JP2010210313A - Surface acoustic wave type vibration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve sensitivity of a surface acoustic wave type vibration sensor in a frequency domain other than a resonance frequency, and to elongate a radio-communicable distance. <P>SOLUTION: In this vibration sensor including SAW (Surface Acoustic Wave) electrodes 3, the shape of a piezoelectric material body 1 having a cantilever structure is a wedged shape corresponding to a desired frequency to be detected, and since a body thickness is changed gradually from a support part 2 toward a tip part of the body, vibration intensity of the SAW electrodes 3 when an external vibration is applied is enhanced by a weight effect generated by change of a distance between the fulcrum and the gravity center of the cantilever structure in comparison with a case of using a piezoelectric material having a rectangular parallelepiped shape, and sensor sensitivity can be improved, and the radio-communicable distance can be elongated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention provides, for example, a structure for increasing the sensitivity of a surface acoustic wave type vibration sensor that detects an AE (Acoustic Emission) wave generated when glass is broken by a surface acoustic wave (SAW) type element. About.

  Patent Document 1 discloses a vibration detection method for detecting a vibration phenomenon of an object and a vibration sensor, which can be used for a glass breakage detection device for earthquake resistance diagnosis of buildings and security security, or for abnormal vibration detection of equipment and machine tools. And a vibration detection system including an interrogator is disclosed.

  As a solution for providing a vibration detection method that enables downsizing of a vibration sensor, the technology disclosed in Patent Document 1 includes a batteryless vibration sensor including a single-port surface acoustic wave (SAW) resonator. It is attached to an object and sends a continuous carrier wave from the interrogator to the batteryless vibration sensor, while the interrogator monitors the reflected waves from the batteryless vibration sensor. When a vibration phenomenon occurs in the object, mechanical vibration is applied to the batteryless vibration sensor, and as a result, a change occurs in the reflected wave, and the vibration phenomenon is detected based on the change.

  The vibration sensor of Patent Document 1 is battery-less, and since the vibration detection process is performed on the interrogator side, the signal processing unit can be omitted from the vibration sensor, and the size can be reduced.

JP 2007-232708 A

  A surface acoustic wave type vibration sensor disclosed in Patent Document 1, that is, a piezoelectric material housing having a surface acoustic wave (SAW) electrode on a main surface has a cantilever structure in which one end of a SAW resonator is fixed to a support portion. It has a rectangular parallelepiped shape with the same thickness on the opposite side to the fixed side. The sensitivity of the sensor is basically determined by how much the vibration of the cantilever structure is large, that is, the intensity of the vibration.

  In the rectangular piezoelectric material housing disclosed in Patent Document 1, when the frequency deviates from the resonance frequency determined by the housing shape, the vibration intensity of the SAW resonator does not increase with respect to that frequency, and the vibration mode deviates from the resonance frequency. On the other hand, there was a problem that improvement in sensitivity could not be expected.

  Further, in the batteryless vibration sensor having the SAW resonator, the SAW electrode and the reflector on the piezoelectric body use, as passive elements, a radio signal modulated based on the vibration of the carrier wave transmitted from the interrogator. Therefore, the fact that the sensitivity is low, the transmission output at this time is also small, and the distance for wireless communication with the interrogator is not increased.

  As a result of intensive investigations to solve the above-mentioned problems, the inventors of the present application have made the casing shape not a rectangular parallelepiped but a substantially trapezoidal shape while setting the resonance frequency determined from the casing shape of the piezoelectric material to a desired frequency. It was found that the intensity of vibration can be increased by adopting the quadrangular prism shape, that is, the wedge shape, and the sensitivity of the surface acoustic wave type vibration sensor can be improved, leading to the present invention.

  According to the present invention, a surface acoustic wave (SAW) electrode and a reflector are provided on the main surface of a housing made of piezoelectric material, and the housing has a substantially trapezoidal quadrangular prism, that is, a wedge shape, and supports the end of the wedge. A surface acoustic wave type vibration sensor having a cantilever structure as a part is obtained.

  The surface acoustic wave (SAW) electrode and the reflector can be composed of comb electrodes (IDT).

  The end of the wedge to be supported may be on the side where the wedge is thick or on the side where the wedge is thin.

  The SAW resonator and the reflector are desirably in the vicinity of the support portion of the main surface on the side where the wedge is supported.

The thickness of the end portion on the thin side of the wedge is a, the thickness of the end portion on the thick side of the wedge is b, and the length from the support portion of the main surface on the side where the wedge is supported to the unsupported end portion As L
It is desirable that 1 <(b / a) ≦ 5 and L = 1 to 10 mm.

  With a SAW resonator, the shape of the piezoelectric material housing that has a cantilever structure is wedge-shaped, depending on the frequency to be detected, so that the thickness of the housing changes from the support to the tip of the housing. Therefore, the distance between the fulcrum of the cantilever beam structure and the center of gravity is different from that when a rectangular parallelepiped piezoelectric material is used, and the weight effect caused by the change causes the vibration of the SAW resonator when external vibration is applied. The strength increases and the sensitivity of the sensor can be improved.

  Thus, in the batteryless glass breakage sensor, the sensitivity of the sensor is improved, so that the reflected output to the interrogator can be increased and the communicable distance can be increased.

1 is a schematic view of a surface acoustic wave type vibration sensor according to the present invention. FIG. 1A is a schematic view when the thick side of the wedge is a support portion, and FIG. 1B is a schematic view when the thin side of the wedge is a support portion. The frequency characteristic of the ratio of the front end side speed of a wedge, and the support part side speed in Embodiment 1. FIG.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic view of a surface acoustic wave type vibration sensor according to the present invention, and FIG. 1 (a) is a schematic view when the thick side of the wedge is a support portion. FIG. 1B is a schematic view when the thin side of the wedge is the support portion. An antenna that contributes to transmission / reception of a carrier wave from the interrogator is not shown.

  A surface acoustic wave vibrator having a piezoelectric material having the shape shown in FIG. 1A or FIG. 1B as a housing is manufactured and evaluated by the following manufacturing procedure and evaluation method.

(About design and structure)
The values of a, b, and L defined as in FIG. 1 (a) and FIG. 1 (b) are respectively set to the thickness a = 0.05 mm at the end on the thin side of the wedge and the end at the thick side of the wedge. Thickness b = 0.25 mm, the length of the main surface (= the length of the piezoelectric material housing) L = 2 mm. The conventional rectangular parallelepiped piezoelectric material housing to be compared has a length of 2 mm, a width of 1 mm, and a thickness of 0.25 mm.

Of the length L of the piezoelectric material housing 1, for example, if 0.5 mm is a portion that is fixed to the support portion 2, the length of the portion that actually vibrates is 1.5 mm. If conventional rectangular parallelepiped, if a material B cut LT (LiTaO _3), the resonant frequency is 60kHz, and the which are included in the frequency range of the AE wave to be detected as the mode of the glass breaking.

  Here, the above-described weight effect will be described. As shown in FIG. 1A, when the thickness on the support side is made thicker than the tip, the position of the center of gravity is on the support side compared to the rectangular parallelepiped shape, and the wavelength of the fundamental vibration mode is short. Therefore, it will be understood that the resonance frequency is shifted to a higher frequency side than the resonance frequency 60 kHz calculated from the rectangular parallelepiped shape. On the other hand, as shown in FIG. 1B, in the case of a structure in which the thickness on the support part side is thinner than the tip part, the center of gravity position is similarly on the tip part side compared to the rectangular parallelepiped shape, so that the resonance frequency Shifts to the low frequency side. Note that the values of the thicknesses a and b need to be determined in consideration of the strength of the resonator itself, and the accurate resonance frequency should be calculated and measured.

(Production procedure)
For example, a piezoelectric material having respective comb-teeth electrodes (IDT) to be the SAW electrodes 3 and the reflectors 4 on the main surface is cut into the shape of FIG. 1A or FIG. The side to be supported is fixed to a pedestal or the like of the package case which is a support portion with an agent or the like.

  Further, if a highly reflective metal film such as gold that reflects laser light is thinly coated on the surface of the resonator, it is easy to observe the vibration with a laser Doppler vibrometer.

(About measurement)
Using a laser Doppler vibrometer, measure the vibration strength of the surface acoustic wave type vibrator when forced vibration is applied by a vibration exciter, that is, the speed ratio, which is the ratio of vibration amplitude between the support and tip sides. Can do.

  As a result of actually producing and evaluating a surface acoustic wave type vibration sensor, the frequency characteristic of the ratio between the speed at the front end side of the wedge and the speed at the support portion side in the embodiment shown in FIG. 2 was obtained.

  According to the present embodiment, when the piezoelectric material is a B-cut LT and has the structure of FIG. 1A, compared to the conventional rectangular parallelepiped structure, the vicinity of 85 kHz indicated by the arrow in FIG. In the vicinity of the resonance frequency, it was found that the ratio of the speed on the tip side to the speed on the support part side was about 6 times. When the impedance change amount of the IDT of the SAW electrode 3 is estimated from this vibration intensity, it is 25 times that of the conventional structure. Furthermore, when the distance that can be wirelessly communicated without a battery is calculated, it is 2.2 times that of the conventional distance.

  In the case where the thin side of the wedge in FIG. 1B is a support portion, that is, in the case of using in the low frequency region, as shown in part in FIG. Is obtained.

In order to expect the above effect, the size of the piezoelectric material housing is preferably in the range of 1 <(b / a) ≦ 5, L = 1 to 10 mm in consideration of the strength of the supporting portion and the frequency of the detected AE wave. It is. As the piezoelectric material, in addition to the above LiTaO ₃ , a piezoelectric single crystal such as quartz or langasite having excellent temperature characteristics, or a sintered body such as PZT (lead zirconate titanate) may be used. It is preferable to select values of a, b and L in consideration of characteristics.

  The present invention can be used as a glass breakage sensor for crime prevention such as home security, ATM, and shop front.

DESCRIPTION OF SYMBOLS 1 Piezoelectric material housing 2 Support part 3 SAW electrode 4 Reflector

Claims (3)

  A cantilever structure comprising a surface acoustic wave electrode and a reflector on the main surface of a housing made of piezoelectric material, the housing having a substantially trapezoidal square pillar, that is, a wedge shape, with the end of the wedge as a support. A surface acoustic wave type vibration sensor.   The surface acoustic wave type vibration sensor according to claim 1, wherein the surface acoustic wave electrode and the reflector are comb electrodes. The thickness of the end portion on the thin side of the wedge is a, the thickness of the end portion on the thick side of the wedge is b, and the length from the support portion of the main surface on the side where the wedge is supported to the unsupported end portion If L is L,
1 <(b / a) ≦ 5, L = 1 to 10 mm
The surface acoustic wave type vibration sensor according to claim 1, wherein the surface acoustic wave type vibration sensor is provided.

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