, sputtering equipment, wafer dicing tools, and chip polishing tools.

    Between 1994 and 2002, LITEF produced more than 30.000 MIOCs with high yield for its FOG products.

    2 FOG-Triad Systems

    At the beginning of the 1990s, the superluminescent light-emitting diode (SLD) light source package was a question of cost for fiber-optic produc- tion. Therefore, the natural decision was to use one SLD light source and distribute the light power to three 10 chips and fiber coils. Such a triad struc- ture is shown in Figure 5; a typical sensor block assembly is shown in Figure 6 -this may be used in the Attitude and Heading Reference System (AHRS) for commercial applications.

    To date, LITEF has developed five different triad configuration systems:

    • LITEF Commercial Reference-92 (LCR-92) , an Attitude Heading Refer- ence System with bubbles as the level sensor for commercial airborne ap- plications

    • LITEF Commercial Reference-93 (LCR-93) , an Attitude and Heading Ref- erence System with integrated silicon accelerometers for commercial air- borne applications

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    Fig. 5. Fiber-optic gyroscope - triade structure.

    Fig. 6. Sensor block micro Attitude and Heading Referencs System.

    • LITEF Transport Reference-97 (LTR-97), an Attitude and Heading Ref- erence System with bubbles as the level sensor for airline and transport application

    • LITEF Land Navigator-GX (LLN-GX), navigator, which integrates the information of the FOGs, of the bubbles, of the odometer of the vehicle, and of a GPS receiver to optimal navigation data

    • LITEF Land Navigator-Gl (LLN-Gl), a navigator similar to LLN-GX but with high accuracy FOGs, with accelerometers instead of bubbles and integrated with self-alignment features to the north

  • Fiber-Optic Gyros and MEMS Accelerometers 279

    The accuracy span from 3° /h to 0.08° /h of those systems is achieved with different coil designs and fiber lengths. The electronics are almost identical. All of these systems have been in production for several years and the quantities produced have increased; for example, between January 2000 and July 2002, more than 3000 Triad Fiber-Optic Systems were produced. Therefore, LITEF has gained much experience in the field of FOG production yield. Where pro- duction yield is concerned, the most critical production test is calibration over the temperature of the system, which is done on a turntable with a climate chamber. Such tests are fully automatic and steered by computers, and four systems can be calibrated simultaneously; however, the test time is between 10 and 36 h and the test equipment is expensive. Today, the production yield in calibration of almost all Fiber-Optic Triad Systems is over 90 %. In addi- tion to the well-known measurement for optical reciprocity and random phase modulation with its auxiliary loops, the main problems to be overcome for that yield were as follows:

    • Electronic noise at the IO modulator that can create bias errors that are difficult to describe with a model. Filtering is very limited because of the required high bandwidth of 100 MHz for the modulation.

    • Effects on bias by the so-called "bunny ears" created by electronic tran- sients in conjunction with the interferometer transfer function. Nonlinear- ities in the detector and amplifier channel can also create bias.

    • Wavelength-selective optical losses in fiber and couplers that create scale factor problems over temperature.

    To always be able to calibrate three FOG axes simultaneously, a high- performance margin is required for the FOGs to achieve the 90% yield in production.

    3 Modular Fiber-Optic System Design

    Modular system design is a general design required for an easy assembly of a system in production. This is also realized in Fiber-Optic Triad Systems. How- ever, a higher level of modularity means that each component of such a system should be testable in its function with high failure elimination, not only for an easy assembly but also for a successful test (e.g., calibration and acceptance test). This is difficult to achieve for Fiber-Optic Gyro Triad Systems, because the total function (e.g., bias or scale factor accuracy) can only be tested with about 50% yield on the component level. In calibration, all parameters of all three axes then have to be tested simultaneously, which creates high require- ments for the material and functionality of components and their integration. However, such a modular system design can easily be realized with single-axis FOGs with internal electronics and a processor which compensates for bias and scale factor over temperature. Each gyro can be calibrated individually

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    Fig. 7. Modular FOG inertial measurement unit (IMU) approach.

    over temperature and later assembled within systems with orthogonal or re- dundant skewed axes. A block diagram of such a modular system is shown in Figure 7.

    A digital synchronous bus IBIS (Intelligent Bus for Inertial Sensors) links the single-axis micro-fiber-opti