JPH01172760A - Piezoelectric type acceleration sensor - Google Patents

Abstract

PURPOSE:To hold a stable output characteristic and frequency characteristic over a long period of time, by providing a vibration plate, a piezoelectric element and a protective film. CONSTITUTION:When the rigidity of a vibration plate 3, that is, the deformation rigidity thereof is made sufficiently larger than that of a piezoelectric element 5, stable output is obtained, because the vibration of a detection part is determined by the vibration mode of the vibration plate 3 and the output characteristic of the piezoelectric element 5 is not damaged. Further, by setting the deformation rigidity of a protective film 6 so as to lower the same to a degree not damaging the vibration mode of the detection part depending on the vibration plate 3, the vibration plate 3 or the piezoelectric element 5 can be protected from external environment without damaging the inherent frequency or output characteristic of the detection part. In addition, the release of the piezoelectric element 5 from the vibration plate 3 is also prevented by the protective film 6.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a piezoelectric acceleration sensor that detects the acceleration of a vibrating body using a polymer-based piezoelectric element, and is particularly suitable for use in low acceleration and low frequency regions. The present invention relates to a suitable piezoelectric acceleration sensor.

[Conventional technology] Detection of acceleration, which is a physical output, is F = Shoulder α (F: force, IR=quality m1α; acceleration) is required according to the applied force.

The method of converting this mechanical quantity of force into electricity m is as follows:
There are piezoelectric types, servo types, strain gauge types, etc., and among these, the piezoelectric type is currently the most popular type used in acceleration sensors.

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) in FIG. 3, depending on the way in which distortion occurs in the piezoelectric element. To briefly explain these, (a) When a 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 is polarized. A "compression type" in which strain occurs in the same direction as the axial direction of the shaft.

(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 applied to the plane in the same direction as the polarization axis direction of the piezoelectric element. "Shear type" that occurs as a deviation.

(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, a strain is generated in a direction perpendicular to the polarization axis direction of the piezoelectric element. A "cantilever beam" type is created.

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,
It has higher detection sensitivity than these and can detect minute vibrations (
The cantilever type shown in c) is used, and different methods are used depending on the frequency, the magnitude of acceleration, the measurement range, etc.

[Problems to be Solved by the Invention] Incidentally, in the case of the cantilever-type detection section that is advantageous mainly for detecting low-frequency acceleration, one end of the piezoelectric element is fixed to the support S. It is difficult to realize the conditions (for example, the condition that the distortion of the fixed part be zero), and as a result, there is a problem that the frequency characteristics and sensitivity are difficult to stabilize. In particular, P b (Z r, T
i) Os-based (abbreviated as PZT), PbTi0a,
l3aTiO, (Pb, La) (Zr.

Although general ceramic piezoelectric materials such as 'l'i) O, (abbreviated as P L ZT) have excellent rigidity, they are brittle and easily chipped, so they have poor workability. In addition, it is weak against impact and easily breaks, so it is unsuitable for the cantilever type.

By the way, in addition to the ceramics mentioned above, piezoelectric materials include polymeric materials such as polyvinylidene fluoride, and compared to ceramics, these polymeric materials have (a) less flexibility; .

(b) It has excellent processability and can be made into a thin film and large area.

(c) Since the dielectric constant is small, the voltage output constant (g) is large.

(d) Excellent insulation properties.

For this reason, it is considered to be suitable for the above-mentioned cantilever type, but it has the disadvantage that it is difficult to obtain stable output due to its low rigidity.

Furthermore, since each of the piezoelectric acceleration sensors mentioned above is constructed with an exposed detection section, it is susceptible to sudden changes in the external environment, such as temperature and humidity, and changes in output characteristics over time due to condensation, etc. There was also the problem that it was easy to wake up and could not withstand long-term use.

The present invention has been made in view of the above circumstances, and aims to take advantage of the features of the "cantilever beam type" described above, overcome its drawbacks, and maintain stable output characteristics and frequency characteristics over a long period of time. The purpose of this research is to provide a highly practical piezoelectric acceleration sensor that can perform

[Means for Solving the Problems] The present invention provides a curved detection section in which the detection section is arranged on a pedestal in which a recess is formed so that its concave surface is directed toward the pedestal side and it runs parallel to the recess. A diaphragm, a polymer-based piezoelectric element bonded to the concave surface of this diaphragm, and a diaphragm and a polymer-based piezoelectric element. 'A protective film that covers the IX strand and protects the diaphragm and the polymer piezoelectric element from the external environment, and one end of which is fixed to the recess of the pedestal and extends through the center of the diaphragm. It is characterized by comprising a column and a fixing part fixed to the other end of the column, and the fixing part fixes the diaphragm to the pedestal in a flat state and storing a spring force. shall be. It is more preferable if the relationship between the Young's modulus EB% moment of inertia of the diaphragm and the second moment of inertia of the Young's modulus Eps of the protective film ip is determined to satisfy the following: .

[Function] In the above configuration, if the stiffness of the diaphragm, that is, the deformation stiffness, is made sufficiently larger than the deformation stiffness of the polymer piezoelectric element, the vibration of the detection unit is determined by the vibration mode of the diaphragm. Therefore, stable output can be obtained, and the output characteristics of the polymer piezoelectric element are not impaired.

Furthermore, by setting the deformation stiffness of the protective film to a low level that does not impair the vibration mode of the detection part that depends on the diaphragm, we can reduce the pressure of the diaphragm and polymer system without impairing the original frequency characteristics and output characteristics of the detection part. ? The FF element can be protected from the external environment. In addition, the protective film also prevents the polymer piezoelectric element from peeling off from the diaphragm.

[Example] The present invention will be described in detail below with reference to FIGS. 1 and 2.

FIG. 1 shows a detection section 1 of an acceleration sensor according to the present invention. In the figure, reference numeral 2 is a thin rectangular parallelepiped base with a rectangular recess 2a formed in the center, and 3 is a rectangular plate spring that is usually curved at a predetermined curvature, with a circular hole 3a in the center.
is a fixed diaphragm.

119 In the center of the recess 2a of the pedestal 2, there is a cylindrical support 4.
However, its axis is perpendicular to the bottom surface of the recess 2a, and it is fixed by a fixing means such as screwing.

The length between the curved ends 3b when the diaphragm 3 is curved (indicated by Q in FIG. 1...hereinafter, this length will be referred to as "crossing")
) is set slightly larger than the length of the recess 2a formed in the base 2. In addition, the concave surface 3 of the diaphragm 3
Two rectangular piezoelectric films (piezoelectric elements) 5, one on each side, are attached to both sides of the circular hole 3a using an adhesive or the like. This piezoelectric film 5 has electrodes coated on both sides of a thin film-like base material made of a polymeric material by a thin film forming method such as vapor deposition.

The serious crime diaphragm 3 is preferably formed of a material with a high Young's modulus, and is mainly made of a single metal such as iron, copper, nickel, etc.
Alternatively, a metal material made of an alloy such as brass or stainless steel is used, but a composite material with plastic such as glass fiber or carbon fiber is also suitable from the viewpoint of a high Young's modulus.

The base material constituting the piezoelectric film 5 is made of a polymeric material as described above, and examples of the material include polyvinyl fluoride, polyvinylidene fluoride (
P V D F ), polyvinyl chloride, nylon 9, nylon 11. Polycarbonate, poly(m-phenylene isophthalamide) vinylidene fluoride-ethylene tetrafluoride copolymer, vinylidene fluoride-vinyl fluoride (polymer, vinylidene fluoride-ethylene trifluoride copolymer, vinylidene cyanide) Vinyl acetate” (polymer, or
Mixtures of two or more of these or mixtures of these and other thermoplastic resins are suitable. In addition to this, P
b(Z r, T i) 03, P bT io a, (+
' b, L aXZ r, T 1) 03, na'l'i
o++, r3a (Zr, Ti) Os, (r3a, Sr)
It is also possible to use a mixture of fine powder of an inorganic piezoelectric material such as 'I'' iol in a polymer such as a thermoplastic resin or a thermosetting resin.

Further, between the diaphragm 3 and the piezoelectric film 5, the diaphragm 3
The following relationship holds true: "Deformation stiffness" of the piezoelectric film 5 "One deformation stiffness of the piezoelectric film 5" where Ell: Young's modulus of the diaphragm, EP: Young's modulus of the piezoelectric film, !B: second moment of area of the diaphragm, ■P: second moment of area of piezoelectric film,
In addition, "deformation stiffness" means Young's modulus EX moment of inertia! The moment of inertia of area (2) is expressed as t3 (d: width of cross section in the bending direction, t: thickness).

Further, the diaphragm 3 to which the piezoelectric film 5 is bonded is able to maintain its diaphragm 3 and the piezoelectric film 5 in an external environment such as temperature
It is covered with a protective film 6 that protects it from sudden changes in humidity and the like. As the material used for this protective film 6, thermosetting resins such as epoxy resins and silicone resins and photocuring resins are used alone or in the form of solvent paints.
In addition to these, we also use thermoplastic resins such as polyethylene, solvent paints of inorganic materials such as 5ins, and thin films made by vapor deposition or sputtering of organic materials such as polyethylene, polyparaxylylene, and inorganic materials such as 5iO1. Good too.

And, between the diaphragm 3 and the protective film 6, the "deformation stiffness" of the diaphragm, the "deformation stiffness" of the protective film, Ea: Young's modulus of the diaphragm, ! ]! : Young's modulus of the protective film, ■B: Second moment of area of the diaphragm, 11: Second moment of area of the protective film. In this case, 1+, that is, the moment of inertia of the area of the protective film 6 is the same as that of the diaphragm 3 or the piezoelectric film 5 described above.

d: width of the protective film in the bending direction; t: thickness of the protective film in the bending direction; This is the value found by

As shown in FIG. The supports tt, 4 are arranged so as to be engaged with each other so as to straddle the recess 2a, and the supports 4 are inserted through the circular holes 3a.
The diaphragm 3 is fixed to the pedestal 2 in a state where it is pressed so as to be ``+l'' by a fixing part 7 fixed to the tip end of the diaphragm 3. As a result, the diaphragm 3 is in a state where spring force is stored.Fixing part 7 is fixed to the support column 4, for example, by a fixing means such as a screw.

The detection unit 1 configured as described above is housed in a conductive shield case 8.

Further, a terminal (not shown) is attached to the electrodes coated on both sides of the piezoelectric film 5 to take out a signal emitted by the vibration of the diaphragm 3, and the output signal from this terminal is As shown in FIG. 2, it is measured via an impedance conversion circuit 9. Note that this impedance conversion circuit 9 and the surface terminal are connected by a low noise cable IO, and the shield case 8 is grounded.

Next, the operation of the two-leg detection section 1 will be explained.

The piezoelectric film 5 receives an external force (vibration) and generates a distortion (deformation) proportional to the acceleration of the force.
However, when the diaphragm 3 and the piezoelectric film 5 are bonded together and integrated with an adhesive, the ratio of the "deformation stiffness" of the diaphragm 3 to the "deformation stiffness" of the piezoelectric film 5 is expressed by the above formula (a
), the diaphragm 3 is connected to the piezoelectric film 5.
Since h < l is 0 times or more, the diaphragm 3 vibrates without being constrained by the piezoelectric film 5, and the piezoelectric film 5 generates an output according to the vibration of the diaphragm 3, and the generated output is It is steeped in stability. In addition, the piezoelectric film 5 made of the polymeric material has a low Young's modulus unlike ceramics, so it can be easily adhered to the diaphragm 3 with an adhesive. No deterioration of output characteristics occurs.

Since the diaphragm 3 and the piezoelectric film 5 are separated by the protective film 6, the diaphragm 3 vibrates under the constraints of the protective film G. 1 enough compared to the "deformation stiffness" of the diaphragm 3: If it is set small, the restriction that the protective film 6 exerts on the vibration of the diaphragm 3 will be negligible, and therefore the addition of the protective film 6 is Pressure caused by? No reduction in the output of the Ili film 5 occurs. In the case of this embodiment, the ratio of "deformation stiffness" is such that the "deformation stiffness" of the protective film 6 is less than 1/20 of the "deformation stiffness" of the vibration temporary 6, as shown in the above equation (b). Therefore, the diaphragm 3 and the piezoelectric film 5 can be exposed to sudden changes in the external environment, such as temperature and humidity, without any loss in the excellent frequency characteristics and output characteristics obtained by laminating the diaphragm 3 and the piezoelectric film 5 together. This makes it possible to protect the detection unit 1 from changes, improve the environmental resistance of the detection unit 1, and prevent the occurrence of changes over time. In addition, the diaphragm 3 is protected by the protective film 6.
Since peeling of the piezoelectric film 5 is also prevented, the life of the detection unit 1 is further extended, and stable frequency characteristics and output characteristics can be obtained over a long period of time.

Next, the piezoelectric acceleration sensor of this embodiment shown in FIG.
After keeping the conditions of the diaphragm 3 and the piezoelectric film 5 constant, 2FIj types were manufactured by changing the deformation stiffness of the protective film 6 within the range of the above formula (b), and as a comparative example, the diaphragm 3 and the piezoelectric film 5 were After maintaining the film 5 under the same conditions as in the example, the film without a protective film and the deformation stiffness were calculated using the above formula (b
) were fabricated, and each sensor was subjected to a prescribed heat cycle test and drop test. The results are explained in Attached Table 1 for 7+C. The test conditions at this time are as follows.

■Sensor manufacturing conditions ・Vibration plate Material: 18-8 stainless steel Dimensions: 100μm thick, 10mm wide.

Width: 20 mm, radius of curvature: 38 mm Young's modulus Ea: 2. OX I O” Pa・Pressure α
Film base material: Polyvinylidene ester stretched film Dimensions: Width 10
mm, length 6 mm, thickness 30 μm pressure U constant ds
+: 20 p c/N Young's modulus Ep: 2.3x
10" Pa・Protective film material: Epoxy resin Young's modulus E+: 3.0 Seikle loss: 5
0 cycle ■ Drop test conditions The sensor is dropped 100 times from a height of 1 m above the concrete floor, and the output after the test is ±5% of the output before the test.
The number of items with the above differences was counted.

■Sensor excitation frequency: 201 (z) As is clear from Attached Table 1, in Comparative Example 1 without a protective film, the output decreased to 70% after the heat cycle test, and it is difficult to resist changes in the external environment. and have poor durability,
It can be seen that the output characteristics inevitably deteriorate during long-term use. In contrast, Example 1 and Example 2 with a protective film
Regarding Comparative Example 2, there was almost no decrease in output even after the heat cycle test, and by covering the diaphragm and piezoelectric film with a protective film, stable output characteristics could be maintained over a long period of time. In addition, in Comparative Example 1 without a protective film, 2 out of 20 had abnormal output after the drop test, whereas with the protective film, no output abnormality occurred; It can be seen that impact resistance has been improved.

Next, FIG. 4 shows the output measurement results when each of the above-mentioned sensors was vibrated by successively changing the frequency within a low frequency region up to a frequency of around 200112.

It was confirmed that in Examples 1 and 2, where the deformation stiffness of the protective film was within the range of formula (b) above, and Comparative Example 1, which did not have a protective film, stable output was maintained in the entire frequency range. On the other hand, in Comparative Example 2, where the deformation stiffness of the protective film exceeds the range of formula (b) above, it can be seen that the protective film adversely affects the vibration mode of the diaphragm, resulting in a deterioration of the frequency characteristics. .

In the above embodiment, the number of piezoelectric films 5 bonded to the diaphragm 3 is two, but the number is not limited to this and may be at least one. It is arbitrary to divide it into pieces.

[Effects of the Invention] As explained above, according to the pressure 7r1ffi acceleration sensor of the present invention, pressure? Since the lli element emits an output in accordance with the vibration of the diaphragm, the stability of the output of the detection section relative to the frequency can be improved.In addition, since the diaphragm and piezoelectric element are covered with a protective film, It has improved environmental resistance, prevents the diaphragm and piezoelectric element from peeling off, improves impact resistance, and maintains stable output and frequency characteristics over a long period of time, making it extremely practical. .

If the relationship between the Young's modulus EB and the moment of inertia of the area 8 of the diaphragm and the Young's modulus E1 and the moment of inertia of the area I+ of the protective film is determined to satisfy the following, the output from the protective film is A piezoelectric acceleration sensor with higher practicality can be provided without causing any deterioration of characteristics or frequency characteristics.

Margin below.

Attached table [1 piece

[Brief explanation of the drawing]

Fig. 1 is a side sectional view showing the detection part of the piezoelectric acceleration sensor of the present invention, Fig. 2 is a circuit diagram in which the detection part is incorporated, and Figs. 3 (a) to (c) show conventional piezoelectric acceleration. FIG. 4 is a cross-sectional view showing the structure of the detection part of the sensor, and is a diagram showing frequency characteristics of an example of the present invention and a comparative example. l...Detection unit, 2...Pedestal, 2a.
...Recess, 3...Diaphragm, 3c...
... Concave surface, 4 ... Support column, 5 ... Piezoelectric film (piezoelectric element), 6 ... Protective film, 7 ...
···Fixed part.

Claims (2)

[Claims] (1) A piezoelectric acceleration sensor that detects acceleration from an amount of electricity generated as a result of distortion of a piezoelectric element provided in a detection part, wherein the detection part a curved diaphragm arranged to face the pedestal side and straddle the recess; a polymer piezoelectric element adhered to the concave surface of the diaphragm; a protective film that protects the diaphragm and the polymer-based piezoelectric element from the external environment; a support whose one end is fixed to the recess of the pedestal and passes through the center of the diaphragm; A piezoelectric acceleration sensor comprising a fixing part fixed to an end part, the fixing part fixing the diaphragm to the base in a flat state and storing a spring force. (2) A relationship such as E_B・I_B/E_I・I_I≧20 exists between the Young's modulus E_B and the second moment of inertia I_B of the diaphragm and the Young's modulus E_I and the second moment of inertia I_I of the protective film. Claim 1 characterized by
The piezoelectric acceleration sensor described in .