Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2003-03-20
2004-11-02
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C073S587000, C073S702000, C073S788000
Reexamination Certificate
active
06810750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to sensors that measure stresses induced on mechanical structures and surface acoustic wave (SAW) communication devices, and more particularly to a strain gauge sensor utilizing a piezoelectric “transmitting” SAW correlator affixed to or embedded into a surface and a “receiving” SAW correlator that is not affixed to the surface which receives a complex RF waveform, wherein the application of stress alters the center frequency of the “transmitting” SAW correlator, and the frequency of the complex RF output waveform and measurement of the center frequency yields an indication of the applied stress.
2. Brief Description of the Prior Art
A surface acoustic wave (SAW) filter, which is well known in the art, is typically comprised of a first set of closely-spaced, electrically conductive traces that resemble fingers (known as an interdigitated transducer), closely spaced from a second set of complimentary, interdigitated output conductive traces (known as output transducers). Both sets of fingers are etched onto the surface of a piezoelectric substrate. In SAW devices, the spacing between the interdigitated transducers affects the frequency of sensitivity.
Surface Acoustic Wave (SAW) detectors and strain gauge devices have also been proposed and implemented to some extent in the prior art; however, most of these devices have been constructed as a surface acoustic wave (SAW) filter sensitive to a particular frequency only, with no encoding.
Surface acoustic wave (SAW) transmitters having a surface acoustic wave resonator and an oscillator are also known in the art. Typically, the resonator section is comprised of a central interdigitated transducer, and a multitude of closely spaced, electrically conductive reflecting strips arranged at both sides of the interdigitated transducer. Both the interdigitated transducer and reflecting strips are deposited onto the surface of a piezoelectric substrate.
Spread Spectrum (SS) radios are well known to provide robust data transfer in varied and/or harsh radio frequency (RF). Spread spectrum (SS) radios were originally developed for military use, but are now becoming popular for commercial applications. Spread spectrum and the derivatives thereof, are quickly becoming the technology of choice for mass communications cellular based technology. Signal spreading provides significant immunity to man-made noise (intentional or unintentional) as well as a reduced probability of interference with existing equipment.
Another important benefit is that spread spectrum (SS) systems offer significant immunity to RF multipath. This is critical for transceivers located in or inside metal objects (i.e. within air frames or space frames). Multipath refers to multiple signal paths due to reflections that may exist between a transmitter and receiver. A classic example of multipath is the “ghost images”, which commonly exist on a standard TV pictures. Multipath occurs when path length differences are exactly (n+½) wavelength. The resulting cancellation can be almost complete for narrow band transmissions. A spread spectrum (SS) signal, which exists over a large frequency or equivalently large wavelength range, may be partially canceled, but other portions will still be received. Since a SS signal contains significant redundancy, complete data can still be extracted from the resulting signal.
Piezoelectric materials are a unique family of materials that can be deformed by an applied electric field, and conversely, a deformation of the material produces an electric field. An antenna will reflect RF energy that is transmitted at its nominal frequency. Further, stress-induced deformation of a SAW-based sensor attached to an antenna will shift the prime frequency response causing the greatest energy to be reflected at a frequency slightly different from the nominal frequency. Therefore, an electric field imposed on the fingers causes a local deformation that launches a wave on the surface of the substrate. This surface wave can be converted back to an electrical signal by being read off another set of interdigitated transducers.
Reeder et al, U.S. Pat. No. 3,978,731 discloses a surface acoustic wave transducer wherein SAW wave are propagated across a thin, flexible diaphragm which is subjected to an applied external pressure. Since the wave velocity and path length vary with diaphragm deformation, the acoustic wave delay time is a function of the applied external pressure. Electroacoustic transducers are fabricated on opposite edges of the diaphragm for electronic excitation and detection of the surface acoustic wave. An electronic feedback path including the two transducers, the wave path, and an electronic amplifier oscillates at a frequency which is determined by the delay time required for acoustic wave propagation over the diaphragm path, and which decreases approximately linearly with applied external pressure. A second acoustic path called the reference path has a length equal to the first path and in the preferred embodiment contains a diaphragm which is subject to a different applied pressure. A second electronic feedback path composed of two transducers, the reference path and a second amplifier oscillates at a second frequency called the reference frequency. By applying a sample of the first and second oscillator voltages to a semiconductor mixer, a difference frequency output is obtained which is proportional to the differential pressure. The difference frequency output is approximately independent of temperature, and is converted to various digital codes by use of standard frequency counter circuits. If only one set of transducers is used, the device can also measure temperature in a digital manner. Stress or strain measurements may also be made by bonding the diaphragms or only the transducers directly on the physical surface to be measured.
Ebata, U.S. Pat. No. 4,249,418 discloses a temperature detector having a transmitting section including an oscillator which has a surface acoustic wave resonator that includes a Y-cut and Z-propagation piezoelectric base plate, of which the frequency characteristic varies responding to the temperature of the base plate, the oscillator generating an oscillation output of a frequency corresponding to the frequency characteristic and an antenna for transmitting the oscillation output. A receiving section includes a receiving antenna, a means for detecting the output of the receiving antenna, and a signal processing circuit for processing the output of the detecting means to generate at least one of the temperature display and control signals of the base plate of the surface acoustic wave device.
Mariani et al, U.S. Pat. No. 5,823,425 discloses a surface acoustic wave (SAW) sensing device for remotely sensing structural integrity of a physical structure. The sensing device includes a piezoelectric substrate with a notch formed part way in the bottom of the substrate and along the width thereof. The substrate is mounted to a physical structure. An antenna is coupled to the RF circuit on the substrate and is capable of receiving and transmitting a RF signal. Interdigital input and output transducers are disposed on the upper surface of the substrate. The input transducer is located adjacent one end of the substrate and the output transducer is located adjacent an opposing end of the substrate. Bus bars connect the input and output transducers. The input transducer provides a complementary first response upon receipt of an RF expanded linear
onlinear FM signal from the antenna and transmits this compressed pulse to the output transducer. The output transducer provides a second response upon receipt of the first response and transmits the same to the antenna via the bus bar. When the substrate is strained beyond a predetermined critical level, the substrate is fractured along the notch and the first response emitted by the input transducer is prevented from being transmitted to the output transducer indicating SAW sensor failu
Ajupova Gulnara
Champaigne Kevin
Kiefer Karl F.
Krug Eric
Ellington Alandra
Invocon, Inc.
Lefkowitz Edward
Roddy Kenneth A.
LandOfFree
Encoded surface acoustic wave based strain sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Encoded surface acoustic wave based strain sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Encoded surface acoustic wave based strain sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3355708