VIBRATION SENSOR

Abstract: A vibration sensor (K) including a piezoelectric vibrator (3) constituted by a pair of plate-like piezoelectric elements (1,2) bonded to each other with the polarization axis aligned in the direction of the thickness. The vibration sensor (K) further includes a vibrator (5) of a bending vibration mode, which is subject to resonance at a predetermined frequency and is locally provided in the piezoelectric vibrator (3) such that a portion of the piezoelectric vibrator (3), disposed peripherally of the vibrator (5) of the bending vibration mode, is fixedly supported.

Description: VIBRATION SENSOR BACKGROUND OF THE INVENTION The present invention generally relates to vibration sensors for detecting elastic vibrations in vibrating members and more particularly, to a resonance type 4 vibration sensor and an acceleration sensor which are excellent in detection sensitivity at a predetermined frequency. Conventionally, rectangular bimorph piezoelectric vibrators of a bending vibration mode, having a cantilever configuration, in which the resonance frequencies are arranged to coincide with natural frequencies of vibrating members or the detection sensitivity is improved as compared with non-resonance type piezoelectric vibrators, and disklike bimorph piezoelectric vibrators in which peripheral portions of the piezoelectric vibrators are fixedly supported through utilization of a surface bending mode of the disks have been widely employed in vibration sensors for detecting elastic vibrations in the vibrating members. Such known rectangular or disk-like piezoelectric vibrators have such advantages that the rectangular or disk-like piezoelectric elements can be manufactured accurately because of their simple shapes and their vibration modes can be easily analyzed and handled. However, the known piezoelectric vibrators have such an inconvenience that, although a portion of each of the piezoelectric vibrators must be stably clamped in order to stabilize their resonance frequencies, supporting members made of metals, etc. for fixedly supporting the portion of each of the piezoelectric vibrators are displaced due to stresses produced mechanically or for various causes such as temperature variations, etc. and thus, it is difficult to d fixedly support the portion of each of the piezoelectric vibrators. Furthermore, the known piezoelectric vibrators have such a disadvantage that, since their resonance frequencies vary widely due to dimensional errors produced in machining or inaccurate assembly of their components during manufacturing processes thereof, adjustments for setting the known piezoelectric vibrators at the predetermined resonance frequencies are required to be made. Accordingly, in the case of the known rectangular bimorph piezoelectric vibrators, lengths of the vibrating portions are properly adjusted through cutting, grinding, etc. Meanwhile, in the case of the known disk-like bimorph piezoelectric vibrators, the above described adjustments are required to be made by replacing the supporting members, etc. Thus, in both of the prior art rectangular and disklike bimorph piezoelectric vibrators, the adjustments for setting them at the predetermined resonance frequencies are complicated and considerably troublesome. Furthermore, the prior art rectangular and disk-like bimorph piezoelectric vibrators have been disadvantageous in that it is difficult to perform the manufacturing operations since clamping conditions of the piezoelectric vibrators vary in accordance with clamping forces of the supporting members, with the result that the piezoelectric elements are damaged in the case where the clamping forces of the supporting members are too large. SUMMARY OF THE INVENTION 4 Accordingly, an essential object of the present invention is to provide an improved vibration sensor which can stably detect elastic vibrations in vibrating members, with substantial elimination of the disadvantages inherent in conventional vibration sensors of this kind. Another important object of the present invention is to provide an improved vibration sensor of the above described type which is simple in structure, highly reliable in actual use and suitable for mass production at low cost. In accomplishing these and other objects according to one preferred embodiment of the present invention, there is provided an improved vibration sensor comprising: a piezoelectric vibrator constituted by a pair of first and second plate-like piezoelectric elements bonded to each other; said first and second piezoelectric elements having a polarization axis aligned in a direction of thickness thereof; and a vibrator of a bending vibration mode, which is subjected to resonance at a predetermined frequency and is locally provided in said piezoelectric vibrator such that a portion of said piezoelectric vibrator, disposed peripherally of said vibrator of the bending vibration mode, is fixedly supported. In accordance with the present invention, since the vibrator of the bending vibration mode is locally provided in the piezoelectric vibrator, the portion of the piezoelectric vibrator, disposed peripherally of the vibrator of the bending vibration mode, can be clamped more securely than those of the conventional piezoelectric vibrators of the cantilever configurations, thus stabilizing the resonance frequency of the vibrator of the bending vibration mode. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the foregoing description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Figs. 1(a), l(b) and l(c) are a top plan view, a cross-sectional view and a bottom plan view of a main portion of a vibration sensor according to a first embodiment of the present invention, respectively; Figs. 2(a) and 2(b) are an exploded perspective view and a cross-sectional view of the vibration sensor of Fig. l(a), respectively; ; Figs. 3(a), 3(b) and 3(c) are a top plan view, a cross-sectional view and a bottom plan view of a main portion of a vibration sensor according to a second embodiment of the present invention, respectively; Figs. 4(a), 4(b) and 4(c) are a top plan view, a cross-sectional view and a bottom plan view of a main portion of a vibration sensor according to a third embodiment of the present invention, respectively; Fig. 5(a) is a perspective view of a rectangular cantilever type vibrator of a bending vibration mode; and Fig. 5(b) is a graph explanatory of surface strain factors against lengths of a vibrating portion of the vibrator of Fig. 5(a). Before the description of the present invention proceeds, it is to noted that like parts are designated by like reference numerals throughout several views of the accompanying drawings. DETAILED DESCRIPTION OF THE DRAWINGS Referring now to the drawings, there is shown in Figs. l(a) to l(c), a vibration sensor K according to a first embodiment of the present invention. As shown in Fig. l(b), the vibration sensor K includes a pair of disklike piezoelectric elements 1 and 2 which are bonded to each other by a pair of sufficiently thin inner electrodes lb and 2b so as to be electrically connected to each other such that the piezoelectric elements 1 and 2 bonded to each other constitute a bonded piezoelectric vibrator 3, with the inner electrodes lb and 2b being provided on inner faces of the piezoelectric elements 1 and 2, respectively. The bonded piezoelectric vibrator 3 has a U-shaped slit 4 formed at a central portion thereof by a laser processing machine, etc. and extending therethrough. A rectangular upper electrode la is provided on an outer face of the piezoelectric element 1 remote from the inner electrode lb so as to be enclosed by the slit 4. Meanwhile, a substantially rectangular lower electrode 2a is provided on an outer face of the piezoelectric element 2 remote from the inner electrode 2b so as to extend linearly and continuously from a portion enclosed by the slit 4 to a peripheral edge portion of the piezoelectric element 2. Namely, the bonded piezoelectric vibrator 3 has the upper electrode la and the lower electrode 2a provided on upper and lower faces thereof, respectively. When a peripheral portion of the bonded piezoelectric vibrator 3, extending from portions disposed peripherally of the slit 4 to a peripheral edge of the bonded piezoelectric vibrator 3, is supported, as a supported portion, by supporting members, a rectangular portion of the bonded piezoelectric vibrator 3, enclosed by the slit 4, functions as a so-called cantilever type vibrator 5 of a bending vibration mode. An output voltage generated through vibration of the vibrator 5 of the bending vibration mode is applied between the upper electrode la and the lower electrode 2a of the bonded piezoelectric vibrator 3 and is further taken out by lead wires 6a and 6b which are, respectively, connected to the upper electrode la and the lower electrode 2a. Assuming that the vibrator 5 of the bending vibration mode has a thickness T and a length S, a resonance frequency Fr of the vibrator 5 is proportional to a value of T/S2. Accordingly, the vibrator 5 can be set at a predetermined resonance frequency by properly determining values of the thickness T and the length S. In the above described arrangement of the vibration sensor K, since the vibrator 5 of the bending vibration mode is locally provided in the bonded piezoelectric vibrator 3, variations of the resonance frequency of the vibrator 5 of the bending vibration mode, resulting from changes in clamping conditions of the supported portion of the bonded piezoelectric vibrator 3, are kept minimal, so that the resonance frequency of the vibrator 5 of the bending vibration mode is made stable. Namely, variations of the resonance frequency of the vibrator 5 of the bending vibration mode, resulting from, for example, displacement of the supported portion of the bonded'piezoelectric vibrator 3 due to stresses produced mechanically or for other causes such as temperature variations, etc., are kept minimal, so that the resonance frequency of the vibrator 5 of the bending vibration mode is made stable. Meanwhile, in the vibration sensor K, since the supporting members are not required to be positioned so accurately, the vibration sensor K can be assembled easily. Furthermore, since the vibrator 5 of the bending vibration mode is locally provided in the bonded piezoelectric vibrator 3, the bonded piezoelectric vibrator 3 is not formed, at the upper and lower faces, with projections which are provided in conventional cantilever type vibrators, so that damage to the vibration sensor K through improper handling thereof can be advantageously eliminated. Moreover, since adjustments for setting the vibrator 5 of the bending vibration mode at the predetermined resonance frequency can be easily made by trimming the vibrating portion with a laser processing machine without the need for partially grinding the vibrating portions in conventional vibration sensors, the vibration sensor K is suitable for mass production. Referring now - to Figs. 2(a) and 2(b), the vibration sensor K includes a metallic supporting member 10 for supporting the bonded piezoelectric vibrator 3, which has a male screw portion 11 formed at one end thereof and threadingly inserted into a vibrating member, a stepped hole 12 formed at the other end thereof remote from the male screw portion ll, and a female screw portion 13 formed at an inlet of the stepped hole 12. The vibration sensor K further includes an annular supporting member 7 and a clamping member 8 having a male screw portion 9 engageable with the female screw portion 13 of the supporting member 10. After the bonded piezoelectric vibrator 3 has been inserted into the stepped hole 12 of the supporting member 10 such that the lower face of the bonded piezoelectric vibrator 3 is brought into contact with an annular intermediate bottom 12a of the stepped hole 12, the supporting member 7 is clamped onto an annular peripheral portion of the upper face of the bonded piezoelectric vibrator 3 by the clamping member 8 through engagement of the male screw portion 9 of the clamping member 8 with the female screw.portion 13 of the supporting member 10, whereby the bonded piezoelectric vibrator 3 is fixedly supported. Since the lower electrode 2a provided on the lower face of the bonded piezoelectric vibrator 3 is fixedly supported by the supporting member 10, the lower electrode 2a is electrically connected to the supporting member 10 and thus, the supporting member 10 is used as one terminal for taking out the output voltage of the vibrator 5 of the bending vibration mode. The lead wire 6a extending from the upper electrode la of the bonded piezoelectric vibrator 3 is used as the other terminal for the vibrator 5 of the bending vibration mode. Referring to Figs. 3(a) to 3(c), there is shown a vibration sensor K' according to a second embodiment of the present invention. The vibration sensor K' includes a bonded piezoelectric vibrator 3' having a pair of U-shaped slits 4' formed radially symmetrically in a side by side relationship with respect to each other such that a pair of rectangular vibrators 5' of a bending vibration mode are enclosed by the slits 4', respectively. The vibration sensor K' further includes a pair of upper electrodes la' and a pair of lower electrodes 2a' which are, respectively, provided on upper and lower faces of the bonded piezoelectric vibrator 3' in the same manner as described in the vibration sensor K. In the vibration sensor K', when a resonance frequency of one vibrator 5' of the bending 4 vibration mode is made equal to that of the other vibrator 5'; its detection sensitivity at a predetermined vibration frequency can be enhanced. Meanwhile, when the vibrators 5' of the bending vibration mode are, respectively, set at different resonance frequencies correlated to each other, its detection accuracy is improved. Referring to Figs. 4(a) to 4(c), there is shown a vibration sensor K'' according to a third embodiment of the present invention. The vibration sensor K'' includes a bonded piezoelectric vibrator 3'' having a substantially triangular slit 4' formed at a central portion thereof such that a vibrator 5'' of a bending vibration mode, enclosed by the slit 4'', is formed into a triangular shape. The vibration sensor K'' further includes a triangular upper electrode la'' and an arrow-shaped lower electrode 2a'' which are, respectively, provided on upper and lower faces of the bonded piezoelectric vibrator 3' ' in the same manner as described in the vibration sensor K. Since other constructions of the vibration sensors K' and K'' are similar to those of the vibration sensor K, description thereof is abbreviated for brevity. Hereinbelow, a rectangular vibrator of a bending vibration mode having a cantilever configuration, will be described with reference to Figs. 5(a) and 5(b). As shown in Fig. 5(a), the vibrator includes a pair of rectangular piezoelectric elements 14 and 15 which are bonded to each other and are fixedly supported, at one end portion thereof, by a pair of rectangular supporting members 16 such that one side face of.each of the supporting members 16 is flush with one end of the vibrator. Assuming that a length L of a vibrating portion of the vibrator for detecting vibrations of a vibrating member is measured from the other side face of each of the supporting members 16 remote from the one end of the vibrator towards the other end of the vibrator and the vibrating portion has a thickness H, surface strain factors at the vibrating portion are illustrated by the curved solid line in Fig. 5(b). Meanwhile, the linear broken line in Fig. 5(b) approximately illustrates the surface strain factors at the vibration portion by connecting opposite ends of the curved solid line in Fig. 5(b). It will be readily understood from Fig. 5(b) that the surface strain factors at the vibration portion of the vibrator of the bending vibration mode reach a maximum value at the time when the length L is zero, namely, at the starting-point of the length L. Consequently, when an extremely large force is applied to the vibrator, a portion of the vibrator, adjacent to the supporting members 16, is most readily damaged. Accordingly, in the present invention, it is so arranged that the slit 411 of the vibration sensor K" is formed on the bonded piezoelectric vibrator 3" by the use of a laser processing machine, etc. such that a vibrating portion of the vibrator 5" of the bending vibration mode for detecting vibrations of a vibrating member has an area 4 proportional to the surface strain factors shown in Fig. 5(b): Namely, since a portion of the vibrating portion of the vibrator 5 11, which has a large surface strain factor, is increased in area while another portion of the vibrating portion of the vibrator 5", which has a small surface strain factor, is decreased in area, the vibrating portion of the vibrator 5'' has a uniform surface strain factor. Accordingly, when a force is applied to a whole of the vibrating portion, a portion of the vibrating portion, adjacent to the supporting members, can be more effectively prevented from being damaged than those of the conventional rectangular vibrators of the bending vibration mode. Furthermore, since the vibrating portion of the vibrator 5" has the uniform surface strain factor, operational characteristics of the vibration sensor K'' can be kept stable for a long time. Meanwhile, numerical control units are widely employed in machine tools such as a laser processing machine, etc. but, in some cases, enable only linear cutting and cutting of circular interpolation, the consequence of which is that considerably complicated programs are required for machining arbitrary curves. In such cases, when the vibrator of the bending vibration mode is so manufactured as to have a surface strain factor corresponding to that shown in the broken line in Fig. 5(b), the surface strain factor of the vibrator of the bending vibration mode can be made more uniform than those of the conventional vibrators of the bending vibration mode. Moreover, when it is so arranged that, in order to increase mechanical strength of the bonded piezoelectric vibrator, the bonded piezoelectric vibrator is constituted by a plate-like piezoelectric element and a metallic plate each having a polarization axis aligned in a direction of thickness thereof or by a pair of plate-like piezoelectric elements each having a polarization axis aligned in a direction of thickness thereof in which direction a metallic plate is interposed therebetween, the same effect as described above can be obtained. In the case where the plate-like piezoelectric element is bonded to the mating metallic plate or the mating plate-like piezoelectric element by the use of organic adhesive in the bonded piezoelectric vibrator, the organic adhesive is carbonized through irradiation of laser beam thereto, so that the carbonized organic adhesive is caused to adhere to opposite wall faces of the slit, thereby incurring a high possibility that the inner electrodes disposed at the bonding faces of the bonded piezoelectric vibrator are short-circuited to the respective upper and lower electrodes of the bonded piezoelectric vibrator. In order to eliminate such a phenomenon, for example, glassy adhesive can be used as inorganic adhesive for bonding the plate-like piezoelectric element to the mating metallic plate or the mating plate-like piezoelectric element. The glassy adhesive has such advantages that it is not 4 carbonized even in a molten state in contrast with the organic adhesive, with the result that the above-described short-circuit between the electrodes of the bonded piezoelectric vibrator does not take place. Furthermore, in place of the glassy adhesive, a thin film of solder can be employed for soldering the inner electrodes to each other. Moreover, when it is so arranged that the bonding faces of either one of the plate-like piezoelectric element and the metallic plate or either one of the plate-like piezoelectric elements, and the other one of the plate-like piezoelectric element and the metallic plate or the other one of the plate-like piezoelectric elements are, respectively, formed as an electrode on which gold is deposited, and as an electrode which is obtained by depositing silicon on a surface of gold such that eutectic bonding of gold and silicon takes place at temperatures of a few hundred degrees centigrade, whereby the plate-like piezoelectric element can be bonded to the mating metallic plate or the mating platelike piezoelectric element. As is clear from the foregoing description, in accordance with the present invention, since the vibrator of the bending vibration mode is locally provided in the bonded piezoelectric vibrator, the portion of the piezoelectric vibrator, disposed peripherally of the vibrator of the bending vibration mode, can be clamped more securely than those of the conventional piezoelectric vibrators of the cantilever configuration, so that the resonance frequency of the vibrator of the bending vibration mode can be kept stable. Furthermore, in accordance with the present invention, the vibration sensor is made suitable for mass production. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as included therein.

Claims: WHAT IS CLAIMED IS: 1. A vibration sensor (K) comprising: a piezoelectric vibrator (3) constituted by a pair of first and second plate-like piezoelectric elements (1,2) bonded to each other; said first and second piezoelectric elements (1,2) having a polarization axis aligned in a direction of thickness thereof; and a vibrator (5) of a bending vibration mode, which is subjected to resonance at a predetermined frequency and is locally provided in said piezoelectric vibrator (3) such that a portion of said piezoelectric vibrator (3), disposed peripherally of said vibrator (5) of the bending vibration mode, is fixedly supported. 2. A vibration sensor (K') as claimed in Claim 1, wherein a plurality of said vibrators (5') of the bending vibration mode are locally provided in said piezoelectric vibrator (3'). 3. A vibration sensor (K") as claimed in Claim 1, wherein said vibrator (5") of the bending vibration mode is so formed as to have, in a longitudinal direction thereof, a uniform surface strain factor. 4. A vibration sensor as claimed in Claim 1, wherein either one of said first and second plate-like piezoelectric elements (1,2) is replaced by a metallic plate. 5. A vibration sensor as claimed in Claim 1, further including a metallic plate which is interposed between said first and second plate-like piezoelectric elements (1,2) in the direction of thickness thereof. 6. A vibration sensor as claimed in Claim 1, wherein said first and second plate-like piezoelectric elements (1,2) are bonded to each other by the use of inorganic adhesive or solder or through eutectic bonding of gold and 4 silicon. 7. A vibration sensor as claimed in Claim 4, wherein the other one of said first and second plate-like piezoelectric elements (1,2) is bonded to said metallic plate by the use of inorganic adhesive or solder or through eutectic bonding of gold and silicon. 8. A vibration sensor as claimed in Claim 5, wherein said first and second plate-like piezoelectric elements (1,2) are bonded to said metallic plate by the use of inorganic adhesive or solder or through eutectic bonding of gold and silicon.

Inventors: FUKADA, TETSUJI, ISE, YUKIHIKO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1983
Main classification: G01H1
IPC codes: G01H11, H01L41, G01H13