Implementation of Multiparameter Monitoring by an LC-Type Passive

168

IEEE INTERNET OF THINGS JOURNAL, VOL. 2, NO. 2, APRIL 2015

Implementation of Multiparameter Monitoring by an LC-Type Passive Wireless Sensor Through Specific Winding Stacked Inductors Lei Dong, Li-Feng Wang, Member, IEEE, and Qing-An Huang, Senior Member, IEEE

Abstract—A traditional LC-type passive wireless sensor has been used for the measurement in sealed environments. Due to the limitation of its operation principle, this method was only suitable for a single parameter monitoring. For most applications, it is desirable for multiparameters to be monitored. In order to keep the chip area of the sensor as small as possible, multilevel inductors may be coaxially stacked. However, the transmitting signals affect each other due to strong mutual coupling between the stacked inductors. This paper presents a novel inductor structure by using a specific winding to achieve the minimized mutual inductance. Using the partial inductance theory, the mutual inductance of two stacked inductors is analyzed. Simulations of the two stacked inductors coaxially aligned, with each connected in a variable capacitor, show that the mutual inductance between the two inductors can be greatly suppressed. This phenomenon has also been verified through multilayer printed circuit board (PCB) inductors. Capacitive temperature and pressure sensors were linked to the two stacked inductor to implement the simultaneous measurements of temperature and pressure. The measurement results indicate that the sensitivity of the temperature sensor is about 41.67 kHz/◦ C between −20 ◦ C and 100 ◦ C, while the sensitivity of the pressure sensor is about −133.33 kHz/kPa between 50 and 110 kPa. Index Terms—Coaxially aligned, inductor, multiparameter, passive wireless sensor.

I. I NTRODUCTION

T

HE LC-type passive wireless sensor has been studied to monitor various parameters of interest in situations where wired connections are difficult or even impossible, such as pressure [1]–[6], strain [7], temperature [8], [9], humidity [10], [11], threshold g-value [12], and pH [13]. This passive wireless sensing method provides the advantages in long lifetime, low cost, small size, and feasibility of installation for devices in the Internet of Things. In addition, the implementation of multiparameter detection without interference would increase the density of sensing and simplify the sophistication of the associated processing, which is the key issue in the scope of

Manuscript received May 03, 2014; revised September 03, 2014; accepted December 21, 2014. Date of publication December 24, 2014; date of current version March 16, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61136006, in part by the National High Technology Research and Development Program of China under Grant 2013AA041101, and in part by the New Teacher Startup Fund of Southeast University under Grant 1106007111. The authors are with the Key Laboratory of MEMS, Ministry of Education, Southeast University, Nanjing 210096, China (e-mail: hqa@seu. edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JIOT.2014.2385798

Fig. 1. LC-type passive wireless sensing system. (a) Schematic diagram. (b) Equivalent circuit.

the “smart world” [14]. This noncontact measurement technique utilizes an LC resonant tank to determine the parameter remotely. The capacitor or the inductor or both of them changes in response to the parameter of interest, resulting in a shift in its resonant frequency. A schematic diagram and an equivalent circuit of an LC-type passive wireless sensing system are shown in Fig. 1, where the sensor element consists of a fixed inductor and a variable capacitor. To wirelessly interrogate the LC-type sensor, an external readout coil is magnetically coupled to the sensor and the resonant frequency of the sensor tank is determined through the readout coil impedance. The resonant frequency of the sensor tank is determined through the readout coil impedance or input return loss, S11 parameter. Equivalent input impedance (Zin ) of the readout coil can be represented by Zin = R0 + jωL0 +

ω2 M 2 RS + jωLS − j ωC1 S

(1)

where ω is the angular frequency, LS , CS , and RS are the inductance, capacitance, and resistance of the sensor tank,

2327-4662 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

DONG et al.: IMPLEMENTATION OF MULTIPARAMETER MONITORING BY LC-TYPE PASSIVE WIRELESS SENSOR

169

respectively, and M is the mutual inductance of the two inductors. The impedance Zin can be rewritten by the real part (Re(Zin )) and the imaginary part (Im(Zin )), and the phase (∠Zin ) of Zin can be expressed as ⎡ ⎤  2 f 1 − fS ⎢ ⎥ Im(Zin ) = 2πf L0⎣1 + k 2 Q2 (2)  2 ⎦ f f 1 + Q2 fS − fS 2

Re(Zin ) = R0 + 2πL0 k Q ∠Zin = arctan

1 + Q2

f ffS  f fS

−

Im(Zin ) Re(Zin )

fS f

2

(3)

(4)

where fS and Q are the resonant frequency and the quality factor of the sensor tank, respectively, and k is the coupling coefficient between the two inductors. The input return loss, S11 parameter, for one port can be expressed as

Zin − Z0

. (5) S11 = Zin + Z0 Z0 =50 Ω In most reported works, the noncontact detection of the resonant frequency of the LC-type passive wireless sensor is achieved through the phase or the S11 dip as the phase (∠Zin ) and the S11 parameter would drop to the minimum at the resonant frequency [15]. The existing LC sensors are limited in applications that require simultaneous detection of multiparameters. The inductors can be stacked for the sake of saving area. However, the transmitting signals affect each other due to the strong mutual coupling between the inductors and the resonant frequencies detected could be shifting or even missing. A method of decreasing overlapping area has been proposed, but the interference still exists [16]. A novel structure for stacked inductors is proposed in this paper for passive wireless multiparameter sensor. The inductor is in a specific winding to achieve a minimized mutual inductance when coaxially aligned stacked in substrate. In Section II, a model for theoretical calculation of self and mutual inductance is developed. In Section III, the simulation of two inductors coaxially aligned each connected with a variable capacitor is introduced. In Section IV, the stacked inductor is realized by a multilayer PCB, and the variable capacitors are replaced by capacitive pressure and temperature sensors. Finally, experimental results are given. The measurement results indicate that the sensitivity of the temperature sensor is about 41.67 kHz/◦ C between −20 ◦ C and 100 ◦ C, while the sensitivity of the pressure sensor is about −133.33 kHz/kPa between 50 and 110 kPa.

Fig. 2. 3-D model of two staked inductors in the proposed structure connected with two sensors, respectively.

Fig. 3. Proposed structure of planar inductor.

inductors can be stacked, in principle, forming more LC sensing circuits. The stacked inductors in Fig. 2 are in specific winding to achieve the minimized mutual inductance between them. The specific winding of the stacked inductors is shown in Fig. 3. The rotation direction of every loop is opposite to the adjacent ones and there must be even turns. If the inductor is taken as an example, the first loop (from outer to inner) is anticlockwise, and the second is clockwise, then the third comes anticlockwise. The rest can be done in the same manner until to the sixth loop. Coils can begin with clockwise or anticlockwise. According to partial inductance theory [17], [18], any geometry conductors could be divided into several rectangular conductor segments. For a very long conductor bar i, the self-inductance of rectangular cross section is given by [19]

II. D ESIGN AND M ODELING In an LC-type passive wireless multiparameter sensor system, the inductors can be stacked in substrate for the sake of saving area as displayed in Fig. 2. The second layer inductor is coaxially placed above the first layer inductor. Two capacitive sensors for different parameter sensing are connected to the corresponding inductors, forming two LC resonant circuits. More

LP ii

 1 µ0 · l 2l ∼ + ln = 2π w+t 2

(6)

where l, w, and t are the length, width, and thickness, respectively.

170

IEEE INTERNET OF THINGS JOURNAL, VOL. 2, NO. 2, APRIL 2015

where the negative sign means the opposite direction of currents flowing on the metal bars, and ⎡ ⎛ ⎞ ⎤   2 2 L L µ0 ⎣ ⎝ L L L ln + + 1⎠ + − + 1⎦ Mk31 = 2π L L L L ⎡ Mk71 =

⎛



2

(11)

⎞

L µ0 ⎣ ⎝ L L ln + + 1⎠ 2π L−s L−s ⎤  2 L−s L−s + − + 1⎦. L L

(12)

In the condition of L  s, Mk71 = Mk31 .

Fig. 4. Two inductor coils coaxially aligned both consist of two loops in the proposed structure.

(13)

Similarly, we have Mutual inductance between two parallel filaments i and j of length l and separation s which are aligned is [19] ⎡ ⎛ ⎞ ⎤    2 2 l s µ0 ⎣ ⎝ l s l ln + + 1⎠ + − + 1⎦ . MP ij = 2π s s l l (7) When l  s, (7) can be written as  2l µ0 l ln − 1 . M= 2π s

Lkk =

i=1

Lkii +

Nk Nk  

Mkij

Mk13 = Mk93

Mk24 = Mk84

Mk26 = Mk86

Mk48 = Mk68

Mk17 = Mk97 .

(14)

Therefore, self-inductance of coil k can be written as Lkk = (Lk11 − Mk91 ) + (Lk22 − Mk82 ) + (Lk33 − Mk73 ) + (Lk44 − Mk64 ) + (Lk66 − Mk46 ) + (Lk77 − Mk37 ) + (Lk88 − Mk28 ) + (Lk99 − Mk19 ) + Lk55 .

(8)

Two loops in the planar inductor in Fig. 3 are taken as an example to calculate the self and mutual inductance as shown in Fig. 4. The two-loop coil is in square shape with the side length L and a separation of s between two adjacent turns with the condition of L  s. Both inductor coils are divided into nine segments and placed coaxially aligned at a distance of D. The self-inductance of coil k can be written as Nk 

Mk42 = Mk62

(9)

i=1 j=1

where Lkii and Mkii represent the self-inductance of single segment and mutual inductance between two bars in coil k. Since there is no flux contribution between perpendicular bars, mutual inductances such as Mk12 and Mk14 are all zero. The mutual inductance between Lk55 and other segments in coil k can be ignored as L  s. Therefore, (9) can be simplified into

(15)

The mutual inductance between coil k and coil n can be written as Mkn =

Nk  Nn 

Mkiinjj =

i=1 j=1

9  9 

Mkiinjj

i=1 j=1

= Mk11n22 + Mk11n44 + Mk11n66 + Mk11n88 + Mk22n11 +Mk22n33 +Mk22n55 +Mk22n77 +Mk22n99 + Mk33n22 + Mk33n44 + Mk33n66 + Mk33n88 + Mk44n11 +Mk44n33 +Mk44n55 +Mk44n77 +Mk44n99 + Mk55n11 + Mk55n33 + Mk55n77 + Mk55n99 + Mk66n11 +Mk66n33 +Mk66n55 +Mk66n77 +Mk66n99 + Mk77n22 + Mk77n44 + Mk77n66 + Mk77n88 + Mk88n11 +Mk88n33 +Mk88n55 +Mk88n77 +Mk88n99 + Mk99n22 + Mk99n44 + Mk99n66 + Mk99n88 . (16) Since L  s, the mutual inductance Mk55njj and Mkiin55 can be ignored.

Lkk =(Lk11 − Mk31 + Mk71 − Mk91 ) + (Lk22 − Mk42 + Mk62 − Mk82 ) + (Lk33 − Mk13 − Mk73 + Mk93 )

A. When D  s

+ (Lk44 − Mk64 − Mk24 + Mk84 ) + (Lk66 − Mk46 + Mk26 − Mk86 ) + (Lk77 + Mk17 − Mk37 − Mk97 )

|Mk11n44 | =

⎡

+ (Lk88 − Mk28 + Mk48 − Mk68 ) + (Lk99 − Mk19 + Mk39 − Mk79 ) + Lk55

(10)

⎛







2

⎞

L µ0 ⎣ ⎝ L L ln + 1⎠ + 2π D D ⎤  2 D D + − + 1⎦ L L

(17)

DONG et al.: IMPLEMENTATION OF MULTIPARAMETER MONITORING BY LC-TYPE PASSIVE WIRELESS SENSOR

171

⎡ ⎛ ⎞  2 L L µ0 ⎣ ⎝ √ L ln √ |Mk11n66 | = + + 1⎠ 2π D2 +s2 D 2 + s2 ⎤  2  √ √  2 2 2 2 D +s D +s ⎥ − + 1⎦ . + L L (18) In the condition of D  s, then |Mk11n66 | − |Mk11n44 | = 0.

(19)

Similarly, we have Mk11n44 = −Mk11n66

Mk11n88 = −Mk11n22

Mk22n33 = −Mk22n77 Mk33n22 = −Mk33n88

Mk22n11 = −Mk22n99 Mk33n44 = −Mk33n66

Mk44n33 = −Mk44n77 Mk66n11 = −Mk66n99 Mk77n22 = −Mk77n88

Mk44n11 = −Mk44n99 Mk66n33 = −Mk66n77 Mk77n44 = −Mk77n66

Mk88n11 = −Mk88n99

Mk88n33 = −Mk88n77

Mk99n44 = −Mk99n66

Mk99n22 = −Mk99n88

Fig. 5. ADS simulation setup.

(20)

where the negative sign means the opposite direction of mutual coupling. Therefore, the mutual inductance between coil k and coil n is Mkn = 0.

(21)

In summary, to suppress the interference of the transmitting signals caused by the mutual inductance, the side length L of the inductor would be designed far greater (at least 10 times) than the separation s with a distance D not greater than the separation.

B. When D = s  L The mutual partial inductance Mk11n66 and Mk11n44 can be calculated by using (8) 2L µ0 2L µ0 L ln − L ln √ 2π D 2π D 2 + s2 √ √ µ0 D 2 + s2 µ0 L ln = L ln 2. = 2π D 2π (22) The mutual inductance Mkn is given by √ √ µ0 µ0 L · ln 2 = L · ln 16. Mkn ∼ (23) = 16 × 2π π

|Mk11n66 | − |Mk11n44 | =

Then the mutual inductance of two coils both with N turns can be derived using (23) √ µ0 µ0 (24) Mkn = 4N L · ln 2 = N L · ln 4. π π C. When D = M · s When the distance D becomes M times of the separation s, the mutual inductance between two coils both consist of N turns can be reasoned using (22) and (24) Mkn = 2N

M +1 µ0 L · ln . π M

(25)

The mutual inductance Mkn will be zero when M becomes very large, which is consistent with (21). The mutual inductance becomes greater when the distance between two inductors is reduced, and reaches the maximum when the distance is equal to the separation s.

III. S IMULATION To demonstrate how the two resonant frequencies of two stacked inductors affect each other due to mutual coupling, Agilent Advanced Design System (ADS) 2011 was used to make electromagnetic (EM) and schematic simulations. As an example of the layout of inductors, the number of turns was set to eight in the proposed structure. The outermost side length was 10 mm and the width was 250 µm with a separation of 120 µm between adjacent loops. The side length is far greater than the separation. The two inductors were coaxially aligned and the distance between them was 120 µm, which is equal to the separation. The mutual inductance was calculated using (25) and the value was 44.36 nH. To capture the two resonant frequencies, there was also a readout coil consisting of two loops in an ordinary structure inductively coupled with the two inductors as shown in Fig. 5. The distance between the readout coil and the two inductors was 2 mm. The inductors and readout coil were modeled in layout and simulated in EM mode. Then the model in layout and the simulation results were imported into schematic window as a component. The readout coil was linked to a term with impedance normalized to 50 Ω. Two discrete capacitors were connected to the two inductors, respectively, forming two resonant circuits. The two variable capacitors were then changed their capacitance in turn to obtain different frequency responses. Fig. 6 shows the S11 parameter versus frequency when capacitor C1 increased from 400 to 600 pF with a step of 50 pF when capacitor C2 kept constant at 150 pF. It is apparent that the resonant frequency f1 corresponding to C1 monotonically

172

IEEE INTERNET OF THINGS JOURNAL, VOL. 2, NO. 2, APRIL 2015

Fig. 6. S11 parameter versus frequency when C1 increases from 400 to 600 pF with capacitor C2 being constant at 150 pF.

Fig. 8. Two stacked inductors in the same size were embedded in a multilayer PCB with a capacitive temperature sensor and a capacitive pressure sensor connected to them, respectively.

Fig. 7. S11 parameter versus frequency when C1 is held at 500 pF with C2 varying from 140 to 200 pF.

decreases from 49.5 to 40 MHz (49.5, 46.5, 44.5, 42, 40 MHz), while resonant frequency f2 corresponding to C2 is kept at 87 MHz. The resonant frequency f2 does not shift when f1 changes as a function of C1. Then C1 was maintained at 500 pF while C2 varied from 140 to 200 pF with a step of 15 pF and the results are given in Fig. 7. It clearly shows that the resonant frequency f2 changes from 90 to 75 MHz (90, 86.5, 81.5, 78, 75 MHz) with f1 at 44.5 MHz all the way. There is no offset in resonant frequency f1 when f2 changes as a function of capacitor C2. It is then confirmed that the interference between resonant frequencies of the two LC tanks due to mutual coupling can be suppressed. Each of the two resonant frequencies corresponds to its own respective capacitors. IV. E XPERIMENTS AND R ESULTS To validate the proposed structure in practical applications of passive wireless multiparameter sensors, two copper inductors in the same size were embedded in PCB, as illustrated in Fig. 8. The PCB has three layers with each layer 320 µm high.

Two inductors were stacked in the first and second layers and capacitive sensors were integrated on the third layer. It is well known that the capacitive sensors have advantages of the low power consumption, low temperature drift, and good long-term stability [20]. The two inductors were both composed of 20 loops in the proposed structure and they were coaxially aligned at a distance of 320 µm. The outermost side length was 15 mm and the line width was 150 µm with a separation of 100 µm between adjacent loops. The mutual inductance was calculated using (25) and the value was 65.26 nH. A capacitive temperature sensor and a capacitive pressure sensor were each connected to one of the two inductors, respectively. The comb electrodes of the capacitive temperature sensor were fabricated by the SOG (silicon-on-glass) process. The sensitive material is polydimethylsiloxane (PDMS) silicone oil. The relative dielectric constant of the silicone oil would change as a function of temperature resulting in the change of capacitance [21]. The capacitive pressure sensor is a commercial product VTI SCB10H which is a silicon absolute pressure sensor. The pressure element consists of two silicon wafers and one glass wafer bonded together by anodic bonding. One silicon wafer forms a diaphragm, which bends as a function of outer pressure. The pressure is proportional to the bending of the diaphragm, and hence to the capacitance between the electrodes. Therefore the capacitance is a function of the outer pressure. As the metal pads of the two electrodes are on the back side of the pressure sensor while the membrane wafer which is used to sense the pressure is on the front side, the pressure sensor has to be embedded in a hole reserved in the PCB. A temperature test chamber was used to control temperature as exhibited in Fig. 9. In order to simultaneously control the

DONG et al.: IMPLEMENTATION OF MULTIPARAMETER MONITORING BY LC-TYPE PASSIVE WIRELESS SENSOR

Fig. 9. Measurement setup.

temperature and the pressure, a portable pressure testing system which can be placed in the temperature chamber is shown in the upper right of Fig. 9. The pressure in the sealed chamber can be monitored by the pressure gauge (Tenmars VC 9200) in real time. The pressure source was a pump that can exhaust and inflate air in the sealed chamber so as to change the pressure. This testing configuration was applied to condition the sensors for testing convenience without losing the fidelity of the sensor performance. The frequency response of the two LC tank was detected by Agilent N5224A PNA network analyzer. The network analyzer was connected with a 15-mm-diameter coil consist of two loops which are served as the readout antenna. During the test, the PCB was placed inside the sealed chamber and the external reader coil was aligned above the sealed chamber on the same axis using a manipulation stage for in situ wireless sensing. The sealed chamber was located in the temperature box. When the temperature varied, the pressure in the sealed chamber was kept to be unchanged through the pressure gauge and the pump. When the pressure was changed, the whole PCB was under constant temperature controlled by the temperature chamber. To verify that there is no interference between the two signals, we have to fix one parameter and change the other one. If the two parameters were both changed simultaneously, than we could not distinguish whether the shift in frequency was caused by the interference or it was just a response of the corresponding parameter. For temperature response, the temperature was raised from −20 ◦ C to 100 ◦ C with a step of 20 ◦ C when the pressure was kept at 100 kPa. Fig. 10 gives the S11 parameter detected and the inset is the resonant frequency versus temperature to show the behavior of two resonant frequencies in detail. The resonant frequency f1 corresponding to temperature monotonically increased from 67 to 72 MHz as the capacitance of temperature sensor decreased. The sensitivity of the temperature sensor is about 41.67 kHz/◦ C. The resonant frequency f2 corresponding to pressure held at 90.5 MHz without shift in frequency.

173

Fig. 10. S11 parameter detected versus frequency when temperature ranged from −20 ◦ C to 100 ◦ C with pressure was kept at 100 kPa.

Fig. 11. S11 parameter detected versus frequency when temperature was controlled at 20 ◦ C and pressure changed from 50 to 110 kPa.

Then the temperature was controlled at 20 ◦ C while pressure changed from 110 to 50 kPa with a step of 10 kPa. Fig. 11 gives the S11 parameter detected versus frequency and the inset shows the resonant frequencies behavior as a function of pressure. It is obvious that resonant frequency f1 remains at 69 MHz while f2 shifts from 89 to 97 MHz because the capacitance of the pressure sensor decreases as a function of outer pressure. The sensitivity of the pressure sensor is about −133.33 kHz/kPa. Every curve in Figs. 10 and 11 can be seen as the monitoring result of one static moment in situations when the temperature and pressure are changed simultaneously. It can be seen that the temperature and pressure can be monitored by their corresponding resonant frequencies without interference due to the mutual coupling between inductors.

174

IEEE INTERNET OF THINGS JOURNAL, VOL. 2, NO. 2, APRIL 2015

V. C ONCLUSION This paper has presented a novel structure of a planar inductor that can be coaxially aligned and stacked for the sake of saving area in integration. A theoretical model for selfinductance and mutual inductance is developed. The mutual inductance would be the same when the first loops of the two inductors rotate in either the same or opposite direction. In addition, the mutual inductance can be greatly suppressed when the two inductors have different even number of loops. Simulations and experiments validate applications in passive wireless multiparameter sensors. Experimental results show that the two resonant frequencies of the two LC sensing circuits have no apparent interference while sensing two environmental parameters: temperature and pressure. It also shows that the two specific winding inductors proposed here have no apparent mutual inductance. Furthermore, inductors in the proposed structure can be added to three or more for more parameters monitoring. R EFERENCES [1] O. Akar, T. Akin, and K. Najafi, “A wireless bath sealed absolute capacitive pressure sensor,” Sens. Actuators A, Phys., vol. 95, no. 1, pp. 29–38, Dec. 2001. [2] M. A. Fonseca, J. M. English, and M. G. Allen, “Wireless micromachined ceramic pressure sensor for high-temperature applications,” J. Microelectromech. Syst., vol. 11, no. 4, pp. 337–343, Aug. 2002. [3] A. Baldi, W. Choi, and B. Ziaie, “A self-resonant frequency-modulated micromachined passive pressure transensor,” IEEE Sensors J., vol. 3, no. 6, pp. 728–733, Dec. 2003. [4] P. J. Chen, D. C. Rodger, S. Saati, M. S. Humayun, and Y. C. Tai, “Microfabricated implantable parylene-based wireless passive intraocular pressure sensors,” J. Microelectromech. Syst., vol. 17, no. 6, pp. 1342– 1351, Dec. 2008. [5] N. Xue, S. P. Chang, and J. B. Lee, “A SU-8-based microfabricated implantable inductively coupled passive RF wireless intraocular pressure sensor,” J. Microelectromech. Syst., vol. 21, no. 6, pp. 1338–1346, Dec. 2012. [6] T. Salpavaara, J. Verho, P. Kumpulainen, and J. Lekkala, “Readout methods for an inductively coupled resonance sensor used in pressure garment application,” Sens. Actuators A, Phys., vol. 172, no. 1, pp. 109–116, Dec. 2011. [7] Y. Jia, K. Sun, F. J. Agosto, and M. T. Quinones, “Design and characterization of a passive wireless strain sensor,” Meas. Sci. Technol., vol. 17, no. 11, pp. 2869–2876, Nov. 2006. [8] Y. Wang, Y. Jia, Q. Chen, and Y. Wang, “A passive wireless temperature sensor for harsh environment application,” Sensors, vol. 8, no. 12, pp. 7982–7995, 2008. [9] S. Emilio and S. Mauro, “Wireless measurement electronics for passive temperature sensor,” IEEE Trans. Instrum. Meas., vol. 61, no. 9, pp. 2354–2361, Sep. 2012. [10] T. Harpster, S. Hauvespre, M. Dokmeci, and K. Najafi, “A passive humidity monitoring system for in situ remote wireless testing of micropackages,” J. Microelectromech. Syst., vol. 11, no. 1, pp. 61–67, Feb. 2002. [11] C. Zhang, J. Q. Huang, and Q. A. Huang, “A passive wireless graphene oxide based humidity sensors and associated portable telemetry unit,” in Proc.17th Int. Conf. Solid-State Sens. Actuators Microsyst. (Transducers’13), Barcelona, Spain, Jun. 16–20, 2013, pp. 278–281. [12] J. C. Kuo et al., “A passive inertial switch using MWCNT hydrogel composite with wireless interrogation capability,” J. Microelectromech. Syst., vol. 22, no. 3, pp. 646–654, Jun. 2013. [13] S. Bhadra, D. S. Y. Tan, D. J. Thomson, M. S. Freund, and G. E. Bridges, “A wireless passive sensor for temperature compensated remote pH monitoring,” IEEE Sensors J., vol. 13, no. 6, pp. 2428–2436, Jun. 2013. [14] J. A. Stankovic, “Research direction for the Internet of Things,” IEEE Internet Things J., vol. 1, no. 1, pp. 3–9, Feb. 2014. [15] R. Nopper, R. Niekrawietz, and L. Reindl, “Wireless readout of passive LC sensors,” IEEE Trans. Instrum. Meas., vol. 59, no. 9, pp. 2450–2457, Sep. 2010.

[16] C. Zhang, J. Q. Huang, and Q. A. Huang, “Design of LC-type passive wireless multi-parameter sensor,” in Proc. 8th Int. Conf. Nano/Micro Eng. Mol. Syst., Suzhou, China, Apr. 7–10, 2013, pp. 256–259. [17] M. A. Reed and W. R. Scott, “Optimized coils for electromagnetic induction systems,” in Proc. SPIE, Jun. 2013, vol. 8709, p. 87090B1-11. [18] A. Ruehli, C. Paul, and J. Garrett, “Inductance calculations using partial inductances and macromodels,” in Proc. IEEE Int. Symp. EMC, Atlanta, GA, USA, Aug. 1995, pp. 23–28. [19] R. Paul, “Partial inductance of conductors of rectangular cross section,” in Inductance: Loop and Partial. Hoboken, NJ, USA: Wiley, 2010, pp. 248–291. [20] R. Puers, “Capacitive sensors when and how to use them,” Sens. Actuators A, Phys., vol. 37–38, pp. 93–105, Jun./Aug. 1993. [21] Q. Y. Ren, L. F. Wang, J. Q. Huang, C. Zhang, and Q. A. Huang, “A novel capacitive temperature sensor for a lab-on-a-chip system,” in Proc. IEEE Sens. Conf., Valencia, Spain, Nov. 2–5, 2014, pp. 436–439.

Lei Dong received the B.S. degree in electronic engineering from Southeast University, Nanjing, China, in 2012, and is currently working toward the Ph.D. degree at the Key Laboratory of MEMS, Ministry of Education, Southeast University. Her research interests include MEMS sensors and inductors, wireless passive microsensor, and inductive telemetry technique.

Li-Feng Wang (M’13) was born in Jiangsu, China, in 1981. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Southeast University, Nanjing, China, in 2003, 2006, and 2013, respectively. From 2006 to 2008, he was with Nanjing Electronic Device Institute, Nanjing, China, where he was involved with silicon micromechanical devices. After graduation, he joined the Faculty of the Department of Electronic Engineering, Southeast University, as an Assistant Professor. His research interests include the design, fabrication, and reliability of wireless microsensors and micromachined RF/microwave switches.

Qing-An Huang (S’89–M’91–SM’95) received the B.S. degree from the Hefei University of Technology, Hefei, China, in 1983, the M.S. degree from Xidian University, Xi’an, China, in 1987, and the Ph.D. degree from Southeast University, Nanjing, China, in 1991, all in electronic engineering. He joined the Faculty of the Department of Electronic Engineering, Southeast University, after graduation, where he became a Full Professor in 1996, and was appointed to Chair Professor of the Chang-Jiang Scholar by the Ministry of Education in 2004. He is currently the Founding Director of the Key Laboratory of MEMS, Ministry of Education, Southeast University. He authored Silicon Micromachining Technology (Sci. Press, 1996), has authored or coauthored over 150 peer-reviewed international journals/conference papers, and holds 40 Chinese patents. Dr. Huang is currently serving as an Editor-in-Chief of the Chinese Journal of Sensors and Actuators, and an Editorial Board Member of the Journal of Micromechanics and Microengineering. He was the Conference Co-Chair of the SPIE-Microfabrication and Micromachining Process Technology and Devices (Proc. SPIE, vol. 4601, 2001), the TPC Co-Chair of the 7th IEEE International Conference on Nano/Micro Engineered and Molecular System (NEMS) (Kyoto, Japan, 2012), the 6th Asia–Pacific Conference of Transducers and Micro/Nano Technologies (Nanjing, China, 2012), a TPC Member of Transducers from 2009 to 2015, and the IEEE Sensors Conference from 2002 to 2014. He has served as the Founding Chairman of the IEEE Electron Devices (ED)-Solid-State Circuits (SSC) Nanjing Chapter. He was a recipient of the National Outstanding Youth Science Foundation Award of China in 2003.