JPH0715485B2 - Piezoelectric mechanical sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a mechanical quantity sensor using a piezoelectric element, and more particularly to a mechanical quantity sensor suitable as an acceleration sensor for a vehicle.

[Prior Art] As an acceleration sensor using a piezoelectric element, for example, there is one disclosed in Japanese Utility Model Publication No. 187462 in 1986.

FIG. 8 is a cross-sectional view showing the overall configuration of the above acceleration sensor,
Ninth is a plan view and a sectional view of the piezoelectric diaphragm.

In FIG. 8, 1 is the piezoelectric diaphragm, 2 is the lower half of the housing, and 3 is the upper half of the housing.
Is attached to the housings 2 and 3 via a support member 4 made of an elastic material such as polyurethane.

In addition, 5 is an electronic circuit board on which the electronic component 6 is mounted, 6 is an electronic component that constitutes an electronic circuit that amplifies or filters the detection signal, 7 is a potting material, and 8 is an external lead wire for extracting the detection signal to the outside. , 13 and 14 are lead wires connecting the piezoelectric element of the piezoelectric diaphragm 1 and an electronic circuit.

Next, in FIG. 9, piezoelectric elements 11 and 12 are adhered to both surfaces of the thin metal plate 10 by an adhesive agent, respectively.

The thin metal plate 10 needs to have the same thermal expansion value as that of the piezoelectric elements 11 and 12, and therefore, for example, as a piezoelectric element.
When PZT is used, it has a similar coefficient of thermal expansion to that of Ni-Fe.
Use an alloy.

The piezoelectric elements 11 and 12 have a smaller area than the metal thin plate 10, and the metal thin plate 10 is exposed in the peripheral portion of the piezoelectric diaphragm 1.

Further, since the piezoelectric element generates voltage in response to temperature change due to pyroelectricity, it is necessary to adopt a so-called bimorph structure in which the two piezoelectric elements are bonded together so as to have opposite output characteristics as described above. Pyroelectric output is removed by.

In addition, 15 to 18 are cushioning materials, and 19 is a small hole that serves as an air passage.

[Problems to be solved by the invention]

However, in such a conventional piezoelectric mechanical quantity sensor, the polarization directions of two thin piezoelectric elements are fixed to a metal thin plate so that the polarization directions are opposite to each other with respect to the mechanical quantity application direction, and the Since the connections were made in series, the noise voltage that should be canceled by the bimorph structure, that is, the noise voltage generated based on the pyroelectric charge peculiar to the piezoelectric element generated in response to the environmental temperature change has two noise voltages. There is a problem that a large noise voltage is generated due to the influence of the capacitance difference and the insulation resistance difference between the piezoelectric elements, and the low frequency mechanical quantity detection accuracy of the mechanical quantity sensor is deteriorated. The details of the above influence will be described later.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the prior art as described above and to provide a piezoelectric mechanical quantity sensor with improved detection accuracy in a low frequency range.

[Means for solving problems]

In order to achieve the above object, in the present invention, the two piezoelectric elements are fixed to both sides of the diaphragm so that the polarization directions of the two piezoelectric elements are the same with respect to the mechanical quantity application direction. Moreover, the two piezoelectric elements are electrically connected in parallel.

With the above-described configuration, even if the bimorph structure is used, the electrical characteristic deviations (pyroelectric current difference,
Of the difference in capacitance and difference in insulation resistance, the two factors of difference in capacitance and difference in insulation resistance can be neglected. Therefore, the noise voltage can be reduced to 1/2 or less compared to series connection. .

Example of Invention

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

FIG. 1 is an embodiment of the present invention, (A) is a sectional view of the piezoelectric diaphragm 100, (B) is an enlarged sectional view of a main part of the piezoelectric diaphragm, and (C) is a piezoelectric element of two sheets. It is a figure which shows the polarization direction. The other parts are the eighth and
It is similar to FIG.

In FIG. 1, 2 fixed on both sides of the thin metal plate 20
The piezoelectric elements 21, 22 are thin electrodes 23, 24, 2 on the surface, respectively.
5,26 are formed. As a material for this electrode, for example, a Ni—Cu alloy is used and is formed by a method such as sputtering or vapor deposition. These electrodes are usually 0.1-0.5μ
m is an extremely thin electrode.

Further, the lead wire for extracting a signal from the piezoelectric diaphragm 100 includes a thin lead wire 27 that electrically connects the outer electrodes 23 and 24 of the two piezoelectric elements 21 and 22, and two piezoelectric elements.
Lead wire 29 for guiding the electric charge generated in the outer electrodes of 21, 22 to the detection circuit, and the bonding side electrode 2 of the two piezoelectric elements 21, 22
Lead wire 2 for guiding the charges generated at 5 and 26 to the detection circuit
It is composed of a total of 3 lead wires such as 8.

The bonding-side electrodes 25 and 26 and the metal thin plate 20 are electrically connected to each other via the convex portion on the surface of the bonding surface, and the lead wire 28 is connected to the metal thin plate 20. Further, as a method of attaching the lead wire to each of the electrodes and the metal thin plate described above, for example, soldering is used, but a conductive adhesive may be used. Reference numeral 31 and 32 in FIG. 1 (B) indicate the solder layer or the adhesive layer.

Further, as shown in FIG. 1 (C), the fixing directions of the two piezoelectric elements 21 and 22 are made of metal so that the polarization directions of the two piezoelectric elements are the same with respect to the mechanical quantity application direction. It adheres to both sides of the thin plate 20.

Next, FIG. 2 is a diagram showing an electrical connection method between the piezoelectric diaphragm 100 and the detection circuit.

As shown in the figure, the two piezoelectric elements 21 of the piezoelectric diaphragm,
22 is electrically connected in parallel, and its output is an operational amplifier 33.
Is output from the output terminal 34 via.

Next, the operation will be described.

FIG. 4 and FIG. 5 are diagrams for comparing and explaining the conventional series connection and the parallel connection of the present invention. FIG. 4 shows the case of the conventional example, and FIG. 5 shows the case of the present invention. Indicates. In both figures, (A) is a diagram showing the polarization direction of the piezoelectric element,
(B) is a diagram showing a charge generation direction when a mechanical quantity is applied, (C) is a diagram showing a pyroelectric charge generation direction due to temperature change, and (D) is an equivalent circuit diagram.

First, the series connection of the conventional example will be described with reference to FIG.

In the serial connection, the signal is taken out only from the outer electrodes of the two piezoelectric elements, and the thin metal plate electrically connected through the surface unevenness and the bonding side electrode of the piezoelectric element are supported by a supporting member made of an insulating material. Insulated from the outside.

The performance as a mechanical quantity sensor is determined by the ratio of the voltage (signal voltage S) generated by the distortion according to the applied mechanical quantity to the pyroelectric voltage (noise voltage N) generated when the environmental temperature changes, and generally S It is called the / N ratio, and the larger this ratio, the better the performance as a sensor.

In the series-connected equivalent circuit shown in FIG. 4D, if the voltage generated when a mechanical quantity is applied is V _F , then in the time domain, Becomes

Here, i _1F and i _2F are currents generated in the two piezoelectric elements corresponding to the applied mechanical quantities. C ₁ and C ₂ are electrostatic capacitances of the piezoelectric element, and IR ₁ and IR ₂ are insulation resistances of the piezoelectric element.

Next, when the environmental temperature changes, as a problem peculiar to the piezoelectric element, a pyroelectric current of i _T = λ _T · dT / dt that is generated in proportion to the temperature change rate occurs. Where λ _T is called the pyroelectric coefficient,
It is a constant representing the generated current with respect to a constant temperature change rate.

The noise voltage V _T generated when the environmental temperature changes in series connection is expressed by the following equation.

Here, if the electrical characteristics of the two piezoelectric elements are completely equal, i _1F = i _2F , i _1T = i _2T , C ₁ = C ₂ , IR ₁ = IR _2, and therefore the signal voltage V _F , The noise voltage V _T is as follows.

However, in reality, the characteristic deviation at the time of manufacturing the piezoelectric element,
The electrical characteristics of the piezoelectric elements slightly differ from element to element due to the stress strain caused by bonding the piezoelectric element to the thin metal plate, the stress strain caused by the support member, etc., which causes noise voltage. Further, as is clear from the above equation, the noise voltage is proportional to the difference in the electrical characteristics of the two piezoelectric elements, so a slight difference in the electrical characteristics of the piezoelectric elements results in a large noise voltage. Aggravates the / N ratio.

There are three main causes of noise voltage in series connection: pyroelectric current difference (i _1T −i _2T ), capacitance difference (C ₁ −C ₂ ), insulation resistance difference (IR ₁ −IR ₂ ). is there.

Next, the parallel connection of the present invention will be described with reference to FIG.

In the parallel connection, the signal is taken out from the electrically connected outer electrode and the thin metal plate which is electrically connected to the bonding side electrode through the surface unevenness.

Also in the equivalent circuit of the parallel connection shown in FIG. 5 (D), when placing the voltage generated when the physical quantity F is applied as V _F, in the time domain, V _F is Becomes Further, the noise voltage V _T due to the environmental temperature change that occurs in parallel connection is expressed by the following equation.

If the two piezoelectric elements have the same electrical characteristics, Becomes

Here, considering the case where the electrical characteristics of the piezoelectric element differ from element to element as described above, as can be seen from the above equation of V _{T (} t ₎ , the parameter affecting the noise voltage V _T is the pyroelectric It is only the current difference (i ₁ −i ₂ ), and is essentially independent of the capacitance difference (C ₁ −C ₂ ) and the insulation resistance difference (IR ₁ −IR ₂ ).

Next, focusing on the signal voltage, the parallel connection reduces the output voltage by half compared to the series connection.

However, as described above, when connected in parallel, two factors among the three electrical characteristic deviations of the piezoelectric element that affect the noise voltage (pyroelectric current i, capacitance C, insulation resistance IR) are ignored. It is possible, and only the difference in pyroelectric current will be affected. Therefore, it is possible to reduce the noise voltage to less than 1/2, so the overall S / N
It is possible to improve the ratio.

The applicant confirmed the effects of the present invention described above by experiments.

FIG. 6 is a diagram showing the configuration of the detection circuit used in the experiment.

First, FIG. 6 (A) shows the case of the conventional series connection. Since the leak resistance 36 has a resistance value of 5,000 MΩ and the electrostatic capacity of the piezoelectric diaphragm 101 is 20,000 pF, the CR product is 100 ( se
c).

Next, FIG. 6 (B) shows a case of parallel connection of the present invention,
Since the leak resistance 37 has a resistance value of 1,250 MΩ and the electrostatic capacity of the piezoelectric diaphragm 102 is 80,000 pF, the CR product is the sixth.
It is 100 (sec) which is the same as in the figure (A).

FIG. 7 shows that, in the sixth detection circuit having the same CR product as described above, the noise voltage due to the environmental temperature change generated when a temperature change of a constant temperature gradient is applied was measured by testing for each of a plurality of samples. Among the results, representative data are shown.

In FIG. 7, (A), (B), and (C) are test results of three samples in parallel connection, (D), (E), and (F).
Are test results of three samples in series connection, and (G) and (H) are characteristics of temperature change applied to the piezoelectric element.

As is clear from FIG. 7, according to the parallel connection of the present invention, the noise voltage is much smaller than that of the conventional series connection, and even considering that the signal voltage becomes 1/2, S
It was confirmed by experiments that the / N ratio was significantly improved compared to the conventional one.

Next, when using a piezoelectric diaphragm as a mechanical quantity sensor and measuring a mechanical quantity up to a low frequency region, a piezoelectric element with high insulation resistance is used to reduce noise voltage, which is extremely large. As shown in FIG. 3, the input terminal of the detection circuit (operational amplifier 33) -GND
A method of connecting a leak resistor 35 between them is used.

This leak resistance allows the mechanical quantity sensor to
The minimum measurable frequency of the mechanical sensor that forms a filter is determined by the product (CR product) of the resistance value and the capacitance of the piezoelectric element when the insulation resistance and the leak resistance of the piezoelectric element are connected in parallel. .

For example, the frequency that attenuates by 3 dB (cutoff frequency fc)
Is expressed by fc = 1 / 2πCR (Hz). That is, if the CR product is increased, the mechanical quantity can be measured without being attenuated to the low frequency region. However, increasing R is as described above. Will increase the noise voltage determined by. Further, in order to enable the mechanical quantity measurement of a practical piezoelectric element up to an ultra low frequency range of about 0.01 (Hz), generally, the required leak resistance value is 1000 (MΩ) to 10,000 (MΩ). However, such ultra-high resistance remains uncertain in terms of stability and reliability when used in a vehicle environment.

Therefore, it is desirable to reduce the leak resistance value as much as possible and increase the electrostatic capacitance in order to be able to measure the low frequency region. In that respect, the parallel connection in the present invention makes it possible to double the capacitance as described above without increasing the number of components, and therefore, the leakage resistance required to secure the same CR product. Since the value can be halved, it is possible to reduce the problems when the high resistance is used as described above.

Further, other effects of the present invention will be described by focusing on the voltage generated in the single piezoelectric element.

In order to impart piezoelectricity to the piezoelectric element after firing, a high electric field is applied to the piezoelectric element to perform polarization (poling) treatment that aligns the crystal orientation in the element in the direction of the electric field, that is, a certain stress (piezoelectricity) or The electric charge generated due to temperature change (pyroelectricity) is determined by this polarization condition. Here, a high voltage of several tens to several hundreds of volts is generated in a single piezoelectric element connected in series when the temperature changes rapidly. On the other hand, in the parallel connection, the pyroelectric currents cancel each other, so that only a voltage of about several volts is generated in the piezoelectric element. When a large voltage is applied to the piezoelectric element,
The electric field generated by this voltage causes a problem that the crystal orientation controlled at the time of manufacture is changed and the sensitivity is changed with respect to the applied mechanical quantity.

For the above reasons, the voltage applied to the piezoelectric element is preferably as low as possible, and in this respect also, the effect of parallel connection in the present invention is great.

〔The invention's effect〕

As described above, according to the present invention, the two thin piezoelectric elements are fixed to both sides of the diaphragm so that the polarization directions thereof are the same with respect to the mechanical quantity application direction,
Moreover, since the electrical connection is configured to be connected in parallel, even if the bimorph structure is used, the electrical characteristic deviation (pyroelectric current difference, Capacitance difference, insulation resistance difference)
Of these, the two factors of capacitance difference and insulation resistance difference can be neglected, and as a result, the noise voltage can be reduced to 1/2 or less compared to series connection, and as a result, the overall S / N ratio can be reduced.
The ratio can be improved, and the effect that good detection sensitivity can be maintained even in the low frequency range can be obtained.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention, FIGS. 2 and 3 are diagrams showing the connection of the present invention, FIG. 4 is an explanatory view of the operation of a conventional series connection, and FIG. 5 is a parallel connection of the present invention. 6 is a circuit diagram of the device used for the performance test, FIG. 7 is a characteristic diagram showing the results of the performance test, FIG. 8 is a sectional view of an example of a conventional device, and FIG.
The drawings are a plan view and a sectional view of an example of a conventional piezoelectric diaphragm. <Explanation of symbols> 20 …… Metal thin plate (diaphragm) 21,22 …… Piezoelectric element, 23,24 …… Outer electrode 25,26 …… Adhesive side electrode, 27,28,29 …… Lead wire 30 …… Support Material, 100 ... Piezoelectric diaphragm

Claims (1)

[Claims] 1. A piezoelectric mechanical quantity sensor using a piezoelectric diaphragm in which a thin plate-shaped piezoelectric element provided with an electrode is fixed to each of both surfaces of a part excluding at least a peripheral portion of a conductive thin plate, The two piezoelectric elements are fixed to the thin plate so that the polarization directions of the two piezoelectric elements are the same with respect to the mechanical quantity application direction, and the two piezoelectric elements are electrically connected. A piezoelectric-type mechanical quantity sensor characterized by connecting in parallel.