JPH032668A - Piezoelectric acceleration sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a piezoelectric acceleration sensor using a film-like piezoelectric material, which has a simple structure and high output, and is particularly suitable for use in the direction perpendicular to the sensing axis. This invention relates to a piezoelectric acceleration sensor whose output due to acceleration is minute.

[Conventional technology]

Conventional piezoelectric acceleration sensor (hereinafter abbreviated as sensor).

) is shown in FIG. 17. This sensor is disclosed in Japanese Patent Application Laid-Open No. 56-10258, in which a disc-shaped vibrating membrane 1 made of a piezoelectric material such as a piezoelectric polymer is fixed to an annular frame 2 at its peripheral edge, and the vibrating membrane l
A load body 3 functioning as an inertial mass is provided on both sides of the center, and a frame body 2 is fixed to a pedestal 4.

In this sensor, the acceleration sensing axis G is an axis that is perpendicular to the membrane surface of the vibrating membrane I and passes through the center of the load body 3.

With a sensor like this, by attaching the pedestal 4 to the object to be measured, it is possible to detect changes in the acceleration of the object to be measured in the direction of the sensing axis G.

[Problem to be solved by the invention]

However, in this sensor, even when acceleration is applied in a direction perpendicular to the direction of the sensing axis G, the load body 3 is displaced in that direction, causing distortion in the vibrating membrane l and producing an electrical output. There was a drawback.

There is also the disadvantage that the structure is complex and manufacturing is troublesome.
Another drawback was that the measurable frequency band was narrow and it was difficult to change it.

In order to eliminate such drawbacks of conventional sensors, the present inventors developed a pedestal that is rigidly attached to the object to be measured, a piezoelectric film that is fixed to a measurement surface perpendicular to the sensing axis of the pedestal, and
It is composed of a rigid load body that is fixed on the membrane piezoelectric body and acts as an inertial mass part, and the planar shape of the membrane piezoelectric body has the sensing axis as the center of symmetry in a plane parallel to the measurement surface. It is point symmetric, and the planar shape of the surface in contact with the membrane piezoelectric material of the load body is a point S, which has the sensing axis as the center of symmetry, and there are countless points passing through the sensing axis and perpendicular to the measurement surface. He has devised a sensor that is characterized by line symmetry with the sensing axis as the axis of symmetry for all cross sections when cut along the plane of , and has previously applied for a patent.

Such a sensor is therefore extremely simple in construction;
It has the advantage that the output when acceleration is applied in a direction perpendicular to the sensing axis is extremely small, and the measurable frequency band is wide.

However, even in this new type of sensor, it was necessary to solve the following disadvantages. In other words, in order to increase the voltage output of the sensor, it is sufficient to increase the thickness of the film piezoelectric material, but with a thick film piezoelectric material, it is difficult to perform poling processing, and the orientation of the dipoles may be affected. Unfortunately, the piezoelectric constant may become small or its value may vary.

Furthermore, in order to increase the output sensitivity as a charge, it is sufficient to increase the area of the membrane piezoelectric material, but increasing the area of the membrane piezoelectric material leads to an increase in the size of the sensor, and it is necessary to make the sensor smaller. It didn't meet my request.

[Means to solve the problem]

This invention includes a pedestal that is rigidly attached to an object to be measured, a piezoelectric film fixed to a measurement surface perpendicular to the sensing axis of the pedestal, and a piezoelectric film fixed to the piezoelectric film that has an inertial mass. The piezoelectric membrane has a load body made of a rigid body that acts as a part, and the membrane piezoelectric body has a planar shape that is point symmetrical with respect to the sensing axis as the center of symmetry in a plane parallel to the measurement surface, and two or more pieces are The load body has a laminated structure and is electrically conductive at the interface between these laminations, and the planar shape of the surface in contact with the membrane piezoelectric material is point symmetrical with the sensing axis as the center of symmetry, and The above-mentioned problem has been solved by a sensor that, when cross-sectioned through countless planes passing through the sensing axis and perpendicular to the measurement surface, has linear symmetry with the sensing axis as the axis of symmetry for all cross-sections.

The present invention will be explained in detail below.

FIG. 1 shows an example of the sensor of the present invention, and the reference numeral 11 in the figure is a pedestal. This pedestal 11 forms the base of the sensor, is rigidly attached to the object to be measured, and is made of a material having sufficient rigidity, such as steel, brass, aluminum, etc. Further, it is desirable that the elastic modulus of the material forming the pedestal 11 is higher than that of the film-like piezoelectric material described later, and the thickness of the pedestal 11 is several times that of the film-like piezoelectric material.

The pedestal 11 here has a cylindrical shape,
The shape is not limited to this, and may be a plate shape, a rectangular parallelepiped, or the like.

One surface of this pedestal 11 is a flat and smooth measurement surface 1.
2. This measurement plane 12 needs to be a vertical plane that is exactly perpendicular to the acceleration sensing axis G of this sensor.

On the measurement surface 12 of this pedestal 11, two membrane-like piezoelectric materials 1 are placed.
3 and 13 are stacked on top of each other and firmly fixed to the base 11.

The film-like piezoelectric body 13 is made of a piezoelectric material and has a thickness of 1
A film-like material with a thickness of 0 to 100 μm, sufficiently uniform in thickness, and sufficiently homogeneous as a whole is used. Piezoelectric materials include polyvinylidene fluoride, polyvinylidene chloride, polyvinyl fluoride, polyvinyl chloride, nylons such as nylon 11 and polymetaphenylene isophthalamide, tetrafluoroethylene, trifluoroethylene, vinyl fluoride, etc. and copolymers of vinylidene fluoride, vinyl acetate, vinyl propionate, vinyl benzoate, etc., and vinylidene heptaanide, blend polymers of polyvinylidene fluoride and polycarbonate, polyvinylidene fluoride and polyvinyl fluoride, etc. In addition to Bommer-based polymers such as blended polymers, piezoelectric materials such as metal titanate, metal titanate zirconate, and other piezoelectric grain powders are added to and dispersed in polymers.

These two film-like piezoelectric bodies 13.13 have the same shape and the same piezoelectric properties, and are made so that their surfaces are electrically conductive using a conductive adhesive or the like. It has a bonded and laminated structure, and electrodes such as aluminum foil (not shown) for outputting output are provided on both sides of the laminated body. The laminate of the two film-like piezoelectric bodies 13.13 is fixed to the pedestal 11 using a hardening adhesive such as an Evoquin adhesive. The thicknesses of the two film-like piezoelectric bodies 13.13 may be the same or different.

The planar shape of the piezoelectric film I3 is important for reducing crosstalk.

Crosstalk in this invention refers to the sensing axis G of the sensor.
The ratio of the output P1 when receiving acceleration in the direction to the output P when receiving acceleration in the direction perpendicular to the sensing axis G, P, /
It is expressed as P.

The planar shape of the piezoelectric film 13 must be point symmetrical with respect to the sensing axis G as the center of symmetry in a plane parallel to the measurement surface 12. The example shown in FIG. 1 is circular, but other planar shapes that satisfy the above conditions include those shown in FIGS. 2 to 7, for example. 2 is a parallelogram, FIG. 3 is a square, FIG. 4 is an ellipse, FIG. 5 is a regular hexagon, FIG. 6 is a hexagon, and FIG. 7 is a torus. In these figures, the symbol G indicates the sensing axis G.

All of these planar shapes are point symmetrical with respect to the sensing axis G as the center of symmetry. Of course, planar shapes other than these can also be used if the above conditions are met.

A load body 14 made of a rigid body and functioning as an inertial mass section is integrally fixed onto the laminate of such film-like piezoelectric bodies 13.13. This load body 14 is displaced in response to acceleration and causes strain or stress on the membrane piezoelectric body 13.13, and its weight is related to the electrical output per unit acceleration force of the sensor, so it is particularly limited. Not that, but
It is set within a range that does not cause creep in the piezoelectric film 13.13. The fixation of the laminate of the load body 14 and the piezoelectric film 13.13 is similar to the fixation of the pedestal 11 and the piezoelectric film 13.

Furthermore, the three-dimensional shape of the load body 14 is important for reducing cross strokes.

First, the surface of the load body 14 that is in contact with the laminate of the piezoelectric film 13.13 (hereinafter referred to as the bottom surface) is exactly perpendicular to the sensing axis G, and the planar shape of the bottom surface is parallel to the sensing axis G. It must be symmetrical about the center of symmetry. Therefore, as the shape that satisfies this condition, the shapes shown in FIGS. 2 to 7, for example, can be adopted, similar to the planar shape of the film-like piezoelectric body 13 described above. However, in the combination of the membrane piezoelectric body 13 and the load body 14, the planar shape of the bottom surface of the load body 14 and the plane shape of the membrane piezoelectric body 13 do not necessarily have to be the same. The planar shape of the load body 14 may be a square, and the bottom surface of the load body 14 may have a circular planar shape, as long as the sensing axes G are the same, as will be described later.

At the same time, when the load body 14 is cross-sectioned by countless planes passing through the sensing axis G and perpendicular to the bottom surface, it is necessary that all the cross sections have line symmetry with the sensing axis G as the axis of symmetry. Figures 8 to 14 satisfy this condition of line symmetry.
There is one shown in the figure.

The one shown in Figure 8 is plate-shaped, the one in Figure 9 is columnar, the one in Figure 10 is cone-shaped, the one in Figure 11 is a sphere cut out on a plane, and the one in Figure 12 is a clear circle. The body is cut out on a plane, the one in Figure 13 has a columnar shape with a space inside, and the one in Figure 14 is a combination of a column and a plate. In these figures, the symbol S indicates the bottom surface, and the symbol G
is the axis of symmetry that coincides with the sensing axis. Furthermore, the load body 14 that satisfies this condition of line symmetry has its center of gravity located on the sensing axis G.

In addition, the load body 14 can be made of a composite material made of different materials in addition to being made of the same material as a whole, but in this case, each material is firmly fixed and the whole is rigid. It is necessary that each of them be unique, and that each of them must not cause different displacements when subjected to acceleration.

Under these conditions, the load body 14 having symmetry has its axis of symmetry aligned with the center of symmetry of the laminate of the membrane piezoelectric bodies 1313. In other words, the membrane piezoelectric bodies 13.13 are aligned on the sensing axis G. The center of symmetry of the laminate and the axis of symmetry of the load body 14 are aligned and fixed.

Such a sensor is used by attaching its pedestal 11 to an object to be measured, thereby creating an opening for measuring acceleration in the direction of its sensing axis G.

15 and 16 are both connection diagrams for extracting the output of the sensor of this example. In the example shown in FIG. 15, the electrodes on one side and the electrodes on the other side of a stack of two membrane piezoelectric bodies 13.13 are used as output output electrodes, respectively. By making these connections, a series connection is achieved, which corresponds to using a film-like piezoelectric material twice the thickness. In addition, the example shown in FIG.
Electrodes on both sides of the laminate of the two membrane piezoelectric bodies 13.13 are connected to each other to form one output extraction electrode, and a conductive adhesive is applied to the laminated interface of the two membrane piezoelectric bodies 13.13. This layer is used as the other output extraction electrode, and by making such connections, it becomes equivalent to using a film-like piezoelectric material with twice the area. In addition, Fig. 15 and Fig. 1
In Figure 6, arrows indicate the direction of the dipole.

In the sensor with this configuration, the pedestal 11 and the film-like piezoelectric material 1
Since the laminate of 3.13 and the load body 14 are simply laminated, the structure is simple, the manufacturing capacity S is easy, and miniaturization is also possible.

Further, the planar shape of the membrane piezoelectric body 13 is point symmetrical with respect to the sensing axis G, and the planar shape of the bottom surface of the load body 14 is point symmetrical with the sensing axis G as the center of symmetry, and at the same time, the load body 1
Since the three-dimensional shapes of No. 4 are all line-symmetrical in a plane passing through the sensing axis G with the sensing axis G as the axis of symmetry, crosstalk is slight.

Generally, when acceleration is applied to a sensor in a direction other than the direction of its sensing axis, it is divided into at least two components, one in the direction perpendicular to the sensing axis and the other in the direction of the sensing axis, according to the law of vector decomposition. This component in the direction perpendicular to the sensing axis acts on the center of gravity of the load body 14, and a bending moment about the center of gravity acts on the load body 14. Therefore, a compressive force acts on a part of the piezoelectric film 13, and a tensile force acts on the remaining part. In the piezoelectric film 13, the compressive force and the tensile force generate charges of opposite signs, or if the amounts of charges are equal, they cancel each other out and no output is output. Therefore, if a compressive force and a tensile force of equal magnitude act on the piezoelectric film 13, the output from the piezoelectric film 13 becomes zero, and acceleration in directions other than the sensing axis direction is no longer detected.

In this invention, since the shapes of the piezoelectric film 13 and the load body 14 have the above-mentioned symmetry, the piezoelectric film 13 is not affected even if acceleration is applied in a direction other than the direction of the sensing axis G. Since compressive force and tensile force of equal magnitude act on the piezoelectric film 13, there is no output from the piezoelectric film 13, and crosstalk becomes extremely small.

Further, this sensor has a high upper limit of its measurable frequency and a wide measurable frequency band.

The upper limit of the measurable frequency of this type of sensor is determined by the sensor's resonant frequency. Due to its structure, the resonance frequency of the sensor in this invention is determined by the resonance frequency of the membrane piezoelectric material 13. +3, adhesive layer,
Since it is proportional to the value obtained by dividing the elastic modulus of the electrode etc. by the quality m of the load body 14, it is higher than the resonant frequency of the conventional vibration model sensor by more than 2 Ni, and is on the order of kilohelp. However, it should be noted that as the elastic modulus of the adhesive layer decreases, the resonance frequency decreases.

Therefore, in the case where an adhesive is used to fix the laminate of the membrane piezoelectric material 13.13 to the pedestal 11 and the load body 14, the elastic modulus of the adhesive layer is EA, the thickness is t, 1
When the elastic modulus of 3 is EP and the thickness is [, it is necessary to satisfy the relationship expressed by the following formula.

(E A/ t A) / (E p/ j p)≧0
1 What this formula means is that the load body 1
This is a condition for the force generated in 4 to be transmitted to the membrane piezoelectric material 13 without being absorbed and relaxed by the adhesive layer, and if the value of the above equation is less than 0.1, the absorption and relaxation by the adhesive layer cannot be ignored. (As mentioned above, the resonant frequency decreases, narrowing the measurable frequency band.

Note that the thickness of the adhesive layer in the above formula refers to the thickness of all adhesive layers existing between the pedestal 11 and the load body 14. In addition, if the type of adhesive is different and the modulus of elasticity is different, calculate the ratio of the modulus of elasticity and thickness of each adhesive layer,
All you have to do is add this up and substitute it into the above formula.

Therefore, the adhesive should be selected from Evoquin, phenol, and cyanoacrylate adhesives that are hardened 142 and have a high elastic modulus, and adhesive types such as Comb adhesives are inappropriate. Moreover, a conductive adhesive can also be used.

Moreover, in this sensor, two membrane-like piezoelectric bodies 13.
13 are laminated in a conductive state, the detection output is large. That is, if the electrodes are connected as shown in FIG.
If they have the same piezoelectric properties, the voltage generated between the output extraction electrodes will be twice that when there is only one sheet. In addition, if the electrodes are connected as shown in Figure 16, they will be connected in parallel, which corresponds to a membrane piezoelectric material with twice the area, and the electric charge generated between the output extraction electrodes will be doubled. It becomes possible to increase the detection output.

Hereinafter, specific examples will be shown to clarify the effects.

(Example 1) An aluminum plate with a thickness of 5 mm was used as a member to serve as a pedestal. Also, the thickness is 100 μm, and one side is 101aI11
Two square piezoelectric films made of polyvinylidene fluoride were bonded together using a conductive Evoquin adhesive, and aluminum vapor-deposited electrodes were provided on both sides. This laminated film-like piezoelectric body was bonded to a pedestal with an epoxy adhesive, and furthermore, a brass load body having a square bottom surface of 10 mm and a weight of Log was bonded thereon with an epoxy adhesive with the axes of symmetry aligned.

Then, wiring was made to take out the output as shown in FIG. 15 to prepare a sensor.

(Example 2) The same procedure as in Example 1 was carried out except that a square load body with a bottom surface of 14 mm and a weight of 120 g made of brass was used, and the wiring for taking out the output was made as shown in Fig. 16. It was used as a sensor.

(Comparative example)) In Example 1, the film-like piezoelectric material had a thickness of 200 μm,
A sensor was prepared in the same manner except that one piece of polyvinylidene fluoride having a square shape of 10 mm on each side was used. however,
Output was extracted using electrodes on both sides of the piezoelectric film.

(Comparative Example 2) In Example 2, the film-like piezoelectric material had a thickness of 100 μm,
A sensor was prepared in the same manner except that one piece of polyvinylidene fluoride having a square shape of 14 mm on one side was used. however,
Output was extracted using electrodes on both sides of the piezoelectric film.

The output of these sensors was measured and evaluated.

For the sensors of Example 1 and Comparative Example 1, the output was connected to an impedance conversion circuit and output as a voltage. Furthermore, regarding the sensors of Example 2 and Comparative Example 2,
The output was output as a charge by a charge amplifier. Twenty sensors of each type were created and evaluated, and the average value and variation of their outputs are shown in the following table.

As is clear from the table, when Example 1 and Comparative Example 1 are compared, Example I is clearly superior in terms of output and variation.

Further, when comparing Example 2 and Comparative Example 2, the performance is the same, but in terms of size, the Example is smaller and superior.

In addition, for both sensors, the crosstalk is in the range of 3 to 5%, and the measurable frequency band is Q, l
It was Hz~2KZ.

〔Effect of the invention〕

As explained above, the piezoelectric acceleration sensor of the present invention includes a pedestal rigidly attached to an object to be measured, a film-like piezoelectric material fixed to a measurement surface perpendicular to the sensing axis of the pedestal, and a piezoelectric film-like It has a load body made of a rigid body that is fixed on the body and acts as an inertial weight part, and the membrane piezoelectric body has a planar shape that is point symmetrical with the sensing axis as the center of symmetry in a plane parallel to the measurement plane. The load body is made of a laminated groove structure in which two or more sheets are laminated, and the interface between these laminations is in a conductive state. The structure is symmetrical with respect to the center, and when cut through countless planes passing through the sensing axis and perpendicular to the measurement surface, all cross sections are line symmetrical with the sensing axis being the fi axis. is simple, easy to miniaturize, provides high output, and has extremely little crosstalk. Furthermore, the measurable frequency band is wide, it is easy to design a measurement application, and there is a large degree of freedom in design.

[Brief explanation of drawings]

FIG. 1 is an exploded perspective view showing an example of a piezoelectric acceleration sensor of the present invention, FIGS. Figures 1 to 14 are all cross-sectional views showing examples of the three-dimensional shape of the load body used in the present invention, and Figures 15 and 16 are cross-sectional views showing examples of the three-dimensional shape of the load body used in this invention. FIG. 17 is a schematic configuration diagram showing an example of a conventional piezoelectric acceleration sensor. Fig. 1 1...Pedestal, 2...Measurement surface,...Sensing axis, 3...Membrane piezoelectric material, 4...Loading body.

Claims (2)

[Claims] (1) A pedestal that is rigidly attached to the object to be measured, a piezoelectric film that is fixed to the measurement surface perpendicular to the sensing axis of the pedestal, and a piezoelectric film that is fixed to the piezoelectric film and acts as an inertial mass part. The membrane piezoelectric material has a load body made of a rigid body, and the planar shape of the film piezoelectric material is point symmetrical with respect to the sensing axis as the center of symmetry in a plane parallel to the measurement surface, and the film piezoelectric material is a laminated layer in which two or more sheets are laminated. The load body has a structure in which the plane shape of the surface in contact with the membrane piezoelectric material is symmetrical with respect to the sensing axis as the center of symmetry, and the plane shape of the surface in contact with the membrane piezoelectric material is symmetrical with respect to the sensing axis. A piezoelectric acceleration sensor, characterized in that, when cut through countless planes perpendicular to the measurement surface, all the cross sections are symmetrical with respect to a sensing axis as an axis of symmetry. (2) In the piezoelectric acceleration sensor according to claim (1), the film-like piezoelectric body is fixed to the pedestal and the load body with an adhesive, the thickness of the adhesive layer is t_A, the elastic modulus is E_A,
A piezoelectric acceleration sensor characterized in that the thickness of the piezoelectric film is t_p and the modulus of elasticity is E_p, and the following relationship is satisfied. (E_A/t_A)/(E_p/t_p)≧0.1