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Gyroscopes based on surface acoustic waves

Gyroscopes based on surface acoustic waves This review gives an overview of the development of surface acoustic wave (SAW) based gyroscopes. Different types of SAW based gyroscope are first presented, which are categorized into standing-wave based or progressive-wave based gyroscopes according to their respective mechanisms. In addition, multi-axis detectable SAW based gyroscopes are also introduced in this review. Different principles, structures, production methods, and control technologies are analyzed. Keywords: Gyroscope; Interference effect; Multi-axis detectable gyroscope; Progressive wave; Standing wave; Surface acoustic wave (SAW) Review transducers (IDT) is deposited on the surface of a piezo- A gyroscope is a sensor for measuring an angular rate or electric substrate, the IDT generates a SAW. The change angle based on the principles of angular momentum. in SAW velocity due to rotation is then detected as a Early gyroscopes (e.g., ball electrostatic and ring laser phase shift between the generated and detected wave gyroscopes) were generally large with poor portability. velocities. In comparison with conventional MEMS gyro- For several decades, micro-gyroscopes based on MEMS scopes, SAW gyroscopes are very attractive for these (Micro electro mechanical systems) technology have reasons. As opposed to the MEMS gyroscope, the SAW been studied, and there has been a steady improvement gyroscope does not need a suspended vibrating mechan- in their performance and in the technology used for ical structure. Therefore, it is more resistant to external their production. Recently, a number of outstanding shocks and vibrations. Frequency matching between the micro-gyroscopes have demonstrated sufficient inertial drive- and sense-mode frequencies in the absence of grade performance to potentially replace fiber-optic and active tuning and feedback control is very easy to achieve. ring laser gyroscopes [1-4]. The micro-gyroscope has Finally, temperature effects that cause variations in the advantages such as a scale of a few millimeters, low- Young’s modulus and residual stress can be almost power consumption, scope for mass production, and it completely eliminated easily. is low cost. However, currently available MEMS gyro- In this review, an overview of the development of scopes have suffered from a low mechanical Q-factor SAW based gyroscopes is provided. According to their due to atmospheric viscosity, production difficulties due functional principles, SAW gyroscopes are categorized to the demand for a three-dimensional construction- into different types such as standing-wave-mode type suspended mechanical structure, and a large susceptibility gyroscopes, progressive-wave-mode type gyroscopes, and to external shock and vibration. multi-axis detectable SAW gyroscopes, all of which are The surface acoustic wave (SAW) gyroscope was pro- introduced in this article [10-19]. posed by Lao in 1980 [5,6]. Several research groups have worked on this concept, and a number of related studies SAW gyroscope using standing wave mode were published between 2000 and 2011 [7-19]. SAW Kurosawa et al. [10] suggested a novel SAW gyroscope gyroscopes detect a change in SAW velocity as a function design concept using the standing wave mode, which of the angular rate of the medium in which the SAW has a metallic dot array within the cavity, as depicted in propagates. When an RF power supply to interdigital Figure 1. When the standing wave is generated by adding two counter-propagating SAWs from the resonators, * Correspondence: ssyang@ajou.ac.kr particles vibrate tangentially to the surface at or near Division of Electrical and computer engineering, Ajou University, Suwon the nodes of the standing wave, whereas those at the 443-749, South Korea Full list of author information is available at the end of the article anti-node vibrate normal to the surface. With rotation © 2015 Oh et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 2 of 10 Figure 1 Schematics and mechanism of the SAW gyroscope with a metallic dot array by Kurosawa et al. [10]. Figure 2 Experimental results of the gyroscope with respect to angular rates. (a) Measured response of the gyroscope and (b) measured response of BEI gyroscope. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 3 of 10 Figure 3 Schematic view and mechanism of the SAW gyroscope using a standing wave mode by Wang et al. Figure 4 Sensitivity evaluation of the constructed SAW gyroscope according to rotation direction. Figure 5 Schematic view and mechanism of the SAW gyroscope with a one-port reflective delay line, from Oh et al. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 4 of 10 Figure 6 Experimental results for the one-port reflective delay line gyroscope. (a) Results of S in the time domain for various rotation velocities. (b). Evaluated phase response with respect to angular rate. about the x-axis, the Coriolis forces act on the perturb- into acoustic waves while the other IDT receives these ation masses perpendicularly, as shown in Figure 1, acoustic waves and converts them back into an output resulting in the generation of a secondary wave that voltage. This reciprocity allows IDTs to be used either as propagates in the orthogonal direction of the standing SAW transmitters or as receivers. For this gyroscope, the wave. In a typical SAW delay line, one of the IDTs acts as IDTs for the sensors are used as receivers to detect the a transmitter that converts the applied voltage variation amplitude of the secondary SAWs, which are created by Table 1 Properties and characteristics of SAW gyroscopes based on standing waves References Author Size (Substrate) Frequency (MHz) Sensitvity Temperature compensation [10] Kurosawa et al. - (128° LiNbO)15 - No [11] Varadan et al. 1×1 mm (128° LiNbO ) 74.2 705 μv/deg/s No [12] Wang et al. 1.2 × 0.75 mm (128° LiNbO ) 80 119 Hz/deg/s Yes [14]Oh et al. 1.2 × 0.8 mm (128° LiNbO ) 80 1.23 deg/deg/s Yes 3 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 5 of 10 Figure 7 SAW MRG schematics and gyroscopic effects on the Rayleigh wave, from Lee et al. [15]. the Coriolis effect when the gyroscope is subject to rota- tion. Despite their design, they cannot obtain any output signals owing to mismatch of the resonance frequencies. Varadan et al. [11] obtained experimental results using this design concept. Figure 2 shows the measured output voltages from the SAW gyroscope for different angular rates. The SAW gyroscope response was evaluated using a rate table and a geophone setup. The gyroscope signal due to the Coriolis force propagates towards the SAWs, which emerge along with the diffracted signals from the resonator. It may be possible to separate the coupled signal from the gyroscope signal using phase-locked detection. Due to the difficulties in driving the rate table at lower rates, the sensitivity of the gyroscope was further measured using a geophone-pendulum setup. From the output voltage level of the BEI gyroscope, it can be seen −1 that the table oscillation at 3 V excitation is 950° h . Wang et al. [12,13] designed another concept SAW gyroscope using a standing wave. This gyroscope con- sists of a two-port SAW resonator, with a vibrating mass array within the cavity, and two SAW oscillators, one of which is used as a sensor and the other as a reference. Figure 3 shows a schematic of the structure and the operating principles of the proposed gyroscope. A stand- ing wave, generated by the two-port resonator, creates a metallic dot array at an antinode of the standing wave Figure 8 Measured output of the SAW MRG depending on vibrating normal to the surface (± z-axis). When the various angular rates. (a). Dynamic responses of the SAWMRG to SAW gyroscope is subjected to an angular rotation several angular rates. (b). Frequency responses of the SAWMRG with about the x-axis, the induced Coriolis force acts on the respect to input angular rate. vibrating mass. The Coriolis force generates a secondary Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 6 of 10 Figure 9 Schematic view and mechanism of the SAW gyroscope based on progressive waves, from Oh et al. SAW in the orthogonal direction of the primary stand- ing wave (± y-axis). This secondary SAW then interferes with the Rayleigh SAW propagating in the sensing device, causing a change in the acoustic velocity of the sensing device, and thus creating the resonant frequency of the SAW oscillator, which is used to detect the phase shift. Consequently, the angular rate can be evaluated by measuring the resonant frequency difference between the sensor oscillator and the reference oscillator. As shown in Figure 4, the sensitivity and linearity of the SAW gyroscope with rotation about the x-axis were measured to be 172 Hz/deg/s and 0.98, respectively. Moreover, negligible frequency differences between two delay lines were observed when rotating about the y- and z-axis, and were of lower magnitude than the white noise level. A superior single sensor directivity in the x-axis was observed. Figure 10 Measured output frequencies for various metallic dots. (a). Frequency responses depending on the position of the metallic dots in the delay line. (b). Frequency responses depending Figure 11 Schematics and mechanism of the wireless and on the mass of the metallic dots. passive SAW gyroscope, from Wang et al. [18]. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 7 of 10 rotation through the Coriolis force, because the particles of the Rayleigh wave are elliptically polarized on the sagittal plane. Therefore, these devices detect the fre- quency rather than the minute amplitude of a SAW and can use a temperature-stable material like ST-cut quartz despite the piezoelectric coupling coefficient. The SAWMRG consists of a delay-line structure, a low temperature co-fired ceramic (LTCC) package, and a cover glass. The oscillator operates at two fundamental frequencies. If the device is subject to rotation around the y-axis, the operating frequency of the oscillator will vary according to the gyroscopic gain factor. Then, the output signals of both oscillators are fed into a multiplier and a band-pass filter to extract the frequency variation. Figure 12 Phase sensitivity evaluation of the wireless SAW Consequently, the angular rate of the SAWMRG can be gyroscope. sensed by measuring the frequency variation. The experi- ment was carried out at rotations of up to 2000 deg/s, and Three years later, Oh et al. [14] proposed a gyroscope the results are shown in Figure 8. The sensitivity of the using a standing wave mode with a one-port reflective device and the residual standard deviation from the delay line. The gyroscope consists of metallic dots, reso- linear fit are 0.431 Hz/deg/s and 46.5 Hz, respectively. nators, and a SAW reflective delay line with an 80 MHz Another type of SAW gyroscope using progressive central frequency. As depicted in Figure 5, the operating waves was proposed by Oh et al. [16,17]. Figure 9 shows principle is similar to the concept of Wang et al. [12,13], the schematics and the mechanism of a SAW gyroscope as previously described, and is as follows. When the based on a progressive wave which consists of a vibrating gyroscope is rotating, the metallic dots induce a Coriolis mass, an absorber, and a two-SAW delay line; one is used force and generate a secondary SAW in the direction as a sensing element and contains a vibrating mass in a orthogonal to the standing wave. This wave interferes cavity, while the other is used as a reference element. with the SAW flowing in the reflective delay line, result- The SAW delay line oscillator operates based on a self- ing in a change in the SAW velocity. Figure 6 shows the excited oscillator that consists of a delay line, a feedback experimental results for the proposed SAW gyroscope, amplifier, and a phase shifter, as shown in Figure 9(a). The with a sensitivity of 1.23 deg/deg/s in phase response at oscillators with a delay line operate at their fundamental an angular rate of up to 2000 deg/s. frequencies, f (reference part) and f (sensing part). A 1 2 Table 1 shows the properties of the SAW gyroscope using progressive wave is generated at the input transducer (left standing waves described above. Although Kurosawa et al. transducer) and propagated to the output transducer [10] failed to obtain experimental results with their device, (right transducer). When the gyroscope is subjected to an they suggested using the standing wave mode and metallic angular rotation, the Coriolis force acts on the vibrating dot array to enhance the sensitivity of the SAW gyroscope. metallic dots. The direction of the force is the same as the Wang et al. [12,13] demonstrated a highly sensitive SAW direction of wave propagation, and the amplitude and vel- gyroscope using a standing wave mode with two SAW ocity of the wave are therefore changed (Δv ), causing a oscillators to eliminate error sources such as temperature, shift in the oscillation frequency (Δf ) of the sensing oscil- pressure, and so on. lator. By measuring the frequency difference between the sensing and reference oscillators, the input rotation can be SAW gyroscope based on progressive wave evaluated without errors. The constructed device has been Lee et al. [15] proposed a SAW micro rate gyroscope tested at an angular rate in the range of up to 1000 deg/s (MRG) using two delay lines in 2007, as shown in Figure 7. and the results are shown in Figure 10. The sensitivity was The rotation vector perpendicular to the propagating axis approximately 62.57 Hz/deg/s when metallic dots were causes a velocity change that is proportional to the input positioned near the output IDT. Table 2 Properties and characteristics of SAW gyroscopes based on progressive waves References Author Size (Substrate) Frequency (MHz) Sensitvity Temperature compensation [15] Lee et al. 9×9 mm (ST-Quartz) 98.6 0.431 Hz/deg/s Yes [16]Oh et al. 1.4 × 0.6 mm (128° LiNbO ) 80.2 52.35 Hz/deg/s Yes [18] Wang et al. - (128° LiNbO ) 434 2.42 deg/deg/s Yes 3 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 8 of 10 In [18], Wang et al. demonstrated a novel wireless and passive SAW gyroscope using two parallel broadband reflective delay lines. As shown in Figure 11, these two SAWs follow reflective delay line patterns propagating in opposite directions. If the device is subjected to a rotation, the SAW gyroscopic effect changes the SAW velocity in the SAW reflective delay line, in both direc- tions. The SAW velocity in one direction increases and decreases in the other, as a result of the opposite rotation, causing a doubling of the sensitivity of the device and compensating for the temperature effects. The phase response of the gyroscope was measured at different rotation rates, up to 900 deg/s along the y-axis, and is depicted in Figure 12. According to their results, the sensitivity and linearity of the proposed device are 2.42 deg/deg/s and 0.956, respectively. Therefore, Wang et al. [18] successfully demonstrated the potential of the wireless and passive SAW gyroscope. Table 2 shows properties of the various SAW gyro- scopes using progressive waves described above. The performance of the SAW gyroscope can be enhanced through the use of two delay lines. Moreover, Lee et al. [15] and Wang et al. [18] have attempted to improve the gyroscope sensitivity by using a differential scheme based on opposing wave propagation directions. Oh et al. [16,17] constructed a SAW gyroscope using a progressive wave with different masses and positions for the metallic dot array on an experimental basis. In particular, Wang et al. [18] successfully demonstrated a wireless and passive SAW gyroscope using two reflective delay lines. Figure 14 Measured output frequency difference with respect to rotation rate. (a) Top element. (b) Bottom element. Other gyroscopes using SAW Figure 13 shows a multi-axis detectable SAW gyroscope utilizing a stacked configuration [19]. It consists of a SAW gyroscopes are bonded with a conductive silver silicon substrate and two SAW gyroscopes using pro- paste, in which a separation gap of about 200 μmis gressive waves, described in more detail in [16], in which formed. When the gyroscope is subjected to an angular the bottom element is used for y-axis detection and the rotation about the y-axis, a Coriolis force in the x-direc- top element is used for x-axis detection. The silicon sub- tion is produced by the vibrating mass. Thus, the bottom strate is used to protect the SAW gyroscope. The four SAW gyroscope is affected by the Coriolis force because sides are completely sealed by JSR photoresists (PR) to the wave propagation direction is in the same direction prevent interference from undesirable factors such as as the Coriolis force. On the other hand, the Coriolis temperature and humidity. The two Si-substrates with force significantly affects the top SAW gyroscope, Figure 13 Cross-sectional view of the multi-axis detectable gyroscope utilizing stacked configuration. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 9 of 10 because the direction of wave propagation for the top respectively. He began working for next generation convergence sensor research center in KETI from 2013. SAW gyroscope is different to the direction of the Coriolis Ki Jung Lee received the BS, MS degrees in electrical engineering from Ajou force. Conversely, when the gyroscope is subjected to an University in 2007 and 2009. His major research areas include SAW sensors angular rotation about the x-axis, the Coriolis force in the and micro mass spectrometer. He began working for Micro-system Lab., Ajou University from 2007 as a PhD candidate. y-direction is produced by a vibrating mass. In this case, Keekeun Lee was born in Seoul, South Korea, in 1968. He received his MS the top SAW gyroscope is only affected by the Coriolis degree from University of Florida, Gainesville, USA, in 1993 and his PhD force. degree in electrical engineering from Arizona State University, Tempe, USA, in 2000. After receiving his PhD degree, he worked as a post doctor and an In this study, as the rotation speed was increased from 0 assistant research professor for 4 years in bioengineering department at to 1000 deg/s along the y-axis at 20°C, the mixed oscillator Arizona State University. In 2004, he joined Ajou University in S. Korea and frequency difference of the bottom element increased currently he is a professor in electronics engineering department. He has published more than 60 papers in internationally renowned journal articles, linearly, as shown in Figure 14(a). However, the mixed mostly regarding wireless surface acoustic wave (SAW) sensors, oscillator frequency of the top element did not change, microstructured neural probe and its systems, organic-based hybrid solar and the measured sensitivity and linearity of the SAW cells, and so on. Sang Sik Yang was born in Korea in 1958. He received his BS and MS gyroscope were found to be 45.32 Hz/deg/s and 0.907, degrees in mechanical engineering from Seoul National University in 1980 respectively. Next, the device was rotated counterclock- and 1983, respectively. In 1988, he received his PhD degree in mechanical wise along the x-axis, from 0 to 1000 deg/s at 20°C. As the engineering from the University of California, Berkeley. He was then a research assistant professor at New Jersey Institute of Technology. Since rotation speed increased, the mixed oscillator frequency 1989, he has been a professor in the Department of Electrical and Computer difference of the top element increased linearly, as shown Engineering at Ajou University. His research interests include the mechanism in Figure 14(b). In this case, the difference in signal from and actuation of microelectromechanical devices, SAW sensors and micro plasma devices. the bottom element is negligible, and the measured sensi- tivity and linearity of the SAW gyroscope were 27.34 Hz/ Acknowledgements (deg/s) and 0.837, respectively. The sensitivity of the top This work was supported by the National Research Foundation of Korea element is lower than that of bottom element. This differ- (NRF), grant funded by the Korean government (MEST) (No. 2009-0081200). ence can be ascribed to the fact that the applied electric Author details energy to the top device is lower than that supplied to the Division of Electrical and computer engineering, Ajou University, Suwon bottom element, because of the different transmission 443-749, South Korea. Next Generation Convergence Sensor Research Center, KETI, Seongnam 463-816, South Korea. lines of the electric signal and the highly resistive metal formed by electroplating. Received: 12 June 2014 Accepted: 20 October 2014 Conclusion References Gyroscopes based on SAW have been reviewed. In com- 1. Wang R, Durgam SK, Hao Z, Vahala LL (2009) A SOI-Based Tuning-Fork parison to existing silicon-based MEMS gyroscopes, a Gyroscope With High Quality Factors. In: Tomizuka M (ed) Proceeding of SAW gyroscope is very attractive for a number of reasons. SPIE Conference on Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, vol 7292. SPIE., p 7292 First, it has no suspended vibrating mechanical structure 2. Wang R, Cheng P, Xie F, Young D, Hao Z (2011) A multiple-beam tuning-fork and is therefore more resistant to external shocks and gyroscope with high quality factors. Sensor Actuat A: Phys 166:22–33 vibrations. Second, frequency matching between the 3. Acar C, Schofield AR, Trusov AA, Costlow LE, Shkel AM (2009) Environmentally robust MEMS vibratory gyroscopes for automotive drive- and sense-mode frequencies in the absence of applications. Sensors 9:1895–1906 active tuning and feedback control is very easy. Finally, 4. Liu K, Zhang W, Chen W, Li K, Dai F, Cui F, Wu X, Ma G, Xiao Q (2009) The the temperature effect that causes a variation in the development of micro-gyroscope technology. J Micromech Microeng 19:113001 Young’s modulus and residual stress can be almost com- 5. Binneg Y, Lao (1983) Surface Acoustic Wave Gyroscope. US patent pletely eliminated. By comparing different structures of 24 May 1983 SAW-based gyroscopes and their mechanisms, we can 6. Lao BY (1980) Gyroscopic Effect in Surface Acoustic Waves. Proceeding of IEEE ultrasonic symposium, In, p 687 see that the SAW gyroscope has the potential to be the 7. Jose KA, Suh WD, Xavier PB, Varadan VK, Varadan VV (2002) Surface acoustic highest performing gyroscope in the near future. wave MEMS gyroscope. Wave Motion 36(4):367–381 8. Woods RC, Kalami H, Johnson B (2002) Evaluation of a novel surface acoustic Competing interests wave gyroscope. IEEE Trans Ultrason Ferroelectr Freq Control 49:136–141 The authors declare that they have no competing interests. 9. Varadan VK, Suh WD, Jose KA, Varadan VV (2001) Hybrid MEMS-IDT-based accelerometer and gyroscope in a single chip. In: Varadan VK (ed) Proceeding Authors’ contributions of SPIE conference on smart structures and materials 2001: smart electronics HO carried out research works and writing of the manuscript. KJL carried out and MEMS, vol.4334. SPIE., p 119 research works and KL carried out a survey on the gyroscope based on 10. Kurosawa M, Fukula Y, Takasaki M, Higuchi T (1998) A surface-acoustic-wave surface acoustic wave. SSY supervised all research works. All authors read gyro sensor. Sensor Actuat A: Phys 66:33–39 and approved the final manuscript. 11. Varadan VK, Suh WD, Xavier PB, Jose KA, Varadan VV (2000) Design and development of a MEMS-IDT gyroscope. Smart Mater Struct 9:898–905 Authors’ information 12. Wang W, Oh H, Lee K, Yoon S, Yang S (2009) Enhanced sensitivity of novel Haekwan Oh was born in Korea in 1981. He received his BS and Ph.D surface acoustic wave microelectromechanical system-interdigital transducer degrees in electrical engineering from Ajou University in 2007 and 2013, gyroscope. Jpn J Appl Phys 48:06FK09 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 10 of 10 13. Oh H, Wang W, Yang S, Lee K (2011) Development of SAW based gyroscope with high shock and thermal stability. Sensor Actuat A: Phys 165:8–15 14. Oh H, Fu C, Yang SS, Wang W, Lee K (2012) A novel shock and heat tolerant gyrosensor utilizing a one-port surface acoustic wave reflective delay line. J Micromech Microeng 22:045007 15. Lee SW, Rhim JW, Park SW, Yang SS (2007) A micro rate gyroscope based on the SAW gyroscope effect. J Micromech Microeng 17:2272–2279 16. Oh H, Yang S, Lee K (2010) Development of SAW-based microgyroscope utilizing progressive wave. Jpn J Appl Phys 49:06GN16 17. Oh H, Lee K, Yang S, Wang W (2011) Enhanced sensitivity of surface acoustic wave gyroscope using progressive wave. J Micromech Microeng 21:075015 18. Wang W, Wang W, Liu J, Liu M, Yang S (2011) Wireless and passive gyroscope based on surface acoustic wave gyroscopic effect. Appl Phys Express 4:086601 19. Oh H, Lee KJ, Yang SS, Lee K (2012) Development of novel dual-axis sensing gyroscope using surface acoustic wave. 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Gyroscopes based on surface acoustic waves

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Springer Journals
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Copyright © 2015 by Oh et al.; licensee Springer.
Subject
Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
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Abstract

This review gives an overview of the development of surface acoustic wave (SAW) based gyroscopes. Different types of SAW based gyroscope are first presented, which are categorized into standing-wave based or progressive-wave based gyroscopes according to their respective mechanisms. In addition, multi-axis detectable SAW based gyroscopes are also introduced in this review. Different principles, structures, production methods, and control technologies are analyzed. Keywords: Gyroscope; Interference effect; Multi-axis detectable gyroscope; Progressive wave; Standing wave; Surface acoustic wave (SAW) Review transducers (IDT) is deposited on the surface of a piezo- A gyroscope is a sensor for measuring an angular rate or electric substrate, the IDT generates a SAW. The change angle based on the principles of angular momentum. in SAW velocity due to rotation is then detected as a Early gyroscopes (e.g., ball electrostatic and ring laser phase shift between the generated and detected wave gyroscopes) were generally large with poor portability. velocities. In comparison with conventional MEMS gyro- For several decades, micro-gyroscopes based on MEMS scopes, SAW gyroscopes are very attractive for these (Micro electro mechanical systems) technology have reasons. As opposed to the MEMS gyroscope, the SAW been studied, and there has been a steady improvement gyroscope does not need a suspended vibrating mechan- in their performance and in the technology used for ical structure. Therefore, it is more resistant to external their production. Recently, a number of outstanding shocks and vibrations. Frequency matching between the micro-gyroscopes have demonstrated sufficient inertial drive- and sense-mode frequencies in the absence of grade performance to potentially replace fiber-optic and active tuning and feedback control is very easy to achieve. ring laser gyroscopes [1-4]. The micro-gyroscope has Finally, temperature effects that cause variations in the advantages such as a scale of a few millimeters, low- Young’s modulus and residual stress can be almost power consumption, scope for mass production, and it completely eliminated easily. is low cost. However, currently available MEMS gyro- In this review, an overview of the development of scopes have suffered from a low mechanical Q-factor SAW based gyroscopes is provided. According to their due to atmospheric viscosity, production difficulties due functional principles, SAW gyroscopes are categorized to the demand for a three-dimensional construction- into different types such as standing-wave-mode type suspended mechanical structure, and a large susceptibility gyroscopes, progressive-wave-mode type gyroscopes, and to external shock and vibration. multi-axis detectable SAW gyroscopes, all of which are The surface acoustic wave (SAW) gyroscope was pro- introduced in this article [10-19]. posed by Lao in 1980 [5,6]. Several research groups have worked on this concept, and a number of related studies SAW gyroscope using standing wave mode were published between 2000 and 2011 [7-19]. SAW Kurosawa et al. [10] suggested a novel SAW gyroscope gyroscopes detect a change in SAW velocity as a function design concept using the standing wave mode, which of the angular rate of the medium in which the SAW has a metallic dot array within the cavity, as depicted in propagates. When an RF power supply to interdigital Figure 1. When the standing wave is generated by adding two counter-propagating SAWs from the resonators, * Correspondence: ssyang@ajou.ac.kr particles vibrate tangentially to the surface at or near Division of Electrical and computer engineering, Ajou University, Suwon the nodes of the standing wave, whereas those at the 443-749, South Korea Full list of author information is available at the end of the article anti-node vibrate normal to the surface. With rotation © 2015 Oh et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 2 of 10 Figure 1 Schematics and mechanism of the SAW gyroscope with a metallic dot array by Kurosawa et al. [10]. Figure 2 Experimental results of the gyroscope with respect to angular rates. (a) Measured response of the gyroscope and (b) measured response of BEI gyroscope. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 3 of 10 Figure 3 Schematic view and mechanism of the SAW gyroscope using a standing wave mode by Wang et al. Figure 4 Sensitivity evaluation of the constructed SAW gyroscope according to rotation direction. Figure 5 Schematic view and mechanism of the SAW gyroscope with a one-port reflective delay line, from Oh et al. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 4 of 10 Figure 6 Experimental results for the one-port reflective delay line gyroscope. (a) Results of S in the time domain for various rotation velocities. (b). Evaluated phase response with respect to angular rate. about the x-axis, the Coriolis forces act on the perturb- into acoustic waves while the other IDT receives these ation masses perpendicularly, as shown in Figure 1, acoustic waves and converts them back into an output resulting in the generation of a secondary wave that voltage. This reciprocity allows IDTs to be used either as propagates in the orthogonal direction of the standing SAW transmitters or as receivers. For this gyroscope, the wave. In a typical SAW delay line, one of the IDTs acts as IDTs for the sensors are used as receivers to detect the a transmitter that converts the applied voltage variation amplitude of the secondary SAWs, which are created by Table 1 Properties and characteristics of SAW gyroscopes based on standing waves References Author Size (Substrate) Frequency (MHz) Sensitvity Temperature compensation [10] Kurosawa et al. - (128° LiNbO)15 - No [11] Varadan et al. 1×1 mm (128° LiNbO ) 74.2 705 μv/deg/s No [12] Wang et al. 1.2 × 0.75 mm (128° LiNbO ) 80 119 Hz/deg/s Yes [14]Oh et al. 1.2 × 0.8 mm (128° LiNbO ) 80 1.23 deg/deg/s Yes 3 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 5 of 10 Figure 7 SAW MRG schematics and gyroscopic effects on the Rayleigh wave, from Lee et al. [15]. the Coriolis effect when the gyroscope is subject to rota- tion. Despite their design, they cannot obtain any output signals owing to mismatch of the resonance frequencies. Varadan et al. [11] obtained experimental results using this design concept. Figure 2 shows the measured output voltages from the SAW gyroscope for different angular rates. The SAW gyroscope response was evaluated using a rate table and a geophone setup. The gyroscope signal due to the Coriolis force propagates towards the SAWs, which emerge along with the diffracted signals from the resonator. It may be possible to separate the coupled signal from the gyroscope signal using phase-locked detection. Due to the difficulties in driving the rate table at lower rates, the sensitivity of the gyroscope was further measured using a geophone-pendulum setup. From the output voltage level of the BEI gyroscope, it can be seen −1 that the table oscillation at 3 V excitation is 950° h . Wang et al. [12,13] designed another concept SAW gyroscope using a standing wave. This gyroscope con- sists of a two-port SAW resonator, with a vibrating mass array within the cavity, and two SAW oscillators, one of which is used as a sensor and the other as a reference. Figure 3 shows a schematic of the structure and the operating principles of the proposed gyroscope. A stand- ing wave, generated by the two-port resonator, creates a metallic dot array at an antinode of the standing wave Figure 8 Measured output of the SAW MRG depending on vibrating normal to the surface (± z-axis). When the various angular rates. (a). Dynamic responses of the SAWMRG to SAW gyroscope is subjected to an angular rotation several angular rates. (b). Frequency responses of the SAWMRG with about the x-axis, the induced Coriolis force acts on the respect to input angular rate. vibrating mass. The Coriolis force generates a secondary Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 6 of 10 Figure 9 Schematic view and mechanism of the SAW gyroscope based on progressive waves, from Oh et al. SAW in the orthogonal direction of the primary stand- ing wave (± y-axis). This secondary SAW then interferes with the Rayleigh SAW propagating in the sensing device, causing a change in the acoustic velocity of the sensing device, and thus creating the resonant frequency of the SAW oscillator, which is used to detect the phase shift. Consequently, the angular rate can be evaluated by measuring the resonant frequency difference between the sensor oscillator and the reference oscillator. As shown in Figure 4, the sensitivity and linearity of the SAW gyroscope with rotation about the x-axis were measured to be 172 Hz/deg/s and 0.98, respectively. Moreover, negligible frequency differences between two delay lines were observed when rotating about the y- and z-axis, and were of lower magnitude than the white noise level. A superior single sensor directivity in the x-axis was observed. Figure 10 Measured output frequencies for various metallic dots. (a). Frequency responses depending on the position of the metallic dots in the delay line. (b). Frequency responses depending Figure 11 Schematics and mechanism of the wireless and on the mass of the metallic dots. passive SAW gyroscope, from Wang et al. [18]. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 7 of 10 rotation through the Coriolis force, because the particles of the Rayleigh wave are elliptically polarized on the sagittal plane. Therefore, these devices detect the fre- quency rather than the minute amplitude of a SAW and can use a temperature-stable material like ST-cut quartz despite the piezoelectric coupling coefficient. The SAWMRG consists of a delay-line structure, a low temperature co-fired ceramic (LTCC) package, and a cover glass. The oscillator operates at two fundamental frequencies. If the device is subject to rotation around the y-axis, the operating frequency of the oscillator will vary according to the gyroscopic gain factor. Then, the output signals of both oscillators are fed into a multiplier and a band-pass filter to extract the frequency variation. Figure 12 Phase sensitivity evaluation of the wireless SAW Consequently, the angular rate of the SAWMRG can be gyroscope. sensed by measuring the frequency variation. The experi- ment was carried out at rotations of up to 2000 deg/s, and Three years later, Oh et al. [14] proposed a gyroscope the results are shown in Figure 8. The sensitivity of the using a standing wave mode with a one-port reflective device and the residual standard deviation from the delay line. The gyroscope consists of metallic dots, reso- linear fit are 0.431 Hz/deg/s and 46.5 Hz, respectively. nators, and a SAW reflective delay line with an 80 MHz Another type of SAW gyroscope using progressive central frequency. As depicted in Figure 5, the operating waves was proposed by Oh et al. [16,17]. Figure 9 shows principle is similar to the concept of Wang et al. [12,13], the schematics and the mechanism of a SAW gyroscope as previously described, and is as follows. When the based on a progressive wave which consists of a vibrating gyroscope is rotating, the metallic dots induce a Coriolis mass, an absorber, and a two-SAW delay line; one is used force and generate a secondary SAW in the direction as a sensing element and contains a vibrating mass in a orthogonal to the standing wave. This wave interferes cavity, while the other is used as a reference element. with the SAW flowing in the reflective delay line, result- The SAW delay line oscillator operates based on a self- ing in a change in the SAW velocity. Figure 6 shows the excited oscillator that consists of a delay line, a feedback experimental results for the proposed SAW gyroscope, amplifier, and a phase shifter, as shown in Figure 9(a). The with a sensitivity of 1.23 deg/deg/s in phase response at oscillators with a delay line operate at their fundamental an angular rate of up to 2000 deg/s. frequencies, f (reference part) and f (sensing part). A 1 2 Table 1 shows the properties of the SAW gyroscope using progressive wave is generated at the input transducer (left standing waves described above. Although Kurosawa et al. transducer) and propagated to the output transducer [10] failed to obtain experimental results with their device, (right transducer). When the gyroscope is subjected to an they suggested using the standing wave mode and metallic angular rotation, the Coriolis force acts on the vibrating dot array to enhance the sensitivity of the SAW gyroscope. metallic dots. The direction of the force is the same as the Wang et al. [12,13] demonstrated a highly sensitive SAW direction of wave propagation, and the amplitude and vel- gyroscope using a standing wave mode with two SAW ocity of the wave are therefore changed (Δv ), causing a oscillators to eliminate error sources such as temperature, shift in the oscillation frequency (Δf ) of the sensing oscil- pressure, and so on. lator. By measuring the frequency difference between the sensing and reference oscillators, the input rotation can be SAW gyroscope based on progressive wave evaluated without errors. The constructed device has been Lee et al. [15] proposed a SAW micro rate gyroscope tested at an angular rate in the range of up to 1000 deg/s (MRG) using two delay lines in 2007, as shown in Figure 7. and the results are shown in Figure 10. The sensitivity was The rotation vector perpendicular to the propagating axis approximately 62.57 Hz/deg/s when metallic dots were causes a velocity change that is proportional to the input positioned near the output IDT. Table 2 Properties and characteristics of SAW gyroscopes based on progressive waves References Author Size (Substrate) Frequency (MHz) Sensitvity Temperature compensation [15] Lee et al. 9×9 mm (ST-Quartz) 98.6 0.431 Hz/deg/s Yes [16]Oh et al. 1.4 × 0.6 mm (128° LiNbO ) 80.2 52.35 Hz/deg/s Yes [18] Wang et al. - (128° LiNbO ) 434 2.42 deg/deg/s Yes 3 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 8 of 10 In [18], Wang et al. demonstrated a novel wireless and passive SAW gyroscope using two parallel broadband reflective delay lines. As shown in Figure 11, these two SAWs follow reflective delay line patterns propagating in opposite directions. If the device is subjected to a rotation, the SAW gyroscopic effect changes the SAW velocity in the SAW reflective delay line, in both direc- tions. The SAW velocity in one direction increases and decreases in the other, as a result of the opposite rotation, causing a doubling of the sensitivity of the device and compensating for the temperature effects. The phase response of the gyroscope was measured at different rotation rates, up to 900 deg/s along the y-axis, and is depicted in Figure 12. According to their results, the sensitivity and linearity of the proposed device are 2.42 deg/deg/s and 0.956, respectively. Therefore, Wang et al. [18] successfully demonstrated the potential of the wireless and passive SAW gyroscope. Table 2 shows properties of the various SAW gyro- scopes using progressive waves described above. The performance of the SAW gyroscope can be enhanced through the use of two delay lines. Moreover, Lee et al. [15] and Wang et al. [18] have attempted to improve the gyroscope sensitivity by using a differential scheme based on opposing wave propagation directions. Oh et al. [16,17] constructed a SAW gyroscope using a progressive wave with different masses and positions for the metallic dot array on an experimental basis. In particular, Wang et al. [18] successfully demonstrated a wireless and passive SAW gyroscope using two reflective delay lines. Figure 14 Measured output frequency difference with respect to rotation rate. (a) Top element. (b) Bottom element. Other gyroscopes using SAW Figure 13 shows a multi-axis detectable SAW gyroscope utilizing a stacked configuration [19]. It consists of a SAW gyroscopes are bonded with a conductive silver silicon substrate and two SAW gyroscopes using pro- paste, in which a separation gap of about 200 μmis gressive waves, described in more detail in [16], in which formed. When the gyroscope is subjected to an angular the bottom element is used for y-axis detection and the rotation about the y-axis, a Coriolis force in the x-direc- top element is used for x-axis detection. The silicon sub- tion is produced by the vibrating mass. Thus, the bottom strate is used to protect the SAW gyroscope. The four SAW gyroscope is affected by the Coriolis force because sides are completely sealed by JSR photoresists (PR) to the wave propagation direction is in the same direction prevent interference from undesirable factors such as as the Coriolis force. On the other hand, the Coriolis temperature and humidity. The two Si-substrates with force significantly affects the top SAW gyroscope, Figure 13 Cross-sectional view of the multi-axis detectable gyroscope utilizing stacked configuration. Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 9 of 10 because the direction of wave propagation for the top respectively. He began working for next generation convergence sensor research center in KETI from 2013. SAW gyroscope is different to the direction of the Coriolis Ki Jung Lee received the BS, MS degrees in electrical engineering from Ajou force. Conversely, when the gyroscope is subjected to an University in 2007 and 2009. His major research areas include SAW sensors angular rotation about the x-axis, the Coriolis force in the and micro mass spectrometer. He began working for Micro-system Lab., Ajou University from 2007 as a PhD candidate. y-direction is produced by a vibrating mass. In this case, Keekeun Lee was born in Seoul, South Korea, in 1968. He received his MS the top SAW gyroscope is only affected by the Coriolis degree from University of Florida, Gainesville, USA, in 1993 and his PhD force. degree in electrical engineering from Arizona State University, Tempe, USA, in 2000. After receiving his PhD degree, he worked as a post doctor and an In this study, as the rotation speed was increased from 0 assistant research professor for 4 years in bioengineering department at to 1000 deg/s along the y-axis at 20°C, the mixed oscillator Arizona State University. In 2004, he joined Ajou University in S. Korea and frequency difference of the bottom element increased currently he is a professor in electronics engineering department. He has published more than 60 papers in internationally renowned journal articles, linearly, as shown in Figure 14(a). However, the mixed mostly regarding wireless surface acoustic wave (SAW) sensors, oscillator frequency of the top element did not change, microstructured neural probe and its systems, organic-based hybrid solar and the measured sensitivity and linearity of the SAW cells, and so on. Sang Sik Yang was born in Korea in 1958. He received his BS and MS gyroscope were found to be 45.32 Hz/deg/s and 0.907, degrees in mechanical engineering from Seoul National University in 1980 respectively. Next, the device was rotated counterclock- and 1983, respectively. In 1988, he received his PhD degree in mechanical wise along the x-axis, from 0 to 1000 deg/s at 20°C. As the engineering from the University of California, Berkeley. He was then a research assistant professor at New Jersey Institute of Technology. Since rotation speed increased, the mixed oscillator frequency 1989, he has been a professor in the Department of Electrical and Computer difference of the top element increased linearly, as shown Engineering at Ajou University. His research interests include the mechanism in Figure 14(b). In this case, the difference in signal from and actuation of microelectromechanical devices, SAW sensors and micro plasma devices. the bottom element is negligible, and the measured sensi- tivity and linearity of the SAW gyroscope were 27.34 Hz/ Acknowledgements (deg/s) and 0.837, respectively. The sensitivity of the top This work was supported by the National Research Foundation of Korea element is lower than that of bottom element. This differ- (NRF), grant funded by the Korean government (MEST) (No. 2009-0081200). ence can be ascribed to the fact that the applied electric Author details energy to the top device is lower than that supplied to the Division of Electrical and computer engineering, Ajou University, Suwon bottom element, because of the different transmission 443-749, South Korea. Next Generation Convergence Sensor Research Center, KETI, Seongnam 463-816, South Korea. lines of the electric signal and the highly resistive metal formed by electroplating. Received: 12 June 2014 Accepted: 20 October 2014 Conclusion References Gyroscopes based on SAW have been reviewed. In com- 1. Wang R, Durgam SK, Hao Z, Vahala LL (2009) A SOI-Based Tuning-Fork parison to existing silicon-based MEMS gyroscopes, a Gyroscope With High Quality Factors. In: Tomizuka M (ed) Proceeding of SAW gyroscope is very attractive for a number of reasons. SPIE Conference on Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, vol 7292. SPIE., p 7292 First, it has no suspended vibrating mechanical structure 2. Wang R, Cheng P, Xie F, Young D, Hao Z (2011) A multiple-beam tuning-fork and is therefore more resistant to external shocks and gyroscope with high quality factors. Sensor Actuat A: Phys 166:22–33 vibrations. Second, frequency matching between the 3. 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Jose KA, Suh WD, Xavier PB, Varadan VK, Varadan VV (2002) Surface acoustic highest performing gyroscope in the near future. wave MEMS gyroscope. Wave Motion 36(4):367–381 8. Woods RC, Kalami H, Johnson B (2002) Evaluation of a novel surface acoustic Competing interests wave gyroscope. IEEE Trans Ultrason Ferroelectr Freq Control 49:136–141 The authors declare that they have no competing interests. 9. Varadan VK, Suh WD, Jose KA, Varadan VV (2001) Hybrid MEMS-IDT-based accelerometer and gyroscope in a single chip. In: Varadan VK (ed) Proceeding Authors’ contributions of SPIE conference on smart structures and materials 2001: smart electronics HO carried out research works and writing of the manuscript. KJL carried out and MEMS, vol.4334. SPIE., p 119 research works and KL carried out a survey on the gyroscope based on 10. Kurosawa M, Fukula Y, Takasaki M, Higuchi T (1998) A surface-acoustic-wave surface acoustic wave. SSY supervised all research works. All authors read gyro sensor. Sensor Actuat A: Phys 66:33–39 and approved the final manuscript. 11. Varadan VK, Suh WD, Xavier PB, Jose KA, Varadan VV (2000) Design and development of a MEMS-IDT gyroscope. Smart Mater Struct 9:898–905 Authors’ information 12. Wang W, Oh H, Lee K, Yoon S, Yang S (2009) Enhanced sensitivity of novel Haekwan Oh was born in Korea in 1981. He received his BS and Ph.D surface acoustic wave microelectromechanical system-interdigital transducer degrees in electrical engineering from Ajou University in 2007 and 2013, gyroscope. Jpn J Appl Phys 48:06FK09 Oh et al. Micro and Nano Systems Letters (2015) 3:1 Page 10 of 10 13. Oh H, Wang W, Yang S, Lee K (2011) Development of SAW based gyroscope with high shock and thermal stability. Sensor Actuat A: Phys 165:8–15 14. Oh H, Fu C, Yang SS, Wang W, Lee K (2012) A novel shock and heat tolerant gyrosensor utilizing a one-port surface acoustic wave reflective delay line. J Micromech Microeng 22:045007 15. Lee SW, Rhim JW, Park SW, Yang SS (2007) A micro rate gyroscope based on the SAW gyroscope effect. J Micromech Microeng 17:2272–2279 16. Oh H, Yang S, Lee K (2010) Development of SAW-based microgyroscope utilizing progressive wave. Jpn J Appl Phys 49:06GN16 17. Oh H, Lee K, Yang S, Wang W (2011) Enhanced sensitivity of surface acoustic wave gyroscope using progressive wave. J Micromech Microeng 21:075015 18. Wang W, Wang W, Liu J, Liu M, Yang S (2011) Wireless and passive gyroscope based on surface acoustic wave gyroscopic effect. Appl Phys Express 4:086601 19. Oh H, Lee KJ, Yang SS, Lee K (2012) Development of novel dual-axis sensing gyroscope using surface acoustic wave. 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Published: Feb 25, 2015

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