Technology: Sensor Tags

In addition to being useful directly as sensors, individually coded SAW devices can be used to provide uniquely identifiable wireless links to external impedance varying or voltage producing sensors. This approach is advantageous in that it would allow the elimination of wiring harnesses to existing sensors on airframes or in test facilities, allowing a small group of SAW interface devices to be qualified for use with a large number of existing qualified sensors.

Work in the area of SAW wireless interface devices dates back more than a decade, although the incorporation of coding into the devices to provide groups of individually identifiable interface devices (known as sensor-tags) is more recent. The following background information in the areas of (uncoded) wireless interface devices for use with external sensors and RFID coded SAW devices (not used as interface devices) is useful in understanding more recent technology development in these areas.

SAW wireless sensor interface device background:

In 2000, Schimetta and colleagues at Siemens AG and the University of Linz, Austria [1] demonstrated the use of SAW reflective delay line devices as wireless links to external sensors. These passive wireless sensor interface devices for measuring pressure consisted of a SAW device with an antenna attached to the transducer port, and multiple reflective taps along the SAW delay line, with an external high-Q capacitive pressure sensor and matching components attached to one of the reflectors. RF signals of the correct frequency range are reflected off of the surface wave device, and their reflection characteristics are modified by changes in the impedance of the attached sensor(s). Amplitude, delay, and phase of the reflections can be used to provide a measure of the attached sensor impedance. Passive wireless sensor interface devices using SAW technology have also been demonstrated by Brocato at Sandia National Labs [2]. One of the significant engineering aspects of implementing SAW devices as wireless links to other sensors is their impedance – typical SAW transducers have characteristic impedances of roughly 50 . External sensors with impedances that vary in a range close to that of the SAW device can be directly connected to the SAW and measured. Sensors with impedances that vary widely from that of the SAW will appear as open or short circuits to the SAW, meaning that variations in the sensor impedance will not cause any measurable change in the SAW device response. Thus, high impedance sensors cannot be connected directly to SAW devices for wireless interrogation. Brocato solved this problem and contributed a useful impedance transformation approach using a zero-bias FET to allow the low impedance SAW device to be matched to sensors with much higher impedance [2]. Using this approach, along with optimized SAW sensor-tag design, almost any passive external sensor can be wirelessly interrogated.

SAW RFID tag background:

Traditional single frequency SAW RFID tags consist of a piezoelectric substrate with a single input/output transducer and a set of reflective taps positioned at various delays on either side of the transducer. The reflective taps are positioned in time “slots” that are separated far enough in time to allow the reflections from any two successive slots to be resolved. The reflected response of a traditional SAW RFID tag consists of a sequence of time domain pulses such that in each successive time slot the existence of a reflected pulse is read as a “1” and the absence of a pulse is read as a “0”. If the code is N time slots long, then the number of unique codes is 2N. Hence the number of reflective tap time slots is directly related to the number of bits in the tag. The use of two-sided single acoustic track device configurations allowed the designer to incorporate more time slots than would be possible with a single-sided layout. In addition to placing constraints on the number of possible codes for a given device size, the time extent of each time slot (with guardbands on each side to prevent misidentification of taps) also limits the time extent of each reflecting tap. This limits the number of reflecting strips in each tap, which reduces the possible reflection for each tap. Another factor limits the number of possible taps as well. Since the taps are located in the same acoustic track, if taps close to the input/output transducer reflect SAW energy, that removes SAW energy from the wave propagating further out in the track, which means that there is less to be received and reflected from the taps farther out from the transducer. The result is that the signals reflected from sequential taps are of decreasing strength. This effect can be compensated for somewhat by increasing the number of electrodes in the taps that are further from the transducer, but this can introduce significant intertap reflection problems. Intertap reflections occur when the SAW reflected by one tap reflects off of a tap closer to the transducer and propagates once again away from the transducer, only to be reflected from taps further away in the SAW path. In order to avoid intertap reflection problems and allow the SAW energy to propagate and be reflected by multiple taps, it is necessary to keep the total reflectivity of each tap low. Low tap reflectivity, however, results in high reflection loss, a significant problem common to all conventional single frequency reflective delay line SAW RFID tags. High loss reduces the S/N ratio and can result in the SAW sensor-tag interfaces encountering difficulty in interpreting the reflected device response. The time domain response of typical SAW RFID tags at 2.45 GHz are generally 55 dB or more with approximately 32 taps [3,4] and can exceed 70 dB for devices with many more tap positions. The smaller the number of reflective taps needed to effect the desired number of codes, the lower the possible insertion loss and vice-versa.

In addition to simple on-off coding involving tap positions, a number of other coding techniques have been applied to SAW RFID tags [5]. Phase shift keying has resulted in a higher signal to noise ratio than simple on-off coding, while advanced techniques such as overlapped pulse position modulation combined with phase offsets and multiple pulses per group have been shown to enable larger codesets with adequate signal levels [3,4]. Pulse position modulation (PPM) is the use of multiple small time slots within each reflector, with a single reflector in one of the slots. Use of PPM allows realization of more codes with fewer total reflective strips than conventional SAW tag coding. The Global SAW Tag uses PPM (with 75 small time slots per reflector) combined with multiple phase shifts within each small time slot to achieve very high code densities. Another method used to avoid intertap reflections and achieve larger codesets is to design devices with multiple parallel acoustic tracks. Input transducers can be connected electrically in series or parallel, with each transducer having a two-sided acoustic track. This reduces the number of reflective taps in each track and allows the designer to use more reflective electrodes in each tap without concern for intertap reflections.

In addition to the reflective delay line approach, resonant tag devices have been investigated [6].

Development of SAW sensor-tags:

Sensor-tags combine the properties of both coded SAW devices and sensor interface devices. Any of the known SAW RFID tag approaches can be implemented on SAW substrates with properties appropriate for use as sensors or as sensor interface devices. ASR&D is a leader in development of SAW wireless sensor-tag interface devices, work that is described further under our Sensor Tage Research page.


Cited works:

  1. Schimetta, G. et. al., “A wireless pressure measurement system using a SAW hybrid sensor”, 2000 IEEE NTT-S Digest, pp. 1407-1410, 2000. U.S.
  2. Brocato, R. W., “Passive Wireless Sensor Tags”, Sandia Report SAND2006-1288, Sandia National Laboratories, March 2006.
  3. L. Reindl et. al., “SAW L. Reindl, W. Ruile, “Programmable Reflectors for SAW-ID-Tags”, Proceedings of the 1993 IEEE Ultrasonics Symposium, p. 125-130.
  4. C. Hartmann et. al., “Anti-Collision Methods for Global SAW RFID Tag Systems”, Proceedings of the 2004 IEEE Ultrasonics Symposium, p. 805-808.
  5. L. Reindl, A. Pohl, G. Scholl, and R. Weigel, “SAW-Based Radio Sensor Systems”, IEEE Sensors Journal, Vol. 1, No. 1, June 2001, pp. 69-78.
  6. M. Rusko, et. al., “Passive Resonator Identification Tag for Narrow-Band Wireless Telemetry”, Proceedings of the 1999 IEEE Ultrasonics Symposium, pp. 377-380.