Patent Application
20100141087
Embodiments are generally related to SAW (Surface Acoustic Wave) sensing devices and applications. Embodiments are also related to semi-synchronous SAW resonators. Embodiments are further related to SAW-based sensor apparatus utilizing semi-synchronous SAW resonators.
Surface Acoustic Wave (SAW) devices are electronic components that generate guided acoustic waves along a surface of the device. The SAW devices can be employed as resonators, filters, oscillators and transformers based on the transduction of the acoustic waves. Such devices are generally fabricated on single crystal anisotropic substrates that are piezoelectric (e.g., quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, etc). SAW devices typically include one or more pairs of interdigital transducers to convert the electrical signals applied to the device into electromechanical surface acoustic waves generated in the device. The surface acoustic wave is an acoustic wave produced on the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate.
SAW resonators have important practical applications as wireless sensors (e.g. pressure, temperature). The desired features of the SAW resonator includes single resonance with precisely controlled frequency, high quality factor, in/out impedance matching and small size. However, it is difficult to achieve all these features simultaneously. For example, a synchronous SAW resonator has typically two resonances at both edges of the stop band and their frequencies depend on reflection coefficient per finger, which in turn depends on surface loading. On the other hand, transducer directivity has to be taken into account in the resonance condition. Directivity is a figure of merit, which measures the power density in the direction of its strongest emission of the acoustic waves generated by the transducer. Therefore, there is a certain degree of uncertainty in prescribing the resonance frequency, its main source being the deposition tolerance of metal such as its thickness. In one prior art SAW sensor, the directivity of the sensor is compensated by shifting the gratings with respect to the transducer, which results in inaccuracies due to determined directivity.
Based on the foregoing, it is believed that a need exists for an improved SAW based sensor apparatus utilizing a semi-synchronous SAW resonator having single resonance with high quality factor, in/out impedance matching and small size simultaneously.
The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for an improved sensing device and applications thereof.
It is another aspect of the present invention to provide for an improved SAW based sensor apparatus.
It is a further aspect of the present invention to provide for improved SAW sensor apparatus utilizing semi-synchronous SAW resonators.
The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A SAW based sensor apparatus utilizing semi-synchronous SAW resonators having a single resonance at Bragg frequency with very high quality factor is disclosed. The semi-synchronous SAW resonators includes at least one inter-digital transducer, which generates and receives surface acoustic wave and a number of grating reflectors, which reflect the surface acoustic wave and generate a standing wave between the reflectors. The inter-digital transducer and the grating reflectors can be fabricated on a substrate (e.g., quartz) by photolithographic process. The resonance condition is independent of transducer directivity and reflection coefficient per finger. Such a SAW based sensor apparatus having three semi-synchronous SAW resonators can be utilized for measuring pressure (PSAW) and temperature (TSAW) and the third one being utilized as a reference (RSAW) for a wireless tire-pressure monitoring system (TPMS).
The design of the SAW resonators is based on a combination of finite-element (FEM) numerical calculations and coupling-of-modes (COM) analytical calculations. The distance between the interdigital transducer and a left grating reflector is λ/2 and the distance between the interdigital transducer and a right grating reflector is 3λ/4. The interdigital transducer can produce a single resonance, which can be independent of the directivity and reflection coefficient. The interdigital transducer and the grating reflectors have the same frequency and the left grating is made synchronous with the interdigital transducer. The quality factor and target values of electrical admittance can be achievable with the small-sized SAW resonators.
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
The interdigital transducer 110 can be made synchronous with the grating reflector 120. D1 and D2 are the distances between the interdigital transducer 110 and the left grating 120 and the right grating 130, respectively. The interdigital transducer 110 and the grating reflectors 120 and 130 have the same frequency and the left grating 120 is made synchronous with the interdigital transducer 110, so that D1=λ/2. The other space D2=3λ/4. The interdigital transducer 110 and reflector gratings 120 and 130 all have the same Bragg frequency to produce a single resonance with high quality factor. The Bragg frequency is the frequency at which the individual reflections from each of the periodically spaced distances add up in phase to maximize the reflection. Such a Bragg frequency occurs at a frequency where the spacing can be 114 of a wavelength on the transmission line. The interdigital transducer 110 can produce a single resonance, which can be independent of the directivity and reflection coefficient. The directivity is the power density of the signal in the direction of the strongest discharge emitted by the transducer 110, whereas the reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave.
The SAW based sensor apparatus 200 further includes acoustic absorbers 220 that can be fabricated on the quartz crystal substrate 210 for absorbing the surface acoustic waves to prevent distortion of the signal. Such SAW based sensor apparatus 200 eliminates possible inaccuracies introduced by the determined directivity. As an external voltage input to the resonator 100, the interdigital transducer 110 converts electrical signals into surface acoustic waves, which then propagate away from the interdigital transducer 110. As the waves hit the metal gratings 120 and 130, they are reflected back to the interdigital transducer 110 and form a resonant cavity.
The frequency response of the SAW resonator 100 can be calculated based on a combination of finite-element (FEM) numerical calculations and coupling-of-modes (COM) analytical calculations. The COM parameters can be, for example: surface wave velocity, reflection parameter, propagation loss, transduction parameter, thin film finger capacitance, finger resistance and other appropriate parameters. The FEM is a technique originally developed for numerical solution of complex problems in structural mechanics. The coupling-of-mode analysis can be utilized in order to analyze the structure, specifically for the film thickness of the interdigital transducer 110, and the FEM can be employed for evaluating the coefficients of COM equations.
The input of the transducer can be connected to the radiofrequency antenna 580. When the antenna 580 receives an electromagnetic signal, this gives rise to waves over the surface of the substrate 505 which are themselves converted into electromagnetic energy on the antenna 580. Thus, the device, consisting of a resonators 540, 550 and 560 connected to the antenna 580, has a response at the resonant frequency of the resonator and it is possible to measure this frequency remotely. It is thus possible to produce remotely interrogable sensors. This possibility is an important advantage of surface acoustic waves and is used in the context of tire pressure sensors 500 since it is advantageous to be able to place the sensor in the tire, while the interrogating electronics are placed in the vehicle.
It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
