JP2007107990A - Acceleration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acceleration sensor having excellent mass productivity and a high output voltage. <P>SOLUTION: At least one or more piezoelectric elements wherein a spontaneous polarization direction is parallel to an electrode respectively are arranged so that the electrode surface of each piezoelectric element becomes parallel. This sensor has a constitution wherein a weight receiving an inertia force by an acceleration is connected to a free end face of the piezoelectric element, and piezoelectricity is generated by applying a shearing stress to the piezoelectric element. A plate material wherein the spontaneous polarization direction is the same as the plate thickness direction is grooved to form an electrode, and thereby a plurality of piezoelectric elements can be formed easily, and an output voltage is heightened by connecting in series the piezoelectric elements. Directivity of the acceleration detection direction can be heightened by using a material classified as a point group of 4 mm or 6 mm as a piezoelectric material. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an acceleration sensor that detects a force acting on a weight by acceleration and extracts it as an electrical signal.

  Conventionally, acceleration sensors are mounted on mobile PCs, game machines, and automobiles, and are widely used as sensors for HDD protection, mechanical quantity monitoring, collision detection, and suspension control when dropped.

  In particular, an acceleration sensor using a piezoelectric body has a simple measurement principle and high reliability, and has a track record of application in heavy duty applications such as a knocking sensor for an automobile engine. Various types of structures are disclosed in these acceleration sensors, but in principle, the force acting on the weight, which is an inertial body by the action of acceleration, is provided on the piezoelectric body constituting the flexible section or the flexible section. Piezoelectricity generated in the piezoelectric body is detected and is roughly classified into three types based on the detection principle of the piezoelectric body.

  The first method is a 33-mode acceleration sensor that detects expansion and contraction in the spontaneous polarization direction of the piezoelectric body as piezoelectricity. As described in Patent Document 1, an electrode is formed perpendicular to the spontaneous polarization direction, and a weight serving as an inertial body vibrates parallel to the spontaneous polarization direction. That is, acceleration in the same (parallel) direction as the spontaneous polarization is detected.

  The second method is a 31 (= 32) mode acceleration sensor that detects in-plane expansion and contraction perpendicular to the spontaneous polarization direction of the piezoelectric body as piezoelectricity. As described in Patent Document 2, a cantilever beam is formed of a piezoelectric body (usually composed of bimorph), and a stress state (compression, tension) generated on the front and back of the beam by a bending moment is detected as piezoelectricity. It is. Also in this case, an electrode is formed perpendicular to the spontaneous polarization direction, and the acceleration in the spontaneous polarization direction is indirectly detected.

  In addition, as disclosed in Patent Document 3, a piezoelectric body may be formed on a support that constitutes a beam.

The third method is a 15 (= 24) mode acceleration sensor that detects the thickness shear vibration of the piezoelectric body. This method is characterized in that an electrode is formed on a surface parallel to the spontaneous polarization direction of the piezoelectric body as shown in Patent Document 4 and Patent Document 5, and a weight is connected to the electrode surface. Similarly, the acceleration in the polarization direction is detected.
JP 05-333045 A Japanese Patent Laid-Open No. 11-183510 JP-A-11-108951 JP 2002-022761 A JP 2002-162408 A

  There are three independent components of a piezoelectric material having a point group of 4 mm, such as PZT, which are currently commercially available, d31 (= d32), d33, and d15 (= d24). In PZT called a low-Q material or soft material having a relatively large d constant, k15 (= k24) shows the highest electromechanical coupling coefficient, and 15 (= 24) using d15 (= d24). ) Detection of acceleration in the mode is very advantageous in terms of improving sensitivity. Therefore, it is desirable to detect acceleration using the third method described above.

  FIG. 6 shows the configuration of a conventional acceleration sensor according to the third method. For convenience of explanation, the uvw axis of the orthogonal coordinate system is as shown in FIG. The piezoelectric element 40 is configured by forming the electrode 30 on the opposing surface of the piezoelectric body 41 having the spontaneous polarization direction as the v-axis direction and parallel to the polarization direction (parallel to the uv plane). Next, when the surface of the electrode 30 on one side of the piezoelectric element 40 is bonded to the substrate 50 and the other surface is bonded to the weight 20 on which the inertial force acts, the acceleration sensor 10 'is completed.

  When the acceleration sensor shown in FIG. 6 moves at an acceleration α in the v-axis direction (positive or negative), a shear stress that balances the inertia force of mα generated by the weight 20 of mass m is generated around the u-axis of the piezoelectric element 40, Piezoelectricity is generated by d15 (= d24), and a voltage is generated between the opposing electrodes 30. Since this voltage is substantially proportional to the acceleration α, it operates as an acceleration sensor.

  However, in the structure of the conventional acceleration sensor, it is necessary to bond the piezoelectric element 40 on which the electrode 30 is formed to the substrate 50 and the weight 20 while performing electrical connection, which has a problem that the manufacturing process becomes complicated. It was.

  Further, when the plate-like piezoelectric element 40 is used, there is a problem in that the capacitance increases and the output voltage decreases. On the other hand, when providing a plurality of sensor elements to increase the output voltage, it is necessary to separately provide wiring for series connection, resulting in an increase in sensor element area and man-hours. From the viewpoint, it was poor in practicality.

  The first aspect of the invention is to realize the detection in the thickness-slip mode of the piezoelectric body, which is the third method described above, while allowing the substrate and the weight to be bonded without going through the electrode surface. is there. That is, it has a weight on which an inertial force acts by acceleration, and a piezoelectric element composed of a piezoelectric body and an electrode and connected to the weight, the electrode being arranged in parallel with the spontaneous polarization direction of the piezoelectric body, Has achieved the object by connecting it to a plane perpendicular to the spontaneous polarization direction of the piezoelectric body.

  The invention described in claim 2 is intended to give the piezoelectric element a deformation in a thickness-shear mode due to a shear stress, not a deformation in a bending or expansion / contraction mode. That is, the weight according to claim 1 is realized by connecting to a plurality of the piezoelectric elements.

  The invention according to claim 3 is for selectively detecting the acceleration in the measurement direction. In the structure of the present invention, the direction of spontaneous polarization of the piezoelectric body is taken as three axes (Z axis), and the piezoelectric component with respect to shear stress around one axis (X axis) and two axes (Y axis) is d15 = d24 and the others If d1m (m = 1, 2, 3, 4, 6) and d2n (n = 1, 2, 3, 5, 6) are 0, only the acceleration in the normal direction of the electrode surface can be detected. It becomes possible. That is, the piezoelectric body constituting the piezoelectric element according to claim 1 and claim 2 is realized by a material mainly composed of crystals classified into point groups of 4 mm or 6 mm by international symbols. Table 1 shows a 4 mm or 6 mm piezoelectric component dij.

  The invention described in claim 4 is for realizing a structure in which a plurality of piezoelectric elements are easily connected in series. That is, in the acceleration sensor according to claims 1 to 3, a plurality of the piezoelectric elements are arranged so that their electrode surfaces are parallel to each other, and the object is achieved by connecting the elements in series.

  The invention according to claim 5 realizes simplification of the manufacturing process and reduction of man-hours. That is, in the acceleration sensor according to claim 4, the object is achieved by arranging a plurality of the piezoelectric elements by grooving the piezoelectric plate polarized in the plate thickness direction.

  The invention described in claim 6 is intended to provide a plurality of piezoelectric strings connected in series in order to enable further increase in output voltage and amplification with differential output. In other words, the acceleration sensor according to claim 4 and 5 is realized by forming a plurality of series of piezoelectric element arrays by forming a plurality of division grooves deeper than the grooves in the normal direction of the electrode surface.

  According to the present invention, since a plurality of piezoelectric elements can be formed and connected in series by a simple process, it is possible to provide a high-sensitivity acceleration sensor excellent in mass productivity at low cost.

  Hereinafter, the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiments.

  As the piezoelectric material, a commercially available HIP-treated PZT material (5D system) classified into a point group of 4 mm by an international symbol was used. A piezoelectric plate having a piezoelectric constant d15 (= d24) of about 800 pC / N and a thickness of 0.8 mm polarized in the thickness direction was used. Of course, even a piezoelectric material such as ZnS, ZnO, CdS, AlN, etc., which is not a ferroelectric material but is classified into a point group of 6 mm where the independence of the piezoelectric components is completely equal, a single crystal material or an orientation material (film) is used. Therefore, it can be applied in principle.

  Further, if the acceleration directivity is not important, there is no particular need for the piezoelectric material having a point group of 6 mm or 4 mm, and any material may be used as long as it generates piezoelectricity by thickness sliding in 15 to 24 modes.

  The Curie temperature of this piezoelectric body was about 300 ° C., but it is necessary to control the process temperature to about 100 ° C. or lower in the electrode forming step and the bonding step described later. At least the aging temperature after the polarization treatment should not be exceeded.

  For the weight receiving the inertial force, PZT of the same material is used from the viewpoint of preventing thermal strain and the density (about 8,000 kg / m3). Of course, other ceramic materials, metal materials, etc. have no problem, but it is necessary to consider insulation from the electrode 30.

  FIG. 1 is a diagram illustrating a schematic configuration of an acceleration sensor 10 according to the first embodiment. For the sake of explanation, the direction of the spontaneous polarization Ps (the sign of the output voltage is only reversed so that it can be positive or negative) is the Z-axis, the acceleration detection direction is the Y-axis, and the orthogonal coordinates where the front side of the page is the X-axis Use the system.

  Electrodes 30 are formed on a pair of side surfaces parallel to the spontaneous polarization direction of a rectangular parallelepiped piezoelectric body 41 polarized in the thickness direction. Next, when the surface perpendicular to the polarization direction is fixed to the non-conductive substrate 50 with an adhesive (not shown) and the weight 20 which is a rigid body is bonded to the opposite surface, the acceleration sensor 10 which is the basis of the present invention. (See FIG. 1A).

  Next, when the acceleration sensor 10 is moved in the Y-axis + direction at an acceleration α, an inertial force mα acts on the mass 20 having a mass m, and a shear stress is generated on the joint surface between the piezoelectric body 41 and the mass 20. As a result, shear deformation (thickness slip) around the X-axis occurs to generate piezoelectricity (see FIG. 1B).

  Here, when the outer dimensions of the piezoelectric body 41 are set to the thickness W / 2 (Z-axis direction), the width W (Y-axis direction), and the depth W (X-axis direction) under the same conditions as in FIG. In the example, it was revealed that the output voltage was about twice as much as the conventional example, the capacitance of the piezoelectric element 40 was about 0.25 times, and the charge amount was 0.5 times.

  In the first embodiment, since the weight 20 is independently connected to the piezoelectric element 40, the structure is a cantilever beam supported by the substrate 50. Therefore, since not only shear deformation but also bending moment acts on the piezoelectric element 40, the linearity of acceleration detection may be deteriorated.

  Further, although the output voltage of one unit of the piezoelectric element 40 is small, if a plurality of piezoelectric elements 40 can be connected in series, the output signal can be increased. A second embodiment that solves these problems will be described below.

  The dry film 12 is laminated on a predetermined portion of the piezoelectric plate 42 that has been polarized in the thickness direction. (See FIG. 2A.) Next, a wall forming groove 14 having a groove width of about 100 μm and a groove depth of about 200 μm is formed in the X-axis direction using a dicing saw, and the wall is formed while being fed at a pitch of 200 μm in the Y-axis direction. When a plurality of formation grooves 14 are formed, the state shown in FIG.

  Thoroughly dry after flowing water ultrasonic cleaning with pure water, and after depositing Ti of the adhesion layer to 0.1 μm (in terms of flat portion precipitation) by sputtering with Ar gas, the conductive layer of Au is 0.3 μm without vacuum breakage. (Flat portion precipitation conversion) is continuously formed. Next, the dry film is swollen and peeled off with an organic solvent, and the metal on the free end face of the piezoelectric element 40 is lifted off together with the dry film 12.

  By this electrode formation and lift-off process, the electrode 30 of the plurality of piezoelectric elements 40, the bridge electrode 31 for connecting the adjacent piezoelectric elements 40 whose electrode surfaces are parallel to each other in series, and the electrode terminal 32 for extracting the signal output can be simultaneously formed. it can.

  In order to remove the electrode material attached to the four outer peripheral surfaces after the lift-off process, the vicinity of the four outer peripheral sides (side surfaces) is again sliced with a dicing saw to complete the piezoelectric element array 43 shown in FIG. FIG. 2D shows a part of the details of the A-A ′ cross section indicated by the alternate long and short dash line in FIG.

  Next, a thin epoxy adhesive (not shown) is applied to the weight 20, and is adhered and cured to the free end wall of the piezoelectric element array 43. FIG. 3E shows a configuration diagram of the completed acceleration sensor 10 according to the second embodiment, and FIG. 3F shows a B-B ′ cross-sectional view in the drawing. With such a configuration, it is possible to produce a piezoelectric element array 43 in which piezoelectric elements 40 as shown in the circuit diagram of FIG. 3G are connected in series at high density, and a high-sensitivity acceleration with a high output voltage. A sensor 10 was obtained.

  In the second embodiment, the piezoelectric plate 42 is provided with a support function without providing the substrate 50. However, the thin piezoelectric plate 42 may be separately attached to the substrate material.

  In the second embodiment, the acceleration sensor in which the piezoelectric elements 40 are connected in series in one row has been described. In the third embodiment, an acceleration sensor in which a plurality of piezoelectric element arrays 43 are formed will be described.

  An implementation method from the piezoelectric element array 43 before the weight 20 shown in FIG. 2 (f) in the process of Example 2 will be described with reference to FIGS.

  FIG. 4A shows the relationship between the piezoelectric element array 43 of Example 2 and the XYZ orthogonal coordinate system. When one or more electrode cutting grooves 16 having a larger depth of cut than the wall forming groove 14, ie, having a deep groove depth, are formed in the Y-axis direction perpendicular to the wall forming groove 14, the X as shown in FIG. A piezoelectric element array unit 44 having a plurality of piezoelectric element arrays 43 formed in the axial direction is completed. A front view (−X axis direction) of FIG. 4B is shown in FIG. 4C, and a side view (Y axis direction) is shown in FIG. 4D.

  When the weight 20 is bonded in the same manner as in the previous embodiment, the acceleration sensor 10 according to the third embodiment (see FIG. 5A) is completed. Note that FIG. 5F is a cross-sectional view taken along the alternate long and short dash line in FIG. 5E. An equivalent circuit of the acceleration sensor 10 of this embodiment is shown in FIG.

  Next, if a plurality of piezoelectric arrays 43 are connected in series, for example, by connecting B1 and A2, B2 and A3,..., Bn-1 and An in FIG. An acceleration sensor capable of extracting an output voltage about n times that of the piezoelectric element array 43 (one row) shown in FIG. 3G was obtained between the terminals.

  Further, for example, by connecting the A2 terminal and the B1 terminal and grounding, and differentially amplifying the voltage generated between the A1 terminal and the B2 terminal, not only the output voltage is high but also the common mode noise is removed. An acceleration sensor that can extract high output signals was obtained.

It is a figure explaining the acceleration sensor by Example 1 of this invention. It is a figure explaining the acceleration sensor by Example 2 of this invention. FIG. 3 is a diagram illustrating an acceleration sensor according to a second embodiment of the present invention following FIG. 2. It is a figure which shows the structure of the acceleration sensor by Example 3 of this invention. FIG. 5 is a diagram illustrating a conventional example according to Example 3 following FIG. 4. It is a figure explaining a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Acceleration sensor 10 'Acceleration sensor 12 Dry film 14 Wall formation groove | channel 16 Electrode cutting groove | channel 20 Weight 30 Electrode 31 Bridge electrode 32 Electrode terminal 40 Piezoelectric element 41 Piezoelectric body 42 Piezoelectric plate 43 Piezoelectric element array 44 Piezoelectric element array unit 50 Substrate

Claims (6)

A weight on which inertial force is applied by acceleration;
A piezoelectric element composed of a piezoelectric body and a pair of electrodes sandwiching the piezoelectric body, and connected to the weight;
The electrode is disposed parallel to the spontaneous polarization direction of the piezoelectric body;
An acceleration sensor in which the weight is connected to a surface perpendicular to the spontaneous polarization direction of the piezoelectric body.   The acceleration sensor according to claim 1, wherein the weight is connected to a plurality of the piezoelectric elements.   The acceleration sensor according to claim 1 or 2, wherein the piezoelectric body constituting the piezoelectric element is mainly composed of crystals classified into point groups of 4 mm or 6 mm by international symbols.   The acceleration sensor according to any one of claims 1 to 3, wherein a plurality of the piezoelectric elements are arranged so that their electrode surfaces are parallel to each other, and the elements are connected in series.   The acceleration sensor according to claim 4, wherein a plurality of the piezoelectric elements are arranged by performing groove processing on the piezoelectric body polarized in the plate thickness direction.   6. The acceleration sensor according to claim 4, wherein a plurality of division grooves deeper than the groove are formed in a direction normal to the electrode surface, and a plurality of series piezoelectric element arrays are formed.

Images