JPH01195368A - Piezoelectric acceleration sensor - Google Patents

Abstract

PURPOSE:To prevent a dew condensation phenomenon at a low temperature by fixing a vibrator to which a piezoelectric element is adhered, to an closed casing in which dry gas is sealed. CONSTITUTION:A detecting part 1 is stored in an electromagnetically shielded closed casing 2. In the detecting part 1, a film-shaped piezoelectric element 4 is adhered to one surface of a disk-shaped diaphragm 3. Electrodes 6 are attached to both faces of the element as one body. Peripheral edge parts of the diaphragm 3 and the piezoelectric element 4 are surrounded with an annular fixed frame 5. Thus, dew condensation does not occur on the diaphragm even at a low temperature, and the temperature characteristic of the sensor output at a low temperature is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a piezoelectric acceleration sensor that prevents dew condensation in low temperature ranges and improves the stability of output signals.

"Prior Art" Detection of acceleration, which is a physical quantity, is determined according to the applied force according to Newton's law, which is expressed as F=m α (where F: force, m: mass, and α: acceleration). Methods for converting this mechanical quantity called force into an electrical quantity include piezoelectric type, servo type, strain gauge type, etc. Among these, piezoelectric type is currently the most popular type used in acceleration sensors. It is a type.

A piezoelectric acceleration sensor utilizes a piezoelectric effect that generates an amount of electricity proportional to the magnitude of the force when an external force is applied to a piezoelectric element provided in a detection section and the piezoelectric element is distorted.

There are roughly three types of detection units, as shown in (a) to (c) of FIG. 3, depending on the way in which distortion occurs in the piezoelectric element. To briefly explain these, (a) When force F is applied to the weight M attached around the support S, the piezoelectric element P arranged between the weight M and the substrate is compressed, and the piezoelectric element P is compressed. [Compression type] where strain occurs in the same direction as the polarization axis.

(b) When force F is applied to the weight M attached around the support S through the piezoelectric element P, the piezoelectric element P receives a shearing force, and the strain is in the same direction as the polarization axis direction of the piezoelectric element P. [shear type] that occurs as a deviation from the

(c) When a piezoelectric element P is attached to a support S in the form of a cantilever, and a force F is applied to a weight M attached to the tip of the piezoelectric element P, distortion occurs in a direction perpendicular to the polarization axis direction of the piezoelectric element. [Cantilever type].

each of them.

For example, to detect the acceleration of a vibrating body at medium and high frequencies,
The compression type (a) or the shear type (b) is used.
When detecting the acceleration of a vibrating body at low frequency, the cantilever type (C), which has higher detection sensitivity and can detect minute vibrations, is used. They are used differently depending on the sheath measurement range, etc.

In addition, piezoelectric materials include P b (Z r, T to s)
Ceramic materials such as polyvinylidene fluoride and polyvinyl chloride are used.Although the former ceramic materials have excellent rigidity, they are brittle and easily chipped, resulting in poor workability and impact resistance. The disadvantage is that it is weak and easily broken. The latter polymer-based material has (a) excellent flexibility compared to ceramic-based materials.

(b) It has excellent processability and can be made into thin films and large areas.

(C) Is the dielectric constant small? Therefore, the voltage output constant is large.

(d) Excellent insulation properties.

It has the following characteristics.

By using these acceleration sensors appropriately, it is possible to obtain electrical signals with stable directional characteristics and frequency characteristics, and by storing the acceleration sensor in a sealed shield case, it is possible to prevent external noise. environmental resistance.

[Problem to be solved by the invention] However, when considering the case where the acceleration sensor is used at low temperatures, water droplets tend to adhere to each part of the piezoelectric element due to condensation of moisture inside the shield case. Fluctuations in the dielectric constant due to water droplets and fluctuations in the mass of the vibrating part tend to increase errors in frequency characteristics, and also cause phenomena such as weakening or stopping of electrical signals due to an increase in leakage current between electrodes.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a piezoelectric acceleration sensor that can obtain stable frequency characteristics and output signals over a wide range including a low temperature range.

"Means for Solving the Problems" In the piezoelectric acceleration sensor according to the present invention, the detection section is housed in a dry gas atmosphere in a sealed casing, and a part of it is fixed to the sealed casing. The device is characterized in that it includes a diaphragm that is bonded to the diaphragm, and a piezoelectric element that is bonded to the surface of the diaphragm.

``Operation'' Creates a dry gas atmosphere inside the sealed casing,
If the practical use range is above the dew point temperature in a low temperature state, water vapor contained in the gas will not condense, and the piezoelectric element will not be affected by moisture. That is, there is no mass variation of the diaphragm in the detection section, the phenomenon that the leakage current between the electrodes increases in a low temperature range is suppressed, and a stable electrical signal can be obtained in the normal operating temperature range.

"Example" Hereinafter, a piezoelectric acceleration sensor according to the present invention will be described based on the drawings.

In one embodiment shown in FIGS. 1 and 2, the detection unit 1
is housed in a sealed casing 2 that has an electromagnetic shielding effect, and the detection unit 1 includes a disc-shaped diaphragm 3.
The film-like piezoelectric element 4 is bonded to one surface of the diaphragm 3 with an adhesive, thereby sandwiching the periphery of the diaphragm 3 and the piezoelectric element 4. an annular fixed frame 5 attached to the piezoelectric element 4; and an electrode 6 integrally attached to both sides of the piezoelectric element 4;
The dry gas atmosphere 7 is formed by the gas filled in the hollow part of the sealed casing 2.

The diaphragm 3 has a circular hole 3a concentric with the fixed frame 5 formed in its center, and vibrates at a portion not in contact with the fixed frame 5. For example, the diaphragm 3 has a thickness of 308 m and a diameter of 15 i+m. , the inner diameter of the circular hole 3a is 9 mm.
copper foil.

The piezoelectric element 4 is made of a stretched vinylidene polybutylene film having a thickness of, for example, 30 μm, and is formed in substantially the same shape as the diaphragm 3.

The electrode 6 is formed by integrally covering both surfaces of the piezoelectric element 4 with a metal conductor by a thin film forming method such as vapor deposition.

The dry gas atmosphere 7 is obtained by replacing the inside of the sealed casing 2 with dry air at, for example, 20° C. and 0.5% humidity. In this case, the dry gas atmosphere 7 is created by replacing the air with an appropriate dry gas with a significantly low water content, such as dry air, during the assembly of the sealed casing 2, etc., but the gas to be replaced is not suitable for long-term stability. It is preferable to use an inert gas such as nitrogen gas or argon gas. The dry gas atmosphere 7 needs to have low humidity no matter which gas is used to prevent condensation from forming inside the sealed casing 2 within the range of practical use of the piezoelectric acceleration sensor. The gas is set so as to satisfy the condition that the dew point of the gas in the sealed casing 2 is lower than the temperature range in which the piezoelectric acceleration sensor is used. Therefore, it is desirable that the drying gas be in an absolutely dry state without any moisture, but it is set so that moisture does not condense in a practical low temperature range (room temperature range), for example, by setting the dew point temperature to -40° C. or lower.

<Experimental Example> Comparative studies were conducted on the piezoelectric acceleration sensors having the structures shown in FIGS. 1 and 2 by changing the dry gas atmosphere.

■As sample 1, the air was replaced with dry air at 20℃ and humidity of 0.5% (dew point -41℃)■As sample 2,
The sample was replaced with nitrogen gas (dew point -50°C) at 20°C with a humidity of 0.17%. The sample was replaced with air with a humidity of 50% at 20°C as comparative sample 3. The sample was replaced with air at 20°C with a humidity of 50% as a comparative sample 4. Replaced with air with 10% humidity (dew point -12℃) As a test method,
The piezoelectric acceleration sensor (2) was subjected to a heat cycle test at 20°C to -35°C for 5 cycles, and the following results were obtained.

Sample 1 operated stably within a range of output fluctuation of 3%.

Sample 2 operated stably within a range of output fluctuation of 3%.

In comparison sample 3, the output fluctuates greatly at temperatures below 5°C.
A fluctuation in output of 50% or more occurred. No output was obtained from the third cycle.

In comparison sample 4, the output became unstable at temperatures below -15° C., and the output fluctuation reached 20%.

From these comparison results, as typified by Comparative Samples 3 and 4, even if measures such as sealing the air between the base pillars inside the case and preventing the flow of air with the outside air are taken,
Although the output fluctuation increases and the low-temperature characteristics deteriorate, it is clear that in a dry gas atmosphere like sample 12, stable frequency characteristics and output signals can be obtained up to the normal operating temperature range. It became.

"Effects of the Invention" As explained above, according to the piezoelectric acceleration sensor according to the present invention, since a dry gas atmosphere with a low dew point is formed in the sealed casing, dew condensation occurs on the diaphragm at low temperatures. This does not occur, and the temperature characteristics of the sensor output at low temperatures can be stabilized. Furthermore, the above-described structure suppresses the occurrence of leakage current between the electrodes in a low temperature range, thereby improving the reliability of the piezoelectric acceleration sensor.

[Brief explanation of the drawing]

FIG. 1 is a front sectional view of a piezoelectric acceleration sensor according to the present invention,
Figure 2 is a view taken along the line ■-■ in Figure 1, and (A) to (A) in Figure 3.
C) is a front sectional view showing an example of a conventional structure of a piezoelectric acceleration sensor. 1...Detection part, 2...Sealed casing, 3...Vibration plate, 3a...Circular hole, 4...Piezoelectric element, 5... Fixed frame, 6... Electrode, 7... Dry gas atmosphere.

Claims (1)

[Claims] (1) A piezoelectric acceleration sensor that detects acceleration from the amount of electricity generated due to distortion of a piezoelectric element provided in a detection section, wherein the detection section is housed in a dry gas atmosphere in a sealed casing. A piezoelectric acceleration sensor comprising: a diaphragm partially fixed to the sealed casing; and a piezoelectric element bonded to the surface of the diaphragm.