JPH0868780A - Sensor with elastic surface wave element - Google Patents

Abstract

PURPOSE: To simplify measurement means for sensor output. CONSTITUTION: On the surface of piezoelectric substrate 21, an input IDT electrode 22 and output IDT electrodes 23 and 24 are provided, and between output IDT electrodes 23 and 24, a functional film 25 with which propagation time of elastic surface wave is changed according to sticking and penetration of a sensing target is provided, and, the input IDT electrode 22 excites pulses of constant cycle with a pulse signal generator 26, and, depending on whether the elastic surface waves propagating on the substrate at that time reach the output IDT electrodes 23 and 24 at the same time or not, amplitude change is generated an the signal combined with the output, then, the change of the amplitude is detected with a signal level detector 27. Thus the degree in sticking and penetration of the sensing target is detected through amplitude changes in signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting humidity and odor using a surface acoustic wave element.

[0002]

2. Description of the Related Art A sensor using a surface acoustic wave element is
(1) A surface acoustic wave filter is used, and (2) A surface acoustic wave resonator is used.

FIG. 3 shows a structure of a surface acoustic wave filter. 10 to 20 pairs of IDT (inter digital transducer) electrodes 2 and 3 are formed on both sides of the piezoelectric substrate 1, and a functional thin film 4 serving as a sensor member is attached to the surface of the substrate 1 between the electrodes. .

The network analyzer 5 applies a signal of a predetermined frequency to the IDT electrode 2, propagates the surface acoustic wave excited by the IDT electrode 2 to the IDT electrode 3, and
The output frequency due to the electromechanical coupling of the T electrode 3 is measured.

FIG. 4 shows a structure using a surface acoustic wave resonator. The IDT electrode 7 is formed in the central portion of the piezoelectric substrate 6, reflectors 8 and 9 are formed on both sides, and the functional thin film 10 is attached to the surface of the substrate 6 across the surfaces of the electrode 7 and the reflectors 8 and 9.

The oscillator 11 excites the IDT electrode 7, and the frequency counter 12 measures the resonance frequency of the surface acoustic wave resonator in response to the excitation.

Here, the functional thin film 4 or 10 is attached to the surface of the piezoelectric substrate to reduce the sound velocity of the surface acoustic wave propagating through the substrate. The degree corresponds to the sensing state of the functional thin film. As a result, the frequency measured by the network analyzer 5 or the frequency counter 12 is
It changes depending on the sensing state of the functional thin film, and this frequency change can be used as a sensor output.

For example, to configure a humidity sensor, the functional thin film is, for example, a polyimide film, an alumina film, or cellulose acetate, and the sound velocity of the substrate 1 is changed according to the degree of water absorption.

Further, the gas sensor detects formaldehyde concentration as a frequency change by using formaldehyde dehydrogenase and its coenzyme as coating films on a quartz substrate.

Further, the odor sensor forms, for example, arachidic acid (LB film), and detects the chemical substance which is the source of the odor as a frequency change by utilizing the high detection sensitivity of the small amount of mass that the quartz oscillator has.

Further, the functional thin films are applied to bitterness sensors and metal corrosion sensors.

In all of the conventional examples up to this point, the functional thin film is provided on the surface of the piezoelectric substrate, but there is also one in which the functional thin film is not provided and only the piezoelectric substrate is provided. In this type, sensing is performed by using a change in frequency associated with a change in characteristics of the substrate itself due to a substance to be sensed being directly attached to the surface of the piezoelectric substrate (change in mass of the substrate portion) or humidity and temperature.

[0013]

In any of the conventional configurations, those having a functional thin film on the piezoelectric substrate or those not having the functional thin film, in either type, in addition to the surface acoustic wave element, measuring means Therefore, devices such as the network analyzer 5, the oscillator 11, and the frequency counter 12 are required.

Further, since the change in the sensing state is detected as the change in frequency, a more complex measuring means is required for the system configuration in which the switch output is obtained when the frequency exceeds a certain value.

It is an object of the present invention to provide a sensor using a surface acoustic wave element that simplifies the sensor output measuring means.

[0016]

In order to solve the above problems, the present invention provides an input IDT electrode provided on the surface of a piezoelectric substrate and excited by a pulse having a constant period, and the piezoelectric substrate. A plurality of output IDT electrodes provided on the surface of the input IDT electrode and spaced from each other in the traveling direction of the surface acoustic wave propagating through the substrate from the input IDT electrode, and signals of the plurality of output IDT electrodes. A signal level detector for detecting an amplitude of a signal obtained by adding the outputs, and detecting a degree of a sensing target substance adhering to the piezoelectric substrate as an amplitude change of an input signal of the signal level detector. To do.

Further, the present invention is characterized in that a functional thin film for changing the propagation velocity of the surface acoustic wave is provided between the output IDT electrodes by the attachment and permeation of the substance to be sensed.

[0018]

When the output surface of the output IDT electrode is added together by the change in the delay time when the surface acoustic wave generated in the input IDT electrode and propagating through the piezoelectric substrate propagates between the plurality of output IDT electrodes. A difference is generated in the level so that the degree of adhesion or permeation of the substance to be sensed corresponds to the amplitude change of the signal.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sensor configuration diagram showing an embodiment of the present invention.

An input IDT electrode 22 is formed on one side of the surface of the piezoelectric substrate 21. This electrode 22 is
About 10 to 20 pairs of normal type IDTs are used.

In addition, a pair of output IDT electrodes 23, 24 are provided on the other side of the piezoelectric substrate 21 with a space in the traveling direction of the surface acoustic wave and are divided at a certain distance. These electrodes 23 and 24 are about 10 to 20 pairs of IDT electrodes. Further, a functional thin film 25 is provided between the IDT electrodes 23 and 24.

A pulse generator 26 as an excitation power source is connected to the input IDT electrode 22, and a pulse having a constant cycle is applied. For this pulse, the input IDT electrode 2
2 generates a surface acoustic wave on the surface of the piezoelectric substrate 21 by electromechanical coupling.

The output IDT electrodes 23 and 24 convert surface acoustic waves propagating on the surface of the piezoelectric substrate 21 into electric signals by electromechanical coupling.

The signal level detector 27 detects whether or not the level of the electric signal output from the output IDT electrodes 23 and 24 has dropped below a set value, and obtains this as a sensor output signal.

The operation of this embodiment will be described below in detail with reference to the waveform chart of each part in FIG.

The input IDT electrode 22 has a pulse generator 2
A continuous pulse (see a in FIG. 2) having a delay time (cycle) T is input from 6.

By this pulse input, a surface acoustic wave having a constant frequency corresponding to the period of the electrode fingers is generated from the input IDT electrode 22 (b in FIG. 2). This surface acoustic wave propagates in the direction indicated by the arrow, that is, toward the output IDT electrodes 23 and 24.

Here, the propagation of the surface acoustic wave from the input IDT electrode 22 is output I when the substance to be sensed is not adhering to or permeating the functional thin film 25.
The delay time T is set so that the next surface acoustic wave reaches the output IDT electrode 23 when the output surface is delayed after reaching the DT electrode 23 and reaches the output IDT electrode 24. This delay time T is designed and adjusted by the division distance of the output IDT electrodes 23 and 24, the surface acoustic wave propagation velocity, and the functional thin film 25.

In this way, the output IDT electrodes 23, 24
When a surface acoustic wave arrives at the same time, the electric outputs from the IDT electrodes 23 and 24 are superposed on the signal level detector 27, and the amplitude becomes maximum (d in FIG. 2).

Next, when the substance to be sensed adheres to and permeates the functional thin film 25 and the surface acoustic wave velocity under the film decreases, before the first surface acoustic wave reaches the output IDT electrode 24. The next surface acoustic wave reaches the output IDT electrode 23.

In this case, the amplitude of the output waveform becomes smaller than that in the case where nothing adheres to or penetrates the functional thin film 25 (c in FIG. 2).

Therefore, the surface acoustic wave propagation velocity between the divided electrodes of the output IDT electrodes 23 and 24 changes according to the degree to which the substance to be sensed adheres to or penetrates the functional thin film 25, and the signal level detector. The amplitude (level) of the signal detected at 27 changes, and by detecting this level, the amount of adhesion / permeation of the sensing target substance can be detected. This level detection has a simpler circuit configuration than the conventional frequency detection.

Further, by setting the threshold level in the signal level detector 27, it is possible to obtain the on / off signal as the adhesion / permeation of the substance to be sensed which exceeds a certain value. In this case, the signal level detector 27
The configuration of is simpler.

The above embodiments show the case where the functional thin film 25 is provided. For example, the functional thin film 25
For this, a humidity sensor, an odor sensor, a gas sensor, and the like belong to this, and a member having excellent performance of trapping a substance to be sensed such as humidity, an odor substance, or a gas is used.

On the other hand, it is possible to obtain the same operation and effect by using the sensor structure in which the functional thin film 25 is not provided. That is, the configuration in which the functional thin film 25 is omitted in the configuration of FIG.

For example, a dust sensor is used to output the IDT electrode 2
The change in the amplitude of the output waveform due to the change in the propagation velocity of the surface wave due to the change in mass due to the attachment and accumulation of dust or the like on the surface of the piezoelectric substrate 21 in the space between 3 and 24 causes the change in the amplitude of the output waveform as in the above-described embodiment. Sensing can be performed.

The humidity sensor function is realized by utilizing the change in the propagation velocity of the surface wave of the piezoelectric substrate 21 due to humidity.

In the above embodiments, the output IDT
Although two electrodes are shown, three or more electrodes may be provided at equal intervals or in equal ratio intervals, and outputs of the respective electrodes may be added together and taken into the signal level detector 27.

In the embodiment, the amplitude of the signal output of the output IDT electrode becomes small when the substance to be sensed adheres to or penetrates the surface of the functional thin film or the piezoelectric substrate. It is also possible to adopt a configuration in which the interval between the output IDT electrodes is changed so that the amplitude of the signal output of the output IDT electrodes becomes large when the electrodes are attached / permeated.

[0040]

As described above, according to the present invention, the output IDT is changed by the change of the delay time when the surface acoustic wave propagating through the piezoelectric substrate propagates between the plurality of output IDT electrodes.
A difference is generated in the level when the outputs of the electrodes are added, and the degree of adhesion / permeation of the sensing target substance is made to correspond to the amplitude change of the signal by this, so the signal level detector or other Anything that can detect the change is sufficient, and there is an effect that it can be greatly simplified compared to the conventional frequency measuring means.

[Brief description of drawings]

FIG. 1 is a sensor configuration diagram showing an embodiment of the present invention.

FIG. 2 is a waveform chart of each part of the embodiment.

FIG. 3 shows a conventional filter configuration example.

FIG. 4 shows a conventional resonator configuration example.

[Explanation of symbols]

 21 ... Piezoelectric substrate 22 ... Input IDT electrode 23, 24 ... Output IDT electrode 25 ... Functional thin film 26 ... Pulse generator 27 ... Signal level detector

Claims (2)

[Claims] 1. An input IDT electrode provided on a surface of a piezoelectric substrate and excited by a pulse having a constant period, and an input IDT electrode provided on the surface of the piezoelectric substrate and propagating through the substrate from the input IDT electrode. A plurality of output IDT electrodes provided at intervals in the traveling direction of the incoming surface acoustic wave; and a signal level detector for detecting the amplitude of a signal obtained by adding the signal outputs of the plurality of output IDT electrodes. A sensor using a surface acoustic wave device, comprising: detecting the degree to which a substance to be sensed adheres to the piezoelectric substrate as a change in amplitude of an input signal of the signal level detector. 2. The surface acoustic wave device according to claim 1, further comprising a functional thin film provided between the output IDT electrodes, the functional thin film changing a propagation velocity of a surface acoustic wave by adhering / permeating a substance to be sensed. The sensor used.