JPH0310163A - Piezoelectric type acceleration sensor - Google Patents

Abstract

PURPOSE:To improve the output of a detected value when electric charges having the different polarities are generated by providing divided electrodes at a piezoelectric element, and collecting the electric charge having the same polarity. CONSTITUTION:Aluminum is deposited by vacuum evaporation on both surfaces of a ceramic-based piezoelectric element 3, and divided electrodes 4A, 4B, 4C and 4D are formed. A circulate hole 5 is provided at the central part. Thus a vibrating body 2 having the laminated structure is provide. The vibrating body 2 is held by a frame- shaped fixing part 5. The center of the hole 5 is arranged coaxially with a vibrating body 6a so as to be stretched. Then, the center of a detecting part 1 is used as the original point, and an arbitrary point (r, theta) in a polar coordinate system is determined on the surface of the element 3. The strain in the direction (r) is made to be epsilonr, and the strain in the direction theta is made to be epsilontheta. The shapes of the electrodes 4A, 4B, 4C and 4D are made to be in planar symmetry which is divided within the ranges where the expression (i) and the expression (ii) hold true. The electrodes are connected in parallel. Then, the generated electric charges are divided and taken out at the surrounding part of the hole 5 and in the vicinity of the fixed part 6. The electric charges Q1 and Q2 can be added and collected with a charge amplifier 8.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a piezoelectric acceleration sensor, and particularly relates to a technique for improving the detection sensitivity of an acceleration sensor and reducing output fluctuations due to the pyroelectric effect. It is something.

"Prior Art" Detection of acceleration, which is a physical quantity, is given by F=mα (where F: force, m: mass, α: acceleration), and is proportional to the applied force.

Acceleration sensors convert a mechanical quantity called force into an electrical quantity for detection, so this conversion method includes piezoelectric type, servo type,
There are strain gauge types. Among these acceleration sensors, the piezoelectric type is currently the most popular.

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 approximately three types of detection portions as shown in (a) to (c) of FIG. 4, 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 placed between the weight M and the substrate is compressed, and the piezoelectric element P is compressed. Strain occurs in the same direction as the axial direction of the polarization axis [compression type 1° (b) When a force F is applied to the weight M attached around the support S via the piezoelectric element P, the piezoelectric element P is sheared. When a force is applied, strain occurs as a deviation from the plane in the same direction as the polarization axis of the piezoelectric element P [shear type].

(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, and when detecting the acceleration of a vibrating body at low frequencies, these are used. The cantilever type (C), which has high sensitivity and can detect minute vibrations, is used, and they are used depending on the frequency, the magnitude of acceleration, the measurement range, etc.

``Problem to be solved by the invention J By the way, the cantilever type is excellent in detecting low frequencies and low accelerations, but in this case, the fixing conditions for fixing one end of the piezoelectric element to the support body must be realized. Therefore, there is a problem that it is difficult to maintain stable frequency characteristics and sensitivity L7.

The present inventors previously proposed in Japanese Patent Application No. 62-258780 and others that a means for supporting a piezoelectric element in a fixed frame has a hole in the center of the vibrating part, as shown in FIG. 4(d). We have proposed a piezoelectric acceleration sensor that can improve detection sensitivity by adding a means to clear the area.

Subsequently, as a result of further research, it was found that in general piezoelectric elements, there is a phenomenon in which the charge generation efficiency changes depending on the position where distortion occurs, and even on the same surface, parts with different polarities of generated charges are generated at the same time. We found that this occurs.

The present invention has been made in view of the above findings, and aims to increase the sensitivity of the negative layer and obtain stability of the detection output by extracting the charges while matching the polarity of the generated charges. The purpose is

A piezoelectric acceleration sensor according to the present invention has a piezoelectric acceleration sensor according to the present invention, as shown in FIG. 1 and FIG. In addition to the basic configuration for detecting acceleration from the ground, the detection unit l includes a thin film-like piezoelectric element 3 with a hole 5 formed in the center supported in a tensioned state in a vibrating type 6a in the fixed part 6. At the same time, the piezoelectric element 3
Split electrodes 4A, 4B, and radially spaced on both sides of the
4C and 4D are integrally provided, and the divided electrodes 4A, 4B,
4C and 4D are detection parts on the surface of the piezoelectric element 3! Take arbitrary points r and θ in a polar coordinate system whose origin is the center of
When, εr+εθ〉 0 ......(i) and ε
r+εoku 0...It is divided into parts where the relationship (ii) holds and is provided in a plane symmetrical state, and the plurality of divided electrodes 4A, 4B, 4C, and 4D are connected in parallel and have the same polarity. It has a configuration for extracting the electric charge.

"Operation" When --like acceleration is applied to the piezoelectric element 3 of the detection part 1 in its thickness direction, distortion occurs in the vibrating part, and the output of the following equation is obtained.

However, ε: strain, S: area, e: piezoelectric strain constant (pc/
N), C: capacitance.

Moreover, the strain that occurs in the piezoelectric element 3 becomes tensile strain or compressive strain depending on the location and direction.

Therefore, on the surface of the piezoelectric element 3, as shown by Q and Q2 in the electrical equivalent circuit model of FIG.
Depending on the strain position of the piezoelectric element 3, that is, the outer electrode 4
Due to the difference in the positions of A and 4B and the inner electrodes 4C and 4D, charges with opposite polarities (positive and negative) are generated at the same time.

As shown in the electrical equivalent circuit model diagram shown in Figure 3, by collecting these two charges Q and -Q with the charge amplifier 8, an amount of electricity corresponding to the total charge Q = QI 102 can be obtained. become.

Here, considering a polar coordinate system with the center of the detection unit 1 as the origin, charges with different signs may be generated in the r-axis direction and the θ-axis direction, and the total charge generated on one surface of the piezoelectric element 3 is When collected at the same electrode, a portion of the charges are canceled out, so that the apparent generated charges tend to decrease.

Therefore, when a force in one direction perpendicular to the surface is applied, εr
The outer electrodes 4A and 4B are divided into a range where the relationship +εθ〉0 holds true and a range where the relationship εr+εθ〈0 holds true.
For example, by dividing and extracting the charges generated in the vicinity of the hole 5 and the vicinity of the fixing part 6, the two charges Q1 are collected so that the entire charge is extracted between the inner electrodes 4C and 4D. - Q2 is added, making it possible to collect the generated charges without waste.

On the other hand, if a charge is generated due to a temperature difference between the front and back surfaces of the piezoelectric element 3 (a so-called pyroelectric effect occurs), the charge generated due to the pyroelectric effect may be transferred to the outer electrodes 4A and 4B, for example. If they have the same polarity, the charges will be added, but on the contrary, they cancel each other out for the inner electrodes 4C and 4D, so if we consider the charges as a whole, roughly speaking, the charges will be added to the outer electrodes 4A and 4B. area and inner electrodes 4c and 4D
Since there is a difference between the area of

"Example" Hereinafter, an example of a piezoelectric acceleration sensor according to the present invention will be described.

1 and 2, reference numeral I is a detection unit, 2 is a vibrating body, 3 is a piezoelectric element, 4A, 4B, 4C, -1D are divided electrodes, 5 is a circular hole (circular hole), 6 is a fixed part (fixed frame), 6a is a vibration hole, and 7A and 7B are terminal conductors.

The vibrating body 2 is made by coating aluminum to a thickness of 0.05 μm on both sides of a ceramic piezoelectric element 3 with a thickness of 30 μm using a vacuum evaporation method.
~ About 1 μm of O was deposited to form divided electrodes 4A, 4B, 4C,
It is a laminated structure formed by forming 4D, with an inner diameter of 2 in the center.
．． A hole (circular hole) 5 of 7 mm is drilled. The vibrating body 2 and the like are sandwiched between circular frame-shaped fixing parts 6 with an inner diameter of 7 mm and an outer diameter of 13 mm, and the center of the hole 5 is arranged concentrically with the circular vibration hole 6a. supported by the state.

Further, as shown in FIG. 1, the divided electrodes 4A, 4B, 4C, and 4D are formed on the front and back surfaces of the piezoelectric element 3 concentrically and in a symmetrical manner.

The outer diameter of the outer divided electrodes 4A and 4B is approximately 7
mm and fits within the vibration hole 6a, the inner diameter is near the boundary of the above-mentioned formulas (i) and (ii) (for example, about 4.8 mm in diameter), and a notch 4e is formed in a part of the annular shape. The terminal conductor 7B is guided in a radial outward direction, and is connected to the terminal conductor 7A on the opposite side. Further, the inner divided electrodes 4G and 4D are formed such that their outer diameters form a small gap with the outer divided electrodes 4A and 4B, and their inner diameters match the inner diameters of the holes 5, and It is connected to the terminal conductor 7B. Such a divided electrode 4A・
4B, as described above, the vapor deposited layer integrally formed on the surface of the piezoelectric element 3 by the aluminum vapor deposition method is processed by chemical etching to form the vapor deposition layer as shown in FIG. This is formed by removing unnecessary parts, and the gap between the divided electrodes 4A and 4B is set to 10 μm.

The fixing part 6 is made by cutting a glass-epoxy resin laminate to form a circular frame shape with an inner diameter of 7 mm and an outer diameter of 13 mm, and is arranged so that the vibrating body 2 is sandwiched in the thickness direction so that the center of the hole 5 is a circle. The vibration hole 6a is arranged concentrically with the vibrating hole 6a, and the vibrating body 2 is supported in a stretched state.

[Experimental Example] Sample #1 based on FIGS. 1 and 2 (Example) and Sample #2, which will be described later and have a similar structure, were prepared for comparison, and differences in vibration characteristics due to electrode shapes were investigated.

<Sample #I> The example shown in Figures 1 and 2. In other words, the outer divided electrodes 4A and 4B are annular ones with an outer diameter of 7 mm and an inner diameter of 48 mm, and have a cutaway part 4e formed in a part, and the inner divided electrodes 4C and 4D are annular ones with an outer diameter of 48 mm. ,8m
m, annular one with an inner diameter of 2.7 mm, and the gaps between the inner and outer divided electrodes 4A, 4B, 4C, and 4D are 50 mm.
μm, with a 2.7 mm hole 5 in the center of the piezoelectric element 3.

<Sample #2> Only the outer divided electrode 4A on the front side of sample #l was provided in a ring shape, electrodes were uniformly provided on the back side of the piezoelectric element 3, and a 2.7 mm hole was made in the center. Something.

For these samples #1 and #2, the following test A
And Test B was conducted.

"Test A" The output was measured when a 100 Hz, IG, sine wave vibration acceleration was applied.

As a result of test A, when the output of sample #I is 100, sample #2
The relative power ratio was 72.

“Test B” The test atmosphere was changed from room temperature (20°C) to 50°C after 30 seconds with 100Hz and IG sinusoidal vibration acceleration applied.
After rapidly heating to a temperature of

Table 1 shows the relative output ratios of Sample #I and Sample #2 of Test B when the initial output is set to 1.

Table 1 To organize and explain these comparison results, sample #1
In other words, it is clear that in a sample that satisfies the conditions of one embodiment of the invention, the generated output itself is increased by several IO% compared to sample #2, and the detection sensitivity can be increased.

In addition, according to Table 1, when examined under conditions where the ambient temperature changes transiently, sample #1 has little change in characteristics, which means that it is advantageous in terms of temperature characteristics and that pyroelectric effects are less likely to appear. However, in sample #2, a characteristic difference, that is, a pyroelectric effect appeared due to a transient temperature change, and the characteristic change tended to increase.

Therefore, combining the results obtained in Test A and Test B, sample #1 can sufficiently increase the detection output and achieve high sensitivity, and is also advantageous in terms of temperature characteristics and pyroelectric effect. Become.

[Other Embodiments] The following embodiments can be adopted in the present invention.

(a) The piezoelectric element should be made of something other than ceramic.

(b) When the temperature dependence of the elastic modulus of the piezoelectric element is large, a backing material L7 is attached to the piezoelectric element to improve its performance as a vibrating body.

(c) When attaching a backing material, the conditions of the following formula must be satisfied.

EvTv3/EpTp3≧5---(iv)
However, Ev: Elastic modulus of the common electrode Tv: Thickness of the common electrode Ep; Elastic modulus of the piezoelectric element Tp: Thickness of the piezoelectric element (d) Each electrode 4A, 4B, 4C is masked by, for example, sputtering or vacuum evaporation. This is formed using . Alternatively, it may be formed by means such as chemical etching.

(e) When reducing the gap between the divided electrodes 4A and 4B, set the limit to 5 μm or more to prevent current leakage between the electrodes.

"Effects of the Invention" As explained above, according to the piezoelectric acceleration sensor according to the present invention, (1) the acceleration detection output can be increased by making holes in the vibrating body.

■Since split electrodes are provided on both sides of the piezoelectric element to collect and utilize charges of both polarities at the same time, it is possible to improve the detection output when charges of different polarities are generated, and also to The detection sensitivity can be improved by preventing the occurrence of a phenomenon in which polar charges cancel each other out when they are generated at the same time.

■Separated electrodes are provided on both sides of the piezoelectric element to collect charges of different polarities that are generated simultaneously on one surface of the piezoelectric element, so the charge that causes an error on one surface of the piezoelectric element due to the pyroelectric effect When this occurs, the erroneous charge increases or decreases with respect to the positive charge and negative charge on one surface, and as a result they cancel each other out, so that the influence of the pyroelectric effect is reduced.

(2) With the above, when a transient temperature difference occurs between the front and back sides of the piezoelectric element, it is possible to reduce the occurrence of errors due to the difference in piezoelectric element characteristics between the front and back sides, and to stabilize the detection output.

It has the following effects.

[Brief explanation of drawings]

Fig. 1 is a partially cutaway plan view showing an embodiment of the piezoelectric acceleration sensor according to the present invention, Fig. 2 is a view taken along the line ■-■ in Fig. 1, and Fig. 3 is a plan view according to the present invention. An electrical equivalent circuit model diagram of a piezoelectric acceleration sensor, (a) to (d) of FIG. 4 are front sectional views showing an example of a conventional structure of a piezoelectric acceleration sensor. ! ...Detection section, 2... Vibrating body, e 4r... 4g, 5... 6... a A... Piezoelectric element, A, 4B, 4C, 4D... ...Split electrode, ...
Separation part, connection conductor, ... fan-shaped part, ... hole (circular hole), - fixed part (fixed frame), vibration hole, 7B...terminal conductor, ... charge amplifier. Figure 1

Claims (1)

[Claims] In a piezoelectric acceleration sensor that detects acceleration from an amount of electricity generated due to distortion of a piezoelectric element provided in the detection section, the detection section (1) includes a vibration hole in a fixed section (6). A thin film piezoelectric element (3) with a hole (5) formed in the center is supported in a stretched state in (6a), and
Split electrodes spaced radially on both sides of the piezoelectric element (
4A, 4B, 4C, 4D) are integrally provided, and the divided electrode takes an arbitrary point (r, θ) on the surface of the piezoelectric element in a polar coordinate system with the origin at the center of the detection part, and When the strain in the r-axis direction at A piezoelectric acceleration sensor characterized in that a plurality of divided electrodes are connected in parallel to extract charges of the same polarity.