Franz Seifert

Mechanical sensors based on surface acoustic waves

Abstract A survey is given of SAW sensor systems. Three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing systems. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance frequency, which are described quantitatively. To show the high resolution of active sensor systems, an electronic spirit level (based on SAW resonators) is described. New SAW chirp sensors allow the dispersive interdigital transducer or reflector geometry to be adjusted to the sensor system resolution and measurement range. In passive systems the sensor device is connected to an antenna. The SAW is excited by an interrogating radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensors information. The time and frequency response of all sensor categories and the possibilities of temperature compensation are discussed.

A Low-Cost Wireless Sensor System and Its Application in Dental Retainers

In this paper, a wireless interrogable sensor device based on an ultra low-power microcontroller for data collection and a radio frequency identification (RFID) interface for data transmission is presented. Wireless sensor systems utilizing RFID transponders offer a new and exciting means of measurement and identification suitable for many biomedical applications. For the majority of these applications, small, inexpensive radio request sensor systems are desirable. In this paper, an ultra-low-power and low-cost wireless temperature data logger system is presented with its application in observing dental retainer use. For this purpose, the retainers ambient temperature is measured by an integrated sensor and recorded using a microcontroller which acts like a temperature data logger storing the thermal history of several months. For a self powered wireless data transmission from the sensor to the interrogation unit a RFID transponder, operating in the 13.56 MHz ISM band, is used. The presented sensor system includes hard- and software power saving modes reducing the sensor idle current consumption to 1 muA. This allows a battery powered operation of the device for up to two years. For dental and biomedical applications, the device is hermetically sealed using a biocompatible polymeric encapsulation. Results of the first clinical trials observing the patients dental retainer usage by a set of retainers equipped with the RFID temperature sensor/data logger device are presented. The stored temperature values are analyzed and a clear temperature characteristic indicating the retainer usage was found.

Comments on Transmission System Using Pseudonoise Modulation of Linear Chirps

The combination of phase shift keying (PSK) with different chirp waveforms is investigated in respect to its signal form and spectrum shape. A reasonable compromise for low probability of exploitation and jam sensitivity is found for a system where the chirp bandwidth is in the vicinity of the bitrate of the pseudonoise code.

State of the art radio interrogation system for passive surface acoustic wave sensors

The radio request of passive wireless interrogable surface acoustic wave (SAW) sensors/transponders, is discussed and a radio interrogation system for passive SAW sensors is introduced. The concept is intended to provide an alternative to interrogators with expensive RF and signal processing effects. Consequently the presented system employs a new concept for the generation of the RF signals and for signal windowing. Considering the inherent limitations, the proposed system presents a simple low cost solution for a wide range of various applications. Additionally we have investigated a new, small size and light weight antenna design used in conjunction with the SAW devices to build the passive sensor/transponder. The antenna structure is based on the concept of folded spiral antennas. Details of this proposed multilayered planar antenna design are described and experimental results are discussed. Numerical simulation results compared with experiments are presented. The system shows satisfactory performance and applicability in a wide range of remote monitoring applications.

A DS/FH Burst Communication Modem based on SAW Elastic Convolvers

This paper presents a burst-type spread spectrum communication modem with binary orthogonal code shift keying (CSK) employed for data modulation. A burst consists of an 8-bit synchronization preamble followed by a block of 1000 message bits, transmitted at 84 kbit/s. The spread spectrum modulation scheme is direct sequence (DS) combined with noncoherent frequency hopping (FH). Each bit of both the preamble and the message portion of a burst is encoded by a different 255-chip Kasami sequence with a chip rate of 21.4 MHz. The hop rate of the carrier is equal to the data rate, the number of frequencies being 13. The total transmission bandwidth is 300 MHz. Receiver signal processing is based on the application of two SAW elastic convolvers to programmable matched filtering. Performance measurements in a simulated communications channel show good agreement with theory.

Drahtlose Abfrage von Oberflächenwellen-Identifikationsmarken und Meßwertwandlern

ZusammenfassungEs wird die drahtlose Abfrage von drei verschiedenen Typen von passiven Oberflächenwellen-(OFW-)Identifikationsmarken und Meßwertwandlern beschrieben, die alle an eine kleine Antenne für den Empfang des Abfragesignals und die Rückstrahlung der verzögerten Antwort angeschlossen sind.(1)Die Abklingfrequenz von OFW-Resonatoren wird während mehrerer Mikrosekunden über eine getastete Phasenregelschleife (PLL) bestimmt.(2)Der Abstand zwischen einigen Antwortimpulsen einer OFW-Laufzeitleitung, die durch akustische Reflektoren in verschiedenen Abständen entstehen, wird mit drei verschiedenen integrativen Auswerteverfahren genau gemessen.(3)Die Amplitude des Antwortimpulses wird vom Wert einer an die OFW-Laufzeitstrecke angeschlossenen Kapazität bestimmt und ausgewertet. Alle Verfahren dienen der drahtlosen Abfrage fester und einstellbarer Identifikationscodes sowie von Meßwerten für Temperatur, Druck, Kraft, Drehmoment, Deformation, elektrische Spannung usw.AbstractWe describe wireless readout of 3 different species of passive surface acoustic wave (SAW) identification tags. and sensors which all are connected to a small antenna for reception of the interrogation signal and transmission of the delayed sensor response.(1)Following the ringing out of SAW resonators using a phase locked loop (PLL) over several microseconds, their resonance-(„Eigne”-)frequencies are determined.(2)The time between some response impulses in a SAW delay line, caused by acoustical reflectors in different distances, accurately is measured by 3 different signal integrating methods.(3)Amplitude variations of the response impulse, due to variations of a capacitance externally connected to the SAW delay line are evaluated. All methods are used for wireless readout of fixed and adjustable identification codes and sensor measurands e.g. temperature, pressure, mechanical force, torque, deformation, electrical voltage etc.

Evolution der akustischen Oberflächenwellen-Bauelemente

ZusammenfassungDer Artikel gibt einen ausgewählten Überblick über die Entwicklung der akustischen Oberflächenwellen (OFW)-Bauelemente seit den siebziger Jahren bis heute. Das Hauptgewicht liegt auf den an der Technischen Universität Wien in Kooperation mit dem Forschungslabor von Siemens (heute EPCOS) in München geleisteten Arbeiten. Zukünftige Möglichkeiten der OFW-Technik werden diskutiert.AbstractA selective survey is given on the development of surface acoustic wave (SAW) devices since the 1970’s. Emphasis is put on research and development delivered by the University of Technology Vienna, Austria, in cooperation with the corporate research of Siemens (now EPCOS) Munich, Germany. Future trends of SAW-technology are discussed.

Message protection by spread spectrum modulation in a packet voice radio link

In spread spectrum communication systems the bandwidth of the transmitted signal is far in excess of the information bandwidth itself. The spectrum spreading is controlled by a pseudonoise (PN) code. Knowledge of this code allows authorized receivers to process the arriving signal with a significant gain in signal-to-noise ratio by correlating it with a locally generated reference waveform. The inherent interference suppression capability has been the primary motivation for the development of spread spectrum techniques /1/. The two most common forms of spread spectrum modulation are direct-sequence (DS) and frequency-hopping (FH), the first of which is considered in this paper. In DS systems the carrier is phase-modulated by a PN code with a code rate (chip rate) much higher than the data rate . The term ‘chip’ is used to distinguish between code and data stream. Although the codes most frequently used are not secure in a cryptographic sense, protection against unauthorized message access is associated with the low power spectral density of the wideband DS signals. This attribute applies even more to systems employing non-repeating spreading codes.