SAW-Based Radio Sensor Systems

Transcription

1 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE SAW-Based Radio Sensor Systems Leonhard M. Reindl, Member, IEEE, Alfred Pohl, Member, IEEE, Gerd Scholl, Member, IEEE, and Robert Weigel, Senior Member, IEEE Abstract Surface acoustic wave (SAW) devices can be used as identification and sensor elements (SAW transponders) for measuring physical quantities such as temperature, pressure, torque, acceleration, humidity, etc., that do not need any power supply and may be accessed wirelessly. The complete wireless sensor system consists of such a SAW transponder and a local radar transceiver. An RF burst transmitted by the radar transceiver is received by the antenna of the SAW transponder. The passive transponder responds with an RF signal like a radar echo which can be received by the front-end of the local transceiver. Amplitude, frequency, phase and time of arrival of this RF response signal carry information about the SAW reflection and propagation mechanisms which in many cases can be directly attributed to the sensor effect for a certain measurand. Usually no intersymbol interference (ISI) due to environmental echoes occur, due to the high delay time of the SAW transponder in the order of some s. The present work reviews the operating principles of such sensor systems and their state-of-the-art performance by way of some examples which include the wireless measurement of temperature, pressure, torque, acceleration, tire-road friction, magnetic field, and water content of soil. Index Terms Acceleration, local radar transceiver, magnetic field, passive SAW transponder for sensing, pressure, temperature, tire-road friction, torque, water content of soil, wireless measurement. I. INTRODUCTION SURFACE acoustic wave (SAW) devices are special microacoustic components consisting of a piezoelectric substrate with metallic structures such as interdigital transducers (IDTs) and reflection or coupling gratings deposited on its plain-polished surface. Due to the piezoelectric effect an RF input signal will stimulate a microacoustic wave propagating on its surface. Vice versa, a SAW wave generates an electrical charge distribution at the receiving IDT and therefore an electrical RF output signal occurs [1], [2]. SAW technology has been exploited for electronic analog signal processing over the past 30 years, with the development of numerous devices and systems Manuscript received September 8, 2000; revised March 12, Many of the results reported in this work were obtained by a common effort of the colleagues and students in the Department ZT MS 1 of Siemens AG. The associate editor coordinating the review of this paper and approving it for publication was Dr. Arnaldo D Amico. L. M. Reindl is with the TU Clausthal, Institute of Electrical Information Technology, D Clausthal-Zellerfeld, Germany ( reindl@iei.tu-clausthal.de). A. Pohl is with Siemens AG Austria, A-1030 Vienna, Austria, and also with the Institute of Industrial Electronics and Material Science, University of Technology Vienna, A-1040 Vienna, Austria ( alfred.pohl@siemens.at; alfred.pohl@tuwien.ac.at). G. Scholl is with Epcos AG, D München, Germany ( gerd.scholl@epcos.com). R. Weigel is with the Institute for Communications and Information Engineering, University of Linz, A-4040 Linz, Austria ( r.weigel@ieee.org). Publisher Item Identifier S X(01) Fig. 1. Schematic of a SAW-based radio-link system. for consumer, commercial, and military applications with an annual rate of some $1 billion today [3] [8]. In recent years, great and important progress has been made in wireless SAW sensor specifications and a variety of innovative applications have appeared. These developments are mainly based upon the invention and implementation of passive SAW identification (ID) tags [9] [12] combined with active oscillator-based SAW sensor techniques [13] [16]. SAW devices for analog signal processing are hermetically sealed in order to be insensitive to external influences, such as humidity or mechanical stress. SAW devices can be used as sensors for physical or chemical parameters if the selected parameters can change the SAW propagation or reflection characteristics. In most cases, this results in a mechanical deformation of the SAW chip in conjunction with a change of the velocity of the SAW. Some physical parameters, like temperature, directly alter the velocity of the SAW. For measuring mechanical parameters like pressure, force, strain, or acceleration, an adapted mounting and appropriate packaging is required. Other parameters, such as gas or vapor concentration, voltage or magnetic fields, can be measured indirectly by coating the chip with a parameter-specific layer [13] [16]. Oscillator-based SAW sensor techniques can be combined with the ID-tag principle to achieve passive sensors which are connected to their request units solely by wireless radio links [17] [24]. The operating principle of such systems is as follows (Fig. 1). A radio frequency electromagnetic (RF) request signal transmitted by a local radar transceiver (Trx) is picked up by the antenna of the passive SAW transponder where an IDT, connected to the antenna, converts the received signal into a SAW. The microacoustic wave propagates toward reflectors distributed in a characteristic barcode-like pattern and is partially reflected at each reflector. The microacoustic wave packets returning to the IDT are reconverted into electrical signals and retransmitted to the radar Trx unit by the transponder antenna. This response contains information about the number and X/01$ IEEE

2 70 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE 2001 location of reflectors as well as the propagation and reflection properties of the SAW wave. Its evaluation in the radar unit may allow for the extraction of the desired information; for example, the sensor effect for a certain measurand and/or a specific ID number. Passive SAW transponders do not require any power supply, and their antennas are usually of a dipole, patch or loop type. Within the VHF/UHF frequency range, the SAW transponder insertion attenuation is in the order of db [10] [12], [23], [24], [30], and the achievable access rate is 10 s. The latter fact allows for communication with fast moving objects or vehicles. Because the distance between the radar Trx and the SAW transponder is unknown or varying, one usually employs differential test arrangements and evaluates differences in amplitude, phase, frequency, and propagation time delay. Long delay times in the microsecond range can be achieved using rather small SAW chips due to the low velocity of practical SAW wave types. Therefore, at VHF/UHF frequencies, environmental echoes caused by electromagnetic multipath propagation phenomena are already safely faded by the time the sensor response arrives at the radar Trx. The sensor response can therefore be separated from the environmental echoes in the time-domain. This fact incorporates a great advantage of wireless SAW-based sensing of distance, linear and angular velocity of targets, compared to conventional radar measurement techniques. The measurement accuracy of those radar values lies within 1% of the electromagnetic free-space wavelength when attained by a SAW transponder. We will first discuss the local radar Trx (request unit). We will then give an overview of some basic SAW transponder principles. Finally, we will illustrate the feasibility of SAW-based radio sensor systems by citing several examples, such as sensors for temperature, for mechanical parameters like pressure, torque, acceleration, or tire-road friction, and impedance sensors like current or water content sensors. II. RADAR TRANSCEIVER Interrogated by an RF radio signal, a linear-distorted version of the requesting signal is retransmitted by the SAW transponder. The signal received by the local Trx carries information about the measurand and is, of course, distorted by noise and interference. Errors occurring during transmission result in additional measurement errors inseparable from the sensor effect. Therefore, special care has to be taken with the radio transmission systems [34]. The request units of wireless SAW sensor systems applications [20], [28] [34] resemble those used in traditional radar [25], [26]. Regardless of which type of passive radio sensor is used, a wireless one-port response measurement has to be performed with time division between the exciting signal and the sensor response. As is the case with radar systems, the receiver usually is located nearby the transmitter (Tx) so that coherent detection is feasible. The actual measurement can be performed in time or frequency domain. For optimal free-space propagation conditions of the electromagnetic waves, the well-known radar equation predicts that the (upper bound) level of the signal received by the request unit Fig. 2. Time-domain sampling technique. decreases with the fourth power of the distance from the SAW transponder. To be more specific, the maximum request distance is given by [25], [26] SNR where electromagnetic wavelength; denotes the transceiver s transmitted power;, respective gains of the Trx and transponder antennas; insertion attenuation of the SAW transponder; relevant thermal noise power (Boltzmann s constant, absolute temperature, system bandwidth, and noise figure ); SNR minimum signal-to-noise ratio required to safely detect the received signal with a specified rate or probability of errors. In Europe, only two frequency bands suitable for SAW devices are allocated to unlicensed low power devices (LPDs) such as industrial, scientific, or medical (ISM) apparatus: MHz and GHz. Adding an additional frequency range ( MHz) is being discussed currently. The allowed equivalent isotropically radiated power (EIRP) in these bands is mw. Typical values for the other parameters appearing in (1) are dbi, dbi, db (depending on frequency and substrate material), and SNR db. Based on these data, one obtains a maximum Trx-transponder distance of only 25 to 30 cm for a single request cycle. In RF-shielded metallic process chambers the operating frequency can be chosen arbitrarily and the distances are larger because the Tx power can be enhanced. Apart from the time domain division of radio request signal and the sensor response due to the SAW delay time, the wireless evaluation of the sensor s reflective characteristic can be done in time and frequency domain. A. Time-Domain Sampling For time-domain sampling (Fig. 2) like pulse or pulse compression radar, the request signal covers the total system bandwidth at once. The duration of the radio request signal is given for nonspread spectrum signals to be /(2B). Each radio request signal causes the corresponding response signal. A wideband sampling has to be done in the receiver. Time-domain sampling is a measurement method by the single- (1)

3 REINDL et al.: SAW-BASED RADIO SENSOR SYSTEMS 71 Fig. 4. Frequency domain sampling technique. Fig. 3. Schematic diagram of a Trx using a pulse radar architecture. TABLE I CHARACTERISTIC DATA OF THE PULSE RADAR REQUEST UNIT Fig. 5. Schematic layout of a reflective delay line. scan, the whole sensor response can be recorded in one radio request cycle. The request rate can be up to 10 /s, because a single read interval takes the length of the SAW sensor response, only a few microseconds. This fact makes this method especially well-suited to measure fast moving objects or fast changing events. The energy content of one request signal is given by the time integral of the transmitted signal power. Due to the wide bandwidth of the impulses and their limited peak power, the energy content is small, thus reducing the maximum request distance. To avoid intersymbol interference (ISI), sampling has to be performed with at least twice the bandwidth of the baseband signals. Using pulse compression methods, can be increased by the time bandwidth product of the pulse compression system thus enlarging the maximum request distance. The maximum and, however, are limited to the maximum basic delay of the SAW sensor (some miscroseconds) and the bandwidth of the SAW device (some megahertz) respectively. Thus, using pulse compression technique a gain of up to 12 db can be achieved, doubling the request distance [20]. At the expense of time resolution the sampled signal can be averaged over several request cycles, thus lowering the measurement system bandwidth but increasing the read out distance Fig. 7. sensor. Fig. 6. Time domain response to the Tx signal. Schematic layout of a passive SAW device combined with an external. According to (1), an averaging factor of 16 doubles the request distance. Using these techniques, request distances of 5 to 10 m in the lower and 1 to 2 m in the upper ISM band have been demonstrated [20]. Fig. 3 shows a schematic diagram of a pulse radar Trx unit. The Rx uses a 70-MHz IF stage and a logarithmic limiter amplifier (Log. Amp.) with a radio signal strength indicator (RSSI) output and second output with a limited signal for phase detection. Using conventional SAW IF bandpass filters, a system bandwidth of 40 MHz is achieved for operation in the 2.45 GHz ISM band. To compensate for the coherent crosstalk in the IF

4 72 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE 2001 Fig. 8. Acoustic reflection factor P of a split-finger IDT as function of its complex load impedance Z (Smith chart representation). The diagram shows lines of constant acoustic reflection. For line n, jp j is n 1 (05 db). Fig. 10. Layout of a SAW device with can be used in combination with an external sensor. Fig. 9. Measured amplitude and phase of the acoustic reflection factor P (Polar chart representation) of a split-finger IDT with capacitive and inductive load impedance Z. stage as well as the DC offset of the mixers and A/D converters, a GaAs FET switch (Null) is included in the Log. Amp. During transmission in the time domain sampling operation, short bursts of variable length are excited by switching the output of a 70-MHz temperature-compensated crystal oscillator (TCXO). With a frequency synthesizer operating between 100 MHz and 2.7 GHz, the bursts are transverted to the RF band. The response signal from a SAW transponder, present within the detection range, is first amplified, then mixed down into the IF band, and then passed to the logarithmic amplifier. Quadrature demodulation is employed to extract the in-phase and quadrature components from the limited signal. After demodulation and digitizing, the data are processed further by a microprocessor. Table I summarizes the achieved characteristics of the system. B. Frequency Domain Sampling For frequency domain sampling or partial band sampling (see Fig. 4) the total bandwidth is scanned in steps in frequency domain. The bandwidth of one step can be rather low to archive a high resolution. The Tx pulse, generated by the system shown in Fig. 3, can last for a relatively long time period (with the resolution bandwidth ) with the basic delay of the sensor as an upper bound. Both, scanning the bandwidth step by step and the long signal duration enhance the SNR (Eb/N0) in the receiver at the expense Fig. 11. Phase as a function of temperature between two selected pulses of a SAW ID tag. of total measurement bandwidth. The method is suitable for quasistationary sensor applications only, but results in high resolution and an enhanced distance range. Magnitude and phase of a narrowband response have to be detected in the Rx, thus lowering the complexity and cost of the sampling unit. Similar to time-domain sampling systems, the RF signals can be derived from a single oscillator for coherent detection. The information about the sensor in time domain is received by a frequency to time transformation, e.g., FFT or more sophisticated algorithms [27]. In order to eliminate the transmitted signal and all environmental echoes, the first 1 2 s of the sensor response can be suppressed in time-domain [29]. Frequency domain sampling is a multiscan measurement technique. To achieve the information of points in time-domain, frequencies have to be scanned. The total measurement time takes more than times the minimum measurement cycle of a time-domain sampling. Frequency domain sampling is therefore well suited for slow and long-distance measurements. It can be performed using a network analyzer or a frequency-modulated continuous wave (FMCW) radar architecture [30], [31].

5 REINDL et al.: SAW-BASED RADIO SENSOR SYSTEMS 73 Fig. 12. Photo of an assembled SAW radio requestable temperature sensor. Fig. 15. Rotor temperature measured with the set up shown in Fig. 14. The red crosses show reference measurements derived with a conventional PT 100. Fig. 13. Time response of the temperature sensor shown in Fig. 12, also displayed in polar coordinates to show the different phases between the reflections. Fig. 16. Schematic drawing of a SAW pressure sensor. Fig. 17. Measured tire pressure, passing a two track railway crossing with adjacent water channel. Fig. 14. Experimental set up for monitoring the rotor temperature in a 11 kw asynchronous motor. Methods for data reduction can help to lower the effort of signal processing, data reduction algorithms can be inserted in the time or the frequency domain, e.g., intersymbol interferences can be evaluated to extract the measurement value [33]. III. SOME BASIC TRANSPONDER PRINCIPLES SAW transponders can easily be designed in the VHF/UHF band to separate data signals from the echoes arising from multipath propagation effects. A SAW reflective delay (RDL) is a very practical choice to attain the required SAW delay time for the response signals (Figs. 5 and 6). The RDL approach is popular because it can be combined easily with ID-tagging [17], [20], [22], [23]. Purely ID-tagging SAW labels typically consist of several reflector tracks incorporating many reflectors. Such systems are already in use, e.g., in a road pricing system in Norway [35] or in German subway systems [23], [36]. Fewer reflectors are required for sensing without tagging. The sensitivity of the RDL transponder can be enhanced by a factor of 10 to 20 using chirped reflectors [22], [23], [37], [38]. One reflector has implemented an up-chirp FM law, and the other a down-chirp FM law. Also non-saw sensors, or switches with a variable impedance, can be addressed wirelessly when combined with a SAW transponder [11]. Here an external sensor is used as an electrical load of an IDT (Fig. 7). The measuring signal changes the value of the load impedance thus changing the reflection behavior of the IDT and thereby of the retransmitted signal of the transponder [11], [23], [39], [40]. Figs. 8 and 9 show the acoustic reflectivity of a split-finger ( ) IDT as a function of its electrical load. To enhance the sensor effect a substrate with a high coupling coefficient should be used. Fig. 10 shows a suitable layout with the SAW device mounted and sealed in a standard DIL 16 package. The device includes four electro-acoustic transducers, three of which are

6 74 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE 2001 Fig. 18. SAW accelerometer configuration using a seismic mass and a flexured cantilever beam. Fig. 20. Deceleration of the dart hitting a target. Fig. 21. Deformation of tread element during road contact. (a) Entering. (b) Contact area. (c) Running out. Fig. 19. SAW accelerometer, fixed to a dart. used as reflectors. One IDT is connected to an external antenna to pick up the RF request signal and generate the RF response. Two reflectors are used for reference purposes. The bus bars of the median reflector are connected to pins of the package and can be wired to an external sensor. The required delay time also can be archived with one-port SAW resonator configurations [21], [32]. An RF burst impulse excites a high-q-resonator. The received RF burst of this resonator can be seen after all multipath echoes have faded [27], [29]. The sensitivity of this resonator-type transponder is equal to that of the RDL transponder, however, with a significant reduction in SAW chip size. The combination of resonant SAW transponders with an external sensor (variable load, similar to RDL) can also be used for pulling the resonator frequency. Because SAW transponders are passive components without any active logic on chip, they cannot be addressed individually. To access more than one transponder a frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), space division multiple access (SDMA), or combinations of them must be implemented. For FDMA, orthogonal frequency bands or sub-bands must be used for each individual transponder. This technique is feasible for several resonant transponders [21]. In combination with TDMA, the number of RDL transponders can be enlarged. When implementing TDMA different time positions for each reflected signal have to be chosen for minimizing ISI. Typically, ten orthogonal TDMA sensors, each with three to four Fig. 22. Mounting of a SAW sensor for tire friction control which can be incorporated into a tire tread element. reflectors, can be implemented. Using CDMA and the corresponding signal processing, again approximately ten code orthogonal transponders can be addressed [41] [43]. For SDMA techniques a space separation of the transponders has to be ensured. Due to the field attenuation of Rx power with, the near far problem limits the multiple access of passive radio transponders. IV. SAW RADIO SENSOR SYSTEMS APPLICATIONS A. Temperature Sensors Lithiumniobate, LiNbO, is an ideal material for temperature sensors because of its large TCD of approx. 85 ppm/ C and its high electroacoustic coupling factor [17], [20], [23]. Fig. 11 shows as an example the phase of two signals as a function of temperature of a RDL. The tag works at 434 MHz and was requested from a distance of 3 m. The time delay between the first and last reflector is 4.55 s. When the phase difference between two pulses from widely separated reflectors (nos. 1 and 11, see Fig. 11) is evaluated, a large phase shift per unit temperature is observed. If the request unit has a phase resolution of 1, the corresponding temperature resolution would be 0.02 C, if the

7 REINDL et al.: SAW-BASED RADIO SENSOR SYSTEMS 75 Fig. 23. Schematic drawing of a SAW magnetic field sensor. Fig. 26. Difference of the amplitude between reflector #2 and reflector #1 as a function of the water content 2 of the soil. Fig. 24. Fig. 23. Sensor characteristics of the wireless SAW current sensor outlined in train slows down entering a station [44] and to monitor the temperature of high-voltage surge arresters [30]. Fig. 14 shows the setup for monitoring the rotor temperature of a 11 kw asynchronous motor [30]. The temperature sensor was mounted on the short-circuiting ring of the rotor and read with a loop antenna mounted on the bearing plate. The motor was loaded with an eddy-current brake. Fig. 15 gives the measured rotor temperature During start-up and operating under load conditions the motor warms up as expected. When the motor is not working the temperature shows an exponential decay. The discrete measured temperature values archieved with a reference thermometer are marked with crosses. There is a slight difference between the measured values because the thermal contacts of the reference thermometer and the SAW device are not identical. Up to about 200 C, standard assembly, interconnect, and packaging techniques can be used, but higher temperatures require new solutions. The same problem arises for the conventional SAW materials like aluminum (electrodes) or LiNbO and quartz (substrate). Based on new material combinations, SAW devices have already been shown to work at temperatures as high as 1000 C [16], [45]. Fig. 25. Schematic drawing of a water-content sensor. SNR is sufficient [34]. However, this accuracy induces an ambiguity when the phase shift exceeds 360. The ambiguity can be eliminated when the phase difference between two pulses from adjacent reflectors (nos. 1 and 2, see Fig. 11) is interpreted, too. Fig. 12 shows the photo of a SAW radio-requestable temperature sensor sealed in a SMD package. On the left side the IDT can be seen and to the right there are four reflectors placed on the chip. Fig. 13 shows corresponding time response; the left curve gives the magnitude in time domain and the right figure plots the time response in polar coordinates to show the different phases between the reflections. SAW radio-requestable temperature sensors have been used to monitor the temperature of the break discs as the B. Mechatronic Sensors Like Pressure Sensors, Torque Sensors, Accelerometers, and Tire Friction Sensors Wireless SAW pressure sensors are formed using a quartz diaphragm that bends under hydrostatic pressure. A reflective delay line is achieved by structuring one surface of the quartz diaphragm (Fig. 16), [47], [48]. An all-quartz package consisting of the delay line chip itself and a lid out of the same material, is expected to produce minimal thermal stresses and therefore reduces the cross-sensitivity to temperature [46]. One obtains a pressure resolution of about 1% of full range. Fig. 17 shows pressure measurements of a sensor in a car tire [44], [49], [50]. With an additional seismic mass, a radio requestable SAW accelerometer can be attained [50], [51]. Fig. 18 shows an accelerometer configuration using a flexured SAW cantilever beam. This sensor was wired to an antenna and fixed to a dart (Fig. 19). With this accelerometer the deceleration of the dart invading the target is shown in Fig. 20.

8 76 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE 2001 TABLE II TYPICAL RESOLUTION OF WIRELESS SAW SENSORS It is possible to monitor the friction coefficient between a car tire and the road surface with an SAW transponder, a key parameter when stabilizing a vehicle in critical situations [24]. The friction coefficient can be measured by evaluating the mechanical strain in the tire surface contacting the road by monitoring the deforming of the tread elements. Fig. 21 shows the deformation of a tread element during road contact. The tread is flattened in the contact region (b) due to the load to the tire. The tread element transfers in the first half of the contact area a propulsion to the road and in the second a breaking. Some slippage of the tread element occurs during contact. If the tire/road contact is lessened, this slippage increases and the gradient of the strain curve is changed. Siemens in cooperation with the Continental AG and the TU Darmstadt (FZD) is developing a SAW sensor for the measurement of the friction coefficient between a car tire and the road surface. A schematic drawing of an experimental encapsulating of a SAW bending beam which can be incorporated into a tire tread element can be seen in Fig. 22. With passive SAW torque transponder slip rings can be omitted [16], [52], [53], and [30]. Like resistive strain gauges the SAW sensors measure the torque indirectly by detecting the strain or stress distribution generated by a torque acting on the shaft. The fact that the strain has an opposite sign in 45 directions relative to the shaft axis can be used for temperature compensation. Quartz and LiNbO have been investigated for use as substrate materials. C. Impedance Varying Sensors Like Current and Water Content Sensors Currents produce magnetic fields that can be detected by SAW devices coated with a magnetostrictive layer [54]. Another possibility is an impedance-type sensor as shown in Fig. 10 with magnetoresistors or giant magnetic impedance (GMI) elements acting as variable load [23], [55]. Again, a differential setup is used to cope with cross-sensitivities: two magnetoresistors exposed to biasing magnetic fields of opposite polarity, each loading an IDT-reflector. The sensor information is calculated from the ratio of the two reflected pulses (Fig. 23). The current resolution of this radio measurement is approximately 5% of full scale (Fig. 24). By connecting an impedance-type sensor to a parallel-wire electrical transmission line with two rods, a passive radio requestable water content sensor can be realized (Fig. 25) [56]. The rods are put into sandy soil. Together, the transmission line and the two rods with the sand-water-mixture between them have a characteristic impedance, which again terminates the second transducer and affect the amplitude and phase of the acoustic reflection. Due to the high permittivity of free water every change of the water content of the soil also changes, which results in a change of the amplitude and phase of the acoustic reflection at the second transducer (Fig. 26). Again the resolution is approximately 5% of full scale. V. CONCLUSION By combining the principles of conventional wired SAW sensors with the radio-request technique, known from SAW ID tags, passive sensors connected to a request unit solely by a radio-frequency link can be designed. Based on standard radar systems operating in the VHF/UHF range, various wireless SAW sensor systems have been tested successfully. In particular, solutions for the noncontact measurements of temperature, mechanic quantities like pressure, torque, acceleration, or tire-road friction, and impedance sensors for current or water content have been developed. These systems also work in harsh environments and at high temperatures. With appropriate compensation for cross-sensitivities, the typical performances of wireless SAW sensor systems are characterized by request distances of 3 10 m. Table II summarizes the typical resolution achievable with wireless SAW sensors.

9 REINDL et al.: SAW-BASED RADIO SENSOR SYSTEMS 77 The circuits of the request units are still quite complex and expensive, which is the main drawback of the SAW-Based Radio Sensor Systems till now. ACKNOWLEDGMENT The authors would like to thank the contributions of W. E. Bulst, C. C. W. Ruppel, F. Schmidt, T. Ostertag, and Dr. W. Ruile. The authors would also like to thank the contributions of their friend and teacher Professor F. Seifert and of the staff members and students of the department of Applied Electronics at the University of Technology Vienna, especially Dr. G. Ostermayer, R. Steindl, M. Brandl and C. Hausleitner. REFERENCES [1] R. M. White and F. W. Voltmer, Direct piezoelectric coupling to surface elastic waves, Appl. Phys. Lett., vol. 7, pp , [2] B. A. Auld, Acoustic Fields and Waves in Solids. New York: Wiley, 1973, vol. II. [3] H. Matthews, Ed., Surface Wave Filters. New York: Wiley, [4] A. A. Oliner, Ed., Acoustic Surface Waves. Berlin, Germany: Springer, [5] D. P. Morgan, Surface-Wave Devices for Signal Processing. Amsterdam, The Netherlands: Elsevier, [6] S. Datta, Surface Acoustic Wave Devices. Englewood Cliffs, NJ: Prentice-Hall, [7] M. Feldmann and J. Hénaff, Surface Acoustic Waves for Signal Processing. Boston, MA: Artech House, [8] C. Campbell, Surface Acoustic Wave Devices and Their Signal Processing Applications. Boston, MA: Academic, [9] P. A. Nysen, H. Skeie, and D. Armstrong, System for interrogating a passive transponder carrying phase-encoded information, U.S. Patents , , , [10] K. Yamanouchi, G. Shimizu, and K. Morishita, 2.5-GHz SAW propagation and reflection characteristics and application to passive electronic tag and matched filter, in Proc. IEEE Ultrasonics Symp., 1993, pp [11] L. Reindl and W. Ruile, Programmable reflectors for SAW-ID-tags, in Proc. IEEE Ultrasonics Symp., 1993, pp [12] V. P. Plessky, S. N. Kondratiev, R. Stierlin, and F. Nyffeler, SAW tags: New ideas, in Proc. IEEE Ultrasonics Symp., 1995, pp [13] M. S. Nieuwenhuizen and A. Venema, Mass-sensitive devices, in Sensors. A Comprehensive Survey, W. Göpel, J. Hesse, J. N. Zemel, T. A. Jones, M. Kleitz, J. Lundström, and T. Seiyama, Eds. Weinheim, Germany: VCH, 1991, vol. 2. [14] G. Fischerauer, Surface acoustic wave devices, in Sensors. A Comprehensive Survey, W. Göpel, J. Hesse, J. N. Zemel, H. Meixner, and R. Jones, Eds. Weinheim, Germany: VCH, 1995, vol. 8. [15] D. S. Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, G. C. Frye, and H. Wohltjen, Acoustic Wave Sensors: Theory, Design, and Physico-Chemical Applications. San Dieg, CA: Academic, [16] U. Wolff, F. L. Dickert, G. Fischerauer, W. Greibl, and C. Ruppel, SAW sensors for harsh environments, IEEE Sensors J., vol. 1, pp. 4 13, June [17] X. Q. Bao, W. Burkhard, V. V. Varadan, and V. K. Varadan, SAW temperature sensor and remote reading system, in Proc. IEEE Ultrasonics Symp., 1987, pp [18] W. Buff, SAW sensors, Sens. Actuators A, vol. 42, pp , [19] F. Seifert, W. E. Bulst, and C. C. W. Ruppel, Mechalical sensors based on surface acoustic waves, Sens. Actuators A, vol. 44, pp , [20] F. Schmidt, O. Sczesny, L. Reindl, and V. Magori, Remote sensing of physical parameters by means of passive surface acoustic wave devices ( ID TAG ), in Proc. IEEE Ultrasonics Symp., 1994, pp [21] W. Buff, F. Plath, O. Schmeckebier, M. Rusko, T. Vandahl, H. Luck, and F. Möller, Remote sensor system using passive SAW sensors, in Proc. IEEE Ultrasonics Symp., 1994, pp [22] L. Reindl, G. Scholl, T. Ostertag, C. C. W. Ruppel, W.-E. Bulst, and F. Seifert, SAW devices as wireless passive sensors, in Proc. IEEE Ultrasonics Symp., 1996, pp [23] L. Reindl, G. Scholl, T. Ostertag, H. Scherr, U. Wolff, and F. Schmidt, Theory and application of passive SAW radio transponders as sensors, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp , Sept [24] A. Pohl, R. Steindl, and L. Reindl, The intelligent tire utilizing passive SAW sensors Measurement of tire friction, IEEE Trans. Instrum. Meas., vol. 48, pp , Dec [25] M. I. Skolnik, Introduction to Radar Systems. New York: McGraw- Hill, [26] C. E. Cook and M. Bernfeld, Radar Signals. Norwood, MA: Artech House, [27] B. D. Rao and K. S. Arun, Model based processing of signals: A state approach, Proc. IEEE, vol. 80, pp , Feb [28] A. Pohl, F. Seifert, L. Reindl, G. Scholl, T. Ostertag, and W. Pietsch, Radio signals for ID tags and sensors in strong electromagnetic interference, in Proc. IEEE Ultrasonics Symp., 1994, pp [29] A. Pohl, G. Ostermayer, C. Hausleitner, F. Seifert, and L. Reindl, Wavelet transform with SAW convolver for sensor application, in Proc. IEEE Ultrasonics Symp., 1995, pp [30] G. Scholl, F. Schmidt, T. Ostertag, L. Reindl, H. Scherr, and U. Wolff, Wireless passive SAW sensor systems for industrial and domestic applications, in Proc. IEEE Freq. Contr. Symp., 1998, pp [31] G. Fischerauer, F. Schmidt, M. Voss, and R. Bader, Mechatronic extension of a tap holder for process monitoring, in Proc. IECON, [32] A. Pohl, G. Ostermayer, and F. Seifert, Wireless sensing using oscillator circuits locked to remote high-q SAW resonators, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp , Sept [33] A. Pohl, A low cost high definition wireless sensor system utilizing intersymbol interference, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp , Sept [34], A review of wireless SAW sensors, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 47, pp , Mar [35] Køfri Unbehindert nach Oslo, Siemens Rev., pp. 8 10, Jan [36] Siemens Transportation Group,, Product description A19100-V700- B535-V [37] L. Reindl, U. Rösler, C. Ruppel, R. Obertreis, and R. Weigel, Chirped SAW devices for wireless passive sensors, in Proc. IEEE Ultrasonics Symp., 1997, pp [38] T. Pankratz, H. Scherr, L. Reindl, C. Ruppel, and R. Weigel, Low TB radio SAW sensors incorporating chirped transducers and reflectors for wireless pressure sensing applications, in Proc. IEEE MTT-S Int. Microwave Symp., 1998, pp [39] R. Steindl, A. Pohl, L. Reindl, and F. Seifert, SAW delay lines for wirelessly requestable conventional sensors, in Proc. IEEE Ultrasonics Symp., 1998, pp [40] R. Steindl, A. Pohl, and F. Seifert, Impedance loaded SAW sensors offer a wide range of measurement opportunities, in Proc. IEEE MTT-S, Anaheim, CA, 1999, pp [41] G. Ostermayer, A. Pohl, L. Reindl, and F. Seifert, Multiple access to SAW sensors using matched filter properties, in Proc. IEEE Ultrasonics Symp., Toronto, ON, Canada, 1997, pp [42] G. Ostermayer, A. Pohl, C. Hausleitner, L. Reindl, and F. Seifert, CDMA for wireless SAW sensor applications, in Proc. IEEE Int. Symp. Spread Spectrum Techn. Applic., 1996, pp [43] G. Ostermayer, A. Pohl, R. Steindl, and F. Seifert, SAW sensors and correlative signal processing A method providing multiple access capability, in Proc. ISSSTA, South Africa, 1998, pp [44] A. Pohl and F. Seifert, Wireless interrogable SAW-sensors for vehicular applications, in Proc. IEEE Instrum. Meas. Conf., 1996, pp [45] L. Reindl, R. Steindl, A. Pohl, J. Hornsteiner, E. Riha, and F. Seifert, Passive SAW sensors for temperature and other measurands, in Proc. Tempmeko, The Netherlands, 1999, pp [46] G. K. Montress, T. E. Parker, and J. Callerame, A miniature hybrid circuit SAW oscillator using an all quartz packaged resonator, in Proc. IEEE Ultrasonics Symp., 1985, pp [47] R. M. Taziev, E. A. Kolosovsky, and A. S. Kozlov, Deformation-sensitive cuts for surface acoustic waves in -quartz, in Proc. IEEE Freq. Contr. Symp., 1993, pp [48] H. Scherr, G. Scholl, F. Seifert, and R. Weigel, Quartz pressure sensor based on SAW reflective delay line, in Proc. IEEE Ultrasonics Symp., 1996, pp [49] A. Pohl, G. Ostermayer, L. Reindl, and F. Seifert, Monitoring the tire pressure at cars using passive SAW sensors, in Proc. IEEE Ultrason. Symp., Toronto, ON, Canada, 1997, pp

10 78 IEEE SENSORS JOURNAL, VOL. 1, NO. 1, JUNE 2001 [50] A. Pohl, A. Springer, L. Reindl, F. Seifert, and R. Weigel, New applications of wirelessly interrogable passive SAW sensors, in Proc. MTT-S, Baltimore, MD, 1998, pp [51] A. Pohl, R. Steindl, and L. Reindl, Measurements of vibration and acceleration utilizing SAW sensors, in SENSOR, vol. 2, Germany, 1999, pp [52] T. Sachs, R. Großmann, J. Michel, and E. Schrüfer, Remote sensing using quartz sensors, Proc. SPIE, vol. 2718, pp , [53] U. Wolff, F. Schmidt, G. Scholl, and V. Magori, Radio accessible SAW sensors for simultaneous noncontact measurement of torque and temperature, in Proc. IEEE Ultrasonics Symp., 1996, pp [54] W. Robbins and A. Hietala, A simple phenomenological model of tunable SAW devices using magnetostrictive thin films, IEEE Trans. Ultrason. Ferroelect., Freq. Contr., vol. 35, pp , Nov [55] R. Steindl, C. Hausleitner, A. Pohl, H. Hauser, and J. Nicolics, Giant magneto-impedance magnetic field sensor with surface acoustic wave technology, in Proc. Eurosensors, Delft, The Netherlands, [56] L. Reindl, C. C. W. Ruppel, A. Kirmayr, N. Stockhausen, and M. Hilhorst, Passive radio requestable SAW water content sensor, in Proc. IEEE Ultrasonics Symp., Gerd Scholl (M 95) was born in Aalen, Germany, in He received the Dipl.-Ing. degree in 1989 and the Dr.-Ing. degree in 1997, both in electrical engineering, from the Technical University of Münich, Germany. He has been with the Microacoustics Group, Siemens Corporate Technology Department, Münich, since He was engaged in the development of SAW resonators and narrowband filters for mobile communication applications. Currently, he is responsible for the development and product launch of wireless SAW sensor and identification systems with Epcos AG, Münich. Leonhard M. Reindl (M 93) was born in Neuburg/Do, Germany, in He received the Dipl.Phys. degree from the Technical University of Munich, Germany, in 1985 and the Dr.Sc.Techn. degree from the University of Technology Vienna, Austria, in From 1985 to 1999, he was with the Microacoustics Group of the Siemens Corporate Technology Department, Münich, Germany, where he has been engaged in research and development on SAW convolvers, dispersive and tapped delay lines, ID-tags, and wireless passive SAW sensors. In 1999, he became Assistant Professor of Communications and Microwave Techniques at the Institute of Electrical Information Technology, Clausthal University of Technology, Germany. He holds 30 patents in SAW devices and wireless passive sensors and has authored 90 papers in the field. Alfred Pohl (M 95) was born in Austria in He received the Dipl.-Ing. and the Dr.Sc.Techn. degree from the University of Technology, Vienna. He then was an Assistant Professor, University of Technology, teaching circuit design and topics related to communication engineering. His research interests are in the field of radio communication, spread spectrum techniques, surface acoustic devices, focused on passive sensors and advanced radio request principles, and new applications of passive sensors. Currently, he is a Senior Expert for radio communication systems with Siemens AG, Vienna. He is also an Associate Professor for applied electronics at the Vienna University of Technology. Robert Weigel (S 88 M 89 SM 95) was born in Ebermannstadt, Germany, in He received the Dr.-Ing. and the Dr.-Ing.Habil. degrees, both in electrical engineering and computer science, from the Münich University of Technology, Germany, in 1989 and 1992, respectively. From 1982 to 1988, he was a Research Engineer, from 1988 to 1994, a Senior Research Engineer, and from 1994 to 1996, a Professor for RF circuits and systems at the Münich University of Technology. In winter , he was a Guest Professor for SAW Technology at Vienna University of Technology, Austria. Since 1996, he has been Director of the Institute for Communications and Information Engineering, University of Linz, Austria. Since 1998, he has been also a Professor for RF engineering at the Tongji University, Shanghai, China. He has been engaged in research and development on microwave theory and techniques, integrated optics, high-temperature superconductivity, SAW technology, and digital and microwave communication systems. In these fields, he has published more than 200 papers and given more than 120 international presentations. His review work includes European research projects and international journals. In August 1999, he co-founded Danube Integrated Circuit Engineering (DICE), Linz, an Infineon Technologies company which is devoted to the design of mobile radio circuits and systems. Dr. Weigel is a member of the Institute for Components and Systems of The Electromagnetics Academy and a member of the German ITG and the Austrian ÖVE. In 1993, he was co-recipient of the MIOP Award.