A Novel Method to Read Remotely Resonant Passive ... - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 8, NO. 1, JANUARY 2008

A Novel Method to Read Remotely Resonant Passive Sensors in Biotelemetric Systems Sérgio Francisco Pichorim and Paulo José Abatti

Abstract—Resonant passive sensors composed by resistive, inductive, and capacitive (RLC) simple sensors are widely used in biotelemetric systems. In this paper, a novel method to read remotely these RLC sensors is presented. The developed method is based on the simultaneous application of three excitation signals of same amplitudes, set at different frequencies, to determine remotely the RLC sensor resonance frequency ( ) and quality factor ( ). Theoretical analysis and experimental results are also presented. Index Terms—Biomedical instrumentation, biomedical telemetry, implantable sensors, quality factor, resistive, inductive, and capacitive (RLC) passive sensor, resonance frequency, wireless telemetry.

I. INTRODUCTION

A

MONG THE biomedical sensors for in vivo applications, the telemetric implantable devices present the advantage of working without wires or cables trespassing the skin, thus reducing biological infection, discomfort, and risks of microshocks [1]. In addition, working with transcutaneous communication by radio frequency (RF) signals, this method is adequate for continuous and/or long-term monitoring [2]. Moreover, an inductive link can be used to power the implantable unit, therefore allowing its application where the battery of the sensor cannot be exchanged or recharged [3], [4]. Another advantage of passive sensors is that they can be constructed with reduced dimensions (few millimeters) allowing their direct injection, using hypodermic needles into the body. This procedure reduces the patient recovering period, since it does not require surgical intervention to place the sensor inside the biological tissue [1]. Of course, for practical purposes, the injectable unit must be encapsulated with biocompatible material. In general, the injectable unit is composed by an antenna (a coil), a capacitor (for tuning), and may contain an electronic integrated circuit. The use of an active circuit (microchip) in a battery-less telemetry improves the system accuracy and sensitivity, but its complexity and the costs of development are the main obstacles to their widespread use [2]. Passive sensors, constructed using only resistive, inductive, and capacitive (RLC) components, are simple, cheaper, and

Manuscript received December 19, 2006; revised July 5, 2007; July 31, 2007. This work was supported in part by CNPq (Brazilian Council for Scientific and Technological Development). The associate editor coordinating the review of this paper and approving it for publication was Dr. Robert Black. The authors are with the CPGEI/UTFPR-Federal University of Technology, Av. Sete de Setembro, 3165 CEP: 80230-901 Curitiba, Paraná, Brazil (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/JSEN.2007.912386

potentially safer. However, they have comparatively poor signal-to-noise ratio, low accuracy, and sensitivity, and its operational distance is intrinsically restricted [2], [4]. Since the classical pressure sensor developed in 1967 by Collins, the implantable passive inductive-capacitive (LC) sensors have been used for in vivo applications in medicine and bioengineering [5]. This sensor has its resonance frequency modified in response to the quantity being measured, which can be detected externally using RF signals [6]. Although the communications distance is still a concern, in many biomedical applications, the range is not more than a few centimeters, and hence the sensor resonance frequency can be easily determined by a sensitive receiver [4]. Recent biomedical applications of implantable/injectable RLC passive sensors include blood flow and pressure [7], intraocular pressure [2], [4], [5], [8], humidity [9], and tendon strain and elasticity [1] biotelemetric systems. These sensors are constructed with silicon micromachining and microelectromechanical systems (MEMS), or even with handmade and discrete components [1]–[10]. Evidently, the sensors reading unit is critical to improve the overall system performance. A traditional receiver to detect the of a passive LC or RLC sensor uses an excitation signal in a “grid-dip” approach [2], [5], [7], [8]. In this technique, a sinusoidal RF voltage with adjustable frequency applied to an excitation coil induces a current in the inductor of the nearby sensor. The loading effect (reflected impedance) of the sensor on excitation coil results in a minimum (dip) in the input voltage when[6]. ever the excitation signal frequency matches the sensor A variation of this approach is the phase-dip technique, since the phase of the input signal due to the reflected impedance shows a “dip” whenever the excitation signal frequency matches the [4]. Although most of the passive biotelemetric syssensor tems employ the “grid-dip” technique, alternative approaches remotely have been developed. For instance, to determine Cho and Asada [3] have been able to determine the capacitance of a remote sensor and the mutual inductance of the bioteleand employing metric system, tracking the estimated sensor the least squares methods to minimize the effect of the noise by using several data points. All the above reading techniques use as an excitation signal a constant amplitude voltage source which can have its frequency adjusted in a “sweep” mode, in a “step” mode, or employing a set of predetermined frequencies values. The sweep mode, usually implemented employing a voltage controlled oscillator (VCO), is intrinsically time consuming, since excitation signal is found. The frequency is incremented continuously until step mode and the use of predetermined frequencies values, usually implemented employing direct digital synthesis (DDS) in-

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PICHORIM AND ABATTI: A NOVEL METHOD TO READ REMOTELY RESONANT PASSIVE SENSORS IN BIOTELEMETRIC SYSTEMS

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Fig. 1. Electric diagram of coils arrangement with inductances and mutual in) and canceling coil ( ) are in front ductances involved. Reading coil ( of and behind of the excitation coil ( ), respectively, with separation distance . Passive RLC sensor with distance of separation from the first coil.

d

L

L

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tegrated circuits, would be comparatively quicker, since excitation signal frequency is incremented in discrete steps. However, it has been recently demonstrated that a sudden modification on excitation signal frequency produces a transient response similar to those produced by a sudden alteration on excitation determination [11]. signal amplitude, introducing errors on To minimize this unexpected effect, it is necessary to measure the circuit response only after the transient component can be neglected. Since for a practical passive sensor this transitory period can be as long as 160 s [11], the main advantage of latter methods, comparing with the former, is not, in fact, significant. In this paper, a novel method of reading the resonance freand quality factor of a passive RLC sensor is quency presented. To avoid problems with sweep time and/or transient components, the sensor is excited using three signals, set at different frequencies, applied simultaneously. The theoretical and experimental aspects of the developed method are discussed in details.

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Fig. 2. Determination of the optimal distance of separation (for a specific microcoil with 5 mm long and 10 mm apart, the best is 8.5 mm). Excitation ) decreases with distance and reading mutual inducmutual inductance ( = ) increases with distance . Bold line indicates tance ( , as shown in (3). the total coupling, proportional to

M

L

M M 0M

M 1M

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and and and (for distance inductance between ) [12], the optimum can be determined. Fig. 2 illustrates and as a function of , for mm, normalized showing that optimum is 8.5 mm [10]. The current at the remote circuit can be written as (2) and are the inductive and where capacitive reactances, respectively, and represents the equivalent inductor and capacitor internal resistances. Substituting (2) in (1) yields (3)

II. THEORETICAL ANALYSIS Fig. 1 shows the developed biotelemetric system simplified and coils are wound in electric circuit. Note that the opposite direction, forming an arrangement known as antiHelmholtz coils, producing a null magnetic field at the central . Thus, the direct coupling plane, i.e., at the excitation coil and output is minimized. Assuming that the between can reading system has a high input impedance be written as

(1) where the mutual inductance (between and ) has (between and ), the mubeen considered equal to (between and ) minus (betual inductance tween and ) is called , and and are the curand at the resonant circuit, respectively. Observe rents at between and , which is equal that as the distance to that between and , increases, also increases, improving the response. However, as increases coupling beand decreases, which, consequently, decreases the tween signal-to-noise ratio. Therefore, there is an optimal for the excitation/reception coils. Considering the distance between and ) equal to 10 mm, and computing the maximum mutual

The proposed method employs only the evaluation of the received voltage. Thus, computing the modulus of the received , using (3) gives voltage

(4) where . In this equation, , and are not known. These parameters represent three passive RLC circuit variables, , , and distance . Rewriting (4) yields (5) Using in (5) the new variables yields

, and

(6) , and is obtained applying (6) for three The solution of different frequencies , and with the same amplitude . This procedure will result in three output voltage signal

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signals quencies , tions system,


, and related to excitation signal fre, and , respectively. Solving the three equaand can be written as

parallel RLC circuit. The conversion between series and parallel RLC circuit can be given [13] by (15)

(7) and (8)

where and are the internal resistances of and in a parallel and series arrangement, respectively. Evidently, a resistive can be applied in parallel or in series with or transducer , respectively, and the influence of the latter should be taken determination from . This can be done into account on using

respectively, where

(16) or (9)

(17)

and (10) III. PRACTICAL IMPLEMENTATION Therefore, combining (7) with (8), the remote circuit resonance frequency can be determined, yielding

(11) where (12) Using (4), for a given excitation signal frequency, for instance, and isolating gives (13) shown at the bottom of the page. Substituting (7) and (8) in (13) and combining the result with (11), after manipulation, yields (14) shown at the bottom of the page. are automatically canceled in (11) Observe that and and (14), so that using only three excitation signal frequencies and can be completely determined. Thus, for instance, considering constant using (11) and afterwards (14), and of the remote sensor can be determined. It must be pointed out that the use of a resistive transducer is not straightforward, since it must be combined with the internal resistances of the inductor and capacitor. In Fig. 1, shown is a series RLC circuit, however, it is possible to obtain an equivalent

In the proposed method, the resonant passive sensor must be , and ) and the responses excited with three frequencies ( , and ) must be read to determine the resonance ( and/or quality factor of the sensor. Fig. 3 frequency shows the implementation of the reading system, which is composed by excitation oscillators, coils, amplifiers, filters, and a microprocessor. In order to have stable excitation frequencies, a quartz oscillator of 8 MHz was used, and its frequency was digitally divided by 15, 14, and 13 (using 74LS193) to obtain three different responses. These signals were also divided by two to yield square waves (duty cycle of 50%) with frequen, and , cies of 267, 286, and 308 kHz, corresponding to that also adds the respectively. The excitation driver for three waves of 10 Vpp was constructed with operational amplifier LM 318, which has a high slew rate (50 V/ s) and opV). Sinusoidal waves in erates with high-voltage supplies ( are obtained by a passive LC filter constructed using . It is important to point out that in the developed equations it has been supposed that the signals have the same amplitude. Fig. 4 spectrum, demonstrating that the indishows the practical vidual signals have approximately the same amplitude. Excitation coil was wound with 20 turns of copper wire with H). Receiving and radius of 200 m (26 AWG and canceling coils were wound with 45 turns of copper wire with radius of 125 m (30 AWG and total auto-inductance

(13)

(14)


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Fig. 3. Block diagram of a circuit which applies the proposed method of reading biotelemetric resonant passive sensors. Direct excitation noise is caused by L and L coils offset (M 6= M ).

set by software. Thus, the use of new technologies as direct digital synthesis (D.D.S.) to generate the excitation frequencies and digital filters by digital signal processor (D.S.P.) could be implemented. IV. EXPERIMENTAL RESULTS

Fig. 4. Practical spectral representation of the excitation signal (V ). Observe that three frequencies f ; f , and f (267, 286, and 308 kHz, respectively) have approximately the same amplitude.

of 230 H). Separation distance was mechanically adjusted to minimize direct coupling of excitation signals in the in was meareceiving coil (a rejection of 63 dB of sured) [10]. In the reception, amplifiers and filters are constructed to separate the responses of passive sensors. Three bandpass filters (second-order filters constructed using TL074 operational amplifiers and parallel LC circuits) with bandwidth of 14 kHz were tuned in the three excitation frequencies. Again, the same voltage gain for amplifiers and filters must be adjusted. The peak detector circuit (half-wave precision rectifier constructed using TL074 operational amplifiers), sample and hold, 8-bit A/D converter (ADC0808 family), and processor (using the parallel interface of an IBM PC compatible) complete the are converted in reading system. Values of response and parameters of the sensor using (11) and (14). Although the developed equations are long, they do not have complex implementation, and the firmware can execute 118 readings of and per second (8.5 ms each reading). However, it must be emphasized that the implemented system was used only to validate the novel method to read remotely resonant RLC sensors, but this is not the best circuitry. A more flexible reading system would be interesting for practical applications, where a wide range of frequencies could be read or

All developed equations in this work were derived assuming (between and ) is that the mutual inductance equal to (between and ). In other words, it was assumed that there was a complete cancellation of excitation in the output signal. However, to have a more realistic prediction about the system performance, a noise, representing the direct dB of ) to values was coupling signal (0.07% or employed on simulation using (11) and (14). Errors less than 0.4% in value of , and for a RLC quality factor of 40, errors up to 10% in were estimated. In this way, other simulations have shown that the determination of has a good immunity to this noise, while errors in decrease only when circuits with moderate quality factor (about ten) are used. In addition, these simulations had shown that the choice of the three excitation frequencies is also very important in the method. Errors less than is located between and , 0.2% have been found when preferably near to . Also, the range between these frequencies cannot be very wide. Finally, due to the proximity between to the three excitation frequencies, an extra noise about of dB, caused by filters limitations is registered in output voltages . However, even with this noise, simulations show errors determination. less than 1.2% in With these theoretical results in mind, a passive RLC circuit was constructed to evaluate the proposed method of reading. A circular coil with radius 12.2 mm and 35 turns (inductance of , and ) was used as 50 H, sensor inductor. A variable capacitor (range from 7 to 8.2 nF) between 277 tuned the sensor with resonance frequency and 302 kHz. A variable resistor (range from 1 k to 10 k ) , change the quality factor between 5 simulating a transducer and 9.5. Distances of 15–20 mm were used during tests. Fig. 5 presents the resonance frequency measured by proposed method as a function of tuned , showing errors less than 1.22% (mean error of 0.4 % and a correlation coefficient of 0.99566). Observe that six different tunings are made, while in each tuning, resistor and distance had been modified. In addition, working with only 20 mm of distance, the correlation coefficient has been improved slightly to 0.9968 (errors less than 1.18% and mean

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Fig. 5. Resonance frequency of the passive sensor measured by the telemetric system in function of tuned frequency. Correlation coefficient of 0.99566.

In addition, as observed in practical experiments, great variareduced the coefficient of correlation in measuretions on and , the proposed ment. However, for limited ranges of method is apparently feasible and applicable for two parameters monitoring with a small passive sensor. The presented method can make possible the determination and almost in real time, having only the delay time beof tween measurement and processing data. However, due to nontotal filter rejection of the other two frequencies, a new error is introduced into the system. Despite the volume of calculations and, in some cases, , this technique necessary to determine can be applied in practical biomedical systems with implantable or injectable sensors [1], [10]. It must be emphasized that the determination of or is independent of the values of distance (keeping the signal-to-noise ratio in an adequate value), amplifiers or filters gain, and passive sensor components, facilitating the system calibration. For situations of low-noise and low-quality factor, knowing the value of only one component of the passive sensor, the other two values , can be obtained. For example, for a known fixed capacitor , and a disusing a NTC thermistor as electrical resistance , two physiological paramplacement sensor as an inductor eters (e.g., temperature and displacement) can be, in principle, simultaneously monitored, using a simple passive sensor with a good accuracy. REFERENCES

Fig. 6. Quality factor of the passive sensor measured by the telemetric system in function of adjusted (by adjusting ) for tuned = 284 kHz and three distances . Correlation coefficient of 0.9890.

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error of 0.3%). Anyway, only a small variation of was obwith respect to and served, showing an insensitivity of variations. Fig. 6 presents measured values of sensor quality factors in functions of adjusted values of . The sensor was set (range from 5–9.5) by adjusting the resistor in six different resistances (range from 1 to 10 k ), and for a tuning of 284 kHz. A mean error of 5.5% (a correlation coefficient of also 0.989) was obtained. If during the measurement of , changes, an extra error is observed and the correlation coefficient decreases to 0.9094. V. DISCUSSION AND CONCLUSION A novel method to read remotely resonant sensors has been presented. The proposed method presents a good accuracy in resonance frequency (about 1%) measurements. For quality factor measurements, some extras cares are necessary, specially the choice of passive circuit range. In high- circuits, errors caused by the noncanceling effects of the coils are significant. For low- circuits, the consequent poor signal-to-noise ratio increases the measurement errors. Anyway, using more selective filters, errors, mainly in measurements can be reduced.

[1] S. F. Pichorim and P. J. Abatti, “Biotelemetric passive sensor injected within tendon for strain and elasticity measurement,” IEEE Trans. Biomed. Eng., vol. 53, no. 5, pp. 921–925, May 2006. [2] S. Lizón-Martínez, R. Giannetti, J. L. Rodríguez-Marrero, and B. Tellini, “Design of a system for continuous intraocular pressure monitoring,” IEEE Trans. Instr. Measur., vol. 54, no. 4, pp. 1534–1540, Aug. 2005. [3] K. J. Cho and H. H. Asada, “A recursive frequency tracking method for passive telemetry sensors,” in IEEE Proc. Amer. Control Conf., Denver, CO, Jun. 4–6, 2003, pp. 4943–4948. [4] 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. [5] R. Puers, G. Vandevoorde, and D. De Bruyker, “Electrodeposited copper inductors for intraocular pressure telemetry,” J. Micromech. Microeng., vol. 10, pp. 124–129, 2000. [6] J. R. Talman, A. J. Fleischman, and S. Roy, “Orthogonal-coil RF probe for implantable passive sensors,” IEEE Trans. Biomed. Eng., vol. 53, no. 3, pp. 538–546, Mar. 2006. [7] K. Takahata, A. DeHennis, K. D. Wise, and Y. B. Gianchandani, “A wireless microsensor for monitoring flow and pressure in a blood vessel utilizing a dual- inductor antenna stent and two pressure sensors,” in Proc. 17th IEEE Int. Conf. Microelectromech. Syst. (MEMS), Jan. 25–29, 2004, pp. 216–219. [8] J. Coosemans, M. Catrysse, and R. Puers, “A readout circuit for an intra-ocular pressure sensor,” Sens. Actuators A, Phys., vol. 110, pp. 432–438, 2004. [9] T. J. Harpster, B. Stark, and K. Najafi, “A passive wireless integrated humidity sensor,” Sens. Actuators A, Phys., vol. 95, pp. 100–107, 2002. [10] S. F. Pichorim, “Biotelemetric passive system using an injectable microsensor for muscular force measurement in tendon,” (in Portuguese) Doctor dissertation, CEFET-PR, Paraná Federal Center of Technological Education, Curitiba, Paraná, Brazil, 2003. [11] R. J. F. de Oliveira and P. J. Abatti, “Analysis of telemetric system based on remote resonant sensing circuit,” Electron. Lett., vol. 42, no. 13, pp. 750–752, June 2006. [12] S. F. Pichorim and P. J. Abatti, “Design of coils for millimeter- and submillimeter-sized biotelemetry,” IEEE Trans. Biomed. Eng., vol. 51, no. 8, pp. 1487–1489, Aug. 2004. [13] F. E. Terman, Radio Engineering Handbook. New York: McGrawHill, 1943.


Sérgio Francisco Pichorim was born in São José dos Pinhais, Paraná, Brazil, on April 23, 1965. He received the Electrical Engineering degree and the M.Sc. and D.Sc. degrees, both in biomedical engineering, from the Paraná Federal Center of Technological Education (CEFET-PR), Paraná, in 1990, 1995, and 2003, respectively. Since 1998, he has been a Professor with the Department of Electrical Engineering, Federal University of Technology—Paraná (UTFPR). His research interests include development of communication techniques and electronic circuits for biotelemetry systems, bioelectromagnetism, implantable biotelemetry systems, and biomedical sensors and instrumentation.

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Paulo José Abatti was born in Curitiba, Paraná, Brazil, on January 3, 1958. He received the B.S. degree in electrical engineering from the Federal University of Paraná, Paraná, in 1980, the M.E.E. degree in biomedical engineering from the State University of Campinas, São Paulo, Brazil, in 1983, and the Doctor of Engineering degree in electrical and electronic engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1991. He has been with the Federal University of Technology—Paraná (UTFPR) since 1977, where he is currently Titular Professor and Pro-Rector for Postgraduation and Research Affairs. His present research activities involve development of communication techniques and electronic circuits for implantable biotelemetry systems and modeling of biological phenomena.