IMAPS Device Packaging Conference 2011 7th International Conference and Exhibition on Device Packaging, Scottsdale/Fountain Hills, Arizona, USA, March 8-10, 2011.
TEMPERATURE AND HUMIDITY EFFECTS ON MEMS VIBRATORY GYROSCOPES 1
Chandradip Patel, 2Patrick McCluskey, and 3David Lemus 1,2 Department of Mechanical Engineering A. James Clark School of Engineering University of Maryland, College Park, MD, USA, 20740 Phone: (301) 405-0279 1 Email:
[email protected] 2 Email:
[email protected] 3 TRX Systems, Inc. 7500 Greenway Center Drive, Suite 820 Greenbelt, MD, USA, 20770 Phone: (301) 313-0053.Ex-30 3 Email:
[email protected]
MEMS vibratory gyroscopes are increasingly used in applications ranging from consumer electronics to aerospace and are now one of the most common MEMS products after accelerometers [1] [2]. Despite their widespread use, the performance of MEMS gyroscopes in harsh environments is still under question. While some studies have been conducted to understand the temperature dependent performance of MEMS gyroscopes, the effects of sustained exposure to temperature combined with other harsh environment stresses have not been well researched [3][4][5]. Thus, it is necessary to quantify MEMS vibratory gyroscope performance under such conditions. MEMS gyroscope is a sensor that measures the rate of change in an angular position (angular velocity) of an object. Majority of the reported MEMS gyroscopes use vibrating mechanical elements to sense angular velocity [6]. A MEMS vibratory gyroscope can be simply visualized as a two degree-of-freedom spring-mass-damper system as shown in Figure 1. The proof mass is suspended above the substrate by use of flexible beams which also work as a mechanical springs. In most of the work on reported MEMS vibratory gyroscopes, the proof mass is subjected to vibration at resonance frequency by use of electrostatic force causing movement in the drive direction. When the gyroscope sensor experiences an angular rotation, a Coriolis force is induced in the direction orthogonal to both drive Figure 1: Schematic illustration of MEMS Vibratory direction (x) and angular rotation axis (z). This rotation induced Gyroscope. Coriolis force causes energy transfer between drive mode and sense mode. The proof mass movement caused by Coriolis force in the sense direction (y) is proportional to the angular rotation applied and can be measured with differential capacitance techniques by use of interdigitated comb electrodes. Typical drive and sense mode resonant frequencies are in the range of few kHz, and sense amplitude is an order of magnitude less than drive amplitude causing a small change in gyroscope parameters leading to large variation in device output. Thus, it is necessary to evaluate the effects of harsh environment stresses on the performance of MEMS vibratory gyroscopes. In this study, we have examined the combined effects of temperature and humidity on gyroscope performance.
Figure 2: Two proof mass-spring-damper system.
The device used in this study is a COTS (commercial of the shelf) single axis MEMS vibratory gyroscope having an operating temperature range from -40˚C to 80˚C. It has two proof masses placed on either side that are driven in opposite directions causing the Coriolis force to induced on the two masses to be in opposite directions as shown in
IMAPS Device Packaging Conference 2011 7th International Conference and Exhibition on Device Packaging, Scottsdale/Fountain Hills, Arizona, USA, March 8-10, 2011. Figure 2. This special arrangement helps to nullify the external inertial inputs caused by undesirable ambient vibration and shock. The single axis MEMS vibratory gyroscopes are in plastic cavity packages and are sealed by use of metal caps. One of the specific applications of these gyroscopes is inertial navigation where single axis MEMS vibratory gyroscopes are combined and mounted on three Cartesian directions (i.e. X, Y and Z) with other electronics. This tracking system requires robust and accurate gyroscopes, the performance of which should be maintained within desired specific range. A gyroscope output drift beyond 0.5˚/s is considered an unacceptable range for the operation of this system [8]. Thus to understand the effects of temperature and humidity on MEMS gyroscope performance, two customized units are used for this study, each of which has three single axis gyroscopes mounted on Cartesian directions. EXPERIMENTAL SET-UP AND PROCEDURE Two units are used for investigating the effects of sustained exposure to temperature and humidity on the performance of a MEMS vibratory gyroscope. The single axis gyroscopes mounted on Cartesian directions are denoted with GyroX, GyroY and GyroZ respectively. A Zero rate output (ZRO) test was performed to evaluate the performance of both units. One of the units was tested without in-situ device calibration where raw data of MEMS vibratory gyroscopes were continuously collected with sampling frequency of 0.25 Hz. The other unit had a calibration mechanism which calibrates Figure 3: Temperature/Humidity Testing Setup. the device every 50 hours during in-situ operation. Both units were tested at 60˚C and 90% RH (Relative humidity) for 500 hours. To keep a track of chamber’s temperature and humidity, two external thermocouples and humidity sensors were placed inside the chamber at different locations. MEMS gyroscope units, thermocouples and humidity sensors were connected to a data acquisition unit which was further connected to a desktop computer as shown in Figure 3. After performing the temperature humidity bias (THB) test for 500 hours, both inertial navigation units were kept at room condition for an extended time to allow any moisture left in the units to come out. This is necessary to determine whether any drift observed during in-situ test is permanent or temporary in its effect on gyroscope performance.
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RESULTS AND ANALYSIS Both units were analyzed for 500 hours at 60˚C Temperature and 90% RH. The in-situ and baseline test data for both units (without calibration and with calibration) are discussed in this section. Unit without calibration: In-Situ Test Result 0.5 Results of the unit tested without in-situ device GyroX calibration is shown in Figure 4. This plot GyroY 0 includes in-situ ZRO outputs of each single axis GyroZ gyroscope mounted on Cartesian directions -0.5 which are denoted with GyroX, GyroY and GyroZ respectively. It is clear that all gyroscopes have -1 undergone in-situ drift over 500 hours the extent of which is summarized in Table 1. -1.5 Gyro Orientation In-situ Drift (˚/s) -2 GyroX 1.3 -2.5 GyroY 2.2 GyroZ 1.7 Table 1: Gyro Orientation vs. In-Situ Drift.
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Figure 4: In-situ test results of the unit tested without device calibration.
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IMAPS Device Packaging Conference 2011 7th International Conference and Exhibition on Device Packaging, Scottsdale/Fountain Hills, Arizona, USA, March 8-10, 2011.
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It can be concluded that gyroscopes tested without inBaseline Test situ device calibration had minimum and maximum 0.5 in-situ drift of 1.3˚/s and 2.2˚/s respectively over 500 0 hours. As expected the drift is of approximately same GyroX amount for all the sensors directions and small -0.5 0 200 400 600 800 1000 1200 variation is due to orientations with respect to its 0 package. In order to determine whether in-situ drift had permanently or temporarily shifted the device -0.5 output, the units were kept at room condition for GyroY about six months. Baseline stationary ZRO test was -1 0 200 400 600 800 1000 1200 performed after six months and it was found that 1 drifts had disappeared leading to conclusion that inGyroZ situ drift was temporary phenomenon related to 0.5 humidity level not any long term degradation of the device or package. The result of baseline stationary 0 0 200 400 600 800 1000 1200 (ZRO) test is shown in Figure 5. This humidity induced in-situ drift was likely due to a moisture Figure 5: Baseline test results of the unit tested without device resulted change in damping in damping coefficient or calibration. vibration mass. Unit with calibration: The unit tested with in-situ device calibration had calibrated all single axis MEMS gyroscopes every 50 hours over 500 hours total during temperature/humidity test. The result is shown in Figure 6. It is clear from Figure 6 that there wasn’t major drift observed when unit was calibrated every 50 hours which had compensated output drift. A small variation was due to time over which unit was not calibrated and the total variation remains within ±0.25˚/s. If this result is compared with the unit tested without calibration in Figure 4, it can be concluded that the in-situ drift caused by temperature and humidity environment can be minimized by Figure 6: In-situ test results of the unit tested with device performing frequent in-situ device calibration. This is calibration. necessary in applications like inertial navigation which requires high sensitivity and robust performance of MEMS vibratory gyroscope. The baseline test conducted after six month of hold at room condition is shown in Figure 7 . This result is quite similar to baseline test performed on unit tested without in-situ calibration. Baseline Test Deg/s
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Figure 7: Baseline test results of the unit tested with device calibration.
IMAPS Device Packaging Conference 2011 7th International Conference and Exhibition on Device Packaging, Scottsdale/Fountain Hills, Arizona, USA, March 8-10, 2011. SUMMARY AND CONCLUSIONS The following have resulted from this study:
It is confirmed that the performance of MEMS vibratory gyroscopes can be affected by sustained exposure to temperature and humidity environment. The unit tested without in-situ device calibration which had three single axis gyroscopes mounted on Cartesian directions, experienced minimum and maximum in-situ drift of 1.3˚/s and 2.2˚/s respectively over 500 hours.
The observed output drift was temporary and drift effects disappeared when tested after six months of hold at room conditions.
It is also observed that if the MEMS vibratory gyroscopes are calibrated frequently over time during harsh environment exposure, output drift can be minimized.
ACKNOWLEDGEMENT The authors would like to acknowledge the financial and technical support of the Maryland Industrial Partnership Program and TRX Systems, Inc. The authors would also like to thank the CALCE Electronic Products and Systems Center for the use of their laboratories in the conduct in this work. REFERENCES [1] N. Yazdi, F. Ayazi, K. Najafi "Micromachined inertial sensors," Proceedings of the IEEE, vol.86, no.8, Pages.1640-1659, Aug-1998. [2] S. Nasiri "A Critical Review of MEMS Gyroscopes Technology and Commercialization Status" InvenSense, Inc. [3] S. H. Choa "Reliability of vacuum packaged MEMS gyroscopes" Microelectronics and Reliability, Volume 45, Issue 2, Pages 361-369, February 2005. [4] C. Patel, F.P McCluskey, D. Lemus, A. Jones, J. Davis " The Temperature Effects on Performance and Reliability of MEMS Gyroscope Sensors" InterPACK'09, The Pacific Rim/ASME International Electronic Packaging Technical Conference, San Francisco, CA,USA July 19-23, 2009. [5] C. Patel, F.P McCluskey, D. Lemus "Performance and reliability of MEMS gyroscopes at high temperatures," Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2010 12th IEEE Intersociety Conference, vol.1, no.5, Pages2-5, June 2010. [6] F.P McCluskey, C. Patel, D. Lemus "Performance and Reliability of MEMS Gyroscopes and Packaging at High Temperatures", High Temperature Electronic, May 2010. [7] C. Acar and A.M. Shkel “MEMS Vibratory Gyroscopes - Structural Approaches to Improve Robustness”. MEMS Reference Shelf Series, Springer. 2009. [8] D. Lemus, C.Patel, F.P McCluskey, D.Westerwick “Temperature-Humidity Lifecycle Report”, TRX System, Inc.
