Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-10-09
2004-10-12
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C073S510000, C073S514280
Reexamination Certificate
active
06803698
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to acceleration sensors and, more particularly, to an acceleration sensor using a surface acoustic wave device (SAW device).
2. Description of the Related Art
Japanese Examined Patent Application Publication No. 4-79419 discloses an acceleration sensor using an SAW device. This acceleration sensor includes a surface acoustic wave device having interdigital (IDT) electrodes arranged on the surfaces of a piezoelectric substrate, and employs, as a means for applying a load on the device under acceleration, a four-point support system which is supported at both ends thereof with an acceleration generating mass attached near the ends thereof.
The surface acoustic wave device is supported at both ends in a case. However, when the case and the surface acoustic device are thermally expanded, stress is generated in the surface acoustic device due to a difference between thermal expansion coefficients thereof. The resonance frequency of the surface acoustic wave device is varied in response to factors other than acceleration, for example, a change in the characteristics of the surface acoustic wave device due to a change in the temperature of the surface acoustic wave device itself. These factors present difficulty in detecting acceleration with sufficient accuracy.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a high-accuracy acceleration sensor free from factors such as temperature changes, except acceleration.
The present invention relates to an acceleration sensor and includes a bimorph type acceleration detection element including a pair of surface acoustic wave resonators coupled to each other with the back surface of one resonator bonded to the back surface of the other resonator. Each resonator includes a piezoelectric substrate and a pair of IDT electrodes which are arranged on the front surface of the piezoelectric substrate. The acceleration detection element is supported at an end thereof such that the acceleration detection element is deflected in the thickness direction of the piezoelectric substrate under acceleration. Acceleration is detected by differentially detecting a frequency change or an impedance change of said two surface acoustic wave resonators which is caused by the deflection of the acceleration detection element.
The acceleration sensor has a bimorph structure in which the two surface acoustic wave resonators are coupled together with the back surface of the one resonator bonded to the back surface of the other resonator. When acceleration acts in the direction of thickness of the acceleration detection element, the acceleration detection element is deflected in the direction of thickness. With the acceleration detection element deflected, tensile stress is generated in the one surface acoustic wave resonator while compressive stress is generated in the other surface acoustic wave resonator. The surface acoustic wave resonator generates a surface acoustic wave (SAW) on the piezoelectric substrate with a signal input between the pair of IDT electrodes. The resonator has a predetermined resonance impedance between the pair of electrodes. The surface acoustic wave travels on only the surface of the piezoelectric substrate, while being immediately attenuated in its travel in the direction of thickness of the substrate. Almost no surface wave travels to the back surface of the substrate having no electrodes. Even if the two surface acoustic wave resonators are laminated with the back surface of the one resonator bonded to the back surface of the other resonator, the vibrations of the two resonators are isolated from each other. Each resonator resonates at its own natural frequency. The frequency of the surface acoustic wave resonator on the tensile stress side becomes low, while the frequency of the surface acoustic wave resonator on the compressive stress side becomes high. Acceleration is thus detected with high gain if the frequency changes of the two resonator or the impedance changes of the two resonators are differentially picked up.
Since the frequency difference or the impedance difference is detected rather than individually picking up the frequency changes of the two resonators or the impedance changes of the two resonator, stresses commonly acting on the two surface acoustic resonators (a stress due to a temperature change, for example) cancel each other out. A high-gain acceleration sensor free from the effect of temperature changes is provided.
When the surface acoustic wave resonators are bonded, an adhesive agent, which is hard in the set state thereof, may be used. Alternatively, an adhesive agent, having a certain degree of softness (elasticity) in the set state thereof, may be used. The surface acoustic wave spreads inwardly while being attenuated at the same time. To prevent the vibrations on the front surface and the back surface from interfering with each other, some thickness is required of the element. The thickness of the element must typically be two to ten times larger than the wavelength of the surface acoustic wave. However, if the element is too thick, the element is difficult to deflect under acceleration. From the standpoint of detection gain of acceleration, a thinner element is better. The bimorph structure having the resonators bonded with the elastic adhesive interposed therebetween substantially attenuates vibration in the adhesive layer. Vibration does not travel between the front surface and the back surface of the element even if the element is thin. The acceleration detection element having a thin overall thickness works.
The adhesive agent may be an elastic adhesive agent such as an epoxy-based adhesive or an acrylic adhesive.
The present invention relates to an acceleration sensor and includes a bimorph type acceleration detection element including two surface acoustic wave resonators which include a single piezoelectric substrate, and a pair of IDT electrodes arranged on each of the front and back surfaces of the piezoelectric substrate. The acceleration detection element is supported at an end thereof such that the acceleration detection element is deflected in the thickness direction of the piezoelectric substrate under acceleration. Acceleration is detected by differentially detecting a frequency change or an impedance change of said two surface acoustic wave resonators which is caused by the deflection of the acceleration detection element.
Two types of surface acoustic wave resonators are typically available: a first type is fabricated of a piezoelectric substrate having IDT electrodes arranged on the surface thereof, and a second type is fabricated of a glass substrate having IDT electrodes arranged on the surface thereof with a piezoelectric film deposited on the IDT electrodes. In the first and second aspects of the present invention, the surface acoustic wave resonator is of the first type, while in third and fourth aspect of the present invention to be discussed later, the surface acoustic wave resonator is of the second type.
The present invention relates to an acceleration sensor and includes a bimorph type acceleration detection element including a pair of surface acoustic wave resonators laminated to each other with the back surface of one resonator bonded to the back surface of the other resonator, wherein each resonator includes a glass substrate, a pair of IDT electrodes which are arranged on the front surface of the glass substrate, and a piezoelectric film which is deposited on the glass substrate including the IDT electrodes. The acceleration detection element is supported at an end thereof such that the acceleration detection element is deflected in the thickness direction of the glass substrate under acceleration. Acceleration is detected by differentially detecting a frequency change or an impedance change of said two surface acoustic wave resonators which is caused by the deflection of the acceleration detection element.
The bimorph type acceleration detection element i
Dougherty Thomas M.
Keating & Bennett LLP
Murata Manufacturing Co. LTD
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