Vibration/acceleration sensor

Abstract: Disclosed is a vibration/acceleration sensor of a structure wherein a basic sensor unit is fixed to a casing (8) mounted on a body whose vibrations are to be detected and is surrounded by a resinous material (7) having a low heat transfer coefficient, the basic sensor unit comprising a vibration detecting unit (3) including a piezoelectric bending vibrator (11), thin plates (4) each consisting of, for example, a resinous material having a low heat transfer coefficient, provided respectively on the upper and lower surfaces of the vibration detecting unit (3), and a signal processing circuitry (5) formed on the upper thin plate (4a).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a vibration/acceleration sensor for detecting the vibration of a substance and the acceleration caused due to such vibration by utilizing the electromechanical conversion characteristics of a piezoelectric material. STATEMENT OF THE RELATED ART In general, conventional piezoelectric type vibration/acceleration sensors for detecting the elastic vibration of a vibrating body include those of a longitudinally effective type which utilize compression and tension in the thicknesswise direction of a piezoelectric element and those of a shear-effect type which utilize a shear force in the piezoelectric element. However, in the case where the resonance frequency is made to coincide with a specific vibration of a vibrating substance to detect only specified frequency components, or in the case where the vibration components in a specified region of frequencies are detected so as to increase the sensitivity in the region of low frequencies, a laterally effective type vibrator, i.e., a cantilever type vibrator utilizing a bending vibration mode has hitherto been widely employed. In case of the vibration/acceleration sensor having the cantilever type vibrator structure, difficulty is encountered in satisfying conditions for fixing one end of the vibrator. In order to solve this problem, one conventional technique such as that disclosed in Japanese Patent Unexamined Publication No. 59-70923 stabilizes the fixing of one end of the vibrator with the use of a cantilever type bending vibrator constituting a vibration detecting portion which is fabricated by forming a slit in a laminated plate-like, such as disc-like piezoelectric element, the bending vibrator being integral, at one end, with the laminated plate-like piezoelectric element, and by fixing the outer surface of the bending vibrator in place and supporting the same, thereby it is possible to stabilize the fixing condition of the vibrator. The above-described prior art piezoelectric vibration/acceleration sensor detects a mechanical force such as acceleration and, at the same time, generates electrical charge in accordance with variations in the ambient temperature. This is due to the pyroelectric effect of a piezoelectric material and the generation of an electrical charge is given by the following formula: dQ/dt = k dT/dt (1) where Q represents the electrical charge, T the temperature, t the time, and k a proportional constant. Namely, the generation of this electrical charge can not be distinguished from the electrical charge produced due to an acceleration desired to be detected, causing a large error in detecting the degree of acceleration. The acceleration sensor of such a structure that two sheets of piezoelectric materials are bonded together can theoretically make mutual cancellation of their generated charges in both types, that is, a serial type in which the two sheets of piezoelectric material are bonded together so that the directions of the polarizing axes may become opposite to each other and that the respective opposing surfaces of the two bonded sheets may serve as signal take-out electrodes, and a parallel type in which the respective opposing surfaces of the two upper/lower bonded sheets are connected together to serve as a signal take-out electrode and the bonded surface electrode is also used as a signal take-out electrode. However, in the conventional acceleration sensor, transfer of heat to the piezoelectric material is non-uniform and in addition the electrical charges generated due to the pyroelectric effect fail to cancel out together under severe temperature conditions, so that output signals are generated from an asymmetrical electrode constituted by a bending vibration mode vibrator serving as the signal take-out portion. This makes it impossible to provide an acceleration sensor capable of highly precise detection. Patent Abstract of Japan, volume 13, No: 409 (p-931)[3757] & Jp-a-1148970 discloses a vibration/acceleration sensor comprising a casing for mounting a basic sensor unit on a body whose vibrations are to be detected, the basic sensor unit including a vibration detecting unit consisting of a piezoelectric vibrator of bending vibration mode and fixing members for fixing the bending vibration mode vibrator therebetween, the fixing members being made of a material having high thermal conductivity. 1 SUMMARY OF THE INVENTION An object of the present invention is to provide a high-precision vibration/acceleration sensor which reduces the level of the output signals generated due to variations in ambient temperature. To attain the above object, the present invention is characterised over other vibration/acceleration sensors in that the fixing members are respectively formed with grooves which define a space for displacement of the vibrator having a depth equal to a displacement of the vibrator for a maximum permissible acceleration, the basic sensor unit also includes thin plates provided respectively on upper and lower surfaces of the detecting unit and a signal processing unit provided on at least one of said plates, and a resinous material enclosing said basic unit. In the vibration detecting unit in which metal fixing members are fixed to the bending vibration mode vibrator on both upper and lower surfaces thereof, the ambient heat is substantially uniformly transferred to the vibrator by way of the fixing members. Therefore, the temperature gradient, which varies depending upon position, becomes moderate. In addition, the thin plates such as those formal of resinous materials having low heat transfer coefficients are provided, respectively, on the top and bottom surfaces of the vibration detecting unit, and the resulting unit is enclosed by resinous materials. Therefore, transfer of heat up to the vibration detecting unit is retarded, resulting in that an undesirable effect upon a sensor output becomes small. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a vibration acceleration sensor in an embodiment form of the present invention; Fig. 2 is an exploded perspective view thereof; Fig. 3 is an exploded perspective view showing the vibration detecting unit; and Fig. 4 is a perspective view showing another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT A vibration/acceleration sensor in an embodiment form of the present invention will now be described in detail with reference to the drawings. Fig. 1 is a sectional view showing a vibration/acceleration sensor in an embodiment form of the present invention, Fig. 2 is an exploded perspective view showing the vibration detecting unit. Fig. 3 is an exploded perspective view illustrating vibration detecting unit. Prepared is a piezoelectric bond element 1 of a structure in which plate-like piezoelectric components 1a, 1b having polarizing axes in the thicknesswise direction and formed with symmetrical electrodes, respectively, on their opposing surfaces are bonded together. And a "U"-like slit 10 is formed in the piezoelectric bond element 1 by, for example, a laser beam machine. Thus, the portion which is enclosed by the slit 10 constitutes a bending vibration mode vibrator 11 having a cantilever type structure. In the case where the bending vibration mode vibrator 11 is of a serial type in which the plate-like piezoelectric members 1a, 1b are bonded together in such a manner that the directions of their polarizing axes are opposite to each other, the upper and lower surface electrodes thereof serve as output take-out electrodes. In the case where the bending vibration mode vibrator 11 is of a parallel type in which the plate-like piezoelectric members 1a, 1b are bonded together with their polarizing axes being directed in the same direction, the upper and lower surface electrodes thereof are connected together and these electrodes and the bonded surface electrodes are used as output signal take-out electrodes. The vibration detecting unit 3 is one which has been prepared by clamping the piezoelectric element 1 constituting the bending vibrator between fixing members 2a, 2b which are respectively formed with grooves 12a, 12b in a displacing section of the bending vibrator 11, outer periphery of the bending vibrator 11 in order to secure the fixing members 2a, 2b being made of copper having a high heat transfer coefficient (approximately 400 W/mk), and then being secured together by an epoxy-based adhesive agent. By making the vibration detecting unit 3 into such a structure as mentioned above, the ambient heat is transferred to the bending vibrator 11 via the fixing members, so that transfer of heat to the bending vibrator 11 constituting the acceleration detecting portion is substantially uniformly effected from the surrounding fixing portions, thus reducing the temperature gradient which varies depending upon positions. In addition, resinous thin plates 4a, 4b, such as print circuit boards, that is glass epoxy based print boards on which there is formed a signal processing circuit 5 including an impedance conversion circuit and a filter amplifier circuit, are mounted, respectively, on the top and bottom surfaces of the vibration detecting unit 3, thereby forming a basic sensor unit 6. The basic sensor unit 6 is fixed onto a casing 8 formed of, for example, metal onto a body whose vibration is to be detected. While variation in heat from the vibration detection body is transmitted by way of the casing 8, a time lag occurs until the heat is transferred up to the vibration detecting unit 3, on account of transfer of heat thereto via the print board having a low heat transfer coefficient. In consequence, signal components due to the pyroelectric effect come to have lower frequencies, and the degree of variation, with time, in temperature shown in the above-mentioned formula (1) becomes lower. This causes suppression of the signal. Thus, the undesirable effect upon the sensor signals can be reduced. Such undesirable effect can be further reduced by charging urethane resin into a space between the sensor cover 9 and the basic sensor unit 6. In this way, the output signals which are produced due to the variation in temperature are suppressed, thereby enabling a highly precise detection of the acceleration. Fig. 4 is a perspective view of a piezoelectric element portion in accordance with another embodiment of the present invention. By using a piezoelectric bond element 1 of a structure wherein plate-like piezoelectric members 1a, 1b are bonded together with intervention, therebetween, of a metallic thin plate 13 made of materials such as cobal, 42% Ni-Fe and the like, having a thermal expansion coefficient equal to, or approximately equal to that of the piezoelectric material, it is possible to increase the mechanical strength against impact such as that occurring at the time of, for example, a fall-down. If the depth of a space defined by the recesses 12a, 12b formed in the fixing members 2a, 2b is made equal to a displacement of the bending vibrator 11 upon permissible acceleration, the bending vibrator can be prevented from being displaced in response to an input greater than the permissible acceleration. This leads to protecting the bending vibrator 11. Thus, the bending vibrator comes to have a higher mechanical strength against impact at the time of, for example, a fall-down. According to the present invention, since the vibration detecting unit permits substantially uniform transfer of the surrounding heat to the vibrator by way of the fixing members, variations in temperature gradient depending upon positions is reduced. Further, the thin plates, each made of a low-heat-transfer-coefficient resinous material or the like, are attached, respectively, to the upper and lower surfaces of the vibration detecting unit and, in addition, such thin plates are covered by resinous material. Thus, time lag is effected until heat reaches the vibration detecting unit. Thus, the undesirable effect upon the sensor output can be reduced.

Claims: A vibration/acceleration sensor comprising a casing (8) for mounting a basic sensor unit (6) on a body whose vibrations are to be detected, the basic sensor unit including a vibration detecting unit (3) consisting of a piezoelectric vibrator (11) of bending vibration mode and fixing members (2a,2b) for fixing the bending vibration mode vibrator (11) therebetween, the fixing members being made of a material having high thermal conductivity, characterised in that the fixing members are respectively formed with grooves (12a,12b) which define a space for displacement of the vibrator (11) having a depth equal to a displacement of the vibrator (11) for a maximum permissible acceleration, the basic sensor unit includes thin plates (4a,4b) provided respectively on upper and lower surfaces of the detecting unit (3), a signal processing unit (5) is provided on at least one of said plates, and a resinous material encloses said basic unit (6). A vibration/acceleration sensor as claimed in claim 1, characterized in that said vibration detecting unit (3) includes a piezoelectric element (1) consisting of two plate-like piezoelectric components (1a,1b) bonded together, each having a polarizing axis in the thicknesswise direction and having electrodes on its upper and lower surfaces, wherein the bending vibration mode vibrator (11) is formed by forming a slit (10) in said piezoelectric element (1). A vibration/acceleration sensor as claimed in claim 1, characterized in that said thin plates (4a,4b) are formed of resinous material having a low heat transfer coefficient, and have print circuit boards formed of glass epoxy resin. A vibration/acceleration sensor as claimed in claim 2, characterized in that said plate-like piezoelectric components (1a,1b) are bonded together with a metallic thin plate (13) there between, whose thermal expansion coefficient is equal to, or approximately equal to, that of said piezoelectric element (1).

Inventors: FUKADA TETSUJI, KAINOU KIKUO, WAKAMIYA MASAYUKI, FUKADA, TETSUJI, KAINOU, KIKUO, WAKAMIYA, MASAYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: G01H11
IPC codes: G01P15, G01H11