AIAA 2005-6466
AIAA Guidance, Navigation, and Control Conference and Exhibit 15 - 18 August 2005, San Francisco, California
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 undeniable interest for use in space. However, in spite of their small size, low mass, low power and low recurring cost, together with their inherent immunity from shock and environmental vibration 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 be suitable for space applications. Various design improvements to enhance the detector performance and to make the terrestrial electronics robust with respect to space environment, e.g. high radiation environment, are required. Recently, a series of feasibility assessments showed that it is now feasible to develop a very low-mass, low-power and lowrecurrent cost three-axis rate measurement sensor for space use based on terrestrial MEMS technology. It has also been shown that there exists a large demand for such a unit in the world space market, including telecom satellite applications. This paper describes the results of several recent ESA-funded market analyses and feasibility studies. Reliability and characterization testing of MEMS gyroscopes have also been completed. The paper also gives the status of the recently initiated development of a European MEMS rate sensor for space applications.
I.
Introduction
M
icro-Electro Mechanical Systems (MEMS) technology progressed rapidly during the last decades. It consists in 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 implementing Deep Reactive Ion Etching (DRIE), combined with Silicon On Insulator (SOI) wafers. The DRIE process enables completely 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] †
1 American Institute of Aeronautics and Astronautics 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 of inertial sensors that has benefited the most from many of the technologies developed for the MEMS. Whereas their mass and size have been drastically reduced, MEMS accelerometers and gyroscopes continuously enhanced their performance. In the 90’s, the automotive industry first recognized the benefits of MEMS devices and first commercialized them on cars, e.g. using MEMS accelerometers in car airbags. Other automotive applications such as vehicle chassis stabilization, safety systems, traction control systems, rollover detection and navigation systems, consolidated the increasing need for reliable, low-cost angular rate sensors. Boosted by this industry, low-cost MEMS accelerometers and gyroscopes are now in large-volume production. They incorporate many functions and their enhanced performance, combined with their inherent reliability, came to a stage where these devices present an undeniable interest 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 electrostatic mechanisms; vibrating beams, vibrating plates, tuning forks and vibrating shells have been deeply analyzed and tested in many research and development projects. Extensive literature1 depicts the various techniques and mechanisms, 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 a vibrating 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 secondary vibration 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 applications but remains not suited for space applications. Thanks to the continuous improvement in the micromachining techniques, it is now believed that MEMS gyroscopes will be capable of providing tactical or even inertial-grade performance, i.e. a bias stability better than 1 to 10 deg/h. Although the performance of these devices are far lower than that required for inertial navigation systems, MEMS gyros are promising for failure detection in large satellites or for attitude propagation and rate determination in microsatellites, telecom satellites, planets landers and rovers.
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II.
MEMS gyroscopes development in Europe
A. European actors and related technologies One 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, with piezoelectric 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 in the 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 rate sensors, with various characteristics depending on the sensor structure (vibrating beams, vibrating plates, tuning forks and vibrating shells) and the excitation / detection mechanisms (piezoresistive, piezoelectric, capacitive, inductive or electrostatic). Company BAE Systems Bosch EADS CRC HSG-IMIT Litef Sagem SA SensoNor Thales
Country UK
Name SiRRS-01
Technology, mechanisms All-silicon vibrating shell, inductive excitation and detection, bias repeatability around 0.1 deg/s Germany Poly-silicon tuning fork of vibrating plates, inductive excitation and detection Germany Tuning fork, with piezoelectric excitation and piezoresistive detection Germany All-silicon vibrating plates, electrostatic excitation, capacitive detection, short term drift around 25 deg/h All-silicon tuning fork of vibrating plates, electrostatic excitation, Germany µCORS-2 capacitive detection France Star Two-axis, capacitive detection Norway SAR10 All-silicon vibrating plates, electrostatic excitation, capacitive detection, noise density around 0.013 deg/s/ Hz 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 of magnitude of the MEMS features. However, it is worth mentioning at this point that drift, bias stability, bias instability, 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 characterization testing.
B. Reliability testing Reliability 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, including thermal cycling, thermal shocks, thermal vacuum, vibration, shock, humidity testing. Between each test, basic performance 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 results as the ones predicted in SiRRS-01 datasheet, e.g. for bias and scale factor thermal sensitivity.
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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 noticeably different behaviors. Construction analyses have been performed on a couple of devices, providing technical lead to explain the observed different behaviors. These analyses, including radiographic inspection, leak tests, Particle Impact Noise Detection (PIND) tests, Scanning Electron Microscope (SEM) tests, material analysis and bond strength tests, allowed highlighting several recommended changes or checks for the manufacturing processes to comply 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, following space standard.
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III.
MEMS gyroscopes for space applications
A. Market analysis A 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) Earth observation, microsatellites and Galileo constellation. In order to quickly identify the space applicability of MEMS gyroscopes, the key performance parameter Angular Random Walk is used to locate the BAE Systems SiRRS-01 performance5 within the various space gyro classes. As described in Fig. 4, two gyro classes have been identified as potential targets for this MEMS gyro package 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 increased performances on bias stability (better than 1 deg/h, ideally close to 0.1 deg/h), on ARW and on scale factor knowledge 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 observation microsatellite is considered as the most likely growth market for the future
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B. Telecom applications 1. 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 tracker replacing earth sensor, a preliminary estimation of the required gyro performance has been completed, which led to the need for an intermediate-class gyro. Nevertheless, the acceptable performance is very often a matter of compromise 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 gyro performance specifications that would make the product compatible with Spacebus4000 platforms application. This study assessed the performance impacts of an intermediate-class gyro on both the baseline Spacebus4000 Attitude and 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 Mode Based 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 any major modification of the AOCS or system design. The only noticeable side effect is a reduction of the target spin rate range: the minimum allowed spin rate shall be 0.5°/s with the MEMS gyro-based design instead of 0.1°/s with the current design. Figure 5 shows the simulation of the different sub-phases of the SAM, using MEMS gyros. The sun search is successful within the required duration and the final performances are guaranteed.
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2nd sun search rotation 1st sun search rotation
SAM in steady state
Sun search successful
Sun search rotation axis stability
Sun pointing error
Fuel consumption stabilised
Figure 5. Spacebus 4000 Sun Acquisition Mode behaviour with MEMs gyros 3. Inertial Attitude Acquisition Mode In 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 system level must be envisaged: in case the star tracker loses tracking during Van Allen belts crossing, the attitude keeping phase only relies on gyros until the star tracker recovers its tracking mode. Therefore, the pointing accuracy specification for the attitude keeping phase during Van Allen belts crossing shall be relaxed and adapted to the MEMS 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 MEMS gyros, the pointing errors induced by the gyro bias will be corrected by the controller, thanks to the star tracker measurement updates. 4. Orbit Raising Mode In 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 fuel consumption due to the high level of gyro noise. However, the analyses showed that this impact can be limited by implementing an adequate filtering of the measurements.
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These first Telecom platform modes (SAM, IAAM and ORM) correspond to the baseline configuration of Spacebus4000. In a second step, it has been analysed how a low-class gyro on-board the Telecom platform can support 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 AOCS design, although the selected star tracker presents this assistance capability. Two different configurations were studied, 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 the duration for helped tracking is elapsed, to perform a reduction of the star catalogue of the star tracker on the basis of the gyro integrated rates. 6. Orbit Raising Mode with MEMS gyros In 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 to consider whether this manoeuvre can be performed with the gyros only. A large delta-V manoeuvre was simulated with the only MEMS gyro in order to assess the impacts in terms of manoeuvre accuracy and fuel consumption. The result was very positive and the mission analysis requirements were respected with the MEMS gyro option, with no restriction 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 roll and 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 order to 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, the maximum duration of utilisation of this degraded back-up configuration is limited to maximum periods of 1700s, in order to comply with the 1deg pointing requirement pointing. However, it has to be noted that such a degraded pointing 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 propulsion The 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 a few weeks, a new orbit topping mode shall be implemented, based on the current IAAM with two major modifications: - 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.
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IV.
European MEMS rate sensor development
A. Three-axis micro gyroscope feasibility study In 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 technical requirements for a MEMS gyro package suited for space applications has been derived and a first concept design has been proposed. The main features of the space qualified three-axis MEMS gyro package have been identified based on the following 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 this process, various layouts were considered, exhibiting various options for accommodation of the main components and for structural architectures. In order then to identify the baseline package layout, a number of technical and feasibility related selection criteria was defined, which allowed completing the trade-off. The selected baseline package concept includes a main central body (rate sensor assembly in Aluminum) that holds the three rate sensors and provides support for the various PCBs. The rate sensors are mounted inward and covered by PCBs. The power supplies are mounted on one external face of the rate sensor assembly. An external casing (in Aluminum) then encloses the assembly.
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Figure 7. Microgyroscope assembly Preliminary stress, thermal, radiation and reliability analyses have been performed that allowed consolidating the compliance of the proposed design with respect to the expected mission requirements.
B. MEMS rate sensor development Following 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 MEMS rate sensor for space applications. The principle is to improve a commercial off-the-shelf MEMS detector, then to design, 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 mode Mass < 0.6 kg Power consumption (in nominal conditions) < 3.5 W < 5 deg/h Bias stability (3 ) over 24 hours, T < ±10oC Angular Random Walk < 0.2 deg/ h Range Up to 20 deg/s Interface RS-422 or MIL-STD-1553B Mission 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 and Assessment Ltd (Bristol), who are also directly responsible for the unit electronics development, with the MEMS detector being provided by BAE SYSTEMS (Plymouth) based on their highly successful automotive/military MEMS devices. The development team also includes SELEX Sensors and Airborne Systems Ltd (Edinburgh) who are 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 with supporting legs, together with a photograph of the unpackaged sensor. More than 7 million of these devices have been produced to date and this wealth of MEMS production experience forms the basis for the planned detector level improvements to meet the target rate sensor specifications. 10 American Institute of Aeronautics and Astronautics
Figure 8. BAE SYSTEMS (Inductive) MEMS Detector Some well-known technical issues are anticipated: vibratory rate microgyroscopes are highly sensitive to spurious oscillations arising from geometric imperfections. The main obstacles in the MEMS rate sensor design rely in the device fabrication, interface and readout circuitry, assembly and packaging, and testing. Keeping the recurring price 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 Complementary Metal Oxyde Semiconductor (CMOS) technology and high quality factor of the structures (less vulnerable to 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 of electronic devices that can provide the required performance while meeting the space radiation environment for an 18 year mission. This is a particularly critical trade-off with fully space hardened components often being expensive to procure and potentially high power consuming devices. Also the required lifetimes are higher than current MEMS applications and there will be a need to design in long term calibration measurement and control to ensure end-oflife performance (mitigating some of the effects discussed above). Despite these constraints and drivers, it has been found from the modeling performed to support the detector improvements study, leading up to the detector Preliminary Design Review (PDR), that the target specification appears achievable. Further this should be possible with an evolution of either the current inductive detector, or the emerging capacitive detector, making full of the established MEMS production processes. The next stage in the development, post PDR, is the prototyping of the selected improved detector concept and the breadboarding of the key electronic circuits.
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V.
Conclusion
The 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 a device, with a low-mass, low-power and low-recurring cost target. The paper also described the results from reliability and characterization testing on various off-the-shelf MEMS rate sensors. A series of design improvements have been defined in order to enhance the off-the-shelf detector performance and to develop radiation hardened electronics, so that the MEMS rate sensor is suitable for space. Following the conclusions from this series of analyses and testing, an ESA-funded development of a three-axis MEMS gyro package has been initiated, leading to a preliminary design of the sensor.
Acknowledgments The 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, BAE SYSTEMS and SELEX Sensors and Airborne Systems Ltd.
References 1
Yazdi, 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. 2 Olivier, B., “MEMS Gyro Market & Feasibility Study,” SEA/02/TR/3910, Iss.1. November 2002. 3 Prezzavento, A., “Micro Gyroscope Feasibility Study,” GNC/TNO/2056, Iss.1 under ESA contract 16331/02/NL/LvH. November 2002. 4 Barthe, 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. 5 Pearce, 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.
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