Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-06-29
2003-03-04
Field, Lynn (Department: 2839)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000
Reexamination Certificate
active
06528927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a piezo actuator of the type having an actuator body and an electrically conductive multi-layer film for a control of expansion and contraction of the actuator body, wherein the multi-layer film has at least one carrier layer, at least one electrical conduction layer and at least one rigid electrical terminal element.
2. Description of the Prior Art
A piezo actuator of the above general type is disclosed, for example, in German PS 197 15 488.
A piezo actuator is constructed, for example, of a number of piezo elements arranged to form a stack-like actuator body. Each piezo element is composed of a piezo-ceramic layer that is provided with metallic electrodes on both sides. When an electrical voltage is applied to these electrodes, which are referred to as inner electrodes, then a surface charge of the piezo-ceramic layer changes. The piezo-ceramic layer reacts to this change of the surface charge with a lattice distortion (piezo-effect). As a result of the lattice distortion, the piezo element, and thus the actuator body expands and contracts in a direction that is defined by the arrangement of the piezo-ceramic layer and the electrodes of a piezo element. A usable change of the dimensions of the stack-like actuator body occurs in conformity with the amount of the expansion and contraction.
German PS 197 15 488 discloses such a piezo actuator having an actuator body in monolithic multi-layer structure. The actuator body is formed by at least one stack of alternating electrodes and piezo-ceramic layers. An electrode layer serves as electrode (inner electrode) for each neighboring piezo-ceramic layer. An electrical contacting of the electrode layers in an alternating polarity ensues for this purpose. The alternating polarity is achieved by means of two outer electrodes. Each outer electrode is formed by a metallization strip and an electrically conductive multi-layer film. An metallization strip is applied to a lateral surface of the actuator body. It thereby extends over a height that is determined by the electrically active layers of the actuator body stacked on top of one another. One metallization strip is electrically conductively connected to every other electrode layer and is electrically insulated from every first electrode layer lying therebetween. In contrast, the second metallization strip is insulated from every other electrode layer and is electrically conductively connected to every first electrode layer. Electrical insulation of an electrode layer relative to a metallization strip is achieved by a recess in the electrode layer. The electrode layer does not extend fully up to the lateral surface of the actuator body to which the metallization strip is attached. Such a region of the actuator body is referred to as piezoelectrically inactive region of the actuator body.
In order to assure the electrical contacting of every individual electrode layer, voltage supply to a metallization strip in the known piezo actuator ensues via a strip-like, electrically conductive multi-layer film in the form of a plastic film laminated with copper. The multi-layer film is thereby soldered to a metallization strip at an edge. The multi-layer film extends over the entire height of the electrically active layers of the actuator body. An outside edge of the multi-layer film facing away from the actuator body is connected to an rigid electrical terminal element. If a tear occurs in a metallization strip, it is electrically bridged by the multi-layer film. As a result, the piezo actuator exhibits a high cycle number, and thus a long service life. A cycle is a one-time expansion and contraction of the piezo actuator or of the actuator body.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a piezo actuator that exhibits an improved electrical contacting compared to known piezo actuators and thus a higher number of cycles and longer service life.
This object is achieved in a piezo actuator according to the invention having an actuator body and an electrically conductive multi-layer film for control of expansion and contraction of the actuator body, wherein the multi-layer film has at least one carrier layer, at least one electrical conduction layer and at least one rigid electrical terminal element. In the multi-layer film the electrical conduction layer has at least one recess. As used herein, “recess” is a generic term which includes, inter alia, depressions, holes, gaps, thinned regions, or any type of intentionally produced removal of material in a localized area of the electrical conduction layer.
As described above, the multi-layer film can be indirectly connected to the inner electrodes of the actuator body via a metallization strip, however, the multi-layer film also can be directly arranged at the inner electrodes. For example, the multi-layer film can be soldered to the inner electrodes. The electrical terminal element serves for electrical contacting of the electrical conduction layer of the multi-layer film. For example, it is soldered to an edge of the multi-layer film facing away from the actuator body so that the terminal element projects beyond the multi-layer film. The terminal element can also be directly applied on a surface of the conduction layer.
Due to the expansion and contraction of the actuator body, mechanical stresses occur in an outer electrode that is secured to the actuator body in the direction of the expansion and contraction. These mechanical stresses can lead to a tear in the outer electrode. Such a tear can occur not only in a metallization strip, but also can occur in an electrically conductive metallization strip, or can continue into the multi-layer film proceeding from the metallization strip. The tear can expand further during the course of operation of the piezo actuator and lead to a failure of the piezoelectrical actuator. As a result, an entire outer electrode with metallization strip and multi-layer film can be separated and the piezo actuator can thus fail.
The basis of the invention is to minimize the mechanical stresses in an electrically conductive multi-layer film during the operation of the piezo actuator. These mechanical stresses are especially large, for example, when a tear has arisen in the piezoelectrically inactive region of the actuator body due to a polarization of the piezo actuator (polarization tear). A relatively large change of the expanse of the actuator body occurs in the expansion and contraction given such a tear. High mechanical stresses occur in the multi-layer film due to this large change, particularly when the multi-layer film is firmly connected to a rigid terminal element. These stresses are reduced with suitable measures.
An electrical conduction layer of the multi-layer film can be, for example, a metal layer. In general, a metal layer is not very flexible and is thus not capable of alleviating mechanical stresses that occur. A particular goal of the invention is therefore to increase the flexibility of the electrical conduction layer.
Above all, the increase in the flexibility is achieved by introducing a recess, or at least a thinned region, into the conduction layer. The conduction layer thus has one or more recesses. For example, each recess can have a diameter that is in the range between 20 and 200 &mgr;m. Shape, size, placement and number of the recesses in the conduction layer are designed such that the multi-layer film is, in particular, flexible in the direction of the expansion and contraction of the actuator body.
The recesses in the conduction layer are, in particular, arranged in a contacting region to which the multi-layer film is attached to the actuator body. Especially large tensile and compressive strains occur in such a contacting region during the operation of the piezo actuator. The recesses, however, alternatively can be distributed over the entire conduction layer. They can also be arranged at the rigid electrical terminal element. The important feature is that the stresses p
Cramer Dieter
Hahn Iris
Lubitz Karl
Scherer Clemens
Schuh Carsten
Field Lynn
Siemens Aktiengesellschaft
Young & Thompson
Zarroli Michael C.
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