A thickness-shear quartz resonator force sensor with ... - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 3, NO. 4, AUGUST 2003

A Thickness-Shear Quartz Resonator Force Sensor With Dual-Mode Temperature Compensation Zheyao Wang, Huizhong Zhu, Yonggui Dong, and Guanping Feng

Abstract—An AT-cut thickness-shear quartz crystal resonator (QXR) has been used as a force sensing and self-temperaturesensing (STS) element to develop a digital output force sensor. The QXR is fixed in a two-line mounting configuration in a cylindrical metal shell by double diaphragms, through which a diametric force proportional to the unknown force is applied to the QXR. The double diaphragms improve the reliability and the mechanical stability of the sensor significantly. In order to increase the measurement range and the sensitivity, the energy trapping-based QXR is cut to a symmetrical, incomplete circular shape to decrease stress concentration. Because operating the QXR in dualmode excitation allows the separation of force change effects from temperature change effects, force measurement and STS are accomplished simultaneously with the same QXR. The structure and the configuration are optimized with theoretical analysis and FEM. The dual-mode STS and temperature compensation are described in detail, as well as a trimming method to reduce activity dips of AT-cut QXRs. Index Terms—Dual-mode oscillation, force sensor, quartz resonator, self-temperature sensing, temperature compensation.

I. INTRODUCTION

B

ULK acoustic wave quartz crystal resonators (QXRs) have a long history as sensors for force/pressure measurement. After it was shown that the resonant frequency changes are proportional to the external force applied to the QXR, i.e., the forcefrequency effect [1], force/pressure sensors were realized by means of measuring the frequency changes induced by unknown force/pressure. The advantages of QXR sensors are high resolution, high accuracy, good long-term stability, low power consumption, and digital output [2]–[4]. QXR pressure sensors have a variety of configurations to allow unknown pressure to be applied to the rim of QXRs [2]–[4]. The unknown pressure is transferred through shells to squeeze QXRs uniformly at their perimeters [5]–[7], or diametrically along a specific crystallgraphic direction [8], [9], or along the opposite edges of rectangular QXRs [10], [11]. Most QXR pressure sensors are in all-quartz structures, constructed entirely from a single piece of crystal to minimize

Manuscript received August 13, 2002; revised November 16, 2002. This work was supported by the China 9th Five-Year-Plan under Grant 96-748-01-04. The associate editor coordinating the review of this paper and approving it for publication was Prof. Chang Liu. Z. Wang is with the Institute of Microelectronics, Tsinghua University, Beijing, China (e-mail: [email protected]). H. Zhu, Y. Dong, and G. Feng are with the Department of Precision Instruments, Tsinghua University, Beijing, China. (e-mail: [email protected]. edu.cn). Digital Object Identifier 10.1109/JSEN.2003.815780

creep, hysteresis, and difference in thermal expansion constants between quartz and the shells. They are widely used in oil and gas exploration because of the excellent performance, including high accuracy, tolerance of high pressure and high temperature, and good long-term stability. Generally, this all-quartz structure is not applicable for force measurement because applying unknown force to the QXR is difficult, and its high cost of manufacturing is unacceptable in force measurement. Due to these difficulties, only several QXR force sensors have been reported. Corbett presented cylindric and surrounding structures to squeeze the diameter of a disc [9], [12], [13]. Loper reported a single-axis crystal-constrained temperature-compensated force sensor [14]. Ramm proposed a structure to clamp the perimeter of the resonator disc like a membrane and apply the force to the center of the disc [15]. Dulmet utilized b- and c- mode to sense force and temperature, respectively [16]. Temperature compensation is of importance for practical QXR sensors because the force sensitivity of QXRs is temperature dependent [17]. Conventional temperature compensation methods make use of separated temperature sensors, placed close to the QXR. This suffers from inaccuracies due to thermal lag stemming from differences in thermal time constants and temperature gradient between the QXR and the temperature sensor. To overcome this limitation, Schodowski has developed dual-c-mode SC-cut crystals to realize self-temperature-sensing (STS) with the same volume of quartz [18]. Dual-c-mode STS can measure the true temperature of QXRs. Filler, Vig, EerNisse, Dulmet, etc., have demonstrated SC-cut crystals with well-behaved performance by using the dual-c-mode excitation and succeeded in implementing their innovations in STS for microprocessor-controlled crystal oscillators, QXR pressure sensors, and microbalances [16], [17], [19]–[25]. However, it was reported that activity dips in AT-cut QXRs produce unpredictable resistance change along temperature range and make them useless in STS [26]. In this paper, we present a batch-produced digital AT-cut QXR force sensor with dual-mode STS and temperature compensation. The main problems we focused on were how to design the sensor structure to make it suitable for force measurement, and how to design the AT-cut QXR to make it free from activity dips in STS application. This paper is organized as follows: In Section II, design and mechanical characteristics of the sensor are presented. Section III gives STS and a trimming technique for eliminating activity dips. In Section IV, specifications and experimental results of STS are presented. Finally, Section V gives the overall conclusions.

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WANG et al.: THICKNESS-SHEAR QUARTZ RESONATOR FORCE SENSOR

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(a) Fig. 1.

Schematic illustration of the hybrid QXR force sensor.

II. SENSOR DESIGN The main consideration in our sensor design was that the structure should have the advantages of simple, low cost of manufacturing, and suitable for force measurement. Because metal structures can fulfill the requirement of force measurement and AT-cut QXRs have the advantages of low cost, ease of fabrication, and high force-frequency sensitivity, a metal shell and an AT-cut QXR were employed to construct a hybrid force sensor. A. Overall Structure Fig. 1 is the schematic illustration of our assembled sensor structure developed for batch production. The hybrid structure consists of two parallel metal diaphragms, a pushrod, an anvil, a cylindrical metal shell, and a QXR. The QXR was inserted into the cylindrical shell and mounted by the anvil and the pushrod. The pushrod was positioned and fixed by the double diaphragms, and they were assembled with glue as one rigid part. The unknown force was applied to the QXR through the pushrod and the double diaphragms. Because the double diaphragms deform when external force acts on the sensor, 17-4 PH stainless steel with excellent elasticity was chosen as the diaphragm material to minimize the nonelastic errors, including creep, hysteresis, and nonrepeatability. was transIt is evident that the vertical unknown force ferred to the shell along two parallel routes: one part of , , was transferred to the base of the shell via the pushrod, the QXR, , was apand the anvil. The other part of , plied to the double diaphragms, and was eventually transferred to the shell through the perimeters of the double diaphragms. that changes the resonant frequency of the QXR, as It was shown in Fig. 2. B. Double-Diaphragm Given the mechanical property that the compressive strength of crystalline quartz is approximately 24 times its tensile strength [11], it is desirable to design a structure to ensure the resonator completely in compression. In practical use, shown in Fig. 1, can destroy inevitable horizontal force, as QXRs, due to tensile stresses, and consequently decrease the reliability of the single-diaphragm sensor significantly, which made practical applications even impossible. To solve this problem, two parallel diaphragms were employed instead of

(b)

Fig. 2. (a) Complete circular QXR with diametric concentrated force. (b) Symmetrical incomplete circular QXR with diametric distribution force.

a single diaphragm to form a double-diaphragm structure to resist the horizontal force and fix the QXR firmly. Fig. 3 shows the FEM simulation results of the effects of 1-N horizontal force on a single-diaphragm and a double-diaphragm structure. Each diaphragm was 0.2-mm thick and 18 mm in diameter. It can be seen from Fig. 3 that under the action of the horizontal force the maximum deformation of the lower diaphragm of the double diaphragm is 0.9 m, whereas the maximum deformation of the single diaphragm is 51 m. Therefore, the horizontal force and tensile stresses in QXRs, which were caused by the deformation of the double-diaphragm structure, were decreased remarkably when compared with those of the single-diaphragm structure. Thus, the double-diaphragm structure can significantly decrease undesired horizontal force and increase the mechanical stability and the reliability. The advantage of the double-diaphragm structure is that it improves mechanical stability, reliability, and the ability of resisting horizontal force, but without decreasing the force sensitivity significantly. The distance between the two diaphragms in the double-diaphragm structure determines its ability to resist horizontal force. Simulation results show that the maximum deformation of the lower diaphragm is inversely proportional to this distance. However, too long a distance increases the bulk of the sensor and results in mechanical instability of the entire structure. After simulation, taking into consideration size, mechanical stability, and ability to reduce horizontal force, the optimal distance was determined to be 4 mm. C. Energy Trapping-Based QXRs 1) Symmetrical Incomplete QXRs: Circular QXRs can only be applied with concentrated force by the pushrod at contact points, as shown in Fig. 2(a). Numerical computation results show that in circular QXRs the stresses at the force applying points are very large and stress concentration is quite serious, as shown in Fig. 4(a). This stress concentration is prone to destroy QXRs by forming micro-cracks. In order to reduce localized stress concentration and enlarge measurement range, circular QXRs were cut to a symmetrical, incomplete circular shape [27]. The advantage of symmetrical incomplete circular QXRs is that they can be applied with distribution force along two parallel flats instead of concentrated force at contact points of circular QXRs, as shown in Fig. 2(b). Fig. 4(b) shows that, with the application of distribution force,

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(a)

(b)

(c)

Fig. 3. FEM simulation results of the deformations of single-and double-diaphragm structures applied with unit horizontal force. Not in scale (a) single-diaphragm structure, (b) double-diaphragm structure, and (c) magnitude of deformations. The maximum deformations of single diaphragm, the upper and the lower diaphragms of the double-diaphragm structure, are 51, 2.1, and 0.9 , respectively.

m

(a)

(b)

Fig. 4. Numerical computation results of T stress distributions of AT-cut QXRs due to the application of external force. (a) Circular QXR with concentrated force; the force applied points were treated as small flats to avoid odd points in computation. (b) Symmetrical, incomplete circular QXR with distribution force.

cut QXRs exhibit more smooth stress distributions, especially along the cut flats. Consequently, the cut QXR can withstand larger force and increase the measurement range definitely. The force-frequency sensitivity of the thickness-shear mode QXR, as shown in Fig. 2, is given by [1] (1) are, respectively, the resonant frequency and where and its changes caused by the external force , and represent times of overtone and the diameter of the QXR, respectively, is Ratajski coefficient, and is a factor relative to mounting configurations. Equation (1) indicates that the applied force is proportional to the frequency changes and can be obtained by measuring the frequency changes. of cut QXRs Numerical computation results show that is larger than that of complete circular QXRs and is inversely linear with the height of the QXRs, as shown in Fig. 5. Therefore, cut QXRs can increase both the measurement range and the force sensitivity, even though it is not possible to cut QXRs to very small dimensions because the smallest height must be larger than the limit determined by the energy trapping to ensure stable resonant frequencies. 2) Energy Trapping: The energy trapping is the basic means by which thickness-shear QXRs obtain highly stable frequencies as a result of confining thickness-shear vibration in electrode zones and restraining undesired vibration modes. Energy

Fig. 5.

Changes of K versus the height of cut QXRs.

trapping makes it possible to mount QXRs at their rims without introducing extraneous effects and acoustic loss through edges and without inducing unstable frequency. Otherwise, destroyed energy trapping would result in mechanical vibration propagating out of electrode zones and quality factors decreasing seriously, in turn causing the instability of frequency and poor measurement accuracy. Energy trapping can be intrinsically accomplished by means such as bevelled plates and proper dimensions of the electrode and diameter of QXRs, because it is primarily determined by the properties of quartz crystals and the configurations of QXRs. When a symmetrical, incomplete circular QXR was clamped by a pushrod and an anvil, the joints approached the electrodes and affected energy trapping. Thus, the major consideration in QXR design was to determine the dimensions of the diameter, the cutting flat, and the electrode so that the joints were moved


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far enough away from the electrode zone to sustain the energy trapping. According to the energy trapping theory equations, dimensions meeting the following inequality enable the thicknessshear wave to achieve energy trapping [28] (2) is the diameter of the electrode, is the thickness of where is the ratio of the eigen-frequency of electrode the QXR, is a function of and zone to that of nonelectrode zone, is the third-order elastic stiffness constants of quartz, and the frequency changes after being electroplated. Equation (2) implies that the energy trapping can be achieved by adjusting the thickness and diameter of the electrode after dimensions of resonators are given. For conventional circular third overtone 10 MHz AT-cut QXRs, the diameters of the electrode and the QXR are, respectively, 6.0 and 12.6 mm to obtain trapped energy and high quality factors. However, the dimension in the -direction influences the energy trapping more seriously than in the -direction. In addition, cut flats reduced the height in -direction, so the diameter of the resonator should be enlarged to a proper dimension to move the joints far enough away from the electrode so that the vibration can be attenuated to zero before reaching joints. After computation and experiments, the diameter of the resonator was specified to be 14.6 mm and the height should be no less than 13 mm to maintain proper distance for energy trapping. Comparison experiments showed that the degradation of quality factors of QXRs with diameter of 12.6 mm, before and after incorporated into the structure, was more serious than that of the QXRs with diameter of 14.6 mm. D. Force-Frequency Sensitivity As external force compresses the pushrod, deformation occurs in the QXR and the double-diaphragm, sequently results in displacement of the pushrod and the top flat of the QXR. It is evident that the displacement of the pushrod is the same as that of the QXR. Theoretical analysis indicates that the deformation of symmetrical incomplete circular QXR under the application of distribution force [ , as shown in Fig. 2(b)] is linear with the magnitude of the force [29] (3) where and are the equivalent stiffness and the deformation of the QXR, respectively. Considering that the deformation of the double-diaphragm structure is smaller than the thickness of the diaphragms, the relation between the external force and the deformation of the double-diaphragm with a rigid core is given as [30] (4) is the radius of diaphragms, is and the thickwhere and are the elastic modulus and ness of diaphragms,

Fig. 6.

K

versus the thickness of different diaphragms.

the Poisson’s ratio of the material, respectively, is the to the radius of the pushrod, and ratio of is the equivalent and stiffness of the double-diaphragm. Note that (3), (4), one can obtain (5) is a constant and defined as force transfer factor. where Combined with (1) and (5), the force-frequency sensitivity of the sensor is (6) Equations (5) and (6) indicate that the sensitivity of the sensor is linear with the sensitivity of the QXR. versus the thickness of Fig. 6 illustrates the curves of the the diaphragm. Curves 1, 2, and 3 represent three diaphragms with different equivalent stiffness and dimensions. Curve 1 rep, curve 2 represents , and curve 3 represents . In fact, curve 2 is the true situation in our resents practical diaphragm configuration. Thin diaphragms make the structure have a large force transfer factor and high sensitivity. However, it should be noted that thin diaphragms cause smaller measurement range and poor ability to resist horizontal force. In addition, the small nonlinear deformation scope of thin diaphragms results in the nonlinearity of QXR sensors. III. DUAL-MODE TEMPERATURE COMPENSATION Separated temperature sensors in conventional temperature compensation result in inaccuracies due to thermal lag (thermal path differences, and unequal thermal-time-constants), thermal gradients, and instabilities of temperature sensors [17]. For example, thermistors used as separate temperature sensors in our experiments had smaller thermal time constants than quartz, so in the process of temperature upward, measured temperature was higher than the true temperature of the QXR. This resulted in over compensation. Similarly, deficient compensation occurred in the case of temperature downward. In order to avoid inaccuracies induced by separate temperature sensors, dual-c-mode STS was developed for AT-cut QXRs based on the idea of [31]. In this paper, we focused on developing a trimming technique to eliminate the activity dips of AT-cut QXRs.

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A. Self-Temperature Sensing Dual-mode excitation is possible because many orders of overtone resonant frequencies can be excited simultaneously. For AT-cut QXRs, both the fundamental and the third overtone frequencies are functions of temperature and force, i.e., they are temperature and force cross sensitive. However, for AT-cut QXRs, one can obtain [31], [32] (7) is the normalized beat frequency changes bewhere tween three times the fundamental frequency and the third overis the temperature changes, is a constant tone frequency, determined by quartz cut and the first order frequency-temperature coefficients of the fundamental and the third overtone frequencies. It can be seen from (7) that the normalized beat frequency is a linear function of temperature and is force independent. Therefore, by exciting dual modes, STS and force measurement can be accomplished simultaneously by measuring the changes of the beat frequency and the changes of the third overtone frequency.

Fig. 7. Changes of motional resistance versus temperature. The cutting flats have significant effect on reducing the activity dips at 12 and 28 C.

Fig. 8. Block diagram of temperature compensation of quartz resonator force sensor.

B. Elimination of Activity Dips Because AT-cut QXRs exhibit more serious activity dips than SC-cut QXRs, the obstacle in the realization of dual-mode STS of AT-cut QXRs was how to eliminate the activity dips. The activity dips prevent AT-cut QXRs from being used in dual-mode STS because they make the frequency-temperature characteristics of AT-cut QXRs unfit for a smooth third-order curve equation and make the valid temperature range shorter than the desired temperature range. It is known that the most likely cause of the activity dips is the coupling of the thickness-shear mode of the AT-cut QXR with multiples of high temperature coefficients of low frequency modes, e.g., the flexural vibration mode [33]. It is evident that the flexural vibration frequencies can be changed by varying plate dimensions, which determine the coupling frequencies and do not change the thickness-shear mode frequency [26], [34]. Therefore, it is possible to make AT-cut QXRs free from activity dips in temperature range from 20 by cutting parallel flats. to 50 Experiments were carried out to compare the effect of different heights on the occurrence of activity dips. For cut and enlarged AT-cut plates with a diameter of 14.6 mm (as described earlier), a variety of QXRs with heights from 13 mm (as described in part B of Section II) to 14.4 mm in intervals of 0.2 mm were used to determine the optimal values of the height with minimal activity dips. The results showed that QXRs with height of 13.2 and 13.4 mm do not show obvious activity . Therefore, the 13.4-mm height was dips from 20 to 50 adopted in the design as the best compromise between energy trapping, sensitivity, and activity dips. Fig. 7 shows the changes of the motional resistance of a typical AT-cut QXR before and after cutting flats. It is obvious that after flat cutting, the and motional resistances (represents of activity dips) at 12 were reduced to reasonable values. 28

C. Annealing for Reducing Residual Stresses There are several reasons that cause residual stresses in QXRs, including flat cutting, polishing, and electroplating. In addition, mounting QXRs with the pushrod and the anvil also induces even more serious stresses in QXRs. Residual stresses were recognized as important impact factors of thermal hysteresis and can be reduced by temperature cycles [35]–[37], so it was necessary to develop an annealing method to reduce residual stresses in QXRs and sensors induced during manufacturing and assembly. First, the resonators were placed in for 20 min, then the temperature was a chamber at 120 for 20 min, and this step was repeated 12 changed to 20 times. Then, the resonators were placed in a chamber at 120 for 48 h for releasing stresses and stabilizing. After annealing, the frequency stability of the sensors was improved [32]. IV. EXPERIMENTAL RESULTS AND DISCUSSION The force sensor used a third overtone 10-MHz (3.3-MHz fundamental) AT-cut QXR as the sensing element. The dual-mode oscillator was realized with two independent oscillators having different eigen-frequencies coupled to each other by the QXR. By exciting the dual modes simultaneously, the frequency shifts of the third overtone gave the magnitude of the force. The beat frequency changes, which were obtained by mixing three times the fundamental frequency with the third overtone frequency, gave the magnitude of temperature. The sensor gave the frequency signal as output, which avoided the A/D converter and complicated analog circuits, and reduced the complexity and cost of the force measurement system. A microprocessor was used to control the measuring system, including force measurement, STS, and temperature compensation, as shown in Fig. 8. The system was supplied with 3-V DC.


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TABLE I SPECIFICATIONS OF QUARTZ RESONATOR SENSORS AND TYPICAL STRAIN-GAUGE SENSORS

diaphragms. The nonrepeatability is due to the mechanical instability of the resonator under the external force because they are two-line mounted, not surrounding mounted. V. CONCLUSION

Fig. 9. Measurement errors of the force sensitivity with different compensation method in temperature changing environment.

A. Temperature Compensation Performance Since the dual-mode STS can decrease the error induced by separate temperature sensors, it was expected to measure temperature more accurately for temperature compensation, especially in dynamic temperature processes. Fig. 9 shows the comparison of the measured errors of the force-frequency sensitivity compensated by a separate temperature sensor and the dual-mode STS separately in the temper. The measureature changing process from 20 to 50 is the sensitivity at referenced temperament error was , ture. The dual-mode STS can reduce the maximum error from 2.55 to 0.21%. Therefore, temperature compensation employed STS as the temperature sensing method reduced the measurement error effectively, and the force sensor met the requirement of practical use. B. Measurement Errors Table I gives the performance of the QXR sensors and typical strain-gauge force sensors. It can be seen that the performance of the QXR reaches the accuracy level of typical strain-gauge sensors except for the repeatability. Several efforts have been attempted to probe the errors and reduce them. Adhesive is definitely the most important contributing factor to the nonelastic errors, including hysteresis, repeatability, and creep. However, the improvement of hysteresis and frequency drift after annealing indicates that residual stresses can deteriorate the performance of the sensor. Theoretical analysis based on the micro-peak-model shows that nonelastic errors are related to the preloaded force and the roughness of the contact surfaces of pushrods. The linearity, in the event of over loading, drops seriously because the diaphragms fall into nonlinear range. In fact, because diaphragms only take a small part of unknown force, the small measurement range of thin diaphragms is due to the fact that they fall into the nonlinearity scope than easily thick

A low cost, low power consumption, and temperature compensated digital force sensor has been developed by employing AT-cut QXRs and a double-diaphragm structure. The sensitivity and the measurement range have been improved by cutting energy trapping-based QXRs in a symmetrical incomplete circular shape. The double-diaphragm structure improves the stability, the repeatability, and the reliability of the sensors by reducing horizontal force. Microprocessor-controlled temperature compensation has been accomplished by developing dual-mode self-temperature-sensing and a trimming technique to avoid activity dips. It eliminates inaccuracies caused by thermal lag when compared with conventional separated sensing elements and improves the measurement accuracy significantly, especially in environments of fluctuating temperature. REFERENCES [1] J. Ratajski, “Force-frequency coefficient of singly rotated vibrating quartz crystals,” IBM J. Res. Dev., vol. 12, pp. 92–99, 1968. [2] E. EerNisse and R. Wiggins, “Review of thickness-shear mode quartz resonator sensors for temperature and pressure,” IEEE Sensors J., vol. 1, pp. 79–87, June 2001. [3] E. Bens, M. Groschl, W. Burger, and M. Schmid, “Sensors based on piezoelectric resonators,” Sens. Actuators A, vol. 48, pp. 1–21, 1995. [4] E. EerNisse, R. Ward, and R. Wiggins, “Survey of quartz bulk resonator sensor technologies,” IEEE Trans. Ultrason. Ferroelect. Freq. Contr., vol. 35, pp. 323–330, May 1988. [5] H. Karrer and J. Leach, “A quartz resonator pressure transducer,” IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-16, pp. 44–50, July 1969. [6] E. EerNisser and R. Ward, “Quartz resonator sensors in extreme environments,” in Proc. 45th Annu. Frequency Control Symp., 1991, pp. 254–260. [7] L. Clayton and E. EerNisse, “Quartz thickness-shear mode pressure sensor design for enhanced sensitivity,” IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., vol. 45, pp. 1196–1203, Sept. 1998. [8] E. Karrer and R. Ward, “A low-range quartz resonator pressure transducer,” ISA Trans., vol. 16, pp. 90–98, 1977. [9] J. Corbett, “Crystal force and pressure transducers,” U.S. Patent 5 424 598, 1995. [10] R. Besson and J. Boyetc, “A dual-mode thickness-shear quartz resonator pressure sensor,” IEEE Trans. Ultrason. Ferroelect. Freq. Contr., vol. 40, pp. 584–591, Sept. 1993. [11] N. Matsumoto, Y. Sudo, B. Sinha, and M. Niwa, “Long-term stability and performance characteristics of crystal quartz gauge at high pressures and temperatures,” IEEE Trans. Ultrason. Ferroelect. Freq. Contr., vol. 47, pp. 346–354, Mar. 2000. [12] J. Corbett, “Oscillating crystal force transducer system,” U.S. Patent 3 541 849, 1970. [13] , “Moveable seat for crystal in an oscillator crystal transducer system,” U.S. Patent 4 126 801, 1978. [14] E. Loper, “Single-axis crystal constrained temperature compensated digital force sensor,” U.S. Patent 4 258 572, 1981. [15] A. Ramm and J. Formaz, “Quartz resonating force and pressure transducer,” U.S. Patent 4 644 804, 1987.

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Zheyao Wang was born in China in 1972. He received the B.S. degree in mechanical engineering and the Ph.D. degree in mechatronics from Tsinghua University, Beijing, China, in 1995 and 2000, respectively. From 2000 to 2002, he was a Postdoctoral Research Fellow with the Institute of Microelectronics, Tsinghua University. He is currently an Assistant Professor at Tsinghua University and his research interests are in wireless sensors and RF MEMS.

Huizhong Zhu was born in China in 1958. He received the M.S. degree in mechanical engineering from the Luoyang Institute of Technology, China, and the Ph.D. degree in mechatronics from Tsinghua University, Beijing, China, in 1988 and 1995, respectively. He is currently an Associate Professor at Tsinghua University, where he is interested in digital force sensors and biosensors, as well as measuring instruments.

Yonggui Dong was born in China in 1965. He received the B.S. degree in mechanical engineering and Ph.D. degree in mechatronics from Tsinghua University, Beijing, China, in 1988 and 1994, respectively. He is currently an Associate Professor at Tsinghua University. His main research work is focused on resonant sensors and their applications, including resonant sensing mechanisms and BAW and SAW chemical sensors.

Guanping Feng was born in China in 1946. He received the B.S. and M.S. degrees in mechanical engineering from Tsinghua University, Beijing, China, in 1970 and 1980, respectively. In 1970, he joined the Department of Precision Instruments, Tsinghua University. As a Full Professor, he is now interested in acoustic resonant sensors, including SAW chemical, physical, and bio sensors, BAW force and bio sensors, as well as precision instruments. He is currently the Assistant President of Tsinghua University and the Operation Dean of the Research Institute of Tsinghua University in Shenzhen. He has authored or coauthored more than 120 papers published in refereed journals and symposia proceedings and he holds ten Chinese and U.S. patents. He has also advised 15 Ph.D. students.