A Novel Method to Read Remotely Resonant Passive Sensors in Biotelemetric Systems

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

A Novel Method to Read Remotely ResonantPassive 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 usedin biotelemetric systems. In this paper, a novel method to readremotely these RLC sensors is presented. The developed methodis based on the simultaneous application of three excitation signalsof same amplitudes, set at different frequencies, to determineremotely the RLC sensor resonance frequency ( ) and qualityfactor ( ). Theoretical analysis and experimental results are alsopresented.

Index Terms—Biomedical instrumentation, biomedicaltelemetry, implantable sensors, quality factor, resistive, inductive,and capacitive (RLC) passive sensor, resonance frequency, wirelesstelemetry.

I. INTRODUCTION

AMONG THE biomedical sensors for in vivo applications,the telemetric implantable devices present the advan-

tage of working without wires or cables trespassing the skin,thus reducing biological infection, discomfort, and risks ofmicroshocks [1]. In addition, working with transcutaneouscommunication by radio frequency (RF) signals, this methodis adequate for continuous and/or long-term monitoring [2].Moreover, an inductive link can be used to power the im-plantable unit, therefore allowing its application where thebattery of the sensor cannot be exchanged or recharged [3], [4].

Another advantage of passive sensors is that they can be con-structed with reduced dimensions (few millimeters) allowingtheir direct injection, using hypodermic needles into the body.This procedure reduces the patient recovering period, since itdoes not require surgical intervention to place the sensor in-side the biological tissue [1]. Of course, for practical purposes,the injectable unit must be encapsulated with biocompatiblematerial.

In general, the injectable unit is composed by an antenna (acoil), a capacitor (for tuning), and may contain an electronic in-tegrated circuit. The use of an active circuit (microchip) in abattery-less telemetry improves the system accuracy and sensi-tivity, but its complexity and the costs of development are themain 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 andTechnological Development). The associate editor coordinating the review ofthis 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 poorsignal-to-noise ratio, low accuracy, and sensitivity, and itsoperational distance is intrinsically restricted [2], [4]. Sincethe classical pressure sensor developed in 1967 by Collins,the implantable passive inductive-capacitive (LC) sensors havebeen used for in vivo applications in medicine and bioengi-neering [5]. This sensor has its resonance frequencymodified in response to the quantity being measured, whichcan be detected externally using RF signals [6]. Although thecommunications distance is still a concern, in many biomedicalapplications, the range is not more than a few centimeters, andhence the sensor resonance frequency can be easily determinedby a sensitive receiver [4]. Recent biomedical applications ofimplantable/injectable RLC passive sensors include blood flowand pressure [7], intraocular pressure [2], [4], [5], [8], humidity[9], and tendon strain and elasticity [1] biotelemetric systems.These sensors are constructed with silicon micromachiningand microelectromechanical systems (MEMS), or even withhandmade and discrete components [1]–[10].

Evidently, the sensors reading unit is critical to improve theoverall 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 sinu-soidal RF voltage with adjustable frequency applied to an exci-tation coil induces a current in the inductor of the nearby sensor.The loading effect (reflected impedance) of the sensor on exci-tation coil results in a minimum (dip) in the input voltage when-ever the excitation signal frequency matches the sensor [6].A variation of this approach is the phase-dip technique, since thephase of the input signal due to the reflected impedance showsa “dip” whenever the excitation signal frequency matches thesensor [4]. Although most of the passive biotelemetric sys-tems employ the “grid-dip” technique, alternative approachesto determine remotely have been developed. For instance,Cho and Asada [3] have been able to determine the capacitanceof a remote sensor and the mutual inductance of the biotele-metric system, tracking the estimated sensor and employingthe least squares methods to minimize the effect of the noise byusing several data points.

All the above reading techniques use as an excitation signal aconstant amplitude voltage source which can have its frequencyadjusted in a “sweep” mode, in a “step” mode, or employinga set of predetermined frequencies values. The sweep mode,usually implemented employing a voltage controlled oscillator(VCO), is intrinsically time consuming, since excitation signalfrequency is incremented continuously until is found. Thestep mode and the use of predetermined frequencies values, usu-ally 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 7

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

tegrated circuits, would be comparatively quicker, since exci-tation signal frequency is incremented in discrete steps. How-ever, it has been recently demonstrated that a sudden modifica-tion on excitation signal frequency produces a transient responsesimilar to those produced by a sudden alteration on excitationsignal amplitude, introducing errors on determination [11].To minimize this unexpected effect, it is necessary to measurethe circuit response only after the transient component can beneglected. Since for a practical passive sensor this transitory pe-riod can be as long as 160 s [11], the main advantage of lattermethods, comparing with the former, is not, in fact, significant.

In this paper, a novel method of reading the resonance fre-quency and quality factor of a passive RLC sensor ispresented. To avoid problems with sweep time and/or transientcomponents, the sensor is excited using three signals, set at dif-ferent frequencies, applied simultaneously. The theoretical andexperimental aspects of the developed method are discussed indetails.

II. THEORETICAL ANALYSIS

Fig. 1 shows the developed biotelemetric system simplifiedelectric circuit. Note that and coils are wound inthe opposite direction, forming an arrangement known as anti-Helmholtz coils, producing a null magnetic field at the centralplane, i.e., at the excitation coil . Thus, the direct couplingbetween and output is minimized. Assuming that thereading system has a high input impedance canbe written as

(1)

where the mutual inductance (between and ) hasbeen considered equal to (between and ), the mu-tual inductance (between and ) minus (be-tween and ) is called , and and are the cur-rents at and at the resonant circuit, respectively. Observethat as the distance between and , which is equalto that between and , increases, also increases,improving the response. However, as increases coupling be-tween and decreases, which, consequently, decreases thesignal-to-noise ratio. Therefore, there is an optimal for the ex-citation/reception coils. Considering the distance between and

) equal to 10 mm, and computing the maximum mutual

Fig. 2. Determination of the optimal distance d of separation (for a specificmicrocoilL with 5 mm long and 10 mm apart, the best d is 8.5 mm). Excitationmutual inductance (M ) decreases with distance d and reading mutual induc-tance (M = M �M ) increases with distance d. Bold line indicatesthe total coupling, proportional to M �M , as shown in (3).

inductance between and and and (for distance) [12], the optimum can be determined. Fig. 2 illustrates

normalized and as a function of , for mm,showing that optimum is 8.5 mm [10].

The current at the remote circuit can be written as

(2)

where and are the inductive andcapacitive reactances, respectively, and represents the equiv-alent inductor and capacitor internal resistances.

Substituting (2) in (1) yields

(3)

The proposed method employs only the evaluation of the re-ceived voltage. Thus, computing the modulus of the receivedvoltage , using (3) gives

(4)

where . In this equation, , andare not known. These parameters represent three passive RLCcircuit variables, , , and distance . Rewriting (4) yields

(5)

Using in (5) the new variables , andyields

(6)

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


signals , and related to excitation signal fre-quencies , , and , respectively. Solving the three equa-tions system, and can be written as

(7)

and

(8)

respectively, where

(9)

and

(10)

Therefore, combining (7) with (8), the remote circuit reso-nance 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 thepage.

Observe that and are automatically canceled in (11)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 internalresistances of the inductor and capacitor. In Fig. 1, shown is aseries RLC circuit, however, it is possible to obtain an equivalent

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

(15)

where and are the internal resistances of and in a par-allel and series arrangement, respectively. Evidently, a resistivetransducer can be applied in parallel or in series with or

, respectively, and the influence of the latter should be takeninto account on determination from . This can be doneusing

(16)

or

(17)

III. PRACTICAL IMPLEMENTATION

In the proposed method, the resonant passive sensor must beexcited with three frequencies ( , and ) and the responses( , and ) must be read to determine the resonancefrequency and/or quality factor of the sensor. Fig. 3shows the implementation of the reading system, which is com-posed by excitation oscillators, coils, amplifiers, filters, and amicroprocessor. In order to have stable excitation frequencies,a quartz oscillator of 8 MHz was used, and its frequency wasdigitally divided by 15, 14, and 13 (using 74LS193) to obtainthree different responses. These signals were also divided bytwo to yield square waves (duty cycle of 50%) with frequen-cies of 267, 286, and 308 kHz, corresponding to , and ,respectively. The excitation driver for that also adds thethree waves of 10 Vpp was constructed with operationalamplifier LM 318, which has a high slew rate (50 V/ s) and op-erates with high-voltage supplies ( V). Sinusoidal waves in

are obtained by a passive LC filter constructed using .It is important to point out that in the developed equations it hasbeen supposed that the signals have the same amplitude. Fig. 4shows the practical spectrum, demonstrating that the indi-vidual signals have approximately the same amplitude.

Excitation coil was wound with 20 turns of copper wire withradius of 200 m (26 AWG and H). Receiving andcanceling coils were wound with 45 turns of copper wire withradius of 125 m (30 AWG and total auto-inductance

(13)

(14)


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

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

of 230 H). Separation distance was mechanically ad-justed to minimize direct coupling of excitation signals in thereceiving coil (a rejection of 63 dB of in was mea-sured) [10].

In the reception, amplifiers and filters are constructed toseparate the responses of passive sensors. Three bandpass fil-ters (second-order filters constructed using TL074 operationalamplifiers and parallel LC circuits) with bandwidth of 14 kHz

were tuned in the three excitationfrequencies. Again, the same voltage gain for amplifiers andfilters must be adjusted.

The peak detector circuit (half-wave precision rectifier con-structed using TL074 operational amplifiers), sample and hold,8-bit A/D converter (ADC0808 family), and processor (usingthe parallel interface of an IBM PC compatible) complete thereading system. Values of response are converted inand parameters of the sensor using (11) and (14). Althoughthe developed equations are long, they do not have complex im-plementation, and the firmware can execute 118 readings ofand per second (8.5 ms each reading).

However, it must be emphasized that the implemented systemwas used only to validate the novel method to read remotelyresonant RLC sensors, but this is not the best circuitry. A moreflexible reading system would be interesting for practical ap-plications, where a wide range of frequencies could be read or

set by software. Thus, the use of new technologies as directdigital synthesis (D.D.S.) to generate the excitation frequenciesand digital filters by digital signal processor (D.S.P.) could beimplemented.

IV. EXPERIMENTAL RESULTS

All developed equations in this work were derived assumingthat the mutual inductance (between and ) isequal to (between and ). In other words, it wasassumed that there was a complete cancellation of excitation inthe output signal. However, to have a more realistic predictionabout the system performance, a noise, representing the directcoupling signal (0.07% or dB of ) to values wasemployed on simulation using (11) and (14). Errors less than0.4% in value of , and for a RLC quality factor of 40, errorsup to 10% in were estimated. In this way, other simulationshave shown that the determination of has a good immunityto this noise, while errors in decrease only when circuits withmoderate quality factor (about ten) are used. In addition, thesesimulations had shown that the choice of the three excitation fre-quencies is also very important in the method. Errors less than0.2% have been found when is located between and ,preferably near to . Also, the range between these frequen-cies cannot be very wide. Finally, due to the proximity betweenthe three excitation frequencies, an extra noise about of to

dB, caused by filters limitations is registered in output volt-ages . However, even with this noise, simulations show errorsless than 1.2% in determination.

With these theoretical results in mind, a passive RLC circuitwas constructed to evaluate the proposed method of reading. Acircular coil with radius 12.2 mm and 35 turns (inductance of50 H, , and ) was used assensor inductor. A variable capacitor (range from 7 to 8.2 nF)tuned the sensor with resonance frequency between 277and 302 kHz. A variable resistor (range from 1 k to 10 k )simulating a transducer , change the quality factor between 5and 9.5. Distances of 15–20 mm were used during tests. Fig. 5presents the resonance frequency measured by proposed methodas a function of tuned , showing errors less than 1.22% (meanerror of 0.4 % and a correlation coefficient of 0.99566). Observethat six different tunings are made, while in each tuning, resistor

and distance had been modified. In addition, workingwith only 20 mm of distance, the correlation coefficient has beenimproved slightly to 0.9968 (errors less than 1.18% and mean


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

Fig. 6. Quality factor of the passive sensor measured by the telemetric systemin function of adjustedQ (by adjustingR ) for tuned fo = 284 kHz and threedistances D. Correlation coefficient of 0.9890.

error of 0.3%). Anyway, only a small variation of was ob-served, showing an insensitivity of with respect to and

variations. Fig. 6 presents measured values of sensor qualityfactors in functions of adjusted values of . The sensor wasset (range from 5–9.5) by adjusting the resistor in six dif-ferent resistances (range from 1 to 10 k ), and for a tuningof 284 kHz. A mean error of 5.5% (a correlation coefficient of0.989) was obtained. If during the measurement of , alsochanges, an extra error is observed and the correlation coeffi-cient decreases to 0.9094.

V. DISCUSSION AND CONCLUSION

A novel method to read remotely resonant sensors has beenpresented. The proposed method presents a good accuracy inresonance frequency (about 1%) measurements. For qualityfactor measurements, some extras cares are necessary, speciallythe choice of passive circuit range. In high- circuits, errorscaused by the noncanceling effects of the coils are significant.For low- circuits, the consequent poor signal-to-noise ratioincreases the measurement errors. Anyway, using more selec-tive filters, errors, mainly in measurements can be reduced.

In addition, as observed in practical experiments, great varia-tions on reduced the coefficient of correlation in measure-ment. However, for limited ranges of and , the proposedmethod is apparently feasible and applicable for two parame-ters monitoring with a small passive sensor.

The presented method can make possible the determinationof and almost in real time, having only the delay time be-tween measurement and processing data. However, due to non-total filter rejection of the other two frequencies, a new error isintroduced into the system. Despite the volume of calculationsnecessary to determine and, in some cases, , this techniquecan be applied in practical biomedical systems with implantableor injectable sensors [1], [10].

It must be emphasized that the determination of or is in-dependent of the values of distance (keeping the signal-to-noiseratio in an adequate value), amplifiers or filters gain, and passivesensor components, facilitating the system calibration. For situ-ations of low-noise and low-quality factor, knowing the value ofonly one component of the passive sensor, the other two valuescan be obtained. For example, for a known fixed capacitor ,using a NTC thermistor as electrical resistance , and a dis-placement sensor as an inductor , two physiological param-eters (e.g., temperature and displacement) can be, in principle,simultaneously monitored, using a simple passive sensor with agood accuracy.

REFERENCES

[1] S. F. Pichorim and P. J. Abatti, “Biotelemetric passive sensor injectedwithin 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 mon-itoring,” 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 forpassive 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-modulatedmicromachined passive pressure transensor,” IEEE Sensors J., vol. 3,no. 6, pp. 728–733, Dec. 2003.

[5] R. Puers, G. Vandevoorde, and D. De Bruyker, “Electrodepositedcopper 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 probefor 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, “Awireless microsensor for monitoring flow and pressure in a bloodvessel utilizing a dual- inductor antenna stent and two pressure sen-sors,” 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 anintra-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 integratedhumidity sensor,” Sens. Actuators A, Phys., vol. 95, pp. 100–107, 2002.

[10] S. F. Pichorim, “Biotelemetric passive system using an injectable mi-crosensor for muscular force measurement in tendon,” (in Portuguese)Doctor dissertation, CEFET-PR, Paraná Federal Center of Technolog-ical Education, Curitiba, Paraná, Brazil, 2003.

[11] R. J. F. de Oliveira and P. J. Abatti, “Analysis of telemetric systembased 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- andsubmillimeter-sized biotelemetry,” IEEE Trans. Biomed. Eng., vol. 51,no. 8, pp. 1487–1489, Aug. 2004.

[13] F. E. Terman, Radio Engineering Handbook. New York: McGraw-Hill, 1943.


Sérgio Francisco Pichorim was born in São Josédos Pinhais, Paraná, Brazil, on April 23, 1965.He received the Electrical Engineering degree andthe M.Sc. and D.Sc. degrees, both in biomedicalengineering, from the Paraná Federal Center ofTechnological Education (CEFET-PR), Paraná, in1990, 1995, and 2003, respectively.

Since 1998, he has been a Professor with theDepartment of Electrical Engineering, FederalUniversity of Technology—Paraná (UTFPR). Hisresearch interests include development of com-

munication techniques and electronic circuits for biotelemetry systems,bioelectromagnetism, implantable biotelemetry systems, and biomedicalsensors and instrumentation.

Paulo José Abatti was born in Curitiba, Paraná,Brazil, on January 3, 1958. He received the B.S.degree in electrical engineering from the FederalUniversity of Paraná, Paraná, in 1980, the M.E.E.degree in biomedical engineering from the StateUniversity of Campinas, São Paulo, Brazil, in 1983,and the Doctor of Engineering degree in electricaland electronic engineering from the Tokyo Instituteof Technology, Tokyo, Japan, in 1991.

He has been with the Federal University of Tech-nology—Paraná (UTFPR) since 1977, where he is

currently Titular Professor and Pro-Rector for Postgraduation and ResearchAffairs. His present research activities involve development of communicationtechniques and electronic circuits for implantable biotelemetry systems andmodeling of biological phenomena.

A Novel Method to Read Remotely Resonant Passive Sensors in Biotelemetric Systems

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