MEM Gyros for Space Applications

American Institute of Aeronautics and Astronautics1

MEMS gyro for space applications

Overview of European activities

Stéphane Dussy*

European Space Agency, Noordwijk – The Netherlands

Dick Durrant†

Systems Engineering & Assessment Ltd, Bristol – United Kingdom

Tony Moy‡

BAE Systems, Plymouth – United Kingdom

Nicolas Perriault and Bruno Célerier§

Alcatel Alenia Space, Cannes – France

The recent technological breakthrough in the area of Micro Electro Mechanical Systems(MEMS) have reached the stage where the MEMS rate sensors now present an undeniableinterest for use in space. However, in spite of their small size, low mass, low power and lowrecurring cost, together with their inherent immunity from shock and environmentalvibration and the lack of a wear out mechanism, the terrestrial off-the-shelf MEMS sensors,currently used for automotive applications, still require major technical adaptations to besuitable for space applications. Various design improvements to enhance the detectorperformance and to make the terrestrial electronics robust with respect to spaceenvironment, e.g. high radiation environment, are required. Recently, a series of feasibilityassessments showed that it is now feasible to develop a very low-mass, low-power and low-recurrent cost three-axis rate measurement sensor for space use based on terrestrial MEMStechnology. It has also been shown that there exists a large demand for such a unit in theworld space market, including telecom satellite applications. This paper describes the resultsof several recent ESA-funded market analyses and feasibility studies. Reliability andcharacterization testing of MEMS gyroscopes have also been completed. The paper alsogives the status of the recently initiated development of a European MEMS rate sensor forspace applications.

I. Introductionicro-Electro Mechanical Systems (MEMS) technology progressed rapidly during the last decades. It consistsin the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate

through micro fabrication techniques. A new degree of effective use of silicon area was achieved by implementingDeep Reactive Ion Etching (DRIE), combined with Silicon On Insulator (SOI) wafers. The DRIE process enablescompletely vertical cuts through the silicon wafer without limitations of silicon crystal orientation. As a result of this

* ESA, Postbox 299, 2200 AG Noordwijk, The Netherlands. [email protected]† SEA, Bristol Business Park, Coldharbour Lane, Bristol BS16 1SU, United Kingdom. [email protected]‡ BAE Systems, Clittaford Road, Plymouth PL6 6DE, United Kingdom. [email protected]§ Alcatel Alenia Space, 100 boulevard du Midi, 06150 Cannes, France. [email protected]

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Copyright © 2005 by European Space Agency. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


technology breakthrough, MEMS devices appeared more and more in many unrelated fields such as biology,microelectronics or communication, through a wide range of heterogeneous applications, such as filters, relays,optical switches, inductors, tunable capacitors or even projection display. However it is the application area ofinertial sensors that has benefited the most from many of the technologies developed for the MEMS. Whereas theirmass and size have been drastically reduced, MEMS accelerometers and gyroscopes continuously enhanced theirperformance.

In the 90’s, the automotive industry first recognized the benefits of MEMS devices and first commercializedthem on cars, e.g. using MEMS accelerometers in car airbags. Other automotive applications such as vehicle chassisstabilization, safety systems, traction control systems, rollover detection and navigation systems, consolidated theincreasing need for reliable, low-cost angular rate sensors. Boosted by this industry, low-cost MEMS accelerometersand gyroscopes are now in large-volume production. They incorporate many functions and their enhancedperformance, combined with their inherent reliability, came to a stage where these devices present an undeniableinterest for a space use.

Various techniques have been developed and manufactured to design silicon and quartz MEMS rate sensors:excitation and detection mechanisms rely either on piezoresistive, piezoelectric, capacitive, inductive or electrostaticmechanisms; vibrating beams, vibrating plates, tuning forks and vibrating shells have been deeply analyzed andtested in many research and development projects. Extensive literature1 depicts the various techniques andmechanisms, and addresses their major advantages and drawbacks.

As described in Fig. 1 for a vibrating shell configuration, the principle of most of the detectors relies on avibrating element (vibrating resonator exciting the carrier mode), which, in the presence of an applied rotation rate,is subjected to Coriolis acceleration and causes the vibration of a resonant response mode. By sensing the secondaryvibration magnitude, the applied rotation rate can be measured.

Figure 1. Excitation and detection principles for a vibrating shell

Until recently, MEMS gyroscopes exhibited performance in the class of rate-grade inertial sensors, i.e.characterized by a bias stability better than 0.5 deg/s. This performance is far sufficient for automotive applicationsbut remains not suited for space applications. Thanks to the continuous improvement in the micromachiningtechniques, it is now believed that MEMS gyroscopes will be capable of providing tactical or even inertial-gradeperformance, i.e. a bias stability better than 1 to 10 deg/h. Although the performance of these devices are far lowerthan that required for inertial navigation systems, MEMS gyros are promising for failure detection in large satellitesor for attitude propagation and rate determination in microsatellites, telecom satellites, planets landers and rovers.


II. MEMS gyroscopes development in Europe

A. European actors and related technologiesOne of the most well-known space-qualified microgyroscopes, flying on Integral, Smart-1 and expected to fly on

Herschel-Planck, is the Systron Donner Quartz Rate Sensor (QRS), relying on a quartz double tuning fork, withpiezoelectric excitation and detection. It is built by etching the structures from single-crystal quartz wafers QRS.ITAR (International Traffic in Arms Regulations) restrictions risk and limited performances of such a device,combined with the technical breakthrough in fabrication techniques with silicon (replacing the quartz), resulted inthe necessity of initiating the development of a European MEMS rate sensor.

As detailed in Table 1, several European companies currently develop or manufacture terrestrial MEMS ratesensors, with various characteristics depending on the sensor structure (vibrating beams, vibrating plates, tuningforks and vibrating shells) and the excitation / detection mechanisms (piezoresistive, piezoelectric, capacitive,inductive or electrostatic).

Company Country Name Technology, mechanismsBAE Systems UK SiRRS-01 All-silicon vibrating shell, inductive excitation and detection, bias

repeatability around 0.1 deg/sBosch Germany Poly-silicon tuning fork of vibrating plates, inductive excitation and

detectionEADS CRC Germany Tuning fork, with piezoelectric excitation and piezoresistive detectionHSG-IMIT Germany All-silicon vibrating plates, electrostatic excitation, capacitive detection,

short term drift around 25 deg/hLitef Germany µCORS-2 All-silicon tuning fork of vibrating plates, electrostatic excitation,

capacitive detectionSagem SA France Star Two-axis, capacitive detectionSensoNor Norway SAR10 All-silicon vibrating plates, electrostatic excitation, capacitive detection,

noise density around 0.013 deg/s/√HzThales France Gyrosil All-silicon, short term drift around 40 deg/h

Table 1. European MEMS rate sensor main manufacturers

Performances are provided as they appear in the sensors’ datasheet and are given to get a rough order ofmagnitude of the MEMS features. However, it is worth mentioning at this point that drift, bias stability, biasinstability, and thermal sensitivity are not addressed the same way by the various MEMS gyro manufacturers.Therefore, such performances cannot be compared as is, and require to be confirmed through characterizationtesting.

B. Reliability testingReliability testing has been performed on a small quantity of BAE Systems SiRRS-015 devices, under ESA

contract.4 The batch of devices underwent a complete reliability testing campaign performed by CNES, includingthermal cycling, thermal shocks, thermal vacuum, vibration, shock, humidity testing. Between each test, basicperformance characterization allows detecting any anomaly, such as fracture or delaminating. As shown in Fig. 2,most of the initial bias and scale factor characterizations, performed by EADS Astrium SAS, exhibit similar resultsas the ones predicted in SiRRS-01 datasheet, e.g. for bias and scale factor thermal sensitivity.


Figure 2. Bias and scale factor thermal sensitivity on BAE Systems SIRRS-01

Figure 3 presents Allan variance plot for one of the tested gyros, exhibiting an Angular Random Walk (ARW)close to the specified 0.2 o/√h. However, some of the gyros procured from different batches can exhibit noticeablydifferent behaviors. Construction analyses have been performed on a couple of devices, providing technical lead toexplain the observed different behaviors. These analyses, including radiographic inspection, leak tests, ParticleImpact Noise Detection (PIND) tests, Scanning Electron Microscope (SEM) tests, material analysis and bondstrength tests, allowed highlighting several recommended changes or checks for the manufacturing processes tocomply with the space quality requirements.

Figure 3. Allan Variance plots on BAE Systems SIRRS-01

Performance characterization testing has also been initiated with Litef MEMS gyroscope µCORS-2. Scale factor,noise, bias stability, repeatability and thermal sensitivity are currently characterized on Litef devices, followingspace standard.


III. MEMS gyroscopes for space applications

A. Market analysisA market survey, aimed at investigating the application potential of MEMS gyroscopes, has been completed,2,3

considering Geostationary Earth Orbit (GEO) platforms, science missions, Low Earth Orbit (LEO) Earthobservation, microsatellites and Galileo constellation.

In order to quickly identify the space applicability of MEMS gyroscopes, the key performance parameterAngular Random Walk is used to locate the BAE Systems SiRRS-01 performance5 within the various space gyroclasses. As described in Fig. 4, two gyro classes have been identified as potential targets for this MEMS gyropackage covering the functions of anomaly detection, initial acquisition and anomaly recovery / safe mode3:

Figure 4. Gyro requirements versus mission phases

Depending on the satellite applications, the associated key objectives of the MEMS rate sensor can vary:- failure detection in large satellite for Earth observation or in Science applications. The key objectives that

could pull the market is a low mass, cost and power- attitude propagation and rate determination in micro-satellites for Earth observation, GEO platforms,

launchers, planets landers and rovers. The key objectives that could pull the market is increasedperformances on bias stability (better than 1 deg/h, ideally close to 0.1 deg/h), on ARW and on scale factorknowledge when gyro is used during a slew

The telecom satellite market is seen as the most credible near-term source of sales whereas the Earth observationmicrosatellite is considered as the most likely growth market for the future


B. Telecom applications1. Spacebus4000 telecom platform

In most of the telecom satellites design, dedicated to GEO platforms, gyroscope is mainly used for:- rate damping, sun search rotation, sun pointing and spin rate control in safe mode,- spacecraft reorientation required by the mission,- spacecraft rate control during orbit raising chemical boosts at high thrust level,- angular attitude estimation if no optical sensor measurement is available,- large angular bias in normal mode, e.g. for antenna mapping.

In the frame of the Alcatel Alenia Space telecom platform (Spacebus4000) development with star trackerreplacing earth sensor, a preliminary estimation of the required gyro performance has been completed, which led tothe need for an intermediate-class gyro. Nevertheless, the acceptable performance is very often a matter ofcompromise between several constraints and more or less complex and cost-effective solutions.

Therefore, in order to assess these compromises, a study has been initiated, which aimed at deriving a set of gyroperformance specifications that would make the product compatible with Spacebus4000 platforms application. Thisstudy assessed the performance impacts of an intermediate-class gyro on both the baseline Spacebus4000 Attitudeand Orbit Control System (AOCS) and the following options:

- star tracking assistance using gyros, - attitude determination based on Earth sensor and gyros,- AOCS with no Earth sensor, assessment of attitude degradation with time, using gyros only,- ORM using gyros only,- ORM with low-thrust plasmic propulsion.

2. Sun Acquisition ModeBased on both detailed analysis and dynamic simulations using in-house AOCS simulator, it came to the

conclusion that the MEMS gyro could be used in safe mode (so-called SAM for Sun Acquisition Mode) without anymajor modification of the AOCS or system design. The only noticeable side effect is a reduction of the target spinrate range: the minimum allowed spin rate shall be 0.5°/s with the MEMS gyro-based design instead of 0.1°/s withthe current design.

Figure 5 shows the simulation of the different sub-phases of the SAM, using MEMS gyros. The sun search issuccessful within the required duration and the final performances are guaranteed.


Sun pointing errorSun searchrotation axis

stability

Sun search successful

SAM in steady state

1st sun search rotation

2nd sun search rotation

Fuel consumption stabilised

Figure 5. Spacebus 4000 Sun Acquisition Mode behaviour with MEMs gyros

3. Inertial Attitude Acquisition ModeIn the reorientation mode (so-called IAAM for Inertial Attitude Acquisition Mode), the performances of the

MEMS gyro-based configuration are compliant with the AOCS requirements. However, a modification at systemlevel must be envisaged: in case the star tracker loses tracking during Van Allen belts crossing, the attitude keepingphase only relies on gyros until the star tracker recovers its tracking mode. Therefore, the pointing accuracyspecification for the attitude keeping phase during Van Allen belts crossing shall be relaxed and adapted to theMEMS gyros capabilities, since during this phase nothing can prevent the attitude from drifting with the gyro bias.This modification is deemed acceptable at system level. At the end of the Van Allen belts crossing with only MEMSgyros, the pointing errors induced by the gyro bias will be corrected by the controller, thanks to the star trackermeasurement updates.

4. Orbit Raising ModeIn the Orbit Raising Mode (ORM), the performances of the MEMS gyro-based configuration are compliant with

the Spacebus4000 AOCS pointing requirements. The most noticeable impact is a reasonable increase of the fuelconsumption due to the high level of gyro noise. However, the analyses showed that this impact can be limited byimplementing an adequate filtering of the measurements.


These first Telecom platform modes (SAM, IAAM and ORM) correspond to the baseline configuration ofSpacebus4000. In a second step, it has been analysed how a low-class gyro on-board the Telecom platform cansupport some additional functionalities or can ease some modes and operations:

5. Star tracking assistance with MEMS gyros (in Normal Mode)The star tracking assistance consists in providing a quaternion to the star tracker while in acquisition mode, in

order to assist it in switching to the tracking mode. This function is not implemented in the Spacebus4000 AOCSdesign, although the selected star tracker presents this assistance capability. Two different configurations werestudied, depending on the star tracker availability, the Failure Detection Isolation and Recovery (FDIR)requirements and the gyro performances:

- permanent use of gyro (gyro-stellar hybridisation),- non-permanent use of gyro (the gyro is dedicated to the star tracking assistance only in case of loss of

tracking.The MEMS gyro performance allows assisting the star tracking, even if the capability offered by such a sensor is

limited in terms of duration. MEMS gyros support can also be envisaged for acquisition assistance, once theduration for helped tracking is elapsed, to perform a reduction of the star catalogue of the star tracker on the basis ofthe gyro integrated rates.

6. Orbit Raising Mode with MEMS gyrosIn the baseline Spacebus4000 AOCS design, for large Delta-V manoeuvres using the 400N apogee boost motors,

the ORM uses both star tracker (attitude determination) and gyro (rate estimation). An interesting option is toconsider whether this manoeuvre can be performed with the gyros only. A large delta-V manoeuvre was simulatedwith the only MEMS gyro in order to assess the impacts in terms of manoeuvre accuracy and fuel consumption. Theresult was very positive and the mission analysis requirements were respected with the MEMS gyro option, with norestriction on the boost duration and with a negligible fuel consumption increase.

7. Attitude determination with Earth sensor and MEMS gyros (in Normal Mode)The baseline Normal Mode provides the satellite customer with the back-up possibility to use the gyro with the

Earth sensor; in such a degraded configuration, yaw is controlled with the gyro (instead of the star tracker), and rolland pitch are controlled with the Earth sensor. However, because of the yaw attitude drifts induced by the gyro bias,the maximum duration of utilisation of this back-up configuration is limited to maximum periods of 2000s, in orderto comply with the 1deg pointing requirement in yaw.

8. Normal Mode with MEMS gyros only (no Earth sensor)It is also possible to operate the Normal Mode in a degraded back-up configuration with only gyros (neither

Earth sensor, nor star tracker). As for the previous option, because of the attitude drift induced by the gyro bias, themaximum duration of utilisation of this degraded back-up configuration is limited to maximum periods of 1700s, inorder to comply with the 1deg pointing requirement pointing. However, it has to be noted that such a degradedpointing accuracy leads to the interruption of the mission and to the switching of the payload in the mute mode.

9. Orbit Raising Mode with plasmic propulsionThe Spacebus4000 AOCS also offers the possibility to use the low-thrust plasmic propulsion in order to

complete the ORM from Geostationary Transfer Orbit (GTO) to GEO. During this long manoeuvre, which can last afew weeks, a new orbit topping mode shall be implemented, based on the current IAAM with two majormodifications:

- the AOCS actuation shall be realised with the reaction wheels,- a new attitude determination function shall be implemented, relying in a Kalman filter hybridisation of

star tracker and gyro measurements.This orbit topping mode is compatible with the use of MEMS gyro, although any loss of tracking during the

boost limits its duration.


IV. European MEMS rate sensor development

A. Three-axis micro gyroscope feasibility studyIn 2002, Astrium UK completed an initial feasibility assessment for a space qualified microgyroscope, under an

ESA contract.3 This study was based on the off-the shelf BAE Systems SiRRS-01.5 Preliminary set of technicalrequirements for a MEMS gyro package suited for space applications has been derived and a first concept design hasbeen proposed.

The main features of the space qualified three-axis MEMS gyro package have been identified based on thefollowing electrical interface design:

Figure 6. Electrical interface design

As described in Fig. 7, an initial design for the microgyroscope packaging layout was also produced. During thisprocess, various layouts were considered, exhibiting various options for accommodation of the main componentsand for structural architectures. In order then to identify the baseline package layout, a number of technical andfeasibility related selection criteria was defined, which allowed completing the trade-off. The selected baselinepackage concept includes a main central body (rate sensor assembly in Aluminum) that holds the three rate sensorsand provides support for the various PCBs. The rate sensors are mounted inward and covered by PCBs. The powersupplies are mounted on one external face of the rate sensor assembly. An external casing (in Aluminum) thenencloses the assembly.


Figure 7. Microgyroscope assembly

Preliminary stress, thermal, radiation and reliability analyses have been performed that allowed consolidating thecompliance of the proposed design with respect to the expected mission requirements.

B. MEMS rate sensor developmentFollowing the market and feasibility analysis, complemented by the reliability and performance characterization

testing presented in previous sections, it has been recently decided to initiate the development of a European MEMSrate sensor for space applications. The principle is to improve a commercial off-the-shelf MEMS detector, then todesign, manufacture and test both a detector prototype and an electronics breadboard.

The target specifications for such a development are:

Configuration 3-axis, rate or integration modeMass < 0.6 kgPower consumption (in nominal conditions) < 3.5 WBias stability (3σ) over 24 hours, ∆T < ±10oC < 5 deg/hAngular Random Walk < 0.2 deg/√hRange Up to 20 deg/sInterface RS-422 or MIL-STD-1553BMission 18 years in GEO

Table 2. MEMS rate sensor target specifications

The MEMS Rate Sensor (MRS) development has been initiated under the lead of Systems Engineering andAssessment Ltd (Bristol), who are also directly responsible for the unit electronics development, with the MEMSdetector being provided by BAE SYSTEMS (Plymouth) based on their highly successful automotive/militaryMEMS devices. The development team also includes SELEX Sensors and Airborne Systems Ltd (Edinburgh) whoare addressing the unit level packaging and sensor production aspects.

The current BAE SYSTEMS, inductive, sensor is illustrated in Fig. 8 showing the MEMS ring structure withsupporting legs, together with a photograph of the unpackaged sensor. More than 7 million of these devices havebeen produced to date and this wealth of MEMS production experience forms the basis for the planned detectorlevel improvements to meet the target rate sensor specifications.


Figure 8. BAE SYSTEMS (Inductive) MEMS Detector

Some well-known technical issues are anticipated: vibratory rate microgyroscopes are highly sensitive tospurious oscillations arising from geometric imperfections. The main obstacles in the MEMS rate sensor design relyin the device fabrication, interface and readout circuitry, assembly and packaging, and testing. Keeping the recurringprice as low as possible is a target that will drive both the parts selection and the gyro package design and testing.

More specifically the main technical design drivers are deemed to be:1. Robust vacuum-packaging techniques, allowing a higher quality factor and lower damping,2. Material uniformity (all-silicon), mixed micromachining fabrication processes on silicon and deep etching

techniques (such as DRIE), combined with SOI technology, ensuring compatibility with ComplementaryMetal Oxyde Semiconductor (CMOS) technology and high quality factor of the structures (less vulnerableto sticking and geometry variation), accurate scale factor over temperature and low stray capacitance,

3. Control loop design- measuring and controlling magnitude and phase of carrier and response modes,- tuning resonant frequencies and ensuring the best matching between resonant frequencies of carrier

(excitation) and response (sensing) modes,- allowing compensation of defects, such as mismatches in suspension and proof mass dimensions,

electrode gaps, asymmetries in fluidic damping, responsible for drifts and long-term effects,- allowing identification of key parameters for an accurate calibration,- maintaining excitation and detection linearity and stability, by tuning adequate force feedback,- controlling carrier mode quadrature errors, by accurately measuring the phase of the excitation mode,

4. Sensing mechanism, e.g. capacitive detection allowing lower thermal sensitivity and better resolution,5. Low-noise and low-drift interface electronic circuitry.

Furthermore the migration of the technology to space brings additional drivers. For example availability ofelectronic devices that can provide the required performance while meeting the space radiation environment for an18 year mission. This is a particularly critical trade-off with fully space hardened components often being expensiveto procure and potentially high power consuming devices. Also the required lifetimes are higher than current MEMSapplications and there will be a need to design in long term calibration measurement and control to ensure end-of-life performance (mitigating some of the effects discussed above).

Despite these constraints and drivers, it has been found from the modeling performed to support the detectorimprovements study, leading up to the detector Preliminary Design Review (PDR), that the target specificationappears achievable. Further this should be possible with an evolution of either the current inductive detector, or theemerging capacitive detector, making full of the established MEMS production processes. The next stage in thedevelopment, post PDR, is the prototyping of the selected improved detector concept and the breadboarding of thekey electronic circuits.


V. ConclusionThe paper presented the results of several market analyses and feasibility studies showing that a consequent

demand exists for a three-axis MEMS rate sensor in space applications, and that it is now feasible to develop such adevice, with a low-mass, low-power and low-recurring cost target. The paper also described the results fromreliability and characterization testing on various off-the-shelf MEMS rate sensors. A series of design improvementshave been defined in order to enhance the off-the-shelf detector performance and to develop radiation hardenedelectronics, so that the MEMS rate sensor is suitable for space. Following the conclusions from this series ofanalyses and testing, an ESA-funded development of a three-axis MEMS gyro package has been initiated, leading toa preliminary design of the sensor.

AcknowledgmentsThe authors would like to acknowledge the colleagues who contributed to the MEMS reliability studies, market

analysis and characterization testing, in particular Ph. Airey, L. Marchand (ESA), F. Pressecq (CNES), J. Busseuil(Alcatel Space), S. Barthe, G. Cros, (EADS Astrium), and teams from Litef, Astrium UK, SEA Ltd, BAESYSTEMS and SELEX Sensors and Airborne Systems Ltd.

References1Yazdi, N., Ayazi, F., and Najafi, K., “Micromahined Inertial SensorsA HARPSS Polysilicon Vibrating Ring Gyroscope,”

Proceedings of the IEEE, Vol. 86, no.8, August 1998, pp. 1640-1659.2Olivier, B., “MEMS Gyro Market & Feasibility Study,” SEA/02/TR/3910, Iss.1. November 2002.3Prezzavento, A., “Micro Gyroscope Feasibility Study,” GNC/TNO/2056, Iss.1 under ESA contract 16331/02/NL/LvH.

November 2002.4Barthe, S., Pressecq, F., and Marchand, L., “MEMS for space applications: a reliability study,” 4th Round Table on

Micro/Nano Technologies for Space, ESA, May 2003.5Pearce, C., “The performance and future development of a MEMS SiVSG and its application to the SiIMU,” AIAA

Guidance, Navigation and Control Conference and Exhibit, Montreal, Canada, August 2001.

MEM Gyros for Space Applications

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