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Introductionthis article serves as a continuation of the previous two articles in sensors magazin[1],[2] on the topic of mems-based systems solutions (mBss). mBss employ single or multiple mems front-end devices including sensors, actuators and/or structures that work in conjunction with electro-nic signal conditioning circuits using embedded controllers with software algorithms, energy harvesting/storage devices, and back-end networking devices all interconnected and packaged. the creation of such a solu-tion is firmly based on the principles of systems engineering, software co-design, reliability analysis and calib-ration/testing.we are familiar through extensive media exposure to accelerometers and gyros for mobile phone applica-tions. these devices have uniquely propelled mems into the commercia-lization mainstream and now achieve top billing for all mems applications thus displacing automotive as the lea-ding application for mems. regrettab-ly however for their producers, these devices sell for approximately $ 0.5 Us (0.33 euro) for three-axis accelerome-ters and $ 2.00 Us (1.3 euro) for three-axis gyros thus pushing profit margins to exceptionally low levels. Compa-nies participating in this application include Bosch, freescale, memsIC, Invensense and stmicroelectronics, silicon Designs, meggitt sensing sys-

tems/endevco and silicon sensing.however, this article will address con-siderations/technologies and repre-sentative products currently available in the market that serve higher-perfor-mance and/or rigorous environmental applications of accelerometers and gyros including oil/gas exploration, structural health monitoring, industri-al control, and guidance/navigation for military and commercial systems es-pecially guided munitions.

Packaging / System Co-Design ConsiderationsProf. Khalil Najafi of the University of michigan who is widely acknowledged as an early investigator of this techno-

logy has co-authored a seminal paper on the evolution of high performance accelerometers and gyros[3]. he stressed that to achieve high per-formance inertial sensors, three fac-tors needed to be considered 1) the sensing mechanism 2) the associated signal conditioning electronic circuits and 3) the package and the sensor / package interface. to support this po-sition, ePack (www.epack.com), a Uni-versity of michigan spin-off has deve-loped a unique packaging strategy to overcome the problems associated with the operation of accelerometers and gyros in extreme vibration/shock and thermal environments. Dr. Jay mitchell, ePack Ceo stated 'our approach to packaging mems gyros and accelerometers minimizes issues including package stress, mate-rial property changes associated with temperature change by mechanically isolating the sensor from the package and the shock and vibration environ-ment and additionally creating the condition of a low and fixed pressure in short, the package controls the environment' (fig. 1).stated Dr. maryAnn maher, Ceo of software provider softmems 'the abi-lity of the designer to better under-stand the interaction of the package with the mems device(s) is critical to optimize the performance of the solu-tion. this concept called co-design has been adopted by many accelero-meter and gyro designers to optimize the device/package system'.

Electronic Circuit Considerationselectronic circuits, especially applica-tion specific integrated circuits (AsICs) are another key element in the mBss approach as applied to high- perfor-mance accelerometers and gyros.

mems-Based systems solutions: high-Performance Accelerometers and Gyros

Analytical instruments offer a great opportunity for MEMS- based systems solu-tions, as introduced in the Sensor Magazin 2-2011[1]. The inherent benefit of MEMS being small, robust, low power and low cost along with surprisingly high performance make them ideally suited to hand-held and portable/field useable instruments. These instruments find their way into a broad assortment of appli-cations for solid, liquid and gas analysis including environmental monitoring, process control quality assurance, homeland security and defense.

▲ ��Fig. 1: Schematic of patented environ-mental resistant package (ERP) which adds value through: i) vibration isola-tion ii) thermal isolation and precise oven control and iii) hermetic/vacuum encapsulation.

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they are widely used to provide cir-cuit functions including A-D conversi-on, excitation/monitoring and control of the position of the device structu-res, built-inself test and calibration. for example si-ware systems (www.siwaresystems.com) has specialized in creating AsICs that address the high-performance accelerometer and gyro products.

Application Examplesthere are many excellent examples of mBss in high-performance accele-rometer and gyro solutions. some of the applications that these products address are shock measurements for

auto crash testing dummy instrumen-tation, inertial measurement of plat-form stability in industrial applica-tions, vibration for health monitoring of machinery and civil engineering structures as well as inertial systems guidance and navigation for avionics. Companies involved in providing the-se products include Goodrich, honey-well, silicon sensing systems, senso-nor and Vtt.Colibrys (www.colibrys.com) provides both open loop (fig. 2) and closed loop accelerometers employing a bulk micromachined capacitive ap-proach. the targeted applications for these devices includes tactical grade

inertial applications, platform stabili-zation, industrial tilt applications and structural health monitoring. to face the future market requirements and offer solutions to support tactical gra-de applications, they have developed a new digital sensor using a high-order sigma-delta loop approach. stated mr. sean Neylan, Colibrys Ceo 'this tech-nique can dramatically improve the sensor nonlinearity and noise level. tests have determined that this ap-proach can achieve a noise floor of 30 ng/square root hz. It also demonstra-tes a noise level of 1.7 ug per square root hz over a 300 hz bandwidth. Long term stability drift measure-ments over 60 hours demonstrates a value of +/-100 ug (+/-10 ppm) in ab-sence of temperature control or algo-rithm compensation. Additionally, mems closed-loop acce-lerometers can substantially reduce the sensitivity to manufacturing tole-rances. tests show that mems accele-rometers which operate in such a high-order sigma-delta closed-loop format have the potential to match the per-formance of expensive amorphous quartz macro electro-mechanical sen-sors and even outperform them in noi-se, robustness and cost."h. P. (www.hp.com) has recently intro-duced a very high- performance acce-lerometer (which measures minute vibrations) which is the “heart” of their wireless autonomous sensor node sys-tem used for land-based oil and gas exploration being developed with shell oil. the novel accelerometer design has exceptional specifications including a sensitivity of <100 ng/square root hertz, >130 dB dynamic range and 0-250 hz. bandwidth in a 7 mm x 7 mm package. At the present time, h. P. engineers are working on integrating the accelerometer with all the other elements in the mBss for use in multiple element autonomous networked sensing applications. h. P. has named this initiative the 'Central Nervous system of the earth (CeNse) and sees future applications to include monitoring of buildings,

▲ ��Fig. 2: MS9000 open-loop accelerometer using a Silicon MEMS bulk micromachined capacitive approach. Shown are the sensor chip, low-power ASIC for signal conditio-ning, temperature sensor and a microcontroller for storage of compensation values.

ANZEIGE

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dams, levees, pipelines and other engineering structures to assess real time structural health status as well as unattended ground sensing including road vibration monitoring for traffic monitoring and control as well as bor-der security. however, h. P. has no interest in selling the accelerometer but rather has established a business approach through which it will solve customers’ applications by providing high value added services vis-à-vis a computational cloud thus providing valuable information to its customers.tronics (www.tronics.com) has over a 10 year history providing high-perfor-mance silicon mems accelerometers and gyros to a wide variety of deman-ding applications including military/aerospace. typical performance of their GYPro product includes gyro based instability (from Allan variance) <0.1 degree/hr. with overall stability of <10 degree/hr. over the full operating temperature range (fig. 3). According to mr. herve Borrel, Direc-tor of marketing 'tronics key ingre-dients to achieving high-performance in our inertial sensors is based on our use of single crystal silicon-on-Insula-tor (soI) technology which provides better stability and repeatability. the device is wafer level packaged in a high and stable vacuum environment. this is combined with a high perfor-mance AsIC which minimizes noise and provides complex signal functions that drive the proof mass and perform proper signal corrections'. the Us Department of Defense (DoD) has a high level of interest in low-cost/high performance inertial systems for use in various missions including dro-ne aircraft navigation/guidance and guided munitions. major DoD con-tractors including Draper Labs, Boe-ing, Lockheed-martin, and Northrop Grumman are pursuing the applica-tion of mems to achieve critical missi-on requirements. the Defense research Projects Agen-cy (DArPA), an agency of the DoD is currently funding three projects who-se goal is to develop inertial sensors

and systems. these projects include Navigation-Grade Integrated micro-Gyroscopes (NGImG), micro Inertial Navigation technology (mINt) and In-formation tethered micro Automated rotary stages (It-mars). According to Dr. Andrei shkel, Program manager in the DArPA microsystems technology office (mto) 'we currently don’t have a complete answer to the question of why does making inertial instruments and clocks small necessarily lead to degradation in performance. however, we are working hard to disprove the convention that high-performance inertial micro instruments are a cont-radiction in terms[4].

Summary / Conclusionsmany different applications exist for high performance accelerometers and gyros and our research has demonst-rated that manufacturers can achieve enhanced profit margins over those associated with low-performance de-vices that address the mobile phone market by providing value added functions. however, extreme care must be taken to select and employ packaging and signal conditioning

electronics compatible with enhanced performance levels.

Want to know moreroger Grace will present a paper and organize a session on 'smart systems Integration in the Us' at the smart sys-tems Integration Conference 2012 that will take place in Zurich switzerland on march 21 / 22, 2012 (www.smartsys-temsintegration.com).

references:1 r. Grace, 'think outside the Chip: mems-

Based systems solutions', seNsor mAGA-ZIN, march 2011.

2 r. Grace, 'mems-Based systems solutions: Analytical Instruments for Gas Analysis', seNsor mAGAZIN, september 2011.

3 N. Yazdi, f. Ayazi, K. Najafi, 'micromachined Inertial sensors', Proceedings of the Ieee, Vol. 86,No. 8, pp. 1640-1659.

4 A. m. shkel, 'microtechnology Comes of Age', GPs world, september 1, 2011.

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Author:roger Grace, President of roger Grace Associates, florida UsA Correspondent, seNsor mAGAZINemail: rgrace@rgrace.comwww.rgrace.com

▲ ��Fig. 3: GYPRO high performance gyro showing Silicon MEMS gyro chip (6 mm. x 7 mm.) and ASIC in a ceramic package.