JP2009281975A - Surface acoustic wave device and sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device capable of obtaining high sensitivity with simple configuration and a sensor using the surface acoustic wave device. <P>SOLUTION: In the stress sensor 1, a first interdigital electrode 15 and a second interdigital electrode 17 have interdigitated finger structure combining up-chirp and down-chirp varying to reduce the breadth of interdigitated finger and spacing of interdigitated finger as each interdigital electrode 15 and 17 departing from the center to right and left directions. In fact, spacing of the interdigitated finger is established to increase from the input side to the central side of the first interdigital electrode 15 (established as up-chirp) and spacing of the interdigitated finger is established to decrease from the central side to the output side of the first interdigital electrode 15 (established as down-chirp). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave element including a comb electrode and a sensor such as a stress sensor including the surface acoustic wave element.

Conventionally, various types of sensors have been studied using surface acoustic wave elements that utilize surface acoustic wave (SAW) characteristics.
The surface acoustic wave element is an element in which a comb electrode is provided on a propagation material made of, for example, a piezoelectric material. The surface acoustic wave element propagates the propagation material by mechanical perturbation applied to the propagation material. The surface acoustic wave characteristics of the For this reason, sensors that detect stress, displacement, etc. have been developed using the characteristics of this surface acoustic wave element.

  Specifically, in a sensor having a surface acoustic wave element in which a comb-shaped electrode on the input side and an output side are provided facing each other on a piezoelectric material substrate, a high-frequency AC signal is applied to the comb-shaped electrode on the input side. The surface acoustic wave is received by the comb electrode on the output side, and the voltage signal corresponding to the received surface acoustic wave is output from the comb electrode on the output side. Displacement etc. is detected.

As a technique related to the above-described sensor using a surface acoustic wave element, for example, a technique of a distributed delay line whose transmission time varies depending on a frequency is disclosed (see Patent Document 1). Moreover, the technique of the following patent documents 2-4 is disclosed as a displacement measuring technique using a surface acoustic wave.
US Pat. No. 614432 JP 2004-51176 A JP-T-2005-526240 JP 2005-156557 A

  Among these, in the techniques of the cited references 2 to 4 described above, the displacement is detected mainly by detecting the change in the resonance frequency, but it is easily affected by noise, and it is not easy to measure the displacement with high accuracy. . As a countermeasure, it is conceivable to use a circuit for removing noise, but this is not preferable because the apparatus becomes expensive.

  On the other hand, although the technique of the cited document 1 employs delay time measurement that is not easily affected by noise, the change in the surface acoustic wave characteristics due to external force is slight, so that detection is not easy. As a countermeasure, a special circuit for detecting a slight change may be used, but this is not preferable because the device becomes expensive.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a surface acoustic wave element that can obtain high sensitivity with a simple configuration and a sensor using the surface acoustic wave element.

  (1) According to the first aspect of the present invention, a first converter comprising a first comb-tooth electrode for exciting a surface acoustic wave and the surface acoustic wave excited by the first comb-tooth electrode are received and the surface acoustic wave is converted into the surface acoustic wave. A surface acoustic wave element in which a second transducer composed of a second comb electrode that outputs a corresponding signal is disposed across a propagation surface through which the surface acoustic wave propagates, wherein at least one of the two comb electrodes Is provided with an up-chirp comb-tooth structure and a down-chirp comb-tooth structure.

In the present invention, at least one of the first comb electrode and the second comb electrode has an up-chirp comb structure and a down-chirp comb structure.
This up-chirp comb-tooth structure is, for example, as illustrated in FIG. 1B, in a direction in which surface acoustic waves are transmitted and received (here, for example, rightward in the left-right direction in the figure). Therefore, it has a structure in which the comb tooth interval (W1 or the like): for example, the interval when measured at the center position of each comb tooth) increases. On the other hand, the down-chirp comb-teeth structure has a structure in which the comb-teeth interval decreases in the right direction in the figure.

  Basically, the frequency characteristics of the surface acoustic wave element are determined by the period of the comb electrode for exciting the surface acoustic wave. That is, when the interval between the comb teeth (W1 or the like) matches the half period of the wavelength λ of the surface acoustic wave, the surface acoustic wave having that period is excited most efficiently.

  Therefore, in the case of the above-described up-chirp and down-chirp comb-tooth structures, the interval between the comb-teeth is set to be different in the transmission / reception direction of the surface acoustic wave, so that the surface acoustic wave that can be excited most efficiently (and hence its frequency: A plurality of these can be set. Specifically, a plurality of excitation frequencies can be set by an up-chirp comb-tooth structure, and a plurality of excitation frequencies can also be set by a down-chirp comb-tooth structure.

  For example, when stress is applied to the surface acoustic wave element, the piezoelectric material is distorted by the stress, so that the comb tooth structure is slightly deformed (specifically, the comb tooth interval is changed), and the frequency of the surface acoustic wave element is increased. The characteristic changes.

  That is, since the frequency characteristics of the surface acoustic wave element change due to mechanical perturbation such as stress, for example, the excitation state (ie, the optimum excitation frequency) of the surface acoustic wave in the first transducer changes. Therefore, for example, since the output state of the received signal (the magnitude of the peak of the voltage signal, etc.) in the second converter changes, for example, stress applied to the surface acoustic wave element can be detected by the output signal of the second converter. it can.

  In particular, in the present invention, a plurality of excitation frequencies (for example, an excitation frequency corresponding to a small comb tooth interval and an excitation frequency corresponding to a large comb tooth interval) can be set respectively by the up-chirp and down-chirp comb-tooth structures. The excitation frequency can be appropriately set so as to be suitable for detection of stress or the like. That is, it is possible to set so that an output signal suitable for detection of stress or the like can be obtained by an up-chirp and down-chirp comb-tooth structure.

  As described later, as described later, the output voltage signal peak delay time is changed according to the stress, or the output voltage signal peak appearance state (appearance appears according to the stress). Number) can be set to change.

Accordingly, a surface acoustic wave device having high sensitivity can be realized with a simple configuration without using an expensive detection circuit.
In the present invention, when only the first comb electrode has the above-described up-chirp and down-chirp comb-tooth structure, only the second comb-tooth electrode has the up-chirp and down-chirp comb-tooth structure, There is a case where both the first comb electrode and the second comb electrode have an up-chirp and down-chirp comb-tooth structure.

In these cases, the up-chirp and down-chirp comb-teeth structures have the same effects as described above regardless of which comb-teeth structure is present.
(2) The invention according to claim 2 is excited by a third converter comprising a third comb electrode for exciting a surface acoustic wave and outputting a signal corresponding to the received surface acoustic wave, and the third comb electrode. A surface acoustic wave element in which a reflector composed of a fourth comb electrode that reflects the surface acoustic wave is disposed across a propagation surface through which the surface acoustic wave propagates, wherein at least one of the comb electrodes Is provided with an up-chirp comb-tooth structure and a down-chirp comb-tooth structure.

In the present invention, a reflector is used in place of the second converter according to the first aspect of the present invention.
In the present invention, since at least one of the third comb electrode and the fourth comb electrode has an up chirp comb structure and a down chirp comb structure, an output signal obtained from the third converter The same effects as those of the first aspect of the invention can be achieved.

  That is, in the present invention, for example, the surface acoustic wave generated by the third transducer is reflected by the reflector and is extracted as an electric signal by the same third transducer. It is the same as the invention.

  In the present invention, when only the third comb-tooth electrode has an up-chirp and down-chirp comb-tooth structure, when only the fourth comb-tooth electrode has an up-chirp and down-chirp comb-tooth structure, There is a case where both the tooth electrode and the fourth comb electrode have an up-chirp and down-chirp comb structure. In addition, the comb-like structure of the up-chirp and the down-chirp has the same effect as the above, regardless of which comb-tooth structure is present.

(3) The invention of claim 3 is characterized in that an electrode that reflects the surface acoustic wave other than the comb-tooth electrode is used instead of the fourth comb-tooth electrode of the reflector.
The present invention exemplifies a case where a comb electrode is not used in the reflector. As an electrode used for the reflector, for example, a metal strip array configured by a metal pattern arranged with a half-wave period of the surface acoustic wave that can efficiently reflect the surface acoustic wave can be adopted.

In this case, the third comb electrode of the third converter has a comb structure in which up chirp and down chirp are combined.
(4) The invention of claim 4 is a sensor comprising the surface acoustic wave element according to any one of claims 1 to 3, wherein a time change between peaks of a voltage signal generated by the surface acoustic wave is detected. Based on this, the state of the measurement object is detected (that is, the sensitivity of the sensor).

  For example, the transmission time of the surface acoustic wave from the first comb electrode to the second comb electrode is strongly excited by which comb tooth of the first comb electrode (a comb tooth having what kind of comb tooth interval). It is different. This is because the positions of the comb teeth that are excited most efficiently are different.

  Therefore, when stress is applied to the surface acoustic wave element, the frequency characteristics of the element change due to the change of the comb-teeth spacing of the comb-teeth electrodes, so that the comb tooth position where the surface acoustic wave is excited more efficiently can be obtained. Because it changes, the transmission time changes. Therefore, for example, there is a time difference (delay time) in the transmission time between the output signal (voltage signal peak) when no stress is applied and the output signal (voltage signal peak) when stress is applied. Therefore, stress and the like can be obtained from the change in the delay time.

  In particular, the present invention has a comb-tooth structure combining up-chirp and down-chirp, so that a plurality of excitation frequencies can be set for each of the comb-tooth structures. Therefore, the transmission time of the voltage peak of the output signal generated by the surface acoustic wave corresponding to each excitation frequency is also different.

  Therefore, if the comb structure of up-chirp and down-chirp is set so that a pair of voltage peaks appear due to a surface acoustic wave with a large frequency and a pair of voltage peaks appear due to a surface wave with a small frequency ( In other words, if the interval between the comb teeth is set), the transmission time between the pair of voltage peaks also changes with the stress change, so that the stress or the like can be detected by measuring the time between the peak voltages.

Even when a reflector is provided, since the transmission time between the third converter and the reflector changes in the same manner, the time between peaks can be similarly set as the sensor sensitivity.
(5) The invention of claim 5 is a sensor comprising the surface acoustic wave element according to any one of claims 1 to 3, and based on the number of peaks of a voltage signal generated by the surface acoustic wave, It is characterized by detecting the state of the measurement object (that is, the sensitivity of the sensor).

  In the present invention, since it has a comb tooth structure combining up-chirp and down-chirp, it is possible to set a plurality of excitation frequencies corresponding to each comb-tooth structure. For example, when a stress is applied to the surface acoustic wave element, the excitation frequency changes due to the change in the electrode spacing between the comb-teeth electrodes, so the output signal (the peak position of the voltage signal) by the surface acoustic wave corresponding to the excitation frequency Also changes.

That is, since the voltage signal peak appearance state changes according to the stress, the stress or the like can be obtained from the voltage peak appearance state (here, the number of peaks).
Similarly, when the reflector is provided, the number of peaks of the voltage signal changes, so that the number of peaks can be used as the sensor sensitivity.

(6) The invention of claim 6 is characterized in that the sensor is a stress sensor for detecting a stress applied to the surface acoustic wave element.
The present invention exemplifies the use of the sensor described above. In addition to the stress, for example, it can be used as a displacement sensor that detects a change in the distance between the first transducer and the second transducer or the distance between the third transducer and the reflector. Furthermore, it can be used for detecting temperature, humidity, strain, and the like.

(7) The invention according to claim 7 is characterized in that the stress sensor is a tire air pressure sensor for detecting tire air pressure from a change in stress generated in the tire.
The present invention illustrates the use of a stress sensor.

(8) The invention of claim 8 has a function of transmitting a signal output from the comb electrode on the receiving side of the surface acoustic wave to the controller by wireless communication.
In the present invention, since data detected by the sensor can be transmitted by wireless communication, there is an advantage that there are few restrictions on the installation location of the sensor.

  For example, if an antenna for outputting to the outside is provided, high-frequency signals can be input / output to / from the surface acoustic wave element in a non-contact manner, so that no cables or contacts are required to input the high-frequency signals. Therefore, it can be installed on a moving part such as a rotating body.

Embodiments to which the present invention is applied will be described below with reference to the drawings.
[First Embodiment]
In the present embodiment, for example, a stress sensor used as a tire pressure sensor for detecting tire pressure will be described as an example.

a) First, the configuration of the stress sensor will be described.
FIG. 1 is an explanatory diagram showing a schematic configuration of the stress sensor 1, and FIG. 2 is an explanatory diagram showing a signal state and the like.

  As shown in FIG. 1 (a), the stress sensor 1 includes a surface acoustic wave element 3 and antennas 5 and 7. The surface acoustic wave element 3 is a mechanical sensor between a surface acoustic wave and an electric signal. A first converter 9 and a second converter 11 that perform electrical conversion are provided.

  Among these, the surface acoustic wave element 3 is a point-symmetrical arrangement of a pair of comb electrodes (the first comb electrode 15 on the input side and the second comb electrode 17 on the output side) on the same surface of the piezoelectric material substrate 13. The first converter 9 is constituted by the first comb electrode 15, and the second converter 17 is constituted by the second comb electrode 17.

Each configuration will be described below.
The antenna 5 is an antenna for receiving a high-frequency signal supplied from the measuring device 19, and a tablet antenna, a patch antenna, or the like can be used. The distance to the measuring device 19, the shape of the mounting portion, or the high-frequency signal to be used. An antenna having an optimal form and characteristics can be used depending on the frequency.

The antenna 7 is for outputting a high-frequency signal output from the second converter 11 to an external measuring device 19, and has the same structure as the antenna 5.
The piezoelectric material substrate 13 is made of, for example, a piezoelectric material such as LiNbO _3, and the first converter 9 and the second converter 11 made of, for example, aluminum are formed on the surface thereof by DC sputtering or the like.

  The 1st converter 9 converts the high frequency signal supplied from the external measuring device 19 into a surface acoustic wave, and converts an electrical signal into a surface acoustic wave by the reverse piezoelectric effect. The first comb electrode 15 is arranged so as to be opposed to each other so that the comb portions of a pair of one-side comb electrodes having a symmetrical shape are fitted into each other, and an electric signal received by the antenna 5 is elastically surfaced. The surface acoustic wave is excited in a direction perpendicular to the comb teeth.

  The second converter 11 converts a surface acoustic wave into an electric signal. The structure of the second converter 11 is the same as that of the first converter 9 and has a point-symmetric shape with respect to the first converter 9. The second converter 11 converts the surface acoustic wave propagating through the piezoelectric material substrate 13 into an electric signal by the second comb electrode 17 and outputs the electric signal to the measuring device 19 via the antenna 7.

The first transducer 9 and the second transducer 11 are arranged on the same surface of the piezoelectric material substrate 13 so as to extend along the propagation direction of the surface acoustic wave (left-right direction in the figure) across the propagation plane. Yes.
In particular, in the present embodiment, the first comb electrode 15 and the second comb electrode 17 are separated from the center of each comb electrode 15, 17 in the horizontal direction of FIG. It has a comb-tooth structure in which up-chirp and down-chirp that change so that the (interval) becomes small.

  For example, as schematically shown in FIG. 1 (b), the first comb-teeth electrode 15 is set so that the comb-teeth interval increases from the left side (input side) to the center of the first comb-teeth electrode 15. It is set so as to decrease from the center of the first comb electrode 15 to the right side of the figure (set to down chirp).

  Specifically, the first comb electrode 15 is composed of comb teeth having three kinds of widths and comb tooth intervals, and the comb tooth interval W1 of the four thinnest comb teeth on the left side of the figure is four on the right side of the figure. Is set to be the same as the comb tooth interval W5 of the thinnest comb tooth. Further, four comb teeth (intermediate thickness comb teeth) each having a width larger than that of the thinnest comb teeth are arranged inside the thinnest left and right comb teeth. The comb tooth intervals W2 and W4 of the comb teeth are the same and are set larger than the comb tooth intervals W1 and W5 of the thinnest comb teeth. Further, four comb teeth having the widest width are arranged between the left and right intermediate comb teeth, and the comb tooth interval W3 is determined by the comb tooth intervals W1, W2, It is set larger than W4 and W5.

In this case, for example, three large, medium, and small patterns of comb teeth with comb teeth intervals of 9.276 μm, 9.476 μm, and 9.676 μm are formed on the piezoelectric material substrate 9 of 128YX-LiNbO ₃ , thereby obtaining a bandwidth of 2. 264 MHz fundamental wave: center frequency 105 MHz, chirp rate 1.19, third harmonic: center frequency 315 MHz, chirp rate 3.56,... Stress sensor capable of obtaining a signal having two peaks (maximum peak) 1 can be configured.

The chirp rate is the slope of the delay time in the frequency characteristics.
In this way, in the present embodiment, the comb-tooth structure described above excites a waveform having two peaks (maximum voltage peaks) having two changing directions (FIG. 5 (c). )), And the value (stress) of the measurement object can be calculated from the relationship between the time difference between the maximum peaks and the external force applied to the surface acoustic wave element 3.

b) Next, the measurement principle in the stress sensor 1 will be described together with a reference example.
2 shows a reference example 1 of a non-dispersed electrode, FIG. 3 shows a reference example 2 of an up-chirped or down-chirped dispersed electrode, and FIG. 4 shows an up-chirp and down-chirped comb tooth structure of this embodiment. The example which has is shown.

  As shown in Reference Example 1 in FIG. 2A, in the case of sensors having the same comb-teeth interval between the input-side comb-teeth electrode (input IDT: interdigital transducer) and the output-side comb-teeth electrode (output IDT) (that is, comb In the case of non-dispersed electrodes having the same tooth spacing, the surface acoustic wave element has frequency characteristics shown in FIG.

  That is, the surface acoustic wave is excited most strongly when the comb tooth interval is ½ of the wavelength λ of the surface acoustic wave, and has a mountain-shaped bandpass filter characteristic centered on the frequency. In FIG. 2B, the solid line represents the insertion loss corresponding to the frequency, and the broken line represents the delay time corresponding to the frequency.

  Therefore, as shown in FIG. 2C, when a high frequency signal having a single frequency is input from the input-side comb-tooth electrode, when no stress is applied to the surface acoustic wave element, the output-side comb-tooth electrode , An output A having a maximum peak corresponding to the comb tooth interval is obtained.

  On the other hand, when stress is applied, the comb tooth interval changes, and the frequency characteristic of each comb tooth changes (shifts to the left and right in FIG. 2B). A signal shifted by a predetermined delay time difference, that is, a signal corresponding to the changed frequency characteristic is obtained.

Since this delay time difference corresponds to the applied stress, the stress can be obtained from the delay time difference, but only a slight delay time difference can be obtained.
Further, as shown in Reference Example 2 of FIG. 3A, in the case of a sensor in which the comb-teeth spacing between the input-side and output-side comb-teeth electrodes is simply increased toward the center of the surface acoustic wave device, When the input side is a dispersive up-chirp electrode and the output side is a dispersive down-chirp electrode, the surface acoustic wave element has frequency characteristics with a wide bandwidth as shown in FIG.

  In other words, by setting the comb-teeth interval of the comb-teeth electrodes to, for example, up-chirp, a group of comb-teeth electrodes having different propagation distances can be formed at a large number of excitation frequencies, so that different delay times can be obtained depending on the frequencies. That is, a delay line having an inclination depending on the electrode shape can be configured.

  Therefore, as shown in FIG. 3C, when a high-frequency signal having a single frequency is input from the input-side comb-tooth electrode, when no stress is applied to the surface acoustic wave element, the output-side comb-tooth electrode Output A is obtained. The output A has a complex waveform with dispersed peaks corresponding to the comb spacing of the up chirp, and corresponds to the comb spacing that can excite the surface acoustic wave most efficiently according to the frequency of the input signal. It has one maximum peak (ie, corresponding to the most efficiently excited surface acoustic wave).

  On the other hand, when stress is applied, as shown in FIG. 4, since the comb tooth interval of the comb electrode changes and the frequency characteristics change, the frequencies (f0, f1,. f2) also changes, and the maximum peak changes accordingly.

  Accordingly, a signal with a predetermined delay time difference shifted, that is, a signal with a delay time difference shifted corresponding to the changed frequency characteristic is obtained as in the output B of FIG. Here, the delay time difference is the maximum peak between output A and output B.

Since this delay time difference corresponds to the applied stress, the stress can be obtained from the delay time difference, but a sufficient delay time difference cannot always be obtained.
In contrast, in the present embodiment, as shown in FIG. 5A, the above-described up-chart and down-chirp comb-teeth electrodes are provided. Therefore, as shown in FIG. A frequency characteristic having a delay line having a large slope can be obtained.

  In the present embodiment, as shown in FIG. 5C, when a high frequency signal having a single frequency is input from the first comb-teeth electrode 15 on the input side, an up-chirp comb-teeth structure of the first comb-teeth electrode 15 Accordingly, the maximum peak appears in the output corresponding to the surface acoustic wave excited most efficiently, and similarly, the surface acoustic wave excited most efficiently by the down-chirp comb structure of the first comb electrode 15. The maximum peak appears in the output corresponding to.

  That is, as shown in FIG. 6A, in the first comb electrode 15 having different comb tooth intervals, when the frequency characteristics coincide with the input frequency (f) (here, for example, the comb tooth intervals W2 and W4 A surface acoustic wave is excited at the same frequency f0), and a maximum peak appears in the output due to the excited surface acoustic wave.

  Therefore, as shown in FIG. 5C, when no stress is applied to the surface acoustic wave element 3, two peaks (maximum peak) are obtained from the up-chirp and down-chirp comb-teeth structures of the first comb-teeth electrode 15. ) Is obtained. Note that the time between the maximum peaks in this case is the basic time.

  The maximum peak on the left side in the output A corresponds to the intermediate comb tooth interval W2 (frequency f0) in the comb structure of the up-chirp of the first comb electrode 15, and the maximum peak on the right side in the output A is the first peak. This corresponds to a similar intermediate comb tooth interval W4 (frequency f0) in the down-chirp comb-tooth structure of one comb-tooth electrode 15. Here, it is set so that the surface acoustic wave corresponding to the center maximum comb tooth interval W3 is not excited most strongly.

  As described above, the position of the maximum peak of the output A is shifted (that is, the delay time is shifted). In the first comb electrode 15, the comb tooth interval corresponding to the frequency at which the surface acoustic wave can be excited most efficiently is obtained. This is because the set positions are shifted like the comb structure of the up chirp and the comb tooth structure of the down chirp.

  On the other hand, when stress is applied (in this case, a stress that is convex on the back side of the paper surface of FIG. 1), the comb-teeth spacing of the comb-teeth electrode changes, and therefore, as shown in FIG. Similarly, the frequency for efficiently exciting the surface acoustic wave also changes. Therefore, as shown in FIG. 5C, the maximum peak of the output signal corresponding to the surface acoustic wave excited according to the frequency also changes, and the output B is obtained. The time between the maximum peaks is taken as the measurement time.

  The maximum peak on the left side in the output B corresponds to the surface acoustic wave excited by the minimum comb teeth of the up-chirp comb structure of the first comb electrode 15, and the maximum peak on the right side is the down-chirp peak. This corresponds to a surface acoustic wave excited by the same smallest comb tooth of the comb tooth structure. As shown in FIG. 6 (c), when stress is applied, the minimum comb tooth excitation frequency is intermediate from the excitation frequency (f2) when no stress is applied to when the stress is not applied. It changes to the excitation frequency (f0) of the comb teeth.

Accordingly, by setting the value obtained by subtracting the basic time from the measurement time as the delay time difference, the delay time difference corresponds to the stress, and therefore the stress can be obtained from the delay time difference.
That is, in this embodiment, when the stress is applied to the stress sensor 1, the structure of the up-chirp and down-chirp comb-teeth electrodes is set so that the measurement time is longer than the basic time. With the configuration, the stress can be detected with higher accuracy than in the past.

c) Next, the measuring device 19 for driving the pressure sensor 1 will be described.
As shown in FIG. 1, the measuring device 19 includes an antenna 21, a high frequency signal generator 23, a switch 25, a voltmeter 27, and a signal processor 29 (see FIG. 7).

  The antenna 21 is an antenna for outputting a high frequency signal generated by the high frequency signal generator 23 to the external pressure sensor 1 and inputting a high frequency signal output from the pressure sensor 1.

  As with the antenna 5, the antenna 21 can be a tablet antenna, a patch antenna, or the like, depending on the distance to the first converter 9 and the second converter 11, the shape of the mounting portion, or the frequency of the high-frequency signal used. An antenna having an optimal form and characteristics can be used.

  The high-frequency signal generator 23 is an oscillator for generating a high-frequency signal for generating a surface acoustic wave in the first comb electrode 15 of the stress sensor 1. The oscillation frequency is, for example, 315 MHz, 434 MHz, 2.45 GHz band, or the like.

  The switch 25 is a switch for supplying or not supplying the high-frequency signal generated by the high-frequency signal generator 23 to the antenna 21, so that the high-frequency signal is supplied to the antenna 21 when turned on and not supplied when turned off. To do.

The voltmeter 27 measures the voltage of the high frequency signal output from the high frequency signal generator 23 and the high frequency signal input from the antenna 21.
As will be described later, the signal processing unit 29 generates a high frequency signal from the high frequency signal generation unit 23 or calculates a stress measured by the stress sensor 1 based on the voltage of the high frequency signal measured by the voltmeter 27. It is.

Here, a method for measuring stress using the stress sensor 1 and the measuring device 19 will be described with reference to FIG.
First, a high frequency signal is output from the measuring device 19 to the stress sensor 1 via the antenna 21. In the stress sensor 1, the high-frequency signal output from the antenna 21 is received by the antenna 5.

  The high frequency signal received by the antenna 5 is supplied to the first comb electrode 15 of the first converter 9, and surface acoustic waves are excited at the first comb electrode 15 by the supplied high frequency signal. The excited surface acoustic wave propagates to the second transducer 11 via the surface (propagation surface) of the piezoelectric material substrate 13.

  In the second converter 11, the surface acoustic wave propagating through the piezoelectric material substrate 13 is acquired, converted into a high frequency signal by the second comb electrode 17, and output from the antenna 7 to the outside. The high frequency signal output from the antenna 7 is received by the antenna 21 of the measuring device 19. The voltage of the high frequency signal received by the antenna 21 is measured by the voltmeter 27.

  Therefore, in the measuring device 19, the measurement time between the two maximum peaks of the measured voltage is detected by a predetermined threshold value, and the basic time stored in advance is subtracted from this measurement time to obtain the delay time difference.

Since this delay time difference corresponds to the stress, the stress can be obtained from the delay time difference using a map or the like obtained in advance by experiments or the like.
d) Next, the detailed configuration and operation of the signal processing unit 29 will be described.
As shown in FIG. 7, the signal processing unit 29 includes an analog circuit unit 31 and a digital circuit unit 33. The analog circuit unit 31 includes an applied voltage generation unit 35 and a time measurement pulse generation unit 37.

The applied voltage generation unit 35 includes an offset circuit 39, a first amplifier circuit 41, a filter circuit 43, and an analog switch 45.
As shown in FIG. 8A, the offset circuit 39 has a pulse waveform of 0 [V] to +5 [V] based on the GND level output from the counter 47 (see FIG. 7) (FIG. 8A). Is applied to the pulse waveform of ± 2.5 [V].

As shown in FIG. 8C, the first amplifier circuit 41 amplifies the pulse waveform of ± 2.5 [V] obtained in the offset circuit 39 and converts it to a pulse waveform of ± 5 [V].
The filter circuit 43 is a low-pass filter, and removes a high-frequency component of the pulse waveform of ± 5 [V] obtained by the first amplifier circuit 41 and has an amplitude of 5 [V] as shown in FIG. Generate a sine wave.

  When the gate pulse is input to the counter 47 as shown in FIG. 8 (o), the analog switch 45 outputs the sine wave generated by the filter circuit 43 to the outside as shown in FIG. 8 (f).

  Next, as shown in FIG. 5, the time measurement pulse generator 37 includes a clip circuit 49, an offset circuit 51, a half-wave rectifier circuit 53, a second amplifier circuit 55, a smoothing circuit 57, and a comparison circuit 59.

  The clip circuit 9 has a detection voltage waveform input from the voltmeter 27 as shown in FIG. 9A, and the maximum value of the output of the operational amplifier (not shown) constituting the circuit as shown in FIG. The peak value of the detected voltage waveform is clipped so that the output value becomes ± 10 [V].

  As shown in FIG. 9C, the offset circuit 51 applies an offset voltage so as to subtract the maximum voltage of noise superimposed on the voltage waveform output from the clip circuit 49.

As shown in FIG. 9D, the half-wave rectifier circuit 53 performs half-wave rectification on the output voltage of the offset circuit 51, and extracts and outputs only the positive-side component of the output voltage.
The second amplifying circuit 55 amplifies the output of the half-wave rectifier circuit 53 to a saturated output value of an operational amplifier (not shown) as shown in FIG.

  The smoothing circuit 57 is a low-pass filter, and as shown in FIG. 9 (f), the high frequency component of the output (pulse waveform) of the second amplifier circuit 55 is removed, and a waveform composed of a plurality of pulse groups rises and falls. Is converted into one pulse waveform which is not sharp.

  As shown in FIG. 9 (G), the comparison circuit 59 compares the output pulse of the smoothing circuit 57 with a predetermined comparison voltage, and generates a constant voltage when the rising voltage of the output pulse exceeds the comparison voltage. The output is stopped when the falling voltage of the output pulse becomes smaller than the comparison voltage.

  In this way, a pulse waveform with a sharp rise and fall output from the smoothing circuit 57 is shaped into a pulse waveform with a sharp rise and fall. In this way, a pulse whose waveform is shaped is output to the outside as a time measurement pulse.

  Therefore, among the time measurement pulses obtained in this way, the time between the time measurement pulses corresponding to the delayed wave (output waveform) is counted by the counter, and the time between the maximum peak of the output waveform is calculated from this counter value. (Delay time difference) can be obtained.

e) As described above, in the present embodiment, at least the first comb-tooth electrode 15 has an up-chirp and down-chirp comb-tooth structure.
Therefore, when stress is applied to the stress sensor 1, the frequency characteristic in the first converter 15 mainly changes, that is, the frequency at which the surface acoustic wave can be excited efficiently changes, so that FIG. As shown, the maximum peak of the output waveform changes, and the measurement time changes greatly with respect to the basic time. Since the delay time difference obtained by subtracting the basic time from the measurement time corresponds to the applied stress, the stress can be obtained from the delay time difference.

In addition, since this delay time difference is larger than the conventional one, there is a remarkable effect that the stress can be accurately measured with a simple circuit.
Further, for example, by sufficiently increasing the peak voltage level in the output waveform by the number of electrode pairs and the comb electrode length, it is possible to arbitrarily set the change amount (delay time difference) due to the comb tooth interval of the first comb electrode 15. Therefore, also from this point, there is an advantage that an inexpensive detection circuit is sufficient.

As a modification of the present embodiment, the first converter may be a third converter (which performs input and output), and the second converter may be a reflector.
That is, as shown in FIG. 10A, in the stress sensor 71 of the modified example, the surface acoustic wave is excited by the third transducer 75 having the third comb-teeth electrode 73 having an up-chirp and down-chirp comb-teeth structure. The surface acoustic wave is reflected by a reflector 79 having a fourth comb electrode 77 having the same structure as that of the third comb electrode 73. Then, the reflected surface acoustic wave is received by the third converter 73 and converted into an output signal, and the output signal is transmitted from the surface acoustic wave element 81 side to the measuring device 83.

Further, as a modified example, as shown in FIG. 10B, the fourth comb electrode 93 of the reflector 91 may be a non-dispersive electrode, and various known electrodes that reflect surface acoustic waves are employed. it can.
In addition, although the stress sensor of this embodiment is used as a tire air pressure sensor which detects tire air pressure, this tire air pressure sensor is attached to the air filling port (muscle) of a tire, for example. Therefore, when the tire air pressure increases, the stress sensor bends like a bridge due to the differential pressure between the tire internal pressure and the reference pressure (atmospheric pressure), and therefore the tire air pressure can be detected by the stress.

[Second Embodiment]
Next, the second embodiment will be described, but the description of the same content as the first embodiment will be omitted.

a) First, the configuration of the stress sensor of this embodiment will be described.
As shown in FIG. 11A, in the stress sensor 101 of this embodiment, the surface acoustic wave element 103 includes a first converter 107 including a first comb electrode 105 and a second comb electrode 109. A second converter 111.

  The first comb electrode 105 and the second comb electrode 109 are each provided with four comb teeth so as to form three types of comb tooth intervals. For example, three types of comb teeth are provided in which the comb tooth interval is changed by 0.1 to several μm.

  Specifically, as shown schematically in FIG. 11B with numbers in the order of the comb tooth spacing, the comb tooth spacing is minimized at the center of each comb electrode 105, 109. Four comb teeth (size: S1) are provided on the left and right, and four comb teeth (S3) are provided on the left and right sides so that the maximum comb tooth spacing is provided, and intermediate comb teeth intervals are provided on the left and right sides. In this way, four comb teeth (S2) are provided, and four comb teeth (S3) are provided on the left and right sides of the comb teeth (S3) so as to have the maximum comb tooth interval.

Specifically, by forming comb teeth (S1 to S3) having three patterns of large, medium, and small sizes with comb tooth intervals of 9.276 μm, 9.476 μm, and 9.673 μm on the piezoelectric material substrate 113 of 128YX-LiNbO ₃ , 4.434 MHz width fundamental signal: center frequency 105 MHz, chirp rate 1.55, third harmonic: center frequency 315 MHz, chirp rate 4.65,... Thus, the stress sensor 101 can be configured.

b) Next, the operation of the stress sensor 101 will be described.
Here, for example, a case will be described in which a stress that becomes convex is applied to the back side of the paper surface of FIG.

  As shown in FIG. 11C, when the stress sensor 101 is not stressed, an output signal waveform of output A is obtained. The maximum peak at the center of the output A corresponds to the center comb tooth (S1) of the stress sensor 101. In other words, when no stress is applied, the comb teeth (S1) having the minimum comb tooth interval corresponding to the frequency corresponding to the frequency of the input signal, that is, the frequency at which the surface acoustic wave can be excited efficiently when no stress is applied. One maximum peak is obtained.

  Next, when the stress increases, the frequency characteristics change according to the increased stress, so the frequency for efficiently exciting the surface acoustic wave also changes. Therefore, two maximum peaks can be obtained by the comb teeth (S2) having the following interdigital spacing corresponding to the frequency at which the surface acoustic wave can be efficiently excited.

  Next, when the stress further increases, the frequency characteristic changes according to the increased stress. Similarly, the frequency for efficiently exciting the surface acoustic wave also changes. Therefore, four maximum peaks are obtained by the comb teeth (S3) having the maximum comb tooth interval corresponding to the frequency for efficiently exciting the surface acoustic wave.

  That is, when a stress is applied to the stress sensor 101, the comb tooth interval of the first comb electrode 105 changes, so that the frequency that is efficiently excited changes, thereby changing the number of maximum peaks in the output waveform. Therefore, the pressure can be obtained by determining the offset value of the voltage and detecting the number of signal peaks.

Here, the maximum peak is a signal corresponding to each comb tooth interval that can be detected by dividing the output signal by a predetermined threshold.
Further, as a method of counting the number of maximum peaks, a pulse having a waveform (ki) shown in FIG. 9 of the first embodiment can be used. That is, since the waveform counting pulse of the delayed wave (output wave) that is not the applied wave corresponds to the maximum number of peaks, the maximum number of peaks may be counted by counting the number of waveform counting pulses.

Thus, in this embodiment, since the maximum number of peaks of the output waveform changes depending on the stress, the stress can be detected by counting the maximum number of peaks.
In addition, a detection circuit for counting the number of waves can be created at a low cost by sufficiently increasing the detection waveform interval or by sufficiently increasing the peak voltage level. Therefore, there is a remarkable effect that the stress can be accurately detected with a simple circuit.

  The measurement range of the stress sensor 101 for counting the maximum number of peaks can be arbitrarily created when the surface acoustic wave element 103 is designed, and can be used for many applications other than the stress sensor 101.

As a modification of the present embodiment, the first converter may be a third converter (which performs input and output), and the second converter may be a reflector.
[Third Embodiment]
Next, a third embodiment will be described, but a description of the same contents as those of the second embodiment will be omitted.

a) First, the configuration of the stress sensor of this embodiment will be described.
As shown in FIG. 12A, in the stress sensor 121 of the present embodiment, the surface acoustic wave element 123 includes a first converter 127 including a first comb electrode 125 and a second comb electrode 129. A second converter 131 is provided.

  In particular, in the present embodiment, the first comb electrode 125 is a non-dispersed electrode having comb teeth with a constant comb tooth interval. Further, the second comb electrode 129 has a down-chirp comb structure in which the comb interval decreases toward the center of the second comb electrode 129 on the left side of FIG. The two comb-tooth electrodes 129 have an up-chirp comb-teeth structure in which the comb-teeth interval increases from the center to the right side.

b) Next, the operation of the stress sensor 121 will be described.
Here, a case where stress is applied from the lower side to the upper side of FIG.
As schematically shown in FIG. 12B, in this embodiment, the output signal waveform changes in the order of output A → output B → output C as the stress applied to the stress sensor 121 increases.

  Specifically, when no stress is applied, the surface acoustic wave is efficiently excited by the small comb tooth interval at the center in response to the input signal. One maximum peak is detected at the output A by the offset voltage (corresponding to the comb spacing).

  After that, when the stress increases, the frequency characteristics change, and the surface acoustic wave is efficiently excited by the outer large comb tooth interval, so that it corresponds to the surface acoustic wave (that is, corresponding to the large comb tooth interval). The two maximum peaks are detected by the offset voltage at the output C. The output B is a waveform on the way from the output A to the output C, and the maximum peak interval is smaller than the output C.

  That is, as in the present embodiment, as the stress increases, the maximum peak number of the output signal increases and the maximum peak interval increases. The second comb electrode 129 has an up chirp and a down chirp. This is because a comb-tooth structure is formed, and the comb teeth for which the surface acoustic waves are excited most efficiently change sequentially according to the stress.

Although the output waveform varies depending on the input wave number, it is necessary to input waves of at least 70 to 80 cycles in order to obtain a continuous waveform response as shown in FIG.
In the present embodiment, the same effects as in the second embodiment can be obtained.

As a modification of the present embodiment, the first converter may be a third converter (which performs input and output), and the second converter may be a reflector.
[Fourth Embodiment]
Next, a fourth embodiment will be described, but the description of the same contents as the second embodiment will be omitted.

a) First, the configuration of the stress sensor of this embodiment will be described.
As shown in FIG. 13A, in the stress sensor 141 of the present embodiment, the surface acoustic wave element 143 includes a first converter 147 including a first comb electrode 145 and a second comb electrode 149. The second converter 151 is provided.

  In particular, in the present embodiment, the first comb electrode 145 is a non-dispersed electrode having comb teeth with a constant comb tooth interval. The second comb electrode 149 has a comb tooth (S1) having a minimum comb tooth interval at the center, like the comb electrode of the second embodiment shown in FIGS. 11 (a) and 11 (b). The comb teeth (S3) having the maximum comb tooth spacing are arranged on both sides of the comb teeth (S1), and the comb teeth having an intermediate comb tooth spacing (S2) are arranged on both sides of the comb teeth (S3). The comb teeth (S3) with the maximum comb tooth spacing are arranged on both sides of the comb teeth (S2).

That is, it has a plurality of up-chirp and down-chirp comb-tooth structures.
b) Next, the operation of the stress sensor 141 will be described.
Here, for example, a case where stress is applied from the bottom to the top in FIG.

  As schematically shown in FIG. 13B, in this embodiment, the greater the stress applied to the stress sensor 141, the more the output signal waveform is output A → output B → output C → output D → output E → output F. It changes as follows.

That is, as in the second embodiment described above, as the stress increases, the number of peaks in the output waveform changes to 1, 2, and 4, and the peak interval increases as the stress increases.
As in the present embodiment, as the stress increases, the maximum number of peaks of the output signal increases and the maximum peak interval increases. The second comb electrode 149 has a plurality of up-chirps and down-chirps. This is because a comb-tooth structure is formed, and thereby the comb-teeth where the surface acoustic wave is most efficiently excited change sequentially according to the stress.

Therefore, the present embodiment has the same effect as the second embodiment.
As a modification of the present embodiment, the first converter may be a third converter (which performs input and output), and the second converter may be a reflector.

In addition, this invention is not limited to the said embodiment at all, As long as it belongs to the technical scope of this invention, it can take a various form.
(1) For example, the stress sensor and the measurement device may be connected by wire instead of transmitting and receiving by wireless communication.

  (2) Moreover, it is applicable not only to a stress sensor such as a tire stress sensor but also to various uses such as a displacement sensor and a temperature sensor.

(A) is explanatory drawing which shows schematic structure of the stress sensor of 1st Embodiment, (b) is explanatory drawing which expands and shows a part of stress sensor. (A) is explanatory drawing which shows schematic structure of the plane of the sensor of the reference example 1, and a side surface, (b) is a graph which shows the frequency characteristic, (c) is a graph which shows the output waveform etc. (A) is explanatory drawing which shows schematic structure of the plane of the sensor of the reference example 2, and a side surface, (b) is a graph which shows the frequency characteristic, (c) is a graph which shows the output waveform etc. It is a graph which shows the relationship between a comb-tooth space | interval and an excitation frequency. (A) is explanatory drawing which shows schematic structure of the plane and side surface of the stress sensor of 1st Embodiment, (b) is a graph which shows the frequency characteristic, (c) is a graph which shows the output waveform etc. (A) is explanatory drawing which shows the relationship between an input frequency and an excitation frequency, (b) is a graph which shows the change of the excitation frequency when stress is added, (c) is the change of the excitation frequency when stress is added. It is a graph shown with a comb tooth. It is a block diagram which shows the schematic structure of a signal processing part. It is explanatory drawing which shows a mode that an applied voltage waveform is obtained from an applied frequency pulse. It is explanatory drawing which shows a mode that a time measurement pulse is acquired from the detection voltage detected with the voltmeter. (A) is explanatory drawing which shows schematic structure of the plane of the stress sensor of a modification, (b) is explanatory drawing which further shows schematic structure of the plane of the modification. (A) is explanatory drawing which shows schematic structure of the plane of the stress sensor of 2nd Embodiment, (b) is explanatory drawing which shows typically the magnitude | size of the comb-tooth space | interval, (c) shows the output waveform etc. It is a graph. (A) is explanatory drawing which shows schematic structure of the side surface of the stress sensor of 3rd Embodiment, (b) is a graph which shows the output waveform. (A) is explanatory drawing which shows schematic structure of the side surface of the stress sensor of 4th Embodiment, (b) is a graph which shows the output waveform etc.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 71, 101, 121, 141 ... Stress sensor 3, 81, 103, 123, 143 ... Surface acoustic wave element 9, 107, 127, 147 ... 1st converter 11, 111, 131, 151 ... 2nd converter DESCRIPTION OF SYMBOLS 13, 113 ... Piezoelectric material board | substrate 15,107,125,141 ... 1st comb-tooth electrode 17,109,129,149 ... 2nd comb-tooth electrode 19,83 ... Measurement apparatus 73 ... 3rd comb-tooth electrode 75 ... 3rd Transducers 77, 93 ... fourth comb electrodes 79, 91 ... reflectors

Claims (8)

A first transducer comprising a first comb electrode for exciting a surface acoustic wave;
A second transducer comprising a second comb electrode that receives the surface acoustic wave excited by the first comb electrode and outputs a signal corresponding to the surface acoustic wave;
A surface acoustic wave element disposed across a propagation surface through which the surface acoustic wave propagates,
The surface acoustic wave device according to claim 1, wherein at least one of the comb electrodes has an up-chirp comb-tooth structure and a down-chirp comb-tooth structure. A third transducer comprising a third comb electrode for exciting a surface acoustic wave and outputting a signal corresponding to the received surface acoustic wave;
A reflector comprising a fourth comb electrode that reflects the surface acoustic wave excited by the third comb electrode;
A surface acoustic wave element disposed across a propagation surface through which the surface acoustic wave propagates,
The surface acoustic wave device according to claim 1, wherein at least one of the comb electrodes has an up-chirp comb-tooth structure and a down-chirp comb-tooth structure.   3. The surface acoustic wave device according to claim 2, wherein an electrode that reflects the surface acoustic wave other than the comb-tooth electrode is used instead of the fourth comb-tooth electrode of the reflector. A sensor comprising the surface acoustic wave device according to any one of claims 1 to 3,
A sensor for detecting a state of an object to be measured based on a time change between peaks of a voltage signal generated by the surface acoustic wave. A sensor comprising the surface acoustic wave device according to any one of claims 1 to 3,
A sensor for detecting a state of an object to be measured based on the number of peaks of a voltage signal generated by the surface acoustic wave.   The sensor according to claim 4, wherein the sensor is a stress sensor that detects a stress applied to the surface acoustic wave element.   The sensor according to claim 6, wherein the stress sensor is a tire air pressure sensor that detects tire air pressure from a change in stress generated in the tire.   The sensor according to any one of claims 4 to 7, wherein the sensor has a function of transmitting a signal output from the comb-tooth electrode on the reception side of the surface acoustic wave to a controller by wireless communication.