1. Field of the Invention
The present invention relates to acceleration sensors and manufacturing methods thereof.
2. Description of the Related Art
A known acceleration sensor utilizing piezoelectric ceramics is described in, for example, Japanese Unexamined Patent Application Publication No. 6-273439. The known acceleration sensor includes a piezoelectric element having a bimorph structure. The bimorph piezoelectric element is formed by bonding a pair of piezoelectric ceramic layers so as to face each other, providing an intermediate electrode between the piezoelectric ceramic layers, and providing signal extraction electrodes on the front and back faces of the bimorph piezoelectric element. The bimorph piezoelectric element is housed in and supported by a double beam structure. In the acceleration sensor, a center portion and both end portions of each of the piezoelectric ceramic layers in the longitudinal direction are polarized in opposite directions. Thus, charge generated in the center portion and both end portions can be externally extracted. As a result, the charge extraction efficiency can be improved.
In the bimorph acceleration sensor, it is necessary to differently polarize the center portion and the end portions of each of the piezoelectric ceramic layer. To this end, surface electrodes which are separated from each other are formed on a surface of the piezoelectric ceramic layer, and, after polarization, a connection electrode which completely covers the surface electrodes is formed, whereby a signal extraction electrode is formed. An acceleration sensor which can reduce the burden of two-step electrode formation is proposed in Japanese Unexamined Patent Application Publication No. 8-166401.
In both of the above-described acceleration sensors, the piezoelectric element is formed by the two piezoelectric ceramic layers, and the capacitance of the piezoelectric element is relatively small. Thus, the charge sensitivity is not very high.
In order to improve the charge sensitivity, an acceleration sensor including a piezoelectric element formed by stacking three piezoelectric ceramic layers is proposed (see Japanese Unexamined Patent Application Publication No. 10-62445). In this case, the capacitance is increased by increasing the number of piezoelectric layers being stacked. As a result, the charge sensitivity can be improved. The structure is, however, limited in that the polarization direction cannot be reversed in the same piezoelectric layer. Thus, charge can be extracted only from the center portion of a piezoelectric crystal, and the charge extraction efficiency is not very high.
Accordingly, it is an object of the present invention to provide an acceleration sensor which can efficiently collect charge generated by applying acceleration and which has a high charge sensitivity and a high detection sensitivity.
It is another object of the present invention to provide a manufacturing method for efficiently manufacturing an acceleration sensor which is thin and small and which has a high detection sensitivity.
The foregoing objects are achieved through provision of an acceleration sensor and a manufacturing method thereof according to the following aspects of the present invention.
According to a first aspect of the present invention, an acceleration sensor is provided including a piezoelectric element which is formed by stacking an even number of piezoelectric layers greater than or equal to six layers; support members for supporting both ends of the piezoelectric element in the longitudinal direction; and electrodes which are provided in between the layers and on the front and back faces of the piezoelectric element. The interlayer electrodes include electrodes which are segmented into portions in the longitudinal direction near inflection points between an expansion stress and a contraction stress applied to the piezoelectric element in response to the application of an acceleration and lead electrodes led to the ends of the piezoelectric element in the longitudinal direction. The two types of interlayer electrodes are alternately stacked with the piezoelectric layers therebetween. The interlayer electrode in the middle of the piezoelectric element in the thickness direction is the lead electrode led to the end in the longitudinal direction. The electrodes on the front and back faces of the piezoelectric element are led to the ends of the piezoelectric element in the longitudinal direction in order to extract generated charge. The piezoelectric layers are polarized in the thickness direction so that, when the acceleration is applied, charge having the same polarity is extracted from the lead electrodes led to the ends in the longitudinal direction in the piezoelectric layers on both sides of the lead electrodes and so that the center portion and both end portions of each piezoelectric layer are polarized in opposite directions.
According to a second aspect of the present invention, an acceleration sensor is provided including a piezoelectric element which is formed by stacking an odd number of piezoelectric layers greater than or equal to five layers; support members for supporting both ends of the piezoelectric element in the longitudinal direction; and electrodes which are formed in between the layers and on the front and back faces of the piezoelectric element. The interlayer electrodes include electrodes which are segmented into portions in the longitudinal direction near inflection points between an expansion stress and a contraction stress applied to the piezoelectric element in response to the application of an acceleration and lead electrodes led to the ends of the piezoelectric element in the longitudinal direction. The interlayer electrodes which are situated on both sides of the piezoelectric layer in the middle of the piezoelectric element in the thickness direction are the lead electrodes led to the ends in the longitudinal direction. The two types of interlayer electrodes are alternately stacked with the piezoelectric layers therebetween, excluding the piezoelectric layer in the middle in the thickness direction. The electrodes on the front and back faces of the piezoelectric element are led to the ends of the piezoelectric element in the longitudinal direction in order to extract generated charge. Among the piezoelectric layers, the piezoelectric layer in the middle in the thickness direction is not polarized. The other piezoelectric layers are polarized in the thickness direction so that, when the acceleration is applied, charge having the same polarity is extracted from the lead electrodes led to the ends in the longitudinal direction in the piezoelectric layers on both sides of the lead electrodes and so that the center portion and both end portions of each piezoelectric layer are polarized in opposite directions.
The acceleration sensor according to the first aspect of the present invention has a structure in which the piezoelectric element includes an even number of piezoelectric layers greater than or equal to six layers. The acceleration sensor according to the second aspect of the present invention has a structure in which the piezoelectric element includes an odd number of piezoelectric layers greater than or equal to five layers. In both structures, the number of piezoelectric layers is greater than that of a two-layer acceleration sensor. Thus, the capacitance can be increased. Because the center portion and both end portions of each of the piezoelectric layers are polarized in opposite directions, charge generated in the center portion and both end portions of the piezoelectric element can be efficiently collected, thus increasing the charge extraction efficiency. As a result, although the acceleration sensor has a multi-layered structure, generated charge can be extracted from both the center portion and the end portions. Thus, the acceleration sensor has a higher charge sensitivity than previously achieved.
When the acceleration sensor has a structure in which both ends of the piezoelectric element are supported, the center portion and both end portions of the piezoelectric element are subjected to different stresses (contraction and expansion) in response to the application of an acceleration. In order to obtain generated charge having the same polarity, it is necessary to reverse the polarization direction within each layer. Specifically, when polarizing each layer, it is necessary to apply voltages having different polarities to the center portion and to both end portions. In order to prevent short-circuits, it is necessary to electrically isolate the surface electrodes and the interlayer electrodes according to each region to which voltage is applied. At the same time, charge can be collected by electrically connecting the electrodes in the regions. Generally, it is necessary to electrically connect the electrodes in the regions after polarization. However, when the electrodes are isolated inside the ceramic, it is technically impossible to connect the electrodes after the polarization in such a layered compact generated by simultaneously firing the electrodes and the piezoelectric ceramic.
According to the present invention, electrodes (segmented electrodes) inside the ceramic and electrodes (lead electrodes) led to the ends of the piezoelectric element in the longitudinal direction are alternately formed, and polarization is performed in between these electrodes, thus achieving a structure in which the center portion and both end portions are polarized in different directions. By collecting charge from the lead electrodes, it is possible to efficiently extract the generated charge.
In a case in which the number of piezoelectric layers is 4n (n is an integer greater than or equal to 2) and a case in which the number of piezoelectric layers is 4n+2 (n is an integer greater than or equal to 1), polarization electrodes formed on the front and back faces of a piezoelectric ceramic fired compact are different in form. Specifically, when the number of piezoelectric layers is 4n+2, it is necessary to form segmented polarization electrodes on the front and back faces of the piezoelectric ceramic fired compact. With the segmented electrodes, the generated charge cannot be extracted from the ends in the longitudinal direction. It is thus necessary to form a connection electrode for connecting the segmented electrodes. Alternatively, the segmented electrodes can be removed, and subsequently new electrodes led to the ends of the piezoelectric element in the longitudinal direction can be formed. As a result, the charge can be extracted.
When the number of piezoelectric layers is 4n, the polarization electrodes are provided by forming lead electrodes led to the ends in the longitudinal direction on the front and back faces of the piezoelectric ceramic fired compact. These electrodes can be used as electrodes from which charge can be extracted.
According to a third aspect of the present invention, a manufacturing method for manufacturing an acceleration sensor is provided. Segmented electrodes and electrodes connected in the longitudinal direction are alternately stacked. Prior to cutting a piezoelectric ceramic fired compact (which functions as a base) into individual elements, the segmented electrodes are externally led. By applying a DC electric field in between the segmented electrodes and the electrodes connected in the longitudinal direction, polarization is performed so that the center portion and both end portions of each piezoelectric layer in the longitudinal direction are polarized in opposite directions. According to the present invention, ceramic green sheets are stacked, and firing of the ceramic green sheets and baking of a conductive paste are simultaneously performed. Although the acceleration sensor has a multi-layered structure, a thin piezoelectric element can be achieved. Thus, the capacitance can be improved. Because the piezoelectric ceramic fired compact is cut into elements after polarization, the manufacturing method is suitable for mass production and is capable of creating uniform piezoelectric elements.
According to an acceleration sensor of the present invention, a piezoelectric element has a structure obtained by stacking an even number of piezoelectric layers greater than or equal to six layers or an odd number of piezoelectric layers greater than or equal to five layers. Charge generated in response to the application of an acceleration can be extracted from the center portion and both end portion of each piezoelectric layer. The capacitance can be increased, and charge can be collected efficiently. As a result, it is possible to achieve an acceleration sensor with a high detection sensitivity.