JPH03108672A - Piezoelectric acceleration sensor - Google Patents

Abstract

PURPOSE:To enable attainment of high sensitivity and stabilization of a detection output by a method wherein a thin-film piezoelectric element is stretched and supported in a vibration hole of a fixing part of a detecting member, split electrodes are provided plane-symmetrically on the opposite surfaces of the element and connected in parallel, and a charge of the same polarity is detected therefrom. CONSTITUTION:A vibrator 2 composed of a piezoelectric element 3, split electrodes 4A to 4D, a backing material 8 and an adhesive layer 9 is stretched and supported by a fixing part 6 so that the central round hole thereof is concentric with a vibration hole 6a of the fixing part 6. The electrodes 4A to 4D are formed on the surface and the back of the element 3 so that they are combined concentrically and plane-symmetrically. According to this constitution, the hole of the vibrator increases an acceleration detection output, the split electrodes improve the detection output at the time of generation of charges being different in polarity, by simultaneous utilization of the charges of both polarities, a canceling phenomenon is prevented at the time of simultaneous generation of the charges of both polarities, and thus detection sensitivity can be improved.

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.

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

Acceleration sensors convert mechanical force (m) into electrical quantity and detect it.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 polarization axis [compression type.

(b) When a 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 a plane.

(c) A pressure 711 rope P is attached to the support S in the form of a cantilever, and a force F is applied to the weight M attached to the tip of the rope P.
When a force is applied, 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 in (a) or the 'JJ cutting type in (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.

``Problems to be solved by the invention'' By the way, the cantilever beam type is excellent in detecting low frequencies and low accelerations, but in this case, it is difficult to realize the fixing conditions when fixing one end of the piezoelectric element to a support. Therefore, there is a problem that frequency characteristics and sensitivity are difficult to stabilize.

The present invention has previously been proposed in Japanese Patent Application No. 62-258780, etc., as shown in FIG. We have proposed a piezoelectric acceleration sensor that can improve detection sensitivity by adding a means to clear the area.

After that, further research was conducted on general piezoelectric elements, and there is a phenomenon in which the piezoelectric constant changes depending on the position and direction of strain. Findings 1. A phenomenon occurs in which portions with different positive and negative charges occur simultaneously on the same surface depending on the position in the circumferential direction or the position in the radial direction on the surface of the piezoelectric element. Ta.

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

"Means for Solving the Problems" The present invention proposes means for solving the above problems. As shown in FIGS. 1 and 2, in addition to the basic configuration of detecting acceleration from the amount of electricity generated due to the distortion of the piezoelectric element 3 provided in the detection section I, the detection section I has the following features:
A thin film piezoelectric element 3 with a hole 5 formed in the center is supported in a tensioned state in a vibration hole 6a in the fixed part 6, and divided electrodes 4A, 4B, 4 are provided on both sides of the piezoelectric element 3.
C and 4D are integrally provided, and the divided electrodes 4A, 4B, and 4
C.4D is an arbitrary point ", θ" on the surface of the piezoelectric element 3 in a polar coordinate system whose origin is the center of the detection part l, and the arbitrary point r,
The piezoelectric constant in the direction where the piezoelectric constant at θ is maximum is ea
The piezoelectric constant in the direction perpendicular to l is Θb, and the radial strain at arbitrary points r and θ in the polar coordinate system is εr.
, when the strain in the circumferential direction is εe, (eacos'θ+Θbsfn'θ)εr+(easi
n'θ+ΘbcO8'θ)e O> O=(i) and (eacos'θ+Θbsin'θ)εr+ (e a
sin'0 + e b cos”θ)5 e <
The electrodes 4A, 4B, 4C, and 4D are provided in plane symmetry with respect to the piezoelectric element 3, and a plurality of divisions are provided. The piezoelectric acceleration sensor is characterized in that electrodes 4A, 4B, 4C, and 4D are connected in parallel to detect charges of the same polarity.

"Operation" When a -like acceleration is applied to the piezoelectric element 37 of the detection unit 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/
m), C: Electrostatic 8-ship.

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 Ql 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 simultaneously.

As shown in the electrical equivalent circuit model diagram in Figure 3, by collecting and processing these two charges Q+''Ql with a charge amplifier IO, an electricity m corresponding to the total charge Q=QI +Qt can be obtained. It turns out.

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 71X pole, a portion of the charges are canceled out, so that the apparent generated charges tend to decrease.

So, pressure 1! Piezoelectric properties when the cord 3 is distorted,
In other words, the piezoelectric constant differs depending on the direction (in the case of anisotropy, the above-mentioned equations i and 11 hold true)
Since the polarity (positive and negative) is different in the range, the angle in the circumferential direction is set and the surface of the piezoelectric element 3 is divided into a part that becomes positive polarity and a part that becomes negative polarity at a certain moment, and the charge Q and the charge Q, and to avoid collecting anisotropic charges at the same electrode.

On the other hand, the front and back of piezoelectric element 3? If a charge is generated due to this temperature difference (a so-called pyroelectric effect occurs), and the charge generated by the pyroelectric effect has the same polarity as, for example, the outer electrodes 4A and 4B. The charges will be added, but on the contrary, they will cancel each other out for the inner electrode 4C-/ID, so if we consider the charge as a whole, it will roughly be the area of the outer electrodes 4A and 4B and the inner electrode 4G. −
/ID will be affected by the difference in area, and the influence of the pyroelectric effect will be reduced, making it possible to reduce the occurrence of errors.

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

In Figs. 1 and 2, numeral 1 is a detection unit, 2 is a vibrating body, 3 is a piezoelectric element, and 4A, 4B, 4C, and 4D are parts 'I'.
IJm pole, 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 includes a piezoelectric element 3 and divided electrodes 4A, 4B, 4.
The product of C・4D, backing It8, and its youthful layer 9 is 7 +
+'l structure, with a hole (circular hole) 5 having an inner diameter of 2.7 mm in the center. The vibrating body 2 is sandwiched between circular frame-shaped fixing parts 6 having an inner diameter of 7fflI11 and an outer diameter of 13 mm, and the center of the hole 5 is arranged concentrically with the circular vibration hole 6a, and the vibrating body 2 is in a stretched state. is supported by

The piezoelectric element 3 is made of a polymeric piezoelectric film (polyvinylidene fluoride material, etc.) and has anisotropy (directivity) in piezoelectric properties. , 15μm thick with epoxy resin adhesive
A backing material 8 made of copper foil is applied, and the space between the piezoelectric element 3 and the backing material 8 is electrically insulated by adhesive B9, and on both sides of the piezoelectric element 3, The divided electrodes 4A, 4B, 4C, and 4D are integrally formed.

That is, as shown in FIG. 1, the divided electrodes 4A, 4B, 4c, and 4D are formed concentrically and symmetrically on the front and back surfaces of the piezoelectric element 3. The divided electrodes 4B and 4D are electrically insulated from the backing material 8 by being covered with the adhesive layer 9 described above.

Such split electrodes 4A and 4B are formed integrally on both sides of the piezoelectric element 3 by aluminum vapor deposition, and have a thickness of 0.05 to 0.
．． A vapor-deposited aluminum layer with a thickness of about 1 μm is formed by chemical etching to remove unnecessary parts, as shown in FIG. , outer split electrodes 4A and 4B
and the inner divided electrodes 4C and 4D are formed in combination, and the rear divided electrodes 4B and 4D are electrically insulated from the backing material 8 by being covered with the adhesive layer 9.

The outer divided electrodes 4A and 4B have an outer diameter of about 7
11Ia so that it fits inside the vibration hole 6a, and the inner diameter is near the boundary of formulas (i) and (ii) described above (for example, the diameter is 4.8 am).
degree), and when the direction where the piezoelectric constant is maximum is 0 degrees, +70 to +110 degrees and -70 to
By etching and separating the range of −110 degrees, a female part 4e is formed in a part of the annular shape, guiding the terminal conductor 7B in a radial outward direction, and on the opposite side, A notch portion 4f is removed, and a connecting conductor 4g is formed by leaving a portion and bulging outward in the radial direction, and the connecting conductor 4g is connected to the terminal conductor 7A.

In addition, the inner divided electrodes 4C and 4D have outer diameters that form a small gap with the outer divided electrodes 4A and 4B, and their inner diameters fit into the annular portion formed to match the inner diameter of the hole 5. , the above-mentioned +70 to +IIO degrees and -70 to -110
A fan-shaped part 4f with the degree range expanded to approximately 7cm outside diameter.
One fan-shaped portion 71 is connected to the terminal conductor 7A.

In addition, the outer part'lIJ? The gap between the Ilt electrodes 4A and 4B and the inner divided electrodes 4C and 4D is set to, for example, 10 μI.

The fixing part 6 is formed 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 designed to sandwich the vibrating body 2 in the thickness direction, so that the center of the hole 5 is a circular frame shape. It is arranged to be concentric with the hole 6a, and the vibrating body 2 is supported in a stretched state.

[Experiment example] Sample #1 based on Fig. 1 and Fig. 2 (Example) and sample #2 and sample #3 described later with similar structures for comparison were prepared, and differences in vibration characteristics due to electrode shape were prepared. It was investigated.

<Sample #I> The example shown in Figures 1 and 2. In other words, the outer divided electrodes 4A and 4B have an outer diameter of 7 mm and an inner diameter of 4.8ffif.
fl annular, +70 to +110 degrees and -70
The one with the range of ~-110 degrees removed and the inner divided electrodes 4C and 4D with an outer diameter of 4.8 mm and an inner diameter of 2.711 nm.
Combining the annular fan-shaped part of + with the addition of 4g and 41, and forming inner and outer divided electrodes 4A, 4B, 4C, 4
The gap D is 50 μm, and the center of the piezoelectric element 3 has a 2.7 μm gap.
One with 5mm holes drilled.

Sample #2> Sample #1 with only the outer divided electrodes 4A and 4B provided and the other parts removed by etching.

<Sample #3> Divided electrodes 4A, 4B-40, in sample #1
A simple annular 7If pole similar to the 4D annular part, i.e.
It is a complete annular electrode that combines an outer electrode with an inner diameter of 4.8 ml and an outer diameter of 7 ml, and an inner electrode with an outer diameter of 4.81 m and an inner diameter of 2.7 ml. which does not take into account the anisotropy of

For these samples #I to #3, the following test A
And Test B was conducted.

[Test A] The output was measured when a sine wave vibration acceleration of 100 tl z, IG was applied.

Results of Test A Table 1 shows the relative output ratios of Sample #2 and Sample #3 when the output of Sample #I is taken as l.

Table 2 Table 1 [Test B 1001-1 z, The test atmosphere was rapidly heated from room temperature (20°C) to 50°C after 30 seconds with the IC sinusoidal vibration acceleration applied. After maintaining the temperature for 30 seconds, the heat was radiated (cooled) so as to return it to room temperature. The output change at that time was measured.

As a result of Test B, Table 2 shows the relative output ratios for Samples #1 to #3 when the initial output is set to 1.

To organize and explain these comparison results, sample #1
1. In other words, in a sample that satisfies the conditions of one embodiment of the invention, compared to sample #2 and sample #3, the generated output itself is several times higher than that of sample #2 and sample #3. It is clear that the detection sensitivity can be increased by approximately θ%.

In addition, according to Table 2, 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 occur. However, in sample #2, a difference in characteristics, that is, a pyroelectric effect, appeared due to a transient temperature change, and sample #3 showed a tendency for the change in characteristics to be larger than that in sample #1.

Therefore, taking together the features shown in Tables 1 and 2, sample #1 can sufficiently increase the detection output and achieve high sensitivity, and is also advantageous in terms of temperature characteristics and pyroelectric effect. .

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

(a) Applying the piezoelectric element to a material other than a polymer-based material and having directionality (anisotropy) in piezoelectric properties.

(b) In the case where the temperature dependence of the elastic modulus in the piezoelectric element is large, the following condition must be satisfied when a metal foil, metal plate, etc. with a high elastic modulus is used as a backing material for the common electrode.

EyTv' / EpTp3≧5-- (iv) However, Ev: Jt: Elastic modulus of current-carrying rod 1゛v: Thickness of common electrode Ep: Elastic modulus of piezoelectric element Tp: Thickness of piezoelectric element (C) Each electrode 4A, 4B, and 4C are formed using a mask by, for example, sputtering or vacuum evaporation. Alternatively, it may be formed by means such as chemical etching.

(d) 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 It is possible to prevent the occurrence of a phenomenon in which polar charges cancel each other out when they are generated at the same time, thereby improving detection sensitivity.

■If the piezoelectric element has anisotropy, the presence of anisotropy can be used to increase the detection output by providing a separate electrode by taking into consideration the angle with respect to the direction in which the piezoelectric constant is maximum. Further improvements can be made by increasing the thickness.

■Split 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 that the pyroelectric effect causes an error in the charge on one surface of the piezoelectric element. If this occurs, the erroneous charges increase or decrease with respect to the positive charges and negative charges on one surface, and are canceled out as a result, so that the influence of the pyroelectric effect is reduced.

■ Due to the above, pressure? 11, when a transient temperature difference occurs between the front and back sides of the 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 diagram 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 examples of conventional #R construction of a piezoelectric acceleration sensor. ! ...detection section, 2 ... vibrating body, 3 ... piezoelectric element, 4A, 4B ... divided electrode, 4C ... common electrode , 4d...notch, 4e...connecting conductor, 4r...sector-shaped portion, 5...hole (circular hole), 6...・Fixed part (fixed frame), 6a... Vibration hole, 7A/7B... Terminal conductor, 8... Backing material, 9... Adhesive layer, 10...Charge amplifier.

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). (6a), a thin film-like piezoelectric element (3) with a hole (5) formed in the center is supported in a stretched state, and divided electrodes (4A, 4B, 4C) are provided on both sides of the piezoelectric element.・4
D) is integrally provided, and the divided electrode is arranged on the surface of the piezoelectric element at an arbitrary point (r,
θ), the piezoelectric constant in the direction where the piezoelectric constant at the arbitrary point is maximum is Θa, the piezoelectric constant in the direction orthogonal to that direction is Θb, and the radial distortion at the arbitrary point in the polar coordinate system is εr, circle When the strain in the circumferential direction is ε_θ, (Θacos＾2θ+Θbsin＾2θ)εr+(Θa
sin^2θ+Θbcos^2θ)ε_θ>0...
...The part where the relationship (i) holds, and (Θacos^2θ+Θbsin^2θ)εr+(Θa
sin^2θ+Θbcos^2θ)ε_θ<0...
...is divided into parts where the relationship (ii) holds, and the divided electrodes are provided in plane symmetry with respect to the piezoelectric element, and the plurality of divided electrodes are connected in parallel to form the same A piezoelectric acceleration sensor that detects polar charges.