JPS61234324A - Stress sensor - Google Patents

Abstract

PURPOSE:To achieve the protection and a higher sensitivity of SAW devices while enabling temperature compensation of a circuit, by combining sets of SAW devices, two per set, with the transduce surfaces thereof facing each other to form an air gap therebetween with two layers of beams. CONSTITUTION:To obtain two differential signals, two SAW devices made almost identical are used. To protect interdigital transducers IDT2a and 2b for transmitting SAWs and those 3a and 3b for receiving SAWs arranged on the surface of the respective devices and a SAW propagation path between the two transducers from external environment, these devices are combined so as to make the transducer surfaces of the two SAW devices face each other while an air gap is formed between the devices through seat members 6a and 6b, so to speak, by building two layers of beams so that the transducers will not contact each other. This enables the protection and a higher sensitivity of the devices along with a temperature compensation.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a mechanical sensor mainly used for measuring bending stress, and in particular to a surface acoustic wave (SAW) that propagates on the surface of a solid material. Surface AcousticWa
The present invention relates to a stress sensor using the pressure-sensitive function of a SAW device using ve).

[Conventional technology]

In a SAW oscillation circuit using a SAW device, the SA
By dynamically distorting the state of the W propagation path, the SAW propagation speed,
Alternatively, SA changes the oscillation frequency unique to the missed circuit by causing a change in the propagation path length, and detects the force or pressure received by the SAW device from the amount of change in this frequency.
There is a W pressure sensor. This SAW sensor has the characteristics of being small, lightweight, highly accurate, and the sensor part is easy to manufacture and has high reproducibility, making it particularly useful as a mechanical sensor for pressure, stress, acceleration, etc. High value.

Furthermore, this SAW sensor is not limited to a mechanical sensor, and most of them constitute a SAW oscillation circuit, and the measured quantity is displayed by a change in the natural oscillation frequency of this oscillation circuit. This is because the SAW device itself is a passive device, and is not a type of device that spontaneously emits energy or converts the emitted energy of the object to be measured into another form and outputs it. Therefore, when using a SAW device as a sensor, it is first necessary to create a state in which the SAW device operates stably, that is, a state in which a closed circuit including the SAW device oscillates steadily at its natural frequency. Also, since this rigid SAW oscillation circuit is sensitive to external temperature changes, it is necessary to perform appropriate temperature compensation unless it is used in a constant temperature state.As a countermeasure, for example,
A widely used method is to detect the difference in frequency between the output signals of two SAW oscillation circuits to cancel the change in oscillation frequency due to temperature change.

On the other hand, a dynamic SAW sensor requires not only high sensitivity to detect a slight external force, but also structural robustness to prevent the SAW device itself from being damaged during use.

These two properties are contradictory, and in addition, in order to realize a practical system, it is necessary to simplify and downsize the structure.

Currently, the structure of a mechanical SAW sensor is one in which one SAW device chip is attached to a metal plate, one chip each on the front and back sides of a metal plate, or one chip or two chips are attached to the back sides of a metal plate. There are structures in which a set of two chips are held together in a cantilever configuration.

[Problem that the invention seeks to solve]

As described above, a mechanical SAW sensor, that is, a stress sensor, requires compensation for the operation of an electric circuit, structural protection of the SAW device, and high sensitivity.

Therefore, the present invention has been made to meet these demands, and despite its simple structure, it achieves both protection and high sensitivity of SAW devices, and also provides temperature compensation for the circuit through differential detection. It is intended to make it possible.

[Means for solving problems]

In this invention, in order to obtain two differential signals, two SAW devices made almost identically are used, and
Interdigital electrodes (IDT) for SAW transmission and reception are placed on the surface of each device.
In order to protect the transducer (transducer) and the SAW propagation path between these two electrodes from the external environment, the two SAW devices are combined so that their electrode surfaces face each other, and these electrodes are in contact with each other. In order to prevent this, we constructed a structure that forms an air gap between the devices via a seat member, that is, a two-layer beam.

[For production]

According to the above configuration, according to the present invention, two SAWs
Due to the simple combination structure of the device, each electrode and SA
It becomes possible to obtain two signals used for protection of the W propagation road surface, temperature compensation of the electric circuit, etc. In addition, if a stress sensor with this structure is used to measure bending stress, two SA
Since forces in opposite directions due to expansion and compression are applied to the electrode surfaces of the W device, these two stresses cause the above two
The oscillation frequencies of the oscillation circuits including the SAW devices shift in different directions. As a result, by detecting the difference in the oscillation frequencies, it is possible to obtain approximately twice the amount of frequency change compared to the case where one SAW device is used, and the amount of change in frequency that occurs within the two SAW devices and the oscillation circuit can be obtained. Frequency drift due to temperature characteristics, changes over time, etc.
Since most of these occur commonly in both circuits, they can be removed by differential detection.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the present invention.

It shows. Transmitting interdigital electrodes IDT 2a, 2b for emitting SAW and receiving interdigital electrodes for receiving SAW are provided on the surfaces of each of the first and second piezoelectric material substrates 1a, lb. IDT 3a, 3b t- are provided to constitute a SAW device. All electrodes on one side of each IDT are grounded, and between the signal input electrode on the ungrounded side of the transmitting IDT and the signal output electrode on the ungrounded side of the receiving IDT. for,
Amplifiers 4a and 4b are connected to each device to configure a SAW oscillation circuit. This amplifier and the SA
An electrical closed loop circuit is configured with the W device,
Self-oscillation occurs at a natural frequency due to frequency selection characteristics caused by the electrode structure of the SAW device and phase delay characteristics of the electric circuit. The SAW excited by the transmitting interdigital electrode IDT during self-excited oscillation advances on the first and second piezoelectric substrates K, as shown by dotted lines in FIG. This SAW operates from the transmitting interdigital electrode IDT to the receiving interdigital electrode I at a constant sound speed determined by the material of the piezoelectric material substrate, which can be said to be a physical constant at a constant temperature.
Propagates to DT. Further, the signal delay time caused by the SAW propagation has a significant effect on the oscillation frequency of the oscillation circuit of the SAW oscillation circuit, and normally, as the propagation path length becomes shorter and the delay time becomes smaller, The oscillation frequency shifts downward. Therefore, if the SAW propagation surfaces of the first and second piezoelectric material substrates are expanded or compressed in the SAW propagation direction by an external force, each of the SAW
The oscillation frequency of the AW oscillation circuit changes in response to this external force.

In the present invention, the two SAW oscillation circuits are configured to shift frequencies in opposite directions due to an external force applied in one direction due to the two-layer beam structure described later, so that the output signals of the two oscillation circuits are received. The amount of change in frequency corresponding to the external force is detected using a comparator circuit 5 that detects the phase difference.

FIG. 2 is a diagram showing an example of the structure of the SAW sensor section. 2
The piezoelectric material substrates 1a and 1b are assembled so that the surfaces on which the electrodes are installed face each other, and a seat member 6a is placed on the end surface of the substrates so as to create an air gap between the two substrates.
6b f via. The example in this figure shows the structure of a stress sensor with a cantilever type. An external force is applied as shown by the arrow. As a result, in the two-layer beam, a compressive force is applied to the electrode surface of the piezoelectric material substrate 1a, and, conversely, a stretching force is applied to the electrode surface of the piezoelectric material substrate 1b. As mentioned above, these forces change the length of the SAW propagation path, and ultimately the oscillation frequency of the SAW oscillation circuit changes. The seat members 6a, 6
In the case of a cantilever structure as shown in the example in this figure, the material installed at the fixed end is hard and difficult to deform, and the member on the end face to which force should be applied is made by bending the piezoelectric material substrate. It is desirable to have some elasticity so as to absorb the misalignment of the upper and lower substrate surfaces that occurs. It is also possible to draw out the lead wires of the electrodes on the piezoelectric substrate using the front and back surfaces and side surfaces of this seat member (actually, print wiring is applied to the seat member with conductive paste, etc.); in this case, requires the use of insulators.

Next, consider a case where forces are applied in opposite directions to both ends and the center of the SAW sensor section. In this case, it is conceivable that both ends are fixed and pressure is applied to the center, or conversely, that the center is fixed and pressure is applied to both ends. Furthermore, hD can also be obtained when pressure is applied to three points without fixing both the center and both ends. This state is shown in FIG. Furthermore, even when both ends are held together and bent, the deformation results in a pressurized state as shown in FIG. 3. In this case, a support member is required to hold the entire SAW sensor section, and the support member has an external shape, for example, as shown in FIG. 4.

In the example shown in FIG. 14, the SAW device, the oscillation circuit, and the comparison circuit are held and covered using an elastic insulating material 8, such as synthetic resin, and the center of the SAW sensor section is easily bent. The structure has a thinner coating.

A stress sensor with this structure can be used in the same way as a conventional strain gauge, that is, it can be attached to a location where deformation is to be measured.

〔Effect of the invention〕

As explained above, in the stress sensor of the present invention, a set of two SAW devices are combined with their electrode surfaces facing each other,
By employing two layers of beams that form an air gap between them, it has become possible to obtain two output signals for use in protecting each electrode and the SAW propagation path, as well as for temperature compensation in the electrical circuit. Furthermore, since the SAW devices combined facing each other are subjected to expansion/contraction forces in opposite directions due to external forces, if the frequency shifts of the two output signals are in opposite directions, the applied force can be determined by measuring the difference between these frequencies. When detecting , the detection sensitivity can now be approximately twice as high as when using a single SAW device. Furthermore, the structure in which two SAW devices are combined face-to-face is suitable for miniaturization, and since the part having a pressure-sensitive function is not exposed on the outer surface of the SAW sensor section after combination,
It also has the practical effect of being easy to coat.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the electrical circuit configuration of the stress sensor of the present invention. FIG. 2 is a diagram showing a SAW sensor section, a support section, a pressurizing point, and a pressurizing state. FIG. 3 is a diagram showing pressurizing points of the SAW sensor section. FIG. 4 shows an overall view of a stress sensor in which the SAW sensor section, oscillation circuit section, and detection circuit section of FIG. 3 are covered. In the figure, la and lb are the first. Second piezoelectric material substrate, 2m. 2b is a transmitting interdigital electrode IDT, 3a and 3b are receiving interdigital electrodes IDT, 4a and 4b are amplifiers, 5 is a comparison circuit, 6a and 6b are seat members, and 7 is a support member. Patent Applicant Anritsu Electric Co., Ltd. Agent Patent Attorney Ryuta Koike 14th Proceeding Amendment Written Voluntary) May 2-2, 1985 1, Display of Case 1985 Patent Application No. 75451 2, Name of Invention Stress Sensor 3, relationship with the amended person case Patent applicant address ■106 5-10-27 Minami-Azabu, Minato-ku, Tokyo Name (057) Anritsu Electric Co., Ltd. Representative Yugo Fujita 4, Agent address ■106 Tokyo Minamiazabu 5-10-27-5, Minato-ku
, date of amendment order Motu 6, subject of amendment (1) Application 7゜Contents of amendment (1) Add the following after the patent application. (Patent application pursuant to the proviso to Article 38 of the Patent Act) (2)
) Add the following to paragraph 2:

Claims (1)

[Claims] 1) A rectangular first piezoelectric material substrate; and a transmission interdigital electrode IDT provided on a first surface of the first piezoelectric material substrate for exciting a surface acoustic wave. ; the first part outputs the excited surface acoustic wave as an electric signal;
a receiving interdigital electrode IDT provided on the surface of; a rectangular second piezoelectric material substrate; and a second piezoelectric substrate of the second piezoelectric material substrate;
a transmitting interdigital electrode IDT provided for exciting surface acoustic waves on the surface of the transmitting interdigital electrode IDT; output from the first receiving interdigital electrode IDT and the second receiving interdigital electrode IDT; a comparison circuit for detecting a phase difference between electric signals; a seat member; the first and second piezoelectric material substrates having a gap therebetween, and the first surface and the second piezoelectric material substrate having a gap therebetween; A stress sensor comprising: two layers of beams stacked one on top of the other with surfaces facing each other; a support member for the two layers of beams; and a force applying means for applying bending stress to the two layers of beams. 2) a rectangular first piezoelectric material substrate; a transmission interdigital electrode IDT provided on a first surface of the first piezoelectric material substrate for exciting a surface acoustic wave; and an excited surface acoustic wave. a receiving interdigital electrode IDT provided on the first surface that outputs waves as electrical signals; a rectangular second piezoelectric material substrate; and a second piezoelectric material substrate of the second piezoelectric material substrate;
a transmitting interdigital electrode IDT provided for exciting surface acoustic waves on the surface of the first surface; inputting the output of the receiving interdigital electrode IDT provided on the first surface; a first oscillation circuit amplifier constituting a first oscillation circuit in addition to the transmitting interdigital electrode IDT provided on the surface of the receiving interdigital electrode IDT provided on the second surface; a second oscillation circuit amplifier that inputs an output and adds the output to the transmission interdigital electrode IDT provided on the second surface to form a second oscillation circuit; the first oscillation circuit; a comparator circuit that detects a phase difference between electric signals output from the second oscillation circuit; a seat member; and a gap between the first and second piezoelectric material substrates via the seat member; , and a two-layer beam in which the first surface and the second surface are stacked facing each other; a support member for the two-layer beam; and a force applying means for applying bending stress to the two-layer beam. A stress sensor consisting of.