F. Seifert

SAW devices as wireless passive sensors

Surface acoustic wave (SAW) radio sensors make it possible to read measurement values from a remote location. The decisive advantage of these SAW sensors lies in their passive operation with no need for a separate power supply, and in the possibility of wireless installation at particularly inaccessible locations. The passive SAW sensors are maintenance free. The physical or chemical properties that shall be detected change the propagation characteristics of the SAW. In this paper we compare the sensitivity of different types of SAW sensors. These are resonators, reflective delay lines, and dispersive structures. Examples for several applications are presented.

Monitoring the tire pressure at cars using passive SAW sensors

In our paper we present the application of surface acoustic wave (SAW) sensors to the continuous monitoring of the tire pressure in road vehicles. With these, the tire pressure can be read out in every phase of driving. We show the implemented prototype setup for measurement of the tire pressure, the applied SAW sensors, improved versions and the interrogation setup. The problems in practical application are discussed. Experimental results measuring the tire pressure during test rides are presented.

Quartz pressure sensor based on SAW reflective delay line

We have developed a wireless SAW (Surface Acoustic Wave) pressure sensor operating in the pressure range from 0 Pa to 250 kPa. In order to minimize the temperature sensitivity the pressure sensor is based on an all quartz package, which has been designed with the Finite Element Method. The package of the pressure sensor consists of a diaphragm and a cover, both made of conventional Y-cut quartz. A blind-hole was structured into the sensor cover. By attaching the cover and the diaphragm with an epoxy-adhesive, this blind-hole forms a closed cavity. The SAW element is a reflective delay line, working at approximately 434 MHz. The delay line consists of ten reflectors and extends over the whole diaphragm. The pressure is determined by evaluating the change of the carrier phase shifts of the reflected impulses at the reflectors. We show that it is possible to minimize the temperature sensitivity and to achieve good linearity by correct positioning of the SAW reflectors. The measurements of the SAW pressure sensor show a deviation from linearity of less than +/-0.7%. The temperature dependence is almost negligible in the range from -20/spl deg/C to 100/spl deg/C.

CDMA for wireless SAW sensor applications

A well known spread spectrum technique is used to get the sensor information from an individually addressed SAW sensor. Coded ID tag type and BPSK type SAW sensors are picked out of a number of sensors by correlating the received signal with the known code of a particular sensor. The ID-tag type and BPSK-coded SAW sensors function is observed as a scaling of time and shape respectively. The time scaling factor is equal to the mechanical extension of the substrate of a sensor due to temperature, etc. This scaling factor is evaluated by use of correlative signal processing techniques. The principal concept, mathematical simulations, numerical estimations and finally an overview of the implemented SAW sensor system are shown. Preliminary results for readout of OOK and BPSK coded passive remote SAW sensors using the wavelet transform processed within a SAW convolver are presented.

A robust ultra broadband wireless communication system using SAW chirped delay lines

The present invention provides an improved grade of polyimide containing an organic phosphorus compound, a film composed thereof and a method of making the aforementioned film. According to the present invention, it is possible to obtain a polyimide film improved in mechanical strength.

Spread spectrum techniques for wirelessly interrogable passive SAW sensors

Recently surface acoustic wave (SAW) sensors for temperature, pressure, strain, etc. were investigated and tested. They use the change in SAW time delay due to the quantity to be measured. Increasing interest was initiated by the possibility of wireless readout. It is shown here, that the well known spread spectrum techniques (chirp impulse compression, CDMA, etc.) allows an increase of both, sensitivity and accuracy of measurement and a discrimination of several sensors interrogated. Some applications (temperature measurement with mK resolution, coded sensors) are demonstrated. We show some mathematical simulations and preliminary experimental results.

Radio signals for SAW ID tags and sensors in strong electromagnetic interference

The applicability of wirelessly interrogated passive ID-tag type SAW sensors for measurements of the temperature of disc brakes of moving railway cars is shown. Here, a short radio channel allows relatively simple radio impulse processing by phase evaluation. Further, using elastic convolver measurement in large steel works, we discuss the feasibility of passive SAW sensors in a severely obstructed radio channel with interference. For such environments radio signals of the spread spectrum type and a receiver with the corresponding correlative SAW structure or a RAKE receiver are appropriate for remote readout

SAW delay lines for wirelessly requestable conventional sensors

Programmable reflectors of SAW ID-tags can be employed as passive transponders for conventional sensors of the capacitive, inductive and resistive type. The programmable reflector is a loadable split finger transducer, other fixed reflectors are used as reference bits. A noncoherent read out signal processing is sufficient, because the load of the programmable reflector influences both the phase and amplitude of the reflected wave. The principle is discussed, setup and results of some actual measurements are presented.

New applications of wirelessly interrogable passive SAW sensors

By applying passive wirelessly interrogable SAW sensors, many physical parameters can be measured. Up to now all SAW sensor applications are performed by taking a snapshot of the sensors response periodically, evaluating the measurand assumed to be quasistationary. Therefore the upper limit for the rate of sampling of a mechanical effect by the sensor is the interrogation rate. Usually it is in the range of 100 kHz or less and measurands with a periodicity of up to a few tenths of kHz can be sampled satisfactory. Even audible vibrations of machine parts can be monitored. Here, the behaviour of the sensors for dynamic measurands is discussed. Advanced applications for the measurement of vibration, acceleration, for dynamic pressure measurement in mechanical engineering, for example for monitoring the tyres of cars are presented. Measurement results from an experimental setup are given.

Wavelet transform with a SAW convolver for sensor application

To obtain the sensor information from an individually addressed SAW sensor, the use of coded ID-tag type SAW sensors is combined with the tool of wavelet transform, a type of correlative signal processing. The function of SAW sensors (ID-tag type) is observed as a scaling of time and shape respectively. The time scaling factor is equal to the mechanical extension of the substrate of a sensor for temperature, etc. This scaling factor is evaluated by calculating the wavelet transform. The fundamental concept, mathematical simulations, numerical estimations and finally, an overview of the implemented SAW sensor system using the wavelet transform processed within a SAW convolver are shown. Preliminary results are presented.