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 rasch.andreasGlitef.de

Fiber-optic gyros (FOGs) and micro-electro-mechanical-systems (MEMS) ac­celerometers are used today in inertial strapdown systems for medium ac­curacy 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 gen­eration 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 Sys­tems 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 micro­machining technique. The technology, the mechanical sensor, and the electron­ics 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.

Accuracy 'lh

0.001

RLG 0.01

0.1 DTG

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10

100

1000

1990 1995 2000 2005 v .....

Fig. 1. Evolution of gyro technologies.

MIOC

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 accu­racies 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 multi­function 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 po­larizing 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 produc­tion. 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 struc­ture 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 Refer­ence System with bubbles as the level sensor for commercial airborne ap­plications

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

278 Andreas Rasch et a!.

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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 Ref­erence 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 pro­duction 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 addi­tion 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 tran­sients in conjunction with the interferometer transfer function. Nonlinear­ities 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 high­performance 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. How­ever, 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 require­ments 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

280 Andreas Rasch et al.

IBIS

+28V +-

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

over temperature and later assembled within systems with orthogonal or re­dundant 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-optic rate sensors (f..t-FORS) and the triad including silicon accelerometers (B-290 Triad) with the processor module. The data rate can be programmed between 5Hz and 8kHz. A power supply can be added if the required voltages (±15 V and ±5 V) are not delivered.

4 Single-Axis ~-t-FORS Family

The f.A,-FORS was developed in 1995 [2] and more than 6000 f.A,-FORS have been produced and delivered to date. f.A,-FORS is a single-axis fiber-optic rate sensor with the necessary optics and electronics in a small housing of 76 x 55 x 20 mm3 .

It requires ±5 V and 2 W and delivers the rate data in digital format via the IBIS bus. Bias and scale factor are compensated over temperature inter­nally by a processor within the digital application specified integrated circuit (ASIC). Many features can be programmed (e.g., the rate range and the out­put data rate) [3]. The f.A,-FORS uses a SLD without Peltier cooler, a binary digital MIOC with integrated digital-to-analog converter (DAC) function, a low-cost detector, and a flash ADC. The main control loop, all auxiliary loops,

Fiber-Optic Gyros and MEMS Accelerometers 281

500 I

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I 400 ~J-FORS-6

___.

-1

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] 6 ' 12 15 18 21 24 27 30 J] 3& 0 ~~~~~~~~~

Fig. 8. Bias statistics of J.t-FORS-36 and J.t-FORS 6.

and the reduced instruction set computer (RISK) processor are to be incor­porated within a digital ASIC with 1.4 Mio transistors. The main bulk of the production quantities are dedicated to ~-t-FORS-36 and ~-t-FORS-6, the bias performance which is shown in Figure 8.

In the meantime, LITEF has developed a ~-t-FORS family with the follow­ing features:

• Compact single-axis rate sensors • Temperature-compensated digital output • Programmable rate range, with higher resolution and data rate • Fiber coil length from 50 m to 500 m • Bias residual errors from 6° /h to 0.03° /h • Scale factor error from 3000 ppm to 100 ppm • Typical size: 76 mm x 55 mm x 20 mm

The major steps for the improvement of bias and scale factor to these limits have been a new digital MIOC with 12-bit electrodes and a new digital ASIC with 4-Mio transistors, which includes the following improvements [4]:

• An improved hardware scale factor control • An optical coil fiber length measurement • Mdulation frequency tracking to the actual fiber length • A bit-weighting compensation algorithm for the digital MIOC • Subsequent noise and resolution reduction • Improved data path for main control loop • ARM 7 RISK processor with RAM/ROM

282 Andreas Rasch et al.

Fig. 9. Optical components of f.l-FORS with a 500-m fiber coil.

Fig. 10. MIOC and electronics board of the !!-FORS.

With the new feature - the tracking of the modulation frequency to the actual fiber length - the bias effects, created by synchronous noise, vanish is a significant improvement.

The smallest t-t-FORS family member is the t-t-FORS-36m. The main chal­lenge was the development of an analog ASIC together with a multichip mod­ule for the detector and an optimized packaging of optics and electronics.

The most accurate t-t-FORS has a 500-m fiber coil and a Peltier stabilized SLD. This t-t-FORS is shown in Figures 9 and 10.

Fiber-Optic Gyros and MEMS Accelerometers 283

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Fig. 12. Bias statistics of a 500-m FOG.

Figure 11 showes the measured bias values of 100 sensors J.L-FORS-36m. It can be seen that all values except three axes are within ±3° /h.

The best performance of 500 m FOGs in production is shown in the bias statistics of Figure 12; 90% of the produced gyros are below 0.04° /h bias and the typical bias (highest peak)is 0.015° /h.

5 MEMS Accelerometer

Most of the MEMS accelerometers principles use elastically supported pen­dulums produced by bulk micromachining. A typical example of that type is LITEF's B-290 accelerometer whose principle is shown in Figure 13.

Upon acceleration, the position of the pendulum and the gaps between the cover wafers are changed. The gaps are used as a capacity bridge for the

284 Andreas Rasch et al.

Acceleration

Pendulum

Hinge

Fig. 13. Principle of the silicon accelerometer B-290.

Fig. 14. Silicon accelerometer chip for B-290.

Cover Wafer

pick-off and as the restoring torquer by means of electrostatic forces. The total silicon chip is shown in Figure 14. The chip is built out of five silicon wafers by silicon direct bonding: Two shield wafers are added to avoid stray capacities.

The opened chip allows the view of the pendulum with the hinges and the electrode with the shielding frame on the other side in Figure 15. The production process of the chip is actually a batch process on 5 wafers for 140 chips which are bonded together at the end of the process and then cut into 140 accelerometer chips.

The sensor electronics switch the voltage at the capacitor bridge and sense the differences in the capacitor bridge by a charge amplifier and an ADC. A

Fiber-Optic Gyros and MEMS Accelerometers 285

Fig. 15. Open accelerometer chip for B-290.

Fig. 16. Open B-290 accelerometer triad.

signal processor performs the linearization and steers the restoring by electro­static forces. The digital acceleration output of the processor is compensated in bias and scale factor over temperature. The electronics for a sensor are integrated within two small hybrids; three sensors and three electronic sets are built into a triad, as shown in Figure 16.

In a cost-reduction program, a mixed signal ASIC was developed for the charge amplifier, different voltage controls, and the ADC used for each sen­sor. Only one signal processor is used for three sensors, which dropped cost and power consumption of the B-290 triad. The new B-290 triad is shown in Figure 17.

Each B-290 triad is calibrated over temperature and scale factor and bias is compensated inside by the signal processor. Typical scale factor and bias repeatability over temperature are shown in Figures 18 and 19.

286 Andreas Rasch et a!.

2!D

100

Fig. 17. New design of the B-290 accelerometer triad.

ATP Bias Repeatability B-290PIN1<3062(AslcJ Samples: 21 TR IADS, 63 axes

Mean: 112 IJ9 1Sig.: 84 IJ9

IS 46 51 52 51 58 EE 68 75 76 77 84 95 ffi 9() 91 93 95 96 111 11:2

Se rial Humber

Fig. 18. Bias repeatability over temperature.

6 Inertial Measurement Unit

Three !!-FORS and one B-290 triad are the inertial sensors for an orthogonal inertial measurement unit (IMU); only the processor and perhaps a separate power supply has to be added according to the block diagram of Figure 7. Due to separation into single-axis FOGs, it is easy to adapt the housing for different geometric requirements. Normally, this packaging is carried out by the manufacturer, but this may also be done by the customer. As an example,

100

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Fiber-Optic Gyros and MEMS Accelerometers 287

ATP SF Repeatabil ity B.aOPIN14311Uf'!ilq Samples: 21 TRIADS, 63 axes

Mean: 192 ppm 1Sig · 58 ppm

-- f-

-r-

=-

IS 'S 51 ~2 51 58 68 68 7S 78 T1 84 8S 89 !:ll 91 1"3 i5 S6 111 U l

Fig. 19. Scale factor repeatability over temperature.

Fig. 20. Example of a modular measurement unit.

a packaged IMU is shown in Figure 20, with integrated IMU processor and power supply. The size is 13xllx7.5cm3 and the weight is l.lkg.

288 Andreas Rasch et al.

7 Conclusion

The chapter presents two generations of FOG systems by LITEF. The first FOG system generation consists of one light source split by a 3x3 coupler to three fiber coils. The second generation of FOG systems uses single-axis FOGs with an internal processor. These fiber-optic rate sensors can easily be calibrated separately in high quantity. The ~J.-FORS family is characterized by bias errors ranging from 6° /h to 0.05° /h. The different bias values are realized by adapting the fiber length on the coil. The other optical parts and the electronis are unchanged.

The mechanical sensor, including electronics, and the underlying technol­ogy are described by the example of the B-290 triad, a typical MEMS ac­celerometer product. Test data for bias repeatability and stability and scale factor accuracy before and after temperature compensation are presented.

Three ~J.-FORS and one B-290 triad are the inertial sensors for an orthog­onal IMU, only the processor and perhaps a seperate power supply has to be added.

References

1. Rasch, A. and Handrich, E., Applications of lithium niobate and potassium titanyl phosphate integrated optic devices, SPIE Proc. 3620, 152-160 (1999).

2. Kemmler, M. et al., Design of a commercial small-volume fiber-optic gyro, SPIE Proc. 2837, 92-97 (1996).

3. Spahlinger, G. et al., Error compensation via signal correlation in high precision closed loop fiber-optic gyros, Fiber-Optic Gyros: 20th Anniversary Conference, Session 3, Denver, 1996.

4. Kunz, J. et al., Design of an ASIC for a commercial small-volume fiber-optic gyro, Fiber-Optic Gyros: 20th Anniversary Conference, Session 3, Denver, 1996.

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